DarkRoom/ui/dr-ui/src/library.rs

//! TRACES: FR-CAT-1 | FR-CAT-4 | FR-NC-3 | NFR-P9
//! Opening a remote library: scan → catalog → grid.
//!
//! This is the wire between three pieces that already worked separately —
//! `dr_sync::scan` walks the tree, `dr_catalog` indexes it, and
//! `dr_decode::preview` turns bytes into pixels. Until now the "Open library"
//! button logged its intent and stopped.
//!
//! # Threading
//!
//! Slint's event loop is single-threaded and must never block (NFR-P9), so
//! every network and decode operation runs on a worker thread and results
//! return through an mpsc channel drained by a Slint timer. That is the same
//! shape the login flow uses; it is repeated rather than shared because the
//! message types differ and a generic version would obscure both.
//!
//! # Why thumbnails are fetched, not derived from the scan
//!
//! A scan yields paths and sizes, nothing visual. Each thumbnail costs its own
//! range request, so they are fetched **only for cells the grid actually
//! wants** — never for the whole library up front. On the reference library
//! that is the difference between a few MB and ~370 GB (ARCH §6.7).

use std::path::PathBuf;
use std::sync::mpsc::{Receiver, Sender};

use dr_catalog::{Catalog, JobKind, Priority};
use dr_sync::{Account, Connection, RemoteBackend, RemoteError, RemoteId, RemotePath};
use dr_thumbs::ThumbStore;

use crate::sidecar_cache::SidecarCache;
use dr_types::FormatFilter;

/// Largest preview worth fetching whole.
///
/// A located preview above this is skipped rather than transferred: past a few
/// MB the saving over the full file stops justifying the wait, and a 256px
/// thumbnail needs nothing like that much detail.
const MAX_PREVIEW_BYTES: u64 = 8 * 1024 * 1024;

/// Longest edge face indexing works at.
///
/// Not the full preview: `index_proxy` needs packed `f32` RGB, which is 12
/// bytes a pixel, so a 24 MP frame would be ~288 MB and the fetch lanes hold
/// one each. 3072 costs ~75 MB at the same moment and still puts a face 2%
/// across the frame at ~61 source pixels, against 5 on a grid thumbnail.
///
/// Raise it if the embedder is ever given a larger input than 112: it is the
/// resolution the *crop* is sampled from, so it bounds face quality directly.
const FACE_SOURCE_EDGE: u32 = 3072;

/// Progress and results from the scan worker.
#[derive(Debug)]
pub enum ScanMessage {
    /// Directories walked so far, and images found.
    Progress {
        directories: usize,
        pruned: usize,
        images: usize,
    },
    /// The scan finished and the catalog is populated.
    ///
    /// `found` counts what this scan *listed*, which on an incremental rescan
    /// is only what changed — pruned directories contribute nothing. `total`
    /// is what the catalog actually holds, which is what the grid shows.
    /// Conflating them made a successful no-op rescan report "0 images" and
    /// blank the library.
    Done {
        found: usize,
        total: usize,
        pruned: usize,
        elapsed_ms: u64,
        /// TRACES: FR-CAT-8 | FR-NC-9
        /// Judgements this scan took *in* from other devices' sidecars.
        ///
        /// Counted and reported rather than left to the log because it is the
        /// only visible sign that a cull made elsewhere has arrived. A grid
        /// that silently gains three hundred stars is indistinguishable from
        /// one that has gone wrong.
        judgements: usize,
    },
    /// The scan could not finish.
    ///
    /// `offline` distinguishes "the server could not be reached" from "the
    /// server refused", and it is carried here rather than re-derived because
    /// the classification is only possible on the worker side: crossing the
    /// channel flattens a [`dr_sync::RemoteError`] into a message, and no
    /// amount of string matching on the far side can reliably recover it.
    /// Without the flag a dead connection and a bad password produce the same
    /// banner, which sends the user to re-enter a credential that was fine.
    ///
    /// `lost_root` is the same idea one step further out, and it is carried
    /// separately from `offline` rather than folded into it because the two
    /// end differently. An offline library comes back when the network does,
    /// with nothing asked of anyone; a library whose root cannot be opened
    /// comes back only when someone restores access to it — a share put back
    /// on the server, a drive plugged in, and in time a document tree granted
    /// again once one can be (FR-PLAT-AND-2). Both show the same grid of what
    /// is stored locally, and they must not offer the same explanation.
    Failed {
        message: String,
        offline: bool,
        lost_root: bool,
    },
}

/// One decoded thumbnail, ready for the grid.
#[derive(Debug)]
pub struct ThumbnailReady {
    /// Index into the grid model this belongs to.
    pub row: usize,
    pub width: u32,
    pub height: u32,
    pub rgba: Vec<u8>,
    /// Whether these pixels came off local disk rather than the server.
    ///
    /// The grid paints both identically, so this exists solely for
    /// reachability: a store hit is evidence about the *cache*, not the
    /// network, and treating one as proof of connectivity clears offline mode
    /// before a single request has been attempted.
    pub from_cache: bool,
}

/// Capture metadata read from the same header the thumbnail needed.
///
/// Free: the header fetch happens either way, so parsing EXIF out of it costs
/// no extra transfer. That is what fills the timeline as the user browses,
/// rather than a separate 6 GB sweep over the library.
#[derive(Debug, Clone)]
pub struct MetadataFound {
    pub image_id: i64,
    pub captured_at: Option<i64>,
    pub captured_offset: Option<i32>,
    pub camera: Option<String>,
    pub lens: Option<String>,
    pub iso: Option<u32>,
}

/// Messages from the thumbnail worker.
#[derive(Debug)]
pub enum ThumbnailMessage {
    Ready(Box<ThumbnailReady>),
    /// No preview could be extracted. The cell stays a placeholder rather than
    /// silently retrying forever.
    Unavailable {
        row: usize,
        reason: String,
    },
    /// How the batch split between the store and the network.
    ///
    /// Sent once, before any fetch. Without it there is no way to tell a
    /// working cache from a broken one — both fill the grid, one just costs
    /// nothing.
    Plan {
        cached: usize,
        fetching: usize,
        dating: usize,
    },
    /// One header-only date read is starting.
    ///
    /// Reported separately from thumbnail progress: this work produces no
    /// visible cell, so without it the window looks idle while it runs.
    DateProgress,
    /// Capture dates were written to the catalog.
    ///
    /// The timeline is rebuilt on this rather than per image — a histogram
    /// that redrew 120 times during a batch would flicker for no benefit.
    DatesRecorded(usize),
    /// TRACES: FR-CAT-9
    /// The server could not be reached while filling this batch.
    ///
    /// Distinct from a run of [`Unavailable`](Self::Unavailable): those are
    /// per-image verdicts ("this file has no extractable preview") and leave
    /// the rest of the library alone, where this is a statement about the
    /// connection. Sent at most once per batch, because a dropped connection
    /// produces one of these per *cell* otherwise and the banner would be
    /// rewritten sixty times.
    Offline {
        reason: String,
    },
}

/// TRACES: FR-CAT-15 | FR-CAT-11
/// What it means for an image to be visible in the library.
///
/// Two exclusions, for two different reasons, and both must appear in *every*
/// query that counts or lists cells — the grid, the timeline, the metadata
/// sweep. A predicate present in four of five places is worse than absent: the
/// counts disagree with the cells and neither looks wrong on its own.
///
/// - `shadowed_by IS NULL` — a JPEG the camera wrote beside its RAW is that
///   same frame, not a second photograph.
/// - `trashed_at IS NULL` — a soft-deleted image has been moved to the trash
///   folder and is listed only by the trash view.
const VISIBLE: &str = "i.shadowed_by IS NULL AND i.trashed_at IS NULL";

/// [`VISIBLE`] for queries that do not alias `images`.
const VISIBLE_UNALIASED: &str = "shadowed_by IS NULL AND trashed_at IS NULL";

/// TRACES: FR-CAT-15
/// What the *trash view* lists: exactly what [`VISIBLE`] excludes on the second
/// clause, and still excludes on the first.
///
/// The inversion is deliberate and only correct on `trashed_at`. A shadowed JPEG
/// is not a separate photograph in the trash any more than it is in the library
/// — trashing a RAW takes its sibling with it, and listing both would offer to
/// restore the same frame twice.
const TRASHED: &str = "i.shadowed_by IS NULL AND i.trashed_at IS NOT NULL";

/// TRACES: FR-CULL-5
/// The clause that hides the frames a collapsed burst is standing in for.
///
/// Subject to exactly the discipline [`VISIBLE`] is under, and for the same
/// reason: the header's count, the scrollbar's size, the run a shift-click
/// resolves and the ordinal a scrub lands on are four answers about one list.
/// A burst folded away in the cells but still counted in the total would leave
/// the grid ending in rows that draw nothing, with no clue why.
///
/// The predicate itself is `dr_catalog::bursts`'s, not this file's, so the
/// interface and the pass that writes the table cannot come to disagree about
/// what collapsed means.
///
/// A function rather than a constant because it has to name the image table,
/// and the grid aliases it as `i` where the timeline's queries do not. `image`
/// is a table name from this file and never anything a user supplied.
fn uncollapsed(image: &str) -> String {
    format!(" AND {}", dr_catalog::bursts::not_collapsed_away(image))
}

/// TRACES: FR-CAT-4
/// The order the grid lists photographs in: when they were taken.
///
/// The file name breaks ties and nothing more — two frames of one burst, or a
/// RAW beside the JPEG the camera wrote with it. What a photographer looks for
/// is the afternoon, not what the camera called the file, and a grid ordered by
/// name interleaves every camera and every card that ever wrote into the same
/// folder.
///
/// Shared rather than spelled out per query, for the same reason [`VISIBLE`] is:
/// the window, the count and the run a shift-click resolves are three answers
/// about one list, and an ordering that drifted between them would select
/// photographs the user never saw without any of it looking wrong.
///
/// Undated images sort last in either direction. EXIF is read as thumbnails
/// load, so a freshly scanned library would otherwise open on the images it
/// knows least about.
const GRID_ORDER: &str = "ORDER BY i.captured_at IS NULL, i.captured_at ASC, i.source_ref ASC";

/// TRACES: FR-CAT-15
/// [`GRID_ORDER`] for the trash, which is ordered by when a thing was deleted —
/// see [`read_trashed_cells`] for why that view answers a different question.
const TRASH_ORDER: &str = "ORDER BY i.trashed_at DESC, i.source_ref ASC";

/// TRACES: FR-CAT-6 | FR-CULL-4
/// What the grid is narrowed to by the rating filter bar.
///
/// Applied in **SQL**, not by filtering the rows after reading them. On a
/// remote library a drawn-then-hidden cell has already cost a thumbnail
/// fetch, which is the transfer FR-NC-3 exists to avoid — and the count in the
/// header has to agree with the cells, which it cannot if the two are computed
/// at different stages.
/// Not `Copy`: [`RatingFilter::people`] is a `Vec`. Every query path already
/// takes this by reference, so the only casualties were two `..*self` struct
/// updates, which clone instead.
#[derive(Debug, Clone, PartialEq, Eq, Default)]
pub struct RatingFilter {
    /// Minimum stars. 0 means no star constraint.
    pub min_rating: u8,
    /// Only images nothing has judged yet — neither starred nor flagged.
    /// This is what lets a culling session resume where it stopped.
    pub unjudged: bool,
    /// `None` for no flag constraint, otherwise exactly that flag.
    pub flag: Option<dr_types::FlagState>,
    /// TRACES: FR-CAT-9
    /// Only images whose original is stored on this device.
    ///
    /// Carried here, beside the rating terms, because every query path already
    /// threads this one struct: adding a parallel parameter to
    /// `read_cells_scoped`, `read_cells_all` and both counts would give four
    /// call sites the chance to disagree about what the grid is showing, and
    /// the count disagreeing with the cells is the specific bug this type's
    /// "filter in SQL" rule exists to prevent.
    pub local_only: bool,
    /// Show only photographs captured within this range, as UTC seconds.
    ///
    /// Half-open ends are meaningful: a `from` with no `to` reads as
    /// "everything since". Undated images are excluded whenever either end is
    /// set — they cannot be placed on the axis the user is narrowing, and
    /// showing them anyway makes the range look broken.
    ///
    /// Here rather than a parallel parameter for the reason `local_only` gives
    /// above: the count and the cells must be narrowed by the same thing.
    pub captured_from: Option<i64>,
    pub captured_to: Option<i64>,
    /// TRACES: FR-CULL-11
    /// Only photographs these people appear in.
    ///
    /// The way back from a face to the pictures it came from, which is the
    /// question the People screen leaves the user holding: they have just
    /// identified someone, and what they want next is *everything with them in
    /// it*. Without this the identification is a dead end.
    ///
    /// Here rather than a grid scope of its own, for the reason `local_only`
    /// gives above — the count and the cells must be narrowed by the same
    /// thing, and this struct is the one narrowing every query path already
    /// threads. It composes with the rest for free: three-star photographs of
    /// Anna from last summer is this term ANDed with two others.
    ///
    /// Suggested faces count, not only confirmed ones. A user who has just
    /// grouped someone and not yet confirmed a single face would otherwise get
    /// an empty grid, which reads as "no photographs of this person" rather
    /// than "you have not ticked anything yet".
    ///
    /// A set rather than one id, because the two questions a photographer
    /// actually asks are "every picture of Anna *or* Bob" and "the pictures
    /// they are *both* in", and the second is not reachable by any sequence of
    /// single-person filters. [`RatingFilter::people_mode`] picks between them.
    pub people: Vec<u64>,
    /// Whether [`RatingFilter::people`] is a union or an intersection.
    pub people_mode: PeopleMode,
}

/// How several people combine when the grid is narrowed by identity.
///
/// TRACES: FR-CULL-11
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
pub enum PeopleMode {
    /// Photographs holding **any** of them — the union.
    ///
    /// The default, and the right one for one person, where the two modes are
    /// identical. It is also the forgiving direction: adding a second person to
    /// a union can only ever show more, so a user who has not noticed the
    /// toggle never ends up staring at an empty grid wondering what they broke.
    #[default]
    Any,
    /// Photographs holding **all** of them — the intersection.
    ///
    /// "Pictures of the two of them together", which is the one worth having a
    /// mode for: it is how you find the photograph you remember rather than
    /// scrolling everything either of them appears in.
    All,
}

impl RatingFilter {
    /// Whether this narrows anything, so the caller can skip the join.
    pub fn is_unfiltered(&self) -> bool {
        self.min_rating == 0
            && !self.unjudged
            && self.flag.is_none()
            && !self.local_only
            && self.captured_from.is_none()
            && self.captured_to.is_none()
            && self.people.is_empty()
    }

    /// Whether a date range is narrowing the grid.
    pub fn has_date_range(&self) -> bool {
        self.captured_from.is_some() || self.captured_to.is_some()
    }

    /// The same filter with the date range lifted.
    ///
    /// The timeline uses this: the histogram is how the range is *chosen*, so
    /// drawing it through the range would collapse the axis onto the current
    /// selection and leave nowhere to widen it back out from.
    pub fn without_date_range(&self) -> Self {
        Self {
            captured_from: None,
            captured_to: None,
            ..self.clone()
        }
    }

    /// The SQL predicate, against an `images` aliased as `i`.
    ///
    /// Returns a `String` of conditions ANDed together, or an empty string
    /// where nothing is constrained. Every branch is built from integers this
    /// code owns — no caller text reaches the SQL, so there is nothing to
    /// escape.
    ///
    /// A correlated subquery per term rather than a join to `versions`: an
    /// image with no version row must still be *findable* as unrated, and an
    /// inner join would silently drop exactly those images — the ones a
    /// library scanned before ratings existed consists entirely of.
    fn sql(&self) -> String {
        let mut terms = Vec::new();

        if self.min_rating > 0 {
            terms.push(format!(
                "coalesce((SELECT dv.rating FROM versions dv
                             WHERE dv.image_id = i.id AND dv.is_default = 1
                             LIMIT 1), 0) >= {}",
                self.min_rating
            ));
        }

        // Integers this code owns, formatted straight in like the rating terms
        // above — no caller text reaches the SQL.
        if let Some(from) = self.captured_from {
            terms.push(format!("i.captured_at >= {from}"));
        }
        if let Some(to) = self.captured_to {
            terms.push(format!("i.captured_at <= {to}"));
        }
        if self.has_date_range() {
            // An undated image cannot be inside or outside a range. Excluding
            // it is the honest answer; the comparisons above would drop it
            // anyway, and saying so keeps that from looking accidental.
            terms.push("i.captured_at IS NOT NULL".to_string());
        }

        if self.unjudged {
            // Both axes: a frame that was picked but never starred has been
            // judged, and re-presenting it would undo the user's decision to
            // move past it.
            terms.push(
                "coalesce((SELECT dv.rating FROM versions dv
                             WHERE dv.image_id = i.id AND dv.is_default = 1
                             LIMIT 1), 0) = 0
                 AND coalesce((SELECT dv.flag FROM versions dv
                             WHERE dv.image_id = i.id AND dv.is_default = 1
                             LIMIT 1), 0) = 0"
                    .to_string(),
            );
        }

        if !self.people.is_empty() {
            // Integers this code owns, like every other term here — the ids
            // come from the catalog, never from typed text, so there is
            // nothing to escape.
            let ids = self
                .people
                .iter()
                .map(|p| p.to_string())
                .collect::<Vec<_>>()
                .join(",");

            terms.push(match self.people_mode {
                // `EXISTS` rather than a join, so a photograph holding three
                // faces of the same person appears once — the grid shows
                // pictures, not faces.
                PeopleMode::Any => format!(
                    "EXISTS (SELECT 1 FROM faces f
                               JOIN face_person fp ON fp.face_id = f.id
                              WHERE f.image_id = i.id AND fp.person_id IN ({ids}))"
                ),
                // Counting *distinct* people rather than ANDing one EXISTS per
                // person: same result, one subquery instead of n, and it does
                // not grow the statement with the selection. `DISTINCT` is
                // what makes it correct — three faces of Anna in one frame
                // must not satisfy a filter asking for Anna and Bob.
                PeopleMode::All => format!(
                    "(SELECT COUNT(DISTINCT fp.person_id) FROM faces f
                        JOIN face_person fp ON fp.face_id = f.id
                       WHERE f.image_id = i.id AND fp.person_id IN ({ids})) = {}",
                    self.people.len()
                ),
            });
        }

        if let Some(flag) = self.flag {
            terms.push(format!(
                "coalesce((SELECT dv.flag FROM versions dv
                             WHERE dv.image_id = i.id AND dv.is_default = 1
                             LIMIT 1), 0) = {}",
                flag_code(flag)
            ));
        }

        if self.local_only {
            // `tier_actual`, not `tier_desired`: the question is what is
            // *here*, not what a pin has promised will be. An image queued for
            // download is exactly the one that cannot be opened yet, so
            // showing it under "on this device" would be the wrong answer to
            // the only question this filter is asked.
            terms.push(format!(
                "EXISTS (SELECT 1 FROM image_cache ic
                          WHERE ic.image_id = i.id AND ic.tier_actual >= {})",
                dr_types::Tier::Original.stored()
            ));
        }

        if terms.is_empty() {
            String::new()
        } else {
            format!(" AND ({})", terms.join(") AND ("))
        }
    }
}

/// The stored integer for a flag, matching `dr_catalog::rating`'s encoding.
fn flag_code(f: dr_types::FlagState) -> i64 {
    match f {
        dr_types::FlagState::Unflagged => 0,
        dr_types::FlagState::Pick => 1,
        dr_types::FlagState::Reject => 2,
    }
}

/// TRACES: FR-CAT-8 | FR-NC-8 | FR-CULL-4 | FR-DEV-6
/// One amendment to one image's sidecar, on its way to the server.
#[derive(Debug, Clone)]
pub struct SidecarWrite {
    /// Remote path of the *image*. The sidecar sits beside it, with the
    /// extension replaced — that adjacency is what makes a sidecar findable
    /// without an index (ARCH §6.12).
    pub image_path: String,
    pub version_uuid: String,
    pub amendment: Amendment,
}

/// What a write changes about the version it names.
///
/// An enum rather than a struct of optional fields because the two are written
/// by different actions with different failure costs, and because a write must
/// never carry a *stale* copy of what it is not changing. A settings write that
/// also carried a rating would have to have read one from somewhere, and the
/// obvious somewhere — the catalog, moments earlier — is exactly how a cull
/// made between the read and the write gets silently reverted.
///
/// Everything not named by the variant is left as the file had it, which is
/// what makes the read-modify-write in [`write_one_sidecar`] a genuine
/// amendment rather than a replacement.
#[derive(Debug, Clone)]
pub enum Amendment {
    /// A star rating and a pick/reject flag — the cull.
    Judgement { rating: u8, flag: u8 },
    /// TRACES: FR-DEV-6
    /// Copied develop settings, applied within `scope`.
    ///
    /// Carries the [`Scope`] rather than a pre-filtered preset so the target's
    /// own framing can be spared *at the file*: excluding framing means
    /// leaving the keys already in the sidecar untouched, which cannot be
    /// expressed by the parameter list alone.
    Settings {
        preset: dr_pipeline::Preset,
        scope: dr_pipeline::Scope,
        /// TRACES: FR-DEV-3f
        /// The film stock, when this is an image's own edit being written back.
        ///
        /// Two levels of `Option`, and both are load-bearing. The outer says
        /// whether this write concerns the film at all — a paste does not,
        /// exactly as it carries no masks. The inner is the choice itself, and
        /// `Some(None)` is a real edit: "develop this normally again". Without
        /// the distinction, clearing a film could never be saved.
        film: Option<Option<dr_pipeline::sidecar::FilmRef>>,
        /// TRACES: FR-DEV-5
        /// The named snapshots, when this is an image's own edit being
        /// written back: the ones the session holds, and the ids it deleted.
        /// A paste carries none — a snapshot is a state of one photograph.
        snapshots: Option<(Vec<dr_pipeline::Version>, Vec<String>)>,
        /// TRACES: FR-DEV-3 | FR-CAT-8
        /// The local adjustments, when this is an image's own edit being
        /// written back rather than a paste onto someone else's.
        ///
        /// A `Preset` is a parameter map, and a mask is not a parameter — it
        /// is a rule about *where*, with a chain of its own. So a save that
        /// carried only the preset wrote the sliders and silently dropped
        /// every local adjustment: the sidecar format has stored masks since
        /// they were added and `Version::apply` restores them, but nothing
        /// ever put any there. The mask survived until the session ended and
        /// then did not exist.
        ///
        /// `None` for a paste, which must not carry the source image's masks
        /// onto the target: a mask is drawn against one photograph and means
        /// nothing on another, and `Scope` cannot express that because it
        /// filters parameters.
        masks: Option<dr_pipeline::mask::MaskStack>,
    },
}

/// Where an image's sidecar lives.
///
/// The image's own path with the extension replaced, not appended: `a.CR2`
/// becomes `a.drsc`, so a RAW and the JPEG beside it share one sidecar and
/// therefore one judgement. That is the intended behaviour — they are the same
/// photograph (FR-CAT-11), and the pairing logic in `dr_catalog::schema`
/// already treats them so.
pub fn sidecar_path(image_path: &str) -> String {
    let stem = match image_path.rsplit_once('.') {
        // Only an extension in the final segment counts; a dot in a directory
        // name must not truncate the path.
        Some((stem, ext)) if !ext.contains('/') => stem,
        _ => image_path,
    };
    format!("{stem}.{}", dr_pipeline::sidecar::EXTENSION)
}

/// TRACES: FR-CAT-8 | FR-CAT-9 | FR-NC-10
/// Persist amendments to sidecars beside their images.
///
/// # Why this reads before it writes
///
/// A sidecar is the authoritative store and may already hold an edit made on
/// this or another device. Writing a fresh document containing only a rating
/// would delete that edit — the exact silent data loss the format's
/// unknown-key preservation exists to prevent. So each file is fetched,
/// parsed, amended, and written back; a fetch that 404s simply means there is
/// no sidecar yet and a new one is created.
///
/// # Why the local write is the commit point
///
/// FR-CAT-9 requires that edits made offline *queue and apply when the source
/// returns*. So every amendment is written to the local cache first and the
/// upload is best-effort: an entry stays marked pending until the server has
/// actually taken it, and [`spawn_outbox_drain`] retries the marked ones later.
///
/// This is what makes `offline` a parameter rather than a reason to skip. It
/// was one: a cull or a paste made with no connection used to be dropped
/// entirely, which for a pasted edit meant it survived nowhere at all — the
/// catalog holds no parameters. Now the two cases differ only in whether the
/// upload is attempted.
///
/// # Why failure here is logged rather than surfaced
///
/// The write has already succeeded locally by the time the network is touched,
/// so nothing is lost by a failure and there is nothing for the user to do
/// about it. Interrupting a cull with an error dialog per frame would be far
/// worse than the risk. The counts are reported once, at the end.
pub fn spawn_sidecar_writes(
    conn: Connection,
    writes: Vec<SidecarWrite>,
    cache_dir: PathBuf,
    offline: bool,
) -> Receiver<SidecarMessage> {
    let (tx, rx) = std::sync::mpsc::channel();

    std::thread::spawn(move || {
        let cache = SidecarCache::open(cache_dir);

        // The runtime and the backend are only needed to *upload*. Offline,
        // neither is built — and a failure to build either is not a failure to
        // record the edit, it just means every write is queued instead.
        let rt = if offline {
            None
        } else {
            match crate::net_runtime::build() {
                Ok(rt) => Some(rt),
                Err(e) => {
                    log::debug!("no runtime for sidecar upload ({e}); queueing");
                    None
                }
            }
        };

        // Every write, recorded locally and queued. The offline path, and the
        // fallback whenever a backend could not be built.
        let queue_all = || {
            let mut report = SidecarReport::default();
            for w in &writes {
                match write_one_sidecar(&cache, w) {
                    Ok(Outcome::Uploaded) => report.written += 1,
                    Ok(Outcome::Queued) => report.queued += 1,
                    Err(e) => {
                        // Warn, not debug. This is unsynced user work — a rating or an
                        // edit that exists only on this device — and the path is
                        // the only thing that says *which* photograph and *where*
                        // the server refused it. Filtered out at the default
                        // level, a 403 on one file is indistinguishable from a
                        // whole library failing.
                        log::warn!("sidecar for {}: {e}", w.image_path);
                        report.last_error = Some(e);
                        report.failed += 1;
                    }
                }
            }
            report
        };

        let report = match rt {
            None => queue_all(),
            Some(rt) => rt.block_on(async {
                match crate::remote::connect(&conn) {
                    Ok(b) => {
                        let mut report = SidecarReport::default();
                        for w in &writes {
                            match write_one_sidecar_online(&*b, &cache, w).await {
                                Ok(Outcome::Uploaded) => report.written += 1,
                                Ok(Outcome::Queued) => report.queued += 1,
                                Err(e) => {
                                    // Warn, not debug. This is unsynced user work — a rating or an
                                    // edit that exists only on this device — and the path is
                                    // the only thing that says *which* photograph and *where*
                                    // the server refused it. Filtered out at the default
                                    // level, a 403 on one file is indistinguishable from a
                                    // whole library failing.
                                    log::warn!("sidecar for {}: {e}", w.image_path);
                                    report.last_error = Some(e);
                                    report.failed += 1;
                                }
                            }
                        }
                        report
                    }
                    // No backend: the edits are still recorded locally and
                    // will go up with the next drain.
                    Err(e) => {
                        log::debug!("no backend for sidecar upload ({e}); queueing");
                        queue_all()
                    }
                }
            }),
        };

        let _ = tx.send(SidecarMessage::Finished {
            written: report.written,
            queued: report.queued,
            failed: report.failed,
            last_error: report.last_error,
        });
    });

    rx
}

/// What one write ended up doing.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum Outcome {
    /// Recorded locally and accepted by the server.
    Uploaded,
    /// Recorded locally, still in the outbox.
    Queued,
}

/// Running totals for a batch, so the loop bodies stay readable.
#[derive(Debug, Default)]
struct SidecarReport {
    written: usize,
    queued: usize,
    failed: usize,
    last_error: Option<String>,
}

/// The outcome of a batch of sidecar writes.
#[derive(Debug)]
pub enum SidecarMessage {
    Finished {
        written: usize,
        /// Recorded locally but not yet on the server — offline, or an upload
        /// that failed. These are retried by [`spawn_outbox_drain`], so this
        /// is a count of *deferred* work rather than of losses.
        queued: usize,
        failed: usize,
        /// Reported once rather than per file: a network that is down fails
        /// every write with the same message, and forty identical lines in the
        /// status bar say nothing forty times.
        last_error: Option<String>,
    },
}

/// Apply an amendment to a document, returning the new one.
///
/// Split out from both write paths so that online and offline produce
/// *identical* documents: the only thing that differs between them is which
/// base was read and whether an upload follows. A second copy of this for the
/// offline case is how the two would come to disagree about what a paste means.
fn amend(base: dr_pipeline::Sidecar, w: &SidecarWrite) -> dr_pipeline::Sidecar {
    let mut sidecar = base;

    // TRACES: FR-NC-8 | FR-NC-9
    // Close any split this file already carries, *before* looking for our own
    // version, and onto the uuid this write is about to use.
    //
    // Every device used to mint its own uuid for the same photograph, so a
    // frame edited on two of them holds two `default = 1` blocks and the
    // lookup below misses both — adding a third rather than amending either.
    // Fusing first folds them into one under `w.version_uuid`, which turns the
    // miss into a hit and makes this an amendment of the other device's work
    // instead of a rival to it.
    //
    // Idempotent: a file with one default and the right uuid is returned
    // byte-identical, so this costs nothing on the ordinary write.
    sidecar.fuse_default_versions(Some(&w.version_uuid));

    // Amend the version this write belongs to, creating it if the file did
    // not have one — a photograph nobody has edited anywhere.
    let mut version = sidecar
        .versions
        .get(&w.version_uuid)
        .cloned()
        .unwrap_or_else(|| dr_pipeline::sidecar::Version {
            uuid: w.version_uuid.clone(),
            name: "Default".to_string(),
            is_default: true,
            revision: 0,
            ..Default::default()
        });

    // Only what the amendment names. Everything else in the version — the
    // rating a settings write must not touch, the crop an adjustments-only
    // paste must spare, the unknown keys of an operation this build lacks —
    // survives because it was read from the file and is written back.
    match &w.amendment {
        Amendment::Judgement { rating, flag } => {
            version.rating = *rating;
            version.flag = *flag;
        }
        Amendment::Settings {
            preset,
            scope,
            masks,
            film,
            ..
        } => {
            preset.amend(&mut version.params, *scope);
            // TRACES: FR-DEV-3f
            // Wholesale, like the masks below and for the same reason: this is
            // the whole of the image's own choice as it stands, so clearing a
            // film has to leave the sidecar too.
            if let Some(film) = film {
                version.film = film.clone();
            }
            // Replaced wholesale rather than merged: this is the whole of the
            // image's local adjustment stack as it stands, so a layer the user
            // deleted has to leave the sidecar too. Cross-device merging of
            // two stacks is `Sidecar::merge`'s job and happens on sync, not
            // here (FR-NC-9).
            if let Some(masks) = masks {
                version.masks = masks.clone();
            }
        }
    }

    // A judgement is an edit as far as the merge is concerned, and so is a
    // paste: without the bump, a device that touched the same frame earlier
    // would win on revision and this write would be discarded at the next sync
    // (FR-NC-9).
    version.revision = version.revision.saturating_add(1);
    version.modified = now_secs();

    sidecar.put(version);
    // TRACES: FR-DEV-5 | FR-NC-9
    // After the version, and against the uuid the fuse settled on: a
    // snapshot points at its edit by uuid, and the edit may have just been
    // renamed onto the canonical one.
    if let Amendment::Settings {
        snapshots: Some((kept, removed)),
        ..
    } = &w.amendment
    {
        sidecar.replace_snapshots(&w.version_uuid, kept.clone(), removed);
    }
    sidecar
}

/// TRACES: FR-CAT-9
/// Record an amendment with no server to send it to.
///
/// The base is whatever the cache holds, which is either what the server last
/// had or what earlier offline writes have already built on top of it. Either
/// way the result is queued, and the drain reconciles it with the server's own
/// copy when the connection returns — that reconciliation is a *merge*
/// (FR-NC-9), not an overwrite, so building on a possibly-stale base here does
/// not cost another device's work.
fn write_one_sidecar(cache: &SidecarCache, w: &SidecarWrite) -> Result<Outcome, String> {
    let path = sidecar_path(&w.image_path);
    let base = cache.load(&path).unwrap_or_default();
    cache.store(&path, &amend(base, w), true)?;
    Ok(Outcome::Queued)
}

/// TRACES: FR-CAT-8 | FR-CAT-9
/// Read-modify-write one sidecar, with a server to read from and send to.
async fn write_one_sidecar_online(
    backend: &dyn RemoteBackend,
    cache: &SidecarCache,
    w: &SidecarWrite,
) -> Result<Outcome, String> {
    let path_str = sidecar_path(&w.image_path);
    let path = RemotePath::new(path_str.clone());
    let id = RemoteId::Path(path.clone());

    // An existing sidecar may hold an edit. Absent is the normal case on a
    // library that has never been edited, and is not an error.
    let existing = backend.get(&id, None).await.ok();

    // A corrupt sidecar is *not* overwritten: that would destroy an edit this
    // build merely failed to understand. Refused before anything is written,
    // locally or remotely, so the cache cannot end up holding a document that
    // silently discarded the file's real contents.
    if let Some(bytes) = existing.as_deref() {
        if !bytes.is_empty() {
            let text = String::from_utf8_lossy(bytes);
            if dr_pipeline::Sidecar::parse(&text).is_err() {
                return Err(format!("sidecar at {path_str} is unreadable"));
            }
        }
    }

    let base = existing
        .as_deref()
        .map(|bytes| String::from_utf8_lossy(bytes).into_owned())
        .and_then(|text| dr_pipeline::Sidecar::parse(&text).ok())
        // No sidecar on the server. The cache may still hold queued offline
        // work for this image, and taking `default()` here would drop it.
        .or_else(|| cache.load(&path_str))
        .unwrap_or_default();

    let sidecar = amend(base, w);

    // Locally first: this is the commit point, and an upload that fails after
    // it leaves the edit queued rather than lost.
    cache.store(&path_str, &sidecar, true)?;

    backend
        .put(&path, sidecar.to_text().into_bytes(), None)
        .await
        .map_err(|e| e.to_string())?;

    // Accepted by the server, so it leaves the outbox. The document stays
    // cached, which is what lets the next offline open still show the edit.
    cache.store(&path_str, &sidecar, false)?;
    Ok(Outcome::Uploaded)
}

/// TRACES: FR-CAT-9 | FR-NC-9 | FR-NC-10
/// Upload everything the outbox is still holding.
///
/// # Why this merges rather than uploads
///
/// A queued edit was built on whatever this device last saw. While it sat in
/// the outbox another device may have edited the same photograph, and simply
/// PUTting the local document would discard that work — the precise failure
/// FR-NC-9's node-level merge exists to prevent. So each entry is reconciled
/// against the server's current copy before it goes up, and disjoint edits
/// (a crop made here, an exposure change made there) both survive.
///
/// # Why an entry stays queued on failure
///
/// The marker is cleared only after the server has taken the bytes. A drain
/// interrupted halfway leaves the rest of the outbox exactly as it was, so
/// nothing depends on this running to completion.
pub fn spawn_outbox_drain(conn: Connection, cache_dir: PathBuf) -> Receiver<SidecarMessage> {
    let (tx, rx) = std::sync::mpsc::channel();

    std::thread::spawn(move || {
        let cache = SidecarCache::open(cache_dir);
        let queued = cache.pending();
        if queued.is_empty() {
            let _ = tx.send(SidecarMessage::Finished {
                written: 0,
                queued: 0,
                failed: 0,
                last_error: None,
            });
            return;
        }
        log::info!("draining {} queued sidecar(s)", queued.len());

        let rt = match crate::net_runtime::build() {
            Ok(rt) => rt,
            Err(e) => {
                let _ = tx.send(SidecarMessage::Finished {
                    written: 0,
                    queued: queued.len(),
                    failed: 0,
                    last_error: Some(e.to_string()),
                });
                return;
            }
        };

        rt.block_on(async {
            let backend = match crate::remote::connect(&conn) {
                Ok(b) => b,
                Err(e) => {
                    let _ = tx.send(SidecarMessage::Finished {
                        written: 0,
                        queued: queued.len(),
                        failed: 0,
                        last_error: Some(e.to_string()),
                    });
                    return;
                }
            };

            let mut report = SidecarReport::default();
            for path_str in &queued {
                match drain_one(&*backend, &cache, path_str).await {
                    Ok(()) => report.written += 1,
                    Err(e) => {
                        // Warn, for the reason the write path does: this is
                        // unsynced work and the path is what makes the failure
                        // actionable.
                        log::warn!("draining {path_str}: {e}");
                        report.last_error = Some(e);
                        report.failed += 1;
                        // Still queued — the marker was never cleared.
                        report.queued += 1;
                    }
                }
            }

            let _ = tx.send(SidecarMessage::Finished {
                written: report.written,
                queued: report.queued,
                failed: report.failed,
                last_error: report.last_error,
            });
        });
    });

    rx
}

/// Reconcile one queued sidecar with the server and upload it.
async fn drain_one(
    backend: &dyn RemoteBackend,
    cache: &SidecarCache,
    path_str: &str,
) -> Result<(), String> {
    let Some(mut local) = cache.load(path_str) else {
        // The document went while the drain was running. Nothing to send.
        return Ok(());
    };

    let path = RemotePath::new(path_str.to_string());
    let id = RemoteId::Path(path.clone());

    // TRACES: FR-NC-6c
    // A miss and a placeholder are not the same answer, and conflating them
    // destroys work. This read decides whether the sidecar already on the
    // remote is merged in; treating "the content is not on this device" as
    // "there is no sidecar" writes a fresh document over an existing one and
    // discards every edit another device put there — the exact loss the
    // format's unknown-key preservation exists to prevent.
    //
    // A sidecar is a few kilobytes, so the right response to a placeholder is
    // to fetch it, not to give up. Where that is impossible — no client
    // running — the entry stays queued, which is what the outbox is for.
    let remote = match backend.get(&id, None).await {
        Ok(bytes) => Some(bytes),
        Err(RemoteError::NotFound(_)) => None,
        Err(RemoteError::NotMaterialised(_)) => {
            backend
                .materialise(&id)
                .await
                .map_err(|e| format!("sidecar is not on this device ({e})"))?;
            match backend.get(&id, None).await {
                Ok(bytes) => Some(bytes),
                Err(e) => return Err(format!("sidecar could not be read ({e})")),
            }
        }
        // Anything else — a refused read, a dead connection — leaves the entry
        // queued rather than resolved by overwriting.
        Err(e) => return Err(format!("sidecar could not be read ({e})")),
    };

    if let Some(bytes) = remote.as_deref() {
        if !bytes.is_empty() {
            let text = String::from_utf8_lossy(bytes);
            match dr_pipeline::Sidecar::parse(&text) {
                Ok(remote) => merge_into(&mut local, &remote),
                // Unreadable on the server. Uploading over it would destroy an
                // edit this build failed to understand, so the entry stays
                // queued rather than being resolved destructively.
                Err(e) => return Err(format!("remote sidecar is unreadable ({e})")),
            }
        }
    }

    // TRACES: FR-NC-8 | FR-NC-9
    // `merge_into` reconciles version by version *by uuid*, so two devices'
    // independently minted defaults pass straight through it and both land in
    // what is about to be uploaded. Fusing here is what stops the outbox from
    // publishing the split rather than resolving it.
    //
    // No canonical uuid: this entry may have been queued by a build that had
    // not derived one yet, and the smallest uuid is device-independent, which
    // is all convergence needs. The next write from either device moves it
    // onto the derived identity.
    local.fuse_default_versions(None);

    backend
        .put(&path, local.to_text().into_bytes(), None)
        .await
        .map_err(|e| e.to_string())?;

    cache.store(path_str, &local, false)
}

/// TRACES: FR-NC-9
/// Merge the server's copy into ours, version by version.
///
/// No common ancestor is available — the outbox stores the result, not the
/// base it was built from — so the merge runs with `None`, which treats every
/// key either side holds as changed. Disjoint keys therefore still both
/// survive, and a key both sides set resolves by revision exactly as it would
/// with a base. What is lost without one is the ability to see a *deletion*:
/// a parameter reset to default on the other device reads as absent rather
/// than as removed, so our value stands. That is the same direction of caution
/// the judgement merge takes — an edit is preserved rather than erased.
fn merge_into(local: &mut dr_pipeline::Sidecar, remote: &dr_pipeline::Sidecar) {
    for (uuid, their_version) in &remote.versions {
        match local.versions.get(uuid).cloned() {
            Some(mut ours) => {
                ours.merge(their_version, None);
                local.put(ours);
            }
            // A version only the server has — another device's virtual copy
            // (FR-CAT-12). Keeping it is what stops one device's upload from
            // deleting another's work.
            None => local.put(their_version.clone()),
        }
    }
}

/// Where the catalog for an account lives.
///
/// Keyed by [`Account::namespace`] so two accounts do not share an index —
/// two servers, two logins on one server, or two folders on one disk. Under
/// the XDG data directory, not cache: the catalog is rebuildable but
/// rebuilding it costs a full rescan, so it is not something to discard on a
/// cache sweep.
///
/// The namespace is the account's to compute, not this function's, because it
/// is also frozen: it names the directory an existing install's catalog,
/// thumbnail shards and un-uploaded sidecars are already in.
pub fn catalog_path(account: &Account) -> PathBuf {
    data_root().join(account.namespace()).join("catalog.sqlite")
}

/// TRACES: FR-UI-8
/// Where this library's last position is remembered.
///
/// Beside the catalog, under the same account namespace, for the reason
/// `catalog_path` gives: a place belongs to one library, and two folders on one
/// disk are two libraries with two positions.
///
/// **The name matches the file that travels.** The copy on the server is
/// `place.json` under `.darkroom-derived/`, and the exchange between them is a
/// straight newest-wins swap of the same bytes — so calling the local one
/// anything else would be one more thing to keep in step for no gain.
///
/// In the data directory rather than the cache one. The consequence is milder
/// here than for the sidecars `data_root` was moved for — losing a place costs
/// a scroll, not a day of culling — but a file the system is free to delete is
/// one that would rarely survive long enough to be read.
pub fn place_path(account: &Account) -> PathBuf {
    data_root().join(account.namespace()).join("place.json")
}

/// The directory every account's data hangs off.
///
/// **Not the cache directory, and on Android that distinction is the whole
/// point.** Neither `XDG_DATA_HOME` nor `HOME` is set there, so this used
/// to fall through to `temp_dir()` — which Android resolves to the app's
/// *cache*, a directory the system deletes without asking under storage
/// pressure.
///
/// What sits beside a catalog is not disposable. `sidecars/` is the
/// commit point for every rating and edit made offline (see
/// `sidecar_cache`), and `outbox/` holds exports the user has been told
/// succeeded. A day of culling on a train, evicted by the OS before it ever
/// reached the server, is the worst failure this application can have, and
/// it would be silent.
///
/// `dr_sync::account::declared_data_dir` is the persistent per-app directory
/// the Android entry point establishes before anything opens a store. A
/// desktop declares nothing and takes the platform's data directory from
/// `dr_plat::dirs` — XDG on Linux, `%LOCALAPPDATA%` on Windows — which keeps
/// the established location on Linux rather than moving anyone's catalog.
fn data_root() -> PathBuf {
    match dr_sync::account::declared_data_dir() {
        Some(declared) => declared.join("darkroom"),
        None => dr_plat::base_dir(dr_plat::Base::Data),
    }
}

/// TRACES: FR-NC-10 | NFR-R1
/// Move an account's data out of the cache directory it used to live in.
///
/// Called once at startup, before anything opens a store. The durable
/// location changed when `catalog_path` stopped falling through to
/// `temp_dir()` on Android, and without this the app would find no catalog,
/// rescan a library of tens of thousands of images over the network, and
/// re-fetch every thumbnail — while the old copy sat in a directory the
/// system was free to delete.
///
/// Worse than the cost: `sidecars/` and `outbox/` hold work that exists
/// nowhere else. Abandoning them would discard offline ratings and edits that
/// had not yet synced, silently, as an upgrade.
///
/// A rename, not a copy: both directories are inside the app's own data on
/// one filesystem, so it is atomic and cannot half-finish. If the destination
/// already exists this does nothing — the migration has run, or this is a
/// fresh install, and in neither case may it overwrite live data.
pub fn migrate_legacy_cache_data(account: &Account) {
    // Only meaningful where the old fallback and the new one differ, which is
    // exactly the platform that had the problem. On a desktop with XDG set,
    // both resolve to the same place and this returns immediately.
    let legacy_base = std::env::temp_dir();
    let Some(current) = catalog_path(account).parent().map(|p| p.to_path_buf()) else {
        return;
    };
    let Some(account) = current.file_name() else {
        return;
    };
    let legacy = legacy_base.join("darkroom").join(account);

    move_account_dir(&legacy, &current);
}

/// The move itself, separated so it can be tested against ordinary
/// directories rather than the platform's idea of a cache.
fn move_account_dir(legacy: &std::path::Path, current: &std::path::Path) {
    if legacy == current || !legacy.is_dir() || current.exists() {
        return;
    }
    if let Some(parent) = current.parent() {
        if let Err(e) = std::fs::create_dir_all(parent) {
            log::warn!("preparing {}: {e}", parent.display());
            return;
        }
    }
    match std::fs::rename(legacy, current) {
        Ok(()) => log::info!(
            "moved library data out of the cache: {} -> {}",
            legacy.display(),
            current.display()
        ),
        // Reported rather than fatal: a failed move leaves the old copy where
        // it was and costs a rescan, which is recoverable. Stopping the app
        // over it would not be.
        Err(e) => log::warn!(
            "could not move {} to {}: {e}",
            legacy.display(),
            current.display()
        ),
    }
}

/// Run a scan on a worker thread, writing results into the catalog.
///
/// Returns the receiver the UI drains. The worker owns its own tokio runtime
/// and backend; nothing here touches the Slint event loop.
pub fn spawn_scan(
    conn: Connection,
    root: String,
    filter: FormatFilter,
    catalog_path: PathBuf,
) -> Receiver<ScanMessage> {
    let (tx, rx) = std::sync::mpsc::channel();

    std::thread::spawn(move || {
        let started = std::time::Instant::now();
        if let Err(e) = run_scan(&tx, conn, root, filter, catalog_path, started) {
            let _ = tx.send(ScanMessage::Failed {
                message: e.message,
                offline: e.offline,
                lost_root: e.lost_root,
            });
        }
    });

    rx
}

/// A scan failure that still knows whether it was a connectivity failure.
///
/// The scan crosses a thread boundary, so the typed error cannot travel with
/// it; this carries the one bit that must survive.
struct ScanFailure {
    message: String,
    offline: bool,
    lost_root: bool,
}

impl ScanFailure {
    /// A failure that is nothing to do with reachability — local I/O, a
    /// runtime that would not start, a catalog that would not open.
    fn local(message: impl std::fmt::Display) -> Self {
        Self {
            message: message.to_string(),
            offline: false,
            lost_root: false,
        }
    }
}

impl From<dr_sync::RemoteError> for ScanFailure {
    fn from(e: dr_sync::RemoteError) -> Self {
        Self {
            offline: e.indicates_offline(),
            lost_root: e.indicates_lost_root(),
            message: e.to_string(),
        }
    }
}

/// TRACES: FR-PLAT-AND-2 | FR-CAT-9
/// Record that a library can no longer be opened, without losing it.
///
/// Called on the worker, before the failure crosses the channel, because this
/// is where the catalog handle is — and because the marking must be durable
/// whether or not anyone is left to draw a banner. A process killed between
/// the failure and the next launch must still come back knowing what it could
/// not reach.
///
/// Nothing is deleted. Every rating, every edit and every row stays exactly
/// where it was; what changes is that the images now say they are offline, so
/// the grid can show them as held-not-here rather than as ordinary
/// photographs whose thumbnails happen to be failing one at a time.
///
/// A root with no row yet is the first scan of a library that has never
/// succeeded, and there is nothing to mark — the failure alone is the whole
/// story, and the launch screen is where it is told.
fn mark_library_offline(catalog: &Catalog, root: &str) {
    let conn = catalog.connection();
    let root_id: Option<i64> = conn
        .query_row(
            "SELECT id FROM roots WHERE label = ?1 AND kind = 'remote'",
            [root],
            |r| r.get(0),
        )
        .ok();
    let Some(root_id) = root_id else {
        log::info!("library {root} has no catalog root yet; nothing to mark offline");
        return;
    };
    match dr_catalog::mark_root_offline(conn, dr_types::RootId(root_id as u64)) {
        Ok(()) => log::warn!("library {root} is unreachable; its images are marked offline"),
        Err(e) => log::error!("could not mark {root} offline: {e}"),
    }
}

fn run_scan(
    tx: &Sender<ScanMessage>,
    conn: Connection,
    root: String,
    filter: FormatFilter,
    catalog_path: PathBuf,
    started: std::time::Instant,
) -> Result<(), ScanFailure> {
    if let Some(dir) = catalog_path.parent() {
        std::fs::create_dir_all(dir)
            .map_err(|e| ScanFailure::local(format!("creating {}: {e}", dir.display())))?;
    }
    let catalog = Catalog::open(&catalog_path).map_err(ScanFailure::local)?;

    let rt = crate::net_runtime::build().map_err(ScanFailure::local)?;

    rt.block_on(async {
        // TRACES: FR-PLAT-AND-2 | FR-CAT-9
        // Classified rather than flattened to a local failure, because the
        // removed-card case never gets as far as a request: the folder
        // connector checks its root when it is constructed, so a library on an
        // ejected card fails here and not in the walk. Reported as an ordinary
        // error it left the grid showing a healthy library of images that
        // could no longer be opened, one silent thumbnail failure at a time.
        let backend = match crate::remote::connect(&conn) {
            Ok(b) => b,
            Err(e) => {
                if e.indicates_lost_root() {
                    mark_library_offline(&catalog, &root);
                }
                return Err(e.into());
            }
        };

        // Stored folder ETags, so an unchanged subtree is skipped whole. On a
        // first run this is empty and the walk is complete; on every run after
        // it is what keeps cost proportional to what changed (ARCH §8.4).
        let known = load_folder_etags(&catalog, &root);

        let scanned = dr_sync::scan(&*backend, &RemotePath::new(&root), &filter, &known, |p| {
            let _ = tx.send(ScanMessage::Progress {
                directories: p.directories_listed,
                pruned: p.directories_pruned,
                images: p.images_found,
            });
        })
        .await;

        // TRACES: FR-PLAT-AND-2 | FR-CAT-9
        // Written before the failure is reported, not after: the banner is a
        // consequence of the catalog state and not the other way round, and a
        // process that dies between the two must come back knowing.
        let result = match scanned {
            Ok(r) => r,
            Err(e) => {
                if e.indicates_lost_root() {
                    mark_library_offline(&catalog, &root);
                }
                return Err(e.into());
            }
        };

        persist(&catalog, &root, &result).map_err(ScanFailure::local)?;

        // TRACES: FR-CAT-8 | FR-NC-9
        // Take in what other devices have judged. After `persist`, because a
        // judgement lands on an image's version row and the image has to be in
        // the catalog first — a sidecar seen in the same listing as a
        // photograph this scan has only just discovered is the ordinary case
        // on a library another device imported.
        //
        // Deliberately not fatal. A pull that fails leaves the ratings this
        // device already had exactly where they were and the sidecar's ETag
        // unrecorded, so the next scan tries again; failing the whole scan over
        // it would blank a grid that was working.
        let judgements = match pull_sidecars(&*backend, &catalog, &root, &result.sidecars).await {
            Ok(n) => n,
            Err(e) => {
                log::warn!("reading sidecars from the library: {e}");
                0
            }
        };

        // Report what the catalog holds, not what this pass listed. An
        // incremental rescan lists only what changed, so its own count is
        // near zero on a healthy library.
        let total = total_images(&catalog).unwrap_or(result.images.len());

        let _ = tx.send(ScanMessage::Done {
            found: result.images.len(),
            total,
            pruned: result.progress.directories_pruned,
            elapsed_ms: started.elapsed().as_millis() as u64,
            judgements,
        });
        Ok(())
    })
}

/// TRACES: FR-CAT-8 | FR-CAT-9 | FR-NC-9
/// Take other devices' judgements out of the library's sidecars.
///
/// # Why this had to exist
///
/// A rating is written to the catalog and to the photograph's sidecar, and the
/// sidecar is the authoritative one (ARCH §6.12). Nothing ever read one back.
/// The scan indexed files, `derived_sync` exchanged thumbnails, collections
/// and keywords, `dr_catalog::merge` reconciled everything in the catalog
/// *except* `versions.rating` and `versions.flag`, and the single sidecar
/// reader ran when one photograph was opened in develop and handed its result
/// to the develop graph. `JobKind::ReadSidecar` had been declared for this
/// since the queue was written and was never enqueued or handled.
///
/// So judgements travelled outward only. The grid draws `versions.rating`, and
/// a cull done on another device could not reach it by any path the app had.
///
/// # What it costs
///
/// Nothing on a library nobody has edited. The sidecars come from listings the
/// walk was making anyway, an unchanged directory is pruned before it is
/// listed at all, and a sidecar whose ETag matches what this device last read
/// is skipped without a request. What is left is one GET per sidecar that
/// genuinely changed — which is the number of photographs somebody edited.
///
/// # Why a judgement can only be added, never withdrawn
///
/// `Version::merge`'s rule, applied here: zero is *unjudged*, and a device that
/// has never rated a frame is indistinguishable from one that deliberately
/// cleared it. Taking a remote zero over a local star would let a device that
/// was never involved erase an afternoon's culling. So a remote zero is
/// ignored, and the cost is that clearing a rating does not propagate.
///
/// Returns how many images gained a judgement.
async fn pull_sidecars(
    backend: &dyn RemoteBackend,
    catalog: &Catalog,
    root: &str,
    seen: &[dr_sync::RemoteEntry],
) -> Result<usize, String> {
    if seen.is_empty() {
        return Ok(0);
    }
    let conn = catalog.connection();
    let root_id: i64 = conn
        .query_row(
            "SELECT id FROM roots WHERE label = ?1 AND kind = 'remote'",
            [root],
            |r| r.get(0),
        )
        .map_err(|e| e.to_string())?;

    let known = load_sidecar_etags(catalog, root_id);
    let mut applied = 0usize;

    for entry in seen {
        let path = entry.path.as_str();
        if known.get(path).is_some_and(|e| *e == entry.validator) {
            continue;
        }

        let bytes = match backend.get(&RemoteId::Path(entry.path.clone()), None).await {
            Ok(b) => b,
            // Gone between the listing and the fetch, or not on this device and
            // not worth materialising a whole library for. Neither is an error,
            // and neither records an ETag — so the next scan tries again.
            Err(e) => {
                log::debug!("reading sidecar {path}: {e}");
                continue;
            }
        };

        let text = String::from_utf8_lossy(&bytes);

        // TRACES: FR-CAT-13
        // A standard XMP beside the photograph — Lightroom's, darktable's,
        // anybody's — takes the other branch: reconciled field by field with
        // the catalog winning, and a disagreement recorded for the reload
        // the requirement asks to be offered. See `xmp_sync`.
        if crate::xmp_sync::is_xmp(path) {
            match crate::xmp_sync::take_in(conn, root_id, path, &text, now_secs()) {
                Ok(taken) => {
                    applied += taken.changed;
                    if !taken.conflicts.is_empty() {
                        log::info!(
                            "xmp sidecar {path} disagrees with the catalog on {:?}; \
                             a reload is offered in Settings",
                            taken.conflicts
                        );
                    }
                    // Recorded only once it reached a photograph: a sidecar
                    // that arrived before its image is read again next time.
                    if taken.described > 0 {
                        record_sidecar_read(conn, root_id, path, &entry.validator);
                    }
                }
                Err(e) => log::warn!("xmp sidecar at {path} is unreadable ({e})"),
            }
            continue;
        }

        let mut sidecar = match dr_pipeline::Sidecar::parse(&text) {
            Ok(s) => s,
            // Unreadable is not empty. Recording the ETag would mean never
            // looking at it again, and a build that understands it may be
            // along; leaving it unrecorded costs one GET per scan and keeps
            // the door open.
            Err(e) => {
                log::warn!("sidecar at {path} is unreadable ({e})");
                continue;
            }
        };
        sidecar.fuse_default_versions(None);

        let Some(version) = sidecar.default_version() else {
            // A sidecar with no default version — someone else's virtual copy
            // and nothing more. Nothing to take, but it *was* read, so its
            // ETag is recorded and it is not fetched again.
            record_sidecar_read(conn, root_id, path, &entry.validator);
            continue;
        };

        match apply_judgement(conn, root_id, path, version.rating, version.flag) {
            Ok(n) => {
                applied += n;
                record_sidecar_read(conn, root_id, path, &entry.validator);
            }
            Err(e) => log::debug!("applying {path}: {e}"),
        }
    }

    if applied > 0 {
        log::info!("{applied} judgement(s) arrived from other devices");
    }
    Ok(applied)
}

/// TRACES: FR-CAT-13 | NFR-R4
/// One image's ratings, label and keywords, on their way to the `.xmp`
/// beside it.
///
/// Distinct from [`SidecarWrite`], which amends DarkRoom's own document
/// through the cache and the outbox. This is best-effort in the other
/// direction: the catalog and the `.drsc` are authoritative, the `.xmp` is a
/// courtesy to whatever else reads the folder, and a write that cannot
/// happen now is written again — from the catalog, whole — the next time
/// anything about the photograph is judged. So nothing is queued.
#[derive(Debug, Clone)]
pub struct XmpWrite {
    pub image_path: String,
    pub record: dr_xmp::Xmp,
}

/// TRACES: FR-CAT-13 | NFR-R4
/// Write each record into the XMP sidecar beside its image.
///
/// An existing sidecar of either spelling is rewritten in place, which is the
/// whole point of `dr_xmp::rewrite`: only the properties DarkRoom owns move,
/// and another application's settings, comments and namespaces come through
/// byte for byte. A photograph with neither gets a new file under Lightroom's
/// name. Reported once at the end, as the judgement writes are.
pub fn spawn_xmp_writes(conn: Connection, writes: Vec<XmpWrite>) -> Receiver<XmpMessage> {
    let (tx, rx) = std::sync::mpsc::channel();
    std::thread::spawn(move || {
        let mut written = 0usize;
        let mut failed = 0usize;
        let mut last_error = None;
        let rt = match crate::net_runtime::build() {
            Ok(rt) => rt,
            Err(e) => {
                let _ = tx.send(XmpMessage::Finished {
                    written: 0,
                    failed: writes.len(),
                    last_error: Some(e.to_string()),
                });
                return;
            }
        };
        rt.block_on(async {
            let backend = match crate::remote::connect(&conn) {
                Ok(b) => b,
                Err(e) => {
                    failed = writes.len();
                    last_error = Some(e.to_string());
                    return;
                }
            };
            for w in &writes {
                match write_one_xmp(&*backend, w).await {
                    Ok(()) => written += 1,
                    Err(e) => {
                        log::warn!("xmp sidecar for {}: {e}", w.image_path);
                        last_error = Some(e);
                        failed += 1;
                    }
                }
            }
        });
        let _ = tx.send(XmpMessage::Finished {
            written,
            failed,
            last_error,
        });
    });
    rx
}

/// The outcome of a batch of XMP writes.
#[derive(Debug)]
pub enum XmpMessage {
    Finished {
        written: usize,
        failed: usize,
        last_error: Option<String>,
    },
}

/// Read-modify-write one image's XMP sidecar on the server.
async fn write_one_xmp(backend: &dyn RemoteBackend, w: &XmpWrite) -> Result<(), String> {
    let [darktable, lightroom] = crate::xmp_sync::candidate_paths(&w.image_path);
    // Whichever exists is the one rewritten; neither existing means the
    // Lightroom spelling is created. An existing file this build cannot
    // parse is left alone rather than replaced — it is somebody else's
    // document, and a refusal is recoverable where an overwrite is not.
    let mut target = lightroom.clone();
    let mut existing: Option<String> = None;
    for path in [&darktable, &lightroom] {
        if let Ok(bytes) = backend
            .get(&RemoteId::Path(RemotePath::new(path.clone())), None)
            .await
        {
            if !bytes.is_empty() {
                target = path.clone();
                existing = Some(String::from_utf8_lossy(&bytes).into_owned());
                break;
            }
        }
    }
    let text = match existing {
        Some(text) => {
            // The file's caption, copyright and hierarchy come through: the
            // catalog has nowhere to keep them, and a rewrite that said
            // nothing about them would remove them.
            let theirs = dr_xmp::Xmp::parse(&text)
                .map_err(|e| format!("{target} is not a sidecar this build can read: {e}"))?;
            let mut record = w.record.clone();
            crate::xmp_sync::carry_through(&mut record, &theirs);
            record
                .rewrite(&text)
                .map_err(|e| format!("{target} is not a sidecar this build can rewrite: {e}"))?
        }
        None => w.record.to_text(),
    };
    backend
        .put(&RemotePath::new(target), text.into_bytes(), None)
        .await
        .map(|_| ())
        .map_err(|e| e.to_string())
}

/// TRACES: FR-CAT-13
/// The offered reload: re-read the sidecars a person chose to trust, with
/// the sidecar winning. One fetch per path, the catalog opened on this
/// thread as the scan opens it.
pub fn spawn_xmp_reload(
    conn: Connection,
    root: String,
    catalog_path: PathBuf,
    paths: Vec<String>,
) -> Receiver<XmpMessage> {
    let (tx, rx) = std::sync::mpsc::channel();
    std::thread::spawn(move || {
        let mut written = 0usize;
        let mut failed = 0usize;
        let mut last_error = None;
        let outcome: Result<(), String> = (|| {
            let catalog = Catalog::open(&catalog_path).map_err(|e| e.to_string())?;
            let root_id: i64 = catalog
                .connection()
                .query_row(
                    "SELECT id FROM roots WHERE label = ?1 AND kind = 'remote'",
                    [&root],
                    |r| r.get(0),
                )
                .map_err(|e| e.to_string())?;
            let rt = crate::net_runtime::build().map_err(|e| e.to_string())?;
            rt.block_on(async {
                let backend = crate::remote::connect(&conn).map_err(|e| e.to_string())?;
                for path in &paths {
                    let fetched = backend
                        .get(&RemoteId::Path(RemotePath::new(path.clone())), None)
                        .await
                        .map_err(|e| e.to_string())
                        .and_then(|bytes| {
                            let text = String::from_utf8_lossy(&bytes);
                            crate::xmp_sync::reload(catalog.connection(), root_id, path, &text)
                        });
                    match fetched {
                        Ok(taken) => written += taken.changed,
                        Err(e) => {
                            log::warn!("reloading {path}: {e}");
                            last_error = Some(e);
                            failed += 1;
                        }
                    }
                }
                Ok(())
            })
        })();
        if let Err(e) = outcome {
            failed = paths.len();
            last_error = Some(e);
        }
        let _ = tx.send(XmpMessage::Finished {
            written,
            failed,
            last_error,
        });
    });
    rx
}

/// What this device has already read, so a pull fetches only what changed.
fn load_sidecar_etags(
    catalog: &Catalog,
    root_id: i64,
) -> std::collections::HashMap<String, dr_sync::Validator> {
    let mut out = std::collections::HashMap::new();
    let Ok(mut stmt) = catalog
        .connection()
        .prepare("SELECT path, etag FROM sidecars WHERE root_id = ?1 AND etag IS NOT NULL")
    else {
        return out;
    };
    if let Ok(rows) = stmt.query_map([root_id], |r| {
        Ok((r.get::<_, String>(0)?, r.get::<_, String>(1)?))
    }) {
        for (path, etag) in rows.flatten() {
            out.insert(path, dr_sync::Validator::new(etag));
        }
    }
    out
}

/// Remember that this sidecar has been taken in at this ETag.
///
/// Written only after the judgement has landed, for the same reason
/// `dr_sync::scan` records a directory's ETag only after listing it: recording
/// it first would let a failure look like work already done, and the edit would
/// never be read again.
fn record_sidecar_read(
    conn: &rusqlite::Connection,
    root_id: i64,
    path: &str,
    etag: &dr_sync::Validator,
) {
    let done = conn.execute(
        "INSERT INTO sidecars(root_id, path, etag, read_at) VALUES (?1, ?2, ?3, ?4)
         ON CONFLICT(root_id, path) DO UPDATE SET
             etag = excluded.etag, read_at = excluded.read_at",
        rusqlite::params![root_id, path, etag.as_str(), now_secs()],
    );
    if let Err(e) = done {
        log::debug!("recording sidecar {path}: {e}");
    }
}

/// Apply one sidecar's judgement to the image or images it describes.
///
/// # Why more than one image
///
/// A sidecar is named for the stem it shares with its photograph, so a RAW and
/// the JPEG the camera wrote beside it — one photograph under FR-CAT-11 — share
/// a document, and both rows have to carry the judgement or the grid disagrees
/// with itself depending on which of the pair it is showing.
///
/// # Why the match is verified in Rust
///
/// The `LIKE` narrows the search to rows sharing the stem, and it is only a
/// filter: `library::sidecar_path` is what actually decides, applied to each
/// candidate. A path holding a `%`, a `_` or a bracket would otherwise match
/// more than it should, and a judgement landing on the wrong photograph is a
/// silent, permanent wrong.
///
/// Returns how many images gained a judgement they did not have.
fn apply_judgement(
    conn: &rusqlite::Connection,
    root_id: i64,
    sidecar: &str,
    rating: u8,
    flag: u8,
) -> Result<usize, String> {
    let stem = sidecar
        .rsplit_once('.')
        .map(|(s, _)| s)
        .unwrap_or(sidecar)
        .to_string();

    // The escape is the point: `%` and `_` are wildcards, and a photographer's
    // folder is entitled to contain both.
    let prefix = stem
        .replace('\\', "\\\\")
        .replace('%', "\\%")
        .replace('_', "\\_");

    let candidates: Vec<(i64, String)> = {
        let mut stmt = conn
            .prepare(
                "SELECT id, source_ref FROM images
                  WHERE root_id = ?1 AND source_ref LIKE ?2 ESCAPE '\\'",
            )
            .map_err(|e| e.to_string())?;
        let rows = stmt
            .query_map(rusqlite::params![root_id, format!("{prefix}.%")], |r| {
                Ok((r.get(0)?, r.get(1)?))
            })
            .map_err(|e| e.to_string())?;
        rows.filter_map(Result::ok)
            .filter(|(_, source): &(i64, String)| sidecar_path(source) == sidecar)
            .collect()
    };

    let mut applied = 0usize;
    for (image, _) in candidates {
        let id = dr_types::ImageId(image as u64);
        let version =
            dr_catalog::rating::default_version_id(conn, id).map_err(|e| e.to_string())?;

        // The sidecar's value is taken, not the larger of the two. It is the
        // authoritative store and the fuse above has already resolved any
        // contest between devices on `revision` — so lowering a rating from
        // four to one on the tablet has to lower it here, and taking a maximum
        // would quietly refuse every demotion the photographer ever made.
        //
        // A zero is the one thing not taken: it means *never judged* rather
        // than "judged zero", so a sidecar that carries none cannot erase a
        // star this device holds. The same asymmetry, and the same direction
        // of caution, as `dr_pipeline::sidecar::merge_judgement`. The cost is
        // the one that rule always carries — clearing a rating does not
        // propagate.
        let changed = conn
            .execute(
                "UPDATE versions
                    SET rating = CASE WHEN ?2 > 0 THEN ?2 ELSE rating END,
                        flag   = CASE WHEN ?3 > 0 THEN ?3 ELSE flag   END
                  WHERE id = ?1
                    AND ((?2 > 0 AND rating <> ?2) OR (?3 > 0 AND flag <> ?3))",
                rusqlite::params![version, rating.min(5) as i64, flag.min(2) as i64],
            )
            .map_err(|e| e.to_string())?;
        applied += changed;
    }
    Ok(applied)
}

/// Read back the folder ETags stored by a previous scan.
///
/// A failure here is not fatal — an empty map simply means no pruning, which
/// is correct but slower. Refusing to scan because the last scan's bookkeeping
/// is unreadable would be the worse outcome.
fn load_folder_etags(
    catalog: &Catalog,
    root: &str,
) -> std::collections::HashMap<RemotePath, dr_sync::Validator> {
    let mut out = std::collections::HashMap::new();
    let sql = "SELECT f.path, f.etag FROM folders f
                 JOIN roots r ON r.id = f.root_id
                WHERE r.label = ?1 AND f.etag IS NOT NULL";

    let Ok(mut stmt) = catalog.connection().prepare(sql) else {
        return out;
    };
    let rows = stmt.query_map([root], |r| {
        Ok((r.get::<_, String>(0)?, r.get::<_, String>(1)?))
    });
    if let Ok(rows) = rows {
        for (path, etag) in rows.flatten() {
            out.insert(RemotePath::new(path), dr_sync::Validator::new(etag));
        }
    }
    out
}

/// Write a scan's findings into the catalog.
///
/// Images insert at `metadata_state = 1` (stat-only): the scan knows name and
/// size but has read no EXIF, and pretending otherwise would make a date
/// filter silently wrong. A `Thumbnail` job is enqueued per image, coalescing
/// with anything already pending.
fn persist(
    catalog: &Catalog,
    root: &str,
    result: &dr_sync::ScanResult,
) -> Result<(), dr_catalog::CatalogError> {
    let conn = catalog.connection();
    let tx = conn.unchecked_transaction()?;

    // One root row per library folder, reused across scans.
    tx.execute(
        "INSERT INTO roots(kind, label, last_seen) VALUES ('remote', ?1, ?2)
         ON CONFLICT DO NOTHING",
        rusqlite::params![root, now_secs()],
    )?;
    let root_id: i64 = tx.query_row(
        "SELECT id FROM roots WHERE label = ?1 AND kind = 'remote'",
        [root],
        |r| r.get(0),
    )?;

    // Folder ETags first — without these persisted, the next scan prunes
    // nothing and walks the whole tree again (ARCH §6.6).
    for (path, validator) in &result.directories {
        tx.execute(
            "INSERT INTO folders(root_id, path, etag) VALUES (?1, ?2, ?3)
             ON CONFLICT(root_id, path) DO UPDATE SET etag = excluded.etag",
            rusqlite::params![root_id, path.as_str(), validator.as_str()],
        )?;
    }

    for entry in &result.images {
        let folder_id: Option<i64> = entry.path.parent().and_then(|p| {
            tx.query_row(
                "SELECT id FROM folders WHERE root_id = ?1 AND path = ?2",
                rusqlite::params![root_id, p.as_str()],
                |r| r.get(0),
            )
            .ok()
        });

        // TRACES: FR-PLAT-AND-2 | FR-CAT-9
        // The `availability` arm is what ends an offline library, and it does
        // it one photograph at a time. 3 is `Availability::Offline` and 0 is
        // `MetadataOnly`, the same code this statement inserts new rows with —
        // so a row that was marked offline when the root became unreachable is
        // returned to exactly the state a fresh scan would have given it, and
        // a row that was never marked is not touched at all.
        //
        // Conditional rather than a blanket reset for the same reason
        // `dr_catalog::walk` restores per file rather than per root: the only
        // thing that may clear "I could not reach this" is having reached it,
        // and this statement runs precisely once per file the scan listed.
        tx.execute(
            "INSERT INTO images(root_id, folder_id, source_ref, format, file_size,
                                availability, metadata_state, added_at)
             VALUES (?1, ?2, ?3, ?4, ?5, 0, 1, ?6)
             ON CONFLICT(root_id, source_ref) DO UPDATE SET
                 file_size = excluded.file_size,
                 folder_id = excluded.folder_id,
                 availability = CASE WHEN images.availability = 3
                                     THEN 0 ELSE images.availability END",
            rusqlite::params![
                root_id,
                folder_id,
                entry.path.as_str(),
                entry
                    .path
                    .name()
                    .rsplit_once('.')
                    .map(|(_, e)| e.to_ascii_lowercase()),
                entry.size as i64,
                now_secs(),
            ],
        )?;

        let image_id: i64 = tx.query_row(
            "SELECT id FROM images WHERE root_id = ?1 AND source_ref = ?2",
            rusqlite::params![root_id, entry.path.as_str()],
            |r| r.get(0),
        )?;

        // Remote identity, keyed on oc:fileid so a server-side move is a move
        // rather than a re-download (FR-NC-5).
        if let RemoteId::Stable(file_id) = entry.id {
            tx.execute(
                "INSERT INTO remote(image_id, file_id, etag, remote_path)
                 VALUES (?1, ?2, ?3, ?4)
                 ON CONFLICT(image_id) DO UPDATE SET
                     etag = excluded.etag, remote_path = excluded.remote_path",
                rusqlite::params![
                    image_id,
                    file_id as i64,
                    entry.validator.as_str(),
                    entry.path.as_str()
                ],
            )?;
        }
    }

    tx.commit()?;

    // Thumbnail jobs after the commit, so a failure mid-insert does not leave
    // jobs pointing at rows that never landed.
    for entry in &result.images {
        if let Ok(image_id) = conn.query_row(
            "SELECT id FROM images WHERE root_id = ?1 AND source_ref = ?2",
            rusqlite::params![root_id, entry.path.as_str()],
            |r| r.get::<_, i64>(0),
        ) {
            let _ = dr_catalog::jobs::enqueue(
                conn,
                JobKind::Thumbnail,
                Some(image_id),
                Priority::Background,
                None,
            );
        }
    }

    Ok(())
}

/// What the grid wants a thumbnail for.
///
/// Carries the `oc:fileid` as well as the path, because that is what the
/// shared store keys on — stable across a server-side move, and the same id
/// every other client sees (FR-NC-5).
#[derive(Debug, Clone)]
pub struct ThumbnailRequest {
    pub row: usize,
    pub path: String,
    /// `None` where the scan found no stable id; such an image is fetched but
    /// not stored, since there is no durable key to store it under.
    pub file_id: Option<u64>,
    /// File length, needed to reject a preview range that points past the end
    /// of the file (NFR-SEC-1).
    pub size: u64,
    /// Catalog row, so EXIF read from the header can be written back.
    pub image_id: i64,
    /// Which resolution this cell needs, from how large it is drawn. A zoomed
    /// grid asks for the large class; a wall of small cells does not.
    ///
    /// Named apart from `size`, which is the file's length in bytes — the two
    /// are unrelated and confusing them would fetch the wrong thing.
    pub thumb_size: dr_thumbs::ThumbSize,
    /// Whether this image still needs its EXIF read. Where false the header is
    /// still fetched — the preview needs it — but nothing is parsed or written.
    pub needs_metadata: bool,
    /// Keep the preview at the resolution it was decoded at, ignoring
    /// `thumb_size`.
    ///
    /// For face indexing, which wants the pixels a thumbnail throws away: a
    /// face 2% across the frame is 5 px on a grid thumbnail and 120 px on the
    /// embedded preview, and 112 is what the embedder samples. Capped by
    /// [`FACE_SOURCE_EDGE`] rather than truly unbounded, because a 24 MP buffer
    /// converted to `f32` RGB is ~288 MB and several lanes hold one at once.
    pub full_resolution: bool,
}

/// Why a full fetch failed, keeping the one bit the UI cannot re-derive.
///
/// The same reasoning as [`ScanFailure`]: the typed error cannot cross the
/// channel, and "offline" versus "refused" decides whether develop shows
/// "you are offline — this image is not stored locally" or a real error.
#[derive(Debug)]
pub struct FetchFailure {
    pub message: String,
    pub offline: bool,
}

impl FetchFailure {
    fn local(message: impl std::fmt::Display) -> Self {
        Self {
            message: message.to_string(),
            offline: false,
        }
    }
}

impl From<dr_sync::RemoteError> for FetchFailure {
    fn from(e: dr_sync::RemoteError) -> Self {
        Self {
            offline: e.indicates_offline(),
            message: e.to_string(),
        }
    }
}

impl std::fmt::Display for FetchFailure {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.write_str(&self.message)
    }
}

/// TRACES: FR-NC-6a
/// Progress from the pin worker.
#[derive(Debug)]
pub enum PinMessage {
    /// How many originals the pin still needs. Sent once, before any transfer.
    Planned {
        total: usize,
    },
    /// One original landed.
    Stored {
        done: usize,
    },
    /// The pin is fully downloaded.
    Done {
        stored: usize,
        bytes: u64,
    },
    Failed {
        message: String,
        offline: bool,
    },
}

/// TRACES: FR-NC-6a
/// Download every original a pin has asked for.
///
/// Whole files, deliberately: a pin exists so the photographs can be *edited*
/// away from the server, and develop needs every photosite. This is the one
/// place in the app that fetches originals in bulk, which is why FR-NC-6
/// makes it opt-in rather than something sync does on its own.
///
/// Sequential rather than parallel. The lanes that make the thumbnail sweep
/// fast are wrong here: these are tens of megabytes each, so concurrency buys
/// little against a single connection's bandwidth and costs a great deal of
/// memory — and it is the same contention that produced 423 Locked in the
/// sweep.
pub fn spawn_pin_fetch(
    conn: Connection,
    catalog_path: PathBuf,
    cache_dir: PathBuf,
    budget: dr_catalog::Budget,
) -> Receiver<PinMessage> {
    let (tx, rx) = std::sync::mpsc::channel();

    std::thread::spawn(move || {
        let (store, catalog) = match (
            dr_catalog::Cache::open(&cache_dir, budget),
            Catalog::open(&catalog_path),
        ) {
            (Ok(s), Ok(c)) => (s, c),
            (Err(e), _) | (_, Err(e)) => {
                let _ = tx.send(PinMessage::Failed {
                    message: e.to_string(),
                    offline: false,
                });
                return;
            }
        };

        let pending = match store.pending_pins(catalog.connection()) {
            Ok(p) => p,
            Err(e) => {
                let _ = tx.send(PinMessage::Failed {
                    message: e.to_string(),
                    offline: false,
                });
                return;
            }
        };

        if tx
            .send(PinMessage::Planned {
                total: pending.len(),
            })
            .is_err()
        {
            return;
        }
        if pending.is_empty() {
            let _ = tx.send(PinMessage::Done {
                stored: 0,
                bytes: 0,
            });
            return;
        }

        let rt = match crate::net_runtime::build() {
            Ok(rt) => rt,
            Err(e) => {
                let _ = tx.send(PinMessage::Failed {
                    message: e.to_string(),
                    offline: false,
                });
                return;
            }
        };

        rt.block_on(async {
            let backend = match crate::remote::connect(&conn) {
                Ok(b) => b,
                Err(e) => {
                    let _ = tx.send(PinMessage::Failed {
                        message: e.to_string(),
                        offline: false,
                    });
                    return;
                }
            };

            let mut stored = 0usize;
            let mut bytes_total = 0u64;

            for image in pending {
                let Some(source_ref) = source_ref_of(&catalog, image) else {
                    // Catalogued and then removed while the pin was pending.
                    continue;
                };

                let id = RemoteId::Path(RemotePath::new(&source_ref));

                // TRACES: FR-NC-6c
                // On a placeholder library "pin" means *keep it downloaded*,
                // not "make a second copy". The original materialises in the
                // library folder itself, so copying it under `originals/`
                // would hold every pinned photograph twice — and the copy
                // would be the half the budget could evict while the real disk
                // cost stayed. Only the bookkeeping is recorded, with no path,
                // so nothing here can ever delete a file inside a synced tree
                // (see `Cache::record_in_place`).
                if backend.capabilities().materialisation.can_materialise() {
                    match backend.materialise(&id).await {
                        Ok(_) => {
                            let bytes = size_of(&catalog, image).unwrap_or(0);
                            if let Err(e) = store.record_in_place(
                                catalog.connection(),
                                image,
                                bytes,
                                true,
                                now_secs(),
                            ) {
                                log::warn!("recording pinned {source_ref}: {e}");
                                continue;
                            }
                            stored += 1;
                            bytes_total += bytes;
                            if tx.send(PinMessage::Stored { done: stored }).is_err() {
                                return;
                            }
                        }
                        Err(e) if e.indicates_offline() => {
                            let _ = tx.send(PinMessage::Failed {
                                message: e.to_string(),
                                offline: true,
                            });
                            return;
                        }
                        Err(e) => log::warn!("pinning {source_ref}: {e}"),
                    }
                    continue;
                }

                match backend.get(&id, None).await {
                    Ok(bytes) => {
                        // `pinned: true` — this is the population the budget
                        // must never evict, which is the entire promise the
                        // user made when they pinned the collection.
                        if let Err(e) = store.store(
                            catalog.connection(),
                            image,
                            &source_ref,
                            &bytes,
                            true,
                            now_secs(),
                        ) {
                            log::warn!("storing pinned {source_ref}: {e}");
                            continue;
                        }
                        stored += 1;
                        bytes_total += bytes.len() as u64;
                        if tx.send(PinMessage::Stored { done: stored }).is_err() {
                            return;
                        }
                    }
                    Err(e) if e.indicates_offline() => {
                        // Stop rather than failing each remaining file against
                        // a dead connection. What was downloaded stays
                        // downloaded, and `pending_pins` resumes from there.
                        let _ = tx.send(PinMessage::Failed {
                            message: e.to_string(),
                            offline: true,
                        });
                        return;
                    }
                    Err(e) => {
                        // One unreadable file must not abandon the whole pin.
                        log::warn!("pinning {source_ref}: {e}");
                    }
                }
            }

            let _ = tx.send(PinMessage::Done {
                stored,
                bytes: bytes_total,
            });
        });
    });

    rx
}

/// TRACES: FR-NC-6c
/// Hand a set of photographs back to the sync client, freeing their disk.
///
/// The other half of pinning on a placeholder library. `Cache::release` drops
/// the bookkeeping and — correctly — deletes nothing, because the rows it
/// holds for a library like this name no file of ours (`record_in_place`).
/// The bytes are in the library folder, and only the client may take them
/// back.
///
/// **This is a dehydration, not a deletion, and the distinction is the whole
/// safety of the feature.** Removing a materialised file inside a synced tree
/// propagates to the server and deletes the photograph everywhere.
///
/// Best effort per image: a file the client refuses to release simply stays,
/// which costs disk and loses nothing.
pub fn spawn_dehydrate(
    conn: Connection,
    catalog_path: PathBuf,
    images: Vec<dr_types::ImageId>,
) -> Receiver<usize> {
    let (tx, rx) = std::sync::mpsc::channel();

    std::thread::spawn(move || {
        let Ok(catalog) = Catalog::open(&catalog_path) else {
            return;
        };
        let Ok(rt) = crate::net_runtime::build() else {
            return;
        };
        rt.block_on(async {
            let Ok(backend) = crate::remote::connect(&conn) else {
                return;
            };
            // Nothing to do where content is not a thing that can be given
            // back — a server library, or a plain folder.
            if !backend.capabilities().materialisation.can_materialise() {
                return;
            }

            let mut released = 0usize;
            for image in images {
                let Some(source_ref) = source_ref_of(&catalog, image) else {
                    continue;
                };
                let id = RemoteId::Path(RemotePath::new(&source_ref));
                match backend.dematerialise(&id).await {
                    Ok(()) => released += 1,
                    Err(e) => log::debug!("releasing {source_ref}: {e}"),
                }
            }
            log::info!("released {released} photograph(s) back to the sync client");
            let _ = tx.send(released);
        });
    });

    rx
}

/// What an image occupies, as the catalog recorded it.
///
/// Zero where the scan could not tell — a placeholder reports no size, because
/// a one-byte stub says nothing about what it stands for (ARCH §9.0a). A pin
/// that cannot state its cost is better than one that states a wrong one.
fn size_of(catalog: &Catalog, image: dr_types::ImageId) -> Option<u64> {
    catalog
        .connection()
        .query_row(
            "SELECT file_size FROM images WHERE id = ?1",
            rusqlite::params![image.0 as i64],
            |r| r.get::<_, Option<i64>>(0),
        )
        .ok()
        .flatten()
        .map(|v| v.max(0) as u64)
}

/// The remote path for a catalogued image.
fn source_ref_of(catalog: &Catalog, image: dr_types::ImageId) -> Option<String> {
    catalog
        .connection()
        .query_row(
            "SELECT source_ref FROM images WHERE id = ?1",
            rusqlite::params![image.0 as i64],
            |r| r.get(0),
        )
        .ok()
}

/// TRACES: FR-NC-6a | FR-CAT-9
/// Where a cached original is kept and how much may be kept.
///
/// Passed in rather than derived here so the caller owns the policy: the
/// budget is a user setting, and this function is on a worker thread with no
/// access to one.
pub struct CacheContext {
    pub dir: PathBuf,
    pub catalog_path: PathBuf,
    pub image: dr_types::ImageId,
    pub budget: dr_catalog::Budget,
    /// Whether a downloaded original is kept.
    ///
    /// Only the write. A cache is always *read*, because bytes already on disk
    /// cost nothing to use and declining them would re-download an image that
    /// is present — including every pinned one, which would leave a pinned
    /// collection unopenable offline the moment this was switched off.
    pub store: bool,
}

/// TRACES: FR-CAT-8 | FR-DEV-6
/// Fetch and parse the sidecar beside one image.
///
/// # Why the edit is read from the file rather than the catalog
///
/// The catalog carries a `graph_hash` and no parameters, and it is
/// *disposable* (ARCH §6.12) — a rebuild would silently return every
/// photograph to neutral. The sidecar is the authoritative store, so it is
/// what an open reads, and that is also what makes an edit pasted on the
/// desktop appear when the same frame is opened on the phone.
///
/// # Why absence and failure are the same answer here
///
/// `None` means "open this image at its defaults", which is right for a
/// photograph that has never been edited — the overwhelmingly common case on a
/// fresh library — and equally right when the network is down. The alternative,
/// refusing to open the image because its sidecar could not be read, would make
/// an unreachable server also mean an unviewable library.
///
/// The one case that is *not* harmless is a sidecar that exists but does not
/// parse. That still opens at defaults, but the write path
/// ([`write_one_sidecar`]) independently refuses to overwrite a file it could
/// not read, so an edit this build failed to understand is never destroyed by
/// having been opened.
pub fn spawn_sidecar_fetch(
    conn: Connection,
    image_path: String,
    cache_dir: PathBuf,
    offline: bool,
) -> Receiver<Option<dr_pipeline::Sidecar>> {
    let (tx, rx) = std::sync::mpsc::channel();

    std::thread::spawn(move || {
        let cache = SidecarCache::open(cache_dir);
        let path_str = sidecar_path(&image_path);

        // TRACES: FR-CAT-9 | FR-NC-10
        // The cache wins outright when it is holding work the server has not
        // seen. Fetching in that state would answer with a document *older*
        // than the edit sitting in the outbox, and opening the photograph
        // would silently show it without the change the user just made —
        // which the next save would then write back over the top of.
        if cache.is_pending(&path_str) {
            log::debug!("{path_str} has queued local edits; opening from the cache");
            let _ = tx.send(cache.load(&path_str));
            return;
        }

        // Offline there is nothing to ask, and the cache is the whole answer.
        let rt = if offline {
            None
        } else {
            match crate::net_runtime::build() {
                Ok(e) => Some(e),
                Err(e) => {
                    log::debug!("sidecar fetch runtime: {e}");
                    None
                }
            }
        };
        let Some(rt) = rt else {
            let _ = tx.send(cache.load(&path_str));
            return;
        };

        rt.block_on(async {
            let backend = match crate::remote::connect(&conn) {
                Ok(b) => b,
                Err(e) => {
                    log::debug!("sidecar fetch backend: {e}");
                    let _ = tx.send(cache.load(&path_str));
                    return;
                }
            };

            let path = RemotePath::new(path_str.clone());
            let id = RemoteId::Path(path.clone());

            // A 404 is the normal case on a library that has never been
            // edited, so this is `ok()` rather than an error path.
            let Ok(bytes) = backend.get(&id, None).await else {
                // Unreachable, or no such file. The cache cannot tell those
                // apart and does not need to: either way it holds the best
                // answer this device has.
                let _ = tx.send(cache.load(&path_str));
                return;
            };

            let text = String::from_utf8_lossy(&bytes).into_owned();
            let parsed = match dr_pipeline::Sidecar::parse(&text) {
                Ok(mut s) => {
                    // TRACES: FR-NC-8 | FR-NC-9
                    // Opening a photograph must show everything that has been
                    // done to it, not whichever of two split default versions
                    // happens to win. Fused in memory with no canonical uuid
                    // to impose — this is a read, and the write path is where
                    // the identity is decided.
                    //
                    // The fused document is what gets cached below, so the
                    // next offline open sees the union too.
                    s.fuse_default_versions(None);
                    Some(s)
                }
                Err(e) => {
                    log::warn!("sidecar at {} is unreadable ({e})", path.as_str());
                    None
                }
            };

            // Populate the cache from what the server said, so the *next*
            // open of this photograph works with no connection. Clean rather
            // than pending: this content came from the server, so there is
            // nothing to send back.
            if let Some(sidecar) = parsed.as_ref() {
                if let Err(e) = cache.store(&path_str, sidecar, false) {
                    log::debug!("caching {path_str}: {e}");
                }
            }

            let _ = tx.send(parsed);
        });
    });

    rx
}

/// Fetch one file in full, for opening it in develop.
///
/// Deliberately *not* the preview path. Browsing fetches a range and decodes
/// an embedded JPEG (FR-NC-3); develop needs every byte, because demosaic
/// needs every photosite. On a RAW file that is tens of megabytes, which is
/// why this is a click-triggered download and not something the grid does.
///
/// # Read-through
///
/// With a `cache`, this checks disk before the network and stores what it
/// downloads. That is what makes opening the same photograph twice cost one
/// transfer, and what leaves a working session's images openable offline
/// without anyone having pinned anything.
///
/// A cache miss is not an error and a cache failure is not fatal: both fall
/// through to the network, which is exactly the behaviour that existed before
/// the cache did.
///
/// Returns the bytes on a channel rather than blocking: the download runs on
/// its own thread and the UI stays live, exactly as thumbnail fetching does.
/// The work itself is [`fetch_original`], which is also what the
/// [`Prefetcher`] runs — one transfer path, so a photograph fetched ahead is
/// stored exactly as one fetched on a click.
pub fn spawn_full_fetch(
    conn: Connection,
    path: String,
    cache: Option<CacheContext>,
) -> Receiver<Result<Vec<u8>, FetchFailure>> {
    let (tx, rx) = std::sync::mpsc::channel();
    std::thread::spawn(move || {
        let _ = tx.send(fetch_original(conn, &path, cache.as_ref()));
    });
    rx
}

/// TRACES: FR-NC-6a
/// Every original transfer that is under way right now, by remote path.
///
/// **One file, one transfer.** The [`Prefetcher`] fetches the photographs
/// beside the open one before they are asked for, and the whole point is that
/// the user then asks for one of them — often while it is still coming down.
/// Without this the click would miss the cache, start a second download of
/// the same file, and the two would halve each other's bandwidth for the rest
/// of the transfer. With it, the click finds the path claimed, waits for the
/// prefetch to store its bytes, and reads them from disk.
///
/// Process-wide rather than passed in, because the property it enforces is
/// process-wide: there is no caller for whom two concurrent downloads of one
/// file is the right answer. Keyed on the path rather than the image id
/// because that is the one name every caller has.
static IN_FLIGHT: std::sync::LazyLock<InFlight> = std::sync::LazyLock::new(InFlight::default);

#[derive(Default)]
struct InFlight {
    busy: std::sync::Mutex<std::collections::HashSet<String>>,
    freed: std::sync::Condvar,
}

impl InFlight {
    /// Take `path` for this thread, or wait for whoever holds it.
    ///
    /// `Some` is a claim, released when the guard drops. `None` means another
    /// thread held the path and has now let it go — so the caller's cache
    /// check is worth repeating, because that thread has very probably just
    /// stored what the caller was about to download.
    fn claim(&self, path: &str) -> Option<InFlightGuard<'_>> {
        let mut busy = self.busy.lock().unwrap_or_else(|e| e.into_inner());
        if busy.insert(path.to_string()) {
            return Some(InFlightGuard {
                of: self,
                path: path.to_string(),
            });
        }
        while busy.contains(path) {
            busy = self.freed.wait(busy).unwrap_or_else(|e| e.into_inner());
        }
        None
    }

    fn release(&self, path: &str) {
        self.busy
            .lock()
            .unwrap_or_else(|e| e.into_inner())
            .remove(path);
        self.freed.notify_all();
    }
}

/// A claim on a path, dropped on every exit from the fetch — a failed
/// download must free the path too, or the click waiting on it never wakes.
struct InFlightGuard<'a> {
    of: &'a InFlight,
    path: String,
}

impl Drop for InFlightGuard<'_> {
    fn drop(&mut self) {
        self.of.release(&self.path);
    }
}

/// The blocking body of [`spawn_full_fetch`]: cache, then in-flight registry,
/// then network, storing what it downloads when the cache says to.
fn fetch_original(
    conn: Connection,
    path: &str,
    cache: Option<&CacheContext>,
) -> Result<Vec<u8>, FetchFailure> {
    // Opened on this thread: `rusqlite::Connection` is not `Send`, and the
    // UI thread's handle cannot be borrowed across the spawn.
    let cached = cache.and_then(|c| {
        let store = dr_catalog::Cache::open(&c.dir, c.budget).ok()?;
        let conn = Catalog::open(&c.catalog_path).ok()?;
        Some((store, conn))
    });

    let from_cache = || {
        let (c, (store, conn)) = (cache?, cached.as_ref()?);
        match store.load(conn.connection(), c.image, now_secs()) {
            Ok(Some(bytes)) => {
                log::info!("{path}: {} bytes from the local cache", bytes.len());
                Some(bytes)
            }
            Ok(None) => None,
            // A cache that cannot be read is a cache miss, not a failure
            // to open the photograph.
            Err(e) => {
                log::debug!("cache lookup for {path}: {e}");
                None
            }
        }
    };

    // Miss, claim, and if the claim had to wait, look again: the thread that
    // held the path has finished with it, and what it fetched is on disk.
    let _claim = loop {
        if let Some(bytes) = from_cache() {
            return Ok(bytes);
        }
        if let Some(claim) = IN_FLIGHT.claim(path) {
            break claim;
        }
    };

    let rt = crate::net_runtime::build().map_err(FetchFailure::local)?;

    rt.block_on(async {
        let backend = crate::remote::connect(&conn).map_err(FetchFailure::local)?;

        let id = RemoteId::Path(RemotePath::new(path));
        let bytes = backend.get(&id, None).await?;

        // Store before returning, so the bytes are on disk by the time the
        // image is on screen. Doing it after would leave a window where
        // closing the app immediately lost the download.
        if let (Some(c), Some((store, conn))) = (cache.filter(|c| c.store), cached.as_ref()) {
            // `pinned: false` — this is the passive population. A pin is
            // something the user asks for explicitly; opening an image is
            // not that, and treating it as one would make the pinned set
            // grow silently and never be evicted.
            if let Err(e) = store.store(conn.connection(), c.image, path, &bytes, false, now_secs())
            {
                log::debug!("caching {path}: {e}");
            } else if let Err(e) = store.enforce(conn.connection()) {
                log::debug!("enforcing the cache budget: {e}");
            }
        }

        Ok(bytes)
    })
}

/// TRACES: FR-NC-6a | FR-UI-4
/// One original to fetch ahead of its being asked for.
pub struct PrefetchJob {
    pub path: String,
    pub cache: CacheContext,
}

/// What the [`Prefetcher`] is doing, for the activity list.
pub enum PrefetchEvent {
    /// A transfer has started for this path.
    Started(String),
    /// And has ended — stored, or not; either way the row can go.
    Ended(String),
}

/// TRACES: FR-NC-6a | FR-UI-4
/// Fetches the photographs beside the open one into the cache, ahead of the
/// step that asks for them.
///
/// **Why this exists.** Walking the photo roll is one click per frame, and
/// without this every click is a download of tens of megabytes with a
/// "Downloading…" line over an empty canvas. A photographer moving between a
/// pair of near-identical frames does that a dozen times. Fetching the two
/// neighbours while the current photograph is being looked at turns the next
/// step into a disk read, which is what makes stepping feel like stepping.
///
/// **One worker, one wish.** A single thread serves the *latest* request and
/// nothing older. Each open replaces the previous wish outright, so a fast
/// walk along the roll does not leave a trail of stale downloads competing
/// with the one the user is actually waiting on; a job already under way is
/// finished rather than abandoned, because the bytes are mostly here. Jobs
/// run in the order given — next before previous, since that is the way a
/// roll is mostly walked — and one at a time, so two neighbours never halve
/// each other's bandwidth.
///
/// **What it never does.** It never fetches into a cache that would not keep
/// the bytes: the caller only hands it jobs whose cache stores, because a
/// prefetch that is discarded on arrival is pure transfer for nothing — and
/// "keep opened originals" being off is the user saying this device is
/// metered or small (FR-NC-6). It never starts while offline, for the same
/// reason. And it fetches only the immediate neighbours: originals are
/// "explicit pin or on-demand open only", and ±1 is as far as "on demand"
/// honestly stretches.
pub struct Prefetcher {
    shared: std::sync::Arc<PrefetchShared>,
    events: Receiver<PrefetchEvent>,
}

#[derive(Default)]
struct PrefetchShared {
    wanted: std::sync::Mutex<Wanted>,
    changed: std::sync::Condvar,
}

/// The latest wish, and a generation so the worker can tell it has been
/// replaced mid-list.
#[derive(Default)]
struct Wanted {
    generation: u64,
    conn: Option<Connection>,
    jobs: Vec<PrefetchJob>,
}

impl Wanted {
    /// Replace whatever was wanted with `jobs`.
    fn replace(&mut self, conn: Connection, jobs: Vec<PrefetchJob>) {
        self.generation += 1;
        self.conn = Some(conn);
        self.jobs = jobs;
    }

    /// Whether a wish taken at `generation` is still the current one.
    fn is_current(&self, generation: u64) -> bool {
        self.generation == generation
    }
}

impl Default for Prefetcher {
    fn default() -> Self {
        Self::new()
    }
}

impl Prefetcher {
    /// Start the worker. It sleeps until the first [`Self::want`].
    pub fn new() -> Self {
        let shared = std::sync::Arc::new(PrefetchShared::default());
        let (tx, events) = std::sync::mpsc::channel();
        let worker = shared.clone();
        std::thread::Builder::new()
            .name("prefetch".into())
            .spawn(move || serve_prefetches(&worker, &tx))
            .expect("spawning the prefetch worker");
        Self { shared, events }
    }

    /// Fetch these, in this order, instead of whatever was asked for before.
    ///
    /// An empty list is a valid wish: it cancels the rest of the previous
    /// one, and is what a photograph with no neighbours in the window asks.
    pub fn want(&self, conn: Connection, jobs: Vec<PrefetchJob>) {
        self.shared
            .wanted
            .lock()
            .unwrap_or_else(|e| e.into_inner())
            .replace(conn, jobs);
        self.shared.changed.notify_one();
    }

    /// Everything the worker has reported since the last poll.
    pub fn poll(&self) -> Vec<PrefetchEvent> {
        std::iter::from_fn(|| self.events.try_recv().ok()).collect()
    }
}

/// The worker: take the current wish, serve it job by job, stop the moment it
/// is superseded, sleep until the next one.
fn serve_prefetches(shared: &PrefetchShared, events: &Sender<PrefetchEvent>) {
    loop {
        let (generation, conn, jobs) = {
            let mut wanted = shared.wanted.lock().unwrap_or_else(|e| e.into_inner());
            while wanted.jobs.is_empty() {
                wanted = shared
                    .changed
                    .wait(wanted)
                    .unwrap_or_else(|e| e.into_inner());
            }
            let jobs = std::mem::take(&mut wanted.jobs);
            let Some(conn) = wanted.conn.clone() else {
                continue;
            };
            (wanted.generation, conn, jobs)
        };

        for job in jobs {
            let current = shared
                .wanted
                .lock()
                .unwrap_or_else(|e| e.into_inner())
                .is_current(generation);
            if !current {
                break;
            }
            if holds_original(&job.cache) {
                continue;
            }

            // A closed channel means the window is gone: nothing to fetch
            // for any more.
            if events
                .send(PrefetchEvent::Started(job.path.clone()))
                .is_err()
            {
                return;
            }
            match fetch_original(conn.clone(), &job.path, Some(&job.cache)) {
                Ok(bytes) => log::info!("{}: {} bytes fetched ahead", job.path, bytes.len()),
                // Not a failure anyone needs to hear about now: the click
                // that wants this photograph will try again and say so.
                Err(e) => log::debug!("fetching {} ahead: {}", job.path, e.message),
            }
            if events.send(PrefetchEvent::Ended(job.path)).is_err() {
                return;
            }
        }
    }
}

/// Whether the cache already has this original — a row check, not a read,
/// so asking costs nothing and touches no `last_used`.
fn holds_original(cache: &CacheContext) -> bool {
    let Ok(store) = dr_catalog::Cache::open(&cache.dir, cache.budget) else {
        return false;
    };
    let Ok(catalog) = Catalog::open(&cache.catalog_path) else {
        return false;
    };
    store.holds_original(catalog.connection(), cache.image)
}

/// Serve thumbnails for a set of rows: store first, network second.
///
/// The store is consulted before any request goes out, so a second launch —
/// or a second device that synced the shards — fills the grid with no transfer
/// at all. Only genuine misses reach the network.
pub fn spawn_thumbnails(
    conn: Connection,
    wanted: Vec<ThumbnailRequest>,
    store_dir: PathBuf,
    catalog_path: PathBuf,
) -> Receiver<ThumbnailMessage> {
    let (tx, rx) = std::sync::mpsc::channel();

    std::thread::spawn(move || {
        let mut store = match ThumbStore::open(&store_dir) {
            Ok(s) => Some(s),
            Err(e) => {
                // A broken store costs speed, never correctness — every
                // thumbnail can still be fetched.
                log::warn!("thumbnail store unavailable, fetching everything: {e}");
                None
            }
        };

        // Split the batch before delivering anything, so the plan can be
        // reported first and the UI knows the shape of the work up front.
        // Decoding happens here rather than in the split, because a corrupt
        // blob turns a hit into a miss.
        let mut hits = Vec::new();
        let mut to_fetch = Vec::new();
        // Images whose thumbnail is cached but whose date is still unknown.
        //
        // These need a header read even though no pixels are wanted. Without
        // this pass an image is dated *only* on the one visit that produced
        // its thumbnail — so a library browsed once before the EXIF code
        // existed, or synced from another device's shards, stays permanently
        // undated and never appears on the timeline.
        let mut metadata_only = Vec::new();

        for req in wanted {
            let stored = req
                .file_id
                .zip(store.as_ref())
                .and_then(|(id, s)| s.get(id, req.thumb_size).ok().flatten());

            match stored.map(|t| dr_thumbs::decode_rgba(&t.bytes)) {
                Some(Ok((width, height, rgba))) => {
                    if req.needs_metadata {
                        metadata_only.push(req.clone());
                    }
                    hits.push(ThumbnailReady {
                        row: req.row,
                        width,
                        height,
                        rgba,
                        from_cache: true,
                    });
                }
                // A corrupt stored blob is a miss, not a failure.
                Some(Err(e)) => {
                    log::debug!("stored thumbnail unreadable, refetching: {e}");
                    to_fetch.push(req);
                }
                None => to_fetch.push(req),
            }
        }

        log::info!(
            "thumbnails: {} from store, {} to fetch{}",
            hits.len(),
            to_fetch.len(),
            if metadata_only.is_empty() {
                String::new()
            } else {
                format!(" · {} dates to read", metadata_only.len())
            }
        );
        if tx
            .send(ThumbnailMessage::Plan {
                cached: hits.len(),
                fetching: to_fetch.len(),
                dating: metadata_only.len(),
            })
            .is_err()
        {
            return;
        }

        for hit in hits {
            if tx.send(ThumbnailMessage::Ready(Box::new(hit))).is_err() {
                return;
            }
        }

        if to_fetch.is_empty() && metadata_only.is_empty() {
            return;
        }

        let rt = match crate::net_runtime::build() {
            Ok(rt) => rt,
            Err(e) => {
                for req in &to_fetch {
                    let _ = tx.send(ThumbnailMessage::Unavailable {
                        row: req.row,
                        reason: e.to_string(),
                    });
                }
                return;
            }
        };

        rt.block_on(async {
            let backend = match crate::remote::connect(&conn) {
                Ok(b) => b,
                Err(e) => {
                    for req in &to_fetch {
                        let _ = tx.send(ThumbnailMessage::Unavailable {
                            row: req.row,
                            reason: e.to_string(),
                        });
                    }
                    return;
                }
            };

            // Batched rather than written per image: one transaction per
            // batch instead of 120, and the grid does not need each date the
            // instant it is read.
            let mut found = Vec::new();

            // Set when the server proves unreachable, which abandons the rest
            // of the batch. The remaining cells would each take a full timeout
            // to reach the same conclusion — on a 120-cell window, minutes of
            // the grid appearing to load against a server that is not there.
            let mut offline = false;

            for req in to_fetch {
                let msg = fetch_one(&*backend, store.as_mut(), &req, &mut found).await;
                offline = matches!(msg, ThumbnailMessage::Offline { .. });
                // A closed channel means the window went away mid-fetch.
                if tx.send(msg).is_err() || offline {
                    break;
                }
            }

            // Dates for images whose pixels were already cached. Header only —
            // no preview range, no decode.
            //
            // Flushed in chunks rather than once at the end: 119 sequential
            // header fetches take tens of seconds, and a single write at the
            // finish loses every one of them if the window closes first. It
            // also lets the timeline appear while the rest are still arriving.
            //
            // Skipped entirely when the connection has already failed: these
            // are network reads too, and there is nothing left to read from.
            const FLUSH_EVERY: usize = 16;
            if !offline {
                log::info!("reading dates for {} image(s)", metadata_only.len());
                for req in metadata_only {
                    if tx.send(ThumbnailMessage::DateProgress).is_err() {
                        break;
                    }
                    read_metadata_only(&*backend, &req, &mut found).await;

                    if found.len() >= FLUSH_EVERY {
                        flush_metadata(&catalog_path, &mut found, &tx);
                    }
                }
            }

            // Always flushed, even when the batch was abandoned: whatever was
            // read before the connection died is still true, and discarding it
            // would mean re-fetching those headers next time.
            flush_metadata(&catalog_path, &mut found, &tx);
        });
    });

    rx
}

async fn fetch_one(
    backend: &dyn RemoteBackend,
    store: Option<&mut ThumbStore>,
    req: &ThumbnailRequest,
    found_metadata: &mut Vec<MetadataFound>,
) -> ThumbnailMessage {
    let preview = match fetch_preview(backend, req, found_metadata).await {
        PreviewOutcome::Ready(p) => p,
        PreviewOutcome::Unavailable(reason) => {
            return ThumbnailMessage::Unavailable {
                row: req.row,
                reason,
            }
        }
        PreviewOutcome::Offline(reason) => return ThumbnailMessage::Offline { reason },
    };

    // Persist for next time, and for every other client that syncs the shard.
    // A store failure is logged and dropped: the pixels are already in hand,
    // and refusing to display them because they could not be cached would be
    // the wrong trade.
    if let (Some(store), Some(file_id)) = (store, req.file_id) {
        if let Some(thumb) = encode_preview(file_id, &preview) {
            store_thumbnail(store, file_id, req.thumb_size, &thumb);
        }
    }

    ThumbnailMessage::Ready(Box::new(ThumbnailReady {
        row: req.row,
        width: preview.width,
        height: preview.height,
        rgba: preview.rgba,
        from_cache: false,
    }))
}

/// What one fetch produced.
///
/// Separate from [`ThumbnailMessage`] because not every caller has a grid row
/// to report against or a store to write through. The whole-library pass
/// ([`spawn_thumbnail_sweep`]) fetches on several lanes at once and stores the
/// results on the one thread that owns the store, so it needs the pixels
/// *before* anything is written or addressed to a cell.
enum PreviewOutcome {
    Ready(dr_decode::Preview),
    /// This image has no usable preview. The batch continues past it.
    Unavailable(String),
    /// The server is unreachable, so nothing after this would succeed either.
    Offline(String),
}

/// Fetch a preview in two stages: header, then the exact preview range.
///
/// This is what FR-NC-3 specifies, and the single-stage version it replaces
/// was wrong in a way that looked like corruption: fetching a fixed prefix cut
/// the embedded JPEG partway through, and decoders render a truncated JPEG as
/// the top fraction of the frame rather than reporting an error.
async fn fetch_preview(
    backend: &dyn RemoteBackend,
    req: &ThumbnailRequest,
    found_metadata: &mut Vec<MetadataFound>,
) -> PreviewOutcome {
    let id = RemoteId::Path(RemotePath::new(&req.path));
    // A connection failure is not this image's verdict. Reported as such so
    // the caller can stop the batch rather than marking sixty cells
    // individually unpreviewable over one dropped connection — a state the
    // grid would then keep until something forced a reload.
    let classify = |e: dr_sync::RemoteError| {
        if e.indicates_offline() {
            PreviewOutcome::Offline(e.to_string())
        } else {
            PreviewOutcome::Unavailable(e.to_string())
        }
    };

    // Stage one: the header, enough to parse the container's IFDs.
    let header = match backend.get(&id, Some(0..dr_decode::HEADER_BYTES)).await {
        Ok(b) => b,
        Err(e) => return classify(e),
    };

    // The same bytes carry EXIF. Reading it here is free — the alternative is
    // a second 256 KB fetch per image over the whole library.
    if req.needs_metadata {
        collect_metadata(&header, req, found_metadata);
    }

    // Read unconditionally, unlike the rest of the EXIF above: `needs_metadata`
    // is false once an image has been catalogued, but a thumbnail can still be
    // regenerated long after that — a cleared cache, a new size — and a
    // thumbnail that came out upright the first time must come out upright
    // every time. This is a header walk, not a decode; see `dr_decode::orientation`.
    let orientation = dr_decode::orientation(&header).unwrap_or_default();

    // A plain JPEG is its own preview; anything else needs locating.
    let bytes = if header.starts_with(&[0xFF, 0xD8, 0xFF]) {
        match backend.get(&id, None).await {
            Ok(b) => b,
            Err(e) => return classify(e),
        }
    } else {
        let Some(loc) = dr_decode::locate_preview(&header, req.size) else {
            // No locatable preview. Declining beats fetching the whole file:
            // that is the 370 GB path FR-NC-3 exists to avoid.
            return PreviewOutcome::Unavailable("no locatable embedded preview".into());
        };
        if loc.len() > MAX_PREVIEW_BYTES {
            return PreviewOutcome::Unavailable(format!(
                "preview is {} bytes, too large",
                loc.len()
            ));
        }

        // Stage two: exactly the preview's bytes.
        match backend.get(&id, Some(loc.range.clone())).await {
            Ok(b) => b,
            Err(e) => return classify(e),
        }
    };

    // Verify before decoding. A truncated JPEG decodes "successfully" into a
    // partial frame, so without this the broken result reaches the cache and
    // the screen looking like a corrupt file.
    if !dr_decode::is_complete_jpeg(&bytes) {
        return PreviewOutcome::Unavailable("preview bytes are incomplete".into());
    }

    // Decode on the worker, never the UI thread.
    let mut preview = match dr_decode::decode_jpeg(&bytes) {
        Ok(p) => p,
        Err(e) => return PreviewOutcome::Unavailable(e.to_string()),
    };
    // Face indexing keeps the detail; every other caller is filling a cell of a
    // known size and the full preview is waste from here on.
    preview.downscale_to(if req.full_resolution {
        FACE_SOURCE_EDGE
    } else {
        req.thumb_size.edge()
    });
    // Turn it the right way up before it is measured, cached or shown. An
    // embedded preview is written in the sensor's orientation, so without this
    // every frame shot in portrait lies on its side in the grid — and, because
    // the cache is keyed by file and size alone, stays that way.
    //
    // After the downscale, so the permutation moves thumbnail-sized bytes
    // rather than the full preview's.
    //
    // **Doing it here is what keeps face geometry honest.** Detection runs on
    // whatever this returns, so returning the photograph rather than the sensor
    // means every box and landmark is already in the space the catalog stores
    // and the develop overlay draws — no second mapping to get backwards, which
    // is the one orientation bug this codebase keeps having. It costs a
    // permutation of a larger buffer for the face path; that is ~15 ms against
    // a decode of ~150 ms, and it buys the whole class of bug.
    preview.apply_orientation(orientation);

    PreviewOutcome::Ready(preview)
}

/// Compress a decoded preview to what the store holds.
///
/// Split from the write so the whole-library pass can do it on the lane that
/// fetched the image: encoding is the one part of storing a thumbnail that
/// costs CPU rather than the store's lock, and it turns a 256 KB RGBA buffer
/// into ~20 KB before the chunk is handed to the single thread that owns the
/// store.
fn encode_preview(file_id: u64, preview: &dr_decode::Preview) -> Option<dr_thumbs::Thumbnail> {
    match dr_thumbs::encode_rgba(preview.width, preview.height, &preview.rgba) {
        Ok(bytes) => Some(dr_thumbs::Thumbnail {
            width: preview.width,
            height: preview.height,
            bytes,
        }),
        Err(e) => {
            log::debug!("encoding thumbnail {file_id}: {e}");
            None
        }
    }
}

/// Put a thumbnail in the store, logging rather than failing.
///
/// A store failure costs a re-fetch next time and nothing else — the pixels
/// are already in hand, and the caller has something to show or count either
/// way (ARCH §6.12: the store is derived, never authoritative).
fn store_thumbnail(
    store: &mut ThumbStore,
    file_id: u64,
    size: dr_thumbs::ThumbSize,
    thumb: &dr_thumbs::Thumbnail,
) -> bool {
    match store.put(file_id, size, thumb) {
        Ok(_) => true,
        Err(e) => {
            log::debug!("storing thumbnail {file_id}: {e}");
            false
        }
    }
}

/// Parse EXIF out of a header and record it.
///
/// Shared by both paths — the thumbnail fetch, which gets the header anyway,
/// and the header-only pass for images whose pixels were already cached.
fn collect_metadata(header: &[u8], req: &ThumbnailRequest, out: &mut Vec<MetadataFound>) {
    let Ok(md) = dr_decode::metadata(header) else {
        return;
    };
    out.push(MetadataFound {
        image_id: req.image_id,
        captured_at: md.captured_at,
        captured_offset: md.captured_offset,
        camera: camera_label(md.make.as_deref(), md.model.as_deref()),
        lens: md.lens.map(|l| l.trim().to_string()),
        iso: md.iso,
    });
}

/// TRACES: FR-CAT-11
/// The camera string the catalog stores, from an EXIF make and model.
///
/// One definition rather than one per caller, because an import's duplicate
/// check compares against what a scan wrote (`dr_catalog::dedup`). Two
/// spellings of the same body would not fail loudly — they would silently
/// disable the cheap tier, and every re-inserted card would transfer in full
/// before the digest caught it.
pub fn camera_label(make: Option<&str>, model: Option<&str>) -> Option<String> {
    match (make, model) {
        // Bodies repeat the make inside the model ("Canon EOS 6D"), so
        // joining unconditionally yields "Canon Canon EOS 6D".
        (Some(make), Some(model)) if model.starts_with(make) => Some(model.trim().to_string()),
        (Some(make), Some(model)) => Some(format!("{} {}", make.trim(), model.trim())),
        (None, Some(model)) => Some(model.trim().to_string()),
        _ => None,
    }
}

/// Write a batch of dates and tell the UI, draining `found`.
///
/// Separate from the loop so the same path serves both the periodic flush and
/// the final one, and so a write failure is reported once rather than being
/// silently swallowed by the caller.
fn flush_metadata(
    catalog_path: &std::path::Path,
    found: &mut Vec<MetadataFound>,
    tx: &Sender<ThumbnailMessage>,
) {
    if found.is_empty() {
        return;
    }
    match Catalog::open(catalog_path) {
        Ok(cat) => match write_metadata(&cat, found) {
            Ok(n) => {
                log::info!("recorded capture dates for {n} of {} image(s)", found.len());
                // Tell the UI so the timeline can appear. Without this the
                // histogram only shows up on the next window load, which on a
                // fully cached library may be never.
                let _ = tx.send(ThumbnailMessage::DatesRecorded(n));
            }
            Err(e) => log::warn!("writing metadata: {e}"),
        },
        Err(e) => log::warn!("opening catalog to write metadata: {e}"),
    }
    found.clear();
}

/// Read only the date for an image whose thumbnail is already cached.
///
/// One 256 KB header request, no preview range and no decode. This is what
/// gets a library dated when its thumbnails came from the store — including
/// shards synced from another device, which carry pixels but no metadata.
/// Read a header for its date.
///
/// Returns whether the file was **reached**, which the caller needs and cannot
/// otherwise tell: a header that carried no EXIF and a fetch that never
/// happened both leave `found` untouched, and recording the second as "this
/// image has no date" would let one lock mark it dateless for good.
async fn read_metadata_only(
    backend: &dyn RemoteBackend,
    req: &ThumbnailRequest,
    found: &mut Vec<MetadataFound>,
) -> bool {
    let id = RemoteId::Path(RemotePath::new(&req.path));

    // Retried, because one failure here is usually a lock rather than a
    // verdict. Nextcloud's file locking answers a plain *read* with 423 under
    // concurrency, and the identical range succeeds moments later — measured
    // against a real server while twelve lanes were running. Without a retry
    // those images sit out the whole pass over a lock that lasted a moment.
    //
    // Bounded and short: a genuinely missing or forbidden file must not cost
    // three round trips before the sweep moves on.
    const ATTEMPTS: usize = 3;
    for attempt in 1..=ATTEMPTS {
        match backend.get(&id, Some(0..dr_decode::HEADER_BYTES)).await {
            Ok(header) => {
                collect_metadata(&header, req, found);
                return true;
            }
            Err(e) if e.is_transient() && attempt < ATTEMPTS => {
                // Backing off at all matters more than the exact interval: the
                // contention that produced the lock is our own lanes, so any
                // pause lets the holder finish.
                tokio::time::sleep(std::time::Duration::from_millis(200 * attempt as u64)).await;
            }
            Err(e) => {
                // Not surfaced: a missing date leaves the image off the
                // timeline rather than breaking anything, and the next sweep
                // retries it regardless.
                log::debug!("reading date for {} ({attempt} attempts): {e}", req.path);
                return false;
            }
        }
    }
    false
}

/// Write capture metadata read during the thumbnail pass.
///
/// Promotes each row from `metadata_state = 1` (stat-only) to 2 (full EXIF),
/// which is what makes it eligible for the timeline. A row whose EXIF was
/// unreadable stays at 1 rather than being marked done with empty fields, so a
/// later attempt can retry it.
///
/// Returns how many rows were promoted.
pub fn write_metadata(
    catalog: &Catalog,
    found: &[MetadataFound],
) -> Result<usize, dr_catalog::CatalogError> {
    let conn = catalog.connection();
    let tx = conn.unchecked_transaction()?;
    let mut promoted = 0;

    for m in found {
        // Only a real timestamp counts as fully read. Camera and lens without
        // a date leave the image unplaceable on a timeline, which is exactly
        // the state the grid needs to distinguish.
        let state = if m.captured_at.is_some() { 2 } else { 1 };

        tx.execute(
            "UPDATE images
                SET captured_at     = coalesce(?2, captured_at),
                    captured_offset = coalesce(?3, captured_offset),
                    camera          = coalesce(?4, camera),
                    lens            = coalesce(?5, lens),
                    iso             = coalesce(?6, iso),
                    metadata_state  = max(metadata_state, ?7)
              WHERE id = ?1",
            rusqlite::params![
                m.image_id,
                m.captured_at,
                m.captured_offset,
                m.camera,
                m.lens,
                m.iso,
                state,
            ],
        )?;
        if state == 2 {
            promoted += 1;
        }
    }

    tx.commit()?;
    Ok(promoted)
}

/// Progress from the whole-library sweep.
#[derive(Debug)]
pub enum SweepMessage {
    /// How many images still need work, counted once at the start.
    Total(usize),
    /// Another chunk finished. Carries cumulative counts.
    Progress {
        done: usize,
        dated: usize,
    },
    Finished {
        dated: usize,
    },
}

/// Await every future concurrently, returning results in order.
///
/// A hand-rolled `join_all` rather than a `futures` dependency for one
/// function. Polling a `Vec` of futures in a loop is exactly what the crate's
/// version does; the ordering guarantee is what lets the caller pair results
/// back to their inputs.
async fn futures_join_all<F>(futures: impl IntoIterator<Item = F>) -> Vec<F::Output>
where
    F: std::future::Future,
{
    use std::pin::Pin;
    use std::task::Poll;

    // Boxed so each future has a stable address while it is polled in place.
    let mut pending: Vec<Option<Pin<Box<F>>>> =
        futures.into_iter().map(|f| Some(Box::pin(f))).collect();
    let mut done: Vec<Option<F::Output>> = (0..pending.len()).map(|_| None).collect();

    std::future::poll_fn(move |cx| {
        let mut all_ready = true;
        for (slot, out) in pending.iter_mut().zip(done.iter_mut()) {
            let Some(fut) = slot else { continue };
            match fut.as_mut().poll(cx) {
                Poll::Ready(v) => {
                    *out = Some(v);
                    // Dropped as soon as it completes, so a long-running lane
                    // does not hold a finished one's resources.
                    *slot = None;
                }
                Poll::Pending => all_ready = false,
            }
        }
        if all_ready {
            Poll::Ready(done.iter_mut().filter_map(Option::take).collect())
        } else {
            Poll::Pending
        }
    })
    .await
}

/// How many images one sweep chunk handles before committing.
///
/// Small enough that a kill loses little, large enough that the catalog is not
/// reopened per image. A multiple of [`SWEEP_LANES`] so every lane gets equal
/// work and no chunk ends with most lanes idle.
const SWEEP_CHUNK: usize = 96;

/// How many fetches the sweep keeps in flight.
///
/// Each is ~0.6 s of round-trip latency and almost no bandwidth — a 256 KB
/// header — so the sequential version spent essentially all its time waiting.
/// Twelve lanes turn ~3 hours into ~15 minutes on the reference library.
///
/// Deliberately bounded rather than unlimited: the grid's own interactive
/// fetches share this server, and a sweep that saturated the connection would
/// make browsing feel broken while it ran.
///
/// **Lowered from twelve after measuring.** Twelve produced 423 Locked on a
/// real server — Nextcloud's file locking answering a plain read under
/// contention we were creating ourselves. Six keeps most of the speedup
/// without provoking it; the retry above covers what still slips through.
const SWEEP_LANES: usize = 6;

/// Date **every** image in the library, not just the ones on screen.
///
/// Thumbnails are deliberately *not* fetched here. A thumbnail needs the
/// mutable store, which cannot be shared across the parallel lanes below, and
/// it costs 1–3 MB against a date's 256 KB. Dating the whole library is what
/// the timeline needs; thumbnails arrive as cells are actually browsed, which
/// is the FR-NC-3 posture anyway.
///
/// The grid's own fetches cover what is on screen; this covers the rest, so the
/// timeline describes the whole library rather than the part that happened to
/// be scrolled past. It is resumable by construction — each pass queries for
/// what is still missing, so a kill mid-sweep costs only the current chunk.
///
/// Runs at the back of the queue by design: it holds no lock the grid needs,
/// and its chunked commits keep write transactions short.
pub fn spawn_sweep(conn: Connection, catalog_path: PathBuf) -> Receiver<SweepMessage> {
    let (tx, rx) = std::sync::mpsc::channel();

    std::thread::spawn(move || {
        let catalog = match Catalog::open(&catalog_path) {
            Ok(c) => c,
            Err(e) => {
                // Silent failure here left the sweep looking like it had run
                // and found nothing: no progress, no error, 17,397 images
                // still unindexed.
                log::warn!(
                    "sweep: cannot open catalog at {}: {e}",
                    catalog_path.display()
                );
                let _ = tx.send(SweepMessage::Finished { dated: 0 });
                return;
            }
        };

        let outstanding = count_outstanding(&catalog).unwrap_or(0);
        if outstanding == 0 {
            let _ = tx.send(SweepMessage::Finished { dated: 0 });
            return;
        }
        log::info!("sweep: {outstanding} image(s) need a date or a thumbnail");
        if tx.send(SweepMessage::Total(outstanding)).is_err() {
            return;
        }

        let rt = match crate::net_runtime::build() {
            Ok(rt) => rt,
            Err(e) => {
                log::warn!("sweep: no runtime: {e}");
                return;
            }
        };

        rt.block_on(async {
            let Ok(backend) = crate::remote::connect(&conn) else {
                return;
            };

            let (mut done, mut dated) = (0usize, 0usize);
            loop {
                // Re-queried each pass rather than held as one long list: the
                // grid is dating images at the same time, and a stale list
                // would refetch what it already covered.
                let chunk = match next_outstanding(&catalog, SWEEP_CHUNK) {
                    Ok(c) if c.is_empty() => break,
                    Ok(c) => c,
                    Err(e) => {
                        log::warn!("sweep: {e}");
                        break;
                    }
                };

                let chunk_started = std::time::Instant::now();
                log::debug!(
                    "sweep: chunk of {} starting at image {}",
                    chunk.len(),
                    chunk[0].image_id
                );

                // Twelve lanes over the chunk. Each lane owns a disjoint slice
                // and its own `found` vector, so nothing is shared and no lock
                // is needed; the results are concatenated after the join.
                //
                // The thumbnail store is the exception — it is `&mut` and
                // cannot be shared — so lanes only *read* metadata and any
                // missing thumbnail is left to the interactive path. Dating the
                // library is what the sweep is for; thumbnails arrive as cells
                // are browsed.
                let lanes: Vec<Vec<&ThumbnailRequest>> = (0..SWEEP_LANES)
                    .map(|lane| chunk.iter().skip(lane).step_by(SWEEP_LANES).collect())
                    .collect();

                let results = futures_join_all(lanes.into_iter().map(|lane| {
                    let backend: &dyn RemoteBackend = &*backend;
                    async move {
                        let mut found = Vec::new();
                        let mut reached = Vec::new();
                        for req in lane {
                            if read_metadata_only(backend, req, &mut found).await {
                                reached.push(req.image_id);
                            }
                        }
                        (found, reached)
                    }
                }))
                .await;

                let mut found = Vec::new();
                let mut reached = std::collections::HashSet::new();
                for (lane_found, lane_reached) in results {
                    found.extend(lane_found);
                    reached.extend(lane_reached);
                }
                done += chunk.len();

                // An image whose header carried no EXIF at all yields nothing
                // to `found`, so nothing marks it examined and the next sweep
                // fetches it again — for ever. Darktable exports strip
                // metadata by default, and 2,188 of them in the reference
                // library meant 2,188 pointless round trips per run.
                //
                // Recorded as examined with no date: the file was read and
                // genuinely has none, which is a different state from "not
                // looked at yet" and must not be confused with it.
                let answered: std::collections::HashSet<i64> =
                    found.iter().map(|m| m.image_id).collect();
                // Only files actually read. One that could not be fetched is
                // left alone so the next pass retries it, rather than being
                // written off over a lock or a dropped connection.
                found.extend(
                    chunk
                        .iter()
                        .filter(|r| {
                            reached.contains(&r.image_id) && !answered.contains(&r.image_id)
                        })
                        .map(|r| MetadataFound {
                            image_id: r.image_id,
                            captured_at: None,
                            captured_offset: None,
                            camera: None,
                            lens: None,
                            iso: None,
                        }),
                );

                dated += found.iter().filter(|m| m.captured_at.is_some()).count();
                let read = answered.len();
                flush_sweep(&catalog, &mut found);
                log::info!(
                    "sweep: {done} done, {dated} dated ({read} read in {:.1}s)",
                    chunk_started.elapsed().as_secs_f64()
                );

                if tx.send(SweepMessage::Progress { done, dated }).is_err() {
                    return;
                }
            }

            log::info!("sweep complete: {dated} date(s) recorded over {done} image(s)");
            let _ = tx.send(SweepMessage::Finished { dated });
        });
    });

    rx
}

/// How many images still lack a date or a thumbnail.
fn count_outstanding(catalog: &Catalog) -> Result<usize, dr_catalog::CatalogError> {
    let n: i64 = catalog.connection().query_row(
        &format!(
            "SELECT count(*) FROM images
               WHERE metadata_state < 2 AND {VISIBLE_UNALIASED}"
        ),
        [],
        |r| r.get(0),
    )?;
    Ok(n as usize)
}

/// The next images needing work.
///
/// Ordered by id so the sweep advances deterministically and a resumed run
/// picks up where it left off rather than revisiting.
fn next_outstanding(
    catalog: &Catalog,
    limit: usize,
) -> Result<Vec<ThumbnailRequest>, dr_catalog::CatalogError> {
    let mut stmt = catalog.connection().prepare(&format!(
        "SELECT i.id, i.source_ref, r.file_id, i.file_size
           FROM images i
           LEFT JOIN remote r ON r.image_id = i.id
          WHERE i.metadata_state < 2 AND {VISIBLE}
          ORDER BY i.id
          LIMIT ?1"
    ))?;
    let rows = stmt
        .query_map([limit as i64], |r| {
            Ok(ThumbnailRequest {
                // The sweep indexes dates, and reads headers only — the size
                // never reaches a fetch, but it must name something.
                thumb_size: dr_thumbs::ThumbSize::Grid,
                // Row index is meaningless here — the sweep touches no grid
                // cell, so nothing consumes it.
                row: 0,
                image_id: r.get(0)?,
                path: r.get(1)?,
                file_id: r.get::<_, Option<i64>>(2)?.map(|v| v as u64),
                size: r.get::<_, Option<i64>>(3)?.unwrap_or(0) as u64,
                needs_metadata: true,
                full_resolution: false,
            })
        })?
        .collect::<Result<Vec<_>, _>>()?;
    Ok(rows)
}

/// Commit a sweep chunk.
///
/// An image whose header yielded no date is still marked done, or the sweep
/// would revisit it forever. `write_metadata` records `metadata_state = 1` for
/// those, so this promotes them explicitly.
fn flush_sweep(catalog: &Catalog, found: &mut Vec<MetadataFound>) {
    if found.is_empty() {
        return;
    }
    if let Err(e) = write_metadata(catalog, found) {
        log::warn!("sweep: writing metadata: {e}");
        found.clear();
        return;
    }

    // Mark the dateless as examined. Without this they stay at state 1 and the
    // sweep loops over them on every pass, never terminating.
    let ids: Vec<i64> = found
        .iter()
        .filter(|m| m.captured_at.is_none())
        .map(|m| m.image_id)
        .collect();
    for id in ids {
        let _ = catalog
            .connection()
            .execute("UPDATE images SET metadata_state = 2 WHERE id = ?1", [id]);
    }
    found.clear();
}

/// TRACES: FR-CAT-3 | FR-NC-3 | NFR-RES-4
/// TRACES: FR-CULL-8
/// Every visible image this model has not been run over.
///
/// **No thumbnail-store filter.** The pass this feeds fetches its own pixels,
/// so an image with no proxy is work to be done rather than work to be skipped
/// — which is the whole difference between indexing a library and indexing the
/// fraction of it that has been browsed.
///
/// **An image whose faces are merely unmeasured is not here.** Schema V14
/// forgot the run marker of every such image so that *something* would look
/// at it again; [`faces_unmeasured`] is that something, and it does the cheap
/// thing — embed the faces already found — where this list would have the
/// whole detection run again. Both lists are drawn from the same catalog in
/// the same sweep, so the exclusion is in the query rather than left to the
/// caller to remember.
fn faces_unindexed(
    catalog: &Catalog,
    model_id: &str,
) -> Result<Vec<ThumbnailRequest>, dr_catalog::CatalogError> {
    let mut stmt = catalog.connection().prepare(&format!(
        "SELECT i.id, i.source_ref, r.file_id, i.file_size
           FROM images i
           JOIN remote r ON r.image_id = i.id
          WHERE r.file_id IS NOT NULL AND {VISIBLE}
            AND NOT EXISTS (
                SELECT 1 FROM face_index fi
                 WHERE fi.image_id = i.id AND {fi_embedder} = ?1
            )
            AND NOT EXISTS (
                SELECT 1 FROM faces f
                 WHERE f.image_id = i.id AND {f_embedder} = ?1 AND f.quality IS NULL
            )
          ORDER BY i.id",
        fi_embedder = dr_catalog::faces::embedder_sql("fi.model_id"),
        f_embedder = dr_catalog::faces::embedder_sql("f.model_id"),
    ))?;
    let rows = stmt
        .query_map([dr_catalog::faces::embedder_of(model_id)], |r| {
            Ok(ThumbnailRequest {
                // Face indexing wants the detail a thumbnail discards.
                full_resolution: true,
                // Named because the field must say something; ignored, because
                // `full_resolution` overrides it.
                thumb_size: dr_thumbs::ThumbSize::Large,
                // No grid cell waits on this.
                row: 0,
                image_id: r.get(0)?,
                path: r.get(1)?,
                file_id: r.get::<_, Option<i64>>(2)?.map(|v| v as u64),
                size: r.get::<_, Option<i64>>(3)?.unwrap_or(0) as u64,
                // The thumbnail sweep owns dating. Reading EXIF here would
                // write the same rows from a second pass for no gain.
                needs_metadata: false,
            })
        })?
        .collect::<Result<Vec<_>, _>>()?;
    Ok(rows)
}

/// Images whose faces have nothing left to be cut out of.
///
/// A face is stored normalised and drawn by cropping the proxy it was found on
/// (`identity::decode_proxy`). Where that proxy is gone the People screen draws
/// "no preview" for every cell and cannot repair itself, because the image
/// already has its `face_index` row and so is not outstanding work.
///
/// Two ways in: an indexing pass that fetched a preview and did not keep it,
/// and an ordinary cache eviction. Both are the same state, and re-running
/// detection over the image fixes it — `record_detections` replaces rather than
/// appends, and carries the user's confirmations across the replacement, so
/// this costs a fetch and loses nothing.
fn faces_without_proxy(
    catalog: &Catalog,
    store: &ThumbStore,
    model_id: &str,
) -> Result<Vec<ThumbnailRequest>, dr_catalog::CatalogError> {
    let mut stmt = catalog.connection().prepare(&format!(
        "SELECT DISTINCT i.id, i.source_ref, r.file_id, i.file_size
           FROM images i
           JOIN remote r ON r.image_id = i.id
           JOIN faces f ON f.image_id = i.id
          WHERE r.file_id IS NOT NULL AND {VISIBLE} AND {embedder} = ?1
          ORDER BY i.id",
        embedder = dr_catalog::faces::embedder_sql("f.model_id"),
    ))?;
    let rows = stmt
        .query_map([dr_catalog::faces::embedder_of(model_id)], |r| {
            Ok(ThumbnailRequest {
                full_resolution: true,
                thumb_size: dr_thumbs::ThumbSize::Large,
                row: 0,
                image_id: r.get(0)?,
                path: r.get(1)?,
                file_id: r.get::<_, Option<i64>>(2)?.map(|v| v as u64),
                size: r.get::<_, Option<i64>>(3)?.unwrap_or(0) as u64,
                needs_metadata: false,
            })
        })?
        .filter_map(Result::ok)
        .filter(|req| {
            req.file_id
                .is_some_and(|id| !store.contains(id, dr_thumbs::ThumbSize::Large))
        })
        .collect();
    Ok(rows)
}

/// TRACES: FR-CULL-9
/// Images holding a face this model found before its quality was kept.
///
/// The work list of the sweep's measuring pass: every stored face whose vector
/// is a unit one (schema V14) is embedded again from the native render, with
/// the landmarks it already has, and the raw vector and its length written
/// over it. Nothing is re-detected and no face changes identity — see
/// `dr_catalog::faces::record_measurements`.
///
/// Costs what the indexing pass costs per image, an original fetched and
/// rendered, because the length exists only at the moment of embedding and
/// there is no embedding without the pixels. What it saves is the detector,
/// and — the part that matters — every suggestion and confirmation on those
/// faces, which a re-detection would rebuild from box overlap.
fn faces_unmeasured(
    catalog: &Catalog,
    model_id: &str,
) -> Result<Vec<ThumbnailRequest>, dr_catalog::CatalogError> {
    let mut stmt = catalog.connection().prepare(&format!(
        "SELECT DISTINCT i.id, i.source_ref, r.file_id, i.file_size
           FROM images i
           JOIN remote r ON r.image_id = i.id
           JOIN faces f ON f.image_id = i.id
          WHERE r.file_id IS NOT NULL AND {VISIBLE}
            AND {embedder} = ?1 AND f.quality IS NULL
          ORDER BY i.id",
        embedder = dr_catalog::faces::embedder_sql("f.model_id"),
    ))?;
    let rows = stmt
        .query_map([dr_catalog::faces::embedder_of(model_id)], |r| {
            Ok(ThumbnailRequest {
                full_resolution: true,
                thumb_size: dr_thumbs::ThumbSize::Large,
                row: 0,
                image_id: r.get(0)?,
                path: r.get(1)?,
                file_id: r.get::<_, Option<i64>>(2)?.map(|v| v as u64),
                size: r.get::<_, Option<i64>>(3)?.unwrap_or(0) as u64,
                needs_metadata: false,
            })
        })?
        .collect::<Result<Vec<_>, _>>()?;
    Ok(rows)
}

/// Images indexed by a detector the chosen one outranks.
///
/// The tail of the sweep's work list: behind everything nothing has looked
/// at, because a photograph with no faces recorded is worth more than one
/// whose faces a weaker detector may have under-counted. `weaker` is
/// `FaceDetector::supersedes` for the current choice, and it is a list of
/// exact ids rather than "anything else sharing the embedder" so the
/// re-detection only ever runs upwards — a device set to the fast detector
/// leaves a peer's thorough pass alone.
///
/// The user's confirmations survive the re-detection by box overlap
/// (`faces::record_detections`), which is what makes this safe to run over a
/// library that is already named.
fn faces_superseded(
    catalog: &Catalog,
    weaker: &[&str],
) -> Result<Vec<ThumbnailRequest>, dr_catalog::CatalogError> {
    if weaker.is_empty() {
        return Ok(Vec::new());
    }
    let placeholders = weaker
        .iter()
        .enumerate()
        .map(|(i, _)| format!("?{}", i + 1))
        .collect::<Vec<_>>()
        .join(", ");
    let mut stmt = catalog.connection().prepare(&format!(
        "SELECT i.id, i.source_ref, r.file_id, i.file_size
           FROM images i
           JOIN remote r ON r.image_id = i.id
          WHERE r.file_id IS NOT NULL AND {VISIBLE}
            AND EXISTS (
                SELECT 1 FROM face_index fi
                 WHERE fi.image_id = i.id AND fi.model_id IN ({placeholders})
            )
          ORDER BY i.id"
    ))?;
    let rows = stmt
        .query_map(rusqlite::params_from_iter(weaker.iter()), |r| {
            Ok(ThumbnailRequest {
                full_resolution: true,
                thumb_size: dr_thumbs::ThumbSize::Large,
                row: 0,
                image_id: r.get(0)?,
                path: r.get(1)?,
                file_id: r.get::<_, Option<i64>>(2)?.map(|v| v as u64),
                size: r.get::<_, Option<i64>>(3)?.unwrap_or(0) as u64,
                needs_metadata: false,
            })
        })?
        .collect::<Result<Vec<_>, _>>()?;
    Ok(rows)
}

/// What the sweep does with one fetched original.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum SweepWork {
    /// Detect and embed from scratch: an image never indexed, or one whose
    /// faces have nothing left to be cut from.
    Detect,
    /// Embed the faces already found, again — see [`faces_unmeasured`].
    Measure,
}

/// TRACES: FR-CULL-8 | FR-EXP-9 | NFR-ARCH-2 | NFR-RES-2
/// Index faces across the **whole** library, at native resolution.
///
/// # The two resolutions, and why they are not one
///
/// FR-CULL-8 asks for a native render, a *reduction* for the detector, and the
/// crop taken back out of the native buffer. That is not three sizes for the
/// sake of it. The detector letterboxes whatever it is handed into a fixed
/// 640×640, so above that its input resolution decides nothing and paying for
/// it is waste; `align::warp` produces the fixed 112×112 ArcFace sees, so
/// *its* input resolution decides everything and economising there is a
/// silent loss. The two stages want opposite things, and a single buffer
/// serving both is how this pass previously came to store 47% of the
/// reference library's faces upsampled (faces.md §7b).
///
/// # Why this replaced a pass that read an embedded preview
///
/// The previous version range-fetched the JPEG preview embedded in each RAW
/// (FR-NC-3) and used it for both stages. It was cheap and it was the tier
/// FR-CULL-8 named at the time. On the reference library that preview tops out
/// at 3072 px against a ~6000 px sensor, which is what put those 8,505 faces
/// below the embedder's 112 px with no way to tell from the catalog that
/// anything was wrong.
///
/// Before that it filtered its work list to images with a `ThumbSize::Large`
/// proxy already on disk, which nothing filled for a whole library, so it
/// reached 220 images out of 23,529.
///
/// # Cost, stated plainly
///
/// **One whole original per un-indexed image, and one full render.** On the
/// reference library that is 412 GB and roughly a hundred minutes of decode —
/// a different order of thing from the byte ranges this used to pay, which is
/// why FR-CULL-8 makes a whole-library pass a transfer under FR-NC-6 rather
/// than something that may start on its own.
///
/// Nothing is kept that was not already wanted: the original is borrowed and
/// given back (ARCH §9.0a), and the only thing written per image is the
/// 1024 px proxy the People screen crops from, and only where a face was
/// found. Resumable by construction — the work list is what the catalog has no
/// `face_index` row for, so a kill costs the images in flight and nothing else.
#[allow(clippy::too_many_arguments)]
pub fn spawn_face_sweep(
    conn: Connection,
    catalog_path: PathBuf,
    store_dir: PathBuf,
    detector_model: PathBuf,
    embedder_model: PathBuf,
    model_id: String,
    supersedes: Vec<String>,
    options: dr_face::DetectOptions,
    gpu: dr_gpu::GpuContext,
) -> Receiver<crate::faces::FaceSweepMessage> {
    use crate::faces::FaceSweepMessage;
    let (tx, rx) = std::sync::mpsc::channel();

    std::thread::spawn(move || {
        let finish_empty = |tx: &Sender<FaceSweepMessage>| {
            let _ = tx.send(FaceSweepMessage::Finished {
                images: 0,
                faces: 0,
                failed: 0,
            });
        };

        let catalog = match Catalog::open(&catalog_path) {
            Ok(c) => c,
            Err(e) => {
                log::warn!("face sweep: cannot open catalog: {e}");
                finish_empty(&tx);
                return;
            }
        };

        // The proxy an indexed face is later *cut from*.
        //
        // `identity::decode_proxy` reads `FACE_TIER` out of this store to draw
        // the People screen, so a face found on a preview that was fetched and
        // dropped has nowhere to come from and the grid shows "no preview" for
        // every cell. Fetching the pixels and not keeping them was the whole
        // bug: they cost a round trip and a decode, and the crop needs them
        // again the moment the user looks at the person.
        let mut store = match ThumbStore::open(&store_dir) {
            Ok(s) => s,
            Err(e) => {
                log::warn!("face sweep: cannot open the thumbnail store: {e}");
                finish_empty(&tx);
                return;
            }
        };

        // Models before the work list: they are the expensive failure, and
        // listing twenty thousand images before discovering the weights are
        // missing helps nobody. A library with no model installed takes this
        // path, so it is a quiet return rather than an error.
        let mut detector = match dr_face::Detector::from_path(&detector_model) {
            Ok(d) => d,
            Err(e) => {
                log::warn!("face sweep: cannot load the detector: {e}");
                finish_empty(&tx);
                return;
            }
        };
        let mut embedder = match dr_face::Embedder::from_path(
            &embedder_model,
            dr_face::ModelId::new(model_id.clone()),
        ) {
            Ok(e) => e,
            Err(e) => {
                log::warn!("face sweep: cannot load the embedder: {e}");
                finish_empty(&tx);
                return;
            }
        };

        // **Repairs first, and the order is the whole point.** These are the
        // images the People screen is drawing *right now* and failing to, and
        // there are a few hundred of them against tens of thousands of
        // un-indexed ones. Appended instead, they sit two hours of fetching
        // down the queue and the screen stays empty for the whole session —
        // which is indistinguishable from the repair not existing.
        let mut wanted: Vec<(ThumbnailRequest, SweepWork)> =
            match faces_without_proxy(&catalog, &store, &model_id) {
                Ok(repair) => {
                    if !repair.is_empty() {
                        log::info!(
                        "face sweep: repairing {} image(s) whose faces have no proxy to crop from",
                        repair.len()
                    );
                    }
                    repair.into_iter().map(|r| (r, SweepWork::Detect)).collect()
                }
                Err(e) => {
                    log::warn!("face sweep: looking for orphaned faces: {e}");
                    Vec::new()
                }
            };

        // Then the faces to measure again. Behind the proxy repair and ahead
        // of the rest for the same reason that one leads: these are faces the
        // People screen is showing and grouping *now*, on vectors the gallery
        // rule cannot act on. An image already queued for a full re-detection
        // gets its quality from that, so it is not queued twice.
        let mut queued: std::collections::HashSet<i64> =
            wanted.iter().map(|(r, _)| r.image_id).collect();
        match faces_unmeasured(&catalog, &model_id) {
            Ok(measure) => {
                let fresh: Vec<_> = measure
                    .into_iter()
                    .filter(|r| queued.insert(r.image_id))
                    .collect();
                if !fresh.is_empty() {
                    log::info!(
                        "face sweep: measuring the faces on {} image(s) stored before their quality was kept",
                        fresh.len()
                    );
                }
                wanted.extend(fresh.into_iter().map(|r| (r, SweepWork::Measure)));
            }
            Err(e) => log::warn!("face sweep: looking for unmeasured faces: {e}"),
        }

        // Disjoint from both of the above by construction: an image with faces
        // recorded is not an image with no `face_index` row, and one whose
        // faces are unmeasured is excluded by the query.
        match faces_unindexed(&catalog, &model_id) {
            Ok(fresh) => wanted.extend(fresh.into_iter().map(|r| (r, SweepWork::Detect))),
            Err(e) => {
                log::warn!("face sweep: {e}");
                if wanted.is_empty() {
                    finish_empty(&tx);
                    return;
                }
            }
        }

        // Last: what a weaker detector already indexed. Everything above is
        // a photograph the People screen cannot show at all; these it shows
        // already, and the re-detection only adds the faces that detector
        // missed. See `faces_superseded`.
        let weaker: Vec<&str> = supersedes.iter().map(String::as_str).collect();
        let mut upgrades = 0usize;
        match faces_superseded(&catalog, &weaker) {
            Ok(older) => {
                let fresh: Vec<_> = older
                    .into_iter()
                    .filter(|r| queued.insert(r.image_id))
                    .collect();
                upgrades = fresh.len();
                wanted.extend(fresh.into_iter().map(|r| (r, SweepWork::Detect)));
            }
            Err(e) => log::warn!("face sweep: looking for images under a weaker detector: {e}"),
        }

        let total = wanted.len();
        if total == 0 {
            log::info!("face sweep: every image has been through this model");
            finish_empty(&tx);
            return;
        }
        if upgrades > 0 {
            log::info!(
                "face sweep: {total} image(s) to index, {upgrades} of them indexed by a weaker detector"
            );
        } else {
            log::info!("face sweep: {total} image(s) to index");
        }
        if tx.send(FaceSweepMessage::Total(total)).is_err() {
            return;
        }

        let rt = match crate::net_runtime::build() {
            Ok(rt) => rt,
            Err(e) => {
                log::warn!("face sweep: no runtime: {e}");
                finish_empty(&tx);
                return;
            }
        };

        rt.block_on(async {
            // Through `remote::connect`, which is the only place in the
            // interface that knows whose backend this is.
            let backend = match crate::remote::connect(&conn) {
                Ok(b) => b,
                Err(e) => {
                    log::warn!("face sweep: {e}");
                    finish_empty(&tx);
                    return;
                }
            };

            let (mut images, mut found, mut failed) = (0usize, 0usize, 0usize);
            let mut offline = false;

            // TRACES: FR-NC-6c
            // The same borrow the thumbnail sweep makes, and deliberately its
            // own pool: the two passes run at different times, so sharing one
            // would keep every file the earlier pass touched hydrated until
            // the later one finished. Each gives its own back (ARCH §9.0a).
            let pool = dr_sync_folder::BorrowPool::new();

            // TRACES: NFR-RES-2
            // **Fetch wide, render narrow.** The chunk is one original per
            // lane and not the 96 the preview sweep used, because the two
            // passes hold different things: that one kept an 8 MB preview per
            // image, this one keeps whole RAWs. Six at ~23 MB is a working set
            // a phone can carry; ninety-six is not.
            //
            // The render is then sequential, and that is not a limitation to
            // be optimised away later. There is one GPU, so concurrent renders
            // would queue on it anyway, and each one materialises a native
            // frame -- 96 MB for a 24 MP photograph. Overlapping them would
            // multiply the one allocation that actually threatens the budget
            // while buying no parallelism that exists.
            for chunk in wanted.chunks(SWEEP_LANES) {
                let fetched = futures_join_all(chunk.iter().map(|(req, work)| {
                    let backend = &*backend;
                    let pool = &pool;
                    async move {
                        let held = match pool.borrow(backend, &RemotePath::new(&req.path)).await {
                            Ok(h) => h,
                            Err(e) if e.indicates_offline() => {
                                log::info!("face sweep: {e}");
                                return (req, *work, Err(FetchOutcome::Offline));
                            }
                            Err(e) => {
                                log::debug!("face sweep: {}: {e}", req.path);
                                return (req, *work, Err(FetchOutcome::Failed));
                            }
                        };

                        // TRACES: FR-CULL-8
                        // The whole file, not FR-NC-3's byte range. The
                        // requirement now asks for a native render and there
                        // is no native render without the original -- which
                        // is why the pass is a transfer under FR-NC-6 and says
                        // so before it starts.
                        let id = RemoteId::Path(RemotePath::new(&req.path));
                        let got = match backend.get(&id, None).await {
                            Ok(b) => Ok(b),
                            Err(e) if e.indicates_offline() => {
                                log::info!("face sweep: server unreachable: {e}");
                                Err(FetchOutcome::Offline)
                            }
                            Err(e) => {
                                log::debug!("face sweep: {}: {e}", req.path);
                                Err(FetchOutcome::Failed)
                            }
                        };
                        drop(held);
                        (req, *work, got)
                    }
                }))
                .await;

                let mut lane_failed = 0usize;
                for (req, work, got) in fetched {
                    let bytes = match got {
                        Ok(b) => b,
                        Err(FetchOutcome::Offline) => {
                            offline = true;
                            continue;
                        }
                        Err(FetchOutcome::Failed) => {
                            lane_failed += 1;
                            continue;
                        }
                    };

                    if work == SweepWork::Measure {
                        let image = dr_types::ImageId(req.image_id as u64);
                        match measure_one_native(
                            &gpu,
                            &mut embedder,
                            &catalog,
                            image,
                            &model_id,
                            &bytes,
                        ) {
                            Ok(n) => {
                                images += 1;
                                found += n;
                                if tx
                                    .send(FaceSweepMessage::Indexed { image, faces: n })
                                    .is_err()
                                {
                                    log::info!("face sweep: cancelled after {images} image(s)");
                                    pool.release_all(&*backend).await;
                                    return;
                                }
                            }
                            Err(e) => {
                                log::debug!("face sweep: measuring {}: {e}", req.path);
                                lane_failed += 1;
                            }
                        }
                        continue;
                    }

                    match index_one_native(&gpu, &mut detector, &mut embedder, &bytes, &options) {
                        Ok((faces, edge, proxy)) => {
                            // Before the detections, so a kill between the two
                            // leaves a proxy with no faces recorded -- which
                            // the next pass simply re-indexes -- rather than
                            // faces with no proxy, which is the state that
                            // draws an empty grid and cannot repair itself.
                            if let (Some(file_id), Some(thumb)) = (req.file_id, proxy) {
                                store_thumbnail(
                                    &mut store,
                                    file_id,
                                    dr_thumbs::ThumbSize::Large,
                                    &thumb,
                                );
                            }
                            match dr_catalog::faces::record_detections(
                                catalog.connection(),
                                dr_types::ImageId(req.image_id as u64),
                                &model_id,
                                edge,
                                &faces,
                            ) {
                                Ok(_) => {
                                    images += 1;
                                    found += faces.len();
                                    if tx
                                        .send(FaceSweepMessage::Indexed {
                                            image: dr_types::ImageId(req.image_id as u64),
                                            faces: faces.len(),
                                        })
                                        .is_err()
                                    {
                                        // Receiver dropped: the screen closed,
                                        // or the user pressed Stop. Everything
                                        // written so far stays written -- and
                                        // everything borrowed is given back.
                                        log::info!("face sweep: cancelled after {images} image(s)");
                                        pool.release_all(&*backend).await;
                                        return;
                                    }
                                }
                                Err(e) => {
                                    log::warn!(
                                        "face sweep: storing faces for {}: {e}",
                                        req.image_id
                                    );
                                    lane_failed += 1;
                                }
                            }
                        }
                        Err(e) => {
                            // A file the decoder cannot open fails the same
                            // way on every pass, and every pass fetched it
                            // first: a 521 MB panorama the decoder refuses
                            // was downloaded once per sweep, on a tablet,
                            // and reported at debug level where nobody saw
                            // it. It is marked examined with nothing found,
                            // so the next pass does not fetch it again; the
                            // zero edge is what says why, and is what a
                            // later "try again with a better decoder" pass
                            // would select on. The count still says it
                            // failed, because it did.
                            log::warn!("face sweep: {}: {e}", req.path);
                            if let Err(e) = dr_catalog::faces::record_detections(
                                catalog.connection(),
                                dr_types::ImageId(req.image_id as u64),
                                &model_id,
                                0,
                                &[],
                            ) {
                                log::warn!(
                                    "face sweep: marking {} as unreadable: {e}",
                                    req.image_id
                                );
                            }
                            lane_failed += 1;
                        }
                    }
                }

                failed += lane_failed;
                if lane_failed > 0
                    && tx
                        .send(FaceSweepMessage::Failed {
                            images: lane_failed,
                        })
                        .is_err()
                {
                    log::info!("face sweep: cancelled after {images} image(s)");
                    pool.release_all(&*backend).await;
                    return;
                }

                if offline {
                    break;
                }
            }

            let returned = pool.release_all(&*backend).await;
            if returned.released > 0 {
                log::info!(
                    "face sweep: released {} borrowed file(s)",
                    returned.released
                );
            }

            log::info!(
                "face sweep: {found} face(s) across {images} image(s), {failed} failed{}",
                if offline { ", server went away" } else { "" }
            );
            let _ = tx.send(FaceSweepMessage::Finished {
                images,
                faces: found,
                failed,
            });
        });
    });

    rx
}

/// TRACES: FR-CULL-8 | FR-EXP-9
/// Open one original for a native render, orientation applied and nothing else.
///
/// The half of [`index_one_native`] that has nothing to do with faces, exposed
/// because measuring what this pass is worth means rendering the same file two
/// ways and comparing the crops — see `examples/face_native.rs`. A tool that
/// had to reimplement the render would be measuring its own reimplementation.
pub fn render_native(gpu: &dr_gpu::GpuContext, bytes: &[u8]) -> Result<dr_export::Frame, String> {
    open_native(gpu, bytes)?.render_for_export(dr_types::ColourSpace::Srgb)
}

/// The session behind [`render_native`], kept private because `DevelopSession`
/// is. The indexing path needs the session itself rather than just its frame:
/// it renders the People screen's proxy from the same open session rather than
/// opening the file twice.
fn open_native(
    gpu: &dr_gpu::GpuContext,
    bytes: &[u8],
) -> Result<crate::develop::DevelopSession, String> {
    // Whatever the header says, or an empty one for a file that has none: the
    // session takes its orientation from it, and remembers the rest for
    // anything that later exports from this session (FR-EXP-8).
    let meta = dr_decode::metadata(bytes).unwrap_or_default();
    crate::open_session(gpu, bytes, &meta)
}

/// Why one original did not arrive.
///
/// Named rather than a bool because the two mean opposite things to the loop:
/// one image failing is one image, and the server going away means nothing
/// after it would have worked either.
enum FetchOutcome {
    Failed,
    Offline,
}

/// TRACES: FR-CULL-8 | FR-EXP-9
/// Render one original at native resolution and index the faces in it.
///
/// The FR-CULL-8 pipeline end to end, for one photograph: decode, render
/// through the same path export uses, detect on a reduction, crop from the
/// native frame. Returns the faces, the native long edge that went into the
/// run marker, and the 1024px proxy the People screen later cuts thumbnails
/// from.
///
/// # Why the stored edit is not applied
///
/// `export::render_from_library` fetches the sidecar and applies it, because
/// an export is of the photograph the user has made. This is not: the face
/// geometry stored in the catalog is normalised to the frame, so applying a
/// crop would record faces against a frame that changes whenever the user
/// changes their mind, and every stored box would silently become wrong. What
/// is applied is the orientation, which is a fact about the file rather than
/// an edit.
fn index_one_native(
    gpu: &dr_gpu::GpuContext,
    detector: &mut dr_face::Detector,
    embedder: &mut dr_face::Embedder,
    bytes: &[u8],
    options: &dr_face::DetectOptions,
) -> Result<
    (
        Vec<dr_catalog::faces::DetectedFace>,
        u32,
        Option<dr_thumbs::Thumbnail>,
    ),
    String,
> {
    let mut session = open_native(gpu, bytes)?;
    let frame = session.render_for_export(dr_types::ColourSpace::Srgb)?;

    let edge = frame.width.max(frame.height);
    let faces = crate::faces::index_native(
        detector,
        embedder,
        &frame.rgba,
        frame.width as usize,
        frame.height as usize,
        options,
    )
    .map_err(|e| e.to_string())?;

    // Only where there is a face to cut out of it. Two thirds of a personal
    // library is landscapes and documents, and those never need a crop -- so
    // this fills the large class for the images the People screen will
    // actually ask about and leaves the rest alone.
    //
    // Rendered rather than downscaled from the frame in hand: the session is
    // still open and `render_thumbnail` is the path the grid's own thumbnails
    // take, so the proxy this writes is the one the store would have had
    // anyway.
    let proxy = if faces.is_empty() {
        None
    } else {
        match session.render_thumbnail(dr_thumbs::ThumbSize::Large.edge()) {
            Ok((w, h, rgba)) => match dr_thumbs::encode_rgba(w, h, &rgba) {
                Ok(bytes) => Some(dr_thumbs::Thumbnail {
                    width: w,
                    height: h,
                    bytes,
                }),
                Err(e) => {
                    log::debug!("encoding a face proxy: {e}");
                    None
                }
            },
            Err(e) => {
                log::debug!("rendering a face proxy: {e}");
                None
            }
        }
    };

    Ok((faces, edge, proxy))
}

/// TRACES: FR-CULL-8 | FR-CULL-9
/// Render one original at native resolution and embed its stored faces again.
///
/// [`index_one_native`]'s sibling for the measuring pass: the same render, no
/// detection, and the result written *over* the faces rather than in place of
/// them. Returns how many faces were measured.
///
/// The proxy is left alone. The faces here were found on a pass that stored
/// one, or [`faces_without_proxy`] would have claimed the image first.
fn measure_one_native(
    gpu: &dr_gpu::GpuContext,
    embedder: &mut dr_face::Embedder,
    catalog: &Catalog,
    image: dr_types::ImageId,
    model_id: &str,
    bytes: &[u8],
) -> Result<usize, String> {
    let faces = dr_catalog::faces::unmeasured_on_image(catalog.connection(), image, model_id)
        .map_err(|e| e.to_string())?;
    if faces.is_empty() {
        return Ok(0);
    }
    let frame = render_native(gpu, bytes)?;
    let edge = frame.width.max(frame.height);
    let measured = crate::faces::measure_native(
        embedder,
        &frame.rgba,
        frame.width as usize,
        frame.height as usize,
        &faces,
    )
    .map_err(|e| e.to_string())?;
    let n = measured.measured.len();
    dr_catalog::faces::record_measurements(
        catalog.connection(),
        image,
        model_id,
        edge,
        &measured.measured,
        &measured.dropped,
    )
    .map_err(|e| e.to_string())?;
    Ok(n)
}

/// The class the whole-library pass fills.
///
/// Grid only, deliberately. The large class is four times the transfer for a
/// detail only a zoomed cell or the loupe asks for — on the reference library
/// that is ~200 MB of shards against ~860 MB, paid by *every* device that
/// syncs them (see [`dr_thumbs::ThumbSize`]). A photograph actually looked at
/// closely still gets its large thumbnail from the interactive path.
const SWEEP_THUMB_SIZE: dr_thumbs::ThumbSize = dr_thumbs::ThumbSize::Grid;

/// Progress from the whole-library thumbnail pass.
#[derive(Debug)]
pub enum ThumbSweepMessage {
    /// How many images still lack a thumbnail, counted once at the start.
    Total(usize),
    /// Another chunk finished. Carries cumulative counts.
    Progress { done: usize, stored: usize },
    Finished {
        stored: usize,
        failed: usize,
        /// Stopped early because the server stopped answering. The pass is
        /// resumable, so this is "come back later", not a failure.
        offline: bool,
    },
}

/// TRACES: FR-CAT-3 | FR-NC-3 | FR-NC-7
/// Thumbnail **every** image in the library, not just the ones browsed.
///
/// # Why this exists next to the grid's own fetching
///
/// The interactive path fills cells as they are scrolled past, which is the
/// right posture for a remote library (FR-NC-3) and the wrong one for handing
/// the result to a second device: a tablet that syncs the shards inherits only
/// the fraction of the library its sibling happened to look at. This is the
/// deliberate, user-launched version of the same work — an hour of range
/// fetches paid once, on the machine that can afford it, so every other client
/// gets a full grid for the cost of a few hundred MB (`derived_sync`).
///
/// # Shape, and why it borrows the metadata sweep's
///
/// Chunked and lane-parallel exactly as [`spawn_sweep`] is, for the same
/// reason: each image is ~0.6 s of round-trip latency and almost no
/// bandwidth, so the sequential version spends its life waiting. What differs
/// is the store — it is `&mut` and cannot be shared across lanes, which is why
/// the metadata sweep skips thumbnails entirely. Here the lanes fetch, decode
/// and *encode*, and only the ~20 KB result crosses back to this thread, which
/// owns the store and writes the chunk in one go. So the parallelism is real
/// and the single-writer rule is never bent.
///
/// Dates arrive free: the header a preview needs is the header EXIF lives in,
/// so an image this pass reaches is dated on the same fetch rather than
/// costing a second one.
///
/// Resumable by construction — the work list is what the store does not have,
/// so a kill costs the chunk in flight and nothing more. An image with no
/// locatable preview is retried on a later run; it is one header fetch, and
/// the alternative is a second piece of state that has to be invalidated when
/// a file is replaced.
pub fn spawn_thumbnail_sweep(
    conn: Connection,
    catalog_path: PathBuf,
    store_dir: PathBuf,
) -> Receiver<ThumbSweepMessage> {
    let (tx, rx) = std::sync::mpsc::channel();

    std::thread::spawn(move || {
        let finish_empty = |tx: &Sender<ThumbSweepMessage>| {
            let _ = tx.send(ThumbSweepMessage::Finished {
                stored: 0,
                failed: 0,
                offline: false,
            });
        };

        let catalog = match Catalog::open(&catalog_path) {
            Ok(c) => c,
            Err(e) => {
                log::warn!(
                    "thumbnail sweep: cannot open catalog at {}: {e}",
                    catalog_path.display()
                );
                finish_empty(&tx);
                return;
            }
        };

        // Unlike the grid's fetch, which carries on without a store and simply
        // shows what it downloaded, a store that will not open ends this: the
        // pass exists to fill it, and running an hour of transfers with
        // nowhere to put them would be worse than not starting.
        let mut store = match ThumbStore::open(&store_dir) {
            Ok(s) => s,
            Err(e) => {
                log::warn!(
                    "thumbnail sweep: cannot open the thumbnail store at {}: {e}",
                    store_dir.display()
                );
                finish_empty(&tx);
                return;
            }
        };

        let wanted = match thumbnails_outstanding(&catalog, &store) {
            Ok(w) => w,
            Err(e) => {
                log::warn!("thumbnail sweep: {e}");
                finish_empty(&tx);
                return;
            }
        };

        let total = wanted.len();
        if total == 0 {
            log::info!("thumbnail sweep: every image already has a thumbnail");
            finish_empty(&tx);
            return;
        }
        log::info!("thumbnail sweep: {total} image(s) need a thumbnail");
        if tx.send(ThumbSweepMessage::Total(total)).is_err() {
            return;
        }

        let rt = match crate::net_runtime::build() {
            Ok(rt) => rt,
            Err(e) => {
                log::warn!("thumbnail sweep: no runtime: {e}");
                finish_empty(&tx);
                return;
            }
        };

        rt.block_on(async {
            let backend = match crate::remote::connect(&conn) {
                Ok(b) => b,
                Err(e) => {
                    log::warn!("thumbnail sweep: {e}");
                    finish_empty(&tx);
                    return;
                }
            };

            let (mut done, mut stored, mut failed) = (0usize, 0usize, 0usize);
            let mut offline = false;
            let mut found = Vec::new();

            // TRACES: FR-NC-6c
            // On a placeholder library the bytes may not be here at all, and
            // this is a pass the user asked for — so it may fetch them, which
            // browsing may not (ARCH §9.0a). Every file is *borrowed*: what
            // this pass downloads it gives back, and what the user already had
            // it leaves alone. Against a server or a plain folder every borrow
            // is a no-op, so there is one code path rather than two.
            let pool = dr_sync_folder::BorrowPool::new();

            for chunk in wanted.chunks(SWEEP_CHUNK) {
                // Each lane owns a disjoint slice and its own output, so
                // nothing is shared and no lock is needed. The store is not
                // touched here — see the note on the function.
                let lanes: Vec<Vec<&ThumbnailRequest>> = (0..SWEEP_LANES)
                    .map(|lane| chunk.iter().skip(lane).step_by(SWEEP_LANES).collect())
                    .collect();

                let results = futures_join_all(lanes.into_iter().map(|lane| {
                    let backend: &dyn RemoteBackend = &*backend;
                    let pool = &pool;
                    async move {
                        let mut made: Vec<(u64, dr_thumbs::Thumbnail)> = Vec::new();
                        let mut found = Vec::new();
                        let mut attempted = 0usize;
                        let mut failed = 0usize;
                        let mut offline = false;
                        for req in lane {
                            // Enforced by the query, which joins `remote`: an
                            // image with no file id has nothing to key the
                            // store on and is not a candidate.
                            let Some(file_id) = req.file_id else { continue };
                            attempted += 1;

                            // Held for this image only. A failure to fetch is
                            // this image's verdict, not the batch's: a client
                            // that cannot reach the server reports it as
                            // offline through the usual path below.
                            let _held =
                                match pool.borrow(backend, &RemotePath::new(&req.path)).await {
                                    Ok(h) => h,
                                    Err(e) if e.indicates_offline() => {
                                        log::info!("thumbnail sweep: {e}");
                                        attempted -= 1;
                                        offline = true;
                                        break;
                                    }
                                    Err(e) => {
                                        log::debug!("thumbnail sweep: {}: {e}", req.path);
                                        failed += 1;
                                        continue;
                                    }
                                };

                            match fetch_preview(backend, req, &mut found).await {
                                PreviewOutcome::Ready(preview) => {
                                    match encode_preview(file_id, &preview) {
                                        Some(thumb) => made.push((file_id, thumb)),
                                        None => failed += 1,
                                    }
                                }
                                PreviewOutcome::Unavailable(reason) => {
                                    log::debug!("thumbnail sweep: {}: {reason}", req.path);
                                    failed += 1;
                                }
                                // Nothing after this would reach the server
                                // either, so the lane stops rather than
                                // spending a timeout per remaining image.
                                PreviewOutcome::Offline(reason) => {
                                    log::info!("thumbnail sweep: server unreachable: {reason}");
                                    attempted -= 1;
                                    offline = true;
                                    break;
                                }
                            }
                        }
                        (made, found, attempted, failed, offline)
                    }
                }))
                .await;

                for (made, lane_found, attempted, lane_failed, lane_offline) in results {
                    done += attempted;
                    failed += lane_failed;
                    offline |= lane_offline;
                    found.extend(lane_found);
                    for (file_id, thumb) in made {
                        if store_thumbnail(&mut store, file_id, SWEEP_THUMB_SIZE, &thumb) {
                            stored += 1;
                        } else {
                            failed += 1;
                        }
                    }
                }

                // Committed per chunk rather than at the end, so a kill keeps
                // every date read so far — the same bargain the metadata sweep
                // makes, and for the same reason.
                flush_sweep(&catalog, &mut found);

                if tx
                    .send(ThumbSweepMessage::Progress { done, stored })
                    .is_err()
                {
                    // Cancelled. Hand back what was borrowed before leaving,
                    // or a stopped pass costs the disk of everything it had
                    // reached and delivers nothing for it.
                    pool.release_all(&*backend).await;
                    return;
                }
                if offline {
                    break;
                }
            }

            flush_sweep(&catalog, &mut found);

            // Give back everything this pass fetched, before reporting done —
            // a user watching the disk should see it return, and a pass that
            // reported success while still holding the library would be
            // lying about what it cost.
            let returned = pool.release_all(&*backend).await;
            if returned.released > 0 {
                log::info!(
                    "thumbnail sweep: released {} borrowed file(s)",
                    returned.released
                );
            }

            log::info!("thumbnail sweep: {stored} stored, {failed} without a usable preview");
            let _ = tx.send(ThumbSweepMessage::Finished {
                stored,
                failed,
                offline,
            });
        });
    });

    rx
}

/// Every visible image on the server that the store has no grid thumbnail for.
///
/// Joined against `remote` rather than left-joined: the store is keyed on
/// Nextcloud's `oc:fileid` (FR-NC-5), so an image the scan recorded without
/// one cannot be stored and is not work this pass can do.
///
/// The whole list is built up front rather than re-queried per chunk, unlike
/// the metadata sweep: "does the store have this" is answered by the store's
/// index, which this thread is also the one writing, so a stale list is not a
/// risk the way a concurrently-dating grid made it one there.
fn thumbnails_outstanding(
    catalog: &Catalog,
    store: &ThumbStore,
) -> Result<Vec<ThumbnailRequest>, dr_catalog::CatalogError> {
    let mut stmt = catalog.connection().prepare(&format!(
        "SELECT i.id, i.source_ref, r.file_id, i.file_size, i.metadata_state
           FROM images i
           JOIN remote r ON r.image_id = i.id
          WHERE r.file_id IS NOT NULL AND {VISIBLE}
          ORDER BY i.id"
    ))?;
    let rows = stmt
        .query_map([], |r| {
            let file_id = r.get::<_, Option<i64>>(2)?.map(|v| v as u64);
            Ok(ThumbnailRequest {
                thumb_size: SWEEP_THUMB_SIZE,
                // No grid cell is waiting on this, so nothing consumes the row.
                row: 0,
                image_id: r.get(0)?,
                path: r.get(1)?,
                file_id,
                size: r.get::<_, Option<i64>>(3)?.unwrap_or(0) as u64,
                // The header this fetch reads is the one EXIF lives in, so an
                // undated image is dated on the way past for nothing.
                needs_metadata: r.get::<_, i64>(4)? < 2,
                full_resolution: false,
            })
        })?
        .filter_map(Result::ok)
        .filter(|req| {
            req.file_id
                .is_some_and(|id| !store.contains(id, SWEEP_THUMB_SIZE))
        })
        .collect();
    Ok(rows)
}

/// Where an account's thumbnail shards live.
///
/// Beside the catalog rather than in the cache directory: these sync to the
/// server and are shared with other clients, so discarding them on a cache
/// sweep would cost a re-download for everyone.
pub fn thumbs_dir(account: &Account) -> PathBuf {
    catalog_path(account)
        .parent()
        .map(|p| p.join("thumbs"))
        .unwrap_or_else(|| std::env::temp_dir().join("darkroom-thumbs"))
}

/// Where the face models live, beside the catalog and the thumbnails.
///
/// **Not shipped with the application** and not a build input: the InsightFace
/// weights carry a non-commercial research grant incompatible with this
/// project's licence, so the user obtains them and the app loads them from here
/// (docs/faces.md §2). An absent directory is the ordinary state of a fresh
/// install, not an error.
pub fn face_models_dir(account: &Account) -> PathBuf {
    catalog_path(account)
        .parent()
        .map(|p| p.join("models"))
        .unwrap_or_else(|| std::env::temp_dir().join("darkroom-models"))
}

/// Where face models live for *every* account on this device.
///
/// Account-independent, unlike the catalog: a model is identified by
/// `faces.model_id` (catalog.md §10.1) and not by who is signed in, so two
/// accounts have no reason to hold two 15 MB copies of the same weights. This
/// is also the only directory an Android build can populate for itself — the
/// entry point extracts the APK's bundled copy here, and no session exists at
/// that point to key a per-account path off.
///
/// **That extraction races the first seconds of a launch and is meant to.** It
/// is 41 MB of copying and it used to happen before the first frame, which on a
/// tablet is an ANR (`darkroom-android`'s `install_bundled_models`). So a
/// lookup here can answer "absent" for a model that is on its way; each file is
/// renamed into place, so what a lookup never sees is a half-written one.
pub fn shared_face_models_dir() -> PathBuf {
    data_root().join("models")
}

/// The detector and embedder files, if both are present.
///
/// Both or neither: an embedder with no detector has nothing to embed, and a
/// detector with no embedder finds faces it cannot tell apart. Reporting the
/// pair missing is more useful than half-starting.
///
/// The names are the **shape-fixed** exports, not what InsightFace ships:
/// `tools/fix-face-model-shapes.sh` has to run over the originals first,
/// because tract cannot parse either graph with a dynamic input.
///
/// Searched in three places, most specific first:
///
/// 1. **The account's own directory.** A library can be pinned to its own
///    weights — a model swap is a `model_id` change and a re-index, and someone
///    mid-migration needs one account's pair to stay put without holding the
///    other back.
/// 2. **The shared user directory.** Where a user drops a pair by hand, and
///    where the Android entry point unpacks the copy the APK carries.
/// 3. **The system directories.** Where a package installs them — the Arch
///    package puts the pair in `/usr/share/darkroom/models`. Last, so anything
///    the user placed themselves outranks what the package shipped.
pub fn face_models(
    account: &Account,
    detector: dr_types::FaceDetector,
) -> Option<(PathBuf, PathBuf)> {
    let pair = |dir: &PathBuf| {
        let detector = dir.join(detector.file_name());
        let embedder = dir.join("arcface_mbf_b1.onnx");
        (detector.is_file() && embedder.is_file()).then_some((detector, embedder))
    };
    let mut searched = vec![face_models_dir(account), shared_face_models_dir()];
    searched.extend(system_face_models_dirs());
    let found = searched.iter().find_map(pair);
    if found.is_none() {
        // The settings page can only say "not installed". This is the line
        // that says where it looked, which is the whole of what a user with
        // the files in the wrong place needs — and the first thing to read
        // when a freshly installed package reports no model.
        log::warn!(
            "face models: no directory holds both {} and arcface_mbf_b1.onnx; searched {}",
            detector.file_name(),
            searched
                .iter()
                .map(|d| d.display().to_string())
                .collect::<Vec<_>>()
                .join(", ")
        );
    }
    found
}

/// The scene model, its vocabulary and its category descriptor, if all three
/// are present.
///
/// All three or none, for the same reason `face_models` insists on its pair:
/// the graph alone decodes to 150 anonymous channels, and a descriptor naming
/// classes a different model does not have is refused by
/// `dr_segment::scene::parse_categories` anyway. Reporting the set missing is
/// more useful than starting and failing at the first inference.
///
/// Searched in the same three places, most specific first — the account's own
/// directory, the shared one, then wherever a package installed them. Android
/// only ever finds the second, which is where `install_bundled_models` unpacks
/// the APK's copy before any store opens.
///
/// Unlike the face weights this model *is* in the repository, so a desktop
/// build from a complete checkout has it. Absent means either a checkout
/// without `git lfs pull` or a package that chose not to carry 24 MB, and the
/// scene tab reports itself unavailable rather than the app refusing to run.
pub fn scene_model(account: &Account) -> Option<(PathBuf, PathBuf, PathBuf)> {
    fn set(dir: PathBuf) -> Option<(PathBuf, PathBuf, PathBuf)> {
        let model = dir.join("yolo26s-sem-ade20k.onnx");
        let classes = dir.join("yolo26s-sem-ade20k.classes.json");
        let categories = dir.join("categories.txt");
        (model.is_file() && classes.is_file() && categories.is_file())
            .then_some((model, classes, categories))
    }
    set(face_models_dir(account))
        .or_else(|| set(shared_face_models_dir()))
        .or_else(|| system_face_models_dirs().into_iter().find_map(set))
}

/// Where a *package* may have installed the models.
///
/// `dr_plat::system_data_dirs` has the rule per platform: `$XDG_DATA_DIRS` on
/// Linux, the executable's own directory on Windows, nothing on Android —
/// there the APK's copy is unpacked into the shared user directory instead,
/// because an asset inside a package is not a path anything can read from
/// (ARCH §6.9). Last in the search order on every platform, so a pair the
/// user placed by hand outranks the installed one.
fn system_face_models_dirs() -> Vec<PathBuf> {
    dr_plat::system_data_dirs()
        .into_iter()
        .map(|d| d.join("models"))
        .collect()
}

/// One grid cell's data, read from the catalog.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct LibraryCell {
    pub image_id: i64,
    pub name: String,
    pub remote_path: String,
    /// `oc:fileid`, the key the shared thumbnail store uses. `None` for an
    /// image the scan found without a stable id.
    pub file_id: Option<u64>,
    /// File length, for bounds-checking a located preview range.
    pub size: u64,
    /// 0 = nothing, 1 = stat-only, 2 = full EXIF.
    pub metadata_state: u8,
    /// UTC seconds, once EXIF has been read.
    pub captured_at: Option<i64>,
}

/// Read a window of cells out of the catalog.
///
/// Windowed rather than wholesale: a 17k-image library must not become 17k
/// rows in a Slint model (FR-CAT-4).
/// The unscoped form, kept as the name the tests and any future caller reach
/// for. The UI goes through [`read_cells_scoped`], because a collection may be
/// selected.
#[cfg(test)]
pub fn read_cells(
    catalog: &Catalog,
    offset: usize,
    limit: usize,
) -> Result<Vec<LibraryCell>, dr_catalog::CatalogError> {
    read_cells_scoped(catalog, None, &RatingFilter::default(), offset, limit)
}

/// Read a window of cells, optionally narrowed to one collection.
///
/// A collection *set* shows its descendants' images too — a parent whose
/// children hold everything would otherwise read as empty, which makes nesting
/// look broken. The id list comes from
/// [`dr_catalog::collections::descendants`], which is depth-guarded.
///
/// Ordering comes from [`grid_order_for`]: manual position for a single manual
/// collection, capture time for a set — because position is only meaningful
/// inside one collection, and this query also serves sets, where two children's
/// positions are unrelated integers.
pub fn read_cells_scoped(
    catalog: &Catalog,
    scope: Option<dr_types::CollectionId>,
    filter: &RatingFilter,
    offset: usize,
    limit: usize,
) -> Result<Vec<LibraryCell>, dr_catalog::CatalogError> {
    let Some(scope) = scope else {
        return read_cells_all(catalog, filter, offset, limit);
    };

    let ids = dr_catalog::collections::descendants(catalog.connection(), scope)?;
    // Placeholders are generated from the *count* of ids, never from user text.
    let placeholders = std::iter::repeat_n("?", ids.len())
        .collect::<Vec<_>>()
        .join(",");
    let rated = filter.sql();
    let folded = uncollapsed("i");
    let (order, order_params) = grid_order_for(catalog, Some(scope));
    let sql = format!(
        "SELECT {CELL_COLUMNS}
           FROM images i
          WHERE {VISIBLE}{rated}{folded}
            AND i.id IN (SELECT image_id FROM collection_members
                          WHERE collection_id IN ({placeholders}))
          {order}
          LIMIT ? OFFSET ?"
    );

    // Bound in the order the `?`s appear: the scope's ids in the WHERE, then
    // whatever the ORDER BY needs, then the window.
    let mut params: Vec<rusqlite::types::Value> = ids
        .iter()
        .map(|c| rusqlite::types::Value::Integer(c.0 as i64))
        .collect();
    params.extend(order_params);
    params.push(rusqlite::types::Value::Integer(limit as i64));
    params.push(rusqlite::types::Value::Integer(offset as i64));

    let mut rows = {
        let mut stmt = catalog.connection().prepare(&sql)?;
        let read = stmt
            .query_map(rusqlite::params_from_iter(params.iter()), row_to_cell)?
            .collect::<Result<Vec<_>, _>>()?;
        read
    };
    attach_file_ids(catalog, &mut rows);
    Ok(rows)
}

fn read_cells_all(
    catalog: &Catalog,
    filter: &RatingFilter,
    offset: usize,
    limit: usize,
) -> Result<Vec<LibraryCell>, dr_catalog::CatalogError> {
    let rated = filter.sql();
    let folded = uncollapsed("i");
    let mut rows = {
        let mut stmt = catalog.connection().prepare(&format!(
            "SELECT {CELL_COLUMNS}
               FROM images i
              WHERE {VISIBLE}{rated}{folded}
              {GRID_ORDER}
              LIMIT ?1 OFFSET ?2"
        ))?;
        let read = stmt
            .query_map(rusqlite::params![limit as i64, offset as i64], row_to_cell)?
            .collect::<Result<Vec<_>, _>>()?;
        read
    };
    attach_file_ids(catalog, &mut rows);
    Ok(rows)
}

/// TRACES: FR-CAT-15
/// Read a window of *trashed* cells, newest deletion first.
///
/// Ordered by when it was trashed rather than by capture time, which is what
/// every other view sorts by. The question in the trash is "what did I just
/// delete?", not "when was this taken" — a mistaken delete is corrected within
/// seconds, and burying it among photographs from the same afternoon would make
/// the one row the user is looking for the hardest one to find.
///
/// The rating filter is deliberately not applied. It narrows a *culling* pass,
/// and a trash that hid rows because of a filter set elsewhere would look like
/// it had lost them.
pub fn read_trashed_cells(
    catalog: &Catalog,
    offset: usize,
    limit: usize,
) -> Result<Vec<LibraryCell>, dr_catalog::CatalogError> {
    let mut rows = {
        let mut stmt = catalog.connection().prepare(&format!(
            "SELECT {CELL_COLUMNS}
               FROM images i
              WHERE {TRASHED}
              {TRASH_ORDER}
              LIMIT ?1 OFFSET ?2"
        ))?;
        let read = stmt
            .query_map(rusqlite::params![limit as i64, offset as i64], row_to_cell)?
            .collect::<Result<Vec<_>, _>>()?;
        read
    };
    attach_file_ids(catalog, &mut rows);
    Ok(rows)
}

/// TRACES: FR-CAT-15
/// How many images the trash view would list.
///
/// Counts exactly what [`read_trashed_cells`] lists — same predicate, no filter
/// — so the scrollbar and the header cannot disagree with the cells.
pub fn total_trashed(catalog: &Catalog) -> Result<usize, dr_catalog::CatalogError> {
    let n: i64 = catalog.connection().query_row(
        &format!("SELECT count(*) FROM images i WHERE {TRASHED}"),
        [],
        |r| r.get(0),
    )?;
    Ok(n as usize)
}

/// TRACES: FR-CAT-5
/// The ids of grid rows `first..=last`, in the order the grid lists them.
///
/// What a shift-click actually means. The gesture names two *ordinals* and the
/// photographs between them are mostly not loaded — the grid is a window of a
/// hundred or so over a library of twenty thousand — so a range answered from
/// the window selected the handful on screen and silently dropped the rest.
/// The catalog knows the whole run, and with the ordering indexed it is one
/// seek rather than a scan.
///
/// Bounded by the same predicates, the same filter and the same [`GRID_ORDER`]
/// the window itself is read with. An ordinal only names a photograph relative
/// to an ordering, so a range taken through any other one is a range through a
/// different library.
///
/// `trash` picks the trash view's list, which is the other thing the grid can
/// be showing and is ordered by deletion time rather than capture time.
///
/// An empty result means the run is empty or the query failed; callers treat
/// the two the same, because both leave the selection where it was.
pub fn read_ids_span(
    catalog: &Catalog,
    scope: Option<dr_types::CollectionId>,
    filter: &RatingFilter,
    trash: bool,
    first: usize,
    last: usize,
) -> Result<Vec<dr_types::ImageId>, dr_catalog::CatalogError> {
    let Some(count) = (last + 1).checked_sub(first) else {
        return Ok(Vec::new());
    };

    let (sql, mut params) = if trash {
        (
            format!("SELECT i.id FROM images i WHERE {TRASHED} {TRASH_ORDER} LIMIT ? OFFSET ?"),
            Vec::new(),
        )
    } else {
        let (clause, mut params) = scope_clause(catalog, scope)?;
        let rated = filter.sql();
        let folded = uncollapsed("i");
        // The same ordering the cells were drawn with, from the same place.
        // A range is a pair of ordinals, and an ordinal read through a
        // different ORDER BY names a different photograph.
        let (order, order_params) = grid_order_for(catalog, scope);
        params.extend(order_params);
        (
            format!(
                "SELECT i.id FROM images i
                  WHERE {VISIBLE}{rated}{folded}{clause}
                  {order}
                  LIMIT ? OFFSET ?"
            ),
            params,
        )
    };
    params.push(rusqlite::types::Value::Integer(count as i64));
    params.push(rusqlite::types::Value::Integer(first as i64));

    let mut stmt = catalog.connection().prepare(&sql)?;
    let ids = stmt
        .query_map(rusqlite::params_from_iter(params.iter()), |r| {
            Ok(dr_types::ImageId(r.get::<_, i64>(0)? as u64))
        })?
        .collect::<Result<Vec<_>, _>>()?;
    Ok(ids)
}

/// TRACES: FR-UI-8
/// Where one photograph sits in the grid, by its remote path.
///
/// The inverse of [`read_ids_span`], and it exists for the same reason that one
/// does: an ordinal only names a photograph relative to an ordering, so it has
/// to be *computed* through the ordering the cells are drawn with rather than
/// guessed at. A restored place that landed on a row read through a different
/// `ORDER BY` would open the library at a photograph the user has never seen,
/// which looks exactly like the position having been forgotten.
///
/// # Why a window function rather than a count
///
/// The obvious implementation is "count the rows that sort before this one",
/// and it would need this file to spell the ordering out a second time — as an
/// inequality, with its own handling of the `captured_at IS NULL` term and its
/// own tie-break. [`grid_order_for`] already warns what a second spelling
/// costs, and a manually ordered collection makes it worse: that ordering is a
/// correlated subquery, and an inequality over it is not something anyone
/// should have to read.
///
/// `row_number() OVER ({order})` takes the ordering *verbatim* from the same
/// function the window read uses, so the two cannot disagree by construction.
/// It is a full pass over the scope rather than an index seek, which is the
/// cost of that guarantee — and it is paid once, at launch, against a query the
/// grid runs several times per screenful of scrolling.
///
/// `Ok(None)` means the path is not in this grid: deleted, trashed, filtered
/// out, or in a collection the place did not name. The caller falls back to
/// when the photograph was taken, which is what makes a place survive its
/// subject.
///
/// # Binding order
///
/// The window's parameters come first, unlike in [`read_ids_span`]. SQLite
/// binds anonymous `?` by their position **in the SQL text**, and here the
/// `OVER (...)` clause is in the select list — ahead of the `WHERE` the scope
/// narrows. Swapping the two silently looks up a collection by an image id.
pub fn ordinal_of_path(
    catalog: &Catalog,
    scope: Option<dr_types::CollectionId>,
    filter: &RatingFilter,
    trash: bool,
    path: &str,
) -> Result<Option<usize>, dr_catalog::CatalogError> {
    let (inner, mut params) = if trash {
        (
            format!(
                "SELECT i.source_ref AS sref,
                        row_number() OVER ({TRASH_ORDER}) - 1 AS ord
                   FROM images i
                  WHERE {TRASHED}"
            ),
            Vec::new(),
        )
    } else {
        let (clause, scope_params) = scope_clause(catalog, scope)?;
        let rated = filter.sql();
        let folded = uncollapsed("i");
        let (order, mut params) = grid_order_for(catalog, scope);
        // The window first, then the scope — see the note above.
        params.extend(scope_params);
        (
            format!(
                "SELECT i.source_ref AS sref,
                        row_number() OVER ({order}) - 1 AS ord
                   FROM images i
                  WHERE {VISIBLE}{rated}{folded}{clause}"
            ),
            params,
        )
    };
    params.push(rusqlite::types::Value::Text(path.to_string()));

    let mut stmt = catalog
        .connection()
        .prepare(&format!("SELECT ord FROM ({inner}) WHERE sref = ?"))?;
    let mut rows = stmt.query(rusqlite::params_from_iter(params.iter()))?;
    match rows.next()? {
        Some(r) => Ok(Some(r.get::<_, i64>(0)?.max(0) as usize)),
        None => Ok(None),
    }
}

/// The columns every windowed read selects, in the order [`row_to_cell`] reads
/// them.
///
/// Named rather than repeated so the three readers cannot drift — and so that
/// the one column that is *not* here stays conspicuous. See
/// [`attach_file_ids`] for why the server's file id is fetched separately.
const CELL_COLUMNS: &str = "i.id, i.source_ref, i.file_size, i.metadata_state, i.captured_at";

/// Shared row mapping, so the scoped and unscoped queries cannot drift.
///
/// `file_id` is left empty here and filled by [`attach_file_ids`].
fn row_to_cell(r: &rusqlite::Row) -> rusqlite::Result<LibraryCell> {
    let path: String = r.get(1)?;
    Ok(LibraryCell {
        image_id: r.get(0)?,
        name: path.rsplit(['/', ':']).next().unwrap_or(&path).to_string(),
        remote_path: path,
        file_id: None,
        size: r.get::<_, Option<i64>>(2)?.unwrap_or(0) as u64,
        metadata_state: r.get::<_, i64>(3)? as u8,
        captured_at: r.get(4)?,
    })
}

/// TRACES: NFR-P5
/// Fill in each cell's server file id, in one query for the whole window.
///
/// # Why this is not a `LEFT JOIN` any more
///
/// It was, and it was the single most expensive thing the grid did while a
/// finger was on it. A window is `ORDER BY ... LIMIT n OFFSET k`, and SQLite
/// answers a join like that by joining *first* and paging after — so reading
/// 280 cells at offset 20,000 meant an index seek into `remote` for all 24,000
/// rows, 23,720 of which were then discarded. Measured at 15.2 ms, inside the
/// scroll handler, against a 16.7 ms frame.
///
/// Paging over `images` alone is 0.36 ms with `images_grid_order` (schema V7),
/// and this fetches the ids for the 280 rows that survived. The same shape the
/// badge and rating reads already use: one query for the window, never one per
/// cell.
///
/// Silent on failure, and cells keep `file_id: None`: that is the same state a
/// photograph the scan has not reached the server for is in, and the callers
/// already treat it as "no cached thumbnail to key on" rather than an error.
fn attach_file_ids(catalog: &Catalog, cells: &mut [LibraryCell]) {
    if cells.is_empty() {
        return;
    }
    let placeholders = std::iter::repeat_n("?", cells.len())
        .collect::<Vec<_>>()
        .join(",");
    let sql = format!("SELECT image_id, file_id FROM remote WHERE image_id IN ({placeholders})");
    let params: Vec<rusqlite::types::Value> = cells
        .iter()
        .map(|c| rusqlite::types::Value::Integer(c.image_id))
        .collect();

    let mut stmt = match catalog.connection().prepare(&sql) {
        Ok(s) => s,
        Err(e) => {
            log::debug!("reading file ids for the window: {e}");
            return;
        }
    };
    let rows = stmt.query_map(rusqlite::params_from_iter(params.iter()), |r| {
        Ok((r.get::<_, i64>(0)?, r.get::<_, Option<i64>>(1)?))
    });
    let found: std::collections::HashMap<i64, Option<i64>> = match rows {
        Ok(rows) => rows.flatten().collect(),
        Err(e) => {
            log::debug!("reading file ids for the window: {e}");
            return;
        }
    };
    for cell in cells {
        cell.file_id = found
            .get(&cell.image_id)
            .copied()
            .flatten()
            .map(|v| v as u64);
    }
}

/// Total images in the catalog, or in one collection and its descendants.
///
/// Counts exactly what [`read_cells_scoped`] would list, filter included. The
/// two must agree: the header says "412 images" and the grid's scrollbar is
/// sized from the same number, so a count that ignored the filter would leave
/// the user scrolling through empty rows.
pub fn total_images_scoped(
    catalog: &Catalog,
    scope: Option<dr_types::CollectionId>,
    filter: &RatingFilter,
) -> Result<usize, dr_catalog::CatalogError> {
    let Some(scope) = scope else {
        return total_images_filtered(catalog, filter);
    };

    let ids = dr_catalog::collections::descendants(catalog.connection(), scope)?;
    let placeholders = std::iter::repeat_n("?", ids.len())
        .collect::<Vec<_>>()
        .join(",");
    let rated = filter.sql();
    // DISTINCT: an image in both a parent and a child is one photograph, and a
    // count that disagrees with the number of cells drawn is worse than either
    // number alone.
    //
    // Counted through `images` rather than over `collection_members` alone, so
    // `VISIBLE` applies — a trashed photograph is still a member row, and
    // counting it made the header claim images the grid would not draw.
    let folded = uncollapsed("i");
    let sql = format!(
        "SELECT count(DISTINCT i.id) FROM images i
          WHERE {VISIBLE}{rated}{folded}
            AND i.id IN (SELECT image_id FROM collection_members
                          WHERE collection_id IN ({placeholders}))"
    );
    let params: Vec<rusqlite::types::Value> = ids
        .iter()
        .map(|c| rusqlite::types::Value::Integer(c.0 as i64))
        .collect();
    let n: i64 =
        catalog
            .connection()
            .query_row(&sql, rusqlite::params_from_iter(params.iter()), |r| {
                r.get(0)
            })?;
    Ok(n as usize)
}

/// TRACES: FR-CAT-7
/// Every member of `scope`, in the order its positions put them.
///
/// The *whole* membership, not the window and not the filtered view. A reorder
/// rewrites positions, and [`dr_catalog::collections::set_order`] only touches
/// the rows it is given — so writing back a filtered subset would leave the
/// images the filter is hiding at their old positions, interleaved with the new
/// ones arbitrarily. The user reorders what they can see; the rows they cannot
/// keep their place relative to it.
pub fn read_member_order(
    catalog: &Catalog,
    scope: dr_types::CollectionId,
) -> Result<Vec<dr_types::ImageId>, dr_catalog::CatalogError> {
    let mut stmt = catalog.connection().prepare(
        "SELECT image_id FROM collection_members
          WHERE collection_id = ?1
          ORDER BY position ASC, image_id ASC",
    )?;
    let ids = stmt
        .query_map([scope.0 as i64], |r| {
            Ok(dr_types::ImageId(r.get::<_, i64>(0)? as u64))
        })?
        .collect::<Result<Vec<_>, _>>()?;
    Ok(ids)
}

/// TRACES: FR-CAT-7
/// `current` with `moving` lifted out and set down beside `target`.
///
/// `target` names a *photograph*, not an index, and that is the point: the grid
/// may be filtered, so the cell the user dropped on sits at one position in
/// what they can see and another in the membership being rewritten. An id
/// survives both. `after` puts the run on the far side of it, which is the only
/// way to name the last place in a collection — there is no cell beyond the
/// last one to drop in front of.
///
/// The run keeps the order `current` has it in rather than the order the
/// selection was built in: the user is looking at the grid, and a selection
/// gathered by tapping the last frame first should not reverse itself on being
/// moved.
///
/// A `target` that is itself being moved leaves the run at the end. There is no
/// gap between a run and itself to land in, so the caller refuses that drop
/// before it gets here; this is what the function does rather than panicking if
/// one ever arrives.
///
/// Pure, so the awkward half of a drag can be tested without a window.
pub fn reordered(
    current: &[dr_types::ImageId],
    moving: &[dr_types::ImageId],
    target: dr_types::ImageId,
    after: bool,
) -> Vec<dr_types::ImageId> {
    let lifting: std::collections::BTreeSet<_> = moving.iter().copied().collect();
    let rest: Vec<_> = current
        .iter()
        .copied()
        .filter(|id| !lifting.contains(id))
        .collect();
    let run: Vec<_> = current
        .iter()
        .copied()
        .filter(|id| lifting.contains(id))
        .collect();

    // Resolved against `rest`, not against `current`: the run has already been
    // lifted, so an index into the original list would be off by however many
    // of it sat ahead of the target.
    let at = match rest.iter().position(|id| *id == target) {
        Some(at) if after => at + 1,
        Some(at) => at,
        None => rest.len(),
    };

    let mut out = Vec::with_capacity(current.len());
    out.extend_from_slice(&rest[..at]);
    out.extend(run);
    out.extend_from_slice(&rest[at..]);
    out
}

/// TRACES: FR-CAT-7
/// The ORDER BY the grid reads `scope` with, and the parameters it binds.
///
/// Manual position where the grid is scoped to a single manual collection with
/// no children; [`GRID_ORDER`] — capture time, then filename — everywhere else.
///
/// **Why the narrowing.** `position` is a column of `collection_members`, so it
/// only exists relative to one collection. A collection *set* shows its
/// descendants' images too, and two children's positions are unrelated integers
/// that would interleave arbitrarily; a smart collection has no member rows to
/// carry a position at all. Outside those cases there is no manual order to
/// read, and falling back is the only honest answer.
///
/// **Why every reader must agree.** An ordinal only names a photograph relative
/// to an ordering. The window read and the span read are two halves of one
/// grid: a shift-click resolved through a different ORDER BY than the cells
/// were drawn with selects a different run than the one on screen, and the user
/// finds out when the export runs. That is the same invariant
/// [`read_ids_span`] already states about `GRID_ORDER`, widened to cover the
/// case where the ordering depends on the scope.
///
/// A correlated subquery rather than a join, so the FROM and WHERE the two
/// readers already share are untouched: position is looked up per row through
/// `collection_members`' primary key, which is `(collection_id, image_id)`.
fn grid_order_for(
    catalog: &Catalog,
    scope: Option<dr_types::CollectionId>,
) -> (String, Vec<rusqlite::types::Value>) {
    let Some(id) = scope else {
        return (GRID_ORDER.to_string(), Vec::new());
    };

    // A set orders by capture time. `descendants` includes the collection
    // itself, so one entry means it has no children.
    let alone = dr_catalog::collections::descendants(catalog.connection(), id)
        .map(|d| d.len() == 1)
        .unwrap_or(false);
    let manual = matches!(
        dr_catalog::collections::kind(catalog.connection(), id),
        Ok(Some(dr_catalog::collections::CollectionKind::Manual))
    );
    if !alone || !manual {
        return (GRID_ORDER.to_string(), Vec::new());
    }

    // `i.id` breaks the tie. Positions are dense after a `set_order`, but a
    // collection that has never been reordered by hand has whatever
    // `add_images` assigned, and two rows can share a position if a merge from
    // another device brought one in — an ordering that is not total is an
    // ordering the window read and the span read can disagree about.
    (
        "ORDER BY (SELECT cm.position FROM collection_members cm
                    WHERE cm.collection_id = ? AND cm.image_id = i.id) ASC,
                  i.id ASC"
            .to_string(),
        vec![rusqlite::types::Value::Integer(id.0 as i64)],
    )
}

/// The SQL restricting a query to `scope` and its descendants, with the bound
/// parameters to go with it.
///
/// Shared by the span and the histogram so the two cannot drift: an axis drawn
/// over one set of images and bars counted over another puts the bars in the
/// wrong place.
fn scope_clause(
    catalog: &Catalog,
    scope: Option<dr_types::CollectionId>,
) -> Result<(String, Vec<rusqlite::types::Value>), dr_catalog::CatalogError> {
    let Some(scope) = scope else {
        return Ok((String::new(), Vec::new()));
    };
    let ids = dr_catalog::collections::descendants(catalog.connection(), scope)?;
    let placeholders = std::iter::repeat_n("?", ids.len())
        .collect::<Vec<_>>()
        .join(",");
    Ok((
        format!(
            " AND i.id IN (SELECT image_id FROM collection_members
                            WHERE collection_id IN ({placeholders}))"
        ),
        ids.iter()
            .map(|c| rusqlite::types::Value::Integer(c.0 as i64))
            .collect(),
    ))
}

/// Earliest and latest capture time within `scope`, honouring the filter.
///
/// The timeline's extent. Taken over the same images the histogram counts, so
/// opening a collection shows that collection's years rather than the whole
/// library's — the axis was previously spanning everything, which left a
/// collection's bars crushed into a sliver of it.
pub fn span_scoped(
    catalog: &Catalog,
    scope: Option<dr_types::CollectionId>,
    filter: &RatingFilter,
) -> Option<(i64, i64)> {
    let (clause, params) = scope_clause(catalog, scope).ok()?;
    // Full extent, not the chosen range — see `without_date_range`.
    let rated = filter.without_date_range().sql();
    let sql = format!(
        "SELECT min(i.captured_at), max(i.captured_at) FROM images i
          WHERE {VISIBLE}{rated} AND i.captured_at IS NOT NULL{clause}"
    );
    catalog
        .connection()
        .query_row(&sql, rusqlite::params_from_iter(params.iter()), |r| {
            Ok((r.get::<_, Option<i64>>(0)?, r.get::<_, Option<i64>>(1)?))
        })
        .ok()
        .and_then(|(lo, hi)| Some((lo?, hi?)))
}

/// TRACES: FR-CAT-6
/// The capture-time histogram as a fixed number of equal bins across
/// `from..=to`, empty ones included.
///
/// # Why not calendar buckets
///
/// [`timeline_scoped`] groups by year, month, day or hour, which has two
/// consequences the axis cannot live with.
///
/// It emits **only the buckets that hold photographs**, and the widget gives
/// every bar an equal slot — so a library with a gap in it drew a February
/// that was six months wide. The position marker, the range band and a click
/// are all linear in time, so on a sparse library they pointed at bars that
/// were somewhere else. Equal bins including the empty ones make a bar's
/// position on the track and the date under it the same quantity.
///
/// And the **count is free to jump by a factor of twelve** between one unit
/// and the next, so each zoom step halved the number of bars until a
/// threshold was crossed: zooming in made the picture coarser, twice out of
/// every three steps. A fixed count re-bins on every zoom instead, which is
/// what makes each step show finer structure rather than the same structure
/// drawn wider.
///
/// The date range is lifted from the filter, like the bars' other terms are
/// not: this histogram is *how a range is chosen*, and drawing through the
/// range would empty every bin outside it and leave nothing to widen into.
///
/// `bins` is clamped to at least one — a zero would be a division by zero in
/// SQL, and the caller's number comes from a hand-editable settings file.
pub fn timeline_uniform(
    catalog: &Catalog,
    scope: Option<dr_types::CollectionId>,
    filter: &RatingFilter,
    from: i64,
    to: i64,
    bins: u32,
) -> Result<Vec<dr_catalog::TimeBucket>, dr_catalog::CatalogError> {
    let bins = bins.max(1) as i64;
    // At least one second, or every photograph lands in bin zero.
    let span = (to - from).max(1);
    let (clause, params) = scope_clause(catalog, scope)?;
    let rated = filter.without_date_range().sql();

    // The bin index is arithmetic on the stored UTC instant, not `strftime` on
    // a local one. A bin is not a calendar unit — it has no local midnight to
    // respect — and the axis it is drawn on is labelled from the same UTC
    // instants, so bucketing the two differently is the one way they could
    // disagree about which bar a photograph belongs to.
    //
    // `min` caps the last edge: an image captured at exactly `to` divides to
    // `bins`, which would be a bin past the end of the axis.
    //
    // Integers this code owns, formatted straight in — the same rule the
    // rating terms follow. They cannot be bound parameters here without
    // ordering them against the scope clause's, which appears later in the
    // text but is bound first.
    let sql = format!(
        "SELECT min({bins} - 1, (i.captured_at - {from}) * {bins} / {span}) AS b,
                count(*) AS n
           FROM images i
          WHERE {VISIBLE}{rated} AND i.captured_at IS NOT NULL{clause}
            AND i.captured_at >= {from} AND i.captured_at <= {to}
          GROUP BY b
          ORDER BY b ASC"
    );

    let conn = catalog.connection();
    let mut stmt = conn.prepare(&sql)?;
    let counted = stmt
        .query_map(rusqlite::params_from_iter(params.iter()), |r| {
            Ok((r.get::<_, i64>(0)?, r.get::<_, i64>(1)? as u32))
        })?
        .collect::<Result<Vec<_>, _>>()?;

    // Every bin, in order, whether or not the query returned one for it. The
    // start is the bin's own left edge rather than the earliest photograph in
    // it: an empty bin has no photograph to take one from, and a bar drawn at
    // its contents' position rather than its bin's would put the axis back
    // where the calendar buckets left it.
    let mut bars: Vec<dr_catalog::TimeBucket> = (0..bins)
        .map(|i| dr_catalog::TimeBucket {
            start: from + (i * span) / bins,
            count: 0,
        })
        .collect();
    for (i, n) in counted {
        if let Some(bar) = bars.get_mut(i.clamp(0, bins - 1) as usize) {
            bar.count = n;
        }
    }
    Ok(bars)
}

/// Total images in the catalog, honouring the rating filter.
fn total_images_filtered(
    catalog: &Catalog,
    filter: &RatingFilter,
) -> Result<usize, dr_catalog::CatalogError> {
    let rated = filter.sql();
    let folded = uncollapsed("i");
    let n: i64 = catalog.connection().query_row(
        &format!("SELECT count(*) FROM images i WHERE {VISIBLE}{rated}{folded}"),
        [],
        |r| r.get(0),
    )?;
    Ok(n as usize)
}

/// TRACES: FR-CAT-9
/// How many visible images have their original stored on this device.
///
/// Whole-library, like the star counts beside it: the chip says what narrowing
/// to it would show, so counting only the current window would make it
/// describe the view it exists to change.
pub fn local_original_count(catalog: &Catalog) -> Result<usize, dr_catalog::CatalogError> {
    let n: i64 = catalog.connection().query_row(
        &format!(
            "SELECT count(*) FROM images i
              WHERE {VISIBLE}
                AND EXISTS (SELECT 1 FROM image_cache ic
                             WHERE ic.image_id = i.id AND ic.tier_actual >= {})",
            dr_types::Tier::Original.stored()
        ),
        [],
        |r| r.get(0),
    )?;
    Ok(n as usize)
}

/// Total images in the catalog, unfiltered.
///
/// What the scan reports and what the sidebar's "all images" row shows — the
/// size of the library itself, not of the current view.
pub fn total_images(catalog: &Catalog) -> Result<usize, dr_catalog::CatalogError> {
    let n: i64 = catalog.connection().query_row(
        &format!("SELECT count(*) FROM images WHERE {VISIBLE_UNALIASED}"),
        [],
        |r| r.get(0),
    )?;
    Ok(n as usize)
}

pub fn now_secs() -> i64 {
    std::time::SystemTime::now()
        .duration_since(std::time::UNIX_EPOCH)
        .map(|d| d.as_secs() as i64)
        .unwrap_or(0)
}

#[cfg(test)]
mod tests {
    use super::*;
    use dr_sync::RemoteEntry;

    #[test]
    fn join_all_preserves_order_regardless_of_completion() {
        // The ordering guarantee is what lets a caller pair results back to
        // their inputs; without it a lane's dates could be attributed to the
        // wrong images.
        let rt = crate::net_runtime::build().unwrap();

        let out = rt.block_on(async {
            futures_join_all(vec![
                Box::pin(async { 1 }) as std::pin::Pin<Box<dyn std::future::Future<Output = i32>>>,
                Box::pin(async {
                    tokio::task::yield_now().await;
                    tokio::task::yield_now().await;
                    2
                }),
                Box::pin(async {
                    tokio::task::yield_now().await;
                    3
                }),
            ])
            .await
        });

        assert_eq!(out, vec![1, 2, 3]);
    }

    #[test]
    fn join_all_of_nothing_completes() {
        let rt = crate::net_runtime::build().unwrap();
        let out: Vec<i32> =
            rt.block_on(async { futures_join_all(Vec::<std::future::Ready<i32>>::new()).await });
        assert!(out.is_empty());
    }

    #[test]
    fn sweep_lanes_divide_a_chunk_without_loss() {
        // Every image in a chunk must land in exactly one lane: a striding
        // split that dropped or duplicated one would silently under- or
        // double-index the library.
        let chunk: Vec<usize> = (0..SWEEP_CHUNK).collect();
        let lanes: Vec<Vec<usize>> = (0..SWEEP_LANES)
            .map(|l| chunk.iter().skip(l).step_by(SWEEP_LANES).copied().collect())
            .collect();

        let mut seen: Vec<usize> = lanes.iter().flatten().copied().collect();
        seen.sort_unstable();
        assert_eq!(seen, chunk);
        // Evenly divided, so no lane sits idle while another finishes.
        assert!(lanes.iter().all(|l| l.len() == SWEEP_CHUNK / SWEEP_LANES));
    }

    #[test]
    fn a_short_chunk_still_covers_every_image() {
        // The last chunk of a library is rarely a full multiple of the lanes.
        let chunk: Vec<usize> = (0..5).collect();
        let lanes: Vec<Vec<usize>> = (0..SWEEP_LANES)
            .map(|l| chunk.iter().skip(l).step_by(SWEEP_LANES).copied().collect())
            .collect();
        let mut seen: Vec<usize> = lanes.iter().flatten().copied().collect();
        seen.sort_unstable();
        assert_eq!(seen, chunk);
    }

    #[test]
    fn a_scrub_ordinal_matches_the_grid_position() {
        // The scrub's count and the grid's window must use *identical*
        // predicates and ordering, or the view lands somewhere else. Counting
        // only dated images against a grid that also shows undated ones put a
        // click near the end of the axis near the top of the library.
        let catalog = Catalog::in_memory().unwrap();
        let c = catalog.connection();
        c.execute(
            "INSERT INTO roots(id, kind, label) VALUES (1, 'remote', 'lib')",
            [],
        )
        .unwrap();

        // A mix: dated, undated, and one shadowed by a RAW sibling.
        for (id, name, captured, shadow) in [
            (1i64, "a.CR2", Some(100i64), None),
            (2, "b.CR2", Some(200), None),
            (3, "b.JPG", Some(200), Some(2i64)),
            (4, "c.CR2", Some(300), None),
            (5, "d.CR2", None, None),
        ] {
            c.execute(
                "INSERT INTO images(id, root_id, source_ref, captured_at, shadowed_by, added_at)
                 VALUES (?1, 1, ?2, ?3, ?4, 0)",
                rusqlite::params![id, name, captured, shadow],
            )
            .unwrap();
        }

        // The grid's own window, in its own order.
        let cells = read_cells(&catalog, 0, 100).unwrap();
        let names: Vec<&str> = cells.iter().map(|c| c.name.as_str()).collect();
        assert_eq!(
            names,
            vec!["a.CR2", "b.CR2", "c.CR2", "d.CR2"],
            "shadowed hidden, undated last"
        );

        // Scrubbing to each image's instant must give its index in that list.
        for (when, expected) in [(100i64, 0usize), (200, 1), (300, 2)] {
            let ordinal: i64 = c
                .query_row(
                    "SELECT count(*) FROM images
                      WHERE shadowed_by IS NULL
                        AND captured_at IS NOT NULL
                        AND captured_at < ?1",
                    [when],
                    |r| r.get(0),
                )
                .unwrap();
            assert_eq!(
                ordinal as usize, expected,
                "scrubbing to {when} must land on grid row {expected}"
            );
        }
    }

    /// The fixture the ordinal tests share: a mix of dated, undated, shadowed
    /// and trashed rows, which is what makes the grid's ordering non-obvious.
    fn a_small_library() -> Catalog {
        let catalog = Catalog::in_memory().unwrap();
        let c = catalog.connection();
        c.execute(
            "INSERT INTO roots(id, kind, label) VALUES (1, 'remote', 'lib')",
            [],
        )
        .unwrap();

        for (id, name, captured, shadow, trashed) in [
            (1i64, "2019/a.CR2", Some(100i64), None, None),
            (2, "2019/b.CR2", Some(200), None, None),
            // Shadowed by its RAW sibling: never a row of the grid.
            (3, "2019/b.JPG", Some(200), Some(2i64), None),
            (4, "2020/c.CR2", Some(300), None, None),
            // Undated sorts last, whatever its name.
            (5, "2018/d.CR2", None, None, None),
            // Trashed: out of the library, and the only row of the trash.
            (6, "2020/e.CR2", Some(400), None, Some(9i64)),
        ] {
            c.execute(
                "INSERT INTO images(id, root_id, source_ref, captured_at, shadowed_by,
                                    trashed_at, added_at)
                 VALUES (?1, 1, ?2, ?3, ?4, ?5, 0)",
                rusqlite::params![id, name, captured, shadow, trashed],
            )
            .unwrap();
        }
        catalog
    }

    #[test]
    fn every_cell_reports_the_ordinal_it_is_drawn_at() {
        // TRACES: FR-UI-8
        // The invariant the whole restore rests on, stated the strongest way
        // there is: walk the window the grid actually draws and ask for each
        // cell's ordinal by path. Anything less — a spot check, or a hand-built
        // expectation — would pass while `ordinal_of_path` and `read_cells`
        // disagreed about undated rows, shadowed rows or the tie-break, which
        // is precisely where an ordering drifts.
        let catalog = a_small_library();
        let filter = RatingFilter::default();
        let cells = read_cells(&catalog, 0, 100).unwrap();
        assert_eq!(cells.len(), 4, "shadowed and trashed are not rows");

        for (i, cell) in cells.iter().enumerate() {
            assert_eq!(
                ordinal_of_path(&catalog, None, &filter, false, &cell.remote_path).unwrap(),
                Some(i),
                "{} is drawn at row {i}",
                cell.remote_path
            );
        }
    }

    #[test]
    fn a_photograph_that_is_not_in_the_grid_reports_nothing() {
        // Deleted, never scanned, or hidden behind its RAW. All three are the
        // same answer, and the caller falls back to the capture time — which is
        // only reachable if this says `None` rather than guessing.
        let catalog = a_small_library();
        let filter = RatingFilter::default();
        assert_eq!(
            ordinal_of_path(&catalog, None, &filter, false, "2019/gone.CR2").unwrap(),
            None,
            "never heard of it"
        );
        assert_eq!(
            ordinal_of_path(&catalog, None, &filter, false, "2019/b.JPG").unwrap(),
            None,
            "shadowed by its RAW"
        );
        assert_eq!(
            ordinal_of_path(&catalog, None, &filter, false, "2020/e.CR2").unwrap(),
            None,
            "trashed"
        );
    }

    #[test]
    fn the_trash_is_ordinalled_through_its_own_list() {
        // The trash orders by deletion time and lists exactly what the library
        // excludes, so an ordinal taken through the library's ordering would
        // name a different photograph — or, here, nothing at all.
        let catalog = a_small_library();
        let filter = RatingFilter::default();
        assert_eq!(
            ordinal_of_path(&catalog, None, &filter, true, "2020/e.CR2").unwrap(),
            Some(0)
        );
        assert_eq!(
            ordinal_of_path(&catalog, None, &filter, true, "2019/a.CR2").unwrap(),
            None,
            "a live photograph is not in the trash"
        );
    }

    #[test]
    fn a_filter_moves_the_ordinal_with_the_grid() {
        // TRACES: FR-UI-8
        // A place carries the filter it was recorded under *and* the position,
        // and the second is only meaningful under the first. Restoring them in
        // the wrong order — position, then filter — would land on a row of a
        // list that no longer exists, which is the bug this pairing exists to
        // rule out.
        let catalog = a_small_library();
        let c = catalog.connection();
        // Three stars on the third photograph only.
        c.execute(
            "INSERT INTO versions(id, image_id, uuid, name, is_default, rating)
             VALUES (1, 4, 'u4', 'default', 1, 3)",
            [],
        )
        .unwrap();

        let unfiltered = RatingFilter::default();
        assert_eq!(
            ordinal_of_path(&catalog, None, &unfiltered, false, "2020/c.CR2").unwrap(),
            Some(2)
        );

        let starred = RatingFilter {
            min_rating: 3,
            ..RatingFilter::default()
        };
        assert_eq!(
            ordinal_of_path(&catalog, None, &starred, false, "2020/c.CR2").unwrap(),
            Some(0),
            "the only survivor of the filter is the first row of it"
        );
        assert_eq!(
            ordinal_of_path(&catalog, None, &starred, false, "2019/a.CR2").unwrap(),
            None,
            "filtered out, so it has no position in this grid"
        );
    }

    #[test]
    fn a_collection_is_ordinalled_through_its_own_scope() {
        // The window's parameters are bound ahead of the scope's — see the note
        // on `ordinal_of_path`. Getting that order wrong looks up a collection
        // by an image id, which fails silently as "not in this grid".
        let catalog = a_small_library();
        let c = catalog.connection();
        let coll =
            dr_catalog::collections::create(c, "Trip", None, dr_catalog::CollectionKind::Manual)
                .unwrap();
        // The second and third photographs, out of order, so position matters.
        dr_catalog::collections::add_images(c, coll, &[dr_types::ImageId(4), dr_types::ImageId(2)])
            .unwrap();

        let filter = RatingFilter::default();
        let cells = read_cells_scoped(&catalog, Some(coll), &filter, 0, 100).unwrap();
        assert_eq!(cells.len(), 2);
        for (i, cell) in cells.iter().enumerate() {
            assert_eq!(
                ordinal_of_path(&catalog, Some(coll), &filter, false, &cell.remote_path).unwrap(),
                Some(i),
                "{} is drawn at row {i} of the collection",
                cell.remote_path
            );
        }
        assert_eq!(
            ordinal_of_path(&catalog, Some(coll), &filter, false, "2019/a.CR2").unwrap(),
            None,
            "not a member"
        );
    }

    #[test]
    fn the_thumbnail_pass_asks_only_for_what_is_missing() {
        // The work list is the whole point of the pass being resumable and of
        // it being safe to press twice: it is derived from what the store
        // lacks, not from a flag in the catalog. Three things it must respect
        // — a thumbnail already stored, a trashed or shadowed image, and an
        // image with no `oc:fileid`, which the store cannot key on at all.
        let catalog = Catalog::in_memory().unwrap();
        let c = catalog.connection();
        c.execute(
            "INSERT INTO roots(id, kind, label) VALUES (1, 'remote', 'lib')",
            [],
        )
        .unwrap();

        // (id, name, file_id, shadowed_by, trashed_at)
        for (id, name, file_id, shadow, trashed) in [
            (1i64, "a.CR2", Some(11i64), None, None),
            (2, "b.CR2", Some(22), None, None),
            (3, "b.JPG", Some(33), Some(2i64), None),
            (4, "c.CR2", Some(44), None, Some(1000i64)),
            // Scanned without a file id: nothing to key the store on.
            (5, "d.CR2", None, None, None),
        ] {
            c.execute(
                "INSERT INTO images(id, root_id, source_ref, shadowed_by, trashed_at, added_at)
                 VALUES (?1, 1, ?2, ?3, ?4, 0)",
                rusqlite::params![id, name, shadow, trashed],
            )
            .unwrap();
            if let Some(file_id) = file_id {
                c.execute(
                    "INSERT INTO remote(image_id, file_id) VALUES (?1, ?2)",
                    rusqlite::params![id, file_id],
                )
                .unwrap();
            }
        }

        let dir = std::env::temp_dir().join(format!("dr-thumb-sweep-{}", std::process::id()));
        let _ = std::fs::remove_dir_all(&dir);
        std::fs::create_dir_all(&dir).unwrap();
        let mut store = ThumbStore::open(&dir).unwrap();

        let all = thumbnails_outstanding(&catalog, &store).unwrap();
        let names: Vec<&str> = all.iter().map(|r| r.path.as_str()).collect();
        assert_eq!(
            names,
            vec!["a.CR2", "b.CR2"],
            "shadowed, trashed and file-id-less images are not work this pass can do"
        );

        // Store one, and it drops out — this is what stops a second run
        // re-fetching an hour of previews.
        store
            .put(
                11,
                SWEEP_THUMB_SIZE,
                &dr_thumbs::Thumbnail {
                    width: 4,
                    height: 4,
                    bytes: vec![0xFF, 0xD8, 0xFF, 0xD9],
                },
            )
            .unwrap();
        let rest = thumbnails_outstanding(&catalog, &store).unwrap();
        let names: Vec<&str> = rest.iter().map(|r| r.path.as_str()).collect();
        assert_eq!(names, vec!["b.CR2"]);

        // The large class is a different key, so filling the grid class does
        // not make the pass think the library is done at another size.
        assert!(!store.contains(11, dr_thumbs::ThumbSize::Large));

        let _ = std::fs::remove_dir_all(&dir);
    }

    /// An account for the path tests, defaulting to the connector every
    /// existing install uses.
    fn account(endpoint: &str, user: &str) -> Account {
        Account::new("nextcloud", endpoint).with_login(user, user)
    }

    #[test]
    fn catalog_paths_separate_accounts() {
        // Two accounts on one machine must not share an index, or one
        // library's images appear in the other.
        let a = catalog_path(&account("https://cloud.example", "duncan"));
        let b = catalog_path(&account("https://cloud.example", "someone"));
        let c = catalog_path(&account("https://other.example", "duncan"));
        assert_ne!(a, b);
        assert_ne!(a, c);
    }

    #[test]
    fn a_folder_library_gets_its_own_catalog() {
        // The same rule across backends: a folder library on this machine
        // must not land in the directory a server account is already using.
        let server = catalog_path(&account("https://cloud.example", "duncan"));
        let folder = catalog_path(&Account::new("folder", "/mnt/photos"));
        assert_ne!(server, folder);
        assert_ne!(
            folder,
            catalog_path(&Account::new("folder", "/mnt/other-photos"))
        );
    }

    #[test]
    fn a_legacy_cache_directory_is_moved_rather_than_abandoned() {
        // The upgrade hazard: `sidecars/` and `outbox/` hold work that exists
        // nowhere else, so leaving them behind in a directory the system may
        // empty would discard unsynced ratings and edits as a side effect of
        // installing a new build.
        let root = std::env::temp_dir().join(format!("dr-migrate-{}", std::process::id()));
        let _ = std::fs::remove_dir_all(&root);
        let legacy = root.join("darkroom").join("cloud-example-duncan");
        std::fs::create_dir_all(legacy.join("sidecars")).unwrap();
        std::fs::write(legacy.join("catalog.sqlite"), b"catalog").unwrap();
        std::fs::write(legacy.join("sidecars").join("a.drsc"), b"an unsynced edit").unwrap();

        let current = root.join("new").join("cloud-example-duncan");
        move_account_dir(&legacy, &current);

        assert!(!legacy.exists(), "the old copy must not be left behind");
        assert_eq!(
            std::fs::read(current.join("catalog.sqlite")).unwrap(),
            b"catalog"
        );
        assert_eq!(
            std::fs::read(current.join("sidecars").join("a.drsc")).unwrap(),
            b"an unsynced edit",
            "an unsynced edit must survive the move"
        );
        let _ = std::fs::remove_dir_all(&root);
    }

    #[test]
    fn a_migration_never_overwrites_live_data() {
        // Running twice, or a fresh install that already has a catalog. The
        // destination wins: it is the one the application is using.
        let root = std::env::temp_dir().join(format!("dr-migrate2-{}", std::process::id()));
        let _ = std::fs::remove_dir_all(&root);
        let legacy = root.join("old").join("acct");
        let current = root.join("new").join("acct");
        std::fs::create_dir_all(&legacy).unwrap();
        std::fs::create_dir_all(&current).unwrap();
        std::fs::write(legacy.join("catalog.sqlite"), b"stale").unwrap();
        std::fs::write(current.join("catalog.sqlite"), b"live").unwrap();

        move_account_dir(&legacy, &current);
        assert_eq!(
            std::fs::read(current.join("catalog.sqlite")).unwrap(),
            b"live"
        );
        let _ = std::fs::remove_dir_all(&root);
    }

    #[test]
    fn durable_data_never_lands_in_a_cache_directory() {
        // The fault this guards against is silent and total: on Android the
        // fallback used to be `temp_dir()`, which resolves to the app's cache
        // — a directory the system empties under storage pressure. Beside this
        // catalog sit `sidecars/`, the commit point for every offline rating
        // and edit, and `outbox/`, holding exports the user was told had
        // succeeded. Losing a day of culling to an OS housekeeping pass, with
        // no error and no trace, is the worst outcome this application has.
        let path = catalog_path(&account("https://cloud.example", "duncan"));
        let text = path.to_string_lossy().to_lowercase();
        assert!(
            !text.contains("/cache/") && !text.contains("/tmp/"),
            "the catalog and everything beside it must be durable, got {}",
            path.display()
        );
    }

    #[test]
    fn the_outbox_and_sidecars_sit_beside_the_catalog() {
        // Stated as a test because three separate call sites derive their
        // location by taking this path's parent, and a change here moves all
        // of them at once — including the two holding unsynced user work.
        let catalog = catalog_path(&account("https://cloud.example", "duncan"));
        let parent = catalog.parent().expect("a parent");
        assert_eq!(
            crate::export::outbox_dir(&account("https://cloud.example", "duncan")),
            parent.join("outbox")
        );
    }

    #[test]
    fn catalog_path_is_filesystem_safe() {
        let p = catalog_path(&account("https://cloud.example.com:8443/nc", "duncan"));
        let s = p.to_string_lossy();
        assert!(!s.contains("://"));
        assert!(!s.contains(':') || cfg!(windows));
    }

    #[test]
    fn persist_inserts_images_and_folder_etags() {
        let catalog = Catalog::in_memory().unwrap();
        let result = dr_sync::ScanResult {
            images: vec![entry("PhotosRaw/a.CR2", 1001, 30_000_000)],
            directories: vec![(RemotePath::new("PhotosRaw"), dr_sync::Validator::new("e1"))],
            progress: Default::default(),
            sidecars: Vec::new(),
        };

        persist(&catalog, "PhotosRaw", &result).unwrap();

        assert_eq!(total_images(&catalog).unwrap(), 1);
        // Folder ETags must persist or the next scan prunes nothing.
        let etag: String = catalog
            .connection()
            .query_row(
                "SELECT etag FROM folders WHERE path = 'PhotosRaw'",
                [],
                |r| r.get(0),
            )
            .unwrap();
        assert_eq!(etag, "e1");
    }

    #[test]
    fn images_land_as_stat_only_not_full_metadata() {
        // The scan read no EXIF. Claiming otherwise would make a date filter
        // silently wrong on a freshly scanned library.
        let catalog = Catalog::in_memory().unwrap();
        let result = dr_sync::ScanResult {
            images: vec![entry("PhotosRaw/a.CR2", 1001, 30_000_000)],
            directories: vec![(RemotePath::new("PhotosRaw"), dr_sync::Validator::new("e1"))],
            progress: Default::default(),
            sidecars: Vec::new(),
        };
        persist(&catalog, "PhotosRaw", &result).unwrap();

        let state: i64 = catalog
            .connection()
            .query_row("SELECT metadata_state FROM images", [], |r| r.get(0))
            .unwrap();
        assert_eq!(state, 1);
    }

    #[test]
    fn rescanning_updates_rather_than_duplicating() {
        let catalog = Catalog::in_memory().unwrap();
        let result = dr_sync::ScanResult {
            images: vec![entry("PhotosRaw/a.CR2", 1001, 30_000_000)],
            directories: vec![(RemotePath::new("PhotosRaw"), dr_sync::Validator::new("e1"))],
            progress: Default::default(),
            sidecars: Vec::new(),
        };

        persist(&catalog, "PhotosRaw", &result).unwrap();
        persist(&catalog, "PhotosRaw", &result).unwrap();

        assert_eq!(total_images(&catalog).unwrap(), 1, "no duplicate rows");
        let roots: i64 = catalog
            .connection()
            .query_row("SELECT count(*) FROM roots", [], |r| r.get(0))
            .unwrap();
        assert_eq!(roots, 1, "no duplicate roots");
    }

    #[test]
    fn stable_file_ids_are_recorded() {
        // FR-NC-5: a server-side move must be a move, not a re-download.
        let catalog = Catalog::in_memory().unwrap();
        let result = dr_sync::ScanResult {
            images: vec![entry("PhotosRaw/a.CR2", 4242, 30_000_000)],
            directories: vec![(RemotePath::new("PhotosRaw"), dr_sync::Validator::new("e1"))],
            progress: Default::default(),
            sidecars: Vec::new(),
        };
        persist(&catalog, "PhotosRaw", &result).unwrap();

        let file_id: i64 = catalog
            .connection()
            .query_row("SELECT file_id FROM remote", [], |r| r.get(0))
            .unwrap();
        assert_eq!(file_id, 4242);
    }

    #[test]
    fn a_thumbnail_job_is_queued_per_image() {
        let catalog = Catalog::in_memory().unwrap();
        let result = dr_sync::ScanResult {
            images: vec![
                entry("PhotosRaw/a.CR2", 1, 30_000_000),
                entry("PhotosRaw/b.CR2", 2, 30_000_000),
            ],
            directories: vec![(RemotePath::new("PhotosRaw"), dr_sync::Validator::new("e1"))],
            progress: Default::default(),
            sidecars: Vec::new(),
        };
        persist(&catalog, "PhotosRaw", &result).unwrap();

        let jobs: i64 = catalog
            .connection()
            .query_row("SELECT count(*) FROM jobs WHERE kind = 2", [], |r| r.get(0))
            .unwrap();
        assert_eq!(jobs, 2);
    }

    #[test]
    fn a_second_scan_does_not_multiply_jobs() {
        let catalog = Catalog::in_memory().unwrap();
        let result = dr_sync::ScanResult {
            images: vec![entry("PhotosRaw/a.CR2", 1, 30_000_000)],
            directories: vec![(RemotePath::new("PhotosRaw"), dr_sync::Validator::new("e1"))],
            progress: Default::default(),
            sidecars: Vec::new(),
        };
        persist(&catalog, "PhotosRaw", &result).unwrap();
        persist(&catalog, "PhotosRaw", &result).unwrap();

        let jobs: i64 = catalog
            .connection()
            .query_row("SELECT count(*) FROM jobs WHERE kind = 2", [], |r| r.get(0))
            .unwrap();
        assert_eq!(jobs, 1, "coalesced, not queued twice");
    }

    #[test]
    fn folder_etags_round_trip_for_the_next_scan() {
        let catalog = Catalog::in_memory().unwrap();
        let result = dr_sync::ScanResult {
            images: vec![],
            directories: vec![
                (RemotePath::new("PhotosRaw"), dr_sync::Validator::new("e1")),
                (
                    RemotePath::new("PhotosRaw/2026"),
                    dr_sync::Validator::new("e2"),
                ),
            ],
            progress: Default::default(),
            sidecars: Vec::new(),
        };
        persist(&catalog, "PhotosRaw", &result).unwrap();

        let known = load_folder_etags(&catalog, "PhotosRaw");
        assert_eq!(known.len(), 2);
        assert_eq!(
            known
                .get(&RemotePath::new("PhotosRaw/2026"))
                .map(|v| v.as_str()),
            Some("e2")
        );
    }

    #[test]
    fn cells_are_windowed_not_wholesale() {
        let catalog = Catalog::in_memory().unwrap();
        let images: Vec<RemoteEntry> = (0..50)
            .map(|i| entry(&format!("PhotosRaw/img{i:03}.CR2"), i as u64, 1000))
            .collect();
        let result = dr_sync::ScanResult {
            images,
            directories: vec![(RemotePath::new("PhotosRaw"), dr_sync::Validator::new("e"))],
            progress: Default::default(),
            sidecars: Vec::new(),
        };
        persist(&catalog, "PhotosRaw", &result).unwrap();

        let page = read_cells(&catalog, 10, 5).unwrap();
        assert_eq!(page.len(), 5);
        assert_eq!(page[0].name, "img010.CR2");
    }

    /// TRACES: NFR-P5
    /// The grid's window read must be answered by walking `images_grid_order`,
    /// never by sorting the library into a temp b-tree.
    ///
    /// This asserts on the *query plan* rather than on a duration, because the
    /// failure has no other symptom: a `GRID_ORDER` edited out of step with the
    /// index in schema V7, or a column added back into the paging query that
    /// drags `remote` in again, both still return the right cells. They just
    /// return them after sorting 24,000 rows, inside the scroll handler — which
    /// is the jitter this pair was introduced to remove, and it would come back
    /// silently.
    #[test]
    fn the_window_read_walks_the_ordering_index() {
        let catalog = with_images(20);
        // Including the burst clause, because the grid includes it: a
        // predicate that quietly cost the ordering index would put the sort
        // back and this is the only place that would notice.
        let folded = uncollapsed("i");
        let plan: Vec<String> = catalog
            .connection()
            .prepare(&format!(
                "EXPLAIN QUERY PLAN
                 SELECT {CELL_COLUMNS} FROM images i
                  WHERE {VISIBLE}{folded}
                  {GRID_ORDER}
                  LIMIT 10 OFFSET 5"
            ))
            .unwrap()
            .query_map([], |r| r.get::<_, String>(3))
            .unwrap()
            .flatten()
            .collect();
        let plan = plan.join(" | ");

        assert!(
            plan.contains("images_grid_order"),
            "the window read is not using the ordering index: {plan}"
        );
        assert!(
            !plan.contains("TEMP B-TREE"),
            "the window read is still sorting the whole library: {plan}"
        );
        assert!(
            !plan.to_lowercase().contains("remote"),
            "the window read is joining `remote` again, which pages the whole \
             library before it discards it: {plan}"
        );
    }

    /// The file ids still arrive, now that they come from a second query.
    #[test]
    fn a_window_still_carries_the_server_file_ids() {
        let catalog = with_images(20);
        let page = read_cells(&catalog, 5, 4).unwrap();
        assert_eq!(page.len(), 4);
        assert!(
            page.iter().all(|c| c.file_id.is_some()),
            "a cell lost its file id when the join was split out"
        );
    }

    /// A catalog with `n` images, ready to file into collections.
    fn with_images(n: usize) -> Catalog {
        let catalog = Catalog::in_memory().unwrap();
        let images: Vec<RemoteEntry> = (0..n)
            .map(|i| entry(&format!("PhotosRaw/img{i:03}.CR2"), i as u64, 1000))
            .collect();
        let result = dr_sync::ScanResult {
            images,
            directories: vec![(RemotePath::new("PhotosRaw"), dr_sync::Validator::new("e"))],
            progress: Default::default(),
            sidecars: Vec::new(),
        };
        persist(&catalog, "PhotosRaw", &result).unwrap();
        catalog
    }

    /// TRACES: FR-CULL-5
    /// A folded burst takes rows out of the cells, the count and the range a
    /// shift-click resolves — all three, together.
    ///
    /// This is the test that would fail if the clause were added to four of
    /// the five queries that need it. That failure has no other symptom: the
    /// header claims images the grid will not draw, the scrollbar sizes itself
    /// for rows that are not there, and neither number looks wrong on its own.
    #[test]
    fn folding_a_burst_takes_the_same_rows_out_of_every_answer() {
        use dr_catalog::bursts::{self, Rules, Signature};

        let catalog = with_images(4);
        // Three of the four are one burst: a second apart, one signature.
        let ids = image_ids(&catalog);
        for (n, id) in ids.iter().enumerate() {
            let hash = if n < 3 { 0xFF00 } else { 0x00FF };
            catalog
                .connection()
                .execute(
                    "UPDATE images SET captured_at = ?2, camera = 'Canon EOS R5',
                                       perceptual_hash = ?3
                      WHERE id = ?1",
                    rusqlite::params![id.0 as i64, 1_000 + n as i64, Signature(hash).to_stored()],
                )
                .unwrap();
        }
        bursts::regroup(catalog.connection(), Rules::default()).unwrap();

        let filter = RatingFilter::default();
        // Open, as a new burst is: nothing has been taken away yet.
        assert_eq!(read_cells(&catalog, 0, 50).unwrap().len(), 4);
        assert_eq!(total_images_filtered(&catalog, &filter).unwrap(), 4);

        bursts::set_expanded(catalog.connection(), ids[0], false).unwrap();

        let cells = read_cells(&catalog, 0, 50).unwrap();
        assert_eq!(cells.len(), 2, "the folded frames are still in the cells");
        assert_eq!(
            total_images_filtered(&catalog, &filter).unwrap(),
            cells.len(),
            "the header's count and the cells disagree"
        );
        assert_eq!(
            read_ids_span(&catalog, None, &filter, false, 0, 49)
                .unwrap()
                .len(),
            cells.len(),
            "a shift-click over the whole grid would select frames it cannot show"
        );
    }

    fn image_ids(catalog: &Catalog) -> Vec<dr_types::ImageId> {
        let mut stmt = catalog
            .connection()
            .prepare("SELECT id FROM images ORDER BY source_ref")
            .unwrap();
        stmt.query_map([], |r| Ok(dr_types::ImageId(r.get::<_, i64>(0)? as u64)))
            .unwrap()
            .map(Result::unwrap)
            .collect()
    }

    /// A face with no proxy left draws "no preview" and cannot repair itself:
    /// the image has its `face_index` row, so it is not outstanding work. The
    /// sweep has to pick it up by a second route.
    #[test]
    fn a_face_whose_proxy_is_gone_is_work_again() {
        let catalog = with_images(3);
        let ids = image_ids(&catalog);
        // An empty store, which is the state the bug lives in: the face is
        // recorded and there is nothing on disk to cut it out of.
        let store_dir =
            std::env::temp_dir().join(format!("dr-face-proxy-test-{}", std::process::id()));
        let _ = std::fs::remove_dir_all(&store_dir);
        std::fs::create_dir_all(&store_dir).unwrap();
        let store = ThumbStore::open(&store_dir).unwrap();

        let face = dr_catalog::faces::DetectedFace {
            x: 0.1,
            y: 0.1,
            w: 0.2,
            h: 0.2,
            landmarks: [(0.0, 0.0); 5],
            confidence: 0.9,
            embedding: vec![0u8; 1024],
            crop_px: 120.0,
            quality: None,
            crop: Vec::new(),
            model_id: "w600k_mbf".into(),
        };
        dr_catalog::faces::record_detections(
            catalog.connection(),
            ids[0],
            "w600k_mbf",
            1024,
            std::slice::from_ref(&face),
        )
        .unwrap();

        // Indexed, so not outstanding — but with nothing to crop from.
        assert!(!faces_unindexed(&catalog, "w600k_mbf")
            .unwrap()
            .iter()
            .any(|r| r.image_id == ids[0].0 as i64));
        let repair = faces_without_proxy(&catalog, &store, "w600k_mbf").unwrap();
        assert_eq!(repair.len(), 1, "the orphaned face was not picked up");
        assert_eq!(repair[0].image_id, ids[0].0 as i64);
        let _ = std::fs::remove_dir_all(&store_dir);
    }

    /// Ordering is load-bearing. A repair queued behind every un-indexed image
    /// in the library is a repair that does not happen inside a session, and
    /// the screen it was meant to fix stays empty.
    #[test]
    fn repairs_are_reached_before_the_rest_of_the_library() {
        let catalog = with_images(50);
        let ids = image_ids(&catalog);
        let store_dir = std::env::temp_dir().join(format!(
            "dr-face-order-test-{}-{:?}",
            std::process::id(),
            std::thread::current().id()
        ));
        let _ = std::fs::remove_dir_all(&store_dir);
        std::fs::create_dir_all(&store_dir).unwrap();
        let store = ThumbStore::open(&store_dir).unwrap();

        // One image late in the library has a face and no proxy.
        let face = dr_catalog::faces::DetectedFace {
            x: 0.1,
            y: 0.1,
            w: 0.2,
            h: 0.2,
            landmarks: [(0.0, 0.0); 5],
            confidence: 0.9,
            embedding: vec![0u8; 1024],
            crop_px: 120.0,
            quality: None,
            crop: Vec::new(),
            model_id: "w600k_mbf".into(),
        };
        let orphan = ids[40];
        dr_catalog::faces::record_detections(
            catalog.connection(),
            orphan,
            "w600k_mbf",
            1024,
            std::slice::from_ref(&face),
        )
        .unwrap();

        let mut wanted = faces_without_proxy(&catalog, &store, "w600k_mbf").unwrap();
        wanted.extend(faces_unindexed(&catalog, "w600k_mbf").unwrap());

        assert_eq!(
            wanted.first().map(|r| r.image_id),
            Some(orphan.0 as i64),
            "the image the screen cannot draw must be fetched first"
        );
        assert_eq!(
            wanted.len(),
            50,
            "49 un-indexed plus the one being repaired"
        );
        let _ = std::fs::remove_dir_all(&store_dir);
    }

    /// The regression this whole pass exists for.
    ///
    /// The previous work list intersected with the thumbnail store, so a
    /// library nobody had zoomed into produced an empty one and "index the
    /// whole library" indexed nothing. Nothing here puts a proxy on disk.
    #[test]
    fn every_unindexed_image_is_work_even_with_no_proxy_anywhere() {
        let catalog = with_images(10);
        let wanted = faces_unindexed(&catalog, "w600k_mbf").unwrap();
        assert_eq!(
            wanted.len(),
            10,
            "a library with no thumbnails is still work"
        );
        assert!(
            wanted.iter().all(|r| r.full_resolution),
            "the pass must keep the detail a thumbnail would throw away"
        );
    }

    /// `face_index` records that detection *ran*, so an image with no face in
    /// it must not come back on the next pass — otherwise a personal library,
    /// which is mostly landscapes and documents, never finishes.
    #[test]
    fn an_image_already_run_over_is_not_work_again() {
        let catalog = with_images(3);
        let ids = image_ids(&catalog);
        dr_catalog::faces::record_detections(catalog.connection(), ids[0], "w600k_mbf", 1024, &[])
            .unwrap();

        let wanted = faces_unindexed(&catalog, "w600k_mbf").unwrap();
        assert_eq!(wanted.len(), 2);
        assert!(!wanted.iter().any(|r| r.image_id == ids[0].0 as i64));

        // A different model has seen none of them.
        assert_eq!(faces_unindexed(&catalog, "other").unwrap().len(), 3);
    }

    /// A detector change keeps the embedder, so it is not a new library: the
    /// images the old detector ran over are not "unindexed" for the new one.
    /// They are an *upgrade*, listed separately and only when the chosen
    /// detector outranks the one that indexed them — never the other way,
    /// or a tablet on the fast detector would undo the desktop's thorough
    /// pass.
    #[test]
    fn a_stronger_detector_upgrades_rather_than_re_indexes() {
        let catalog = with_images(3);
        let ids = image_ids(&catalog);
        let conn = catalog.connection();
        dr_catalog::faces::record_detections(conn, ids[0], "w600k_mbf", 1024, &[]).unwrap();
        dr_catalog::faces::record_detections(conn, ids[1], "scrfd_10g+w600k_mbf", 1024, &[])
            .unwrap();

        let fresh = faces_unindexed(&catalog, "scrfd_10g+w600k_mbf").unwrap();
        assert_eq!(
            fresh.len(),
            1,
            "the earlier detector's images were re-queued"
        );
        assert_eq!(fresh[0].image_id, ids[2].0 as i64);

        // Thorough re-runs what Fast did; Fast leaves what Thorough did alone.
        let up = faces_superseded(&catalog, &["w600k_mbf", "scrfd_2.5g+w600k_mbf"]).unwrap();
        assert_eq!(up.len(), 1);
        assert_eq!(up[0].image_id, ids[0].0 as i64);
        assert!(faces_superseded(&catalog, &[]).unwrap().is_empty());
    }

    /// The state schema V14 leaves: a face with no quality and an image with
    /// no marker. It is the measuring pass's work, and *only* that pass's — a
    /// full re-detection of the same image would throw away every suggestion
    /// on it for nothing.
    #[test]
    fn an_unmeasured_face_is_measured_rather_than_re_detected() {
        let catalog = with_images(3);
        let ids = image_ids(&catalog);
        let stored = |quality: Option<f32>| dr_catalog::faces::DetectedFace {
            x: 0.1,
            y: 0.1,
            w: 0.2,
            h: 0.2,
            landmarks: [(0.0, 0.0); 5],
            confidence: 0.9,
            embedding: vec![0u8; 1024],
            crop_px: 120.0,
            quality,
            crop: Vec::new(),
            model_id: "w600k_mbf".into(),
        };
        let conn = catalog.connection();
        dr_catalog::faces::record_detections(conn, ids[0], "w600k_mbf", 4000, &[stored(None)])
            .unwrap();
        dr_catalog::faces::record_detections(
            conn,
            ids[1],
            "w600k_mbf",
            4000,
            &[stored(Some(18.0))],
        )
        .unwrap();
        // What V14 does to the first: the marker goes, the face stays.
        dr_catalog::faces::clear_index_marker(conn, ids[0], "w600k_mbf").unwrap();

        let measure = faces_unmeasured(&catalog, "w600k_mbf").unwrap();
        assert_eq!(measure.len(), 1);
        assert_eq!(measure[0].image_id, ids[0].0 as i64);
        assert!(measure[0].full_resolution);

        // Not re-detected, marker or no marker; the third image, never seen,
        // still is.
        let detect = faces_unindexed(&catalog, "w600k_mbf").unwrap();
        assert_eq!(detect.len(), 1);
        assert_eq!(detect[0].image_id, ids[2].0 as i64);
    }

    #[test]
    fn a_scoped_grid_shows_only_that_collections_images() {
        use dr_catalog::collections::{self as coll, CollectionKind};

        let catalog = with_images(10);
        let ids = image_ids(&catalog);
        let c = coll::create(
            catalog.connection(),
            "Selects",
            None,
            CollectionKind::Manual,
        )
        .unwrap();
        coll::add_images(catalog.connection(), c, &ids[2..5]).unwrap();

        let cells = read_cells_scoped(&catalog, Some(c), &RatingFilter::default(), 0, 120).unwrap();
        assert_eq!(cells.len(), 3);
        assert_eq!(
            total_images_scoped(&catalog, Some(c), &RatingFilter::default()).unwrap(),
            3
        );
        // Unscoped is still the whole library.
        assert_eq!(
            total_images_scoped(&catalog, None, &RatingFilter::default()).unwrap(),
            10
        );
    }

    #[test]
    fn a_collection_set_shows_its_childrens_images() {
        // A parent whose children hold everything must not read as empty —
        // that is what makes nesting look broken.
        use dr_catalog::collections::{self as coll, CollectionKind};

        let catalog = with_images(10);
        let ids = image_ids(&catalog);
        let trips =
            coll::create(catalog.connection(), "Trips", None, CollectionKind::Manual).unwrap();
        let iceland = coll::create(
            catalog.connection(),
            "Iceland",
            Some(trips),
            CollectionKind::Manual,
        )
        .unwrap();
        coll::add_images(catalog.connection(), iceland, &ids[0..4]).unwrap();

        // The parent itself has no direct members at all.
        let cells =
            read_cells_scoped(&catalog, Some(trips), &RatingFilter::default(), 0, 120).unwrap();
        assert_eq!(cells.len(), 4, "the set shows what its children hold");
        assert_eq!(
            total_images_scoped(&catalog, Some(trips), &RatingFilter::default()).unwrap(),
            4
        );
    }

    #[test]
    fn an_image_in_both_a_parent_and_a_child_is_shown_once() {
        // The count and the number of cells drawn must agree, or neither is
        // believable.
        use dr_catalog::collections::{self as coll, CollectionKind};

        let catalog = with_images(10);
        let ids = image_ids(&catalog);
        let trips =
            coll::create(catalog.connection(), "Trips", None, CollectionKind::Manual).unwrap();
        let iceland = coll::create(
            catalog.connection(),
            "Iceland",
            Some(trips),
            CollectionKind::Manual,
        )
        .unwrap();
        coll::add_images(catalog.connection(), trips, &ids[0..2]).unwrap();
        coll::add_images(catalog.connection(), iceland, &ids[0..3]).unwrap();

        let cells =
            read_cells_scoped(&catalog, Some(trips), &RatingFilter::default(), 0, 120).unwrap();
        assert_eq!(cells.len(), 3, "images 0..3, each once");
        assert_eq!(
            total_images_scoped(&catalog, Some(trips), &RatingFilter::default()).unwrap(),
            3
        );
    }

    #[test]
    fn a_scoped_window_still_pages() {
        // FR-CAT-4 applies inside a collection too: a 5,000-image collection
        // must not become 5,000 rows.
        use dr_catalog::collections::{self as coll, CollectionKind};

        let catalog = with_images(30);
        let ids = image_ids(&catalog);
        let c = coll::create(catalog.connection(), "Big", None, CollectionKind::Manual).unwrap();
        coll::add_images(catalog.connection(), c, &ids).unwrap();

        let page = read_cells_scoped(&catalog, Some(c), &RatingFilter::default(), 10, 5).unwrap();
        assert_eq!(page.len(), 5);
        assert_eq!(page[0].name, "img010.CR2");
    }

    #[test]
    fn an_empty_collection_reads_as_empty_rather_than_as_the_whole_library() {
        // The failure that would make scoping useless: an empty IN-list
        // matching everything.
        use dr_catalog::collections::{self as coll, CollectionKind};

        let catalog = with_images(10);
        let c = coll::create(catalog.connection(), "Empty", None, CollectionKind::Manual).unwrap();

        assert!(
            read_cells_scoped(&catalog, Some(c), &RatingFilter::default(), 0, 120)
                .unwrap()
                .is_empty()
        );
        assert_eq!(
            total_images_scoped(&catalog, Some(c), &RatingFilter::default()).unwrap(),
            0
        );
    }

    #[test]
    fn cells_carry_the_file_id_the_thumbnail_store_keys_on() {
        // Without this the store can never be hit: every launch would refetch
        // every thumbnail over the network.
        let catalog = Catalog::in_memory().unwrap();
        let result = dr_sync::ScanResult {
            images: vec![entry("PhotosRaw/a.CR2", 7777, 30_000_000)],
            directories: vec![(RemotePath::new("PhotosRaw"), dr_sync::Validator::new("e"))],
            progress: Default::default(),
            sidecars: Vec::new(),
        };
        persist(&catalog, "PhotosRaw", &result).unwrap();

        let cells = read_cells(&catalog, 0, 10).unwrap();
        assert_eq!(cells[0].file_id, Some(7777));
    }

    #[test]
    fn a_stored_thumbnail_survives_a_restart() {
        // The end-to-end property the store exists for: encode, persist,
        // reopen, decode. A second launch must not re-fetch.
        let dir = std::env::temp_dir().join(format!("dr-ui-thumbs-{}", std::process::id()));
        let _ = std::fs::remove_dir_all(&dir);

        let rgba: Vec<u8> = std::iter::repeat_n([90u8, 140, 200, 255], 32 * 32)
            .flatten()
            .collect();
        {
            let mut store = ThumbStore::open(&dir).unwrap();
            let bytes = dr_thumbs::encode_rgba(32, 32, &rgba).unwrap();
            store
                .put(
                    4242,
                    dr_thumbs::ThumbSize::Grid,
                    &dr_thumbs::Thumbnail {
                        width: 32,
                        height: 32,
                        bytes,
                    },
                )
                .unwrap();
        }

        let store = ThumbStore::open(&dir).unwrap();
        let stored = store
            .get(4242, dr_thumbs::ThumbSize::Grid)
            .unwrap()
            .expect("persisted");
        let (w, h, out) = dr_thumbs::decode_rgba(&stored.bytes).unwrap();
        assert_eq!((w, h), (32, 32));
        // Lossy, so compare approximately — a blue-ish pixel must stay blue.
        assert!(out[2] > out[0], "channel order survived the round trip");
    }

    #[test]
    fn a_store_hit_is_not_evidence_the_server_is_reachable() {
        // The regression this guards: store hits were delivered as the same
        // `Ready` the network path sends, and the UI took any `Ready` as proof
        // of connectivity. A mostly-cached window then declared "back online"
        // against a server that was down — clearing the banner and kicking off
        // a sweep that immediately failed, on every scroll.
        let dir = std::env::temp_dir().join(format!("dr-ui-provenance-{}", std::process::id()));
        let _ = std::fs::remove_dir_all(&dir);

        let rgba: Vec<u8> = std::iter::repeat_n([10u8, 20, 30, 255], 8 * 8)
            .flatten()
            .collect();
        let mut store = ThumbStore::open(&dir).unwrap();
        let bytes = dr_thumbs::encode_rgba(8, 8, &rgba).unwrap();
        store
            .put(
                99,
                dr_thumbs::ThumbSize::Grid,
                &dr_thumbs::Thumbnail {
                    width: 8,
                    height: 8,
                    bytes,
                },
            )
            .unwrap();

        // The split in `spawn_thumbnails`: a hit decodes off local disk and is
        // reported with `from_cache` set, which is what the reachability gate
        // keys on.
        let stored = store
            .get(99, dr_thumbs::ThumbSize::Grid)
            .unwrap()
            .expect("stored");
        let (width, height, rgba) = dr_thumbs::decode_rgba(&stored.bytes).unwrap();
        let hit = ThumbnailReady {
            row: 0,
            width,
            height,
            rgba,
            from_cache: true,
        };

        assert!(
            hit.from_cache,
            "a thumbnail read from the store must not be mistaken for a fetch"
        );

        // And the mechanism it feeds: an offline tracker must survive it.
        let mut reach = dr_sync::Reachability::new();
        let now = std::time::Instant::now();
        reach.mark_unreachable("network error".into(), now);

        if !hit.from_cache {
            reach.mark_reachable(now);
        }
        assert!(
            reach.is_offline(),
            "replaying cached thumbnails must leave offline mode intact"
        );
    }

    #[test]
    fn thumbs_live_beside_the_catalog_not_in_the_cache() {
        // They sync to the server and are shared with other clients, so a
        // cache sweep must not discard them.
        let cat = catalog_path(&account("https://cloud.example", "duncan"));
        let thumbs = thumbs_dir(&account("https://cloud.example", "duncan"));
        assert_eq!(thumbs.parent(), cat.parent());
    }

    // --- the trash view (FR-CAT-15) ---------------------------------------

    /// A catalog with `n` scanned images, none trashed.
    fn scanned(n: u64) -> Catalog {
        let catalog = Catalog::in_memory().unwrap();
        let images = (1..=n)
            .map(|i| entry(&format!("PhotosRaw/IMG_{i:04}.CR2"), 1000 + i, 30_000_000))
            .collect();
        persist(
            &catalog,
            "PhotosRaw",
            &dr_sync::ScanResult {
                images,
                directories: vec![(RemotePath::new("PhotosRaw"), dr_sync::Validator::new("e1"))],
                progress: Default::default(),
                sidecars: Vec::new(),
            },
        )
        .unwrap();
        catalog
    }

    /// Mark one image trashed, at a given instant.
    fn trash_at(catalog: &Catalog, path: &str, when: i64) {
        let n = catalog
            .connection()
            .execute(
                "UPDATE images SET trashed_at = ?1, trashed_from = source_ref
                  WHERE source_ref = ?2",
                rusqlite::params![when, path],
            )
            .unwrap();
        assert_eq!(n, 1, "fixture should have trashed exactly {path}");
    }

    #[test]
    fn the_trash_lists_what_the_library_hides() {
        // The whole point of the view: these rows exist and no other query in
        // the application will show them.
        let catalog = scanned(3);
        trash_at(&catalog, "PhotosRaw/IMG_0002.CR2", 100);

        let trashed = read_trashed_cells(&catalog, 0, 50).unwrap();
        assert_eq!(trashed.len(), 1);
        assert_eq!(trashed[0].remote_path, "PhotosRaw/IMG_0002.CR2");

        // And it has left the library in the same move.
        let live = read_cells_all(&catalog, &RatingFilter::default(), 0, 50).unwrap();
        assert_eq!(live.len(), 2);
        assert!(!live.iter().any(|c| c.remote_path.contains("IMG_0002")));
    }

    #[test]
    fn an_untrashed_library_has_an_empty_trash() {
        let catalog = scanned(3);
        assert!(read_trashed_cells(&catalog, 0, 50).unwrap().is_empty());
        assert_eq!(total_trashed(&catalog).unwrap(), 0);
    }

    #[test]
    fn the_trash_count_agrees_with_the_cells_it_lists() {
        // The header and the scrollbar are sized from the count while the grid
        // draws the cells. Two predicates that drift leave the user scrolling
        // through rows that are not there.
        let catalog = scanned(5);
        for (i, when) in [(1, 100), (3, 200), (5, 300)] {
            trash_at(&catalog, &format!("PhotosRaw/IMG_{i:04}.CR2"), when);
        }

        assert_eq!(total_trashed(&catalog).unwrap(), 3);
        assert_eq!(read_trashed_cells(&catalog, 0, 50).unwrap().len(), 3);
    }

    // --- manual order within a collection (FR-CAT-7) ------------------------

    fn ids(n: &[u64]) -> Vec<dr_types::ImageId> {
        n.iter().copied().map(dr_types::ImageId).collect()
    }

    #[test]
    fn a_run_moved_forward_lands_before_the_photograph_it_was_dropped_on() {
        let current = ids(&[1, 2, 3, 4, 5]);
        assert_eq!(
            reordered(&current, &ids(&[4]), dr_types::ImageId(2), false),
            ids(&[1, 4, 2, 3, 5])
        );
    }

    #[test]
    fn a_run_moved_backward_lands_before_it_too() {
        // The direction of travel must not change what "before this one" means,
        // or the same drop would land in two different places depending on
        // where the photograph came from.
        let current = ids(&[1, 2, 3, 4, 5]);
        assert_eq!(
            reordered(&current, &ids(&[2]), dr_types::ImageId(5), false),
            ids(&[1, 3, 4, 2, 5])
        );
    }

    #[test]
    fn the_trailing_half_of_the_last_cell_is_how_the_end_is_reached() {
        // There is no cell beyond the last one to drop in front of, so without
        // `after` the final position is unreachable — which is exactly the
        // place a "put this at the end" drag is aiming for.
        let current = ids(&[1, 2, 3]);
        assert_eq!(
            reordered(&current, &ids(&[1]), dr_types::ImageId(3), true),
            ids(&[2, 3, 1])
        );
    }

    #[test]
    fn a_moved_run_keeps_the_order_the_grid_shows_it_in() {
        // Not the order the selection was built in. A user who tapped the last
        // frame first has said nothing about how the run should be arranged —
        // only about where it should go.
        let current = ids(&[1, 2, 3, 4, 5]);
        assert_eq!(
            reordered(&current, &ids(&[5, 1]), dr_types::ImageId(3), false),
            ids(&[2, 1, 5, 3, 4])
        );
    }

    #[test]
    fn a_run_dropped_on_one_of_its_own_members_stays_together() {
        // The caller refuses this drop, so it is only reachable if that guard
        // is ever lost. It must not lose photographs when it is.
        let current = ids(&[1, 2, 3, 4]);
        let moved = reordered(&current, &ids(&[2, 3]), dr_types::ImageId(3), false);
        assert_eq!(moved.len(), current.len(), "nothing was dropped");
        let mut sorted = moved.clone();
        sorted.sort();
        assert_eq!(sorted, ids(&[1, 2, 3, 4]), "and nothing was invented");
    }

    #[test]
    fn a_reorder_never_loses_or_duplicates_a_member() {
        // The property that matters most: this writes the whole membership
        // back, so a run that dropped one image would delete it from the
        // collection.
        let current = ids(&[1, 2, 3, 4, 5, 6]);
        for target in [1u64, 2, 3, 4, 5, 6] {
            for after in [false, true] {
                let moved = reordered(&current, &ids(&[2, 5]), dr_types::ImageId(target), after);
                let mut sorted = moved.clone();
                sorted.sort();
                assert_eq!(
                    sorted,
                    ids(&[1, 2, 3, 4, 5, 6]),
                    "target {target}, after {after}"
                );
            }
        }
    }

    /// The scoped grid and the range a shift-click resolves are two halves of
    /// one ordering. This is the assertion that keeps them one: an ordinal read
    /// through a different ORDER BY names a different photograph, and the user
    /// finds out when the export runs.
    #[test]
    fn a_manual_collection_is_read_and_spanned_in_the_order_it_was_given() {
        let catalog = with_images(5);
        let all = image_ids(&catalog);
        let id = dr_catalog::collections::create(
            catalog.connection(),
            "Trip",
            None,
            dr_catalog::collections::CollectionKind::Manual,
        )
        .unwrap();
        dr_catalog::collections::add_images(catalog.connection(), id, &all).unwrap();

        // Reversed, so position and capture time disagree about everything.
        let wanted: Vec<_> = all.iter().rev().copied().collect();
        dr_catalog::collections::set_order(catalog.connection(), id, &wanted).unwrap();

        let cells = read_cells_scoped(&catalog, Some(id), &RatingFilter::default(), 0, 50).unwrap();
        let drawn: Vec<_> = cells
            .iter()
            .map(|c| dr_types::ImageId(c.image_id as u64))
            .collect();
        assert_eq!(drawn, wanted, "the grid draws the order that was written");

        let spanned =
            read_ids_span(&catalog, Some(id), &RatingFilter::default(), false, 0, 4).unwrap();
        assert_eq!(spanned, wanted, "and a range resolves through the same one");

        assert_eq!(
            read_member_order(&catalog, id).unwrap(),
            wanted,
            "and so does the membership a reorder rewrites"
        );
    }

    #[test]
    fn a_collection_with_children_falls_back_to_capture_time() {
        // A set draws its descendants' images too, and two children's positions
        // are unrelated integers. Ordering by them interleaves the two
        // arbitrarily, which is worse than an order that at least means
        // something.
        let catalog = with_images(4);
        let all = image_ids(&catalog);
        let parent = dr_catalog::collections::create(
            catalog.connection(),
            "Iceland",
            None,
            dr_catalog::collections::CollectionKind::Manual,
        )
        .unwrap();
        dr_catalog::collections::create(
            catalog.connection(),
            "Day one",
            Some(parent),
            dr_catalog::collections::CollectionKind::Manual,
        )
        .unwrap();
        dr_catalog::collections::add_images(catalog.connection(), parent, &all).unwrap();
        let reversed: Vec<_> = all.iter().rev().copied().collect();
        dr_catalog::collections::set_order(catalog.connection(), parent, &reversed).unwrap();

        let cells =
            read_cells_scoped(&catalog, Some(parent), &RatingFilter::default(), 0, 50).unwrap();
        let drawn: Vec<_> = cells
            .iter()
            .map(|c| dr_types::ImageId(c.image_id as u64))
            .collect();
        assert_eq!(drawn, all, "capture time, not the positions that were set");
    }

    #[test]
    fn a_smart_collection_has_no_manual_order_to_read() {
        // No member rows at all, so `position` is not a column any of its
        // images have. Falling back is the only thing there is to do.
        let catalog = with_images(3);
        let id = dr_catalog::collections::create(
            catalog.connection(),
            "Picks",
            None,
            dr_catalog::collections::CollectionKind::Smart,
        )
        .unwrap();
        let (order, params) = grid_order_for(&catalog, Some(id));
        assert_eq!(order, GRID_ORDER);
        assert!(params.is_empty());
    }

    #[test]
    fn the_most_recently_trashed_image_is_listed_first() {
        // A mistaken delete is corrected within seconds, so the row the user
        // wants is the one they just made — not the oldest photograph.
        let catalog = scanned(3);
        trash_at(&catalog, "PhotosRaw/IMG_0001.CR2", 100);
        trash_at(&catalog, "PhotosRaw/IMG_0002.CR2", 300);
        trash_at(&catalog, "PhotosRaw/IMG_0003.CR2", 200);

        let order: Vec<String> = read_trashed_cells(&catalog, 0, 50)
            .unwrap()
            .into_iter()
            .map(|c| c.remote_path)
            .collect();
        assert_eq!(
            order,
            vec![
                "PhotosRaw/IMG_0002.CR2".to_string(),
                "PhotosRaw/IMG_0003.CR2".to_string(),
                "PhotosRaw/IMG_0001.CR2".to_string(),
            ]
        );
    }

    #[test]
    fn a_shadowed_jpeg_is_not_listed_beside_the_raw_it_belongs_to() {
        // The inversion applies to `trashed_at` only. Trashing a RAW takes its
        // sibling JPEG with it, and listing both would offer to restore the
        // same frame twice.
        let catalog = scanned(2);
        let c = catalog.connection();
        let raw: i64 = c
            .query_row(
                "SELECT id FROM images WHERE source_ref = 'PhotosRaw/IMG_0001.CR2'",
                [],
                |r| r.get(0),
            )
            .unwrap();
        c.execute(
            "UPDATE images SET shadowed_by = ?1 WHERE source_ref = 'PhotosRaw/IMG_0002.CR2'",
            [raw],
        )
        .unwrap();

        trash_at(&catalog, "PhotosRaw/IMG_0001.CR2", 100);
        trash_at(&catalog, "PhotosRaw/IMG_0002.CR2", 100);

        let trashed = read_trashed_cells(&catalog, 0, 50).unwrap();
        assert_eq!(trashed.len(), 1, "one photograph, not two");
        assert_eq!(trashed[0].remote_path, "PhotosRaw/IMG_0001.CR2");
        assert_eq!(total_trashed(&catalog).unwrap(), 1, "and the count agrees");
    }

    #[test]
    fn the_trash_window_pages_like_the_grid_does() {
        // The trash uses the same windowed read as the library, so a large one
        // must not try to draw itself in a single query.
        let catalog = scanned(6);
        for i in 1..=6 {
            trash_at(
                &catalog,
                &format!("PhotosRaw/IMG_{i:04}.CR2"),
                100 + i as i64,
            );
        }

        let first = read_trashed_cells(&catalog, 0, 2).unwrap();
        let second = read_trashed_cells(&catalog, 2, 2).unwrap();
        assert_eq!(first.len(), 2);
        assert_eq!(second.len(), 2);
        assert!(
            first
                .iter()
                .all(|a| !second.iter().any(|b| b.image_id == a.image_id)),
            "pages must not overlap"
        );
    }

    #[test]
    fn a_rating_filter_does_not_hide_anything_in_the_trash() {
        // The filter narrows a culling pass. A trash that dropped rows because
        // of a filter set elsewhere would look like it had lost them.
        let catalog = scanned(3);
        trash_at(&catalog, "PhotosRaw/IMG_0001.CR2", 100);

        // Nothing here is rated, so a four-star library filter would empty any
        // view that honoured it.
        let strict = RatingFilter {
            min_rating: 4,
            ..Default::default()
        };
        assert!(read_cells_all(&catalog, &strict, 0, 50).unwrap().is_empty());
        assert_eq!(read_trashed_cells(&catalog, 0, 50).unwrap().len(), 1);
    }

    #[test]
    fn a_date_range_narrows_the_grid_and_the_count_together() {
        // The whole reason the range lives on `RatingFilter`: every query path
        // threads that one struct, so the header cannot claim a total the grid
        // does not draw.
        let catalog = scanned(3);
        let conn = catalog.connection();
        for (n, at) in [(1, 1_000), (2, 5_000), (3, 9_000)] {
            conn.execute(
                "UPDATE images SET captured_at = ?2 WHERE source_ref LIKE ?1",
                rusqlite::params![format!("%IMG_000{n}%"), at],
            )
            .unwrap();
        }

        let ranged = RatingFilter {
            captured_from: Some(4_000),
            captured_to: Some(6_000),
            ..Default::default()
        };
        assert_eq!(read_cells_all(&catalog, &ranged, 0, 50).unwrap().len(), 1);
        assert_eq!(total_images_filtered(&catalog, &ranged).unwrap(), 1);
    }

    #[test]
    fn an_undated_image_is_not_shown_inside_a_date_range() {
        // It cannot be in or out of a span. Drawing it anyway makes a range the
        // user just chose look as though it had not applied.
        let catalog = scanned(2);
        catalog
            .connection()
            .execute("UPDATE images SET captured_at = NULL", [])
            .unwrap();

        let ranged = RatingFilter {
            captured_from: Some(0),
            captured_to: Some(i64::MAX),
            ..Default::default()
        };
        assert!(read_cells_all(&catalog, &ranged, 0, 50).unwrap().is_empty());
    }

    #[test]
    fn a_span_reads_the_whole_run_whether_or_not_it_is_loaded() {
        // The shift-click this exists for. The grid holds a window of five and
        // the user names a run of twelve, so seven of them have no cell and no
        // id anywhere in the UI — but they are still what was asked for, and
        // the catalog is what knows them.
        let catalog = scanned(12);
        let filter = RatingFilter::default();

        let loaded = read_cells_all(&catalog, &filter, 0, 5).unwrap();
        assert_eq!(loaded.len(), 5, "the window is smaller than the run");

        let whole: Vec<_> = read_cells_all(&catalog, &filter, 0, 50)
            .unwrap()
            .iter()
            .map(|c| dr_types::ImageId(c.image_id as u64))
            .collect();
        let span = read_ids_span(&catalog, None, &filter, false, 0, 11).unwrap();

        assert_eq!(span.len(), 12);
        assert_eq!(span, whole, "the run is the grid's own list, in its order");
    }

    #[test]
    fn a_span_starts_and_ends_where_it_was_asked_to() {
        // Ordinals index the grid's list, so a run has to be exactly the slice
        // of it the two ends name — one off at either end selects a
        // photograph the user did not point at.
        let catalog = scanned(12);
        let filter = RatingFilter::default();
        let whole: Vec<_> = read_cells_all(&catalog, &filter, 0, 50)
            .unwrap()
            .iter()
            .map(|c| dr_types::ImageId(c.image_id as u64))
            .collect();

        let span = read_ids_span(&catalog, None, &filter, false, 4, 6).unwrap();
        assert_eq!(span, whole[4..=6], "ordinals 4..=6, inclusive at both ends");
    }

    #[test]
    fn a_span_is_ordered_by_capture_time_rather_than_by_name() {
        // A card written by two cameras interleaves names that have nothing to
        // do with each other. What a photographer means by "everything between
        // these two" is a stretch of an afternoon, so the run has to be taken
        // through capture time — the ordering the grid draws them in.
        let catalog = scanned(3);
        let conn = catalog.connection();
        for (n, at) in [(1, 9_000), (2, 5_000), (3, 1_000)] {
            conn.execute(
                "UPDATE images SET captured_at = ?2 WHERE source_ref LIKE ?1",
                rusqlite::params![format!("%IMG_000{n}%"), at],
            )
            .unwrap();
        }

        let span = read_ids_span(&catalog, None, &RatingFilter::default(), false, 0, 2).unwrap();
        let names: Vec<String> = span
            .iter()
            .map(|id| {
                conn.query_row(
                    "SELECT source_ref FROM images WHERE id = ?1",
                    [id.0 as i64],
                    |r| r.get::<_, String>(0),
                )
                .unwrap()
            })
            .collect();

        assert!(
            names[0].ends_with("IMG_0003.CR2")
                && names[1].ends_with("IMG_0002.CR2")
                && names[2].ends_with("IMG_0001.CR2"),
            "earliest first, which here is the reverse of the file names: {names:?}"
        );
    }

    #[test]
    fn the_histogram_ignores_the_range_it_is_used_to_choose() {
        // Drawing the axis through the chosen range would collapse it onto the
        // selection, leaving nowhere to widen back out from.
        let catalog = dated_three();
        let ranged = RatingFilter {
            captured_from: Some(4_000),
            captured_to: Some(6_000),
            ..Default::default()
        };
        assert_eq!(
            span_scoped(&catalog, None, &ranged),
            Some((1_000, 9_000)),
            "the axis must keep describing the whole extent"
        );
    }

    /// Three photographs at 1000, 5000 and 9000 seconds.
    fn dated_three() -> Catalog {
        let catalog = scanned(3);
        let conn = catalog.connection();
        for (n, at) in [(1, 1_000), (2, 5_000), (3, 9_000)] {
            conn.execute(
                "UPDATE images SET captured_at = ?2 WHERE source_ref LIKE ?1",
                rusqlite::params![format!("%IMG_000{n}%"), at],
            )
            .unwrap();
        }
        catalog
    }

    #[test]
    fn the_histogram_has_the_number_of_bins_it_was_asked_for() {
        // Fixed, whatever the span holds. The axis draws one bar per bin and
        // positions it by index, so a query that returned only the occupied
        // ones would put the bars at the wrong dates.
        let catalog = dated_three();
        let filter = RatingFilter::default();
        for bins in [1_u32, 8, 32, 64] {
            let bars = timeline_uniform(&catalog, None, &filter, 1_000, 9_000, bins).unwrap();
            assert_eq!(bars.len() as u32, bins);
            assert_eq!(
                bars.iter().map(|b| b.count).sum::<u32>(),
                3,
                "every photograph is counted exactly once"
            );
        }
    }

    #[test]
    fn a_bin_starts_where_the_axis_says_it_does() {
        // The bar's start is its bin's left edge, not the earliest photograph
        // in it. It is what the position marker and the range band are drawn
        // against, and an empty bin has no photograph to borrow a date from.
        let catalog = dated_three();
        let bars = timeline_uniform(&catalog, None, &RatingFilter::default(), 0, 8_000, 8).unwrap();
        for (i, bar) in bars.iter().enumerate() {
            assert_eq!(bar.start, i as i64 * 1_000);
        }
        // 1000 and 5000 land in their own bins, 9000 is past the end.
        assert_eq!(bars[1].count, 1);
        assert_eq!(bars[5].count, 1);
        assert_eq!(bars.iter().map(|b| b.count).sum::<u32>(), 2);
    }

    #[test]
    fn the_last_bin_holds_a_photograph_taken_at_the_very_end() {
        // The division puts an image captured at exactly `to` one bin past the
        // axis. Uncapped it would be dropped from the histogram — and it is
        // precisely the image that defines the extent, so it would go missing
        // on every unzoomed library.
        let catalog = dated_three();
        let bars =
            timeline_uniform(&catalog, None, &RatingFilter::default(), 1_000, 9_000, 4).unwrap();
        assert_eq!(bars.len(), 4);
        assert_eq!(bars[3].count, 1, "the image at 9000 is in the last bin");
        assert_eq!(bars[0].count, 1);
    }

    #[test]
    fn the_bins_ignore_the_range_they_are_used_to_choose() {
        // Same rule as the extent: the bars outside the band are what the
        // range is widened back into, so counting through the range would
        // leave every one of them empty.
        let catalog = dated_three();
        let ranged = RatingFilter {
            captured_from: Some(4_000),
            captured_to: Some(6_000),
            ..Default::default()
        };
        let bars = timeline_uniform(&catalog, None, &ranged, 1_000, 9_000, 4).unwrap();
        assert_eq!(
            bars.iter().map(|b| b.count).sum::<u32>(),
            3,
            "all three, not just the one inside the range"
        );
    }

    #[test]
    fn the_bins_still_honour_every_other_filter() {
        // A histogram of the five-star frames is a fair question, and the bars
        // have to agree with the grid beneath them.
        let catalog = dated_three();
        let strict = RatingFilter {
            min_rating: 4,
            ..Default::default()
        };
        let bars = timeline_uniform(&catalog, None, &strict, 1_000, 9_000, 4).unwrap();
        assert_eq!(bars.len(), 4, "the axis keeps its shape");
        assert_eq!(
            bars.iter().map(|b| b.count).sum::<u32>(),
            0,
            "nothing here is rated"
        );
    }

    fn entry(path: &str, file_id: u64, size: u64) -> RemoteEntry {
        RemoteEntry {
            id: RemoteId::Stable(file_id),
            path: RemotePath::new(path),
            kind: dr_sync::EntryKind::File,
            validator: dr_sync::Validator::new("v"),
            size,
            modified: None,
            has_preview: false,
            materialised: true,
        }
    }

    // --- the local-only filter (FR-CAT-9) ---------------------------------

    /// Record that an image's original is held locally at `tier`.
    fn cache_at(catalog: &Catalog, id: dr_types::ImageId, tier: dr_types::Tier) {
        catalog
            .connection()
            .execute(
                "INSERT INTO image_cache (image_id, tier_actual, bytes)
                 VALUES (?1, ?2, 0)",
                rusqlite::params![id.0 as i64, tier.stored()],
            )
            .unwrap();
    }

    #[test]
    fn local_only_shows_just_the_images_held_here() {
        let catalog = with_images(10);
        let ids = image_ids(&catalog);
        for id in &ids[0..3] {
            cache_at(&catalog, *id, dr_types::Tier::Original);
        }

        let filter = RatingFilter {
            local_only: true,
            ..Default::default()
        };
        let cells = read_cells_all(&catalog, &filter, 0, 120).unwrap();
        assert_eq!(cells.len(), 3);
        // The count the header shows must agree with the cells drawn, which is
        // the whole reason the predicate lives in SQL rather than in a
        // post-filter over the rows.
        assert_eq!(total_images_filtered(&catalog, &filter).unwrap(), 3);
        assert_eq!(local_original_count(&catalog).unwrap(), 3);
    }

    #[test]
    fn a_cached_preview_is_not_a_local_original() {
        // The filter answers "can I open this in develop right now", and a
        // preview cannot. Counting it would put images in the offline set that
        // fail the moment they are clicked.
        let catalog = with_images(5);
        let ids = image_ids(&catalog);
        cache_at(&catalog, ids[0], dr_types::Tier::Preview);
        cache_at(&catalog, ids[1], dr_types::Tier::Original);

        let filter = RatingFilter {
            local_only: true,
            ..Default::default()
        };
        assert_eq!(read_cells_all(&catalog, &filter, 0, 120).unwrap().len(), 1);
        assert_eq!(local_original_count(&catalog).unwrap(), 1);
    }

    #[test]
    fn local_only_composes_with_the_rating_filter() {
        // "Five-star frames I can actually edit on this train" is one filter,
        // not a mode that replaces the others.
        let catalog = with_images(6);
        let ids = image_ids(&catalog);
        for id in &ids[0..4] {
            cache_at(&catalog, *id, dr_types::Tier::Original);
        }
        // Rate two of the cached ones, and one that is not cached.
        for id in [ids[0], ids[1], ids[5]] {
            dr_catalog::rating::set_rating(catalog.connection(), id, 5).unwrap();
        }

        let filter = RatingFilter {
            min_rating: 5,
            local_only: true,
            ..Default::default()
        };
        let cells = read_cells_all(&catalog, &filter, 0, 120).unwrap();
        assert_eq!(cells.len(), 2, "five-starred AND held locally");
        assert_eq!(total_images_filtered(&catalog, &filter).unwrap(), 2);
    }

    #[test]
    fn an_empty_cache_is_not_an_empty_library() {
        // The unfiltered grid must not depend on the cache table having rows —
        // a library nothing has been downloaded from is still a full library.
        let catalog = with_images(4);
        assert_eq!(local_original_count(&catalog).unwrap(), 0);
        assert_eq!(
            read_cells_all(&catalog, &RatingFilter::default(), 0, 120)
                .unwrap()
                .len(),
            4
        );
    }

    #[test]
    fn local_only_counts_as_a_narrowing_filter() {
        // `is_unfiltered` gates the "filtered" indicator. Reporting this one as
        // unfiltered would leave a narrowed grid looking like the whole
        // library, which is the state the indicator exists to prevent.
        assert!(RatingFilter::default().is_unfiltered());
        assert!(!RatingFilter {
            local_only: true,
            ..Default::default()
        }
        .is_unfiltered());
    }

    // ── narrowing the grid by identity ────────────────────────────────────

    /// Put `person` on the given images, as a suggestion.
    fn assign(
        catalog: &Catalog,
        person: dr_catalog::faces::PersonId,
        images: &[dr_types::ImageId],
    ) {
        for img in images {
            let face = dr_catalog::faces::DetectedFace {
                x: 0.1,
                y: 0.1,
                w: 0.2,
                h: 0.2,
                landmarks: [(0.0, 0.0); 5],
                confidence: 0.9,
                embedding: vec![0u8; 1024],
                crop_px: 120.0,
                quality: None,
                crop: Vec::new(),
                model_id: "w600k_mbf".into(),
            };
            // Appends rather than replaces across calls for *different*
            // people, because `record_detections` clears the image first —
            // so the second person's face is added by hand.
            let existing: Vec<i64> = {
                let mut q = catalog
                    .connection()
                    .prepare("SELECT id FROM faces WHERE image_id = ?1")
                    .unwrap();
                q.query_map([img.0 as i64], |r| r.get(0))
                    .unwrap()
                    .map(Result::unwrap)
                    .collect()
            };
            let id = if existing.is_empty() {
                dr_catalog::faces::record_detections(
                    catalog.connection(),
                    *img,
                    "w600k_mbf",
                    1024,
                    std::slice::from_ref(&face),
                )
                .unwrap()[0]
            } else {
                catalog
                    .connection()
                    .execute(
                        "INSERT INTO faces
                             (image_id, x, y, w, h, landmarks, detector_confidence,
                              embedding, crop_px, model_id, detected_at)
                         VALUES (?1, 0.5, 0.5, 0.2, 0.2, X'00', 0.9, X'00', 120.0, 'w600k_mbf', 0)",
                        [img.0 as i64],
                    )
                    .unwrap();
                dr_catalog::faces::FaceId(catalog.connection().last_insert_rowid() as u64)
            };
            dr_catalog::faces::suggest(catalog.connection(), id, person, 0.9).unwrap();
        }
    }

    /// The two questions a photographer actually asks, and the reason the
    /// filter holds a set rather than one id: the intersection is not reachable
    /// by any sequence of single-person filters.
    #[test]
    fn people_narrow_the_grid_as_a_union_or_an_intersection() {
        let catalog = with_images(4);
        let ids = image_ids(&catalog);
        let anna = dr_catalog::faces::create_person(catalog.connection(), "Anna").unwrap();
        let bob = dr_catalog::faces::create_person(catalog.connection(), "Bob").unwrap();

        // 0: Anna. 1: both. 2: Bob. 3: neither.
        assign(&catalog, anna, &[ids[0], ids[1]]);
        assign(&catalog, bob, &[ids[1], ids[2]]);

        let count = |people: Vec<u64>, mode: PeopleMode| {
            let f = RatingFilter {
                people,
                people_mode: mode,
                ..Default::default()
            };
            total_images_scoped(&catalog, None, &f).unwrap()
        };

        assert_eq!(count(vec![anna.0], PeopleMode::Any), 2, "Anna alone");
        assert_eq!(count(vec![bob.0], PeopleMode::Any), 2, "Bob alone");
        assert_eq!(
            count(vec![anna.0, bob.0], PeopleMode::Any),
            3,
            "the union should hold every picture either is in"
        );
        assert_eq!(
            count(vec![anna.0, bob.0], PeopleMode::All),
            1,
            "the intersection should hold only the picture they share"
        );
    }

    /// One person is the same filter either way, and the UI leans on that to
    /// hide the toggle until there are two.
    #[test]
    fn one_person_reads_the_same_in_both_modes() {
        let catalog = with_images(3);
        let ids = image_ids(&catalog);
        let anna = dr_catalog::faces::create_person(catalog.connection(), "Anna").unwrap();
        assign(&catalog, anna, &[ids[0], ids[1]]);

        for mode in [PeopleMode::Any, PeopleMode::All] {
            let f = RatingFilter {
                people: vec![anna.0],
                people_mode: mode,
                ..Default::default()
            };
            assert_eq!(total_images_scoped(&catalog, None, &f).unwrap(), 2);
        }
    }

    /// Three faces of one person in a frame must not satisfy "Anna and Bob".
    /// This is what `COUNT(DISTINCT ...)` is for, and it is the intersection's
    /// one real trap.
    #[test]
    fn repeated_faces_of_one_person_do_not_satisfy_an_intersection() {
        let catalog = with_images(2);
        let ids = image_ids(&catalog);
        let anna = dr_catalog::faces::create_person(catalog.connection(), "Anna").unwrap();
        let bob = dr_catalog::faces::create_person(catalog.connection(), "Bob").unwrap();

        // Two separate faces, both Anna, in the same photograph.
        assign(&catalog, anna, &[ids[0]]);
        assign(&catalog, anna, &[ids[0]]);

        let f = RatingFilter {
            people: vec![anna.0, bob.0],
            people_mode: PeopleMode::All,
            ..Default::default()
        };
        assert_eq!(total_images_scoped(&catalog, None, &f).unwrap(), 0);
    }

    #[test]
    fn no_people_narrows_nothing() {
        let catalog = with_images(3);
        let f = RatingFilter::default();
        assert!(f.is_unfiltered());
        assert_eq!(total_images_scoped(&catalog, None, &f).unwrap(), 3);
    }
}

/// TRACES: FR-CAT-8 | FR-NC-9
/// Taking another device's judgement out of a sidecar and into the grid.
#[cfg(test)]
mod reading_judgements_back {
    use super::*;

    /// A remote library holding the given files, indexed as a scan leaves it.
    fn library(files: &[&str]) -> Catalog {
        let cat = Catalog::in_memory().unwrap();
        let c = cat.connection();
        c.execute(
            "INSERT INTO roots(id, kind, label) VALUES (1, 'remote', 'lib')",
            [],
        )
        .unwrap();
        for (i, f) in files.iter().enumerate() {
            c.execute(
                "INSERT INTO images(root_id, source_ref, added_at) VALUES (1, ?1, 0)",
                [f],
            )
            .unwrap();
            let image = c.last_insert_rowid();
            c.execute(
                "INSERT INTO remote(image_id, file_id) VALUES (?1, ?2)",
                rusqlite::params![image, 1000 + i as i64],
            )
            .unwrap();
        }
        dr_catalog::rating::ensure_default_versions(c).unwrap();
        cat
    }

    fn judgements(cat: &Catalog) -> Vec<(String, i64, i64)> {
        let c = cat.connection();
        let mut stmt = c
            .prepare(
                "SELECT i.source_ref, v.rating, v.flag FROM images i
                   JOIN versions v ON v.image_id = i.id AND v.is_default = 1
                  ORDER BY i.source_ref",
            )
            .unwrap();
        let v = stmt
            .query_map([], |r| Ok((r.get(0)?, r.get(1)?, r.get(2)?)))
            .unwrap()
            .map(Result::unwrap)
            .collect();
        v
    }

    /// The regression, in one line: a rating in a sidecar reaches the grid.
    /// Before this there was no path by which it could.
    #[test]
    fn a_rating_from_another_device_reaches_the_catalog() {
        let cat = library(&["2026/a.CR2"]);
        let n = apply_judgement(cat.connection(), 1, "2026/a.drsc", 4, 1).unwrap();

        assert_eq!(n, 1);
        assert_eq!(judgements(&cat), vec![("2026/a.CR2".to_string(), 4, 1)]);
    }

    /// A RAW and the JPEG beside it are one photograph (FR-CAT-11) sharing one
    /// sidecar, so both rows have to carry the judgement — otherwise the grid
    /// disagrees with itself depending which of the pair it draws.
    #[test]
    fn both_halves_of_a_raw_and_jpeg_pair_are_judged() {
        let cat = library(&["2026/a.CR2", "2026/a.jpg"]);
        let n = apply_judgement(cat.connection(), 1, "2026/a.drsc", 3, 0).unwrap();

        assert_eq!(n, 2);
        assert_eq!(
            judgements(&cat),
            vec![
                ("2026/a.CR2".to_string(), 3, 0),
                ("2026/a.jpg".to_string(), 3, 0)
            ]
        );
    }

    /// A demotion has to travel. Taking the larger of the two would refuse
    /// every rating the photographer ever lowered — and lowering one is most
    /// of what a second pass over a shoot does.
    #[test]
    fn a_lowered_rating_travels() {
        let cat = library(&["2026/a.CR2"]);
        apply_judgement(cat.connection(), 1, "2026/a.drsc", 4, 0).unwrap();
        apply_judgement(cat.connection(), 1, "2026/a.drsc", 1, 0).unwrap();

        assert_eq!(judgements(&cat)[0].1, 1);
    }

    /// But a zero is *unjudged*, not "judged zero". A device that never culled
    /// the frame must not erase the stars of one that did.
    #[test]
    fn an_unjudged_sidecar_does_not_erase_a_local_rating() {
        let cat = library(&["2026/a.CR2"]);
        apply_judgement(cat.connection(), 1, "2026/a.drsc", 5, 2).unwrap();

        assert_eq!(
            apply_judgement(cat.connection(), 1, "2026/a.drsc", 0, 0).unwrap(),
            0
        );
        assert_eq!(judgements(&cat), vec![("2026/a.CR2".to_string(), 5, 2)]);
    }

    /// Idempotent: a scan runs repeatedly, and a sidecar whose judgement is
    /// already in the catalog must report no change or the status line claims
    /// work that did not happen.
    #[test]
    fn applying_the_same_judgement_twice_changes_nothing() {
        let cat = library(&["2026/a.CR2"]);
        assert_eq!(
            apply_judgement(cat.connection(), 1, "2026/a.drsc", 4, 1).unwrap(),
            1
        );
        assert_eq!(
            apply_judgement(cat.connection(), 1, "2026/a.drsc", 4, 1).unwrap(),
            0
        );
    }

    /// The `LIKE` is a filter and not the decision. A stem holding a wildcard
    /// would otherwise reach photographs it has nothing to do with, and a
    /// rating landing on the wrong frame is silent and permanent.
    #[test]
    fn a_wildcard_in_a_path_does_not_reach_another_photograph() {
        // `_` is LIKE's single-character wildcard, so an unescaped `a_b` stem
        // would also match `axb`.
        let cat = library(&["2026/a_b.CR2", "2026/axb.CR2"]);
        apply_judgement(cat.connection(), 1, "2026/a_b.drsc", 5, 0).unwrap();

        assert_eq!(
            judgements(&cat),
            vec![
                ("2026/a_b.CR2".to_string(), 5, 0),
                ("2026/axb.CR2".to_string(), 0, 0)
            ]
        );
    }

    /// A stem that is a prefix of another must not spill onto it: `a.drsc`
    /// describes `a.CR2`, never `ab.CR2`.
    #[test]
    fn a_shared_prefix_is_not_a_shared_sidecar() {
        let cat = library(&["2026/a.CR2", "2026/ab.CR2"]);
        apply_judgement(cat.connection(), 1, "2026/a.drsc", 5, 0).unwrap();

        assert_eq!(
            judgements(&cat),
            vec![
                ("2026/a.CR2".to_string(), 5, 0),
                ("2026/ab.CR2".to_string(), 0, 0)
            ]
        );
    }

    /// A sidecar for a photograph this device has not indexed is not an error:
    /// the scan may have pruned the folder, or the file may be a format this
    /// device does not accept.
    #[test]
    fn a_sidecar_with_no_photograph_here_is_not_a_failure() {
        let cat = library(&["2026/a.CR2"]);
        assert_eq!(
            apply_judgement(cat.connection(), 1, "2026/elsewhere.drsc", 5, 0).unwrap(),
            0
        );
    }
}

/// TRACES: FR-NC-8 | FR-NC-9
/// What a write does to a sidecar another device has already edited.
#[cfg(test)]
mod amending_across_devices {
    use super::*;

    fn judgement(uuid: &str, rating: u8) -> SidecarWrite {
        SidecarWrite {
            image_path: "PhotosRaw/incoming/a.CR2".to_string(),
            version_uuid: uuid.to_string(),
            amendment: Amendment::Judgement { rating, flag: 0 },
        }
    }

    fn version(uuid: &str, revision: u64, modified: i64, rating: u8) -> dr_pipeline::Version {
        dr_pipeline::sidecar::Version {
            uuid: uuid.to_string(),
            name: "Default".to_string(),
            is_default: true,
            revision,
            modified,
            rating,
            ..Default::default()
        }
    }

    /// The regression. A sidecar already holding two independently minted
    /// defaults used to gain a *third* on the next write, because the lookup
    /// is by uuid and neither of the two was ours.
    #[test]
    fn a_write_onto_a_split_sidecar_does_not_add_a_third_version() {
        let mut base = dr_pipeline::Sidecar::new();
        base.put(version("tablet-uuid", 1, 100, 4));
        base.put(version("laptop-uuid", 2, 200, 1));

        let out = amend(base, &judgement("derived-uuid", 5));

        assert_eq!(out.versions.len(), 1, "the split must be closed, not grown");
        assert!(out.versions.contains_key("derived-uuid"));
        assert_eq!(out.versions["derived-uuid"].rating, 5);
    }

    /// The other device's work has to survive the fold, or closing the split
    /// would be the same data loss by a different route.
    #[test]
    fn the_other_devices_edit_survives_the_write() {
        let mut theirs = version("tablet-uuid", 3, 300, 4);
        theirs
            .params
            .insert(("exposure".into(), "exposure".into()), 0.75);
        let mut base = dr_pipeline::Sidecar::new();
        base.put(theirs);

        // Ours is a rating, which touches no parameter at all.
        let out = amend(base, &judgement("derived-uuid", 2));
        let v = &out.versions["derived-uuid"];

        assert_eq!(
            v.params.get(&("exposure".into(), "exposure".into())),
            Some(&0.75),
            "the tablet's exposure was dropped by our rating"
        );
        assert_eq!(v.rating, 2, "and our own judgement did not land");
    }

    /// A sidecar this device has already written must not be disturbed: the
    /// ordinary case is one default under the right uuid, and fusing it has to
    /// be a no-op beyond the amendment itself.
    #[test]
    fn the_ordinary_write_is_unaffected() {
        let mut base = dr_pipeline::Sidecar::new();
        base.put(version("derived-uuid", 7, 700, 3));

        let out = amend(base, &judgement("derived-uuid", 5));

        assert_eq!(out.versions.len(), 1);
        assert_eq!(out.versions["derived-uuid"].rating, 5);
        assert_eq!(
            out.versions["derived-uuid"].revision, 8,
            "one bump for one edit"
        );
    }

    /// A virtual copy is not a rival default and must be left where it is.
    #[test]
    fn a_named_version_is_not_folded_into_the_default() {
        let mut copy = version("for-print", 4, 400, 5);
        copy.is_default = false;
        copy.name = "For print".to_string();
        let mut base = dr_pipeline::Sidecar::new();
        base.put(version("tablet-uuid", 1, 100, 4));
        base.put(copy);

        let out = amend(base, &judgement("derived-uuid", 2));

        assert_eq!(out.versions.len(), 2);
        assert_eq!(out.versions["for-print"].name, "For print");
        assert_eq!(out.versions["for-print"].rating, 5);
    }
}

#[cfg(test)]
mod fetching_ahead {
    use super::*;

    /// The whole reason the registry exists: a click on a photograph that is
    /// being fetched ahead waits for that transfer rather than starting its
    /// own, and is told so — `None` — so it looks in the cache again.
    #[test]
    fn a_second_claim_waits_for_the_first_to_be_released() {
        let registry = std::sync::Arc::new(InFlight::default());
        let first = registry.claim("shoot/one.CR2");
        assert!(first.is_some(), "an unclaimed path is claimed outright");

        let (tx, rx) = std::sync::mpsc::channel();
        let waiter = {
            let registry = registry.clone();
            std::thread::spawn(move || {
                tx.send(()).unwrap();
                registry.claim("shoot/one.CR2").is_some()
            })
        };
        rx.recv().unwrap();
        // The waiter is blocked on the first claim. Not provable without a
        // sleep, but a release that reaches it proves the wait ended there.
        assert!(
            !waiter.is_finished(),
            "the second claim must not return while the first is held"
        );

        drop(first);
        let claimed = waiter.join().unwrap();
        assert!(
            !claimed,
            "after waiting, the caller is told to recheck the cache"
        );

        assert!(
            registry.claim("shoot/one.CR2").is_some(),
            "and once nobody holds the path it can be claimed again"
        );
    }

    /// Different photographs never wait on each other.
    #[test]
    fn distinct_paths_are_claimed_independently() {
        let registry = InFlight::default();
        let _a = registry.claim("a.CR2");
        assert!(registry.claim("b.CR2").is_some());
    }

    /// A newer wish supersedes an older one mid-list; the worker checks this
    /// between jobs, and it is what keeps a fast walk along the roll from
    /// queueing every neighbour it passed.
    #[test]
    fn a_new_wish_supersedes_the_one_being_served() {
        let mut wanted = Wanted::default();
        let conn = test_connection();
        wanted.replace(conn.clone(), Vec::new());
        let taken = wanted.generation;
        assert!(wanted.is_current(taken));

        wanted.replace(conn, Vec::new());
        assert!(
            !wanted.is_current(taken),
            "the list taken before the replacement is stale"
        );
    }

    fn test_connection() -> Connection {
        Connection::new(
            Account::new("nextcloud", "https://cloud.example").with_login("d", "d"),
            None,
        )
    }
}