DarkRoom/core/dr-sync-folder/src/borrow.rs

// TRACES: FR-NC-6c | FR-NC-6a
//! Hydrating a file for as long as it is needed, and no longer.
//!
//! A pass over a library — thumbnails, face indexing — needs each photograph's
//! bytes for a moment and never again. On a virtual-filesystem folder those
//! bytes may not be here, and fetching them is whole-file: hydrating a 17,000
//! image library to index it would land the entire library on a disk the user
//! deliberately keeps most of it off (ARCH §9.0).
//!
//! So hydration is a **borrow**. Ask for a file, use it, give it back. Peak
//! disk becomes the working set rather than the library, and the transfer is
//! paid once for a thumbnail that is then kept for ever — and pushed to the
//! server for other devices, which never pay it at all.
//!
//! # The rule that makes it safe
//!
//! **A file is returned to the state it was found in.** If it was already
//! downloaded — the user pinned it, opened it yesterday, or never uses VFS —
//! the borrow leaves it downloaded. Only what this pass hydrated is released.
//! Anything else silently undoes a choice the user made, and "my pinned trip
//! evaporated after an indexing run" is the kind of failure that makes people
//! stop trusting the feature.
//!
//! # Why it is reference counted
//!
//! Lanes run concurrently and two of them meet on the same file: the
//! thumbnail pass and the face pass want the same RAW. Without counting, the
//! first to finish dehydrates the file the second is reading. With it, the
//! transfer is paid once and the release happens when the last borrower is
//! done.

use std::collections::HashMap;
use std::sync::{Arc, Mutex};

use dr_sync::{RemoteBackend, RemoteError, RemoteId, RemotePath};

/// What a borrow is holding, per path.
#[derive(Debug, Default)]
struct Held {
    /// How many borrowers are using it now.
    borrowers: usize,
    /// Whether *we* brought it here. False means it was already downloaded
    /// and must be left that way.
    ours: bool,
}

/// Tracks what has been hydrated and by whom.
///
/// Cheap to clone — every worker holds one and they share the same state.
#[derive(Clone, Default, Debug)]
pub struct BorrowPool {
    held: Arc<Mutex<HashMap<RemotePath, Held>>>,
}

/// What a completed borrow did, for reporting a pass's real cost.
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
pub struct BorrowStats {
    /// Files that were already here. These cost nothing.
    pub already_local: usize,
    /// Files this pass downloaded.
    pub hydrated: usize,
    /// Files released again afterwards.
    pub released: usize,
    /// Files left downloaded because they were already so.
    pub kept: usize,
}

impl BorrowPool {
    pub fn new() -> Self {
        Self::default()
    }

    /// Borrow a file's content for the life of the returned guard.
    ///
    /// Downloads it if it is a placeholder; does nothing if it is already
    /// here. The guard releases it on drop, but only if this pool hydrated it
    /// and nothing else still holds it.
    ///
    /// A backend without [`Materialisation::OnDemand`] short-circuits: the
    /// borrow succeeds and does nothing, so a caller written for a VFS library
    /// runs unchanged against a server or a plain folder.
    ///
    /// [`Materialisation::OnDemand`]: dr_sync::Materialisation::OnDemand
    pub async fn borrow<'p>(
        &'p self,
        backend: &dyn RemoteBackend,
        path: &RemotePath,
    ) -> Result<Borrowed<'p>, RemoteError> {
        if !backend.capabilities().materialisation.can_materialise() {
            return Ok(Borrowed {
                pool: None,
                path: path.clone(),
                hydrated: false,
            });
        }

        // Another borrower already has it: join them rather than asking the
        // client a second time.
        {
            let mut held = self.lock();
            if let Some(entry) = held.get_mut(path) {
                entry.borrowers += 1;
                return Ok(Borrowed {
                    pool: Some(self),
                    path: path.clone(),
                    hydrated: false,
                });
            }
        }

        // The backend answers whether *it* fetched the content, because it had
        // to look before deciding. Determining that here instead would cost a
        // directory listing per file, and getting it wrong in the wrong
        // direction releases a file the user pinned.
        let ours = backend.materialise(&RemoteId::Path(path.clone())).await?;

        self.lock()
            .insert(path.clone(), Held { borrowers: 1, ours });

        Ok(Borrowed {
            pool: Some(self),
            path: path.clone(),
            hydrated: ours,
        })
    }

    /// Release everything this pool still holds that it hydrated.
    ///
    /// The end-of-pass sweep. A guard dropped on a panicking worker cannot run
    /// its async release, so the pool is drained deliberately at the end
    /// rather than trusted to unwind cleanly.
    pub async fn release_all(&self, backend: &dyn RemoteBackend) -> BorrowStats {
        let ours: Vec<RemotePath> = {
            let held = self.lock();
            held.iter()
                .filter(|(_, h)| h.ours)
                .map(|(p, _)| p.clone())
                .collect()
        };

        let mut stats = BorrowStats::default();
        for path in ours {
            match backend.dematerialise(&RemoteId::Path(path.clone())).await {
                Ok(()) => stats.released += 1,
                // Not fatal, and not worth failing a completed pass over: the
                // content stays, which costs disk and loses nothing.
                Err(e) => log::debug!("releasing {path}: {e}"),
            }
        }
        self.lock().clear();
        stats
    }

    /// How many paths are currently held.
    pub fn held(&self) -> usize {
        self.lock().len()
    }

    fn lock(&self) -> std::sync::MutexGuard<'_, HashMap<RemotePath, Held>> {
        // A poisoned lock means a worker panicked while holding it. The map is
        // bookkeeping, not a resource — carrying on with it is better than
        // taking the whole pass down.
        self.held.lock().unwrap_or_else(|e| e.into_inner())
    }
}

/// A file held local for as long as this lives.
///
/// Dropping it marks the borrow finished. The actual release happens in
/// [`BorrowPool::release_all`], because dropping cannot await.
#[derive(Debug)]
pub struct Borrowed<'p> {
    pool: Option<&'p BorrowPool>,
    path: RemotePath,
    /// Whether this borrow was the one that downloaded it.
    hydrated: bool,
}

impl Borrowed<'_> {
    /// Whether this borrow paid for a download.
    pub fn hydrated(&self) -> bool {
        self.hydrated
    }

    pub fn path(&self) -> &RemotePath {
        &self.path
    }
}

impl Drop for Borrowed<'_> {
    fn drop(&mut self) {
        let Some(pool) = self.pool else { return };
        let mut held = pool.lock();
        if let Some(entry) = held.get_mut(&self.path) {
            entry.borrowers = entry.borrowers.saturating_sub(1);
            // Left in the map even at zero borrowers: `release_all` needs to
            // know it was ours, and a file wanted again a moment later should
            // not be downloaded twice.
            if entry.borrowers == 0 && !entry.ours {
                held.remove(&self.path);
            }
        }
    }
}

#[cfg(test)]
mod tests;