DarkRoom/docs/storage.md

# Storage backends

How DarkRoom talks to wherever a library lives, and what it takes to add
somewhere new.

This document is the contract. `docs/architecture.md` §8 says why sync is built
on capability negotiation rather than a common denominator; this says what the
seam actually is, where each piece lives, and what a third connector has to do.

---

## 1. What "pluggable" has to mean

A trait alone does not make storage pluggable. `RemoteBackend` existed from the
first release and every layer above it still knew it was talking to Nextcloud:
seven files in `dr-ui` constructed a `NextcloudBackend` directly, ten functions
took one by concrete type, the account model was a server URL beside a DAV user
id, and the local cache directory was named after a hostname. The abstraction
was real and bought nothing, because everything that *reached* a backend was
still shaped like one product.

Pluggable means all four of these, not just the first:

1. **Operations** — what a backend can do. `RemoteBackend`.
2. **Capabilities** — what it can do *cheaply*, so the engine adapts instead of
   assuming. `Capabilities`.
3. **Configuration** — what an account is, with no server in it. `Account`.
4. **Registration** — how the application discovers a connector at all, without
   naming it. `BackendProvider` + `BackendRegistry`.

Two connectors ship. Nextcloud is unchanged and keeps every one of its
peculiarities — those are the point of the capability model, not an
embarrassment it has to hide. The folder connector serves a plain directory and
exists partly because it is genuinely useful and partly because a second
implementation is the only way to find out whether the first was an
abstraction.

---

## 2. Where each piece lives

```
core/dr-sync/                 the contract, and nothing that speaks a protocol
  ├─ types.rs                 RemotePath, RemoteId, RemoteEntry, Validator, …
  ├─ capability.rs            Capabilities, ChangeDetection, ServerPreviews
  ├─ error.rs                 RemoteError — the one error every caller handles
  ├─ account.rs               Account, AccountStore, Secret, Connection
  ├─ provider.rs              BackendProvider, BackendRegistry, SignIn
  ├─ lib.rs                   RemoteBackend, SyncStrategy
  ├─ scan.rs                  the walk, driven by capabilities
  ├─ upload.rs                where an original is placed
  └─ reachability.rs          online/offline, inferred from observed results

core/dr-sync-nextcloud/       WebDAV, oc:fileid, chunked upload v2, Login Flow v2
core/dr-sync-folder/          a directory on a filesystem

ui/dr-ui/src/remote.rs        the registry — the ONLY file above dr-sync that
                              names a connector
```

`dr-sync` depends on no connector. That is deliberate and load-bearing: a build
that only wants a folder library must not compile a TLS stack to get one, and
the registry therefore lives in the crate that already depends on everything —
the interface.

---

## 3. The four traits and types a connector meets

### 3.1 `RemoteBackend` — operations

```rust
#[async_trait]
pub trait RemoteBackend: Send + Sync {
    fn capabilities(&self) -> &Capabilities;
    fn name(&self) -> &str;

    // discovery
    async fn list(&self, dir: &RemotePath, since: Option<&Validator>)
        -> Result<Vec<RemoteEntry>, RemoteError>;
    async fn dir_validator(&self, dir: &RemotePath) -> Result<Validator, RemoteError>;
    async fn delta(&self, cursor: &Cursor)
        -> Result<(Vec<RemoteChange>, Cursor), RemoteError>;

    // transfer
    async fn get(&self, id: &RemoteId, range: Option<Range<u64>>)
        -> Result<Vec<u8>, RemoteError>;
    async fn put(&self, path: &RemotePath, body: Vec<u8>, precond: Option<Precondition>)
        -> Result<Validator, RemoteError>;
    async fn put_many(&self, items: Vec<(RemotePath, Vec<u8>)>)   // defaulted
        -> Result<Vec<Result<Validator, RemoteError>>, RemoteError>;
    async fn delete(&self, id: &RemoteId, precond: Option<Precondition>)
        -> Result<(), RemoteError>;
    async fn move_to(&self, from: &RemoteId, to: &RemotePath) -> Result<(), RemoteError>;
    async fn create_dir(&self, path: &RemotePath) -> Result<(), RemoteError>;

    // optional
    async fn thumbnail(&self, id: &RemoteId, size: u32)   // defaulted to None
        -> Result<Option<Vec<u8>>, RemoteError>;
}
```

Rules that are not obvious from the signatures:

- **`dir_validator` and `delta` are capability-gated.** Return
  `RemoteError::Unsupported` unless your `ChangeDetection` is
  `PropagatingEtags` or `DeltaCursor` respectively. Answering
  `dir_validator` with something that does not actually propagate is worse than
  refusing: it lets a caller prune a subtree whose contents changed, and hides
  those changes for as long as the folder list holds still.
- **`get` takes an optional range, and it is a hint.** A backend without cheap
  ranges may return the whole object; the caller slices. Correctness holds
  either way and `Capabilities::range_reads` says whether it was cheap.
- **Chunked upload is not in the trait.** It is an implementation detail of
  `put`, chosen by body size. Exposing it would leak one server's protocol.
- **`move_to` must preserve identity where the backend has stable ids.** This
  is what a soft delete uses (`FR-CAT-15`): a move implemented as copy + delete
  allocates a new id, orphaning the thumbnail shard and turning a restore into a
  full re-download.
- **`create_dir` makes parents and succeeds if the directory exists.** Callers
  use it to guarantee a destination, not to claim they created one.

### 3.2 `Capabilities` — what is cheap

The engine reads these once at connect time and picks a `SyncStrategy`. See
ARCH §8.1–8.2 for the tiers. The two that change behaviour rather than speed:

| Absent | Consequence the engine handles |
|---|---|
| `range_reads` | Embedded-preview extraction is impossible; browsing falls back to server previews or full download, and is refused on a metered connection |
| `conditional_write` | Sidecar conflict detection falls back to revision counters inside the sidecar — narrows the race, does not close it. Reported as a reduced-safety mode |

**Declare what is true, not what is flattering.** A backend claiming
`PropagatingEtags` it does not have does not merely run slowly; it silently
hides changes.

### 3.3 `Account` — configuration with no server in it

```rust
pub struct Account {
    pub backend: String,     // BackendProvider::id; defaults to "nextcloud" on load
    pub endpoint: String,    // stored as "server" — a URL, a path, a bucket
    pub login: String,       // empty where the connector has no notion of a user
    pub user_id: String,     // connector-defined sub-address; Nextcloud's DAV segment
    pub root: String,        // the folder chosen as the library root
    pub formats: Vec<String>,
    pub last_scan: Option<i64>,
}
```

Everything but `backend` is the connector's to interpret. Code above `dr-sync`
reads these for display and for cache keys, never for meaning.

Two properties are load-bearing:

- **The on-disk form is backwards compatible.** `backend` defaults to
  `"nextcloud"` and `endpoint` is stored under its historical key `server`, so
  every account written before there was a choice loads unchanged. A config the
  app refuses to parse is an account the user has to set up again.
- **`Account::namespace()` is frozen for Nextcloud.** It names the directory
  holding the catalog, the thumbnail shards, the sidecar spool and the export
  outbox. Changing it does not lose that data, it *abandons* it — silently, as
  an upgrade — and costs a full rescan on top. The Nextcloud form is reproduced
  byte for byte from what `catalog_path` computed before; every other backend is
  prefixed by its connector id, and long endpoints are truncated with a hash
  tail so two deep paths cannot collide inside one filesystem's 255-byte
  component limit.

### 3.4 `Connection` and `Secret` — the credential split

```rust
pub struct Connection { pub account: Account, pub secret: Option<Secret> }
```

Credentials go to platform secure storage (`FR-NC-2`, `NFR-SEC-2`). Never the
catalog, never the config file, never a log line. `AccountStore` writes the
account as plain JSON and the secret to the keyring, which is what lets the app
show "signed in as duncan, watching /PhotosRaw" before it has touched the
keyring at all.

`Secret`'s inner string is reachable only through `expose()`, and its `Debug`
prints `Secret(***)`. That closes the indirect leak — a `{:?}` on any struct
that happens to hold a connection — by construction rather than by review.

`Connection` is also what replaced a pair of arguments (credentials, user id)
threaded together through fifteen signatures in an order that could be swapped.

### 3.5 `BackendProvider` — registration

```rust
pub trait BackendProvider: Send + Sync {
    fn id(&self) -> &'static str;               // written to Account::backend
    fn display_name(&self) -> &'static str;
    fn endpoint_label(&self) -> &'static str;   // "Server" / "Folder"
    fn endpoint_placeholder(&self) -> &'static str;
    fn sign_in(&self) -> SignIn;
    fn normalise_endpoint(&self, input: &str) -> Result<String, String>;
    fn account_for(&self, endpoint: &str) -> Result<Account, RemoteError>;  // defaulted
    fn connect(&self, conn: &Connection) -> Result<Box<dyn RemoteBackend>, RemoteError>;
}

pub enum SignIn {
    /// A handshake the user completes outside the app, yielding a credential.
    Browser,
    /// The endpoint is the whole account. No credential, no waiting state.
    EndpointOnly,
}
```

- **`id` is on-disk configuration.** Changing it after anyone has an account
  orphans that account. Pick it once.
- **`normalise_endpoint` is where a bad endpoint is *rejected*,** before an
  account is written for a library that does not exist. Its error string is
  shown to the user, so it says what to fix rather than naming a type. The
  Nextcloud provider upgrades `http://` to `https://` here (`NFR-SEC-3`); the
  folder provider canonicalises the path, so two spellings of one directory do
  not become two accounts indexing the same photographs.
- **`connect` is synchronous and cheap.** It validates configuration and builds
  a client; it does not talk to the remote. Workers call it per task.
- **`SignIn` is a shape, not a method.** It would be tidier to expose
  `async fn sign_in()`, and wrong: Login Flow v2 is a browser handshake the user
  completes elsewhere while the app polls, so it is not one call, it does not
  finish on our schedule, and the screen has to render a URL and a waiting state
  in the middle of it. `SignIn` tells the launch screen which of the two shapes
  to draw; the flow stays where its protocol is.

**Credentials are deliberately not abstracted.** An app password, an OAuth
token and a bucket key pair have no useful common shape, and inventing one
before a third backend exists would produce a wrong answer confidently. The
general form is `Connection` — an account plus an opaque secret — and each
connector translates that into what its protocol needs
(`NextcloudProvider::credentials`).

---

## 4. Adding a backend

1. **Implement `RemoteBackend`** over your protocol, in a new
   `core/dr-sync-<name>` crate depending on `dr-sync` and nothing else of ours.
2. **Declare `Capabilities` honestly.** Start from `Capabilities::minimal()` and
   raise only what you can actually deliver.
3. **Implement `BackendProvider`** beside it.
4. **Register it** in `ui/dr-ui/src/remote.rs::registry()` and add the crate to
   `ui/dr-ui/Cargo.toml`.

That is the whole list. Nothing else in `dr-ui` changes, because nothing else in
`dr-ui` names a connector.

**Two things to get right, because they are silent when wrong:**

- **Identity.** `RemoteEntry::id` should be `RemoteId::Stable(u64)` wherever you
  can produce a `u64` that names the same photograph on every device looking at
  the same library. The catalog keys the thumbnail shards and the face index on
  it (`catalog.md` §10.1), and an entry without one gets neither. Set
  `Capabilities::stable_ids` only if that id also survives a rename — the two
  are different questions and only the second is a capability.
- **Path safety.** A `RemotePath` is built from names on the remote and from a
  catalog another device wrote. If you resolve one against a real filesystem,
  reject `..` before you open anything.

Register a test double the same way — `BackendRegistry::register` replaces an
existing id rather than shadowing it — so an integration test can stand a fake
server behind `"nextcloud"` without the registry knowing it happened.

---

## 5. The connectors that ship

### 5.1 Nextcloud (`dr-sync-nextcloud`, id `"nextcloud"`)

Unchanged by the abstraction, peculiarities intact — see ARCH §8.4 for the full
mapping. What matters here is that none of them had to be given up to make room
for a second backend:

| | |
|---|---|
| `change_detection` | `PropagatingEtags` — the one-request no-op sync |
| `stable_ids` | yes, `oc:fileid`, survives server-side rename and move |
| `range_reads` | yes, detected by `206` vs `200`, never `HEAD` |
| `chunked_upload` | v2, 5 MB – 5 GB, `MKCOL` → `PUT` chunks → `MOVE .file` |
| `bulk_upload` | yes, `POST /remote.php/dav/bulk` |
| `conditional_write` | yes, `If-Match` |
| `server_previews` | `CommonFormatsOnly` — stock Nextcloud renders no RAW |
| sign-in | `SignIn::Browser`, Login Flow v2, system browser, app password |

Also kept: the `oc:permissions` probe on a refused `PUT`, which is what
distinguishes a create-only share from a bad credential; the `423 Locked`
retry classification; and the bundled ISRG Root YE certificate.

### 5.2 Folder (`dr-sync-folder`, id `"folder"`)

A local disk, an NFS or SMB mount, an external drive, or the directory a
Nextcloud desktop client already syncs. No server, no account, no credential —
which makes it the route that works on a machine with no secrets daemon at all.

| | |
|---|---|
| `change_detection` | `LocalEtags` — see below |
| `stable_ids` | **no** — the id is a path hash and does not survive a rename |
| `range_reads` | yes, `seek` + `take` |
| `chunked_upload` | none; a write is a write |
| `bulk_upload` | no |
| `conditional_write` | yes, with a documented residual race |
| `server_previews` | `None` |
| sign-in | `SignIn::EndpointOnly` |

**Why `LocalEtags` and not `PropagatingEtags`.** A POSIX directory's mtime
changes when its own entry list changes and at no other time — not when a
child's contents are edited, and not for a grandchild. There is nothing to
propagate, so `dir_validator` returns `Unsupported` and the engine walks the
tree every scan. Which costs almost nothing, because the walk that was expensive
was expensive for a reason this backend does not have.

**Measured 2026-08-28**, `cargo run -p dr-sync-folder --example scan`: a full
uncached walk of 2,299 images across 233 directories completed in **137 ms**,
and 380 images across 13 directories in **29 ms** — the same engine, the same
`Depth: 1`-per-directory walk, with no pruning at all. The Nextcloud connector's
comparable figure is 34.1 s for 17,185 RAWs across 334 directories *with*
pruning available (ARCH §8.4). The capability model is what lets one engine
drive both at the speed each actually runs at, instead of forcing the fast one
down to the slow one's interface.

**Identity is a hash of the path relative to the library root**, FNV-1a 64
(written out, because `DefaultHasher` is explicitly unstable between Rust
releases and this value is written into the catalog). It gives the catalog a
`u64` that names a photograph, is the same on every device looking at the same
folder, and does not change when the file is edited. It does not survive a
rename, and `stable_ids: false` says so: a moved photograph is seen as a delete
and an add, and its thumbnail is derived again.

The alternative — keying on the inode — is stable across a rename but *differs
between devices* and is reused by the filesystem after a delete. Two machines
would disagree about which photograph a thumbnail belonged to, and a recycled
inode would silently attach an old thumbnail to a new image. Re-deriving a
thumbnail is a cost; showing the wrong one is a bug.

**Conditional writes.** `IfAbsent` is genuinely atomic (`O_CREAT | O_EXCL`).
`IfMatch` is compare-then-swap: a `stat`, then a write to a temporary beside the
destination and a `rename` over it. A POSIX filesystem has no compare-and-swap,
so the race is narrowed to the microseconds between the two syscalls rather than
closed — still far tighter than the fallback the engine uses for a backend that
declares no conditional write at all, which spans a whole read-modify-write.
The capability is declared, and the residual race is documented at the call
site.

**Two deliberate divergences from WebDAV semantics:**

- **`delete` is not recursive.** A folder library is the user's own photographs
  on their own disk with no server-side trash behind it, so a caller that passed
  the wrong path would have no way back. Deleting a non-empty directory returns
  `RemoteError::Configuration`. Nothing in the engine deletes a directory — the
  soft delete is a `move_to` into the trash folder — so the guard is free.
- **Every filesystem call runs on the blocking pool.** On a local disk that is
  overkill; on the NFS mount this backend is most useful over, a stalled server
  would otherwise wedge the async worker that made the call and every other
  request sharing it.

**Failure classification** matters as much as the operations. A vanished mount
(`ESTALE`, `ENOTCONN`, `EIO`) maps to `RemoteError::Network`, which is what puts
the app into offline mode and leaves the catalog readable — exactly as a dead
server does. A permissions problem maps to `PermissionDenied` and does *not*,
because going offline over one forbidden file would hide a fixable problem
behind a network banner. An endpoint that is not a directory at all maps to
`RemoteError::Configuration`: nothing was unreachable and no credential was
wrong, so neither of the other two would send the user anywhere useful.

---

## 6. What the abstraction does not yet cover

Stated so the next person does not have to rediscover it.

- **Multiple accounts at once.** `AccountStore` holds a list and the launch
  screen uses the most recent. Nothing in the model prevents two open libraries;
  the interface has no place to show them.
- **Per-backend settings.** A connector has no way to contribute a settings
  page. Anything configurable is on the `Account` or is not configurable.
- **Capability probing at runtime.** `Capabilities` is fixed at construction.
  Nextcloud's `server_previews` should really be probed per account — a server
  with `camerarawpreviews` installed can render RAW — and today it is assumed to
  be `CommonFormatsOnly`.
- **A general notion of an account.** Credentials stay connector-specific on
  purpose (§3.5). A third connector with an OAuth flow will need a third `SignIn`
  variant, and that is the right place for it to appear.