README.md

@@ -1070,23 +1070,24 @@ photos newest first). `lib.subscribe({ onChange })` delivers a `LibraryChange`
   directory, and fails at once with an error whose `code` is `ELOCKED` while
   another backup of it runs. A caller can instead take the lock itself, before
   it opens its library, as `quak backup` does: `await lockBackupDirectory(dir)`
-  takes it, failing the same way, and returns the function that releases it, and
+  takes it, failing the same way, and returns a `BackupLock`, whose `release()`
-  the caller passes `lockHeld: true` to the backup. It waits for a refresh as
+  removes the lock unless it was lost and whose `lost` holds why it was lost.
-  `fresh()` does, puts every in-scope original not already at its save path
+  The caller passes it to the backup as `lock`, and releases it once the backup
-  there as `photo.download()` does (and, with `includeThumbnails`, fetches
+  ends. It waits for a refresh as `fresh()` does, puts every in-scope original
-  thumbnails) through the content cache, waits for an ML data fetch, and
+  not already at its save path there as `photo.download()` does (and, with
-  rebuilds the on-disk backup tree with a durable failure ledger. Each file's
+  `includeThumbnails`, fetches thumbnails) through the content cache, waits for
-  JSON holds its ML data and, for an image (for a live photo, its image), its
+  an ML data fetch, and rebuilds the on-disk backup tree with a durable failure
-  original's EXIF and XMP, with its dimensions for a JPEG only; a video's JSON
+  ledger. Each file's JSON holds its ML data and, for an image (for a live
-  holds none of these three. A fetched original is written straight to its save
+  photo, its image), its original's EXIF and XMP, with its dimensions for a JPEG
-  path and not into the cache, which then counts it as present; one the cache
+  only; a video's JSON holds none of these three. A fetched original is written
-  already held is copied from there. `BackupOptions`: `downloadDirectory` (falls
+  straight to its save path and not into the cache, which then counts it as
-  back to the library's), `includeOriginals` (default `true`),
+  present; one the cache already held is copied from there. `BackupOptions`:
-  `includeThumbnails` (default `false`), `onlyAlbumNames`, `verify` (default
+  `downloadDirectory` (falls back to the library's), `includeOriginals` (default
-  `false`), `onProgress`, and `lockHeld` (default `false`). See Backup layout
+  `true`), `includeThumbnails` (default `false`), `onlyAlbumNames`, `verify`
-  above for the tree it writes. Whether the backup took the lock or its caller
+  (default `false`), `onProgress`, and `lock`. See Backup layout above for the
-  did, once the lock is lost the backup rejects with
+  tree it writes. Whether the backup took the lock or its caller did, once the
-  `lost the lock on <dir>: <reason>` before it starts another file.
+  lock is lost the backup rejects with `lost the lock on <dir>: <reason>` before
+  it starts another file.
 
 ### Request pools
 
@@ -1146,7 +1147,7 @@ from a very old client, is stored unchecked.
 - `src/library/mlsearch.ts`: `MLDataAPI`, `SimilarResult`
 - `src/exif.ts`: `ExifTags`, `PhotoExif`
 - `src/library/pools.ts`: `RequestPools`, `RequestPoolsOptions`, `BoundedPool`
-- `src/backup.ts`: `BackupOptions`, `BackupResult`, `BackupError`
+- `src/backup.ts`: `BackupLock`, `BackupOptions`, `BackupResult`, `BackupError`
 - `src/client.ts`: `Client`, `LoginOptions`, `ClientSnapshot`
 - `src/api/client.ts`: `ApiClient`, `ApiClientOptions`, `StreamOptions`
 - `src/errors.ts`: `ApiError`, `TruncatedStreamError`

TODO.md

@@ -27,12 +27,13 @@ declares one.
 
 - 2026-10-14: A backup whose lock is lost no longer dies with a stack trace
   (issue 184). `lockBackupDirectory` gives proper-lockfile an `onCompromised`
-  that records why the lock was lost instead of throwing from its timer. The
+  that records why the lock was lost instead of throwing from its timer, and
-  backup then stops before it starts another original, and before it writes
+  returns the lock, which `quak backup` passes to `lib.backup()` as `lock`. The
-  `failures.json`, with `lost the lock on DIR: REASON`, which `quak backup`
+  backup then stops before it starts another file with
-  prints as one line before it exits 1. Releasing a lost lock leaves
+  `lost the lock on DIR: REASON`, which `quak backup` prints as one line before
-  `backup.lock` alone. Tests in `test/cli/backup-end-to-end.test.ts` remove the
+  it exits 1. Releasing a lost lock leaves `backup.lock` alone. Tests in
-  lock, or put another backup's in its place, while a download is held open.
+  `test/cli/backup-end-to-end.test.ts` remove the lock, or put another backup's
+  in its place, while a download is held open.
 
 - 2026-10-14: An album or file quak cannot decrypt, or whose metadata is not
   JSON, no longer fails the refresh, so one bad record no longer stops

src/backup.ts

@@ -1,7 +1,7 @@
 // The backup command, rebuilt on the library API (issue #51).
 //
 // `lib.backup()` takes the lock in `downloadDirectory` (unless its caller
-// already holds it), and fails at once when another backup of it holds the
+// passes one in), and fails at once when another backup of it holds the
 // lock; when the lock is lost while it runs, it fails before it starts another
 // file. It waits for a completed refresh of the library (a failed one fails the
 // backup before any file is touched), then, for every file in scope, puts its
@@ -97,6 +97,14 @@ import type { Collection, EnteFile, FileMetadata } from "./model/types.js";
 
 export type ProgressCallback = (message: string) => void;
 
+// The lock in a backup directory, as `lockBackupDirectory` returns it.
+export interface BackupLock {
+    // Removes `backup.lock`, unless the lock was lost.
+    release(): Promise<void>;
+    // Why the lock was lost, once it is.
+    lost?: Error;
+}
+
 export interface BackupOptions {
     // Where the backup tree lives. `lib.backup()` defaults it to the library's
     // download directory; `runBackup` with none throws before any network
@@ -113,11 +121,11 @@ export interface BackupOptions {
     // bytes do not match the content hash its metadata records. Default false.
     verify?: boolean;
     onProgress?: ProgressCallback;
-    // The caller already holds the lock in `downloadDirectory`, taken with
+    // The lock in `downloadDirectory`, when the caller took it with
-    // `lockBackupDirectory`, and releases it itself, so the backup does not
+    // `lockBackupDirectory` and releases it itself. The backup then does not
-    // take it, but still fails once that lock is lost. `quak backup` takes it
+    // take the lock, and fails once this one is lost. `quak backup` takes it
     // before it opens its library.
-    lockHeld?: boolean;
+    lock?: BackupLock;
 }
 
 export interface BackupError {
@@ -512,23 +520,24 @@ const writeAlbumJSON = (
     writeFileSync(path, JSON.stringify(album, null, 2));
 };
 
-// The backup itself, which `runBackup` below runs while the lock is held.
+// The backup itself, which `runBackup` below runs while `lock` is held. Once
+// the lock is lost, another backup may be writing to `downloadDirectory`, so
+// this one fails before it starts another file.
 const runLockedBackup = async (
     lib: BackupLibrary,
     opts: BackupOptions,
     downloadDirectory: string,
+    lock: BackupLock,
 ): Promise<BackupResult> => {
     const includeOriginals = opts.includeOriginals ?? true;
     const includeThumbnails = opts.includeThumbnails ?? false;
     const log = opts.onProgress ?? (() => {});
     const only = opts.onlyAlbumNames ? new Set(opts.onlyAlbumNames) : undefined;
     const verify = opts.verify ?? false;
-    // The lock `lockBackupDirectory` took for this backup. Once it is lost,
-    // another backup may be writing to `downloadDirectory`.
-    const lock = heldLocks.get(resolve(downloadDirectory));
 
     log("Refreshing library...");
     await lib.refresh();
+    if (lock.lost) throw lock.lost;
 
     const collectionsDir = join(downloadDirectory, "collections");
     const thumbnailsDir = join(downloadDirectory, "thumbnails");
@@ -647,7 +656,7 @@ const runLockedBackup = async (
     // that cannot be read is recorded as failed and left where it is.
     if (includeOriginals) {
         for (const [fileID, file] of distinct) {
-            if (lock?.lost) throw lock.lost;
+            if (lock.lost) throw lock.lost;
             let stored = storedAtSavePath(downloadDirectory, file);
             let mismatch = false;
             if (stored !== undefined && verify) {
@@ -727,6 +736,7 @@ const runLockedBackup = async (
 
     if (includeThumbnails) {
         for (const [fileID, file] of distinct) {
+            if (lock.lost) throw lock.lost;
             const dest = join(thumbnailsDir, `${fileID}.jpg`);
             if (isPresent(dest)) continue;
             try {
@@ -759,6 +769,7 @@ const runLockedBackup = async (
             log(`FAILED ML data: ${mlDataError}`);
         }
         for (const file of distinct.values()) {
+            if (lock.lost) throw lock.lost;
             const stored = storedAtSavePath(downloadDirectory, file);
             if (stored === undefined) continue;
             const path = withExtension(
@@ -802,6 +813,7 @@ const runLockedBackup = async (
     const albumDirNames = new Map(
         allCollections.map((c, i) => [c.id, dirNames[i]!]),
     );
+    if (lock.lost) throw lock.lost;
     try {
         removeStaleAlbumDirs(
             collectionsDir,
@@ -813,6 +825,7 @@ const runLockedBackup = async (
     }
 
     for (const c of collections) {
+        if (lock.lost) throw lock.lost;
         const colDirName = albumDirNames.get(c.id)!;
         const colDir = join(collectionsDir, colDirName);
         mkdirSync(colDir, { recursive: true });
@@ -872,7 +885,7 @@ const runLockedBackup = async (
     // stale entry would keep the exit code non-zero forever — a single
     // since-deleted photo would fail every future scheduled backup. A backup
     // whose lock was lost leaves the ledger as it is.
-    if (lock?.lost) throw lock.lost;
+    if (lock.lost) throw lock.lost;
     for (const fileID of [...ledger.keys()]) {
         if (!failedThisRun.has(fileID)) ledger.delete(fileID);
     }
@@ -891,29 +904,30 @@ const runLockedBackup = async (
     };
 };
 
-// The lock this process holds on each backup directory, by the directory's
-// absolute path, until it is released. `lost` is set once the lock is lost.
-const heldLocks = new Map<string, { lost?: Error }>();
-
 // Only one backup of a directory runs at a time, in this process or another.
 // Creates `downloadDirectory` if it is missing, takes the lock in it and
-// returns the function that releases it; while another backup holds the lock,
+// returns it; while another backup holds the lock, fails at once with an error
-// fails at once with an error whose `code` is `ELOCKED`. The lock is the
+// whose `code` is `ELOCKED`. The lock is the directory `backup.lock`, whose
-// directory `backup.lock`, whose modification time proper-lockfile keeps
+// modification time proper-lockfile keeps current while it is held. One it has
-// current while it is held. One it has not touched for 10 seconds was left by
+// not touched for 10 seconds was left by a run that could not remove it, and
-// a run that could not remove it, and is taken over.
+// is taken over.
 //
 // proper-lockfile finds the lock lost when `backup.lock` is gone or is another
 // backup's, and calls `onCompromised` from a timer, where throwing would end
-// the process. The backup holding the lock stops before it starts another file
+// the process. The `onCompromised` given here sets the lock's `lost` instead,
-// instead (see `runLockedBackup`), and releasing a lost lock leaves
+// and the backup holding the lock stops before it starts another file (see
-// `backup.lock` alone.
+// `runLockedBackup`). Releasing a lost lock leaves `backup.lock` alone, since
+// it may be another backup's.
 export const lockBackupDirectory = async (
     downloadDirectory: string,
-): Promise<() => Promise<void>> => {
+): Promise<BackupLock> => {
     mkdirSync(downloadDirectory, { recursive: true });
-    const lock: { lost?: Error } = {};
     let release: () => Promise<void>;
+    const lock: BackupLock = {
+        release: async () => {
+            if (lock.lost === undefined) await release();
+        },
+    };
     try {
         release = await lockfile.lock(downloadDirectory, {
             lockfilePath: join(downloadDirectory, "backup.lock"),
@@ -930,18 +944,11 @@ export const lockBackupDirectory = async (
             { code: "ELOCKED" },
         );
     }
-    const path = resolve(downloadDirectory);
+    return lock;
-    heldLocks.set(path, lock);
-    return async () => {
-        // Once this lock was lost, another backup in this process may have
-        // taken the directory.
-        if (heldLocks.get(path) === lock) heldLocks.delete(path);
-        if (lock.lost === undefined) await release();
-    };
 };
 
 // Runs the backup holding the lock, which it takes and releases itself unless
-// the caller already holds it (`opts.lockHeld`).
+// the caller passes the lock it took (`opts.lock`).
 export const runBackup = async (
     lib: BackupLibrary,
     opts: BackupOptions,
@@ -953,13 +960,13 @@ export const runBackup = async (
                 "open the library with one)",
         );
     }
-    if (opts.lockHeld) {
+    if (opts.lock) {
-        return runLockedBackup(lib, opts, downloadDirectory);
+        return runLockedBackup(lib, opts, downloadDirectory, opts.lock);
     }
-    const release = await lockBackupDirectory(downloadDirectory);
+    const lock = await lockBackupDirectory(downloadDirectory);
     try {
-        return await runLockedBackup(lib, opts, downloadDirectory);
+        return await runLockedBackup(lib, opts, downloadDirectory, lock);
     } finally {
-        await release();
+        await lock.release();
     }
 };

src/cli-commands.ts

@@ -20,7 +20,7 @@ import {
     type ClientSnapshot,
     type LoginOptions,
 } from "./client.js";
-import { lockBackupDirectory } from "./backup.js";
+import { lockBackupDirectory, type BackupLock } from "./backup.js";
 import { init } from "./crypto/index.js";
 import {
     defaultCacheDirectory,
@@ -416,9 +416,9 @@ export const backupCommand = async (
     ctx.stderr.write("Starting backup...\n");
     // The lock is taken before the library opens and starts its refresh, so a
     // run refused while another backup of `dir` runs sends no request.
-    let release: () => Promise<void>;
+    let lock: BackupLock;
     try {
-        release = await lockBackupDirectory(dir);
+        lock = await lockBackupDirectory(dir);
     } catch (err) {
         if ((err as NodeJS.ErrnoException).code !== "ELOCKED") throw err;
         ctx.stderr.write(`quak: ${(err as Error).message}\n`);
@@ -440,7 +440,7 @@ export const backupCommand = async (
         try {
             const result = await lib.backup({
                 downloadDirectory: dir,
-                lockHeld: true,
+                lock,
                 verify: opts.verify,
                 onProgress: (msg) => {
                     if (!opts.json) ctx.stderr.write(msg + "\n");
@@ -477,7 +477,7 @@ export const backupCommand = async (
             await lib.close();
         }
     } finally {
-        await release();
+        await lock.release();
     }
 };
 

src/index.ts

@@ -69,6 +69,7 @@ export {
     type EnsureEvent,
     lockBackupDirectory,
     runBackup,
+    type BackupLock,
     type BackupOptions,
     type BackupResult,
     type BackupError,

src/library/index.ts

@@ -96,6 +96,7 @@ import { runBackup, type BackupOptions, type BackupResult } from "../backup.js";
 export {
     lockBackupDirectory,
     runBackup,
+    type BackupLock,
     type BackupOptions,
     type BackupResult,
     type BackupError,
@@ -570,10 +571,10 @@ export class Library {
 
     // Back up every in-scope file to `opts.downloadDirectory`, or else the
     // library's, each original at its save path, with a durable failure
-    // ledger (issue #51). Takes the lock in that directory first, unless
+    // ledger (issue #51). Takes the lock in that directory first, unless the
-    // `opts.lockHeld` says the caller holds it, failing at once while another
+    // caller passes the lock it took as `opts.lock`, failing at once while
-    // backup of it runs, and before it starts another file once the lock is
+    // another backup of it runs, and before it starts another file once the
-    // lost (see `lockBackupDirectory`). Waits for a
+    // lock is lost (see `lockBackupDirectory`). Waits for a
     // completed refresh, as `fresh()` does, joining one already running, and
     // rejects before touching any file but the lock when it fails. Then puts
     // pending originals at their save paths as `Photo.download()` does (and

test/cli/backup-end-to-end.test.ts

@@ -806,9 +806,10 @@ describe("a backup whose lock is lost", () => {
     });
 
     // A program can take the lock before it opens its library, as `quak
-    // backup` does. `lockBackupDirectory` returns the lock; the program passes
+    // backup` does: `lockBackupDirectory` returns the lock, which the program
-    // it to `lib.backup()`, which checks it as it checks a lock it took, and
+    // passes to `lib.backup()` and releases once the backup ends. The backup
-    // releases it once the backup ends. The lock's `lost` says why it was lost.
+    // checks it as it checks a lock it took, and the lock's `lost` says why it
+    // was lost.
     it("rejects lib.backup() given its caller's lock once that lock is lost", async () => {
         const lock = await lockBackupDirectory(dir);
         const lib = await openLibrary();