quak backup refuses to run while another backup of the same directory runs, exit 2 (closes #169)

lib.backup() takes a lock, backup.lock in its download directory, made
with proper-lockfile, before its refresh, and removes it when it ends. A
second backup of the directory fails at once with an error naming it;
quak backup prints it as one line and exits 2. A lock untouched for 10
seconds, left by a killed run, is taken over.

Deviation: yarn.lock was regenerated by yarn add in the pinned node image.
Judgement call: a run failing at its refresh leaves the directory, empty.
Judgement call: a lock removed mid-run stops that run with an uncaught
error, the library's default.

Model: opus-5-5

README.md

@@ -524,10 +524,10 @@ stack trace. All three exit with status 3, which means the user must run
 `quak login` again. `quak logout` is the exception: with no file it says there
 is no session and exits 0, and it handles a corrupt file or a failed server call
 as described below. `quak backup` meets an expired session on the refresh that
-starts every run, before it touches any file. A session that stops working
-partway through a backup instead fails each remaining file into `failures.json`,
-so that run exits 1 and the next one stops at its refresh with status 3. No
-command but `quak login` ever prompts.
+starts every run, before it touches any file but its lock (see "Backup layout").
+A session that stops working partway through a backup instead fails each
+remaining file into `failures.json`, so that run exits 1 and the next one stops
+at its refresh with status 3. No command but `quak login` ever prompts.
 
 `quak logout` ends the session on the server, so the token in `session.json`
 stops working even in a copy of the file, and then deletes the file. If the
@@ -621,6 +621,7 @@ the smallest does not.
                                     (symlink)
         <name>.json                 collection metadata + file list
     account.json                    the account's email and user ID
+    backup.lock                     the lock a running backup holds (see below)
     failures.json                   files that failed and have not yet succeeded
 ```
 
@@ -665,6 +666,22 @@ succeeds, or once it is no longer in the library or in the backup's scope. The
 library's `lib.backup({ includeThumbnails: true })` also writes
 `thumbnails/<fileID>.jpg` beside `collections/`; `quak backup` does not.
 
+`backup.lock` keeps two backups of the same directory from running at once, such
+as a cron run that starts while the previous one is still going. A backup
+creates `<dir>` if it is missing and takes the lock before its refresh, and
+removes the lock when it ends, whether it succeeds or fails. The lock is a
+directory that
+[proper-lockfile](https://github.com/moxystudio/node-proper-lockfile) creates
+and keeps touching while the backup runs. A second backup of the directory, from
+another process or the same one, fails at once: `quak backup` prints
+`quak: another backup of <dir> is running` and exits with status 2. It opens its
+library before the backup takes the lock, so a refused run still waits for the
+refresh that opening the library starts before it exits. A run that is killed
+leaves the lock behind; once it has gone 10 seconds untouched, the next run
+takes it over, so nobody has to remove it. The lock is outside the date folders
+and `collections/`, so it is never taken for an original, and the removal of old
+album directories never touches it.
+
 A collection's directory and JSON are named after the collection, and a symlink
 after the file's title, both with unsafe characters replaced. When two
 collections would get the same name, or two symlinks in one collection the same
@@ -912,18 +929,19 @@ photos newest first). `lib.subscribe({ onChange })` delivers a `LibraryChange`
   `SimilarResult[]` (`{ fileID, score }`, cosine similarity, most similar first,
   default limit 20). quak bundles no text encoder, so `searchByEmbedding` takes
   a query vector the caller produced elsewhere.
-- `await lib.backup(opts?)` → `BackupResult`. It waits for a refresh as
-  `fresh()` does, puts every in-scope original not already at its save path
-  there as `photo.download()` does (and, with `includeThumbnails`, fetches
-  thumbnails) through the content cache, waits for an ML data fetch, and
-  rebuilds the on-disk backup tree, each file's JSON with its ML data and its
-  original's EXIF, XMP and dimensions, with a durable failure ledger. A fetched
-  original is written straight to its save path and not into the cache, which
-  then counts it as present; one the cache already held is copied from there.
-  `BackupOptions`: `downloadDirectory` (falls back to the library's),
-  `includeOriginals` (default `true`), `includeThumbnails` (default `false`),
-  `onlyAlbumNames`, and `onProgress`. See Backup layout above for the tree it
-  writes.
+- `await lib.backup(opts?)` → `BackupResult`. It takes the lock in the download
+  directory, and fails at once with an error whose `code` is `ELOCKED` while
+  another backup of it runs. It waits for a refresh as `fresh()` does, puts
+  every in-scope original not already at its save path there as
+  `photo.download()` does (and, with `includeThumbnails`, fetches thumbnails)
+  through the content cache, waits for an ML data fetch, and rebuilds the
+  on-disk backup tree, each file's JSON with its ML data and its original's
+  EXIF, XMP and dimensions, with a durable failure ledger. A fetched original is
+  written straight to its save path and not into the cache, which then counts it
+  as present; one the cache already held is copied from there. `BackupOptions`:
+  `downloadDirectory` (falls back to the library's), `includeOriginals` (default
+  `true`), `includeThumbnails` (default `false`), `onlyAlbumNames`, and
+  `onProgress`. See Backup layout above for the tree it writes.
 
 ### Request pools
 

TODO.md

@@ -25,6 +25,15 @@ declares one.
 
 # Completed Steps
 
+- 2026-10-06: Two backups of the same directory never run at once (issue 169).
+  `lib.backup()` takes a lock, `backup.lock` in its download directory, made
+  with `proper-lockfile`, before its refresh, and removes it when it ends,
+  whether it succeeds or fails. A second backup of the directory, from another
+  process or the same one, fails at once with an error naming the directory;
+  `quak backup` prints it as one line and exits 2. A lock that has gone 10
+  seconds untouched, left by a run that was killed, is taken over by the next
+  run.
+
 - 2026-10-06: `quak backup` writes each original's EXIF, XMP and dimensions into
   the file's JSON as `imageMetadata`, what `backup-metadata --exif` records
   (issue 167): for a live photo from its image, for a video nothing, and `{}`

package.json

@@ -36,6 +36,7 @@
     "devDependencies": {
         "@eslint/js": "9.38.0",
         "@types/node": "22.18.13",
+        "@types/proper-lockfile": "4.1.4",
         "eslint": "9.38.0",
         "prettier": "3.8.1",
         "typescript": "5.9.3",
@@ -50,6 +51,7 @@
         "fast-srp-hap": "2.0.4",
         "fflate": "0.8.3",
         "jpeg-js": "0.4.4",
-        "libsodium-wrappers-sumo": "0.8.4"
+        "libsodium-wrappers-sumo": "0.8.4",
+        "proper-lockfile": "4.1.2"
     }
 }

src/backup.ts

@@ -1,11 +1,12 @@
 // The backup command, rebuilt on the library API (issue #51).
 //
-// `lib.backup()` 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 original at its save path under `downloadDirectory`, as
-// `Photo.download()` does, waits for an ML data fetch, and rebuilds the derived
-// views (per-file sidecars, per-collection symlink trees, per-collection JSON)
-// from the model. The on-disk layout:
+// `lib.backup()` takes the lock in `downloadDirectory`, and fails at once when
+// another backup of it holds the lock. 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 original at its save path under
+// `downloadDirectory`, as `Photo.download()` does, waits for an ML data fetch,
+// and rebuilds the derived views (per-file sidecars, per-collection symlink
+// trees, per-collection JSON) from the model. The on-disk layout:
 //
 //     <downloadDirectory>/
 //         YYYY/YYYY-MM/YYYY-MM-DD/
@@ -17,6 +18,7 @@
 //         collections/<name>/<title>       symlink to the original
 //         collections/<name>.json          per-collection metadata
 //         account.json                     the account's email and user ID
+//         backup.lock                      the lock, while a backup runs
 //         failures.json                    durable ledger of unresolved failures
 //
 // A live photo's original is its image and its video, each with its own
@@ -60,6 +62,7 @@ import {
 } from "node:fs";
 import { readFile } from "node:fs/promises";
 import { dirname, extname, join, relative, resolve } from "node:path";
+import lockfile from "proper-lockfile";
 
 import { removeLeftoverTempFiles } from "./download/index.js";
 import { sanitizeFileName, withExtension } from "./filename.js";
@@ -450,17 +453,12 @@ const writeAlbumJSON = (
     writeFileSync(path, JSON.stringify(album, null, 2));
 };
 
-export const runBackup = async (
+// The backup itself, which `runBackup` below runs while it holds the lock.
+const runLockedBackup = async (
     lib: BackupLibrary,
     opts: BackupOptions,
+    downloadDirectory: string,
 ): Promise<BackupResult> => {
-    const downloadDirectory = opts.downloadDirectory;
-    if (!downloadDirectory) {
-        throw new Error(
-            "backup requires a downloadDirectory (pass one to backup() or " +
-                "open the library with one)",
-        );
-    }
     const includeOriginals = opts.includeOriginals ?? true;
     const includeThumbnails = opts.includeThumbnails ?? false;
     const log = opts.onProgress ?? (() => {});
@@ -736,3 +734,39 @@ export const runBackup = async (
         errors,
     };
 };
+
+// Only one backup of a directory runs at a time, in this process or another:
+// a second one fails at once with an error whose `code` is `ELOCKED`. The lock
+// is the directory `backup.lock`, whose modification time proper-lockfile
+// keeps current while the backup runs. One it has not touched for 10 seconds
+// was left by a run that was killed, and is taken over.
+export const runBackup = async (
+    lib: BackupLibrary,
+    opts: BackupOptions,
+): Promise<BackupResult> => {
+    const downloadDirectory = opts.downloadDirectory;
+    if (!downloadDirectory) {
+        throw new Error(
+            "backup requires a downloadDirectory (pass one to backup() or " +
+                "open the library with one)",
+        );
+    }
+    mkdirSync(downloadDirectory, { recursive: true });
+    let release: () => Promise<void>;
+    try {
+        release = await lockfile.lock(downloadDirectory, {
+            lockfilePath: join(downloadDirectory, "backup.lock"),
+        });
+    } catch (err) {
+        if ((err as NodeJS.ErrnoException).code !== "ELOCKED") throw err;
+        throw Object.assign(
+            new Error(`another backup of ${downloadDirectory} is running`),
+            { code: "ELOCKED" },
+        );
+    }
+    try {
+        return await runLockedBackup(lib, opts, downloadDirectory);
+    } finally {
+        await release();
+    }
+};

src/cli-commands.ts

@@ -22,6 +22,7 @@ import {
 } from "./client.js";
 import { init } from "./crypto/index.js";
 import {
+    type BackupResult,
     defaultCacheDirectory,
     Library,
     type LibraryClient,
@@ -418,12 +419,20 @@ export const backupCommand = async (
         precacheOriginals: false,
     });
     try {
-        const result = await lib.backup({
-            downloadDirectory: dir,
-            onProgress: (msg) => {
-                if (!opts.json) ctx.stderr.write(msg + "\n");
-            },
-        });
+        let result: BackupResult;
+        try {
+            result = await lib.backup({
+                downloadDirectory: dir,
+                onProgress: (msg) => {
+                    if (!opts.json) ctx.stderr.write(msg + "\n");
+                },
+            });
+        } catch (err) {
+            // Another backup of `dir` is running and holds its lock.
+            if ((err as NodeJS.ErrnoException).code !== "ELOCKED") throw err;
+            ctx.stderr.write(`quak: ${(err as Error).message}\n`);
+            return 2;
+        }
 
         if (opts.json) {
             ctx.stdout.write(JSON.stringify(result, null, 2) + "\n");

src/library/index.ts

@@ -566,13 +566,15 @@ 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). Waits for a completed refresh first, as `fresh()`
-    // does, joining one already running, and rejects before touching any file
-    // when it fails. Then puts pending originals at their save paths as
-    // `Photo.download()` does (and optional thumbnails) through the content
-    // cache and pools, waits for an ML data fetch, and rebuilds the derived
-    // symlink/JSON views from the model, each file's JSON with its ML data,
-    // beside an `account.json` with the account's email and user ID.
+    // ledger (issue #51). Takes the lock in that directory first, failing at
+    // once while another backup of it runs (see `runBackup`). 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
+    // optional thumbnails) through the content cache and pools, waits for an
+    // ML data fetch, and rebuilds the derived symlink/JSON views from the
+    // model, each file's JSON with its ML data, beside an `account.json` with
+    // the account's email and user ID.
     // Throws before any network work when no content cache backs the
     // originals it must fetch.
     backup(opts?: BackupOptions): Promise<BackupResult> {

test/cli/backup.test.ts

@@ -43,6 +43,7 @@ import {
     readlinkSync,
     rmSync,
     symlinkSync,
+    utimesSync,
     writeFileSync,
 } from "node:fs";
 import { spawnSync } from "node:child_process";
@@ -830,7 +831,81 @@ describe("the refresh before a backup", () => {
         );
 
         expect(source.originalCalls).toBe(0);
-        expect(existsSync(outDir)).toBe(false);
+        // The backup made the directory for its lock, and removed the lock.
+        expect(readdirSync(outDir)).toEqual([]);
+        await lib.close();
+    });
+});
+
+describe("the backup lock", () => {
+    const lockPath = (outDir: string): string => join(outDir, "backup.lock");
+
+    it("refuses a second backup of the directory while one runs", async () => {
+        await fillCache();
+        const client = new HeldClient();
+        const lib = await openLibrary(stubSource(), client);
+        const outDir = join(root, "backup");
+        // The first backup holds the lock once it starts its refresh, which
+        // `HeldClient` keeps from finishing.
+        let refreshing!: () => void;
+        const started = new Promise<void>((resolve) => {
+            refreshing = resolve;
+        });
+        const first = lib.backup({
+            downloadDirectory: outDir,
+            onProgress: (msg) => {
+                if (msg === "Refreshing library...") refreshing();
+            },
+        });
+        await started;
+
+        await expect(lib.backup({ downloadDirectory: outDir })).rejects.toThrow(
+            `another backup of ${outDir} is running`,
+        );
+
+        client.release();
+        expect((await first).failed).toBe(0);
+        await lib.close();
+    });
+
+    it("releases the lock after a backup succeeds", async () => {
+        const lib = await openLibrary(stubSource());
+        const outDir = join(root, "backup");
+
+        await lib.backup({ downloadDirectory: outDir });
+
+        expect(existsSync(lockPath(outDir))).toBe(false);
+        // So the next backup of the directory runs.
+        expect((await lib.backup({ downloadDirectory: outDir })).skipped).toBe(
+            3,
+        );
+        await lib.close();
+    });
+
+    it("releases the lock after a backup fails", async () => {
+        const lib = await openLibrary(stubSource(), new FailingClient());
+        const outDir = join(root, "backup");
+
+        await expect(lib.backup({ downloadDirectory: outDir })).rejects.toThrow(
+            "HTTP 401 from server",
+        );
+
+        expect(existsSync(lockPath(outDir))).toBe(false);
+        await lib.close();
+    });
+
+    it("takes over a lock left by a run that was killed", async () => {
+        const outDir = join(root, "backup");
+        // A killed run's lock, last touched a minute ago.
+        mkdirSync(lockPath(outDir), { recursive: true });
+        const minuteAgo = new Date(Date.now() - 60_000);
+        utimesSync(lockPath(outDir), minuteAgo, minuteAgo);
+        const lib = await openLibrary(stubSource());
+
+        const result = await lib.backup({ downloadDirectory: outDir });
+
+        expect(result.downloaded).toBe(3);
+        expect(existsSync(lockPath(outDir))).toBe(false);
         await lib.close();
     });
 });

test/cli/commands.test.ts

@@ -18,6 +18,7 @@ import {
     readFileSync,
     rmSync,
     statSync,
+    utimesSync,
     writeFileSync,
 } from "node:fs";
 import { join } from "node:path";
@@ -792,7 +793,7 @@ describe("backup", () => {
         expect(code).toBe(1);
         expect(runText).toBe("quak: HTTP 503 from server\n");
         expect(stderr.text).toBe("Starting backup...\nRefreshing library...\n");
-        expect(existsSync(dir)).toBe(false);
+        expect(readdirSync(dir)).toEqual([]);
     });
 
     // A real saved session, read back by `loadSession`, whose server answers
@@ -823,7 +824,23 @@ describe("backup", () => {
             `quak: the saved session is no longer valid; run "quak login"\n`,
         );
         expect(stderr.text).toBe("Starting backup...\nRefreshing library...\n");
-        expect(existsSync(dir)).toBe(false);
+        expect(readdirSync(dir)).toEqual([]);
+    });
+
+    it("exits 2 with one line naming the directory while another backup of it runs", async () => {
+        const dir = join(root, "backup");
+        // The lock another backup holds. Its modification time is set an hour
+        // ahead, so it stays current however long this test takes.
+        const lock = join(dir, "backup.lock");
+        mkdirSync(lock, { recursive: true });
+        const hourAhead = new Date(Date.now() + 3_600_000);
+        utimesSync(lock, hourAhead, hourAhead);
+
+        expect(await backupCommand(context(), dir, {})).toBe(2);
+        expect(stderr.text).toBe(
+            `Starting backup...\nquak: another backup of ${dir} is running\n`,
+        );
+        expect(readdirSync(dir)).toEqual(["backup.lock"]);
     });
 });
 

yarn.lock

@@ -535,6 +535,18 @@
   dependencies:
     undici-types "~6.21.0"
 
+"@types/proper-lockfile@4.1.4":
+  version "4.1.4"
+  resolved "https://registry.yarnpkg.com/@types/proper-lockfile/-/proper-lockfile-4.1.4.tgz#cd9fab92bdb04730c1ada542c356f03620f84008"
+  integrity sha512-uo2ABllncSqg9F1D4nugVl9v93RmjxF6LJzQLMLDdPaXCUIDPeOJ21Gbqi43xNKzBi/WQ0Q0dICqufzQbMjipQ==
+  dependencies:
+    "@types/retry" "*"
+
+"@types/retry@*":
+  version "0.12.5"
+  resolved "https://registry.yarnpkg.com/@types/retry/-/retry-0.12.5.tgz#f090ff4bd8d2e5b940ff270ab39fd5ca1834a07e"
+  integrity sha512-3xSjTp3v03X/lSQLkczaN9UIEwJMoMCA1+Nb5HfbJEQWogdeQIyVtTvxPXDQjZ5zws8rFQfVfRdz03ARihPJgw==
+
 "@typescript-eslint/eslint-plugin@8.46.2":
   version "8.46.2"
   resolved "https://registry.yarnpkg.com/@typescript-eslint/eslint-plugin/-/eslint-plugin-8.46.2.tgz#dc4ab93ee3d7e6c8e38820a0d6c7c93c7183e2dc"
@@ -1140,6 +1152,11 @@ globals@^14.0.0:
   resolved "https://registry.yarnpkg.com/globals/-/globals-14.0.0.tgz#898d7413c29babcf6bafe56fcadded858ada724e"
   integrity sha512-oahGvuMGQlPw/ivIYBjVSrWAfWLBeku5tpPE2fOPLi+WHffIWbuh2tCjhyQhTBPMf5E9jDEH4FOmTYgYwbKwtQ==
 
+graceful-fs@^4.2.4:
+  version "4.2.11"
+  resolved "https://registry.yarnpkg.com/graceful-fs/-/graceful-fs-4.2.11.tgz#4183e4e8bf08bb6e05bbb2f7d2e0c8f712ca40e3"
+  integrity sha512-RbJ5/jmFcNNCcDV5o9eTnBLJ/HszWV0P73bc+Ff4nS/rJj+YaS6IGyiOL0VoBYX+l1Wrl3k63h/KrH+nhJ0XvQ==
+
 graphemer@^1.4.0:
   version "1.4.0"
   resolved "https://registry.yarnpkg.com/graphemer/-/graphemer-1.4.0.tgz#fb2f1d55e0e3a1849aeffc90c4fa0dd53a0e66c6"
@@ -1414,6 +1431,15 @@ prettier@3.8.1:
   resolved "https://registry.yarnpkg.com/prettier/-/prettier-3.8.1.tgz#edf48977cf991558f4fcbd8a3ba6015ba2a3a173"
   integrity sha512-UOnG6LftzbdaHZcKoPFtOcCKztrQ57WkHDeRD9t/PTQtmT0NHSeWWepj6pS0z/N7+08BHFDQVUrfmfMRcZwbMg==
 
+proper-lockfile@4.1.2:
+  version "4.1.2"
+  resolved "https://registry.yarnpkg.com/proper-lockfile/-/proper-lockfile-4.1.2.tgz#c8b9de2af6b2f1601067f98e01ac66baa223141f"
+  integrity sha512-TjNPblN4BwAWMXU8s9AEz4JmQxnD1NNL7bNOY/AKUzyamc379FWASUhc/K1pL2noVb+XmZKLL68cjzLsiOAMaA==
+  dependencies:
+    graceful-fs "^4.2.4"
+    retry "^0.12.0"
+    signal-exit "^3.0.2"
+
 punycode@^2.1.0:
   version "2.3.1"
   resolved "https://registry.yarnpkg.com/punycode/-/punycode-2.3.1.tgz#027422e2faec0b25e1549c3e1bd8309b9133b6e5"
@@ -1429,6 +1455,11 @@ resolve-from@^4.0.0:
   resolved "https://registry.yarnpkg.com/resolve-from/-/resolve-from-4.0.0.tgz#4abcd852ad32dd7baabfe9b40e00a36db5f392e6"
   integrity sha512-pb/MYmXstAkysRFx8piNI1tGFNQIFA3vkE3Gq4EuA1dF6gHp/+vgZqsCGJapvy8N3Q+4o7FwvquPJcnZ7RYy4g==
 
+retry@^0.12.0:
+  version "0.12.0"
+  resolved "https://registry.yarnpkg.com/retry/-/retry-0.12.0.tgz#1b42a6266a21f07421d1b0b54b7dc167b01c013b"
+  integrity sha512-9LkiTwjUh6rT555DtE9rTX+BKByPfrMzEAtnlEtdEwr3Nkffwiihqe2bWADg+OQRjt9gl6ICdmB/ZFDCGAtSow==
+
 reusify@^1.0.4:
   version "1.1.0"
   resolved "https://registry.yarnpkg.com/reusify/-/reusify-1.1.0.tgz#0fe13b9522e1473f51b558ee796e08f11f9b489f"
@@ -1502,6 +1533,11 @@ siginfo@^2.0.0:
   resolved "https://registry.yarnpkg.com/siginfo/-/siginfo-2.0.0.tgz#32e76c70b79724e3bb567cb9d543eb858ccfaf30"
   integrity sha512-ybx0WO1/8bSBLEWXZvEd7gMW3Sn3JFlW3TvX1nREbDLRNQNaeNN8WK0meBwPdAaOI7TtRRRJn/Es1zhrrCHu7g==
 
+signal-exit@^3.0.2:
+  version "3.0.7"
+  resolved "https://registry.yarnpkg.com/signal-exit/-/signal-exit-3.0.7.tgz#a9a1767f8af84155114eaabd73f99273c8f59ad9"
+  integrity sha512-wnD2ZE+l+SPC/uoS0vXeE9L1+0wuaMqKlfz9AMUo38JsyLSBWSFcHR1Rri62LZc12vLr1gb3jl7iwQhgwpAbGQ==
+
 signal-exit@^4.1.0:
   version "4.1.0"
   resolved "https://registry.yarnpkg.com/signal-exit/-/signal-exit-4.1.0.tgz#952188c1cbd546070e2dd20d0f41c0ae0530cb04"