README: running quak backup from cron, and its exit codes (closes #170)

A new README section shows how to run quak backup unattended: log in once
as the job's user, a crontab with a backup every night and --verify on
Sundays, output appended to a log file, and the HOME and XDG_DATA_HOME the
job needs to find the saved session. A table gives each exit code as the
code returns it, and says which failures the next run retries. The
introduction now lists everything the backup keeps for each file. It, the
backup layout tree and the lib.backup() entry say EXIF and XMP are kept for
an image only, and dimensions for a JPEG only.

Judgement call: the --verify run takes Sunday's slot rather than a second
job that night, since an overlapping run would exit 2.

Model: opus-5-5

README.md

@@ -10,9 +10,12 @@ account into a deduplicated local directory tree, skipping files that already
 exist on disk and continuing past individual download failures instead of
 crashing. For each file it persists the basic metadata fields quak keeps (title,
 file type, creation and modification time, latitude, longitude, content hash),
-and the private and public magic metadata in full. A helper subcommand can
-detect and regenerate missing thumbnails, encrypting and uploading them back to
-the server.
+its update time, the private and public magic metadata in full, Ente's ML data
+for it when there is any, and, for an image (for a live photo, its image), its
+original's EXIF and XMP, with its dimensions for a JPEG only; for a video it
+keeps none of these three. It runs unattended from cron (see "Running the backup
+from cron"). A helper subcommand can detect and regenerate missing thumbnails,
+encrypting and uploading them back to the server.
 
 ## Getting Started
 
@@ -80,6 +83,64 @@ await lib.close();
 The lower-level `Client` (login, session serialization, and the raw
 enumeration/download calls) is exported too and documented under Design below.
 
+## Running the backup from cron
+
+`quak backup` never prompts, so cron can run it. `make install` builds quak as a
+single binary and copies it to `~/bin/quak`. Log in once with it, as the user
+the cron job will run as; the session is saved in that user's data directory
+(see "Session handling"):
+
+```bash
+~/bin/quak login
+```
+
+Then add the backup to that user's crontab with `crontab -e`. These two lines
+back up the account to `~/photos-backup` at 03:30 every night, with `--verify`
+on Sundays, and append all output to `~/quak-backup.log`:
+
+```
+30 3 * * 1-6 $HOME/bin/quak backup $HOME/photos-backup >> $HOME/quak-backup.log 2>&1
+30 3 * * 0   $HOME/bin/quak backup --verify $HOME/photos-backup >> $HOME/quak-backup.log 2>&1
+```
+
+A run with `--verify` does all a plain run does, and also checks each original
+already in the backup against the content hash Ente records for it, and replaces
+any that do not match (see "Backup layout"). Sunday's run is the `--verify` one,
+not a second job that night, because a backup that starts while another backup
+of the same directory is running exits 2 without backing anything up.
+
+Cron runs the job with a short `PATH`, usually `/usr/bin:/bin`, so the crontab
+names quak by its full path. To find the saved session, quak needs the same
+`HOME` as when you logged in, which cron sets from the password file, and on
+Linux the same `XDG_DATA_HOME`: quak looks for the session in
+`$XDG_DATA_HOME/quak`, or in `~/.local/share/quak` when that is not set. Cron
+does not set `XDG_DATA_HOME`, so if your login shell does, set it at the top of
+the crontab too. Cron does not expand variables in such a line, so give the full
+path:
+
+```
+XDG_DATA_HOME=/home/you/.data
+```
+
+On macOS the session is in `~/Library/Application Support/quak`, and only `HOME`
+matters.
+
+### Exit codes
+
+| Code | Meaning                                                                                                                                                     |
+| ---- | ----------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| `0`  | The backup is complete: every original is at its save path, and no file failed.                                                                             |
+| `1`  | The run finished with files in `failures.json`, or it stopped on another error, printed as one line `quak: <message>`. The next run tries again.            |
+| `2`  | Another backup of the same directory is running. This run sent no request and changed nothing.                                                              |
+| `3`  | There is no usable session: none is saved, the saved one is corrupt, or the server no longer accepts it. Run `quak login` as the user the cron job runs as. |
+
+After a `1`, the next run fetches each missing original again, fetches the ML
+data that is not cached, and rebuilds the album links. An original that a
+`--verify` run could not read, or put back with bytes that still do not match,
+stays at its save path. Only a later `--verify` run checks it again: a run
+without `--verify` leaves it as it is and takes the file out of `failures.json`,
+so that run can exit 0.
+
 ## Examples
 
 `examples/download-albums.ts` downloads every album's photos and their metadata
@@ -517,8 +578,17 @@ The CLI stores the snapshot at the platform-appropriate data directory via
 `0600`. The key material is stored in cleartext in the JSON; treat this file as
 you would treat the password itself. A missing file is reported as "not logged
 in"; a file that exists but is corrupt is reported as such, naming the bad
-field. Both exit with status 1, except that `quak logout` with no file says
-there is no session and exits 0.
+field. When the refresh a command starts with gets HTTP 401 from the server,
+because it no longer accepts the saved session's token, the command prints one
+line, `quak: the saved session is no longer valid; run "quak login"`, with no
+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 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
@@ -540,7 +610,7 @@ quak collections [--json]                                 list all collections
 quak files --collection <id> [--json]                     list files in a collection
 quak get <fileID> [--out path] [--collection]             download and decrypt a file
 quak get-thumb <fileID> [--out] [--collection]            download and decrypt a thumbnail
-quak backup <dir> [--json]                                full incremental backup
+quak backup <dir> [--json] [--verify]                     full incremental backup
 quak backup-metadata <dir> [--exif]                       dump the metadata quak keeps as JSON
 quak helper list-missing-thumbnails [--json]              find files with missing thumbnails
 quak helper fix-missing-thumbnails [--file ids] [--json]  generate + upload missing thumbnails
@@ -552,8 +622,9 @@ library. The read commands — `collections`, `files`, `get`, `get-thumb`,
 `helper fix-missing-thumbnails` — force a fresh server round-trip before they
 answer, so they report current account state rather than whatever the cache last
 held. If that round-trip fails, the command prints the error on one line and
-exits 1. `--cache-dir` overrides where the cache lives; without it each account
-gets its own directory under the per-user cache path.
+exits 1, or 3 when the server no longer accepts the saved session (see "Session
+handling"). `--cache-dir` overrides where the cache lives; without it each
+account gets its own directory under the per-user cache path.
 
 `get` and `get-thumb` resolve the file by ID directly, so `--collection` is
 accepted for backward compatibility but ignored. For a live photo, `get` writes
@@ -572,6 +643,10 @@ reads no tag from is recorded, base64, as `exifRaw`, with the reason in
 `exifError`. `collections`, `files`, `backup`, `helper list-missing-thumbnails`
 and `helper fix-missing-thumbnails` take `--json` for machine-readable output.
 
+`backup --verify` also hashes the originals already in the backup and replaces
+any that do not match the content hash Ente records; one it cannot replace goes
+into `failures.json` (see "Backup layout").
+
 `backup-metadata` fetches ML data in requests of up to 200 files. When a request
 fails, the error is logged, each of its files is written with the reason in an
 `mlDataError` field instead of `mlData`, and the dump goes on. The exit code is
@@ -601,8 +676,11 @@ the smallest does not.
                                     below)
         YYYY-MM-DD.<fileID>.json    the file's basic metadata fields quak
                                     keeps, its update time, its private and
-                                    public magic metadata, its ML data, and
-                                    its original's EXIF, XMP and dimensions
+                                    public magic metadata, its ML data, and,
+                                    for an image (for a live photo, its
+                                    image), its original's EXIF and XMP, with
+                                    dimensions for a JPEG only; none of these
+                                    three for a video
         YYYY-MM-DD.<fileID>.livephoto.json
                                     which of a live photo's two files is which
     collections/
@@ -611,6 +689,7 @@ 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
 ```
 
@@ -655,6 +734,23 @@ 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; `quak backup` takes
+it before it opens its library, so a refused run sends no request. 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. A run
+stopped with Ctrl-C or `kill` removes the lock as it exits. Only a run that
+cannot, such as one killed with SIGKILL or cut off by a crash or power loss,
+leaves it 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
@@ -687,6 +783,25 @@ if any files failed. `quak backup` opens its library with the thumbnail and
 originals precache off, so the only file content it fetches is the originals the
 backup stores.
 
+With `--verify`, or `lib.backup({ verify: true })`, a run also hashes each
+original already at its save path the way a download is checked (see "On-disk
+cache layout" below): its bytes, read in chunks, or a live photo's image and
+video, joined as `<imageHash>:<videoHash>`. An original that matches the content
+hash its metadata records is left as it is. One that does not is logged on one
+line naming the file, deleted (a live photo's image and video both), and put
+back in the same run like a missing one: downloaded, or copied from the cache if
+the cache holds it. What is put back is hashed too, because a copy from the
+cache is not checked as a download is. If it still does not match, it stays at
+its save path and the file goes into `failures.json`, as it does when the
+download fails. A file whose metadata records no hash is left as it is and
+counted as unchecked. A stored original that cannot be read is left as it is and
+counts as failed. The summary and `--json` add the counts `verified`,
+`mismatched` and `unchecked`, all of originals that were already stored; one
+first downloaded in this run is in none of them. A mismatch that was put back
+with matching bytes does not make the exit code non-zero. Without `--verify`
+nothing is hashed, the summary is unchanged, and the three counts are 0 in
+`--json`.
+
 Each original is written to a temporary file in the same directory, synced to
 disk, and renamed into place, so an original is either complete or absent, even
 after a power cut. A downloaded original's temporary file is named
@@ -902,18 +1017,25 @@ 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
+- `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. 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
+  the caller passes `lockHeld: true` to the backup. 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.
+  rebuilds the on-disk backup tree with a durable failure ledger. Each file's
+  JSON holds its ML data and, for an image (for a live photo, its image), its
+  original's EXIF and XMP, with its dimensions for a JPEG only; a video's JSON
+  holds none of these three. 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`, `verify` (default
+  `false`), `onProgress`, and `lockHeld` (default `false`). See Backup layout
+  above for the tree it writes.
 
 ### Request pools
 

TODO.md

@@ -25,6 +25,35 @@ declares one.
 
 # Completed Steps
 
+- 2026-10-06: The README says how to run `quak backup` from cron (issue 170):
+  log in once as the job's user, a crontab with a backup every night and
+  `--verify` on Sundays appending to a log file, and the `HOME` and
+  `XDG_DATA_HOME` the job needs to find the saved session. A table gives each
+  exit code: 0, the backup is complete; 1, files are in `failures.json` or
+  another error stopped the run; 2, another backup of the directory is running;
+  3, there is no usable session. The introduction lists everything the backup
+  keeps for each file.
+
+- 2026-10-06: `quak backup --verify` and `lib.backup({ verify: true })` hash
+  each original already at its save path as the download check does, streamed, a
+  live photo as `<imageHash>:<videoHash>` (issue 168). One that does not match
+  the content hash its metadata records is logged, removed (both files of a live
+  photo) and put back in the same run, downloaded or copied from the cache, and
+  what is put back is hashed too. One that still does not match, or whose
+  download fails, goes into `failures.json`. One with no recorded hash is left
+  alone. The result, `--json` and the summary gain `verified`, `mismatched` and
+  `unchecked`. Without `--verify` nothing is hashed.
+
+- 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` takes the lock before it opens its library, so a refused run
+  sends no request; it prints the error as one line and exits 2. A lock that has
+  gone 10 seconds untouched, left by a run that could not remove it, 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 `{}`
@@ -34,6 +63,16 @@ declares one.
   field is taken from that JSON. The hand-built JPEGs moved to
   `test/exif-jpeg.ts`, beside the HEIC.
 
+- 2026-10-06: When the server answers HTTP 401 and that ends a command that
+  loads the saved session, because the server no longer accepts its token, the
+  command prints one line,
+  `quak: the saved session is no longer valid; run "quak login"`, and exits 3
+  (issue 164). `quak backup` meets that 401 on the refresh it starts with,
+  before it touches any file. For the same commands, a missing or corrupt
+  session file keeps its message and also exits 3, so a cron job can tell that
+  the user must log in again. `quak logout` is unchanged. `run` in
+  `src/cli-run.ts` recognises the 401, which reaches it unchanged.
+
 - 2026-10-06: `quak backup` retries a failed request for longer than the other
   commands do (issue 165). `src/retry.ts` exports `UNATTENDED_RETRY_OPTIONS`
   beside the unchanged default: 10 attempts, a 1 s base delay and a 60 s cap, so

bin/quak.ts

@@ -130,9 +130,13 @@ program
     )
     .argument("<dir>", "Output directory")
     .option("--json", "Print result as JSON instead of human-readable summary")
+    .option(
+        "--verify",
+        "Re-hash stored originals and download again any that do not match",
+    )
     // A backup usually runs from cron with nobody watching, so every request
     // it makes retries for longer than the other commands' requests do.
-    .action((dir: string, opts: { json?: boolean }) =>
+    .action((dir: string, opts: { json?: boolean; verify?: boolean }) =>
         run(
             backupCommand(
                 {

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 +1,13 @@
 // 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` (unless its caller
+// already holds it), 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 +19,7 @@
 //         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
@@ -35,6 +38,11 @@
 // why they could not be read), so that a run does not read every stored
 // original again; a sidecar without them gets them read from the original.
 //
+// With `verify`, each original already at its save path is hashed as the
+// download check hashes it, and one that does not match the content hash its
+// metadata records is removed and fetched again in the same run. What is put
+// back is hashed too, and recorded as failed if it still does not match.
+//
 // Resilience (issue #8): no per-file condition aborts the run. A failed
 // download, a failed symlink, or ML data missing because the ML data fetch
 // failed is caught, recorded in `failures.json` with a classification, a
@@ -47,6 +55,7 @@
 // code.
 
 import {
+    createReadStream,
     lstatSync,
     mkdirSync,
     readdirSync,
@@ -60,7 +69,14 @@ 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 {
+    chunkHashFinal,
+    chunkHashInit,
+    chunkHashUpdate,
+    init,
+} from "./crypto/index.js";
 import { removeLeftoverTempFiles } from "./download/index.js";
 import { sanitizeFileName, withExtension } from "./filename.js";
 import {
@@ -88,7 +104,14 @@ export interface BackupOptions {
     includeThumbnails?: boolean;
     // Restrict the backup to albums with these names; others are left untouched.
     onlyAlbumNames?: string[];
+    // Hash each original already at its save path, and fetch again any whose
+    // 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
+    // `lockBackupDirectory`, and releases it itself, so the backup does not
+    // take it. `quak backup` takes it before it opens its library.
+    lockHeld?: boolean;
 }
 
 export interface BackupError {
@@ -105,6 +128,13 @@ export interface BackupResult {
     downloaded: number;
     // Originals already at their save path and left untouched.
     skipped: number;
+    // With `verify`, the originals already at their save path whose hash
+    // matched, those whose hash did not (each removed and fetched again), and
+    // those whose metadata records no hash (left as they are). All three are
+    // zero without `verify`.
+    verified: number;
+    mismatched: number;
+    unchecked: number;
     // Files with an unresolved failure after this run (the ledger size); the
     // CLI exits non-zero while this is above zero. A file can be both
     // downloaded and failed if its bytes landed but its symlink did not.
@@ -361,6 +391,26 @@ const saveLedger = (path: string, ledger: Map<number, FailureEntry>): void => {
     );
 };
 
+// The content hash of the original stored at `stored`, computed as the download
+// check computes it: over each file's bytes, read in chunks, and for a live
+// photo `<imageHash>:<videoHash>`.
+const storedHash = async (stored: {
+    path: string;
+    videoPath?: string;
+}): Promise<string> => {
+    await init();
+    const hashFile = async (path: string): Promise<string> => {
+        const state = chunkHashInit();
+        for await (const chunk of createReadStream(path)) {
+            chunkHashUpdate(state, chunk as Buffer);
+        }
+        return chunkHashFinal(state);
+    };
+    const hash = await hashFile(stored.path);
+    if (stored.videoPath === undefined) return hash;
+    return `${hash}:${await hashFile(stored.videoPath)}`;
+};
+
 // A file's EXIF, XMP and dimensions as its JSON holds them: what
 // `extractImageMetadata` found in its original, or why the original could not
 // be read.
@@ -450,21 +500,17 @@ const writeAlbumJSON = (
     writeFileSync(path, JSON.stringify(album, null, 2));
 };
 
-export const runBackup = async (
+// The backup itself, which `runBackup` below runs while the lock is held.
+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 ?? (() => {});
     const only = opts.onlyAlbumNames ? new Set(opts.onlyAlbumNames) : undefined;
+    const verify = opts.verify ?? false;
 
     log("Refreshing library...");
     await lib.refresh();
@@ -523,6 +569,9 @@ export const runBackup = async (
     const storedThisRun = new Set<number>();
     let downloaded = 0;
     let skipped = 0;
+    let verified = 0;
+    let mismatched = 0;
+    let unchecked = 0;
 
     const recordFailure = (
         file: EnteFile,
@@ -554,10 +603,47 @@ export const runBackup = async (
 
     // Phase 1: get the bytes. Put each pending original at its save path
     // through the content cache/pools, as `Photo.download()` does, and fetch
-    // the optional thumbnails; a present file is left as is.
+    // the optional thumbnails; a present file is left as is. With `verify`, a
+    // present original is hashed first, and one that does not match the hash
+    // its metadata records is removed and fetched like a missing one. One
+    // that cannot be read is recorded as failed and left where it is.
     if (includeOriginals) {
         for (const [fileID, file] of distinct) {
-            if (storedAtSavePath(downloadDirectory, file) !== undefined) {
+            let stored = storedAtSavePath(downloadDirectory, file);
+            let mismatch = false;
+            if (stored !== undefined && verify) {
+                try {
+                    if (file.metadata.hash === undefined) {
+                        unchecked++;
+                    } else if (
+                        (await storedHash(stored)) === file.metadata.hash
+                    ) {
+                        verified++;
+                    } else {
+                        log(
+                            `MISMATCH original ${file.metadata.title} (${fileID}): its bytes do not match its content hash`,
+                        );
+                        mismatched++;
+                        mismatch = true;
+                        rmSync(stored.path);
+                        if (stored.videoPath !== undefined) {
+                            rmSync(stored.videoPath);
+                        }
+                        stored = undefined;
+                    }
+                } catch (err) {
+                    log(
+                        `FAILED verifying original ${file.metadata.title}: ${errorMessage(err)}`,
+                    );
+                    recordFailure(
+                        file,
+                        collectionName.get(file.collectionID) ?? "",
+                        err,
+                    );
+                    continue;
+                }
+            }
+            if (stored !== undefined) {
                 skipped++;
                 continue;
             }
@@ -566,10 +652,24 @@ export const runBackup = async (
                 // A fetched original is written straight to its save path (a
                 // live photo beside it); only one that was already cached
                 // elsewhere is copied.
-                await placeOriginal(downloadDirectory, file, (dest) =>
-                    lib.original(fileID, dest),
+                const placed = await placeOriginal(
+                    downloadDirectory,
+                    file,
+                    (dest) => lib.original(fileID, dest),
                 );
                 storedThisRun.add(fileID);
+                // A copy from the cache is not checked as a download is and can
+                // hold the same bad bytes, so what is put back after a
+                // mismatch is hashed too. A bad copy stays where it is and the
+                // file fails.
+                if (
+                    mismatch &&
+                    (await storedHash(placed)) !== file.metadata.hash
+                ) {
+                    throw new Error(
+                        "the original put back does not match its content hash either",
+                    );
+                }
                 downloaded++;
             } catch (err) {
                 log(
@@ -732,7 +832,58 @@ export const runBackup = async (
         totalFiles: distinct.size,
         downloaded,
         skipped,
+        verified,
+        mismatched,
+        unchecked,
         failed: ledger.size,
         errors,
     };
 };
+
+// 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,
+// 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 it is held. One it has not touched for 10 seconds was left by
+// a run that could not remove it, and is taken over.
+export const lockBackupDirectory = async (
+    downloadDirectory: string,
+): Promise<() => Promise<void>> => {
+    mkdirSync(downloadDirectory, { recursive: true });
+    try {
+        return 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" },
+        );
+    }
+};
+
+// Runs the backup holding the lock, which it takes and releases itself unless
+// the caller already holds it (`opts.lockHeld`).
+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)",
+        );
+    }
+    if (opts.lockHeld) {
+        return runLockedBackup(lib, opts, downloadDirectory);
+    }
+    const release = await lockBackupDirectory(downloadDirectory);
+    try {
+        return await runLockedBackup(lib, opts, downloadDirectory);
+    } finally {
+        await release();
+    }
+};

src/cli-commands.ts

@@ -20,6 +20,7 @@ import {
     type ClientSnapshot,
     type LoginOptions,
 } from "./client.js";
+import { lockBackupDirectory } from "./backup.js";
 import { init } from "./crypto/index.js";
 import {
     defaultCacheDirectory,
@@ -73,7 +74,9 @@ export const saveSession = (
     );
 };
 
-// The saved client, or undefined after telling the user why there is none.
+// The saved client, or undefined after telling the user why there is none. The
+// command then exits 3, the code for "log in again", as `run` in `cli-run.ts`
+// does when the server no longer accepts the saved session.
 const requireSession = (ctx: CliContext): Client | undefined => {
     let client: Client | null;
     try {
@@ -150,7 +153,7 @@ export const loginCommand = async (ctx: CliContext): Promise<number> => {
 export const whoamiCommand = async (ctx: CliContext): Promise<number> => {
     await init();
     const client = requireSession(ctx);
-    if (!client) return 1;
+    if (!client) return 3;
     const info = client.whoami();
     ctx.stdout.write(JSON.stringify(info) + "\n");
     return 0;
@@ -201,7 +204,7 @@ export const collectionsCommand = async (
 ): Promise<number> => {
     await init();
     const client = requireSession(ctx);
-    if (!client) return 1;
+    if (!client) return 3;
     const lib = await openReadLibrary(ctx, client);
     try {
         // Force a server round-trip and list in enumeration order (issue #36
@@ -243,7 +246,7 @@ export const filesCommand = async (
 ): Promise<number> => {
     await init();
     const client = requireSession(ctx);
-    if (!client) return 1;
+    if (!client) return 3;
     const collectionID = Number(opts.collection);
     if (!Number.isFinite(collectionID)) {
         ctx.stderr.write("Invalid collection ID\n");
@@ -285,7 +288,7 @@ export const getCommand = async (
 ): Promise<number> => {
     await init();
     const client = requireSession(ctx);
-    if (!client) return 1;
+    if (!client) return 3;
     const fileID = Number(fileIDStr);
     if (!Number.isFinite(fileID)) {
         ctx.stderr.write("Invalid file ID\n");
@@ -343,7 +346,7 @@ export const getThumbCommand = async (
 ): Promise<number> => {
     await init();
     const client = requireSession(ctx);
-    if (!client) return 1;
+    if (!client) return 3;
     const fileID = Number(fileIDStr);
     if (!Number.isFinite(fileID)) {
         ctx.stderr.write("Invalid file ID\n");
@@ -380,7 +383,7 @@ export const backupMetadataCommand = async (
 ): Promise<number> => {
     await init();
     const client = requireSession(ctx);
-    if (!client) return 1;
+    if (!client) return 3;
     const lib = await openReadLibrary(ctx, client);
     try {
         // Refresh first so the dump holds current account state, not what the
@@ -399,51 +402,72 @@ export const backupMetadataCommand = async (
 export const backupCommand = async (
     ctx: CliContext,
     dir: string,
-    opts: { json?: boolean },
+    opts: { json?: boolean; verify?: boolean },
 ): Promise<number> => {
     await init();
     const client = requireSession(ctx);
-    if (!client) return 1;
+    if (!client) return 3;
 
     ctx.stderr.write("Starting backup...\n");
-    // The precache is off: the backup fetches what it needs, and must not
-    // also fill the cache with every thumbnail and the recent originals.
-    const lib = await Library.open({
-        client,
-        downloadDirectory: dir,
-        cacheDirectory: ctx.cacheDir,
-        precacheThumbnails: false,
-        precacheOriginals: false,
-    });
+    // 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>;
     try {
-        const result = await lib.backup({
+        release = await lockBackupDirectory(dir);
+    } catch (err) {
+        if ((err as NodeJS.ErrnoException).code !== "ELOCKED") throw err;
+        ctx.stderr.write(`quak: ${(err as Error).message}\n`);
+        return 2;
+    }
+    try {
+        // The precache is off: the backup fetches what it needs, and must not
+        // also fill the cache with every thumbnail and the recent originals.
+        const lib = await Library.open({
+            client,
             downloadDirectory: dir,
-            onProgress: (msg) => {
-                if (!opts.json) ctx.stderr.write(msg + "\n");
-            },
+            cacheDirectory: ctx.cacheDir,
+            precacheThumbnails: false,
+            precacheOriginals: false,
         });
+        try {
+            const result = await lib.backup({
+                downloadDirectory: dir,
+                lockHeld: true,
+                verify: opts.verify,
+                onProgress: (msg) => {
+                    if (!opts.json) ctx.stderr.write(msg + "\n");
+                },
+            });
 
-        if (opts.json) {
-            ctx.stdout.write(JSON.stringify(result, null, 2) + "\n");
-        } else {
-            ctx.stderr.write("\n--- Backup complete ---\n");
-            ctx.stderr.write(`  Total files: ${result.totalFiles}\n`);
-            ctx.stderr.write(`  Downloaded:  ${result.downloaded}\n`);
-            ctx.stderr.write(`  Skipped:     ${result.skipped}\n`);
-            ctx.stderr.write(`  Failed:      ${result.failed}\n`);
-            if (result.errors.length > 0) {
-                ctx.stderr.write("\nFailed files:\n");
-                for (const e of result.errors) {
-                    ctx.stderr.write(
-                        `  [${e.collection}] ${e.title} (id ${e.fileID}): ${e.error}\n`,
-                    );
+            if (opts.json) {
+                ctx.stdout.write(JSON.stringify(result, null, 2) + "\n");
+            } else {
+                ctx.stderr.write("\n--- Backup complete ---\n");
+                ctx.stderr.write(`  Total files: ${result.totalFiles}\n`);
+                ctx.stderr.write(`  Downloaded:  ${result.downloaded}\n`);
+                ctx.stderr.write(`  Skipped:     ${result.skipped}\n`);
+                if (opts.verify) {
+                    ctx.stderr.write(`  Verified:    ${result.verified}\n`);
+                    ctx.stderr.write(`  Mismatched:  ${result.mismatched}\n`);
+                    ctx.stderr.write(`  Unchecked:   ${result.unchecked}\n`);
+                }
+                ctx.stderr.write(`  Failed:      ${result.failed}\n`);
+                if (result.errors.length > 0) {
+                    ctx.stderr.write("\nFailed files:\n");
+                    for (const e of result.errors) {
+                        ctx.stderr.write(
+                            `  [${e.collection}] ${e.title} (id ${e.fileID}): ${e.error}\n`,
+                        );
+                    }
                 }
             }
-        }
 
-        return result.failed > 0 ? 1 : 0;
+            return result.failed > 0 ? 1 : 0;
+        } finally {
+            await lib.close();
+        }
     } finally {
-        await lib.close();
+        await release();
     }
 };
 
@@ -453,7 +477,7 @@ export const listMissingThumbnailsCommand = async (
 ): Promise<number> => {
     await init();
     const client = requireSession(ctx);
-    if (!client) return 1;
+    if (!client) return 3;
     const lib = await openReadLibrary(ctx, client);
     try {
         // Refresh first so files added since the cache was written are
@@ -491,7 +515,7 @@ export const fixMissingThumbnailsCommand = async (
 ): Promise<number> => {
     await init();
     const client = requireSession(ctx);
-    if (!client) return 1;
+    if (!client) return 3;
     const lib = await openReadLibrary(ctx, client);
     try {
         // Refresh first so files added since the cache was written are found;

src/cli-run.ts

@@ -1,12 +1,15 @@
 // Runs one CLI command for `bin/quak.ts` and exits with its code.
 
 import type { Writable } from "node:stream";
+import { ApiError } from "./api/client.js";
 
 // Run a command and exit with its code once stdout/stderr have drained.
 // Exiting before the drain can truncate piped output, and the library can keep
 // the event loop alive after a command returns, so a plain return could hang.
 // An error the command throws is printed as one `quak: MESSAGE` line, without
-// the stack trace, and exits 1.
+// the stack trace, and exits 1. A 401 from the server means it no longer
+// accepts the saved session: that prints one line saying to log in again and
+// exits 3, as a missing or corrupt session file does.
 export const run = async (
     command: Promise<number>,
     stdout: Writable,
@@ -17,10 +20,17 @@ export const run = async (
     try {
         code = await command;
     } catch (err) {
-        stderr.write(
-            `quak: ${err instanceof Error ? err.message : String(err)}\n`,
-        );
-        code = 1;
+        if (err instanceof ApiError && err.status === 401) {
+            stderr.write(
+                `quak: the saved session is no longer valid; run "quak login"\n`,
+            );
+            code = 3;
+        } else {
+            stderr.write(
+                `quak: ${err instanceof Error ? err.message : String(err)}\n`,
+            );
+            code = 1;
+        }
     }
     const pending = [stdout, stderr].filter((s) => s.writableLength > 0);
     if (pending.length === 0) {

src/index.ts

@@ -67,6 +67,7 @@ export {
     type EnsureOptions,
     type EnsureResult,
     type EnsureEvent,
+    lockBackupDirectory,
     runBackup,
     type BackupOptions,
     type BackupResult,

src/library/index.ts

@@ -94,6 +94,7 @@ import type { Collection, EnteFile } from "../model/types.js";
 import { runBackup, type BackupOptions, type BackupResult } from "../backup.js";
 
 export {
+    lockBackupDirectory,
     runBackup,
     type BackupOptions,
     type BackupResult,
@@ -566,13 +567,16 @@ 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, unless
+    // `opts.lockHeld` says the caller holds it, 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";
@@ -61,6 +62,7 @@ import { HEIC_WITH_EXIF } from "../exif-heic.js";
 import { JPEG_WITH_EXIF } from "../exif-jpeg.js";
 import {
     asLivePhoto,
+    blake2b,
     cdnSource,
     IMAGE,
     livePhotoHash,
@@ -830,7 +832,81 @@ 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();
     });
 });
@@ -1196,6 +1272,181 @@ describe("image metadata in each file's JSON", () => {
     });
 });
 
+// With `verify`, each original already stored is hashed, and one that does not
+// match the content hash its metadata records is downloaded again.
+describe("backup with verify", () => {
+    // MockClient's files, each recording the hash of what `stubSource` writes
+    // for it, except diagram.png (200), which records none.
+    class HashedClient extends MockClient {
+        override async filesSince(args: {
+            collectionID: number;
+        }): Promise<FilesPage> {
+            const page = await super.filesSince(args);
+            const files = page.files.map((f) =>
+                f.id === 200
+                    ? f
+                    : {
+                          ...f,
+                          metadata: {
+                              ...f.metadata,
+                              hash: blake2b(Buffer.alloc(SIZE_BY_ID[f.id]!)),
+                          },
+                      },
+            );
+            return { ...page, files };
+        }
+    }
+
+    // A backup of the account in `root/backup`, the library that made it, and
+    // the source it fetched from.
+    const backedUp = async (): Promise<{
+        lib: Library;
+        source: StubSource;
+        outDir: string;
+    }> => {
+        const source = stubSource();
+        const lib = await openLibrary(source, new HashedClient());
+        const outDir = join(root, "backup");
+        await lib.backup({ downloadDirectory: outDir });
+        return { lib, source, outDir };
+    };
+
+    // What `stubSource` writes for beach.jpg (100), and other bytes of the
+    // same length.
+    const good = Buffer.alloc(SIZE_BY_ID[100]!);
+    const corrupt = Buffer.alloc(SIZE_BY_ID[100]!, 1);
+
+    it("leaves an original that matches its hash, and one with no hash, as they are", async () => {
+        const { lib, source, outDir } = await backedUp();
+        const calls = source.originalCalls;
+
+        const result = await lib.backup({
+            downloadDirectory: outDir,
+            verify: true,
+        });
+
+        expect(result).toMatchObject({
+            downloaded: 0,
+            skipped: 3,
+            verified: 2,
+            mismatched: 0,
+            unchecked: 1,
+            failed: 0,
+        });
+        expect(source.originalCalls).toBe(calls);
+        await lib.close();
+    });
+
+    it("downloads again an original that does not match its hash", async () => {
+        const { lib, outDir } = await backedUp();
+        writeFileSync(saved(outDir, "100.jpg"), corrupt);
+        const log: string[] = [];
+
+        const result = await lib.backup({
+            downloadDirectory: outDir,
+            verify: true,
+            onProgress: (msg) => log.push(msg),
+        });
+
+        expect(result).toMatchObject({
+            downloaded: 1,
+            skipped: 2,
+            verified: 1,
+            mismatched: 1,
+            unchecked: 1,
+            failed: 0,
+        });
+        expect(readFileSync(saved(outDir, "100.jpg"))).toEqual(good);
+        expect(log.filter((msg) => msg.startsWith("MISMATCH"))).toEqual([
+            "MISMATCH original beach.jpg (100): its bytes do not match its content hash",
+        ]);
+        await lib.close();
+    });
+
+    it("records a failed download in failures.json, with the original removed", async () => {
+        const { lib, source, outDir } = await backedUp();
+        writeFileSync(saved(outDir, "100.jpg"), corrupt);
+        source.failID = 100;
+
+        const result = await lib.backup({
+            downloadDirectory: outDir,
+            verify: true,
+        });
+
+        expect(result).toMatchObject({
+            downloaded: 0,
+            mismatched: 1,
+            failed: 1,
+        });
+        expect(Object.keys(readLedger(outDir).files)).toEqual(["100"]);
+        expect(existsSync(saved(outDir, "100.jpg"))).toBe(false);
+        await lib.close();
+    });
+
+    it("records in failures.json an original the cache puts back with the same bad bytes", async () => {
+        const lib = await openLibrary(stubSource(), new HashedClient());
+        const outDir = join(root, "backup");
+        // The cache holds a bad copy, and a backup copies an original the
+        // cache holds to its save path.
+        const cached = await lib.photos.byID({ fileID: 100 })!.original();
+        writeFileSync(cached.path, corrupt);
+        await lib.backup({ downloadDirectory: outDir });
+
+        const result = await lib.backup({
+            downloadDirectory: outDir,
+            verify: true,
+        });
+
+        expect(result).toMatchObject({
+            downloaded: 0,
+            mismatched: 1,
+            failed: 1,
+        });
+        expect(result.errors.map((e) => e.error)).toEqual([
+            "the original put back does not match its content hash either",
+        ]);
+        expect(Object.keys(readLedger(outDir).files)).toEqual(["100"]);
+        expect(readFileSync(saved(outDir, "100.jpg"))).toEqual(corrupt);
+        await lib.close();
+    });
+
+    it("hashes nothing without verify", async () => {
+        const { lib, outDir } = await backedUp();
+        writeFileSync(saved(outDir, "100.jpg"), corrupt);
+
+        const result = await lib.backup({ downloadDirectory: outDir });
+
+        expect(result).toMatchObject({
+            downloaded: 0,
+            skipped: 3,
+            verified: 0,
+            mismatched: 0,
+            unchecked: 0,
+            failed: 0,
+        });
+        expect(readFileSync(saved(outDir, "100.jpg"))).toEqual(corrupt);
+        await lib.close();
+    });
+
+    // Root ignores file permissions, so this fails when run as root. The
+    // `test` phase of the `Dockerfile` runs as the `node` user.
+    it("records an original it cannot read as failed, and leaves it", async () => {
+        const { lib, outDir } = await backedUp();
+        const original = saved(outDir, "100.jpg");
+        chmodSync(original, 0o000);
+
+        const result = await lib
+            .backup({ downloadDirectory: outDir, verify: true })
+            .finally(() => chmodSync(original, 0o600));
+
+        expect(result).toMatchObject({ downloaded: 0, verified: 1, failed: 1 });
+        expect(result.errors.map((e) => e.fileID)).toEqual([100]);
+        expect(result.errors[0]!.error).toMatch(/EACCES/);
+        expect(readFileSync(original)).toEqual(good);
+        await lib.close();
+    });
+});
+
 // Every entry under collections/, one level of directories deep, with each
 // symlink's target.
 const tree = (outDir: string): string[] => {
@@ -1691,6 +1942,64 @@ describe("backup of live photos", () => {
         },
     );
 
+    it("verifies a live photo's image and video together, and downloads it again when one does not match", async () => {
+        const { file: live, body } = await asLivePhoto(
+            file(500, 10, "IMG_0500.HEIC"),
+        );
+        const lib = await open([live], new Map([[500, body]]));
+        const outDir = join(root, "backup");
+        await lib.backup({ downloadDirectory: outDir });
+
+        const good = await lib.backup({
+            downloadDirectory: outDir,
+            verify: true,
+        });
+        expect(good).toMatchObject({ skipped: 1, verified: 1, mismatched: 0 });
+
+        const video = saved(outDir, "500.mov");
+        writeFileSync(video, "another few seconds of video");
+        const bad = await lib.backup({
+            downloadDirectory: outDir,
+            verify: true,
+        });
+
+        expect(bad).toMatchObject({
+            downloaded: 1,
+            verified: 0,
+            mismatched: 1,
+            failed: 0,
+        });
+        expect(readFileSync(saved(outDir, "500.heic"))).toEqual(
+            Buffer.from(IMAGE),
+        );
+        expect(readFileSync(video)).toEqual(Buffer.from(VIDEO));
+        expect(tree(outDir)).toEqual(linked);
+        await lib.close();
+    });
+
+    it("removes both files of a live photo that does not match its hash when downloading it again fails", async () => {
+        const { file: live, body } = await asLivePhoto(
+            file(500, 10, "IMG_0500.HEIC"),
+        );
+        const bodies = new Map([[500, body]]);
+        const lib = await open([live], bodies);
+        const outDir = join(root, "backup");
+        await lib.backup({ downloadDirectory: outDir });
+        writeFileSync(saved(outDir, "500.mov"), "another few seconds of video");
+        bodies.delete(500);
+
+        const result = await lib.backup({
+            downloadDirectory: outDir,
+            verify: true,
+        });
+
+        expect(result).toMatchObject({ mismatched: 1, failed: 1 });
+        expect(Object.keys(readLedger(outDir).files)).toEqual(["500"]);
+        expect(existsSync(saved(outDir, "500.heic"))).toBe(false);
+        expect(existsSync(saved(outDir, "500.mov"))).toBe(false);
+        await lib.close();
+    });
+
     it("serves a live photo the backup stored to a library reading the backup", async () => {
         const { file: live, body } = await asLivePhoto(
             file(500, 10, "IMG_0500.HEIC"),

test/cli/commands.test.ts

@@ -18,6 +18,7 @@ import {
     readFileSync,
     rmSync,
     statSync,
+    utimesSync,
     writeFileSync,
 } from "node:fs";
 import { join } from "node:path";
@@ -52,13 +53,14 @@ import {
 import { run } from "../../src/cli-run.js";
 import { loadSession } from "../../src/cli-session.js";
 import type { Client, ClientSnapshot, LoginOptions } from "../../src/client.js";
-import type { ContentSource } from "../../src/library/content.js";
+import { savePath, type ContentSource } from "../../src/library/content.js";
 import type { Collection, EnteFile } from "../../src/model/types.js";
 import { init, toBase64 } from "../../src/crypto/index.js";
 import { defaultCacheDirectory } from "../../src/library/index.js";
 import { HEIC_WITH_EXIF } from "../exif-heic.js";
 import {
     asLivePhoto,
+    blake2b,
     cdnSource,
     IMAGE,
     livePhotoHash,
@@ -230,20 +232,30 @@ describe("session file", () => {
         expect(JSON.parse(readFileSync(path, "utf-8"))).toEqual(snapshot);
     });
 
-    it("a missing session exits 1 with 'Not logged in'", async () => {
+    it("a missing session exits 3 with 'Not logged in' from every command that needs one", async () => {
         const ctx = { ...context(), loadSession };
-        expect(await whoamiCommand(ctx)).toBe(1);
-        expect(stderr.text).toBe(
+        const dir = join(root, "backup");
+        expect(await whoamiCommand(ctx)).toBe(3);
+        expect(await collectionsCommand(ctx, {})).toBe(3);
+        expect(await filesCommand(ctx, { collection: "1" })).toBe(3);
+        expect(await getCommand(ctx, "100", {})).toBe(3);
+        expect(await getThumbCommand(ctx, "100", {})).toBe(3);
+        expect(await backupMetadataCommand(ctx, dir, {})).toBe(3);
+        expect(await backupCommand(ctx, dir, {})).toBe(3);
+        expect(await listMissingThumbnailsCommand(ctx, {})).toBe(3);
+        expect(await fixMissingThumbnailsCommand(ctx, {})).toBe(3);
+        const notLoggedIn =
             `Not logged in. Run "quak login" first.\n` +
-                `Session file: ${join(ctx.sessionDir, "session.json")}\n`,
-        );
+            `Session file: ${join(ctx.sessionDir, "session.json")}\n`;
+        expect(stderr.text).toBe(notLoggedIn.repeat(9));
         expect(stdout.text).toBe("");
+        expect(existsSync(dir)).toBe(false);
     });
 
-    it("a corrupt session exits 1 and says it is corrupt", async () => {
+    it("a corrupt session exits 3 and says it is corrupt", async () => {
         const ctx = { ...context(), loadSession };
         saveSession(ctx.sessionDir, snapshot);
-        expect(await collectionsCommand(ctx, {})).toBe(1);
+        expect(await collectionsCommand(ctx, {})).toBe(3);
         expect(stderr.text).toContain("is corrupt");
         expect(stderr.text).toContain(
             `Run "quak logout" and then "quak login" to replace it.\n`,
@@ -693,6 +705,7 @@ describe("backup", () => {
                 "  Failed:      0\n",
         );
         expect(stdout.text).toBe("");
+        expect(existsSync(join(dir, "backup.lock"))).toBe(false);
     });
 
     // The backup opens its library with the precache off: it fetches the
@@ -732,6 +745,61 @@ describe("backup", () => {
         expect(stderr.text).toBe("Starting backup...\n");
     });
 
+    it("--verify downloads again an original that does not match its hash, prints the counts, and exits 0", async () => {
+        // Each file records the hash of the original the fake writes for it.
+        const client = {
+            ...fakeClient(),
+            filesSince: async (args: { collectionID: number }) => ({
+                files: (FILES[args.collectionID] ?? []).map((f) => ({
+                    ...f,
+                    metadata: {
+                        ...f.metadata,
+                        hash: blake2b(Buffer.alloc(7, f.id & 0xff)),
+                    },
+                })),
+                deleted: [],
+                cursor: 1,
+            }),
+        } as unknown as Client;
+        const ctx = context(client);
+        const dir = join(root, "backup");
+        expect(await backupCommand(ctx, dir, {})).toBe(0);
+        writeFileSync(savePath(dir, FILES[1]![0]!), "corrupt");
+
+        expect(await backupCommand(ctx, dir, { verify: true })).toBe(0);
+
+        expect(stderr.text).toContain(
+            "MISMATCH original beach.jpg (100): its bytes do not match its content hash\n",
+        );
+        expect(stderr.text).toContain(
+            "  Downloaded:  1\n" +
+                "  Skipped:     2\n" +
+                "  Verified:    2\n" +
+                "  Mismatched:  1\n" +
+                "  Unchecked:   0\n" +
+                "  Failed:      0\n",
+        );
+    });
+
+    it("--verify --json adds the verified, mismatched and unchecked counts", async () => {
+        const dir = join(root, "backup");
+        expect(await backupCommand(context(), dir, {})).toBe(0);
+
+        const code = await backupCommand(context(), dir, {
+            verify: true,
+            json: true,
+        });
+
+        expect(code).toBe(0);
+        expect(JSON.parse(stdout.text)).toMatchObject({
+            skipped: 3,
+            verified: 0,
+            mismatched: 0,
+            unchecked: 3,
+            failed: 0,
+        });
+    });
+
     it("exits 1 and lists each file when the ML data fetch fails", async () => {
         const client = {
             ...fakeClient(),
@@ -748,15 +816,12 @@ describe("backup", () => {
         );
     });
 
-    it("exits 1 with the error on one line when the refresh fails", async () => {
-        const client = {
-            ...fakeClient(),
-            collectionsSince: async () => {
-                throw new Error("HTTP 401 from server");
-            },
-        } as unknown as Client;
-        const dir = join(root, "backup");
-        // Through `run`, as `bin/quak.ts` does, which prints a thrown error.
+    // Runs `backup` through `run`, as `bin/quak.ts` does, which prints a thrown
+    // error; returns the exit code and what `run` printed.
+    const backupThroughRun = async (
+        ctx: CliContext,
+        dir: string,
+    ): Promise<{ code: number; runText: string }> => {
         const runStderr = new PassThrough();
         let runText = "";
         runStderr.on("data", (chunk: Buffer) => {
@@ -764,16 +829,90 @@ describe("backup", () => {
         });
         const code = await new Promise<number>((resolve) => {
             void run(
-                backupCommand(context(client), dir, {}),
+                backupCommand(ctx, dir, {}),
                 new PassThrough(),
                 runStderr,
                 resolve,
             );
         });
+        return { code, runText };
+    };
+
+    it("exits 1 with the error on one line when the refresh fails", async () => {
+        const client = {
+            ...fakeClient(),
+            collectionsSince: async () => {
+                throw new Error("HTTP 503 from server");
+            },
+        } as unknown as Client;
+        const dir = join(root, "backup");
+        const { code, runText } = await backupThroughRun(context(client), dir);
         expect(code).toBe(1);
-        expect(runText).toBe("quak: HTTP 401 from server\n");
+        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
+    // every request with 401, as it does once it no longer accepts the token.
+    // The context's prompts throw, so a prompt would end the run with another
+    // line and exit 1.
+    it("exits 3 with one line saying to log in again when the server answers 401", async () => {
+        const key = toBase64(new Uint8Array(32));
+        saveSession(join(root, "session"), {
+            email: "cli@example.com",
+            userID: USER_ID,
+            token: "expired",
+            masterKey: key,
+            secretKey: key,
+            publicKey: key,
+        });
+        const unauthorized = async (): Promise<Response> =>
+            new Response(null, { status: 401 });
+        const ctx = {
+            ...context(),
+            loadSession: (path: string) =>
+                loadSession(path, { fetch: unauthorized }),
+        };
+        const dir = join(root, "backup");
+        const { code, runText } = await backupThroughRun(ctx, dir);
+        expect(code).toBe(3);
+        expect(runText).toBe(
+            `quak: the saved session is no longer valid; run "quak login"\n`,
+        );
+        expect(stderr.text).toBe("Starting backup...\nRefreshing library...\n");
+        expect(readdirSync(dir)).toEqual([]);
+    });
+
+    it("exits 2 with one line naming the directory, sending no request, 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);
+        // A client whose refresh never finishes, so a run that started one
+        // before it exits would never return.
+        let requests = 0;
+        const never = (): Promise<never> => {
+            requests++;
+            return new Promise(() => {});
+        };
+        const client = {
+            ...fakeClient(),
+            collectionsSince: never,
+            filesSince: never,
+        } as unknown as Client;
+
+        expect(await backupCommand(context(client), dir, {})).toBe(2);
+        expect(requests).toBe(0);
+        expect(stderr.text).toBe(
+            `Starting backup...\nquak: another backup of ${dir} is running\n`,
+        );
+        expect(readdirSync(dir)).toEqual(["backup.lock"]);
+        // Opening the library would have made its cache directory.
+        expect(existsSync(join(root, "cache"))).toBe(false);
     });
 });
 

test/cli/run.test.ts

@@ -5,6 +5,7 @@
 import { PassThrough } from "node:stream";
 import { describe, it, expect } from "vitest";
 
+import { ApiError } from "../../src/api/client.js";
 import { run } from "../../src/cli-run.js";
 
 // A stream whose written text is kept in `text`; writes finish at once, so
@@ -55,4 +56,26 @@ describe("run", () => {
             stderr: "quak: offline\n",
         });
     });
+
+    it("on a 401 from the server says to run quak login, on one line, and exits 3", async () => {
+        const result = await runToExit(
+            Promise.reject(new ApiError("unauthorized", 401)),
+        );
+        expect(result).toEqual({
+            code: 3,
+            stdout: "",
+            stderr: `quak: the saved session is no longer valid; run "quak login"\n`,
+        });
+    });
+
+    it("prints another HTTP error as it is and exits 1", async () => {
+        const result = await runToExit(
+            Promise.reject(new ApiError("forbidden", 403)),
+        );
+        expect(result).toEqual({
+            code: 1,
+            stdout: "",
+            stderr: "quak: forbidden\n",
+        });
+    });
 });

test/live-photo.ts

@@ -30,7 +30,8 @@ export const livePhotoZip = (
     },
 ): Uint8Array => zipSync(entries);
 
-const blake2b = (bytes: Uint8Array): string =>
+// The content hash Ente's clients record for an original's bytes.
+export const blake2b = (bytes: Uint8Array): string =>
     createHash("blake2b512").update(bytes).digest("base64");
 
 // The hash Ente's clients record for a live photo: the unkeyed BLAKE2b-512 of

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"