quak backup works on up to the content pool's limit of originals at once (closes #182)

The backup awaited each original before starting the next, so it used
one of the content pool's five slots. lib.backup(), and so quak backup,
now runs as many workers as the content pool's limit (5 by default, or
the contentConcurrency of the pools the library was opened with), each
taking the next file from one shared list, so the limit covers the
--verify hashing too. Once a worker has thrown or the lock is lost, no
worker takes another file; the backup waits for the files under way and
checks its lock before the ML data fetch. runBackup rejects a
contentConcurrency that is not a positive integer before it takes the
lock. Failed files are listed in the order they failed.

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 +83,66 @@ 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 an album it could not decrypt, 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 album quak could not
+decrypt, or a file in `failures.json` because quak could not decrypt it, keeps
+the exit code at `1` until the server sends a copy that decrypts, or deletes it
+(see "Backup layout"). 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
@@ -524,10 +587,10 @@ 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
@@ -549,7 +612,7 @@ 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
@@ -562,8 +625,17 @@ library. The read commands — `collections`, `files`, `get`, `get-thumb`,
 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, 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.
+handling"). An album or file quak cannot decrypt does not fail it; the command
+answers from what did decrypt (see "Default reads vs. fresh reads").
+`--cache-dir` overrides where the cache lives; without it each account gets its
+own directory under the per-user cache path.
+
+An ID given to `files --collection`, `get`, `get-thumb` or
+`helper fix-missing-thumbnails --file` is written in decimal digits, no larger
+than 9007199254740991. Anything else, such as `0x10`, `1e3` or `-1`, is refused
+before any request with one line, `Invalid file ID: <arg>` or
+`Invalid collection ID: <arg>`, and exit status 1. One bad `--file` ID refuses
+the whole command, so nothing is fixed.
 
 `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
@@ -582,6 +654,10 @@ 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
@@ -611,8 +687,11 @@ 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/
@@ -621,6 +700,7 @@ 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 +745,46 @@ 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.
 
+A file in the backup's scope that the library could not decrypt (see "Default
+reads vs. fresh reads") goes into `failures.json` with an error that starts
+`could not be decrypted:` and gives the reason. If a copy of it in another album
+decrypted, it does not: its original is stored from that copy, and the album
+where it does not decrypt neither links nor lists it. An album the library could
+not decrypt has no name quak can read and no files it can list. A run without
+`--json` prints one line for it,
+`FAILED album <id>: could not be decrypted: <reason>`, and `--json` and the
+`BackupResult` list its ID in `undecryptableAlbums`. Such a file in
+`failures.json`, or such an album, keeps the exit code at 1, run after run,
+until the server sends a copy that decrypts or deletes it; the next run then
+drops it.
+
+`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, unless it lost the
+lock (see below); `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 running backup loses its lock when someone removes `backup.lock`, or when
+another backup takes it over because the running one left it untouched for 10
+seconds, as a stopped or stalled process does. The backup finds this out the
+next time it goes to touch the lock, which it does every 5 seconds, and then
+stops before it starts another file. The originals it is downloading at that
+moment are still stored. It leaves `failures.json` and `backup.lock` as they
+are, since another backup may be using them. `quak backup` prints
+`quak: lost the lock on <dir>: <reason>` as one line and exits 1, and
+`lib.backup()` rejects with `lost the lock on <dir>: <reason>`.
+
 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
@@ -684,9 +804,12 @@ both, each named after the title with that file's extension (`IMG_0001.heic` and
 
 Each run removes the symlinks into the date folders that no longer belong in
 their collection's directory, and the directories (and JSON) of collections that
-were deleted or renamed. Nothing else in `collections/` is touched: a file or a
-symlink you put there stays, and a directory that still holds one after its
-symlinks are removed stays too, with its JSON.
+were deleted or renamed. An album quak can no longer decrypt loses its directory
+and JSON the same way, and a file whose copy in an album quak can no longer
+decrypt loses its symlink there and its entry in that album's JSON; the
+originals stay at their save paths. Nothing else in `collections/` is touched: a
+file or a symlink you put there stays, and a directory that still holds one
+after its symlinks are removed stays too, with its JSON.
 
 Each file is downloaded exactly once regardless of how many collections it
 appears in, and written once: straight to its save path, with no copy left in
@@ -695,7 +818,27 @@ subsequent runs, existing originals are skipped. If a download fails, the error
 is logged and the backup continues with the next file. The exit code is non-zero
 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.
+backup stores. It also opens it with no background timer, so it asks the server
+for changes only as it starts, not again while it downloads.
+
+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
@@ -738,7 +881,8 @@ The library's primary surface is the cache-backed `Library`; the lower-level
 `Client` sits underneath it and is covered by the Design sections above. The
 test suite is the canonical, executable documentation — `test/library/` and
 `test/client/usage.test.ts` walk most operations, `test/cli/backup.test.ts`
-walks `lib.backup()`, and `yarn test` verifies them.
+walks `lib.backup()`, `test/cli/backup-end-to-end.test.ts` runs `quak backup`
+against a fake of the Ente HTTP API, and `yarn test` verifies them.
 
 ### Opening a library
 
@@ -756,7 +900,7 @@ background, so an unreachable server does not block opening.
 | `client`                 | required                                           | the account client (a `Client`, or any `LibraryClient`)              |
 | `cacheDirectory`         | `quak/<userID>` under the per-user cache directory | where `metadata.json` and the content cache live                     |
 | `downloadDirectory`      | `photos` in the working directory                  | root of the save paths; an original stored there counts as cached    |
-| `refreshIntervalSeconds` | `3`                                                | background refresh cadence                                           |
+| `refreshIntervalSeconds` | `3`                                                | background refresh cadence; `Infinity` for no background timer       |
 | `precacheThumbnails`     | `true`                                             | prefetch every thumbnail, newest first                               |
 | `precacheOriginals`      | `true`                                             | prefetch the favorites album and the latest-window originals         |
 | `precacheOriginalsDays`  | `7`                                                | length in days of that latest window                                 |
@@ -801,6 +945,16 @@ rejects the caller (default reads stay silent and keep serving the last good
 copy). The CLI's read commands use fresh reads (issue
 https://git.eeqj.de/sneak/quak/issues/75).
 
+An album or file in the server's answer that quak cannot decrypt, or whose
+metadata is not JSON, does not fail the refresh. The refresh leaves it out, an
+album with all its files, and drops any older copy the cache held, so reads
+answer from what did decrypt. The library keeps a list of what it left out, with
+the reason, in `metadata.json`: the next refresh asks the server only for what
+changed, so it would not see the record again. A record stays on that list until
+a later refresh gets a copy that decrypts or learns that the server deleted it.
+`lib.backup()` reports each such album, and each such file in its scope that has
+no copy that decrypted (see "Backup layout").
+
 ### Read surface
 
 - `lib.albums.list()` → `Album[]`, newest-updated first.
@@ -912,18 +1066,28 @@ 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. 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 a `BackupLock`, whose `release()`
+  removes the lock unless it was lost and whose `lost` holds why it was lost.
+  The caller passes it to the backup as `lock`, and releases it once the backup
+  ends. 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 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 `lock`. See Backup layout above for the
+  tree it writes. Whether the backup took the lock or its caller did, once the
+  lock is lost the backup rejects with `lost the lock on <dir>: <reason>` before
+  it starts another file.
 
 ### Request pools
 
@@ -932,6 +1096,11 @@ photos newest first). `lib.subscribe({ onChange })` delivers a `LibraryChange`
 Each pool orders on-demand work ahead of background/precache work and dedups
 in-flight fetches by key, and an idle pool never lends its slots to a busy one.
 
+`lib.backup()`, and so `quak backup`, works on up to the content pool's limit of
+originals at once, 5 unless the library was opened with other `pools`. The limit
+covers everything it does for one original, hashing it for `verify` included, so
+however large the account is, no more originals than that are open at once.
+
 ### On-disk cache layout
 
 Under `cacheDirectory`:
@@ -983,7 +1152,7 @@ from a very old client, is stored unchecked.
 - `src/library/mlsearch.ts`: `MLDataAPI`, `SimilarResult`
 - `src/exif.ts`: `ExifTags`, `PhotoExif`
 - `src/library/pools.ts`: `RequestPools`, `RequestPoolsOptions`, `BoundedPool`
-- `src/backup.ts`: `BackupOptions`, `BackupResult`, `BackupError`
+- `src/backup.ts`: `BackupLock`, `BackupOptions`, `BackupResult`, `BackupError`
 - `src/client.ts`: `Client`, `LoginOptions`, `ClientSnapshot`
 - `src/api/client.ts`: `ApiClient`, `ApiClientOptions`, `StreamOptions`
 - `src/errors.ts`: `ApiError`, `TruncatedStreamError`

TODO.md

@@ -17,14 +17,109 @@ pre-1.0
 
 # Next Step
 
-None: no implementation work is open. The cache design,
-https://git.eeqj.de/sneak/quak/issues/36, waits on sneak's review.
+None: no implementation work is open. The check of quak against a live Ente
+account, https://git.eeqj.de/sneak/quak/issues/187, needs an account only sneak
+has, so it waits on him.
 
 Tagging and releases are decided by sneak alone, and happen only when he
 declares one.
 
 # Completed Steps
 
+- 2026-10-14: `lib.backup()`, and so `quak backup`, works on up to the content
+  pool's limit of originals at once, 5 by default, instead of one at a time
+  (issue 182). That many workers take files from one shared list, each doing all
+  the work for one original, its `--verify` hashing included, so no more
+  originals than that are open at once. `BackupLibrary` says the limit as
+  `contentConcurrency`. A test holds each download at the fake Ente server to
+  count those in flight. Once a worker has thrown, as one does when `onProgress`
+  throws, or the lock is lost, no worker takes another file; the backup fails
+  once the originals under way are in place, and a lost lock stops it before it
+  fetches ML data.
+
+- 2026-10-14: Next Step said the cache design, issue 36, waits on sneak's
+  review, though that issue closed on 2026-10-03 (issue 194). It now names the
+  issues open on the tracker.
+
+- 2026-10-14: A backup whose lock is lost no longer dies with a stack trace
+  (issue 184). `lockBackupDirectory` gives proper-lockfile an `onCompromised`
+  that records why the lock was lost instead of throwing from its timer, and
+  returns the lock, which `quak backup` passes to `lib.backup()` as `lock`. The
+  backup then stops before it starts another file with
+  `lost the lock on DIR: REASON`, which `quak backup` prints as one line before
+  it exits 1. Releasing a lost lock leaves `backup.lock` alone. Tests in
+  `test/cli/backup-end-to-end.test.ts` remove the lock, or put another backup's
+  in its place, while a download is held open.
+
+- 2026-10-14: An album or file quak cannot decrypt, or whose metadata is not
+  JSON, no longer fails the refresh, so one bad record no longer stops
+  `quak backup` from storing anything (issue 181). The refresh leaves it out, an
+  album with its files, and the library keeps it with the reason in
+  `metadata.json` until the server sends a copy that decrypts or deletes it.
+  `quak backup` stores every other file, records each such file in
+  `failures.json` with an error starting `could not be decrypted:`, names each
+  such album's ID in a printed line, or with `--json` in `undecryptableAlbums`,
+  and exits 1 while any remain. A file whose copy in another album decrypts is
+  not recorded: its original is stored from that copy, and the album where it
+  does not decrypt neither links nor lists it. The read commands answer from
+  what decrypted.
+
+- 2026-10-14: `quak backup` no longer asks the server for changes every 3
+  seconds while it runs (issue 183). It opens its library with
+  `refreshIntervalSeconds: Infinity`, which the library now takes as no
+  background timer, so the backup refreshes only as it starts and no timed
+  refresh starts another ML data pass during the run. A test holds a backup's
+  download open while two hours pass and counts its `/collections/v2` requests.
+
+- 2026-10-14: `test/cli/backup-end-to-end.test.ts` runs `quak backup` as cron
+  does (issue 186): `backupCommand` and `run`, the real `loadSession` reading a
+  session saved after logging in, and every request answered by a fake of the
+  Ente HTTP API serving data encrypted with libsodium. It covers the backup tree
+  of an account with two albums of its own and one shared with it, a second run
+  that downloads nothing, a download that succeeds after two 503s, a 404 that
+  lands in `failures.json` with exit 1, a refresh answered 401 with exit 3, and
+  `--verify` putting back an original changed on disk. The fake server and the
+  account it logs in moved from the backup-metadata test into
+  `test/ente-server.ts`, unchanged.
+
+- 2026-10-14: `files --collection`, `get`, `get-thumb` and
+  `helper fix-missing-thumbnails --file` take an ID only when it is written in
+  decimal digits and is a safe integer (issue 185). Before, `Number()` read
+  `0x10` as 16, `1e3` as 1000 and an empty string as 0, and `--file` dropped an
+  ID it could not read without a word. Anything else now prints
+  `Invalid file ID: <arg>` or `Invalid collection ID: <arg>` and exits 1 before
+  any request; one bad `--file` ID fails the whole command. `parseID` in
+  `src/cli-commands.ts` does the check for all four.
+
+- 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 `{}`

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 +1,14 @@
 // 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
+// passes one in), and fails at once when another backup of it holds the
+// lock; when the lock is lost while it runs, it fails before it starts another
+// file. It waits for a completed refresh of the library (a failed one fails the
+// backup before any file is touched), then, for every file in scope, puts its
+// original at its save path under `downloadDirectory`, as `Photo.download()`
+// does, up to the content pool's limit of them at once, 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 +20,7 @@
 //         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
@@ -35,10 +39,18 @@
 // 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
 // running attempt count, and the last-tried time, and the run continues.
+// A file the library could not decrypt is recorded there too, unless a copy of
+// it in another album decrypted; an album it could not decrypt is logged by ID
+// and listed in `result.undecryptableAlbums`, as its name is not known.
 // `result.failed` — and thus the CLI's exit code — stays non-zero while any
 // failure remains unresolved and clears once every one succeeds. Each run
 // reconciles the ledger against the files it attempted, so an entry for a file
@@ -47,6 +59,7 @@
 // code.
 
 import {
+    createReadStream,
     lstatSync,
     mkdirSync,
     readdirSync,
@@ -60,7 +73,14 @@ 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 {
@@ -70,12 +90,21 @@ import {
     storedAtSavePath,
 } from "./library/content.js";
 import { representative } from "./library/records.js";
+import type { Undecryptable } from "./library/store.js";
 import { extractImageMetadata } from "./metadata-backup.js";
 import type { MLData } from "./mldata-fetch.js";
 import type { Collection, EnteFile, FileMetadata } from "./model/types.js";
 
 export type ProgressCallback = (message: string) => void;
 
+// The lock in a backup directory, as `lockBackupDirectory` returns it.
+export interface BackupLock {
+    // Removes `backup.lock`, unless the lock was lost.
+    release(): Promise<void>;
+    // Why the lock was lost, once it is.
+    lost?: Error;
+}
+
 export interface BackupOptions {
     // Where the backup tree lives. `lib.backup()` defaults it to the library's
     // download directory; `runBackup` with none throws before any network
@@ -88,7 +117,15 @@ 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 lock in `downloadDirectory`, when the caller took it with
+    // `lockBackupDirectory` and releases it itself. The backup then does not
+    // take the lock, and fails once this one is lost. `quak backup` takes it
+    // before it opens its library.
+    lock?: BackupLock;
 }
 
 export interface BackupError {
@@ -105,12 +142,22 @@ 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.
     failed: number;
     // This run's per-file errors, in encounter order.
     errors: BackupError[];
+    // The IDs of the albums the library could not decrypt, whose files it
+    // could not list; the CLI exits non-zero while there are any.
+    undecryptableAlbums: number[];
 }
 
 // The slice of the library that backup drives. `Library` implements it; a test
@@ -121,6 +168,9 @@ export interface BackupLibrary {
     refresh(): Promise<void>;
     listCollections(): Collection[];
     listFiles(collectionID: number): EnteFile[];
+    // The albums, and files in albums, that the library could not decrypt,
+    // which `listCollections` and `listFiles` leave out.
+    undecryptable(): Undecryptable[];
     // Get an original's bytes onto disk through the content cache/pools,
     // returning where they landed: `destination` when they were fetched now,
     // otherwise wherever they already were (the cache, or the library's save
@@ -131,6 +181,9 @@ export interface BackupLibrary {
         destination: string,
     ): Promise<{ path: string; videoPath?: string }>;
     thumbnail(fileID: number): Promise<{ path: string }>;
+    // How many originals the backup works on at once: the limit of the
+    // library's content pool.
+    contentConcurrency: number;
     // Wait for an ML data fetch to complete, joining one already running or
     // starting one. Rejects with the reason when it fails; resolves at once
     // when the library cannot fetch ML data.
@@ -361,6 +414,26 @@ 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.
@@ -450,24 +523,24 @@ const writeAlbumJSON = (
     writeFileSync(path, JSON.stringify(album, null, 2));
 };
 
-export const runBackup = async (
+// The backup itself, which `runBackup` below runs while `lock` is held. Once
+// the lock is lost, another backup may be writing to `downloadDirectory`, so
+// this one fails before it starts another file.
+const runLockedBackup = async (
     lib: BackupLibrary,
     opts: BackupOptions,
+    downloadDirectory: string,
+    lock: BackupLock,
 ): Promise<BackupResult> => {
-    const 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();
+    if (lock.lost) throw lock.lost;
 
     const collectionsDir = join(downloadDirectory, "collections");
     const thumbnailsDir = join(downloadDirectory, "thumbnails");
@@ -523,69 +596,172 @@ 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,
+        fileID: number,
+        title: string,
         collection: string,
         err: unknown,
     ): void => {
         // Count at most one attempt per file per run: a file whose original
         // and thumbnail both fail this run must not double its attempt count
         // or appear twice in errors.
-        if (failedThisRun.has(file.id)) return;
+        if (failedThisRun.has(fileID)) return;
         const error = errorMessage(err);
-        errors.push({
-            fileID: file.id,
-            title: file.metadata.title,
-            collection,
-            error,
-        });
-        const prior = ledger.get(file.id);
-        ledger.set(file.id, {
-            fileID: file.id,
-            title: file.metadata.title,
+        errors.push({ fileID, title, collection, error });
+        const prior = ledger.get(fileID);
+        ledger.set(fileID, {
+            fileID,
+            title,
             classification: classify(err),
             attempts: (prior?.attempts ?? 0) + 1,
             lastTriedAt: now,
             error,
         });
-        failedThisRun.add(file.id);
+        failedThisRun.add(fileID);
     };
 
+    // What the library could not decrypt. An album is logged by its ID, as
+    // its name is not known, so a run limited to some albums logs it too. A
+    // file in an album in scope is backed up from a copy of it in another
+    // album that decrypted, even an album out of scope. With no such copy it
+    // is recorded as failed, with an empty title as its metadata is not known.
+    const scope = new Set(collections.map((c) => c.id));
+    const undecryptableAlbums: number[] = [];
+    for (const record of lib.undecryptable()) {
+        const error = `could not be decrypted: ${record.error}`;
+        if (record.fileID === undefined) {
+            log(`FAILED album ${record.collectionID}: ${error}`);
+            undecryptableAlbums.push(record.collectionID);
+        } else if (scope.has(record.collectionID)) {
+            const copies = memberships.get(record.fileID);
+            if (copies) {
+                distinct.set(record.fileID, representative(copies));
+            } else {
+                recordFailure(
+                    record.fileID,
+                    "",
+                    collectionName.get(record.collectionID) ?? "",
+                    new Error(error),
+                );
+            }
+        }
+    }
+
     // 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.
-    if (includeOriginals) {
-        for (const [fileID, file] of distinct) {
-            if (storedAtSavePath(downloadDirectory, file) !== undefined) {
+    // 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.
+    // `storeOriginal` does this for one original, and records its failure.
+    const storeOriginal = async (file: EnteFile): Promise<void> => {
+        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 ${file.metadata.title} (${file.id}): 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.id,
+                    file.metadata.title,
+                    collectionName.get(file.collectionID) ?? "",
+                    err,
+                );
+                return;
+            }
+        }
+        if (stored !== undefined) {
             skipped++;
-                continue;
+            return;
         }
         try {
-                log(`Fetching original ${file.metadata.title} (${fileID})...`);
+            log(`Fetching original ${file.metadata.title} (${file.id})...`);
             // 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(file.id, dest),
             );
-                storedThisRun.add(fileID);
+            storedThisRun.add(file.id);
+            // 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(
-                    `FAILED original ${file.metadata.title}: ${errorMessage(err)}`,
-                );
+            log(`FAILED original ${file.metadata.title}: ${errorMessage(err)}`);
             recordFailure(
-                    file,
+                file.id,
+                file.metadata.title,
                 collectionName.get(file.collectionID) ?? "",
                 err,
             );
         }
+    };
+
+    // Up to the content pool's limit of originals are worked on at once,
+    // their `verify` hashing included, so however large the account is, no
+    // more than that many are open. Each worker takes the next file from the
+    // iterator they all share until none is left. Once a worker has thrown,
+    // as one does when `onProgress` throws, or the lock is lost, no worker
+    // takes another file. The backup fails only once every worker has
+    // stopped, so none writes into the backup after it has ended. It checks
+    // the lock then too, so it does not go on to the thumbnails or the ML
+    // data fetch without it.
+    if (includeOriginals) {
+        const pending = distinct.values();
+        let workerThrew = false;
+        const worker = async (): Promise<void> => {
+            for (const file of pending) {
+                if (workerThrew || lock.lost) return;
+                try {
+                    await storeOriginal(file);
+                } catch (err) {
+                    workerThrew = true;
+                    throw err;
                 }
             }
+        };
+        const workers = await Promise.allSettled(
+            Array.from({ length: lib.contentConcurrency }, worker),
+        );
+        for (const w of workers) {
+            if (w.status === "rejected") throw w.reason;
+        }
+        if (lock.lost) throw lock.lost;
+    }
 
     if (includeThumbnails) {
         for (const [fileID, file] of distinct) {
+            if (lock.lost) throw lock.lost;
             const dest = join(thumbnailsDir, `${fileID}.jpg`);
             if (isPresent(dest)) continue;
             try {
@@ -593,7 +769,8 @@ export const runBackup = async (
                 await copyAtomic(path, dest);
             } catch (err) {
                 recordFailure(
-                    file,
+                    file.id,
+                    file.metadata.title,
                     collectionName.get(file.collectionID) ?? "",
                     err,
                 );
@@ -617,6 +794,7 @@ export const runBackup = async (
             log(`FAILED ML data: ${mlDataError}`);
         }
         for (const file of distinct.values()) {
+            if (lock.lost) throw lock.lost;
             const stored = storedAtSavePath(downloadDirectory, file);
             if (stored === undefined) continue;
             const path = withExtension(
@@ -633,7 +811,8 @@ export const runBackup = async (
             if (mlData === undefined && mlDataError !== undefined) {
                 writeSidecar(path, file, { mlDataError }, image);
                 recordFailure(
-                    file,
+                    file.id,
+                    file.metadata.title,
                     collectionName.get(file.collectionID) ?? "",
                     new Error(`ML data: ${mlDataError}`),
                 );
@@ -659,6 +838,7 @@ export const runBackup = async (
     const albumDirNames = new Map(
         allCollections.map((c, i) => [c.id, dirNames[i]!]),
     );
+    if (lock.lost) throw lock.lost;
     try {
         removeStaleAlbumDirs(
             collectionsDir,
@@ -670,6 +850,7 @@ export const runBackup = async (
     }
 
     for (const c of collections) {
+        if (lock.lost) throw lock.lost;
         const colDirName = albumDirNames.get(c.id)!;
         const colDir = join(collectionsDir, colDirName);
         mkdirSync(colDir, { recursive: true });
@@ -706,7 +887,12 @@ export const runBackup = async (
                 log(
                     `FAILED symlink ${c.name}/${linkName}: ${errorMessage(err)}`,
                 );
-                recordFailure(link.file, c.name, err);
+                recordFailure(
+                    link.file.id,
+                    link.file.metadata.title,
+                    c.name,
+                    err,
+                );
             }
         }
 
@@ -722,7 +908,9 @@ export const runBackup = async (
     // its failure resolved; a file gone from the library (deleted) or outside
     // this run's scope is not something this run can resolve, so keeping its
     // stale entry would keep the exit code non-zero forever — a single
-    // since-deleted photo would fail every future scheduled backup.
+    // since-deleted photo would fail every future scheduled backup. A backup
+    // whose lock was lost leaves the ledger as it is.
+    if (lock.lost) throw lock.lost;
     for (const fileID of [...ledger.keys()]) {
         if (!failedThisRun.has(fileID)) ledger.delete(fileID);
     }
@@ -732,7 +920,86 @@ export const runBackup = async (
         totalFiles: distinct.size,
         downloaded,
         skipped,
+        verified,
+        mismatched,
+        unchecked,
         failed: ledger.size,
         errors,
+        undecryptableAlbums,
     };
 };
+
+// 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 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.
+//
+// proper-lockfile finds the lock lost when `backup.lock` is gone or is another
+// backup's, and calls `onCompromised` from a timer, where throwing would end
+// the process. The `onCompromised` given here sets the lock's `lost` instead,
+// and the backup holding the lock stops before it starts another file (see
+// `runLockedBackup`). Releasing a lost lock leaves `backup.lock` alone, since
+// it may be another backup's.
+export const lockBackupDirectory = async (
+    downloadDirectory: string,
+): Promise<BackupLock> => {
+    mkdirSync(downloadDirectory, { recursive: true });
+    let release: () => Promise<void>;
+    const lock: BackupLock = {
+        release: async () => {
+            if (lock.lost === undefined) await release();
+        },
+    };
+    try {
+        release = await lockfile.lock(downloadDirectory, {
+            lockfilePath: join(downloadDirectory, "backup.lock"),
+            onCompromised: (err) => {
+                lock.lost = new Error(
+                    `lost the lock on ${downloadDirectory}: ${err.message}`,
+                );
+            },
+        });
+    } catch (err) {
+        if ((err as NodeJS.ErrnoException).code !== "ELOCKED") throw err;
+        throw Object.assign(
+            new Error(`another backup of ${downloadDirectory} is running`),
+            { code: "ELOCKED" },
+        );
+    }
+    return lock;
+};
+
+// Runs the backup holding the lock, which it takes and releases itself unless
+// the caller passes the lock it took (`opts.lock`).
+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)",
+        );
+    }
+    // As `BoundedPool` does. A limit below 1, or one that is not a number,
+    // would back up no original and end with no error.
+    const { contentConcurrency } = lib;
+    if (!Number.isInteger(contentConcurrency) || contentConcurrency < 1) {
+        throw new RangeError(
+            `contentConcurrency must be a positive integer, got ${contentConcurrency}`,
+        );
+    }
+    if (opts.lock) {
+        return runLockedBackup(lib, opts, downloadDirectory, opts.lock);
+    }
+    const lock = await lockBackupDirectory(downloadDirectory);
+    try {
+        return await runLockedBackup(lib, opts, downloadDirectory, lock);
+    } finally {
+        await lock.release();
+    }
+};

src/cli-commands.ts

@@ -20,6 +20,7 @@ import {
     type ClientSnapshot,
     type LoginOptions,
 } from "./client.js";
+import { lockBackupDirectory, type BackupLock } from "./backup.js";
 import { init } from "./crypto/index.js";
 import {
     defaultCacheDirectory,
@@ -96,6 +97,20 @@ const requireSession = (ctx: CliContext): Client | undefined => {
     return client;
 };
 
+// `text` as an ID, or undefined after telling the user it is not one. An ID is
+// written in decimal digits and is a safe integer; `Number()` alone would read
+// `0x10` as 16, `1e3` as 1000 and "" as 0.
+const parseID = (
+    ctx: CliContext,
+    kind: "file" | "collection",
+    text: string,
+): number | undefined => {
+    const id = Number(text);
+    if (/^[0-9]+$/.test(text) && Number.isSafeInteger(id)) return id;
+    ctx.stderr.write(`Invalid ${kind} ID: ${text}\n`);
+    return undefined;
+};
+
 // A library client that omits `fetchMLData`, so the point commands below do not
 // kick the library's background ML backfill: they read metadata, or fetch one
 // file's content, and exit. `backup` and `backup-metadata` handle ML on their
@@ -246,11 +261,8 @@ export const filesCommand = async (
     await init();
     const client = requireSession(ctx);
     if (!client) return 3;
-    const collectionID = Number(opts.collection);
-    if (!Number.isFinite(collectionID)) {
-        ctx.stderr.write("Invalid collection ID\n");
-        return 1;
-    }
+    const collectionID = parseID(ctx, "collection", opts.collection);
+    if (collectionID === undefined) return 1;
 
     const lib = await openReadLibrary(ctx, client);
     try {
@@ -288,11 +300,8 @@ export const getCommand = async (
     await init();
     const client = requireSession(ctx);
     if (!client) return 3;
-    const fileID = Number(fileIDStr);
-    if (!Number.isFinite(fileID)) {
-        ctx.stderr.write("Invalid file ID\n");
-        return 1;
-    }
+    const fileID = parseID(ctx, "file", fileIDStr);
+    if (fileID === undefined) return 1;
 
     const lib = await openReadLibrary(ctx, client);
     try {
@@ -346,11 +355,8 @@ export const getThumbCommand = async (
     await init();
     const client = requireSession(ctx);
     if (!client) return 3;
-    const fileID = Number(fileIDStr);
-    if (!Number.isFinite(fileID)) {
-        ctx.stderr.write("Invalid file ID\n");
-        return 1;
-    }
+    const fileID = parseID(ctx, "file", fileIDStr);
+    if (fileID === undefined) return 1;
 
     const lib = await openReadLibrary(ctx, client);
     try {
@@ -401,25 +407,41 @@ 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 3;
 
     ctx.stderr.write("Starting backup...\n");
+    // The lock is taken before the library opens and starts its refresh, so a
+    // run refused while another backup of `dir` runs sends no request.
+    let lock: BackupLock;
+    try {
+        lock = 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.
+        // No refresh starts on a timer: the backup lists its files once, from
+        // the refresh it waits for, and can run for hours.
         const lib = await Library.open({
             client,
             downloadDirectory: dir,
             cacheDirectory: ctx.cacheDir,
+            refreshIntervalSeconds: Infinity,
             precacheThumbnails: false,
             precacheOriginals: false,
         });
         try {
             const result = await lib.backup({
                 downloadDirectory: dir,
+                lock,
+                verify: opts.verify,
                 onProgress: (msg) => {
                     if (!opts.json) ctx.stderr.write(msg + "\n");
                 },
@@ -432,6 +454,11 @@ export const backupCommand = async (
                 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");
@@ -443,10 +470,15 @@ export const backupCommand = async (
                 }
             }
 
-        return result.failed > 0 ? 1 : 0;
+            return result.failed > 0 || result.undecryptableAlbums.length > 0
+                ? 1
+                : 0;
         } finally {
             await lib.close();
         }
+    } finally {
+        await lock.release();
+    }
 };
 
 export const listMissingThumbnailsCommand = async (
@@ -494,15 +526,19 @@ export const fixMissingThumbnailsCommand = async (
     await init();
     const client = requireSession(ctx);
     if (!client) return 3;
+    // One bad --file ID fails the whole command, before anything is fetched.
+    let fileIDs: number[] = [];
+    for (const text of opts.file ?? []) {
+        const fileID = parseID(ctx, "file", text);
+        if (fileID === undefined) return 1;
+        fileIDs.push(fileID);
+    }
     const lib = await openReadLibrary(ctx, client);
     try {
         // Refresh first so files added since the cache was written are found;
         // a failed refresh throws.
         await lib.fresh();
-        let fileIDs: number[];
-        if (opts.file && opts.file.length > 0) {
-            fileIDs = opts.file.map(Number).filter(Number.isFinite);
-        } else {
+        if (fileIDs.length === 0) {
             ctx.stderr.write("Scanning for missing thumbnails...\n");
             const missing = await listMissingThumbnails(lib, client, (msg) => {
                 if (!opts.json) ctx.stderr.write(msg + "\n");

src/client.ts

@@ -44,20 +44,28 @@ export interface ClientSnapshot {
 
 // The result of a resumable enumeration. Live decrypted records and deleted
 // ids are kept apart on purpose: a tombstone carries no key or metadata to
-// decrypt, so it is a bare id rather than a hollowed-out record. `cursor` is
-// the max `updationTime` seen, to pass back into the next call.
+// decrypt, so it is a bare id rather than a hollowed-out record. A live row
+// that cannot be decrypted, or whose metadata cannot be parsed, is in
+// `undecryptable` with the reason instead, so one bad row does not fail the
+// call; absent means there was none. `cursor` is the max `updationTime` seen,
+// those rows included, to pass back into the next call.
 export interface CollectionsPage {
     collections: Collection[];
     deleted: number[];
+    undecryptable?: { id: number; error: string }[];
     cursor: number;
 }
 
 export interface FilesPage {
     files: EnteFile[];
     deleted: number[];
+    undecryptable?: { id: number; error: string }[];
     cursor: number;
 }
 
+const errorMessage = (err: unknown): string =>
+    err instanceof Error ? err.message : String(err);
+
 export class Client {
     private readonly api: ApiClient;
     private readonly email: string;
@@ -237,11 +245,12 @@ export class Client {
     }
 
     // Enumerate collections changed since `sinceTime`. Live collections are
-    // decrypted; tombstoned ones (isDeleted) are surfaced as bare ids. The
-    // returned cursor is the max `updationTime` seen — including tombstones, so
-    // the next sync resumes past them — falling back to `sinceTime` when the
-    // response is empty. `/collections/v2` returns the whole changed set in one
-    // response, so there is no pagination here.
+    // decrypted; tombstoned ones (isDeleted) are surfaced as bare ids, and
+    // ones that do not decrypt as ids with the reason. The returned cursor is
+    // the max `updationTime` seen — including tombstones, so the next sync
+    // resumes past them — falling back to `sinceTime` when the response is
+    // empty. `/collections/v2` returns the whole changed set in one response,
+    // so there is no pagination here.
     async collectionsSince(args: {
         sinceTime: number;
     }): Promise<CollectionsPage> {
@@ -254,11 +263,13 @@ export class Client {
 
         const collections: Collection[] = [];
         const deleted: number[] = [];
+        const undecryptable: { id: number; error: string }[] = [];
         let cursor = args.sinceTime;
         for (const raw of raws) {
             if (raw.isDeleted) {
                 deleted.push(raw.id);
             } else {
+                try {
                     collections.push(
                         decryptCollection(
                             raw,
@@ -270,15 +281,22 @@ export class Client {
                             this.userID,
                         ),
                     );
+                } catch (err) {
+                    undecryptable.push({
+                        id: raw.id,
+                        error: errorMessage(err),
+                    });
+                }
             }
             if (raw.updationTime > cursor) cursor = raw.updationTime;
         }
-        return { collections, deleted, cursor };
+        return { collections, deleted, undecryptable, cursor };
     }
 
     // Enumerate a collection's files changed since `sinceTime`, paginating the
     // diff from that cursor. Live rows are decrypted; tombstoned ones are
-    // surfaced as bare ids. Returns the final cursor to resume from.
+    // surfaced as bare ids, and ones that do not decrypt or parse as ids with
+    // the reason. Returns the final cursor to resume from.
     async filesSince(args: {
         collectionID: number;
         collectionKey: Uint8Array;
@@ -288,6 +306,7 @@ export class Client {
         const { collectionID, collectionKey } = args;
         const files: EnteFile[] = [];
         const deleted: number[] = [];
+        const undecryptable: { id: number; error: string }[] = [];
         let cursor = args.sinceTime;
         for (;;) {
             const { diff, hasMore } = await this.api.getJSON<{
@@ -300,7 +319,14 @@ export class Client {
                 if (raw.isDeleted) {
                     deleted.push(raw.id);
                 } else {
+                    try {
                         files.push(decryptFile(raw, collectionKey));
+                    } catch (err) {
+                        undecryptable.push({
+                            id: raw.id,
+                            error: errorMessage(err),
+                        });
+                    }
                 }
                 if (raw.updationTime > pageMax) pageMax = raw.updationTime;
             }
@@ -321,18 +347,19 @@ export class Client {
             }
             cursor = pageMax;
         }
-        return { files, deleted, cursor };
+        return { files, deleted, undecryptable, cursor };
     }
 
-    // Whole-account listing: every live collection, deletions hidden. A thin
-    // wrapper over `collectionsSince` from the beginning of time.
+    // Whole-account listing: every live collection that decrypts, deletions
+    // hidden. A thin wrapper over `collectionsSince` from the beginning of
+    // time.
     async listCollections(): Promise<Collection[]> {
         const { collections } = await this.collectionsSince({ sinceTime: 0 });
         return collections;
     }
 
-    // Every live file in a collection, deletions hidden. A thin wrapper over
-    // `filesSince` from the beginning of time.
+    // Every live file in a collection that decrypts, deletions hidden. A thin
+    // wrapper over `filesSince` from the beginning of time.
     async listFiles(
         collectionID: number,
         collectionKey: Uint8Array,

src/index.ts

@@ -67,7 +67,9 @@ export {
     type EnsureOptions,
     type EnsureResult,
     type EnsureEvent,
+    lockBackupDirectory,
     runBackup,
+    type BackupLock,
     type BackupOptions,
     type BackupResult,
     type BackupError,

src/library/index.ts

@@ -94,7 +94,9 @@ import type { Collection, EnteFile } from "../model/types.js";
 import { runBackup, type BackupOptions, type BackupResult } from "../backup.js";
 
 export {
+    lockBackupDirectory,
     runBackup,
+    type BackupLock,
     type BackupOptions,
     type BackupResult,
     type BackupError,
@@ -178,6 +180,9 @@ export interface LibraryOptions {
     // directory at open. The content cache treats an original already stored
     // at its save path as present.
     downloadDirectory?: string;
+    // How long after one refresh ends the background timer starts the next.
+    // `Infinity` starts none: the library then refreshes only as it opens and
+    // when `fresh()` or `backup()` asks.
     refreshIntervalSeconds?: number;
     onProgress?: RefreshProgressCallback;
     // The bounded request pools (issue #45), shared by the ML-data fetch (the
@@ -566,13 +571,17 @@ 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 the
+    // caller passes the lock it took as `opts.lock`, failing at once while
+    // another backup of it runs, and before it starts another file once the
+    // lock is lost (see `lockBackupDirectory`). Waits for a
+    // completed refresh, as `fresh()` does, joining one already running, and
+    // rejects before touching any file but the lock when it fails. Then puts
+    // pending originals at their save paths as `Photo.download()` does (and
+    // 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> {
@@ -594,9 +603,11 @@ export class Library {
                 refresh: () => this.refreshNow(),
                 listCollections: () => this.store.listCollections(),
                 listFiles: (id) => this.store.listFiles(id),
+                undecryptable: () => this.store.listUndecryptable(),
                 original: (fileID, destination) =>
                     cache!.backupOriginal(fileID, destination),
                 thumbnail: (fileID) => cache!.thumbnail(fileID),
+                contentConcurrency: this.pools.content.concurrency,
                 fetchMLData: () => this.fetchMLDataNow(),
                 mlData: (fileID) => this.mldata.forFile({ fileID }),
             },
@@ -623,7 +634,8 @@ export class Library {
     }
 
     private scheduleNext(): void {
-        if (this.closed) return;
+        // `setTimeout` would turn an infinite delay into 1 ms.
+        if (this.closed || this.intervalMs === Infinity) return;
         this.timer = setTimeout(() => {
             void this.runRefresh().then(() => this.scheduleNext());
         }, this.intervalMs);
@@ -695,7 +707,10 @@ export class Library {
 
     // Fetch every change since the stored cursor, then commit. All network
     // reads happen before any store mutation, so a fetch that throws leaves the
-    // store untouched and the previous snapshot intact.
+    // store untouched and the previous snapshot intact. A collection or file
+    // that does not decrypt is left out instead, as if deleted, with the reason
+    // kept in the store: the cursor moves past it, so no later refresh sees it
+    // again until the server's copy changes.
     private async refreshOnce(): Promise<void> {
         const page = await this.client.collectionsSince({
             sinceTime: this.store.collectionsSinceTime,
@@ -732,9 +747,17 @@ export class Library {
                 this.store.deleteCollection(id);
                 changed = true;
             }
+            if (this.store.deleteUndecryptable(id)) changed = true;
+        }
+        // A collection left out takes its files with it.
+        for (const { id, error } of page.undecryptable ?? []) {
+            this.store.deleteCollection(id);
+            this.store.putUndecryptable({ collectionID: id, error });
+            changed = true;
         }
         for (const collection of page.collections) {
             this.store.putCollection(collection);
+            this.store.deleteUndecryptable(collection.id);
             changed = true;
         }
 
@@ -744,9 +767,22 @@ export class Library {
                     this.store.deleteFile(collectionID, id);
                     changed = true;
                 }
+                if (this.store.deleteUndecryptable(collectionID, id)) {
+                    changed = true;
+                }
+            }
+            for (const { id, error } of filePage.undecryptable ?? []) {
+                this.store.deleteFile(collectionID, id);
+                this.store.putUndecryptable({
+                    collectionID,
+                    fileID: id,
+                    error,
+                });
+                changed = true;
             }
             for (const f of filePage.files) {
                 this.store.putFile(f);
+                this.store.deleteUndecryptable(collectionID, f.id);
                 changed = true;
             }
         }

src/library/store.ts

@@ -2,10 +2,11 @@
 //
 // The store keeps one `metadata.json` file holding the account's server
 // state: the user id, a schema version, the cursor for the incremental
-// collections listing, and the decrypted collection and file records. The
-// whole file is read into RAM on load and rewritten as a whole on save; there
-// is no partial update and no lock file. A separate refresh unit populates the
-// store from the server — this module only stores what it is given.
+// collections listing, the decrypted collection and file records, and the
+// collections and files the refresh could not decrypt. The whole file is read
+// into RAM on load and rewritten as a whole on save; there is no partial
+// update and no lock file. A separate refresh unit populates the store from
+// the server — this module only stores what it is given.
 //
 // The file is a cache, so it is never trusted to exist or to be intact: a
 // missing or unreadable file loads as an empty store rather than an error, and
@@ -33,12 +34,22 @@ const FILE_MODE = 0o600;
 type StoredCollection = Omit<Collection, "key"> & { key: string };
 type StoredFile = Omit<EnteFile, "key"> & { key: string };
 
+// A collection, or with `fileID` a file in it, that the last refresh to fetch
+// it could not decrypt or parse, and why. It is kept until a later refresh
+// fetches it again, because a refresh fetches only what changed.
+export interface Undecryptable {
+    collectionID: number;
+    fileID?: number;
+    error: string;
+}
+
 interface StoredMetadata {
     schemaVersion: number;
     userID: number;
     collectionsSinceTime: Microseconds;
     collections: StoredCollection[];
     files: StoredFile[];
+    undecryptable: Undecryptable[];
 }
 
 const encodeKey = (key: Uint8Array): string =>
@@ -67,6 +78,7 @@ export class MetadataStore {
 
     private readonly collections = new Map<number, Collection>();
     private readonly files = new Map<string, EnteFile>();
+    private undecryptable: Undecryptable[] = [];
 
     private constructor(path: string) {
         this.path = path;
@@ -107,6 +119,12 @@ export class MetadataStore {
                 };
                 store.files.set(fileKey(file.collectionID, file.id), file);
             }
+            for (const record of parsed.undecryptable) {
+                if (typeof record.collectionID !== "number") {
+                    throw new Error("damaged record of what did not decrypt");
+                }
+                store.undecryptable.push(record);
+            }
         } catch {
             // Any corruption discards the partial result: a half-read cache is
             // worse than an empty one, since the refresh unit will rebuild it.
@@ -134,6 +152,7 @@ export class MetadataStore {
                 ...f,
                 key: encodeKey(f.key),
             })),
+            undecryptable: this.undecryptable,
         };
 
         const dir = dirname(this.path);
@@ -165,7 +184,8 @@ export class MetadataStore {
         this.collections.set(collection.id, collection);
     }
 
-    // Removing a collection also drops its file memberships: a file record is
+    // Removing a collection also drops its file memberships, and any record
+    // that it or one of its files could not be decrypted: a file record is
     // only meaningful as part of a collection the cache still knows about.
     deleteCollection(id: number): void {
         this.collections.delete(id);
@@ -174,6 +194,9 @@ export class MetadataStore {
                 this.files.delete(key);
             }
         }
+        this.undecryptable = this.undecryptable.filter(
+            (record) => record.collectionID !== id,
+        );
     }
 
     getFile(collectionID: number, fileID: number): EnteFile | undefined {
@@ -205,4 +228,26 @@ export class MetadataStore {
     deleteFile(collectionID: number, fileID: number): void {
         this.files.delete(fileKey(collectionID, fileID));
     }
+
+    listUndecryptable(): Undecryptable[] {
+        return [...this.undecryptable];
+    }
+
+    // Replaces any earlier record of the same collection or file.
+    putUndecryptable(record: Undecryptable): void {
+        this.deleteUndecryptable(record.collectionID, record.fileID);
+        this.undecryptable.push(record);
+    }
+
+    // Forgets the record of the collection, or with `fileID` of that file in
+    // it. Returns whether there was one.
+    deleteUndecryptable(collectionID: number, fileID?: number): boolean {
+        const before = this.undecryptable.length;
+        this.undecryptable = this.undecryptable.filter(
+            (record) =>
+                record.collectionID !== collectionID ||
+                record.fileID !== fileID,
+        );
+        return this.undecryptable.length < before;
+    }
 }

test/cli/backup-concurrency.test.ts

@@ -0,0 +1,608 @@
+/**
+ * How many originals `lib.backup()` works on at once.
+ *
+ * The backup puts up to the content pool's limit of originals at their save
+ * paths at the same time: 5 for a library opened with the default
+ * `RequestPools`, as `quak backup` opens it, or the `contentConcurrency` of the
+ * pools a library was opened with. Everything the backup does for one original
+ * counts against that limit, hashing it for `verify` included, so however large
+ * the account is, the backup has no more than that many originals open at once.
+ *
+ * As in `backup-end-to-end.test.ts`, the real `Client` and `Library` run
+ * against `buildMetaFetch` from `test/ente-server.ts`, a fake of the Ente HTTP
+ * API serving data this file encrypts with libsodium. Here a test can also hold
+ * each original's download until it lets it go, to see how many the backup has
+ * in flight at once.
+ *
+ * The account, user 42, has one album of its own, Holiday, of eight photos,
+ * all taken at noon on 1 March 2024, each with an original of its own.
+ */
+
+import {
+    existsSync,
+    lstatSync,
+    mkdtempSync,
+    readdirSync,
+    readFileSync,
+    readlinkSync,
+    rmSync,
+    writeFileSync,
+} from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import sodium from "libsodium-wrappers-sumo";
+import { afterAll, beforeAll, describe, expect, it, vi } from "vitest";
+
+import type { BackupOptions } from "../../src/backup.js";
+import { Client, type ClientSnapshot } from "../../src/client.js";
+import { init, toBase64 } from "../../src/crypto/index.js";
+import { Library } from "../../src/library/index.js";
+import { RequestPools } from "../../src/library/pools.js";
+import {
+    buildAccount,
+    buildMetaFetch,
+    encryptSecretbox,
+    encryptStreamBlob,
+} from "../ente-server.js";
+import { blake2b } from "../live-photo.js";
+
+const EMAIL = "backup@example.com";
+const PASSWORD = "backup-password";
+// The fake server logs every account in as user 42.
+const USER_ID = 42;
+
+// Noon on 1 March 2024 in the time zone of the machine running the test, in
+// microseconds, as Ente records times.
+const TAKEN = new Date(2024, 2, 1, 12).getTime() * 1000;
+
+// The eight photos' IDs. Photo `id` is titled `photo-<id>.jpg`, and its
+// original is a few bytes naming it.
+const PHOTO_IDS = [101, 102, 103, 104, 105, 106, 107, 108];
+const originalOf = (id: number): Uint8Array =>
+    new TextEncoder().encode(`the photo ${id}`);
+
+// Where a backup to `dir` saves photo `id`'s original.
+const savedAt = (dir: string, id: number): string =>
+    join(dir, "2024/2024-03/2024-03-01", `2024-03-01.${id}.jpg`);
+
+// The fake server, the session it logs the account in with, and the
+// directory every backup below is made in.
+let server: typeof globalThis.fetch;
+let session: ClientSnapshot;
+let root: string;
+
+// The fake server, holding the account encrypted as Ente's clients upload it.
+const buildServer = async () => {
+    const account = await buildAccount(PASSWORD);
+
+    // The album as `/collections/v2` lists it: its key encrypted with the
+    // account's master key, its name with its own key.
+    const albumKey = sodium.crypto_secretbox_keygen();
+    const encryptedAlbumKey = encryptSecretbox(albumKey, account.masterKey);
+    const encryptedName = encryptSecretbox(
+        new TextEncoder().encode("Holiday"),
+        albumKey,
+    );
+    const album = {
+        id: 10,
+        owner: { id: USER_ID },
+        encryptedKey: toBase64(encryptedAlbumKey.ciphertext),
+        keyDecryptionNonce: toBase64(encryptedAlbumKey.nonce),
+        encryptedName: toBase64(encryptedName.ciphertext),
+        nameDecryptionNonce: toBase64(encryptedName.nonce),
+        type: "album",
+        updationTime: TAKEN,
+    };
+
+    // Each photo as the album's `/collections/v2/diff` lists it: its key
+    // encrypted with the album's key, and its metadata, holding the hash of
+    // its original, encrypted with its own key. Its original, encrypted with
+    // its own key too, is what the files CDN serves for it.
+    const ciphertexts: Record<number, Uint8Array> = {};
+    const photos = PHOTO_IDS.map((id) => {
+        const key = sodium.crypto_secretstream_xchacha20poly1305_keygen();
+        const original = encryptStreamBlob(originalOf(id), key);
+        ciphertexts[id] = original.ciphertext;
+        const encryptedKey = encryptSecretbox(key, albumKey);
+        const metadata = encryptStreamBlob(
+            new TextEncoder().encode(
+                JSON.stringify({
+                    title: `photo-${id}.jpg`,
+                    fileType: 0,
+                    hash: blake2b(originalOf(id)),
+                    creationTime: TAKEN,
+                    modificationTime: TAKEN,
+                }),
+            ),
+            key,
+        );
+        return {
+            id,
+            collectionID: 10,
+            ownerID: USER_ID,
+            encryptedKey: toBase64(encryptedKey.ciphertext),
+            keyDecryptionNonce: toBase64(encryptedKey.nonce),
+            metadata: {
+                encryptedData: toBase64(metadata.ciphertext),
+                decryptionHeader: toBase64(metadata.header),
+            },
+            file: { decryptionHeader: toBase64(original.header) },
+            thumbnail: {
+                decryptionHeader: toBase64(sodium.randombytes_buf(24)),
+            },
+            updationTime: TAKEN,
+        };
+    });
+
+    return buildMetaFetch({
+        verifier: account.verifier,
+        srpAttributes: account.srpAttributes,
+        keyAttributes: account.keyAttributes,
+        encryptedToken: account.encryptedToken,
+        collections: [album],
+        filesByCollection: { 10: photos },
+        encryptedMLData: {},
+        fileCiphertexts: ciphertexts,
+    });
+};
+
+beforeAll(async () => {
+    await init();
+    await sodium.ready;
+    root = mkdtempSync(join(tmpdir(), "quak-backup-concurrency-"));
+    server = await buildServer();
+    const client = await Client.login({
+        email: EMAIL,
+        password: PASSWORD,
+        apiOptions: { fetch: server },
+    });
+    session = client.toJSON();
+});
+
+afterAll(() => {
+    rmSync(root, { recursive: true, force: true });
+});
+
+// The ID of the photo whose original a request downloads, or undefined for any
+// other request. quak downloads an original from
+// `https://files.ente.io/?fileID=<id>`.
+const downloadOf = (input: RequestInfo | URL): number | undefined => {
+    const url = new URL(String(input));
+    return url.hostname === "files.ente.io"
+        ? Number(url.searchParams.get("fileID"))
+        : undefined;
+};
+
+// The fake server, except that it answers 404 to every download of photo
+// `failing`'s original.
+const failingFor =
+    (failing: number): typeof globalThis.fetch =>
+    async (input, init) =>
+        downloadOf(input) === failing
+            ? new Response("not found", { status: 404 })
+            : server(input, init);
+
+// The fake server, holding each download of an original until the test lets
+// it go. `held` lists the downloads waiting, the first to arrive first;
+// `most()` is the largest number that ever waited at once. A held download of
+// photo `failing`'s original is answered 404 once it is let go.
+const holdingServer = (failing?: number) => {
+    const held: { id: number; release: () => void }[] = [];
+    let most = 0;
+    const fetch: typeof globalThis.fetch = async (input, init) => {
+        const id = downloadOf(input);
+        if (id !== undefined) {
+            await new Promise<void>((release) => {
+                held.push({ id, release });
+                most = Math.max(most, held.length);
+            });
+            if (id === failing) {
+                return new Response("not found", { status: 404 });
+            }
+        }
+        return server(input, init);
+    };
+    return { fetch, held, most: () => most };
+};
+
+// Let the held downloads go one at a time, the first to arrive first, each
+// once `limit` of them wait, or all the downloads still to come when there
+// are fewer. Each of the eight photos is downloaded once. With a backup that
+// downloads fewer than `limit` at once, this fails waiting for them. Each wait
+// allows five seconds, for a loaded machine's disk to sync the files written.
+const releaseOneAtATime = async (
+    held: { release: () => void }[],
+    limit: number,
+): Promise<void> => {
+    for (let left = PHOTO_IDS.length; left > 0; left--) {
+        await vi.waitFor(
+            () => expect(held).toHaveLength(Math.min(limit, left)),
+            { interval: 5, timeout: 5000 },
+        );
+        held.shift()!.release();
+    }
+};
+
+// A library of the account opened as `quak backup` opens one, with no
+// precache and no background refresh, sending its requests through `fetch`.
+// It backs up to `dir` and keeps its cache beside it. Its request pools are
+// `pools`, or by default the library's own.
+const openLibrary = (
+    dir: string,
+    fetch: typeof globalThis.fetch,
+    pools?: RequestPools,
+): Promise<Library> =>
+    Library.open({
+        client: Client.fromJSON(session, { fetch }),
+        downloadDirectory: dir,
+        cacheDirectory: `${dir}-cache`,
+        refreshIntervalSeconds: Infinity,
+        precacheThumbnails: false,
+        precacheOriginals: false,
+        pools,
+    });
+
+// Every entry under `dir`, relative to it, sorted, a symlink as
+// `name -> target`.
+const listTree = (dir: string): string[] =>
+    readdirSync(dir, { recursive: true })
+        .sort()
+        .map((name) => {
+            const path = join(dir, name);
+            return lstatSync(path).isSymbolicLink()
+                ? `${name} -> ${readlinkSync(path)}`
+                : name;
+        });
+
+const expectStored = (dir: string, ids: number[]): void => {
+    for (const id of ids) {
+        expect(readFileSync(savedAt(dir, id))).toEqual(
+            Buffer.from(originalOf(id)),
+        );
+    }
+};
+
+describe("lib.backup()", () => {
+    // A library opened with the default pools, as `quak backup` opens one,
+    // downloads up to five originals at once. As soon as one finishes, the
+    // backup starts on the next photo, so five stay in flight until fewer
+    // than five are left.
+    it("downloads five originals at once, and never more, with the default pools", async () => {
+        const dir = mkdtempSync(join(root, "backup-"));
+        const holding = holdingServer();
+        const lib = await openLibrary(dir, holding.fetch);
+        try {
+            const backup = lib.backup();
+            await releaseOneAtATime(holding.held, 5);
+            const result = await backup;
+
+            expect(holding.most()).toBe(5);
+            expect(result).toMatchObject({
+                totalFiles: 8,
+                downloaded: 8,
+                failed: 0,
+            });
+            expectStored(dir, PHOTO_IDS);
+        } finally {
+            await lib.close();
+        }
+    });
+
+    // The limit is the content pool's, so a library opened with pools of
+    // another `contentConcurrency` backs up that many at once.
+    it("downloads as many originals at once as the content pool of the pools it was opened with allows", async () => {
+        const dir = mkdtempSync(join(root, "backup-"));
+        const holding = holdingServer();
+        const lib = await openLibrary(
+            dir,
+            holding.fetch,
+            new RequestPools({ contentConcurrency: 2 }),
+        );
+        try {
+            const backup = lib.backup();
+            await releaseOneAtATime(holding.held, 2);
+            const result = await backup;
+
+            expect(holding.most()).toBe(2);
+            expect(result).toMatchObject({ downloaded: 8, failed: 0 });
+            expectStored(dir, PHOTO_IDS);
+        } finally {
+            await lib.close();
+        }
+    });
+
+    // With `verify`, hashing an original already stored is part of the work
+    // on it, and counts against the limit too: a large backup never has more
+    // than five originals open for hashing. Here every stored original has
+    // changed on disk, so each is hashed, found not to match, and downloaded
+    // again. While five downloads wait, the backup has hashed only those five
+    // photos' originals; it opens the next one once a download finishes.
+    it("with verify, hashes no more originals ahead of their downloads than the limit", async () => {
+        const dir = mkdtempSync(join(root, "backup-"));
+        const first = await openLibrary(dir, server);
+        try {
+            await first.backup();
+        } finally {
+            await first.close();
+        }
+        for (const id of PHOTO_IDS) {
+            writeFileSync(savedAt(dir, id), "not the photo any more");
+        }
+
+        const holding = holdingServer();
+        const lib = await openLibrary(dir, holding.fetch);
+        const lines: string[] = [];
+        const mismatches = () =>
+            lines.filter((line) => line.startsWith("MISMATCH"));
+        try {
+            const backup = lib.backup({
+                verify: true,
+                onProgress: (line) => lines.push(line),
+            });
+            await vi.waitFor(() => expect(holding.held).toHaveLength(5), {
+                interval: 5,
+                timeout: 5000,
+            });
+            expect(mismatches()).toHaveLength(5);
+
+            await releaseOneAtATime(holding.held, 5);
+            const result = await backup;
+
+            expect(holding.most()).toBe(5);
+            expect(mismatches()).toHaveLength(8);
+            expect(result).toMatchObject({
+                downloaded: 8,
+                verified: 0,
+                mismatched: 8,
+                failed: 0,
+            });
+            expectStored(dir, PHOTO_IDS);
+        } finally {
+            await lib.close();
+        }
+    });
+
+    // The server answers 404 for photo 103's original. A 404 is not retried,
+    // so that download fails at once, while other downloads are still in
+    // flight. The backup records the failure and goes on: the downloads
+    // beside it finish, the photos after it are downloaded, and only photo
+    // 103 is missing from the backup and in failures.json.
+    it("goes on with the other originals when one fails", async () => {
+        const dir = mkdtempSync(join(root, "backup-"));
+        const holding = holdingServer(103);
+        const lib = await openLibrary(dir, holding.fetch);
+        try {
+            const backup = lib.backup();
+            await releaseOneAtATime(holding.held, 5);
+            const result = await backup;
+
+            expect(result).toMatchObject({
+                totalFiles: 8,
+                downloaded: 7,
+                failed: 1,
+            });
+            expect(result.errors.map((e) => e.fileID)).toEqual([103]);
+            expectStored(
+                dir,
+                PHOTO_IDS.filter((id) => id !== 103),
+            );
+            expect(existsSync(savedAt(dir, 103))).toBe(false);
+            const failures = JSON.parse(
+                readFileSync(join(dir, "failures.json"), "utf-8"),
+            );
+            expect(Object.keys(failures.files)).toEqual(["103"]);
+        } finally {
+            await lib.close();
+        }
+    });
+
+    // A throw from `onProgress` fails the backup: `lib.backup()` rejects with
+    // it. From then on the backup starts no further original. It finishes the
+    // ones it is working on, and rejects only once they are in place, so
+    // nothing is written into the backup after it has ended and removed its
+    // lock. Here the first five downloads wait. Photo 103's is let go first
+    // and answered 404, and the line reporting that failure throws while the
+    // other four wait. Only those five downloads are held: a backup that went
+    // on would store the other three photos.
+    it("when onProgress throws, starts no further original, and rejects once the originals it was working on are in place", async () => {
+        const dir = mkdtempSync(join(root, "backup-"));
+        const downloads: number[] = [];
+        const held: { id: number; release: () => void }[] = [];
+        const fetch: typeof globalThis.fetch = async (input, init) => {
+            const id = downloadOf(input);
+            if (id !== undefined) {
+                downloads.push(id);
+                if (downloads.length <= 5) {
+                    await new Promise<void>((release) =>
+                        held.push({ id, release }),
+                    );
+                }
+                if (id === 103) {
+                    return new Response("not found", { status: 404 });
+                }
+            }
+            return server(input, init);
+        };
+        const lib = await openLibrary(dir, fetch);
+        const lines: string[] = [];
+        try {
+            const backup = lib.backup({
+                onProgress: (line) => {
+                    lines.push(line);
+                    if (line.startsWith("FAILED")) {
+                        throw new Error("the progress output is closed");
+                    }
+                },
+            });
+            // The photos whose originals are in place, and whether the lock
+            // is there, at the moment `lib.backup()` rejects.
+            let storedWhenRejected: number[] = [];
+            let lockedWhenRejected = true;
+            const rejection = backup.catch((err: unknown) => {
+                storedWhenRejected = PHOTO_IDS.filter((id) =>
+                    existsSync(savedAt(dir, id)),
+                );
+                lockedWhenRejected = existsSync(join(dir, "backup.lock"));
+                return err;
+            });
+
+            await vi.waitFor(() => expect(held).toHaveLength(5), {
+                interval: 5,
+                timeout: 5000,
+            });
+            const at103 = held.findIndex((h) => h.id === 103);
+            held.splice(at103, 1)[0]!.release();
+            await vi.waitFor(
+                () =>
+                    expect(
+                        lines.some((line) => line.startsWith("FAILED")),
+                    ).toBe(true),
+                { interval: 5, timeout: 5000 },
+            );
+            for (const download of held) download.release();
+
+            expect(await rejection).toEqual(
+                new Error("the progress output is closed"),
+            );
+            expect(downloads.sort()).toEqual([101, 102, 103, 104, 105]);
+            expect(storedWhenRejected).toEqual([101, 102, 104, 105]);
+            expect(lockedWhenRejected).toBe(false);
+        } finally {
+            await lib.close();
+        }
+    });
+
+    // Once the backup's lock is lost, another backup may be writing to the
+    // directory, so the backup starts no further original. It finishes the
+    // ones it is working on, and `lib.backup()` rejects once they are all in
+    // place. Here `backup.lock` is removed while the first five downloads
+    // wait, and proper-lockfile's timer finds it gone. As in
+    // `backup-end-to-end.test.ts`, only `setTimeout` and `clearTimeout` are
+    // faked, so the test can fire that timer without waiting 5 seconds. Only
+    // the first five downloads are held: a backup that went on would store
+    // the other three photos.
+    it("when its lock is lost, starts no further original, and rejects once the originals it was working on are in place", async () => {
+        const dir = mkdtempSync(join(root, "backup-"));
+        const lockPath = join(dir, "backup.lock");
+        const downloads: number[] = [];
+        const held: (() => void)[] = [];
+        const fetch: typeof globalThis.fetch = async (input, init) => {
+            const id = downloadOf(input);
+            if (id !== undefined) {
+                downloads.push(id);
+                if (downloads.length <= 5) {
+                    await new Promise<void>((release) => held.push(release));
+                }
+            }
+            return server(input, init);
+        };
+        const lib = await openLibrary(dir, fetch);
+        vi.useFakeTimers({ toFake: ["setTimeout", "clearTimeout"] });
+        try {
+            const backup = lib.backup();
+            // The photos whose originals are in place at the moment
+            // `lib.backup()` rejects.
+            let storedWhenRejected: number[] = [];
+            const rejection = backup.catch((err: unknown) => {
+                storedWhenRejected = PHOTO_IDS.filter((id) =>
+                    existsSync(savedAt(dir, id)),
+                );
+                return err;
+            });
+
+            await vi.waitFor(() => expect(held).toHaveLength(5), {
+                interval: 5,
+                timeout: 5000,
+            });
+            rmSync(lockPath, { recursive: true });
+            // Fire proper-lockfile's timer, and wait for the look at the lock
+            // on disk that it starts to finish: Node lists a file system call
+            // in progress as an `FSReqCallback`.
+            vi.advanceTimersByTime(5_000);
+            while (process.getActiveResourcesInfo().includes("FSReqCallback")) {
+                await new Promise((resolve) => setImmediate(resolve));
+            }
+            for (const release of held) release();
+
+            expect(await rejection).toEqual(
+                new Error(
+                    `lost the lock on ${dir}: ENOENT: no such file or directory, stat '${lockPath}'`,
+                ),
+            );
+            expect(downloads.sort()).toEqual([101, 102, 103, 104, 105]);
+            expect(storedWhenRejected).toEqual([101, 102, 103, 104, 105]);
+        } finally {
+            vi.useRealTimers();
+            await lib.close();
+        }
+    });
+
+    // Working on several originals at once changes only how long a backup
+    // takes. The same two runs, with a content pool of one, which backs up
+    // one original at a time, and with the default pools, end the same way:
+    // the same result, the same progress lines (in another order), the same
+    // failures.json, and the same tree. The first run stores every photo but
+    // 103, whose download is answered 404. Then two stored originals change
+    // on disk, and a run with `verify` puts them back and tries 103 again.
+    it("ends as a backup of one original at a time does", async () => {
+        const twoRuns = async (pools: RequestPools) => {
+            const dir = mkdtempSync(join(root, "backup-"));
+            const lines: string[] = [];
+            const backup = async (opts: BackupOptions) => {
+                const lib = await openLibrary(dir, failingFor(103), pools);
+                try {
+                    return await lib.backup({
+                        ...opts,
+                        onProgress: (line) => lines.push(line),
+                    });
+                } finally {
+                    await lib.close();
+                }
+            };
+            const first = await backup({});
+            writeFileSync(savedAt(dir, 101), "not the photo any more");
+            writeFileSync(savedAt(dir, 105), "not the photo any more");
+            const second = await backup({ verify: true });
+
+            // Each entry of failures.json without the time it was last
+            // tried, which differs from run to run.
+            const { files } = JSON.parse(
+                readFileSync(join(dir, "failures.json"), "utf-8"),
+            ) as { files: Record<string, { lastTriedAt?: number }> };
+            for (const entry of Object.values(files)) delete entry.lastTriedAt;
+
+            return {
+                first,
+                second,
+                lines: lines.sort(),
+                failures: files,
+                tree: listTree(dir),
+            };
+        };
+
+        const oneAtATime = await twoRuns(
+            new RequestPools({ contentConcurrency: 1 }),
+        );
+        const fiveAtATime = await twoRuns(new RequestPools());
+
+        expect(oneAtATime.first).toMatchObject({
+            totalFiles: 8,
+            downloaded: 7,
+            skipped: 0,
+            failed: 1,
+        });
+        expect(oneAtATime.second).toMatchObject({
+            totalFiles: 8,
+            downloaded: 2,
+            skipped: 5,
+            verified: 5,
+            mismatched: 2,
+            unchecked: 0,
+            failed: 1,
+        });
+        expect(oneAtATime.failures).toEqual({
+            "103": expect.objectContaining({ fileID: 103, attempts: 2 }),
+        });
+        expect(fiveAtATime).toEqual(oneAtATime);
+    });
+});

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

@@ -0,0 +1,835 @@
+/**
+ * `quak backup`, end to end.
+ *
+ * Each test runs the command as cron runs it: `backupCommand` reads the
+ * session file that logging in saved, opens the library on the real `Client`
+ * and backs the whole account up into a directory, and `run` turns the outcome
+ * into the exit code, as `bin/quak.ts` does. Only the server is a stand-in:
+ * `buildMetaFetch` from `test/ente-server.ts` answers the Ente HTTP API from
+ * data this file encrypts with libsodium, as Ente's clients encrypt it. So
+ * quak's own code fetches each original, decrypts it, checks it against the
+ * hash its uploader recorded and unpacks a live photo, and quak's own retries
+ * repeat a failed request. Two tests run `lib.backup()` instead, as a program
+ * using quak as a library does.
+ *
+ * The account, user 42, has three albums:
+ *
+ *     Trips              its own: beach.jpg
+ *     Family             its own: beach.jpg and the live photo IMG_0002.HEIC
+ *     Friend's Wedding   shared with it by user 99: toast.mp4
+ *
+ * beach.jpg is one file in two albums. It holds EXIF (a Canon EOS R5), and
+ * Ente has ML data for it. All three files were taken at noon on 1 March 2024.
+ */
+
+import {
+    existsSync,
+    lstatSync,
+    mkdirSync,
+    mkdtempSync,
+    readdirSync,
+    readFileSync,
+    readlinkSync,
+    rmSync,
+    writeFileSync,
+} from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { Writable } from "node:stream";
+import { gzipSync } from "node:zlib";
+import sodium from "libsodium-wrappers-sumo";
+import {
+    afterAll,
+    afterEach,
+    beforeAll,
+    beforeEach,
+    describe,
+    expect,
+    it,
+    vi,
+} from "vitest";
+
+import {
+    backupCommand,
+    saveSession,
+    type CliContext,
+} from "../../src/cli-commands.js";
+import { run } from "../../src/cli-run.js";
+import { loadSession } from "../../src/cli-session.js";
+import { Client } from "../../src/client.js";
+import { init, toBase64 } from "../../src/crypto/index.js";
+import { Library, lockBackupDirectory } from "../../src/library/index.js";
+import { UNATTENDED_RETRY_OPTIONS } from "../../src/retry.js";
+import {
+    buildAccount,
+    buildMetaFetch,
+    encryptSecretbox,
+    encryptStreamBlob,
+} from "../ente-server.js";
+import { HEIC_WITH_EXIF } from "../exif-heic.js";
+import { JPEG_WITH_EXIF } from "../exif-jpeg.js";
+import { blake2b, livePhotoHash, livePhotoZip, VIDEO } from "../live-photo.js";
+
+const EMAIL = "backup@example.com";
+const PASSWORD = "backup-password";
+// The fake server logs every account in as user 42.
+const USER_ID = 42;
+const FRIEND_ID = 99;
+
+// Noon on 1 March 2024 in the time zone of the machine running the test, the
+// one quak dates save paths in, in microseconds, as Ente records times.
+const TAKEN = new Date(2024, 2, 1, 12).getTime() * 1000;
+// The date folder every original is saved in.
+const DAY = "2024/2024-03/2024-03-01";
+
+const TOAST = new TextEncoder().encode("a few seconds of the wedding toast");
+
+// Ente's ML data for beach.jpg: a face it found, and the photo's CLIP
+// embedding.
+const ML_DATA = {
+    face: { faces: [{ faceID: "100_1", score: 0.98 }] },
+    clip: { embedding: [0.5, 0.6, 0.7] },
+};
+
+// Each file's ID, its owner, its original as uploaded, and the metadata its
+// uploader recorded. `fileType` is Ente's number for the kind: 0 an image, 1 a
+// video, 2 a live photo, whose original is a ZIP of its image and its video.
+// `hash` is the content hash a download must match.
+const BEACH = {
+    id: 100,
+    ownerID: USER_ID,
+    original: JPEG_WITH_EXIF,
+    metadata: {
+        title: "beach.jpg",
+        fileType: 0,
+        hash: blake2b(JPEG_WITH_EXIF),
+    },
+};
+const TOAST_VIDEO = {
+    id: 200,
+    ownerID: FRIEND_ID,
+    original: TOAST,
+    metadata: { title: "toast.mp4", fileType: 1, hash: blake2b(TOAST) },
+};
+const LIVE_PHOTO = {
+    id: 300,
+    ownerID: USER_ID,
+    original: livePhotoZip({
+        "image.heic": HEIC_WITH_EXIF,
+        "video.mov": VIDEO,
+    }),
+    metadata: {
+        title: "IMG_0002.HEIC",
+        fileType: 2,
+        hash: livePhotoHash(HEIC_WITH_EXIF, VIDEO),
+    },
+};
+
+// The fake Ente server, as the `fetch` quak sends its requests through.
+let server: typeof globalThis.fetch;
+let root: string;
+let sessionDir: string;
+
+// The fake server, holding the account encrypted as Ente's clients upload it.
+const buildServer = async () => {
+    const account = await buildAccount(PASSWORD);
+
+    // An album as `/collections/v2` lists it, its name encrypted with its own
+    // key. An album of the account's own carries that key encrypted with the
+    // master key; an album shared with it carries the key sealed to its public
+    // key, and no nonce.
+    const album = (id: number, name: string, ownerID: number) => {
+        const key = sodium.crypto_secretbox_keygen();
+        const encryptedName = encryptSecretbox(
+            new TextEncoder().encode(name),
+            key,
+        );
+        let keyFields: { encryptedKey: string; keyDecryptionNonce?: string };
+        if (ownerID === USER_ID) {
+            const encryptedKey = encryptSecretbox(key, account.masterKey);
+            keyFields = {
+                encryptedKey: toBase64(encryptedKey.ciphertext),
+                keyDecryptionNonce: toBase64(encryptedKey.nonce),
+            };
+        } else {
+            const sealedKey = sodium.crypto_box_seal(key, account.publicKey);
+            keyFields = { encryptedKey: toBase64(sealedKey) };
+        }
+        const row = {
+            id,
+            owner: { id: ownerID },
+            ...keyFields,
+            encryptedName: toBase64(encryptedName.ciphertext),
+            nameDecryptionNonce: toBase64(encryptedName.nonce),
+            type: "album",
+            updationTime: TAKEN,
+        };
+        return { key, row };
+    };
+    const trips = album(10, "Trips", USER_ID);
+    const family = album(20, "Family", USER_ID);
+    const wedding = album(30, "Friend's Wedding", FRIEND_ID);
+
+    // Each file's original is encrypted with the file's own key, as one
+    // secretstream, which the server serves whole.
+    const encrypted = (file: typeof BEACH) => {
+        const key = sodium.crypto_secretstream_xchacha20poly1305_keygen();
+        return { file, key, content: encryptStreamBlob(file.original, key) };
+    };
+    const beach = encrypted(BEACH);
+    const toast = encrypted(TOAST_VIDEO);
+    const livePhoto = encrypted(LIVE_PHOTO);
+
+    // A file as an album's `/collections/v2/diff` lists it: its key encrypted
+    // with the album's key, its metadata encrypted with its own key, and the
+    // header that starts the decryption of its original.
+    const inAlbum = (
+        { file, key, content }: ReturnType<typeof encrypted>,
+        collectionID: number,
+        albumKey: Uint8Array,
+    ) => {
+        const encryptedKey = encryptSecretbox(key, albumKey);
+        const metadata = encryptStreamBlob(
+            new TextEncoder().encode(
+                JSON.stringify({
+                    ...file.metadata,
+                    creationTime: TAKEN,
+                    modificationTime: TAKEN,
+                }),
+            ),
+            key,
+        );
+        return {
+            id: file.id,
+            collectionID,
+            ownerID: file.ownerID,
+            encryptedKey: toBase64(encryptedKey.ciphertext),
+            keyDecryptionNonce: toBase64(encryptedKey.nonce),
+            metadata: {
+                encryptedData: toBase64(metadata.ciphertext),
+                decryptionHeader: toBase64(metadata.header),
+            },
+            file: { decryptionHeader: toBase64(content.header) },
+            thumbnail: {
+                decryptionHeader: toBase64(sodium.randombytes_buf(24)),
+            },
+            updationTime: TAKEN,
+        };
+    };
+
+    // ML data is gzipped JSON, encrypted with the file's key.
+    const mlData = encryptStreamBlob(
+        gzipSync(JSON.stringify(ML_DATA)),
+        beach.key,
+    );
+
+    return buildMetaFetch({
+        verifier: account.verifier,
+        srpAttributes: account.srpAttributes,
+        keyAttributes: account.keyAttributes,
+        encryptedToken: account.encryptedToken,
+        collections: [trips.row, family.row, wedding.row],
+        filesByCollection: {
+            10: [inAlbum(beach, 10, trips.key)],
+            20: [
+                inAlbum(beach, 20, family.key),
+                inAlbum(livePhoto, 20, family.key),
+            ],
+            30: [inAlbum(toast, 30, wedding.key)],
+        },
+        encryptedMLData: {
+            100: {
+                encryptedData: toBase64(mlData.ciphertext),
+                decryptionHeader: toBase64(mlData.header),
+            },
+        },
+        fileCiphertexts: {
+            100: beach.content.ciphertext,
+            200: toast.content.ciphertext,
+            300: livePhoto.content.ciphertext,
+        },
+    });
+};
+
+beforeAll(async () => {
+    await init();
+    await sodium.ready;
+    root = mkdtempSync(join(tmpdir(), "quak-backup-end-to-end-"));
+    server = await buildServer();
+
+    // Log in once, as `quak login` does: the session file it saves is what
+    // every backup below reads.
+    sessionDir = join(root, "session");
+    const client = await Client.login({
+        email: EMAIL,
+        password: PASSWORD,
+        apiOptions: { fetch: server },
+    });
+    saveSession(sessionDir, client.toJSON());
+});
+
+afterAll(() => {
+    rmSync(root, { recursive: true, force: true });
+});
+
+// Run `quak backup <dir>` as `bin/quak.ts` runs it, sending its requests
+// through `fetch`, and return its exit code and everything it printed on
+// stderr. Its requests are retried as those of `quak backup` are, except that
+// a retry does not wait. Its cache is beside `dir`, so each run of one
+// directory finds the cache the run before it left, as cron's runs find the
+// user's.
+const backup = async (
+    dir: string,
+    opts: { verify?: boolean } = {},
+    fetch = server,
+): Promise<{ code: number; stderr: string }> => {
+    let printed = "";
+    const stderr = new Writable({
+        write(chunk: Buffer, _encoding, done) {
+            printed += chunk.toString();
+            done();
+        },
+    });
+    const stdout = new Writable({
+        write(_chunk, _encoding, done) {
+            done();
+        },
+    });
+    // A backup never prompts: any of these ends the run with exit 1.
+    const prompted = async (): Promise<never> => {
+        throw new Error("quak backup prompted");
+    };
+    const ctx: CliContext = {
+        stdout,
+        stderr,
+        sessionDir,
+        cacheDir: `${dir}-cache`,
+        loadSession: (path) =>
+            loadSession(path, {
+                fetch,
+                retry: { ...UNATTENDED_RETRY_OPTIONS, sleep: async () => {} },
+            }),
+        login: prompted,
+        prompt: prompted,
+        promptSecret: prompted,
+    };
+    const code = await new Promise<number>((resolve) => {
+        void run(backupCommand(ctx, dir, opts), stdout, stderr, resolve);
+    });
+    return { code, stderr: printed };
+};
+
+// The ID of the file whose original a request downloads, or undefined for any
+// other request. quak downloads an original from
+// `https://files.ente.io/?fileID=<id>`.
+const downloadOf = (input: RequestInfo | URL): number | undefined => {
+    const url = new URL(String(input));
+    return url.hostname === "files.ente.io"
+        ? Number(url.searchParams.get("fileID"))
+        : undefined;
+};
+
+// The fake server, pushing onto `downloads` the ID of each file whose original
+// is downloaded.
+const recordingDownloads =
+    (downloads: number[]): typeof globalThis.fetch =>
+    async (input, init) => {
+        const fileID = downloadOf(input);
+        if (fileID !== undefined) downloads.push(fileID);
+        return server(input, init);
+    };
+
+// Every entry under `dir`, relative to it, sorted, a symlink as
+// `name -> target`.
+const listTree = (dir: string): string[] =>
+    readdirSync(dir, { recursive: true })
+        .sort()
+        .map((name) => {
+            const path = join(dir, name);
+            return lstatSync(path).isSymbolicLink()
+                ? `${name} -> ${readlinkSync(path)}`
+                : name;
+        });
+
+const readJSON = (path: string) => JSON.parse(readFileSync(path, "utf-8"));
+
+describe("the first backup of the account", () => {
+    let dir: string;
+    let code: number;
+    const downloads: number[] = [];
+
+    beforeAll(async () => {
+        dir = mkdtempSync(join(root, "backup-"));
+        ({ code } = await backup(dir, {}, recordingDownloads(downloads)));
+    });
+
+    it("exits 0, having downloaded each file once", () => {
+        expect(code).toBe(0);
+        // beach.jpg is in two albums but is downloaded, and stored, once.
+        expect(downloads.sort()).toEqual([100, 200, 300]);
+    });
+
+    // The tree "Backup layout" in the README describes. Each original is at
+    // its save path, `YYYY/YYYY-MM/YYYY-MM-DD/YYYY-MM-DD.<fileID>.<ext>`, with
+    // the extension of its title; the live photo is its image and its video,
+    // each with the extension it had inside the ZIP, and a `.livephoto.json`
+    // naming the two. Beside each original is its JSON. Each album is a
+    // folder of links named after the files' titles, with a JSON file beside
+    // it. No file failed, so there is no `failures.json`, and the run removed
+    // its lock, `backup.lock`, as it ended.
+    it("lays the backup out as the README describes", () => {
+        expect(listTree(dir)).toEqual([
+            "2024",
+            "2024/2024-03",
+            "2024/2024-03/2024-03-01",
+            "2024/2024-03/2024-03-01/2024-03-01.100.jpg",
+            "2024/2024-03/2024-03-01/2024-03-01.100.json",
+            "2024/2024-03/2024-03-01/2024-03-01.200.json",
+            "2024/2024-03/2024-03-01/2024-03-01.200.mp4",
+            "2024/2024-03/2024-03-01/2024-03-01.300.heic",
+            "2024/2024-03/2024-03-01/2024-03-01.300.json",
+            "2024/2024-03/2024-03-01/2024-03-01.300.livephoto.json",
+            "2024/2024-03/2024-03-01/2024-03-01.300.mov",
+            "account.json",
+            "collections",
+            "collections/Family",
+            "collections/Family.json",
+            "collections/Family/IMG_0002.heic -> ../../2024/2024-03/2024-03-01/2024-03-01.300.heic",
+            "collections/Family/IMG_0002.mov -> ../../2024/2024-03/2024-03-01/2024-03-01.300.mov",
+            "collections/Family/beach.jpg -> ../../2024/2024-03/2024-03-01/2024-03-01.100.jpg",
+            "collections/Friend's Wedding",
+            "collections/Friend's Wedding.json",
+            "collections/Friend's Wedding/toast.mp4 -> ../../2024/2024-03/2024-03-01/2024-03-01.200.mp4",
+            "collections/Trips",
+            "collections/Trips.json",
+            "collections/Trips/beach.jpg -> ../../2024/2024-03/2024-03-01/2024-03-01.100.jpg",
+        ]);
+    });
+
+    it("stores each original decrypted, a live photo as its image and its video", () => {
+        const day = join(dir, DAY);
+        expect(readFileSync(join(day, "2024-03-01.100.jpg"))).toEqual(
+            Buffer.from(JPEG_WITH_EXIF),
+        );
+        expect(readFileSync(join(day, "2024-03-01.200.mp4"))).toEqual(
+            Buffer.from(TOAST),
+        );
+        expect(readFileSync(join(day, "2024-03-01.300.heic"))).toEqual(
+            Buffer.from(HEIC_WITH_EXIF),
+        );
+        expect(readFileSync(join(day, "2024-03-01.300.mov"))).toEqual(
+            Buffer.from(VIDEO),
+        );
+        expect(readJSON(join(day, "2024-03-01.300.livephoto.json"))).toEqual({
+            image: "2024-03-01.300.heic",
+            video: "2024-03-01.300.mov",
+        });
+    });
+
+    // A file's JSON holds what Ente records for it, decrypted, and what quak
+    // read from its original: its ML data when Ente has any, and, for an
+    // image, its EXIF. A video gets no image metadata.
+    it("writes each file's JSON with its ML data and image metadata", () => {
+        const day = join(dir, DAY);
+
+        const beach = readJSON(join(day, "2024-03-01.100.json"));
+        expect(beach).toMatchObject({
+            id: 100,
+            ownerID: USER_ID,
+            metadata: {
+                title: "beach.jpg",
+                fileType: "image",
+                creationTime: TAKEN,
+                hash: BEACH.metadata.hash,
+            },
+            updationTime: TAKEN,
+        });
+        expect(beach.mlData).toEqual(ML_DATA);
+        expect(beach.imageMetadata.exif.Make.description).toBe("Canon");
+        expect(beach.imageMetadata.exif.Model.description).toBe("EOS R5");
+
+        const toast = readJSON(join(day, "2024-03-01.200.json"));
+        expect(toast).toMatchObject({
+            id: 200,
+            ownerID: FRIEND_ID,
+            metadata: { title: "toast.mp4", fileType: "video" },
+        });
+        expect(toast).not.toHaveProperty("mlData");
+        expect(toast).not.toHaveProperty("imageMetadata");
+        expect(toast).not.toHaveProperty("imageMetadataError");
+
+        // A live photo's image metadata is read from its image.
+        const livePhoto = readJSON(join(day, "2024-03-01.300.json"));
+        expect(livePhoto).toMatchObject({
+            id: 300,
+            ownerID: USER_ID,
+            metadata: {
+                title: "IMG_0002.HEIC",
+                fileType: "livePhoto",
+                hash: LIVE_PHOTO.metadata.hash,
+            },
+        });
+        expect(livePhoto.imageMetadata.exif.Make.description).toBe("Canon");
+    });
+
+    it("writes each album's JSON and the account's", () => {
+        const albumJSON = (name: string) =>
+            readJSON(join(dir, "collections", `${name}.json`));
+        const fileIDs = (album: { files: { id: number }[] }) =>
+            album.files.map((f) => f.id);
+
+        const trips = albumJSON("Trips");
+        expect(trips).toMatchObject({
+            id: 10,
+            name: "Trips",
+            type: "album",
+            ownerID: USER_ID,
+            isShared: false,
+            updationTime: TAKEN,
+        });
+        expect(fileIDs(trips)).toEqual([100]);
+
+        const family = albumJSON("Family");
+        expect(family).toMatchObject({ id: 20, isShared: false });
+        expect(fileIDs(family)).toEqual([100, 300]);
+
+        const wedding = albumJSON("Friend's Wedding");
+        expect(wedding).toMatchObject({
+            id: 30,
+            ownerID: FRIEND_ID,
+            isShared: true,
+        });
+        expect(fileIDs(wedding)).toEqual([200]);
+
+        expect(readJSON(join(dir, "account.json"))).toEqual({
+            email: EMAIL,
+            userID: USER_ID,
+        });
+    });
+});
+
+describe("quak backup", () => {
+    // An original already at its save path is complete, so a later run, such
+    // as the next night's from cron, only rewrites the JSON files and the
+    // album links.
+    it("downloads nothing on a second run, and exits 0", async () => {
+        const dir = mkdtempSync(join(root, "backup-"));
+        expect((await backup(dir)).code).toBe(0);
+        const firstTree = listTree(dir);
+
+        const downloads: number[] = [];
+        const { code } = await backup(dir, {}, recordingDownloads(downloads));
+        expect(code).toBe(0);
+        expect(downloads).toEqual([]);
+        expect(listTree(dir)).toEqual(firstTree);
+    });
+
+    // The files CDN is briefly unavailable for toast.mp4: its first two
+    // downloads answer 503. A 5xx is worth repeating, so quak tries again,
+    // here without waiting, and the third download succeeds.
+    it("stores a file whose download fails twice with 503 and then succeeds, and exits 0", async () => {
+        let attempts = 0;
+        const fetch: typeof globalThis.fetch = async (input, init) => {
+            if (downloadOf(input) === 200 && ++attempts <= 2) {
+                return new Response("unavailable", { status: 503 });
+            }
+            return server(input, init);
+        };
+        const dir = mkdtempSync(join(root, "backup-"));
+
+        const { code } = await backup(dir, {}, fetch);
+
+        expect(code).toBe(0);
+        expect(attempts).toBe(3);
+        expect(readFileSync(join(dir, DAY, "2024-03-01.200.mp4"))).toEqual(
+            Buffer.from(TOAST),
+        );
+        expect(existsSync(join(dir, "failures.json"))).toBe(false);
+    });
+
+    // The server answers 404 for toast.mp4 on every try. A 404 is an answer,
+    // not a hiccup, so quak does not repeat it: the file goes into
+    // `failures.json` with the kind of failure, how many times it has been
+    // tried and why, the backup goes on with the other files, and the exit
+    // code is 1, so the next run tries again.
+    it("records a file whose download always answers 404 in failures.json, stores the rest, and exits 1", async () => {
+        let attempts = 0;
+        const fetch: typeof globalThis.fetch = async (input, init) => {
+            if (downloadOf(input) === 200) {
+                attempts++;
+                return new Response("not found", { status: 404 });
+            }
+            return server(input, init);
+        };
+        const dir = mkdtempSync(join(root, "backup-"));
+
+        const { code, stderr } = await backup(dir, {}, fetch);
+
+        expect(code).toBe(1);
+        expect(attempts).toBe(1);
+        const failures = readJSON(join(dir, "failures.json"));
+        expect(Object.keys(failures.files)).toEqual(["200"]);
+        expect(failures.files["200"]).toMatchObject({
+            fileID: 200,
+            title: "toast.mp4",
+            classification: "permanent",
+            attempts: 1,
+        });
+        expect(failures.files["200"].error).toContain("404");
+        expect(stderr).toContain("[Friend's Wedding] toast.mp4 (id 200): ");
+
+        // The rest of the account is backed up; the video is not, and its
+        // album folder has no link to it.
+        expect(readFileSync(join(dir, DAY, "2024-03-01.100.jpg"))).toEqual(
+            Buffer.from(JPEG_WITH_EXIF),
+        );
+        expect(readFileSync(join(dir, DAY, "2024-03-01.300.heic"))).toEqual(
+            Buffer.from(HEIC_WITH_EXIF),
+        );
+        expect(existsSync(join(dir, DAY, "2024-03-01.200.mp4"))).toBe(false);
+        expect(
+            readdirSync(join(dir, "collections", "Friend's Wedding")),
+        ).toEqual([]);
+    });
+
+    // The server no longer accepts the saved session's token, as after
+    // `quak logout` elsewhere, and answers every request with 401. The run
+    // stops at the refresh it starts with, says to log in again, and exits 3.
+    // It took its lock first, so every request was sent while the lock was
+    // there, and it removed the lock as it ended: the directory is left as it
+    // was, empty.
+    it("exits 3 when the refresh is answered with 401, writing nothing but the lock", async () => {
+        const dir = mkdtempSync(join(root, "backup-"));
+        const lockAtEachRequest: boolean[] = [];
+        const fetch: typeof globalThis.fetch = async () => {
+            lockAtEachRequest.push(existsSync(join(dir, "backup.lock")));
+            return new Response(null, { status: 401 });
+        };
+
+        const { code, stderr } = await backup(dir, {}, fetch);
+
+        expect(code).toBe(3);
+        expect(stderr).toBe(
+            "Starting backup...\n" +
+                "Refreshing library...\n" +
+                `quak: the saved session is no longer valid; run "quak login"\n`,
+        );
+        expect(lockAtEachRequest).not.toHaveLength(0);
+        expect(lockAtEachRequest).not.toContain(false);
+        expect(readdirSync(dir)).toEqual([]);
+    });
+
+    // Something changed beach.jpg on disk after it was backed up. A run with
+    // --verify hashes every stored original, finds that one no longer matches
+    // the hash Ente recorded, says so, and downloads it again; it put back the
+    // right bytes, so the exit code is 0.
+    it("--verify puts back an original that changed on disk, and exits 0", async () => {
+        const dir = mkdtempSync(join(root, "backup-"));
+        expect((await backup(dir)).code).toBe(0);
+        const beach = join(dir, DAY, "2024-03-01.100.jpg");
+        writeFileSync(beach, "not the photo any more");
+
+        const downloads: number[] = [];
+        const { code, stderr } = await backup(
+            dir,
+            { verify: true },
+            recordingDownloads(downloads),
+        );
+
+        expect(code).toBe(0);
+        expect(downloads).toEqual([100]);
+        expect(readFileSync(beach)).toEqual(Buffer.from(JPEG_WITH_EXIF));
+        expect(stderr).toContain(
+            "MISMATCH original beach.jpg (100): its bytes do not match its content hash",
+        );
+        expect(stderr).toContain("  Verified:    2\n  Mismatched:  1\n");
+    });
+});
+
+// `backup.lock` is a directory that proper-lockfile touches every 5 seconds,
+// from a timer, while the backup runs. Each time, it first looks at the lock
+// on disk. The lock is lost when it is gone, or when another backup has made
+// its own in its place, as one does with a lock nobody touched for 10
+// seconds. Another backup may then be writing to the directory, so the backup
+// stops before it starts another file.
+//
+// The fake server holds one file's download open until the test lets it go,
+// so the lock can be lost while the backup runs: beach.jpg's, the first file
+// backed up, unless the test names another. Only `setTimeout` and
+// `clearTimeout` are faked, so a test can fire proper-lockfile's timer without
+// waiting 5 seconds.
+describe("a backup whose lock is lost", () => {
+    let dir: string;
+    let lockPath: string;
+    let downloads: number[];
+    let heldFile: number;
+    let heldRequested: Promise<void>;
+    let releaseHeld: () => void;
+    let fetch: typeof globalThis.fetch;
+
+    beforeEach(() => {
+        dir = mkdtempSync(join(root, "backup-"));
+        lockPath = join(dir, "backup.lock");
+        downloads = [];
+        heldFile = BEACH.id;
+        let requested!: () => void;
+        heldRequested = new Promise((resolve) => {
+            requested = resolve;
+        });
+        const released = new Promise<void>((resolve) => {
+            releaseHeld = resolve;
+        });
+        const recording = recordingDownloads(downloads);
+        fetch = async (input, init) => {
+            if (downloadOf(input) === heldFile) {
+                requested();
+                await released;
+            }
+            return recording(input, init);
+        };
+        vi.useFakeTimers({ toFake: ["setTimeout", "clearTimeout"] });
+    });
+
+    afterEach(() => {
+        vi.useRealTimers();
+    });
+
+    // Fire proper-lockfile's timer, and wait for the look at the lock on disk
+    // that it starts to finish: Node lists a file system call in progress as
+    // an `FSReqCallback`.
+    const fireLockTimer = async (): Promise<void> => {
+        vi.advanceTimersByTime(5_000);
+        while (process.getActiveResourcesInfo().includes("FSReqCallback")) {
+            await new Promise((resolve) => setImmediate(resolve));
+        }
+    };
+
+    // The backup downloads up to five originals at once, so it has started
+    // all three when the lock is lost, and beach.jpg is stored. The backup
+    // then stops before it fetches ML data. `quak backup` prints why as one
+    // line, with no stack trace, and exits 1.
+    it("stops quak backup before it fetches ML data when backup.lock is removed, and exits 1", async () => {
+        const result = backup(dir, {}, fetch);
+        await heldRequested;
+        rmSync(lockPath, { recursive: true });
+        await fireLockTimer();
+        releaseHeld();
+        const { code, stderr } = await result;
+
+        expect(code).toBe(1);
+        expect(stderr).toBe(
+            "Starting backup...\n" +
+                "Refreshing library...\n" +
+                "Fetching original beach.jpg (100)...\n" +
+                "Fetching original IMG_0002.HEIC (300)...\n" +
+                "Fetching original toast.mp4 (200)...\n" +
+                `quak: lost the lock on ${dir}: ENOENT: no such file or directory, stat '${lockPath}'\n`,
+        );
+        expect(downloads.sort()).toEqual([100, 200, 300]);
+        expect(readFileSync(join(dir, DAY, "2024-03-01.100.jpg"))).toEqual(
+            Buffer.from(JPEG_WITH_EXIF),
+        );
+    });
+
+    // Here the lock is lost while toast.mp4, the last original, downloads.
+    // All three originals are stored, beside what the backup wrote before it
+    // started them, but no file's JSON and no album is written.
+    it("stops quak backup before any file's JSON when the lock is lost during the last original", async () => {
+        heldFile = TOAST_VIDEO.id;
+        const result = backup(dir, {}, fetch);
+        await heldRequested;
+        rmSync(lockPath, { recursive: true });
+        await fireLockTimer();
+        releaseHeld();
+        const { code, stderr } = await result;
+
+        expect(code).toBe(1);
+        expect(stderr).toContain(`quak: lost the lock on ${dir}: `);
+        expect(listTree(dir)).toEqual([
+            "2024",
+            "2024/2024-03",
+            "2024/2024-03/2024-03-01",
+            "2024/2024-03/2024-03-01/2024-03-01.100.jpg",
+            "2024/2024-03/2024-03-01/2024-03-01.200.mp4",
+            "2024/2024-03/2024-03-01/2024-03-01.300.heic",
+            "2024/2024-03/2024-03-01/2024-03-01.300.livephoto.json",
+            "2024/2024-03/2024-03-01/2024-03-01.300.mov",
+            "account.json",
+            "collections",
+        ]);
+    });
+
+    // Nothing touches the lock but quak, which finds it as it left it each
+    // time it looks, so the backup runs to the end and removes its lock.
+    it("lets quak backup run to the end while its lock stays", async () => {
+        const result = backup(dir, {}, fetch);
+        await heldRequested;
+        await fireLockTimer();
+        await fireLockTimer();
+        releaseHeld();
+        const { code } = await result;
+
+        expect(code).toBe(0);
+        expect(downloads.sort()).toEqual([100, 200, 300]);
+        expect(existsSync(lockPath)).toBe(false);
+    });
+
+    // The library a program using quak opens to back `dir` up.
+    const openLibrary = () =>
+        Library.open({
+            client: loadSession(join(sessionDir, "session.json"), { fetch })!,
+            cacheDirectory: `${dir}-cache`,
+            downloadDirectory: dir,
+            refreshIntervalSeconds: Infinity,
+            precacheThumbnails: false,
+            precacheOriginals: false,
+        });
+
+    // A program using quak as a library sees the same: `lib.backup()` rejects
+    // with the reason. Here another backup has made its own lock in place of
+    // quak's, and that lock is left where it is.
+    it("rejects lib.backup() when another backup has taken the lock over", async () => {
+        const lib = await openLibrary();
+        try {
+            const result = lib.backup();
+            await heldRequested;
+            rmSync(lockPath, { recursive: true });
+            mkdirSync(lockPath);
+            await fireLockTimer();
+            releaseHeld();
+
+            await expect(result).rejects.toThrow(
+                `lost the lock on ${dir}: Unable to update lock within the stale threshold`,
+            );
+            expect(downloads.sort()).toEqual([100, 200, 300]);
+            expect(existsSync(lockPath)).toBe(true);
+        } finally {
+            await lib.close();
+        }
+    });
+
+    // A program can take the lock before it opens its library, as `quak
+    // backup` does: `lockBackupDirectory` returns the lock, which the program
+    // passes to `lib.backup()` and releases once the backup ends. The backup
+    // checks it as it checks a lock it took, and the lock's `lost` says why it
+    // was lost.
+    it("rejects lib.backup() given its caller's lock once that lock is lost", async () => {
+        const lock = await lockBackupDirectory(dir);
+        const lib = await openLibrary();
+        try {
+            const result = lib.backup({ lock });
+            await heldRequested;
+            rmSync(lockPath, { recursive: true });
+            await fireLockTimer();
+            releaseHeld();
+
+            const lost = `lost the lock on ${dir}: ENOENT: no such file or directory, stat '${lockPath}'`;
+            await expect(result).rejects.toThrow(lost);
+            expect(lock.lost?.message).toBe(lost);
+            expect(downloads.sort()).toEqual([100, 200, 300]);
+        } finally {
+            await lib.close();
+            await lock.release();
+        }
+    });
+});

test/cli/backup-undecryptable.test.ts

@@ -0,0 +1,554 @@
+/**
+ * `quak backup` over an account holding an album or a file quak cannot decrypt
+ * (issue 181).
+ *
+ * The server sends every album and file encrypted, and a refresh decrypts each
+ * one it fetches. A record whose key no key of the account opens, or whose
+ * metadata is not JSON, must not fail the refresh: a refresh that fails commits
+ * nothing, so `quak backup` would store nothing, on every run, until the
+ * server's copy changed. Instead the refresh leaves that record out, and the
+ * library keeps a list of what it left out in its cache. It has to: the next
+ * refresh asks the server only for what changed since the last one, so it will
+ * not see the record again.
+ *
+ * The backup stores every other file. It records each file it could not
+ * decrypt in `failures.json`, prints one line for each album it could not
+ * decrypt, and exits 1 while any remain. Once the server sends a copy that
+ * decrypts, or deletes the record, the next run drops it and can exit 0. The
+ * read commands answer from what did decrypt.
+ *
+ * These tests run the commands with a real saved session over a stand-in Ente
+ * server, which serves albums, files and originals encrypted with libsodium as
+ * the real server does. A record is broken the way a corrupt one arrives: its
+ * key replaced by bytes no key opens, or its metadata text that is not JSON.
+ */
+
+import {
+    existsSync,
+    mkdtempSync,
+    readdirSync,
+    readFileSync,
+    rmSync,
+} from "node:fs";
+import { join } from "node:path";
+import { tmpdir } from "node:os";
+import sodium from "libsodium-wrappers-sumo";
+import { afterEach, beforeAll, beforeEach, describe, expect, it } from "vitest";
+
+import {
+    backupCommand,
+    collectionsCommand,
+    filesCommand,
+    saveSession,
+    type CliContext,
+} from "../../src/cli-commands.js";
+import { loadSession } from "../../src/cli-session.js";
+import { Client, type ClientSnapshot } from "../../src/client.js";
+import { encryptBlob, init, toBase64 } from "../../src/crypto/index.js";
+import { Library } from "../../src/library/index.js";
+
+const USER_ID = 42;
+
+// Every file is taken at noon local time on 2026-03-01, in microseconds as Ente
+// stores times, so its original is saved in the folder `DAY` whatever the time
+// zone of the machine running the tests.
+const TAKEN = new Date(2026, 2, 1, 12).getTime() * 1000;
+const DAY = join("2026", "2026-03", "2026-03-01");
+
+type Row = { id: number; updationTime: number } & Record<string, unknown>;
+
+// `plaintext` sealed under `key` with a fresh nonce, both base64, as Ente
+// sends a key or an album name.
+const secretbox = (
+    plaintext: Uint8Array,
+    key: Uint8Array,
+): { ciphertext: string; nonce: string } => {
+    const nonce = sodium.randombytes_buf(sodium.crypto_secretbox_NONCEBYTES);
+    return {
+        ciphertext: toBase64(
+            sodium.crypto_secretbox_easy(plaintext, nonce, key),
+        ),
+        nonce: toBase64(nonce),
+    };
+};
+
+// What a corrupt row carries in place of its sealed key: random bytes of the
+// same length, which no key opens.
+const corruptKey = (): string =>
+    toBase64(
+        sodium.randombytes_buf(
+            sodium.crypto_secretbox_KEYBYTES + sodium.crypto_secretbox_MACBYTES,
+        ),
+    );
+
+const json = (body: unknown): Response =>
+    new Response(JSON.stringify(body), {
+        status: 200,
+        headers: { "content-type": "application/json" },
+    });
+
+// A stand-in Ente server for one account. It holds each album, and each file in
+// an album, as the row `/collections/v2` and `/collections/v2/diff` serve, and
+// answers a request with the rows whose `updationTime` is past the request's
+// `sinceTime`, as the real server does. A change to a file also moves its
+// album's `updationTime`, which is what tells a client to list the album's
+// files again. Originals are served by file ID, as the files CDN serves them.
+class FakeEnte {
+    private readonly masterKey = sodium.crypto_secretbox_keygen();
+    private readonly keyPair = sodium.crypto_box_keypair();
+    private readonly albums = new Map<number, Row>();
+    private readonly albumKeys = new Map<number, Uint8Array>();
+    // Each album's file rows, by album ID and then file ID.
+    private readonly files = new Map<number, Map<number, Row>>();
+    // Each file's key and its original as served, by file ID. A file in two
+    // albums is one original under one key, sealed under each album's key.
+    private readonly originals = new Map<
+        number,
+        { key: Uint8Array; header: string; body: Uint8Array }
+    >();
+
+    // The session `quak login` would have saved for the account.
+    snapshot(): ClientSnapshot {
+        return {
+            email: "user@example.com",
+            userID: USER_ID,
+            token: "token",
+            masterKey: toBase64(this.masterKey),
+            secretKey: toBase64(this.keyPair.privateKey),
+            publicKey: toBase64(this.keyPair.publicKey),
+        };
+    }
+
+    // Album `id`, named `name`, as of time `t`. With `corrupt`, its key cannot
+    // be decrypted. An album keeps its key from one call to the next, so its
+    // files' rows still decrypt once it is put back without `corrupt`.
+    putAlbum(id: number, name: string, t: number, corrupt = false): void {
+        const key = this.albumKeys.get(id) ?? sodium.crypto_secretbox_keygen();
+        this.albumKeys.set(id, key);
+        const sealedKey = secretbox(key, this.masterKey);
+        const sealedName = secretbox(new TextEncoder().encode(name), key);
+        this.albums.set(id, {
+            id,
+            owner: { id: USER_ID },
+            encryptedKey: corrupt ? corruptKey() : sealedKey.ciphertext,
+            keyDecryptionNonce: sealedKey.nonce,
+            encryptedName: sealedName.ciphertext,
+            nameDecryptionNonce: sealedName.nonce,
+            type: "album",
+            updationTime: t,
+        });
+        if (!this.files.has(id)) this.files.set(id, new Map());
+    }
+
+    // File `id`, titled `photo-<id>.jpg`, in album `albumID` as of time `t`.
+    // With `corruptKey`, its key cannot be decrypted. With `metadata`, its
+    // metadata decrypts to that text instead of the JSON a client uploads.
+    putFile(
+        albumID: number,
+        id: number,
+        t: number,
+        opts: { corruptKey?: boolean; metadata?: string } = {},
+    ): void {
+        const original = this.originalOf(id);
+        const sealedKey = secretbox(original.key, this.albumKeys.get(albumID)!);
+        const metadata =
+            opts.metadata ??
+            JSON.stringify({
+                title: `photo-${id}.jpg`,
+                fileType: 0,
+                creationTime: TAKEN,
+                modificationTime: TAKEN,
+            });
+        const sealedMetadata = encryptBlob(
+            new TextEncoder().encode(metadata),
+            original.key,
+        );
+        this.files.get(albumID)!.set(id, {
+            id,
+            collectionID: albumID,
+            ownerID: USER_ID,
+            encryptedKey: opts.corruptKey ? corruptKey() : sealedKey.ciphertext,
+            keyDecryptionNonce: sealedKey.nonce,
+            metadata: {
+                encryptedData: toBase64(sealedMetadata.ciphertext),
+                decryptionHeader: toBase64(sealedMetadata.header),
+            },
+            file: { decryptionHeader: original.header },
+            thumbnail: { decryptionHeader: original.header },
+            updationTime: t,
+        });
+        this.moveAlbum(albumID, t);
+    }
+
+    // A deleted record stays on the server as a tombstone holding only its
+    // ID, its time and `isDeleted`.
+    deleteFile(albumID: number, id: number, t: number): void {
+        this.files
+            .get(albumID)!
+            .set(id, { id, updationTime: t, isDeleted: true });
+        this.moveAlbum(albumID, t);
+    }
+
+    deleteAlbum(id: number, t: number): void {
+        this.albums.set(id, { id, updationTime: t, isDeleted: true });
+    }
+
+    // The `fetch` the saved session's client is given.
+    readonly fetch = (async (input: string | URL | Request) => {
+        const url = new URL(input instanceof Request ? input.url : input);
+        const since = Number(url.searchParams.get("sinceTime"));
+        const changed = (rows: Iterable<Row>): Row[] =>
+            [...rows].filter((row) => row.updationTime > since);
+        switch (url.pathname) {
+            case "/collections/v2":
+                return json({ collections: changed(this.albums.values()) });
+            case "/collections/v2/diff": {
+                const albumID = Number(url.searchParams.get("collectionID"));
+                const rows = this.files.get(albumID)?.values() ?? [];
+                return json({ diff: changed(rows), hasMore: false });
+            }
+            case "/files/data/fetch":
+                // No file has ML data.
+                return json({ data: [] });
+            default: {
+                const fileID = Number(url.searchParams.get("fileID"));
+                const original = this.originals.get(fileID);
+                return original
+                    ? new Response(original.body)
+                    : new Response("not found", { status: 404 });
+            }
+        }
+    }) as typeof globalThis.fetch;
+
+    private originalOf(id: number): {
+        key: Uint8Array;
+        header: string;
+        body: Uint8Array;
+    } {
+        const known = this.originals.get(id);
+        if (known) return known;
+        const key = sodium.crypto_secretbox_keygen();
+        const { header, ciphertext } = encryptBlob(
+            new TextEncoder().encode(`original of ${id}`),
+            key,
+        );
+        const original = { key, header: toBase64(header), body: ciphertext };
+        this.originals.set(id, original);
+        return original;
+    }
+
+    private moveAlbum(albumID: number, t: number): void {
+        this.albums.set(albumID, {
+            ...this.albums.get(albumID)!,
+            updationTime: t,
+        });
+    }
+}
+
+// Album 1, "Trip", holds file 10, which decrypts, file 11, whose key does not,
+// and file 12, whose metadata is not JSON. Album 2, "Family", holds file 20,
+// but the album's own key does not decrypt. All of it as of time 1.
+const brokenAccount = (): FakeEnte => {
+    const ente = new FakeEnte();
+    ente.putAlbum(1, "Trip", 1);
+    ente.putFile(1, 10, 1);
+    ente.putFile(1, 11, 1, { corruptKey: true });
+    ente.putFile(1, 12, 1, { metadata: "not json" });
+    ente.putAlbum(2, "Family", 1, true);
+    ente.putFile(2, 20, 1);
+    return ente;
+};
+
+// Album 1, "Trip", holds file 10, and album 2, "Family", holds file 20. Both
+// files decrypt, but album 2's own key does not, so the album is the only
+// record left out. All of it as of time 1.
+const brokenAlbumAccount = (): FakeEnte => {
+    const ente = new FakeEnte();
+    ente.putAlbum(1, "Trip", 1);
+    ente.putFile(1, 10, 1);
+    ente.putAlbum(2, "Family", 1, true);
+    ente.putFile(2, 20, 1);
+    return ente;
+};
+
+// Collects everything written to it.
+class Output {
+    text = "";
+    write(text: string): void {
+        this.text += text;
+    }
+}
+
+let root: string;
+let stdout: Output;
+let stderr: Output;
+
+beforeAll(async () => {
+    await init();
+    await sodium.ready;
+});
+
+beforeEach(() => {
+    root = mkdtempSync(join(tmpdir(), "quak-undecryptable-"));
+});
+
+afterEach(() => {
+    rmSync(root, { recursive: true, force: true });
+});
+
+// A context for one command, with fresh output, whose saved session talks to
+// `ente`. Every run shares one cache directory, as runs of `quak` do.
+const context = (ente: FakeEnte): CliContext => {
+    stdout = new Output();
+    stderr = new Output();
+    saveSession(join(root, "session"), ente.snapshot());
+    return {
+        stdout,
+        stderr,
+        sessionDir: join(root, "session"),
+        cacheDir: join(root, "cache"),
+        loadSession: (path) =>
+            loadSession(path, { fetch: ente.fetch, retry: { attempts: 1 } }),
+        login: async () => {
+            throw new Error("login not expected");
+        },
+        prompt: async () => {
+            throw new Error("prompt not expected");
+        },
+        promptSecret: async () => {
+            throw new Error("prompt not expected");
+        },
+    };
+};
+
+const backupDir = (): string => join(root, "backup");
+
+// Runs `quak backup` into the backup directory and returns its exit code.
+const backup = (ente: FakeEnte, opts: { json?: boolean } = {}) =>
+    backupCommand(context(ente), backupDir(), opts);
+
+// The backup's `failures.json` entries by file ID, or {} when it has none.
+const failures = (): Record<string, { error: string; attempts: number }> => {
+    const path = join(backupDir(), "failures.json");
+    return existsSync(path)
+        ? JSON.parse(readFileSync(path, "utf-8")).files
+        : {};
+};
+
+// Whether file `id`'s original is at its save path in the backup.
+const stored = (id: number): boolean =>
+    existsSync(join(backupDir(), DAY, `2026-03-01.${id}.jpg`));
+
+// The IDs of the files that album `name`'s JSON lists.
+const albumFiles = (name: string): number[] =>
+    JSON.parse(
+        readFileSync(join(backupDir(), "collections", `${name}.json`), "utf-8"),
+    ).files.map((f: { id: number }) => f.id);
+
+describe("quak backup with records it cannot decrypt", () => {
+    it("backs up every other file, records each file it cannot decrypt in failures.json, names each such album, and exits 1", async () => {
+        expect(await backup(brokenAccount())).toBe(1);
+
+        expect(stored(10)).toBe(true);
+        // Without its album's key, file 20 cannot even be listed.
+        expect(stored(20)).toBe(false);
+        expect(failures()).toEqual({
+            "11": expect.objectContaining({
+                fileID: 11,
+                error: expect.stringMatching(
+                    /^could not be decrypted: .*wrong secret key/,
+                ),
+            }),
+            "12": expect.objectContaining({
+                fileID: 12,
+                error: expect.stringMatching(/^could not be decrypted: .*JSON/),
+            }),
+        });
+        expect(stderr.text).toMatch(
+            /^FAILED album 2: could not be decrypted: .*wrong secret key/m,
+        );
+        expect(stderr.text).toContain("  Failed:      2\n");
+    });
+
+    it("names each album it cannot decrypt in the --json result", async () => {
+        expect(await backup(brokenAccount(), { json: true })).toBe(1);
+
+        expect(JSON.parse(stdout.text)).toMatchObject({
+            downloaded: 1,
+            failed: 2,
+            undecryptableAlbums: [2],
+        });
+    });
+
+    it("still reports them on the next run, though the server does not send them again", async () => {
+        const ente = brokenAccount();
+        await backup(ente);
+
+        // Nothing changed on the server, so this run's refresh gets no rows at
+        // all. What it reports, it remembered from the first run.
+        expect(await backup(ente)).toBe(1);
+
+        expect(Object.keys(failures())).toEqual(["11", "12"]);
+        expect(failures()["11"]!.attempts).toBe(2);
+        expect(stderr.text).toMatch(/^FAILED album 2: could not be decrypted/m);
+    });
+
+    it("drops each once the server sends a copy that decrypts, and then exits 0", async () => {
+        const ente = brokenAccount();
+        await backup(ente);
+
+        // At time 2 files 11 and 12 and album 2 change on the server, and now
+        // each decrypts. Album 2 brings file 20 with it.
+        ente.putFile(1, 11, 2);
+        ente.putFile(1, 12, 2);
+        ente.putAlbum(2, "Family", 2);
+        expect(await backup(ente)).toBe(0);
+
+        expect([11, 12, 20].map(stored)).toEqual([true, true, true]);
+        expect(existsSync(join(backupDir(), "failures.json"))).toBe(false);
+        expect(stderr.text).not.toContain("FAILED");
+    });
+
+    it("drops each once the server deletes it, and then exits 0", async () => {
+        const ente = brokenAccount();
+        await backup(ente);
+
+        ente.deleteFile(1, 11, 2);
+        ente.deleteFile(1, 12, 2);
+        ente.deleteAlbum(2, 2);
+        expect(await backup(ente)).toBe(0);
+
+        expect(existsSync(join(backupDir(), "failures.json"))).toBe(false);
+        expect(stderr.text).not.toContain("FAILED");
+    });
+
+    it("exits 1 when the only record it cannot decrypt is an album, and 0 once the album decrypts", async () => {
+        const ente = brokenAlbumAccount();
+
+        // No file is in `failures.json`: the album alone keeps the exit code
+        // at 1.
+        expect(await backup(ente)).toBe(1);
+        expect(failures()).toEqual({});
+
+        ente.putAlbum(2, "Family", 2);
+        expect(await backup(ente)).toBe(0);
+        expect(stored(20)).toBe(true);
+    });
+
+    it("exits 1 when the only record it cannot decrypt is an album, and 0 once the album is deleted", async () => {
+        const ente = brokenAlbumAccount();
+
+        expect(await backup(ente)).toBe(1);
+        expect(failures()).toEqual({});
+
+        ente.deleteAlbum(2, 2);
+        expect(await backup(ente)).toBe(0);
+    });
+
+    it("removes what an earlier run linked for an album or a file that no longer decrypts, and keeps the originals", async () => {
+        // At time 1 everything decrypts: the first run links files 10 and 11
+        // in Trip and file 20 in Family.
+        const ente = new FakeEnte();
+        ente.putAlbum(1, "Trip", 1);
+        ente.putFile(1, 10, 1);
+        ente.putFile(1, 11, 1);
+        ente.putAlbum(2, "Family", 1);
+        ente.putFile(2, 20, 1);
+        expect(await backup(ente)).toBe(0);
+        const collections = join(backupDir(), "collections");
+        expect(readdirSync(join(collections, "Trip")).sort()).toEqual([
+            "photo-10.jpg",
+            "photo-11.jpg",
+        ]);
+        expect(readdirSync(join(collections, "Family"))).toEqual([
+            "photo-20.jpg",
+        ]);
+
+        // At time 2 the server's copy of file 11 in Trip, and album 2 itself,
+        // no longer decrypt. A run builds the album folders, their JSON and
+        // their links from what the library holds, so it removes them as if
+        // the album and the file had been deleted: Family's folder and JSON
+        // go, and Trip neither links nor lists file 11. The originals stay at
+        // their save paths, and the run exits 1.
+        ente.putFile(1, 11, 2, { corruptKey: true });
+        ente.putAlbum(2, "Family", 2, true);
+        expect(await backup(ente)).toBe(1);
+
+        expect(readdirSync(collections).sort()).toEqual(["Trip", "Trip.json"]);
+        expect(readdirSync(join(collections, "Trip"))).toEqual([
+            "photo-10.jpg",
+        ]);
+        expect(albumFiles("Trip")).toEqual([10]);
+        expect([10, 11, 20].map(stored)).toEqual([true, true, true]);
+        expect(Object.keys(failures())).toEqual(["11"]);
+    });
+
+    it("backs up a file that does not decrypt in one album but does in another, and does not report it", async () => {
+        // File 30 is in albums 1 and 3, each holding its key sealed under the
+        // album's own key. In album 1 that is corrupt; in album 3 it is not,
+        // and one copy that decrypts is all the backup needs.
+        const ente = new FakeEnte();
+        ente.putAlbum(1, "Trip", 1);
+        ente.putFile(1, 30, 1, { corruptKey: true });
+        ente.putAlbum(3, "Favorites", 1);
+        ente.putFile(3, 30, 1);
+
+        expect(await backup(ente)).toBe(0);
+
+        expect(stored(30)).toBe(true);
+        expect(failures()).toEqual({});
+        // Album 1 has no copy of file 30 that decrypts, so it neither links
+        // the original nor lists the file; album 3 lists it.
+        expect(readdirSync(join(backupDir(), "collections", "Trip"))).toEqual(
+            [],
+        );
+        expect(albumFiles("Trip")).toEqual([]);
+        expect(albumFiles("Favorites")).toEqual([30]);
+    });
+
+    it("backs up that file in a run limited to the album where it does not decrypt", async () => {
+        // The same account, backed up by `lib.backup()` limited to album 1,
+        // "Trip", as `quak backup` cannot be. File 30 is in Trip, so the run
+        // must store it, from its copy in album 3 that the run otherwise
+        // skips.
+        const ente = new FakeEnte();
+        ente.putAlbum(1, "Trip", 1);
+        ente.putFile(1, 30, 1, { corruptKey: true });
+        ente.putAlbum(3, "Favorites", 1);
+        ente.putFile(3, 30, 1);
+        const lib = await Library.open({
+            client: Client.fromJSON(ente.snapshot(), {
+                fetch: ente.fetch,
+                retry: { attempts: 1 },
+            }),
+            downloadDirectory: backupDir(),
+            cacheDirectory: join(root, "cache"),
+            precacheThumbnails: false,
+            precacheOriginals: false,
+        });
+        try {
+            const result = await lib.backup({
+                downloadDirectory: backupDir(),
+                onlyAlbumNames: ["Trip"],
+            });
+            expect(result).toMatchObject({ downloaded: 1, failed: 0 });
+        } finally {
+            await lib.close();
+        }
+
+        expect(stored(30)).toBe(true);
+        expect(failures()).toEqual({});
+    });
+});
+
+describe("the read commands with records quak cannot decrypt", () => {
+    it("quak collections and quak files answer from what decrypts", async () => {
+        const ente = brokenAccount();
+
+        expect(await collectionsCommand(context(ente), {})).toBe(0);
+        expect(stdout.text).toBe("1\talbum\tTrip\n");
+
+        expect(await filesCommand(context(ente), { collection: "1" })).toBe(0);
+        expect(stdout.text).toBe("10\timage\tphoto-10.jpg\n");
+    });
+});

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,
@@ -830,11 +832,106 @@ 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();
+    });
+
+    // `runBackup` takes a library object from its caller. With a
+    // `contentConcurrency` below 1, or one that is not a number, it would
+    // back up no original and report no error. It rejects any that is not a
+    // positive integer, as `BoundedPool` does, before it takes the lock,
+    // which would create the download directory, and before it asks the
+    // library for anything: the object here has nothing else.
+    it("rejects a library whose contentConcurrency is not a positive integer, before taking the lock", async () => {
+        const outDir = join(root, "backup");
+        for (const contentConcurrency of [undefined, 0, -1, 1.5, NaN]) {
+            const lib = { contentConcurrency } as unknown as BackupLibrary;
+
+            await expect(
+                runBackup(lib, { downloadDirectory: outDir }),
+            ).rejects.toThrow(
+                `contentConcurrency must be a positive integer, got ${contentConcurrency}`,
+            );
+
+            expect(existsSync(outDir)).toBe(false);
+        }
+    });
+});
+
 // Also serves ML data: file 100 has `ML_PAYLOAD`, the other two have none.
 // While `mlError` is set, every ML data request fails with it.
 const ML_PAYLOAD = { face: { faces: [] }, clip: { embedding: [0.5, 0.25] } };
@@ -1196,6 +1293,181 @@ 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[] => {
@@ -1231,6 +1503,7 @@ describe("backup album folders", () => {
         listCollections: () => albums.map((a) => a.collection),
         listFiles: (id) =>
             albums.find((a) => a.collection.id === id)?.files ?? [],
+        undecryptable: () => [],
         original: async (fileID) => {
             const path = join(root, `source-${fileID}`);
             writeFileSync(path, `original ${fileID}`);
@@ -1239,6 +1512,7 @@ describe("backup album folders", () => {
         thumbnail: async () => {
             throw new Error("no thumbnails in this stand-in");
         },
+        contentConcurrency: 5,
         fetchMLData: async () => {},
         mlData: async () => undefined,
         whoami: () => ({ email: "backup@example.com", userID: USER_ID }),
@@ -1691,6 +1965,64 @@ 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

@@ -18,12 +18,14 @@ import {
     readFileSync,
     rmSync,
     statSync,
+    utimesSync,
     writeFileSync,
 } from "node:fs";
 import { join } from "node:path";
 import { tmpdir } from "node:os";
 import { PassThrough } from "node:stream";
 import * as jpegJs from "jpeg-js";
+import sodium from "libsodium-wrappers-sumo";
 import {
     describe,
     it,
@@ -52,13 +54,14 @@ 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 { encryptBlob, 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,
@@ -509,11 +512,6 @@ describe("files", () => {
         expect(await filesCommand(context(), { collection: "9" })).toBe(1);
         expect(stderr.text).toBe("Collection 9 not found\n");
     });
-
-    it("exits 1 for a collection ID that is not a number", async () => {
-        expect(await filesCommand(context(), { collection: "abc" })).toBe(1);
-        expect(stderr.text).toBe("Invalid collection ID\n");
-    });
 });
 
 describe("get and get-thumb", () => {
@@ -544,12 +542,6 @@ describe("get and get-thumb", () => {
         expect(stderr.text).toBe("File 999 not found\n");
         expect(existsSync(out)).toBe(false);
     });
-
-    it("both exit 1 for a file ID that is not a number", async () => {
-        expect(await getCommand(context(), "abc", {})).toBe(1);
-        expect(await getThumbCommand(context(), "abc", {})).toBe(1);
-        expect(stderr.text).toBe("Invalid file ID\nInvalid file ID\n");
-    });
 });
 
 // A live photo, which Ente stores as one ZIP, is written as its image and its
@@ -703,6 +695,7 @@ 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
@@ -742,6 +735,61 @@ 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(),
@@ -792,7 +840,7 @@ 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 +871,178 @@ 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, 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);
+    });
+
+    // A backup runs for as long as its downloads take, hours on a large
+    // account. It lists its files from the refresh it waits for as it starts,
+    // so it must not ask the server for changes again on a timer while it
+    // runs: each refresh is a request, and each also starts a pass that asks
+    // again for the ML data of every file Ente holds none for.
+    //
+    // A saved session is read back by `loadSession`, its `fetch` a fake of the
+    // Ente HTTP API serving one album with one photo, encrypted as the server
+    // serves them. The fake holds the photo's download open while the clock
+    // moves on by two hours, then serves it. Only `setTimeout` and
+    // `clearTimeout` are faked: the library's refresh timer runs on them.
+    it("asks the server for changes only as it starts, however long it runs", async () => {
+        const masterKey = new Uint8Array(32).fill(1);
+        const albumKey = new Uint8Array(32).fill(2);
+        const photoKey = new Uint8Array(32).fill(3);
+        saveSession(join(root, "session"), {
+            email: "cli@example.com",
+            userID: USER_ID,
+            token: "token",
+            masterKey: toBase64(masterKey),
+            secretKey: toBase64(new Uint8Array(32)),
+            publicKey: toBase64(new Uint8Array(32)),
+        });
+        // A secretbox as the server sends one: ciphertext and nonce, base64.
+        const box = (plaintext: Uint8Array, key: Uint8Array) => {
+            const nonce = sodium.randombytes_buf(
+                sodium.crypto_secretbox_NONCEBYTES,
+            );
+            return {
+                ciphertext: toBase64(
+                    sodium.crypto_secretbox_easy(plaintext, nonce, key),
+                ),
+                nonce: toBase64(nonce),
+            };
+        };
+        const albumKeyBox = box(albumKey, masterKey);
+        const albumName = box(new TextEncoder().encode("Vacation"), albumKey);
+        const photoKeyBox = box(photoKey, albumKey);
+        const metadata = encryptBlob(
+            new TextEncoder().encode(
+                JSON.stringify({
+                    title: "beach.jpg",
+                    fileType: 0,
+                    creationTime: 1_700_000_000_000_000,
+                    modificationTime: 1_700_000_000_000_000,
+                }),
+            ),
+            photoKey,
+        );
+        const content = encryptBlob(
+            new TextEncoder().encode("the photo's bytes"),
+            photoKey,
+        );
+        const album = {
+            id: 1,
+            owner: { id: USER_ID },
+            encryptedKey: albumKeyBox.ciphertext,
+            keyDecryptionNonce: albumKeyBox.nonce,
+            encryptedName: albumName.ciphertext,
+            nameDecryptionNonce: albumName.nonce,
+            type: "album",
+            updationTime: 1,
+        };
+        const photo = {
+            id: 100,
+            collectionID: 1,
+            ownerID: USER_ID,
+            encryptedKey: photoKeyBox.ciphertext,
+            keyDecryptionNonce: photoKeyBox.nonce,
+            metadata: {
+                encryptedData: toBase64(metadata.ciphertext),
+                decryptionHeader: toBase64(metadata.header),
+            },
+            file: { decryptionHeader: toBase64(content.header) },
+            // The backup fetches no thumbnail.
+            thumbnail: { decryptionHeader: "" },
+            updationTime: 1,
+        };
+
+        // Each refresh starts with `collectionsSince`, which sends one
+        // `/collections/v2` request.
+        let refreshes = 0;
+        let downloadRequested!: () => void;
+        const requested = new Promise<void>((resolve) => {
+            downloadRequested = resolve;
+        });
+        let releaseDownload!: () => void;
+        const released = new Promise<void>((resolve) => {
+            releaseDownload = resolve;
+        });
+        const json = (body: unknown): Response =>
+            new Response(JSON.stringify(body), {
+                headers: { "content-type": "application/json" },
+            });
+        const fetch = async (url: string | URL): Promise<Response> => {
+            const { hostname, pathname } = new URL(String(url));
+            if (hostname === "files.ente.io") {
+                downloadRequested();
+                await released;
+                return new Response(content.ciphertext);
+            }
+            if (pathname === "/collections/v2") {
+                refreshes++;
+                return json({ collections: [album] });
+            }
+            if (pathname === "/collections/v2/diff") {
+                return json({ diff: [photo], hasMore: false });
+            }
+            // Ente holds no ML data for the photo.
+            if (pathname === "/files/data/fetch") return json({ data: [] });
+            return new Response("not found", { status: 404 });
+        };
+        const ctx = {
+            ...context(),
+            // The download's idle deadline is set past the two hours, or quak
+            // would give up the held download as stalled.
+            loadSession: (path: string) =>
+                loadSession(path, {
+                    fetch: fetch as typeof globalThis.fetch,
+                    downloadTimeoutMs: 3 * 3_600_000,
+                }),
+        };
+
+        vi.useFakeTimers({ toFake: ["setTimeout", "clearTimeout"] });
+        try {
+            const backup = backupCommand(ctx, join(root, "backup"), {});
+            await requested;
+            vi.advanceTimersByTime(2 * 3_600_000);
+            releaseDownload();
+            expect(await backup).toBe(0);
+        } finally {
+            vi.useRealTimers();
+        }
+
+        // Two refreshes, both before the download: the library opens onto the
+        // empty cache with one, and the backup waits for the other before it
+        // lists its files.
+        expect(refreshes).toBe(2);
     });
 });
 
@@ -946,3 +1165,130 @@ describe("backup-metadata and the thumbnail helpers refresh first", () => {
         expect(stdout.text).toBe("");
     });
 });
+
+// `files --collection`, `get`, `get-thumb` and `helper fix-missing-thumbnails
+// --file` take an ID only when it is written in decimal digits and is a safe
+// integer. `Number()` alone reads `0x10` as 16, `1e3` as 1000 and an empty
+// string as 0, so the command would act on a file the user never named.
+// Anything else is refused with one line naming it, and exit 1, before any
+// request.
+//
+// These run on a real client, read back from a saved session, over a fake Ente
+// server holding an empty account, which records each request it receives. With
+// a good ID the command goes on to ask the server, and finds nothing there.
+describe("an ID given on the command line", () => {
+    const NOT_IDS = [
+        "0x10",
+        "1e3",
+        "1.5",
+        "-1",
+        "",
+        "abc",
+        // One past Number.MAX_SAFE_INTEGER: from here on a number cannot
+        // hold every integer, so two IDs could read as one.
+        "9007199254740992",
+    ];
+
+    const requests: Request[] = [];
+
+    const emptyAccount = (): CliContext => {
+        const ctx = context();
+        const key = toBase64(new Uint8Array(32));
+        saveSession(ctx.sessionDir, {
+            email: "cli@example.com",
+            userID: USER_ID,
+            token: "token",
+            masterKey: key,
+            secretKey: key,
+            publicKey: key,
+        });
+        return {
+            ...ctx,
+            loadSession: (path) =>
+                loadSession(path, {
+                    fetch: async (url, init) => {
+                        requests.push(new Request(url, init));
+                        return new Response(
+                            JSON.stringify({ collections: [] }),
+                            { headers: { "content-type": "application/json" } },
+                        );
+                    },
+                }),
+        };
+    };
+
+    beforeEach(() => {
+        requests.length = 0;
+    });
+
+    it("files looks up a decimal collection ID", async () => {
+        const ctx = emptyAccount();
+        expect(await filesCommand(ctx, { collection: "1" })).toBe(1);
+        expect(stderr.text).toBe("Collection 1 not found\n");
+        expect(requests).not.toHaveLength(0);
+    });
+
+    it.each(NOT_IDS)(
+        "files refuses --collection %j before any request",
+        async (arg) => {
+            const ctx = emptyAccount();
+            expect(await filesCommand(ctx, { collection: arg })).toBe(1);
+            expect(stderr.text).toBe(`Invalid collection ID: ${arg}\n`);
+            expect(stdout.text).toBe("");
+            expect(requests).toHaveLength(0);
+        },
+    );
+
+    it("get looks up a decimal file ID", async () => {
+        expect(await getCommand(emptyAccount(), "100", {})).toBe(1);
+        expect(stderr.text).toBe("File 100 not found\n");
+        expect(requests).not.toHaveLength(0);
+    });
+
+    it.each(NOT_IDS)("get refuses %j before any request", async (arg) => {
+        expect(await getCommand(emptyAccount(), arg, {})).toBe(1);
+        expect(stderr.text).toBe(`Invalid file ID: ${arg}\n`);
+        expect(stdout.text).toBe("");
+        expect(requests).toHaveLength(0);
+    });
+
+    it("get-thumb looks up a decimal file ID", async () => {
+        expect(await getThumbCommand(emptyAccount(), "100", {})).toBe(1);
+        expect(stderr.text).toBe("File 100 not found\n");
+        expect(requests).not.toHaveLength(0);
+    });
+
+    it.each(NOT_IDS)("get-thumb refuses %j before any request", async (arg) => {
+        expect(await getThumbCommand(emptyAccount(), arg, {})).toBe(1);
+        expect(stderr.text).toBe(`Invalid file ID: ${arg}\n`);
+        expect(stdout.text).toBe("");
+        expect(requests).toHaveLength(0);
+    });
+
+    // The fixer reports each file it was given; on this account neither is
+    // found, which is a failure, so it exits 1.
+    it("fix-missing-thumbnails looks up each decimal --file ID", async () => {
+        const ctx = emptyAccount();
+        const opts = { file: ["100", "200"], json: true };
+        expect(await fixMissingThumbnailsCommand(ctx, opts)).toBe(1);
+        expect(JSON.parse(stdout.text)).toMatchObject([
+            { fileID: 100, reason: "file not found in any collection" },
+            { fileID: 200, reason: "file not found in any collection" },
+        ]);
+        expect(requests).not.toHaveLength(0);
+    });
+
+    // One bad ID fails the whole command, so the good one before it is not
+    // fixed either.
+    it.each(NOT_IDS)(
+        "fix-missing-thumbnails refuses --file 100 %j before any request",
+        async (arg) => {
+            const ctx = emptyAccount();
+            const opts = { file: ["100", arg] };
+            expect(await fixMissingThumbnailsCommand(ctx, opts)).toBe(1);
+            expect(stderr.text).toBe(`Invalid file ID: ${arg}\n`);
+            expect(stdout.text).toBe("");
+            expect(requests).toHaveLength(0);
+        },
+    );
+});

test/cli/metadata-backup.test.ts

@@ -37,15 +37,8 @@ import {
 import { join } from "node:path";
 import { tmpdir } from "node:os";
 import sodium from "libsodium-wrappers-sumo";
-import { SRP, SrpServer } from "fast-srp-hap";
 import { beforeAll, afterAll, describe, expect, it, vi } from "vitest";
-import {
-    init,
-    toBase64,
-    deriveKEK,
-    deriveLoginSubkey,
-    encryptBlob,
-} from "../../src/crypto/index.js";
+import { init, toBase64, encryptBlob } from "../../src/crypto/index.js";
 import * as jpegJs from "jpeg-js";
 import { Client } from "../../src/client.js";
 import { Library } from "../../src/library/index.js";
@@ -54,7 +47,13 @@ import {
     type MetadataBackupOptions,
 } from "../../src/metadata-backup.js";
 import { backupMetadataCommand } from "../../src/cli-commands.js";
-import type { KeyAttributes } from "../../src/auth/types.js";
+import {
+    buildAccount,
+    buildMetaFetch,
+    encryptSecretbox,
+    encryptStreamBlob,
+    type MetaMockState,
+} from "../ente-server.js";
 
 // One file per ML data request, so the two files of the mock account are
 // fetched in two requests and one of them can fail on its own.
@@ -65,80 +64,13 @@ vi.mock("../../src/mldata-fetch.js", async (importOriginal) => ({
 
 const TEST_EMAIL = "metabackup@example.com";
 const TEST_PASSWORD = "metapass";
-const TEST_OPS = 2;
-const TEST_MEM = 64 * 1024 * 1024;
-
-interface MetaMockState {
-    verifier: Buffer;
-    srpAttributes: Record<string, unknown>;
-    keyAttributes: KeyAttributes;
-    encryptedToken: string;
-    collections: Record<string, unknown>[];
-    filesByCollection: Record<number, Record<string, unknown>[]>;
-    // For ML data and EXIF tests
-    encryptedMLData: Record<
-        number,
-        { encryptedData: string; decryptionHeader: string }
-    >;
-    fileCiphertexts: Record<number, Uint8Array>;
-}
 
 let mock: MetaMockState;
 let testDir: string;
 
-const encryptSecretbox = (plaintext: Uint8Array, key: Uint8Array) => {
-    const nonce = sodium.randombytes_buf(sodium.crypto_secretbox_NONCEBYTES);
-    const ciphertext = sodium.crypto_secretbox_easy(plaintext, nonce, key);
-    return { ciphertext, nonce };
-};
-
-const encryptStreamBlob = (plaintext: Uint8Array, key: Uint8Array) => {
-    const push = sodium.crypto_secretstream_xchacha20poly1305_init_push(key);
-    const ciphertext = sodium.crypto_secretstream_xchacha20poly1305_push(
-        push.state,
-        plaintext,
-        null,
-        sodium.crypto_secretstream_xchacha20poly1305_TAG_FINAL,
-    );
-    return { ciphertext, header: push.header };
-};
-
 const buildMetaMock = async (): Promise<MetaMockState> => {
-    const kekSalt = sodium.randombytes_buf(sodium.crypto_pwhash_SALTBYTES);
-    const kek = await deriveKEK(TEST_PASSWORD, kekSalt, TEST_OPS, TEST_MEM);
-    const loginSubKeyBytes = deriveLoginSubkey(kek);
-    const srpUserID = "meta-srp";
-    const srpSalt = sodium.randombytes_buf(16);
-    const verifier = SRP.computeVerifier(
-        SRP.params["4096"],
-        Buffer.from(srpSalt),
-        Buffer.from(srpUserID),
-        Buffer.from(loginSubKeyBytes),
-    );
-
-    const masterKey = sodium.randombytes_buf(32);
-    const { ciphertext: encMK, nonce: mkNonce } = encryptSecretbox(
-        masterKey,
-        kek,
-    );
-    const kp = sodium.crypto_box_keypair();
-    const { ciphertext: encSK, nonce: skNonce } = encryptSecretbox(
-        kp.privateKey,
-        masterKey,
-    );
-    const tokenBytes = sodium.randombytes_buf(32);
-    const encToken = sodium.crypto_box_seal(tokenBytes, kp.publicKey);
-
-    const keyAttributes: KeyAttributes = {
-        kekSalt: toBase64(kekSalt),
-        encryptedKey: toBase64(encMK),
-        keyDecryptionNonce: toBase64(mkNonce),
-        publicKey: toBase64(kp.publicKey),
-        encryptedSecretKey: toBase64(encSK),
-        secretKeyDecryptionNonce: toBase64(skNonce),
-        memLimit: TEST_MEM,
-        opsLimit: TEST_OPS,
-    };
+    const account = await buildAccount(TEST_PASSWORD);
+    const masterKey = account.masterKey;
 
     // Collection 1: "Vacation" with collection-level pubMagicMetadata
     const ck1 = sodium.crypto_secretbox_keygen();
@@ -332,17 +264,10 @@ const buildMetaMock = async (): Promise<MetaMockState> => {
     rawFile1.file.decryptionHeader = toBase64(filePush1.header);
 
     return {
-        verifier,
-        srpAttributes: {
-            srpUserID,
-            srpSalt: toBase64(srpSalt),
-            memLimit: TEST_MEM,
-            opsLimit: TEST_OPS,
-            kekSalt: toBase64(kekSalt),
-            isEmailMFAEnabled: false,
-        },
-        keyAttributes,
-        encryptedToken: toBase64(encToken),
+        verifier: account.verifier,
+        srpAttributes: account.srpAttributes,
+        keyAttributes: account.keyAttributes,
+        encryptedToken: account.encryptedToken,
         collections: [rawColl1, rawColl2],
         filesByCollection: { 10: [rawFile1], 20: [rawFile2] },
         encryptedMLData: {
@@ -355,90 +280,6 @@ const buildMetaMock = async (): Promise<MetaMockState> => {
     };
 };
 
-// `failMLDataFor`: answer 500 to every ML data request that asks for this file.
-const buildMetaFetch = (m: MetaMockState, failMLDataFor?: number) => {
-    let srpServer: SrpServer;
-    return (async (
-        input: RequestInfo | URL,
-        init?: RequestInit,
-    ): Promise<Response> => {
-        const url =
-            typeof input === "string"
-                ? input
-                : input instanceof URL
-                  ? input.href
-                  : input.url;
-        const path = new URL(url).pathname;
-        const json = (body: unknown) =>
-            new Response(JSON.stringify(body), {
-                status: 200,
-                headers: { "content-type": "application/json" },
-            });
-
-        if (path === "/users/srp/attributes")
-            return json({ attributes: m.srpAttributes });
-        if (path === "/users/srp/create-session") {
-            const body = JSON.parse(init?.body as string);
-            const serverKey = await SRP.genKey();
-            srpServer = new SrpServer(
-                SRP.params["4096"],
-                m.verifier,
-                serverKey,
-            );
-            const B = srpServer.computeB();
-            srpServer.setA(Buffer.from(body.srpA, "base64"));
-            return json({ sessionID: "s1", srpB: B.toString("base64") });
-        }
-        if (path === "/users/srp/verify-session") {
-            const body = JSON.parse(init?.body as string);
-            srpServer.checkM1(Buffer.from(body.srpM1, "base64"));
-            return json({
-                srpM2: srpServer.computeM2().toString("base64"),
-                id: 42,
-                keyAttributes: m.keyAttributes,
-                encryptedToken: m.encryptedToken,
-            });
-        }
-        if (path === "/collections/v2")
-            return json({ collections: m.collections });
-        if (path === "/collections/v2/diff") {
-            const collID = Number(
-                new URL(url).searchParams.get("collectionID"),
-            );
-            return json({
-                diff: m.filesByCollection[collID] ?? [],
-                hasMore: false,
-            });
-        }
-        if (path === "/files/data/fetch") {
-            const body = JSON.parse(init?.body as string);
-            if ((body.fileIDs as number[]).includes(failMLDataFor!))
-                return new Response("server error", { status: 500 });
-            const data = (body.fileIDs as number[])
-                .filter((id: number) => m.encryptedMLData[id])
-                .map((id: number) => ({
-                    fileID: id,
-                    ...m.encryptedMLData[id],
-                    updatedAt: 1700000000000000,
-                }));
-            return json({ data });
-        }
-        if (
-            url.includes("files.ente.io") ||
-            path.startsWith("/files/download/")
-        ) {
-            const parsed = new URL(url);
-            const fileID = Number(
-                parsed.searchParams.get("fileID") ?? path.split("/").pop(),
-            );
-            const ct = m.fileCiphertexts[fileID];
-            if (ct) return new Response(ct, { status: 200 });
-            return new Response("not found", { status: 404 });
-        }
-        return new Response("not found", { status: 404 });
-    }) as typeof globalThis.fetch;
-};
-
 beforeAll(async () => {
     await init();
     await sodium.ready;

test/client/enumeration.test.ts

@@ -10,7 +10,9 @@
  * The return shape keeps live records and tombstones apart — `collections` /
  * `files` are decrypted live records, `deleted` is a plain list of the ids the
  * server tombstoned. A tombstone carries no decryptable key or metadata, so it
- * is a bare id rather than a hollowed-out `Collection` / `EnteFile`.
+ * is a bare id rather than a hollowed-out `Collection` / `EnteFile`. A live row
+ * that does not decrypt, or whose metadata is not JSON, is listed under
+ * `undecryptable` with the reason, so one bad row does not fail the call.
  *
  * All tests inject a fake `fetch` and drive a real `Client` (built with
  * `Client.fromJSON`) so the decryption path runs for real. Live rows are built
@@ -91,27 +93,38 @@ const ownedCollectionRow = (
     };
 };
 
-/** A live file row inside a collection, keyed under that collection's key. */
+/**
+ * A live file row inside a collection, keyed under that collection's key. Its
+ * metadata decrypts to `metadata` when given, otherwise to the JSON a client
+ * uploads for `title`.
+ */
 const fileRow = (
     collectionKey: Uint8Array,
-    opts: { id: number; title: string; updationTime: number },
+    opts: {
+        id: number;
+        title: string;
+        updationTime: number;
+        metadata?: string;
+    },
 ): Record<string, unknown> => {
     const fileKey = sodium.crypto_secretbox_keygen();
     const { ciphertext: encFileKey, nonce: fileKeyNonce } = secretboxEncrypt(
         fileKey,
         collectionKey,
     );
-    const metadata = {
+    const metadata =
+        opts.metadata ??
+        JSON.stringify({
             title: opts.title,
             fileType: 0,
             creationTime: opts.updationTime,
             modificationTime: opts.updationTime,
-    };
+        });
     const push =
         sodium.crypto_secretstream_xchacha20poly1305_init_push(fileKey);
     const encMeta = sodium.crypto_secretstream_xchacha20poly1305_push(
         push.state,
-        new TextEncoder().encode(JSON.stringify(metadata)),
+        new TextEncoder().encode(metadata),
         null,
         sodium.crypto_secretstream_xchacha20poly1305_TAG_FINAL,
     );
@@ -131,6 +144,21 @@ const fileRow = (
     };
 };
 
+/**
+ * `row` with its sealed key replaced by random bytes of the same length, as a
+ * corrupt row arrives: no key opens it, so the row cannot be decrypted.
+ */
+const withCorruptKey = (
+    row: Record<string, unknown>,
+): Record<string, unknown> => ({
+    ...row,
+    encryptedKey: toBase64(
+        sodium.randombytes_buf(
+            sodium.crypto_secretbox_KEYBYTES + sodium.crypto_secretbox_MACBYTES,
+        ),
+    ),
+});
+
 /** A tombstone row. Never decrypted, so only these fields are ever read. */
 const tombstoneRow = (
     id: number,
@@ -278,6 +306,58 @@ describe("Client.filesSince", () => {
         // It gave up after the first page rather than looping.
         expect(sinceTimes).toEqual(["50"]);
     });
+
+    it("leaves out each file it cannot decrypt or parse, with the reason, and decrypts the rest", async () => {
+        const keys = buildKeys();
+        const collectionKey = sodium.crypto_secretbox_keygen();
+
+        // File 1002's key is sealed under no key quak holds, and file 1003's
+        // metadata decrypts to text that is not JSON. Neither may cost the
+        // caller file 1001: each is left out of `files` and reported under
+        // `undecryptable` with the reason instead of failing the call.
+        const { fetch } = recordingFetch(
+            jsonResponse({
+                diff: [
+                    fileRow(collectionKey, {
+                        id: 1001,
+                        title: "fine.jpg",
+                        updationTime: 100,
+                    }),
+                    withCorruptKey(
+                        fileRow(collectionKey, {
+                            id: 1002,
+                            title: "bad-key.jpg",
+                            updationTime: 200,
+                        }),
+                    ),
+                    fileRow(collectionKey, {
+                        id: 1003,
+                        title: "bad-metadata.jpg",
+                        updationTime: 300,
+                        metadata: "not json",
+                    }),
+                ],
+                hasMore: false,
+            }),
+        );
+        const client = Client.fromJSON(snapshotFor(keys), { fetch });
+
+        const { files, undecryptable, cursor } = await client.filesSince({
+            collectionID: 1,
+            collectionKey,
+            sinceTime: 0,
+        });
+
+        expect(files.map((f) => f.id)).toEqual([1001]);
+        expect(undecryptable).toEqual([
+            { id: 1002, error: expect.stringContaining("wrong secret key") },
+            { id: 1003, error: expect.stringContaining("JSON") },
+        ]);
+        // The cursor moves past the two left out, as past any other row, so
+        // the server does not send them again until its copy changes. The
+        // caller has to keep what it was told here.
+        expect(cursor).toBe(300);
+    });
 });
 
 describe("Client.collectionsSince", () => {
@@ -331,6 +411,42 @@ describe("Client.collectionsSince", () => {
         expect(result.cursor).toBe(150);
         expect(sinceTimes).toEqual(["150"]);
     });
+
+    it("leaves out an album it cannot decrypt, with the reason, and decrypts the rest", async () => {
+        const keys = buildKeys();
+
+        // Album 2's key is sealed under no key quak holds. Without its key
+        // neither its name nor its files can be read, so it is left out and
+        // reported under `undecryptable`; album 1 is unaffected.
+        const { fetch } = recordingFetch(
+            jsonResponse({
+                collections: [
+                    ownedCollectionRow(keys.masterKey, {
+                        id: 1,
+                        name: "Vacation",
+                        updationTime: 100,
+                    }),
+                    withCorruptKey(
+                        ownedCollectionRow(keys.masterKey, {
+                            id: 2,
+                            name: "Broken",
+                            updationTime: 200,
+                        }),
+                    ),
+                ],
+            }),
+        );
+        const client = Client.fromJSON(snapshotFor(keys), { fetch });
+
+        const { collections, undecryptable, cursor } =
+            await client.collectionsSince({ sinceTime: 0 });
+
+        expect(collections.map((c) => c.name)).toEqual(["Vacation"]);
+        expect(undecryptable).toEqual([
+            { id: 2, error: expect.stringContaining("wrong secret key") },
+        ]);
+        expect(cursor).toBe(200);
+    });
 });
 
 describe("Client list wrappers still hide deletions", () => {

test/ente-server.ts

@@ -0,0 +1,197 @@
+/**
+ * A fake of the Ente HTTP API, for tests that run quak's real `Client` against
+ * a server.
+ *
+ * `buildMetaFetch` is a `fetch` that answers the requests quak makes, from a
+ * `MetaMockState` the test builds: the SRP login, `/collections/v2`, each
+ * collection's `/collections/v2/diff`, the ML data of `/files/data/fetch`, and
+ * each file's content from the files CDN, a 404 for a file it holds none for.
+ * The test encrypts everything in that state with libsodium, as Ente's clients
+ * do, so quak decrypts it with its own code. Nothing hits the network.
+ *
+ * `buildAccount` builds what the server holds to log an account in, and the
+ * keys a test encrypts that account's collections with. Call `init()` from
+ * `src/crypto` and await `sodium.ready` before using either.
+ */
+
+import sodium from "libsodium-wrappers-sumo";
+import { SRP, SrpServer } from "fast-srp-hap";
+import { toBase64, deriveKEK, deriveLoginSubkey } from "../src/crypto/index.js";
+import type { KeyAttributes } from "../src/auth/types.js";
+
+const TEST_OPS = 2;
+const TEST_MEM = 64 * 1024 * 1024;
+
+export interface MetaMockState {
+    verifier: Buffer;
+    srpAttributes: Record<string, unknown>;
+    keyAttributes: KeyAttributes;
+    encryptedToken: string;
+    collections: Record<string, unknown>[];
+    filesByCollection: Record<number, Record<string, unknown>[]>;
+    // For ML data and EXIF tests
+    encryptedMLData: Record<
+        number,
+        { encryptedData: string; decryptionHeader: string }
+    >;
+    fileCiphertexts: Record<number, Uint8Array>;
+}
+
+export const encryptSecretbox = (plaintext: Uint8Array, key: Uint8Array) => {
+    const nonce = sodium.randombytes_buf(sodium.crypto_secretbox_NONCEBYTES);
+    const ciphertext = sodium.crypto_secretbox_easy(plaintext, nonce, key);
+    return { ciphertext, nonce };
+};
+
+export const encryptStreamBlob = (plaintext: Uint8Array, key: Uint8Array) => {
+    const push = sodium.crypto_secretstream_xchacha20poly1305_init_push(key);
+    const ciphertext = sodium.crypto_secretstream_xchacha20poly1305_push(
+        push.state,
+        plaintext,
+        null,
+        sodium.crypto_secretstream_xchacha20poly1305_TAG_FINAL,
+    );
+    return { ciphertext, header: push.header };
+};
+
+// An account whose password is `password`. The first four fields are those of
+// `MetaMockState`; with the master key the test encrypts the keys of the
+// account's own collections, and to the public key it seals the keys of the
+// collections other accounts share with it.
+export const buildAccount = async (password: string) => {
+    const kekSalt = sodium.randombytes_buf(sodium.crypto_pwhash_SALTBYTES);
+    const kek = await deriveKEK(password, kekSalt, TEST_OPS, TEST_MEM);
+    const loginSubKeyBytes = deriveLoginSubkey(kek);
+    const srpUserID = "meta-srp";
+    const srpSalt = sodium.randombytes_buf(16);
+    const verifier = SRP.computeVerifier(
+        SRP.params["4096"],
+        Buffer.from(srpSalt),
+        Buffer.from(srpUserID),
+        Buffer.from(loginSubKeyBytes),
+    );
+
+    const masterKey = sodium.randombytes_buf(32);
+    const { ciphertext: encMK, nonce: mkNonce } = encryptSecretbox(
+        masterKey,
+        kek,
+    );
+    const kp = sodium.crypto_box_keypair();
+    const { ciphertext: encSK, nonce: skNonce } = encryptSecretbox(
+        kp.privateKey,
+        masterKey,
+    );
+    const tokenBytes = sodium.randombytes_buf(32);
+    const encToken = sodium.crypto_box_seal(tokenBytes, kp.publicKey);
+
+    const keyAttributes: KeyAttributes = {
+        kekSalt: toBase64(kekSalt),
+        encryptedKey: toBase64(encMK),
+        keyDecryptionNonce: toBase64(mkNonce),
+        publicKey: toBase64(kp.publicKey),
+        encryptedSecretKey: toBase64(encSK),
+        secretKeyDecryptionNonce: toBase64(skNonce),
+        memLimit: TEST_MEM,
+        opsLimit: TEST_OPS,
+    };
+
+    return {
+        verifier,
+        srpAttributes: {
+            srpUserID,
+            srpSalt: toBase64(srpSalt),
+            memLimit: TEST_MEM,
+            opsLimit: TEST_OPS,
+            kekSalt: toBase64(kekSalt),
+            isEmailMFAEnabled: false,
+        },
+        keyAttributes,
+        encryptedToken: toBase64(encToken),
+        masterKey,
+        publicKey: kp.publicKey,
+    };
+};
+
+// `failMLDataFor`: answer 500 to every ML data request that asks for this file.
+export const buildMetaFetch = (m: MetaMockState, failMLDataFor?: number) => {
+    let srpServer: SrpServer;
+    return (async (
+        input: RequestInfo | URL,
+        init?: RequestInit,
+    ): Promise<Response> => {
+        const url =
+            typeof input === "string"
+                ? input
+                : input instanceof URL
+                  ? input.href
+                  : input.url;
+        const path = new URL(url).pathname;
+        const json = (body: unknown) =>
+            new Response(JSON.stringify(body), {
+                status: 200,
+                headers: { "content-type": "application/json" },
+            });
+
+        if (path === "/users/srp/attributes")
+            return json({ attributes: m.srpAttributes });
+        if (path === "/users/srp/create-session") {
+            const body = JSON.parse(init?.body as string);
+            const serverKey = await SRP.genKey();
+            srpServer = new SrpServer(
+                SRP.params["4096"],
+                m.verifier,
+                serverKey,
+            );
+            const B = srpServer.computeB();
+            srpServer.setA(Buffer.from(body.srpA, "base64"));
+            return json({ sessionID: "s1", srpB: B.toString("base64") });
+        }
+        if (path === "/users/srp/verify-session") {
+            const body = JSON.parse(init?.body as string);
+            srpServer.checkM1(Buffer.from(body.srpM1, "base64"));
+            return json({
+                srpM2: srpServer.computeM2().toString("base64"),
+                id: 42,
+                keyAttributes: m.keyAttributes,
+                encryptedToken: m.encryptedToken,
+            });
+        }
+        if (path === "/collections/v2")
+            return json({ collections: m.collections });
+        if (path === "/collections/v2/diff") {
+            const collID = Number(
+                new URL(url).searchParams.get("collectionID"),
+            );
+            return json({
+                diff: m.filesByCollection[collID] ?? [],
+                hasMore: false,
+            });
+        }
+        if (path === "/files/data/fetch") {
+            const body = JSON.parse(init?.body as string);
+            if ((body.fileIDs as number[]).includes(failMLDataFor!))
+                return new Response("server error", { status: 500 });
+            const data = (body.fileIDs as number[])
+                .filter((id: number) => m.encryptedMLData[id])
+                .map((id: number) => ({
+                    fileID: id,
+                    ...m.encryptedMLData[id],
+                    updatedAt: 1700000000000000,
+                }));
+            return json({ data });
+        }
+        if (
+            url.includes("files.ente.io") ||
+            path.startsWith("/files/download/")
+        ) {
+            const parsed = new URL(url);
+            const fileID = Number(
+                parsed.searchParams.get("fileID") ?? path.split("/").pop(),
+            );
+            const ct = m.fileCiphertexts[fileID];
+            if (ct) return new Response(ct, { status: 200 });
+            return new Response("not found", { status: 404 });
+        }
+        return new Response("not found", { status: 404 });
+    }) as typeof globalThis.fetch;
+};

test/library/store.test.ts

@@ -172,6 +172,29 @@ describe("MetadataStore", () => {
         expect(store.listCollections()).toEqual([]);
     });
 
+    it("loads an empty store when the list of what the refresh could not decrypt is damaged", async () => {
+        // A backup reads that list on every run, so a damaged one that loaded
+        // would fail every backup until the cache was deleted. A string is
+        // damaged too, though JavaScript can loop over its characters.
+        mkdirSync(join(dir, "cache"), { recursive: true });
+        for (const undecryptable of [{}, "not a list", [1]]) {
+            writeFileSync(
+                path,
+                JSON.stringify({
+                    schemaVersion: METADATA_SCHEMA_VERSION,
+                    userID: 42,
+                    collectionsSinceTime: 0,
+                    collections: [],
+                    files: [],
+                    undecryptable,
+                }),
+            );
+            const store = await MetadataStore.load(path);
+            expect(store.userID).toBe(0);
+            expect(store.listUndecryptable()).toEqual([]);
+        }
+    });
+
     it("creates the directory 0700 and the file 0600", async () => {
         const store = await MetadataStore.load(path);
         store.putCollection(sampleCollection());

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"