Names quak builds from server data fit in 255 bytes (closes #205)

The default names of `quak get`, `quak get-thumb`, `downloadFile` and `downloadThumbnail`, and the album folders of `backup-metadata`, could be over 255 bytes, which a file system refuses. `defaultFileName` and `shortenKeepingExtension` in `src/filename.ts` now cut such a name short, keeping its extension when it fits; a live photo's image and video get their own extension before the cut. A cut name never starts with a dot and is never a Windows device name. `--out` and `outPath` are kept as given. A `backup-metadata` folder keeps its `<id>-`. `safeExtension` gives `.bin` for an extension of more than 16 letters and digits, so a save path's name always fits.

Model: opus-5-5

README.md

@@ -683,27 +683,41 @@ 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.
 
+Without `--out`, `get` writes the file to the working directory under its title,
+and `get-thumb` writes the thumbnail there under `thumb_` followed by the title,
+each with unsafe characters replaced. A name over 255 bytes, which most file
+systems refuse, is cut short between two characters before its extension, so the
+extension is kept whenever it fits. An extension too long to leave room for the
+name's first character is cut short too, so the name never starts with a dot.
+
 `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
 its image and its video, each named after the title with its own extension, as
-Ente's clients name them (`IMG_0001.heic` and `IMG_0001.mov`). With
-`--out PATH`, the image is written to `PATH` and the video beside it, with
-`PATH`'s name and the video's extension; a `PATH` with the video's extension is
-refused. `backup-metadata --exif` (alias `--all`) additionally fetches each
-file's original through the cache and records, from it or a live photo's image,
-its XMP metadata, its EXIF metadata and, for a JPEG, its dimensions. EXIF is
-read with [exifreader](https://github.com/mattiasw/ExifReader) from any image
-format it reads, JPEG, HEIC/HEIF, AVIF, PNG and WebP among them. The record's
-`exif` field is exifreader's EXIF tag output: each tag by name, with its
-`value`, `description` and `computed` value. An EXIF block exifreader finds but
-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.
+Ente's clients name them (`IMG_0001.heic` and `IMG_0001.mov`), and each name is
+cut short as above once its own extension is in it. With `--out PATH`, the image
+is written to `PATH` and the video beside it, with `PATH`'s name and the video's
+extension; a `PATH` with the video's extension is refused.
+`backup-metadata --exif` (alias `--all`) additionally fetches each file's
+original through the cache and records, from it or a live photo's image, its XMP
+metadata, its EXIF metadata and, for a JPEG, its dimensions. EXIF is read with
+[exifreader](https://github.com/mattiasw/ExifReader) from any image format it
+reads, JPEG, HEIC/HEIF, AVIF, PNG and WebP among them. The record's `exif` field
+is exifreader's EXIF tag output: each tag by name, with its `value`,
+`description` and `computed` value. An EXIF block exifreader finds but 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 stays
 as it was and goes into `failures.json` (see "Backup layout").
 
+`backup-metadata <dir>` writes `account.json` and, for each collection, a
+directory `collections/<id>-<name>/` holding `_collection.json` and a
+`<fileID>.json` for each of its files. The collection's name has unsafe
+characters replaced and is cut short between two characters so that the
+directory's name fits in 255 bytes; the ID keeps the directories apart.
+
 `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. A file whose ML
@@ -756,9 +770,10 @@ Each original is saved at its save path, the same path `photo.savePath` gives
 and `photo.download()` writes (see Read surface below). The date is the photo's
 `takenAt` (the date set in Ente if it was edited, else its creation time) in the
 time zone of the machine running quak. The extension is the one in the file's
-name as uploaded, case kept, or `.bin` when it has none or it holds anything but
-letters and digits. When the date or the time zone changes, the next run saves
-the original at its new path and leaves the old copy where it is.
+name as uploaded, case kept, or `.bin` when it has none, it holds anything but
+letters and digits, or it is more than 16 of them long. When the date or the
+time zone changes, the next run saves the original at its new path and leaves
+the old copy where it is.
 
 `account.json` holds the account's `email` and `userID`, as `backup-metadata`
 writes it. A collection's JSON holds its `id`, `name`, `type`, `ownerID`,
@@ -851,11 +866,12 @@ gets its ID added as well: beside an album `x`, whose JSON is `x.json`, an album
 named `x.json` gets the directory `x.json (12)/`. A name too long for a file
 name, which most file systems limit to 255 bytes, gets its ID added too. A name
 with its ID added is cut short between two characters in the part before the ID,
-just far enough that it fits. When a symlink's extension alone leaves no room
-for the ID, the extension is cut short and the part before the ID keeps only the
-bytes that cut leaves over. A collection's directory name also leaves room for
-the `.json` its JSON adds. IDs never change, so a name stays the same from run
-to run until such a clash appears or goes away.
+just far enough that it fits. When a symlink's extension leaves no room for the
+ID and the first character of the name, the extension is cut short, and the part
+before the ID keeps that character and any bytes the cut leaves over. A
+collection's directory name also leaves room for the `.json` its JSON adds. IDs
+never change, so a name stays the same from run to run until such a clash
+appears or goes away.
 
 A collection whose directory or JSON cannot be written, such as one whose
 directory name is taken by a file you put in `collections/`, does not stop the

TODO.md

@@ -17,15 +17,32 @@ pre-1.0
 
 # Next Step
 
-One bug is open: https://git.eeqj.de/sneak/quak/issues/205. 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.
+No bug is open. Two changes sneak asked for are open:
+https://git.eeqj.de/sneak/quak/issues/211 and
+https://git.eeqj.de/sneak/quak/issues/212. 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
 
+- 2026-10-18: Names quak builds from an album name or a title fit in a file name
+  (https://git.eeqj.de/sneak/quak/issues/205). `shortenKeepingExtension` in
+  `src/filename.ts` cuts a name short to a number of bytes, keeping its
+  extension when it fits and always its first character, and can put text right
+  before the extension; the backup's `withID` uses it for an ID.
+  `defaultFileName` there gives the name a file is saved under when the caller
+  gives no path: a prefix such as `thumb_` and the title with unsafe characters
+  replaced, cut short with it to 255 bytes, and, for a live photo's image or
+  video, with that part's own extension before the cut. `quak get`,
+  `quak get-thumb`, `downloadFile` and `downloadThumbnail` use it without
+  `--out` or `outPath`. `backup-metadata` cuts an album's name short so that its
+  folder, `<id>-<name>`, fits in 255 bytes. `safeExtension` gives `.bin` for an
+  extension of more than 16 letters and digits, so a save path's name always
+  fits.
+
 - 2026-10-18: `quak collections` no longer waits for the files of every
   collection (issue 210). On an empty cache it opened its library with a refresh
   that fetched and decrypted the files of each collection in turn, printing

src/backup.ts

@@ -82,6 +82,7 @@ import { describeError } from "./errors.js";
 import {
     MAX_NAME_BYTES,
     sanitizeFileName,
+    shortenKeepingExtension,
     shortenToBytes,
     withExtension,
 } from "./filename.js";
@@ -289,9 +290,10 @@ const rebuildSymlink = (linkPath: string, target: string): void => {
 // case (two names that differ only in case are one entry on a case-insensitive
 // file system), or it is too long for a file name; then it gets ` (<id>)`,
 // right before the extension when `beforeExtension` is set. To fit, the part
-// before the ID is cut short as far as it must be. When the extension alone
-// leaves no room for the ID, the extension is cut short and the part before
-// the ID keeps only the bytes that cut leaves over. With `withJSON`, each
+// before the ID is cut short as far as it must be. When the extension leaves
+// no room for the ID and the first character of the name, the extension is
+// cut short, and the part before the ID keeps that character and any bytes
+// the cut leaves over (see `shortenKeepingExtension`). With `withJSON`, each
 // entry also has the JSON file `<name>.json` beside it, as an album does: a
 // name then leaves room for `.json`, and one that is another entry's JSON name
 // gets its ID added too.
@@ -305,11 +307,9 @@ const uniqueNames = (
     const maxBytes = MAX_NAME_BYTES - (opts.withJSON ? ".json".length : 0);
     const withID = (id: number, name: string): string => {
         const tag = ` (${id})`;
-        const fullExt = opts.beforeExtension ? extname(name) : "";
-        const stem = name.slice(0, name.length - fullExt.length);
-        const ext = shortenToBytes(fullExt, maxBytes - Buffer.byteLength(tag));
-        const room = maxBytes - Buffer.byteLength(tag + ext);
-        return shortenToBytes(stem, room) + tag + ext;
+        return opts.beforeExtension
+            ? shortenKeepingExtension(name, maxBytes, tag)
+            : shortenToBytes(name, maxBytes - Buffer.byteLength(tag)) + tag;
     };
     const names = entries.map((e) => e.name);
     const suffixed = new Set<number>();

src/cli-commands.ts

@@ -332,8 +332,8 @@ export const getCommand = async (
         const result = await photo.original();
         // Default name is the file's own title, as the pre-library CLI used
         // (not the editedName-preferring projection title) (issue #52).
-        const outPath = opts.out ?? originalName(file);
         if (result.videoPath === undefined) {
+            const outPath = opts.out ?? originalName(file);
             copyFileSync(result.path, outPath);
             ctx.stderr.write(`${result.bytes} bytes -> ${outPath}\n`);
             return 0;
@@ -341,9 +341,13 @@ export const getCommand = async (
         // A live photo is written as its image and its video, each named after
         // the title with its own extension, as Ente's clients name them. With
         // --out, the image goes there and the video beside it.
-        const imageOut =
-            opts.out ?? withExtension(outPath, extname(result.path));
-        const videoOut = withExtension(outPath, extname(result.videoPath));
+        const imageExt = extname(result.path);
+        const videoExt = extname(result.videoPath);
+        const imageOut = opts.out ?? originalName(file, imageExt);
+        const videoOut =
+            opts.out === undefined
+                ? originalName(file, videoExt)
+                : withExtension(opts.out, videoExt);
         if (imageOut.toLowerCase() === videoOut.toLowerCase()) {
             ctx.stderr.write(
                 `File ${fileID} is a live photo, and its video would also be written to ${imageOut}\n`,

src/cli-output.ts

@@ -9,7 +9,7 @@
 // `metadata.title`, and issue #52 requires that output stay byte-identical, so
 // the commands shape their output from the raw `EnteFile` through here.
 
-import { sanitizeFileName } from "./filename.js";
+import { defaultFileName } from "./filename.js";
 import type { EnteFile, FileType, Microseconds } from "./model/types.js";
 
 // One row of `quak files --json`.
@@ -33,11 +33,13 @@ export const fileListRow = (file: EnteFile): FileListRow => ({
 export const fileListLine = (file: EnteFile): string =>
     `${file.id}\t${file.metadata.fileType}\t${file.metadata.title}`;
 
-// Default output path for `quak get` when `--out` is not given. The title comes
-// from the server, so it is sanitized; `--out` is the user's and is used as is.
-export const originalName = (file: EnteFile): string =>
-    sanitizeFileName(file.metadata.title, `file-${file.id}`);
+// Default output path for `quak get` when `--out` is not given, made from the
+// title (see `defaultFileName`), with `ext` in place of its extension for a
+// live photo's image or video; `--out` is the user's and is used as is.
+export const originalName = (file: EnteFile, ext?: string): string =>
+    defaultFileName(file.metadata.title, file.id, { ext });
 
-// Default output path for `quak get-thumb` when `--out` is not given.
+// Default output path for `quak get-thumb` when `--out` is not given: `thumb_`
+// and the title.
 export const thumbnailName = (file: EnteFile): string =>
-    `thumb_${originalName(file)}`;
+    defaultFileName(file.metadata.title, file.id, { prefix: "thumb_" });

src/download/index.ts

@@ -16,7 +16,7 @@ import {
     streamTagFinal,
 } from "../crypto/index.js";
 import { TruncatedStreamError } from "../errors.js";
-import { safeExtension, sanitizeFileName, withExtension } from "../filename.js";
+import { defaultFileName, safeExtension, withExtension } from "../filename.js";
 import { withRetry } from "../retry.js";
 import type { ApiClient } from "../api/client.js";
 import type { EnteFile } from "../model/types.js";
@@ -365,10 +365,14 @@ const openPart = async (
 // name starts with `image` as the image and the one whose name starts with
 // `video` as the video, and refuses a ZIP holding a second of either. It is
 // written unpacked: each part is named `destination` with the extension
-// replaced by its own entry's, and the two must differ ignoring case. When the
-// file records a hash, `<imageHash>:<videoHash>` must match it, each over that
-// part's own bytes. Only then are the image, then the video, renamed into
-// place; on any failure neither is stored.
+// replaced by its own entry's, and the two must differ ignoring case. When
+// `defaultName` is set, `destination` is the name `defaultFileName` made from
+// the title rather than a path the caller gave, and each part is named as
+// `defaultFileName` names it with its own extension, so that the name is cut
+// short to fit with that extension. When the file records a hash,
+// `<imageHash>:<videoHash>` must match it, each over that part's own bytes.
+// Only then are the image, then the video, renamed into place; on any failure
+// neither is stored.
 //
 // The ZIP is chosen by its uploader and may expand enormously, so each part is
 // written as it decompresses and never held, and the ZIP is refused once the
@@ -388,6 +392,7 @@ const decryptLivePhoto = async (
     key: Uint8Array,
     onProgress: ProgressCallback | undefined,
     file: EnteFile,
+    defaultName: boolean,
 ): Promise<DownloadResult> => {
     const sliceSize = 4096;
     const dir = dirname(destination);
@@ -486,8 +491,12 @@ const decryptLivePhoto = async (
                 `live photo's image and video have the same extension, ${video.ext}`,
             );
         }
-        const path = withExtension(destination, image.ext);
-        const videoPath = withExtension(destination, video.ext);
+        const partPath = (ext: string): string =>
+            defaultName
+                ? defaultFileName(file.metadata.title, file.id, { ext })
+                : withExtension(destination, ext);
+        const path = partPath(image.ext);
+        const videoPath = partPath(video.ext);
         for (const part of parts) {
             await part.handle.sync();
             await part.handle.close();
@@ -531,6 +540,9 @@ const decryptLivePhoto = async (
 // and then removes its own temp file, and only the attempt that reaches
 // TAG_FINAL renames one into place, so a download that needed three tries still
 // performs exactly one rename over the destination.
+//
+// `defaultName` says `destination` is the name `defaultFileName` made, not a
+// path the caller gave (see `decryptLivePhoto`).
 const fetchAndDecrypt = async (
     api: ApiClient,
     openStream: () => Promise<ReadableStream<Uint8Array>>,
@@ -539,6 +551,7 @@ const fetchAndDecrypt = async (
     destination: string,
     onProgress?: ProgressCallback,
     original?: EnteFile,
+    defaultName = false,
 ): Promise<DownloadResult> =>
     withRetry(async () => {
         const stream = await openStream();
@@ -551,6 +564,7 @@ const fetchAndDecrypt = async (
                     key,
                     onProgress,
                     original,
+                    defaultName,
                 );
             }
             const bytesWritten = await decryptToTemp(
@@ -582,10 +596,11 @@ export const downloadFile = async (
     outPath?: string,
     onProgress?: ProgressCallback,
 ): Promise<DownloadResult> => {
-    // `outPath` is the caller's and is used as is; the title is the server's
-    // and is sanitized so it can only name a file in the current directory.
+    // `outPath` is the caller's and is used as is. Without it, the file is
+    // written in the current directory under the name `defaultFileName` makes
+    // from the title, which is the server's.
     const resolvedPath =
-        outPath ?? sanitizeFileName(file.metadata.title, `file-${file.id}`);
+        outPath ?? defaultFileName(file.metadata.title, file.id);
     const header = fromBase64(file.file.decryptionHeader);
     return fetchAndDecrypt(
         api,
@@ -595,6 +610,7 @@ export const downloadFile = async (
         resolvedPath,
         onProgress,
         file,
+        outPath === undefined,
     );
 };
 
@@ -604,9 +620,10 @@ export const downloadThumbnail = async (
     outPath?: string,
     onProgress?: ProgressCallback,
 ): Promise<DownloadResult> => {
+    // As in `downloadFile`, with `thumb_` before the title.
     const resolvedPath =
         outPath ??
-        `thumb_${sanitizeFileName(file.metadata.title, `file-${file.id}`)}`;
+        defaultFileName(file.metadata.title, file.id, { prefix: "thumb_" });
     const header = fromBase64(file.thumbnail.decryptionHeader);
     return fetchAndDecrypt(
         api,

src/filename.ts

@@ -5,7 +5,9 @@
 // used on disk goes through here, so it can only ever name one file inside the
 // directory the caller chose: never a path, never `..`, never hidden, never a
 // Windows device name. Such a name can still be too long for a file name, and
-// `shortenToBytes` cuts one short.
+// `shortenToBytes` or `shortenKeepingExtension` cuts one short.
+// `defaultFileName` does both for the name a file is saved under when the
+// caller gives no path.
 //
 // A path the user typed (`--out`, `outPath`) is not passed through here: the
 // caller is trusted, the server is not.
@@ -30,11 +32,11 @@ export const sanitizeFileName = (name: string, fallback: string): string => {
     return RESERVED_DEVICE_NAME.test(cleaned) ? `_${cleaned}` : cleaned;
 };
 
-// The extension of `title` (".jpg"), or ".bin" when it has none or it holds
-// anything but letters and digits.
+// The extension of `title` (".jpg"), or ".bin" when it has none, it holds
+// anything but letters and digits, or it is more than 16 of them long.
 export const safeExtension = (title: string): string => {
     const ext = extname(title);
-    return /^\.[A-Za-z0-9]+$/.test(ext) ? ext : ".bin";
+    return /^\.[A-Za-z0-9]{1,16}$/.test(ext) ? ext : ".bin";
 };
 
 // `name` with its extension, if it has one, replaced by `ext` (".mov").
@@ -57,3 +59,53 @@ export const shortenToBytes = (name: string, maxBytes: number): string => {
     }
     return name.slice(0, end);
 };
+
+// `name` cut short to fit in `maxBytes` bytes with `insert` put right before
+// its extension (`IMG_0001 (12).jpg`), each cut made as `shortenToBytes`
+// makes it. The part before the extension is cut first, so the extension is
+// kept whenever it fits, but that part always keeps its first character:
+// when the extension leaves no room for `insert` and that character, the
+// extension is cut short too, and the part before it keeps the bytes that cut
+// leaves over. So a name that does not start with a dot never comes back
+// starting with one, which would hide it. `insert` is never cut. With no
+// `insert`, a name that fits comes back unchanged.
+export const shortenKeepingExtension = (
+    name: string,
+    maxBytes: number,
+    insert = "",
+): string => {
+    const fullExt = extname(name);
+    const stem = name.slice(0, name.length - fullExt.length);
+    const first = Array.from(stem)[0] ?? "";
+    const ext = shortenToBytes(
+        fullExt,
+        maxBytes - Buffer.byteLength(insert + first),
+    );
+    const room = maxBytes - Buffer.byteLength(insert + ext);
+    return shortenToBytes(stem, room) + insert + ext;
+};
+
+// The name a file is saved under in the working directory when the caller
+// gives no path (`quak get` without `--out`, `downloadFile` without
+// `outPath`): `prefix` (`thumb_` for a thumbnail), then the file's title with
+// unsafe characters replaced, or `file-<id>` when the title is empty, cut
+// short so that the whole fits in a file name, keeping the title's extension
+// whenever it fits. `ext` (".mov"), when given, takes the place of the
+// title's extension before the name is cut short; a live photo's image and
+// video are each named with their own. The cut can leave a device name
+// (`CONSOLE.<long extension>` becomes `CON.<long extension>`); that name gets
+// a leading `_`, as `sanitizeFileName` gives one, and is cut a byte shorter to
+// leave room for it.
+export const defaultFileName = (
+    title: string,
+    fileID: number,
+    opts: { prefix?: string; ext?: string } = {},
+): string => {
+    const { prefix = "", ext } = opts;
+    const safe = sanitizeFileName(title, `file-${fileID}`);
+    const name = ext === undefined ? safe : withExtension(safe, ext);
+    const room = MAX_NAME_BYTES - Buffer.byteLength(prefix);
+    const cut = prefix + shortenKeepingExtension(name, room);
+    if (!RESERVED_DEVICE_NAME.test(cut)) return cut;
+    return `_${prefix}${shortenKeepingExtension(name, room - 1)}`;
+};

src/metadata-backup.ts

@@ -5,7 +5,11 @@ import type { Client } from "./client.js";
 import { describeError } from "./errors.js";
 import { readExifTags } from "./exif.js";
 import type { Library, Photo } from "./library/index.js";
-import { sanitizeFileName } from "./filename.js";
+import {
+    MAX_NAME_BYTES,
+    sanitizeFileName,
+    shortenToBytes,
+} from "./filename.js";
 import {
     fetchMLDataBatch,
     MLDATA_BATCH_SIZE,
@@ -127,7 +131,15 @@ export const runMetadataBackup = async (
         const col = lib.getCollection(album.collectionID);
         if (!col) continue;
 
-        const dirName = `${col.id}-${sanitizeFileName(col.name, "unnamed")}`;
+        // The album's ID keeps the folders apart, so the name after it can be
+        // cut short to fit in a file name.
+        const prefix = `${col.id}-`;
+        const dirName =
+            prefix +
+            shortenToBytes(
+                sanitizeFileName(col.name, "unnamed"),
+                MAX_NAME_BYTES - prefix.length,
+            );
         const colDir = join(outDir, "collections", dirName);
         mkdirSync(colDir, { recursive: true });
 

test/cli/backup.test.ts

@@ -1795,12 +1795,11 @@ describe("backup album folders", () => {
         expect(readLedger(outDir).files["5"]).toMatchObject({ attempts: 1 });
     });
 
-    // The extensions of these titles leave no room for the ID, so each is cut
-    // short and the name before the ID keeps only the bytes that cut leaves
-    // over. File 1's extension is 303 bytes and is cut to 251, which leaves
-    // none. File 2's is 271 bytes and is cut to 250, since "€" takes three
-    // bytes, which leaves one, for the "a". Neither extension is only letters
-    // and digits, so both originals are saved with `.bin`, a name that fits.
+    // The extensions of these titles leave no room for the ID and the "a"
+    // before them, so each is cut short to 250 bytes, which leaves one, for
+    // the "a". File 1's extension is 303 bytes and file 2's 271. Neither
+    // extension is only letters and digits, so both originals are saved with
+    // `.bin`, a name that fits.
     it("cuts the extension of a title short when it leaves no room for the ID", async () => {
         const outDir = join(root, "backup");
         const lib = libraryOf([
@@ -1815,7 +1814,7 @@ describe("backup album folders", () => {
 
         const result = await runBackup(lib, { downloadDirectory: outDir });
 
-        const link = ` (1).${"L".repeat(250)}`;
+        const link = `a (1).${"L".repeat(249)}`;
         const link2 = `a (2).${"€".repeat(83)}`;
         expect(Buffer.byteLength(link)).toBe(255);
         expect(Buffer.byteLength(link2)).toBe(255);
@@ -1830,9 +1829,9 @@ describe("backup album folders", () => {
 
     // Files 1 and 3 share a title, so each gets its ID before the
     // extension. File 2's title is too long, and its extension leaves no
-    // room for the ID. Were the ID put at the end of file 2's title cut
-    // short, its link would have file 1's name, and the link made second
-    // would replace the first.
+    // room for the ID and the "x" before it. Were the ID put at the end of
+    // file 2's title cut short, its link would have file 1's name, and the
+    // link made second would replace the first.
     it("keeps names unique when a title's extension leaves no room for the ID", async () => {
         const outDir = join(root, "backup");
         const shared = `x.${"Q".repeat(245)} (2)`;
@@ -1852,8 +1851,8 @@ describe("backup album folders", () => {
         expect(result.failed).toBe(0);
         expect(tree(outDir)).toEqual([
             "Trip/",
-            `Trip/ (2).${"Q".repeat(250)} -> ${linkTo("2.bin")}`,
             `Trip/x (1).${"Q".repeat(245)} (2) -> ${linkTo("1.bin")}`,
+            `Trip/x (2).${"Q".repeat(249)} -> ${linkTo("2.bin")}`,
             `Trip/x (3).${"Q".repeat(245)} (2) -> ${linkTo("3.bin")}`,
             "Trip.json",
         ]);

test/cli/commands.test.ts

@@ -21,7 +21,7 @@ import {
     utimesSync,
     writeFileSync,
 } from "node:fs";
-import { join } from "node:path";
+import { basename, join } from "node:path";
 import { tmpdir } from "node:os";
 import { PassThrough } from "node:stream";
 import * as jpegJs from "jpeg-js";
@@ -187,6 +187,9 @@ const FETCHED_BOTH_ALBUMS =
     "Fetched the files of 1 of 2 collections\n" +
     "Fetched the files of 2 of 2 collections\n";
 
+// The same for an account with one album.
+const FETCHED_ONE_ALBUM = "Fetched the files of 1 of 1 collections\n";
+
 // Collects everything written to it.
 class Output {
     text = "";
@@ -562,17 +565,109 @@ describe("get and get-thumb", () => {
     });
 });
 
+// A file system refuses a file name over 255 bytes. The account here is one
+// album, whose name is 300 bytes, holding one file, 700, with the title given.
+describe("names from an album name or a title over 255 bytes", () => {
+    const longNames = (title: string): Client =>
+        ({
+            ...fakeClient(),
+            collectionsSince: async () => ({
+                collections: [collection(7, "A".repeat(300))],
+                deleted: [],
+                cursor: 1,
+            }),
+            filesSince: async () => ({
+                files: [file(700, 7, title)],
+                deleted: [],
+                cursor: 1,
+            }),
+        }) as unknown as Client;
+    const longTitle = `${"t".repeat(296)}.jpg`;
+
+    // Run `command` in a new directory, where `get` and `get-thumb` without
+    // `--out` write, and return its exit code and the directory.
+    const inNewDirectory = async (
+        command: () => Promise<number>,
+    ): Promise<{ code: number; dir: string }> => {
+        const dir = mkdtempSync(join(root, "cwd-"));
+        const previous = process.cwd();
+        process.chdir(dir);
+        try {
+            return { code: await command(), dir };
+        } finally {
+            process.chdir(previous);
+        }
+    };
+
+    it("get cuts the title short to 255 bytes, keeping its extension, and exits 0", async () => {
+        const { code, dir } = await inNewDirectory(() =>
+            getCommand(context(longNames(longTitle)), "700", {}),
+        );
+
+        const name = `${"t".repeat(251)}.jpg`;
+        expect(Buffer.byteLength(name)).toBe(255);
+        expect(code).toBe(0);
+        expect(readdirSync(dir)).toEqual([name]);
+        expect(stderr.text).toBe(FETCHED_ONE_ALBUM + `7 bytes -> ${name}\n`);
+    });
+
+    it("get-thumb cuts the title short so that thumb_ and it fit in 255 bytes, and exits 0", async () => {
+        const { code, dir } = await inNewDirectory(() =>
+            getThumbCommand(context(longNames(longTitle)), "700", {}),
+        );
+
+        const name = `thumb_${"t".repeat(245)}.jpg`;
+        expect(Buffer.byteLength(name)).toBe(255);
+        expect(code).toBe(0);
+        expect(readdirSync(dir)).toEqual([name]);
+        expect(stderr.text).toBe(FETCHED_ONE_ALBUM + `3 bytes -> ${name}\n`);
+    });
+
+    it("backup-metadata cuts the album name short so that its folder's name fits in 255 bytes, and exits 0", async () => {
+        const dir = join(root, "dump");
+
+        const code = await backupMetadataCommand(
+            context(longNames("beach.jpg")),
+            dir,
+            {},
+        );
+
+        const folder = `7-${"A".repeat(253)}`;
+        expect(Buffer.byteLength(folder)).toBe(255);
+        expect(code).toBe(0);
+        expect(readdirSync(join(dir, "collections"))).toEqual([folder]);
+        expect(readdirSync(join(dir, "collections", folder)).sort()).toEqual([
+            "700.json",
+            "_collection.json",
+        ]);
+    });
+
+    // The title's extension is 300 letters, too long for the save path's
+    // name, so the original is saved with `.bin`.
+    it("backup saves a title with a 300-letter extension as .bin, and exits 0", async () => {
+        const dir = join(root, "backup");
+        const title = `b.${"e".repeat(300)}`;
+
+        const code = await backupCommand(context(longNames(title)), dir, {});
+
+        const path = savePath(dir, file(700, 7, title));
+        expect(code).toBe(0);
+        expect(basename(path)).toMatch(/^\d{4}-\d{2}-\d{2}\.700\.bin$/);
+        expect(readFileSync(path)).toEqual(Buffer.alloc(7, 700 & 0xff));
+    });
+});
+
 // A live photo, which Ente stores as one ZIP, is written as its image and its
 // video, which a photo viewer can open. The account here is Vacation holding
 // one live photo, 300, downloaded through the real download layer
 // (test/live-photo.ts).
 describe("a live photo", () => {
-    // What `get` prints on stderr while its refresh fetches Vacation's files.
-    const FETCHED_ONE_ALBUM = "Fetched the files of 1 of 1 collections\n";
-
-    const livePhotoClient = async (image = IMAGE): Promise<Client> => {
+    const livePhotoClient = async (
+        image = IMAGE,
+        title = "IMG_0300.HEIC",
+    ): Promise<Client> => {
         const { file: live, body } = await asLivePhoto(
-            file(300, 1, "IMG_0300.HEIC"),
+            file(300, 1, title),
             livePhotoZip({ "image.heic": image, "video.mov": VIDEO }),
             livePhotoHash(image, VIDEO),
         );
@@ -620,6 +715,44 @@ describe("a live photo", () => {
         );
     });
 
+    // Run `get` without `--out` in a new directory for a live photo titled
+    // `title`, and return its exit code and the names it wrote.
+    const getInNewDirectory = async (
+        title: string,
+    ): Promise<{ code: number; names: string[] }> => {
+        const ctx = context(await livePhotoClient(IMAGE, title));
+        const dir = mkdtempSync(join(root, "cwd-"));
+        const previous = process.cwd();
+        process.chdir(dir);
+        try {
+            const code = await getCommand(ctx, "300", {});
+            return { code, names: readdirSync(dir).sort() };
+        } finally {
+            process.chdir(previous);
+        }
+    };
+    const imageName = `${"t".repeat(250)}.heic`;
+    const videoName = `${"t".repeat(251)}.mov`;
+
+    // The title fits, but ".heic" is one byte longer than ".jpg", so the
+    // image's name is cut short once its extension is in it.
+    it("get cuts its image's name short for a 255-byte title ending in .jpg, and exits 0", async () => {
+        const title = `${"t".repeat(251)}.jpg`;
+
+        const result = await getInNewDirectory(title);
+
+        expect(Buffer.byteLength(title)).toBe(255);
+        expect(Buffer.byteLength(imageName)).toBe(255);
+        expect(Buffer.byteLength(videoName)).toBe(255);
+        expect(result).toEqual({ code: 0, names: [imageName, videoName] });
+    });
+
+    it("get cuts the names of its image and video short for a 300-byte title with no extension, and exits 0", async () => {
+        const result = await getInNewDirectory("t".repeat(300));
+
+        expect(result).toEqual({ code: 0, names: [imageName, videoName] });
+    });
+
     it("get --out writes the image there and the video beside it", async () => {
         const out = join(root, "photo.jpg");
         const video = join(root, "photo.mov");

test/download/download.test.ts

@@ -658,6 +658,26 @@ describe("downloadFile", () => {
         expect(readdirSync(parent)).toEqual(["cwd"]);
     });
 
+    // A file system refuses a file name over 255 bytes.
+    it("cuts a title too long for a file name short, keeping its extension", async () => {
+        const plaintext = new Uint8Array([1, 2, 3]);
+        const key = sodium.crypto_secretstream_xchacha20poly1305_keygen();
+        const { header, ciphertext } = encryptFileBody(plaintext, key);
+        const thumbPush =
+            sodium.crypto_secretstream_xchacha20poly1305_init_push(key);
+        const file = buildMockEnteFile(key, header, thumbPush.header);
+        file.metadata.title = `${"t".repeat(296)}.jpg`;
+        const dir = mkdtempSync(join(testDir, "long-title-"));
+
+        const api = new ApiClient({ fetch: mockFetchForBody(ciphertext) });
+        const result = await inDirectory(dir, () => downloadFile(api, file));
+
+        const name = `${"t".repeat(251)}.jpg`;
+        expect(Buffer.byteLength(name)).toBe(255);
+        expect(result.path).toBe(name);
+        expect(readdirSync(dir)).toEqual([name]);
+    });
+
     it("uses an explicit outPath verbatim, even one with ..", async () => {
         // The caller is trusted: its path is not sanitized.
         const { api, file } = fixtureFor(
@@ -751,6 +771,28 @@ describe("downloadThumbnail", () => {
         expect(result.path).toBe("thumb__etc_passwd");
         expect(readdirSync(dir)).toEqual(["thumb__etc_passwd"]);
     });
+
+    it("leaves room for thumb_ when it cuts a title too long for a file name short", async () => {
+        const plaintext = new Uint8Array([0xff, 0xd8, 0xff, 0xe0]);
+        const key = sodium.crypto_secretstream_xchacha20poly1305_keygen();
+        const filePush =
+            sodium.crypto_secretstream_xchacha20poly1305_init_push(key);
+        const { header: thumbHeader, ciphertext: thumbCipher } =
+            encryptFileBody(plaintext, key);
+        const file = buildMockEnteFile(key, filePush.header, thumbHeader);
+        file.metadata.title = `${"t".repeat(296)}.jpg`;
+        const dir = mkdtempSync(join(testDir, "long-thumb-title-"));
+
+        const api = new ApiClient({ fetch: mockFetchForBody(thumbCipher) });
+        const result = await inDirectory(dir, () =>
+            downloadThumbnail(api, file),
+        );
+
+        const name = `thumb_${"t".repeat(245)}.jpg`;
+        expect(Buffer.byteLength(name)).toBe(255);
+        expect(result.path).toBe(name);
+        expect(readdirSync(dir)).toEqual([name]);
+    });
 });
 
 // ---------------------------------------------------------------------------
@@ -1881,6 +1923,53 @@ describe("downloadFile live photos", () => {
         ]);
     });
 
+    // A file system refuses a file name over 255 bytes. Each name is cut
+    // short once its own extension is in it.
+    const imageName = `${"t".repeat(250)}.heic`;
+    const videoName = `${"t".repeat(251)}.mov`;
+
+    // The title fits, but ".heic" is one byte longer than ".jpg".
+    it("cuts the image's name short for a 255-byte title ending in .jpg when no outPath is given", async () => {
+        const title = `${"t".repeat(251)}.jpg`;
+        const t = setup(livePhotoZip(), { ...livePhoto, title });
+
+        const result = await inDirectory(t.dir, () =>
+            downloadFile(t.api, t.file),
+        );
+
+        expect(Buffer.byteLength(title)).toBe(255);
+        expect(Buffer.byteLength(imageName)).toBe(255);
+        expect(Buffer.byteLength(videoName)).toBe(255);
+        expect(result.path).toBe(imageName);
+        expect(result.videoPath).toBe(videoName);
+        expect(readdirSync(t.dir).sort()).toEqual([imageName, videoName]);
+    });
+
+    it("cuts the names of the image and the video short for a 300-byte title with no extension when no outPath is given", async () => {
+        const t = setup(livePhotoZip(), {
+            ...livePhoto,
+            title: "t".repeat(300),
+        });
+
+        const result = await inDirectory(t.dir, () =>
+            downloadFile(t.api, t.file),
+        );
+
+        expect(result.path).toBe(imageName);
+        expect(result.videoPath).toBe(videoName);
+        expect(readdirSync(t.dir).sort()).toEqual([imageName, videoName]);
+    });
+
+    // The caller's path is used as is; only the extension is each part's own.
+    it("keeps an outPath too long for a file name as given", async () => {
+        const t = setup(livePhotoZip(), livePhoto);
+        const outPath = join(t.dir, `${"t".repeat(251)}.jpg`);
+
+        await expect(downloadFile(t.api, t.file, outPath)).rejects.toThrow(
+            /ENAMETOOLONG/,
+        );
+    });
+
     it("gives each part its own extension from the ZIP, letters and digits only", async () => {
         const zip = livePhotoZip({ "image.JPG": IMAGE, "video.m-4v": VIDEO });
         const t = setup(zip, livePhoto);

test/filename/filename.test.ts

@@ -15,8 +15,10 @@
 import { describe, expect, it } from "vitest";
 
 import {
+    defaultFileName,
     safeExtension,
     sanitizeFileName,
+    shortenKeepingExtension,
     shortenToBytes,
 } from "../../src/filename.js";
 
@@ -94,6 +96,12 @@ describe("safeExtension", () => {
         expect(safeExtension("x.jp g")).toBe(".bin");
         expect(safeExtension("x.jpg\0")).toBe(".bin");
     });
+
+    it("keeps an extension of 16 letters and digits, and uses .bin for a longer one", () => {
+        expect(safeExtension(`x.${"e".repeat(16)}`)).toBe(`.${"e".repeat(16)}`);
+        expect(safeExtension(`x.${"e".repeat(17)}`)).toBe(".bin");
+        expect(safeExtension(`b.${"e".repeat(300)}`)).toBe(".bin");
+    });
 });
 
 describe("shortenToBytes", () => {
@@ -118,3 +126,103 @@ describe("shortenToBytes", () => {
         );
     });
 });
+
+describe("shortenKeepingExtension", () => {
+    it("returns a name that fits unchanged", () => {
+        expect(shortenKeepingExtension("beach.jpg", 9)).toBe("beach.jpg");
+        expect(shortenKeepingExtension("beach", 5)).toBe("beach");
+    });
+
+    it("cuts the part before the extension short and keeps the extension", () => {
+        expect(shortenKeepingExtension(`${"t".repeat(296)}.jpg`, 255)).toBe(
+            `${"t".repeat(251)}.jpg`,
+        );
+        expect(shortenKeepingExtension("é".repeat(150), 255)).toBe(
+            "é".repeat(127),
+        );
+    });
+
+    // A name cut down to its extension would start with a dot, and so be
+    // hidden.
+    it("keeps the first character before the extension, cutting the extension short to leave room for it", () => {
+        expect(shortenKeepingExtension(`b.${"e".repeat(300)}`, 255)).toBe(
+            `b.${"e".repeat(253)}`,
+        );
+        expect(shortenKeepingExtension(`ab.${"e".repeat(300)}`, 255)).toBe(
+            `a.${"e".repeat(253)}`,
+        );
+        expect(shortenKeepingExtension(`é.${"e".repeat(300)}`, 255)).toBe(
+            `é.${"e".repeat(252)}`,
+        );
+        // "€" takes three bytes, so the extension is cut to 253 and the
+        // part before it keeps the two bytes left over, for the "a".
+        expect(shortenKeepingExtension(`a.${"€".repeat(90)}`, 255)).toBe(
+            `a.${"€".repeat(84)}`,
+        );
+    });
+
+    it("puts `insert` right before the extension and leaves room for it", () => {
+        expect(shortenKeepingExtension("beach.jpg", 255, " (7)")).toBe(
+            "beach (7).jpg",
+        );
+        expect(
+            shortenKeepingExtension(`${"t".repeat(296)}.jpg`, 255, " (7)"),
+        ).toBe(`${"t".repeat(247)} (7).jpg`);
+        expect(
+            shortenKeepingExtension(`b.${"e".repeat(300)}`, 255, " (7)"),
+        ).toBe(`b (7).${"e".repeat(249)}`);
+    });
+});
+
+describe("defaultFileName", () => {
+    it("is the title when it is safe and fits", () => {
+        expect(defaultFileName("IMG_0001.HEIC", 42)).toBe("IMG_0001.HEIC");
+    });
+
+    it("replaces unsafe characters, and is file-<id> for an empty title", () => {
+        expect(defaultFileName("../x.jpg", 42)).toBe("__x.jpg");
+        expect(defaultFileName("", 42)).toBe("file-42");
+    });
+
+    it("cuts a title over 255 bytes short, keeping its extension", () => {
+        expect(defaultFileName(`${"t".repeat(296)}.jpg`, 42)).toBe(
+            `${"t".repeat(251)}.jpg`,
+        );
+        expect(defaultFileName(`b.${"e".repeat(300)}`, 42)).toBe(
+            `b.${"e".repeat(253)}`,
+        );
+    });
+
+    it("puts `prefix` before the title and leaves room for it", () => {
+        expect(defaultFileName("beach.jpg", 42, { prefix: "thumb_" })).toBe(
+            "thumb_beach.jpg",
+        );
+        expect(
+            defaultFileName(`${"t".repeat(296)}.jpg`, 42, { prefix: "thumb_" }),
+        ).toBe(`thumb_${"t".repeat(245)}.jpg`);
+    });
+
+    // A live photo's image and video are each named with their own
+    // extension, and the name is cut short once that extension is in it.
+    it("puts `ext` in place of the title's extension before cutting the name short", () => {
+        expect(defaultFileName("IMG_0001.HEIC", 42, { ext: ".mov" })).toBe(
+            "IMG_0001.mov",
+        );
+        expect(defaultFileName("", 42, { ext: ".mov" })).toBe("file-42.mov");
+        expect(
+            defaultFileName(`${"t".repeat(251)}.jpg`, 42, { ext: ".heic" }),
+        ).toBe(`${"t".repeat(250)}.heic`);
+        expect(defaultFileName("t".repeat(300), 42, { ext: ".heic" })).toBe(
+            `${"t".repeat(250)}.heic`,
+        );
+    });
+
+    // Cutting `CONSOLE` or `NULL` short before a long extension leaves `CON`
+    // or `NUL`, so the name gets the `_` `sanitizeFileName` gives a device
+    // name, and the part before the extension gives up a byte for it.
+    it("never returns a Windows device name, even once cut short", () => {
+        const ext = `.${"e".repeat(251)}`;
+        expect(defaultFileName(`CONSOLE${ext}`, 42)).toBe(`_CO${ext}`);
+        expect(defaultFileName(`NULL${ext}`, 42)).toBe(`_NU${ext}`);
+    });
+});