quak/src/backup.ts

// The backup command, rebuilt on the library API (issue #51).
//
// `lib.backup()` takes the lock in `downloadDirectory`, and fails at once when
// another backup of it holds the lock. It waits for a completed refresh of the
// library (a failed one fails the backup before any file is touched), then, for
// every file in scope, puts its original at its save path under
// `downloadDirectory`, as `Photo.download()` does, waits for an ML data fetch,
// and rebuilds the derived views (per-file sidecars, per-collection symlink
// trees, per-collection JSON) from the model. The on-disk layout:
//
//     <downloadDirectory>/
//         YYYY/YYYY-MM/YYYY-MM-DD/
//             YYYY-MM-DD.<fileID>.<ext>    the decrypted bytes (the save path)
//             YYYY-MM-DD.<fileID>.json     per-file metadata sidecar, with
//                                          the file's ML data and its
//                                          original's EXIF, XMP and
//                                          dimensions
//         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
// extension, beside `YYYY-MM-DD.<fileID>.livephoto.json` naming them; its album
// folders link both.
//
// Crash-safety rests on two properties. Bytes are present-means-complete: an
// original appears at its save path only via the content layer's atomic
// temp-then-rename, so a file that exists is whole and is never re-fetched — an
// interrupted run resumes by looking at the save paths. The derived views hold
// no unique state, so they are rebuilt every run; that repairs stale sidecars
// and missing or broken symlinks left by an earlier crash. A rebuild also
// removes the symlinks to originals that no longer belong to an album, and the
// directories of albums that no longer exist. The one thing a sidecar takes
// from the sidecar it replaces is its original's EXIF, XMP and dimensions (or
// 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.
//
// 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.
// `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
// that has since left the library (deleted) or this run's scope is dropped
// rather than counted forever, which would poison a scheduled backup's exit
// code.

import {
    lstatSync,
    mkdirSync,
    readdirSync,
    readFileSync,
    readlinkSync,
    rmdirSync,
    rmSync,
    statSync,
    symlinkSync,
    writeFileSync,
} from "node:fs";
import { readFile } from "node:fs/promises";
import { dirname, extname, join, relative, resolve } from "node:path";
import lockfile from "proper-lockfile";

import { removeLeftoverTempFiles } from "./download/index.js";
import { sanitizeFileName, withExtension } from "./filename.js";
import {
    copyAtomic,
    placeOriginal,
    savePath,
    storedAtSavePath,
} from "./library/content.js";
import { representative } from "./library/records.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;

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
    // traffic.
    downloadDirectory?: string;
    // Fetch and store full-resolution originals. Default true.
    includeOriginals?: boolean;
    // Also fetch and store thumbnails under `thumbnails/<fileID>.jpg`. Default
    // false.
    includeThumbnails?: boolean;
    // Restrict the backup to albums with these names; others are left untouched.
    onlyAlbumNames?: string[];
    onProgress?: ProgressCallback;
}

export interface BackupError {
    fileID: number;
    title: string;
    collection: string;
    error: string;
}

export interface BackupResult {
    // Distinct files in scope this run.
    totalFiles: number;
    // Originals fetched (or copied from the cache) this run.
    downloaded: number;
    // Originals already at their save path and left untouched.
    skipped: 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 slice of the library that backup drives. `Library` implements it; a test
// can drive backup with a stand-in.
export interface BackupLibrary {
    // The account the library belongs to.
    whoami(): { email: string; userID: number };
    refresh(): Promise<void>;
    listCollections(): Collection[];
    listFiles(collectionID: number): EnteFile[];
    // 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
    // path). A live photo lands as its image and its video, fetched now beside
    // `destination`.
    original(
        fileID: number,
        destination: string,
    ): Promise<{ path: string; videoPath?: string }>;
    thumbnail(fileID: number): Promise<{ path: string }>;
    // 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.
    fetchMLData(): Promise<void>;
    // A file's cached ML data, as `lib.mldata.forFile()` returns it, or
    // undefined when none is cached.
    mlData(fileID: number): Promise<MLData | undefined>;
}

type FailureClass = "transient" | "permanent" | "unknown";

interface FailureEntry {
    fileID: number;
    title: string;
    classification: FailureClass;
    attempts: number;
    lastTriedAt: number;
    error: string;
}

const LEDGER_VERSION = 1;

// A regular file with content is treated as complete. A zero-byte file is not:
// it is the shape an aborted write leaves and must be re-fetched.
const isPresent = (path: string): boolean => {
    try {
        const s = statSync(path);
        return s.isFile() && s.size > 0;
    } catch {
        return false;
    }
};

// Best-effort classification for the ledger. Retryable server/network problems
// are transient; refusals and local filesystem/decrypt errors are permanent;
// anything else is unknown. Both the error code and message are inspected.
const classify = (err: unknown): FailureClass => {
    const e = err as NodeJS.ErrnoException;
    const text =
        `${e?.code ?? ""} ${err instanceof Error ? err.message : String(err)}`.toLowerCase();
    if (
        /timeout|timed out|econnreset|econnrefused|econnaborted|network|socket|eai_again|throttl|temporarily|429|500|502|503|504/.test(
            text,
        )
    ) {
        return "transient";
    }
    if (
        /enoent|eacces|eperm|eexist|eisdir|enotempty|erofs|enospc|not found|forbidden|unauthor|decrypt|truncat|401|403|404/.test(
            text,
        )
    ) {
        return "permanent";
    }
    return "unknown";
};

const errorMessage = (err: unknown): string =>
    err instanceof Error ? err.message : String(err);

// Ensure `linkPath` is a symlink to `target`, rebuilding a missing, wrong, or
// non-symlink entry. Throws on failure (a directory in the way, no permission)
// so the caller records it and moves on rather than aborting the run.
const rebuildSymlink = (linkPath: string, target: string): void => {
    try {
        const st = lstatSync(linkPath);
        if (st.isSymbolicLink() && readlinkSync(linkPath) === target) return;
    } catch {
        // Nothing there (or unreadable): fall through to create it.
    }
    // Remove a wrong symlink or stray file. `force` ignores a missing path but
    // still refuses a directory (no `recursive`), which surfaces as a failure.
    rmSync(linkPath, { force: true });
    symlinkSync(target, linkPath);
};

// The on-disk names for the entries of one directory, in entry order. Each
// name is used as is unless another entry would get the same name, ignoring
// case (two names that differ only in case are one entry on a case-insensitive
// file system); then every entry sharing it gets ` (<id>)`, before the
// extension when `beforeExtension` is set. A name with an ID added can match
// another entry's own name (`IMG (6).JPG`), so this repeats until no name is
// shared. IDs are stable, so the names are too.
const uniqueNames = (
    entries: { id: number; name: string }[],
    beforeExtension: boolean,
): string[] => {
    const withID = (id: number, name: string): string => {
        const ext = beforeExtension ? extname(name) : "";
        const stem = name.slice(0, name.length - ext.length);
        return `${stem} (${id})${ext}`;
    };
    const names = entries.map((e) => e.name);
    const suffixed = new Set<number>();
    for (;;) {
        const counts = new Map<string, number>();
        for (const name of names) {
            const key = name.toLowerCase();
            counts.set(key, (counts.get(key) ?? 0) + 1);
        }
        let changed = false;
        for (const [i, { id, name }] of entries.entries()) {
            if (suffixed.has(i)) continue;
            if (counts.get(name.toLowerCase()) === 1) continue;
            names[i] = withID(id, name);
            suffixed.add(i);
            changed = true;
        }
        if (!changed) return names;
    }
};

// The links a file gets in its album's folder: one named after its title, to
// its original if that is stored. A stored live photo gets two, to its image
// and its video, each named after the title with that file's extension.
const linksFor = (
    file: EnteFile,
    stored: { path: string; videoPath?: string } | undefined,
): { id: number; name: string; file: EnteFile; target?: string }[] => {
    const name = sanitizeFileName(file.metadata.title, `file-${file.id}`);
    if (stored?.videoPath === undefined) {
        return [{ id: file.id, name, file, target: stored?.path }];
    }
    return [stored.path, stored.videoPath].map((target) => ({
        id: file.id,
        name: withExtension(name, extname(target)),
        file,
        target,
    }));
};

// Every date folder (`YYYY/YYYY-MM/YYYY-MM-DD/`) under `root`, whether or not a
// file in this backup is saved there. A folder that cannot be read is skipped.
const dateFolders = (root: string): string[] => {
    const subfolders = (dir: string, name: RegExp): string[] => {
        try {
            return readdirSync(dir, { withFileTypes: true })
                .filter((e) => e.isDirectory() && name.test(e.name))
                .map((e) => join(dir, e.name));
        } catch {
            return [];
        }
    };
    return subfolders(root, /^\d{4}$/)
        .flatMap((year) => subfolders(year, /^\d{4}-\d\d$/))
        .flatMap((month) => subfolders(month, /^\d{4}-\d\d-\d\d$/));
};

// Whether the entry at `path` is a symlink a backup to `root` made: one to an
// original in a `YYYY/YYYY-MM/YYYY-MM-DD/` folder of `root`.
const linksToOriginal = (path: string, root: string): boolean => {
    if (!lstatSync(path).isSymbolicLink()) return false;
    const target = relative(root, resolve(dirname(path), readlinkSync(path)));
    return /^\d{4}\/\d{4}-\d\d\/\d{4}-\d\d-\d\d\/[^/]+$/.test(target);
};

// Remove the symlinks in the album directory `dir` that point to an original
// in `root` and are not named in `keep`. Nothing else in the directory is
// touched: anything else there was put there by the user.
const removeStaleLinks = (
    dir: string,
    keep: Set<string>,
    root: string,
): void => {
    for (const name of readdirSync(dir)) {
        if (keep.has(name)) continue;
        const path = join(dir, name);
        if (linksToOriginal(path, root)) rmSync(path);
    }
};

// Remove the directories under `collectionsDir` that an earlier run wrote for
// an album that is gone or renamed: a directory not named in `current` with a
// `<name>.json` beside it holding an album ID, which is what a run writes. Its
// symlinks to originals in `root` are removed; if that leaves it empty, it and
// its JSON are deleted, otherwise both stay for what the user put there.
const removeStaleAlbumDirs = (
    collectionsDir: string,
    current: Set<string>,
    root: string,
): void => {
    for (const entry of readdirSync(collectionsDir, { withFileTypes: true })) {
        if (!entry.isDirectory() || current.has(entry.name)) continue;
        const jsonPath = join(collectionsDir, `${entry.name}.json`);
        try {
            const album = JSON.parse(readFileSync(jsonPath, "utf-8")) as {
                id?: unknown;
            };
            if (typeof album.id !== "number") continue;
        } catch {
            continue;
        }
        const dir = join(collectionsDir, entry.name);
        removeStaleLinks(dir, new Set(), root);
        if (readdirSync(dir).length > 0) continue;
        rmdirSync(dir);
        rmSync(jsonPath);
    }
};

const loadLedger = (path: string): Map<number, FailureEntry> => {
    const ledger = new Map<number, FailureEntry>();
    try {
        const parsed = JSON.parse(readFileSync(path, "utf-8")) as {
            files?: Record<string, FailureEntry>;
        };
        for (const entry of Object.values(parsed.files ?? {})) {
            if (entry && typeof entry.fileID === "number") {
                ledger.set(entry.fileID, entry);
            }
        }
    } catch {
        // No ledger yet, or an unreadable one: start clean.
    }
    return ledger;
};

const saveLedger = (path: string, ledger: Map<number, FailureEntry>): void => {
    if (ledger.size === 0) {
        rmSync(path, { force: true });
        return;
    }
    const files: Record<string, FailureEntry> = {};
    for (const [fileID, entry] of ledger) files[String(fileID)] = entry;
    writeFileSync(
        path,
        JSON.stringify({ version: LEDGER_VERSION, files }, null, 2),
    );
};

// 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.
interface ImageMetadata {
    imageMetadata?: Record<string, unknown>;
    imageMetadataError?: string;
}

// The image metadata for the file whose original is at `originalPath` (for a
// live photo, its image) and whose JSON is at `jsonPath`. A video gets none,
// as `photo.exif()` reads none. An original stored before this run is not read
// again when its JSON already holds image metadata: that is kept. A failed
// read gives the reason, and fails neither the file nor the run. An original
// with no EXIF, XMP or JPEG dimensions gets `{}`, so it is not read again.
const imageMetadataFor = async (
    file: EnteFile,
    originalPath: string,
    jsonPath: string,
    storedThisRun: boolean,
): Promise<ImageMetadata> => {
    if (file.metadata.fileType === "video") return {};
    if (!storedThisRun) {
        try {
            const { imageMetadata, imageMetadataError } = JSON.parse(
                readFileSync(jsonPath, "utf-8"),
            ) as ImageMetadata;
            if (imageMetadata !== undefined || imageMetadataError !== undefined)
                return { imageMetadata, imageMetadataError };
        } catch {
            // No JSON yet, or one that cannot be parsed: read the original.
        }
    }
    try {
        const bytes = await readFile(originalPath);
        return { imageMetadata: extractImageMetadata(bytes) ?? {} };
    } catch (err) {
        return { imageMetadataError: errorMessage(err) };
    }
};

// The file's JSON: its basic fields, its magic metadata, its ML data or the
// reason the ML data is missing, and its image metadata.
const writeSidecar = (
    path: string,
    file: EnteFile,
    ml: { mlData?: MLData; mlDataError?: string },
    image: ImageMetadata,
): void => {
    const meta: Record<string, unknown> = {
        id: file.id,
        collectionID: file.collectionID,
        ownerID: file.ownerID,
        metadata: file.metadata,
        updationTime: file.updationTime,
    };
    if (file.magicMetadata) meta.magicMetadata = file.magicMetadata;
    if (file.pubMagicMetadata) meta.pubMagicMetadata = file.pubMagicMetadata;
    if (ml.mlData) meta.mlData = ml.mlData;
    if (ml.mlDataError) meta.mlDataError = ml.mlDataError;
    if (image.imageMetadata) meta.imageMetadata = image.imageMetadata;
    if (image.imageMetadataError) {
        meta.imageMetadataError = image.imageMetadataError;
    }
    writeFileSync(path, JSON.stringify(meta, null, 2));
};

// The album's JSON: its basic fields, its magic metadata, and its files.
const writeAlbumJSON = (
    path: string,
    c: Collection,
    files: { id: number; metadata: FileMetadata }[],
): void => {
    const album: Record<string, unknown> = {
        id: c.id,
        name: c.name,
        type: c.type,
        ownerID: c.ownerID,
        isShared: c.isShared,
        updationTime: c.updationTime,
    };
    if (c.magicMetadata) album.magicMetadata = c.magicMetadata;
    if (c.pubMagicMetadata) album.pubMagicMetadata = c.pubMagicMetadata;
    if (c.sharedMagicMetadata) {
        album.sharedMagicMetadata = c.sharedMagicMetadata;
    }
    album.files = files;
    writeFileSync(path, JSON.stringify(album, null, 2));
};

// The backup itself, which `runBackup` below runs while it holds the lock.
const runLockedBackup = async (
    lib: BackupLibrary,
    opts: BackupOptions,
    downloadDirectory: string,
): Promise<BackupResult> => {
    const includeOriginals = opts.includeOriginals ?? true;
    const includeThumbnails = opts.includeThumbnails ?? false;
    const log = opts.onProgress ?? (() => {});
    const only = opts.onlyAlbumNames ? new Set(opts.onlyAlbumNames) : undefined;

    log("Refreshing library...");
    await lib.refresh();

    const collectionsDir = join(downloadDirectory, "collections");
    const thumbnailsDir = join(downloadDirectory, "thumbnails");
    mkdirSync(collectionsDir, { recursive: true });
    const { email, userID } = lib.whoami();
    writeFileSync(
        join(downloadDirectory, "account.json"),
        JSON.stringify({ email, userID }, null, 2),
    );
    if (includeThumbnails) mkdirSync(thumbnailsDir, { recursive: true });
    removeLeftoverTempFiles(thumbnailsDir);
    for (const dir of dateFolders(downloadDirectory)) {
        removeLeftoverTempFiles(dir);
    }

    const ledgerPath = join(downloadDirectory, "failures.json");
    const ledger = loadLedger(ledgerPath);
    const now = Date.now();

    // Collections in scope, and the distinct files across them (a file shared
    // by two albums is one original). Each file is the membership
    // `representative` picks from all of its albums, in scope or not, so it is
    // saved at the path `photo.savePath` names.
    const allCollections = lib.listCollections();
    const collections = allCollections.filter((c) =>
        only ? only.has(c.name) : true,
    );
    const collectionName = new Map<number, string>();
    for (const c of allCollections) collectionName.set(c.id, c.name);

    const memberships = new Map<number, EnteFile[]>();
    const filesByCollection = new Map<number, EnteFile[]>();
    for (const c of allCollections) {
        const files = lib.listFiles(c.id);
        filesByCollection.set(c.id, files);
        for (const f of files) {
            const arr = memberships.get(f.id);
            if (arr) arr.push(f);
            else memberships.set(f.id, [f]);
        }
    }
    const distinct = new Map<number, EnteFile>();
    for (const c of collections) {
        for (const f of filesByCollection.get(c.id)!) {
            if (!distinct.has(f.id)) {
                distinct.set(f.id, representative(memberships.get(f.id)!));
            }
        }
    }

    const errors: BackupError[] = [];
    const failedThisRun = new Set<number>();
    const storedThisRun = new Set<number>();
    let downloaded = 0;
    let skipped = 0;

    const recordFailure = (
        file: EnteFile,
        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;
        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,
            classification: classify(err),
            attempts: (prior?.attempts ?? 0) + 1,
            lastTriedAt: now,
            error,
        });
        failedThisRun.add(file.id);
    };

    // 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) {
                skipped++;
                continue;
            }
            try {
                log(`Fetching original ${file.metadata.title} (${fileID})...`);
                // 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),
                );
                storedThisRun.add(fileID);
                downloaded++;
            } catch (err) {
                log(
                    `FAILED original ${file.metadata.title}: ${errorMessage(err)}`,
                );
                recordFailure(
                    file,
                    collectionName.get(file.collectionID) ?? "",
                    err,
                );
            }
        }
    }

    if (includeThumbnails) {
        for (const [fileID, file] of distinct) {
            const dest = join(thumbnailsDir, `${fileID}.jpg`);
            if (isPresent(dest)) continue;
            try {
                const { path } = await lib.thumbnail(fileID);
                await copyAtomic(path, dest);
            } catch (err) {
                recordFailure(
                    file,
                    collectionName.get(file.collectionID) ?? "",
                    err,
                );
            }
        }
    }

    // Phase 2: rebuild the derived views from the model. Sidecars first, for
    // every present original (this repairs stale ones), each with the file's
    // ML data once an ML data fetch has completed, and its image metadata.
    // When the fetch fails, a file with no cached ML data gets the reason
    // instead and is recorded as failed. The next run fetches its ML data
    // again because none is cached.
    if (includeOriginals) {
        let mlDataError: string | undefined;
        try {
            log("Fetching ML data...");
            await lib.fetchMLData();
        } catch (err) {
            mlDataError = errorMessage(err);
            log(`FAILED ML data: ${mlDataError}`);
        }
        for (const file of distinct.values()) {
            const stored = storedAtSavePath(downloadDirectory, file);
            if (stored === undefined) continue;
            const path = withExtension(
                savePath(downloadDirectory, file),
                ".json",
            );
            const image = await imageMetadataFor(
                file,
                stored.path,
                path,
                storedThisRun.has(file.id),
            );
            const mlData = await lib.mlData(file.id);
            if (mlData === undefined && mlDataError !== undefined) {
                writeSidecar(path, file, { mlDataError }, image);
                recordFailure(
                    file,
                    collectionName.get(file.collectionID) ?? "",
                    new Error(`ML data: ${mlDataError}`),
                );
            } else {
                writeSidecar(path, file, { mlData }, image);
            }
        }
    }

    // Then the per-collection symlink trees and JSON. Directory names are
    // chosen across every album, not just those in scope, so a scoped run
    // names an album the same as a full one and never takes the directory of
    // an album it skipped. Stale entries are removed before anything is
    // rebuilt, so on a case-insensitive file system removing an old name can
    // never remove the new one.
    const dirNames = uniqueNames(
        allCollections.map((c) => ({
            id: c.id,
            name: sanitizeFileName(c.name, `collection-${c.id}`),
        })),
        false,
    );
    const albumDirNames = new Map(
        allCollections.map((c, i) => [c.id, dirNames[i]!]),
    );
    try {
        removeStaleAlbumDirs(
            collectionsDir,
            new Set(dirNames),
            downloadDirectory,
        );
    } catch (err) {
        log(`FAILED removing old album directories: ${errorMessage(err)}`);
    }

    for (const c of collections) {
        const colDirName = albumDirNames.get(c.id)!;
        const colDir = join(collectionsDir, colDirName);
        mkdirSync(colDir, { recursive: true });

        // Every album links the one original, saved from the file's entry in
        // `distinct`.
        const files = filesByCollection.get(c.id) ?? [];
        const links = files.flatMap((f) =>
            linksFor(
                f,
                storedAtSavePath(downloadDirectory, distinct.get(f.id)!),
            ),
        );
        const linkNames = uniqueNames(links, true);
        try {
            removeStaleLinks(colDir, new Set(linkNames), downloadDirectory);
        } catch (err) {
            log(`FAILED removing old links in ${c.name}: ${errorMessage(err)}`);
        }

        const metaFiles = files.map((f) => ({
            id: f.id,
            metadata: f.metadata,
        }));
        for (const [i, link] of links.entries()) {
            if (!includeOriginals || link.target === undefined) continue;
            const linkName = linkNames[i]!;
            try {
                rebuildSymlink(
                    join(colDir, linkName),
                    relative(colDir, link.target),
                );
            } catch (err) {
                log(
                    `FAILED symlink ${c.name}/${linkName}: ${errorMessage(err)}`,
                );
                recordFailure(link.file, c.name, err);
            }
        }

        writeAlbumJSON(
            join(collectionsDir, `${colDirName}.json`),
            c,
            metaFiles,
        );
    }

    // Reconcile the ledger against what this run actually attempted: an entry
    // survives only for a file that failed this run. A file that succeeded had
    // 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.
    for (const fileID of [...ledger.keys()]) {
        if (!failedThisRun.has(fileID)) ledger.delete(fileID);
    }
    saveLedger(ledgerPath, ledger);

    return {
        totalFiles: distinct.size,
        downloaded,
        skipped,
        failed: ledger.size,
        errors,
    };
};

// Only one backup of a directory runs at a time, in this process or another:
// a second one fails at once with an error whose `code` is `ELOCKED`. The lock
// is the directory `backup.lock`, whose modification time proper-lockfile
// keeps current while the backup runs. One it has not touched for 10 seconds
// was left by a run that was killed, and is taken over.
export const runBackup = async (
    lib: BackupLibrary,
    opts: BackupOptions,
): Promise<BackupResult> => {
    const downloadDirectory = opts.downloadDirectory;
    if (!downloadDirectory) {
        throw new Error(
            "backup requires a downloadDirectory (pass one to backup() or " +
                "open the library with one)",
        );
    }
    mkdirSync(downloadDirectory, { recursive: true });
    let release: () => Promise<void>;
    try {
        release = await lockfile.lock(downloadDirectory, {
            lockfilePath: join(downloadDirectory, "backup.lock"),
        });
    } catch (err) {
        if ((err as NodeJS.ErrnoException).code !== "ELOCKED") throw err;
        throw Object.assign(
            new Error(`another backup of ${downloadDirectory} is running`),
            { code: "ELOCKED" },
        );
    }
    try {
        return await runLockedBackup(lib, opts, downloadDirectory);
    } finally {
        await release();
    }
};