quak backup --verify re-hashes stored originals and downloads again any that do not match (closes #168)

`--verify`, or `lib.backup({ verify: true })`, hashes each original already
at its save path as the download check does, streamed, a live photo as
`<imageHash>:<videoHash>`. A mismatch is logged, removed and fetched again in
the same run; a failed fetch goes into `failures.json`. A file with no
recorded hash counts as unchecked. The result, the summary and `--json` gain
`verified`, `mismatched` and `unchecked`.

Judgement call: a stored original that cannot be read for hashing is recorded as failed and left in place.
Judgement call: the summary prints the three counts only with `--verify`.
Judgement call: the README's `BackupOptions` list does not name `verify`, to stay clear of #178's edit of that paragraph; Backup layout documents it.

Model: opus-5-5

src/backup.ts

@@ -35,6 +35,10 @@
 // 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.
+//
 // Resilience (issue #8): no per-file condition aborts the run. A failed
 // download, a failed symlink, or ML data missing because the ML data fetch
 // failed is caught, recorded in `failures.json` with a classification, a
@@ -47,6 +51,7 @@
 // code.
 
 import {
+    createReadStream,
     lstatSync,
     mkdirSync,
     readdirSync,
@@ -61,6 +66,12 @@ import {
 import { readFile } from "node:fs/promises";
 import { dirname, extname, join, relative, resolve } from "node:path";
 
+import {
+    chunkHashFinal,
+    chunkHashInit,
+    chunkHashUpdate,
+    init,
+} from "./crypto/index.js";
 import { removeLeftoverTempFiles } from "./download/index.js";
 import { sanitizeFileName, withExtension } from "./filename.js";
 import {
@@ -88,6 +99,9 @@ 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;
 }
 
@@ -105,6 +119,13 @@ export interface BackupResult {
     downloaded: number;
     // Originals already at their save path and left untouched.
     skipped: number;
+    // With `verify`, the originals already at their save path whose hash
+    // matched, those whose hash did not (each removed and fetched again), and
+    // those whose metadata records no hash (left as they are). All three are
+    // zero without `verify`.
+    verified: number;
+    mismatched: number;
+    unchecked: number;
     // Files with an unresolved failure after this run (the ledger size); the
     // CLI exits non-zero while this is above zero. A file can be both
     // downloaded and failed if its bytes landed but its symlink did not.
@@ -361,6 +382,26 @@ 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.
@@ -465,6 +506,7 @@ export const runBackup = async (
     const includeThumbnails = opts.includeThumbnails ?? false;
     const log = opts.onProgress ?? (() => {});
     const only = opts.onlyAlbumNames ? new Set(opts.onlyAlbumNames) : undefined;
+    const verify = opts.verify ?? false;
 
     log("Refreshing library...");
     await lib.refresh();
@@ -523,6 +565,9 @@ export const runBackup = async (
     const storedThisRun = new Set<number>();
     let downloaded = 0;
     let skipped = 0;
+    let verified = 0;
+    let mismatched = 0;
+    let unchecked = 0;
 
     const recordFailure = (
         file: EnteFile,
@@ -554,10 +599,45 @@ export const runBackup = async (
 
     // Phase 1: get the bytes. Put each pending original at its save path
     // through the content cache/pools, as `Photo.download()` does, and fetch
-    // the optional thumbnails; a present file is left as is.
+    // the optional thumbnails; a present file is left as is. With `verify`, a
+    // present original is hashed first, and one that does not match the hash
+    // its metadata records is removed and fetched like a missing one. One
+    // that cannot be read is recorded as failed and left where it is.
     if (includeOriginals) {
         for (const [fileID, file] of distinct) {
-            if (storedAtSavePath(downloadDirectory, file) !== undefined) {
+            let stored = storedAtSavePath(downloadDirectory, file);
+            if (stored !== undefined && verify) {
+                try {
+                    if (file.metadata.hash === undefined) {
+                        unchecked++;
+                    } else if (
+                        (await storedHash(stored)) === file.metadata.hash
+                    ) {
+                        verified++;
+                    } else {
+                        log(
+                            `MISMATCH original ${file.metadata.title} (${fileID}): its bytes do not match its content hash`,
+                        );
+                        mismatched++;
+                        rmSync(stored.path);
+                        if (stored.videoPath !== undefined) {
+                            rmSync(stored.videoPath);
+                        }
+                        stored = undefined;
+                    }
+                } catch (err) {
+                    log(
+                        `FAILED verifying original ${file.metadata.title}: ${errorMessage(err)}`,
+                    );
+                    recordFailure(
+                        file,
+                        collectionName.get(file.collectionID) ?? "",
+                        err,
+                    );
+                    continue;
+                }
+            }
+            if (stored !== undefined) {
                 skipped++;
                 continue;
             }
@@ -732,6 +812,9 @@ export const runBackup = async (
         totalFiles: distinct.size,
         downloaded,
         skipped,
+        verified,
+        mismatched,
+        unchecked,
         failed: ledger.size,
         errors,
     };