Harden the backup tree's atomic copy (closes #22)

The backup copy now fsyncs its temp file before the rename and the
directory after it, using the download writer's new fsyncPath helper.
Each backup run deletes .quak-backup-*.tmp files whose process is no
longer running, leaving those of a concurrent backup alone. The rename
sites and the README backup layout state that a symlink at the
destination is replaced and the new file takes the temp file's
permissions, and the README names the temp files. Adds tests for a
missing and an unwritable destination directory for downloadFile and
downloadThumbnail.

Model: opus-5-5

test/cli/backup.test.ts

@@ -42,15 +42,44 @@ import {
     rmSync,
     writeFileSync,
 } from "node:fs";
+import { spawnSync } from "node:child_process";
 import { join } from "node:path";
 import { tmpdir } from "node:os";
-import { describe, it, expect, beforeEach, afterEach } from "vitest";
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
 
 import { Library } from "../../src/library/index.js";
 import type { ContentSource } from "../../src/library/content.js";
 import type { CollectionsPage, FilesPage } from "../../src/client.js";
 import type { Collection, EnteFile } from "../../src/model/types.js";
 
+// `open` and `rename` are wrapped to record, in order, every fsync and rename,
+// so a test can pin the sequence "fsync the temp file, rename, fsync the
+// directory" that makes a copied original survive a power cut. `vi.hoisted`
+// because `vi.mock` factories run before module-level constants exist.
+const fsEvents = vi.hoisted(() => [] as string[]);
+
+vi.mock("node:fs/promises", async (importOriginal) => {
+    const actual = await importOriginal<typeof import("node:fs/promises")>();
+    return {
+        ...actual,
+        open: async (
+            ...args: Parameters<typeof actual.open>
+        ): Promise<Awaited<ReturnType<typeof actual.open>>> => {
+            const handle = await actual.open(...args);
+            const realSync = handle.sync.bind(handle);
+            handle.sync = async (): Promise<void> => {
+                fsEvents.push(`sync:${String(args[0])}`);
+                await realSync();
+            };
+            return handle;
+        },
+        rename: async (from: string, to: string): Promise<void> => {
+            fsEvents.push(`rename:${to}`);
+            await actual.rename(from, to);
+        },
+    };
+});
+
 const USER_ID = 42;
 
 // Decrypted-byte length each stub original writes, keyed by fileID.
@@ -530,4 +559,44 @@ describe("lib.backup", () => {
         expect(readLedger(outDir).files["101"]!.attempts).toBe(1);
         lib.close();
     });
+
+    it("fsyncs a copied original before the rename and its directory after", async () => {
+        const lib = await openLibrary(stubSource());
+        const outDir = join(root, "backup");
+        const originals = join(outDir, "originals");
+        const dest = join(originals, "100.jpg");
+        fsEvents.length = 0;
+
+        await lib.backup({ downloadDirectory: outDir });
+
+        const at = fsEvents.indexOf(`rename:${dest}`);
+        expect(at).toBeGreaterThan(0);
+        expect(fsEvents[at - 1]).toMatch(
+            /^sync:.*\/\.quak-backup-100\.jpg-\d+-[0-9a-z]*\.tmp$/,
+        );
+        expect(fsEvents[at + 1]).toBe(`sync:${originals}`);
+        lib.close();
+    });
+
+    it("removes temp files left by a killed backup but not those of one still running", async () => {
+        const outDir = join(root, "backup");
+        const originals = join(outDir, "originals");
+        mkdirSync(originals, { recursive: true });
+        // A child that has already exited: its process ID is not running.
+        const exitedPID = spawnSync(process.execPath, ["-e", ""]).pid;
+        const leftover = `.quak-backup-100.jpg-${exitedPID}-abc123.tmp`;
+        // This test's own process stands in for a backup running at the same
+        // time.
+        const inProgress = `.quak-backup-101.jpg-${process.pid}-def456.tmp`;
+        writeFileSync(join(originals, leftover), "partial");
+        writeFileSync(join(originals, inProgress), "partial");
+        const lib = await openLibrary(stubSource());
+
+        await lib.backup({ downloadDirectory: outDir });
+
+        const names = readdirSync(originals);
+        expect(names).not.toContain(leftover);
+        expect(names).toContain(inProgress);
+        lib.close();
+    });
 });