quak backup refuses to run while another backup of the same directory runs, exit 2 (closes #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. A
second backup of the directory fails at once with an error naming it.
quak backup takes the lock itself before it opens its library and passes
lockHeld to the backup, so a refused run sends no request; it prints the
error as one line and exits 2. A lock untouched for 10 seconds, left by a
run that could not remove it, is taken over.

Deviation: yarn.lock was regenerated by yarn add in the pinned node image.
Judgement call: a run failing at its refresh leaves the directory, empty.
Judgement call: a lock removed mid-run stops that run with an uncaught
error, the library's default.

Model: opus-5-5

src/cli-commands.ts

@@ -20,6 +20,7 @@ import {
     type ClientSnapshot,
     type LoginOptions,
 } from "./client.js";
+import { lockBackupDirectory } from "./backup.js";
 import { init } from "./crypto/index.js";
 import {
     defaultCacheDirectory,
@@ -408,44 +409,59 @@ export const backupCommand = async (
     if (!client) return 3;
 
     ctx.stderr.write("Starting backup...\n");
-    // The precache is off: the backup fetches what it needs, and must not
-    // also fill the cache with every thumbnail and the recent originals.
-    const lib = await Library.open({
-        client,
-        downloadDirectory: dir,
-        cacheDirectory: ctx.cacheDir,
-        precacheThumbnails: false,
-        precacheOriginals: false,
-    });
+    // 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 release: () => Promise<void>;
     try {
-        const result = await lib.backup({
+        release = 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.
+        const lib = await Library.open({
+            client,
             downloadDirectory: dir,
-            onProgress: (msg) => {
-                if (!opts.json) ctx.stderr.write(msg + "\n");
-            },
+            cacheDirectory: ctx.cacheDir,
+            precacheThumbnails: false,
+            precacheOriginals: false,
         });
+        try {
+            const result = await lib.backup({
+                downloadDirectory: dir,
+                lockHeld: true,
+                onProgress: (msg) => {
+                    if (!opts.json) ctx.stderr.write(msg + "\n");
+                },
+            });
 
-        if (opts.json) {
-            ctx.stdout.write(JSON.stringify(result, null, 2) + "\n");
-        } else {
-            ctx.stderr.write("\n--- Backup complete ---\n");
-            ctx.stderr.write(`  Total files: ${result.totalFiles}\n`);
-            ctx.stderr.write(`  Downloaded:  ${result.downloaded}\n`);
-            ctx.stderr.write(`  Skipped:     ${result.skipped}\n`);
-            ctx.stderr.write(`  Failed:      ${result.failed}\n`);
-            if (result.errors.length > 0) {
-                ctx.stderr.write("\nFailed files:\n");
-                for (const e of result.errors) {
-                    ctx.stderr.write(
-                        `  [${e.collection}] ${e.title} (id ${e.fileID}): ${e.error}\n`,
-                    );
+            if (opts.json) {
+                ctx.stdout.write(JSON.stringify(result, null, 2) + "\n");
+            } else {
+                ctx.stderr.write("\n--- Backup complete ---\n");
+                ctx.stderr.write(`  Total files: ${result.totalFiles}\n`);
+                ctx.stderr.write(`  Downloaded:  ${result.downloaded}\n`);
+                ctx.stderr.write(`  Skipped:     ${result.skipped}\n`);
+                ctx.stderr.write(`  Failed:      ${result.failed}\n`);
+                if (result.errors.length > 0) {
+                    ctx.stderr.write("\nFailed files:\n");
+                    for (const e of result.errors) {
+                        ctx.stderr.write(
+                            `  [${e.collection}] ${e.title} (id ${e.fileID}): ${e.error}\n`,
+                        );
+                    }
                 }
             }
-        }
 
-        return result.failed > 0 ? 1 : 0;
+            return result.failed > 0 ? 1 : 0;
+        } finally {
+            await lib.close();
+        }
     } finally {
-        await lib.close();
+        await release();
     }
 };