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 prints it as one line and exits 2. A lock untouched for 10
seconds, left by a killed run, 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

test/cli/commands.test.ts

@@ -18,6 +18,7 @@ import {
     readFileSync,
     rmSync,
     statSync,
+    utimesSync,
     writeFileSync,
 } from "node:fs";
 import { join } from "node:path";
@@ -792,7 +793,7 @@ describe("backup", () => {
         expect(code).toBe(1);
         expect(runText).toBe("quak: HTTP 503 from server\n");
         expect(stderr.text).toBe("Starting backup...\nRefreshing library...\n");
-        expect(existsSync(dir)).toBe(false);
+        expect(readdirSync(dir)).toEqual([]);
     });
 
     // A real saved session, read back by `loadSession`, whose server answers
@@ -823,7 +824,23 @@ describe("backup", () => {
             `quak: the saved session is no longer valid; run "quak login"\n`,
         );
         expect(stderr.text).toBe("Starting backup...\nRefreshing library...\n");
-        expect(existsSync(dir)).toBe(false);
+        expect(readdirSync(dir)).toEqual([]);
+    });
+
+    it("exits 2 with one line naming the directory while another backup of it runs", async () => {
+        const dir = join(root, "backup");
+        // The lock another backup holds. Its modification time is set an hour
+        // ahead, so it stays current however long this test takes.
+        const lock = join(dir, "backup.lock");
+        mkdirSync(lock, { recursive: true });
+        const hourAhead = new Date(Date.now() + 3_600_000);
+        utimesSync(lock, hourAhead, hourAhead);
+
+        expect(await backupCommand(context(), dir, {})).toBe(2);
+        expect(stderr.text).toBe(
+            `Starting backup...\nquak: another backup of ${dir} is running\n`,
+        );
+        expect(readdirSync(dir)).toEqual(["backup.lock"]);
     });
 });