row 251 step 1: T27's lists print the timing and the agent state when they fail (#453)

A failed list now names when addKey started and resolved and when each list ran, on hrtime and
/proc/uptime, with how far into its second each fell, and whether the agent and its socket were
still there. The test's checks are unchanged.

packages/bus/tests/host-run.test.mjs

@@ -458,17 +458,32 @@ test('T27: ssh-add -t drops the key from an agent the test keeps alive', async (
   const agent = spawn('ssh-agent', ['-D', '-a', sock], { stdio: 'ignore' });
   t.after(() => agent.kill('SIGKILL'));
   assert.ok(await until(() => existsSync(sock)));
+  // A failed list names its cause: when each step ran on the monotonic clock (hrtime) and on
+  // /proc/uptime (CLOCK_BOOTTIME, which ssh-agent's lifetime counts in whole seconds), and
+  // whether the agent and its socket were still there.
+  const marks = [];
+  const mark = (name) => marks.push({ name, ns: process.hrtime.bigint(), uptime: readFileSync('/proc/uptime', 'utf8').split(' ')[0] });
+  const timing = () => {
+    const at = (m) => `${m.name} +${(Number(m.ns - marks[0].ns) / 1e6).toFixed(1)} ms (hrtime ${(Number(m.ns % 1_000_000_000n) / 1e9).toFixed(4)} s into its second, uptime ${m.uptime})`;
+    const alive = agent.exitCode === null && agent.signalCode === null;
+    return `${marks.map(at).join('; ')}; agent ${alive ? 'alive' : `gone (${agent.exitCode ?? agent.signalCode})`}, socket ${existsSync(sock) ? 'present' : 'missing'}`;
+  };
+  mark('addKey start');
   await addKey(sock, readFileSync(join(dir, 'k'), 'utf8'), 1);
-  const list = () => {
+  mark('addKey resolved');
+  const list = (name) => {
+    mark(name);
     try {
       return execFileSync('ssh-add', ['-l'], { env: { PATH: '/usr/bin:/bin', SSH_AUTH_SOCK: sock }, encoding: 'utf8' });
     } catch (e) {
       return e.stdout;
     }
   };
-  assert.match(list(), /SHA256:/);
+  const first = list('first list');
+  assert.match(first, /SHA256:/, `first list got ${JSON.stringify(first)}: ${timing()}`);
   await sleep(2500);
-  assert.match(list(), /no identities/);
+  const second = list('second list');
+  assert.match(second, /no identities/, `second list got ${JSON.stringify(second)}: ${timing()}`);
 });
 
 // The test's own limit makes a hung addKey (M37a, M37b) fail in seconds, not at the batch timeout.