row 251: T27 adds its key with -t 2 and sleeps 3.5 s, because ssh-agent counts lifetimes in whole seconds (#453)

ssh-agent sets a key's death to whole seconds of CLOCK_BOOTTIME plus the lifetime and reaps on the
same whole seconds (OpenSSH 10.5p1 ssh-agent.c:1315, :1064, misc.c:1828), so -t 1 can end within
milliseconds of an add made late in a second. -t 2 keeps the key over 1 s; the sleep keeps its 1.5 s
margin past the 2 s ceiling. Both checks stay. host-run.mjs is unchanged: production adds with
timeout + 30. Evidence and the slot plan are in agents/darkwing/work/queue-251/BUILD.md.

agents/darkwing/work/queue-251/BUILD.md

@@ -0,0 +1,77 @@
+# Row 251 build: T27 lists no identities right after `ssh-add -t 1` (Vikunja #453)
+
+Brief `docs/plans/2026-10-21_t27-ssh-add-lifetime.md` (rev 2931), Mos ruling mos-1185 item 2, assigned in
+sage-2075. Owner Darkwing, reviewer Dewey, gate owner Sage. Branch `darkwing/row251` from 3b0aef995,
+claimed in-progress at rev 2938.
+
+## Step 1: the failure names its cause (e5e4b2e5a)
+
+T27's two lists now carry a message that prints only when they fail. It gives the time from addKey's
+start to its resolve and to each `ssh-add -l` call, the hrtime at each with how far into its second it
+fell, `/proc/uptime` at each, and whether the agent process and its socket were still there. The checks
+themselves did not change. The commit holds nothing else.
+
+## Step 2: the cause
+
+ssh-agent counts a key's lifetime in whole seconds of CLOCK_BOOTTIME, at the add and at the reap. I read
+OpenSSH 10.5p1, the version on dragon-lin (`ssh -V`: OpenSSH_10.5p1), at tag V_10_5_P1 (ssh-agent.c
+sha256 873691c4, misc.c sha256 ee467f43):
+
+- `misc.c:1790-1798` and `:1828`: `monotime()` returns `ts.tv_sec` of `clock_gettime(CLOCK_BOOTTIME)`,
+  falling back to MONOTONIC. The fraction of the second is dropped.
+- `ssh-agent.c:1315`: the `-t` constraint sets `*deathp = monotime() + seconds`. Lines 1410 and 1598
+  do the same for the agent's default lifetime.
+- `ssh-agent.c:1064`: the reaper drops a key when `now >= id->death`, `now` being `monotime()` again.
+- `ssh-agent.c:2650`: the main loop runs `reaper()` after every poll, before it reads requests. A list
+  that arrives after the boundary sees the key gone.
+
+So a key added at second s plus a fraction f dies at s + N, and `-t N` keeps it N - f seconds, between
+N - 1 and N. With `-t 1`, an add late in a second leaves the key milliseconds to live. That matches
+Sage's probe in the brief: 8 of 80 `-t 1` adds missed, each one made at 0.9947-0.9973 of a monotonic
+second and finished at 0.998-0.0008, and none earlier in the second missed. It also matches proof 5's
+failure, where the first list ran 31.9 ms into the test and found no identities.
+
+Node's hrtime is CLOCK_MONOTONIC. BOOTTIME differs from it by the time the host spent suspended. On
+dragon-lin the difference read 0.000001 s at 19:09Z, so a monotonic second boundary is a BOOTTIME one
+here.
+
+`addKey` and the agent start are not the cause. `addKey` passes the lifetime through as `-t`
+(`host-run.mjs:90`), and the agent starts with no default lifetime (`:175`). Production adds the key
+with `timeout + 30` (`:180`), where losing up to a second makes no difference. The only caller that
+can't afford the lost fraction is T27, which asks for 1 s. The fix belongs in the test's lifetime choice,
+and `host-run.mjs` doesn't change.
+
+The slot confirms this against T27 itself (below), not just from the source.
+
+## Step 3: the fix (this commit)
+
+T27 adds the key with `-t 2`. The agent then keeps it more than 1 s and at most 2 s. The first list
+runs milliseconds after the add, so it has over a second of margin even under load. The sleep before
+the second list grows from 2.5 s to 3.5 s, which keeps the old 1.5 s margin past the new 2 s ceiling.
+Both checks stay: listed after the add, gone after the lifetime. A comment above the add says why the
+lifetime is 2.
+
+I considered `-t 3` with a 4.5 s sleep for a 2 s floor. One second of floor is far above anything the
+first list has needed (31.9 ms in proof 5), and the concurrent-suite run below tests it under load, so I
+took the smaller change.
+
+## Slot plan
+
+All scripts are in `/mnt/storage/scratch/darkwing-251-out/`, launched from `/`, each behind the busy
+checks and chained with `&&`, TMPDIR under `/mnt/storage/scratch/tmp`.
+
+1. Confirmation. `align-hooks.mjs` is a load hook, outside the repo, that serves `host-run.mjs` with
+   `addKey` wrapped to start `ssh-add` at a chosen fraction of a monotonic second. T27 runs unchanged
+   under it, alone (`--test-name-pattern`). It runs at fractions 0.990, 0.993, 0.995, 0.996, 0.997 and
+   0.998, three times each:
+   - on e5e4b2e5a (step 1, `-t 1`). Expected: some first lists fail, and the step 1 message shows the
+     add's resolve and the list on either side of a second boundary, with the agent alive and the
+     socket present. These reds are the point of the run.
+   - on the fix commit (`-t 2`). Expected: all 18 pass.
+2. `host-run.test.mjs` whole, once, on the fix commit.
+3. The bus suite whole, three times in a row.
+4. The bus suite whole in two clean worktrees at the same commit, started together.
+
+## Slot results
+
+Not run yet.

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

@@ -468,8 +468,12 @@ test('T27: ssh-add -t drops the key from an agent the test keeps alive', async (
     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'}`;
   };
+  // ssh-agent sets a key's death to the whole second of CLOCK_BOOTTIME at the add plus the
+  // lifetime, and drops it once that whole second is reached. So `-t N` keeps a key between N - 1
+  // and N seconds: `-t 1` added just before a second boundary was gone within milliseconds (#453).
+  // `-t 2` keeps it at least 1 s, and the sleep waits past the 2 s ceiling.
   mark('addKey start');
-  await addKey(sock, readFileSync(join(dir, 'k'), 'utf8'), 1);
+  await addKey(sock, readFileSync(join(dir, 'k'), 'utf8'), 2);
   mark('addKey resolved');
   const list = (name) => {
     mark(name);
@@ -481,7 +485,7 @@ test('T27: ssh-add -t drops the key from an agent the test keeps alive', async (
   };
   const first = list('first list');
   assert.match(first, /SHA256:/, `first list got ${JSON.stringify(first)}: ${timing()}`);
-  await sleep(2500);
+  await sleep(3500);
   const second = list('second list');
   assert.match(second, /no identities/, `second list got ${JSON.stringify(second)}: ${timing()}`);
 });