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cancel-async-tasks — one subtest from PASS; verified the right scenario with the wrong fixture

Status: FAIL 3/3 trials across both 27b fast runs (fast__qwen3.6-27b__20260706-124744 dk74haS @129s; fast__qwen3.6-27b__20260706-132725 ytDudum @83s + tF7m98S @82s). No fast-cap involvement (cap 180s, no fast-timeout.json). All three verdicts are identical and deterministic: 5/6 subtests pass, only test_tasks_cancel_above_max_concurrent fails — stdout.count("Cleaned up.") == 0 where 2 expected (Task started. count is correct at 2). Contrast the 35b MoE trial in fast__…113104: it deadlocked its own smoke test and was cut at the cap with 1/6; the dense 27b writes plausible code, self-verifies, and finishes clean in ~80s — a genuine near-miss.

What the verifier probes

test.py's task cleanup is itself async:

finally:
    await asyncio.sleep(1)
    print("Cleaned up.")

The failing subtest runs 3 tasks with max_concurrent=2, SIGINTs at 500ms, and expects the 2 started tasks to still print Cleaned up. The docstring calls out the gotcha: "asyncio.gather doesn't properly cancel existing tasks if there are still tasks in the queue."

The bug — a second CancelledError lands inside the async cleanup

All three trials wrote the same shape: semaphore + create_task per task, then

try:
    await asyncio.gather(*handles)
except (KeyboardInterrupt, asyncio.CancelledError):
    for h in handles:
        h.cancel()                                   # ← the fatal line
    await asyncio.gather(*handles, return_exceptions=True)
    raise

Mechanism on the container's Python 3.13 (asyncio.Runner turns the first SIGINT into a cancel of the main task, not a raw KeyboardInterrupt):

  1. Main task cancelled → its await gather cancels the GatheringFuture, which already delivers cancel #1 to every child.
  2. Task 3 is parked at semaphore.acquire() with nothing to clean up, so it finishes cancelled instantly — and a non-return_exceptions gather completes on the first failed child, waking the main task while tasks 1–2 are still inside finally: await asyncio.sleep(1).
  3. The handler's manual h.cancel() is cancel #2, delivered into that cleanup sleep(1) → the finally block aborts before printing. 0 cleanups.

This is exactly why the below/at-max subtests pass with the same code: with no queued task, every child has the slow async cleanup, so the gather can't complete until a child has finished cleaning up — the re-cancel loop then finds t.done() and is harmless. The queued third task is what wakes the handler early.

dk74haS's variant caught only KeyboardInterrupt, so its handler never fires at all (the Runner delivers CancelledError) — same 0-cleanup outcome via a different path: the main task dies immediately and asyncio.run teardown (_cancel_all_tasks) delivers cancel #2 into the in-flight cleanups. Same root-cause family: control escapes while cleanup is still in flight, and whoever runs next re-cancels.

Reproduced host-side (agent's exact run.py + task test.py, SIGINT at 500ms): 2/3 → cleaned=2, 2/2 → cleaned=2, 3/2 → cleaned=0 — byte-for-byte the benchmark failure. Deleting the re-cancel loop alone flips 3/2 to cleaned=2 (the gather cancellation already propagated; you only need to await the children once). The reference asyncio.TaskGroup solution passes for the same reason: cancel once, await, never re-cancel.

The verification story — RULE 2 fired, fixture fidelity missed

This is the interesting part for the harness. The model did verify (4 bash commands in tF7m98S, 7 in dk74haS): a signature check, a concurrency smoke test, and — remarkably — the exact failing scenario: 3 tasks, max_concurrent=2, cancelled mid-flight, asserting cleanup ran. It observed ['A start', 'B start', 'A cleanup', 'B cleanup'] → "All cleanup code ran!" and declared done.

Its fixture's finally was synchronous (results.append(...)) — and a sync finally executes atomically, so cancel #2 has no await point to land on. The bug is only observable when the cleanup itself awaits, which is precisely what the hidden test.py does (await asyncio.sleep(1) before the print). Right scenario, wrong fixture: self-verification passed while the verifier failed.

Verdict attribution

Model capability near-miss, harness clean. No length-stops, no write-guard or recovery triggers, no timeout shaping — the trial ends with the model's own (honestly obtained, wrongly reassuring) green self-test. This lands in the known "hidden-criterion near-miss" false-success class, with a sharper edge: RULE 2's "verify with real commands" can't help when the model's test fixture lacks the one property the hidden test exercises. No mechanical fix proposed — detecting "your fixture is weaker than the hidden criterion" is task knowledge, not harness ground truth.

Notes