During gateway shutdown, a message arriving while
cancel_background_tasks is mid-await (inside asyncio.gather) spawns
a fresh _process_message_background task via handle_message and adds
it to self._background_tasks. The original implementation's
_background_tasks.clear() at the end of cancel_background_tasks
dropped the reference; the task ran untracked against a disconnecting
adapter, logged send-failures, and lingered until it completed on
its own.
Fix: wrap the cancel+gather in a bounded loop (MAX_DRAIN_ROUNDS=5).
If new tasks appeared during the gather, cancel them in the next
round. The .clear() at the end is preserved as a safety net for
any task that appeared after MAX_DRAIN_ROUNDS — but in practice the
drain stabilizes in 1-2 rounds.
Tests: tests/gateway/test_cancel_background_drain.py — 3 cases.
- test_cancel_background_tasks_drains_late_arrivals: spawn M1, start
cancel, inject M2 during M1's shielded cleanup, verify M2 is
cancelled.
- test_cancel_background_tasks_handles_no_tasks: no-op path still
terminates cleanly.
- test_cancel_background_tasks_bounded_rounds: baseline — single
task cancels in one round, loop terminates.
Regression-guard validated: against the unpatched implementation,
the late-arrival test fails with exactly the expected message
('task leaked'). With the fix it passes.
Blast radius is shutdown-only; the audit classified this as MED.
Shipping because the fix is small and the hygiene is worth it.
While investigating the audit's other MEDs (busy-handler double-ack,
Discord ExecApprovalView double-resolve, UpdatePromptView
double-resolve), I verified all three were false positives — the
check-and-set patterns have no await between them, so they're
atomic on single-threaded asyncio. No fix needed for those.
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