`ThreadParticipationTracker._save` (gateway/platforms/helpers.py) ends in
`atomic_json_write` -> `os.replace`, whose duration is unbounded under
filesystem pressure. All five call sites are coroutines on the inbound-message
or slash-command path:
plugins/platforms/matrix/adapter.py _resolve_message_context
create_handoff_thread
plugins/platforms/discord/adapter.py _handle_message (x2)
_handle_thread_create_slash
So that rename was paid inline on the running loop, stalling every other
adapter's polling, every in-flight turn and every heartbeat for as long as it
took.
The fix is at the CHOKE POINT rather than at five call sites:
- `mark_async` does the in-memory insert synchronously and offloads only the
persist via `asyncio.to_thread`. The insert must stay synchronous because
both adapters gate on `thread_id in self._threads` immediately after
marking; deferring it would make mention-gating depend on executor
availability.
- all five coroutine call sites now await it.
- `mark` keeps its exact synchronous contract for the non-loop callers.
- an RLock is added in the SAME commit that introduces the concurrency: the
event loop used to serialize every caller by accident, and
`atomic_json_write` makes each write atomic without making
check/insert/trim/write atomic. Without it two concurrent marks lose one.
Enforcement is an AST class sweep, not an inventory: no `async def` under
gateway/ or plugins/ may call `<x>._threads.mark(...)`, and every
`mark_async(...)` must be awaited -- an un-awaited one never runs at all, so
the thread is neither persisted nor recorded in memory and mention-gating
re-prompts forever in a thread the bot already joined. A new adapter fails the
gate without anyone remembering a list.
tests/gateway/test_discord_thread_slash_expired_defer.py stubbed the tracker
with `SimpleNamespace(mark=...)`; it now uses the real tracker against a
tmp_path, so the test cannot rot silently the next time this surface moves, and
it additionally asserts the thread really was recorded.
Verified on this exact head, PYTHONPATH pinned to the worktree:
tests/gateway/test_thread_tracker_mark_off_loop.py 7 passed
expired-defer + admission-exemption + off-loop 10 passed
-k "discord or matrix or thread" over tests/gateway 1136 passed, 13 failed
The 13 failures are INHERITED: a clean worktree at upstream/main 75e9567ca7
with none of these changes fails the identical 13.
Every guard is gate-proven -- reverting the RLock, the to_thread, one call
site, one `await`, the sync insert, or the dedupe short-circuit each fails its
own test and only its own.
(cherry picked from commit 6fc8037d292ec051fe76ad213903b2ee5534080a)
ee8a3ded96