Inbound Feishu messages arriving during brief windows when the adapter
loop is unavailable (startup/restart transitions, network-flap reconnect)
were silently dropped with a WARNING log. This matches the symptom in
issue #5499 — and users have reported seeing only a subset of their
messages reach the agent.
Fix: queue pending events in a thread-safe list and spawn a single
drainer thread that replays them once the loop becomes ready. Covers
these scenarios:
* Queue events instead of dropping when loop is None/closed
* Single drainer handles the full queue (not thread-per-event)
* Thread-safe with threading.Lock on the queue and schedule flag
* Handles mid-drain bursts (new events arrive while drainer is working)
* Handles RuntimeError if loop closes between check and submit
* Depth cap (1000) prevents unbounded growth during extended outages
* Drops queue cleanly on disconnect rather than holding forever
* Safety timeout (120s) prevents infinite retention on broken adapters
Based on the approach proposed in #4789 by milkoor, rewritten for
thread-safety and correctness.
Test plan:
* 5 new unit tests (TestPendingInboundQueue) — all passing
* E2E test with real asyncio loop + fake WS thread: 10-event burst
before loop ready → all 10 delivered in order
* E2E concurrent burst test: 20 events queued, 20 more arrive during
drainer dispatch → all 40 delivered, no loss, no duplicates
* All 111 existing feishu tests pass
Related: #5499, #4789
Co-authored-by: milkoor <milkoor@users.noreply.github.com>
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