Two related race conditions in gateway/platforms/base.py that could
produce duplicate agent runs or silently drop messages. Neither is
specific to any one platform — all adapters inherit this logic.
R5 (HIGH) — duplicate agent spawn on turn chain
In _process_message_background, the pending-drain path deleted
_active_sessions[session_key] before awaiting typing_task.cancel()
and then recursively awaiting _process_message_background for the
queued event. During the typing_task await, a fresh inbound message
M3 could pass the Level-1 guard (entry now missing), set its own
Event, and spawn a second _process_message_background for the same
session_key — two agents running simultaneously, duplicate responses,
duplicate tool calls.
Fix: keep the _active_sessions entry populated and only clear() the
Event. The guard stays live, so any concurrent inbound message takes
the busy-handler path (queue + interrupt) as intended.
R6 (MED-HIGH) — message dropped during finally cleanup
The finally block has two await points (typing_task, stop_typing)
before it deletes _active_sessions. A message arriving in that
window passes the guard (entry still live), lands in
_pending_messages via the busy-handler — and then the unconditional
del removes the guard with that message still queued. Nothing
drains it; the user never gets a reply.
Fix: before deleting _active_sessions in finally, pop any late
pending_messages entry and spawn a drain task for it. Only delete
_active_sessions when no pending is waiting.
Tests: tests/gateway/test_pending_drain_race.py — three regression
cases. Validated: without the fix, two of the three fail exactly
where the races manifest (duplicate-spawn guard loses identity,
late-arrival 'LATE' message not in processed list).
