Follow-up to the salvaged #99912 rescue. The original helper left the
rescued orphan IN the adapter slot while the caller also swapped it in as
the current turn, so the post-turn _dequeue_pending_event ran the same
follow-up a second time (live repro: TURNS=['Sent','C','C','D']). The
helper now pops the oldest orphan and returns it to run as this turn,
stages the NEXT orphan in the slot so the drain continues the chain in
arrival order, and the call site parks the incoming message behind the
chain via _enqueue_fifo (slot when free, overflow otherwise) instead of
always appending to overflow. The rescued event's own source drives the
turn so reply anchors point at the message actually being answered.
Tests: contract updated for the new return type; added the 2-orphan chain
case and the single-orphan-then-new-message slot case (both fail against
the original helper shape).
When a follow-up is demoted to /queue during compression-in-flight,
it lands in SessionState.conversation.queued_events (overflow) with
the slot event in adapter._pending_messages. After the slot's turn
completes, _promote_queued_event should move the overflow head into
the slot for the recursive drain. When that drain never runs — the
#99882 shape: busy window ended through an exit that skipped the
promotion site — the overflow is silently orphaned: never dispatched,
never persisted, never logged. A 170-char Telegram follow-up vanished
without a trace; its re-send also vanished for the same reason.
Fix: _rescue_orphaned_overflow stages one orphan into the empty slot
on the next idle arrival, and the new message is enqueued behind it
so FIFO order (#28503) holds — oldest orphan runs as this turn, the
rest drain in order, the new message last. The helper is best-effort
(slot occupied or no overflow → no-op) and logs at WARNING when it
fires so a future drain regression is visible.
Tests (tests/gateway/test_fifo_overflow_rescue.py, 4 cases on the real
GatewayRunner FIFO):
- moves overflow head to empty slot
- no-op when slot occupied
- no-op when no overflow
- FIFO preserved: orphan-1, orphan-2, new-msg in exact arrival order
Existing queue suites pass unchanged (test_queue_consumption — 5 passed).
Fixes#99882
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