Follow-up to the cherry-picked PR #17447. The original flush spawned a
bare threading.Thread for the buffer-flush path, overwriting
self._sync_thread — which is aliased to the long-lived writer thread.
Two consequences:
1. No serialization with the writer queue. If old-session retains were
still queued in _retain_queue, the flush ran concurrently with the
writer and both threads could call aretain_batch against the same
document_id.
2. The pre-spawn 'self._sync_thread.join(timeout=5.0)' tried to join the
long-lived writer, which never exits, so the join was a no-op that
just timed out — never actually serialized anything.
Fix: enqueue the flush closure on _retain_queue via _ensure_writer +
put(). Natural FIFO ordering behind any pending retains, no new thread,
no broken join. Shutdown-aware so it doesn't enqueue after teardown.
Tests updated to drain via _retain_queue.join() instead of the stale
_sync_thread.join(). Added regression guard
test_flush_serializes_behind_pending_retains_via_writer_queue that
blocks the writer mid-retain to prove the flush waits in FIFO behind
the old retain.
Also seeds _retain_queue / _shutting_down / stubbed _ensure_writer on
the bare-object test helper in test_memory_session_switch.py so that
path doesn't blow up under the new queue-enqueue.
tests/plugins/memory/test_hindsight_provider.py + tests/agent/test_memory_session_switch.py: 103/103 passing.
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