sync_turn's bounded join could drop a still-alive previous worker by
replacing the single _sync_thread slot. The dropped worker kept POSTing
under the old sid but was no longer visible to on_session_end /
on_session_switch, so the commit could fire while orphaned writes were
still in flight — those writes landed past the commit boundary and were
never extracted.
Replace the single _sync_thread slot with _inflight_writers:
Dict[sid, Set[Thread]]. Writers self-register on spawn (sync_turn,
on_memory_write) and self-deregister on exit. The commit path drains
_drain_writers(sid, 10.0) and skips the commit if any writer for that
sid is still alive after the bounded budget.
Also trim inline review-rationale comments to short invariants per
reviewer style ask: "commit only after session writes drain" and
"drop prefetch results from older switch generations."
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
(cherry picked from commit 7537ee6f5b9ffa4c0f7b79af053aab449caf5af5)
91e9459e10