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Sanjay Santhanam
fb822239ea
The originally-proposed fix (gate _ensureAllMessagesLoaded on the existing _loadingOlder flag) does not actually close the race. By the time the prefetch reaches its post-await body, it has already cleared the entry- gate that reads _loadingOlder, so a same-flag check inside the resolved callback would be a no-op for an in-flight request. The actual fix is two-pronged: 1. New module-scoped _messagesGeneration counter, bumped every time S.messages is wholesale-replaced. _loadOlderMessages snapshots it BEFORE its await and re-checks after — if it changed, the prepend is aborted. This is the canonical async-invalidation pattern. 2. _ensureAllMessagesLoaded now claims the _loadingOlder mutex around its body so a new prefetch cannot start mid-replace and concurrent ensure-all calls (rapid double-click on Start) serialize cleanly. It bumps the generation token before mutating S.messages, yields until any in-flight prefetch finishes, and resets _oldestIdx so a subsequent prefetch cannot request stale older messages. Also adds the same-session / _loadingSessionId guards that the original ensure-all body was missing post-await — if the user switched sessions mid-flight, the old code would happily overwrite the new session's messages with the previous session's full history. 12 new regression tests in tests/test_issue1937_endless_scroll_jumpstart_race.py lock in: generation token declaration, bump-helper presence, snapshot- before-await ordering, post-await-abort behaviour, mutex acquisition and finally-release, yield-then-claim ordering when a prefetch is in flight, generation bump during the wait phase, _oldestIdx reset, and the new session-switch guard. Closes #1937.
