Simultaneous game start by both partners created two divergent active sessions (TOCTOU: a
non-transactional check-then-create in GameSessionManager.startGameWithCouple). Each partner
ended up in a separate session with different questions → no shared reveal.
Fix: QuestionSessionRepository.startSessionAtomically runs a Firestore transaction on a
per-couple pointer doc (couples/{cid}/sessions/_active). It reads the pointer (+ the pointed
session) and either returns AlreadyActive (caller joins the existing session) or atomically
creates the new session and re-points the lock. Concurrent starts contend on the one pointer,
so the loser's transaction retries, sees the now-set pointer, and joins instead of duplicating.
The pointer self-heals (checks the pointed session's status) so no clear-on-finish is needed,
and it carries no status/completedAt so it's invisible to the active/history queries.
GameSessionManager routes all 7 games through it. firestore.rules adds member-write for
sessions/_active (deployed).
Verified live on both emulators: atomic create → 1 session + pointer; sequential 2nd start →
joins (1 session); literal parallel-tap race → 1 session (was 2); 0 FATAL.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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