Races 5 concurrent HeartbeatQueueTicket calls, all at the same expected
revision, against a real PostgreSQL instance -- a genuinely plausible
client scenario (slow-response retry, duplicate tab/process), and one
the existing sequential stale-revision test can't exercise since it
only calls the second heartbeat after the first has already
committed. Asserts exactly one wins, the durable revision ends at
exactly 1 (not higher, which a stale winner slipping through would
produce), and every loser fails cleanly rather than hanging or
returning a raw serialization error. Stable across 6 runs with -race,
plus the full integration and unit suites.
Josh Creek
270ab00c52