The gate's "backfill and prune never interleave" guarantee was only argued
structurally: the MailBackfiller/MailPruner unit tests each build a throwaway,
uncontended gate, so the serialization is never exercised. Add MailMaintenanceGateTest:
- oneGateSerializesConcurrentCriticalSections: 50 coroutines contend on one gate;
an overlap counter must never exceed 1 (guards against a per-access mutex).
- aConcurrentPruneWaitsForAnInFlightBackfillToReleaseTheGate: a real MailBackfiller
parks inside the gate (its fetchOlderThan suspended) while a real MailPruner is
launched concurrently; the two share ONLY the gate, and the prune provably cannot
enter its critical section until the backfill releases.
Turns the defence-in-depth guarantee from argued to asserted.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
JMR-devandClaude Opus 4.8
1d4103747f