Module commit_waiter 

Synchronous commit waiter (PLAN.md Phase 11.4 — ack_n).

Bridges the primary’s commit path with replica ACKs. The commit caller picks a target_lsn (the LSN it just made durable locally) and asks the waiter “block until at least N replicas have ack’d this LSN, or the timeout expires.” Replica ACK RPCs call record_replica_ack which signals every waiter whose threshold is now met.

Thread safety

The waiter uses a Mutex<State> + Condvar so the await_acks call blocks the caller’s thread without spinning. Acks bump a per-replica last_durable_lsn map and broadcast on the condvar. Waiters wake, recompute the count of replicas at or past their target, and either return Ok(count) or re-wait.

Runtime integration

Public mutation surfaces call RedDBRuntime::enforce_commit_policy after successful writes. That runtime method maps ack_n to this waiter and decides whether a timeout is soft telemetry or a hard client-visible failure based on RED_COMMIT_FAIL_ON_TIMEOUT.

Structs

CommitWaiter

CommitWaiterMetrics

Outcome counters for /metrics. PLAN.md Phase 11.4 — operators alert on timed_out rising (commit policy is too tight or replicas are stalled) and watch last_wait_micros for the p95.

Enums

AwaitOutcome