A periodic reconcile can synthesize a repair transition whose matching live
transition is still buffered in the alarm lease; both then broadcast as
indistinguishable duplicates on StreamAlarms and the dashboard hub. Nothing
serializes the two paths, and a correct serialization needs a worker-side
high-water mark on QueryActiveAlarms (proto + worker change + a stall path),
so this closes the common case with a local best-effort dedup instead: both
paths already carry the same worker-derived identity — the worker stamps
record.TransitionTimestampUtc into both OnAlarmTransitionEvent's
transition_timestamp and ActiveAlarmSnapshot.last_transition_timestamp — so
ApplyTransition suppresses a live transition whose (timestamp, resulting
state) the cache already carries from a repair. The Clear leg has no cache
entry left to compare, so ApplyReconcile tombstones each synthesized Clear by
the instance's original_raise_timestamp for one reconcile generation; a
matching live Clear consumes the tombstone, while a new raise/clear cycle
carries a newer raise timestamp and passes. Suppression fires only on a
positive marker match — unset timestamps keep today's behavior — so the
documented at-least-once consumer contract stands (gateway.md, Sessions.md
updated in the same change).
Tests: two new regressions drive the exact race through the GWC-26 harness
(repair-then-buffered-live for Raise and for Clear, each with a genuine
follow-up transition proving no over-suppression); GatewayAlarmMonitor
suites 18/18.
Code-review follow-up on fix/gwc-26-27-alarm-attach.
ApplyReconcile's snapshot-derived feed repairs are at-least-once, not
exactly-once: a reconcile reads the worker's current state while the matching
live transition may still be buffered in the monitor's lease, so both broadcast
and the duplicates are indistinguishable on the alarm feed. This pre-dates the
acked-state delta — the Raise/Clear presence repair has always had it, since
nothing serializes a reconcile pass against the in-flight live stream — so
closing it (serialization or timestamp dedup) stays out of scope for a P2 fix.
Documented instead, with the consumer contract stated explicitly (apply
transitions idempotently, never as an increment or toggle):
- ApplyReconcile gains a "Delivery semantics" comment.
- gateway.md softens the "defense in depth" prose to state the semantics.
- docs/Sessions.md carries the same caveat on the alarm-feed description.
- Tracker change-log records it as a known pre-existing characteristic and a
candidate finding for the next review cycle.
Also hoists the ChannelWorkerClient fake — duplicated across the three alarm
test files — into TestSupport/, dropping the usings it took with it.
Seeding the cache with a live Raise transition raced the first reconcile: when
the reconcile snapshot populated the cache first, the still-buffered live Raise
was applied — and broadcast — after the test's feed subscriber had registered,
so the exactly-one-transition assertion saw two. Seed through a reconcile pass
instead (forced by a provider-mode probe, as the acked step already did) so no
live transition is ever in flight. Verified: 5 consecutive full alarm-monitor
runs green, and the test still fails (timeout) with the ApplyReconcile
acked-delta branch removed.
RunMonitorAsync issued SubscribeAlarms and the first reconcile before the
internal distributor subscriber was attached (via ISessionManager
.ReadAlarmEventsAsync). The pump has been running since MarkReady started the
dashboard mirror and only fans to subscribers registered at fan-out time, so
every transition raised in that two-round-trip window bypassed the alarm feed —
and a missed Acknowledge was never repaired, because ApplyReconcile broadcast
presence deltas only.
- The monitor now takes the internal lease directly from its session BEFORE
SubscribeAlarms and drains it after the first reconcile; window transitions
buffer in the lease's bounded channel. Processing them after ApplyReconcile is
order-safe (ApplyTransition handles alarms the snapshot already placed).
- ISessionManager.ReadAlarmEventsAsync removed — zero remaining callers.
- ApplyReconcile broadcasts an Acknowledge feed transition when a both-present
alarm's state advanced to ActiveAcked. This is a feed-level repair on the
AlarmFeedMessage/StreamAlarms surface rebuilt from the worker's own snapshot,
not MxEvent emission, so the "never synthesize events" rule is untouched;
the reasoning is recorded on ApplyReconcile.
The alarm-monitor test fakes now hand the monitor a real Ready GatewaySession
with a dashboard mirror, which is what makes the window reproducible.
Docs: docs/Sessions.md and gateway.md alarm-monitor ordering notes.
Refs: archreview/2026-07-12/remediation/10-gateway-core.md GWC-26
Joseph Doherty