Option A from the issue — the cheap consistency check, no interface change.
The desired ref set was re-applied on a Connected TRANSITION (`ResubscribeDesired`)
and on a deploy's `SetDesiredSubscriptions`, and nowhere else. A subscription lost
while the driver STAYED Connected — a subscribe that failed and was never retried,
a handle dropped down an error path — left the actor subscribed to nothing while
still reporting Healthy, indefinitely.
`ReconcileSubscription` now rides the existing 30 s `health-poll` tick: while
Connected, a driver holding a desired set but no live handle is re-subscribed.
Gated on `ISubscribable`. A driver that cannot subscribe at all can still be handed
a desired set, and without the guard it would be told `Subscribe` every tick and
fail every tick, forever.
This is a state-machine consistency check, NOT a probe — it can only see that this
actor holds no handle, never that a handle is present but dead server-side, because
`ISubscriptionHandle` is opaque. That is option B, and it stays deferred until a
real occurrence shows the handle outliving the subscription; it would need a
liveness member implemented across all eight drivers with different subscription
mechanics.
A redundant `Subscribe` is possible (a tick queued ahead of an already-queued one
observes the same no-handle state) and is harmless: `HandleSubscribeAsync` is a
`ReceiveAsync`, so no tick is processed mid-subscribe, and it drops any prior
subscription before establishing the new one. Suppressing it would need a
pending-subscribe flag threaded through every self-tell site — more state than the
race is worth.
`healthPollInterval` becomes an optional Props parameter, mirroring the existing
`rediscoverInterval` seam, so a test can drive the reconcile without waiting 30 s.
Three tests. The reconcile itself is proven by a stub whose FIRST subscribe throws:
nothing else would ever retry it, so a second attempt can only come from the
reconcile — disabling `ReconcileSubscription` turns it red. The two guard tests are
absence assertions and are bounded by a positive control rather than a sleep: a
counting health publisher makes ticks PROCESSED observable, so "still one subscribe"
means the reconcile ran and declined, not that the timer had not fired.
Explicitly not deploy-time drift (#486) — re-asserting an already-correct desired
set would not have helped there.
Joseph Doherty
4b9bcddbcc