visit2rahul opened a new issue, #4497:
URL: https://github.com/apache/polaris/issues/4497
## Summary
`InMemoryBufferEventListener` has a race between `UnicastProcessor.onNext()`
(called from `processEvent`) and `UnicastProcessor.onComplete()` (called from
the Caffeine eviction listener and shutdown). At the eviction boundary, events
can be silently dropped.
## Background
Surfaced during review of #4487 by @nandorKollar, who noted that
`UnicastProcessor.onNext()` is declared `public synchronized void` in
smallrye-mutiny but `onComplete()` is not synchronized. @adutra agreed it
warrants its own PR.
## The race
When a per-realm processor is evicted after ~1 hour of inactivity:
1. Caffeine's eviction listener fires `processor.onComplete()` on its
cleanup thread
2. Concurrently, a new event may arrive for the same realm
3. `processEvent` retrieves the (about-to-be-evicted) processor reference
and calls `processor.onNext(event)`
`onNext` is method-synchronized so it acquires the processor's intrinsic
monitor. `onComplete` is not synchronized and runs without acquiring that
monitor. The two methods can therefore interleave; an event arriving at the
wrong moment can be silently dropped (either by `onNext`'s `isDoneOrCancelled`
check seeing `done = true` from a concurrent `onComplete`, or by a mid-drain
termination).
## Practical impact
Race window is small (microseconds at eviction boundary), only manifests
after ~1 hour of inactivity per realm. But silent drops on an audit/event-log
path are unacceptable.
## Proposed fix
Wrap both `onComplete()` call sites (the Caffeine eviction listener and the
`shutdown()` loop) in `synchronized (processor) { ... }` blocks so they acquire
the same intrinsic monitor that `onNext()` uses. `processEvent` itself does not
change.
I will open a PR with this fix shortly.
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