He-Pin commented on code in PR #3096:
URL: https://github.com/apache/pekko/pull/3096#discussion_r3444075901
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actor/src/main/scala/org/apache/pekko/actor/FSM.scala:
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@@ -779,11 +779,25 @@ trait FSM[S, D] extends Actor with Listeners with
ActorLogging {
* transition handling
*/
private var transitionEvent: List[TransitionHandler] = Nil
+ private val watchedListeners: mutable.Set[ActorRef] = mutable.Set.empty
private def handleTransition(prev: S, next: S): Unit = {
Review Comment:
🟡 **Suggestion: listener leak when FSM already watches the listener actor**
When `isWatching(actorRef)` returns `true` (the FSM's business logic already
called `context.watch()` on this actor), `watchWith` is skipped and the actor
is not added to `watchedListeners`. If that actor later terminates, the FSM
receives a plain `Terminated` instead of `ListenerTerminated`, and the listener
is never removed from `listeners`.
Adding a `case Terminated(actorRef) => listeners.remove(actorRef)` handler
to the `receive` block would be a **breaking change** (it would intercept
`Terminated` before it reaches user state functions via `processMsg`). So this
edge case may be acceptable as-is.
Consider adding a log warning when `isWatching` is true at subscription
time, so users know the listener won't be auto-cleaned:
```scala
private def addListener(actorRef: ActorRef): Unit = {
if (listeners.add(actorRef)) {
if (!context.asInstanceOf[ActorCell].isWatching(actorRef)) {
context.watchWith(actorRef, ListenerTerminated(actorRef))
watchedListeners += actorRef
} else {
log.warning(
"Listener [{}] is already watched by this FSM; it will not be " +
"automatically removed on termination. Use
UnsubscribeTransitionCallBack explicitly.",
actorRef)
}
}
}
```
Not blocking — the common case is handled correctly.
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