He-Pin opened a new issue, #3258:
URL: https://github.com/apache/pekko/issues/3258
### Motivation
The fix for timer message loss during stash/unstash (`Timers.scala:58`) only
covers actors using the `Stash` trait. Actors using `UnboundedStash` or
`UnrestrictedStash` still lose timer messages because the `currentMessage`
rewrite is skipped.
### Current Behavior
`actor/src/main/scala/org/apache/pekko/actor/Timers.scala:58-61`:
```scala
if (this.isInstanceOf[Stash]) {
actorCell.currentMessage = actorCell.currentMessage.copy(message = m)
}
```
The trait hierarchy (`Stash.scala:71-82`):
```
Stash extends UnrestrictedStash (with RequiresMessageQueue[...])
UnboundedStash extends UnrestrictedStash (with RequiresMessageQueue[...])
UnrestrictedStash extends Actor with StashSupport
```
`Stash` and `UnboundedStash` are **siblings** — neither extends the other.
The `isInstanceOf[Stash]` guard returns `false` for actors mixing in
`UnboundedStash` or `UnrestrictedStash` directly.
### What Happens Without the Fix
1. Timer fires, wraps user message in `TimerMsg`
2. `aroundReceive` unwraps `TimerMsg`, calls user's `receive` with the
actual message
3. User calls `stash()` — but `StashSupport.stash()` reads
`actorCell.currentMessage` which still contains the `TimerMsg` wrapper
4. On `unstashAll()`, the `TimerMsg` wrapper is re-enqueued
5. `aroundReceive` intercepts it again, `interceptTimerMsg` discards it
(single-shot timer already removed, or stale generation for repeating)
6. **Message is lost**
### Fix
Change `this.isInstanceOf[Stash]` to `this.isInstanceOf[StashSupport]` to
cover all three public stash traits.
### Not Affected
Typed `StashBuffer` is not affected — timer messages are unwrapped at
`ActorAdapter.scala:136-138` before reaching the behavior.
### Environment
- Pekko Actor (any version)
- `Timers.scala:58`
### References
None
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