fix(actor-typed): discard stale ReceiveTimeout after cancelReceiveTimeout to avoid NPE

[He-Pin]

Motivation

When context.cancelReceiveTimeout() is called while a classic ReceiveTimeout message has already been enqueued in the mailbox by the scheduler, the typed actor crashes with a NullPointerException inside InterceptorImpl.receive.

Root cause trace:

1. setReceiveTimeout(d, msg) → sets receiveTimeoutMsg = msg, schedules classic.ReceiveTimeout via scheduleOnce
2. Before the actor processes the enqueued ReceiveTimeout, another message triggers cancelReceiveTimeout()
3. cancelReceiveTimeout() sets receiveTimeoutMsg = null and calls classicActorContext.setReceiveTimeout(Duration.Undefined)
4. The already-enqueued classic.ReceiveTimeout cannot be retracted from the mailbox (the Cancellable.cancel() only prevents future scheduling, not already-delivered messages)
5. ActorAdapter.aroundReceive matches classic.ReceiveTimeout and calls handleMessage(ctx.receiveTimeoutMsg) — with null
6. The null message propagates through Behavior.interpretMessage → InterceptorImpl.receive → msg.getClass throws NPE

This bug was observed in both 2.8.3 and 2.6.21 of the upstream project. A reproduction is available at https://github.com/lolboxen/akka-receive-timeout-bug.

Modification

• ActorAdapter.aroundReceive: null-check ctx.receiveTimeoutMsg before forwarding to handleMessage. If null (timeout was cancelled), the stale classic.ReceiveTimeout is silently discarded.
• Added CancelReceiveTimeoutSpec: a deterministic regression test that directly sends classic.ReceiveTimeout to a typed actor after cancelReceiveTimeout() has been called, verifying the actor survives without NPE. The test uses an interceptor in the behavior stack to exercise the exact InterceptorImpl.receive code path where the NPE manifests.

Result

Typed actors no longer crash when cancelReceiveTimeout() races with an already-enqueued stale ReceiveTimeout. The fix is a single null-guard at the adapter layer — the typed behavior stack never sees a null message.

Tests

sbt "actor-typed-tests / Test / testOnly org.apache.pekko.actor.typed.CancelReceiveTimeoutSpec"
→ All tests passed

sbt "actor-typed-tests / Test / testOnly org.apache.pekko.actor.typed.ActorContextSpec org.apache.pekko.actor.typed.TimerSpec org.apache.pekko.actor.typed.CancelReceiveTimeoutSpec"
→ 14 tests passed, 0 failed

The regression test was verified to fail without the fix (NPE at InterceptorImpl.scala:94) and pass with the fix.

References

• Reproduction: https://github.com/lolboxen/akka-receive-timeout-bug

[pjfanning]

lgtm