fix: discard ReceiveTimeout when timeout message is null #32015 (#32016)

* fix: discard ReceiveTimeout when timeout message is null #32015

* removed unused code

Author: lolboxen

akka-actor-typed-tests/src/test/scala/akka/actor/typed/internal/adpater/ActorAdapterSpec.scala

@@ -0,0 +1,53 @@
+/*
+ * Copyright (C) 2023 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package akka.actor.typed.internal.adpater
+
+import scala.concurrent.duration.DurationInt
+import org.scalatest.wordspec.AnyWordSpecLike
+import akka.actor.testkit.typed.scaladsl._
+import akka.actor.typed._
+import akka.actor.typed.scaladsl.Behaviors
+import akka.actor.typed.scaladsl.adapter._
+
+class ActorAdapterSpec extends ScalaTestWithActorTestKit with AnyWordSpecLike {
+
+  sealed trait Command
+  case object Begin extends Command
+  case object End extends Command
+  case object Timeout extends Command
+
+  sealed trait Event
+  case object Ended extends Event
+
+  def evidence(monitor: ActorRef[Event]): Behavior[Command] =
+    Behaviors.setup { context =>
+      Behaviors.receiveMessage[Command] {
+        case Begin =>
+          context.setReceiveTimeout(100.milliseconds, Timeout)
+          context.cancelReceiveTimeout()
+          context.self.toClassic ! akka.actor.ReceiveTimeout
+          context.self ! End
+          Behaviors.same
+
+        case Timeout =>
+          Behaviors.stopped
+
+        case End =>
+          monitor ! Ended
+          Behaviors.stopped
+      }
+    }
+
+  "An ActorAdapter" must {
+    "not cause actor crash after recieve timeout was cancelled with ReceiveTimeout waiting in mailbox" in {
+      val probe = TestProbe[Event]("evt")
+      val behavior = evidence(probe.ref)
+
+      val ref = spawn(behavior)
+      ref ! Begin
+      probe.expectMessage(Ended)
+    }
+  }
+}

akka-actor-typed/src/main/scala/akka/actor/typed/internal/adapter/ActorAdapter.scala

@@ -90,7 +90,9 @@ import akka.util.OptionVal
             } else Terminated(ActorRefAdapter(ref))
           handleSignal(msg)
         case classic.ReceiveTimeout =>
-          handleMessage(ctx.receiveTimeoutMsg)
+          // discard when null as timeout was cancelled after RecieveTimeout was already enqueued into the mailbox
+          if (ctx.receiveTimeoutMsg != null)
+            handleMessage(ctx.receiveTimeoutMsg)
         case wrapped: AdaptMessage[Any, T] @unchecked =>
           withSafelyAdapted(() => wrapped.adapt()) {
             case AdaptWithRegisteredMessageAdapter(msg) =>