redo ChildTerminated() processing, see #2391"

the previous patch of including the UID in the Terminated message did
suffer from the system vs. normal message ordering problem, hence I
reverted the previous fix and replaced it by sending a NullMessage after
ChildTerminated, exactly like in the Supervise case.

akka-actor-tests/src/test/scala/akka/actor/ActorSystemSpec.scala

@@ -214,7 +214,7 @@ class ActorSystemSpec extends AkkaSpec("""akka.extensions = ["akka.actor.TestExt
       EventFilter[Exception]("hello", occurrences = 1) intercept {
         a ! "die"
       }
-      val t = probe.expectMsg(Terminated(a)(true, 0))
+      val t = probe.expectMsg(Terminated(a)(true))
       t.existenceConfirmed must be(true)
     }
 

akka-actor/src/main/scala/akka/actor/Actor.scala

@@ -63,10 +63,7 @@ case object Kill extends Kill {
  * When Death Watch is used, the watcher will receive a Terminated(watched) message when watched is terminated.
  */
 @SerialVersionUID(1L)
-case class Terminated private[akka] (@BeanProperty actor: ActorRef)(
-  @BeanProperty val existenceConfirmed: Boolean,
-  private[akka] val uid: Int)
-  extends AutoReceivedMessage
+case class Terminated private[akka] (@BeanProperty actor: ActorRef)(@BeanProperty val existenceConfirmed: Boolean) extends AutoReceivedMessage
 
 abstract class ReceiveTimeout extends PossiblyHarmful
 

akka-actor/src/main/scala/akka/actor/ActorCell.scala

@@ -376,13 +376,7 @@ private[akka] class ActorCell(
 
     msg.message match {
       case Failed(cause, uid)       ⇒ handleFailure(sender, cause, uid)
-      case t @ Terminated(actor) ⇒
-        getChildByRef(actor) match {
-          case Some(crs) if crs.uid == t.uid ⇒ removeChild(actor)
-          case _                             ⇒
-        }
-        watchedActorTerminated(t.actor)
-        receiveMessage(t)
+      case t: Terminated            ⇒ watchedActorTerminated(t.actor); receiveMessage(t)
       case Kill                     ⇒ throw new ActorKilledException("Kill")
       case PoisonPill               ⇒ self.stop()
       case SelectParent(m)          ⇒ parent.tell(m, msg.sender)

akka-actor/src/main/scala/akka/actor/ActorRef.scala

@@ -441,7 +441,7 @@ private[akka] class EmptyLocalActorRef(override val provider: ActorRefProvider,
   protected def specialHandle(msg: Any): Boolean = msg match {
     case w: Watch ⇒
       if (w.watchee == this && w.watcher != this)
-        w.watcher ! Terminated(w.watchee)(existenceConfirmed = false, 0)
+        w.watcher ! Terminated(w.watchee)(existenceConfirmed = false)
       true
     case _: Unwatch ⇒ true // Just ignore
     case _          ⇒ false
@@ -466,9 +466,10 @@ private[akka] class DeadLetterActorRef(_provider: ActorRefProvider,
   override protected def specialHandle(msg: Any): Boolean = msg match {
     case w: Watch ⇒
       if (w.watchee != this && w.watcher != this)
-        w.watcher ! Terminated(w.watchee)(existenceConfirmed = false, 0)
+        w.watcher ! Terminated(w.watchee)(existenceConfirmed = false)
       true
     case w: Unwatch  ⇒ true // Just ignore
+    case NullMessage ⇒ true
     case _           ⇒ false
   }
 

akka-actor/src/main/scala/akka/actor/cell/Children.scala

@@ -94,7 +94,7 @@ private[akka] trait Children { this: ActorCell ⇒
     swapChildrenRefs(c, c.shallDie(ref)) || shallDie(ref)
   }
 
-  @tailrec final protected def removeChild(ref: ActorRef): ChildrenContainer = {
+  @tailrec final private def removeChild(ref: ActorRef): ChildrenContainer = {
     val c = childrenRefs
     val n = c.remove(ref)
     if (swapChildrenRefs(c, n)) n

akka-actor/src/main/scala/akka/actor/cell/DeathWatch.scala

@@ -36,7 +36,7 @@ private[akka] trait DeathWatch { this: ActorCell ⇒
 
   protected def tellWatchersWeDied(actor: Actor): Unit = {
     if (!watchedBy.isEmpty) {
-      val terminated = Terminated(self)(existenceConfirmed = true, uid)
+      val terminated = Terminated(self)(existenceConfirmed = true)
       try {
         watchedBy foreach {
           watcher ⇒

akka-actor/src/main/scala/akka/actor/cell/FaultHandling.scala

@@ -11,6 +11,7 @@ import akka.event.Logging.{ Warning, Error, Debug }
 import scala.util.control.NonFatal
 import akka.dispatch.SystemMessage
 import akka.event.Logging
+import akka.dispatch.NullMessage
 
 private[akka] trait FaultHandling { this: ActorCell ⇒
 
@@ -157,6 +158,7 @@ private[akka] trait FaultHandling { this: ActorCell ⇒
     try if (a ne null) a.postStop()
     finally try dispatcher.detach(this)
     finally try parent.sendSystemMessage(ChildTerminated(self))
+    finally try parent.tell(NullMessage) // read ScalaDoc of NullMessage to see why
     finally try tellWatchersWeDied(a)
     finally try unwatchWatchedActors(a)
     finally {

akka-actor/src/main/scala/akka/pattern/AskSupport.scala

@@ -262,7 +262,7 @@ private[akka] final class PromiseActorRef private (val provider: ActorRefProvide
     case _: Terminate ⇒ stop()
     case Watch(watchee, watcher) ⇒
       if (watchee == this && watcher != this) {
-        if (!addWatcher(watcher)) watcher ! Terminated(watchee)(existenceConfirmed = true, 0)
+        if (!addWatcher(watcher)) watcher ! Terminated(watchee)(existenceConfirmed = true)
       } else System.err.println("BUG: illegal Watch(%s,%s) for %s".format(watchee, watcher, this))
     case Unwatch(watchee, watcher) ⇒
       if (watchee == this && watcher != this) remWatcher(watcher)
@@ -281,7 +281,7 @@ private[akka] final class PromiseActorRef private (val provider: ActorRefProvide
       result tryComplete Failure(new ActorKilledException("Stopped"))
       val watchers = clearWatchers()
       if (!watchers.isEmpty) {
-        val termination = Terminated(this)(existenceConfirmed = true, 0)
+        val termination = Terminated(this)(existenceConfirmed = true)
         watchers foreach { w ⇒ try w.tell(termination, this) catch { case NonFatal(t) ⇒ /* FIXME LOG THIS */ } }
       }
     }