add NullMessage after Supervise to prevent race, see #2418

- parent just checked for system messages and finds none
- Supervise is enqueued to parent
- Failed is enqueued to the parent
- Failed is processed before Supervise and thus dropped due to
  non-matching UID

remedy is to enqueue NullMessage after enqueuing Supervise but before
really starting the actor, so that if NullMessage overtakes Supervise
the loop in processMailbox will then pick up Supervise next

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

@@ -5,16 +5,15 @@
 package akka.actor
 
 import java.io.{ ObjectOutputStream, NotSerializableException }
-
 import scala.annotation.tailrec
 import scala.collection.immutable.TreeSet
 import scala.concurrent.util.Duration
 import scala.util.control.NonFatal
-
 import akka.actor.cell.ChildrenContainer
 import akka.dispatch.{ Watch, Unwatch, Terminate, SystemMessage, Suspend, Supervise, Resume, Recreate, NoMessage, MessageDispatcher, Envelope, Create, ChildTerminated }
 import akka.event.Logging.{ LogEvent, Debug, Error }
 import akka.japi.Procedure
+import akka.dispatch.NullMessage
 
 /**
  * The actor context - the view of the actor cell from the actor.
@@ -365,6 +364,7 @@ private[akka] class ActorCell(
       publish(Debug(self.path.toString, clazz(actor), "received AutoReceiveMessage " + msg))
 
     msg.message match {
+      case NullMessage        ⇒ // do nothing
       case Failed(cause, uid) ⇒ handleFailure(sender, cause, uid)
       case t @ Terminated(actor) ⇒
         getChildByRef(actor) match {

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

@@ -360,6 +360,7 @@ class LocalActorRefProvider(
 
     override def !(message: Any)(implicit sender: ActorRef = null): Unit = stopped.ifOff(message match {
       case Failed(ex, _) if sender ne null ⇒ causeOfTermination = Some(ex); sender.asInstanceOf[InternalActorRef].stop()
+      case NullMessage                     ⇒ // do nothing
       case _                               ⇒ log.error(this + " received unexpected message [" + message + "]")
     })
 

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

@@ -10,6 +10,7 @@ import akka.dispatch.{ Terminate, SystemMessage, Suspend, Resume, Recreate, Mess
 import akka.event.Logging.Error
 import akka.util.Unsafe
 import scala.util.control.NonFatal
+import akka.dispatch.NullMessage
 
 private[akka] trait Dispatch { this: ActorCell ⇒
 
@@ -56,6 +57,7 @@ private[akka] trait Dispatch { this: ActorCell ⇒
     if (sendSupervise) {
       // ➡➡➡ NEVER SEND THE SAME SYSTEM MESSAGE OBJECT TO TWO ACTORS ⬅⬅⬅
       parent.sendSystemMessage(akka.dispatch.Supervise(self, uid))
+      parent.tell(NullMessage)
     }
 
     // This call is expected to start off the actor by scheduling its mailbox.

akka-actor/src/main/scala/akka/dispatch/AbstractDispatcher.scala

@@ -37,6 +37,13 @@ object Envelope {
   }
 }
 
+/**
+ * This message is sent directly after the Supervise system message in order
+ * to form a barrier wrt. the first real message sent by the child, so that e.g.
+ * Failed() cannot overtake Supervise(). Processing this does nothing.
+ */
+case object NullMessage extends AutoReceivedMessage
+
 /**
  * INTERNAL API
  */