From f0c370cf873a5de1bcc378e14fa31fb39751f10c Mon Sep 17 00:00:00 2001
From: Roland <rk@rkuhn.info>
Date: Tue, 21 Aug 2012 13:43:57 +0200
Subject: [PATCH] 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       | 4 ++--
 .../src/main/scala/akka/actor/ActorRefProvider.scala       | 1 +
 akka-actor/src/main/scala/akka/actor/cell/Dispatch.scala   | 2 ++
 .../src/main/scala/akka/dispatch/AbstractDispatcher.scala  | 7 +++++++
 4 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/akka-actor/src/main/scala/akka/actor/ActorCell.scala b/akka-actor/src/main/scala/akka/actor/ActorCell.scala
index 90500cfc88..a07e60ea45 100644
--- a/akka-actor/src/main/scala/akka/actor/ActorCell.scala
+++ b/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 {
diff --git a/akka-actor/src/main/scala/akka/actor/ActorRefProvider.scala b/akka-actor/src/main/scala/akka/actor/ActorRefProvider.scala
index 5c64e5554b..e71782869d 100644
--- a/akka-actor/src/main/scala/akka/actor/ActorRefProvider.scala
+++ b/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 + "]")
     })
 
diff --git a/akka-actor/src/main/scala/akka/actor/cell/Dispatch.scala b/akka-actor/src/main/scala/akka/actor/cell/Dispatch.scala
index 794c051eed..5702ee91cb 100644
--- a/akka-actor/src/main/scala/akka/actor/cell/Dispatch.scala
+++ b/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.
diff --git a/akka-actor/src/main/scala/akka/dispatch/AbstractDispatcher.scala b/akka-actor/src/main/scala/akka/dispatch/AbstractDispatcher.scala
index 3792748b02..45aac0279f 100644
--- a/akka-actor/src/main/scala/akka/dispatch/AbstractDispatcher.scala
+++ b/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
  */