Making sure that you cannot go from Mailbox.Closed to anything else

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

@@ -10,7 +10,8 @@ import akka.util._
 import java.util.Queue
 import akka.actor.ActorContext
 import java.util.concurrent._
-import atomic.AtomicReferenceFieldUpdater
+import atomic.{ AtomicInteger, AtomicReferenceFieldUpdater }
+import annotation.tailrec
 
 class MessageQueueAppendFailedException(message: String, cause: Throwable = null) extends AkkaException(message, cause)
 
@@ -31,16 +32,24 @@ abstract class Mailbox extends MessageQueue with SystemMessageQueue with Runnabl
    * Internal implementation of MessageDispatcher uses these, don't touch or rely on
    */
   final val dispatcherLock = new SimpleLock(startLocked = false)
-  @volatile
-  var _status: Status = Open //Must be named _status because of the updater
+  val _status: AtomicInteger = new AtomicInteger(Open) //Must be named _status because of the updater
 
-  final def status: Mailbox.Status = _status //mailboxStatusUpdater.get(this)
+  final def status: Mailbox.Status = _status.get() //mailboxStatusUpdater.get(this)
 
   final def isSuspended: Boolean = status == Suspended
   final def isClosed: Boolean = status == Closed
   final def isOpen: Boolean = status == Open
 
-  def become(newStatus: Status) = _status = newStatus //mailboxStatusUpdater.set(this, newStatus)
+  def become(newStatus: Status): Boolean = {
+    @tailrec
+    def transcend(): Boolean = {
+      val current = _status.get()
+      if (current == Closed) { _status.set(Closed); false } //Ensure that the write is always performed
+      else if (_status.compareAndSet(current, newStatus)) true
+      else transcend()
+    }
+    transcend()
+  } //mailboxStatusUpdater.set(this, newStatus)
 
   def shouldBeRegisteredForExecution(hasMessageHint: Boolean, hasSystemMessageHint: Boolean): Boolean = status match {
     case `Open`      ⇒ hasMessageHint || hasSystemMessageHint || hasSystemMessages || hasMessages

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

@@ -87,7 +87,6 @@ abstract class MessageDispatcher extends Serializable {
   object DeadLetterMailbox extends Mailbox {
     dispatcherLock.tryLock()
     become(Mailbox.Closed)
-    override def become(newStatus: Mailbox.Status) { super.become(Mailbox.Closed) } //Always transcend to CLOSED to preserve the volatile write
     override def dispatcher = null //MessageDispatcher.this
     override def enqueue(envelope: Envelope) { envelope.channel sendException new ActorKilledException("Actor has been stopped") }
     override def dequeue() = null