add debug helper for CallingThreadDispatcherModelSpec, see #2821

- the problem is that the user guardian gets blocked for more than 3
  seconds, and I have no clue who blocks it where
- therefore print all stack traces two seconds into the test if still
  not finished

akka-actor-tests/src/test/scala/akka/actor/dispatch/ActorModelSpec.scala

@@ -420,12 +420,21 @@ abstract class ActorModelSpec(config: String) extends AkkaSpec(config) with Defa
         val f5 = try { a ? Interrupt } catch { case ie: InterruptedException ⇒ Promise.failed(new ActorInterruptedException(ie)).future }
         val f6 = a ? Reply("bar2")
 
+        val c = system.scheduler.scheduleOnce(2.seconds) {
+          import collection.JavaConverters._
+          Thread.getAllStackTraces().asScala foreach {
+            case (thread, stack) ⇒
+              println(s"$thread:")
+              stack foreach (s => println(s"\t$s"))
+          }
+        }
         assert(Await.result(f1, remaining) === "foo")
         assert(Await.result(f2, remaining) === "bar")
         assert(Await.result(f4, remaining) === "foo2")
         assert(intercept[ActorInterruptedException](Await.result(f3, remaining)).getCause.getMessage === "Ping!")
         assert(Await.result(f6, remaining) === "bar2")
         assert(intercept[ActorInterruptedException](Await.result(f5, remaining)).getCause.getMessage === "Ping!")
+        c.cancel()
       }
     }
 

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

@@ -212,7 +212,7 @@ private[akka] class UnstartedCell(val systemImpl: ActorSystemImpl,
         } else if (!queue.offer(msg)) {
           system.eventStream.publish(Warning(self.path.toString, getClass, "dropping message of type " + msg.message.getClass + " due to enqueue failure"))
           system.deadLetters ! DeadLetter(msg.message, msg.sender, self)
-        }
+        } else if (Mailbox.debug) println(s"$self temp queueing ${msg.message} from ${msg.sender}")
       } finally lock.unlock()
     } else {
       system.eventStream.publish(Warning(self.path.toString, getClass, "dropping message of type" + msg.message.getClass + " due to lock timeout"))
@@ -243,7 +243,7 @@ private[akka] class UnstartedCell(val systemImpl: ActorSystemImpl,
           if (!wasEnqueued) {
             system.eventStream.publish(Warning(self.path.toString, getClass, "dropping system message " + msg + " due to enqueue failure"))
             system.deadLetters ! DeadLetter(msg, self, self)
-          }
+          } else if (Mailbox.debug) println(s"$self temp queueing system $msg")
         }
       } finally lock.unlock()
     } else {