Make provider registration of PromiseActorRefs lazy

By only registering a PromiseActorRef on access to `path`
we can shave off about 30% of the overhead involved in the common,
purely local `ask` use case.
Unfortunately `path` is accessed not only for ActorRef serialization
but also during `toString`, `equals` and `hashCode`, so one should
be careful how to handle PromiseActorRefs in hot paths.

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

@@ -3,13 +3,14 @@
  */
 package akka.pattern
 
-import java.util.concurrent.atomic.AtomicBoolean
+import java.util.concurrent.atomic.AtomicReference
 import java.util.concurrent.TimeoutException
 import akka.actor.{ Terminated, Status, MinimalActorRef, InternalActorRef, ActorRef, ActorPath }
 import akka.dispatch.{ Promise, Terminate, SystemMessage, Future }
 import akka.event.DeathWatch
 import akka.actor.ActorRefProvider
 import akka.util.Timeout
+import annotation.tailrec
 
 /**
  * This is what is used to complete a Future that is returned from an ask/? call,
@@ -159,12 +160,50 @@ trait AskSupport {
    */
   private[akka] final class PromiseActorRef(
     val provider: ActorRefProvider,
-    val path: ActorPath,
     override val getParent: InternalActorRef,
     val result: Promise[Any],
     val deathWatch: DeathWatch) extends MinimalActorRef {
 
-    final val running = new AtomicBoolean(true)
+    /**
+     * As an optimization for the common (local) case we only register this PromiseActorRef
+     * with the provider when the `path` member is actually queried, which happens during
+     * serialization (but also during a simple `toString`!).
+     *
+     * Defined states:
+     * null             => started, path not yet created
+     * path: ActorPath  => path is available and was registered
+     * RefStopped(path) => stopped, path available
+     * 'stopped         => stopped, path not yet created
+     * 'busy            => another thread is in a state change
+     */
+    final val state = new AtomicReference[Any]
+
+    /**
+     * Contract of this method:
+     * Must always return a valid path string, which must have
+     * been registered if we haven't been stopped yet.
+     */
+    @tailrec
+    def path: ActorPath = state.get match {
+      case null ⇒
+        if (state.compareAndSet(null, 'busy)) {
+          // TODO: are there any exceptions "expected" from the following two lines that we need to take care of?
+          val p = provider.tempPath()
+          provider.registerTempActor(this, p)
+          state.set(p)
+          p
+        } else path
+      case p: ActorPath  ⇒ p
+      case x: RefStopped ⇒ x.path
+      case 'stopped ⇒
+        if (state.compareAndSet('stopped, 'busy)) {
+          // TODO: are there any exceptions "expected" from the `tempPath` call that we need to take care of?
+          state.set(new RefStopped(provider.tempPath()))
+        }
+        path
+      case 'busy ⇒ path
+      case x     ⇒ throw new IllegalStateException(x.toString)
+    }
 
     override def !(message: Any)(implicit sender: ActorRef = null): Unit = {
       try {
@@ -185,8 +224,23 @@ trait AskSupport {
 
     override def isTerminated = result.isCompleted
 
-    override def stop(): Unit = if (running.getAndSet(false)) {
-      deathWatch.publish(Terminated(this))
+    @tailrec
+    override def stop(): Unit = state.get match {
+      case null ⇒
+        if (state.compareAndSet(null, 'busy)) {
+          state.set('stopped)
+          // TODO: are there any exceptions "expected" from the following line that we need to take care of?
+          deathWatch.publish(Terminated(this))
+        } else stop()
+      case p: ActorPath ⇒
+        if (state.compareAndSet(p, 'busy)) {
+          state.set(new RefStopped(p))
+          // TODO: are there any exceptions "expected" from the following two lines that we need to take care of?
+          deathWatch.publish(Terminated(this))
+          provider.unregisterTempActor(p)
+        } else stop()
+      case 'busy ⇒ stop()
+      case _     ⇒ // 'stopped and RefStopped do not require any action here
     }
   }
 
@@ -194,15 +248,14 @@ trait AskSupport {
    * INTERNAL AKKA USE ONLY
    */
   private[akka] def createAsker(provider: ActorRefProvider, timeout: Timeout): PromiseActorRef = {
-    val path = provider.tempPath()
     val result = Promise[Any]()(provider.dispatcher)
-    val a = new PromiseActorRef(provider, path, provider.tempContainer, result, provider.deathWatch)
-    provider.registerTempActor(a, path)
+    val a = new PromiseActorRef(provider, provider.tempContainer, result, provider.deathWatch)
     val f = provider.scheduler.scheduleOnce(timeout.duration) { result.tryComplete(Left(new AskTimeoutException("Timed out"))) }
     result onComplete { _ ⇒
-      try { try a.stop() finally f.cancel() }
-      finally { provider.unregisterTempActor(path) }
+      try a.stop() finally f.cancel()
     }
     a
   }
 }
+
+private[akka] final class RefStopped(val path: ActorPath)