=act #13970 Send transistion on goto(CurrentState) in FSM

When in `A`:
* `goto(A)` will trigger `onTransition(A -> A)`
* `stay()` will NOT trigger `onTransition`

Includes:
* migration guide
* docs updates
* test

akka-actor-tests/src/test/scala/akka/actor/FSMTransitionSpec.scala

@@ -32,13 +32,15 @@ object FSMTransitionSpec {
   class OtherFSM(target: ActorRef) extends Actor with FSM[Int, Int] {
     startWith(0, 0)
     when(0) {
-      case Event("tick", _) ⇒ goto(1) using (1)
+      case Event("tick", _) ⇒ goto(1) using 1
+      case Event("stay", _) ⇒ stay()
     }
     when(1) {
-      case _ ⇒ stay
+      case _ ⇒ goto(1)
     }
     onTransition {
       case 0 -> 1 ⇒ target ! ((stateData, nextStateData))
+      case 1 -> 1 ⇒ target ! ((stateData, nextStateData))
     }
   }
 
@@ -78,7 +80,7 @@ class FSMTransitionSpec extends AkkaSpec with ImplicitSender {
         expectMsg(FSM.CurrentState(fsm, 0))
         akka.pattern.gracefulStop(forward, 5 seconds)
         fsm ! "tick"
-        expectNoMsg
+        expectNoMsg()
       }
     }
   }
@@ -93,6 +95,36 @@ class FSMTransitionSpec extends AkkaSpec with ImplicitSender {
       }
     }
 
+    "trigger transition event when goto() the same state" in {
+      import FSM.Transition
+      val forward = system.actorOf(Props(new Forwarder(testActor)))
+      val fsm = system.actorOf(Props(new OtherFSM(testActor)))
+
+      within(1 second) {
+        fsm ! FSM.SubscribeTransitionCallBack(forward)
+        expectMsg(FSM.CurrentState(fsm, 0))
+        fsm ! "tick"
+        expectMsg((0, 1))
+        expectMsg(Transition(fsm, 0, 1))
+        fsm ! "tick"
+        expectMsg((1, 1))
+        expectMsg(Transition(fsm, 1, 1))
+      }
+    }
+
+    "not trigger transition event on stay()" in {
+      import FSM.Transition
+      val forward = system.actorOf(Props(new Forwarder(testActor)))
+      val fsm = system.actorOf(Props(new OtherFSM(testActor)))
+
+      within(1 second) {
+        fsm ! FSM.SubscribeTransitionCallBack(forward)
+        expectMsg(FSM.CurrentState(fsm, 0))
+        fsm ! "stay"
+        expectNoMsg()
+      }
+    }
+
     "not leak memory in nextState" in {
       val fsmref = system.actorOf(Props(new Actor with FSM[Int, ActorRef] {
         startWith(0, null)
@@ -105,11 +137,11 @@ class FSMTransitionSpec extends AkkaSpec with ImplicitSender {
         when(1) {
           case Event("test", _) ⇒
             try {
-              sender() ! s"failed: ${nextStateData}"
+              sender() ! s"failed: $nextStateData"
             } catch {
               case _: IllegalStateException ⇒ sender() ! "ok"
             }
-            stay
+            stay()
         }
       }))
       fsmref ! "switch"

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

@@ -123,7 +123,14 @@ object FSM {
    * name, the state data, possibly custom timeout, stop reason and replies
    * accumulated while processing the last message.
    */
-  final case class State[S, D](stateName: S, stateData: D, timeout: Option[FiniteDuration] = None, stopReason: Option[Reason] = None, replies: List[Any] = Nil) {
+  final case class State[S, D](stateName: S, stateData: D, timeout: Option[FiniteDuration] = None, stopReason: Option[Reason] = None, replies: List[Any] = Nil)(private[akka] val notifies: Boolean = true) {
+
+    /**
+     * Copy object and update values if needed.
+     */
+    private[akka] def copy(stateName: S = stateName, stateData: D = stateData, timeout: Option[FiniteDuration] = timeout, stopReason: Option[Reason] = stopReason, replies: List[Any] = replies, notifies: Boolean = notifies): State[S, D] = {
+      State(stateName, stateData, timeout, stopReason, replies)(notifies)
+    }
 
     /**
      * Modify state transition descriptor to include a state timeout for the
@@ -160,7 +167,12 @@ object FSM {
     private[akka] def withStopReason(reason: Reason): State[S, D] = {
       copy(stopReason = Some(reason))
     }
+
+    private[akka] def withNotification(notifies: Boolean): State[S, D] = {
+      copy(notifies = notifies)
+    }
   }
+
   /**
    * All messages sent to the [[akka.actor.FSM]] will be wrapped inside an
    * `Event`, which allows pattern matching to extract both state and data.
@@ -179,7 +191,6 @@ object FSM {
  * Finite State Machine actor trait. Use as follows:
  *
  * <pre>
- *   object A {
  *     trait State
  *     case class One extends State
  *     case class Two extends State
@@ -312,24 +323,30 @@ trait FSM[S, D] extends Actor with Listeners with ActorLogging {
    * @param timeout state timeout for the initial state, overriding the default timeout for that state
    */
   final def startWith(stateName: S, stateData: D, timeout: Timeout = None): Unit =
-    currentState = FSM.State(stateName, stateData, timeout)
+    currentState = FSM.State(stateName, stateData, timeout)()
 
   /**
-   * Produce transition to other state. Return this from a state function in
+   * Produce transition to other state.
-   * order to effect the transition.
+   * Return this from a state function in order to effect the transition.
+   *
+   * This method always triggers transition events, even for `A -> A` transitions.
+   * If you want to stay in the same state without triggering an state transition event use [[#stay]] instead.
    *
    * @param nextStateName state designator for the next state
    * @return state transition descriptor
    */
-  final def goto(nextStateName: S): State = FSM.State(nextStateName, currentState.stateData)
+  final def goto(nextStateName: S): State = FSM.State(nextStateName, currentState.stateData)()
 
   /**
-   * Produce "empty" transition descriptor. Return this from a state function
+   * Produce "empty" transition descriptor.
-   * when no state change is to be effected.
+   * Return this from a state function when no state change is to be effected.
+   *
+   * No transition event will be triggered by [[#stay]].
+   * If you want to trigger an event like `S -> S` for [[#onTransition]] to handle use [[#goto]] instead.
    *
    * @return descriptor for staying in current state
    */
-  final def stay(): State = goto(currentState.stateName) // cannot directly use currentState because of the timeout field
+  final def stay(): State = goto(currentState.stateName).withNotification(false) // cannot directly use currentState because of the timeout field
 
   /**
    * Produce change descriptor to stop this FSM actor with reason "Normal".
@@ -624,7 +641,7 @@ trait FSM[S, D] extends Actor with Listeners with ActorLogging {
       terminate(stay withStopReason Failure("Next state %s does not exist".format(nextState.stateName)))
     } else {
       nextState.replies.reverse foreach { r ⇒ sender() ! r }
-      if (currentState.stateName != nextState.stateName) {
+      if (currentState.stateName != nextState.stateName || nextState.notifies) {
         this.nextState = nextState
         handleTransition(currentState.stateName, nextState.stateName)
         gossip(Transition(self, currentState.stateName, nextState.stateName))

akka-docs/rst/project/migration-guide-2.3.x-2.4.x.rst

@@ -38,6 +38,19 @@ If you have been creating EventStreams manually, you now have to provide an acto
 Please note that this change affects you only if you have implemented your own busses, Akka's own ``context.eventStream``
 is still there and does not require any attention from you concerning this change.
 
+FSM notifies on same state transitions
+======================================
+When changing states in an Finite-State-Machine Actor (``FSM``), state transition events are emitted and can be handled by the user
+either by registering ``onTransition`` handlers or by subscribing to these events by sending it an ``SubscribeTransitionCallBack`` message.
+
+Previously in ``2.3.x`` when an ``FSM`` was in state ``A`` and performed an ``goto(A)`` transition, no state transition notification would be sent.
+This is because it would effectively stay in the same state, and was deemed to be semantically equivalent to calling ``stay()``.
+
+In ``2.4.x`` when an ``FSM`` performs a any ``goto(X)`` transition, it will always trigger state transition events.
+Which turns out to be useful in many systems where same-state transitions actually should have an effect.
+
+In case you do *not* want to trigger a state transition event when effectively performing an ``X->X`` transition, use ``stay()`` instead.
+
 Removed Deprecated Features
 ===========================
 

akka-docs/rst/scala/fsm.rst

@@ -102,6 +102,13 @@ you of the direction of the state change which is being matched. During the
 state change, the old state data is available via ``stateData`` as shown, and
 the new state data would be available as ``nextStateData``.
 
+.. note::
+  Same-state transitions can be implemented (when currently in state ``S``) using
+  ``goto(S)`` or ``stay()``. The difference between those being that ``goto(S)`` will
+  emit an event ``S->S`` event that can be handled by ``onTransition``,
+  whereas ``stay()`` will *not*.
+
+
 To verify that this buncher actually works, it is quite easy to write a test
 using the :ref:`akka-testkit`, which is conveniently bundled with ScalaTest traits
 into ``AkkaSpec``:
@@ -327,8 +334,16 @@ External actors may be registered to be notified of state transitions by
 sending a message :class:`SubscribeTransitionCallBack(actorRef)`. The named
 actor will be sent a :class:`CurrentState(self, stateName)` message immediately
 and will receive :class:`Transition(actorRef, oldState, newState)` messages
-whenever a new state is reached. External monitors may be unregistered by
+whenever a state change is triggered.
-sending :class:`UnsubscribeTransitionCallBack(actorRef)` to the FSM actor.
+
+Please note that a state change includes the action of performing an ``goto(S)``, while
+already being state ``S``. In that case the monitoring actor will be notified with an
+``Transition(ref,S,S)`` message. This may be useful if your ``FSM`` should
+react on all (also same-state) transitions. In case you'd rather not emit events for same-state
+transitions use ``stay()`` instead of ``goto(S)``.
+
+External monitors may be unregistered by sending
+:class:`UnsubscribeTransitionCallBack(actorRef)` to the ``FSM`` actor.
 
 Stopping a listener without unregistering will not remove the listener from the
 subscription list; use :class:`UnsubscribeTransitionCallback` before stopping

akka-testkit/src/main/scala/akka/testkit/TestFSMRef.scala

@@ -62,7 +62,7 @@ class TestFSMRef[S, D, T <: Actor](
    * and stop handling.
    */
   def setState(stateName: S = fsm.stateName, stateData: D = fsm.stateData, timeout: FiniteDuration = null, stopReason: Option[FSM.Reason] = None) {
-    fsm.applyState(FSM.State(stateName, stateData, Option(timeout), stopReason))
+    fsm.applyState(FSM.State(stateName, stateData, Option(timeout), stopReason)())
   }
 
   /**