=per #15937 Allow PersistentActor to be used as a stackable modification

* PersistentActor correctly calls `super.around*` and allows to be mixed into e.g. ActorSubscriber
 * Tests have been added on PersistentActor and Processor to verify the stackable behavior of `around*` and `pre*` methods
 * Delegation in Processor has been simplified

akka-persistence/src/test/scala/akka/persistence/PersistentActorSpec.scala

@@ -13,6 +13,7 @@ import akka.testkit.EventFilter
 import akka.testkit.TestProbe
 import java.util.concurrent.atomic.AtomicInteger
 import scala.util.Random
+import scala.util.control.NoStackTrace
 
 object PersistentActorSpec {
   final case class Cmd(data: Any)
@@ -403,6 +404,95 @@ object PersistentActorSpec {
     }
   }
 
+  class StackableTestPersistentActor(val probe: ActorRef) extends StackableTestPersistentActor.BaseActor with PersistentActor with StackableTestPersistentActor.MixinActor {
+    override def persistenceId: String = "StackableTestPersistentActor"
+
+    def receiveCommand = {
+      case "restart" ⇒ throw new Exception("triggering restart") with NoStackTrace { override def toString = "Boom!" }
+    }
+
+    def receiveRecover = {
+      case _ ⇒ ()
+    }
+
+    override def preStart(): Unit = {
+      probe ! "preStart"
+      super.preStart()
+    }
+
+    override def postStop(): Unit = {
+      probe ! "postStop"
+      super.postStop()
+    }
+
+    override def preRestart(reason: Throwable, message: Option[Any]): Unit = {
+      probe ! "preRestart"
+      super.preRestart(reason, message)
+    }
+
+    override def postRestart(reason: Throwable): Unit = {
+      probe ! "postRestart"
+      super.postRestart(reason)
+    }
+
+  }
+
+  object StackableTestPersistentActor {
+    trait BaseActor extends Actor { this: StackableTestPersistentActor ⇒
+      override protected[akka] def aroundPreStart() = {
+        probe ! "base aroundPreStart"
+        super.aroundPreStart()
+      }
+
+      override protected[akka] def aroundPostStop() = {
+        probe ! "base aroundPostStop"
+        super.aroundPostStop()
+      }
+
+      override protected[akka] def aroundPreRestart(reason: Throwable, message: Option[Any]) = {
+        probe ! "base aroundPreRestart"
+        super.aroundPreRestart(reason, message)
+      }
+
+      override protected[akka] def aroundPostRestart(reason: Throwable) = {
+        probe ! "base aroundPostRestart"
+        super.aroundPostRestart(reason)
+      }
+
+      override protected[akka] def aroundReceive(receive: Receive, message: Any) = {
+        probe ! "base aroundReceive"
+        super.aroundReceive(receive, message)
+      }
+    }
+
+    trait MixinActor extends Actor { this: StackableTestPersistentActor ⇒
+      override protected[akka] def aroundPreStart() = {
+        probe ! "mixin aroundPreStart"
+        super.aroundPreStart()
+      }
+
+      override protected[akka] def aroundPostStop() = {
+        probe ! "mixin aroundPostStop"
+        super.aroundPostStop()
+      }
+
+      override protected[akka] def aroundPreRestart(reason: Throwable, message: Option[Any]) = {
+        probe ! "mixin aroundPreRestart"
+        super.aroundPreRestart(reason, message)
+      }
+
+      override protected[akka] def aroundPostRestart(reason: Throwable) = {
+        probe ! "mixin aroundPostRestart"
+        super.aroundPostRestart(reason)
+      }
+
+      override protected[akka] def aroundReceive(receive: Receive, message: Any) = {
+        if (message == "restart" && recoveryFinished) { probe ! "mixin aroundReceive" }
+        super.aroundReceive(receive, message)
+      }
+    }
+  }
+
 }
 
 abstract class PersistentActorSpec(config: Config) extends AkkaSpec(config) with PersistenceSpec with ImplicitSender {
@@ -770,7 +860,35 @@ abstract class PersistentActorSpec(config: Config) extends AkkaSpec(config) with
       processor2 ! GetState
       expectMsg(List("a-1", "a-2", "b-41", "b-42", "c-41", "c-42", RecoveryCompleted))
     }
+    "be used as a stackable modification" in {
+      val persistentActor = system.actorOf(Props(classOf[StackableTestPersistentActor], testActor))
+      expectMsg("mixin aroundPreStart")
+      expectMsg("base aroundPreStart")
+      expectMsg("preStart")
+
+      persistentActor ! "restart"
+      expectMsg("mixin aroundReceive")
+      expectMsg("base aroundReceive")
+
+      expectMsg("mixin aroundPreRestart")
+      expectMsg("base aroundPreRestart")
+      expectMsg("preRestart")
+      expectMsg("postStop")
+
+      expectMsg("mixin aroundPostRestart")
+      expectMsg("base aroundPostRestart")
+      expectMsg("postRestart")
+      expectMsg("preStart")
+
+      persistentActor ! PoisonPill
+      expectMsg("mixin aroundPostStop")
+      expectMsg("base aroundPostStop")
+      expectMsg("postStop")
+
+      expectNoMsg(100.millis)
+    }
   }
+
 }
 
 class LeveldbPersistentActorSpec extends PersistentActorSpec(PersistenceSpec.config("leveldb", "LeveldbPersistentActorSpec"))