Merge pull request #18772 from ktoso/wip-persist-nil-ktoso

=per #18728 allow persistAll(Nil)

akka-persistence/src/main/scala/akka/persistence/Eventsourced.scala

@@ -303,11 +303,13 @@ private[persistence] trait Eventsourced extends Snapshotter with Stash with Stas
    * @param handler handler for each persisted `events`
    */
   def persistAll[A](events: immutable.Seq[A])(handler: A ⇒ Unit): Unit = {
-    events.foreach { event ⇒
-      pendingStashingPersistInvocations += 1
-      pendingInvocations addLast StashingHandlerInvocation(event, handler.asInstanceOf[Any ⇒ Unit])
+    if (events.nonEmpty) {
+      events.foreach { event ⇒
+        pendingStashingPersistInvocations += 1
+        pendingInvocations addLast StashingHandlerInvocation(event, handler.asInstanceOf[Any ⇒ Unit])
+      }
+      eventBatch = AtomicWrite(events.map(PersistentRepr.apply(_, sender = sender()))) :: eventBatch
     }
-    eventBatch = AtomicWrite(events.map(PersistentRepr.apply(_, sender = sender()))) :: eventBatch
   }
 
   @deprecated("use persistAll instead", "2.4")
@@ -350,12 +352,13 @@ private[persistence] trait Eventsourced extends Snapshotter with Stash with Stas
    * @param events events to be persisted
    * @param handler handler for each persisted `events`
    */
-  def persistAllAsync[A](events: immutable.Seq[A])(handler: A ⇒ Unit): Unit = {
-    events.foreach { event ⇒
-      pendingInvocations addLast AsyncHandlerInvocation(event, handler.asInstanceOf[Any ⇒ Unit])
+  def persistAllAsync[A](events: immutable.Seq[A])(handler: A ⇒ Unit): Unit =
+    if (events.nonEmpty) {
+      events.foreach { event ⇒
+        pendingInvocations addLast AsyncHandlerInvocation(event, handler.asInstanceOf[Any ⇒ Unit])
+      }
+      eventBatch = AtomicWrite(events.map(PersistentRepr.apply(_, sender = sender()))) :: eventBatch
     }
-    eventBatch = AtomicWrite(events.map(PersistentRepr.apply(_, sender = sender()))) :: eventBatch
-  }
 
   @deprecated("use persistAllAsync instead", "2.4")
   def persistAsync[A](events: immutable.Seq[A])(handler: A ⇒ Unit): Unit =

akka-persistence/src/main/scala/akka/persistence/Persistent.scala

@@ -4,13 +4,13 @@
 
 package akka.persistence
 
-import scala.collection.immutable
 import java.lang.{ Iterable ⇒ JIterable }
 import java.util.{ List ⇒ JList }
-import akka.actor.{ ActorContext, ActorRef }
-import akka.pattern.PromiseActorRef
+
+import akka.actor.{ ActorRef, NoSerializationVerificationNeeded }
 import akka.persistence.serialization.Message
-import akka.actor.NoSerializationVerificationNeeded
+
+import scala.collection.immutable
 
 /**
  * INTERNAL API
@@ -38,10 +38,11 @@ object AtomicWrite {
 }
 
 final case class AtomicWrite(payload: immutable.Seq[PersistentRepr]) extends PersistentEnvelope with Message {
+  require(payload.nonEmpty, "payload of AtomicWrite must not be empty!")
 
   // only check that all persistenceIds are equal when there's more than one in the Seq
   if (payload match {
-    case l: List[PersistentRepr]   ⇒ l.tail.nonEmpty
+    case l: List[PersistentRepr]   ⇒ l.tail.nonEmpty // avoids calling .size
     case v: Vector[PersistentRepr] ⇒ v.size > 1
     case _                         ⇒ true // some other collection type, let's just check
   }) require(payload.forall(_.persistenceId == payload.head.persistenceId),

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

@@ -248,6 +248,22 @@ object PersistentActorSpec {
         persistAsync(event) { evt ⇒ sender() ! s"${evt.data}-b-${sendMsgCounter.incrementAndGet()}" }
     }
   }
+  class PersistAllNilPersistentActor(name: String) extends ExamplePersistentActor(name) {
+
+    val receiveCommand: Receive = commonBehavior orElse {
+      case Cmd(data: String) if data contains "defer" ⇒
+        deferAsync("before-nil")(sender() ! _)
+        persistAll(Nil)(_ ⇒ sender() ! "Nil")
+        deferAsync("after-nil")(sender() ! _)
+        sender() ! data
+
+      case Cmd(data: String) if data contains "persist" ⇒
+        persist("before-nil")(sender() ! _)
+        persistAll(Nil)(_ ⇒ sender() ! "Nil")
+        deferAsync("after-nil")(sender() ! _)
+        sender() ! data
+    }
+  }
   class AsyncPersistAndPersistMixedSyncAsyncSyncPersistentActor(name: String) extends ExamplePersistentActor(name) {
 
     var counter = 0
@@ -785,6 +801,17 @@ abstract class PersistentActorSpec(config: Config) extends PersistenceSpec(confi
       expectMsg("x-b-2")
       expectNoMsg(100.millis)
     }
+    "support calling persistAll with Nil" in {
+      val persistentActor = namedPersistentActor[PersistAllNilPersistentActor]
+      persistentActor ! Cmd("defer-x")
+      expectMsg("before-nil")
+      expectMsg("after-nil")
+      expectMsg("defer-x")
+      persistentActor ! Cmd("persist-x")
+      expectMsg("persist-x")
+      expectMsg("before-nil")
+      expectMsg("after-nil")
+    }
     "support a mix of persist calls (sync, async, sync) and persist calls in expected order" in {
       val persistentActor = namedPersistentActor[AsyncPersistAndPersistMixedSyncAsyncSyncPersistentActor]
       persistentActor ! Cmd("a")