Adds ActorRef.ignore (#28630)

akka-actor-testkit-typed/src/main/scala/akka/actor/testkit/typed/internal/ActorSystemStub.scala

@@ -73,7 +73,9 @@ import org.slf4j.LoggerFactory
   def isTerminated: Boolean = whenTerminated.isCompleted
 
   val deadLettersInbox = new DebugRef[Any](path.parent / "deadLetters", true)
-  override def deadLetters[U]: akka.actor.typed.ActorRef[U] = deadLettersInbox
+  override def deadLetters[U]: ActorRef[U] = deadLettersInbox
+
+  override def ignoreRef[U]: ActorRef[U] = deadLettersInbox
 
   val controlledExecutor = new ControlledExecutor
   implicit override def executionContext: scala.concurrent.ExecutionContextExecutor = controlledExecutor

akka-actor-typed-tests/src/test/java/jdocs/akka/typed/InteractionPatternsTest.java

@@ -99,6 +99,11 @@ public class InteractionPatternsTest extends JUnitSuite {
         // #request-response-send
         cookieFabric.tell(new CookieFabric.Request("give me cookies", context.getSelf()));
         // #request-response-send
+
+        // #ignore-reply
+        cookieFabric.tell(
+            new CookieFabric.Request("don't send cookies back", context.getSystem().ignoreRef()));
+        // #ignore-reply
       }
     }
 

akka-actor-typed-tests/src/test/scala/akka/actor/typed/ActorRefIgnoreSpec.scala

@@ -0,0 +1,147 @@
+/*
+ * Copyright (C) 2019-2020 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package akka.actor.typed
+
+import java.util.concurrent.TimeoutException
+
+import akka.actor.typed.scaladsl.Behaviors
+import org.scalatest.wordspec.AnyWordSpecLike
+import org.scalatest.concurrent.PatienceConfiguration.{ Timeout => PatienceTimeout }
+import akka.actor.testkit.typed.scaladsl.{ ScalaTestWithActorTestKit, TestProbe }
+import akka.actor.typed.scaladsl.adapter._
+import akka.actor.typed.scaladsl.AskPattern._
+import akka.util.Timeout
+
+import scala.concurrent.duration._
+import scala.util.{ Failure, Success }
+
+class ActorRefIgnoreSpec extends ScalaTestWithActorTestKit() with AnyWordSpecLike {
+
+  case class Request(replyTo: ActorRef[Int])
+
+  // this Actor behavior receives simple request and answers back total number of
+  // messages it received so far
+  val askMeActorBehavior: Behavior[Request] = {
+    def internalBehavior(counter: Int): Behavior[Request] =
+      Behaviors.receiveMessage[Request] {
+        case Request(replyTo) =>
+          val newCounter = counter + 1
+          replyTo ! newCounter
+          internalBehavior(newCounter)
+      }
+
+    internalBehavior(0)
+  }
+
+  /**
+   * This actor sends a ask to 'askMeRef' at bootstrap and forward the answer to the probe.
+   * We will use it through out this test.
+   */
+  def behavior(askMeRef: ActorRef[Request], probe: TestProbe[Int]) = Behaviors.setup[Int] { context =>
+    implicit val timeout: Timeout = 1.second
+
+    // send a message to interactWithRef
+    context.ask(askMeRef, Request) {
+      case Success(res) => res
+      case Failure(ex)  => throw ex
+    }
+
+    Behaviors.receiveMessage { num =>
+      // receive response from interactWithRef and sent to prob
+      probe.ref ! num
+      Behaviors.same
+    }
+  }
+
+  "IgnoreActorRef instance" should {
+
+    "ignore all incoming messages" in {
+
+      val askMeRef = testKit.spawn(askMeActorBehavior)
+
+      val probe = testKit.createTestProbe[Int]("response-probe")
+      askMeRef ! Request(probe.ref)
+      probe.expectMessage(1)
+
+      // this is more a compile-time proof
+      // since the reply is ignored, we can't check that a message was sent to it
+      askMeRef ! Request(testKit.system.ignoreRef)
+      probe.expectNoMessage()
+
+      // but we do check that the counter has increased when we used the ActorRef.ignore
+      askMeRef ! Request(probe.ref)
+      probe.expectMessage(3)
+    }
+
+    // this is kind of obvious, the Future won't complete because the ignoreRef is used
+    "make a Future timeout when used in a 'ask'" in {
+
+      implicit val timeout: Timeout = 500.millis
+      val askMeRef = testKit.spawn(askMeActorBehavior)
+
+      val failedAsk =
+        askMeRef
+          .ask { _: ActorRef[Request] =>
+            Request(testKit.system.ignoreRef) // <- pass the ignoreRef instead, so Future never completes
+          }
+          .failed
+          .futureValue(PatienceTimeout(1.second))
+
+      failedAsk shouldBe a[TimeoutException]
+    }
+
+    // similar to above, but using actor-to-actor interaction
+    "ignore messages when used in actor-to-actor interaction ('ask')" in {
+
+      val probe = testKit.createTestProbe[Int]("probe-response")
+
+      // this prove that the machinery works, probe will receive a response
+      val askMeRef = testKit.spawn(askMeActorBehavior)
+      testKit.spawn(behavior(askMeRef, probe))
+      probe.expectMessage(1)
+
+      // new interaction using ignoreRef, probe won't receive anything
+      val ignoreRef = testKit.system.ignoreRef[Request]
+      testKit.spawn(behavior(ignoreRef, probe))
+      probe.expectNoMessage()
+
+    }
+
+    "be watchable from another actor without throwing an exception" in {
+
+      val probe = testKit.createTestProbe[String]("probe-response")
+
+      val forwardMessageRef =
+        Behaviors.setup[String] { ctx =>
+          ctx.watch(testKit.system.ignoreRef[String])
+
+          Behaviors.receiveMessage { str =>
+            probe.ref ! str
+            Behaviors.same
+          }
+        }
+
+      // this proves that the actor started and is operational and 'watch' didn't impact it
+      val ref = testKit.spawn(forwardMessageRef)
+      ref ! "abc"
+      probe.expectMessage("abc")
+    }
+
+    "be a singleton" in {
+      withClue("using the same type") {
+        testKit.system.ignoreRef[String] shouldBe theSameInstanceAs(testKit.system.ignoreRef[String])
+      }
+
+      withClue("using different types") {
+        testKit.system.ignoreRef[String] shouldBe theSameInstanceAs(testKit.system.ignoreRef[Int])
+      }
+    }
+
+    "be adaptable back and forth to classic" in {
+      testKit.system.ignoreRef[String].toClassic.toTyped[String]
+    }
+
+  }
+}

akka-actor-typed-tests/src/test/scala/docs/akka/typed/InteractionPatternsSpec.scala

@@ -82,11 +82,19 @@ class InteractionPatternsSpec extends ScalaTestWithActorTestKit with AnyWordSpec
       val context = new {
         def self = probe.ref
       }
+
       // #request-response-send
       cookieFabric ! CookieFabric.Request("give me cookies", context.self)
       // #request-response-send
 
       probe.receiveMessage()
+
+      Behaviors.setup[Nothing] { context =>
+        // #ignore-reply
+        cookieFabric ! CookieFabric.Request("don't send cookies back", context.system.ignoreRef)
+        // #ignore-reply
+        Behaviors.empty
+      }
     }
 
     "contain a sample for adapted response" in {

akka-actor-typed/src/main/mima-filters/2.6.3.backwards.excludes/28630-ActorRef-ignore.excludes

@@ -0,0 +1,2 @@
+# Typed ActorRef that silently ignores all messages #25306 - PR #28630
+ProblemFilters.exclude[ReversedMissingMethodProblem]("akka.actor.typed.ActorSystem.ignoreRef")

akka-actor-typed/src/main/scala/akka/actor/typed/ActorRef.scala

@@ -7,7 +7,6 @@ package akka.actor.typed
 import akka.annotation.DoNotInherit
 import akka.{ actor => classic }
 import scala.annotation.unchecked.uncheckedVariance
-
 import akka.actor.typed.internal.InternalRecipientRef
 
 /**
@@ -66,7 +65,6 @@ object ActorRef {
      */
     def !(msg: T): Unit = ref.tell(msg)
   }
-
 }
 
 /**

akka-actor-typed/src/main/scala/akka/actor/typed/ActorRefResolver.scala

@@ -4,10 +4,8 @@
 
 package akka.actor.typed
 
-import akka.actor.ActorRefWithCell
-import akka.actor.ExtendedActorSystem
-import akka.annotation.DoNotInherit
-import akka.annotation.InternalApi
+import akka.actor.{ ActorRefWithCell, ExtendedActorSystem }
+import akka.annotation.{ DoNotInherit, InternalApi }
 
 object ActorRefResolver extends ExtensionId[ActorRefResolver] {
   def get(system: ActorSystem[_]): ActorRefResolver = apply(system)
@@ -75,8 +73,13 @@ abstract class ActorRefResolver extends Extension {
     }
   }
 
-  override def resolveActorRef[T](serializedActorRef: String): ActorRef[T] =
-    classicSystem.provider.resolveActorRef(serializedActorRef)
+  override def resolveActorRef[T](serializedActorRef: String): ActorRef[T] = {
+    val ref = classicSystem.provider.resolveActorRef(serializedActorRef)
+    if (ref eq classicSystem.provider.ignoreRef)
+      classicSystem.toTyped.ignoreRef
+    else
+      ref
+  }
 }
 
 object ActorRefResolverSetup {

akka-actor-typed/src/main/scala/akka/actor/typed/ActorSystem.scala

@@ -6,12 +6,12 @@ package akka.actor.typed
 
 import java.util.concurrent.{ CompletionStage, ThreadFactory }
 
-import akka.actor.{ Address, BootstrapSetup, ClassicActorSystemProvider }
 import akka.actor.setup.ActorSystemSetup
 import akka.actor.typed.eventstream.EventStream
-import akka.actor.typed.internal.{ EventStreamExtension, InternalRecipientRef }
 import akka.actor.typed.internal.adapter.{ ActorSystemAdapter, GuardianStartupBehavior, PropsAdapter }
+import akka.actor.typed.internal.{ EventStreamExtension, InternalRecipientRef }
 import akka.actor.typed.receptionist.Receptionist
+import akka.actor.{ Address, BootstrapSetup, ClassicActorSystemProvider }
 import akka.annotation.DoNotInherit
 import akka.util.Helpers.Requiring
 import akka.{ Done, actor => classic }
@@ -141,6 +141,11 @@ abstract class ActorSystem[-T] extends ActorRef[T] with Extensions with ClassicA
    */
   def deadLetters[U]: ActorRef[U]
 
+  /**
+   * An ActorRef that ignores any incoming messages.
+   */
+  def ignoreRef[U]: ActorRef[U]
+
   /**
    * Create a string representation of the actor hierarchy within this system
    * for debugging purposes.

akka-actor-typed/src/main/scala/akka/actor/typed/internal/adapter/ActorSystemAdapter.scala

@@ -76,6 +76,10 @@ import org.slf4j.{ Logger, LoggerFactory }
 
   // Members declared in akka.actor.typed.ActorSystem
   override def deadLetters[U]: ActorRef[U] = ActorRefAdapter(system.deadLetters)
+
+  private val cachedIgnoreRef: ActorRef[Nothing] = ActorRefAdapter(provider.ignoreRef)
+  override def ignoreRef[U]: ActorRef[U] = cachedIgnoreRef.unsafeUpcast[U]
+
   override def dispatchers: Dispatchers = new Dispatchers {
     override def lookup(selector: DispatcherSelector): ExecutionContextExecutor =
       selector match {
@@ -116,6 +120,7 @@ import org.slf4j.{ Logger, LoggerFactory }
   }
 
   override def address: Address = system.provider.getDefaultAddress
+
 }
 
 private[akka] object ActorSystemAdapter {

akka-actor/src/main/mima-filters/2.6.3.backwards.excludes/28630-ActorRef-ignore.excludes

@@ -0,0 +1,2 @@
+# Typed ActorRef that silently ignores all messages #25306 - PR #28630
+ProblemFilters.exclude[ReversedMissingMethodProblem]("akka.actor.ActorRefProvider.ignoreRef")

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

@@ -368,16 +368,26 @@ final class ChildActorPath private[akka] (val parent: ActorPath, val name: Strin
   }
 
   override def toStringWithAddress(addr: Address): String = {
-    val diff = addressStringLengthDiff(addr)
-    val length = toStringLength + diff
-    buildToString(new JStringBuilder(length), length, diff, _.toStringWithAddress(addr)).toString
+    if (IgnoreActorRef.isIgnoreRefPath(this)) {
+      // we never change address for IgnoreActorRef
+      this.toString
+    } else {
+      val diff = addressStringLengthDiff(addr)
+      val length = toStringLength + diff
+      buildToString(new JStringBuilder(length), length, diff, _.toStringWithAddress(addr)).toString
+    }
   }
 
   override def toSerializationFormatWithAddress(addr: Address): String = {
-    val diff = addressStringLengthDiff(addr)
-    val length = toStringLength + diff
-    val sb = buildToString(new JStringBuilder(length + 12), length, diff, _.toStringWithAddress(addr))
-    appendUidFragment(sb).toString
+    if (IgnoreActorRef.isIgnoreRefPath(this)) {
+      // we never change address for IgnoreActorRef
+      this.toString
+    } else {
+      val diff = addressStringLengthDiff(addr)
+      val length = toStringLength + diff
+      val sb = buildToString(new JStringBuilder(length + 12), length, diff, _.toStringWithAddress(addr))
+      appendUidFragment(sb).toString
+    }
   }
 
   private def addressStringLengthDiff(address: Address): Int = {

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

@@ -7,10 +7,10 @@ package akka.actor
 import java.util.concurrent.ConcurrentHashMap
 
 import akka.annotation.InternalApi
+
 import scala.annotation.tailrec
 import scala.collection.immutable
 import scala.util.control.NonFatal
-
 import akka.annotation.DoNotInherit
 import akka.dispatch._
 import akka.dispatch.sysmsg._
@@ -476,6 +476,53 @@ private[akka] trait MinimalActorRef extends InternalActorRef with LocalRef {
   protected def writeReplace(): AnyRef = SerializedActorRef(this)
 }
 
+/**
+ * An ActorRef that ignores any incoming messages.
+ *
+ * INTERNAL API
+ */
+@InternalApi private[akka] final class IgnoreActorRef(override val provider: ActorRefProvider) extends MinimalActorRef {
+
+  override val path: ActorPath = IgnoreActorRef.path
+
+  @throws(classOf[java.io.ObjectStreamException])
+  override protected def writeReplace(): AnyRef = SerializedIgnore
+}
+
+/**
+ * INTERNAL API
+ */
+@InternalApi private[akka] object IgnoreActorRef {
+
+  private val fakeSystemName = "local"
+
+  val path: ActorPath =
+    RootActorPath(Address("akka", IgnoreActorRef.fakeSystemName)) / "ignore"
+
+  private val pathString = path.toString
+
+  /**
+   * Check if the passed `otherPath` is the same as IgnoreActorRef.path
+   */
+  def isIgnoreRefPath(otherPath: String): Boolean =
+    pathString == otherPath
+
+  /**
+   * Check if the passed `otherPath` is the same as IgnoreActorRef.path
+   */
+  def isIgnoreRefPath(otherPath: ActorPath): Boolean =
+    path == otherPath
+
+}
+
+/**
+ * INTERNAL API
+ */
+@InternalApi @SerialVersionUID(1L) private[akka] object SerializedIgnore extends Serializable {
+  @throws(classOf[java.io.ObjectStreamException])
+  private def readResolve(): AnyRef = IgnoreActorRef
+}
+
 /**
  * Subscribe to this class to be notified about all [[DeadLetter]] (also the suppressed ones)
  * and [[Dropped]].

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

@@ -10,12 +10,12 @@ import akka.routing._
 import akka.event._
 import akka.util.Helpers
 import akka.util.Collections.EmptyImmutableSeq
+
 import scala.util.control.NonFatal
 import java.util.concurrent.atomic.AtomicLong
 
 import scala.concurrent.{ ExecutionContextExecutor, Future, Promise }
 import scala.annotation.implicitNotFound
-
 import akka.ConfigurationException
 import akka.annotation.DoNotInherit
 import akka.annotation.InternalApi
@@ -58,6 +58,9 @@ import akka.util.OptionVal
    */
   def deadLetters: ActorRef
 
+  /** INTERNAL API */
+  @InternalApi private[akka] def ignoreRef: ActorRef
+
   /**
    * The root path for all actors within this actor system, not including any remote address information.
    */
@@ -391,6 +394,8 @@ private[akka] class LocalActorRefProvider private[akka] (
       .getOrElse((p: ActorPath) => new DeadLetterActorRef(this, p, eventStream))
       .apply(rootPath / "deadLetters")
 
+  override val ignoreRef: ActorRef = new IgnoreActorRef(this)
+
   private[this] final val terminationPromise: Promise[Terminated] = Promise[Terminated]()
 
   def terminationFuture: Future[Terminated] = terminationPromise.future

akka-cluster-typed/src/test/scala/akka/cluster/typed/ActorRefIgnoreSerializationSpec.scala

@@ -0,0 +1,115 @@
+/*
+ * Copyright (C) 2020 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package akka.cluster.typed
+
+import akka.actor.typed.scaladsl.Behaviors
+import akka.actor.typed.scaladsl.adapter._
+import akka.actor.typed.{ ActorRef, ActorRefResolver, ActorSystem }
+import akka.actor.{ ExtendedActorSystem, IgnoreActorRef }
+import com.typesafe.config.ConfigFactory
+import org.scalatest.BeforeAndAfterAll
+import org.scalatest.concurrent.ScalaFutures
+import org.scalatest.matchers.should.Matchers
+import org.scalatest.wordspec.AnyWordSpec
+import akka.{ actor => classic }
+
+class ActorRefIgnoreSerializationSpec extends AnyWordSpec with ScalaFutures with Matchers with BeforeAndAfterAll {
+
+  private var system1: ActorSystem[String] = _
+  private var system2: ActorSystem[String] = _
+
+  val config = ConfigFactory.parseString(s"""
+      akka {
+        loglevel = debug
+        actor.provider = cluster
+        remote.classic.netty.tcp.port = 0
+        remote.artery {
+          canonical {
+            hostname = 127.0.0.1
+            port = 0
+          }
+        }
+      }
+      """)
+
+  override protected def beforeAll(): Unit = {
+    system1 = ActorSystem(Behaviors.empty[String], "sys1", config)
+    system2 = ActorSystem(Behaviors.empty[String], "sys2", config)
+  }
+
+  override protected def afterAll(): Unit = {
+    system1.terminate()
+    system2.terminate()
+  }
+
+  "ActorSystem.ignoreRef (in typed)" should {
+
+    "return a serializable ActorRef that can be sent between two ActorSystems using remote" in {
+
+      val ignoreRef = system1.ignoreRef[String]
+      val remoteRefStr = ActorRefResolver(system1).toSerializationFormat(ignoreRef)
+
+      withClue("check ActorRef path stays untouched, ie: /local/ignore") {
+        remoteRefStr shouldBe IgnoreActorRef.path.toString
+      }
+
+      withClue("check ActorRef path stays untouched when deserialized by another actor system") {
+        val deserRef: ActorRef[String] = ActorRefResolver(system2).resolveActorRef[String](remoteRefStr)
+        deserRef.path shouldBe IgnoreActorRef.path
+        (deserRef should be).theSameInstanceAs(system2.ignoreRef[String])
+      }
+    }
+
+    "return same instance when deserializing it twice (IgnoreActorRef is cached)" in {
+
+      val resolver = ActorRefResolver(system1)
+
+      withClue("using the same type") {
+        val deserRef1 = resolver.resolveActorRef[String](resolver.toSerializationFormat(system1.ignoreRef[String]))
+        val deserRef2 = resolver.resolveActorRef[String](resolver.toSerializationFormat(system1.ignoreRef[String]))
+        (deserRef1 should be).theSameInstanceAs(deserRef2)
+      }
+
+      withClue("using different types") {
+        val deserRef1 = resolver.resolveActorRef[String](resolver.toSerializationFormat(system1.ignoreRef[String]))
+        val deserRef2 = resolver.resolveActorRef[Int](resolver.toSerializationFormat(system1.ignoreRef[Int]))
+        (deserRef1 should be).theSameInstanceAs(deserRef2)
+      }
+
+    }
+  }
+
+  "IgnoreActorRef (in classic)" should {
+
+    "return a serializable ActorRef that can be sent between two ActorSystems using remote (akka classic)" in {
+
+      val ignoreRef = system1.ignoreRef[String].toClassic
+      val remoteRefStr = ignoreRef.path.toSerializationFormatWithAddress(system1.address)
+
+      withClue("check ActorRef path stays untouched, ie: /local/ignore") {
+        remoteRefStr shouldBe IgnoreActorRef.path.toString
+      }
+
+      withClue("check ActorRef path stays untouched when deserialized by another actor system") {
+        val providerSys2 = system2.classicSystem.asInstanceOf[ExtendedActorSystem].provider
+        val deserRef: classic.ActorRef = providerSys2.resolveActorRef(remoteRefStr)
+        deserRef.path shouldBe IgnoreActorRef.path
+        (deserRef should be).theSameInstanceAs(system2.ignoreRef[String].toClassic)
+      }
+    }
+
+    "return same instance when deserializing it twice (IgnoreActorRef is cached)" in {
+
+      val ignoreRef = system1.ignoreRef[String].toClassic
+      val remoteRefStr = ignoreRef.path.toSerializationFormat
+      val providerSys1 = system1.classicSystem.asInstanceOf[ExtendedActorSystem].provider
+
+      val deserRef1 = providerSys1.resolveActorRef(remoteRefStr)
+      val deserRef2 = providerSys1.resolveActorRef(remoteRefStr)
+      (deserRef1 should be).theSameInstanceAs(deserRef2)
+    }
+
+  }
+}

akka-docs/src/main/paradox/typed/interaction-patterns.md

@@ -76,7 +76,7 @@ Java
 :  @@snip [InteractionPatternsTest.java](/akka-actor-typed-tests/src/test/java/jdocs/akka/typed/InteractionPatternsTest.java) { #request-response-protocol }
 
 
-The sender would use its own @scala[`ActorRef[Response]`]@java[`ActorRef<Response>`], which it can access through @scala[`ActorContext.self`]@java[`ActorContext.getSelf()`], for the `respondTo`. 
+The sender would use its own @scala[`ActorRef[Response]`]@java[`ActorRef<Response>`], which it can access through @scala[`ActorContext.self`]@java[`ActorContext.getSelf()`], for the `replyTo`. 
 
 Scala
 :  @@snip [InteractionPatternsSpec.scala](/akka-actor-typed-tests/src/test/scala/docs/akka/typed/InteractionPatternsSpec.scala) { #request-response-send }
@@ -225,6 +225,31 @@ Java
  * There can only be a single response to one `ask` (see @ref:[per session child Actor](#per-session-child-actor))
  * When `ask` times out, the receiving actor does not know and may still process it to completion, or even start processing it after the fact
 
+## Ignoring replies
+
+In some situations an actor has a response for a particular request message but you are not interested in the response. In this case you can pass @scala[`system.ignoreRef`]@java[`system.ignoreRef()`] turning the request-response into a fire-and-forget.
+
+@scala[`system.ignoreRef`]@java[`system.ignoreRef()`], as the name indicates, returns an `ActorRef` that ignores any message sent to it.
+
+With the same protocol as the @ref[request response](#request-response) above, if the sender would prefer to ignore the reply it could pass @scala[`system.ignoreRef`]@java[`system.ignoreRef()`] for the `replyTo`, which it can access through @scala[`ActorContext.system.ignoreRef`]@java[`ActorContext.getSystem().ignoreRef()`]. 
+
+Scala
+:  @@snip [InteractionPatternsSpec.scala](/akka-actor-typed-tests/src/test/scala/docs/akka/typed/InteractionPatternsSpec.scala) { #ignore-reply }
+
+Java
+:  @@snip [InteractionPatternsTest.java](/akka-actor-typed-tests/src/test/java/jdocs/akka/typed/InteractionPatternsTest.java) { #ignore-reply }
+
+**Useful when:**
+
+  * Sending a message for which the protocol defines a reply, but you are not interested in getting the reply
+
+**Problems:**
+
+The returned `ActorRef` ignores all messages sent to it, therefore it should be used carefully.
+ * Passing it around inadvertently as if it was a normal `ActorRef` may result in broken actor-to-actor interactions.
+ * Using it when performing an `ask` from outside the Actor System will cause the @scala[`Future`]@java[`CompletionStage`] returned by the `ask` to timeout since it will never complete.
+ * Finally, it's legal to `watch` it, but since it's of a special kind, it never terminates and therefore you will never receive a `Terminated` signal from it.
+
 ## Send Future result to self
 
 When using an API that returns a @scala[`Future`]@java[`CompletionStage`] from an actor it's common that you would

akka-remote/src/main/scala/akka/remote/RemoteActorRefProvider.scala

@@ -187,6 +187,7 @@ private[akka] class RemoteActorRefProvider(
 
   override def rootPath: ActorPath = local.rootPath
   override def deadLetters: InternalActorRef = local.deadLetters
+  override def ignoreRef: ActorRef = local.ignoreRef
 
   // these are only available after init()
   override def rootGuardian: InternalActorRef = local.rootGuardian
@@ -524,6 +525,9 @@ private[akka] class RemoteActorRefProvider(
    * public `resolveActorRef(path: String)`.
    */
   private[akka] def internalResolveActorRef(path: String): ActorRef = path match {
+
+    case p if IgnoreActorRef.isIgnoreRefPath(p) => this.ignoreRef
+
     case ActorPathExtractor(address, elems) =>
       if (hasAddress(address)) local.resolveActorRef(rootGuardian, elems)
       else {
@@ -542,6 +546,7 @@ private[akka] class RemoteActorRefProvider(
             new EmptyLocalActorRef(this, rootPath, eventStream)
         }
       }
+
     case _ =>
       log.debug("Resolve (deserialization) of unknown (invalid) path [{}], using deadLetters.", path)
       deadLetters

build.sbt

@@ -455,7 +455,10 @@ lazy val actorTestkitTyped = akkaModule("akka-actor-testkit-typed")
   .settings(Dependencies.actorTestkitTyped)
 
 lazy val actorTypedTests = akkaModule("akka-actor-typed-tests")
-  .dependsOn(actorTyped, actorTestkitTyped % "compile->compile;test->test")
+  .dependsOn(
+    actorTyped,
+    actorTestkitTyped % "compile->compile;test->test"
+  )
   .settings(AkkaBuild.mayChangeSettings)
   .disablePlugins(MimaPlugin)
   .enablePlugins(NoPublish)