feat: Add SchedulerTask which will be notified once cancelled.

actor-tests/src/test/scala/org/apache/pekko/actor/SchedulerSpec.scala

@@ -131,6 +131,17 @@ trait SchedulerSpec extends BeforeAndAfterEach with DefaultTimeout with Implicit
         task.isCancelled should ===(true)
       }
 
+      "notify callback if cancel is performed before execution" taggedAs TimingTest in {
+        val latch = new CountDownLatch(1)
+        val task = system.scheduler.scheduleOnce(100 millis,
+          new SchedulerTask {
+            override def run(): Unit = ()
+            override def cancelled(): Unit = latch.countDown()
+          })
+        task.cancel()
+        latch.await(100, TimeUnit.MILLISECONDS) should ===(true)
+      }
+
       "not be canceled if cancel is performed after execution" taggedAs TimingTest in {
         val latch = TestLatch(1)
         val task = collectCancellable(system.scheduler.scheduleOnce(10.millis)(latch.countDown()))
@@ -334,6 +345,23 @@ trait SchedulerSpec extends BeforeAndAfterEach with DefaultTimeout with Implicit
           ticks.get should ===(1)
         }
 
+        "notify callback if cancel is performed after initial delay" taggedAs TimingTest in {
+          val latch = new CountDownLatch(1)
+          val initialDelay = 90.millis.dilated
+          val delay = 500.millis.dilated
+          val task = system.scheduler.scheduleWithFixedDelay(
+            initialDelay,
+            delay)(
+            new SchedulerTask {
+              override def run(): Unit = ()
+              override def cancelled(): Unit = latch.countDown()
+            })
+
+          Thread.sleep((initialDelay + 200.millis.dilated).toMillis)
+          task.cancel()
+          latch.await(100, TimeUnit.MILLISECONDS) should ===(true)
+        }
+
         /**
          * ticket #307
          */

actor/src/main/scala/org/apache/pekko/actor/LightArrayRevolverScheduler.scala

@@ -139,7 +139,7 @@ class LightArrayRevolverScheduler(config: Config, log: LoggingAdapter, threadFac
       final override protected def scheduledFirst(): Cancellable =
         schedule(
           executor,
-          new AtomicLong(clock() + initialDelay.toNanos) with Runnable {
+          new AtomicLong(clock() + initialDelay.toNanos) with SchedulerTask {
             override def run(): Unit = {
               try {
                 runnable.run()
@@ -150,6 +150,11 @@ class LightArrayRevolverScheduler(config: Config, log: LoggingAdapter, threadFac
                 case _: SchedulerException => // ignore failure to enqueue or terminated target actor
               }
             }
+
+            override def cancelled(): Unit = runnable match {
+              case task: SchedulerTask => task.cancelled()
+              case _                   =>
+            }
           },
           roundUp(initialDelay))
     }
@@ -390,7 +395,18 @@ object LightArrayRevolverScheduler {
 
     override def cancel(): Boolean = extractTask(CancelledTask) match {
       case ExecutedTask | CancelledTask => false
-      case _                            => true
+      case task: SchedulerTask =>
+        notifyCancellation(task)
+        true
+      case _ => true
+    }
+
+    private def notifyCancellation(task: SchedulerTask): Unit = {
+      try {
+        executionContext.execute(() => task.cancelled())
+      } catch {
+        case NonFatal(e) => executionContext.reportFailure(e)
+      }
     }
 
     override def isCancelled: Boolean = task eq CancelledTask

actor/src/main/scala/org/apache/pekko/actor/Scheduler.scala

@@ -85,7 +85,7 @@ trait Scheduler {
     final override protected def scheduledFirst(): Cancellable =
       scheduleOnce(
         initialDelay,
-        new Runnable {
+        new SchedulerTask {
           override def run(): Unit = {
             try {
               runnable.run()
@@ -97,6 +97,11 @@ trait Scheduler {
               case e: IllegalStateException if e.getCause != null && e.getCause.isInstanceOf[SchedulerException] =>
             }
           }
+
+          override def cancelled(): Unit = runnable match {
+            case task: SchedulerTask => task.cancelled()
+            case _                   =>
+          }
         })
   }
 
@@ -498,6 +503,18 @@ trait Scheduler {
 // this one is just here so we can present a nice AbstractScheduler for Java
 abstract class AbstractSchedulerBase extends Scheduler
 
+/**
+ * A Task that will be notified when it is cancelled.
+ */
+trait SchedulerTask extends Runnable {
+
+  /**
+   * Called for [[SchedulerTask]]s that are successfully canceled via [[Cancellable#cancel]].
+   * Overriding this method allows to for example run some cleanup.
+   */
+  def cancelled(): Unit = ()
+}
+
 /**
  * Signifies something that can be cancelled
  * There is no strict guarantee that the implementation is thread-safe,