feat: Add support for watching actor termination

This change adds support for tracking termination of actors in the
system.

5 files changed, 35 insertions, 4 deletions

diff --git a/odcsim-core/src/main/kotlin/com/atlarge/odcsim/ActorContext.kt b/odcsim-core/src/main/kotlin/com/atlarge/odcsim/ActorContext.kt
index 3a704f32..8ea4a09d 100644
--- a/odcsim-core/src/main/kotlin/com/atlarge/odcsim/ActorContext.kt
+++ b/odcsim-core/src/main/kotlin/com/atlarge/odcsim/ActorContext.kt
@@ -84,6 +84,23 @@ interface ActorContext<T : Any> {
     fun stop(child: ActorRef<*>): Boolean
 
     /**
+     * Watch the specified [ActorRef] for termination of the referenced actor. On termination of the watched actor,
+     * a [Terminated] signal is sent to this actor.
+     *
+     * @param target The target actor to watch.
+     */
+    fun watch(target: ActorRef<*>)
+
+    /**
+     * Revoke the registration established by [watch].
+     *
+     * In case there exists no registration for the specified [target], no action will be performed.
+     *
+     * @param target The target actor to unwatch.
+     */
+    fun unwatch(target: ActorRef<*>)
+
+    /**
      * Synchronize the local virtual time of this target with the other referenced actor's local virtual time.
      *
      * By default, actors are not guaranteed to be synchronized, meaning that for some implementations, virtual time may
diff --git a/odcsim-core/src/main/kotlin/com/atlarge/odcsim/Behavior.kt b/odcsim-core/src/main/kotlin/com/atlarge/odcsim/Behavior.kt
index 8298ed28..9ad7f83f 100644
--- a/odcsim-core/src/main/kotlin/com/atlarge/odcsim/Behavior.kt
+++ b/odcsim-core/src/main/kotlin/com/atlarge/odcsim/Behavior.kt
@@ -167,7 +167,7 @@ val <T : Any> Behavior<T>.isAlive get() = this !is StoppedBehavior
 /**
  * A flag to indicate whether the last message/signal went unhandled.
  */
-val <T : Any> Behavior<T>.isUnhandled get() = this !is UnhandledBehavior
+val <T : Any> Behavior<T>.isUnhandled get() = this is UnhandledBehavior
 
 // The special behaviors are kept in this file as to be able to seal the Behavior class to prevent users from extending
 // it.
diff --git a/odcsim-core/src/main/kotlin/com/atlarge/odcsim/Signals.kt b/odcsim-core/src/main/kotlin/com/atlarge/odcsim/Signals.kt
index 35aad18b..e393c6a1 100644
--- a/odcsim-core/src/main/kotlin/com/atlarge/odcsim/Signals.kt
+++ b/odcsim-core/src/main/kotlin/com/atlarge/odcsim/Signals.kt
@@ -48,4 +48,12 @@ object PostStop : Signal
  *
  * @property target The target object that has timed out.
  */
-class Timeout(val target: Any) : Signal
+data class Timeout(val target: Any) : Signal
+
+/**
+ * A lifecycle signal to indicate that an actor that was watched has terminated.
+ *
+ * @property ref The reference to the actor that has terminated.
+ * @property failure  The failure that caused the termination, or `null` on graceful termination.
+ */
+data class Terminated(val ref: ActorRef<*>, val failure: Throwable? = null) : Signal
diff --git a/odcsim-core/src/main/kotlin/com/atlarge/odcsim/coroutines/dsl/Timeout.kt b/odcsim-core/src/main/kotlin/com/atlarge/odcsim/coroutines/dsl/Timeout.kt
index 1d91f5b8..694a107c 100644
--- a/odcsim-core/src/main/kotlin/com/atlarge/odcsim/coroutines/dsl/Timeout.kt
+++ b/odcsim-core/src/main/kotlin/com/atlarge/odcsim/coroutines/dsl/Timeout.kt
@@ -28,6 +28,7 @@ import com.atlarge.odcsim.Duration
 import com.atlarge.odcsim.Timeout
 import com.atlarge.odcsim.coroutines.SuspendingActorContext
 import com.atlarge.odcsim.coroutines.suspendWithBehavior
+import com.atlarge.odcsim.internal.sendSignal
 import com.atlarge.odcsim.receiveSignal
 import com.atlarge.odcsim.setup
 import com.atlarge.odcsim.unhandled
@@ -43,7 +44,7 @@ suspend fun <T : Any> SuspendingActorContext<T>.timeout(after: Duration) =
         setup { ctx ->
             val target = this
             @Suppress("UNCHECKED_CAST")
-            ctx.send(ctx.self, Timeout(target) as T, after)
+            ctx.sendSignal(ctx.self, Timeout(target), after)
             receiveSignal { _, signal ->
                 if (signal is Timeout && signal.target == target) {
                     cont.resume(Unit)
diff --git a/odcsim-core/src/main/kotlin/com/atlarge/odcsim/internal/Coroutines.kt b/odcsim-core/src/main/kotlin/com/atlarge/odcsim/internal/Coroutines.kt
index 8d533e9e..a9e20d0e 100644
--- a/odcsim-core/src/main/kotlin/com/atlarge/odcsim/internal/Coroutines.kt
+++ b/odcsim-core/src/main/kotlin/com/atlarge/odcsim/internal/Coroutines.kt
@@ -68,7 +68,8 @@ interface SuspendingActorContextImpl<T : Any> : SuspendingActorContext<T> {
  */
 internal class SuspendingBehaviorImpl<T : Any>(
     private var actorContext: ActorContext<T>,
-    initialBehavior: SuspendingBehavior<T>) : ReceivingBehavior<T>(), SuspendingActorContextImpl<T> {
+    initialBehavior: SuspendingBehavior<T>
+) : ReceivingBehavior<T>(), SuspendingActorContextImpl<T> {
 
     /**
      * The next behavior to use.
@@ -106,6 +107,10 @@ internal class SuspendingBehaviorImpl<T : Any>(
 
     override fun stop(child: ActorRef<*>): Boolean = actorContext.stop(child)
 
+    override fun watch(target: ActorRef<*>) = actorContext.watch(target)
+
+    override fun unwatch(target: ActorRef<*>) = actorContext.unwatch(target)
+
     override fun sync(target: ActorRef<*>) = actorContext.sync(target)
 
     override fun unsync(target: ActorRef<*>) = actorContext.unsync(target)