feat: Add consistent exception handling in actor systems

This change makes exception handling in actor systems consistent.
Implementors should not propagate unhandled exceptions in actors to the
(one of the) threads running the engine, but instead should stop the
offending actor and continue running.

Supervision of actors should be implemented within actor behavior
instead. Since BehaviorInterpreter will propagate exceptions thrown by
an actor, consumers of that API can use it to catch unhandled exceptions in an
actor.

1 files changed, 17 insertions, 3 deletions

diff --git a/odcsim-engine-omega/src/main/kotlin/com/atlarge/odcsim/engine/omega/OmegaActorSystem.kt b/odcsim-engine-omega/src/main/kotlin/com/atlarge/odcsim/engine/omega/OmegaActorSystem.kt
index 56e3020f..c4a9b35f 100644
--- a/odcsim-engine-omega/src/main/kotlin/com/atlarge/odcsim/engine/omega/OmegaActorSystem.kt
+++ b/odcsim-engine-omega/src/main/kotlin/com/atlarge/odcsim/engine/omega/OmegaActorSystem.kt
@@ -35,6 +35,7 @@ import com.atlarge.odcsim.PostStop
 import com.atlarge.odcsim.PreStart
 import com.atlarge.odcsim.Signal
 import com.atlarge.odcsim.internal.BehaviorInterpreter
+import mu.KotlinLogging
 import java.util.PriorityQueue
 import kotlin.math.max
 
@@ -75,6 +76,11 @@ class OmegaActorSystem<in T : Any>(root: Behavior<T>, override val name: String)
      */
     private val registry: MutableMap<ActorPath, Actor<*>> = HashMap()
 
+    /**
+     * A [KotlinLogging] instance that writes logs to a SLF4J implementation.
+     */
+    private val logger = KotlinLogging.logger {}
+
     init {
         registry[path] = Actor(this, root)
         schedule(this, PreStart, .0)
@@ -100,8 +106,16 @@ class OmegaActorSystem<in T : Any>(root: Behavior<T>, override val name: String)
             time = delivery
             queue.poll()
 
-            // Notice that messages for unknown/terminated actors are ignored for now
-            registry[envelope.destination]?.interpretMessage(envelope.message)
+            val actor = registry[envelope.destination] ?: continue
+            try {
+                // Notice that messages for unknown/terminated actors are ignored for now
+                actor.interpretMessage(envelope.message)
+            } catch (e: Exception) {
+                // Forcefully stop the actor if it crashed
+                actor.stop()
+
+                logger.error(e) { "Unhandled exception in actor ${envelope.destination}" }
+            }
         }
 
         // Jump forward in time as the caller expects the system to have run until the specified instant
@@ -171,7 +185,7 @@ class OmegaActorSystem<in T : Any>(root: Behavior<T>, override val name: String)
          * Terminate this actor and its children.
          */
         fun terminate() {
-            children.forEach { it.terminate() }
+            children.forEach { it.stop() }
             registry.remove(self.path)
         }