fixed racy ActorModelSpec (#4976)

fixed `A_dispatcher_must_handle_queuing_from_multiple_threads` - we were using the wrong message type the entire time, and the previous instance caused `Thread.Sleep` to be called repeatedly.

src/core/Akka.Tests/Actor/Dispatch/ActorModelSpec.cs

@@ -21,12 +21,17 @@
 using Akka.Util;
 using Akka.Util.Internal;
 using Xunit;
+using Xunit.Abstractions;
 
 namespace Akka.Tests.Actor.Dispatch
 {
     public abstract class ActorModelSpec : AkkaSpec
     {
-        protected ActorModelSpec(Config hocon) : base(hocon) { }
+        private readonly ITestOutputHelper _testOutputHelper;
+        protected ActorModelSpec(Config hocon, ITestOutputHelper output = null) : base(hocon, output)
+        {
+            _testOutputHelper = output;
+        }
 
         interface IActorModelMessage : INoSerializationVerificationNeeded { }
 
@@ -168,6 +173,13 @@ private DoubleStop() { }
             public static readonly DoubleStop Instance = new DoubleStop();
         }
 
+        private class GetStats : IActorModelMessage
+        {
+            private GetStats(){}
+
+            public static readonly GetStats Instance = new GetStats();
+        }
+
         sealed class ThrowException : IActorModelMessage
         {
             public ThrowException(Exception e)
@@ -184,7 +196,7 @@ public ThrowException(Exception e)
         class DispatcherActor : ReceiveActor
         {
             private Switch _busy = new Switch(false);
-
+            private readonly ILoggingAdapter _log = Context.GetLogger();
             private MessageDispatcherInterceptor _interceptor = Context.Dispatcher.AsInstanceOf<MessageDispatcherInterceptor>();
 
             private void Ack()
@@ -222,6 +234,11 @@ public DispatcherActor()
                 Receive<InterruptNicely>(interrupt => { Ack(); Sender.Tell(interrupt.Expect); _busy.SwitchOff(); });
                 Receive<ThrowException>(throwEx => { Ack(); _busy.SwitchOff(); throw throwEx.E; }, throwEx => true);
                 Receive<DoubleStop>(doubleStop => { Ack(); Context.Stop(Self); Context.Stop(Self); _busy.SwitchOff(); });
+                Receive<GetStats>(stats => {
+                    Ack();
+                    Sender.Tell(_interceptor.GetStats(Self));
+                    _busy.SwitchOff();
+                });
             }
         }
 
@@ -474,7 +491,7 @@ public void A_dispatcher_must_process_messages_one_at_a_time()
             AssertRefDefaultZero(a, registers: 1, msgsReceived: 3, msgsProcessed: 3, unregisters: 1, dispatcher: dispatcher);
         }
 
-        [Fact(Skip = "Racy on Azure DevOps")]
+        [Fact]
         public void A_dispatcher_must_handle_queuing_from_multiple_threads()
         {
             var dispatcher = InterceptedDispatcher();
@@ -487,14 +504,26 @@ public void A_dispatcher_must_handle_queuing_from_multiple_threads()
                 {
                     foreach (var c in Enumerable.Range(1, 20))
                     {
-                        a.Tell(new WaitAck(1, counter));
+                        a.Tell(new CountDown(counter));
                     }
                 });
             }
 
-            AssertCountdown(counter, (int)Dilated(TimeSpan.FromSeconds(3.0)).TotalMilliseconds, "Should process 200 messages");
-            AssertRefDefaultZero(a, dispatcher, registers: 1, msgsReceived: 200, msgsProcessed: 200);
-            Sys.Stop(a);
+            try
+            {
+                AssertCountdown(counter, (int)Dilated(TimeSpan.FromSeconds(3.0)).TotalMilliseconds,
+                    "Should process 200 messages");
+                AssertRefDefaultZero(a, dispatcher, registers: 1, msgsReceived: 200, msgsProcessed: 200);
+            }
+            finally
+            {
+                var stats = a.Ask<InterceptorStats>(GetStats.Instance).Result;
+                _testOutputHelper.WriteLine("Observed stats: {0}", stats);
+
+                Sys.Stop(a);
+            }
+
+
         }
 
         [Fact]
@@ -643,7 +672,7 @@ public class DispatcherModelSpec : ActorModelSpec
 
         ";
 
-        public DispatcherModelSpec() : base(DispatcherHocon) { }
+        public DispatcherModelSpec(ITestOutputHelper output) : base(DispatcherHocon, output) { }
 
         protected override MessageDispatcherInterceptor InterceptedDispatcher()
         {