Fix leaky coordinated shutdown

src/core/Akka/Actor/ActorSystem.cs

@@ -364,7 +364,7 @@ private static ActorSystem CreateAndStartSystem(string name, Config withFallback
         /// </returns>
         public abstract Task Terminate();
 
-        internal abstract void FinalTerminate();
+        internal abstract Task FinalTerminate();
 
         /// <summary>
         /// Returns a task which will be completed after the <see cref="ActorSystem"/> has been

src/core/Akka/Actor/CoordinatedShutdown.cs

@@ -251,7 +251,7 @@ private ClusterJoinUnsuccessfulReason() { }
         /// <summary>
         /// The <see cref="ActorSystem"/>
         /// </summary>
-        public ExtendedActorSystem System { get; }
+        public ExtendedActorSystem System { get; private set; }
 
         /// <summary>
         /// The set of named <see cref="Phase"/>s that will be executed during coordinated shutdown.
@@ -261,7 +261,7 @@ private ClusterJoinUnsuccessfulReason() { }
         /// <summary>
         /// INTERNAL API
         /// </summary>
-        internal ILoggingAdapter Log { get; }
+        internal ILoggingAdapter Log { get; private set; }
 
         private readonly HashSet<string> _knownPhases;
 
@@ -270,7 +270,7 @@ private ClusterJoinUnsuccessfulReason() { }
         /// </summary>
         internal readonly List<string> OrderedPhases;
 
-        private readonly ConcurrentBag<Func<Task<Done>>> _clrShutdownTasks = new ConcurrentBag<Func<Task<Done>>>();
+        private readonly ConcurrentSet<Func<Task<Done>>> _clrShutdownTasks = new ConcurrentSet<Func<Task<Done>>>();
         private readonly ConcurrentDictionary<string, ImmutableList<(string, Func<Task<Done>>)>> _tasks = new ConcurrentDictionary<string, ImmutableList<(string, Func<Task<Done>>)>>();
         private readonly AtomicReference<Reason> _runStarted = new AtomicReference<Reason>(null);
         private readonly AtomicBoolean _clrHooksStarted = new AtomicBoolean(false);
@@ -335,7 +335,7 @@ internal void AddClrShutdownHook(Func<Task<Done>> hook)
         {
             if (!_clrHooksStarted)
             {
-                _clrShutdownTasks.Add(hook);
+                _clrShutdownTasks.TryAdd(hook);
             }
         }
 
@@ -653,13 +653,16 @@ internal static void InitPhaseActorSystemTerminate(ActorSystem system, Config co
 
                     if (terminateActorSystem)
                     {
-                        system.FinalTerminate();
-                        return system.Terminate().ContinueWith(tr =>
+                        return system.FinalTerminate().ContinueWith(tr =>
                         {
                             if (exitClr && !coord._runningClrHook)
                             {
                                 Environment.Exit(0);
                             }
+
+                            coord.System = null;
+                            coord.Log = null;
+                            coord._tasks.Clear(); // Clear the dictionary, just in case it is retained in memory
                             return Done.Instance;
                         });
                     }
@@ -691,7 +694,11 @@ internal static void InitClrHook(ActorSystem system, Config conf, CoordinatedShu
                 var exitTask = TerminateOnClrExit(coord);
                 // run all hooks during termination sequence
                 AppDomain.CurrentDomain.ProcessExit += exitTask;
-                system.WhenTerminated.ContinueWith(tr => { AppDomain.CurrentDomain.ProcessExit -= exitTask; });
+                system.WhenTerminated.ContinueWith(tr =>
+                {
+                    AppDomain.CurrentDomain.ProcessExit -= exitTask;
+                    coord._clrShutdownTasks.Clear(); // Clear the tasks, just in case it is retained in memory
+                });
 
                 coord.AddClrShutdownHook(() =>
                 {

src/core/Akka/Actor/Internal/ActorSystemImpl.cs

@@ -525,20 +525,19 @@ public override Task Terminate()
         {
             if(Settings.CoordinatedShutdownRunByActorSystemTerminate)
             {
-                CoordinatedShutdown.Get(this).Run(CoordinatedShutdown.ActorSystemTerminateReason.Instance);
-            } else
-            {
-                FinalTerminate();
+                return CoordinatedShutdown.Get(this)
+                    .Run(CoordinatedShutdown.ActorSystemTerminateReason.Instance);
             }
-            return WhenTerminated;
+            return FinalTerminate();
         }
 
-        internal override void FinalTerminate()
+        internal override Task FinalTerminate()
         {
             Log.Debug("System shutdown initiated");
             if (!Settings.LogDeadLettersDuringShutdown && _logDeadLetterListener != null)
                 Stop(_logDeadLetterListener);
             _provider.Guardian.Stop();
+            return WhenTerminated;
         }
 
         /// <summary>