Fix PeriodicTimer scheduler deadlock during start (#6946)

Co-authored-by: Aaron Stannard <[email protected]>

src/core/Akka/Actor/Scheduler/HashedWheelTimerScheduler.cs

@@ -153,9 +153,7 @@ private void Start()
                     var t = TimeSpan.FromTicks(_tickDuration);
                     _timer = new PeriodicTimer(t);
 
-#pragma warning disable CS4014 // Intentional, running detached async Task
-                    RunAsync(_cts.Token); // start the clock
-#pragma warning restore CS4014 // Intentional, running detached async Task
+                    Task.Run(() => RunAsync(_cts.Token)); // start the clock
                 }
             }
             else if (_workerState == WORKER_STATE_SHUTDOWN)
@@ -502,39 +500,46 @@ private Task<IEnumerable<SchedulerRegistration>> Stop()
 
         public void Dispose()
         {
-#if NET6_0_OR_GREATER
-            _timer?.Dispose();
-#endif
-            var stopped = Stop();
-            if (!stopped.Wait(_shutdownTimeout))
-            {
-                Log.Warning("Failed to shutdown scheduler within {0}", _shutdownTimeout);
-                return;
-            }
-
-            // Execute all outstanding work items
-            foreach (var task in stopped.Result)
+            try
             {
-                try
+                var stopped = Stop();
+                if (!stopped.Wait(_shutdownTimeout))
                 {
-                    task.Action.Run();
+                    Log.Warning("Failed to shutdown scheduler within {0}", _shutdownTimeout);
+                    return;
                 }
-                catch (SchedulerException)
-                {
-                    // ignore, this is from terminated actors
-                }
-                catch (Exception ex)
-                {
-                    Log.Error(ex, "Exception while executing timer task.");
-                }
-                finally
+
+                // Execute all outstanding work items
+                foreach (var task in stopped.Result)
                 {
-                    // free the object from bucket
-                    task.Reset();
+                    try
+                    {
+                        task.Action.Run();
+                    }
+                    catch (SchedulerException)
+                    {
+                        // ignore, this is from terminated actors
+                    }
+                    catch (Exception ex)
+                    {
+                        Log.Error(ex, "Exception while executing timer task.");
+                    }
+                    finally
+                    {
+                        // free the object from bucket
+                        task.Reset();
+                    }
                 }
             }
-
-            _unprocessedRegistrations.Clear();
+            finally
+            {
+                _unprocessedRegistrations.Clear();
+
+#if NET6_0_OR_GREATER
+                _timer?.Dispose();
+                _cts.Dispose();
+#endif
+            }
         }
 
         /// <summary>