Prevent races between orchestration-invoker and user code (#26)

Author: davidmrdavid

src/DurableEngine/SharedMemory.cs

@@ -65,5 +65,15 @@ public bool YieldToUserCodeThread(WaitHandle completionHandle)
             var shouldStop = index == 0;
             return shouldStop;
         }
+
+        /// <summary>
+        /// Blocks user code thread if the orchestrator-invoker thread is currently running.
+        /// This guarantees that the user-code thread and the orchestration-invoker thread run one
+        /// at a time after this point.
+        /// </summary>
+        public void GuaranteeUserCodeTurn()
+        {
+            userCodeThreadTurn.WaitOne();
+        }
     }
 }

src/DurableEngine/Tasks/DurableTask.cs

@@ -49,6 +49,10 @@ internal OrchestrationAction GetOrCreateAction()
         /// <param name="writeErr">Function to write an exception to the pipeline.</param>
         public void Execute(Action<object> write, Action<ErrorRecord> writeErr)
         {
+            // Ensure that a DurableTask in the usercode thread
+            // only executes while the orchestration-invoker thread is blocked.
+            OrchestrationContext.SharedMemory.GuaranteeUserCodeTurn();
+
             DurableTask task = this;
 
             if (NoWait)