Remove redundant and error-prone call to .Reset() in our AutoResetEvents (#68)

Author: davidmrdavid

src/DurableEngine/OrchestrationInvoker.cs

@@ -76,7 +76,7 @@ internal Hashtable Invoke(IPowerShellServices powerShellServices)
                 while (true)
                 {
                     // block this thread until user-code thread (the PS orchestrator) invokes a DF CmdLet or completes.
-                    var orchestratorReturned = context.SharedMemory.YieldToUserCodeThread(orchestratorReturnedHandle);
+                    var orchestratorReturned = context.SharedMemory.WaitForInvokerThreadTurn(orchestratorReturnedHandle);
                     if (orchestratorReturned)
                     {
                         // The PS orchestrator has a return value, there's no more DF APIs to await.
@@ -109,6 +109,12 @@ internal Hashtable Invoke(IPowerShellServices powerShellServices)
                         await task.GetDTFxTask();
                     } // Exceptions are ignored at this point, they will be re-surfaced by the PS code if left unhandled.
                     catch { }
+
+                    // Wake up user-code thread. For a small moment, both the user code thread and the invoker thread
+                    // will be running at the same time.
+                    // However, the invoker thread will block itself again at the start of the next loop until the user-code
+                    // thread yields control.
+                    context.SharedMemory.WakeUserCodeThread();
                 }
             };
 

src/DurableEngine/SharedMemory.cs

@@ -40,25 +40,16 @@ public void YieldToInvokerThread()
         {
             // Wake invoker thread.
             invokerThreadTurn.Set();
-
-            // Block user-code thread.
-            userCodeThreadTurn.Reset();
             userCodeThreadTurn.WaitOne();
         }
 
         /// <summary>
-        /// Blocks Orchestration-invoker thread, wakes up user-code thread.
-        /// This is usually used after the invoker has a result for the PS orchestrator.
+        /// Blocks Orchestration-invoker thread until the user-code thread completes or yields.
         /// </summary>
         /// <param name="completionHandle">The WaitHandle tracking if the user-code thread completed.</param>
         /// <returns>True if the user-code thread completed, False if it requests an API to be awaited.</returns>
-        public bool YieldToUserCodeThread(WaitHandle completionHandle)
+        public bool WaitForInvokerThreadTurn(WaitHandle completionHandle)
         {
-            // Wake user-code thread
-            userCodeThreadTurn.Set();
-
-            // Get invoker thread ready to block
-            invokerThreadTurn.Reset();
 
             // Wake up when either the user-code returns, or when we're yielded-to for `await`'ing.
             var index = WaitHandle.WaitAny(new[] { completionHandle, invokerThreadTurn });
@@ -67,13 +58,12 @@ public bool YieldToUserCodeThread(WaitHandle completionHandle)
         }
 
         /// <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.
+        /// Wakes up the user-code thread without blocking the invoker thread.
+        /// The invoker thread should block itself afterwards to prevent races.
         /// </summary>
-        public void GuaranteeUserCodeTurn()
+        public void WakeUserCodeThread()
         {
-            userCodeThreadTurn.WaitOne();
+            userCodeThreadTurn.Set();
         }
     }
 }

src/DurableEngine/Tasks/DurableTask.cs

@@ -49,10 +49,6 @@ 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)