Merge pull request #1938 from stephentoub/fix_asyncoperation_ordering

Fix race condition in AsyncOperation setting of completion flag

Author: stephentoub

src/System.ComponentModel.EventBasedAsync/src/System/ComponentModel/AsyncOperation.cs

@@ -53,23 +53,37 @@ public SynchronizationContext SynchronizationContext
 
         public void Post(SendOrPostCallback d, object arg)
         {
-            VerifyNotCompleted();
-            VerifyDelegateNotNull(d);
-            _syncContext.Post(d, arg);
+            PostCore(d, arg, markCompleted: false);
         }
 
         public void PostOperationCompleted(SendOrPostCallback d, object arg)
         {
-            Post(d, arg);
+            PostCore(d, arg, markCompleted: true);
             OperationCompletedCore();
         }
 
         public void OperationCompleted()
         {
             VerifyNotCompleted();
+            _alreadyCompleted = true;
             OperationCompletedCore();
         }
 
+        private void PostCore(SendOrPostCallback d, object arg, bool markCompleted)
+        {
+            VerifyNotCompleted();
+            VerifyDelegateNotNull(d);
+            if (markCompleted)
+            {
+                // This call is in response to a PostOperationCompleted.  As such, we need to mark
+                // _alreadyCompleted as true so that subsequent attempts to use this instance will
+                // fail appropriately.  And we need to do that before we queue the callback, as
+                // that callback could itself trigger additional attempts to use this instance.
+                _alreadyCompleted = true;
+            }
+            _syncContext.Post(d, arg);
+        }
+
         private void OperationCompletedCore()
         {
             try
@@ -78,7 +92,6 @@ private void OperationCompletedCore()
             }
             finally
             {
-                _alreadyCompleted = true;
                 GC.SuppressFinalize(this);
             }
         }