Preserve order: didn't (quite); fixed

StackExchange.Redis/StackExchange/Redis/CompletionManager.cs

@@ -110,8 +110,14 @@ private static void ProcessAsyncCompletionQueue(object state)
         }
 
         partial void OnCompletedAsync();
+
+        int activeAsyncWorkerThread = 0;
         private void ProcessAsyncCompletionQueueImpl()
         {
+            int currentThread = Environment.CurrentManagedThreadId;
+            try
+            {
+                if (Interlocked.CompareExchange(ref activeAsyncWorkerThread, currentThread, 0) != 0) return;
                 int total = 0;
                 do
                 {
@@ -121,32 +127,41 @@ private void ProcessAsyncCompletionQueueImpl()
                         next = asyncCompletionQueue.Count == 0 ? null
                             : asyncCompletionQueue.Dequeue();
                     }
-                if(next == null && Thread.Yield()) // give it a moment and try again
+                    if (next == null)
                     {
+                        // give it a moment and try again, noting that we might lose the battle
+                        // when we pause
+                        Interlocked.CompareExchange(ref activeAsyncWorkerThread, 0, currentThread);
+                        if (Thread.Yield() && Interlocked.CompareExchange(ref activeAsyncWorkerThread, currentThread, 0) == 0)
+                        {
+                            // we paused, and we got the lock back; anything else?
                             lock (asyncCompletionQueue)
                             {
                                 next = asyncCompletionQueue.Count == 0 ? null
                                     : asyncCompletionQueue.Dequeue();
                             }
                         }
-                if (next == null) break; // nothing to do
+                    }
+                    if (next == null) break; // nothing to do <===== exit point
                     try
                     {
                         multiplexer.Trace("Completing async (ordered): " + next, name);
                         next.TryComplete(true);
                         Interlocked.Increment(ref completedAsync);
                     }
-                catch(Exception ex)
+                    catch (Exception ex)
                     {
                         multiplexer.Trace("Async completion error: " + ex.Message, name);
                         Interlocked.Increment(ref failedAsync);
                     }
                     total++;
                 } while (true);
-
                 multiplexer.Trace("Async completion worker processed " + total + " operations", name);
             }
-
-
+            finally
+            {
+                Interlocked.CompareExchange(ref activeAsyncWorkerThread, 0, currentThread);
+            }
+        }
     }
 }