Unregister receive handler fix

src/Microsoft.Azure.EventHubs/Amqp/AmqpPartitionReceiver.cs

@@ -130,10 +130,10 @@ protected override void OnSetReceiveHandler(IPartitionReceiveHandler newReceiveH
         {
             lock (this.receivePumpLock)
             {
-                if (this.receiveHandler != null)
+                if (newReceiveHandler != null && this.receiveHandler != null)
                 {
                     // Notify existing handler first (but don't wait).
-                    this.receiveHandler.ProcessErrorAsync(new OperationCanceledException("New handler has registered for this receiver.")); // .Fork();
+                    this.receiveHandler.ProcessErrorAsync(new OperationCanceledException("New handler has registered for this receiver."));
                 }
 
                 this.receiveHandler = newReceiveHandler;

test/Microsoft.Azure.EventHubs.UnitTests/EventHubClientTests.cs

@@ -532,7 +532,13 @@ async Task PartitionReceiverSetReceiveHandler()
 
                 EventWaitHandle dataReceivedEvent = new EventWaitHandle(false, EventResetMode.ManualReset);
                 var handler = new TestPartitionReceiveHandler();
-                handler.ErrorReceived += (s, e) => Log($"TestPartitionReceiveHandler.ProcessError {e.GetType().Name}: {e.Message}");
+
+                // Not expecting any errors.
+                handler.ErrorReceived += (s, e) =>
+                {
+                    throw new Exception($"TestPartitionReceiveHandler.ProcessError {e.GetType().Name}: {e.Message}");
+                };
+
                 handler.EventsReceived += (s, eventDatas) =>
                 {
                     int count = eventDatas != null ? eventDatas.Count() : 0;
@@ -564,6 +570,10 @@ async Task PartitionReceiverSetReceiveHandler()
             }
             finally
             {
+                // Unregister handler.
+                partitionReceiver.SetReceiveHandler(null);
+
+                // Close clients.
                 await partitionSender.CloseAsync();
                 await partitionReceiver.CloseAsync();
             }