Question

While attempting to send a message for a queue through the BeginSend call seem te behave as a blocking call.

Specificly I have:

public void Send(MyMessage message)
{
  lock(SEND_LOCK){
    var state = ...
    try {
      log.Info("Begin Sending...");
      socket.BeginSend(message.AsBytes(),0, message.ByteLength, SocketFlags.None, 
         (r) => EndSend(r), state);
      log.Info("Begin Send Complete.");
    }
    catch (SocketException e) {
      ...
    }
  }
}

The callback would be something like this:

  private void EndSend(IAsyncResult result) {
     log.Info("EndSend: Ending send.");
     var state = (MySendState) result.AsyncState;
     ...

     state.Socket.EndSend(result, out code);

     log.Info("EndSend: Send ended.");

     WaitUntilNewMessageInQueue();
     SendNextMessage();
  }

Most of the time this works fine, but sometimes it hangs. Logging indicates this happens when BeginSend en EndSend are excecuted on the same Thread. The WaitUntilNewMessageInQueue blocks until there is a new message in the queue, so when there is no new message it can wait quit a while.

As far as I can tell this should not really be a problem, but in the some cases BeginSend blocks causing a deadlock situation where EndSend is blocking on WaitUntilNewMessageInQueue (expected), but Send is blocking on BeginSend in return as it seems te be waiting for the EndSend callback te return (not expected).

This behaviour was not what I was expecting. Why does BeginSend sometimes block if the callback does not return in timely fashion?

Answer 1

First of all, why are you locking in your Send method? The lock will be released before the send is complete since you are using BeginSend. The result is that multiple sends can be executing at the same time.

Secondly, do not write (r) => EndSend(r), just write EndSend (without any parameters).

Thrid: You do not need to include the socket in your state. Your EndSend method is working like any other instance method. You can therefore access the socket field directly.

As for your deadlocks, it’s hard to tell. You delegate may have something to do with it (optimizations by the compiler / runner). But I have no knowledge in that area.

Need more help? Post more code. but I suggest that you fix the issues above (all four of them) and try again first.