SynchronizationLockException in user-defined mailboxes

[Aaronontheweb]

Akka.NET Version: Unknown

Found an issue reported by an end-user on StackOverflow: https://stackoverflow.com/questions/50292073/synchronizationlockexception-in-akka-net-mailboxes

They're using a custom mailbox

public class MyActorMailBox: UnboundedPriorityMailbox
{
    public MyActorMailBox(Settings settings, Config config) : base(settings, config)
    {}

    protected override int PriorityGenerator(object message)
    {
        if (message is MyActorMailBox.ErroredMessage)
            return 0;
        return 1;
    }
}

Configured with the following HOCON:

  <!-- language: lang-xml -->
  <akka>
    <hocon>
      <![CDATA[
      akka
      {...}
      myactor-mailbox
      {     
         mailbox-type="MyActorMailBox, MyNamespace"
      }
      ]]>
    </hocon>
  </akka>

And initialized with the following code:

  ActorSystem.ActorOf(
        Props.Create(() => new MyActor()).WithMailbox("myactor-mailbox"), "MyActor");

Which throws the following exception:

Swallowing exception during message send
System.Threading.SynchronizationLockException: Object synchronization method was called from an unsynchronized block of code.
   at Akka.Dispatch.MessageQueues.BlockingMessageQueue.Enqueue(IActorRef receiver, Envelope envelope)
   at Akka.Dispatch.MessageDispatcher.Dispatch(ActorCell cell, Envelope envelope)
   at Akka.Actor.ActorCell.SendMessage(Envelope message)

Need to determine why this is being thrown and under what version of Akka.NET this occurs.

[Aaronontheweb]

Ok, figured this one out.

Looks like we're not checking to see if the TryEnter method succeeds or not when we move onto the next operation:

akka.net/src/core/Akka/Dispatch/MessageQueues/BlockingMessageQueue.cs

Lines 46 to 60 in 56c284b

	public int Count
	{
	get
	{
	Monitor.TryEnter(_lock, BlockTimeOut);
	try
	{
	return LockedCount;
	}
	finally
	{
	Monitor.Exit(_lock);
	}
	}
	}

We need to do a check to see if the lock entry failed or succeeded, given that the queue is blocking. If the lock entry timed out, we need to do something other than try to enter. This is a bug.