DistributedPubSub: publishing to topics that have no subscribers doesn't produce any dead letters

[Aaronontheweb]

Version Information
Version of Akka.NET? 1.4.27
Which Akka.NET Modules? Akka.Cluster.Tools

Describe the bug
Once the final subscriber to a topic unsubscribes, no one is available to receive these messages published to this topic. Yet, when messages are published no DeadLetters are produced.

To Reproduce
Steps to reproduce the behavior:

1. Subscribe to topic "foo" with one IActorRef
2. Unsubscribe IActorRef from "foo"
3. Wait for UnsubscribeAck
4. Query CurrentTopics from Mediator - "foo" still appears.

This appears to be because:

akka.net/src/contrib/cluster/Akka.Cluster.Tools/PublishSubscribe/DistributedPubSubMediator.cs

Lines 248 to 253 in 838b6a3

	Receive<NoMoreSubscribers>(msg =>
	{
	var key = Internal.Utils.MakeKey(Sender);
	_buffer.InitializeGrouping(key);
	Sender.Tell(TerminateRequest.Instance);
	});

And the Prune operation never un-register any empty buckets once they've been created.

Expected behavior
Topic registrations should eventually be removed when not in use - or should publish to dead letters when there are no subscribers.

Actual behavior
If no one can receive a message published to a topic, whatever the reason, it should be logged as such - usually a DeadLetter.

[Aaronontheweb]

More context on this bug - the behavior Akka.NET exposes when publishing to a topic that never had any subscribers is different: we always publish a deadletter when this happens.

[Aaronontheweb]

Let me simplify this:

can you follow the steps above, publish to this topic, and not get a DeadLetter?