QQs: support if-empty and if-unused deletion parameters

[Zerpet]

QQs: support if-empty and if-unused deletion parameters

[ikavgo]

It appears as if IfUnused and IfEmpty are ignored for streams - https://github.com/rabbitmq/rabbitmq-server/blob/main/deps/rabbit/src/rabbit_stream_queue.erl#L197.

Regarding user experience. The numbers shown have different source depending on the queue type. Classic queues go thru process dictionary for all ch records and use the same mechanics for implementing IfUnused. Quorum queues show consumers via queue_metrics and ets:lookup_element. Stream queue uses yet another way of getting consumer count - it enumerates queue nodes and does rpc.

So regarding guarantees and behaviours I have these question:

1. does it make sense to let streams respect IfUnused/IfEmpty?
2. does it make sense to use the same mechanics from info/2 that used to generate values displayed to user to implement IfUnused/IfEmpty for streams and quorum queues?
3. Quorum queues go to metrics for consumer counts and streams enumerate nodes and also doing ets calls. Shouldn't they look the same?

[Zerpet]

does it make sense to let streams respect IfUnused/IfEmpty?

I don't think so, specially if they are just ignored at the moment. Streams are expected to have data all the time when, at least, one message is published. Perhaps it could support if-unused, I don't have a strong opinion on that regard. My intention with this issue was to focus primarily on Quorum Queues.

I can't comment on your other questions since I have little-to-none context on those areas 🙃