Channel.basic_consume doesn't support consume-ok/completion callback

Channel.basic_consume doesn’t provide an argument for specifying the completion callback to be called upon consume-ok. (the callback arg that it presently supports is for consuming incoming messages).

A client’s logic might need to know that the basic-consume request succeeded before initiating other operations (e.g., when requesting exclusive access). Lack of the completion callback prevents the application from doing so easily.

Implementation notes:

1. Rename Channel.basic_consume’s consumer callback (presently named callback) to consumer_callback, message_callback, message_rx_callback, or similar, and name the completion callback callback to follow the recently-established convention for naming of completion callbacks. Also rename the consumer callback arg in BlockingChannel.basic_consume for consistency with Channel.basic_consume.
2. If completion callback isn’t specified, auto-generate and return the consumer tag following the legacy behavior. If completion callback is specified, return None from Channel.basic_consume and let the server generate the consumer tag and return it via consume-ok. Update the docstring in Channel.basic_consume to reflect this.
3. Make sure pika’s Channel._generate_consumer_tag doesn’t generate consumer tags that can potentially collide with server-generated ones by prefixing “pika” into the consumer tag name that Channel._generate_consumer_tag generates.

[vitaly-krugl]

I’m wondering about how I can replace the existing behavior with regard to consumer tag with a server-generated one …

@lukebakken, I supposed the easiest for now is to continue to generate the consumer tag in pika as we do presently.

In theory, you could cache the consumer callbacks in a list and then match them with tags returned in consumer-ok, which should return in the given order. Another option could be to have an API method that the app could call to register the consumer callback for a consumer tag once it is known. Still, continuing to generate the consumer tag in pika seems like the safest and simplest approach with no apparent downside.

[vitaly-krugl]

cc @lukebakken It would be great to get this API change in before pika 1.0 as it has implications on the callback arg name changed in your recent PR #941