CustomOnionMessageHandler::handle_message doesn't expose the reply_path

[TheBlueMatt]

I'm trying to implement an onion message handler that does some work async and then eventually replies to an onion message. However, handle_message currently only exposes the message itself but not the reply path, which would be needed to be able to respond async. To avoid cloneing the BlindedPath in such a case, maybe it makes sense to drop the whole reply-inline logic and force users to reply via release_pending_custom_messages?

[jkczyz]

Hmm... possibly though it would be nice to have some optionality here. We could pass the handler an object that has methods like:

• respond for handling the current case
• respond_with_reply_path for giving a reply path in the response (needed for proper invoice error handling still)
• defer (or something) to consume the object and obtain the reply path, path_id, etc.

[TheBlueMatt]

That seems sensible, and then the method would return a PendingOnionMessage<Self::CustomMessage> rather than a Self::CustomMessage?

[jkczyz]

Not 100% sure. I was thinking those first two methods would essentially act as callbacks to OnionMessenger::handle_onion_message_response. (Note that we'll need the second method for OffersMessageHandler when responding with an invoice plus reply path, so this isn't only applicable to custom messages.) Then the message handler would no longer have a return value since responding async wouldn't require one. But I can see there being other ways of formulating this.

[TheBlueMatt]

Yea, I don't really have much of an opinion on how this should look, any of those seem reasonable to me.

[shaavan]

Quick heads up, I'm actively tackling this issue now. Will update as soon as I make some good progress!