Faster remote room joins: add an error code to /members when in partial state

[MatMaul]

For now the request is just stalled until the join completes, which is not ideal since clients are not aware of what is happening.

This requires a MSC to define the error code.
For backward compatibility we also probably need to add a parameter to /members so that the client can opt-in to the specific error code.
Or perhaps returning a 5xx is enough ? https://stackoverflow.com/questions/9794696/which-http-status-code-means-not-ready-yet-try-again-later has several suggestions on that subject.

[DMRobertson]

Note that Olivier has proposed a return code for federation APIs: matrix-org/matrix-spec-proposals#3895

[erikjohnston]

Is this something we've decided we want to do? From a backwards compatibility PoV it feels a bit tricky to do well.

[MatMaul]

It's more an idea than a decision.

For backwards compatibility we could either introduce a parameter for that, or consider that clients should be able to retry on 5xx and use that.

First approach should be on the safe side I believe ?

[erikjohnston]

I think we decided to drop this for now, and see if there was a use case for it?