While this is not documented in the spec (but should be), Riot (and other clients) revoke 3PID invites by sending a m.room.third_party_invite event with an empty ({}) content to the room's state.
When the invited 3PID gets associated with a MXID, the identity server (which doesn't know about revocations) sends down to the MXID's homeserver all of the undelivered invites it has for this 3PID. The homeserver then tries to talk to the inviting homeserver in order to exchange these invite for m.room.member events.
When one of the invite is revoked, the inviting homeserver responds with a 500 error because it tries to extract a 'display_name' property from the content, which is empty. This might cause the invited server to consider that the server is down and not try to exchange other, valid invites (or at least delay it).
This fix handles the case of revoked invites by avoiding trying to fetch a 'display_name' from the original invite's content, and letting the m.room.member event fail the auth rules (because, since the original invite's content is empty, it doesn't have public keys), which results in sending a 403 with the correct error message to the invited server.
Doing a password reset via SMS has never worked, and in any case is a silly
idea because msisdn recycling is a thing.
See also matrix-org/matrix-doc#2303.
Pull the checkers out to their own classes, rather than having them lost in a
massive 1000-line class which does everything.
This is also preparation for some more intelligent advertising of flows, as per #6100
We don't actually care about what happens in `get_user_by_req` and
having it as a separate span means that the entity tag isn't added to
the servlet spans, making it harder to search.
because, frankly, it looked like it was written by an axe-murderer.
This should be a non-functional change, except that where `m.login.dummy` was
previously advertised *before* `m.login.terms`, it will now be advertised
afterwards. AFAICT that should have no effect, and will be more consistent with
the flows that involve passing a 3pid.
