Federated Invites: It appears that there is no way to retract an invite such that the invitee is made aware of it

[reivilibre]

If you invite a user over federation and then change your mind (retract the invite, by setting their m.room.member membership to leave), there doesn't seem to be any mechanism to notify the invitee's server of that.

The result is that the invitee is left with a stale invite and when they try to redeem it, they get some generic error like 'Failed to make_join via any server' (Synapse example).

This is probably limited to the case where there aren't other users in the room on the same server as the invitee, otherwise the m.room.member event would be sent to the invitee's homeserver (by coincidence) and that probably leads to the right thing happening.

It seems there should be:

• a Federation endpoint to notify about a retracted invite, but since that can't be guaranteed to be reliable, we also need...
• an error code on make_join and send_join somehow informing the invitee that their invite has since been retracted, so clients can show an appropriate error message

Very similar in spirit to #146

[richvdh]

also closely related: #1017