Joining by invite to empty room

[slipeer]

Problem reproduction

If user A has invited user B to a new room, but he did not wait when B accenpt invite and leave room.
User B see invite, but when trying to accept it, the error "no known servers"

I believe that synapse must more correct handle this situation:

• for example return error like "everyone left the room".
• or accept user join into room in which there is already no one (maybe there are messages for user).

Second variant of behavior seems more logical to me.

Probably requires changes in _is_host_in_room method in synapse/handlers/room_member.py - this method return false if all local users leave room.

This method is called from _update_membership (same handler) and if it return False then used method _remote_join. I think that call to _remote_join is not entirely correct whent local user try to join local room in which there is already no one.

[richvdh]

this is basically a duplicate of matrix-org/matrix-spec-proposals#1533 and https://github.com/matrix-org/matrix-doc/issues/534.

[ara4n]

Another solution would be to for the invite to be cancelled when the inviter leaves, as per https://github.com/matrix-org/matrix-doc/issues/1966