fix(MSC3911): Avoid partial room creation due to errors in room avatars with respect to media restriction

[jason-famedly]

Media that is already restricted will cause the creation of a room avatar to fail, leading to a partial room.

Check before saving any room data that the room avatar will not cause this situation

This is basically a duplicate of the existing condition block inside of _send_events_for_new_room(), with the exception that the media restrictions are not collected for attachment

[itsoyou]

hmm would it better if we have another restrictions type: profile_room_id or something like that?

[jason-famedly]

Probably not? This just needs to make sure the room avatar isn't going to break room creation after part of it has already happened. Like for instance, if the requested room avatar is already attached to another room, then they messed up on the request somehow. Instead of having a partial room with only one person and no other events or state, this just aborts the whole process.

[jason-famedly]

To be fair, the correct thing to do here would be to make the entire process of creating a room atomic. That is a lot of rearranging of existing code to "prevalidate" that all events will be valid before they are persisted. Then there is the situation where the persisting of the events can fail part way through.

It's a heavy lift, and way out of scope for this part of the project

[nico-famedly]

Seems reasonable, although we probably should dedup the code some more after we fixed all the bugs :)