Overly aggressive purging of get_users_in_room cache?

[clokep]

We invalidate the get_users_in_room cache (and a few others) as part of _invalidate_state_caches, we calculate a members_changed with the comment:

Figure out the changes of membership to invalidate the get_rooms_for_user cache.

But don't seem to be using it in the opposite direction (get_users_for_room).

tl;dr I think we are invalidating this cache in situations we do not need to.