We're up to schema v47 on develop now, so this will have to go in there to have
an effect.
This might cause an error if somebody has already run it in the v46 guise, and
runs it again in the v47 guise, because it will cause a duplicate entry in the
bbackground_updates table. On the other hand, the entry is removed once it is
complete, and it is unlikely that anyone other than matrix.org has run it on
v46. The update itself is harmless to re-run because it deliberately copes with
the index already existing.
Create the url_cache index on local_media_repository as a background update, so
that we can detect whether we are on sqlite or not and create a partial or
complete index accordingly.
To avoid running the cleanup job before we have built the index, add a bailout
which will defer the cleanup if the bg updates are still running.
Fixes https://github.com/matrix-org/synapse/issues/2572.
Prevent group API access to non-members for private groups
Also make all the group code paths consistent with `requester_user_id` always being the User ID of the requesting user.
