This fixes#3518, and ensures that we get useful logs and metrics for lots of
things that happen in the background.
(There are certainly more things that happen in the background; these are just
the common ones I've found running a single-process synapse locally).
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.
Add db_conn parameters to the `__init__` methods of the *Store classes, so that
they are all consistent, which makes the multiple inheritance work correctly
(and so that we can later extract mixins which can be used in the slavedstores)
background_updates was using `call_later` in a way that leaked the logcontext
into the reactor.
We could have rewritten it to do it properly, but given that we weren't using
the fancier facilities provided by `call_later`, we might as well just use
`async.sleep`, which does the logcontext stuff properly.
A bit of a cleanup for background_updates, and make sure that the real
background updates have run before we start the unit tests, so that they don't
interfere with the tests.
user_ips is kinda big, so really we want to add the index in the background
once we're running. Replace the schema delta with one which will do that.
I've done this in a way that's reasonably easy to reuse as there a few other
indexes I need, and I don't suppose they will be the last.
The progress for each background update is stored as a JSON blob in the
database. Each background update is broken up into separate batches.
The batch size is automatically tuned to try avoid blocking single
threaded databases for too long.
