Sometimes the filtering function can return a pruned version of an event (on top of either the event itself or an empty list), if it thinks the user should be able to see that there's an event there but not the content of that event. Therefore, the previous logic of 'if filtered is empty then we can use the event we retrieved from the database' is flawed, and we should use the event returned by the filtering function.
have_events was a map from event_id to rejection reason (or None) for events
which are in our local database. It was used as filter on the list of
event_ids being passed into get_events_as_list. However, since
get_events_as_list will ignore any event_ids that are unknown or rejected, we
can equivalently just leave it to get_events_as_list to do the filtering.
That means that we don't have to keep `have_events` up-to-date, and can use
`have_seen_events` instead of `get_seen_events_with_rejection` in the one place
we do need it.
Implement part [MSC2228](https://github.com/matrix-org/matrix-doc/pull/2228). The parts that differ are:
* the feature is hidden behind a configuration flag (`enable_ephemeral_messages`)
* self-destruction doesn't happen for state events
* only implement support for the `m.self_destruct_after` field (not the `m.self_destruct` one)
* doesn't send synthetic redactions to clients because for this specific case we consider the clients to be able to destroy an event themselves, instead we just censor it (by pruning its JSON) in the database
Purge jobs don't delete the latest event in a room in order to keep the forward extremity and not break the room. On the other hand, get_state_events, when given an at_token argument calls filter_events_for_client to know if the user can see the event that matches that (sync) token. That function uses the retention policies of the events it's given to filter out those that are too old from a client's view.
Some clients, such as Riot, when loading a room, request the list of members for the latest sync token it knows about, and get confused to the point of refusing to send any message if the server tells it that it can't get that information. This can happen very easily with the message retention feature turned on and a room with low activity so that the last event sent becomes too old according to the room's retention policy.
An easy and clean fix for that issue is to discard the room's retention policies when retrieving state.
We were doing this in a number of places which meant that some login
code paths incremented the counter multiple times.
It was also applying ratelimiting to UIA endpoints, which was probably
not intentional.
In particular, some custom auth modules were calling
`check_user_exists`, which incremented the counters, meaning that people
would fail to login sometimes.
Fixes a bug where rejected events were persisted with the wrong state group.
Also fixes an occasional internal-server-error when receiving events over
federation which are rejected and (possibly because they are
backwards-extremities) have no prev_group.
Fixes#6289.
* Raise an exception if accessing state for rejected events
Add some sanity checks on accessing state_group etc for
rejected events.
* Skip calculating push actions for rejected events
It didn't actually cause any bugs, because rejected events get filtered out at
various later points, but there's not point in trying to calculate the push
actions for a rejected event.
When the `/keys/query` API is hit on client_reader worker Synapse may
decide that it needs to resync some remote deivces. Usually this happens
on master, and then gets cached. However, that fails on workers and so
it falls back to fetching devices from remotes directly, which may in
turn fail if the remote is down.
