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
=============================
Bugfixes
--------
- Fix a bug which could cause the background database update hander for event labels to get stuck in a loop raising exceptions. ([\#6407](https://github.com/matrix-org/synapse/issues/6407))
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Merge tag 'v1.6.0rc2' into develop
Synapse 1.6.0rc2 (2019-11-25)
=============================
Bugfixes
--------
- Fix a bug which could cause the background database update hander for event labels to get stuck in a loop raising exceptions. ([\#6407](https://github.com/matrix-org/synapse/issues/6407))
Doing a SELECT DISTINCT when paginating is quite expensive, because it requires the engine to do sorting on the entire events table. However, we only need to run it if we're filtering on 2+ labels, so this PR is changing the request so that DISTINCT is only used then.
