Co-authored-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

This should have been in #17045. Whoops.

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Forget a line, and an empty batch is trivially linear.

c.f. #17064

PR #16942 removed an invalid optimisation that avoided pulling out state
for non-gappy syncs. This causes a large increase in DB usage. c.f.
#16941 for why that optimisation was wrong.

However, we can still optimise in the simple case where the events in
the timeline are a linear chain without any branching/merging of the
DAG.

cc. @richvdh

Before we were pulling out *all* read receipts for a user for every
event we pushed. Instead let's only pull out the relevant receipts.

This also pulled out the event rows for each receipt, causing load on
the events table.

Unfortunately, the optimisation we applied here for non-gappy syncs is
not actually valid.

Fixes https://github.com/element-hq/synapse/issues/16941.

~~Based on https://github.com/element-hq/synapse/pull/16930.~~
Requires https://github.com/matrix-org/sytest/pull/1374.

Fix a long-standing issue which could cause state to be omitted from the
sync response if the last event was filtered out.

Fixes: https://github.com/element-hq/synapse/issues/16928

This PR fixes a very, very niche edge-case, but I've got some more work
coming which will otherwise make the problem worse.

The bug happens when the syncing user leaves a room, and has a sync
filter which includes "left" rooms, but sets the timeline limit to 0. In
that case, the state returned in the `state` section is calculated
incorrectly.

The fix is to pass a token corresponding to the point that the user
leaves the room through to `compute_state_delta`.

This was causing sequential scans when using refresh tokens.

Since these queries are duplicated in two places.

Follow on from #17037

Requests may require a User-Agent header, and the change in #16972
accidentally removed it, resulting in requests getting rejected causing
login to fail.

Fixes https://github.com/element-hq/synapse/issues/16680, as well as a
related bug, where servers which we had *never* successfully sent an
event to would not be retried.

In order to fix the case of pending to-device messages, we hook into the
existing `wake_destinations_needing_catchup` process, by extending it to
look for destinations that have pending to-device messages. The
federation transmission loop then attempts to send the pending to-device
messages as normal.

When running unit tests, we patch the database connection pool so that
it runs queries "synchronously". This is ok, except that if any queries
are launched before we do the patching, those queries get left in limbo
and never complete.

To fix this, let's change the way we do the switcheroo, by patching out
the method which creates the connection pool in the first place.