Currently background proccesses stream the events stream use the "minimum persisted position" (i.e. `get_current_token()`) rather than the vector clock style tokens. This is broadly fine as it doesn't matter if the background processes lag a small amount. However, in extreme cases (i.e. SyTests) where we only write to one event persister the background processes will never make progress.

This PR changes it so that the `MultiWriterIDGenerator` keeps the current position of a given instance as up to date as possible (i.e using the latest token it sees if its not in the process of persisting anything), and then periodically announces that over replication. This then allows the "minimum persisted position" to advance, albeit with a small lag.

We call `_update_stream_positions_table_txn` a lot, which is an UPSERT
that can conflict in `REPEATABLE READ` isolation level. Instead of doing
a transaction consisting of a single query we may as well run it outside
of a transaction.

We call `_update_stream_positions_table_txn` a lot, which is an UPSERT
that can conflict in `REPEATABLE READ` isolation level. Instead of doing
a transaction consisting of a single query we may as well run it outside
of a transaction.

This is so we can tell what is going on when things are taking a while to start up.

The main change here is to ensure that transactions that are created during startup get correctly logged like normal transactions.

The idea is that in future tokens will encode a mapping of instance to position. However, we don't want to include the full instance name in the string representation, so instead we'll have a mapping between instance name and an immutable integer ID in the DB that we can use instead. We'll then do the lookup when we serialize/deserialize the token (we could alternatively pass around an `Instance` type that includes both the name and ID, but that turns out to be a lot more invasive).

This was a bit unweildy for what I wanted: in particular, I wanted to assign
each measurement straight into a bucket, rather than storing an intermediate
Counter which didn't do any bucketing at all.

I've replaced it with something that is hopefully a bit easier to use.

(I'm not entirely sure what the difference between a HistogramMetricFamily and
a GaugeHistogramMetricFamily is, but given our counters can go down as well as
up the latter *sounds* more accurate?)

* Fix table scan of events on worker startup.

This happened because we assumed "new" writers had an initial stream
position of 0, so the replication code tried to fetch all events written
by the instance between 0 and the current position.

Instead, set the initial position of new writers to the current
persisted up to position, on the assumption that new writers won't have
written anything before that point.

* Consider old writers coming back as "new".

Otherwise we'd try and fetch entries between the old stale token and the
current position, even though it won't have written any rows.

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

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

This is an attempt to fix #8403.

On startup `MultiWriteIdGenerator` fetches the maximum stream ID for
each instance from the table and uses that as its initial "current
position" for each writer. This is problematic as a) it involves either
a scan of events table or an index (neither of which is ideal), and b)
if rows are being persisted out of order elsewhere while the process
restarts then using the maximum stream ID is not correct. This could
theoretically lead to race conditions where e.g. events that are
persisted out of order are not sent down sync streams.

We fix this by creating a new table that tracks the current positions of
each writer to the stream, and update it each time we finish persisting
a new entry. This is a relatively small overhead when persisting events.
However for the cache invalidation stream this is a much bigger relative
overhead, so instead we note that for invalidation we don't actually
care about reliability over restarts (as there's no caches to
invalidate) and simply don't bother reading and writing to the new table
in that particular case.

This will allow us to hit the DB after we've finished using the generated stream ID.

This converts calls like super(Foo, self) -> super().

Generated with:

    sed -i "" -Ee 's/super\([^\(]+\)/super()/g' **/*.py

It did not correctly handle IDs finishing being persisted out of
order, resulting in the `current_position` lagging until new IDs are
persisted.

This is so that we can use it for the backfill events stream.

... and to show that it does something slightly different to
`_get_e2e_device_keys_txn`.

`include_all_devices` and `include_deleted_devices` were never used (and
`include_deleted_devices` was broken, since that would cause `None`s in the
result which were not handled in the loop below.

Add some typing too.

This was forgotten in #8164.

Don't fail /submit_token requests on incorrect session ID if request_token_inhibit_3pid_errors is turned on (#7991)

* Don't raise session_id errors on submit_token if request_token_inhibit_3pid_errors is set

* Changelog

* Also wait some time before responding to /requestToken

* Incorporate review

* Update synapse/storage/databases/main/registration.py

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

* Incorporate review

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

The function is used for two purposes: 1) for subscribers of streams to
get a token they can use to get further updates with, and 2) for
replication to track position of the writers of the stream.

For streams with a single writer the two scenarios produce the same
result, however the situation becomes complicated for streams with
multiple writers. The current `MultiWriterIdGenerator` does not
correctly handle the first case (which is not an issue as its only used
for the `caches` stream which nothing subscribes to outside of
replication).

I think this would have caught all the cases in
https://github.com/matrix-org/synapse/issues/7642 - and I think a 500 makes
more sense here than a 403