Minor corrections and advice... Should help beginners.

The main use case is to see how many requests are being made, and how
many are second/third/etc attempts. If there are large number of retries
then that likely indicates a delivery problem.

If the script fails (or is CTRL-C'ed) between porting some of the events table and copying of the sequences then the port script will immediately die if run again due to the postgres DB having inconsistencies between sequences and tables.

The fix is to move the porting of sequences to before porting the tables, so that there is never a period where the Postgres DB is inconsistent. To do that we need to change how we port the sequences so that it calculates the values from the SQLite DB rather than the Postgres DB.

Fixes #8619 

This should hopefully speed up `get_auth_chain_difference` a bit in the case of repeated state res on the same rooms.

`get_auth_chain_difference` does a breadth first walk of the auth graphs by repeatedly looking up events' auth events. Different state resolutions on the same room will end up doing a lot of the same event to auth events lookups, so by caching them we should speed things up in cases of repeated state resolutions on the same room.

`adbapi.ConnectionPool` let's you turn on auto reconnect of DB connections. This is off by default.
As far as I can tell if its not enabled dead connections never get removed from the pool.

Maybe helps #8574

Fixes #8618

* Check support room has only two users

* Create 8728.bugfix

* Update synapse/server_notices/server_notices_manager.py

Co-authored-by: Erik Johnston <erik@matrix.org>

Co-authored-by: Erik Johnston <erik@matrix.org>

Signed-off-by: Nicolai Søborg <git@xn--sb-lka.org>

If SSO login is used (e.g. SAML) in a multi worker setup, it should be mentioned that currently all SAML logins must run on the same worker, see https://github.com/matrix-org/synapse/issues/7530

Also, if you are using different ports (for example 443 and 8448) in a reverse proxy for client and federation, the path `/_matrix/media` on the client and federation port must point to the listener of the `media_repository` worker, otherwise you'll get a 404 on the federation port for the path `/_matrix/media`, if a remote server is trying to get the media object on federation port, see https://github.com/matrix-org/synapse/issues/8695

This PR adds some documentation that:

* Describes who the audience for the `docs/`, `docs/dev/` and `docs/admin/` directories are, as well as Synapse's wiki page.
* Stresses that we'd like all documentation to be down in markdown.

Fixes:

```
builtins.TypeError: _reload_logging_config() takes 1 positional argument but 2 were given
```

Add `GET /_synapse/admin/v1/statistics/users/media` to get statisics about local media usage by users.
Related to #6094
It is the first API for statistics.
Goal is to avoid/reduce usage of sql queries like [Wiki analyzing Synapse](https://github.com/matrix-org/synapse/wiki/SQL-for-analyzing-Synapse-PostgreSQL-database-stats

)

Signed-off-by: Dirk Klimpel <dirk@klimpel.org>

Add `displayname` to Shared-Secret Registration for admins to `POST /_synapse/admin/v1/register`

I idly noticed that these lists were out of sync with each other, causing us to miss a table in a test case (`local_invites`). Let's consolidate this list instead to prevent this from happening in the future.

Fixes #8550

Fixes #4042

This PR fixes two things:

* Corrects the copy/paste error of telling the client their displayname is wrong when they are submitting an `avatar_url`.
* Returns a `M_INVALID_PARAM` instead of `M_UNKNOWN` for non-str type parameters.

Reported by @t3chguy.

This could be customised to trigger a different kind of notification in the future, but for now it's a normal non-highlight one.

This bumps us closer to current Python without going all the way to 3.9.

Fixes #8674

Signed-off-by: Dan Callahan <danc@element.io>

We do a `SELECT MAX(stream_id) FROM e2e_cross_signing_keys` on startup.

Fix serialisation errors when using third-party event rules.

* Tie together matches_user_in_member_list and get_users_in_room

* changelog

* Remove type to fix mypy

* Add `on_invalidate` to the function signature in the hopes that may make things work well

* Remove **kwargs

* Update 8676.bugfix

We don't always need the full power of a DeferredCache.

Fixes #6755

Missed in #8671.