The purging code already cleans unreferenced state groups up
the graph when purging, ie. previous state groups that are not
referenced anymore because we are deleting their children.

However, some state groups are completely unreferenced, and
thus never cleaned, because no code even looks for them. They
could be detected and cleaned on a regular basis, yet because
they cause the room purging code to spam the database with
useless de-delta-ed lines, it would be nice to detect and clean
them early before things go wrong.

See https://github.com/element-hq/synapse/issues/9406 for details
about the issue as seen in the wild.

We are interested in state groups *down* the graph from those
we delete, which are the ones we might accidently de-delta. Hence
we replace the code that climbs up the graph with a broader search
for state groups to delete, browsing all linked groups, up and down
the graph.

Climbing up the state group graph was looking for previous state
groups, but actually fetched the state groups themselves instead.

State group purging during history purging is a rather complex
process. It took us a while to reverse engineer it and find
some basic and more complex issues. This merely add the comments
that would have helped us dive faster into the codebase.

This release enables support for
[MSC4186](https://github.com/matrix-org/matrix-spec-proposals/pull/4186) —
Simplified Sliding Sync. This allows using the upcoming releases of the Element
X mobile apps without having to run a Sliding Sync Proxy.

- Enable native sliding sync support ([MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575) and [MSC4186](https://github.com/matrix-org/matrix-spec-proposals/pull/4186)) by default. ([\#17648](https://github.com/element-hq/synapse/issues/17648))

- Fix regression in v1.114.0rc2 that caused workers to fail to start. ([\#17626](https://github.com/element-hq/synapse/issues/17626))

- Improve cross-signing upload when using [MSC3861](https://github.com/matrix-org/matrix-spec-proposals/pull/3861) to use a custom UIA flow stage, with web fallback support. ([\#17509](https://github.com/element-hq/synapse/issues/17509))
- Make `hash_password` script accept password input from stdin. ([\#17608](https://github.com/element-hq/synapse/issues/17608))

- Fix hierarchy returning 403 when room is accessible through federation. Contributed by Krishan (@kfiven). ([\#17194](https://github.com/element-hq/synapse/issues/17194))
- Fix content-length on federation `/thumbnail` responses. ([\#17532](https://github.com/element-hq/synapse/issues/17532))
- Fix authenticated media responses using a wrong limit when following redirects over federation. ([\#17543](https://github.com/element-hq/synapse/issues/17543))

- MSC3861: load the issuer and account management URLs from OIDC discovery. ([\#17407](https://github.com/element-hq/synapse/issues/17407))
- Refactor sliding sync class into multiple files. ([\#17595](https://github.com/element-hq/synapse/issues/17595))
- Store sliding sync per-connection state in the database. ([\#17599](https://github.com/element-hq/synapse/issues/17599))
- Make the sliding sync `PerConnectionState` class immutable. ([\#17600](https://github.com/element-hq/synapse/issues/17600))
- Add support to `@tag_args` for standalone functions. ([\#17604](https://github.com/element-hq/synapse/issues/17604))
- Speed up incremental syncs in sliding sync by adding some more caching. ([\#17606](https://github.com/element-hq/synapse/issues/17606))
- Always return the user's own read receipts in sliding sync. ([\#17617](https://github.com/element-hq/synapse/issues/17617))
- Replace `isort` and `black` with `ruff`. ([\#17620](https://github.com/element-hq/synapse/issues/17620))
- Refactor sliding sync code to move room list logic out into a separate class. ([\#17622](https://github.com/element-hq/synapse/issues/17622))

* Bump attrs from 23.2.0 to 24.2.0. ([\#17609](https://github.com/element-hq/synapse/issues/17609))
* Bump cryptography from 42.0.8 to 43.0.0. ([\#17584](https://github.com/element-hq/synapse/issues/17584))
* Bump phonenumbers from 8.13.43 to 8.13.44. ([\#17610](https://github.com/element-hq/synapse/issues/17610))
* Bump pygithub from 2.3.0 to 2.4.0. ([\#17612](https://github.com/element-hq/synapse/issues/17612))
* Bump pyyaml from 6.0.1 to 6.0.2. ([\#17611](https://github.com/element-hq/synapse/issues/17611))
* Bump sentry-sdk from 2.12.0 to 2.13.0. ([\#17585](https://github.com/element-hq/synapse/issues/17585))
* Bump serde from 1.0.206 to 1.0.208. ([\#17581](https://github.com/element-hq/synapse/issues/17581))
* Bump serde from 1.0.208 to 1.0.209. ([\#17613](https://github.com/element-hq/synapse/issues/17613))
* Bump serde_json from 1.0.124 to 1.0.125. ([\#17582](https://github.com/element-hq/synapse/issues/17582))
* Bump serde_json from 1.0.125 to 1.0.127. ([\#17614](https://github.com/element-hq/synapse/issues/17614))
* Bump types-jsonschema from 4.23.0.20240712 to 4.23.0.20240813. ([\#17583](https://github.com/element-hq/synapse/issues/17583))
* Bump types-setuptools from 71.1.0.20240726 to 71.1.0.20240818. ([\#17586](https://github.com/element-hq/synapse/issues/17586))

- Add a flag to `/versions`, `org.matrix.simplified_msc3575`, to indicate whether experimental sliding sync support has been enabled. ([\#17571](https://github.com/element-hq/synapse/issues/17571))
- Handle changes in `timeline_limit` in experimental sliding sync. ([\#17579](https://github.com/element-hq/synapse/issues/17579))
- Correctly track read receipts that should be sent down in experimental sliding sync. ([\#17575](https://github.com/element-hq/synapse/issues/17575), [\#17589](https://github.com/element-hq/synapse/issues/17589), [\#17592](https://github.com/element-hq/synapse/issues/17592))

- Start handlers for new media endpoints when media resource configured. ([\#17483](https://github.com/element-hq/synapse/issues/17483))
- Fix timeline ordering (using `stream_ordering` instead of topological ordering) in experimental [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575) Sliding Sync `/sync` endpoint. ([\#17510](https://github.com/element-hq/synapse/issues/17510))
- Fix experimental sliding sync implementation to remember any updates in rooms that were not sent down immediately. ([\#17535](https://github.com/element-hq/synapse/issues/17535))
- Better exclude partially stated rooms if we must await full state in experimental [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575) Sliding Sync `/sync` endpoint. ([\#17538](https://github.com/element-hq/synapse/issues/17538))
- Handle lower-case http headers in `_Mulitpart_Parser_Protocol`. ([\#17545](https://github.com/element-hq/synapse/issues/17545))
- Fix fetching federation signing keys from servers that omit `old_verify_keys`. Contributed by @tulir @ Beeper. ([\#17568](https://github.com/element-hq/synapse/issues/17568))
- Fix bug where we would respond with an error when a remote server asked for media that had a length of 0, using the new multipart federation media endpoint. ([\#17570](https://github.com/element-hq/synapse/issues/17570))

- Clarify default behaviour of the
  [`auto_accept_invites.worker_to_run_on`](https://element-hq.github.io/synapse/develop/usage/configuration/config_documentation.html#auto-accept-invites)
  option. ([\#17515](https://github.com/element-hq/synapse/issues/17515))
- Improve docstrings for profile methods. ([\#17559](https://github.com/element-hq/synapse/issues/17559))

- Add more tracing to experimental [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575) Sliding Sync `/sync` endpoint. ([\#17514](https://github.com/element-hq/synapse/issues/17514))
- Fixup comment in sliding sync implementation. ([\#17531](https://github.com/element-hq/synapse/issues/17531))
- Replace override of deprecated method `HTTPAdapter.get_connection` with `get_connection_with_tls_context`. ([\#17536](https://github.com/element-hq/synapse/issues/17536))
- Fix performance of device lists in `/key/changes` and sliding sync. ([\#17537](https://github.com/element-hq/synapse/issues/17537), [\#17548](https://github.com/element-hq/synapse/issues/17548))
- Bump setuptools from 67.6.0 to 72.1.0. ([\#17542](https://github.com/element-hq/synapse/issues/17542))
- Add a utility function for generating random event IDs. ([\#17557](https://github.com/element-hq/synapse/issues/17557))
- Speed up responding to media requests. ([\#17558](https://github.com/element-hq/synapse/issues/17558), [\#17561](https://github.com/element-hq/synapse/issues/17561), [\#17564](https://github.com/element-hq/synapse/issues/17564), [\#17566](https://github.com/element-hq/synapse/issues/17566), [\#17567](https://github.com/element-hq/synapse/issues/17567), [\#17569](https://github.com/element-hq/synapse/issues/17569))
- Test github token before running release script steps. ([\#17562](https://github.com/element-hq/synapse/issues/17562))
- Reduce log spam of multipart files. ([\#17563](https://github.com/element-hq/synapse/issues/17563))
- Refactor per-connection state in experimental sliding sync handler. ([\#17574](https://github.com/element-hq/synapse/issues/17574))
- Add histogram metrics for sliding sync processing time. ([\#17593](https://github.com/element-hq/synapse/issues/17593))

* Bump bytes from 1.6.1 to 1.7.1. ([\#17526](https://github.com/element-hq/synapse/issues/17526))
* Bump lxml from 5.2.2 to 5.3.0. ([\#17550](https://github.com/element-hq/synapse/issues/17550))
* Bump phonenumbers from 8.13.42 to 8.13.43. ([\#17551](https://github.com/element-hq/synapse/issues/17551))
* Bump regex from 1.10.5 to 1.10.6. ([\#17527](https://github.com/element-hq/synapse/issues/17527))
* Bump sentry-sdk from 2.10.0 to 2.12.0. ([\#17553](https://github.com/element-hq/synapse/issues/17553))
* Bump serde from 1.0.204 to 1.0.206. ([\#17556](https://github.com/element-hq/synapse/issues/17556))
* Bump serde_json from 1.0.122 to 1.0.124. ([\#17555](https://github.com/element-hq/synapse/issues/17555))
* Bump sigstore/cosign-installer from 3.5.0 to 3.6.0. ([\#17549](https://github.com/element-hq/synapse/issues/17549))
* Bump types-pyyaml from 6.0.12.20240311 to 6.0.12.20240808. ([\#17552](https://github.com/element-hq/synapse/issues/17552))
* Bump types-requests from 2.31.0.20240406 to 2.32.0.20240712. ([\#17524](https://github.com/element-hq/synapse/issues/17524))

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

Regressed in #17543.

The `max_download_size` config is not available on workers that don't
load the media repo.

Besides, we should honour the max_size param that was passed into the
function.

Ruff now has decent parity with black and isort, so this is going to just save us a bunch of time

This will help mitigating any discrepancies between the issuer
configured and the one returned by the OIDC provider.

This also removes the need for configuring the `account_management_url`
explicitely, as it will now be loaded from the OIDC discovery, as per
MSC2965.

Because we may now fetch stuff for the .well-known/matrix/client
endpoint, this also transforms the client well-known resource to be
asynchronous.

Rather than 501 M_UNRECOGNISED

Client side implementation at
https://github.com/matrix-org/matrix-react-sdk/pull/12892/

Based on #17600

---------

Co-authored-by: Eric Eastwood <eric.eastwood@beta.gouv.fr>

This is so that we can cache it.

We also move the sliding sync types to
`synapse/types/handlers/sliding_sync.py`. This is mainly in-prep for

The only change in behaviour is that
`RoomSyncConfig.combine_sync_config(..)` now returns a new room sync
config rather than mutating in-place.

Reviewable commit-by-commit.

---------

Co-authored-by: Eric Eastwood <eric.eastwood@beta.gouv.fr>

When returning receipts in sliding sync for initial rooms we should
always include our own receipts in the room (even if they don't match
any timeline events).

Reviewable commit-by-commit.

---------

Co-authored-by: Eric Eastwood <eric.eastwood@beta.gouv.fr>

Otherwise we use the `expected_size` from the initial federation
request, which might be far too low.

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters)

)

---------

Co-authored-by: Erik Johnston <erikj@element.io>

Move calculating of the room lists out of the core handler. This should
make it easier to switch things around to start using the tables in
#17512.

This is just moving code between files and methods.

Reviewable commit-by-commit

`hash_password` now actually accepts password from stdin. The `getpass`
reads from TTY, and does NOT accept stdin in any way.

The manpage has been updated to reflect that.

The idea is to engineer it so that the vast majority of the rooms can
stay in the cache, so we can just ignore them.

The decorator assumed we were always wrapping function methods

That file was getting long.

The changes are non functional, and simply split things up into:
- the main class
- the connection store
- the extensions
- the types