The PR was cherrypicked into v1.51.0rc1.

Synapse 1.51.0rc1 (2022-01-21)
==============================

Features
--------

- Add `track_puppeted_user_ips` config flag to record client IP addresses against puppeted users, and include the puppeted users in monthly active user counts. ([\#11561](https://github.com/matrix-org/synapse/issues/11561), [\#11749](https://github.com/matrix-org/synapse/issues/11749), [\#11757](https://github.com/matrix-org/synapse/issues/11757))
- Include whether the requesting user has participated in a thread when generating a summary for [MSC3440](https://github.com/matrix-org/matrix-doc/pull/3440). ([\#11577](https://github.com/matrix-org/synapse/issues/11577))
- Return an `M_FORBIDDEN` error code instead of `M_UNKNOWN` when a spam checker module prevents a user from creating a room. ([\#11672](https://github.com/matrix-org/synapse/issues/11672))
- Add a flag to the `synapse_review_recent_signups` script to ignore and filter appservice users. ([\#11675](https://github.com/matrix-org/synapse/issues/11675), [\#11770](https://github.com/matrix-org/synapse/issues/11770))

Bugfixes
--------

- Fix a long-standing issue which could cause Synapse to incorrectly accept data in the unsigned field of events
  received over federation. ([\#11530](https://github.com/matrix-org/synapse/issues/11530))
- Fix a long-standing bug where Synapse wouldn't cache a response indicating that a remote user has no devices. ([\#11587](https://github.com/matrix-org/synapse/issues/11587))
- Fix an error that occurs whilst trying to get the federation status of a destination server that was working normally. This admin API was newly introduced in Synapse v1.49.0. ([\#11593](https://github.com/matrix-org/synapse/issues/11593))
- Fix bundled aggregations not being included in the `/sync` response, per [MSC2675](https://github.com/matrix-org/matrix-doc/pull/2675). ([\#11612](https://github.com/matrix-org/synapse/issues/11612), [\#11659](https://github.com/matrix-org/synapse/issues/11659), [\#11791](https://github.com/matrix-org/synapse/issues/11791))
- Fix the `/_matrix/client/v1/room/{roomId}/hierarchy` endpoint returning incorrect fields which have been present since Synapse 1.49.0. ([\#11667](https://github.com/matrix-org/synapse/issues/11667))
- Fix preview of some GIF URLs (like tenor.com). Contributed by Philippe Daouadi. ([\#11669](https://github.com/matrix-org/synapse/issues/11669))
- Fix a bug where only the first 50 rooms from a space were returned from the `/hierarchy` API. This has existed since the introduction of the API in Synapse v1.41.0. ([\#11695](https://github.com/matrix-org/synapse/issues/11695))
- Fix a bug introduced in Synapse v1.18.0 where password reset and address validation emails would not be sent if their subject was configured to use the 'app' template variable. Contributed by @br4nnigan. ([\#11710](https://github.com/matrix-org/synapse/issues/11710), [\#11745](https://github.com/matrix-org/synapse/issues/11745))
- Make the 'List Rooms' Admin API sort stable. Contributed by Daniël Sonck. ([\#11737](https://github.com/matrix-org/synapse/issues/11737))
- Fix a long-standing bug where space hierarchy over federation would only work correctly some of the time. ([\#11775](https://github.com/matrix-org/synapse/issues/11775))
- Fix a bug introduced in Synapse v1.46.0 that prevented `on_logged_out` module callbacks from being correctly awaited by Synapse. ([\#11786](https://github.com/matrix-org/synapse/issues/11786))

Improved Documentation
----------------------

- Warn against using a Let's Encrypt certificate for TLS/DTLS TURN server client connections, and suggest using ZeroSSL certificate instead. This works around client-side connectivity errors caused by WebRTC libraries that reject Let's Encrypt certificates. Contibuted by @AndrewFerr. ([\#11686](https://github.com/matrix-org/synapse/issues/11686))
- Document the new `SYNAPSE_TEST_PERSIST_SQLITE_DB` environment variable in the contributing guide. ([\#11715](https://github.com/matrix-org/synapse/issues/11715))
- Document that the minimum supported PostgreSQL version is now 10. ([\#11725](https://github.com/matrix-org/synapse/issues/11725))
- Fix typo in demo docs: differnt. ([\#11735](https://github.com/matrix-org/synapse/issues/11735))
- Update room spec URL in config files. ([\#11739](https://github.com/matrix-org/synapse/issues/11739))
- Mention `python3-venv` and `libpq-dev` dependencies in the contribution guide. ([\#11740](https://github.com/matrix-org/synapse/issues/11740))
- Update documentation for configuring login with Facebook. ([\#11755](https://github.com/matrix-org/synapse/issues/11755))
- Update installation instructions to note that Python 3.6 is no longer supported. ([\#11781](https://github.com/matrix-org/synapse/issues/11781))

Deprecations and Removals
-------------------------

- Remove the unstable `/send_relation` endpoint. ([\#11682](https://github.com/matrix-org/synapse/issues/11682))
- Remove `python_twisted_reactor_pending_calls` Prometheus metric. ([\#11724](https://github.com/matrix-org/synapse/issues/11724))
- Remove the `password_hash` field from the response dictionaries of the [Users Admin API](https://matrix-org.github.io/synapse/latest/admin_api/user_admin_api.html). ([\#11576](https://github.com/matrix-org/synapse/issues/11576))
- Deprecate support for `webclient` listeners and non-HTTP(S) `web_client_location` configuration. ([\#11774](https://github.com/matrix-org/synapse/issues/11774), [\#11783](https://github.com/matrix-org/synapse/issues/11783))

Internal Changes
----------------

- Run `pyupgrade --py37-plus --keep-percent-format` on Synapse. ([\#11685](https://github.com/matrix-org/synapse/issues/11685))
- Use buildkit's cache feature to speed up docker builds. ([\#11691](https://github.com/matrix-org/synapse/issues/11691))
- Use `auto_attribs` and native type hints for attrs classes. ([\#11692](https://github.com/matrix-org/synapse/issues/11692), [\#11768](https://github.com/matrix-org/synapse/issues/11768))
- Remove debug logging for #4422, which has been closed since Synapse 0.99. ([\#11693](https://github.com/matrix-org/synapse/issues/11693))
- Remove fallback code for Python 2. ([\#11699](https://github.com/matrix-org/synapse/issues/11699))
- Add a test for [an edge case](https://github.com/matrix-org/synapse/pull/11532#discussion_r769104461) in the `/sync` logic. ([\#11701](https://github.com/matrix-org/synapse/issues/11701))
- Add the option to write SQLite test dbs to disk when running tests. ([\#11702](https://github.com/matrix-org/synapse/issues/11702))
- Improve Complement test output for Gitub Actions. ([\#11707](https://github.com/matrix-org/synapse/issues/11707))
- Fix docstring on `add_account_data_for_user`. ([\#11716](https://github.com/matrix-org/synapse/issues/11716))
- Complement environment variable name change and update `.gitignore`. ([\#11718](https://github.com/matrix-org/synapse/issues/11718))
- Simplify calculation of Prometheus metrics for garbage collection. ([\#11723](https://github.com/matrix-org/synapse/issues/11723))
- Improve accuracy of `python_twisted_reactor_tick_time` Prometheus metric. ([\#11724](https://github.com/matrix-org/synapse/issues/11724), [\#11771](https://github.com/matrix-org/synapse/issues/11771))
- Minor efficiency improvements when inserting many values into the database. ([\#11742](https://github.com/matrix-org/synapse/issues/11742))
- Invite PR authors to give themselves credit in the changelog. ([\#11744](https://github.com/matrix-org/synapse/issues/11744))
- Add optional debugging to investigate [issue 8631](https://github.com/matrix-org/synapse/issues/8631). ([\#11760](https://github.com/matrix-org/synapse/issues/11760))
- Remove `log_function` utility function and its uses. ([\#11761](https://github.com/matrix-org/synapse/issues/11761))
- Add a unit test that checks both `client` and `webclient` resources will function when simultaneously enabled. ([\#11765](https://github.com/matrix-org/synapse/issues/11765))
- Allow overriding complement commit using `COMPLEMENT_REF`. ([\#11766](https://github.com/matrix-org/synapse/issues/11766))
- Add some comments and type annotations for `_update_outliers_txn`. ([\#11776](https://github.com/matrix-org/synapse/issues/11776))

... and start populating them for new events

This is a follow-up to #10565.

Preparation for dropping this table altogether. Part of #6574.

This is more efficient, since we only have to look up `state_key` in the event
dict once, rather than three (!) times.

Make the `get_global_account_data_by_type_for_user` cache be a tree-cache whose key is prefixed with the user ID (#11788)

To not change the behaviour during the deprecation period.

Follow-up to #11774.

#11595 dropped support for python 3.6, but forgot to update this doc.

This changes the behaviour of the root endpoint to redirect
directly to the configuration of `web_client_location` if it is
given an HTTP(S) URL.

Debug for #8631.

I'm having a hard time tracking down what's going wrong in that issue.
In the reported example, I could see server A sending federation traffic
to server B and all was well. Yet B reports out-of-sync device updates
from A.

I couldn't see what was _in_ the events being sent from A to B. So I
have added some crude logging to track

- when we have updates to send to a remote HS
- the edus we actually accumulate to send
- when a federation transaction includes a device list update edu
- when such an EDU is received

This is a bit of a sledgehammer.

Updates complement.sh to read the ref from an environment
variable (defaulting to master) when downloading a complement
bundle for testing.

`FederationClient.get_room_hierarchy()` caches its return values, so
refactor the code to avoid modifying the returned room summary.

A couple of surprises for me here, so thought I'd document them

By scraping Open Graph information from the HTML even
when an autodiscovery endpoint is found. The results are
then combined to capture as much information as possible
from the page.

Per updates to MSC3440.

This is implement as a separate method since it needs to be cached
on a per-user basis, instead of a per-thread basis.

I've never found this terribly useful. I think it was added in the early days
of Synapse, without much thought as to what would actually be useful to log,
and has just been cargo-culted ever since.

Rather, it tends to clutter up debug logs with useless information.

Add a flag to the `synapse_review_recent_signups` script to ignore and filter appservice users. (#11675)

* Warn against using Let's Encrypt certs for encrypted TURN

This helps to avoid client-side issues:
* https://github.com/vector-im/element-android/issues/1533
* https://github.com/vector-im/element-ios/issues/2712



Signed-off-by: Andrew Ferrazzutti <fair@miscworks.net>

Fixes #11741

... and a minor thinko fix in the sample config.