This PR adds a Dockerfile and some supporting files to the `docker/` directory. The Dockerfile's intention is to spin up a container with:

* A Synapse main process.
* Any desired worker processes, defined by a `SYNAPSE_WORKERS` environment variable supplied at runtime.
* A redis for worker communication.
* A nginx for routing traffic.
* A supervisord to start all worker processes and monitor them if any go down.

Note that **this is not currently intended to be used in production**. If you'd like to use Synapse workers with Docker, instead make use of the official image, with one worker per container. The purpose of this dockerfile is currently to allow testing Synapse in worker mode with the [Complement](https://github.com/matrix-org/complement/) test suite.

`configure_workers_and_start.py` is where most of the magic happens in this PR. It reads from environment variables (documented in the file) and creates all necessary config files for the processes. It is the entrypoint of the Dockerfile, and thus is run any time the docker container is spun up, recreating all config files in case you want to use a different set of workers. One can specify which workers they'd like to use by setting the `SYNAPSE_WORKERS` environment variable (as a comma-separated list of arbitrary worker names) or by setting it to `*` for all worker processes. We will be using the latter in CI.

Huge thanks to @MatMaul for helping get this all working  This PR is paired with its equivalent on the Complement side: https://github.com/matrix-org/complement/pull/62.

Note, for the purpose of testing this PR before it's merged: You'll need to (re)build the base Synapse docker image for everything to work (`matrixdotorg/synapse:latest`). Then build the worker-based docker image on top (`matrixdotorg/synapse:workers`). 

Context is in https://github.com/matrix-org/synapse/issues/9764#issuecomment-818615894.

I struggled to find a more official link for this. The problem occurs when using WSL1 instead of WSL2, which some Windows platforms (at least Server 2019) still don't have. Docker have updated their documentation to paint a much happier picture now given WSL2's support.

The last sentence here can probably be removed once WSL1 is no longer around... though that will likely not be for a very long time.

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

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

* Drop Python 3.5 from Trove classifier metadata.

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

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

This change ensures that the appservice registration behaviour follows the spec. We decided to do this for Dendrite, so it made sense to also make a PR for synapse to correct the behaviour.

There's no point logging this twice.

By providing the additional build tag for `msc2946`.

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

Related: #8334
Deprecated in: #9429 - Synapse 1.28.0 (2021-02-25)

`GET /_synapse/admin/v1/users/<user_id>` has no
- unit tests
- documentation

API in v2 is available (#5925 - 12/2019, v1.7.0).
API is misleading. It expects `user_id` and returns a list of all users.

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

Part of #9366

Adds in fixes for B006 and B008, both relating to mutable parameter lint errors.

Signed-off-by: Jonathan de Jong <jonathan@automatia.nl>

Require py36, Postgres 9.6, and sqlite 3.22

Fix incompatibility with bionic

We pull all destinations requiring catchup from the DB in batches.
However, if all those destinations get filtered out (due to the
federation sender being sharded), then the `last_processed` destination
doesn't get updated, and we keep requesting the same set repeatedly.

Apparently on tox 2.5, `usedevelop` overrides `skip_install`, so we end up
trying to install the full dependencies even for the `-old` environment.

They don't make any sense on the intermediate builder image. The final
images needs them to be of use for anyone.

Signed-off-by: Johannes Wienke <languitar@semipol.de>

When joining a room with join rules set to 'restricted', check if the
user is a member of the spaces defined in the 'allow' key of the join rules.

This only applies to an experimental room version, as defined in MSC3083.

Records additional request information into the structured logs,
e.g. the requester, IP address, etc.

This PR modifies `GaugeBucketCollector` to only report data once it has been updated, rather than initially reporting a value of 0. Fixes zero values being reported for some metrics on startup until a background job to update the metric's value runs later.

At the moment, if you'd like to share presence between local or remote users, those users must be sharing a room together. This isn't always the most convenient or useful situation though.

This PR adds a module to Synapse that will allow deployments to set up extra logic on where presence updates should be routed. The module must implement two methods, `get_users_for_states` and `get_interested_users`. These methods are given presence updates or user IDs and must return information that Synapse will use to grant passing presence updates around.

A method is additionally added to `ModuleApi` which allows triggering a set of users to receive the current, online presence information for all users they are considered interested in. This is the equivalent of that user receiving presence information during an initial sync. 

The goal of this module is to be fairly generic and useful for a variety of applications, with hard requirements being:

* Sending state for a specific set or all known users to a defined set of local and remote users.
* The ability to trigger an initial sync for specific users, so they receive all current state.

The `remote_media_cache_thumbnails_media_origin_media_id_thumbna_key`
constraint is superceded by
`remote_media_repository_thumbn_media_origin_id_width_height_met` (which adds
`thumbnail_method` to the unique key).

PR #7124 made an attempt to remove the old constraint, but got the name wrong,
so it didn't work. Here we update the bg update and rerun it.

Fixes #8649.

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