* Split ShardedWorkerHandlingConfig

This is so that we have a type level understanding of when it is safe to
call `get_instance(..)` (as opposed to `should_handle(..)`).

* Remove special cases in ShardedWorkerHandlingConfig.

`ShardedWorkerHandlingConfig` tried to handle the various different ways
it was possible to configure federation senders and pushers. This led to
special cases that weren't hit during testing.

To fix this the handling of the different cases is moved from there and
`generic_worker` into the worker config class. This allows us to have
the logic in one place and allows the rest of the code to ignore the
different cases.

The idea here is to stop people forgetting to call `check_consistency`. Folks can still just pass in `None` to the new args in `build_sequence_generator`, but hopefully they won't.

This confused me for a while.

And ensure the consistency of `event_auth_chain_id`.

`uploads_path` was a thing that was never used; most of it was removed in #6628
but a few vestiges remained.

It should be possible to reload `synapse.target` to have the reload propagate
to all the synapse units.

This PR remove the cache for the `get_shared_rooms_for_users` storage method (the db method driving the experimental "what rooms do I share with this user?" feature: [MSC2666](https://github.com/matrix-org/matrix-doc/pull/2666)). Currently subsequent requests to the endpoint will return the same result, even if your shared rooms with that user have changed.

The cache was added in https://github.com/matrix-org/synapse/pull/7785, but we forgot to ensure it was invalidated appropriately.

Upon attempting to invalidate it, I found that the cache had to be entirely invalidated whenever a user (remote or local) joined or left a room. This didn't make for a very useful cache, especially for a function that may or may not be called very often. Thus, I've opted to remove it instead of invalidating it.

The user directory sample config section was a little messy, and didn't adhere to our [recommended config format guidelines](https://github.com/matrix-org/synapse/blob/develop/docs/code_style.md#configuration-file-format).

This PR cleans that up a bit.

Synapse 1.28.0rc1 (2021-02-19)
==============================

Note that this release drops support for ARMv7 in the official Docker images, due to repeated problems building for ARMv7 (and the associated maintenance burden this entails).

This release also fixes the documentation included in v1.27.0 around the callback URI for SAML2 identity providers. If your server is configured to use single sign-on via a SAML2 IdP, you may need to make configuration changes. Please review [UPGRADE.rst](UPGRADE.rst) for more details on these changes.

Removal warning
---------------

The v1 list accounts API is deprecated and will be removed in a future release.
This API was undocumented and misleading. It can be replaced by the
[v2 list accounts API](https://github.com/matrix-org/synapse/blob/release-v1.28.0/docs/admin_api/user_admin_api.rst#list-accounts),
which has been available since Synapse 1.7.0 (2019-12-13).

Please check if you're using any scripts which use the admin API and replace
`GET /_synapse/admin/v1/users/<user_id>` with `GET /_synapse/admin/v2/users`.

Features
--------

- New admin API to get the context of an event: `/_synapse/admin/rooms/{roomId}/context/{eventId}`. ([\#9150](https://github.com/matrix-org/synapse/issues/9150))
- Further improvements to the user experience of registration via single sign-on. ([\#9300](https://github.com/matrix-org/synapse/issues/9300), [\#9301](https://github.com/matrix-org/synapse/issues/9301))
- Add hook to spam checker modules that allow checking file uploads and remote downloads. ([\#9311](https://github.com/matrix-org/synapse/issues/9311))
- Add support for receiving OpenID Connect authentication responses via form `POST`s rather than `GET`s. ([\#9376](https://github.com/matrix-org/synapse/issues/9376))
- Add the shadow-banning status to the admin API for user info. ([\#9400](https://github.com/matrix-org/synapse/issues/9400))

Bugfixes
--------

- Fix long-standing bug where sending email notifications would fail for rooms that the server had since left. ([\#9257](https://github.com/matrix-org/synapse/issues/9257))
- Fix bug in Synapse 1.27.0rc1 which meant the "session expired" error page during SSO registration was badly formatted. ([\#9296](https://github.com/matrix-org/synapse/issues/9296))
- Assert a maximum length for some parameters for spec compliance. ([\#9321](https://github.com/matrix-org/synapse/issues/9321), [\#9393](https://github.com/matrix-org/synapse/issues/9393))
- Fix additional errors when previewing URLs: "AttributeError 'NoneType' object has no attribute 'xpath'" and "ValueError: Unicode strings with encoding declaration are not supported. Please use bytes input or XML fragments without declaration.". ([\#9333](https://github.com/matrix-org/synapse/issues/9333))
- Fix a bug causing Synapse to impose the wrong type constraints on fields when processing responses from appservices to `/_matrix/app/v1/thirdparty/user/{protocol}`. ([\#9361](https://github.com/matrix-org/synapse/issues/9361))
- Fix bug where Synapse would occasionally stop reconnecting to Redis after the connection was lost. ([\#9391](https://github.com/matrix-org/synapse/issues/9391))
- Fix a long-standing bug when upgrading a room: "TypeError: '>' not supported between instances of 'NoneType' and 'int'". ([\#9395](https://github.com/matrix-org/synapse/issues/9395))
- Reduce the amount of memory used when generating the URL preview of a file that is larger than the `max_spider_size`. ([\#9421](https://github.com/matrix-org/synapse/issues/9421))
- Fix a long-standing bug in the deduplication of old presence, resulting in no deduplication. ([\#9425](https://github.com/matrix-org/synapse/issues/9425))
- The `ui_auth.session_timeout` config option can now be specified in terms of number of seconds/minutes/etc/. Contributed by Rishabh Arya. ([\#9426](https://github.com/matrix-org/synapse/issues/9426))
- Fix a bug introduced in v1.27.0: "TypeError: int() argument must be a string, a bytes-like object or a number, not 'NoneType." related to the user directory. ([\#9428](https://github.com/matrix-org/synapse/issues/9428))

Updates to the Docker image
---------------------------

- Drop support for ARMv7 in Docker images. ([\#9433](https://github.com/matrix-org/synapse/issues/9433))

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

- Reorganize CHANGELOG.md. ([\#9281](https://github.com/matrix-org/synapse/issues/9281))
- Add note to `auto_join_rooms` config option explaining existing rooms must be publicly joinable. ([\#9291](https://github.com/matrix-org/synapse/issues/9291))
- Correct name of Synapse's service file in TURN howto. ([\#9308](https://github.com/matrix-org/synapse/issues/9308))
- Fix the braces in the `oidc_providers` section of the sample config. ([\#9317](https://github.com/matrix-org/synapse/issues/9317))
- Update installation instructions on Fedora. ([\#9322](https://github.com/matrix-org/synapse/issues/9322))
- Add HTTP/2 support to the nginx example configuration. Contributed by David Vo. ([\#9390](https://github.com/matrix-org/synapse/issues/9390))
- Update docs for using Gitea as OpenID provider. ([\#9404](https://github.com/matrix-org/synapse/issues/9404))
- Document that pusher instances are shardable. ([\#9407](https://github.com/matrix-org/synapse/issues/9407))
- Fix erroneous documentation from v1.27.0 about updating the SAML2 callback URL. ([\#9434](https://github.com/matrix-org/synapse/issues/9434))

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

- Deprecate old admin API `GET /_synapse/admin/v1/users/<user_id>`. ([\#9429](https://github.com/matrix-org/synapse/issues/9429))

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

- Fix 'object name reserved for internal use' errors with recent versions of SQLite. ([\#9003](https://github.com/matrix-org/synapse/issues/9003))
- Add experimental support for running Synapse with PyPy. ([\#9123](https://github.com/matrix-org/synapse/issues/9123))
- Deny access to additional IP addresses by default. ([\#9240](https://github.com/matrix-org/synapse/issues/9240))
- Update the `Cursor` type hints to better match PEP 249. ([\#9299](https://github.com/matrix-org/synapse/issues/9299))
- Add debug logging for SRV lookups. Contributed by @Bubu. ([\#9305](https://github.com/matrix-org/synapse/issues/9305))
- Improve logging for OIDC login flow. ([\#9307](https://github.com/matrix-org/synapse/issues/9307))
- Share the code for handling required attributes between the CAS and SAML handlers. ([\#9326](https://github.com/matrix-org/synapse/issues/9326))
- Clean up the code to load the metadata for OpenID Connect identity providers. ([\#9362](https://github.com/matrix-org/synapse/issues/9362))
- Convert tests to use `HomeserverTestCase`. ([\#9377](https://github.com/matrix-org/synapse/issues/9377), [\#9396](https://github.com/matrix-org/synapse/issues/9396))
- Update the version of black used to 20.8b1. ([\#9381](https://github.com/matrix-org/synapse/issues/9381))
- Allow OIDC config to override discovered values. ([\#9384](https://github.com/matrix-org/synapse/issues/9384))
- Remove some dead code from the acceptance of room invites path. ([\#9394](https://github.com/matrix-org/synapse/issues/9394))
- Clean up an unused method in the presence handler code. ([\#9408](https://github.com/matrix-org/synapse/issues/9408))

This PR attempts to eliminate unnecessary presence sending work when your local server joins a room, or when a remote server joins a room your server is participating in by processing state deltas in chunks rather than individually.

---

When your server joins a room for the first time, it requests the historical state as well. This chunk of new state is passed to the presence handler which, after filtering that state down to only membership joins, will send presence updates to homeservers for each join processed.

It turns out that we were being a bit naive and processing each event individually, and sending out presence updates for every one of those joins. Even if many different joins were users on the same server (hello IRC bridges), we'd send presence to that same homeserver for every remote user join we saw.

This PR attempts to deduplicate all of that by processing the entire batch of state deltas at once, instead of only doing each join individually. We process the joins and note down which servers need which presence:

* If it was a local user join, send that user's latest presence to all servers in the room
* If it was a remote user join, send the presence for all local users in the room to that homeserver

We deduplicate by inserting all of those pending updates into a dictionary of the form:

```
{
  server_name1: {presence_update1, ...},
  server_name2: {presence_update1, presence_update2, ...}
}
```

Only after building this dict do we then start sending out presence updates.

This PR adds a homeserver config option, `user_directory.prefer_local_users`, that when enabled will show local users higher in user directory search results than remote users. This option is off by default.

Note that turning this on doesn't necessarily mean that remote users will always be put below local users, but they should be assuming all other ranking factors (search query match, profile information present etc) are identical.

This is useful for, say, University networks that are openly federating, but want to prioritise local students and staff in the user directory over other random users.

Add off-by-default configuration settings to:
- disable putting an invitee's profile info in invite events
- disable profile lookup via federation

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

Updates documentation from #9289 and removes a deprecated
endpoint which didn't work as expected.

This reverts commit 8ad4676f.

It's proving incredibly hard to build in CircleCI infra.

This API was undocumented and nonsensical.

This reduces the memory usage of previewing media files which
end up larger than the `max_spider_size` by avoiding buffering
content internally in treq.

It also checks the `Content-Length` header in additional places
instead of streaming the content to check the body length.

As the comment says, this guard was there for when the
initial user directory update has yet to happen.

This reverts commit a8878960.