When we hit an unknown room in the space tree, see if there are other servers that we might be able to poll to get the data.

Fixes: #9447

This fixes an error ("Cannot determine consistent method resolution order (MRO)")
when running mypy with a cache.

It's legitimate behaviour to try and join a bunch of rooms at once.

Builds on the work done in #9643 to add a federation API for space summaries.

There's a bit of refactoring of the existing client-server code first, to avoid too much duplication.

Addresses https://github.com/matrix-org/synapse-dinsic/issues/70

This PR causes `ProxyAgent` to attempt to extract credentials from an `HTTPS_PROXY` env var. If credentials are found, a `Proxy-Authorization` header ([details](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Proxy-Authorization)) is sent to the proxy server to authenticate against it. The headers are *not* passed to the remote server.

Also added some type hints.

Cf. https://github.com/opencontainers/image-spec/blob/master/annotations.md#pre-defined-annotation-keys



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

- Merge 'isinstance' calls.
- Remove unnecessary dict call outside of comprehension.
- Use 'sys.exit()' calls.

Implement MSC3026: busy presence state

This is very bare-bones for now: federation will come soon, while pagination is descoped for now but will come later.

Currently federation catchup will send the last *local* event that we
failed to send to the remote. This can cause issues for large rooms
where lots of servers have sent events while the remote server was down,
as when it comes back up again it'll be flooded with events from various
points in the DAG.

Instead, let's make it so that all the servers send the most recent
events, even if its not theirs. The remote should deduplicate the
events, so there shouldn't be much overhead in doing this.
Alternatively, the servers could only send local events if they were
also extremities and hope that the other server will send the event
over, but that is a bit risky.

This bug was discovered by DINUM. We were modifying `serialized_event["content"]`, which - if you've got `USE_FROZEN_DICTS` turned on or are [using a third party rules module](https://github.com/matrix-org/synapse/blob/17cd48fe5171d50da4cb59db647b993168e7dfab/synapse/events/third_party_rules.py#L73-L76) - will raise a 500 if you try to a edit a reply to a message.

`serialized_event["content"]` could be set to the edit event's content, instead of a copy of it, which is bad as we attempt to modify it. Instead, we also end up modifying the original event's content. DINUM uses a third party rules module, which meant the event's content got frozen and thus an exception was raised.

To be clear, the problem is not that the event's content was frozen. In fact doing so helped us uncover the fact we weren't copying event content correctly.

By splitting this to two separate methods the callers know
what methods they can expect on the handler.

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

We had two functions named `get_forward_extremities_for_room` and
`get_forward_extremeties_for_room` that took different paramters. We
rename one of them to avoid confusion.

* Populate `internal_metadata.outlier` based on `events` table

Rather than relying on `outlier` being in the `internal_metadata` column,
populate it based on the `events.outlier` column.

* Move `outlier` out of InternalMetadata._dict

Ultimately, this will allow us to stop writing it to the database. For now, we
have to grandfather it back in so as to maintain compatibility with older
versions of Synapse.

* Adds B00 to ignored checks.
* Fixes remaining issues.

Allows limiting who can login using OIDC via the claims
made from the IdP.

Instead of if the user does not have a password hash. This allows a SSO
user to add a password to their account, but only if the local password
database is configured.