Introduce `Clock.call_when_running(...)` to wrap startup code in a
logcontext, ensuring we can identify which server generated the logs.
Background:
> Ideally, nothing from the Synapse homeserver would be logged against the `sentinel`
> logcontext as we want to know which server the logs came from. In practice, this is not
> always the case yet especially outside of request handling.
>
> Global things outside of Synapse (e.g. Twisted reactor code) should run in the
> `sentinel` logcontext. It's only when it calls into application code that a logcontext
> gets activated. This means the reactor should be started in the `sentinel` logcontext,
> and any time an awaitable yields control back to the reactor, it should reset the
> logcontext to be the `sentinel` logcontext. This is important to avoid leaking the
> current logcontext to the reactor (which would then get picked up and associated with
> the next thing the reactor does).
>
> *-- `docs/log_contexts.md`
Also adds a lint to prefer `Clock.call_when_running(...)` over
`reactor.callWhenRunning(...)`
Part of https://github.com/element-hq/synapse/issues/18905
This can help ensure that the rooms are eventually purged if the other
local users also forget them. Synapse already clears some of the room
information as part of the `_background_remove_left_rooms` background
task, but this doesn't catch `events`, `event_json`, etc.
During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
This improves load times for push rules:
| Version | Time per user | Time for 1k users |
| -------------------- | ------------- | ----------------- |
| Before | 138 µs | 138ms |
| Now (with custom) | 2.11 µs | 2.11ms |
| Now (without custom) | 49.7 ns | 0.05 ms |
This therefore has a large impact on send times for rooms
with large numbers of local users in the room.
There are a bunch of places we call get_success on an immediate value, which is unnecessary. Let's rip them out, and remove the redundant functionality in get_success and friends.
