Spawning from
https://github.com/matrix-org/synapse/pull/12588#discussion_r865843321
> It turns out `Deferred.cancel()` is a lot like
`Deferred.callback()`/`errback()` in that it will trash the logging
context:
> it can resume a coroutine, which will restore its own logging context,
then run:
>
> - until it blocks, setting the sentinel context
> - or until it terminates, setting the context it was started with
>
> So we need to wrap it in `with PreserveLoggingContext():`, like we do
with `.callback()`:
>
> ```python
> with PreserveLoggingContext():
> self.render_deferred.cancel()
> ```
>
> *-- @squahtx,
https://github.com/matrix-org/synapse/pull/12588#discussion_r865843321*
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