Fix potential lost logcontext when PerDestinationQueue.shutdown(...)

[MadLittleMods]

Fix potential lost logcontext when PerDestinationQueue.shutdown(...)

Spawning from looking at the logs in #19165 (comment) which mention the federation_transaction_transmission_loop. I don't think it's the source of the lost logcontext that person in the issue is experiencing because this only applies when you try to shutdown the homeserver.

Problem code introduced in #18828

To explain the fix, see the Deferred callbacks section of our logcontext docs for more info (specifically using solution 2).

Heads-up, I wrote the docs too so it's my assumptions/understanding all the way down. Apply your own scrutiny.

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[MadLittleMods]

To explain the fix, see the Deferred callbacks section of our logcontext docs for more info (specifically using solution 2).

Heads-up, I wrote the docs too so it's my assumptions/understanding all the way down. Apply your own scrutiny.

[devonh]

Based on my understanding, and your docs this change makes sense to me.

Though I agree it shouldn't fix the issue reported as normal Synapse usage never calls shutdown.

[MadLittleMods]

Thanks for the review @devonh 🐄