Move start_doing_background_updates() to SynapseHomeServer.start_background_tasks()

[MadLittleMods]

Move start_doing_background_updates() to SynapseHomeServer.start_background_tasks() (more sane standard location for this sort of thing)

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[MadLittleMods]

The one difference here is that previously, start_doing_background_updates() only ran on the main Synapse instance.

But start_background_tasks() runs on the worker that supposed to run_background_tasks.

synapse/synapse/app/_base.py

Lines 683 to 684 in 47fb4b4

	if hs.config.worker.run_background_tasks:
	hs.start_background_tasks()

Is that okay? Nothing is immediately coming to mind on why this would be bad.

[reivilibre]

seems OK to me too

[MadLittleMods]

Actually, I think there is a flaw with this setup.

SynapseHomeServer is for the main Synapse instance. We have SynapseHomeServer.start_background_tasks() setup to call start_doing_background_updates() but if the run_background_tasks_on is set to anything else besides main, then we will never run SynapseHomeServer.start_background_tasks() specifically.

What we actually need to do is just move this to HomeServer.start_background_tasks() instead. We want start_doing_background_updates() to run on whatever homeserver is configured to run_background_tasks_on

[MadLittleMods]

This is being addressed in #19057

[MadLittleMods]

Deciding to revert this PR in the meantime -> #19059

While I figure out all of the intricacies in the test infrastructure from moving it to the correct spot. Will re-introduce things in #19057

[MadLittleMods]

Thanks for the review @reivilibre 🦧