An attempt to type tests.util.caches

[DMRobertson]

This wasn't very satisfactory: I had to resort to a lot of type-ignores here.

I struggle to understand our caching descriptor machinery at the best of times, but I think that the annotations for @cached, plus whatever we do in the Synapse mypy plugin isn't quite working right in these tests.

It's not chopped up into meaningful commits; I am happy to rebase this to do so if it would be helpful.

LMK what you think. Is this good enough to land, or should we look for a different approach?

[DMRobertson]

should we look for a different approach?

I think sticking some defer.ensureDeferred might remove the need for type-ignores---though it's still about the same amount of effort to make mypy accept runtime-valid source code.

[clokep]

This sounds like the mypy plugin for cached isn't working at all then? This makes me very nervous to ignore TBH.

[DMRobertson]

I think it works okay in Synapse proper, when @cached decorates an async def and is awaited. I don't think it works well if you're trying to get at the underlying Deferred machinery, as we are here.

[clokep]

Does it Just Work™️ if we use async def instead?

[DMRobertson]

Haven't got the time to look at this right now. Going to close it for now since we weren't very happy with how the mypy plugin handles @cached wrapping a function that returns a deferred

[DMRobertson]

Superseded by #15659 and #15658