Convert runInteraction to async/await

[clokep]

This converts runInteraction to async/await.

This should be the last "main" part of #7988.

[clokep]

This has a conflict already so I'll likely rebase this after #8140 is merged.

[clokep]

@erikjohnston previously stated that we might consider it an anti-pattern to return Awaitable instead of awaiting on it. I don't know if we want to "fix" that as part of this work or leave it to a future piece of work? We could do this module-by-module or so if we wanted (which might also be a way to make this PR smaller).

[clokep]

I'm putting review on this, but it's much larger than I'd like. Thoughts / ideas of how to slim it down and make it more reviewable would be appreciated!

[anoadragon453]

While it would roughly 2x the size of this PR, it's not too much more of a mental exercise to review converting to Await + converting the method itself to async and adding an await to the return statement.

If you did that and then proposed review by modules I think that could be manageable.

[clokep]

I thought a bit more about this and I think it will make sense to do this method by method, e.g. make simple_select_one into an async function and chase down all the users of that. Then after all those are done, do runInteraction directly (which should essentially just be this PR, but with a bunch of stuff removed).

[clokep]

There's a few PRs that were spun out of this. They don't really depend on this, but it would be nice to merge them first so that the comments and type hints are all correct in referring to awaitables.

[clokep]

I think this can be:

Suggested change

	async def runInteraction(
	self, desc: str, func: Callable, *args: Any, **kwargs: Any
	) -> Any:
	async def runInteraction(
	self, desc: str, func: "Callable[..., R]", *args: Any, **kwargs: Any
	) -> R:

But mypy then complains:

synapse/storage/database.py:550: error: Incompatible return value type (got "R", expected "R") [return-value]

I suspect something isn't getting piped through runWithConnection / new_transaction properly?

[erikjohnston]

So I think the disconnect is happening because mypy doesn't know that the return type of self.new_transaction is the same R as returned by func, due to the indirection of runWithConnection.

The signature runWithConnection(func, *args, **kwargs) means that when we call it with runWithConnection(self.new_transaction, func) mypy only sees us calling self.new_transaction(*args) as the typing of func is lost due to the variadic parameters.

Changing it to runWithConnection(lambda conn: self.new_transaction(conn, func)) works, but it may be simpler to do a result = cast(R, result) instead.

[clokep]

Woot! Thanks. See ca31cfe for this.