Drop run and run_in_threadpool

[lovelydinosaur]

We're no longer using the backend methods run and run_in_threadpool within the httpx codebase, so this PR removes them.

We are however using them within the test cases in order to perform the server restarts.

I've changed things around so that .restart() is a standard blocking function (It may as well be, and it runs from a different thread from the server, so it won't block the server itself from shutting down.)

However, the tests that use server.restart() are currently failing in the asyncio case. (With h11 errors that the server close event is occuring at a point that it should not.) Really not clear to me why we'd see that with asyncio but not with trio. I think that are resolution here might be that restart should perform a full shutdown, and reopen the server on an entirely new thread, to keep things cleaner?

Any thoughts on this @florimondmanca?

[florimondmanca]

Yes, I remember server restarts have already been the cause of some headaches in the past… #276 (comment)

As far as understanding the error message, it's already been discussed in https://github.com/encode/httpx/issues/96 — basically the server abruptly closed the connection (which it did). We're making a blocking call in the main thread via time.sleep(), which seems to be OK for trio, but I'd suspect that's what causing failures on asyncio.

Actually, if we turn restart() into a coroutine again, and have it call asyncio.sleep() instead of time.sleep(), the test runs fine on asyncio — but it fails on trio, obviously, because of the asyncio-specific call.

Luckily, we have an agnostic sleep() implementation in tests/concurrency.py, so since there are no other asyncio-specific async calls in restart(), we can probably call sleep(backend, ...), and things will work on trio too?

Lemme push a commit to demonstrate the idea…

[florimondmanca]

Note that these connection pool tests run faster now, because we don't have to setup/teardown a thread on each restart. 😄

[lovelydinosaur]

@florimondmanca Thanks, great stuff!

I've now also dropped AsyncioBackend.loop, since it's not used anywhere.

Also cleaned up the tests/concurrency.py util functions to use a more direct style than functools.single_dispatch - we don't really need to be that clever.

[florimondmanca]

💯