Fix async cancellation behaviour

[lovelydinosaur]

Closes #564.

I haven't come up with a good test case for this yet, but it resolves the example provided in that issue, by ensuring the request queue modifications happen outside of async context switches.

[Kludex]

Should we wait for a test to be added, or may I ask instructions on how to test it?

[lovelydinosaur]

Within a clean directory...

example.py

import trio
import httpcore


async def worker(client, nurse):
    r = await client.request('HEAD', 'https://google.com')
    print(r)
    nurse.cancel_scope.cancel()


async def main():
    async with httpcore.AsyncConnectionPool() as client, trio.open_nursery() as nurse:
        for _ in range(3):
            nurse.start_soon(worker, client, nurse)


if __name__ == "__main__":
    trio.run(main)

Using trio

Now...

$ python3 -m venv venv
$ venv/bin/pip install trio
$ venv/bin/pip install httpcore
$ venv/bin/python ./example.py 
<Response [301]>
Traceback (most recent call last):
  File "/Users/tomchristie/Temp/./example.py", line 18, in <module>
    trio.run(main)
  File "/Users/tomchristie/Temp/venv/lib/python3.9/site-packages/trio/_core/_run.py", line 1946, in run
    raise runner.main_task_outcome.error
  File "/Users/tomchristie/Temp/./example.py", line 14, in main
    nurse.start_soon(worker, client, nurse)
  File "/Users/tomchristie/Temp/venv/lib/python3.9/site-packages/httpcore/_async/connection_pool.py", line 326, in __aexit__
    await self.aclose()
  File "/Users/tomchristie/Temp/venv/lib/python3.9/site-packages/httpcore/_async/connection_pool.py", line 312, in aclose
    raise RuntimeError(
RuntimeError: The connection pool was closed while 2 HTTP requests/responses were still in-flight.

Or with this branch...

$ python3 -m venv venv
$ venv/bin/pip install trio
$ venv/bin/pip install git+https://github.com/encode/httpcore.git@fix-async-cancellation-behaviour
$ venv/bin/python ./example.py 
/Users/tomchristie/Temp/venv/lib/python3.9/site-packages/anyio/_backends/_trio.py:164: TrioDeprecationWarning: trio.MultiError is deprecated since Trio 0.22.0; use BaseExceptionGroup (on Python 3.11 and later) or exceptiongroup.BaseExceptionGroup (earlier versions) instead (https://github.com/python-trio/trio/issues/2211)
  class ExceptionGroup(BaseExceptionGroup, trio.MultiError):
<Response [301]>

Ooops, we've run into agronholm/anyio#470...

Downgrading trio should avoid that (the upcoming anyio 4.0 should resolve it too).

$ python3 -m venv venv
$ venv/bin/pip install trio==0.21.0
$ venv/bin/pip install git+https://github.com/encode/httpcore.git@fix-async-cancellation-behaviour
$ venv/bin/python ./example.py 
<Response [301]>

Using asyncio, with anyio for TaskGroups

example.py

import anyio
import httpcore


async def worker(client, nurse):
    r = await client.request('HEAD', 'https://google.com')
    print(r)
    nurse.cancel_scope.cancel()


async def main():
    async with httpcore.AsyncConnectionPool() as client, anyio.create_task_group() as tg:
        for _ in range(3):
            tg.start_soon(worker, client, nurse)


if __name__ == "__main__":
    anyio.run(main)

Before...

$ python3 -m venv venv
$ venv/bin/pip install httpcore
$ venv/bin/python ./example.py 
<Response [301]>
Traceback (most recent call last):
  File "/Users/tomchristie/Temp/./example.py", line 18, in <module>
    anyio.run(main)
  File "/Users/tomchristie/Temp/venv/lib/python3.9/site-packages/anyio/_core/_eventloop.py", line 70, in run
    return asynclib.run(func, *args, **backend_options)
  File "/Users/tomchristie/Temp/venv/lib/python3.9/site-packages/anyio/_backends/_asyncio.py", line 292, in run
    return native_run(wrapper(), debug=debug)
  File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/asyncio/base_events.py", line 642, in run_until_complete
    return future.result()
  File "/Users/tomchristie/Temp/venv/lib/python3.9/site-packages/anyio/_backends/_asyncio.py", line 287, in wrapper
    return await func(*args)
  File "/Users/tomchristie/Temp/./example.py", line 14, in main
    nurse.start_soon(worker, client, nurse)
  File "/Users/tomchristie/Temp/venv/lib/python3.9/site-packages/httpcore/_async/connection_pool.py", line 326, in __aexit__
    await self.aclose()
  File "/Users/tomchristie/Temp/venv/lib/python3.9/site-packages/httpcore/_async/connection_pool.py", line 312, in aclose
    raise RuntimeError(
RuntimeError: The connection pool was closed while 2 HTTP requests/responses were still in-flight.

After...

$ python3 -m venv venv
$ venv/bin/pip install git+https://github.com/encode/httpcore.git@fix-async-cancellation-behaviour
$ venv/bin/python ./example.py 
<Response [301]>

[lovelydinosaur]

Using Python 3.11 TaskGroups, looks like the scheduling is a little different, and I can't reproduce the issue.

import asyncio
import httpcore


async def worker(client, tasks):
    r = await client.request('HEAD', 'https://google.com')
    print(r)
    [task.cancel() for task in tasks]


async def main():
    async with httpcore.AsyncConnectionPool() as client, asyncio.TaskGroup() as tg:
        tasks = []
        for _ in range(3):
            task = tg.create_task(worker(client, tasks))
            tasks.append(task)


if __name__ == "__main__":
    asyncio.run(main())

Both httpcore 0.15.0 and httpcore master result in the same...

$ venv/bin/python ./example.py 
<Response [301]>