RuntimeError: The connection pool was closed while 1 HTTP requests/responses were still in-flight.

[guyskk]

When request cancelled, AsyncClient raise RuntimeError on exiting,

httpx: both 0.22.0 and master branch
Python: 3.9.7
OS: Mac OS

Reproduce:

I run into the issue when send many parallel requests, some requests may timeout, and I see the RuntimeError occasionally.

Below is a simple reproduce, when task2 timeout, asyncio.gather will cancel task1, but it seems AsyncClient not proper close the request.

import asyncio
import traceback

import httpx


async def task1(client: httpx.AsyncClient):
    try:
        return await client.get("https://httpbin.org/delay/1")
    except BaseException as ex:
        print("task1", type(ex), repr(ex))
        traceback.print_exc()
        print("-" * 60)
        raise


async def task2():
    # will raise asyncio.exceptions.TimeoutError in 0.5 seconds
    await asyncio.wait_for(asyncio.sleep(1), 0.5)


async def main():
    async with httpx.AsyncClient(timeout=10) as client:
        coro1 = task1(client)
        coro2 = task2()
        await asyncio.gather(coro1, coro2)


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

Output:

task1 <class 'asyncio.exceptions.CancelledError'> CancelledError()
Traceback (most recent call last):
  File "/Users/kk/.pyenv/versions/3.9.7/envs/httpx/lib/python3.9/site-packages/httpcore/_async/connection_pool.py", line 237, in handle_async_request
    response = await connection.handle_async_request(request)
  File "/Users/kk/.pyenv/versions/3.9.7/envs/httpx/lib/python3.9/site-packages/httpcore/_async/http_proxy.py", line 283, in handle_async_request
    stream = await stream.start_tls(**kwargs)
  File "/Users/kk/.pyenv/versions/3.9.7/envs/httpx/lib/python3.9/site-packages/httpcore/backends/asyncio.py", line 72, in start_tls
    raise exc
  File "/Users/kk/.pyenv/versions/3.9.7/envs/httpx/lib/python3.9/site-packages/httpcore/backends/asyncio.py", line 63, in start_tls
    ssl_stream = await anyio.streams.tls.TLSStream.wrap(
  File "/Users/kk/.pyenv/versions/3.9.7/envs/httpx/lib/python3.9/site-packages/anyio/streams/tls.py", line 100, in wrap
    await wrapper._call_sslobject_method(ssl_object.do_handshake)
  File "/Users/kk/.pyenv/versions/3.9.7/envs/httpx/lib/python3.9/site-packages/anyio/streams/tls.py", line 115, in _call_sslobject_method
    data = await self.transport_stream.receive()
  File "/Users/kk/.pyenv/versions/3.9.7/envs/httpx/lib/python3.9/site-packages/anyio/_backends/_asyncio.py", line 1112, in receive
    raise ClosedResourceError from None
anyio.ClosedResourceError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/kk/dev/zhuwen/httpx/http_error.py", line 9, in task1
    return await client.get("https://httpbin.org/delay/1")
  File "/Users/kk/dev/zhuwen/httpx/httpx/_client.py", line 1732, in get
    return await self.request(
  File "/Users/kk/dev/zhuwen/httpx/httpx/_client.py", line 1509, in request
    return await self.send(request, auth=auth, follow_redirects=follow_redirects)
  File "/Users/kk/dev/zhuwen/httpx/httpx/_client.py", line 1596, in send
    response = await self._send_handling_auth(
  File "/Users/kk/dev/zhuwen/httpx/httpx/_client.py", line 1624, in _send_handling_auth
    response = await self._send_handling_redirects(
  File "/Users/kk/dev/zhuwen/httpx/httpx/_client.py", line 1661, in _send_handling_redirects
    response = await self._send_single_request(request)
  File "/Users/kk/dev/zhuwen/httpx/httpx/_client.py", line 1698, in _send_single_request
    response = await transport.handle_async_request(request)
  File "/Users/kk/dev/zhuwen/httpx/httpx/_transports/default.py", line 353, in handle_async_request
    resp = await self._pool.handle_async_request(req)
  File "/Users/kk/.pyenv/versions/3.9.7/envs/httpx/lib/python3.9/site-packages/httpcore/_async/connection_pool.py", line 252, in handle_async_request
    await self.response_closed(status)
  File "/Users/kk/.pyenv/versions/3.9.7/envs/httpx/lib/python3.9/site-packages/anyio/_core/_synchronization.py", line 138, in acquire
    await cancel_shielded_checkpoint()
  File "/Users/kk/.pyenv/versions/3.9.7/envs/httpx/lib/python3.9/site-packages/anyio/lowlevel.py", line 57, in cancel_shielded_checkpoint
    await get_asynclib().cancel_shielded_checkpoint()
  File "/Users/kk/.pyenv/versions/3.9.7/envs/httpx/lib/python3.9/site-packages/anyio/_backends/_asyncio.py", line 464, in cancel_shielded_checkpoint
    await sleep(0)
  File "/Users/kk/.pyenv/versions/3.9.7/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/asyncio/tasks.py", line 644, in sleep
    await __sleep0()
  File "/Users/kk/.pyenv/versions/3.9.7/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/asyncio/tasks.py", line 633, in __sleep0
    yield
asyncio.exceptions.CancelledError
------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/kk/.pyenv/versions/3.9.7/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/asyncio/tasks.py", line 655, in sleep
    return await future
asyncio.exceptions.CancelledError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/kk/.pyenv/versions/3.9.7/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/asyncio/tasks.py", line 492, in wait_for
    fut.result()
asyncio.exceptions.CancelledError

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/kk/dev/zhuwen/httpx/http_error.py", line 26, in main
    await asyncio.gather(coro1, coro2)
  File "/Users/kk/dev/zhuwen/httpx/http_error.py", line 19, in task2
    await asyncio.wait_for(asyncio.sleep(1), 0.5)
  File "/Users/kk/.pyenv/versions/3.9.7/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/asyncio/tasks.py", line 494, in wait_for
    raise exceptions.TimeoutError() from exc
asyncio.exceptions.TimeoutError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/kk/dev/zhuwen/httpx/http_error.py", line 30, in <module>
    asyncio.run(main())
  File "/Users/kk/.pyenv/versions/3.9.7/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/Users/kk/.pyenv/versions/3.9.7/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/kk/dev/zhuwen/httpx/http_error.py", line 26, in main
    await asyncio.gather(coro1, coro2)
  File "/Users/kk/dev/zhuwen/httpx/httpx/_client.py", line 1981, in __aexit__
    await proxy.__aexit__(exc_type, exc_value, traceback)
  File "/Users/kk/dev/zhuwen/httpx/httpx/_transports/default.py", line 332, in __aexit__
    await self._pool.__aexit__(exc_type, exc_value, traceback)
  File "/Users/kk/.pyenv/versions/3.9.7/envs/httpx/lib/python3.9/site-packages/httpcore/_async/connection_pool.py", line 326, in __aexit__
    await self.aclose()
  File "/Users/kk/.pyenv/versions/3.9.7/envs/httpx/lib/python3.9/site-packages/httpcore/_async/connection_pool.py", line 312, in aclose
    raise RuntimeError(
RuntimeError: The connection pool was closed while 1 HTTP requests/responses were still in-flight.

[guyskk]

I got the reason: asyncio.gather will not cancel task1 when task2 timeout, so task1 is pending when AsyncClient close. And task1 seems cancelled by asyncio loop or GC when asyncio.run ending.

https://docs.python.org/3/library/asyncio-task.html#asyncio.gather
Other awaitables in the aws sequence won’t be cancelled and will continue to run.

The solution is manually cancel task1 before AsyncClient close.