PoolTimeout when num tasks in asyncio.gather() exceeds client max_connections

Checklist

• Reproducible on 0.13.3
• This issue seems similar but it’s closed and was supposedly fixed

Describe the bug

If the number of tasks executed via asyncio.gather(...) is greater than max_connections, i get a PoolTimeout. It seems like maybe this is happening because the tasks that have completed aren’t releasing their connections upon completion.

I’m new to asyncio so it’s possible I’m doing something wrong, but haven’t been able to find any documentation or issues that cover this case definitively.

To reproduce

import asyncio
import httpx

async def main() -> None:
    url = "https://www.example.com"
    max_connections = 2
    timeout = httpx.Timeout(5.0, pool=2.0)
    limits = httpx.Limits(max_connections=2)
    client = httpx.AsyncClient(timeout=timeout, pool_limits=limits)

    async with client:
        tasks = []
        for _ in range(max_connections + 1):
            tasks.append(client.get(url))
        await asyncio.gather(*tasks)

if __name__ == "__main__":
    loop = asyncio.get_event_loop()
    try:
        loop.run_until_complete(main())
    finally:
        loop.close()

Expected behavior

I would expect all tasks to complete, rather than getting a PoolTimeout on the nth task, where n = max_connections + 1.

Actual behavior

Getting a PoolTimeout on the nth task, where n = max_connections + 1.

Debugging material

Traceback (most recent call last):
  File "test_async.py", line 21, in <module>
    loop.run_until_complete(main())
  File "/Users/redacted/.pyenv/versions/3.6.9/lib/python3.6/asyncio/base_events.py", line 484, in run_until_complete
    return future.result()
  File "test_async.py", line 16, in main
    await asyncio.gather(*tasks)
  File "/Users/redacted/.pyenv/versions/3.6.9/lib/python3.6/site-packages/httpx/_client.py", line 1416, in get
    timeout=timeout,
  File "/Users/redacted/.pyenv/versions/3.6.9/lib/python3.6/site-packages/httpx/_client.py", line 1242, in request
    request, auth=auth, allow_redirects=allow_redirects, timeout=timeout,
  File "/Users/redacted/.pyenv/versions/3.6.9/lib/python3.6/site-packages/httpx/_client.py", line 1273, in send
    request, auth=auth, timeout=timeout, allow_redirects=allow_redirects,
  File "/Users/redacted/.pyenv/versions/3.6.9/lib/python3.6/site-packages/httpx/_client.py", line 1302, in _send_handling_redirects
    request, auth=auth, timeout=timeout, history=history
  File "/Users/redacted/.pyenv/versions/3.6.9/lib/python3.6/site-packages/httpx/_client.py", line 1338, in _send_handling_auth
    response = await self._send_single_request(request, timeout)
  File "/Users/redacted/.pyenv/versions/3.6.9/lib/python3.6/site-packages/httpx/_client.py", line 1374, in _send_single_request
    timeout=timeout.as_dict(),
  File "/Users/redacted/.pyenv/versions/3.6.9/lib/python3.6/contextlib.py", line 99, in __exit__
    self.gen.throw(type, value, traceback)
  File "/Users/redacted/.pyenv/versions/3.6.9/lib/python3.6/site-packages/httpx/_exceptions.py", line 359, in map_exceptions
    raise mapped_exc(message, **kwargs) from None  # type: ignore
httpx._exceptions.PoolTimeout

Environment

• OS: macOS 10.14.6
• Python version: 3.6.9
• HTTPX version: 0.13.3
• Async environment: asyncio
• HTTP proxy: no
• Custom certificates: no

Additional context

I commented on this issue, but it’s closed so figured it would be better to create a new one.