Honor max concurrent streams

[tomchristie]

Closes #85

Work based off #38

[yeraydiazdiaz]

Awesome! Left a couple of specific comments in the code.

Checking out the branch and testing against this very simple script:

import asyncio

import httpx


async def main(url, n):
    async with httpx.AsyncClient(
        timeout=httpx.Timeout(5.0),
        http2=True,
    ) as client:
        await asyncio.gather(*[request(client, url, i) for i in range(n)])


async def request(client, url, i):
    response = await client.get(url)
    assert response.status_code == 200
    assert response.http_version == "HTTP/2"


if __name__ == "__main__":
    asyncio.run(main("https://example.org", 200))

It fails around 1/2 to 1/3 of the time with the following traceback:

Traceback (most recent call last):
  File ".dev/gather_http2_reuse.py", line 23, in <module>
    asyncio.run(main("https://example.org", 200))
  File "/Users/yeray/.pyenv/versions/3.8.2/lib/python3.8/asyncio/runners.py", line 43, in run
    return loop.run_until_complete(main)
  File "/Users/yeray/.pyenv/versions/3.8.2/lib/python3.8/asyncio/base_events.py", line 616, in run_until_complete
    return future.result()
  File ".dev/gather_http2_reuse.py", line 13, in main
    await asyncio.gather(*[request(client, url, i) for i in range(n)])
  File ".dev/gather_http2_reuse.py", line 17, in request
    response = await client.get(url)
  File "/Users/yeray/code/personal/_forks/httpx/httpx/_client.py", line 1275, in get
    return await self.request(
  File "/Users/yeray/code/personal/_forks/httpx/httpx/_client.py", line 1117, in request
    response = await self.send(
  File "/Users/yeray/code/personal/_forks/httpx/httpx/_client.py", line 1138, in send
    response = await self.send_handling_redirects(
  File "/Users/yeray/code/personal/_forks/httpx/httpx/_client.py", line 1165, in send_handling_redirects
    response = await self.send_handling_auth(
  File "/Users/yeray/code/personal/_forks/httpx/httpx/_client.py", line 1202, in send_handling_auth
    response = await self.send_single_request(request, timeout)
  File "/Users/yeray/code/personal/_forks/httpx/httpx/_client.py", line 1234, in send_single_request
    ) = await dispatcher.request(
  File "/Users/yeray/code/personal/_forks/httpcore/httpcore/_async/connection_pool.py", line 152, in request
    response = await connection.request(
  File "/Users/yeray/code/personal/_forks/httpcore/httpcore/_async/connection.py", line 78, in request
    return await self.connection.request(method, url, headers, stream, timeout)
  File "/Users/yeray/code/personal/_forks/httpcore/httpcore/_async/http2.py", line 116, in request
    return await h2_stream.request(method, url, headers, stream, timeout)
  File "/Users/yeray/code/personal/_forks/httpcore/httpcore/_async/http2.py", line 286, in request
    status_code, headers = await self.receive_response(timeout)
  File "/Users/yeray/code/personal/_forks/httpcore/httpcore/_async/http2.py", line 338, in receive_response
    event = await self.connection.wait_for_event(self.stream_id, timeout)
  File "/Users/yeray/code/personal/_forks/httpcore/httpcore/_async/http2.py", line 192, in wait_for_event
    await self.receive_events(timeout)
  File "/Users/yeray/code/personal/_forks/httpcore/httpcore/_async/http2.py", line 206, in receive_events
    raise ProtocolError(event)
httpcore._exceptions.ProtocolError: <StreamReset stream_id:363, error_code:ErrorCodes.REFUSED_STREAM, remote_reset:True>

Not really sure why it's only sometimes though, curious to know if you're also getting it.

[tomchristie]

I’ve not seen that no - is that before or after this change, or both?

[yeraydiazdiaz]

That happens on master as well, but usually fails with a "no more streams available" error which your change fixes.

[tomchristie]

I've addressed the error_code:ErrorCodes.REFUSED_STREAM issue now.
The semaphore release in _response_closed needs to occur after the network close.

[tomchristie]

Righty. I think we're there now.
Thanks so much for your review on this one - it was invaluable.

[yeraydiazdiaz]

My pleasure! Awesome work 🚀