HTTP/2 does not honor max connections

[mbrancato]

When forcing HTTP/2 usage, I noticed max_connections is not honored.

This is likely due to httpcore sets SettingCodes.MAX_CONCURRENT_STREAMS: 100 in the H2Connection class. It seems httpx should control that value. It might be an issue with httpcore, I just haven't dug into it.

The client object looks like: AsyncClient(http1=False, http2=True, limits=Limits(max_connections=200, max_keepalive_connections=40).

Traceback (most recent call last):
  File "/usr/local/lib/python3.10/site-packages/anyio/_core/_synchronization.py", line 314, in acquire
    self.acquire_nowait()
  File "/usr/local/lib/python3.10/site-packages/anyio/_core/_synchronization.py", line 342, in acquire_nowait
    raise WouldBlock
anyio.WouldBlock

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.10/site-packages/httpcore/_async/connection_pool.py", line 237, in handle_async_request
    response = await connection.handle_async_request(request)
  File "/usr/local/lib/python3.10/site-packages/httpcore/_async/connection.py", line 90, in handle_async_request
    return await self._connection.handle_async_request(request)
  File "/usr/local/lib/python3.10/site-packages/httpcore/_async/http2.py", line 96, in handle_async_request
    await self._max_streams_semaphore.acquire()
  File "/usr/local/lib/python3.10/site-packages/httpcore/_synchronization.py", line 46, in acquire
    await self._semaphore.acquire()
  File "/usr/local/lib/python3.10/site-packages/anyio/_core/_synchronization.py", line 319, in acquire
    await event.wait()
  File "/usr/local/lib/python3.10/asyncio/locks.py", line 214, in wait
    await fut
asyncio.exceptions.CancelledError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
...
    response = await self.session.request(
  File "/usr/local/lib/python3.10/site-packages/httpx/_client.py", line 1527, in request
    return await self.send(request, auth=auth, follow_redirects=follow_redirects)
  File "/usr/local/lib/python3.10/site-packages/httpx/_client.py", line 1614, in send
    response = await self._send_handling_auth(
  File "/usr/local/lib/python3.10/site-packages/httpx/_client.py", line 1642, in _send_handling_auth
    response = await self._send_handling_redirects(
  File "/usr/local/lib/python3.10/site-packages/httpx/_client.py", line 1679, in _send_handling_redirects
    response = await self._send_single_request(request)
  File "/usr/local/lib/python3.10/site-packages/httpx/_client.py", line 1716, in _send_single_request
    response = await transport.handle_async_request(request)
  File "/usr/local/lib/python3.10/site-packages/httpx/_transports/default.py", line 353, in handle_async_request
    resp = await self._pool.handle_async_request(req)
  File "/usr/local/lib/python3.10/site-packages/httpcore/_async/connection_pool.py", line 252, in handle_async_request
    await self.response_closed(status)
asyncio.exceptions.CancelledError

...

I also noticed there are no tests today for HTTP/2 - and the test server is uvicorn which does not support HTTP/2.

[dunkmann00]

In HTTP/2 streams and connections are two different things. A connection is as it is named, a connection between a client and a server. A stream represents a specific request in a connection. If the client and server support it, a connection can have multiple streams. The max_connections limit that is configurable withLimits(max_connections=200, max_keepalive_connections=40) is for the number of connections, not streams.

So I do not think that is a bug, however, this then gets into the question, #2416, I just brought up a few days ago. As I said there, AFAICT, you can't configure the H2Connection object to change the max streams limit (also, just to correct a slight error, H2Connection is a part of the python-hyper/h2 library, not httpcore). This functionality seems like something that would be useful to be able to do in httpx.

[ruanimal]

There is likely to be a misunderstanding here: max_concurrency != max_connections * MAX_CONCURRENT_STREAMS , because for the same origin (domain), there will only be one connection in a connecttion pool.