Since 0.21.0 httpx behaves differently if it just receives a 304 response and no other data (hangs indefinitely)

[johtso]

I've just been updating httpx-caching to work with httpx 0.22 and came across a strange issue where a test was hanging indefinitely.

With 0.20 it works fine, but with 0.21 that specific test hangs.

My test server is just responding with a 304 status.

Is this change in behaviour just httpx now being stricter? Possibly there's no newline being returned after the 304 header?

https://github.com/johtso/httpx-caching/blob/httpx-v0.22/tests/_async/test_expires_heuristics.py#L59
https://github.com/johtso/httpx-caching/blob/httpx-v0.22/tests/conftest.py#L66

EDIT:

I've confirmed that the server is responding with this:

b'HTTP/1.1 304 Not Modified\r\nDate: Wed, 16 Mar 2022 22:27:30 GMT\r\nServer: 127.0.0.1\r\n\r\n'

Don't see why this would cause the client to hang..

EDIT2:

I've tried unsuccessfully to isolate the problem in a test case..

import socket

HOST = "127.0.0.1"  # Standard loopback interface address (localhost)
PORT = 65432  # Port to listen on (non-privileged ports are > 1023)

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.bind((HOST, PORT))
    s.listen()
    conn, addr = s.accept()
    with conn:
        print(f"Connected by {addr}")
        while True:
            data = conn.recv(1024)
            if not data:
                break
            conn.sendall(b'HTTP/1.1 304 Not Modified\r\nDate: Wed, 16 Mar 2022 22:27:30 GMT\r\nServer: 127.0.0.1\r\n\r\n')

import httpx

async def main():
    async with httpx.AsyncClient() as client:
        print(await client.get('http://127.0.0.1:65432'))


if __name__ == '__main__':
    import asyncio
    asyncio.run(main())

The above works fine..

EDIT3:

It seems to be caused by this in the httpx-caching transport:
https://github.com/johtso/httpx-caching/blob/httpx-v0.22/httpx_caching/_async/_transport.py#L110

Previously this worked fine, now it seems to hang indefinitely if the response doesn't have any body..