http.client hangs reading a 304 or 204 response that has Transfer-Encoding: chunked

[DrVelvetFog]

Bug description:

HTTPResponse.read() blocks when a 304 (or 204) response carries Transfer-Encoding: chunked. With no timeout set it waits forever.

RFC 9112 §6.1 explicitly allows this header on a 304: "A server MAY send a Transfer-Encoding header field ... in a 304 (Not Modified) response to a GET request, neither of which includes a message body". So a compliant server can trigger it. requests and urllib3 go through http.client, so they hang the same way.

import http.client, socket, threading

srv = socket.create_server(("127.0.0.1", 0))

def serve():
    conn, _ = srv.accept()
    conn.recv(65536)
    conn.sendall(b"HTTP/1.1 304 Not Modified\r\n"
                 b"Transfer-Encoding: chunked\r\n\r\n")
    conn.recv(1)  # keep the connection open, as a keep-alive server would

threading.Thread(target=serve, daemon=True).start()

c = http.client.HTTPConnection(*srv.getsockname(), timeout=5)
c.request("GET", "/")
r = c.getresponse()
print(r.status)  # 304
print(r.read())  # expected b'', but blocks until the timeout (forever without one)

Cause: HTTPResponse.begin() sets self.length = 0 for 1xx, 204, 304 and HEAD, but leaves self.chunked = True. read() short-circuits for HEAD only, so for 204/304 it goes into _read_chunked() and waits for a chunk that never comes. HEAD hit the same problem in bpo-6312 (gh-50561) and got the HEAD-specific check in read(). The other bodiless statuses never did.

Suggested fix: set self.chunked = False alongside self.length = 0 in that block of begin(). I have a patch with a test and I'm happy to open the PR.

CPython versions tested on:

3.14, CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-157951: Don't read a chunked body for 1xx, 204 and 304 responses in http.client #157952