httptools: Connection: Close does not close the connection

[8dazo]

Discussed in #3071

^{Originally posted by 8dazo August 17, 2026}

Initial Checks

• I confirm this was discussed, and the maintainers suggest I open an issue.
• I'm aware that if I created this issue without a discussion, it may be closed without a response.

Discussion Link

#3071

Description

On the httptools HTTP implementation (what --http auto selects when httptools is installed), Uvicorn only honors a request Connection: close when the header value is exactly lowercase close.

Connection: Close and Connection: keep-alive, close leave the TCP connection open and do not echo Connection: close on the response.

This is a bug, not intentional:

• docs/server-behavior.md says a Connection: Close header must close the connection, and that HTTP headers are case-insensitive.
• RFC 9112 §9.6: a server that receives a close connection option MUST close after the final response, and SHOULD send Connection: close on that response. Tokens are case-insensitive (RFC 9110 §7.6.1).
• httptools already treats the response Connection value as case-insensitive (value.lower() == b"close"). The request check is an exact tuple match on (b"connection", b"close"). Existing tests only send lowercase close.

As noted in the discussion, the exact-match is not local to httptools. CLOSE_HEADER in flow_control.py is used the same way in httptools_impl.py, h11_impl.py, and zttp_impl.py. h11 and zttp still close via their own state machines, but they also fail to echo Connection: close for mixed-case / multi-token values (RFC SHOULD). Patching only httptools would leave that echo gap.

Lowercasing the whole header value is not enough for keep-alive, close. Connection is a comma-separated token list. The same tokenisation already exists in _get_upgrade() (split, strip, lower, membership test). A small helper next to CLOSE_HEADER (split / strip / lowercase each token) should be used for the echo in all implementations, and for httptools' keep-alive decision.

Actual (0.52.3 / main):

h11        close | Close | keep-alive, close   closing=True  echo=True for lowercase only
httptools  close                               closing=True  echo=True
httptools  Close                               closing=False echo=False
httptools  keep-alive, close                   closing=False echo=False
zttp       close                               closing=True  echo=True
zttp       Close / keep-alive, close           closing=True  echo=False

Expected: all three implementations close after the response and echo Connection: close whenever the request Connection header contains a close token, regardless of case, surrounding whitespace, or sibling tokens.

Example Code

import asyncio

from tests.protocols.test_http import get_connected_protocol
from tests.response import Response
from uvicorn.protocols.http.h11_impl import H11Protocol
from uvicorn.protocols.http.httptools_impl import HttpToolsProtocol
from uvicorn.protocols.http.zttp_impl import ZttpProtocol

REQUESTS = {
    "lower": b"GET / HTTP/1.1\r\nHost: example.org\r\nConnection: close\r\n\r\n",
    "upper": b"GET / HTTP/1.1\r\nHost: example.org\r\nConnection: Close\r\n\r\n",
    "mixed": b"GET / HTTP/1.1\r\nHost: example.org\r\nConnection: keep-alive, close\r\n\r\n",
}

async def main():
    for name, cls in [
        ("h11", H11Protocol),
        ("httptools", HttpToolsProtocol),
        ("zttp", ZttpProtocol),
    ]:
        for key, req in REQUESTS.items():
            protocol = get_connected_protocol(
                Response("Hello, world", media_type="text/plain"),
                cls,
                access_log=False,
            )
            protocol.data_received(req)
            await protocol.loop.run_one()
            buf = protocol.transport.buffer.lower()
            print(
                name,
                key,
                "closing=",
                protocol.transport.is_closing(),
                "echo=",
                b"connection: close" in buf,
            )

asyncio.run(main())

Python, Uvicorn & OS Version

Running uvicorn 0.52.3 with CPython 3.12.13 on Darwin