uvicorn may respond to requests sent after the client asks for the connection to be closed

[Kludex]

Discussed in #2234

^{Originally posted by kenballus January 28, 2024}

Describe the bug

From RFC 9112, section 9.6:

A server that receives a "close" connection option MUST initiate closure of the connection (see below) after it sends the final response to the request that contained the "close" connection option. The server SHOULD send a "close" connection option in its final response on that connection. The server MUST NOT process any further requests received on that connection.

When uvicorn receives a pipeline with a request containing Connection: close, followed by an invalid request, uvicorn responds only to the second (invalid) request, even though the standard requires that uvicorn respond only to the first one.

To Reproduce

1. Start the example server from the README.
2. Send it a pipeline consisting of a valid request with Connection: close set, followed by an invalid request:

printf 'GET / HTTP/1.1\r\nConnection: close\r\n\r\nInvalid\r\n\r\n' | nc localhost 8080

3. Observe that the only response received is intended for the invalid request:

HTTP/1.1 400 Bad Request
content-type: text/plain; charset=utf-8
Transfer-Encoding: chunked
Connection: close

1e
Invalid HTTP request received.
0

Expected behavior

The server should respond only to the first request, and then close the connection.

Logs/tracebacks

INFO:     127.0.0.1:51922 - "GET / HTTP/1.1" 200 OK
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "/usr/local/lib/python3.11/dist-packages/uvicorn/protocols/http/h11_impl.py", line 404, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/uvicorn/middleware/proxy_headers.py", line 84, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/example.py", line 4, in app
    await send({
  File "/usr/local/lib/python3.11/dist-packages/uvicorn/protocols/http/h11_impl.py", line 486, in send
    output = self.conn.send(event=response)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/h11/_connection.py", line 512, in send
    data_list = self.send_with_data_passthrough(event)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/h11/_connection.py", line 537, in send_with_data_passthrough
    self._process_event(self.our_role, event)
  File "/usr/local/lib/python3.11/dist-packages/h11/_connection.py", line 272, in _process_event
    self._cstate.process_event(role, type(event), server_switch_event)
  File "/usr/local/lib/python3.11/dist-packages/h11/_state.py", line 293, in process_event
    self._fire_event_triggered_transitions(role, _event_type)
  File "/usr/local/lib/python3.11/dist-packages/h11/_state.py", line 311, in _fire_event_triggered_transitions
    raise LocalProtocolError(
h11._util.LocalProtocolError: can't handle event type Response when role=SERVER and state=MUST_CLOSE

Python Version

$ python --version
Python 3.11.2

uvicorn Version

$ python -m pip show uvicorn
Name: uvicorn
Version: 0.27.0
Summary: The lightning-fast ASGI server.
Home-page:
Author:
Author-email: Tom Christie <tom@tomchristie.com>
License:
Location: /usr/local/lib/python3.11/dist-packages
Requires: click, h11
Required-by:

h11 Version

$ python -m pip show h11
Name: h11
Version: 0.14.0
Summary: A pure-Python, bring-your-own-I/O implementation of HTTP/1.1
Home-page: https://github.com/python-hyper/h11
Author: Nathaniel J. Smith
Author-email: njs@pobox.com
License: MIT
Location: /usr/local/lib/python3.11/dist-packages
Requires:
Required-by: uvicorn

OS

Debian 12 (running in Docker on Arch Linux)
Linux 6.7.2

Additional context

Some other HTTP implementations that handle this correctly:
Apache httpd, Boost::Beast, Daphne, H2O, Lighttpd, Nginx, Tornado, OpenWrt uhttpd, Waitress

Some other HTTP implementations that also have this bug:
Mongoose, aiohttp

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[kenballus]

The repro that I provided above has a typo (the first request is missing a Host header).

Here's a better pipeline that actually demonstrates the bug:

printf 'GET / HTTP/1.1\r\nConnection: close\r\nHost: a\r\n\r\nGET / HTTP/1.1\r\n\r\n' | nc localhost 8080

There are two requests in this pipeline; the first is valid and should close the connection. The second is invalid (missing Host header) and should receive no response at all.

What Uvicorn does is respond to the second request and not the first.

[kenballus]

same error

I can't tell whether you're supporting this issue or trying to poke a hole in it :)

That error isn't necessarily confirmation of the bug; you'd have to look at the server's HTTP response in order to see the bad behavior.

[Kludex]

Confirmed. PR welcome.

[Massakera]

@Kludex do you have a guess about where's the file that needs to be changed? I was thinking on h11_impl.py but looking at it

        if self.response_complete:
            if self.conn.our_state is h11.MUST_CLOSE or not self.keep_alive:
                self.conn.send(event=h11.ConnectionClosed())
                self.transport.close()
            self.on_response()

it looks ok to me so I'm bit confused

[berkio3x]

@kenballus

The repro that I provided above has a typo (the first request is missing a Host header).

Here's a better pipeline that actually demonstrates the bug:

printf 'GET / HTTP/1.1\r\nConnection: close\r\nHost: a\r\n\r\nGET / HTTP/1.1\r\n\r\n' | nc localhost 8080

There are two requests in this pipeline; the first is valid and should close the connection. The second is invalid (missing Host header) and should receive no response at all.

What Uvicorn does is respond to the second request and not the first.

I get below in response. has this issue been fixed already?

date: Sun, 18 Feb 2024 17:34:35 GMT
server: uvicorn
content-type: text/plain
connection: close
transfer-encoding: chunked

d
Hello, world!
0

[kenballus]

The issue has not been fixed. I just reproduced it again on the current master branch (latest commit is 1e5f1be at the time of writing).

When I send that pipeline, I get the following:

HTTP/1.1 400 Bad Request
content-type: text/plain; charset=utf-8
Transfer-Encoding: chunked
Connection: close

1e
Invalid HTTP request received.
0

[nayanmanojgupta]

+1 to what @berkio3x said, got below response when I tried to run it.

HTTP/1.1 200 OK
date: Wed, 28 Feb 2024 18:09:27 GMT
server: uvicorn
content-type: text/plain
connection: close
transfer-encoding: chunked

d
Hello, world!
0

[Kludex]

We don't need further confirmation.

[kenballus]

We don't need further confirmation.

Agreed, though I am curious about why others are not able to reproduce the issue.

[JustAPyro]

I'd be willing to take a shot at fixing this, if nobody else is working on it. I haven't contributed to Uvicorn before though, would there be anything to keep in mind apart from what's in https://www.uvicorn.org/contributing/?