New submission from Vlad Starostin <>:

Long story short, if you have a TCP asyncio-server (implementing 
asyncio.Protocol) that sends something to socket in connection_made callback, 
it will leak fds and won't be able to handle some consequent requests, if you 
will send to it RST right after TCP-handshake.

In my case, these requests are sent by keepalived demon (it makes TCP-checks 
that way), but they can be easily hand-crafted.

Example server:

import asyncio

class EchoServerClientProtocol(asyncio.Protocol):
    def connection_made(self, transport):
        self.transport = transport
        self.transport.write(b'Say something')

    def data_received(self, data):

loop = asyncio.get_event_loop()
coro = loop.create_server(EchoServerClientProtocol, '', 8888)
server = loop.run_until_complete(coro)

Example client:

import socket
import struct
import time

# first request
sock = socket.socket()
l_onoff = 1
l_linger = 0
sock.setsockopt(socket.SOL_SOCKET, socket.SO_LINGER, struct.pack('ii', l_onoff, 
sock.connect(('', 8022))


# second request
sock = socket.socket()
sock.connect(('', 8888))
print('Got', sock.recv(1024))
sock.sendall(b'Hi there')
print('Got', sock.recv(1024))  # <-- Will hang here

Why is this happening?
1. First request
  1.1. _SelectorSocketTransport.__init__ schedules connection_made and 
  1.2. connection_made tries to send data to the socket, it fails, 
_SelectorTransport._fatal_error is called, then _SelectorTransport._force_close 
is called.
  1.3. _SelectorTransport._force_close removes the reader from the loop (there 
is none, it's ok), and schedules closing the socket in 
  1.4. loop.add_reader is called (scheduled on step 1), it calls epoll_ctl, no 
error is returned because the socket isn't closed yet.
  1.5. the socket is closed by _SelectorTransport._call_connection_lost 
(scheduled on step 3). The corresponding entry in _SelectorMapping remains and 
isn't cleaned up.

2. Second request
  2.1. FD from the first request is reused by the OS
  2.2. BaseSelectorEventLoop._add_reader sees that the FD is already in 
BaseSelectorEventLoop._selector, so it calls _selector.modify
  2.3 _BaseSelectorImpl.modify sees that the events mask is the same, so it 
uses a shortcut, changing only the callback, but without calling epoll_ctl. The 
socket won't be polled.

I think the source of the error is in step 1.4, we shouldn't add the reader if 
we are already closing the transport. I've made a simple PR fixing this, but 
maybe there are other options here.

components: asyncio
messages: 315196
nosy: asvetlov, drtyrsa, giampaolo.rodola, yselivanov
priority: normal
severity: normal
status: open
title: Asyncio server enters an invalid state after a request with SO_LINGER
type: behavior
versions: Python 3.6, Python 3.7, Python 3.8

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