Connection reset by peer in Python when socket listen backlog is small

Question

I have the following code, run with Python 2.7 (on Mac OS) (needed because it's going into a legacy application). If LISTEN_BACKLOG is less than the number of clients, then after a few iterations I will get a "Connection reset by peer" on one of the clients during the recv() call, and sometimes during the connect().

For example, if you run 1 instance of the server and 2 instances of the client, with LISTEN_BACKLOG = 1, then you will get the issue very quickly.

server.py

import socket

LISTEN_BACKLOG = 1

exit_condition = False
listener = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
listener.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
listener.bind(('127.0.0.1', 12345))
listener.settimeout(0.2)
listener.listen(LISTEN_BACKLOG)

while not exit_condition:
    try:
        server = listener.accept()[0]
        server.send(' ')
        server.close()
    except socket.timeout:
        # Timed-out connection so go around and wait again
        pass

client.py

while True:
    client = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    client.connect(('127.0.0.1', 12345))
    print "Connected"
    data = client.recv(1) # <-- Get "Connection reset by peer" here sometimes
    if data == ' ':
        print "Got Data"
    else:
        print "ERROR: Received data is wrong: ", data, "\n"
        break

    client.close()
    print "Closed"

I am using a small socket timeout for the socket accept on the server, because I need to periodically check an exit condition for the while loop in the larger application. I know this is not ideal, but regardless I would expect it to work.

Can you please help me understand why the "Connection reset by peer" occurs?

EDIT: I have also tried an updated version of my code with Python3 and the behaviour is the same: "Connection reset by peer" at the recv() line.

Increase the backlog, of course. Why wouldn't you? The usual settings are 5, 50, or 500. But yiou can't get 'connection reset' from connect(): you get 'connection refused'. 'Connection reset' from recv() is caused by a bug in the code, and not in the code you posted: possibly in the peer code. — user207421
In the real application there are many more clients, and I use a bigger backlog but the behaviour is still the same - I get “connection reset by peer” as soon as the number of clients is larger than the listen backlog. I would expect “connection refused” from the connect, but certainly not “connection reset by peer” from recv. — user1592096

user1592096 user1592096 · Accepted Answer · 2020-10-28T01:54:09

If anyone else is coming across the same issue, I believe it is only a problem on MacOS. Perhaps the python socket implementation has a bug in MacOS. I've tried on Linux and I cannot reproduce the issue there.

Connection reset by peer in Python when socket listen backlog is small

1 Answers