DatagramServer can be started multiple time on same port without Exception

[bobatsar]

• gevent version: 1.2.1
• Python version: 2.7.13
• Operating System: Linux Mint: Linux 4.2.0-42-generic Monkey patch Python's signal.signal or make it work #49~14.04.1-Ubuntu 64Bit

Description:

It seems it is possible to start two Datagram servers in the same script on the same port. I thought that should not be possible. If I do the same for a StreamServer I get an error as expected (socket.error: [Errno 98] Address already in use: ('127.0.0.1', 8000)).

The second started server can receive data, the first does not get anything. After stopping the second, no data is received at all.

If a normal UDP Server is already started, I get an exception (as expected).

What I've run:

from gevent.server import DatagramServer
from gevent import sleep, spawn, Timeout
import random
import argparse
import socket
import sys


class UDPServer(object):

    def __init__(self, host):
        self.udp_server = DatagramServer(host, handle=self.handler)
        self.udp_server.start()

    def __repr__(self):
        return "<UDPServer id={} started: {}>".format(id(self),
                                                      self.udp_server.started)

    def handler(self, data, address):
        print "{}: got data {} from {}".format(self, data, address)

    def stop(self):
        self.udp_server.close()


def sender(host, timeout=1):
    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

    print "start sending data to", host
    with Timeout(timeout, exception=False):
        while(True):
            rand = random.random()
            print "send", rand
            sock.sendto(str(rand), host)
            sleep(.1)
    print "sender finished"

def plain_datagram_server(host, port, time):
    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

    # Bind the socket to the port
    print "start plain UDP server for {}s".format(time)
    server_address = (host, port)
    print 'starting up on %s port %s' % server_address
    sock.bind(server_address)
    print "started plain datagram server"
    sleep(time)
    print "stop plain datagram server"


def test_datagram_server(host):
    print "DatagramServer test on", host
    print "Start udp1 on {} and send data".format(host)
    udp1 = UDPServer(host)
    udp2 = None
    print "udp1", udp1
    print "udp2", udp2
    sender_greenlet = spawn(sender, host)
    sender_greenlet.join()

    print "\nStart udp2 on {} and send data".format(host)
    udp2 = UDPServer(host)
    print "udp1", udp1
    print "udp2", udp2
    sender_greenlet = spawn(sender, host)
    sender_greenlet.join()

    print "\nStop udp2 and send data"
    udp2.stop()
    print "udp1", udp1
    print "udp2", udp2
    sender_greenlet = spawn(sender, host)
    sender_greenlet.join()


if __name__ == '__main__':

    parser = argparse.ArgumentParser()
    parser.add_argument("mode", choices=["send", "test", "test_plain",
                                         "receive_plain"])
    parser.add_argument("--time", default=10)
    parser.add_argument("--port", default=8000)
    parser.add_argument("--host", default="localhost")

    args = parser.parse_args()
    if args.mode == "send":
        sender((args.host, args.port), args.time)
    elif args.mode == "test":
        test_datagram_server((args.host, args.port))
    elif args.mode == "test_plain":
        spawn(plain_datagram_server, args.host, args.port, args.time)
        sleep()
        test_datagram_server((args.host, args.port))
    elif args.mode == "receive_plain":
        plain_datagram_server(args.host, args.port, args.time)

If you run my script with mode "test", it shows the described behaviour. If you run it with mode "test_plain", a normal UDP Server is started on a socket and then the DatagramServer fails.

[jamadden]

I believe you're seeing the effects of SO_REUSEADDR, which is set to True by default. Its effects are highly system dependent (and maybe even family---INET vs DGRAM) dependent. It seems to make no difference at all on OS X (I can't "double bind" ever), while on Linux 4.4.21 I can unless I set reuse_addr to False.

This is poorly documented, perhaps because the effects vary so widely. But you can change it for your particular system:

class MyDS(DatagramServer):
  reuse_addr = 0

[bobatsar]

sorry that I didn't respond until now.
I changed my code to the following but the behaviour didn't change.

I can work around this problem but I was wondering if this is a bug.

class UDPServer(object):

    def __init__(self, host):
        self.udp_server = DatagramServer(host, handle=self.handler)
        self.udp_server.reuse_addr = 0
        self.udp_server.start()