master-cronjob

#!/usr/bin/env python
#
# Copyright(C) 2010 Simon Howard
#
# This program is free software; you can redistribute it and/or
# modify it under the terms of the GNU General Public License
# as published by the Free Software Foundation; either version 2
# of the License, or (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
# 02111-1307, USA.
#
#
# Cronjob script that checks the master server is running.
#

import socket
import sys
import struct
from select import select
from time import time

NET_MASTER_PACKET_TYPE_ADD = 0
NET_MASTER_PACKET_TYPE_ADD_RESPONSE = 1
NET_MASTER_PACKET_TYPE_QUERY = 2
NET_MASTER_PACKET_TYPE_QUERY_RESPONSE = 3

QUERY_ATTEMPTS = 5

MASTER_SERVER = "master.chocolate-doom.org"
UDP_PORT = 2342

def send_message(sock, addr, message_type, payload=None):
    header = struct.pack(">h", message_type)
    packet = header

    if payload is not None:
        packet += payload

    sock.sendto(packet, addr)

def get_response(sock, addr, expected_type, timeout):
    """ Wait for a response packet to be received. """

    start_time = time()

    while time() - start_time < timeout:
        r, w, x = select([sock], [], [], 1)

        if sock in r:
            packet, remote_addr = sock.recvfrom(1024)

            type, = struct.unpack(">h", packet[0:2])

            if expected_type == type:
                return type, packet[2:]

    raise Exception("No response received from server in %i seconds"
                    % (time() - start_time))

def read_string(packet):
    terminator = struct.pack("b", 0)
    strlen = packet.index(terminator)

    result = struct.unpack("%ss" % strlen, packet[0:strlen])

    return packet[strlen + 1:], result

def parse_query_response(packet):
    servers = []

    while len(packet) > 0:
        packet, addr_str = read_string(packet)

        servers.append(addr_str)

    return servers

def query_master(addr_str):
    """ Query a master server for its list of server IP addresses. """

    addr = (socket.gethostbyname(addr_str), UDP_PORT)
    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

    send_message(sock, addr, NET_MASTER_PACKET_TYPE_QUERY)

    type, response = get_response(sock, addr,
                                  NET_MASTER_PACKET_TYPE_QUERY_RESPONSE,
                                  5)

    servers = parse_query_response(response)

# Try several times, as there might be packet loss.

failures = 0

while failures < QUERY_ATTEMPTS:
    try:
        query_master(MASTER_SERVER)
        break
    except:
        failures += 1
else:
    sys.stderr.write("No response from master server after %i attempts.\n"
                     % failures)