watcher.py

tinyos-2.x.rar

# Copyright (c) 2006-2007 Chad Metcalf <[e-mail]>
#
# Permission is hereby granted, free of charge, to any person obtaining a 
# copy of this software and associated documentation files (the "Software"),
# to deal in the Software without restriction, including without limitation
# the rights to use, copy, modify, merge, publish, distribute, sublicense,
# and/or sell copies of the Software, and to permit persons to whom the
# Software is furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR 
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.
#
# Author: Chad Metcalf <[e-mail]>
#

import os
import sys
import signal

from tinyos.utils.Singleton import Singleton

class Watcher(Singleton):
    """ As seen in:
    http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/496735
    
    This class solves two problems with multithreaded
    programs in Python, (1) a signal might be delivered
    to any thread (which is just a malfeature) and (2) if
    the thread that gets the signal is waiting, the signal
    is ignored (which is a bug).

    The watcher is a concurrent process (not thread) that
    waits for a signal and the process that contains the
    threads.  See Appendix A of The Little Book of Semaphores.
    http://greenteapress.com/semaphores/
    """
    
    def __init__(self):
        """ Creates a child thread, which returns.  The parent
            thread waits for a KeyboardInterrupt and then kills
            the child thread.
        """  
        Singleton.__init__(self)
        
        self.child = os.fork()
        if self.child != 0:
            self.watch()

    def watch(self):
        try:
            os.wait()
        except KeyboardInterrupt:
            # I put the capital B in KeyBoardInterrupt so I can
            # tell when the Watcher gets the SIGINT
            print 'KeyBoardInterrupt'
            self.kill()
        sys.exit()

    def kill(self):
        try:
            os.kill(self.child, signal.SIGKILL)
        except OSError, x: 
            print "os.kill failed"
            print x