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applicationinstance.py

#    Back In Time
#    Copyright (C) 2008-2009 Oprea Dan
#
#    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.,
#    51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.


import os
import os.path
import time


#class used to handle one application instance mechanism
class ApplicationInstance:

      #specify the file used to save the application instance pid
      def __init__( self, pid_file, auto_exit = True ):
            self.pid_file = pid_file

            if auto_exit:
                  if self.check( True ):
                        self.start_application()

      #check if the current application is already running, returns True if this is the application instance
      def check( self, auto_exit = False ):
            #check if the pidfile exists
            if not os.path.isfile( self.pid_file ):
                  return True

            #read the pid from the file
            pid = 0
            try:
                  file = open( self.pid_file, 'rt' )
                  data = file.read()
                  file.close()
                  pid = int( data )
            except:
                  pass

            #check if the process with specified by pid exists
            if 0 == pid:
                  return True

            try:
                  os.kill( pid, 0 ) #this will raise an exception if the pid is not valid
            except:
                  return True

            if auto_exit:
                  #exit the application
                  print "The application is already running !"
                  exit(0) #exit raise an exception so don't put it in a try/except block

            return False

      #called when the single instance starts to save it's pid
      def start_application( self ):
            file = open( self.pid_file, 'wt' )
            file.write( str( os.getpid() ) )
            file.close()

      #called when the single instance exit ( remove pid file )
      def exit_application( self ):
            try:
                  os.remove( self.pid_file )
            except:
                  pass


if __name__ == '__main__':
      #create application instance
      app_instance = ApplicationInstance( '/tmp/myapp.pid' )

      #do something here
      print "Start MyApp"
      time.sleep(5)     #sleep 5 seconds
      print "End MyApp"

      #remove pid file
      app_instance.exit_application()


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