applicationinstance.py

#    Back In Time
#    Copyright (C) 2008-2021 Oprea Dan, Bart de Koning, Richard Bailey, Germar Reitze
#
#    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 fcntl
import errno

import logger
import tools

class ApplicationInstance:
    """
    Class used to handle one application instance mechanism.

    Args:
        pidFile (str):      full path of file used to save pid and procname
        autoExit (bool):    automatically call sys.exit if there is an other
                            instance running
        flock (bool):       use file-locks to make sure only one instance
                            is checking at the same time
    """

    def __init__(self, pidFile, autoExit = True, flock = False):
        self.pidFile = pidFile
        self.pid = 0
        self.procname = ''
        self.flock = None
        if flock:
            self.flockExclusiv()

        if autoExit:
            if self.check(True):
                self.startApplication()

    def __del__(self):
        self.flockUnlock()

    def check(self, autoExit = False):
        """
        Check if the current application is already running

        Args:
            autoExit (bool):   automatically call sys.exit if there is an other
                                instance running

        Returns:
            bool:               ``True`` if this is the only application
                                instance
        """
        #check if the pidfile exists
        if not os.path.isfile(self.pidFile):
            return True

        self.pid, self.procname = self.readPidFile()

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

        if not tools.processAlive(self.pid):
            return True

        #check if the process has the same procname
        #check cmdline for backwards compatibility
        if self.procname and                                    \
           self.procname != tools.processName(self.pid) and    \
           self.procname != tools.processCmdline(self.pid):
                return True

        if autoExit:
            #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

    def busy(self):
        """
        Check if one application with this instance is currently running.

        Returns:
            bool:       ``True`` if an other instance is currently running.
        """
        return not self.check()

    def startApplication(self):
        """
        Called when the single instance starts to save its pid
        """
        pid = os.getpid()
        procname = tools.processName(pid)

        try:
            with open(self.pidFile, 'wt') as f:
                f.write('{}\n{}'.format(pid, procname))
        except OSError as e:
            logger.error('Failed to write PID file %s: [%s] %s' %(e.filename, e.errno, e.strerror))

        self.flockUnlock()

    def exitApplication(self):
        """
        Called when the single instance exit (remove pid file)
        """
        try:
            os.remove(self.pidFile)
        except:
            pass

    def flockExclusiv(self):
        """
        Create an exclusive lock to block a second instance while
        the first instance is starting.
        """
        try:
            self.flock = open(self.pidFile + '.flock', 'w')
            fcntl.flock(self.flock, fcntl.LOCK_EX)
        except OSError as e:
            logger.error('Failed to write flock file %s: [%s] %s' %(e.filename, e.errno, e.strerror))

    def flockUnlock(self):
        """
        Remove the exclusive lock. Second instance can now continue
        but should find it self to be obsolete.
        """
        if self.flock:
            fcntl.fcntl(self.flock, fcntl.LOCK_UN)
            self.flock.close()
            try:
                os.remove(self.flock.name)
            except:
                #an other instance was faster
                #race condition while using 'if os.path.exists(...)'
                pass
        self.flock = None

    def readPidFile(self):
        """
        Read the pid and procname from the file

        Returns:
            tuple:  tuple of (pid(int), procname(str))
        """
        pid = 0
        procname = ''
        try:
            with open(self.pidFile, 'rt') as f:
                data = f.read()
            data = data.split('\n', 1)
            if data[0].isdigit():
                pid = int(data[0])
            if len(data) > 1:
                procname = data[1].strip('\n')
        except OSError as e:
            logger.warning('Failed to read PID and process name from %s: [%s] %s' %(e.filename, e.errno, e.strerror))
        except ValueError as e:
            logger.warning('Failed to extract PID and process name from %s: %s'
                            %(self.pidFile, str(e)))
        return (pid, procname)

if __name__ == '__main__':
    import time

    #create application instance
    appInstance = ApplicationInstance('/tmp/myapp.pid')

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

    #remove pid file
    appInstance.exitApplication()