stdd/stddlib/daemon.py

# -*- coding: utf-8-*-
#
# This file is part of stdd, the simple time display daemon,
# written by Helmut Pozimski <helmut@pozimski.eu>,
# licensed under the 3-clause BSD license

""" Generic module to implement a daemon in python, takes care of the creation
of the PID files and the usual daemonizing stuff.
"""

from __future__ import unicode_literals

import os
import sys


class Daemon(object):
    """ Tries to implement a well behaving unix daemon in a generic way,
    so the code could be used in different projects.

    """

    def __init__(self, pfile_path, pfile_name):
        """ Initializes the object. """
        self._pfile_path = pfile_path
        self._pfile_name = pfile_name
        self._daemon = False

    def daemonize(self):
        """ Turns the calling prozess into a daemon running on it's own """

        try:
            # Fork for the first time
            pid = os.fork()
        except OSError:
            sys.exit(os.EX_OSERR)
        else:
            if pid > 0:
                sys.exit(os.EX_OK)
        # Become session and group leader
        os.setsid()
        try:
            # Fork for the second time
            pid = os.fork()
        except OSError:
            sys.exit(os.EX_OSERR)
        else:
            if pid > 0:
                sys.exit(os.EX_OK)
        # Change cwd to / to avoid interfering with other mounted file systems
        os.chdir("/")
        # Reset the umask
        os.umask(0o22)

        # Close possibly open file descriptors
        os.close(0)
        os.close(1)
        os.close(2)

        # And redirect them to /dev/null
        os.open("/dev/null", 0)
        os.open("/dev/null", 1)
        os.open("/dev/null", 2)

        self._daemon = True

    def drop_privileges(self, user, group):
        """ If the daemon is running as root user, drop privileges and continue
        running as the defined unprivileged user. """
        pid_file_path = os.path.join(self._pfile_path, self._pfile_name)
        passwd_file = open("/etc/passwd", "r")
        group_file = open("/etc/group", "r")
        uid = ""
        gid = ""
        for line in passwd_file:
            if user in line:
                uid = line.split(":")[2]
                break
        for line in group_file:
            if group in line.split(":")[0]:
                gid = line.split(":")[2]
                break
        passwd_file.close()
        group_file.close()
        if os.getuid() == 0:
            os.chown(pid_file_path, int(uid), int(gid))
            os.setgid(int(gid))
            os.setuid(int(uid))

    @staticmethod
    def set_name(name, cmdline):
        """ Sets the name of the process shown visible in ps and top,
        this allows to make your daemon look more like a standalone
        program instead of a python script.

        """
        try:
            # First check if prctl is available, otherwise this function
            # does nothing
            import prctl
        except ImportError:
            return False
        else:
            prctl.set_name(name)
            prctl.set_proctitle(cmdline)
            return True

    def start(self):
        """ Performs the operations needed to "start" the daemon """
        if self._daemon is True:
            if os.access(self._pfile_path, os.F_OK & os.W_OK):
                pidfile = open(os.path.join(self._pfile_path,
                                            self._pfile_name), "w")
                pidfile.write(str(os.getpid()) + "\n")
                pidfile.close()
                return True
            else:
                return False
        else:
            return False

    def stop(self):
        """ Performs the operations needed to stop the daemon """
        if self._daemon is True:
            try:
                os.remove(os.path.join(self._pfile_path, self._pfile_name))
            except OSError:
                return False
            else:
                return True
        else:
            return True