daemon.py

import sys
import os
import time
import atexit
from signal import SIGTERM


class Daemon:
    """
    A generic daemon class.
    Usage: subclass the Daemon class and override the run() method
    """
    def __init__(self, pidfile: str,
                 stdin: str = '/dev/null', stdout: str = '/dev/null', stderr: str = '/dev/null') -> None:
        self.stdin = stdin
        self.stdout = stdout
        self.stderr = stderr
        self.pidfile = pidfile

    def daemonize(self) -> None:
        try:
            pid = os.fork()
            if pid > 0:
                # exit first parent
                sys.exit(0)
        except OSError as e:
            sys.stderr.write("fork #1 failed: {errno} ({strerror})\n".format(errno=e.errno, strerror=e.strerror))
            sys.exit(1)

        # decouple from parent environment
        os.chdir("/")
        os.setsid()
        os.umask(0)

        # do second fork
        try:
            pid = os.fork()
            if pid > 0:
                # exit from second parent
                sys.exit(0)
        except OSError as e:
            sys.stderr.write("fork #2 failed: {errno} ({strerror})\n".format(errno=e.errno, strerror=e.strerror))
            sys.exit(1)

        # redirect standard file descriptors
        sys.stdout.flush()
        sys.stderr.flush()

        si = open(self.stdin, 'r')
        so = open(self.stdout, 'a+')
        se = open(self.stderr, 'a+', 0)

        os.dup2(si.fileno(), sys.stdin.fileno())
        os.dup2(so.fileno(), sys.stdout.fileno())
        os.dup2(se.fileno(), sys.stderr.fileno())

        # write pidfile
        atexit.register(self.delpid)
        pid = str(os.getpid())
        open(self.pidfile, 'w+').write("{pid}\n".format(pid=pid))

    def delpid(self) -> None:
        os.remove(self.pidfile)

    def start(self) -> None:
        try:
            pf = open(self.pidfile, 'r')
            pid = int(pf.read().strip())
            pf.close()
        except IOError:
            pid = None

        if pid:
            message = "pidfile {pidfile} already exist. Daemon already running?\n"
            sys.stderr.write(message.format(pidfile=self.pidfile))
            sys.exit(1)

        # Start the daemon
        self.daemonize()
        self.run()

    def stop(self) -> None:
        # Get the pid from the pidfile
        try:
            pf = open(self.pidfile, 'r')
            pid = int(pf.read().strip())
            pf.close()
        except IOError as e:
            pid = None
            print(str(e))

        if not pid:
            message = "pidfile {pidfile} does not exist. Daemon not running?\n"
            sys.stderr.write(message.format(pidfile=self.pidfile))
            return  # not an error in a restart

        # Try killing the daemon process
        try:
            while 1:
                os.kill(pid, SIGTERM)
                time.sleep(0.1)
        except OSError as err:
            err = str(err)
            if err.find("No such process") > 0:
                if os.path.exists(self.pidfile):
                    os.remove(self.pidfile)
            else:
                print(str(err))
                sys.exit(1)

    def restart(self) -> None:
        self.stop()
        self.start()

    def run(self) -> None:
        """
        You should override this method when you subclass Daemon. It will be called after the process has been
        daemonized by start() or restart().
        """