py-tools/src/tools/daemon.py

# Shamelessly stolen from https://gist.github.com/josephernest/77fdb0012b72ebdf4c9d19d6256a1119
#
# From "A simple unix/linux daemon in Python" by Sander Marechal
# See http://stackoverflow.com/a/473702/1422096 and
# http://web.archive.org/web/20131017130434/http://www.jejik.com/articles/2007/02/a_simple_unix_linux_daemon_in_python/
#
# Modified to add quit() that allows to run some code before closing the daemon
# See http://stackoverflow.com/a/40423758/1422096
#
# Modified for Python 3
# (see also: http://web.archive.org/web/20131017130434/http://www.jejik.com/files/examples/daemon3x.py)
#
# Joseph Ernest, 20200507_1220

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

DEV_NULL = "/dev/null"


class Daemon:
    """
    A generic daemon class.

    Usage: subclass the Daemon class and override the run() method
    """

    def __init__(
        self, pidfile="_.pid", stdin=DEV_NULL, stdout=DEV_NULL, stderr=DEV_NULL
    ):
        self.stdin = stdin
        self.stdout = stdout
        self.stderr = stderr
        self.pidfile = pidfile

    def daemonize(self):
        """
        do the UNIX double-fork magic, see Stevens' "Advanced
        Programming in the UNIX Environment" for details (ISBN 0201563177)
        http://www.erlenstar.demon.co.uk/unix/faq_2.html#SEC16
        """
        try:
            pid = os.fork()
            if pid > 0:
                # exit first parent
                sys.exit(0)
        except OSError as e:
            sys.stderr.write("fork #1 failed: %d (%s)\n" % (e.errno, e.strerror))
            sys.exit(1)

        # decouple from parent environment
        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: %d (%s)\n" % (e.errno, e.strerror))
            sys.exit(1)

        # redirect standard file descriptors
        sys.stdout.flush()
        sys.stderr.flush()
        si = open(os.devnull, "r")
        so = open(os.devnull, "a+")
        se = open(os.devnull, "a+")
        os.dup2(si.fileno(), sys.stdin.fileno())
        os.dup2(so.fileno(), sys.stdout.fileno())
        os.dup2(se.fileno(), sys.stderr.fileno())

        atexit.register(self.onstop)
        signal(SIGTERM, lambda signum, stack_frame: exit())

        # write pidfile
        pid = str(os.getpid())
        open(self.pidfile, "w+").write("%s\n" % pid)

    def onstop(self):
        self.quit()
        os.remove(self.pidfile)

    def start(self):
        """
        Start the daemon
        """
        # Check for a pidfile to see if the daemon already runs
        try:
            pf = open(self.pidfile, "r")
            pid = int(pf.read().strip())
            pf.close()
        except IOError:
            pid = None

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

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

    def stop(self):
        """
        Stop the daemon
        """
        # Get the pid from the pidfile
        try:
            pf = open(self.pidfile, "r")
            pid = int(pf.read().strip())
            pf.close()
        except IOError:
            pid = None

        if not pid:
            message = "pidfile %s does not exist. Daemon not running?\n"
            sys.stderr.write(message % 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):
        """
        Restart the daemon
        """
        self.stop()
        self.start()

    def run(self):
        """
        You should override this method when you subclass Daemon. It will be called after the process has been
        daemonized by start() or restart().
        """

    def quit(self):
        """
        You should override this method when you subclass Daemon. It will be called before the process is stopped.
        """