Daemon em Python

Postado em 24.05.2009 00:00

Já usei mais de uma vez o código abaixo para criar pequenos [daemons](http://en.wikipedia.org/wiki/Daemon_(computer_software)) para Linux. Achei [aqui](https://www.jejik.com/articles/2007/02/a_simple_unix_linux_daemon_in_python/) e como ele foi [liberado em domínio público](https://www.jejik.com/articles/2007/02/a_simple_unix_linux_daemon_in_python/#c6), use como melhor convir.

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import sys, os, time, 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, 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, e:
            sys.stderr.write("fork #1 failed: %d (%s)\n" % (e.errno, 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, 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 = file(self.stdin, 'r')
        so = file(self.stdout, 'a+')
        se = file(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())
        file(self.pidfile,'w+').write("%s\n" % pid)

    def delpid(self):
        os.remove(self.pidfile)

    def start(self):
        """
        Start the daemon
        """
        # Check for a pidfile to see if the daemon already runs
        try:
            pf = file(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 = file(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, 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().
        """

Resolvi adicionar a funcionalidade de verificar o status do processo também, daí adicionei o seguinte método à classe acima:

def status(self):
    try:
        pf = file(self.pidfile, 'r')
        pid = int(pf.read().strip())
        pf.close()
    except IOError:
        pid = None

    try:
        procfile = file("/proc/%d/status" % pid, 'r')
        procfile.close()
    except IOError:
        sys.stdout.write("there is not a process with the PID specified in %s\n" % self.pidfile)
        sys.exit(0)
    except TypeError:
        sys.stdout.write("pidfile %s does not exist\n" % self.pidfile)
        sys.exit(0)

    sys.stdout.write("the process with the PID %d is running\n" % pid)

Não ficou muito bonito, mas funciona. Lembrando o que já está comentado no [código original](http://www.jejik.com/articles/2007/02/a_simple_unix_linux_daemon_in_python): para usar a classe acima para "daemonizar" seu programa, crie uma classe filha de `Daemon` e sobrescreva o método `run()`. Ele será chamado após o processo de "daemonização" por `start()` e `restart()`.

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