如何寫一個daemon程序

在實際的服務器後臺程序開發時，有時須要將某個服務 daemon 化來完成一些定時任務，好比往 KV 系統中刷新數據。

有兩個問題須要提早弄清楚:

• daemon 程序中的 pid file 有什麼做用?
• 爲何要二次fork，只fork一次能夠嗎？

Daemon class

這是一個 Daemon class，繼承此 class 的類須要 override 裏面的 run 方法。

#!/usr/bin/env python

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


class Daemon:
    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):
        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 ssecond 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):
        print "here1"
        """
        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().
        """

A daemon instance

#!/usr/bin/env python
from daemon import Daemon
import time


class DaemonInstance(Daemon):
    def run(self):
        while(True):
            print "run in deamon instance"
            time.sleep(2)


if __name__ == '__main__':
    daemon = DaemonInstance("/home/huangyi/Practice/Python/daemon.pid")
    daemon.start()

前面所提問題見 daemon 進程爲何要fork兩次 ， Daemon程序中的pidfile。

參考

http://www.jejik.com/articles/2007/02/a_simple_unix_linux_daemon_in_python/

http://dram.me/blog/2010/08/14/pidfile-in-daemon.html

APUE