daemon.py

# -*- coding: utf-8 -*-
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#
# Portions taken from twistd:
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from __future__ import absolute_import

import os
import sys
import errno


def basic_daemonize(silence=True):
    # See http://www.erlenstar.demon.co.uk/unix/faq_toc.html#TOC16
    if os.fork():  # launch child and...
        os._exit(0)  # kill off parent
    os.setsid()
    if os.fork():  # launch child and...
        os._exit(0)  # kill off parent again.
    os.umask(0o022)  # Don't allow others to write

    if not silence:
        print("daemonize.basic_daemonize: process NOT silenced, for debugging")
    else:
        null = os.open('/dev/null', os.O_RDWR)
        for i in range(3):
            try:
                os.dup2(null, i)
            except OSError as e:
                if e.errno != errno.EBADF:
                    raise
        os.close(null)


def writePID(pidfile):
    open(pidfile, 'w').write(str(os.getpid()))
    if not os.path.exists(pidfile):
        raise Exception("pidfile %s does not exist" % pidfile)


def checkPID(pidfile):
    if not pidfile:
        return
    if os.path.exists(pidfile):
        try:
            pid = int(open(pidfile).read())
        except ValueError:
            sys.exit('Pidfile %s contains non-numeric value' % pidfile)
        try:
            os.kill(pid, 0)
        except OSError as why:
            if why.args[0] == errno.ESRCH:
                # The pid doesnt exists.
                print(('Removing stale pidfile %s' % pidfile))
                os.remove(pidfile)
            else:
                sys.exit("Can't check status of PID %s from pidfile %s: %s" %
                         (pid, pidfile, why.args[1]))
        else:
            sys.exit("Another server is running, PID %s\n" % pid)


def daemonize(pidfile):
    checkPID(pidfile)
    # CRITICAL: DO NOT set silence=False in production.  It hangs starting the sage server
    # properly, which breaks things badly for users (e.g., their first worksheet
    # never works).
    basic_daemonize(silence=True)
    writePID(pidfile)