"""Disk And Execution MONitor (Daemon)

Configurable daemon behaviors:

   1.) The current working directory set to the "/" directory.
   2.) The current file creation mode mask set to 0.
   3.) Close all open files (1024).
   4.) Redirect standard I/O streams to "/dev/null".

A failed call to fork() now raises an exception.

References:
   1) Advanced Programming in the Unix Environment: W. Richard Stevens
   2) Unix Programming Frequently Asked Questions:
         http://www.erlenstar.demon.co.uk/unix/faq_toc.html
"""
from os import _exit  # circumvent PyLint's protected-access warning
import os
import sys

__author__ = "Chad J. Schroeder"
__copyright__ = "Copyright (C) 2005 Chad J. Schroeder"

__revision__ = "$Id$"
__version__ = "0.2"

# Default daemon parameters.
# File mode creation mask of the daemon.
UMASK = 0

# Default working directory for the daemon.
WORKDIR = "/"

# Default maximum for the number of available file descriptors.
MAXFD = 1024

# The standard I/O file descriptors are redirected to /dev/null by default.
if hasattr(os, "devnull"):
    REDIRECT_TO = os.devnull
else:
    REDIRECT_TO = "/dev/null"


def create_daemon():
    """Detach a process from the controlling terminal and run it in the
    background as a daemon.
    """

    try:
        # Fork a child process so the parent can exit.  This returns control to
        # the command-line or shell.  It also guarantees that the child will not
        # be a process group leader, since the child receives a new process ID
        # and inherits the parent's process group ID.  This step is required
        # to insure that the next call to os.setsid is successful.
        pid = os.fork()
    except OSError as err:
        raise Exception("%s [%d]" % (err.strerror, err.errno))

    if pid == 0:  # The first child.
        # To become the session leader of this new session and the process group
        # leader of the new process group, we call os.setsid().  The process is
        # also guaranteed not to have a controlling terminal.
        os.setsid()

        # Is ignoring SIGHUP necessary?
        #
        # It's often suggested that the SIGHUP signal should be ignored before
        # the second fork to avoid premature termination of the process.  The
        # reason is that when the first child terminates, all processes, e.g.
        # the second child, in the orphaned group will be sent a SIGHUP.
        #
        # "However, as part of the session management system, there are exactly
        # two cases where SIGHUP is sent on the death of a process:
        #
        #   1) When the process that dies is the session leader of a session that
        #      is attached to a terminal device, SIGHUP is sent to all processes
        #      in the foreground process group of that terminal device.
        #   2) When the death of a process causes a process group to become
        #      orphaned, and one or more processes in the orphaned group are
        #      stopped, then SIGHUP and SIGCONT are sent to all members of the
        #      orphaned group." [2]
        #
        # The first case can be ignored since the child is guaranteed not to have
        # a controlling terminal.  The second case isn't so easy to dismiss.
        # The process group is orphaned when the first child terminates and
        # POSIX.1 requires that every STOPPED process in an orphaned process
        # group be sent a SIGHUP signal followed by a SIGCONT signal.  Since the
        # second child is not STOPPED though, we can safely forego ignoring the
        # SIGHUP signal.  In any case, there are no ill-effects if it is ignored.
        #
        # import signal           # Set handlers for asynchronous events.
        # signal.signal(signal.SIGHUP, signal.SIG_IGN)

        try:
            # Fork a second child and exit immediately to prevent zombies.  This
            # causes the second child process to be orphaned, making the init
            # process responsible for its cleanup.  And, since the first child is
            # a session leader without a controlling terminal, it's possible for
            # it to acquire one by opening a terminal in the future (System V-
            # based systems).  This second fork guarantees that the child is no
            # longer a session leader, preventing the daemon from ever acquiring
            # a controlling terminal.
            pid = os.fork()  # Fork a second child.
        except OSError as err:
            raise Exception("%s [%d]" % (err.strerror, err.errno))

        if pid == 0:  # The second child.
            # Since the current working directory may be a mounted filesystem, we
            # avoid the issue of not being able to unmount the filesystem at
            # shutdown time by changing it to the root directory.
            os.chdir(WORKDIR)
            # We probably don't want the file mode creation mask inherited from
            # the parent, so we give the child complete control over permissions.
            os.umask(UMASK)
        else:
            # exit() or _exit()?  See below.
            _exit(0)  # Exit parent (the first child) of the second child.
    else:
        # exit() or _exit()?
        # _exit is like exit(), but it doesn't call any functions registered
        # with atexit (and on_exit) or any registered signal handlers.  It also
        # closes any open file descriptors.  Using exit() may cause all stdio
        # streams to be flushed twice and any temporary files may be unexpectedly
        # removed.  It's therefore recommended that child branches of a fork()
        # and the parent branch(es) of a daemon use _exit().
        _exit(0)  # Exit parent of the first child.

    # Close all open file descriptors.  This prevents the child from keeping
    # open any file descriptors inherited from the parent.  There is a variety
    # of methods to accomplish this task.  Three are listed below.
    #
    # Try the system configuration variable, SC_OPEN_MAX, to obtain the maximum
    # number of open file descriptors to close.  If it doesn't exists, use
    # the default value (configurable).
    #
    # try:
    #    maxfd = os.sysconf("SC_OPEN_MAX")
    # except (AttributeError, ValueError):
    #    maxfd = MAXFD
    #
    # OR
    #
    # if os.sysconf_names.has_key("SC_OPEN_MAX"):
    #    maxfd = os.sysconf("SC_OPEN_MAX")
    # else:
    #    maxfd = MAXFD
    #
    # OR
    #
    # Use the getrlimit method to retrieve the maximum file descriptor number
    # that can be opened by this process.  If there is not limit on the
    # resource, use the default value.
    #
    import resource  # Resource usage information.
    maxfd = resource.getrlimit(resource.RLIMIT_NOFILE)[1]
    if maxfd == resource.RLIM_INFINITY:
        maxfd = MAXFD

    # Iterate through and close all file descriptors.
    for file_desc in range(0, maxfd):
        try:
            os.close(file_desc)
        except OSError:  # file descriptor wasn't open to begin with (ignored)
            pass

    # Redirect the standard I/O file descriptors to the specified file.  Since
    # the daemon has no controlling terminal, most daemons redirect stdin,
    # stdout, and stderr to /dev/null.  This is done to prevent side-effects
    # from reads and writes to the standard I/O file descriptors.

    # This call to open is guaranteed to return the lowest file descriptor,
    # which will be 0 (stdin), since it was closed above.
    os.open(REDIRECT_TO, os.O_RDWR)  # standard input (0)

    # Duplicate standard input to standard output and standard error.
    os.dup2(0, 1)  # standard output (1)
    os.dup2(0, 2)  # standard error (2)

    return 0


if __name__ == "__main__":
    RET_CODE = create_daemon()

    # The code, as is, will create a new file in the root directory, when
    # executed with superuser privileges.  The file will contain the following
    # daemon related process parameters: return code, process ID, parent
    # process group ID, session ID, user ID, effective user ID, real group ID,
    # and the effective group ID.  Notice the relationship between the daemon's
    # process ID, process group ID, and its parent's process ID.

    PROC_PARAMS = """
    return code = %s
    process ID = %s
    parent process ID = %s
    process group ID = %s
    session ID = %s
    user ID = %s
    effective user ID = %s
    real group ID = %s
    effective group ID = %s
    """ % (RET_CODE, os.getpid(), os.getppid(), os.getpgrp(), os.getsid(0),
           os.getuid(), os.geteuid(), os.getgid(), os.getegid())

    open("createDaemon.log", "w").write(PROC_PARAMS + "\n")

    sys.exit(RET_CODE)