191 lines
7.8 KiB
Python
191 lines
7.8 KiB
Python
"""
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Python Deamonizing helper
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Configurable daemon behaviors:
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1.) The current working directory set to the "/" directory.
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2.) The current file creation mode mask set to 0.
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3.) Close all open files (1024).
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4.) Redirect standard I/O streams to "/dev/null".
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A failed call to fork() now raises an exception.
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References:
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1) Advanced Programming in the Unix Environment: W. Richard Stevens
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2) Unix Programming Frequently Asked Questions:
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http://www.erlenstar.demon.co.uk/unix/faq_toc.html
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Modified to allow a function to be daemonized and return for
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bitbake use by Richard Purdie
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"""
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__author__ = "Chad J. Schroeder"
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__copyright__ = "Copyright (C) 2005 Chad J. Schroeder"
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__version__ = "0.2"
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# Standard Python modules.
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import os # Miscellaneous OS interfaces.
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import sys # System-specific parameters and functions.
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# Default daemon parameters.
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# File mode creation mask of the daemon.
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# For BitBake's children, we do want to inherit the parent umask.
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UMASK = None
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# Default maximum for the number of available file descriptors.
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MAXFD = 1024
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# The standard I/O file descriptors are redirected to /dev/null by default.
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if (hasattr(os, "devnull")):
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REDIRECT_TO = os.devnull
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else:
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REDIRECT_TO = "/dev/null"
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def createDaemon(function, logfile):
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"""
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Detach a process from the controlling terminal and run it in the
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background as a daemon, returning control to the caller.
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"""
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try:
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# Fork a child process so the parent can exit. This returns control to
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# the command-line or shell. It also guarantees that the child will not
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# be a process group leader, since the child receives a new process ID
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# and inherits the parent's process group ID. This step is required
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# to insure that the next call to os.setsid is successful.
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pid = os.fork()
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except OSError as e:
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raise Exception("%s [%d]" % (e.strerror, e.errno))
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if (pid == 0): # The first child.
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# To become the session leader of this new session and the process group
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# leader of the new process group, we call os.setsid(). The process is
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# also guaranteed not to have a controlling terminal.
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os.setsid()
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# Is ignoring SIGHUP necessary?
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#
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# It's often suggested that the SIGHUP signal should be ignored before
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# the second fork to avoid premature termination of the process. The
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# reason is that when the first child terminates, all processes, e.g.
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# the second child, in the orphaned group will be sent a SIGHUP.
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#
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# "However, as part of the session management system, there are exactly
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# two cases where SIGHUP is sent on the death of a process:
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#
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# 1) When the process that dies is the session leader of a session that
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# is attached to a terminal device, SIGHUP is sent to all processes
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# in the foreground process group of that terminal device.
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# 2) When the death of a process causes a process group to become
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# orphaned, and one or more processes in the orphaned group are
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# stopped, then SIGHUP and SIGCONT are sent to all members of the
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# orphaned group." [2]
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#
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# The first case can be ignored since the child is guaranteed not to have
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# a controlling terminal. The second case isn't so easy to dismiss.
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# The process group is orphaned when the first child terminates and
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# POSIX.1 requires that every STOPPED process in an orphaned process
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# group be sent a SIGHUP signal followed by a SIGCONT signal. Since the
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# second child is not STOPPED though, we can safely forego ignoring the
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# SIGHUP signal. In any case, there are no ill-effects if it is ignored.
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#
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# import signal # Set handlers for asynchronous events.
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# signal.signal(signal.SIGHUP, signal.SIG_IGN)
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try:
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# Fork a second child and exit immediately to prevent zombies. This
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# causes the second child process to be orphaned, making the init
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# process responsible for its cleanup. And, since the first child is
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# a session leader without a controlling terminal, it's possible for
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# it to acquire one by opening a terminal in the future (System V-
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# based systems). This second fork guarantees that the child is no
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# longer a session leader, preventing the daemon from ever acquiring
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# a controlling terminal.
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pid = os.fork() # Fork a second child.
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except OSError as e:
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raise Exception("%s [%d]" % (e.strerror, e.errno))
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if (pid == 0): # The second child.
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# We probably don't want the file mode creation mask inherited from
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# the parent, so we give the child complete control over permissions.
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if UMASK is not None:
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os.umask(UMASK)
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else:
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# Parent (the first child) of the second child.
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os._exit(0)
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else:
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# exit() or _exit()?
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# _exit is like exit(), but it doesn't call any functions registered
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# with atexit (and on_exit) or any registered signal handlers. It also
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# closes any open file descriptors. Using exit() may cause all stdio
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# streams to be flushed twice and any temporary files may be unexpectedly
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# removed. It's therefore recommended that child branches of a fork()
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# and the parent branch(es) of a daemon use _exit().
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return
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# Close all open file descriptors. This prevents the child from keeping
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# open any file descriptors inherited from the parent. There is a variety
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# of methods to accomplish this task. Three are listed below.
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#
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# Try the system configuration variable, SC_OPEN_MAX, to obtain the maximum
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# number of open file descriptors to close. If it doesn't exists, use
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# the default value (configurable).
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#
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# try:
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# maxfd = os.sysconf("SC_OPEN_MAX")
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# except (AttributeError, ValueError):
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# maxfd = MAXFD
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#
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# OR
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#
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# if (os.sysconf_names.has_key("SC_OPEN_MAX")):
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# maxfd = os.sysconf("SC_OPEN_MAX")
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# else:
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# maxfd = MAXFD
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#
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# OR
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#
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# Use the getrlimit method to retrieve the maximum file descriptor number
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# that can be opened by this process. If there is not limit on the
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# resource, use the default value.
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#
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import resource # Resource usage information.
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maxfd = resource.getrlimit(resource.RLIMIT_NOFILE)[1]
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if (maxfd == resource.RLIM_INFINITY):
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maxfd = MAXFD
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# Iterate through and close all file descriptors.
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# for fd in range(0, maxfd):
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# try:
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# os.close(fd)
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# except OSError: # ERROR, fd wasn't open to begin with (ignored)
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# pass
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# Redirect the standard I/O file descriptors to the specified file. Since
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# the daemon has no controlling terminal, most daemons redirect stdin,
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# stdout, and stderr to /dev/null. This is done to prevent side-effects
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# from reads and writes to the standard I/O file descriptors.
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# This call to open is guaranteed to return the lowest file descriptor,
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# which will be 0 (stdin), since it was closed above.
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# os.open(REDIRECT_TO, os.O_RDWR) # standard input (0)
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# Duplicate standard input to standard output and standard error.
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# os.dup2(0, 1) # standard output (1)
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# os.dup2(0, 2) # standard error (2)
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si = file('/dev/null', 'r')
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so = file(logfile, 'w')
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se = so
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# Replace those fds with our own
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os.dup2(si.fileno(), sys.stdin.fileno())
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os.dup2(so.fileno(), sys.stdout.fileno())
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os.dup2(se.fileno(), sys.stderr.fileno())
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function()
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os._exit(0)
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