poky/bitbake/lib/bb/daemonize.py

#
# Copyright BitBake Contributors
#
# SPDX-License-Identifier: GPL-2.0-only
#

"""
Python Daemonizing helper

Originally based on code Copyright (C) 2005 Chad J. Schroeder but now heavily modified
to allow a function to be daemonized and return for bitbake use by Richard Purdie
"""

import os
import sys
import io
import traceback

import bb

def createDaemon(function, logfile):
    """
    Detach a process from the controlling terminal and run it in the
    background as a daemon, returning control to the caller.
    """

    # Ensure stdout/stderror are flushed before forking to avoid duplicate output
    sys.stdout.flush()
    sys.stderr.flush()

    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 e:
        raise Exception("%s [%d]" % (e.strerror, e.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()
        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 e:
            raise Exception("%s [%d]" % (e.strerror, e.errno))

        if (pid != 0):
            # Parent (the first child) of the second child.
            # 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, but doesn't flush any buffered output.
            # Using exit() may cause all any temporary files to be unexpectedly
            # removed.  It's therefore recommended that child branches of a fork()
            # and the parent branch(es) of a daemon use _exit().
            os._exit(0)
    else:
        os.waitpid(pid, 0)
        return

    # The second child.

    # Replace standard fds with our own
    with open('/dev/null', 'r') as si:
        os.dup2(si.fileno(), sys.stdin.fileno())

    with open(logfile, 'a+') as so:
        try:
            os.dup2(so.fileno(), sys.stdout.fileno())
            os.dup2(so.fileno(), sys.stderr.fileno())
        except io.UnsupportedOperation:
            sys.stdout = so

        # Have stdout and stderr be the same so log output matches chronologically
        # and there aren't two separate buffers
        sys.stderr = sys.stdout

        try:
            function()
        except Exception as e:
            traceback.print_exc()
        finally:
            bb.event.print_ui_queue()
            # os._exit() doesn't flush open files like os.exit() does. Manually flush
            # stdout and stderr so that any logging output will be seen, particularly
            # exception tracebacks.
            sys.stdout.flush()
            sys.stderr.flush()
            os._exit(0)