| #!/usr/bin/env python2.6 |
| |
| ''' |
| Licensed to the Apache Software Foundation (ASF) under one |
| or more contributor license agreements. See the NOTICE file |
| distributed with this work for additional information |
| regarding copyright ownership. The ASF licenses this file |
| to you under the Apache License, Version 2.0 (the |
| "License"); you may not use this file except in compliance |
| with the License. You may obtain a copy of the License at |
| |
| http://www.apache.org/licenses/LICENSE-2.0 |
| |
| Unless required by applicable law or agreed to in writing, software |
| distributed under the License is distributed on an "AS IS" BASIS, |
| WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| See the License for the specific language governing permissions and |
| limitations under the License. |
| ''' |
| |
| """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 |
| """ |
| |
| __author__ = "Chad J. Schroeder" |
| __copyright__ = "Copyright (C) 2005 Chad J. Schroeder" |
| |
| __revision__ = "$Id$" |
| __version__ = "0.2" |
| |
| # Standard Python modules. |
| import os # Miscellaneous OS interfaces. |
| import sys # System-specific parameters and functions. |
| |
| # Default daemon parameters. |
| # File mode creation mask of the daemon. |
| UMASK = 0022 |
| |
| # 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 createDaemon(): |
| """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, 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() |
| |
| # 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, e: |
| raise Exception, "%s [%d]" % (e.strerror, e.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. |
| os._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(). |
| os._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 fd in range(0, maxfd): |
| try: |
| os.close(fd) |
| except OSError: # ERROR, fd 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__": |
| |
| retCode = createDaemon() |
| |
| sys.exit(retCode) |
