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1"""
2Python Deamonizing helper
3
4Configurable daemon behaviors:
5
6 1.) The current working directory set to the "/" directory.
7 2.) The current file creation mode mask set to 0.
8 3.) Close all open files (1024).
9 4.) Redirect standard I/O streams to "/dev/null".
10
11A failed call to fork() now raises an exception.
12
13References:
14 1) Advanced Programming in the Unix Environment: W. Richard Stevens
15 2) Unix Programming Frequently Asked Questions:
16 http://www.erlenstar.demon.co.uk/unix/faq_toc.html
17
18Modified to allow a function to be daemonized and return for
19bitbake use by Richard Purdie
20"""
21
22__author__ = "Chad J. Schroeder"
23__copyright__ = "Copyright (C) 2005 Chad J. Schroeder"
24__version__ = "0.2"
25
26# Standard Python modules.
27import os # Miscellaneous OS interfaces.
28import sys # System-specific parameters and functions.
29
30# Default daemon parameters.
31# File mode creation mask of the daemon.
32# For BitBake's children, we do want to inherit the parent umask.
33UMASK = None
34
35# Default maximum for the number of available file descriptors.
36MAXFD = 1024
37
38# The standard I/O file descriptors are redirected to /dev/null by default.
39if (hasattr(os, "devnull")):
40 REDIRECT_TO = os.devnull
41else:
42 REDIRECT_TO = "/dev/null"
43
44def createDaemon(function, logfile):
45 """
46 Detach a process from the controlling terminal and run it in the
47 background as a daemon, returning control to the caller.
48 """
49
50 try:
51 # Fork a child process so the parent can exit. This returns control to
52 # the command-line or shell. It also guarantees that the child will not
53 # be a process group leader, since the child receives a new process ID
54 # and inherits the parent's process group ID. This step is required
55 # to insure that the next call to os.setsid is successful.
56 pid = os.fork()
57 except OSError as e:
58 raise Exception("%s [%d]" % (e.strerror, e.errno))
59
60 if (pid == 0): # The first child.
61 # To become the session leader of this new session and the process group
62 # leader of the new process group, we call os.setsid(). The process is
63 # also guaranteed not to have a controlling terminal.
64 os.setsid()
65
66 # Is ignoring SIGHUP necessary?
67 #
68 # It's often suggested that the SIGHUP signal should be ignored before
69 # the second fork to avoid premature termination of the process. The
70 # reason is that when the first child terminates, all processes, e.g.
71 # the second child, in the orphaned group will be sent a SIGHUP.
72 #
73 # "However, as part of the session management system, there are exactly
74 # two cases where SIGHUP is sent on the death of a process:
75 #
76 # 1) When the process that dies is the session leader of a session that
77 # is attached to a terminal device, SIGHUP is sent to all processes
78 # in the foreground process group of that terminal device.
79 # 2) When the death of a process causes a process group to become
80 # orphaned, and one or more processes in the orphaned group are
81 # stopped, then SIGHUP and SIGCONT are sent to all members of the
82 # orphaned group." [2]
83 #
84 # The first case can be ignored since the child is guaranteed not to have
85 # a controlling terminal. The second case isn't so easy to dismiss.
86 # The process group is orphaned when the first child terminates and
87 # POSIX.1 requires that every STOPPED process in an orphaned process
88 # group be sent a SIGHUP signal followed by a SIGCONT signal. Since the
89 # second child is not STOPPED though, we can safely forego ignoring the
90 # SIGHUP signal. In any case, there are no ill-effects if it is ignored.
91 #
92 # import signal # Set handlers for asynchronous events.
93 # signal.signal(signal.SIGHUP, signal.SIG_IGN)
94
95 try:
96 # Fork a second child and exit immediately to prevent zombies. This
97 # causes the second child process to be orphaned, making the init
98 # process responsible for its cleanup. And, since the first child is
99 # a session leader without a controlling terminal, it's possible for
100 # it to acquire one by opening a terminal in the future (System V-
101 # based systems). This second fork guarantees that the child is no
102 # longer a session leader, preventing the daemon from ever acquiring
103 # a controlling terminal.
104 pid = os.fork() # Fork a second child.
105 except OSError as e:
106 raise Exception("%s [%d]" % (e.strerror, e.errno))
107
108 if (pid == 0): # The second child.
109 # We probably don't want the file mode creation mask inherited from
110 # the parent, so we give the child complete control over permissions.
111 if UMASK is not None:
112 os.umask(UMASK)
113 else:
114 # Parent (the first child) of the second child.
115 os._exit(0)
116 else:
117 # exit() or _exit()?
118 # _exit is like exit(), but it doesn't call any functions registered
119 # with atexit (and on_exit) or any registered signal handlers. It also
120 # closes any open file descriptors. Using exit() may cause all stdio
121 # streams to be flushed twice and any temporary files may be unexpectedly
122 # removed. It's therefore recommended that child branches of a fork()
123 # and the parent branch(es) of a daemon use _exit().
124 return
125
126 # Close all open file descriptors. This prevents the child from keeping
127 # open any file descriptors inherited from the parent. There is a variety
128 # of methods to accomplish this task. Three are listed below.
129 #
130 # Try the system configuration variable, SC_OPEN_MAX, to obtain the maximum
131 # number of open file descriptors to close. If it doesn't exists, use
132 # the default value (configurable).
133 #
134 # try:
135 # maxfd = os.sysconf("SC_OPEN_MAX")
136 # except (AttributeError, ValueError):
137 # maxfd = MAXFD
138 #
139 # OR
140 #
141 # if (os.sysconf_names.has_key("SC_OPEN_MAX")):
142 # maxfd = os.sysconf("SC_OPEN_MAX")
143 # else:
144 # maxfd = MAXFD
145 #
146 # OR
147 #
148 # Use the getrlimit method to retrieve the maximum file descriptor number
149 # that can be opened by this process. If there is not limit on the
150 # resource, use the default value.
151 #
152 import resource # Resource usage information.
153 maxfd = resource.getrlimit(resource.RLIMIT_NOFILE)[1]
154 if (maxfd == resource.RLIM_INFINITY):
155 maxfd = MAXFD
156
157 # Iterate through and close all file descriptors.
158# for fd in range(0, maxfd):
159# try:
160# os.close(fd)
161# except OSError: # ERROR, fd wasn't open to begin with (ignored)
162# pass
163
164 # Redirect the standard I/O file descriptors to the specified file. Since
165 # the daemon has no controlling terminal, most daemons redirect stdin,
166 # stdout, and stderr to /dev/null. This is done to prevent side-effects
167 # from reads and writes to the standard I/O file descriptors.
168
169 # This call to open is guaranteed to return the lowest file descriptor,
170 # which will be 0 (stdin), since it was closed above.
171# os.open(REDIRECT_TO, os.O_RDWR) # standard input (0)
172
173 # Duplicate standard input to standard output and standard error.
174# os.dup2(0, 1) # standard output (1)
175# os.dup2(0, 2) # standard error (2)
176
177
178 si = file('/dev/null', 'r')
179 so = file(logfile, 'w')
180 se = so
181
182
183 # Replace those fds with our own
184 os.dup2(si.fileno(), sys.stdin.fileno())
185 os.dup2(so.fileno(), sys.stdout.fileno())
186 os.dup2(se.fileno(), sys.stderr.fileno())
187
188 function()
189
190 os._exit(0)