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PEP 3143 -- Standard daemon process library

PEP: 3143
Title: Standard daemon process library
Author: Ben Finney <ben+python at benfinney.id.au>
Status: Deferred
Type: Standards Track
Created: 2009-01-26
Python-Version: 3.x
Post-History:

Abstract

Writing a program to become a well-behaved Unix daemon is somewhat complex and tricky to get right, yet the steps are largely similar for any daemon regardless of what else the program may need to do.

This PEP introduces a package to the Python standard library that provides a simple interface to the task of becoming a daemon process.

PEP Deferral

Further exploration of the concepts covered in this PEP has been deferred for lack of a current champion interested in promoting the goals of the PEP and collecting and incorporating feedback, and with sufficient available time to do so effectively.

Specification

Example usage

Simple example of direct DaemonContext usage:

import daemon

from spam import do_main_program

with daemon.DaemonContext():
    do_main_program()

More complex example usage:

import os
import grp
import signal
import daemon
import lockfile

from spam import (
    initial_program_setup,
    do_main_program,
    program_cleanup,
    reload_program_config,
    )

context = daemon.DaemonContext(
    working_directory='/var/lib/foo',
    umask=0o002,
    pidfile=lockfile.FileLock('/var/run/spam.pid'),
    )

context.signal_map = {
    signal.SIGTERM: program_cleanup,
    signal.SIGHUP: 'terminate',
    signal.SIGUSR1: reload_program_config,
    }

mail_gid = grp.getgrnam('mail').gr_gid
context.gid = mail_gid

important_file = open('spam.data', 'w')
interesting_file = open('eggs.data', 'w')
context.files_preserve = [important_file, interesting_file]

initial_program_setup()

with context:
    do_main_program()

Interface

A new package, daemon , is added to the standard library.

A class, DaemonContext , is defined to represent the settings and process context for the program running as a daemon process.

DaemonContext objects

A DaemonContext instance represents the behaviour settings and process context for the program when it becomes a daemon. The behaviour and environment is customised by setting options on the instance, before calling the open method.

Each option can be passed as a keyword argument to the DaemonContext constructor, or subsequently altered by assigning to an attribute on the instance at any time prior to calling open . That is, for options named wibble and wubble , the following invocation:

foo = daemon.DaemonContext(wibble=bar, wubble=baz)
foo.open()

is equivalent to:

foo = daemon.DaemonContext()
foo.wibble = bar
foo.wubble = baz
foo.open()

The following options are defined.

files_preserve
Default: None

List of files that should not be closed when starting the daemon. If None , all open file descriptors will be closed.

Elements of the list are file descriptors (as returned by a file object's fileno() method) or Python file objects. Each specifies a file that is not to be closed during daemon start.

chroot_directory
Default: None

Full path to a directory to set as the effective root directory of the process. If None , specifies that the root directory is not to be changed.

working_directory
Default: '/'

Full path of the working directory to which the process should change on daemon start.

Since a filesystem cannot be unmounted if a process has its current working directory on that filesystem, this should either be left at default or set to a directory that is a sensible “home directory” for the daemon while it is running.

umask
Default: 0

File access creation mask (“umask”) to set for the process on daemon start.

Since a process inherits its umask from its parent process, starting the daemon will reset the umask to this value so that files are created by the daemon with access modes as it expects.

pidfile
Default: None

Context manager for a PID lock file. When the daemon context opens and closes, it enters and exits the pidfile context manager.

detach_process
Default: None

If True , detach the process context when opening the daemon context; if False , do not detach.

If unspecified ( None ) during initialisation of the instance, this will be set to True by default, and False only if detaching the process is determined to be redundant; for example, in the case when the process was started by init , by initd , or by inetd .

signal_map
Default: system-dependent

Mapping from operating system signals to callback actions.

The mapping is used when the daemon context opens, and determines the action for each signal's signal handler:

  • A value of None will ignore the signal (by setting the signal action to signal.SIG_IGN ).
  • A string value will be used as the name of an attribute on the DaemonContext instance. The attribute's value will be used as the action for the signal handler.
  • Any other value will be used as the action for the signal handler.

The default value depends on which signals are defined on the running system. Each item from the list below whose signal is actually defined in the signal module will appear in the default map:

  • signal.SIGTTIN : None
  • signal.SIGTTOU : None
  • signal.SIGTSTP : None
  • signal.SIGTERM : 'terminate'

Depending on how the program will interact with its child processes, it may need to specify a signal map that includes the signal.SIGCHLD signal (received when a child process exits). See the specific operating system's documentation for more detail on how to determine what circumstances dictate the need for signal handlers.

uid
Default: os.getuid()
gid
Default: os.getgid()

The user ID (“UID”) value and group ID (“GID”) value to switch the process to on daemon start.

The default values, the real UID and GID of the process, will relinquish any effective privilege elevation inherited by the process.

prevent_core
Default: True

If true, prevents the generation of core files, in order to avoid leaking sensitive information from daemons run as root .

stdin
Default: None
stdout
Default: None
stderr
Default: None

Each of stdin , stdout , and stderr is a file-like object which will be used as the new file for the standard I/O stream sys.stdin , sys.stdout , and sys.stderr respectively. The file should therefore be open, with a minimum of mode 'r' in the case of stdin , and mode 'w+' in the case of stdout and stderr .

If the object has a fileno() method that returns a file descriptor, the corresponding file will be excluded from being closed during daemon start (that is, it will be treated as though it were listed in files_preserve ).

If None , the corresponding system stream is re-bound to the file named by os.devnull .

The following methods are defined.

open()
Return: None

Open the daemon context, turning the current program into a daemon process. This performs the following steps:

  • If this instance's is_open property is true, return immediately. This makes it safe to call open multiple times on an instance.

  • If the prevent_core attribute is true, set the resource limits for the process to prevent any core dump from the process.

  • If the chroot_directory attribute is not None , set the effective root directory of the process to that directory (via os.chroot ).

    This allows running the daemon process inside a “chroot gaol” as a means of limiting the system's exposure to rogue behaviour by the process. Note that the specified directory needs to already be set up for this purpose.

  • Set the process UID and GID to the uid and gid attribute values.

  • Close all open file descriptors. This excludes those listed in the files_preserve attribute, and those that correspond to the stdin , stdout , or stderr attributes.

  • Change current working directory to the path specified by the working_directory attribute.

  • Reset the file access creation mask to the value specified by the umask attribute.

  • If the detach_process option is true, detach the current process into its own process group, and disassociate from any controlling terminal.

  • Set signal handlers as specified by the signal_map attribute.

  • If any of the attributes stdin , stdout , stderr are not None , bind the system streams sys.stdin , sys.stdout , and/or sys.stderr to the files represented by the corresponding attributes. Where the attribute has a file descriptor, the descriptor is duplicated (instead of re-binding the name).

  • If the pidfile attribute is not None , enter its context manager.

  • Mark this instance as open (for the purpose of future open and close calls).

  • Register the close method to be called during Python's exit processing.

When the function returns, the running program is a daemon process.

close()
Return: None

Close the daemon context. This performs the following steps:

  • If this instance's is_open property is false, return immediately. This makes it safe to call close multiple times on an instance.
  • If the pidfile attribute is not None , exit its context manager.
  • Mark this instance as closed (for the purpose of future open and close calls).
is_open
Return: True if the instance is open, False otherwise.

This property exposes the state indicating whether the instance is currently open. It is True if the instance's open method has been called and the close method has not subsequently been called.

terminate(signal_number, stack_frame)
Return: None

Signal handler for the signal.SIGTERM signal. Performs the following step:

  • Raise a SystemExit exception explaining the signal.

The class also implements the context manager protocol via __enter__ and __exit__ methods.

__enter__()
Return: The DaemonContext instance

Call the instance's open() method, then return the instance.

__exit__(exc_type, exc_value, exc_traceback)
Return: True or False as defined by the context manager protocol

Call the instance's close() method, then return True if the exception was handled or False if it was not.

Motivation

The majority of programs written to be Unix daemons either implement behaviour very similar to that in the specification , or are poorly-behaved daemons by the correct daemon behaviour .

Since these steps should be much the same in most implementations but are very particular and easy to omit or implement incorrectly, they are a prime target for a standard well-tested implementation in the standard library.

Rationale

Correct daemon behaviour

According to Stevens in [stevens] §2.6, a program should perform the following steps to become a Unix daemon process.

  • Close all open file descriptors.
  • Change current working directory.
  • Reset the file access creation mask.
  • Run in the background.
  • Disassociate from process group.
  • Ignore terminal I/O signals.
  • Disassociate from control terminal.
  • Don't reacquire a control terminal.
  • Correctly handle the following circumstances:
    • Started by System V init process.
    • Daemon termination by SIGTERM signal.
    • Children generate SIGCLD signal.

The daemon tool [slack-daemon] lists (in its summary of features) behaviour that should be performed when turning a program into a well-behaved Unix daemon process. It differs from this PEP's intent in that it invokes a separate program as a daemon process. The following features are appropriate for a daemon that starts itself once the program is already running:

  • Sets up the correct process context for a daemon.
  • Behaves sensibly when started by initd(8) or inetd(8) .
  • Revokes any suid or sgid privileges to reduce security risks in case daemon is incorrectly installed with special privileges.
  • Prevents the generation of core files to prevent leaking sensitive information from daemons run as root (optional).
  • Names the daemon by creating and locking a PID file to guarantee that only one daemon with the given name can execute at any given time (optional).
  • Sets the user and group under which to run the daemon (optional, root only).
  • Creates a chroot gaol (optional, root only).
  • Captures the daemon's stdout and stderr and directs them to syslog (optional).

A daemon is not a service

This PEP addresses only Unix-style daemons, for which the above correct behaviour is relevant, as opposed to comparable behaviours on other operating systems.

There is a related concept in many systems, called a “service”. A service differs from the model in this PEP, in that rather than having the current program continue to run as a daemon process, a service starts an additional process to run in the background, and the current process communicates with that additional process via some defined channels.

The Unix-style daemon model in this PEP can be used, among other things, to implement the background-process part of a service; but this PEP does not address the other aspects of setting up and managing a service.

Reference Implementation

The python-daemon package [python-daemon] .

Other daemon implementations

Prior to this PEP, several existing third-party Python libraries or tools implemented some of this PEP's correct daemon behaviour .

The reference implementation is a fairly direct successor from the following implementations:

Other Python daemon implementations that differ from this PEP:

  • The zdaemon tool [zdaemon] was written for the Zope project. Like [slack-daemon] , it differs from this specification because it is used to run another program as a daemon process.
  • The Python library daemon [clapper-daemon] is (according to its homepage) no longer maintained. As of version 1.0.1, it implements the basic steps from [stevens] .
  • The daemonize library [seutter-daemonize] also implements the basic steps from [stevens] .
  • Ray Burr's daemon.py module [burr-daemon] provides the [stevens] procedure as well as PID file handling and redirection of output to syslog.
  • Twisted [twisted] includes, perhaps unsurprisingly, an implementation of a process daemonisation API that is integrated with the rest of the Twisted framework; it differs significantly from the API in this PEP.
  • The Python initd library [dagitses-initd] , which uses [clapper-daemon] , implements an equivalent of Unix initd(8) for controlling a daemon process.

References

[stevens] ( 1 , 2 , 3 , 4 ) Unix Network Programming , W. Richard Stevens, 1994 Prentice Hall.
[slack-daemon] ( 1 , 2 ) The (non-Python) “libslack” implementation of a daemon tool http://www.libslack.org/daemon/ by “raf” < raf@raf.org >.
[python-daemon] ( 1 , 2 ) The python-daemon library http://pypi.python.org/pypi/python-daemon/ by Ben Finney et al.
[cookbook-66012] ( 1 , 2 ) Python Cookbook recipe 66012, “Fork a daemon process on Unix” http://code.activestate.com/recipes/66012/ .
[cookbook-278731] Python Cookbook recipe 278731, “Creating a daemon the Python way” http://code.activestate.com/recipes/278731/ .
[bda.daemon] The bda.daemon library http://pypi.python.org/pypi/bda.daemon/ by Robert Niederreiter et al.
[zdaemon] The zdaemon tool http://pypi.python.org/pypi/zdaemon/ by Guido van Rossum et al.
[clapper-daemon] ( 1 , 2 ) The daemon library http://pypi.python.org/pypi/daemon/ by Brian Clapper.
[seutter-daemonize] The daemonize library http://daemonize.sourceforge.net/ by Jerry Seutter.
[burr-daemon] The daemon.py module http://www.nightmare.com/~ryb/code/daemon.py by Ray Burr.
[twisted] The Twisted application framework http://pypi.python.org/pypi/Twisted/ by Glyph Lefkowitz et al.
[dagitses-initd] The Python initd library http://pypi.python.org/pypi/initd/ by Michael Andreas Dagitses.
Source: https://github.com/python/peps/blob/master/pep-3143.txt