[Python-ideas] Draft PEP: Standard daemon process library
Ben Finney
ben+python at benfinney.id.au
Sun Jan 25 23:11:10 CET 2009
Howdy all,
I am preparing a PEP, and corresponding reference implementation, for
a standard implementation of the steps needed to turn a program into a
well-behaved Unix daemon.
This message is a call for comments, support, and suggestions, prior
to submitting this PEP to the editor.
:PEP: XXX
:Title: Standard daemon process library
:Version: 0.1
:Last-Modified: 2009-01-26 08:44
:Author: Ben Finney <ben+python at benfinney.id.au>
:Status: Draft
:Type: Standards Track
:Content-Type: text/x-rst
:Created: 2009-01-26
:Python-Version: 3.1
: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 module to the Python standard library that
provides a simple interface to the task of becoming a daemon process.
.. contents::
..
Table of Contents:
Abstract
Specification
Example usage
Interface
``Daemon`` objects
``DaemonError`` objects
Motivation
Rationale
Correct daemon behaviour
Reference Implementation
References
Copyright
=============
Specification
=============
Example usage
=============
Simple example of usage::
import daemon
from spam import do_main_program
this_daemon = daemon.Daemon()
this_daemon.start()
do_main_program()
More complex example usage::
import os
import grp
import signal
import daemon
from spam import (
initial_program_setup,
do_main_program,
program_cleanup,
reload_program_config,
)
initial_program_setup()
important_file = open('spam.data', 'w')
interesting_file = open('eggs.data', 'w')
this_daemon = daemon.Daemon()
this_daemon.files_preserve = [important_file, interesting_file]
this_daemon.working_directory = '/var/lib/foo'
this_daemon.umask = 0o002
mail_gid = grp.getgrnam('mail').gr_gid
this_daemon.gid = mail_gid
this_daemon.terminate_callback = program_cleanup
this_daemon.reload_callback = reload_program_config
this_daemon.reload_signals = [signal.SIGHUP, signal.SIGUSR1]
this_daemon.start()
do_main_program()
Interface
=========
A new module, `daemon`, is added to the standard library.
The module defines a class, `Daemon`, used to represent the settings
for a daemon process.
An exception class, `DaemonError`, is defined for exceptions raised
from the module.
``Daemon`` objects
==================
A `Daemon` instance represents the behaviour settings for the process
when it becomes a daemon. The behaviour is customised by setting
attributes on the instance, before calling the `start` method.
The following attributes 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.
`lockfile_directory`
:Default: ``'/var/run'``
Absolute directory path to contain the daemon's lockfile. If
``None``, the lockfile behaviour for this daemon is skipped.
`lockfile_name`
:Default: ``None``
Base name of the lockfile for this daemon, without directory or
extension. If ``None``, the name is derived from the process
command line.
`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.
`ignore_signals`
:Default: ``[signal.SIGTTOU, signal.SIGTTIN, signal.SIGTSTP]``
List of signals that the process should ignore (by setting the
signal action to ``signal.SIG_IGN``) on daemon start.
`terminate_signals`
:Default: ``[signal.SIGTERM]``
List of signals that the process should interpret as a request to
terminate cleanly.
`terminate_callback`
:Default: ``None``
Callable to invoke when the process receives any of the
`terminate_signals` signals, before then terminating the process.
`reload_signals`
:Default: ``[signal.SIGHUP]``
List of signals that the process should interpret as a request to
reload runtime configuration.
`reload_callback`
:Default: ``None``
Callable to invoke when the process receives any of the
`reload_signals` signals.
`uid`
:Default: ``None``
The user ID (“uid”) value to switch the process to on daemon start.
`gid`
:Default: ``None``
The group ID (“gid”) value to switch the process to on daemon start.
`prevent_core`
:Default: ``True``
If true, prevents the generation of core files, in order to avoid
leaking sensitive information from daemons run as `root`.
`stdout`
:Default: ``None``
File-like object, open for writing (in append mode, 'w+'), that
will be used as the new value of `sys.stdout`. If it represents an
actual file, it should be listed in `files_preserve` to prevent it
being closed during daemon start. If ``None``, then `sys.stdout`
is not re-bound.
`stderr`
:Default: ``None``
File-like object, open for writing (in append mode, 'w+'), that
will be used as the new value of `sys.stderr`. If it represents an
actual file, it should be listed in `files_preserve` to prevent it
being closed during daemon start. If ``None``, then `sys.stderr`
is not re-bound.
The following methods are defined.
`start()`
:Return: ``None``
Start the daemon. This performs the following steps:
* 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.
* If the `lockfile_directory` attribute is not ``None``:
* Look in that directory for a file named '`lockfile_name`.pid';
if it exists, raise a `DaemonError` to prevent multiple
instances of the daemon process.
* Close all open file descriptors, excluding those listed in the
`files_preserve` attribute.
* 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.
* Detach the current process into its own process group, and
disassociate from any controlling terminal.
This step is skipped if it is determined to be redundant: if the
process was started by `init`, by `initd`, or by `inetd`.
* Set signal handlers as specified by the `ignore_signals`,
`terminate_signals`, `terminate_callback`, `reload_signals`,
`reload_callback` attributes.
* If the `prevent_core` attribute is true:
* Set the resource limits for the process to prevent any core
dump from the process.
* Set the process uid and gid to the true uid and gid of the
process, to relinquish any elevated privilege.
* If the `lockfile_directory` attribute is not ``None``:
* Create the lockfile for this daemon in that directory, by
writing a text line containing the current process ID (“pid”)
to a file named '`lockfile_name`.pid'.
* If either of the attributes `uid` or `gid` are not ``None``:
* Set the process uid and/or gid to the specified values.
* If either of the attributes `stdout` or `stderr` are not
``None``:
* Bind the names `sys.stdout` and/or `sys.stderr` to the
corresponding objects.
`reload()`
:Return: ``None``
Reload the daemon configuration. The meaning of this is entirely
defined by the customisation of this daemon: if the
`reload_callback` attribute is not ``None``, call that object. The
return value is discarded.
`stop()`
:Return: ``None``
Stop the daemon. This performs the following steps:
* If the `terminate_callback` attribute is not ``None``:
* Call that object. The return value is discarded.
* If the `lockfile_directory` attribute is not ``None``:
* Delete the lockfile for this daemon.
* Raise a `SystemExit` exception.
``DaemonError`` objects
=======================
The `DaemonError` class inherits from `Exception`. The module
implementation will raise an instance of `DaemonError` when an error
occurs in processing daemon behaviour.
==========
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 `Rationale`_.
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` package [daemon]_ lists (in its summary of features)
behaviour that should be performed when turning a program into a
well-behaved Unix daemon process. The following are appropriate for a
daemon started 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).
========================
Reference Implementation
========================
The `python-daemon` package [python-daemon]_.
As of 2009-01-26, the package is under active development and is not
yet a full implementation of this PEP.
==========
References
==========
.. [stevens]
`Unix Network Programming`, W. Richard Stevens, 1994 Prentice
Hall.
.. [daemon]
The (non-Python) `daemon` package,
`<http://www.libslack.org/daemon/>`_ by “raf” <raf at raf.org>.
.. [python-daemon]
The `python-daemon` package at the Python Package Index,
`<http://pypi.python.org/pypi/python-daemon>`_. This is the
successor to [bda.daemon]_.
.. [bda.daemon]
The `bda.daemon` package at the Python Package Index,
`<http://pypi.python.org/pypi/bda.daemon>`_. This is an
implementation of [cookbook-66012]_.
.. [cookbook-66012]
Many good ideas were contributed by the community to Python
cookbook recipe 66012, “Fork a daemon process on Unix”
`<http://code.activestate.com/recipes/66012/>`_.
=========
Copyright
=========
This work is hereby placed in the public domain. To the extent that
placing a work in the public domain is not legally possible, the
copyright holder hereby grants to all recipients of this work all
rights and freedoms that would otherwise be restricted by copyright.
--
\ “Pinky, are you pondering what I'm pondering?” “Well, I think |
`\ so (hiccup), but Kevin Costner with an English accent?” —_Pinky |
_o__) and The Brain_ |
Ben Finney
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