[Python-ideas] Draft PEP: Standard daemon process library

Ben Finney ben+python at benfinney.id.au
Thu Mar 12 04:57:11 CET 2009

Howdy all,

Significant changes in this release:

* Name the daemon process context class `DaemonContext`, since it
  doesn't actually represent a separate daemon. (The reference
  implementation will also have a `DaemonRunner` class, but that's
  outside the scope of this PEP.)

* Implement the context manager protocol, allowing use as a ‘with’
  context manager or via explicit ‘open’ and ‘close’ calls.

* Delegate PID file handling to a `pidfile` object handed to the
  `DaemonContext` instance, and used simply as a context manager.

* Simplify the set of options by using a mapping for signal handlers.

* Target Python 3.2, since the reference implementation will very
  likely not be complete in time for anything earlier.

:PEP:               XXX
:Title:             Standard daemon process library
:Version:           0.5
:Last-Modified:     2009-03-12 14:50
: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:    3.2


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.

..  contents::
    Table of Contents: 
      Example usage
      ``DaemonContext`` objects
      ``DaemonError`` objects
      Correct daemon behaviour
      A daemon is not a service
    Reference Implementation
      Other daemon implementations


Example usage

Simple example of direct `DaemonContext` usage::

    import daemon

    from spam import do_main_program

    with daemon.DaemonContext() as daemon_context:

More complex example usage::

    import os
    import grp
    import signal
    import daemon
    import lockfile

    from spam import (

    context = daemon.DaemonContext(

    context.signal_map = {
        signal.SIGTERM: program_cleanup,
        signal.SIGHUP: 'close',
        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]


    with context:


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

An exception class, `DaemonError`, is defined for exceptions raised
from the package.

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)

is equivalent to::

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

The following options are defined.

    :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.

    :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.

    :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.

    :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.

    :Default: ``None``

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

    :Default: ``{signal.SIGTOU: None, signal.SIGTTIN: None,
        signal.SIGTSTP: None, signal.SIGTERM: 'close'}``

    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

    :Default: ``None``

    The user ID (“uid”) value to switch the process to on daemon start.

    :Default: ``None``

    The group ID (“gid”) value to switch the process to on daemon start.

    :Default: ``True``

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

    :Default: ``None``

    :Default: ``None``

    :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.

    :Return: ``None``

    Open the daemon context, turning the current program into a daemon
    process. 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.

    * 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.

    * 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 `signal_map` attribute.

    * 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 `pidfile` attribute is not ``None``, enter its context

    * If either of the attributes `uid` or `gid` are not ``None``, set
      the process uid and/or gid to the specified values.

    * 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).

    When the function returns, the running program is a daemon

    :Return: ``None``

    Terminate the daemon context. This performs the following step:

    * If the `pidfile` attribute is not ``None``, exit its context

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

    :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

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

``DaemonError`` objects

The `DaemonError` class inherits from `Exception`. The `daemon`
package implementation will raise an instance of `DaemonError` when an
error occurs in processing daemon behaviour.


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.


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

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]_.

As of `python-daemon` version 1.3 (2009-03-12), the package is under
active development and is not yet a full implementation of this PEP.

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:

* Many good ideas were contributed by the community to Python cookbook
  recipes #66012 [cookbook-66012]_ and #278731 [cookbook-278731]_.

* The `bda.daemon` library [bda.daemon]_ is an implementation of
  [cookbook-66012]_. It is the predecessor of [python-daemon]_.

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

* 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.


..  [stevens]

    `Unix Network Programming`, W. Richard Stevens, 1994 Prentice

..  [slack-daemon]

    The (non-Python) “libslack” implementation of a `daemon` tool
    `<http://www.libslack.org/daemon/>`_ by “raf” <raf at raf.org>.

..  [python-daemon]

    The `python-daemon` library
    `<http://pypi.python.org/pypi/python-daemon/>`_ by Ben Finney et

..  [cookbook-66012]

    Python Cookbook recipe 66012, “Fork a daemon process on Unix”

..  [cookbook-278731]

    Python Cookbook recipe 278731, “Creating a daemon the Python way”

..  [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]

    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

..  [dagitses-initd]

    The Python `initd` library `<http://pypi.python.org/pypi/initd/>`_
    by Michael Andreas Dagitses.


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.

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