PEP: 3143 Title: Standard daemon process library Version: $Revision$
Last-Modified: $Date$ Author: Ben Finney <ben+python@benfinney.id.au>
Status: Deferred Type: Standards Track Content-Type: text/x-rst Created:
26-Jan-2009 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:

-   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
    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

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.

bda.daemon

    The bda.daemon library http://pypi.python.org/pypi/bda.daemon/ by
    Robert Niederreiter et al.

burr-daemon

    The daemon.py module http://www.nightmare.com/~ryb/code/daemon.py by
    Ray Burr.

clapper-daemon

    The daemon library http://pypi.python.org/pypi/daemon/ by Brian
    Clapper.

cookbook-278731

    Python Cookbook recipe 278731, “Creating a daemon the Python way”
    http://code.activestate.com/recipes/278731/.

cookbook-66012

    Python Cookbook recipe 66012, “Fork a daemon process on Unix”
    http://code.activestate.com/recipes/66012/.

dagitses-initd

    The Python initd library http://pypi.python.org/pypi/initd/ by
    Michael Andreas Dagitses.

python-daemon

    The python-daemon library http://pypi.python.org/pypi/python-daemon/
    by Ben Finney et al.

seutter-daemonize

    The daemonize library http://daemonize.sourceforge.net/ by Jerry
    Seutter.

slack-daemon

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

stevens

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

twisted

    The Twisted application framework
    http://pypi.python.org/pypi/Twisted/ by Glyph Lefkowitz et al.

zdaemon

    The zdaemon tool http://pypi.python.org/pypi/zdaemon/ by Guido van
    Rossum et al.