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Framework for writing daemons, with API similar to threading and multiprocessing.

Project description

pandaemonium
============

n. the abode of all the daemons [1]_

pandaemonium provides a framework for writing daemons in Python. The API is
based on the threading/multiprocessing model [2]_ [3]_, so the primary way
of creating your own daemon is to either subclass and override the ``run``
method, or provide a function as the ``target`` to the ``Daemon`` class.

Besides ``Daemon`` there is also a locking pid file -- ``PidLockFile``.
``PidLockFile`` can either be used manually, or, if a complete path and file
name are provided to ``Daemon``, used automatically.


simple usage
------------

from pandaemonium import Daemon

def DoSomethingInteresting():
"Just like it says ;)"
pass

daemon = Daemon(target=DoSomethingInteresting)
daemon.start()
#
# daemon.output will contain any stdout output generated during the
# daemonizing process, up to the stdin/stdout/stderr redirection
#
# daemon.error contains anything sent to the daemon's stderr -- which
# most likely means the daemon died due to an exception
#
# both can be parsed, examined, ignored, etc.


or:

from pandaemonium import Daemon

class MyDaemon(Daemon):
def run():
# do some interesting stuff

md = MyDaemon().start()

The sequence of events that takes place when `start()` is called (adapted from
The Linux Programming Interface by Michael Kerrisk) is:

- detach from the current process, creating a new session
- turn off core dumps
- set uid and gid
- set umask
- set working directory
- create pid file
- set signal handlers
- close inherited file handles
- redirect stdin/stdout/stderr

If any exceptions occur or if any feedback is generated during the `start`
process it will be available as the `error` and `output` attributes of the
daemon instance, where the parent process can analyze, print, etc before
quiting.

Note: Most guides on writing daemons specify setting the umask to 0, but
this creates a security hole as all files become world readable/writable by
default. Pandaemonium sets the umask to 077, but that can be changed if
desired.


advanced usage
--------------

If more control is needed than what is provided by the parameters of Daemon
then one has a couple options available:

- if certain set up / initialization steps need to happen somewhere in the
`start()` sequence, such as after setting the umask and before changing
the working directory::

Daemon.stage4()
# stages 1-4 have now been completed
# do custom steps here
Daemon.start()
# stages 5-9 have now been completed, and run() called

- one can also override any of the stages in a subclass (make sure and
decorate with `check_stage`:

class MyDaemon(Daemon):
def run(self, ip):
# do stuff
@check_stage
def stage7(self):
# do some custom stuff with signals set up

md = MyDaemon('192.168.11.1')
md.start()

- or, to simplify between foreground and daemon operation:

foreground = sys.argv[2:3] == ['--foreground']
pid_file = PidLockFile('/some/path/to/lock.pid')
pid_file.acquire()
if foreground:
pid_file.seal()
else:
daemon = Daemon()
daemon.pid_file = pid_file
daemon.activate()
# at this point, in either foreground or daemon mode, the pid file has
# been sealed (has our correct pid written to it, and it has been
# closed)
run_main_program()

If one's desire is to start the daemon and automatically have any output
printed to screen, one can use `daemon.report()` which prints whatever was
received from the daemon and then quits.


Daemon
------

``Daemon(target=None, args=None, kwargs=None, working_directory='/', umask=0,
prevent_core=True, process_ids=None, inherit_files=None,
signal_map=None, stdin=None, stdout=None, stderr=None)``

target
function to call when daemonized

args
positional args to provide to target

kwargs
keyword args to provide to target

detach
`None` (default) means figure it out, `True` means yes, `False` means no.
Figuring it out means if the parent process is `init`, or a `super
server`, do not detach

working_directory
directory to change to (relative to chroot, if one is in effect)

umask
mask to use when creating files

prevent_core
prevent core dump files from being created

process_ids
tuple of (uid, gid) to switch process to (use (None, None) to disable)

pid_file
`None` (default), or
a PidLockFile instance, or
the string of where to create a PidLockFile

inherit_files
list of open files or file descriptors to keep open

signal_map
dictionary of signal names or numbers to method names or functions

stdin / stdout / stderr
streams to map the standard streams to
default is `None` which is mapped to ``os.devnull``


``Daemon.run()``
''''''''''''''''
Method representing the daemon's activity.

You may override this method in a subclass. The standard ``run``
method invokes the callable object passed to the object's constructor as
the `target` argument, if any, with sequential and keyword arguments taken
from the `args` and `kwargs` arguments, respectively.

``Daemon.start()``
''''''''''''''''''
Start the daemon's activity.

This may be called at most once per daemon object. It arranges for the
object's ``run`` method to be invoked as a daemon process.

``Daemon.monitor()``
''''''''''''''''''''
Collects stdout and stderr from Daemon process until stage 9 and attaches
it to the daemon instance as ``output`` and ``error``. Can be overridden
if one wants to do more interesting stuff with the daemon's output

``Daemon.stage[1-9]()``
''''''''''''''''''''''''''
One can override the various stages for even more customizations options.
Make sure and decorate such functions with ``check_stage``.


PidLockFile
-----------

``PidLockFile(file_name, timeout)``

file_name
full path and name of file to use for locking

timeout
how long to wait before concluding that an existing held lock is not
going to be released (default: -1, meaning conclude immediately)

``PidLockFile.acquire(timeout=None)``
''''''''''''''''''''''''''''''''''''''
attempt to capture the lock file; if timeout is `None` use the time out
specified when PidLockFile was created.

``PidLockFile.seal()``
''''''''''''''''''''''
write the current process' PID into the acquired file and close it --
should only be called by the daemon process or the stored PID will not be
correct.

``PidLockFile.release()``
'''''''''''''''''''''''''
remove the lock file, releasing the lock.



[1] http://dictionary.reference.com/browse/pandemonium
[2] https://docs.python.org/2/library/threading.html#threading.Thread
[3] https://docs.python.org/2/library/multiprocessing.html#multiprocessing.Process


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