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

Project description


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 ;)"

daemon = Daemon(target=DoSomethingInteresting)
# 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.


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

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

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::

# stages 1-4 have now been completed
# do custom steps here
# 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
def stage7(self):
# do some custom stuff with signals set up

md = MyDaemon('')

- or, to simplify between foreground and daemon operation:

foreground = sys.argv[2:3] == ['--foreground']
pid_file = PidLockFile('/some/path/to/')
if foreground:
daemon = Daemon()
daemon.pid_file = pid_file
# 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)

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


``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)``

function to call when daemonized

positional args to provide to target

keyword args to provide to target

`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

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

mask to use when creating files

prevent core dump files from being created

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

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

list of open files or file descriptors to keep open

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``

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.

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.

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

One can override the various stages for even more customizations options.
Make sure and decorate such functions with ``check_stage``.


``PidLockFile(file_name, timeout)``

full path and name of file to use for locking

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

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

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

remove the lock file, releasing the lock.


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