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From: Thomas Courbon on 30 Apr 2010 11:13
Hello there,

I currently have a little project which involve the writing of a
server that launch tasks when requested by the user (through a web
interface for instance).

I would like to turn my server script into a Linux/Unix daemon
(launched at boot time by init, dunno if that matter) using the nice
python-daemon package by Ben Finley et al (http://pypi.python.org/pypi/
python-daemon/). This package comes with a class DaemonRunner that
seems to fit almost exactly my need but I still have some
interrogation.

My main class looks like the following:
class SIGTERM_Received(Exception):
pass

class MyDaemon(object):
def __init__(self):
self.pidfile = '/path/to/the/file'
self.pidfile_timeout = 5
self.stdin_path = None
self.stdout_path = None
self.stderr_path = None

def run(self):
self.init()
self.main_loop()
self.terminate()

def init(self):
#doing some initialisation here eg: database connection, logging,
connection establishing and such...
signal.signal (signal.SIGTERM, lambda: raise SIGTERM_Received()) #
used to interrupt the connection listening loop

def main_loop(self):
#This is the "server" loop
try:
while True:
rcv = self.some_connection.receive() # this may hang until
something was written into the connection by a client.
# do something with rcv that involving processes spawning,
database interaction and so on...
except SIGTERM_Received:
return

def terminate(self):
#some clean up like connection closing and child processes
joining.
pass

And this is the script I intend to drop into /etc/rc.d (for an
Archlinux-style initd):
#! /usr/bin/python
from daemon.runner import DaemonRunner
from foo import MyDaemon
daemon_runner = DaemonRunner(MyDaemon())
daemon_runner.parse_args()
daemon_runner.do_action()

The DaemonRunner class expects my class to have stdin_path,
stdout_path, stderr_path attributes and after reading the code it
seems they have to be valid paths. Is that ok for a Daemon to redirect
those stream to /dev/null for example ? I would prefer to alter the
DaemonRunner class to accept None as value since DaemonContext, the
underlying class, seems to accept None for those parameters.

Also, the DaemonRunner use os.kill(pid, signal.SIGTERM) to stop the
daemon. I wonder if with my signal handling I'll be able to terminate
correctly the daemon (joining children, flushing buffers, closing
connections...). If that's relevant, the connection I use is a
Listener/Client connection from the standard multiprocessing module.

I'm quite neophyte in Unix daemon programming so please forgive me if
my question are obvious.

Thank for reading,
Thomas
From: Ben Finney on 1 May 2010 06:55
Thomas Courbon <thcourbon(a)gmail.com> writes:

> I would like to turn my server script into a Linux/Unix daemon
> (launched at boot time by init, dunno if that matter) using the nice
> python-daemon package by Ben Finley et al

I resemble that name :-)

> This package comes with a class DaemonRunner that seems to fit almost
> exactly my need but I still have some interrogation.

Yes, that class is unlikely to grow much beyond a simple example of how
to use 'DaemonContext'. You should feel free to take 'DaemonRunner'
under the granted license and adapt it to be closer to what you want.

> The DaemonRunner class expects my class to have stdin_path,
> stdout_path, stderr_path attributes and after reading the code it
> seems they have to be valid paths. Is that ok for a Daemon to redirect
> those stream to /dev/null for example ? I would prefer to alter the
> DaemonRunner class to accept None as value since DaemonContext, the
> underlying class, seems to accept None for those parameters.

This is a godd idea, allowing a more minimal use of 'DaemonRunner'. I'll
take this as a feature request for a future revision of the class.

> Also, the DaemonRunner use os.kill(pid, signal.SIGTERM) to stop the
> daemon. I wonder if with my signal handling I'll be able to terminate
> correctly the daemon (joining children, flushing buffers, closing
> connections...). If that's relevant, the connection I use is a
> Listener/Client connection from the standard multiprocessing module.

I'll be interested to know too :-)

> I'm quite neophyte in Unix daemon programming so please forgive me if
> my question are obvious.

Thank you for your feedback, and I hope the 'python-daemon' library
continues to meet your needs.

--
\ “In the long run, the utility of all non-Free software |
`\ approaches zero. All non-Free software is a dead end.” —Mark |
_o__) Pilgrim, 2006 |
Ben Finney
 | 
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