Running code when closing a Python daemon

Question

I'm using the famous code referenced here or here to do a daemon in Python, like this:

import sys, daemon

class test(daemon.Daemon):
    def run(self):
        self.db = somedb.connect()            # connect to a DB
        self.blah = 127
        with open('blah0.txt', 'w') as f:
            f.write(self.blah)
        # doing lots of things here, modifying self.blah

    def before_stop(self):
        self.db.close()                      # properly close the DB (sync to disk, etc.)
        with open('blah1.txt', 'w') as f:
            f.write(self.blah)

daemon = test(pidfile='_.pid')

if 'start' == sys.argv[1]: 
    daemon.start()
elif 'stop' == sys.argv[1]: 
    daemon.before_stop()               # AttributeError: test instance has no attribute 'blah'
    daemon.stop()

The problem is that when calling ./myscript.py stop and thus daemon.before_stop(), there is no reference anymore to self.blah!

AttributeError: test instance has no attribute 'blah'

Thus with this daemonization method, it's impossible to have access to the daemon's variables before stopping the daemon...

Question: how to have access to the daemon class' variables just before:

• stopping with ./myscript.py stop

• being stopped with SIGTERM

• (being killed?)

---

EDIT: solved, and here is a working daemon code with a quit() method.

Answer 1

The Daemon code sends a SIGTERM signal to the daemon process to ask it to stop. If you want that something is run by the daemon process itself, it must be run from a signal handler or from an atexit.register called method.

The daemonize method already installs such a method, just call beforestop from there:

# this one could be either in a subclass or in a modified base daemeon class
def delpid(self):
        if hasattr(self, 'before_stop'):
                self.before_stop()
        os.remove(self.pidfile)

# this one should be in subclass
def before_stop(self):
    self.db.close()                      # properly close the DB (sync to disk, etc.)
    with open('blah1.txt', 'w') as f:
        f.write(self.blah)

But that is not enough! The Python Standard Library documentation says about atexit:

The functions registered via this module are not called when the program is killed by a signal not handled by Python

As the process is expected to receive a SIGTERM signal, you have to install a handler. As shown in one active state recipe, it is damned simple: just ask the program to stop if it receives the signal:

...
from signal import signal, SIGTERM
...

atexit.register(self.delpid)
signal(SIGTERM, lambda signum, stack_frame: exit(1))

Answer 2

The Daemon class does not retrieve the former object when you call before_stop(), it just find the process by PID and kill with a SIGTERM. If you need some instance attribute, either initialize them in __init__() or write them down in some temporary file to read them later in your before_stop() implementation.

As a general rule you should always initialize all instance bound attributes inside __init__() anyway.