xenstore watches not firing for external-to-xend code using xen.xend.xenstore classes

[Charles Duffy <cduffy@spamcop.net>]

I'm trying to write a simple xenstore-based watchdog, as discussed 
previously on this list (quite some time ago -- it got pushed to the 
back of my priority list for a bit).

In any event, I'm trying to use the below code to test the ability of a 
program external to xend to place watches on xenstore. However, the 
callbacks aren't firing except on initial startup, either when I use 
xenstore-write to update /tool/watchdog/test or when I start or shutdown 
domains (to invoke the @introduceDomain or @releaseDomain triggers). 
Obviously I'm missing something here. If this is a simple error on my 
part, or if I'm expecting behaviour that wasn't designed for, I'd 
appreciate a pointer or two.

Thanks!

---------

#!/usr/bin/env python

from xen.xend.xenstore.xstransact import xstransact
from xen.xend.xenstore.xswatch import xswatch

from pprint import pprint
import time, logging
logging.basicConfig(level=logging.DEBUG)

TROOT = '/tool/watchdog'

class XenWatchdog:
   def __init__(self):
     xswatch('@releaseDomain', self.callback, 'releaseDomain')
     xswatch('@introduceDomain', self.callback, 'introduceDomain')
     xswatch('%(TROOT)s/test' % locals(), self.callback, 'test')
   def callback(self, *args, **kwargs):
     pprint([ 'onReleaseDomain', args, kwargs ])