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Re: [libvirt] AttributeError in virConnect.__del__ at program exit



On 06/13/2013 03:10 AM, Sandro Bonazzola wrote:
> Il 13/06/2013 08:54, Sandro Bonazzola ha scritto:
>> Il 12/06/2013 19:31, Cole Robinson ha scritto:
>>> On 06/12/2013 04:10 AM, Sandro Bonazzola wrote:
>>>> Il 11/06/2013 18:21, Cole Robinson ha scritto:
>>>>> On 06/11/2013 07:58 AM, Sandro Bonazzola wrote:
>>>>>> Hi,
>>>>>> using vdsm python code, I've the following error at program exit that
>>>>>> seems to be related to libvirt python code, something wrong in a destructor:
>>>>>>
>>>>>> Exception AttributeError: AttributeError("virConnect instance has  no
>>>>>> attribute 'domainEventCallbacks'",) in <bound method  virConnect.__del__
>>>>>> of <libvirt.virConnect instance at  0x4280f38>> ignored
>>>>>>
>>>>>> I'm using libvirt 1.0.6
>>>>>>
>>>>>> Is it a known issue? Is there any workaround / fix ?
>>>>>>
>>>>> I've seen this too, sometimes via the virtinst test suite. Once upon a time I
>>>>> tracked it down to whether the virtinst code did 'import selinux' or not, so
>>>>> maybe it's a just some weird race, or a side effect of something other
>>>>> libraries do in their cleanup path. By inspection alone the __del__ handler
>>>>> doesn't seem to be doing anything wrong.
>>>>>
>>>>> - Cole
>>>> It seems related only to domainEventCallbacks so maybe it appears only
>>>> after a domain creation / modification.
>>>>
>>> Oh, sorry, now that I look at the code it makes a bit more sense. The error I
>>> was recalling was a __del__ Exception but wasn't involving domain events.
>>>
>>> But I still don't see why __del__ is complaining, since the whole thing is
>>> wrapped in try: ... except AttributeError. So not sure what the issue is.
>>>
>>> - Cole
>>
>> According to the python documentation, you can't handle exception in
>> __del__:
>> http://docs.python.org/2/reference/datamodel.html#object.__del__
>>
>> Warning
>>
>> Due to the precarious circumstances under which __del__()
>> <http://docs.python.org/2/reference/datamodel.html#object.__del__>
>> methods are invoked, exceptions that occur during their execution are
>> ignored, and a warning is printed to sys.stderr instead.
>>
>> So you have to test the existence of the attribute before using it and
>> not try to use it and try to handle an exception that can't be handled.
> 
> You can use hasattr for checking if the object has the needed attribute.
> 

$ cat test.py
class foo(object):
    def __del__(self):
        print "in del"
        try:
            print self.foo
        except AttributeError:
            print "caught"

f = foo()
del f
[crobinso colepc ~]$ python test.py
in del
caught

So I don't think that interpretation is entirely correct. I think it's just
talking about uncaught exceptions.

Either way, using hasattr should be fine, I'd make the change to libvirt.py so
that your issue doesn't reproduce anymore, then transfer that change to
libvirt.git/python/libvirt-override-virConnect.py and submit it.

- Cole

> 
>>
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> 
> -- 
> Sandro Bonazzola
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