[ZODB-Dev] TypeError: ('object.__new__(SyncedLanguages) is not safe, use Persistence.Persistent.__new__()', ...

[Godefroid Chapelle]

Le 31/12/10 04:08, Marius Gedminas a écrit :
> On Fri, Dec 31, 2010 at 02:02:36AM +0100, Godefroid Chapelle wrote:
>> I have been fighting all day to understand a bug with the unregistration
>> of a utility from a local site.
>>
>> After unregistration, some instance is left in the _subscribers list of
>> the utilities AdapterRegistry in my local site.
>
> Sounds like https://bugs.launchpad.net/zope.component/+bug/551049
>
>> This avoids me to remove the code as it does leave a broken object.
>>
>> I have come to the conclusion that the code that first registered the
>> utility is the culprit : the utility itself is not persistent.
>>
>> It seems that the utility instance is unserialized at least two times
>> during unregistration, which leads (if I understand well) to two
>> different objects in memory.
>>
>> This breaks the unsubscription algorithm that is based on object identity.
>
> Actually, half of it is based on identity, and half on equality; when
> these disagree, you end up with inconsistent state.

Yup

> Your example is a good argument for making both halves use equality
> rather than identity.
>
>> I naively tried to introduce the Persistent base class into the utility
>> calss. When I try to access the utility instance again, I get the error
>> mentioned in the subject of this mail :
>>
>> TypeError: ('object.__new__(SyncedLanguages) is not safe, use
>> Persistence.Persistent.__new__()',<function _reconstructor at
>> 0x1004297d0>, (<class
>> 'Products.LinguaPlone.vocabulary.SyncedLanguages'>,<type 'object'>, None))
>>
>> If I understand well, the a posteriori introduction of Persistent cannot
>> happen alone.
>>
>> Can anyone tell me what I should do add to the class I try to make it
>> actually persistent ?
>
> I think a more fruitful approach would be to fix the zope.component bug
> and then unregister the utility in the normal way.

Marius, did you already try to fix zope.component ? Is it because it is 
hard that you accepted the Won't fix ? Or because of lack of time ?

> Alternatively, you could hack up a __new__ method to always return the
> same instance, so you can unregister it cleanly before removing the
> code.  This will not work if you have more than one instance in the DB.

Could that __new__ method insert Persistent as a base class if not present ?

> Fixing up _subscribers directly is also a possible workaround, if you're
> feeling brave.
>
> Marius Gedminas

Thanks for your help.

-- 
Godefroid Chapelle (aka __gotcha) http://bubblenet.be