Re: Migrating to BTrees 4.x

Hey David.



I don't remember the details, but you might have some luck with the
copy_reg module [0].


Usually that only allows you to overwrite the per-type pickle  function
for extension types and not the unpickle one. But IIRC the actual bytes
of a extension type pickle contain something like `copy_reg.__newobj__`
as the actual function to invoke, with the type as an argument. So you
might be able to monkey patch the `__newobj__` function in copy_reg.


Of course this is quite a horrible hack ;-)



Hanno



[0] https://docs.python.org/2/library/copy_reg.html



On Sat, Nov 19, 2016, at 21:21, David Glick wrote:

> I'm working on updating Plone to support ZODB 4 and 5.

> 

> So far most of the blockers are because BTrees 4.x no longer allows
> using keys whose ordering is not well-defined, including None, and
> there are various places where Plone was using None as a key. I
> understand the reason for the restriction and am making progress on
> making Plone no longer do this.
> 

> However I'm looking for suggestions on how to handle migrating
> existing BTrees to remove the None key. Plone users are used to first
> updating the software and then running upgrade steps to make any
> necessary changes to the database. However, because of the new
> restriction, existing BTrees with None as a key cannot be unpickled:
> 

>   File "/Users/davisagli/.buildout/eggs/ZODB-4.4.3-
>   py2.7.egg/ZODB/Connection.py", line 899, in setstate
>     self._setstate(obj, oid)

>   File "/Users/davisagli/.buildout/eggs/ZODB-4.4.3-
>   py2.7.egg/ZODB/Connection.py", line 956, in _setstate
>     self._reader.setGhostState(obj, p)

>   File "/Users/davisagli/.buildout/eggs/ZODB-4.4.3-
>   py2.7.egg/ZODB/serialize.py", line 623, in setGhostState
>     obj.__setstate__(state)

> TypeError: Object has default comparison

> 

> Any ideas about how I can write a migration script to fix these BTree
> instances *after* the BTrees package has been updated? I guess I'm
> looking for a hook to adjust the state for a particular class before
> __setstate__ is called, since BTrees are extension types and I can't
> override __setstate__.
> 

> thanks,

> David

> 



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