Migrate Plone site from Zope2 2.13 to Zope2 4.0b3.dev0

[bsuttor]

python version : 2.7.14

Steps to reproduce :

• create Plone site on Zope2 2.13 (Plone 5.1) (with buildout.coredev)
• restart buildout with "-c plip-zope4.cfg" option (use Zope2 4.0b3)
• start instance

2017-10-21 11:34:22 ERROR ZODB.Connection Couldn't load state for OFS.Application.Application 0x01 
Traceback (most recent call last): 
  File "/home/bsuttor/.buildout/eggs/ZODB-5.3.0-py2.7.egg/ZODB/Connection.py", line 800, in setstate
    self._reader.setGhostState(obj, p) 
  File "/home/bsuttor/.buildout/eggs/ZODB-5.3.0-py2.7.egg/ZODB/serialize.py", line 622, in setGhostState
    state = self.getState(pickle)
  File "/home/bsuttor/.buildout/eggs/ZODB-5.3.0-py2.7.egg/ZODB/serialize.py", line 615, in getState
    return unpickler.load()         
TypeError: ('__init__() takes exactly 2 arguments (1 given)', <class 'ZPublisher.BeforeTraverse.NameCaller'>, ())  

[icemac]

Expect the same error you got for ZPublisher.BeforeTraverse.NameCaller for the following classes, too:

• ZPublisher.BeforeTraverse.MultiHook
• Shared.DC.Scripts.Signature.FuncCode
• Shared.DC.Scripts.Bindings.NameAssignments

Instances of these classes are stored in pickles in the ZODB. They have a constructor (__init__ method) which seems to be called during unpicking.

This behaviour is independent from the ZODB:

>>> class A:
...     def __init__(self, x):
...         self.x = x

>>> a = A(42)
>>> import pickle
>>> pickle.dumps(a)
"(i__main__\nA\np0\n(dp1\nS'x'\np2\nI42\nsb."

When trying to read this pickle in a different Python 2.7 interpreter you get:

>>> class A(object):
...     def __init__(self, x):
...         self.x = x
...
>>> import pickle
>>> pickle.loads("(i__main__\nA\np0\n(dp1\nS'x'\np2\nI42\nsb.")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/pickle.py", line 1388, in loads
    return Unpickler(file).load()
  File "/usr/lib/python2.7/pickle.py", line 864, in load
    dispatch[key](self)
  File "/usr/lib/python2.7/pickle.py", line 1076, in load_inst
    self._instantiate(klass, self.marker())
  File "/usr/lib/python2.7/pickle.py", line 1066, in _instantiate
    value = klass(*args)
TypeError: in constructor for A: __init__() takes exactly 2 arguments (1 given)

A solution could be to make the argument of __init__ optional:

>>> class A(object):
...     def __init__(self, x=None):
...         self.x = x
...
>>> import pickle
>>> a2 = pickle.loads("(i__main__\nA\np0\n(dp1\nS'x'\np2\nI42\nsb.")
>>> a2.x
42
>>> pickle.dumps(a2)
"ccopy_reg\n_reconstructor\np0\n(c__main__\nA\np1\nc__builtin__\nobject\np2\nNtp3\nRp4\n(dp5\nS'x'\np6\nI42\nsb."
>>> pickle.loads(pickle.dumps(a2)).x
42

The pickle changes by switching from old-style classes to new-style classes, but it can be read again. So this seems to be okay.

[icemac]

@bsuttor I sketched a possible solution in #208. Could you please try with the branch used there and report back if it helps?

[icemac]

Maybe there are other classes besides the ones mentioned in this ticket which have the same problem.

[bsuttor]

@icemac thank you,
I will try tomorrow or after tomorrow

[bsuttor]

I just try it, and it works,
Now Zope2 4.0b3 can start with a Plone site created on Zope2 2.13.

Thank you