PicklingError after migrating from Plone 4.2 to Plone 4.3

[hvelarde]

A site running under Plone 4.2 and using collective.syndication was recently migrated to Plone 4.3.

Before migrating it, collective.syndication was uninstalled as all is code base is only a backport of the Plone 4.3 syndication feature.

Everything looked fine after we tried to edit an content type instance with some related items, then the following error showed up:

2015-04-22T16:12:05 ERROR Zope.SiteErrorLog 1429729925.20.196064521565 http://localhost:8481/Plone/economia/outro-teste-2852.html/@@edit
Traceback (innermost last):
  Module ZPublisher.Publish, line 146, in publish
  Module Zope2.App.startup, line 301, in commit
  Module transaction._manager, line 89, in commit
  Module transaction._transaction, line 329, in commit
  Module transaction._transaction, line 443, in _commitResources
  Module ZODB.Connection, line 567, in commit
  Module ZODB.Connection, line 623, in _commit
  Module ZODB.Connection, line 658, in _store_objects
  Module ZODB.serialize, line 422, in serialize
  Module ZODB.serialize, line 431, in _dump
PicklingError: Can't pickle <class 'collective.syndication.interfaces.ISyndicatable'>: import of module collective.syndication.interfaces failed

My workaround was based on what was discussed in this issue: http://stackoverflow.com/q/29787760/644075

So I ended up listing the package in the eggs part but, because collective.syndication cannot be installed on a site with Plone 4.3, I had to modify the egg information, which is nasty.

This seems indeed related with this other issue: http://stackoverflow.com/q/20290361/644075

[hvelarde]

@vangheem what do you think? how can we avoid this on the future?

[vangheem]

recommend uninstalling before you upgrade to 4.2. uninstall should clean the database.

[hvelarde]

@vangheem that's what I did: I uninstalled before upgrading to 4.3

[vangheem]

Is there an uninstall step that actually cleans the database? I didn't write any of collective.syndication.

[vangheem]

Could we add an alias_module to plone.app.upgrade for collective.syndication?

https://github.com/plone/plone.app.upgrade/blob/master/plone/app/upgrade/__init__.py

There should be a recipe to configure these through buildout or something.

[hvelarde]

@vangheem you're speaking like in another language to me :-)

@tcurvelo do you want to help on this? BTW, was your pull request finally merged?

[vangheem]

Use this: https://pypi.python.org/pypi/wildcard.fixmissing/1.0

[vangheem]

https://github.com/collective/collective.easyslider/blob/master/collective/easyslider/importexport.py#L28 is an example of removing marker interfaces.

[hvelarde]

just to document the workaround for this issue, the following code must be added to a policy package or similar:

# BBB: avoid PicklingError in Plone 4.3
import sys
from Products.CMFPlone.interfaces import syndication
sys.modules['collective.syndication.interfaces'] = sys.modules[syndication.__name__]

@ebrehault @jensens @thet @frapell @tisto @bloodbare can we add this fix to Plone 4.3 as @vangheem suggested?