Success on a one year old problem installing CacheFu

I've upgraded CacheFu (Products.CacheSetup) to the latest 1.2 for some of the websites that I manage, in a Plone 2.5 cluster. One of them had a problem that I haven't been able to track previously, due to limited time: on a reinstall of CacheSetup, due to product upgrades, CacheFu couldn't be installed anymore. The traceback was something like:

this product has already been installed without Quickinstaller!failed:
Traceback (most recent call last):

  File "/home/zope/z29/Products/CMFQuickInstallerTool/", line 330, in installProduct

  File "/home/zope/p25/parts/zope2/lib/python/Products/ExternalMethod/", line 225, in __call__
    try: return f(*args, **kw)

  File "/home/zope/p25/eggs/Products.CacheSetup-1.2-py2.4.egg/Products/CacheSetup/Extensions/", line 35, in install
    policy_utils.addCachePolicies(self, out)

  File "/home/zope/p25/eggs/Products.CacheSetup-1.2-py2.4.egg/Products/CacheSetup/Extensions/", line 72, in addCachePolicies

  File "/home/zope/p25/eggs/Products.CacheSetup-1.2-py2.4.egg/Products/CacheSetup/Extensions/", line 13, in addCacheRules
    rules.invokeFactory(id=id, type_name='PolicyHTTPCacheManagerCacheRule')

  File "/home/zope/z29/Products/CMFCore/", line 408, in invokeFactory

  File "/home/zope/z29/Products/CMFCore/", line 934, in constructContent

  File "/home/zope/z29/Products/CMFCore/", line 343, in constructInstance

  File "/home/zope/z29/Products/CMFCore/", line 574, in _constructInstance

  File "", line 6, in addPolicyHTTPCacheManagerCacheRule

  File "/home/zope/p25/parts/zope2/lib/python/OFS/", line 301, in _setObject
    v = self._checkId(id)

  File "/home/zope/z29/Products/CMFCore/", line 223, in _checkId

  File "/home/zope/p25/parts/zope2/lib/python/OFS/", line 95, in checkValidId
    raise BadRequest, (

BadRequest: The id "httpcache" is invalid - it is already in use.

One other weird thing are the paths in this traceback: /home/zope/z29 doesn't exist anymore, the database was moved from a different server. I think it's related to the persistent product entries in the Control_Panels, which can be cleared. Not a big problem. In the log, there was also an entry related to this traceback:

2009-02-28 17:01:18 CRITICAL txn.-1223480432 A storage error occurred during the second phase of the two-phase commit.  Resources may be in an inconsistent state

Now, the solution is really simple, but I needed to debug the policy_2 module to find this: 

rules.invokeFactory(id=id, type_name='PolicyHTTPCacheManagerCacheRule')

didn't work because there was a document in the root called "rules". Nobody expects the spamish Acquisition! And I didn't either... Lesson? Zope 2 was designed to be too smart for its own good, thus violating the KISS principle. Still love it, though.