[Zope-CMF] traversal and __before_publishing_traverse__ conflict

Wichert Akkerman wichert at wiggy.net
Sun Feb 21 15:21:01 EST 2010

Apologies for the cross post, but since there are many components 
involved here I'm not sure what the right list is.

Plone uses plone.theme to set an IBrowserSkinType on the request that 
matches the currently selected CMF skin. This is very useful since it
allows you to register browser views and other components for a CMF 
skin. plone.theme does this by using the __before_publishing_traverse__ 
hook on the site root.

For one site I want to offer a different skin to people coming in via a 
separate URL. I do this by proxying to the site and adding a ++skin++ 
traverser to set the skin layer.

However this does not work due to how Zope 2 publication works. What 
happens is this:

1. ZPublisher traverses over the site root and calls the
    CMF __before_publishing_traverse__ hooks. Via a IBeforeTraverseEvent
    event plone.theme gets called and adds a IBrowserSkinType layer
    for the current CMF skin.
2. ZPublisher handles ++skin++name, which sets an IBrowserSkinType layer
    for my new skin, replacing the layer set by plone.theme. The
    traverser returns the same object, our site root, again.
3. ZPublishes procees by calling the __before_publishing_traverse__ for
    the site root again, which ends up calling plone.theme again and
    replacing my skin layer again.

The problem here is that a ++skin++ traverser always returns the same 
object again, and ZPublisher will happily treat it as a new object and 
call all hooks for it again.

The only way to avoid this that I can see is to make ZPublisher check if 
a traversal step returns the same object again, and if so skip
the __before_publishing_traverse__ hook in that case. This feels like a 
safe change, but I am not sure if this would break anything.


Wichert Akkerman <wichert at wiggy.net>   It is simple to make things.
http://www.wiggy.net/                  It is hard to make things simple.

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