[Zope3-dev] Heads Up: Problem with queryNextSiteManager

[Stephan Richter]

Hello everyone,

the following helper functions in zope.app.component do not work well with the 
new registry concept of bases: getNextSiteManager, queryNextSiteManager, 
getNextUtility, queryNextUtility.

Those methods expect a simple lookup tree, where the first base of a local 
registry is the registry its parent's site manager's registry or the global 
registry; in other words, always the first base is selected (line 45). Since 
I started using the baseregistry, I commonly add the base registries at the 
first position, since I want them to override the global and other parent 
registries. Doing so breaks all components that use the above helper 
functions. Here is the solution I am going to check in later today:

- get/queryNextSiteManager

These functions will be deprecated, since they simply make no sense with the 
current design of the registries. I have checked the core code and they are 
not used anywhere except for queryNextUtility. I suspect the same to be true 
for all 3rd-party code as well, so that deprecating them should not really 
hurt anyone.

- get/queryNextUtility

They are still very useful and their extensive use in the Zope 3 core code is 
proof of that. Luckily their fix is simple. Instead of using 
get/queryNextSiteManager to look up the next registry, we simply iterate 
through all the bases of the closest site manager until an answer is found. 
This way the lookup pattern will work as before and supports the new usage 
pattern. I have tested this fix and it works for my code well.

The really big question is:  Is that change worth porting to the Zope 3.3 
branch? It really is a serious bug that will cause people painful error 
messages that are hard to understand. Also, it prevents people from using 
Zope 3.3 and z3c.baseregistry.

Thanks for reading and any comments you might have!

Regards,
Stephan
-- 
Stephan Richter
CBU Physics & Chemistry (B.S.) / Tufts Physics (Ph.D. student)
Web2k - Web Software Design, Development and Training