profile sharing and directory service

See http://www.mozilla.org/projects/embedding/shared_profiles.html for 
background on the problem.

One issue here is that some data my be desirable to share within a given 
profile and some may not. As an example of data that is not, much of 
what's stored in localstore.rdf (window state) is not. The necko cache 
is another, but for a different reason - it's difficult to implement 
sharing of the cache in the first pass at this ;-) Until this problem is 
overcome, the Cache dir would not be shared.

Consumers of profile data get their file locations through directory 
service. The provider for the profile-relative data is the profile mgr. 
There needs to be a way for a consumer of a file location to get a file 
location specific to its private partition of a profile. One possibility 
is this: If the directory service key was prefixed by a proc ID string 
(constant always for that app), say "%Proc:ProgramX%", directory service 
could separate the given string into the proc ID and the location key. 
Given these two pieces of data, directory service could QI to (yet 
another, sorry) nsIDirectoryServiceProvider iface. That provider would 
return the location for that given app's file or directory.

Yes, it's unfortunate to have another type of 
nsIDirectoryServiceProvider but, on the other hand, it would allow 
partitioning of profile date into shared and private data with minimal 
change to directory service consumers.

Feedback?

-Conrad