Discussing need for an ordered directoryservice or categorymanager

[Benjamin Smedberg]

I am posting this to npm.xpcom, so feel free to reply there or by email 
directly... this isn't quite ready for a bug report, because I don't 
know what the API might look like.

I have recently encountered the need for something like a 
directoryservice or categorymanager that can return ordered lists of 
files/directories, but which can be dynamically updated at runtime. Here 
are a couple use-cases:

Both the prefs engine and the chrome registry should register default 
prefs/chrome lists from the GRE and the application... in addition, 
various extensions need to install chrome and default prefs.

My original thought was to use a category "default-prefs" with entries 
pointing to file paths. Unfortunately, default prefs need to be read in 
an ordered way: GRE -> application -> extensions  and the category 
manager can't do ordered lists.

My next thought was to use the directoryservice. This provides an 
ordered list (through directoryservice2.getFiles) but it does not allow 
the various pieces (GRE/application/extensions) to *add* paths to the 
list... only *replace* the list through a new directoryserviceprovider 
entry. In addition, it is not persistent... extensions would need to 
register a directoryserviceprovider.

So I am pondering something like a "orderedcategorymanager", but I'm 
having trouble designing it... do you have suggestions for an interface 
that does what I have outlined?

--BDS