If the resolver attribute of the buildnumber task point to a chained resolver the buildnumber task will not find any existing revision.
I will attach a unittest to show the problem.
I've debuged into this point:
The Buildnubmer task uses SearchEngine.listModules whcih in turn uses
Resolver.listTokenValues. The Doc says ListTokenValue must not return values for child resolvers.
So litTokenValues can not return values for child resolvers either. I don't know how to fix.