[Scons-dev] Chained DynamicSourceGenerators: Chicken-and-egg problem

[eKiwi]

Hey everyone,

I am working on a project that uses SCons as its build system.
My problem right now is that I am trying to have several 
DynamicSourceGenerators [1] chained together.

My Dependency Chain (from bottom up) looks something like this:
(I marked every action that I have implemented as a builder right now 
with [B(#)])

Config File that contains name of device this library is to be built for

=> [B(1)] with the device name one can search for a xml device file 
describing the properties of the device/the properties od the family the 
device belongs to (the search starts with the most specific device file 
possible and then searches for less specific ones)

=> [B(2)] the device file (output from B[1]) specifies the drivers that 
need to be built. Every driver has its own xml file. These are assigned 
to [B(3)]

=> [B(3)] takes the driver xml file as well as some data in a dictionary 
(handed to it from [B(2)] via the local environment and marked as a 
dependency by this builders emitter). From this it can derive a list of 
files template files that need to be processed (by a template Builder) 
and static files that just need to be copied to a 'generated' directory

The Files that are marked to be generated/copied by [B(3)] are cpp files 
that need to be compiled as well as hpp files that some files outside of 
the 'generated' directory include.

The problem I am having with this approach right now is a chicken and 
egg problem.

When scons starts without any saved data, it simply can not know how to 
build any file in the 'generated' directory, because the Builders 
mentioned above are never executed. This is because the Builders were 
never executed.

So for initial build the system above is useless (it took me a while to 
understand that).

But as far as rebuilds are concerned it all makes a lot of sense in my 
opinion: If the xml device file is changed, [B(1)], [B(2)], [B(3)] and 
all Template/Copy/Program Builders need to be run. If one driver xml 
file changes [B(2)] only needs to be run for the specific driver ans 
only the template/static files of that driver need to be rebuilt.
If one driver template file changes, only that certain file and any 
other file that might depend on it needs to be rebuilt.

So I was wondering: Does anyone here (who might know a lot more about 
the internal workings of SCons) have any suggestion on how to solve my 
Chicken-and-egg problem?

What I would basically need is something like a Builder that has a file 
as source, no specific target and outputs dependencies (that is what I 
am doing with a Builder right now, but SCons is not really "aware 
enough" of that).

I already thought about building a Scanner, but these work the other way 
around: They specify dependencies of their source file. In my case the 
output depends on my source.

So I am looking forward to any suggestion.

Best Regards
Kevin

[1] http://www.scons.org/wiki/DynamicSourceGenerator