[cmake-developers] Tools for handling cross project dependencies

[Raffi Enficiaud]

Hi CMake,

I am currently working on a toy cmake prototype for Boost. I am not sure 
it will ever be released but I believe it might be useful beyond this 
project.

The idea is to include several "packages" (one package ~ one project in 
Boost) and make those available to the build, exactly as for a regular 
CMakeLists.txt that adds several directories or subprojects through a 
sequence of calls to "add_directories".

However, the difference here is that "packages" have inter-dependencies, 
and the order of the "add_directories" should honor those dependencies.

I developed a rather small CMake script that
* encodes those dependencies in a very simple manner, just by stating 
parent projects in the DAG of dependencies in an appropriate cmake variable,
* runs the ordered inclusion of those dependencies inside the master 
project,

It works like this:
1/ a small "dependencies.cmake" declares the provided components and 
dependencies:
    * a variable that indicates the provided components (say "core", 
"doc" and "test"). Each component defines a CMakeLists.txt
    * a variable that indicates the dependencies to another module (with 
or without components)

2/ a function parses a build tree (not necessarily flat) and detect all 
those dependencies.cmake, loads the containing variables and extract the 
module and components

3/ another function reads the variables set by step 2/ and adds the 
components in the right order

Additional filtering can happen after 2 (already implemented in my 
project) to remove all the packages/components that are not needed for 
the current build, but 3/ will bring any necessary dependency to the build.

A possible example, that can of course be adapted, is given below.

My inquiry there is the following:
* would this be of any interest for the community? I know that some 
projects are struggling with this or provide solutions that, as a 
developer, dislike
* is there already any such mechanism in cmake? I failed to see any, and 
the problem statement is a different than the cmake-package facility 
provided by cmake.

I believe it would also be possible to enhance step 3/ by eg. having a 
logic that checks out the package sources instead of doing an 
"add_directory", and by the possible intermediate filter after step 2/, 
then checking out only what is needed for the end-user/developer.

Thanks!
Raffi



------------------ Example for Boost.test ----

set(_current_package "TEST")
# this declares the components
set(BOOST_LIB_${_current_package}_COMPONENTS "build" "doc" "test")

# this declares the dependencies for the component "build"
set(BOOST_LIB_${_current_package}_COMPONENTS_BUILD_DEPENDENCY
   "system:build"
   "core:build"
   "config:build"
   "predef:build"
)

# same for the component "DOC"
set(BOOST_LIB_${_current_package}_COMPONENTS_DOC_DEPENDENCY )
# same for the component "TEST" (or the module TEST)
set(BOOST_LIB_${_current_package}_COMPONENTS_TEST_DEPENDENCY "test:build")