[CMake] FetchContent on missing submodule

[nim65s]

Hi,

This proposition allows users of CMake >= 3.14 to dynamically fetch jrl-cmakemodules when the submodule is not already available.

This also allows eigenpy to be used in other projects with FetchContent, because FetchContent is not able to handle submodules at all.

eg:

cmake_minimum_required(VERSION 3.14)

project(test-eigenpy)

include(FetchContent)
FetchContent_Declare("eigenpy"
  GIT_REPOSITORY "https://github.com/nim65s/eigenpy"
  GIT_TAG "devel")
FetchContent_MakeAvailable("eigenpy")
FetchContent_GetProperties("eigenpy" SOURCE_DIR EIGENPY_SOURCE_DIR)

add_library(test_eigenpy SHARED test_eigenpy.cpp)
add_dependencies(test_eigenpy eigenpy)
target_link_libraries(test_eigenpy PRIVATE eigenpy)
target_include_directories(test_eigenpy PRIVATE "${EIGENPY_SOURCE_DIR}/include")

Additional CMake code can also be used to FetchContent only if eigenpy is not already found, with something like:

find_package(eigenpy QUIET)
if(eigenpy_FOUND)
  target_link_libraries(test_eigenpy PRIVATE eigenpy::eigenpy)
else()
  FetchContent_Declare(eigenpy …)
  FetchContent_MakeAvailable(eigenpy)
  target_link_libraries(test_eigenpy PRIVATE eigenpy)
  target_include_directories(test_eigenpy PRIVATE "${EIGENPY_SOURCE_DIR}/include")
endif()

[jcarpent]

Could you add a unit test in the CI for this feature?

[nim65s]

@jcarpent : done

[jcarpent]

Perfect! Thanks a lot.

[jmirabel]

Ideally, you should pull the correct commit too. I don't think it is too hard to get the hash. I am not sure if it is wise to do so.

[nim65s]

GIT_TAG can easily be added, with a branch, a tag, or a hash. But I'm not sure about whether this is best to add one and having to manage it, or if we can just be happy with whatever last version we'll get this way.

[jmirabel]

I had in mind getting the expected hash from some git command when git is available and the command succeeds and fall back to master when everything you tried failed.

No, don't add the hash explicitly in the file.