Export rmw_fastrtps_cpp target

[sloretz]

This exports the target rmw_fastrtps_cpp::rmw_fastrtps_cpp and appends it to the CMake variable ${rmw_fastrtps_cpp_LIBRARIES}. Exporting the target gives downstream users transitive dependencies automatically. Appending it to ..._LIBRARIES causes the target to be linked against downstream targets who use ament_target_dependencies(some_downstream_target "rmw_fastrtps_cpp").

This still requires CONFIG_EXTRAS_POST. I have not come up with a way to eliminate it. I looked at extending ament_export_libraries(...) to automatically append exported targets to $..._LIBRARIES, or get ament_target_dependencies(...) to automatically link against exported targets, but didn't come up with anything I feel is reasonable. I confirmed making ament_export_libraries(...) recursively export things in INTERFACE_LINK_LIBRARIES works for everything in ros2.repos, but I doubt my implementation would work for the general case because I explicitly ignored things beginning with a - (ex: -lpthread ) or a $ (targets using generator expressions).

connects to #149

[sloretz]

In progress while CI runs

• Linux
• Linux-aarch64
• macOS
• Windows

[sloretz]

This made the installed package non-relocatable because the exported targets file contains absolute paths. I don't see an easy fix. I'm just documenting what I found in case I'm curious about this again in the future.

Here's part of the installed exported target with this PR.

set_target_properties(rmw_fastrtps_cpp::rmw_fastrtps_cpp PROPERTIES
  INTERFACE_INCLUDE_DIRECTORIES "/workspace/ros2_ws/install/rcutils/include;/workspace/ros2_ws/install/rosidl_typesupport_interface/include;/workspace/ros2_ws/install/rosidl_generator_c/include;/workspace/ros2_ws/install/rosidl_typesupport_introspection_c/include;/workspace/ros2_ws/install/rosidl_generator_cpp/include;/workspace/ros2_ws/install/rosidl_typesupport_introspection_cpp/include;/workspace/ros2_ws/install/rmw/include"
  INTERFACE_LINK_LIBRARIES "fastcdr;fastrtps;/workspace/ros2_ws/install/rosidl_typesupport_introspection_c/lib/librosidl_typesupport_introspection_c.so;/workspace/ros2_ws/install/rosidl_typesupport_introspection_cpp/lib/librosidl_typesupport_introspection_cpp.so;/workspace/ros2_ws/install/rcutils/lib/librcutils.so;/workspace/ros2_ws/install/rmw/lib/librmw.so;/workspace/ros2_ws/install/rosidl_generator_c/lib/librosidl_generator_c.so"
)

Exported targets can be relocatable if all paths given to target_xxx() functions are relative paths.

However, the absolute paths in this case are files in dependencies. Relative paths from dependencies hard-coded in the exported targets file might work for the ros2 packaging job because all the packages are shipped together, but it doesn't work if the user has any dependency installed with a different install prefix.

The right solution is to have any paths to the dependency's files come from <dependency>Config.cmake. The modern cmake way to do this is to have all dependencies export targets, and then target_link_libraries() to them. If ament_cmake used exported targets then the above snippet would look like this.

set_target_properties(rmw_fastrtps_cpp::rmw_fastrtps_cpp PROPERTIES
  INTERFACE_INCLUDE_DIRECTORIES ""
  INTERFACE_LINK_LIBRARIES "fastcdr;fastrtps;rosidl_typesupport_introspection_c::rosidl_typesupport_introspection_c;rosidl_typesupport_introspection_cpp::rosidl_typesupport_introspection_cpp;rcutils::rcutils;rmw::rmw;rosidl_generator_c::rosidl_generator_c"
)

However ament_cmake makes relocatable packages using cmake variables that contain absolute paths. If a target uses variables from a dependency using ament_cmake then the target cannot be relocatable because any INTERFACE_xxx properties of it would contain those absolute paths. In this PR the absolute paths come from this call to ament_target_dependencies()

rmw_fastrtps/rmw_fastrtps_cpp/CMakeLists.txt

Lines 85 to 91 in a9c33de

	ament_target_dependencies(rmw_fastrtps_cpp
	"rcutils"
	"rosidl_typesupport_introspection_c"
	"rosidl_typesupport_introspection_cpp"
	"rmw"
	"rosidl_generator_c"
	)

This calls target_include_directories() and target_link_libraries() using the content of ${<dependency>_INCLUDE_DIRS} and ${<dependency>_LIBRARIES} for each dependency. Since these dependencies are all using ament_cmake the include directories and libraries are populated by ament_export_include_directories() and ament_export_libraries().

When ament_export_include_directories() is called it stores a path as a string prefixed by ${<dependency>_DIR}. That variable contains the dependency's CMAKE_INSTALL_PREFIX. When the dependency is find_package()'d that string is evaluated as an absolute path and stored in ${<dependency>_INCLUDE_DIR}.

ament_export_libraries() is different but the result is the same. In this PR's case all the dependencies were target names. When the dependency is find_packaged() the library names are resolved to absolute paths using find_library().

Since ament_cmake does export a variable ${<dependency_DIR} it might be possible to allow downstream packages to create relocatable exported targets if the exported target could be made to look like the following. I have no idea how to achieve this though.

set_target_properties(rmw_fastrtps_cpp::rmw_fastrtps_cpp PROPERTIES
  INTERFACE_INCLUDE_DIRECTORIES "${rcutils_DIR}/../../../include${rosidl_typesupport_interface_DIR}/../../../include;${rosidl_generator_c_DIR}/../../../include;${rosidl_typesupport_introspection_c_DIR}/../../../include;${rosidl_generator_cpp_DIR}/../../../include;${rosidl_typesupport_introspection_cpp_DIR}/../../../include;${rmw_DIR}/../../../include"
  INTERFACE_LINK_LIBRARIES "fastcdr;fastrtps;${rosidl_typesupport_introspection_c_DIR}/../../../lib/librosidl_typesupport_introspection_c.so;${rosidl_typesupport_introspection_cpp_DIR}/../../../lib/librosidl_typesupport_introspection_cpp.so;${rcutils_DIR}/../../../lib/librcutils.so;${rmw_DIR}/../../../lib/librmw.so;${rosidl_generator_c_DIR}/../../../lib/librosidl_generator_c.so"
)