[ament_target_dependencies] pick up imported interfaces

[stonier]

Problem to Solve

Allow this function to work with packages that only use the more recent export/import target style via ament_export_interfaces and ament_export_dependencies.

[sloretz]

Is more needed for transitive dependencies? It looks like ament_export_dependencies() doesn't combine ..._INTERFACES from the packages it find_package()s. If A depends on B which depends on C, and A calls ament_target_depdencies(a_target B), A will target_link_libraries(a_target ${B_INTERFACES}), but I think ${B_INTERFACES} won't contain ${C_INTERFACES}.

[stonier]

Not sure I'm quite catching your concern.

The exporting/importing of targets is going via ament_export_interfaces (ament_cmake_export_interfaces), not ament_export_dependencies.

Transitive dependencies should be fine - all defns, include paths, libraries, compiler flags get wrapped up in the imported targets and passed along via the target_link_libraries call (admittedly, it's initially confusing that target_link_libraries handles it all now). This is different to the older cmake style where you had to manage them all independently.

[sloretz]

Not sure I'm quite catching your concern.

I mistakenly thought ament_export_dependencies() would need to make the ${PROJECT_NAME}_INTERFACES variable include targets from transitive dependencies so they too would get passed to the target_link_libraries() call, but it looks like that's not necessary. The targets from transitive dependencies just need to exist. I wrote an example to convince myself: sloretz/ament_export_interfaces_example.

The exporting/importing of targets is going via ament_export_interfaces (ament_cmake_export_interfaces), not ament_export_dependencies.

It looks like ament_export_dependencies() is still required when using ament_export_interfaces(). The variables it creates aren't needed, but the find_package() call it adds makes sure targets from transitive dependencies exist.

[sloretz]

CI (building all packages to make sure no use of ament_target_dependencies() errors, testing just ament_cmake_target_dependencies for cmake linter)

• Linux
• Linux-aarch64
• macOS
• Windows

[sloretz]

CI failed with:

10:32:18 CMake Error at CMakeLists.txt:20 (add_library):
10:32:18   Target "resource_retriever" links to target
10:32:18   "ament_index_cpp::export_ament_index_cpp" but the target was not found.
10:32:18   Perhaps a find_package() call is missing for an IMPORTED target, or an
10:32:18   ALIAS target is missing?

Which is caused by ament_index_cpp calling ament_export_interfaces() with an interface name that it doesn't actually export. That will need to be fixed too. However this brings up another problem that the ..._INTERFACES variable only has valid target names if the "export" name is the same as the target name, and there's just one export per target #183.

This PR seems like the right thing to do, but I think it should go in after a fix for #183. Otherwise it may cause configure-time failures of projects that are currently ignoring the ..._INTERFACES variable of their dependencies.

[dirk-thomas]

@stonier @sloretz What is the status of this ticket?

[sloretz]

What is the status of this ticket?

@dirk-thomas It looks like this is blocked by #183, and there's been no progress on that as far as I know.

[dirk-thomas]

Thanks for creating this PR. #232 implemented the same change just slightly different.