[ament_export_targets] Use IMPORTED_TARGETS directory property when CMake version is 3.21+

[sloretz]

Currently ament_export_targets(<export_name>) is given the name of the export which export targets, but it doesn't actually know what targets that export name is going to be used to export. When a package using ament_export_targets() is find_package()'d by a downstream package, it will search for matches to a regex on the generated export file to get a list of targets it exports. These targets are then added to a package_TARGETS variable. This is a little fragile as any upstream changes to CMake could break the regex matching

ament_cmake/ament_cmake_export_targets/cmake/ament_cmake_export_targets-extras.cmake.in

Line 12 in efcbe32

set(_regex "foreach\\(_expectedTarget (.+)\\)")

Example in an export file the above code matches

foreach(_expectedTarget rcutils::rcutils)

CMake 3.21 adds a directory property IMPORTED_TARGETS that would allow the info to be queried directly. If one queries IMPORTED_TARGETS before and after including the export file, then all the new imported targets can be added to package_TARGETS.

ament_cmake/ament_cmake_export_targets/cmake/ament_cmake_export_targets-extras.cmake.in

Line 12 in efcbe32

set(_regex "foreach\\(_expectedTarget (.+)\\)")

[topin89]

This is a little fragile as any upstream changes to CMake could break the regex matching

Yup, a cat knocked the code off a table about 4 days ago

[fake-name]

This caused multiple of my colleagues to waste several hours with mysterious build errors today, and I just spent several hours getting to the point where I (finally) found this issue.

If this was a known-going-to-break, at least fail on cmake >= 3.24, rather then wasting so much time.

[topin89]

If this was a known-going-to-break, at least fail on cmake >= 3.24, rather then wasting so much time.

Well, it was broken for only four days, and fix is on its way. It may took longer for LTS releases, of course.

Wait, fix is being here for four days as well. Now I'm not sure how long it will be before it's merged.