feat: add option to export libraries as cmake targets

[VRichardJP]

Add EXPORT_LIBRARY_TARGETS flag to ament_auto_package() macro.
When the flag is provided, libraries are exported using their cmake target instead of going through ament home-made library export logic.

Although exporting libraries this way is stricter, it makes cmake invocations faster on downstream packages.

Example on Autoware's behavior path planner:

Before: 112s

After: 26s

Note: In my original experimentation, cmake time on this package went down to 17s. I have not checked exactly what is slowing things down yet. ament_ex macros are still way faster (8s on this package).

This feature has been experimented on this autoware branch: autowarefoundation/autoware_universe@f3b6226

Note that this experimental branch also leverages extra macros that are not included in this PR, found here: 450b6de

See #453 for more details

[clalancette]

We are a bit concerned that making things stricter is going to break quite a lot of downstream packages. To get at least some idea of what would happen, here is a full CI run of the ROS 2 core with this in place:

• Linux
• Linux-aarch64
• Windows

[VRichardJP]

Thank you for investigating this.

To be precise, I don't think this PR should break anything dowstream itself. There are only 2 changes here:

• the EXPORT_LIBRARY_TARGETS flag is not enabled by default. Without the flag the code behavior should remain unchanged.
• ament_auto_add_library now use the installed headers instead of the ones in the source directory. Since ament_auto macros have always installed headers I don't think it changes anything.

If this PR was to break anything, I think it would show at build time, no during tests.

[clalancette]

Belated I realized that this will have almost no effect on the ROS 2 core, since we don't use ament_auto there. We would need to do a more comprehensive test of downstream packages that do use ament_auto to see if there is fallout from this.

[romainreignier]

With the deprecation of ament_target_dependencies() (#572) and the migration message encouraging to use the CMake targets syntax, I think that this PR should be merged to be able to keep using ament_cmake_auto with the new ROS2 releases.

What is missing to get this merged?

[VRichardJP]

If anyone is interested I have updated the branch. However I don't do much ROS recently so I don't have a test example at hand.

[wjwwood]

@VRichardJP I know you said you don't have the ability to test this and that you've moved on, but if you have the time and context still, does this conditional install need to be outside of the above if-else?

As I read the code, one thing that definitely changed is that we try to install ${${PROJECT_NAME}_TARGETS} even if you don't use EXPORT_LIBRARY_TARGETS. If we were able to move this block up into the if (_ARG_AMENT_AUTO_PACKAGE_EXPORT_LIBRARY_TARGETS) clause, then this change becomes a very safe one (EXPORT_LIBRARY_TARGETS gives you a new behavior and without it you get the old behavior as the default).

But if you think it needs to be outside the clause then I'll have to study it closer. I don't think anything else is using ${${PROJECT_NAME}_TARGETS}, but I need to spend more time on it.

[VRichardJP]

I understand your point.

As I look back at the code, I think I have implemented it this way because it is simply more future proof. If you consider that more scripts may add targets to ${PROJECT_NAME}_TARGETS in the future (e.g. binaries as target too), and that targets should only be exported once (if any), then I think it makes more sense to keep the target exports separate.

Basically, this PR adds support for target export, and uses that new feature to implement a new export library as target option.