Handle the no-target case.

[olivier-stasse]

Goal

Handling package with no targets (typically data packages)
Issue #398

Principle

It is done through a COMPUTE_TARGETS_LIST in package-config.cmake
Targets are gathered by going through the root and the subdirectories
of the root and collect the BUILDSYSTEM_TARGET property for directory.
From this list, standard targets are removed.

Then if there is no user target the Target.cmake file
is not generated and not included in the Config.cmake.in file.

Cons:

There is a hard coded list of targets which are filtered out. They are generated probably by a call to CTest or equivalent. That is probably the weaker point of this proposal. However I did not find the source of such targets. And they do not appear in the list of targets generated by the INSTALL(export) call.
With respect to what it is intended here I believe that there is no impact on most of our use cases.

Tests:

It was tested over talos_data (https://github.com/olivier-stasse/talos_data/tree/topic/no-target-optional) and jrl-walkgen.

To make sure it does not affect usual packages it was tested over dynamic-graph:
https://github.com/olivier-stasse/dynamic-graph/tree/Mac
And together with dynamc-graph-python:
https://github.com/olivier-stasse/dynamic-graph-python/tree/topic/no-target-optional

[jmirabel]

Cannot we let user #398 say that this package has no target ?
It is simple to implement, and fairly simple to use. Something that says that you want a Config.cmake file without a Targets.cmake file.

I am not sure this will cover all cases anyway. There are packages which have targets that aren't meant to be exported.

[jmirabel]

What is the scope of variable VALID_TARGETS_EXPORT_NAME in the configured Config.cmake ? Will it interfere with previous included config files ?

[olivier-stasse]

Good point.

[gergondet]

I agree with @jmirabel I think it's much simpler to add an option to opt-out of targets installation (and checking for the file existence before including in the generated Config.cmake) than trying to list all targets in the project. Plus this listing doesn't check that any targets are actually exported so it's prone to be a partial solution at best.

[olivier-stasse]

But then how do you handle the case where some packages are optional ?
EDITED to clarify:
The package can either export targets if a package is present.
Or the package is not present and there is no target.

The user has to handle both cases ?

[olivier-stasse]

Cannot we let user #398 say that this package has no target ?
It is simple to implement, and fairly simple to use. Something that says that you want a Config.cmake file without a Targets.cmake file.

I am not sure this will cover all cases anyway. There are packages which have targets that aren't meant to be exported.

This indeed would be a better solution to avoid the problem with the scope of TARGETS_LIST_LENGTH.
Still we need something to check if a target is present or not when dealing with optional packages.

[jmirabel]

The user has to handle both cases ?

I think I understand where you need it. On your side, it is going to be a
set(variable_that_says_whether_you_want_an_export ${optional_dep_FOUND}).

There may be a cleverer way of doing this but I think letting the user handle it is preferable over relying on targets being created or not.

[jmirabel]

Closing in favor of #408