[test] Add test creating a target without namespaces (CMake)

[jgsogo]

Changelog: omit
Docs: omit

Adds a test using build_modules to create a target without namespace. This target consumes the targets provided by Conan.

closes #7615 - At this moment we are not implementing anything else in the model to create targets without namespaces (it is very CMake specific and cpp_info should have some level of abstraction over different build-systems). ping @madebr @SpaceIm

[SpaceIm]

What happen if a consumer relies on another generator than cmake_find_package(_multi) (cmake for example)? library::library doesn't exist anymore, so I guess target_link_libraries(nonamespace INTERFACE library::library) will fail?

Maybe this?

Suggested change

	if(NOT TARGET nonamespace)
	add_library(nonamespace INTERFACE IMPORTED)
	target_link_libraries(nonamespace INTERFACE library::library)
	endif()
	if(NOT TARGET nonamespace AND TARGET library::library)
	add_library(nonamespace INTERFACE IMPORTED)
	target_link_libraries(nonamespace INTERFACE library::library)
	endif()

[jgsogo]

I wrote a comment, but it went away with some of the commits 🤷 .

As you can see here, I can specify a different build-module per generator (f2163c6), otherwise it is the recipe author the one that needs to implement the required logic inside the build-module, of course, as you have suggested.

[SpaceIm]

ok, so I understand that we could already (conan 1.32.0) provide non namespaced CMake imported targets in CCI recipe? conan 1.32.0 doesn't allow build_modules per generator but with some tricks it should work.

[jgsogo]

Limitation in 1.32 is that build_modules are included before the Conan targets. It was changed here: #8130

[SpaceIm]

Surprinsingly, it works in 1.32: conan-io/conan-center-index#4258
If I add message() in the build_modules file, I see that it's called twice: one before targets found (in cmake_find_package_multi) and another one after.

[jgsogo]

Dammit! 😅 It should be some kind of (evil) CMake laziness resolution...

[madebr]

So the idea is to create cmake build modules that are aliases to the conan generated ones?
This is supposed to happen in build, package or copied through from exports_sources?

Sounds good to me! 💯

My next question is to add a build helper, as this is going to be often needed in cci.

Something like:

from conan.tools.cmake import cmake_tools

def package(self):
    cmake_tools.write_alias_module
        {
            "library": "library::library",
            "library2": "library::library2",
        },
        filename=None,  # default is os.path.join("lib", "cmake", "build-modules.cmake"),
        generators=None,  # (generators' default is `cmake_find_package` and `cmake_find_package_multi`)
    )

def package_info(self):
    # ...
    cmake_tools.append_alias_module_filename(
        self.cpp_info,
        filename=None,  # default is os.path.join("lib", "cmake", "build-modules.cmake"),
        generators=None,  # default is os.path.join("lib", "cmake", "build-modules.cmake"),
    )

The idea is to need no extra arguments by default.

[prince-chrismc]

Please add a helper ❤️

[SpaceIm]

Dammit! 😅 It should be some kind of (evil) CMake laziness resolution...

Actually, it seems to work in conan 1.32 if there are no components in the recipe. It fails in opencv recipe (lot of components): conan-io/conan-center-index#4267

[jgsogo]

About the helper, something for a feature request, not for this PR. Here we only wanted to check that adding targets without namespaces was possible.

[memsharded]

why not using pytest fixtures? I have started to used them and so far look great, if we are going pytest, probably we want to start adopting its practices?

[jgsogo]

because the purpose here is to execute these lines once for all the tests in this class (I could use class-scoped fixture).

A fixture typically provides a way to execute (and inject) something before/after tests, but IMHO something intended to be reusable by many tests in the test suite. This one probably applies only to this test-class, I think these class methods are better than fixtures when they apply only to one specific test.

[SpaceIm]

Is there a test with CMake imported executables targets?

I've tried to add this in geographiclib recipe:
https://github.com/SpaceIm/conan-geographiclib/blob/a143f702c9fb369651356e7bd28b590032044e1e/conanfile.py#L142
https://github.com/SpaceIm/conan-geographiclib/blob/a143f702c9fb369651356e7bd28b590032044e1e/test_package/CMakeLists.txt#L16

And it works... https://github.com/SpaceIm/conan-geographiclib/actions/runs/494023796

[jgsogo]

Hi! I would say it should be more a built-in feature than something for a custom build-module. I'd vote for this issue (#7240) and then cmake generators can create those targets as well.

[SpaceIm]

It seems that set_property(TARGET nonamespace PROPERTY INTERFACE_LINK_LIBRARIES library::library) has wider support across CMake versions (because nonamespace is an IMPORTED target I guess):
conan-io/conan-center-index#4369