[bug] tools.cmake.cmaketoolchain:user_toolchain set by build_requires not consistently seen

[puetzk]

Environment Details (include every applicable attribute)

• Operating System+version: Ubuntu 20.04
• Compiler+version: 9.3.0-17ubuntu1~20.04
• Conan version: 1.40
• Python version: 3.8.10

Steps to reproduce (Include if Applicable)

I have a package (represending a Yocto cross-compiler, further wrapped by winegcc) wants to inject custom embedded toolchain's CMAKE_TOOLCHAIN_FILE

def package_info(self):
    ...
    self.conf_info["tools.cmake.cmaketoolchain:user_toolchain"] = os.path.join(self.package_folder,"EmbeddedToolchain.cmake")

Consumers (or really a profiles) would build_require this package target the system described by those settings.

This works if the consumer uses

from conan.tools.cmake import CMakeToolchain
...
def generate(self):
     CMakeToolchain(self).generate()

but not if it uses

     generators = "CMakeToolchain"

the latter does not get the include(/path/to/EmbeddedTollchain.cmake) line in conan_toolchain.cmake (the user_toolchain block is missing).

I think the issue is that _receive_conf called (to integrate the info from build_requires) during write_toolchain, which runs after the self._generator_manager.write_generators, so generates which run via the generators = attribute will not see the effects of any build_requires' conf_info, wheras the same generator invoked from generate() will have these values. This seems like an undesirable inconsistency.

conan/conans/client/generators/__init__.py

Lines 229 to 236 in ff7b8e6

	_receive_conf(conanfile)
	if hasattr(conanfile, "generate"):
	output.highlight("Calling generate()")
	mkdir(path)
	with chdir(path):
	with conanfile_exception_formatter(str(conanfile), "generate"):
	conanfile.generate()

conan/conans/client/installer.py

Lines 133 to 137 in ff7b8e6

	self._generator_manager.write_generators(conanfile, conanfile.build_folder,
	conanfile.generators_folder, self._output)
	logger.info("TOOLCHAIN: Writing toolchain")
	write_toolchain(conanfile, conanfile.generators_folder, self._output)

[memsharded]

Thanks for the report, and the detailed investigation. Indeed a bug, fixed in #9573, and scheduled for next patch release 1.40.1.

Beware that this is quite experimental, there are quite a few unknowns, check the very simple implementation with a few TODOS:

def _receive_conf(conanfile):
    """  collect conf_info from the immediate build_requires, aggregate it and injects/update
    current conf
    """
    # TODO: Open question 1: Only build_requires can define config?
    # TODO: Only direct build_requires?
    # TODO: Is really the best mechanism to define this info? Better than env-vars?
    # Conf only for first level build_requires
    for build_require in conanfile.dependencies.direct_build.values():
        if build_require.conf_info:
            conanfile.conf.compose(build_require.conf_info)

[puetzk]

Beware that this is quite experimental, there are quite a few unknowns

Yeah :-). I just figured it was perhaps far enough along that I should try getting one of our cross-compiler/toolchain setups expressed using the new way and see what did/didn't work :-)

[memsharded]

Great, please try again in a few days, when we release 1.40.1, many thanks for trying the new features! :)

[memsharded]

Fixed by #9573, to be released in 1.40.1