[bug] CONAN_CMAKE_SYSTEM_PROCESSOR is not applied

[madebr]

The CONAN_CMAKE_SYSTEM_PROCESSOR environment variable is not applied to a cmake build,
instead CMAKE_SYSTEM_PROCESSOR is set to self.settings.arch.

Reading the documentation about cmake related environment variables, it seems to me that by settings the CONAN_CMAKE_SYSTEM_PROCESSOR, the cmake variable CMAKE_SYSTEM_PROCESSOR will be set automatically.

The following script reproduces my problem/question:

#!/usr/bin/sh

cd /tmp

cat >buildreq.py <<EOF
from conans import ConanFile

class BuildReq(ConanFile):
    name = "buildreq"
    version = "0.1"
    def package_info(self):
        self.env_info.CONAN_CMAKE_SYSTEM_PROCESSOR = "awesomecpu"
EOF

cat >consumer.py <<EOF
from conans import CMake, ConanFile, tools
import textwrap

class Consumer(ConanFile):
    name = "consumer"
    version = "0.1"
    generators = "cmake"
    def build_requirements(self):
        self.build_requires("buildreq/0.1")
    def build(self):
        tools.save("CMakeLists.txt", textwrap.dedent("""\
            cmake_minimum_required(VERSION 3.0)
            project(consumer C)
            include("\${CMAKE_BINARY_DIR}/conanbuildinfo.cmake")
            conan_basic_setup()
            message(STATUS "CMAKE_SYSTEM_PROCESSOR: \${CMAKE_SYSTEM_PROCESSOR}")
            message(STATUS "CONAN_CMAKE_SYSTEM_PROCESSOR: \$ENV{CONAN_CMAKE_SYSTEM_PROCESSOR}")
        """))
        cmake = CMake(self)
        cmake.configure()
EOF

conan create buildreq.py -tf None
conan create consumer.py -tf None

The output of the cmake configure step is:

-- CMAKE_SYSTEM_PROCESSOR: x86_64
-- CONAN_CMAKE_SYSTEM_PROCESSOR: awesomecpu

Where I expect it to contain awesomecpu twice.

The Android NDK requires setting CMAKE_SYSTEM_PROCESSOR to a known and valid architecture.
I added CONAN_CMAKE_SYSTEM_PROCESSOR to the android-ndk recipe in conan-io/conan-center-index#7474,
but it looks to me that conan is missing functionality here.

[martin-schulze-vireso]

I dug a bit deeper into conan to see where this invalid setting is coming from and I think I found what is described here to be the culprit (emphasis mine):

If user_toolchain is not defined and Conan detects it is cross-building, because the build and host profiles contain different OS or architecture, it will try to define the following variables:
[...]
CMAKE_SYSTEM_PROCESSOR: tools.cmake.cmaketoolchain:system_processor conf if defined, otherwise arch setting (host) if defined

conan/conan/tools/cmake/toolchain.py

Lines 630 to 638 in 14d01da

	system_name, system_version, system_processor = self._get_cross_build()
	return {"compiler": compiler,
	"toolset": toolset,
	"generator_platform": generator_platform,
	"build_type": build_type,
	"cmake_system_name": system_name,
	"cmake_system_version": system_version,
	"cmake_system_processor": system_processor}

With _get_cross_build containing

conan/conan/tools/cmake/toolchain.py

Line 581 in 14d01da

system_processor = self._conanfile.conf["tools.cmake.cmaketoolchain:system_processor"]

and if hasattr(self._conanfile, "settings_build"):

conan/conan/tools/cmake/toolchain.py

Lines 608 to 610 in 14d01da

	if system_name is not None and system_processor is None:
	if arch != arch_build:
	system_processor = arch

so arch (host) is taken.

CONAN_CMAKE_SYSTEM_PROCESSOR is used in

conan/conans/client/build/cmake_flags.py

Line 226 in a620e5b

cmake_system_processor = os.getenv("CONAN_CMAKE_SYSTEM_PROCESSOR")

[memsharded]

Closing as outdated, referring to legacy generators removed in 2.0