[bug] GnuToolchain can not be generated when cross-building

[uilianries]

DISCLAIMER: GnuToolchain is not documented yet and is considered under incubating state.

Describe the bug

Description

When trying to consume a Conan package using the GnuToolchain and the host profile is cross-building, the Conan client breaks with:

ERROR: Error in generator 'GnuToolchain': 'ConanFile' object has no attribute 'conf_build'

Environment

Conan version: 2.11.0
OS: GNU Linux 6.8.0
Linux Distro: Linux Mint 21.3 Virginia
Python: 3.12

Steps to reproduce

1. Prepare a cross-build environment

In my case, I downloaded directly from ARM page: https://developer.arm.com/downloads/-/arm-gnu-toolchain-downloads. But it's not really needed, in case using a minimal Linux distribution, you still can use the system package manager to install a cross-toolchain

2. Adapt your profile to use your toolchain

I created a separate Conan profile to build, but you can see it in the log below.

3. Try to install zlib/1.3.1 using this new profile and with GnuToolchain. It also works for any other package.

   conan install -r conancenter --requires=zlib/1.3.1 -pr armv8 -g GnuToolchain

Issues logs

This log represents my environment, including the profile used and target package.

$ conan install --requires=zlib/1.3.1 -pr armv8 -of conan -g GnuToolchain

======== Input profiles ========
Profile host:
[settings]
arch=armv8
build_type=Release
compiler=gcc
compiler.cppstd=gnu17
compiler.libcxx=libstdc++11
compiler.version=13
os=Linux
[conf]
tools.build:compiler_executables={'c': '/opt/arm-gnu-toolchain-13/bin/aarch64-none-linux-gnu-gcc', 'cpp': '/opt/arm-gnu-toolchain-13/bin/aarch64-none-linux-gnu-g++', 'ar': '/opt/arm-gnu-toolchain-13/bin/aarch64-none-linux-gnu-ar', 'ld': '/opt/arm-gnu-toolchain-13/bin/aarch64-none-linux-gnu-ld', 'fortran': '/opt/arm-gnu-toolchain-13/bin/aarch64-none-linux-gnu-fortran'}
tools.build:verbosity=verbose
tools.cmake.cmaketoolchain:generator=Ninja
tools.compilation:verbosity=verbose
tools.system.package_manager:mode=install
tools.system.package_manager:sudo=true
[buildenv]
STRIP=/opt/arm-gnu-toolchain-13/bin/aarch64-none-linux-gnu-strip

Profile build:
[settings]
arch=x86_64
build_type=Release
compiler=gcc
compiler.cppstd=gnu17
compiler.libcxx=libstdc++11
compiler.version=11
os=Linux
[conf]
tools.build:verbosity=verbose
tools.cmake.cmaketoolchain:generator=Ninja
tools.compilation:verbosity=verbose
tools.system.package_manager:mode=install
tools.system.package_manager:sudo=true


======== Computing dependency graph ========
Graph root
    cli
Requirements
    zlib/1.3.1#b8bc2603263cf7eccbd6e17e66b0ed76 - Cache

======== Computing necessary packages ========
Requirements
    zlib/1.3.1#b8bc2603263cf7eccbd6e17e66b0ed76:258056170a5add18e564ac86c77027634e725af3#2c689c81c1c8c14f176d40e79d558de7 - Cache

======== Installing packages ========
zlib/1.3.1: Already installed! (1 of 1)
WARN: deprecated: Usage of deprecated Conan 1.X features that will be removed in Conan 2.X:
WARN: deprecated:     'cpp_info.names' used in: zlib/1.3.1

======== Finalizing install (deploy, generators) ========
cli: Writing generators to /tmp/conan
cli: ERROR: Traceback (most recent call last):
  File "/opt/pyenv/versions/3.12.4/lib/python3.12/site-packages/conan/internal/api/install/generators.py", line 104, in write_generators
    generator = generator_class(conanfile)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/pyenv/versions/3.12.4/lib/python3.12/site-packages/conan/tools/gnu/gnutoolchain.py", line 108, in __init__
    self._initialize_default_extra_env()
  File "/opt/pyenv/versions/3.12.4/lib/python3.12/site-packages/conan/tools/gnu/gnutoolchain.py", line 215, in _initialize_default_extra_env
    extra_env_vars.update(self._resolve_compilers_mapping_variables())
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/pyenv/versions/3.12.4/lib/python3.12/site-packages/conan/tools/gnu/gnutoolchain.py", line 189, in _resolve_compilers_mapping_variables
    if self._is_cross_building and self._conanfile.conf_build:
                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'ConanFile' object has no attribute 'conf_build'

ERROR: Error in generator 'GnuToolchain': 'ConanFile' object has no attribute 'conf_build'

How to reproduce it

No response

[franramirez688]

Good catch! Thanks @uilianries

Also, this is just a reminder (for myself): #17470
I need to include those changes in the GnuToolchain too.

[valgur]

I have been using GnuToolchain for the better cross-compilation features with the latest Conan client in quite a few recipes without encountering this:
https://github.com/search?q=repo%3Avalgur%2Fconan-center-index+gnutoolchain&type=code

I wonder what you are doing differently. Could it be the command-line generator invocation?