twister: Fix toolchain assignment for posix

[keith-zephyr]

twister was unconditionally setting the toolchain to "host" for the "posix" and "unit" architectures, clobbering the toolchain setup in the environment.

[nashif]

twister was unconditionally setting the toolchain to "host" for the "posix" and "unit" architectures, clobbering the toolchain setup in the environment.

what environment? you mean you set export ZEPHYR_TOOLCHAIN_VARIANT=llvm and it still uses host?

[keith-zephyr]

twister was unconditionally setting the toolchain to "host" for the "posix" and "unit" architectures, clobbering the toolchain setup in the environment.

what environment? you mean you set export ZEPHYR_TOOLCHAIN_VARIANT=llvm and it still uses host?

That's correct. Here's a snippet of a testplan.json file generated:

{                                                                                                                                           
    "environment":{                                                                                                                         
        "os":"posix",
        "zephyr_version":"v2.7.99-52011-g166fb5dd61f5",
        "toolchain":"llvm",
...
    "testsuites":[
        {
            "name":"drivers/drivers.ap_mux_control",
            "arch":"posix",
            "platform":"native_sim/native",
            "path":"../../../platform/ec/zephyr/test/drivers",
            "run_id":"8f1313c1ffcbd3ee543b908d665c37a6",
            "runnable":true,
            "retries":0,
            "toolchain":"host",
...

[thughes]

Ah, this explains why I was not seeing clang in the build.log when trying to build with LLVM:

ZEPHYR_TOOLCHAIN_VARIANT=llvm VERBOSE=1 ./scripts/twister -c -p native_sim -s cpp.libcxx.host

With this patch, I now see it using clang instead of gcc.

It also looks like this was affecting the Build with Clang/LLVM workflow, which sets ZEPHYR_TOOLCHAIN_VARIANT=llvm. On my recent PR it's showing <host> toolchain when running. The run for this PR correctly shows <llvm>.

[keith-zephyr]

Looking at the CI failures, I see that verify-toolchain.cmake falls back to the zephyr toolchain when ZEPHYR_TOOLCHAIN_VARIANT isn't set by the environment. Because of the fallback, self.env.toolchain always has a value.