Sysbuild uses wrong Python env

[leonwip]

I’m setting up a dev shell using the flake attached below. Running west build works fine for “regular” Zephyr applications in this setup. However, when using sysbuild (for example, to integrate MCUboot), the build fails due to Python import errors.

My current understanding is that the initial CMake invocation uses the Python environment passed from west via -DWEST_PYTHON=..., which is the correct environment with all dependencies installed through zephyr-nix. That explains why building “regular” applications works.

With sysbuild, however, a second CMake invocation is spawned for MCUboot without -DWEST_PYTHON=.... As a result, it falls back to using the Python found via PATH. In theory this should also work, but in practice the PATH used there differs from the one I get when running echo $PATH inside the dev shell, and it does not contain the correct Python environment.

Using print-style debugging, I found that PATH is already incorrect in west before the Zephyr build even starts. I suspect this is caused by something in the west wrapper binary that is installed into the Python environment’s bin directory.

As a workaround, invoking west as python -m west ... instead of west ... avoids the issue.

flake.nix

{
    inputs = {
        nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";

        zephyr.url = "github:zephyrproject-rtos/zephyr/v4.3.0";
        zephyr.flake = false;

        zephyr-nix.url = "github:nix-community/zephyr-nix";
        zephyr-nix.inputs.nixpkgs.follows = "nixpkgs";
        zephyr-nix.inputs.zephyr.follows = "zephyr";
    };

    outputs = { self, nixpkgs, zephyr-nix, ... }:
    let
        pkgs = nixpkgs.legacyPackages.x86_64-linux;
        zephyr = zephyr-nix.packages.x86_64-linux;
    in {
        devShells.x86_64-linux.default = pkgs.mkShell {
            packages = [
                (zephyr.sdk-0_17.override {
                    targets = [
                        "arm-zephyr-eabi"
                    ];
                })
                zephyr.pythonEnv
                zephyr.hosttools-nix
                pkgs.cmake
                pkgs.ninja
            ];
        };
    };
}

[urob]

I had a similar issue which which I fixed setting the PYTHONPATH for the devshell:

pkgs.mkShell {
    packages = [ ]:
    env = {
        PYTHONPATH = "${zephyr.pythonEnv}/${zephyr.pythonEnv.sitePackages}";
    };
};

Might be worth trying if that helps here.

[oryjkov]

Wow, just spend a few hours debugging the same issue here.

I found an issue in zephyr zephyrproject-rtos/zephyr#70258 which seems relevant, although it looks like they did not consider sysbuild :(

Thanks for the idea to run it as python -m west .... This works for me. PYTHONPATH did not work - failure happens in sysbuild while building mcuboot and west is not passing through PYTHONPATH to that cmake invocation.

[urob]

I found an issue in zephyr zephyrproject-rtos/zephyr#70258 which seems relevant, although it looks like they did not consider sysbuild :(

The linked issue suggests that the problem is caused by nix setting CMAKE_PREFIX_PATH (to the wrong python env), which is then picked up by the zephyr-patched cmake. Running the flake posted by OP, this does not seem to be the case (CMAKE_PREFIX_PATH is not set in my testing).

Nevertheless, I wonder if setting it to the correct value might work around the issue.

pkgs.mkShell {
 packages = [
   zephyr.pythonEnv
   # ...
 ];
 env = {
   PYTHONPATH = "${zephyr.pythonEnv}/${zephyr.pythonEnv.sitePackages}";
 };
 shellHook = ''
   export "NIXPKGS_CMAKE_PREFIX_PATH=$NIXPKGS_CMAKE_PREFIX_PATH:$PYTHONPATH" # Probably redundant but just to be sure.
   export "CMAKE_PREFIX_PATH=$NIXPKGS_CMAKE_PREFIX_PATH"
 '';
};

[oryjkov]

@urob sorry, I just tried your suggestion in the second post and it works too. Seems to be enough to just set PYTHONPATH.

Thanks!