Android Kotlin build fails: cpython_archive select() missing default for cfg:no-san configuration

[homeup-ai]

Description

When building an Android Kotlin app using system_demo_toolchains(), the build fails because the cpython_archive target's sha256 attribute uses a select() without a default value, and the Android build configuration (cfg:no-san) sets CPU to x86_32 which doesn't match any of the available options.

Environment

• Buck2 version: 2026-01-14-27ba7841b62d77ea13dbee5f69fa5999cfe18a90
• OS: macOS (Darwin 25.1.0, Apple Silicon arm64)
• Java: OpenJDK 17 via Homebrew

Steps to Reproduce

1. Create a new Buck2 project with buck2 init
2. Add an android_binary target with Kotlin source files using android_library with language = "KOTLIN"
3. Run buck2 build //path/to:android_target

Error Message

BUILD FAILED
Error running analysis for `root//app:app_debug (prelude//platforms:default#...)`

Caused by:
    0: Error in configured node dependency, dependency chain follows:
              root//app:app_debug
           -> root//app:core (cfg:no-san#...)
           -> toolchains//:kotlin_for_android
           -> prelude//toolchains/android/.../kotlincd_tool
           -> toolchains//:java_bootstrap
           -> prelude//java/tools:fat_jar
           -> toolchains//:python_bootstrap
           -> toolchains//:cpython
           -> toolchains//:cpython_archive
       
    1: configuring attr `sha256`
    2: None of 2 conditions matched configuration `cfg:no-san#...` and no default was set:
         prelude//cpu:arm64
         prelude//cpu:x86_64

Analysis

The cpython_archive target in system_demo_toolchains() uses a nested select based on OS and CPU:

sha256 = select({
    "prelude//os:macos": select({
        "prelude//cpu:arm64": "...",
        "prelude//cpu:x86_64": "...",
    }),
    ...
})

When building for Android, the configuration cfg:no-san is created with:

• prelude//cpu/constraints:x86_32 (Android target architecture)
• prelude//os/constraints:macos (host OS)

The select for CPU only has arm64 and x86_64 options, so x86_32 doesn't match, causing the failure.

Expected Behavior

The cpython_archive should use the host execution platform's CPU (arm64 on Apple Silicon) rather than the target platform's CPU (x86_32 for Android), since Python is needed for build tooling, not for running on the Android device.

Suggested Fix

Either:

1. Add a default value to the sha256 select that falls back to the host platform
2. Use exec_compatible_with or execution platform constraints to ensure cpython_archive resolves based on the execution platform, not the target platform
3. Document that system_demo_toolchains() doesn't support Android Kotlin builds

Workaround

Currently, building Java-only Android resources works:

buck2 build //app:res  # Works
buck2 build //app:app_debug  # Fails (Kotlin)