ISSUE

  Bazel unconditionally downloads and registers Python toolchains for all 7 platforms (Linux x86{_}64, ARM64, PPC64LE, s390x, macOS x8664, ARM64, Windows x8664) regardless of the host platform. This causes:

 

  1. Windows Build Failures (CRITICAL):

     - Linux Python toolchains contain symlinks in terminfo database

     - Windows NTFS cannot extract these symlinks

     - Bazel analysis phase fails with "ERROR: circular symlinks detected"

     - Affects 3 Windows variants: windows-compile-required, windows-2022-compile-required, windows-2022

 

  2. Bandwidth and Storage Waste (HIGH):

     - All platforms download ~729MB of toolchains

     - Each platform only needs ~90-120MB (their specific toolchain)

     - 88-93% waste per build

     - ~22GB unnecessary downloads per full CI run

     - Affects 32 Linux/macOS variants

 

  ROOT CAUSE

  In MODULE.bazel (lines 226-234), all 7 Python toolchains are registered unconditionally:

 

  register{_}toolchains(

      "@py_linux_arm64//:python_toolchain",

      "@py_linux_ppc64le//:python_toolchain",

      "@py_linux_s390x//:python_toolchain",

      "@py_linux_x86_64//:python_toolchain",

      "@py_macos_arm64//:python_toolchain",

      "@py_macos_x86_64//:python_toolchain",

      "@py_windows_x86_64//:python_toolchain",

  )

 

  Bazel's module extension system downloads and evaluates all registered toolchains during the analysis phase, even if they will never be used on the current platform.

 

  IMPACT

  - 3 Windows variants: Build failures (cannot proceed)

  - 32 Linux/macOS variants: 80-93% bandwidth/storage waste

  - Total affected: 35 build variants across all platforms

 

  SOLUTION

  Implement platform-conditional toolchain registration in bazel/toolchains/python/python{_}toolchain.bzl:

 

  1. Detect host platform in module extension context (os.name, os.arch)

  2. Create stub repositories for non-matching platforms

     - Stub repos satisfy MODULE.bazel's register{_}toolchains() requirements

     - Use impossible constraint (@platforms//:incompatible) so never selected

     - No actual downloads occur for stub repos

  3. Only create full toolchain repository for matching host platform

 

  This ensures each platform only downloads the single toolchain it needs.