refactor: make bzlmod pass platform mapping to host repo creation

Author: rickeylev

python/private/python.bzl

@@ -298,7 +298,9 @@ def _python_impl(module_ctx):
             _internal_bzlmod_toolchain_call = True,
             **kwargs
         )
-        host_compatible = []
+        host_platforms = []
+        host_os_names = {}
+        host_archs = {}
         for repo_name, (platform_name, platform_info) in register_result.impl_repos.items():
             toolchain_impls.append(struct(
                 # str: The base name to use for the toolchain() target
@@ -317,12 +319,17 @@ def _python_impl(module_ctx):
                 set_python_version_constraint = is_last,
             ))
             if _is_compatible_with_host(module_ctx, platform_info):
-                host_compatible.append(platform_name)
+                host_key = str(len(host_platforms))
+                host_platforms.append(platform_name)
+                host_os_names[host_key] = platform_info.os_name
+                host_archs[host_key] = platform_info.arch
 
         host_toolchain(
             name = toolchain_info.name + "_host",
             # NOTE: Order matters. The first found to be compatible is (usually) used.
-            platforms = host_compatible,
+            platforms = host_platforms,
+            os_names = host_os_names,
+            archs = host_archs,
             python_version = full_python_version,
         )
 

python/private/toolchains_repo.bzl

@@ -388,6 +388,16 @@ this repo causes an eager fetch of the toolchain for the host platform.
     attrs = {
         "platforms": attr.string_list(mandatory = True),
         "python_version": attr.string(mandatory = True),
+        "os_names": attr.string_dict(
+            doc = """
+If set, overrides the platform metadata. Keyed by index in `platforms`
+""",
+        ),
+        "archs": attr.string_dict(
+            doc = """
+If set, overrides the platform metadata. Keyed by index in `platforms`
+""",
+        ),
         "_rule_name": attr.string(default = "host_toolchain"),
         "_rules_python_workspace": attr.label(default = Label("//:WORKSPACE")),
     },
@@ -436,9 +446,20 @@ def _get_host_platform(*, rctx, logger, python_version, os_name, cpu_name, platf
     Returns:
         The host platform.
     """
+    if rctx.attr.os_names:
+        platform_map = {}
+        for i, platform_name in enumerate(platforms):
+            key = str(i)
+            platform_map[platform_name] = struct(
+                os_name = rctx.attr.os_names[key],
+                arch = rctx.attr.archs[key],
+            )
+    else:
+        platform_map = PLATFORMS
+
     candidates = []
     for platform in platforms:
-        meta = PLATFORMS[platform]
+        meta = platform_map[platform]
 
         if meta.os_name == os_name and meta.arch == cpu_name:
             candidates.append(platform)