refactor: make bzlmod pass platform mapping to host repo creation (#2889)

This makes bzlmod pass the platform metadata to the host_toolchain rule
instead of
the host toolchain rule using the fixed PLATFORMS global.

This allows the bzlmod extension to modify the platforms that are
available, where the
fixed PLATFORM global can't be changed.

Work towards #2081

---------

Co-authored-by: Ignas Anikevicius <[email protected]>

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,
+            arch_names = host_archs,
             python_version = full_python_version,
         )
 

python/private/toolchains_repo.bzl

@@ -386,6 +386,16 @@ toolchain_aliases repo because referencing the `python` interpreter target from
 this repo causes an eager fetch of the toolchain for the host platform.
     """,
     attrs = {
+        "arch_names": attr.string_dict(
+            doc = """
+If set, overrides the platform metadata. Keyed by index in `platforms`
+""",
+        ),
+        "os_names": attr.string_dict(
+            doc = """
+If set, overrides the platform metadata. Keyed by index in `platforms`
+""",
+        ),
         "platforms": attr.string_list(mandatory = True),
         "python_version": attr.string(mandatory = True),
         "_rule_name": attr.string(default = "host_toolchain"),
@@ -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.arch_names[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)