refactor: make bzlmod create host repos

Author: rickeylev

python/private/python.bzl

@@ -315,6 +315,12 @@ def _python_impl(module_ctx):
                 # The last toolchain is the default; it can't have version constraints
                 set_python_version_constraint = is_last,
             ))
+            if _is_compatible_with_host(mctx, platform_info):
+                host_toolchain(
+                    name = toolchain_info.name,
+                    platforms = [platform_name],
+                    python_version = full_python_version,
+                )
 
     # List of the base names ("python_3_10") for the toolchain repos
     base_toolchain_repo_names = []
@@ -406,6 +412,11 @@ def _python_impl(module_ctx):
     else:
         return None
 
+def _is_compatible_with_host(mctx, platform_info):
+    os_name = repo_utils.get_platforms_os_name(rctx)
+    cpu_name = repo_utils.get_platforms_cpu_name(rctx)
+    return platform_info.os_name == os_name and platform_info.arch == cpu_name
+
 def _one_or_the_same(first, second, *, onerror = None):
     if not first:
         return second

python/private/python_register_toolchains.bzl

@@ -170,26 +170,26 @@ def python_register_toolchains(
                 platform = platform,
             ))
 
-    host_toolchain(
-        name = name + "_host",
-        platforms = loaded_platforms,
-        python_version = python_version,
-    )
-
     toolchain_aliases(
         name = name,
         python_version = python_version,
         user_repository_name = name,
         platforms = loaded_platforms,
     )
 
-    # in bzlmod we write out our own toolchain repos
+    # in bzlmod we write out our own toolchain repos and host repos
     if bzlmod_toolchain_call:
         return struct(
             # dict[str name, tuple[str platform_name, platform_info]]
             impl_repos = impl_repos,
         )
 
+    host_toolchain(
+        name = name + "_host",
+        platforms = loaded_platforms,
+        python_version = python_version,
+    )
+
     toolchains_repo(
         name = toolchain_repo_name,
         python_version = python_version,