test(bzlmod): add python.toolchain unit tests (bazelbuild#2204)

With this PR we get rudimentary unit tests for the `python` bzlmod
extension
which allows us to unit test the override behaviour that will become
more
complex soon.

Summary:
- refactor: inline the python_register_toolchains
- refactor: use toolchain_info to call python_register_toolchains
- refactor: move the registration out of the main loop
- refactor: split the parsing of the modules to a separate function
- test(bzlmod): add python.toolchain module parsing tests

Work towards bazelbuild#2081

---------

Co-authored-by: Richard Levasseur <richardlev@gmail.com>

python/private/python.bzl

@@ -28,22 +28,21 @@ load(":util.bzl", "IS_BAZEL_6_4_OR_HIGHER")
 _MAX_NUM_TOOLCHAINS = 9999
 _TOOLCHAIN_INDEX_PAD_LENGTH = len(str(_MAX_NUM_TOOLCHAINS))
 
-def _python_register_toolchains(name, toolchain_attr, module, ignore_root_user_error):
-    """Calls python_register_toolchains and returns a struct used to collect the toolchains.
+def parse_modules(module_ctx):
+    """Parse the modules and return a struct for registrations.
+
+    Args:
+        module_ctx: {type}`module_ctx` module context.
+
+    Returns:
+        A struct with the following attributes:
+            * `toolchains`: The list of toolchains to register. The last
+              element is special and is treated as the default toolchain.
+            * `defaults`: The default `kwargs` passed to
+              {bzl:obj}`python_register_toolchains`.
+            * `debug_info`: {type}`None | dict` extra information to be passed
+              to the debug repo.
     """
-    python_register_toolchains(
-        name = name,
-        python_version = toolchain_attr.python_version,
-        register_coverage_tool = toolchain_attr.configure_coverage_tool,
-        ignore_root_user_error = ignore_root_user_error,
-    )
-    return struct(
-        python_version = toolchain_attr.python_version,
-        name = name,
-        module = struct(name = module.name, is_root = module.is_root),
-    )
-
-def _python_impl(module_ctx):
     if module_ctx.os.environ.get("RULES_PYTHON_BZLMOD_DEBUG", "0") == "1":
         debug_info = {
             "toolchains_registered": [],
@@ -61,7 +60,7 @@ def _python_impl(module_ctx):
     # This is a toolchain_info struct.
     default_toolchain = None
 
-    # Map of string Major.Minor to the toolchain_info struct
+    # Map of version string to the toolchain_info struct
     global_toolchain_versions = {}
 
     ignore_root_user_error = None
@@ -139,11 +138,11 @@ def _python_impl(module_ctx):
                     )
                 toolchain_info = None
             else:
-                toolchain_info = _python_register_toolchains(
-                    toolchain_name,
-                    toolchain_attr,
-                    module = mod,
-                    ignore_root_user_error = ignore_root_user_error,
+                toolchain_info = struct(
+                    python_version = toolchain_attr.python_version,
+                    name = toolchain_name,
+                    register_coverage_tool = toolchain_attr.configure_coverage_tool,
+                    module = struct(name = mod.name, is_root = mod.is_root),
                 )
                 global_toolchain_versions[toolchain_version] = toolchain_info
                 if debug_info:
@@ -184,39 +183,67 @@ def _python_impl(module_ctx):
     if len(toolchains) > _MAX_NUM_TOOLCHAINS:
         fail("more than {} python versions are not supported".format(_MAX_NUM_TOOLCHAINS))
 
+    return struct(
+        toolchains = [
+            struct(
+                python_version = t.python_version,
+                name = t.name,
+                register_coverage_tool = t.register_coverage_tool,
+            )
+            for t in toolchains
+        ],
+        debug_info = debug_info,
+        default_python_version = toolchains[-1].python_version,
+        defaults = {
+            "ignore_root_user_error": ignore_root_user_error,
+        },
+    )
+
+def _python_impl(module_ctx):
+    py = parse_modules(module_ctx)
+
+    for toolchain_info in py.toolchains:
+        python_register_toolchains(
+            name = toolchain_info.name,
+            python_version = toolchain_info.python_version,
+            register_coverage_tool = toolchain_info.register_coverage_tool,
+            **py.defaults
+        )
+
     # Create the pythons_hub repo for the interpreter meta data and the
     # the various toolchains.
     hub_repo(
         name = "pythons_hub",
-        default_python_version = default_toolchain.python_version,
+        # Last toolchain is default
+        default_python_version = py.default_python_version,
         toolchain_prefixes = [
             render.toolchain_prefix(index, toolchain.name, _TOOLCHAIN_INDEX_PAD_LENGTH)
-            for index, toolchain in enumerate(toolchains)
+            for index, toolchain in enumerate(py.toolchains)
         ],
-        toolchain_python_versions = [t.python_version for t in toolchains],
+        toolchain_python_versions = [t.python_version for t in py.toolchains],
         # The last toolchain is the default; it can't have version constraints
         # Despite the implication of the arg name, the values are strs, not bools
         toolchain_set_python_version_constraints = [
-            "True" if i != len(toolchains) - 1 else "False"
-            for i in range(len(toolchains))
+            "True" if i != len(py.toolchains) - 1 else "False"
+            for i in range(len(py.toolchains))
         ],
-        toolchain_user_repository_names = [t.name for t in toolchains],
+        toolchain_user_repository_names = [t.name for t in py.toolchains],
     )
 
     # This is require in order to support multiple version py_test
     # and py_binary
     multi_toolchain_aliases(
         name = "python_versions",
         python_versions = {
-            version: toolchain.name
-            for version, toolchain in global_toolchain_versions.items()
+            toolchain.python_version: toolchain.name
+            for toolchain in py.toolchains
         },
     )
 
-    if debug_info != None:
+    if py.debug_info != None:
         _debug_repo(
             name = "rules_python_bzlmod_debug",
-            debug_info = json.encode_indent(debug_info),
+            debug_info = json.encode_indent(py.debug_info),
         )
 
     if bazel_features.external_deps.extension_metadata_has_reproducible:

tests/python/BUILD.bazel

@@ -0,0 +1,17 @@
+# Copyright 2024 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+load(":python_tests.bzl", "python_test_suite")
+
+python_test_suite(name = "python_tests")