refactor: read migrated native flags through a centralized accessor function (bazel-contrib#3290)

This lets Python logic support either Starlark- or native-defined flags,
based on Bazel support:

- Pre-Bazel 9.0: always use native flags
- Bazel 9.0+: use Starlark flags if [incompatible
change](bazelbuild/bazel#27056) that disables
`ctx.fragments.py` and `ctx.fragments.bazel_py` is set
- Include a developer override to trigger Starlark versions while
testing Starlarkification. Developers enable this by updating .bzl code
in their local workspace. This can be removed as soon as
Starlarkification is complete

Also check in a Starlark version of
`--experimental_python_import_all_repositories` for testing.

**This is a no-op. New flag sources of truth don't take effect without a
supporting bazel version that sets appropriate incompatible flags.**

**Caveats:**

- May not work with `configuration_field` as implemented. We can loosen
the incompatible flag lockdown if that's an issue.

For bazel-contrib#3252

TODO list

* [ ] Add docs to docs/api/rules_python/python/config_settings/index.md
for starlarkified flags

Author: gregestren

python/config_settings/BUILD.bazel

@@ -1,4 +1,4 @@
-load("@bazel_skylib//rules:common_settings.bzl", "string_flag")
+load("@bazel_skylib//rules:common_settings.bzl", "bool_flag", "string_flag")
 load("@pythons_hub//:versions.bzl", "DEFAULT_PYTHON_VERSION", "MINOR_MAPPING", "PYTHON_VERSIONS")
 load(
     "//python/private:flags.bzl",
@@ -240,3 +240,9 @@ label_flag(
     # NOTE: Only public because it is used in pip hub repos.
     visibility = ["//visibility:public"],
 )
+
+bool_flag(
+    name = "experimental_python_import_all_repositories",
+    build_setting_default = True,
+    visibility = ["//visibility:public"],
+)

python/private/flags.bzl

@@ -21,6 +21,59 @@ unnecessary files when all that are needed are flag definitions.
 load("@bazel_skylib//rules:common_settings.bzl", "BuildSettingInfo")
 load(":enum.bzl", "FlagEnum", "enum")
 
+# Maps "--myflag" to a tuple of:
+#
+# - the flag's ctx.fragments native API accessor
+# -"native|starlark": which definition to use if the flag is available both
+#      from ctx.fragments and Starlark
+#
+# Builds that set --incompatible_remove_ctx_py_fragment or
+# --incompatible_remove_ctx_bazel_py_fragment disable ctx.fragments. These
+# builds assume flags are solely defined in Starlark.
+#
+# The "native|starlark" override is only for devs who are testing flag
+# Starlarkification. If ctx.fragments.[py|bazel_py] is available and
+# a flag is set to "starlark", we exclusively read its starlark version.
+#
+# See https://github.com/bazel-contrib/rules_python/issues/3252.
+_POSSIBLY_NATIVE_FLAGS = {
+    "build_python_zip": (lambda ctx: ctx.fragments.py.build_python_zip, "native"),
+    "default_to_explicit_init_py": (lambda ctx: ctx.fragments.py.default_to_explicit_init_py, "native"),
+    "disable_py2": (lambda ctx: ctx.fragments.py.disable_py2, "native"),
+    "python_import_all_repositories": (lambda ctx: ctx.fragments.bazel_py.python_import_all_repositories, "native"),
+    "python_path": (lambda ctx: ctx.fragments.bazel_py.python_path, "native"),
+}
+
+def read_possibly_native_flag(ctx, flag_name):
+    """
+    Canonical API for reading a Python build flag.
+
+    Flags might be defined in Starlark or native-Bazel. This function reasd flags
+    from tbe correct source based on supporting Bazel version and --incompatible*
+    flags that disable native references.
+
+    Args:
+        ctx: Rule's configuration context.
+        flag_name: Name of the flag to read, without preceding "--".
+
+    Returns:
+        The flag's value.
+    """
+
+    # Bazel 9.0+ can disable these fragments with --incompatible_remove_ctx_py_fragment and
+    # --incompatible_remove_ctx_bazel_py_fragment. Disabling them means bazel expects
+    # Python to read Starlark flags.
+    use_native_def = hasattr(ctx.fragments, "py") and hasattr(ctx.fragments, "bazel_py")
+
+    # Developer override to force the Starlark definition for testing.
+    if _POSSIBLY_NATIVE_FLAGS[flag_name][1] == "starlark":
+        use_native_def = False
+    if use_native_def:
+        return _POSSIBLY_NATIVE_FLAGS[flag_name][0](ctx)
+    else:
+        # Starlark definition of "--foo" is assumed to be a label dependency named "_foo".
+        return getattr(ctx.attr, "_" + flag_name)[BuildSettingInfo].value
+
 def _AddSrcsToRunfilesFlag_is_enabled(ctx):
     value = ctx.attr._add_srcs_to_runfiles_flag[BuildSettingInfo].value
     if value == AddSrcsToRunfilesFlag.AUTO:

python/private/py_executable.bzl

@@ -53,7 +53,7 @@ load(
     "target_platform_has_any_constraint",
 )
 load(":common_labels.bzl", "labels")
-load(":flags.bzl", "BootstrapImplFlag", "VenvsUseDeclareSymlinkFlag")
+load(":flags.bzl", "BootstrapImplFlag", "VenvsUseDeclareSymlinkFlag", "read_possibly_native_flag")
 load(":precompile.bzl", "maybe_precompile")
 load(":py_cc_link_params_info.bzl", "PyCcLinkParamsInfo")
 load(":py_executable_info.bzl", "PyExecutableInfo")
@@ -293,7 +293,7 @@ def _get_stamp_flag(ctx):
 
 def _should_create_init_files(ctx):
     if ctx.attr.legacy_create_init == -1:
-        return not ctx.fragments.py.default_to_explicit_init_py
+        return not read_possibly_native_flag(ctx, "default_to_explicit_init_py")
     else:
         return bool(ctx.attr.legacy_create_init)
 
@@ -381,7 +381,7 @@ def _create_executable(
     extra_files_to_build = []
 
     # NOTE: --build_python_zip defaults to true on Windows
-    build_zip_enabled = ctx.fragments.py.build_python_zip
+    build_zip_enabled = read_possibly_native_flag(ctx, "build_python_zip")
 
     # When --build_python_zip is enabled, then the zip file becomes
     # one of the default outputs.
@@ -587,7 +587,7 @@ def _create_venv(ctx, output_prefix, imports, runtime_details):
         output = site_init,
         substitutions = {
             "%coverage_tool%": _get_coverage_tool_runfiles_path(ctx, runtime),
-            "%import_all%": "True" if ctx.fragments.bazel_py.python_import_all_repositories else "False",
+            "%import_all%": "True" if read_possibly_native_flag(ctx, "python_import_all_repositories") else "False",
             "%site_init_runfiles_path%": "{}/{}".format(ctx.workspace_name, site_init.short_path),
             "%workspace_name%": ctx.workspace_name,
         },
@@ -668,7 +668,7 @@ def _create_stage2_bootstrap(
         output = output,
         substitutions = {
             "%coverage_tool%": _get_coverage_tool_runfiles_path(ctx, runtime),
-            "%import_all%": "True" if ctx.fragments.bazel_py.python_import_all_repositories else "False",
+            "%import_all%": "True" if read_possibly_native_flag(ctx, "python_import_all_repositories") else "False",
             "%imports%": ":".join(imports.to_list()),
             "%main%": main_py_path,
             "%main_module%": ctx.attr.main_module,
@@ -755,7 +755,7 @@ def _create_stage1_bootstrap(
             template = ctx.file._bootstrap_template
 
         subs["%coverage_tool%"] = coverage_tool_runfiles_path
-        subs["%import_all%"] = ("True" if ctx.fragments.bazel_py.python_import_all_repositories else "False")
+        subs["%import_all%"] = ("True" if read_possibly_native_flag(ctx, "python_import_all_repositories") else "False")
         subs["%imports%"] = ":".join(imports.to_list())
         subs["%main%"] = "{}/{}".format(ctx.workspace_name, main_py.short_path)
 
@@ -1135,7 +1135,7 @@ def _get_runtime_details(ctx, semantics):
     #
     # TOOD(bazelbuild/bazel#7901): Remove this once --python_path flag is removed.
 
-    flag_interpreter_path = ctx.fragments.bazel_py.python_path
+    flag_interpreter_path = read_possibly_native_flag(ctx, "python_path")
     toolchain_runtime, effective_runtime = _maybe_get_runtime_from_ctx(ctx)
     if not effective_runtime:
         # Clear these just in case

python/private/py_runtime_pair_rule.bzl

@@ -17,6 +17,7 @@
 load("@bazel_skylib//rules:common_settings.bzl", "BuildSettingInfo")
 load("//python:py_runtime_info.bzl", "PyRuntimeInfo")
 load(":common_labels.bzl", "labels")
+load(":flags.bzl", "read_possibly_native_flag")
 load(":reexports.bzl", "BuiltinPyRuntimeInfo")
 load(":util.bzl", "IS_BAZEL_7_OR_HIGHER")
 
@@ -69,7 +70,7 @@ def _is_py2_disabled(ctx):
     # TODO: Remove this once all supported Balze versions have this flag.
     if not hasattr(ctx.fragments.py, "disable_py"):
         return False
-    return ctx.fragments.py.disable_py2
+    return read_possibly_native_flag(ctx, "disable_py2")
 
 _MaybeBuiltinPyRuntimeInfo = [[BuiltinPyRuntimeInfo]] if BuiltinPyRuntimeInfo != None else []