python: More fixes for Windows, part 4

* Normalize the interpreter path. When the path is coming from another repo,
  it starts with "../", which the launcher resolution code doesn't know
  how to handle. Normalizing converts e.g. "workspace/../foo/bar" to "foo/bar".
* Implement the magic "py2 interpreter path sentinel overrides toolchain"
  logic. This logic is triggered on Windows to get "python" as the interpreter
  path that is put into the native launcher.

Work towards bazelbuild#15897

PiperOrigin-RevId: 505265140
Change-Id: I7beb6109739e23b884b818537c6948450458e2f7

src/main/starlark/builtins_bzl/common/python/py_executable.bzl

@@ -301,6 +301,16 @@ def _maybe_get_runtime_from_ctx(ctx):
     """
     if ctx.fragments.py.use_toolchains:
         toolchain = ctx.toolchains[TOOLCHAIN_TYPE]
+
+        # Hack around the fact that the autodetecting Python toolchain, which is
+        # automatically registered, does not yet support Windows. In this case,
+        # we want to return null so that _get_interpreter_path falls back on
+        # --python_path. See tools/python/toolchain.bzl.
+        # TODO(#7844): Remove this hack when the autodetecting toolchain has a
+        # Windows implementation.
+        if toolchain.py2_runtime and toolchain.py2_runtime.interpreter_path == "/_magic_pyruntime_sentinel_do_not_use":
+            return None, None
+
         if not hasattr(toolchain, "py3_runtime"):
             fail("Python toolchain field 'py3_runtime' is missing")
         if not toolchain.py3_runtime:

src/main/starlark/builtins_bzl/common/python/py_executable_bazel.bzl

@@ -397,7 +397,7 @@ def _get_zip_runfiles_path(path, workspace_name, legacy_external_runfiles):
         # NOTE: External runfiles (artifacts in other repos) will have a leading
         # path component of "../" so that they refer outside the main workspace
         # directory and into the runfiles root. By normalizing, we simplify e.g.
-        # "workspace/../foo/bar" to simply "foo/bar" and
+        # "workspace/../foo/bar" to simply "foo/bar".
         zip_runfiles_path = paths.normalize("{}/{}".format(workspace_name, path))
     return "{}/{}".format(_ZIP_RUNFILES_DIRECTORY_NAME, zip_runfiles_path)
 
@@ -427,7 +427,6 @@ def _get_cc_details_for_binary(ctx, extra_deps):
     )
 
 def _get_interpreter_path(ctx, *, runtime, flag_interpreter_path):
-    _ = ctx  # @unused
     if runtime:
         if runtime.interpreter_path:
             interpreter_path = runtime.interpreter_path
@@ -436,6 +435,14 @@ def _get_interpreter_path(ctx, *, runtime, flag_interpreter_path):
                 ctx.workspace_name,
                 runtime.interpreter.short_path,
             )
+
+            # NOTE: External runfiles (artifacts in other repos) will have a
+            # leading path component of "../" so that they refer outside the
+            # main workspace directory and into the runfiles root. By
+            # normalizing, we simplify e.g. "workspace/../foo/bar" to simply
+            # "foo/bar"
+            interpreter_path = paths.normalize(interpreter_path)
+
     elif flag_interpreter_path:
         interpreter_path = flag_interpreter_path
     else: