conditional support for the bootstrap=script

Author: aignas

python/config_settings/BUILD.bazel

@@ -13,6 +13,7 @@ load(
     "VenvsUseDeclareSymlinkFlag",
     rp_string_flag = "string_flag",
 )
+load("//python/private:util.bzl", "IS_BAZEL_8_OR_HIGHER")
 load(
     "//python/private/pypi:flags.bzl",
     "UniversalWhlFlag",
@@ -90,7 +91,7 @@ string_flag(
 
 rp_string_flag(
     name = "bootstrap_impl",
-    build_setting_default = BootstrapImplFlag.SCRIPT,
+    build_setting_default = BootstrapImplFlag.SCRIPT if IS_BAZEL_8_OR_HIGHER else BootstrapImplFlag.SYSTEM_PYTHON,
     override = select({
         # Windows doesn't yet support bootstrap=script, so force disable it
         ":_is_windows": BootstrapImplFlag.SYSTEM_PYTHON,

python/private/util.bzl

@@ -99,6 +99,9 @@ def define_bazel_6_provider(doc, fields, **kwargs):
         return provider("Stub, not used", fields = []), None
     return provider(doc = doc, fields = fields, **kwargs)
 
+# Only bazel 8 can distinguish symlinks within the file depsets.
+IS_BAZEL_8_OR_HIGHER = hasattr(config, "none")
+
 IS_BAZEL_7_4_OR_HIGHER = hasattr(native, "legacy_globals")
 
 IS_BAZEL_7_OR_HIGHER = hasattr(native, "starlark_doc_extract")