feat: make bootstrap=script default for linux

Author: rickeylev

python/config_settings/BUILD.bazel

@@ -11,6 +11,7 @@ load(
     "PrecompileSourceRetentionFlag",
     "VenvsSitePackages",
     "VenvsUseDeclareSymlinkFlag",
+    rp_string_flag = "string_flag",
 )
 load(
     "//python/private/pypi:flags.bzl",
@@ -87,14 +88,26 @@ string_flag(
     visibility = ["//visibility:public"],
 )
 
-string_flag(
+rp_string_flag(
     name = "bootstrap_impl",
-    build_setting_default = BootstrapImplFlag.SYSTEM_PYTHON,
+    build_setting_default = BootstrapImplFlag.SCRIPT,
+    override = select({
+        ":_is_windows": BootstrapImplFlag.SYSTEM_PYTHON,
+        "//conditions:default": "",
+    }),
     values = sorted(BootstrapImplFlag.__members__.values()),
     # NOTE: Only public because it's an implicit dependency
     visibility = ["//visibility:public"],
 )
 
+# For some reason, @platforms//os:windows can't be directly used
+# in the select() for the flag. But it can be used when put behind
+# a config_setting().
+config_setting(
+    name = "_is_windows",
+    constraint_values = ["@platforms//os:windows"],
+)
+
 # This is used for pip and hermetic toolchain resolution.
 string_flag(
     name = "py_linux_libc",

python/private/flags.bzl

@@ -35,8 +35,38 @@ AddSrcsToRunfilesFlag = FlagEnum(
     is_enabled = _AddSrcsToRunfilesFlag_is_enabled,
 )
 
+def _string_flag_impl(ctx):
+    if ctx.attr.override:
+        value = ctx.attr.override
+    else:
+        value = ctx.build_setting_value
+
+    if value not in ctx.attr.values:
+        fail((
+            "Invalid value for {name}: got {value}, must " +
+            "be one of {allowed}"
+        ).format(
+            name = ctx.label,
+            value = value,
+            allowed = ctx.attr.values,
+        ))
+
+    return [
+        BuildSettingInfo(value = value),
+        config_common.FeatureFlagInfo(value = value),
+    ]
+
+string_flag = rule(
+    implementation = _string_flag_impl,
+    build_setting = config.string(flag = True),
+    attrs = {
+        "override": attr.string(),
+        "values": attr.string_list(),
+    },
+)
+
 def _bootstrap_impl_flag_get_value(ctx):
-    return ctx.attr._bootstrap_impl_flag[BuildSettingInfo].value
+    return ctx.attr._bootstrap_impl_flag[config_common.FeatureFlagInfo].value
 
 # buildifier: disable=name-conventions
 BootstrapImplFlag = enum(