Don't check for native protobuf flags if the proto fragment doesn't exist. (#25803)

PiperOrigin-RevId: 867872524

Co-authored-by: Protobuf Team Bot <protobuf-github-bot@google.com>

Author: anandolee

bazel/flags/flags.bzl

@@ -50,7 +50,7 @@ _FLAGS = {
 }
 
 def get_flag_value(ctx, flag_name):
-    """Returns the value of the given flag in Starlark if it's set, otherwise reads the Java flag value.
+    """Returns the value of the given flag in Starlark if it's set, otherwise reads the Java flag value, if the proto fragment exists.
 
     Args:
         ctx: The rule context.
@@ -64,10 +64,19 @@ def get_flag_value(ctx, flag_name):
     if flag_name not in _FLAGS:
         return getattr(ctx.attr, "_" + flag_name)
 
+    starlark_flag = getattr(ctx.attr, "_" + flag_name)
+
     # Label flags don't have a BuildSettingInfo, just get the value.
-    if "toolchain" in flag_name and getattr(ctx.attr, "_" + flag_name).label != _FLAGS[flag_name].default:
-        return getattr(ctx.attr, "_" + flag_name)
-    elif getattr(ctx.attr, "_" + flag_name)[BuildSettingInfo].value != _FLAGS[flag_name].default:
-        return getattr(ctx.attr, "_" + flag_name)[BuildSettingInfo].value
+    if "toolchain" in flag_name:
+        starlark_flag_is_set = starlark_flag.label != _FLAGS[flag_name].default
+        starlark_value = starlark_flag
+    else:
+        starlark_flag_is_set = starlark_flag[BuildSettingInfo].value != _FLAGS[flag_name].default
+        starlark_value = starlark_flag[BuildSettingInfo].value
+
+    # Starlark flags take precedence over native flags.
+    # Also of course, use the Starlark value if the proto fragment no longer exists.
+    if starlark_flag_is_set or not hasattr(ctx.fragments, "proto"):
+        return starlark_value
     else:
         return _FLAGS[flag_name].native(ctx)