Check that there are unknown fields before calling MutableUnknownFiel…

…ds to

clear it. Otherwise, we end up creating the unknown field set just to call
Clear on it, which wastes memory.

PiperOrigin-RevId: 691844650

src/google/protobuf/reflection_ops.cc

@@ -20,6 +20,7 @@
 #include "google/protobuf/descriptor.pb.h"
 #include "google/protobuf/map_field.h"
 #include "google/protobuf/map_field_inl.h"
+#include "google/protobuf/port.h"
 #include "google/protobuf/unknown_field_set.h"
 
 // Must be included last.
@@ -320,7 +321,9 @@ static bool IsMapValueMessageTyped(const FieldDescriptor* map_field) {
 void ReflectionOps::DiscardUnknownFields(Message* message) {
   const Reflection* reflection = GetReflectionOrDie(*message);
 
-  reflection->MutableUnknownFields(message)->Clear();
+  if (reflection->GetUnknownFields(*message).field_count() != 0) {
+    reflection->MutableUnknownFields(message)->Clear();
+  }
 
   // Walk through the fields of this message and DiscardUnknownFields on any
   // messages present.

src/google/protobuf/reflection_ops_unittest.cc

@@ -274,6 +274,15 @@ TEST(ReflectionOpsTest, DiscardUnknownFields) {
             message.repeated_nested_message(0).unknown_fields().field_count());
 }
 
+TEST(ReflectionOpsTest, DiscardUnknownFieldsIsANoopIfNotPresent) {
+  unittest::TestAllTypes message;
+  EXPECT_EQ(&message.unknown_fields(),
+            &google::protobuf::UnknownFieldSet::default_instance());
+  ReflectionOps::DiscardUnknownFields(&message);
+  EXPECT_EQ(&message.unknown_fields(),
+            &google::protobuf::UnknownFieldSet::default_instance());
+}
+
 TEST(ReflectionOpsTest, DiscardUnknownExtensions) {
   unittest::TestAllExtensions message;
   TestUtil::SetAllExtensions(&message);