Add upb_Message_ClearOneOf

upb users currently need to manually fetch a oneof field in order to clear it.
In this CL, we add a convenience method to do that in one fell swoop.

PiperOrigin-RevId: 627796728

upb/message/accessors.h

@@ -74,6 +74,20 @@ UPB_API_INLINE uint32_t upb_Message_WhichOneofFieldNumber(
   return UPB_PRIVATE(_upb_Message_GetOneofCase)(message, oneof_field);
 }
 
+UPB_API_INLINE void upb_Message_ClearOneof(upb_Message* msg,
+                                           const upb_MiniTable* m,
+                                           const upb_MiniTableField* f) {
+  uint32_t field_number = upb_Message_WhichOneofFieldNumber(msg, f);
+  if (field_number == 0) {
+    // if the field_number is zero, the field is not set
+    return;
+  }
+
+  const upb_MiniTableField* field =
+      upb_MiniTable_FindFieldByNumber(m, field_number);
+  upb_Message_ClearBaseField(msg, field);
+}
+
 // NOTE: The default_val is only used for fields that support presence.
 // For repeated/map fields, the resulting upb_Array*/upb_Map* can be NULL if a
 // upb_Array/upb_Map has not been allocated yet. Array/map fields do not have

upb/message/accessors_test.cc

@@ -477,5 +477,22 @@ TEST(GeneratedCode, EnumClosedCheck) {
   EXPECT_TRUE(upb_MiniTableField_IsClosedEnum(closedEnumField));
   upb_Arena_Free(arena);
 }
+TEST(GeneratedCode, OneofClear) {
+  upb_Arena* arena = upb_Arena_New();
+
+  protobuf_test_messages_proto2_TestAllTypesProto2* msg =
+      protobuf_test_messages_proto2_TestAllTypesProto2_new(arena);
+
+  const upb_MiniTable* x =
+      &protobuf_0test_0messages__proto2__TestAllTypesProto2_msg_init;
+
+  const upb_MiniTableField* oneofField =
+      upb_MiniTable_FindFieldByNumber(x, 111);
+  EXPECT_TRUE(upb_MiniTableField_IsInOneof(oneofField));
+  upb_Message_ClearOneof((upb_Message*)msg, x, oneofField);
+  EXPECT_FALSE(UPB_PRIVATE(_upb_MiniTableField_HasHasbit)(oneofField));
+
+  upb_Arena_Free(arena);
+}
 
 }  // namespace