Rework MDL enum constants values

--- sql/mdl.h

+++ sql/mdl.h

@@ -125,6 +125,11 @@ class MDL_context_owner

 enum enum_mdl_type {

   /* This means that the MDL_request is not initialized */

+  /*

+    TODO (newbie): should be MDL_NOT_INITIALIZED= 0, as it is strange

+    that not-inited request has MDL_INTENTION_EXCLUSIVE.

+    Must fix tests, as at least mysql_rm_table_no_locks() depends on this.

+  */

   MDL_NOT_INITIALIZED= -1,

   /*

     An intention exclusive metadata lock (IX). Used only for scoped locks.

@@ -381,6 +386,10 @@ struct MDL_key

     Note that although there isn't metadata locking on triggers,

     it's necessary to have a separate namespace for them since

     MDL_key is also used outside of the MDL subsystem.

+

+    TODO (newbie): NOT_INITIALIZED=0 as default bzero() sets wrongly type to

+    BACKUP. But dozens switch() cases for NOT_INITIALIZED must be added to

+    pacify the compiler.

   */

   enum enum_mdl_namespace { BACKUP=0,

                             SCHEMA,

--- sql/sql_parse.cc

+++ sql/sql_parse.cc

@@ -8509,6 +8509,8 @@ TABLE_LIST *st_select_lex::add_table_to_list(THD *thd,

     MDL_REQUEST_INIT(&ptr->mdl_request, MDL_key::TABLE, ptr->db.str,

                      ptr->table_name.str, mdl_type, MDL_TRANSACTION);

   }

+  else

+    ptr->mdl_request.type= MDL_NOT_INITIALIZED;

   DBUG_RETURN(ptr);

 }