[Maria-developers] updating a string session variable from a storage engine

i have a MYSQL_THDVAR_STR session variable in my storage engine. i want to update its value from the storage engine. it was suggested that i use "THDVAR(x)=new_value" to do it. i noticed that update_func_str is NOT called, so i wonder about the contract. is the caller responsible for memory management? i used PLUGIN_VAR_MEMALLOC for the var def. thanks