Use default character set for expressions

- Force usage of () around complex DEFAULT expressions
- Give error if DEFAULT expression contains invalid characters
- Don't use const_charset_conversion for stored Item_func_sysconf expressions
  as the result is not constaint over different executions
- Fixed Item_func_user() to not store calculated value in str_value

sql/sql_class.cc

@@ -870,7 +870,7 @@ THD::THD(my_thread_id id, bool is_wsrep_applier)
    is_fatal_sub_stmt_error(false),
    rand_used(0),
    time_zone_used(0),
-   in_lock_tables(0),
+   in_lock_tables(0), in_stored_expression(0),
    bootstrap(0),
    derived_tables_processing(FALSE),
    waiting_on_group_commit(FALSE), has_waiter(FALSE),
@@ -1417,7 +1417,7 @@ void THD::init(void)
 			TL_WRITE);
   tx_isolation= (enum_tx_isolation) variables.tx_isolation;
   tx_read_only= variables.tx_read_only;
-  update_charset();
+  update_charset();             // plugin_thd_var() changed character sets
   reset_current_stmt_binlog_format_row();
   reset_binlog_local_stmt_filter();
   set_status_var_init();
@@ -2311,12 +2311,19 @@ bool THD::convert_string(LEX_STRING *to, CHARSET_INFO *to_cs,
 {
   DBUG_ENTER("THD::convert_string");
   size_t new_length= to_cs->mbmaxlen * from_length;
-  uint dummy_errors;
+  uint errors;
   if (alloc_lex_string(to, new_length + 1))
     DBUG_RETURN(true);                          // EOM
   to->length= copy_and_convert((char*) to->str, new_length, to_cs,
-			       from, from_length, from_cs, &dummy_errors);
+			       from, from_length, from_cs, &errors);
   to->str[to->length]= 0;                       // Safety
+  if (errors && in_stored_expression)
+  {
+    my_error(ER_BAD_DATA, MYF(0),
+             ErrConvString(from, from_length, from_cs).ptr(),
+             to_cs->csname);
+    DBUG_RETURN(true);
+  }
   DBUG_RETURN(false);
 }