BUG#19770958: WRONG RESULTS WITH STORED PROGRAMS AND NULLABLE SYSTEM

VARIABLES

null_value was not being reset between invocations of
Item_func_get_system_var::val_str().

When a stored procedure evaluates a system variable it calls
sp_eval_expr() which in turn calls
Item::save_in_field(). Item::save_in_field() calls
Item_func_get_system_var::val_str() but ignores the returned value if
null_value is true. As a result it continued to return NULL even if
the value of the system variable had been updated.

Normal execution was not affected because Item::send() does not call
Item::save_in_field(), but calls val_str() directly and ignores the
null_value variable.

Patch modifies Item_func_get_system_var::val_str() so that null_value
is reset to FALSE before checking if the variable value is NULL.

Author: Dyre Tjeldvoll

mysql-test/r/sp-vars.result

@@ -1204,3 +1204,20 @@ CALL vctest();
 Warnings:
 Warning	1292	Truncated incorrect DOUBLE value: 'string'
 DROP PROCEDURE vctest;
+
+---------------------------------------------------------------
+BUG#19770958
+---------------------------------------------------------------
+
+CREATE TABLE t (a INT PRIMARY KEY, b TEXT, FULLTEXT(b)) ENGINE=InnoDB;
+CREATE FUNCTION f() RETURNS TEXT RETURN @@GLOBAL.innodb_ft_aux_table;
+SELECT @@GLOBAL.innodb_ft_aux_table, f();
+@@GLOBAL.innodb_ft_aux_table	f()
+NULL	NULL
+SET GLOBAL innodb_ft_aux_table="test/t";
+SELECT @@GLOBAL.innodb_ft_aux_table, f();
+@@GLOBAL.innodb_ft_aux_table	f()
+test/t	test/t
+SET GLOBAL innodb_ft_aux_table= default;
+DROP FUNCTION f;
+DROP TABLE t;

mysql-test/t/sp-vars.test

@@ -1476,3 +1476,28 @@ DELIMITER ;|
 
 CALL vctest();
 DROP PROCEDURE vctest;
+
+
+###########################################################################
+#
+# Test case for BUG#19770958 WRONG RESULTS WITH STORED PROGRAMS AND 
+# NULLABLE SYSTEM VARIABLES: Create a function which returns a system
+# variable which initially is NULL. Then update the variable and call function
+# again to see that it returns the updated value.
+#
+###########################################################################
+
+--echo
+--echo ---------------------------------------------------------------
+--echo BUG#19770958
+--echo ---------------------------------------------------------------
+--echo
+
+CREATE TABLE t (a INT PRIMARY KEY, b TEXT, FULLTEXT(b)) ENGINE=InnoDB; 
+CREATE FUNCTION f() RETURNS TEXT RETURN @@GLOBAL.innodb_ft_aux_table;
+SELECT @@GLOBAL.innodb_ft_aux_table, f();
+SET GLOBAL innodb_ft_aux_table="test/t";
+SELECT @@GLOBAL.innodb_ft_aux_table, f();
+SET GLOBAL innodb_ft_aux_table= default;
+DROP FUNCTION f;
+DROP TABLE t;

sql/item_func.cc

@@ -7122,6 +7122,7 @@ String* Item_func_get_system_var::val_str(String* str)
   }
 
   str= &cached_strval;
+  null_value= FALSE;
   switch (var->show_type())
   {
     case SHOW_CHAR:
@@ -7167,7 +7168,7 @@ String* Item_func_get_system_var::val_str(String* str)
 
     default:
       my_error(ER_VAR_CANT_BE_READ, MYF(0), var->name.str);
-      str= NULL;
+      str= error_str();
       break;
   }