Bug #42778: delete order by null global variable causes

            assertion .\filesort.cc, line 797

A query with the "ORDER BY @@some_system_variable" clause,
where @@some_system_variable is NULL, causes assertion
failure in the filesort procedures.

The reason of the failure is in the value of
Item_func_get_system_var::maybe_null: it was unconditionally
set to false even if the value of a variable was NULL.


mysql-test/r/variables.result:
  Added test case for bug #42778.
mysql-test/suite/sys_vars/r/innodb_data_home_dir_basic.result:
  Updated test case for bug #42778:
  system variables were NOT NULL, now they are nullable.
mysql-test/suite/sys_vars/r/innodb_flush_method_basic.result:
  Updated test case for bug #42778:
  system variables were NOT NULL, now they are nullable.
mysql-test/suite/sys_vars/r/rpl_init_slave_func.result:
  Updated test case for bug #42778:
  system variables were NOT NULL, now they are nullable.
mysql-test/suite/sys_vars/r/ssl_capath_basic.result:
  Updated test case for bug #42778:
  system variables were NOT NULL, now they are nullable.
mysql-test/suite/sys_vars/r/ssl_cipher_basic.result:
  Updated test case for bug #42778:
  system variables were NOT NULL, now they are nullable.
mysql-test/suite/sys_vars/t/innodb_data_home_dir_basic.test:
  Updated test case for bug #42778:
  system variables were NOT NULL, now they are nullable.
mysql-test/suite/sys_vars/t/innodb_flush_method_basic.test:
  Updated test case for bug #42778:
  system variables were NOT NULL, now they are nullable.
mysql-test/suite/sys_vars/t/ssl_capath_basic.test:
  Updated test case for bug #42778:
  system variables were NOT NULL, now they are nullable.
mysql-test/suite/sys_vars/t/ssl_cipher_basic.test:
  Updated test case for bug #42778:
  system variables were NOT NULL, now they are nullable.
mysql-test/t/variables.test:
  Added test case for bug #42778.
sql/item.cc:
  Bug #42778: delete order by null global variable causes
              assertion .\filesort.cc, line 797
  
  The longlong_from_string_with_check function has been modified
  to skip unwanted warnings: now it uses the THD::no_errors
  flag to suppress warnings.
  The Item_func_get_system_var::update_null_value method
  sets the no_error flag.
sql/item_func.cc:
  Bug #42778: delete order by null global variable causes
              assertion .\filesort.cc, line 797
  
  1. The Item_func_get_system_var::fix_length_and_dec method
     has been modified to make system variables truly nullable.
  
  2. The Item_func_get_system_var::update_null_value method
     method has been overloaded with a simple wrapper (like
     Item_field::update_null_value) to suppress unwanted warnings
     from Item_func_get_system_var::val_int() calls on non-numeric
     variable values: the Item_func_get_system_var::update_null_value
     method sets and restores THD::no_errors flag for a nested
     call of the longlong_from_string_with_check function.
sql/item_func.h:
  Bug #42778: delete order by null global variable causes
              assertion .\filesort.cc, line 797
  
  The Item_func_get_system_var::update_null_value method
  method has been overloaded.

mysql-test/r/variables.result

@@ -1436,7 +1436,7 @@ Warnings:
 Warning	1292	Truncated incorrect auto_increment_offset value: '0'
 select @@storage_engine;
 Catalog	Database	Table	Table_alias	Column	Column_alias	Type	Length	Max length	Is_null	Flags	Decimals	Charsetnr
-def					@@storage_engine	253	6	6	N	1	31	8
+def					@@storage_engine	253	6	6	Y	0	31	8
 @@storage_engine
 MyISAM
 SET @old_server_id = @@GLOBAL.server_id;
@@ -1467,4 +1467,23 @@ SELECT @@GLOBAL.server_id;
 @@GLOBAL.server_id
 0
 SET GLOBAL server_id = @old_server_id;
+SELECT @@GLOBAL.INIT_FILE, @@GLOBAL.INIT_FILE IS NULL;
+@@GLOBAL.INIT_FILE	@@GLOBAL.INIT_FILE IS NULL
+NULL	1
+SELECT @@GLOBAL.REPORT_HOST, @@GLOBAL.REPORT_HOST IS NULL;
+@@GLOBAL.REPORT_HOST	@@GLOBAL.REPORT_HOST IS NULL
+NULL	1
+SELECT @@GLOBAL.REPORT_PASSWORD, @@GLOBAL.REPORT_PASSWORD IS NULL;
+@@GLOBAL.REPORT_PASSWORD	@@GLOBAL.REPORT_PASSWORD IS NULL
+NULL	1
+SELECT @@GLOBAL.REPORT_USER, @@GLOBAL.REPORT_USER IS NULL;
+@@GLOBAL.REPORT_USER	@@GLOBAL.REPORT_USER IS NULL
+NULL	1
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES ();
+SET @bug42778= @@sql_safe_updates;
+SET @@sql_safe_updates= 0;
+DELETE FROM t1 ORDER BY (@@GLOBAL.INIT_FILE) ASC LIMIT 10;
+SET @@sql_safe_updates= @bug42778;
+DROP TABLE t1;
 End of 5.1 tests

mysql-test/suite/sys_vars/r/innodb_data_home_dir_basic.result

 '#---------------------BS_STVARS_025_01----------------------#'
 SELECT COUNT(@@GLOBAL.innodb_data_home_dir);
 COUNT(@@GLOBAL.innodb_data_home_dir)
-1
-1 Expected
+0
+0 Expected
 '#---------------------BS_STVARS_025_02----------------------#'
 SET @@GLOBAL.innodb_data_home_dir=1;
 ERROR HY000: Variable 'innodb_data_home_dir' is a read only variable
 Expected error 'Read only variable'
 SELECT COUNT(@@GLOBAL.innodb_data_home_dir);
 COUNT(@@GLOBAL.innodb_data_home_dir)
-1
-1 Expected
+0
+0 Expected
 '#---------------------BS_STVARS_025_03----------------------#'
 SELECT @@GLOBAL.innodb_data_home_dir = VARIABLE_VALUE
 FROM INFORMATION_SCHEMA.GLOBAL_VARIABLES
@@ -20,8 +20,8 @@ NULL
 1 Expected
 SELECT COUNT(@@GLOBAL.innodb_data_home_dir);
 COUNT(@@GLOBAL.innodb_data_home_dir)
-1
-1 Expected
+0
+0 Expected
 SELECT COUNT(VARIABLE_VALUE)
 FROM INFORMATION_SCHEMA.GLOBAL_VARIABLES 
 WHERE VARIABLE_NAME='innodb_data_home_dir';
@@ -36,8 +36,8 @@ NULL
 '#---------------------BS_STVARS_025_05----------------------#'
 SELECT COUNT(@@innodb_data_home_dir);
 COUNT(@@innodb_data_home_dir)
-1
-1 Expected
+0
+0 Expected
 SELECT COUNT(@@local.innodb_data_home_dir);
 ERROR HY000: Variable 'innodb_data_home_dir' is a GLOBAL variable
 Expected error 'Variable is a GLOBAL variable'
@@ -46,8 +46,8 @@ ERROR HY000: Variable 'innodb_data_home_dir' is a GLOBAL variable
 Expected error 'Variable is a GLOBAL variable'
 SELECT COUNT(@@GLOBAL.innodb_data_home_dir);
 COUNT(@@GLOBAL.innodb_data_home_dir)
-1
-1 Expected
+0
+0 Expected
 SELECT innodb_data_home_dir = @@SESSION.innodb_data_home_dir;
 ERROR 42S22: Unknown column 'innodb_data_home_dir' in 'field list'
 Expected error 'Readonly variable'

mysql-test/suite/sys_vars/r/innodb_flush_method_basic.result

 '#---------------------BS_STVARS_029_01----------------------#'
 SELECT COUNT(@@GLOBAL.innodb_flush_method);
 COUNT(@@GLOBAL.innodb_flush_method)
-1
-1 Expected
+0
+0 Expected
 '#---------------------BS_STVARS_029_02----------------------#'
 SET @@GLOBAL.innodb_flush_method=1;
 ERROR HY000: Variable 'innodb_flush_method' is a read only variable
 Expected error 'Read only variable'
 SELECT COUNT(@@GLOBAL.innodb_flush_method);
 COUNT(@@GLOBAL.innodb_flush_method)
-1
-1 Expected
+0
+0 Expected
 '#---------------------BS_STVARS_029_03----------------------#'
 SELECT @@GLOBAL.innodb_flush_method = VARIABLE_VALUE
 FROM INFORMATION_SCHEMA.GLOBAL_VARIABLES
@@ -20,8 +20,8 @@ NULL
 1 Expected
 SELECT COUNT(@@GLOBAL.innodb_flush_method);
 COUNT(@@GLOBAL.innodb_flush_method)
-1
-1 Expected
+0
+0 Expected
 SELECT COUNT(VARIABLE_VALUE)
 FROM INFORMATION_SCHEMA.GLOBAL_VARIABLES 
 WHERE VARIABLE_NAME='innodb_flush_method';
@@ -36,8 +36,8 @@ NULL
 '#---------------------BS_STVARS_029_05----------------------#'
 SELECT COUNT(@@innodb_flush_method);
 COUNT(@@innodb_flush_method)
-1
-1 Expected
+0
+0 Expected
 SELECT COUNT(@@local.innodb_flush_method);
 ERROR HY000: Variable 'innodb_flush_method' is a GLOBAL variable
 Expected error 'Variable is a GLOBAL variable'
@@ -46,8 +46,8 @@ ERROR HY000: Variable 'innodb_flush_method' is a GLOBAL variable
 Expected error 'Variable is a GLOBAL variable'
 SELECT COUNT(@@GLOBAL.innodb_flush_method);
 COUNT(@@GLOBAL.innodb_flush_method)
-1
-1 Expected
+0
+0 Expected
 SELECT innodb_flush_method = @@SESSION.innodb_flush_method;
 ERROR 42S22: Unknown column 'innodb_flush_method' in 'field list'
 Expected error 'Readonly variable'

mysql-test/suite/sys_vars/r/rpl_init_slave_func.result

@@ -12,7 +12,7 @@ DROP TABLE IF EXISTS t1;
 CREATE TEMPORARY TABLE t1 AS SELECT @@global.init_slave AS my_column;
 DESCRIBE t1;
 Field	Type	Null	Key	Default	Extra
-my_column	varchar(59)	NO			
+my_column	varchar(59)	YES		NULL	
 DROP TABLE t1;
 SELECT @@global.init_slave = 'SET @@global.max_connections = @@global.max_connections + 1';
 @@global.init_slave = 'SET @@global.max_connections = @@global.max_connections + 1'

mysql-test/suite/sys_vars/r/ssl_capath_basic.result

 '#---------------------BS_STVARS_046_01----------------------#'
 SELECT COUNT(@@GLOBAL.ssl_capath);
 COUNT(@@GLOBAL.ssl_capath)
-1
-1 Expected
+0
+0 Expected
 '#---------------------BS_STVARS_046_02----------------------#'
 SET @@GLOBAL.ssl_capath=1;
 ERROR HY000: Variable 'ssl_capath' is a read only variable
 Expected error 'Read only variable'
 SELECT COUNT(@@GLOBAL.ssl_capath);
 COUNT(@@GLOBAL.ssl_capath)
-1
-1 Expected
+0
+0 Expected
 '#---------------------BS_STVARS_046_03----------------------#'
 SELECT @@GLOBAL.ssl_capath = VARIABLE_VALUE
 FROM INFORMATION_SCHEMA.GLOBAL_VARIABLES
@@ -20,8 +20,8 @@ NULL
 1 Expected
 SELECT COUNT(@@GLOBAL.ssl_capath);
 COUNT(@@GLOBAL.ssl_capath)
-1
-1 Expected
+0
+0 Expected
 SELECT COUNT(VARIABLE_VALUE)
 FROM INFORMATION_SCHEMA.GLOBAL_VARIABLES 
 WHERE VARIABLE_NAME='ssl_capath';
@@ -36,8 +36,8 @@ NULL
 '#---------------------BS_STVARS_046_05----------------------#'
 SELECT COUNT(@@ssl_capath);
 COUNT(@@ssl_capath)
-1
-1 Expected
+0
+0 Expected
 SELECT COUNT(@@local.ssl_capath);
 ERROR HY000: Variable 'ssl_capath' is a GLOBAL variable
 Expected error 'Variable is a GLOBAL variable'
@@ -46,8 +46,8 @@ ERROR HY000: Variable 'ssl_capath' is a GLOBAL variable
 Expected error 'Variable is a GLOBAL variable'
 SELECT COUNT(@@GLOBAL.ssl_capath);
 COUNT(@@GLOBAL.ssl_capath)
-1
-1 Expected
+0
+0 Expected
 SELECT ssl_capath = @@SESSION.ssl_capath;
 ERROR 42S22: Unknown column 'ssl_capath' in 'field list'
 Expected error 'Readonly variable'

mysql-test/suite/sys_vars/r/ssl_cipher_basic.result

 '#---------------------BS_STVARS_048_01----------------------#'
 SELECT COUNT(@@GLOBAL.ssl_cipher);
 COUNT(@@GLOBAL.ssl_cipher)
-1
-1 Expected
+0
+0 Expected
 '#---------------------BS_STVARS_048_02----------------------#'
 SET @@GLOBAL.ssl_cipher=1;
 ERROR HY000: Variable 'ssl_cipher' is a read only variable
 Expected error 'Read only variable'
 SELECT COUNT(@@GLOBAL.ssl_cipher);
 COUNT(@@GLOBAL.ssl_cipher)
-1
-1 Expected
+0
+0 Expected
 '#---------------------BS_STVARS_048_03----------------------#'
 SELECT @@GLOBAL.ssl_cipher = VARIABLE_VALUE
 FROM INFORMATION_SCHEMA.GLOBAL_VARIABLES
@@ -20,8 +20,8 @@ NULL
 1 Expected
 SELECT COUNT(@@GLOBAL.ssl_cipher);
 COUNT(@@GLOBAL.ssl_cipher)
-1
-1 Expected
+0
+0 Expected
 SELECT COUNT(VARIABLE_VALUE)
 FROM INFORMATION_SCHEMA.GLOBAL_VARIABLES 
 WHERE VARIABLE_NAME='ssl_cipher';
@@ -36,8 +36,8 @@ NULL
 '#---------------------BS_STVARS_048_05----------------------#'
 SELECT COUNT(@@ssl_cipher);
 COUNT(@@ssl_cipher)
-1
-1 Expected
+0
+0 Expected
 SELECT COUNT(@@local.ssl_cipher);
 ERROR HY000: Variable 'ssl_cipher' is a GLOBAL variable
 Expected error 'Variable is a GLOBAL variable'
@@ -46,8 +46,8 @@ ERROR HY000: Variable 'ssl_cipher' is a GLOBAL variable
 Expected error 'Variable is a GLOBAL variable'
 SELECT COUNT(@@GLOBAL.ssl_cipher);
 COUNT(@@GLOBAL.ssl_cipher)
-1
-1 Expected
+0
+0 Expected
 SELECT ssl_cipher = @@SESSION.ssl_cipher;
 ERROR 42S22: Unknown column 'ssl_cipher' in 'field list'
 Expected error 'Readonly variable'

mysql-test/suite/sys_vars/t/innodb_data_home_dir_basic.test

@@ -29,7 +29,7 @@
 #   Displaying default value                                       #
 ####################################################################
 SELECT COUNT(@@GLOBAL.innodb_data_home_dir);
---echo 1 Expected
+--echo 0 Expected
 
 
 --echo '#---------------------BS_STVARS_025_02----------------------#'
@@ -42,7 +42,7 @@ SET @@GLOBAL.innodb_data_home_dir=1;
 --echo Expected error 'Read only variable'
 
 SELECT COUNT(@@GLOBAL.innodb_data_home_dir);
---echo 1 Expected
+--echo 0 Expected
 
 
 
@@ -58,7 +58,7 @@ WHERE VARIABLE_NAME='innodb_data_home_dir';
 --echo 1 Expected
 
 SELECT COUNT(@@GLOBAL.innodb_data_home_dir);
---echo 1 Expected
+--echo 0 Expected
 
 SELECT COUNT(VARIABLE_VALUE)
 FROM INFORMATION_SCHEMA.GLOBAL_VARIABLES 
@@ -82,7 +82,7 @@ SELECT @@innodb_data_home_dir = @@GLOBAL.innodb_data_home_dir;
 ################################################################################
 
 SELECT COUNT(@@innodb_data_home_dir);
---echo 1 Expected
+--echo 0 Expected
 
 --Error ER_INCORRECT_GLOBAL_LOCAL_VAR
 SELECT COUNT(@@local.innodb_data_home_dir);
@@ -93,7 +93,7 @@ SELECT COUNT(@@SESSION.innodb_data_home_dir);
 --echo Expected error 'Variable is a GLOBAL variable'
 
 SELECT COUNT(@@GLOBAL.innodb_data_home_dir);
---echo 1 Expected
+--echo 0 Expected
 
 --Error ER_BAD_FIELD_ERROR
 SELECT innodb_data_home_dir = @@SESSION.innodb_data_home_dir;

mysql-test/suite/sys_vars/t/innodb_flush_method_basic.test

@@ -29,7 +29,7 @@
 #   Displaying default value                                       #
 ####################################################################
 SELECT COUNT(@@GLOBAL.innodb_flush_method);
---echo 1 Expected
+--echo 0 Expected
 
 
 --echo '#---------------------BS_STVARS_029_02----------------------#'
@@ -42,7 +42,7 @@ SET @@GLOBAL.innodb_flush_method=1;
 --echo Expected error 'Read only variable'
 
 SELECT COUNT(@@GLOBAL.innodb_flush_method);
---echo 1 Expected
+--echo 0 Expected
 
 
 
@@ -58,7 +58,7 @@ WHERE VARIABLE_NAME='innodb_flush_method';
 --echo 1 Expected
 
 SELECT COUNT(@@GLOBAL.innodb_flush_method);
---echo 1 Expected
+--echo 0 Expected
 
 SELECT COUNT(VARIABLE_VALUE)
 FROM INFORMATION_SCHEMA.GLOBAL_VARIABLES 
@@ -82,7 +82,7 @@ SELECT @@innodb_flush_method = @@GLOBAL.innodb_flush_method;
 ################################################################################
 
 SELECT COUNT(@@innodb_flush_method);
---echo 1 Expected
+--echo 0 Expected
 
 --Error ER_INCORRECT_GLOBAL_LOCAL_VAR
 SELECT COUNT(@@local.innodb_flush_method);
@@ -93,7 +93,7 @@ SELECT COUNT(@@SESSION.innodb_flush_method);
 --echo Expected error 'Variable is a GLOBAL variable'
 
 SELECT COUNT(@@GLOBAL.innodb_flush_method);
---echo 1 Expected
+--echo 0 Expected
 
 --Error ER_BAD_FIELD_ERROR
 SELECT innodb_flush_method = @@SESSION.innodb_flush_method;

mysql-test/suite/sys_vars/t/ssl_capath_basic.test

@@ -27,7 +27,7 @@
 #   Displaying default value                                       #
 ####################################################################
 SELECT COUNT(@@GLOBAL.ssl_capath);
---echo 1 Expected
+--echo 0 Expected
 
 
 --echo '#---------------------BS_STVARS_046_02----------------------#'
@@ -40,7 +40,7 @@ SET @@GLOBAL.ssl_capath=1;
 --echo Expected error 'Read only variable'
 
 SELECT COUNT(@@GLOBAL.ssl_capath);
---echo 1 Expected
+--echo 0 Expected
 
 
 
@@ -56,7 +56,7 @@ WHERE VARIABLE_NAME='ssl_capath';
 --echo 1 Expected
 
 SELECT COUNT(@@GLOBAL.ssl_capath);
---echo 1 Expected
+--echo 0 Expected
 
 SELECT COUNT(VARIABLE_VALUE)
 FROM INFORMATION_SCHEMA.GLOBAL_VARIABLES 
@@ -80,7 +80,7 @@ SELECT @@ssl_capath = @@GLOBAL.ssl_capath;
 ################################################################################
 
 SELECT COUNT(@@ssl_capath);
---echo 1 Expected
+--echo 0 Expected
 
 --Error ER_INCORRECT_GLOBAL_LOCAL_VAR
 SELECT COUNT(@@local.ssl_capath);
@@ -91,7 +91,7 @@ SELECT COUNT(@@SESSION.ssl_capath);
 --echo Expected error 'Variable is a GLOBAL variable'
 
 SELECT COUNT(@@GLOBAL.ssl_capath);
---echo 1 Expected
+--echo 0 Expected
 
 --Error ER_BAD_FIELD_ERROR
 SELECT ssl_capath = @@SESSION.ssl_capath;

mysql-test/suite/sys_vars/t/ssl_cipher_basic.test

@@ -27,7 +27,7 @@
 #   Displaying default value                                       #
 ####################################################################
 SELECT COUNT(@@GLOBAL.ssl_cipher);
---echo 1 Expected
+--echo 0 Expected
 
 
 --echo '#---------------------BS_STVARS_048_02----------------------#'
@@ -40,7 +40,7 @@ SET @@GLOBAL.ssl_cipher=1;
 --echo Expected error 'Read only variable'
 
 SELECT COUNT(@@GLOBAL.ssl_cipher);
---echo 1 Expected
+--echo 0 Expected
 
 
 
@@ -56,7 +56,7 @@ WHERE VARIABLE_NAME='ssl_cipher';
 --echo 1 Expected
 
 SELECT COUNT(@@GLOBAL.ssl_cipher);
---echo 1 Expected
+--echo 0 Expected
 
 SELECT COUNT(VARIABLE_VALUE)
 FROM INFORMATION_SCHEMA.GLOBAL_VARIABLES 
@@ -80,7 +80,7 @@ SELECT @@ssl_cipher = @@GLOBAL.ssl_cipher;
 ################################################################################
 
 SELECT COUNT(@@ssl_cipher);
---echo 1 Expected
+--echo 0 Expected
 
 --Error ER_INCORRECT_GLOBAL_LOCAL_VAR
 SELECT COUNT(@@local.ssl_cipher);
@@ -91,7 +91,7 @@ SELECT COUNT(@@SESSION.ssl_cipher);
 --echo Expected error 'Variable is a GLOBAL variable'
 
 SELECT COUNT(@@GLOBAL.ssl_cipher);
---echo 1 Expected
+--echo 0 Expected
 
 --Error ER_BAD_FIELD_ERROR
 SELECT ssl_cipher = @@SESSION.ssl_cipher;

mysql-test/t/variables.test

@@ -1201,4 +1201,23 @@ SET GLOBAL server_id = -1;
 SELECT @@GLOBAL.server_id;
 SET GLOBAL server_id = @old_server_id;
 
+#
+# Bug #42778: delete order by null global variable causes 
+#             assertion .\filesort.cc, line 797
+#
+
+SELECT @@GLOBAL.INIT_FILE, @@GLOBAL.INIT_FILE IS NULL;
+SELECT @@GLOBAL.REPORT_HOST, @@GLOBAL.REPORT_HOST IS NULL;
+SELECT @@GLOBAL.REPORT_PASSWORD, @@GLOBAL.REPORT_PASSWORD IS NULL;
+SELECT @@GLOBAL.REPORT_USER, @@GLOBAL.REPORT_USER IS NULL;
+
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES ();
+SET @bug42778= @@sql_safe_updates;
+SET @@sql_safe_updates= 0;
+DELETE FROM t1 ORDER BY (@@GLOBAL.INIT_FILE) ASC LIMIT 10;
+SET @@sql_safe_updates= @bug42778;
+
+DROP TABLE t1;
+
 --echo End of 5.1 tests

sql/item.cc

@@ -2472,8 +2472,9 @@ longlong_from_string_with_check (CHARSET_INFO *cs, const char *cptr, char *end)
     TODO: Give error if we wanted a signed integer and we got an unsigned
     one
   */
-  if (err > 0 ||
-      (end != org_end && !check_if_only_end_space(cs, end, org_end)))
+  if (!current_thd->no_errors &&
+      (err > 0 ||
+       (end != org_end && !check_if_only_end_space(cs, end, org_end))))
   {
     push_warning_printf(current_thd, MYSQL_ERROR::WARN_LEVEL_WARN,
                         ER_TRUNCATED_WRONG_VALUE,

sql/item_func.cc

@@ -4880,10 +4880,20 @@ bool Item_func_get_system_var::is_written_to_binlog()
 }
 
 
+void Item_func_get_system_var::update_null_value()
+{
+  THD *thd= current_thd;
+  int save_no_errors= thd->no_errors;
+  thd->no_errors= TRUE;
+  Item::update_null_value();
+  thd->no_errors= save_no_errors;
+}
+
+
 void Item_func_get_system_var::fix_length_and_dec()
 {
   char *cptr;
-  maybe_null=0;
+  maybe_null= TRUE;
   max_length= 0;
 
   if (var->check_type(var_type))

sql/item_func.h

@@ -1455,6 +1455,7 @@ public:
                            LEX_STRING *component_arg, const char *name_arg,
                            size_t name_len_arg);
   enum Functype functype() const { return GSYSVAR_FUNC; }
+  void update_null_value();
   void fix_length_and_dec();
   void print(String *str, enum_query_type query_type);
   bool const_item() const { return true; }