Bug#12363

character_set_results is nullable, but value_ptr returns string "NULL"

set_var.cc:
  Create Item_null instead of Item_string for NULL values

variables.result, variables.test:
  adding test case

mysql-test/r/variables.result

@@ -491,3 +491,8 @@ SHOW VARIABLES LIKE 'table_cache';
 Variable_name	Value
 table_cache	1
 SET GLOBAL table_cache=DEFAULT;
+set character_set_results=NULL;
+select ifnull(@@character_set_results,"really null");
+ifnull(@@character_set_results,"really null")
+really null
+set names latin1;

mysql-test/t/variables.test

@@ -380,4 +380,12 @@ SET GLOBAL table_cache=-1;
 SHOW VARIABLES LIKE 'table_cache';
 SET GLOBAL table_cache=DEFAULT;
 
+#
+# Bugs12363: character_set_results is nullable,
+# but value_ptr returns string "NULL"
+#
+set character_set_results=NULL;
+select ifnull(@@character_set_results,"really null");
+set names latin1;
+
 # End of 4.1 tests

sql/set_var.cc

@@ -1598,11 +1598,17 @@ Item *sys_var::item(THD *thd, enum_var_type var_type, LEX_STRING *base)
     return new Item_int((int32) *(my_bool*) value_ptr(thd, var_type, base),1);
   case SHOW_CHAR:
   {
-    Item_string *tmp;
+    Item *tmp;
     pthread_mutex_lock(&LOCK_global_system_variables);
     char *str= (char*) value_ptr(thd, var_type, base);
-    tmp= new Item_string(str, strlen(str),
-                         system_charset_info, DERIVATION_SYSCONST);
+    if (str)
+      tmp= new Item_string(str, strlen(str),
+                           system_charset_info, DERIVATION_SYSCONST);
+    else
+    {
+      tmp= new Item_null();
+      tmp->collation.set(system_charset_info, DERIVATION_SYSCONST);
+    }
     pthread_mutex_unlock(&LOCK_global_system_variables);
     return tmp;
   }
@@ -1892,7 +1898,7 @@ byte *sys_var_character_set::value_ptr(THD *thd, enum_var_type type,
 				       LEX_STRING *base)
 {
   CHARSET_INFO *cs= ci_ptr(thd,type)[0];
-  return cs ? (byte*) cs->csname : (byte*) "NULL";
+  return cs ? (byte*) cs->csname : (byte*) NULL;
 }