Fix for bug#11775 Variable character_set_system does not exist (sometimes).

  Variable character_set_system is selectable now
  More accurate error message results during 
  update of character_set_system and some other read only variables

mysql-test/r/variables.result

@@ -545,3 +545,10 @@ select @@max_heap_table_size > 0;
 select @@have_innodb;
 @@have_innodb
 #
+select @@character_set_system;
+@@character_set_system
+utf8
+set global character_set_system = latin1;
+ERROR HY000: Variable 'character_set_system' is a read only variable
+set @@global.version_compile_os='234';
+ERROR HY000: Variable 'version_compile_os' is a read only variable

mysql-test/t/variables.test

@@ -435,3 +435,12 @@ select @@max_heap_table_size > 0;
 
 --replace_column 1 #
 select @@have_innodb;
+
+#
+# Bug #11775 Variable character_set_system does not exist (sometimes)
+#
+select @@character_set_system;
+--error 1238
+set global character_set_system = latin1;
+--error 1238
+set @@global.version_compile_os='234';

sql/set_var.cc

@@ -142,10 +142,7 @@ sys_var_long_ptr	sys_binlog_cache_size("binlog_cache_size",
 sys_var_thd_ulong	sys_bulk_insert_buff_size("bulk_insert_buffer_size",
 						  &SV::bulk_insert_buff_size);
 sys_var_character_set_server	sys_character_set_server("character_set_server");
-sys_var_str			sys_charset_system("character_set_system",
-				    sys_check_charset,
-				    sys_update_charset,
-				    sys_set_default_charset,
+sys_var_const_str       sys_charset_system("character_set_system",
                                            (char *)my_charset_utf8_general_ci.name);
 sys_var_character_set_database	sys_character_set_database("character_set_database");
 sys_var_character_set_client  sys_character_set_client("character_set_client");
@@ -569,6 +566,7 @@ sys_var *sys_variables[]=
   &sys_character_set_client,
   &sys_character_set_connection,
   &sys_character_set_results,
+  &sys_charset_system,
   &sys_collation_connection,
   &sys_collation_database,
   &sys_collation_server,
@@ -1117,27 +1115,6 @@ static void sys_default_ftb_syntax(THD *thd, enum_var_type type)
 	  sizeof(ft_boolean_syntax)-1);
 }
 
-/*
-  The following 3 functions need to be changed in 4.1 when we allow
-  one to change character sets
-*/
-
-static int sys_check_charset(THD *thd, set_var *var)
-{
-  return 0;
-}
-
-
-static bool sys_update_charset(THD *thd, set_var *var)
-{
-  return 0;
-}
-
-
-static void sys_set_default_charset(THD *thd, enum_var_type type)
-{
-}
-
 
 /*
   If one sets the LOW_PRIORIY UPDATES flag, we also must change the

sql/set_var.h

@@ -190,6 +190,7 @@ public:
     return 1;
   }
   bool check_default(enum_var_type type) { return 1; }
+  bool is_readonly() const { return 1; }
 };
 
 
@@ -900,7 +901,7 @@ int sql_set_variables(THD *thd, List<set_var_base> *var_list);
 bool not_all_support_one_shot(List<set_var_base> *var_list);
 void fix_delay_key_write(THD *thd, enum_var_type type);
 ulong fix_sql_mode(ulong sql_mode);
-extern sys_var_str sys_charset_system;
+extern sys_var_const_str sys_charset_system;
 extern sys_var_str sys_init_connect;
 extern sys_var_str sys_init_slave;
 extern sys_var_thd_time_zone sys_time_zone;