Bug #30651 Problems with thread_handling system variable

Changed thread_handling variable to a global only, read only variable, as it is currently used.

mysql-test/r/no-threads.result

@@ -7,3 +7,7 @@ select 1+2;
 SHOW GLOBAL VARIABLES LIKE 'thread_handling';
 Variable_name	Value
 thread_handling	no-threads
+select @@session.thread_handling;
+ERROR HY000: Variable 'thread_handling' is a GLOBAL variable
+set GLOBAL thread_handling='one-thread';
+ERROR HY000: Variable 'thread_handling' is a read only variable

mysql-test/t/no-threads.test

@@ -4,3 +4,13 @@
 select 1+1;
 select 1+2;
 SHOW GLOBAL VARIABLES LIKE 'thread_handling';
+
+#
+# Bug #30651	Problems with thread_handling system variable
+#
+
+--error ER_INCORRECT_GLOBAL_LOCAL_VAR
+select @@session.thread_handling;
+
+--error ER_INCORRECT_GLOBAL_LOCAL_VAR
+set GLOBAL thread_handling='one-thread';

sql/set_var.cc

@@ -384,7 +384,7 @@ static sys_var_thd_ulong	sys_trans_alloc_block_size(&vars, "transaction_alloc_bl
 static sys_var_thd_ulong	sys_trans_prealloc_size(&vars, "transaction_prealloc_size",
 						&SV::trans_prealloc_size,
 						0, fix_trans_mem_root);
-sys_var_thd_enum        sys_thread_handling(&vars, "thread_handling",
+sys_var_enum_const      sys_thread_handling(&vars, "thread_handling",
                                             &SV::thread_handling,
                                             &thread_handling_typelib,
                                             NULL);
@@ -1183,6 +1183,13 @@ uchar *sys_var_enum::value_ptr(THD *thd, enum_var_type type, LEX_STRING *base)
   return (uchar*) enum_names->type_names[*value];
 }
 
+
+uchar *sys_var_enum_const::value_ptr(THD *thd, enum_var_type type,
+                                     LEX_STRING *base)
+{
+  return (uchar*) enum_names->type_names[global_system_variables.*offset];
+}
+
 bool sys_var_thd_ulong::check(THD *thd, set_var *var)
 {
   return (sys_var_thd::check(thd, var) ||

sql/set_var.h

@@ -305,6 +305,24 @@ public:
 };
 
 
+class sys_var_enum_const :public sys_var
+{
+  ulong SV::*offset;
+  TYPELIB *enum_names;
+public:
+  sys_var_enum_const(sys_var_chain *chain, const char *name_arg, ulong SV::*offset_arg,
+      TYPELIB *typelib, sys_after_update_func func)
+    :sys_var(name_arg,func), offset(offset_arg), enum_names(typelib)
+  { chain_sys_var(chain); }
+  bool check(THD *thd, set_var *var) { return 1; }
+  bool update(THD *thd, set_var *var) { return 1; }
+  SHOW_TYPE show_type() { return SHOW_CHAR; }
+  bool check_update_type(Item_result type) { return 1; }
+  bool is_readonly() const { return 1; }
+  uchar *value_ptr(THD *thd, enum_var_type type, LEX_STRING *base);
+};
+
+
 class sys_var_thd :public sys_var
 {
 public: