a fix (bug #10339: Incorrect error is displayed if try to set value of internal ' warning_count ').

mysql-test/r/variables.result

@@ -526,3 +526,7 @@ show warnings;
 Level	Code	Message
 Warning	1329	No data to FETCH
 drop table t1;
+set @@warning_count=1;
+ERROR HY000: Variable 'warning_count' is a read only variable
+set @@global.error_count=1;
+ERROR HY000: Variable 'error_count' is a read only variable

mysql-test/t/variables.test

@@ -395,3 +395,11 @@ select a into @x from t1;
 show warnings;
 drop table t1;
 
+#
+# Bug #10339: read only variables.
+#
+
+--error 1238
+set @@warning_count=1;
+--error 1238
+set @@global.error_count=1;

sql/set_var.cc

@@ -2964,6 +2964,11 @@ bool not_all_support_one_shot(List<set_var_base> *var_list)
 
 int set_var::check(THD *thd)
 {
+  if (var->is_readonly())
+  {
+    my_error(ER_INCORRECT_GLOBAL_LOCAL_VAR, MYF(0), var->name, "read only");
+    return -1;
+  }
   if (var->check_type(type))
   {
     int err= type == OPT_GLOBAL ? ER_LOCAL_VARIABLE : ER_GLOBAL_VARIABLE;

sql/set_var.h

@@ -75,6 +75,7 @@ public:
   { return option_limits == 0; }
   Item *item(THD *thd, enum_var_type type, LEX_STRING *base);
   virtual bool is_struct() { return 0; }
+  virtual bool is_readonly() const { return 0; }
 };
 
 
@@ -699,6 +700,7 @@ public:
     return (*value_ptr_func)(thd);
   }
   SHOW_TYPE type() { return show_type; }
+  bool is_readonly() const { return 1; }
 };
 
 class sys_var_thd_time_zone :public sys_var_thd