Fix for Bug#3754 "SET GLOBAL myisam_max_sort_file_size doesn't

work as expected": precision-losing conversion removed from 
sys_var_thd_ulonglong.

mysql-test/r/variables.result

@@ -377,3 +377,7 @@ select 1;
 1
 select @@session.key_buffer_size;
 Variable 'key_buffer_size' is a GLOBAL variable
+set global myisam_max_sort_file_size=4294967296;
+show global variables like 'myisam_max_sort_file_size';
+Variable_name	Value
+myisam_max_sort_file_size	4294967296

mysql-test/t/variables.test

@@ -269,3 +269,9 @@ select 1;
 
 --error 1238
 select @@session.key_buffer_size;
+
+# Bug#3754 SET GLOBAL myisam_max_sort_file_size doesn't work as
+# expected: check that there is no overflow when 64-bit unsigned
+# variables are set
+set global myisam_max_sort_file_size=4294967296;
+show global variables like 'myisam_max_sort_file_size';

sql/set_var.cc

@@ -965,11 +965,11 @@ bool sys_var_thd_ulonglong::update(THD *thd,  set_var *var)
 {
   ulonglong tmp= var->value->val_int();
 
-  if ((ulonglong) tmp > max_system_variables.*offset)
+  if (tmp > max_system_variables.*offset)
     tmp= max_system_variables.*offset;
 
   if (option_limits)
-    tmp= (ulong) getopt_ull_limit_value(tmp, option_limits);
+    tmp= getopt_ull_limit_value(tmp, option_limits);
   if (var->type == OPT_GLOBAL)
   {
     /* Lock is needed to make things safe on 32 bit systems */