A fix and a test case for Bug#9359 "Prepared statements take snapshot

 of system vars at PREPARE time": implement a special Item
to handle system variables. This item substitutes itself with 
a basic constant containing variable value at fix_fields.

mysql-test/r/ps.result

@@ -676,3 +676,28 @@ ERROR 42000: You have an error in your SQL syntax; check the manual that corresp
 select ? from t1;
 ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '? from t1' at line 1
 drop table t1;
+prepare stmt from "select @@time_zone";
+execute stmt;
+@@time_zone
+SYSTEM
+set @@time_zone:='Japan';
+execute stmt;
+@@time_zone
+Japan
+prepare stmt from "select @@tx_isolation";
+execute stmt;
+@@tx_isolation
+REPEATABLE-READ
+set transaction isolation level read committed;
+execute stmt;
+@@tx_isolation
+READ-COMMITTED
+set transaction isolation level serializable;
+execute stmt;
+@@tx_isolation
+SERIALIZABLE
+set @@tx_isolation=default;
+execute stmt;
+@@tx_isolation
+REPEATABLE-READ
+deallocate prepare stmt;

mysql-test/t/ps.test

@@ -702,4 +702,20 @@ select ??;
 select ? from t1;
 --enable_ps_protocol
 drop table t1;
-# 
+#
+# Bug#9359 "Prepared statements take snapshot of system vars at PREPARE
+# time"
+#
+prepare stmt from "select @@time_zone";
+execute stmt;
+set @@time_zone:='Japan';
+execute stmt;
+prepare stmt from "select @@tx_isolation";
+execute stmt;
+set transaction isolation level read committed;
+execute stmt;
+set transaction isolation level serializable;
+execute stmt;
+set @@tx_isolation=default;
+execute stmt;
+deallocate prepare stmt;

sql/item_func.cc

@@ -3031,6 +3031,36 @@ bool Item_func_get_user_var::eq(const Item *item, bool binary_cmp) const
 }
 
 
+Item_func_get_system_var::
+Item_func_get_system_var(sys_var *var_arg, enum_var_type var_type_arg,
+                       LEX_STRING *component_arg, const char *name_arg,
+                       size_t name_len_arg)
+  :var(var_arg), var_type(var_type_arg), component(*component_arg)
+{
+  /* set_name() will allocate the name */
+  set_name(name_arg, name_len_arg, system_charset_info);
+}
+
+
+bool
+Item_func_get_system_var::fix_fields(THD *thd, TABLE_LIST *tables, Item **ref)
+{
+  Item *item= var->item(thd, var_type, &component);
+  DBUG_ENTER("Item_func_get_system_var::fix_fields");
+  /*
+    Evaluate the system variable and substitute the result (a basic constant)
+    instead of this item. If the variable can not be evaluated,
+    the error is reported in sys_var::item().
+  */
+  if (item == 0)
+    DBUG_RETURN(1);                             // Impossible
+  item->set_name(name, 0, system_charset_info); // don't allocate a new name
+  thd->change_item_tree(ref, item);
+
+  DBUG_RETURN(0);
+}
+
+
 longlong Item_func_inet_aton::val_int()
 {
   DBUG_ASSERT(fixed == 1);
@@ -3375,22 +3405,21 @@ longlong Item_func_bit_xor::val_int()
     0	error
     #	constant item
 */
-  
+
 
 Item *get_system_var(THD *thd, enum_var_type var_type, LEX_STRING name,
 		     LEX_STRING component)
 {
+  sys_var *var;
+  char buff[MAX_SYS_VAR_LENGTH*2+4+8], *pos;
+  LEX_STRING *base_name, *component_name;
+
   if (component.str == 0 &&
       !my_strcasecmp(system_charset_info, name.str, "VERSION"))
     return new Item_string("@@VERSION", server_version,
 			   (uint) strlen(server_version),
 			   system_charset_info, DERIVATION_SYSCONST);
 
-  Item *item;
-  sys_var *var;
-  char buff[MAX_SYS_VAR_LENGTH*2+4+8], *pos;
-  LEX_STRING *base_name, *component_name;
-
   if (component.str)
   {
     base_name= &component;
@@ -3412,9 +3441,8 @@ Item *get_system_var(THD *thd, enum_var_type var_type, LEX_STRING name,
       return 0;
     }
   }
-  if (!(item=var->item(thd, var_type, component_name)))
-    return 0;					// Impossible
   thd->lex->uncacheable(UNCACHEABLE_SIDEEFFECT);
+
   buff[0]='@';
   buff[1]='@';
   pos=buff+2;
@@ -3435,28 +3463,8 @@ Item *get_system_var(THD *thd, enum_var_type var_type, LEX_STRING name,
   memcpy(pos, base_name->str, base_name->length);
   pos+= base_name->length;
 
-  // set_name() will allocate the name
-  item->set_name(buff,(uint) (pos-buff), system_charset_info);
-  return item;
-}
-
-
-Item *get_system_var(THD *thd, enum_var_type var_type, const char *var_name,
-		     uint length, const char *item_name)
-{
-  Item *item;
-  sys_var *var;
-  LEX_STRING null_lex_string;
-
-  null_lex_string.str= 0;
-
-  var= find_sys_var(var_name, length);
-  DBUG_ASSERT(var != 0);
-  if (!(item=var->item(thd, var_type, &null_lex_string)))
-    return 0;						// Impossible
-  thd->lex->uncacheable(UNCACHEABLE_SIDEEFFECT);
-  item->set_name(item_name, 0, system_charset_info);	// Will use original name
-  return item;
+  return new Item_func_get_system_var(var, var_type, component_name,
+                                      buff, pos - buff);
 }
 
 

sql/item_func.h

@@ -988,6 +988,29 @@ class Item_func_get_user_var :public Item_func
 };
 
 
+/* A system variable */
+
+class Item_func_get_system_var :public Item_func
+{
+  sys_var *var;
+  enum_var_type var_type;
+  LEX_STRING component;
+public:
+  Item_func_get_system_var(sys_var *var_arg, enum_var_type var_type_arg,
+                           LEX_STRING *component_arg, const char *name_arg,
+                           size_t name_len_arg);
+  bool fix_fields(THD *thd, TABLE_LIST *tables, Item **ref);
+  /*
+    Stubs for pure virtual methods. Should never be called: this
+    item is always substituted with a constant in fix_fields().
+  */
+  double val()              { DBUG_ASSERT(0); return 0.0; }
+  longlong val_int()        { DBUG_ASSERT(0); return 0; }
+  String* val_str(String*)  { DBUG_ASSERT(0); return 0; }
+  void fix_length_and_dec() { DBUG_ASSERT(0); }
+};
+
+
 class Item_func_inet_aton : public Item_int_func
 {
 public:

sql/mysql_priv.h

@@ -358,6 +358,11 @@ inline THD *_current_thd(void)
 #include "protocol.h"
 #include "sql_udf.h"
 class user_var_entry;
+enum enum_var_type
+{
+  OPT_DEFAULT, OPT_SESSION, OPT_GLOBAL
+};
+class sys_var;
 #include "item.h"
 typedef Comp_creator* (*chooser_compare_func_creator)(bool invert);
 /* sql_parse.cc */
@@ -1119,12 +1124,9 @@ extern bool sql_cache_init();
 extern void sql_cache_free();
 extern int sql_cache_hit(THD *thd, char *inBuf, uint length);
 
-/* item.cc */
+/* item_func.cc */
 Item *get_system_var(THD *thd, enum_var_type var_type, LEX_STRING name,
 		     LEX_STRING component);
-Item *get_system_var(THD *thd, enum_var_type var_type, const char *var_name,
-		     uint length, const char *item_name);
-/* item_func.cc */
 int get_var_with_binlog(THD *thd, LEX_STRING &name,
                         user_var_entry **out_entry);
 /* log.cc */

sql/set_var.cc

@@ -1551,15 +1551,7 @@ bool sys_var::check_set(THD *thd, set_var *var, TYPELIB *enum_names)
 
 /*
   Return an Item for a variable.  Used with @@[global.]variable_name
-
   If type is not given, return local value if exists, else global
-
-  We have to use netprintf() instead of my_error() here as this is
-  called on the parsing stage.
-
-  TODO:
-    With prepared statements/stored procedures this has to be fixed
-    to create an item that gets the current value at fix_fields() stage.
 */
 
 Item *sys_var::item(THD *thd, enum_var_type var_type, LEX_STRING *base)
@@ -1568,8 +1560,8 @@ Item *sys_var::item(THD *thd, enum_var_type var_type, LEX_STRING *base)
   {
     if (var_type != OPT_DEFAULT)
     {
-      net_printf(thd, ER_INCORRECT_GLOBAL_LOCAL_VAR,
-		 name, var_type == OPT_GLOBAL ? "SESSION" : "GLOBAL");
+      my_error(ER_INCORRECT_GLOBAL_LOCAL_VAR, MYF(0),
+               name, var_type == OPT_GLOBAL ? "SESSION" : "GLOBAL");
       return 0;
     }
     /* As there was no local variable, return the global value */
@@ -1613,7 +1605,7 @@ Item *sys_var::item(THD *thd, enum_var_type var_type, LEX_STRING *base)
     return tmp;
   }
   default:
-    net_printf(thd, ER_VAR_CANT_BE_READ, name);
+    my_error(ER_VAR_CANT_BE_READ, MYF(0), name);
   }
   return 0;
 }

sql/set_var.h

@@ -30,11 +30,6 @@ class set_var;
 typedef struct system_variables SV;
 extern TYPELIB bool_typelib, delay_key_write_typelib, sql_mode_typelib;
 
-enum enum_var_type
-{
-  OPT_DEFAULT, OPT_SESSION, OPT_GLOBAL
-};
-
 typedef int (*sys_check_func)(THD *,  set_var *);
 typedef bool (*sys_update_func)(THD *, set_var *);
 typedef void (*sys_after_update_func)(THD *,enum_var_type);