Fix for bug#4105 "Server crash on attempt to prepare a statement with

character set introducer": add new item type to be returned before from 
Item_param until it's value is set.
This way items like Item_bool_func2 and udf_handler won't treat this item
as constant literal when statement is prepared.

mysql-test/r/ps.result

@@ -113,3 +113,9 @@ execute stmt1 using @ivar;
 ?
 1234
 drop table t1,t2;
+PREPARE stmt1 FROM "select _utf8 'A' collate utf8_bin = ?";
+set @var='A';
+EXECUTE stmt1 USING @var;
+_utf8 'A' collate utf8_bin = ?
+1
+DEALLOCATE PREPARE stmt1;

mysql-test/t/ps.test

@@ -116,4 +116,11 @@ prepare stmt1 from @str2;
 execute stmt1 using @ivar;
 
 drop table t1,t2;
-
+#
+# Bug #4105: Server crash on attempt to prepare a statement with character
+# set introducer
+#
+PREPARE stmt1 FROM "select _utf8 'A' collate utf8_bin = ?";
+set @var='A';
+EXECUTE stmt1 USING @var;
+DEALLOCATE PREPARE stmt1;

sql/item.cc

@@ -628,7 +628,8 @@ default_set_param_func(Item_param *param,
 Item_param::Item_param(unsigned pos_in_query_arg) :
   state(NO_VALUE),
   item_result_type(STRING_RESULT),
-  item_type(STRING_ITEM),
+  /* Don't pretend to be a literal unless value for this item is set. */
+  item_type(PARAM_ITEM),
   param_type(MYSQL_TYPE_STRING),
   pos_in_query(pos_in_query_arg),
   set_param_func(default_set_param_func)
@@ -827,6 +828,15 @@ void Item_param::reset()
   state= NO_VALUE;
   maybe_null= 1;
   null_value= 0;
+  /*
+    Don't reset item_type to PARAM_ITEM: it's only needed to guard
+    us from item optimizations at prepare stage, when item doesn't yet
+    contain a literal of some kind.
+    In all other cases when this object is accessed its value is
+    set (this assumption is guarded by 'state' and
+    DBUG_ASSERTS(state != NO_VALUE) in all Item_param::get_*
+    methods).
+  */
 }
 
 

sql/item.h

@@ -98,7 +98,8 @@ class Item {
 	     COPY_STR_ITEM, FIELD_AVG_ITEM, DEFAULT_VALUE_ITEM,
 	     PROC_ITEM,COND_ITEM, REF_ITEM, FIELD_STD_ITEM,
 	     FIELD_VARIANCE_ITEM, INSERT_VALUE_ITEM,
-             SUBSELECT_ITEM, ROW_ITEM, CACHE_ITEM, TYPE_HOLDER};
+             SUBSELECT_ITEM, ROW_ITEM, CACHE_ITEM, TYPE_HOLDER,
+             PARAM_ITEM};
 
   enum cond_result { COND_UNDEF,COND_OK,COND_TRUE,COND_FALSE };