Bug #26303: Reserve is not called before qs_append().

This may lead to buffer overflow.
The String::qs_append() function will append a string
without checking if there's enough space.
So qs_append() must be called beforehand to ensure 
there's enough space in the buffer for the subsequent 
qs_append() calls.
Fixed Item_case_expr::print() to make sure there's
enough space before appending data by adding a call to 
String::reserve() to make sure qs_append() will have 
enough space.

mysql-test/r/sp-code.result

@@ -621,3 +621,20 @@ Pos	Instruction
 0	stmt 2 "CREATE INDEX idx ON t1 (c1)"
 DROP PROCEDURE p1;
 End of 5.0 tests.
+CREATE PROCEDURE p1() 
+BEGIN 
+DECLARE dummy int default 0;
+CASE 12 
+WHEN 12 
+THEN SET dummy = 0;
+END CASE;
+END//
+SHOW PROCEDURE CODE p1;
+Pos	Instruction
+0	set dummy@0 0
+1	set_case_expr (6) 0 12
+2	jump_if_not 5(6) (case_expr@0 = 12)
+3	set dummy@0 0
+4	jump 6
+5	error 1339
+DROP PROCEDURE p1;

mysql-test/t/sp-code.test

@@ -447,3 +447,21 @@ DROP PROCEDURE p1;
 
 
 --echo End of 5.0 tests.
+
+#
+# Bug #26303: reserve() not called before qs_append() may lead to buffer
+# overflow
+#
+DELIMITER //;
+CREATE PROCEDURE p1() 
+BEGIN 
+  DECLARE dummy int default 0;
+
+  CASE 12 
+    WHEN 12 
+    THEN SET dummy = 0;
+  END CASE;
+END//
+DELIMITER ;//
+SHOW PROCEDURE CODE p1;
+DROP PROCEDURE p1;

sql/item.cc

@@ -1088,7 +1088,7 @@ bool Item_splocal::set_value(THD *thd, sp_rcontext *ctx, Item **it)
   Item_case_expr methods
 *****************************************************************************/
 
-Item_case_expr::Item_case_expr(int case_expr_id)
+Item_case_expr::Item_case_expr(uint case_expr_id)
   :Item_sp_variable( C_STRING_WITH_LEN("case_expr")),
    m_case_expr_id(case_expr_id)
 {
@@ -1125,6 +1125,8 @@ Item_case_expr::this_item_addr(THD *thd, Item **)
 
 void Item_case_expr::print(String *str)
 {
+  if (str->reserve(MAX_INT_WIDTH + sizeof("case_expr@")))
+    return;                                    /* purecov: inspected */
   VOID(str->append(STRING_WITH_LEN("case_expr@")));
   str->qs_append(m_case_expr_id);
 }

sql/item.h

@@ -1116,7 +1116,7 @@ inline Item_result Item_splocal::result_type() const
 class Item_case_expr :public Item_sp_variable
 {
 public:
-  Item_case_expr(int case_expr_id);
+  Item_case_expr(uint case_expr_id);
 
 public:
   Item *this_item();
@@ -1135,7 +1135,7 @@ class Item_case_expr :public Item_sp_variable
   void print(String *str);
 
 private:
-  int m_case_expr_id;
+  uint m_case_expr_id;
 };
 
 /*****************************************************************************