BUG#11766720 - setting storage engine to null segfaults mysqld

MONTHNAME(0) claims that it is about to return NOT NULL
value, whereas it actually returns NULL.

As a result storage_engine variable (which cannot be NULL)
protection was bypassed and NULL value was accepted, causing
server crash.

Fixed MONTHNAME(0) to report valid NULL flag.

mysql-test/r/func_time.result:
  A test case for BUG#11766720.
mysql-test/t/func_time.test:
  A test case for BUG#11766720.
sql/item_timefunc.cc:
  MONTHNAME(0) must report NULL, as opposed to base class
  MONTH(0) which is NOT NULL.
  
  Fixed Item_func_monthname to inherit from Item_str_func
  instead of Item_func_month.
sql/item_timefunc.h:
  MONTHNAME(0) must report NULL, as opposed to base class
  MONTH(0) which is NOT NULL.
  
  Fixed Item_func_monthname to inherit from Item_str_func
  instead of Item_func_month.

1 files changed, 6 insertions, 3 deletions

diff --git a/sql/item_timefunc.h b/sql/item_timefunc.h
index 9732e8dc360..92e24d75111 100644
--- a/sql/item_timefunc.h
+++ b/sql/item_timefunc.h
@@ -163,16 +163,19 @@ public:
 };
 
 
-class Item_func_monthname :public Item_func_month
+class Item_func_monthname :public Item_str_func
 {
   MY_LOCALE *locale;
 public:
-  Item_func_monthname(Item *a) :Item_func_month(a) {}
+  Item_func_monthname(Item *a) :Item_str_func(a) {}
   const char *func_name() const { return "monthname"; }
   String *val_str(String *str);
-  enum Item_result result_type () const { return STRING_RESULT; }
   void fix_length_and_dec();
   bool check_partition_func_processor(uchar *int_arg) {return TRUE;}
+  bool check_valid_arguments_processor(uchar *int_arg)
+  {
+    return !has_date_args();
+  }
 };