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.

mysql-test/r/func_time.result

@@ -136,7 +136,7 @@ dayname("1962-03-03")	dayname("1962-03-03")+0
 Saturday	5
 select monthname("1972-03-04"),monthname("1972-03-04")+0;
 monthname("1972-03-04")	monthname("1972-03-04")+0
-March	3
+March	0
 select time_format(19980131000000,'%H|%I|%k|%l|%i|%p|%r|%S|%T');
 time_format(19980131000000,'%H|%I|%k|%l|%i|%p|%r|%S|%T')
 00|12|0|12|00|AM|12:00:00 AM|00|00:00:00
@@ -1368,3 +1368,11 @@ SELECT SUBDATE(STR_TO_DATE(NULL,0), INTERVAL 1 HOUR);
 SUBDATE(STR_TO_DATE(NULL,0), INTERVAL 1 HOUR)
 NULL
 #
+# BUG#59895 - setting storage engine to null segfaults mysqld
+#
+SELECT MONTHNAME(0), MONTHNAME(0) IS NULL, MONTHNAME(0) + 1;
+MONTHNAME(0)	MONTHNAME(0) IS NULL	MONTHNAME(0) + 1
+NULL	1	NULL
+SET storage_engine=NULL;
+ERROR 42000: Variable 'storage_engine' can't be set to the value of 'NULL'
+#

mysql-test/t/func_time.test

@@ -881,4 +881,11 @@ SELECT WEEK(STR_TO_DATE(NULL,0));
 SELECT SUBDATE(STR_TO_DATE(NULL,0), INTERVAL 1 HOUR);
 
 --echo #
+--echo # BUG#59895 - setting storage engine to null segfaults mysqld
+--echo #
+SELECT MONTHNAME(0), MONTHNAME(0) IS NULL, MONTHNAME(0) + 1;
+--error ER_WRONG_VALUE_FOR_VAR
+SET storage_engine=NULL;
+
+--echo #
 

sql/item_timefunc.cc

@@ -1133,16 +1133,13 @@ String* Item_func_monthname::val_str(String* str)
 {
   DBUG_ASSERT(fixed == 1);
   const char *month_name;
-  uint month= (uint) val_int();
   uint err;
+  MYSQL_TIME ltime;
 
-  if (null_value || !month)
-  {
-    null_value=1;
-    return (String*) 0;
-  }
-  null_value=0;
-  month_name= locale->month_names->type_names[month-1];
+  if ((null_value= (get_arg0_date(&ltime, TIME_FUZZY_DATE) || !ltime.month)))
+    return (String *) 0;
+
+  month_name= locale->month_names->type_names[ltime.month - 1];
   str->copy(month_name, (uint) strlen(month_name), &my_charset_utf8_bin,
 	    collation.collation, &err);
   return str;

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();
+  }
 };