Fix for bug #32137: prepared statement crash with str_to_date in update clause

Problem: calling non-constant argument's val_xxx() methods 
in the ::fix_length_and_dec() is inadmissible.

Fix: call the method only for constant arguments.

mysql-test/r/ps.result

@@ -1109,4 +1109,9 @@ a
 13
 DEALLOCATE PREPARE st1;
 DROP TABLE t1;
+create table t1 (a int, b tinyint);
+prepare st1 from 'update t1 set b= (str_to_date(a, a))';
+execute st1;
+deallocate prepare st1;
+drop table t1;
 End of 4.1 tests.

mysql-test/t/ps.test

@@ -1146,4 +1146,13 @@ EXECUTE st1;
 DEALLOCATE PREPARE st1;
 DROP TABLE t1;
 
+#
+# Bug #32137: prepared statement crash with str_to_date in update clause
+#
+create table t1 (a int, b tinyint);
+prepare st1 from 'update t1 set b= (str_to_date(a, a))';
+execute st1;
+deallocate prepare st1;
+drop table t1;
+
 --echo End of 4.1 tests.

sql/item_timefunc.cc

@@ -2958,16 +2958,17 @@ Field *Item_func_str_to_date::tmp_table_field(TABLE *t_arg)
 
 void Item_func_str_to_date::fix_length_and_dec()
 {
-  char format_buff[64];
-  String format_str(format_buff, sizeof(format_buff), &my_charset_bin);
-  String *format;
   maybe_null= 1;
   decimals=0;
   cached_field_type= MYSQL_TYPE_STRING;
   max_length= MAX_DATETIME_FULL_WIDTH*MY_CHARSET_BIN_MB_MAXLEN;
   cached_timestamp_type= MYSQL_TIMESTAMP_NONE;
-  format= args[1]->val_str(&format_str);
-  if (!args[1]->null_value && (const_item= args[1]->const_item()))
+  if ((const_item= args[1]->const_item()))
+  {
+    char format_buff[64];
+    String format_str(format_buff, sizeof(format_buff), &my_charset_bin);
+    String *format= args[1]->val_str(&format_str);
+    if (!args[1]->null_value)
     {
       cached_format_type= get_date_time_result_type(format->ptr(),
                                                     format->length());
@@ -2975,13 +2976,13 @@ void Item_func_str_to_date::fix_length_and_dec()
       case DATE_ONLY:
         cached_timestamp_type= MYSQL_TIMESTAMP_DATE;
         cached_field_type= MYSQL_TYPE_DATE; 
-      max_length= MAX_DATE_WIDTH*MY_CHARSET_BIN_MB_MAXLEN;
+        max_length= MAX_DATE_WIDTH * MY_CHARSET_BIN_MB_MAXLEN;
         break;
       case TIME_ONLY:
       case TIME_MICROSECOND:
         cached_timestamp_type= MYSQL_TIMESTAMP_TIME;
         cached_field_type= MYSQL_TYPE_TIME; 
-      max_length= MAX_TIME_WIDTH*MY_CHARSET_BIN_MB_MAXLEN;
+        max_length= MAX_TIME_WIDTH * MY_CHARSET_BIN_MB_MAXLEN;
         break;
       default:
         cached_timestamp_type= MYSQL_TIMESTAMP_DATETIME;
@@ -2989,8 +2990,10 @@ void Item_func_str_to_date::fix_length_and_dec()
         break;
       }
     }
+  }
 }
 
+
 bool Item_func_str_to_date::get_date(TIME *ltime, uint fuzzy_date)
 {
   DATE_TIME_FORMAT date_time_format;