Fix Item_func_abs::fix_length_and_dec() to set maybe_null properly. (Bug #14009)

mysql-test/r/func_math.result:
  Add new results
mysql-test/t/func_math.test:
  Add new regression test
sql/item_func.cc:
  Set maybe_null in Item_func_abs::fix_length_and_dec().

mysql-test/r/func_math.result

@@ -145,3 +145,9 @@ insert into t1 values (1);
 select rand(i) from t1;
 ERROR HY000: Incorrect arguments to RAND
 drop table t1;
+create table t1 (a varchar(90), ts datetime not null, index (a)) engine=innodb default charset=utf8;
+insert into t1 values ('http://www.foo.com/', now());
+select a from t1 where a='http://www.foo.com/' order by abs(timediff(ts, 0));
+a
+http://www.foo.com/
+drop table t1;

mysql-test/t/func_math.test

@@ -86,4 +86,14 @@ insert into t1 values (1);
 select rand(i) from t1;
 drop table t1;
 
+#
+# Bug #14009: use of abs() on null value causes problems with filesort
+#
+# InnoDB is required to reproduce the fault, but it is okay if we default to
+# MyISAM when testing.
+create table t1 (a varchar(90), ts datetime not null, index (a)) engine=innodb default charset=utf8;
+insert into t1 values ('http://www.foo.com/', now());
+select a from t1 where a='http://www.foo.com/' order by abs(timediff(ts, 0));
+drop table t1;
+
 # End of 4.1 tests

sql/item_func.cc

@@ -766,6 +766,7 @@ void Item_func_abs::fix_length_and_dec()
   hybrid_type= REAL_RESULT;
   if (args[0]->result_type() == INT_RESULT)
     hybrid_type= INT_RESULT;
+  maybe_null= 1;
 }