Bug #20987: str_to_date doesn't accept user variable for specification

str_to_date() would sometimes render NULL if %D was used as rule other than last.
since this was due to two pointers getting mixed up in the server, this behaviour
seemed somewhat non-deterministic at SQL level.

mysql-test/r/func_time.result

@@ -695,3 +695,37 @@ t1	CREATE TABLE `t1` (
   `from_unixtime(1) + 0` double(23,6) default NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 drop table t1;
+SET @df:="%M %D, %Y";
+SELECT DATE_FORMAT('2005-10-31', "%M %D, %Y");
+DATE_FORMAT('2005-10-31', "%M %D, %Y")
+October 31st, 2005
+SELECT DATE_FORMAT('2005-10-31', @df);
+DATE_FORMAT('2005-10-31', @df)
+October 31st, 2005
+SELECT STR_TO_DATE('October 31st, 2005', "%M %D, %Y");
+STR_TO_DATE('October 31st, 2005', "%M %D, %Y")
+2005-10-31
+SELECT STR_TO_DATE('October 31st, 2005', @df);
+STR_TO_DATE('October 31st, 2005', @df)
+2005-10-31
+CREATE TABLE `dt` (
+d datetime,
+ds char(30)
+);
+INSERT INTO `dt` (d) VALUES ('2005-10-31'), ('2005-11-30');
+SET @df:="%M %D, %Y";
+UPDATE dt SET ds = DATE_FORMAT(d, @df);
+SELECT * FROM dt;
+d	ds
+2005-10-31 00:00:00	October 31st, 2005
+2005-11-30 00:00:00	November 30th, 2005
+SELECT d, ds, STR_TO_DATE(ds, @df) FROM dt;
+d	ds	STR_TO_DATE(ds, @df)
+2005-10-31 00:00:00	October 31st, 2005	2005-10-31
+2005-11-30 00:00:00	November 30th, 2005	2005-11-30
+SELECT d, ds, STR_TO_DATE(ds, "%M %D, %Y") FROM dt;
+d	ds	STR_TO_DATE(ds, "%M %D, %Y")
+2005-10-31 00:00:00	October 31st, 2005	2005-10-31
+2005-11-30 00:00:00	November 30th, 2005	2005-11-30
+DROP TABLE `dt`;
+End of 4.1 tests

mysql-test/t/func_time.test

@@ -368,4 +368,34 @@ create table t1 select now() - now(), curtime() - curtime(),
 show create table t1;
 drop table t1;
 
-# End of 4.1 tests
+
+
+#
+# Bug #20987: str_to_date doesn't accept user variable for specification
+#
+
+SET @df:="%M %D, %Y";
+SELECT DATE_FORMAT('2005-10-31', "%M %D, %Y"); 
+SELECT DATE_FORMAT('2005-10-31', @df); 
+SELECT STR_TO_DATE('October 31st, 2005', "%M %D, %Y");
+SELECT STR_TO_DATE('October 31st, 2005', @df);
+
+CREATE TABLE `dt` (
+  d datetime,
+  ds char(30)
+);
+
+INSERT INTO `dt` (d) VALUES ('2005-10-31'), ('2005-11-30');
+SET @df:="%M %D, %Y";
+
+UPDATE dt SET ds = DATE_FORMAT(d, @df);
+
+SELECT * FROM dt;
+SELECT d, ds, STR_TO_DATE(ds, @df) FROM dt;
+SELECT d, ds, STR_TO_DATE(ds, "%M %D, %Y") FROM dt;
+
+DROP TABLE `dt`;
+
+
+
+--echo End of 4.1 tests

sql/item_timefunc.cc

@@ -223,7 +223,7 @@ static bool extract_date_time(DATE_TIME_FORMAT *format,
 	tmp= (char*) val + min(2, val_len);
 	l_time->day= (int) my_strtoll10(val, &tmp, &error);
 	/* Skip 'st, 'nd, 'th .. */
-	val= tmp + min((int) (end-tmp), 2);
+	val= tmp + min((int) (val_end-tmp), 2);
 	break;
 
 	/* Hour */