MDEV-17414: MyROCKS order desc limit 1 fails : Backport to 10.2

- Use the correct range bounds when doing a reverse-ordered range scan
  (this was already done for HA_READ_PREFIX_LAST_OR_PREV but not for
   HA_READ_BEFORE_KEY).

storage/rocksdb/ha_rocksdb.cc

@@ -8560,9 +8560,17 @@ int ha_rocksdb::index_read_map_impl(uchar *const buf, const uchar *const key,
                        packed_size);
 
   uint end_key_packed_size = 0;
+  /*
+    In MariaDB, the end_key is always the bigger end of the range.
+    If we are doing a reverse-ordered scan (that is, walking from the bigger
+    key values to smaller), we should use the smaller end of range as end_key.
+  */
   const key_range *cur_end_key= end_key;
-  if (find_flag == HA_READ_PREFIX_LAST_OR_PREV)
+  if (find_flag == HA_READ_PREFIX_LAST_OR_PREV ||
+      find_flag == HA_READ_BEFORE_KEY)
+  {
     cur_end_key= m_start_range;
+  }
 
   const uint eq_cond_len =
       calc_eq_cond_len(kd, find_flag, slice, bytes_changed_by_succ, cur_end_key,

storage/rocksdb/mysql-test/rocksdb/r/rocksdb_range2.result

@@ -9,3 +9,20 @@ explain select c1 from t1 where c1 > 5 limit 10;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	range	i	i	9	NULL	#	Using where; Using index
 drop table t1;
+#
+# MDEV-17414: MyROCKS order desc limit 1 fails
+#
+create table t1 (date date);
+insert into t1 values ('2018-10-04'), ('2018-10-05');
+select * from t1 where date < '2018-10-09' order by date desc limit 1;
+date
+2018-10-05
+alter table t1 add index date_index (date);
+select * from t1 where date < '2018-10-05' order by date desc limit 1;
+date
+2018-10-04
+# this should not produce an empty set:
+select * from t1 where date < '2018-10-09' order by date desc limit 1;
+date
+2018-10-05
+drop table t1;

storage/rocksdb/mysql-test/rocksdb/t/rocksdb_range2.test

@@ -19,3 +19,15 @@ select count(*) from t1;
 explain select c1 from t1 where c1 > 5 limit 10;
 drop table t1;
 
+--echo #
+--echo # MDEV-17414: MyROCKS order desc limit 1 fails
+--echo #
+create table t1 (date date);
+insert into t1 values ('2018-10-04'), ('2018-10-05');
+select * from t1 where date < '2018-10-09' order by date desc limit 1; # Works as expected
+alter table t1 add index date_index (date);
+select * from t1 where date < '2018-10-05' order by date desc limit 1; # Works as expected
+--echo # this should not produce an empty set:
+select * from t1 where date < '2018-10-09' order by date desc limit 1;
+drop table t1;
+