back connection will not release after response

[yanhuqing666]

steps:

si is not a sharding key

mysql> select si from sharding_two_node group by si;
+-----+
| si  |
+-----+
|   1 |
|   5 |
| 514 |
+-----+
3 rows in set (0.02 sec)

use manager port to see backend,borrowed is still true:

mysql> show @@backend;
+------------+------+---------+--------------+------+----------------+--------+---------+----------------+--------+----------+------------+-----------+----------------------+----------------------+-----------------------+--------------------+------------+---------------+----------------+
| processor  | ID   | MYSQLID | HOST         | PORT | LOACL_TCP_PORT | NET_IN | NET_OUT | ACTIVE_TIME(S) | CLOSED | BORROWED | SEND_QUEUE | SCHEMA    | CHARACTER_SET_CLIENT | COLLATION_CONNECTION | CHARACTER_SET_RESULTS | TX_ISOLATION_LEVEL | AUTOCOMMIT | SYS_VARIABLES | USER_VARIABLES |
+------------+------+---------+--------------+------+----------------+--------+---------+----------------+--------+----------+------------+-----------+----------------------+----------------------+-----------------------+--------------------+------------+---------------+----------------+
| Processor0 |   44 |   10374 | 10.186.24.63 | 3320 |          59316 |    214 |     229 |              9 | false  | true     |          0 | ares_test | utf8                 | utf8_general_ci      | utf8                  | 3			 | true       |               |                |
| Processor1 |   45 |   22585 | 10.186.24.64 | 3320 |          59317 |    214 |     229 |              9 | false  | true     |          0 | ares_test | utf8	             | utf8_general_ci      | utf8                  | 3			 | true       |               |                |
+------------+------+---------+--------------+------+----------------+--------+---------+----------------+--------+----------+------------+-----------+----------------------+----------------------+-----------------------+--------------------+------------+---------------+----------------+
2 rows in set (0.00 sec)