BUG #14230: Wrong timeline returned by pg_stop_backup on a standby

The following bug has been logged on the website:

Bug reference: 14230
Logged by: Francesco Canovai
Email address: francesco(dot)canovai(at)2ndquadrant(dot)it
PostgreSQL version: 9.6beta2
Operating system: Linux
Description:

I'm taking a concurrent backup from a standby in PostgreSQL beta2 and I get
the wrong timeline from pg_stop_backup(false).

This is what I'm doing:

1) I set up an environment with a primary server and a replica in streaming
replication.

2) On the replica, I run

postgres=# SELECT pg_start_backup('test_backup', true, false);
pg_start_backup
-----------------
0/3000A00
(1 row)

3) When I run pg_stop_backup, it returns a start wal location belonging to a
file with timeline 0.

postgres=# SELECT pg_stop_backup(false);
pg_stop_backup

---------------------------------------------------------------------------
(0/3000AE0,"START WAL LOCATION: 0/3000A00 (file
000000000000000000000003)+
CHECKPOINT LOCATION: 0/3000A38
+
BACKUP METHOD: streamed
+
BACKUP FROM: standby
+
START TIME: 2016-07-06 16:44:31 CEST
+
LABEL: test_backup
+
","")
(1 row)

The timeline returned is fine (is 1) when running the same commands on the
master.

An incorrect backup label doesn't prevent PostgreSQL from starting up, but
it affects the tools using that information.