BUG #15073: Bad error message for when pg_basebackup fails due toincorrect wal_level

The following bug has been logged on the website:

Bug reference:      15073
Logged by:          Franz Beltran
Email address:      franz@kalibrr.com
PostgreSQL version: 9.6.6
Operating system:   Linux
Description:

Hi,

I was trying to perform a repmgr standby clone but it gets stuck on
pg_basebackup:

$ /usr/bin/repmgr -h <primary_DB_server_IP> -D /var/lib/postgresql/9.6/main
-d repmgr -U repmgr --upstream-node-id=<primary_DB_node_ID> -f
/etc/postgresql/9.6/main/repmgr.conf standby clone -v --log-level DEBUG
NOTICE: using provided configuration file
"/etc/postgresql/9.6/main/repmgr.conf"
NOTICE: destination directory "/var/lib/postgresql/9.6/main" provided
...
...
...
HINT: this may take some time; consider using the -c/--fast-checkpoint
option
INFO: executing:
  pg_basebackup -l "repmgr base backup"  -D /var/lib/postgresql/9.6/main -h
<primary_DB_server_IP> -p <primary_DB_server_port> -U repmgr -X stream 

Upon checking the logs on the upstream server, the following error is
encountered:

user=repmgr,db=[unknown],app=pg_basebackup,client=<standby_IP>LOG:  could
not send data to client: Broken pipe
user=repmgr,db=[unknown],app=pg_basebackup,client=<standby_IP>ERROR:  base
backup could not send data, aborting backup
user=repmgr,db=[unknown],app=pg_basebackup,client=<standby_IP>FATAL:
connection to client lost

After some tinkering, we found that updating the following line in the
primary's postgresql.conf from:

wal_level = archive

to:

wal_level = hot_standby

resolves the issue. 

It just seems like a bad user experience and a more meaningful error message
would've helped.

Thanks!

--
Franz Beltran