pg_receivexlog does not report flush position with --synchronous

Hi guys,

while adding synchronous WAL streaming to Barman, I noticed that
pg_receivexlog - unless a replication slot is specified and --synchronous
is passed - does not become a synchronous receiver (if the application_name
is in the synchronous_standby_names value). I was a bit surprised by this
behaviour.

By reading the pg_receivexlog documentation, I assumed that:

1) if I set application_name properly for pg_receivexlog (let's say
'barman_receive_wal')
2) then I set synchronous_standby_names so that barman_receive_wal is first
in the list
3) then I run pg_receivexlog with --synchronous

I would find the pg_receivexlog in 'sync' state in the
pg_stat_replication view on the master.

However, I kept receiving the 'async' state. After looking at the
documentation once more, I noticed that '--synchronous' was mentioned also
in the '--slot-name' option but the explanation - at least to me - was not
very clear.

I tried again by creating a replication slot and passing it to
pg_receivexlog and this time I could see 'sync' as streaming state.

Looking up the code in more details I see that, unless replication slot
are enabled, pg_receivexlog does not report the flush position (this is a
precaution that's been taken when '--synchronous' was probably not around).
Please find this very short patch which - in case replication slots are not
present but synchronous is - reports the flush position.

I am not sure if it is a bug or not. I any case I guess we should
probably improve the documentation - it's a bit late here so maybe I can
try better tomorrow with a fresher mind. :)

Thanks,
Gabriele
--
Gabriele Bartolini - 2ndQuadrant Italia - Director
PostgreSQL Training, Services and Support
gabriele(dot)bartolini(at)2ndQuadrant(dot)it | www.2ndQuadrant.it