Re: [HACKERS] Create replication slot in pg_basebackup if requestedand not yet present

Hi,

with the default configuration improvements so far for 10, it seems to
be very easy to setup streaming replication (at least locally):

$ initdb --pgdata=data1
$ pg_ctl --pgdata=data1 start
$ pg_basebackup --pgdata=data2 --write-recovery-conf
$ sed -i -e 's/^#port.=.5432/port = 5433/' \
> -e 's/^#hot_standby.=.off/hot_standby = on/' \
> data2/postgresql.conf
$ pg_ctl --pgdata=data2 start

(there might be a case for having hot_standby=on by default, but I think
that got discussed elsewhere and is anyway a different thread).

However, the above does not use replication slots, and if you want to do
so, you get:

$ pg_basebackup --pgdata=data2 --write-recovery-conf --slot=pg2
2017-03-19 11:04:37.978 CET [25362] ERROR:  replication slot "pg2" does
not exist
pg_basebackup: could not send replication command "START_REPLICATION":
ERROR:  replication slot "pg2" does not exist
pg_basebackup: child process exited with error 1
pg_basebackup: removing data directory "data2"

The error message is clear enough, but I wonder whether people will
start writing streaming replication tutorials just glossing over this
because it's one more step to run CREATE_REPLICATION_SLOT manually.

So I propose the attached tiny patch to just create the slot (if it does
not exist already) in pg_basebackup, somewhat similar to what
pg_receivewal does, albeit unconditionally, if the user explicitly
requested a slot:

$ pg_basebackup --pgdata=data2 --write-recovery-conf --slot=pg2
$ echo $?
0
$ psql -c "DROP_REPLICATION_SLOT pg2" "host=127.0.0.1 port=5432 replication=database user=mba dbname=postgres"
SELECT
$

This would get us somewhat closer to near zero-config replication, in my
opinion. Pardon me if that was discussed already and shot down

Comments?