RE: POC: enable logical decoding when wal_level = 'replica' without a server restart

Dear Sawada-san,

> I'm concerned that users could be confused if two different names
> refer to substantially the same thing.
>
> Having said that, I guess that we need to drastically change the
> messages. For example, I think that the wal_level worker should say
> something like "successfully made 'logical' wal_level effective"
> instead of saying something like "changed wal_level value". Also,
> users might not need gradual messages when increasing 'minimal' to
> 'logical' or decreasing 'logical' to 'minimal'.

+1 for something like "successfully made 'logical' wal_level effective", and
removing gradual messages.

> > 6.
> > With the patch present, the wal_level can be changed to the minimal even when
> the
> > streaming replication is going. If we do that, the walsender exits immediately
> and
> > the below FATAL appears periodically until the standby stops. Same things can
> be
> > said for the logical replication:
> >
> > ```
> > FATAL: streaming replication receiver "walreceiver" could not connect to the
> primary server:
> > connection to server on socket "/tmp/.s.PGSQL.oooo" failed:
> > FATAL: WAL senders require "wal_level" to be "replica" or "logical
> > ```
> >
> > I know this is not a perfect, but can we avoid the issue by reject the GUC update
> > if the walsender exists? Another approach is not to update the value when
> replication
> > slots need to be invalidated.
>
> Does it mean that we reject the config file from being reloaded in
> that case? I have no idea how to reject it in a case where the
> wal_level in postgresql.conf changed and the user did 'pg_ctl reload'.

I imagined like attached. When I modified wal_level to minimal and send SIGHUP,
postmaster reported below lines and failed to update wal_level.

```
LOG: received SIGHUP, reloading configuration files
LOG: wal_level cannot be set to "minimal" while walsender exists
LOG: configuration file "...postgresql.conf" contains errors; unaffected changes were applied
```

Best regards,
Hayato Kuroda
FUJITSU LIMITED