Re: SIGSEGV from START_REPLICATION 0/XXXXXXX in XLogSendPhysical () at walsender.c:2762

Hi,

On 2020-06-02 14:23:50 +0900, Fujii Masao wrote:
> On 2020/06/02 13:24, Michael Paquier wrote:
> > On Fri, May 29, 2020 at 06:09:06PM +0900, Masahiko Sawada wrote:
> > > Yes. Conversely, if we start logical replication in a physical
> > > replication connection (i.g. replication=true) we got an error before
> > > staring replication:
> > >
> > > ERROR: logical decoding requires a database connection
> > >
> > > I think we can prevent that SEGV in a similar way.
> >
> > Still unconvinced as this restriction stands for logical decoding
> > requiring a database connection but it is not necessarily true now as
> > physical replication has less restrictions than a logical one.
>
> Could you tell me what the benefit for supporting physical replication on
> logical rep connection is? If it's only for "undocumented"
> backward-compatibility, IMO it's better to reject such "tricky" set up.
> But if there are some use cases for that, I'm ok to support that.

I don't think we should prohibit this. For one, it'd probably break some
clients, without a meaningful need.

But I think it's also actually quite useful to be able to access
catalogs before streaming data. You e.g. can look up configuration of
the primary before streaming WAL. With a second connection that's
actually harder to do reliably in some cases, because you need to be
sure that you actually reached the right server (consider a pooler,
automatic failover etc).

Greetings,

Andres Freund