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

On Fri, 29 May 2020 at 17:57, Kyotaro Horiguchi <horikyota.ntt@gmail.com> wrote:
>
> At Fri, 29 May 2020 16:21:38 +0900, Michael Paquier <michael@paquier.xyz> wrote in
> > On Thu, May 28, 2020 at 06:11:39PM +0900, Kyotaro Horiguchi wrote:
> > > Mmm. It is not the proper way to use physical replication and it's
> > > totally accidental that that worked (or even it might be a bug). The
> > > documentation is saying as the follows, as more-or-less the same for
> > > all versions since 9.4.
> > >
> > > https://www.postgresql.org/docs/13/protocol-replication.html
> >
> > +       if (am_db_walsender)
> > +               ereport(ERROR,
> > +                       (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
> > +                                errmsg("cannot initiate physical
> > replication on a logical replication connection")));
> >
> > I don't agree with this change.  The only restriction that we have in
> > place now in walsender.c regarding MyDatabaseId not being set is to
> > prevent the execution of SQL commands.  Note that it is possible to
> > start physical replication even if MyDatabaseId is set in a
> > replication connection, so you could break cases that have been valid
> > until now.
>
> It donesn't check MyDatabase, but whether the connection parameter
> "repliation" is "true" or "database".  The documentation is telling
> that "replication" should be "true" for a connection that is to be
> used for physical replication, and "replication" should literally be
> "database" for a connection that is for logical replication.  We need
> to revise the documentation if we are going to allow physical
> replication on a conection with "replication = database".
>

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.

Regards,

-- 
Masahiko Sawada            http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services