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

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.

Looking at the code, I think that there is some confusion with the
fake WAL reader used as base reference in InitWalSender() where we
assume that it could only be used in the context of a non-database WAL
sender.  However, this initialization happens when the WAL sender
connection is initialized, and what I think this misses is that we 
should try to initialize a WAL reader when actually going through a
START_REPLICATION command.

I can note as well that StartLogicalReplication() moves in this sense
by setting xlogreader to be the one from logical_decoding_ctx once the
decoding context has been created.

This results in the attached.  The extra test from upthread to check
that logical decoding is not allowed in a non-database WAL sender is a
good idea, so I have kept it.
--
Michael