Subject: Re: SIGSEGV from START_REPLICATION 0/XXXXXXX in XLogSendPhysical ()
 at walsender.c:2762
To: Michael Paquier <michael@paquier.xyz>,
 Masahiko Sawada <masahiko.sawada@2ndquadrant.com>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>, sitnikov.vladimir@gmail.com,
 PostgreSQL-development <pgsql-hackers@postgresql.org>
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 <20200528.181139.1631626601933187948.horikyota.ntt@gmail.com>
 <20200529072138.GG44192@paquier.xyz>
 <20200529.175653.1322451159722162490.horikyota.ntt@gmail.com>
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 <20200602042456.GA3594@paquier.xyz>
From: Fujii Masao <masao.fujii@oss.nttdata.com>
Message-ID: <b7b1fcaa-2532-a1e9-6533-d74954608003@oss.nttdata.com>
Date: Tue, 2 Jun 2020 14:23:50 +0900
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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.

> 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.

Yes. Also we should add the test to check if physical replication can work
fine even on logical rep connection?

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION