Re: [BUGS] Bug in Physical Replication Slots (at least 9.5)?

From: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
To: michael(dot)paquier(at)gmail(dot)com
Cc: nag1010(at)gmail(dot)com, jdnelson(at)dyn(dot)com, pgsql-hackers(at)postgresql(dot)org
Subject: Re: [BUGS] Bug in Physical Replication Slots (at least 9.5)?
Date: 2017-09-04 06:51:51
Message-ID: 20170904.155151.168399829.horiguchi.kyotaro@lab.ntt.co.jp
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Hello,

Thank you for reviewing this.

At Mon, 28 Aug 2017 20:14:54 +0900, Michael Paquier <michael(dot)paquier(at)gmail(dot)com> wrote in <CAB7nPqT03+uaHXun3ft4LJWNDviKTgWSZDsXiqyNdtcCfeqcgg(at)mail(dot)gmail(dot)com>
> On Mon, Aug 28, 2017 at 8:02 PM, Kyotaro HORIGUCHI
> <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
> > The first patch (0001-) fixes this problem, preventing the
> > problematic state of WAL segments by retarding restart LSN of a
> > physical replication slot in a certain condition.
>
> FWIW, I have this patch marked on my list of things to look at, so you
> can count me as a reviewer. There are also some approaches that I
> would like to test because I rely on replication slots for some
> infrastructure. Still...

This test patch modifies the code for easiness. The window for
this bug to occur is from receiving the first record of a segment
to in most cases receiving the second record or after receiving
several records. Intentionally emitting a record spanning two or
more segments would work?

> + if (oldFlushPtr != InvalidXLogRecPtr &&
> + (restartLSN == InvalidXLogRecPtr ?
> + oldFlushPtr / XLOG_SEG_SIZE != flushPtr / XLOG_SEG_SIZE :
> + restartLSN / XLOG_BLCKSZ != flushPtr / XLOG_BLCKSZ))
> I find such code patterns not readable.

Yeah, I agree. I rewrote there and the result in the attached
patch is far cleaner than the blob.

regards,

--
Kyotaro Horiguchi
NTT Open Source Software Center

Attachment Content-Type Size
0001-Retard-restart-LSN-of-a-slot-when-a-segment-starts-w.patch text/x-patch 7.7 KB

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