Re: [HACKERS] Restricting maximum keep segments by repslots

From: Michael Paquier <michael(at)paquier(dot)xyz>
To: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
Cc: sawada(dot)mshk(at)gmail(dot)com, peter(dot)eisentraut(at)2ndquadrant(dot)com, pgsql-hackers(at)lists(dot)postgresql(dot)org, thomas(dot)munro(at)enterprisedb(dot)com, sk(at)zsrv(dot)org, michael(dot)paquier(at)gmail(dot)com, andres(at)anarazel(dot)de
Subject: Re: [HACKERS] Restricting maximum keep segments by repslots
Date: 2018-11-19 04:39:58
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On Fri, Oct 26, 2018 at 11:26:36AM +0900, Kyotaro HORIGUCHI wrote:
> The reason for doing that in the fucntion is it can happen also
> for physical replication when walsender is active but far
> behind. The removed(renamed)-but-still-open segment may be
> recycled and can be overwritten while reading, and it will be
> caught by page/record validation. It is substantially lost in
> that sense. I don't think the strictness is useful for anything..

I was just coming by to look at bit at the patch series, and bumped
into that:

> + /*
> + * checkpoint can remove the segment currently looking for. make sure the
> + * current segment is still exists. We check this only once per record.
> + */
> + XLByteToSeg(targetPagePtr, targetSegNo, state->wal_segment_size);
> + if (targetSegNo <= XLogGetLastRemovedSegno())
> + ereport(ERROR,
> + (errcode(ERRCODE_NO_DATA),
> + errmsg("WAL record at %X/%X no longer available",
> + (uint32)(RecPtr >> 32), (uint32) RecPtr),
> + errdetail("The segment for the record has been removed.")));
> +

ereport should not be called within xlogreader.c as a base rule:
* This file is compiled as both front-end and backend code, so it
* may not use ereport, server-defined static variables, etc.

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