Subject: Re: prevent immature WAL streaming
To: Andres Freund <andres@anarazel.de>,
 Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Pg Hackers <pgsql-hackers@lists.postgresql.org>,
 "Bossart, Nathan" <bossartn@amazon.com>,
 Kyotaro Horiguchi <horikyota.ntt@gmail.com>,
 =?UTF-8?B?6JSh5qKm5aifKOeOiuS6jik=?= <mengjuan.cmj@alibaba-inc.com>,
 Jakub Wartak <Jakub.Wartak@tomtom.com>
References: <20210831042949.52eqp5xwbxgrfank@alap3.anarazel.de>
 <202108311356.sl33wcpcz5x6@alvherre.pgsql>
 <20210831155333.2vv2zdf7v4nhh2m2@alap3.anarazel.de>
From: Fujii Masao <masao.fujii@oss.nttdata.com>
Message-ID: <bb15b918-6dad-b01d-23b9-17b5dfa7e6a2@oss.nttdata.com>
Date: Wed, 1 Sep 2021 11:34:34 +0900
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On 2021/09/01 0:53, Andres Freund wrote:
> I was thinking that on a normal WAL write we'd do nothing. Instead we would
> have dedicated code at the end of recovery that, if the WAL ends in a partial
> record, changes the page following the "valid" portion of the WAL to indicate
> that an incomplete record is to be skipped.

Agreed!


> Of course, we need to be careful to not weaken WAL validity checking too
> much. How about the following:
> 
> If we're "aborting" a continued record, we set XLP_FIRST_IS_ABORTED_PARTIAL on
> the page at which we do so (i.e. the page after the valid end of the WAL).

When do you expect that XLP_FIRST_IS_ABORTED_PARTIAL is set? It's set
when recovery finds a a partially-flushed segment-spanning record?
But maybe we cannot do that (i.e., cannot overwrite the page) because
the page that the flag is set in might have already been archived. No?


> I think we can just read the WAL and see if it ends with a partial
> record. It'd add a bit of complication to the error checking in xlogreader,
> because we'd likely want to treat verification from page headers a bit
> different from verification due to record data. But that seems doable.

Yes.


> Does this make sense?

Yes, I think!


Regards,

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