| From: | Michael Paquier <michael(at)paquier(dot)xyz> |
|---|---|
| To: | Dilip Kumar <dilipbalaut(at)gmail(dot)com> |
| Cc: | vignesh C <vignesh21(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Alexander Korotkov <aekorotkov(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Melanie Plageman <melanieplageman(at)gmail(dot)com> |
| Subject: | Re: pg_logical_slot_get_changes waits continously for a partial WAL record spanning across 2 pages |
| Date: | 2025-06-26 06:17:32 |
| Message-ID: | aFzl_A36PKjXmOG9@paquier.xyz |
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| Lists: | pgsql-hackers |
On Thu, Jun 26, 2025 at 08:48:32AM +0530, Dilip Kumar wrote:
> On Thu, Jun 26, 2025 at 6:22 AM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
>> So you are suggesting the addition of an extra ReadPageInternal() that
>> forces a read of only the read, perform the checks on the header, then
>> read the rest. After reading SizeOfXLogShortPHD worth of data,
>> shouldn't the checks on xlp_rem_len be done a bit earlier than what
>> you are proposing in this patch?
>
> I did not get the point, IMHO it has to be validated after the record
> on the next page has been read.
As proposed in the patch, the first ReadPageInternal() reads
SizeOfXLogShortPHD worth of data (right?), so we should have access to
XLogPageHeaderData.xlp_rem_len before attempting the second
ReadPageInternal() worth Min(total_len - gotlen + SizeOfXLogShortPHD,
XLOG_BLCKSZ) of data. The check that fails with a "invalid contrecord
length %u" relies on xlp_rem_len, total_len, RecPtr and gotlen. We
know all this information before the second ReadPageInternal().
--
Michael
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