Re: Infinite loop in XLogPageRead() on standby

Kyotaro Horiguchi <horikyota.ntt@gmail.com>

From: Kyotaro Horiguchi <horikyota.ntt@gmail.com>

To: cyberdemn@gmail.com

Cc: michael@paquier.xyz, pgsql-hackers@postgresql.org, thomas.munro@gmail.com

Date: 2024-03-06T08:57:44Z

Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

Fix header check for continuation records where standbys could be stuck
- 0f0431e919f4 13.19 landed
- a2d4f806c4b9 14.16 landed
- 26554faccc97 15.11 landed
- 2c2e1d4f42c0 16.7 landed
- e6767c0ed16f 17.3 landed
- 6cf1647d87e7 18.0 landed
Move routines to manipulate WAL into PostgreSQL::Test::Cluster
- c9e50ce2a044 13.19 landed
- 50406b15540c 14.16 landed
- e5d113057d5f 15.11 landed
- 9420f9bb61e6 16.7 landed
- 149ed87e22ce 17.3 landed
- 32a18cc0a73d 18.0 landed
Prevent archive recovery from scanning non-existent WAL files.
- 4bd0ad9e44be 13.0 cited
Fix scenario where streaming standby gets stuck at a continuation record.
- 066871980183 11.0 cited

At Tue, 5 Mar 2024 09:36:44 +0100, Alexander Kukushkin <cyberdemn@gmail.com> wrote in 
> Please find attached the patch fixing the problem and the updated TAP test
> that addresses Nit.

Record-level retries happen when the upper layer detects errors. In my
previous mail, I cited code that is intended to prevent this at
segment boundaries. However, the resulting code applies to all page
boundaries, since we judged that the difference doen't significanty
affects the outcome.

> * Check the page header immediately, so that we can retry immediately if
> * it's not valid. This may seem unnecessary, because ReadPageInternal()
> * validates the page header anyway, and would propagate the failure up to

So, the following (tentative) change should also work.

xlogrecovery.c:
@@ -3460,8 +3490,10 @@ retry:
 	 * responsible for the validation.
 	 */
 	if (StandbyMode &&
+		targetPagePtr % 0x100000 == 0 &&
 		!XLogReaderValidatePageHeader(xlogreader, targetPagePtr, readBuf))
 	{

Thus, I managed to reproduce precisely the same situation as you
described utilizing your script with modifications and some core
tweaks, and with the change above, I saw that the behavior was
fixed. However, for reasons unclear to me, it shows another issue, and
I am running out of time and need more caffeine. I'll continue
investigating this tomorrow.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center