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Fujii Masao 8d85cb889a bufmgr: Fix race in LockBufferForCleanup() 
LockBufferForCleanup() acquires the exclusive content lock, checks the
buffer's shared pin count, and, if other pins remain, registers itself
as the BM_PIN_COUNT_WAITER before waiting for an unpin notification.

Since commits 5310fac6e0 and c75ebc657f, however, a shared buffer
pin can be released while BM_LOCKED is set, introducing the following
race:

- LockBufferForCleanup() observes a refcount greater than one.
- Before it sets BM_PIN_COUNT_WAITER, another backend releases the
  last conflicting pin.
- Since BM_PIN_COUNT_WAITER is not yet set, no wakeup is sent.
- LockBufferForCleanup() then sets BM_PIN_COUNT_WAITER and goes to
  sleep, even though only its own pin remains.

As a result, LockBufferForCleanup() can sleep indefinitely because
the wakeup corresponding to the last conflicting unpin has already been
missed.

Fix this by setting BM_PIN_COUNT_WAITER while holding the buffer
header lock, then rechecking the refcount before releasing the content
lock. If only our pin remains, clear the waiter state and proceed
without sleeping. Otherwise, wait as before.

This issue was reported by buildfarm member skink, where it manifested
as intermittent timeouts in 048_vacuum_horizon_floor.pl.

Backpatch to v19, where commits 5310fac6e0 and c75ebc657f
introduced the race.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Author: Xuneng Zhou <xunengzhou@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/7685519a-0bf9-4e17-93ca-7e3aa10fa29c@gmail.com
Backpatch-through: 19
2026-06-30 10:36:41 +09:00
.github ci: Improve ccache handling 2026-06-08 15:26:47 -04:00
config Update config.guess and config.sub 2026-04-09 11:26:14 +02:00
contrib Run pgperltidy 2026-06-29 15:27:44 -04:00
doc Stamp HEAD as 20devel. 2026-06-29 16:29:11 -04:00
src bufmgr: Fix race in LockBufferForCleanup() 2026-06-30 10:36:41 +09:00
.dir-locals.el Make Emacs perl-mode indent more like perltidy. 2019-01-13 11:32:31 -08:00
.editorconfig Update .editorconfig and .gitattributes for postgresql.conf.sample. 2025-11-18 10:28:36 -06:00
.git-blame-ignore-revs Add previous 2 commits to .git-blame-ignore-revs. 2026-06-29 15:33:52 -04:00
.gitattributes Update .editorconfig and .gitattributes for postgresql.conf.sample. 2025-11-18 10:28:36 -06:00
.gitignore Update top-level .gitignore. 2022-12-04 15:23:00 -05:00
.mailmap Add a Git .mailmap file 2024-11-05 13:56:02 +01:00
aclocal.m4 autoconf: Move export_dynamic determination to configure 2022-12-06 18:55:28 -08:00
configure Stamp HEAD as 20devel. 2026-06-29 16:29:11 -04:00
configure.ac Stamp HEAD as 20devel. 2026-06-29 16:29:11 -04:00
COPYRIGHT Update copyright for 2026 2026-01-01 13:24:10 -05:00
GNUmakefile.in Allow selecting the git revision to be packaged by "make dist". 2024-05-03 11:08:50 -04:00
HISTORY Canonicalize some URLs 2020-02-10 20:47:50 +01:00
Makefile Restore AIX support. 2026-02-23 13:34:22 -05:00
meson.build Stamp HEAD as 20devel. 2026-06-29 16:29:11 -04:00
meson_options.txt Revert "Add built-in fuzzing harnesses for security testing." 2026-04-10 09:53:58 -04:00
README.md Revise the style of a paragraph in README.md. 2024-03-21 10:16:41 -05:00

README.md

PostgreSQL Database Management System

This directory contains the source code distribution of the PostgreSQL database management system.

PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings.

Copyright and license information can be found in the file COPYRIGHT.

General documentation about this version of PostgreSQL can be found at https://www.postgresql.org/docs/devel/. In particular, information about building PostgreSQL from the source code can be found at https://www.postgresql.org/docs/devel/installation.html.

The latest version of this software, and related software, may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.