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Peter Geoghegan 40aa5ddea1 Make _bt_killitems drop pins it acquired itself. 
Teach nbtree's _bt_killitems to leave the so->currPos page that it sets
LP_DEAD items on in whatever state it was in when _bt_killitems was
called.  In particular, make sure that so->dropPin scans don't acquire a
pin whose reference is saved in so->currPos.buf.

Allowing _bt_killitems to change so->currPos.buf like this is wrong.
The immediate consequence of allowing it is that code in _bt_steppage
(that copies so->currPos into so->markPos) will behave as if the scan is
a !so->dropPin scan.  so->markPos will therefore retain the buffer pin
indefinitely, even though _bt_killitems only needs to acquire a pin
(along with a lock) for long enough to mark known-dead items LP_DEAD.

This issue came to light following a report of a failure of an assertion
from recent commit e6eed40e.  The test case in question involves the use
of mark and restore.  An initial call to _bt_killitems takes place that
leaves so->currPos.buf in a state that is inconsistent with the scan
being so->dropPin.  A subsequent call to _bt_killitems for the same
position (following so->currPos being saved in so->markPos, and then
restored as so->currPos) resulted in the failure of an assertion that
tests that so->currPos.buf is InvalidBuffer when the scan is so->dropPin
(non-assert builds got a "resource was not closed" WARNING instead).

The same problem exists on earlier releases, though the issue is far
more subtle there.  Recent commit e6eed40e introduced the so->dropPin
field as a partial replacement for testing so->currPos.buf directly.
Earlier releases won't get an assertion failure (or buffer pin leak),
but they will allow the second _bt_killitems call from the test case to
behave as if a buffer pin was consistently held since the original call
to _bt_readpage.  This is wrong; there will have been an initial window
during which no pin was held on the so->currPos page, and yet the second
_bt_killitems call will neglect to check if so->currPos.lsn continues to
match the page's now-current LSN.

As a result of all this, it's just about possible that _bt_killitems
will set the wrong items LP_DEAD (on release branches).  This could only
happen with merge joins (the sole user of nbtree mark/restore support),
when a concurrently inserted index tuple used a recently-recycled TID
(and only when the new tuple was inserted onto the same page as a
distinct concurrently-removed tuple with the same TID).  This is exactly
the scenario that _bt_killitems' check of the page's now-current LSN
against the LSN stashed in currPos was supposed to prevent.

A follow-up commit will make nbtree completely stop conditioning whether
or not a position's pin needs to be dropped on whether the 'buf' field
is set.  All call sites that might need to drop a still-held pin will be
taught to rely on the scan-level so->dropPin field recently introduced
by commit e6eed40e.  That will make bugs of the same general nature as
this one impossible (or make them much easier to detect, at least).

Author: Peter Geoghegan <pg@bowt.ie>
Reported-By: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/545be1e5-3786-439a-9257-a90d30f8b849@gmail.com
Backpatch-through: 13
2025-06-11 09:17:33 -04:00
config jit: Remove {llvm-config,clang}-N configure probes. 2024-05-16 13:58:25 +12:00
contrib pg_prewarm: Allow autoprewarm to use more than 1GB to dump blocks. 2025-06-06 08:18:26 -04:00
doc Doc: you must own the target object to use SECURITY LABEL. 2025-06-05 11:29:33 -04:00
src Make _bt_killitems drop pins it acquired itself. 2025-06-11 09:17:33 -04:00
.cirrus.star Remove duplicate words in docs and code comments. 2023-10-09 09:18:47 +05:30
.cirrus.tasks.yml ci: Upgrade FreeBSD image 2025-03-05 10:29:08 -05:00
.cirrus.yml ci: Make compute resources for CI configurable 2023-08-23 15:15:28 -07:00
.dir-locals.el Make Emacs perl-mode indent more like perltidy. 2019-01-13 11:32:31 -08:00
.editorconfig Add .editorconfig 2019-12-18 09:13:13 +01:00
.git-blame-ignore-revs Add commit 7229ebe011 to .git-blame-ignore-revs. 2024-09-14 20:17:30 +02:00
.gitattributes Exclude LLVM files from whitespace checks 2024-11-27 11:09:27 +01:00
.gitignore Update top-level .gitignore. 2022-12-04 15:23:00 -05:00
aclocal.m4 autoconf: Move export_dynamic determination to configure 2022-12-06 18:55:28 -08:00
configure Make our usage of memset_s() conform strictly to the C11 standard. 2025-05-18 12:45:55 -04:00
configure.ac Make our usage of memset_s() conform strictly to the C11 standard. 2025-05-18 12:45:55 -04:00
COPYRIGHT Align organization wording in copyright statement 2025-05-16 11:20:07 -04:00
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 Adapt REL_17_STABLE to its new status as a stable branch 2024-07-01 08:05:35 +09:00
meson.build Make our usage of memset_s() conform strictly to the C11 standard. 2025-05-18 12:45:55 -04:00
meson_options.txt Allow selecting the git revision to be packaged by "make dist". 2024-05-03 11:08:50 -04:00
README.md Adapt REL_17_STABLE to its new status as a stable branch 2024-07-01 08:05:35 +09:00

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/17/. In particular, information about building PostgreSQL from the source code can be found at https://www.postgresql.org/docs/17/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/.