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Masahiko Sawada 57048d6e17 Fix a race condition in updating procArray->replication_slot_xmin. 
Previously, ReplicationSlotsComputeRequiredXmin() computed the oldest
xmin across all slots without holding ProcArrayLock (when
already_locked is false), acquiring the lock just before updating the
replication slot xmin.

This could lead to a race condition: if a backend created a new slot
and updates the global replication slot xmin, another backend
concurrently running ReplicationSlotsComputeRequiredXmin() could
overwrite that update with an invalid or stale value. This happens
because the concurrent backend might have computed the aggregate xmin
before the new slot was accounted for, but applied the update after
the new slot had already updated the global value.

In the reported failure, a walsender for an apply worker computed
InvalidTransactionId as the oldest xmin and overwrote a valid
replication slot xmin value computed by a walsender for a tablesync
worker. Consequently, the tablesync worker computed a transaction ID
via GetOldestSafeDecodingTransactionId() effectively without
considering the replication slot xmin. This led to the error "cannot
build an initial slot snapshot as oldest safe xid %u follows
snapshot's xmin %u", which was an assertion failure prior to commit
240e0dbacd.

To fix this, we acquire ReplicationSlotControlLock in exclusive mode
during slot creation to perform the initial update of the slot
xmin. In ReplicationSlotsComputeRequiredXmin(), we hold
ReplicationSlotControlLock in shared mode until the global slot xmin
is updated in ProcArraySetReplicationSlotXmin(). This prevents
concurrent computations and updates of the global xmin by other
backends during the initial slot xmin update process, while still
permitting concurrent calls to ReplicationSlotsComputeRequiredXmin().

Backpatch to all supported versions.

Author: Zhijie Hou <houzj.fnst@fujitsu.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Pradeep Kumar <spradeepkumar29@gmail.com>
Reviewed-by: Hayato Kuroda (Fujitsu) <kuroda.hayato@fujitsu.com>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CAA4eK1L8wYcyTPxNzPGkhuO52WBGoOZbT0A73Le=ZUWYAYmdfw@mail.gmail.com
Backpatch-through: 14
2025-12-30 10:56:18 -08:00
config Don't put library-supplied -L/-I switches before user-supplied ones. 2025-07-29 15:17:41 -04:00
contrib Fix multibyte issue in ltree_strncasecmp(). 2025-12-16 10:38:15 -08:00
doc doc: Remove duplicate word in ECPG description 2025-12-26 15:26:11 +09:00
src Fix a race condition in updating procArray->replication_slot_xmin. 2025-12-30 10:56:18 -08:00
.abi-compliance-history Update .abi-compliance-history for PrepareToInvalidateCacheTuple(). 2025-12-17 09:48:56 -08:00
.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 previous commit to .git-blame-ignore-revs. 2025-10-21 10:02:19 -05:00
.gitattributes Exclude LLVM files from whitespace checks 2024-11-27 11:10:00 +01:00
.gitignore Add portlock directory to .gitignore 2022-11-26 07:47:06 -05:00
aclocal.m4 Probe $PROVE not $PERL while checking for modules needed by TAP tests. 2021-11-22 12:54:52 -05:00
configure Stamp 14.20. 2025-11-10 16:58:00 -05:00
configure.ac Stamp 14.20. 2025-11-10 16:58:00 -05:00
COPYRIGHT Align organization wording in copyright statement 2025-05-16 11:20:07 -04:00
GNUmakefile.in Remove temporary portlock directory during make [dist]clean. 2022-11-26 10:30:53 -05:00
HISTORY Canonicalize some URLs 2020-02-10 20:47:50 +01:00
Makefile Don't unset MAKEFLAGS in non-GNU Makefile. 2019-06-25 09:36:21 +12:00
README Canonicalize some URLs 2020-02-10 20:47:50 +01:00
README.git Canonicalize some URLs 2020-02-10 20:47:50 +01:00

README 

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.

PostgreSQL has many language interfaces, many of which are listed here:

	https://www.postgresql.org/download/

See the file INSTALL for instructions on how to build and install
PostgreSQL.  That file also lists supported operating systems and
hardware platforms and contains information regarding any other
software packages that are required to build or run the PostgreSQL
system.  Copyright and license information can be found in the
file COPYRIGHT.  A comprehensive documentation set is included in this
distribution; it can be read as described in the installation
instructions.

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