From: Peter Geoghegan <pg@bowt.ie>
To: pgsql-committers@lists.postgresql.org
Subject: pgsql: Fix snapshot import xmin ProcArrayLock bug.
Date: Thu, 20 Aug 2026 23:48:11 +0000
Message-ID: <E1wxCUU-00000001QWS-0O0A@gemulon.postgresql.org> (raw)
Fix snapshot import xmin ProcArrayLock bug.
ProcArrayInstallImportedXmin verifies that the source transaction (the
transaction whose snapshot we're importing) is still running, and then
installs the caller's imported xmin. These steps have to be atomic.
But it was just about possible for VACUUM to fail to observe the
imported xmin in either the source proc or the importing one. This
could result in VACUUM pruning away deleted tuples that were still
visible to the imported snapshot.
To fix, take ProcArrayLock in exclusive mode while importing an exported
snapshot's xmin within ProcArrayInstallImportedXmin. That guarantees
that a concurrent VACUUM's OldestXmin cannot advance past the xmin (one
proc or the other always advertises an xmin that holds it back).
Author: Chee Wooson <chee.wooson@gmail.com>
Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/20260730042128.714201-1-chee.wooson@gmail.com
Backpatch-through: 14
Branch
------
REL_16_STABLE
Details
-------
https://git.postgresql.org/pg/commitdiff/7a392b0f6530236b71f15c936ba3f813c0720897
Modified Files
--------------
src/backend/storage/ipc/procarray.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-committers@postgresql.org
Cc: pg@bowt.ie, pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: Fix snapshot import xmin ProcArrayLock bug.
In-Reply-To: <E1wxCUU-00000001QWS-0O0A@gemulon.postgresql.org>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox