[PATCH] Fix TOCTOU race in ReplicationSlotsComputeRequiredLSN()

JoongHyuk Shin <sjh910805@gmail.com>

From: JoongHyuk Shin <sjh910805@gmail.com>

To: pgsql-hackers@lists.postgresql.org

Date: 2026-04-17T06:50:01Z

Lists: pgsql-hackers

Attachments

0001-Fix-TOCTOU-race-in-ReplicationSlotsComputeRequiredLS.patch (application/octet-stream) patch 0001

Hi,

Commit 2a5225b99d7 fixed a race in ReplicationSlotsComputeRequiredXmin()
where ReplicationSlotControlLock was released before the global xmin
update, allowing a concurrent backend to overwrite a correct value with
a stale one.

ReplicationSlotsComputeRequiredLSN() has the same problem, it releases the
lock before calling XLogSetReplicationSlotMinimumLSN(), so a stale minimum
LSN can overwrite a correct (lower) one, potentially leading to premature
WAL removal.

The attached patch moves LWLockRelease() to after the LSN update,
matching the xmin fix. Since 2a5225b99d7 was backpatched to all supported
versions, I believe this should be as well.

Commits

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

Fix a race condition in updating procArray->replication_slot_xmin.
- 2a5225b99d76 19 (unreleased) cited