From 5a6002215fb8ebeaf1dde120e5f6706bca7b62ae Mon Sep 17 00:00:00 2001 From: Antonin Houska Date: Mon, 19 Jan 2026 16:07:45 +0100 Subject: [PATCH] Fix race conditions during the setup of logical decoding. Although it's rather unlikely, it can happen that the snapshot builder considers transaction committed (according to WAL) before the commit could be recorded in CLOG. In an extreme case, snapshot can even be created and used in between. Since both snapshot and CLOG are needed for visibility checks, this inconsistency can make them work incorrectly. The typical symptom is that a transaction that the snapshot considers not running anymore is (per CLOG) considered aborted instead of committed. Thus a new tuple version can be evaluated as invisible (if xmin is incorrectly considered aborted) or a deleted tuple version can be evaluated as visible (if xmax is incorrectly considered aborted). This patch fixes the problem by checking if all the XIDs that the new snapshot considers committed are really committed per CLOG. If at least one is not, the check is repeated after a short delay. However, a single check is sufficient in almost all cases, so the performance impact should be minimal. --- src/backend/replication/logical/snapbuild.c | 41 +++++++++++++++++++ .../utils/activity/wait_event_names.txt | 1 + 2 files changed, 42 insertions(+) diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 9b09dc8eac1..c02d08f3417 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -400,6 +400,47 @@ SnapBuildBuildSnapshot(SnapBuild *builder) snapshot->xmin = builder->xmin; snapshot->xmax = builder->xmax; + /* + * Although it's very unlikely, it's possible that a commit WAL record was + * decoded but CLOG is not aware of the commit yet. Should the CLOG update + * be delayed even more, visibility checks that use this snapshot could + * work incorrectly. Therefore we check the CLOG status here. + */ + while (true) + { + bool found = false; + + for (int i = 0; i < builder->committed.xcnt; i++) + { + /* + * XXX Is it worth remembering the XIDs that appear to be + * committed per CLOG and skipping them in the next iteration of + * the outer loop? Not sure it's worth the effort - a single + * iteration is enough in most cases. + */ + if (unlikely(!TransactionIdDidCommit(builder->committed.xip[i]))) + { + found = true; + + /* + * Wait a bit before going to the next iteration of the outer + * loop. The race conditions we address here is pretty rare, + * so we shouldn't need to wait too long. + */ + (void) WaitLatch(MyLatch, + WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH, + 10L, + WAIT_EVENT_SNAPBUILD_CLOG); + ResetLatch(MyLatch); + + break; + } + } + + if (!found) + break; + } + /* store all transactions to be treated as committed by this snapshot */ snapshot->xip = (TransactionId *) ((char *) snapshot + sizeof(SnapshotData)); diff --git a/src/backend/utils/activity/wait_event_names.txt b/src/backend/utils/activity/wait_event_names.txt index 5537a2d2530..b6318b0cf37 100644 --- a/src/backend/utils/activity/wait_event_names.txt +++ b/src/backend/utils/activity/wait_event_names.txt @@ -181,6 +181,7 @@ PG_SLEEP "Waiting due to a call to pg_sleep or a sibling fu RECOVERY_APPLY_DELAY "Waiting to apply WAL during recovery because of a delay setting." RECOVERY_RETRIEVE_RETRY_INTERVAL "Waiting during recovery when WAL data is not available from any source (pg_wal, archive or stream)." REGISTER_SYNC_REQUEST "Waiting while sending synchronization requests to the checkpointer, because the request queue is full." +SNAPBUILD_CLOG "Waiting for CLOG update before building snapshot." SPIN_DELAY "Waiting while acquiring a contended spinlock." VACUUM_DELAY "Waiting in a cost-based vacuum delay point." VACUUM_TRUNCATE "Waiting to acquire an exclusive lock to truncate off any empty pages at the end of a table vacuumed." -- 2.47.3