From ad5eaf390c58294e2e4c1509aa87bf13261a5d15 Mon Sep 17 00:00:00 2001 From: Amit Kapila Date: Mon, 19 May 2025 12:13:06 +0530 Subject: [PATCH] Don't retreat slot's confirmed_flush LSN. Prevent moving the confirmed_flush backwards, as this could lead to data duplication issues caused by replicating already replicated changes. This can happen when a client acknowledges an LSN it doesn't have to do anything for, and thus didn't store persistently. After a restart, the client can send the prior LSN that it stored persistently as an acknowledgement, but we need to ignore such an LSN to avoid retreating confirm_flush LSN. Diagnosed-by: Zhijie Hou Author: shveta malik Reviewed-by: Amit Kapila Reviewed-by: Dilip Kumar Tested-by: Nisha Moond Backpatch-through: 13 Discussion: https://postgr.es/m/CAJpy0uDZ29P=BYB1JDWMCh-6wXaNqMwG1u1mB4=10Ly0x7HhwQ@mail.gmail.com Discussion: https://postgr.es/m/OS0PR01MB57164AB5716AF2E477D53F6F9489A@OS0PR01MB5716.jpnprd01.prod.outlook.com --- src/backend/replication/logical/logical.c | 23 +++++++++++++++++++++-- 1 file changed, 21 insertions(+), 2 deletions(-) diff --git a/src/backend/replication/logical/logical.c b/src/backend/replication/logical/logical.c index a8d2e024d34..1d56d0c4ef3 100644 --- a/src/backend/replication/logical/logical.c +++ b/src/backend/replication/logical/logical.c @@ -1828,7 +1828,19 @@ LogicalConfirmReceivedLocation(XLogRecPtr lsn) SpinLockAcquire(&MyReplicationSlot->mutex); - MyReplicationSlot->data.confirmed_flush = lsn; + /* + * Prevent moving the confirmed_flush backwards, as this could lead to + * data duplication issues caused by replicating already replicated + * changes. + * + * This can happen when a client acknowledges an LSN it doesn't have + * to do anything for, and thus didn't store persistently. After a + * restart, the client can send the prior LSN that it stored + * persistently as an acknowledgement, but we need to ignore such an + * LSN. See similar case handling in CreateDecodingContext. + */ + if (lsn > MyReplicationSlot->data.confirmed_flush) + MyReplicationSlot->data.confirmed_flush = lsn; /* if we're past the location required for bumping xmin, do so */ if (MyReplicationSlot->candidate_xmin_lsn != InvalidXLogRecPtr && @@ -1893,7 +1905,14 @@ LogicalConfirmReceivedLocation(XLogRecPtr lsn) else { SpinLockAcquire(&MyReplicationSlot->mutex); - MyReplicationSlot->data.confirmed_flush = lsn; + + /* + * Prevent moving the confirmed_flush backwards. See comments above + * for the details. + */ + if (lsn > MyReplicationSlot->data.confirmed_flush) + MyReplicationSlot->data.confirmed_flush = lsn; + SpinLockRelease(&MyReplicationSlot->mutex); } } -- 2.30.2