postgres.git / summary / log / commit / refs

commit    e323d9df002debb103a58ba2479368c46804a9b6
Author:   Amit Kapila <akapila@postgresql.org>
Date:     Mon May 19 05:34:39 2025 +0000
First released in: 18.0, 13.22, 14.19, 15.14, 16.10, 17.6

    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 <houzj.fnst@fujitsu.com>
    Author: shveta malik <shveta.malik@gmail.com>
    Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
    Reviewed-by: Dilip Kumar <dilipbalaut@gmail.com>
    Tested-by: Nisha Moond <nisha.moond412@gmail.com>
    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 6f1b3702470..08e38c29fa6 100644
--- a/src/backend/replication/logical/logical.c
+++ b/src/backend/replication/logical/logical.c
@@ -1037,7 +1037,19 @@ 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 &&
@@ -1102,7 +1114,14 @@ 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);
 	}
 }


[parent: 9bc2d37ccaab]