0003-Prevent-snapshot-builder-xmin-from-going-backwards.patch

From ae60b52ae0ca96bc14169cd507f101fbb5dfdf52 Mon Sep 17 00:00:00 2001
From: Petr Jelinek <pjmodos@pjmodos.net>
Date: Sun, 26 Feb 2017 01:07:33 +0100
Subject: [PATCH 3/5] Prevent snapshot builder xmin from going backwards

Logical decoding snapshot builder may encounter xl_running_xacts with
older xmin than the xmin of the builder. This can happen because
LogStandbySnapshot() sometimes sees already committed transactions as
running (there is difference between "running" in terms for WAL and in
terms of ProcArray). When this happens we must make sure that the xmin
of snapshot builder won't go back otherwise the resulting snapshot would
show some transaction as running even though they have already
committed.
---
 src/backend/replication/logical/snapbuild.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c
index ada618d..3e34f75 100644
--- a/src/backend/replication/logical/snapbuild.c
+++ b/src/backend/replication/logical/snapbuild.c
@@ -1165,7 +1165,8 @@ SnapBuildProcessRunningXacts(SnapBuild *builder, XLogRecPtr lsn, xl_running_xact
 	 * looking, it's correct and actually more efficient this way since we hit
 	 * fast paths in tqual.c.
 	 */
-	builder->xmin = running->oldestRunningXid;
+	if (TransactionIdFollowsOrEquals(running->oldestRunningXid, builder->xmin))
+		builder->xmin = running->oldestRunningXid;
 
 	/* Remove transactions we don't need to keep track off anymore */
 	SnapBuildPurgeCommittedTxn(builder);
-- 
2.7.4