From: Robert Haas <rhaas@postgresql.org>
Date: Wed, 27 Oct 2021 21:03:44 +0000 (-0400)
Subject: Don't set ThisTimeLineID when there's no reason to do so.
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Don't set ThisTimeLineID when there's no reason to do so.

In slotfuncs.c, pg_replication_slot_advance() needs to determine
the LSN up to which the slot should be advanced, but that doesn't
require us to update ThisTimeLineID, because none of the code called
from here depends on it. If the replication slot is logical,
pg_logical_replication_slot_advance will call read_local_xlog_page,
which does use ThisTimeLineID, but also takes care of making sure
it's up to date. If the replication slot is physical, the timeline
isn't used for anything at all.

In logicalfuncs.c, pg_logical_slot_get_changes_guts() has the same
issue: the only code we're going to run that cares about timelines
is in or downstream of read_local_xlog_page, which already makes
sure that the correct value gets set. Hence, don't do it here.
---

diff --git a/src/backend/replication/logical/logicalfuncs.c b/src/backend/replication/logical/logicalfuncs.c
index e59939aad1..5a7fae4a87 100644
--- a/src/backend/replication/logical/logicalfuncs.c
+++ b/src/backend/replication/logical/logicalfuncs.c
@@ -208,13 +208,12 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool bin
 	rsinfo->setDesc = p->tupdesc;
 
 	/*
-	 * Compute the current end-of-wal and maintain ThisTimeLineID.
-	 * RecoveryInProgress() will update ThisTimeLineID on promotion.
+	 * Compute the current end-of-wal.
 	 */
 	if (!RecoveryInProgress())
 		end_of_wal = GetFlushRecPtr();
 	else
-		end_of_wal = GetXLogReplayRecPtr(&ThisTimeLineID);
+		end_of_wal = GetXLogReplayRecPtr(NULL);
 
 	ReplicationSlotAcquire(NameStr(*name), true);
 
diff --git a/src/backend/replication/slotfuncs.c b/src/backend/replication/slotfuncs.c
index 17df99c2ac..877a006d50 100644
--- a/src/backend/replication/slotfuncs.c
+++ b/src/backend/replication/slotfuncs.c
@@ -627,7 +627,7 @@ pg_replication_slot_advance(PG_FUNCTION_ARGS)
 	if (!RecoveryInProgress())
 		moveto = Min(moveto, GetFlushRecPtr());
 	else
-		moveto = Min(moveto, GetXLogReplayRecPtr(&ThisTimeLineID));
+		moveto = Min(moveto, GetXLogReplayRecPtr(NULL));
 
 	/* Acquire the slot so we "own" it */
 	ReplicationSlotAcquire(NameStr(*slotname), true);