v3-0001-Improve-the-documentation-for-pg_copy_logical_rep.patch

From 94d10dabc00b13624909b0b658ef9201118a3b52 Mon Sep 17 00:00:00 2001
From: Hou Zhijie <houzj.fnst@cn.fujitsu.com>
Date: Mon, 24 Feb 2025 15:36:11 +0800
Subject: [PATCH v3 1/2] Improve the documentation for
 pg_copy_logical_replication_slot

This commit documents that the failover option is not copied when using the
pg_copy_logical_replication_slot function, and it defaults to false.
Additionally, the comments within the function is improved to provide a clear
explanation of the reason behind this behavior.
---
 doc/src/sgml/func.sgml              |  3 +++
 src/backend/replication/slotfuncs.c | 19 ++++++++++++-------
 2 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 9f60a476eb..a29c9500ba 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -29374,6 +29374,9 @@ postgres=# SELECT '0/0'::pg_lsn + pd.segment_number * ps.setting::int + :offset
         from the same <acronym>LSN</acronym> as the source logical slot.  Both
         <parameter>temporary</parameter> and <parameter>plugin</parameter> are
         optional; if they are omitted, the values of the source slot are used.
+        The <parameter>failover</parameter> option of source logical slot is
+        not copied and is set to <literal>false</literal> by default to avoid
+        potential issues with the slot synchronization.
        </para></entry>
       </row>
 
diff --git a/src/backend/replication/slotfuncs.c b/src/backend/replication/slotfuncs.c
index f652ec8a73..8222e5a109 100644
--- a/src/backend/replication/slotfuncs.c
+++ b/src/backend/replication/slotfuncs.c
@@ -695,13 +695,18 @@ copy_replication_slot(FunctionCallInfo fcinfo, bool logical_slot)
 		 * hence pass find_startpoint false.  confirmed_flush will be set
 		 * below, by copying from the source slot.
 		 *
-		 * To avoid potential issues with the slot synchronization where the
-		 * restart_lsn of a replication slot can go backward, we set the
-		 * failover option to false here.  This situation occurs when a slot
-		 * on the primary server is dropped and immediately replaced with a
-		 * new slot of the same name, created by copying from another existing
-		 * slot.  However, the slot synchronization will only observe the
-		 * restart_lsn of the same slot going backward.
+		 * Copying the failover option is not allowed to prevent potential
+		 * issues with slot synchronization. For instance, if a slot was
+		 * synchronized to the standby, then dropped on the primary, and
+		 * immediately recreated by copying from another existing slot with
+		 * much earlier restart_lsn and confirmed_flush_lsn, the slot
+		 * synchronization would only observe the LSN of the same slot moving
+		 * backward. As slot synchronization does not copy the restart_lsn and
+		 * confirmed_flush_lsn backward (see update_local_synced_slot() for
+		 * details), if a failover happens before the primary's slot catches
+		 * up, logical replication cannot continue using the synchronized slot
+		 * on the promoted standby because the slot retains the restart_lsn and
+		 * confirmed_flush_lsn that are much later than expected.
 		 */
 		create_logical_replication_slot(NameStr(*dst_name),
 										plugin,
-- 
2.30.0.windows.2