Clean-ups for 776621a and 7329240.

Following are a few clean-ups related to failover option support in slots:
1. Improve the documentation in create_subscription.sgml.
2. Remove the spurious blank line in subscriptioncmds.c.
3. Remove the NOTICE for alter_replication_slot in subscriptioncmds.c as
we would sometimes print it even when nothing has changed. One can find
the change by enabling log_replication_commands on the publisher.
4. Optimize ReplicationSlotAlter() function to prevent disk flushing when
the slot's data remains unchanged.

Author: Hou Zhijie
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/514f6f2f-6833-4539-39f1-96cd1e011f23@enterprisedb.com
Discussion: https://postgr.es/m/OS0PR01MB57164904651FB588A518E98894472@OS0PR01MB5716.jpnprd01.prod.outlook.com

Author: Amit Kapila

doc/src/sgml/ref/create_subscription.sgml

@@ -117,9 +117,8 @@ CREATE SUBSCRIPTION <replaceable class="parameter">subscription_name</replaceabl
           command should connect to the publisher at all.  The default
           is <literal>true</literal>.  Setting this to
           <literal>false</literal> will force the values of
-          <literal>create_slot</literal>, <literal>enabled</literal>,
-          <literal>copy_data</literal>, and <literal>failover</literal>
-          to <literal>false</literal>.
+          <literal>create_slot</literal>, <literal>enabled</literal> and
+          <literal>copy_data</literal> to <literal>false</literal>.
           (You cannot combine setting <literal>connect</literal>
           to <literal>false</literal> with
           setting <literal>create_slot</literal>, <literal>enabled</literal>,

doc/src/sgml/ref/pg_dump.sgml

@@ -1591,9 +1591,7 @@ CREATE DATABASE foo WITH TEMPLATE template0;
    information might have to be changed.  If the subscription needs to
    be enabled for
    <link linkend="sql-createsubscription-params-with-failover"><literal>failover</literal></link>,
-   then same needs to be done by executing
-   <link linkend="sql-altersubscription-params-set">
-   <literal>ALTER SUBSCRIPTION ... SET (failover = true)</literal></link>
+   execute <link linkend="sql-altersubscription-params-set"><literal>ALTER SUBSCRIPTION ... SET (failover = true)</literal></link>
    after the slot has been created.  It might also be appropriate to
    truncate the target tables before initiating a new full table copy.  If users
    intend to copy initial data during refresh they must create the slot with

src/backend/commands/subscriptioncmds.c

@@ -73,7 +73,6 @@
 #define SUBOPT_LSN					0x00004000
 #define SUBOPT_ORIGIN				0x00008000
 
-
 /* check if the 'val' has 'bits' set */
 #define IsSet(val, bits)  (((val) & (bits)) == (bits))
 
@@ -852,9 +851,6 @@ CreateSubscription(ParseState *pstate, CreateSubscriptionStmt *stmt,
 					 (opts.failover || walrcv_server_version(wrconn) >= 170000))
 			{
 				walrcv_alter_slot(wrconn, opts.slot_name, opts.failover);
-				ereport(NOTICE,
-						(errmsg("changed the failover state of replication slot \"%s\" on publisher to %s",
-								opts.slot_name, opts.failover ? "true" : "false")));
 			}
 		}
 		PG_FINALLY();
@@ -1547,10 +1543,6 @@ AlterSubscription(ParseState *pstate, AlterSubscriptionStmt *stmt,
 		PG_TRY();
 		{
 			walrcv_alter_slot(wrconn, sub->slotname, opts.failover);
-
-			ereport(NOTICE,
-					(errmsg("changed the failover state of replication slot \"%s\" on publisher to %s",
-							sub->slotname, opts.failover ? "true" : "false")));
 		}
 		PG_FINALLY();
 		{

src/backend/replication/slot.c

@@ -696,12 +696,16 @@ ReplicationSlotAlter(const char *name, bool failover)
 				errmsg("cannot use %s with a physical replication slot",
 					   "ALTER_REPLICATION_SLOT"));
 
-	SpinLockAcquire(&MyReplicationSlot->mutex);
-	MyReplicationSlot->data.failover = failover;
-	SpinLockRelease(&MyReplicationSlot->mutex);
+	if (MyReplicationSlot->data.failover != failover)
+	{
+		SpinLockAcquire(&MyReplicationSlot->mutex);
+		MyReplicationSlot->data.failover = failover;
+		SpinLockRelease(&MyReplicationSlot->mutex);
+
+		ReplicationSlotMarkDirty();
+		ReplicationSlotSave();
+	}
 
-	ReplicationSlotMarkDirty();
-	ReplicationSlotSave();
 	ReplicationSlotRelease();
 }