standby follow: don't restart server (PostgreSQL 13 and later)

As of PostgreSQL 13, changes to the fundamental replication
configuration can be applied with a simple SIGHUP, no restart
required.

In case the old behaviour is desired, i.e. a full restart to apply
the configuration changes, the new configuration parameter
"standby_follow_restart" can be set. This parameter has no effect
in PostgreSQL 12 and earlier.

configdata.c

@@ -316,6 +316,16 @@ struct ConfigFileSetting config_file_settings[] =
 		{},
 		{}
 	},
+	/* standby_follow_restart */
+	{
+		"standby_follow_restart",
+		CONFIG_BOOL,
+		{ .boolptr = &config_file_options.standby_follow_restart },
+		{ .booldefault = DEFAULT_STANDBY_FOLLOW_RESTART },
+		{},
+		{},
+		{}
+	},
 
 	/* ===========================
 	 * standby switchover settings

configfile.h

@@ -164,6 +164,7 @@ typedef struct
 	/* standby follow settings */
 	int			primary_follow_timeout;
 	int			standby_follow_timeout;
+	bool		standby_follow_restart;
 
 	/* standby switchover settings */
 	int			shutdown_check_timeout;

doc/appendix-release-notes.xml

@@ -79,11 +79,24 @@
             </para>
           </listitem>
 
+          <listitem>
+            <para>
+              <link linkend="repmgr-standby-follow">repmgr standby follow</link>:
+              In PostgreSQL 13 and later, a standby no longer requires a restart to
+              follow a new upstream node.
+            </para>
+            <para>
+              The old behaviour (always restarting the standby to follow a new node)
+              can be restored by setting the configuration file parameter
+              <varname>standby_follow_restart</varname> to <literal>true</literal>.
+            </para>
+          </listitem>
+
           <listitem>
             <para>
               <link linkend="repmgr-node-check">repmgr node check</link>:
-              option <option>--db-connection</option> added to check if &repmgr; can connect to the database on the
-              local node.
+              option <option>--db-connection</option> added to check if &repmgr;
+              can connect to the database on the local node.
             </para>
           </listitem>
           <listitem>

doc/repmgr-standby-follow.xml

@@ -47,7 +47,15 @@
     </para>
 
     <para>
-      This command will force a restart of PostgreSQL on the standby node.
+      In PostgreSQL 12 and earlier, this command will force a restart of PostgreSQL on the standby node.
+    </para>
+
+    <para>
+      In PostgreSQL 13 and later, by default this command will signal PostgreSQL to reload its
+      configuration, which will cause PostgreSQL to follow the new upstream without
+      a restart. If this behaviour is not desired for whatever reason, the configuration
+      file parameter <varname>standby_follow_restart</varname> can be set <literal>true</literal>
+      to always force a restart.
     </para>
 
     <para>

repmgr-action-standby.c

@@ -3361,9 +3361,27 @@ do_standby_follow_internal(PGconn *primary_conn, PGconn *follow_target_conn, t_n
 
 		if (server_up == true)
 		{
-			/* no "service_restart_command" defined - stop and start using pg_ctl*/
+
+			if (PQserverVersion(primary_conn) >= 130000 && config_file_options.standby_follow_restart == false)
+			{
+				/* PostgreSQL 13 and later: we'll send SIGHUP via pg_ctl */
+				get_server_action(ACTION_RELOAD, server_command, config_file_options.data_directory);
+
+				success = local_command(server_command, &output_buf);
+
+				if (success == true)
+				{
+					goto cleanup;
+				}
+
+				/* In the unlikley event that fails, we'll fall back to a restart */
+				log_warning(_("unable to reload server configuration"));
+			}
+
 			if (config_file_options.service_restart_command[0] == '\0')
 			{
+				/* no "service_restart_command" defined - stop and start using pg_ctl */
+
 				action = "stopp"; /* sic */
 				get_server_action(ACTION_STOP_WAIT, server_command, config_file_options.data_directory);
 
@@ -3445,6 +3463,7 @@ do_standby_follow_internal(PGconn *primary_conn, PGconn *follow_target_conn, t_n
 		}
 	}
 
+cleanup:
 	/*
 	 * If replication slots are in use, and an inactive one for this node
 	 * exists on the former upstream, drop it.

repmgr.conf.sample

@@ -238,7 +238,8 @@ ssh_options='-q -o ConnectTimeout=10'	# Options to append to "ssh"
 					# for the new primary to become available
 #standby_follow_timeout=15		# The max length of time (in seconds) to wait
 					# for the standby to connect to the primary
-
+#standby_follow_restart=false		# Restart the standby instead of sending a SIGHUP
+					# (only for PostgreSQL 13 and later)
 
 #------------------------------------------------------------------------------
 # "standby switchover" settings

repmgr.h

@@ -116,6 +116,7 @@
 #define DEFAULT_PROMOTE_CHECK_INTERVAL       1   /* seconds */
 #define DEFAULT_PRIMARY_FOLLOW_TIMEOUT       60  /* seconds */
 #define DEFAULT_STANDBY_FOLLOW_TIMEOUT       30  /* seconds */
+#define DEFAULT_STANDBY_FOLLOW_RESTART       false
 #define DEFAULT_SHUTDOWN_CHECK_TIMEOUT       60  /* seconds */
 #define DEFAULT_STANDBY_RECONNECT_TIMEOUT    60  /* seconds */
 #define DEFAULT_NODE_REJOIN_TIMEOUT          60  /* seconds */