Set fallback_application_name for a walreceiver to cluster_name

By default, the fallback_application_name for a physical walreceiver
is "walreceiver".  This means that multiple standbys cannot be
distinguished easily on a primary, for example in pg_stat_activity or
synchronous_standby_names.

If cluster_name is set, use that for fallback_application_name in the
walreceiver.  (If it's not set, it remains "walreceiver".)  If someone
set cluster_name to identify their instance, we might as well use that
by default to identify the node remotely as well.  It's still possible
to specify another application_name in primary_conninfo explicitly.
Reviewed-by: Euler Taveira <euler@timbira.com.br>
Discussion: https://www.postgresql.org/message-id/flat/1257eaee-4874-e791-e83a-46720c72cac7@2ndquadrant.com

doc/src/sgml/config.sgml

@@ -3659,7 +3659,8 @@ restore_command = 'copy "C:\\server\\archivedir\\%f" "%p"'  # Windows
         <varname>application_name</varname> setting of the standby, as set in the
         standby's connection information.  In case of a physical replication
         standby, this should be set in the <varname>primary_conninfo</varname>
-        setting; the default is <literal>walreceiver</literal>.
+        setting; the default is the setting of <xref linkend="guc-cluster-name"/>
+        if set, else <literal>walreceiver</literal>.
         For logical replication, this can be set in the connection
         information of the subscription, and it defaults to the
         subscription name.  For other replication stream consumers,
@@ -6560,8 +6561,15 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
       </term>
       <listitem>
        <para>
-        Sets the cluster name that appears in the process title for all
-        server processes in this cluster. The name can be any string of less
+        Sets a name that identifies this database cluster (instance) for
+        various purposes.  The cluster name appears in the process title for
+        all server processes in this cluster.  Moreover, it is the default
+        application name for a standby connection (see <xref
+        linkend="guc-synchronous-standby-names"/>.)
+       </para>
+
+       <para>
+        The name can be any string of less
         than <symbol>NAMEDATALEN</symbol> characters (64 characters in a standard
         build). Only printable ASCII characters may be used in the
         <varname>cluster_name</varname> value. Other characters will be

src/backend/replication/walreceiver.c

@@ -293,7 +293,7 @@ WalReceiverMain(void)
 
 	/* Establish the connection to the primary for XLOG streaming */
 	EnableWalRcvImmediateExit();
-	wrconn = walrcv_connect(conninfo, false, "walreceiver", &err);
+	wrconn = walrcv_connect(conninfo, false, cluster_name[0] ? cluster_name : "walreceiver", &err);
 	if (!wrconn)
 		ereport(ERROR,
 				(errmsg("could not connect to the primary server: %s", err)));