repmgr: automatically create slot name if missing

It's possible that a node was registered with "use_replication_slots=false"
but that was later changed to "use_replication_slots=true". If the node
was not subsequently re-registered, the node record will contain an empty
slot name, which will cause any slot creation operation during
"standby follow" or "node rejoin" to fail.

To prevent this happening, check for an empty slot name and automatically
set before proceeding.

Addresses GitHub #343.

dbutils.c

@@ -2592,6 +2592,36 @@ truncate_node_records(PGconn *conn)
 	return true;
 }
 
+
+bool
+update_node_record_slot_name(PGconn *primary_conn, int node_id, char *slot_name)
+{
+	PQExpBufferData query;
+	PGresult   *res = NULL;
+
+	initPQExpBuffer(&query);
+
+	appendPQExpBuffer(&query,
+					  " UPDATE repmgr.nodes "
+					  "    SET slot_name = '%s' "
+					  "  WHERE node_id = %i ",
+					  slot_name,
+					  node_id);
+	res = PQexec(primary_conn, query.data);
+	termPQExpBuffer(&query);
+
+	if (PQresultStatus(res) != PGRES_COMMAND_OK)
+	{
+		log_error(_("unable to set node record slot name:\n  %s"),
+				  PQerrorMessage(primary_conn));
+		PQclear(res);
+		return false;
+	}
+
+	PQclear(res);
+	return true;
+}
+
 void
 get_node_replication_stats(PGconn *conn, int server_version_num, t_node_info *node_info)
 {
@@ -3350,6 +3380,14 @@ get_event_records(PGconn *conn, int node_id, const char *node_name, const char *
 /* replication slot functions */
 /* ========================== */
 
+
+void
+create_slot_name(char *slot_name, int node_id)
+{
+	maxlen_snprintf(slot_name, "repmgr_slot_%i", node_id);
+}
+
+
 bool
 create_replication_slot(PGconn *conn, char *slot_name, int server_version_num, PQExpBufferData *error_msg)
 {