Prevent "invalid LSN returned from node..." infinite loop

Currently in repmgrd3, if a repmgrd enters failover, but one or more other
repmgrds do not (e.g. partial primary invisibility), the repmgrd in failover
may enter an infinite loop waiting for the repmgrd(s) not in failover to
update shared memory.

dbutils.c

@@ -2095,3 +2095,48 @@ get_data_checksum_version(const char *data_directory)
 
 	return (int)control_file.data_checksum_version;
 }
+
+
+
+/* ========================== */
+/* backported from repmgr 4.x */
+/* ========================== */
+
+XLogRecPtr
+parse_lsn(const char *str)
+{
+	XLogRecPtr		ptr = InvalidXLogRecPtr;
+	uint32			high,
+					low;
+
+	if (sscanf(str, "%x/%x", &high, &low) == 2)
+		ptr = (((XLogRecPtr) high) << 32) + (XLogRecPtr) low;
+
+	return ptr;
+}
+
+
+XLogRecPtr
+get_last_wal_receive_location(PGconn *conn)
+{
+	PGresult   *res = NULL;
+	XLogRecPtr	ptr = InvalidXLogRecPtr;
+
+	if (PQserverVersion(conn) >= 100000)
+	{
+		res = PQexec(conn, "SELECT pg_catalog.pg_last_wal_receive_lsn()");
+	}
+	else
+	{
+		res = PQexec(conn, "SELECT pg_catalog.pg_last_xlog_receive_location()");
+	}
+
+	if (PQresultStatus(res) == PGRES_TUPLES_OK)
+	{
+		ptr = parse_lsn(PQgetvalue(res, 0, 0));
+	}
+
+	PQclear(res);
+
+	return ptr;
+}