hostgw: Check existence of and compare routes before attempting to add/update them

Closes a bug where hostgw.network.rl (route list) would be out of
sync with the host system in case of startup with existing routes.

Gives hostgw the ability to update existing routes that does not
match configuration.

Improves usability by avoiding to log non-errors to the user.

backend/hostgw/network.go

@@ -92,7 +92,37 @@ func (n *network) handleSubnetEvents(batch []subnet.Event) {
 				Gw:        evt.Lease.Attrs.PublicIP.ToIP(),
 				LinkIndex: n.linkIndex,
 			}
-			if err := netlink.RouteAdd(&route); err != nil {
+
+			// Check if route exists before attempting to add it
+			//   Search for route with same Dst Address and Interface
+			routeAf := netlink.FAMILY_V4
+
+			// XXX Comment this in as soon as we have net library with IP.To4()
+			// 
+			// routeAf := netlink.FAMILY_V6
+			// if route.Dst.To4() != nil {
+			//	routeAf = netlink.FAMILY_V4
+			// }
+			routeList, err := netlink.RouteListFiltered(routeAf, &netlink.Route{
+				Dst:       route.Dst,
+				LinkIndex: route.LinkIndex,
+			}, netlink.RT_FILTER_DST|netlink.RT_FILTER_OIF)
+			if err != nil {
+				log.Warningf("Unable to list routes: %v", err)
+			}
+			//   Check match on Dst for match on Gw
+			if len(routeList) > 0 && !routeList[0].Gw.Equal(route.Gw) {
+				// Same Dst different Gw. Remove it, correct route will be added below.
+				log.Warningf("Replacing existing route to %v via %v with %v via %v.", evt.Lease.Subnet, routeList[0].Gw, evt.Lease.Subnet, evt.Lease.Attrs.PublicIP)
+				if err := netlink.RouteDel(&route); err != nil {
+					log.Errorf("Error deleting route to %v: %v", evt.Lease.Subnet, err)
+					continue
+				}
+			}
+			if len(routeList) > 0 && routeList[0].Gw.Equal(route.Gw) {
+				// Same Dst and same Gw, keep it and do not attempt to add it.
+				log.Infof("Route to %v via %v allready exists, skipping.", evt.Lease.Subnet, evt.Lease.Attrs.PublicIP)
+			} else if err := netlink.RouteAdd(&route); err != nil {
 				log.Errorf("Error adding route to %v via %v: %v", evt.Lease.Subnet, evt.Lease.Attrs.PublicIP, err)
 				continue
 			}