Fix handling of retractions with no next-hop again.

RFC 8966 Section 3.2.6 says that the route table is indexed by
triples of the form (prefix, plen, neigh).  Our indexing used to
be incorrect, we'd sometimes include the next hop address, which
made it impossible to interpret retractions with no next hop.

message.c

@@ -728,9 +728,13 @@ parse_packet(const unsigned char *from, struct interface *ifp,
                    format_prefix(prefix, plen),
                    format_address(from), ifp->name);
             if(message[2] == 1) {
-                if(!have_v4_nh)
-                    goto fail;
+                if(have_v4_nh) {
                     nh = v4_nh;
+                } else {
+                    if(metric < INFINITY)
+                        goto fail;
+                    nh = NULL;
+                }
             } else if(have_v6_nh) {
                 nh = v6_nh;
             } else {

route.c

@@ -130,7 +130,7 @@ find_route_slot(const unsigned char *prefix, unsigned char plen,
 struct babel_route *
 find_route(const unsigned char *prefix, unsigned char plen,
            const unsigned char *src_prefix, unsigned char src_plen,
-           struct neighbour *neigh, const unsigned char *nexthop)
+           struct neighbour *neigh)
 {
     struct babel_route *route;
     int i = find_route_slot(prefix, plen, src_prefix, src_plen, NULL);
@@ -141,7 +141,7 @@ find_route(const unsigned char *prefix, unsigned char plen,
     route = routes[i];
 
     while(route) {
-        if(route->neigh == neigh && memcmp(route->nexthop, nexthop, 16) == 0)
+        if(route->neigh == neigh)
             return route;
         route = route->next;
     }
@@ -885,7 +885,8 @@ update_route(const unsigned char *id,
     if(martian_prefix(prefix, plen) || martian_prefix(src_prefix, src_plen)) {
         fprintf(stderr, "Rejecting martian route to %s from %s through %s.\n",
                 format_prefix(prefix, plen),
-                format_prefix(src_prefix, src_plen), format_address(nexthop));
+                format_prefix(src_prefix, src_plen),
+                nexthop ? format_address(nexthop) : "(unknown)");
         return NULL;
     }
 
@@ -898,7 +899,7 @@ update_route(const unsigned char *id,
     if(add_metric >= INFINITY)
         return NULL;
 
-    route = find_route(prefix, plen, src_prefix, src_plen, neigh, nexthop);
+    route = find_route(prefix, plen, src_prefix, src_plen, neigh);
 
     if(refmetric >= INFINITY && !route) {
         /* Somebody's retracting a route that we've never seen. */
@@ -996,6 +997,12 @@ update_route(const unsigned char *id,
         if(refmetric >= INFINITY)
             /* Somebody's retracting a route we never saw. */
             return NULL;
+        if(nexthop == NULL) {
+            fprintf(stderr,
+                    "No nexthop in update_route, this shouldn't happen.\n");
+            return NULL;
+        }
+
         if(!feasible) {
             send_unfeasible_request(neigh, 0, seqno, metric, src);
         }

route.h

@@ -69,7 +69,7 @@ route_metric_noninterfering(const struct babel_route *route)
 
 struct babel_route *find_route(const unsigned char *prefix, unsigned char plen,
                         const unsigned char *src_prefix, unsigned char src_plen,
-                        struct neighbour *neigh, const unsigned char *nexthop);
+                        struct neighbour *neigh);
 struct babel_route *find_installed_route(const unsigned char *prefix,
                         unsigned char plen, const unsigned char *src_prefix,
                         unsigned char src_plen);