route: return NULL if destination is unreachable.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/route.c

@@ -61,16 +61,20 @@ u64 route_score_cheaper(u32 distance,
 	return (costs << 32) + distance;
 }
 
-struct route **route_from_dijkstra(const struct gossmap *map,
+struct route **route_from_dijkstra(const tal_t *ctx,
+				   const struct gossmap *map,
 				   const struct dijkstra *dij,
 				   const struct gossmap_node *cur)
 {
-	struct route **path = tal_arr(map, struct route *, 0);
+	struct route **path = tal_arr(ctx, struct route *, 0);
 	u32 curidx = gossmap_node_idx(map, cur);
 
 	while (dijkstra_distance(dij, curidx) != 0) {
 		struct route *r;
 
+		if (dijkstra_distance(dij, curidx) == UINT_MAX)
+			return tal_free(path);
+
 		r = tal(path, struct route);
 		r->c = dijkstra_best_chan(dij, curidx);
 		if (r->c->half[0].nodeidx == curidx) {

common/route.h

@@ -36,8 +36,9 @@ u64 route_score_cheaper(u32 distance,
 			struct amount_msat cost,
 			struct amount_msat risk);
 
-/* Extract route tal_arr from completed dijkstra */
+/* Extract route tal_arr from completed dijkstra: NULL if none. */
-struct route **route_from_dijkstra(const struct gossmap *map,
+struct route **route_from_dijkstra(const tal_t *ctx,
+				   const struct gossmap *map,
 				   const struct dijkstra *dij,
 				   const struct gossmap_node *cur);
 #endif /* LIGHTNING_COMMON_ROUTE_H */

devtools/route.c

@@ -69,7 +69,7 @@ static struct route **least_cost(struct gossmap *map,
 		return NULL;
 	}
 
-	path = route_from_dijkstra(map, dij, src);
+	path = route_from_dijkstra(map, map, dij, src);
 	printf("# path length %zu\n", tal_count(path));
 	/* We don't pay fee on first hop! */
 	if (!amount_msat_sub(&fee,

devtools/topology.c

@@ -209,7 +209,7 @@ static bool measure_least_cost(struct gossmap *map,
 		return false;
 	}
 
-	path = route_from_dijkstra(map, dij, src);
+	path = route_from_dijkstra(map, map, dij, src);
 	printf("# path length %zu\n", tal_count(path));
 	if (!amount_msat_sub(&fee, dijkstra_amount(dij, srcidx), sent))
 		abort();