xpay: don't give up when the onion will not fit

.msggen.json

@@ -2249,6 +2249,7 @@
             "GetRoutes.layers[]": 4,
             "GetRoutes.maxdelay": 8,
             "GetRoutes.maxfee_msat": 5,
+            "GetRoutes.maxhops": 10,
             "GetRoutes.maxparts": 9,
             "GetRoutes.source": 1
         },
@@ -8816,6 +8817,10 @@
             "added": "v24.08",
             "deprecated": null
         },
+        "GetRoutes.maxhops": {
+            "added": "v26.09",
+            "deprecated": null
+        },
         "GetRoutes.maxparts": {
             "added": "v25.09",
             "deprecated": null

contrib/msggen/msggen/schema.json

@@ -17251,7 +17251,7 @@
       "title": "Command for routing a payment",
       "added": "v24.08",
       "description": [
-        "The **getroutes** RPC command attempts to find the best set of paths for the payment from *source* to *destination* of *amount_msat*, using the given *layers* on top of the gossip information.  The result is constrained by *maxfee*, and will arrive at the destination with *final_cltv*.",
+        "The **getroutes** RPC command attempts to find the best set of paths for the payment from *source* to *destination* of *amount_msat*, using the given *layers* on top of the gossip information.  The result is constrained by *maxfee*, and will arrive at the destination with *final_cltv*.  If *maxhops* is set, no returned path is longer than that.",
         "",
         "NOTE: The returned paths are a different format then *getroute*, being more appropriate for creating intermediary onion layers.",
         "",
@@ -17327,6 +17327,14 @@
               "Maximum number of routes in the solution."
             ],
             "default": "100"
+          },
+          "maxhops": {
+            "type": "u32",
+            "added": "v26.09",
+            "description": [
+              "Maximum hops in any returned path. 0 means no limit."
+            ],
+            "default": "0"
           }
         }
       },

doc/schemas/getroutes.json

@@ -5,7 +5,7 @@
   "title": "Command for routing a payment",
   "added": "v24.08",
   "description": [
-    "The **getroutes** RPC command attempts to find the best set of paths for the payment from *source* to *destination* of *amount_msat*, using the given *layers* on top of the gossip information.  The result is constrained by *maxfee*, and will arrive at the destination with *final_cltv*.",
+    "The **getroutes** RPC command attempts to find the best set of paths for the payment from *source* to *destination* of *amount_msat*, using the given *layers* on top of the gossip information.  The result is constrained by *maxfee*, and will arrive at the destination with *final_cltv*.  If *maxhops* is set, no returned path is longer than that.",
     "",
     "NOTE: The returned paths are a different format then *getroute*, being more appropriate for creating intermediary onion layers.",
     "",
@@ -81,6 +81,14 @@
           "Maximum number of routes in the solution."
         ],
         "default": "100"
+      },
+      "maxhops": {
+        "type": "u32",
+        "added": "v26.09",
+        "description": [
+          "Maximum hops in any returned path. 0 means no limit."
+        ],
+        "default": "0"
       }
     }
   },

plugins/askrene/askrene.c

@@ -359,6 +359,8 @@ struct getroutes_info {
 	/* Non-NULL if we are told to use "auto.localchans" */
 	struct layer *local_layer;
 	u32 maxparts;
+	/* 0 means no hop limit. */
+	size_t maxhops;
 };
 
 static void add_layer(const struct layer ***layers,
@@ -717,7 +719,7 @@ static struct command_result *do_getroutes(struct command *cmd,
 			  info->dev_algo == ALGO_SINGLE_PATH,
 			  deadline, srcnode, dstnode, info->amount,
 			  info->maxfee, info->finalcltv, info->maxdelay, info->maxparts,
-			  include_fees,
+			  info->maxhops, include_fees,
 			  cmd->idstr, cmd->filter,
 			  include_next_node_id,
 			  include_amount_msat,
@@ -907,7 +909,7 @@ static struct command_result *json_getroutes(struct command *cmd,
 	struct amount_msat *amount, *maxfee;
 	u32 *finalcltv, *maxdelay;
 	enum algorithm *dev_algo;
-	u32 *maxparts;
+	u32 *maxparts, *maxhops;
 
 	if (!param_check(cmd, buffer, params,
 			 p_req("source", param_node_id, &source),
@@ -920,6 +922,7 @@ static struct command_result *json_getroutes(struct command *cmd,
 				   maxdelay_allowed),
 			 p_opt_def("maxparts", param_u32, &maxparts,
 				   default_maxparts),
+			 p_opt_def("maxhops", param_u32, &maxhops, 0),
 			 p_opt_dev("dev_algorithm", param_algorithm,
 				   &dev_algo, ALGO_DEFAULT),
 			 NULL))
@@ -961,6 +964,7 @@ static struct command_result *json_getroutes(struct command *cmd,
 	info->dev_algo = *dev_algo;
 	info->additional_costs = new_htable(info, additional_cost_htable);
 	info->maxparts = *maxparts;
+	info->maxhops = *maxhops;
 
 	if (askrene->num_live_requests >= askrene->max_children) {
 		cmd_log(tmpctx, cmd, LOG_INFORM,

plugins/askrene/child/algorithm.c

@@ -75,9 +75,10 @@ bool dijkstra_path(const tal_t *ctx, const struct graph *graph,
 		   const struct node source, const struct node destination,
 		   bool prune, const s64 *capacity, const s64 cap_threshold,
 		   const s64 *cost, const s64 *potential, struct arc *prev,
-		   s64 *distance)
+		   s64 *distance, size_t max_hops)
 {
 	bool target_found = false;
+	u16 *hops;
 	assert(graph);
 	const size_t max_num_arcs = graph_max_num_arcs(graph);
 	const size_t max_num_nodes = graph_max_num_nodes(graph);
@@ -105,6 +106,14 @@ bool dijkstra_path(const tal_t *ctx, const struct graph *graph,
 	for (size_t i = 0; i < max_num_nodes; ++i)
 		prev[i].idx = INVALID_INDEX;
 
+	hops = NULL;
+	if (max_hops) {
+		hops = tal_arr(this_ctx, u16, max_num_nodes);
+		for (size_t i = 0; i < max_num_nodes; ++i)
+			hops[i] = UINT16_MAX;
+		hops[source.idx] = 0;
+	}
+
 	struct priorityqueue *q;
 	q = priorityqueue_new(this_ctx, max_num_nodes);
 	const s64 *const dijkstra_distance = priorityqueue_value(q);
@@ -143,13 +152,18 @@ bool dijkstra_path(const tal_t *ctx, const struct graph *graph,
 			/* Dijkstra only works with non-negative weights */
 			assert(cij >= 0);
 
+			if (max_hops && hops[cur] >= max_hops)
+				continue;
+
 			if (dijkstra_distance[next.idx] <=
 			    dijkstra_distance[cur] + cij)
 				continue;
 
 			priorityqueue_update(q, next.idx,
 					     dijkstra_distance[cur] + cij);
 			prev[next.idx] = arc;
+			if (max_hops)
+				hops[next.idx] = hops[cur] + 1;
 		}
 	}
 	for (size_t i = 0; i < max_num_nodes; i++)

plugins/askrene/child/algorithm.h

@@ -62,11 +62,12 @@ bool BFS_path(const tal_t *ctx, const struct graph *graph,
  * cost[i]>=0
  * if prune is true the destination must be valid
  * */
+/* max_hops is the arc limit on the reconstructed path.  0 means none. */
 bool dijkstra_path(const tal_t *ctx, const struct graph *graph,
 		   const struct node source, const struct node destination,
 		   bool prune, const s64 *capacity, const s64 cap_threshold,
 		   const s64 *cost, const s64 *potential, struct arc *prev,
-		   s64 *distance);
+		   s64 *distance, size_t max_hops);
 
 
 /* Finds any flow that satisfy the capacity constraints:

plugins/askrene/child/child.c

@@ -219,8 +219,8 @@ void run_child(const struct gossmap *gossmap,
 	       const struct gossmap_node *srcnode,
 	       const struct gossmap_node *dstnode,
 	       struct amount_msat amount, struct amount_msat maxfee,
-	       u32 finalcltv, u32 maxdelay, size_t maxparts,
-              bool include_fees,
+	       u32 finalcltv, u32 maxdelay, size_t maxparts, size_t maxhops,
+	       bool include_fees,
 	       const char *cmd_id,
 	       struct json_filter *cmd_filter,
 	       bool include_next_node_id,
@@ -243,11 +243,13 @@ void run_child(const struct gossmap *gossmap,
 	if (single_path) {
 		err = single_path_routes(rq, rq, deadline, srcnode, dstnode,
 					 amount, maxfee, finalcltv,
-					 maxdelay, &flows, &probability, &ecode);
+					 maxdelay, maxhops, &flows,
+					 &probability, &ecode);
 	} else {
 		err = default_routes(rq, rq, deadline, srcnode, dstnode,
 				     amount, maxfee, finalcltv, maxdelay,
-				     maxparts, &flows, &probability, &ecode);
+				     maxparts, maxhops, &flows, &probability,
+				     &ecode);
 	}
 	if (err) {
 		write_all(replyfd, &ecode, sizeof(ecode));

plugins/askrene/child/child.h

@@ -27,7 +27,7 @@ void run_child(const struct gossmap *gossmap,
 	       const struct gossmap_node *srcnode,
 	       const struct gossmap_node *dstnode,
 	       struct amount_msat amount, struct amount_msat maxfee,
-	       u32 finalcltv, u32 maxdelay, size_t maxparts,
+	       u32 finalcltv, u32 maxdelay, size_t maxparts, size_t maxhops,
 	       bool include_fees,
 	       const char *cmd_id,
 	       struct json_filter *cmd_filter,

plugins/askrene/child/mcf.c

@@ -951,7 +951,8 @@ static struct flow **minflow(const tal_t *ctx,
 			     const struct gossmap_node *target,
 			     struct amount_msat amount,
 			     u32 mu,
-			     double delay_feefactor)
+			     double delay_feefactor,
+			     size_t maxhops)
 {
 	struct flow **flow_paths;
 	/* We allocate everything off this, and free it at the end,
@@ -986,6 +987,7 @@ static struct flow **minflow(const tal_t *ctx,
 
 	params->delay_feefactor = delay_feefactor;
 	params->base_fee_penalty = base_fee_penalty_estimate(amount);
+	(void)maxhops;
 
 	// build the uncertainty network with linearization and residual arcs
 	struct graph *graph;
@@ -1171,7 +1173,7 @@ static struct flow **single_path_flow(const tal_t *ctx, const struct route_query
 				      const struct gossmap_node *source,
 				      const struct gossmap_node *target,
 				      struct amount_msat amount, u32 mu,
-				      double delay_feefactor)
+				      double delay_feefactor, size_t maxhops)
 {
 	struct flow **flow_paths;
 	/* We allocate everything off this, and free it at the end,
@@ -1216,7 +1218,7 @@ static struct flow **single_path_flow(const tal_t *ctx, const struct route_query
 	if (!dijkstra_path(working_ctx, graph, src, dst,
 			   /* prune = */ true, arc_capacity,
 			   /*threshold = */ 1, arc_cost, potential, prev,
-			   distance)) {
+			   distance, maxhops)) {
                 /* This might fail if we are unable to find a suitable route, it
                  * doesn't mean the plugin is broken, that's why we LOG_INFORM. */
 		child_log(tmpctx, LOG_INFORM,
@@ -1348,13 +1350,14 @@ linear_routes(const tal_t *ctx, struct route_query *rq,
 	      const struct gossmap_node *srcnode,
 	      const struct gossmap_node *dstnode, struct amount_msat amount,
 	      struct amount_msat maxfee, u32 finalcltv, u32 maxdelay,
-	      size_t maxparts,
+	      size_t maxparts, size_t maxhops,
 	      struct flow ***flows, double *probability,
 	      enum jsonrpc_errcode *ecode,
 	      struct flow **(*solver)(const tal_t *, const struct route_query *,
 				      const struct gossmap_node *,
 				      const struct gossmap_node *,
-				      struct amount_msat, u32, double))
+				      struct amount_msat, u32, double,
+				      size_t))
 {
 	const tal_t *working_ctx = tal(ctx, tal_t);
 	const char *error_message;
@@ -1399,11 +1402,13 @@ linear_routes(const tal_t *ctx, struct route_query *rq,
 					SINGLE_PATH_THRESHOLD)) {
 			new_flows = single_path_flow(working_ctx, rq, srcnode,
 						     dstnode, amount_to_deliver,
-						     mu, delay_feefactor);
+						     mu, delay_feefactor,
+						     maxhops);
 		} else {
 			new_flows =
 			    solver(working_ctx, rq, srcnode, dstnode,
-				   amount_to_deliver, mu, delay_feefactor);
+				   amount_to_deliver, mu, delay_feefactor,
+				   maxhops);
 		}
 
 		if (!new_flows) {
@@ -1505,6 +1510,26 @@ linear_routes(const tal_t *ctx, struct route_query *rq,
 			}
 		}
 
+		/* Caller asked for a hop cap.  Disable the first hop and retry. */
+		if (maxhops) {
+			bool too_long = false;
+			for (size_t i = 0; i < tal_count(new_flows); i++) {
+				if (tal_count(new_flows[i]->path) <= maxhops)
+					continue;
+				too_long = true;
+				bitmap_set_bit(rq->disabled_chans,
+					       gossmap_chan_idx(rq->gossmap,
+								new_flows[i]->path[0]) * 2
+					       + new_flows[i]->dirs[0]);
+			}
+			if (too_long) {
+				child_log(tmpctx, LOG_INFORM,
+					  "A flow exceeds maxhops %zu, disabling its first hop and retrying",
+					  maxhops);
+				continue;
+			}
+		}
+
 		/* Too much delay? */
 		if (finalcltv + flows_worst_delay(new_flows) > maxdelay) {
 			if (delay_feefactor > 10) {
@@ -1639,13 +1664,13 @@ const char *default_routes(const tal_t *ctx, struct route_query *rq,
 			   const struct gossmap_node *dstnode,
 			   struct amount_msat amount, struct amount_msat maxfee,
 			   u32 finalcltv, u32 maxdelay, size_t maxparts,
-			   struct flow ***flows,
+			   size_t maxhops, struct flow ***flows,
 			   double *probability,
 			   enum jsonrpc_errcode *ecode)
 {
 	return linear_routes(ctx, rq, deadline, srcnode, dstnode, amount, maxfee,
-			     finalcltv, maxdelay, maxparts, flows, probability, ecode,
-			     minflow);
+			     finalcltv, maxdelay, maxparts, maxhops, flows,
+			     probability, ecode, minflow);
 }
 
 const char *single_path_routes(const tal_t *ctx, struct route_query *rq,
@@ -1654,11 +1679,11 @@ const char *single_path_routes(const tal_t *ctx, struct route_query *rq,
 			       const struct gossmap_node *dstnode,
 			       struct amount_msat amount,
 			       struct amount_msat maxfee, u32 finalcltv,
-			       u32 maxdelay, struct flow ***flows,
-			       double *probability,
+			       u32 maxdelay, size_t maxhops,
+			       struct flow ***flows, double *probability,
 			       enum jsonrpc_errcode *ecode)
 {
 	return linear_routes(ctx, rq, deadline, srcnode, dstnode, amount, maxfee,
-			     finalcltv, maxdelay, 1, flows, probability, ecode,
-			     single_path_flow);
+			     finalcltv, maxdelay, 1, maxhops, flows, probability,
+			     ecode, single_path_flow);
 }

plugins/askrene/child/mcf.h

@@ -18,8 +18,8 @@ const char *default_routes(const tal_t *ctx, struct route_query *rq,
 			   const struct gossmap_node *dstnode,
 			   struct amount_msat amount,
 			   struct amount_msat maxfee, u32 finalcltv,
-			   u32 maxdelay, size_t maxparts, struct flow ***flows,
-			   double *probability,
+			   u32 maxdelay, size_t maxparts, size_t maxhops,
+			   struct flow ***flows, double *probability,
 			   enum jsonrpc_errcode *ecode);
 
 /* A wrapper to the single-path constrained solver. */
@@ -29,8 +29,8 @@ const char *single_path_routes(const tal_t *ctx, struct route_query *rq,
 			       const struct gossmap_node *dstnode,
 			       struct amount_msat amount,
 			       struct amount_msat maxfee, u32 finalcltv,
-			       u32 maxdelay, struct flow ***flows,
-			       double *probability,
+			       u32 maxdelay, size_t maxhops,
+			       struct flow ***flows, double *probability,
 			       enum jsonrpc_errcode *ecode);
 
 #endif /* LIGHTNING_PLUGINS_ASKRENE_CHILD_MCF_H */

plugins/askrene/test/run-dijkstra.c

@@ -84,7 +84,7 @@ int main(int argc, char *argv[])
 	struct node dst = {.idx = 6};
 
 	bool result = dijkstra_path(ctx, graph, src, dst, false, capacity, 1,
-				    cost, potential, prev, distance);
+				    cost, potential, prev, distance, 0);
 	CHECK(result);
 
 	int pathlen = 0;