Askrene: fix constraints

plugins/askrene/askrene.c

@@ -564,6 +564,13 @@ static struct command_result *do_getroutes(struct command *cmd,
 	/* we temporarily apply localmods */
 	gossmap_apply_localmods(askrene->gossmap, localmods);
 
+	/* I want to be able to disable channels while working on this query.
+	 * Layers are for user interaction and cannot be used for this purpose.
+	 */
+	rq->disabled_chans =
+	    tal_arrz(rq, bitmap,
+		     2 * BITMAP_NWORDS(gossmap_max_chan_idx(askrene->gossmap)));
+
 	/* localmods can add channels, so we need to allocate biases array
 	 * *afterwards* */
 	rq->biases =

plugins/askrene/askrene.h

@@ -2,6 +2,7 @@
 #define LIGHTNING_PLUGINS_ASKRENE_ASKRENE_H
 #include "config.h"
 #include <bitcoin/short_channel_id.h>
+#include <ccan/bitmap/bitmap.h>
 #include <ccan/htable/htable_type.h>
 #include <ccan/list/list.h>
 #include <common/amount.h>
@@ -60,6 +61,9 @@ struct route_query {
 
 	/* Additional per-htlc cost for local channels */
 	const struct additional_cost_htable *additional_costs;
+
+	/* channels we disable during computation to meet constraints */
+	bitmap *disabled_chans;
 };
 
 /* Given a gossmap channel, get the current known min/max */

plugins/askrene/flow.c

@@ -32,17 +32,24 @@ struct amount_msat flowset_delivers(struct plugin *plugin,
 	return final;
 }
 
-static double edge_probability(struct amount_msat sent,
-			       struct amount_msat mincap,
-			       struct amount_msat maxcap,
-			       struct amount_msat used)
+/* Stolen whole-cloth from @Lagrang3 in renepay's flow.c.  Wrong
+ * because of htlc overhead in reservations! */
+static double edge_probability(const struct route_query *rq,
+			       const struct short_channel_id_dir *scidd,
+			       struct amount_msat sent)
 {
 	struct amount_msat numerator, denominator;
+	struct amount_msat mincap, maxcap, additional;
+	const struct gossmap_chan *c = gossmap_find_chan(rq->gossmap, &scidd->scid);
 
-	if (!amount_msat_sub(&mincap, mincap, used))
-		mincap = AMOUNT_MSAT(0);
-	if (!amount_msat_sub(&maxcap, maxcap, used))
-		maxcap = AMOUNT_MSAT(0);
+	get_constraints(rq, c, scidd->dir, &mincap, &maxcap);
+
+	/* We add an extra per-htlc reservation for the *next* HTLC, so we "over-reserve"
+	 * on local channels.  Undo that! */
+	additional = get_additional_per_htlc_cost(rq, scidd);
+	if (!amount_msat_accumulate(&mincap, additional)
+	    || !amount_msat_accumulate(&maxcap, additional))
+		abort();
 
 	if (amount_msat_less_eq(sent, mincap))
 		return 1.0;
@@ -129,10 +136,11 @@ double flow_probability(const struct flow *flow,
 
 	for (int i = (int)pathlen - 1; i >= 0; i--) {
 		const struct half_chan *h = flow_edge(flow, i);
-		struct amount_msat mincap, maxcap;
+		struct short_channel_id_dir scidd;
+		scidd.scid = gossmap_chan_scid(rq->gossmap, flow->path[i]);
+		scidd.dir = flow->dirs[i];
 
-		get_constraints(rq, flow->path[i], flow->dirs[i], &mincap, &maxcap);
-		prob *= edge_probability(spend, mincap, maxcap, AMOUNT_MSAT(0));
+		prob *= edge_probability(rq, &scidd, spend);
 
 		if (!amount_msat_add_fee(&spend, h->base_fee,
 					 h->proportional_fee)) {