askrene: rework the caller of the MCF solver

We use a wrapper around the MCF solver that takes care of finding the
best linearization parameters and fixing the flow values to meet the
htlc_min and htlc_max constraints.
We have reworked the current implementation and made it a bit more
similar to renepay's version.

Out of 50000 simulated payment situations distributed accross payment
amounts of 1e2, 1e3, 1e4, 1e5 and 1e6 sats, we find that 133 failed
cases in the master branch turn to success with the current changes,
while only 3 success cases in the master are not solved by the changes.

                master
            +-------+------+
            | S     | F    |
        +---+-------+------+
        | S | 46329 | 133  |
changes +---+-------+------+
        | F | 3     | 3535 |
        +---+-------+------+

Out of the 133 cases that flipped from failure to success the failed
reasons were:

122 -> "Could not find route without excessive cost"
5   -> "We couldn't quite afford it"
5   -> "Amount *msat below minimum"
1   -> tripped an HTLC min check

Changelog-None.

Signed-off-by: Lagrang3 <[email protected]>

Author: Lagrang3

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)) {