renepay: flow routes limited by htlc_max and htlc_min

The flow routes returned by minflow are bounded by the htlc_min and
htlc_max along the route whenever possible.

Signed-off-by: Lagrang3 <lagrang3@protonmail.com>

plugins/renepay/flow.h

@@ -322,4 +322,16 @@ void commit_flow_set(
 		struct chan_extra_map *chan_extra_map,
 		struct flow **flows);
 
+static inline struct amount_msat channel_htlc_max(
+		const struct gossmap_chan *chan,
+		const int dir)
+{
+	return amount_msat( fp16_to_u64(chan->half[dir].htlc_max) );
+}
+static inline struct amount_msat channel_htlc_min(
+		const struct gossmap_chan *chan,
+		const int dir)
+{
+	return amount_msat( fp16_to_u64(chan->half[dir].htlc_min) );
+}
 #endif /* LIGHTNING_PLUGINS_RENEPAY_FLOW_H */

plugins/renepay/mcf.c

@@ -3,6 +3,7 @@
 #include <ccan/list/list.h>
 #include <ccan/lqueue/lqueue.h>
 #include <ccan/tal/tal.h>
+#include <common/pseudorand.h>
 #include <common/type_to_string.h>
 #include <math.h>
 #include <plugins/renepay/dijkstra.h>
@@ -190,6 +191,7 @@
 static const double CHANNEL_PIVOTS[]={0,0.5,0.8,0.95};
 
 static const s64 INFINITE = INT64_MAX;
+static const u64 INFINITE_MSAT = UINT64_MAX;
 static const u32 INVALID_INDEX=0xffffffff;
 static const s64 MU_MAX = 128;
 
@@ -1105,6 +1107,12 @@ struct list_data
 	struct flow *flow_path;
 };
 
+static inline uint64_t pseudorand_interval(uint64_t a, uint64_t b)
+{
+	assert(b>a);
+	return a + pseudorand(b-a);
+}
+
 /* Given a flow in the residual network, build a set of payment flows in the
  * gossmap that corresponds to this flow. */
 static struct flow **
@@ -1181,6 +1189,9 @@ static struct flow **
 			u32 final_idx = find_positive_balance(gossmap,chan_flow,node_idx,balance,
 							prev_chan,prev_dir,prev_idx);
 
+			struct amount_msat sup_htlc_min = AMOUNT_MSAT_INIT(0),
+				inf_htlc_max = AMOUNT_MSAT_INIT(INFINITE_MSAT);
+
 			s64 delta=-balance[node_idx];
 			int length = 0;
 			delta = MIN(delta,balance[final_idx]);
@@ -1200,13 +1211,32 @@ static struct flow **
 				delta=MIN(delta,chan_flow[c_idx].half[dir]);
 				length++;
 
-				// TODO(eduardo) does htlc_max has any relevance
-				// here?
-				// HINT: delta=MIN(delta,htlc_max);
-				// however this might not work because often we
-				// move delta+fees
+				/* obtain the supremum htlc_min along the route */
+				sup_htlc_min = amount_msat_max(sup_htlc_min, channel_htlc_min(c,dir));
+
+				/* obtain the infimum htlc_max along the route */
+				inf_htlc_max = amount_msat_min(inf_htlc_max, channel_htlc_max(c,dir));
 			}
 
+			s64 htlc_max = inf_htlc_max.millisatoshis/1000; /* Raw: need htlc_max in sats to do arithmetic operations. */
+			s64 htlc_min = (sup_htlc_min.millisatoshis+999)/1000 /* Raw: need htlc_min in stas to do arithmetic operations. */;
+
+			if(htlc_min>htlc_max)
+			{
+				/* htlc_min is too big or htlc_max is too small, we
+				 * cannot send `delta` along this route.
+				 *
+				 * FIXME: We try anyways because failing channels will
+				 * be blacklisted downstream. */
+				htlc_min = 0;
+			}
+
+			/* If we divide this route into different flows make it
+			 * random to avoid routing nodes making correlations. */
+			if(delta>htlc_max)
+			{
+				delta = pseudorand_interval(MAX(htlc_min,(htlc_max*95)/100),htlc_max);
+			}
 
 			struct flow *fp = tal(this_ctx,struct flow);
 			fp->path = tal_arr(fp,const struct gossmap_chan *,length);