paymod: Track how many HTLCs each channel can still add

It turns out that by aggressively splitting payments we may end up exhausting
the number of HTLCs we can add to a channel quickly. By tracking the number of
HTLCs we can still add, and excluding the channels to which we cannot add any
more we increase the route diversity, and avoid quickly exhausting the HTLC
budget.

In the next commit we'll also implement an early abort if we've exhausted all
channels, so we don't end up splitting indefinitely and we can also optimize
the initial split to not run afoul of that limit.

plugins/libplugin-pay.c

@@ -198,6 +198,7 @@ static void payment_exclude_most_expensive(struct payment *p)
 	hint.scid.scid = e->channel_id;
 	hint.scid.dir = e->direction;
 	hint.enabled = false;
+	hint.local = false;
 	tal_arr_expand(&root->channel_hints, hint);
 }
 
@@ -218,6 +219,7 @@ static void payment_exclude_longest_delay(struct payment *p)
 	hint.scid.scid = e->channel_id;
 	hint.scid.dir = e->direction;
 	hint.enabled = false;
+	hint.local = false;
 	tal_arr_expand(&root->channel_hints, hint);
 }
 
@@ -278,6 +280,14 @@ static void payment_chanhints_apply_route(struct payment *p, bool remove)
 			if (short_channel_id_eq(&curhint->scid.scid,
 						&curhop->channel_id) &&
 			    curhint->scid.dir == curhop->direction) {
+
+				/* Update the number of htlcs for any local
+				 * channel in the route */
+				if (curhint->local && remove)
+					curhint->htlc_budget++;
+				else if (curhint->local)
+					curhint->htlc_budget--;
+
 				if (remove && !amount_msat_add(
 						  &curhint->estimated_capacity,
 						  curhint->estimated_capacity,
@@ -392,6 +402,10 @@ payment_get_excluded_channels(const tal_t *ctx, struct payment *p)
 		else if (amount_msat_greater_eq(p->amount,
 						hint->estimated_capacity))
 			tal_arr_expand(&res, hint->scid);
+		else if (hint->local && hint->htlc_budget == 0)
+			/* If we cannot add any HTLCs to the channel we
+			 * shouldn't look for a route through that channel */
+			tal_arr_expand(&res, hint->scid);
 	}
 	return res;
 }
@@ -1603,7 +1617,8 @@ static struct command_result *
 local_channel_hints_listpeers(struct command *cmd, const char *buffer,
 			      const jsmntok_t *toks, struct payment *p)
 {
-	const jsmntok_t *peers, *peer, *channels, *channel, *spendsats, *scid, *dir, *connected;
+	const jsmntok_t *peers, *peer, *channels, *channel, *spendsats, *scid,
+	    *dir, *connected, *max_htlc, *htlcs;
 	size_t i, j;
 	peers = json_get_member(buffer, toks, "peers");
 
@@ -1622,7 +1637,12 @@ local_channel_hints_listpeers(struct command *cmd, const char *buffer,
 			spendsats = json_get_member(buffer, channel, "spendable_msat");
 			scid = json_get_member(buffer, channel, "short_channel_id");
 			dir = json_get_member(buffer, channel, "direction");
-			if(spendsats == NULL || scid == NULL || dir == NULL)
+			max_htlc = json_get_member(buffer, channel, "max_accepted_htlcs");
+			htlcs = json_get_member(buffer, channel, "htlcs");
+			if (spendsats == NULL || scid == NULL || dir == NULL ||
+			    max_htlc == NULL ||
+			    max_htlc->type != JSMN_PRIMITIVE || htlcs == NULL ||
+			    htlcs->type != JSMN_ARRAY)
 				continue;
 
 			json_to_bool(buffer, connected, &h.enabled);
@@ -1630,6 +1650,16 @@ local_channel_hints_listpeers(struct command *cmd, const char *buffer,
 			json_to_int(buffer, dir, &h.scid.dir);
 
 			json_to_msat(buffer, spendsats, &h.estimated_capacity);
+
+			/* Take the configured number of max_htlcs and
+			 * subtract any HTLCs that might already be added to
+			 * the channel. This is a best effort estimate and
+			 * mostly considers stuck htlcs, concurrent payments
+			 * may throw us off a bit. */
+			json_to_u16(buffer, max_htlc, &h.htlc_budget);
+			h.htlc_budget -= htlcs->size;
+			h.local = true;
+
 			tal_arr_expand(&p->channel_hints, h);
 		}
 	}

plugins/libplugin-pay.h

@@ -69,6 +69,14 @@ struct channel_hint {
 
 	/* Is the channel enabled? */
 	bool enabled;
+
+	/* True if we are one endpoint of this channel */
+	bool local;
+
+	/* How many more htlcs can we send over this channel? Only set if this
+	 * is a local channel, because those are the channels we have exact
+	 * numbers on, and they are the bottleneck onto the network. */
+	u16 htlc_budget;
 };
 
 /* Each payment goes through a number of steps that are always processed in