Relax the channel saturation limit if we can't find enough paths

In order to avoid failing to find paths due to the new channel
saturation limit, if we fail to find enough paths, we simply
disable the saturation limit for further path finding iterations.

Because we can now increase the maximum sent over a given channel
during routefinding, we may now generate redundant paths for the
same payment. Because this is wasteful in the network, we add an
additional pass during routefinding to merge redundant paths.

Note that two tests which previously attempted to send exactly the
available liquidity over a channel which charged an absolute fee
need updating - in those cases the router will first collect a path
that is saturation-limited, then attempt to collect a second path
without a saturation limit while stil honoring the existing
utilized capacity on the channel, causing failure as the absolute
fee must be included.

Author: TheBlueMatt

lightning/src/ln/functional_tests.rs

@@ -1823,9 +1823,12 @@ fn test_channel_reserve_holding_cell_htlcs() {
 
 	// attempt to send amt_msat > their_max_htlc_value_in_flight_msat
 	{
-		let (mut route, our_payment_hash, _, our_payment_secret) = get_route_and_payment_hash!(nodes[0], nodes[2], recv_value_0);
+		let payment_params = PaymentParameters::from_node_id(nodes[2].node.get_our_node_id())
+			.with_features(InvoiceFeatures::known()).with_max_channel_saturation_power_of_half(0);
+		let (mut route, our_payment_hash, _, our_payment_secret) = get_route_and_payment_hash!(nodes[0], nodes[2], payment_params, recv_value_0, TEST_FINAL_CLTV);
 		route.paths[0].last_mut().unwrap().fee_msat += 1;
 		assert!(route.paths[0].iter().rev().skip(1).all(|h| h.fee_msat == feemsat));
+
 		unwrap_send_err!(nodes[0].node.send_payment(&route, our_payment_hash, &Some(our_payment_secret)), true, APIError::ChannelUnavailable { ref err },
 			assert!(regex::Regex::new(r"Cannot send value that would put us over the max HTLC value in flight our peer will accept \(\d+\)").unwrap().is_match(err)));
 		assert!(nodes[0].node.get_and_clear_pending_msg_events().is_empty());
@@ -1844,7 +1847,12 @@ fn test_channel_reserve_holding_cell_htlcs() {
 		if stat01.value_to_self_msat < stat01.channel_reserve_msat + commit_tx_fee_all_htlcs + ensure_htlc_amounts_above_dust_buffer + amt_msat {
 			break;
 		}
-		send_payment(&nodes[0], &vec![&nodes[1], &nodes[2]][..], recv_value_0);
+
+		let payment_params = PaymentParameters::from_node_id(nodes[2].node.get_our_node_id())
+			.with_features(InvoiceFeatures::known()).with_max_channel_saturation_power_of_half(0);
+		let route = get_route!(nodes[0], payment_params, recv_value_0, TEST_FINAL_CLTV).unwrap();
+		let (payment_preimage, ..) = send_along_route(&nodes[0], route, &[&nodes[1], &nodes[2]], recv_value_0);
+		claim_payment(&nodes[0], &[&nodes[1], &nodes[2]], payment_preimage);
 
 		let (stat01_, stat11_, stat12_, stat22_) = (
 			get_channel_value_stat!(nodes[0], chan_1.2),

lightning/src/ln/payment_tests.rs

@@ -942,7 +942,7 @@ fn failed_probe_yields_event() {
 
 	let payment_params = PaymentParameters::from_node_id(nodes[2].node.get_our_node_id());
 
-	let (route, _, _, _) = get_route_and_payment_hash!(&nodes[0], nodes[2], &payment_params, 9_999_000, 42);
+	let (route, _, _, _) = get_route_and_payment_hash!(&nodes[0], nodes[2], &payment_params, 9_998_000, 42);
 
 	let (payment_hash, payment_id) = nodes[0].node.send_probe(route.paths[0].clone()).unwrap();
 

lightning/src/routing/router.rs

@@ -230,6 +230,10 @@ pub struct PaymentParameters {
 	/// channel, as a power of 1/2. A higher value prefers to send the payment using more MPP parts
 	/// whereas a lower value prefers to send larger MPP parts, potentially saturating channels.
 	///
+	/// Note that this restriction will be relaxed during pathfinding after paths which meet this
+	/// restriction have been found. While paths which meet this criteria will be searched for, it
+	/// is ultimately up to the scorer to select them over other paths.
+	///
 	/// A value of 0 will allow payments up to and including a channel's total announced usable
 	/// capacity, a value of one will only use up to half its capacity, two 1/4, etc.
 	///
@@ -257,10 +261,6 @@ impl PaymentParameters {
 			expiry_time: None,
 			max_total_cltv_expiry_delta: DEFAULT_MAX_TOTAL_CLTV_EXPIRY_DELTA,
 			max_path_count: DEFAULT_MAX_PATH_COUNT,
-			#[cfg(test)] // Many tests were written prior to the introduction of this parameter, so
-			             // we leave it as 0 by default in tests, and change it for a few.
-			max_channel_saturation_power_of_half: 0,
-			#[cfg(not(test))]
 			max_channel_saturation_power_of_half: 1,
 		}
 	}
@@ -304,6 +304,13 @@ impl PaymentParameters {
 	pub fn with_max_path_count(self, max_path_count: u8) -> Self {
 		Self { max_path_count, ..self }
 	}
+
+	/// Includes a limit for the maximum number of payment paths that may be used.
+	///
+	/// (C-not exported) since bindings don't support move semantics
+	pub fn with_max_channel_saturation_power_of_half(self, max_channel_saturation_power_of_half: u8) -> Self {
+		Self { max_channel_saturation_power_of_half, ..self }
+	}
 }
 
 /// A list of hops along a payment path terminating with a channel to the recipient.
@@ -488,6 +495,17 @@ fn max_htlc_from_capacity(capacity: EffectiveCapacity, max_channel_saturation_po
 	}
 }
 
+fn iter_equal<I1: Iterator, I2: Iterator>(mut iter_a: I1, mut iter_b: I2)
+-> bool where I1::Item: PartialEq<I2::Item> {
+	loop {
+		let a = iter_a.next();
+		let b = iter_b.next();
+		if a.is_none() && b.is_none() { return true; }
+		if a.is_none() || b.is_none() { return false; }
+		if a.unwrap().ne(&b.unwrap()) { return false; }
+	}
+}
+
 /// It's useful to keep track of the hops associated with the fees required to use them,
 /// so that we can choose cheaper paths (as per Dijkstra's algorithm).
 /// Fee values should be updated only in the context of the whole path, see update_value_and_recompute_fees.
@@ -595,10 +613,9 @@ impl<'a> PaymentPath<'a> {
 	// to the fees being paid not lining up with the actual limits.
 	//
 	// Note that this function is not aware of the available_liquidity limit, and thus does not
-	// support increasing the value being transferred.
+	// support increasing the value being transferred beyond what was selected during the initial
+	// routing passes.
 	fn update_value_and_recompute_fees(&mut self, value_msat: u64) {
-		assert!(value_msat <= self.hops.last().unwrap().0.fee_msat);
-
 		let mut total_fee_paid_msat = 0 as u64;
 		for i in (0..self.hops.len()).rev() {
 			let last_hop = i == self.hops.len() - 1;
@@ -906,6 +923,11 @@ where L::Target: Logger {
 		final_value_msat
 	};
 
+	// When we start collecting routes we enforce the max_channel_saturation_power_of_half
+	// requirement strictly. After we've collected enough (or if we fail to find new routes) we
+	// drop the requirement by setting this to 0.
+	let mut channel_saturation_pow_half = payment_params.max_channel_saturation_power_of_half;
+
 	// Keep track of how much liquidity has been used in selected channels. Used to determine
 	// if the channel can be used by additional MPP paths or to inform path finding decisions. It is
 	// aware of direction *only* to ensure that the correct htlc_maximum_msat value is used. Hence,
@@ -959,7 +981,7 @@ where L::Target: Logger {
 			if $src_node_id != $dest_node_id {
 				let short_channel_id = $candidate.short_channel_id();
 				let effective_capacity = $candidate.effective_capacity();
-				let htlc_maximum_msat = max_htlc_from_capacity(effective_capacity, payment_params.max_channel_saturation_power_of_half);
+				let htlc_maximum_msat = max_htlc_from_capacity(effective_capacity, channel_saturation_pow_half);
 
 				// It is tricky to subtract $next_hops_fee_msat from available liquidity here.
 				// It may be misleading because we might later choose to reduce the value transferred
@@ -1531,8 +1553,7 @@ where L::Target: Logger {
 						.and_modify(|used_liquidity_msat| *used_liquidity_msat += spent_on_hop_msat)
 						.or_insert(spent_on_hop_msat);
 					let hop_capacity = hop.candidate.effective_capacity();
-					let hop_max_msat = max_htlc_from_capacity(hop_capacity,
-						payment_params.max_channel_saturation_power_of_half);
+					let hop_max_msat = max_htlc_from_capacity(hop_capacity, channel_saturation_pow_half);
 					if *used_liquidity_msat == hop_max_msat {
 						// If this path used all of this channel's available liquidity, we know
 						// this path will not be selected again in the next loop iteration.
@@ -1579,6 +1600,10 @@ where L::Target: Logger {
 		}
 
 		if !allow_mpp {
+			if !found_new_path && channel_saturation_pow_half != 0 {
+				channel_saturation_pow_half = 0;
+				continue 'paths_collection;
+			}
 			// If we don't support MPP, no use trying to gather more value ever.
 			break 'paths_collection;
 		}
@@ -1588,7 +1613,9 @@ where L::Target: Logger {
 		// iteration.
 		// In the latter case, making another path finding attempt won't help,
 		// because we deterministically terminated the search due to low liquidity.
-		if already_collected_value_msat >= recommended_value_msat || !found_new_path {
+		if !found_new_path && channel_saturation_pow_half != 0 {
+			channel_saturation_pow_half = 0;
+		} else if already_collected_value_msat >= recommended_value_msat || !found_new_path {
 			log_trace!(logger, "Have now collected {} msat (seeking {} msat) in paths. Last path loop {} a new path.",
 				already_collected_value_msat, recommended_value_msat, if found_new_path { "found" } else { "did not find" });
 			break 'paths_collection;
@@ -1704,8 +1731,32 @@ where L::Target: Logger {
 	// Step (9).
 	// Select the best route by lowest total cost.
 	drawn_routes.sort_unstable_by_key(|paths| paths.iter().map(|path| path.get_cost_msat()).sum::<u64>());
+	let selected_route = drawn_routes.first_mut().unwrap();
+
+	// Sort by the path itself and combine redundant paths.
+	// Note that we sort by SCIDs alone as its simpler but when combining we have to ensure we
+	// compare both SCIDs and NodeIds as individual nodes may use random aliases causing collisions
+	// across nodes.
+	selected_route.sort_unstable_by_key(|path| {
+		let mut key = [0u64; MAX_PATH_LENGTH_ESTIMATE as usize];
+		debug_assert!(path.hops.len() <= key.len());
+		for (scid, key) in path.hops.iter().map(|h| h.0.candidate.short_channel_id()).zip(key.iter_mut()) {
+			*key = scid;
+		}
+		key
+	});
+	for idx in 0..(selected_route.len() - 1) {
+		if idx + 1 >= selected_route.len() { break; }
+		if iter_equal(selected_route[idx    ].hops.iter().map(|h: &(PathBuildingHop<'_>, _)| (h.0.candidate.short_channel_id(), h.0.node_id)),
+		              selected_route[idx + 1].hops.iter().map(|h: &(PathBuildingHop<'_>, _)| (h.0.candidate.short_channel_id(), h.0.node_id))) {
+			let new_value = selected_route[idx].get_value_msat() + selected_route[idx + 1].get_value_msat();
+			selected_route[idx].update_value_and_recompute_fees(new_value);
+			selected_route.remove(idx + 1);
+		}
+	}
+
 	let mut selected_paths = Vec::<Vec<Result<RouteHop, LightningError>>>::new();
-	for payment_path in drawn_routes.first().unwrap() {
+	for payment_path in selected_route {
 		let mut path = payment_path.hops.iter().map(|(payment_hop, node_features)| {
 			Ok(RouteHop {
 				pubkey: PublicKey::from_slice(payment_hop.node_id.as_slice()).map_err(|_| LightningError{err: format!("Public key {:?} is invalid", &payment_hop.node_id), action: ErrorAction::IgnoreAndLog(Level::Trace)})?,
@@ -4791,17 +4842,18 @@ mod tests {
 
 		// Get a route for 100 sats and check that we found the MPP route no problem and didn't
 		// overpay at all.
-		let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 100_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
+		let mut route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 100_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
 		assert_eq!(route.paths.len(), 2);
-		// Paths are somewhat randomly ordered, but:
-		// * the first is channel 2 (1 msat fee) -> channel 4 -> channel 42
-		// * the second is channel 1 (0 fee, but 99 sat maximum) -> channel 3 -> channel 42
-		assert_eq!(route.paths[0][0].short_channel_id, 2);
-		assert_eq!(route.paths[0][0].fee_msat, 1);
-		assert_eq!(route.paths[0][2].fee_msat, 1_000);
-		assert_eq!(route.paths[1][0].short_channel_id, 1);
-		assert_eq!(route.paths[1][0].fee_msat, 0);
-		assert_eq!(route.paths[1][2].fee_msat, 99_000);
+		route.paths.sort_by_key(|path| path[0].short_channel_id);
+		// Paths are manually ordered ordered by SCID, so:
+		// * the first is channel 1 (0 fee, but 99 sat maximum) -> channel 3 -> channel 42
+		// * the second is channel 2 (1 msat fee) -> channel 4 -> channel 42
+		assert_eq!(route.paths[0][0].short_channel_id, 1);
+		assert_eq!(route.paths[0][0].fee_msat, 0);
+		assert_eq!(route.paths[0][2].fee_msat, 99_000);
+		assert_eq!(route.paths[1][0].short_channel_id, 2);
+		assert_eq!(route.paths[1][0].fee_msat, 1);
+		assert_eq!(route.paths[1][2].fee_msat, 1_000);
 		assert_eq!(route.get_total_fees(), 1);
 		assert_eq!(route.get_total_amount(), 100_000);
 	}
@@ -4815,7 +4867,8 @@ mod tests {
 		let scorer = test_utils::TestScorer::with_penalty(0);
 		let keys_manager = test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
 		let random_seed_bytes = keys_manager.get_secure_random_bytes();
-		let payment_params = PaymentParameters::from_node_id(nodes[2]).with_features(InvoiceFeatures::known());
+		let payment_params = PaymentParameters::from_node_id(nodes[2]).with_features(InvoiceFeatures::known())
+			.with_max_channel_saturation_power_of_half(0);
 
 		// We need a route consisting of 3 paths:
 		// From our node to node2 via node0, node7, node1 (three paths one hop each).
@@ -5263,12 +5316,13 @@ mod tests {
 			assert_eq!(route.paths[0].len(), 1);
 			assert_eq!(route.paths[1].len(), 1);
 
+			assert!((route.paths[0][0].short_channel_id == 3 && route.paths[1][0].short_channel_id == 2) ||
+				(route.paths[0][0].short_channel_id == 2 && route.paths[1][0].short_channel_id == 3));
+
 			assert_eq!(route.paths[0][0].pubkey, nodes[0]);
-			assert_eq!(route.paths[0][0].short_channel_id, 3);
 			assert_eq!(route.paths[0][0].fee_msat, 50_000);
 
 			assert_eq!(route.paths[1][0].pubkey, nodes[0]);
-			assert_eq!(route.paths[1][0].short_channel_id, 2);
 			assert_eq!(route.paths[1][0].fee_msat, 50_000);
 		}