Router: fix blinded route hint path_min_htlc calc

See previous commit, we had a bug where we would find invalid paths due to
underestimating the min_htlc of downstream hops when adding upstream route
candidates.

Author: valentinewallace

lightning/src/routing/router.rs

@@ -2061,9 +2061,19 @@ where L::Target: Logger {
 						Some(fee) => fee,
 						None => continue
 					};
+					let path_min = {
+						let blinded_path_min = candidate.htlc_minimum_msat();
+						let path_min_fees = match compute_fees(blinded_path_min, candidate.fees()) {
+							Some(fee) => fee,
+							None => continue
+						};
+						match blinded_path_min.checked_add(path_min_fees) {
+							Some(min) => min,
+							None => continue
+						}
+					};
 					add_entry!(first_hop_candidate, our_node_id, intro_node_id, blinded_path_fee,
-						path_contribution_msat, candidate.htlc_minimum_msat(), 0_u64,
-						candidate.cltv_expiry_delta(),
+						path_contribution_msat, path_min, 0_u64, candidate.cltv_expiry_delta(),
 						candidate.blinded_path().map_or(1, |bp| bp.blinded_hops.len() as u8));
 				}
 			}
@@ -7190,6 +7200,10 @@ mod tests {
 
 	#[test]
 	fn min_htlc_overpay_violates_max_htlc() {
+		do_min_htlc_overpay_violates_max_htlc(true);
+		do_min_htlc_overpay_violates_max_htlc(false);
+	}
+	fn do_min_htlc_overpay_violates_max_htlc(blinded_payee: bool) {
 		// Test that if overpaying to meet a later hop's min_htlc and causes us to violate an earlier
 		// hop's max_htlc, we discard that invalid path. Previously we would consider the path to be
 		// valid and hit a debug panic asserting that the used liquidity for a hop was less than its
@@ -7211,21 +7225,45 @@ mod tests {
 			first_hop_outbound_capacity
 		)];
 
-		let route_hint = RouteHint(vec![RouteHintHop {
-			src_node_id: nodes[0],
-			short_channel_id: 44,
-			fees: RoutingFees {
-				base_msat: 1_6778_3453,
-				proportional_millionths: 0,
-			},
-			cltv_expiry_delta: 10,
-			htlc_minimum_msat: Some(2_5165_8240),
-			htlc_maximum_msat: None,
-		}]);
+		let base_fee = 1_6778_3453;
+		let htlc_min = 2_5165_8240;
+		let payment_params = if blinded_payee {
+			let blinded_path = BlindedPath {
+				introduction_node_id: nodes[0],
+				blinding_point: ln_test_utils::pubkey(42),
+				blinded_hops: vec![
+					BlindedHop { blinded_node_id: ln_test_utils::pubkey(42 as u8), encrypted_payload: Vec::new() },
+					BlindedHop { blinded_node_id: ln_test_utils::pubkey(42 as u8), encrypted_payload: Vec::new() }
+				],
+			};
+			let blinded_payinfo = BlindedPayInfo {
+				fee_base_msat: base_fee,
+				fee_proportional_millionths: 0,
+				htlc_minimum_msat: htlc_min,
+				htlc_maximum_msat: htlc_min * 1000,
+				cltv_expiry_delta: 0,
+				features: BlindedHopFeatures::empty(),
+			};
+			let bolt12_features: Bolt12InvoiceFeatures = channelmanager::provided_invoice_features(&config).to_context();
+			PaymentParameters::blinded(vec![(blinded_payinfo, blinded_path)])
+				.with_bolt12_features(bolt12_features.clone()).unwrap()
+		} else {
+			let route_hint = RouteHint(vec![RouteHintHop {
+				src_node_id: nodes[0],
+				short_channel_id: 44,
+				fees: RoutingFees {
+					base_msat: base_fee,
+					proportional_millionths: 0,
+				},
+				cltv_expiry_delta: 10,
+				htlc_minimum_msat: Some(htlc_min),
+				htlc_maximum_msat: None,
+			}]);
 
-		let payment_params = PaymentParameters::from_node_id(nodes[1], 42)
-			.with_route_hints(vec![route_hint]).unwrap()
-			.with_bolt11_features(channelmanager::provided_invoice_features(&config)).unwrap();
+			PaymentParameters::from_node_id(nodes[1], 42)
+				.with_route_hints(vec![route_hint]).unwrap()
+				.with_bolt11_features(channelmanager::provided_invoice_features(&config)).unwrap()
+		};
 
 		let netgraph = network_graph.read_only();
 		let route_params = RouteParameters::from_payment_params_and_value(
@@ -7240,6 +7278,11 @@ mod tests {
 
 	#[test]
 	fn previously_used_liquidity_violates_max_htlc() {
+		do_previously_used_liquidity_violates_max_htlc(true);
+		do_previously_used_liquidity_violates_max_htlc(false);
+
+	}
+	fn do_previously_used_liquidity_violates_max_htlc(blinded_payee: bool) {
 		// Test that if a candidate hop would cause us to violate an upstream hop's available
 		// contribution amount due to a previously found path, we will not consider that candidate in
 		// path construction. Previously we would construct an invalid path and hit a debug panic
@@ -7261,51 +7304,50 @@ mod tests {
 			Some(122), nodes[0], channelmanager::provided_init_features(&config), 179_5000
 		)];
 
-		let route_hints = vec![RouteHint(vec![RouteHintHop {
-			src_node_id: nodes[0],
-			short_channel_id: 42,
-			fees: RoutingFees {
-				base_msat: 0,
-				proportional_millionths: 0,
-			},
-			cltv_expiry_delta: 10,
-			htlc_minimum_msat: Some(1_4392),
-			htlc_maximum_msat: Some(143_9200),
-		}]), RouteHint(vec![RouteHintHop {
-			src_node_id: nodes[0],
-			short_channel_id: 43,
-			fees: RoutingFees {
-				base_msat: 425_9840,
-				proportional_millionths: 0,
-			},
-			cltv_expiry_delta: 10,
-			htlc_minimum_msat: Some(19_7401),
-			htlc_maximum_msat: Some(1974_0100),
-		}]), RouteHint(vec![RouteHintHop {
-			src_node_id: nodes[0],
-			short_channel_id: 44,
-			fees: RoutingFees {
-				base_msat: 0,
-				proportional_millionths: 0,
-			},
-			cltv_expiry_delta: 10,
-			htlc_minimum_msat: Some(1027),
-			htlc_maximum_msat: Some(10_2700),
-		}]), RouteHint(vec![RouteHintHop {
-			src_node_id: nodes[0],
-			short_channel_id: 45,
-			fees: RoutingFees {
-				base_msat: 0,
-				proportional_millionths: 0,
-			},
-			cltv_expiry_delta: 10,
-			htlc_minimum_msat: Some(6_5535),
-			htlc_maximum_msat: Some(655_3500),
-		}])];
-
-		let payment_params = PaymentParameters::from_node_id(nodes[1], 42)
-			.with_route_hints(route_hints).unwrap()
-			.with_bolt11_features(channelmanager::provided_invoice_features(&config)).unwrap();
+		let base_fees = [0, 425_9840, 0, 0];
+		let htlc_mins = [1_4392, 19_7401, 1027, 6_5535];
+		let payment_params = if blinded_payee {
+			let blinded_path = BlindedPath {
+				introduction_node_id: nodes[0],
+				blinding_point: ln_test_utils::pubkey(42),
+				blinded_hops: vec![
+					BlindedHop { blinded_node_id: ln_test_utils::pubkey(42 as u8), encrypted_payload: Vec::new() },
+					BlindedHop { blinded_node_id: ln_test_utils::pubkey(42 as u8), encrypted_payload: Vec::new() }
+				],
+			};
+			let mut blinded_hints = Vec::new();
+			for (base_fee, htlc_min) in base_fees.iter().zip(htlc_mins.iter()) {
+				blinded_hints.push((BlindedPayInfo {
+					fee_base_msat: *base_fee,
+					fee_proportional_millionths: 0,
+					htlc_minimum_msat: *htlc_min,
+					htlc_maximum_msat: htlc_min * 100,
+					cltv_expiry_delta: 10,
+					features: BlindedHopFeatures::empty(),
+				}, blinded_path.clone()));
+			}
+			let bolt12_features: Bolt12InvoiceFeatures = channelmanager::provided_invoice_features(&config).to_context();
+			PaymentParameters::blinded(blinded_hints.clone())
+				.with_bolt12_features(bolt12_features.clone()).unwrap()
+		} else {
+			let mut route_hints = Vec::new();
+			for (idx, (base_fee, htlc_min)) in base_fees.iter().zip(htlc_mins.iter()).enumerate() {
+				route_hints.push(RouteHint(vec![RouteHintHop {
+					src_node_id: nodes[0],
+					short_channel_id: 42 + idx as u64,
+					fees: RoutingFees {
+						base_msat: *base_fee,
+						proportional_millionths: 0,
+					},
+					cltv_expiry_delta: 10,
+					htlc_minimum_msat: Some(*htlc_min),
+					htlc_maximum_msat: Some(htlc_min * 100),
+				}]));
+			}
+			PaymentParameters::from_node_id(nodes[1], 42)
+				.with_route_hints(route_hints).unwrap()
+				.with_bolt11_features(channelmanager::provided_invoice_features(&config)).unwrap()
+		};
 
 		let netgraph = network_graph.read_only();
 		let route_params = RouteParameters::from_payment_params_and_value(