f fix mpp with multiple matching intro nodes

Author: valentinewallace

lightning/src/routing/router.rs

@@ -770,6 +770,7 @@ impl Payee {
 			_ => None,
 		}
 	}
+	// This method must always return the hints in the same order.
 	fn blinded_route_hints(&self) -> &[(BlindedPayInfo, BlindedPath)] {
 		match self {
 			Self::Blinded { route_hints, .. } => &route_hints[..],
@@ -2307,12 +2308,13 @@ where L::Target: Logger {
 	for results_vec in selected_paths {
 		let mut hops = Vec::with_capacity(results_vec.len());
 		for res in results_vec { hops.push(res?); }
-		let blinded_path = payment_params.payee.blinded_route_hints().iter()
-			.find(|(_, p)| {
-				let intro_node_idx = if p.blinded_hops.len() == 1 { hops.len() - 1 }
-					else { hops.len().saturating_sub(2) };
-				p.introduction_node_id == hops[intro_node_idx].pubkey
-			}).map(|(_, p)| p.clone());
+
+		let blinded_path = payment_params.payee.blinded_route_hints().iter().enumerate()
+			.find(|(idx, (_, p))| {
+				DUMMY_BLINDED_SCID + *idx as u64 == hops[hops.len() - 1].short_channel_id ||
+					(p.blinded_hops.len() == 1 && p.introduction_node_id == hops[hops.len() - 1].pubkey)
+			})
+			.map(|(_, (_, p))| p.clone());
 		let blinded_tail = if let Some(BlindedPath { blinded_hops, blinding_point, .. }) = blinded_path {
 			let num_blinded_hops = blinded_hops.len();
 			Some(BlindedTail {
@@ -6355,6 +6357,65 @@ mod tests {
 			_ => panic!("Expected error")
 		}
 	}
+
+	#[test]
+	fn matching_intro_node_paths_provided() {
+		// Check that if multiple blinded paths with the same intro node are provided in payment
+		// parameters, we'll return the correct paths in the resulting MPP route.
+		let (secp_ctx, network, _, _, logger) = build_graph();
+		let (_, our_id, _, nodes) = get_nodes(&secp_ctx);
+		let network_graph = network.read_only();
+
+		let scorer = ln_test_utils::TestScorer::new();
+		let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
+		let random_seed_bytes = keys_manager.get_secure_random_bytes();
+		let config = UserConfig::default();
+
+		let bolt12_features: Bolt12InvoiceFeatures = channelmanager::provided_invoice_features(&config).to_context();
+		let blinded_path_1 = BlindedPath {
+			introduction_node_id: nodes[2],
+			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_1 = BlindedPayInfo {
+			fee_base_msat: 0,
+			fee_proportional_millionths: 0,
+			htlc_minimum_msat: 0,
+			htlc_maximum_msat: 30_000,
+			cltv_expiry_delta: 0,
+			features: BlindedHopFeatures::empty(),
+		};
+
+		let mut blinded_path_2 = blinded_path_1.clone();
+		blinded_path_2.blinding_point = ln_test_utils::pubkey(43);
+		let mut blinded_payinfo_2 = blinded_payinfo_1.clone();
+		blinded_payinfo_2.htlc_maximum_msat = 70_000;
+
+		let blinded_hints = vec![
+			(blinded_payinfo_1.clone(), blinded_path_1.clone()),
+			(blinded_payinfo_2.clone(), blinded_path_2.clone()),
+		];
+		let payment_params = PaymentParameters::blinded(blinded_hints.clone())
+			.with_bolt12_features(bolt12_features.clone()).unwrap();
+
+		let route = get_route(&our_id, &payment_params, &network_graph, None,
+			100_000, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
+		assert_eq!(route.paths.len(), 2);
+		let mut total_amount_paid_msat = 0;
+		for path in route.paths.into_iter() {
+			assert_eq!(path.hops.last().unwrap().pubkey, nodes[2]);
+			if let Some(bt) = &path.blinded_tail {
+				assert_eq!(bt.blinding_point,
+					blinded_hints.iter().find(|(p, _)| p.htlc_maximum_msat == path.final_value_msat())
+						.map(|(_, bp)| bp.blinding_point).unwrap());
+			} else { panic!(); }
+			total_amount_paid_msat += path.final_value_msat();
+		}
+		assert_eq!(total_amount_paid_msat, 100_000);
+	}
 }
 
 #[cfg(all(test, not(feature = "no-std")))]