Introduce payment dummy hops in DefaultRouter

Author: shaavan

lightning-dns-resolver/src/lib.rs

@@ -175,6 +175,7 @@ mod test {
 	use lightning::onion_message::messenger::{
 		AOnionMessenger, Destination, MessageRouter, OnionMessagePath, OnionMessenger,
 	};
+	use lightning::routing::router::DEFAULT_PAYMENT_DUMMY_HOPS;
 	use lightning::sign::{KeysManager, NodeSigner, ReceiveAuthKey, Recipient};
 	use lightning::types::features::InitFeatures;
 	use lightning::types::payment::PaymentHash;
@@ -419,6 +420,11 @@ mod test {
 		let updates = get_htlc_update_msgs(&nodes[0], &payee_id);
 		nodes[1].node.handle_update_add_htlc(payer_id, &updates.update_add_htlcs[0]);
 		do_commitment_signed_dance(&nodes[1], &nodes[0], &updates.commitment_signed, false, false);
+
+		for _ in 0..DEFAULT_PAYMENT_DUMMY_HOPS {
+			nodes[1].node.process_pending_htlc_forwards();
+		}
+
 		expect_and_process_pending_htlcs(&nodes[1], false);
 
 		let claimable_events = nodes[1].node.get_and_clear_pending_events();

lightning/src/ln/async_payments_tests.rs

@@ -55,7 +55,7 @@ use crate::onion_message::messenger::{
 use crate::onion_message::offers::OffersMessage;
 use crate::onion_message::packet::ParsedOnionMessageContents;
 use crate::prelude::*;
-use crate::routing::router::{Payee, PaymentParameters};
+use crate::routing::router::{Payee, PaymentParameters, DEFAULT_PAYMENT_DUMMY_HOPS};
 use crate::sign::NodeSigner;
 use crate::sync::Mutex;
 use crate::types::features::Bolt12InvoiceFeatures;
@@ -1858,6 +1858,13 @@ fn expired_static_invoice_payment_path() {
 		blinded_path
 			.advance_path_by_one(&nodes[1].keys_manager, &nodes[1].node, &secp_ctx)
 			.unwrap();
+
+		for _ in 0..DEFAULT_PAYMENT_DUMMY_HOPS {
+			blinded_path
+				.advance_path_by_one(&nodes[2].keys_manager, &nodes[2].node, &secp_ctx)
+				.unwrap();
+		}
+
 		match blinded_path.decrypt_intro_payload(&nodes[2].keys_manager).unwrap().0 {
 			BlindedPaymentTlvs::Receive(tlvs) => tlvs.payment_constraints.max_cltv_expiry,
 			_ => panic!(),

lightning/src/ln/functional_test_utils.rs

@@ -39,7 +39,9 @@ use crate::ln::peer_handler::IgnoringMessageHandler;
 use crate::ln::types::ChannelId;
 use crate::onion_message::messenger::OnionMessenger;
 use crate::routing::gossip::{NetworkGraph, NetworkUpdate, P2PGossipSync};
-use crate::routing::router::{self, PaymentParameters, Route, RouteParameters};
+use crate::routing::router::{
+	self, PaymentParameters, Route, RouteParameters, DEFAULT_PAYMENT_DUMMY_HOPS,
+};
 use crate::sign::{EntropySource, RandomBytes};
 use crate::types::features::ChannelTypeFeatures;
 use crate::types::features::InitFeatures;
@@ -3550,6 +3552,15 @@ pub fn do_pass_along_path<'a, 'b, 'c>(args: PassAlongPathArgs) -> Option<Event>
 			node.node.process_pending_htlc_forwards();
 		}
 
+		if is_last_hop {
+			// At the final hop, the incoming packet contains N dummy-hop layers
+			// before the real HTLC. Each call to `process_pending_htlc_forwards`
+			// strips exactly one dummy layer, so we call it N times.
+			for _ in 0..dummy_hop_override.unwrap_or(DEFAULT_PAYMENT_DUMMY_HOPS) {
+				node.node.process_pending_htlc_forwards();
+			}
+		}
+
 		if is_last_hop && clear_recipient_events {
 			let events_2 = node.node.get_and_clear_pending_events();
 			if payment_claimable_expected {

lightning/src/ln/offers_tests.rs

@@ -1432,7 +1432,20 @@ fn creates_offer_with_blinded_path_using_unannounced_introduction_node() {
 	route_bolt12_payment(bob, &[alice], &invoice);
 	expect_recent_payment!(bob, RecentPaymentDetails::Pending, payment_id);
 
-	claim_bolt12_payment(bob, &[alice], payment_context, &invoice);
+	// When the payer is the introduction node of a blinded path, LDK doesn't
+	// subtract the forward fee for the `payer -> next_hop` channel (see
+	// `BlindedPaymentPath::advance_path_by_one`). This keeps fee logic simple,
+	// at the cost of a small, intentional overpayment.
+	//
+	// In the old two-hop case (payer as introduction node → payee), this never
+	// surfaced because the payer simply wasn’t charged the forward fee.
+	//
+	// With dummy hops in LDK v0.3, even a real two-node path can appear as a
+	// longer blinded route, so the overpayment shows up in tests.
+	//
+	// Until the fee-handling trade-off is revisited, we pass an expected extra
+	// fee here so tests can compensate for it.
+	claim_bolt12_payment_with_extra_fees(bob, &[alice], payment_context, &invoice, Some(1000));
 	expect_recent_payment!(bob, RecentPaymentDetails::Fulfilled, payment_id);
 }
 
@@ -2447,9 +2460,9 @@ fn rejects_keysend_to_non_static_invoice_path() {
 		.expect_failure(HTLCHandlingFailureType::Receive { payment_hash });
 	do_pass_along_path(args);
 	let mut updates = get_htlc_update_msgs(&nodes[1], &nodes[0].node.get_our_node_id());
-	nodes[0].node.handle_update_fail_htlc(nodes[1].node.get_our_node_id(), &updates.update_fail_htlcs[0]);
+	nodes[0].node.handle_update_fail_malformed_htlc(nodes[1].node.get_our_node_id(), &updates.update_fail_malformed_htlcs[0]);
 	do_commitment_signed_dance(&nodes[0], &nodes[1], &updates.commitment_signed, false, false);
-	expect_payment_failed_conditions(&nodes[0], payment_hash, true, PaymentFailedConditions::new());
+	expect_payment_failed_conditions(&nodes[0], payment_hash, false, PaymentFailedConditions::new());
 }
 
 #[test]

lightning/src/routing/router.rs

@@ -74,6 +74,9 @@ pub struct DefaultRouter<
 	score_params: SP,
 }
 
+/// The number of dummy hops included in [`BlindedPaymentPath`]s created by [`DefaultRouter`].
+pub const DEFAULT_PAYMENT_DUMMY_HOPS: usize = 3;
+
 impl<
 		G: Deref<Target = NetworkGraph<L>>,
 		L: Deref,
@@ -198,9 +201,9 @@ where
 				})
 			})
 			.map(|forward_node| {
-				BlindedPaymentPath::new(
-					&[forward_node], recipient, local_node_receive_key, tlvs.clone(), u64::MAX, MIN_FINAL_CLTV_EXPIRY_DELTA,
-					&*self.entropy_source, secp_ctx
+				BlindedPaymentPath::new_with_dummy_hops(
+					&[forward_node], recipient, DEFAULT_PAYMENT_DUMMY_HOPS, local_node_receive_key, tlvs.clone(), u64::MAX,
+					MIN_FINAL_CLTV_EXPIRY_DELTA, &*self.entropy_source, secp_ctx
 				)
 			})
 			.take(MAX_PAYMENT_PATHS)
@@ -210,9 +213,9 @@ where
 			Ok(paths) if !paths.is_empty() => Ok(paths),
 			_ => {
 				if network_graph.nodes().contains_key(&NodeId::from_pubkey(&recipient)) {
-					BlindedPaymentPath::new(
-						&[], recipient, local_node_receive_key, tlvs, u64::MAX, MIN_FINAL_CLTV_EXPIRY_DELTA, &*self.entropy_source,
-						secp_ctx
+					BlindedPaymentPath::new_with_dummy_hops(
+						&[], recipient, DEFAULT_PAYMENT_DUMMY_HOPS, local_node_receive_key, tlvs, u64::MAX,
+						MIN_FINAL_CLTV_EXPIRY_DELTA, &*self.entropy_source, secp_ctx
 					).map(|path| vec![path])
 				} else {
 					Err(())