Allow failing back intercepted HTLCs

Co-authored-by: John Cantrell <johncantrell97@gmail.com>
Co-authored-by: Valentine Wallace <vwallace@protonmail.com>

lightning/src/ln/channelmanager.rs

@@ -3059,7 +3059,8 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 	///
 	/// To make use of intercepted HTLCs, use [`ChannelManager::get_intercept_scid`] to generate short
 	/// channel id(s) to put in the receiver's invoice route hints. These route hints will signal to
-	/// LDK to generate an [`HTLCIntercepted`] event when it receives the forwarded HTLC.
+	/// LDK to generate an [`HTLCIntercepted`] event when it receives the forwarded HTLC, and this
+	/// method or [`ChannelManager::fail_intercepted_htlc`] MUST be called in response to the event.
 	///
 	/// Note that LDK does not enforce fee requirements in `amt_to_forward_msat`, and will not stop
 	/// you from forwarding more than you received.
@@ -3102,6 +3103,35 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 		Ok(())
 	}
 
+	/// Fails the intercepted HTLC indicated by intercept_id. Should only be called in response to
+	/// an [`HTLCIntercepted`] event. See [`ChannelManager::forward_intercepted_htlc`].
+	///
+	/// [`HTLCIntercepted`]: events::Event::HTLCIntercepted
+	pub fn fail_intercepted_htlc(&self, intercept_id: InterceptId) -> Result<(), APIError> {
+		let _persistence_guard = PersistenceNotifierGuard::notify_on_drop(&self.total_consistency_lock, &self.persistence_notifier);
+
+		let payment = self.pending_intercepted_htlcs.lock().unwrap().remove(&intercept_id)
+			.ok_or_else(|| APIError::APIMisuseError {
+				err: format!("Payment with InterceptId {:?} not found", intercept_id)
+			})?;
+
+		if let PendingHTLCRouting::Forward { short_channel_id, .. } = payment.forward_info.routing {
+			let htlc_source = HTLCSource::PreviousHopData(HTLCPreviousHopData {
+				short_channel_id: payment.prev_short_channel_id,
+				outpoint: payment.prev_funding_outpoint,
+				htlc_id: payment.prev_htlc_id,
+				incoming_packet_shared_secret: payment.forward_info.incoming_shared_secret,
+				phantom_shared_secret: None,
+			});
+
+			let failure_reason = HTLCFailReason::Reason { failure_code: 0x4000 | 10, data: Vec::new() };
+			let destination = HTLCDestination::UnknownNextHop { requested_forward_scid: short_channel_id };
+			self.fail_htlc_backwards_internal(htlc_source, &payment.forward_info.payment_hash, failure_reason, destination);
+		} else { unreachable!() } // Only `PendingHTLCRouting::Forward`s are intercepted
+
+		Ok(())
+	}
+
 	/// Processes HTLCs which are pending waiting on random forward delay.
 	///
 	/// Should only really ever be called in response to a PendingHTLCsForwardable event.

lightning/src/ln/payment_tests.rs

@@ -1374,9 +1374,15 @@ fn test_holding_cell_inflight_htlcs() {
 }
 
 #[test]
-fn forward_intercepted_payment() {
+fn intercepted_payment() {
 	// Test that detecting an intercept scid on payment forward will signal LDK to generate an
-	// intercept event, which the LSP can then use to open a JIT channel to forward the payment.
+	// intercept event, which the LSP can then use to either (a) open a JIT channel to forward the
+	// payment or (b) fail the payment.
+	do_test_intercepted_payment(false);
+	do_test_intercepted_payment(true);
+}
+
+fn do_test_intercepted_payment(fail_intercept: bool) {
 	let chanmon_cfgs = create_chanmon_cfgs(3);
 	let node_cfgs = create_node_cfgs(3, &chanmon_cfgs);
 	let mut chan_config = test_default_channel_config();
@@ -1449,49 +1455,69 @@ fn forward_intercepted_payment() {
 	let unknown_chan_id_err = nodes[1].node.forward_intercepted_htlc(intercept_id, &[42; 32], nodes[2].node.get_our_node_id(), expected_outbound_amount_msat).unwrap_err();
 	assert_eq!(unknown_chan_id_err , APIError::APIMisuseError { err: format!("Channel with id {:?} not found", [42; 32]) });
 
-	// Open the just-in-time channel so the payment can then be forwarded.
-	let (_, channel_id) = open_zero_conf_channel(&nodes[1], &nodes[2], None);
-
-	// Check for unknown intercept id error.
-	let unknown_intercept_id = InterceptId([42; 32]);
-	let unknown_intercept_id_err = nodes[1].node.forward_intercepted_htlc(unknown_intercept_id, &channel_id, nodes[2].node.get_our_node_id(), expected_outbound_amount_msat).unwrap_err();
-	assert_eq!(unknown_intercept_id_err , APIError::APIMisuseError { err: format!("Payment with intercept id {:?} not found", unknown_intercept_id.0) });
-
-	// Finally, forward the intercepted payment through and claim it.
-	nodes[1].node.forward_intercepted_htlc(intercept_id, &channel_id, nodes[2].node.get_our_node_id(), expected_outbound_amount_msat).unwrap();
-	expect_pending_htlcs_forwardable!(nodes[1]);
-
-	let payment_event = {
-		{
-			let mut added_monitors = nodes[1].chain_monitor.added_monitors.lock().unwrap();
-			assert_eq!(added_monitors.len(), 1);
-			added_monitors.clear();
+	if fail_intercept {
+		// Ensure we can fail the intercepted payment back.
+		nodes[1].node.fail_intercepted_htlc(intercept_id).unwrap();
+		expect_pending_htlcs_forwardable_and_htlc_handling_failed_ignore!(nodes[1], vec![HTLCDestination::UnknownNextHop { requested_forward_scid: intercept_scid }]);
+		nodes[1].node.process_pending_htlc_forwards();
+		let update_fail = get_htlc_update_msgs!(nodes[1], nodes[0].node.get_our_node_id());
+		check_added_monitors!(&nodes[1], 1);
+		assert!(update_fail.update_fail_htlcs.len() == 1);
+		let fail_msg = update_fail.update_fail_htlcs[0].clone();
+		nodes[0].node.handle_update_fail_htlc(&nodes[1].node.get_our_node_id(), &fail_msg);
+		commitment_signed_dance!(nodes[0], nodes[1], update_fail.commitment_signed, false);
+
+		// Ensure the payment fails with the expected error.
+		let fail_conditions = PaymentFailedConditions::new()
+			.blamed_scid(intercept_scid)
+			.blamed_chan_closed(true)
+			.expected_htlc_error_data(0x4000 | 10, &[]);
+		expect_payment_failed_conditions(&nodes[0], payment_hash, false, fail_conditions);
+	} else {
+		// Open the just-in-time channel so the payment can then be forwarded.
+		let (_, channel_id) = open_zero_conf_channel(&nodes[1], &nodes[2], None);
+
+		// Check for unknown intercept id error.
+		let unknown_intercept_id = InterceptId([42; 32]);
+		let unknown_intercept_id_err = nodes[1].node.forward_intercepted_htlc(unknown_intercept_id, &channel_id, nodes[2].node.get_our_node_id(), expected_outbound_amount_msat).unwrap_err();
+		assert_eq!(unknown_intercept_id_err , APIError::APIMisuseError { err: format!("Payment with intercept id {:?} not found", unknown_intercept_id.0) });
+
+		// Finally, forward the intercepted payment through and claim it.
+		nodes[1].node.forward_intercepted_htlc(intercept_id, &channel_id, nodes[2].node.get_our_node_id(), expected_outbound_amount_msat).unwrap();
+		expect_pending_htlcs_forwardable!(nodes[1]);
+
+		let payment_event = {
+			{
+				let mut added_monitors = nodes[1].chain_monitor.added_monitors.lock().unwrap();
+				assert_eq!(added_monitors.len(), 1);
+				added_monitors.clear();
+			}
+			let mut events = nodes[1].node.get_and_clear_pending_msg_events();
+			assert_eq!(events.len(), 1);
+			SendEvent::from_event(events.remove(0))
+		};
+		nodes[2].node.handle_update_add_htlc(&nodes[1].node.get_our_node_id(), &payment_event.msgs[0]);
+		commitment_signed_dance!(nodes[2], nodes[1], &payment_event.commitment_msg, false, true);
+		expect_pending_htlcs_forwardable!(nodes[2]);
+
+		let payment_preimage = nodes[2].node.get_payment_preimage(payment_hash, payment_secret).unwrap();
+		expect_payment_received!(&nodes[2], payment_hash, payment_secret, amt_msat, Some(payment_preimage), nodes[2].node.get_our_node_id());
+		do_claim_payment_along_route(&nodes[0], &vec!(&vec!(&nodes[1], &nodes[2])[..]), false, payment_preimage);
+		let events = nodes[0].node.get_and_clear_pending_events();
+		assert_eq!(events.len(), 2);
+		match events[0] {
+			Event::PaymentSent { payment_preimage: ref ev_preimage, payment_hash: ref ev_hash, ref fee_paid_msat, .. } => {
+				assert_eq!(payment_preimage, *ev_preimage);
+				assert_eq!(payment_hash, *ev_hash);
+				assert_eq!(fee_paid_msat, &Some(1000));
+			},
+			_ => panic!("Unexpected event")
+		}
+		match events[1] {
+			Event::PaymentPathSuccessful { payment_hash: hash, .. } => {
+				assert_eq!(hash, Some(payment_hash));
+			},
+			_ => panic!("Unexpected event")
 		}
-		let mut events = nodes[1].node.get_and_clear_pending_msg_events();
-		assert_eq!(events.len(), 1);
-		SendEvent::from_event(events.remove(0))
-	};
-	nodes[2].node.handle_update_add_htlc(&nodes[1].node.get_our_node_id(), &payment_event.msgs[0]);
-	commitment_signed_dance!(nodes[2], nodes[1], &payment_event.commitment_msg, false, true);
-	expect_pending_htlcs_forwardable!(nodes[2]);
-
-	let payment_preimage = nodes[2].node.get_payment_preimage(payment_hash, payment_secret).unwrap();
-	expect_payment_received!(&nodes[2], payment_hash, payment_secret, amt_msat, Some(payment_preimage), nodes[2].node.get_our_node_id());
-	do_claim_payment_along_route(&nodes[0], &vec!(&vec!(&nodes[1], &nodes[2])[..]), false, payment_preimage);
-	let events = nodes[0].node.get_and_clear_pending_events();
-	assert_eq!(events.len(), 2);
-	match events[0] {
-		Event::PaymentSent { payment_preimage: ref ev_preimage, payment_hash: ref ev_hash, ref fee_paid_msat, .. } => {
-			assert_eq!(payment_preimage, *ev_preimage);
-			assert_eq!(payment_hash, *ev_hash);
-			assert_eq!(fee_paid_msat, &Some(1000));
-		},
-		_ => panic!("Unexpected event")
-	}
-	match events[1] {
-		Event::PaymentPathSuccessful { payment_hash: hash, .. } => {
-			assert_eq!(hash, Some(payment_hash));
-		},
-		_ => panic!("Unexpected event")
 	}
 }

lightning/src/util/events.rs

@@ -614,7 +614,13 @@ pub enum Event {
 	/// you've encoded an intercept scid in the receiver's invoice route hints using
 	/// [`ChannelManager::get_intercept_scid`].
 	///
+	/// [`ChannelManager::forward_intercepted_htlc`] or
+	/// [`ChannelManager::fail_intercepted_htlc`] MUST be called in response to this event. See
+	/// their docs for more information.
+	///
 	/// [`ChannelManager::get_intercept_scid`]: crate::ln::channelmanager::ChannelManager::get_intercept_scid
+	/// [`ChannelManager::forward_intercepted_htlc`]: crate::ln::channelmanager::ChannelManager::forward_intercepted_htlc
+	/// [`ChannelManager::fail_intercepted_htlc`]: crate::ln::channelmanager::ChannelManager::fail_intercepted_htlc
 	HTLCIntercepted {
 		/// An id to help LDK identify which HTLC is being forwarded or failed.
 		intercept_id: InterceptId,