Expose the outgoing HTLC's CLTV expiry in Event::HTLCIntercepted

In the previous commit our interception documentation noted that in
some cases devs may wish to validate the CLTV expiry of HTLCs
before forwarding. Here we enable that by exposing the next-hop's
CLTV value in `Event::HTLCIntercepted`

Author: TheBlueMatt

lightning/src/events/mod.rs

@@ -1287,6 +1287,13 @@ pub enum Event {
 		/// [`Self::HTLCIntercepted::inbound_amount_msat`]) or subtract it as required. Further,
 		/// LDK will not stop you from forwarding more than you received.
 		expected_outbound_amount_msat: u64,
+		/// The block height at which the forwarded HTLC sent to our peer will time out. In
+		/// practice, LDK will refuse to forward an HTLC several blocks before this height (as if
+		/// we attempted to forward an HTLC at this height we'd run some risk that our peer
+		/// force-closes the channel immediately).
+		///
+		/// This will only be `None` for events generated or serialized by LDK 0.2 or prior.
+		outgoing_htlc_expiry_block_height: Option<u32>,
 	},
 	/// Used to indicate that an output which you should know how to spend was confirmed on chain
 	/// and is now spendable.
@@ -2017,11 +2024,13 @@ impl Writeable for Event {
 				inbound_amount_msat,
 				expected_outbound_amount_msat,
 				intercept_id,
+				outgoing_htlc_expiry_block_height,
 			} => {
 				6u8.write(writer)?;
 				let intercept_scid = InterceptNextHop::FakeScid { requested_next_hop_scid };
 				write_tlv_fields!(writer, {
 					(0, intercept_id, required),
+					(1, outgoing_htlc_expiry_block_height, option),
 					(2, intercept_scid, required),
 					(4, payment_hash, required),
 					(6, inbound_amount_msat, required),
@@ -2526,8 +2535,10 @@ impl MaybeReadable for Event {
 					InterceptNextHop::FakeScid { requested_next_hop_scid: 0 };
 				let mut inbound_amount_msat = 0;
 				let mut expected_outbound_amount_msat = 0;
+				let mut outgoing_htlc_expiry_block_height = None;
 				read_tlv_fields!(reader, {
 					(0, intercept_id, required),
+					(1, outgoing_htlc_expiry_block_height, option),
 					(2, requested_next_hop_scid, required),
 					(4, payment_hash, required),
 					(6, inbound_amount_msat, required),
@@ -2542,6 +2553,7 @@ impl MaybeReadable for Event {
 					inbound_amount_msat,
 					expected_outbound_amount_msat,
 					intercept_id,
+					outgoing_htlc_expiry_block_height,
 				}))
 			},
 			7u8 => {

lightning/src/ln/channelmanager.rs

@@ -3409,6 +3409,7 @@ fn create_htlc_intercepted_event(
 		inbound_amount_msat,
 		expected_outbound_amount_msat: pending_add.forward_info.outgoing_amt_msat,
 		intercept_id,
+		outgoing_htlc_expiry_block_height: Some(pending_add.forward_info.outgoing_cltv_value),
 	})
 }
 

lightning/src/ln/payment_tests.rs

@@ -2277,17 +2277,20 @@ fn do_test_intercepted_payment(test: InterceptTest) {
 	// Check that we generate the PaymentIntercepted event when an intercept forward is detected.
 	let events = nodes[1].node.get_and_clear_pending_events();
 	assert_eq!(events.len(), 1);
+	let expected_cltv = nodes[0].best_block_info().1 + TEST_FINAL_CLTV + 1;
 	let (intercept_id, outbound_amt) = match events[0] {
 		crate::events::Event::HTLCIntercepted {
 			intercept_id,
 			expected_outbound_amount_msat,
 			payment_hash,
 			inbound_amount_msat,
 			requested_next_hop_scid: short_channel_id,
+			outgoing_htlc_expiry_block_height,
 		} => {
 			assert_eq!(payment_hash, hash);
 			assert_eq!(inbound_amount_msat, route.get_total_amount() + route.get_total_fees());
 			assert_eq!(short_channel_id, intercept_scid);
+			assert_eq!(outgoing_htlc_expiry_block_height.unwrap(), expected_cltv);
 			(intercept_id, expected_outbound_amount_msat)
 		},
 		_ => panic!(),
@@ -2356,6 +2359,7 @@ fn do_test_intercepted_payment(test: InterceptTest) {
 			assert_eq!(events.len(), 1);
 			SendEvent::from_event(events.remove(0))
 		};
+		assert_eq!(payment_event.msgs[0].cltv_expiry, expected_cltv);
 		nodes[2].node.handle_update_add_htlc(node_b_id, &payment_event.msgs[0]);
 		let commitment = &payment_event.commitment_msg;
 		do_commitment_signed_dance(&nodes[2], &nodes[1], commitment, false, true);