Report the sending peer in Event::OnionMessageIntercepted

When the OnionMessenger intercepts an onion message to forward, it now reports
which peer sent us the message via a new `prev_hop` field, so handlers can
apply source-based policy when deciding whether to forward.

`prev_hop` is `None` when the forward is enqueued by a message handler (the
BOLT 12 static-invoice-server flow), which isn't given the sending node;
otherwise it is the node we received the message from.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: jkczyz

lightning-tests/src/upgrade_downgrade_tests.rs

@@ -761,7 +761,9 @@ fn test_onion_message_intercepted_upgrade_from_0_2() {
 	let deserialized = <Event as MaybeReadable>::read(&mut reader).unwrap().unwrap();
 
 	match deserialized {
-		Event::OnionMessageIntercepted { next_hop, message } => {
+		Event::OnionMessageIntercepted { prev_hop, next_hop, message } => {
+			// LDK 0.2 did not write a `prev_hop`, so it must default to `None`.
+			assert_eq!(prev_hop, None);
 			assert_eq!(next_hop, NextMessageHop::NodeId(pubkey));
 			assert_eq!(message, dummy_onion_message());
 		},
@@ -773,11 +775,14 @@ fn test_onion_message_intercepted_upgrade_from_0_2() {
 fn test_onion_message_intercepted_node_id_downgrade_to_0_2() {
 	// Ensure that an `Event::OnionMessageIntercepted` with a `NodeId` next hop serialized by
 	// the current version can be deserialized by LDK 0.2 (which expects `peer_node_id` in TLV
-	// field 0).
+	// field 0 and ignores the newer `prev_hop` in TLV field 3).
 	let pubkey =
 		PublicKey::from_secret_key(&Secp256k1::new(), &SecretKey::from_slice(&[42; 32]).unwrap());
+	let prev_hop =
+		PublicKey::from_secret_key(&Secp256k1::new(), &SecretKey::from_slice(&[43; 32]).unwrap());
 
 	let event = Event::OnionMessageIntercepted {
+		prev_hop: Some(prev_hop),
 		next_hop: NextMessageHop::NodeId(pubkey),
 		message: dummy_onion_message(),
 	};
@@ -802,6 +807,7 @@ fn test_onion_message_intercepted_scid_downgrade_to_0_2() {
 	// serialized by the current version cannot be deserialized by LDK 0.2, since the
 	// `peer_node_id` field (0) is not written for SCID variants and LDK 0.2 requires it.
 	let event = Event::OnionMessageIntercepted {
+		prev_hop: None,
 		next_hop: NextMessageHop::ShortChannelId(42),
 		message: dummy_onion_message(),
 	};

lightning/src/events/mod.rs

@@ -1856,6 +1856,12 @@ pub enum Event {
 	///
 	/// [`OnionMessenger::new_with_offline_peer_interception`]: crate::onion_message::messenger::OnionMessenger::new_with_offline_peer_interception
 	OnionMessageIntercepted {
+		/// The node id of the peer that sent the message, if known.
+		///
+		/// This is `None` when the forward is enqueued by a message handler (e.g. the BOLT 12
+		/// static-invoice-server flow), which isn't given the sending node. Otherwise it is the
+		/// node we received the message from.
+		prev_hop: Option<PublicKey>,
 		/// The next hop (offline peer or unknown SCID).
 		next_hop: NextMessageHop,
 		/// The onion message intended to be forwarded to the offline peer or via the unknown
@@ -2441,25 +2447,20 @@ impl Writeable for Event {
 				35u8.write(writer)?;
 				// Never write ConnectionNeeded events as buffered onion messages aren't serialized.
 			},
-			&Event::OnionMessageIntercepted { ref next_hop, ref message } => {
+			&Event::OnionMessageIntercepted { ref prev_hop, ref next_hop, ref message } => {
 				37u8.write(writer)?;
-				match next_hop {
-					NextMessageHop::NodeId(peer_node_id) => {
-						// If we have the node_id, we keep writing it for backwards compatibility.
-						write_tlv_fields!(writer, {
-							(0, peer_node_id, required),
-							(1, next_hop, required),
-							(2, message, required),
-						});
-					},
-					NextMessageHop::ShortChannelId(_) => {
-						write_tlv_fields!(writer, {
-							// 0 used to be peer_node_id in LDK v0.2 and prior.
-							(1, next_hop, required),
-							(2, message, required),
-						});
-					},
-				}
+				// 0 used to be peer_node_id in LDK v0.2 and prior; we keep writing it when the next
+				// hop is a node id for backwards compatibility.
+				let legacy_peer_node_id = match next_hop {
+					NextMessageHop::NodeId(node_id) => Some(node_id),
+					NextMessageHop::ShortChannelId(_) => None,
+				};
+				write_tlv_fields!(writer, {
+					(0, legacy_peer_node_id, option),
+					(1, next_hop, required),
+					(2, message, required),
+					(3, prev_hop, option),
+				});
 			},
 			&Event::OnionMessagePeerConnected { ref peer_node_id } => {
 				39u8.write(writer)?;
@@ -3090,12 +3091,14 @@ impl MaybeReadable for Event {
 						(0, peer_node_id, option),
 						(1, next_hop, option),
 						(2, message, required),
+						(3, prev_hop, option),
 					});
 
 					let next_hop = next_hop
 						.or(peer_node_id.map(NextMessageHop::NodeId))
 						.ok_or(msgs::DecodeError::InvalidValue)?;
 					Ok(Some(Event::OnionMessageIntercepted {
+						prev_hop,
 						next_hop,
 						message: message.0.unwrap(),
 					}))

lightning/src/onion_message/functional_tests.rs

@@ -1154,7 +1154,8 @@ fn intercept_offline_peer_oms() {
 	let mut events = release_events(&nodes[1]);
 	assert_eq!(events.len(), 1);
 	let onion_message = match events.remove(0) {
-		Event::OnionMessageIntercepted { next_hop, message } => {
+		Event::OnionMessageIntercepted { prev_hop, next_hop, message } => {
+			assert_eq!(prev_hop, Some(nodes[0].node_id));
 			if let NextMessageHop::NodeId(peer_node_id) = next_hop {
 				assert_eq!(peer_node_id, final_node_vec[0].node_id);
 				message
@@ -1240,7 +1241,8 @@ fn intercept_unknown_scid_oms() {
 	let mut events = release_events(&nodes[1]);
 	assert_eq!(events.len(), 1);
 	let onion_message = match events.remove(0) {
-		Event::OnionMessageIntercepted { next_hop, message } => {
+		Event::OnionMessageIntercepted { prev_hop, next_hop, message } => {
+			assert_eq!(prev_hop, Some(nodes[0].node_id));
 			if let NextMessageHop::ShortChannelId(intercepted_scid) = next_hop {
 				assert_eq!(intercepted_scid, scid);
 				message

lightning/src/onion_message/messenger.rs

@@ -1556,6 +1556,7 @@ impl<
 
 		let result = if is_forward {
 			self.enqueue_forwarded_onion_message(
+				None,
 				NextMessageHop::NodeId(first_node_id),
 				onion_message,
 				log_suffix,
@@ -1671,7 +1672,8 @@ impl<
 	}
 
 	fn enqueue_forwarded_onion_message(
-		&self, next_hop: NextMessageHop, onion_message: OnionMessage, log_suffix: fmt::Arguments,
+		&self, prev_hop: Option<PublicKey>, next_hop: NextMessageHop, onion_message: OnionMessage,
+		log_suffix: fmt::Arguments,
 	) -> Result<(), SendError> {
 		let next_node_id = match next_hop {
 			NextMessageHop::NodeId(pubkey) => pubkey,
@@ -1686,6 +1688,7 @@ impl<
 							log_suffix
 						);
 						self.enqueue_intercepted_event(Event::OnionMessageIntercepted {
+							prev_hop,
 							next_hop,
 							message: onion_message,
 						});
@@ -1734,6 +1737,7 @@ impl<
 					log_suffix
 				);
 				self.enqueue_intercepted_event(Event::OnionMessageIntercepted {
+					prev_hop,
 					next_hop,
 					message: onion_message,
 				});
@@ -2315,6 +2319,7 @@ impl<
 			},
 			Ok(PeeledOnion::Forward(next_hop, onion_message)) => {
 				let _ = self.enqueue_forwarded_onion_message(
+					Some(peer_node_id),
 					next_hop,
 					onion_message,
 					format_args!("when forwarding peeled onion message from {}", peer_node_id),