BlindedPath with unannounced introduction node

When creating blinded paths for receiving onion messages, allow using
the recipient's only peer as the introduction node when the recipient is
unannounced. This allows for sending messages without going through an
intermediary, which is useful for testing or when only connected to an
LSP with an empty NetworkGraph.

Author: jkczyz

lightning/src/blinded_path/message.rs

@@ -30,7 +30,7 @@ use core::mem;
 use core::ops::Deref;
 
 /// An intermediate node, and possibly a short channel id leading to the next node.
-#[derive(Clone, Debug, Hash, PartialEq, Eq)]
+#[derive(Clone, Copy, Debug, Hash, PartialEq, Eq)]
 pub struct ForwardNode {
 	/// This node's pubkey.
 	pub node_id: PublicKey,

lightning/src/ln/offers_tests.rs

@@ -865,43 +865,75 @@ fn pays_for_refund_without_blinded_paths() {
 	expect_recent_payment!(bob, RecentPaymentDetails::Fulfilled, payment_id);
 }
 
-/// Fails creating an offer when a blinded path cannot be created without exposing the node's id.
+/// Checks that an offer can be created using an unannounced node as a blinded path's introduction
+/// node. This is only preferred if there are no other options which may indicated either the offer
+/// is intended for the unannounced node or that the node is actually announced (e.g., an LSP) but
+/// the recipient doesn't have a network graph.
 #[test]
-fn fails_creating_offer_without_blinded_paths() {
+fn creates_offer_with_blinded_path_using_unannounced_introduction_node() {
 	let chanmon_cfgs = create_chanmon_cfgs(2);
 	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
 	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
 	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
 
 	create_unannounced_chan_between_nodes_with_value(&nodes, 0, 1, 10_000_000, 1_000_000_000);
 
-	match nodes[0].node.create_offer_builder(None) {
-		Ok(_) => panic!("Expected error"),
-		Err(e) => assert_eq!(e, Bolt12SemanticError::MissingPaths),
+	let alice = &nodes[0];
+	let alice_id = alice.node.get_our_node_id();
+	let bob = &nodes[1];
+	let bob_id = bob.node.get_our_node_id();
+
+	let offer = alice.node
+		.create_offer_builder(None).unwrap()
+		.amount_msats(10_000_000)
+		.build().unwrap();
+	assert_ne!(offer.signing_pubkey(), Some(alice_id));
+	assert!(!offer.paths().is_empty());
+	for path in offer.paths() {
+		assert_eq!(path.introduction_node, IntroductionNode::NodeId(bob_id));
 	}
+
+	let payment_id = PaymentId([1; 32]);
+	bob.node.pay_for_offer(&offer, None, None, None, payment_id, Retry::Attempts(0), None).unwrap();
+	expect_recent_payment!(bob, RecentPaymentDetails::AwaitingInvoice, payment_id);
+
+	let onion_message = bob.onion_messenger.next_onion_message_for_peer(alice_id).unwrap();
+	alice.onion_messenger.handle_onion_message(&bob_id, &onion_message);
+
+	let (_, reply_path) = extract_invoice_request(alice, &onion_message);
+	assert_eq!(reply_path.introduction_node, IntroductionNode::NodeId(alice_id));
 }
 
-/// Fails creating a refund when a blinded path cannot be created without exposing the node's id.
+/// Checks that a refund can be created using an unannounced node as a blinded path's introduction
+/// node. This is only preferred if there are no other options which may indicated either the refund
+/// is intended for the unannounced node or that the node is actually announced (e.g., an LSP) but
+/// the sender doesn't have a network graph.
 #[test]
-fn fails_creating_refund_without_blinded_paths() {
+fn creates_refund_with_blinded_path_using_unannounced_introduction_node() {
 	let chanmon_cfgs = create_chanmon_cfgs(2);
 	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
 	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
 	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
 
 	create_unannounced_chan_between_nodes_with_value(&nodes, 0, 1, 10_000_000, 1_000_000_000);
 
+	let alice = &nodes[0];
+	let alice_id = alice.node.get_our_node_id();
+	let bob = &nodes[1];
+	let bob_id = bob.node.get_our_node_id();
+
 	let absolute_expiry = Duration::from_secs(u64::MAX);
 	let payment_id = PaymentId([1; 32]);
-
-	match nodes[0].node.create_refund_builder(
-		10_000, absolute_expiry, payment_id, Retry::Attempts(0), None
-	) {
-		Ok(_) => panic!("Expected error"),
-		Err(e) => assert_eq!(e, Bolt12SemanticError::MissingPaths),
+	let refund = bob.node
+		.create_refund_builder(10_000_000, absolute_expiry, payment_id, Retry::Attempts(0), None)
+		.unwrap()
+		.build().unwrap();
+	assert_ne!(refund.payer_id(), bob_id);
+	assert!(!refund.paths().is_empty());
+	for path in refund.paths() {
+		assert_eq!(path.introduction_node, IntroductionNode::NodeId(alice_id));
 	}
-
-	assert!(nodes[0].node.list_recent_payments().is_empty());
+	expect_recent_payment!(bob, RecentPaymentDetails::AwaitingInvoice, payment_id);
 }
 
 /// Fails creating or paying an offer when a blinded path cannot be created because no peers are
@@ -1080,8 +1112,7 @@ fn fails_sending_invoice_with_unsupported_chain_for_refund() {
 	}
 }
 
-/// Fails creating an invoice request when a blinded reply path cannot be created without exposing
-/// the node's id.
+/// Fails creating an invoice request when a blinded reply path cannot be created.
 #[test]
 fn fails_creating_invoice_request_without_blinded_reply_path() {
 	let chanmon_cfgs = create_chanmon_cfgs(6);
@@ -1098,7 +1129,7 @@ fn fails_creating_invoice_request_without_blinded_reply_path() {
 	let (alice, bob, charlie, david) = (&nodes[0], &nodes[1], &nodes[2], &nodes[3]);
 
 	disconnect_peers(alice, &[charlie, david, &nodes[4], &nodes[5]]);
-	disconnect_peers(david, &[bob, &nodes[4], &nodes[5]]);
+	disconnect_peers(david, &[bob, charlie, &nodes[4], &nodes[5]]);
 
 	let offer = alice.node
 		.create_offer_builder(None).unwrap()

lightning/src/onion_message/functional_tests.rs

@@ -492,8 +492,9 @@ fn async_response_with_reply_path_fails() {
 	let path_id = Some([2; 32]);
 	let reply_path = BlindedPath::new_for_message(&[], bob.node_id, &*bob.entropy_source, &secp_ctx).unwrap();
 
-	// Alice tries to asynchronously respond to Bob, but fails because the nodes are unannounced.
-	// Therefore, the reply_path cannot be used for the response.
+	// Alice tries to asynchronously respond to Bob, but fails because the nodes are unannounced and
+	// disconnected. Thus, a reply path could no be created for the response.
+	disconnect_peers(alice, bob);
 	let responder = Responder::new(reply_path, path_id);
 	alice.custom_message_handler.expect_message_and_response(message.clone());
 	let response_instruction = alice.custom_message_handler.handle_custom_message(message, Some(responder));

lightning/src/onion_message/messenger.rs

@@ -483,7 +483,7 @@ where
 	}
 
 	fn create_blinded_paths_from_iter<
-		I: Iterator<Item = ForwardNode>,
+		I: ExactSizeIterator<Item = ForwardNode>,
 		T: secp256k1::Signing + secp256k1::Verification
 	>(
 		&self, recipient: PublicKey, peers: I, secp_ctx: &Secp256k1<T>, compact_paths: bool
@@ -499,13 +499,18 @@ where
 		let is_recipient_announced =
 			network_graph.nodes().contains_key(&NodeId::from_pubkey(&recipient));
 
+		let has_one_peer = peers.len() == 1;
 		let mut peer_info = peers
-			// Limit to peers with announced channels
+			// Limit to peers with announced channels unless the recipient is unannounced.
 			.filter_map(|peer|
 				network_graph
 					.node(&NodeId::from_pubkey(&peer.node_id))
 					.filter(|info| info.channels.len() >= MIN_PEER_CHANNELS)
 					.map(|info| (peer, info.is_tor_only(), info.channels.len()))
+					// Allow messages directly with the only peer when unannounced.
+					.or_else(|| (!is_recipient_announced && has_one_peer)
+						.then(|| (peer, false, 0))
+					)
 			)
 			// Exclude Tor-only nodes when the recipient is announced.
 			.filter(|(_, is_tor_only, _)| !(*is_tor_only && is_recipient_announced))