Blinded payments to unannounced introduction node

When creating blinded paths for receiving onion payments, allow using
the recipient's only peer as the introduction node when the recipient is
unannounced. This allows for sending payments 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/ln/offers_tests.rs

@@ -900,8 +900,18 @@ fn creates_offer_with_blinded_path_using_unannounced_introduction_node() {
 	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);
+	let (invoice_request, reply_path) = extract_invoice_request(alice, &onion_message);
+	assert_ne!(invoice_request.payer_id(), bob_id);
 	assert_eq!(reply_path.introduction_node, IntroductionNode::NodeId(alice_id));
+
+	let onion_message = alice.onion_messenger.next_onion_message_for_peer(bob_id).unwrap();
+
+	let invoice = extract_invoice(bob, &onion_message);
+	assert_ne!(invoice.signing_pubkey(), alice_id);
+	assert!(!invoice.payment_paths().is_empty());
+	for (_, path) in invoice.payment_paths() {
+		assert_eq!(path.introduction_node, IntroductionNode::NodeId(bob_id));
+	}
 }
 
 /// Checks that a refund can be created using an unannounced node as a blinded path's introduction
@@ -934,6 +944,18 @@ fn creates_refund_with_blinded_path_using_unannounced_introduction_node() {
 		assert_eq!(path.introduction_node, IntroductionNode::NodeId(alice_id));
 	}
 	expect_recent_payment!(bob, RecentPaymentDetails::AwaitingInvoice, payment_id);
+
+	let expected_invoice = alice.node.request_refund_payment(&refund).unwrap();
+
+	let onion_message = alice.onion_messenger.next_onion_message_for_peer(bob_id).unwrap();
+
+	let invoice = extract_invoice(bob, &onion_message);
+	assert_eq!(invoice, expected_invoice);
+	assert_ne!(invoice.signing_pubkey(), alice_id);
+	assert!(!invoice.payment_paths().is_empty());
+	for (_, path) in invoice.payment_paths() {
+		assert_eq!(path.introduction_node, IntroductionNode::NodeId(bob_id));
+	}
 }
 
 /// Fails creating or paying an offer when a blinded path cannot be created because no peers are

lightning/src/routing/router.rs

@@ -100,7 +100,20 @@ impl<G: Deref<Target = NetworkGraph<L>> + Clone, L: Deref, ES: Deref, S: Deref,
 		// recipient's node_id.
 		const MIN_PEER_CHANNELS: usize = 3;
 
+		let has_one_peer = first_hops
+			.first()
+			.map(|details| details.counterparty.node_id)
+			.map(|node_id| first_hops
+				.iter()
+				.skip(1)
+				.all(|details| details.counterparty.node_id == node_id)
+			)
+			.unwrap_or(false);
+
 		let network_graph = self.network_graph.deref().read_only();
+		let is_recipient_announced =
+			network_graph.nodes().contains_key(&NodeId::from_pubkey(&recipient));
+
 		let paths = first_hops.into_iter()
 			.filter(|details| details.counterparty.features.supports_route_blinding())
 			.filter(|details| amount_msats <= details.inbound_capacity_msat)
@@ -109,7 +122,8 @@ impl<G: Deref<Target = NetworkGraph<L>> + Clone, L: Deref, ES: Deref, S: Deref,
 			.filter(|details| network_graph
 					.node(&NodeId::from_pubkey(&details.counterparty.node_id))
 					.map(|node_info| node_info.channels.len() >= MIN_PEER_CHANNELS)
-					.unwrap_or(false)
+					// Allow payments directly with the only peer when unannounced.
+					.unwrap_or(!is_recipient_announced && has_one_peer)
 			)
 			.filter_map(|details| {
 				let short_channel_id = match details.get_inbound_payment_scid() {