Onion message routing to immediate peers.

DefaultMessageRouter always fails. Update it so that it can route to a
directly connected peer. This is needed for an Offers minimum viable
product.

Author: jkczyz

lightning/src/ln/channelmanager.rs

@@ -7170,6 +7170,11 @@ where
 	/// the node must be announced. Otherwise, there is no way to find a path to the introduction
 	/// node in order to send the [`InvoiceRequest`].
 	///
+	/// # Limitations
+	///
+	/// Requires a direct connection to the introduction node in the responding [`InvoiceRequest`]'s
+	/// reply path.
+	///
 	/// [`Offer`]: crate::offers::offer::Offer
 	/// [`InvoiceRequest`]: crate::offers::invoice_request::InvoiceRequest
 	pub fn create_offer_builder(
@@ -7210,13 +7215,19 @@ where
 	/// node must be announced. Otherwise, there is no way to find a path to the introduction node
 	/// in order to send the [`Bolt12Invoice`].
 	///
+	/// # Limitations
+	///
+	/// Requires a direct connection to an introduction node in the responding
+	/// [`Bolt12Invoice::payment_paths`].
+	///
 	/// # Errors
 	///
 	/// Errors if a duplicate `payment_id` is provided given the caveats in the aforementioned link
 	/// or if `amount_msats` is invalid.
 	///
 	/// [`Refund`]: crate::offers::refund::Refund
 	/// [`Bolt12Invoice`]: crate::offers::invoice::Bolt12Invoice
+	/// [`Bolt12Invoice::payment_paths`]: crate::offers::invoice::Bolt12Invoice::payment_paths
 	pub fn create_refund_builder(
 		&self, description: String, amount_msats: u64, absolute_expiry: Duration,
 		payment_id: PaymentId, retry_strategy: Retry, max_total_routing_fee_msat: Option<u64>
@@ -7273,6 +7284,12 @@ where
 	/// node must be announced. Otherwise, there is no way to find a path to the introduction node
 	/// in order to send the [`Bolt12Invoice`].
 	///
+	/// # Limitations
+	///
+	/// Requires a direct connection to an introduction node in [`Offer::paths`] or to
+	/// [`Offer::signing_pubkey`], if empty. A similar restriction applies to the responding
+	/// [`Bolt12Invoice::payment_paths`].
+	///
 	/// # Errors
 	///
 	/// Errors if a duplicate `payment_id` is provided given the caveats in the aforementioned link
@@ -7283,6 +7300,7 @@ where
 	/// [`InvoiceRequest::payer_note`]: crate::offers::invoice_request::InvoiceRequest::payer_note
 	/// [`InvoiceRequestBuilder`]: crate::offers::invoice_request::InvoiceRequestBuilder
 	/// [`Bolt12Invoice`]: crate::offers::invoice::Bolt12Invoice
+	/// [`Bolt12Invoice::payment_paths`]: crate::offers::invoice::Bolt12Invoice::payment_paths
 	/// [Avoiding Duplicate Payments]: #avoiding-duplicate-payments
 	pub fn pay_for_offer(
 		&self, offer: &Offer, quantity: Option<u64>, amount_msats: Option<u64>,
@@ -7352,6 +7370,11 @@ where
 	/// [`BlindedPath`] containing the [`PaymentSecret`] needed to reconstruct the corresponding
 	/// [`PaymentPreimage`].
 	///
+	/// # Limitations
+	///
+	/// Requires a direct connection to an introduction node in [`Refund::paths`] or to
+	/// [`Refund::payer_id`], if empty.
+	///
 	/// [`Bolt12Invoice`]: crate::offers::invoice::Bolt12Invoice
 	pub fn request_refund_payment(&self, refund: &Refund) -> Result<(), Bolt12SemanticError> {
 		let expanded_key = &self.inbound_payment_key;

lightning/src/onion_message/messenger.rs

@@ -177,14 +177,18 @@ pub trait MessageRouter {
 	) -> Result<OnionMessagePath, ()>;
 }
 
-/// A [`MessageRouter`] that always fails.
+/// A [`MessageRouter`] that can only route to a directly connected [`Destination`].
 pub struct DefaultMessageRouter;
 
 impl MessageRouter for DefaultMessageRouter {
 	fn find_path(
-		&self, _sender: PublicKey, _peers: Vec<PublicKey>, _destination: Destination
+		&self, _sender: PublicKey, peers: Vec<PublicKey>, destination: Destination
 	) -> Result<OnionMessagePath, ()> {
-		Err(())
+		if peers.contains(&destination.first_node()) {
+			Ok(OnionMessagePath { intermediate_nodes: vec![], destination })
+		} else {
+			Err(())
+		}
 	}
 }
 
@@ -214,6 +218,13 @@ impl Destination {
 			Destination::BlindedPath(BlindedPath { blinded_hops, .. }) => blinded_hops.len(),
 		}
 	}
+
+	fn first_node(&self) -> PublicKey {
+		match self {
+			Destination::Node(node_id) => *node_id,
+			Destination::BlindedPath(BlindedPath { introduction_node_id: node_id, .. }) => *node_id,
+		}
+	}
 }
 
 /// Errors that may occur when [sending an onion message].