Return the invoice when requesting a refund

When sending an invoice for a refund, information from the invoice may
be useful for caller. For instance, the payment_hash can be used to
track whether the refund was paid. Return the invoice to facilitate this
use case.

Author: jkczyz

lightning/src/ln/channelmanager.rs

@@ -7934,7 +7934,7 @@ where
 	///
 	/// The resulting invoice uses a [`PaymentHash`] recognized by the [`ChannelManager`] and a
 	/// [`BlindedPath`] containing the [`PaymentSecret`] needed to reconstruct the corresponding
-	/// [`PaymentPreimage`].
+	/// [`PaymentPreimage`]. It is returned purely for informational purposes.
 	///
 	/// # Limitations
 	///
@@ -7951,7 +7951,9 @@ where
 	///   the invoice.
 	///
 	/// [`Bolt12Invoice`]: crate::offers::invoice::Bolt12Invoice
-	pub fn request_refund_payment(&self, refund: &Refund) -> Result<(), Bolt12SemanticError> {
+	pub fn request_refund_payment(
+		&self, refund: &Refund
+	) -> Result<Bolt12Invoice, Bolt12SemanticError> {
 		let expanded_key = &self.inbound_payment_key;
 		let entropy = &*self.entropy_source;
 		let secp_ctx = &self.secp_ctx;
@@ -7990,7 +7992,7 @@ where
 				let mut pending_offers_messages = self.pending_offers_messages.lock().unwrap();
 				if refund.paths().is_empty() {
 					let message = new_pending_onion_message(
-						OffersMessage::Invoice(invoice),
+						OffersMessage::Invoice(invoice.clone()),
 						Destination::Node(refund.payer_id()),
 						Some(reply_path),
 					);
@@ -8006,7 +8008,7 @@ where
 					}
 				}
 
-				Ok(())
+				Ok(invoice)
 			},
 			Err(()) => Err(Bolt12SemanticError::InvalidAmount),
 		}

lightning/src/ln/offers_tests.rs

@@ -473,7 +473,7 @@ fn creates_and_pays_for_refund_using_two_hop_blinded_path() {
 	}
 	expect_recent_payment!(david, RecentPaymentDetails::AwaitingInvoice, payment_id);
 
-	alice.node.request_refund_payment(&refund).unwrap();
+	let expected_invoice = alice.node.request_refund_payment(&refund).unwrap();
 
 	connect_peers(alice, charlie);
 
@@ -484,6 +484,8 @@ fn creates_and_pays_for_refund_using_two_hop_blinded_path() {
 	david.onion_messenger.handle_onion_message(&charlie_id, &onion_message);
 
 	let invoice = extract_invoice(david, &onion_message);
+	assert_eq!(invoice, expected_invoice);
+
 	assert_eq!(invoice.amount_msats(), 10_000_000);
 	assert_ne!(invoice.signing_pubkey(), alice_id);
 	assert!(!invoice.payment_paths().is_empty());
@@ -589,12 +591,14 @@ fn creates_and_pays_for_refund_using_one_hop_blinded_path() {
 	}
 	expect_recent_payment!(bob, RecentPaymentDetails::AwaitingInvoice, payment_id);
 
-	alice.node.request_refund_payment(&refund).unwrap();
+	let expected_invoice = alice.node.request_refund_payment(&refund).unwrap();
 
 	let onion_message = alice.onion_messenger.next_onion_message_for_peer(bob_id).unwrap();
 	bob.onion_messenger.handle_onion_message(&alice_id, &onion_message);
 
 	let invoice = extract_invoice(bob, &onion_message);
+	assert_eq!(invoice, expected_invoice);
+
 	assert_eq!(invoice.amount_msats(), 10_000_000);
 	assert_ne!(invoice.signing_pubkey(), alice_id);
 	assert!(!invoice.payment_paths().is_empty());
@@ -681,12 +685,14 @@ fn pays_for_refund_without_blinded_paths() {
 	assert!(refund.paths().is_empty());
 	expect_recent_payment!(bob, RecentPaymentDetails::AwaitingInvoice, payment_id);
 
-	alice.node.request_refund_payment(&refund).unwrap();
+	let expected_invoice = alice.node.request_refund_payment(&refund).unwrap();
 
 	let onion_message = alice.onion_messenger.next_onion_message_for_peer(bob_id).unwrap();
 	bob.onion_messenger.handle_onion_message(&alice_id, &onion_message);
 
 	let invoice = extract_invoice(bob, &onion_message);
+	assert_eq!(invoice, expected_invoice);
+
 	route_bolt12_payment(bob, &[alice], &invoice);
 	expect_recent_payment!(bob, RecentPaymentDetails::Pending, payment_id);