Set max path len on receipt of static invoice.

Because we may receive a static invoice to pay days before the recipient
actually comes back online to receive the payment, it's good to do as many
checks as we can up-front. Here we ensure that the blinded paths provided
in the invoice won't cause us to exceed the maximum onion packet size.

Author: valentinewallace

lightning/src/ln/channelmanager.rs

@@ -4326,9 +4326,11 @@ where
 	) -> Result<(), Bolt12PaymentError> {
 		let mut res = Ok(());
 		PersistenceNotifierGuard::optionally_notify(self, || {
+			let best_block_height = self.best_block.read().unwrap().height;
 			let features = self.bolt12_invoice_features();
 			let outbound_pmts_res = self.pending_outbound_payments.static_invoice_received(
-				invoice, payment_id, features, &*self.entropy_source, &self.pending_events
+				invoice, payment_id, features, best_block_height, &*self.entropy_source,
+				&self.pending_events
 			);
 			let payment_release_secret = match outbound_pmts_res {
 				Ok(secret) => secret,

lightning/src/ln/outbound_payment.rs

@@ -937,7 +937,7 @@ impl OutboundPayments {
 	#[cfg(async_payments)]
 	pub(super) fn static_invoice_received<ES: Deref>(
 		&self, invoice: &StaticInvoice, payment_id: PaymentId, features: Bolt12InvoiceFeatures,
-		entropy_source: ES,
+		best_block_height: u32, entropy_source: ES,
 		pending_events: &Mutex<VecDeque<(events::Event, Option<EventCompletionAction>)>>
 	) -> Result<[u8; 32], Bolt12PaymentError> where ES::Target: EntropySource {
 		macro_rules! abandon_with_entry {
@@ -988,6 +988,15 @@ impl OutboundPayments {
 					let pay_params = PaymentParameters::from_static_invoice(invoice);
 					let mut route_params = RouteParameters::from_payment_params_and_value(pay_params, amount_msat);
 					route_params.max_total_routing_fee_msat = *max_total_routing_fee_msat;
+
+					if let Err(()) = onion_utils::set_max_path_length(
+						&mut route_params, &RecipientOnionFields::spontaneous_empty(), Some(keysend_preimage),
+						best_block_height
+					) {
+						abandon_with_entry!(entry, PaymentFailureReason::RouteNotFound);
+						return Err(Bolt12PaymentError::SendingFailed(RetryableSendFailure::OnionPacketSizeExceeded))
+					}
+
 					*entry.into_mut() = PendingOutboundPayment::StaticInvoiceReceived {
 						payment_hash,
 						keysend_preimage,