Async receive: update static invoices when stale

diff --git a/lightning/src/ln/async_payments_tests.rs b/lightning/src/ln/async_payments_tests.rs
index 4da3145..d868eee 100644
--- a/lightning/src/ln/async_payments_tests.rs
+++ b/lightning/src/ln/async_payments_tests.rs
@@ -28,7 +28,7 @@ use crate::ln::outbound_payment::{
 	PendingOutboundPayment, Retry, TEST_ASYNC_PAYMENT_TIMEOUT_RELATIVE_EXPIRY,
 };
 use crate::offers::async_receive_offer_cache::{
-	TEST_MAX_CACHED_OFFERS_TARGET, TEST_MAX_UPDATE_ATTEMPTS,
+	TEST_INVOICE_REFRESH_THRESHOLD, TEST_MAX_CACHED_OFFERS_TARGET, TEST_MAX_UPDATE_ATTEMPTS,
 	TEST_MIN_OFFER_PATHS_RELATIVE_EXPIRY_SECS, TEST_OFFER_REFRESH_THRESHOLD,
 };
 use crate::offers::flow::{
@@ -1715,11 +1715,53 @@ fn offer_cache_round_trip_ser() {
 	assert_eq!(cached_offers_pre_ser, cached_offers_post_ser);
 }
 
+#[test]
+fn refresh_static_invoices_for_pending_offers() {
+	// Check that an invoice for an  offer that is pending persistence with the server will be updated
+	// every timer tick.
+	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, None]);
+	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+	create_unannounced_chan_between_nodes_with_value(&nodes, 0, 1, 1_000_000, 0);
+	let server = &nodes[0];
+	let recipient = &nodes[1];
+
+	let recipient_id = vec![42; 32];
+	let inv_server_paths =
+		server.node.blinded_paths_for_async_recipient(recipient_id.clone(), None).unwrap();
+	recipient.node.set_paths_to_static_invoice_server(inv_server_paths).unwrap();
+	expect_offer_paths_requests(&nodes[1], &[&nodes[0]]);
+
+	// Set up the recipient to have one offer pending with the static invoice server.
+	invoice_flow_up_to_send_serve_static_invoice(server, recipient);
+
+	// Every timer tick, we'll send a fresh invoice to the server.
+	for _ in 0..10 {
+		recipient.node.timer_tick_occurred();
+		let pending_oms = recipient.onion_messenger.release_pending_msgs();
+		pending_oms
+			.get(&server.node.get_our_node_id())
+			.unwrap()
+			.iter()
+			.find(|msg| match server.onion_messenger.peel_onion_message(&msg).unwrap() {
+				PeeledOnion::AsyncPayments(AsyncPaymentsMessage::ServeStaticInvoice(_), _, _) => {
+					true
+				},
+				PeeledOnion::AsyncPayments(AsyncPaymentsMessage::OfferPathsRequest(_), _, _) => {
+					false
+				},
+				_ => panic!("Unexpected message"),
+			})
+			.unwrap();
+	}
+}
+
 #[cfg_attr(feature = "std", ignore)]
 #[test]
-fn refresh_static_invoices() {
-	// Check that an invoice for a particular offer stored with the server will be updated once per
-	// timer tick.
+fn refresh_static_invoices_for_used_offers() {
+	// Check that an invoice for a used offer stored with the server will be updated every
+	// INVOICE_REFRESH_THRESHOLD.
 	let chanmon_cfgs = create_chanmon_cfgs(3);
 	let node_cfgs = create_node_cfgs(3, &chanmon_cfgs);
 
@@ -1744,25 +1786,26 @@ fn refresh_static_invoices() {
 	// Set up the recipient to have one offer and an invoice with the static invoice server.
 	let flow_res = pass_static_invoice_server_messages(server, recipient, recipient_id.clone());
 	let original_invoice = flow_res.invoice;
-	// Mark the offer as used so we'll update the invoice on timer tick.
+	// Mark the offer as used so we'll update the invoice after INVOICE_REFRESH_THRESHOLD.
 	let _offer = recipient.node.get_async_receive_offer().unwrap();
 
 	// Force the server and recipient to send OMs directly to each other for testing simplicity.
 	server.message_router.peers_override.lock().unwrap().push(recipient.node.get_our_node_id());
 	recipient.message_router.peers_override.lock().unwrap().push(server.node.get_our_node_id());
 
-	assert!(recipient
-		.onion_messenger
-		.next_onion_message_for_peer(server.node.get_our_node_id())
-		.is_none());
+	// Prior to INVOICE_REFRESH_THRESHOLD, we won't refresh the invoice.
+	advance_time_by(TEST_INVOICE_REFRESH_THRESHOLD, recipient);
+	recipient.node.timer_tick_occurred();
+	expect_offer_paths_requests(&nodes[2], &[&nodes[0], &nodes[1]]);
 
-	// Check that we'll refresh the invoice on the next timer tick.
+	// After INVOICE_REFRESH_THRESHOLD, we will refresh the invoice.
+	advance_time_by(Duration::from_secs(1), recipient);
 	recipient.node.timer_tick_occurred();
 	let pending_oms = recipient.onion_messenger.release_pending_msgs();
 	let serve_static_invoice_om = pending_oms
 		.get(&server.node.get_our_node_id())
 		.unwrap()
-		.into_iter()
+		.iter()
 		.find(|msg| match server.onion_messenger.peel_onion_message(&msg).unwrap() {
 			PeeledOnion::AsyncPayments(AsyncPaymentsMessage::ServeStaticInvoice(_), _, _) => true,
 			PeeledOnion::AsyncPayments(AsyncPaymentsMessage::OfferPathsRequest(_), _, _) => false,
diff --git a/lightning/src/offers/async_receive_offer_cache.rs b/lightning/src/offers/async_receive_offer_cache.rs
index 7c57296..346c7a8 100644
--- a/lightning/src/offers/async_receive_offer_cache.rs
+++ b/lightning/src/offers/async_receive_offer_cache.rs
@@ -199,6 +199,10 @@ const MAX_UPDATE_ATTEMPTS: u8 = 3;
 #[cfg(async_payments)]
 const OFFER_REFRESH_THRESHOLD: Duration = Duration::from_secs(2 * 60 * 60);
 
+/// Invoices stored with the static invoice server may become stale due to outdated channel and fee
+/// info, so they should be updated regularly.
+const INVOICE_REFRESH_THRESHOLD: Duration = Duration::from_secs(2 * 60 * 60);
+
 // Require offer paths that we receive to last at least 3 months.
 #[cfg(async_payments)]
 const MIN_OFFER_PATHS_RELATIVE_EXPIRY_SECS: u64 = 3 * 30 * 24 * 60 * 60;
@@ -210,6 +214,8 @@ pub(crate) const TEST_MAX_UPDATE_ATTEMPTS: u8 = MAX_UPDATE_ATTEMPTS;
 #[cfg(all(test, async_payments))]
 pub(crate) const TEST_OFFER_REFRESH_THRESHOLD: Duration = OFFER_REFRESH_THRESHOLD;
 #[cfg(all(test, async_payments))]
+pub(crate) const TEST_INVOICE_REFRESH_THRESHOLD: Duration = INVOICE_REFRESH_THRESHOLD;
+#[cfg(all(test, async_payments))]
 pub(crate) const TEST_MIN_OFFER_PATHS_RELATIVE_EXPIRY_SECS: u64 =
 	MIN_OFFER_PATHS_RELATIVE_EXPIRY_SECS;
 
@@ -416,13 +422,21 @@ impl AsyncReceiveOfferCache {
 	/// Returns an iterator over the list of cached offers where we need to send an updated invoice to
 	/// the static invoice server.
 	pub(super) fn offers_needing_invoice_refresh(
-		&self,
+		&self, duration_since_epoch: Duration,
 	) -> impl Iterator<Item = (&Offer, Nonce, u16, &Responder)> {
 		// For any offers which are either in use or pending confirmation by the server, we should send
 		// them a fresh invoice on each timer tick.
-		self.offers_with_idx().filter_map(|(idx, offer)| {
-			let needs_invoice_update =
-				matches!(offer.status, OfferStatus::Used { .. } | OfferStatus::Pending);
+		self.offers_with_idx().filter_map(move |(idx, offer)| {
+			let needs_invoice_update = match offer.status {
+				OfferStatus::Used { invoice_created_at } => {
+					invoice_created_at.saturating_add(INVOICE_REFRESH_THRESHOLD)
+						< duration_since_epoch
+				},
+				OfferStatus::Pending => true,
+				// Don't bother updating `Ready` offers' invoices on a timer because the offers themselves
+				// are regularly rotated anyway.
+				OfferStatus::Ready { .. } => false,
+			};
 			if needs_invoice_update {
 				let offer_slot = idx.try_into().unwrap_or(u16::MAX);
 				Some((
diff --git a/lightning/src/offers/flow.rs b/lightning/src/offers/flow.rs
index d576e2c..107c720 100644
--- a/lightning/src/offers/flow.rs
+++ b/lightning/src/offers/flow.rs
@@ -1333,8 +1333,9 @@ where
 	{
 		let mut serve_static_invoice_msgs = Vec::new();
 		{
+			let duration_since_epoch = self.duration_since_epoch();
 			let cache = self.async_receive_offer_cache.lock().unwrap();
-			for offer_and_metadata in cache.offers_needing_invoice_refresh() {
+			for offer_and_metadata in cache.offers_needing_invoice_refresh(duration_since_epoch) {
 				let (offer, offer_nonce, slot_number, update_static_invoice_path) =
 					offer_and_metadata;