Avoid repeated refreshes for persisted async invoices

When a used async receive offer's refreshed static invoice is persisted, advance the recorded invoice creation time. This keeps the refresh threshold anchored to the newest invoice instead of making the offer look stale on every timer tick.

Add coverage that a used offer does not enqueue another ServeStaticInvoice immediately after the server confirms the refresh.

Co-Authored-By: HAL 9000

This finding was discovered by Project Loupe

Author: tnull

lightning/src/ln/async_payments_tests.rs

@@ -2450,6 +2450,25 @@ fn refresh_static_invoices_for_used_offers() {
 		.handle_onion_message(server.node.get_our_node_id(), &invoice_persisted_om);
 	assert_eq!(recipient.node.flow.test_get_async_receive_offers().len(), 1);
 
+	// The invoice was just refreshed and persisted. A later timer tick must wait until the next
+	// refresh threshold before generating another invoice for the same offer.
+	recipient.node.timer_tick_occurred();
+	let pending_oms_after = recipient.onion_messenger.release_pending_msgs();
+	let mut extra_serve_invoices = 0;
+	if let Some(msgs) = pending_oms_after.get(&server.node.get_our_node_id()) {
+		for msg in msgs {
+			if let PeeledOnion::AsyncPayments(AsyncPaymentsMessage::ServeStaticInvoice(_), _, _) =
+				server.onion_messenger.peel_onion_message(&msg).unwrap()
+			{
+				extra_serve_invoices += 1;
+			}
+		}
+	}
+	assert_eq!(
+		extra_serve_invoices, 0,
+		"used offer invoice was refreshed again immediately after a successful refresh"
+	);
+
 	// Remove the peer restriction added above.
 	server.message_router.peers_override.lock().unwrap().clear();
 	recipient.message_router.peers_override.lock().unwrap().clear();

lightning/src/offers/async_receive_offer_cache.rs

@@ -491,7 +491,7 @@ impl AsyncReceiveOfferCache {
 			match offer.status {
 				OfferStatus::Used { invoice_created_at: ref mut inv_created_at }
 				| OfferStatus::Ready { invoice_created_at: ref mut inv_created_at } => {
-					*inv_created_at = core::cmp::min(invoice_created_at, *inv_created_at);
+					*inv_created_at = core::cmp::max(invoice_created_at, *inv_created_at);
 				},
 				OfferStatus::Pending => offer.status = OfferStatus::Ready { invoice_created_at },
 			}