Include invoice_slot in OfferPathsRequest message

In the initially-merged version of the static invoice server protocol, the
static invoice server would sometimes have to find a specific static invoice
based on (recipient_id, invoice_slot) and sometimes based on (recipient_id,
invoice_id). This made the API harder to use in terms of how the server would
index into the KVStore.

We'd like to transition to the server always finding a specific invoice based on
(recipient_id, invoice_slot) and get rid of the invoice_id concept.

Now that the invoice_slot is in the initial paths request, the server will be
able to include the slot in the offer paths that they create in response,
allowing the slot to be surfaced instead of the invoice_id when the invoice
request comes in, in upcoming commits.

Author: valentinewallace

lightning/src/offers/async_receive_offer_cache.rs

@@ -246,10 +246,11 @@ impl AsyncReceiveOfferCache {
 			.ok_or(())
 	}
 
-	/// Remove expired offers from the cache, returning whether new offers are needed.
+	/// Remove expired offers from the cache, returning the first slot number in the cache that needs
+	/// a new offer, if any exist.
 	pub(super) fn prune_expired_offers(
 		&mut self, duration_since_epoch: Duration, force_reset_request_attempts: bool,
-	) -> bool {
+	) -> Option<u16> {
 		// Remove expired offers from the cache.
 		let mut offer_was_removed = false;
 		for offer_opt in self.offers.iter_mut() {
@@ -268,8 +269,14 @@ impl AsyncReceiveOfferCache {
 			self.reset_offer_paths_request_attempts()
 		}
 
-		self.needs_new_offer_idx(duration_since_epoch).is_some()
-			&& self.offer_paths_request_attempts < MAX_UPDATE_ATTEMPTS
+		if self.offer_paths_request_attempts >= MAX_UPDATE_ATTEMPTS {
+			return None;
+		}
+
+		self.needs_new_offer_idx(duration_since_epoch).and_then(|idx| {
+			debug_assert!(idx < MAX_CACHED_OFFERS_TARGET);
+			idx.try_into().ok()
+		})
 	}
 
 	/// Returns whether the new paths we've just received from the static invoice server should be used

lightning/src/offers/flow.rs

@@ -1266,11 +1266,11 @@ where
 
 		// Update the cache to remove expired offers, and check to see whether we need new offers to be
 		// interactively built with the static invoice server.
-		let needs_new_offers =
-			cache.prune_expired_offers(duration_since_epoch, timer_tick_occurred);
-		if !needs_new_offers {
-			return Ok(());
-		}
+		let needs_new_offer_slot =
+			match cache.prune_expired_offers(duration_since_epoch, timer_tick_occurred) {
+				Some(idx) => idx,
+				None => return Ok(()),
+			};
 
 		// If we need new offers, send out offer paths request messages to the static invoice server.
 		let context = MessageContext::AsyncPayments(AsyncPaymentsContext::OfferPaths {
@@ -1289,7 +1289,9 @@ where
 
 		let mut pending_async_payments_messages =
 			self.pending_async_payments_messages.lock().unwrap();
-		let message = AsyncPaymentsMessage::OfferPathsRequest(OfferPathsRequest {});
+		let message = AsyncPaymentsMessage::OfferPathsRequest(OfferPathsRequest {
+			invoice_slot: needs_new_offer_slot,
+		});
 		enqueue_onion_message_with_reply_paths(
 			message,
 			cache.paths_to_static_invoice_server(),

lightning/src/onion_message/async_payments.rs

@@ -131,7 +131,13 @@ pub enum AsyncPaymentsMessage {
 ///
 /// [`Offer::paths`]: crate::offers::offer::Offer::paths
 #[derive(Clone, Debug)]
-pub struct OfferPathsRequest {}
+pub struct OfferPathsRequest {
+	/// The "slot" in the static invoice server's database that this invoice should go into. This
+	/// allows us as the recipient to replace a specific invoice that is stored by the server, which
+	/// is useful for limiting the number of invoices stored by the server while also keeping all the
+	/// invoices persisted with the server fresh.
+	pub invoice_slot: u16,
+}
 
 /// [`BlindedMessagePath`]s to be included in an async recipient's [`Offer::paths`], sent by a
 /// static invoice server in response to an [`OfferPathsRequest`].
@@ -233,7 +239,9 @@ impl OnionMessageContents for ReleaseHeldHtlc {
 	}
 }
 
-impl_writeable_tlv_based!(OfferPathsRequest, {});
+impl_writeable_tlv_based!(OfferPathsRequest, {
+	(0, invoice_slot, required),
+});
 
 impl_writeable_tlv_based!(OfferPaths, {
 	(0, paths, required_vec),