Cache offer on StaticInvoicePersisted onion message

As an async recipient, we need to interactively build a static invoice that an
always-online node will serve on our behalf.

Once this invoice is built and persisted by the static invoice server, they
will send us a confirmation onion message. At this time, cache the
corresponding offer and mark it as ready to receive async payments.

Author: valentinewallace

lightning/src/ln/channelmanager.rs

@@ -12384,7 +12384,17 @@ where
 
 	fn handle_static_invoice_persisted(
 		&self, _message: StaticInvoicePersisted, _context: AsyncPaymentsContext,
-	) {}
+	) {
+		#[cfg(async_payments)] {
+			let should_persist = self.flow.handle_static_invoice_persisted(_context);
+			let _persistence_guard = PersistenceNotifierGuard::optionally_notify(self, || {
+				match should_persist {
+					true => NotifyOption::DoPersist,
+					false => NotifyOption::SkipPersistNoEvents,
+				}
+			});
+		}
+	}
 
 	fn handle_held_htlc_available(
 		&self, _message: HeldHtlcAvailable, _context: AsyncPaymentsContext,

lightning/src/offers/async_receive_offer_cache.rs

@@ -16,11 +16,14 @@ use crate::io::Read;
 use crate::ln::msgs::DecodeError;
 use crate::offers::nonce::Nonce;
 use crate::offers::offer::Offer;
-#[cfg(async_payments)]
-use crate::onion_message::async_payments::OfferPaths;
 use crate::onion_message::messenger::Responder;
 use crate::util::ser::{Readable, Writeable, Writer};
 use core::time::Duration;
+#[cfg(async_payments)]
+use {
+	crate::blinded_path::message::AsyncPaymentsContext,
+	crate::onion_message::async_payments::OfferPaths,
+};
 
 struct AsyncReceiveOffer {
 	offer: Offer,
@@ -183,6 +186,112 @@ impl AsyncReceiveOfferCache {
 		self.offer_paths_request_attempts = 0;
 		self.last_offer_paths_request_timestamp = Duration::from_secs(0);
 	}
+
+	/// Should be called when we receive a [`StaticInvoicePersisted`] message from the static invoice
+	/// server, which indicates that a new offer was persisted by the server and they are ready to
+	/// serve the corresponding static invoice to payers on our behalf.
+	///
+	/// Returns a bool indicating whether an offer was added/updated and re-persistence of the cache
+	/// is needed.
+	pub(super) fn static_invoice_persisted(
+		&mut self, context: AsyncPaymentsContext, duration_since_epoch: Duration,
+	) -> bool {
+		let (
+			candidate_offer,
+			candidate_offer_nonce,
+			offer_created_at,
+			update_static_invoice_path,
+			static_invoice_absolute_expiry,
+		) = match context {
+			AsyncPaymentsContext::StaticInvoicePersisted {
+				offer,
+				offer_nonce,
+				offer_created_at,
+				update_static_invoice_path,
+				static_invoice_absolute_expiry,
+				..
+			} => (
+				offer,
+				offer_nonce,
+				offer_created_at,
+				update_static_invoice_path,
+				static_invoice_absolute_expiry,
+			),
+			_ => return false,
+		};
+
+		if candidate_offer.is_expired_no_std(duration_since_epoch) {
+			return false;
+		}
+		if static_invoice_absolute_expiry < duration_since_epoch {
+			return false;
+		}
+
+		// If the candidate offer is known, either this is a duplicate message or we updated the
+		// corresponding static invoice that is stored with the server.
+		if let Some(existing_offer) =
+			self.offers.iter_mut().find(|cached_offer| cached_offer.offer == candidate_offer)
+		{
+			// The blinded path used to update the static invoice corresponding to an offer should never
+			// change because we reuse the same path every time we update.
+			debug_assert_eq!(existing_offer.update_static_invoice_path, update_static_invoice_path);
+			debug_assert_eq!(existing_offer.offer_nonce, candidate_offer_nonce);
+
+			let needs_persist =
+				existing_offer.static_invoice_absolute_expiry != static_invoice_absolute_expiry;
+
+			// Since this is the most recent update we've received from the static invoice server, assume
+			// that the invoice that was just persisted is the only invoice that the server has stored
+			// corresponding to this offer.
+			existing_offer.static_invoice_absolute_expiry = static_invoice_absolute_expiry;
+			existing_offer.invoice_update_attempts = 0;
+
+			return needs_persist;
+		}
+
+		let candidate_offer = AsyncReceiveOffer {
+			offer: candidate_offer,
+			offer_nonce: candidate_offer_nonce,
+			offer_created_at,
+			update_static_invoice_path,
+			static_invoice_absolute_expiry,
+			invoice_update_attempts: 0,
+		};
+
+		// An offer with 2 2-hop blinded paths has ~700 bytes, so this cache limit would allow up to
+		// ~100 offers of that size.
+		const MAX_CACHE_SIZE: usize = (1 << 10) * 70; // 70KiB
+		const MAX_OFFERS: usize = 100;
+		// If we have room in the cache, go ahead and add this new offer so we have more options. We
+		// should generally never get close to the cache limit because we limit the number of requests
+		// for offer persistence that are sent to begin with.
+		if self.offers.len() < MAX_OFFERS && self.serialized_length() < MAX_CACHE_SIZE {
+			self.offers.push(candidate_offer);
+			return true;
+		}
+
+		// Swap out our lowest expiring offer for this candidate offer if needed. Otherwise we'd be
+		// risking a situation where all of our existing offers expire soon but we still ignore this one
+		// even though it's fresh.
+		const NEVER_EXPIRES: Duration = Duration::from_secs(u64::MAX);
+		let (soonest_expiring_offer_idx, soonest_offer_expiry) = self
+			.offers
+			.iter()
+			.map(|offer| offer.offer.absolute_expiry().unwrap_or(NEVER_EXPIRES))
+			.enumerate()
+			.min_by(|(_, offer_exp_a), (_, offer_exp_b)| offer_exp_a.cmp(offer_exp_b))
+			.unwrap_or_else(|| {
+				debug_assert!(false);
+				(0, NEVER_EXPIRES)
+			});
+
+		if soonest_offer_expiry < candidate_offer.offer.absolute_expiry().unwrap_or(NEVER_EXPIRES) {
+			self.offers[soonest_expiring_offer_idx] = candidate_offer;
+			return true;
+		}
+
+		false
+	}
 }
 
 impl Writeable for AsyncReceiveOfferCache {

lightning/src/offers/flow.rs

@@ -1257,4 +1257,37 @@ where
 
 		Ok((ServeStaticInvoice { invoice: static_invoice }, reply_path_context))
 	}
+
+	/// Handles an incoming [`StaticInvoicePersisted`] onion message from the static invoice server.
+	/// Returns a bool indicating whether the async receive offer cache needs to be re-persisted.
+	///
+	/// [`StaticInvoicePersisted`]: crate::onion_message::async_payments::StaticInvoicePersisted
+	#[cfg(async_payments)]
+	pub(crate) fn handle_static_invoice_persisted(&self, context: AsyncPaymentsContext) -> bool {
+		let expanded_key = &self.inbound_payment_key;
+		let duration_since_epoch = self.duration_since_epoch();
+
+		if let AsyncPaymentsContext::StaticInvoicePersisted {
+			nonce,
+			hmac,
+			path_absolute_expiry,
+			..
+		} = context
+		{
+			if let Err(()) =
+				signer::verify_static_invoice_persisted_context(nonce, hmac, expanded_key)
+			{
+				return false;
+			}
+
+			if duration_since_epoch > path_absolute_expiry {
+				return false;
+			}
+		} else {
+			return false;
+		}
+
+		let mut cache = self.async_receive_offer_cache.lock().unwrap();
+		cache.static_invoice_persisted(context, duration_since_epoch)
+	}
 }

lightning/src/offers/signer.rs

@@ -602,3 +602,14 @@ pub(crate) fn hmac_for_static_invoice_persisted_context(
 
 	Hmac::from_engine(hmac)
 }
+
+#[cfg(async_payments)]
+pub(crate) fn verify_static_invoice_persisted_context(
+	nonce: Nonce, hmac: Hmac<Sha256>, expanded_key: &ExpandedKey,
+) -> Result<(), ()> {
+	if hmac_for_static_invoice_persisted_context(nonce, expanded_key) == hmac {
+		Ok(())
+	} else {
+		Err(())
+	}
+}