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

@@ -12759,7 +12759,35 @@ where
 
 	fn handle_static_invoice_persisted(
 		&self, _message: StaticInvoicePersisted, _context: AsyncPaymentsContext,
-	) {}
+	) {
+		#[cfg(async_payments)] {
+			let expanded_key = &self.inbound_payment_key;
+			let duration_since_epoch = self.duration_since_epoch();
+
+			let mut new_offer = match _context {
+				AsyncPaymentsContext::StaticInvoicePersisted {
+					offer, nonce, hmac, path_absolute_expiry
+				} => {
+					if let Err(()) = signer::verify_static_invoice_persisted_context(nonce, hmac, expanded_key) {
+						return
+					}
+					if duration_since_epoch > path_absolute_expiry { return }
+					if offer.is_expired_no_std(duration_since_epoch) { return }
+					Some(offer)
+				},
+				_ => return
+			};
+
+			PersistenceNotifierGuard::optionally_notify(self, || {
+				let mut offer_cache = self.async_receive_offer_cache.lock().unwrap();
+				if !offer_cache.should_refresh_offer(duration_since_epoch) {
+					return NotifyOption::SkipPersistNoEvents
+				}
+				offer_cache.offer = new_offer.take();
+				NotifyOption::DoPersist
+			});
+		}
+	}
 
 	fn handle_held_htlc_available(
 		&self, _message: HeldHtlcAvailable, _context: AsyncPaymentsContext,

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(())
+	}
+}