Add API to retrieve cached async receive offers

Over the past several commits we've implemented interactively building an async
receive offer with a static invoice server that will service invoice requests
on our behalf as an async recipient.

Here we add an API to retrieve the resulting offers so we can receive payments
when we're offline.

Author: valentinewallace

lightning/src/ln/channelmanager.rs

@@ -10356,9 +10356,23 @@ where
 	#[cfg(c_bindings)]
 	create_refund_builder!(self, RefundMaybeWithDerivedMetadataBuilder);
 
+	/// Retrieve our cached [`Offer`]s for receiving async payments as an often-offline recipient.
+	/// Will only be set if [`UserConfig::paths_to_static_invoice_server`] is set and we succeeded in
+	/// interactively building a [`StaticInvoice`] with the static invoice server.
+	///
+	/// Useful for posting offers to receive payments later, such as posting an offer on a website.
+	#[cfg(async_payments)]
+	pub fn get_cached_async_receive_offers(&self) -> Vec<Offer> {
+		self.flow.get_cached_async_receive_offers()
+	}
+
 	/// Create an offer for receiving async payments as an often-offline recipient.
 	///
-	/// Because we may be offline when the payer attempts to request an invoice, you MUST:
+	/// Instead of using this method, it is preferable to set
+	/// [`UserConfig::paths_to_static_invoice_server`] and retrieve the automatically built offer via
+	/// [`Self::get_cached_async_receive_offers`].
+	///
+	/// If you want to build the [`StaticInvoice`] manually using this method instead, you MUST:
 	/// 1. Provide at least 1 [`BlindedMessagePath`] terminating at an always-online node that will
 	///    serve the [`StaticInvoice`] created from this offer on our behalf.
 	/// 2. Use [`Self::create_static_invoice_builder`] to create a [`StaticInvoice`] from this
@@ -10374,6 +10388,10 @@ where
 	/// Creates a [`StaticInvoiceBuilder`] from the corresponding [`Offer`] and [`Nonce`] that were
 	/// created via [`Self::create_async_receive_offer_builder`]. If `relative_expiry` is unset, the
 	/// invoice's expiry will default to [`STATIC_INVOICE_DEFAULT_RELATIVE_EXPIRY`].
+	///
+	/// Instead of using this method to manually build the invoice, it is preferable to set
+	/// [`UserConfig::paths_to_static_invoice_server`] and retrieve the automatically built offer via
+	/// [`Self::get_cached_async_receive_offers`].
 	#[cfg(async_payments)]
 	pub fn create_static_invoice_builder<'a>(
 		&self, offer: &'a Offer, offer_nonce: Nonce, relative_expiry: Option<Duration>

lightning/src/offers/async_receive_offer_cache.rs

@@ -93,6 +93,26 @@ impl AsyncReceiveOfferCache {
 	// invoice before giving up.
 	const MAX_UPDATE_ATTEMPTS: u8 = 3;
 
+	/// Retrieve our cached [`Offer`]s for receiving async payments as an often-offline recipient.
+	pub fn offers(&self, duration_since_epoch: Duration) -> Vec<Offer> {
+		const NEVER_EXPIRES: Duration = Duration::from_secs(u64::MAX);
+
+		self.offers
+			.iter()
+			.filter_map(|offer| {
+				if offer.static_invoice_absolute_expiry < duration_since_epoch {
+					None
+				} else if offer.offer.absolute_expiry().unwrap_or(NEVER_EXPIRES)
+					< duration_since_epoch
+				{
+					None
+				} else {
+					Some(offer.offer.clone())
+				}
+			})
+			.collect()
+	}
+
 	/// Remove expired offers from the cache.
 	pub(super) fn prune_expired_offers(&mut self, duration_since_epoch: Duration) {
 		// Remove expired offers from the cache.

lightning/src/offers/flow.rs

@@ -1115,6 +1115,14 @@ where
 		core::mem::take(&mut self.pending_dns_onion_messages.lock().unwrap())
 	}
 
+	/// Retrieve our cached [`Offer`]s for receiving async payments as an often-offline recipient.
+	/// Will only be set if [`UserConfig::paths_to_static_invoice_server`] is set and we succeeded in
+	/// interactively building a [`StaticInvoice`] with the static invoice server.
+	#[cfg(async_payments)]
+	pub(crate) fn get_cached_async_receive_offers(&self) -> Vec<Offer> {
+		self.async_receive_offer_cache.lock().unwrap().offers(self.duration_since_epoch())
+	}
+
 	/// Sends out [`OfferPathsRequest`] and [`ServeStaticInvoice`] onion messages if we are an
 	/// often-offline recipient and are configured to interactively build offers and static invoices
 	/// with a static invoice server.