Add static invoice server messages and boilerplate

Because async recipients are not online to respond to invoice requests,
the plan is for another node on the network that is always-online to serve
static invoices on their behalf.

The protocol is as follows:
- Recipient is configured with blinded message paths to reach the static invoice
  server
- On startup, recipient requests blinded message paths for inclusion in their
  offer from the static invoice server over the configured paths
- Server replies with offer paths for the recipient
- Recipient builds their offer using these paths and the corresponding static
  invoice and replies with the invoice
- Server persists the invoice and confirms that they've persisted it, causing
  the recipient to cache the interactively built offer for use

At pay-time, the payer sends an invoice request to the static invoice server,
who replies with the static invoice after forwarding the invreq to the
recipient (to give them a chance to provide a fresh invoice in case they're
online).

Here we add the requisite trait methods and onion messages to support this
protocol.

An alterate design could be for the async recipient to publish static invoices
directly without a preceding offer, e.g. on their website. Some drawbacks of
this design include:
1) No fallback to regular BOLT 12 in the case that the recipient happens to be online
at pay-time. Falling back to regular BOLT 12 allows the recipient to provide a fresh
invoice and regain the proof-of-payment property
2) Static invoices don't fit in a QR code
3) No automatic rotation of the static invoice, which is useful in the case that payment
paths become outdated due to changing fees, etc

Author: valentinewallace

fuzz/src/onion_message.rs

@@ -15,7 +15,8 @@ use lightning::ln::peer_handler::IgnoringMessageHandler;
 use lightning::ln::script::ShutdownScript;
 use lightning::offers::invoice::UnsignedBolt12Invoice;
 use lightning::onion_message::async_payments::{
-	AsyncPaymentsMessageHandler, HeldHtlcAvailable, ReleaseHeldHtlc,
+	AsyncPaymentsMessageHandler, HeldHtlcAvailable, OfferPaths, OfferPathsRequest, ReleaseHeldHtlc,
+	ServeStaticInvoice, StaticInvoicePersisted,
 };
 use lightning::onion_message::messenger::{
 	CustomOnionMessageHandler, Destination, MessageRouter, MessageSendInstructions,
@@ -124,6 +125,30 @@ impl OffersMessageHandler for TestOffersMessageHandler {
 struct TestAsyncPaymentsMessageHandler {}
 
 impl AsyncPaymentsMessageHandler for TestAsyncPaymentsMessageHandler {
+	fn handle_offer_paths_request(
+		&self, _message: OfferPathsRequest, _context: AsyncPaymentsContext,
+		responder: Option<Responder>,
+	) -> Option<(OfferPaths, ResponseInstruction)> {
+		let responder = match responder {
+			Some(resp) => resp,
+			None => return None,
+		};
+		Some((OfferPaths { paths: Vec::new(), paths_absolute_expiry: None }, responder.respond()))
+	}
+	fn handle_offer_paths(
+		&self, _message: OfferPaths, _context: AsyncPaymentsContext, _responder: Option<Responder>,
+	) -> Option<(ServeStaticInvoice, ResponseInstruction)> {
+		None
+	}
+	fn handle_serve_static_invoice(
+		&self, _message: ServeStaticInvoice, _context: AsyncPaymentsContext,
+		_responder: Option<Responder>,
+	) {
+	}
+	fn handle_static_invoice_persisted(
+		&self, _message: StaticInvoicePersisted, _context: AsyncPaymentsContext,
+	) {
+	}
 	fn handle_held_htlc_available(
 		&self, _message: HeldHtlcAvailable, _context: AsyncPaymentsContext,
 		responder: Option<Responder>,

lightning/src/ln/channelmanager.rs

@@ -98,7 +98,8 @@ use crate::offers::parse::Bolt12SemanticError;
 use crate::offers::refund::Refund;
 use crate::offers::signer;
 use crate::onion_message::async_payments::{
-	AsyncPaymentsMessage, AsyncPaymentsMessageHandler, HeldHtlcAvailable, ReleaseHeldHtlc,
+	AsyncPaymentsMessage, AsyncPaymentsMessageHandler, HeldHtlcAvailable, OfferPaths,
+	OfferPathsRequest, ReleaseHeldHtlc, ServeStaticInvoice, StaticInvoicePersisted,
 };
 use crate::onion_message::dns_resolution::HumanReadableName;
 use crate::onion_message::messenger::{
@@ -13374,6 +13375,30 @@ where
 	MR::Target: MessageRouter,
 	L::Target: Logger,
 {
+	fn handle_offer_paths_request(
+		&self, _message: OfferPathsRequest, _context: AsyncPaymentsContext,
+		_responder: Option<Responder>,
+	) -> Option<(OfferPaths, ResponseInstruction)> {
+		None
+	}
+
+	fn handle_offer_paths(
+		&self, _message: OfferPaths, _context: AsyncPaymentsContext, _responder: Option<Responder>,
+	) -> Option<(ServeStaticInvoice, ResponseInstruction)> {
+		None
+	}
+
+	fn handle_serve_static_invoice(
+		&self, _message: ServeStaticInvoice, _context: AsyncPaymentsContext,
+		_responder: Option<Responder>,
+	) {
+	}
+
+	fn handle_static_invoice_persisted(
+		&self, _message: StaticInvoicePersisted, _context: AsyncPaymentsContext,
+	) {
+	}
+
 	fn handle_held_htlc_available(
 		&self, _message: HeldHtlcAvailable, _context: AsyncPaymentsContext,
 		_responder: Option<Responder>,

lightning/src/ln/peer_handler.rs

@@ -31,7 +31,8 @@ use crate::ln::types::ChannelId;
 use crate::ln::wire;
 use crate::ln::wire::{Encode, Type};
 use crate::onion_message::async_payments::{
-	AsyncPaymentsMessageHandler, HeldHtlcAvailable, ReleaseHeldHtlc,
+	AsyncPaymentsMessageHandler, HeldHtlcAvailable, OfferPaths, OfferPathsRequest, ReleaseHeldHtlc,
+	ServeStaticInvoice, StaticInvoicePersisted,
 };
 use crate::onion_message::dns_resolution::{
 	DNSResolverMessage, DNSResolverMessageHandler, DNSSECProof, DNSSECQuery,
@@ -212,6 +213,26 @@ impl OffersMessageHandler for IgnoringMessageHandler {
 	}
 }
 impl AsyncPaymentsMessageHandler for IgnoringMessageHandler {
+	fn handle_offer_paths_request(
+		&self, _message: OfferPathsRequest, _context: AsyncPaymentsContext,
+		_responder: Option<Responder>,
+	) -> Option<(OfferPaths, ResponseInstruction)> {
+		None
+	}
+	fn handle_offer_paths(
+		&self, _message: OfferPaths, _context: AsyncPaymentsContext, _responder: Option<Responder>,
+	) -> Option<(ServeStaticInvoice, ResponseInstruction)> {
+		None
+	}
+	fn handle_serve_static_invoice(
+		&self, _message: ServeStaticInvoice, _context: AsyncPaymentsContext,
+		_responder: Option<Responder>,
+	) {
+	}
+	fn handle_static_invoice_persisted(
+		&self, _message: StaticInvoicePersisted, _context: AsyncPaymentsContext,
+	) {
+	}
 	fn handle_held_htlc_available(
 		&self, _message: HeldHtlcAvailable, _context: AsyncPaymentsContext,
 		_responder: Option<Responder>,