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.

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,
@@ -121,6 +122,29 @@ 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() }, 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,
+	) {
+	}
+	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

@@ -74,7 +74,10 @@ use crate::offers::offer::{Offer, OfferBuilder};
 use crate::offers::parse::Bolt12SemanticError;
 use crate::offers::refund::{Refund, RefundBuilder};
 use crate::offers::signer;
-use crate::onion_message::async_payments::{AsyncPaymentsMessage, HeldHtlcAvailable, ReleaseHeldHtlc, AsyncPaymentsMessageHandler};
+use crate::onion_message::async_payments::{
+	AsyncPaymentsMessage, AsyncPaymentsMessageHandler, HeldHtlcAvailable, OfferPaths,
+	OfferPathsRequest, ReleaseHeldHtlc, ServeStaticInvoice, StaticInvoicePersisted
+};
 use crate::onion_message::dns_resolution::HumanReadableName;
 use crate::onion_message::messenger::{Destination, MessageRouter, Responder, ResponseInstruction, MessageSendInstructions};
 use crate::onion_message::offers::{OffersMessage, OffersMessageHandler};
@@ -12559,6 +12562,27 @@ 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,
+	) {}
+
+	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

@@ -29,7 +29,7 @@ use crate::util::ser::{VecWriter, Writeable, Writer};
 use crate::ln::peer_channel_encryptor::{PeerChannelEncryptor, NextNoiseStep, MessageBuf, MSG_BUF_ALLOC_SIZE};
 use crate::ln::wire;
 use crate::ln::wire::{Encode, Type};
-use crate::onion_message::async_payments::{AsyncPaymentsMessageHandler, HeldHtlcAvailable, ReleaseHeldHtlc};
+use crate::onion_message::async_payments::{AsyncPaymentsMessageHandler, HeldHtlcAvailable, OfferPaths, OfferPathsRequest, ServeStaticInvoice, ReleaseHeldHtlc, StaticInvoicePersisted};
 use crate::onion_message::dns_resolution::{DNSResolverMessageHandler, DNSResolverMessage, DNSSECProof, DNSSECQuery};
 use crate::onion_message::messenger::{CustomOnionMessageHandler, Responder, ResponseInstruction, MessageSendInstructions};
 use crate::onion_message::offers::{OffersMessage, OffersMessageHandler};
@@ -151,6 +151,22 @@ 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
+	) {}
+	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/offers/static_invoice.rs

@@ -33,7 +33,7 @@ use crate::offers::offer::{
 };
 use crate::offers::parse::{Bolt12ParseError, Bolt12SemanticError, ParsedMessage};
 use crate::types::features::{Bolt12InvoiceFeatures, OfferFeatures};
-use crate::util::ser::{CursorReadable, Iterable, WithoutLength, Writeable, Writer};
+use crate::util::ser::{CursorReadable, Iterable, Readable, WithoutLength, Writeable, Writer};
 use crate::util::string::PrintableString;
 use bitcoin::address::Address;
 use bitcoin::constants::ChainHash;
@@ -529,6 +529,13 @@ impl InvoiceContents {
 	}
 }
 
+impl Readable for StaticInvoice {
+	fn read<R: io::Read>(reader: &mut R) -> Result<Self, DecodeError> {
+		let bytes: WithoutLength<Vec<u8>> = Readable::read(reader)?;
+		Self::try_from(bytes.0).map_err(|_| DecodeError::InvalidValue)
+	}
+}
+
 impl Writeable for StaticInvoice {
 	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), io::Error> {
 		WithoutLength(&self.bytes).write(writer)