Forward invoice requests to async recipient

As a static invoice server, if we receive an invoice request on
behalf of an often-offline recipient we will reply to the sender
with the static invoice previously provided by the async recipient.

Here, in addition to doing that we'll forward the invoice request
received to the async recipient to give it a chance to reply with
a fresh invoice in case it is online.

Author: elnosh

lightning/src/events/mod.rs

@@ -18,7 +18,7 @@ pub mod bump_transaction;
 
 pub use bump_transaction::BumpTransactionEvent;
 
-use crate::blinded_path::message::OffersContext;
+use crate::blinded_path::message::{BlindedMessagePath, OffersContext};
 use crate::blinded_path::payment::{
 	Bolt12OfferContext, Bolt12RefundContext, PaymentContext, PaymentContextRef,
 };
@@ -28,6 +28,7 @@ use crate::ln::channelmanager::{InterceptId, PaymentId, RecipientOnionFields};
 use crate::ln::types::ChannelId;
 use crate::ln::{msgs, LocalHTLCFailureReason};
 use crate::offers::invoice::Bolt12Invoice;
+use crate::offers::invoice_request::InvoiceRequest;
 use crate::offers::static_invoice::StaticInvoice;
 use crate::onion_message::messenger::Responder;
 use crate::routing::gossip::NetworkUpdate;
@@ -1654,6 +1655,15 @@ pub enum Event {
 		/// The invoice that should be persisted and later provided to payers when handling a future
 		/// [`Event::StaticInvoiceRequested`].
 		invoice: StaticInvoice,
+		/// The path to where invoice requests will be forwarded. As a static invoice
+		/// server, if we receive an invoice request on behalf of an async recipient, a static
+		/// invoice will be provided to the payer. However, we'll also forward the invoice
+		/// request to this path to the async recipient in case it is online so that the
+		/// recipient can provide a new invoice. This path should be persisted and later
+		/// provided to [`ChannelManager::send_response_static_invoice_request`].
+		///
+		/// [`ChannelManager::send_response_static_invoice_request`]: crate::ln::channelmanager::ChannelManager::send_response_static_invoice_request
+		invoice_request_path: BlindedMessagePath,
 		/// Useful for the recipient to replace a specific invoice stored by us as the static invoice
 		/// server.
 		///
@@ -1686,12 +1696,14 @@ pub enum Event {
 	///
 	/// If we previously persisted a [`StaticInvoice`] from an [`Event::PersistStaticInvoice`] that
 	/// matches the below `recipient_id` and `invoice_slot`, that invoice should be retrieved now
-	/// and forwarded to the payer via [`ChannelManager::send_static_invoice`].
+	/// and forwarded to the payer via [`ChannelManager::send_response_static_invoice_request`].
+	/// The invoice request path previously persisted from [`Event::PersistStaticInvoice`] should
+	/// also be provided in [`ChannelManager::send_response_static_invoice_request`].
 	///
 	/// [`ChannelManager::blinded_paths_for_async_recipient`]: crate::ln::channelmanager::ChannelManager::blinded_paths_for_async_recipient
 	/// [`ChannelManager::set_paths_to_static_invoice_server`]: crate::ln::channelmanager::ChannelManager::set_paths_to_static_invoice_server
 	/// [`InvoiceRequest`]: crate::offers::invoice_request::InvoiceRequest
-	/// [`ChannelManager::send_static_invoice`]: crate::ln::channelmanager::ChannelManager::send_static_invoice
+	/// [`ChannelManager::send_response_static_invoice_request`]: crate::ln::channelmanager::ChannelManager::send_response_static_invoice_request
 	StaticInvoiceRequested {
 		/// An identifier for the recipient previously surfaced in
 		/// [`Event::PersistStaticInvoice::recipient_id`]. Useful when paired with the `invoice_slot` to
@@ -1702,10 +1714,15 @@ pub enum Event {
 		/// retrieve the [`StaticInvoice`] requested by the payer.
 		invoice_slot: u16,
 		/// The path over which the [`StaticInvoice`] will be sent to the payer, which should be
-		/// provided to [`ChannelManager::send_static_invoice`] along with the invoice.
+		/// provided to [`ChannelManager::send_response_static_invoice_request`] along with the invoice.
 		///
-		/// [`ChannelManager::send_static_invoice`]: crate::ln::channelmanager::ChannelManager::send_static_invoice
+		/// [`ChannelManager::send_response_static_invoice_request`]: crate::ln::channelmanager::ChannelManager::send_response_static_invoice_request
 		reply_path: Responder,
+		/// The invoice request that will be forwarded to the async recipient to give it a
+		/// chance to provide an invoice in case it is online. It should be provided to [`ChannelManager::send_response_static_invoice_request`].
+		///
+		/// [`ChannelManager::send_response_static_invoice_request`]: crate::ln::channelmanager::ChannelManager::send_response_static_invoice_request
+		invoice_request: InvoiceRequest,
 	},
 	/// Indicates that a channel funding transaction constructed interactively is ready to be
 	/// signed. This event will only be triggered if at least one input was contributed.