Include Refund context in blinded payment paths

When requesting a payment for a refund, include a context in the
Bolt12Invoice's blinded payment paths indicated it is for such. When the
eventual payment is received, the user can use the payment hash to
correlate it with the corresponding Refund.

Author: jkczyz

lightning/src/blinded_path/payment.rs

@@ -114,6 +114,11 @@ pub enum PaymentContext {
 	///
 	/// [`Offer`]: crate::offers::offer::Offer
 	Bolt12Offer(Bolt12OfferContext),
+
+	/// The payment was made for an invoice sent for a BOLT 12 [`Refund`].
+	///
+	/// [`Refund`]: crate::offers::refund::Refund
+	Bolt12Refund(Bolt12RefundContext),
 }
 
 /// An unknown payment context.
@@ -131,6 +136,12 @@ pub struct Bolt12OfferContext {
 	pub offer_id: OfferId,
 }
 
+/// The context of a payment made for an invoice sent for a BOLT 12 [`Refund`].
+///
+/// [`Refund`]: crate::offers::refund::Refund
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct Bolt12RefundContext {}
+
 impl PaymentContext {
 	pub(crate) fn unknown() -> Self {
 		PaymentContext::Unknown(UnknownPaymentContext(()))
@@ -358,6 +369,7 @@ impl_writeable_tlv_based_enum!(PaymentContext,
 	;
 	(0, Unknown),
 	(1, Bolt12Offer),
+	(2, Bolt12Refund),
 );
 
 impl Writeable for UnknownPaymentContext {
@@ -376,6 +388,8 @@ impl_writeable_tlv_based!(Bolt12OfferContext, {
 	(0, offer_id, required),
 });
 
+impl_writeable_tlv_based!(Bolt12RefundContext, {});
+
 #[cfg(test)]
 mod tests {
 	use bitcoin::secp256k1::PublicKey;

lightning/src/ln/channelmanager.rs

@@ -32,7 +32,7 @@ use bitcoin::secp256k1::Secp256k1;
 use bitcoin::{secp256k1, Sequence};
 
 use crate::blinded_path::{BlindedPath, NodeIdLookUp};
-use crate::blinded_path::payment::{Bolt12OfferContext, PaymentConstraints, PaymentContext, ReceiveTlvs};
+use crate::blinded_path::payment::{Bolt12OfferContext, Bolt12RefundContext, PaymentConstraints, PaymentContext, ReceiveTlvs};
 use crate::chain;
 use crate::chain::{Confirm, ChannelMonitorUpdateStatus, Watch, BestBlock};
 use crate::chain::chaininterface::{BroadcasterInterface, ConfirmationTarget, FeeEstimator, LowerBoundedFeeEstimator};
@@ -8826,7 +8826,7 @@ where
 
 		match self.create_inbound_payment(Some(amount_msats), relative_expiry, None) {
 			Ok((payment_hash, payment_secret)) => {
-				let payment_context = PaymentContext::unknown();
+				let payment_context = PaymentContext::Bolt12Refund(Bolt12RefundContext {});
 				let payment_paths = self.create_blinded_payment_paths(
 					amount_msats, payment_secret, payment_context
 				)