Add a payment_metadata field to RecipientOnionFields

This adds the new `payment_metadata` to `RecipientOnionFields`,
passing the metadata from BOLT11 invoices through the send pipeline
and finally copying them info the onion when sending HTLCs.

This completes send-side support for the new payment metadata
feature.

lightning-invoice/src/payment.rs

@@ -145,8 +145,10 @@ fn pay_invoice_using_amount<P: Deref>(
 	payer: P
 ) -> Result<(), PaymentError> where P::Target: Payer {
 	let payment_hash = PaymentHash((*invoice.payment_hash()).into_inner());
-	let payment_secret = Some(*invoice.payment_secret());
-	let recipient_onion = RecipientOnionFields { payment_secret };
+	let recipient_onion = RecipientOnionFields {
+		payment_secret: Some(*invoice.payment_secret()),
+		payment_metadata: invoice.payment_metadata().map(|v| v.clone()),
+	};
 	let mut payment_params = PaymentParameters::from_node_id(invoice.recover_payee_pub_key(),
 		invoice.min_final_cltv_expiry_delta() as u32)
 		.with_expiry_time(expiry_time_from_unix_epoch(invoice).as_secs())

lightning/src/ln/channelmanager.rs

@@ -7603,6 +7603,7 @@ where
 										session_privs: [session_priv_bytes].iter().map(|a| *a).collect(),
 										payment_hash: htlc.payment_hash,
 										payment_secret: None, // only used for retries, and we'll never retry on startup
+										payment_metadata: None, // only used for retries, and we'll never retry on startup
 										keysend_preimage: None, // only used for retries, and we'll never retry on startup
 										pending_amt_msat: path_amt,
 										pending_fee_msat: Some(path_fee),

lightning/src/ln/onion_utils.rs

@@ -170,7 +170,7 @@ pub(super) fn build_onion_payloads(path: &Vec<RouteHop>, total_msat: u64, mut re
 							total_msat,
 						})
 					} else { None },
-					payment_metadata: None,
+					payment_metadata: recipient_onion.payment_metadata.take(),
 					keysend_preimage: *keysend_preimage,
 				}
 			} else {

lightning/src/ln/outbound_payment.rs

@@ -46,6 +46,7 @@ pub(crate) enum PendingOutboundPayment {
 		session_privs: HashSet<[u8; 32]>,
 		payment_hash: PaymentHash,
 		payment_secret: Option<PaymentSecret>,
+		payment_metadata: Option<Vec<u8>>,
 		keysend_preimage: Option<PaymentPreimage>,
 		pending_amt_msat: u64,
 		/// Used to track the fee paid. Only present if the payment was serialized on 0.0.103+.
@@ -422,6 +423,19 @@ pub struct RecipientOnionFields {
 	/// receives, thus you should generally never be providing a secret here for spontaneous
 	/// payments.
 	pub payment_secret: Option<PaymentSecret>,
+	/// The payment metadata serves a similar purpose as [`Self::payment_secret`] but is of
+	/// arbitrary length. This gives recipients substantially more flexibility to receive
+	/// additional data.
+	///
+	/// In LDK, while the [`Self::payment_secret`] is fixed based on an internal authentication
+	/// scheme to authenticate received payments against expected payments and invoices, this field
+	/// is not used in LDK for received payments, and can be used to store arbitrary data in
+	/// invoices which will be received with the payment.
+	///
+	/// Note that this field was added to the lightning specification more recently than
+	/// [`Self::payment_secret`] and while nearly all lightning senders support secrets, metadata
+	/// may not be supported as universally.
+	pub payment_metadata: Option<Vec<u8>>,
 }
 
 impl RecipientOnionFields {
@@ -429,7 +443,7 @@ impl RecipientOnionFields {
 	/// set of onion fields for today's BOLT11 invoices - most nodes require a [`PaymentSecret`]
 	/// but do not require or provide any further data.
 	pub fn secret_only(payment_secret: PaymentSecret) -> Self {
-		Self { payment_secret: Some(payment_secret) }
+		Self { payment_secret: Some(payment_secret), payment_metadata: None }
 	}
 
 	/// Creates a new [`RecipientOnionFields`] with no fields. This generally does not create
@@ -438,7 +452,7 @@ impl RecipientOnionFields {
 	///
 	/// [`ChannelManager::send_spontaneous_payment`]: super::channelmanager::ChannelManager::send_spontaneous_payment
 	pub fn spontaneous_empty() -> Self {
-		Self { payment_secret: None }
+		Self { payment_secret: None, payment_metadata: None }
 	}
 }
 
@@ -719,7 +733,7 @@ impl OutboundPayments {
 				hash_map::Entry::Occupied(mut payment) => {
 					let res = match payment.get() {
 						PendingOutboundPayment::Retryable {
-							total_msat, keysend_preimage, payment_secret, pending_amt_msat, ..
+							total_msat, keysend_preimage, payment_secret, payment_metadata, pending_amt_msat, ..
 						} => {
 							let retry_amt_msat: u64 = route.paths.iter().map(|path| path.last().unwrap().fee_msat).sum();
 							if retry_amt_msat + *pending_amt_msat > *total_msat * (100 + RETRY_OVERFLOW_PERCENTAGE) / 100 {
@@ -729,6 +743,7 @@ impl OutboundPayments {
 							}
 							(*total_msat, RecipientOnionFields {
 									payment_secret: *payment_secret,
+									payment_metadata: payment_metadata.clone(),
 								}, *keysend_preimage)
 						},
 						PendingOutboundPayment::Legacy { .. } => {
@@ -912,6 +927,7 @@ impl OutboundPayments {
 					pending_fee_msat: Some(0),
 					payment_hash,
 					payment_secret: recipient_onion.payment_secret,
+					payment_metadata: recipient_onion.payment_metadata,
 					keysend_preimage,
 					starting_block_height: best_block_height,
 					total_msat: route.get_total_amount(),
@@ -1345,6 +1361,7 @@ impl_writeable_tlv_based_enum_upgradable!(PendingOutboundPayment,
 		(4, payment_secret, option),
 		(5, keysend_preimage, option),
 		(6, total_msat, required),
+		(7, payment_metadata, option),
 		(8, pending_amt_msat, required),
 		(10, starting_block_height, required),
 		(not_written, retry_strategy, (static_value, None)),