Allow for variable amount payments

bindings/ldk_node.udl

@@ -87,6 +87,8 @@ interface LDKNode {
 	Bolt11Invoice receive_variable_amount_payment([ByRef]string description, u32 expiry_secs);
 	[Throws=NodeError]
 	Bolt11Invoice receive_payment_via_jit_channel(u64 amount_msat, [ByRef]string description, u32 expiry_secs, u64? max_lsp_fee_limit_msat);
+	[Throws=NodeError]
+	Bolt11Invoice receive_variable_amount_payment_via_jit_channel([ByRef]string description, u32 expiry_secs, u64? max_proportional_lsp_fee_limit_ppm_msat);
 	PaymentDetails? payment([ByRef]PaymentHash payment_hash);
 	[Throws=NodeError]
 	void remove_payment([ByRef]PaymentHash payment_hash);
@@ -174,6 +176,7 @@ enum PaymentStatus {
 
 dictionary LSPFeeLimits {
 	u64? max_total_opening_fee_msat;
+	u64? max_proportional_opening_fee_ppm_msat;
 };
 
 dictionary PaymentDetails {

src/event.rs

@@ -26,6 +26,7 @@ use lightning::util::ser::{Readable, ReadableArgs, Writeable, Writer};
 use bitcoin::blockdata::locktime::absolute::LockTime;
 use bitcoin::secp256k1::PublicKey;
 use bitcoin::OutPoint;
+use lightning_liquidity::lsps2::utils::compute_opening_fee;
 use rand::{thread_rng, Rng};
 use std::collections::VecDeque;
 use std::ops::Deref;
@@ -381,8 +382,18 @@ where
 						return;
 					}
 
-					let max_total_opening_fee_msat =
-						info.lsp_fee_limits.and_then(|l| l.max_total_opening_fee_msat).unwrap_or(0);
+					let max_total_opening_fee_msat = info
+						.lsp_fee_limits
+						.and_then(|l| {
+							l.max_total_opening_fee_msat.or_else(|| {
+								l.max_proportional_opening_fee_ppm_msat.and_then(|max_prop_fee| {
+									// If it's a variable amount payment, compute the actual fee.
+									compute_opening_fee(amount_msat, 0, max_prop_fee)
+								})
+							})
+						})
+						.unwrap_or(0);
+
 					if counterparty_skimmed_fee_msat > max_total_opening_fee_msat {
 						log_info!(
 							self.logger,

src/lib.rs

@@ -1614,12 +1614,38 @@ impl<K: KVStore + Sync + Send + 'static> Node<K> {
 			description,
 			expiry_secs,
 			max_total_lsp_fee_limit_msat,
+			None,
+		)
+	}
+
+	/// Returns a payable invoice that can be used to request a variable amount payment (also known
+	/// as "zero-amount" invoice) and receive it via a newly created just-in-time (JIT) channel.
+	///
+	/// When the returned invoice is paid, the configured [LSPS2]-compliant LSP will open a channel
+	/// to us, supplying just-in-time inbound liquidity.
+	///
+	/// If set, `max_proportional_lsp_fee_limit_ppm_msat` will limit how much proportional fee, in
+	/// parts-per-million millisatoshis, we allow the LSP to take for opening the channel to us.
+	/// We'll use its cheapest offer otherwise.
+	///
+	/// [LSPS2]: https://github.com/BitcoinAndLightningLayerSpecs/lsp/blob/main/LSPS2/README.md
+	pub fn receive_variable_amount_payment_via_jit_channel(
+		&self, description: &str, expiry_secs: u32,
+		max_proportional_lsp_fee_limit_ppm_msat: Option<u64>,
+	) -> Result<Bolt11Invoice, Error> {
+		self.receive_payment_via_jit_channel_inner(
+			None,
+			description,
+			expiry_secs,
+			None,
+			max_proportional_lsp_fee_limit_ppm_msat,
 		)
 	}
 
 	fn receive_payment_via_jit_channel_inner(
 		&self, amount_msat: Option<u64>, description: &str, expiry_secs: u32,
 		max_total_lsp_fee_limit_msat: Option<u64>,
+		max_proportional_lsp_fee_limit_ppm_msat: Option<u64>,
 	) -> Result<Bolt11Invoice, Error> {
 		let liquidity_source =
 			self.liquidity_source.as_ref().ok_or(Error::LiquiditySourceUnavailable)?;
@@ -1649,29 +1675,38 @@ impl<K: KVStore + Sync + Send + 'static> Node<K> {
 		log_info!(self.logger, "Connected to LSP {}@{}. ", peer_info.node_id, peer_info.address);
 
 		let liquidity_source = Arc::clone(&liquidity_source);
-		let (invoice, lsp_total_opening_fee) = tokio::task::block_in_place(move || {
-			runtime.block_on(async move {
-				if let Some(amount_msat) = amount_msat {
-					liquidity_source
-						.lsps2_receive_to_jit_channel(
-							amount_msat,
-							description,
-							expiry_secs,
-							max_total_lsp_fee_limit_msat,
-						)
-						.await
-						.map(|(invoice, total_fee)| (invoice, Some(total_fee)))
-				} else {
-					// TODO: will be implemented in the next commit
-					Err(Error::LiquidityRequestFailed)
-				}
-			})
-		})?;
+		let (invoice, lsp_total_opening_fee, lsp_prop_opening_fee) =
+			tokio::task::block_in_place(move || {
+				runtime.block_on(async move {
+					if let Some(amount_msat) = amount_msat {
+						liquidity_source
+							.lsps2_receive_to_jit_channel(
+								amount_msat,
+								description,
+								expiry_secs,
+								max_total_lsp_fee_limit_msat,
+							)
+							.await
+							.map(|(invoice, total_fee)| (invoice, Some(total_fee), None))
+					} else {
+						liquidity_source
+							.lsps2_receive_variable_amount_to_jit_channel(
+								description,
+								expiry_secs,
+								max_proportional_lsp_fee_limit_ppm_msat,
+							)
+							.await
+							.map(|(invoice, prop_fee)| (invoice, None, Some(prop_fee)))
+					}
+				})
+			})?;
 
 		// Register payment in payment store.
 		let payment_hash = PaymentHash(invoice.payment_hash().to_byte_array());
-		let lsp_fee_limits =
-			Some(LSPFeeLimits { max_total_opening_fee_msat: lsp_total_opening_fee });
+		let lsp_fee_limits = Some(LSPFeeLimits {
+			max_total_opening_fee_msat: lsp_total_opening_fee,
+			max_proportional_opening_fee_ppm_msat: lsp_prop_opening_fee,
+		});
 		let payment = PaymentDetails {
 			hash: payment_hash,
 			preimage: None,

src/liquidity.rs

@@ -249,6 +249,49 @@ where
 		Ok((invoice, min_total_fee_msat))
 	}
 
+	pub(crate) async fn lsps2_receive_variable_amount_to_jit_channel(
+		&self, description: &str, expiry_secs: u32,
+		max_proportional_lsp_fee_limit_ppm_msat: Option<u64>,
+	) -> Result<(Bolt11Invoice, u64), Error> {
+		let fee_response = self.lsps2_request_opening_fee_params().await?;
+
+		let (min_prop_fee_ppm_msat, min_opening_params) = fee_response
+			.opening_fee_params_menu
+			.into_iter()
+			.map(|params| (params.proportional as u64, params))
+			.min_by_key(|p| p.0)
+			.ok_or_else(|| {
+				log_error!(self.logger, "Failed to handle response from liquidity service",);
+				Error::LiquidityRequestFailed
+			})?;
+
+		if let Some(max_proportional_lsp_fee_limit_ppm_msat) =
+			max_proportional_lsp_fee_limit_ppm_msat
+		{
+			if min_prop_fee_ppm_msat > max_proportional_lsp_fee_limit_ppm_msat {
+				log_error!(self.logger,
+					"Failed to request inbound JIT channel as LSP's requested proportional opening fee of {} ppm msat exceeds our fee limit of {} ppm msat",
+					min_prop_fee_ppm_msat,
+					max_proportional_lsp_fee_limit_ppm_msat
+				);
+				return Err(Error::LiquidityFeeTooHigh);
+			}
+		}
+
+		log_debug!(
+			self.logger,
+			"Choosing cheapest liquidity offer, will pay {}ppm msat in proportional LSP fees",
+			min_prop_fee_ppm_msat
+		);
+
+		let buy_response = self.lsps2_send_buy_request(None, min_opening_params).await?;
+		let invoice =
+			self.lsps2_create_jit_invoice(buy_response, None, description, expiry_secs)?;
+
+		log_info!(self.logger, "JIT-channel invoice created: {}", invoice);
+		Ok((invoice, min_prop_fee_ppm_msat))
+	}
+
 	async fn lsps2_request_opening_fee_params(&self) -> Result<LSPS2FeeResponse, Error> {
 		let lsps2_service = self.lsps2_service.as_ref().ok_or(Error::LiquiditySourceUnavailable)?;
 

src/payment_store.rs

@@ -92,10 +92,14 @@ pub struct LSPFeeLimits {
 	/// The maximal total amount we allow any configured LSP withhold from us when forwarding the
 	/// payment.
 	pub max_total_opening_fee_msat: Option<u64>,
+	/// The maximal proportional fee, in parts-per-million millisatoshi, we allow any configured
+	/// LSP withhold from us when forwarding the payment.
+	pub max_proportional_opening_fee_ppm_msat: Option<u64>,
 }
 
 impl_writeable_tlv_based!(LSPFeeLimits, {
 	(0, max_total_opening_fee_msat, option),
+	(2, max_proportional_opening_fee_ppm_msat, option),
 });
 
 #[derive(Clone, Debug, PartialEq, Eq)]