Allow for variable amount payments

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 +382,20 @@ where
 						return;
 					}
 
-					let max_total_lsp_fee_limit_msat =
-						info.max_total_lsp_fee_limit_msat.unwrap_or(0);
+					let max_total_lsp_fee_limit_msat = if let Some(max_total_lsp_fee_limit_msat) =
+						info.max_total_lsp_fee_limit_msat
+					{
+						max_total_lsp_fee_limit_msat
+					} else if let Some(max_proportional_lsp_fee_limit_ppm_msat) =
+						info.max_proportional_lsp_fee_limit_ppm_msat
+					{
+						// If it's a variable amount payment, compute the actual total opening fee.
+						compute_opening_fee(amount_msat, 0, max_proportional_lsp_fee_limit_ppm_msat)
+							.unwrap_or(0)
+					} else {
+						0
+					};
+
 					if counterparty_skimmed_fee_msat > max_total_lsp_fee_limit_msat {
 						log_info!(
 							self.logger,
@@ -497,6 +510,7 @@ where
 							direction: PaymentDirection::Inbound,
 							status: PaymentStatus::Succeeded,
 							max_total_lsp_fee_limit_msat: None,
+							max_proportional_lsp_fee_limit_ppm_msat: None,
 						};
 
 						match self.payment_store.insert(payment) {

src/lib.rs

@@ -1198,6 +1198,7 @@ impl<K: KVStore + Sync + Send + 'static> Node<K> {
 					direction: PaymentDirection::Outbound,
 					status: PaymentStatus::Pending,
 					max_total_lsp_fee_limit_msat: None,
+					max_proportional_lsp_fee_limit_ppm_msat: None,
 				};
 				self.payment_store.insert(payment)?;
 
@@ -1218,6 +1219,7 @@ impl<K: KVStore + Sync + Send + 'static> Node<K> {
 							direction: PaymentDirection::Outbound,
 							status: PaymentStatus::Failed,
 							max_total_lsp_fee_limit_msat: None,
+							max_proportional_lsp_fee_limit_ppm_msat: None,
 						};
 
 						self.payment_store.insert(payment)?;
@@ -1306,6 +1308,7 @@ impl<K: KVStore + Sync + Send + 'static> Node<K> {
 					direction: PaymentDirection::Outbound,
 					status: PaymentStatus::Pending,
 					max_total_lsp_fee_limit_msat: None,
+					max_proportional_lsp_fee_limit_ppm_msat: None,
 				};
 				self.payment_store.insert(payment)?;
 
@@ -1327,6 +1330,7 @@ impl<K: KVStore + Sync + Send + 'static> Node<K> {
 							direction: PaymentDirection::Outbound,
 							status: PaymentStatus::Failed,
 							max_total_lsp_fee_limit_msat: None,
+							max_proportional_lsp_fee_limit_ppm_msat: None,
 						};
 						self.payment_store.insert(payment)?;
 
@@ -1382,6 +1386,7 @@ impl<K: KVStore + Sync + Send + 'static> Node<K> {
 					direction: PaymentDirection::Outbound,
 					amount_msat: Some(amount_msat),
 					max_total_lsp_fee_limit_msat: None,
+					max_proportional_lsp_fee_limit_ppm_msat: None,
 				};
 				self.payment_store.insert(payment)?;
 
@@ -1403,6 +1408,7 @@ impl<K: KVStore + Sync + Send + 'static> Node<K> {
 							direction: PaymentDirection::Outbound,
 							amount_msat: Some(amount_msat),
 							max_total_lsp_fee_limit_msat: None,
+							max_proportional_lsp_fee_limit_ppm_msat: None,
 						};
 
 						self.payment_store.insert(payment)?;
@@ -1577,6 +1583,7 @@ impl<K: KVStore + Sync + Send + 'static> Node<K> {
 			direction: PaymentDirection::Inbound,
 			status: PaymentStatus::Pending,
 			max_total_lsp_fee_limit_msat: None,
+			max_proportional_lsp_fee_limit_ppm_msat: None,
 		};
 
 		self.payment_store.insert(payment)?;
@@ -1603,12 +1610,38 @@ 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)?;
@@ -1638,18 +1671,20 @@ 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_opening_fee) = tokio::task::block_in_place(move || {
-			runtime.block_on(async move {
-				liquidity_source
-					.lsps2_receive_to_jit_channel(
-						amount_msat,
-						description,
-						expiry_secs,
-						max_total_lsp_fee_limit_msat,
-					)
-					.await
-			})
-		})?;
+		let (invoice, lsp_total_opening_fee, lsp_prop_opening_fee) =
+			tokio::task::block_in_place(move || {
+				runtime.block_on(async move {
+					liquidity_source
+						.lsps2_receive_to_jit_channel(
+							amount_msat,
+							description,
+							expiry_secs,
+							max_total_lsp_fee_limit_msat,
+							max_proportional_lsp_fee_limit_ppm_msat,
+						)
+						.await
+				})
+			})?;
 
 		// Register payment in payment store.
 		let payment_hash = PaymentHash(invoice.payment_hash().to_byte_array());
@@ -1660,7 +1695,8 @@ impl<K: KVStore + Sync + Send + 'static> Node<K> {
 			amount_msat,
 			direction: PaymentDirection::Inbound,
 			status: PaymentStatus::Pending,
-			max_total_lsp_fee_limit_msat: Some(lsp_opening_fee),
+			max_total_lsp_fee_limit_msat: lsp_total_opening_fee,
+			max_proportional_lsp_fee_limit_ppm_msat: lsp_prop_opening_fee,
 		};
 
 		self.payment_store.insert(payment)?;

src/liquidity.rs

@@ -191,54 +191,86 @@ where
 	pub(crate) async fn lsps2_receive_to_jit_channel(
 		&self, amount_msat: Option<u64>, description: &str, expiry_secs: u32,
 		max_total_lsp_fee_limit_msat: Option<u64>,
-	) -> Result<(Bolt11Invoice, u64), Error> {
+		max_proportional_lsp_fee_limit_ppm_msat: Option<u64>,
+	) -> Result<(Bolt11Invoice, Option<u64>, Option<u64>), Error> {
 		let lsps2_service = self.lsps2_service.as_ref().ok_or(Error::LiquiditySourceUnavailable)?;
 
 		let fee_response = self.request_opening_fee_params().await?;
 
-		if let Some(amount_msat) = amount_msat {
+		let (min_total_fee_msat, min_prop_fee_ppm_msat, min_opening_params) = if let Some(
+			amount_msat,
+		) = amount_msat
+		{
+			// `MPP+fixed-invoice` mode
 			if amount_msat < fee_response.min_payment_size_msat
 				|| amount_msat > fee_response.max_payment_size_msat
 			{
 				log_error!(self.logger, "Failed to request inbound JIT channel as the payment of {}msat doesn't meet LSP limits (min: {}msat, max: {}msat)", amount_msat, fee_response.min_payment_size_msat, fee_response.max_payment_size_msat);
 				return Err(Error::LiquidityRequestFailed);
 			}
-		}
 
-		// If it's variable amount, we pick the cheapest opening fee with a dummy value.
-		let fee_computation_amount = amount_msat.unwrap_or(1_000_000);
-		let (min_opening_fee_msat, min_opening_params) = fee_response
-			.opening_fee_params_menu
-			.iter()
-			.flat_map(|params| {
-				if let Some(fee) = compute_opening_fee(
-					fee_computation_amount,
-					params.min_fee_msat,
-					params.proportional as u64,
-				) {
-					Some((fee, params))
-				} else {
-					None
+			let (min_total_fee_msat, min_params) = fee_response
+				.opening_fee_params_menu
+				.iter()
+				.flat_map(|params| {
+					if let Some(fee) = compute_opening_fee(
+						amount_msat,
+						params.min_fee_msat,
+						params.proportional as u64,
+					) {
+						Some((fee, params))
+					} else {
+						None
+					}
+				})
+				.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_total_lsp_fee_limit_msat) = max_total_lsp_fee_limit_msat {
+				if min_total_fee_msat > max_total_lsp_fee_limit_msat {
+					log_error!(self.logger, "Failed to request inbound JIT channel as LSP's requested total opening fee of {}msat exceeds our fee limit of {}msat", min_total_fee_msat, max_total_lsp_fee_limit_msat);
+					return Err(Error::LiquidityFeeTooHigh);
 				}
-			})
-			.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_total_lsp_fee_limit_msat) = max_total_lsp_fee_limit_msat {
-			if min_opening_fee_msat > max_total_lsp_fee_limit_msat {
-				log_error!(self.logger, "Failed to request inbound JIT channel as LSP's requested opening fee of {}msat exceeds our fee limit of {}msat", min_opening_fee_msat, max_total_lsp_fee_limit_msat);
-				return Err(Error::LiquidityFeeTooHigh);
+			log_debug!(
+				self.logger,
+				"Choosing cheapest liquidity offer, will pay {}msat in total LSP fees",
+				min_total_fee_msat
+			);
+
+			(Some(min_total_fee_msat), None, min_params)
+		} else {
+			// `no-MPP+var-invoice` mode
+			let (min_prop_fee_ppm_msat, min_params) = fee_response
+				.opening_fee_params_menu
+				.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 {}msat in LSP fees",
-			min_opening_fee_msat
-		);
+			log_debug!(
+				self.logger,
+				"Choosing cheapest liquidity offer, will pay {}ppm msat in proportional LSP fees",
+				min_prop_fee_ppm_msat
+			);
+			(None, Some(min_prop_fee_ppm_msat), min_params)
+		};
 
 		let buy_response = self.send_buy_request(amount_msat, min_opening_params.clone()).await?;
 
@@ -291,7 +323,7 @@ where
 			})?;
 
 		log_info!(self.logger, "JIT-channel invoice created: {}", invoice);
-		Ok((invoice, min_opening_fee_msat))
+		Ok((invoice, min_total_fee_msat, min_prop_fee_ppm_msat))
 	}
 
 	async fn request_opening_fee_params(&self) -> Result<LSPS2FeeResponse, Error> {

src/payment_store.rs

@@ -40,12 +40,23 @@ pub struct PaymentDetails {
 	///
 	/// [`LdkChannelConfig::accept_underpaying_htlcs`]: lightning::util::config::ChannelConfig::accept_underpaying_htlcs
 	pub max_total_lsp_fee_limit_msat: Option<u64>,
+	/// The maximal proportional fee, in parts-per-million millisatoshi, we allow any configured
+	/// LSP withhold from us when forwarding the payment.
+	///
+	/// This is usually only `Some` for payments received via a JIT-channel, in which case the first
+	/// inbound payment will pay for the LSP's channel opening fees.
+	///
+	/// See [`LdkChannelConfig::accept_underpaying_htlcs`] for more information.
+	///
+	/// [`LdkChannelConfig::accept_underpaying_htlcs`]: lightning::util::config::ChannelConfig::accept_underpaying_htlcs
+	pub max_proportional_lsp_fee_limit_ppm_msat: Option<u64>,
 }
 
 impl_writeable_tlv_based!(PaymentDetails, {
 	(0, hash, required),
 	(1, max_total_lsp_fee_limit_msat, option),
 	(2, preimage, required),
+	(3, max_proportional_lsp_fee_limit_ppm_msat, option),
 	(4, secret, required),
 	(6, amount_msat, required),
 	(8, direction, required),
@@ -265,6 +276,7 @@ mod tests {
 			direction: PaymentDirection::Inbound,
 			status: PaymentStatus::Pending,
 			max_total_lsp_fee_limit_msat: None,
+			max_proportional_lsp_fee_limit_ppm_msat: None,
 		};
 
 		assert_eq!(Ok(false), payment_store.insert(payment.clone()));