Make max_mpp_path_count configurable.

Author: tnull

lightning/src/ln/channelmanager.rs

@@ -2529,12 +2529,6 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 		if route.paths.len() < 1 {
 			return Err(PaymentSendFailure::ParameterError(APIError::RouteError{err: "There must be at least one path to send over"}));
 		}
-		if route.paths.len() > 10 {
-			// This limit is completely arbitrary - there aren't any real fundamental path-count
-			// limits. After we support retrying individual paths we should likely bump this, but
-			// for now more than 10 paths likely carries too much one-path failure.
-			return Err(PaymentSendFailure::ParameterError(APIError::RouteError{err: "Sending over more than 10 paths is not currently supported"}));
-		}
 		if payment_secret.is_none() && route.paths.len() > 1 {
 			return Err(PaymentSendFailure::ParameterError(APIError::APIMisuseError{err: "Payment secret is required for multi-path payments".to_string()}));
 		}

lightning/src/routing/router.rs

@@ -176,6 +176,12 @@ impl_writeable_tlv_based!(RouteParameters, {
 /// Maximum total CTLV difference we allow for a full payment path.
 pub const DEFAULT_MAX_TOTAL_CLTV_EXPIRY_DELTA: u32 = 1008;
 
+/// Maximum number of paths we allow an MPP payment to have.
+// The default limit is currently set rather arbitrary - there aren't any real fundamental path-count
+// limits. After we support retrying individual paths we should likely bump this, but
+// for now more than 10 paths likely carries too much one-path failure.
+pub const DEFAULT_MAX_MPP_PATH_COUNT: u8 = 10;
+
 // The median hop CLTV expiry delta currently seen in the network.
 const MEDIAN_HOP_CLTV_EXPIRY_DELTA: u32 = 40;
 
@@ -215,12 +221,16 @@ pub struct PaymentParameters {
 
 	/// The maximum total CLTV delta we accept for the route.
 	pub max_total_cltv_expiry_delta: u32,
+
+	/// The maximum number of paths that may be used by MPP payments.
+	pub max_mpp_path_count: u8,
 }
 
 impl_writeable_tlv_based!(PaymentParameters, {
 	(0, payee_pubkey, required),
 	(1, max_total_cltv_expiry_delta, (default_value, DEFAULT_MAX_TOTAL_CLTV_EXPIRY_DELTA)),
 	(2, features, option),
+	(3, max_mpp_path_count, (default_value, DEFAULT_MAX_MPP_PATH_COUNT)),
 	(4, route_hints, vec_type),
 	(6, expiry_time, option),
 });
@@ -234,6 +244,7 @@ impl PaymentParameters {
 			route_hints: vec![],
 			expiry_time: None,
 			max_total_cltv_expiry_delta: DEFAULT_MAX_TOTAL_CLTV_EXPIRY_DELTA,
+			max_mpp_path_count: DEFAULT_MAX_MPP_PATH_COUNT,
 		}
 	}
 
@@ -269,6 +280,13 @@ impl PaymentParameters {
 	pub fn with_max_total_cltv_expiry_delta(self, max_total_cltv_expiry_delta: u32) -> Self {
 		Self { max_total_cltv_expiry_delta, ..self }
 	}
+
+	/// Includes a limit for the maximum number of payment paths that may be used by MPP.
+	///
+	/// (C-not exported) since bindings don't support move semantics
+	pub fn with_max_mpp_path_count(self, max_mpp_path_count: u8) -> Self {
+		Self { max_mpp_path_count, ..self }
+	}
 }
 
 /// A list of hops along a payment path terminating with a channel to the recipient.
@@ -790,6 +808,11 @@ where L::Target: Logger {
 			node_info.features.supports_basic_mpp()
 		} else { false }
 	} else { false };
+
+	if allow_mpp && payment_params.max_mpp_path_count == 0 {
+		return Err(LightningError{err: "Can't find an MPP route with no paths allowed.".to_owned(), action: ErrorAction::IgnoreError});
+	}
+
 	log_trace!(logger, "Searching for a route from payer {} to payee {} {} MPP and {} first hops {}overriding the network graph", our_node_pubkey,
 		payment_params.payee_pubkey, if allow_mpp { "with" } else { "without" },
 		first_hops.map(|hops| hops.len()).unwrap_or(0), if first_hops.is_some() { "" } else { "not " });
@@ -917,12 +940,10 @@ where L::Target: Logger {
 					// Taking too many smaller paths also increases the chance of payment failure.
 					// Thus to avoid this effect, we require from our collected links to provide
 					// at least a minimal contribution to the recommended value yet-to-be-fulfilled.
-					// This requirement is currently 10% of the value.
-
-					// Derive the minimal liquidity contribution with a ratio of 10 (10%, rounded up)
-					// or 100% if we're not allowed to do multipath payments.
+					// This requirement is currently set to be 1/max_mpp_path_count of the payment
+					// value to ensure we only ever return routes that do not violate this limit.
 					let minimal_value_contribution_msat: u64 = if allow_mpp {
-						(final_value_msat + 9) / 10
+						(final_value_msat + (payment_params.max_mpp_path_count as u64 - 1)) / payment_params.max_mpp_path_count as u64
 					} else {
 						final_value_msat
 					};
@@ -1669,6 +1690,8 @@ where L::Target: Logger {
 		});
 		selected_paths.push(path);
 	}
+	// Make sure we would never create a route with more paths than we allow.
+	assert!(selected_paths.len() <= payment_params.max_mpp_path_count.into());
 
 	if let Some(features) = &payment_params.features {
 		for path in selected_paths.iter_mut() {