Add Node::find_route accessor (#29)

Public wrapper around the previously private `_router` so callers can
resolve a route to a destination without dispatching a payment. The
intended consumer is mdk's max-withdrawable estimator, which needs a
real route to invert per-hop fees against payout destinations.

The signature takes `Option<RouteParametersConfig>`. With `None` the
node-wide `Config::route_parameters` applies, matching the fallback
already used by `SpontaneousPayment::send_inner`. Keeps both call
sites consistent.

`Error::RouteNotFound` is a new variant. This path performs lookup only
and never dispatches anything, and a missing route under current
liquidity is a routine outcome. The router's `&'static str` reason
is logged at debug level and dropped; callers already get the typed
error.

`Route` and `RouteParametersConfig` are re-exported from `lib.rs` for
Rust consumers. No UDL bindings in this commit: the `uniffi` build is
currently broken on this branch from drift left over by the
rust-lightning upgrade in e935695, so adding more UDL surface would
fly blind. A follow-up that fixes the existing drift can layer
`Route` bindings on top.

The returned `Route` may contain multiple `Path` entries when
`max_path_count > 1`, since the router is free to split the amount
across MPP paths. Callers wanting a single-path response should set
`max_path_count: 1` in the supplied config.

Author: amackillop

src/error.rs

@@ -39,6 +39,8 @@ pub enum Error {
 	InvalidCustomTlvs,
 	/// Sending a payment probe has failed.
 	ProbeSendingFailed,
+	/// A route to the given destination could not be found.
+	RouteNotFound,
 	/// A channel could not be opened.
 	ChannelCreationFailed,
 	/// A channel could not be closed.
@@ -145,6 +147,7 @@ impl fmt::Display for Error {
 			Self::PaymentSendingFailed => write!(f, "Failed to send the given payment."),
 			Self::InvalidCustomTlvs => write!(f, "Failed to construct payment with custom TLVs."),
 			Self::ProbeSendingFailed => write!(f, "Failed to send the given payment probe."),
+			Self::RouteNotFound => write!(f, "Failed to find a route to the destination."),
 			Self::ChannelCreationFailed => write!(f, "Failed to create channel."),
 			Self::ChannelClosingFailed => write!(f, "Failed to close channel."),
 			Self::ChannelSplicingFailed => write!(f, "Failed to splice channel."),

src/lib.rs

@@ -144,6 +144,8 @@ use lightning::ln::channelmanager::PaymentId;
 use lightning::ln::funding::SpliceContribution;
 use lightning::ln::msgs::SocketAddress;
 use lightning::routing::gossip::NodeAlias;
+use lightning::routing::router::{PaymentParameters, RouteParameters};
+pub use lightning::routing::router::{Route, RouteParametersConfig};
 use lightning::util::persist::KVStoreSync;
 use lightning_background_processor::process_events_async;
 use liquidity::{LSPS1Liquidity, LiquiditySource};
@@ -152,7 +154,7 @@ use payment::asynchronous::om_mailbox::OnionMessageMailbox;
 use payment::asynchronous::static_invoice_store::StaticInvoiceStore;
 use payment::{
 	Bolt11Payment, Bolt12Payment, OnchainPayment, PaymentDetails, SpontaneousPayment,
-	UnifiedQrPayment,
+	UnifiedQrPayment, LDK_DEFAULT_FINAL_CLTV_EXPIRY_DELTA,
 };
 use peer_store::{PeerInfo, PeerStore};
 use rand::Rng;
@@ -1888,6 +1890,59 @@ impl Node {
 			.to_sat_per_kwu()
 	}
 
+	/// Finds a route to `payee` for `amount_msat` using the node's internal router.
+	///
+	/// Intended for callers that need to introspect routing fees ahead of
+	/// time (e.g. computing a max-sendable estimate by inverting per-hop fees).
+	///
+	/// `params` overrides the node-wide [`RouteParametersConfig`] when `Some`; otherwise
+	/// the value from [`Config::route_parameters`] is used.
+	///
+	/// [`Config::route_parameters`]: crate::config::Config::route_parameters
+	pub fn find_route(
+		&self, payee: PublicKey, amount_msat: u64, params: Option<RouteParametersConfig>,
+	) -> Result<Route, Error> {
+		if !*self.is_running.read().unwrap() {
+			return Err(Error::NotRunning);
+		}
+
+		let mut route_params = RouteParameters::from_payment_params_and_value(
+			PaymentParameters::from_node_id(payee, LDK_DEFAULT_FINAL_CLTV_EXPIRY_DELTA),
+			amount_msat,
+		);
+
+		if let Some(RouteParametersConfig {
+			max_total_routing_fee_msat,
+			max_total_cltv_expiry_delta,
+			max_path_count,
+			max_channel_saturation_power_of_half,
+		}) = params.as_ref().or(self.config.route_parameters.as_ref())
+		{
+			route_params.max_total_routing_fee_msat = *max_total_routing_fee_msat;
+			route_params.payment_params.max_total_cltv_expiry_delta = *max_total_cltv_expiry_delta;
+			route_params.payment_params.max_path_count = *max_path_count;
+			route_params.payment_params.max_channel_saturation_power_of_half =
+				*max_channel_saturation_power_of_half;
+		}
+
+		let payer = self.channel_manager.get_our_node_id();
+		let first_hops = self.channel_manager.list_usable_channels();
+		let first_hops_refs: Vec<&LdkChannelDetails> = first_hops.iter().collect();
+		let inflight_htlcs = self.channel_manager.compute_inflight_htlcs();
+
+		lightning::routing::router::Router::find_route(
+			&*self._router,
+			&payer,
+			&route_params,
+			Some(&first_hops_refs),
+			inflight_htlcs,
+		)
+		.map_err(|e| {
+			log_debug!(self.logger, "Failed to find route to {}: {}", payee, e);
+			Error::RouteNotFound
+		})
+	}
+
 	/// Exports the current state of the scorer. The result can be shared with and merged by light nodes that only have
 	/// a limited view of the network.
 	pub fn export_pathfinding_scores(&self) -> Result<Vec<u8>, Error> {

src/payment/mod.rs

@@ -19,6 +19,7 @@ pub use bolt11::Bolt11Payment;
 pub use bolt12::Bolt12Payment;
 pub use onchain::OnchainPayment;
 pub use spontaneous::SpontaneousPayment;
+pub(crate) use spontaneous::LDK_DEFAULT_FINAL_CLTV_EXPIRY_DELTA;
 pub use store::{
 	ConfirmationStatus, LSPFeeLimits, PaymentDetails, PaymentDirection, PaymentKind, PaymentStatus,
 };

src/payment/spontaneous.rs

@@ -22,7 +22,7 @@ use crate::payment::store::{PaymentDetails, PaymentDirection, PaymentKind, Payme
 use crate::types::{ChannelManager, CustomTlvRecord, KeysManager, PaymentStore};
 
 // The default `final_cltv_expiry_delta` we apply when not set.
-const LDK_DEFAULT_FINAL_CLTV_EXPIRY_DELTA: u32 = 144;
+pub(crate) const LDK_DEFAULT_FINAL_CLTV_EXPIRY_DELTA: u32 = 144;
 
 /// A payment handler allowing to send spontaneous ("keysend") payments.
 ///