Return an error for an inverted probe amount range

A `debug_assert!` let an inverted range through in release builds. Reject
it with `BuildError::InvalidProbeAmountRange` instead, from both the Rust
builder and the uniffi one, like the other fallible builder setters.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: joschisan

bindings/ldk_node.udl

@@ -40,6 +40,7 @@ interface ProbingConfigBuilder {
 	void set_max_locked_msat(u64 max_msat);
 	void set_diversity_penalty_msat(u64 penalty_msat);
 	void set_cooldown(u64 secs);
+	[Throws=BuildError]
 	void set_amount_range_msat(u64 min_msat, u64 max_msat);
 	ProbingConfig build();
 };

src/builder.rs

@@ -192,6 +192,8 @@ pub enum BuildError {
 	InvalidTorProxyAddress,
 	/// The provided alias is invalid.
 	InvalidNodeAlias,
+	/// The given probe amount range is invalid, i.e., its minimum exceeds its maximum.
+	InvalidProbeAmountRange,
 	/// An attempt to setup a runtime has failed.
 	RuntimeSetupFailed,
 	/// We failed to read data from the [`KVStore`].
@@ -254,6 +256,7 @@ impl fmt::Display for BuildError {
 			Self::LoggerSetupFailed => write!(f, "Failed to setup the logger."),
 			Self::ChainSourceSetupFailed => write!(f, "Failed to setup the chain source."),
 			Self::InvalidNodeAlias => write!(f, "Given node alias is invalid."),
+			Self::InvalidProbeAmountRange => write!(f, "Given probe amount range is invalid."),
 			Self::NetworkMismatch => {
 				write!(f, "Given network does not match the node's previously configured network.")
 			},

src/probing.rs

@@ -78,6 +78,7 @@ use lightning::routing::router::{
 use lightning_invoice::DEFAULT_MIN_FINAL_CLTV_EXPIRY_DELTA;
 use lightning_types::features::{ChannelFeatures, NodeFeatures};
 
+use crate::builder::BuildError;
 use crate::config::{
 	DEFAULT_MAX_PROBE_AMOUNT_MSAT, DEFAULT_MAX_PROBE_LOCKED_MSAT, DEFAULT_MIN_PROBE_AMOUNT_MSAT,
 	DEFAULT_PROBED_NODE_COOLDOWN_SECS, DEFAULT_PROBING_INTERVAL_SECS, MIN_PROBING_INTERVAL,
@@ -270,11 +271,18 @@ impl ProbingConfigBuilder {
 	/// more liquidity per in-flight probe — size `max_locked_msat` accordingly.
 	///
 	/// Defaults to 1 000 000 - 10 000 000 msat (1k - 10k sats).
-	pub fn amount_range_msat(&mut self, min_msat: u64, max_msat: u64) -> &mut Self {
-		debug_assert!(min_msat <= max_msat, "min_msat must not exceed max_msat");
+	///
+	/// Returns [`BuildError::InvalidProbeAmountRange`] if `min_msat` exceeds `max_msat`.
+	pub fn amount_range_msat(
+		&mut self, min_msat: u64, max_msat: u64,
+	) -> Result<&mut Self, BuildError> {
+		if min_msat > max_msat {
+			return Err(BuildError::InvalidProbeAmountRange);
+		}
+
 		self.min_amount_msat = min_msat;
 		self.max_amount_msat = max_msat;
-		self
+		Ok(self)
 	}
 
 	/// Builds the [`ProbingConfig`].
@@ -364,8 +372,10 @@ impl ArcedProbingConfigBuilder {
 	/// own probe amounts when building paths.
 	///
 	/// Defaults to 1 000 000 - 10 000 000 msat (1k - 10k sats).
-	pub fn set_amount_range_msat(&self, min_msat: u64, max_msat: u64) {
-		self.inner.write().expect("lock").amount_range_msat(min_msat, max_msat);
+	///
+	/// Returns [`BuildError::InvalidProbeAmountRange`] if `min_msat` exceeds `max_msat`.
+	pub fn set_amount_range_msat(&self, min_msat: u64, max_msat: u64) -> Result<(), BuildError> {
+		self.inner.write().expect("lock").amount_range_msat(min_msat, max_msat).map(|_| ())
 	}
 
 	/// Builds the [`ProbingConfig`].
@@ -879,3 +889,23 @@ pub(crate) async fn run_prober(prober: Arc<Prober>, mut stop_rx: tokio::sync::wa
 		}
 	}
 }
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+
+	#[test]
+	fn amount_range_rejects_min_above_max() {
+		let mut builder = ProbingConfigBuilder::random_walk(3);
+
+		assert_eq!(
+			builder.amount_range_msat(2_000, 1_000).err(),
+			Some(BuildError::InvalidProbeAmountRange)
+		);
+
+		builder.amount_range_msat(1_000, 1_000).expect("an equal minimum and maximum is valid");
+
+		let config = builder.build();
+		assert_eq!((config.min_amount_msat, config.max_amount_msat), (1_000, 1_000));
+	}
+}