Move value contrib. calculation and improve docs.

We move the calculation of the value contribution out of the
`add_entry!` macro and improve/update documentation in a few places.

Author: tnull

lightning/src/routing/router.rs

@@ -178,8 +178,7 @@ 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.
+// limits, 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.
@@ -220,9 +219,11 @@ pub struct PaymentParameters {
 	pub expiry_time: Option<u64>,
 
 	/// The maximum total CLTV delta we accept for the route.
+	/// Defaults to [`DEFAULT_MAX_TOTAL_CLTV_EXPIRY_DELTA`].
 	pub max_total_cltv_expiry_delta: u32,
 
 	/// The maximum number of paths that may be used by MPP payments.
+	/// Defaults to [`DEFAULT_MAX_MPP_PATH_COUNT`].
 	pub max_mpp_path_count: u8,
 }
 
@@ -863,6 +864,21 @@ where L::Target: Logger {
 	let recommended_value_msat = final_value_msat * ROUTE_CAPACITY_PROVISION_FACTOR as u64;
 	let mut path_value_msat = final_value_msat;
 
+	// Routing Fragmentation Mitigation heuristic:
+	//
+	// Routing fragmentation across many payment paths increases the overall routing
+	// fees as you have irreducible routing fees per-link used (`fee_base_msat`).
+	// 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 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 + (payment_params.max_mpp_path_count as u64 - 1)) / payment_params.max_mpp_path_count as u64
+	} else {
+		final_value_msat
+	};
+
 	// Keep track of how much liquidity has been used in selected channels. Used to determine
 	// if the channel can be used by additional MPP paths or to inform path finding decisions. It is
 	// aware of direction *only* to ensure that the correct htlc_maximum_msat value is used. Hence,
@@ -933,20 +949,6 @@ where L::Target: Logger {
 							*used_liquidity_msat
 						});
 
-					// Routing Fragmentation Mitigation heuristic:
-					//
-					// Routing fragmentation across many payment paths increases the overall routing
-					// fees as you have irreducible routing fees per-link used (`fee_base_msat`).
-					// 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 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 + (payment_params.max_mpp_path_count as u64 - 1)) / payment_params.max_mpp_path_count as u64
-					} else {
-						final_value_msat
-					};
 					// Verify the liquidity offered by this channel complies to the minimal contribution.
 					let contributes_sufficient_value = available_value_contribution_msat >= minimal_value_contribution_msat;
 					// Do not consider candidate hops that would exceed the maximum path length.
@@ -1691,7 +1693,7 @@ 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());
+	debug_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() {