Consider offset in pathfinding. Revisit max_path_offset limit.

Author: tnull

lightning/src/routing/router.rs

@@ -861,7 +861,10 @@ where L::Target: Logger {
 					let contributes_sufficient_value = available_value_contribution_msat >= minimal_value_contribution_msat;
 
 					// Do not consider candidates that exceed the maximum total cltv expiry limit.
-					let max_total_cltv_expiry_delta = payment_params.max_total_cltv_expiry_delta;
+					// We subtract 2*40 here in order to account for some of the privacy-enhancing
+					// random CLTV expiry delta offset we add on top later.
+					let max_total_cltv_expiry_delta = payment_params.max_total_cltv_expiry_delta
+						.checked_sub(2*40).unwrap_or(payment_params.max_total_cltv_expiry_delta);
 					let hop_total_cltv_delta = ($next_hops_cltv_delta as u32)
 						.checked_add($candidate.cltv_expiry_delta())
 						.unwrap_or(u32::max_value());
@@ -1602,12 +1605,14 @@ fn add_random_cltv_offset(route: &mut Route, payment_params: &PaymentParameters,
 		const MAX_SHADOW_CLTV_EXPIRY_DELTA_OFFSET: u32 = 3*144;
 		shadow_ctlv_expiry_delta_offset = cmp::min(shadow_ctlv_expiry_delta_offset, MAX_SHADOW_CLTV_EXPIRY_DELTA_OFFSET);
 
-		// Limit the offset so we never exceed the max_total_cltv_expiry_delta
-		let max_path_offset = payment_params.max_total_cltv_expiry_delta
+		// Limit the offset so we never exceed the max_total_cltv_expiry_delta. To improve plausibility,
+		// we choose the limit to be the largest possible multiple of 40.
+		let mut max_path_offset = payment_params.max_total_cltv_expiry_delta
 			.checked_sub(path.iter().map(|h| h.cltv_expiry_delta).sum())
-			.unwrap_or(shadow_ctlv_expiry_delta_offset);
-		shadow_ctlv_expiry_delta_offset = cmp::min(shadow_ctlv_expiry_delta_offset,
-			max_path_offset.checked_sub(max_path_offset.wrapping_rem(40)).unwrap_or(max_path_offset));
+			.unwrap_or(0);
+		max_path_offset = max_path_offset.wrapping_sub(max_path_offset.wrapping_rem(40))
+			.max(max_path_offset.wrapping_rem(40));
+		shadow_ctlv_expiry_delta_offset = cmp::min(shadow_ctlv_expiry_delta_offset, max_path_offset);
 
 		// Add 'shadow' CLTV offset to the final hop
 		if let Some(last_hop) = path.last_mut() {