Limit the maximum shadow offset, but do not fail.

Author: tnull

lightning/src/routing/router.rs

@@ -839,8 +839,7 @@ 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.
-					// We subtract 2*40 here in order to account for the privacy-enhancing random CLTV delta offset we add on top later
-					let max_total_cltv_expiry_delta = payment_params.max_total_cltv_expiry_delta.checked_sub(80).unwrap_or(0);
+					let max_total_cltv_expiry_delta = 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());
@@ -1569,20 +1568,22 @@ where L::Target: Logger {
 			shadow_ctlv_expiry_delta_offset = random_walk_length.wrapping_mul(40);
 		}
 
-		// Limit the offset to reduce the payment failure probability
+		// Limit the total offset to reduce the worst-case locked liquidity timevalue
 		const MAX_SHADOW_CLTV_EXPIRY_DELTA_OFFSET: u32 = 432; // 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
+			.checked_sub(path.iter().fold(0, |acc,b| b.as_ref().unwrap().cltv_expiry_delta.max(acc)))
+			.unwrap_or(shadow_ctlv_expiry_delta_offset);
+		shadow_ctlv_expiry_delta_offset = cmp::min(shadow_ctlv_expiry_delta_offset, max_path_offset);
+
 		// Add 'shadow' CLTV offset to all hops but the final one
 		for h in path {
 			if let Ok(hop) = h {
 				if hop.pubkey != payment_params.payee_pubkey {
 					hop.cltv_expiry_delta = hop.cltv_expiry_delta
 						.checked_add(shadow_ctlv_expiry_delta_offset).unwrap_or(hop.cltv_expiry_delta);
-					let max_total_cltv_expiry_delta = payment_params.max_total_cltv_expiry_delta;
-					if hop.cltv_expiry_delta > max_total_cltv_expiry_delta { 
-						return Err(LightningError{err: "Chosen path exceeds max total CLTV delta limit".to_owned(), action: ErrorAction::IgnoreError});
-					}
 				}
 			}
 		}