Account for extra fees in value contribution

When overpaying to meet `htlc_minimum_msat`, we update the path value
and recompute fees accordingly. However, so far we didn't account for
the extra paid fees in `value_contribution_msat`, which leads to it being
slightly off, potentially having us hit a debug assertion when later
checking the route's total value matches
`already_collected_value_msat`. Here, we therefore add the extra fees to
`value_contribution_msat`.

Author: tnull

lightning/src/routing/router.rs

@@ -1265,7 +1265,8 @@ impl<'a> PaymentPath<'a> {
 	// Note that this function is not aware of the available_liquidity limit, and thus does not
 	// support increasing the value being transferred beyond what was selected during the initial
 	// routing passes.
-	fn update_value_and_recompute_fees(&mut self, value_msat: u64) {
+	fn update_value_and_recompute_fees(&mut self, value_msat: u64) -> u64 {
+		let mut extra_contribution_msat = 0;
 		let mut total_fee_paid_msat = 0 as u64;
 		for i in (0..self.hops.len()).rev() {
 			let last_hop = i == self.hops.len() - 1;
@@ -1297,6 +1298,12 @@ impl<'a> PaymentPath<'a> {
 				cur_hop_transferred_amount_msat += extra_fees_msat;
 				total_fee_paid_msat += extra_fees_msat;
 				cur_hop_fees_msat += extra_fees_msat;
+
+				// We remember and return the extra fees on the final hop to allow accounting for
+				// them in the path's value contribution.
+				if last_hop {
+					extra_contribution_msat = extra_fees_msat;
+				}
 			}
 
 			if last_hop {
@@ -1324,6 +1331,7 @@ impl<'a> PaymentPath<'a> {
 				}
 			}
 		}
+		extra_contribution_msat
 	}
 }
 
@@ -2321,7 +2329,7 @@ where L::Target: Logger {
 				// underpaid htlc_minimum_msat with fees.
 				debug_assert_eq!(payment_path.get_value_msat(), value_contribution_msat);
 				value_contribution_msat = cmp::min(value_contribution_msat, final_value_msat);
-				payment_path.update_value_and_recompute_fees(value_contribution_msat);
+				value_contribution_msat += payment_path.update_value_and_recompute_fees(value_contribution_msat);
 
 				// Since a path allows to transfer as much value as
 				// the smallest channel it has ("bottleneck"), we should recompute