Add excess fees, not total fees, to the htlc dust exposure

We previously added all the fees of the candidate incoming htlc to the
htlc dust exposure. In-line with the documentation, only add the fees in
excess of our local fee estimates.

Author: tankyleo

lightning/src/ln/channel.rs

@@ -7333,14 +7333,17 @@ impl<SP: Deref> FundedChannel<SP> where
 				return Err(("Exceeded our dust exposure limit on counterparty commitment tx", 0x1000|7))
 			}
 		} else {
-			let htlc_dust_exposure_msat =
-				per_outbound_htlc_counterparty_commit_tx_fee_msat(self.context.feerate_per_kw, &self.context.channel_type);
-			let counterparty_tx_dust_exposure =
-				htlc_stats.on_counterparty_tx_dust_exposure_msat.saturating_add(htlc_dust_exposure_msat);
-			if counterparty_tx_dust_exposure > max_dust_htlc_exposure_msat {
-				log_info!(logger, "Cannot accept value that would put our exposure to tx fee dust at {} over the limit {} on counterparty commitment tx",
-					counterparty_tx_dust_exposure, max_dust_htlc_exposure_msat);
-				return Err(("Exceeded our tx fee dust exposure limit on counterparty commitment tx", 0x1000|7))
+			let excess_feerate_opt = self.context.feerate_per_kw.checked_sub(dust_exposure_limiting_feerate);
+			if let Some(excess_feerate) = excess_feerate_opt {
+				let htlc_dust_exposure_msat =
+					per_outbound_htlc_counterparty_commit_tx_fee_msat(excess_feerate, &self.context.channel_type);
+				let counterparty_tx_dust_exposure =
+					htlc_stats.on_counterparty_tx_dust_exposure_msat.saturating_add(htlc_dust_exposure_msat);
+				if counterparty_tx_dust_exposure > max_dust_htlc_exposure_msat {
+					log_info!(logger, "Cannot accept value that would put our exposure to tx fee dust at {} over the limit {} on counterparty commitment tx",
+						counterparty_tx_dust_exposure, max_dust_htlc_exposure_msat);
+					return Err(("Exceeded our tx fee dust exposure limit on counterparty commitment tx", 0x1000|7))
+				}
 			}
 		}