Account for dust in closing channel fee

Before we were ignoring dust when creating the closing channel fee. Now
if an output would be dust, we will recalculate the fee with the missing
output. This should fix some force closes that happen with lnd nodes.

Author: benthecarman

lightning/src/ln/channel.rs

@@ -4396,6 +4396,22 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 		}
 	}
 
+	fn calculate_proposed_fee_limits(&self, proposed_feerate: u32, normal_feerate: u32, proposed_max_feerate: u32, a_scriptpubkey: Option<&Script>, b_scriptpubkey: Option<&Script>) -> (u64, u64) {
+		let tx_weight = self.get_closing_transaction_weight(a_scriptpubkey, b_scriptpubkey);
+		let proposed_total_fee_satoshis = proposed_feerate as u64 * tx_weight / 1000;
+		let proposed_max_total_fee_satoshis = if self.is_outbound() {
+			// We always add force_close_avoidance_max_fee_satoshis to our normal
+			// feerate-calculated fee, but allow the max to be overridden if we're using a
+			// target feerate-calculated fee.
+			cmp::max(normal_feerate as u64 * tx_weight / 1000 + self.config.options.force_close_avoidance_max_fee_satoshis,
+			         proposed_max_feerate as u64 * tx_weight / 1000)
+		} else {
+			self.channel_value_satoshis - (self.value_to_self_msat + 999) / 1000
+		};
+
+		(proposed_total_fee_satoshis, proposed_max_total_fee_satoshis)
+	}
+
 	/// Calculates and returns our minimum and maximum closing transaction fee amounts, in whole
 	/// satoshis. The amounts remain consistent unless a peer disconnects/reconnects or we restart,
 	/// at which point they will be recalculated.
@@ -4424,24 +4440,20 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 			proposed_max_feerate = cmp::max(proposed_max_feerate, min_feerate);
 		}
 
-		// Note that technically we could end up with a lower minimum fee if one sides' balance is
-		// below our dust limit, causing the output to disappear. We don't bother handling this
-		// case, however, as this should only happen if a channel is closed before any (material)
-		// payments have been made on it. This may cause slight fee overpayment and/or failure to
-		// come to consensus with our counterparty on appropriate fees, however it should be a
-		// relatively rare case. We can revisit this later, though note that in order to determine
-		// if the funders' output is dust we have to know the absolute fee we're going to use.
-		let tx_weight = self.get_closing_transaction_weight(Some(&self.get_closing_scriptpubkey()), Some(self.counterparty_shutdown_scriptpubkey.as_ref().unwrap()));
-		let proposed_total_fee_satoshis = proposed_feerate as u64 * tx_weight / 1000;
-		let proposed_max_total_fee_satoshis = if self.is_outbound() {
-				// We always add force_close_avoidance_max_fee_satoshis to our normal
-				// feerate-calculated fee, but allow the max to be overridden if we're using a
-				// target feerate-calculated fee.
-				cmp::max(normal_feerate as u64 * tx_weight / 1000 + self.config.options.force_close_avoidance_max_fee_satoshis,
-					proposed_max_feerate as u64 * tx_weight / 1000)
-			} else {
-				self.channel_value_satoshis - (self.value_to_self_msat + 999) / 1000
-			};
+		let (mut proposed_total_fee_satoshis, mut proposed_max_total_fee_satoshis) =
+			self.calculate_proposed_fee_limits(proposed_feerate, normal_feerate, proposed_max_feerate, Some(&self.get_closing_scriptpubkey()), Some(self.counterparty_shutdown_scriptpubkey.as_ref().unwrap()));
+
+		let (closing_tx, _) = self.build_closing_transaction(proposed_total_fee_satoshis, false);
+
+		// if one of the outputs is dust, we need to recalculate the fee limits to account for the
+		// dust output being removed
+		if closing_tx.to_holder_value_sat() == 0 {
+			(proposed_total_fee_satoshis, proposed_max_total_fee_satoshis) =
+				self.calculate_proposed_fee_limits(proposed_feerate, normal_feerate, proposed_max_feerate, Some(&self.get_closing_scriptpubkey()), None);
+		} else if closing_tx.to_counterparty_value_sat() == 0 {
+			(proposed_total_fee_satoshis, proposed_max_total_fee_satoshis) =
+				self.calculate_proposed_fee_limits(proposed_feerate, normal_feerate, proposed_max_feerate, None, Some(self.counterparty_shutdown_scriptpubkey.as_ref().unwrap()));
+		}
 
 		self.closing_fee_limits = Some((proposed_total_fee_satoshis, proposed_max_total_fee_satoshis));
 		self.closing_fee_limits.clone().unwrap()