Add NextCommitmentStats::get_balances_including_fee

`NextCommitmentStats` provides the commitment transaction fee as a
separate value to assist with applying a multiplier on it in
`can_accept_incoming_htlc`.

Nonetheless in most cases, we want the balances to include the
commitment transaction fee, so here we add a helper that gives us these
balances.

Also make the style of `tx_builder::subtract_addl_outputs` consistent
with `get_balances_including_fee`.

Author: tankyleo

lightning/src/sign/tx_builder.rs

@@ -35,6 +35,7 @@ impl HTLCAmountDirection {
 }
 
 pub(crate) struct NextCommitmentStats {
+	pub is_outbound_from_holder: bool,
 	pub inbound_htlcs_count: usize,
 	pub inbound_htlcs_value_msat: u64,
 	pub holder_balance_before_fee_msat: Option<u64>,
@@ -48,6 +49,26 @@ pub(crate) struct NextCommitmentStats {
 	pub extra_nondust_htlc_on_counterparty_tx_dust_exposure_msat: Option<u64>,
 }
 
+impl NextCommitmentStats {
+	pub(crate) fn get_balances_including_fee_msat(&self) -> (Option<u64>, Option<u64>) {
+		if self.is_outbound_from_holder {
+			(
+				self.holder_balance_before_fee_msat.and_then(|balance_msat| {
+					balance_msat.checked_sub(self.commit_tx_fee_sat * 1000)
+				}),
+				self.counterparty_balance_before_fee_msat,
+			)
+		} else {
+			(
+				self.holder_balance_before_fee_msat,
+				self.counterparty_balance_before_fee_msat.and_then(|balance_msat| {
+					balance_msat.checked_sub(self.commit_tx_fee_sat * 1000)
+				}),
+			)
+		}
+	}
+}
+
 fn excess_fees_on_counterparty_tx_dust_exposure_msat(
 	next_commitment_htlcs: &[HTLCAmountDirection], dust_buffer_feerate: u32, excess_feerate: u32,
 	counterparty_dust_limit_satoshis: u64, dust_htlc_exposure_msat: u64,
@@ -126,21 +147,19 @@ fn subtract_addl_outputs(
 	// commitment transaction *before* checking whether the remote party's balance is enough to
 	// cover the total anchor sum.
 
-	let local_balance_before_fee_msat = if is_outbound_from_holder {
-		value_to_self_after_htlcs_msat
-			.and_then(|balance_msat| balance_msat.checked_sub(total_anchors_sat * 1000))
-	} else {
-		value_to_self_after_htlcs_msat
-	};
-
-	let remote_balance_before_fee_msat = if !is_outbound_from_holder {
-		value_to_remote_after_htlcs_msat
-			.and_then(|balance_msat| balance_msat.checked_sub(total_anchors_sat * 1000))
+	if is_outbound_from_holder {
+		(
+			value_to_self_after_htlcs_msat
+				.and_then(|balance_msat| balance_msat.checked_sub(total_anchors_sat * 1000)),
+			value_to_remote_after_htlcs_msat,
+		)
 	} else {
-		value_to_remote_after_htlcs_msat
-	};
-
-	(local_balance_before_fee_msat, remote_balance_before_fee_msat)
+		(
+			value_to_self_after_htlcs_msat,
+			value_to_remote_after_htlcs_msat
+				.and_then(|balance_msat| balance_msat.checked_sub(total_anchors_sat * 1000)),
+		)
+	}
 }
 
 fn get_dust_buffer_feerate(feerate_per_kw: u32) -> u32 {
@@ -280,6 +299,7 @@ impl TxBuilder for SpecTxBuilder {
 			};
 
 		NextCommitmentStats {
+			is_outbound_from_holder,
 			inbound_htlcs_count,
 			inbound_htlcs_value_msat,
 			holder_balance_before_fee_msat,