-f Constify HTLC buffer

Author: Antoine Riard

lightning/src/ln/channel.rs

@@ -355,6 +355,18 @@ pub const FEE_SPIKE_BUFFER_FEE_INCREASE_MULTIPLE: u64 = 2;
 #[cfg(not(fuzzing))]
 const FEE_SPIKE_BUFFER_FEE_INCREASE_MULTIPLE: u64 = 2;
 
+
+/// In case of a concurrent update_add_htlc proposed by our counterparty, we might
+/// not have enough balance value remaining to cover the onchain cost of this new
+/// HTLC weight. If this happens, our counterparty fails the reception of our
+/// commitment_signed including this new HTLC due to infringement on the channel
+/// reserve.
+/// To prevent this case, we compute our outbound update_fee with an HTLC buffer of
+/// size 2. However, if the number of concurrent update_add_htlc is higher, this still
+/// leads to a channel force-close. Ultimately, this is an issue coming from the
+/// design of LN state machines, allowing asynchronous updates.
+const CONCURRENT_INBOUND_HTLC_FEE_BUFFER: u32 = 2;
+
 // TODO: We should refactor this to be an Inbound/OutboundChannel until initial setup handshaking
 // has been completed, and then turn into a Channel to get compiler-time enforcement of things like
 // calling channel_id() before we're set up or things like get_outbound_funding_signed on an
@@ -2974,16 +2986,7 @@ impl<Signer: Sign> Channel<Signer> {
 		// Before proposing a feerate update, check that we can actually afford the new fee.
 		let inbound_stats = self.get_inbound_pending_htlc_stats(Some(feerate_per_kw));
 		let outbound_stats = self.get_outbound_pending_htlc_stats(Some(feerate_per_kw));
-		// In case of a concurrent update_add_htlc proposed by our counterparty, we might
-		// not have enough balance value remaining to cover the onchain cost of this new
-		// HTLC weight. If this happens, our counterparty fails the reception of our
-		// commitment_signed including this new HTLC due to infringement on the channel
-		// reserve.
-		// To prevent this case, we compute our outbound update_fee with an HTLC buffer of
-		// size 2. However, if the number of concurrent update_add_htlc is higher, this still
-		// leads to a channel force-close. Ultimately, this is an issue coming from the
-		// design of LN state machines, allowing asynchronous updates.
-		let total_fee_sat = Channel::<Signer>::commit_tx_fee_sat(feerate_per_kw, (inbound_stats.pending_htlcs + /* HTLC feerate buffer */ 2 + outbound_stats.pending_htlcs) as usize);
+		let total_fee_sat = Channel::<Signer>::commit_tx_fee_sat(feerate_per_kw, (inbound_stats.pending_htlcs + CONCURRENT_INBOUND_HTLC_FEE_BUFFER + outbound_stats.pending_htlcs) as usize);
 		let keys = if let Ok(keys) = self.build_holder_transaction_keys(self.cur_holder_commitment_transaction_number) { keys } else { return None; };
 		let holder_balance_msat = self.build_commitment_transaction(self.cur_holder_commitment_transaction_number, &keys, true, true, logger).4;
 		if holder_balance_msat * 1000 < total_fee_sat + self.counterparty_selected_channel_reserve_satoshis.unwrap() {