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Account for dust in closing channel fee #2344

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Account for dust in closing channel fee #2344

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48 changes: 30 additions & 18 deletions lightning/src/ln/channel.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -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.
Expand Down Expand Up @@ -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()
Expand Down