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Add splice-out support
Update SpliceContribution with a variant used to support splice-out (i.e., removing funds from a channel). The TxOut values must not exceed the users channel balance after accounting for fees and the reserve requirement.
1 parent 3e65d51 commit 16af388

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4 files changed

+165
-74
lines changed

4 files changed

+165
-74
lines changed

lightning/src/ln/channel.rs

Lines changed: 126 additions & 63 deletions
Original file line numberDiff line numberDiff line change
@@ -24,9 +24,9 @@ use bitcoin::hashes::Hash;
2424
use bitcoin::secp256k1::constants::PUBLIC_KEY_SIZE;
2525
use bitcoin::secp256k1::{ecdsa::Signature, Secp256k1};
2626
use bitcoin::secp256k1::{PublicKey, SecretKey};
27-
#[cfg(splicing)]
28-
use bitcoin::Sequence;
2927
use bitcoin::{secp256k1, sighash, TxIn};
28+
#[cfg(splicing)]
29+
use bitcoin::{FeeRate, Sequence};
3030

3131
use crate::chain::chaininterface::{
3232
fee_for_weight, ConfirmationTarget, FeeEstimator, LowerBoundedFeeEstimator,
@@ -5878,20 +5878,53 @@ fn get_v2_channel_reserve_satoshis(channel_value_satoshis: u64, dust_limit_satos
58785878
cmp::min(channel_value_satoshis, cmp::max(q, dust_limit_satoshis))
58795879
}
58805880

5881+
#[cfg(splicing)]
5882+
fn check_splice_contribution_sufficient(
5883+
channel_balance: Amount, contribution: &SpliceContribution, is_initiator: bool,
5884+
funding_feerate: FeeRate,
5885+
) -> Result<Amount, ChannelError> {
5886+
let contribution_amount = contribution.value();
5887+
if contribution_amount < SignedAmount::ZERO {
5888+
let estimated_fee = Amount::from_sat(estimate_v2_funding_transaction_fee(
5889+
is_initiator,
5890+
1, // spends the previous funding output
5891+
Weight::from_wu(FUNDING_TRANSACTION_WITNESS_WEIGHT),
5892+
contribution.outputs(),
5893+
funding_feerate.to_sat_per_kwu() as u32,
5894+
));
5895+
5896+
if channel_balance > contribution_amount.unsigned_abs() + estimated_fee {
5897+
Ok(estimated_fee)
5898+
} else {
5899+
Err(ChannelError::Warn(format!(
5900+
"Available channel balance {} is lower than needed for splicing out {}, considering fees of {}",
5901+
channel_balance, contribution_amount.unsigned_abs(), estimated_fee,
5902+
)))
5903+
}
5904+
} else {
5905+
check_v2_funding_inputs_sufficient(
5906+
contribution_amount.to_sat(),
5907+
contribution.inputs(),
5908+
is_initiator,
5909+
true,
5910+
funding_feerate.to_sat_per_kwu() as u32,
5911+
)
5912+
.map(Amount::from_sat)
5913+
}
5914+
}
5915+
58815916
/// Estimate our part of the fee of the new funding transaction.
58825917
/// input_count: Number of contributed inputs.
58835918
/// witness_weight: The witness weight for contributed inputs.
58845919
#[allow(dead_code)] // TODO(dual_funding): TODO(splicing): Remove allow once used.
58855920
#[rustfmt::skip]
58865921
fn estimate_v2_funding_transaction_fee(
5887-
is_initiator: bool, input_count: usize, witness_weight: Weight,
5922+
is_initiator: bool, input_count: usize, witness_weight: Weight, outputs: &[TxOut],
58885923
funding_feerate_sat_per_1000_weight: u32,
58895924
) -> u64 {
5890-
// Inputs
58915925
let mut weight = (input_count as u64) * BASE_INPUT_WEIGHT;
5892-
5893-
// Witnesses
58945926
weight = weight.saturating_add(witness_weight.to_wu());
5927+
weight = weight.saturating_add(outputs.iter().map(|txout| txout.weight().to_wu()).sum());
58955928

58965929
// If we are the initiator, we must pay for weight of all common fields in the funding transaction.
58975930
if is_initiator {
@@ -5929,7 +5962,7 @@ fn check_v2_funding_inputs_sufficient(
59295962
funding_inputs_len += 1;
59305963
total_input_witness_weight += Weight::from_wu(FUNDING_TRANSACTION_WITNESS_WEIGHT);
59315964
}
5932-
let estimated_fee = estimate_v2_funding_transaction_fee(is_initiator, funding_inputs_len, total_input_witness_weight, funding_feerate_sat_per_1000_weight);
5965+
let estimated_fee = estimate_v2_funding_transaction_fee(is_initiator, funding_inputs_len, total_input_witness_weight, &[], funding_feerate_sat_per_1000_weight);
59335966

59345967
let mut total_input_sats = 0u64;
59355968
for FundingTxInput { utxo, .. } in funding_inputs.iter() {
@@ -5976,6 +6009,9 @@ pub(super) struct FundingNegotiationContext {
59766009
/// The funding inputs we will be contributing to the channel.
59776010
#[allow(dead_code)] // TODO(dual_funding): Remove once contribution to V2 channels is enabled.
59786011
pub our_funding_inputs: Vec<FundingTxInput>,
6012+
/// The funding outputs we will be contributing to the channel.
6013+
#[allow(dead_code)] // TODO(dual_funding): Remove once contribution to V2 channels is enabled.
6014+
pub our_funding_outputs: Vec<TxOut>,
59796015
/// The change output script. This will be used if needed or -- if not set -- generated using
59806016
/// `SignerProvider::get_destination_script`.
59816017
#[allow(dead_code)] // TODO(splicing): Remove once splicing is enabled.
@@ -6005,45 +6041,46 @@ impl FundingNegotiationContext {
60056041
debug_assert!(matches!(context.channel_state, ChannelState::NegotiatingFunding(_)));
60066042
}
60076043

6008-
// Add output for funding tx
60096044
// Note: For the error case when the inputs are insufficient, it will be handled after
60106045
// the `calculate_change_output_value` call below
6011-
let mut funding_outputs = Vec::new();
60126046

60136047
let shared_funding_output = TxOut {
60146048
value: Amount::from_sat(funding.get_value_satoshis()),
60156049
script_pubkey: funding.get_funding_redeemscript().to_p2wsh(),
60166050
};
60176051

60186052
// Optionally add change output
6019-
if self.our_funding_contribution > SignedAmount::ZERO {
6020-
let change_value_opt = calculate_change_output_value(
6053+
let change_value_opt = if self.our_funding_contribution > SignedAmount::ZERO {
6054+
calculate_change_output_value(
60216055
&self,
60226056
self.shared_funding_input.is_some(),
60236057
&shared_funding_output.script_pubkey,
6024-
&funding_outputs,
60256058
context.holder_dust_limit_satoshis,
6026-
)?;
6027-
if let Some(change_value) = change_value_opt {
6028-
let change_script = if let Some(script) = self.change_script {
6029-
script
6030-
} else {
6031-
signer_provider
6032-
.get_destination_script(context.channel_keys_id)
6033-
.map_err(|_err| AbortReason::InternalError("Error getting change script"))?
6034-
};
6035-
let mut change_output =
6036-
TxOut { value: Amount::from_sat(change_value), script_pubkey: change_script };
6037-
let change_output_weight = get_output_weight(&change_output.script_pubkey).to_wu();
6038-
let change_output_fee =
6039-
fee_for_weight(self.funding_feerate_sat_per_1000_weight, change_output_weight);
6040-
let change_value_decreased_with_fee =
6041-
change_value.saturating_sub(change_output_fee);
6042-
// Check dust limit again
6043-
if change_value_decreased_with_fee > context.holder_dust_limit_satoshis {
6044-
change_output.value = Amount::from_sat(change_value_decreased_with_fee);
6045-
funding_outputs.push(change_output);
6046-
}
6059+
)?
6060+
} else {
6061+
None
6062+
};
6063+
6064+
let mut funding_outputs = self.our_funding_outputs;
6065+
6066+
if let Some(change_value) = change_value_opt {
6067+
let change_script = if let Some(script) = self.change_script {
6068+
script
6069+
} else {
6070+
signer_provider
6071+
.get_destination_script(context.channel_keys_id)
6072+
.map_err(|_err| AbortReason::InternalError("Error getting change script"))?
6073+
};
6074+
let mut change_output =
6075+
TxOut { value: Amount::from_sat(change_value), script_pubkey: change_script };
6076+
let change_output_weight = get_output_weight(&change_output.script_pubkey).to_wu();
6077+
let change_output_fee =
6078+
fee_for_weight(self.funding_feerate_sat_per_1000_weight, change_output_weight);
6079+
let change_value_decreased_with_fee = change_value.saturating_sub(change_output_fee);
6080+
// Check dust limit again
6081+
if change_value_decreased_with_fee > context.holder_dust_limit_satoshis {
6082+
change_output.value = Amount::from_sat(change_value_decreased_with_fee);
6083+
funding_outputs.push(change_output);
60476084
}
60486085
}
60496086

@@ -10634,44 +10671,66 @@ where
1063410671
if our_funding_contribution > SignedAmount::MAX_MONEY {
1063510672
return Err(APIError::APIMisuseError {
1063610673
err: format!(
10637-
"Channel {} cannot be spliced; contribution exceeds total bitcoin supply: {}",
10674+
"Channel {} cannot be spliced in; contribution exceeds total bitcoin supply: {}",
1063810675
self.context.channel_id(),
1063910676
our_funding_contribution,
1064010677
),
1064110678
});
1064210679
}
1064310680

10644-
if our_funding_contribution < SignedAmount::ZERO {
10681+
if our_funding_contribution < -SignedAmount::MAX_MONEY {
1064510682
return Err(APIError::APIMisuseError {
1064610683
err: format!(
10647-
"TODO(splicing): Splice-out not supported, only splice in; channel ID {}, contribution {}",
10648-
self.context.channel_id(), our_funding_contribution,
10649-
),
10684+
"Channel {} cannot be spliced out; contribution exhausts total bitcoin supply: {}",
10685+
self.context.channel_id(),
10686+
our_funding_contribution,
10687+
),
1065010688
});
1065110689
}
1065210690

10653-
// TODO(splicing): Once splice-out is supported, check that channel balance does not go below 0
10654-
// (or below channel reserve)
10655-
1065610691
// Note: post-splice channel value is not yet known at this point, counterparty contribution is not known
1065710692
// (Cannot test for miminum required post-splice channel value)
1065810693

10659-
// Check that inputs are sufficient to cover our contribution.
10660-
let _fee = check_v2_funding_inputs_sufficient(
10661-
our_funding_contribution.to_sat(),
10662-
contribution.inputs(),
10663-
true,
10664-
true,
10665-
funding_feerate_per_kw,
10694+
let channel_balance = Amount::from_sat(self.funding.get_value_to_self_msat() / 1000);
10695+
let fees = check_splice_contribution_sufficient(
10696+
channel_balance,
10697+
&contribution,
10698+
true, // is_initiator
10699+
FeeRate::from_sat_per_kwu(funding_feerate_per_kw as u64),
1066610700
)
10667-
.map_err(|err| APIError::APIMisuseError {
10668-
err: format!(
10669-
"Insufficient inputs for splicing; channel ID {}, err {}",
10670-
self.context.channel_id(),
10671-
err,
10672-
),
10701+
.map_err(|e| {
10702+
let splice_type = if our_funding_contribution < SignedAmount::ZERO {
10703+
"spliced out"
10704+
} else {
10705+
"spliced in"
10706+
};
10707+
APIError::APIMisuseError {
10708+
err: format!(
10709+
"Channel {} cannot be {}; {}",
10710+
self.context.channel_id(),
10711+
splice_type,
10712+
e,
10713+
),
10714+
}
1067310715
})?;
1067410716

10717+
// Fees for splice-out are paid from the channel balance whereas fees for splice-in are paid
10718+
// by the funding inputs.
10719+
let adjusted_funding_contribution = if our_funding_contribution < SignedAmount::ZERO {
10720+
let adjusted_funding_contribution = our_funding_contribution
10721+
- fees.to_signed().expect("fees should never exceed splice-out value");
10722+
10723+
// TODO(splicing): Check that channel balance does not go below the channel reserve
10724+
let _post_channel_balance = AddSigned::checked_add_signed(
10725+
channel_balance.to_sat(),
10726+
adjusted_funding_contribution.to_sat(),
10727+
);
10728+
10729+
adjusted_funding_contribution
10730+
} else {
10731+
our_funding_contribution
10732+
};
10733+
1067510734
for FundingTxInput { utxo, prevtx, .. } in contribution.inputs().iter() {
1067610735
const MESSAGE_TEMPLATE: msgs::TxAddInput = msgs::TxAddInput {
1067710736
channel_id: ChannelId([0; 32]),
@@ -10693,14 +10752,15 @@ where
1069310752
}
1069410753

1069510754
let prev_funding_input = self.funding.to_splice_funding_input();
10696-
let (our_funding_inputs, change_script) = contribution.into_tx_parts();
10755+
let (our_funding_inputs, our_funding_outputs, change_script) = contribution.into_tx_parts();
1069710756
let funding_negotiation_context = FundingNegotiationContext {
1069810757
is_initiator: true,
10699-
our_funding_contribution,
10758+
our_funding_contribution: adjusted_funding_contribution,
1070010759
funding_tx_locktime: LockTime::from_consensus(locktime),
1070110760
funding_feerate_sat_per_1000_weight: funding_feerate_per_kw,
1070210761
shared_funding_input: Some(prev_funding_input),
1070310762
our_funding_inputs,
10763+
our_funding_outputs,
1070410764
change_script,
1070510765
};
1070610766

@@ -10716,7 +10776,7 @@ where
1071610776

1071710777
Ok(msgs::SpliceInit {
1071810778
channel_id: self.context.channel_id,
10719-
funding_contribution_satoshis: our_funding_contribution.to_sat(),
10779+
funding_contribution_satoshis: adjusted_funding_contribution.to_sat(),
1072010780
funding_feerate_per_kw,
1072110781
locktime,
1072210782
funding_pubkey,
@@ -10825,6 +10885,7 @@ where
1082510885
funding_feerate_sat_per_1000_weight: msg.funding_feerate_per_kw,
1082610886
shared_funding_input: Some(prev_funding_input),
1082710887
our_funding_inputs: Vec::new(),
10888+
our_funding_outputs: Vec::new(),
1082810889
change_script: None,
1082910890
};
1083010891

@@ -12523,6 +12584,7 @@ where
1252312584
funding_feerate_sat_per_1000_weight,
1252412585
shared_funding_input: None,
1252512586
our_funding_inputs: funding_inputs,
12587+
our_funding_outputs: Vec::new(),
1252612588
change_script: None,
1252712589
};
1252812590
let chan = Self {
@@ -12677,6 +12739,7 @@ where
1267712739
funding_feerate_sat_per_1000_weight: msg.funding_feerate_sat_per_1000_weight,
1267812740
shared_funding_input: None,
1267912741
our_funding_inputs: our_funding_inputs.clone(),
12742+
our_funding_outputs: Vec::new(),
1268012743
change_script: None,
1268112744
};
1268212745
let shared_funding_output = TxOut {
@@ -12702,7 +12765,7 @@ where
1270212765
inputs_to_contribute,
1270312766
shared_funding_input: None,
1270412767
shared_funding_output: SharedOwnedOutput::new(shared_funding_output, our_funding_contribution_sats),
12705-
outputs_to_contribute: Vec::new(),
12768+
outputs_to_contribute: funding_negotiation_context.our_funding_outputs.clone(),
1270612769
}
1270712770
).map_err(|err| {
1270812771
let reason = ClosureReason::ProcessingError { err: err.to_string() };
@@ -15871,31 +15934,31 @@ mod tests {
1587115934

1587215935
// 2 inputs with weight 300, initiator, 2000 sat/kw feerate
1587315936
assert_eq!(
15874-
estimate_v2_funding_transaction_fee(true, 2, Weight::from_wu(300), 2000),
15937+
estimate_v2_funding_transaction_fee(true, 2, Weight::from_wu(300), &[], 2000),
1587515938
1668
1587615939
);
1587715940

1587815941
// higher feerate
1587915942
assert_eq!(
15880-
estimate_v2_funding_transaction_fee(true, 2, Weight::from_wu(300), 3000),
15943+
estimate_v2_funding_transaction_fee(true, 2, Weight::from_wu(300), &[], 3000),
1588115944
2502
1588215945
);
1588315946

1588415947
// only 1 input
1588515948
assert_eq!(
15886-
estimate_v2_funding_transaction_fee(true, 1, Weight::from_wu(300), 2000),
15949+
estimate_v2_funding_transaction_fee(true, 1, Weight::from_wu(300), &[], 2000),
1588715950
1348
1588815951
);
1588915952

1589015953
// 0 input weight
1589115954
assert_eq!(
15892-
estimate_v2_funding_transaction_fee(true, 1, Weight::from_wu(0), 2000),
15955+
estimate_v2_funding_transaction_fee(true, 1, Weight::from_wu(0), &[], 2000),
1589315956
748
1589415957
);
1589515958

1589615959
// not initiator
1589715960
assert_eq!(
15898-
estimate_v2_funding_transaction_fee(false, 1, Weight::from_wu(0), 2000),
15961+
estimate_v2_funding_transaction_fee(false, 1, Weight::from_wu(0), &[], 2000),
1589915962
320
1590015963
);
1590115964
}

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