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Dual funding extension: begin_interactive_funding_tx_construction #3443

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Apr 2, 2025
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Dual funding extension: begin_interactive_funding_tx_construction #3443

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0f6c1d0
Add begin_interactive_funding_tx_construction()
optout21 Mar 9, 2025
f1a8602
Remove OutputOwned::SharedControlFullyOwned option
optout21 Apr 1, 2025
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118 changes: 108 additions & 10 deletions lightning/src/ln/channel.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -10,7 +10,7 @@
use bitcoin::amount::Amount;
use bitcoin::constants::ChainHash;
use bitcoin::script::{Script, ScriptBuf, Builder, WScriptHash};
use bitcoin::transaction::{Transaction, TxIn};
use bitcoin::transaction::{Transaction, TxIn, TxOut};
use bitcoin::sighash::EcdsaSighashType;
use bitcoin::consensus::encode;
use bitcoin::absolute::LockTime;
Expand All @@ -30,9 +30,9 @@ use crate::ln::types::ChannelId;
use crate::types::payment::{PaymentPreimage, PaymentHash};
use crate::types::features::{ChannelTypeFeatures, InitFeatures};
use crate::ln::interactivetxs::{
get_output_weight, HandleTxCompleteValue, HandleTxCompleteResult, InteractiveTxConstructor,
InteractiveTxConstructorArgs, InteractiveTxSigningSession, InteractiveTxMessageSendResult,
TX_COMMON_FIELDS_WEIGHT,
calculate_change_output_value, get_output_weight, AbortReason, HandleTxCompleteValue, HandleTxCompleteResult, InteractiveTxConstructor,
InteractiveTxConstructorArgs, InteractiveTxMessageSend, InteractiveTxSigningSession, InteractiveTxMessageSendResult,
OutputOwned, SharedOwnedOutput, TX_COMMON_FIELDS_WEIGHT,
};
use crate::ln::msgs;
use crate::ln::msgs::{ClosingSigned, ClosingSignedFeeRange, DecodeError, OnionErrorPacket};
Expand Down Expand Up @@ -2220,6 +2220,95 @@ impl<SP: Deref> InitialRemoteCommitmentReceiver<SP> for FundedChannel<SP> where
}

impl<SP: Deref> PendingV2Channel<SP> where SP::Target: SignerProvider {
/// Prepare and start interactive transaction negotiation.
/// `change_destination_opt` - Optional destination for optional change; if None,
/// default destination address is used.
/// If error occurs, it is caused by our side, not the counterparty.
#[allow(dead_code)] // TODO(dual_funding): Remove once contribution to V2 channels is enabled
fn begin_interactive_funding_tx_construction<ES: Deref>(
&mut self, signer_provider: &SP, entropy_source: &ES, holder_node_id: PublicKey,
change_destination_opt: Option<ScriptBuf>,
) -> Result<Option<InteractiveTxMessageSend>, AbortReason>
where ES::Target: EntropySource
{
debug_assert!(matches!(self.context.channel_state, ChannelState::NegotiatingFunding(_)));
debug_assert!(self.interactive_tx_constructor.is_none());

let mut funding_inputs = Vec::new();
mem::swap(&mut self.dual_funding_context.our_funding_inputs, &mut funding_inputs);

// TODO(splicing): Add prev funding tx as input, must be provided as a parameter

// Add output for funding tx
// Note: For the error case when the inputs are insufficient, it will be handled after
// the `calculate_change_output_value` call below
let mut funding_outputs = Vec::new();
let mut expected_remote_shared_funding_output = None;

let shared_funding_output = TxOut {
value: Amount::from_sat(self.funding.get_value_satoshis()),
script_pubkey: self.funding.get_funding_redeemscript().to_p2wsh(),
};

if self.funding.is_outbound() {
funding_outputs.push(
OutputOwned::Shared(SharedOwnedOutput::new(
shared_funding_output, self.dual_funding_context.our_funding_satoshis,
))
);
} else {
let TxOut { value, script_pubkey } = shared_funding_output;
expected_remote_shared_funding_output = Some((script_pubkey, value.to_sat()));
}

// Optionally add change output
let change_script = if let Some(script) = change_destination_opt {
script
} else {
signer_provider.get_destination_script(self.context.channel_keys_id)
.map_err(|_err| AbortReason::InternalError("Error getting destination script"))?
};
let change_value_opt = calculate_change_output_value(
self.funding.is_outbound(), self.dual_funding_context.our_funding_satoshis,
&funding_inputs, &funding_outputs,
self.dual_funding_context.funding_feerate_sat_per_1000_weight,
change_script.minimal_non_dust().to_sat(),
)?;
if let Some(change_value) = change_value_opt {
let mut change_output = TxOut {
value: Amount::from_sat(change_value),
script_pubkey: change_script,
};
let change_output_weight = get_output_weight(&change_output.script_pubkey).to_wu();
let change_output_fee = fee_for_weight(self.dual_funding_context.funding_feerate_sat_per_1000_weight, change_output_weight);
let change_value_decreased_with_fee = change_value.saturating_sub(change_output_fee);
// Check dust limit again
if change_value_decreased_with_fee > self.context.holder_dust_limit_satoshis {
change_output.value = Amount::from_sat(change_value_decreased_with_fee);
funding_outputs.push(OutputOwned::Single(change_output));
}
}

let constructor_args = InteractiveTxConstructorArgs {
entropy_source,
holder_node_id,
counterparty_node_id: self.context.counterparty_node_id,
channel_id: self.context.channel_id(),
feerate_sat_per_kw: self.dual_funding_context.funding_feerate_sat_per_1000_weight,
is_initiator: self.funding.is_outbound(),
funding_tx_locktime: self.dual_funding_context.funding_tx_locktime,
inputs_to_contribute: funding_inputs,
outputs_to_contribute: funding_outputs,
expected_remote_shared_funding_output,
};
let mut tx_constructor = InteractiveTxConstructor::new(constructor_args)?;
let msg = tx_constructor.take_initiator_first_message();

self.interactive_tx_constructor = Some(tx_constructor);

Ok(msg)
}

pub fn tx_add_input(&mut self, msg: &msgs::TxAddInput) -> InteractiveTxMessageSendResult {
InteractiveTxMessageSendResult(match &mut self.interactive_tx_constructor {
Some(ref mut tx_constructor) => tx_constructor.handle_tx_add_input(msg).map_err(
Expand Down Expand Up @@ -4849,6 +4938,9 @@ fn check_v2_funding_inputs_sufficient(
pub(super) struct DualFundingChannelContext {
/// The amount in satoshis we will be contributing to the channel.
pub our_funding_satoshis: u64,
/// The amount in satoshis our counterparty will be contributing to the channel.
#[allow(dead_code)] // TODO(dual_funding): Remove once contribution to V2 channels is enabled.
pub their_funding_satoshis: Option<u64>,
/// The funding transaction locktime suggested by the initiator. If set by us, it is always set
/// to the current block height to align incentives against fee-sniping.
pub funding_tx_locktime: LockTime,
Expand All @@ -4860,6 +4952,8 @@ pub(super) struct DualFundingChannelContext {
/// Note that the `our_funding_satoshis` field is equal to the total value of `our_funding_inputs`
/// minus any fees paid for our contributed weight. This means that change will never be generated
/// and the maximum value possible will go towards funding the channel.
///
/// Note that this field may be emptied once the interactive negotiation has been started.
#[allow(dead_code)] // TODO(dual_funding): Remove once contribution to V2 channels is enabled.
pub our_funding_inputs: Vec<(TxIn, TransactionU16LenLimited)>,
}
Expand Down Expand Up @@ -9829,16 +9923,19 @@ impl<SP: Deref> PendingV2Channel<SP> where SP::Target: SignerProvider {
unfunded_channel_age_ticks: 0,
holder_commitment_point: HolderCommitmentPoint::new(&context.holder_signer, &context.secp_ctx),
};
let dual_funding_context = DualFundingChannelContext {
our_funding_satoshis: funding_satoshis,
// TODO(dual_funding) TODO(splicing) Include counterparty contribution, once that's enabled
their_funding_satoshis: None,
funding_tx_locktime,
funding_feerate_sat_per_1000_weight,
our_funding_inputs: funding_inputs,
};
let chan = Self {
funding,
context,
unfunded_context,
dual_funding_context: DualFundingChannelContext {
our_funding_satoshis: funding_satoshis,
funding_tx_locktime,
funding_feerate_sat_per_1000_weight,
our_funding_inputs: funding_inputs,
},
dual_funding_context,
interactive_tx_constructor: None,
interactive_tx_signing_session: None,
};
Expand Down Expand Up @@ -9980,6 +10077,7 @@ impl<SP: Deref> PendingV2Channel<SP> where SP::Target: SignerProvider {

let dual_funding_context = DualFundingChannelContext {
our_funding_satoshis: our_funding_satoshis,
their_funding_satoshis: Some(msg.common_fields.funding_satoshis),
funding_tx_locktime: LockTime::from_consensus(msg.locktime),
funding_feerate_sat_per_1000_weight: msg.funding_feerate_sat_per_1000_weight,
our_funding_inputs: our_funding_inputs.clone(),
Expand Down
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