[Custom Transactions] Define the TxBuilder trait

This commit defines the `TxBuilder` trait to give users the ability to
customize the build of the lightning commitment transactions.

The `TxBuilder` trait has a single method,
`build_commitment_transaction`, which builds the commitment transaction,
and populates the output indices of the HTLCs passed in. Note that the
method itself does not impose any sorting.

Cargo.toml

@@ -67,4 +67,5 @@ check-cfg = [
     "cfg(splicing)",
     "cfg(async_payments)",
     "cfg(dual_funding)",
+    "cfg(custom_tx)",
 ]

ci/ci-tests.sh

@@ -135,4 +135,6 @@ RUSTFLAGS="--cfg=trampoline" cargo test --verbose --color always -p lightning
 [ "$CI_MINIMIZE_DISK_USAGE" != "" ] && cargo clean
 RUSTFLAGS="--cfg=async_payments" cargo test --verbose --color always -p lightning
 [ "$CI_MINIMIZE_DISK_USAGE" != "" ] && cargo clean
+RUSTFLAGS="--cfg=custom_tx" cargo test --verbose --color always -p lightning
+[ "$CI_MINIMIZE_DISK_USAGE" != "" ] && cargo clean
 RUSTFLAGS="--cfg=lsps1_service" cargo test --verbose --color always -p lightning-liquidity

lightning/src/sign/mod.rs

@@ -68,6 +68,8 @@ use crate::prelude::*;
 use crate::sign::ecdsa::EcdsaChannelSigner;
 #[cfg(taproot)]
 use crate::sign::taproot::TaprootChannelSigner;
+#[cfg(custom_tx)]
+use crate::sign::tx_builder::SpecTxBuilder;
 use crate::types::features::ChannelTypeFeatures;
 use crate::util::atomic_counter::AtomicCounter;
 use core::convert::TryInto;
@@ -81,6 +83,8 @@ pub(crate) mod type_resolver;
 pub mod ecdsa;
 #[cfg(taproot)]
 pub mod taproot;
+#[cfg(custom_tx)]
+pub mod tx_builder;
 
 /// Information about a spendable output to a P2WSH script.
 ///
@@ -816,6 +820,10 @@ pub trait ChannelSigner {
 	///
 	/// channel_parameters.is_populated() MUST be true.
 	fn provide_channel_parameters(&mut self, channel_parameters: &ChannelTransactionParameters);
+
+	/// Derive a `TxBuilder`
+	#[cfg(custom_tx)]
+	fn derive_tx_builder(&self) -> SpecTxBuilder;
 }
 
 /// Specifies the recipient of an invoice.
@@ -1400,6 +1408,11 @@ impl ChannelSigner for InMemorySigner {
 		assert!(channel_parameters.is_populated(), "Channel parameters must be fully populated");
 		self.channel_parameters = Some(channel_parameters.clone());
 	}
+
+	#[cfg(custom_tx)]
+	fn derive_tx_builder(&self) -> SpecTxBuilder {
+		SpecTxBuilder::default()
+	}
 }
 
 const MISSING_PARAMS_ERR: &'static str =

lightning/src/sign/tx_builder.rs

@@ -0,0 +1,81 @@
+//! Defines the `TxBuilder` trait, and the `SpecTxBuilder` type
+#![allow(dead_code)]
+#![allow(unused_variables)]
+
+use bitcoin::secp256k1::{self, PublicKey, Secp256k1};
+use bitcoin::{Amount, Transaction};
+
+use crate::ln::chan_utils::{ChannelTransactionParameters, HTLCOutputInCommitment, TxCreationKeys};
+use crate::prelude::*;
+
+/// A trait for types that can build commitment transactions, both for the holder, and the counterparty.
+pub trait TxBuilder {
+	/// Set the counterparty static channel data, including basepoints,
+	/// `counterparty_selected`/`holder_selected_contest_delay` and funding outpoint.
+	///
+	/// This data is static, and will never change for a channel once set.
+	///
+	/// channel_parameters.is_populated() MUST be true.
+	fn provide_channel_parameters(&mut self, channel_parameters: &ChannelTransactionParameters);
+	/// Build a commitment transaction, and populate the elements of `htlcs` with their output indices.
+	/// Do not sort `htlcs`; this will be done by the caller as needed.
+	/// This method will be called only after all the channel parameters have been provided via `provide_channel_parameters`.
+	fn build_commitment_transaction(
+		&self, is_holder_tx: bool, commitment_number: u64, per_commitment_point: &PublicKey,
+		to_broadcaster_value_sat: Amount, to_countersignatory_value_sat: Amount,
+		trimmed_value_sat: Amount, htlcs: Vec<&mut HTLCOutputInCommitment>,
+		secp_ctx: &Secp256k1<secp256k1::All>,
+	) -> Transaction;
+}
+
+/// A type that builds commitment transactions according to the Lightning Specification.
+#[derive(Clone, Debug, Default)]
+pub struct SpecTxBuilder {
+	channel_parameters: Option<ChannelTransactionParameters>,
+}
+
+impl TxBuilder for SpecTxBuilder {
+	fn provide_channel_parameters(&mut self, channel_parameters: &ChannelTransactionParameters) {
+		assert!(
+			self.channel_parameters.is_none()
+				|| self.channel_parameters.as_ref().unwrap() == channel_parameters
+		);
+		if self.channel_parameters.is_some() {
+			// The channel parameters were already set and they match, return early.
+			return;
+		}
+		assert!(channel_parameters.is_populated(), "Channel parameters must be fully populated");
+		self.channel_parameters = Some(channel_parameters.clone());
+	}
+	fn build_commitment_transaction(
+		&self, is_holder_tx: bool, commitment_number: u64, per_commitment_point: &PublicKey,
+		to_broadcaster_value_sat: Amount, to_countersignatory_value_sat: Amount,
+		trimmed_value_sat: Amount, htlcs: Vec<&mut HTLCOutputInCommitment>,
+		secp_ctx: &Secp256k1<secp256k1::All>,
+	) -> Transaction {
+		let params = if is_holder_tx {
+			self.channel_parameters.as_ref().unwrap().as_holder_broadcastable()
+		} else {
+			self.channel_parameters.as_ref().unwrap().as_counterparty_broadcastable()
+		};
+		let keys = TxCreationKeys::from_channel_static_keys(
+			per_commitment_point,
+			params.broadcaster_pubkeys(),
+			params.countersignatory_pubkeys(),
+			secp_ctx,
+		);
+		/*
+			let (obscured_commitment_transaction_number, txins) =
+				internal_build_inputs(commitment_number, &params);
+			let txouts = internal_build_outputs(
+				&keys,
+				to_broadcaster_value_sat,
+				to_countersignatory_value_sat,
+				htlcs,
+				&params,
+			);
+			make_transaction(obscured_commitment_transaction_number, txins, txouts)
+		*/
+		todo!();
+	}
+}

lightning/src/util/test_channel_signer.rs

@@ -15,6 +15,8 @@ use crate::ln::channel::{ANCHOR_OUTPUT_VALUE_SATOSHI, MIN_CHAN_DUST_LIMIT_SATOSH
 use crate::ln::channel_keys::HtlcKey;
 use crate::ln::msgs;
 use crate::sign::ecdsa::EcdsaChannelSigner;
+#[cfg(custom_tx)]
+use crate::sign::tx_builder::SpecTxBuilder;
 use crate::sign::{ChannelSigner, InMemorySigner};
 use crate::types::payment::PaymentPreimage;
 
@@ -227,6 +229,11 @@ impl ChannelSigner for TestChannelSigner {
 	fn provide_channel_parameters(&mut self, channel_parameters: &ChannelTransactionParameters) {
 		self.inner.provide_channel_parameters(channel_parameters)
 	}
+
+	#[cfg(custom_tx)]
+	fn derive_tx_builder(&self) -> SpecTxBuilder {
+		SpecTxBuilder::default()
+	}
 }
 
 impl EcdsaChannelSigner for TestChannelSigner {