Add splice-out support #3979

jkczyz · 2025-07-31T16:11:16Z

Splice-in support was added in #3736. This PR expands ChannelManager::splice_channel to support splice-out (i.e., removing funds from a channel). This is accomplished by adding a FundingTxContributions enum to cover both use cases.

Depends on #3736.

ldk-reviews-bot · 2025-07-31T16:11:18Z

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codecov · 2025-07-31T16:27:41Z

Codecov Report

❌ Patch coverage is 70.17544% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.93%. Comparing base (1e1d300) to head (381fba6).

Files with missing lines	Patch %	Lines
lightning/src/ln/msgs.rs	29.41%	15 Missing and 9 partials ⚠️
lightning/src/ln/channel.rs	77.77%	7 Missing and 1 partial ⚠️
lightning/src/ln/channelmanager.rs	0.00%	1 Missing ⚠️
lightning/src/ln/interactivetxs.rs	97.61%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3979      +/-   ##
==========================================
- Coverage   88.94%   88.93%   -0.02%     
==========================================
  Files         174      174              
  Lines      124200   124211      +11     
  Branches   124200   124211      +11     
==========================================
- Hits       110471   110462       -9     
- Misses      11253    11261       +8     
- Partials     2476     2488      +12

Flag	Coverage Δ
fuzzing	`22.63% <2.53%> (-0.01%)`	⬇️
tests	`88.75% <68.42%> (-0.02%)`	⬇️

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Splice contributions should never exceed the total bitcoin supply. This check prevents a potential overflow when converting the contribution from sats to msats. The commit additionally begins to store the contribution using SignedAmount.

…text Once the counterparty supplies their funding contribution, there is no longer a need to store it in FundingNegotiationContext as it will have already been used to create a FundingScope.

Specify what inputs and outputs to contribute to a funding transaction using an enum. This will make the splice-in and upcoming splice-out use cases explicit.

TransactionU16LenLimited was used to limit Transaction serialization size to u16::MAX. This was because messages can not be longer than u16::MAX bytes when serialized for the transport layer. However, this limit doesn't take into account other fields in a message containing a Transaction, including the length of the transaction itself. Remove TransactionU16LenLimited and instead check any user supplied transactions in the context of the enclosing message (e.g. TxAddInput).

In preparation for having ChannelManager::splice_channel take FundingTxContributions, add a weight to the FundingTxContributions::InputsOnly, which supports the splice-in use case.

Using FundingTxContributions for ChannelManager::splice_channel allows supporting both splice-in and splice-out using the same method, with a descriptive enum for the contributions.

Update FundingTxContributions 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.

ldk-reviews-bot · 2025-08-04T00:01:04Z

🔔 1st Reminder

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wpaulino · 2025-08-04T19:09:02Z

lightning/src/ln/channel.rs

+pub enum FundingTxContributions {
+	/// When only inputs -- except for a possible change output -- are contributed to the funding
+	/// transaction. This must correspond to a positive contribution amount.
+	InputsOnly {


We should also include the contribution amount in each variant as they'll be slightly different: InputsOnly must be > 0, OutputsOnly must be < 0, and the mixed case can be SignedAmount.

wpaulino · 2025-08-04T19:12:06Z

lightning/src/ln/channel.rs

@@ -6071,6 +6074,30 @@ impl FundingNegotiationContext {
 	}
 }

+/// The components of a funding transaction that are contributed by one party.
+pub enum FundingTxContributions {


Does this mean we'll have higher-level variants for splicing and dual-funding that map to this one? I would like being able to name these SpliceIn/Out for splicing.

wpaulino · 2025-08-04T19:15:47Z

lightning/src/ln/channel.rs

+				shared_input_txid: None,
+			};
+			let message_len = MESSAGE_TEMPLATE.serialized_length() + tx.serialized_length();
+			if message_len > u16::MAX as usize {


Nit: use LN_MAX_MSG_LEN

wpaulino · 2025-08-04T19:16:09Z

lightning/src/ln/channel.rs

+			let message_len = MESSAGE_TEMPLATE.serialized_length() + tx.serialized_length();
+			if message_len > u16::MAX as usize {
+				return Err(APIError::APIMisuseError {
+					err: format!("Funding input's prevtx is too large for tx_add_input"),


Nit: include the index or outpoint in the error

wpaulino · 2025-08-04T19:19:38Z

lightning/src/ln/channel.rs

@@ -6079,7 +6081,7 @@ pub enum FundingTxContributions {
 	InputsOnly {
 		/// The inputs used to meet the contributed amount. Any excess amount will be sent to a
 		/// change output.
-		inputs: Vec<(TxIn, Transaction)>,
+		inputs: Vec<(TxIn, Transaction, Weight)>,


Let's make this a struct now so we can properly document each field. The weight here should be the witness_weight for the input to be spent. We may even be able to reuse bump_transaction::Utxo.

wpaulino · 2025-08-04T19:22:14Z

lightning/src/ln/interactivetxs.rs

@@ -1894,6 +1894,7 @@ pub(super) fn calculate_change_output_value(
 			.get(txin.previous_output.vout as usize)
 			.ok_or(AbortReason::PrevTxOutInvalid)?;
 		total_input_satoshis = total_input_satoshis.saturating_add(output.value.to_sat());
+		// FIXME: Can we use the Weight from context.funding_tx_contributions.inputs()?


Yes, but that weight is only for the witness, so we'd also have to account for BASE_INPUT_WEIGHT and EMPTY_SCRIPT_SIG_WEIGHT for each input.

wpaulino · 2025-08-04T19:24:21Z

lightning/src/ln/channel.rs

+	},
+	/// When both inputs and outputs are contributed to the funding transaction.
+	#[cfg(test)]
+	InputsAndOutputs {


I think we should wait to add this as a follow-up

wpaulino · 2025-08-04T19:26:57Z

lightning/src/ln/channel.rs

+				our_funding_contribution_satoshis,
+			);
+			// FIXME: Should we check value_to_self instead? Do HTLCs need to be accounted for?
+			// FIXME: Check that we can pay for the outputs from the channel value?


We probably don't even need our_funding_contribution for a splice out, we can just assume it from adding all the output values

wpaulino · 2025-08-04T19:28:18Z

lightning/src/ln/channel.rs

+				self.funding.get_value_satoshis(),
+				our_funding_contribution_satoshis,
+			);
+			// FIXME: Should we check value_to_self instead? Do HTLCs need to be accounted for?


Definitely value_to_self, and we need to make sure we stay above the reserve and can still afford fees if we're the channel initiator

ldk-reviews-bot · 2025-08-04T19:28:52Z

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jkczyz requested a review from wpaulino July 31, 2025 16:11

jkczyz marked this pull request as draft July 31, 2025 16:11

jkczyz added this to Weekly Goals Jul 31, 2025

jkczyz self-assigned this Jul 31, 2025

jkczyz mentioned this pull request Jul 31, 2025

Dual-funded channels and Splicing Project Tracking #1621

Open

7 tasks

jkczyz force-pushed the 2025-07-splice-out branch 2 times, most recently from acab158 to 2d39059 Compare July 31, 2025 22:15

jkczyz added 7 commits July 31, 2025 19:22

Remove their_funding_contribution_satoshis from FundingNegotiationCon…

fe6f003

…text Once the counterparty supplies their funding contribution, there is no longer a need to store it in FundingNegotiationContext as it will have already been used to create a FundingScope.

Use a FundingTxContributions enum for funding iputs and change

95e4ac3

Specify what inputs and outputs to contribute to a funding transaction using an enum. This will make the splice-in and upcoming splice-out use cases explicit.

Include prevtx weight in FundingTxContributions::InputsOnly

283e3c6

In preparation for having ChannelManager::splice_channel take FundingTxContributions, add a weight to the FundingTxContributions::InputsOnly, which supports the splice-in use case.

Pass FundingTxContributions to ChannelManager::splice_channel

0b187cf

Using FundingTxContributions for ChannelManager::splice_channel allows supporting both splice-in and splice-out using the same method, with a descriptive enum for the contributions.

Add splice-out support

381fba6

Update FundingTxContributions 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.

jkczyz force-pushed the 2025-07-splice-out branch from 2d39059 to 381fba6 Compare August 1, 2025 00:23

jkczyz marked this pull request as ready for review August 1, 2025 00:23

wpaulino reviewed Aug 4, 2025

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