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 81.42857% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.94%. Comparing base (1e1d300) to head (56ed8ba).
⚠️ Report is 36 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/msgs.rs	29.41%	15 Missing and 9 partials ⚠️
lightning/src/ln/channel.rs	98.00%	0 Missing and 1 partial ⚠️
lightning/src/ln/interactivetxs.rs	97.61%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3979      +/-   ##
==========================================
- Coverage   88.94%   88.94%   -0.01%     
==========================================
  Files         174      174              
  Lines      124200   124575     +375     
  Branches   124200   124575     +375     
==========================================
+ Hits       110471   110797     +326     
- Misses      11253    11277      +24     
- Partials     2476     2501      +25

Flag	Coverage Δ
fuzzing	`22.18% <22.00%> (-0.45%)`	⬇️
tests	`88.76% <80.00%> (-0.01%)`	⬇️

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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.

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.

Done. Just used Amount for both variant and added a value method to return a SignedAmount for internal use. A fixup removes Amount from SpliceOut since it can be summed from the outputs as mentioned in another comment.

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.

Discussed offline. Dropped FundingTxContributions in favor of SplicingContribution with SpliceIn and SpliceOut variants. FundingNegotiationContext now holds the inputs as before as well as the outputs.

lightning/src/ln/channel.rs

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.

We may even be able to reuse bump_transaction::Utxo.

Hmm... that doesn't have a TxIn. We also need the full previous transaction in InteractiveTxConstructor to use as prevtx in tx_add_input.

We would just need to track TxIn::sequence and prevtx separately if we choose to go with it

lightning/src/ln/interactivetxs.rs

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

Added earlier because the tests needed a FundingTxContributions with both inputs and outputs. But no longer needed now that FundingNegotiationContext has individual fields.

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

Ah, good point. For paying for the outputs, seems we should actually use check_v2_funding_inputs_sufficient for both splice-in and splice-out and account for outputs?

Ah, actually that only looks at the input from the previous funding transaction for weight, not amount.

Re-wrote this to ensure fees for the funding outputs are accounted for. One thing of note is that our_funding_contribution for a splice-out needs to be adjusted by the fees to be paid (i.e., fees need to be subtracted from the negative our_funding_contribution to increase the amount being removed from the channel).

Also, a bit of an oversight, but we don't yet support accepting a splice-out. Technically, it could contain both inputs and outputs even if the net amount is negative. But I think we shouldn't care as long as we check everything is sane upon exchanging tx_complete.

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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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).

ChannelManager::splice_channel takes witness weights with the funding inputs. Storing these in FundingNegotiationContext allows us to use them when calculating the change output and include them in a common struct used for initiating a splice-in. In preparation for having ChannelManager::splice_channel take FundingTxContributions, add a weight to the FundingTxContributions::InputsOnly, which supports the splice-in use case.

The funding inputs used for splicing and v2 channel establishment are passed as a tuple of txin, prevtx, and witness weight. Add a struct so that the items included can be better documented.

ChannelManager::splice_channel takes individual parameters to support splice-in. Change these to an enum such that it can be used for splice-out as well.

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.

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 2 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.

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

View reviewed changes

jkczyz added 3 commits August 6, 2025 14:02

f - use LN_MAX_MSG_LEN

dc0b7b7

f - log outpoint

63413b0

jkczyz force-pushed the 2025-07-splice-out branch from 381fba6 to 396948e Compare August 7, 2025 13:39

jkczyz added 10 commits August 7, 2025 16:05

f - fix lint warning

229086d

f - drop FIXME

bd86078

Replace funding input tuple with struct

c9db499

The funding inputs used for splicing and v2 channel establishment are passed as a tuple of txin, prevtx, and witness weight. Add a struct so that the items included can be better documented.

f - use witness_weight

5b15f9b

f - fix doc links

f5933cb

f - fix lint warnings

4913b30

Use a SpliceContribution enum for passing splice-in params

5839ae4

ChannelManager::splice_channel takes individual parameters to support splice-in. Change these to an enum such that it can be used for splice-out as well.

f - rephrase docs

009ffa2

Add splice-out support

2b57d4c

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.

jkczyz added 3 commits August 7, 2025 16:20

f - drop SpliceOut::value

b68efb9

f - use get_value_to_self_msat

d9f10e4

f - check that splice-out outputs can be paid for by channel balance

59fbd7e

jkczyz force-pushed the 2025-07-splice-out branch from 396948e to 59fbd7e Compare August 7, 2025 23:38

jkczyz added 2 commits August 7, 2025 19:05

f - include output weights

e8dd4b6

f - correctly adjust contribution for fees

56ed8ba

Add splice-out support #3979

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Add splice-out support #3979

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