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 74.21053% with 49 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.76%. Comparing base (6d47f0a) to head (ce203f2).
⚠️ Report is 94 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/funding.rs	45.71%	18 Missing and 1 partial ⚠️
lightning/src/ln/channel.rs	91.93%	1 Missing and 4 partials ⚠️
lightning/src/ln/interactivetxs.rs	97.91%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3979      +/-   ##
==========================================
+ Coverage   88.74%   88.76%   +0.01%     
==========================================
  Files         173      176       +3     
  Lines      124899   127944    +3045     
  Branches   124899   127944    +3045     
==========================================
+ Hits       110841   113564    +2723     
- Misses      11631    11808     +177     
- Partials     2427     2572     +145

Flag	Coverage Δ
fuzzing	`21.69% <2.34%> (?)`
tests	`88.59% <73.15%> (-0.16%)`	⬇️

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

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lightning/src/ln/channel.rs

lightning/src/ln/interactivetxs.rs

lightning/src/ln/channel.rs

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

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ldk-reviews-bot · 2025-08-11T00:00:21Z

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jkczyz · 2025-08-11T16:41:48Z

Rebased to resolve merge conflicts.

jkczyz · 2025-08-11T22:22:26Z

Squashed as requested.

lightning/src/ln/channel.rs

lightning/src/ln/channelmanager.rs

wpaulino · 2025-08-12T17:05:38Z

lightning/src/ln/channel.rs

+			funding_feerate.to_sat_per_kwu() as u32,
+		));
+
+		if channel_balance > contribution_amount.unsigned_abs() + estimated_fee {


This isn't enough, we need to make sure the balance is still above the reserve and has enough to pay for fees on the commitment transaction if they're the channel initiator

Right, the reserve check will be at the call site as a separate check. I hadn't considered the fees for the commitment transaction. For the splice-in case, is it assumed it is sufficient because it was sufficient with the previous funding contributions?

For the splice-in case, is it assumed it is sufficient because it was sufficient with the previous funding contributions?

Most likely yeah, but it's better to be safe than sorry. We already need to always check it for the counterparty and for our splice-outs, so might as well always do it for splice-ins.

Leaving this for @tankyleo.

See other comments.

lightning/src/ln/channel.rs

jkczyz · 2025-08-14T22:20:13Z

Squashed as requested.

wpaulino

We should also make sure a FundingTransactionReadyForSigning event gets pushed when doing splice outs

lightning/src/ln/channelmanager.rs

lightning/src/ln/interactivetxs.rs

lightning/src/ln/channel.rs

wpaulino · 2025-08-14T23:14:32Z

lightning/src/ln/channelmanager.rs

+	/// When funds are added to a channel.
+	SpliceIn {
+		/// The amount to contribute to the splice.
+		value: Amount,


Another variant could be to consume the full input set, but we can revisit that later

I was thinking we could have a constructor that took inputs and gave back a SpliceContribution::SpliceIn with amount. However, we'd need to provide information about the balance in order to compute that. So maybe best to use a dedicated variant as you said.

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.

jkczyz

Fixed CI and squashed commits as requested.

jkczyz · 2025-08-20T21:58:02Z

Pushed a fix for CI.

When splicing a channel, the previous funding output is spent and fees for it are paid by the splice initiator. However, the witness weight was not including EMPTY_SCRIPT_SIG_WEIGHT. Fix this and update the variable name to make clear the weight needed is the input satisfaction.

jkczyz · 2025-08-21T16:07:32Z

Sorry, early CI failures were masking another one. Think it should be fixed in latest push. Also, pulled out the EMPTY_SCRIPT_SIG_WEIGHT into a separate commit.

wpaulino · 2025-08-21T23:46:45Z

lightning/src/ln/funding.rs

+
+impl FundingTxInput {
+	fn new<F: FnOnce(&bitcoin::Script) -> bool>(
+		prevtx: Transaction, vout: u32, sequence: Sequence, witness_weight: Weight,


Nit: sequence could be made optional with a default value of ENABLE_RBF_NO_LOCKTIME

What do you think about always setting it to ENABLE_RBF_NO_LOCKTIME and providing a mutator to override it? Would be nice to avoid passing an Option here if possible.

Sure, let's link to it in the FundingTxInput::new docs though

That's private, so will need to link each of the four public functions.

lightning/src/ln/channel.rs

ldk-reviews-bot · 2025-08-23T00:00:52Z

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ldk-reviews-bot · 2025-08-25T00:00:24Z

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ldk-reviews-bot · 2025-08-25T00:01:32Z

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wpaulino · 2025-08-25T16:16:39Z

Feel free to squash on your next push

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.

When a counterparty sends splice_init with a negative contribution, they are requesting to remove funds from a channel. Remove conditions guarding against this and check that they have enough channel balance to cover the removed funds.

jkczyz · 2025-08-25T17:34:08Z

Feel free to squash on your next push

Done. PTAL

tankyleo

Three small nits we can take care of in followups

tankyleo · 2025-08-25T19:12:50Z

lightning/src/ln/channel.rs

-			funding_negotiation_context.our_funding_contribution_satoshis;
-		let their_funding_contribution_satoshis = msg.funding_contribution_satoshis;
+		let our_funding_contribution = funding_negotiation_context.our_funding_contribution;
+		debug_assert!(our_funding_contribution <= SignedAmount::MAX_MONEY);


Check that the abs is smaller than MAX_MONEY ?

tankyleo · 2025-08-25T19:14:18Z

lightning/src/ln/funding.rs

+		/// The outputs to include in the splice's funding transaction. The total value of all
+		/// outputs will be the amount that is removed.


"We will remove the total value of the outputs plus the transaction fees" ?

tankyleo · 2025-08-25T19:15:01Z

lightning/src/ln/funding.rs

+		/// The amount to contribute to the splice.
+		value: Amount,
+
+		/// The inputs included in the splice's funding transaction to meet the contributed amount.


"to meet the contributed amount plus the fees" ?

Follow-ups for #3979

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

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jkczyz force-pushed the 2025-07-splice-out branch 3 times, most recently from 2d39059 to 381fba6 Compare August 1, 2025 00:23

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jkczyz force-pushed the 2025-07-splice-out branch from fde2f61 to f12c993 Compare August 11, 2025 16:39

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Simplify Quiescence and prepare for splice integration #4007

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wpaulino requested a review from tankyleo August 14, 2025 23:24

jkczyz added 2 commits August 20, 2025 14:37

jkczyz force-pushed the 2025-07-splice-out branch from 7a38617 to 29c61bd Compare August 20, 2025 19:37

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jkczyz force-pushed the 2025-07-splice-out branch 2 times, most recently from 567442d to ef24de4 Compare August 22, 2025 21:06

jkczyz requested a review from wpaulino August 22, 2025 21:07

jkczyz added 4 commits August 25, 2025 12:20

Replace funding input tuple with struct

bdb8d5d

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.

Use a SpliceContribution enum for passing splice-in params

ae58a4f

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.

Add splice-out support

3245ebf

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.

Support accepting splice-out

ce203f2

When a counterparty sends splice_init with a negative contribution, they are requesting to remove funds from a channel. Remove conditions guarding against this and check that they have enough channel balance to cover the removed funds.

jkczyz force-pushed the 2025-07-splice-out branch from ef24de4 to ce203f2 Compare August 25, 2025 17:32

wpaulino approved these changes Aug 25, 2025

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tankyleo approved these changes Aug 25, 2025

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wpaulino merged commit 5f73b70 into lightningdevkit:main Aug 25, 2025
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github-project-automation bot moved this to Done in Weekly Goals Aug 25, 2025

jkczyz mentioned this pull request Aug 25, 2025

Follow-ups for #3979 #4028

Merged

jkczyz added a commit that referenced this pull request Aug 26, 2025

Merge pull request #4028 from jkczyz/2025-08-splice-out-fixups

a18f362

Follow-ups for #3979

		/// The outputs to include in the splice's funding transaction. The total value of all
		/// outputs will be the amount that is removed.

Add splice-out support #3979

Add splice-out support #3979

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