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

👋 Thanks for assigning @tankyleo as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

codecov · 2025-07-31T16:27:41Z

Codecov Report

❌ Patch coverage is 74.86339% with 46 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.77%. Comparing base (6d47f0a) to head (8a596fb).
⚠️ Report is 79 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	51.42%	16 Missing and 1 partial ⚠️
lightning/src/ln/channel.rs	92.72%	2 Missing and 2 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%   88.77%   +0.02%     
==========================================
  Files         173      176       +3     
  Lines      124899   127934    +3035     
  Branches   124899   127934    +3035     
==========================================
+ Hits       110841   113571    +2730     
- Misses      11631    11796     +165     
- Partials     2427     2567     +140

Flag	Coverage Δ
fuzzing	`22.09% <2.38%> (?)`
tests	`88.60% <73.77%> (-0.14%)`	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

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

🔔 1st Reminder

Hey @wpaulino! This PR has been waiting for your review.
Please take a look when you have a chance. If you're unable to review, please let us know so we can find another reviewer.

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

lightning/src/ln/channel.rs

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.

lightning/src/ln/channel.rs

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

👋 The first review has been submitted!

Do you think this PR is ready for a second reviewer? If so, click here to assign a second reviewer.

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

🔔 1st Reminder

Hey @wpaulino! This PR has been waiting for your review.
Please take a look when you have a chance. If you're unable to review, please let us know so we can find another reviewer.

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.

ldk-reviews-bot · 2025-08-18T00:00:15Z

🔔 1st Reminder

Hey @wpaulino @tankyleo! This PR has been waiting for your review.
Please take a look when you have a chance. If you're unable to review, please let us know so we can find another reviewer.

ldk-reviews-bot · 2025-08-18T00:00:19Z

🔔 1st Reminder

Hey @wpaulino @tankyleo! This PR has been waiting for your review.
Please take a look when you have a chance. If you're unable to review, please let us know so we can find another reviewer.

lightning/src/ln/channel.rs

tankyleo · 2025-08-18T07:51:03Z

lightning/src/ln/channel.rs

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


See other comments.

jkczyz

Addressed @tankyleo's comments, leaving some for #4011. Let me know if I missed something that should have been addressed here.

lightning/src/ln/channel.rs

jkczyz · 2025-08-18T22:10:38Z

lightning/src/ln/channelmanager.rs

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


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.

lightning/src/ln/channel.rs

tankyleo · 2025-08-19T17:30:18Z

lightning/src/ln/channel.rs

 	let mut weight = (input_count as u64) * BASE_INPUT_WEIGHT;
-
-	// Witnesses
 	weight = weight.saturating_add(witness_weight.to_wu());
+	weight = weight.saturating_add(outputs.iter().map(|txout| txout.weight().to_wu()).sum());


Seems we are missing the weight of the empty script sig, ie EMPTY_SCRIPT_SIG_WEIGHT ?

Isn't that only for inputs? Outputs added to the transaction will have a script_pubkey that needs to be satisfied when spent.

Right I mean that BASE_INPUT_WEIGHT on the very first line does not include EMPTY_SCRIPT_SIG_WEIGHT and then on the second line we add the Witness weight only.

Ah, that parameter should be renamed similar to #3979 (comment). But it looks like we do need to add EMPTY_SCRIPT_SIG_WEIGHT at call sites when adding FUNDING_TRANSACTION_WITNESS_WEIGHT, which AFAICT doesn't include it. (cc @wpaulino)

Updated estimate_v2_funding_transaction_fee to do all the weight calculations so as to avoid this.

lightning/src/ln/channel.rs

ldk-reviews-bot · 2025-08-20T00:01:07Z

🔔 2nd Reminder

Hey @wpaulino @tankyleo! This PR has been waiting for your review.
Please take a look when you have a chance. If you're unable to review, please let us know so we can find another reviewer.

tankyleo

See discussion comments :)

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.

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.

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 · 2025-08-21T16:07:32Z

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.

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 3 times, most recently 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 force-pushed the 2025-07-splice-out branch 2 times, most recently from 396948e to 59fbd7e Compare August 7, 2025 23:38

jkczyz requested a review from wpaulino August 8, 2025 13:42

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

jkczyz force-pushed the 2025-07-splice-out branch from f12c993 to 806220e Compare August 11, 2025 22:22

wpaulino reviewed Aug 12, 2025

View reviewed changes

jkczyz force-pushed the 2025-07-splice-out branch from 806220e to 9c2f266 Compare August 13, 2025 23:58

jkczyz requested a review from wpaulino August 13, 2025 23:58

jkczyz force-pushed the 2025-07-splice-out branch from 9c2f266 to ef1aee7 Compare August 14, 2025 22:19

wpaulino mentioned this pull request Aug 14, 2025

Simplify Quiescence and prepare for splice integration #4007

Merged

wpaulino reviewed Aug 14, 2025

View reviewed changes

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

tankyleo reviewed Aug 18, 2025

View reviewed changes

TheBlueMatt mentioned this pull request Aug 18, 2025

Integrate Splicing with Quiescence #4019

Draft

jkczyz force-pushed the 2025-07-splice-out branch from 4a02574 to a39a294 Compare August 18, 2025 22:15

jkczyz commented Aug 18, 2025

View reviewed changes

jkczyz requested a review from tankyleo August 18, 2025 22:17

tankyleo reviewed Aug 19, 2025

View reviewed changes

jkczyz force-pushed the 2025-07-splice-out branch from a39a294 to 8ad0b83 Compare August 19, 2025 23:39

jkczyz requested a review from tankyleo August 19, 2025 23:40

tankyleo reviewed Aug 20, 2025

View reviewed changes

jkczyz force-pushed the 2025-07-splice-out branch 2 times, most recently from d4c2cd8 to 7a38617 Compare August 20, 2025 16:37

jkczyz added 4 commits August 20, 2025 14:37

Remove their_funding_contribution_satoshis from FundingNegotiationCon…

c9ddcbf

…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 7a38617 to 29c61bd Compare August 20, 2025 19:37

jkczyz commented Aug 20, 2025

View reviewed changes

jkczyz requested a review from tankyleo August 20, 2025 19:38

jkczyz force-pushed the 2025-07-splice-out branch from 29c61bd to f2aa7fe Compare August 20, 2025 21:57

jkczyz added 5 commits August 21, 2025 10:42

Replace funding input tuple with struct

7c936e2

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

3e04691

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

20b6809

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

8a596fb

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 f2aa7fe to 8a596fb Compare August 21, 2025 16:05

Add splice-out support #3979

Are you sure you want to change the base?

Add splice-out support #3979

Uh oh!

Conversation

jkczyz commented Jul 31, 2025

Uh oh!

ldk-reviews-bot commented Jul 31, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

codecov bot commented Jul 31, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Codecov Report

Uh oh!

ldk-reviews-bot commented Aug 4, 2025

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Uh oh!

ldk-reviews-bot commented Aug 4, 2025

Uh oh!

ldk-reviews-bot commented Aug 11, 2025

Uh oh!

jkczyz commented Aug 11, 2025

Uh oh!

jkczyz commented Aug 11, 2025

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Uh oh!

jkczyz commented Aug 14, 2025

Uh oh!

wpaulino left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

ldk-reviews-bot commented Aug 18, 2025

Uh oh!

ldk-reviews-bot commented Aug 18, 2025

Uh oh!

Uh oh!

Uh oh!

Uh oh!

ldk-reviews-bot commented Jul 31, 2025 •

edited

Loading

codecov bot commented Jul 31, 2025 •

edited

Loading