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Add splice-out support - #3979

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Add splice-out support#3979
wpaulino merged 9 commits into
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jkczyz:2025-07-splice-out

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@jkczyzjkczyz commented Jul 31, 2025

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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.

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❌ 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 1400 commits behind head on main.

Files with missing linesPatch %Lines
lightning/src/ln/msgs.rs29.41%15 Missing and 9 partials ⚠️
lightning/src/ln/funding.rs45.71%18 Missing and 1 partial ⚠️
lightning/src/ln/channel.rs91.93%1 Missing and 4 partials ⚠️
lightning/src/ln/interactivetxs.rs97.91%0 Missing and 1 partial ⚠️
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@@ 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
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Comment threadlightning/src/ln/channel.rs Outdated
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Comment threadlightning/src/ln/channel.rs Outdated
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Comment threadlightning/src/ln/interactivetxs.rs Outdated
Comment threadlightning/src/ln/channel.rs Outdated
Comment threadlightning/src/ln/channel.rs Outdated
Comment threadlightning/src/ln/channel.rs Outdated
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Rebased to resolve merge conflicts.

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Squashed as requested.

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Comment threadlightning/src/ln/channel.rs
Comment threadlightning/src/ln/channelmanager.rs Outdated
Comment threadlightning/src/ln/channelmanager.rs Outdated
Comment threadlightning/src/ln/channelmanager.rs Outdated
Comment threadlightning/src/ln/channelmanager.rs Outdated
Comment threadlightning/src/ln/channel.rs Outdated
funding_feerate.to_sat_per_kwu() as u32,
));

if channel_balance > contribution_amount.unsigned_abs() + estimated_fee {

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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

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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?

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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.

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Leaving this for @tankyleo.

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See other comments.

Comment threadlightning/src/ln/channel.rs
@jkczyz
jkczyzforce-pushed the 2025-07-splice-out branch from 806220e to 9c2f266CompareAugust 13, 2025 23:58
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Squashed as requested.

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

Comment threadlightning/src/ln/channelmanager.rs Outdated
Comment threadlightning/src/ln/interactivetxs.rs Outdated
Comment threadlightning/src/ln/channel.rs Outdated
Comment threadlightning/src/ln/channelmanager.rs Outdated
/// When funds are added to a channel.
SpliceIn {
/// The amount to contribute to the splice.
value: Amount,

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

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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.

@wpaulino
wpaulino requested a review from tankyleoAugust 14, 2025 23:24
@jkczyz
Remove TransactionU16LenLimited
70929ae
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).
@jkczyz
Include witness weights in FundingNegotiationContext
3327382
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.
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jkczyzforce-pushed the 2025-07-splice-out branch from 7a38617 to 29c61bdCompareAugust 20, 2025 19:37
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Fixed CI and squashed commits as requested.

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jkczyz requested a review from tankyleoAugust 20, 2025 19:38
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ghost commented Aug 20, 2025

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Pushed a fix for CI.

Jeffrey Czyz
Account for shared input EMPTY_SCRIPT_SIG_WEIGHT
75b7e80
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.
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jkczyzforce-pushed the 2025-07-splice-out branch from f2aa7fe to 8a596fbCompareAugust 21, 2025 16:05
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ghost commented Aug 21, 2025

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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
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Comment threadlightning/src/ln/funding.rs Outdated

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

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

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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.

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

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

Comment threadlightning/src/ln/channel.rs
Comment threadlightning/src/ln/channel.rs Outdated
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Feel free to squash on your next push

Jeffrey Czyz added 4 commits August 25, 2025 12:20
Jeffrey Czyz
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.
Jeffrey Czyz
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.
Jeffrey Czyz
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.
Jeffrey Czyz
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
jkczyzforce-pushed the 2025-07-splice-out branch from ef24de4 to ce203f2CompareAugust 25, 2025 17:32
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Feel free to squash on your next push

Done. PTAL

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Three small nits we can take care of in followups

Comment threadlightning/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);

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Check that the abs is smaller than MAX_MONEY ?

Comment threadlightning/src/ln/funding.rs
Comment on lines +37 to +38
/// The outputs to include in the splice's funding transaction. The total value of all
/// outputs will be the amount that is removed.

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

Comment threadlightning/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.

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"to meet the contributed amount plus the fees" ?

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wpaulino merged commit 5f73b70 into lightningdevkit:mainAug 25, 2025
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Add splice-out support by jkczyz · Pull Request #3979 · lightningdevkit/rust-lightning · GitHub
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Add splice-out support - #3979
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Add splice-out support#3979
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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.
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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 1400 commits behind head on main.
Files with missing linesPatch %Lines
lightning/src/ln/msgs.rs29.41%15 Missing and 9 partials ⚠️
lightning/src/ln/funding.rs45.71%18 Missing and 1 partial ⚠️
lightning/src/ln/channel.rs91.93%1 Missing and 4 partials ⚠️
lightning/src/ln/interactivetxs.rs97.91%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 
FlagCoverage Δ
fuzzing21.69% <2.34%> (?)
tests88.59% <73.15%> (-0.16%)⬇️
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Rebased to resolve merge conflicts.
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Squashed as requested.
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Comment threadlightning/src/ln/channel.rs
Comment threadlightning/src/ln/channelmanager.rs
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Comment threadlightning/src/ln/channelmanager.rs
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Comment threadlightning/src/ln/channel.rs
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 funding_feerate.to_sat_per_kwu() as u32,
 ));

 if channel_balance > contribution_amount.unsigned_abs() + estimated_fee {
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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
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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?
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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.
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Leaving this for @tankyleo.
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Comment threadlightning/src/ln/channel.rs
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Squashed as requested.
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We should also make sure a FundingTransactionReadyForSigning event gets pushed when doing splice outs
Comment threadlightning/src/ln/channelmanager.rs
Outdated

Comment threadlightning/src/ln/interactivetxs.rs
Outdated

Comment threadlightning/src/ln/channel.rs
Outdated

Comment threadlightning/src/ln/channelmanager.rs
Outdated

	/// When funds are added to a channel.
	SpliceIn {
 /// The amount to contribute to the splice.
 value: Amount,
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Another variant could be to consume the full input set, but we can revisit that later
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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.
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@jkczyz
Remove TransactionU16LenLimited
70929ae
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).
@jkczyz
Include witness weights in FundingNegotiationContext
3327382
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.
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Fixed CI and squashed commits as requested.
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Pushed a fix for CI.
Jeffrey Czyz
Account for shared input EMPTY_SCRIPT_SIG_WEIGHT
75b7e80
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.
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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.
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Comment threadlightning/src/ln/funding.rs
Outdated


impl FundingTxInput {
	fn new<F: FnOnce(&bitcoin::Script) -> bool>(
 prevtx: Transaction, vout: u32, sequence: Sequence, witness_weight: Weight,
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Nit: sequence could be made optional with a default value of ENABLE_RBF_NO_LOCKTIME
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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.
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Sure, let's link to it in the FundingTxInput::new docs though
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That's private, so will need to link each of the four public functions.
Comment threadlightning/src/ln/channel.rs
Comment threadlightning/src/ln/channel.rs
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Feel free to squash on your next push
Jeffrey Czyz
added 4 commits
August 25, 2025 12:20
Jeffrey Czyz
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.
Jeffrey Czyz
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.
Jeffrey Czyz
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.
Jeffrey Czyz
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.
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Feel free to squash on your next push
Done. PTAL
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Three small nits we can take care of in followups
Comment threadlightning/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);
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Check that the abs is smaller than MAX_MONEY ?
Comment threadlightning/src/ln/funding.rs

Comment on lines

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 /// The outputs to include in the splice's funding transaction. The total value of all
 /// outputs will be the amount that is removed.
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"We will remove the total value of the outputs plus the transaction fees" ?
Comment threadlightning/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.
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"to meet the contributed amount plus the fees" ?
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Add splice-out support by jkczyz · Pull Request #3979 · lightningdevkit/rust-lightning · GitHub
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Add splice-out support#3979
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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.
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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 1400 commits behind head on main.
Files with missing linesPatch %Lines
lightning/src/ln/msgs.rs29.41%15 Missing and 9 partials ⚠️
lightning/src/ln/funding.rs45.71%18 Missing and 1 partial ⚠️
lightning/src/ln/channel.rs91.93%1 Missing and 4 partials ⚠️
lightning/src/ln/interactivetxs.rs97.91%0 Missing and 1 partial ⚠️
Additional details and impacted files
@@ Coverage Diff @@## main #3979 +/- ##
==========================================
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==========================================
Files 173 176 +3 Lines 124899 127944 +3045 Branches 124899 127944 +3045 ==========================================
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Rebased to resolve merge conflicts.
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Squashed as requested.
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Comment threadlightning/src/ln/channel.rs
Comment threadlightning/src/ln/channelmanager.rs
Outdated

Comment threadlightning/src/ln/channelmanager.rs
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Comment threadlightning/src/ln/channelmanager.rs
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Comment threadlightning/src/ln/channelmanager.rs
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Comment threadlightning/src/ln/channel.rs
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 funding_feerate.to_sat_per_kwu() as u32,
 ));

 if channel_balance > contribution_amount.unsigned_abs() + estimated_fee {
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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
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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?
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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.
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Leaving this for @tankyleo.
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See other comments.
Comment threadlightning/src/ln/channel.rs
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We should also make sure a FundingTransactionReadyForSigning event gets pushed when doing splice outs
Comment threadlightning/src/ln/channelmanager.rs
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Comment threadlightning/src/ln/interactivetxs.rs
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Comment threadlightning/src/ln/channel.rs
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Comment threadlightning/src/ln/channelmanager.rs
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	/// When funds are added to a channel.
	SpliceIn {
 /// The amount to contribute to the splice.
 value: Amount,
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Another variant could be to consume the full input set, but we can revisit that later
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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.
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@jkczyz
Remove TransactionU16LenLimited
70929ae
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).
@jkczyz
Include witness weights in FundingNegotiationContext
3327382
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.
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Fixed CI and squashed commits as requested.
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Pushed a fix for CI.
Jeffrey Czyz
Account for shared input EMPTY_SCRIPT_SIG_WEIGHT
75b7e80
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.
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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.
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Comment threadlightning/src/ln/funding.rs
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impl FundingTxInput {
	fn new<F: FnOnce(&bitcoin::Script) -> bool>(
 prevtx: Transaction, vout: u32, sequence: Sequence, witness_weight: Weight,
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Nit: sequence could be made optional with a default value of ENABLE_RBF_NO_LOCKTIME
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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.
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Sure, let's link to it in the FundingTxInput::new docs though
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That's private, so will need to link each of the four public functions.
Comment threadlightning/src/ln/channel.rs
Comment threadlightning/src/ln/channel.rs
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Feel free to squash on your next push
Jeffrey Czyz
added 4 commits
August 25, 2025 12:20
Jeffrey Czyz
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.
Jeffrey Czyz
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.
Jeffrey Czyz
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.
Jeffrey Czyz
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.
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Feel free to squash on your next push
Done. PTAL
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Three small nits we can take care of in followups
Comment threadlightning/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);
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Check that the abs is smaller than MAX_MONEY ?
Comment threadlightning/src/ln/funding.rs

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 /// The outputs to include in the splice's funding transaction. The total value of all
 /// outputs will be the amount that is removed.
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"We will remove the total value of the outputs plus the transaction fees" ?
Comment threadlightning/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.
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"to meet the contributed amount plus the fees" ?
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Add splice-out support by jkczyz · Pull Request #3979 · lightningdevkit/rust-lightning · GitHub
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Add splice-out support - #3979
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Add splice-out support#3979
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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.
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❌ 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 1400 commits behind head on main.
Files with missing linesPatch %Lines
lightning/src/ln/msgs.rs29.41%15 Missing and 9 partials ⚠️
lightning/src/ln/funding.rs45.71%18 Missing and 1 partial ⚠️
lightning/src/ln/channel.rs91.93%1 Missing and 4 partials ⚠️
lightning/src/ln/interactivetxs.rs97.91%0 Missing and 1 partial ⚠️
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@@ Coverage Diff @@## main #3979 +/- ##
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Rebased to resolve merge conflicts.
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Squashed as requested.
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Comment threadlightning/src/ln/channel.rs
Comment threadlightning/src/ln/channelmanager.rs
Outdated

Comment threadlightning/src/ln/channelmanager.rs
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Comment threadlightning/src/ln/channelmanager.rs
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Comment threadlightning/src/ln/channelmanager.rs
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Comment threadlightning/src/ln/channel.rs
Outdated

 funding_feerate.to_sat_per_kwu() as u32,
 ));

 if channel_balance > contribution_amount.unsigned_abs() + estimated_fee {
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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
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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?
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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.
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Leaving this for @tankyleo.
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See other comments.
Comment threadlightning/src/ln/channel.rs
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Squashed as requested.
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We should also make sure a FundingTransactionReadyForSigning event gets pushed when doing splice outs
Comment threadlightning/src/ln/channelmanager.rs
Outdated

Comment threadlightning/src/ln/interactivetxs.rs
Outdated

Comment threadlightning/src/ln/channel.rs
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Comment threadlightning/src/ln/channelmanager.rs
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	/// When funds are added to a channel.
	SpliceIn {
 /// The amount to contribute to the splice.
 value: Amount,
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Another variant could be to consume the full input set, but we can revisit that later
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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.
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@jkczyz
Remove TransactionU16LenLimited
70929ae
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).
@jkczyz
Include witness weights in FundingNegotiationContext
3327382
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.
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Fixed CI and squashed commits as requested.
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Pushed a fix for CI.
Jeffrey Czyz
Account for shared input EMPTY_SCRIPT_SIG_WEIGHT
75b7e80
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.
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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.
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Comment threadlightning/src/ln/funding.rs
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impl FundingTxInput {
	fn new<F: FnOnce(&bitcoin::Script) -> bool>(
 prevtx: Transaction, vout: u32, sequence: Sequence, witness_weight: Weight,
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Nit: sequence could be made optional with a default value of ENABLE_RBF_NO_LOCKTIME
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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.
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Sure, let's link to it in the FundingTxInput::new docs though
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That's private, so will need to link each of the four public functions.
Comment threadlightning/src/ln/channel.rs
Comment threadlightning/src/ln/channel.rs
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Feel free to squash on your next push
Jeffrey Czyz
added 4 commits
August 25, 2025 12:20
Jeffrey Czyz
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.
Jeffrey Czyz
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.
Jeffrey Czyz
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.
Jeffrey Czyz
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.
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Feel free to squash on your next push
Done. PTAL
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Three small nits we can take care of in followups
Comment threadlightning/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);
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Check that the abs is smaller than MAX_MONEY ?
Comment threadlightning/src/ln/funding.rs

Comment on lines

+37
 to 
+38

 /// The outputs to include in the splice's funding transaction. The total value of all
 /// outputs will be the amount that is removed.
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"We will remove the total value of the outputs plus the transaction fees" ?
Comment threadlightning/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.
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"to meet the contributed amount plus the fees" ?
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Add splice-out support by jkczyz · Pull Request #3979 · lightningdevkit/rust-lightning · GitHub
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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.
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Rebased to resolve merge conflicts.
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Squashed as requested.
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Comment threadlightning/src/ln/channel.rs
Comment threadlightning/src/ln/channelmanager.rs
Outdated

Comment threadlightning/src/ln/channelmanager.rs
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Comment threadlightning/src/ln/channelmanager.rs
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Comment threadlightning/src/ln/channel.rs
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 funding_feerate.to_sat_per_kwu() as u32,
 ));

 if channel_balance > contribution_amount.unsigned_abs() + estimated_fee {
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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
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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?
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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.
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Leaving this for @tankyleo.
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See other comments.
Comment threadlightning/src/ln/channel.rs
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Squashed as requested.
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We should also make sure a FundingTransactionReadyForSigning event gets pushed when doing splice outs
Comment threadlightning/src/ln/channelmanager.rs
Outdated

Comment threadlightning/src/ln/interactivetxs.rs
Outdated

Comment threadlightning/src/ln/channel.rs
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Comment threadlightning/src/ln/channelmanager.rs
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	/// When funds are added to a channel.
	SpliceIn {
 /// The amount to contribute to the splice.
 value: Amount,
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Another variant could be to consume the full input set, but we can revisit that later
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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.
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@jkczyz
Remove TransactionU16LenLimited
70929ae
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).
@jkczyz
Include witness weights in FundingNegotiationContext
3327382
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.
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Fixed CI and squashed commits as requested.
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Pushed a fix for CI.
Jeffrey Czyz
Account for shared input EMPTY_SCRIPT_SIG_WEIGHT
75b7e80
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.
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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.
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Comment threadlightning/src/ln/funding.rs
Outdated


impl FundingTxInput {
	fn new<F: FnOnce(&bitcoin::Script) -> bool>(
 prevtx: Transaction, vout: u32, sequence: Sequence, witness_weight: Weight,
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Nit: sequence could be made optional with a default value of ENABLE_RBF_NO_LOCKTIME
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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.
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Sure, let's link to it in the FundingTxInput::new docs though
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That's private, so will need to link each of the four public functions.
Comment threadlightning/src/ln/channel.rs
Comment threadlightning/src/ln/channel.rs
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Feel free to squash on your next push
Jeffrey Czyz
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August 25, 2025 12:20
Jeffrey Czyz
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.
Jeffrey Czyz
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.
Jeffrey Czyz
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.
Jeffrey Czyz
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.
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Feel free to squash on your next push
Done. PTAL
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Three small nits we can take care of in followups
Comment threadlightning/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);
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Check that the abs is smaller than MAX_MONEY ?
Comment threadlightning/src/ln/funding.rs

Comment on lines

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+38

 /// The outputs to include in the splice's funding transaction. The total value of all
 /// outputs will be the amount that is removed.
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"We will remove the total value of the outputs plus the transaction fees" ?
Comment threadlightning/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.
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"to meet the contributed amount plus the fees" ?
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Add splice-out support by jkczyz · Pull Request #3979 · lightningdevkit/rust-lightning · GitHub
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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.
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❌ 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 1400 commits behind head on main.
Files with missing linesPatch %Lines
lightning/src/ln/msgs.rs29.41%15 Missing and 9 partials ⚠️
lightning/src/ln/funding.rs45.71%18 Missing and 1 partial ⚠️
lightning/src/ln/channel.rs91.93%1 Missing and 4 partials ⚠️
lightning/src/ln/interactivetxs.rs97.91%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 
FlagCoverage Δ
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Rebased to resolve merge conflicts.
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Comment threadlightning/src/ln/channel.rs
Comment threadlightning/src/ln/channelmanager.rs
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 funding_feerate.to_sat_per_kwu() as u32,
 ));

 if channel_balance > contribution_amount.unsigned_abs() + estimated_fee {
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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
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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?
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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.
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Leaving this for @tankyleo.
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See other comments.
Comment threadlightning/src/ln/channel.rs
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We should also make sure a FundingTransactionReadyForSigning event gets pushed when doing splice outs
Comment threadlightning/src/ln/channelmanager.rs
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Comment threadlightning/src/ln/channelmanager.rs
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	/// When funds are added to a channel.
	SpliceIn {
 /// The amount to contribute to the splice.
 value: Amount,
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Another variant could be to consume the full input set, but we can revisit that later
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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.
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@jkczyz
Remove TransactionU16LenLimited
70929ae
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).
@jkczyz
Include witness weights in FundingNegotiationContext
3327382
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.
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Fixed CI and squashed commits as requested.
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Pushed a fix for CI.
Jeffrey Czyz
Account for shared input EMPTY_SCRIPT_SIG_WEIGHT
75b7e80
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.
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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.
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Comment threadlightning/src/ln/funding.rs
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impl FundingTxInput {
	fn new<F: FnOnce(&bitcoin::Script) -> bool>(
 prevtx: Transaction, vout: u32, sequence: Sequence, witness_weight: Weight,
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Nit: sequence could be made optional with a default value of ENABLE_RBF_NO_LOCKTIME
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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.
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Sure, let's link to it in the FundingTxInput::new docs though
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That's private, so will need to link each of the four public functions.
Comment threadlightning/src/ln/channel.rs
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Feel free to squash on your next push
Jeffrey Czyz
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August 25, 2025 12:20
Jeffrey Czyz
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.
Jeffrey Czyz
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.
Jeffrey Czyz
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.
Jeffrey Czyz
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.
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Feel free to squash on your next push
Done. PTAL
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Three small nits we can take care of in followups
Comment threadlightning/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);
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Check that the abs is smaller than MAX_MONEY ?
Comment threadlightning/src/ln/funding.rs

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 /// The outputs to include in the splice's funding transaction. The total value of all
 /// outputs will be the amount that is removed.
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"We will remove the total value of the outputs plus the transaction fees" ?
Comment threadlightning/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.
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"to meet the contributed amount plus the fees" ?
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Add splice-out support by jkczyz · Pull Request #3979 · lightningdevkit/rust-lightning · GitHub
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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.
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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 1400 commits behind head on main.
Files with missing linesPatch %Lines
lightning/src/ln/msgs.rs29.41%15 Missing and 9 partials ⚠️
lightning/src/ln/funding.rs45.71%18 Missing and 1 partial ⚠️
lightning/src/ln/channel.rs91.93%1 Missing and 4 partials ⚠️
lightning/src/ln/interactivetxs.rs97.91%0 Missing and 1 partial ⚠️
Additional details and impacted files
@@ Coverage Diff @@## main #3979 +/- ##
==========================================
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==========================================
Files 173 176 +3 Lines 124899 127944 +3045 Branches 124899 127944 +3045 ==========================================
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Rebased to resolve merge conflicts.
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Comment threadlightning/src/ln/channel.rs
Comment threadlightning/src/ln/channelmanager.rs
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Comment threadlightning/src/ln/channel.rs
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 funding_feerate.to_sat_per_kwu() as u32,
 ));

 if channel_balance > contribution_amount.unsigned_abs() + estimated_fee {
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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
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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?
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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.
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Leaving this for @tankyleo.
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See other comments.
Comment threadlightning/src/ln/channel.rs
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We should also make sure a FundingTransactionReadyForSigning event gets pushed when doing splice outs
Comment threadlightning/src/ln/channelmanager.rs
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Comment threadlightning/src/ln/channel.rs
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Comment threadlightning/src/ln/channelmanager.rs
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	/// When funds are added to a channel.
	SpliceIn {
 /// The amount to contribute to the splice.
 value: Amount,
@wpaulinowpaulinoAug 14, 2025

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Another variant could be to consume the full input set, but we can revisit that later
@jkczyzjkczyzAug 18, 2025

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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.
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@jkczyz
Remove TransactionU16LenLimited
70929ae
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).
@jkczyz
Include witness weights in FundingNegotiationContext
3327382
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.
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Fixed CI and squashed commits as requested.
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Pushed a fix for CI.
Jeffrey Czyz
Account for shared input EMPTY_SCRIPT_SIG_WEIGHT
75b7e80
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.
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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.
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Comment threadlightning/src/ln/funding.rs
Outdated


impl FundingTxInput {
	fn new<F: FnOnce(&bitcoin::Script) -> bool>(
 prevtx: Transaction, vout: u32, sequence: Sequence, witness_weight: Weight,
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Nit: sequence could be made optional with a default value of ENABLE_RBF_NO_LOCKTIME
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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.
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Sure, let's link to it in the FundingTxInput::new docs though
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That's private, so will need to link each of the four public functions.
Comment threadlightning/src/ln/channel.rs
Comment threadlightning/src/ln/channel.rs
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Feel free to squash on your next push
Jeffrey Czyz
added 4 commits
August 25, 2025 12:20
Jeffrey Czyz
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.
Jeffrey Czyz
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.
Jeffrey Czyz
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.
Jeffrey Czyz
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.
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Feel free to squash on your next push
Done. PTAL
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Three small nits we can take care of in followups
Comment threadlightning/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);
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Check that the abs is smaller than MAX_MONEY ?
Comment threadlightning/src/ln/funding.rs

Comment on lines

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 /// The outputs to include in the splice's funding transaction. The total value of all
 /// outputs will be the amount that is removed.
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"We will remove the total value of the outputs plus the transaction fees" ?
Comment threadlightning/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.
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"to meet the contributed amount plus the fees" ?
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Add splice-out support by jkczyz · Pull Request #3979 · lightningdevkit/rust-lightning · GitHub
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Add splice-out support - #3979
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Add splice-out support#3979
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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.
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❌ Patch coverage is 74.21053% with 49 lines in your changes missing coverage. Please review.

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⚠️ Report is 1400 commits behind head on main.
Files with missing linesPatch %Lines
lightning/src/ln/msgs.rs29.41%15 Missing and 9 partials ⚠️
lightning/src/ln/funding.rs45.71%18 Missing and 1 partial ⚠️
lightning/src/ln/channel.rs91.93%1 Missing and 4 partials ⚠️
lightning/src/ln/interactivetxs.rs97.91%0 Missing and 1 partial ⚠️
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@@ Coverage Diff @@## main #3979 +/- ##
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Comment threadlightning/src/ln/channel.rs
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Rebased to resolve merge conflicts.
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Squashed as requested.
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Comment threadlightning/src/ln/channel.rs
Comment threadlightning/src/ln/channelmanager.rs
Outdated

Comment threadlightning/src/ln/channelmanager.rs
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Comment threadlightning/src/ln/channelmanager.rs
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Comment threadlightning/src/ln/channelmanager.rs
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Comment threadlightning/src/ln/channel.rs
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 funding_feerate.to_sat_per_kwu() as u32,
 ));

 if channel_balance > contribution_amount.unsigned_abs() + estimated_fee {
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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
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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?
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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.
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Leaving this for @tankyleo.
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See other comments.
Comment threadlightning/src/ln/channel.rs
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Squashed as requested.
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We should also make sure a FundingTransactionReadyForSigning event gets pushed when doing splice outs
Comment threadlightning/src/ln/channelmanager.rs
Outdated

Comment threadlightning/src/ln/interactivetxs.rs
Outdated

Comment threadlightning/src/ln/channel.rs
Outdated

Comment threadlightning/src/ln/channelmanager.rs
Outdated

	/// When funds are added to a channel.
	SpliceIn {
 /// The amount to contribute to the splice.
 value: Amount,
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Another variant could be to consume the full input set, but we can revisit that later
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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.
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@jkczyz
Remove TransactionU16LenLimited
70929ae
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).
@jkczyz
Include witness weights in FundingNegotiationContext
3327382
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.
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Fixed CI and squashed commits as requested.
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Pushed a fix for CI.
Jeffrey Czyz
Account for shared input EMPTY_SCRIPT_SIG_WEIGHT
75b7e80
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.
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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.
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Comment threadlightning/src/ln/funding.rs
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impl FundingTxInput {
	fn new<F: FnOnce(&bitcoin::Script) -> bool>(
 prevtx: Transaction, vout: u32, sequence: Sequence, witness_weight: Weight,
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Nit: sequence could be made optional with a default value of ENABLE_RBF_NO_LOCKTIME
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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.
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Sure, let's link to it in the FundingTxInput::new docs though
ghostAug 25, 2025

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That's private, so will need to link each of the four public functions.
Comment threadlightning/src/ln/channel.rs
Comment threadlightning/src/ln/channel.rs
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🔔 1st Reminder
Hey @wpaulino@tankyleo! This PR has been waiting for your review.

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🔔 1st Reminder
Hey @wpaulino@tankyleo! This PR has been waiting for your review.

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🔔 2nd Reminder
Hey @wpaulino@tankyleo! This PR has been waiting for your review.

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Feel free to squash on your next push
Jeffrey Czyz
added 4 commits
August 25, 2025 12:20
Jeffrey Czyz
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.
Jeffrey Czyz
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.
Jeffrey Czyz
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.
Jeffrey Czyz
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.
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Feel free to squash on your next push
Done. PTAL
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Three small nits we can take care of in followups
Comment threadlightning/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);
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Check that the abs is smaller than MAX_MONEY ?
Comment threadlightning/src/ln/funding.rs

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 /// The outputs to include in the splice's funding transaction. The total value of all
 /// outputs will be the amount that is removed.
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"We will remove the total value of the outputs plus the transaction fees" ?
Comment threadlightning/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.
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"to meet the contributed amount plus the fees" ?
@wpaulino
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lightningdevkit:mainAug 25, 2025
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mentioned this pull request
Aug 25, 2025
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