Support manually selecting inputs consuming their entire value by wpaulino · Pull Request #4575 · lightningdevkit/rust-lightning

wpaulino · 2026-04-23T17:08:48Z

This commit introduces an alternative way of splicing in funds without coin selection by requiring the full UTXO to be provided. Each UTXO's entire value (minus fees) is allocated towards the channel, which provides unified balance wallets a more intuitive API when splicing funds into the channel, as they don't particularly care about maintaining a portion of their balance onchain.

To simplify the implementation, we require that contributions are not allowed to mix coin-selected inputs with manually-selected ones. Users will need to start a fresh contribution if they want to change the funding input mode.

ldk-reviews-bot · 2026-04-23T17:08:52Z

👋 Thanks for assigning @TheBlueMatt 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.

ldk-claude-review-bot · 2026-04-23T17:16:47Z

 		);

-		if !self.inputs.is_empty() {
+		if !self.inputs.is_empty() && self.input_mode == Some(FundingInputMode::CoinSelected) {


Bug: Backwards compatibility issue with persisted FundingContribution objects.

FundingContribution is persisted in PendingFunding.contributions (channel.rs:2917). When deserializing contributions created before this PR, input_mode will be None (it's a new option TLV field). Old coin-selected contributions with inputs will have input_mode == None, causing this condition to be false even though they should take the coin-selected branch.

This causes two problems for old persisted coin-selected contributions:

Wrong fee buffer calculation: Uses holder_balance + net_value_without_fee instead of estimated_fee + change_value, potentially allowing or rejecting feerate adjustments incorrectly.

Change output silently dropped: compute_feerate_adjustment returns None for change, and at_feerate sets change_output = None, losing the change value.

This is reachable via for_acceptor_at_feerate / for_initiator_at_feerate called on contributions loaded from pending_splice.contributions (channel.rs lines 12504, 12944, 13127, 13145).

Fix: use self.input_mode != Some(FundingInputMode::Manual) instead of self.input_mode == Some(FundingInputMode::CoinSelected) to preserve old behavior for contributions where input_mode is None:

Suggested change

if !self.inputs.is_empty() && self.input_mode == Some(FundingInputMode::CoinSelected) {

if !self.inputs.is_empty() && self.input_mode != Some(FundingInputMode::Manual) {

There is no backwards compatibility concern because the serialized object has not been included in a release yet.

ldk-claude-review-bot · 2026-04-23T17:17:33Z

+		if let Some(PriorContribution { contribution: prior_contribution, .. }) =
+			self.prior_contribution.as_ref()
+		{
+			if prior_contribution.input_mode == Some(FundingInputMode::CoinSelected)


Minor: Same backwards-compat pattern as line 880. Old persisted coin-selected contributions will have input_mode == None, so this guard won't fire for them. In practice this is mostly mitigated by the check at line 1309 (value_added > 0 && manually_selected_inputs non-empty), but it could miss edge cases where the old prior had value_added() == 0 (inputs exactly covered outputs + fees).

Consider using != Some(FundingInputMode::Manual) combined with a non-empty inputs check:

Suggested change

if prior_contribution.input_mode == Some(FundingInputMode::CoinSelected)

if prior_contribution.input_mode != Some(FundingInputMode::Manual)

&& !prior_contribution.inputs.is_empty()

ldk-claude-review-bot · 2026-04-23T17:18:44Z

I've completed a thorough review of the entire PR diff. The two backwards-compatibility issues flagged in the prior review are still present and are the only material bugs. No new issues were found.

Review Summary

Previously Flagged Issues (still open)

These are the only issues in this PR and were already flagged in the prior review:

lightning/src/ln/funding.rs:866 — Old persisted FundingContribution objects have input_mode == None, causing == Some(CoinSelected) to route them into the wrong branch of compute_feerate_adjustment. Should use != Some(FundingInputMode::Manual).
lightning/src/ln/funding.rs:1307 — Same backwards-compat pattern for the guard preventing manual inputs on a coin-selected prior. Old priors with input_mode == None bypass this check.

New Issues

No new issues found. The rest of the implementation is correct:

The ManuallySelected flow through amend_without_coin_selection correctly replaces inputs, drops change output, and adjusts feerate.
The fee buffer calculation for manual inputs in compute_feerate_adjustment (lines 913-934) correctly uses net_value_without_fee + holder_balance as the available buffer.
splice_in_inputs correctly appends to prior manual inputs via FundingBuilder::new copying prior inputs + add_inputs extending.
Validation properly rejects mixing manual inputs with coin-selected value (line 1300) and manual inputs on coin-selected priors (line 1306-1313).
ManuallySelectedInputsInsufficient errors do not fall through to coin selection in AsyncFundingBuilder::build / SyncFundingBuilder::build.
The spliceable_balance check in try_build_without_coin_selection (lines 1234-1243) correctly validates that the channel balance can cover any net-negative contribution.
The input_mode TLV option field (tag 15) is properly serialized and handles None for new contributions without inputs.

codecov · 2026-04-23T18:30:17Z

Codecov Report

❌ Patch coverage is 96.37931% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.20%. Comparing base (2313bd5) to head (228cae8).
⚠️ Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/funding.rs	96.28%	12 Missing and 9 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4575      +/-   ##
==========================================
+ Coverage   87.08%   87.20%   +0.11%     
==========================================
  Files         161      161              
  Lines      109255   109662     +407     
  Branches   109255   109662     +407     
==========================================
+ Hits        95147    95626     +479     
+ Misses      11627    11557      -70     
+ Partials     2481     2479       -2

Flag	Coverage Δ
fuzzing-fake-hashes	`31.14% <12.10%> (+0.16%)`	⬆️
fuzzing-real-hashes	`22.89% <0.00%> (+0.23%)`	⬆️
tests	`86.26% <96.03%> (+0.09%)`	⬆️

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Alkamal01 · 2026-04-23T18:41:13Z

The root problem seems to be relying on == Some(CoinSelected) for persisted data. For legacy contributions, None effectively behaved like coin selection when inputs were present, so switching to != Some(Manual) (with an inputs non-empty check where needed) would preserve the old behavior.

One additional edge case in request_matches_prior: (Some(CoinSelected), None) currently falls into _ => false, so an RBF with the same value on a legacy prior won't be treated as a match and skips the feerate-adjustment path. Not strictly a correctness issue, but it becomes inconsistent if the other branches get fixed.

So overall, it'd be good to make the handling of None consistent across all three paths.

TheBlueMatt

All LGTM. One question.

ldk-reviews-bot · 2026-04-27T00:00:09Z

🔔 1st Reminder

Hey @jkczyz! 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.

This commit introduces an alternative way of splicing in funds without coin selection by requiring the full UTXO to be provided. Each UTXO's entire value (minus fees) is allocated towards the channel, which provides unified balance wallets a more intuitive API when splicing funds into the channel, as they don't particularly care about maintaining a portion of their balance onchain. To simplify the implementation, we require that contributions are not allowed to mix coin-selected inputs with manually-selected ones. Users will need to start a fresh contribution if they want to change the funding input mode.

There's no reason not to do so, and it allows us to fail earlier when the user's net contribution exceeds their spliceable balance.

ldk-reviews-bot · 2026-04-29T00:01:00Z

🔔 2nd Reminder

Hey @jkczyz! 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.

TheBlueMatt

thanks

jkczyz · 2026-04-30T02:38:44Z

+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+enum FundingInputMode {
+	CoinSelected,
+	Manual,


Could we make this ManuallySelected to match the one in FundingInputs?

jkczyz · 2026-04-30T02:39:46Z

+	/// Whether this contribution currently uses coin-selected or manual-input semantics.
+	///
+	/// This is `None` when the contribution has no inputs. Builders resuming from a prior
+	/// contribution use the next non-empty input source to establish the mode again.


"next non-empty input source" is a little confusing. What is "next" in reference to?

jkczyz · 2026-04-30T03:15:14Z

 	value_added: Amount,
+	manually_selected_inputs: Vec<FundingTxInput>,


Seems making these an enum would simplify some of the checks? Or is there a good argument for keeping these separate fields? It would essentially be changing FundingInputs to own the manual inputs and using that here.

jkczyz · 2026-04-30T03:46:12Z

+					(Some(value_added), contribution.inputs, Some(FundingInputMode::CoinSelected))
+				},
+				FundingInputs::ManuallySelected { inputs } => {
+					(None, inputs.to_vec(), Some(FundingInputMode::Manual))


FWIW, with the enum approach we could avoid this allocation.

elnosh

Not that familiar with splicing-related code in the codebase but looking at this for learning purposes. After much staring, changes make sense although agree with @jkczyz comment on making an enum since those fields (value_added and manually_selected_inputs) are mutually exclusive in the ways inputs can be provided.

elnosh · 2026-04-30T22:25:07Z

+/// used if the request cannot be satisfied by reusing a prior contribution, by using only
+/// manually selected inputs, or by building a pure splice-out directly.


I'm not sure I follow this comment. If we have a builder with either sync or async CoinSelectionSource then manually added inputs are not allowed.

elnosh · 2026-04-30T22:30:50Z

 			return Err(FundingContributionError::InvalidSpliceValue);
 		}

+		validate_inputs(&self.manually_selected_inputs)?;


shouldn't happen but wouldn't hurt for this to check for duplicate inputs provided?

wpaulino added this to the 0.3 milestone Apr 23, 2026

wpaulino requested review from TheBlueMatt and jkczyz April 23, 2026 17:08

wpaulino self-assigned this Apr 23, 2026

wpaulino marked this pull request as ready for review April 23, 2026 17:08

ldk-claude-review-bot reviewed Apr 23, 2026

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wpaulino force-pushed the funding-contribution-builder-manual-inputs branch from 12e5994 to 412ec3d Compare April 23, 2026 17:19

TheBlueMatt reviewed Apr 24, 2026

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Comment thread lightning/src/ln/funding.rs Outdated

wpaulino added 2 commits April 28, 2026 14:43

Include spliceable balance in every FundingTemplate

228cae8

There's no reason not to do so, and it allows us to fail earlier when the user's net contribution exceeds their spliceable balance.

wpaulino force-pushed the funding-contribution-builder-manual-inputs branch from 412ec3d to 228cae8 Compare April 28, 2026 22:00

wpaulino requested a review from TheBlueMatt April 29, 2026 16:58

TheBlueMatt approved these changes Apr 29, 2026

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jkczyz reviewed Apr 30, 2026

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elnosh reviewed Apr 30, 2026

View reviewed changes

	if !self.inputs.is_empty() && self.input_mode == Some(FundingInputMode::CoinSelected) {
	if !self.inputs.is_empty() && self.input_mode != Some(FundingInputMode::Manual) {

	if prior_contribution.input_mode == Some(FundingInputMode::CoinSelected)
	if prior_contribution.input_mode != Some(FundingInputMode::Manual)
	&& !prior_contribution.inputs.is_empty()

		value_added: Amount,
		manually_selected_inputs: Vec<FundingTxInput>,

		/// used if the request cannot be satisfied by reusing a prior contribution, by using only
		/// manually selected inputs, or by building a pure splice-out directly.

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Review Summary

Previously Flagged Issues (still open)

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