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

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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 and verified the current code state. All previously flagged issues have been checked, and I've searched for any new issues.

Review Summary

Previously Flagged Issues — Status

FIXED — lightning/src/ln/funding.rs — .take() destroying funding_inputs on coin selection fallback path is now .clone() at line 1229.
Still open — lightning/src/ln/funding.rs:933 — Backwards-compat: old persisted coin-selected contributions have input_mode == None, causing them to take the manual-inputs fee buffer branch instead of the coin-selected branch in compute_feerate_adjustment. This drops the change output and miscalculates the fee buffer.
Still open — lightning/src/ln/funding.rs:1401-1412 — Backwards-compat: old coin-selected priors with input_mode == None get funding_inputs = None in builder init, silently discarding their inputs when the request doesn't match and amend_without_coin_selection is called.
Still open — lightning/src/ln/funding.rs:1375-1381 — Duplicate-output validation (by script_pubkey) shadows MAX_MONEY sum tests since funding_output_sats() always produces the same script_pubkey.

No New Issues Found

The new code is otherwise correct. Specifically verified:

FundingInputs/FundingInputMode enums and their serialization
Builder add_input/add_inputs/remove_input methods correctly enforce mode exclusivity
splice_in_inputs convenience method correctly appends to prior manual inputs
ManuallySelectedInputsInsufficient error correctly does NOT fall through to coin selection in both sync and async builders
Fee buffer calculation for manual inputs in compute_feerate_adjustment is correct for positive and negative net_value_without_fee
validate_inputs duplicate detection covers the combined set (prior + new) of manually selected inputs
validate_contribution_parameters runs before build_from_prior_contribution, catching duplicates and MAX_MONEY violations
request_matches_prior correctly compares outpoints for manual inputs and value for coin-selected inputs
net_value_without_fee and net_value_at_feerate arithmetic is sound for manual inputs
Spliceable balance check in try_build_without_coin_selection correctly gates net-negative manual contributions

codecov · 2026-04-23T18:30:17Z

Codecov Report

❌ Patch coverage is 96.14874% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.58%. Comparing base (f408b17) to head (dcca939).
⚠️ Report is 33 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/funding.rs	96.07%	15 Missing and 14 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4575      +/-   ##
==========================================
+ Coverage   86.15%   86.58%   +0.43%     
==========================================
  Files         157      159       +2     
  Lines      109096   110419    +1323     
  Branches   109096   110419    +1323     
==========================================
+ Hits        93989    95611    +1622     
+ Misses      12485    12275     -210     
+ Partials     2622     2533      -89

Flag	Coverage Δ
fuzzing-fake-hashes	`6.57% <0.00%> (?)`
fuzzing-real-hashes	`23.17% <3.67%> (?)`
tests	`86.23% <96.14%> (+0.07%)`	⬆️

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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.

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

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.

Manually selected inputs are still allowed as long as the added_value is 0.

TheBlueMatt · 2026-05-07T21:31:01Z

Needs rebase again :/

ldk-claude-review-bot · 2026-05-13T18:47:37Z

+		for (idx, output) in self.outputs.iter().enumerate() {
+			if self.outputs[..idx]
+				.iter()
+				.any(|existing_output| existing_output.script_pubkey == output.script_pubkey)
+			{
+				return Err(FundingContributionError::InvalidSpliceValue);


Nit / test gap: this new duplicate-output check fires before the value_removed > MAX_MONEY accumulation below. Since funding_output_sats() always returns the same script_pubkey (new_p2wpkh(&WPubkeyHash::all_zeros())), the existing tests test_build_funding_contribution_validates_max_money ("splice_out with multiple outputs summing > MAX_MONEY", lines 2723-2734) and test_funding_builder_validates_mixed_request_max_money ("outputs summing > MAX_MONEY", lines 2755-2769) now hit this duplicate check first. They still pass because both paths yield InvalidSpliceValue, but the MAX_MONEY sum-of-outputs path has lost test coverage.

Consider using distinct script_pubkey values for each output in those tests.

TheBlueMatt · 2026-05-17T23:02:22Z

+		let value_added =
+			inner.funding_inputs.as_ref().map_or(Amount::ZERO, FundingInputs::value_added);
+
 		match inner.build_without_coin_selection() {


Its quite awkward that we call this here and it takes several fields from inner internally but then we reuse inner a few lines down as well as later. It probably makes sense to pay the penalty of either calling prepare_coin_selection_request upfront so we can make build_without_coin_selection take mut self without ref or cloneing internally to avoid ripping out parts of inner before we reuse it. Seems like too much opportunity for stuff to go wrong later.

Alright just decided to clone internally

Sorry, this wasn't the only place, we also take the prior_contribution in try_build_without_coin_selection and mutate inner that way too.

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.

While this is already enforced when we get to the interactive negotiation phase, we choose to fail early anyway.

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 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

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jkczyz reviewed Apr 30, 2026

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

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

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

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

elnosh reviewed Apr 30, 2026

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wpaulino dismissed TheBlueMatt’s stale review via 17c864b May 5, 2026 23:36

wpaulino force-pushed the funding-contribution-builder-manual-inputs branch 3 times, most recently from c3da7fe to 6e16493 Compare May 6, 2026 19:27

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

wpaulino mentioned this pull request May 8, 2026

Drop QuiescentError::with_negotiation_failure_reason #4608

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wpaulino force-pushed the funding-contribution-builder-manual-inputs branch from 6e16493 to c240f1b Compare May 13, 2026 18:40

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TheBlueMatt previously approved these changes May 17, 2026

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wpaulino dismissed TheBlueMatt’s stale review via ed757b2 May 18, 2026 20:13

wpaulino force-pushed the funding-contribution-builder-manual-inputs branch from c240f1b to ed757b2 Compare May 18, 2026 20:13

wpaulino requested review from TheBlueMatt and jkczyz May 18, 2026 20:14

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wpaulino force-pushed the funding-contribution-builder-manual-inputs branch from ed757b2 to a9fa610 Compare May 18, 2026 22:04

wpaulino added 3 commits May 19, 2026 10:58

Include spliceable balance in every FundingTemplate

740ebb1

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.

Detect duplicate inputs and outputs upon building FundingBuilder

dcca939

While this is already enforced when we get to the interactive negotiation phase, we choose to fail early anyway.

wpaulino force-pushed the funding-contribution-builder-manual-inputs branch from a9fa610 to dcca939 Compare May 19, 2026 18:00

wpaulino requested a review from TheBlueMatt May 19, 2026 18:00

TheBlueMatt approved these changes May 19, 2026

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jkczyz approved these changes May 19, 2026

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jkczyz merged commit e47a231 into lightningdevkit:main May 19, 2026
24 checks passed

wpaulino deleted the funding-contribution-builder-manual-inputs branch May 19, 2026 20:08

	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()

		/// 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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wpaulino commented Apr 23, 2026

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Review Summary

Previously Flagged Issues — Status

No New Issues Found

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