Add Shared Input support in interactive TX construction by optout21 · Pull Request #3842 · lightningdevkit/rust-lightning

optout21 · 2025-06-10T15:40:49Z

In interactive TX construction, add support for shared input:

if we expect a shared input, make sure that the remote node provides it
if we provide a shared input, make sure that it's accounted appropriately

Additionally, the prevtx field of the TxAddInput message is changed to Optional, as it should not be set for the shared input (it cannot, as the full funding transaction is not stored on the acceptor side) (spec discussion: lightning/bolts#1160 (comment))

To be used by splicing, see #3736 .

Note: this PR does not include splicing negotiation.

ldk-reviews-bot · 2025-06-10T15:40:52Z

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ldk-reviews-bot · 2025-06-14T00:00:19Z

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optout21 · 2025-06-16T12:45:46Z

Great, can you push those changes here then and rework the shared funding input work to follow a similar approach?

I managed to do it: first I refactored the shared output support as discussed, then added shared input support in a similar style. Although this PR does not include spicing, I also checked the changes with splicing (shared input is used there).

optout21 · 2025-06-17T03:49:12Z

@wpaulino thanks for the comments. I will process them (I've re-requested review accidentally, please ignore that).

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codecov · 2025-06-19T08:28:59Z

Codecov Report

Attention: Patch coverage is 89.61240% with 67 lines in your changes missing coverage. Please review.

Project coverage is 90.10%. Comparing base (bed20cf) to head (e4dd8be).
Report is 34 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/interactivetxs.rs	90.30%	51 Missing and 7 partials ⚠️
lightning/src/ln/channel.rs	54.54%	5 Missing ⚠️
lightning/src/util/ser.rs	63.63%	4 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #3842 +/- ##
==========================================
+ Coverage 89.72% 90.10% +0.38% 
==========================================
Files 164 165 +1 Lines 133359 132400 -959 Branches 133359 132400 -959 ==========================================
- Hits 119651 119296 -355 + Misses 11037 10728 -309 + Partials 2671 2376 -295

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optout21 · 2025-07-02T20:40:45Z

Review comments addressed, some minor (doc) changes applied.
I've squashed the fixups into the 2 main commits.
Ready for review from my part.

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ghost · 2025-07-07T18:51:03Z

+ if is_initiator {
 our_funding_inputs_weight =
- our_funding_inputs_weight.saturating_add(estimate_input_weight(output).to_wu());
- } else {
- return Err(AbortReason::PrevTxOutInvalid);
+ our_funding_inputs_weight.saturating_add(P2WSH_INPUT_WEIGHT_LOWER_BOUND);


As a reader of the method though, it seems clearer to keep all the additional weight tracking when we're the initiator in one place.

ghost · 2025-07-09T00:02:35Z

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ghost

Taking this PR on since @optout21 is unavailable for a week.

ghost · 2025-07-10T16:20:06Z

+ if is_initiator {
 our_funding_inputs_weight =
- our_funding_inputs_weight.saturating_add(estimate_input_weight(output).to_wu());
- } else {
- return Err(AbortReason::PrevTxOutInvalid);
+ our_funding_inputs_weight.saturating_add(P2WSH_INPUT_WEIGHT_LOWER_BOUND);


I ended up moving this since doing so is more consistent with how the shared output is included in the weight. That is, shared output weight is not included in our_funding_outputs_weight, so including the shared input in our_funding_inputs_weight is not consistent.

ghost

LGTM after squash

ghost · 2025-07-10T22:04:59Z

+ script_pubkey: funding.get_funding_redeemscript().to_p2wsh(),
+ };

 let interactive_tx_constructor = Some(InteractiveTxConstructor::new(


Pre-existing, but it looks like we shouldn't be initializing a constructor now, and should be relying on begin_interactive_funding_tx_construction to do it instead. Let's leave it for a follow-up.

This simplifies tracking separately the expected and actual shared output. In the initiator case, we can just provide the shared output separately, instead of including it within other outputs, and marking which one is the output. We can use the same field for the intended shared output in the initiator case, and the expected one in the acceptor case.

In interactivetxs, add support for shared inputs, similar to shared outputs. A shared input is optional, and is used in case of splicing to add the current funding as an input.

ghost · 2025-07-12T00:00:32Z

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ghost · 2025-07-14T00:01:00Z

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Instead of calculating the fee for the entire weight contributed by the counterparty, each portion of the weight was calculated individually before summed. That caused a rounding error that is avoided by summing the weights first.

ghost · 2025-07-15T21:32:06Z

Merging with one approval since @jkczyz took over and his is implicit.

optout21 mentioned this pull request Jun 10, 2025
[Splicing] Tx negotiation during splicing #3736
Merged

optout21 force-pushed the shared-input branch from 660dd6b to f63c8a4Compare June 11, 2025 18:53

optout21 marked this pull request as ready for review June 11, 2025 19:03

optout21 requested review from dunxen, jkczyz and wpaulino June 11, 2025 19:03

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Comment threadlightning/src/ln/msgs.rs Outdated
Comment threadlightning/src/ln/msgs.rs
Comment threadlightning/src/ln/interactivetxs.rs Outdated
Comment threadlightning/src/ln/interactivetxs.rs Outdated
Comment threadlightning/src/ln/interactivetxs.rs Outdated

optout21 force-pushed the shared-input branch from f63c8a4 to 9151a72Compare June 16, 2025 12:43

optout21 force-pushed the shared-input branch 5 times, most recently from 166e364 to b2e0da8Compare June 16, 2025 13:47

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optout21 force-pushed the shared-input branch from 967dfa3 to 5fb333dCompare June 19, 2025 08:12

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Comment threadlightning/src/ln/interactivetxs.rs Outdated
Comment threadlightning/src/ln/interactivetxs.rs Outdated

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Comment threadlightning/src/ln/interactivetxs.rs

jkczyz force-pushed the shared-input branch from ee896e7 to f45fe4fCompare July 10, 2025 21:48

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Comment threadlightning/src/ln/interactivetxs.rs

optout added 2 commits July 11, 2025 09:45

Add Shared Input support
c18422d
In interactivetxs, add support for shared inputs, similar to shared outputs. A shared input is optional, and is used in case of splicing to add the current funding as an input.

jkczyz reviewed Jul 11, 2025
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Comment threadlightning/src/ln/interactivetxs.rs Outdated

jkczyz force-pushed the shared-input branch from 92f5e21 to c18422dCompare July 11, 2025 15:13

Fix rounding bug when checking counterparty fees
cd2ffaf
Instead of calculating the fee for the entire weight contributed by the counterparty, each portion of the weight was calculated individually before summed. That caused a rounding error that is avoided by summing the weights first.

graphite-appBot reviewed Jul 15, 2025
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Comment threadlightning/src/ln/interactivetxs.rs

wpaulino approved these changes Jul 15, 2025
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wpaulino merged commit 1d507f2 into lightningdevkit:mainJul 15, 2025

github-project-automationBot moved this to Done in Weekly Goals Jul 15, 2025

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