chancloser: account for aux close outputs in initial coop close fee baseline

[darioAnongba]

Problem

Custom channel coop closes can fail with: unable to process close msg: insufficient fee.

Root cause

For taproot/custom channels, ChanCloser computed the initial coop close fee (idealFeeSat) from only local+remote outputs, but the final close tx can include auxiliary extra outputs.

That mismatch can underprice the first accepted close offer (notably in taproot fast-close), causing mempool rejection and peer disconnect.

Fix

Extend coop close fee weight calculation to include optional extra outputs.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical issue where cooperative channel closes, particularly for taproot and custom channels with auxiliary outputs, could fail due to insufficient fee estimation. The ChanCloser mechanism now accurately accounts for these additional outputs when calculating the initial cooperative close fee, preventing transactions from being underpriced and subsequently rejected by the mempool. This ensures smoother and more reliable channel closures for advanced channel types.

Highlights

• Cooperative Close Fee Calculation Enhancement: The calcCoopCloseFee function was updated to accept and incorporate optional auxiliary transaction outputs into its fee weight estimation, ensuring more accurate fee calculations for cooperative channel closes.
• Iterative Fee Estimation for Auxiliary Outputs: A new estimateCloseFee function was introduced to handle the complexity of auxiliary outputs, which may depend on the close fee itself. This function iteratively calculates the fee to ensure consistency between the fee and the set of auxiliary outputs.
• Improved Initial Fee Baseline: The initFeeBaseline method now utilizes the new estimateCloseFee function, preventing underpriced cooperative closes for taproot and custom channels that include auxiliary outputs, thereby avoiding mempool rejection and peer disconnects.
• Comprehensive Testing: New test cases, including TestInitFeeBaselineWithAuxCloseOutputs, were added to validate the correct accounting of auxiliary close outputs in the fee baseline calculation, ensuring the robustness of the fix.
• Release Note Update: A release note entry was added to document the fix for cooperative close fee baseline issues when channels include auxiliary close outputs.

🧠 New Feature in Public Preview: You can now enable Memory to help Gemini Code Assist learn from your team's feedback. This makes future code reviews more consistent and personalized to your project's style. Click here to enable Memory in your admin console.

Changelog

• docs/release-notes/release-notes-0.21.0.md
  • Updated bullet point formatting from '*' to '-' for consistency.
  • Added a release note entry detailing the fix for cooperative close fee baseline calculation to include auxiliary outputs.
• lnwallet/chancloser/chancloser.go
  • Modified calcCoopCloseFee to accept an extraOutputs parameter for including auxiliary transaction outputs in fee weight estimation.
  • Implemented a loop within calcCoopCloseFee to add the weights of extraOutputs to the total transaction weight.
  • Updated SimpleCoopFeeEstimator.EstimateFee to pass nil for extraOutputs when calling calcCoopCloseFee.
  • Introduced estimateCloseFee to handle iterative fee calculation for auxiliary close outputs, ensuring self-consistent fee and output sets.
  • Changed the signature of initFeeBaseline to return an error.
  • Modified initFeeBaseline to use estimateCloseFee for calculating idealFeeSat and maxFee, incorporating auxiliary output considerations.
  • Added error handling for estimateCloseFee calls within initFeeBaseline.
  • Updated BeginNegotiation to handle the potential error returned by initFeeBaseline.
• lnwallet/chancloser/chancloser_test.go
  • Imported wallettypes package.
  • Added mockAuxChanCloser struct and its associated methods (ShutdownBlob, AuxCloseOutputs, FinalizeClose) to simulate auxiliary channel closer behavior for testing.
  • Updated TestMaxFeeClamp to use require.NoError when calling chanCloser.initFeeBaseline().
  • Added TestInitFeeBaselineWithAuxCloseOutputs to specifically verify that auxiliary close outputs are correctly factored into the initial fee baseline calculation.
  • Updated TestTaprootFastClose to use require.NoError for initFeeBaseline() calls.

Activity

• The pull request was created by darioAnongba to address a known issue with cooperative channel closes failing due to incorrect fee estimation for channels with auxiliary outputs.
• The changes involve modifying core fee calculation logic and adding new functions to handle iterative fee estimation, along with comprehensive test coverage.

Using Gemini Code Assist

The full guide for Gemini Code Assist can be found on our documentation page, here are some quick tips.

Invoking Gemini

You can request assistance from Gemini at any point by creating a comment using either /gemini <command> or @gemini-code-assist <command>. Below is a summary of the supported commands on the current page.

Feature	Command	Description
Code Review	/gemini review	Performs a code review for the current pull request in its current state.
Pull Request Summary	/gemini summary	Provides a summary of the current pull request in its current state.
Comment	@gemini-code-assist	Responds in comments when explicitly tagged, both in pull request comments and review comments.
Help	/gemini help	Displays a list of available commands.

Customization

To customize Gemini Code Assist for GitHub experience, repository maintainers can create a configuration file and/or provide a custom code review style guide (such as PEP-8 for Python) by creating and adding files to a .gemini/ folder in the base of the repository. Detailed instructions can be found here.

Limitations & Feedback

Gemini Code Assist may make mistakes. Please leave feedback on any instances where its feedback is incorrect or counter productive. You can react with 👍 and 👎 on @gemini-code-assist comments. If you're interested in giving your feedback about your experience with Gemini Code Assist for Github and other Google products, sign up here.

You can also get AI-powered code generation, chat, as well as code reviews directly in the IDE at no cost with the Gemini Code Assist IDE Extension.

Footnotes

1. Review the Privacy Notices, Generative AI Prohibited Use Policy, Terms of Service, and learn how to configure Gemini Code Assist in GitHub here. Gemini can make mistakes, so double check it and use code with caution. ↩

[gemini-code-assist]

Code Review

This pull request addresses a problem where custom channel cooperative closes could fail due to insufficient fee estimation when auxiliary close outputs were present. The changes correctly extend the fee weight calculation to include these extra outputs, preventing underpriced close offers. The pull request also includes updates to release notes and new test cases to validate the fix. Overall, the changes improve the robustness of the channel closing mechanism for taproot and custom channels.

[GeorgeTsagk]

Nice, cACK

A few things to help with context:

• How did we discover this edge case / behavior? Does it change now?
• Will this be accompanied by a tapd itest?

[jtobin]

This looks pretty solid. I made one comment re: adding a test for the fee-bump-yields-different-outputs logic that I believe is not strongly exercised at present. What do you think?

[github-actions]

🔴 PR Severity: CRITICAL

Automated classification | 3 files | 200 lines changed

🔴 Critical (1 file)

• lnwallet/chancloser/chancloser.go - wallet operations / channel closing logic (lnwallet/*)

🟢 Low (1 file, 1 excluded)

• docs/release-notes/release-notes-0.21.0.md - release notes (docs/*.md)
• lnwallet/chancloser/chancloser_test.go — excluded (*_test.go)

Analysis

This PR modifies lnwallet/chancloser/chancloser.go, which falls under the lnwallet/* package — a CRITICAL package covering wallet operations, channel funding, signing, and commitment transactions. The change is focused on the cooperative channel-closing state machine (chancloser), which handles the negotiation and signing of closing transactions. Any defects here can result in funds being locked or lost.

The PR is small in scope (82 additions / 9 deletions in the production file) and is accompanied by a test file update, but the sensitivity of the code path warrants expert review regardless of size.

No bump rules were triggered:

• Non-test, non-generated files changed: 2 (< 20)
• Non-test, non-generated lines changed: ~97 (< 500)
• Only one distinct critical package touched

---

_{To override, add a severity-override-{critical,high,medium,low} label.}
<!-- pr-severity-bot -->

[darioAnongba]

See regression test here: lightninglabs/taproot-assets#2091

[GeorgeTsagk]

lgtm, tACK

[jtobin]

LGTM. 👌

[Roasbeef]

LGTM 🐧

[ziggie1984]

I find this quite fragile and cannot fully understand reason why we need to do 2 passes here, can you elaborate ?

[lightninglabs-deploy]

@hieblmi: review reminder
@darioAnongba, remember to re-request review from reviewers when ready