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.

[lightninglabs-deploy]

🔴 PR Severity: CRITICAL

Automated classification | 2 files | 98 lines changed

🔴 Critical (1 file)

• lnwallet/chancloser/chancloser.go - Falls under lnwallet/*: wallet operations, channel funding, and commitment transactions — requires expert review

🟢 Low (1 file)

• docs/release-notes/release-notes-0.21.0.md - Release notes documentation

Analysis

This PR modifies lnwallet/chancloser/chancloser.go, which is part of the lnwallet/* package — a CRITICAL severity category covering wallet operations, channel funding, signing, and commitment transactions. Any changes here directly affect on-chain fund safety and cooperative channel closing logic, warranting expert review.

Severity bump check: No bump needed — only 2 non-test files changed (~98 lines), and a single critical package is touched.

The test file (lnwallet/chancloser/chancloser_test.go) and the release notes (docs/release-notes/release-notes-0.21.0.md) were excluded from the line/file count per policy.

---

_{To override, add a severity-override-{critical,high,medium,low} label.}

[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.

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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.

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[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.

[gemini-code-assist]

The function signature for calcCoopCloseFee has been updated to include extraOutputs. Please update the function comment to reflect this new parameter and its purpose, as per the style guide rule that every function must be commented with its purpose and assumptions (Repository Style Guide, line 17).

[gemini-code-assist]

The initFeeBaseline function now returns an error. The function comment should be updated to reflect this change, explaining when an error might be returned. This aligns with the style guide's requirement for function comments to be complete sentences and describe the function's purpose and assumptions (Repository Style Guide, lines 17, 19).