Add Configurable Fee Functions to Sweeper with Dynamic Selection

[gitsofaryan]

Description

This PR enhances the LND UtxoSweeper by introducing configurable fee functions—LinearFeeFunction, CubicDelayFeeFunction, and CubicEagerFeeFunction—to optimize transaction fee estimation for sweeping UTXOs. Users can now specify the fee function type (linear, cubic_delay, or cubic_eager) and a base fee rate via configuration options (--sweeper.feefunctiontype and --sweeper.basefeerate). Additionally, a dynamic selection mechanism chooses the optimal fee function based on value density ((\rho)) and confirmation target (deadline_delta), derived from analysis in optimization.ipynb (gist: [pending upload, replace with link if available]).

The motivation is to improve fee efficiency: cubic_eager accelerates fee increases for urgent, high-value sweeps; cubic_delay delays increases for less urgent, low-value sweeps; and linear remains a balanced default. This is a feature enhancement and not tied to a specific issue, though it aligns with broader efforts to refine fee management in LND.

Steps to Test

1. Build the Code:

   git checkout feature/sweeper-fee-optimization
   make build

2. Run LND with Custom Fee Function:
   • Start LND with a specific fee function, e.g., cubic_eager:

     ./lnd --sweeper.feefunctiontype=cubic_eager --sweeper.basefeerate=10

   • Check logs for fee rate adjustments (e.g., Fee rate increased from X to Y at position Z).
3. Simulate a Sweep:
   • Force close a channel:

     lncli closechannel --force <channel_point>

   • Monitor sweep transaction via lncli pendingchannels or logs.
4. Test Dynamic Selection:
   • Run with default settings (unset or linear):

     ./lnd

   • Use a test script or manual channel close with varying UTXO values and deadlines to trigger:
     • cubic_eager ((\rho > 9008.22), deadline_delta < 13).
     • cubic_delay (deadline_delta >= 40 or >= 26 with (\rho < 65983.36)).
5. Run Unit Tests:

   make unit

   (Note: Existing tests should pass; new tests for fee functions are pending.)

Pull Request Checklist

Testing

• Your PR passes all CI checks. (Pending CI run after push; make fmt-check and make unit pass locally.)
• Tests covering the positive and negative (error paths) are included. (TODO: Unit tests for CubicDelayFeeFunction and CubicEagerFeeFunction, and integration tests for dynamic selection are pending.)
• Bug fixes contain tests triggering the bug to prevent regressions. (N/A; this is a feature addition, not a bug fix.)

Code Style and Documentation

• The change is not insubstantial. Typo fixes are not accepted to fight bot spam. (This is a substantial feature enhancement.)
• The change obeys the Code Documentation and Commenting guidelines, and lines wrap at 80. (Exported fields documented; struct tags exceed 80 chars due to descriptions, consistent with LND style.)
• Commits follow the Ideal Git Commit Structure. (Commit: feat: add configurable fee functions to sweeper with dynamic selection)
• Any new logging statements use an appropriate subsystem and logging level. (Uses log.Debugf and log.Errorf in sweep package, matching existing conventions.)
• Any new lncli commands have appropriate tags in the comments for the rpc in the proto file. (N/A; no new lncli commands added.)
• There is a change description in the release notes, or [skip ci] in the commit message for small changes. (TODO: Add release notes entry under docs/release-notes for this feature.)

📝 Please see our Contribution Guidelines for further guidance.

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[gitsofaryan]

Hey @ziggie1984 sir,
I’ve submitted a PR to enhance the LND UtxoSweeper with configurable fee functions (linear, cubic_delay, cubic_eager) and dynamic selection based on value density and deadline_delta. Could you please review it? I’ve included a detailed description and test steps, but unit/integration tests and release notes are still pending—I’d appreciate guidance on those. Thanks!🙏

[ziggie1984]

I guess you refer to the SoB project proposal here: LND - Sweeper Fee Function

Please complete the Competency Tests and apply properly via the SoB program.