chainfee: make relay fee floor configurable via --fee.min-relay-feerate

[Kilombino]

Summary

LND currently hardcodes a minimum fee rate floor of 253 sat/kw (~1 sat/vb) in the chainfee package. This floor is applied in three places:

• minFeeManager: clamps the fee returned by the chain backend
• WebAPIEstimator.EstimateFeePerKW: clamps the cached API result
• sweep/walletsweep.go: silently bumps 1 sat/vb inputs to 253 sat/kw

This prevents operators who run a Bitcoin backend with a lower minrelaytxfee (e.g. a personal mining setup, signet, regtest, or a node connected directly to a cooperative miner) from opening channels or sweeping funds below 1 sat/vb.

This PR introduces a new --fee.min-relay-feerate config option (sat/vb) that lets operators lower the floor when their setup permits it. The default value is unchanged (1 sat/vb = 253 sat/kw), so existing nodes are unaffected.

Changes

File	Change
lnwallet/chainfee/minfeemanager.go	feeFloor SatPerKWeight field + param; replaces constant clamp
lnwallet/chainfee/estimator.go	feeFloor field on BtcdEstimator, BitcoindEstimator, WebAPIEstimator; constructors updated; hardcoded FeePerKwFloor references replaced
sweep/walletsweep.go	Remove unconditional 250→253 bump; the existing minFeeRate check downstream already enforces the relay floor
chainreg/chainregistry.go	Pass cfg.Fee.FeeFloorKW() to all three estimator constructors
lncfg/fee.go	MinRelayFeeRate chainfee.SatPerVByte field + Validate() + FeeFloorKW() helper
sample-lnd.conf	Document the new option
lnwallet/chainfee/minfeemanager_test.go	Update existing test; add two new tests for custom floor behaviour
lnwallet/chainfee/estimator_test.go	Update NewWebAPIEstimator calls for new signature

Usage

[fee]
# Lower the floor for a node with minrelaytxfee=0 in bitcoin.conf
fee.min-relay-feerate=0

Safety

The commit message and sample-lnd.conf include a clear warning:

Lowering this value below 1 sat/vb means your node will create transactions that standard Bitcoin nodes will not relay. Only use this option if your backend has a matching minrelaytxfee and you have a direct path to a miner's mempool.

Test plan

• go test ./lnwallet/chainfee/... — existing tests pass, new floor tests pass
• go test ./sweep/... — walletsweep tests unaffected
• go build ./... — compiles cleanly
• Start lnd with fee.min-relay-feerate=0; open a channel specifying a fee rate of 100 sat/kw; verify the channel opens without the old "fee too low" error

---

Fixes: operators running nodes with minrelaytxfee=0 or sub-1 sat/vb mempool policies unable to transact at their configured fee rates.

[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 enables greater flexibility for LND node operators running on custom Bitcoin backends with non-standard mempool policies. By introducing a configurable minimum relay fee rate, operators can now transact at lower fee rates than the default 1 sat/vb, provided their infrastructure supports it. The changes maintain backward compatibility by defaulting to the existing 1 sat/vb floor.

Highlights

• Configurable Fee Floor: Introduced a new configuration option --fee.min-relay-feerate allowing operators to lower the minimum relay fee rate floor below the default 1 sat/vb (253 sat/kw).
• Refactored Fee Estimation: Updated minFeeManager and fee estimators (BtcdEstimator, BitcoindEstimator, WebAPIEstimator) to support a configurable fee floor instead of relying on hardcoded constants.
• Removed Hardcoded Bumps: Removed the unconditional 250→253 sat/kw bump in walletsweep.go, allowing downstream enforcement of the configured relay floor.

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[github-actions]

PR Severity: CRITICAL. Automated classification | 6 files | 127 lines changed (excl tests). Critical files: lnwallet/chainfee/estimator.go, lnwallet/chainfee/minfeemanager.go (lnwallet/* package), sweep/walletsweep.go (sweep/* package). Medium files: chainreg/chainregistry.go, lncfg/fee.go, sample-lnd.conf. Two distinct critical packages touched. Expert review required. <!-- pr-severity-bot -->

[gemini-code-assist]

Code Review

This pull request introduces a configurable minimum relay fee rate floor, allowing operators to specify values below the default 1 sat/vb. The changes update the configuration struct, fee estimators, and the minimum fee manager to respect the new setting. Review feedback identifies that using zero as a sentinel value in the configuration and estimator logic prevents the implementation of an actual zero floor. Additionally, a regression was noted in the wallet sweep logic regarding the handling of 1 sat/vb fee rates, and a documentation error was found in the fee rate conversion description.

[lightninglabs-deploy]

@Kilombino, remember to re-request review from reviewers when ready