lnd+sweep: move DetermineFeePerKw into config

This commit makes `DetermineFeePerKw` configurable on sweeper so it's
easier to write unit tests for it.

server.go

@@ -1060,6 +1060,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 
 	s.sweeper = sweep.New(&sweep.UtxoSweeperConfig{
 		FeeEstimator:      cc.FeeEstimator,
+		DetermineFeePerKw: sweep.DetermineFeePerKw,
 		GenSweepScript:    newSweepPkScriptGen(cc.Wallet),
 		Signer:            cc.Wallet.Cfg.Signer,
 		Wallet:            newSweeperWallet(cc.Wallet),

sweep/sweeper.go

@@ -237,12 +237,21 @@ type UtxoSweeper struct {
 	wg   sync.WaitGroup
 }
 
+// feeDeterminer defines an alias to the function signature of
+// `DetermineFeePerKw`.
+type feeDeterminer func(chainfee.Estimator,
+	FeePreference) (chainfee.SatPerKWeight, error)
+
 // UtxoSweeperConfig contains dependencies of UtxoSweeper.
 type UtxoSweeperConfig struct {
 	// GenSweepScript generates a P2WKH script belonging to the wallet where
 	// funds can be swept.
 	GenSweepScript func() ([]byte, error)
 
+	// DetermineFeePerKw determines the fee in sat/kw based on the given
+	// estimator and fee preference.
+	DetermineFeePerKw feeDeterminer
+
 	// FeeEstimator is used when crafting sweep transactions to estimate
 	// the necessary fee relative to the expected size of the sweep
 	// transaction.
@@ -470,7 +479,9 @@ func (s *UtxoSweeper) feeRateForPreference(
 		return 0, ErrNoFeePreference
 	}
 
-	feeRate, err := DetermineFeePerKw(s.cfg.FeeEstimator, feePreference)
+	feeRate, err := s.cfg.DetermineFeePerKw(
+		s.cfg.FeeEstimator, feePreference,
+	)
 	if err != nil {
 		return 0, err
 	}
@@ -1599,7 +1610,7 @@ func (s *UtxoSweeper) handleUpdateReq(req *updateReq, bestHeight int32) (
 func (s *UtxoSweeper) CreateSweepTx(inputs []input.Input, feePref FeePreference,
 	currentBlockHeight uint32) (*wire.MsgTx, error) {
 
-	feePerKw, err := DetermineFeePerKw(s.cfg.FeeEstimator, feePref)
+	feePerKw, err := s.cfg.DetermineFeePerKw(s.cfg.FeeEstimator, feePref)
 	if err != nil {
 		return nil, err
 	}

sweep/sweeper_test.go

@@ -149,6 +149,7 @@ func createSweeperTestContext(t *testing.T) *sweeperTestContext {
 		},
 		MaxFeeRate:        DefaultMaxFeeRate,
 		FeeRateBucketSize: DefaultFeeRateBucketSize,
+		DetermineFeePerKw: DetermineFeePerKw,
 	})
 
 	ctx.sweeper.Start()