sweep+lnrpc: add new interface FeePreference

This commit adds a new interface `FeePreference` which makes it easier
to write unit tests and allows more customized implementation in
following commits.

contractcourt/commit_sweep_resolver_test.go

@@ -1,6 +1,7 @@
 package contractcourt
 
 import (
+	"fmt"
 	"testing"
 	"time"
 
@@ -127,9 +128,15 @@ func (s *mockSweeper) SweepInput(input input.Input, params sweep.Params) (
 
 	s.sweptInputs <- input
 
+	// TODO(yy): use `mock.Mock` to avoid the conversion.
+	fee, ok := params.Fee.(sweep.FeeEstimateInfo)
+	if !ok {
+		return nil, fmt.Errorf("unexpected fee type: %T", params.Fee)
+	}
+
 	// Update the deadlines used if it's set.
-	if params.Fee.ConfTarget != 0 {
-		s.deadlines = append(s.deadlines, int(params.Fee.ConfTarget))
+	if fee.ConfTarget != 0 {
+		s.deadlines = append(s.deadlines, int(fee.ConfTarget))
 	}
 
 	result := make(chan sweep.Result, 1)

lnrpc/walletrpc/walletkit_server.go

@@ -884,7 +884,13 @@ func (w *WalletKit) PendingSweeps(ctx context.Context,
 		broadcastAttempts := uint32(pendingInput.BroadcastAttempts)
 		nextBroadcastHeight := uint32(pendingInput.NextBroadcastHeight)
 
-		requestedFee := pendingInput.Params.Fee
+		feePref := pendingInput.Params.Fee
+		requestedFee, ok := feePref.(sweep.FeeEstimateInfo)
+		if !ok {
+			return nil, fmt.Errorf("unknown fee preference type: "+
+				"%v", feePref)
+		}
+
 		requestedFeeRate := uint64(requestedFee.FeeRate.FeePerVByte())
 
 		rpcPendingSweeps = append(rpcPendingSweeps, &PendingSweep{

sweep/mocks.go

@@ -0,0 +1,29 @@
+package sweep
+
+import (
+	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
+	"github.com/stretchr/testify/mock"
+)
+
+type MockFeePreference struct {
+	mock.Mock
+}
+
+// Compile-time constraint to ensure MockFeePreference implements FeePreference.
+var _ FeePreference = (*MockFeePreference)(nil)
+
+func (m *MockFeePreference) String() string {
+	return "mock fee preference"
+}
+
+func (m *MockFeePreference) Estimate(estimator chainfee.Estimator,
+	maxFeeRate chainfee.SatPerKWeight) (chainfee.SatPerKWeight, error) {
+
+	args := m.Called(estimator, maxFeeRate)
+
+	if args.Get(0) == nil {
+		return 0, args.Error(1)
+	}
+
+	return args.Get(0).(chainfee.SatPerKWeight), args.Error(1)
+}

sweep/sweeper.go

@@ -68,7 +68,7 @@ type Params struct {
 	// Fee is the fee preference of the client who requested the input to be
 	// swept. If a confirmation target is specified, then we'll map it into
 	// a fee rate whenever we attempt to cluster inputs for a sweep.
-	Fee FeeEstimateInfo
+	Fee FeePreference
 
 	// Force indicates whether the input should be swept regardless of
 	// whether it is economical to do so.
@@ -84,7 +84,7 @@ type ParamsUpdate struct {
 	// Fee is the fee preference of the client who requested the input to be
 	// swept. If a confirmation target is specified, then we'll map it into
 	// a fee rate whenever we attempt to cluster inputs for a sweep.
-	Fee FeeEstimateInfo
+	Fee FeePreference
 
 	// Force indicates whether the input should be swept regardless of
 	// whether it is economical to do so.

sweep/sweeper_test.go

@@ -1,6 +1,7 @@
 package sweep
 
 import (
+	"errors"
 	"os"
 	"reflect"
 	"runtime/pprof"
@@ -335,7 +336,9 @@ func TestSuccess(t *testing.T) {
 	ctx := createSweeperTestContext(t)
 
 	// Sweeping an input without a fee preference should result in an error.
-	_, err := ctx.sweeper.SweepInput(spendableInputs[0], Params{})
+	_, err := ctx.sweeper.SweepInput(spendableInputs[0], Params{
+		Fee: &FeeEstimateInfo{},
+	})
 	if err != ErrNoFeePreference {
 		t.Fatalf("expected ErrNoFeePreference, got %v", err)
 	}
@@ -1100,7 +1103,9 @@ func TestBumpFeeRBF(t *testing.T) {
 	ctx.estimator.blocksToFee[highFeePref.ConfTarget] = highFeeRate
 
 	// We should expect to see an error if a fee preference isn't provided.
-	_, err = ctx.sweeper.UpdateParams(*input.OutPoint(), ParamsUpdate{})
+	_, err = ctx.sweeper.UpdateParams(*input.OutPoint(), ParamsUpdate{
+		Fee: &FeeEstimateInfo{},
+	})
 	if err != ErrNoFeePreference {
 		t.Fatalf("expected ErrNoFeePreference, got %v", err)
 	}
@@ -2141,9 +2146,12 @@ func TestSweeperShutdownHandling(t *testing.T) {
 func TestClusterByLockTime(t *testing.T) {
 	t.Parallel()
 
+	// Create a mock FeePreference.
+	mockFeePref := &MockFeePreference{}
+
 	// Create a test param with a dummy fee preference. This is needed so
 	// `feeRateForPreference` won't throw an error.
-	param := Params{Fee: FeeEstimateInfo{ConfTarget: 1}}
+	param := Params{Fee: mockFeePref}
 
 	// We begin the test by creating three clusters of inputs, the first
 	// cluster has a locktime of 1, the second has a locktime of 2, and the
@@ -2222,15 +2230,11 @@ func TestClusterByLockTime(t *testing.T) {
 	// minFeeRate will cause an error to be returned.
 	s.relayFeeRate = minFeeRate
 
-	// applyFeeRate takes a testing fee rate and makes a mocker over
-	// DetermineFeePerKw that always return the testing fee rate. This
-	// mocked method is then attached to the sweeper.
-	applyFeeRate := func(feeRate chainfee.SatPerKWeight) {
-		// TODO(yy): fix the test here.
-	}
-
 	testCases := []struct {
-		name                    string
+		name string
+		// setupMocker takes a testing fee rate and makes a mocker over
+		// `Estimate` that always return the testing fee rate.
+		setupMocker             func()
 		testFeeRate             chainfee.SatPerKWeight
 		expectedClusters        []inputCluster
 		expectedRemainingInputs pendingInputs
@@ -2240,6 +2244,14 @@ func TestClusterByLockTime(t *testing.T) {
 			// are created and the no-locktime cluster is returned
 			// as the remaining inputs.
 			name: "successfully create clusters",
+			setupMocker: func() {
+				mockFeePref.On("Estimate",
+					s.cfg.FeeEstimator,
+					s.cfg.MaxFeeRate.FeePerKWeight(),
+				// Expect the four inputs with locktime to call
+				// this method.
+				).Return(minFeeRate+1, nil).Times(4)
+			},
 			// Use a fee rate above the min value so we don't hit
 			// an error when performing fee estimation.
 			//
@@ -2267,6 +2279,14 @@ func TestClusterByLockTime(t *testing.T) {
 			// Test that when the input is skipped when the fee
 			// estimation returns an error.
 			name: "error from fee estimation",
+			setupMocker: func() {
+				mockFeePref.On("Estimate",
+					s.cfg.FeeEstimator,
+					s.cfg.MaxFeeRate.FeePerKWeight(),
+				).Return(chainfee.SatPerKWeight(0),
+					errors.New("dummy")).Times(4)
+			},
+
 			// Use a fee rate below the min value so we hit an
 			// error when performing fee estimation.
 			testFeeRate:      minFeeRate - 1,
@@ -2283,7 +2303,10 @@ func TestClusterByLockTime(t *testing.T) {
 		t.Run(tc.name, func(t *testing.T) {
 			// Apply the test fee rate so `feeRateForPreference` is
 			// mocked to return the specified value.
-			applyFeeRate(tc.testFeeRate)
+			tc.setupMocker()
+
+			// Assert the mocked methods are called as expeceted.
+			defer mockFeePref.AssertExpectations(t)
 
 			// Call the method under test.
 			clusters, remainingInputs := s.clusterByLockTime(inputs)

sweep/walletsweep.go

@@ -33,6 +33,24 @@ var (
 	ErrFeePreferenceConflict = errors.New("fee preference conflict")
 )
 
+// FeePreference defines an interface that allows the caller to specify how the
+// fee rate should be handled. Depending on the implementation, the fee rate
+// can either be specified directly, or via a conf target which relies on the
+// chain backend(`bitcoind`) to give a fee estimation, or a customized fee
+// function which handles fee calculation based on the specified
+// urgency(deadline).
+type FeePreference interface {
+	// String returns a human-readable string of the fee preference.
+	String() string
+
+	// Estimate takes a fee estimator and a max allowed fee rate and
+	// returns a fee rate for the given fee preference. It ensures that the
+	// fee rate respects the bounds of the relay fee and the specified max
+	// fee rates.
+	Estimate(chainfee.Estimator,
+		chainfee.SatPerKWeight) (chainfee.SatPerKWeight, error)
+}
+
 // FeeEstimateInfo allows callers to express their time value for inclusion of
 // a transaction into a block via either a confirmation target, or a fee rate.
 type FeeEstimateInfo struct {
@@ -45,12 +63,16 @@ type FeeEstimateInfo struct {
 	FeeRate chainfee.SatPerKWeight
 }
 
+// Compile-time constraint to ensure FeeEstimateInfo implements FeePreference.
+var _ FeePreference = (*FeeEstimateInfo)(nil)
+
 // String returns a human-readable string of the fee preference.
-func (p FeeEstimateInfo) String() string {
-	if p.ConfTarget != 0 {
-		return fmt.Sprintf("%v blocks", p.ConfTarget)
+func (f FeeEstimateInfo) String() string {
+	if f.ConfTarget != 0 {
+		return fmt.Sprintf("%v blocks", f.ConfTarget)
 	}
-	return p.FeeRate.String()
+
+	return f.FeeRate.String()
 }
 
 // Estimate returns a fee rate for the given fee preference. It ensures that