sweep: prevent default fee preference fallback

We want to make sure clients are aware of their own fee preferences,
rather than relying on defaults.

sweep/sweeper.go

@@ -46,6 +46,10 @@ var (
 	// for the configured max number of attempts.
 	ErrTooManyAttempts = errors.New("sweep failed after max attempts")
 
+	// ErrNoFeePreference is returned when we attempt to satisfy a sweep
+	// request from a client whom did not specify a fee preference.
+	ErrNoFeePreference = errors.New("no fee preference specified")
+
 	// ErrSweeperShuttingDown is an error returned when a client attempts to
 	// make a request to the UtxoSweeper, but it is unable to handle it as
 	// it is/has already been stoppepd.
@@ -394,6 +398,12 @@ func (s *UtxoSweeper) SweepInput(input input.Input,
 func (s *UtxoSweeper) feeRateForPreference(
 	feePreference FeePreference) (lnwallet.SatPerKWeight, error) {
 
+	// Ensure a type of fee preference is specified to prevent using a
+	// default below.
+	if feePreference.FeeRate == 0 && feePreference.ConfTarget == 0 {
+		return 0, ErrNoFeePreference
+	}
+
 	feeRate, err := DetermineFeePerKw(s.cfg.FeeEstimator, feePreference)
 	if err != nil {
 		return 0, err

sweep/sweeper_test.go

@@ -372,6 +372,12 @@ func assertTxFeeRate(t *testing.T, tx *wire.MsgTx,
 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], FeePreference{})
+	if err != ErrNoFeePreference {
+		t.Fatalf("expected ErrNoFeePreference, got %v", err)
+	}
+
 	resultChan, err := ctx.sweeper.SweepInput(
 		spendableInputs[0], defaultFeePref,
 	)