sweep: make sure max fee rate can be reached

Previously we don't allow confTarget to be 0, which ended up the final
position being never reached. We fix it here by allowing confTarget to
be 0 in case the deadline has already been passed for a given input.

sweep/fee_function.go

@@ -113,9 +113,15 @@ var _ FeeFunction = (*LinearFeeFunction)(nil)
 func NewLinearFeeFunction(maxFeeRate chainfee.SatPerKWeight, confTarget uint32,
 	estimator chainfee.Estimator) (*LinearFeeFunction, error) {
 
-	// Sanity check conf target.
+	// If the deadline has already been reached, there's nothing the fee
+	// function can do. In this case, we'll use the max fee rate
+	// immediately.
 	if confTarget == 0 {
-		return nil, fmt.Errorf("width must be greater than zero")
+		return &LinearFeeFunction{
+			startingFeeRate: maxFeeRate,
+			endingFeeRate:   maxFeeRate,
+			currentFeeRate:  maxFeeRate,
+		}, nil
 	}
 
 	l := &LinearFeeFunction{
@@ -193,11 +199,6 @@ func (l *LinearFeeFunction) Increment() (bool, error) {
 //
 // NOTE: part of the FeeFunction interface.
 func (l *LinearFeeFunction) IncreaseFeeRate(confTarget uint32) (bool, error) {
-	// If the new position is already at the end, we return an error.
-	if confTarget == 0 {
-		return false, ErrMaxPosition
-	}
-
 	newPosition := uint32(0)
 
 	// Only calculate the new position when the conf target is less than