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_test.go

@@ -22,10 +22,16 @@ func TestLinearFeeFunctionNew(t *testing.T) {
 	minRelayFeeRate := chainfee.SatPerKWeight(100)
 	confTarget := uint32(6)
 
-	// Assert init fee function with zero conf value returns an error.
+	// Assert init fee function with zero conf value will end up using the
+	// max fee rate.
 	f, err := NewLinearFeeFunction(maxFeeRate, 0, estimator)
-	rt.ErrorContains(err, "width must be greater than zero")
-	rt.Nil(f)
+	rt.NoError(err)
+	rt.NotNil(f)
+
+	// Assert the internal state.
+	rt.Equal(maxFeeRate, f.startingFeeRate)
+	rt.Equal(maxFeeRate, f.endingFeeRate)
+	rt.Equal(maxFeeRate, f.currentFeeRate)
 
 	// When the fee estimator returns an error, it's returned.
 	//
@@ -240,11 +246,6 @@ func TestLinearFeeFunctionIncreaseFeeRate(t *testing.T) {
 	rt.NoError(err)
 	rt.False(increased)
 
-	// Test that when we use a conf target of 0, we get an error.
-	increased, err = f.IncreaseFeeRate(0)
-	rt.ErrorIs(err, ErrMaxPosition)
-	rt.False(increased)
-
 	// We now increase the fee rate from conf target 8 to 1 and assert we
 	// get no error and true.
 	for i := uint32(1); i < confTarget; i++ {
@@ -262,4 +263,12 @@ func TestLinearFeeFunctionIncreaseFeeRate(t *testing.T) {
 		// Check public method returns the expected fee rate.
 		rt.Equal(estimatedFeeRate+delta, f.FeeRate())
 	}
+
+	// Test that when we use a conf target of 0, we get the ending fee
+	// rate.
+	increased, err = f.IncreaseFeeRate(0)
+	rt.NoError(err)
+	rt.True(increased)
+	rt.Equal(confTarget, f.position)
+	rt.Equal(maxFeeRate, f.currentFeeRate)
 }