lnwallet+sweep: cap conf target used in fee estimator

sweep/fee_function.go

@@ -276,6 +276,16 @@ func (l *LinearFeeFunction) estimateFeeRate(
 		ConfTarget: confTarget,
 	}
 
+	// If the conf target is greater or equal to the max allowed value
+	// (1008), we will use the min relay fee instead.
+	if confTarget >= chainfee.MaxBlockTarget {
+		minFeeRate := l.estimator.RelayFeePerKW()
+		log.Debugf("Conf target %v is greater than max block target, "+
+			"using min relay fee rate %v", confTarget, minFeeRate)
+
+		return minFeeRate, nil
+	}
+
 	// endingFeeRate comes from budget/txWeight, which means the returned
 	// fee rate will always be capped by this value, hence we don't need to
 	// worry about overpay.

sweep/fee_function_test.go

@@ -19,6 +19,7 @@ func TestLinearFeeFunctionNew(t *testing.T) {
 	// Create testing params.
 	maxFeeRate := chainfee.SatPerKWeight(10000)
 	estimatedFeeRate := chainfee.SatPerKWeight(500)
+	minRelayFeeRate := chainfee.SatPerKWeight(100)
 	confTarget := uint32(6)
 
 	// Assert init fee function with zero conf value returns an error.
@@ -62,6 +63,23 @@ func TestLinearFeeFunctionNew(t *testing.T) {
 	rt.ErrorContains(err, "fee rate delta is zero")
 	rt.Nil(f)
 
+	// When the conf target is >= 1008, the min relay fee should be used.
+	//
+	// Mock the fee estimator to reutrn the fee rate.
+	estimator.On("RelayFeePerKW").Return(minRelayFeeRate).Once()
+
+	largeConf := uint32(1008)
+	f, err = NewLinearFeeFunction(maxFeeRate, largeConf, estimator)
+	rt.NoError(err)
+	rt.NotNil(f)
+
+	// Assert the internal state.
+	rt.Equal(minRelayFeeRate, f.startingFeeRate)
+	rt.Equal(maxFeeRate, f.endingFeeRate)
+	rt.Equal(minRelayFeeRate, f.currentFeeRate)
+	rt.NotZero(f.deltaFeeRate)
+	rt.Equal(largeConf, f.width)
+
 	// Check a successfully created fee function.
 	//
 	// Mock the fee estimator to return the fee rate.