htlcswitch: add receiver-side inbound fee support

channeldb/models/channel.go

@@ -115,6 +115,9 @@ type ForwardingPolicy struct {
 	// used to compute the required fee for a given HTLC.
 	FeeRate lnwire.MilliSatoshi
 
+	// InboundFee is the fee that must be paid for incoming HTLCs.
+	InboundFee InboundFee
+
 	// TimeLockDelta is the absolute time-lock value, expressed in blocks,
 	// that will be subtracted from an incoming HTLC's timelock value to
 	// create the time-lock value for the forwarded outgoing HTLC. The

channeldb/models/inbound_fee.go

@@ -0,0 +1,53 @@
+package models
+
+import "github.com/lightningnetwork/lnd/lnwire"
+
+const (
+	// maxFeeRate is the maximum fee rate that we allow. It is set to allow
+	// a variable fee component of up to 10x the payment amount.
+	maxFeeRate = 10 * feeRateParts
+)
+
+type InboundFee struct {
+	Base int32
+	Rate int32
+}
+
+// NewInboundFeeFromWire constructs an inbound fee structure from a wire fee.
+func NewInboundFeeFromWire(fee lnwire.Fee) InboundFee {
+	return InboundFee{
+		Base: fee.BaseFee,
+		Rate: fee.FeeRate,
+	}
+}
+
+// ToWire converts the inbound fee to a wire fee structure.
+func (i *InboundFee) ToWire() lnwire.Fee {
+	return lnwire.Fee{
+		BaseFee: i.Base,
+		FeeRate: i.Rate,
+	}
+}
+
+// CalcFee calculates what the inbound fee should minimally be for forwarding
+// the given amount. This amount is the total of the outgoing amount plus the
+// outbound fee, which is what the inbound fee is based on.
+func (i *InboundFee) CalcFee(amt lnwire.MilliSatoshi) int64 {
+	fee := int64(i.Base)
+	rate := int64(i.Rate)
+
+	// Cap the rate to prevent overflows.
+	switch {
+	case rate > maxFeeRate:
+		rate = maxFeeRate
+
+	case rate < -maxFeeRate:
+		rate = -maxFeeRate
+	}
+
+	// Calculate proportional component. To keep the integer math simple,
+	// positive fees are rounded down while negative fees are rounded up.
+	fee += rate * int64(amt) / feeRateParts
+
+	return fee
+}

channeldb/models/inbound_fee_test.go

@@ -0,0 +1,33 @@
+package models
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestInboundFee(t *testing.T) {
+	t.Parallel()
+
+	// Test positive fee.
+	i := InboundFee{
+		Base: 5,
+		Rate: 500000,
+	}
+
+	require.Equal(t, int64(6), i.CalcFee(2))
+
+	// Expect fee to be rounded down.
+	require.Equal(t, int64(6), i.CalcFee(3))
+
+	// Test negative fee.
+	i = InboundFee{
+		Base: -5,
+		Rate: -500000,
+	}
+
+	require.Equal(t, int64(-6), i.CalcFee(2))
+
+	// Expect fee to be rounded up.
+	require.Equal(t, int64(-6), i.CalcFee(3))
+}