multi: Inbound fees are retained when not provided

Fixes the problem that inbound base fee and fee rate are overwritten
with 0 if they are not specified in PolicyUpdateRequest. This ensures
backward compatibility with older rpc clients that do not yet support
the inbound feature.

cmd/lncli/commands.go

@@ -2387,12 +2387,28 @@ func updateChannelPolicy(ctx *cli.Context) error {
 		return errors.New("inbound_fee_rate_ppm out of range")
 	}
 
+	// Inbound fees are optional. However, if an update is required,
+	// both the base fee and the fee rate must be provided.
+	var inboundFee *lnrpc.InboundFee
+	if ctx.IsSet("inbound_base_fee_msat") !=
+		ctx.IsSet("inbound_fee_rate_ppm") {
+
+		return errors.New("both parameters must be provided: " +
+			"inbound_base_fee_msat and inbound_fee_rate_ppm")
+	}
+
+	if ctx.IsSet("inbound_fee_rate_ppm") {
+		inboundFee = &lnrpc.InboundFee{
+			BaseFeeMsat: int32(inboundBaseFeeMsat),
+			FeeRatePpm:  int32(inboundFeeRatePpm),
+		}
+	}
+
 	req := &lnrpc.PolicyUpdateRequest{
-		BaseFeeMsat:        baseFee,
-		TimeLockDelta:      uint32(timeLockDelta),
-		MaxHtlcMsat:        ctx.Uint64("max_htlc_msat"),
-		InboundBaseFeeMsat: int32(inboundBaseFeeMsat),
-		InboundFeeRatePpm:  int32(inboundFeeRatePpm),
+		BaseFeeMsat:   baseFee,
+		TimeLockDelta: uint32(timeLockDelta),
+		MaxHtlcMsat:   ctx.Uint64("max_htlc_msat"),
+		InboundFee:    inboundFee,
 	}
 
 	if ctx.IsSet("min_htlc_msat") {

itest/lnd_multi-hop-payments_test.go

@@ -257,9 +257,11 @@ func updateChannelPolicy(ht *lntest.HarnessTest, hn *node.HarnessNode,
 		Scope: &lnrpc.PolicyUpdateRequest_ChanPoint{
 			ChanPoint: chanPoint,
 		},
-		MaxHtlcMsat:        maxHtlc,
-		InboundBaseFeeMsat: inboundBaseFee,
-		InboundFeeRatePpm:  inboundFeeRate,
+		MaxHtlcMsat: maxHtlc,
+		InboundFee: &lnrpc.InboundFee{
+			BaseFeeMsat: inboundBaseFee,
+			FeeRatePpm:  inboundFeeRate,
+		},
 	}
 
 	hn.RPC.UpdateChannelPolicy(updateFeeReq)

lnrpc/lightning.proto

@@ -4369,6 +4369,16 @@ message FeeReportResponse {
     uint64 month_fee_sum = 4;
 }
 
+message InboundFee {
+    // The inbound base fee charged regardless of the number of milli-satoshis
+    // received in the channel. By default, only negative values are accepted.
+    int32 base_fee_msat = 1;
+
+    // The effective inbound fee rate in micro-satoshis (parts per million).
+    // By default, only negative values are accepted.
+    int32 fee_rate_ppm = 2;
+}
+
 message PolicyUpdateRequest {
     oneof scope {
         // If set, then this update applies to all currently active channels.
@@ -4402,8 +4412,9 @@ message PolicyUpdateRequest {
     // If true, min_htlc_msat is applied.
     bool min_htlc_msat_specified = 8;
 
-    int32 inbound_base_fee_msat = 10;
-    int32 inbound_fee_rate_ppm = 11;
+    // Optional inbound fee. If unset, the previously set value will be
+    // retained [EXPERIMENTAL].
+    InboundFee inbound_fee = 10;
 }
 
 enum UpdateFailure {

lnrpc/lightning.swagger.json

@@ -5297,6 +5297,21 @@
         }
       }
     },
+    "lnrpcInboundFee": {
+      "type": "object",
+      "properties": {
+        "base_fee_msat": {
+          "type": "integer",
+          "format": "int32",
+          "description": "The inbound base fee charged regardless of the number of milli-satoshis\nreceived in the channel. By default, only negative values are accepted."
+        },
+        "fee_rate_ppm": {
+          "type": "integer",
+          "format": "int32",
+          "description": "The effective inbound fee rate in micro-satoshis (parts per million).\nBy default, only negative values are accepted."
+        }
+      }
+    },
     "lnrpcInitiator": {
       "type": "string",
       "enum": [
@@ -6585,13 +6600,9 @@
           "type": "boolean",
           "description": "If true, min_htlc_msat is applied."
         },
-        "inbound_base_fee_msat": {
-          "type": "integer",
-          "format": "int32"
-        },
-        "inbound_fee_rate_ppm": {
-          "type": "integer",
-          "format": "int32"
+        "inbound_fee": {
+          "$ref": "#/definitions/lnrpcInboundFee",
+          "description": "Optional inbound fee. If unset, the previously set value will be\nretained [EXPERIMENTAL]."
         }
       }
     },

routing/localchans/manager.go

@@ -9,6 +9,7 @@ import (
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/channeldb/models"
 	"github.com/lightningnetwork/lnd/discovery"
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/kvdb"
 	"github.com/lightningnetwork/lnd/lnrpc"
 	"github.com/lightningnetwork/lnd/lnwire"
@@ -105,6 +106,14 @@ func (r *Manager) UpdatePolicy(newSchema routing.ChannelPolicy,
 			Edge: edge,
 		})
 
+		// Extract inbound fees from the ExtraOpaqueData.
+		var inboundWireFee lnwire.Fee
+		_, err = edge.ExtraOpaqueData.ExtractRecords(&inboundWireFee)
+		if err != nil {
+			return err
+		}
+		inboundFee := models.NewInboundFeeFromWire(inboundWireFee)
+
 		// Add updated policy to list of policies to send to switch.
 		policiesToUpdate[info.ChannelPoint] = models.ForwardingPolicy{
 			BaseFee:       edge.FeeBaseMSat,
@@ -112,7 +121,7 @@ func (r *Manager) UpdatePolicy(newSchema routing.ChannelPolicy,
 			TimeLockDelta: uint32(edge.TimeLockDelta),
 			MinHTLCOut:    edge.MinHTLC,
 			MaxHTLC:       edge.MaxHTLC,
-			InboundFee:    newSchema.InboundFee,
+			InboundFee:    inboundFee,
 		}
 
 		return nil
@@ -182,8 +191,15 @@ func (r *Manager) updateEdge(tx kvdb.RTx, chanPoint wire.OutPoint,
 		newSchema.FeeRate,
 	)
 
-	inboundFee := newSchema.InboundFee.ToWire()
-	if err := edge.ExtraOpaqueData.PackRecords(&inboundFee); err != nil {
+	// If inbound fees are set, we update the edge with them.
+	err := fn.MapOptionZ(newSchema.InboundFee,
+		func(f models.InboundFee) error {
+			inboundWireFee := f.ToWire()
+			return edge.ExtraOpaqueData.PackRecords(
+				&inboundWireFee,
+			)
+		})
+	if err != nil {
 		return err
 	}
 

routing/router.go

@@ -22,6 +22,7 @@ import (
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/channeldb/models"
 	"github.com/lightningnetwork/lnd/clock"
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/htlcswitch"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/kvdb"
@@ -290,7 +291,7 @@ type FeeSchema struct {
 
 	// InboundFee is the inbound fee schedule that applies to forwards
 	// coming in through a channel to which this FeeSchema pertains.
-	InboundFee models.InboundFee
+	InboundFee fn.Option[models.InboundFee]
 }
 
 // ChannelPolicy holds the parameters that determine the policy we enforce

rpcserver.go

@@ -46,6 +46,7 @@ import (
 	"github.com/lightningnetwork/lnd/contractcourt"
 	"github.com/lightningnetwork/lnd/discovery"
 	"github.com/lightningnetwork/lnd/feature"
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/funding"
 	"github.com/lightningnetwork/lnd/htlcswitch"
 	"github.com/lightningnetwork/lnd/htlcswitch/hop"
@@ -7216,27 +7217,35 @@ func (r *rpcServer) UpdateChannelPolicy(ctx context.Context,
 	}
 
 	// By default, positive inbound fees are rejected.
-	if !r.cfg.AcceptPositiveInboundFees {
-		if req.InboundBaseFeeMsat > 0 {
+	if !r.cfg.AcceptPositiveInboundFees && req.InboundFee != nil {
+		if req.InboundFee.BaseFeeMsat > 0 {
 			return nil, fmt.Errorf("positive values for inbound "+
 				"base fee msat are not supported: %v",
-				req.InboundBaseFeeMsat)
+				req.InboundFee.BaseFeeMsat)
 		}
-		if req.InboundFeeRatePpm > 0 {
+		if req.InboundFee.FeeRatePpm > 0 {
 			return nil, fmt.Errorf("positive values for inbound "+
 				"fee rate ppm are not supported: %v",
-				req.InboundFeeRatePpm)
+				req.InboundFee.FeeRatePpm)
 		}
 	}
 
+	// If no inbound fees have been specified, we indicate with an empty
+	// option that the previous inbound fee should be retained during the
+	// edge update.
+	inboundFee := fn.None[models.InboundFee]()
+	if req.InboundFee != nil {
+		inboundFee = fn.Some(models.InboundFee{
+			Base: req.InboundFee.BaseFeeMsat,
+			Rate: req.InboundFee.FeeRatePpm,
+		})
+	}
+
 	baseFeeMsat := lnwire.MilliSatoshi(req.BaseFeeMsat)
 	feeSchema := routing.FeeSchema{
-		BaseFee: baseFeeMsat,
-		FeeRate: feeRateFixed,
-		InboundFee: models.InboundFee{
-			Base: req.InboundBaseFeeMsat,
-			Rate: req.InboundFeeRatePpm,
-		},
+		BaseFee:    baseFeeMsat,
+		FeeRate:    feeRateFixed,
+		InboundFee: inboundFee,
 	}
 
 	maxHtlc := lnwire.MilliSatoshi(req.MaxHtlcMsat)