From 420001a98cf210fa36db9e8ce5a579bad16794a4 Mon Sep 17 00:00:00 2001
From: Elle Mouton <elle.mouton@gmail.com>
Date: Wed, 4 Jun 2025 12:08:19 +0200
Subject: [PATCH] lnwire: add InboundFee TLV record to ChannelUpdate

---
 lnwire/channel_update.go | 34 +++++++++++++++++++++++++++++--
 lnwire/test_message.go   | 43 +++++++++++++++++++++++++++++++++++++++-
 2 files changed, 74 insertions(+), 3 deletions(-)

diff --git a/lnwire/channel_update.go b/lnwire/channel_update.go
index 55d9d3181..558d81a9e 100644
--- a/lnwire/channel_update.go
+++ b/lnwire/channel_update.go
@@ -6,6 +6,7 @@ import (
 	"io"
 
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
+	"github.com/lightningnetwork/lnd/tlv"
 )
 
 // ChanUpdateMsgFlags is a bitfield that signals whether optional fields are
@@ -114,6 +115,10 @@ type ChannelUpdate1 struct {
 	// HtlcMaximumMsat is the maximum HTLC value which will be accepted.
 	HtlcMaximumMsat MilliSatoshi
 
+	// InboundFee is an optional TLV record that contains the fee
+	// information for incoming HTLCs.
+	InboundFee tlv.OptionalRecordT[tlv.TlvType55555, Fee]
+
 	// ExtraData is the set of data that was appended to this message to
 	// fill out the full maximum transport message size. These fields can
 	// be used to specify optional data such as custom TLV fields.
@@ -156,12 +161,27 @@ func (a *ChannelUpdate1) Decode(r io.Reader, _ uint32) error {
 		}
 	}
 
-	err = a.ExtraOpaqueData.Decode(r)
+	var tlvRecords ExtraOpaqueData
+	if err := ReadElements(r, &tlvRecords); err != nil {
+		return err
+	}
+
+	var inboundFee = a.InboundFee.Zero()
+	typeMap, err := tlvRecords.ExtractRecords(&inboundFee)
 	if err != nil {
 		return err
 	}
 
-	return a.ExtraOpaqueData.ValidateTLV()
+	val, ok := typeMap[a.InboundFee.TlvType()]
+	if ok && val == nil {
+		a.InboundFee = tlv.SomeRecordT(inboundFee)
+	}
+
+	if len(tlvRecords) != 0 {
+		a.ExtraOpaqueData = tlvRecords
+	}
+
+	return nil
 }
 
 // Encode serializes the target ChannelUpdate into the passed io.Writer
@@ -218,6 +238,16 @@ func (a *ChannelUpdate1) Encode(w *bytes.Buffer, pver uint32) error {
 		}
 	}
 
+	recordProducers := make([]tlv.RecordProducer, 0, 1)
+	a.InboundFee.WhenSome(func(fee tlv.RecordT[tlv.TlvType55555, Fee]) {
+		recordProducers = append(recordProducers, &fee)
+	})
+
+	err := EncodeMessageExtraData(&a.ExtraOpaqueData, recordProducers...)
+	if err != nil {
+		return err
+	}
+
 	// Finally, append any extra opaque data.
 	return WriteBytes(w, a.ExtraOpaqueData)
 }
diff --git a/lnwire/test_message.go b/lnwire/test_message.go
index 8b3d98400..9d086c868 100644
--- a/lnwire/test_message.go
+++ b/lnwire/test_message.go
@@ -12,6 +12,7 @@ import (
 	"github.com/lightningnetwork/lnd/fn/v2"
 	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 	"github.com/lightningnetwork/lnd/tlv"
+	"github.com/stretchr/testify/require"
 	"pgregory.net/rapid"
 )
 
@@ -406,6 +407,45 @@ func (a *ChannelUpdate1) RandTestMessage(t *rapid.T) Message {
 		maxHtlc = 0
 	}
 
+	// Randomly decide if an inbound fee should be included.
+	// By default, our extra opaque data will just be random TLV but if we
+	// include an inbound fee, then we will also set the record in the
+	// extra opaque data.
+	var (
+		customRecords, _ = RandCustomRecords(t, nil, false)
+		inboundFee       tlv.OptionalRecordT[tlv.TlvType55555, Fee]
+	)
+	includeInboundFee := rapid.Bool().Draw(t, "includeInboundFee")
+	if includeInboundFee {
+		if customRecords == nil {
+			customRecords = make(CustomRecords)
+		}
+
+		inFeeBase := int32(
+			rapid.IntRange(-1000, 1000).Draw(t, "inFeeBase"),
+		)
+		inFeeProp := int32(
+			rapid.IntRange(-1000, 1000).Draw(t, "inFeeProp"),
+		)
+		fee := Fee{
+			BaseFee: inFeeBase,
+			FeeRate: inFeeProp,
+		}
+		inboundFee = tlv.SomeRecordT(
+			tlv.NewRecordT[tlv.TlvType55555, Fee](fee),
+		)
+
+		var b bytes.Buffer
+		feeRecord := fee.Record()
+		err := feeRecord.Encode(&b)
+		require.NoError(t, err)
+
+		customRecords[uint64(FeeRecordType)] = b.Bytes()
+	}
+
+	extraBytes, err := customRecords.Serialize()
+	require.NoError(t, err)
+
 	return &ChannelUpdate1{
 		Signature:      RandSignature(t),
 		ChainHash:      hash,
@@ -428,7 +468,8 @@ func (a *ChannelUpdate1) RandTestMessage(t *rapid.T) Message {
 			t, "feeRate"),
 		),
 		HtlcMaximumMsat: maxHtlc,
-		ExtraOpaqueData: RandExtraOpaqueData(t, nil),
+		InboundFee:      inboundFee,
+		ExtraOpaqueData: extraBytes,
 	}
 }