lnwire: add custom records field to type UpdateFulfillHtlc

- Introduce the field `CustomRecords` to the type `UpdateFulfillHtlc`.
- Encode and decode the new field into the `ExtraData` field of the
`update_fulfill_htlc` wire message.
- Empty `ExtraData` field is set to `nil`.

htlcswitch/payment_result_test.go

@@ -38,7 +38,6 @@ func TestNetworkResultSerialization(t *testing.T) {
 		ChanID:          chanID,
 		ID:              2,
 		PaymentPreimage: preimage,
-		ExtraData:       make([]byte, 0),
 	}
 
 	fail := &lnwire.UpdateFailHTLC{

lnwire/update_fulfill_htlc.go

@@ -2,7 +2,10 @@ package lnwire
 
 import (
 	"bytes"
+	"fmt"
 	"io"
+
+	"github.com/lightningnetwork/lnd/tlv"
 )
 
 // UpdateFulfillHTLC is sent by Alice to Bob when she wishes to settle a
@@ -23,6 +26,10 @@ type UpdateFulfillHTLC struct {
 	// HTLC.
 	PaymentPreimage [32]byte
 
+	// CustomRecords maps TLV types to byte slices, storing arbitrary data
+	// intended for inclusion in the ExtraData field.
+	CustomRecords CustomRecords
+
 	// 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.
@@ -49,12 +56,75 @@ var _ Message = (*UpdateFulfillHTLC)(nil)
 //
 // This is part of the lnwire.Message interface.
 func (c *UpdateFulfillHTLC) Decode(r io.Reader, pver uint32) error {
-	return ReadElements(r,
+	// msgExtraData is a temporary variable used to read the message extra
+	// data field from the reader.
+	var msgExtraData ExtraOpaqueData
+
+	if err := ReadElements(r,
 		&c.ChanID,
 		&c.ID,
 		c.PaymentPreimage[:],
-		&c.ExtraData,
+		&msgExtraData,
+	); err != nil {
+		return err
+	}
+
+	// Extract TLV records from the message extra data field.
+	extraDataTlvMap, err := msgExtraData.ExtractRecords()
+	if err != nil {
+		return err
+	}
+
+	// Any records from the extra data TLV map which are in the custom
+	// records TLV type range will be included in the custom records field
+	// and removed from the extra data field.
+	customRecordsTlvMap := make(tlv.TypeMap, len(extraDataTlvMap))
+	for k, v := range extraDataTlvMap {
+		// Skip records that are not in the custom records TLV type
+		// range.
+		if k < MinCustomRecordsTlvType {
+			continue
+		}
+
+		// Include the record in the custom records map.
+		customRecordsTlvMap[k] = v
+
+		// Now that the record is included in the custom records map,
+		// we can remove it from the extra data TLV map.
+		delete(extraDataTlvMap, k)
+	}
+
+	// Set the custom records field to the TLV record map.
+	customRecords, err := NewCustomRecordsFromTlvTypeMap(
+		customRecordsTlvMap,
 	)
+	if err != nil {
+		return err
+	}
+	c.CustomRecords = customRecords
+
+	// Set custom records to nil if we didn't parse anything out of it so
+	// that we can use assert.Equal in tests.
+	if len(customRecordsTlvMap) == 0 {
+		c.CustomRecords = nil
+	}
+
+	// Set extra data to nil if we didn't parse anything out of it so that
+	// we can use assert.Equal in tests.
+	if len(extraDataTlvMap) == 0 {
+		c.ExtraData = nil
+		return nil
+	}
+
+	// Encode the remaining records back into the extra data field. These
+	// records are not in the custom records TLV type range and do not
+	// have associated fields in the UpdateFulfillHTLC struct.
+	c.ExtraData, err = NewExtraOpaqueDataFromTlvTypeMap(extraDataTlvMap)
+	if err != nil {
+		return err
+	}
+
+	return nil
 }
 
 // Encode serializes the target UpdateFulfillHTLC into the passed io.Writer
@@ -74,7 +144,34 @@ func (c *UpdateFulfillHTLC) Encode(w *bytes.Buffer, pver uint32) error {
 		return err
 	}
 
-	return WriteBytes(w, c.ExtraData)
+	// Construct a slice of all the records that we should include in the
+	// message extra data field. We will start by including any records from
+	// the extra data field.
+	msgExtraDataRecords, err := c.ExtraData.RecordProducers()
+	if err != nil {
+		return err
+	}
+
+	// Include custom records in the extra data wire field if they are
+	// present. Ensure that the custom records are validated before encoding
+	// them.
+	if err := c.CustomRecords.Validate(); err != nil {
+		return fmt.Errorf("custom records validation error: %w", err)
+	}
+
+	// Extend the message extra data records slice with TLV records from the
+	// custom records field.
+	customTlvRecords := c.CustomRecords.RecordProducers()
+	msgExtraDataRecords = append(msgExtraDataRecords, customTlvRecords...)
+
+	// We will now construct the message extra data field that will be
+	// encoded into the byte writer.
+	var msgExtraData ExtraOpaqueData
+	if err := msgExtraData.PackRecords(msgExtraDataRecords...); err != nil {
+		return err
+	}
+
+	return WriteBytes(w, msgExtraData)
 }
 
 // MsgType returns the integer uniquely identifying this message type on the

peer/brontide.go

@@ -2033,8 +2033,9 @@ func messageSummary(msg lnwire.Message) string {
 			msg.ID, msg.Reason)
 
 	case *lnwire.UpdateFulfillHTLC:
-		return fmt.Sprintf("chan_id=%v, id=%v, pre_image=%x",
-			msg.ChanID, msg.ID, msg.PaymentPreimage[:])
+		return fmt.Sprintf("chan_id=%v, id=%v, pre_image=%x, "+
+			"custom_records=%v", msg.ChanID, msg.ID,
+			msg.PaymentPreimage[:], msg.CustomRecords)
 
 	case *lnwire.CommitSig:
 		return fmt.Sprintf("chan_id=%v, num_htlcs=%v", msg.ChanID,