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`.

lnwire/update_fulfill_htlc_test.go

@@ -0,0 +1,129 @@
+package lnwire
+
+import (
+	"bytes"
+	"fmt"
+	"testing"
+
+	"github.com/lightningnetwork/lnd/tlv"
+	"github.com/stretchr/testify/require"
+)
+
+// testCaseUpdateFulfill is a test case for the UpdateFulfillHTLC message.
+type testCaseUpdateFulfill struct {
+	// Msg is the message to be encoded and decoded.
+	Msg UpdateFulfillHTLC
+
+	// ExpectEncodeError is a flag that indicates whether we expect the
+	// encoding of the message to fail.
+	ExpectEncodeError bool
+}
+
+// generateTestCases generates a set of UpdateFulfillHTLC message test cases.
+func generateUpdateFulfillTestCases(t *testing.T) []testCaseUpdateFulfill {
+	// Firstly, we'll set basic values for the message fields.
+	//
+	// Generate random channel ID.
+	chanIDBytes, err := generateRandomBytes(32)
+	require.NoError(t, err)
+
+	var chanID ChannelID
+	copy(chanID[:], chanIDBytes)
+
+	// Generate random payment preimage.
+	paymentPreimageBytes, err := generateRandomBytes(32)
+	require.NoError(t, err)
+
+	var paymentPreimage [32]byte
+	copy(paymentPreimage[:], paymentPreimageBytes)
+
+	// Define custom records.
+	recordKey1 := uint64(MinCustomRecordsTlvType + 1)
+	recordValue1, err := generateRandomBytes(10)
+	require.NoError(t, err)
+
+	recordKey2 := uint64(MinCustomRecordsTlvType + 2)
+	recordValue2, err := generateRandomBytes(10)
+	require.NoError(t, err)
+
+	customRecords := CustomRecords{
+		recordKey1: recordValue1,
+		recordKey2: recordValue2,
+	}
+
+	// Construct an instance of extra data that contains records with TLV
+	// types below the minimum custom records threshold and that lack
+	// corresponding fields in the message struct. Content should persist in
+	// the extra data field after encoding and decoding.
+	var (
+		recordBytes45 = []byte("recordBytes45")
+		tlvRecord45   = tlv.NewPrimitiveRecord[tlv.TlvType45](
+			recordBytes45,
+		)
+
+		recordBytes55 = []byte("recordBytes55")
+		tlvRecord55   = tlv.NewPrimitiveRecord[tlv.TlvType55](
+			recordBytes55,
+		)
+	)
+
+	var extraData ExtraOpaqueData
+	err = extraData.PackRecords(
+		[]tlv.RecordProducer{&tlvRecord45, &tlvRecord55}...,
+	)
+	require.NoError(t, err)
+
+	return []testCaseUpdateFulfill{
+		{
+			Msg: UpdateFulfillHTLC{
+				ChanID:          chanID,
+				ID:              42,
+				PaymentPreimage: paymentPreimage,
+			},
+		},
+		{
+			Msg: UpdateFulfillHTLC{
+				ChanID:          chanID,
+				ID:              42,
+				PaymentPreimage: paymentPreimage,
+				CustomRecords:   customRecords,
+				ExtraData:       extraData,
+			},
+		},
+	}
+}
+
+// TestUpdateFulfillHtlcEncodeDecode tests UpdateFulfillHTLC message encoding
+// and decoding for all supported field values.
+func TestUpdateFulfillHtlcEncodeDecode(t *testing.T) {
+	t.Parallel()
+
+	// Generate test cases.
+	testCases := generateUpdateFulfillTestCases(t)
+
+	// Execute test cases.
+	for tcIdx, tc := range testCases {
+		t.Run(fmt.Sprintf("testcase-%d", tcIdx), func(t *testing.T) {
+			// Encode test case message.
+			var buf bytes.Buffer
+			err := tc.Msg.Encode(&buf, 0)
+
+			// Check if we expect an encoding error.
+			if tc.ExpectEncodeError {
+				require.Error(t, err)
+				return
+			}
+
+			require.NoError(t, err)
+
+			// Decode the encoded message bytes message.
+			var actualMsg UpdateFulfillHTLC
+			decodeReader := bytes.NewReader(buf.Bytes())
+			err = actualMsg.Decode(decodeReader, 0)
+			require.NoError(t, err)
+
+			// Compare the two messages to ensure equality.
+			require.Equal(t, tc.Msg, actualMsg)
+		})
+	}
+}