lnwire: add OutPoint type with TLV encoding support

diff --git a/lnwire/outpoint.go b/lnwire/outpoint.go
new file mode 100644
index 0000000..ec893ee
--- /dev/null
+++ b/lnwire/outpoint.go
@@ -0,0 +1,53 @@
+package lnwire
+
+import (
+	"bytes"
+	"io"
+
+	"github.com/btcsuite/btcd/wire"
+	"github.com/lightningnetwork/lnd/tlv"
+)
+
+// OutPoint describes an outpoint of a transaction via its transaction hash
+// and the outpoint index. This is a thin wrapper around the wire.OutPoint to
+// provide TLV encoding/decoding.
+type OutPoint wire.OutPoint
+
+// Record returns a TLV record that can be used to encode/decode the OutPoint.
+//
+// NOTE: this is part of the tlv.RecordProducer interface.
+func (o *OutPoint) Record() tlv.Record {
+	return tlv.MakeStaticRecord(0, o, 34, outpointEncoder, outpointDecoder)
+}
+
+// outpointEncoder is a TLV encoder for OutPoint.
+func outpointEncoder(w io.Writer, val any, _ *[8]byte) error {
+	if v, ok := val.(*OutPoint); ok {
+		buf := bytes.NewBuffer(nil)
+		err := WriteOutPoint(buf, wire.OutPoint(*v))
+		if err != nil {
+			return err
+		}
+		_, err = w.Write(buf.Bytes())
+
+		return err
+	}
+
+	return tlv.NewTypeForEncodingErr(val, "OutPoint")
+}
+
+// outpointDecoder is a TLV decoder for OutPoint.
+func outpointDecoder(r io.Reader, val any, _ *[8]byte, l uint64) error {
+	if v, ok := val.(*OutPoint); ok {
+		var o wire.OutPoint
+		if err := ReadElement(r, &o); err != nil {
+			return err
+		}
+
+		*v = OutPoint(o)
+
+		return nil
+	}
+
+	return tlv.NewTypeForDecodingErr(val, "OutPoint", l, 34)
+}
diff --git a/lnwire/outpoint_test.go b/lnwire/outpoint_test.go
new file mode 100644
index 0000000..b8ac1a7
--- /dev/null
+++ b/lnwire/outpoint_test.go
@@ -0,0 +1,107 @@
+package lnwire
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
+	"github.com/btcsuite/btcd/wire"
+	"github.com/lightningnetwork/lnd/tlv"
+	"github.com/stretchr/testify/require"
+	"pgregory.net/rapid"
+)
+
+// TestOutPointTLVEncoding tests the TLV encoding and decoding of OutPoint
+// structs using the Record interface.
+func TestOutPointTLVEncoding(t *testing.T) {
+	t.Parallel()
+
+	testOutPoint := OutPoint(wire.OutPoint{
+		Hash: chainhash.Hash{
+			0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08,
+			0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f, 0x10,
+			0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18,
+			0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f, 0x20,
+		},
+		Index: 12345,
+	})
+
+	var extraData ExtraOpaqueData
+	require.NoError(t, extraData.PackRecords(&testOutPoint))
+
+	var decodedOutPoint OutPoint
+	tlvs, err := extraData.ExtractRecords(&decodedOutPoint)
+	require.NoError(t, err)
+
+	require.Contains(t, tlvs, tlv.Type(0))
+	require.Equal(t, testOutPoint, decodedOutPoint)
+}
+
+// TestOutPointRecord tests the TLV Record interface of OutPoint
+// by directly encoding and decoding using the Record method.
+func TestOutPointRecord(t *testing.T) {
+	t.Parallel()
+
+	testOutPoint := OutPoint(wire.OutPoint{
+		Hash: chainhash.Hash{
+			0xff, 0xfe, 0xfd, 0xfc, 0xfb, 0xfa, 0xf9, 0xf8,
+			0xf7, 0xf6, 0xf5, 0xf4, 0xf3, 0xf2, 0xf1, 0xf0,
+			0xef, 0xee, 0xed, 0xec, 0xeb, 0xea, 0xe9, 0xe8,
+			0xe7, 0xe6, 0xe5, 0xe4, 0xe3, 0xe2, 0xe1, 0xe0,
+		},
+		Index: 65535,
+	})
+
+	var buf bytes.Buffer
+	record := testOutPoint.Record()
+	require.NoError(t, record.Encode(&buf))
+
+	var decodedOutPoint OutPoint
+	decodedRecord := decodedOutPoint.Record()
+	require.NoError(t, decodedRecord.Decode(&buf, uint64(buf.Len())))
+
+	require.Equal(t, testOutPoint, decodedOutPoint)
+}
+
+// TestOutPointProperty uses property-based testing to verify that OutPoint
+// TLV encoding and decoding is correct for random OutPoint values.
+func TestOutPointProperty(t *testing.T) {
+	t.Parallel()
+
+	scenario := func(t *rapid.T) {
+		wireOutPoint := RandOutPoint(t)
+		lnOutPoint := OutPoint(wireOutPoint)
+
+		var buf bytes.Buffer
+		record := lnOutPoint.Record()
+		err := record.Encode(&buf)
+		require.NoError(t, err)
+
+		var decodedOutPoint OutPoint
+		decodedRecord := decodedOutPoint.Record()
+		err = decodedRecord.Decode(&buf, uint64(buf.Len()))
+		require.NoError(t, err)
+
+		require.Equal(t, lnOutPoint, decodedOutPoint)
+		require.Equal(t, wireOutPoint, wire.OutPoint(decodedOutPoint))
+	}
+
+	rapid.Check(t, scenario)
+}
+
+// TestOutPointZeroValues tests that OutPoint handles zero values correctly.
+func TestOutPointZeroValues(t *testing.T) {
+	t.Parallel()
+
+	zeroOutPoint := OutPoint(wire.OutPoint{})
+
+	var buf bytes.Buffer
+	record := zeroOutPoint.Record()
+	require.NoError(t, record.Encode(&buf))
+
+	var decodedOutPoint OutPoint
+	decodedRecord := decodedOutPoint.Record()
+	require.NoError(t, decodedRecord.Decode(&buf, uint64(buf.Len())))
+
+	require.Equal(t, zeroOutPoint, decodedOutPoint)
+}