lnwire: introduce new explicit ChannelType TLV record

In this commit, we add a new TLV record that's intended to be used as an
explicit channel commitment type for a new form of funding negotiation,
and later on a dynamic commitment upgrade protocol. As defined, we have
3 channel types: base (the OG), tweakless, and anchors w/ zero fee
HTLCs. We omit the original variant of anchors as it was never truly
deployed from the PoV of lnd.

lnwire/channel_type.go

@@ -0,0 +1,58 @@
+package lnwire
+
+import (
+	"io"
+
+	"github.com/lightningnetwork/lnd/tlv"
+)
+
+const (
+	// ChannelTypeRecordType is the type of the experimental record used
+	// to denote which channel type is being negotiated.
+	ChannelTypeRecordType tlv.Type = 1
+)
+
+// ChannelType represents a specific channel type as a set of feature bits that
+// comprise it.
+type ChannelType RawFeatureVector
+
+// featureBitLen returns the length in bytes of the encoded feature bits.
+func (c ChannelType) featureBitLen() uint64 {
+	fv := RawFeatureVector(c)
+	return uint64(fv.SerializeSize())
+}
+
+// Record returns a TLV record that can be used to encode/decode the channel
+// type from a given TLV stream.
+func (c *ChannelType) Record() tlv.Record {
+	return tlv.MakeDynamicRecord(
+		ChannelTypeRecordType, c, c.featureBitLen, channelTypeEncoder,
+		channelTypeDecoder,
+	)
+}
+
+// channelTypeEncoder is a custom TLV encoder for the ChannelType record.
+func channelTypeEncoder(w io.Writer, val interface{}, buf *[8]byte) error {
+	if v, ok := val.(*ChannelType); ok {
+		// Encode the feature bits as a byte slice without its length
+		// prepended, as that's already taken care of by the TLV record.
+		fv := RawFeatureVector(*v)
+		return fv.encode(w, fv.SerializeSize(), 8)
+	}
+
+	return tlv.NewTypeForEncodingErr(val, "lnwire.ChannelType")
+}
+
+// channelTypeDecoder is a custom TLV decoder for the ChannelType record.
+func channelTypeDecoder(r io.Reader, val interface{}, buf *[8]byte, l uint64) error {
+	if v, ok := val.(*ChannelType); ok {
+		fv := NewRawFeatureVector()
+		if err := fv.decode(r, int(l), 8); err != nil {
+			return err
+		}
+		*v = ChannelType(*fv)
+		return nil
+	}
+
+	return tlv.NewTypeForEncodingErr(val, "lnwire.ChannelType")
+}

lnwire/channel_type_test.go

@@ -0,0 +1,28 @@
+package lnwire
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// TestChannelTypeEncodeDecode tests that we're able to properly encode and
+// decode channel types within TLV streams.
+func TestChannelTypeEncodeDecode(t *testing.T) {
+	t.Parallel()
+
+	chanType := ChannelType(*NewRawFeatureVector(
+		StaticRemoteKeyRequired,
+		AnchorsZeroFeeHtlcTxRequired,
+	))
+
+	var extraData ExtraOpaqueData
+	require.NoError(t, extraData.PackRecords(chanType.Record()))
+
+	var chanType2 ChannelType
+	tlvs, err := extraData.ExtractRecords(chanType2.Record())
+	require.NoError(t, err)
+
+	require.Contains(t, tlvs, ChannelTypeRecordType)
+	require.Equal(t, chanType, chanType2)
+}