lnwire: introduce PureTLVMessage

diff --git a/lnwire/pure_tlv.go b/lnwire/pure_tlv.go
new file mode 100644
index 0000000..8e6f7bd
--- /dev/null
+++ b/lnwire/pure_tlv.go
@@ -0,0 +1,105 @@
+package lnwire
+
+import (
+	"bytes"
+
+	"github.com/lightningnetwork/lnd/tlv"
+)
+
+const (
+	// pureTLVUnsignedRangeOneStart defines the start of the first unsigned
+	// TLV range used for pure TLV messages. The range is inclusive of this
+	// number.
+	pureTLVUnsignedRangeOneStart = 160
+
+	// pureTLVSignedSecondRangeStart defines the start of the second signed
+	// TLV range used for pure TLV messages. The range is inclusive of this
+	// number. Note that the first range is the inclusive range of 0-159.
+	pureTLVSignedSecondRangeStart = 1000000000
+
+	// pureTLVUnsignedRangeTwoStart defines the start of the second unsigned
+	// TLV range used for pure TLV message.
+	pureTLVUnsignedRangeTwoStart = 3000000000
+)
+
+// PureTLVMessage describes an LN message that is a pure TLV stream. If the
+// message includes a signature, it will sign all the TLV records in the
+// inclusive ranges: 0 to 159 and 1000000000 to 2999999999.
+type PureTLVMessage interface {
+	// AllRecords returns all the TLV records for the message. This will
+	// include all the records we know about along with any that we don't
+	// know about but that fall in the signed TLV range.
+	AllRecords() []tlv.Record
+}
+
+// EncodePureTLVMessage encodes the given PureTLVMessage to the given buffer.
+func EncodePureTLVMessage(msg PureTLVMessage, buf *bytes.Buffer) error {
+	return EncodeRecordsTo(buf, msg.AllRecords())
+}
+
+// SerialiseFieldsToSign serialises all the records from the given
+// PureTLVMessage that fall within the signed TLV range.
+func SerialiseFieldsToSign(msg PureTLVMessage) ([]byte, error) {
+	// Filter out all the fields not in the signed ranges.
+	var signedRecords []tlv.Record
+	for _, record := range msg.AllRecords() {
+		if InUnsignedRange(record.Type()) {
+			continue
+		}
+
+		signedRecords = append(signedRecords, record)
+	}
+
+	var buf bytes.Buffer
+	if err := EncodeRecordsTo(&buf, signedRecords); err != nil {
+		return nil, err
+	}
+
+	return buf.Bytes(), nil
+}
+
+// InUnsignedRange returns true if the given TLV type falls outside the TLV
+// ranges that the signature of a pure TLV message will cover.
+func InUnsignedRange(t tlv.Type) bool {
+	return (t >= pureTLVUnsignedRangeOneStart &&
+		t < pureTLVSignedSecondRangeStart) ||
+		t >= pureTLVUnsignedRangeTwoStart
+}
+
+// ExtraSignedFields is a type that stores a map from TLV types in the signed
+// range (for PureMessages) to their corresponding serialised values. This type
+// can be used to keep around data that we don't yet understand but that we need
+// for re-composing the wire message since the signature covers these fields.
+type ExtraSignedFields map[uint64][]byte
+
+// ExtraSignedFieldsFromTypeMap is a helper that can be used alongside calls to
+// the tlv.Stream DecodeWithParsedTypesP2P or DecodeWithParsedTypes methods to
+// extract the tlv type and value pairs in the defined PureTLVMessage signed
+// range which we have not handled with any of our defined Records. These
+// methods will return a tlv.TypeMap containing the records that were extracted
+// from an io.Reader. If the record was know and handled by a defined record,
+// then the value accompanying the record's type in the map will be nil.
+// Otherwise, if the record was unhandled, it will be non-nil.
+func ExtraSignedFieldsFromTypeMap(m tlv.TypeMap) ExtraSignedFields {
+	extraFields := make(ExtraSignedFields)
+	for t, v := range m {
+		// If the value in the type map is nil, then it indicates that
+		// we know this type, and it was handled by one of the records
+		// we passed to the decode function vai the TLV stream.
+		if v == nil {
+			continue
+		}
+
+		// No need to keep this field if it is unknown to us and is not
+		// in the sign range.
+		if InUnsignedRange(t) {
+			continue
+		}
+
+		// Otherwise, this is an un-handled type, so we keep track of
+		// it for signature validation and re-encoding later on.
+		extraFields[uint64(t)] = v
+	}
+
+	return extraFields
+}
diff --git a/lnwire/pure_tlv_test.go b/lnwire/pure_tlv_test.go
new file mode 100644
index 0000000..a81a89e
--- /dev/null
+++ b/lnwire/pure_tlv_test.go
@@ -0,0 +1,389 @@
+package lnwire
+
+import (
+	"bytes"
+	"io"
+	"testing"
+
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/lightningnetwork/lnd/tlv"
+	"github.com/stretchr/testify/require"
+)
+
+// TestPureTLVMessages tests the forwards compatibility of two versions of the
+// same Lightning Network message that uses the Pure TLV format. This in essence
+// tests that and older client is able to verify the signature over relevant
+// data in a newer client's message.
+func TestPureTLVMessage(t *testing.T) {
+	t.Parallel()
+
+	var (
+		_, pkA   = btcec.PrivKeyFromBytes([]byte{1})
+		_, pkB   = btcec.PrivKeyFromBytes([]byte{2})
+		capacity = MilliSatoshi(100)
+	)
+
+	// Test encode and decode of MsgV1 as is.
+	t.Run("Encode and Decode of MsgV1", func(t *testing.T) {
+		t.Parallel()
+
+		msgOld := newMsgV1(pkA, &capacity)
+
+		buf := bytes.NewBuffer(nil)
+		require.NoError(t, msgOld.Encode(buf, 0))
+
+		var msgOld2 MsgV1
+		require.NoError(t, msgOld2.Decode(buf, 0))
+
+		require.Equal(t, msgOld, &msgOld2)
+	})
+
+	// Test encode and decode of MsgV2 as is.
+	t.Run("Encode and Decode of MsgV2", func(t *testing.T) {
+		t.Parallel()
+
+		msgNew := newMsgV2(
+			pkA, &capacity, pkB, []byte{1, 2, 3, 4}, 90, 100, true,
+		)
+
+		buf := bytes.NewBuffer(nil)
+		require.NoError(t, msgNew.Encode(buf, 0))
+
+		var msgNew2 MsgV2
+		require.NoError(t, msgNew2.Decode(buf, 0))
+
+		require.Equal(t, msgNew, &msgNew2)
+	})
+
+	// Create a MsgV2 and decode it into a MsgV1. Both the new client
+	// (MsgV2) and old client (MsgV1) should be able to generate the same
+	// digest that will be used to create and validate the signture.
+	t.Run("Encode MsgV2 and decode via MsgV1", func(t *testing.T) {
+		t.Parallel()
+
+		var (
+			buf   = bytes.NewBuffer(nil)
+			msgV2 = newMsgV2(
+				pkA, &capacity, pkB, []byte{1, 2, 3, 4}, 100,
+				90, true,
+			)
+		)
+		require.NoError(t, msgV2.Encode(buf, 0))
+
+		// Get the serialised bytes that would be signed for msgV2.
+		signData1, err := SerialiseFieldsToSign(msgV2)
+		require.NoError(t, err)
+
+		// Decoding via the old message should store some of the extra
+		// fields.
+		var msgV1 MsgV1
+		require.NoError(t, msgV1.Decode(buf, 0))
+		require.NotEmpty(t, msgV1.ExtraSignedFields)
+
+		// Show that the extra fields map contains unknown fields in the
+		// signed range but not unknown fields in the unsigned range.
+		_, ok := msgV1.ExtraSignedFields[uint64(msgV2.Num.TlvType())] //nolint:ll
+		require.True(t, ok)
+		_, ok = msgV1.ExtraSignedFields[uint64(msgV2.Other.TlvType())] //nolint:ll
+		require.False(t, ok)
+
+		// The serialised bytes to verify the signature against should
+		// be the same though.
+		signData2, err := SerialiseFieldsToSign(&msgV1)
+		require.NoError(t, err)
+
+		require.Equal(t, signData1, signData2)
+
+		// Re-encoding via the old message should keep the extra fields.
+		buf = bytes.NewBuffer(nil)
+		require.NoError(t, msgV1.Encode(buf, 0))
+
+		var msgV1ReEncoded MsgV1
+		require.NoError(t, msgV1ReEncoded.Decode(buf, 0))
+
+		require.Equal(t, &msgV1, &msgV1ReEncoded)
+	})
+}
+
+// MsgV1 represents a more minimal, first version of a Lightning Network
+// message.
+type MsgV1 struct {
+	// Two known fields in the signed range.
+	NodeKey  tlv.RecordT[tlv.TlvType0, *btcec.PublicKey]
+	Capacity tlv.OptionalRecordT[tlv.TlvType1, MilliSatoshi]
+
+	// Signature in the unsigned range.
+	Signature tlv.RecordT[tlv.TlvType160, Sig]
+
+	// Any extra fields in the signed range that we do not yet know about,
+	// but we need to keep them for signature validation and to produce a
+	// valid message.
+	ExtraSignedFields
+}
+
+var _ Message = (*MsgV1)(nil)
+var _ PureTLVMessage = (*MsgV1)(nil)
+
+// newMsgV1 is a constructor for MsgV1.
+func newMsgV1(nodeKey *btcec.PublicKey, capacity *MilliSatoshi) *MsgV1 {
+	newMsg := &MsgV1{
+		NodeKey: tlv.NewPrimitiveRecord[tlv.TlvType0](
+			nodeKey,
+		),
+		Signature: tlv.NewRecordT[tlv.TlvType160](
+			testSchnorrSig,
+		),
+		ExtraSignedFields: make(ExtraSignedFields),
+	}
+
+	if capacity != nil {
+		newMsg.Capacity = tlv.SomeRecordT(
+			tlv.NewPrimitiveRecord[tlv.TlvType1](*capacity),
+		)
+	}
+
+	return newMsg
+}
+
+// Decode deserializes a serialized MsgV1 in the passed io.Reader.
+//
+// This is part of the lnwire.Message interface.
+func (g *MsgV1) Decode(r io.Reader, _ uint32) error {
+	var capacity = tlv.ZeroRecordT[tlv.TlvType1, MilliSatoshi]()
+	stream, err := tlv.NewStream(
+		ProduceRecordsSorted(
+			&g.NodeKey,
+			&capacity,
+			&g.Signature,
+		)...,
+	)
+	if err != nil {
+		return err
+	}
+	g.Signature.Val.ForceSchnorr()
+
+	typeMap, err := stream.DecodeWithParsedTypesP2P(r)
+	if err != nil {
+		return err
+	}
+
+	if _, ok := typeMap[g.Capacity.TlvType()]; ok {
+		g.Capacity = tlv.SomeRecordT(capacity)
+	}
+
+	g.ExtraSignedFields = ExtraSignedFieldsFromTypeMap(typeMap)
+
+	return nil
+}
+
+// Encode serializes the target MsgV1 into the passed buffer.
+//
+// This is part of the lnwire.Message interface.
+func (g *MsgV1) Encode(buf *bytes.Buffer, _ uint32) error {
+	return EncodePureTLVMessage(g, buf)
+}
+
+// MsgType returns the integer uniquely identifying this message type on the
+// wire.
+//
+// This is part of the lnwire.Message interface.
+func (g *MsgV1) MsgType() MessageType {
+	return 7777
+}
+
+// AllRecords returns all the TLV records for the message. This will
+// include all the records we know about along with any that we don't
+// know about but that fall in the signed TLV range.
+//
+// This is part of the PureTLVMessage interface.
+func (g *MsgV1) AllRecords() []tlv.Record {
+	recordProducers := []tlv.RecordProducer{
+		&g.NodeKey,
+		&g.Signature,
+	}
+	recordProducers = append(
+		recordProducers,
+		RecordsAsProducers(
+			tlv.MapToRecords(g.ExtraSignedFields),
+		)...,
+	)
+
+	g.Capacity.WhenSome(
+		func(capacity tlv.RecordT[tlv.TlvType1, MilliSatoshi]) {
+			recordProducers = append(recordProducers, &capacity)
+		},
+	)
+
+	return ProduceRecordsSorted(recordProducers...)
+}
+
+// MsgV2 represents a newer version of MsgV1 which contains more fields both in
+// the unsigned and signed TLV ranges.
+type MsgV2 struct {
+	NodeKey  tlv.RecordT[tlv.TlvType0, *btcec.PublicKey]
+	Capacity tlv.OptionalRecordT[tlv.TlvType1, MilliSatoshi]
+
+	// An additional fields (optional) in the signed range.
+	BitcoinKey tlv.OptionalRecordT[tlv.TlvType3, *btcec.PublicKey]
+
+	// A zero length TLV in the signed range.
+	SecondPeer tlv.OptionalRecordT[tlv.TlvType5, TrueBoolean]
+
+	// Signature in the unsigned range.
+	Signature tlv.RecordT[tlv.TlvType160, Sig]
+
+	// Another field in the unsigned range. An older node can throw this
+	// away.
+	SPVProof tlv.RecordT[tlv.TlvType161, []byte]
+
+	// A new field in the second signed range. An older node should keep
+	// this since it is part of the serialised message that is signed.
+	Num tlv.RecordT[tlv.TlvType1000000000, uint8]
+
+	// Another field in the second unsigned-range. Older nodes may throw
+	// this away and it won't affect the digest used for signature creation
+	// and validation.
+	Other tlv.RecordT[tlv.TlvType3000000000, uint8]
+
+	// Any extra fields in the signed range that we do not yet know about,
+	// but we need to keep them for signature validation and to produce a
+	// valid message.
+	ExtraSignedFields
+}
+
+// newMsgV2 is a constructor for MsgV2.
+func newMsgV2(nodeKey *btcec.PublicKey, capacity *MilliSatoshi,
+	btcKey *btcec.PublicKey, spvProof []byte, num, other uint8,
+	secondPeer bool) *MsgV2 {
+
+	newMsg := &MsgV2{
+		NodeKey:  tlv.NewPrimitiveRecord[tlv.TlvType0](nodeKey),
+		SPVProof: tlv.NewPrimitiveRecord[tlv.TlvType161](spvProof),
+		Num:      tlv.NewPrimitiveRecord[tlv.TlvType1000000000](num),
+		Other:    tlv.NewPrimitiveRecord[tlv.TlvType3000000000](num),
+		Signature: tlv.NewRecordT[tlv.TlvType160](
+			testSchnorrSig,
+		),
+		ExtraSignedFields: make(ExtraSignedFields),
+	}
+
+	if secondPeer {
+		newMsg.SecondPeer = tlv.SomeRecordT(
+			tlv.NewRecordT[tlv.TlvType5](TrueBoolean{}),
+		)
+	}
+
+	if capacity != nil {
+		newMsg.Capacity = tlv.SomeRecordT(
+			tlv.NewPrimitiveRecord[tlv.TlvType1](*capacity),
+		)
+	}
+
+	if btcKey != nil {
+		newMsg.BitcoinKey = tlv.SomeRecordT(
+			tlv.NewPrimitiveRecord[tlv.TlvType3](btcKey),
+		)
+	}
+
+	return newMsg
+}
+
+// Decode deserializes a serialized MsgV2 in the passed io.Reader.
+//
+// This is part of the lnwire.Message interface.
+func (g *MsgV2) Decode(r io.Reader, _ uint32) error {
+	var (
+		capacity   = tlv.ZeroRecordT[tlv.TlvType1, MilliSatoshi]()
+		btcKey     = tlv.ZeroRecordT[tlv.TlvType3, *btcec.PublicKey]()
+		secondPeer = tlv.ZeroRecordT[tlv.TlvType5, TrueBoolean]()
+	)
+
+	stream, err := tlv.NewStream(
+		ProduceRecordsSorted(
+			&g.NodeKey,
+			&capacity,
+			&btcKey,
+			&secondPeer,
+			&g.Signature,
+			&g.SPVProof,
+			&g.Num,
+			&g.Other,
+		)...,
+	)
+	if err != nil {
+		return err
+	}
+	g.Signature.Val.ForceSchnorr()
+
+	typeMap, err := stream.DecodeWithParsedTypesP2P(r)
+	if err != nil {
+		return err
+	}
+
+	if _, ok := typeMap[g.Capacity.TlvType()]; ok {
+		g.Capacity = tlv.SomeRecordT(capacity)
+	}
+
+	if _, ok := typeMap[g.SecondPeer.TlvType()]; ok {
+		g.SecondPeer = tlv.SomeRecordT(secondPeer)
+	}
+
+	if _, ok := typeMap[g.BitcoinKey.TlvType()]; ok {
+		g.BitcoinKey = tlv.SomeRecordT(btcKey)
+	}
+
+	g.ExtraSignedFields = ExtraSignedFieldsFromTypeMap(typeMap)
+
+	return nil
+}
+
+// Encode serializes the target MsgV2 into the passed buffer.
+//
+// This is part of the lnwire.Message interface.
+func (g *MsgV2) Encode(buf *bytes.Buffer, _ uint32) error {
+	return EncodePureTLVMessage(g, buf)
+}
+
+// MsgType returns the integer uniquely identifying this message type on the
+// wire.
+//
+// This is part of the lnwire.Message interface.
+func (g *MsgV2) MsgType() MessageType {
+	return 7779
+}
+
+// AllRecords returns all the TLV records for the message. This will
+// include all the records we know about along with any that we don't
+// know about but that fall in the signed TLV range.
+//
+// This is part of the PureTLVMessage interface.
+func (g *MsgV2) AllRecords() []tlv.Record {
+	recordProducers := []tlv.RecordProducer{
+		&g.NodeKey,
+		&g.Signature,
+		&g.SPVProof,
+		&g.Num,
+		&g.Other,
+	}
+	recordProducers = append(recordProducers, RecordsAsProducers(
+		tlv.MapToRecords(g.ExtraSignedFields),
+	)...)
+
+	g.Capacity.WhenSome(
+		func(c tlv.RecordT[tlv.TlvType1, MilliSatoshi]) {
+			recordProducers = append(recordProducers, &c)
+		},
+	)
+	g.BitcoinKey.WhenSome(
+		func(key tlv.RecordT[tlv.TlvType3, *btcec.PublicKey]) {
+			recordProducers = append(recordProducers, &key)
+		},
+	)
+	g.SecondPeer.WhenSome(
+		func(second tlv.RecordT[tlv.TlvType5, TrueBoolean]) {
+			recordProducers = append(recordProducers, &second)
+		},
+	)
+
+	return ProduceRecordsSorted(recordProducers...)
+}