lnwire: introduce PureTLVMessage

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.

A comprehensive test is added that shows how two versions of the same
message remain forward compatible.

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
+}