lnwire: add new ExtraOpaqueData field to gossip messages

In this commit, we add a new field to all the existing gossip messages:
ExtraOpqueData. We do this, as before this commit, if we came across a
ChannelUpdate message with a set of optional fields, then we wouldn't be
able to properly parse the signatures related to the message. If we
never corrected this behavior, then we would violate the forwards
compatible principle we use when parsing existing messages.

As these messages can now be padded out to the max message size, we've
increased the MaxPayloadLength value for all of these messages.

Fixes #1814.

lnwire/channel_update.go

@@ -3,6 +3,7 @@ package lnwire
 import (
 	"bytes"
 	"io"
+	"io/ioutil"
 
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 )
@@ -73,6 +74,14 @@ type ChannelUpdate struct {
 	// FeeRate is the fee rate that will be charged per millionth of a
 	// satoshi.
 	FeeRate uint32
+
+	// ExtraOpaqueData is the set of data that was appended to this
+	// message, some of which we may not actually know how to iterate or
+	// parse. By holding onto this data, we ensure that we're able to
+	// properly validate the set of signatures that cover these new fields,
+	// and ensure we're able to make upgrades to the network in a forwards
+	// compatible manner.
+	ExtraOpaqueData []byte
 }
 
 // A compile time check to ensure ChannelUpdate implements the lnwire.Message
@@ -84,7 +93,7 @@ var _ Message = (*ChannelUpdate)(nil)
 //
 // This is part of the lnwire.Message interface.
 func (a *ChannelUpdate) Decode(r io.Reader, pver uint32) error {
-	return readElements(r,
+	err := readElements(r,
 		&a.Signature,
 		a.ChainHash[:],
 		&a.ShortChannelID,
@@ -95,6 +104,23 @@ func (a *ChannelUpdate) Decode(r io.Reader, pver uint32) error {
 		&a.BaseFee,
 		&a.FeeRate,
 	)
+	if err != nil {
+		return err
+	}
+
+	// Now that we've read out all the fields that we explicitly know of,
+	// we'll collect the remainder into the ExtraOpaqueData field. If there
+	// aren't any bytes, then we'll snip off the slice to avoid carrying
+	// around excess capacity.
+	a.ExtraOpaqueData, err = ioutil.ReadAll(r)
+	if err != nil {
+		return err
+	}
+	if len(a.ExtraOpaqueData) == 0 {
+		a.ExtraOpaqueData = nil
+	}
+
+	return nil
 }
 
 // Encode serializes the target ChannelUpdate into the passed io.Writer
@@ -112,6 +138,7 @@ func (a *ChannelUpdate) Encode(w io.Writer, pver uint32) error {
 		a.HtlcMinimumMsat,
 		a.BaseFee,
 		a.FeeRate,
+		a.ExtraOpaqueData,
 	)
 }
 
@@ -128,36 +155,7 @@ func (a *ChannelUpdate) MsgType() MessageType {
 //
 // This is part of the lnwire.Message interface.
 func (a *ChannelUpdate) MaxPayloadLength(pver uint32) uint32 {
-	var length uint32
-
-	// Signature - 64 bytes
-	length += 64
-
-	// ChainHash - 64 bytes
-	length += 32
-
-	// ShortChannelID - 8 bytes
-	length += 8
-
-	// Timestamp - 4 bytes
-	length += 4
-
-	// Flags - 2 bytes
-	length += 2
-
-	// Expiry - 2 bytes
-	length += 2
-
-	// HtlcMinimumMstat - 8 bytes
-	length += 8
-
-	// FeeBaseMstat - 4 bytes
-	length += 4
-
-	// FeeProportionalMillionths - 4 bytes
-	length += 4
-
-	return length
+	return 65533
 }
 
 // DataToSign is used to retrieve part of the announcement message which should
@@ -175,6 +173,7 @@ func (a *ChannelUpdate) DataToSign() ([]byte, error) {
 		a.HtlcMinimumMsat,
 		a.BaseFee,
 		a.FeeRate,
+		a.ExtraOpaqueData,
 	)
 	if err != nil {
 		return nil, err