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/announcement_signatures.go

@@ -1,6 +1,9 @@
 package lnwire
 
-import "io"
+import (
+	"io"
+	"io/ioutil"
+)
 
 // AnnounceSignatures this is a direct message between two endpoints of a
 // channel and serves as an opt-in mechanism to allow the announcement of
@@ -30,6 +33,14 @@ type AnnounceSignatures struct {
 	// bitcoin key and and creating the reverse reference bitcoin_key ->
 	// node_key.
 	BitcoinSignature Sig
+
+	// 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 AnnounceSignatures implements the
@@ -41,12 +52,29 @@ var _ Message = (*AnnounceSignatures)(nil)
 //
 // This is part of the lnwire.Message interface.
 func (a *AnnounceSignatures) Decode(r io.Reader, pver uint32) error {
-	return readElements(r,
+	err := readElements(r,
 		&a.ChannelID,
 		&a.ShortChannelID,
 		&a.NodeSignature,
 		&a.BitcoinSignature,
 	)
+	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 AnnounceSignatures into the passed io.Writer
@@ -59,6 +87,7 @@ func (a *AnnounceSignatures) Encode(w io.Writer, pver uint32) error {
 		a.ShortChannelID,
 		a.NodeSignature,
 		a.BitcoinSignature,
+		a.ExtraOpaqueData,
 	)
 }
 
@@ -75,19 +104,5 @@ func (a *AnnounceSignatures) MsgType() MessageType {
 //
 // This is part of the lnwire.Message interface.
 func (a *AnnounceSignatures) MaxPayloadLength(pver uint32) uint32 {
-	var length uint32
-
-	// ChannelID - 36 bytes
-	length += 36
-
-	// ShortChannelID - 8 bytes
-	length += 8
-
-	// NodeSignatures - 64 bytes
-	length += 64
-
-	// BitcoinSignatures - 64 bytes
-	length += 64
-
-	return length
+	return 65533
 }