lnwire: prep wire messages for TLV extensions

Messages:
- UpdateFulfillHTLC
- UpdateFee
- UpdateFailMalformedHTLC
- UpdateFailHTLC
- UpdateAddHTLC
- Shutdown
- RevokeAndAck
- ReplyShortChanIDsEnd
- ReplyChannelRange
- QueryShortChanIDs
- QueryChannelRange
- NodeAnnouncement
- Init
- GossipTimestampRange
- FundingSigned
- FundingLocked
- FundingCreated
- CommitSig
- ClosingSigned
- ChannelUpdate
- ChannelReestablish
- ChannelAnnouncement
- AnnounceSignatures

lnwire: update quickcheck tests, use constant for Error

multi: update unit tests to pass deep equal assertions with messages

In this commit, we update a series of unit tests in the code base to now
pass due to the new wire message encode/decode logic. In many instances,
we'll now manually set the extra bytes to an empty byte slice to avoid
comparisons that fail due to one message having an empty byte slice and
the other having a nil pointer.

lnwire/channel_update.go

@@ -4,7 +4,6 @@ import (
 	"bytes"
 	"fmt"
 	"io"
-	"io/ioutil"
 
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 )
@@ -115,13 +114,10 @@ type ChannelUpdate struct {
 	// HtlcMaximumMsat is the maximum HTLC value which will be accepted.
 	HtlcMaximumMsat MilliSatoshi
 
-	// 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
+	// ExtraData is the set of data that was appended to this message to
+	// fill out the full maximum transport message size. These fields can
+	// be used to specify optional data such as custom TLV fields.
+	ExtraOpaqueData ExtraOpaqueData
 }
 
 // A compile time check to ensure ChannelUpdate implements the lnwire.Message
@@ -156,19 +152,7 @@ func (a *ChannelUpdate) Decode(r io.Reader, pver uint32) error {
 		}
 	}
 
-	// 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
+	return a.ExtraOpaqueData.Decode(r)
 }
 
 // Encode serializes the target ChannelUpdate into the passed io.Writer
@@ -201,7 +185,7 @@ func (a *ChannelUpdate) Encode(w io.Writer, pver uint32) error {
 	}
 
 	// Finally, append any extra opaque data.
-	return WriteElements(w, a.ExtraOpaqueData)
+	return a.ExtraOpaqueData.Encode(w)
 }
 
 // MsgType returns the integer uniquely identifying this message type on the
@@ -217,7 +201,7 @@ func (a *ChannelUpdate) MsgType() MessageType {
 //
 // This is part of the lnwire.Message interface.
 func (a *ChannelUpdate) MaxPayloadLength(pver uint32) uint32 {
-	return 65533
+	return MaxMsgBody
 }
 
 // DataToSign is used to retrieve part of the announcement message which should
@@ -250,7 +234,7 @@ func (a *ChannelUpdate) DataToSign() ([]byte, error) {
 	}
 
 	// Finally, append any extra opaque data.
-	if err := WriteElements(&w, a.ExtraOpaqueData); err != nil {
+	if err := a.ExtraOpaqueData.Encode(&w); err != nil {
 		return nil, err
 	}