lnwire: Add upfront shutdown messages and feature bit

This commit adds the feature bit and additional fields
required in `open_channel` and `accept_channel` wire
messages for `option_upfront_shutdown_script`.

lnwire/accept_channel.go

@@ -86,6 +86,12 @@ type AcceptChannel struct {
 	// base point in order to derive the revocation keys that are placed
 	// within the commitment transaction of the sender.
 	FirstCommitmentPoint *btcec.PublicKey
+
+	// UpfrontShutdownScript is the script to which the channel funds should
+	// be paid when mutually closing the channel. This field is optional, and
+	// and has a length prefix, so a zero will be written if it is not set
+	// and its length followed by the script will be written if it is set.
+	UpfrontShutdownScript DeliveryAddress
 }
 
 // A compile time check to ensure AcceptChannel implements the lnwire.Message
@@ -113,6 +119,7 @@ func (a *AcceptChannel) Encode(w io.Writer, pver uint32) error {
 		a.DelayedPaymentPoint,
 		a.HtlcPoint,
 		a.FirstCommitmentPoint,
+		a.UpfrontShutdownScript,
 	)
 }
 
@@ -122,7 +129,8 @@ func (a *AcceptChannel) Encode(w io.Writer, pver uint32) error {
 //
 // This is part of the lnwire.Message interface.
 func (a *AcceptChannel) Decode(r io.Reader, pver uint32) error {
-	return ReadElements(r,
+	// Read all the mandatory fields in the accept message.
+	err := ReadElements(r,
 		a.PendingChannelID[:],
 		&a.DustLimit,
 		&a.MaxValueInFlight,
@@ -138,6 +146,17 @@ func (a *AcceptChannel) Decode(r io.Reader, pver uint32) error {
 		&a.HtlcPoint,
 		&a.FirstCommitmentPoint,
 	)
+	if err != nil {
+		return err
+	}
+
+	// Check for the optional upfront shutdown script field. If it is not there,
+	// silence the EOF error.
+	err = ReadElement(r, &a.UpfrontShutdownScript)
+	if err != nil && err != io.EOF {
+		return err
+	}
+	return nil
 }
 
 // MsgType returns the MessageType code which uniquely identifies this message
@@ -154,5 +173,10 @@ func (a *AcceptChannel) MsgType() MessageType {
 // This is part of the lnwire.Message interface.
 func (a *AcceptChannel) MaxPayloadLength(uint32) uint32 {
 	// 32 + (8 * 4) + (4 * 1) + (2 * 2) + (33 * 6)
-	return 270
+	var length uint32 = 270 // base length
+
+	// Upfront shutdown script max length.
+	length += 2 + deliveryAddressMaxSize
+
+	return length
 }