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/open_channel.go

@@ -122,6 +122,12 @@ type OpenChannel struct {
 	// Currently, the least significant bit of this bit field indicates the
 	// initiator of the channel wishes to advertise this channel publicly.
 	ChannelFlags FundingFlag
+
+	// 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 OpenChannel implements the lnwire.Message
@@ -153,6 +159,7 @@ func (o *OpenChannel) Encode(w io.Writer, pver uint32) error {
 		o.HtlcPoint,
 		o.FirstCommitmentPoint,
 		o.ChannelFlags,
+		o.UpfrontShutdownScript,
 	)
 }
 
@@ -162,7 +169,7 @@ func (o *OpenChannel) Encode(w io.Writer, pver uint32) error {
 //
 // This is part of the lnwire.Message interface.
 func (o *OpenChannel) Decode(r io.Reader, pver uint32) error {
-	return ReadElements(r,
+	if err := ReadElements(r,
 		o.ChainHash[:],
 		o.PendingChannelID[:],
 		&o.FundingAmount,
@@ -181,7 +188,18 @@ func (o *OpenChannel) Decode(r io.Reader, pver uint32) error {
 		&o.HtlcPoint,
 		&o.FirstCommitmentPoint,
 		&o.ChannelFlags,
-	)
+	); err != nil {
+		return err
+	}
+
+	// Check for the optional upfront shutdown script field. If it is not there,
+	// silence the EOF error.
+	err := ReadElement(r, &o.UpfrontShutdownScript)
+	if err != nil && err != io.EOF {
+		return err
+	}
+
+	return nil
 }
 
 // MsgType returns the MessageType code which uniquely identifies this message
@@ -198,5 +216,10 @@ func (o *OpenChannel) MsgType() MessageType {
 // This is part of the lnwire.Message interface.
 func (o *OpenChannel) MaxPayloadLength(uint32) uint32 {
 	// (32 * 2) + (8 * 6) + (4 * 1) + (2 * 2) + (33 * 6) + 1
-	return 319
+	var length uint32 = 319 // base length
+
+	// Upfront shutdown script max length.
+	length += 2 + deliveryAddressMaxSize
+
+	return length
 }