lnwire: convert the delivery addr in [Open+Accept]Channel to a TLV type in ExtraData

In this commit, we convert the delivery address in the open and accept
channel methods to be a TLV type. This works as an "empty" delivery
address is encoded using a two zero bytes (uint16 length zero), and a
tlv type of 0 is encoded in the same manner (byte for type, byte for
zero length). This change allows us to easily extend these messages in
the future, in a uniform manner.

When decoding the message we snip the bytes from the read TLV data.
Similarly, when encoding we concatenate the TLV record for the shutdown
script with the rest of the TLV data.

lnwire/open_channel.go

@@ -128,6 +128,17 @@ type OpenChannel struct {
 	// 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
+
+	// 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.
+	//
+	// NOTE: Since the upfront shutdown script MUST be present (though can
+	// be zero-length) if any TLV data is available, the script will be
+	// extracted and removed from this blob when decoding. ExtraData will
+	// contain all TLV records _except_ the DeliveryAddress record in that
+	// case.
+	ExtraData ExtraOpaqueData
 }
 
 // A compile time check to ensure OpenChannel implements the lnwire.Message
@@ -140,6 +151,15 @@ var _ Message = (*OpenChannel)(nil)
 //
 // This is part of the lnwire.Message interface.
 func (o *OpenChannel) Encode(w io.Writer, pver uint32) error {
+	// Since the upfront script is encoded as a TLV record, concatenate it
+	// with the ExtraData, and write them as one.
+	tlvRecords, err := packShutdownScript(
+		o.UpfrontShutdownScript, o.ExtraData,
+	)
+	if err != nil {
+		return err
+	}
+
 	return WriteElements(w,
 		o.ChainHash[:],
 		o.PendingChannelID[:],
@@ -159,7 +179,7 @@ func (o *OpenChannel) Encode(w io.Writer, pver uint32) error {
 		o.HtlcPoint,
 		o.FirstCommitmentPoint,
 		o.ChannelFlags,
-		o.UpfrontShutdownScript,
+		tlvRecords,
 	)
 }
 
@@ -169,7 +189,8 @@ 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 {
-	if err := ReadElements(r,
+	// Read all the mandatory fields in the open message.
+	err := ReadElements(r,
 		o.ChainHash[:],
 		o.PendingChannelID[:],
 		&o.FundingAmount,
@@ -188,14 +209,23 @@ func (o *OpenChannel) Decode(r io.Reader, pver uint32) error {
 		&o.HtlcPoint,
 		&o.FirstCommitmentPoint,
 		&o.ChannelFlags,
-	); err != nil {
+	)
+	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, &o.UpfrontShutdownScript)
-	if err != nil && err != io.EOF {
+	// For backwards compatibility, the optional extra data blob for
+	// OpenChannel must contain an entry for the upfront shutdown script.
+	// We'll read it out and attempt to parse it.
+	var tlvRecords ExtraOpaqueData
+	if err := ReadElements(r, &tlvRecords); err != nil {
+		return err
+	}
+
+	o.UpfrontShutdownScript, o.ExtraData, err = parseShutdownScript(
+		tlvRecords,
+	)
+	if err != nil {
 		return err
 	}
 
@@ -215,11 +245,5 @@ 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
-	var length uint32 = 319 // base length
-
-	// Upfront shutdown script max length.
-	length += 2 + deliveryAddressMaxSize
-
-	return length
+	return MaxMsgBody
 }