lnwire: add new SerializedSize method to all wire messages

This'll be useful for the bandwidth based rate limiting we'll implement
in the next commit.

lnwire/message.go

@@ -12,6 +12,10 @@ import (
 	"io"
 )
 
+// MessageTypeSize is the size in bytes of the message type field in the header
+// of all messages.
+const MessageTypeSize = 2
+
 // MessageType is the unique 2 byte big-endian integer that indicates the type
 // of message on the wire. All messages have a very simple header which
 // consists simply of 2-byte message type. We omit a length field, and checksum
@@ -234,6 +238,31 @@ type LinkUpdater interface {
 	TargetChanID() ChannelID
 }
 
+// SizeableMessage is an interface that extends the base Message interface with
+// a method to calculate the serialized size of a message.
+type SizeableMessage interface {
+	Message
+
+	// SerializedSize returns the serialized size of the message in bytes.
+	// The returned size includes the message type header bytes.
+	SerializedSize() (uint32, error)
+}
+
+// MessageSerializedSize calculates the serialized size of a message in bytes.
+// This is a helper function that can be used by all message types to implement
+// the SerializedSize method.
+func MessageSerializedSize(msg Message) (uint32, error) {
+	var buf bytes.Buffer
+
+	// Encode the message to the buffer.
+	if err := msg.Encode(&buf, 0); err != nil {
+		return 0, err
+	}
+
+	// Add the size of the message type.
+	return uint32(buf.Len()) + MessageTypeSize, nil
+}
+
 // makeEmptyMessage creates a new empty message of the proper concrete type
 // based on the passed message type.
 func makeEmptyMessage(msgType MessageType) (Message, error) {
@@ -337,6 +366,12 @@ func makeEmptyMessage(msgType MessageType) (Message, error) {
 	return msg, nil
 }
 
+// MakeEmptyMessage creates a new empty message of the proper concrete type
+// based on the passed message type. This is exported to be used in tests.
+func MakeEmptyMessage(msgType MessageType) (Message, error) {
+	return makeEmptyMessage(msgType)
+}
+
 // WriteMessage writes a lightning Message to a buffer including the necessary
 // header information and returns the number of bytes written. If any error is
 // encountered, the buffer passed will be reset to its original state since we