BIP0144+wire: add a MessageEncoding variant for serialization/deserialization

This commit modifies the existing wire.Message interface to introduce a
new MessageEncoding variant which dictates the exact encoding to be
used when serializing and deserializing messages. Such an option is now
necessary due to the segwit soft-fork package, as btcd will need to be
able to optionally encode transactions/blocks without witness data to
un-upgraded peers.

Two new functions have been introduced: ReadMessageWithEncodingN and
WriteMessageWithEncodingN which wrap BtcDecode/BtcEncode with the
desired encoding format.

wire/message.go

@@ -53,13 +53,31 @@ const (
 	CmdFeeFilter   = "feefilter"
 )
 
+// WireEncoding represents the wire message encoding format to be used.
+type MessageEncoding uint32
+
+const (
+	// BaseEncoding encodes all messages in the default format specified
+	// for the Bitcoin wire protocol.
+	BaseEncoding MessageEncoding = 1 << iota
+
+	// WitnessEncoding encodes all messages other than transaction messages
+	// using the default Bitcoin wire protocol specification. For transaction
+	// messages, the new encoding format detailed in BIP0144 will be used.
+	WitnessEncoding
+)
+
+// LatestEncoding is the most recently specified encoding for the Bitcoin wire
+// protocol.
+var LatestEncoding = WitnessEncoding
+
 // Message is an interface that describes a bitcoin message.  A type that
 // implements Message has complete control over the representation of its data
 // and may therefore contain additional or fewer fields than those which
 // are used directly in the protocol encoded message.
 type Message interface {
-	BtcDecode(io.Reader, uint32) error
-	BtcEncode(io.Writer, uint32) error
+	BtcDecode(io.Reader, uint32, MessageEncoding) error
+	BtcEncode(io.Writer, uint32, MessageEncoding) error
 	Command() string
 	MaxPayloadLength(uint32) uint32
 }
@@ -200,6 +218,27 @@ func discardInput(r io.Reader, n uint32) {
 // information and returns the number of bytes written.    This function is the
 // same as WriteMessage except it also returns the number of bytes written.
 func WriteMessageN(w io.Writer, msg Message, pver uint32, btcnet BitcoinNet) (int, error) {
+	return WriteMessageWithEncodingN(w, msg, pver, btcnet, BaseEncoding)
+}
+
+// WriteMessage writes a bitcoin Message to w including the necessary header
+// information.  This function is the same as WriteMessageN except it doesn't
+// doesn't return the number of bytes written.  This function is mainly provided
+// for backwards compatibility with the original API, but it's also useful for
+// callers that don't care about byte counts.
+func WriteMessage(w io.Writer, msg Message, pver uint32, btcnet BitcoinNet) error {
+	_, err := WriteMessageN(w, msg, pver, btcnet)
+	return err
+}
+
+// WriteMessageWithEncodingN writes a bitcoin Message to w including the
+// necessary header information and returns the number of bytes written.
+// This function is the same as WriteMessageN except it also allows the caller
+// to specify the message encoding format to be used when serializing wire
+// messages.
+func WriteMessageWithEncodingN(w io.Writer, msg Message, pver uint32,
+	btcnet BitcoinNet, encoding MessageEncoding) (int, error) {
+
 	totalBytes := 0
 
 	// Enforce max command size.
@@ -214,7 +253,7 @@ func WriteMessageN(w io.Writer, msg Message, pver uint32, btcnet BitcoinNet) (in
 
 	// Encode the message payload.
 	var bw bytes.Buffer
-	err := msg.BtcEncode(&bw, pver)
+	err := msg.BtcEncode(&bw, pver, encoding)
 	if err != nil {
 		return totalBytes, err
 	}
@@ -264,22 +303,15 @@ func WriteMessageN(w io.Writer, msg Message, pver uint32, btcnet BitcoinNet) (in
 	return totalBytes, err
 }
 
-// WriteMessage writes a bitcoin Message to w including the necessary header
-// information.  This function is the same as WriteMessageN except it doesn't
-// doesn't return the number of bytes written.  This function is mainly provided
-// for backwards compatibility with the original API, but it's also useful for
-// callers that don't care about byte counts.
-func WriteMessage(w io.Writer, msg Message, pver uint32, btcnet BitcoinNet) error {
-	_, err := WriteMessageN(w, msg, pver, btcnet)
-	return err
-}
+// ReadMessageWithEncodingN reads, validates, and parses the next bitcoin Message
+// from r for the provided protocol version and bitcoin network.  It returns the
+// number of bytes read in addition to the parsed Message and raw bytes which
+// comprise the message.  This function is the same as ReadMessageN except it
+// allows the caller to specify which message encoding is to to consult when
+// decoding wire messages.
+func ReadMessageWithEncodingN(r io.Reader, pver uint32, btcnet BitcoinNet,
+	enc MessageEncoding) (int, Message, []byte, error) {
 
-// ReadMessageN reads, validates, and parses the next bitcoin Message from r for
-// the provided protocol version and bitcoin network.  It returns the number of
-// bytes read in addition to the parsed Message and raw bytes which comprise the
-// message.  This function is the same as ReadMessage except it also returns the
-// number of bytes read.
-func ReadMessageN(r io.Reader, pver uint32, btcnet BitcoinNet) (int, Message, []byte, error) {
 	totalBytes := 0
 	n, hdr, err := readMessageHeader(r)
 	totalBytes += n
@@ -351,7 +383,7 @@ func ReadMessageN(r io.Reader, pver uint32, btcnet BitcoinNet) (int, Message, []
 	// Unmarshal message.  NOTE: This must be a *bytes.Buffer since the
 	// MsgVersion BtcDecode function requires it.
 	pr := bytes.NewBuffer(payload)
-	err = msg.BtcDecode(pr, pver)
+	err = msg.BtcDecode(pr, pver, enc)
 	if err != nil {
 		return totalBytes, nil, nil, err
 	}
@@ -359,6 +391,15 @@ func ReadMessageN(r io.Reader, pver uint32, btcnet BitcoinNet) (int, Message, []
 	return totalBytes, msg, payload, nil
 }
 
+// ReadMessageN reads, validates, and parses the next bitcoin Message from r for
+// the provided protocol version and bitcoin network.  It returns the number of
+// bytes read in addition to the parsed Message and raw bytes which comprise the
+// message.  This function is the same as ReadMessage except it also returns the
+// number of bytes read.
+func ReadMessageN(r io.Reader, pver uint32, btcnet BitcoinNet) (int, Message, []byte, error) {
+	return ReadMessageWithEncodingN(r, pver, btcnet, BaseEncoding)
+}
+
 // ReadMessage reads, validates, and parses the next bitcoin Message from r for
 // the provided protocol version and bitcoin network.  It returns the parsed
 // Message and raw bytes which comprise the message.  This function only differs