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/msgblock.go

@@ -60,7 +60,7 @@ func (msg *MsgBlock) ClearTransactions() {
 // This is part of the Message interface implementation.
 // See Deserialize for decoding blocks stored to disk, such as in a database, as
 // opposed to decoding blocks from the wire.
-func (msg *MsgBlock) BtcDecode(r io.Reader, pver uint32) error {
+func (msg *MsgBlock) BtcDecode(r io.Reader, pver uint32, enc MessageEncoding) error {
 	err := readBlockHeader(r, pver, &msg.Header)
 	if err != nil {
 		return err
@@ -83,7 +83,7 @@ func (msg *MsgBlock) BtcDecode(r io.Reader, pver uint32) error {
 	msg.Transactions = make([]*MsgTx, 0, txCount)
 	for i := uint64(0); i < txCount; i++ {
 		tx := MsgTx{}
-		err := tx.BtcDecode(r, pver)
+		err := tx.BtcDecode(r, pver, enc)
 		if err != nil {
 			return err
 		}
@@ -106,7 +106,19 @@ func (msg *MsgBlock) Deserialize(r io.Reader) error {
 	// At the current time, there is no difference between the wire encoding
 	// at protocol version 0 and the stable long-term storage format.  As
 	// a result, make use of BtcDecode.
-	return msg.BtcDecode(r, 0)
+	//
+	// Passing an encoding type of WitnessEncoding to BtcEncode for the
+	// MessageEncoding parameter indicates that the transactions within the
+	// block are expected to be serialized according to the new
+	// serialization structure defined in BIP0141.
+	return msg.BtcDecode(r, 0, WitnessEncoding)
+}
+
+// DeserializeWitness decodes a block from r into the receiver similar to
+// Deserialize, however DeserializeWitness strips all (if any) witness data
+// from the transactions within the block before encoding them.
+func (msg *MsgBlock) DeserializeNoWitness(r io.Reader) error {
+	return msg.BtcDecode(r, 0, BaseEncoding)
 }
 
 // DeserializeTxLoc decodes r in the same manner Deserialize does, but it takes
@@ -160,7 +172,7 @@ func (msg *MsgBlock) DeserializeTxLoc(r *bytes.Buffer) ([]TxLoc, error) {
 // This is part of the Message interface implementation.
 // See Serialize for encoding blocks to be stored to disk, such as in a
 // database, as opposed to encoding blocks for the wire.
-func (msg *MsgBlock) BtcEncode(w io.Writer, pver uint32) error {
+func (msg *MsgBlock) BtcEncode(w io.Writer, pver uint32, enc MessageEncoding) error {
 	err := writeBlockHeader(w, pver, &msg.Header)
 	if err != nil {
 		return err
@@ -172,7 +184,7 @@ func (msg *MsgBlock) BtcEncode(w io.Writer, pver uint32) error {
 	}
 
 	for _, tx := range msg.Transactions {
-		err = tx.BtcEncode(w, pver)
+		err = tx.BtcEncode(w, pver, enc)
 		if err != nil {
 			return err
 		}
@@ -194,7 +206,20 @@ func (msg *MsgBlock) Serialize(w io.Writer) error {
 	// At the current time, there is no difference between the wire encoding
 	// at protocol version 0 and the stable long-term storage format.  As
 	// a result, make use of BtcEncode.
-	return msg.BtcEncode(w, 0)
+	//
+	// Passing WitnessEncoding as the encoding type here indicates that
+	// each of the transactions should be serialized using the witness
+	// serialization structure defined in BIP0141.
+	return msg.BtcEncode(w, 0, WitnessEncoding)
+}
+
+// SerializeWitness encodes a block to w using an identical format to Serialize,
+// with all (if any) witness data stripped from all transactions.
+// This method is provided in additon to the regular Serialize, in order to
+// allow one to selectively encode transaction witness data to non-upgraded
+// peers which are unaware of the new encoding.
+func (msg *MsgBlock) SerializeNoWitness(w io.Writer) error {
+	return msg.BtcEncode(w, 0, BaseEncoding)
 }
 
 // SerializeSize returns the number of bytes it would take to serialize the