dcrd/wire/msgtx.go

// Copyright (c) 2013-2016 The btcsuite developers
// Copyright (c) 2015-2020 The Decred developers
// Use of this source code is governed by an ISC
// license that can be found in the LICENSE file.

package wire

import (
	"bytes"
	"fmt"
	"io"
	"strconv"

	"github.com/decred/dcrd/chaincfg/chainhash"
)

const (
	// TxVersion is the initial transaction version.
	TxVersion uint16 = 1

	// TxVersionSeqLock is the transaction version that enables sequence
	// locks.
	TxVersionSeqLock uint16 = 2

	// TxVersionTreasury is the transaction version that enables the
	// decentralized treasury features.
	TxVersionTreasury uint16 = 3

	// MaxTxInSequenceNum is the maximum sequence number the sequence field
	// of a transaction input can be.
	MaxTxInSequenceNum uint32 = 0xffffffff

	// MaxPrevOutIndex is the maximum index the index field of a previous
	// outpoint can be.
	MaxPrevOutIndex uint32 = 0xffffffff

	// NoExpiryValue is the value of expiry that indicates the transaction
	// has no expiry.
	NoExpiryValue uint32 = 0

	// NullValueIn is a null value for an input witness.
	NullValueIn int64 = -1

	// NullBlockHeight is the null value for an input witness. It references
	// the genesis block.
	NullBlockHeight uint32 = 0x00000000

	// NullBlockIndex is the null transaction index in a block for an input
	// witness.
	NullBlockIndex uint32 = 0xffffffff

	// DefaultPkScriptVersion is the default pkScript version, referring to
	// extended Decred script.
	DefaultPkScriptVersion uint16 = 0x0000

	// TxTreeUnknown is the value returned for a transaction tree that is
	// unknown.  This is typically because the transaction has not been
	// inserted into a block yet.
	TxTreeUnknown int8 = -1

	// TxTreeRegular is the value for a normal transaction tree for a
	// transaction's location in a block.
	TxTreeRegular int8 = 0

	// TxTreeStake is the value for a stake transaction tree for a
	// transaction's location in a block.
	TxTreeStake int8 = 1

	// SequenceLockTimeDisabled is a flag that if set on a transaction
	// input's sequence number, the sequence number will not be interpreted
	// as a relative locktime.
	SequenceLockTimeDisabled = 1 << 31

	// SequenceLockTimeIsSeconds is a flag that if set on a transaction
	// input's sequence number, the relative locktime has units of 512
	// seconds.
	SequenceLockTimeIsSeconds = 1 << 22

	// SequenceLockTimeMask is a mask that extracts the relative locktime
	// when masked against the transaction input sequence number.
	SequenceLockTimeMask = 0x0000ffff

	// SequenceLockTimeGranularity is the defined time based granularity
	// for seconds-based relative time locks.  When converting from seconds
	// to a sequence number, the value is right shifted by this amount,
	// therefore the granularity of relative time locks in 512 or 2^9
	// seconds.  Enforced relative lock times are multiples of 512 seconds.
	SequenceLockTimeGranularity = 9
)

const (
	// defaultTxInOutAlloc is the default size used for the backing array
	// for transaction inputs and outputs.  The array will dynamically grow
	// as needed, but this figure is intended to provide enough space for
	// the number of inputs and outputs in a typical transaction without
	// needing to grow the backing array multiple times.
	defaultTxInOutAlloc = 15

	// minTxInPayload is the minimum payload size for a transaction input.
	// PreviousOutPoint.Hash + PreviousOutPoint.Index 4 bytes +
	// PreviousOutPoint.Tree 1 byte + Varint for SignatureScript length 1
	// byte + Sequence 4 bytes.
	minTxInPayload = 11 + chainhash.HashSize

	// maxTxInPerMessage is the maximum number of transactions inputs that
	// a transaction which fits into a message could possibly have.
	maxTxInPerMessage = (MaxMessagePayload / minTxInPayload) + 1

	// minTxOutPayload is the minimum payload size for a transaction output.
	// Value 8 bytes + Varint for PkScript length 1 byte.
	minTxOutPayload = 9

	// maxTxOutPerMessage is the maximum number of transactions outputs that
	// a transaction which fits into a message could possibly have.
	maxTxOutPerMessage = (MaxMessagePayload / minTxOutPayload) + 1

	// minTxPayload is the minimum payload size for any full encoded
	// (prefix and witness transaction). Note that any realistically
	// usable transaction must have at least one input or output, but
	// that is a rule enforced at a higher layer, so it is intentionally
	// not included here.
	// Version 4 bytes + Varint number of transaction inputs 1 byte + Varint
	// number of transaction outputs 1 byte + Varint representing the number
	// of transaction signatures + LockTime 4 bytes + Expiry 4 bytes + min
	// input payload + min output payload.
	minTxPayload = 4 + 1 + 1 + 1 + 4 + 4

	// freeListMaxScriptSize is the size of each buffer in the free list
	// that is used for deserializing scripts from the wire before they are
	// concatenated into a single contiguous buffer.  This value was chosen
	// because it is slightly more than twice the size of the vast majority
	// of all "standard" scripts.  Larger scripts are still deserialized
	// properly as the free list will simply be bypassed for them.
	freeListMaxScriptSize = 512

	// freeListMaxItems is the number of buffers to keep in the free list
	// to use for script deserialization.  This value allows up to 100
	// scripts per transaction being simultaneously deserialized by 125
	// peers.  Thus, the peak usage of the free list is 12,500 * 512 =
	// 6,400,000 bytes.
	freeListMaxItems = 12500
)

// TxSerializeType represents the serialized type of a transaction.
type TxSerializeType uint16

const (
	// TxSerializeFull indicates a transaction be serialized with the prefix
	// and all witness data.
	TxSerializeFull TxSerializeType = iota

	// TxSerializeNoWitness indicates a transaction be serialized with only
	// the prefix.
	TxSerializeNoWitness

	// TxSerializeOnlyWitness indicates a transaction be serialized with
	// only the witness data.
	TxSerializeOnlyWitness
)

// scriptFreeList defines a free list of byte slices (up to the maximum number
// defined by the freeListMaxItems constant) that have a cap according to the
// freeListMaxScriptSize constant.  It is used to provide temporary buffers for
// deserializing scripts in order to greatly reduce the number of allocations
// required.
//
// The caller can obtain a buffer from the free list by calling the Borrow
// function and should return it via the Return function when done using it.
type scriptFreeList chan []byte

// Borrow returns a byte slice from the free list with a length according the
// provided size.  A new buffer is allocated if there are any items available.
//
// When the size is larger than the max size allowed for items on the free list
// a new buffer of the appropriate size is allocated and returned.  It is safe
// to attempt to return said buffer via the Return function as it will be
// ignored and allowed to go the garbage collector.
func (c scriptFreeList) Borrow(size uint64) []byte {
	if size > freeListMaxScriptSize {
		return make([]byte, size)
	}

	var buf []byte
	select {
	case buf = <-c:
	default:
		buf = make([]byte, freeListMaxScriptSize)
	}
	return buf[:size]
}

// Return puts the provided byte slice back on the free list when it has a cap
// of the expected length.  The buffer is expected to have been obtained via
// the Borrow function.  Any slices that are not of the appropriate size, such
// as those whose size is greater than the largest allowed free list item size
// are simply ignored so they can go to the garbage collector.
func (c scriptFreeList) Return(buf []byte) {
	// Ignore any buffers returned that aren't the expected size for the
	// free list.
	if cap(buf) != freeListMaxScriptSize {
		return
	}

	// Return the buffer to the free list when it's not full.  Otherwise let
	// it be garbage collected.
	select {
	case c <- buf:
	default:
		// Let it go to the garbage collector.
	}
}

// Create the concurrent safe free list to use for script deserialization.  As
// previously described, this free list is maintained to significantly reduce
// the number of allocations.
var scriptPool scriptFreeList = make(chan []byte, freeListMaxItems)

// readScript reads a variable length byte array that represents a transaction
// script.  It is encoded as a varInt containing the length of the array
// followed by the bytes themselves.  An error is returned if the length is
// greater than the passed maxAllowed parameter which helps protect against
// memory exhaustion attacks and forced panics through malformed messages.  The
// fieldName parameter is only used for the error message so it provides more
// context in the error.
func readScript(r io.Reader, pver uint32, maxAllowed uint32, fieldName string) ([]byte, error) {
	const op = "readScript"
	count, err := ReadVarInt(r, pver)
	if err != nil {
		return nil, err
	}

	// Prevent byte array larger than the max message size.  It would
	// be possible to cause memory exhaustion and panics without a sane
	// upper bound on this count.
	if count > uint64(maxAllowed) {
		msg := fmt.Sprintf("%s is larger than the max allowed size "+
			"[count %d, max %d]", fieldName, count, maxAllowed)
		return nil, messageError(op, ErrVarBytesTooLong, msg)
	}

	b := scriptPool.Borrow(count)
	_, err = io.ReadFull(r, b)
	if err != nil {
		scriptPool.Return(b)
		return nil, err
	}
	return b, nil
}

// OutPoint defines a Decred data type that is used to track previous
// transaction outputs.
type OutPoint struct {
	Hash  chainhash.Hash
	Index uint32
	Tree  int8
}

// NewOutPoint returns a new Decred transaction outpoint point with the
// provided hash and index.
func NewOutPoint(hash *chainhash.Hash, index uint32, tree int8) *OutPoint {
	return &OutPoint{
		Hash:  *hash,
		Index: index,
		Tree:  tree,
	}
}

// String returns the OutPoint in the human-readable form "hash:index".
func (o OutPoint) String() string {
	// Allocate enough for hash string, colon, and 10 digits.  Although
	// at the time of writing, the number of digits can be no greater than
	// the length of the decimal representation of maxTxOutPerMessage, the
	// maximum message payload may increase in the future and this
	// optimization may go unnoticed, so allocate space for 10 decimal
	// digits, which will fit any uint32.
	buf := make([]byte, 2*chainhash.HashSize+1, 2*chainhash.HashSize+1+10)
	copy(buf, o.Hash.String())
	buf[2*chainhash.HashSize] = ':'
	buf = strconv.AppendUint(buf, uint64(o.Index), 10)
	return string(buf)
}

// TxIn defines a Decred transaction input.
type TxIn struct {
	// Non-witness
	PreviousOutPoint OutPoint
	Sequence         uint32

	// Witness
	ValueIn         int64
	BlockHeight     uint32
	BlockIndex      uint32
	SignatureScript []byte
}

// SerializeSizePrefix returns the number of bytes it would take to serialize
// the transaction input for a prefix.
func (t *TxIn) SerializeSizePrefix() int {
	// Outpoint Hash 32 bytes + Outpoint Index 4 bytes + Outpoint Tree 1 byte +
	// Sequence 4 bytes.
	return 41
}

// SerializeSizeWitness returns the number of bytes it would take to serialize the
// transaction input for a witness.
func (t *TxIn) SerializeSizeWitness() int {
	// ValueIn (8 bytes) + BlockHeight (4 bytes) + BlockIndex (4 bytes) +
	// serialized varint size for the length of SignatureScript +
	// SignatureScript bytes.
	return 8 + 4 + 4 + VarIntSerializeSize(uint64(len(t.SignatureScript))) +
		len(t.SignatureScript)
}

// NewTxIn returns a new Decred transaction input with the provided
// previous outpoint point and signature script with a default sequence of
// MaxTxInSequenceNum.
func NewTxIn(prevOut *OutPoint, valueIn int64, signatureScript []byte) *TxIn {
	return &TxIn{
		PreviousOutPoint: *prevOut,
		Sequence:         MaxTxInSequenceNum,
		SignatureScript:  signatureScript,
		ValueIn:          valueIn,
		BlockHeight:      NullBlockHeight,
		BlockIndex:       NullBlockIndex,
	}
}

// TxOut defines a Decred transaction output.
type TxOut struct {
	Value    int64
	Version  uint16
	PkScript []byte
}

// SerializeSize returns the number of bytes it would take to serialize the
// the transaction output.
func (t *TxOut) SerializeSize() int {
	// Value 8 bytes + Version 2 bytes + serialized varint size for
	// the length of PkScript + PkScript bytes.
	return 8 + 2 + VarIntSerializeSize(uint64(len(t.PkScript))) + len(t.PkScript)
}

// NewTxOut returns a new Decred transaction output with the provided
// transaction value and public key script.
func NewTxOut(value int64, pkScript []byte) *TxOut {
	return &TxOut{
		Value:    value,
		Version:  DefaultPkScriptVersion,
		PkScript: pkScript,
	}
}

// MsgTx implements the Message interface and represents a Decred tx message.
// It is used to deliver transaction information in response to a getdata
// message (MsgGetData) for a given transaction.
//
// Use the AddTxIn and AddTxOut functions to build up the list of transaction
// inputs and outputs.
type MsgTx struct {
	CachedHash *chainhash.Hash
	SerType    TxSerializeType
	Version    uint16
	TxIn       []*TxIn
	TxOut      []*TxOut
	LockTime   uint32
	Expiry     uint32 // In blocks
}

// AddTxIn adds a transaction input to the message.
func (msg *MsgTx) AddTxIn(ti *TxIn) {
	msg.TxIn = append(msg.TxIn, ti)
}

// AddTxOut adds a transaction output to the message.
func (msg *MsgTx) AddTxOut(to *TxOut) {
	msg.TxOut = append(msg.TxOut, to)
}

// serialize returns the serialization of the transaction for the provided
// serialization type without modifying the original transaction.
func (msg *MsgTx) serialize(serType TxSerializeType) ([]byte, error) {
	// Shallow copy so the serialization type can be changed without
	// modifying the original transaction.
	mtxCopy := *msg
	mtxCopy.SerType = serType
	buf := bytes.NewBuffer(make([]byte, 0, mtxCopy.SerializeSize()))
	err := mtxCopy.Serialize(buf)
	if err != nil {
		return nil, err
	}
	return buf.Bytes(), nil
}

// mustSerialize returns the serialization of the transaction for the provided
// serialization type without modifying the original transaction.  It will panic
// if any errors occur.
func (msg *MsgTx) mustSerialize(serType TxSerializeType) []byte {
	serialized, err := msg.serialize(serType)
	if err != nil {
		panic(fmt.Sprintf("MsgTx failed serializing for type %v",
			serType))
	}
	return serialized
}

// TxHash generates the hash for the transaction prefix.  Since it does not
// contain any witness data, it is not malleable and therefore is stable for
// use in unconfirmed transaction chains.
func (msg *MsgTx) TxHash() chainhash.Hash {
	// TxHash should always calculate a non-witnessed hash.
	return chainhash.HashH(msg.mustSerialize(TxSerializeNoWitness))
}

// CachedTxHash is equivalent to calling TxHash, however it caches the result so
// subsequent calls do not have to recalculate the hash.  It can be recalculated
// later with RecacheTxHash.
func (msg *MsgTx) CachedTxHash() *chainhash.Hash {
	if msg.CachedHash == nil {
		h := msg.TxHash()
		msg.CachedHash = &h
	}

	return msg.CachedHash
}

// RecacheTxHash is equivalent to calling TxHash, however it replaces the cached
// result so future calls to CachedTxHash will return this newly calculated
// hash.
func (msg *MsgTx) RecacheTxHash() *chainhash.Hash {
	h := msg.TxHash()
	msg.CachedHash = &h

	return msg.CachedHash
}

// TxHashWitness generates the hash for the transaction witness.
func (msg *MsgTx) TxHashWitness() chainhash.Hash {
	// TxHashWitness should always calculate a witnessed hash.
	return chainhash.HashH(msg.mustSerialize(TxSerializeOnlyWitness))
}

// TxHashFull generates the hash for the transaction prefix || witness. It first
// obtains the hashes for both the transaction prefix and witness, then
// concatenates them and hashes the result.
func (msg *MsgTx) TxHashFull() chainhash.Hash {
	// Note that the inputs to the hashes, the serialized prefix and
	// witness, have different serialized versions because the serialized
	// encoding of the version includes the real transaction version in the
	// lower 16 bits and the transaction serialization type in the upper 16
	// bits.  The real transaction version (lower 16 bits) will be the same
	// in both serializations.
	concat := make([]byte, chainhash.HashSize*2)
	prefixHash := msg.TxHash()
	witnessHash := msg.TxHashWitness()
	copy(concat[0:], prefixHash[:])
	copy(concat[chainhash.HashSize:], witnessHash[:])

	return chainhash.HashH(concat)
}

// Copy creates a deep copy of a transaction so that the original does not get
// modified when the copy is manipulated.
func (msg *MsgTx) Copy() *MsgTx {
	// Create new tx and start by copying primitive values and making space
	// for the transaction inputs and outputs.
	newTx := MsgTx{
		SerType:  msg.SerType,
		Version:  msg.Version,
		TxIn:     make([]*TxIn, 0, len(msg.TxIn)),
		TxOut:    make([]*TxOut, 0, len(msg.TxOut)),
		LockTime: msg.LockTime,
		Expiry:   msg.Expiry,
	}

	// Deep copy the old TxIn data.
	for _, oldTxIn := range msg.TxIn {
		// Deep copy the old previous outpoint.
		oldOutPoint := oldTxIn.PreviousOutPoint
		newOutPoint := OutPoint{}
		newOutPoint.Hash.SetBytes(oldOutPoint.Hash[:])
		newOutPoint.Index = oldOutPoint.Index
		newOutPoint.Tree = oldOutPoint.Tree

		// Deep copy the old signature script.
		var newScript []byte
		oldScript := oldTxIn.SignatureScript
		oldScriptLen := len(oldScript)
		if oldScriptLen > 0 {
			newScript = make([]byte, oldScriptLen)
			copy(newScript, oldScript[:oldScriptLen])
		}

		// Create new txIn with the deep copied data and append it to
		// new Tx.
		newTxIn := TxIn{
			PreviousOutPoint: newOutPoint,
			Sequence:         oldTxIn.Sequence,
			ValueIn:          oldTxIn.ValueIn,
			BlockHeight:      oldTxIn.BlockHeight,
			BlockIndex:       oldTxIn.BlockIndex,
			SignatureScript:  newScript,
		}
		newTx.TxIn = append(newTx.TxIn, &newTxIn)
	}

	// Deep copy the old TxOut data.
	for _, oldTxOut := range msg.TxOut {
		// Deep copy the old PkScript
		var newScript []byte
		oldScript := oldTxOut.PkScript
		oldScriptLen := len(oldScript)
		if oldScriptLen > 0 {
			newScript = make([]byte, oldScriptLen)
			copy(newScript, oldScript[:oldScriptLen])
		}

		// Create new txOut with the deep copied data and append it to
		// new Tx.
		newTxOut := TxOut{
			Value:    oldTxOut.Value,
			Version:  oldTxOut.Version,
			PkScript: newScript,
		}
		newTx.TxOut = append(newTx.TxOut, &newTxOut)
	}

	return &newTx
}

// writeTxScriptsToMsgTx allocates the memory for variable length fields in a
// MsgTx TxIns, TxOuts, or both as a contiguous chunk of memory, then fills
// in these fields for the MsgTx by copying to a contiguous piece of memory
// and setting the pointer.
//
// NOTE: It is no longer valid to return any previously borrowed script
// buffers after this function has run because it is already done and the
// scripts in the transaction inputs and outputs no longer point to the
// buffers.
func writeTxScriptsToMsgTx(msg *MsgTx, totalScriptSize uint64, serType TxSerializeType) {
	// Create a single allocation to house all of the scripts and set each
	// input signature scripts and output public key scripts to the
	// appropriate subslice of the overall contiguous buffer.  Then, return
	// each individual script buffer back to the pool so they can be reused
	// for future deserializations.  This is done because it significantly
	// reduces the number of allocations the garbage collector needs to track,
	// which in turn improves performance and drastically reduces the amount
	// of runtime overhead that would otherwise be needed to keep track of
	// millions of small allocations.
	//
	// Closures around writing the TxIn and TxOut scripts are used in Decred
	// because, depending on the serialization type desired, only input or
	// output scripts may be required.
	var offset uint64
	scripts := make([]byte, totalScriptSize)
	writeTxIns := func() {
		for i := 0; i < len(msg.TxIn); i++ {
			// Copy the signature script into the contiguous buffer at the
			// appropriate offset.
			signatureScript := msg.TxIn[i].SignatureScript
			copy(scripts[offset:], signatureScript)

			// Reset the signature script of the transaction input to the
			// slice of the contiguous buffer where the script lives.
			scriptSize := uint64(len(signatureScript))
			end := offset + scriptSize
			msg.TxIn[i].SignatureScript = scripts[offset:end:end]
			offset += scriptSize

			// Return the temporary script buffer to the pool.
			scriptPool.Return(signatureScript)
		}
	}
	writeTxOuts := func() {
		for i := 0; i < len(msg.TxOut); i++ {
			// Copy the public key script into the contiguous buffer at the
			// appropriate offset.
			pkScript := msg.TxOut[i].PkScript
			copy(scripts[offset:], pkScript)

			// Reset the public key script of the transaction output to the
			// slice of the contiguous buffer where the script lives.
			scriptSize := uint64(len(pkScript))
			end := offset + scriptSize
			msg.TxOut[i].PkScript = scripts[offset:end:end]
			offset += scriptSize

			// Return the temporary script buffer to the pool.
			scriptPool.Return(pkScript)
		}
	}

	// Handle the serialization types accordingly.
	switch serType {
	case TxSerializeNoWitness:
		writeTxOuts()
	case TxSerializeOnlyWitness:
		fallthrough
	case TxSerializeFull:
		writeTxIns()
		writeTxOuts()
	}
}

// decodePrefix decodes a transaction prefix and stores the contents
// in the embedded msgTx.
func (msg *MsgTx) decodePrefix(r io.Reader, pver uint32) (uint64, error) {
	const op = "MsgTx.decodePrefix"
	count, err := ReadVarInt(r, pver)
	if err != nil {
		return 0, err
	}

	// Prevent more input transactions than could possibly fit into a
	// message.  It would be possible to cause memory exhaustion and panics
	// without a sane upper bound on this count.
	if count > uint64(maxTxInPerMessage) {
		msg := fmt.Sprintf("too many input transactions to fit into max "+
			"message size [count %d, max %d]", count, maxTxInPerMessage)
		return 0, messageError(op, ErrTooManyTxs, msg)
	}

	// TxIns.
	txIns := make([]TxIn, count)
	msg.TxIn = make([]*TxIn, count)
	for i := uint64(0); i < count; i++ {
		// The pointer is set now in case a script buffer is borrowed
		// and needs to be returned to the pool on error.
		ti := &txIns[i]
		msg.TxIn[i] = ti
		err = readTxInPrefix(r, pver, msg.SerType, msg.Version, ti)
		if err != nil {
			return 0, err
		}
	}

	count, err = ReadVarInt(r, pver)
	if err != nil {
		return 0, err
	}

	// Prevent more output transactions than could possibly fit into a
	// message.  It would be possible to cause memory exhaustion and panics
	// without a sane upper bound on this count.
	if count > uint64(maxTxOutPerMessage) {
		msg := fmt.Sprintf("too many output transactions to fit into "+
			"max message size [count %d, max %d]", count, maxTxOutPerMessage)
		return 0, messageError(op, ErrTooManyTxs, msg)
	}

	// TxOuts.
	var totalScriptSize uint64
	txOuts := make([]TxOut, count)
	msg.TxOut = make([]*TxOut, count)
	for i := uint64(0); i < count; i++ {
		// The pointer is set now in case a script buffer is borrowed
		// and needs to be returned to the pool on error.
		to := &txOuts[i]
		msg.TxOut[i] = to
		err = readTxOut(r, pver, msg.Version, to)
		if err != nil {
			return 0, err
		}
		totalScriptSize += uint64(len(to.PkScript))
	}

	// Locktime and expiry.
	msg.LockTime, err = binarySerializer.Uint32(r, littleEndian)
	if err != nil {
		return 0, err
	}

	msg.Expiry, err = binarySerializer.Uint32(r, littleEndian)
	if err != nil {
		return 0, err
	}

	return totalScriptSize, nil
}

func (msg *MsgTx) decodeWitness(r io.Reader, pver uint32, isFull bool) (uint64, error) {
	const op = "MsgTx.decodeWitness"

	// Witness only; generate the TxIn list and fill out only the
	// sigScripts.
	var totalScriptSize uint64
	if !isFull {
		count, err := ReadVarInt(r, pver)
		if err != nil {
			return 0, err
		}

		// Prevent more input transactions than could possibly fit into a
		// message.  It would be possible to cause memory exhaustion and panics
		// without a sane upper bound on this count.
		if count > uint64(maxTxInPerMessage) {
			str := fmt.Sprintf("too many input transactions to fit into "+
				"max message size [count %d, max %d]", count, maxTxInPerMessage)
			return 0, messageError(op, ErrTooManyTxs, str)
		}

		txIns := make([]TxIn, count)
		msg.TxIn = make([]*TxIn, count)
		for i := uint64(0); i < count; i++ {
			// The pointer is set now in case a script buffer is borrowed
			// and needs to be returned to the pool on error.
			ti := &txIns[i]
			msg.TxIn[i] = ti
			err = readTxInWitness(r, pver, msg.Version, ti)
			if err != nil {
				return 0, err
			}
			totalScriptSize += uint64(len(ti.SignatureScript))
		}
		msg.TxOut = make([]*TxOut, 0)
	} else {
		// We're decoding witnesses from a full transaction, so read in
		// the number of signature scripts, check to make sure it's the
		// same as the number of TxIns we currently have, then fill in
		// the signature scripts.
		count, err := ReadVarInt(r, pver)
		if err != nil {
			return 0, err
		}

		// Don't allow the deserializer to panic by accessing memory
		// that doesn't exist.
		if int(count) != len(msg.TxIn) {
			msg := fmt.Sprintf("non equal witness and prefix txin quantities "+
				"(witness %v, prefix %v)", count, len(msg.TxIn))
			return 0, messageError(op, ErrMismatchedWitnessCount, msg)
		}

		// Prevent more input transactions than could possibly fit into a
		// message.  It would be possible to cause memory exhaustion and panics
		// without a sane upper bound on this count.
		if count > uint64(maxTxInPerMessage) {
			msg := fmt.Sprintf("too many input transactions to fit into "+
				"max message size [count %d, max %d]", count, maxTxInPerMessage)
			return 0, messageError(op, ErrTooManyTxs, msg)
		}

		// Read in the witnesses, and copy them into the already generated
		// by decodePrefix TxIns.
		txIns := make([]TxIn, count)
		for i := uint64(0); i < count; i++ {
			ti := &txIns[i]
			err = readTxInWitness(r, pver, msg.Version, ti)
			if err != nil {
				return 0, err
			}
			totalScriptSize += uint64(len(ti.SignatureScript))

			msg.TxIn[i].ValueIn = ti.ValueIn
			msg.TxIn[i].BlockHeight = ti.BlockHeight
			msg.TxIn[i].BlockIndex = ti.BlockIndex
			msg.TxIn[i].SignatureScript = ti.SignatureScript
		}
	}

	return totalScriptSize, nil
}

// BtcDecode decodes r using the Decred protocol encoding into the receiver.
// This is part of the Message interface implementation.
// See Deserialize for decoding transactions stored to disk, such as in a
// database, as opposed to decoding transactions from the wire.
func (msg *MsgTx) BtcDecode(r io.Reader, pver uint32) error {
	const op = "MsgTx.BtcDecode"

	// The serialized encoding of the version includes the real transaction
	// version in the lower 16 bits and the transaction serialization type
	// in the upper 16 bits.
	version, err := binarySerializer.Uint32(r, littleEndian)
	if err != nil {
		return err
	}
	msg.Version = uint16(version & 0xffff)
	msg.SerType = TxSerializeType(version >> 16)

	// returnScriptBuffers is a closure that returns any script buffers that
	// were borrowed from the pool when there are any deserialization
	// errors.  This is only valid to call before the final step which
	// replaces the scripts with the location in a contiguous buffer and
	// returns them.
	returnScriptBuffers := func() {
		for _, txIn := range msg.TxIn {
			if txIn == nil || txIn.SignatureScript == nil {
				continue
			}
			scriptPool.Return(txIn.SignatureScript)
		}
		for _, txOut := range msg.TxOut {
			if txOut == nil || txOut.PkScript == nil {
				continue
			}
			scriptPool.Return(txOut.PkScript)
		}
	}

	// Serialize the transactions depending on their serialization
	// types.  Write the transaction scripts at the end of each
	// serialization procedure using the more efficient contiguous
	// memory allocations, which reduces the amount of memory that
	// must be handled by the GC tremendously.  If any of these
	// serializations fail, free the relevant memory.
	switch txSerType := msg.SerType; txSerType {
	case TxSerializeNoWitness:
		totalScriptSize, err := msg.decodePrefix(r, pver)
		if err != nil {
			returnScriptBuffers()
			return err
		}
		writeTxScriptsToMsgTx(msg, totalScriptSize, txSerType)

	case TxSerializeOnlyWitness:
		totalScriptSize, err := msg.decodeWitness(r, pver, false)
		if err != nil {
			returnScriptBuffers()
			return err
		}
		writeTxScriptsToMsgTx(msg, totalScriptSize, txSerType)

	case TxSerializeFull:
		totalScriptSizeIns, err := msg.decodePrefix(r, pver)
		if err != nil {
			returnScriptBuffers()
			return err
		}
		totalScriptSizeOuts, err := msg.decodeWitness(r, pver, true)
		if err != nil {
			returnScriptBuffers()
			return err
		}
		writeTxScriptsToMsgTx(msg, totalScriptSizeIns+
			totalScriptSizeOuts, txSerType)

	default:
		return messageError(op, ErrUnknownTxType, "unsupported transaction type")
	}

	return nil
}

// Deserialize decodes a transaction from r into the receiver using a format
// that is suitable for long-term storage such as a database while respecting
// the Version field in the transaction.  This function differs from BtcDecode
// in that BtcDecode decodes from the Decred wire protocol as it was sent
// across the network.  The wire encoding can technically differ depending on
// the protocol version and doesn't even really need to match the format of a
// stored transaction at all.  As of the time this comment was written, the
// encoded transaction is the same in both instances, but there is a distinct
// difference and separating the two allows the API to be flexible enough to
// deal with changes.
func (msg *MsgTx) 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)
}

// FromBytes deserializes a transaction byte slice.
func (msg *MsgTx) FromBytes(b []byte) error {
	r := bytes.NewReader(b)
	return msg.Deserialize(r)
}

// encodePrefix encodes a transaction prefix into a writer.
func (msg *MsgTx) encodePrefix(w io.Writer, pver uint32) error {
	count := uint64(len(msg.TxIn))
	err := WriteVarInt(w, pver, count)
	if err != nil {
		return err
	}

	for _, ti := range msg.TxIn {
		err = writeTxInPrefix(w, pver, msg.Version, ti)
		if err != nil {
			return err
		}
	}

	count = uint64(len(msg.TxOut))
	err = WriteVarInt(w, pver, count)
	if err != nil {
		return err
	}

	for _, to := range msg.TxOut {
		err = writeTxOut(w, pver, msg.Version, to)
		if err != nil {
			return err
		}
	}

	err = binarySerializer.PutUint32(w, littleEndian, msg.LockTime)
	if err != nil {
		return err
	}

	return binarySerializer.PutUint32(w, littleEndian, msg.Expiry)
}

// encodeWitness encodes a transaction witness into a writer.
func (msg *MsgTx) encodeWitness(w io.Writer, pver uint32) error {
	count := uint64(len(msg.TxIn))
	err := WriteVarInt(w, pver, count)
	if err != nil {
		return err
	}

	for _, ti := range msg.TxIn {
		err = writeTxInWitness(w, pver, msg.Version, ti)
		if err != nil {
			return err
		}
	}

	return nil
}

// BtcEncode encodes the receiver to w using the Decred protocol encoding.
// This is part of the Message interface implementation.
// See Serialize for encoding transactions to be stored to disk, such as in a
// database, as opposed to encoding transactions for the wire.
func (msg *MsgTx) BtcEncode(w io.Writer, pver uint32) error {
	// The serialized encoding of the version includes the real transaction
	// version in the lower 16 bits and the transaction serialization type
	// in the upper 16 bits.
	serializedVersion := uint32(msg.Version) | uint32(msg.SerType)<<16
	err := binarySerializer.PutUint32(w, littleEndian, serializedVersion)
	if err != nil {
		return err
	}

	switch msg.SerType {
	case TxSerializeNoWitness:
		err := msg.encodePrefix(w, pver)
		if err != nil {
			return err
		}

	case TxSerializeOnlyWitness:
		err := msg.encodeWitness(w, pver)
		if err != nil {
			return err
		}

	case TxSerializeFull:
		err := msg.encodePrefix(w, pver)
		if err != nil {
			return err
		}
		err = msg.encodeWitness(w, pver)
		if err != nil {
			return err
		}

	default:
		return messageError("MsgTx.BtcEncode", ErrUnknownTxType,
			"unsupported transaction type")
	}

	return nil
}

// Serialize encodes the transaction to w using a format that suitable for
// long-term storage such as a database while respecting the Version field in
// the transaction.  This function differs from BtcEncode in that BtcEncode
// encodes the transaction to the Decred wire protocol in order to be sent
// across the network.  The wire encoding can technically differ depending on
// the protocol version and doesn't even really need to match the format of a
// stored transaction at all.  As of the time this comment was written, the
// encoded transaction is the same in both instances, but there is a distinct
// difference and separating the two allows the API to be flexible enough to
// deal with changes.
func (msg *MsgTx) 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)
}

// Bytes returns the serialized form of the transaction in bytes.
func (msg *MsgTx) Bytes() ([]byte, error) {
	buf := bytes.NewBuffer(make([]byte, 0, msg.SerializeSize()))
	err := msg.Serialize(buf)
	if err != nil {
		return nil, err
	}
	return buf.Bytes(), nil
}

// BytesPrefix returns the serialized form of the transaction prefix in bytes.
func (msg *MsgTx) BytesPrefix() ([]byte, error) {
	return msg.serialize(TxSerializeNoWitness)
}

// BytesWitness returns the serialized form of the transaction prefix in bytes.
func (msg *MsgTx) BytesWitness() ([]byte, error) {
	return msg.serialize(TxSerializeOnlyWitness)
}

// SerializeSize returns the number of bytes it would take to serialize the
// the transaction.
func (msg *MsgTx) SerializeSize() int {
	// Unknown type return 0.
	n := 0
	switch msg.SerType {
	case TxSerializeNoWitness:
		// Version 4 bytes + LockTime 4 bytes + Expiry 4 bytes +
		// Serialized varint size for the number of transaction
		// inputs and outputs.
		n = 12 + VarIntSerializeSize(uint64(len(msg.TxIn))) +
			VarIntSerializeSize(uint64(len(msg.TxOut)))

		for _, txIn := range msg.TxIn {
			n += txIn.SerializeSizePrefix()
		}
		for _, txOut := range msg.TxOut {
			n += txOut.SerializeSize()
		}

	case TxSerializeOnlyWitness:
		// Version 4 bytes + Serialized varint size for the
		// number of transaction signatures.
		n = 4 + VarIntSerializeSize(uint64(len(msg.TxIn)))

		for _, txIn := range msg.TxIn {
			n += txIn.SerializeSizeWitness()
		}

	case TxSerializeFull:
		// Version 4 bytes + LockTime 4 bytes + Expiry 4 bytes + Serialized
		// varint size for the number of transaction inputs (x2) and
		// outputs. The number of inputs is added twice because it's
		// encoded once in both the witness and the prefix.
		n = 12 + VarIntSerializeSize(uint64(len(msg.TxIn))) +
			VarIntSerializeSize(uint64(len(msg.TxIn))) +
			VarIntSerializeSize(uint64(len(msg.TxOut)))

		for _, txIn := range msg.TxIn {
			n += txIn.SerializeSizePrefix()
		}
		for _, txIn := range msg.TxIn {
			n += txIn.SerializeSizeWitness()
		}
		for _, txOut := range msg.TxOut {
			n += txOut.SerializeSize()
		}
	}

	return n
}

// Command returns the protocol command string for the message.  This is part
// of the Message interface implementation.
func (msg *MsgTx) Command() string {
	return CmdTx
}

// MaxPayloadLength returns the maximum length the payload can be for the
// receiver.  This is part of the Message interface implementation.
func (msg *MsgTx) MaxPayloadLength(pver uint32) uint32 {
	// Protocol version 3 and lower have a different max block payload.
	if pver <= 3 {
		return MaxBlockPayloadV3
	}

	return MaxBlockPayload
}

// PkScriptLocs returns a slice containing the start of each public key script
// within the raw serialized transaction.  The caller can easily obtain the
// length of each script by using len on the script available via the
// appropriate transaction output entry.
// TODO: Make this work for all serialization types, not just the full
// serialization type.
func (msg *MsgTx) PkScriptLocs() []int {
	// Return nil for witness-only tx.
	numTxOut := len(msg.TxOut)
	if numTxOut == 0 {
		return nil
	}

	// The starting offset in the serialized transaction of the first
	// transaction output is:
	//
	// Version 4 bytes + serialized varint size for the number of
	// transaction inputs and outputs + serialized size of each transaction
	// input.
	n := 4 + VarIntSerializeSize(uint64(len(msg.TxIn))) +
		VarIntSerializeSize(uint64(numTxOut))
	for _, txIn := range msg.TxIn {
		n += txIn.SerializeSizePrefix()
	}

	// Calculate and set the appropriate offset for each public key script.
	pkScriptLocs := make([]int, numTxOut)
	for i, txOut := range msg.TxOut {
		// The offset of the script in the transaction output is:
		//
		// Value 8 bytes + version 2 bytes + serialized varint size
		// for the length of PkScript.
		n += 8 + 2 + VarIntSerializeSize(uint64(len(txOut.PkScript)))
		pkScriptLocs[i] = n
		n += len(txOut.PkScript)
	}

	return pkScriptLocs
}

// NewMsgTx returns a new Decred tx message that conforms to the Message
// interface.  The return instance has a default version of TxVersion and there
// are no transaction inputs or outputs.  Also, the lock time is set to zero
// to indicate the transaction is valid immediately as opposed to some time in
// future.
func NewMsgTx() *MsgTx {
	return &MsgTx{
		SerType: TxSerializeFull,
		Version: TxVersion,
		TxIn:    make([]*TxIn, 0, defaultTxInOutAlloc),
		TxOut:   make([]*TxOut, 0, defaultTxInOutAlloc),
	}
}

// ReadOutPoint reads the next sequence of bytes from r as an OutPoint.
func ReadOutPoint(r io.Reader, pver uint32, version uint16, op *OutPoint) error {
	_, err := io.ReadFull(r, op.Hash[:])
	if err != nil {
		return err
	}

	op.Index, err = binarySerializer.Uint32(r, littleEndian)
	if err != nil {
		return err
	}

	tree, err := binarySerializer.Uint8(r)
	if err != nil {
		return err
	}
	op.Tree = int8(tree)

	return nil
}

// WriteOutPoint encodes op to the Decred protocol encoding for an OutPoint
// to w.
func WriteOutPoint(w io.Writer, pver uint32, version uint16, op *OutPoint) error {
	_, err := w.Write(op.Hash[:])
	if err != nil {
		return err
	}

	err = binarySerializer.PutUint32(w, littleEndian, op.Index)
	if err != nil {
		return err
	}

	return binarySerializer.PutUint8(w, uint8(op.Tree))
}

// readTxInPrefix reads the next sequence of bytes from r as a transaction input
// (TxIn) in the transaction prefix.
func readTxInPrefix(r io.Reader, pver uint32, serType TxSerializeType, version uint16, ti *TxIn) error {
	if serType == TxSerializeOnlyWitness {
		return messageError("readTxInPrefix", ErrReadInPrefixFromWitnessOnlyTx,
			"tried to read a prefix input for a witness only tx")
	}

	// Outpoint.
	err := ReadOutPoint(r, pver, version, &ti.PreviousOutPoint)
	if err != nil {
		return err
	}

	// Sequence.
	ti.Sequence, err = binarySerializer.Uint32(r, littleEndian)
	return err
}

// readTxInWitness reads the next sequence of bytes from r as a transaction input
// (TxIn) in the transaction witness.
func readTxInWitness(r io.Reader, pver uint32, version uint16, ti *TxIn) error {
	// ValueIn.
	valueIn, err := binarySerializer.Uint64(r, littleEndian)
	if err != nil {
		return err
	}
	ti.ValueIn = int64(valueIn)

	// BlockHeight.
	ti.BlockHeight, err = binarySerializer.Uint32(r, littleEndian)
	if err != nil {
		return err
	}

	// BlockIndex.
	ti.BlockIndex, err = binarySerializer.Uint32(r, littleEndian)
	if err != nil {
		return err
	}

	// Signature script.
	ti.SignatureScript, err = readScript(r, pver, MaxMessagePayload,
		"transaction input signature script")
	return err
}

// writeTxInPrefixs encodes ti to the Decred protocol encoding for a transaction
// input (TxIn) prefix to w.
func writeTxInPrefix(w io.Writer, pver uint32, version uint16, ti *TxIn) error {
	err := WriteOutPoint(w, pver, version, &ti.PreviousOutPoint)
	if err != nil {
		return err
	}

	return binarySerializer.PutUint32(w, littleEndian, ti.Sequence)
}

// writeTxWitness encodes ti to the Decred protocol encoding for a transaction
// input (TxIn) witness to w.
func writeTxInWitness(w io.Writer, pver uint32, version uint16, ti *TxIn) error {
	// ValueIn.
	err := binarySerializer.PutUint64(w, littleEndian, uint64(ti.ValueIn))
	if err != nil {
		return err
	}

	// BlockHeight.
	err = binarySerializer.PutUint32(w, littleEndian, ti.BlockHeight)
	if err != nil {
		return err
	}

	// BlockIndex.
	err = binarySerializer.PutUint32(w, littleEndian, ti.BlockIndex)
	if err != nil {
		return err
	}

	// Write the signature script.
	return WriteVarBytes(w, pver, ti.SignatureScript)
}

// readTxOut reads the next sequence of bytes from r as a transaction output
// (TxOut).
func readTxOut(r io.Reader, pver uint32, version uint16, to *TxOut) error {
	value, err := binarySerializer.Uint64(r, littleEndian)
	if err != nil {
		return err
	}
	to.Value = int64(value)

	to.Version, err = binarySerializer.Uint16(r, littleEndian)
	if err != nil {
		return err
	}

	to.PkScript, err = readScript(r, pver, MaxMessagePayload,
		"transaction output public key script")
	return err
}

// writeTxOut encodes to into the Decred protocol encoding for a transaction
// output (TxOut) to w.
func writeTxOut(w io.Writer, pver uint32, version uint16, to *TxOut) error {
	err := binarySerializer.PutUint64(w, littleEndian, uint64(to.Value))
	if err != nil {
		return err
	}

	err = binarySerializer.PutUint16(w, littleEndian, to.Version)
	if err != nil {
		return err
	}

	return WriteVarBytes(w, pver, to.PkScript)
}