btcd/blockchain/utxoviewpoint.go

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

package blockchain

import (
	"fmt"

	"github.com/btcsuite/btcd/btcutil"
	"github.com/btcsuite/btcd/chaincfg/chainhash"
	"github.com/btcsuite/btcd/database"
	"github.com/btcsuite/btcd/txscript"
	"github.com/btcsuite/btcd/wire"
)

// txoFlags is a bitmask defining additional information and state for a
// transaction output in a utxo view.
type txoFlags uint8

const (
	// tfCoinBase indicates that a txout was contained in a coinbase tx.
	tfCoinBase txoFlags = 1 << iota

	// tfSpent indicates that a txout is spent.
	tfSpent

	// tfModified indicates that a txout has been modified since it was
	// loaded.
	tfModified

	// tfFresh indicates that the entry is fresh.  This means that the parent
	// view never saw this entry.  Note that tfFresh is a performance
	// optimization with which we can erase entries that are fully spent if we
	// know we do not need to commit them.  It is always safe to not mark
	// tfFresh if that condition is not guaranteed.
	tfFresh
)

// UtxoEntry houses details about an individual transaction output in a utxo
// view such as whether or not it was contained in a coinbase tx, the height of
// the block that contains the tx, whether or not it is spent, its public key
// script, and how much it pays.
type UtxoEntry struct {
	// NOTE: Additions, deletions, or modifications to the order of the
	// definitions in this struct should not be changed without considering
	// how it affects alignment on 64-bit platforms.  The current order is
	// specifically crafted to result in minimal padding.  There will be a
	// lot of these in memory, so a few extra bytes of padding adds up.

	amount      int64
	pkScript    []byte // The public key script for the output.
	blockHeight int32  // Height of block containing tx.

	// packedFlags contains additional info about output such as whether it
	// is a coinbase, whether it is spent, and whether it has been modified
	// since it was loaded.  This approach is used in order to reduce memory
	// usage since there will be a lot of these in memory.
	packedFlags txoFlags
}

// isModified returns whether or not the output has been modified since it was
// loaded.
func (entry *UtxoEntry) isModified() bool {
	return entry.packedFlags&tfModified == tfModified
}

// isFresh returns whether or not it's certain the output has never previously
// been stored in the database.
func (entry *UtxoEntry) isFresh() bool {
	return entry.packedFlags&tfFresh == tfFresh
}

// memoryUsage returns the memory usage in bytes of for the utxo entry.
// It returns 0 for a nil entry.
func (entry *UtxoEntry) memoryUsage() uint64 {
	if entry == nil {
		return 0
	}

	return baseEntrySize + uint64(cap(entry.pkScript))
}

// IsCoinBase returns whether or not the output was contained in a coinbase
// transaction.
func (entry *UtxoEntry) IsCoinBase() bool {
	return entry.packedFlags&tfCoinBase == tfCoinBase
}

// BlockHeight returns the height of the block containing the output.
func (entry *UtxoEntry) BlockHeight() int32 {
	return entry.blockHeight
}

// IsSpent returns whether or not the output has been spent based upon the
// current state of the unspent transaction output view it was obtained from.
func (entry *UtxoEntry) IsSpent() bool {
	return entry.packedFlags&tfSpent == tfSpent
}

// Spend marks the output as spent.  Spending an output that is already spent
// has no effect.
func (entry *UtxoEntry) Spend() {
	// Nothing to do if the output is already spent.
	if entry.IsSpent() {
		return
	}

	// Mark the output as spent and modified.
	entry.packedFlags |= tfSpent | tfModified
}

// Amount returns the amount of the output.
func (entry *UtxoEntry) Amount() int64 {
	return entry.amount
}

// PkScript returns the public key script for the output.
func (entry *UtxoEntry) PkScript() []byte {
	return entry.pkScript
}

// Clone returns a shallow copy of the utxo entry.
func (entry *UtxoEntry) Clone() *UtxoEntry {
	if entry == nil {
		return nil
	}

	return &UtxoEntry{
		amount:      entry.amount,
		pkScript:    entry.pkScript,
		blockHeight: entry.blockHeight,
		packedFlags: entry.packedFlags,
	}
}

// NewUtxoEntry returns a new UtxoEntry built from the arguments.
func NewUtxoEntry(
	txOut *wire.TxOut, blockHeight int32, isCoinbase bool) *UtxoEntry {
	var cbFlag txoFlags
	if isCoinbase {
		cbFlag |= tfCoinBase
	}

	return &UtxoEntry{
		amount:      txOut.Value,
		pkScript:    txOut.PkScript,
		blockHeight: blockHeight,
		packedFlags: cbFlag,
	}
}

// UtxoViewpoint represents a view into the set of unspent transaction outputs
// from a specific point of view in the chain.  For example, it could be for
// the end of the main chain, some point in the history of the main chain, or
// down a side chain.
//
// The unspent outputs are needed by other transactions for things such as
// script validation and double spend prevention.
type UtxoViewpoint struct {
	entries  map[wire.OutPoint]*UtxoEntry
	bestHash chainhash.Hash
}

// BestHash returns the hash of the best block in the chain the view currently
// respresents.
func (view *UtxoViewpoint) BestHash() *chainhash.Hash {
	return &view.bestHash
}

// SetBestHash sets the hash of the best block in the chain the view currently
// respresents.
func (view *UtxoViewpoint) SetBestHash(hash *chainhash.Hash) {
	view.bestHash = *hash
}

// LookupEntry returns information about a given transaction output according to
// the current state of the view.  It will return nil if the passed output does
// not exist in the view or is otherwise not available such as when it has been
// disconnected during a reorg.
func (view *UtxoViewpoint) LookupEntry(outpoint wire.OutPoint) *UtxoEntry {
	return view.entries[outpoint]
}

// FetchPrevOutput fetches the previous output referenced by the passed
// outpoint. This is identical to the LookupEntry method, but it returns a
// wire.TxOut instead.
//
// NOTE: This is an implementation of the txscript.PrevOutputFetcher interface.
func (view *UtxoViewpoint) FetchPrevOutput(op wire.OutPoint) *wire.TxOut {
	prevOut := view.entries[op]
	if prevOut == nil {
		return nil
	}

	return &wire.TxOut{
		Value:    prevOut.amount,
		PkScript: prevOut.PkScript(),
	}
}

// addTxOut adds the specified output to the view if it is not provably
// unspendable.  When the view already has an entry for the output, it will be
// marked unspent.  All fields will be updated for existing entries since it's
// possible it has changed during a reorg.
func (view *UtxoViewpoint) addTxOut(outpoint wire.OutPoint, txOut *wire.TxOut, isCoinBase bool, blockHeight int32) {
	// Don't add provably unspendable outputs.
	if txscript.IsUnspendable(txOut.PkScript) {
		return
	}

	// Update existing entries.  All fields are updated because it's
	// possible (although extremely unlikely) that the existing entry is
	// being replaced by a different transaction with the same hash.  This
	// is allowed so long as the previous transaction is fully spent.
	entry := view.LookupEntry(outpoint)
	if entry == nil {
		entry = new(UtxoEntry)
		view.entries[outpoint] = entry
	}

	entry.amount = txOut.Value
	entry.pkScript = txOut.PkScript
	entry.blockHeight = blockHeight
	entry.packedFlags = tfFresh | tfModified
	if isCoinBase {
		entry.packedFlags |= tfCoinBase
	}
}

// AddTxOut adds the specified output of the passed transaction to the view if
// it exists and is not provably unspendable.  When the view already has an
// entry for the output, it will be marked unspent.  All fields will be updated
// for existing entries since it's possible it has changed during a reorg.
func (view *UtxoViewpoint) AddTxOut(tx *btcutil.Tx, txOutIdx uint32, blockHeight int32) {
	// Can't add an output for an out of bounds index.
	if txOutIdx >= uint32(len(tx.MsgTx().TxOut)) {
		return
	}

	// Update existing entries.  All fields are updated because it's
	// possible (although extremely unlikely) that the existing entry is
	// being replaced by a different transaction with the same hash.  This
	// is allowed so long as the previous transaction is fully spent.
	prevOut := wire.OutPoint{Hash: *tx.Hash(), Index: txOutIdx}
	txOut := tx.MsgTx().TxOut[txOutIdx]
	view.addTxOut(prevOut, txOut, IsCoinBase(tx), blockHeight)
}

// AddTxOuts adds all outputs in the passed transaction which are not provably
// unspendable to the view.  When the view already has entries for any of the
// outputs, they are simply marked unspent.  All fields will be updated for
// existing entries since it's possible it has changed during a reorg.
func (view *UtxoViewpoint) AddTxOuts(tx *btcutil.Tx, blockHeight int32) {
	// Loop all of the transaction outputs and add those which are not
	// provably unspendable.
	isCoinBase := IsCoinBase(tx)
	prevOut := wire.OutPoint{Hash: *tx.Hash()}
	for txOutIdx, txOut := range tx.MsgTx().TxOut {
		// Update existing entries.  All fields are updated because it's
		// possible (although extremely unlikely) that the existing
		// entry is being replaced by a different transaction with the
		// same hash.  This is allowed so long as the previous
		// transaction is fully spent.
		prevOut.Index = uint32(txOutIdx)
		view.addTxOut(prevOut, txOut, isCoinBase, blockHeight)
	}
}

// connectTransaction updates the view by adding all new utxos created by the
// passed transaction and marking all utxos that the transactions spend as
// spent.  In addition, when the 'stxos' argument is not nil, it will be updated
// to append an entry for each spent txout.  An error will be returned if the
// view does not contain the required utxos.
func (view *UtxoViewpoint) connectTransaction(tx *btcutil.Tx, blockHeight int32, stxos *[]SpentTxOut) error {
	// Coinbase transactions don't have any inputs to spend.
	if IsCoinBase(tx) {
		// Add the transaction's outputs as available utxos.
		view.AddTxOuts(tx, blockHeight)
		return nil
	}

	// Spend the referenced utxos by marking them spent in the view and,
	// if a slice was provided for the spent txout details, append an entry
	// to it.
	for _, txIn := range tx.MsgTx().TxIn {
		// Ensure the referenced utxo exists in the view.  This should
		// never happen unless there is a bug is introduced in the code.
		entry := view.entries[txIn.PreviousOutPoint]
		if entry == nil {
			return AssertError(fmt.Sprintf("view missing input %v",
				txIn.PreviousOutPoint))
		}

		// Only create the stxo details if requested.
		if stxos != nil {
			// Populate the stxo details using the utxo entry.
			var stxo = SpentTxOut{
				Amount:     entry.Amount(),
				PkScript:   entry.PkScript(),
				Height:     entry.BlockHeight(),
				IsCoinBase: entry.IsCoinBase(),
			}
			*stxos = append(*stxos, stxo)
		}

		// Mark the entry as spent.  This is not done until after the
		// relevant details have been accessed since spending it might
		// clear the fields from memory in the future.
		entry.Spend()
	}

	// Add the transaction's outputs as available utxos.
	view.AddTxOuts(tx, blockHeight)
	return nil
}

// connectTransactions updates the view by adding all new utxos created by all
// of the transactions in the passed block, marking all utxos the transactions
// spend as spent, and setting the best hash for the view to the passed block.
// In addition, when the 'stxos' argument is not nil, it will be updated to
// append an entry for each spent txout.
func (view *UtxoViewpoint) connectTransactions(block *btcutil.Block, stxos *[]SpentTxOut) error {
	for _, tx := range block.Transactions() {
		err := view.connectTransaction(tx, block.Height(), stxos)
		if err != nil {
			return err
		}
	}

	// Update the best hash for view to include this block since all of its
	// transactions have been connected.
	view.SetBestHash(block.Hash())
	return nil
}

// fetchEntryByHash attempts to find any available utxo for the given hash by
// searching the entire set of possible outputs for the given hash.  It checks
// the view first and then falls back to the database if needed.
func (view *UtxoViewpoint) fetchEntryByHash(db database.DB, hash *chainhash.Hash) (*UtxoEntry, error) {
	// First attempt to find a utxo with the provided hash in the view.
	prevOut := wire.OutPoint{Hash: *hash}
	for idx := uint32(0); idx < MaxOutputsPerBlock; idx++ {
		prevOut.Index = idx
		entry := view.LookupEntry(prevOut)
		if entry != nil {
			return entry, nil
		}
	}

	// Check the database since it doesn't exist in the view.  This will
	// often by the case since only specifically referenced utxos are loaded
	// into the view.
	var entry *UtxoEntry
	err := db.View(func(dbTx database.Tx) error {
		var err error
		entry, err = dbFetchUtxoEntryByHash(dbTx, hash)
		return err
	})
	return entry, err
}

// disconnectTransactions updates the view by removing all of the transactions
// created by the passed block, restoring all utxos the transactions spent by
// using the provided spent txo information, and setting the best hash for the
// view to the block before the passed block.
func (view *UtxoViewpoint) disconnectTransactions(db database.DB, block *btcutil.Block, stxos []SpentTxOut) error {
	// Sanity check the correct number of stxos are provided.
	if len(stxos) != countSpentOutputs(block) {
		return AssertError("disconnectTransactions called with bad " +
			"spent transaction out information")
	}

	// Loop backwards through all transactions so everything is unspent in
	// reverse order.  This is necessary since transactions later in a block
	// can spend from previous ones.
	stxoIdx := len(stxos) - 1
	transactions := block.Transactions()
	for txIdx := len(transactions) - 1; txIdx > -1; txIdx-- {
		tx := transactions[txIdx]

		// All entries will need to potentially be marked as a coinbase.
		var packedFlags txoFlags
		isCoinBase := txIdx == 0
		if isCoinBase {
			packedFlags |= tfCoinBase
		}

		// Mark all of the spendable outputs originally created by the
		// transaction as spent.  It is instructive to note that while
		// the outputs aren't actually being spent here, rather they no
		// longer exist, since a pruned utxo set is used, there is no
		// practical difference between a utxo that does not exist and
		// one that has been spent.
		//
		// When the utxo does not already exist in the view, add an
		// entry for it and then mark it spent.  This is done because
		// the code relies on its existence in the view in order to
		// signal modifications have happened.
		txHash := tx.Hash()
		prevOut := wire.OutPoint{Hash: *txHash}
		for txOutIdx, txOut := range tx.MsgTx().TxOut {
			if txscript.IsUnspendable(txOut.PkScript) {
				continue
			}

			prevOut.Index = uint32(txOutIdx)
			entry := view.entries[prevOut]
			if entry == nil {
				entry = &UtxoEntry{
					amount:      txOut.Value,
					pkScript:    txOut.PkScript,
					blockHeight: block.Height(),
					packedFlags: packedFlags,
				}

				view.entries[prevOut] = entry
			}

			entry.Spend()
		}

		// Loop backwards through all of the transaction inputs (except
		// for the coinbase which has no inputs) and unspend the
		// referenced txos.  This is necessary to match the order of the
		// spent txout entries.
		if isCoinBase {
			continue
		}
		for txInIdx := len(tx.MsgTx().TxIn) - 1; txInIdx > -1; txInIdx-- {
			// Ensure the spent txout index is decremented to stay
			// in sync with the transaction input.
			stxo := &stxos[stxoIdx]
			stxoIdx--

			// When there is not already an entry for the referenced
			// output in the view, it means it was previously spent,
			// so create a new utxo entry in order to resurrect it.
			originOut := &tx.MsgTx().TxIn[txInIdx].PreviousOutPoint
			entry := view.entries[*originOut]
			if entry == nil {
				entry = new(UtxoEntry)
				view.entries[*originOut] = entry
			}

			// The legacy v1 spend journal format only stored the
			// coinbase flag and height when the output was the last
			// unspent output of the transaction.  As a result, when
			// the information is missing, search for it by scanning
			// all possible outputs of the transaction since it must
			// be in one of them.
			//
			// It should be noted that this is quite inefficient,
			// but it realistically will almost never run since all
			// new entries include the information for all outputs
			// and thus the only way this will be hit is if a long
			// enough reorg happens such that a block with the old
			// spend data is being disconnected.  The probability of
			// that in practice is extremely low to begin with and
			// becomes vanishingly small the more new blocks are
			// connected.  In the case of a fresh database that has
			// only ever run with the new v2 format, this code path
			// will never run.
			if stxo.Height == 0 {
				utxo, err := view.fetchEntryByHash(db, txHash)
				if err != nil {
					return err
				}
				if utxo == nil {
					return AssertError(fmt.Sprintf("unable "+
						"to resurrect legacy stxo %v",
						*originOut))
				}

				stxo.Height = utxo.BlockHeight()
				stxo.IsCoinBase = utxo.IsCoinBase()
			}

			// Restore the utxo using the stxo data from the spend
			// journal and mark it as modified.
			entry.amount = stxo.Amount
			entry.pkScript = stxo.PkScript
			entry.blockHeight = stxo.Height
			entry.packedFlags = tfModified
			if stxo.IsCoinBase {
				entry.packedFlags |= tfCoinBase
			}
		}
	}

	// Update the best hash for view to the previous block since all of the
	// transactions for the current block have been disconnected.
	view.SetBestHash(&block.MsgBlock().Header.PrevBlock)
	return nil
}

// RemoveEntry removes the given transaction output from the current state of
// the view.  It will have no effect if the passed output does not exist in the
// view.
func (view *UtxoViewpoint) RemoveEntry(outpoint wire.OutPoint) {
	delete(view.entries, outpoint)
}

// Entries returns the underlying map that stores of all the utxo entries.
func (view *UtxoViewpoint) Entries() map[wire.OutPoint]*UtxoEntry {
	return view.entries
}

// commit prunes all entries marked modified that are now fully spent and marks
// all entries as unmodified.
func (view *UtxoViewpoint) commit() {
	for outpoint, entry := range view.entries {
		if entry == nil || (entry.isModified() && entry.IsSpent()) {
			delete(view.entries, outpoint)
			continue
		}

		entry.packedFlags ^= tfModified
	}
}

// fetchUtxosMain fetches unspent transaction output data about the provided
// set of outpoints from the point of view of the end of the main chain at the
// time of the call.
//
// Upon completion of this function, the view will contain an entry for each
// requested outpoint.  Spent outputs, or those which otherwise don't exist,
// will result in a nil entry in the view.
func (view *UtxoViewpoint) fetchUtxosMain(db database.DB, outpoints []wire.OutPoint) error {
	// Nothing to do if there are no requested outputs.
	if len(outpoints) == 0 {
		return nil
	}

	// Load the requested set of unspent transaction outputs from the point
	// of view of the end of the main chain.
	//
	// NOTE: Missing entries are not considered an error here and instead
	// will result in nil entries in the view.  This is intentionally done
	// so other code can use the presence of an entry in the store as a way
	// to unnecessarily avoid attempting to reload it from the database.
	return db.View(func(dbTx database.Tx) error {
		utxoBucket := dbTx.Metadata().Bucket(utxoSetBucketName)
		for i := range outpoints {
			entry, err := dbFetchUtxoEntry(dbTx, utxoBucket, outpoints[i])
			if err != nil {
				return err
			}

			view.entries[outpoints[i]] = entry
		}

		return nil
	})
}

// fetchUtxosFromCache fetches unspent transaction output data about the provided
// set of outpoints from the point of view of the end of the main chain at the
// time of the call.  It attempts to fetch them from the cache and whatever entries
// that were not in the cache will be attempted to be fetched from the database and
// it'll be cached.
//
// Upon completion of this function, the view will contain an entry for each
// requested outpoint.  Spent outputs, or those which otherwise don't exist,
// will result in a nil entry in the view.
func (view *UtxoViewpoint) fetchUtxosFromCache(cache *utxoCache, outpoints []wire.OutPoint) error {
	// Nothing to do if there are no requested outputs.
	if len(outpoints) == 0 {
		return nil
	}

	// Load the requested set of unspent transaction outputs from the point
	// of view of the end of the main chain.  Any missing entries will be
	// fetched from the database and be cached.
	//
	// NOTE: Missing entries are not considered an error here and instead
	// will result in nil entries in the view.  This is intentionally done
	// so other code can use the presence of an entry in the store as a way
	// to unnecessarily avoid attempting to reload it from the database.
	entries, err := cache.fetchEntries(outpoints)
	if err != nil {
		return err
	}
	for i, entry := range entries {
		view.entries[outpoints[i]] = entry.Clone()
	}
	return nil
}

// fetchUtxos loads the unspent transaction outputs for the provided set of
// outputs into the view from the database as needed unless they already exist
// in the view in which case they are ignored.
func (view *UtxoViewpoint) fetchUtxos(cache *utxoCache, outpoints []wire.OutPoint) error {
	// Nothing to do if there are no requested outputs.
	if len(outpoints) == 0 {
		return nil
	}

	// Filter entries that are already in the view.
	needed := make([]wire.OutPoint, 0, len(outpoints))
	for i := range outpoints {
		// Already loaded into the current view.
		if _, ok := view.entries[outpoints[i]]; ok {
			continue
		}

		needed = append(needed, outpoints[i])
	}

	// Request the input utxos from the database.
	return view.fetchUtxosFromCache(cache, needed)
}

// findInputsToFetch goes through all the blocks and returns all the outpoints of
// the entries that need to be fetched in order to validate the block.  Outpoints
// for the entries that are already in the block are not included in the returned
// outpoints.
func (view *UtxoViewpoint) findInputsToFetch(block *btcutil.Block) []wire.OutPoint {
	// Build a map of in-flight transactions because some of the inputs in
	// this block could be referencing other transactions earlier in this
	// block which are not yet in the chain.
	txInFlight := map[chainhash.Hash]int{}
	transactions := block.Transactions()
	for i, tx := range transactions {
		txInFlight[*tx.Hash()] = i
	}

	// Loop through all of the transaction inputs (except for the coinbase
	// which has no inputs) collecting them into sets of what is needed and
	// what is already known (in-flight).
	needed := make([]wire.OutPoint, 0, len(transactions))
	for i, tx := range transactions[1:] {
		for _, txIn := range tx.MsgTx().TxIn {
			// It is acceptable for a transaction input to reference
			// the output of another transaction in this block only
			// if the referenced transaction comes before the
			// current one in this block.  Add the outputs of the
			// referenced transaction as available utxos when this
			// is the case.  Otherwise, the utxo details are still
			// needed.
			//
			// NOTE: The >= is correct here because i is one less
			// than the actual position of the transaction within
			// the block due to skipping the coinbase.
			originHash := &txIn.PreviousOutPoint.Hash
			if inFlightIndex, ok := txInFlight[*originHash]; ok &&
				i >= inFlightIndex {

				originTx := transactions[inFlightIndex]
				view.AddTxOuts(originTx, block.Height())
				continue
			}

			// Don't request entries that are already in the view
			// from the database.
			if _, ok := view.entries[txIn.PreviousOutPoint]; ok {
				continue
			}

			needed = append(needed, txIn.PreviousOutPoint)
		}
	}

	return needed
}

// fetchInputUtxos loads the unspent transaction outputs for the inputs
// referenced by the transactions in the given block into the view from the
// database or the cache as needed.  In particular, referenced entries that
// are earlier in the block are added to the view and entries that are already
// in the view are not modified.
func (view *UtxoViewpoint) fetchInputUtxos(db database.DB, cache *utxoCache, block *btcutil.Block) error {
	if cache != nil {
		return view.fetchUtxosFromCache(cache, view.findInputsToFetch(block))
	}
	// Request the input utxos from the cache.
	return view.fetchUtxosMain(db, view.findInputsToFetch(block))
}

// NewUtxoViewpoint returns a new empty unspent transaction output view.
func NewUtxoViewpoint() *UtxoViewpoint {
	return &UtxoViewpoint{
		entries: make(map[wire.OutPoint]*UtxoEntry),
	}
}

// FetchUtxoView loads unspent transaction outputs for the inputs referenced by
// the passed transaction from the point of view of the end of the main chain.
// It also attempts to fetch the utxos for the outputs of the transaction itself
// so the returned view can be examined for duplicate transactions.
//
// This function is safe for concurrent access however the returned view is NOT.
func (b *BlockChain) FetchUtxoView(tx *btcutil.Tx) (*UtxoViewpoint, error) {
	// Create a set of needed outputs based on those referenced by the
	// inputs of the passed transaction and the outputs of the transaction
	// itself.
	neededLen := len(tx.MsgTx().TxOut)
	if !IsCoinBase(tx) {
		neededLen += len(tx.MsgTx().TxIn)
	}
	needed := make([]wire.OutPoint, 0, neededLen)
	prevOut := wire.OutPoint{Hash: *tx.Hash()}
	for txOutIdx := range tx.MsgTx().TxOut {
		prevOut.Index = uint32(txOutIdx)
		needed = append(needed, prevOut)
	}
	if !IsCoinBase(tx) {
		for _, txIn := range tx.MsgTx().TxIn {
			needed = append(needed, txIn.PreviousOutPoint)
		}
	}

	// Request the utxos from the point of view of the end of the main
	// chain.
	view := NewUtxoViewpoint()
	b.chainLock.RLock()
	err := view.fetchUtxosFromCache(b.utxoCache, needed)
	b.chainLock.RUnlock()
	return view, err
}

// FetchUtxoEntry loads and returns the requested unspent transaction output
// from the point of view of the end of the main chain.
//
// NOTE: Requesting an output for which there is no data will NOT return an
// error.  Instead both the entry and the error will be nil.  This is done to
// allow pruning of spent transaction outputs.  In practice this means the
// caller must check if the returned entry is nil before invoking methods on it.
//
// This function is safe for concurrent access however the returned entry (if
// any) is NOT.
func (b *BlockChain) FetchUtxoEntry(outpoint wire.OutPoint) (*UtxoEntry, error) {
	b.chainLock.RLock()
	defer b.chainLock.RUnlock()

	entries, err := b.utxoCache.fetchEntries([]wire.OutPoint{outpoint})
	if err != nil {
		return nil, err
	}

	return entries[0], nil
}