tendermint/mempool/mempool.go

package mempool

import (
	"bytes"
	"container/list"
	"crypto/sha256"
	"fmt"
	"sync"
	"sync/atomic"
	"time"

	"github.com/pkg/errors"

	abci "github.com/tendermint/tendermint/abci/types"
	cfg "github.com/tendermint/tendermint/config"
	auto "github.com/tendermint/tendermint/libs/autofile"
	"github.com/tendermint/tendermint/libs/clist"
	cmn "github.com/tendermint/tendermint/libs/common"
	"github.com/tendermint/tendermint/libs/log"
	"github.com/tendermint/tendermint/proxy"
	"github.com/tendermint/tendermint/types"
)

// PreCheckFunc is an optional filter executed before CheckTx and rejects
// transaction if false is returned. An example would be to ensure that a
// transaction doesn't exceeded the block size.
type PreCheckFunc func(types.Tx) error

// PostCheckFunc is an optional filter executed after CheckTx and rejects
// transaction if false is returned. An example would be to ensure a
// transaction doesn't require more gas than available for the block.
type PostCheckFunc func(types.Tx, *abci.ResponseCheckTx) error

// TxInfo are parameters that get passed when attempting to add a tx to the
// mempool.
type TxInfo struct {
	// We don't use p2p.ID here because it's too big. The gain is to store max 2
	// bytes with each tx to identify the sender rather than 20 bytes.
	PeerID uint16
}

/*

The mempool pushes new txs onto the proxyAppConn.
It gets a stream of (req, res) tuples from the proxy.
The mempool stores good txs in a concurrent linked-list.

Multiple concurrent go-routines can traverse this linked-list
safely by calling .NextWait() on each element.

So we have several go-routines:
1. Consensus calling Update() and Reap() synchronously
2. Many mempool reactor's peer routines calling CheckTx()
3. Many mempool reactor's peer routines traversing the txs linked list
4. Another goroutine calling GarbageCollectTxs() periodically

To manage these goroutines, there are three methods of locking.
1. Mutations to the linked-list is protected by an internal mtx (CList is goroutine-safe)
2. Mutations to the linked-list elements are atomic
3. CheckTx() calls can be paused upon Update() and Reap(), protected by .proxyMtx

Garbage collection of old elements from mempool.txs is handlde via
the DetachPrev() call, which makes old elements not reachable by
peer broadcastTxRoutine() automatically garbage collected.

TODO: Better handle abci client errors. (make it automatically handle connection errors)

*/

var (
	// ErrTxInCache is returned to the client if we saw tx earlier
	ErrTxInCache = errors.New("Tx already exists in cache")

	// ErrTxTooLarge means the tx is too big to be sent in a message to other peers
	ErrTxTooLarge = fmt.Errorf("Tx too large. Max size is %d", maxTxSize)
)

// ErrMempoolIsFull means Tendermint & an application can't handle that much load
type ErrMempoolIsFull struct {
	numTxs int
	maxTxs int

	txsBytes    int64
	maxTxsBytes int64
}

func (e ErrMempoolIsFull) Error() string {
	return fmt.Sprintf(
		"Mempool is full: number of txs %d (max: %d), total txs bytes %d (max: %d)",
		e.numTxs, e.maxTxs,
		e.txsBytes, e.maxTxsBytes)
}

// ErrPreCheck is returned when tx is too big
type ErrPreCheck struct {
	Reason error
}

func (e ErrPreCheck) Error() string {
	return e.Reason.Error()
}

// IsPreCheckError returns true if err is due to pre check failure.
func IsPreCheckError(err error) bool {
	_, ok := err.(ErrPreCheck)
	return ok
}

// PreCheckAminoMaxBytes checks that the size of the transaction plus the amino
// overhead is smaller or equal to the expected maxBytes.
func PreCheckAminoMaxBytes(maxBytes int64) PreCheckFunc {
	return func(tx types.Tx) error {
		// We have to account for the amino overhead in the tx size as well
		// NOTE: fieldNum = 1 as types.Block.Data contains Txs []Tx as first field.
		// If this field order ever changes this needs to updated here accordingly.
		// NOTE: if some []Tx are encoded without a parenting struct, the
		// fieldNum is also equal to 1.
		aminoOverhead := types.ComputeAminoOverhead(tx, 1)
		txSize := int64(len(tx)) + aminoOverhead
		if txSize > maxBytes {
			return fmt.Errorf("Tx size (including amino overhead) is too big: %d, max: %d",
				txSize, maxBytes)
		}
		return nil
	}
}

// PostCheckMaxGas checks that the wanted gas is smaller or equal to the passed
// maxGas. Returns nil if maxGas is -1.
func PostCheckMaxGas(maxGas int64) PostCheckFunc {
	return func(tx types.Tx, res *abci.ResponseCheckTx) error {
		if maxGas == -1 {
			return nil
		}
		if res.GasWanted < 0 {
			return fmt.Errorf("gas wanted %d is negative",
				res.GasWanted)
		}
		if res.GasWanted > maxGas {
			return fmt.Errorf("gas wanted %d is greater than max gas %d",
				res.GasWanted, maxGas)
		}
		return nil
	}
}

// TxID is the hex encoded hash of the bytes as a types.Tx.
func TxID(tx []byte) string {
	return fmt.Sprintf("%X", types.Tx(tx).Hash())
}

// Mempool is an ordered in-memory pool for transactions before they are proposed in a consensus
// round. Transaction validity is checked using the CheckTx abci message before the transaction is
// added to the pool. The Mempool uses a concurrent list structure for storing transactions that
// can be efficiently accessed by multiple concurrent readers.
type Mempool struct {
	config *cfg.MempoolConfig

	proxyMtx     sync.Mutex
	proxyAppConn proxy.AppConnMempool
	txs          *clist.CList // concurrent linked-list of good txs
	// map for quick access to txs
	// Used in CheckTx to record the tx sender.
	txsMap               map[[sha256.Size]byte]*clist.CElement
	height               int64           // the last block Update()'d to
	rechecking           int32           // for re-checking filtered txs on Update()
	recheckCursor        *clist.CElement // next expected response
	recheckEnd           *clist.CElement // re-checking stops here
	notifiedTxsAvailable bool
	txsAvailable         chan struct{} // fires once for each height, when the mempool is not empty
	preCheck             PreCheckFunc
	postCheck            PostCheckFunc

	// Atomic integers

	// Used to check if the mempool size is bigger than the allowed limit.
	// See TxsBytes
	txsBytes int64

	// Keep a cache of already-seen txs.
	// This reduces the pressure on the proxyApp.
	cache txCache

	// A log of mempool txs
	wal *auto.AutoFile

	logger log.Logger

	metrics *Metrics
}

// MempoolOption sets an optional parameter on the Mempool.
type MempoolOption func(*Mempool)

// NewMempool returns a new Mempool with the given configuration and connection to an application.
func NewMempool(
	config *cfg.MempoolConfig,
	proxyAppConn proxy.AppConnMempool,
	height int64,
	options ...MempoolOption,
) *Mempool {
	mempool := &Mempool{
		config:        config,
		proxyAppConn:  proxyAppConn,
		txs:           clist.New(),
		txsMap:        make(map[[sha256.Size]byte]*clist.CElement),
		height:        height,
		rechecking:    0,
		recheckCursor: nil,
		recheckEnd:    nil,
		logger:        log.NewNopLogger(),
		metrics:       NopMetrics(),
	}
	if config.CacheSize > 0 {
		mempool.cache = newMapTxCache(config.CacheSize)
	} else {
		mempool.cache = nopTxCache{}
	}
	proxyAppConn.SetResponseCallback(mempool.resCb)
	for _, option := range options {
		option(mempool)
	}
	return mempool
}

// EnableTxsAvailable initializes the TxsAvailable channel,
// ensuring it will trigger once every height when transactions are available.
// NOTE: not thread safe - should only be called once, on startup
func (mem *Mempool) EnableTxsAvailable() {
	mem.txsAvailable = make(chan struct{}, 1)
}

// SetLogger sets the Logger.
func (mem *Mempool) SetLogger(l log.Logger) {
	mem.logger = l
}

// WithPreCheck sets a filter for the mempool to reject a tx if f(tx) returns
// false. This is ran before CheckTx.
func WithPreCheck(f PreCheckFunc) MempoolOption {
	return func(mem *Mempool) { mem.preCheck = f }
}

// WithPostCheck sets a filter for the mempool to reject a tx if f(tx) returns
// false. This is ran after CheckTx.
func WithPostCheck(f PostCheckFunc) MempoolOption {
	return func(mem *Mempool) { mem.postCheck = f }
}

// WithMetrics sets the metrics.
func WithMetrics(metrics *Metrics) MempoolOption {
	return func(mem *Mempool) { mem.metrics = metrics }
}

// InitWAL creates a directory for the WAL file and opens a file itself.
//
// *panics* if can't create directory or open file.
// *not thread safe*
func (mem *Mempool) InitWAL() {
	walDir := mem.config.WalDir()
	err := cmn.EnsureDir(walDir, 0700)
	if err != nil {
		panic(errors.Wrap(err, "Error ensuring Mempool WAL dir"))
	}
	af, err := auto.OpenAutoFile(walDir + "/wal")
	if err != nil {
		panic(errors.Wrap(err, "Error opening Mempool WAL file"))
	}
	mem.wal = af
}

// CloseWAL closes and discards the underlying WAL file.
// Any further writes will not be relayed to disk.
func (mem *Mempool) CloseWAL() {
	mem.proxyMtx.Lock()
	defer mem.proxyMtx.Unlock()

	if err := mem.wal.Close(); err != nil {
		mem.logger.Error("Error closing WAL", "err", err)
	}
	mem.wal = nil
}

// Lock locks the mempool. The consensus must be able to hold lock to safely update.
func (mem *Mempool) Lock() {
	mem.proxyMtx.Lock()
}

// Unlock unlocks the mempool.
func (mem *Mempool) Unlock() {
	mem.proxyMtx.Unlock()
}

// Size returns the number of transactions in the mempool.
func (mem *Mempool) Size() int {
	return mem.txs.Len()
}

// TxsBytes returns the total size of all txs in the mempool.
func (mem *Mempool) TxsBytes() int64 {
	return atomic.LoadInt64(&mem.txsBytes)
}

// FlushAppConn flushes the mempool connection to ensure async reqResCb calls are
// done. E.g. from CheckTx.
func (mem *Mempool) FlushAppConn() error {
	return mem.proxyAppConn.FlushSync()
}

// Flush removes all transactions from the mempool and cache
func (mem *Mempool) Flush() {
	mem.proxyMtx.Lock()
	defer mem.proxyMtx.Unlock()

	mem.cache.Reset()

	for e := mem.txs.Front(); e != nil; e = e.Next() {
		mem.txs.Remove(e)
		e.DetachPrev()
	}

	mem.txsMap = make(map[[sha256.Size]byte]*clist.CElement)
	_ = atomic.SwapInt64(&mem.txsBytes, 0)
}

// TxsFront returns the first transaction in the ordered list for peer
// goroutines to call .NextWait() on.
func (mem *Mempool) TxsFront() *clist.CElement {
	return mem.txs.Front()
}

// TxsWaitChan returns a channel to wait on transactions. It will be closed
// once the mempool is not empty (ie. the internal `mem.txs` has at least one
// element)
func (mem *Mempool) TxsWaitChan() <-chan struct{} {
	return mem.txs.WaitChan()
}

// CheckTx executes a new transaction against the application to determine its validity
// and whether it should be added to the mempool.
// It blocks if we're waiting on Update() or Reap().
// cb: A callback from the CheckTx command.
//     It gets called from another goroutine.
// CONTRACT: Either cb will get called, or err returned.
func (mem *Mempool) CheckTx(tx types.Tx, cb func(*abci.Response)) (err error) {
	return mem.CheckTxWithInfo(tx, cb, TxInfo{PeerID: UnknownPeerID})
}

// CheckTxWithInfo performs the same operation as CheckTx, but with extra meta data about the tx.
// Currently this metadata is the peer who sent it,
// used to prevent the tx from being gossiped back to them.
func (mem *Mempool) CheckTxWithInfo(tx types.Tx, cb func(*abci.Response), txInfo TxInfo) (err error) {
	mem.proxyMtx.Lock()
	// use defer to unlock mutex because application (*local client*) might panic
	defer mem.proxyMtx.Unlock()

	var (
		memSize  = mem.Size()
		txsBytes = mem.TxsBytes()
	)
	if memSize >= mem.config.Size ||
		int64(len(tx))+txsBytes > mem.config.MaxTxsBytes {
		return ErrMempoolIsFull{
			memSize, mem.config.Size,
			txsBytes, mem.config.MaxTxsBytes}
	}

	// The size of the corresponding amino-encoded TxMessage
	// can't be larger than the maxMsgSize, otherwise we can't
	// relay it to peers.
	if len(tx) > maxTxSize {
		return ErrTxTooLarge
	}

	if mem.preCheck != nil {
		if err := mem.preCheck(tx); err != nil {
			return ErrPreCheck{err}
		}
	}

	// CACHE
	if !mem.cache.Push(tx) {
		// record the sender
		e, ok := mem.txsMap[sha256.Sum256(tx)]
		// The check is needed because tx may be in cache, but not in the mempool.
		// E.g. after we've committed a block, txs are removed from the mempool,
		// but not from the cache.
		if ok {
			memTx := e.Value.(*mempoolTx)
			if _, loaded := memTx.senders.LoadOrStore(txInfo.PeerID, true); loaded {
				// TODO: consider punishing peer for dups,
				// its non-trivial since invalid txs can become valid,
				// but they can spam the same tx with little cost to them atm.
			}
		}

		return ErrTxInCache
	}
	// END CACHE

	// WAL
	if mem.wal != nil {
		// TODO: Notify administrators when WAL fails
		_, err := mem.wal.Write([]byte(tx))
		if err != nil {
			mem.logger.Error("Error writing to WAL", "err", err)
		}
		_, err = mem.wal.Write([]byte("\n"))
		if err != nil {
			mem.logger.Error("Error writing to WAL", "err", err)
		}
	}
	// END WAL

	// NOTE: proxyAppConn may error if tx buffer is full
	if err = mem.proxyAppConn.Error(); err != nil {
		return err
	}
	reqRes := mem.proxyAppConn.CheckTxAsync(tx)
	if cb != nil {
		composedCallback := func(res *abci.Response) {
			mem.reqResCb(tx, txInfo.PeerID)(res)
			cb(res)
		}
		reqRes.SetCallback(composedCallback)
	} else {
		reqRes.SetCallback(mem.reqResCb(tx, txInfo.PeerID))
	}

	return nil
}

// Global callback, which is called in the absence of the specific callback.
//
// In recheckTxs because no reqResCb (specific) callback is set, this callback
// will be called.
func (mem *Mempool) resCb(req *abci.Request, res *abci.Response) {
	if mem.recheckCursor == nil {
		return
	}

	mem.metrics.RecheckTimes.Add(1)
	mem.resCbRecheck(req, res)

	// update metrics
	mem.metrics.Size.Set(float64(mem.Size()))
}

// Specific callback, which allows us to incorporate local information, like
// the peer that sent us this tx, so we can avoid sending it back to the same
// peer.
//
// Used in CheckTxWithInfo to record PeerID who sent us the tx.
func (mem *Mempool) reqResCb(tx []byte, peerID uint16) func(res *abci.Response) {
	return func(res *abci.Response) {
		if mem.recheckCursor != nil {
			return
		}

		mem.resCbFirstTime(tx, peerID, res)

		// update metrics
		mem.metrics.Size.Set(float64(mem.Size()))
	}
}

func (mem *Mempool) addTx(memTx *mempoolTx) {
	e := mem.txs.PushBack(memTx)
	mem.txsMap[sha256.Sum256(memTx.tx)] = e
	atomic.AddInt64(&mem.txsBytes, int64(len(memTx.tx)))
	mem.metrics.TxSizeBytes.Observe(float64(len(memTx.tx)))
}

func (mem *Mempool) removeTx(tx types.Tx, elem *clist.CElement, removeFromCache bool) {
	mem.txs.Remove(elem)
	elem.DetachPrev()
	delete(mem.txsMap, sha256.Sum256(tx))
	atomic.AddInt64(&mem.txsBytes, int64(-len(tx)))

	if removeFromCache {
		mem.cache.Remove(tx)
	}
}

// callback, which is called after the app checked the tx for the first time.
//
// The case where the app checks the tx for the second and subsequent times is
// handled by the resCbRecheck callback.
func (mem *Mempool) resCbFirstTime(tx []byte, peerID uint16, res *abci.Response) {
	switch r := res.Value.(type) {
	case *abci.Response_CheckTx:
		var postCheckErr error
		if mem.postCheck != nil {
			postCheckErr = mem.postCheck(tx, r.CheckTx)
		}
		if (r.CheckTx.Code == abci.CodeTypeOK) && postCheckErr == nil {
			memTx := &mempoolTx{
				height:    mem.height,
				gasWanted: r.CheckTx.GasWanted,
				tx:        tx,
			}
			memTx.senders.Store(peerID, true)
			mem.addTx(memTx)
			mem.logger.Info("Added good transaction",
				"tx", TxID(tx),
				"res", r,
				"height", memTx.height,
				"total", mem.Size(),
			)
			mem.notifyTxsAvailable()
		} else {
			// ignore bad transaction
			mem.logger.Info("Rejected bad transaction", "tx", TxID(tx), "res", r, "err", postCheckErr)
			mem.metrics.FailedTxs.Add(1)
			// remove from cache (it might be good later)
			mem.cache.Remove(tx)
		}
	default:
		// ignore other messages
	}
}

// callback, which is called after the app rechecked the tx.
//
// The case where the app checks the tx for the first time is handled by the
// resCbFirstTime callback.
func (mem *Mempool) resCbRecheck(req *abci.Request, res *abci.Response) {
	switch r := res.Value.(type) {
	case *abci.Response_CheckTx:
		tx := req.GetCheckTx().Tx
		memTx := mem.recheckCursor.Value.(*mempoolTx)
		if !bytes.Equal(tx, memTx.tx) {
			panic(fmt.Sprintf(
				"Unexpected tx response from proxy during recheck\nExpected %X, got %X",
				memTx.tx,
				tx))
		}
		var postCheckErr error
		if mem.postCheck != nil {
			postCheckErr = mem.postCheck(tx, r.CheckTx)
		}
		if (r.CheckTx.Code == abci.CodeTypeOK) && postCheckErr == nil {
			// Good, nothing to do.
		} else {
			// Tx became invalidated due to newly committed block.
			mem.logger.Info("Tx is no longer valid", "tx", TxID(tx), "res", r, "err", postCheckErr)
			// NOTE: we remove tx from the cache because it might be good later
			mem.removeTx(tx, mem.recheckCursor, true)
		}
		if mem.recheckCursor == mem.recheckEnd {
			mem.recheckCursor = nil
		} else {
			mem.recheckCursor = mem.recheckCursor.Next()
		}
		if mem.recheckCursor == nil {
			// Done!
			atomic.StoreInt32(&mem.rechecking, 0)
			mem.logger.Info("Done rechecking txs")

			// incase the recheck removed all txs
			if mem.Size() > 0 {
				mem.notifyTxsAvailable()
			}
		}
	default:
		// ignore other messages
	}
}

// TxsAvailable returns a channel which fires once for every height,
// and only when transactions are available in the mempool.
// NOTE: the returned channel may be nil if EnableTxsAvailable was not called.
func (mem *Mempool) TxsAvailable() <-chan struct{} {
	return mem.txsAvailable
}

func (mem *Mempool) notifyTxsAvailable() {
	if mem.Size() == 0 {
		panic("notified txs available but mempool is empty!")
	}
	if mem.txsAvailable != nil && !mem.notifiedTxsAvailable {
		// channel cap is 1, so this will send once
		mem.notifiedTxsAvailable = true
		select {
		case mem.txsAvailable <- struct{}{}:
		default:
		}
	}
}

// ReapMaxBytesMaxGas reaps transactions from the mempool up to maxBytes bytes total
// with the condition that the total gasWanted must be less than maxGas.
// If both maxes are negative, there is no cap on the size of all returned
// transactions (~ all available transactions).
func (mem *Mempool) ReapMaxBytesMaxGas(maxBytes, maxGas int64) types.Txs {
	mem.proxyMtx.Lock()
	defer mem.proxyMtx.Unlock()

	for atomic.LoadInt32(&mem.rechecking) > 0 {
		// TODO: Something better?
		time.Sleep(time.Millisecond * 10)
	}

	var totalBytes int64
	var totalGas int64
	// TODO: we will get a performance boost if we have a good estimate of avg
	// size per tx, and set the initial capacity based off of that.
	// txs := make([]types.Tx, 0, cmn.MinInt(mem.txs.Len(), max/mem.avgTxSize))
	txs := make([]types.Tx, 0, mem.txs.Len())
	for e := mem.txs.Front(); e != nil; e = e.Next() {
		memTx := e.Value.(*mempoolTx)
		// Check total size requirement
		aminoOverhead := types.ComputeAminoOverhead(memTx.tx, 1)
		if maxBytes > -1 && totalBytes+int64(len(memTx.tx))+aminoOverhead > maxBytes {
			return txs
		}
		totalBytes += int64(len(memTx.tx)) + aminoOverhead
		// Check total gas requirement.
		// If maxGas is negative, skip this check.
		// Since newTotalGas < masGas, which
		// must be non-negative, it follows that this won't overflow.
		newTotalGas := totalGas + memTx.gasWanted
		if maxGas > -1 && newTotalGas > maxGas {
			return txs
		}
		totalGas = newTotalGas
		txs = append(txs, memTx.tx)
	}
	return txs
}

// ReapMaxTxs reaps up to max transactions from the mempool.
// If max is negative, there is no cap on the size of all returned
// transactions (~ all available transactions).
func (mem *Mempool) ReapMaxTxs(max int) types.Txs {
	mem.proxyMtx.Lock()
	defer mem.proxyMtx.Unlock()

	if max < 0 {
		max = mem.txs.Len()
	}

	for atomic.LoadInt32(&mem.rechecking) > 0 {
		// TODO: Something better?
		time.Sleep(time.Millisecond * 10)
	}

	txs := make([]types.Tx, 0, cmn.MinInt(mem.txs.Len(), max))
	for e := mem.txs.Front(); e != nil && len(txs) <= max; e = e.Next() {
		memTx := e.Value.(*mempoolTx)
		txs = append(txs, memTx.tx)
	}
	return txs
}

// Update informs the mempool that the given txs were committed and can be discarded.
// NOTE: this should be called *after* block is committed by consensus.
// NOTE: unsafe; Lock/Unlock must be managed by caller
func (mem *Mempool) Update(
	height int64,
	txs types.Txs,
	preCheck PreCheckFunc,
	postCheck PostCheckFunc,
) error {
	// Set height
	mem.height = height
	mem.notifiedTxsAvailable = false

	if preCheck != nil {
		mem.preCheck = preCheck
	}
	if postCheck != nil {
		mem.postCheck = postCheck
	}

	// Add committed transactions to cache (if missing).
	for _, tx := range txs {
		_ = mem.cache.Push(tx)
	}

	// Remove committed transactions.
	txsLeft := mem.removeTxs(txs)

	// Either recheck non-committed txs to see if they became invalid
	// or just notify there're some txs left.
	if len(txsLeft) > 0 {
		if mem.config.Recheck {
			mem.logger.Info("Recheck txs", "numtxs", len(txsLeft), "height", height)
			mem.recheckTxs(txsLeft)
			// At this point, mem.txs are being rechecked.
			// mem.recheckCursor re-scans mem.txs and possibly removes some txs.
			// Before mem.Reap(), we should wait for mem.recheckCursor to be nil.
		} else {
			mem.notifyTxsAvailable()
		}
	}

	// Update metrics
	mem.metrics.Size.Set(float64(mem.Size()))

	return nil
}

func (mem *Mempool) removeTxs(txs types.Txs) []types.Tx {
	// Build a map for faster lookups.
	txsMap := make(map[string]struct{}, len(txs))
	for _, tx := range txs {
		txsMap[string(tx)] = struct{}{}
	}

	txsLeft := make([]types.Tx, 0, mem.txs.Len())
	for e := mem.txs.Front(); e != nil; e = e.Next() {
		memTx := e.Value.(*mempoolTx)
		// Remove the tx if it's already in a block.
		if _, ok := txsMap[string(memTx.tx)]; ok {
			// NOTE: we don't remove committed txs from the cache.
			mem.removeTx(memTx.tx, e, false)

			continue
		}
		txsLeft = append(txsLeft, memTx.tx)
	}
	return txsLeft
}

// NOTE: pass in txs because mem.txs can mutate concurrently.
func (mem *Mempool) recheckTxs(txs []types.Tx) {
	if len(txs) == 0 {
		return
	}
	atomic.StoreInt32(&mem.rechecking, 1)
	mem.recheckCursor = mem.txs.Front()
	mem.recheckEnd = mem.txs.Back()

	// Push txs to proxyAppConn
	// NOTE: reqResCb may be called concurrently.
	for _, tx := range txs {
		mem.proxyAppConn.CheckTxAsync(tx)
	}
	mem.proxyAppConn.FlushAsync()
}

//--------------------------------------------------------------------------------

// mempoolTx is a transaction that successfully ran
type mempoolTx struct {
	height    int64    // height that this tx had been validated in
	gasWanted int64    // amount of gas this tx states it will require
	senders   sync.Map // ids of peers who've sent us this tx (as a map for quick lookups)
	tx        types.Tx //
}

// Height returns the height for this transaction
func (memTx *mempoolTx) Height() int64 {
	return atomic.LoadInt64(&memTx.height)
}

//--------------------------------------------------------------------------------

type txCache interface {
	Reset()
	Push(tx types.Tx) bool
	Remove(tx types.Tx)
}

// mapTxCache maintains a LRU cache of transactions. This only stores the hash
// of the tx, due to memory concerns.
type mapTxCache struct {
	mtx  sync.Mutex
	size int
	map_ map[[sha256.Size]byte]*list.Element
	list *list.List
}

var _ txCache = (*mapTxCache)(nil)

// newMapTxCache returns a new mapTxCache.
func newMapTxCache(cacheSize int) *mapTxCache {
	return &mapTxCache{
		size: cacheSize,
		map_: make(map[[sha256.Size]byte]*list.Element, cacheSize),
		list: list.New(),
	}
}

// Reset resets the cache to an empty state.
func (cache *mapTxCache) Reset() {
	cache.mtx.Lock()
	cache.map_ = make(map[[sha256.Size]byte]*list.Element, cache.size)
	cache.list.Init()
	cache.mtx.Unlock()
}

// Push adds the given tx to the cache and returns true. It returns
// false if tx is already in the cache.
func (cache *mapTxCache) Push(tx types.Tx) bool {
	cache.mtx.Lock()
	defer cache.mtx.Unlock()

	// Use the tx hash in the cache
	txHash := sha256.Sum256(tx)
	if moved, exists := cache.map_[txHash]; exists {
		cache.list.MoveToBack(moved)
		return false
	}

	if cache.list.Len() >= cache.size {
		popped := cache.list.Front()
		poppedTxHash := popped.Value.([sha256.Size]byte)
		delete(cache.map_, poppedTxHash)
		if popped != nil {
			cache.list.Remove(popped)
		}
	}
	e := cache.list.PushBack(txHash)
	cache.map_[txHash] = e
	return true
}

// Remove removes the given tx from the cache.
func (cache *mapTxCache) Remove(tx types.Tx) {
	cache.mtx.Lock()
	txHash := sha256.Sum256(tx)
	popped := cache.map_[txHash]
	delete(cache.map_, txHash)
	if popped != nil {
		cache.list.Remove(popped)
	}

	cache.mtx.Unlock()
}

type nopTxCache struct{}

var _ txCache = (*nopTxCache)(nil)

func (nopTxCache) Reset()             {}
func (nopTxCache) Push(types.Tx) bool { return true }
func (nopTxCache) Remove(types.Tx)    {}