path: root/core/tx_pool.go

// Copyright 2014 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

package core

import (
    "errors"
    "fmt"
    "math/big"
    "sort"
    "sync"
    "time"

    "github.com/ethereum/go-ethereum/common"
    "github.com/ethereum/go-ethereum/core/state"
    "github.com/ethereum/go-ethereum/core/types"
    "github.com/ethereum/go-ethereum/event"
    "github.com/ethereum/go-ethereum/log"
    "github.com/ethereum/go-ethereum/metrics"
    "github.com/ethereum/go-ethereum/params"
    "gopkg.in/karalabe/cookiejar.v2/collections/prque"
)

var (
    // ErrInvalidSender is returned if the transaction contains an invalid signature.
    ErrInvalidSender = errors.New("invalid sender")

    // ErrNonceTooLow is returned if the nonce of a transaction is lower than the
    // one present in the local chain.
    ErrNonceTooLow = errors.New("nonce too low")

    // ErrUnderpriced is returned if a transaction's gas price is below the minimum
    // configured for the transaction pool.
    ErrUnderpriced = errors.New("transaction underpriced")

    // ErrReplaceUnderpriced is returned if a transaction is attempted to be replaced
    // with a different one without the required price bump.
    ErrReplaceUnderpriced = errors.New("replacement transaction underpriced")

    // ErrInsufficientFunds is returned if the total cost of executing a transaction
    // is higher than the balance of the user's account.
    ErrInsufficientFunds = errors.New("insufficient funds for gas * price + value")

    // ErrIntrinsicGas is returned if the transaction is specified to use less gas
    // than required to start the invocation.
    ErrIntrinsicGas = errors.New("intrinsic gas too low")

    // ErrGasLimit is returned if a transaction's requested gas limit exceeds the
    // maximum allowance of the current block.
    ErrGasLimit = errors.New("exceeds block gas limit")

    // ErrNegativeValue is a sanity error to ensure noone is able to specify a
    // transaction with a negative value.
    ErrNegativeValue = errors.New("negative value")

    // ErrOversizedData is returned if the input data of a transaction is greater
    // than some meaningful limit a user might use. This is not a consensus error
    // making the transaction invalid, rather a DOS protection.
    ErrOversizedData = errors.New("oversized data")
)

var (
    evictionInterval    = time.Minute     // Time interval to check for evictable transactions
    statsReportInterval = 8 * time.Second // Time interval to report transaction pool stats
)

var (
    // Metrics for the pending pool
    pendingDiscardCounter   = metrics.NewCounter("txpool/pending/discard")
    pendingReplaceCounter   = metrics.NewCounter("txpool/pending/replace")
    pendingRateLimitCounter = metrics.NewCounter("txpool/pending/ratelimit") // Dropped due to rate limiting
    pendingNofundsCounter   = metrics.NewCounter("txpool/pending/nofunds")   // Dropped due to out-of-funds

    // Metrics for the queued pool
    queuedDiscardCounter   = metrics.NewCounter("txpool/queued/discard")
    queuedReplaceCounter   = metrics.NewCounter("txpool/queued/replace")
    queuedRateLimitCounter = metrics.NewCounter("txpool/queued/ratelimit") // Dropped due to rate limiting
    queuedNofundsCounter   = metrics.NewCounter("txpool/queued/nofunds")   // Dropped due to out-of-funds

    // General tx metrics
    invalidTxCounter     = metrics.NewCounter("txpool/invalid")
    underpricedTxCounter = metrics.NewCounter("txpool/underpriced")
)

type stateFn func() (*state.StateDB, error)

// TxPoolConfig are the configuration parameters of the transaction pool.
type TxPoolConfig struct {
    NoLocals bool // Whether local transaction handling should be disabled

    PriceLimit uint64 // Minimum gas price to enforce for acceptance into the pool
    PriceBump  uint64 // Minimum price bump percentage to replace an already existing transaction (nonce)

    AccountSlots uint64 // Minimum number of executable transaction slots guaranteed per account
    GlobalSlots  uint64 // Maximum number of executable transaction slots for all accounts
    AccountQueue uint64 // Maximum number of non-executable transaction slots permitted per account
    GlobalQueue  uint64 // Maximum number of non-executable transaction slots for all accounts

    Lifetime time.Duration // Maximum amount of time non-executable transaction are queued
}

// DefaultTxPoolConfig contains the default configurations for the transaction
// pool.
var DefaultTxPoolConfig = TxPoolConfig{
    PriceLimit: 1,
    PriceBump:  10,

    AccountSlots: 16,
    GlobalSlots:  4096,
    AccountQueue: 64,
    GlobalQueue:  1024,

    Lifetime: 3 * time.Hour,
}

// sanitize checks the provided user configurations and changes anything that's
// unreasonable or unworkable.
func (config *TxPoolConfig) sanitize() TxPoolConfig {
    conf := *config
    if conf.PriceLimit < 1 {
        log.Warn("Sanitizing invalid txpool price limit", "provided", conf.PriceLimit, "updated", DefaultTxPoolConfig.PriceLimit)
        conf.PriceLimit = DefaultTxPoolConfig.PriceLimit
    }
    if conf.PriceBump < 1 {
        log.Warn("Sanitizing invalid txpool price bump", "provided", conf.PriceBump, "updated", DefaultTxPoolConfig.PriceBump)
        conf.PriceBump = DefaultTxPoolConfig.PriceBump
    }
    return conf
}

// TxPool contains all currently known transactions. Transactions
// enter the pool when they are received from the network or submitted
// locally. They exit the pool when they are included in the blockchain.
//
// The pool separates processable transactions (which can be applied to the
// current state) and future transactions. Transactions move between those
// two states over time as they are received and processed.
type TxPool struct {
    config       TxPoolConfig
    chainconfig  *params.ChainConfig
    currentState stateFn // The state function which will allow us to do some pre checks
    pendingState *state.ManagedState
    gasLimit     func() *big.Int // The current gas limit function callback
    gasPrice     *big.Int
    eventMux     *event.TypeMux
    events       *event.TypeMuxSubscription
    locals       *accountSet
    signer       types.Signer
    mu           sync.RWMutex

    pending map[common.Address]*txList         // All currently processable transactions
    queue   map[common.Address]*txList         // Queued but non-processable transactions
    beats   map[common.Address]time.Time       // Last heartbeat from each known account
    all     map[common.Hash]*types.Transaction // All transactions to allow lookups
    priced  *txPricedList                      // All transactions sorted by price

    wg   sync.WaitGroup // for shutdown sync
    quit chan struct{}

    homestead bool
}

// NewTxPool creates a new transaction pool to gather, sort and filter inbound
// trnsactions from the network.
func NewTxPool(config TxPoolConfig, chainconfig *params.ChainConfig, eventMux *event.TypeMux, currentStateFn stateFn, gasLimitFn func() *big.Int) *TxPool {
    // Sanitize the input to ensure no vulnerable gas prices are set
    config = (&config).sanitize()

    // Create the transaction pool with its initial settings
    pool := &TxPool{
        config:       config,
        chainconfig:  chainconfig,
        signer:       types.NewEIP155Signer(chainconfig.ChainId),
        pending:      make(map[common.Address]*txList),
        queue:        make(map[common.Address]*txList),
        beats:        make(map[common.Address]time.Time),
        all:          make(map[common.Hash]*types.Transaction),
        eventMux:     eventMux,
        currentState: currentStateFn,
        gasLimit:     gasLimitFn,
        gasPrice:     new(big.Int).SetUint64(config.PriceLimit),
        pendingState: nil,
        events:       eventMux.Subscribe(ChainHeadEvent{}, RemovedTransactionEvent{}),
        quit:         make(chan struct{}),
    }
    pool.locals = newAccountSet(pool.signer)
    pool.priced = newTxPricedList(&pool.all)
    pool.resetState()

    // Start the various events loops and return
    pool.wg.Add(2)
    go pool.eventLoop()
    go pool.expirationLoop()

    return pool
}

func (pool *TxPool) eventLoop() {
    defer pool.wg.Done()

    // Start a ticker and keep track of interesting pool stats to report
    var prevPending, prevQueued, prevStales int

    report := time.NewTicker(statsReportInterval)
    defer report.Stop()

    // Track chain events. When a chain events occurs (new chain canon block)
    // we need to know the new state. The new state will help us determine
    // the nonces in the managed state
    for {
        select {
        // Handle any events fired by the system
        case ev, ok := <-pool.events.Chan():
            if !ok {
                return
            }
            switch ev := ev.Data.(type) {
            case ChainHeadEvent:
                pool.mu.Lock()
                if ev.Block != nil {
                    if pool.chainconfig.IsHomestead(ev.Block.Number()) {
                        pool.homestead = true
                    }
                }
                pool.resetState()
                pool.mu.Unlock()

            case RemovedTransactionEvent:
                pool.addTxs(ev.Txs, false)
            }

        // Handle stats reporting ticks
        case <-report.C:
            pool.mu.RLock()
            pending, queued := pool.stats()
            stales := pool.priced.stales
            pool.mu.RUnlock()

            if pending != prevPending || queued != prevQueued || stales != prevStales {
                log.Debug("Transaction pool status report", "executable", pending, "queued", queued, "stales", stales)
                prevPending, prevQueued, prevStales = pending, queued, stales
            }
        }
    }
}

func (pool *TxPool) resetState() {
    currentState, err := pool.currentState()
    if err != nil {
        log.Error("Failed reset txpool state", "err", err)
        return
    }
    pool.pendingState = state.ManageState(currentState)

    // validate the pool of pending transactions, this will remove
    // any transactions that have been included in the block or
    // have been invalidated because of another transaction (e.g.
    // higher gas price)
    pool.demoteUnexecutables(currentState)

    // Update all accounts to the latest known pending nonce
    for addr, list := range pool.pending {
        txs := list.Flatten() // Heavy but will be cached and is needed by the miner anyway
        pool.pendingState.SetNonce(addr, txs[len(txs)-1].Nonce()+1)
    }
    // Check the queue and move transactions over to the pending if possible
    // or remove those that have become invalid
    pool.promoteExecutables(currentState, nil)
}

// Stop terminates the transaction pool.
func (pool *TxPool) Stop() {
    pool.events.Unsubscribe()
    close(pool.quit)
    pool.wg.Wait()

    log.Info("Transaction pool stopped")
}

// GasPrice returns the current gas price enforced by the transaction pool.
func (pool *TxPool) GasPrice() *big.Int {
    pool.mu.RLock()
    defer pool.mu.RUnlock()

    return new(big.Int).Set(pool.gasPrice)
}

// SetGasPrice updates the minimum price required by the transaction pool for a
// new transaction, and drops all transactions below this threshold.
func (pool *TxPool) SetGasPrice(price *big.Int) {
    pool.mu.Lock()
    defer pool.mu.Unlock()

    pool.gasPrice = price
    for _, tx := range pool.priced.Cap(price, pool.locals) {
        pool.removeTx(tx.Hash())
    }
    log.Info("Transaction pool price threshold updated", "price", price)
}

// State returns the virtual managed state of the transaction pool.
func (pool *TxPool) State() *state.ManagedState {
    pool.mu.RLock()
    defer pool.mu.RUnlock()

    return pool.pendingState
}

// Stats retrieves the current pool stats, namely the number of pending and the
// number of queued (non-executable) transactions.
func (pool *TxPool) Stats() (int, int) {
    pool.mu.RLock()
    defer pool.mu.RUnlock()

    return pool.stats()
}

// stats retrieves the current pool stats, namely the number of pending and the
// number of queued (non-executable) transactions.
func (pool *TxPool) stats() (int, int) {
    pending := 0
    for _, list := range pool.pending {
        pending += list.Len()
    }
    queued := 0
    for _, list := range pool.queue {
        queued += list.Len()
    }
    return pending, queued
}

// Content retrieves the data content of the transaction pool, returning all the
// pending as well as queued transactions, grouped by account and sorted by nonce.
func (pool *TxPool) Content() (map[common.Address]types.Transactions, map[common.Address]types.Transactions) {
    pool.mu.RLock()
    defer pool.mu.RUnlock()

    pending := make(map[common.Address]types.Transactions)
    for addr, list := range pool.pending {
        pending[addr] = list.Flatten()
    }
    queued := make(map[common.Address]types.Transactions)
    for addr, list := range pool.queue {
        queued[addr] = list.Flatten()
    }
    return pending, queued
}

// Pending retrieves all currently processable transactions, groupped by origin
// account and sorted by nonce. The returned transaction set is a copy and can be
// freely modified by calling code.
func (pool *TxPool) Pending() (map[common.Address]types.Transactions, error) {
    pool.mu.Lock()
    defer pool.mu.Unlock()

    pending := make(map[common.Address]types.Transactions)
    for addr, list := range pool.pending {
        pending[addr] = list.Flatten()
    }
    return pending, nil
}

// validateTx checks whether a transaction is valid according to the consensus
// rules and adheres to some heuristic limits of the local node (price and size).
func (pool *TxPool) validateTx(tx *types.Transaction, local bool) error {
    // Heuristic limit, reject transactions over 32KB to prevent DOS attacks
    if tx.Size() > 32*1024 {
        return ErrOversizedData
    }
    // Transactions can't be negative. This may never happen using RLP decoded
    // transactions but may occur if you create a transaction using the RPC.
    if tx.Value().Sign() < 0 {
        return ErrNegativeValue
    }
    // Ensure the transaction doesn't exceed the current block limit gas.
    if pool.gasLimit().Cmp(tx.Gas()) < 0 {
        return ErrGasLimit
    }
    // Make sure the transaction is signed properly
    from, err := types.Sender(pool.signer, tx)
    if err != nil {
        return ErrInvalidSender
    }
    // Drop non-local transactions under our own minimal accepted gas price
    local = local || pool.locals.contains(from) // account may be local even if the transaction arrived from the network
    if !local && pool.gasPrice.Cmp(tx.GasPrice()) > 0 {
        return ErrUnderpriced
    }
    // Ensure the transaction adheres to nonce ordering
    currentState, err := pool.currentState()
    if err != nil {
        return err
    }
    if currentState.GetNonce(from) > tx.Nonce() {
        return ErrNonceTooLow
    }
    // Transactor should have enough funds to cover the costs
    // cost == V + GP * GL
    if currentState.GetBalance(from).Cmp(tx.Cost()) < 0 {
        return ErrInsufficientFunds
    }
    intrGas := IntrinsicGas(tx.Data(), tx.To() == nil, pool.homestead)
    if tx.Gas().Cmp(intrGas) < 0 {
        return ErrIntrinsicGas
    }
    return nil
}

// add validates a transaction and inserts it into the non-executable queue for
// later pending promotion and execution. If the transaction is a replacement for
// an already pending or queued one, it overwrites the previous and returns this
// so outer code doesn't uselessly call promote.
//
// If a newly added transaction is marked as local, its sending account will be
// whitelisted, preventing any associated transaction from being dropped out of
// the pool due to pricing constraints.
func (pool *TxPool) add(tx *types.Transaction, local bool) (bool, error) {
    // If the transaction is already known, discard it
    hash := tx.Hash()
    if pool.all[hash] != nil {
        log.Trace("Discarding already known transaction", "hash", hash)
        return false, fmt.Errorf("known transaction: %x", hash)
    }
    // If the transaction fails basic validation, discard it
    if err := pool.validateTx(tx, local); err != nil {
        log.Trace("Discarding invalid transaction", "hash", hash, "err", err)
        invalidTxCounter.Inc(1)
        return false, err
    }
    // If the transaction pool is full, discard underpriced transactions
    if uint64(len(pool.all)) >= pool.config.GlobalSlots+pool.config.GlobalQueue {
        // If the new transaction is underpriced, don't accept it
        if pool.priced.Underpriced(tx, pool.locals) {
            log.Trace("Discarding underpriced transaction", "hash", hash, "price", tx.GasPrice())
            underpricedTxCounter.Inc(1)
            return false, ErrUnderpriced
        }
        // New transaction is better than our worse ones, make room for it
        drop := pool.priced.Discard(len(pool.all)-int(pool.config.GlobalSlots+pool.config.GlobalQueue-1), pool.locals)
        for _, tx := range drop {
            log.Trace("Discarding freshly underpriced transaction", "hash", tx.Hash(), "price", tx.GasPrice())
            underpricedTxCounter.Inc(1)
            pool.removeTx(tx.Hash())
        }
    }
    // If the transaction is replacing an already pending one, do directly
    from, _ := types.Sender(pool.signer, tx) // already validated
    if list := pool.pending[from]; list != nil && list.Overlaps(tx) {
        // Nonce already pending, check if required price bump is met
        inserted, old := list.Add(tx, pool.config.PriceBump)
        if !inserted {
            pendingDiscardCounter.Inc(1)
            return false, ErrReplaceUnderpriced
        }
        // New transaction is better, replace old one
        if old != nil {
            delete(pool.all, old.Hash())
            pool.priced.Removed()
            pendingReplaceCounter.Inc(1)
        }
        pool.all[tx.Hash()] = tx
        pool.priced.Put(tx)

        log.Trace("Pooled new executable transaction", "hash", hash, "from", from, "to", tx.To())
        return old != nil, nil
    }
    // New transaction isn't replacing a pending one, push into queue and potentially mark local
    replace, err := pool.enqueueTx(hash, tx)
    if err != nil {
        return false, err
    }
    if local {
        pool.locals.add(from)
    }
    log.Trace("Pooled new future transaction", "hash", hash, "from", from, "to", tx.To())
    return replace, nil
}

// enqueueTx inserts a new transaction into the non-executable transaction queue.
//
// Note, this method assumes the pool lock is held!
func (pool *TxPool) enqueueTx(hash common.Hash, tx *types.Transaction) (bool, error) {
    // Try to insert the transaction into the future queue
    from, _ := types.Sender(pool.signer, tx) // already validated
    if pool.queue[from] == nil {
        pool.queue[from] = newTxList(false)
    }
    inserted, old := pool.queue[from].Add(tx, pool.config.PriceBump)
    if !inserted {
        // An older transaction was better, discard this
        queuedDiscardCounter.Inc(1)
        return false, ErrReplaceUnderpriced
    }
    // Discard any previous transaction and mark this
    if old != nil {
        delete(pool.all, old.Hash())
        pool.priced.Removed()
        queuedReplaceCounter.Inc(1)
    }
    pool.all[hash] = tx
    pool.priced.Put(tx)
    return old != nil, nil
}

// promoteTx adds a transaction to the pending (processable) list of transactions.
//
// Note, this method assumes the pool lock is held!
func (pool *TxPool) promoteTx(addr common.Address, hash common.Hash, tx *types.Transaction) {
    // Try to insert the transaction into the pending queue
    if pool.pending[addr] == nil {
        pool.pending[addr] = newTxList(true)
    }
    list := pool.pending[addr]

    inserted, old := list.Add(tx, pool.config.PriceBump)
    if !inserted {
        // An older transaction was better, discard this
        delete(pool.all, hash)
        pool.priced.Removed()

        pendingDiscardCounter.Inc(1)
        return
    }
    // Otherwise discard any previous transaction and mark this
    if old != nil {
        delete(pool.all, old.Hash())
        pool.priced.Removed()

        pendingReplaceCounter.Inc(1)
    }
    // Failsafe to work around direct pending inserts (tests)
    if pool.all[hash] == nil {
        pool.all[hash] = tx
        pool.priced.Put(tx)
    }
    // Set the potentially new pending nonce and notify any subsystems of the new tx
    pool.beats[addr] = time.Now()
    pool.pendingState.SetNonce(addr, tx.Nonce()+1)
    go pool.eventMux.Post(TxPreEvent{tx})
}

// AddLocal enqueues a single transaction into the pool if it is valid, marking
// the sender as a local one in the mean time, ensuring it goes around the local
// pricing constraints.
func (pool *TxPool) AddLocal(tx *types.Transaction) error {
    return pool.addTx(tx, !pool.config.NoLocals)
}

// AddRemote enqueues a single transaction into the pool if it is valid. If the
// sender is not among the locally tracked ones, full pricing constraints will
// apply.
func (pool *TxPool) AddRemote(tx *types.Transaction) error {
    return pool.addTx(tx, false)
}

// AddLocals enqueues a batch of transactions into the pool if they are valid,
// marking the senders as a local ones in the mean time, ensuring they go around
// the local pricing constraints.
func (pool *TxPool) AddLocals(txs []*types.Transaction) error {
    return pool.addTxs(txs, !pool.config.NoLocals)
}

// AddRemotes enqueues a batch of transactions into the pool if they are valid.
// If the senders are not among the locally tracked ones, full pricing constraints
// will apply.
func (pool *TxPool) AddRemotes(txs []*types.Transaction) error {
    return pool.addTxs(txs, false)
}

// addTx enqueues a single transaction into the pool if it is valid.
func (pool *TxPool) addTx(tx *types.Transaction, local bool) error {
    pool.mu.Lock()
    defer pool.mu.Unlock()

    // Try to inject the transaction and update any state
    replace, err := pool.add(tx, local)
    if err != nil {
        return err
    }
    // If we added a new transaction, run promotion checks and return
    if !replace {
        state, err := pool.currentState()
        if err != nil {
            return err
        }
        from, _ := types.Sender(pool.signer, tx) // already validated
        pool.promoteExecutables(state, []common.Address{from})
    }
    return nil
}

// addTxs attempts to queue a batch of transactions if they are valid.
func (pool *TxPool) addTxs(txs []*types.Transaction, local bool) error {
    pool.mu.Lock()
    defer pool.mu.Unlock()

    // Add the batch of transaction, tracking the accepted ones
    dirty := make(map[common.Address]struct{})
    for _, tx := range txs {
        if replace, err := pool.add(tx, local); err == nil {
            if !replace {
                from, _ := types.Sender(pool.signer, tx) // already validated
                dirty[from] = struct{}{}
            }
        }
    }
    // Only reprocess the internal state if something was actually added
    if len(dirty) > 0 {
        state, err := pool.currentState()
        if err != nil {
            return err
        }
        addrs := make([]common.Address, 0, len(dirty))
        for addr, _ := range dirty {
            addrs = append(addrs, addr)
        }
        pool.promoteExecutables(state, addrs)
    }
    return nil
}

// Get returns a transaction if it is contained in the pool
// and nil otherwise.
func (pool *TxPool) Get(hash common.Hash) *types.Transaction {
    pool.mu.RLock()
    defer pool.mu.RUnlock()

    return pool.all[hash]
}

// Remove removes the transaction with the given hash from the pool.
func (pool *TxPool) Remove(hash common.Hash) {
    pool.mu.Lock()
    defer pool.mu.Unlock()

    pool.removeTx(hash)
}

// RemoveBatch removes all given transactions from the pool.
func (pool *TxPool) RemoveBatch(txs types.Transactions) {
    pool.mu.Lock()
    defer pool.mu.Unlock()

    for _, tx := range txs {
        pool.removeTx(tx.Hash())
    }
}

// removeTx removes a single transaction from the queue, moving all subsequent
// transactions back to the future queue.
func (pool *TxPool) removeTx(hash common.Hash) {
    // Fetch the transaction we wish to delete
    tx, ok := pool.all[hash]
    if !ok {
        return
    }
    addr, _ := types.Sender(pool.signer, tx) // already validated during insertion

    // Remove it from the list of known transactions
    delete(pool.all, hash)
    pool.priced.Removed()

    // Remove the transaction from the pending lists and reset the account nonce
    if pending := pool.pending[addr]; pending != nil {
        if removed, invalids := pending.Remove(tx); removed {
            // If no more transactions are left, remove the list
            if pending.Empty() {
                delete(pool.pending, addr)
                delete(pool.beats, addr)
            } else {
                // Otherwise postpone any invalidated transactions
                for _, tx := range invalids {
                    pool.enqueueTx(tx.Hash(), tx)
                }
            }
            // Update the account nonce if needed
            if nonce := tx.Nonce(); pool.pendingState.GetNonce(addr) > nonce {
                pool.pendingState.SetNonce(addr, nonce)
            }
            return
        }
    }
    // Transaction is in the future queue
    if future := pool.queue[addr]; future != nil {
        future.Remove(tx)
        if future.Empty() {
            delete(pool.queue, addr)
        }
    }
}

// promoteExecutables moves transactions that have become processable from the
// future queue to the set of pending transactions. During this process, all
// invalidated transactions (low nonce, low balance) are deleted.
func (pool *TxPool) promoteExecutables(state *state.StateDB, accounts []common.Address) {
    gaslimit := pool.gasLimit()

    // Gather all the accounts potentially needing updates
    if accounts == nil {
        accounts = make([]common.Address, 0, len(pool.queue))
        for addr, _ := range pool.queue {
            accounts = append(accounts, addr)
        }
    }
    // Iterate over all accounts and promote any executable transactions
    for _, addr := range accounts {
        list := pool.queue[addr]
        if list == nil {
            continue // Just in case someone calls with a non existing account
        }
        // Drop all transactions that are deemed too old (low nonce)
        for _, tx := range list.Forward(state.GetNonce(addr)) {
            hash := tx.Hash()
            log.Trace("Removed old queued transaction", "hash", hash)
            delete(pool.all, hash)
            pool.priced.Removed()
        }
        // Drop all transactions that are too costly (low balance or out of gas)
        drops, _ := list.Filter(state.GetBalance(addr), gaslimit)
        for _, tx := range drops {
            hash := tx.Hash()
            log.Trace("Removed unpayable queued transaction", "hash", hash)
            delete(pool.all, hash)
            pool.priced.Removed()
            queuedNofundsCounter.Inc(1)
        }
        // Gather all executable transactions and promote them
        for _, tx := range list.Ready(pool.pendingState.GetNonce(addr)) {
            hash := tx.Hash()
            log.Trace("Promoting queued transaction", "hash", hash)
            pool.promoteTx(addr, hash, tx)
        }
        // Drop all transactions over the allowed limit
        if !pool.locals.contains(addr) {
            for _, tx := range list.Cap(int(pool.config.AccountQueue)) {
                hash := tx.Hash()
                delete(pool.all, hash)
                pool.priced.Removed()
                queuedRateLimitCounter.Inc(1)
                log.Trace("Removed cap-exceeding queued transaction", "hash", hash)
            }
        }
        // Delete the entire queue entry if it became empty.
        if list.Empty() {
            delete(pool.queue, addr)
        }
    }
    // If the pending limit is overflown, start equalizing allowances
    pending := uint64(0)
    for _, list := range pool.pending {
        pending += uint64(list.Len())
    }
    if pending > pool.config.GlobalSlots {
        pendingBeforeCap := pending
        // Assemble a spam order to penalize large transactors first
        spammers := prque.New()
        for addr, list := range pool.pending {
            // Only evict transactions from high rollers
            if !pool.locals.contains(addr) && uint64(list.Len()) > pool.config.AccountSlots {
                spammers.Push(addr, float32(list.Len()))
            }
        }
        // Gradually drop transactions from offenders
        offenders := []common.Address{}
        for pending > pool.config.GlobalSlots && !spammers.Empty() {
            // Retrieve the next offender if not local address
            offender, _ := spammers.Pop()
            offenders = append(offenders, offender.(common.Address))

            // Equalize balances until all the same or below threshold
            if len(offenders) > 1 {
                // Calculate the equalization threshold for all current offenders
                threshold := pool.pending[offender.(common.Address)].Len()

                // Iteratively reduce all offenders until below limit or threshold reached
                for pending > pool.config.GlobalSlots && pool.pending[offenders[len(offenders)-2]].Len() > threshold {
                    for i := 0; i < len(offenders)-1; i++ {
                        list := pool.pending[offenders[i]]
                        for _, tx := range list.Cap(list.Len() - 1) {
                            // Drop the transaction from the global pools too
                            hash := tx.Hash()
                            delete(pool.all, hash)
                            pool.priced.Removed()

                            // Update the account nonce to the dropped transaction
                            if nonce := tx.Nonce(); pool.pendingState.GetNonce(offenders[i]) > nonce {
                                pool.pendingState.SetNonce(offenders[i], nonce)
                            }
                            log.Trace("Removed fairness-exceeding pending transaction", "hash", hash)
                        }
                        pending--
                    }
                }
            }
        }
        // If still above threshold, reduce to limit or min allowance
        if pending > pool.config.GlobalSlots && len(offenders) > 0 {
            for pending > pool.config.GlobalSlots && uint64(pool.pending[offenders[len(offenders)-1]].Len()) > pool.config.AccountSlots {
                for _, addr := range offenders {
                    list := pool.pending[addr]
                    for _, tx := range list.Cap(list.Len() - 1) {
                        // Drop the transaction from the global pools too
                        hash := tx.Hash()
                        delete(pool.all, hash)
                        pool.priced.Removed()

                        // Update the account nonce to the dropped transaction
                        if nonce := tx.Nonce(); pool.pendingState.GetNonce(addr) > nonce {
                            pool.pendingState.SetNonce(addr, nonce)
                        }
                        log.Trace("Removed fairness-exceeding pending transaction", "hash", hash)
                    }
                    pending--
                }
            }
        }
        pendingRateLimitCounter.Inc(int64(pendingBeforeCap - pending))
    }
    // If we've queued more transactions than the hard limit, drop oldest ones
    queued := uint64(0)
    for _, list := range pool.queue {
        queued += uint64(list.Len())
    }
    if queued > pool.config.GlobalQueue {
        // Sort all accounts with queued transactions by heartbeat
        addresses := make(addresssByHeartbeat, 0, len(pool.queue))
        for addr := range pool.queue {
            if !pool.locals.contains(addr) { // don't drop locals
                addresses = append(addresses, addressByHeartbeat{addr, pool.beats[addr]})
            }
        }
        sort.Sort(addresses)

        // Drop transactions until the total is below the limit or only locals remain
        for drop := queued - pool.config.GlobalQueue; drop > 0 && len(addresses) > 0; {
            addr := addresses[len(addresses)-1]
            list := pool.queue[addr.address]

            addresses = addresses[:len(addresses)-1]

            // Drop all transactions if they are less than the overflow
            if size := uint64(list.Len()); size <= drop {
                for _, tx := range list.Flatten() {
                    pool.removeTx(tx.Hash())
                }
                drop -= size
                queuedRateLimitCounter.Inc(int64(size))
                continue
            }
            // Otherwise drop only last few transactions
            txs := list.Flatten()
            for i := len(txs) - 1; i >= 0 && drop > 0; i-- {
                pool.removeTx(txs[i].Hash())
                drop--
                queuedRateLimitCounter.Inc(1)
            }
        }
    }
}

// demoteUnexecutables removes invalid and processed transactions from the pools
// executable/pending queue and any subsequent transactions that become unexecutable
// are moved back into the future queue.
func (pool *TxPool) demoteUnexecutables(state *state.StateDB) {
    gaslimit := pool.gasLimit()

    // Iterate over all accounts and demote any non-executable transactions
    for addr, list := range pool.pending {
        nonce := state.GetNonce(addr)

        // Drop all transactions that are deemed too old (low nonce)
        for _, tx := range list.Forward(nonce) {
            hash := tx.Hash()
            log.Trace("Removed old pending transaction", "hash", hash)
            delete(pool.all, hash)
            pool.priced.Removed()
        }
        // Drop all transactions that are too costly (low balance or out of gas), and queue any invalids back for later
        drops, invalids := list.Filter(state.GetBalance(addr), gaslimit)
        for _, tx := range drops {
            hash := tx.Hash()
            log.Trace("Removed unpayable pending transaction", "hash", hash)
            delete(pool.all, hash)
            pool.priced.Removed()
            pendingNofundsCounter.Inc(1)
        }
        for _, tx := range invalids {
            hash := tx.Hash()
            log.Trace("Demoting pending transaction", "hash", hash)
            pool.enqueueTx(hash, tx)
        }
        // Delete the entire queue entry if it became empty.
        if list.Empty() {
            delete(pool.pending, addr)
            delete(pool.beats, addr)
        }
    }
}

// expirationLoop is a loop that periodically iterates over all accounts with
// queued transactions and drop all that have been inactive for a prolonged amount
// of time.
func (pool *TxPool) expirationLoop() {
    defer pool.wg.Done()

    evict := time.NewTicker(evictionInterval)
    defer evict.Stop()

    for {
        select {
        case <-evict.C:
            pool.mu.Lock()
            for addr := range pool.queue {
                // Skip local transactions from the eviction mechanism
                if pool.locals.contains(addr) {
                    continue
                }
                // Any non-locals old enough should be removed
                if time.Since(pool.beats[addr]) > pool.config.Lifetime {
                    for _, tx := range pool.queue[addr].Flatten() {
                        pool.removeTx(tx.Hash())
                    }
                }
            }
            pool.mu.Unlock()

        case <-pool.quit:
            return
        }
    }
}

// addressByHeartbeat is an account address tagged with its last activity timestamp.
type addressByHeartbeat struct {
    address   common.Address
    heartbeat time.Time
}

type addresssByHeartbeat []addressByHeartbeat

func (a addresssByHeartbeat) Len() int           { return len(a) }
func (a addresssByHeartbeat) Less(i, j int) bool { return a[i].heartbeat.Before(a[j].heartbeat) }
func (a addresssByHeartbeat) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }

// accountSet is simply a set of addresses to check for existance, and a signer
// capable of deriving addresses from transactions.
type accountSet struct {
    accounts map[common.Address]struct{}
    signer   types.Signer
}

// newAccountSet creates a new address set with an associated signer for sender
// derivations.
func newAccountSet(signer types.Signer) *accountSet {
    return &accountSet{
        accounts: make(map[common.Address]struct{}),
        signer:   signer,
    }
}

// contains checks if a given address is contained within the set.
func (as *accountSet) contains(addr common.Address) bool {
    _, exist := as.accounts[addr]
    return exist
}

// containsTx checks if the sender of a given tx is within the set. If the sender
// cannot be derived, this method returns false.
func (as *accountSet) containsTx(tx *types.Transaction) bool {
    if addr, err := types.Sender(as.signer, tx); err == nil {
        return as.contains(addr)
    }
    return false
}

// add inserts a new address into the set to track.
func (as *accountSet) add(addr common.Address) {
    as.accounts[addr] = struct{}{}
}