path: root/core/chain_manager.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 implements the Ethereum consensus protocol.
package core

import (
    "errors"
    "fmt"
    "io"
    "math/big"
    "runtime"
    "sync"
    "sync/atomic"
    "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/logger"
    "github.com/ethereum/go-ethereum/logger/glog"
    "github.com/ethereum/go-ethereum/metrics"
    "github.com/ethereum/go-ethereum/pow"
    "github.com/hashicorp/golang-lru"
)

var (
    chainlogger = logger.NewLogger("CHAIN")
    jsonlogger  = logger.NewJsonLogger()

    blockInsertTimer = metrics.NewTimer("chain/inserts")

    ErrNoGenesis = errors.New("Genesis not found in chain")
)

const (
    blockCacheLimit     = 256
    maxFutureBlocks     = 256
    maxTimeFutureBlocks = 30
    checkpointLimit     = 200
)

type ChainManager struct {
    //eth          EthManager
    chainDb      common.Database
    processor    types.BlockProcessor
    eventMux     *event.TypeMux
    genesisBlock *types.Block
    // Last known total difficulty
    mu      sync.RWMutex
    chainmu sync.RWMutex
    tsmu    sync.RWMutex

    checkpoint      int // checkpoint counts towards the new checkpoint
    td              *big.Int
    currentBlock    *types.Block
    lastBlockHash   common.Hash
    currentGasLimit *big.Int

    cache        *lru.Cache // cache is the LRU caching
    futureBlocks *lru.Cache // future blocks are blocks added for later processing

    quit    chan struct{}
    running int32 // running must be called automically
    // procInterrupt must be atomically called
    procInterrupt int32 // interrupt signaler for block processing
    wg            sync.WaitGroup

    pow pow.PoW
}

func NewChainManager(chainDb common.Database, pow pow.PoW, mux *event.TypeMux) (*ChainManager, error) {
    cache, _ := lru.New(blockCacheLimit)
    bc := &ChainManager{
        chainDb:  chainDb,
        eventMux: mux,
        quit:     make(chan struct{}),
        cache:    cache,
        pow:      pow,
    }

    bc.genesisBlock = bc.GetBlockByNumber(0)
    if bc.genesisBlock == nil {
        reader, err := NewDefaultGenesisReader()
        if err != nil {
            return nil, err
        }
        bc.genesisBlock, err = WriteGenesisBlock(chainDb, reader)
        if err != nil {
            return nil, err
        }
        glog.V(logger.Info).Infoln("WARNING: Wrote default ethereum genesis block")
    }

    if err := bc.setLastState(); err != nil {
        return nil, err
    }

    // Check the current state of the block hashes and make sure that we do not have any of the bad blocks in our chain
    for hash, _ := range BadHashes {
        if block := bc.GetBlock(hash); block != nil {
            glog.V(logger.Error).Infof("Found bad hash. Reorganising chain to state %x\n", block.ParentHash().Bytes()[:4])
            block = bc.GetBlock(block.ParentHash())
            if block == nil {
                glog.Fatal("Unable to complete. Parent block not found. Corrupted DB?")
            }
            bc.SetHead(block)

            glog.V(logger.Error).Infoln("Chain reorg was successfull. Resuming normal operation")
        }
    }

    // Take ownership of this particular state

    bc.futureBlocks, _ = lru.New(maxFutureBlocks)
    bc.makeCache()

    go bc.update()

    return bc, nil
}

func (bc *ChainManager) SetHead(head *types.Block) {
    bc.mu.Lock()
    defer bc.mu.Unlock()

    for block := bc.currentBlock; block != nil && block.Hash() != head.Hash(); block = bc.GetBlock(block.ParentHash()) {
        bc.removeBlock(block)
    }

    bc.cache, _ = lru.New(blockCacheLimit)
    bc.currentBlock = head
    bc.makeCache()

    bc.setTotalDifficulty(head.Td)
    bc.insert(head)
    bc.setLastState()
}

func (self *ChainManager) Td() *big.Int {
    self.mu.RLock()
    defer self.mu.RUnlock()

    return new(big.Int).Set(self.td)
}

func (self *ChainManager) GasLimit() *big.Int {
    self.mu.RLock()
    defer self.mu.RUnlock()

    return self.currentBlock.GasLimit()
}

func (self *ChainManager) LastBlockHash() common.Hash {
    self.mu.RLock()
    defer self.mu.RUnlock()

    return self.lastBlockHash
}

func (self *ChainManager) CurrentBlock() *types.Block {
    self.mu.RLock()
    defer self.mu.RUnlock()

    return self.currentBlock
}

func (self *ChainManager) Status() (td *big.Int, currentBlock common.Hash, genesisBlock common.Hash) {
    self.mu.RLock()
    defer self.mu.RUnlock()

    return new(big.Int).Set(self.td), self.currentBlock.Hash(), self.genesisBlock.Hash()
}

func (self *ChainManager) SetProcessor(proc types.BlockProcessor) {
    self.processor = proc
}

func (self *ChainManager) State() *state.StateDB {
    return state.New(self.CurrentBlock().Root(), self.chainDb)
}

func (bc *ChainManager) recover() bool {
    data, _ := bc.chainDb.Get([]byte("checkpoint"))
    if len(data) != 0 {
        block := bc.GetBlock(common.BytesToHash(data))
        if block != nil {
            err := bc.chainDb.Put([]byte("LastBlock"), block.Hash().Bytes())
            if err != nil {
                glog.Fatalln("db write err:", err)
            }

            bc.currentBlock = block
            bc.lastBlockHash = block.Hash()
            return true
        }
    }
    return false
}

func (bc *ChainManager) setLastState() error {
    data, _ := bc.chainDb.Get([]byte("LastBlock"))
    if len(data) != 0 {
        block := bc.GetBlock(common.BytesToHash(data))
        if block != nil {
            bc.currentBlock = block
            bc.lastBlockHash = block.Hash()
        } else {
            glog.Infof("LastBlock (%x) not found. Recovering...\n", data)
            if bc.recover() {
                glog.Infof("Recover successful")
            } else {
                glog.Fatalf("Recover failed. Please report")
            }
        }
    } else {
        bc.Reset()
    }
    bc.td = bc.currentBlock.Td
    bc.currentGasLimit = CalcGasLimit(bc.currentBlock)

    if glog.V(logger.Info) {
        glog.Infof("Last block (#%v) %x TD=%v\n", bc.currentBlock.Number(), bc.currentBlock.Hash(), bc.td)
    }

    return nil
}

func (bc *ChainManager) makeCache() {
    bc.cache, _ = lru.New(blockCacheLimit)
    // load in last `blockCacheLimit` - 1 blocks. Last block is the current.
    bc.cache.Add(bc.genesisBlock.Hash(), bc.genesisBlock)
    for _, block := range bc.GetBlocksFromHash(bc.currentBlock.Hash(), blockCacheLimit) {
        bc.cache.Add(block.Hash(), block)
    }
}

func (bc *ChainManager) Reset() {
    bc.mu.Lock()
    defer bc.mu.Unlock()

    for block := bc.currentBlock; block != nil; block = bc.GetBlock(block.ParentHash()) {
        bc.removeBlock(block)
    }

    bc.cache, _ = lru.New(blockCacheLimit)

    // Prepare the genesis block
    err := WriteBlock(bc.chainDb, bc.genesisBlock)
    if err != nil {
        glog.Fatalln("db err:", err)
    }

    bc.insert(bc.genesisBlock)
    bc.currentBlock = bc.genesisBlock
    bc.makeCache()

    bc.setTotalDifficulty(common.Big("0"))
}

func (bc *ChainManager) removeBlock(block *types.Block) {
    bc.chainDb.Delete(append(blockHashPre, block.Hash().Bytes()...))
}

func (bc *ChainManager) ResetWithGenesisBlock(gb *types.Block) {
    bc.mu.Lock()
    defer bc.mu.Unlock()

    for block := bc.currentBlock; block != nil; block = bc.GetBlock(block.ParentHash()) {
        bc.removeBlock(block)
    }

    // Prepare the genesis block
    gb.Td = gb.Difficulty()
    bc.genesisBlock = gb

    err := WriteBlock(bc.chainDb, bc.genesisBlock)
    if err != nil {
        glog.Fatalln("db err:", err)
    }

    bc.insert(bc.genesisBlock)
    bc.currentBlock = bc.genesisBlock
    bc.makeCache()
    bc.td = gb.Difficulty()
}

// Export writes the active chain to the given writer.
func (self *ChainManager) Export(w io.Writer) error {
    if err := self.ExportN(w, uint64(0), self.currentBlock.NumberU64()); err != nil {
        return err
    }
    return nil
}

// ExportN writes a subset of the active chain to the given writer.
func (self *ChainManager) ExportN(w io.Writer, first uint64, last uint64) error {
    self.mu.RLock()
    defer self.mu.RUnlock()

    if first > last {
        return fmt.Errorf("export failed: first (%d) is greater than last (%d)", first, last)
    }

    glog.V(logger.Info).Infof("exporting %d blocks...\n", last-first+1)

    for nr := first; nr <= last; nr++ {
        block := self.GetBlockByNumber(nr)
        if block == nil {
            return fmt.Errorf("export failed on #%d: not found", nr)
        }

        if err := block.EncodeRLP(w); err != nil {
            return err
        }
    }

    return nil
}

// insert injects a block into the current chain block chain. Note, this function
// assumes that the `mu` mutex is held!
func (bc *ChainManager) insert(block *types.Block) {
    err := WriteHead(bc.chainDb, block)
    if err != nil {
        glog.Fatal("db write fail:", err)
    }

    bc.checkpoint++
    if bc.checkpoint > checkpointLimit {
        err = bc.chainDb.Put([]byte("checkpoint"), block.Hash().Bytes())
        if err != nil {
            glog.Fatal("db write fail:", err)
        }

        bc.checkpoint = 0
    }

    bc.currentBlock = block
    bc.lastBlockHash = block.Hash()
}

// Accessors
func (bc *ChainManager) Genesis() *types.Block {
    return bc.genesisBlock
}

// Block fetching methods
func (bc *ChainManager) HasBlock(hash common.Hash) bool {
    if bc.cache.Contains(hash) {
        return true
    }

    data, _ := bc.chainDb.Get(append(blockHashPre, hash[:]...))
    return len(data) != 0
}

func (self *ChainManager) GetBlockHashesFromHash(hash common.Hash, max uint64) (chain []common.Hash) {
    block := self.GetBlock(hash)
    if block == nil {
        return
    }
    // XXX Could be optimised by using a different database which only holds hashes (i.e., linked list)
    for i := uint64(0); i < max; i++ {
        block = self.GetBlock(block.ParentHash())
        if block == nil {
            break
        }

        chain = append(chain, block.Hash())
        if block.Number().Cmp(common.Big0) <= 0 {
            break
        }
    }

    return
}

func (self *ChainManager) GetBlock(hash common.Hash) *types.Block {
    if block, ok := self.cache.Get(hash); ok {
        return block.(*types.Block)
    }

    block := GetBlockByHash(self.chainDb, hash)
    if block == nil {
        return nil
    }

    // Add the block to the cache
    self.cache.Add(hash, (*types.Block)(block))

    return (*types.Block)(block)
}

func (self *ChainManager) GetBlockByNumber(num uint64) *types.Block {
    self.mu.RLock()
    defer self.mu.RUnlock()

    return self.getBlockByNumber(num)

}

// GetBlocksFromHash returns the block corresponding to hash and up to n-1 ancestors.
func (self *ChainManager) GetBlocksFromHash(hash common.Hash, n int) (blocks []*types.Block) {
    for i := 0; i < n; i++ {
        block := self.GetBlock(hash)
        if block == nil {
            break
        }
        blocks = append(blocks, block)
        hash = block.ParentHash()
    }
    return
}

// non blocking version
func (self *ChainManager) getBlockByNumber(num uint64) *types.Block {
    return GetBlockByNumber(self.chainDb, num)
}

func (self *ChainManager) GetUnclesInChain(block *types.Block, length int) (uncles []*types.Header) {
    for i := 0; block != nil && i < length; i++ {
        uncles = append(uncles, block.Uncles()...)
        block = self.GetBlock(block.ParentHash())
    }

    return
}

// setTotalDifficulty updates the TD of the chain manager. Note, this function
// assumes that the `mu` mutex is held!
func (bc *ChainManager) setTotalDifficulty(td *big.Int) {
    bc.td = new(big.Int).Set(td)
}

func (bc *ChainManager) Stop() {
    if !atomic.CompareAndSwapInt32(&bc.running, 0, 1) {
        return
    }
    close(bc.quit)
    atomic.StoreInt32(&bc.procInterrupt, 1)

    bc.wg.Wait()

    glog.V(logger.Info).Infoln("Chain manager stopped")
}

type queueEvent struct {
    queue          []interface{}
    canonicalCount int
    sideCount      int
    splitCount     int
}

func (self *ChainManager) procFutureBlocks() {
    blocks := make([]*types.Block, self.futureBlocks.Len())
    for i, hash := range self.futureBlocks.Keys() {
        block, _ := self.futureBlocks.Get(hash)
        blocks[i] = block.(*types.Block)
    }
    if len(blocks) > 0 {
        types.BlockBy(types.Number).Sort(blocks)
        self.InsertChain(blocks)
    }
}

type writeStatus byte

const (
    NonStatTy writeStatus = iota
    CanonStatTy
    SplitStatTy
    SideStatTy
)

// WriteBlock writes the block to the chain (or pending queue)
func (self *ChainManager) WriteBlock(block *types.Block, queued bool) (status writeStatus, err error) {
    self.wg.Add(1)
    defer self.wg.Done()

    cblock := self.currentBlock
    // Compare the TD of the last known block in the canonical chain to make sure it's greater.
    // At this point it's possible that a different chain (fork) becomes the new canonical chain.
    if block.Td.Cmp(self.Td()) > 0 {
        // chain fork
        if block.ParentHash() != cblock.Hash() {
            // during split we merge two different chains and create the new canonical chain
            err := self.merge(cblock, block)
            if err != nil {
                return NonStatTy, err
            }

            status = SplitStatTy
        }

        self.mu.Lock()
        self.setTotalDifficulty(block.Td)
        self.insert(block)
        self.mu.Unlock()

        status = CanonStatTy
    } else {
        status = SideStatTy
    }

    err = WriteBlock(self.chainDb, block)
    if err != nil {
        glog.Fatalln("db err:", err)
    }
    // Delete from future blocks
    self.futureBlocks.Remove(block.Hash())

    return
}

// InsertChain will attempt to insert the given chain in to the canonical chain or, otherwise, create a fork. It an error is returned
// it will return the index number of the failing block as well an error describing what went wrong (for possible errors see core/errors.go).
func (self *ChainManager) InsertChain(chain types.Blocks) (int, error) {
    self.wg.Add(1)
    defer self.wg.Done()

    self.chainmu.Lock()
    defer self.chainmu.Unlock()

    // A queued approach to delivering events. This is generally
    // faster than direct delivery and requires much less mutex
    // acquiring.
    var (
        queue      = make([]interface{}, len(chain))
        queueEvent = queueEvent{queue: queue}
        stats      struct{ queued, processed, ignored int }
        tstart     = time.Now()

        nonceDone    = make(chan nonceResult, len(chain))
        nonceQuit    = make(chan struct{})
        nonceChecked = make([]bool, len(chain))
    )

    // Start the parallel nonce verifier.
    go verifyNonces(self.pow, chain, nonceQuit, nonceDone)
    defer close(nonceQuit)

    txcount := 0
    for i, block := range chain {
        if atomic.LoadInt32(&self.procInterrupt) == 1 {
            glog.V(logger.Debug).Infoln("Premature abort during chain processing")
            break
        }

        bstart := time.Now()
        // Wait for block i's nonce to be verified before processing
        // its state transition.
        for !nonceChecked[i] {
            r := <-nonceDone
            nonceChecked[r.i] = true
            if !r.valid {
                block := chain[r.i]
                return r.i, &BlockNonceErr{Hash: block.Hash(), Number: block.Number(), Nonce: block.Nonce()}
            }
        }

        if BadHashes[block.Hash()] {
            err := fmt.Errorf("Found known bad hash in chain %x", block.Hash())
            blockErr(block, err)
            return i, err
        }

        // Setting block.Td regardless of error (known for example) prevents errors down the line
        // in the protocol handler
        block.Td = new(big.Int).Set(CalcTD(block, self.GetBlock(block.ParentHash())))

        // Call in to the block processor and check for errors. It's likely that if one block fails
        // all others will fail too (unless a known block is returned).
        logs, receipts, err := self.processor.Process(block)
        if err != nil {
            if IsKnownBlockErr(err) {
                stats.ignored++
                continue
            }

            if err == BlockFutureErr {
                // Allow up to MaxFuture second in the future blocks. If this limit
                // is exceeded the chain is discarded and processed at a later time
                // if given.
                if max := uint64(time.Now().Unix()) + maxTimeFutureBlocks; block.Time() > max {
                    return i, fmt.Errorf("%v: BlockFutureErr, %v > %v", BlockFutureErr, block.Time(), max)
                }

                self.futureBlocks.Add(block.Hash(), block)
                stats.queued++
                continue
            }

            if IsParentErr(err) && self.futureBlocks.Contains(block.ParentHash()) {
                self.futureBlocks.Add(block.Hash(), block)
                stats.queued++
                continue
            }

            blockErr(block, err)

            go ReportBlock(block, err)

            return i, err
        }

        txcount += len(block.Transactions())

        // write the block to the chain and get the status
        status, err := self.WriteBlock(block, true)
        if err != nil {
            return i, err
        }
        switch status {
        case CanonStatTy:
            if glog.V(logger.Debug) {
                glog.Infof("[%v] inserted block #%d (%d TXs %v G %d UNCs) (%x...). Took %v\n", time.Now().UnixNano(), block.Number(), len(block.Transactions()), block.GasUsed(), len(block.Uncles()), block.Hash().Bytes()[0:4], time.Since(bstart))
            }
            queue[i] = ChainEvent{block, block.Hash(), logs}
            queueEvent.canonicalCount++

            // This puts transactions in a extra db for rpc
            PutTransactions(self.chainDb, block, block.Transactions())
            // store the receipts
            PutReceipts(self.chainDb, receipts)
        case SideStatTy:
            if glog.V(logger.Detail) {
                glog.Infof("inserted forked block #%d (TD=%v) (%d TXs %d UNCs) (%x...). Took %v\n", block.Number(), block.Difficulty(), len(block.Transactions()), len(block.Uncles()), block.Hash().Bytes()[0:4], time.Since(bstart))
            }
            queue[i] = ChainSideEvent{block, logs}
            queueEvent.sideCount++
        case SplitStatTy:
            queue[i] = ChainSplitEvent{block, logs}
            queueEvent.splitCount++
        }
        if err := PutBlockReceipts(self.chainDb, block, receipts); err != nil {
            glog.V(logger.Warn).Infoln("error writing block receipts:", err)
        }

        stats.processed++
    }

    if (stats.queued > 0 || stats.processed > 0 || stats.ignored > 0) && bool(glog.V(logger.Info)) {
        tend := time.Since(tstart)
        start, end := chain[0], chain[len(chain)-1]
        glog.Infof("imported %d block(s) (%d queued %d ignored) including %d txs in %v. #%v [%x / %x]\n", stats.processed, stats.queued, stats.ignored, txcount, tend, end.Number(), start.Hash().Bytes()[:4], end.Hash().Bytes()[:4])
    }

    go self.eventMux.Post(queueEvent)

    return 0, nil
}

// diff takes two blocks, an old chain and a new chain and will reconstruct the blocks and inserts them
// to be part of the new canonical chain.
func (self *ChainManager) diff(oldBlock, newBlock *types.Block) (types.Blocks, error) {
    var (
        newChain    types.Blocks
        commonBlock *types.Block
        oldStart    = oldBlock
        newStart    = newBlock
    )

    // first reduce whoever is higher bound
    if oldBlock.NumberU64() > newBlock.NumberU64() {
        // reduce old chain
        for oldBlock = oldBlock; oldBlock != nil && oldBlock.NumberU64() != newBlock.NumberU64(); oldBlock = self.GetBlock(oldBlock.ParentHash()) {
        }
    } else {
        // reduce new chain and append new chain blocks for inserting later on
        for newBlock = newBlock; newBlock != nil && newBlock.NumberU64() != oldBlock.NumberU64(); newBlock = self.GetBlock(newBlock.ParentHash()) {
            newChain = append(newChain, newBlock)
        }
    }
    if oldBlock == nil {
        return nil, fmt.Errorf("Invalid old chain")
    }
    if newBlock == nil {
        return nil, fmt.Errorf("Invalid new chain")
    }

    numSplit := newBlock.Number()
    for {
        if oldBlock.Hash() == newBlock.Hash() {
            commonBlock = oldBlock
            break
        }
        newChain = append(newChain, newBlock)

        oldBlock, newBlock = self.GetBlock(oldBlock.ParentHash()), self.GetBlock(newBlock.ParentHash())
        if oldBlock == nil {
            return nil, fmt.Errorf("Invalid old chain")
        }
        if newBlock == nil {
            return nil, fmt.Errorf("Invalid new chain")
        }
    }

    if glog.V(logger.Debug) {
        commonHash := commonBlock.Hash()
        glog.Infof("Chain split detected @ %x. Reorganising chain from #%v %x to %x", commonHash[:4], numSplit, oldStart.Hash().Bytes()[:4], newStart.Hash().Bytes()[:4])
    }

    return newChain, nil
}

// merge merges two different chain to the new canonical chain
func (self *ChainManager) merge(oldBlock, newBlock *types.Block) error {
    newChain, err := self.diff(oldBlock, newBlock)
    if err != nil {
        return fmt.Errorf("chain reorg failed: %v", err)
    }

    // insert blocks. Order does not matter. Last block will be written in ImportChain itself which creates the new head properly
    self.mu.Lock()
    for _, block := range newChain {
        // insert the block in the canonical way, re-writing history
        self.insert(block)
        // write canonical receipts and transactions
        PutTransactions(self.chainDb, block, block.Transactions())
        PutReceipts(self.chainDb, GetBlockReceipts(self.chainDb, block.Hash()))

    }
    self.mu.Unlock()

    return nil
}

func (self *ChainManager) update() {
    events := self.eventMux.Subscribe(queueEvent{})
    futureTimer := time.Tick(5 * time.Second)
out:
    for {
        select {
        case ev := <-events.Chan():
            switch ev := ev.(type) {
            case queueEvent:
                for _, event := range ev.queue {
                    switch event := event.(type) {
                    case ChainEvent:
                        // We need some control over the mining operation. Acquiring locks and waiting for the miner to create new block takes too long
                        // and in most cases isn't even necessary.
                        if self.lastBlockHash == event.Hash {
                            self.currentGasLimit = CalcGasLimit(event.Block)
                            self.eventMux.Post(ChainHeadEvent{event.Block})
                        }
                    }

                    self.eventMux.Post(event)
                }
            }
        case <-futureTimer:
            self.procFutureBlocks()
        case <-self.quit:
            break out
        }
    }
}

func blockErr(block *types.Block, err error) {
    h := block.Header()
    glog.V(logger.Error).Infof("Bad block #%v (%x)\n", h.Number, h.Hash().Bytes())
    glog.V(logger.Error).Infoln(err)
    glog.V(logger.Debug).Infoln(verifyNonces)
}

type nonceResult struct {
    i     int
    valid bool
}

// block verifies nonces of the given blocks in parallel and returns
// an error if one of the blocks nonce verifications failed.
func verifyNonces(pow pow.PoW, blocks []*types.Block, quit <-chan struct{}, done chan<- nonceResult) {
    // Spawn a few workers. They listen for blocks on the in channel
    // and send results on done. The workers will exit in the
    // background when in is closed.
    var (
        in       = make(chan int)
        nworkers = runtime.GOMAXPROCS(0)
    )
    defer close(in)
    if len(blocks) < nworkers {
        nworkers = len(blocks)
    }
    for i := 0; i < nworkers; i++ {
        go func() {
            for i := range in {
                done <- nonceResult{i: i, valid: pow.Verify(blocks[i])}
            }
        }()
    }
    // Feed block indices to the workers.
    for i := range blocks {
        select {
        case in <- i:
            continue
        case <-quit:
            return
        }
    }
}