path: root/core/chain_manager.go

package core

import (
    "bytes"
    "fmt"
    "io"
    "math/big"
    "os"
    "runtime"
    "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/logger"
    "github.com/ethereum/go-ethereum/logger/glog"
    "github.com/ethereum/go-ethereum/params"
    "github.com/ethereum/go-ethereum/pow"
    "github.com/ethereum/go-ethereum/rlp"
)

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

    blockHashPre = []byte("block-hash-")
    blockNumPre  = []byte("block-num-")
)

const (
    blockCacheLimit     = 10000
    maxFutureBlocks     = 256
    maxTimeFutureBlocks = 30
)

func CalcDifficulty(block, parent *types.Header) *big.Int {
    diff := new(big.Int)

    adjust := new(big.Int).Div(parent.Difficulty, params.DifficultyBoundDivisor)
    if big.NewInt(int64(block.Time)-int64(parent.Time)).Cmp(params.DurationLimit) < 0 {
        diff.Add(parent.Difficulty, adjust)
    } else {
        diff.Sub(parent.Difficulty, adjust)
    }

    if diff.Cmp(params.MinimumDifficulty) < 0 {
        return params.MinimumDifficulty
    }

    return diff
}

func CalcTD(block, parent *types.Block) *big.Int {
    if parent == nil {
        return block.Difficulty()
    }

    td := new(big.Int).Add(parent.Td, block.Header().Difficulty)

    return td
}

func CalcGasLimit(parent *types.Block) *big.Int {
    decay := new(big.Int).Div(parent.GasLimit(), params.GasLimitBoundDivisor)
    contrib := new(big.Int).Mul(parent.GasUsed(), big.NewInt(3))
    contrib = contrib.Div(contrib, big.NewInt(2))
    contrib = contrib.Div(contrib, params.GasLimitBoundDivisor)

    gl := new(big.Int).Sub(parent.GasLimit(), decay)
    gl = gl.Add(gl, contrib)
    gl = gl.Add(gl, big.NewInt(1))
    gl = common.BigMax(gl, params.MinGasLimit)

    if gl.Cmp(params.GenesisGasLimit) < 0 {
        gl2 := new(big.Int).Add(parent.GasLimit(), decay)
        return common.BigMin(params.GenesisGasLimit, gl2)
    }
    return gl
}

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

    td              *big.Int
    currentBlock    *types.Block
    lastBlockHash   common.Hash
    currentGasLimit *big.Int

    transState *state.StateDB
    txState    *state.ManagedState

    cache        *BlockCache
    futureBlocks *BlockCache

    quit chan struct{}
    wg   sync.WaitGroup

    pow pow.PoW
}

func NewChainManager(blockDb, stateDb common.Database, pow pow.PoW, mux *event.TypeMux) *ChainManager {
    bc := &ChainManager{
        blockDb:      blockDb,
        stateDb:      stateDb,
        genesisBlock: GenesisBlock(stateDb),
        eventMux:     mux,
        quit:         make(chan struct{}),
        cache:        NewBlockCache(blockCacheLimit),
        pow:          pow,
    }
    bc.setLastState()

    // 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")
        }
    }

    bc.transState = bc.State().Copy()
    // Take ownership of this particular state
    bc.txState = state.ManageState(bc.State().Copy())

    bc.futureBlocks = NewBlockCache(maxFutureBlocks)
    bc.makeCache()

    go bc.update()

    return bc
}

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.Header().ParentHash) {
        bc.removeBlock(block)
    }

    bc.cache = NewBlockCache(blockCacheLimit)
    bc.currentBlock = head
    bc.makeCache()

    statedb := state.New(head.Root(), bc.stateDb)
    bc.txState = state.ManageState(statedb)
    bc.transState = statedb.Copy()
    bc.setTotalDifficulty(head.Td)
    bc.insert(head)
    bc.setLastState()
}

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

    return self.td
}

func (self *ChainManager) GasLimit() *big.Int {
    // return self.currentGasLimit
    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 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.stateDb)
}

func (self *ChainManager) TransState() *state.StateDB {
    self.tsmu.RLock()
    defer self.tsmu.RUnlock()

    return self.transState
}

func (self *ChainManager) setTransState(statedb *state.StateDB) {
    self.transState = statedb
}

func (bc *ChainManager) setLastState() {
    data, _ := bc.blockDb.Get([]byte("LastBlock"))
    if len(data) != 0 {
        block := bc.GetBlock(common.BytesToHash(data))
        if block != nil {
            bc.currentBlock = block
            bc.lastBlockHash = block.Hash()
        } else { // TODO CLEAN THIS UP TMP CODE
            block = bc.GetBlockByNumber(400000)
            if block == nil {
                fmt.Println("Fatal. LastBlock not found. Report this issue")
                os.Exit(1)
            }
            bc.currentBlock = block
            bc.lastBlockHash = block.Hash()
            bc.insert(block)
        }
    } 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)
    }
}

func (bc *ChainManager) makeCache() {
    if bc.cache == nil {
        bc.cache = NewBlockCache(blockCacheLimit)
    }
    // load in last `blockCacheLimit` - 1 blocks. Last block is the current.
    ancestors := bc.GetAncestors(bc.currentBlock, blockCacheLimit-1)
    ancestors = append(ancestors, bc.currentBlock)
    for _, block := range ancestors {
        bc.cache.Push(block)
    }
}

// Block creation & chain handling
func (bc *ChainManager) NewBlock(coinbase common.Address) *types.Block {
    bc.mu.RLock()
    defer bc.mu.RUnlock()

    var (
        root       common.Hash
        parentHash common.Hash
    )

    if bc.currentBlock != nil {
        root = bc.currentBlock.Header().Root
        parentHash = bc.lastBlockHash
    }

    block := types.NewBlock(
        parentHash,
        coinbase,
        root,
        common.BigPow(2, 32),
        0,
        nil)
    block.SetUncles(nil)
    block.SetTransactions(nil)
    block.SetReceipts(nil)

    parent := bc.currentBlock
    if parent != nil {
        header := block.Header()
        header.Difficulty = CalcDifficulty(block.Header(), parent.Header())
        header.Number = new(big.Int).Add(parent.Header().Number, common.Big1)
        header.GasLimit = CalcGasLimit(parent)
    }

    return block
}

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

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

    if bc.cache == nil {
        bc.cache = NewBlockCache(blockCacheLimit)
    }

    // Prepare the genesis block
    bc.write(bc.genesisBlock)
    bc.insert(bc.genesisBlock)
    bc.currentBlock = bc.genesisBlock
    bc.makeCache()

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

func (bc *ChainManager) removeBlock(block *types.Block) {
    bc.blockDb.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.Header().ParentHash) {
        bc.removeBlock(block)
    }

    // Prepare the genesis block
    gb.Td = gb.Difficulty()
    bc.genesisBlock = gb
    bc.write(bc.genesisBlock)
    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 {
    self.mu.RLock()
    defer self.mu.RUnlock()
    glog.V(logger.Info).Infof("exporting %v blocks...\n", self.currentBlock.Header().Number)

    last := self.currentBlock.NumberU64()

    for nr := uint64(1); 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
}

func (bc *ChainManager) insert(block *types.Block) {
    key := append(blockNumPre, block.Number().Bytes()...)
    bc.blockDb.Put(key, block.Hash().Bytes())

    bc.blockDb.Put([]byte("LastBlock"), block.Hash().Bytes())
    bc.currentBlock = block
    bc.lastBlockHash = block.Hash()
}

func (bc *ChainManager) write(block *types.Block) {
    enc, _ := rlp.EncodeToBytes((*types.StorageBlock)(block))
    key := append(blockHashPre, block.Hash().Bytes()...)
    bc.blockDb.Put(key, enc)
    // Push block to cache
    bc.cache.Push(block)
}

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

// Block fetching methods
func (bc *ChainManager) HasBlock(hash common.Hash) bool {
    data, _ := bc.blockDb.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 := self.cache.Get(hash); block != nil {
            return block
        }
    */

    data, _ := self.blockDb.Get(append(blockHashPre, hash[:]...))
    if len(data) == 0 {
        return nil
    }
    var block types.StorageBlock
    if err := rlp.Decode(bytes.NewReader(data), &block); err != nil {
        glog.V(logger.Error).Infof("invalid block RLP for hash %x: %v", hash, err)
        return nil
    }
    return (*types.Block)(&block)
}

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

    return self.getBlockByNumber(num)

}

// non blocking version
func (self *ChainManager) getBlockByNumber(num uint64) *types.Block {
    key, _ := self.blockDb.Get(append(blockNumPre, big.NewInt(int64(num)).Bytes()...))
    if len(key) == 0 {
        return nil
    }

    return self.GetBlock(common.BytesToHash(key))
}

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
}

func (self *ChainManager) GetAncestors(block *types.Block, length int) (blocks []*types.Block) {
    for i := 0; i < length; i++ {
        block = self.GetBlock(block.ParentHash())
        if block == nil {
            break
        }

        blocks = append(blocks, block)
    }

    return
}

func (bc *ChainManager) setTotalDifficulty(td *big.Int) {
    //bc.blockDb.Put([]byte("LTD"), td.Bytes())
    bc.td = td
}

func (self *ChainManager) CalcTotalDiff(block *types.Block) (*big.Int, error) {
    parent := self.GetBlock(block.Header().ParentHash)
    if parent == nil {
        return nil, fmt.Errorf("Unable to calculate total diff without known parent %x", block.Header().ParentHash)
    }

    parentTd := parent.Td

    uncleDiff := new(big.Int)
    for _, uncle := range block.Uncles() {
        uncleDiff = uncleDiff.Add(uncleDiff, uncle.Difficulty)
    }

    td := new(big.Int)
    td = td.Add(parentTd, uncleDiff)
    td = td.Add(td, block.Header().Difficulty)

    return td, nil
}

func (bc *ChainManager) Stop() {
    close(bc.quit)

    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() {
    var blocks []*types.Block
    self.futureBlocks.Each(func(i int, block *types.Block) {
        blocks = append(blocks, block)
    })
    if len(blocks) > 0 {
        types.BlockBy(types.Number).Sort(blocks)
        self.InsertChain(blocks)
    }
}

// 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)

    for i, block := range chain {
        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[i]
                return i, ValidationError("Block (#%v / %x) nonce is invalid (= %x)", block.Number(), block.Hash(), 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, 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 := time.Now().Unix() + maxTimeFutureBlocks; block.Time() > max {
                    return i, fmt.Errorf("%v: BlockFutureErr, %v > %v", BlockFutureErr, block.Time(), max)
                }

                block.SetQueued(true)
                self.futureBlocks.Push(block)
                stats.queued++
                continue
            }

            if IsParentErr(err) && self.futureBlocks.Has(block.ParentHash()) {
                block.SetQueued(true)
                self.futureBlocks.Push(block)
                stats.queued++
                continue
            }

            blockErr(block, err)

            return i, err
        }

        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 i, err
                }

                queue[i] = ChainSplitEvent{block, logs}
                queueEvent.splitCount++
            }

            self.setTotalDifficulty(block.Td)
            self.insert(block)

            jsonlogger.LogJson(&logger.EthChainNewHead{
                BlockHash:     block.Hash().Hex(),
                BlockNumber:   block.Number(),
                ChainHeadHash: cblock.Hash().Hex(),
                BlockPrevHash: block.ParentHash().Hex(),
            })

            self.setTransState(state.New(block.Root(), self.stateDb))
            self.txState.SetState(state.New(block.Root(), self.stateDb))

            queue[i] = ChainEvent{block, block.Hash(), logs}
            queueEvent.canonicalCount++

            if glog.V(logger.Debug) {
                glog.Infof("[%v] inserted block #%d (%d TXs %d UNCs) (%x...). Took %v\n", time.Now().UnixNano(), block.Number(), len(block.Transactions()), len(block.Uncles()), block.Hash().Bytes()[0:4], time.Since(bstart))
            }
        } else {
            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++
        }
        // Write block to database. Eventually we'll have to improve on this and throw away blocks that are
        // not in the canonical chain.
        self.write(block)
        // Delete from future blocks
        self.futureBlocks.Delete(block.Hash())

        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) in %v. #%v [%x / %x]\n", stats.processed, stats.queued, stats.ignored, 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.Info) {
        commonHash := commonBlock.Hash()
        glog.Infof("Fork 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
    for _, block := range newChain {
        self.insert(block)
    }

    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
        }
    }
}