path: root/core/headerchain.go

// Copyright 2015 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 (
    crand "crypto/rand"
    "errors"
    "fmt"
    "math"
    "math/big"
    mrand "math/rand"
    "reflect"
    "sync/atomic"
    "time"

    dexCore "github.com/dexon-foundation/dexon-consensus/core"
    coreCrypto "github.com/dexon-foundation/dexon-consensus/core/crypto"
    coreTypes "github.com/dexon-foundation/dexon-consensus/core/types"
    lru "github.com/hashicorp/golang-lru"

    "github.com/dexon-foundation/dexon/common"
    "github.com/dexon-foundation/dexon/consensus"
    "github.com/dexon-foundation/dexon/consensus/dexcon"
    "github.com/dexon-foundation/dexon/core/rawdb"
    "github.com/dexon-foundation/dexon/core/types"
    "github.com/dexon-foundation/dexon/core/vm"
    "github.com/dexon-foundation/dexon/crypto"
    "github.com/dexon-foundation/dexon/ethdb"
    "github.com/dexon-foundation/dexon/log"
    "github.com/dexon-foundation/dexon/params"
    "github.com/dexon-foundation/dexon/rlp"
    "github.com/dexon-foundation/dexon/trie"
)

const (
    headerCacheLimit = 512
    tdCacheLimit     = 1024
    numberCacheLimit = 2048
)

// HeaderChain implements the basic block header chain logic that is shared by
// core.BlockChain and light.LightChain. It is not usable in itself, only as
// a part of either structure.
// It is not thread safe either, the encapsulating chain structures should do
// the necessary mutex locking/unlocking.
type HeaderChain struct {
    config *params.ChainConfig

    chainDb       ethdb.Database
    genesisHeader *types.Header

    currentHeader     atomic.Value // Current head of the header chain (may be above the block chain!)
    currentHeaderHash common.Hash  // Hash of the current head of the header chain (prevent recomputing all the time)

    headerCache *lru.Cache // Cache for the most recent block headers
    tdCache     *lru.Cache // Cache for the most recent block total difficulties
    numberCache *lru.Cache // Cache for the most recent block numbers

    procInterrupt func() bool

    rand   *mrand.Rand
    engine consensus.Engine
}

// NewHeaderChain creates a new HeaderChain structure.
//  getValidator should return the parent's validator
//  procInterrupt points to the parent's interrupt semaphore
//  wg points to the parent's shutdown wait group
func NewHeaderChain(chainDb ethdb.Database, config *params.ChainConfig, engine consensus.Engine, procInterrupt func() bool) (*HeaderChain, error) {
    headerCache, _ := lru.New(headerCacheLimit)
    tdCache, _ := lru.New(tdCacheLimit)
    numberCache, _ := lru.New(numberCacheLimit)

    // Seed a fast but crypto originating random generator
    seed, err := crand.Int(crand.Reader, big.NewInt(math.MaxInt64))
    if err != nil {
        return nil, err
    }

    hc := &HeaderChain{
        config:        config,
        chainDb:       chainDb,
        headerCache:   headerCache,
        tdCache:       tdCache,
        numberCache:   numberCache,
        procInterrupt: procInterrupt,
        rand:          mrand.New(mrand.NewSource(seed.Int64())),
        engine:        engine,
    }

    hc.genesisHeader = hc.GetHeaderByNumber(0)
    if hc.genesisHeader == nil {
        return nil, ErrNoGenesis
    }

    hc.currentHeader.Store(hc.genesisHeader)
    if head := rawdb.ReadHeadBlockHash(chainDb); head != (common.Hash{}) {
        if chead := hc.GetHeaderByHash(head); chead != nil {
            hc.currentHeader.Store(chead)
        }
    }
    hc.currentHeaderHash = hc.CurrentHeader().Hash()

    return hc, nil
}

// GetBlockNumber retrieves the block number belonging to the given hash
// from the cache or database
func (hc *HeaderChain) GetBlockNumber(hash common.Hash) *uint64 {
    if cached, ok := hc.numberCache.Get(hash); ok {
        number := cached.(uint64)
        return &number
    }
    number := rawdb.ReadHeaderNumber(hc.chainDb, hash)
    if number != nil {
        hc.numberCache.Add(hash, *number)
    }
    return number
}

// WriteHeader writes a header into the local chain, given that its parent is
// already known. If the total difficulty of the newly inserted header becomes
// greater than the current known TD, the canonical chain is re-routed.
//
// Note: This method is not concurrent-safe with inserting blocks simultaneously
// into the chain, as side effects caused by reorganisations cannot be emulated
// without the real blocks. Hence, writing headers directly should only be done
// in two scenarios: pure-header mode of operation (light clients), or properly
// separated header/block phases (non-archive clients).
func (hc *HeaderChain) WriteHeader(header *types.Header) (status WriteStatus, err error) {
    // Cache some values to prevent constant recalculation
    var (
        hash   = header.Hash()
        number = header.Number.Uint64()
    )
    // Calculate the total difficulty of the header
    ptd := hc.GetTd(header.ParentHash, number-1)
    if ptd == nil {
        return NonStatTy, consensus.ErrUnknownAncestor
    }
    localTd := hc.GetTd(hc.currentHeaderHash, hc.CurrentHeader().Number.Uint64())
    externTd := new(big.Int).Add(header.Difficulty, ptd)

    // Irrelevant of the canonical status, write the td and header to the database
    if err := hc.WriteTd(hash, number, externTd); err != nil {
        log.Crit("Failed to write header total difficulty", "err", err)
    }
    rawdb.WriteHeader(hc.chainDb, header)

    // If the total difficulty is higher than our known, add it to the canonical chain
    // Second clause in the if statement reduces the vulnerability to selfish mining.
    // Please refer to http://www.cs.cornell.edu/~ie53/publications/btcProcFC.pdf
    if externTd.Cmp(localTd) > 0 || (externTd.Cmp(localTd) == 0 && mrand.Float64() < 0.5) {
        // Delete any canonical number assignments above the new head
        batch := hc.chainDb.NewBatch()
        for i := number + 1; ; i++ {
            hash := rawdb.ReadCanonicalHash(hc.chainDb, i)
            if hash == (common.Hash{}) {
                break
            }
            rawdb.DeleteCanonicalHash(batch, i)
        }
        batch.Write()

        // Overwrite any stale canonical number assignments
        var (
            headHash   = header.ParentHash
            headNumber = header.Number.Uint64() - 1
            headHeader = hc.GetHeader(headHash, headNumber)
        )
        for rawdb.ReadCanonicalHash(hc.chainDb, headNumber) != headHash {
            rawdb.WriteCanonicalHash(hc.chainDb, headHash, headNumber)

            headHash = headHeader.ParentHash
            headNumber = headHeader.Number.Uint64() - 1
            headHeader = hc.GetHeader(headHash, headNumber)
        }
        // Extend the canonical chain with the new header
        rawdb.WriteCanonicalHash(hc.chainDb, hash, number)
        rawdb.WriteHeadHeaderHash(hc.chainDb, hash)

        hc.currentHeaderHash = hash
        hc.currentHeader.Store(types.CopyHeader(header))

        status = CanonStatTy
    } else {
        status = SideStatTy
    }

    hc.headerCache.Add(hash, header)
    hc.numberCache.Add(hash, number)

    return
}

// WhCallback is a callback function for inserting individual headers.
// A callback is used for two reasons: first, in a LightChain, status should be
// processed and light chain events sent, while in a BlockChain this is not
// necessary since chain events are sent after inserting blocks. Second, the
// header writes should be protected by the parent chain mutex individually.
type WhCallback func(*types.Header) error

func (hc *HeaderChain) ValidateHeaderChain(chain []*types.Header, checkFreq int) (int, error) {
    // Do a sanity check that the provided chain is actually ordered and linked
    for i := 1; i < len(chain); i++ {
        if chain[i].Number.Uint64() != chain[i-1].Number.Uint64()+1 || chain[i].ParentHash != chain[i-1].Hash() {
            // Chain broke ancestry, log a message (programming error) and skip insertion
            log.Error("Non contiguous header insert", "number", chain[i].Number, "hash", chain[i].Hash(),
                "parent", chain[i].ParentHash, "prevnumber", chain[i-1].Number, "prevhash", chain[i-1].Hash())

            return 0, fmt.Errorf("non contiguous insert: item %d is #%d [%x…], item %d is #%d [%x…] (parent [%x…])", i-1, chain[i-1].Number,
                chain[i-1].Hash().Bytes()[:4], i, chain[i].Number, chain[i].Hash().Bytes()[:4], chain[i].ParentHash[:4])
        }
    }

    // Generate the list of seal verification requests, and start the parallel verifier
    seals := make([]bool, len(chain))
    for i := 0; i < len(seals)/checkFreq; i++ {
        index := i*checkFreq + hc.rand.Intn(checkFreq)
        if index >= len(seals) {
            index = len(seals) - 1
        }
        seals[index] = true
    }
    seals[len(seals)-1] = true // Last should always be verified to avoid junk

    abort, results := hc.engine.VerifyHeaders(hc, chain, seals)
    defer close(abort)

    // Iterate over the headers and ensure they all check out
    for i, header := range chain {
        // If the chain is terminating, stop processing blocks
        if hc.procInterrupt() {
            log.Debug("Premature abort during headers verification")
            return 0, errors.New("aborted")
        }
        // If the header is a banned one, straight out abort
        if BadHashes[header.Hash()] {
            return i, ErrBlacklistedHash
        }
        // Otherwise wait for headers checks and ensure they pass
        if err := <-results; err != nil {
            return i, err
        }
    }

    return 0, nil
}

// InsertHeaderChain attempts to insert the given header chain in to the local
// chain, possibly creating a reorg. If an error is returned, it will return the
// index number of the failing header as well an error describing what went wrong.
//
// The verify parameter can be used to fine tune whether nonce verification
// should be done or not. The reason behind the optional check is because some
// of the header retrieval mechanisms already need to verfy nonces, as well as
// because nonces can be verified sparsely, not needing to check each.
func (hc *HeaderChain) InsertHeaderChain(chain []*types.Header, writeHeader WhCallback, start time.Time) (int, error) {
    // Collect some import statistics to report on
    stats := struct{ processed, ignored int }{}
    // All headers passed verification, import them into the database
    for i, header := range chain {
        // Short circuit insertion if shutting down
        if hc.procInterrupt() {
            log.Debug("Premature abort during headers import")
            return i, errors.New("aborted")
        }
        // If the header's already known, skip it, otherwise store
        if hc.HasHeader(header.Hash(), header.Number.Uint64()) {
            stats.ignored++
            continue
        }
        if err := writeHeader(header); err != nil {
            return i, err
        }
        stats.processed++
    }
    // Report some public statistics so the user has a clue what's going on
    last := chain[len(chain)-1]

    context := []interface{}{
        "count", stats.processed, "elapsed", common.PrettyDuration(time.Since(start)),
        "number", last.Number, "hash", last.Hash(),
    }
    if timestamp := time.Unix(int64(last.Time), 0); time.Since(timestamp) > time.Minute {
        context = append(context, []interface{}{"age", common.PrettyAge(timestamp)}...)
    }
    if stats.ignored > 0 {
        context = append(context, []interface{}{"ignored", stats.ignored}...)
    }
    log.Info("Imported new block headers", context...)

    return 0, nil
}

func (hc *HeaderChain) WriteDexonHeader(header *types.HeaderWithGovState) (status WriteStatus, err error) {
    // Cache some values to prevent constant recalculation
    var (
        hash   = header.Hash()
        number = header.Number.Uint64()
    )
    // Calculate the total difficulty of the header
    ptd := hc.GetTd(header.ParentHash, number-1)
    if ptd == nil {
        return NonStatTy, consensus.ErrUnknownAncestor
    }
    localTd := hc.GetTd(hc.currentHeaderHash, hc.CurrentHeader().Number.Uint64())
    externTd := new(big.Int).Add(header.Difficulty, ptd)

    // Irrelevant of the canonical status, write the td and header to the database
    if err := hc.WriteTd(hash, number, externTd); err != nil {
        log.Crit("Failed to write header total difficulty", "err", err)
    }
    rawdb.WriteHeader(hc.chainDb, header.Header)

    // If the total difficulty is higher than our known, add it to the canonical chain
    // Second clause in the if statement reduces the vulnerability to selfish mining.
    // Please refer to http://www.cs.cornell.edu/~ie53/publications/btcProcFC.pdf
    if externTd.Cmp(localTd) > 0 || (externTd.Cmp(localTd) == 0 && mrand.Float64() < 0.5) {
        // Delete any canonical number assignments above the new head
        batch := hc.chainDb.NewBatch()
        for i := number + 1; ; i++ {
            hash := rawdb.ReadCanonicalHash(hc.chainDb, i)
            if hash == (common.Hash{}) {
                break
            }
            rawdb.DeleteCanonicalHash(batch, i)
        }
        batch.Write()

        // Overwrite any stale canonical number assignments
        var (
            headHash   = header.ParentHash
            headNumber = header.Number.Uint64() - 1
            headHeader = hc.GetHeader(headHash, headNumber)
        )
        for rawdb.ReadCanonicalHash(hc.chainDb, headNumber) != headHash {
            rawdb.WriteCanonicalHash(hc.chainDb, headHash, headNumber)

            headHash = headHeader.ParentHash
            headNumber = headHeader.Number.Uint64() - 1
            headHeader = hc.GetHeader(headHash, headNumber)
        }
        // Extend the canonical chain with the new header
        rawdb.WriteCanonicalHash(hc.chainDb, hash, number)
        rawdb.WriteHeadHeaderHash(hc.chainDb, hash)

        hc.currentHeaderHash = hash
        hc.currentHeader.Store(types.CopyHeader(header.Header))

        status = CanonStatTy
    } else {
        status = SideStatTy
    }

    hc.headerCache.Add(hash, header.Header)
    hc.numberCache.Add(hash, number)

    // Store the govState
    if govState := header.GovState; govState != nil {
        batch := hc.chainDb.NewBatch()
        for _, node := range govState.Proof {
            batch.Put(crypto.Keccak256(node), node)
        }
        if err := batch.Write(); err != nil {
            panic(fmt.Errorf("DB write error: %v", err))
        }

        triedb := trie.NewDatabase(hc.chainDb)
        t, err := trie.New(common.Hash{}, triedb)
        if err != nil {
            panic(err)
        }

        for _, kv := range govState.Storage {
            t.TryUpdate(kv[0], kv[1])
        }
        t.Commit(nil)
        triedb.Commit(t.Hash(), false)
    }
    return
}

type Wh2Callback func(*types.HeaderWithGovState) error

type headerVerifierCache struct {
    verifierCache  *dexCore.TSigVerifierCache
    gov            dexcon.GovernanceStateFetcher
    stateCache     *lru.Cache
    nodeOwnerCache *lru.Cache
    configCache    *lru.Cache
}

func newHeaderVerifierCache(
    verifierCache *dexCore.TSigVerifierCache,
    gov dexcon.GovernanceStateFetcher) *headerVerifierCache {
    stateCache, _ := lru.New(5)
    nodeOwnerCache, _ := lru.New(5)
    configCache, _ := lru.New(5)
    return &headerVerifierCache{
        verifierCache:  verifierCache,
        gov:            gov,
        stateCache:     stateCache,
        nodeOwnerCache: nodeOwnerCache,
        configCache:    configCache,
    }
}

func (c *headerVerifierCache) state(round uint64) *vm.GovernanceState {
    if state, exist := c.stateCache.Get(round); exist {
        return state.(*vm.GovernanceState)
    }
    state := c.gov.GetStateForConfigAtRound(round)
    c.stateCache.Add(round, state)
    return state
}

func (c *headerVerifierCache) getNodeOwnerByID(round uint64, ID coreTypes.NodeID) (
    common.Address, error) {
    nodeOwner, exist := c.nodeOwnerCache.Get(round)
    if !exist {
        nodeOwner = make(map[coreTypes.NodeID]interface{})
        c.nodeOwnerCache.Add(round, nodeOwner)
    }
    nodeOwnerMap := nodeOwner.(map[coreTypes.NodeID]interface{})
    if owner, exist := nodeOwnerMap[ID]; exist {
        if addr, ok := owner.(common.Address); ok {
            return addr, nil
        }
        return common.Address{}, owner.(error)
    }
    node, err := c.state(round).GetNodeByID(ID)
    if err != nil {
        nodeOwnerMap[ID] = err
        return common.Address{}, err
    }
    nodeOwnerMap[ID] = node.Owner
    return node.Owner, nil

}

func (c *headerVerifierCache) configuration(round uint64) *params.DexconConfig {
    if cfg, exist := c.configCache.Get(round); exist {
        return cfg.(*params.DexconConfig)
    }
    cfg := c.state(round).Configuration()
    c.configCache.Add(round, cfg)
    return cfg
}

func (hc *HeaderChain) ValidateDexonHeaderChain(chain []*types.HeaderWithGovState,
    gov dexcon.GovernanceStateFetcher,
    verifierCache *dexCore.TSigVerifierCache, validator Validator) (int, error) {
    // Do a sanity check that the provided chain is actually ordered and linked
    for i := 1; i < len(chain); i++ {
        if chain[i].Number.Uint64() != chain[i-1].Number.Uint64()+1 || chain[i].ParentHash != chain[i-1].Hash() {
            // Chain broke ancestry, log a message (programming error) and skip insertion
            log.Error("Non contiguous header insert", "number", chain[i].Number, "hash", chain[i].Hash(),
                "parent", chain[i].ParentHash, "prevnumber", chain[i-1].Number, "prevhash", chain[i-1].Hash())

            return 0, fmt.Errorf("non contiguous insert: item %d is #%d [%x…], item %d is #%d [%x…] (parent [%x…])", i-1, chain[i-1].Number,
                chain[i-1].Hash().Bytes()[:4], i, chain[i].Number, chain[i].Hash().Bytes()[:4], chain[i].ParentHash[:4])
        }
    }

    // If the last TSig pass the verification, we don't need to verify others.
    cache := newHeaderVerifierCache(verifierCache, gov)
    verifyTSig := false
    if err := hc.verifyDexonHeader(chain[len(chain)-1].Header, gov, cache, true); err != nil {
        verifyTSig = true
    }
    // Iterate over the headers and ensure they all check out
    for i, header := range chain {
        // If the chain is terminating, stop processing blocks
        if hc.procInterrupt() {
            log.Debug("Premature abort during headers verification")
            return 0, errors.New("aborted")
        }

        if i == 0 {
            log.Debug("validate header chain", "parent", header.ParentHash.String(), "number", header.Number.Uint64()-1)
            if parent := hc.GetHeader(header.ParentHash, header.Number.Uint64()-1); parent == nil {
                return 0, consensus.ErrUnknownAncestor
            }
        }

        if err := hc.verifyDexonHeader(header.Header, gov, cache, verifyTSig); err != nil {
            return i, err
        }

        // Verify witness
        var coreBlock coreTypes.Block
        if err := rlp.DecodeBytes(header.DexconMeta, &coreBlock); err != nil {
            return i, err
        }

        if !coreBlock.IsEmpty() {
            var witnessBlockHash common.Hash
            if err := rlp.DecodeBytes(coreBlock.Witness.Data, &witnessBlockHash); err != nil {
                log.Error("decode witness data fail", "err", err)
                return i, err
            }

            index := int64(coreBlock.Witness.Height) - int64(chain[0].Number.Uint64())
            if index < 0 {
                if err := validator.ValidateWitnessData(
                    coreBlock.Witness.Height, witnessBlockHash); err != nil {
                    return i, err
                }
            } else {
                if witnessBlockHash != chain[index].Hash() {
                    return i, consensus.ErrWitnessMismatch
                }
            }
        }
    }
    return 0, nil
}

func (hc *HeaderChain) VerifyDexonHeader(header *types.Header,
    gov dexcon.GovernanceStateFetcher,
    verifierCache *dexCore.TSigVerifierCache, validator Validator) error {

    if parent := hc.GetHeader(header.ParentHash, header.Number.Uint64()-1); parent == nil {
        return consensus.ErrUnknownAncestor
    }
    cache := newHeaderVerifierCache(verifierCache, gov)
    if err := hc.verifyDexonHeader(header, gov, cache, true); err != nil {
        return err
    }

    // Verify witness
    var coreBlock coreTypes.Block
    if err := rlp.DecodeBytes(header.DexconMeta, &coreBlock); err != nil {
        return err
    }

    if !coreBlock.IsEmpty() {
        var witnessBlockHash common.Hash
        if err := rlp.DecodeBytes(coreBlock.Witness.Data, &witnessBlockHash); err != nil {
            log.Error("decode witness data fail", "err", err)
            return err
        }

        if err := validator.ValidateWitnessData(
            coreBlock.Witness.Height, witnessBlockHash); err != nil {
            return err
        }
    }
    return nil
}

func (hc *HeaderChain) verifyDexonHeader(header *types.Header,
    gov dexcon.GovernanceStateFetcher,
    cache *headerVerifierCache, verifyTSig bool) error {

    // If the header is a banned one, straight out abort
    if BadHashes[header.Hash()] {
        return ErrBlacklistedHash
    }

    // Difficulty should always be 1.
    if header.Difficulty.Cmp(big.NewInt(1)) != 0 {
        return fmt.Errorf("difficulty should be 1")
    }

    // Verify fields that should be same as dexcon meta.
    var coreBlock coreTypes.Block
    if err := rlp.DecodeBytes(header.DexconMeta, &coreBlock); err != nil {
        return fmt.Errorf("decode dexcon meta fail, number=%d, err=%v",
            header.Number.Uint64(), err)
    }

    if verifyTSig {
        if err := hc.verifyTSig(&coreBlock, cache.verifierCache); err != nil {
            log.Debug("verify header sig fail, number=%d, err=%v",
                header.Number.Uint64(), err)
        }
    }

    if coreBlock.IsEmpty() {
        if header.Coinbase != (common.Address{}) {
            return fmt.Errorf("coinbase should be nil for empty block")
        }
    } else {
        owner, err := cache.getNodeOwnerByID(header.Round, coreBlock.ProposerID)
        if err != nil {
            return err
        }

        if header.Coinbase != owner {
            return fmt.Errorf("coinbase mismatch")
        }
    }

    if header.Time != uint64(coreBlock.Timestamp.UnixNano()/1000000) {
        return fmt.Errorf("timestamp mismatch")
    }

    if header.Number.Uint64() != coreBlock.Position.Height {
        return fmt.Errorf("height mismatch")
    }

    if !reflect.DeepEqual(header.Randomness, coreBlock.Randomness) {
        return fmt.Errorf("randomness mismatch")
    }

    if header.Round != coreBlock.Position.Round {
        return fmt.Errorf("round mismatch")
    }

    config := cache.configuration(header.Round)
    if header.GasLimit != config.BlockGasLimit {
        return fmt.Errorf("block gas limit mismatch")
    }
    return nil
}

func (hc *HeaderChain) verifyTSig(coreBlock *coreTypes.Block,
    verifierCache *dexCore.TSigVerifierCache) error {

    round := coreBlock.Position.Round
    randomness := coreBlock.Randomness

    if round == 0 {
        return nil
    }

    // Verify threshold signature
    v, ok, err := verifierCache.UpdateAndGet(round)
    if err != nil {
        panic(err)
    }

    if !ok {
        panic(fmt.Errorf("DKG of round %d is not finished", round))
    }

    if !v.VerifySignature(coreBlock.Hash, coreCrypto.Signature{
        Type:      "bls",
        Signature: randomness}) {
        return fmt.Errorf("signature invalid")
    }
    return nil
}

// InsertDexonHeaderChain attempts to insert the given header chain in to the local
// chain, possibly creating a reorg. If an error is returned, it will return the
// index number of the failing header as well an error describing what went wrong.
//
// The verify parameter can be used to fine tune whether nonce verification
// should be done or not. The reason behind the optional check is because some
// of the header retrieval mechanisms already need to verfy nonces, as well as
// because nonces can be verified sparsely, not needing to check each.
func (hc *HeaderChain) InsertDexonHeaderChain(chain []*types.HeaderWithGovState, writeHeader Wh2Callback, start time.Time) (int, error) {
    // Collect some import statistics to report on
    stats := struct{ processed, ignored int }{}
    // All headers passed verification, import them into the database
    for i, header := range chain {
        // Short circuit insertion if shutting down
        if hc.procInterrupt() {
            log.Debug("Premature abort during headers import")
            return i, errors.New("aborted")
        }
        // If the header's already known, skip it, otherwise store
        if hc.HasHeader(header.Hash(), header.Number.Uint64()) {
            stats.ignored++
            continue
        }
        if err := writeHeader(header); err != nil {
            return i, err
        }
        stats.processed++
    }
    // Report some public statistics so the user has a clue what's going on
    last := chain[len(chain)-1]
    log.Info("Imported new block headers", "count", stats.processed, "elapsed", common.PrettyDuration(time.Since(start)),
        "number", last.Number, "hash", last.Hash(), "ignored", stats.ignored)

    return 0, nil
}

// GetBlockHashesFromHash retrieves a number of block hashes starting at a given
// hash, fetching towards the genesis block.
func (hc *HeaderChain) GetBlockHashesFromHash(hash common.Hash, max uint64) []common.Hash {
    // Get the origin header from which to fetch
    header := hc.GetHeaderByHash(hash)
    if header == nil {
        return nil
    }
    // Iterate the headers until enough is collected or the genesis reached
    chain := make([]common.Hash, 0, max)
    for i := uint64(0); i < max; i++ {
        next := header.ParentHash
        if header = hc.GetHeader(next, header.Number.Uint64()-1); header == nil {
            break
        }
        chain = append(chain, next)
        if header.Number.Sign() == 0 {
            break
        }
    }
    return chain
}

// GetAncestor retrieves the Nth ancestor of a given block. It assumes that either the given block or
// a close ancestor of it is canonical. maxNonCanonical points to a downwards counter limiting the
// number of blocks to be individually checked before we reach the canonical chain.
//
// Note: ancestor == 0 returns the same block, 1 returns its parent and so on.
func (hc *HeaderChain) GetAncestor(hash common.Hash, number, ancestor uint64, maxNonCanonical *uint64) (common.Hash, uint64) {
    if ancestor > number {
        return common.Hash{}, 0
    }
    if ancestor == 1 {
        // in this case it is cheaper to just read the header
        if header := hc.GetHeader(hash, number); header != nil {
            return header.ParentHash, number - 1
        } else {
            return common.Hash{}, 0
        }
    }
    for ancestor != 0 {
        if rawdb.ReadCanonicalHash(hc.chainDb, number) == hash {
            number -= ancestor
            return rawdb.ReadCanonicalHash(hc.chainDb, number), number
        }
        if *maxNonCanonical == 0 {
            return common.Hash{}, 0
        }
        *maxNonCanonical--
        ancestor--
        header := hc.GetHeader(hash, number)
        if header == nil {
            return common.Hash{}, 0
        }
        hash = header.ParentHash
        number--
    }
    return hash, number
}

// GetTd retrieves a block's total difficulty in the canonical chain from the
// database by hash and number, caching it if found.
func (hc *HeaderChain) GetTd(hash common.Hash, number uint64) *big.Int {
    // Short circuit if the td's already in the cache, retrieve otherwise
    if cached, ok := hc.tdCache.Get(hash); ok {
        return cached.(*big.Int)
    }
    td := rawdb.ReadTd(hc.chainDb, hash, number)
    if td == nil {
        return nil
    }
    // Cache the found body for next time and return
    hc.tdCache.Add(hash, td)
    return td
}

// GetTdByHash retrieves a block's total difficulty in the canonical chain from the
// database by hash, caching it if found.
func (hc *HeaderChain) GetTdByHash(hash common.Hash) *big.Int {
    number := hc.GetBlockNumber(hash)
    if number == nil {
        return nil
    }
    return hc.GetTd(hash, *number)
}

// WriteTd stores a block's total difficulty into the database, also caching it
// along the way.
func (hc *HeaderChain) WriteTd(hash common.Hash, number uint64, td *big.Int) error {
    rawdb.WriteTd(hc.chainDb, hash, number, td)
    hc.tdCache.Add(hash, new(big.Int).Set(td))
    return nil
}

// GetHeader retrieves a block header from the database by hash and number,
// caching it if found.
func (hc *HeaderChain) GetHeader(hash common.Hash, number uint64) *types.Header {
    // Short circuit if the header's already in the cache, retrieve otherwise
    if header, ok := hc.headerCache.Get(hash); ok {
        return header.(*types.Header)
    }
    header := rawdb.ReadHeader(hc.chainDb, hash, number)
    if header == nil {
        return nil
    }
    // Cache the found header for next time and return
    hc.headerCache.Add(hash, header)
    return header
}

// GetHeaderByHash retrieves a block header from the database by hash, caching it if
// found.
func (hc *HeaderChain) GetHeaderByHash(hash common.Hash) *types.Header {
    number := hc.GetBlockNumber(hash)
    if number == nil {
        return nil
    }
    return hc.GetHeader(hash, *number)
}

// HasHeader checks if a block header is present in the database or not.
func (hc *HeaderChain) HasHeader(hash common.Hash, number uint64) bool {
    if hc.numberCache.Contains(hash) || hc.headerCache.Contains(hash) {
        return true
    }
    return rawdb.HasHeader(hc.chainDb, hash, number)
}

// GetHeaderByNumber retrieves a block header from the database by number,
// caching it (associated with its hash) if found.
func (hc *HeaderChain) GetHeaderByNumber(number uint64) *types.Header {
    hash := rawdb.ReadCanonicalHash(hc.chainDb, number)
    if hash == (common.Hash{}) {
        return nil
    }
    return hc.GetHeader(hash, number)
}

// CurrentHeader retrieves the current head header of the canonical chain. The
// header is retrieved from the HeaderChain's internal cache.
func (hc *HeaderChain) CurrentHeader() *types.Header {
    return hc.currentHeader.Load().(*types.Header)
}

// SetCurrentHeader sets the current head header of the canonical chain.
func (hc *HeaderChain) SetCurrentHeader(head *types.Header) {
    rawdb.WriteHeadHeaderHash(hc.chainDb, head.Hash())

    hc.currentHeader.Store(head)
    hc.currentHeaderHash = head.Hash()
}

// DeleteCallback is a callback function that is called by SetHead before
// each header is deleted.
type DeleteCallback func(rawdb.DatabaseDeleter, common.Hash, uint64)

// SetHead rewinds the local chain to a new head. Everything above the new head
// will be deleted and the new one set.
func (hc *HeaderChain) SetHead(head uint64, delFn DeleteCallback) {
    height := uint64(0)

    if hdr := hc.CurrentHeader(); hdr != nil {
        height = hdr.Number.Uint64()
    }
    batch := hc.chainDb.NewBatch()
    for hdr := hc.CurrentHeader(); hdr != nil && hdr.Number.Uint64() > head; hdr = hc.CurrentHeader() {
        hash := hdr.Hash()
        num := hdr.Number.Uint64()
        if delFn != nil {
            delFn(batch, hash, num)
        }
        rawdb.DeleteHeader(batch, hash, num)
        rawdb.DeleteTd(batch, hash, num)

        hc.currentHeader.Store(hc.GetHeader(hdr.ParentHash, hdr.Number.Uint64()-1))
    }
    // Roll back the canonical chain numbering
    for i := height; i > head; i-- {
        rawdb.DeleteCanonicalHash(batch, i)
    }
    batch.Write()

    // Clear out any stale content from the caches
    hc.headerCache.Purge()
    hc.tdCache.Purge()
    hc.numberCache.Purge()

    if hc.CurrentHeader() == nil {
        hc.currentHeader.Store(hc.genesisHeader)
    }
    hc.currentHeaderHash = hc.CurrentHeader().Hash()

    rawdb.WriteHeadHeaderHash(hc.chainDb, hc.currentHeaderHash)
}

// SetGenesis sets a new genesis block header for the chain
func (hc *HeaderChain) SetGenesis(head *types.Header) {
    hc.genesisHeader = head
}

// Config retrieves the header chain's chain configuration.
func (hc *HeaderChain) Config() *params.ChainConfig { return hc.config }

// Engine retrieves the header chain's consensus engine.
func (hc *HeaderChain) Engine() consensus.Engine { return hc.engine }

// GetBlock implements consensus.ChainReader, and returns nil for every input as
// a header chain does not have blocks available for retrieval.
func (hc *HeaderChain) GetBlock(hash common.Hash, number uint64) *types.Block {
    return nil
}