path: root/eth/downloader/downloader.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 downloader contains the manual full chain synchronisation.
package downloader

import (
    "errors"
    "fmt"
    "math"
    "math/big"
    "strings"
    "sync"
    "sync/atomic"
    "time"

    "github.com/ethereum/go-ethereum/common"
    "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"
)

var (
    MaxHashFetch    = 512 // Amount of hashes to be fetched per retrieval request
    MaxBlockFetch   = 128 // Amount of blocks to be fetched per retrieval request
    MaxHeaderFetch  = 192 // Amount of block headers to be fetched per retrieval request
    MaxBodyFetch    = 128 // Amount of block bodies to be fetched per retrieval request
    MaxStateFetch   = 384 // Amount of node state values to allow fetching per request
    MaxReceiptFetch = 256 // Amount of transaction receipts to allow fetching per request

    hashTTL        = 5 * time.Second    // [eth/61] Time it takes for a hash request to time out
    blockSoftTTL   = 3 * time.Second    // [eth/61] Request completion threshold for increasing or decreasing a peer's bandwidth
    blockHardTTL   = 3 * blockSoftTTL   // [eth/61] Maximum time allowance before a block request is considered expired
    headerTTL      = 5 * time.Second    // [eth/62] Time it takes for a header request to time out
    bodySoftTTL    = 3 * time.Second    // [eth/62] Request completion threshold for increasing or decreasing a peer's bandwidth
    bodyHardTTL    = 3 * bodySoftTTL    // [eth/62] Maximum time allowance before a block body request is considered expired
    receiptSoftTTL = 3 * time.Second    // [eth/63] Request completion threshold for increasing or decreasing a peer's bandwidth
    receiptHardTTL = 3 * receiptSoftTTL // [eth/63] Maximum time allowance before a block body request is considered expired

    maxQueuedHashes   = 256 * 1024 // [eth/61] Maximum number of hashes to queue for import (DOS protection)
    maxQueuedHeaders  = 256 * 1024 // [eth/62] Maximum number of headers to queue for import (DOS protection)
    maxResultsProcess = 256        // Number of download results to import at once into the chain

    headerCheckFrequency = 64   // Verification frequency of the downloaded headers during fast sync
    minCheckedHeaders    = 1024 // Number of headers to verify fully when approaching the chain head
    minFullBlocks        = 1024 // Number of blocks to retrieve fully even in fast sync
)

var (
    errBusy               = errors.New("busy")
    errUnknownPeer        = errors.New("peer is unknown or unhealthy")
    errBadPeer            = errors.New("action from bad peer ignored")
    errStallingPeer       = errors.New("peer is stalling")
    errNoPeers            = errors.New("no peers to keep download active")
    errPendingQueue       = errors.New("pending items in queue")
    errTimeout            = errors.New("timeout")
    errEmptyHashSet       = errors.New("empty hash set by peer")
    errEmptyHeaderSet     = errors.New("empty header set by peer")
    errPeersUnavailable   = errors.New("no peers available or all tried for download")
    errAlreadyInPool      = errors.New("hash already in pool")
    errInvalidChain       = errors.New("retrieved hash chain is invalid")
    errInvalidBlock       = errors.New("retrieved block is invalid")
    errInvalidBody        = errors.New("retrieved block body is invalid")
    errInvalidReceipt     = errors.New("retrieved receipt is invalid")
    errCancelHashFetch    = errors.New("hash download canceled (requested)")
    errCancelBlockFetch   = errors.New("block download canceled (requested)")
    errCancelHeaderFetch  = errors.New("block header download canceled (requested)")
    errCancelBodyFetch    = errors.New("block body download canceled (requested)")
    errCancelReceiptFetch = errors.New("receipt download canceled (requested)")
    errNoSyncActive       = errors.New("no sync active")
)

// headerCheckFn is a callback type for verifying a header's presence in the local chain.
type headerCheckFn func(common.Hash) bool

// blockCheckFn is a callback type for verifying a block's presence in the local chain.
type blockCheckFn func(common.Hash) bool

// headerRetrievalFn is a callback type for retrieving a header from the local chain.
type headerRetrievalFn func(common.Hash) *types.Header

// blockRetrievalFn is a callback type for retrieving a block from the local chain.
type blockRetrievalFn func(common.Hash) *types.Block

// headHeaderRetrievalFn is a callback type for retrieving the head header from the local chain.
type headHeaderRetrievalFn func() *types.Header

// headBlockRetrievalFn is a callback type for retrieving the head block from the local chain.
type headBlockRetrievalFn func() *types.Block

// tdRetrievalFn is a callback type for retrieving the total difficulty of a local block.
type tdRetrievalFn func(common.Hash) *big.Int

// headerChainInsertFn is a callback type to insert a batch of headers into the local chain.
type headerChainInsertFn func([]*types.Header, bool) (int, error)

// blockChainInsertFn is a callback type to insert a batch of blocks into the local chain.
type blockChainInsertFn func(types.Blocks) (int, error)

// receiptChainInsertFn is a callback type to insert a batch of receipts into the local chain.
type receiptChainInsertFn func(types.Blocks, []types.Receipts) (int, error)

// peerDropFn is a callback type for dropping a peer detected as malicious.
type peerDropFn func(id string)

// dataPack is a data message returned by a peer for some query.
type dataPack interface {
    PeerId() string
    Empty() bool
    Stats() string
}

// hashPack is a batch of block hashes returned by a peer (eth/61).
type hashPack struct {
    peerId string
    hashes []common.Hash
}

// blockPack is a batch of blocks returned by a peer (eth/61).
type blockPack struct {
    peerId string
    blocks []*types.Block
}

// headerPack is a batch of block headers returned by a peer.
type headerPack struct {
    peerId  string
    headers []*types.Header
}

// bodyPack is a batch of block bodies returned by a peer.
type bodyPack struct {
    peerId       string
    transactions [][]*types.Transaction
    uncles       [][]*types.Header
}

// PeerId retrieves the origin peer who sent this block body packet.
func (p *bodyPack) PeerId() string { return p.peerId }

// Empty returns whether the no block bodies were delivered.
func (p *bodyPack) Empty() bool { return len(p.transactions) == 0 || len(p.uncles) == 0 }

// Stats creates a textual stats report for logging purposes.
func (p *bodyPack) Stats() string { return fmt.Sprintf("%d:%d", len(p.transactions), len(p.uncles)) }

// receiptPack is a batch of receipts returned by a peer.
type receiptPack struct {
    peerId   string
    receipts [][]*types.Receipt
}

// PeerId retrieves the origin peer who sent this receipt packet.
func (p *receiptPack) PeerId() string { return p.peerId }

// Empty returns whether the no receipts were delivered.
func (p *receiptPack) Empty() bool { return len(p.receipts) == 0 }

// Stats creates a textual stats report for logging purposes.
func (p *receiptPack) Stats() string { return fmt.Sprintf("%d", len(p.receipts)) }

type Downloader struct {
    mode SyncMode       // Synchronisation mode defining the strategies used
    mux  *event.TypeMux // Event multiplexer to announce sync operation events

    queue *queue   // Scheduler for selecting the hashes to download
    peers *peerSet // Set of active peers from which download can proceed

    interrupt int32 // Atomic boolean to signal termination

    // Statistics
    syncStatsOrigin uint64       // Origin block number where syncing started at
    syncStatsHeight uint64       // Highest block number known when syncing started
    syncStatsLock   sync.RWMutex // Lock protecting the sync stats fields

    // Callbacks
    hasHeader      headerCheckFn         // Checks if a header is present in the chain
    hasBlock       blockCheckFn          // Checks if a block is present in the chain
    getHeader      headerRetrievalFn     // Retrieves a header from the chain
    getBlock       blockRetrievalFn      // Retrieves a block from the chain
    headHeader     headHeaderRetrievalFn // Retrieves the head header from the chain
    headBlock      headBlockRetrievalFn  // Retrieves the head block from the chain
    getTd          tdRetrievalFn         // Retrieves the TD of a block from the chain
    insertHeaders  headerChainInsertFn   // Injects a batch of headers into the chain
    insertBlocks   blockChainInsertFn    // Injects a batch of blocks into the chain
    insertReceipts receiptChainInsertFn  // Injects a batch of blocks and their receipts into the chain
    dropPeer       peerDropFn            // Drops a peer for misbehaving

    // Status
    synchroniseMock func(id string, hash common.Hash) error // Replacement for synchronise during testing
    synchronising   int32
    processing      int32
    notified        int32

    // Channels
    newPeerCh     chan *peer
    hashCh        chan hashPack   // [eth/61] Channel receiving inbound hashes
    blockCh       chan blockPack  // [eth/61] Channel receiving inbound blocks
    headerCh      chan headerPack // [eth/62] Channel receiving inbound block headers
    bodyCh        chan dataPack   // [eth/62] Channel receiving inbound block bodies
    receiptCh     chan dataPack   // [eth/63] Channel receiving inbound receipts
    blockWakeCh   chan bool       // [eth/61] Channel to signal the block fetcher of new tasks
    bodyWakeCh    chan bool       // [eth/62] Channel to signal the block body fetcher of new tasks
    receiptWakeCh chan bool       // [eth/63] Channel to signal the receipt fetcher of new tasks

    cancelCh   chan struct{} // Channel to cancel mid-flight syncs
    cancelLock sync.RWMutex  // Lock to protect the cancel channel in delivers

    // Testing hooks
    syncInitHook     func(uint64, uint64)  // Method to call upon initiating a new sync run
    bodyFetchHook    func([]*types.Header) // Method to call upon starting a block body fetch
    receiptFetchHook func([]*types.Header) // Method to call upon starting a receipt fetch
    chainInsertHook  func([]*fetchResult)  // Method to call upon inserting a chain of blocks (possibly in multiple invocations)
}

// New creates a new downloader to fetch hashes and blocks from remote peers.
func New(mode SyncMode, mux *event.TypeMux, hasHeader headerCheckFn, hasBlock blockCheckFn, getHeader headerRetrievalFn, getBlock blockRetrievalFn,
    headHeader headHeaderRetrievalFn, headBlock headBlockRetrievalFn, getTd tdRetrievalFn, insertHeaders headerChainInsertFn, insertBlocks blockChainInsertFn,
    insertReceipts receiptChainInsertFn, dropPeer peerDropFn) *Downloader {

    return &Downloader{
        mode:           mode,
        mux:            mux,
        queue:          newQueue(),
        peers:          newPeerSet(),
        hasHeader:      hasHeader,
        hasBlock:       hasBlock,
        getHeader:      getHeader,
        getBlock:       getBlock,
        headHeader:     headHeader,
        headBlock:      headBlock,
        getTd:          getTd,
        insertHeaders:  insertHeaders,
        insertBlocks:   insertBlocks,
        insertReceipts: insertReceipts,
        dropPeer:       dropPeer,
        newPeerCh:      make(chan *peer, 1),
        hashCh:         make(chan hashPack, 1),
        blockCh:        make(chan blockPack, 1),
        headerCh:       make(chan headerPack, 1),
        bodyCh:         make(chan dataPack, 1),
        receiptCh:      make(chan dataPack, 1),
        blockWakeCh:    make(chan bool, 1),
        bodyWakeCh:     make(chan bool, 1),
        receiptWakeCh:  make(chan bool, 1),
    }
}

// Boundaries retrieves the synchronisation boundaries, specifically the origin
// block where synchronisation started at (may have failed/suspended) and the
// latest known block which the synchonisation targets.
func (d *Downloader) Boundaries() (uint64, uint64) {
    d.syncStatsLock.RLock()
    defer d.syncStatsLock.RUnlock()

    return d.syncStatsOrigin, d.syncStatsHeight
}

// Synchronising returns whether the downloader is currently retrieving blocks.
func (d *Downloader) Synchronising() bool {
    return atomic.LoadInt32(&d.synchronising) > 0
}

// RegisterPeer injects a new download peer into the set of block source to be
// used for fetching hashes and blocks from.
func (d *Downloader) RegisterPeer(id string, version int, head common.Hash,
    getRelHashes relativeHashFetcherFn, getAbsHashes absoluteHashFetcherFn, getBlocks blockFetcherFn, // eth/61 callbacks, remove when upgrading
    getRelHeaders relativeHeaderFetcherFn, getAbsHeaders absoluteHeaderFetcherFn, getBlockBodies blockBodyFetcherFn, getReceipts receiptFetcherFn) error {

    glog.V(logger.Detail).Infoln("Registering peer", id)
    if err := d.peers.Register(newPeer(id, version, head, getRelHashes, getAbsHashes, getBlocks, getRelHeaders, getAbsHeaders, getBlockBodies, getReceipts)); err != nil {
        glog.V(logger.Error).Infoln("Register failed:", err)
        return err
    }
    return nil
}

// UnregisterPeer remove a peer from the known list, preventing any action from
// the specified peer. An effort is also made to return any pending fetches into
// the queue.
func (d *Downloader) UnregisterPeer(id string) error {
    glog.V(logger.Detail).Infoln("Unregistering peer", id)
    if err := d.peers.Unregister(id); err != nil {
        glog.V(logger.Error).Infoln("Unregister failed:", err)
        return err
    }
    d.queue.Revoke(id)
    return nil
}

// Synchronise tries to sync up our local block chain with a remote peer, both
// adding various sanity checks as well as wrapping it with various log entries.
func (d *Downloader) Synchronise(id string, head common.Hash, td *big.Int) {
    glog.V(logger.Detail).Infof("Attempting synchronisation: %v, head [%x…], TD %v", id, head[:4], td)

    switch err := d.synchronise(id, head, td); err {
    case nil:
        glog.V(logger.Detail).Infof("Synchronisation completed")

    case errBusy:
        glog.V(logger.Detail).Infof("Synchronisation already in progress")

    case errTimeout, errBadPeer, errStallingPeer, errEmptyHashSet, errEmptyHeaderSet, errPeersUnavailable, errInvalidChain:
        glog.V(logger.Debug).Infof("Removing peer %v: %v", id, err)
        d.dropPeer(id)

    case errPendingQueue:
        glog.V(logger.Debug).Infoln("Synchronisation aborted:", err)

    default:
        glog.V(logger.Warn).Infof("Synchronisation failed: %v", err)
    }
}

// synchronise will select the peer and use it for synchronising. If an empty string is given
// it will use the best peer possible and synchronize if it's TD is higher than our own. If any of the
// checks fail an error will be returned. This method is synchronous
func (d *Downloader) synchronise(id string, hash common.Hash, td *big.Int) error {
    // Mock out the synchonisation if testing
    if d.synchroniseMock != nil {
        return d.synchroniseMock(id, hash)
    }
    // Make sure only one goroutine is ever allowed past this point at once
    if !atomic.CompareAndSwapInt32(&d.synchronising, 0, 1) {
        return errBusy
    }
    defer atomic.StoreInt32(&d.synchronising, 0)

    // Post a user notification of the sync (only once per session)
    if atomic.CompareAndSwapInt32(&d.notified, 0, 1) {
        glog.V(logger.Info).Infoln("Block synchronisation started")
    }
    // Abort if the queue still contains some leftover data
    if d.queue.GetHeadResult() != nil {
        return errPendingQueue
    }
    // Reset the queue, peer set and wake channels to clean any internal leftover state
    d.queue.Reset()
    d.peers.Reset()

    for _, ch := range []chan bool{d.blockWakeCh, d.bodyWakeCh, d.receiptWakeCh} {
        select {
        case <-ch:
        default:
        }
    }
    // Create cancel channel for aborting mid-flight
    d.cancelLock.Lock()
    d.cancelCh = make(chan struct{})
    d.cancelLock.Unlock()

    // Retrieve the origin peer and initiate the downloading process
    p := d.peers.Peer(id)
    if p == nil {
        return errUnknownPeer
    }
    return d.syncWithPeer(p, hash, td)
}

/*
// Has checks if the downloader knows about a particular hash, meaning that its
// either already downloaded of pending retrieval.
func (d *Downloader) Has(hash common.Hash) bool {
    return d.queue.Has(hash)
}
*/
// syncWithPeer starts a block synchronization based on the hash chain from the
// specified peer and head hash.
func (d *Downloader) syncWithPeer(p *peer, hash common.Hash, td *big.Int) (err error) {
    d.mux.Post(StartEvent{})
    defer func() {
        // reset on error
        if err != nil {
            d.cancel()
            d.mux.Post(FailedEvent{err})
        } else {
            d.mux.Post(DoneEvent{})
        }
    }()

    glog.V(logger.Debug).Infof("Synchronising with the network using: %s [eth/%d]", p.id, p.version)
    defer glog.V(logger.Debug).Infof("Synchronisation terminated")

    switch {
    case p.version == 61:
        // Look up the sync boundaries: the common ancestor and the target block
        latest, err := d.fetchHeight61(p)
        if err != nil {
            return err
        }
        origin, err := d.findAncestor61(p)
        if err != nil {
            return err
        }
        d.syncStatsLock.Lock()
        if d.syncStatsHeight <= origin || d.syncStatsOrigin > origin {
            d.syncStatsOrigin = origin
        }
        d.syncStatsHeight = latest
        d.syncStatsLock.Unlock()

        // Initiate the sync using a  concurrent hash and block retrieval algorithm
        if d.syncInitHook != nil {
            d.syncInitHook(origin, latest)
        }
        d.queue.Prepare(origin+1, 1)

        errc := make(chan error, 2)
        go func() { errc <- d.fetchHashes61(p, td, origin+1) }()
        go func() { errc <- d.fetchBlocks61(origin + 1) }()

        // If any fetcher fails, cancel the other
        if err := <-errc; err != nil {
            d.cancel()
            <-errc
            return err
        }
        return <-errc

    case p.version >= 62:
        // Look up the sync boundaries: the common ancestor and the target block
        latest, err := d.fetchHeight(p)
        if err != nil {
            return err
        }
        origin, err := d.findAncestor(p)
        if err != nil {
            return err
        }
        d.syncStatsLock.Lock()
        if d.syncStatsHeight <= origin || d.syncStatsOrigin > origin {
            d.syncStatsOrigin = origin
        }
        d.syncStatsHeight = latest
        d.syncStatsLock.Unlock()

        // Initiate the sync using a  concurrent header and content retrieval algorithm
        parts := 1
        if d.mode == FastSync {
            parts = 2 // receipts are fetched too
        }
        d.queue.Prepare(origin+1, parts)

        if d.syncInitHook != nil {
            d.syncInitHook(origin, latest)
        }
        errc := make(chan error, 3)
        go func() { errc <- d.fetchHeaders(p, td, origin+1) }() // Headers are always retrieved
        go func() { errc <- d.fetchReceipts(origin + 1) }()     // Receipts are retrieved during fast sync
        go func() { errc <- d.fetchBodies(origin + 1) }()       // Bodies are retrieved during normal sync

        // If any fetcher fails, cancel the others
        var fail error
        for i := 0; i < cap(errc); i++ {
            if err := <-errc; err != nil {
                if fail == nil {
                    fail = err
                    d.cancel()
                }
            }
        }
        return fail

    default:
        // Something very wrong, stop right here
        glog.V(logger.Error).Infof("Unsupported eth protocol: %d", p.version)
        return errBadPeer
    }
    return nil
}

// cancel cancels all of the operations and resets the queue. It returns true
// if the cancel operation was completed.
func (d *Downloader) cancel() {
    // Close the current cancel channel
    d.cancelLock.Lock()
    if d.cancelCh != nil {
        select {
        case <-d.cancelCh:
            // Channel was already closed
        default:
            close(d.cancelCh)
        }
    }
    d.cancelLock.Unlock()

    // Reset the queue
    d.queue.Reset()
}

// Terminate interrupts the downloader, canceling all pending operations.
func (d *Downloader) Terminate() {
    atomic.StoreInt32(&d.interrupt, 1)
    d.cancel()
}

// fetchHeight61 retrieves the head block of the remote peer to aid in estimating
// the total time a pending synchronisation would take.
func (d *Downloader) fetchHeight61(p *peer) (uint64, error) {
    glog.V(logger.Debug).Infof("%v: retrieving remote chain height", p)

    // Request the advertised remote head block and wait for the response
    go p.getBlocks([]common.Hash{p.head})

    timeout := time.After(blockSoftTTL)
    for {
        select {
        case <-d.cancelCh:
            return 0, errCancelBlockFetch

        case <-d.headerCh:
            // Out of bounds eth/62 block headers received, ignore them

        case <-d.bodyCh:
            // Out of bounds eth/62 block bodies received, ignore them

        case <-d.hashCh:
            // Out of bounds hashes received, ignore them

        case blockPack := <-d.blockCh:
            // Discard anything not from the origin peer
            if blockPack.peerId != p.id {
                glog.V(logger.Debug).Infof("Received blocks from incorrect peer(%s)", blockPack.peerId)
                break
            }
            // Make sure the peer actually gave something valid
            blocks := blockPack.blocks
            if len(blocks) != 1 {
                glog.V(logger.Debug).Infof("%v: invalid number of head blocks: %d != 1", p, len(blocks))
                return 0, errBadPeer
            }
            return blocks[0].NumberU64(), nil

        case <-timeout:
            glog.V(logger.Debug).Infof("%v: head block timeout", p)
            return 0, errTimeout
        }
    }
}

// findAncestor61 tries to locate the common ancestor block of the local chain and
// a remote peers blockchain. In the general case when our node was in sync and
// on the correct chain, checking the top N blocks should already get us a match.
// In the rare scenario when we ended up on a long reorganization (i.e. none of
// the head blocks match), we do a binary search to find the common ancestor.
func (d *Downloader) findAncestor61(p *peer) (uint64, error) {
    glog.V(logger.Debug).Infof("%v: looking for common ancestor", p)

    // Request out head blocks to short circuit ancestor location
    head := d.headBlock().NumberU64()
    from := int64(head) - int64(MaxHashFetch) + 1
    if from < 0 {
        from = 0
    }
    go p.getAbsHashes(uint64(from), MaxHashFetch)

    // Wait for the remote response to the head fetch
    number, hash := uint64(0), common.Hash{}
    timeout := time.After(hashTTL)

    for finished := false; !finished; {
        select {
        case <-d.cancelCh:
            return 0, errCancelHashFetch

        case hashPack := <-d.hashCh:
            // Discard anything not from the origin peer
            if hashPack.peerId != p.id {
                glog.V(logger.Debug).Infof("Received hashes from incorrect peer(%s)", hashPack.peerId)
                break
            }
            // Make sure the peer actually gave something valid
            hashes := hashPack.hashes
            if len(hashes) == 0 {
                glog.V(logger.Debug).Infof("%v: empty head hash set", p)
                return 0, errEmptyHashSet
            }
            // Check if a common ancestor was found
            finished = true
            for i := len(hashes) - 1; i >= 0; i-- {
                if d.hasBlock(hashes[i]) {
                    number, hash = uint64(from)+uint64(i), hashes[i]
                    break
                }
            }

        case <-d.blockCh:
            // Out of bounds blocks received, ignore them

        case <-d.headerCh:
            // Out of bounds eth/62 block headers received, ignore them

        case <-d.bodyCh:
            // Out of bounds eth/62 block bodies received, ignore them

        case <-timeout:
            glog.V(logger.Debug).Infof("%v: head hash timeout", p)
            return 0, errTimeout
        }
    }
    // If the head fetch already found an ancestor, return
    if !common.EmptyHash(hash) {
        glog.V(logger.Debug).Infof("%v: common ancestor: #%d [%x…]", p, number, hash[:4])
        return number, nil
    }
    // Ancestor not found, we need to binary search over our chain
    start, end := uint64(0), head
    for start+1 < end {
        // Split our chain interval in two, and request the hash to cross check
        check := (start + end) / 2

        timeout := time.After(hashTTL)
        go p.getAbsHashes(uint64(check), 1)

        // Wait until a reply arrives to this request
        for arrived := false; !arrived; {
            select {
            case <-d.cancelCh:
                return 0, errCancelHashFetch

            case hashPack := <-d.hashCh:
                // Discard anything not from the origin peer
                if hashPack.peerId != p.id {
                    glog.V(logger.Debug).Infof("Received hashes from incorrect peer(%s)", hashPack.peerId)
                    break
                }
                // Make sure the peer actually gave something valid
                hashes := hashPack.hashes
                if len(hashes) != 1 {
                    glog.V(logger.Debug).Infof("%v: invalid search hash set (%d)", p, len(hashes))
                    return 0, errBadPeer
                }
                arrived = true

                // Modify the search interval based on the response
                block := d.getBlock(hashes[0])
                if block == nil {
                    end = check
                    break
                }
                if block.NumberU64() != check {
                    glog.V(logger.Debug).Infof("%v: non requested hash #%d [%x…], instead of #%d", p, block.NumberU64(), block.Hash().Bytes()[:4], check)
                    return 0, errBadPeer
                }
                start = check

            case <-d.blockCh:
                // Out of bounds blocks received, ignore them

            case <-d.headerCh:
                // Out of bounds eth/62 block headers received, ignore them

            case <-d.bodyCh:
                // Out of bounds eth/62 block bodies received, ignore them

            case <-timeout:
                glog.V(logger.Debug).Infof("%v: search hash timeout", p)
                return 0, errTimeout
            }
        }
    }
    return start, nil
}

// fetchHashes61 keeps retrieving hashes from the requested number, until no more
// are returned, potentially throttling on the way.
func (d *Downloader) fetchHashes61(p *peer, td *big.Int, from uint64) error {
    glog.V(logger.Debug).Infof("%v: downloading hashes from #%d", p, from)

    // Create a timeout timer, and the associated hash fetcher
    request := time.Now()       // time of the last fetch request
    timeout := time.NewTimer(0) // timer to dump a non-responsive active peer
    <-timeout.C                 // timeout channel should be initially empty
    defer timeout.Stop()

    getHashes := func(from uint64) {
        glog.V(logger.Detail).Infof("%v: fetching %d hashes from #%d", p, MaxHashFetch, from)

        go p.getAbsHashes(from, MaxHashFetch)
        request = time.Now()
        timeout.Reset(hashTTL)
    }
    // Start pulling hashes, until all are exhausted
    getHashes(from)
    gotHashes := false

    for {
        select {
        case <-d.cancelCh:
            return errCancelHashFetch

        case <-d.headerCh:
            // Out of bounds eth/62 block headers received, ignore them

        case <-d.bodyCh:
            // Out of bounds eth/62 block bodies received, ignore them

        case hashPack := <-d.hashCh:
            // Make sure the active peer is giving us the hashes
            if hashPack.peerId != p.id {
                glog.V(logger.Debug).Infof("Received hashes from incorrect peer(%s)", hashPack.peerId)
                break
            }
            hashReqTimer.UpdateSince(request)
            timeout.Stop()

            // If no more hashes are inbound, notify the block fetcher and return
            if len(hashPack.hashes) == 0 {
                glog.V(logger.Debug).Infof("%v: no available hashes", p)

                select {
                case d.blockWakeCh <- false:
                case <-d.cancelCh:
                }
                // If no hashes were retrieved at all, the peer violated it's TD promise that it had a
                // better chain compared to ours. The only exception is if it's promised blocks were
                // already imported by other means (e.g. fecher):
                //
                // R <remote peer>, L <local node>: Both at block 10
                // R: Mine block 11, and propagate it to L
                // L: Queue block 11 for import
                // L: Notice that R's head and TD increased compared to ours, start sync
                // L: Import of block 11 finishes
                // L: Sync begins, and finds common ancestor at 11
                // L: Request new hashes up from 11 (R's TD was higher, it must have something)
                // R: Nothing to give
                if !gotHashes && td.Cmp(d.getTd(d.headBlock().Hash())) > 0 {
                    return errStallingPeer
                }
                return nil
            }
            gotHashes = true

            // Otherwise insert all the new hashes, aborting in case of junk
            glog.V(logger.Detail).Infof("%v: scheduling %d hashes from #%d", p, len(hashPack.hashes), from)

            inserts := d.queue.Schedule61(hashPack.hashes, true)
            if len(inserts) != len(hashPack.hashes) {
                glog.V(logger.Debug).Infof("%v: stale hashes", p)
                return errBadPeer
            }
            // Notify the block fetcher of new hashes, but stop if queue is full
            if d.queue.PendingBlocks() < maxQueuedHashes {
                // We still have hashes to fetch, send continuation wake signal (potential)
                select {
                case d.blockWakeCh <- true:
                default:
                }
            } else {
                // Hash limit reached, send a termination wake signal (enforced)
                select {
                case d.blockWakeCh <- false:
                case <-d.cancelCh:
                }
                return nil
            }
            // Queue not yet full, fetch the next batch
            from += uint64(len(hashPack.hashes))
            getHashes(from)

        case <-timeout.C:
            glog.V(logger.Debug).Infof("%v: hash request timed out", p)
            hashTimeoutMeter.Mark(1)
            return errTimeout
        }
    }
}

// fetchBlocks61 iteratively downloads the scheduled hashes, taking any available
// peers, reserving a chunk of blocks for each, waiting for delivery and also
// periodically checking for timeouts.
func (d *Downloader) fetchBlocks61(from uint64) error {
    glog.V(logger.Debug).Infof("Downloading blocks from #%d", from)
    defer glog.V(logger.Debug).Infof("Block download terminated")

    // Create a timeout timer for scheduling expiration tasks
    ticker := time.NewTicker(100 * time.Millisecond)
    defer ticker.Stop()

    update := make(chan struct{}, 1)

    // Fetch blocks until the hash fetcher's done
    finished := false
    for {
        select {
        case <-d.cancelCh:
            return errCancelBlockFetch

        case <-d.headerCh:
            // Out of bounds eth/62 block headers received, ignore them

        case <-d.bodyCh:
            // Out of bounds eth/62 block bodies received, ignore them

        case blockPack := <-d.blockCh:
            // If the peer was previously banned and failed to deliver it's pack
            // in a reasonable time frame, ignore it's message.
            if peer := d.peers.Peer(blockPack.peerId); peer != nil {
                // Deliver the received chunk of blocks, and demote in case of errors
                err := d.queue.Deliver61(blockPack.peerId, blockPack.blocks)
                switch err {
                case nil:
                    // If no blocks were delivered, demote the peer (need the delivery above)
                    if len(blockPack.blocks) == 0 {
                        peer.Demote()
                        peer.SetBlocksIdle()
                        glog.V(logger.Detail).Infof("%s: no blocks delivered", peer)
                        break
                    }
                    // All was successful, promote the peer and potentially start processing
                    peer.Promote()
                    peer.SetBlocksIdle()
                    glog.V(logger.Detail).Infof("%s: delivered %d blocks", peer, len(blockPack.blocks))
                    go d.process()

                case errInvalidChain:
                    // The hash chain is invalid (blocks are not ordered properly), abort
                    return err

                case errNoFetchesPending:
                    // Peer probably timed out with its delivery but came through
                    // in the end, demote, but allow to to pull from this peer.
                    peer.Demote()
                    peer.SetBlocksIdle()
                    glog.V(logger.Detail).Infof("%s: out of bound delivery", peer)

                case errStaleDelivery:
                    // Delivered something completely else than requested, usually
                    // caused by a timeout and delivery during a new sync cycle.
                    // Don't set it to idle as the original request should still be
                    // in flight.
                    peer.Demote()
                    glog.V(logger.Detail).Infof("%s: stale delivery", peer)

                default:
                    // Peer did something semi-useful, demote but keep it around
                    peer.Demote()
                    peer.SetBlocksIdle()
                    glog.V(logger.Detail).Infof("%s: delivery partially failed: %v", peer, err)
                    go d.process()
                }
            }
            // Blocks arrived, try to update the progress
            select {
            case update <- struct{}{}:
            default:
            }

        case cont := <-d.blockWakeCh:
            // The hash fetcher sent a continuation flag, check if it's done
            if !cont {
                finished = true
            }
            // Hashes arrive, try to update the progress
            select {
            case update <- struct{}{}:
            default:
            }

        case <-ticker.C:
            // Sanity check update the progress
            select {
            case update <- struct{}{}:
            default:
            }

        case <-update:
            // Short circuit if we lost all our peers
            if d.peers.Len() == 0 {
                return errNoPeers
            }
            // Check for block request timeouts and demote the responsible peers
            for _, pid := range d.queue.Expire61(blockHardTTL) {
                if peer := d.peers.Peer(pid); peer != nil {
                    peer.Demote()
                    glog.V(logger.Detail).Infof("%s: block delivery timeout", peer)
                }
            }
            // If there's nothing more to fetch, wait or terminate
            if d.queue.PendingBlocks() == 0 {
                if d.queue.InFlight() == 0 && finished {
                    glog.V(logger.Debug).Infof("Block fetching completed")
                    return nil
                }
                break
            }
            // Send a download request to all idle peers, until throttled
            throttled := false
            idles, total := d.peers.BlockIdlePeers(61)

            for _, peer := range idles {
                // Short circuit if throttling activated
                if d.queue.ThrottleBlocks() {
                    throttled = true
                    break
                }
                // Reserve a chunk of hashes for a peer. A nil can mean either that
                // no more hashes are available, or that the peer is known not to
                // have them.
                request := d.queue.Reserve61(peer, peer.BlockCapacity())
                if request == nil {
                    continue
                }
                if glog.V(logger.Detail) {
                    glog.Infof("%s: requesting %d blocks", peer, len(request.Hashes))
                }
                // Fetch the chunk and make sure any errors return the hashes to the queue
                if err := peer.Fetch61(request); err != nil {
                    glog.V(logger.Error).Infof("%v: fetch failed, rescheduling", peer)
                    d.queue.Cancel61(request)
                }
            }
            // Make sure that we have peers available for fetching. If all peers have been tried
            // and all failed throw an error
            if !throttled && d.queue.InFlight() == 0 && len(idles) == total {
                return errPeersUnavailable
            }
        }
    }
}

// fetchHeight retrieves the head header of the remote peer to aid in estimating
// the total time a pending synchronisation would take.
func (d *Downloader) fetchHeight(p *peer) (uint64, error) {
    glog.V(logger.Debug).Infof("%v: retrieving remote chain height", p)

    // Request the advertised remote head block and wait for the response
    go p.getRelHeaders(p.head, 1, 0, false)

    timeout := time.After(headerTTL)
    for {
        select {
        case <-d.cancelCh:
            return 0, errCancelBlockFetch

        case headerPack := <-d.headerCh:
            // Discard anything not from the origin peer
            if headerPack.peerId != p.id {
                glog.V(logger.Debug).Infof("Received headers from incorrect peer(%s)", headerPack.peerId)
                break
            }
            // Make sure the peer actually gave something valid
            headers := headerPack.headers
            if len(headers) != 1 {
                glog.V(logger.Debug).Infof("%v: invalid number of head headers: %d != 1", p, len(headers))
                return 0, errBadPeer
            }
            return headers[0].Number.Uint64(), nil

        case <-d.bodyCh:
            // Out of bounds block bodies received, ignore them

        case <-d.hashCh:
            // Out of bounds eth/61 hashes received, ignore them

        case <-d.blockCh:
            // Out of bounds eth/61 blocks received, ignore them

        case <-timeout:
            glog.V(logger.Debug).Infof("%v: head header timeout", p)
            return 0, errTimeout
        }
    }
}

// findAncestor tries to locate the common ancestor link of the local chain and
// a remote peers blockchain. In the general case when our node was in sync and
// on the correct chain, checking the top N links should already get us a match.
// In the rare scenario when we ended up on a long reorganization (i.e. none of
// the head links match), we do a binary search to find the common ancestor.
func (d *Downloader) findAncestor(p *peer) (uint64, error) {
    glog.V(logger.Debug).Infof("%v: looking for common ancestor", p)

    // Request our head headers to short circuit ancestor location
    head := d.headHeader().Number.Uint64()
    if d.mode == FullSync {
        head = d.headBlock().NumberU64()
    }
    from := int64(head) - int64(MaxHeaderFetch) + 1
    if from < 0 {
        from = 0
    }
    go p.getAbsHeaders(uint64(from), MaxHeaderFetch, 0, false)

    // Wait for the remote response to the head fetch
    number, hash := uint64(0), common.Hash{}
    timeout := time.After(hashTTL)

    for finished := false; !finished; {
        select {
        case <-d.cancelCh:
            return 0, errCancelHashFetch

        case headerPack := <-d.headerCh:
            // Discard anything not from the origin peer
            if headerPack.peerId != p.id {
                glog.V(logger.Debug).Infof("Received headers from incorrect peer(%s)", headerPack.peerId)
                break
            }
            // Make sure the peer actually gave something valid
            headers := headerPack.headers
            if len(headers) == 0 {
                glog.V(logger.Debug).Infof("%v: empty head header set", p)
                return 0, errEmptyHeaderSet
            }
            // Check if a common ancestor was found
            finished = true
            for i := len(headers) - 1; i >= 0; i-- {
                if (d.mode == FullSync && d.hasBlock(headers[i].Hash())) || (d.mode != FullSync && d.hasHeader(headers[i].Hash())) {
                    number, hash = headers[i].Number.Uint64(), headers[i].Hash()
                    break
                }
            }

        case <-d.bodyCh:
            // Out of bounds block bodies received, ignore them

        case <-d.hashCh:
            // Out of bounds eth/61 hashes received, ignore them

        case <-d.blockCh:
            // Out of bounds eth/61 blocks received, ignore them

        case <-timeout:
            glog.V(logger.Debug).Infof("%v: head header timeout", p)
            return 0, errTimeout
        }
    }
    // If the head fetch already found an ancestor, return
    if !common.EmptyHash(hash) {
        glog.V(logger.Debug).Infof("%v: common ancestor: #%d [%x…]", p, number, hash[:4])
        return number, nil
    }
    // Ancestor not found, we need to binary search over our chain
    start, end := uint64(0), head
    for start+1 < end {
        // Split our chain interval in two, and request the hash to cross check
        check := (start + end) / 2

        timeout := time.After(hashTTL)
        go p.getAbsHeaders(uint64(check), 1, 0, false)

        // Wait until a reply arrives to this request
        for arrived := false; !arrived; {
            select {
            case <-d.cancelCh:
                return 0, errCancelHashFetch

            case headerPack := <-d.headerCh:
                // Discard anything not from the origin peer
                if headerPack.peerId != p.id {
                    glog.V(logger.Debug).Infof("Received headers from incorrect peer(%s)", headerPack.peerId)
                    break
                }
                // Make sure the peer actually gave something valid
                headers := headerPack.headers
                if len(headers) != 1 {
                    glog.V(logger.Debug).Infof("%v: invalid search header set (%d)", p, len(headers))
                    return 0, errBadPeer
                }
                arrived = true

                // Modify the search interval based on the response
                if (d.mode == FullSync && !d.hasBlock(headers[0].Hash())) || (d.mode != FullSync && !d.hasHeader(headers[0].Hash())) {
                    end = check
                    break
                }
                header := d.getHeader(headers[0].Hash()) // Independent of sync mode, header surely exists
                if header.Number.Uint64() != check {
                    glog.V(logger.Debug).Infof("%v: non requested header #%d [%x…], instead of #%d", p, header.Number, header.Hash().Bytes()[:4], check)
                    return 0, errBadPeer
                }
                start = check

            case <-d.bodyCh:
                // Out of bounds block bodies received, ignore them

            case <-d.hashCh:
                // Out of bounds eth/61 hashes received, ignore them

            case <-d.blockCh:
                // Out of bounds eth/61 blocks received, ignore them

            case <-timeout:
                glog.V(logger.Debug).Infof("%v: search header timeout", p)
                return 0, errTimeout
            }
        }
    }
    return start, nil
}

// fetchHeaders keeps retrieving headers from the requested number, until no more
// are returned, potentially throttling on the way.
//
// The queue parameter can be used to switch between queuing headers for block
// body download too, or directly import as pure header chains.
func (d *Downloader) fetchHeaders(p *peer, td *big.Int, from uint64) error {
    glog.V(logger.Debug).Infof("%v: downloading headers from #%d", p, from)
    defer glog.V(logger.Debug).Infof("%v: header download terminated", p)

    // Create a timeout timer, and the associated hash fetcher
    request := time.Now()       // time of the last fetch request
    timeout := time.NewTimer(0) // timer to dump a non-responsive active peer
    <-timeout.C                 // timeout channel should be initially empty
    defer timeout.Stop()

    getHeaders := func(from uint64) {
        glog.V(logger.Detail).Infof("%v: fetching %d headers from #%d", p, MaxHeaderFetch, from)

        go p.getAbsHeaders(from, MaxHeaderFetch, 0, false)
        request = time.Now()
        timeout.Reset(headerTTL)
    }
    // Start pulling headers, until all are exhausted
    getHeaders(from)
    gotHeaders := false

    for {
        select {
        case <-d.cancelCh:
            return errCancelHeaderFetch

        case <-d.hashCh:
            // Out of bounds eth/61 hashes received, ignore them

        case <-d.blockCh:
            // Out of bounds eth/61 blocks received, ignore them

        case headerPack := <-d.headerCh:
            // Make sure the active peer is giving us the headers
            if headerPack.peerId != p.id {
                glog.V(logger.Debug).Infof("Received headers from incorrect peer (%s)", headerPack.peerId)
                break
            }
            headerReqTimer.UpdateSince(request)
            timeout.Stop()

            // If no more headers are inbound, notify the content fetchers and return
            if len(headerPack.headers) == 0 {
                glog.V(logger.Debug).Infof("%v: no available headers", p)

                for _, ch := range []chan bool{d.bodyWakeCh, d.receiptWakeCh} {
                    select {
                    case ch <- false:
                    case <-d.cancelCh:
                    }
                }
                // If no headers were retrieved at all, the peer violated it's TD promise that it had a
                // better chain compared to ours. The only exception is if it's promised blocks were
                // already imported by other means (e.g. fecher):
                //
                // R <remote peer>, L <local node>: Both at block 10
                // R: Mine block 11, and propagate it to L
                // L: Queue block 11 for import
                // L: Notice that R's head and TD increased compared to ours, start sync
                // L: Import of block 11 finishes
                // L: Sync begins, and finds common ancestor at 11
                // L: Request new headers up from 11 (R's TD was higher, it must have something)
                // R: Nothing to give
                if !gotHeaders && td.Cmp(d.getTd(d.headBlock().Hash())) > 0 {
                    return errStallingPeer
                }
                return nil
            }
            gotHeaders = true

            // Otherwise insert all the new headers, aborting in case of junk
            glog.V(logger.Detail).Infof("%v: schedule %d headers from #%d", p, len(headerPack.headers), from)

            if d.mode == FullSync || d.mode == FastSync {
                inserts := d.queue.Schedule(headerPack.headers, from, d.mode == FastSync)
                if len(inserts) != len(headerPack.headers) {
                    glog.V(logger.Debug).Infof("%v: stale headers", p)
                    return errBadPeer
                }
            } else {
                if n, err := d.insertHeaders(headerPack.headers, true); err != nil {
                    glog.V(logger.Debug).Infof("%v: invalid header #%d [%x…]: %v", p, headerPack.headers[n].Number, headerPack.headers[n].Hash().Bytes()[:4], err)
                    return errInvalidChain
                }
            }
            // Notify the content fetchers of new headers, but stop if queue is full
            cont := d.queue.PendingBlocks() < maxQueuedHeaders || d.queue.PendingReceipts() < maxQueuedHeaders
            for _, ch := range []chan bool{d.bodyWakeCh, d.receiptWakeCh} {
                if cont {
                    // We still have headers to fetch, send continuation wake signal (potential)
                    select {
                    case ch <- true:
                    default:
                    }
                } else {
                    // Header limit reached, send a termination wake signal (enforced)
                    select {
                    case ch <- false:
                    case <-d.cancelCh:
                    }
                    return nil
                }
            }
            // Queue not yet full, fetch the next batch
            from += uint64(len(headerPack.headers))
            getHeaders(from)

        case <-timeout.C:
            // Header retrieval timed out, consider the peer bad and drop
            glog.V(logger.Debug).Infof("%v: header request timed out", p)
            headerTimeoutMeter.Mark(1)
            d.dropPeer(p.id)

            // Finish the sync gracefully instead of dumping the gathered data though
            for _, ch := range []chan bool{d.bodyWakeCh, d.receiptWakeCh} {
                select {
                case ch <- false:
                case <-d.cancelCh:
                }
            }
            return nil
        }
    }
}

// fetchBodies iteratively downloads the scheduled block bodies, taking any
// available peers, reserving a chunk of blocks for each, waiting for delivery
// and also periodically checking for timeouts.
func (d *Downloader) fetchBodies(from uint64) error {
    glog.V(logger.Debug).Infof("Downloading block bodies from #%d", from)

    var (
        deliver = func(packet interface{}) error {
            pack := packet.(*bodyPack)
            return d.queue.DeliverBlocks(pack.peerId, pack.transactions, pack.uncles)
        }
        expire   = func() []string { return d.queue.ExpireBlocks(bodyHardTTL) }
        fetch    = func(p *peer, req *fetchRequest) error { return p.FetchBodies(req) }
        capacity = func(p *peer) int { return p.BlockCapacity() }
        getIdles = func() ([]*peer, int) { return d.peers.BlockIdlePeers(62) }
        setIdle  = func(p *peer) { p.SetBlocksIdle() }
    )
    err := d.fetchParts(from, errCancelBodyFetch, d.bodyCh, deliver, d.bodyWakeCh, expire,
        d.queue.PendingBlocks, d.queue.ThrottleBlocks, d.queue.ReserveBlocks, d.bodyFetchHook,
        fetch, d.queue.CancelBlocks, capacity, getIdles, setIdle, "Body")

    glog.V(logger.Debug).Infof("Block body download terminated: %v", err)
    return err
}

// fetchReceipts iteratively downloads the scheduled block receipts, taking any
// available peers, reserving a chunk of receipts for each, waiting for delivery
// and also periodically checking for timeouts.
func (d *Downloader) fetchReceipts(from uint64) error {
    glog.V(logger.Debug).Infof("Downloading receipts from #%d", from)

    var (
        deliver = func(packet interface{}) error {
            pack := packet.(*receiptPack)
            return d.queue.DeliverReceipts(pack.peerId, pack.receipts)
        }
        expire   = func() []string { return d.queue.ExpireReceipts(bodyHardTTL) }
        fetch    = func(p *peer, req *fetchRequest) error { return p.FetchReceipts(req) }
        capacity = func(p *peer) int { return p.ReceiptCapacity() }
        setIdle  = func(p *peer) { p.SetReceiptsIdle() }
    )
    err := d.fetchParts(from, errCancelReceiptFetch, d.receiptCh, deliver, d.receiptWakeCh, expire,
        d.queue.PendingReceipts, d.queue.ThrottleReceipts, d.queue.ReserveReceipts, d.receiptFetchHook,
        fetch, d.queue.CancelReceipts, capacity, d.peers.ReceiptIdlePeers, setIdle, "Receipt")

    glog.V(logger.Debug).Infof("Receipt download terminated: %v", err)
    return err
}

// fetchParts iteratively downloads scheduled block parts, taking any available
// peers, reserving a chunk of fetch requests for each, waiting for delivery and
// also periodically checking for timeouts.
func (d *Downloader) fetchParts(from uint64, errCancel error, deliveryCh chan dataPack, deliver func(packet interface{}) error, wakeCh chan bool,
    expire func() []string, pending func() int, throttle func() bool, reserve func(*peer, int) (*fetchRequest, bool, error), fetchHook func([]*types.Header),
    fetch func(*peer, *fetchRequest) error, cancel func(*fetchRequest), capacity func(*peer) int, idle func() ([]*peer, int), setIdle func(*peer), kind string) error {

    // Create a ticker to detect expired retreival tasks
    ticker := time.NewTicker(100 * time.Millisecond)
    defer ticker.Stop()

    update := make(chan struct{}, 1)

    // Prepare the queue and fetch block parts until the block header fetcher's done
    finished := false
    for {
        select {
        case <-d.cancelCh:
            return errCancel

        case <-d.hashCh:
            // Out of bounds eth/61 hashes received, ignore them

        case <-d.blockCh:
            // Out of bounds eth/61 blocks received, ignore them

        case packet := <-deliveryCh:
            // If the peer was previously banned and failed to deliver it's pack
            // in a reasonable time frame, ignore it's message.
            if peer := d.peers.Peer(packet.PeerId()); peer != nil {
                // Deliver the received chunk of data, and demote in case of errors
                switch err := deliver(packet); err {
                case nil:
                    // If no blocks were delivered, demote the peer (need the delivery above to clean internal queue!)
                    if packet.Empty() {
                        peer.Demote()
                        setIdle(peer)
                        glog.V(logger.Detail).Infof("%s: no %s delivered", peer, strings.ToLower(kind))
                        break
                    }
                    // All was successful, promote the peer and potentially start processing
                    peer.Promote()
                    setIdle(peer)
                    glog.V(logger.Detail).Infof("%s: delivered %s %s(s)", peer, packet.Stats(), strings.ToLower(kind))
                    go d.process()

                case errInvalidChain:
                    // The hash chain is invalid (blocks are not ordered properly), abort
                    return err

                case errInvalidBody, errInvalidReceipt:
                    // The peer delivered something very bad, drop immediately
                    glog.V(logger.Error).Infof("%s: delivered invalid %s, dropping", peer, strings.ToLower(kind))
                    d.dropPeer(peer.id)

                case errNoFetchesPending:
                    // Peer probably timed out with its delivery but came through
                    // in the end, demote, but allow to to pull from this peer.
                    peer.Demote()
                    setIdle(peer)
                    glog.V(logger.Detail).Infof("%s: out of bound %s delivery", peer, strings.ToLower(kind))

                case errStaleDelivery:
                    // Delivered something completely else than requested, usually
                    // caused by a timeout and delivery during a new sync cycle.
                    // Don't set it to idle as the original request should still be
                    // in flight.
                    peer.Demote()
                    glog.V(logger.Detail).Infof("%s: %s stale delivery", peer, strings.ToLower(kind))

                default:
                    // Peer did something semi-useful, demote but keep it around
                    peer.Demote()
                    setIdle(peer)
                    glog.V(logger.Detail).Infof("%s: %s delivery partially failed: %v", peer, strings.ToLower(kind), err)
                    go d.process()
                }
            }
            // Blocks assembled, try to update the progress
            select {
            case update <- struct{}{}:
            default:
            }

        case cont := <-wakeCh:
            // The header fetcher sent a continuation flag, check if it's done
            if !cont {
                finished = true
            }
            // Headers arrive, try to update the progress
            select {
            case update <- struct{}{}:
            default:
            }

        case <-ticker.C:
            // Sanity check update the progress
            select {
            case update <- struct{}{}:
            default:
            }

        case <-update:
            // Short circuit if we lost all our peers
            if d.peers.Len() == 0 {
                return errNoPeers
            }
            // Check for fetch request timeouts and demote the responsible peers
            for _, pid := range expire() {
                if peer := d.peers.Peer(pid); peer != nil {
                    peer.Demote()
                    glog.V(logger.Detail).Infof("%s: %s delivery timeout", peer, strings.ToLower(kind))
                }
            }
            // If there's nothing more to fetch, wait or terminate
            if pending() == 0 {
                if d.queue.InFlight() == 0 && finished {
                    glog.V(logger.Debug).Infof("%s fetching completed", kind)
                    return nil
                }
                break
            }
            // Send a download request to all idle peers, until throttled
            progressed, throttled := false, false
            idles, total := idle()

            for _, peer := range idles {
                // Short circuit if throttling activated
                if throttle() {
                    throttled = true
                    break
                }
                // Reserve a chunk of fetches for a peer. A nil can mean either that
                // no more headers are available, or that the peer is known not to
                // have them.
                request, progress, err := reserve(peer, capacity(peer))
                if err != nil {
                    return err
                }
                if progress {
                    progressed = true
                    go d.process()
                }
                if request == nil {
                    continue
                }
                if glog.V(logger.Detail) {
                    glog.Infof("%s: requesting %d %s(s), first at #%d", peer, len(request.Headers), strings.ToLower(kind), request.Headers[0].Number)
                }
                // Fetch the chunk and make sure any errors return the hashes to the queue
                if fetchHook != nil {
                    fetchHook(request.Headers)
                }
                if err := fetch(peer, request); err != nil {
                    glog.V(logger.Error).Infof("%v: %s fetch failed, rescheduling", peer, strings.ToLower(kind))
                    cancel(request)
                }
            }
            // Make sure that we have peers available for fetching. If all peers have been tried
            // and all failed throw an error
            if !progressed && !throttled && d.queue.InFlight() == 0 && len(idles) == total {
                return errPeersUnavailable
            }
        }
    }
}

// process takes fetch results from the queue and tries to import them into the
// chain. The type of import operation will depend on the result contents:
//  -
//
// The algorithmic flow is as follows:
//  - The `processing` flag is swapped to 1 to ensure singleton access
//  - The current `cancel` channel is retrieved to detect sync abortions
//  - Blocks are iteratively taken from the cache and inserted into the chain
//  - When the cache becomes empty, insertion stops
//  - The `processing` flag is swapped back to 0
//  - A post-exit check is made whether new blocks became available
//     - This step is important: it handles a potential race condition between
//       checking for no more work, and releasing the processing "mutex". In
//       between these state changes, a block may have arrived, but a processing
//       attempt denied, so we need to re-enter to ensure the block isn't left
//       to idle in the cache.
func (d *Downloader) process() {
    // Make sure only one goroutine is ever allowed to process blocks at once
    if !atomic.CompareAndSwapInt32(&d.processing, 0, 1) {
        return
    }
    // If the processor just exited, but there are freshly pending items, try to
    // reenter. This is needed because the goroutine spinned up for processing
    // the fresh results might have been rejected entry to to this present thread
    // not yet releasing the `processing` state.
    defer func() {
        if atomic.LoadInt32(&d.interrupt) == 0 && d.queue.GetHeadResult() != nil {
            d.process()
        }
    }()
    // Release the lock upon exit (note, before checking for reentry!)
    // the import statistics to zero.
    defer atomic.StoreInt32(&d.processing, 0)

    // Repeat the processing as long as there are results to process
    for {
        // Fetch the next batch of results
        results := d.queue.TakeResults()
        if len(results) == 0 {
            return
        }
        if d.chainInsertHook != nil {
            d.chainInsertHook(results)
        }
        // Actually import the blocks
        if glog.V(logger.Debug) {
            first, last := results[0].Header, results[len(results)-1].Header
            glog.V(logger.Debug).Infof("Inserting chain with %d items (#%d [%x…] - #%d [%x…])", len(results), first.Number, first.Hash().Bytes()[:4], last.Number, last.Hash().Bytes()[:4])
        }
        for len(results) != 0 {
            // Check for any termination requests
            if atomic.LoadInt32(&d.interrupt) == 1 {
                return
            }
            // Retrieve the a batch of results to import
            var (
                headers  = make([]*types.Header, 0, maxResultsProcess)
                blocks   = make([]*types.Block, 0, maxResultsProcess)
                receipts = make([]types.Receipts, 0, maxResultsProcess)
            )
            items := int(math.Min(float64(len(results)), float64(maxResultsProcess)))
            for _, result := range results[:items] {
                switch {
                case d.mode == FullSync:
                    blocks = append(blocks, types.NewBlockWithHeader(result.Header).WithBody(result.Transactions, result.Uncles))
                case d.mode == FastSync:
                    blocks = append(blocks, types.NewBlockWithHeader(result.Header).WithBody(result.Transactions, result.Uncles))
                    receipts = append(receipts, result.Receipts)
                case d.mode == LightSync:
                    headers = append(headers, result.Header)
                }
            }
            // Try to process the results, aborting if there's an error
            var (
                err   error
                index int
            )
            switch {
            case d.mode == FullSync:
                index, err = d.insertBlocks(blocks)
            case d.mode == FastSync:
                index, err = d.insertReceipts(blocks, receipts)
            case d.mode == LightSync:
                index, err = d.insertHeaders(headers, true)
            }
            if err != nil {
                glog.V(logger.Debug).Infof("Result #%d [%x…] processing failed: %v", results[index].Header.Number, results[index].Header.Hash(), err)
                d.cancel()
                return
            }
            // Shift the results to the next batch
            results = results[items:]
        }
    }
}

// DeliverHashes61 injects a new batch of hashes received from a remote node into
// the download schedule. This is usually invoked through the BlockHashesMsg by
// the protocol handler.
func (d *Downloader) DeliverHashes61(id string, hashes []common.Hash) (err error) {
    // Update the delivery metrics for both good and failed deliveries
    hashInMeter.Mark(int64(len(hashes)))
    defer func() {
        if err != nil {
            hashDropMeter.Mark(int64(len(hashes)))
        }
    }()
    // Make sure the downloader is active
    if atomic.LoadInt32(&d.synchronising) == 0 {
        return errNoSyncActive
    }
    // Deliver or abort if the sync is canceled while queuing
    d.cancelLock.RLock()
    cancel := d.cancelCh
    d.cancelLock.RUnlock()

    select {
    case d.hashCh <- hashPack{id, hashes}:
        return nil

    case <-cancel:
        return errNoSyncActive
    }
}

// DeliverBlocks61 injects a new batch of blocks received from a remote node.
// This is usually invoked through the BlocksMsg by the protocol handler.
func (d *Downloader) DeliverBlocks61(id string, blocks []*types.Block) (err error) {
    // Update the delivery metrics for both good and failed deliveries
    blockInMeter.Mark(int64(len(blocks)))
    defer func() {
        if err != nil {
            blockDropMeter.Mark(int64(len(blocks)))
        }
    }()
    // Make sure the downloader is active
    if atomic.LoadInt32(&d.synchronising) == 0 {
        return errNoSyncActive
    }
    // Deliver or abort if the sync is canceled while queuing
    d.cancelLock.RLock()
    cancel := d.cancelCh
    d.cancelLock.RUnlock()

    select {
    case d.blockCh <- blockPack{id, blocks}:
        return nil

    case <-cancel:
        return errNoSyncActive
    }
}

// DeliverHeaders injects a new batch of blck headers received from a remote
// node into the download schedule.
func (d *Downloader) DeliverHeaders(id string, headers []*types.Header) (err error) {
    // Update the delivery metrics for both good and failed deliveries
    headerInMeter.Mark(int64(len(headers)))
    defer func() {
        if err != nil {
            headerDropMeter.Mark(int64(len(headers)))
        }
    }()
    // Make sure the downloader is active
    if atomic.LoadInt32(&d.synchronising) == 0 {
        return errNoSyncActive
    }
    // Deliver or abort if the sync is canceled while queuing
    d.cancelLock.RLock()
    cancel := d.cancelCh
    d.cancelLock.RUnlock()

    select {
    case d.headerCh <- headerPack{id, headers}:
        return nil

    case <-cancel:
        return errNoSyncActive
    }
}

// DeliverBodies injects a new batch of block bodies received from a remote node.
func (d *Downloader) DeliverBodies(id string, transactions [][]*types.Transaction, uncles [][]*types.Header) (err error) {
    // Update the delivery metrics for both good and failed deliveries
    bodyInMeter.Mark(int64(len(transactions)))
    defer func() {
        if err != nil {
            bodyDropMeter.Mark(int64(len(transactions)))
        }
    }()
    // Make sure the downloader is active
    if atomic.LoadInt32(&d.synchronising) == 0 {
        return errNoSyncActive
    }
    // Deliver or abort if the sync is canceled while queuing
    d.cancelLock.RLock()
    cancel := d.cancelCh
    d.cancelLock.RUnlock()

    select {
    case d.bodyCh <- &bodyPack{id, transactions, uncles}:
        return nil

    case <-cancel:
        return errNoSyncActive
    }
}

// DeliverReceipts injects a new batch of receipts received from a remote node.
func (d *Downloader) DeliverReceipts(id string, receipts [][]*types.Receipt) (err error) {
    // Update the delivery metrics for both good and failed deliveries
    receiptInMeter.Mark(int64(len(receipts)))
    defer func() {
        if err != nil {
            receiptDropMeter.Mark(int64(len(receipts)))
        }
    }()
    // Make sure the downloader is active
    if atomic.LoadInt32(&d.synchronising) == 0 {
        return errNoSyncActive
    }
    // Deliver or abort if the sync is canceled while queuing
    d.cancelLock.RLock()
    cancel := d.cancelCh
    d.cancelLock.RUnlock()

    select {
    case d.receiptCh <- &receiptPack{id, receipts}:
        return nil

    case <-cancel:
        return errNoSyncActive
    }
}