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 (
    "crypto/rand"
    "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/ethdb"
    "github.com/ethereum/go-ethereum/event"
    "github.com/ethereum/go-ethereum/logger"
    "github.com/ethereum/go-ethereum/logger/glog"
    "github.com/rcrowley/go-metrics"
)

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
    MaxReceiptFetch = 256 // Amount of transaction receipts to allow fetching per request
    MaxStateFetch   = 384 // Amount of node state values 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 receipt request is considered expired
    stateSoftTTL   = 2 * time.Second    // [eth/63] Request completion threshold for increasing or decreasing a peer's bandwidth
    stateHardTTL   = 3 * stateSoftTTL   // [eth/63] Maximum time allowance before a node data 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)
    maxQueuedStates   = 256 * 1024 // [eth/63] Maximum number of state requests to queue (DOS protection)
    maxResultsProcess = 256        // Number of download results to import at once into the chain

    fsHeaderCheckFrequency = 100  // Verification frequency of the downloaded headers during fast sync
    fsHeaderSafetyNet      = 2048 // Number of headers to discard in case a chain violation is detected
    fsHeaderForceVerify    = 24   // Number of headers to verify before and after the pivot to accept it
    fsPivotInterval        = 512  // Number of headers out of which to randomize the pivot point
    fsMinFullBlocks        = 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)")
    errCancelStateFetch   = errors.New("state data download canceled (requested)")
    errNoSyncActive       = errors.New("no sync active")
)

type Downloader struct {
    mode   SyncMode       // Synchronisation mode defining the strategy used (per sync cycle)
    noFast bool           // Flag to disable fast syncing in case of a security error
    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
    syncStatsChainOrigin uint64       // Origin block number where syncing started at
    syncStatsChainHeight uint64       // Highest block number known when syncing started
    syncStatsStateTotal  uint64       // Total number of node state entries known so far
    syncStatsStateDone   uint64       // Number of state trie entries already pulled
    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
    headFastBlock   headFastBlockRetrievalFn // Retrieves the head fast-sync block from the chain
    commitHeadBlock headBlockCommitterFn     // Commits a manually assembled block as the chain head
    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
    rollback        chainRollbackFn          // Removes a batch of recently added chain links
    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 dataPack // [eth/61] Channel receiving inbound hashes
    blockCh       chan dataPack // [eth/61] Channel receiving inbound blocks
    headerCh      chan dataPack // [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
    stateCh       chan dataPack // [eth/63] Channel receiving inbound node state data
    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
    stateWakeCh   chan bool     // [eth/63] Channel to signal the state 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(stateDb ethdb.Database, mux *event.TypeMux, hasHeader headerCheckFn, hasBlock blockCheckFn, getHeader headerRetrievalFn,
    getBlock blockRetrievalFn, headHeader headHeaderRetrievalFn, headBlock headBlockRetrievalFn, headFastBlock headFastBlockRetrievalFn,
    commitHeadBlock headBlockCommitterFn, getTd tdRetrievalFn, insertHeaders headerChainInsertFn, insertBlocks blockChainInsertFn,
    insertReceipts receiptChainInsertFn, rollback chainRollbackFn, dropPeer peerDropFn) *Downloader {

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

// Progress retrieves the synchronisation boundaries, specifically the origin
// block where synchronisation started at (may have failed/suspended); the block
// or header sync is currently at; and the latest known block which the sync targets.
func (d *Downloader) Progress() (uint64, uint64, uint64) {
    d.syncStatsLock.RLock()
    defer d.syncStatsLock.RUnlock()

    current := uint64(0)
    switch d.mode {
    case FullSync:
        current = d.headBlock().NumberU64()
    case FastSync:
        current = d.headFastBlock().NumberU64()
    case LightSync:
        current = d.headHeader().Number.Uint64()
    }
    return d.syncStatsChainOrigin, current, d.syncStatsChainHeight
}

// Synchronising returns whether the downloader is currently retrieving blocks.
func (d *Downloader) Synchronising() bool {
    return atomic.LoadInt32(&d.synchronising) > 0 || atomic.LoadInt32(&d.processing) > 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, getNodeData stateFetcherFn) 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, getNodeData)); 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, mode SyncMode) {
    glog.V(logger.Detail).Infof("Attempting synchronisation: %v, head [%x…], TD %v", id, head[:4], td)

    switch err := d.synchronise(id, head, td, mode); 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, mode SyncMode) 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, d.stateWakeCh} {
        select {
        case <-ch:
        default:
        }
    }
    // Reset and ephemeral sync statistics
    d.syncStatsLock.Lock()
    d.syncStatsStateTotal = 0
    d.syncStatsStateDone = 0
    d.syncStatsLock.Unlock()

    // Create cancel channel for aborting mid-flight
    d.cancelLock.Lock()
    d.cancelCh = make(chan struct{})
    d.cancelLock.Unlock()

    // Set the requested sync mode, unless it's forbidden
    d.mode = mode
    if d.mode == FastSync && d.noFast {
        d.mode = FullSync
    }
    // 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)
}

// 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 func(start time.Time) {
        glog.V(logger.Debug).Infof("Synchronisation terminated after %v", time.Since(start))
    }(time.Now())

    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.syncStatsChainHeight <= origin || d.syncStatsChainOrigin > origin {
            d.syncStatsChainOrigin = origin
        }
        d.syncStatsChainHeight = 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, d.mode, 0)

        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.syncStatsChainHeight <= origin || d.syncStatsChainOrigin > origin {
            d.syncStatsChainOrigin = origin
        }
        d.syncStatsChainHeight = latest
        d.syncStatsLock.Unlock()

        // Initiate the sync using a concurrent header and content retrieval algorithm
        pivot := uint64(0)
        switch d.mode {
        case LightSync:
            pivot = latest

        case FastSync:
            // Calculate the new fast/slow sync pivot point
            pivotOffset, err := rand.Int(rand.Reader, big.NewInt(int64(fsPivotInterval)))
            if err != nil {
                panic(fmt.Sprintf("Failed to access crypto random source: %v", err))
            }
            if latest > uint64(fsMinFullBlocks)+pivotOffset.Uint64() {
                pivot = latest - uint64(fsMinFullBlocks) - pivotOffset.Uint64()
            }
            // If the point is below the origin, move origin back to ensure state download
            if pivot < origin {
                if pivot > 0 {
                    origin = pivot - 1
                } else {
                    origin = 0
                }
            }
            glog.V(logger.Debug).Infof("Fast syncing until pivot block #%d", pivot)
        }
        d.queue.Prepare(origin+1, d.mode, pivot)

        if d.syncInitHook != nil {
            d.syncInitHook(origin, latest)
        }
        errc := make(chan error, 4)
        go func() { errc <- d.fetchHeaders(p, td, origin+1) }() // Headers are always retrieved
        go func() { errc <- d.fetchBodies(origin + 1) }()       // Bodies are retrieved during normal and fast sync
        go func() { errc <- d.fetchReceipts(origin + 1) }()     // Receipts are retrieved during fast sync
        go func() { errc <- d.fetchNodeData() }()               // Node state data is retrieved during fast 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 packet := <-d.blockCh:
            // Discard anything not from the origin peer
            if packet.PeerId() != p.id {
                glog.V(logger.Debug).Infof("Received blocks from incorrect peer(%s)", packet.PeerId())
                break
            }
            // Make sure the peer actually gave something valid
            blocks := packet.(*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 packet := <-d.hashCh:
            // Discard anything not from the origin peer
            if packet.PeerId() != p.id {
                glog.V(logger.Debug).Infof("Received hashes from incorrect peer(%s)", packet.PeerId())
                break
            }
            // Make sure the peer actually gave something valid
            hashes := packet.(*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 packet := <-d.hashCh:
                // Discard anything not from the origin peer
                if packet.PeerId() != p.id {
                    glog.V(logger.Debug).Infof("Received hashes from incorrect peer(%s)", packet.PeerId())
                    break
                }
                // Make sure the peer actually gave something valid
                hashes := packet.(*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 packet := <-d.hashCh:
            // Make sure the active peer is giving us the hashes
            if packet.PeerId() != p.id {
                glog.V(logger.Debug).Infof("Received hashes from incorrect peer(%s)", packet.PeerId())
                break
            }
            hashReqTimer.UpdateSince(request)
            timeout.Stop()

            // If no more hashes are inbound, notify the block fetcher and return
            if packet.Items() == 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
            hashes := packet.(*hashPack).hashes

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

            inserts := d.queue.Schedule61(hashes, true)
            if len(inserts) != len(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(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 packet := <-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(packet.PeerId()); peer != nil {
                // Deliver the received chunk of blocks, and demote in case of errors
                blocks := packet.(*blockPack).blocks
                err := d.queue.DeliverBlocks(peer.id, blocks)
                switch err {
                case nil:
                    // If no blocks were delivered, demote the peer (need the delivery above)
                    if len(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(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.ExpireBlocks(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.InFlightBlocks() && 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()

            for _, peer := range idles {
                // Short circuit if throttling activated
                if d.queue.ShouldThrottleBlocks() {
                    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.ReserveBlocks(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 {
                    // Although we could try and make an attempt to fix this, this error really
                    // means that we've double allocated a fetch task to a peer. If that is the
                    // case, the internal state of the downloader and the queue is very wrong so
                    // better hard crash and note the error instead of silently accumulating into
                    // a much bigger issue.
                    panic(fmt.Sprintf("%v: fetch assignment failed, hard panic", peer))
                    d.queue.CancelBlocks(request) // noop for now
                }
            }
            // 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.InFlightBlocks() && 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 packet := <-d.headerCh:
            // Discard anything not from the origin peer
            if packet.PeerId() != p.id {
                glog.V(logger.Debug).Infof("Received headers from incorrect peer(%s)", packet.PeerId())
                break
            }
            // Make sure the peer actually gave something valid
            headers := packet.(*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()
    } else if d.mode == FastSync {
        head = d.headFastBlock().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 packet := <-d.headerCh:
            // Discard anything not from the origin peer
            if packet.PeerId() != p.id {
                glog.V(logger.Debug).Infof("Received headers from incorrect peer(%s)", packet.PeerId())
                break
            }
            // Make sure the peer actually gave something valid
            headers := packet.(*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 != LightSync && d.hasBlock(headers[i].Hash())) || (d.mode == LightSync && 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 packer := <-d.headerCh:
                // Discard anything not from the origin peer
                if packer.PeerId() != p.id {
                    glog.V(logger.Debug).Infof("Received headers from incorrect peer(%s)", packer.PeerId())
                    break
                }
                // Make sure the peer actually gave something valid
                headers := packer.(*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)

    // Calculate the pivoting point for switching from fast to slow sync
    pivot := d.queue.FastSyncPivot()

    // Keep a count of uncertain headers to roll back
    rollback := []*types.Header{}
    defer func() {
        if len(rollback) > 0 {
            // Flatten the headers and roll them back
            hashes := make([]common.Hash, len(rollback))
            for i, header := range rollback {
                hashes[i] = header.Hash()
            }
            lh, lfb, lb := d.headHeader().Number, d.headFastBlock().Number(), d.headBlock().Number()
            d.rollback(hashes)
            glog.V(logger.Warn).Infof("Rolled back %d headers (LH: %d->%d, FB: %d->%d, LB: %d->%d)",
                len(hashes), lh, d.headHeader().Number, lfb, d.headFastBlock().Number(), lb, d.headBlock().Number())

            // If we're already past the pivot point, this could be an attack, disable fast sync
            if rollback[len(rollback)-1].Number.Uint64() > pivot {
                d.noFast = true
            }
        }
    }()

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

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

                for _, ch := range []chan bool{d.bodyWakeCh, d.receiptWakeCh, d.stateWakeCh} {
                    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
                }
                // If fast or light syncing, ensure promised headers are indeed delivered. This is
                // needed to detect scenarios where an attacker feeds a bad pivot and then bails out
                // of delivering the post-pivot blocks that would flag the invalid content.
                //
                // This check cannot be executed "as is" for full imports, since blocks may still be
                // queued for processing when the header download completes. However, as long as the
                // peer gave us something useful, we're already happy/progressed (above check).
                if d.mode == FastSync || d.mode == LightSync {
                    if td.Cmp(d.getTd(d.headHeader().Hash())) > 0 {
                        return errStallingPeer
                    }
                }
                rollback = nil
                return nil
            }
            gotHeaders = true
            headers := packet.(*headerPack).headers

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

            if d.mode == FastSync || d.mode == LightSync {
                // Collect the yet unknown headers to mark them as uncertain
                unknown := make([]*types.Header, 0, len(headers))
                for _, header := range headers {
                    if !d.hasHeader(header.Hash()) {
                        unknown = append(unknown, header)
                    }
                }
                // If we're importing pure headers, verify based on their recentness
                frequency := fsHeaderCheckFrequency
                if headers[len(headers)-1].Number.Uint64()+uint64(fsHeaderForceVerify) > pivot {
                    frequency = 1
                }
                if n, err := d.insertHeaders(headers, frequency); err != nil {
                    glog.V(logger.Debug).Infof("%v: invalid header #%d [%x…]: %v", p, headers[n].Number, headers[n].Hash().Bytes()[:4], err)
                    return errInvalidChain
                }
                // All verifications passed, store newly found uncertain headers
                rollback = append(rollback, unknown...)
                if len(rollback) > fsHeaderSafetyNet {
                    rollback = append(rollback[:0], rollback[len(rollback)-fsHeaderSafetyNet:]...)
                }
            }
            if d.mode == FullSync || d.mode == FastSync {
                inserts := d.queue.Schedule(headers, from)
                if len(inserts) != len(headers) {
                    glog.V(logger.Debug).Infof("%v: stale headers", p)
                    return errBadPeer
                }
            }
            // 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, d.stateWakeCh} {
                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(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, d.stateWakeCh} {
                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 dataPack) error {
            pack := packet.(*bodyPack)
            return d.queue.DeliverBodies(pack.peerId, pack.transactions, pack.uncles)
        }
        expire   = func() []string { return d.queue.ExpireBodies(bodyHardTTL) }
        fetch    = func(p *peer, req *fetchRequest) error { return p.FetchBodies(req) }
        capacity = func(p *peer) int { return p.BlockCapacity() }
        setIdle  = func(p *peer) { p.SetBodiesIdle() }
    )
    err := d.fetchParts(errCancelBodyFetch, d.bodyCh, deliver, d.bodyWakeCh, expire,
        d.queue.PendingBlocks, d.queue.InFlightBlocks, d.queue.ShouldThrottleBlocks, d.queue.ReserveBodies,
        d.bodyFetchHook, fetch, d.queue.CancelBodies, capacity, d.peers.BodyIdlePeers, 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 dataPack) error {
            pack := packet.(*receiptPack)
            return d.queue.DeliverReceipts(pack.peerId, pack.receipts)
        }
        expire   = func() []string { return d.queue.ExpireReceipts(receiptHardTTL) }
        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(errCancelReceiptFetch, d.receiptCh, deliver, d.receiptWakeCh, expire,
        d.queue.PendingReceipts, d.queue.InFlightReceipts, d.queue.ShouldThrottleReceipts, 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
}

// fetchNodeData iteratively downloads the scheduled state trie nodes, taking any
// available peers, reserving a chunk of nodes for each, waiting for delivery and
// also periodically checking for timeouts.
func (d *Downloader) fetchNodeData() error {
    glog.V(logger.Debug).Infof("Downloading node state data")

    var (
        deliver = func(packet dataPack) error {
            start := time.Now()
            return d.queue.DeliverNodeData(packet.PeerId(), packet.(*statePack).states, func(err error, delivered int) {
                if err != nil {
                    // If the node data processing failed, the root hash is very wrong, abort
                    glog.V(logger.Error).Infof("peer %d: state processing failed: %v", packet.PeerId(), err)
                    d.cancel()
                    return
                }
                // Processing succeeded, notify state fetcher and processor of continuation
                if d.queue.PendingNodeData() == 0 {
                    go d.process()
                } else {
                    select {
                    case d.stateWakeCh <- true:
                    default:
                    }
                }
                // Log a message to the user and return
                d.syncStatsLock.Lock()
                defer d.syncStatsLock.Unlock()

                d.syncStatsStateDone += uint64(delivered)
                glog.V(logger.Info).Infof("imported %d state entries in %v: processed %d in total", delivered, time.Since(start), d.syncStatsStateDone)
            })
        }
        expire   = func() []string { return d.queue.ExpireNodeData(stateHardTTL) }
        throttle = func() bool { return false }
        reserve  = func(p *peer, count int) (*fetchRequest, bool, error) {
            return d.queue.ReserveNodeData(p, count), false, nil
        }
        fetch    = func(p *peer, req *fetchRequest) error { return p.FetchNodeData(req) }
        capacity = func(p *peer) int { return p.NodeDataCapacity() }
        setIdle  = func(p *peer) { p.SetNodeDataIdle() }
    )
    err := d.fetchParts(errCancelStateFetch, d.stateCh, deliver, d.stateWakeCh, expire,
        d.queue.PendingNodeData, d.queue.InFlightNodeData, throttle, reserve, nil, fetch,
        d.queue.CancelNodeData, capacity, d.peers.NodeDataIdlePeers, setIdle, "State")

    glog.V(logger.Debug).Infof("Node state data 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(errCancel error, deliveryCh chan dataPack, deliver func(packet dataPack) error, wakeCh chan bool,
    expire func() []string, pending func() int, inFlight func() bool, 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 retrieval 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.Items() == 0 {
                        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 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()
                    setIdle(peer)
                    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 !inFlight() && 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, running := false, false, inFlight()
            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) {
                    if len(request.Headers) > 0 {
                        glog.Infof("%s: requesting %d %s(s), first at #%d", peer, len(request.Headers), strings.ToLower(kind), request.Headers[0].Number)
                    } else {
                        glog.Infof("%s: requesting %d %s(s)", peer, len(request.Hashes), strings.ToLower(kind))
                    }
                }
                // 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 {
                    // Although we could try and make an attempt to fix this, this error really
                    // means that we've double allocated a fetch task to a peer. If that is the
                    // case, the internal state of the downloader and the queue is very wrong so
                    // better hard crash and note the error instead of silently accumulating into
                    // a much bigger issue.
                    panic(fmt.Sprintf("%v: %s fetch assignment failed, hard panic", peer, strings.ToLower(kind)))
                    cancel(request) // noop for now
                }
                running = true
            }
            // Make sure that we have peers available for fetching. If all peers have been tried
            // and all failed throw an error
            if !progressed && !throttled && !running && len(idles) == total && pending() > 0 {
                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
        pivot := d.queue.FastSyncPivot() // Fetch pivot before results to prevent reset race
        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.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 (
                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))
                    if result.Header.Number.Uint64() <= pivot {
                        receipts = append(receipts, result.Receipts)
                    }
                }
            }
            // Try to process the results, aborting if there's an error
            var (
                err   error
                index int
            )
            switch {
            case len(receipts) > 0:
                index, err = d.insertReceipts(blocks, receipts)
                if err == nil && blocks[len(blocks)-1].NumberU64() == pivot {
                    glog.V(logger.Debug).Infof("Committing block #%d [%x…] as the new head", blocks[len(blocks)-1].Number(), blocks[len(blocks)-1].Hash().Bytes()[:4])
                    index, err = len(blocks)-1, d.commitHeadBlock(blocks[len(blocks)-1].Hash())
                }
            default:
                index, err = d.insertBlocks(blocks)
            }
            if err != nil {
                glog.V(logger.Debug).Infof("Result #%d [%x…] processing failed: %v", results[index].Header.Number, results[index].Header.Hash().Bytes()[:4], err)
                d.cancel()
                return
            }
            // Shift the results to the next batch
            results = results[items:]
        }
    }
}

// DeliverHashes 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) DeliverHashes(id string, hashes []common.Hash) (err error) {
    return d.deliver(id, d.hashCh, &hashPack{id, hashes}, hashInMeter, hashDropMeter)
}

// DeliverBlocks 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) DeliverBlocks(id string, blocks []*types.Block) (err error) {
    return d.deliver(id, d.blockCh, &blockPack{id, blocks}, blockInMeter, blockDropMeter)
}

// 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) {
    return d.deliver(id, d.headerCh, &headerPack{id, headers}, headerInMeter, headerDropMeter)
}

// 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) {
    return d.deliver(id, d.bodyCh, &bodyPack{id, transactions, uncles}, bodyInMeter, bodyDropMeter)
}

// DeliverReceipts injects a new batch of receipts received from a remote node.
func (d *Downloader) DeliverReceipts(id string, receipts [][]*types.Receipt) (err error) {
    return d.deliver(id, d.receiptCh, &receiptPack{id, receipts}, receiptInMeter, receiptDropMeter)
}

// DeliverNodeData injects a new batch of node state data received from a remote node.
func (d *Downloader) DeliverNodeData(id string, data [][]byte) (err error) {
    return d.deliver(id, d.stateCh, &statePack{id, data}, stateInMeter, stateDropMeter)
}

// deliver injects a new batch of data received from a remote node.
func (d *Downloader) deliver(id string, destCh chan dataPack, packet dataPack, inMeter, dropMeter metrics.Meter) (err error) {
    // Update the delivery metrics for both good and failed deliveries
    inMeter.Mark(int64(packet.Items()))
    defer func() {
        if err != nil {
            dropMeter.Mark(int64(packet.Items()))
        }
    }()
    // 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 destCh <- packet:
        return nil

    case <-cancel:
        return errNoSyncActive
    }
}