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"
    "sync"
    "sync/atomic"
    "time"

    ethereum "github.com/ethereum/go-ethereum"
    "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/log"
    "github.com/ethereum/go-ethereum/params"
    "github.com/ethereum/go-ethereum/trie"
    "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
    MaxSkeletonSize = 128 // Number of header fetches to need for a skeleton assembly
    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

    MaxForkAncestry  = 3 * params.EpochDuration // Maximum chain reorganisation
    rttMinEstimate   = 2 * time.Second          // Minimum round-trip time to target for download requests
    rttMaxEstimate   = 20 * time.Second         // Maximum rount-trip time to target for download requests
    rttMinConfidence = 0.1                      // Worse confidence factor in our estimated RTT value
    ttlScaling       = 3                        // Constant scaling factor for RTT -> TTL conversion
    ttlLimit         = time.Minute              // Maximum TTL allowance to prevent reaching crazy timeouts

    qosTuningPeers   = 5    // Number of peers to tune based on (best peers)
    qosConfidenceCap = 10   // Number of peers above which not to modify RTT confidence
    qosTuningImpact  = 0.25 // Impact that a new tuning target has on the previous value

    maxQueuedHeaders  = 32 * 1024 // [eth/62] Maximum number of headers to queue for import (DOS protection)
    maxHeadersProcess = 2048      // Number of header download results to import at once into the chain
    maxResultsProcess = 2048      // Number of content 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        = 256        // Number of headers out of which to randomize the pivot point
    fsMinFullBlocks        = 64         // Number of blocks to retrieve fully even in fast sync
    fsCriticalTrials       = uint32(32) // Number of times to retry in the cricical section before bailing
)

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")
    errTimeout                 = errors.New("timeout")
    errEmptyHeaderSet          = errors.New("empty header set by peer")
    errPeersUnavailable        = errors.New("no peers available or all tried for download")
    errInvalidAncestor         = errors.New("retrieved ancestor is invalid")
    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")
    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)")
    errCancelHeaderProcessing  = errors.New("header processing canceled (requested)")
    errCancelContentProcessing = errors.New("content processing canceled (requested)")
    errNoSyncActive            = errors.New("no sync active")
    errTooOld                  = errors.New("peer doesn't speak recent enough protocol version (need version >= 62)")
)

type Downloader struct {
    mode SyncMode       // Synchronisation mode defining the strategy used (per sync cycle)
    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

    fsPivotLock  *types.Header // Pivot header on critical section entry (cannot change between retries)
    fsPivotFails uint32        // Number of subsequent fast sync failures in the critical section

    rttEstimate   uint64 // Round trip time to target for download requests
    rttConfidence uint64 // Confidence in the estimated RTT (unit: millionths to allow atomic ops)

    // Statistics
    syncStatsChainOrigin uint64       // Origin block number where syncing started at
    syncStatsChainHeight uint64       // Highest block number known when syncing started
    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
    hasBlockAndState blockAndStateCheckFn     // Checks if a block and associated state 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
    notified        int32

    // Channels
    newPeerCh     chan *peer
    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
    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
    headerProcCh  chan []*types.Header // [eth/62] Channel to feed the header processor new tasks

    // Cancellation and termination
    cancelPeer string        // Identifier of the peer currently being used as the master (cancel on drop)
    cancelCh   chan struct{} // Channel to cancel mid-flight syncs
    cancelLock sync.RWMutex  // Lock to protect the cancel channel and peer in delivers

    quitCh   chan struct{} // Quit channel to signal termination
    quitLock sync.RWMutex  // Lock to prevent double closes

    // 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, stateDb ethdb.Database, mux *event.TypeMux, hasHeader headerCheckFn, hasBlockAndState blockAndStateCheckFn,
    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 {

    dl := &Downloader{
        mode:             mode,
        mux:              mux,
        queue:            newQueue(stateDb),
        peers:            newPeerSet(),
        rttEstimate:      uint64(rttMaxEstimate),
        rttConfidence:    uint64(1000000),
        hasHeader:        hasHeader,
        hasBlockAndState: hasBlockAndState,
        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),
        headerCh:         make(chan dataPack, 1),
        bodyCh:           make(chan dataPack, 1),
        receiptCh:        make(chan dataPack, 1),
        stateCh:          make(chan dataPack, 1),
        bodyWakeCh:       make(chan bool, 1),
        receiptWakeCh:    make(chan bool, 1),
        stateWakeCh:      make(chan bool, 1),
        headerProcCh:     make(chan []*types.Header, 1),
        quitCh:           make(chan struct{}),
    }
    go dl.qosTuner()
    return dl
}

// 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.
//
// In addition, during the state download phase of fast synchronisation the number
// of processed and the total number of known states are also returned. Otherwise
// these are zero.
func (d *Downloader) Progress() ethereum.SyncProgress {
    // Fetch the pending state count outside of the lock to prevent unforeseen deadlocks
    pendingStates := uint64(d.queue.PendingNodeData())

    // Lock the current stats and return the progress
    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 ethereum.SyncProgress{
        StartingBlock: d.syncStatsChainOrigin,
        CurrentBlock:  current,
        HighestBlock:  d.syncStatsChainHeight,
        PulledStates:  d.syncStatsStateDone,
        KnownStates:   d.syncStatsStateDone + pendingStates,
    }
}

// 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, currentHead currentHeadRetrievalFn,
    getRelHeaders relativeHeaderFetcherFn, getAbsHeaders absoluteHeaderFetcherFn, getBlockBodies blockBodyFetcherFn,
    getReceipts receiptFetcherFn, getNodeData stateFetcherFn) error {

    logger := log.New("peer", id)
    logger.Trace("Registering sync peer")
    if err := d.peers.Register(newPeer(id, version, currentHead, getRelHeaders, getAbsHeaders, getBlockBodies, getReceipts, getNodeData, logger)); err != nil {
        logger.Error("Failed to register sync peer", "err", err)
        return err
    }
    d.qosReduceConfidence()

    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 {
    // Unregister the peer from the active peer set and revoke any fetch tasks
    logger := log.New("peer", id)
    logger.Trace("Unregistering sync peer")
    if err := d.peers.Unregister(id); err != nil {
        logger.Error("Failed to unregister sync peer", "err", err)
        return err
    }
    d.queue.Revoke(id)

    // If this peer was the master peer, abort sync immediately
    d.cancelLock.RLock()
    master := id == d.cancelPeer
    d.cancelLock.RUnlock()

    if master {
        d.cancel()
    }
    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) error {
    err := d.synchronise(id, head, td, mode)
    switch err {
    case nil:
    case errBusy:

    case errTimeout, errBadPeer, errStallingPeer,
        errEmptyHeaderSet, errPeersUnavailable, errTooOld,
        errInvalidAncestor, errInvalidChain:
        log.Warn("Synchronisation failed, dropping peer", "peer", id, "err", err)
        d.dropPeer(id)

    default:
        log.Warn("Synchronisation failed, retrying", "err", err)
    }
    return 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 synchronisation 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) {
        log.Info("Block synchronisation started")
    }
    // 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.bodyWakeCh, d.receiptWakeCh, d.stateWakeCh} {
        select {
        case <-ch:
        default:
        }
    }
    for _, ch := range []chan dataPack{d.headerCh, d.bodyCh, d.receiptCh, d.stateCh} {
        for empty := false; !empty; {
            select {
            case <-ch:
            default:
                empty = true
            }
        }
    }
    for empty := false; !empty; {
        select {
        case <-d.headerProcCh:
        default:
            empty = true
        }
    }
    // Create cancel channel for aborting mid-flight and mark the master peer
    d.cancelLock.Lock()
    d.cancelCh = make(chan struct{})
    d.cancelPeer = id
    d.cancelLock.Unlock()

    defer d.cancel() // No matter what, we can't leave the cancel channel open

    // Set the requested sync mode, unless it's forbidden
    d.mode = mode
    if d.mode == FastSync && atomic.LoadUint32(&d.fsPivotFails) >= fsCriticalTrials {
        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.mux.Post(FailedEvent{err})
        } else {
            d.mux.Post(DoneEvent{})
        }
    }()
    if p.version < 62 {
        return errTooOld
    }

    log.Debug("Synchronising with the network", "peer", p.id, "eth", p.version, "head", hash, "td", td, "mode", d.mode)
    defer func(start time.Time) {
        log.Debug("Synchronisation terminated", "elapsed", time.Since(start))
    }(time.Now())

    // Look up the sync boundaries: the common ancestor and the target block
    latest, err := d.fetchHeight(p)
    if err != nil {
        return err
    }
    height := latest.Number.Uint64()

    origin, err := d.findAncestor(p, height)
    if err != nil {
        return err
    }
    d.syncStatsLock.Lock()
    if d.syncStatsChainHeight <= origin || d.syncStatsChainOrigin > origin {
        d.syncStatsChainOrigin = origin
    }
    d.syncStatsChainHeight = height
    d.syncStatsLock.Unlock()

    // Initiate the sync using a concurrent header and content retrieval algorithm
    pivot := uint64(0)
    switch d.mode {
    case LightSync:
        pivot = height
    case FastSync:
        // Calculate the new fast/slow sync pivot point
        if d.fsPivotLock == nil {
            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 height > uint64(fsMinFullBlocks)+pivotOffset.Uint64() {
                pivot = height - uint64(fsMinFullBlocks) - pivotOffset.Uint64()
            }
        } else {
            // Pivot point locked in, use this and do not pick a new one!
            pivot = d.fsPivotLock.Number.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
            }
        }
        log.Debug("Fast syncing until pivot block", "pivot", pivot)
    }
    d.queue.Prepare(origin+1, d.mode, pivot, latest)
    if d.syncInitHook != nil {
        d.syncInitHook(origin, height)
    }
    return d.spawnSync(origin+1,
        func() error { return d.fetchHeaders(p, origin+1) },    // Headers are always retrieved
        func() error { return d.processHeaders(origin+1, td) }, // Headers are always retrieved
        func() error { return d.fetchBodies(origin + 1) },      // Bodies are retrieved during normal and fast sync
        func() error { return d.fetchReceipts(origin + 1) },    // Receipts are retrieved during fast sync
        func() error { return d.fetchNodeData() },              // Node state data is retrieved during fast sync
    )
}

// spawnSync runs d.process and all given fetcher functions to completion in
// separate goroutines, returning the first error that appears.
func (d *Downloader) spawnSync(origin uint64, fetchers ...func() error) error {
    var wg sync.WaitGroup
    errc := make(chan error, len(fetchers)+1)
    wg.Add(len(fetchers) + 1)
    go func() { defer wg.Done(); errc <- d.processContent() }()
    for _, fn := range fetchers {
        fn := fn
        go func() { defer wg.Done(); errc <- fn() }()
    }
    // Wait for the first error, then terminate the others.
    var err error
    for i := 0; i < len(fetchers)+1; i++ {
        if i == len(fetchers) {
            // Close the queue when all fetchers have exited.
            // This will cause the block processor to end when
            // it has processed the queue.
            d.queue.Close()
        }
        if err = <-errc; err != nil {
            break
        }
    }
    d.queue.Close()
    d.cancel()
    wg.Wait()

    // If sync failed in the critical section, bump the fail counter
    if err != nil && d.mode == FastSync && d.fsPivotLock != nil {
        atomic.AddUint32(&d.fsPivotFails, 1)
    }
    return err
}

// 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()
}

// Terminate interrupts the downloader, canceling all pending operations.
// The downloader cannot be reused after calling Terminate.
func (d *Downloader) Terminate() {
    // Close the termination channel (make sure double close is allowed)
    d.quitLock.Lock()
    select {
    case <-d.quitCh:
    default:
        close(d.quitCh)
    }
    d.quitLock.Unlock()

    // Cancel any pending download requests
    d.cancel()
}

// 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) (*types.Header, error) {
    p.logger.Debug("Retrieving remote chain height")

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

    ttl := d.requestTTL()
    timeout := time.After(ttl)
    for {
        select {
        case <-d.cancelCh:
            return nil, errCancelBlockFetch

        case packet := <-d.headerCh:
            // Discard anything not from the origin peer
            if packet.PeerId() != p.id {
                log.Debug("Received headers from incorrect peer", "peer", packet.PeerId())
                break
            }
            // Make sure the peer actually gave something valid
            headers := packet.(*headerPack).headers
            if len(headers) != 1 {
                p.logger.Debug("Multiple headers for single request", "headers", len(headers))
                return nil, errBadPeer
            }
            head := headers[0]
            p.logger.Debug("Remote head header identified", "number", head.Number, "hash", head.Hash())
            return head, nil

        case <-timeout:
            p.logger.Debug("Waiting for head header timed out", "elapsed", ttl)
            return nil, errTimeout

        case <-d.bodyCh:
        case <-d.stateCh:
        case <-d.receiptCh:
            // Out of bounds delivery, ignore
        }
    }
}

// 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 reorganisation (i.e. none of
// the head links match), we do a binary search to find the common ancestor.
func (d *Downloader) findAncestor(p *peer, height uint64) (uint64, error) {
    // Figure out the valid ancestor range to prevent rewrite attacks
    floor, ceil := int64(-1), d.headHeader().Number.Uint64()

    p.logger.Debug("Looking for common ancestor", "local", ceil, "remote", height)
    if d.mode == FullSync {
        ceil = d.headBlock().NumberU64()
    } else if d.mode == FastSync {
        ceil = d.headFastBlock().NumberU64()
    }
    if ceil >= MaxForkAncestry {
        floor = int64(ceil - MaxForkAncestry)
    }
    // Request the topmost blocks to short circuit binary ancestor lookup
    head := ceil
    if head > height {
        head = height
    }
    from := int64(head) - int64(MaxHeaderFetch)
    if from < 0 {
        from = 0
    }
    // Span out with 15 block gaps into the future to catch bad head reports
    limit := 2 * MaxHeaderFetch / 16
    count := 1 + int((int64(ceil)-from)/16)
    if count > limit {
        count = limit
    }
    go p.getAbsHeaders(uint64(from), count, 15, false)

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

    ttl := d.requestTTL()
    timeout := time.After(ttl)

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

        case packet := <-d.headerCh:
            // Discard anything not from the origin peer
            if packet.PeerId() != p.id {
                log.Debug("Received headers from incorrect peer", "peer", packet.PeerId())
                break
            }
            // Make sure the peer actually gave something valid
            headers := packet.(*headerPack).headers
            if len(headers) == 0 {
                p.logger.Warn("Empty head header set")
                return 0, errEmptyHeaderSet
            }
            // Make sure the peer's reply conforms to the request
            for i := 0; i < len(headers); i++ {
                if number := headers[i].Number.Int64(); number != from+int64(i)*16 {
                    p.logger.Warn("Head headers broke chain ordering", "index", i, "requested", from+int64(i)*16, "received", number)
                    return 0, errInvalidChain
                }
            }
            // Check if a common ancestor was found
            finished = true
            for i := len(headers) - 1; i >= 0; i-- {
                // Skip any headers that underflow/overflow our requested set
                if headers[i].Number.Int64() < from || headers[i].Number.Uint64() > ceil {
                    continue
                }
                // Otherwise check if we already know the header or not
                if (d.mode == FullSync && d.hasBlockAndState(headers[i].Hash())) || (d.mode != FullSync && d.hasHeader(headers[i].Hash())) {
                    number, hash = headers[i].Number.Uint64(), headers[i].Hash()

                    // If every header is known, even future ones, the peer straight out lied about its head
                    if number > height && i == limit-1 {
                        p.logger.Warn("Lied about chain head", "reported", height, "found", number)
                        return 0, errStallingPeer
                    }
                    break
                }
            }

        case <-timeout:
            p.logger.Debug("Waiting for head header timed out", "elapsed", ttl)
            return 0, errTimeout

        case <-d.bodyCh:
        case <-d.stateCh:
        case <-d.receiptCh:
            // Out of bounds delivery, ignore
        }
    }
    // If the head fetch already found an ancestor, return
    if !common.EmptyHash(hash) {
        if int64(number) <= floor {
            p.logger.Warn("Ancestor below allowance", "number", number, "hash", hash, "allowance", floor)
            return 0, errInvalidAncestor
        }
        p.logger.Debug("Found common ancestor", "number", number, "hash", hash)
        return number, nil
    }
    // Ancestor not found, we need to binary search over our chain
    start, end := uint64(0), head
    if floor > 0 {
        start = uint64(floor)
    }
    for start+1 < end {
        // Split our chain interval in two, and request the hash to cross check
        check := (start + end) / 2

        ttl := d.requestTTL()
        timeout := time.After(ttl)

        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, errCancelHeaderFetch

            case packer := <-d.headerCh:
                // Discard anything not from the origin peer
                if packer.PeerId() != p.id {
                    log.Debug("Received headers from incorrect peer", "peer", packer.PeerId())
                    break
                }
                // Make sure the peer actually gave something valid
                headers := packer.(*headerPack).headers
                if len(headers) != 1 {
                    p.logger.Debug("Multiple headers for single request", "headers", len(headers))
                    return 0, errBadPeer
                }
                arrived = true

                // Modify the search interval based on the response
                if (d.mode == FullSync && !d.hasBlockAndState(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 {
                    p.logger.Debug("Received non requested header", "number", header.Number, "hash", header.Hash(), "request", check)
                    return 0, errBadPeer
                }
                start = check

            case <-timeout:
                p.logger.Debug("Waiting for search header timed out", "elapsed", ttl)
                return 0, errTimeout

            case <-d.bodyCh:
            case <-d.stateCh:
            case <-d.receiptCh:
                // Out of bounds delivery, ignore
            }
        }
    }
    // Ensure valid ancestry and return
    if int64(start) <= floor {
        p.logger.Warn("Ancestor below allowance", "number", start, "hash", hash, "allowance", floor)
        return 0, errInvalidAncestor
    }
    p.logger.Debug("Found common ancestor", "number", start, "hash", hash)
    return start, nil
}

// fetchHeaders keeps retrieving headers concurrently from the number
// requested, until no more are returned, potentially throttling on the way. To
// facilitate concurrency but still protect against malicious nodes sending bad
// headers, we construct a header chain skeleton using the "origin" peer we are
// syncing with, and fill in the missing headers using anyone else. Headers from
// other peers are only accepted if they map cleanly to the skeleton. If no one
// can fill in the skeleton - not even the origin peer - it's assumed invalid and
// the origin is dropped.
func (d *Downloader) fetchHeaders(p *peer, from uint64) error {
    p.logger.Debug("Directing header downloads", "origin", from)
    defer p.logger.Debug("Header download terminated")

    // Create a timeout timer, and the associated header fetcher
    skeleton := true            // Skeleton assembly phase or finishing up
    request := time.Now()       // time of the last skeleton 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()

    var ttl time.Duration
    getHeaders := func(from uint64) {
        request = time.Now()

        ttl = d.requestTTL()
        timeout.Reset(ttl)

        if skeleton {
            p.logger.Trace("Fetching skeleton headers", "count", MaxHeaderFetch, "from", from)
            go p.getAbsHeaders(from+uint64(MaxHeaderFetch)-1, MaxSkeletonSize, MaxHeaderFetch-1, false)
        } else {
            p.logger.Trace("Fetching full headers", "count", MaxHeaderFetch, "from", from)
            go p.getAbsHeaders(from, MaxHeaderFetch, 0, false)
        }
    }
    // Start pulling the header chain skeleton until all is done
    getHeaders(from)

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

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

            // If the skeleton's finished, pull any remaining head headers directly from the origin
            if packet.Items() == 0 && skeleton {
                skeleton = false
                getHeaders(from)
                continue
            }
            // If no more headers are inbound, notify the content fetchers and return
            if packet.Items() == 0 {
                p.logger.Debug("No more headers available")
                select {
                case d.headerProcCh <- nil:
                    return nil
                case <-d.cancelCh:
                    return errCancelHeaderFetch
                }
            }
            headers := packet.(*headerPack).headers

            // If we received a skeleton batch, resolve internals concurrently
            if skeleton {
                filled, proced, err := d.fillHeaderSkeleton(from, headers)
                if err != nil {
                    p.logger.Debug("Skeleton chain invalid", "err", err)
                    return errInvalidChain
                }
                headers = filled[proced:]
                from += uint64(proced)
            }
            // Insert all the new headers and fetch the next batch
            if len(headers) > 0 {
                p.logger.Trace("Scheduling new headers", "count", len(headers), "from", from)
                select {
                case d.headerProcCh <- headers:
                case <-d.cancelCh:
                    return errCancelHeaderFetch
                }
                from += uint64(len(headers))
            }
            getHeaders(from)

        case <-timeout.C:
            // Header retrieval timed out, consider the peer bad and drop
            p.logger.Debug("Header request timed out", "elapsed", ttl)
            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:
                }
            }
            select {
            case d.headerProcCh <- nil:
            case <-d.cancelCh:
            }
            return errBadPeer
        }
    }
}

// fillHeaderSkeleton concurrently retrieves headers from all our available peers
// and maps them to the provided skeleton header chain.
//
// Any partial results from the beginning of the skeleton is (if possible) forwarded
// immediately to the header processor to keep the rest of the pipeline full even
// in the case of header stalls.
//
// The method returs the entire filled skeleton and also the number of headers
// already forwarded for processing.
func (d *Downloader) fillHeaderSkeleton(from uint64, skeleton []*types.Header) ([]*types.Header, int, error) {
    log.Debug("Filling up skeleton", "from", from)
    d.queue.ScheduleSkeleton(from, skeleton)

    var (
        deliver = func(packet dataPack) (int, error) {
            pack := packet.(*headerPack)
            return d.queue.DeliverHeaders(pack.peerId, pack.headers, d.headerProcCh)
        }
        expire   = func() map[string]int { return d.queue.ExpireHeaders(d.requestTTL()) }
        throttle = func() bool { return false }
        reserve  = func(p *peer, count int) (*fetchRequest, bool, error) {
            return d.queue.ReserveHeaders(p, count), false, nil
        }
        fetch    = func(p *peer, req *fetchRequest) error { return p.FetchHeaders(req.From, MaxHeaderFetch) }
        capacity = func(p *peer) int { return p.HeaderCapacity(d.requestRTT()) }
        setIdle  = func(p *peer, accepted int) { p.SetHeadersIdle(accepted) }
    )
    err := d.fetchParts(errCancelHeaderFetch, d.headerCh, deliver, d.queue.headerContCh, expire,
        d.queue.PendingHeaders, d.queue.InFlightHeaders, throttle, reserve,
        nil, fetch, d.queue.CancelHeaders, capacity, d.peers.HeaderIdlePeers, setIdle, "headers")

    log.Debug("Skeleton fill terminated", "err", err)

    filled, proced := d.queue.RetrieveHeaders()
    return filled, proced, err
}

// 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 {
    log.Debug("Downloading block bodies", "origin", from)

    var (
        deliver = func(packet dataPack) (int, error) {
            pack := packet.(*bodyPack)
            return d.queue.DeliverBodies(pack.peerId, pack.transactions, pack.uncles)
        }
        expire   = func() map[string]int { return d.queue.ExpireBodies(d.requestTTL()) }
        fetch    = func(p *peer, req *fetchRequest) error { return p.FetchBodies(req) }
        capacity = func(p *peer) int { return p.BlockCapacity(d.requestRTT()) }
        setIdle  = func(p *peer, accepted int) { p.SetBodiesIdle(accepted) }
    )
    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, "bodies")

    log.Debug("Block body download terminated", "err", 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 {
    log.Debug("Downloading transaction receipts", "origin", from)

    var (
        deliver = func(packet dataPack) (int, error) {
            pack := packet.(*receiptPack)
            return d.queue.DeliverReceipts(pack.peerId, pack.receipts)
        }
        expire   = func() map[string]int { return d.queue.ExpireReceipts(d.requestTTL()) }
        fetch    = func(p *peer, req *fetchRequest) error { return p.FetchReceipts(req) }
        capacity = func(p *peer) int { return p.ReceiptCapacity(d.requestRTT()) }
        setIdle  = func(p *peer, accepted int) { p.SetReceiptsIdle(accepted) }
    )
    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, "receipts")

    log.Debug("Transaction receipt download terminated", "err", 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 {
    log.Debug("Downloading node state data")

    var (
        deliver = func(packet dataPack) (int, error) {
            start := time.Now()
            return d.queue.DeliverNodeData(packet.PeerId(), packet.(*statePack).states, func(delivered int, progressed bool, err error) {
                // If the peer returned old-requested data, forgive
                if err == trie.ErrNotRequested {
                    log.Debug("Forgiving reply to stale state request", "peer", packet.PeerId())
                    return
                }
                if err != nil {
                    // If the node data processing failed, the root hash is very wrong, abort
                    log.Error("State processing failed", "peer", packet.PeerId(), "err", err)
                    d.cancel()
                    return
                }
                // Processing succeeded, notify state fetcher of continuation
                pending := d.queue.PendingNodeData()
                if pending > 0 {
                    select {
                    case d.stateWakeCh <- true:
                    default:
                    }
                }
                d.syncStatsLock.Lock()
                d.syncStatsStateDone += uint64(delivered)
                syncStatsStateDone := d.syncStatsStateDone // Thread safe copy for the log below
                d.syncStatsLock.Unlock()

                // If real database progress was made, reset any fast-sync pivot failure
                if progressed && atomic.LoadUint32(&d.fsPivotFails) > 1 {
                    log.Debug("Fast-sync progressed, resetting fail counter", "previous", atomic.LoadUint32(&d.fsPivotFails))
                    atomic.StoreUint32(&d.fsPivotFails, 1) // Don't ever reset to 0, as that will unlock the pivot block
                }
                // Log a message to the user and return
                if delivered > 0 {
                    log.Info("Imported new state entries", "count", delivered, "elapsed", common.PrettyDuration(time.Since(start)), "processed", syncStatsStateDone, "pending", pending)
                }
            })
        }
        expire   = func() map[string]int { return d.queue.ExpireNodeData(d.requestTTL()) }
        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(d.requestRTT()) }
        setIdle  = func(p *peer, accepted int) { p.SetNodeDataIdle(accepted) }
    )
    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, "states")

    log.Debug("Node state data download terminated", "err", 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.
//
// As the scheduling/timeout logic mostly is the same for all downloaded data
// types, this method is used by each for data gathering and is instrumented with
// various callbacks to handle the slight differences between processing them.
//
// The instrumentation parameters:
//  - errCancel:   error type to return if the fetch operation is cancelled (mostly makes logging nicer)
//  - deliveryCh:  channel from which to retrieve downloaded data packets (merged from all concurrent peers)
//  - deliver:     processing callback to deliver data packets into type specific download queues (usually within `queue`)
//  - wakeCh:      notification channel for waking the fetcher when new tasks are available (or sync completed)
//  - expire:      task callback method to abort requests that took too long and return the faulty peers (traffic shaping)
//  - pending:     task callback for the number of requests still needing download (detect completion/non-completability)
//  - inFlight:    task callback for the number of in-progress requests (wait for all active downloads to finish)
//  - throttle:    task callback to check if the processing queue is full and activate throttling (bound memory use)
//  - reserve:     task callback to reserve new download tasks to a particular peer (also signals partial completions)
//  - fetchHook:   tester callback to notify of new tasks being initiated (allows testing the scheduling logic)
//  - fetch:       network callback to actually send a particular download request to a physical remote peer
//  - cancel:      task callback to abort an in-flight download request and allow rescheduling it (in case of lost peer)
//  - capacity:    network callback to retrieve the estimated type-specific bandwidth capacity of a peer (traffic shaping)
//  - idle:        network callback to retrieve the currently (type specific) idle peers that can be assigned tasks
//  - setIdle:     network callback to set a peer back to idle and update its estimated capacity (traffic shaping)
//  - kind:        textual label of the type being downloaded to display in log mesages
func (d *Downloader) fetchParts(errCancel error, deliveryCh chan dataPack, deliver func(dataPack) (int, error), wakeCh chan bool,
    expire func() map[string]int, 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, int), 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 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 check chain validity
                accepted, err := deliver(packet)
                if err == errInvalidChain {
                    return err
                }
                // Unless a peer delivered something completely else than requested (usually
                // caused by a timed out request which came through in the end), set it to
                // idle. If the delivery's stale, the peer should have already been idled.
                if err != errStaleDelivery {
                    setIdle(peer, accepted)
                }
                // Issue a log to the user to see what's going on
                switch {
                case err == nil && packet.Items() == 0:
                    peer.logger.Trace("Requested data not delivered", "type", kind)
                case err == nil:
                    peer.logger.Trace("Delivered new batch of data", "type", kind, "count", packet.Stats())
                default:
                    peer.logger.Trace("Failed to deliver retrieved data", "type", kind, "err", err)
                }
            }
            // 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, fails := range expire() {
                if peer := d.peers.Peer(pid); peer != nil {
                    // If a lot of retrieval elements expired, we might have overestimated the remote peer or perhaps
                    // ourselves. Only reset to minimal throughput but don't drop just yet. If even the minimal times
                    // out that sync wise we need to get rid of the peer.
                    //
                    // The reason the minimum threshold is 2 is because the downloader tries to estimate the bandwidth
                    // and latency of a peer separately, which requires pushing the measures capacity a bit and seeing
                    // how response times reacts, to it always requests one more than the minimum (i.e. min 2).
                    if fails > 2 {
                        peer.logger.Trace("Data delivery timed out", "type", kind)
                        setIdle(peer, 0)
                    } else {
                        peer.logger.Debug("Stalling delivery, dropping", "type", kind)
                        d.dropPeer(pid)
                    }
                }
            }
            // If there's nothing more to fetch, wait or terminate
            if pending() == 0 {
                if !inFlight() && finished {
                    log.Debug("Data fetching completed", "type", 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
                }
                if request == nil {
                    continue
                }
                if request.From > 0 {
                    peer.logger.Trace("Requesting new batch of data", "type", kind, "from", request.From)
                } else if len(request.Headers) > 0 {
                    peer.logger.Trace("Requesting new batch of data", "type", kind, "count", len(request.Headers), "from", request.Headers[0].Number)
                } else {
                    peer.logger.Trace("Requesting new batch of data", "type", kind, "count", len(request.Hashes))
                }
                // 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", peer, kind))
                }
                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
            }
        }
    }
}

// processHeaders takes batches of retrieved headers from an input channel and
// keeps processing and scheduling them into the header chain and downloader's
// queue until the stream ends or a failure occurs.
func (d *Downloader) processHeaders(origin uint64, td *big.Int) error {
    // 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()
            }
            lastHeader, lastFastBlock, lastBlock := d.headHeader().Number, common.Big0, common.Big0
            if d.headFastBlock != nil {
                lastFastBlock = d.headFastBlock().Number()
            }
            if d.headBlock != nil {
                lastBlock = d.headBlock().Number()
            }
            d.rollback(hashes)
            curFastBlock, curBlock := common.Big0, common.Big0
            if d.headFastBlock != nil {
                curFastBlock = d.headFastBlock().Number()
            }
            if d.headBlock != nil {
                curBlock = d.headBlock().Number()
            }
            log.Warn("Rolled back headers", "count", len(hashes),
                "header", fmt.Sprintf("%d->%d", lastHeader, d.headHeader().Number),
                "fast", fmt.Sprintf("%d->%d", lastFastBlock, curFastBlock),
                "block", fmt.Sprintf("%d->%d", lastBlock, curBlock))

            // If we're already past the pivot point, this could be an attack, thread carefully
            if rollback[len(rollback)-1].Number.Uint64() > pivot {
                // If we didn't ever fail, lock in te pivot header (must! not! change!)
                if atomic.LoadUint32(&d.fsPivotFails) == 0 {
                    for _, header := range rollback {
                        if header.Number.Uint64() == pivot {
                            log.Warn("Fast-sync critical section failure, locked pivot to header", "number", pivot, "hash", header.Hash())
                            d.fsPivotLock = header
                        }
                    }
                }
            }
        }
    }()

    // Wait for batches of headers to process
    gotHeaders := false

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

        case headers := <-d.headerProcCh:
            // Terminate header processing if we synced up
            if len(headers) == 0 {
                // Notify everyone that headers are fully processed
                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 d.mode != LightSync {
                    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
                    }
                }
                // Disable any rollback and return
                rollback = nil
                return nil
            }
            // Otherwise split the chunk of headers into batches and process them
            gotHeaders = true

            for len(headers) > 0 {
                // Terminate if something failed in between processing chunks
                select {
                case <-d.cancelCh:
                    return errCancelHeaderProcessing
                default:
                }
                // Select the next chunk of headers to import
                limit := maxHeadersProcess
                if limit > len(headers) {
                    limit = len(headers)
                }
                chunk := headers[:limit]

                // In case of header only syncing, validate the chunk immediately
                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 chunk {
                        if !d.hasHeader(header.Hash()) {
                            unknown = append(unknown, header)
                        }
                    }
                    // If we're importing pure headers, verify based on their recentness
                    frequency := fsHeaderCheckFrequency
                    if chunk[len(chunk)-1].Number.Uint64()+uint64(fsHeaderForceVerify) > pivot {
                        frequency = 1
                    }
                    if n, err := d.insertHeaders(chunk, frequency); err != nil {
                        // If some headers were inserted, add them too to the rollback list
                        if n > 0 {
                            rollback = append(rollback, chunk[:n]...)
                        }
                        log.Debug("Invalid header encountered", "number", chunk[n].Number, "hash", chunk[n].Hash(), "err", 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 we're fast syncing and just pulled in the pivot, make sure it's the one locked in
                if d.mode == FastSync && d.fsPivotLock != nil && chunk[0].Number.Uint64() <= pivot && chunk[len(chunk)-1].Number.Uint64() >= pivot {
                    if pivot := chunk[int(pivot-chunk[0].Number.Uint64())]; pivot.Hash() != d.fsPivotLock.Hash() {
                        log.Warn("Pivot doesn't match locked in one", "remoteNumber", pivot.Number, "remoteHash", pivot.Hash(), "localNumber", d.fsPivotLock.Number, "localHash", d.fsPivotLock.Hash())
                        return errInvalidChain
                    }
                }
                // Unless we're doing light chains, schedule the headers for associated content retrieval
                if d.mode == FullSync || d.mode == FastSync {
                    // If we've reached the allowed number of pending headers, stall a bit
                    for d.queue.PendingBlocks() >= maxQueuedHeaders || d.queue.PendingReceipts() >= maxQueuedHeaders {
                        select {
                        case <-d.cancelCh:
                            return errCancelHeaderProcessing
                        case <-time.After(time.Second):
                        }
                    }
                    // Otherwise insert the headers for content retrieval
                    inserts := d.queue.Schedule(chunk, origin)
                    if len(inserts) != len(chunk) {
                        log.Debug("Stale headers")
                        return errBadPeer
                    }
                }
                headers = headers[limit:]
                origin += uint64(limit)
            }
            // Signal the content downloaders of the availablility of new tasks
            for _, ch := range []chan bool{d.bodyWakeCh, d.receiptWakeCh, d.stateWakeCh} {
                select {
                case ch <- true:
                default:
                }
            }
        }
    }
}

// processContent 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.
func (d *Downloader) processContent() error {
    pivot := d.queue.FastSyncPivot()
    for {
        results := d.queue.WaitResults()
        if len(results) == 0 {
            return nil // queue empty
        }
        if d.chainInsertHook != nil {
            d.chainInsertHook(results)
        }
        // Actually import the blocks
        first, last := results[0].Header, results[len(results)-1].Header
        log.Debug("Inserting downloaded chain", "items", len(results),
            "firstnum", first.Number, "firsthash", first.Hash(),
            "lastnum", last.Number, "lasthash", last.Hash(),
        )
        for len(results) != 0 {
            // Check for any termination requests
            select {
            case <-d.quitCh:
                return errCancelContentProcessing
            default:
            }
            // 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 {
                    log.Debug("Committing block as new head", "number", blocks[len(blocks)-1].Number(), "hash", blocks[len(blocks)-1].Hash())
                    index, err = len(blocks)-1, d.commitHeadBlock(blocks[len(blocks)-1].Hash())
                }
            default:
                index, err = d.insertBlocks(blocks)
            }
            if err != nil {
                log.Debug("Downloaded item processing failed", "number", results[index].Header.Number, "hash", results[index].Header.Hash(), "err", err)
                return errInvalidChain
            }
            // Shift the results to the next batch
            results = results[items:]
        }
    }
}

// DeliverHeaders injects a new batch of block 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()))
        }
    }()
    // Deliver or abort if the sync is canceled while queuing
    d.cancelLock.RLock()
    cancel := d.cancelCh
    d.cancelLock.RUnlock()
    if cancel == nil {
        return errNoSyncActive
    }
    select {
    case destCh <- packet:
        return nil
    case <-cancel:
        return errNoSyncActive
    }
}

// qosTuner is the quality of service tuning loop that occasionally gathers the
// peer latency statistics and updates the estimated request round trip time.
func (d *Downloader) qosTuner() {
    for {
        // Retrieve the current median RTT and integrate into the previoust target RTT
        rtt := time.Duration(float64(1-qosTuningImpact)*float64(atomic.LoadUint64(&d.rttEstimate)) + qosTuningImpact*float64(d.peers.medianRTT()))
        atomic.StoreUint64(&d.rttEstimate, uint64(rtt))

        // A new RTT cycle passed, increase our confidence in the estimated RTT
        conf := atomic.LoadUint64(&d.rttConfidence)
        conf = conf + (1000000-conf)/2
        atomic.StoreUint64(&d.rttConfidence, conf)

        // Log the new QoS values and sleep until the next RTT
        log.Debug("Recalculated downloader QoS values", "rtt", rtt, "confidence", float64(conf)/1000000.0, "ttl", d.requestTTL())
        select {
        case <-d.quitCh:
            return
        case <-time.After(rtt):
        }
    }
}

// qosReduceConfidence is meant to be called when a new peer joins the downloader's
// peer set, needing to reduce the confidence we have in out QoS estimates.
func (d *Downloader) qosReduceConfidence() {
    // If we have a single peer, confidence is always 1
    peers := uint64(d.peers.Len())
    if peers == 1 {
        atomic.StoreUint64(&d.rttConfidence, 1000000)
        return
    }
    // If we have a ton of peers, don't drop confidence)
    if peers >= uint64(qosConfidenceCap) {
        return
    }
    // Otherwise drop the confidence factor
    conf := atomic.LoadUint64(&d.rttConfidence) * (peers - 1) / peers
    if float64(conf)/1000000 < rttMinConfidence {
        conf = uint64(rttMinConfidence * 1000000)
    }
    atomic.StoreUint64(&d.rttConfidence, conf)

    rtt := time.Duration(atomic.LoadUint64(&d.rttEstimate))
    log.Debug("Relaxed downloader QoS values", "rtt", rtt, "confidence", float64(conf)/1000000.0, "ttl", d.requestTTL())
}

// requestRTT returns the current target round trip time for a download request
// to complete in.
//
// Note, the returned RTT is .9 of the actually estimated RTT. The reason is that
// the downloader tries to adapt queries to the RTT, so multiple RTT values can
// be adapted to, but smaller ones are preffered (stabler download stream).
func (d *Downloader) requestRTT() time.Duration {
    return time.Duration(atomic.LoadUint64(&d.rttEstimate)) * 9 / 10
}

// requestTTL returns the current timeout allowance for a single download request
// to finish under.
func (d *Downloader) requestTTL() time.Duration {
    var (
        rtt  = time.Duration(atomic.LoadUint64(&d.rttEstimate))
        conf = float64(atomic.LoadUint64(&d.rttConfidence)) / 1000000.0
    )
    ttl := time.Duration(ttlScaling) * time.Duration(float64(rtt)/conf)
    if ttl > ttlLimit {
        ttl = ttlLimit
    }
    return ttl
}