path: root/eth/downloader/downloader.go

// Copyright 2015 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

// Package downloader contains the manual full chain synchronisation.
package downloader

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

    "github.com/ethereum/go-ethereum/common"
    "github.com/ethereum/go-ethereum/core"
    "github.com/ethereum/go-ethereum/core/types"
    "github.com/ethereum/go-ethereum/event"
    "github.com/ethereum/go-ethereum/logger"
    "github.com/ethereum/go-ethereum/logger/glog"
    "gopkg.in/fatih/set.v0"
)

const (
    eth61 = 61 // Constant to check for old protocol support
    eth62 = 62 // Constant to check for new protocol support
)

var (
    MinHashFetch     = 512 // Minimum amount of hashes to not consider a peer stalling
    MaxHashFetch     = 512 // Amount of hashes to be fetched per retrieval request
    MaxBlockFetch    = 128 // Amount of blocks to be fetched per retrieval request
    MaxHeaderFetch   = 256 // Amount of block headers to be fetched per retrieval request
    MaxStateFetch    = 384 // Amount of node state values to allow fetching per request
    MaxReceiptsFetch = 384 // Amount of transaction receipts to allow fetching per request

    hashTTL         = 5 * time.Second  // Time it takes for a hash request to time out
    blockSoftTTL    = 3 * time.Second  // Request completion threshold for increasing or decreasing a peer's bandwidth
    blockHardTTL    = 3 * blockSoftTTL // Maximum time allowance before a block request is considered expired
    crossCheckCycle = time.Second      // Period after which to check for expired cross checks

    maxQueuedHashes = 256 * 1024 // Maximum number of hashes to queue for import (DOS protection)
    maxBannedHashes = 4096       // Number of bannable hashes before phasing old ones out
    maxBlockProcess = 256        // Number of blocks to import at once into the chain
)

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")
    errBannedHead       = errors.New("peer head hash already banned")
    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")
    errPeersUnavailable = errors.New("no peers available or all peers tried for block download process")
    errAlreadyInPool    = errors.New("hash already in pool")
    errInvalidChain     = errors.New("retrieved hash chain is invalid")
    errCrossCheckFailed = errors.New("block cross-check failed")
    errCancelHashFetch  = errors.New("hash fetching canceled (requested)")
    errCancelBlockFetch = errors.New("block downloading canceled (requested)")
    errNoSyncActive     = errors.New("no sync active")
)

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

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

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

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

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

type blockPack struct {
    peerId string
    blocks []*types.Block
}

type hashPack struct {
    peerId string
    hashes []common.Hash
}

type crossCheck struct {
    expire time.Time
    parent common.Hash
}

type Downloader struct {
    mux *event.TypeMux

    queue  *queue                      // Scheduler for selecting the hashes to download
    peers  *peerSet                    // Set of active peers from which download can proceed
    checks map[common.Hash]*crossCheck // Pending cross checks to verify a hash chain
    banned *set.Set                    // Set of hashes we've received and banned

    interrupt int32 // Atomic boolean to signal termination

    // Statistics
    importStart time.Time // Instance when the last blocks were taken from the cache
    importQueue []*Block  // Previously taken blocks to check import progress
    importDone  int       // Number of taken blocks already imported from the last batch
    importLock  sync.Mutex

    // Callbacks
    hasBlock    hashCheckFn      // Checks if a block is present in the chain
    getBlock    blockRetrievalFn // Retrieves a block from the chain
    headBlock   headRetrievalFn  // Retrieves the head block from the chain
    insertChain chainInsertFn    // Injects a batch of blocks into the chain
    dropPeer    peerDropFn       // Drops a peer for misbehaving

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

    // Channels
    newPeerCh chan *peer
    hashCh    chan hashPack  // Channel receiving inbound hashes
    blockCh   chan blockPack // Channel receiving inbound blocks
    processCh chan bool      // Channel to signal the block fetcher of new or finished work

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

// Block is an origin-tagged blockchain block.
type Block struct {
    RawBlock   *types.Block
    OriginPeer string
}

// New creates a new downloader to fetch hashes and blocks from remote peers.
func New(mux *event.TypeMux, hasBlock hashCheckFn, getBlock blockRetrievalFn, headBlock headRetrievalFn, insertChain chainInsertFn, dropPeer peerDropFn) *Downloader {
    // Create the base downloader
    downloader := &Downloader{
        mux:         mux,
        queue:       newQueue(),
        peers:       newPeerSet(),
        hasBlock:    hasBlock,
        getBlock:    getBlock,
        headBlock:   headBlock,
        insertChain: insertChain,
        dropPeer:    dropPeer,
        newPeerCh:   make(chan *peer, 1),
        hashCh:      make(chan hashPack, 1),
        blockCh:     make(chan blockPack, 1),
        processCh:   make(chan bool, 1),
    }
    // Inject all the known bad hashes
    downloader.banned = set.New()
    for hash, _ := range core.BadHashes {
        downloader.banned.Add(hash)
    }
    return downloader
}

// Stats retrieves the current status of the downloader.
func (d *Downloader) Stats() (pending int, cached int, importing int, estimate time.Duration) {
    // Fetch the download status
    pending, cached = d.queue.Size()

    // Figure out the import progress
    d.importLock.Lock()
    defer d.importLock.Unlock()

    for len(d.importQueue) > 0 && d.hasBlock(d.importQueue[0].RawBlock.Hash()) {
        d.importQueue = d.importQueue[1:]
        d.importDone++
    }
    importing = len(d.importQueue)

    // Make an estimate on the total sync
    estimate = 0
    if d.importDone > 0 {
        estimate = time.Since(d.importStart) / time.Duration(d.importDone) * time.Duration(pending+cached+importing)
    }
    return
}

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

// RegisterPeer injects a new download peer into the set of block source to be
// used for fetching hashes and blocks from.
func (d *Downloader) RegisterPeer(id string, version int, head common.Hash, getRelHashes relativeHashFetcherFn, getAbsHashes absoluteHashFetcherFn, getBlocks blockFetcherFn) error {
    // If the peer wants to send a banned hash, reject
    if d.banned.Has(head) {
        glog.V(logger.Debug).Infoln("Register rejected, head hash banned:", id)
        return errBannedHead
    }
    // Otherwise try to construct and register the peer
    glog.V(logger.Detail).Infoln("Registering peer", id)
    if err := d.peers.Register(newPeer(id, version, head, getRelHashes, getAbsHashes, getBlocks)); 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.
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
    }
    return nil
}

// Synchronise tries to sync up our local block chain with a remote peer, both
// adding various sanity checks as well as wrapping it with various log entries.
func (d *Downloader) Synchronise(id string, head common.Hash, td *big.Int) {
    glog.V(logger.Detail).Infof("Attempting synchronisation: %v, head 0x%x, TD %v", id, head[:4], td)

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

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

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

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

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

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

    // If the head hash is banned, terminate immediately
    if d.banned.Has(hash) {
        return errBannedHead
    }
    // 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 _, cached := d.queue.Size(); cached > 0 && d.queue.GetHeadBlock() != nil {
        return errPendingQueue
    }
    // Reset the queue and peer set to clean any internal leftover state
    d.queue.Reset()
    d.peers.Reset()
    d.checks = make(map[common.Hash]*crossCheck)

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

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

// Has checks if the downloader knows about a particular hash, meaning that its
// either already downloaded of pending retrieval.
func (d *Downloader) Has(hash common.Hash) bool {
    return d.queue.Has(hash)
}

// syncWithPeer starts a block synchronization based on the hash chain from the
// specified peer and head hash.
func (d *Downloader) syncWithPeer(p *peer, hash common.Hash, td *big.Int) (err error) {
    d.mux.Post(StartEvent{})
    defer func() {
        // reset on error
        if err != nil {
            d.cancel()
            d.mux.Post(FailedEvent{err})
        } else {
            d.mux.Post(DoneEvent{})
        }
    }()

    glog.V(logger.Debug).Infof("Synchronizing with the network using: %s, eth/%d", p.id, p.version)
    switch p.version {
    case eth61:
        // Old eth/61, use forward, concurrent hash and block retrieval algorithm
        number, err := d.findAncestor(p)
        if err != nil {
            return err
        }
        errc := make(chan error, 2)
        go func() { errc <- d.fetchHashes(p, td, number+1) }()
        go func() { errc <- d.fetchBlocks(number + 1) }()

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

    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()
}

// findAncestor 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 soft fork (i.e. none of the
// head blocks 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 out head blocks to short circuit ancestor location
    head := d.headBlock().NumberU64()
    from := int64(head) - int64(MaxHashFetch)
    if from < 0 {
        from = 0
    }
    go p.getAbsHashes(uint64(from), MaxHashFetch)

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

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

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

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

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

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

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

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

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

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

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

// fetchHashes keeps retrieving hashes from the requested number, until no more
// are returned, potentially throttling on the way.
func (d *Downloader) fetchHashes(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
    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)
        timeout.Reset(hashTTL)
    }
    // Start pulling hashes, until all are exhausted
    getHashes(from)
    gotHashes := false

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

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

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

                select {
                case d.processCh <- 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.headBlock().Td) > 0 {
                    return errStallingPeer
                }
                return nil
            }
            gotHashes = true

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

            inserts := d.queue.Insert(hashPack.hashes, true)
            if len(inserts) != len(hashPack.hashes) {
                glog.V(logger.Debug).Infof("%v: stale hashes", p)
                return errBadPeer
            }
            // Notify the block fetcher of new hashes, but stop if queue is full
            cont := d.queue.Pending() < maxQueuedHashes
            select {
            case d.processCh <- cont:
            default:
            }
            if !cont {
                return nil
            }
            // Queue not yet full, fetch the next batch
            from += uint64(len(hashPack.hashes))
            getHashes(from)

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

// fetchBlocks 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) fetchBlocks(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)

    // Prepare the queue and fetch blocks until the hash fetcher's done
    d.queue.Prepare(from)
    finished := false

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

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

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

                case errNoFetchesPending:
                    // Peer probably timed out with its delivery but came through
                    // in the end, demote, but allow to to pull from this peer.
                    peer.Demote()
                    peer.SetIdle()
                    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.SetIdle()
                    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.processCh:
            // 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.Expire(blockHardTTL) {
                if peer := d.peers.Peer(pid); peer != nil {
                    peer.Demote()
                    glog.V(logger.Detail).Infof("%s: block delivery timeout", peer)
                }
            }
            // If there's noting more to fetch, wait or terminate
            if d.queue.Pending() == 0 {
                if d.queue.InFlight() == 0 && finished {
                    glog.V(logger.Debug).Infof("Block fetching completed")
                    return nil
                }
                break
            }
            // Send a download request to all idle peers, until throttled
            for _, peer := range d.peers.IdlePeers() {
                // Short circuit if throttling activated
                if d.queue.Throttle() {
                    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.Reserve(peer, peer.Capacity())
                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.Fetch(request); err != nil {
                    glog.V(logger.Error).Infof("%v: fetch failed, rescheduling", peer)
                    d.queue.Cancel(request)
                }
            }
            // Make sure that we have peers available for fetching. If all peers have been tried
            // and all failed throw an error
            if !d.queue.Throttle() && d.queue.InFlight() == 0 {
                return errPeersUnavailable
            }
        }
    }
}

// process takes blocks from the queue and tries to import them into the chain.
//
// 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 blocks 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.GetHeadBlock() != nil {
            d.process()
        }
    }()
    // Release the lock upon exit (note, before checking for reentry!), and set
    // the import statistics to zero.
    defer func() {
        d.importLock.Lock()
        d.importQueue = nil
        d.importDone = 0
        d.importLock.Unlock()

        atomic.StoreInt32(&d.processing, 0)
    }()
    // Repeat the processing as long as there are blocks to import
    for {
        // Fetch the next batch of blocks
        blocks := d.queue.TakeBlocks()
        if len(blocks) == 0 {
            return
        }
        // Reset the import statistics
        d.importLock.Lock()
        d.importStart = time.Now()
        d.importQueue = blocks
        d.importDone = 0
        d.importLock.Unlock()

        // Actually import the blocks
        glog.V(logger.Debug).Infof("Inserting chain with %d blocks (#%v - #%v)\n", len(blocks), blocks[0].RawBlock.Number(), blocks[len(blocks)-1].RawBlock.Number())
        for len(blocks) != 0 {
            // Check for any termination requests
            if atomic.LoadInt32(&d.interrupt) == 1 {
                return
            }
            // Retrieve the first batch of blocks to insert
            max := int(math.Min(float64(len(blocks)), float64(maxBlockProcess)))
            raw := make(types.Blocks, 0, max)
            for _, block := range blocks[:max] {
                raw = append(raw, block.RawBlock)
            }
            // Try to inset the blocks, drop the originating peer if there's an error
            index, err := d.insertChain(raw)
            if err != nil {
                glog.V(logger.Debug).Infof("Block #%d import failed: %v", raw[index].NumberU64(), err)
                d.dropPeer(blocks[index].OriginPeer)
                d.cancel()
                return
            }
            blocks = blocks[max:]
        }
    }
}

// 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) error {
    // Make sure the downloader is active
    if atomic.LoadInt32(&d.synchronising) == 0 {
        return errNoSyncActive
    }
    // Deliver or abort if the sync is canceled while queuing
    d.cancelLock.RLock()
    cancel := d.cancelCh
    d.cancelLock.RUnlock()

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

    case <-cancel:
        return errNoSyncActive
    }
}

// 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) error {
    // Make sure the downloader is active
    if atomic.LoadInt32(&d.synchronising) == 0 {
        return errNoSyncActive
    }
    // Deliver or abort if the sync is canceled while queuing
    d.cancelLock.RLock()
    cancel := d.cancelCh
    d.cancelLock.RUnlock()

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

    case <-cancel:
        return errNoSyncActive
    }
}