path: root/eth/downloader/downloader.go

package downloader

import (
    "bytes"
    "errors"
    "math"
    "math/rand"
    "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"
)

var (
    MinHashFetch  = 512  // Minimum amount of hashes to not consider a peer stalling
    MaxHashFetch  = 2048 // Amount of hashes to be fetched per retrieval request
    MaxBlockFetch = 128  // Amount of blocks to be fetched per retrieval 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

    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)")
    errCancelChainImport = errors.New("chain importing 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

// 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
    insertChain chainInsertFn    // Injects a batch of blocks into the chain
    dropPeer    peerDropFn       // Retrieved the TD of our own chain

    // 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
    blockCh   chan blockPack

    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, insertChain chainInsertFn, dropPeer peerDropFn) *Downloader {
    // Create the base downloader
    downloader := &Downloader{
        mux:         mux,
        queue:       newQueue(),
        peers:       newPeerSet(),
        hasBlock:    hasBlock,
        getBlock:    getBlock,
        insertChain: insertChain,
        dropPeer:    dropPeer,
        newPeerCh:   make(chan *peer, 1),
        hashCh:      make(chan hashPack, 1),
        blockCh:     make(chan blockPack, 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, head common.Hash, getHashes hashFetcherFn, 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, head, getHashes, 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) {
    glog.V(logger.Detail).Infof("Attempting synchronisation: %v, 0x%x", id, head)

    switch err := d.synchronise(id, head); 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) 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)
}

// 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) (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).Infoln("Synchronizing with the network using:", p.id)
    if err = d.fetchHashes(p, hash); err != nil {
        return err
    }
    if err = d.fetchBlocks(); err != nil {
        return err
    }
    glog.V(logger.Debug).Infoln("Synchronization completed")

    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()
}

// fetchHahes starts retrieving hashes backwards from a specific peer and hash,
// up until it finds a common ancestor. If the source peer times out, alternative
// ones are tried for continuation.
func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
    var (
        start  = time.Now()
        active = p             // active peer will help determine the current active peer
        head   = common.Hash{} // common and last hash

        timeout     = time.NewTimer(0)                // timer to dump a non-responsive active peer
        attempted   = make(map[string]bool)           // attempted peers will help with retries
        crossTicker = time.NewTicker(crossCheckCycle) // ticker to periodically check expired cross checks
    )
    defer crossTicker.Stop()
    defer timeout.Stop()

    glog.V(logger.Debug).Infof("Downloading hashes (%x) from %s", h[:4], p.id)
    <-timeout.C // timeout channel should be initially empty.

    getHashes := func(from common.Hash) {
        go active.getHashes(from)
        timeout.Reset(hashTTL)
    }

    // Add the hash to the queue, and start hash retrieval.
    d.queue.Insert([]common.Hash{h})
    getHashes(h)

    attempted[p.id] = true
    for finished := false; !finished; {
        select {
        case <-d.cancelCh:
            return errCancelHashFetch

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

            // Make sure the peer actually gave something valid
            if len(hashPack.hashes) == 0 {
                glog.V(logger.Debug).Infof("Peer (%s) responded with empty hash set", active.id)
                return errEmptyHashSet
            }
            for index, hash := range hashPack.hashes {
                if d.banned.Has(hash) {
                    glog.V(logger.Debug).Infof("Peer (%s) sent a known invalid chain", active.id)

                    d.queue.Insert(hashPack.hashes[:index+1])
                    if err := d.banBlocks(active.id, hash); err != nil {
                        glog.V(logger.Debug).Infof("Failed to ban batch of blocks: %v", err)
                    }
                    return errInvalidChain
                }
            }
            // Determine if we're done fetching hashes (queue up all pending), and continue if not done
            done, index := false, 0
            for index, head = range hashPack.hashes {
                if d.hasBlock(head) || d.queue.GetBlock(head) != nil {
                    glog.V(logger.Debug).Infof("Found common hash %x", head[:4])
                    hashPack.hashes = hashPack.hashes[:index]
                    done = true
                    break
                }
            }
            // Insert all the new hashes, but only continue if got something useful
            inserts := d.queue.Insert(hashPack.hashes)
            if len(inserts) == 0 && !done {
                glog.V(logger.Debug).Infof("Peer (%s) responded with stale hashes", active.id)
                return errBadPeer
            }
            if !done {
                // Check that the peer is not stalling the sync
                if len(inserts) < MinHashFetch {
                    return errStallingPeer
                }
                // Try and fetch a random block to verify the hash batch
                // Skip the last hash as the cross check races with the next hash fetch
                cross := rand.Intn(len(inserts) - 1)
                origin, parent := inserts[cross], inserts[cross+1]
                glog.V(logger.Detail).Infof("Cross checking (%s) with %x/%x", active.id, origin, parent)

                d.checks[origin] = &crossCheck{
                    expire: time.Now().Add(blockSoftTTL),
                    parent: parent,
                }
                go active.getBlocks([]common.Hash{origin})

                // Also fetch a fresh batch of hashes
                getHashes(head)
                continue
            }
            // We're done, prepare the download cache and proceed pulling the blocks
            offset := 0
            if block := d.getBlock(head); block != nil {
                offset = int(block.NumberU64() + 1)
            }
            d.queue.Prepare(offset)
            finished = true

        case blockPack := <-d.blockCh:
            // Cross check the block with the random verifications
            if blockPack.peerId != active.id || len(blockPack.blocks) != 1 {
                continue
            }
            block := blockPack.blocks[0]
            if check, ok := d.checks[block.Hash()]; ok {
                if block.ParentHash() != check.parent {
                    return errCrossCheckFailed
                }
                delete(d.checks, block.Hash())
            }

        case <-crossTicker.C:
            // Iterate over all the cross checks and fail the hash chain if they're not verified
            for hash, check := range d.checks {
                if time.Now().After(check.expire) {
                    glog.V(logger.Debug).Infof("Cross check timeout for %x", hash)
                    return errCrossCheckFailed
                }
            }

        case <-timeout.C:
            glog.V(logger.Debug).Infof("Peer (%s) didn't respond in time for hash request", p.id)

            var p *peer // p will be set if a peer can be found
            // Attempt to find a new peer by checking inclusion of peers best hash in our
            // already fetched hash list. This can't guarantee 100% correctness but does
            // a fair job. This is always either correct or false incorrect.
            for _, peer := range d.peers.AllPeers() {
                if d.queue.Has(peer.head) && !attempted[peer.id] {
                    p = peer
                    break
                }
            }
            // if all peers have been tried, abort the process entirely or if the hash is
            // the zero hash.
            if p == nil || (head == common.Hash{}) {
                return errTimeout
            }
            // set p to the active peer. this will invalidate any hashes that may be returned
            // by our previous (delayed) peer.
            active = p
            getHashes(head)
            glog.V(logger.Debug).Infof("Hash fetching switched to new peer(%s)", p.id)
        }
    }
    glog.V(logger.Debug).Infof("Downloaded hashes (%d) in %v", d.queue.Pending(), time.Since(start))

    return nil
}

// fetchBlocks iteratively downloads the entire schedules block-chain, taking
// any available peers, reserving a chunk of blocks for each, wait for delivery
// and periodically checking for timeouts.
func (d *Downloader) fetchBlocks() error {
    glog.V(logger.Debug).Infoln("Downloading", d.queue.Pending(), "block(s)")
    start := time.Now()

    // Start a ticker to continue throttled downloads and check for bad peers
    ticker := time.NewTicker(20 * time.Millisecond)
    defer ticker.Stop()

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

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

        case blockPack := <-d.blockCh:
            // Short circuit if it's a stale cross check
            if len(blockPack.blocks) == 1 {
                block := blockPack.blocks[0]
                if _, ok := d.checks[block.Hash()]; ok {
                    delete(d.checks, block.Hash())
                    break
                }
            }
            // 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()
                }
            }

        case <-ticker.C:
            // 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
            badPeers := d.queue.Expire(blockHardTTL)
            for _, pid := range badPeers {
                if peer := d.peers.Peer(pid); peer != nil {
                    peer.Demote()
                    glog.V(logger.Detail).Infof("%s: block delivery timeout", peer)
                }
            }
            // If there are unrequested hashes left start fetching from the available peers
            if d.queue.Pending() > 0 {
                // Throttle the download if block cache is full and waiting processing
                if d.queue.Throttle() {
                    break
                }
                // Send a download request to all idle peers, until throttled
                idlePeers := d.peers.IdlePeers()
                for _, peer := range idlePeers {
                    // Short circuit if throttling activated since above
                    if d.queue.Throttle() {
                        break
                    }
                    // Get a possible chunk. If nil is returned no chunk
                    // could be returned due to no hashes available.
                    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 check for error. If the peer was somehow
                    // already fetching a chunk due to a bug, it will be returned to
                    // the queue
                    if err := peer.Fetch(request); err != nil {
                        glog.V(logger.Error).Infof("Peer %s received double work", peer.id)
                        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.InFlight() == 0 {
                    return errPeersUnavailable
                }

            } else if d.queue.InFlight() == 0 {
                // When there are no more queue and no more in flight, We can
                // safely assume we're done. Another part of the process will  check
                // for parent errors and will re-request anything that's missing
                break out
            }
        }
    }
    glog.V(logger.Detail).Infoln("Downloaded block(s) in", time.Since(start))
    return nil
}

// banBlocks retrieves a batch of blocks from a peer feeding us invalid hashes,
// and bans the head of the retrieved batch.
//
// This method only fetches one single batch as the goal is not ban an entire
// (potentially long) invalid chain - wasting a lot of time in the meanwhile -,
// but rather to gradually build up a blacklist if the peer keeps reconnecting.
func (d *Downloader) banBlocks(peerId string, head common.Hash) error {
    glog.V(logger.Debug).Infof("Banning a batch out of %d blocks from %s", d.queue.Pending(), peerId)

    // Ask the peer being banned for a batch of blocks from the banning point
    peer := d.peers.Peer(peerId)
    if peer == nil {
        return nil
    }
    request := d.queue.Reserve(peer, MaxBlockFetch)
    if request == nil {
        return nil
    }
    if err := peer.Fetch(request); err != nil {
        return err
    }
    // Wait a bit for the reply to arrive, and ban if done so
    timeout := time.After(blockHardTTL)
    for {
        select {
        case <-d.cancelCh:
            return errCancelBlockFetch

        case <-timeout:
            return errTimeout

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

        case blockPack := <-d.blockCh:
            blocks := blockPack.blocks

            // Short circuit if it's a stale cross check
            if len(blocks) == 1 {
                block := blocks[0]
                if _, ok := d.checks[block.Hash()]; ok {
                    delete(d.checks, block.Hash())
                    break
                }
            }
            // Short circuit if it's not from the peer being banned
            if blockPack.peerId != peerId {
                break
            }
            // Short circuit if no blocks were returned
            if len(blocks) == 0 {
                return errors.New("no blocks returned to ban")
            }
            // Reconstruct the original chain order and ensure we're banning the correct blocks
            types.BlockBy(types.Number).Sort(blocks)
            if bytes.Compare(blocks[0].Hash().Bytes(), head.Bytes()) != 0 {
                return errors.New("head block not the banned one")
            }
            index := 0
            for _, block := range blocks[1:] {
                if bytes.Compare(block.ParentHash().Bytes(), blocks[index].Hash().Bytes()) != 0 {
                    break
                }
                index++
            }
            // Ban the head hash and phase out any excess
            d.banned.Add(blocks[index].Hash())
            for d.banned.Size() > maxBannedHashes {
                var evacuate common.Hash

                d.banned.Each(func(item interface{}) bool {
                    // Skip any hard coded bans
                    if core.BadHashes[item.(common.Hash)] {
                        return true
                    }
                    evacuate = item.(common.Hash)
                    return false
                })
                d.banned.Remove(evacuate)
            }
            glog.V(logger.Debug).Infof("Banned %d blocks from: %s", index+1, peerId)
            return nil
        }
    }
}

// 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() (err error) {
    // 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 err == nil && d.queue.GetHeadBlock() != nil {
            err = 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 nil
        }
        // 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 errCancelChainImport
            }
            // 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 errCancelChainImport
            }
            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
    }
}