path: root/eth/downloader/downloader.go

package downloader

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

    "github.com/ethereum/go-ethereum/common"
    "github.com/ethereum/go-ethereum/core/types"
    "github.com/ethereum/go-ethereum/event"
    "github.com/ethereum/go-ethereum/logger"
    "github.com/ethereum/go-ethereum/logger/glog"
)

const (
    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

    peerCountTimeout = 12 * time.Second // Amount of time it takes for the peer handler to ignore minDesiredPeerCount
    hashTTL          = 5 * time.Second  // Time it takes for a hash request to time out
)

var (
    blockTTL            = 5 * time.Second // Time it takes for a block request to time out
    crossCheckCycle     = time.Second     // Period after which to check for expired cross checks
    minDesiredPeerCount = 5               // Amount of peers desired to start syncing
)

var (
    errLowTd            = errors.New("peers TD is too low")
    ErrBusy             = errors.New("busy")
    errUnknownPeer      = errors.New("peer is unknown or unhealthy")
    ErrBadPeer          = errors.New("action from bad peer ignored")
    ErrStallingPeer     = errors.New("peer is stalling")
    errNoPeers          = errors.New("no peers to keep download active")
    ErrPendingQueue     = errors.New("pending items in queue")
    ErrTimeout          = errors.New("timeout")
    errEmptyHashSet     = errors.New("empty hash set by peer")
    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 cancelled (requested)")
    errCancelBlockFetch = errors.New("block downloading cancelled (requested)")
    errNoSyncActive     = errors.New("no sync active")
)

type hashCheckFn func(common.Hash) bool
type getBlockFn func(common.Hash) *types.Block
type chainInsertFn func(types.Blocks) (int, error)
type hashIterFn func() (common.Hash, error)

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

    mu     sync.RWMutex
    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

    // Callbacks
    hasBlock hashCheckFn
    getBlock getBlockFn

    // Status
    synchronising 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
}

func New(mux *event.TypeMux, hasBlock hashCheckFn, getBlock getBlockFn) *Downloader {
    downloader := &Downloader{
        mux:       mux,
        queue:     newQueue(),
        peers:     newPeerSet(),
        hasBlock:  hasBlock,
        getBlock:  getBlock,
        newPeerCh: make(chan *peer, 1),
        hashCh:    make(chan hashPack, 1),
        blockCh:   make(chan blockPack, 1),
    }
    return downloader
}

func (d *Downloader) Stats() (current int, max int) {
    return d.queue.Size()
}

// Synchronising returns the state of the downloader
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 {
    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 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 {
    // Make sure only one goroutine is ever allowed past this point at once
    if !atomic.CompareAndSwapInt32(&d.synchronising, 0, 1) {
        return ErrBusy
    }
    defer atomic.StoreInt32(&d.synchronising, 0)

    // Post a user notification of the sync (only once per session)
    if atomic.CompareAndSwapInt32(&d.notified, 0, 1) {
        glog.V(logger.Info).Infoln("Block synchronisation started")
    }

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

    // 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)

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

    return d.syncWithPeer(p, hash)
}

// TakeBlocks takes blocks from the queue and yields them to the caller.
func (d *Downloader) TakeBlocks() types.Blocks {
    return d.queue.TakeBlocks()
}

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() bool {
    // If we're not syncing just return.
    hs, bs := d.queue.Size()
    if atomic.LoadInt32(&d.synchronising) == 0 && hs == 0 && bs == 0 {
        return false
    }
    // Close the current cancel channel
    d.cancelLock.Lock()
    select {
    case <-d.cancelCh:
        // Channel was already closed
    default:
        close(d.cancelCh)
    }
    d.cancelLock.Unlock()

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

    return true
}

// XXX Make synchronous
func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
    glog.V(logger.Debug).Infof("Downloading hashes (%x) from %s", h[:4], p.id)

    start := time.Now()

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

    var (
        active = p             // active peer will help determine the current active peer
        head   = common.Hash{} // common and last hash

        timeout     = time.NewTimer(hashTTL)          // 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()

    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)\n", hashPack.peerId)
                break
            }
            timeout.Reset(hashTTL)

            // 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\n", active.id)
                return errEmptyHashSet
            }
            // 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\n", 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\n", 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(blockTTL),
                    parent: parent,
                }
                active.getBlocks([]common.Hash{origin})

                // Also fetch a fresh
                active.getHashes(head)
                continue
            }
            // We're done, allocate the download cache and proceed pulling the blocks
            offset := 0
            if block := d.getBlock(head); block != nil {
                offset = int(block.NumberU64() + 1)
            }
            d.queue.Alloc(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\n", 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
            p.getHashes(head)
            glog.V(logger.Debug).Infof("Hash fetching switched to new peer(%s)\n", p.id)
        }
    }
    glog.V(logger.Debug).Infof("Downloaded hashes (%d) in %v\n", 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()

    // default ticker for re-fetching blocks every now and then
    ticker := time.NewTicker(20 * time.Millisecond)
out:
    for {
        select {
        case <-d.cancelCh:
            return errCancelBlockFetch

        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())
                    continue
                }
            }
            // 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
                if err := d.queue.Deliver(blockPack.peerId, blockPack.blocks); err != nil {
                    if err == ErrInvalidChain {
                        // The hash chain is invalid (blocks are not ordered properly), abort
                        return err
                    }
                    // Peer did deliver, but some blocks were off, penalize
                    glog.V(logger.Debug).Infof("Failed delivery for peer %s: %v\n", blockPack.peerId, err)
                    peer.Demote()
                    break
                }
                if glog.V(logger.Debug) {
                    glog.Infof("Added %d blocks from: %s\n", len(blockPack.blocks), blockPack.peerId)
                }
                // Promote the peer and update it's idle state
                peer.Promote()
                peer.SetIdle()
            }
        case <-ticker.C:
            // Check for bad peers. Bad peers may indicate a peer not responding
            // to a `getBlocks` message. A timeout of 5 seconds is set. Peers
            // that badly or poorly behave are removed from the peer set (not banned).
            // Bad peers are excluded from the available peer set and therefor won't be
            // reused. XXX We could re-introduce peers after X time.
            badPeers := d.queue.Expire(blockTTL)
            for _, pid := range badPeers {
                // XXX We could make use of a reputation system here ranking peers
                // in their performance
                // 1) Time for them to respond;
                // 2) Measure their speed;
                // 3) Amount and availability.
                if peer := d.peers.Peer(pid); peer != nil {
                    peer.Demote()
                }
            }
            // After removing bad peers make sure we actually have sufficient peer left to keep downloading
            if d.peers.Len() == 0 {
                return errNoPeers
            }
            // 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() {
                    continue
                }
                // 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, MaxBlockFetch)
                    if request == nil {
                        continue
                    }
                    // 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\n", 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
}

// 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
    }
}