github.com/annchain/OG@v0.0.9/og/downloader/downloader.go

// Copyright © 2019 Annchain Authors <EMAIL ADDRESS>
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package downloader

import (
	"errors"
	"fmt"
	"github.com/annchain/OG/arefactor/common/goroutine"
	types2 "github.com/annchain/OG/arefactor/og/types"
	"github.com/annchain/OG/metrics"
	"github.com/annchain/OG/og/protocol/dagmessage"
	"github.com/annchain/OG/og/types"
	"github.com/sirupsen/logrus"
	"sync"
	"sync/atomic"
	"time"
)

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

	rttMinEstimate   = 2 * time.Second  // Minimum round-trip time to target for download requests
	rttMaxEstimate   = 20 * time.Second // Maximum round-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 // [og/01] 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
	fsHeaderContCheck      = 3 * time.Second // Time interval to check for header continuations during state download
	fsMinFullBlocks        = 64
	MaxForkAncestry        = 3 * uint64(3000) // Number of blocks to retrieve fully even in fast sync
)

var (
	errBusy                    = errors.New("busy")
	errUnknownPeer             = errors.New("peer is unknown or unhealthy")
	errBadPeer                 = errors.New("action from bad peer ignored")
	errStallingPeer            = errors.New("peer is stalling")
	errNoPeers                 = errors.New("no peers to keep download active")
	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 >= 01)")
	errNotAccepet              = errors.New("skeleton filling not accepted")
)

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

	queue *queue   // Scheduler for selecting the hashes to download
	peers *peerSet // Set of active peers from which download can proceed

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

	dag IDag

	insertTxs insertTxsFn
	// Callbacks
	dropPeer peerDropFn // Drops a peer for misbehaving

	// Status
	synchroniseMock func(id string, hash types2.Hash) error // Replacement for synchronise during testing
	synchronising   int32
	notified        int32
	committed       int32

	// Channels
	headerCh      chan dataPack                      // [og/01] Channel receiving inbound block headers
	bodyCh        chan dataPack                      // [og/01] Channel receiving inbound block bodies
	receiptCh     chan dataPack                      // [eth/63] Channel receiving inbound receipts
	bodyWakeCh    chan bool                          // [og/01] Channel to signal the block body fetcher of new tasks
	receiptWakeCh chan bool                          // [eth/63] Channel to signal the receipt fetcher of new tasks
	headerProcCh  chan []*dagmessage.SequencerHeader // [og/01] Channel to feed the header processor new tasks

	// for stateFetcher
	stateCh chan dataPack // [eth/63] Channel receiving inbound node state data

	// 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
	cancelWg   sync.WaitGroup // Make sure all fetcher goroutines have exited.

	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([]*dagmessage.SequencerHeader) // Method to call upon starting a block body fetch
	chainInsertHook func([]*fetchResult)                // Method to call upon inserting a chain of blocks (possibly in multiple invocations)
}

type IDag interface {
	LatestSequencer() *types.Sequencer
	GetSequencer(hash types2.Hash, id uint64) *types.Sequencer
}

// New creates a new downloader to fetch hashes and blocks from remote peers.
func New(mode SyncMode, dag IDag, dropPeer peerDropFn, insertTxs insertTxsFn) *Downloader {

	dl := &Downloader{
		mode:          mode,
		queue:         newQueue(),
		peers:         newPeerSet(),
		rttEstimate:   uint64(rttMaxEstimate),
		rttConfidence: uint64(1000000),
		dag:           dag,
		dropPeer:      dropPeer,
		insertTxs:     insertTxs,
		headerCh:      make(chan dataPack, 1),
		bodyCh:        make(chan dataPack, 1),
		receiptCh:     make(chan dataPack, 1),
		bodyWakeCh:    make(chan bool, 1),
		receiptWakeCh: make(chan bool, 1),
		headerProcCh:  make(chan []*dagmessage.SequencerHeader, 1),
		quitCh:        make(chan struct{}),
		stateCh:       make(chan dataPack),
	}
	return dl
}

func (d *Downloader) Start() {
	goroutine.New(d.qosTuner)
}

// 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, peer Peer) error {

	log.WithField("id ", id).Trace("Registering sync peer")
	if err := d.peers.Register(newPeerConnection(id, version, peer)); err != nil {
		log.WithField("id ", id).WithError(err).Error("Failed to register sync peer")
		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

	log.WithField("id ", id).Trace("Unregistering sync peer")
	if err := d.peers.Unregister(id); err != nil {
		log.WithField("id ", id).WithError(err).Error("Failed to unregister sync peer")
		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 types2.Hash, seqId uint64, mode SyncMode) error {
	err := d.synchronise(id, head, seqId, mode)
	switch err {
	case nil:
	case errBusy:
		log.WithError(err).Debug("Synchronisation is busy, retrying")
	case errTimeout, errBadPeer, errStallingPeer,
		errEmptyHeaderSet, errPeersUnavailable, errTooOld,
		errInvalidAncestor, errInvalidChain, errNotAccepet:
		log.WithError(err).WithField("id", id).Warn("Synchronisation failed, dropping peer")
		if d.dropPeer == nil {
			// The dropPeer method is nil when `--copydb` is used for a local copy.
			// Timeouts can occur if e.g. compaction hits at the wrong time, and can be ignored
			log.WithField("peer", id).Warn("Downloader wants to drop peer, but peerdrop-function is not set")
		} else {
			d.dropPeer(id)
		}
	default:
		log.WithError(err).Warn("Synchronisation failed, retrying")
	}
	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 its 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 types2.Hash, seqId uint64, 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} {
		select {
		case <-ch:
		default:
		}
	}
	for _, ch := range []chan dataPack{d.headerCh, d.bodyCh} {
		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

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

// syncWithPeer starts a block synchronization based on the hash chain from the
// specified peer and head hash.
func (d *Downloader) syncWithPeer(p *peerConnection, hash types2.Hash, seqId uint64) (err error) {

	if p.version < 1 {
		return errTooOld
	}
	log.WithField("peer", p.id).WithField("og", p.version).WithField("head", hash).WithField(
		"seqid", seqId).WithField("mode", d.mode).Debug("Synchronising with the network")
	defer func(start time.Time) {
		log.WithField("elapsed", time.Since(start)).Debug("Synchronisation terminated")
	}(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.SequencerId()
	origin, err := d.findAncestor(p, height)
	if err != nil {
		return err
	}

	//ancestor is smaller than our height
	ourHeight := d.dag.LatestSequencer().Number()
	if ourHeight > origin {
		origin = ourHeight
	}
	// Ensure our origin point is below any fast sync pivot point
	pivot := uint64(0)
	if d.mode == FastSync {
		if height <= uint64(fsMinFullBlocks) {
			origin = 0
		} else {
			pivot = height - uint64(fsMinFullBlocks)
			if pivot <= origin {
				origin = pivot - 1
			}
		}
	}
	d.committed = 1
	if d.mode == FastSync && pivot != 0 {
		d.committed = 0
	}
	// Initiate the sync using a concurrent header and content retrieval algorithm
	d.queue.Prepare(origin+1, d.mode)
	if d.syncInitHook != nil {
		d.syncInitHook(origin, height)
	}

	fetchers := []func() error{
		func() error { return d.fetchHeaders(p, origin+1, pivot) }, // Headers are always retrieved
		func() error { return d.fetchBodies(origin + 1) },          // Bodies are retrieved during normal and fast sync
		// Receipts are retrieved during fast sync
		func() error { return d.processHeaders(origin+1, pivot, seqId) },
	}
	if d.mode == FastSync {
		fetchers = append(fetchers, func() error { return d.processFastSyncContent(latest) })
	} else if d.mode == FullSync {
		fetchers = append(fetchers, d.processFullSyncContent)
	}
	return d.spawnSync(fetchers)
}

// spawnSync runs d.process and all given fetcher functions to completion in
// separate goroutines, returning the first error that appears.
func (d *Downloader) spawnSync(fetchers []func() error) error {
	errc := make(chan error, len(fetchers))
	d.cancelWg.Add(len(fetchers))
	for _, fn := range fetchers {
		fn := fn
		function := func() { defer d.cancelWg.Done(); errc <- fn() }
		goroutine.New(function)
	}
	// Wait for the first error, then terminate the others.
	var err error
	for i := 0; i < len(fetchers); i++ {
		if i == len(fetchers)-1 {
			// 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()
	return err
}

// cancel aborts all of the operations and resets the queue. However, cancel does
// not wait for the running download goroutines to finish. This method should be
// used when cancelling the downloads from inside the downloader.
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()
}

// Cancel aborts all of the operations and waits for all download goroutines to
// finish before returning.
func (d *Downloader) Cancel() {
	d.cancel()
	d.cancelWg.Wait()
}

// Terminate interrupts the downloader, canceling all pending operations.
// The downloader cannot be reused after calling Terminate.
func (d *Downloader) Terminate() {
	log.Info("downloader terminate")
	// Close the termination channel (make sure double close is allowed)
	d.quitLock.Lock()
	select {
	case <-d.quitCh:
		log.Debug("got d.quitCh")
	default:
		log.Debug("close d.quitCh")
		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 *peerConnection) (*dagmessage.SequencerHeader, error) {
	log.Debug("Retrieving remote chain height")

	// Request the advertised remote head block and wait for the response
	head, _ := p.peer.Head()
	function := func() { p.peer.RequestHeadersByHash(head, 1, 0, false) }
	goroutine.New(function)

	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.WithField("peer", packet.PeerId()).Info("Received headers from incorrect peer")
				break
			}
			// Make sure the peer actually gave something valid
			headers := packet.(*headerPack).headers
			if len(headers) != 1 {
				log.WithField("headers", len(headers)).Info("Multiple headers for single request")
				return nil, errBadPeer
			}
			head := headers[0]
			log.WithField("number", head.SequencerId()).WithField("hash", head.GetHash()).Debug(
				"Remote head header identified")
			return head, nil

		case <-timeout:
			log.WithField("peer ", p.id).Info("Waiting for head header timed out", "elapsed", ttl)
			return nil, errTimeout

		case <-d.bodyCh:
		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 *peerConnection, height uint64) (uint64, error) {
	// Figure out the valid ancestor range to prevent rewrite attacks
	floor, ceil := int64(-1), d.dag.LatestSequencer().Number()
	if d.mode == FullSync {
		ceil = d.dag.LatestSequencer().Number()
	} else if d.mode == FastSync {
		ceil = d.dag.LatestSequencer().Number()
	}
	if ceil >= MaxForkAncestry {
		floor = int64(ceil - MaxForkAncestry)
	}
	log.WithField("local", ceil).WithField("remote", height).Debug("Looking for common ancestor")

	// 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
	}
	function := func() { p.peer.RequestHeadersByNumber(uint64(from), count, 15, false) }
	goroutine.New(function)

	// Wait for the remote response to the head fetch
	number, hash := uint64(0), types2.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.WithField("peer", packet.PeerId()).Debug("Received headers from incorrect peer")
				break
			}
			// Make sure the peer actually gave something valid
			headers := packet.(*headerPack).headers
			if len(headers) == 0 {
				log.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].SequencerId(); int64(number) != from+int64(i)*16 {
					log.WithField("index", i).WithField("requested", from+int64(i)*16).WithField("received", number).Warn(
						"Head headers broke chain ordering")
					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].SequencerId() < uint64(from) || headers[i].SequencerId() > ceil {
					continue
				}
				// Otherwise check if we already know the header or not
				if d.mode == FullSync && d.dag.GetSequencer(headers[i].GetHash(), headers[i].SequencerId()) != nil {
					number, hash = headers[i].SequencerId(), headers[i].GetHash()

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

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

		case <-d.bodyCh:
		case <-d.receiptCh:
			// Out of bounds delivery, ignore
		}
	}
	// If the head fetch already found an ancestor, return
	if !hash.Empty() {
		if int64(number) <= floor {
			log.WithField("number", number).WithField("hash", hash).WithField(
				"allowance", floor).Warn("Ancestor below allowance")
			return 0, errInvalidAncestor
		}
		log.WithField("number", number).WithField("hash", hash.String()).Debug("Found common ancestor")
		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)

		function := func() { p.peer.RequestHeadersByNumber(check, 1, 0, false) }
		goroutine.New(function)

		// 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.WithField("peer", packer.PeerId()).Info("Received headers from incorrect peer")
					break
				}
				// Make sure the peer actually gave something valid
				headers := packer.(*headerPack).headers
				if len(headers) != 1 {
					log.WithField("headers", len(headers)).Info("Multiple headers for single request")
					return 0, errBadPeer
				}
				arrived = true

				// Modify the search interval based on the response
				if d.mode == FullSync && d.dag.GetSequencer(headers[0].GetHash(), headers[0].SequencerId()) == nil {
					end = check
					break
				}

				start = check

			case <-timeout:
				log.WithField("peer ", p.id).WithField("elapsed", ttl).Info("Waiting for search header timed out")
				return 0, errTimeout

			case <-d.bodyCh:
			case <-d.receiptCh:
				// Out of bounds delivery, ignore
			}
		}
	}
	// Ensure valid ancestry and return
	if int64(start) <= floor {
		log.WithField("number", start).WithField("hash", hash).WithField(
			"allowance", floor).Warn("Ancestor below allowance")
		return 0, errInvalidAncestor
	}
	log.WithField("number", start).WithField("hash", hash).Debug("Found common ancestor")
	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 *peerConnection, from uint64, pivot uint64) error {
	log.WithField("origin", from).Debug("Directing header downloads")
	defer log.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

	//skeleton = false
	getHeaders := func(from uint64) {
		request = time.Now()

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

		if skeleton {
			log.WithField("count", MaxHeaderFetch).WithField("from", from).Trace("Fetching skeleton headers")
			function := func() {
				p.peer.RequestHeadersByNumber(from+uint64(MaxHeaderFetch)-1,
					MaxSkeletonSize, MaxHeaderFetch-1, false)
			}
			goroutine.New(function)
		} else {
			log.WithField("count", MaxHeaderFetch).WithField("from", from).Trace("Fetching full headers")
			function := func() {
				p.peer.RequestHeadersByNumber(from, MaxHeaderFetch, 0, false)

			}
			goroutine.New(function)
		}

	}
	// 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.WithField("peer", packet.PeerId()).Debug("Received skeleton from incorrect peer")
				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 {
				// Don't abort header fetches while the pivot is downloading
				if atomic.LoadInt32(&d.committed) == 0 && pivot <= from {
					log.Debug("No headers, waiting for pivot commit")
					select {
					case <-time.After(fsHeaderContCheck):
						getHeaders(from)
						continue
					case <-d.cancelCh:
						return errCancelHeaderFetch
					}
				}
				// Pivot done (or not in fast sync) and no more headers, terminate the process
				log.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 {
					log.WithError(err).Warn("Skeleton chain invalid")
					return errInvalidChain
				}
				headers = filled[proced:]
				from += uint64(proced)
			}
			// Insert all the new headers and fetch the next batch
			if len(headers) > 0 {
				log.WithField("from", from).WithField("count", len(headers)).Trace("Scheduling new headers")
				select {
				case d.headerProcCh <- headers:
				case <-d.cancelCh:
					return errCancelHeaderFetch
				}
				from += uint64(len(headers))
			}
			getHeaders(from)

		case <-timeout.C:

			if d.dropPeer == nil {
				// The dropPeer method is nil when `--copydb` is used for a local copy.
				// Timeouts can occur if e.g. compaction hits at the wrong time, and can be ignored
				log.WithField("peer", p.id).Warn("Downloader wants to drop peer, but peerdrop-function is not set")
				break
			}
			// Header retrieval timed out, consider the peer bad and drop
			log.WithField("peer ", p.id).WithField("elapsed", ttl).Debug("Header request timed out")
			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} {
				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 returns the entire filled skeleton and also the number of headers
// already forwarded for processing.
func (d *Downloader) fillHeaderSkeleton(from uint64, skeleton []*dagmessage.SequencerHeader) ([]*dagmessage.SequencerHeader, int, error) {
	log.WithField("from", from).Debug("Filling up skeleton")
	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 *peerConnection, count int) (*fetchRequest, bool, error) {
			return d.queue.ReserveHeaders(p, count), false, nil
		}
		fetch    = func(p *peerConnection, req *fetchRequest) error { return p.FetchHeaders(req.From, MaxHeaderFetch) }
		capacity = func(p *peerConnection) int { return p.HeaderCapacity(d.requestRTT()) }
		setIdle  = func(p *peerConnection, 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")
	if err != nil {
		log.WithError(err).Warn("Skeleton fill terminated")
	} else {
		log.Debug("Skeleton fill terminated")
	}

	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.WithField("origin", from).Debug("Downloading block bodies")

	var (
		deliver = func(packet dataPack) (int, error) {

			pack := packet.(*bodyPack)
			return d.queue.DeliverBodies(pack.peerID, pack.transactions, pack.sequencers)
		}
		expire   = func() map[string]int { return d.queue.ExpireBodies(d.requestTTL()) }
		fetch    = func(p *peerConnection, req *fetchRequest) error { return p.FetchBodies(req) }
		capacity = func(p *peerConnection) int { return p.BlockCapacity(d.requestRTT()) }
		setIdle  = func(p *peerConnection, 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")

	if err != nil {
		log.WithError(err).Warn("Block body download terminated")
	} else {
		log.Debug("Block body download terminated")
	}
	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(*peerConnection, int) (*fetchRequest, bool, error),
	fetchHook func([]*dagmessage.SequencerHeader), fetch func(*peerConnection, *fetchRequest) error, cancel func(*fetchRequest), capacity func(*peerConnection) int,
	idle func() ([]*peerConnection, int), setIdle func(*peerConnection, 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 its pack
			// in a reasonable time frame, ignore its 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:
					log.WithField("type", kind).Warn("Requested data not delivered")
				case err == nil:
					log.WithField("type", kind).WithField("count", packet.Stats()).Trace("Delivered new batch of data")
				default:
					log.WithField("type", kind).WithError(err).Warn("Failed to deliver retrieved data")
				}
			}
			// 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 {
						log.WithField("type", kind).Debug("Data delivery timed out")
						setIdle(peer, 0)
					} else {
						log.WithField("type", kind).Debug("Stalling delivery, dropping")
						if d.dropPeer == nil {
							// The dropPeer method is nil when `--copydb` is used for a local copy.
							// Timeouts can occur if e.g. compaction hits at the wrong time, and can be ignored
							log.WithField("peer", pid).Warn("Downloader wants to drop peer, but peerdrop-function is not set")
						} else {
							d.dropPeer(pid)
						}
					}
				}
			}
			// If there's nothing more to fetch, wait or terminate
			if pending() == 0 {
				if !inFlight() && finished {
					log.WithField("type", kind).Debug("data fetching completed")
					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
				}
				// Short circuit if there is no more available task.
				if pending() == 0 {
					break
				}
				_, id := peer.peer.Head()
				if id < d.dag.LatestSequencer().Number() {
					log.WithField("peer head ", id).WithField("peer", peer.id).Debug("peer head is behind")
					continue
				}
				// 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 {
					log.WithField("type", kind).WithField("from", request.From).Trace(
						"Requesting new batch of data")
				} else {
					log.WithField("from", request.Headers[0].SequencerId()).WithField(
						"type", kind).WithField("count", len(request.Headers)).Trace(
						"Requesting new batch of data")
				}
				// 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 {
				log.WithField("progressed", progressed).WithField("throttled", throttled).WithField(
					"running", running).WithField("len idesls", len(idles)).WithField("total", total).WithField("pending", pending()).Debug("fetch error")
				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, pivot uint64, seqId uint64) error {
	// Keep a count of uncertain headers to roll back
	// 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} {
					select {
					case ch <- false:
					case <-d.cancelCh:
					}
				}

				// If no headers were retrieved at all, the peer violated its TD promise that it had a
				// better chain compared to ours. The only exception is if its promised blocks were
				// already imported by other means (e.g. fetcher):
				//
				// 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 {
					head := d.dag.LatestSequencer()
					if !gotHeaders && head.Number() > seqId {
						return errStallingPeer
					}
				}
				// Disable any rollback and return
				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]

				// 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 {
						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} {
				select {
				case ch <- true:
				default:
				}
			}
		}
	}
}

// processFullSyncContent takes fetch results from the queue and imports them into the chain.
func (d *Downloader) processFullSyncContent() error {
	for {
		results := d.queue.Results(true)
		if len(results) == 0 {
			return nil
		}
		if d.chainInsertHook != nil {
			d.chainInsertHook(results)
		}
		if err := d.importBlockResults(results); err != nil {
			return err
		}
	}
}

func (d *Downloader) importBlockResults(results []*fetchResult) error {
	// Check for any early termination requests
	if len(results) == 0 {
		return nil
	}
	select {
	case <-d.quitCh:
		log.Debug("got d.quitch")
		return errCancelContentProcessing
	default:
	}
	// Retrieve the a batch of results to import
	first, last := results[0].Header, results[len(results)-1].Header

	log.WithField("items", len(results)).WithField("firstnum", first.SequencerId()).WithField(
		"firsthash", first.GetHash()).WithField("lastnum", last.SequencerId()).WithField("lasthash", last.GetHash()).Debug(
		"Inserting downloaded txs")

	for _, result := range results {
		log.WithField("len txs", len(result.Transactions)).WithField("seq", result.Sequencer).Debug("Inserting downloaded txs")
		err := d.insertTxs(result.Sequencer, result.Transactions)
		if err != nil {
			return err
		}

	}

	return nil
}

// processFastSyncContent takes fetch results from the queue and writes them to the
// database. It also controls the synchronisation of state nodes of the pivot block.
func (d *Downloader) processFastSyncContent(latest *dagmessage.SequencerHeader) error {

	// Figure out the ideal pivot block. Note, that this goalpost may move if the
	// sync takes long enough for the chain head to move significantly.
	pivot := uint64(0)
	if height := latest.SequencerId(); height > uint64(fsMinFullBlocks) {
		pivot = height - uint64(fsMinFullBlocks)
	}
	// To cater for moving pivot points, track the pivot block and subsequently
	// accumulated download results separately.
	var (
		oldPivot *fetchResult   // Locked in pivot block, might change eventually
		oldTail  []*fetchResult // Downloaded content after the pivot
	)
	for {
		// Wait for the next batch of downloaded data to be available, and if the pivot
		// block became stale, move the goalpost
		results := d.queue.Results(oldPivot == nil) // Block if we're not monitoring pivot staleness
		if len(results) == 0 {
			// If pivot sync is done, stop
			if oldPivot == nil {
				return nil
			}
			// If sync failed, stop
			select {
			case <-d.cancelCh:
				return nil
			default:
			}
		}
		if d.chainInsertHook != nil {
			d.chainInsertHook(results)
		}
		if oldPivot != nil {
			results = append(append([]*fetchResult{oldPivot}, oldTail...), results...)
		}
		// Split around the pivot block and process the two sides via fast/full sync
		if atomic.LoadInt32(&d.committed) == 0 {
			latest = results[len(results)-1].Header
			if height := latest.SequencerId(); height > pivot+2*uint64(fsMinFullBlocks) {
				log.WithField("old", pivot).WithField("new", height-uint64(fsMinFullBlocks)).Warn("Pivot became stale, moving")
				pivot = height - uint64(fsMinFullBlocks)
			}
		}

		// Fast sync done, pivot commit done, full import
		if err := d.importBlockResults(results); err != nil {
			return err
		}
	}
}

// DeliverHeaders injects a new batch of block headers received from a remote
// node into the download schedule.
func (d *Downloader) DeliverHeaders(id string, headers []*dagmessage.SequencerHeader) (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.Txis, sequencers []*types.Sequencer) (err error) {
	return d.deliver(id, d.bodyCh, &bodyPack{id, transactions, sequencers}, bodyInMeter, bodyDropMeter)
}

// 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((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.WithFields(logrus.Fields{
			"rtt":        rtt,
			"confidence": float64(conf) / 1000000.0,
			"ttl":        d.requestTTL(),
		}).Debug("Recalculated downloader QoS values")
		select {
		case <-d.quitCh:
			log.Debug("got 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 == 0 {
		// Ensure peer connectivity races don't catch us off guard
		return
	}
	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.WithFields(logrus.Fields{
		"rtt":        rtt,
		"confidence": float64(conf) / 1000000.0,
		"ttl":        d.requestTTL(),
	}).Debug("Relaxed downloader QoS values")
}

// 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 preferred (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
}