path: root/eth/downloader/queue.go

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

// Contains the block download scheduler to collect download tasks and schedule
// them in an ordered, and throttled way.

package downloader

import (
    "errors"
    "fmt"
    "sync"
    "time"

    "github.com/ethereum/go-ethereum/common"
    "github.com/ethereum/go-ethereum/core/types"
    "github.com/ethereum/go-ethereum/log"
    "github.com/ethereum/go-ethereum/metrics"
    "gopkg.in/karalabe/cookiejar.v2/collections/prque"
)

var (
    blockCacheItems      = 8192             // Maximum number of blocks to cache before throttling the download
    blockCacheMemory     = 64 * 1024 * 1024 // Maximum amount of memory to use for block caching
    blockCacheSizeWeight = 0.1              // Multiplier to approximate the average block size based on past ones
)

var (
    errNoFetchesPending = errors.New("no fetches pending")
    errStaleDelivery    = errors.New("stale delivery")
)

// fetchRequest is a currently running data retrieval operation.
type fetchRequest struct {
    Peer    *peerConnection // Peer to which the request was sent
    From    uint64          // [eth/62] Requested chain element index (used for skeleton fills only)
    Headers []*types.Header // [eth/62] Requested headers, sorted by request order
    Time    time.Time       // Time when the request was made
}

// fetchResult is a struct collecting partial results from data fetchers until
// all outstanding pieces complete and the result as a whole can be processed.
type fetchResult struct {
    Pending int         // Number of data fetches still pending
    Hash    common.Hash // Hash of the header to prevent recalculating

    Header       *types.Header
    Uncles       []*types.Header
    Transactions types.Transactions
    Receipts     types.Receipts
}

// queue represents hashes that are either need fetching or are being fetched
type queue struct {
    mode SyncMode // Synchronisation mode to decide on the block parts to schedule for fetching

    // Headers are "special", they download in batches, supported by a skeleton chain
    headerHead      common.Hash                    // [eth/62] Hash of the last queued header to verify order
    headerTaskPool  map[uint64]*types.Header       // [eth/62] Pending header retrieval tasks, mapping starting indexes to skeleton headers
    headerTaskQueue *prque.Prque                   // [eth/62] Priority queue of the skeleton indexes to fetch the filling headers for
    headerPeerMiss  map[string]map[uint64]struct{} // [eth/62] Set of per-peer header batches known to be unavailable
    headerPendPool  map[string]*fetchRequest       // [eth/62] Currently pending header retrieval operations
    headerResults   []*types.Header                // [eth/62] Result cache accumulating the completed headers
    headerProced    int                            // [eth/62] Number of headers already processed from the results
    headerOffset    uint64                         // [eth/62] Number of the first header in the result cache
    headerContCh    chan bool                      // [eth/62] Channel to notify when header download finishes

    // All data retrievals below are based on an already assembles header chain
    blockTaskPool  map[common.Hash]*types.Header // [eth/62] Pending block (body) retrieval tasks, mapping hashes to headers
    blockTaskQueue *prque.Prque                  // [eth/62] Priority queue of the headers to fetch the blocks (bodies) for
    blockPendPool  map[string]*fetchRequest      // [eth/62] Currently pending block (body) retrieval operations
    blockDonePool  map[common.Hash]struct{}      // [eth/62] Set of the completed block (body) fetches

    receiptTaskPool  map[common.Hash]*types.Header // [eth/63] Pending receipt retrieval tasks, mapping hashes to headers
    receiptTaskQueue *prque.Prque                  // [eth/63] Priority queue of the headers to fetch the receipts for
    receiptPendPool  map[string]*fetchRequest      // [eth/63] Currently pending receipt retrieval operations
    receiptDonePool  map[common.Hash]struct{}      // [eth/63] Set of the completed receipt fetches

    resultCache  []*fetchResult     // Downloaded but not yet delivered fetch results
    resultOffset uint64             // Offset of the first cached fetch result in the block chain
    resultSize   common.StorageSize // Approximate size of a block (exponential moving average)

    lock   *sync.Mutex
    active *sync.Cond
    closed bool
}

// newQueue creates a new download queue for scheduling block retrieval.
func newQueue() *queue {
    lock := new(sync.Mutex)
    return &queue{
        headerPendPool:   make(map[string]*fetchRequest),
        headerContCh:     make(chan bool),
        blockTaskPool:    make(map[common.Hash]*types.Header),
        blockTaskQueue:   prque.New(),
        blockPendPool:    make(map[string]*fetchRequest),
        blockDonePool:    make(map[common.Hash]struct{}),
        receiptTaskPool:  make(map[common.Hash]*types.Header),
        receiptTaskQueue: prque.New(),
        receiptPendPool:  make(map[string]*fetchRequest),
        receiptDonePool:  make(map[common.Hash]struct{}),
        resultCache:      make([]*fetchResult, blockCacheItems),
        active:           sync.NewCond(lock),
        lock:             lock,
    }
}

// Reset clears out the queue contents.
func (q *queue) Reset() {
    q.lock.Lock()
    defer q.lock.Unlock()

    q.closed = false
    q.mode = FullSync

    q.headerHead = common.Hash{}
    q.headerPendPool = make(map[string]*fetchRequest)

    q.blockTaskPool = make(map[common.Hash]*types.Header)
    q.blockTaskQueue.Reset()
    q.blockPendPool = make(map[string]*fetchRequest)
    q.blockDonePool = make(map[common.Hash]struct{})

    q.receiptTaskPool = make(map[common.Hash]*types.Header)
    q.receiptTaskQueue.Reset()
    q.receiptPendPool = make(map[string]*fetchRequest)
    q.receiptDonePool = make(map[common.Hash]struct{})

    q.resultCache = make([]*fetchResult, blockCacheItems)
    q.resultOffset = 0
}

// Close marks the end of the sync, unblocking WaitResults.
// It may be called even if the queue is already closed.
func (q *queue) Close() {
    q.lock.Lock()
    q.closed = true
    q.lock.Unlock()
    q.active.Broadcast()
}

// PendingHeaders retrieves the number of header requests pending for retrieval.
func (q *queue) PendingHeaders() int {
    q.lock.Lock()
    defer q.lock.Unlock()

    return q.headerTaskQueue.Size()
}

// PendingBlocks retrieves the number of block (body) requests pending for retrieval.
func (q *queue) PendingBlocks() int {
    q.lock.Lock()
    defer q.lock.Unlock()

    return q.blockTaskQueue.Size()
}

// PendingReceipts retrieves the number of block receipts pending for retrieval.
func (q *queue) PendingReceipts() int {
    q.lock.Lock()
    defer q.lock.Unlock()

    return q.receiptTaskQueue.Size()
}

// InFlightHeaders retrieves whether there are header fetch requests currently
// in flight.
func (q *queue) InFlightHeaders() bool {
    q.lock.Lock()
    defer q.lock.Unlock()

    return len(q.headerPendPool) > 0
}

// InFlightBlocks retrieves whether there are block fetch requests currently in
// flight.
func (q *queue) InFlightBlocks() bool {
    q.lock.Lock()
    defer q.lock.Unlock()

    return len(q.blockPendPool) > 0
}

// InFlightReceipts retrieves whether there are receipt fetch requests currently
// in flight.
func (q *queue) InFlightReceipts() bool {
    q.lock.Lock()
    defer q.lock.Unlock()

    return len(q.receiptPendPool) > 0
}

// Idle returns if the queue is fully idle or has some data still inside.
func (q *queue) Idle() bool {
    q.lock.Lock()
    defer q.lock.Unlock()

    queued := q.blockTaskQueue.Size() + q.receiptTaskQueue.Size()
    pending := len(q.blockPendPool) + len(q.receiptPendPool)
    cached := len(q.blockDonePool) + len(q.receiptDonePool)

    return (queued + pending + cached) == 0
}

// ShouldThrottleBlocks checks if the download should be throttled (active block (body)
// fetches exceed block cache).
func (q *queue) ShouldThrottleBlocks() bool {
    q.lock.Lock()
    defer q.lock.Unlock()

    return q.resultSlots(q.blockPendPool, q.blockDonePool) <= 0
}

// ShouldThrottleReceipts checks if the download should be throttled (active receipt
// fetches exceed block cache).
func (q *queue) ShouldThrottleReceipts() bool {
    q.lock.Lock()
    defer q.lock.Unlock()

    return q.resultSlots(q.receiptPendPool, q.receiptDonePool) <= 0
}

// resultSlots calculates the number of results slots available for requests
// whilst adhering to both the item and the memory limit too of the results
// cache.
func (q *queue) resultSlots(pendPool map[string]*fetchRequest, donePool map[common.Hash]struct{}) int {
    // Calculate the maximum length capped by the memory limit
    limit := len(q.resultCache)
    if common.StorageSize(len(q.resultCache))*q.resultSize > common.StorageSize(blockCacheMemory) {
        limit = int((common.StorageSize(blockCacheMemory) + q.resultSize - 1) / q.resultSize)
    }
    // Calculate the number of slots already finished
    finished := 0
    for _, result := range q.resultCache[:limit] {
        if result == nil {
            break
        }
        if _, ok := donePool[result.Hash]; ok {
            finished++
        }
    }
    // Calculate the number of slots currently downloading
    pending := 0
    for _, request := range pendPool {
        for _, header := range request.Headers {
            if header.Number.Uint64() < q.resultOffset+uint64(limit) {
                pending++
            }
        }
    }
    // Return the free slots to distribute
    return limit - finished - pending
}

// ScheduleSkeleton adds a batch of header retrieval tasks to the queue to fill
// up an already retrieved header skeleton.
func (q *queue) ScheduleSkeleton(from uint64, skeleton []*types.Header) {
    q.lock.Lock()
    defer q.lock.Unlock()

    // No skeleton retrieval can be in progress, fail hard if so (huge implementation bug)
    if q.headerResults != nil {
        panic("skeleton assembly already in progress")
    }
    // Schedule all the header retrieval tasks for the skeleton assembly
    q.headerTaskPool = make(map[uint64]*types.Header)
    q.headerTaskQueue = prque.New()
    q.headerPeerMiss = make(map[string]map[uint64]struct{}) // Reset availability to correct invalid chains
    q.headerResults = make([]*types.Header, len(skeleton)*MaxHeaderFetch)
    q.headerProced = 0
    q.headerOffset = from
    q.headerContCh = make(chan bool, 1)

    for i, header := range skeleton {
        index := from + uint64(i*MaxHeaderFetch)

        q.headerTaskPool[index] = header
        q.headerTaskQueue.Push(index, -float32(index))
    }
}

// RetrieveHeaders retrieves the header chain assemble based on the scheduled
// skeleton.
func (q *queue) RetrieveHeaders() ([]*types.Header, int) {
    q.lock.Lock()
    defer q.lock.Unlock()

    headers, proced := q.headerResults, q.headerProced
    q.headerResults, q.headerProced = nil, 0

    return headers, proced
}

// Schedule adds a set of headers for the download queue for scheduling, returning
// the new headers encountered.
func (q *queue) Schedule(headers []*types.Header, from uint64) []*types.Header {
    q.lock.Lock()
    defer q.lock.Unlock()

    // Insert all the headers prioritised by the contained block number
    inserts := make([]*types.Header, 0, len(headers))
    for _, header := range headers {
        // Make sure chain order is honoured and preserved throughout
        hash := header.Hash()
        if header.Number == nil || header.Number.Uint64() != from {
            log.Warn("Header broke chain ordering", "number", header.Number, "hash", hash, "expected", from)
            break
        }
        if q.headerHead != (common.Hash{}) && q.headerHead != header.ParentHash {
            log.Warn("Header broke chain ancestry", "number", header.Number, "hash", hash)
            break
        }
        // Make sure no duplicate requests are executed
        if _, ok := q.blockTaskPool[hash]; ok {
            log.Warn("Header  already scheduled for block fetch", "number", header.Number, "hash", hash)
            continue
        }
        if _, ok := q.receiptTaskPool[hash]; ok {
            log.Warn("Header already scheduled for receipt fetch", "number", header.Number, "hash", hash)
            continue
        }
        // Queue the header for content retrieval
        q.blockTaskPool[hash] = header
        q.blockTaskQueue.Push(header, -float32(header.Number.Uint64()))

        if q.mode == FastSync {
            q.receiptTaskPool[hash] = header
            q.receiptTaskQueue.Push(header, -float32(header.Number.Uint64()))
        }
        inserts = append(inserts, header)
        q.headerHead = hash
        from++
    }
    return inserts
}

// Results retrieves and permanently removes a batch of fetch results from
// the cache. the result slice will be empty if the queue has been closed.
func (q *queue) Results(block bool) []*fetchResult {
    q.lock.Lock()
    defer q.lock.Unlock()

    // Count the number of items available for processing
    nproc := q.countProcessableItems()
    for nproc == 0 && !q.closed {
        if !block {
            return nil
        }
        q.active.Wait()
        nproc = q.countProcessableItems()
    }
    // Since we have a batch limit, don't pull more into "dangling" memory
    if nproc > maxResultsProcess {
        nproc = maxResultsProcess
    }
    results := make([]*fetchResult, nproc)
    copy(results, q.resultCache[:nproc])
    if len(results) > 0 {
        // Mark results as done before dropping them from the cache.
        for _, result := range results {
            hash := result.Header.Hash()
            delete(q.blockDonePool, hash)
            delete(q.receiptDonePool, hash)
        }
        // Delete the results from the cache and clear the tail.
        copy(q.resultCache, q.resultCache[nproc:])
        for i := len(q.resultCache) - nproc; i < len(q.resultCache); i++ {
            q.resultCache[i] = nil
        }
        // Advance the expected block number of the first cache entry.
        q.resultOffset += uint64(nproc)

        // Recalculate the result item weights to prevent memory exhaustion
        for _, result := range results {
            size := result.Header.Size()
            for _, uncle := range result.Uncles {
                size += uncle.Size()
            }
            for _, receipt := range result.Receipts {
                size += receipt.Size()
            }
            for _, tx := range result.Transactions {
                size += tx.Size()
            }
            q.resultSize = common.StorageSize(blockCacheSizeWeight)*size + (1-common.StorageSize(blockCacheSizeWeight))*q.resultSize
        }
    }
    return results
}

// countProcessableItems counts the processable items.
func (q *queue) countProcessableItems() int {
    for i, result := range q.resultCache {
        if result == nil || result.Pending > 0 {
            return i
        }
    }
    return len(q.resultCache)
}

// ReserveHeaders reserves a set of headers for the given peer, skipping any
// previously failed batches.
func (q *queue) ReserveHeaders(p *peerConnection, count int) *fetchRequest {
    q.lock.Lock()
    defer q.lock.Unlock()

    // Short circuit if the peer's already downloading something (sanity check to
    // not corrupt state)
    if _, ok := q.headerPendPool[p.id]; ok {
        return nil
    }
    // Retrieve a batch of hashes, skipping previously failed ones
    send, skip := uint64(0), []uint64{}
    for send == 0 && !q.headerTaskQueue.Empty() {
        from, _ := q.headerTaskQueue.Pop()
        if q.headerPeerMiss[p.id] != nil {
            if _, ok := q.headerPeerMiss[p.id][from.(uint64)]; ok {
                skip = append(skip, from.(uint64))
                continue
            }
        }
        send = from.(uint64)
    }
    // Merge all the skipped batches back
    for _, from := range skip {
        q.headerTaskQueue.Push(from, -float32(from))
    }
    // Assemble and return the block download request
    if send == 0 {
        return nil
    }
    request := &fetchRequest{
        Peer: p,
        From: send,
        Time: time.Now(),
    }
    q.headerPendPool[p.id] = request
    return request
}

// ReserveBodies reserves a set of body fetches for the given peer, skipping any
// previously failed downloads. Beside the next batch of needed fetches, it also
// returns a flag whether empty blocks were queued requiring processing.
func (q *queue) ReserveBodies(p *peerConnection, count int) (*fetchRequest, bool, error) {
    isNoop := func(header *types.Header) bool {
        return header.TxHash == types.EmptyRootHash && header.UncleHash == types.EmptyUncleHash
    }
    q.lock.Lock()
    defer q.lock.Unlock()

    return q.reserveHeaders(p, count, q.blockTaskPool, q.blockTaskQueue, q.blockPendPool, q.blockDonePool, isNoop)
}

// ReserveReceipts reserves a set of receipt fetches for the given peer, skipping
// any previously failed downloads. Beside the next batch of needed fetches, it
// also returns a flag whether empty receipts were queued requiring importing.
func (q *queue) ReserveReceipts(p *peerConnection, count int) (*fetchRequest, bool, error) {
    isNoop := func(header *types.Header) bool {
        return header.ReceiptHash == types.EmptyRootHash
    }
    q.lock.Lock()
    defer q.lock.Unlock()

    return q.reserveHeaders(p, count, q.receiptTaskPool, q.receiptTaskQueue, q.receiptPendPool, q.receiptDonePool, isNoop)
}

// reserveHeaders reserves a set of data download operations for a given peer,
// skipping any previously failed ones. This method is a generic version used
// by the individual special reservation functions.
//
// Note, this method expects the queue lock to be already held for writing. The
// reason the lock is not obtained in here is because the parameters already need
// to access the queue, so they already need a lock anyway.
func (q *queue) reserveHeaders(p *peerConnection, count int, taskPool map[common.Hash]*types.Header, taskQueue *prque.Prque,
    pendPool map[string]*fetchRequest, donePool map[common.Hash]struct{}, isNoop func(*types.Header) bool) (*fetchRequest, bool, error) {
    // Short circuit if the pool has been depleted, or if the peer's already
    // downloading something (sanity check not to corrupt state)
    if taskQueue.Empty() {
        return nil, false, nil
    }
    if _, ok := pendPool[p.id]; ok {
        return nil, false, nil
    }
    // Calculate an upper limit on the items we might fetch (i.e. throttling)
    space := q.resultSlots(pendPool, donePool)

    // Retrieve a batch of tasks, skipping previously failed ones
    send := make([]*types.Header, 0, count)
    skip := make([]*types.Header, 0)

    progress := false
    for proc := 0; proc < space && len(send) < count && !taskQueue.Empty(); proc++ {
        header := taskQueue.PopItem().(*types.Header)
        hash := header.Hash()

        // If we're the first to request this task, initialise the result container
        index := int(header.Number.Int64() - int64(q.resultOffset))
        if index >= len(q.resultCache) || index < 0 {
            common.Report("index allocation went beyond available resultCache space")
            return nil, false, errInvalidChain
        }
        if q.resultCache[index] == nil {
            components := 1
            if q.mode == FastSync {
                components = 2
            }
            q.resultCache[index] = &fetchResult{
                Pending: components,
                Hash:    hash,
                Header:  header,
            }
        }
        // If this fetch task is a noop, skip this fetch operation
        if isNoop(header) {
            donePool[hash] = struct{}{}
            delete(taskPool, hash)

            space, proc = space-1, proc-1
            q.resultCache[index].Pending--
            progress = true
            continue
        }
        // Otherwise unless the peer is known not to have the data, add to the retrieve list
        if p.Lacks(hash) {
            skip = append(skip, header)
        } else {
            send = append(send, header)
        }
    }
    // Merge all the skipped headers back
    for _, header := range skip {
        taskQueue.Push(header, -float32(header.Number.Uint64()))
    }
    if progress {
        // Wake WaitResults, resultCache was modified
        q.active.Signal()
    }
    // Assemble and return the block download request
    if len(send) == 0 {
        return nil, progress, nil
    }
    request := &fetchRequest{
        Peer:    p,
        Headers: send,
        Time:    time.Now(),
    }
    pendPool[p.id] = request

    return request, progress, nil
}

// CancelHeaders aborts a fetch request, returning all pending skeleton indexes to the queue.
func (q *queue) CancelHeaders(request *fetchRequest) {
    q.cancel(request, q.headerTaskQueue, q.headerPendPool)
}

// CancelBodies aborts a body fetch request, returning all pending headers to the
// task queue.
func (q *queue) CancelBodies(request *fetchRequest) {
    q.cancel(request, q.blockTaskQueue, q.blockPendPool)
}

// CancelReceipts aborts a body fetch request, returning all pending headers to
// the task queue.
func (q *queue) CancelReceipts(request *fetchRequest) {
    q.cancel(request, q.receiptTaskQueue, q.receiptPendPool)
}

// Cancel aborts a fetch request, returning all pending hashes to the task queue.
func (q *queue) cancel(request *fetchRequest, taskQueue *prque.Prque, pendPool map[string]*fetchRequest) {
    q.lock.Lock()
    defer q.lock.Unlock()

    if request.From > 0 {
        taskQueue.Push(request.From, -float32(request.From))
    }
    for _, header := range request.Headers {
        taskQueue.Push(header, -float32(header.Number.Uint64()))
    }
    delete(pendPool, request.Peer.id)
}

// Revoke cancels all pending requests belonging to a given peer. This method is
// meant to be called during a peer drop to quickly reassign owned data fetches
// to remaining nodes.
func (q *queue) Revoke(peerId string) {
    q.lock.Lock()
    defer q.lock.Unlock()

    if request, ok := q.blockPendPool[peerId]; ok {
        for _, header := range request.Headers {
            q.blockTaskQueue.Push(header, -float32(header.Number.Uint64()))
        }
        delete(q.blockPendPool, peerId)
    }
    if request, ok := q.receiptPendPool[peerId]; ok {
        for _, header := range request.Headers {
            q.receiptTaskQueue.Push(header, -float32(header.Number.Uint64()))
        }
        delete(q.receiptPendPool, peerId)
    }
}

// ExpireHeaders checks for in flight requests that exceeded a timeout allowance,
// canceling them and returning the responsible peers for penalisation.
func (q *queue) ExpireHeaders(timeout time.Duration) map[string]int {
    q.lock.Lock()
    defer q.lock.Unlock()

    return q.expire(timeout, q.headerPendPool, q.headerTaskQueue, headerTimeoutMeter)
}

// ExpireBodies checks for in flight block body requests that exceeded a timeout
// allowance, canceling them and returning the responsible peers for penalisation.
func (q *queue) ExpireBodies(timeout time.Duration) map[string]int {
    q.lock.Lock()
    defer q.lock.Unlock()

    return q.expire(timeout, q.blockPendPool, q.blockTaskQueue, bodyTimeoutMeter)
}

// ExpireReceipts checks for in flight receipt requests that exceeded a timeout
// allowance, canceling them and returning the responsible peers for penalisation.
func (q *queue) ExpireReceipts(timeout time.Duration) map[string]int {
    q.lock.Lock()
    defer q.lock.Unlock()

    return q.expire(timeout, q.receiptPendPool, q.receiptTaskQueue, receiptTimeoutMeter)
}

// expire is the generic check that move expired tasks from a pending pool back
// into a task pool, returning all entities caught with expired tasks.
//
// Note, this method expects the queue lock to be already held. The
// reason the lock is not obtained in here is because the parameters already need
// to access the queue, so they already need a lock anyway.
func (q *queue) expire(timeout time.Duration, pendPool map[string]*fetchRequest, taskQueue *prque.Prque, timeoutMeter metrics.Meter) map[string]int {
    // Iterate over the expired requests and return each to the queue
    expiries := make(map[string]int)
    for id, request := range pendPool {
        if time.Since(request.Time) > timeout {
            // Update the metrics with the timeout
            timeoutMeter.Mark(1)

            // Return any non satisfied requests to the pool
            if request.From > 0 {
                taskQueue.Push(request.From, -float32(request.From))
            }
            for _, header := range request.Headers {
                taskQueue.Push(header, -float32(header.Number.Uint64()))
            }
            // Add the peer to the expiry report along the the number of failed requests
            expiries[id] = len(request.Headers)
        }
    }
    // Remove the expired requests from the pending pool
    for id := range expiries {
        delete(pendPool, id)
    }
    return expiries
}

// DeliverHeaders injects a header retrieval response into the header results
// cache. This method either accepts all headers it received, or none of them
// if they do not map correctly to the skeleton.
//
// If the headers are accepted, the method makes an attempt to deliver the set
// of ready headers to the processor to keep the pipeline full. However it will
// not block to prevent stalling other pending deliveries.
func (q *queue) DeliverHeaders(id string, headers []*types.Header, headerProcCh chan []*types.Header) (int, error) {
    q.lock.Lock()
    defer q.lock.Unlock()

    // Short circuit if the data was never requested
    request := q.headerPendPool[id]
    if request == nil {
        return 0, errNoFetchesPending
    }
    headerReqTimer.UpdateSince(request.Time)
    delete(q.headerPendPool, id)

    // Ensure headers can be mapped onto the skeleton chain
    target := q.headerTaskPool[request.From].Hash()

    accepted := len(headers) == MaxHeaderFetch
    if accepted {
        if headers[0].Number.Uint64() != request.From {
            log.Trace("First header broke chain ordering", "peer", id, "number", headers[0].Number, "hash", headers[0].Hash(), request.From)
            accepted = false
        } else if headers[len(headers)-1].Hash() != target {
            log.Trace("Last header broke skeleton structure ", "peer", id, "number", headers[len(headers)-1].Number, "hash", headers[len(headers)-1].Hash(), "expected", target)
            accepted = false
        }
    }
    if accepted {
        for i, header := range headers[1:] {
            hash := header.Hash()
            if want := request.From + 1 + uint64(i); header.Number.Uint64() != want {
                log.Warn("Header broke chain ordering", "peer", id, "number", header.Number, "hash", hash, "expected", want)
                accepted = false
                break
            }
            if headers[i].Hash() != header.ParentHash {
                log.Warn("Header broke chain ancestry", "peer", id, "number", header.Number, "hash", hash)
                accepted = false
                break
            }
        }
    }
    // If the batch of headers wasn't accepted, mark as unavailable
    if !accepted {
        log.Trace("Skeleton filling not accepted", "peer", id, "from", request.From)

        miss := q.headerPeerMiss[id]
        if miss == nil {
            q.headerPeerMiss[id] = make(map[uint64]struct{})
            miss = q.headerPeerMiss[id]
        }
        miss[request.From] = struct{}{}

        q.headerTaskQueue.Push(request.From, -float32(request.From))
        return 0, errors.New("delivery not accepted")
    }
    // Clean up a successful fetch and try to deliver any sub-results
    copy(q.headerResults[request.From-q.headerOffset:], headers)
    delete(q.headerTaskPool, request.From)

    ready := 0
    for q.headerProced+ready < len(q.headerResults) && q.headerResults[q.headerProced+ready] != nil {
        ready += MaxHeaderFetch
    }
    if ready > 0 {
        // Headers are ready for delivery, gather them and push forward (non blocking)
        process := make([]*types.Header, ready)
        copy(process, q.headerResults[q.headerProced:q.headerProced+ready])

        select {
        case headerProcCh <- process:
            log.Trace("Pre-scheduled new headers", "peer", id, "count", len(process), "from", process[0].Number)
            q.headerProced += len(process)
        default:
        }
    }
    // Check for termination and return
    if len(q.headerTaskPool) == 0 {
        q.headerContCh <- false
    }
    return len(headers), nil
}

// DeliverBodies injects a block body retrieval response into the results queue.
// The method returns the number of blocks bodies accepted from the delivery and
// also wakes any threads waiting for data delivery.
func (q *queue) DeliverBodies(id string, txLists [][]*types.Transaction, uncleLists [][]*types.Header) (int, error) {
    q.lock.Lock()
    defer q.lock.Unlock()

    reconstruct := func(header *types.Header, index int, result *fetchResult) error {
        if types.DeriveSha(types.Transactions(txLists[index])) != header.TxHash || types.CalcUncleHash(uncleLists[index]) != header.UncleHash {
            return errInvalidBody
        }
        result.Transactions = txLists[index]
        result.Uncles = uncleLists[index]
        return nil
    }
    return q.deliver(id, q.blockTaskPool, q.blockTaskQueue, q.blockPendPool, q.blockDonePool, bodyReqTimer, len(txLists), reconstruct)
}

// DeliverReceipts injects a receipt retrieval response into the results queue.
// The method returns the number of transaction receipts accepted from the delivery
// and also wakes any threads waiting for data delivery.
func (q *queue) DeliverReceipts(id string, receiptList [][]*types.Receipt) (int, error) {
    q.lock.Lock()
    defer q.lock.Unlock()

    reconstruct := func(header *types.Header, index int, result *fetchResult) error {
        if types.DeriveSha(types.Receipts(receiptList[index])) != header.ReceiptHash {
            return errInvalidReceipt
        }
        result.Receipts = receiptList[index]
        return nil
    }
    return q.deliver(id, q.receiptTaskPool, q.receiptTaskQueue, q.receiptPendPool, q.receiptDonePool, receiptReqTimer, len(receiptList), reconstruct)
}

// deliver injects a data retrieval response into the results queue.
//
// Note, this method expects the queue lock to be already held for writing. The
// reason the lock is not obtained in here is because the parameters already need
// to access the queue, so they already need a lock anyway.
func (q *queue) deliver(id string, taskPool map[common.Hash]*types.Header, taskQueue *prque.Prque,
    pendPool map[string]*fetchRequest, donePool map[common.Hash]struct{}, reqTimer metrics.Timer,
    results int, reconstruct func(header *types.Header, index int, result *fetchResult) error) (int, error) {

    // Short circuit if the data was never requested
    request := pendPool[id]
    if request == nil {
        return 0, errNoFetchesPending
    }
    reqTimer.UpdateSince(request.Time)
    delete(pendPool, id)

    // If no data items were retrieved, mark them as unavailable for the origin peer
    if results == 0 {
        for _, header := range request.Headers {
            request.Peer.MarkLacking(header.Hash())
        }
    }
    // Assemble each of the results with their headers and retrieved data parts
    var (
        accepted int
        failure  error
        useful   bool
    )
    for i, header := range request.Headers {
        // Short circuit assembly if no more fetch results are found
        if i >= results {
            break
        }
        // Reconstruct the next result if contents match up
        index := int(header.Number.Int64() - int64(q.resultOffset))
        if index >= len(q.resultCache) || index < 0 || q.resultCache[index] == nil {
            failure = errInvalidChain
            break
        }
        if err := reconstruct(header, i, q.resultCache[index]); err != nil {
            failure = err
            break
        }
        hash := header.Hash()

        donePool[hash] = struct{}{}
        q.resultCache[index].Pending--
        useful = true
        accepted++

        // Clean up a successful fetch
        request.Headers[i] = nil
        delete(taskPool, hash)
    }
    // Return all failed or missing fetches to the queue
    for _, header := range request.Headers {
        if header != nil {
            taskQueue.Push(header, -float32(header.Number.Uint64()))
        }
    }
    // Wake up WaitResults
    if accepted > 0 {
        q.active.Signal()
    }
    // If none of the data was good, it's a stale delivery
    switch {
    case failure == nil || failure == errInvalidChain:
        return accepted, failure
    case useful:
        return accepted, fmt.Errorf("partial failure: %v", failure)
    default:
        return accepted, errStaleDelivery
    }
}

// Prepare configures the result cache to allow accepting and caching inbound
// fetch results.
func (q *queue) Prepare(offset uint64, mode SyncMode) {
    q.lock.Lock()
    defer q.lock.Unlock()

    // Prepare the queue for sync results
    if q.resultOffset < offset {
        q.resultOffset = offset
    }
    q.mode = mode
}