path: root/dex/fetcher/fetcher.go

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

// Package fetcher contains the block announcement based synchronisation.
package fetcher

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

    "github.com/dexon-foundation/dexon/common"
    "github.com/dexon-foundation/dexon/common/prque"
    "github.com/dexon-foundation/dexon/consensus"
    "github.com/dexon-foundation/dexon/core/types"
    "github.com/dexon-foundation/dexon/log"
)

const (
    arriveTimeout = 500 * time.Millisecond // Time allowance before an announced block is explicitly requested
    gatherSlack   = 100 * time.Millisecond // Interval used to collate almost-expired announces with fetches
    fetchTimeout  = 5 * time.Second        // Maximum allotted time to return an explicitly requested block
    maxUncleDist  = 7                      // Maximum allowed backward distance from the chain head
    maxQueueDist  = 32                     // Maximum allowed distance from the chain head to queue
    hashLimit     = 256                    // Maximum number of unique blocks a peer may have announced
    blockLimit    = 64                     // Maximum number of unique blocks a peer may have delivered
)

var (
    errTerminated = errors.New("terminated")
)

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

// headerRequesterFn is a callback type for sending a header retrieval request.
type headerRequesterFn func(common.Hash) error

// bodyRequesterFn is a callback type for sending a body retrieval request.
type bodyRequesterFn func([]common.Hash) error

// headerVerifierFn is a callback type to verify a block's header for fast propagation.
type headerVerifierFn func(header *types.Header) error

// blockBroadcasterFn is a callback type for broadcasting a block to connected peers.
type blockBroadcasterFn func(block *types.Block, propagate bool)

// chainHeightFn is a callback type to retrieve the current chain height.
type chainHeightFn func() uint64

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

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

// announce is the hash notification of the availability of a new block in the
// network.
type announce struct {
    hash   common.Hash   // Hash of the block being announced
    number uint64        // Number of the block being announced (0 = unknown | old protocol)
    header *types.Header // Header of the block partially reassembled (new protocol)
    time   time.Time     // Timestamp of the announcement

    origin string // Identifier of the peer originating the notification

    fetchHeader headerRequesterFn // Fetcher function to retrieve the header of an announced block
    fetchBodies bodyRequesterFn   // Fetcher function to retrieve the body of an announced block
}

// headerFilterTask represents a batch of headers needing fetcher filtering.
type headerFilterTask struct {
    peer    string          // The source peer of block headers
    headers []*types.Header // Collection of headers to filter
    time    time.Time       // Arrival time of the headers
}

// bodyFilterTask represents a batch of block bodies (transactions and uncles)
// needing fetcher filtering.
type bodyFilterTask struct {
    peer         string                 // The source peer of block bodies
    transactions [][]*types.Transaction // Collection of transactions per block bodies
    uncles       [][]*types.Header      // Collection of uncles per block bodies
    time         time.Time              // Arrival time of the blocks' contents
}

// inject represents a schedules import operation.
type inject struct {
    origin string
    block  *types.Block
}

// Fetcher is responsible for accumulating block announcements from various peers
// and scheduling them for retrieval.
type Fetcher struct {
    // Various event channels
    notify chan *announce
    inject chan *inject

    blockFilter  chan chan []*types.Block
    headerFilter chan chan *headerFilterTask
    bodyFilter   chan chan *bodyFilterTask

    done chan common.Hash
    quit chan struct{}

    // Announce states
    announces  map[string]int              // Per peer announce counts to prevent memory exhaustion
    announced  map[common.Hash][]*announce // Announced blocks, scheduled for fetching
    fetching   map[common.Hash]*announce   // Announced blocks, currently fetching
    fetched    map[common.Hash][]*announce // Blocks with headers fetched, scheduled for body retrieval
    completing map[common.Hash]*announce   // Blocks with headers, currently body-completing

    // Block cache
    queue  *prque.Prque            // Queue containing the import operations (block number sorted)
    queues map[string]int          // Per peer block counts to prevent memory exhaustion
    queued map[common.Hash]*inject // Set of already queued blocks (to dedupe imports)

    // Callbacks
    getBlock       blockRetrievalFn   // Retrieves a block from the local chain
    verifyHeader   headerVerifierFn   // Checks if a block's headers have a valid proof of work
    broadcastBlock blockBroadcasterFn // Broadcasts a block to connected peers
    chainHeight    chainHeightFn      // Retrieves the current chain's height
    insertChain    chainInsertFn      // Injects a batch of blocks into the chain
    dropPeer       peerDropFn         // Drops a peer for misbehaving

    // Testing hooks
    announceChangeHook func(common.Hash, bool) // Method to call upon adding or deleting a hash from the announce list
    queueChangeHook    func(common.Hash, bool) // Method to call upon adding or deleting a block from the import queue
    fetchingHook       func([]common.Hash)     // Method to call upon starting a block (eth/61) or header (eth/62) fetch
    completingHook     func([]common.Hash)     // Method to call upon starting a block body fetch (eth/62)
    importedHook       func(*types.Block)      // Method to call upon successful block import (both eth/61 and eth/62)
}

// New creates a block fetcher to retrieve blocks based on hash announcements.
func New(getBlock blockRetrievalFn, verifyHeader headerVerifierFn, broadcastBlock blockBroadcasterFn, chainHeight chainHeightFn, insertChain chainInsertFn, dropPeer peerDropFn) *Fetcher {
    return &Fetcher{
        notify:         make(chan *announce),
        inject:         make(chan *inject),
        blockFilter:    make(chan chan []*types.Block),
        headerFilter:   make(chan chan *headerFilterTask),
        bodyFilter:     make(chan chan *bodyFilterTask),
        done:           make(chan common.Hash),
        quit:           make(chan struct{}),
        announces:      make(map[string]int),
        announced:      make(map[common.Hash][]*announce),
        fetching:       make(map[common.Hash]*announce),
        fetched:        make(map[common.Hash][]*announce),
        completing:     make(map[common.Hash]*announce),
        queue:          prque.New(nil),
        queues:         make(map[string]int),
        queued:         make(map[common.Hash]*inject),
        getBlock:       getBlock,
        verifyHeader:   verifyHeader,
        broadcastBlock: broadcastBlock,
        chainHeight:    chainHeight,
        insertChain:    insertChain,
        dropPeer:       dropPeer,
    }
}

// Start boots up the announcement based synchroniser, accepting and processing
// hash notifications and block fetches until termination requested.
func (f *Fetcher) Start() {
    go f.loop()
}

// Stop terminates the announcement based synchroniser, canceling all pending
// operations.
func (f *Fetcher) Stop() {
    close(f.quit)
}

// Notify announces the fetcher of the potential availability of a new block in
// the network.
func (f *Fetcher) Notify(peer string, hash common.Hash, number uint64, time time.Time,
    headerFetcher headerRequesterFn, bodyFetcher bodyRequesterFn) error {
    block := &announce{
        hash:        hash,
        number:      number,
        time:        time,
        origin:      peer,
        fetchHeader: headerFetcher,
        fetchBodies: bodyFetcher,
    }
    select {
    case f.notify <- block:
        return nil
    case <-f.quit:
        return errTerminated
    }
}

// Enqueue tries to fill gaps the fetcher's future import queue.
func (f *Fetcher) Enqueue(peer string, block *types.Block) error {
    op := &inject{
        origin: peer,
        block:  block,
    }
    select {
    case f.inject <- op:
        return nil
    case <-f.quit:
        return errTerminated
    }
}

// FilterHeaders extracts all the headers that were explicitly requested by the fetcher,
// returning those that should be handled differently.
func (f *Fetcher) FilterHeaders(peer string, headers []*types.Header, time time.Time) []*types.Header {
    log.Trace("Filtering headers", "peer", peer, "headers", len(headers))

    // Send the filter channel to the fetcher
    filter := make(chan *headerFilterTask)

    select {
    case f.headerFilter <- filter:
    case <-f.quit:
        return nil
    }
    // Request the filtering of the header list
    select {
    case filter <- &headerFilterTask{peer: peer, headers: headers, time: time}:
    case <-f.quit:
        return nil
    }
    // Retrieve the headers remaining after filtering
    select {
    case task := <-filter:
        return task.headers
    case <-f.quit:
        return nil
    }
}

// FilterBodies extracts all the block bodies that were explicitly requested by
// the fetcher, returning those that should be handled differently.
func (f *Fetcher) FilterBodies(peer string, transactions [][]*types.Transaction, uncles [][]*types.Header, time time.Time) ([][]*types.Transaction, [][]*types.Header) {
    log.Trace("Filtering bodies", "peer", peer, "txs", len(transactions), "uncles", len(uncles))

    // Send the filter channel to the fetcher
    filter := make(chan *bodyFilterTask)

    select {
    case f.bodyFilter <- filter:
    case <-f.quit:
        return nil, nil
    }
    // Request the filtering of the body list
    select {
    case filter <- &bodyFilterTask{peer: peer, transactions: transactions, uncles: uncles, time: time}:
    case <-f.quit:
        return nil, nil
    }
    // Retrieve the bodies remaining after filtering
    select {
    case task := <-filter:
        return task.transactions, task.uncles
    case <-f.quit:
        return nil, nil
    }
}

// Loop is the main fetcher loop, checking and processing various notification
// events.
func (f *Fetcher) loop() {
    // Iterate the block fetching until a quit is requested
    fetchTimer := time.NewTimer(0)
    completeTimer := time.NewTimer(0)

    for {
        // Clean up any expired block fetches
        for hash, announce := range f.fetching {
            if time.Since(announce.time) > fetchTimeout {
                f.forgetHash(hash)
            }
        }
        // Import any queued blocks that could potentially fit
        height := f.chainHeight()
        for !f.queue.Empty() {
            op := f.queue.PopItem().(*inject)
            hash := op.block.Hash()
            if f.queueChangeHook != nil {
                f.queueChangeHook(hash, false)
            }
            // If too high up the chain or phase, continue later
            number := op.block.NumberU64()
            if number > height+1 {
                f.queue.Push(op, -int64(number))
                if f.queueChangeHook != nil {
                    f.queueChangeHook(hash, true)
                }
                break
            }
            // Otherwise if fresh and still unknown, try and import
            if number+maxUncleDist < height || f.getBlock(hash) != nil {
                f.forgetBlock(hash)
                continue
            }
            f.insert(op.origin, op.block)
        }
        // Wait for an outside event to occur
        select {
        case <-f.quit:
            // Fetcher terminating, abort all operations
            return

        case notification := <-f.notify:
            // A block was announced, make sure the peer isn't DOSing us
            propAnnounceInMeter.Mark(1)

            count := f.announces[notification.origin] + 1
            if count > hashLimit {
                log.Debug("Peer exceeded outstanding announces", "peer", notification.origin, "limit", hashLimit)
                propAnnounceDOSMeter.Mark(1)
                break
            }
            // If we have a valid block number, check that it's potentially useful
            if notification.number > 0 {
                if dist := int64(notification.number) - int64(f.chainHeight()); dist < -maxUncleDist || dist > maxQueueDist {
                    log.Debug("Peer discarded announcement", "peer", notification.origin, "number", notification.number, "hash", notification.hash, "distance", dist)
                    propAnnounceDropMeter.Mark(1)
                    break
                }
            }
            // All is well, schedule the announce if block's not yet downloading
            if _, ok := f.fetching[notification.hash]; ok {
                break
            }
            if _, ok := f.completing[notification.hash]; ok {
                break
            }
            f.announces[notification.origin] = count
            f.announced[notification.hash] = append(f.announced[notification.hash], notification)
            if f.announceChangeHook != nil && len(f.announced[notification.hash]) == 1 {
                f.announceChangeHook(notification.hash, true)
            }
            if len(f.announced) == 1 {
                f.rescheduleFetch(fetchTimer)
            }

        case op := <-f.inject:
            // A direct block insertion was requested, try and fill any pending gaps
            propBroadcastInMeter.Mark(1)
            f.enqueue(op.origin, op.block)

        case hash := <-f.done:
            // A pending import finished, remove all traces of the notification
            f.forgetHash(hash)
            f.forgetBlock(hash)

        case <-fetchTimer.C:
            // At least one block's timer ran out, check for needing retrieval
            request := make(map[string][]common.Hash)

            for hash, announces := range f.announced {
                if time.Since(announces[0].time) > arriveTimeout-gatherSlack {
                    // Pick a random peer to retrieve from, reset all others
                    announce := announces[rand.Intn(len(announces))]
                    f.forgetHash(hash)

                    // If the block still didn't arrive, queue for fetching
                    if f.getBlock(hash) == nil {
                        request[announce.origin] = append(request[announce.origin], hash)
                        f.fetching[hash] = announce
                    }
                }
            }
            // Send out all block header requests
            for peer, hashes := range request {
                log.Trace("Fetching scheduled headers", "peer", peer, "list", hashes)

                // Create a closure of the fetch and schedule in on a new thread
                fetchHeader, hashes := f.fetching[hashes[0]].fetchHeader, hashes
                go func() {
                    if f.fetchingHook != nil {
                        f.fetchingHook(hashes)
                    }
                    for _, hash := range hashes {
                        headerFetchMeter.Mark(1)
                        fetchHeader(hash) // Suboptimal, but protocol doesn't allow batch header retrievals
                    }
                }()
            }
            // Schedule the next fetch if blocks are still pending
            f.rescheduleFetch(fetchTimer)

        case <-completeTimer.C:
            // At least one header's timer ran out, retrieve everything
            request := make(map[string][]common.Hash)

            for hash, announces := range f.fetched {
                // Pick a random peer to retrieve from, reset all others
                announce := announces[rand.Intn(len(announces))]
                f.forgetHash(hash)

                // If the block still didn't arrive, queue for completion
                if f.getBlock(hash) == nil {
                    request[announce.origin] = append(request[announce.origin], hash)
                    f.completing[hash] = announce
                }
            }
            // Send out all block body requests
            for peer, hashes := range request {
                log.Trace("Fetching scheduled bodies", "peer", peer, "list", hashes)

                // Create a closure of the fetch and schedule in on a new thread
                if f.completingHook != nil {
                    f.completingHook(hashes)
                }
                bodyFetchMeter.Mark(int64(len(hashes)))
                go f.completing[hashes[0]].fetchBodies(hashes)
            }
            // Schedule the next fetch if blocks are still pending
            f.rescheduleComplete(completeTimer)

        case filter := <-f.headerFilter:
            // Headers arrived from a remote peer. Extract those that were explicitly
            // requested by the fetcher, and return everything else so it's delivered
            // to other parts of the system.
            var task *headerFilterTask
            select {
            case task = <-filter:
            case <-f.quit:
                return
            }
            headerFilterInMeter.Mark(int64(len(task.headers)))

            // Split the batch of headers into unknown ones (to return to the caller),
            // known incomplete ones (requiring body retrievals) and completed blocks.
            unknown, incomplete, complete := []*types.Header{}, []*announce{}, []*types.Block{}
            for _, header := range task.headers {
                hash := header.Hash()

                // Filter fetcher-requested headers from other synchronisation algorithms
                if announce := f.fetching[hash]; announce != nil && announce.origin == task.peer && f.fetched[hash] == nil && f.completing[hash] == nil && f.queued[hash] == nil {
                    // If the delivered header does not match the promised number, drop the announcer
                    if header.Number.Uint64() != announce.number {
                        log.Trace("Invalid block number fetched", "peer", announce.origin, "hash", header.Hash(), "announced", announce.number, "provided", header.Number)
                        f.dropPeer(announce.origin)
                        f.forgetHash(hash)
                        continue
                    }
                    // Only keep if not imported by other means
                    if f.getBlock(hash) == nil {
                        announce.header = header
                        announce.time = task.time

                        // If the block is empty (header only), short circuit into the final import queue
                        if header.TxHash == types.DeriveSha(types.Transactions{}) && header.UncleHash == types.CalcUncleHash([]*types.Header{}) {
                            log.Trace("Block empty, skipping body retrieval", "peer", announce.origin, "number", header.Number, "hash", header.Hash())

                            block := types.NewBlockWithHeader(header)
                            block.ReceivedAt = task.time

                            complete = append(complete, block)
                            f.completing[hash] = announce
                            continue
                        }
                        // Otherwise add to the list of blocks needing completion
                        incomplete = append(incomplete, announce)
                    } else {
                        log.Trace("Block already imported, discarding header", "peer", announce.origin, "number", header.Number, "hash", header.Hash())
                        f.forgetHash(hash)
                    }
                } else {
                    // Fetcher doesn't know about it, add to the return list
                    unknown = append(unknown, header)
                }
            }
            headerFilterOutMeter.Mark(int64(len(unknown)))
            select {
            case filter <- &headerFilterTask{headers: unknown, time: task.time}:
            case <-f.quit:
                return
            }
            // Schedule the retrieved headers for body completion
            for _, announce := range incomplete {
                hash := announce.header.Hash()
                if _, ok := f.completing[hash]; ok {
                    continue
                }
                f.fetched[hash] = append(f.fetched[hash], announce)
                if len(f.fetched) == 1 {
                    f.rescheduleComplete(completeTimer)
                }
            }
            // Schedule the header-only blocks for import
            for _, block := range complete {
                if announce := f.completing[block.Hash()]; announce != nil {
                    f.enqueue(announce.origin, block)
                }
            }

        case filter := <-f.bodyFilter:
            // Block bodies arrived, extract any explicitly requested blocks, return the rest
            var task *bodyFilterTask
            select {
            case task = <-filter:
            case <-f.quit:
                return
            }
            bodyFilterInMeter.Mark(int64(len(task.transactions)))

            blocks := []*types.Block{}
            for i := 0; i < len(task.transactions) && i < len(task.uncles); i++ {
                // Match up a body to any possible completion request
                matched := false

                for hash, announce := range f.completing {
                    if f.queued[hash] == nil {
                        txnHash := types.DeriveSha(types.Transactions(task.transactions[i]))
                        uncleHash := types.CalcUncleHash(task.uncles[i])

                        if txnHash == announce.header.TxHash && uncleHash == announce.header.UncleHash && announce.origin == task.peer {
                            // Mark the body matched, reassemble if still unknown
                            matched = true

                            if f.getBlock(hash) == nil {
                                block := types.NewBlockWithHeader(announce.header).WithBody(task.transactions[i], task.uncles[i])
                                block.ReceivedAt = task.time

                                blocks = append(blocks, block)
                            } else {
                                f.forgetHash(hash)
                            }
                        }
                    }
                }
                if matched {
                    task.transactions = append(task.transactions[:i], task.transactions[i+1:]...)
                    task.uncles = append(task.uncles[:i], task.uncles[i+1:]...)
                    i--
                    continue
                }
            }

            bodyFilterOutMeter.Mark(int64(len(task.transactions)))
            select {
            case filter <- task:
            case <-f.quit:
                return
            }
            // Schedule the retrieved blocks for ordered import
            for _, block := range blocks {
                if announce := f.completing[block.Hash()]; announce != nil {
                    f.enqueue(announce.origin, block)
                }
            }
        }
    }
}

// rescheduleFetch resets the specified fetch timer to the next announce timeout.
func (f *Fetcher) rescheduleFetch(fetch *time.Timer) {
    // Short circuit if no blocks are announced
    if len(f.announced) == 0 {
        return
    }
    // Otherwise find the earliest expiring announcement
    earliest := time.Now()
    for _, announces := range f.announced {
        if earliest.After(announces[0].time) {
            earliest = announces[0].time
        }
    }
    fetch.Reset(arriveTimeout - time.Since(earliest))
}

// rescheduleComplete resets the specified completion timer to the next fetch timeout.
func (f *Fetcher) rescheduleComplete(complete *time.Timer) {
    // Short circuit if no headers are fetched
    if len(f.fetched) == 0 {
        return
    }
    // Otherwise find the earliest expiring announcement
    earliest := time.Now()
    for _, announces := range f.fetched {
        if earliest.After(announces[0].time) {
            earliest = announces[0].time
        }
    }
    complete.Reset(gatherSlack - time.Since(earliest))
}

// enqueue schedules a new future import operation, if the block to be imported
// has not yet been seen.
func (f *Fetcher) enqueue(peer string, block *types.Block) {
    hash := block.Hash()

    // Ensure the peer isn't DOSing us
    count := f.queues[peer] + 1
    if count > blockLimit {
        log.Debug("Discarded propagated block, exceeded allowance", "peer", peer, "number", block.Number(), "hash", hash, "limit", blockLimit)
        propBroadcastDOSMeter.Mark(1)
        f.forgetHash(hash)
        return
    }
    // Discard any past or too distant blocks
    if dist := int64(block.NumberU64()) - int64(f.chainHeight()); dist < -maxUncleDist || dist > maxQueueDist {
        log.Debug("Discarded propagated block, too far away", "peer", peer, "number", block.Number(), "hash", hash, "distance", dist)
        propBroadcastDropMeter.Mark(1)
        f.forgetHash(hash)
        return
    }
    // Schedule the block for future importing
    if _, ok := f.queued[hash]; !ok {
        op := &inject{
            origin: peer,
            block:  block,
        }
        f.queues[peer] = count
        f.queued[hash] = op
        f.queue.Push(op, -int64(block.NumberU64()))
        if f.queueChangeHook != nil {
            f.queueChangeHook(op.block.Hash(), true)
        }
        log.Debug("Queued propagated block", "peer", peer, "number", block.Number(), "hash", hash, "queued", f.queue.Size())
    }
}

// insert spawns a new goroutine to run a block insertion into the chain. If the
// block's number is at the same height as the current import phase, it updates
// the phase states accordingly.
func (f *Fetcher) insert(peer string, block *types.Block) {
    hash := block.Hash()

    // Run the import on a new thread
    log.Debug("Importing propagated block", "peer", peer, "number", block.Number(), "hash", hash)
    go func() {
        defer func() { f.done <- hash }()

        // If the parent's unknown, abort insertion
        parent := f.getBlock(block.ParentHash())
        if parent == nil {
            log.Debug("Unknown parent of propagated block", "peer", peer, "number", block.Number(), "hash", hash, "parent", block.ParentHash())
            return
        }
        // Quickly validate the header and propagate the block if it passes
        switch err := f.verifyHeader(block.Header()); err {
        case nil:
            // All ok, quickly propagate to our peers
            propBroadcastOutTimer.UpdateSince(block.ReceivedAt)
            go f.broadcastBlock(block, true)

        case consensus.ErrFutureBlock:
            // Weird future block, don't fail, but neither propagate

        default:
            // Something went very wrong, drop the peer
            log.Debug("Propagated block verification failed", "peer", peer, "number", block.Number(), "hash", hash, "err", err)
            f.dropPeer(peer)
            return
        }
        // Run the actual import and log any issues
        if _, err := f.insertChain(types.Blocks{block}); err != nil {
            log.Debug("Propagated block import failed", "peer", peer, "number", block.Number(), "hash", hash, "err", err)
            return
        }
        // If import succeeded, broadcast the block
        propAnnounceOutTimer.UpdateSince(block.ReceivedAt)
        go f.broadcastBlock(block, false)

        // Invoke the testing hook if needed
        if f.importedHook != nil {
            f.importedHook(block)
        }
    }()
}

// forgetHash removes all traces of a block announcement from the fetcher's
// internal state.
func (f *Fetcher) forgetHash(hash common.Hash) {
    // Remove all pending announces and decrement DOS counters
    for _, announce := range f.announced[hash] {
        f.announces[announce.origin]--
        if f.announces[announce.origin] == 0 {
            delete(f.announces, announce.origin)
        }
    }
    delete(f.announced, hash)
    if f.announceChangeHook != nil {
        f.announceChangeHook(hash, false)
    }
    // Remove any pending fetches and decrement the DOS counters
    if announce := f.fetching[hash]; announce != nil {
        f.announces[announce.origin]--
        if f.announces[announce.origin] == 0 {
            delete(f.announces, announce.origin)
        }
        delete(f.fetching, hash)
    }

    // Remove any pending completion requests and decrement the DOS counters
    for _, announce := range f.fetched[hash] {
        f.announces[announce.origin]--
        if f.announces[announce.origin] == 0 {
            delete(f.announces, announce.origin)
        }
    }
    delete(f.fetched, hash)

    // Remove any pending completions and decrement the DOS counters
    if announce := f.completing[hash]; announce != nil {
        f.announces[announce.origin]--
        if f.announces[announce.origin] == 0 {
            delete(f.announces, announce.origin)
        }
        delete(f.completing, hash)
    }
}

// forgetBlock removes all traces of a queued block from the fetcher's internal
// state.
func (f *Fetcher) forgetBlock(hash common.Hash) {
    if insert := f.queued[hash]; insert != nil {
        f.queues[insert.origin]--
        if f.queues[insert.origin] == 0 {
            delete(f.queues, insert.origin)
        }
        delete(f.queued, hash)
    }
}