path: root/eth/fetcher/fetcher.go

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

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

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

const (
    arriveTimeout = 500 * time.Millisecond // Time allowance before an announced block is explicitly requested
    fetchTimeout  = 5 * time.Second        // Maximum alloted time to return an explicitly requested block
)

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

// hashCheckFn is a callback type for verifying a hash's presence in the local chain.
type hashCheckFn func(common.Hash) bool

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

// blockImporterFn is a callback type for trying to inject a block into the local chain.
type blockImporterFn func(peer string, block *types.Block) error

// 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
    time time.Time   // Timestamp of the announcement

    origin string           // Identifier of the peer originating the notification
    fetch  blockRequesterFn // Fetcher function to retrieve
}

// Fetcher is responsible for accumulating block announcements from various peers
// and scheduling them for retrieval.
type Fetcher struct {
    // Various event channels
    notify chan *announce
    filter chan chan []*types.Block
    quit   chan struct{}

    // Callbacks
    hasBlock    hashCheckFn     // Checks if a block is present in the chain
    importBlock blockImporterFn // Injects a block from an origin peer into the chain
}

// New creates a block fetcher to retrieve blocks based on hash announcements.
func New(hasBlock hashCheckFn, importBlock blockImporterFn) *Fetcher {
    return &Fetcher{
        notify:      make(chan *announce),
        filter:      make(chan chan []*types.Block),
        quit:        make(chan struct{}),
        hasBlock:    hasBlock,
        importBlock: importBlock,
    }
}

// Start boots up the announcement based synchoniser, 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, time time.Time, fetcher blockRequesterFn) error {
    block := &announce{
        hash:   hash,
        time:   time,
        origin: peer,
        fetch:  fetcher,
    }
    select {
    case f.notify <- block:
        return nil
    case <-f.quit:
        return errTerminated
    }
}

// Filter extracts all the blocks that were explicitly requested by the fetcher,
// returning those that should be handled differently.
func (f *Fetcher) Filter(blocks types.Blocks) types.Blocks {
    // Send the filter channel to the fetcher
    filter := make(chan []*types.Block)

    select {
    case f.filter <- filter:
    case <-f.quit:
        return nil
    }
    // Request the filtering of the block list
    select {
    case filter <- blocks:
    case <-f.quit:
        return nil
    }
    // Retrieve the blocks remaining after filtering
    select {
    case blocks := <-filter:
        return blocks
    case <-f.quit:
        return nil
    }
}

// Loop is the main fetcher loop, checking and processing various notification
// events.
func (f *Fetcher) loop() {
    announced := make(map[common.Hash][]*announce)
    fetching := make(map[common.Hash]*announce)
    fetch := time.NewTimer(0)
    done := make(chan common.Hash)

    // Iterate the block fetching until a quit is requested
    for {
        // Clean up any expired block fetches
        for hash, announce := range fetching {
            if time.Since(announce.time) > fetchTimeout {
                delete(announced, hash)
                delete(fetching, hash)
            }
        }
        // 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, schedule if it's not yet downloading
            glog.V(logger.Debug).Infof("Peer %s: scheduling %x", notification.origin, notification.hash[:4])
            if _, ok := fetching[notification.hash]; ok {
                break
            }
            if len(announced) == 0 {
                fetch.Reset(arriveTimeout)
            }
            announced[notification.hash] = append(announced[notification.hash], notification)

        case hash := <-done:
            // A pending import finished, remove all traces of the notification
            delete(announced, hash)
            delete(fetching, hash)

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

            for hash, announces := range announced {
                if time.Since(announces[0].time) > arriveTimeout {
                    announce := announces[rand.Intn(len(announces))]
                    if !f.hasBlock(hash) {
                        request[announce.origin] = append(request[announce.origin], hash)
                        fetching[hash] = announce
                    }
                    delete(announced, hash)
                }
            }
            // Send out all block requests
            for peer, hashes := range request {
                glog.V(logger.Debug).Infof("Peer %s: explicitly fetching %d blocks", peer, len(hashes))
                go fetching[hashes[0]].fetch(hashes)
            }
            // Schedule the next fetch if blocks are still pending
            if len(announced) > 0 {
                nearest := time.Now()
                for _, announces := range announced {
                    if nearest.Before(announces[0].time) {
                        nearest = announces[0].time
                    }
                }
                fetch.Reset(arriveTimeout + time.Since(nearest))
            }

        case filter := <-f.filter:
            // Blocks arrived, extract any explicit fetches, return all else
            var blocks types.Blocks
            select {
            case blocks = <-filter:
            case <-f.quit:
                return
            }

            explicit, download := []*types.Block{}, []*types.Block{}
            for _, block := range blocks {
                hash := block.Hash()

                // Filter explicitly requested blocks from hash announcements
                if _, ok := fetching[hash]; ok {
                    // Discard if already imported by other means
                    if !f.hasBlock(hash) {
                        explicit = append(explicit, block)
                    } else {
                        delete(fetching, hash)
                    }
                } else {
                    download = append(download, block)
                }
            }

            select {
            case filter <- download:
            case <-f.quit:
                return
            }
            // Create a closure with the retrieved blocks and origin peers
            peers := make([]string, 0, len(explicit))
            blocks = make([]*types.Block, 0, len(explicit))
            for _, block := range explicit {
                hash := block.Hash()
                if announce := fetching[hash]; announce != nil {
                    // Drop the block if it surely cannot fit
                    if f.hasBlock(hash) || !f.hasBlock(block.ParentHash()) {
                        // delete(fetching, hash) // if we drop, it will re-fetch it, wait for timeout?
                        continue
                    }
                    // Otherwise accumulate for import
                    peers = append(peers, announce.origin)
                    blocks = append(blocks, block)
                }
            }
            // If any explicit fetches were replied to, import them
            if count := len(blocks); count > 0 {
                glog.V(logger.Debug).Infof("Importing %d explicitly fetched blocks", len(blocks))
                go func() {
                    // Make sure all hashes are cleaned up
                    for _, block := range blocks {
                        hash := block.Hash()
                        defer func() { done <- hash }()
                    }
                    // Try and actually import the blocks
                    for i := 0; i < len(blocks); i++ {
                        if err := f.importBlock(peers[i], blocks[i]); err != nil {
                            glog.V(logger.Detail).Infof("Failed to import explicitly fetched block: %v", err)
                            return
                        }
                    }
                }()
            }
        }
    }
}