path: root/eth/downloader/queue.go

// 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/logger"
    "github.com/ethereum/go-ethereum/logger/glog"
    "gopkg.in/karalabe/cookiejar.v2/collections/prque"
)

const (
    blockCacheLimit = 8 * MaxBlockFetch // Maximum number of blocks to cache before throttling the download
)

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

// fetchRequest is a currently running block retrieval operation.
type fetchRequest struct {
    Peer   *peer               // Peer to which the request was sent
    Hashes map[common.Hash]int // Requested hashes with their insertion index (priority)
    Time   time.Time           // Time when the request was made
}

// queue represents hashes that are either need fetching or are being fetched
type queue struct {
    hashPool    map[common.Hash]int // Pending hashes, mapping to their insertion index (priority)
    hashQueue   *prque.Prque        // Priority queue of the block hashes to fetch
    hashCounter int                 // Counter indexing the added hashes to ensure retrieval order

    pendPool map[string]*fetchRequest // Currently pending block retrieval operations

    blockPool   map[common.Hash]int // Hash-set of the downloaded data blocks, mapping to cache indexes
    blockCache  []*Block            // Downloaded but not yet delivered blocks
    blockOffset int                 // Offset of the first cached block in the block-chain

    lock sync.RWMutex
}

// newQueue creates a new download queue for scheduling block retrieval.
func newQueue() *queue {
    return &queue{
        hashPool:   make(map[common.Hash]int),
        hashQueue:  prque.New(),
        pendPool:   make(map[string]*fetchRequest),
        blockPool:  make(map[common.Hash]int),
        blockCache: make([]*Block, blockCacheLimit),
    }
}

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

    q.hashPool = make(map[common.Hash]int)
    q.hashQueue.Reset()
    q.hashCounter = 0

    q.pendPool = make(map[string]*fetchRequest)

    q.blockPool = make(map[common.Hash]int)
    q.blockOffset = 0
    q.blockCache = make([]*Block, blockCacheLimit)
}

// Size retrieves the number of hashes in the queue, returning separately for
// pending and already downloaded.
func (q *queue) Size() (int, int) {
    q.lock.RLock()
    defer q.lock.RUnlock()

    return len(q.hashPool), len(q.blockPool)
}

// Pending retrieves the number of hashes pending for retrieval.
func (q *queue) Pending() int {
    q.lock.RLock()
    defer q.lock.RUnlock()

    return q.hashQueue.Size()
}

// InFlight retrieves the number of fetch requests currently in flight.
func (q *queue) InFlight() int {
    q.lock.RLock()
    defer q.lock.RUnlock()

    return len(q.pendPool)
}

// Throttle checks if the download should be throttled (active block fetches
// exceed block cache).
func (q *queue) Throttle() bool {
    q.lock.RLock()
    defer q.lock.RUnlock()

    // Calculate the currently in-flight block requests
    pending := 0
    for _, request := range q.pendPool {
        pending += len(request.Hashes)
    }
    // Throttle if more blocks are in-flight than free space in the cache
    return pending >= len(q.blockCache)-len(q.blockPool)
}

// Has checks if a hash is within the download queue or not.
func (q *queue) Has(hash common.Hash) bool {
    q.lock.RLock()
    defer q.lock.RUnlock()

    if _, ok := q.hashPool[hash]; ok {
        return true
    }
    if _, ok := q.blockPool[hash]; ok {
        return true
    }
    return false
}

// Insert adds a set of hashes for the download queue for scheduling, returning
// the new hashes encountered.
func (q *queue) Insert(hashes []common.Hash) []common.Hash {
    q.lock.Lock()
    defer q.lock.Unlock()

    // Insert all the hashes prioritized in the arrival order
    inserts := make([]common.Hash, 0, len(hashes))
    for _, hash := range hashes {
        // Skip anything we already have
        if old, ok := q.hashPool[hash]; ok {
            glog.V(logger.Warn).Infof("Hash %x already scheduled at index %v", hash, old)
            continue
        }
        // Update the counters and insert the hash
        q.hashCounter = q.hashCounter + 1
        inserts = append(inserts, hash)

        q.hashPool[hash] = q.hashCounter
        q.hashQueue.Push(hash, float32(q.hashCounter)) // Highest gets schedules first
    }
    return inserts
}

// GetHeadBlock retrieves the first block from the cache, or nil if it hasn't
// been downloaded yet (or simply non existent).
func (q *queue) GetHeadBlock() *Block {
    q.lock.RLock()
    defer q.lock.RUnlock()

    if len(q.blockCache) == 0 {
        return nil
    }
    return q.blockCache[0]
}

// GetBlock retrieves a downloaded block, or nil if non-existent.
func (q *queue) GetBlock(hash common.Hash) *Block {
    q.lock.RLock()
    defer q.lock.RUnlock()

    // Short circuit if the block hasn't been downloaded yet
    index, ok := q.blockPool[hash]
    if !ok {
        return nil
    }
    // Return the block if it's still available in the cache
    if q.blockOffset <= index && index < q.blockOffset+len(q.blockCache) {
        return q.blockCache[index-q.blockOffset]
    }
    return nil
}

// TakeBlocks retrieves and permanently removes a batch of blocks from the cache.
func (q *queue) TakeBlocks() []*Block {
    q.lock.Lock()
    defer q.lock.Unlock()

    // Accumulate all available blocks
    blocks := []*Block{}
    for _, block := range q.blockCache {
        if block == nil {
            break
        }
        blocks = append(blocks, block)
        delete(q.blockPool, block.RawBlock.Hash())
    }
    // Delete the blocks from the slice and let them be garbage collected
    // without this slice trick the blocks would stay in memory until nil
    // would be assigned to q.blocks
    copy(q.blockCache, q.blockCache[len(blocks):])
    for k, n := len(q.blockCache)-len(blocks), len(q.blockCache); k < n; k++ {
        q.blockCache[k] = nil
    }
    q.blockOffset += len(blocks)

    return blocks
}

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

    // Short circuit if the pool has been depleted, or if the peer's already
    // downloading something (sanity check not to corrupt state)
    if q.hashQueue.Empty() {
        return nil
    }
    if _, ok := q.pendPool[p.id]; ok {
        return nil
    }
    // Calculate an upper limit on the hashes we might fetch (i.e. throttling)
    space := len(q.blockCache) - len(q.blockPool)
    for _, request := range q.pendPool {
        space -= len(request.Hashes)
    }
    // Retrieve a batch of hashes, skipping previously failed ones
    send := make(map[common.Hash]int)
    skip := make(map[common.Hash]int)

    for proc := 0; proc < space && len(send) < count && !q.hashQueue.Empty(); proc++ {
        hash, priority := q.hashQueue.Pop()
        if p.ignored.Has(hash) {
            skip[hash.(common.Hash)] = int(priority)
        } else {
            send[hash.(common.Hash)] = int(priority)
        }
    }
    // Merge all the skipped hashes back
    for hash, index := range skip {
        q.hashQueue.Push(hash, float32(index))
    }
    // Assemble and return the block download request
    if len(send) == 0 {
        return nil
    }
    request := &fetchRequest{
        Peer:   p,
        Hashes: send,
        Time:   time.Now(),
    }
    q.pendPool[p.id] = request

    return request
}

// Cancel aborts a fetch request, returning all pending hashes to the queue.
func (q *queue) Cancel(request *fetchRequest) {
    q.lock.Lock()
    defer q.lock.Unlock()

    for hash, index := range request.Hashes {
        q.hashQueue.Push(hash, float32(index))
    }
    delete(q.pendPool, request.Peer.id)
}

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

    // Iterate over the expired requests and return each to the queue
    peers := []string{}
    for id, request := range q.pendPool {
        if time.Since(request.Time) > timeout {
            for hash, index := range request.Hashes {
                q.hashQueue.Push(hash, float32(index))
            }
            peers = append(peers, id)
        }
    }
    // Remove the expired requests from the pending pool
    for _, id := range peers {
        delete(q.pendPool, id)
    }
    return peers
}

// Deliver injects a block retrieval response into the download queue.
func (q *queue) Deliver(id string, blocks []*types.Block) (err error) {
    q.lock.Lock()
    defer q.lock.Unlock()

    // Short circuit if the blocks were never requested
    request := q.pendPool[id]
    if request == nil {
        return errNoFetchesPending
    }
    delete(q.pendPool, id)

    // If no blocks were retrieved, mark them as unavailable for the origin peer
    if len(blocks) == 0 {
        for hash, _ := range request.Hashes {
            request.Peer.ignored.Add(hash)
        }
    }
    // Iterate over the downloaded blocks and add each of them
    errs := make([]error, 0)
    for _, block := range blocks {
        // Skip any blocks that were not requested
        hash := block.Hash()
        if _, ok := request.Hashes[hash]; !ok {
            errs = append(errs, fmt.Errorf("non-requested block %x", hash))
            continue
        }
        // If a requested block falls out of the range, the hash chain is invalid
        index := int(block.NumberU64()) - q.blockOffset
        if index >= len(q.blockCache) || index < 0 {
            return ErrInvalidChain
        }
        // Otherwise merge the block and mark the hash block
        q.blockCache[index] = &Block{
            RawBlock:   block,
            OriginPeer: id,
        }
        delete(request.Hashes, hash)
        delete(q.hashPool, hash)
        q.blockPool[hash] = int(block.NumberU64())
    }
    // Return all failed or missing fetches to the queue
    for hash, index := range request.Hashes {
        q.hashQueue.Push(hash, float32(index))
    }
    // If none of the blocks were good, it's a stale delivery
    if len(errs) != 0 {
        if len(errs) == len(blocks) {
            return errStaleDelivery
        }
        return fmt.Errorf("multiple failures: %v", errs)
    }
    return nil
}

// Prepare configures the block cache offset to allow accepting inbound blocks.
func (q *queue) Prepare(offset int) {
    q.lock.Lock()
    defer q.lock.Unlock()

    if q.blockOffset < offset {
        q.blockOffset = offset
    }
}