eth: added a cancel method for the downloader

Added a cancel method to the downloader which gracefully shuts down any
active syncing process (hash fetching or block downloading) and resets
the queue and remove any pending blocks.

Issue with the downloader which would stall because of an active ongoing
process when an invalid block was found.

eth/downloader/downloader.go

@@ -34,6 +34,9 @@ var (
 	errPeersUnavailable    = errors.New("no peers available or all peers tried for block download process")
 	errAlreadyInPool       = errors.New("hash already in pool")
 	errBlockNumberOverflow = errors.New("received block which overflows")
+	errCancelHashFetch     = errors.New("hash fetching cancelled (requested)")
+	errCancelBlockFetch    = errors.New("block downloading cancelled (requested)")
+	errNoSyncActive        = errors.New("no sync active")
 )
 
 type hashCheckFn func(common.Hash) bool
@@ -74,6 +77,7 @@ type Downloader struct {
 	newPeerCh chan *peer
 	hashCh    chan hashPack
 	blockCh   chan blockPack
+	cancelCh  chan struct{}
 }
 
 func New(hasBlock hashCheckFn, getBlock getBlockFn) *Downloader {
@@ -129,6 +133,9 @@ func (d *Downloader) Synchronise(id string, hash common.Hash) error {
 	}
 	defer atomic.StoreInt32(&d.synchronising, 0)
 
+	// Create cancel channel for aborting midflight
+	d.cancelCh = make(chan struct{})
+
 	// Abort if the queue still contains some leftover data
 	if _, cached := d.queue.Size(); cached > 0 && d.queue.GetHeadBlock() != nil {
 		return errPendingQueue
@@ -161,7 +168,6 @@ func (d *Downloader) Has(hash common.Hash) bool {
 }
 
 func (d *Downloader) getFromPeer(p *peer, hash common.Hash, ignoreInitial bool) (err error) {
-
 	d.activePeer = p.id
 	defer func() {
 		// reset on error
@@ -191,6 +197,42 @@ func (d *Downloader) getFromPeer(p *peer, hash common.Hash, ignoreInitial bool)
 	return nil
 }
 
+// Cancel cancels all of the operations and resets the queue. It returns true
+// if the cancel operation was completed.
+func (d *Downloader) Cancel() bool {
+	hs, bs := d.queue.Size()
+	// If we're not syncing just return.
+	if atomic.LoadInt32(&d.synchronising) == 0 && hs == 0 && bs == 0 {
+		return false
+	}
+
+	close(d.cancelCh)
+
+	// clean up
+hashDone:
+	for {
+		select {
+		case <-d.hashCh:
+		default:
+			break hashDone
+		}
+	}
+
+blockDone:
+	for {
+		select {
+		case <-d.blockCh:
+		default:
+			break blockDone
+		}
+	}
+
+	// reset the queue
+	d.queue.Reset()
+
+	return true
+}
+
 // XXX Make synchronous
 func (d *Downloader) startFetchingHashes(p *peer, h common.Hash, ignoreInitial bool) error {
 	glog.V(logger.Debug).Infof("Downloading hashes (%x) from %s", h[:4], p.id)
@@ -217,6 +259,8 @@ func (d *Downloader) startFetchingHashes(p *peer, h common.Hash, ignoreInitial b
 out:
 	for {
 		select {
+		case <-d.cancelCh:
+			return errCancelHashFetch
 		case hashPack := <-d.hashCh:
 			// Make sure the active peer is giving us the hashes
 			if hashPack.peerId != activePeer.id {
@@ -305,6 +349,8 @@ func (d *Downloader) startFetchingBlocks(p *peer) error {
 out:
 	for {
 		select {
+		case <-d.cancelCh:
+			return errCancelBlockFetch
 		case blockPack := <-d.blockCh:
 			// If the peer was previously banned and failed to deliver it's pack
 			// in a reasonable time frame, ignore it's message.
@@ -394,11 +440,23 @@ out:
 
 // Deliver a chunk to the downloader. This is usually done through the BlocksMsg by
 // the protocol handler.
-func (d *Downloader) DeliverChunk(id string, blocks []*types.Block) {
+func (d *Downloader) DeliverChunk(id string, blocks []*types.Block) error {
+	// Make sure the downloader is active
+	if atomic.LoadInt32(&d.synchronising) == 0 {
+		return errNoSyncActive
+	}
+
 	d.blockCh <- blockPack{id, blocks}
+
+	return nil
 }
 
 func (d *Downloader) AddHashes(id string, hashes []common.Hash) error {
+	// Make sure the downloader is active
+	if atomic.LoadInt32(&d.synchronising) == 0 {
+		return errNoSyncActive
+	}
+
 	// make sure that the hashes that are being added are actually from the peer
 	// that's the current active peer. hashes that have been received from other
 	// peers are dropped and ignored.

eth/downloader/downloader_test.go

@@ -182,6 +182,49 @@ func TestTaking(t *testing.T) {
 	}
 }
 
+func TestInactiveDownloader(t *testing.T) {
+	targetBlocks := 1000
+	hashes := createHashes(0, targetBlocks)
+	blocks := createBlocksFromHashSet(createHashSet(hashes))
+	tester := newTester(t, hashes, nil)
+
+	err := tester.downloader.AddHashes("bad peer 001", hashes)
+	if err != errNoSyncActive {
+		t.Error("expected no sync error, got", err)
+	}
+
+	err = tester.downloader.DeliverChunk("bad peer 001", blocks)
+	if err != errNoSyncActive {
+		t.Error("expected no sync error, got", err)
+	}
+}
+
+func TestCancel(t *testing.T) {
+	minDesiredPeerCount = 4
+	blockTtl = 1 * time.Second
+
+	targetBlocks := 1000
+	hashes := createHashes(0, targetBlocks)
+	blocks := createBlocksFromHashes(hashes)
+	tester := newTester(t, hashes, blocks)
+
+	tester.newPeer("peer1", big.NewInt(10000), hashes[0])
+
+	err := tester.sync("peer1", hashes[0])
+	if err != nil {
+		t.Error("download error", err)
+	}
+
+	if !tester.downloader.Cancel() {
+		t.Error("cancel operation unsuccessfull")
+	}
+
+	hashSize, blockSize := tester.downloader.queue.Size()
+	if hashSize > 0 || blockSize > 0 {
+		t.Error("block (", blockSize, ") or hash (", hashSize, ") not 0")
+	}
+}
+
 func TestThrottling(t *testing.T) {
 	minDesiredPeerCount = 4
 	blockTtl = 1 * time.Second

eth/sync.go

@@ -63,6 +63,9 @@ func (pm *ProtocolManager) processBlocks() error {
 		max := int(math.Min(float64(len(blocks)), float64(blockProcAmount)))
 		_, err := pm.chainman.InsertChain(blocks[:max])
 		if err != nil {
+			// cancel download process
+			pm.downloader.Cancel()
+
 			return err
 		}
 		blocks = blocks[max:]