downloader: hash downloading recovery

If a peer fails to respond (disconnect, etc) during hash downloading
switch to a different peer which has it's current_hash in the queue's
peer set.

Author: obscuren

eth/downloader/downloader.go

@@ -53,6 +53,11 @@ type syncPack struct {
 	ignoreInitial bool
 }
 
+type hashPack struct {
+	peerId string
+	hashes []common.Hash
+}
+
 type Downloader struct {
 	mu         sync.RWMutex
 	queue      *queue
@@ -69,7 +74,7 @@ type Downloader struct {
 
 	// Channels
 	newPeerCh chan *peer
-	hashCh    chan []common.Hash
+	hashCh    chan hashPack
 	blockCh   chan blockPack
 }
 
@@ -80,7 +85,7 @@ func New(hasBlock hashCheckFn, getBlock getBlockFn) *Downloader {
 		hasBlock:  hasBlock,
 		getBlock:  getBlock,
 		newPeerCh: make(chan *peer, 1),
-		hashCh:    make(chan []common.Hash, 1),
+		hashCh:    make(chan hashPack, 1),
 		blockCh:   make(chan blockPack, 1),
 	}
 
@@ -235,38 +240,50 @@ func (d *Downloader) getFromPeer(p *peer, hash common.Hash, ignoreInitial bool)
 }
 
 // XXX Make synchronous
-func (d *Downloader) startFetchingHashes(p *peer, hash common.Hash, ignoreInitial bool) error {
+func (d *Downloader) startFetchingHashes(p *peer, h common.Hash, ignoreInitial bool) error {
 	atomic.StoreInt32(&d.fetchingHashes, 1)
 	defer atomic.StoreInt32(&d.fetchingHashes, 0)
 
-	if d.queue.has(hash) {
+	if d.queue.has(h) {
 		return errAlreadyInPool
 	}
 
-	glog.V(logger.Debug).Infof("Downloading hashes (%x) from %s", hash.Bytes()[:4], p.id)
+	glog.V(logger.Debug).Infof("Downloading hashes (%x) from %s", h[:4], p.id)
 
 	start := time.Now()
 
 	// We ignore the initial hash in some cases (e.g. we received a block without it's parent)
 	// In such circumstances we don't need to download the block so don't add it to the queue.
 	if !ignoreInitial {
 		// Add the hash to the queue first
-		d.queue.hashPool.Add(hash)
+		d.queue.hashPool.Add(h)
 	}
 	// Get the first batch of hashes
-	p.getHashes(hash)
+	p.getHashes(h)
 
-	failureResponseTimer := time.NewTimer(hashTtl)
+	var (
+		failureResponseTimer = time.NewTimer(hashTtl)
+		attemptedPeers       = make(map[string]bool) // attempted peers will help with retries
+		activePeer           = p                     // active peer will help determine the current active peer
+		hash                 common.Hash             // common and last hash
+	)
+	attemptedPeers[p.id] = true
 
 out:
 	for {
 		select {
-		case hashes := <-d.hashCh:
+		case hashPack := <-d.hashCh:
+			// make sure the active peer is giving us the hashes
+			if hashPack.peerId != activePeer.id {
+				glog.V(logger.Debug).Infof("Received hashes from incorrect peer(%s)\n", hashPack.peerId)
+				break
+			}
+
 			failureResponseTimer.Reset(hashTtl)
 
 			var (
-				done bool        // determines whether we're done fetching hashes (i.e. common hash found)
-				hash common.Hash // current and common hash
+				hashes = hashPack.hashes
+				done   bool // determines whether we're done fetching hashes (i.e. common hash found)
 			)
 			hashSet := set.New()
 			for _, hash = range hashes {
@@ -283,13 +300,13 @@ out:
 
 			// Add hashes to the chunk set
 			if len(hashes) == 0 { // Make sure the peer actually gave you something valid
-				glog.V(logger.Debug).Infof("Peer (%s) responded with empty hash set\n", p.id)
+				glog.V(logger.Debug).Infof("Peer (%s) responded with empty hash set\n", activePeer.id)
 				d.queue.reset()
 
 				return errEmptyHashSet
 			} else if !done { // Check if we're done fetching
 				// Get the next set of hashes
-				p.getHashes(hashes[len(hashes)-1])
+				activePeer.getHashes(hash)
 			} else { // we're done
 				// The offset of the queue is determined by the highest known block
 				var offset int
@@ -303,12 +320,30 @@ out:
 			}
 		case <-failureResponseTimer.C:
 			glog.V(logger.Debug).Infof("Peer (%s) didn't respond in time for hash request\n", p.id)
-			// TODO instead of reseting the queue select a new peer from which we can start downloading hashes.
-			// 1. check for peer's best hash to be included in the current hash set;
-			// 2. resume from last point (hashes[len(hashes)-1]) using the newly selected peer.
-			d.queue.reset()
 
-			return errTimeout
+			var p *peer // p will be set if a peer can be found
+			// Attempt to find a new peer by checking inclusion of peers best hash in our
+			// already fetched hash list. This can't guarantee 100% correctness but does
+			// a fair job. This is always either correct or false incorrect.
+			for id, peer := range d.peers {
+				if d.queue.hashPool.Has(peer.recentHash) && !attemptedPeers[id] {
+					p = peer
+					break
+				}
+			}
+
+			// if all peers have been tried, abort the process entirely or if the hash is
+			// the zero hash.
+			if p == nil || (hash == common.Hash{}) {
+				d.queue.reset()
+				return errTimeout
+			}
+
+			// set p to the active peer. this will invalidate any hashes that may be returned
+			// by our previous (delayed) peer.
+			activePeer = p
+			p.getHashes(hash)
+			glog.V(logger.Debug).Infof("Hash fetching switched to new peer(%s)\n", p.id)
 		}
 	}
 	glog.V(logger.Detail).Infof("Downloaded hashes (%d) in %v\n", d.queue.hashPool.Size(), time.Since(start))
@@ -454,7 +489,7 @@ func (d *Downloader) AddHashes(id string, hashes []common.Hash) error {
 		glog.Infof("adding %d (T=%d) hashes [ %x / %x ] from: %s\n", len(hashes), d.queue.hashPool.Size(), from[:4], to[:4], id)
 	}
 
-	d.hashCh <- hashes
+	d.hashCh <- hashPack{id, hashes}
 
 	return nil
 }

eth/downloader/downloader_test.go

@@ -42,12 +42,13 @@ func createBlocksFromHashes(hashes []common.Hash) map[common.Hash]*types.Block {
 }
 
 type downloadTester struct {
-	downloader *Downloader
-	hashes     []common.Hash
-	blocks     map[common.Hash]*types.Block
-	t          *testing.T
-	pcount     int
-	done       chan bool
+	downloader   *Downloader
+	hashes       []common.Hash
+	blocks       map[common.Hash]*types.Block
+	t            *testing.T
+	pcount       int
+	done         chan bool
+	activePeerId string
 }
 
 func newTester(t *testing.T, hashes []common.Hash, blocks map[common.Hash]*types.Block) *downloadTester {
@@ -58,6 +59,11 @@ func newTester(t *testing.T, hashes []common.Hash, blocks map[common.Hash]*types
 	return tester
 }
 
+func (dl *downloadTester) sync(peerId string, hash common.Hash) error {
+	dl.activePeerId = peerId
+	return dl.downloader.Synchronise(peerId, hash)
+}
+
 func (dl *downloadTester) hasBlock(hash common.Hash) bool {
 	if knownHash == hash {
 		return true
@@ -70,7 +76,7 @@ func (dl *downloadTester) getBlock(hash common.Hash) *types.Block {
 }
 
 func (dl *downloadTester) getHashes(hash common.Hash) error {
-	dl.downloader.hashCh <- dl.hashes
+	dl.downloader.AddHashes(dl.activePeerId, dl.hashes)
 	return nil
 }
 
@@ -115,8 +121,9 @@ func TestDownload(t *testing.T) {
 	tester.newPeer("peer2", big.NewInt(0), common.Hash{})
 	tester.badBlocksPeer("peer3", big.NewInt(0), common.Hash{})
 	tester.badBlocksPeer("peer4", big.NewInt(0), common.Hash{})
+	tester.activePeerId = "peer1"
 
-	err := tester.downloader.Synchronise("peer1", hashes[0])
+	err := tester.sync("peer1", hashes[0])
 	if err != nil {
 		t.Error("download error", err)
 	}
@@ -139,7 +146,7 @@ func TestMissing(t *testing.T) {
 	hashes = append(extraHashes, hashes[:len(hashes)-1]...)
 	tester.newPeer("peer2", big.NewInt(0), common.Hash{})
 
-	err := tester.downloader.Synchronise("peer1", hashes[0])
+	err := tester.sync("peer1", hashes[0])
 	if err != nil {
 		t.Error("download error", err)
 	}
@@ -164,7 +171,7 @@ func TestTaking(t *testing.T) {
 	tester.badBlocksPeer("peer3", big.NewInt(0), common.Hash{})
 	tester.badBlocksPeer("peer4", big.NewInt(0), common.Hash{})
 
-	err := tester.downloader.Synchronise("peer1", hashes[0])
+	err := tester.sync("peer1", hashes[0])
 	if err != nil {
 		t.Error("download error", err)
 	}