Merge pull request #835 from obscuren/handler_errors

eth, eth/downloader: error handlers and td checks

9 files changed, 547 insertions, 318 deletions

diff --git a/cmd/geth/main.go b/cmd/geth/main.go
index ef007051c..0f5ebf5be 100644
--- a/cmd/geth/main.go
+++ b/cmd/geth/main.go
@@ -47,7 +47,7 @@ import _ "net/http/pprof"
 
 const (
 	ClientIdentifier = "Geth"
-	Version          = "0.9.14"
+	Version          = "0.9.15"
 )
 
 var (
diff --git a/core/chain_manager.go b/core/chain_manager.go
index 4fdb2edce..29830188e 100644
--- a/core/chain_manager.go
+++ b/core/chain_manager.go
@@ -49,6 +49,10 @@ func CalcDifficulty(block, parent *types.Header) *big.Int {
 }
 
 func CalculateTD(block, parent *types.Block) *big.Int {
+	if parent == nil {
+		return block.Difficulty()
+	}
+
 	td := new(big.Int).Add(parent.Td, block.Header().Difficulty)
 
 	return td
@@ -89,6 +93,7 @@ type ChainManager struct {
 	futureBlocks *BlockCache
 
 	quit chan struct{}
+	wg   sync.WaitGroup
 }
 
 func NewChainManager(blockDb, stateDb common.Database, mux *event.TypeMux) *ChainManager {
@@ -478,6 +483,10 @@ func (self *ChainManager) CalcTotalDiff(block *types.Block) (*big.Int, error) {
 
 func (bc *ChainManager) Stop() {
 	close(bc.quit)
+
+	bc.wg.Wait()
+
+	glog.V(logger.Info).Infoln("Chain manager stopped")
 }
 
 type queueEvent struct {
@@ -500,22 +509,30 @@ func (self *ChainManager) procFutureBlocks() {
 // InsertChain will attempt to insert the given chain in to the canonical chain or, otherwise, create a fork. It an error is returned
 // it will return the index number of the failing block as well an error describing what went wrong (for possible errors see core/errors.go).
 func (self *ChainManager) InsertChain(chain types.Blocks) (int, error) {
+	self.wg.Add(1)
+	defer self.wg.Done()
+
 	// A queued approach to delivering events. This is generally faster than direct delivery and requires much less mutex acquiring.
 	var (
 		queue      = make([]interface{}, len(chain))
 		queueEvent = queueEvent{queue: queue}
-		stats      struct{ queued, processed int }
+		stats      struct{ queued, processed, ignored int }
 		tstart     = time.Now()
 	)
 	for i, block := range chain {
 		if block == nil {
 			continue
 		}
+		// Setting block.Td regardless of error (known for example) prevents errors down the line
+		// in the protocol handler
+		block.Td = new(big.Int).Set(CalculateTD(block, self.GetBlock(block.ParentHash())))
+
 		// Call in to the block processor and check for errors. It's likely that if one block fails
 		// all others will fail too (unless a known block is returned).
 		logs, err := self.processor.Process(block)
 		if err != nil {
 			if IsKnownBlockErr(err) {
+				stats.ignored++
 				continue
 			}
 
@@ -545,8 +562,6 @@ func (self *ChainManager) InsertChain(chain types.Blocks) (int, error) {
 			return i, err
 		}
 
-		block.Td = new(big.Int).Set(CalculateTD(block, self.GetBlock(block.ParentHash())))
-
 		self.mu.Lock()
 		{
 			cblock := self.currentBlock
@@ -589,7 +604,7 @@ func (self *ChainManager) InsertChain(chain types.Blocks) (int, error) {
 				queueEvent.canonicalCount++
 
 				if glog.V(logger.Debug) {
-					glog.Infof("inserted block #%d (%d TXs %d UNCs) (%x...)\n", block.Number(), len(block.Transactions()), len(block.Uncles()), block.Hash().Bytes()[0:4])
+					glog.Infof("[%v] inserted block #%d (%d TXs %d UNCs) (%x...)\n", time.Now().UnixNano(), block.Number(), len(block.Transactions()), len(block.Uncles()), block.Hash().Bytes()[0:4])
 				}
 			} else {
 				if glog.V(logger.Detail) {
@@ -607,10 +622,10 @@ func (self *ChainManager) InsertChain(chain types.Blocks) (int, error) {
 
 	}
 
-	if (stats.queued > 0 || stats.processed > 0) && bool(glog.V(logger.Info)) {
+	if (stats.queued > 0 || stats.processed > 0 || stats.ignored > 0) && bool(glog.V(logger.Info)) {
 		tend := time.Since(tstart)
 		start, end := chain[0], chain[len(chain)-1]
-		glog.Infof("imported %d block(s) %d queued in %v. #%v [%x / %x]\n", stats.processed, stats.queued, tend, end.Number(), start.Hash().Bytes()[:4], end.Hash().Bytes()[:4])
+		glog.Infof("imported %d block(s) (%d queued %d ignored) in %v. #%v [%x / %x]\n", stats.processed, stats.queued, stats.ignored, tend, end.Number(), start.Hash().Bytes()[:4], end.Hash().Bytes()[:4])
 	}
 
 	go self.eventMux.Post(queueEvent)
@@ -654,7 +669,7 @@ func (self *ChainManager) merge(oldBlock, newBlock *types.Block) {
 
 func (self *ChainManager) update() {
 	events := self.eventMux.Subscribe(queueEvent{})
-	futureTimer := time.NewTicker(5 * time.Second)
+	futureTimer := time.Tick(5 * time.Second)
 out:
 	for {
 		select {
@@ -681,7 +696,7 @@ out:
 					self.eventMux.Post(event)
 				}
 			}
-		case <-futureTimer.C:
+		case <-futureTimer:
 			self.procFutureBlocks()
 		case <-self.quit:
 			break out
diff --git a/eth/backend.go b/eth/backend.go
index c5fa328b0..a8b0fae50 100644
--- a/eth/backend.go
+++ b/eth/backend.go
@@ -219,7 +219,7 @@ func New(config *Config) (*Ethereum, error) {
 	}
 
 	eth.chainManager = core.NewChainManager(blockDb, stateDb, eth.EventMux())
-	eth.downloader = downloader.New(eth.chainManager.HasBlock, eth.chainManager.InsertChain)
+	eth.downloader = downloader.New(eth.chainManager.HasBlock, eth.chainManager.GetBlock)
 	eth.pow = ethash.New(eth.chainManager)
 	eth.txPool = core.NewTxPool(eth.EventMux(), eth.chainManager.State, eth.chainManager.GasLimit)
 	eth.blockProcessor = core.NewBlockProcessor(stateDb, extraDb, eth.pow, eth.txPool, eth.chainManager, eth.EventMux())
@@ -455,6 +455,7 @@ func (s *Ethereum) Stop() {
 	s.txSub.Unsubscribe() // quits txBroadcastLoop
 
 	s.protocolManager.Stop()
+	s.chainManager.Stop()
 	s.txPool.Stop()
 	s.eventMux.Stop()
 	if s.whisper != nil {
diff --git a/eth/downloader/downloader.go b/eth/downloader/downloader.go
index 4cd927fd5..15f4cb0a3 100644
--- a/eth/downloader/downloader.go
+++ b/eth/downloader/downloader.go
@@ -3,14 +3,11 @@ package downloader
 import (
 	"errors"
 	"fmt"
-	"math"
-	"math/big"
 	"sync"
 	"sync/atomic"
 	"time"
 
 	"github.com/ethereum/go-ethereum/common"
-	"github.com/ethereum/go-ethereum/core"
 	"github.com/ethereum/go-ethereum/core/types"
 	"github.com/ethereum/go-ethereum/logger"
 	"github.com/ethereum/go-ethereum/logger/glog"
@@ -27,16 +24,21 @@ var (
 	minDesiredPeerCount = 5                // Amount of peers desired to start syncing
 	blockTtl            = 20 * time.Second // The amount of time it takes for a block request to time out
 
-	errLowTd            = errors.New("peer's TD is too low")
-	errBusy             = errors.New("busy")
-	errUnknownPeer      = errors.New("peer's unknown or unhealthy")
-	errBadPeer          = errors.New("action from bad peer ignored")
-	errTimeout          = errors.New("timeout")
-	errEmptyHashSet     = errors.New("empty hash set by peer")
-	errPeersUnavailable = errors.New("no peers available or all peers tried for block download process")
+	errLowTd               = errors.New("peer's TD is too low")
+	errBusy                = errors.New("busy")
+	errUnknownPeer         = errors.New("peer's unknown or unhealthy")
+	ErrBadPeer             = errors.New("action from bad peer ignored")
+	errNoPeers             = errors.New("no peers to keep download active")
+	errPendingQueue        = errors.New("pending items in queue")
+	errTimeout             = errors.New("timeout")
+	errEmptyHashSet        = errors.New("empty hash set by peer")
+	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")
 )
 
 type hashCheckFn func(common.Hash) bool
+type getBlockFn func(common.Hash) *types.Block
 type chainInsertFn func(types.Blocks) (int, error)
 type hashIterFn func() (common.Hash, error)
 
@@ -51,6 +53,11 @@ type syncPack struct {
 	ignoreInitial bool
 }
 
+type hashPack struct {
+	peerId string
+	hashes []common.Hash
+}
+
 type Downloader struct {
 	mu         sync.RWMutex
 	queue      *queue
@@ -58,29 +65,28 @@ type Downloader struct {
 	activePeer string
 
 	// Callbacks
-	hasBlock    hashCheckFn
-	insertChain chainInsertFn
+	hasBlock hashCheckFn
+	getBlock getBlockFn
 
 	// Status
 	fetchingHashes    int32
 	downloadingBlocks int32
-	processingBlocks  int32
 
 	// Channels
 	newPeerCh chan *peer
-	hashCh    chan []common.Hash
+	hashCh    chan hashPack
 	blockCh   chan blockPack
 }
 
-func New(hasBlock hashCheckFn, insertChain chainInsertFn) *Downloader {
+func New(hasBlock hashCheckFn, getBlock getBlockFn) *Downloader {
 	downloader := &Downloader{
-		queue:       newqueue(),
-		peers:       make(peers),
-		hasBlock:    hasBlock,
-		insertChain: insertChain,
-		newPeerCh:   make(chan *peer, 1),
-		hashCh:      make(chan []common.Hash, 1),
-		blockCh:     make(chan blockPack, 1),
+		queue:     newqueue(),
+		peers:     make(peers),
+		hasBlock:  hasBlock,
+		getBlock:  getBlock,
+		newPeerCh: make(chan *peer, 1),
+		hashCh:    make(chan hashPack, 1),
+		blockCh:   make(chan blockPack, 1),
 	}
 
 	return downloader
@@ -126,6 +132,12 @@ func (d *Downloader) Synchronise(id string, hash common.Hash) error {
 		return errBusy
 	}
 
+	// When a synchronisation attempt is made while the queue stil
+	// contains items we abort the sync attempt
+	if d.queue.size() > 0 {
+		return errPendingQueue
+	}
+
 	// Fetch the peer using the id or throw an error if the peer couldn't be found
 	p := d.peers[id]
 	if p == nil {
@@ -138,30 +150,87 @@ func (d *Downloader) Synchronise(id string, hash common.Hash) error {
 		return err
 	}
 
-	return d.process(p)
+	return nil
+}
+
+// Done lets the downloader know that whatever previous hashes were taken
+// are processed. If the block count reaches zero and done is called
+// we reset the queue for the next batch of incoming hashes and blocks.
+func (d *Downloader) Done() {
+	d.queue.mu.Lock()
+	defer d.queue.mu.Unlock()
+
+	if len(d.queue.blocks) == 0 {
+		d.queue.resetNoTS()
+	}
 }
 
-func (d *Downloader) getFromPeer(p *peer, hash common.Hash, ignoreInitial bool) error {
+// TakeBlocks takes blocks from the queue and yields them to the blockTaker handler
+// it's possible it yields no blocks
+func (d *Downloader) TakeBlocks() types.Blocks {
+	d.queue.mu.Lock()
+	defer d.queue.mu.Unlock()
+
+	var blocks types.Blocks
+	if len(d.queue.blocks) > 0 {
+		// Make sure the parent hash is known
+		if d.queue.blocks[0] != nil && !d.hasBlock(d.queue.blocks[0].ParentHash()) {
+			return nil
+		}
+
+		for _, block := range d.queue.blocks {
+			if block == nil {
+				break
+			}
+
+			blocks = append(blocks, block)
+		}
+		d.queue.blockOffset += len(blocks)
+		// 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 d.queue.blocks
+		copy(d.queue.blocks, d.queue.blocks[len(blocks):])
+		for k, n := len(d.queue.blocks)-len(blocks), len(d.queue.blocks); k < n; k++ {
+			d.queue.blocks[k] = nil
+		}
+		d.queue.blocks = d.queue.blocks[:len(d.queue.blocks)-len(blocks)]
+
+		//d.queue.blocks = d.queue.blocks[len(blocks):]
+		if len(d.queue.blocks) == 0 {
+			d.queue.blocks = nil
+		}
+
+	}
+
+	return blocks
+}
+
+func (d *Downloader) Has(hash common.Hash) bool {
+	return d.queue.has(hash)
+}
+
+func (d *Downloader) getFromPeer(p *peer, hash common.Hash, ignoreInitial bool) (err error) {
 	d.activePeer = p.id
+	defer func() {
+		// reset on error
+		if err != nil {
+			d.queue.reset()
+		}
+	}()
 
 	glog.V(logger.Detail).Infoln("Synchronising with the network using:", p.id)
 	// Start the fetcher. This will block the update entirely
 	// interupts need to be send to the appropriate channels
 	// respectively.
-	if err := d.startFetchingHashes(p, hash, ignoreInitial); err != nil {
-		// handle error
-		glog.V(logger.Debug).Infoln("Error fetching hashes:", err)
-		// XXX Reset
+	if err = d.startFetchingHashes(p, hash, ignoreInitial); err != nil {
 		return err
 	}
 
 	// Start fetching blocks in paralel. The strategy is simple
 	// take any available peers, seserve a chunk for each peer available,
 	// let the peer deliver the chunkn and periodically check if a peer
-	// has timedout. When done downloading, process blocks.
-	if err := d.startFetchingBlocks(p); err != nil {
-		glog.V(logger.Debug).Infoln("Error downloading blocks:", err)
-		// XXX reset
+	// has timedout.
+	if err = d.startFetchingBlocks(p); err != nil {
 		return err
 	}
 
@@ -171,11 +240,15 @@ 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)
 
-	glog.V(logger.Debug).Infof("Downloading hashes (%x) from %s", hash.Bytes()[:4], p.id)
+	if d.queue.has(h) {
+		return errAlreadyInPool
+	}
+
+	glog.V(logger.Debug).Infof("Downloading hashes (%x) from %s", h[:4], p.id)
 
 	start := time.Now()
 
@@ -183,23 +256,38 @@ func (d *Downloader) startFetchingHashes(p *peer, hash common.Hash, ignoreInitia
 	// 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)
+			var (
+				hashes = hashPack.hashes
+				done   bool // determines whether we're done fetching hashes (i.e. common hash found)
+			)
 			hashSet := set.New()
-			for _, hash := range hashes {
-				if d.hasBlock(hash) {
+			for _, hash = range hashes {
+				if d.hasBlock(hash) || d.queue.blockHashes.Has(hash) {
 					glog.V(logger.Debug).Infof("Found common hash %x\n", hash[:4])
 
 					done = true
@@ -212,24 +300,50 @@ 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
+				if block := d.getBlock(hash); block != nil {
+					offset = int(block.NumberU64() + 1)
+				}
+				// allocate proper size for the queueue
+				d.queue.alloc(offset, d.queue.hashPool.Size())
+
 				break 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))
@@ -257,11 +371,27 @@ out:
 			// If the peer was previously banned and failed to deliver it's pack
 			// in a reasonable time frame, ignore it's message.
 			if d.peers[blockPack.peerId] != nil {
+				err := d.queue.deliver(blockPack.peerId, blockPack.blocks)
+				if err != nil {
+					glog.V(logger.Debug).Infof("deliver failed for peer %s: %v\n", blockPack.peerId, err)
+					// FIXME d.UnregisterPeer(blockPack.peerId)
+					break
+				}
+
+				if glog.V(logger.Debug) {
+					glog.Infof("adding %d blocks from: %s\n", len(blockPack.blocks), blockPack.peerId)
+				}
 				d.peers[blockPack.peerId].promote()
-				d.queue.deliver(blockPack.peerId, blockPack.blocks)
 				d.peers.setState(blockPack.peerId, idleState)
 			}
 		case <-ticker.C:
+			// after removing bad peers make sure we actually have suffucient peer left to keep downlading
+			if len(d.peers) == 0 {
+				d.queue.reset()
+
+				return errNoPeers
+			}
+
 			// If there are unrequested hashes left start fetching
 			// from the available peers.
 			if d.queue.hashPool.Size() > 0 {
@@ -310,7 +440,7 @@ out:
 					if time.Since(chunk.itime) > blockTtl {
 						badPeers = append(badPeers, pid)
 						// remove peer as good peer from peer list
-						//d.UnregisterPeer(pid)
+						// FIXME d.UnregisterPeer(pid)
 					}
 				}
 				d.queue.mu.Unlock()
@@ -354,112 +484,14 @@ func (d *Downloader) AddHashes(id string, hashes []common.Hash) error {
 		return fmt.Errorf("received hashes from %s while active peer is %s", id, d.activePeer)
 	}
 
-	d.hashCh <- hashes
-
-	return nil
-}
-
-// Add an (unrequested) block to the downloader. This is usually done through the
-// NewBlockMsg by the protocol handler.
-// Adding blocks is done synchronously. if there are missing blocks, blocks will be
-// fetched first. If the downloader is busy or if some other processed failed an error
-// will be returned.
-func (d *Downloader) AddBlock(id string, block *types.Block, td *big.Int) error {
-	hash := block.Hash()
-
-	if d.hasBlock(hash) {
-		return fmt.Errorf("known block %x", hash.Bytes()[:4])
-	}
-
-	peer := d.peers.getPeer(id)
-	// if the peer is in our healthy list of peers; update the td
-	// and add the block. Otherwise just ignore it
-	if peer == nil {
-		glog.V(logger.Detail).Infof("Ignored block from bad peer %s\n", id)
-		return errBadPeer
-	}
-
-	peer.mu.Lock()
-	peer.recentHash = block.Hash()
-	peer.mu.Unlock()
-	peer.promote()
-
-	glog.V(logger.Detail).Infoln("Inserting new block from:", id)
-	d.queue.addBlock(id, block)
-
-	// if neither go ahead to process
-	if d.isBusy() {
-		return errBusy
-	}
-
-	// Check if the parent of the received block is known.
-	// If the block is not know, request it otherwise, request.
-	phash := block.ParentHash()
-	if !d.hasBlock(phash) {
-		glog.V(logger.Detail).Infof("Missing parent %x, requires fetching\n", phash.Bytes()[:4])
-
-		// Get the missing hashes from the peer (synchronously)
-		err := d.getFromPeer(peer, peer.recentHash, true)
-		if err != nil {
-			return err
-		}
+	if glog.V(logger.Detail) && len(hashes) != 0 {
+		from, to := hashes[0], hashes[len(hashes)-1]
+		glog.Infof("adding %d (T=%d) hashes [ %x / %x ] from: %s\n", len(hashes), d.queue.hashPool.Size(), from[:4], to[:4], id)
 	}
 
-	return d.process(peer)
-}
-
-func (d *Downloader) process(peer *peer) error {
-	atomic.StoreInt32(&d.processingBlocks, 1)
-	defer atomic.StoreInt32(&d.processingBlocks, 0)
+	d.hashCh <- hashPack{id, hashes}
 
-	// XXX this will move when optimised
-	// Sort the blocks by number. This bit needs much improvement. Right now
-	// it assumes full honesty form peers (i.e. it's not checked when the blocks
-	// link). We should at least check whihc queue match. This code could move
-	// to a seperate goroutine where it periodically checks for linked pieces.
-	types.BlockBy(types.Number).Sort(d.queue.blocks)
-	if len(d.queue.blocks) == 0 {
-		return nil
-	}
-
-	var (
-		blocks = d.queue.blocks
-		err    error
-	)
-	glog.V(logger.Debug).Infof("Inserting chain with %d blocks (#%v - #%v)\n", len(blocks), blocks[0].Number(), blocks[len(blocks)-1].Number())
-
-	// Loop untill we're out of blocks
-	for len(blocks) != 0 {
-		max := int(math.Min(float64(len(blocks)), 256))
-		// TODO check for parent error. When there's a parent error we should stop
-		// processing and start requesting the `block.hash` so that it's parent and
-		// grandparents can be requested and queued.
-		var i int
-		i, err = d.insertChain(blocks[:max])
-		if err != nil && core.IsParentErr(err) {
-			// Ignore the missing blocks. Handler should take care of anything that's missing.
-			glog.V(logger.Debug).Infof("Ignored block with missing parent (%d)\n", i)
-			blocks = blocks[i+1:]
-
-			continue
-		} else if err != nil {
-			// immediatly unregister the false peer but do not disconnect
-			d.UnregisterPeer(d.activePeer)
-			// Reset chain completely. This needs much, much improvement.
-			// instead: check all blocks leading down to this block false block and remove it
-			blocks = nil
-			break
-		}
-		blocks = blocks[max:]
-	}
-
-	// This will allow the GC to remove the in memory blocks
-	if len(blocks) == 0 {
-		d.queue.blocks = nil
-	} else {
-		d.queue.blocks = blocks
-	}
-	return err
+	return nil
 }
 
 func (d *Downloader) isFetchingHashes() bool {
@@ -470,12 +502,8 @@ func (d *Downloader) isDownloadingBlocks() bool {
 	return atomic.LoadInt32(&d.downloadingBlocks) == 1
 }
 
-func (d *Downloader) isProcessing() bool {
-	return atomic.LoadInt32(&d.processingBlocks) == 1
-}
-
 func (d *Downloader) isBusy() bool {
-	return d.isFetchingHashes() || d.isDownloadingBlocks() || d.isProcessing()
+	return d.isFetchingHashes() || d.isDownloadingBlocks()
 }
 
 func (d *Downloader) IsBusy() bool {
diff --git a/eth/downloader/downloader_test.go b/eth/downloader/downloader_test.go
index 5518163ca..872ea02eb 100644
--- a/eth/downloader/downloader_test.go
+++ b/eth/downloader/downloader_test.go
@@ -8,8 +8,6 @@ import (
 
 	"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"
 )
 
 var knownHash = common.Hash{1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
@@ -25,36 +23,47 @@ func createHashes(start, amount int) (hashes []common.Hash) {
 	return
 }
 
+func createBlock(i int, prevHash, hash common.Hash) *types.Block {
+	header := &types.Header{Number: big.NewInt(int64(i))}
+	block := types.NewBlockWithHeader(header)
+	block.HeaderHash = hash
+	block.ParentHeaderHash = knownHash
+	return block
+}
+
 func createBlocksFromHashes(hashes []common.Hash) map[common.Hash]*types.Block {
 	blocks := make(map[common.Hash]*types.Block)
+
 	for i, hash := range hashes {
-		header := &types.Header{Number: big.NewInt(int64(i))}
-		blocks[hash] = types.NewBlockWithHeader(header)
-		blocks[hash].HeaderHash = hash
+		blocks[hash] = createBlock(len(hashes)-i, knownHash, hash)
 	}
 
 	return blocks
 }
 
 type downloadTester struct {
-	downloader *Downloader
-	hashes     []common.Hash
-	blocks     map[common.Hash]*types.Block
-	t          *testing.T
-	pcount     int
-	done       chan bool
-
-	insertedBlocks int
+	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 {
 	tester := &downloadTester{t: t, hashes: hashes, blocks: blocks, done: make(chan bool)}
-	downloader := New(tester.hasBlock, tester.insertChain)
+	downloader := New(tester.hasBlock, tester.getBlock)
 	tester.downloader = downloader
 
 	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
@@ -62,14 +71,12 @@ func (dl *downloadTester) hasBlock(hash common.Hash) bool {
 	return false
 }
 
-func (dl *downloadTester) insertChain(blocks types.Blocks) (int, error) {
-	dl.insertedBlocks += len(blocks)
-
-	return 0, nil
+func (dl *downloadTester) getBlock(hash common.Hash) *types.Block {
+	return dl.blocks[knownHash]
 }
 
 func (dl *downloadTester) getHashes(hash common.Hash) error {
-	dl.downloader.hashCh <- dl.hashes
+	dl.downloader.AddHashes(dl.activePeerId, dl.hashes)
 	return nil
 }
 
@@ -102,9 +109,6 @@ func (dl *downloadTester) badBlocksPeer(id string, td *big.Int, hash common.Hash
 }
 
 func TestDownload(t *testing.T) {
-	glog.SetV(logger.Detail)
-	glog.SetToStderr(true)
-
 	minDesiredPeerCount = 4
 	blockTtl = 1 * time.Second
 
@@ -117,21 +121,20 @@ 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)
 	}
 
-	if tester.insertedBlocks != targetBlocks {
-		t.Error("expected", targetBlocks, "have", tester.insertedBlocks)
+	inqueue := len(tester.downloader.queue.blocks)
+	if inqueue != targetBlocks {
+		t.Error("expected", targetBlocks, "have", inqueue)
 	}
 }
 
 func TestMissing(t *testing.T) {
-	glog.SetV(logger.Detail)
-	glog.SetToStderr(true)
-
 	targetBlocks := 1000
 	hashes := createHashes(0, 1000)
 	extraHashes := createHashes(1001, 1003)
@@ -143,12 +146,38 @@ 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)
+	}
+
+	inqueue := len(tester.downloader.queue.blocks)
+	if inqueue != targetBlocks {
+		t.Error("expected", targetBlocks, "have", inqueue)
+	}
+}
+
+func TestTaking(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])
+	tester.newPeer("peer2", big.NewInt(0), common.Hash{})
+	tester.badBlocksPeer("peer3", big.NewInt(0), common.Hash{})
+	tester.badBlocksPeer("peer4", big.NewInt(0), common.Hash{})
+
+	err := tester.sync("peer1", hashes[0])
 	if err != nil {
 		t.Error("download error", err)
 	}
 
-	if tester.insertedBlocks != targetBlocks {
-		t.Error("expected", targetBlocks, "have", tester.insertedBlocks)
+	bs1 := tester.downloader.TakeBlocks()
+	if len(bs1) != 1000 {
+		t.Error("expected to take 1000, got", len(bs1))
 	}
 }
diff --git a/eth/downloader/queue.go b/eth/downloader/queue.go
index a21a44706..1b63a5ffb 100644
--- a/eth/downloader/queue.go
+++ b/eth/downloader/queue.go
@@ -1,6 +1,7 @@
 package downloader
 
 import (
+	"fmt"
 	"math"
 	"sync"
 	"time"
@@ -18,7 +19,9 @@ type queue struct {
 
 	mu       sync.Mutex
 	fetching map[string]*chunk
-	blocks   []*types.Block
+
+	blockOffset int
+	blocks      []*types.Block
 }
 
 func newqueue() *queue {
@@ -34,6 +37,10 @@ func (c *queue) reset() {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 
+	c.resetNoTS()
+}
+func (c *queue) resetNoTS() {
+	c.blockOffset = 0
 	c.hashPool.Clear()
 	c.fetchPool.Clear()
 	c.blockHashes.Clear()
@@ -41,6 +48,10 @@ func (c *queue) reset() {
 	c.fetching = make(map[string]*chunk)
 }
 
+func (c *queue) size() int {
+	return c.hashPool.Size() + c.blockHashes.Size() + c.fetchPool.Size()
+}
+
 // reserve a `max` set of hashes for `p` peer.
 func (c *queue) get(p *peer, max int) *chunk {
 	c.mu.Lock()
@@ -89,22 +100,27 @@ func (c *queue) get(p *peer, max int) *chunk {
 }
 
 func (c *queue) has(hash common.Hash) bool {
-	return c.hashPool.Has(hash) || c.fetchPool.Has(hash)
+	return c.hashPool.Has(hash) || c.fetchPool.Has(hash) || c.blockHashes.Has(hash)
 }
 
-func (c *queue) addBlock(id string, block *types.Block) {
+func (c *queue) getBlock(hash common.Hash) *types.Block {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 
-	// when adding a block make sure it doesn't already exist
-	if !c.blockHashes.Has(block.Hash()) {
-		c.hashPool.Remove(block.Hash())
-		c.blocks = append(c.blocks, block)
+	if !c.blockHashes.Has(hash) {
+		return nil
+	}
+
+	for _, block := range c.blocks {
+		if block.Hash() == hash {
+			return block
+		}
 	}
+	return nil
 }
 
 // deliver delivers a chunk to the queue that was requested of the peer
-func (c *queue) deliver(id string, blocks []*types.Block) {
+func (c *queue) deliver(id string, blocks []*types.Block) (err error) {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 
@@ -119,16 +135,45 @@ func (c *queue) deliver(id string, blocks []*types.Block) {
 			chunk.peer.ignored.Merge(chunk.hashes)
 		}
 
+		// Add the blocks
+		for i, block := range blocks {
+			// See (1) for future limitation
+			n := int(block.NumberU64()) - c.blockOffset
+			if n > len(c.blocks) || n < 0 {
+				// set the error and set the blocks which could be processed
+				// abort the rest of the blocks (FIXME this could be improved)
+				err = fmt.Errorf("received block which overflow (N=%v O=%v)", block.Number(), c.blockOffset)
+				blocks = blocks[:i]
+				break
+			}
+			c.blocks[n] = block
+		}
 		// seperate the blocks and the hashes
 		blockHashes := chunk.fetchedHashes(blocks)
 		// merge block hashes
 		c.blockHashes.Merge(blockHashes)
-		// Add the blocks
-		c.blocks = append(c.blocks, blocks...)
 		// Add back whatever couldn't be delivered
 		c.hashPool.Merge(chunk.hashes)
+		// Remove the hashes from the fetch pool
 		c.fetchPool.Separate(chunk.hashes)
 	}
+
+	return
+}
+
+func (c *queue) alloc(offset, size int) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	if c.blockOffset < offset {
+		c.blockOffset = offset
+	}
+
+	// (1) XXX at some point we could limit allocation to memory and use the disk
+	// to store future blocks.
+	if len(c.blocks) < size {
+		c.blocks = append(c.blocks, make([]*types.Block, size)...)
+	}
 }
 
 // puts puts sets of hashes on to the queue for fetching
diff --git a/eth/downloader/queue_test.go b/eth/downloader/queue_test.go
new file mode 100644
index 000000000..b163bd9c7
--- /dev/null
+++ b/eth/downloader/queue_test.go
@@ -0,0 +1,62 @@
+package downloader
+
+import (
+	"testing"
+
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/core/types"
+	"gopkg.in/fatih/set.v0"
+)
+
+func createHashSet(hashes []common.Hash) *set.Set {
+	hset := set.New()
+
+	for _, hash := range hashes {
+		hset.Add(hash)
+	}
+
+	return hset
+}
+
+func createBlocksFromHashSet(hashes *set.Set) []*types.Block {
+	blocks := make([]*types.Block, hashes.Size())
+
+	var i int
+	hashes.Each(func(v interface{}) bool {
+		blocks[i] = createBlock(i, common.Hash{}, v.(common.Hash))
+		i++
+		return true
+	})
+
+	return blocks
+}
+
+func TestChunking(t *testing.T) {
+	queue := newqueue()
+	peer1 := newPeer("peer1", common.Hash{}, nil, nil)
+	peer2 := newPeer("peer2", common.Hash{}, nil, nil)
+
+	// 99 + 1 (1 == known genesis hash)
+	hashes := createHashes(0, 99)
+	hashSet := createHashSet(hashes)
+	queue.put(hashSet)
+
+	chunk1 := queue.get(peer1, 99)
+	if chunk1 == nil {
+		t.Errorf("chunk1 is nil")
+		t.FailNow()
+	}
+	chunk2 := queue.get(peer2, 99)
+	if chunk2 == nil {
+		t.Errorf("chunk2 is nil")
+		t.FailNow()
+	}
+
+	if chunk1.hashes.Size() != 99 {
+		t.Error("expected chunk1 hashes to be 99, got", chunk1.hashes.Size())
+	}
+
+	if chunk2.hashes.Size() != 1 {
+		t.Error("expected chunk1 hashes to be 1, got", chunk2.hashes.Size())
+	}
+}
diff --git a/eth/handler.go b/eth/handler.go
index fecd71632..1e0663816 100644
--- a/eth/handler.go
+++ b/eth/handler.go
@@ -1,39 +1,5 @@
 package eth
 
-// XXX Fair warning, most of the code is re-used from the old protocol. Please be aware that most of this will actually change
-// The idea is that most of the calls within the protocol will become synchronous.
-// Block downloading and block processing will be complete seperate processes
-/*
-# Possible scenarios
-
-// Synching scenario
-// Use the best peer to synchronise
-blocks, err := pm.downloader.Synchronise()
-if err != nil {
-	// handle
-	break
-}
-pm.chainman.InsertChain(blocks)
-
-// Receiving block with known parent
-if parent_exist {
-	if err := pm.chainman.InsertChain(block); err != nil {
-		// handle
-		break
-	}
-	pm.BroadcastBlock(block)
-}
-
-// Receiving block with unknown parent
-blocks, err := pm.downloader.SynchroniseWithPeer(peer)
-if err != nil {
-	// handle
-	break
-}
-pm.chainman.InsertChain(blocks)
-
-*/
-
 import (
 	"fmt"
 	"math"
@@ -54,7 +20,9 @@ import (
 
 const (
 	peerCountTimeout    = 12 * time.Second // Amount of time it takes for the peer handler to ignore minDesiredPeerCount
-	minDesiredPeerCount = 5                // Amount of peers desired to start syncing
+	blockProcTimer      = 500 * time.Millisecond
+	minDesiredPeerCount = 5 // Amount of peers desired to start syncing
+	blockProcAmount     = 256
 )
 
 func errResp(code errCode, format string, v ...interface{}) error {
@@ -91,6 +59,10 @@ type ProtocolManager struct {
 
 	newPeerCh chan *peer
 	quitSync  chan struct{}
+	// wait group is used for graceful shutdowns during downloading
+	// and processing
+	wg   sync.WaitGroup
+	quit bool
 }
 
 // NewProtocolManager returns a new ethereum sub protocol manager. The Ethereum sub protocol manages peers capable
@@ -122,60 +94,11 @@ func NewProtocolManager(protocolVersion, networkId int, mux *event.TypeMux, txpo
 	return manager
 }
 
-func (pm *ProtocolManager) syncHandler() {
-	// itimer is used to determine when to start ignoring `minDesiredPeerCount`
-	itimer := time.NewTimer(peerCountTimeout)
-out:
-	for {
-		select {
-		case <-pm.newPeerCh:
-			// Meet the `minDesiredPeerCount` before we select our best peer
-			if len(pm.peers) < minDesiredPeerCount {
-				break
-			}
-
-			// Find the best peer
-			peer := getBestPeer(pm.peers)
-			if peer == nil {
-				glog.V(logger.Debug).Infoln("Sync attempt cancelled. No peers available")
-			}
-
-			itimer.Stop()
-			go pm.synchronise(peer)
-		case <-itimer.C:
-			// The timer will make sure that the downloader keeps an active state
-			// in which it attempts to always check the network for highest td peers
-			// Either select the peer or restart the timer if no peers could
-			// be selected.
-			if peer := getBestPeer(pm.peers); peer != nil {
-				go pm.synchronise(peer)
-			} else {
-				itimer.Reset(5 * time.Second)
-			}
-		case <-pm.quitSync:
-			break out
-		}
-	}
-}
-
-func (pm *ProtocolManager) synchronise(peer *peer) {
-	// Make sure the peer's TD is higher than our own. If not drop.
-	if peer.td.Cmp(pm.chainman.Td()) <= 0 {
-		return
-	}
-	// Check downloader if it's busy so it doesn't show the sync message
-	// for every attempty
-	if pm.downloader.IsBusy() {
-		return
-	}
-
-	glog.V(logger.Info).Infof("Synchronisation attempt using %s TD=%v\n", peer.id, peer.td)
-	// Get the hashes from the peer (synchronously)
-	err := pm.downloader.Synchronise(peer.id, peer.recentHash)
-	if err != nil {
-		// handle error
-		glog.V(logger.Debug).Infoln("error downloading:", err)
-	}
+func (pm *ProtocolManager) removePeer(peer *peer) {
+	pm.pmu.Lock()
+	defer pm.pmu.Unlock()
+	pm.downloader.UnregisterPeer(peer.id)
+	delete(pm.peers, peer.id)
 }
 
 func (pm *ProtocolManager) Start() {
@@ -187,18 +110,26 @@ func (pm *ProtocolManager) Start() {
 	pm.minedBlockSub = pm.eventMux.Subscribe(core.NewMinedBlockEvent{})
 	go pm.minedBroadcastLoop()
 
-	// sync handler
-	go pm.syncHandler()
+	go pm.update()
 }
 
 func (pm *ProtocolManager) Stop() {
+	// Showing a log message. During download / process this could actually
+	// take between 5 to 10 seconds and therefor feedback is required.
+	glog.V(logger.Info).Infoln("Stopping ethereum protocol handler...")
+
+	pm.quit = true
 	pm.txSub.Unsubscribe()         // quits txBroadcastLoop
 	pm.minedBlockSub.Unsubscribe() // quits blockBroadcastLoop
 	close(pm.quitSync)             // quits the sync handler
+
+	// Wait for any process action
+	pm.wg.Wait()
+
+	glog.V(logger.Info).Infoln("Ethereum protocol handler stopped")
 }
 
 func (pm *ProtocolManager) newPeer(pv, nv int, p *p2p.Peer, rw p2p.MsgReadWriter) *peer {
-
 	td, current, genesis := pm.chainman.Status()
 
 	return newPeer(pv, nv, genesis, current, td, p, rw)
@@ -214,10 +145,7 @@ func (pm *ProtocolManager) handle(p *peer) error {
 
 	pm.downloader.RegisterPeer(p.id, p.recentHash, p.requestHashes, p.requestBlocks)
 	defer func() {
-		pm.pmu.Lock()
-		defer pm.pmu.Unlock()
-		delete(pm.peers, p.id)
-		pm.downloader.UnregisterPeer(p.id)
+		pm.removePeer(p)
 	}()
 
 	// propagate existing transactions. new transactions appearing
@@ -352,6 +280,9 @@ func (self *ProtocolManager) handleMsg(p *peer) error {
 		// Add the block hash as a known hash to the peer. This will later be used to determine
 		// who should receive this.
 		p.blockHashes.Add(hash)
+		// update the peer info
+		p.recentHash = hash
+		p.td = request.TD
 
 		_, chainHead, _ := self.chainman.Status()
 
@@ -376,24 +307,24 @@ func (self *ProtocolManager) handleMsg(p *peer) error {
 
 		// Attempt to insert the newly received by checking if the parent exists.
 		// if the parent exists we process the block and propagate to our peers
-		// if the parent does not exists we delegate to the downloader.
+		// otherwise synchronise with the peer
 		if self.chainman.HasBlock(request.Block.ParentHash()) {
 			if _, err := self.chainman.InsertChain(types.Blocks{request.Block}); err != nil {
-				// handle error
+				glog.V(logger.Error).Infoln("removed peer (", p.id, ") due to block error")
+
+				self.removePeer(p)
+
+				return nil
+			}
+
+			if err := self.verifyTd(p, request); err != nil {
+				glog.V(logger.Error).Infoln(err)
+				// XXX for now return nil so it won't disconnect (we should in the future)
 				return nil
 			}
 			self.BroadcastBlock(hash, request.Block)
 		} else {
-			// adding blocks is synchronous
-			go func() {
-				// TODO check parent error
-				err := self.downloader.AddBlock(p.id, request.Block, request.TD)
-				if err != nil {
-					glog.V(logger.Detail).Infoln("downloader err:", err)
-					return
-				}
-				self.BroadcastBlock(hash, request.Block)
-			}()
+			go self.synchronise(p)
 		}
 	default:
 		return errResp(ErrInvalidMsgCode, "%v", msg.Code)
@@ -401,6 +332,16 @@ func (self *ProtocolManager) handleMsg(p *peer) error {
 	return nil
 }
 
+func (pm *ProtocolManager) verifyTd(peer *peer, request newBlockMsgData) error {
+	if request.Block.Td.Cmp(request.TD) != 0 {
+		glog.V(logger.Detail).Infoln(peer)
+
+		return fmt.Errorf("invalid TD on block(%v) from peer(%s): block.td=%v, request.td=%v", request.Block.Number(), peer.id, request.Block.Td, request.TD)
+	}
+
+	return nil
+}
+
 // BroadcastBlock will propagate the block to its connected peers. It will sort
 // out which peers do not contain the block in their block set and will do a
 // sqrt(peers) to determine the amount of peers we broadcast to.
@@ -421,7 +362,7 @@ func (pm *ProtocolManager) BroadcastBlock(hash common.Hash, block *types.Block)
 	for _, peer := range peers {
 		peer.sendNewBlock(block)
 	}
-	glog.V(logger.Detail).Infoln("broadcast block to", len(peers), "peers. Total propagation time:", time.Since(block.ReceivedAt))
+	glog.V(logger.Detail).Infoln("broadcast block to", len(peers), "peers. Total processing time:", time.Since(block.ReceivedAt))
 }
 
 // BroadcastTx will propagate the block to its connected peers. It will sort
diff --git a/eth/sync.go b/eth/sync.go
new file mode 100644
index 000000000..9e8b21a7c
--- /dev/null
+++ b/eth/sync.go
@@ -0,0 +1,108 @@
+package eth
+
+import (
+	"math"
+	"time"
+
+	"github.com/ethereum/go-ethereum/eth/downloader"
+	"github.com/ethereum/go-ethereum/logger"
+	"github.com/ethereum/go-ethereum/logger/glog"
+)
+
+// Sync contains all synchronisation code for the eth protocol
+
+func (pm *ProtocolManager) update() {
+	// itimer is used to determine when to start ignoring `minDesiredPeerCount`
+	itimer := time.NewTimer(peerCountTimeout)
+	// btimer is used for picking of blocks from the downloader
+	btimer := time.Tick(blockProcTimer)
+
+	for {
+		select {
+		case <-pm.newPeerCh:
+			// Meet the `minDesiredPeerCount` before we select our best peer
+			if len(pm.peers) < minDesiredPeerCount {
+				break
+			}
+
+			// Find the best peer
+			peer := getBestPeer(pm.peers)
+			if peer == nil {
+				glog.V(logger.Debug).Infoln("Sync attempt cancelled. No peers available")
+			}
+
+			itimer.Stop()
+			go pm.synchronise(peer)
+		case <-itimer.C:
+			// The timer will make sure that the downloader keeps an active state
+			// in which it attempts to always check the network for highest td peers
+			// Either select the peer or restart the timer if no peers could
+			// be selected.
+			if peer := getBestPeer(pm.peers); peer != nil {
+				go pm.synchronise(peer)
+			} else {
+				itimer.Reset(5 * time.Second)
+			}
+		case <-btimer:
+			go pm.processBlocks()
+		case <-pm.quitSync:
+			return
+		}
+	}
+}
+
+// processBlocks will attempt to reconstruct a chain by checking the first item and check if it's
+// a known parent. The first block in the chain may be unknown during downloading. When the
+// downloader isn't downloading blocks will be dropped with an unknown parent until either it
+// has depleted the list or found a known parent.
+func (pm *ProtocolManager) processBlocks() error {
+	pm.wg.Add(1)
+	defer pm.wg.Done()
+
+	blocks := pm.downloader.TakeBlocks()
+	if len(blocks) == 0 {
+		return nil
+	}
+	defer pm.downloader.Done()
+
+	glog.V(logger.Debug).Infof("Inserting chain with %d blocks (#%v - #%v)\n", len(blocks), blocks[0].Number(), blocks[len(blocks)-1].Number())
+
+	for len(blocks) != 0 && !pm.quit {
+		max := int(math.Min(float64(len(blocks)), float64(blockProcAmount)))
+		_, err := pm.chainman.InsertChain(blocks[:max])
+		if err != nil {
+			return err
+		}
+		blocks = blocks[max:]
+	}
+	return nil
+}
+
+func (pm *ProtocolManager) synchronise(peer *peer) {
+	// Make sure the peer's TD is higher than our own. If not drop.
+	if peer.td.Cmp(pm.chainman.Td()) <= 0 {
+		return
+	}
+	// Check downloader if it's busy so it doesn't show the sync message
+	// for every attempty
+	if pm.downloader.IsBusy() {
+		return
+	}
+
+	// FIXME if we have the hash in our chain and the TD of the peer is
+	// much higher than ours, something is wrong with us or the peer.
+	// Check if the hash is on our own chain
+	if pm.chainman.HasBlock(peer.recentHash) {
+		return
+	}
+
+	// Get the hashes from the peer (synchronously)
+	err := pm.downloader.Synchronise(peer.id, peer.recentHash)
+	if err != nil && err == downloader.ErrBadPeer {
+		glog.V(logger.Debug).Infoln("removed peer from peer set due to bad action")
+		pm.removePeer(peer)
+	} else if err != nil {
+		// handle error
+		glog.V(logger.Detail).Infoln("error downloading:", err)
+	}
+}