Merge pull request #1243 from karalabe/instrument-downloader-sync

eth, eth/downloader: separate concerns, clean up test suite

9 files changed, 646 insertions, 468 deletions

diff --git a/eth/backend.go b/eth/backend.go
index d2ec0cc62..4ebf21811 100644
--- a/eth/backend.go
+++ b/eth/backend.go
@@ -193,7 +193,6 @@ type Ethereum struct {
 	whisper         *whisper.Whisper
 	pow             *ethash.Ethash
 	protocolManager *ProtocolManager
-	downloader      *downloader.Downloader
 	SolcPath        string
 	solc            *compiler.Solidity
 
@@ -290,14 +289,13 @@ func New(config *Config) (*Ethereum, error) {
 	if err != nil {
 		return nil, err
 	}
-	eth.downloader = downloader.New(eth.EventMux(), eth.chainManager.HasBlock, eth.chainManager.GetBlock)
 	eth.txPool = core.NewTxPool(eth.EventMux(), eth.chainManager.State, eth.chainManager.GasLimit)
 	eth.blockProcessor = core.NewBlockProcessor(stateDb, extraDb, eth.pow, eth.chainManager, eth.EventMux())
 	eth.chainManager.SetProcessor(eth.blockProcessor)
+	eth.protocolManager = NewProtocolManager(config.ProtocolVersion, config.NetworkId, eth.eventMux, eth.txPool, eth.chainManager)
+
 	eth.miner = miner.New(eth, eth.EventMux(), eth.pow)
 	eth.miner.SetGasPrice(config.GasPrice)
-
-	eth.protocolManager = NewProtocolManager(config.ProtocolVersion, config.NetworkId, eth.eventMux, eth.txPool, eth.chainManager, eth.downloader)
 	if config.Shh {
 		eth.whisper = whisper.New()
 		eth.shhVersionId = int(eth.whisper.Version())
@@ -447,7 +445,7 @@ func (s *Ethereum) ClientVersion() string                { return s.clientVersio
 func (s *Ethereum) EthVersion() int                      { return s.ethVersionId }
 func (s *Ethereum) NetVersion() int                      { return s.netVersionId }
 func (s *Ethereum) ShhVersion() int                      { return s.shhVersionId }
-func (s *Ethereum) Downloader() *downloader.Downloader   { return s.downloader }
+func (s *Ethereum) Downloader() *downloader.Downloader   { return s.protocolManager.downloader }
 
 // Start the ethereum
 func (s *Ethereum) Start() error {
diff --git a/eth/downloader/downloader.go b/eth/downloader/downloader.go
index f0a515d12..306c4fd2d 100644
--- a/eth/downloader/downloader.go
+++ b/eth/downloader/downloader.go
@@ -3,6 +3,7 @@ package downloader
 import (
 	"bytes"
 	"errors"
+	"math"
 	"math/rand"
 	"sync"
 	"sync/atomic"
@@ -28,32 +29,40 @@ var (
 	crossCheckCycle = time.Second      // Period after which to check for expired cross checks
 
 	maxBannedHashes = 4096 // Number of bannable hashes before phasing old ones out
+	maxBlockProcess = 256  // Number of blocks to import at once into the chain
 )
 
 var (
-	errLowTd            = errors.New("peers TD is too low")
-	ErrBusy             = errors.New("busy")
-	errUnknownPeer      = errors.New("peer is unknown or unhealthy")
-	ErrBadPeer          = errors.New("action from bad peer ignored")
-	ErrStallingPeer     = errors.New("peer is stalling")
-	errBannedHead       = errors.New("peer head hash already banned")
-	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")
-	ErrInvalidChain     = errors.New("retrieved hash chain is invalid")
-	ErrCrossCheckFailed = errors.New("block cross-check failed")
-	errCancelHashFetch  = errors.New("hash fetching cancelled (requested)")
-	errCancelBlockFetch = errors.New("block downloading cancelled (requested)")
-	errNoSyncActive     = errors.New("no sync active")
+	errBusy              = errors.New("busy")
+	errUnknownPeer       = errors.New("peer is unknown or unhealthy")
+	errBadPeer           = errors.New("action from bad peer ignored")
+	errStallingPeer      = errors.New("peer is stalling")
+	errBannedHead        = errors.New("peer head hash already banned")
+	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")
+	errInvalidChain      = errors.New("retrieved hash chain is invalid")
+	errCrossCheckFailed  = errors.New("block cross-check failed")
+	errCancelHashFetch   = errors.New("hash fetching canceled (requested)")
+	errCancelBlockFetch  = errors.New("block downloading canceled (requested)")
+	errCancelChainImport = errors.New("chain importing canceled (requested)")
+	errNoSyncActive      = errors.New("no sync active")
 )
 
+// hashCheckFn is a callback type for verifying a hash's presence in the local chain.
 type hashCheckFn func(common.Hash) bool
-type getBlockFn func(common.Hash) *types.Block
+
+// blockRetrievalFn is a callback type for retrieving a block from the local chain.
+type blockRetrievalFn func(common.Hash) *types.Block
+
+// chainInsertFn is a callback type to insert a batch of blocks into the local chain.
 type chainInsertFn func(types.Blocks) (int, error)
-type hashIterFn func() (common.Hash, error)
+
+// peerDropFn is a callback type for dropping a peer detected as malicious.
+type peerDropFn func(id string)
 
 type blockPack struct {
 	peerId string
@@ -85,12 +94,16 @@ type Downloader struct {
 	importLock  sync.Mutex
 
 	// Callbacks
-	hasBlock hashCheckFn
-	getBlock getBlockFn
+	hasBlock    hashCheckFn      // Checks if a block is present in the chain
+	getBlock    blockRetrievalFn // Retrieves a block from the chain
+	insertChain chainInsertFn    // Injects a batch of blocks into the chain
+	dropPeer    peerDropFn       // Retrieved the TD of our own chain
 
 	// Status
-	synchronising int32
-	notified      int32
+	synchroniseMock func(id string, hash common.Hash) error // Replacement for synchronise during testing
+	synchronising   int32
+	processing      int32
+	notified        int32
 
 	// Channels
 	newPeerCh chan *peer
@@ -107,17 +120,20 @@ type Block struct {
 	OriginPeer string
 }
 
-func New(mux *event.TypeMux, hasBlock hashCheckFn, getBlock getBlockFn) *Downloader {
+// New creates a new downloader to fetch hashes and blocks from remote peers.
+func New(mux *event.TypeMux, hasBlock hashCheckFn, getBlock blockRetrievalFn, insertChain chainInsertFn, dropPeer peerDropFn) *Downloader {
 	// Create the base downloader
 	downloader := &Downloader{
-		mux:       mux,
-		queue:     newQueue(),
-		peers:     newPeerSet(),
-		hasBlock:  hasBlock,
-		getBlock:  getBlock,
-		newPeerCh: make(chan *peer, 1),
-		hashCh:    make(chan hashPack, 1),
-		blockCh:   make(chan blockPack, 1),
+		mux:         mux,
+		queue:       newQueue(),
+		peers:       newPeerSet(),
+		hasBlock:    hasBlock,
+		getBlock:    getBlock,
+		insertChain: insertChain,
+		dropPeer:    dropPeer,
+		newPeerCh:   make(chan *peer, 1),
+		hashCh:      make(chan hashPack, 1),
+		blockCh:     make(chan blockPack, 1),
 	}
 	// Inject all the known bad hashes
 	downloader.banned = set.New()
@@ -150,7 +166,7 @@ func (d *Downloader) Stats() (pending int, cached int, importing int, estimate t
 	return
 }
 
-// Synchronising returns the state of the downloader
+// Synchronising returns whether the downloader is currently retrieving blocks.
 func (d *Downloader) Synchronising() bool {
 	return atomic.LoadInt32(&d.synchronising) > 0
 }
@@ -183,19 +199,47 @@ func (d *Downloader) UnregisterPeer(id string) error {
 	return nil
 }
 
-// Synchronise will select the peer and use it for synchronising. If an empty string is given
+// Synchronise tries to sync up our local block chain with a remote peer, both
+// adding various sanity checks as well as wrapping it with various log entries.
+func (d *Downloader) Synchronise(id string, head common.Hash) {
+	glog.V(logger.Detail).Infof("Attempting synchronisation: %v, 0x%x", id, head)
+
+	switch err := d.synchronise(id, head); err {
+	case nil:
+		glog.V(logger.Detail).Infof("Synchronisation completed")
+
+	case errBusy:
+		glog.V(logger.Detail).Infof("Synchronisation already in progress")
+
+	case errTimeout, errBadPeer, errStallingPeer, errBannedHead, errEmptyHashSet, errPeersUnavailable, errInvalidChain, errCrossCheckFailed:
+		glog.V(logger.Debug).Infof("Removing peer %v: %v", id, err)
+		d.dropPeer(id)
+
+	case errPendingQueue:
+		glog.V(logger.Debug).Infoln("Synchronisation aborted:", err)
+
+	default:
+		glog.V(logger.Warn).Infof("Synchronisation failed: %v", err)
+	}
+}
+
+// synchronise will select the peer and use it for synchronising. If an empty string is given
 // it will use the best peer possible and synchronize if it's TD is higher than our own. If any of the
 // checks fail an error will be returned. This method is synchronous
-func (d *Downloader) Synchronise(id string, hash common.Hash) error {
+func (d *Downloader) synchronise(id string, hash common.Hash) error {
+	// Mock out the synchonisation if testing
+	if d.synchroniseMock != nil {
+		return d.synchroniseMock(id, hash)
+	}
 	// Make sure only one goroutine is ever allowed past this point at once
 	if !atomic.CompareAndSwapInt32(&d.synchronising, 0, 1) {
-		return ErrBusy
+		return errBusy
 	}
 	defer atomic.StoreInt32(&d.synchronising, 0)
 
 	// If the head hash is banned, terminate immediately
 	if d.banned.Has(hash) {
-		return ErrInvalidChain
+		return errBannedHead
 	}
 	// Post a user notification of the sync (only once per session)
 	if atomic.CompareAndSwapInt32(&d.notified, 0, 1) {
@@ -209,7 +253,7 @@ func (d *Downloader) Synchronise(id string, hash common.Hash) error {
 
 	// Abort if the queue still contains some leftover data
 	if _, cached := d.queue.Size(); cached > 0 && d.queue.GetHeadBlock() != nil {
-		return ErrPendingQueue
+		return errPendingQueue
 	}
 	// Reset the queue and peer set to clean any internal leftover state
 	d.queue.Reset()
@@ -225,19 +269,6 @@ func (d *Downloader) Synchronise(id string, hash common.Hash) error {
 	return d.syncWithPeer(p, hash)
 }
 
-// TakeBlocks takes blocks from the queue and yields them to the caller.
-func (d *Downloader) TakeBlocks() []*Block {
-	blocks := d.queue.TakeBlocks()
-	if len(blocks) > 0 {
-		d.importLock.Lock()
-		d.importStart = time.Now()
-		d.importQueue = blocks
-		d.importDone = 0
-		d.importLock.Unlock()
-	}
-	return blocks
-}
-
 // Has checks if the downloader knows about a particular hash, meaning that its
 // either already downloaded of pending retrieval.
 func (d *Downloader) Has(hash common.Hash) bool {
@@ -272,34 +303,26 @@ func (d *Downloader) syncWithPeer(p *peer, hash common.Hash) (err error) {
 
 // Cancel cancels all of the operations and resets the queue. It returns true
 // if the cancel operation was completed.
-func (d *Downloader) Cancel() bool {
-	// If we're not syncing just return.
-	hs, bs := d.queue.Size()
-	if atomic.LoadInt32(&d.synchronising) == 0 && hs == 0 && bs == 0 {
-		return false
-	}
+func (d *Downloader) Cancel() {
 	// Close the current cancel channel
 	d.cancelLock.Lock()
-	select {
-	case <-d.cancelCh:
-		// Channel was already closed
-	default:
-		close(d.cancelCh)
+	if d.cancelCh != nil {
+		select {
+		case <-d.cancelCh:
+			// Channel was already closed
+		default:
+			close(d.cancelCh)
+		}
 	}
 	d.cancelLock.Unlock()
 
-	// Reset the queue and import statistics
+	// Reset the queue
 	d.queue.Reset()
-
-	d.importLock.Lock()
-	d.importQueue = nil
-	d.importDone = 0
-	d.importLock.Unlock()
-
-	return true
 }
 
-// XXX Make synchronous
+// fetchHahes starts retrieving hashes backwards from a specific peer and hash,
+// up until it finds a common ancestor. If the source peer times out, alternative
+// ones are tried for continuation.
 func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
 	var (
 		start  = time.Now()
@@ -317,7 +340,7 @@ func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
 	<-timeout.C // timeout channel should be initially empty.
 
 	getHashes := func(from common.Hash) {
-		active.getHashes(from)
+		go active.getHashes(from)
 		timeout.Reset(hashTTL)
 	}
 
@@ -342,7 +365,7 @@ func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
 			// Make sure the peer actually gave something valid
 			if len(hashPack.hashes) == 0 {
 				glog.V(logger.Debug).Infof("Peer (%s) responded with empty hash set", active.id)
-				return ErrEmptyHashSet
+				return errEmptyHashSet
 			}
 			for index, hash := range hashPack.hashes {
 				if d.banned.Has(hash) {
@@ -352,7 +375,7 @@ func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
 					if err := d.banBlocks(active.id, hash); err != nil {
 						glog.V(logger.Debug).Infof("Failed to ban batch of blocks: %v", err)
 					}
-					return ErrInvalidChain
+					return errInvalidChain
 				}
 			}
 			// Determine if we're done fetching hashes (queue up all pending), and continue if not done
@@ -369,12 +392,12 @@ func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
 			inserts := d.queue.Insert(hashPack.hashes)
 			if len(inserts) == 0 && !done {
 				glog.V(logger.Debug).Infof("Peer (%s) responded with stale hashes", active.id)
-				return ErrBadPeer
+				return errBadPeer
 			}
 			if !done {
 				// Check that the peer is not stalling the sync
 				if len(inserts) < MinHashFetch {
-					return ErrStallingPeer
+					return errStallingPeer
 				}
 				// Try and fetch a random block to verify the hash batch
 				// Skip the last hash as the cross check races with the next hash fetch
@@ -386,9 +409,9 @@ func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
 					expire: time.Now().Add(blockSoftTTL),
 					parent: parent,
 				}
-				active.getBlocks([]common.Hash{origin})
+				go active.getBlocks([]common.Hash{origin})
 
-				// Also fetch a fresh
+				// Also fetch a fresh batch of hashes
 				getHashes(head)
 				continue
 			}
@@ -408,7 +431,7 @@ func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
 			block := blockPack.blocks[0]
 			if check, ok := d.checks[block.Hash()]; ok {
 				if block.ParentHash() != check.parent {
-					return ErrCrossCheckFailed
+					return errCrossCheckFailed
 				}
 				delete(d.checks, block.Hash())
 			}
@@ -418,7 +441,7 @@ func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
 			for hash, check := range d.checks {
 				if time.Now().After(check.expire) {
 					glog.V(logger.Debug).Infof("Cross check timeout for %x", hash)
-					return ErrCrossCheckFailed
+					return errCrossCheckFailed
 				}
 			}
 
@@ -438,7 +461,7 @@ func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
 			// if all peers have been tried, abort the process entirely or if the hash is
 			// the zero hash.
 			if p == nil || (head == common.Hash{}) {
-				return ErrTimeout
+				return errTimeout
 			}
 			// set p to the active peer. this will invalidate any hashes that may be returned
 			// by our previous (delayed) peer.
@@ -495,12 +518,13 @@ out:
 						glog.V(logger.Detail).Infof("%s: no blocks delivered", peer)
 						break
 					}
-					// All was successful, promote the peer
+					// All was successful, promote the peer and potentially start processing
 					peer.Promote()
 					peer.SetIdle()
 					glog.V(logger.Detail).Infof("%s: delivered %d blocks", peer, len(blockPack.blocks))
+					go d.process()
 
-				case ErrInvalidChain:
+				case errInvalidChain:
 					// The hash chain is invalid (blocks are not ordered properly), abort
 					return err
 
@@ -617,7 +641,7 @@ func (d *Downloader) banBlocks(peerId string, head common.Hash) error {
 			return errCancelBlockFetch
 
 		case <-timeout:
-			return ErrTimeout
+			return errTimeout
 
 		case <-d.hashCh:
 			// Out of bounds hashes received, ignore them
@@ -674,6 +698,92 @@ func (d *Downloader) banBlocks(peerId string, head common.Hash) error {
 	}
 }
 
+// process takes blocks from the queue and tries to import them into the chain.
+//
+// The algorithmic flow is as follows:
+//  - The `processing` flag is swapped to 1 to ensure singleton access
+//  - The current `cancel` channel is retrieved to detect sync abortions
+//  - Blocks are iteratively taken from the cache and inserted into the chain
+//  - When the cache becomes empty, insertion stops
+//  - The `processing` flag is swapped back to 0
+//  - A post-exit check is made whether new blocks became available
+//     - This step is important: it handles a potential race condition between
+//       checking for no more work, and releasing the processing "mutex". In
+//       between these state changes, a block may have arrived, but a processing
+//       attempt denied, so we need to re-enter to ensure the block isn't left
+//       to idle in the cache.
+func (d *Downloader) process() (err error) {
+	// Make sure only one goroutine is ever allowed to process blocks at once
+	if !atomic.CompareAndSwapInt32(&d.processing, 0, 1) {
+		return
+	}
+	// If the processor just exited, but there are freshly pending items, try to
+	// reenter. This is needed because the goroutine spinned up for processing
+	// the fresh blocks might have been rejected entry to to this present thread
+	// not yet releasing the `processing` state.
+	defer func() {
+		if err == nil && d.queue.GetHeadBlock() != nil {
+			err = d.process()
+		}
+	}()
+	// Release the lock upon exit (note, before checking for reentry!), and set
+	// the import statistics to zero.
+	defer func() {
+		d.importLock.Lock()
+		d.importQueue = nil
+		d.importDone = 0
+		d.importLock.Unlock()
+
+		atomic.StoreInt32(&d.processing, 0)
+	}()
+
+	// Fetch the current cancel channel to allow termination
+	d.cancelLock.RLock()
+	cancel := d.cancelCh
+	d.cancelLock.RUnlock()
+
+	// Repeat the processing as long as there are blocks to import
+	for {
+		// Fetch the next batch of blocks
+		blocks := d.queue.TakeBlocks()
+		if len(blocks) == 0 {
+			return nil
+		}
+		// Reset the import statistics
+		d.importLock.Lock()
+		d.importStart = time.Now()
+		d.importQueue = blocks
+		d.importDone = 0
+		d.importLock.Unlock()
+
+		// Actually import the blocks
+		glog.V(logger.Debug).Infof("Inserting chain with %d blocks (#%v - #%v)\n", len(blocks), blocks[0].RawBlock.Number(), blocks[len(blocks)-1].RawBlock.Number())
+		for len(blocks) != 0 { // TODO: quit
+			// Check for any termination requests
+			select {
+			case <-cancel:
+				return errCancelChainImport
+			default:
+			}
+			// Retrieve the first batch of blocks to insert
+			max := int(math.Min(float64(len(blocks)), float64(maxBlockProcess)))
+			raw := make(types.Blocks, 0, max)
+			for _, block := range blocks[:max] {
+				raw = append(raw, block.RawBlock)
+			}
+			// Try to inset the blocks, drop the originating peer if there's an error
+			index, err := d.insertChain(raw)
+			if err != nil {
+				glog.V(logger.Debug).Infof("Block #%d import failed: %v", raw[index].NumberU64(), err)
+				d.dropPeer(blocks[index].OriginPeer)
+				d.Cancel()
+				return errCancelChainImport
+			}
+			blocks = blocks[max:]
+		}
+	}
+}
+
 // DeliverBlocks injects a new batch of blocks received from a remote node.
 // This is usually invoked through the BlocksMsg by the protocol handler.
 func (d *Downloader) DeliverBlocks(id string, blocks []*types.Block) error {
diff --git a/eth/downloader/downloader_test.go b/eth/downloader/downloader_test.go
index 5f10fb41f..f71c16237 100644
--- a/eth/downloader/downloader_test.go
+++ b/eth/downloader/downloader_test.go
@@ -2,7 +2,10 @@ package downloader
 
 import (
 	"encoding/binary"
+	"errors"
+	"fmt"
 	"math/big"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -13,21 +16,29 @@ import (
 )
 
 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}
-	unknownHash = common.Hash{9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9}
-	bannedHash  = common.Hash{5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5}
+	knownHash   = common.Hash{1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1}
+	unknownHash = common.Hash{2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2}
+	bannedHash  = common.Hash{3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3}
+
+	genesis = createBlock(1, common.Hash{}, knownHash)
 )
 
-func createHashes(start, amount int) (hashes []common.Hash) {
+// idCounter is used by the createHashes method the generate deterministic but unique hashes
+var idCounter = int64(2) // #1 is the genesis block
+
+// createHashes generates a batch of hashes rooted at a specific point in the chain.
+func createHashes(amount int, root common.Hash) (hashes []common.Hash) {
 	hashes = make([]common.Hash, amount+1)
-	hashes[len(hashes)-1] = knownHash
+	hashes[len(hashes)-1] = root
 
-	for i := range hashes[:len(hashes)-1] {
-		binary.BigEndian.PutUint64(hashes[i][:8], uint64(start+i+2))
+	for i := 0; i < len(hashes)-1; i++ {
+		binary.BigEndian.PutUint64(hashes[i][:8], uint64(idCounter))
+		idCounter++
 	}
 	return
 }
 
+// createBlock assembles a new block at the given chain height.
 func createBlock(i int, parent, hash common.Hash) *types.Block {
 	header := &types.Header{Number: big.NewInt(int64(i))}
 	block := types.NewBlockWithHeader(header)
@@ -36,6 +47,11 @@ func createBlock(i int, parent, hash common.Hash) *types.Block {
 	return block
 }
 
+// copyBlock makes a deep copy of a block suitable for local modifications.
+func copyBlock(block *types.Block) *types.Block {
+	return createBlock(int(block.Number().Int64()), block.ParentHeaderHash, block.HeaderHash)
+}
+
 func createBlocksFromHashes(hashes []common.Hash) map[common.Hash]*types.Block {
 	blocks := make(map[common.Hash]*types.Block)
 	for i := 0; i < len(hashes); i++ {
@@ -51,181 +67,171 @@ func createBlocksFromHashes(hashes []common.Hash) map[common.Hash]*types.Block {
 type downloadTester struct {
 	downloader *Downloader
 
-	hashes []common.Hash                // Chain of hashes simulating
-	blocks map[common.Hash]*types.Block // Blocks associated with the hashes
-	chain  []common.Hash                // Block-chain being constructed
+	ownHashes  []common.Hash                           // Hash chain belonging to the tester
+	ownBlocks  map[common.Hash]*types.Block            // Blocks belonging to the tester
+	peerHashes map[string][]common.Hash                // Hash chain belonging to different test peers
+	peerBlocks map[string]map[common.Hash]*types.Block // Blocks belonging to different test peers
 
 	maxHashFetch int // Overrides the maximum number of retrieved hashes
-
-	t            *testing.T
-	done         chan bool
-	activePeerId string
 }
 
-func newTester(t *testing.T, hashes []common.Hash, blocks map[common.Hash]*types.Block) *downloadTester {
+func newTester() *downloadTester {
 	tester := &downloadTester{
-		t: t,
-
-		hashes: hashes,
-		blocks: blocks,
-		chain:  []common.Hash{knownHash},
-
-		done: make(chan bool),
+		ownHashes:  []common.Hash{knownHash},
+		ownBlocks:  map[common.Hash]*types.Block{knownHash: genesis},
+		peerHashes: make(map[string][]common.Hash),
+		peerBlocks: make(map[string]map[common.Hash]*types.Block),
 	}
 	var mux event.TypeMux
-	downloader := New(&mux, tester.hasBlock, tester.getBlock)
+	downloader := New(&mux, tester.hasBlock, tester.getBlock, tester.insertChain, tester.dropPeer)
 	tester.downloader = downloader
 
 	return tester
 }
 
-// sync is a simple wrapper around the downloader to start synchronisation and
-// block until it returns
-func (dl *downloadTester) sync(peerId string, head common.Hash) error {
-	dl.activePeerId = peerId
-	return dl.downloader.Synchronise(peerId, head)
+// sync starts synchronizing with a remote peer, blocking until it completes.
+func (dl *downloadTester) sync(id string) error {
+	err := dl.downloader.synchronise(id, dl.peerHashes[id][0])
+	for atomic.LoadInt32(&dl.downloader.processing) == 1 {
+		time.Sleep(time.Millisecond)
+	}
+	return err
+}
+
+// hasBlock checks if a block is pres	ent in the testers canonical chain.
+func (dl *downloadTester) hasBlock(hash common.Hash) bool {
+	return dl.getBlock(hash) != nil
 }
 
-// syncTake is starts synchronising with a remote peer, but concurrently it also
-// starts fetching blocks that the downloader retrieved. IT blocks until both go
-// routines terminate.
-func (dl *downloadTester) syncTake(peerId string, head common.Hash) ([]*Block, error) {
-	// Start a block collector to take blocks as they become available
-	done := make(chan struct{})
-	took := []*Block{}
-	go func() {
-		for running := true; running; {
-			select {
-			case <-done:
-				running = false
-			default:
-				time.Sleep(time.Millisecond)
-			}
-			// Take a batch of blocks and accumulate
-			took = append(took, dl.downloader.TakeBlocks()...)
-		}
-		done <- struct{}{}
-	}()
-	// Start the downloading, sync the taker and return
-	err := dl.sync(peerId, head)
+// getBlock retrieves a block from the testers canonical chain.
+func (dl *downloadTester) getBlock(hash common.Hash) *types.Block {
+	return dl.ownBlocks[hash]
+}
 
-	done <- struct{}{}
-	<-done
+// insertChain injects a new batch of blocks into the simulated chain.
+func (dl *downloadTester) insertChain(blocks types.Blocks) (int, error) {
+	for i, block := range blocks {
+		if _, ok := dl.ownBlocks[block.ParentHash()]; !ok {
+			return i, errors.New("unknown parent")
+		}
+		dl.ownHashes = append(dl.ownHashes, block.Hash())
+		dl.ownBlocks[block.Hash()] = block
+	}
+	return len(blocks), nil
+}
 
-	return took, err
+// newPeer registers a new block download source into the downloader.
+func (dl *downloadTester) newPeer(id string, hashes []common.Hash, blocks map[common.Hash]*types.Block) error {
+	return dl.newSlowPeer(id, hashes, blocks, 0)
 }
 
-func (dl *downloadTester) hasBlock(hash common.Hash) bool {
-	for _, h := range dl.chain {
-		if h == hash {
-			return true
+// newSlowPeer registers a new block download source into the downloader, with a
+// specific delay time on processing the network packets sent to it, simulating
+// potentially slow network IO.
+func (dl *downloadTester) newSlowPeer(id string, hashes []common.Hash, blocks map[common.Hash]*types.Block, delay time.Duration) error {
+	err := dl.downloader.RegisterPeer(id, hashes[0], dl.peerGetHashesFn(id, delay), dl.peerGetBlocksFn(id, delay))
+	if err == nil {
+		// Assign the owned hashes and blocks to the peer (deep copy)
+		dl.peerHashes[id] = make([]common.Hash, len(hashes))
+		copy(dl.peerHashes[id], hashes)
+
+		dl.peerBlocks[id] = make(map[common.Hash]*types.Block)
+		for hash, block := range blocks {
+			dl.peerBlocks[id][hash] = copyBlock(block)
 		}
 	}
-	return false
+	return err
 }
 
-func (dl *downloadTester) getBlock(hash common.Hash) *types.Block {
-	return dl.blocks[knownHash]
-}
-
-// getHashes retrieves a batch of hashes for reconstructing the chain.
-func (dl *downloadTester) getHashes(head common.Hash) error {
-	limit := MaxHashFetch
-	if dl.maxHashFetch > 0 {
-		limit = dl.maxHashFetch
-	}
-	// Gather the next batch of hashes
-	hashes := make([]common.Hash, 0, limit)
-	for i, hash := range dl.hashes {
-		if hash == head {
-			i++
-			for len(hashes) < cap(hashes) && i < len(dl.hashes) {
-				hashes = append(hashes, dl.hashes[i])
+// dropPeer simulates a hard peer removal from the connection pool.
+func (dl *downloadTester) dropPeer(id string) {
+	delete(dl.peerHashes, id)
+	delete(dl.peerBlocks, id)
+
+	dl.downloader.UnregisterPeer(id)
+}
+
+// peerGetBlocksFn constructs a getHashes function associated with a particular
+// peer in the download tester. The returned function can be used to retrieve
+// batches of hashes from the particularly requested peer.
+func (dl *downloadTester) peerGetHashesFn(id string, delay time.Duration) func(head common.Hash) error {
+	return func(head common.Hash) error {
+		time.Sleep(delay)
+
+		limit := MaxHashFetch
+		if dl.maxHashFetch > 0 {
+			limit = dl.maxHashFetch
+		}
+		// Gather the next batch of hashes
+		hashes := dl.peerHashes[id]
+		result := make([]common.Hash, 0, limit)
+		for i, hash := range hashes {
+			if hash == head {
 				i++
+				for len(result) < cap(result) && i < len(hashes) {
+					result = append(result, hashes[i])
+					i++
+				}
+				break
 			}
-			break
 		}
+		// Delay delivery a bit to allow attacks to unfold
+		go func() {
+			time.Sleep(time.Millisecond)
+			dl.downloader.DeliverHashes(id, result)
+		}()
+		return nil
 	}
-	// Delay delivery a bit to allow attacks to unfold
-	id := dl.activePeerId
-	go func() {
-		time.Sleep(time.Millisecond)
-		dl.downloader.DeliverHashes(id, hashes)
-	}()
-	return nil
 }
 
-func (dl *downloadTester) getBlocks(id string) func([]common.Hash) error {
+// peerGetBlocksFn constructs a getBlocks function associated with a particular
+// peer in the download tester. The returned function can be used to retrieve
+// batches of blocks from the particularly requested peer.
+func (dl *downloadTester) peerGetBlocksFn(id string, delay time.Duration) func([]common.Hash) error {
 	return func(hashes []common.Hash) error {
-		blocks := make([]*types.Block, 0, len(hashes))
+		time.Sleep(delay)
+
+		blocks := dl.peerBlocks[id]
+		result := make([]*types.Block, 0, len(hashes))
 		for _, hash := range hashes {
-			if block, ok := dl.blocks[hash]; ok {
-				blocks = append(blocks, block)
+			if block, ok := blocks[hash]; ok {
+				result = append(result, block)
 			}
 		}
-		go dl.downloader.DeliverBlocks(id, blocks)
+		go dl.downloader.DeliverBlocks(id, result)
 
 		return nil
 	}
 }
 
-// newPeer registers a new block download source into the syncer.
-func (dl *downloadTester) newPeer(id string, td *big.Int, hash common.Hash) error {
-	return dl.downloader.RegisterPeer(id, hash, dl.getHashes, dl.getBlocks(id))
-}
-
 // Tests that simple synchronization, without throttling from a good peer works.
 func TestSynchronisation(t *testing.T) {
 	// Create a small enough block chain to download and the tester
 	targetBlocks := blockCacheLimit - 15
-	hashes := createHashes(0, targetBlocks)
+	hashes := createHashes(targetBlocks, knownHash)
 	blocks := createBlocksFromHashes(hashes)
 
-	tester := newTester(t, hashes, blocks)
-	tester.newPeer("peer", big.NewInt(10000), hashes[0])
+	tester := newTester()
+	tester.newPeer("peer", hashes, blocks)
 
 	// Synchronise with the peer and make sure all blocks were retrieved
-	if err := tester.sync("peer", hashes[0]); err != nil {
-		t.Fatalf("failed to synchronise blocks: %v", err)
-	}
-	if queued := len(tester.downloader.queue.blockPool); queued != targetBlocks {
-		t.Fatalf("synchronised block mismatch: have %v, want %v", queued, targetBlocks)
-	}
-}
-
-// Tests that the synchronized blocks can be correctly retrieved.
-func TestBlockTaking(t *testing.T) {
-	// Create a small enough block chain to download and the tester
-	targetBlocks := blockCacheLimit - 15
-	hashes := createHashes(0, targetBlocks)
-	blocks := createBlocksFromHashes(hashes)
-
-	tester := newTester(t, hashes, blocks)
-	tester.newPeer("peer", big.NewInt(10000), hashes[0])
-
-	// Synchronise with the peer and test block retrieval
-	if err := tester.sync("peer", hashes[0]); err != nil {
+	if err := tester.sync("peer"); err != nil {
 		t.Fatalf("failed to synchronise blocks: %v", err)
 	}
-	if took := tester.downloader.TakeBlocks(); len(took) != targetBlocks {
-		t.Fatalf("took block mismatch: have %v, want %v", len(took), targetBlocks)
+	if imported := len(tester.ownBlocks); imported != targetBlocks+1 {
+		t.Fatalf("synchronised block mismatch: have %v, want %v", imported, targetBlocks+1)
 	}
 }
 
 // Tests that an inactive downloader will not accept incoming hashes and blocks.
 func TestInactiveDownloader(t *testing.T) {
-	// Create a small enough block chain to download and the tester
-	targetBlocks := blockCacheLimit - 15
-	hashes := createHashes(0, targetBlocks)
-	blocks := createBlocksFromHashSet(createHashSet(hashes))
-
-	tester := newTester(t, nil, nil)
+	tester := newTester()
 
 	// Check that neither hashes nor blocks are accepted
-	if err := tester.downloader.DeliverHashes("bad peer", hashes); err != errNoSyncActive {
+	if err := tester.downloader.DeliverHashes("bad peer", []common.Hash{}); err != errNoSyncActive {
 		t.Errorf("error mismatch: have %v, want %v", err, errNoSyncActive)
 	}
-	if err := tester.downloader.DeliverBlocks("bad peer", blocks); err != errNoSyncActive {
+	if err := tester.downloader.DeliverBlocks("bad peer", []*types.Block{}); err != errNoSyncActive {
 		t.Errorf("error mismatch: have %v, want %v", err, errNoSyncActive)
 	}
 }
@@ -234,27 +240,27 @@ func TestInactiveDownloader(t *testing.T) {
 func TestCancel(t *testing.T) {
 	// Create a small enough block chain to download and the tester
 	targetBlocks := blockCacheLimit - 15
-	hashes := createHashes(0, targetBlocks)
+	hashes := createHashes(targetBlocks, knownHash)
 	blocks := createBlocksFromHashes(hashes)
 
-	tester := newTester(t, hashes, blocks)
-	tester.newPeer("peer", big.NewInt(10000), hashes[0])
+	tester := newTester()
+	tester.newPeer("peer", hashes, blocks)
 
+	// Make sure canceling works with a pristine downloader
+	tester.downloader.Cancel()
+	hashCount, blockCount := tester.downloader.queue.Size()
+	if hashCount > 0 || blockCount > 0 {
+		t.Errorf("block or hash count mismatch: %d hashes, %d blocks, want 0", hashCount, blockCount)
+	}
 	// Synchronise with the peer, but cancel afterwards
-	if err := tester.sync("peer", hashes[0]); err != nil {
+	if err := tester.sync("peer"); err != nil {
 		t.Fatalf("failed to synchronise blocks: %v", err)
 	}
-	if !tester.downloader.Cancel() {
-		t.Fatalf("cancel operation failed")
-	}
-	// Make sure the queue reports empty and no blocks can be taken
-	hashCount, blockCount := tester.downloader.queue.Size()
+	tester.downloader.Cancel()
+	hashCount, blockCount = tester.downloader.queue.Size()
 	if hashCount > 0 || blockCount > 0 {
 		t.Errorf("block or hash count mismatch: %d hashes, %d blocks, want 0", hashCount, blockCount)
 	}
-	if took := tester.downloader.TakeBlocks(); len(took) != 0 {
-		t.Errorf("taken blocks mismatch: have %d, want %d", len(took), 0)
-	}
 }
 
 // Tests that if a large batch of blocks are being downloaded, it is throttled
@@ -262,98 +268,167 @@ func TestCancel(t *testing.T) {
 func TestThrottling(t *testing.T) {
 	// Create a long block chain to download and the tester
 	targetBlocks := 8 * blockCacheLimit
-	hashes := createHashes(0, targetBlocks)
+	hashes := createHashes(targetBlocks, knownHash)
 	blocks := createBlocksFromHashes(hashes)
 
-	tester := newTester(t, hashes, blocks)
-	tester.newPeer("peer", big.NewInt(10000), hashes[0])
+	tester := newTester()
+	tester.newPeer("peer", hashes, blocks)
 
+	// Wrap the importer to allow stepping
+	done := make(chan int)
+	tester.downloader.insertChain = func(blocks types.Blocks) (int, error) {
+		n, err := tester.insertChain(blocks)
+		done <- n
+		return n, err
+	}
 	// Start a synchronisation concurrently
 	errc := make(chan error)
 	go func() {
-		errc <- tester.sync("peer", hashes[0])
+		errc <- tester.sync("peer")
 	}()
 	// Iteratively take some blocks, always checking the retrieval count
-	for total := 0; total < targetBlocks; {
-		// Wait a bit for sync to complete
+	for len(tester.ownBlocks) < targetBlocks+1 {
+		// Wait a bit for sync to throttle itself
+		var cached int
 		for start := time.Now(); time.Since(start) < 3*time.Second; {
 			time.Sleep(25 * time.Millisecond)
-			if len(tester.downloader.queue.blockPool) == blockCacheLimit {
+
+			cached = len(tester.downloader.queue.blockPool)
+			if cached == blockCacheLimit || len(tester.ownBlocks)+cached == targetBlocks+1 {
 				break
 			}
 		}
-		// Fetch the next batch of blocks
-		took := tester.downloader.TakeBlocks()
-		if len(took) != blockCacheLimit {
-			t.Fatalf("block count mismatch: have %v, want %v", len(took), blockCacheLimit)
+		// Make sure we filled up the cache, then exhaust it
+		time.Sleep(25 * time.Millisecond) // give it a chance to screw up
+		if cached != blockCacheLimit && len(tester.ownBlocks)+cached < targetBlocks+1 {
+			t.Fatalf("block count mismatch: have %v, want %v", cached, blockCacheLimit)
 		}
-		total += len(took)
-		if total > targetBlocks {
-			t.Fatalf("target block count mismatch: have %v, want %v", total, targetBlocks)
+		<-done // finish previous blocking import
+		for cached > maxBlockProcess {
+			cached -= <-done
 		}
+		time.Sleep(25 * time.Millisecond) // yield to the insertion
+	}
+	<-done // finish the last blocking import
+
+	// Check that we haven't pulled more blocks than available
+	if len(tester.ownBlocks) > targetBlocks+1 {
+		t.Fatalf("target block count mismatch: have %v, want %v", len(tester.ownBlocks), targetBlocks+1)
 	}
 	if err := <-errc; err != nil {
 		t.Fatalf("block synchronization failed: %v", err)
 	}
 }
 
+// Tests that synchronisation from multiple peers works as intended (multi thread sanity test).
+func TestMultiSynchronisation(t *testing.T) {
+	// Create various peers with various parts of the chain
+	targetPeers := 16
+	targetBlocks := targetPeers*blockCacheLimit - 15
+
+	hashes := createHashes(targetBlocks, knownHash)
+	blocks := createBlocksFromHashes(hashes)
+
+	tester := newTester()
+	for i := 0; i < targetPeers; i++ {
+		id := fmt.Sprintf("peer #%d", i)
+		tester.newPeer(id, hashes[i*blockCacheLimit:], blocks)
+	}
+	// Synchronise with the middle peer and make sure half of the blocks were retrieved
+	id := fmt.Sprintf("peer #%d", targetPeers/2)
+	if err := tester.sync(id); err != nil {
+		t.Fatalf("failed to synchronise blocks: %v", err)
+	}
+	if imported := len(tester.ownBlocks); imported != len(tester.peerHashes[id]) {
+		t.Fatalf("synchronised block mismatch: have %v, want %v", imported, len(tester.peerHashes[id]))
+	}
+	// Synchronise with the best peer and make sure everything is retrieved
+	if err := tester.sync("peer #0"); err != nil {
+		t.Fatalf("failed to synchronise blocks: %v", err)
+	}
+	if imported := len(tester.ownBlocks); imported != targetBlocks+1 {
+		t.Fatalf("synchronised block mismatch: have %v, want %v", imported, targetBlocks+1)
+	}
+}
+
+// Tests that synchronising with a peer who's very slow at network IO does not
+// stall the other peers in the system.
+func TestSlowSynchronisation(t *testing.T) {
+	tester := newTester()
+
+	// Create a batch of blocks, with a slow and a full speed peer
+	targetCycles := 2
+	targetBlocks := targetCycles*blockCacheLimit - 15
+	targetIODelay := 500 * time.Millisecond
+
+	hashes := createHashes(targetBlocks, knownHash)
+	blocks := createBlocksFromHashes(hashes)
+
+	tester.newSlowPeer("fast", hashes, blocks, 0)
+	tester.newSlowPeer("slow", hashes, blocks, targetIODelay)
+
+	// Try to sync with the peers (pull hashes from fast)
+	start := time.Now()
+	if err := tester.sync("fast"); err != nil {
+		t.Fatalf("failed to synchronise blocks: %v", err)
+	}
+	if imported := len(tester.ownBlocks); imported != targetBlocks+1 {
+		t.Fatalf("synchronised block mismatch: have %v, want %v", imported, targetBlocks+1)
+	}
+	// Check that the slow peer got hit at most once per block-cache-size import
+	limit := time.Duration(targetCycles+1) * targetIODelay
+	if delay := time.Since(start); delay >= limit {
+		t.Fatalf("synchronisation exceeded delay limit: have %v, want %v", delay, limit)
+	}
+}
+
 // Tests that if a peer returns an invalid chain with a block pointing to a non-
 // existing parent, it is correctly detected and handled.
 func TestNonExistingParentAttack(t *testing.T) {
+	tester := newTester()
+
 	// Forge a single-link chain with a forged header
-	hashes := createHashes(0, 1)
+	hashes := createHashes(1, knownHash)
 	blocks := createBlocksFromHashes(hashes)
+	tester.newPeer("valid", hashes, blocks)
 
-	forged := blocks[hashes[0]]
-	forged.ParentHeaderHash = unknownHash
+	hashes = createHashes(1, knownHash)
+	blocks = createBlocksFromHashes(hashes)
+	blocks[hashes[0]].ParentHeaderHash = unknownHash
+	tester.newPeer("attack", hashes, blocks)
 
 	// Try and sync with the malicious node and check that it fails
-	tester := newTester(t, hashes, blocks)
-	tester.newPeer("attack", big.NewInt(10000), hashes[0])
-	if err := tester.sync("attack", hashes[0]); err != nil {
-		t.Fatalf("failed to synchronise blocks: %v", err)
-	}
-	bs := tester.downloader.TakeBlocks()
-	if len(bs) != 1 {
-		t.Fatalf("retrieved block mismatch: have %v, want %v", len(bs), 1)
+	if err := tester.sync("attack"); err == nil {
+		t.Fatalf("block synchronization succeeded")
 	}
-	if tester.hasBlock(bs[0].RawBlock.ParentHash()) {
-		t.Fatalf("tester knows about the unknown hash")
+	if tester.hasBlock(hashes[0]) {
+		t.Fatalf("tester accepted unknown-parent block: %v", blocks[hashes[0]])
 	}
-	tester.downloader.Cancel()
-
-	// Reconstruct a valid chain, and try to synchronize with it
-	forged.ParentHeaderHash = knownHash
-	tester.newPeer("valid", big.NewInt(20000), hashes[0])
-	if err := tester.sync("valid", hashes[0]); err != nil {
+	// Try to synchronize with the valid chain and make sure it succeeds
+	if err := tester.sync("valid"); err != nil {
 		t.Fatalf("failed to synchronise blocks: %v", err)
 	}
-	bs = tester.downloader.TakeBlocks()
-	if len(bs) != 1 {
-		t.Fatalf("retrieved block mismatch: have %v, want %v", len(bs), 1)
-	}
-	if !tester.hasBlock(bs[0].RawBlock.ParentHash()) {
-		t.Fatalf("tester doesn't know about the origin hash")
+	if !tester.hasBlock(tester.peerHashes["valid"][0]) {
+		t.Fatalf("tester didn't accept known-parent block: %v", tester.peerBlocks["valid"][hashes[0]])
 	}
 }
 
 // Tests that if a malicious peers keeps sending us repeating hashes, we don't
 // loop indefinitely.
-func TestRepeatingHashAttack(t *testing.T) {
+func TestRepeatingHashAttack(t *testing.T) { // TODO: Is this thing valid??
+	tester := newTester()
+
 	// Create a valid chain, but drop the last link
-	hashes := createHashes(0, blockCacheLimit)
+	hashes := createHashes(blockCacheLimit, knownHash)
 	blocks := createBlocksFromHashes(hashes)
-	forged := hashes[:len(hashes)-1]
+	tester.newPeer("valid", hashes, blocks)
+	tester.newPeer("attack", hashes[:len(hashes)-1], blocks)
 
 	// Try and sync with the malicious node
-	tester := newTester(t, forged, blocks)
-	tester.newPeer("attack", big.NewInt(10000), forged[0])
-
 	errc := make(chan error)
 	go func() {
-		errc <- tester.sync("attack", hashes[0])
+		errc <- tester.sync("attack")
 	}()
-
 	// Make sure that syncing returns and does so with a failure
 	select {
 	case <-time.After(time.Second):
@@ -364,9 +439,7 @@ func TestRepeatingHashAttack(t *testing.T) {
 		}
 	}
 	// Ensure that a valid chain can still pass sync
-	tester.hashes = hashes
-	tester.newPeer("valid", big.NewInt(20000), hashes[0])
-	if err := tester.sync("valid", hashes[0]); err != nil {
+	if err := tester.sync("valid"); err != nil {
 		t.Fatalf("failed to synchronise blocks: %v", err)
 	}
 }
@@ -374,23 +447,22 @@ func TestRepeatingHashAttack(t *testing.T) {
 // Tests that if a malicious peers returns a non-existent block hash, it should
 // eventually time out and the sync reattempted.
 func TestNonExistingBlockAttack(t *testing.T) {
+	tester := newTester()
+
 	// Create a valid chain, but forge the last link
-	hashes := createHashes(0, blockCacheLimit)
+	hashes := createHashes(blockCacheLimit, knownHash)
 	blocks := createBlocksFromHashes(hashes)
-	origin := hashes[len(hashes)/2]
+	tester.newPeer("valid", hashes, blocks)
 
 	hashes[len(hashes)/2] = unknownHash
+	tester.newPeer("attack", hashes, blocks)
 
 	// Try and sync with the malicious node and check that it fails
-	tester := newTester(t, hashes, blocks)
-	tester.newPeer("attack", big.NewInt(10000), hashes[0])
-	if err := tester.sync("attack", hashes[0]); err != errPeersUnavailable {
+	if err := tester.sync("attack"); err != errPeersUnavailable {
 		t.Fatalf("synchronisation error mismatch: have %v, want %v", err, errPeersUnavailable)
 	}
 	// Ensure that a valid chain can still pass sync
-	hashes[len(hashes)/2] = origin
-	tester.newPeer("valid", big.NewInt(20000), hashes[0])
-	if err := tester.sync("valid", hashes[0]); err != nil {
+	if err := tester.sync("valid"); err != nil {
 		t.Fatalf("failed to synchronise blocks: %v", err)
 	}
 }
@@ -398,30 +470,28 @@ func TestNonExistingBlockAttack(t *testing.T) {
 // Tests that if a malicious peer is returning hashes in a weird order, that the
 // sync throttler doesn't choke on them waiting for the valid blocks.
 func TestInvalidHashOrderAttack(t *testing.T) {
+	tester := newTester()
+
 	// Create a valid long chain, but reverse some hashes within
-	hashes := createHashes(0, 4*blockCacheLimit)
+	hashes := createHashes(4*blockCacheLimit, knownHash)
 	blocks := createBlocksFromHashes(hashes)
+	tester.newPeer("valid", hashes, blocks)
 
 	chunk1 := make([]common.Hash, blockCacheLimit)
 	chunk2 := make([]common.Hash, blockCacheLimit)
 	copy(chunk1, hashes[blockCacheLimit:2*blockCacheLimit])
 	copy(chunk2, hashes[2*blockCacheLimit:3*blockCacheLimit])
 
-	reverse := make([]common.Hash, len(hashes))
-	copy(reverse, hashes)
-	copy(reverse[2*blockCacheLimit:], chunk1)
-	copy(reverse[blockCacheLimit:], chunk2)
+	copy(hashes[2*blockCacheLimit:], chunk1)
+	copy(hashes[blockCacheLimit:], chunk2)
+	tester.newPeer("attack", hashes, blocks)
 
 	// Try and sync with the malicious node and check that it fails
-	tester := newTester(t, reverse, blocks)
-	tester.newPeer("attack", big.NewInt(10000), reverse[0])
-	if _, err := tester.syncTake("attack", reverse[0]); err != ErrInvalidChain {
-		t.Fatalf("synchronisation error mismatch: have %v, want %v", err, ErrInvalidChain)
+	if err := tester.sync("attack"); err != errInvalidChain {
+		t.Fatalf("synchronisation error mismatch: have %v, want %v", err, errInvalidChain)
 	}
 	// Ensure that a valid chain can still pass sync
-	tester.hashes = hashes
-	tester.newPeer("valid", big.NewInt(20000), hashes[0])
-	if _, err := tester.syncTake("valid", hashes[0]); err != nil {
+	if err := tester.sync("valid"); err != nil {
 		t.Fatalf("failed to synchronise blocks: %v", err)
 	}
 }
@@ -429,17 +499,24 @@ func TestInvalidHashOrderAttack(t *testing.T) {
 // Tests that if a malicious peer makes up a random hash chain and tries to push
 // indefinitely, it actually gets caught with it.
 func TestMadeupHashChainAttack(t *testing.T) {
+	tester := newTester()
 	blockSoftTTL = 100 * time.Millisecond
 	crossCheckCycle = 25 * time.Millisecond
 
 	// Create a long chain of hashes without backing blocks
-	hashes := createHashes(0, 1024*blockCacheLimit)
+	hashes := createHashes(4*blockCacheLimit, knownHash)
+	blocks := createBlocksFromHashes(hashes)
+
+	tester.newPeer("valid", hashes, blocks)
+	tester.newPeer("attack", createHashes(1024*blockCacheLimit, knownHash), nil)
 
 	// Try and sync with the malicious node and check that it fails
-	tester := newTester(t, hashes, nil)
-	tester.newPeer("attack", big.NewInt(10000), hashes[0])
-	if _, err := tester.syncTake("attack", hashes[0]); err != ErrCrossCheckFailed {
-		t.Fatalf("synchronisation error mismatch: have %v, want %v", err, ErrCrossCheckFailed)
+	if err := tester.sync("attack"); err != errCrossCheckFailed {
+		t.Fatalf("synchronisation error mismatch: have %v, want %v", err, errCrossCheckFailed)
+	}
+	// Ensure that a valid chain can still pass sync
+	if err := tester.sync("valid"); err != nil {
+		t.Fatalf("failed to synchronise blocks: %v", err)
 	}
 }
 
@@ -449,14 +526,14 @@ func TestMadeupHashChainAttack(t *testing.T) {
 // one by one prevents reliable block/parent verification.
 func TestMadeupHashChainDrippingAttack(t *testing.T) {
 	// Create a random chain of hashes to drip
-	hashes := createHashes(0, 16*blockCacheLimit)
-	tester := newTester(t, hashes, nil)
+	hashes := createHashes(16*blockCacheLimit, knownHash)
+	tester := newTester()
 
 	// Try and sync with the attacker, one hash at a time
 	tester.maxHashFetch = 1
-	tester.newPeer("attack", big.NewInt(10000), hashes[0])
-	if _, err := tester.syncTake("attack", hashes[0]); err != ErrStallingPeer {
-		t.Fatalf("synchronisation error mismatch: have %v, want %v", err, ErrStallingPeer)
+	tester.newPeer("attack", hashes, nil)
+	if err := tester.sync("attack"); err != errStallingPeer {
+		t.Fatalf("synchronisation error mismatch: have %v, want %v", err, errStallingPeer)
 	}
 }
 
@@ -470,7 +547,7 @@ func TestMadeupBlockChainAttack(t *testing.T) {
 	crossCheckCycle = 25 * time.Millisecond
 
 	// Create a long chain of blocks and simulate an invalid chain by dropping every second
-	hashes := createHashes(0, 16*blockCacheLimit)
+	hashes := createHashes(16*blockCacheLimit, knownHash)
 	blocks := createBlocksFromHashes(hashes)
 
 	gapped := make([]common.Hash, len(hashes)/2)
@@ -478,18 +555,17 @@ func TestMadeupBlockChainAttack(t *testing.T) {
 		gapped[i] = hashes[2*i]
 	}
 	// Try and sync with the malicious node and check that it fails
-	tester := newTester(t, gapped, blocks)
-	tester.newPeer("attack", big.NewInt(10000), gapped[0])
-	if _, err := tester.syncTake("attack", gapped[0]); err != ErrCrossCheckFailed {
-		t.Fatalf("synchronisation error mismatch: have %v, want %v", err, ErrCrossCheckFailed)
+	tester := newTester()
+	tester.newPeer("attack", gapped, blocks)
+	if err := tester.sync("attack"); err != errCrossCheckFailed {
+		t.Fatalf("synchronisation error mismatch: have %v, want %v", err, errCrossCheckFailed)
 	}
 	// Ensure that a valid chain can still pass sync
 	blockSoftTTL = defaultBlockTTL
 	crossCheckCycle = defaultCrossCheckCycle
 
-	tester.hashes = hashes
-	tester.newPeer("valid", big.NewInt(20000), hashes[0])
-	if _, err := tester.syncTake("valid", hashes[0]); err != nil {
+	tester.newPeer("valid", hashes, blocks)
+	if err := tester.sync("valid"); err != nil {
 		t.Fatalf("failed to synchronise blocks: %v", err)
 	}
 }
@@ -498,6 +574,8 @@ func TestMadeupBlockChainAttack(t *testing.T) {
 // attacker make up a valid hashes for random blocks, but also forges the block
 // parents to point to existing hashes.
 func TestMadeupParentBlockChainAttack(t *testing.T) {
+	tester := newTester()
+
 	defaultBlockTTL := blockSoftTTL
 	defaultCrossCheckCycle := crossCheckCycle
 
@@ -505,25 +583,24 @@ func TestMadeupParentBlockChainAttack(t *testing.T) {
 	crossCheckCycle = 25 * time.Millisecond
 
 	// Create a long chain of blocks and simulate an invalid chain by dropping every second
-	hashes := createHashes(0, 16*blockCacheLimit)
+	hashes := createHashes(16*blockCacheLimit, knownHash)
 	blocks := createBlocksFromHashes(hashes)
-	forges := createBlocksFromHashes(hashes)
-	for hash, block := range forges {
-		block.ParentHeaderHash = hash // Simulate pointing to already known hash
+	tester.newPeer("valid", hashes, blocks)
+
+	for _, block := range blocks {
+		block.ParentHeaderHash = knownHash // Simulate pointing to already known hash
 	}
+	tester.newPeer("attack", hashes, blocks)
+
 	// Try and sync with the malicious node and check that it fails
-	tester := newTester(t, hashes, forges)
-	tester.newPeer("attack", big.NewInt(10000), hashes[0])
-	if _, err := tester.syncTake("attack", hashes[0]); err != ErrCrossCheckFailed {
-		t.Fatalf("synchronisation error mismatch: have %v, want %v", err, ErrCrossCheckFailed)
+	if err := tester.sync("attack"); err != errCrossCheckFailed {
+		t.Fatalf("synchronisation error mismatch: have %v, want %v", err, errCrossCheckFailed)
 	}
 	// Ensure that a valid chain can still pass sync
 	blockSoftTTL = defaultBlockTTL
 	crossCheckCycle = defaultCrossCheckCycle
 
-	tester.blocks = blocks
-	tester.newPeer("valid", big.NewInt(20000), hashes[0])
-	if _, err := tester.syncTake("valid", hashes[0]); err != nil {
+	if err := tester.sync("valid"); err != nil {
 		t.Fatalf("failed to synchronise blocks: %v", err)
 	}
 }
@@ -532,68 +609,81 @@ func TestMadeupParentBlockChainAttack(t *testing.T) {
 // the downloader, it will not keep refetching the same chain indefinitely, but
 // gradually block pieces of it, until it's head is also blocked.
 func TestBannedChainStarvationAttack(t *testing.T) {
-	// Construct a valid chain, but ban one of the hashes in it
-	hashes := createHashes(0, 8*blockCacheLimit)
-	hashes[len(hashes)/2+23] = bannedHash // weird index to have non multiple of ban chunk size
+	// Create the tester and ban the selected hash
+	tester := newTester()
+	tester.downloader.banned.Add(bannedHash)
 
+	// Construct a valid chain, for it and ban the fork
+	hashes := createHashes(8*blockCacheLimit, knownHash)
 	blocks := createBlocksFromHashes(hashes)
+	tester.newPeer("valid", hashes, blocks)
 
-	// Create the tester and ban the selected hash
-	tester := newTester(t, hashes, blocks)
-	tester.downloader.banned.Add(bannedHash)
+	fork := len(hashes)/2 - 23
+	hashes = append(createHashes(4*blockCacheLimit, bannedHash), hashes[fork:]...)
+	blocks = createBlocksFromHashes(hashes)
+	tester.newPeer("attack", hashes, blocks)
 
 	// Iteratively try to sync, and verify that the banned hash list grows until
 	// the head of the invalid chain is blocked too.
-	tester.newPeer("attack", big.NewInt(10000), hashes[0])
 	for banned := tester.downloader.banned.Size(); ; {
 		// Try to sync with the attacker, check hash chain failure
-		if _, err := tester.syncTake("attack", hashes[0]); err != ErrInvalidChain {
-			t.Fatalf("synchronisation error mismatch: have %v, want %v", err, ErrInvalidChain)
+		if err := tester.sync("attack"); err != errInvalidChain {
+			if tester.downloader.banned.Has(hashes[0]) && err == errBannedHead {
+				break
+			}
+			t.Fatalf("synchronisation error mismatch: have %v, want %v", err, errInvalidChain)
 		}
 		// Check that the ban list grew with at least 1 new item, or all banned
 		bans := tester.downloader.banned.Size()
 		if bans < banned+1 {
-			if tester.downloader.banned.Has(hashes[0]) {
-				break
-			}
 			t.Fatalf("ban count mismatch: have %v, want %v+", bans, banned+1)
 		}
 		banned = bans
 	}
 	// Check that after banning an entire chain, bad peers get dropped
-	if err := tester.newPeer("new attacker", big.NewInt(10000), hashes[0]); err != errBannedHead {
+	if err := tester.newPeer("new attacker", hashes, blocks); err != errBannedHead {
 		t.Fatalf("peer registration mismatch: have %v, want %v", err, errBannedHead)
 	}
-	if peer := tester.downloader.peers.Peer("net attacker"); peer != nil {
+	if peer := tester.downloader.peers.Peer("new attacker"); peer != nil {
 		t.Fatalf("banned attacker registered: %v", peer)
 	}
+	// Ensure that a valid chain can still pass sync
+	if err := tester.sync("valid"); err != nil {
+		t.Fatalf("failed to synchronise blocks: %v", err)
+	}
 }
 
 // Tests that if a peer sends excessively many/large invalid chains that are
 // gradually banned, it will have an upper limit on the consumed memory and also
 // the origin bad hashes will not be evacuated.
 func TestBannedChainMemoryExhaustionAttack(t *testing.T) {
+	// Create the tester and ban the selected hash
+	tester := newTester()
+	tester.downloader.banned.Add(bannedHash)
+
 	// Reduce the test size a bit
+	defaultMaxBlockFetch := MaxBlockFetch
+	defaultMaxBannedHashes := maxBannedHashes
+
 	MaxBlockFetch = 4
 	maxBannedHashes = 256
 
 	// Construct a banned chain with more chunks than the ban limit
-	hashes := createHashes(0, maxBannedHashes*MaxBlockFetch)
-	hashes[len(hashes)-1] = bannedHash // weird index to have non multiple of ban chunk size
-
+	hashes := createHashes(8*blockCacheLimit, knownHash)
 	blocks := createBlocksFromHashes(hashes)
+	tester.newPeer("valid", hashes, blocks)
 
-	// Create the tester and ban the selected hash
-	tester := newTester(t, hashes, blocks)
-	tester.downloader.banned.Add(bannedHash)
+	fork := len(hashes)/2 - 23
+	hashes = append(createHashes(maxBannedHashes*MaxBlockFetch, bannedHash), hashes[fork:]...)
+	blocks = createBlocksFromHashes(hashes)
+	tester.newPeer("attack", hashes, blocks)
 
 	// Iteratively try to sync, and verify that the banned hash list grows until
 	// the head of the invalid chain is blocked too.
-	tester.newPeer("attack", big.NewInt(10000), hashes[0])
 	for {
 		// Try to sync with the attacker, check hash chain failure
-		if _, err := tester.syncTake("attack", hashes[0]); err != ErrInvalidChain {
-			t.Fatalf("synchronisation error mismatch: have %v, want %v", err, ErrInvalidChain)
+		if err := tester.sync("attack"); err != errInvalidChain {
+			t.Fatalf("synchronisation error mismatch: have %v, want %v", err, errInvalidChain)
 		}
 		// Short circuit if the entire chain was banned
 		if tester.downloader.banned.Has(hashes[0]) {
@@ -609,4 +699,91 @@ func TestBannedChainMemoryExhaustionAttack(t *testing.T) {
 			}
 		}
 	}
+	// Ensure that a valid chain can still pass sync
+	MaxBlockFetch = defaultMaxBlockFetch
+	maxBannedHashes = defaultMaxBannedHashes
+
+	if err := tester.sync("valid"); err != nil {
+		t.Fatalf("failed to synchronise blocks: %v", err)
+	}
+}
+
+// Tests that misbehaving peers are disconnected, whilst behaving ones are not.
+func TestHashAttackerDropping(t *testing.T) {
+	// Define the disconnection requirement for individual hash fetch errors
+	tests := []struct {
+		result error
+		drop   bool
+	}{
+		{nil, false},                  // Sync succeeded, all is well
+		{errBusy, false},              // Sync is already in progress, no problem
+		{errUnknownPeer, false},       // Peer is unknown, was already dropped, don't double drop
+		{errBadPeer, true},            // Peer was deemed bad for some reason, drop it
+		{errStallingPeer, true},       // Peer was detected to be stalling, drop it
+		{errBannedHead, true},         // Peer's head hash is a known bad hash, drop it
+		{errNoPeers, false},           // No peers to download from, soft race, no issue
+		{errPendingQueue, false},      // There are blocks still cached, wait to exhaust, no issue
+		{errTimeout, true},            // No hashes received in due time, drop the peer
+		{errEmptyHashSet, true},       // No hashes were returned as a response, drop as it's a dead end
+		{errPeersUnavailable, true},   // Nobody had the advertised blocks, drop the advertiser
+		{errInvalidChain, true},       // Hash chain was detected as invalid, definitely drop
+		{errCrossCheckFailed, true},   // Hash-origin failed to pass a block cross check, drop
+		{errCancelHashFetch, false},   // Synchronisation was canceled, origin may be innocent, don't drop
+		{errCancelBlockFetch, false},  // Synchronisation was canceled, origin may be innocent, don't drop
+		{errCancelChainImport, false}, // Synchronisation was canceled, origin may be innocent, don't drop
+	}
+	// Run the tests and check disconnection status
+	tester := newTester()
+	for i, tt := range tests {
+		// Register a new peer and ensure it's presence
+		id := fmt.Sprintf("test %d", i)
+		if err := tester.newPeer(id, []common.Hash{knownHash}, nil); err != nil {
+			t.Fatalf("test %d: failed to register new peer: %v", i, err)
+		}
+		if _, ok := tester.peerHashes[id]; !ok {
+			t.Fatalf("test %d: registered peer not found", i)
+		}
+		// Simulate a synchronisation and check the required result
+		tester.downloader.synchroniseMock = func(string, common.Hash) error { return tt.result }
+
+		tester.downloader.Synchronise(id, knownHash)
+		if _, ok := tester.peerHashes[id]; !ok != tt.drop {
+			t.Errorf("test %d: peer drop mismatch for %v: have %v, want %v", i, tt.result, !ok, tt.drop)
+		}
+	}
+}
+
+// Tests that feeding bad blocks will result in a peer drop.
+func TestBlockAttackerDropping(t *testing.T) {
+	// Define the disconnection requirement for individual block import errors
+	tests := []struct {
+		failure bool
+		drop    bool
+	}{{true, true}, {false, false}}
+
+	// Run the tests and check disconnection status
+	tester := newTester()
+	for i, tt := range tests {
+		// Register a new peer and ensure it's presence
+		id := fmt.Sprintf("test %d", i)
+		if err := tester.newPeer(id, []common.Hash{common.Hash{}}, nil); err != nil {
+			t.Fatalf("test %d: failed to register new peer: %v", i, err)
+		}
+		if _, ok := tester.peerHashes[id]; !ok {
+			t.Fatalf("test %d: registered peer not found", i)
+		}
+		// Assemble a good or bad block, depending of the test
+		raw := createBlock(1, knownHash, common.Hash{})
+		if tt.failure {
+			raw = createBlock(1, unknownHash, common.Hash{})
+		}
+		block := &Block{OriginPeer: id, RawBlock: raw}
+
+		// Simulate block processing and check the result
+		tester.downloader.queue.blockCache[0] = block
+		tester.downloader.process()
+		if _, ok := tester.peerHashes[id]; !ok != tt.drop {
+			t.Errorf("test %d: peer drop mismatch for %v: have %v, want %v", i, tt.failure, !ok, tt.drop)
+		}
+	}
 }
diff --git a/eth/downloader/peer.go b/eth/downloader/peer.go
index 9614a6951..f36e133e4 100644
--- a/eth/downloader/peer.go
+++ b/eth/downloader/peer.go
@@ -74,7 +74,7 @@ func (p *peer) Fetch(request *fetchRequest) error {
 	for hash, _ := range request.Hashes {
 		hashes = append(hashes, hash)
 	}
-	p.getBlocks(hashes)
+	go p.getBlocks(hashes)
 
 	return nil
 }
diff --git a/eth/downloader/queue.go b/eth/downloader/queue.go
index 7abbd42fd..903f043eb 100644
--- a/eth/downloader/queue.go
+++ b/eth/downloader/queue.go
@@ -320,7 +320,7 @@ func (q *queue) Deliver(id string, blocks []*types.Block) (err error) {
 		// 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
+			return errInvalidChain
 		}
 		// Otherwise merge the block and mark the hash block
 		q.blockCache[index] = &Block{
diff --git a/eth/downloader/queue_test.go b/eth/downloader/queue_test.go
deleted file mode 100644
index ee6141f71..000000000
--- a/eth/downloader/queue_test.go
+++ /dev/null
@@ -1,30 +0,0 @@
-package downloader
-
-import (
-	"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
-}
diff --git a/eth/handler.go b/eth/handler.go
index f002727f3..ec4f2d53a 100644
--- a/eth/handler.go
+++ b/eth/handler.go
@@ -68,12 +68,11 @@ type ProtocolManager struct {
 
 // NewProtocolManager returns a new ethereum sub protocol manager. The Ethereum sub protocol manages peers capable
 // with the ethereum network.
-func NewProtocolManager(protocolVersion, networkId int, mux *event.TypeMux, txpool txPool, chainman *core.ChainManager, downloader *downloader.Downloader) *ProtocolManager {
+func NewProtocolManager(protocolVersion, networkId int, mux *event.TypeMux, txpool txPool, chainman *core.ChainManager) *ProtocolManager {
 	manager := &ProtocolManager{
 		eventMux:   mux,
 		txpool:     txpool,
 		chainman:   chainman,
-		downloader: downloader,
 		peers:      newPeerSet(),
 		newPeerCh:  make(chan *peer, 1),
 		newHashCh:  make(chan []*blockAnnounce, 1),
@@ -81,6 +80,7 @@ func NewProtocolManager(protocolVersion, networkId int, mux *event.TypeMux, txpo
 		txsyncCh:   make(chan *txsync),
 		quitSync:   make(chan struct{}),
 	}
+	manager.downloader = downloader.New(manager.eventMux, manager.chainman.HasBlock, manager.chainman.GetBlock, manager.chainman.InsertChain, manager.removePeer)
 	manager.SubProtocol = p2p.Protocol{
 		Name:    "eth",
 		Version: uint(protocolVersion),
diff --git a/eth/protocol_test.go b/eth/protocol_test.go
index bbea9fc11..69d487c71 100644
--- a/eth/protocol_test.go
+++ b/eth/protocol_test.go
@@ -11,7 +11,6 @@ import (
 	"github.com/ethereum/go-ethereum/core"
 	"github.com/ethereum/go-ethereum/core/types"
 	"github.com/ethereum/go-ethereum/crypto"
-	"github.com/ethereum/go-ethereum/eth/downloader"
 	"github.com/ethereum/go-ethereum/ethdb"
 	"github.com/ethereum/go-ethereum/event"
 	"github.com/ethereum/go-ethereum/p2p"
@@ -168,8 +167,7 @@ func newProtocolManagerForTesting(txAdded chan<- []*types.Transaction) *Protocol
 		db, _    = ethdb.NewMemDatabase()
 		chain, _ = core.NewChainManager(core.GenesisBlock(0, db), db, db, core.FakePow{}, em)
 		txpool   = &fakeTxPool{added: txAdded}
-		dl       = downloader.New(em, chain.HasBlock, chain.GetBlock)
-		pm       = NewProtocolManager(ProtocolVersion, 0, em, txpool, chain, dl)
+		pm       = NewProtocolManager(ProtocolVersion, 0, em, txpool, chain)
 	)
 	pm.Start()
 	return pm
diff --git a/eth/sync.go b/eth/sync.go
index 8fee21d7b..a3b177a4d 100644
--- a/eth/sync.go
+++ b/eth/sync.go
@@ -1,14 +1,11 @@
 package eth
 
 import (
-	"math"
 	"math/rand"
-	"sync/atomic"
 	"time"
 
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/core/types"
-	"github.com/ethereum/go-ethereum/eth/downloader"
 	"github.com/ethereum/go-ethereum/logger"
 	"github.com/ethereum/go-ethereum/logger/glog"
 	"github.com/ethereum/go-ethereum/p2p/discover"
@@ -16,12 +13,10 @@ import (
 
 const (
 	forceSyncCycle      = 10 * time.Second       // Time interval to force syncs, even if few peers are available
-	blockProcCycle      = 500 * time.Millisecond // Time interval to check for new blocks to process
 	notifyCheckCycle    = 100 * time.Millisecond // Time interval to allow hash notifies to fulfill before hard fetching
 	notifyArriveTimeout = 500 * time.Millisecond // Time allowance before an announced block is explicitly requested
 	notifyFetchTimeout  = 5 * time.Second        // Maximum alloted time to return an explicitly requested block
 	minDesiredPeerCount = 5                      // Amount of peers desired to start syncing
-	blockProcAmount     = 256
 
 	// This is the target size for the packs of transactions sent by txsyncLoop.
 	// A pack can get larger than this if a single transactions exceeds this size.
@@ -176,7 +171,7 @@ func (pm *ProtocolManager) fetcher() {
 			// Send out all block requests
 			for peer, hashes := range request {
 				glog.V(logger.Debug).Infof("Explicitly fetching %d blocks from %s", len(hashes), peer.id)
-				peer.requestBlocks(hashes)
+				go peer.requestBlocks(hashes)
 			}
 			request = make(map[*peer][]common.Hash)
 
@@ -219,7 +214,7 @@ func (pm *ProtocolManager) fetcher() {
 				if announce := pending[hash]; announce != nil {
 					// Drop the block if it surely cannot fit
 					if pm.chainman.HasBlock(hash) || !pm.chainman.HasBlock(block.ParentHash()) {
-						delete(pending, hash)
+						// delete(pending, hash) // if we drop, it will re-fetch it, wait for timeout?
 						continue
 					}
 					// Otherwise accumulate for import
@@ -255,10 +250,10 @@ func (pm *ProtocolManager) fetcher() {
 // syncer is responsible for periodically synchronising with the network, both
 // downloading hashes and blocks as well as retrieving cached ones.
 func (pm *ProtocolManager) syncer() {
-	forceSync := time.Tick(forceSyncCycle)
-	blockProc := time.Tick(blockProcCycle)
-	blockProcPend := int32(0)
+	// Abort any pending syncs if we terminate
+	defer pm.downloader.Cancel()
 
+	forceSync := time.Tick(forceSyncCycle)
 	for {
 		select {
 		case <-pm.newPeerCh:
@@ -272,55 +267,12 @@ func (pm *ProtocolManager) syncer() {
 			// Force a sync even if not enough peers are present
 			go pm.synchronise(pm.peers.BestPeer())
 
-		case <-blockProc:
-			// Try to pull some blocks from the downloaded
-			if atomic.CompareAndSwapInt32(&blockProcPend, 0, 1) {
-				go func() {
-					pm.processBlocks()
-					atomic.StoreInt32(&blockProcPend, 0)
-				}()
-			}
-
 		case <-pm.quitSync:
 			return
 		}
 	}
 }
 
-// processBlocks retrieves downloaded blocks from the download cache and tries
-// to construct the local block chain with it. Note, since the block retrieval
-// order matters, access to this function *must* be synchronized/serialized.
-func (pm *ProtocolManager) processBlocks() error {
-	pm.wg.Add(1)
-	defer pm.wg.Done()
-
-	// Short circuit if no blocks are available for insertion
-	blocks := pm.downloader.TakeBlocks()
-	if len(blocks) == 0 {
-		return nil
-	}
-	glog.V(logger.Debug).Infof("Inserting chain with %d blocks (#%v - #%v)\n", len(blocks), blocks[0].RawBlock.Number(), blocks[len(blocks)-1].RawBlock.Number())
-
-	for len(blocks) != 0 && !pm.quit {
-		// Retrieve the first batch of blocks to insert
-		max := int(math.Min(float64(len(blocks)), float64(blockProcAmount)))
-		raw := make(types.Blocks, 0, max)
-		for _, block := range blocks[:max] {
-			raw = append(raw, block.RawBlock)
-		}
-		// Try to inset the blocks, drop the originating peer if there's an error
-		index, err := pm.chainman.InsertChain(raw)
-		if err != nil {
-			glog.V(logger.Debug).Infoln("Downloaded block import failed:", err)
-			pm.removePeer(blocks[index].OriginPeer)
-			pm.downloader.Cancel()
-			return err
-		}
-		blocks = blocks[max:]
-	}
-	return nil
-}
-
 // synchronise tries to sync up our local block chain with a remote peer, both
 // adding various sanity checks as well as wrapping it with various log entries.
 func (pm *ProtocolManager) synchronise(peer *peer) {
@@ -332,33 +284,6 @@ func (pm *ProtocolManager) synchronise(peer *peer) {
 	if peer.Td().Cmp(pm.chainman.Td()) <= 0 {
 		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
-	head := peer.Head()
-	if pm.chainman.HasBlock(head) {
-		glog.V(logger.Debug).Infoln("Synchronisation canceled: head already known")
-		return
-	}
-	// Get the hashes from the peer (synchronously)
-	glog.V(logger.Detail).Infof("Attempting synchronisation: %v, 0x%x", peer.id, head)
-
-	err := pm.downloader.Synchronise(peer.id, head)
-	switch err {
-	case nil:
-		glog.V(logger.Detail).Infof("Synchronisation completed")
-
-	case downloader.ErrBusy:
-		glog.V(logger.Detail).Infof("Synchronisation already in progress")
-
-	case downloader.ErrTimeout, downloader.ErrBadPeer, downloader.ErrEmptyHashSet, downloader.ErrInvalidChain, downloader.ErrCrossCheckFailed:
-		glog.V(logger.Debug).Infof("Removing peer %v: %v", peer.id, err)
-		pm.removePeer(peer.id)
-
-	case downloader.ErrPendingQueue:
-		glog.V(logger.Debug).Infoln("Synchronisation aborted:", err)
-
-	default:
-		glog.V(logger.Warn).Infof("Synchronisation failed: %v", err)
-	}
+	// Otherwise try to sync with the downloader
+	pm.downloader.Synchronise(peer.id, peer.Head())
 }