Merge pull request #1275 from karalabe/optimise-fetcher

eth/fetcher: separate the announce based sync into its own package

2 files changed, 765 insertions, 0 deletions

diff --git a/eth/fetcher/fetcher.go b/eth/fetcher/fetcher.go
new file mode 100644
index 000000000..98170cf79
--- /dev/null
+++ b/eth/fetcher/fetcher.go
@@ -0,0 +1,368 @@
+// Package fetcher contains the block announcement based synchonisation.
+package fetcher
+
+import (
+	"errors"
+	"math/rand"
+	"time"
+
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/logger"
+	"github.com/ethereum/go-ethereum/logger/glog"
+	"gopkg.in/karalabe/cookiejar.v2/collections/prque"
+)
+
+const (
+	arriveTimeout = 500 * time.Millisecond // Time allowance before an announced block is explicitly requested
+	fetchTimeout  = 5 * time.Second        // Maximum alloted time to return an explicitly requested block
+	maxUncleDist  = 7                      // Maximum allowed backward distance from the chain head
+	maxQueueDist  = 256                    // Maximum allowed distance from the chain head to queue
+)
+
+var (
+	errTerminated = errors.New("terminated")
+)
+
+// blockRetrievalFn is a callback type for retrieving a block from the local chain.
+type blockRetrievalFn func(common.Hash) *types.Block
+
+// blockRequesterFn is a callback type for sending a block retrieval request.
+type blockRequesterFn func([]common.Hash) error
+
+// blockValidatorFn is a callback type to verify a block's header for fast propagation.
+type blockValidatorFn func(block *types.Block, parent *types.Block) error
+
+// blockBroadcasterFn is a callback type for broadcasting a block to connected peers.
+type blockBroadcasterFn func(block *types.Block, propagate bool)
+
+// chainHeightFn is a callback type to retrieve the current chain height.
+type chainHeightFn func() uint64
+
+// chainInsertFn is a callback type to insert a batch of blocks into the local chain.
+type chainInsertFn func(types.Blocks) (int, error)
+
+// peerDropFn is a callback type for dropping a peer detected as malicious.
+type peerDropFn func(id string)
+
+// announce is the hash notification of the availability of a new block in the
+// network.
+type announce struct {
+	hash common.Hash // Hash of the block being announced
+	time time.Time   // Timestamp of the announcement
+
+	origin string           // Identifier of the peer originating the notification
+	fetch  blockRequesterFn // Fetcher function to retrieve
+}
+
+// inject represents a schedules import operation.
+type inject struct {
+	origin string
+	block  *types.Block
+}
+
+// Fetcher is responsible for accumulating block announcements from various peers
+// and scheduling them for retrieval.
+type Fetcher struct {
+	// Various event channels
+	notify chan *announce
+	inject chan *inject
+	filter chan chan []*types.Block
+	done   chan common.Hash
+	quit   chan struct{}
+
+	// Announce states
+	announced map[common.Hash][]*announce // Announced blocks, scheduled for fetching
+	fetching  map[common.Hash]*announce   // Announced blocks, currently fetching
+
+	// Block cache
+	queue  *prque.Prque             // Queue containing the import operations (block number sorted)
+	queued map[common.Hash]struct{} // Presence set of already queued blocks (to dedup imports)
+
+	// Callbacks
+	getBlock       blockRetrievalFn   // Retrieves a block from the local chain
+	validateBlock  blockValidatorFn   // Checks if a block's headers have a valid proof of work
+	broadcastBlock blockBroadcasterFn // Broadcasts a block to connected peers
+	chainHeight    chainHeightFn      // Retrieves the current chain's height
+	insertChain    chainInsertFn      // Injects a batch of blocks into the chain
+	dropPeer       peerDropFn         // Drops a peer for misbehaving
+}
+
+// New creates a block fetcher to retrieve blocks based on hash announcements.
+func New(getBlock blockRetrievalFn, validateBlock blockValidatorFn, broadcastBlock blockBroadcasterFn, chainHeight chainHeightFn, insertChain chainInsertFn, dropPeer peerDropFn) *Fetcher {
+	return &Fetcher{
+		notify:         make(chan *announce),
+		inject:         make(chan *inject),
+		filter:         make(chan chan []*types.Block),
+		done:           make(chan common.Hash),
+		quit:           make(chan struct{}),
+		announced:      make(map[common.Hash][]*announce),
+		fetching:       make(map[common.Hash]*announce),
+		queue:          prque.New(),
+		queued:         make(map[common.Hash]struct{}),
+		getBlock:       getBlock,
+		validateBlock:  validateBlock,
+		broadcastBlock: broadcastBlock,
+		chainHeight:    chainHeight,
+		insertChain:    insertChain,
+		dropPeer:       dropPeer,
+	}
+}
+
+// Start boots up the announcement based synchoniser, accepting and processing
+// hash notifications and block fetches until termination requested.
+func (f *Fetcher) Start() {
+	go f.loop()
+}
+
+// Stop terminates the announcement based synchroniser, canceling all pending
+// operations.
+func (f *Fetcher) Stop() {
+	close(f.quit)
+}
+
+// Notify announces the fetcher of the potential availability of a new block in
+// the network.
+func (f *Fetcher) Notify(peer string, hash common.Hash, time time.Time, fetcher blockRequesterFn) error {
+	block := &announce{
+		hash:   hash,
+		time:   time,
+		origin: peer,
+		fetch:  fetcher,
+	}
+	select {
+	case f.notify <- block:
+		return nil
+	case <-f.quit:
+		return errTerminated
+	}
+}
+
+// Enqueue tries to fill gaps the the fetcher's future import queue.
+func (f *Fetcher) Enqueue(peer string, block *types.Block) error {
+	op := &inject{
+		origin: peer,
+		block:  block,
+	}
+	select {
+	case f.inject <- op:
+		return nil
+	case <-f.quit:
+		return errTerminated
+	}
+}
+
+// Filter extracts all the blocks that were explicitly requested by the fetcher,
+// returning those that should be handled differently.
+func (f *Fetcher) Filter(blocks types.Blocks) types.Blocks {
+	// Send the filter channel to the fetcher
+	filter := make(chan []*types.Block)
+
+	select {
+	case f.filter <- filter:
+	case <-f.quit:
+		return nil
+	}
+	// Request the filtering of the block list
+	select {
+	case filter <- blocks:
+	case <-f.quit:
+		return nil
+	}
+	// Retrieve the blocks remaining after filtering
+	select {
+	case blocks := <-filter:
+		return blocks
+	case <-f.quit:
+		return nil
+	}
+}
+
+// Loop is the main fetcher loop, checking and processing various notification
+// events.
+func (f *Fetcher) loop() {
+	// Iterate the block fetching until a quit is requested
+	fetch := time.NewTimer(0)
+	for {
+		// Clean up any expired block fetches
+		for hash, announce := range f.fetching {
+			if time.Since(announce.time) > fetchTimeout {
+				delete(f.announced, hash)
+				delete(f.fetching, hash)
+			}
+		}
+		// Import any queued blocks that could potentially fit
+		height := f.chainHeight()
+		for !f.queue.Empty() {
+			op := f.queue.PopItem().(*inject)
+			number := op.block.NumberU64()
+
+			// If too high up the chain or phase, continue later
+			if number > height+1 {
+				f.queue.Push(op, -float32(op.block.NumberU64()))
+				break
+			}
+			// Otherwise if fresh and still unknown, try and import
+			if number+maxUncleDist < height || f.getBlock(op.block.Hash()) != nil {
+				continue
+			}
+			f.insert(op.origin, op.block)
+		}
+		// Wait for an outside event to occur
+		select {
+		case <-f.quit:
+			// Fetcher terminating, abort all operations
+			return
+
+		case notification := <-f.notify:
+			// A block was announced, schedule if it's not yet downloading
+			if _, ok := f.fetching[notification.hash]; ok {
+				break
+			}
+			f.announced[notification.hash] = append(f.announced[notification.hash], notification)
+			if len(f.announced) == 1 {
+				f.reschedule(fetch)
+			}
+
+		case op := <-f.inject:
+			// A direct block insertion was requested, try and fill any pending gaps
+			f.enqueue(op.origin, op.block)
+
+		case hash := <-f.done:
+			// A pending import finished, remove all traces of the notification
+			delete(f.announced, hash)
+			delete(f.fetching, hash)
+			delete(f.queued, hash)
+
+		case <-fetch.C:
+			// At least one block's timer ran out, check for needing retrieval
+			request := make(map[string][]common.Hash)
+
+			for hash, announces := range f.announced {
+				if time.Since(announces[0].time) > arriveTimeout {
+					announce := announces[rand.Intn(len(announces))]
+					if f.getBlock(hash) == nil {
+						request[announce.origin] = append(request[announce.origin], hash)
+						f.fetching[hash] = announce
+					}
+					delete(f.announced, hash)
+				}
+			}
+			// Send out all block requests
+			for _, hashes := range request {
+				go f.fetching[hashes[0]].fetch(hashes)
+			}
+			// Schedule the next fetch if blocks are still pending
+			f.reschedule(fetch)
+
+		case filter := <-f.filter:
+			// Blocks arrived, extract any explicit fetches, return all else
+			var blocks types.Blocks
+			select {
+			case blocks = <-filter:
+			case <-f.quit:
+				return
+			}
+
+			explicit, download := []*types.Block{}, []*types.Block{}
+			for _, block := range blocks {
+				hash := block.Hash()
+
+				// Filter explicitly requested blocks from hash announcements
+				if _, ok := f.fetching[hash]; ok {
+					// Discard if already imported by other means
+					if f.getBlock(hash) == nil {
+						explicit = append(explicit, block)
+					} else {
+						delete(f.fetching, hash)
+					}
+				} else {
+					download = append(download, block)
+				}
+			}
+
+			select {
+			case filter <- download:
+			case <-f.quit:
+				return
+			}
+			// Schedule the retrieved blocks for ordered import
+			for _, block := range explicit {
+				if announce := f.fetching[block.Hash()]; announce != nil {
+					f.enqueue(announce.origin, block)
+				}
+			}
+		}
+	}
+}
+
+// reschedule resets the specified fetch timer to the next announce timeout.
+func (f *Fetcher) reschedule(fetch *time.Timer) {
+	// Short circuit if no blocks are announced
+	if len(f.announced) == 0 {
+		return
+	}
+	// Otherwise find the earliest expiring announcement
+	earliest := time.Now()
+	for _, announces := range f.announced {
+		if earliest.After(announces[0].time) {
+			earliest = announces[0].time
+		}
+	}
+	fetch.Reset(arriveTimeout - time.Since(earliest))
+}
+
+// enqueue schedules a new future import operation, if the block to be imported
+// has not yet been seen.
+func (f *Fetcher) enqueue(peer string, block *types.Block) {
+	hash := block.Hash()
+
+	// Discard any past or too distant blocks
+	if dist := int64(block.NumberU64()) - int64(f.chainHeight()); dist < -maxUncleDist || dist > maxQueueDist {
+		glog.V(logger.Detail).Infof("Peer %s: discarded block #%d [%x], distance %d", peer, block.NumberU64(), hash.Bytes()[:4], dist)
+		return
+	}
+	// Schedule the block for future importing
+	if _, ok := f.queued[hash]; !ok {
+		f.queued[hash] = struct{}{}
+		f.queue.Push(&inject{origin: peer, block: block}, -float32(block.NumberU64()))
+
+		if glog.V(logger.Debug) {
+			glog.Infof("Peer %s: queued block #%d [%x], total %v", peer, block.NumberU64(), hash.Bytes()[:4], f.queue.Size())
+		}
+	}
+}
+
+// insert spawns a new goroutine to run a block insertion into the chain. If the
+// block's number is at the same height as the current import phase, if updates
+// the phase states accordingly.
+func (f *Fetcher) insert(peer string, block *types.Block) {
+	hash := block.Hash()
+
+	// Run the import on a new thread
+	glog.V(logger.Debug).Infof("Peer %s: importing block #%d [%x]", peer, block.NumberU64(), hash[:4])
+	go func() {
+		defer func() { f.done <- hash }()
+
+		// If the parent's unknown, abort insertion
+		parent := f.getBlock(block.ParentHash())
+		if parent == nil {
+			return
+		}
+		// Quickly validate the header and propagate the block if it passes
+		if err := f.validateBlock(block, parent); err != nil {
+			glog.V(logger.Debug).Infof("Peer %s: block #%d [%x] verification failed: %v", peer, block.NumberU64(), hash[:4], err)
+			f.dropPeer(peer)
+			return
+		}
+		go f.broadcastBlock(block, true)
+
+		// Run the actual import and log any issues
+		if _, err := f.insertChain(types.Blocks{block}); err != nil {
+			glog.V(logger.Warn).Infof("Peer %s: block #%d [%x] import failed: %v", peer, block.NumberU64(), hash[:4], err)
+			return
+		}
+		// If import succeeded, broadcast the block
+		go f.broadcastBlock(block, false)
+	}()
+}
diff --git a/eth/fetcher/fetcher_test.go b/eth/fetcher/fetcher_test.go
new file mode 100644
index 000000000..0d069ac65
--- /dev/null
+++ b/eth/fetcher/fetcher_test.go
@@ -0,0 +1,397 @@
+package fetcher
+
+import (
+	"encoding/binary"
+	"errors"
+	"math/big"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/core/types"
+)
+
+var (
+	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)
+)
+
+// 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] = root
+
+	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)
+	block.HeaderHash = hash
+	block.ParentHeaderHash = parent
+	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)
+}
+
+// createBlocksFromHashes assembles a collection of blocks, each having a correct
+// place in the given hash chain.
+func createBlocksFromHashes(hashes []common.Hash) map[common.Hash]*types.Block {
+	blocks := make(map[common.Hash]*types.Block)
+	for i := 0; i < len(hashes); i++ {
+		parent := knownHash
+		if i < len(hashes)-1 {
+			parent = hashes[i+1]
+		}
+		blocks[hashes[i]] = createBlock(len(hashes)-i, parent, hashes[i])
+	}
+	return blocks
+}
+
+// fetcherTester is a test simulator for mocking out local block chain.
+type fetcherTester struct {
+	fetcher *Fetcher
+
+	hashes []common.Hash                // Hash chain belonging to the tester
+	blocks map[common.Hash]*types.Block // Blocks belonging to the tester
+
+	lock sync.RWMutex
+}
+
+// newTester creates a new fetcher test mocker.
+func newTester() *fetcherTester {
+	tester := &fetcherTester{
+		hashes: []common.Hash{knownHash},
+		blocks: map[common.Hash]*types.Block{knownHash: genesis},
+	}
+	tester.fetcher = New(tester.getBlock, tester.verifyBlock, tester.broadcastBlock, tester.chainHeight, tester.insertChain, tester.dropPeer)
+	tester.fetcher.Start()
+
+	return tester
+}
+
+// getBlock retrieves a block from the tester's block chain.
+func (f *fetcherTester) getBlock(hash common.Hash) *types.Block {
+	f.lock.RLock()
+	defer f.lock.RUnlock()
+
+	return f.blocks[hash]
+}
+
+// verifyBlock is a nop placeholder for the block header verification.
+func (f *fetcherTester) verifyBlock(block *types.Block, parent *types.Block) error {
+	return nil
+}
+
+// broadcastBlock is a nop placeholder for the block broadcasting.
+func (f *fetcherTester) broadcastBlock(block *types.Block, propagate bool) {
+}
+
+// chainHeight retrieves the current height (block number) of the chain.
+func (f *fetcherTester) chainHeight() uint64 {
+	f.lock.RLock()
+	defer f.lock.RUnlock()
+
+	return f.blocks[f.hashes[len(f.hashes)-1]].NumberU64()
+}
+
+// insertChain injects a new blocks into the simulated chain.
+func (f *fetcherTester) insertChain(blocks types.Blocks) (int, error) {
+	f.lock.Lock()
+	defer f.lock.Unlock()
+
+	for i, block := range blocks {
+		// Make sure the parent in known
+		if _, ok := f.blocks[block.ParentHash()]; !ok {
+			return i, errors.New("unknown parent")
+		}
+		// Discard any new blocks if the same height already exists
+		if block.NumberU64() <= f.blocks[f.hashes[len(f.hashes)-1]].NumberU64() {
+			return i, nil
+		}
+		// Otherwise build our current chain
+		f.hashes = append(f.hashes, block.Hash())
+		f.blocks[block.Hash()] = block
+	}
+	return 0, nil
+}
+
+// dropPeer is a nop placeholder for the peer removal.
+func (f *fetcherTester) dropPeer(peer string) {
+}
+
+// peerFetcher retrieves a fetcher associated with a simulated peer.
+func (f *fetcherTester) makeFetcher(blocks map[common.Hash]*types.Block) blockRequesterFn {
+	// Copy all the blocks to ensure they are not tampered with
+	closure := make(map[common.Hash]*types.Block)
+	for hash, block := range blocks {
+		closure[hash] = copyBlock(block)
+	}
+	// Create a function that returns blocks from the closure
+	return func(hashes []common.Hash) error {
+		// Gather the blocks to return
+		blocks := make([]*types.Block, 0, len(hashes))
+		for _, hash := range hashes {
+			if block, ok := closure[hash]; ok {
+				blocks = append(blocks, block)
+			}
+		}
+		// Return on a new thread
+		go f.fetcher.Filter(blocks)
+
+		return nil
+	}
+}
+
+// Tests that a fetcher accepts block announcements and initiates retrievals for
+// them, successfully importing into the local chain.
+func TestSequentialAnnouncements(t *testing.T) {
+	// Create a chain of blocks to import
+	targetBlocks := 24
+	hashes := createHashes(targetBlocks, knownHash)
+	blocks := createBlocksFromHashes(hashes)
+
+	tester := newTester()
+	fetcher := tester.makeFetcher(blocks)
+
+	// Iteratively announce blocks until all are imported
+	for i := len(hashes) - 1; i >= 0; i-- {
+		tester.fetcher.Notify("valid", hashes[i], time.Now().Add(-arriveTimeout), fetcher)
+		time.Sleep(50 * time.Millisecond)
+	}
+	if imported := len(tester.blocks); imported != targetBlocks+1 {
+		t.Fatalf("synchronised block mismatch: have %v, want %v", imported, targetBlocks+1)
+	}
+}
+
+// Tests that if blocks are announced by multiple peers (or even the same buggy
+// peer), they will only get downloaded at most once.
+func TestConcurrentAnnouncements(t *testing.T) {
+	// Create a chain of blocks to import
+	targetBlocks := 24
+	hashes := createHashes(targetBlocks, knownHash)
+	blocks := createBlocksFromHashes(hashes)
+
+	// Assemble a tester with a built in counter for the requests
+	tester := newTester()
+	fetcher := tester.makeFetcher(blocks)
+
+	counter := uint32(0)
+	wrapper := func(hashes []common.Hash) error {
+		atomic.AddUint32(&counter, uint32(len(hashes)))
+		return fetcher(hashes)
+	}
+	// Iteratively announce blocks until all are imported
+	for i := len(hashes) - 1; i >= 0; i-- {
+		tester.fetcher.Notify("first", hashes[i], time.Now().Add(-arriveTimeout), wrapper)
+		tester.fetcher.Notify("second", hashes[i], time.Now().Add(-arriveTimeout+time.Millisecond), wrapper)
+		tester.fetcher.Notify("second", hashes[i], time.Now().Add(-arriveTimeout-time.Millisecond), wrapper)
+
+		time.Sleep(50 * time.Millisecond)
+	}
+	if imported := len(tester.blocks); imported != targetBlocks+1 {
+		t.Fatalf("synchronised block mismatch: have %v, want %v", imported, targetBlocks+1)
+	}
+	// Make sure no blocks were retrieved twice
+	if int(counter) != targetBlocks {
+		t.Fatalf("retrieval count mismatch: have %v, want %v", counter, targetBlocks)
+	}
+}
+
+// Tests that announcements arriving while a previous is being fetched still
+// results in a valid import.
+func TestOverlappingAnnouncements(t *testing.T) {
+	// Create a chain of blocks to import
+	targetBlocks := 24
+	hashes := createHashes(targetBlocks, knownHash)
+	blocks := createBlocksFromHashes(hashes)
+
+	tester := newTester()
+	fetcher := tester.makeFetcher(blocks)
+
+	// Iteratively announce blocks, but overlap them continuously
+	delay, overlap := 50*time.Millisecond, time.Duration(5)
+	for i := len(hashes) - 1; i >= 0; i-- {
+		tester.fetcher.Notify("valid", hashes[i], time.Now().Add(-arriveTimeout+overlap*delay), fetcher)
+		time.Sleep(delay)
+	}
+	time.Sleep(overlap * delay)
+
+	if imported := len(tester.blocks); imported != targetBlocks+1 {
+		t.Fatalf("synchronised block mismatch: have %v, want %v", imported, targetBlocks+1)
+	}
+}
+
+// Tests that announces already being retrieved will not be duplicated.
+func TestPendingDeduplication(t *testing.T) {
+	// Create a hash and corresponding block
+	hashes := createHashes(1, knownHash)
+	blocks := createBlocksFromHashes(hashes)
+
+	// Assemble a tester with a built in counter and delayed fetcher
+	tester := newTester()
+	fetcher := tester.makeFetcher(blocks)
+
+	delay := 50 * time.Millisecond
+	counter := uint32(0)
+	wrapper := func(hashes []common.Hash) error {
+		atomic.AddUint32(&counter, uint32(len(hashes)))
+
+		// Simulate a long running fetch
+		go func() {
+			time.Sleep(delay)
+			fetcher(hashes)
+		}()
+		return nil
+	}
+	// Announce the same block many times until it's fetched (wait for any pending ops)
+	for tester.getBlock(hashes[0]) == nil {
+		tester.fetcher.Notify("repeater", hashes[0], time.Now().Add(-arriveTimeout), wrapper)
+		time.Sleep(time.Millisecond)
+	}
+	time.Sleep(delay)
+
+	// Check that all blocks were imported and none fetched twice
+	if imported := len(tester.blocks); imported != 2 {
+		t.Fatalf("synchronised block mismatch: have %v, want %v", imported, 2)
+	}
+	if int(counter) != 1 {
+		t.Fatalf("retrieval count mismatch: have %v, want %v", counter, 1)
+	}
+}
+
+// Tests that announcements retrieved in a random order are cached and eventually
+// imported when all the gaps are filled in.
+func TestRandomArrivalImport(t *testing.T) {
+	// Create a chain of blocks to import, and choose one to delay
+	targetBlocks := 24
+	hashes := createHashes(targetBlocks, knownHash)
+	blocks := createBlocksFromHashes(hashes)
+	skip := targetBlocks / 2
+
+	tester := newTester()
+	fetcher := tester.makeFetcher(blocks)
+
+	// Iteratively announce blocks, skipping one entry
+	for i := len(hashes) - 1; i >= 0; i-- {
+		if i != skip {
+			tester.fetcher.Notify("valid", hashes[i], time.Now().Add(-arriveTimeout), fetcher)
+			time.Sleep(50 * time.Millisecond)
+		}
+	}
+	// Finally announce the skipped entry and check full import
+	tester.fetcher.Notify("valid", hashes[skip], time.Now().Add(-arriveTimeout), fetcher)
+	time.Sleep(50 * time.Millisecond)
+
+	if imported := len(tester.blocks); imported != targetBlocks+1 {
+		t.Fatalf("synchronised block mismatch: have %v, want %v", imported, targetBlocks+1)
+	}
+}
+
+// Tests that direct block enqueues (due to block propagation vs. hash announce)
+// are correctly schedule, filling and import queue gaps.
+func TestQueueGapFill(t *testing.T) {
+	// Create a chain of blocks to import, and choose one to not announce at all
+	targetBlocks := 24
+	hashes := createHashes(targetBlocks, knownHash)
+	blocks := createBlocksFromHashes(hashes)
+	skip := targetBlocks / 2
+
+	tester := newTester()
+	fetcher := tester.makeFetcher(blocks)
+
+	// Iteratively announce blocks, skipping one entry
+	for i := len(hashes) - 1; i >= 0; i-- {
+		if i != skip {
+			tester.fetcher.Notify("valid", hashes[i], time.Now().Add(-arriveTimeout), fetcher)
+			time.Sleep(50 * time.Millisecond)
+		}
+	}
+	// Fill the missing block directly as if propagated
+	tester.fetcher.Enqueue("valid", blocks[hashes[skip]])
+	time.Sleep(50 * time.Millisecond)
+
+	if imported := len(tester.blocks); imported != targetBlocks+1 {
+		t.Fatalf("synchronised block mismatch: have %v, want %v", imported, targetBlocks+1)
+	}
+}
+
+// Tests that blocks arriving from various sources (multiple propagations, hash
+// announces, etc) do not get scheduled for import multiple times.
+func TestImportDeduplication(t *testing.T) {
+	// Create two blocks to import (one for duplication, the other for stalling)
+	hashes := createHashes(2, knownHash)
+	blocks := createBlocksFromHashes(hashes)
+
+	// Create the tester and wrap the importer with a counter
+	tester := newTester()
+	fetcher := tester.makeFetcher(blocks)
+
+	counter := uint32(0)
+	tester.fetcher.insertChain = func(blocks types.Blocks) (int, error) {
+		atomic.AddUint32(&counter, uint32(len(blocks)))
+		return tester.insertChain(blocks)
+	}
+	// Announce the duplicating block, wait for retrieval, and also propagate directly
+	tester.fetcher.Notify("valid", hashes[0], time.Now().Add(-arriveTimeout), fetcher)
+	time.Sleep(50 * time.Millisecond)
+
+	tester.fetcher.Enqueue("valid", blocks[hashes[0]])
+	tester.fetcher.Enqueue("valid", blocks[hashes[0]])
+	tester.fetcher.Enqueue("valid", blocks[hashes[0]])
+
+	// Fill the missing block directly as if propagated, and check import uniqueness
+	tester.fetcher.Enqueue("valid", blocks[hashes[1]])
+	time.Sleep(50 * time.Millisecond)
+
+	if imported := len(tester.blocks); imported != 3 {
+		t.Fatalf("synchronised block mismatch: have %v, want %v", imported, 3)
+	}
+	if counter != 2 {
+		t.Fatalf("import invocation count mismatch: have %v, want %v", counter, 2)
+	}
+}
+
+// Tests that blocks with numbers much lower or higher than out current head get
+// discarded no prevent wasting resources on useless blocks from faulty peers.
+func TestDistantDiscarding(t *testing.T) {
+	// Create a long chain to import
+	hashes := createHashes(3*maxQueueDist, knownHash)
+	blocks := createBlocksFromHashes(hashes)
+
+	head := hashes[len(hashes)/2]
+
+	// Create a tester and simulate a head block being the middle of the above chain
+	tester := newTester()
+	tester.hashes = []common.Hash{head}
+	tester.blocks = map[common.Hash]*types.Block{head: blocks[head]}
+
+	// Ensure that a block with a lower number than the threshold is discarded
+	tester.fetcher.Enqueue("lower", blocks[hashes[0]])
+	time.Sleep(10 * time.Millisecond)
+	if !tester.fetcher.queue.Empty() {
+		t.Fatalf("fetcher queued stale block")
+	}
+	// Ensure that a block with a higher number than the threshold is discarded
+	tester.fetcher.Enqueue("higher", blocks[hashes[len(hashes)-1]])
+	time.Sleep(10 * time.Millisecond)
+	if !tester.fetcher.queue.Empty() {
+		t.Fatalf("fetcher queued future block")
+	}
+}