eth/downloader: separate state sync from queue (#14460)

* eth/downloader: separate state sync from queue

Scheduling of state node downloads hogged the downloader queue lock when
new requests were scheduled. This caused timeouts for other requests.
With this change, state sync is fully independent of all other downloads
and doesn't involve the queue at all.

State sync is started and checked on in processContent. This is slightly
awkward because processContent doesn't have a select loop. Instead, the
queue is closed by an auxiliary goroutine when state sync fails. We
tried several alternatives to this but settled on the current approach
because it's the least amount of change overall.

Handling of the pivot block has changed slightly: the queue previously
prevented import of pivot block receipts before the state of the pivot
block was available. In this commit, the receipt will be imported before
the state. This causes an annoyance where the pivot block is committed
as fast block head even when state downloads fail. Stay tuned for more
updates in this area ;)

* eth/downloader: remove cancelTimeout channel

* eth/downloader: retry state requests on timeout

* eth/downloader: improve comment

* eth/downloader: mark peers idle when state sync is done

* eth/downloader: move pivot block splitting to processContent

This change also ensures that pivot block receipts aren't imported
before the pivot block itself.

* eth/downloader: limit state node retries

* eth/downloader: improve state node error handling and retry check

* eth/downloader: remove maxStateNodeRetries

It fails the sync too much.

* eth/downloader: remove last use of cancelCh in statesync.go

Fixes TestDeliverHeadersHang*Fast and (hopefully)
the weird cancellation behaviour at the end of fast sync.

* eth/downloader: fix leak in runStateSync

* eth/downloader: don't run processFullSyncContent in LightSync mode

* eth/downloader: improve comments

* eth/downloader: fix vet, megacheck

* eth/downloader: remove unrequested tasks anyway

* eth/downloader, trie: various polishes around duplicate items

This commit explicitly tracks duplicate and unexpected state
delieveries done against a trie Sync structure, also adding
there to import info logs.

The commit moves the db batch used to commit trie changes one
level deeper so its flushed after every node insertion. This
is needed to avoid a lot of duplicate retrievals caused by
inconsistencies between Sync internals and database. A better
approach is to track not-yet-written states in trie.Sync and
flush on commit, but I'm focuing on correctness first now.

The commit fixes a regression around pivot block fail count.
The counter previously was reset to 1 if and only if a sync
cycle progressed (inserted at least 1 entry to the database).
The current code reset it already if a node was delivered,
which is not stong enough, because unless it ends up written
to disk, an attacker can just loop and attack ad infinitum.

The commit also fixes a regression around state deliveries
and timeouts. The old downloader tracked if a delivery is
stale (none of the deliveries were requestedt), in which
case it didn't mark the node idle and did not send further
requests, since it signals a past timeout. The current code
did mark it idle even on stale deliveries, which eventually
caused two requests to be in flight at the same time, making
the deliveries always stale and mass duplicating retrievals
between multiple peers.

* eth/downloader: fix state request leak

This commit fixes the hang seen sometimes while doing the state
sync. The cause of the hang was a rare combination of events:
request state data from peer, peer drops and reconnects almost
immediately. This caused a new download task to be assigned to
the peer, overwriting the old one still waiting for a timeout,
which in turned leaked the requests out, never to be retried.
The fix is to ensure that a task assignment moves any pending
one back into the retry queue.

The commit also fixes a regression with peer dropping due to
stalls. The current code considered a peer stalling if they
timed out delivering 1 item. However, the downloader never
requests only one, the minimum is 2 (attempt to fine tune
estimated latency/bandwidth). The fix is simply to drop if
a timeout is detected at 2 items.

Apart from the above bugfixes, the commit contains some code
polishes I made while debugging the hang.

* core, eth, trie: support batched trie sync db writes

* trie: rename SyncMemCache to syncMemBatch

1 files changed, 178 insertions, 156 deletions

diff --git a/eth/downloader/downloader.go b/eth/downloader/downloader.go
index 839969f03..e4d1392d0 100644
--- a/eth/downloader/downloader.go
+++ b/eth/downloader/downloader.go
@@ -34,7 +34,6 @@ import (
 	"github.com/ethereum/go-ethereum/event"
 	"github.com/ethereum/go-ethereum/log"
 	"github.com/ethereum/go-ethereum/params"
-	"github.com/ethereum/go-ethereum/trie"
 	"github.com/rcrowley/go-metrics"
 )
 
@@ -99,8 +98,9 @@ type Downloader struct {
 	mode SyncMode       // Synchronisation mode defining the strategy used (per sync cycle)
 	mux  *event.TypeMux // Event multiplexer to announce sync operation events
 
-	queue *queue   // Scheduler for selecting the hashes to download
-	peers *peerSet // Set of active peers from which download can proceed
+	queue   *queue   // Scheduler for selecting the hashes to download
+	peers   *peerSet // Set of active peers from which download can proceed
+	stateDB ethdb.Database
 
 	fsPivotLock  *types.Header // Pivot header on critical section entry (cannot change between retries)
 	fsPivotFails uint32        // Number of subsequent fast sync failures in the critical section
@@ -109,9 +109,9 @@ type Downloader struct {
 	rttConfidence uint64 // Confidence in the estimated RTT (unit: millionths to allow atomic ops)
 
 	// Statistics
-	syncStatsChainOrigin uint64       // Origin block number where syncing started at
-	syncStatsChainHeight uint64       // Highest block number known when syncing started
-	syncStatsStateDone   uint64       // Number of state trie entries already pulled
+	syncStatsChainOrigin uint64 // Origin block number where syncing started at
+	syncStatsChainHeight uint64 // Highest block number known when syncing started
+	syncStatsState       stateSyncStats
 	syncStatsLock        sync.RWMutex // Lock protecting the sync stats fields
 
 	// Callbacks
@@ -136,16 +136,18 @@ type Downloader struct {
 	notified        int32
 
 	// Channels
-	newPeerCh     chan *peer
 	headerCh      chan dataPack        // [eth/62] Channel receiving inbound block headers
 	bodyCh        chan dataPack        // [eth/62] Channel receiving inbound block bodies
 	receiptCh     chan dataPack        // [eth/63] Channel receiving inbound receipts
-	stateCh       chan dataPack        // [eth/63] Channel receiving inbound node state data
 	bodyWakeCh    chan bool            // [eth/62] Channel to signal the block body fetcher of new tasks
 	receiptWakeCh chan bool            // [eth/63] Channel to signal the receipt fetcher of new tasks
-	stateWakeCh   chan bool            // [eth/63] Channel to signal the state fetcher of new tasks
 	headerProcCh  chan []*types.Header // [eth/62] Channel to feed the header processor new tasks
 
+	// for stateFetcher
+	stateSyncStart chan *stateSync
+	trackStateReq  chan *stateReq
+	stateCh        chan dataPack // [eth/63] Channel receiving inbound node state data
+
 	// Cancellation and termination
 	cancelPeer string        // Identifier of the peer currently being used as the master (cancel on drop)
 	cancelCh   chan struct{} // Channel to cancel mid-flight syncs
@@ -170,8 +172,9 @@ func New(mode SyncMode, stateDb ethdb.Database, mux *event.TypeMux, hasHeader he
 	dl := &Downloader{
 		mode:             mode,
 		mux:              mux,
-		queue:            newQueue(stateDb),
+		queue:            newQueue(),
 		peers:            newPeerSet(),
+		stateDB:          stateDb,
 		rttEstimate:      uint64(rttMaxEstimate),
 		rttConfidence:    uint64(1000000),
 		hasHeader:        hasHeader,
@@ -188,18 +191,20 @@ func New(mode SyncMode, stateDb ethdb.Database, mux *event.TypeMux, hasHeader he
 		insertReceipts:   insertReceipts,
 		rollback:         rollback,
 		dropPeer:         dropPeer,
-		newPeerCh:        make(chan *peer, 1),
 		headerCh:         make(chan dataPack, 1),
 		bodyCh:           make(chan dataPack, 1),
 		receiptCh:        make(chan dataPack, 1),
-		stateCh:          make(chan dataPack, 1),
 		bodyWakeCh:       make(chan bool, 1),
 		receiptWakeCh:    make(chan bool, 1),
-		stateWakeCh:      make(chan bool, 1),
 		headerProcCh:     make(chan []*types.Header, 1),
 		quitCh:           make(chan struct{}),
+		// for stateFetcher
+		stateSyncStart: make(chan *stateSync),
+		trackStateReq:  make(chan *stateReq),
+		stateCh:        make(chan dataPack),
 	}
 	go dl.qosTuner()
+	go dl.stateFetcher()
 	return dl
 }
 
@@ -211,9 +216,6 @@ func New(mode SyncMode, stateDb ethdb.Database, mux *event.TypeMux, hasHeader he
 // of processed and the total number of known states are also returned. Otherwise
 // these are zero.
 func (d *Downloader) Progress() ethereum.SyncProgress {
-	// Fetch the pending state count outside of the lock to prevent unforeseen deadlocks
-	pendingStates := uint64(d.queue.PendingNodeData())
-
 	// Lock the current stats and return the progress
 	d.syncStatsLock.RLock()
 	defer d.syncStatsLock.RUnlock()
@@ -231,8 +233,8 @@ func (d *Downloader) Progress() ethereum.SyncProgress {
 		StartingBlock: d.syncStatsChainOrigin,
 		CurrentBlock:  current,
 		HighestBlock:  d.syncStatsChainHeight,
-		PulledStates:  d.syncStatsStateDone,
-		KnownStates:   d.syncStatsStateDone + pendingStates,
+		PulledStates:  d.syncStatsState.processed,
+		KnownStates:   d.syncStatsState.processed + d.syncStatsState.pending,
 	}
 }
 
@@ -324,13 +326,13 @@ func (d *Downloader) synchronise(id string, hash common.Hash, td *big.Int, mode
 	d.queue.Reset()
 	d.peers.Reset()
 
-	for _, ch := range []chan bool{d.bodyWakeCh, d.receiptWakeCh, d.stateWakeCh} {
+	for _, ch := range []chan bool{d.bodyWakeCh, d.receiptWakeCh} {
 		select {
 		case <-ch:
 		default:
 		}
 	}
-	for _, ch := range []chan dataPack{d.headerCh, d.bodyCh, d.receiptCh, d.stateCh} {
+	for _, ch := range []chan dataPack{d.headerCh, d.bodyCh, d.receiptCh} {
 		for empty := false; !empty; {
 			select {
 			case <-ch:
@@ -439,30 +441,40 @@ func (d *Downloader) syncWithPeer(p *peer, hash common.Hash, td *big.Int) (err e
 	if d.syncInitHook != nil {
 		d.syncInitHook(origin, height)
 	}
-	return d.spawnSync(origin+1,
-		func() error { return d.fetchHeaders(p, origin+1) },    // Headers are always retrieved
-		func() error { return d.processHeaders(origin+1, td) }, // Headers are always retrieved
-		func() error { return d.fetchBodies(origin + 1) },      // Bodies are retrieved during normal and fast sync
-		func() error { return d.fetchReceipts(origin + 1) },    // Receipts are retrieved during fast sync
-		func() error { return d.fetchNodeData() },              // Node state data is retrieved during fast sync
-	)
+
+	fetchers := []func() error{
+		func() error { return d.fetchHeaders(p, origin+1) }, // Headers are always retrieved
+		func() error { return d.fetchBodies(origin + 1) },   // Bodies are retrieved during normal and fast sync
+		func() error { return d.fetchReceipts(origin + 1) }, // Receipts are retrieved during fast sync
+		func() error { return d.processHeaders(origin+1, td) },
+	}
+	if d.mode == FastSync {
+		fetchers = append(fetchers, func() error { return d.processFastSyncContent(latest) })
+	} else if d.mode == FullSync {
+		fetchers = append(fetchers, d.processFullSyncContent)
+	}
+	err = d.spawnSync(fetchers)
+	if err != nil && d.mode == FastSync && d.fsPivotLock != nil {
+		// If sync failed in the critical section, bump the fail counter.
+		atomic.AddUint32(&d.fsPivotFails, 1)
+	}
+	return err
 }
 
 // spawnSync runs d.process and all given fetcher functions to completion in
 // separate goroutines, returning the first error that appears.
-func (d *Downloader) spawnSync(origin uint64, fetchers ...func() error) error {
+func (d *Downloader) spawnSync(fetchers []func() error) error {
 	var wg sync.WaitGroup
-	errc := make(chan error, len(fetchers)+1)
-	wg.Add(len(fetchers) + 1)
-	go func() { defer wg.Done(); errc <- d.processContent() }()
+	errc := make(chan error, len(fetchers))
+	wg.Add(len(fetchers))
 	for _, fn := range fetchers {
 		fn := fn
 		go func() { defer wg.Done(); errc <- fn() }()
 	}
 	// Wait for the first error, then terminate the others.
 	var err error
-	for i := 0; i < len(fetchers)+1; i++ {
-		if i == len(fetchers) {
+	for i := 0; i < len(fetchers); i++ {
+		if i == len(fetchers)-1 {
 			// Close the queue when all fetchers have exited.
 			// This will cause the block processor to end when
 			// it has processed the queue.
@@ -475,11 +487,6 @@ func (d *Downloader) spawnSync(origin uint64, fetchers ...func() error) error {
 	d.queue.Close()
 	d.Cancel()
 	wg.Wait()
-
-	// If sync failed in the critical section, bump the fail counter
-	if err != nil && d.mode == FastSync && d.fsPivotLock != nil {
-		atomic.AddUint32(&d.fsPivotFails, 1)
-	}
 	return err
 }
 
@@ -552,7 +559,6 @@ func (d *Downloader) fetchHeight(p *peer) (*types.Header, error) {
 			return nil, errTimeout
 
 		case <-d.bodyCh:
-		case <-d.stateCh:
 		case <-d.receiptCh:
 			// Out of bounds delivery, ignore
 		}
@@ -649,7 +655,6 @@ func (d *Downloader) findAncestor(p *peer, height uint64) (uint64, error) {
 			return 0, errTimeout
 
 		case <-d.bodyCh:
-		case <-d.stateCh:
 		case <-d.receiptCh:
 			// Out of bounds delivery, ignore
 		}
@@ -714,7 +719,6 @@ func (d *Downloader) findAncestor(p *peer, height uint64) (uint64, error) {
 				return 0, errTimeout
 
 			case <-d.bodyCh:
-			case <-d.stateCh:
 			case <-d.receiptCh:
 				// Out of bounds delivery, ignore
 			}
@@ -827,7 +831,7 @@ func (d *Downloader) fetchHeaders(p *peer, from uint64) error {
 			d.dropPeer(p.id)
 
 			// Finish the sync gracefully instead of dumping the gathered data though
-			for _, ch := range []chan bool{d.bodyWakeCh, d.receiptWakeCh, d.stateWakeCh} {
+			for _, ch := range []chan bool{d.bodyWakeCh, d.receiptWakeCh} {
 				select {
 				case ch <- false:
 				case <-d.cancelCh:
@@ -927,68 +931,6 @@ func (d *Downloader) fetchReceipts(from uint64) error {
 	return err
 }
 
-// fetchNodeData iteratively downloads the scheduled state trie nodes, taking any
-// available peers, reserving a chunk of nodes for each, waiting for delivery and
-// also periodically checking for timeouts.
-func (d *Downloader) fetchNodeData() error {
-	log.Debug("Downloading node state data")
-
-	var (
-		deliver = func(packet dataPack) (int, error) {
-			start := time.Now()
-			return d.queue.DeliverNodeData(packet.PeerId(), packet.(*statePack).states, func(delivered int, progressed bool, err error) {
-				// If the peer returned old-requested data, forgive
-				if err == trie.ErrNotRequested {
-					log.Debug("Forgiving reply to stale state request", "peer", packet.PeerId())
-					return
-				}
-				if err != nil {
-					// If the node data processing failed, the root hash is very wrong, abort
-					log.Error("State processing failed", "peer", packet.PeerId(), "err", err)
-					d.Cancel()
-					return
-				}
-				// Processing succeeded, notify state fetcher of continuation
-				pending := d.queue.PendingNodeData()
-				if pending > 0 {
-					select {
-					case d.stateWakeCh <- true:
-					default:
-					}
-				}
-				d.syncStatsLock.Lock()
-				d.syncStatsStateDone += uint64(delivered)
-				syncStatsStateDone := d.syncStatsStateDone // Thread safe copy for the log below
-				d.syncStatsLock.Unlock()
-
-				// If real database progress was made, reset any fast-sync pivot failure
-				if progressed && atomic.LoadUint32(&d.fsPivotFails) > 1 {
-					log.Debug("Fast-sync progressed, resetting fail counter", "previous", atomic.LoadUint32(&d.fsPivotFails))
-					atomic.StoreUint32(&d.fsPivotFails, 1) // Don't ever reset to 0, as that will unlock the pivot block
-				}
-				// Log a message to the user and return
-				if delivered > 0 {
-					log.Info("Imported new state entries", "count", delivered, "elapsed", common.PrettyDuration(time.Since(start)), "processed", syncStatsStateDone, "pending", pending)
-				}
-			})
-		}
-		expire   = func() map[string]int { return d.queue.ExpireNodeData(d.requestTTL()) }
-		throttle = func() bool { return false }
-		reserve  = func(p *peer, count int) (*fetchRequest, bool, error) {
-			return d.queue.ReserveNodeData(p, count), false, nil
-		}
-		fetch    = func(p *peer, req *fetchRequest) error { return p.FetchNodeData(req) }
-		capacity = func(p *peer) int { return p.NodeDataCapacity(d.requestRTT()) }
-		setIdle  = func(p *peer, accepted int) { p.SetNodeDataIdle(accepted) }
-	)
-	err := d.fetchParts(errCancelStateFetch, d.stateCh, deliver, d.stateWakeCh, expire,
-		d.queue.PendingNodeData, d.queue.InFlightNodeData, throttle, reserve, nil, fetch,
-		d.queue.CancelNodeData, capacity, d.peers.NodeDataIdlePeers, setIdle, "states")
-
-	log.Debug("Node state data download terminated", "err", err)
-	return err
-}
-
 // fetchParts iteratively downloads scheduled block parts, taking any available
 // peers, reserving a chunk of fetch requests for each, waiting for delivery and
 // also periodically checking for timeouts.
@@ -1229,7 +1171,7 @@ func (d *Downloader) processHeaders(origin uint64, td *big.Int) error {
 			// Terminate header processing if we synced up
 			if len(headers) == 0 {
 				// Notify everyone that headers are fully processed
-				for _, ch := range []chan bool{d.bodyWakeCh, d.receiptWakeCh, d.stateWakeCh} {
+				for _, ch := range []chan bool{d.bodyWakeCh, d.receiptWakeCh} {
 					select {
 					case ch <- false:
 					case <-d.cancelCh:
@@ -1341,7 +1283,7 @@ func (d *Downloader) processHeaders(origin uint64, td *big.Int) error {
 				origin += uint64(limit)
 			}
 			// Signal the content downloaders of the availablility of new tasks
-			for _, ch := range []chan bool{d.bodyWakeCh, d.receiptWakeCh, d.stateWakeCh} {
+			for _, ch := range []chan bool{d.bodyWakeCh, d.receiptWakeCh} {
 				select {
 				case ch <- true:
 				default:
@@ -1351,71 +1293,151 @@ func (d *Downloader) processHeaders(origin uint64, td *big.Int) error {
 	}
 }
 
-// processContent takes fetch results from the queue and tries to import them
-// into the chain. The type of import operation will depend on the result contents.
-func (d *Downloader) processContent() error {
-	pivot := d.queue.FastSyncPivot()
+// processFullSyncContent takes fetch results from the queue and imports them into the chain.
+func (d *Downloader) processFullSyncContent() error {
 	for {
 		results := d.queue.WaitResults()
 		if len(results) == 0 {
-			return nil // queue empty
+			return nil
 		}
 		if d.chainInsertHook != nil {
 			d.chainInsertHook(results)
 		}
-		// Actually import the blocks
-		first, last := results[0].Header, results[len(results)-1].Header
+		if err := d.importBlockResults(results); err != nil {
+			return err
+		}
+	}
+}
+
+func (d *Downloader) importBlockResults(results []*fetchResult) error {
+	for len(results) != 0 {
+		// Check for any termination requests. This makes clean shutdown faster.
+		select {
+		case <-d.quitCh:
+			return errCancelContentProcessing
+		default:
+		}
+		// Retrieve the a batch of results to import
+		items := int(math.Min(float64(len(results)), float64(maxResultsProcess)))
+		first, last := results[0].Header, results[items-1].Header
 		log.Debug("Inserting downloaded chain", "items", len(results),
 			"firstnum", first.Number, "firsthash", first.Hash(),
 			"lastnum", last.Number, "lasthash", last.Hash(),
 		)
-		for len(results) != 0 {
-			// Check for any termination requests
-			select {
-			case <-d.quitCh:
-				return errCancelContentProcessing
-			default:
-			}
-			// Retrieve the a batch of results to import
-			var (
-				blocks   = make([]*types.Block, 0, maxResultsProcess)
-				receipts = make([]types.Receipts, 0, maxResultsProcess)
-			)
-			items := int(math.Min(float64(len(results)), float64(maxResultsProcess)))
-			for _, result := range results[:items] {
-				switch {
-				case d.mode == FullSync:
-					blocks = append(blocks, types.NewBlockWithHeader(result.Header).WithBody(result.Transactions, result.Uncles))
-				case d.mode == FastSync:
-					blocks = append(blocks, types.NewBlockWithHeader(result.Header).WithBody(result.Transactions, result.Uncles))
-					if result.Header.Number.Uint64() <= pivot {
-						receipts = append(receipts, result.Receipts)
-					}
-				}
-			}
-			// Try to process the results, aborting if there's an error
-			var (
-				err   error
-				index int
-			)
-			switch {
-			case len(receipts) > 0:
-				index, err = d.insertReceipts(blocks, receipts)
-				if err == nil && blocks[len(blocks)-1].NumberU64() == pivot {
-					log.Debug("Committing block as new head", "number", blocks[len(blocks)-1].Number(), "hash", blocks[len(blocks)-1].Hash())
-					index, err = len(blocks)-1, d.commitHeadBlock(blocks[len(blocks)-1].Hash())
-				}
-			default:
-				index, err = d.insertBlocks(blocks)
+		blocks := make([]*types.Block, items)
+		for i, result := range results[:items] {
+			blocks[i] = types.NewBlockWithHeader(result.Header).WithBody(result.Transactions, result.Uncles)
+		}
+		if index, err := d.insertBlocks(blocks); err != nil {
+			log.Debug("Downloaded item processing failed", "number", results[index].Header.Number, "hash", results[index].Header.Hash(), "err", err)
+			return errInvalidChain
+		}
+		// Shift the results to the next batch
+		results = results[items:]
+	}
+	return nil
+}
+
+// processFastSyncContent takes fetch results from the queue and writes them to the
+// database. It also controls the synchronisation of state nodes of the pivot block.
+func (d *Downloader) processFastSyncContent(latest *types.Header) error {
+	// Start syncing state of the reported head block.
+	// This should get us most of the state of the pivot block.
+	stateSync := d.syncState(latest.Root)
+	defer stateSync.Cancel()
+	go func() {
+		if err := stateSync.Wait(); err != nil {
+			d.queue.Close() // wake up WaitResults
+		}
+	}()
+
+	pivot := d.queue.FastSyncPivot()
+	for {
+		results := d.queue.WaitResults()
+		if len(results) == 0 {
+			return stateSync.Cancel()
+		}
+		if d.chainInsertHook != nil {
+			d.chainInsertHook(results)
+		}
+		P, beforeP, afterP := splitAroundPivot(pivot, results)
+		if err := d.commitFastSyncData(beforeP, stateSync); err != nil {
+			return err
+		}
+		if P != nil {
+			stateSync.Cancel()
+			if err := d.commitPivotBlock(P); err != nil {
+				return err
 			}
-			if err != nil {
-				log.Debug("Downloaded item processing failed", "number", results[index].Header.Number, "hash", results[index].Header.Hash(), "err", err)
-				return errInvalidChain
+		}
+		if err := d.importBlockResults(afterP); err != nil {
+			return err
+		}
+	}
+}
+
+func splitAroundPivot(pivot uint64, results []*fetchResult) (p *fetchResult, before, after []*fetchResult) {
+	for _, result := range results {
+		num := result.Header.Number.Uint64()
+		switch {
+		case num < pivot:
+			before = append(before, result)
+		case num == pivot:
+			p = result
+		default:
+			after = append(after, result)
+		}
+	}
+	return p, before, after
+}
+
+func (d *Downloader) commitFastSyncData(results []*fetchResult, stateSync *stateSync) error {
+	for len(results) != 0 {
+		// Check for any termination requests.
+		select {
+		case <-d.quitCh:
+			return errCancelContentProcessing
+		case <-stateSync.done:
+			if err := stateSync.Wait(); err != nil {
+				return err
 			}
-			// Shift the results to the next batch
-			results = results[items:]
+		default:
+		}
+		// Retrieve the a batch of results to import
+		items := int(math.Min(float64(len(results)), float64(maxResultsProcess)))
+		first, last := results[0].Header, results[items-1].Header
+		log.Debug("Inserting fast-sync blocks", "items", len(results),
+			"firstnum", first.Number, "firsthash", first.Hash(),
+			"lastnumn", last.Number, "lasthash", last.Hash(),
+		)
+		blocks := make([]*types.Block, items)
+		receipts := make([]types.Receipts, items)
+		for i, result := range results[:items] {
+			blocks[i] = types.NewBlockWithHeader(result.Header).WithBody(result.Transactions, result.Uncles)
+			receipts[i] = result.Receipts
+		}
+		if index, err := d.insertReceipts(blocks, receipts); err != nil {
+			log.Debug("Downloaded item processing failed", "number", results[index].Header.Number, "hash", results[index].Header.Hash(), "err", err)
+			return errInvalidChain
 		}
+		// Shift the results to the next batch
+		results = results[items:]
+	}
+	return nil
+}
+
+func (d *Downloader) commitPivotBlock(result *fetchResult) error {
+	b := types.NewBlockWithHeader(result.Header).WithBody(result.Transactions, result.Uncles)
+	// Sync the pivot block state. This should complete reasonably quickly because
+	// we've already synced up to the reported head block state earlier.
+	if err := d.syncState(b.Root()).Wait(); err != nil {
+		return err
+	}
+	log.Debug("Committing fast sync pivot as new head", "number", b.Number(), "hash", b.Hash())
+	if _, err := d.insertReceipts([]*types.Block{b}, []types.Receipts{result.Receipts}); err != nil {
+		return err
 	}
+	return d.commitHeadBlock(b.Hash())
 }
 
 // DeliverHeaders injects a new batch of block headers received from a remote