eth/downloader: concurrent receipt and state processing

15 files changed, 516 insertions, 259 deletions

diff --git a/core/block_processor.go b/core/block_processor.go
index ba6350805..60f0258c4 100644
--- a/core/block_processor.go
+++ b/core/block_processor.go
@@ -383,6 +383,15 @@ func (sm *BlockProcessor) ValidateHeader(header *types.Header, checkPow, uncle b
 	}
 }
 
+// ValidateHeaderWithParent verifies the validity of a header, relying on the database and
+// POW behind the block processor.
+func (sm *BlockProcessor) ValidateHeaderWithParent(header, parent *types.Header, checkPow, uncle bool) error {
+	if sm.bc.HasHeader(header.Hash()) {
+		return nil
+	}
+	return ValidateHeader(sm.Pow, header, parent, checkPow, uncle)
+}
+
 // See YP section 4.3.4. "Block Header Validity"
 // Validates a header. Returns an error if the header is invalid.
 func ValidateHeader(pow pow.PoW, header *types.Header, parent *types.Header, checkPow, uncle bool) error {
@@ -425,7 +434,7 @@ func ValidateHeader(pow pow.PoW, header *types.Header, parent *types.Header, che
 	if checkPow {
 		// Verify the nonce of the header. Return an error if it's not valid
 		if !pow.Verify(types.NewBlockWithHeader(header)) {
-			return ValidationError("Header's nonce is invalid (= %x)", header.Nonce)
+			return &BlockNonceErr{Hash: header.Hash(), Number: header.Number, Nonce: header.Nonce.Uint64()}
 		}
 	}
 	return nil
diff --git a/core/blockchain.go b/core/blockchain.go
index 6c8a24751..3e7dfa9ee 100644
--- a/core/blockchain.go
+++ b/core/blockchain.go
@@ -22,6 +22,8 @@ import (
 	"fmt"
 	"io"
 	"math/big"
+	"math/rand"
+	"runtime"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -671,7 +673,7 @@ func (self *BlockChain) writeHeader(header *types.Header) error {
 // should be done or not. The reason behind the optional check is because some
 // of the header retrieval mechanisms already need to verfy nonces, as well as
 // because nonces can be verified sparsely, not needing to check each.
-func (self *BlockChain) InsertHeaderChain(chain []*types.Header, verify bool) (int, error) {
+func (self *BlockChain) InsertHeaderChain(chain []*types.Header, checkFreq int) (int, error) {
 	self.wg.Add(1)
 	defer self.wg.Done()
 
@@ -683,16 +685,85 @@ func (self *BlockChain) InsertHeaderChain(chain []*types.Header, verify bool) (i
 	stats := struct{ processed, ignored int }{}
 	start := time.Now()
 
-	// Start the parallel nonce verifier, with a fake nonce if not requested
-	verifier := self.pow
-	if !verify {
-		verifier = FakePow{}
+	// Generate the list of headers that should be POW verified
+	verify := make([]bool, len(chain))
+	for i := 0; i < len(verify)/checkFreq; i++ {
+		index := i*checkFreq + rand.Intn(checkFreq)
+		if index >= len(verify) {
+			index = len(verify) - 1
+		}
+		verify[index] = true
 	}
-	nonceAbort, nonceResults := verifyNoncesFromHeaders(verifier, chain)
-	defer close(nonceAbort)
+	verify[len(verify)-1] = true // Last should always be verified to avoid junk
+
+	// Create the header verification task queue and worker functions
+	tasks := make(chan int, len(chain))
+	for i := 0; i < len(chain); i++ {
+		tasks <- i
+	}
+	close(tasks)
 
-	// Iterate over the headers, inserting any new ones
-	complete := make([]bool, len(chain))
+	errs, failed := make([]error, len(tasks)), int32(0)
+	process := func(worker int) {
+		for index := range tasks {
+			header, hash := chain[index], chain[index].Hash()
+
+			// Short circuit insertion if shutting down or processing failed
+			if atomic.LoadInt32(&self.procInterrupt) == 1 {
+				return
+			}
+			if atomic.LoadInt32(&failed) > 0 {
+				return
+			}
+			// Short circuit if the header is bad or already known
+			if BadHashes[hash] {
+				errs[index] = BadHashError(hash)
+				atomic.AddInt32(&failed, 1)
+				return
+			}
+			if self.HasHeader(hash) {
+				continue
+			}
+			// Verify that the header honors the chain parameters
+			checkPow := verify[index]
+
+			var err error
+			if index == 0 {
+				err = self.processor.ValidateHeader(header, checkPow, false)
+			} else {
+				err = self.processor.ValidateHeaderWithParent(header, chain[index-1], checkPow, false)
+			}
+			if err != nil {
+				errs[index] = err
+				atomic.AddInt32(&failed, 1)
+				return
+			}
+		}
+	}
+	// Start as many worker threads as goroutines allowed
+	pending := new(sync.WaitGroup)
+	for i := 0; i < runtime.GOMAXPROCS(0); i++ {
+		pending.Add(1)
+		go func(id int) {
+			defer pending.Done()
+			process(id)
+		}(i)
+	}
+	pending.Wait()
+
+	// If anything failed, report
+	if atomic.LoadInt32(&self.procInterrupt) == 1 {
+		glog.V(logger.Debug).Infoln("premature abort during receipt chain processing")
+		return 0, nil
+	}
+	if failed > 0 {
+		for i, err := range errs {
+			if err != nil {
+				return i, err
+			}
+		}
+	}
+	// All headers passed verification, import them into the database
 	for i, header := range chain {
 		// Short circuit insertion if shutting down
 		if atomic.LoadInt32(&self.procInterrupt) == 1 {
@@ -701,24 +772,7 @@ func (self *BlockChain) InsertHeaderChain(chain []*types.Header, verify bool) (i
 		}
 		hash := header.Hash()
 
-		// Accumulate verification results until the next header is verified
-		for !complete[i] {
-			if res := <-nonceResults; res.valid {
-				complete[res.index] = true
-			} else {
-				header := chain[res.index]
-				return res.index, &BlockNonceErr{
-					Hash:   header.Hash(),
-					Number: new(big.Int).Set(header.Number),
-					Nonce:  header.Nonce.Uint64(),
-				}
-			}
-		}
-		if BadHashes[hash] {
-			glog.V(logger.Error).Infof("bad header %d [%x…], known bad hash", header.Number, hash)
-			return i, BadHashError(hash)
-		}
-		// Write the header to the chain and get the status
+		// If the header's already known, skip it, otherwise store
 		if self.HasHeader(hash) {
 			stats.ignored++
 			continue
@@ -743,76 +797,116 @@ func (self *BlockChain) InsertReceiptChain(blockChain types.Blocks, receiptChain
 	defer self.wg.Done()
 
 	// Collect some import statistics to report on
-	stats := struct{ processed, ignored int }{}
+	stats := struct{ processed, ignored int32 }{}
 	start := time.Now()
 
-	// Iterate over the blocks and receipts, inserting any new ones
+	// Create the block importing task queue and worker functions
+	tasks := make(chan int, len(blockChain))
 	for i := 0; i < len(blockChain) && i < len(receiptChain); i++ {
-		block, receipts := blockChain[i], receiptChain[i]
+		tasks <- i
+	}
+	close(tasks)
 
-		// Short circuit insertion if shutting down
-		if atomic.LoadInt32(&self.procInterrupt) == 1 {
-			glog.V(logger.Debug).Infoln("premature abort during receipt chain processing")
-			break
-		}
-		// Short circuit if the owner header is unknown
-		if !self.HasHeader(block.Hash()) {
-			glog.V(logger.Debug).Infof("containing header #%d [%x…] unknown", block.Number(), block.Hash().Bytes()[:4])
-			return i, fmt.Errorf("containing header #%d [%x…] unknown", block.Number(), block.Hash().Bytes()[:4])
-		}
-		// Skip if the entire data is already known
-		if self.HasBlock(block.Hash()) {
-			stats.ignored++
-			continue
-		}
-		// Compute all the non-consensus fields of the receipts
-		transactions, logIndex := block.Transactions(), uint(0)
-		for j := 0; j < len(receipts); j++ {
-			// The transaction hash can be retrieved from the transaction itself
-			receipts[j].TxHash = transactions[j].Hash()
-
-			// The contract address can be derived from the transaction itself
-			if MessageCreatesContract(transactions[j]) {
-				from, _ := transactions[j].From()
-				receipts[j].ContractAddress = crypto.CreateAddress(from, transactions[j].Nonce())
+	errs, failed := make([]error, len(tasks)), int32(0)
+	process := func(worker int) {
+		for index := range tasks {
+			block, receipts := blockChain[index], receiptChain[index]
+
+			// Short circuit insertion if shutting down or processing failed
+			if atomic.LoadInt32(&self.procInterrupt) == 1 {
+				return
 			}
-			// The used gas can be calculated based on previous receipts
-			if j == 0 {
-				receipts[j].GasUsed = new(big.Int).Set(receipts[j].CumulativeGasUsed)
-			} else {
-				receipts[j].GasUsed = new(big.Int).Sub(receipts[j].CumulativeGasUsed, receipts[j-1].CumulativeGasUsed)
+			if atomic.LoadInt32(&failed) > 0 {
+				return
 			}
-			// The derived log fields can simply be set from the block and transaction
-			for k := 0; k < len(receipts[j].Logs); k++ {
-				receipts[j].Logs[k].BlockNumber = block.NumberU64()
-				receipts[j].Logs[k].BlockHash = block.Hash()
-				receipts[j].Logs[k].TxHash = receipts[j].TxHash
-				receipts[j].Logs[k].TxIndex = uint(j)
-				receipts[j].Logs[k].Index = logIndex
-				logIndex++
+			// Short circuit if the owner header is unknown
+			if !self.HasHeader(block.Hash()) {
+				errs[index] = fmt.Errorf("containing header #%d [%x…] unknown", block.Number(), block.Hash().Bytes()[:4])
+				atomic.AddInt32(&failed, 1)
+				return
 			}
+			// Skip if the entire data is already known
+			if self.HasBlock(block.Hash()) {
+				atomic.AddInt32(&stats.ignored, 1)
+				continue
+			}
+			// Compute all the non-consensus fields of the receipts
+			transactions, logIndex := block.Transactions(), uint(0)
+			for j := 0; j < len(receipts); j++ {
+				// The transaction hash can be retrieved from the transaction itself
+				receipts[j].TxHash = transactions[j].Hash()
+
+				// The contract address can be derived from the transaction itself
+				if MessageCreatesContract(transactions[j]) {
+					from, _ := transactions[j].From()
+					receipts[j].ContractAddress = crypto.CreateAddress(from, transactions[j].Nonce())
+				}
+				// The used gas can be calculated based on previous receipts
+				if j == 0 {
+					receipts[j].GasUsed = new(big.Int).Set(receipts[j].CumulativeGasUsed)
+				} else {
+					receipts[j].GasUsed = new(big.Int).Sub(receipts[j].CumulativeGasUsed, receipts[j-1].CumulativeGasUsed)
+				}
+				// The derived log fields can simply be set from the block and transaction
+				for k := 0; k < len(receipts[j].Logs); k++ {
+					receipts[j].Logs[k].BlockNumber = block.NumberU64()
+					receipts[j].Logs[k].BlockHash = block.Hash()
+					receipts[j].Logs[k].TxHash = receipts[j].TxHash
+					receipts[j].Logs[k].TxIndex = uint(j)
+					receipts[j].Logs[k].Index = logIndex
+					logIndex++
+				}
+			}
+			// Write all the data out into the database
+			if err := WriteBody(self.chainDb, block.Hash(), &types.Body{block.Transactions(), block.Uncles()}); err != nil {
+				errs[index] = fmt.Errorf("failed to write block body: %v", err)
+				atomic.AddInt32(&failed, 1)
+				glog.Fatal(errs[index])
+				return
+			}
+			if err := PutBlockReceipts(self.chainDb, block.Hash(), receipts); err != nil {
+				errs[index] = fmt.Errorf("failed to write block receipts: %v", err)
+				atomic.AddInt32(&failed, 1)
+				glog.Fatal(errs[index])
+				return
+			}
+			atomic.AddInt32(&stats.processed, 1)
 		}
-		// Write all the data out into the database
-		if err := WriteBody(self.chainDb, block.Hash(), &types.Body{block.Transactions(), block.Uncles()}); err != nil {
-			glog.Fatalf("failed to write block body: %v", err)
-			return i, err
-		}
-		if err := PutBlockReceipts(self.chainDb, block.Hash(), receipts); err != nil {
-			glog.Fatalf("failed to write block receipts: %v", err)
-			return i, err
-		}
-		// Update the head fast sync block if better
-		self.mu.Lock()
-		if self.GetTd(self.currentFastBlock.Hash()).Cmp(self.GetTd(block.Hash())) < 0 {
-			if err := WriteHeadFastBlockHash(self.chainDb, block.Hash()); err != nil {
-				glog.Fatalf("failed to update head fast block hash: %v", err)
+	}
+	// Start as many worker threads as goroutines allowed
+	pending := new(sync.WaitGroup)
+	for i := 0; i < runtime.GOMAXPROCS(0); i++ {
+		pending.Add(1)
+		go func(id int) {
+			defer pending.Done()
+			process(id)
+		}(i)
+	}
+	pending.Wait()
+
+	// If anything failed, report
+	if atomic.LoadInt32(&self.procInterrupt) == 1 {
+		glog.V(logger.Debug).Infoln("premature abort during receipt chain processing")
+		return 0, nil
+	}
+	if failed > 0 {
+		for i, err := range errs {
+			if err != nil {
+				return i, err
 			}
-			self.currentFastBlock = block
 		}
-		self.mu.Unlock()
-
-		stats.processed++
 	}
+	// Update the head fast sync block if better
+	self.mu.Lock()
+	head := blockChain[len(errs)-1]
+	if self.GetTd(self.currentFastBlock.Hash()).Cmp(self.GetTd(head.Hash())) < 0 {
+		if err := WriteHeadFastBlockHash(self.chainDb, head.Hash()); err != nil {
+			glog.Fatalf("failed to update head fast block hash: %v", err)
+		}
+		self.currentFastBlock = head
+	}
+	self.mu.Unlock()
+
 	// Report some public statistics so the user has a clue what's going on
 	first, last := blockChain[0], blockChain[len(blockChain)-1]
 	glog.V(logger.Info).Infof("imported %d receipt(s) (%d ignored) in %v. #%d [%x… / %x…]", stats.processed, stats.ignored,
diff --git a/core/blockchain_test.go b/core/blockchain_test.go
index 93c2128bc..a614aaa2f 100644
--- a/core/blockchain_test.go
+++ b/core/blockchain_test.go
@@ -94,7 +94,7 @@ func testFork(t *testing.T, processor *BlockProcessor, i, n int, full bool, comp
 		}
 	} else {
 		headerChainB = makeHeaderChain(processor2.bc.CurrentHeader(), n, db, forkSeed)
-		if _, err := processor2.bc.InsertHeaderChain(headerChainB, true); err != nil {
+		if _, err := processor2.bc.InsertHeaderChain(headerChainB, 1); err != nil {
 			t.Fatalf("failed to insert forking chain: %v", err)
 		}
 	}
@@ -415,7 +415,9 @@ func TestChainMultipleInsertions(t *testing.T) {
 
 type bproc struct{}
 
-func (bproc) Process(*types.Block) (vm.Logs, types.Receipts, error) { return nil, nil, nil }
+func (bproc) Process(*types.Block) (vm.Logs, types.Receipts, error)                   { return nil, nil, nil }
+func (bproc) ValidateHeader(*types.Header, bool, bool) error                          { return nil }
+func (bproc) ValidateHeaderWithParent(*types.Header, *types.Header, bool, bool) error { return nil }
 
 func makeHeaderChainWithDiff(genesis *types.Block, d []int, seed byte) []*types.Header {
 	blocks := makeBlockChainWithDiff(genesis, d, seed)
@@ -492,8 +494,8 @@ func testReorg(t *testing.T, first, second []int, td int64, full bool) {
 		bc.InsertChain(makeBlockChainWithDiff(genesis, first, 11))
 		bc.InsertChain(makeBlockChainWithDiff(genesis, second, 22))
 	} else {
-		bc.InsertHeaderChain(makeHeaderChainWithDiff(genesis, first, 11), false)
-		bc.InsertHeaderChain(makeHeaderChainWithDiff(genesis, second, 22), false)
+		bc.InsertHeaderChain(makeHeaderChainWithDiff(genesis, first, 11), 1)
+		bc.InsertHeaderChain(makeHeaderChainWithDiff(genesis, second, 22), 1)
 	}
 	// Check that the chain is valid number and link wise
 	if full {
@@ -543,7 +545,7 @@ func testBadHashes(t *testing.T, full bool) {
 	} else {
 		headers := makeHeaderChainWithDiff(genesis, []int{1, 2, 4}, 10)
 		BadHashes[headers[2].Hash()] = true
-		_, err = bc.InsertHeaderChain(headers, true)
+		_, err = bc.InsertHeaderChain(headers, 1)
 	}
 	if !IsBadHashError(err) {
 		t.Errorf("error mismatch: want: BadHashError, have: %v", err)
@@ -575,7 +577,7 @@ func testReorgBadHashes(t *testing.T, full bool) {
 		BadHashes[blocks[3].Header().Hash()] = true
 		defer func() { delete(BadHashes, blocks[3].Header().Hash()) }()
 	} else {
-		if _, err := bc.InsertHeaderChain(headers, true); err != nil {
+		if _, err := bc.InsertHeaderChain(headers, 1); err != nil {
 			t.Fatalf("failed to import headers: %v", err)
 		}
 		if bc.CurrentHeader().Hash() != headers[3].Hash() {
@@ -631,6 +633,8 @@ func testInsertNonceError(t *testing.T, full bool) {
 			failHash = blocks[failAt].Hash()
 
 			processor.bc.pow = failPow{failNum}
+			processor.Pow = failPow{failNum}
+
 			failRes, err = processor.bc.InsertChain(blocks)
 		} else {
 			headers := makeHeaderChain(processor.bc.CurrentHeader(), i, db, 0)
@@ -640,7 +644,9 @@ func testInsertNonceError(t *testing.T, full bool) {
 			failHash = headers[failAt].Hash()
 
 			processor.bc.pow = failPow{failNum}
-			failRes, err = processor.bc.InsertHeaderChain(headers, true)
+			processor.Pow = failPow{failNum}
+
+			failRes, err = processor.bc.InsertHeaderChain(headers, 1)
 		}
 		// Check that the returned error indicates the nonce failure.
 		if failRes != failAt {
@@ -714,12 +720,13 @@ func TestFastVsFullChains(t *testing.T) {
 	fastDb, _ := ethdb.NewMemDatabase()
 	WriteGenesisBlockForTesting(fastDb, GenesisAccount{address, funds})
 	fast, _ := NewBlockChain(fastDb, FakePow{}, new(event.TypeMux))
+	fast.SetProcessor(NewBlockProcessor(fastDb, FakePow{}, fast, new(event.TypeMux)))
 
 	headers := make([]*types.Header, len(blocks))
 	for i, block := range blocks {
 		headers[i] = block.Header()
 	}
-	if n, err := fast.InsertHeaderChain(headers, true); err != nil {
+	if n, err := fast.InsertHeaderChain(headers, 1); err != nil {
 		t.Fatalf("failed to insert header %d: %v", n, err)
 	}
 	if n, err := fast.InsertReceiptChain(blocks, receipts); err != nil {
@@ -796,12 +803,13 @@ func TestLightVsFastVsFullChainHeads(t *testing.T) {
 	fastDb, _ := ethdb.NewMemDatabase()
 	WriteGenesisBlockForTesting(fastDb, GenesisAccount{address, funds})
 	fast, _ := NewBlockChain(fastDb, FakePow{}, new(event.TypeMux))
+	fast.SetProcessor(NewBlockProcessor(fastDb, FakePow{}, fast, new(event.TypeMux)))
 
 	headers := make([]*types.Header, len(blocks))
 	for i, block := range blocks {
 		headers[i] = block.Header()
 	}
-	if n, err := fast.InsertHeaderChain(headers, true); err != nil {
+	if n, err := fast.InsertHeaderChain(headers, 1); err != nil {
 		t.Fatalf("failed to insert header %d: %v", n, err)
 	}
 	if n, err := fast.InsertReceiptChain(blocks, receipts); err != nil {
@@ -813,8 +821,9 @@ func TestLightVsFastVsFullChainHeads(t *testing.T) {
 	lightDb, _ := ethdb.NewMemDatabase()
 	WriteGenesisBlockForTesting(lightDb, GenesisAccount{address, funds})
 	light, _ := NewBlockChain(lightDb, FakePow{}, new(event.TypeMux))
+	light.SetProcessor(NewBlockProcessor(lightDb, FakePow{}, light, new(event.TypeMux)))
 
-	if n, err := light.InsertHeaderChain(headers, true); err != nil {
+	if n, err := light.InsertHeaderChain(headers, 1); err != nil {
 		t.Fatalf("failed to insert header %d: %v", n, err)
 	}
 	assert(t, "light", light, height, 0, 0)
diff --git a/core/chain_makers.go b/core/chain_makers.go
index 31b2627af..1d41b4091 100644
--- a/core/chain_makers.go
+++ b/core/chain_makers.go
@@ -239,7 +239,7 @@ func newCanonical(n int, full bool) (ethdb.Database, *BlockProcessor, error) {
 	}
 	// Header-only chain requested
 	headers := makeHeaderChain(genesis.Header(), n, db, canonicalSeed)
-	_, err := blockchain.InsertHeaderChain(headers, true)
+	_, err := blockchain.InsertHeaderChain(headers, 1)
 	return db, processor, err
 }
 
diff --git a/core/error.go b/core/error.go
index 5e32124a7..af42cd19c 100644
--- a/core/error.go
+++ b/core/error.go
@@ -111,7 +111,7 @@ type BlockNonceErr struct {
 }
 
 func (err *BlockNonceErr) Error() string {
-	return fmt.Sprintf("block %d (%v) nonce is invalid (got %d)", err.Number, err.Hash, err.Nonce)
+	return fmt.Sprintf("nonce for #%d [%x…] is invalid (got %d)", err.Number, err.Hash, err.Nonce)
 }
 
 // IsBlockNonceErr returns true for invalid block nonce errors.
diff --git a/core/state/sync.go b/core/state/sync.go
index e9bebe8ee..5a388886c 100644
--- a/core/state/sync.go
+++ b/core/state/sync.go
@@ -21,78 +21,51 @@ import (
 	"math/big"
 
 	"github.com/ethereum/go-ethereum/common"
-	"github.com/ethereum/go-ethereum/crypto/sha3"
 	"github.com/ethereum/go-ethereum/ethdb"
 	"github.com/ethereum/go-ethereum/rlp"
 	"github.com/ethereum/go-ethereum/trie"
 )
 
-type StateSync struct {
-	db          ethdb.Database
-	sync        *trie.TrieSync
-	codeReqs    map[common.Hash]struct{} // requested but not yet written to database
-	codeReqList []common.Hash            // requested since last Missing
-}
+// StateSync is the main state  synchronisation scheduler, which provides yet the
+// unknown state hashes to retrieve, accepts node data associated with said hashes
+// and reconstructs the state database step by step until all is done.
+type StateSync trie.TrieSync
 
-var sha3_nil = common.BytesToHash(sha3.NewKeccak256().Sum(nil))
+// NewStateSync create a new state trie download scheduler.
+func NewStateSync(root common.Hash, database ethdb.Database) *StateSync {
+	// Pre-declare the result syncer t
+	var syncer *trie.TrieSync
 
-func NewStateSync(root common.Hash, db ethdb.Database) *StateSync {
-	ss := &StateSync{
-		db:       db,
-		codeReqs: make(map[common.Hash]struct{}),
-	}
-	ss.codeReqs[sha3_nil] = struct{}{} // never request the nil hash
-	ss.sync = trie.NewTrieSync(root, db, ss.leafFound)
-	return ss
-}
+	callback := func(leaf []byte, parent common.Hash) error {
+		var obj struct {
+			Nonce    uint64
+			Balance  *big.Int
+			Root     common.Hash
+			CodeHash []byte
+		}
+		if err := rlp.Decode(bytes.NewReader(leaf), &obj); err != nil {
+			return err
+		}
+		syncer.AddSubTrie(obj.Root, 64, parent, nil)
+		syncer.AddRawEntry(common.BytesToHash(obj.CodeHash), 64, parent)
 
-func (self *StateSync) leafFound(leaf []byte, parent common.Hash) error {
-	var obj struct {
-		Nonce    uint64
-		Balance  *big.Int
-		Root     common.Hash
-		CodeHash []byte
-	}
-	if err := rlp.Decode(bytes.NewReader(leaf), &obj); err != nil {
-		return err
+		return nil
 	}
-	self.sync.AddSubTrie(obj.Root, 64, parent, nil)
+	syncer = trie.NewTrieSync(root, database, callback)
+	return (*StateSync)(syncer)
+}
 
-	codehash := common.BytesToHash(obj.CodeHash)
-	if _, ok := self.codeReqs[codehash]; !ok {
-		code, _ := self.db.Get(obj.CodeHash)
-		if code == nil {
-			self.codeReqs[codehash] = struct{}{}
-			self.codeReqList = append(self.codeReqList, codehash)
-		}
-	}
-	return nil
+// Missing retrieves the known missing nodes from the state trie for retrieval.
+func (s *StateSync) Missing(max int) []common.Hash {
+	return (*trie.TrieSync)(s).Missing(max)
 }
 
-func (self *StateSync) Missing(max int) []common.Hash {
-	cr := len(self.codeReqList)
-	gh := 0
-	if max != 0 {
-		if cr > max {
-			cr = max
-		}
-		gh = max - cr
-	}
-	list := append(self.sync.Missing(gh), self.codeReqList[:cr]...)
-	self.codeReqList = self.codeReqList[cr:]
-	return list
+// Process injects a batch of retrieved trie nodes data.
+func (s *StateSync) Process(list []trie.SyncResult) (int, error) {
+	return (*trie.TrieSync)(s).Process(list)
 }
 
-func (self *StateSync) Process(list []trie.SyncResult) error {
-	for i := 0; i < len(list); i++ {
-		if _, ok := self.codeReqs[list[i].Hash]; ok { // code data, not a node
-			self.db.Put(list[i].Hash[:], list[i].Data)
-			delete(self.codeReqs, list[i].Hash)
-			list[i] = list[len(list)-1]
-			list = list[:len(list)-1]
-			i--
-		}
-	}
-	_, err := self.sync.Process(list)
-	return err
+// Pending returns the number of state entries currently pending for download.
+func (s *StateSync) Pending() int {
+	return (*trie.TrieSync)(s).Pending()
 }
diff --git a/core/state/sync_test.go b/core/state/sync_test.go
index f6afe8bd8..f0376d484 100644
--- a/core/state/sync_test.go
+++ b/core/state/sync_test.go
@@ -115,8 +115,8 @@ func testIterativeStateSync(t *testing.T, batch int) {
 			}
 			results[i] = trie.SyncResult{hash, data}
 		}
-		if err := sched.Process(results); err != nil {
-			t.Fatalf("failed to process results: %v", err)
+		if index, err := sched.Process(results); err != nil {
+			t.Fatalf("failed to process result #%d: %v", index, err)
 		}
 		queue = append(queue[:0], sched.Missing(batch)...)
 	}
@@ -145,8 +145,8 @@ func TestIterativeDelayedStateSync(t *testing.T) {
 			}
 			results[i] = trie.SyncResult{hash, data}
 		}
-		if err := sched.Process(results); err != nil {
-			t.Fatalf("failed to process results: %v", err)
+		if index, err := sched.Process(results); err != nil {
+			t.Fatalf("failed to process result #%d: %v", index, err)
 		}
 		queue = append(queue[len(results):], sched.Missing(0)...)
 	}
@@ -183,8 +183,8 @@ func testIterativeRandomStateSync(t *testing.T, batch int) {
 			results = append(results, trie.SyncResult{hash, data})
 		}
 		// Feed the retrieved results back and queue new tasks
-		if err := sched.Process(results); err != nil {
-			t.Fatalf("failed to process results: %v", err)
+		if index, err := sched.Process(results); err != nil {
+			t.Fatalf("failed to process result #%d: %v", index, err)
 		}
 		queue = make(map[common.Hash]struct{})
 		for _, hash := range sched.Missing(batch) {
@@ -226,8 +226,8 @@ func TestIterativeRandomDelayedStateSync(t *testing.T) {
 			}
 		}
 		// Feed the retrieved results back and queue new tasks
-		if err := sched.Process(results); err != nil {
-			t.Fatalf("failed to process results: %v", err)
+		if index, err := sched.Process(results); err != nil {
+			t.Fatalf("failed to process result #%d: %v", index, err)
 		}
 		for _, hash := range sched.Missing(0) {
 			queue[hash] = struct{}{}
diff --git a/core/types/common.go b/core/types/common.go
index 29019a1b4..fe682f98a 100644
--- a/core/types/common.go
+++ b/core/types/common.go
@@ -20,4 +20,6 @@ import "github.com/ethereum/go-ethereum/core/vm"
 
 type BlockProcessor interface {
 	Process(*Block) (vm.Logs, Receipts, error)
+	ValidateHeader(*Header, bool, bool) error
+	ValidateHeaderWithParent(*Header, *Header, bool, bool) error
 }
diff --git a/eth/downloader/downloader.go b/eth/downloader/downloader.go
index 96177ae8a..e19b70dfd 100644
--- a/eth/downloader/downloader.go
+++ b/eth/downloader/downloader.go
@@ -830,7 +830,7 @@ func (d *Downloader) fetchBlocks61(from uint64) error {
 			}
 			// If there's nothing more to fetch, wait or terminate
 			if d.queue.PendingBlocks() == 0 {
-				if d.queue.InFlight() == 0 && finished {
+				if !d.queue.InFlightBlocks() && finished {
 					glog.V(logger.Debug).Infof("Block fetching completed")
 					return nil
 				}
@@ -864,7 +864,7 @@ func (d *Downloader) fetchBlocks61(from uint64) error {
 			}
 			// Make sure that we have peers available for fetching. If all peers have been tried
 			// and all failed throw an error
-			if !throttled && d.queue.InFlight() == 0 && len(idles) == total {
+			if !throttled && !d.queue.InFlightBlocks() && len(idles) == total {
 				return errPeersUnavailable
 			}
 		}
@@ -1124,7 +1124,7 @@ func (d *Downloader) fetchHeaders(p *peer, td *big.Int, from uint64) error {
 			glog.V(logger.Detail).Infof("%v: schedule %d headers from #%d", p, len(headers), from)
 
 			if d.mode == FastSync || d.mode == LightSync {
-				if n, err := d.insertHeaders(headers, false); err != nil {
+				if n, err := d.insertHeaders(headers, headerCheckFrequency); err != nil {
 					glog.V(logger.Debug).Infof("%v: invalid header #%d [%x…]: %v", p, headers[n].Number, headers[n].Hash().Bytes()[:4], err)
 					return errInvalidChain
 				}
@@ -1194,8 +1194,8 @@ func (d *Downloader) fetchBodies(from uint64) error {
 		setIdle  = func(p *peer) { p.SetBlocksIdle() }
 	)
 	err := d.fetchParts(errCancelBodyFetch, d.bodyCh, deliver, d.bodyWakeCh, expire,
-		d.queue.PendingBlocks, d.queue.ThrottleBlocks, d.queue.ReserveBodies, d.bodyFetchHook,
-		fetch, d.queue.CancelBodies, capacity, getIdles, setIdle, "Body")
+		d.queue.PendingBlocks, d.queue.InFlightBlocks, d.queue.ThrottleBlocks, d.queue.ReserveBodies,
+		d.bodyFetchHook, fetch, d.queue.CancelBodies, capacity, getIdles, setIdle, "Body")
 
 	glog.V(logger.Debug).Infof("Block body download terminated: %v", err)
 	return err
@@ -1218,8 +1218,8 @@ func (d *Downloader) fetchReceipts(from uint64) error {
 		setIdle  = func(p *peer) { p.SetReceiptsIdle() }
 	)
 	err := d.fetchParts(errCancelReceiptFetch, d.receiptCh, deliver, d.receiptWakeCh, expire,
-		d.queue.PendingReceipts, d.queue.ThrottleReceipts, d.queue.ReserveReceipts, d.receiptFetchHook,
-		fetch, d.queue.CancelReceipts, capacity, d.peers.ReceiptIdlePeers, setIdle, "Receipt")
+		d.queue.PendingReceipts, d.queue.InFlightReceipts, d.queue.ThrottleReceipts, d.queue.ReserveReceipts,
+		d.receiptFetchHook, fetch, d.queue.CancelReceipts, capacity, d.peers.ReceiptIdlePeers, setIdle, "Receipt")
 
 	glog.V(logger.Debug).Infof("Receipt download terminated: %v", err)
 	return err
@@ -1234,15 +1234,29 @@ func (d *Downloader) fetchNodeData() error {
 	var (
 		deliver = func(packet dataPack) error {
 			start := time.Now()
-			done, found, err := d.queue.DeliverNodeData(packet.PeerId(), packet.(*statePack).states)
-
-			d.syncStatsLock.Lock()
-			totalDone, totalKnown := d.syncStatsStateDone+uint64(done), d.syncStatsStateTotal+uint64(found)
-			d.syncStatsStateDone, d.syncStatsStateTotal = totalDone, totalKnown
-			d.syncStatsLock.Unlock()
+			return d.queue.DeliverNodeData(packet.PeerId(), packet.(*statePack).states, func(err error, delivered int) {
+				if err != nil {
+					// If the node data processing failed, the root hash is very wrong, abort
+					glog.V(logger.Error).Infof("peer %d: state processing failed: %v", packet.PeerId(), err)
+					d.cancel()
+					return
+				}
+				// Processing succeeded, notify state fetcher and processor of continuation
+				if d.queue.PendingNodeData() == 0 {
+					go d.process()
+				} else {
+					select {
+					case d.stateWakeCh <- true:
+					default:
+					}
+				}
+				// Log a message to the user and return
+				d.syncStatsLock.Lock()
+				defer d.syncStatsLock.Unlock()
 
-			glog.V(logger.Info).Infof("imported %d [%d / %d] state entries in %v.", done, totalDone, totalKnown, time.Since(start))
-			return err
+				d.syncStatsStateDone += uint64(delivered)
+				glog.V(logger.Info).Infof("imported %d state entries in %v: processed %d in total", delivered, time.Since(start), d.syncStatsStateDone)
+			})
 		}
 		expire   = func() []string { return d.queue.ExpireNodeData(stateHardTTL) }
 		throttle = func() bool { return false }
@@ -1254,8 +1268,8 @@ func (d *Downloader) fetchNodeData() error {
 		setIdle  = func(p *peer) { p.SetNodeDataIdle() }
 	)
 	err := d.fetchParts(errCancelReceiptFetch, d.stateCh, deliver, d.stateWakeCh, expire,
-		d.queue.PendingNodeData, throttle, reserve, nil, fetch, d.queue.CancelNodeData,
-		capacity, d.peers.ReceiptIdlePeers, setIdle, "State")
+		d.queue.PendingNodeData, d.queue.InFlightNodeData, throttle, reserve, nil, fetch,
+		d.queue.CancelNodeData, capacity, d.peers.ReceiptIdlePeers, setIdle, "State")
 
 	glog.V(logger.Debug).Infof("Node state data download terminated: %v", err)
 	return err
@@ -1265,8 +1279,9 @@ func (d *Downloader) fetchNodeData() error {
 // peers, reserving a chunk of fetch requests for each, waiting for delivery and
 // also periodically checking for timeouts.
 func (d *Downloader) fetchParts(errCancel error, deliveryCh chan dataPack, deliver func(packet dataPack) error, wakeCh chan bool,
-	expire func() []string, pending func() int, throttle func() bool, reserve func(*peer, int) (*fetchRequest, bool, error), fetchHook func([]*types.Header),
-	fetch func(*peer, *fetchRequest) error, cancel func(*fetchRequest), capacity func(*peer) int, idle func() ([]*peer, int), setIdle func(*peer), kind string) error {
+	expire func() []string, pending func() int, inFlight func() bool, throttle func() bool, reserve func(*peer, int) (*fetchRequest, bool, error),
+	fetchHook func([]*types.Header), fetch func(*peer, *fetchRequest) error, cancel func(*fetchRequest), capacity func(*peer) int,
+	idle func() ([]*peer, int), setIdle func(*peer), kind string) error {
 
 	// Create a ticker to detect expired retreival tasks
 	ticker := time.NewTicker(100 * time.Millisecond)
@@ -1378,14 +1393,14 @@ func (d *Downloader) fetchParts(errCancel error, deliveryCh chan dataPack, deliv
 			}
 			// If there's nothing more to fetch, wait or terminate
 			if pending() == 0 {
-				if d.queue.InFlight() == 0 && finished {
+				if !inFlight() && finished {
 					glog.V(logger.Debug).Infof("%s fetching completed", kind)
 					return nil
 				}
 				break
 			}
 			// Send a download request to all idle peers, until throttled
-			progressed, throttled := false, false
+			progressed, throttled, running := false, false, inFlight()
 			idles, total := idle()
 
 			for _, peer := range idles {
@@ -1423,10 +1438,11 @@ func (d *Downloader) fetchParts(errCancel error, deliveryCh chan dataPack, deliv
 					glog.V(logger.Error).Infof("%v: %s fetch failed, rescheduling", peer, strings.ToLower(kind))
 					cancel(request)
 				}
+				running = true
 			}
 			// Make sure that we have peers available for fetching. If all peers have been tried
 			// and all failed throw an error
-			if !progressed && !throttled && d.queue.InFlight() == 0 && len(idles) == total {
+			if !progressed && !throttled && !running && len(idles) == total && pending() > 0 {
 				return errPeersUnavailable
 			}
 		}
@@ -1514,12 +1530,12 @@ func (d *Downloader) process() {
 			)
 			switch {
 			case len(headers) > 0:
-				index, err = d.insertHeaders(headers, true)
+				index, err = d.insertHeaders(headers, headerCheckFrequency)
 
 			case len(receipts) > 0:
 				index, err = d.insertReceipts(blocks, receipts)
 				if err == nil && blocks[len(blocks)-1].NumberU64() == d.queue.fastSyncPivot {
-					err = d.commitHeadBlock(blocks[len(blocks)-1].Hash())
+					index, err = len(blocks)-1, d.commitHeadBlock(blocks[len(blocks)-1].Hash())
 				}
 			default:
 				index, err = d.insertBlocks(blocks)
diff --git a/eth/downloader/downloader_test.go b/eth/downloader/downloader_test.go
index 8944ae4b0..0e60371b3 100644
--- a/eth/downloader/downloader_test.go
+++ b/eth/downloader/downloader_test.go
@@ -268,7 +268,7 @@ func (dl *downloadTester) getTd(hash common.Hash) *big.Int {
 }
 
 // insertHeaders injects a new batch of headers into the simulated chain.
-func (dl *downloadTester) insertHeaders(headers []*types.Header, verify bool) (int, error) {
+func (dl *downloadTester) insertHeaders(headers []*types.Header, checkFreq int) (int, error) {
 	dl.lock.Lock()
 	defer dl.lock.Unlock()
 
@@ -1262,7 +1262,7 @@ func testForkedSyncBoundaries(t *testing.T, protocol int, mode SyncMode) {
 	pending.Wait()
 
 	// Simulate a successful sync above the fork
-	tester.downloader.syncStatsOrigin = tester.downloader.syncStatsHeight
+	tester.downloader.syncStatsChainOrigin = tester.downloader.syncStatsChainHeight
 
 	// Synchronise with the second fork and check boundary resets
 	tester.newPeer("fork B", protocol, hashesB, headersB, blocksB, receiptsB)
diff --git a/eth/downloader/queue.go b/eth/downloader/queue.go
index 942ed0d63..bb8d892cd 100644
--- a/eth/downloader/queue.go
+++ b/eth/downloader/queue.go
@@ -23,6 +23,7 @@ import (
 	"errors"
 	"fmt"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/ethereum/go-ethereum/common"
@@ -93,8 +94,10 @@ type queue struct {
 	stateTaskQueue *prque.Prque             // [eth/63] Priority queue of the hashes to fetch the node data for
 	statePendPool  map[string]*fetchRequest // [eth/63] Currently pending node data retrieval operations
 
-	stateDatabase  ethdb.Database   // [eth/63] Trie database to populate during state reassembly
-	stateScheduler *state.StateSync // [eth/63] State trie synchronisation scheduler and integrator
+	stateDatabase   ethdb.Database   // [eth/63] Trie database to populate during state reassembly
+	stateScheduler  *state.StateSync // [eth/63] State trie synchronisation scheduler and integrator
+	stateProcessors int32            // [eth/63] Number of currently running state processors
+	stateSchedLock  sync.RWMutex     // [eth/63] Lock serializing access to the state scheduler
 
 	resultCache  []*fetchResult // Downloaded but not yet delivered fetch results
 	resultOffset uint64         // Offset of the first cached fetch result in the block-chain
@@ -175,18 +178,40 @@ func (q *queue) PendingReceipts() int {
 
 // PendingNodeData retrieves the number of node data entries pending for retrieval.
 func (q *queue) PendingNodeData() int {
+	q.stateSchedLock.RLock()
+	defer q.stateSchedLock.RUnlock()
+
+	if q.stateScheduler != nil {
+		return q.stateScheduler.Pending()
+	}
+	return 0
+}
+
+// InFlightBlocks retrieves whether there are block fetch requests currently in
+// flight.
+func (q *queue) InFlightBlocks() bool {
 	q.lock.RLock()
 	defer q.lock.RUnlock()
 
-	return q.stateTaskQueue.Size()
+	return len(q.blockPendPool) > 0
 }
 
-// InFlight retrieves the number of fetch requests currently in flight.
-func (q *queue) InFlight() int {
+// InFlightReceipts retrieves whether there are receipt fetch requests currently
+// in flight.
+func (q *queue) InFlightReceipts() bool {
 	q.lock.RLock()
 	defer q.lock.RUnlock()
 
-	return len(q.blockPendPool) + len(q.receiptPendPool) + len(q.statePendPool)
+	return len(q.receiptPendPool) > 0
+}
+
+// InFlightNodeData retrieves whether there are node data entry fetch requests
+// currently in flight.
+func (q *queue) InFlightNodeData() bool {
+	q.lock.RLock()
+	defer q.lock.RUnlock()
+
+	return len(q.statePendPool)+int(atomic.LoadInt32(&q.stateProcessors)) > 0
 }
 
 // Idle returns if the queue is fully idle or has some data still inside. This
@@ -199,6 +224,12 @@ func (q *queue) Idle() bool {
 	pending := len(q.blockPendPool) + len(q.receiptPendPool) + len(q.statePendPool)
 	cached := len(q.blockDonePool) + len(q.receiptDonePool)
 
+	q.stateSchedLock.RLock()
+	if q.stateScheduler != nil {
+		queued += q.stateScheduler.Pending()
+	}
+	q.stateSchedLock.RUnlock()
+
 	return (queued + pending + cached) == 0
 }
 
@@ -299,12 +330,9 @@ func (q *queue) Schedule(headers []*types.Header, from uint64) []*types.Header {
 		}
 		if q.mode == FastSync && header.Number.Uint64() == q.fastSyncPivot {
 			// Pivoting point of the fast sync, retrieve the state tries
+			q.stateSchedLock.Lock()
 			q.stateScheduler = state.NewStateSync(header.Root, q.stateDatabase)
-			for _, hash := range q.stateScheduler.Missing(0) {
-				q.stateTaskPool[hash] = q.stateTaskIndex
-				q.stateTaskQueue.Push(hash, -float32(q.stateTaskIndex))
-				q.stateTaskIndex++
-			}
+			q.stateSchedLock.Unlock()
 		}
 		inserts = append(inserts, header)
 		q.headerHead = hash
@@ -325,8 +353,13 @@ func (q *queue) GetHeadResult() *fetchResult {
 	if q.resultCache[0].Pending > 0 {
 		return nil
 	}
-	if q.mode == FastSync && q.resultCache[0].Header.Number.Uint64() == q.fastSyncPivot && len(q.stateTaskPool) > 0 {
-		return nil
+	if q.mode == FastSync && q.resultCache[0].Header.Number.Uint64() == q.fastSyncPivot {
+		if len(q.stateTaskPool) > 0 {
+			return nil
+		}
+		if q.PendingNodeData() > 0 {
+			return nil
+		}
 	}
 	return q.resultCache[0]
 }
@@ -345,8 +378,13 @@ func (q *queue) TakeResults() []*fetchResult {
 			break
 		}
 		// The fast sync pivot block may only be processed after state fetch completes
-		if q.mode == FastSync && result.Header.Number.Uint64() == q.fastSyncPivot && len(q.stateTaskPool) > 0 {
-			break
+		if q.mode == FastSync && result.Header.Number.Uint64() == q.fastSyncPivot {
+			if len(q.stateTaskPool) > 0 {
+				break
+			}
+			if q.PendingNodeData() > 0 {
+				break
+			}
 		}
 		// If we've just inserted the fast sync pivot, stop as the following batch needs different insertion
 		if q.mode == FastSync && result.Header.Number.Uint64() == q.fastSyncPivot+1 && len(results) > 0 {
@@ -373,26 +411,34 @@ func (q *queue) TakeResults() []*fetchResult {
 // ReserveBlocks reserves a set of block hashes for the given peer, skipping any
 // previously failed download.
 func (q *queue) ReserveBlocks(p *peer, count int) *fetchRequest {
-	return q.reserveHashes(p, count, q.hashQueue, q.blockPendPool, len(q.resultCache)-len(q.blockDonePool))
+	return q.reserveHashes(p, count, q.hashQueue, nil, q.blockPendPool, len(q.resultCache)-len(q.blockDonePool))
 }
 
 // ReserveNodeData reserves a set of node data hashes for the given peer, skipping
 // any previously failed download.
 func (q *queue) ReserveNodeData(p *peer, count int) *fetchRequest {
-	return q.reserveHashes(p, count, q.stateTaskQueue, q.statePendPool, 0)
+	// Create a task generator to fetch status-fetch tasks if all schedules ones are done
+	generator := func(max int) {
+		q.stateSchedLock.Lock()
+		defer q.stateSchedLock.Unlock()
+
+		for _, hash := range q.stateScheduler.Missing(max) {
+			q.stateTaskPool[hash] = q.stateTaskIndex
+			q.stateTaskQueue.Push(hash, -float32(q.stateTaskIndex))
+			q.stateTaskIndex++
+		}
+	}
+	return q.reserveHashes(p, count, q.stateTaskQueue, generator, q.statePendPool, count)
 }
 
 // reserveHashes reserves a set of hashes for the given peer, skipping previously
 // failed ones.
-func (q *queue) reserveHashes(p *peer, count int, taskQueue *prque.Prque, pendPool map[string]*fetchRequest, maxPending int) *fetchRequest {
+func (q *queue) reserveHashes(p *peer, count int, taskQueue *prque.Prque, taskGen func(int), pendPool map[string]*fetchRequest, maxPending int) *fetchRequest {
 	q.lock.Lock()
 	defer q.lock.Unlock()
 
-	// Short circuit if the pool has been depleted, or if the peer's already
-	// downloading something (sanity check not to corrupt state)
-	if taskQueue.Empty() {
-		return nil
-	}
+	// Short circuit if the peer's already downloading something (sanity check not
+	// to corrupt state)
 	if _, ok := pendPool[p.id]; ok {
 		return nil
 	}
@@ -403,6 +449,13 @@ func (q *queue) reserveHashes(p *peer, count int, taskQueue *prque.Prque, pendPo
 			allowance -= len(request.Hashes)
 		}
 	}
+	// If there's a task generator, ask it to fill our task queue
+	if taskGen != nil && taskQueue.Size() < allowance {
+		taskGen(allowance - taskQueue.Size())
+	}
+	if taskQueue.Empty() {
+		return nil
+	}
 	// Retrieve a batch of hashes, skipping previously failed ones
 	send := make(map[common.Hash]int)
 	skip := make(map[common.Hash]int)
@@ -809,14 +862,14 @@ func (q *queue) deliver(id string, taskPool map[common.Hash]*types.Header, taskQ
 }
 
 // DeliverNodeData injects a node state data retrieval response into the queue.
-func (q *queue) DeliverNodeData(id string, data [][]byte) (int, int, error) {
+func (q *queue) DeliverNodeData(id string, data [][]byte, callback func(error, int)) error {
 	q.lock.Lock()
 	defer q.lock.Unlock()
 
 	// Short circuit if the data was never requested
 	request := q.statePendPool[id]
 	if request == nil {
-		return 0, 0, errNoFetchesPending
+		return errNoFetchesPending
 	}
 	stateReqTimer.UpdateSince(request.Time)
 	delete(q.statePendPool, id)
@@ -829,7 +882,7 @@ func (q *queue) DeliverNodeData(id string, data [][]byte) (int, int, error) {
 	}
 	// Iterate over the downloaded data and verify each of them
 	errs := make([]error, 0)
-	processed := 0
+	process := []trie.SyncResult{}
 	for _, blob := range data {
 		// Skip any blocks that were not requested
 		hash := common.BytesToHash(crypto.Sha3(blob))
@@ -837,41 +890,58 @@ func (q *queue) DeliverNodeData(id string, data [][]byte) (int, int, error) {
 			errs = append(errs, fmt.Errorf("non-requested state data %x", hash))
 			continue
 		}
-		// Inject the next state trie item into the database
-		if err := q.stateScheduler.Process([]trie.SyncResult{{hash, blob}}); err != nil {
-			errs = []error{err}
-			break
-		}
-		processed++
+		// Inject the next state trie item into the processing queue
+		process = append(process, trie.SyncResult{hash, blob})
 
 		delete(request.Hashes, hash)
 		delete(q.stateTaskPool, hash)
 	}
+	// Start the asynchronous node state data injection
+	atomic.AddInt32(&q.stateProcessors, 1)
+	go func() {
+		defer atomic.AddInt32(&q.stateProcessors, -1)
+		q.deliverNodeData(process, callback)
+	}()
 	// Return all failed or missing fetches to the queue
 	for hash, index := range request.Hashes {
 		q.stateTaskQueue.Push(hash, float32(index))
 	}
-	// Also enqueue any newly required state trie nodes
-	discovered := 0
-	if len(q.stateTaskPool) < maxQueuedStates {
-		for _, hash := range q.stateScheduler.Missing(4 * MaxStateFetch) {
-			q.stateTaskPool[hash] = q.stateTaskIndex
-			q.stateTaskQueue.Push(hash, -float32(q.stateTaskIndex))
-			q.stateTaskIndex++
-			discovered++
-		}
-	}
 	// If none of the data items were good, it's a stale delivery
 	switch {
 	case len(errs) == 0:
-		return processed, discovered, nil
+		return nil
 
 	case len(errs) == len(request.Hashes):
-		return processed, discovered, errStaleDelivery
+		return errStaleDelivery
 
 	default:
-		return processed, discovered, fmt.Errorf("multiple failures: %v", errs)
+		return fmt.Errorf("multiple failures: %v", errs)
+	}
+}
+
+// deliverNodeData is the asynchronous node data processor that injects a batch
+// of sync results into the state scheduler.
+func (q *queue) deliverNodeData(results []trie.SyncResult, callback func(error, int)) {
+	// Process results one by one to permit task fetches in between
+	for i, result := range results {
+		q.stateSchedLock.Lock()
+
+		if q.stateScheduler == nil {
+			// Syncing aborted since this async delivery started, bail out
+			q.stateSchedLock.Unlock()
+			callback(errNoFetchesPending, i)
+			return
+		}
+		if _, err := q.stateScheduler.Process([]trie.SyncResult{result}); err != nil {
+			// Processing a state result failed, bail out
+			q.stateSchedLock.Unlock()
+			callback(err, i)
+			return
+		}
+		// Item processing succeeded, release the lock (temporarily)
+		q.stateSchedLock.Unlock()
 	}
+	callback(nil, len(results))
 }
 
 // Prepare configures the result cache to allow accepting and caching inbound
diff --git a/eth/downloader/types.go b/eth/downloader/types.go
index 221ef38f6..60d9a2b12 100644
--- a/eth/downloader/types.go
+++ b/eth/downloader/types.go
@@ -52,7 +52,7 @@ type headBlockCommitterFn func(common.Hash) error
 type tdRetrievalFn func(common.Hash) *big.Int
 
 // headerChainInsertFn is a callback type to insert a batch of headers into the local chain.
-type headerChainInsertFn func([]*types.Header, bool) (int, error)
+type headerChainInsertFn func([]*types.Header, int) (int, error)
 
 // blockChainInsertFn is a callback type to insert a batch of blocks into the local chain.
 type blockChainInsertFn func(types.Blocks) (int, error)
diff --git a/ethdb/memory_database.go b/ethdb/memory_database.go
index 81911f23f..330834fa4 100644
--- a/ethdb/memory_database.go
+++ b/ethdb/memory_database.go
@@ -18,6 +18,7 @@ package ethdb
 
 import (
 	"fmt"
+	"sync"
 
 	"github.com/ethereum/go-ethereum/common"
 )
@@ -26,29 +27,42 @@ import (
  * This is a test memory database. Do not use for any production it does not get persisted
  */
 type MemDatabase struct {
-	db map[string][]byte
+	db   map[string][]byte
+	lock sync.RWMutex
 }
 
 func NewMemDatabase() (*MemDatabase, error) {
-	db := &MemDatabase{db: make(map[string][]byte)}
-
-	return db, nil
+	return &MemDatabase{
+		db: make(map[string][]byte),
+	}, nil
 }
 
 func (db *MemDatabase) Put(key []byte, value []byte) error {
+	db.lock.Lock()
+	defer db.lock.Unlock()
+
 	db.db[string(key)] = common.CopyBytes(value)
 	return nil
 }
 
 func (db *MemDatabase) Set(key []byte, value []byte) {
+	db.lock.Lock()
+	defer db.lock.Unlock()
+
 	db.Put(key, value)
 }
 
 func (db *MemDatabase) Get(key []byte) ([]byte, error) {
+	db.lock.RLock()
+	defer db.lock.RUnlock()
+
 	return db.db[string(key)], nil
 }
 
 func (db *MemDatabase) Keys() [][]byte {
+	db.lock.RLock()
+	defer db.lock.RUnlock()
+
 	keys := [][]byte{}
 	for key, _ := range db.db {
 		keys = append(keys, []byte(key))
@@ -65,12 +79,17 @@ func (db *MemDatabase) GetKeys() []*common.Key {
 */
 
 func (db *MemDatabase) Delete(key []byte) error {
-	delete(db.db, string(key))
+	db.lock.Lock()
+	defer db.lock.Unlock()
 
+	delete(db.db, string(key))
 	return nil
 }
 
 func (db *MemDatabase) Print() {
+	db.lock.RLock()
+	defer db.lock.RUnlock()
+
 	for key, val := range db.db {
 		fmt.Printf("%x(%d): ", key, len(key))
 		node := common.NewValueFromBytes(val)
@@ -83,11 +102,9 @@ func (db *MemDatabase) Close() {
 
 func (db *MemDatabase) LastKnownTD() []byte {
 	data, _ := db.Get([]byte("LastKnownTotalDifficulty"))
-
 	if len(data) == 0 || data == nil {
 		data = []byte{0x0}
 	}
-
 	return data
 }
 
@@ -100,16 +117,26 @@ type kv struct{ k, v []byte }
 type memBatch struct {
 	db     *MemDatabase
 	writes []kv
+	lock   sync.RWMutex
 }
 
-func (w *memBatch) Put(key, value []byte) error {
-	w.writes = append(w.writes, kv{key, common.CopyBytes(value)})
+func (b *memBatch) Put(key, value []byte) error {
+	b.lock.Lock()
+	defer b.lock.Unlock()
+
+	b.writes = append(b.writes, kv{key, common.CopyBytes(value)})
 	return nil
 }
 
-func (w *memBatch) Write() error {
-	for _, kv := range w.writes {
-		w.db.db[string(kv.k)] = kv.v
+func (b *memBatch) Write() error {
+	b.lock.RLock()
+	defer b.lock.RUnlock()
+
+	b.db.lock.RLock()
+	defer b.db.lock.RUnlock()
+
+	for _, kv := range b.writes {
+		b.db.db[string(kv.k)] = kv.v
 	}
 	return nil
 }
diff --git a/trie/sync.go b/trie/sync.go
index 65cfd6ed8..bb112fb62 100644
--- a/trie/sync.go
+++ b/trie/sync.go
@@ -20,6 +20,7 @@ import (
 	"fmt"
 
 	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/ethdb"
 	"gopkg.in/karalabe/cookiejar.v2/collections/prque"
 )
 
@@ -50,15 +51,15 @@ type TrieSyncLeafCallback func(leaf []byte, parent common.Hash) error
 
 // TrieSync is the main state trie synchronisation scheduler, which provides yet
 // unknown trie hashes to retrieve, accepts node data associated with said hashes
-// and reconstructs the trie steb by step until all is done.
+// and reconstructs the trie step by step until all is done.
 type TrieSync struct {
-	database Database                 // State database for storing all the assembled node data
+	database ethdb.Database           // State database for storing all the assembled node data
 	requests map[common.Hash]*request // Pending requests pertaining to a key hash
 	queue    *prque.Prque             // Priority queue with the pending requests
 }
 
 // NewTrieSync creates a new trie data download scheduler.
-func NewTrieSync(root common.Hash, database Database, callback TrieSyncLeafCallback) *TrieSync {
+func NewTrieSync(root common.Hash, database ethdb.Database, callback TrieSyncLeafCallback) *TrieSync {
 	ts := &TrieSync{
 		database: database,
 		requests: make(map[common.Hash]*request),
@@ -70,10 +71,14 @@ func NewTrieSync(root common.Hash, database Database, callback TrieSyncLeafCallb
 
 // AddSubTrie registers a new trie to the sync code, rooted at the designated parent.
 func (s *TrieSync) AddSubTrie(root common.Hash, depth int, parent common.Hash, callback TrieSyncLeafCallback) {
-	// Short circuit if the trie is empty
+	// Short circuit if the trie is empty or already known
 	if root == emptyRoot {
 		return
 	}
+	blob, _ := s.database.Get(root.Bytes())
+	if local, err := decodeNode(blob); local != nil && err == nil {
+		return
+	}
 	// Assemble the new sub-trie sync request
 	node := node(hashNode(root.Bytes()))
 	req := &request{
@@ -94,6 +99,35 @@ func (s *TrieSync) AddSubTrie(root common.Hash, depth int, parent common.Hash, c
 	s.schedule(req)
 }
 
+// AddRawEntry schedules the direct retrieval of a state entry that should not be
+// interpreted as a trie node, but rather accepted and stored into the database
+// as is. This method's goal is to support misc state metadata retrievals (e.g.
+// contract code).
+func (s *TrieSync) AddRawEntry(hash common.Hash, depth int, parent common.Hash) {
+	// Short circuit if the entry is empty or already known
+	if hash == emptyState {
+		return
+	}
+	if blob, _ := s.database.Get(hash.Bytes()); blob != nil {
+		return
+	}
+	// Assemble the new sub-trie sync request
+	req := &request{
+		hash:  hash,
+		depth: depth,
+	}
+	// If this sub-trie has a designated parent, link them together
+	if parent != (common.Hash{}) {
+		ancestor := s.requests[parent]
+		if ancestor == nil {
+			panic(fmt.Sprintf("raw-entry ancestor not found: %x", parent))
+		}
+		ancestor.deps++
+		req.parents = append(req.parents, ancestor)
+	}
+	s.schedule(req)
+}
+
 // Missing retrieves the known missing nodes from the trie for retrieval.
 func (s *TrieSync) Missing(max int) []common.Hash {
 	requests := []common.Hash{}
@@ -111,6 +145,12 @@ func (s *TrieSync) Process(results []SyncResult) (int, error) {
 		if request == nil {
 			return i, fmt.Errorf("not requested: %x", item.Hash)
 		}
+		// If the item is a raw entry request, commit directly
+		if request.object == nil {
+			request.data = item.Data
+			s.commit(request, nil)
+			continue
+		}
 		// Decode the node data content and update the request
 		node, err := decodeNode(item.Data)
 		if err != nil {
@@ -125,7 +165,7 @@ func (s *TrieSync) Process(results []SyncResult) (int, error) {
 			return i, err
 		}
 		if len(requests) == 0 && request.deps == 0 {
-			s.commit(request)
+			s.commit(request, nil)
 			continue
 		}
 		request.deps += len(requests)
@@ -136,6 +176,11 @@ func (s *TrieSync) Process(results []SyncResult) (int, error) {
 	return 0, nil
 }
 
+// Pending returns the number of state entries currently pending for download.
+func (s *TrieSync) Pending() int {
+	return len(s.requests)
+}
+
 // schedule inserts a new state retrieval request into the fetch queue. If there
 // is already a pending request for this node, the new request will be discarded
 // and only a parent reference added to the old one.
@@ -213,9 +258,16 @@ func (s *TrieSync) children(req *request) ([]*request, error) {
 // commit finalizes a retrieval request and stores it into the database. If any
 // of the referencing parent requests complete due to this commit, they are also
 // committed themselves.
-func (s *TrieSync) commit(req *request) error {
+func (s *TrieSync) commit(req *request, batch ethdb.Batch) (err error) {
+	// Create a new batch if none was specified
+	if batch == nil {
+		batch = s.database.NewBatch()
+		defer func() {
+			err = batch.Write()
+		}()
+	}
 	// Write the node content to disk
-	if err := s.database.Put(req.hash[:], req.data); err != nil {
+	if err := batch.Put(req.hash[:], req.data); err != nil {
 		return err
 	}
 	delete(s.requests, req.hash)
@@ -224,7 +276,7 @@ func (s *TrieSync) commit(req *request) error {
 	for _, parent := range req.parents {
 		parent.deps--
 		if parent.deps == 0 {
-			if err := s.commit(parent); err != nil {
+			if err := s.commit(parent, batch); err != nil {
 				return err
 			}
 		}
diff --git a/trie/trie.go b/trie/trie.go
index aa8d39fe2..a3a383fb5 100644
--- a/trie/trie.go
+++ b/trie/trie.go
@@ -24,6 +24,7 @@ import (
 	"hash"
 
 	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/crypto"
 	"github.com/ethereum/go-ethereum/crypto/sha3"
 	"github.com/ethereum/go-ethereum/logger"
 	"github.com/ethereum/go-ethereum/logger/glog"
@@ -35,8 +36,12 @@ const defaultCacheCapacity = 800
 var (
 	// The global cache stores decoded trie nodes by hash as they get loaded.
 	globalCache = newARC(defaultCacheCapacity)
+
 	// This is the known root hash of an empty trie.
 	emptyRoot = common.HexToHash("56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421")
+
+	// This is the known hash of an empty state trie entry.
+	emptyState = crypto.Sha3Hash(nil)
 )
 
 var ErrMissingRoot = errors.New("missing root node")