major rewrite, reorg of blockpool + new features

- blockpool moves to its own package
- uses errs pkg for its own coded errors
- publicly settable config of params (time intervals and batchsizes)
- test helpers in subpackage
- optional TD in blocks used now to update peers chain info
- major improvement in algorithm
- fix fragility and sync/parallelisation bugs
- implement status for reporting on sync status (peers/hashes/blocks etc)
- several tests added and further corner cases covered

1 files changed, 749 insertions, 0 deletions

diff --git a/blockpool/blockpool.go b/blockpool/blockpool.go
new file mode 100644
index 000000000..0126d734c
--- /dev/null
+++ b/blockpool/blockpool.go
@@ -0,0 +1,749 @@
+package blockpool
+
+import (
+	"bytes"
+	"fmt"
+	"math/big"
+	"sync"
+	"time"
+
+	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/errs"
+	ethlogger "github.com/ethereum/go-ethereum/logger"
+	"github.com/ethereum/go-ethereum/pow"
+)
+
+var plog = ethlogger.NewLogger("Blockpool")
+
+var (
+	// max number of block hashes sent in one request
+	blockHashesBatchSize = 512
+	// max number of blocks sent in one request
+	blockBatchSize = 64
+	// interval between two consecutive block checks (and requests)
+	blocksRequestInterval = 3 * time.Second
+	// level of redundancy in block requests sent
+	blocksRequestRepetition = 1
+	// interval between two consecutive block hash checks (and requests)
+	blockHashesRequestInterval = 3 * time.Second
+	// max number of idle iterations, ie., check through a section without new blocks coming in
+	blocksRequestMaxIdleRounds = 100
+	// timeout interval: max time allowed for peer without sending a block hash
+	blockHashesTimeout = 60 * time.Second
+	// timeout interval: max time allowed for peer without sending a block
+	blocksTimeout = 120 * time.Second
+)
+
+// config embedded in components, by default fall back to constants
+// by default all resolved to local
+type Config struct {
+	BlockHashesBatchSize       int
+	BlockBatchSize             int
+	BlocksRequestRepetition    int
+	BlocksRequestMaxIdleRounds int
+	BlockHashesRequestInterval time.Duration
+	BlocksRequestInterval      time.Duration
+	BlockHashesTimeout         time.Duration
+	BlocksTimeout              time.Duration
+}
+
+// blockpool errors
+const (
+	ErrInvalidBlock = iota
+	ErrInvalidPoW
+	ErrUnrequestedBlock
+	ErrInsufficientChainInfo
+)
+
+var errorToString = map[int]string{
+	ErrInvalidBlock:          "Invalid block",
+	ErrInvalidPoW:            "Invalid PoW",
+	ErrUnrequestedBlock:      "Unrequested block",
+	ErrInsufficientChainInfo: "Insufficient chain info",
+}
+
+// init initialises all your laundry
+func (self *Config) init() {
+	if self.BlockHashesBatchSize == 0 {
+		self.BlockHashesBatchSize = blockHashesBatchSize
+	}
+	if self.BlockBatchSize == 0 {
+		self.BlockBatchSize = blockBatchSize
+	}
+	if self.BlocksRequestRepetition == 0 {
+		self.BlocksRequestRepetition = blocksRequestRepetition
+	}
+	if self.BlocksRequestMaxIdleRounds == 0 {
+		self.BlocksRequestMaxIdleRounds = blocksRequestMaxIdleRounds
+	}
+	if self.BlockHashesRequestInterval == 0 {
+		self.BlockHashesRequestInterval = blockHashesRequestInterval
+	}
+	if self.BlocksRequestInterval == 0 {
+		self.BlocksRequestInterval = blocksRequestInterval
+	}
+	if self.BlockHashesTimeout == 0 {
+		self.BlockHashesTimeout = blockHashesTimeout
+	}
+	if self.BlocksTimeout == 0 {
+		self.BlocksTimeout = blocksTimeout
+	}
+}
+
+// node is the basic unit of the internal model of block chain/tree in the blockpool
+type node struct {
+	lock    sync.RWMutex
+	hash    []byte
+	block   *types.Block
+	hashBy  string
+	blockBy string
+	td      *big.Int
+}
+
+type index struct {
+	int
+}
+
+// entry is the struct kept and indexed in the pool
+type entry struct {
+	node    *node
+	section *section
+	index   *index
+}
+
+type BlockPool struct {
+	Config *Config
+
+	// the minimal interface with blockchain
+	hasBlock    func(hash []byte) bool
+	insertChain func(types.Blocks) error
+	verifyPoW   func(pow.Block) bool
+
+	pool  map[string]*entry
+	peers *peers
+
+	lock      sync.RWMutex
+	chainLock sync.RWMutex
+	// alloc-easy pool of hash slices
+	hashSlicePool chan [][]byte
+
+	status *status
+
+	quit    chan bool
+	wg      sync.WaitGroup
+	running bool
+}
+
+// public constructor
+func New(
+	hasBlock func(hash []byte) bool,
+	insertChain func(types.Blocks) error,
+	verifyPoW func(pow.Block) bool,
+) *BlockPool {
+
+	return &BlockPool{
+		Config:      &Config{},
+		hasBlock:    hasBlock,
+		insertChain: insertChain,
+		verifyPoW:   verifyPoW,
+	}
+}
+
+// allows restart
+func (self *BlockPool) Start() {
+	self.lock.Lock()
+	defer self.lock.Unlock()
+
+	if self.running {
+		return
+	}
+
+	self.Config.init()
+	self.hashSlicePool = make(chan [][]byte, 150)
+	self.status = newStatus()
+	self.quit = make(chan bool)
+	self.pool = make(map[string]*entry)
+	self.running = true
+
+	self.peers = &peers{
+		errors: &errs.Errors{
+			Package: "Blockpool",
+			Errors:  errorToString,
+		},
+		peers:  make(map[string]*peer),
+		status: self.status,
+		bp:     self,
+	}
+	timer := time.NewTicker(3 * time.Second)
+	go func() {
+		for {
+			select {
+			case <-self.quit:
+				return
+			case <-timer.C:
+				plog.Debugf("status:\n%v", self.Status())
+			}
+		}
+	}()
+	plog.Infoln("Started")
+}
+
+func (self *BlockPool) Stop() {
+	self.lock.Lock()
+	if !self.running {
+		self.lock.Unlock()
+		return
+	}
+	self.running = false
+
+	self.lock.Unlock()
+
+	plog.Infoln("Stopping...")
+
+	close(self.quit)
+
+	self.lock.Lock()
+	self.peers = nil
+	self.pool = nil
+	self.lock.Unlock()
+
+	plog.Infoln("Stopped")
+}
+
+// Wait blocks until active processes finish
+func (self *BlockPool) Wait(t time.Duration) {
+	self.lock.Lock()
+	if !self.running {
+		self.lock.Unlock()
+		return
+	}
+	self.lock.Unlock()
+
+	plog.Infoln("Waiting for processes to complete...")
+	w := make(chan bool)
+	go func() {
+		self.wg.Wait()
+		close(w)
+	}()
+
+	select {
+	case <-w:
+		plog.Infoln("Processes complete")
+	case <-time.After(t):
+		plog.Warnf("Timeout")
+	}
+}
+
+/*
+AddPeer is called by the eth protocol instance running on the peer after
+the status message has been received with total difficulty and current block hash
+Called a second time with the same peer id, it is used to update chain info for a peer. This is used when a new (mined) block message is received.
+RemovePeer needs to be called when the peer disconnects.
+Peer info is currently not persisted across disconnects (or sessions)
+*/
+func (self *BlockPool) AddPeer(
+
+	td *big.Int, currentBlockHash []byte,
+	peerId string,
+	requestBlockHashes func([]byte) error,
+	requestBlocks func([][]byte) error,
+	peerError func(*errs.Error),
+
+) (best bool) {
+
+	return self.peers.addPeer(td, currentBlockHash, peerId, requestBlockHashes, requestBlocks, peerError)
+}
+
+// RemovePeer needs to be called when the peer disconnects
+func (self *BlockPool) RemovePeer(peerId string) {
+	self.peers.removePeer(peerId)
+}
+
+/*
+AddBlockHashes
+
+Entry point for eth protocol to add block hashes received via BlockHashesMsg
+
+only hashes from the best peer are handled
+
+initiates further hash requests until a known parent is reached (unless cancelled by a peerSwitch event, i.e., when a better peer becomes best peer)
+launches all block request processes on each chain section
+
+the first argument is an iterator function. Using this block hashes are decoded from the rlp message payload on demand. As a result, AddBlockHashes needs to run synchronously for one peer since the message is discarded if the caller thread returns.
+*/
+func (self *BlockPool) AddBlockHashes(next func() ([]byte, bool), peerId string) {
+
+	bestpeer, best := self.peers.getPeer(peerId)
+	if !best {
+		return
+	}
+	// bestpeer is still the best peer
+
+	self.wg.Add(1)
+
+	defer func() { self.wg.Done() }()
+
+	self.status.lock.Lock()
+	self.status.activePeers[bestpeer.id]++
+	self.status.lock.Unlock()
+
+	var n int
+	var hash []byte
+	var ok, headSection, peerswitch bool
+	var sec, child, parent *section
+	var entry *entry
+	var nodes []*node
+
+	hash, ok = next()
+	bestpeer.lock.Lock()
+
+	plog.Debugf("AddBlockHashes: peer <%s> starting from [%s] (peer head: %s)", peerId, hex(bestpeer.parentHash), hex(bestpeer.currentBlockHash))
+
+	// first check if we are building the head section of a peer's chain
+	if bytes.Equal(bestpeer.parentHash, hash) {
+		if self.hasBlock(bestpeer.currentBlockHash) {
+			return
+		}
+		/*
+		 when peer is promoted in switchPeer, a new header section process is launched
+		 as the head section skeleton is actually created here, it is signaled to the process
+		 so that it can quit
+		 in the special case that the node for parent of the head block is found in the blockpool
+		 (with or without fetched block)
+		*/
+		headSection = true
+		if entry := self.get(bestpeer.currentBlockHash); entry == nil {
+			plog.DebugDetailf("AddBlockHashes: peer <%s> (head: %s) head section starting from [%s] ", peerId, hex(bestpeer.currentBlockHash), hex(bestpeer.parentHash))
+			// if head block is not yet in the pool, create entry and start node list for section
+			node := &node{
+				hash:    bestpeer.currentBlockHash,
+				block:   bestpeer.currentBlock,
+				hashBy:  peerId,
+				blockBy: peerId,
+			}
+			// nodes is a list of nodes in one section ordered top-bottom (old to young)
+			nodes = append(nodes, node)
+			n++
+		} else {
+			// otherwise set child section iff found node is the root of a section
+			// this is a possible scenario when a singleton head section was created
+			// on an earlier occasion this peer or another with the same block was best peer
+			if entry.node == entry.section.bottom {
+				child = entry.section
+				plog.DebugDetailf("AddBlockHashes: peer <%s>: connects to child section root %s", peerId, hex(bestpeer.currentBlockHash))
+			}
+		}
+	} else {
+		// otherwise : we are not building the head section of the peer
+		plog.DebugDetailf("AddBlockHashes: peer <%s> (head: %s) section starting from [%s] ", peerId, hex(bestpeer.currentBlockHash), hex(hash))
+	}
+	// the switch channel signals peerswitch event
+	switchC := bestpeer.switchC
+	bestpeer.lock.Unlock()
+
+	// iterate over hashes coming from peer (first round we have hash set above)
+LOOP:
+	for ; ok; hash, ok = next() {
+
+		select {
+		case <-self.quit:
+			// global quit for blockpool
+			return
+
+		case <-switchC:
+			// if the peer is demoted, no more hashes read
+			plog.DebugDetailf("AddBlockHashes: demoted peer <%s> (head: %s)", peerId, hex(bestpeer.currentBlockHash), hex(hash))
+			peerswitch = true
+			break LOOP
+		default:
+		}
+
+		// if we reach the blockchain we stop reading more
+		if self.hasBlock(hash) {
+			// check if known block connecting the downloaded chain to our blockchain
+			plog.DebugDetailf("AddBlockHashes: peer <%s> (head: %s) found block %s in the blockchain", peerId, hex(bestpeer.currentBlockHash), hex(hash))
+			if len(nodes) == 1 {
+				// create new section if needed and push it to the blockchain
+				sec = self.newSection(nodes)
+				sec.addSectionToBlockChain(bestpeer)
+			} else {
+
+				/*
+					 not added hash yet but according to peer child section built
+					earlier chain connects with blockchain
+					this maybe a potential vulnarability
+					the root block arrives (or already there but its parenthash was not pointing to known block in the blockchain)
+					we start inserting -> error -> remove the entire chain
+					instead of punishing this peer
+					solution: when switching peers always make sure best peers own head block
+					and td together with blockBy are recorded on the node
+				*/
+				if len(nodes) == 0 && child != nil {
+					child.addSectionToBlockChain(bestpeer)
+				}
+			}
+			break LOOP
+		}
+
+		// look up node in the pool
+		entry = self.get(hash)
+		if entry != nil {
+			// reached a known chain in the pool
+			if entry.node == entry.section.bottom && n == 1 {
+				/*
+					the first block hash received is an orphan in the pool
+					this also supports clients that (despite the spec) include <from> hash in their
+					response to hashes request. Note that by providing <from> we can link sections
+					without having to wait for the root block of the child section to arrive, so it allows for superior performance
+				*/
+				plog.DebugDetailf("AddBlockHashes: peer <%s> (head: %s) found head block [%s] as root of connecting child section [%s] skipping", peerId, hex(bestpeer.currentBlockHash), hex(hash), sectionhex(entry.section))
+				// record the entry's chain section as child section
+				child = entry.section
+				continue LOOP
+			}
+			// otherwise record entry's chain section as parent connecting it to the pool
+			plog.DebugDetailf("AddBlockHashes: peer <%s> (head: %s) found block [%s] in section [%s]. Connected to pool.", peerId, hex(bestpeer.currentBlockHash), hex(hash), sectionhex(entry.section))
+			parent = entry.section
+			break LOOP
+		}
+
+		// finally if node for block hash does not exist, create it and append node to section nodes
+		node := &node{
+			hash:   hash,
+			hashBy: peerId,
+		}
+		nodes = append(nodes, node)
+	} //for
+
+	/*
+		we got here if
+		- run out of hashes (parent = nil) sent by our best peer
+		- our peer is demoted (peerswitch = true)
+		- reached blockchain or blockpool
+		- quitting
+	*/
+	self.chainLock.Lock()
+
+	plog.DebugDetailf("AddBlockHashes: peer <%s> (head: %s): %v nodes in new section", peerId, hex(bestpeer.currentBlockHash), len(nodes))
+	/*
+	  handle forks where connecting node is mid-section
+	  by splitting section at fork
+	  no splitting needed if connecting node is head of a section
+	*/
+	if parent != nil && entry != nil && entry.node != parent.top && len(nodes) > 0 {
+		plog.DebugDetailf("AddBlockHashes: peer <%s> (head: %s): fork after %s", peerId, hex(bestpeer.currentBlockHash), hex(hash))
+
+		self.splitSection(parent, entry)
+
+		self.status.lock.Lock()
+		self.status.values.Forks++
+		self.status.lock.Unlock()
+	}
+
+	/*
+	  if new section is created, link it to parent/child sections
+	  and launch section process fetching blocks and further hashes
+	*/
+	sec = self.linkSections(nodes, parent, child)
+
+	self.status.lock.Lock()
+	self.status.values.BlockHashes += len(nodes)
+	self.status.lock.Unlock()
+	plog.DebugDetailf("AddBlockHashes: peer <%s> (head: %s): section [%s] created", peerId, hex(bestpeer.currentBlockHash), sectionhex(sec))
+
+	self.chainLock.Unlock()
+
+	/*
+		if a blockpool node is reached (parent section is not nil),
+		activate section (unless our peer is demoted by now).
+		this can be the bottom half of a newly split section in case of a fork.
+		bestPeer is nil if we got here after our peer got demoted while processing.
+		in this case no activation should happen
+	*/
+	if parent != nil && !peerswitch {
+		self.activateChain(parent, bestpeer, nil)
+		plog.DebugDetailf("AddBlockHashes: peer <%s> (head: %s): parent section [%s]", peerId, hex(bestpeer.currentBlockHash), sectionhex(parent))
+	}
+
+	/*
+	  if a new section was created,
+	  register section iff head section or no child known
+	  activate it with this peer
+	*/
+	if sec != nil {
+		// switch on section process (it is paused by switchC)
+		if !peerswitch {
+			if headSection || child == nil {
+				bestpeer.lock.Lock()
+				bestpeer.sections = append(bestpeer.sections, sec.top.hash)
+				bestpeer.lock.Unlock()
+			}
+			/*
+			  request next block hashes for parent section here.
+			  but only once, repeating only when bottom block arrives,
+			  otherwise no way to check if it arrived
+			*/
+			bestpeer.requestBlockHashes(sec.bottom.hash)
+			plog.DebugDetailf("AddBlockHashes: peer <%s> (head: %s): start requesting blocks for section [%s]", peerId, hex(bestpeer.currentBlockHash), sectionhex(sec))
+			sec.activate(bestpeer)
+		} else {
+			plog.DebugDetailf("AddBlockHashes: peer <%s> (head: %s) no longer best: delay requesting blocks for section [%s]", peerId, hex(bestpeer.currentBlockHash), sectionhex(sec))
+			sec.deactivate()
+		}
+	}
+
+	// if we are processing peer's head section, signal it to headSection process that it is created
+
+	if headSection {
+		plog.DebugDetailf("AddBlockHashes: peer <%s> (head: %s) head section registered on head section process", peerId, hex(bestpeer.currentBlockHash))
+
+		var headSec *section
+		switch {
+		case sec != nil:
+			headSec = sec
+		case child != nil:
+			headSec = child
+		default:
+			headSec = parent
+		}
+		if !peerswitch {
+			plog.DebugDetailf("AddBlockHashes: peer <%s> (head: %s) head section [%s] created signalled to head section process", peerId, hex(bestpeer.currentBlockHash), sectionhex(headSec))
+			bestpeer.headSectionC <- headSec
+		}
+	}
+}
+
+/*
+	AddBlock is the entry point for the eth protocol to call when blockMsg is received.
+
+	It has a strict interpretation of the protocol in that if the block received has not been requested, it results in an error
+
+	At the same time it is opportunistic in that if a requested block may be provided by any peer.
+
+	The received block is checked for PoW. Only the first PoW-valid block for a hash is considered legit.
+*/
+func (self *BlockPool) AddBlock(block *types.Block, peerId string) {
+	hash := block.Hash()
+
+	sender, _ := self.peers.getPeer(peerId)
+	if sender == nil {
+		return
+	}
+
+	self.status.lock.Lock()
+	self.status.activePeers[peerId]++
+	self.status.lock.Unlock()
+
+	entry := self.get(hash)
+
+	// a peer's current head block is appearing the first time
+	if bytes.Equal(hash, sender.currentBlockHash) {
+		if sender.currentBlock == nil {
+			plog.Debugf("AddBlock: add head block %s for peer <%s> (head: %s)", hex(hash), peerId, hex(sender.currentBlockHash))
+			sender.setChainInfoFromBlock(block)
+			// sender.currentBlockC <- block
+
+			self.status.lock.Lock()
+			self.status.values.Blocks++
+			self.status.values.BlocksInPool++
+			self.status.lock.Unlock()
+		} else {
+			plog.DebugDetailf("AddBlock: head block %s for peer <%s> (head: %s) already known", hex(hash), peerId, hex(sender.currentBlockHash))
+		}
+	} else {
+
+		plog.DebugDetailf("AddBlock: block %s received from peer <%s> (head: %s)", hex(hash), peerId, hex(sender.currentBlockHash))
+
+		sender.lock.Lock()
+		// update peer chain info if more recent than what we registered
+		if block.Td != nil && block.Td.Cmp(sender.td) > 0 {
+			sender.td = block.Td
+			sender.currentBlockHash = block.Hash()
+			sender.parentHash = block.ParentHash()
+			sender.currentBlock = block
+			sender.headSection = nil
+		}
+		sender.lock.Unlock()
+
+		if entry == nil {
+			// penalise peer for sending what we have not asked
+			plog.DebugDetailf("AddBlock: unrequested block %s received from peer <%s> (head: %s)", hex(hash), peerId, hex(sender.currentBlockHash))
+			sender.addError(ErrUnrequestedBlock, "%x", hash)
+
+			self.status.lock.Lock()
+			self.status.badPeers[peerId]++
+			self.status.lock.Unlock()
+			return
+		}
+	}
+	if entry == nil {
+		return
+	}
+
+	node := entry.node
+	node.lock.Lock()
+	defer node.lock.Unlock()
+
+	// check if block already present
+	if node.block != nil {
+		plog.DebugDetailf("AddBlock: block %s from peer <%s> (head: %s) already sent by <%s> ", hex(hash), peerId, hex(sender.currentBlockHash), node.blockBy)
+		return
+	}
+
+	// check if block is already inserted in the blockchain
+	if self.hasBlock(hash) {
+		plog.DebugDetailf("AddBlock: block %s from peer <%s> (head: %s) already in the blockchain", hex(hash), peerId, hex(sender.currentBlockHash))
+		return
+	}
+
+	// validate block for PoW
+	if !self.verifyPoW(block) {
+		plog.Warnf("AddBlock: invalid PoW on block %s from peer  <%s> (head: %s)", hex(hash), peerId, hex(sender.currentBlockHash))
+		sender.addError(ErrInvalidPoW, "%x", hash)
+
+		self.status.lock.Lock()
+		self.status.badPeers[peerId]++
+		self.status.lock.Unlock()
+
+		return
+	}
+
+	node.block = block
+	node.blockBy = peerId
+	node.td = block.Td // optional field
+
+	self.status.lock.Lock()
+	self.status.values.Blocks++
+	self.status.values.BlocksInPool++
+	self.status.lock.Unlock()
+
+}
+
+/*
+  iterates down a chain section by section
+  activating section process on incomplete sections with peer
+  relinking orphaned sections with their parent if root block (and its parent hash) is known)
+*/
+func (self *BlockPool) activateChain(sec *section, p *peer, connected map[string]*section) {
+
+	p.lock.RLock()
+	switchC := p.switchC
+	p.lock.RUnlock()
+
+	var i int
+
+LOOP:
+	for sec != nil {
+		parent := self.getParent(sec)
+		plog.DebugDetailf("activateChain: section [%s] activated by peer <%s>", sectionhex(sec), p.id)
+		sec.activate(p)
+		if i > 0 && connected != nil {
+			connected[string(sec.top.hash)] = sec
+		}
+		/*
+		  we need to relink both complete and incomplete sections
+		  the latter could have been blockHashesRequestsComplete before being delinked from its parent
+		*/
+		if parent == nil && sec.bottom.block != nil {
+			if entry := self.get(sec.bottom.block.ParentHash()); entry != nil {
+				parent = entry.section
+				plog.DebugDetailf("activateChain: [%s]-[%s] relink", sectionhex(parent), sectionhex(sec))
+				link(parent, sec)
+			}
+		}
+		sec = parent
+
+		// stop if peer got demoted
+		select {
+		case <-switchC:
+			break LOOP
+		case <-self.quit:
+			break LOOP
+		default:
+		}
+	}
+}
+
+// must run in separate go routine, otherwise
+// switchpeer -> activateChain -> activate deadlocks on section process select and peers.lock
+func (self *BlockPool) requestBlocks(attempts int, hashes [][]byte) {
+	self.wg.Add(1)
+	go func() {
+		self.peers.requestBlocks(attempts, hashes)
+		self.wg.Done()
+	}()
+}
+
+// convenience methods to access adjacent sections
+func (self *BlockPool) getParent(sec *section) *section {
+	self.chainLock.RLock()
+	defer self.chainLock.RUnlock()
+	return sec.parent
+}
+
+func (self *BlockPool) getChild(sec *section) *section {
+	self.chainLock.RLock()
+	defer self.chainLock.RUnlock()
+	return sec.child
+}
+
+// accessor and setter for entries in the pool
+func (self *BlockPool) get(hash []byte) *entry {
+	self.lock.RLock()
+	defer self.lock.RUnlock()
+	return self.pool[string(hash)]
+}
+
+func (self *BlockPool) set(hash []byte, e *entry) {
+	self.lock.Lock()
+	defer self.lock.Unlock()
+	self.pool[string(hash)] = e
+}
+
+func (self *BlockPool) remove(sec *section) {
+	// delete node entries from pool index under pool lock
+	self.lock.Lock()
+	defer self.lock.Unlock()
+	for _, node := range sec.nodes {
+		delete(self.pool, string(node.hash))
+	}
+}
+
+func (self *BlockPool) getHashSlice() (s [][]byte) {
+	select {
+	case s = <-self.hashSlicePool:
+	default:
+		s = make([][]byte, self.Config.BlockBatchSize)
+	}
+	return
+}
+
+// Return returns a Client to the pool.
+func (self *BlockPool) putHashSlice(s [][]byte) {
+	if len(s) == self.Config.BlockBatchSize {
+		select {
+		case self.hashSlicePool <- s:
+		default:
+		}
+	}
+}
+
+// pretty prints hash (byte array) with first 4 bytes in hex
+func hex(hash []byte) (name string) {
+	if hash == nil {
+		name = ""
+	} else {
+		name = fmt.Sprintf("%x", hash[:4])
+	}
+	return
+}
+
+// pretty prints a section using first 4 bytes in hex of bottom and top blockhash of the section
+func sectionhex(section *section) (name string) {
+	if section == nil {
+		name = ""
+	} else {
+		name = fmt.Sprintf("%x-%x", section.bottom.hash[:4], section.top.hash[:4])
+	}
+	return
+}