path: root/blockpool/blockpool.go

package blockpool

import (
    "fmt"
    "math/big"
    "sync"
    "time"

    "github.com/ethereum/go-ethereum/common"
    "github.com/ethereum/go-ethereum/core"
    "github.com/ethereum/go-ethereum/core/types"
    "github.com/ethereum/go-ethereum/errs"
    "github.com/ethereum/go-ethereum/event"
    ethlogger "github.com/ethereum/go-ethereum/logger"
    "github.com/ethereum/go-ethereum/logger/glog"
    "github.com/ethereum/go-ethereum/pow"
)

var plog = ethlogger.NewLogger("Blockpool")

var (
    // max number of block hashes sent in one request
    blockHashesBatchSize = 256
    // 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 = 20
    // 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 = 60 * time.Second
    // timeout interval: max time allowed for best peer to remain idle (not send new block after sync complete)
    idleBestPeerTimeout = 120 * time.Second
    // duration of suspension after peer fatal error during which peer is not allowed to reconnect
    peerSuspensionInterval = 300 * time.Second
    // status is logged every statusUpdateInterval
    statusUpdateInterval = 3 * time.Second
)

// blockpool config, values default to constants
type Config struct {
    BlockHashesBatchSize       int
    BlockBatchSize             int
    BlocksRequestRepetition    int
    BlocksRequestMaxIdleRounds int
    BlockHashesRequestInterval time.Duration
    BlocksRequestInterval      time.Duration
    BlockHashesTimeout         time.Duration
    BlocksTimeout              time.Duration
    IdleBestPeerTimeout        time.Duration
    PeerSuspensionInterval     time.Duration
    StatusUpdateInterval       time.Duration
}

// blockpool errors
const (
    ErrInvalidBlock = iota
    ErrInvalidPoW
    ErrInsufficientChainInfo
    ErrIdleTooLong
    ErrIncorrectTD
    ErrUnrequestedBlock
)

// error descriptions
var errorToString = map[int]string{
    ErrInvalidBlock:          "Invalid block",              // fatal
    ErrInvalidPoW:            "Invalid PoW",                // fatal
    ErrInsufficientChainInfo: "Insufficient chain info",    // fatal
    ErrIdleTooLong:           "Idle too long",              // fatal
    ErrIncorrectTD:           "Incorrect Total Difficulty", // fatal
    ErrUnrequestedBlock:      "Unrequested block",
}

// error severity
func severity(code int) ethlogger.LogLevel {
    switch code {
    case ErrUnrequestedBlock:
        return ethlogger.WarnLevel
    default:
        return ethlogger.ErrorLevel
    }
}

// init initialises the Config, zero values fall back to constants
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
    }
    if self.IdleBestPeerTimeout == 0 {
        self.IdleBestPeerTimeout = idleBestPeerTimeout
    }
    if self.PeerSuspensionInterval == 0 {
        self.PeerSuspensionInterval = peerSuspensionInterval
    }
    if self.StatusUpdateInterval == 0 {
        self.StatusUpdateInterval = statusUpdateInterval
    }
}

// node is the basic unit of the internal model of block chain/tree in the blockpool
type node struct {
    lock    sync.RWMutex
    hash    common.Hash
    block   *types.Block
    hashBy  string
    blockBy string
    peers   map[string]bool
    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 manager
    hasBlock    func(hash common.Hash) bool // query if block is known
    insertChain func(types.Blocks) error    // add section to blockchain
    verifyPoW   func(pow.Block) bool        // soft PoW verification
    chainEvents *event.TypeMux              // ethereum eventer for chainEvents

    tdSub event.Subscription // subscription to core.ChainHeadEvent
    td    *big.Int           // our own total difficulty

    pool  map[common.Hash]*entry // the actual blockpool
    peers *peers                 // peers manager in peers.go

    status *status // info about blockpool (UI interface) in status.go

    lock      sync.RWMutex
    chainLock sync.RWMutex
    // alloc-easy pool of hash slices
    hashSlicePool chan []common.Hash

    // waitgroup is used in tests to wait for result-critical routines
    // as well as in determining idle / syncing status
    wg      sync.WaitGroup //
    quit    chan bool      // chan used for quitting parallel routines
    running bool           //
}

// public constructor
// after blockpool returned, config can be set
// BlockPool.Start will call Config.init to set missing values
func New(
    hasBlock func(hash common.Hash) bool,
    insertChain func(types.Blocks) error,
    verifyPoW func(pow.Block) bool,
    chainEvents *event.TypeMux,
    td *big.Int,
) *BlockPool {

    return &BlockPool{
        Config:      &Config{},
        hasBlock:    hasBlock,
        insertChain: insertChain,
        verifyPoW:   verifyPoW,
        chainEvents: chainEvents,
        td:          td,
    }
}

// allows restart
func (self *BlockPool) Start() {
    self.lock.Lock()
    defer self.lock.Unlock()

    if self.running {
        return
    }

    // set missing values
    self.Config.init()

    self.hashSlicePool = make(chan []common.Hash, 150)
    self.status = newStatus()
    self.quit = make(chan bool)
    self.pool = make(map[common.Hash]*entry)
    self.running = true

    self.peers = &peers{
        errors: &errs.Errors{
            Package: "Blockpool",
            Errors:  errorToString,
            Level:   severity,
        },
        peers:     make(map[string]*peer),
        blacklist: make(map[string]time.Time),
        status:    self.status,
        bp:        self,
    }

    // subscribe and listen to core.ChainHeadEvent{} for uptodate TD
    self.tdSub = self.chainEvents.Subscribe(core.ChainHeadEvent{})

    // status update interval
    timer := time.NewTicker(self.Config.StatusUpdateInterval)
    go func() {
        for {
            select {
            case <-self.quit:
                return
            case event := <-self.tdSub.Chan():
                if ev, ok := event.(core.ChainHeadEvent); ok {
                    td := ev.Block.Td
                    var height *big.Int
                    if (ev.Block.HeaderHash == common.Hash{}) {
                        height = ev.Block.Header().Number
                    }
                    plog.DebugDetailf("ChainHeadEvent: height: %v, td: %v, hash: %s", height, td, hex(ev.Block.Hash()))
                    self.setTD(td)
                    self.peers.lock.Lock()

                    if best := self.peers.best; best != nil {
                        // only switch if we strictly go above otherwise we may stall if only
                        if td.Cmp(best.td) > 0 {
                            self.peers.best = nil
                            self.switchPeer(best, nil)
                        }
                    }
                    self.peers.lock.Unlock()
                }
            case <-timer.C:
                plog.DebugDetailf("status:\n%v", self.Status())
            }
        }
    }()
    glog.V(ethlogger.Info).Infoln("Blockpool started")
}

func (self *BlockPool) Stop() {
    self.lock.Lock()
    if !self.running {
        self.lock.Unlock()
        return
    }
    self.running = false

    self.lock.Unlock()

    plog.Infoln("Stopping...")

    self.tdSub.Unsubscribe()
    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) except for suspension

*/
func (self *BlockPool) AddPeer(

    td *big.Int, currentBlockHash common.Hash,
    peerId string,
    requestBlockHashes func(common.Hash) error,
    requestBlocks func([]common.Hash) error,
    peerError func(*errs.Error),

) (best bool, suspended 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() (common.Hash, 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 common.Hash
    var ok, headSection, peerswitch bool
    var sec, child, parent *section
    var entry *entry
    var nodes []*node

    hash, ok = next()
    bestpeer.lock.RLock()

    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 bestpeer.parentHash == hash {
        if self.hasBlock(bestpeer.currentBlockHash) {
            bestpeer.lock.RUnlock()
            return
        }
        /*
         When peer is promoted in switchPeer, a new header section process is launched.
         Once 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), a singleton section containing only the head block node is created.
        */
        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,
                td:      bestpeer.td,
            }
            // 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 when 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.RUnlock()

    // 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 further blockhashes
        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 {
                plog.DebugDetailf("AddBlockHashes: singleton section pushed to blockchain peer <%s> (head: %s) found block %s in the blockchain", peerId, hex(bestpeer.currentBlockHash), hex(hash))

                // 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 {
                    plog.DebugDetailf("AddBlockHashes: child section [%s] pushed to blockchain peer <%s> (head: %s) found block %s in the blockchain", sectionhex(child), peerId, hex(bestpeer.currentBlockHash), hex(hash))

                    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 node 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.
    sec = self.linkSections(nodes, parent, child)

    if sec != nil {
        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 {
        bestpeer.lock.RLock()
        self.activateChain(parent, bestpeer, bestpeer.switchC, nil)
        plog.DebugDetailf("AddBlockHashes: peer <%s> (head: %s): parent section [%s]", peerId, hex(bestpeer.currentBlockHash), sectionhex(parent))
        bestpeer.lock.RUnlock()
    }

    /*
      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 another batch of older block hashes for parent section here.
              But only once, repeating only when the section's root 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.

    If the block received is the head block of the current best peer, signal it to the head section process
*/
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)
    blockIsCurrentHead := false
    sender.lock.RLock()
    currentBlockHash := sender.currentBlockHash
    currentBlock := sender.currentBlock
    currentBlockC := sender.currentBlockC
    switchC := sender.switchC
    sender.lock.RUnlock()

    // a peer's current head block is appearing the first time
    if hash == currentBlockHash {
        // this happens when block came in a newblock message but
        // also if sent in a blockmsg (for instance, if we requested, only if we
        // dont apply on blockrequests the restriction of flood control)
        blockIsCurrentHead = true
        if currentBlock == nil {
            sender.lock.Lock()
            sender.setChainInfoFromBlock(block)
            sender.lock.Unlock()

            self.status.lock.Lock()
            self.status.values.BlockHashes++
            self.status.values.Blocks++
            self.status.values.BlocksInPool++
            self.status.lock.Unlock()
            // signal to head section process
            select {
            case currentBlockC <- block:
            case <-switchC:
            }
        } else {
            plog.DebugDetailf("AddBlock: head block %s for peer <%s> (head: %s) already known", hex(hash), peerId, hex(currentBlockHash))
        }
    } else {

        plog.DebugDetailf("AddBlock: block %s received from peer <%s> (head: %s)", hex(hash), peerId, hex(currentBlockHash))

        /* @zelig !!!
           requested 5 hashes from both A & B. A responds sooner then B, process blocks. Close section.
           delayed B sends you block ... UNREQUESTED. Blocked
             if entry == nil {
               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 {
        // FIXME: here check the cache find or create node -
        // put peer as blockBy!
        return
    }

    node := entry.node
    node.lock.Lock()
    defer node.lock.Unlock()

    // register peer on node as source
    if node.peers == nil {
        node.peers = make(map[string]bool)
    }
    FoundBlockCurrentHead, found := node.peers[sender.id]
    if !found || FoundBlockCurrentHead {
        // if found but not FoundBlockCurrentHead, then no update
        // necessary (||)
        node.peers[sender.id] = blockIsCurrentHead
        // for those that are false, TD will update their head
        // for those that are true, TD is checked !
        // this is checked at the time of TD calculation in checkTD
    }
    // check if block already received
    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)
    }

    // 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
    }

    /*
        @zelig needs discussing
        Viktor: pow check can be delayed in a go routine and therefore cache
        creation is not blocking
            // 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

    self.status.lock.Lock()
    self.status.values.Blocks++
    self.status.values.BlocksInPool++
    self.status.lock.Unlock()

}

/*
  activateChain iterates down a chain section by section.
  It activates the section process on incomplete sections with peer.
  It relinks orphaned sections with their parent if root block (and its parent hash) is known.
*/
func (self *BlockPool) activateChain(sec *section, p *peer, switchC chan bool, connected map[common.Hash]*section) {

    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[sec.top.hash] = sec
        }
        /*
          Need to relink both complete and incomplete sections
          An incomplete section could have been blockHashesRequestsComplete before being delinked from its parent.
        */
        if parent == nil {
            if sec.bottom.block != nil {
                if entry := self.get(sec.bottom.block.ParentHash()); entry != nil {
                    parent = entry.section
                    plog.DebugDetailf("activateChain: [%s]-[%s] link", sectionhex(parent), sectionhex(sec))
                    link(parent, sec)
                }
            } else {
                plog.DebugDetailf("activateChain: section [%s] activated by peer <%s> has missing root block", sectionhex(sec), p.id)
            }
        }
        sec = parent

        // stop if peer got demoted or global quit
        select {
        case <-switchC:
            break LOOP
        case <-self.quit:
            break LOOP
        default:
        }
    }
}

// check if block's actual TD (calculated after successful insertChain) is identical to TD advertised for peer's head block.
func (self *BlockPool) checkTD(nodes ...*node) {
    for _, n := range nodes {
        // skip check if queued future block
        if n.td != nil && !n.block.Queued() {
            plog.DebugDetailf("peer td %v =?= block td %v", n.td, n.block.Td)
            if n.td.Cmp(n.block.Td) != 0 {
                self.peers.peerError(n.blockBy, ErrIncorrectTD, "on block %x", n.hash)
                self.status.lock.Lock()
                self.status.badPeers[n.blockBy]++
                self.status.lock.Unlock()
            }
        }
    }
}

// requestBlocks 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 []common.Hash) {
    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 common.Hash) *entry {
    self.lock.RLock()
    defer self.lock.RUnlock()
    return self.pool[hash]
}

func (self *BlockPool) set(hash common.Hash, e *entry) {
    self.lock.Lock()
    defer self.lock.Unlock()
    self.pool[hash] = e
}

// accessor and setter for total difficulty
func (self *BlockPool) getTD() *big.Int {
    self.lock.RLock()
    defer self.lock.RUnlock()
    return self.td
}

func (self *BlockPool) setTD(td *big.Int) {
    self.lock.Lock()
    defer self.lock.Unlock()
    self.td = td
}

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, node.hash)
    }
    if sec.initialised && sec.poolRootIndex != 0 {
        self.status.lock.Lock()
        self.status.values.BlocksInPool -= len(sec.nodes) - sec.missing
        self.status.lock.Unlock()
    }
}

// get/put for optimised allocation similar to sync.Pool
func (self *BlockPool) getHashSlice() (s []common.Hash) {
    select {
    case s = <-self.hashSlicePool:
    default:
        s = make([]common.Hash, self.Config.BlockBatchSize)
    }
    return
}

func (self *BlockPool) putHashSlice(s []common.Hash) {
    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 common.Hash) (name string) {
    if (hash == common.Hash{}) {
        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
}