blockmanager.go

// NOTE: THIS API IS UNSTABLE RIGHT NOW AND WILL GO MOSTLY PRIVATE SOON.

package neutrino

import (
	"bytes"
	"container/list"
	"fmt"
	"math/big"
	"sync"
	"sync/atomic"
	"time"

	"github.com/btcsuite/btcd/blockchain"
	"github.com/btcsuite/btcd/chaincfg"
	"github.com/btcsuite/btcd/chaincfg/chainhash"
	"github.com/btcsuite/btcd/txscript"
	"github.com/btcsuite/btcd/wire"
	"github.com/btcsuite/btcutil"
	"github.com/btcsuite/btcutil/gcs"
	"github.com/btcsuite/btcutil/gcs/builder"
	"github.com/lightninglabs/neutrino/headerfs"
	"github.com/lightninglabs/neutrino/headerlist"
)

const (
	// maxTimeOffset is the maximum duration a block time is allowed to be
	// ahead of the curent time. This is currently 2 hours.
	maxTimeOffset = 2 * time.Hour

	// numMaxMemHeaders is the max number of headers to store in memory for
	// a particular peer. By bounding this value, we're able to closely
	// control our effective memory usage during initial sync and re-org
	// handling. This value should be set a "sane" re-org size, such that
	// we're able to properly handle re-orgs in size strictly less than
	// this value.
	numMaxMemHeaders = 10000
)

// filterStoreLookup
type filterStoreLookup func(*ChainService) *headerfs.FilterHeaderStore

var (
	// filterTypes is a map of filter types to synchronize to a lookup
	// function for the service's store for that filter type.
	filterTypes = map[wire.FilterType]filterStoreLookup{
		wire.GCSFilterRegular: func(
			s *ChainService) *headerfs.FilterHeaderStore {

			return s.RegFilterHeaders
		},
	}
)

// zeroHash is the zero value hash (all zeros).  It is defined as a convenience.
var zeroHash chainhash.Hash

// newPeerMsg signifies a newly connected peer to the block handler.
type newPeerMsg struct {
	peer *ServerPeer
}

// invMsg packages a bitcoin inv message and the peer it came from together
// so the block handler has access to that information.
type invMsg struct {
	inv  *wire.MsgInv
	peer *ServerPeer
}

// headersMsg packages a bitcoin headers message and the peer it came from
// together so the block handler has access to that information.
type headersMsg struct {
	headers *wire.MsgHeaders
	peer    *ServerPeer
}

// donePeerMsg signifies a newly disconnected peer to the block handler.
type donePeerMsg struct {
	peer *ServerPeer
}

// txMsg packages a bitcoin tx message and the peer it came from together
// so the block handler has access to that information.
type txMsg struct {
	tx   *btcutil.Tx
	peer *ServerPeer
}

// blockManager provides a concurrency safe block manager for handling all
// incoming blocks.
type blockManager struct {
	started  int32
	shutdown int32

	// blkHeaderProgressLogger is a progress logger that we'll use to
	// update the number of blocker headers we've processed in the past 10
	// seconds within the log.
	blkHeaderProgressLogger *headerProgressLogger

	// fltrHeaderProgessLogger is a process logger similar to the one
	// above, but we'll use it to update the progress of the set of filter
	// headers that we've verified in the past 10 seconds.
	fltrHeaderProgessLogger *headerProgressLogger

	// headerTip will be set to the current block header tip at all times.
	// Callers MUST hold the lock below each time they read/write from
	// this field.
	headerTip uint32

	// headerTipHash will be set to the hash of the current block header
	// tip at all times.  Callers MUST hold the lock below each time they
	// read/write from this field.
	headerTipHash chainhash.Hash

	// newHeadersMtx is the mutex that should be held when reading/writing
	// the headerTip variable above.
	newHeadersMtx sync.RWMutex

	// newHeadersSignal is condition variable which will be used to notify
	// any waiting callers (via Broadcast()) that the tip of the current
	// chain has changed. This is useful when callers need to know we have
	// a new tip, but not necessarily each block that was connected during
	// switch over.
	newHeadersSignal *sync.Cond

	// filterHeaderTip will be set to the current filter header tip at all
	// times.  Callers MUST hold the lock below each time they read/write
	// from this field.
	filterHeaderTip uint32

	// filterHeaderTipHash will be set to the current block hash of the
	// fitler header tip at all times.  Callers MUST hold the lock below
	// each time they read/write from this field.
	filterHeaderTipHash chainhash.Hash

	// newFilterHeadersMtx is the mutex that should be held when
	// reading/writing the filterHeaderTip variable above.
	newFilterHeadersMtx sync.RWMutex

	// newFilterHeadersSignal is condition variable which will be used to
	// notify any waiting callers (via Broadcast()) that the tip of the
	// current filter header chain has changed. This is useful when callers
	// need to know we have a new tip, but not necessarily each filter
	// header that was connected during switch over.
	newFilterHeadersSignal *sync.Cond

	// syncPeer points to the peer that we're currently syncing block
	// headers from.
	syncPeer *ServerPeer

	// syncPeerMutex protects the above syncPeer pointer at all times.
	syncPeerMutex sync.RWMutex

	// server is a pointer to the main p2p server for Neutrino, we'll use
	// this pointer at times to do things like access the database, etc
	server *ChainService

	// peerChan is a channel for messages that come from peers
	peerChan chan interface{}

	wg   sync.WaitGroup
	quit chan struct{}

	headerList     headerlist.Chain
	reorgList      headerlist.Chain
	startHeader    *headerlist.Node
	nextCheckpoint *chaincfg.Checkpoint
	lastRequested  chainhash.Hash

	minRetargetTimespan int64 // target timespan / adjustment factor
	maxRetargetTimespan int64 // target timespan * adjustment factor
	blocksPerRetarget   int32 // target timespan / target time per block
}

// newBlockManager returns a new bitcoin block manager.  Use Start to begin
// processing asynchronous block and inv updates.
func newBlockManager(s *ChainService) (*blockManager, error) {
	targetTimespan := int64(s.chainParams.TargetTimespan / time.Second)
	targetTimePerBlock := int64(s.chainParams.TargetTimePerBlock / time.Second)
	adjustmentFactor := s.chainParams.RetargetAdjustmentFactor

	bm := blockManager{
		server:   s,
		peerChan: make(chan interface{}, MaxPeers*3),
		blkHeaderProgressLogger: newBlockProgressLogger(
			"Processed", "block", log,
		),
		fltrHeaderProgessLogger: newBlockProgressLogger(
			"Verified", "filter header", log,
		),
		headerList: headerlist.NewBoundedMemoryChain(
			numMaxMemHeaders,
		),
		reorgList: headerlist.NewBoundedMemoryChain(
			numMaxMemHeaders,
		),
		quit:                make(chan struct{}),
		blocksPerRetarget:   int32(targetTimespan / targetTimePerBlock),
		minRetargetTimespan: targetTimespan / adjustmentFactor,
		maxRetargetTimespan: targetTimespan * adjustmentFactor,
	}

	// Next we'll create the two signals that goroutines will use to wait
	// on a particular header chain height before starting their normal
	// duties.
	bm.newHeadersSignal = sync.NewCond(&bm.newHeadersMtx)
	bm.newFilterHeadersSignal = sync.NewCond(&bm.newFilterHeadersMtx)

	// Initialize the next checkpoint based on the current height.
	header, height, err := s.BlockHeaders.ChainTip()
	if err != nil {
		return nil, err
	}
	bm.nextCheckpoint = bm.findNextHeaderCheckpoint(int32(height))
	bm.headerList.ResetHeaderState(headerlist.Node{
		Header: *header,
		Height: int32(height),
	})
	bm.headerTip = height
	bm.headerTipHash = header.BlockHash()

	// Finally, we'll set the filter header tip so any goroutines waiting
	// on the condition obtain the correct initial state.
	_, bm.filterHeaderTip, err = s.RegFilterHeaders.ChainTip()
	if err != nil {
		return nil, err
	}
	bm.filterHeaderTipHash = header.BlockHash()

	return &bm, nil
}

// Start begins the core block handler which processes block and inv messages.
func (b *blockManager) Start() {
	// Already started?
	if atomic.AddInt32(&b.started, 1) != 1 {
		return
	}

	log.Trace("Starting block manager")
	b.wg.Add(2)
	go b.blockHandler()
	go b.cfHandler()
}

// Stop gracefully shuts down the block manager by stopping all asynchronous
// handlers and waiting for them to finish.
func (b *blockManager) Stop() error {
	if atomic.AddInt32(&b.shutdown, 1) != 1 {
		log.Warnf("Block manager is already in the process of " +
			"shutting down")
		return nil
	}

	// We'll send out update signals before the quit to ensure that any
	// goroutines waiting on them will properly exit.
	done := make(chan struct{})
	go func() {
		ticker := time.NewTicker(time.Millisecond * 50)
		defer ticker.Stop()

		for {
			select {
			case <-done:
				return
			case <-ticker.C:
			}

			b.newHeadersSignal.Broadcast()
			b.newFilterHeadersSignal.Broadcast()
		}
	}()

	log.Infof("Block manager shutting down")
	close(b.quit)
	b.wg.Wait()

	close(done)
	return nil
}

// NewPeer informs the block manager of a newly active peer.
func (b *blockManager) NewPeer(sp *ServerPeer) {
	// Ignore if we are shutting down.
	if atomic.LoadInt32(&b.shutdown) != 0 {
		return
	}

	select {
	case b.peerChan <- &newPeerMsg{peer: sp}:
	case <-b.quit:
		return
	}
}

// handleNewPeerMsg deals with new peers that have signalled they may be
// considered as a sync peer (they have already successfully negotiated).  It
// also starts syncing if needed.  It is invoked from the syncHandler
// goroutine.
func (b *blockManager) handleNewPeerMsg(peers *list.List, sp *ServerPeer) {
	// Ignore if in the process of shutting down.
	if atomic.LoadInt32(&b.shutdown) != 0 {
		return
	}

	log.Infof("New valid peer %s (%s)", sp, sp.UserAgent())

	// Ignore the peer if it's not a sync candidate.
	if !b.isSyncCandidate(sp) {
		return
	}

	// Add the peer as a candidate to sync from.
	peers.PushBack(sp)

	// If we're current with our sync peer and the new peer is advertising
	// a higher block than the newest one we know of, request headers from
	// the new peer.
	_, height, err := b.server.BlockHeaders.ChainTip()
	if err != nil {
		log.Criticalf("Couldn't retrieve block header chain tip: %s",
			err)
		return
	}
	if b.IsCurrent() && height < uint32(sp.StartingHeight()) {
		locator, err := b.server.BlockHeaders.LatestBlockLocator()
		if err != nil {
			log.Criticalf("Couldn't retrieve latest block "+
				"locator: %s", err)
			return
		}
		stopHash := &zeroHash
		sp.PushGetHeadersMsg(locator, stopHash)
	}

	// Start syncing by choosing the best candidate if needed.
	b.startSync(peers)
}

// DonePeer informs the blockmanager that a peer has disconnected.
func (b *blockManager) DonePeer(sp *ServerPeer) {
	// Ignore if we are shutting down.
	if atomic.LoadInt32(&b.shutdown) != 0 {
		return
	}

	select {
	case b.peerChan <- &donePeerMsg{peer: sp}:
	case <-b.quit:
		return
	}
}

// handleDonePeerMsg deals with peers that have signalled they are done.  It
// removes the peer as a candidate for syncing and in the case where it was the
// current sync peer, attempts to select a new best peer to sync from.  It is
// invoked from the syncHandler goroutine.
func (b *blockManager) handleDonePeerMsg(peers *list.List, sp *ServerPeer) {
	// Remove the peer from the list of candidate peers.
	for e := peers.Front(); e != nil; e = e.Next() {
		if e.Value == sp {
			peers.Remove(e)
			break
		}
	}

	log.Infof("Lost peer %s", sp)

	// Attempt to find a new peer to sync from if the quitting peer is the
	// sync peer.  Also, reset the header state.
	if b.SyncPeer() != nil && b.SyncPeer() == sp {
		b.syncPeerMutex.Lock()
		b.syncPeer = nil
		b.syncPeerMutex.Unlock()
		header, height, err := b.server.BlockHeaders.ChainTip()
		if err != nil {
			return
		}
		b.headerList.ResetHeaderState(headerlist.Node{
			Header: *header,
			Height: int32(height),
		})
		b.startSync(peers)
	}
}

// cfHandler is the cfheader download handler for the block manager. It must be
// run as a goroutine. It requests and processes cfheaders messages in a
// separate goroutine from the peer handlers.
func (b *blockManager) cfHandler() {
	// If a loop ends with a quit, we want to signal that the goroutine is
	// done.
	defer func() {
		log.Trace("Committed filter header handler done")
		b.wg.Done()
	}()

	log.Infof("Waiting for block headers to sync, then will start " +
		"cfheaders sync...")

	// We'll wait until the main header sync is mostly finished before we
	// actually start to sync the set of cfheaders. We do this to speed up
	// the sync, as the check pointed sync is faster, then fetching each
	// header from each peer during the normal "at tip" syncing.
	b.newHeadersSignal.L.Lock()
	for !b.IsCurrent() {
		b.newHeadersSignal.Wait()

		// While we're awake, we'll quickly check to see if we need to
		// quit early.
		select {
		case <-b.quit:
			b.newHeadersSignal.L.Unlock()
			return
		default:

		}
	}
	b.newHeadersSignal.L.Unlock()

	// Now that we know the header sync is mostly finished, we'll grab the
	// current chain tip so we can base our header sync off of that.
	lastHeader, lastHeight, err := b.server.BlockHeaders.ChainTip()
	if err != nil {
		log.Critical(err)
		return
	}
	lastHash := lastHeader.BlockHash()

	log.Infof("Starting cfheaders sync at block_height=%v, hash=%v", lastHeight,
		lastHeader.BlockHash())

	// We'll sync the headers and checkpoints for all filter types in
	// parallel, by using a goroutine for each filter type.
	var wg sync.WaitGroup
	wg.Add(len(filterTypes))
	for fType, storeLookup := range filterTypes {
		// Launch a goroutine to get all of the filter headers for this
		// filter type.
		go func(fType wire.FilterType, storeLookup func(
			s *ChainService) *headerfs.FilterHeaderStore) {

			defer wg.Done()

			log.Infof("Starting cfheaders sync for "+
				"filter_type=%v", fType)

			var (
				goodCheckpoints []*chainhash.Hash
				err             error
			)

			// Get the header store for this filter type.
			store := storeLookup(b.server)

			// We're current as we received on startCFHeaderSync.
			// If we have less than a full checkpoint's worth of
			// blocks, such as on simnet, we don't really need to
			// request checkpoints as we'll get 0 from all peers.
			// We can go on and just request the cfheaders.
			for len(goodCheckpoints) == 0 &&
				lastHeight >= wire.CFCheckptInterval {

				// Quit if requested.
				select {
				case <-b.quit:
					return
				default:
				}

				// Try to get all checkpoints from current
				// peers.
				allCheckpoints := b.getCheckpts(&lastHash, fType)
				if len(allCheckpoints) == 0 {
					log.Warnf("Unable to fetch set of " +
						"candidate checkpoints, trying again...")

					time.Sleep(QueryTimeout)
					continue
				}

				// See if we can detect which checkpoint list
				// is correct. If not, we will cycle again.
				goodCheckpoints, err = b.resolveConflict(
					allCheckpoints, store, fType,
				)
				if err != nil {
					log.Debugf("got error attempting "+
						"to determine correct cfheader"+
						" checkpoints: %v, trying "+
						"again", err)
				}
				if len(goodCheckpoints) == 0 {
					time.Sleep(QueryTimeout)
				}
			}

			// Get all the headers up to the last known good
			// checkpoint.
			b.getCheckpointedCFHeaders(
				goodCheckpoints, store, fType,
			)

			log.Infof("Fully caught up with cfheaders at height "+
				"%v, waiting at tip for new blocks", lastHeight)

			// Now that we've been fully caught up to the tip of
			// the current header chain, we'll wait here for a
			// signal that more blocks have been connected. If this
			// happens then we'll do another round to fetch the new
			// set of filter new  set of filter headers
			for {
				// We'll wait until the filter header tip and
				// the header tip are mismatched.
				//
				// NOTE: We can grab the filterHeaderTipHash
				// here without a lock, as this is the only
				// goroutine that can modify this value.
				b.newHeadersSignal.L.Lock()
				for b.filterHeaderTipHash == b.headerTipHash {
					// We'll wait here until we're woken up
					// by the broadcast signal.
					b.newHeadersSignal.Wait()

					// Before we proceed, we'll check if we
					// need to exit at all
					select {
					case <-b.quit:
						b.newHeadersSignal.L.Unlock()
						return
					default:
					}
				}

				b.newHeadersSignal.L.Unlock()

				// At this point, we know that there're a set
				// of new filter headers to fetch, so we'll
				// grab them now.
				if err = b.getUncheckpointedCFHeaders(
					store, fType,
				); err != nil {
					log.Debugf("couldn't get "+
						"uncheckpointed headers for "+
						"%v: %v", fType, err)
				}

				// Quit if requested.
				select {
				case <-b.quit:
					return
				default:
				}
			}

		}(fType, storeLookup)
	}
	wg.Wait()
}

// getUncheckpointedCFHeaders gets the next batch of cfheaders from the
// network, if it can, and resolves any conflicts between them. It then writes
// any verified headers to the store.
func (b *blockManager) getUncheckpointedCFHeaders(
	store *headerfs.FilterHeaderStore, fType wire.FilterType) error {

	// Get the filter header store's chain tip.
	_, filtHeight, err := store.ChainTip()
	if err != nil {
		return fmt.Errorf("error getting filter chain tip: %v", err)
	}
	blockHeader, blockHeight, err := b.server.BlockHeaders.ChainTip()
	if err != nil {
		return fmt.Errorf("error getting block chain tip: %v", err)
	}

	// If the block height is somehow before the filter height, then this
	// means that we may still be handling a re-org, so we'll bail our so
	// we can retry after a timeout.
	if blockHeight < filtHeight {
		return fmt.Errorf("reorg in progress, waiting to get "+
			"uncheckpointed cfheaders (block height %d, filter "+
			"height %d", blockHeight, filtHeight)
	}

	// If the heights match, then we're fully synced, so we don't need to
	// do anything from there.
	if blockHeight == filtHeight {
		log.Tracef("cfheaders already caught up to blocks")
		return nil
	}

	log.Infof("Attempting to fetch set of un-checkpointed filters "+
		"at height=%v, hash=%v", blockHeight, blockHeader.BlockHash())

	// Query all peers for the responses.
	startHeight := filtHeight + 1
	headers := b.getCFHeadersForAllPeers(startHeight, fType)
	if len(headers) == 0 {
		return fmt.Errorf("couldn't get cfheaders from peers")
	}

	// For each header, go through and check whether all headers messages
	// have the same filter hash. If we find a difference, get the block,
	// calculate the filter, and throw out any mismatching peers.
	for i := 0; i < wire.MaxCFHeadersPerMsg; i++ {
		if checkForCFHeaderMismatch(headers, i) {
			targetHeight := startHeight + uint32(i)

			log.Warnf("Detected cfheader mismatch at "+
				"height=%v!!!", targetHeight)

			// Get the block header for this height, along with the
			// block as well.
			header, err := b.server.BlockHeaders.FetchHeaderByHeight(
				targetHeight,
			)
			if err != nil {
				return err
			}
			block, err := b.server.GetBlock(header.BlockHash())
			if err != nil {
				return err
			}

			log.Warnf("Attempting to reconcile cfheader mismatch "+
				"amongst %v peers", len(headers))

			// We'll also fetch each of the filters from the peers
			// that reported check points, as we may need this in
			// order to determine which peers are faulty.
			filtersFromPeers := b.fetchFilterFromAllPeers(
				targetHeight, header.BlockHash(), fType,
			)
			badPeers, err := resolveCFHeaderMismatch(
				block.MsgBlock(), fType, filtersFromPeers,
			)
			if err != nil {
				return err
			}

			log.Warnf("Banning %v peers due to invalid filter "+
				"headers", len(badPeers))

			for _, peer := range badPeers {
				log.Infof("Banning peer=%v for invalid filter "+
					"headers", peer)

				sp := b.server.PeerByAddr(peer)
				if sp != nil {
					b.server.BanPeer(sp)
					sp.Disconnect()
				}

				delete(headers, peer)
			}
		}
	}

	// Get the longest filter hash chain and write it to the store.
	key, maxLen := "", 0
	for peer, msg := range headers {
		if len(msg.FilterHashes) > maxLen {
			key, maxLen = peer, len(msg.FilterHashes)
		}
	}

	// We'll now fetch the set of pristine headers from the map. If ALL the
	// peers were banned, then we won't have a set of headers at all. We'll
	// return nil so we can go to the top of the loop and fetch from a new
	// set of peers.
	pristineHeaders, ok := headers[key]
	if !ok {
		return fmt.Errorf("All peers served bogus headers! Retrying " +
			"with new set")
	}

	_, err = b.writeCFHeadersMsg(pristineHeaders, store)
	return err
}

// getCheckpointedCFHeaders catches a filter header store up with the
// checkpoints we got from the network. It assumes that the filter header store
// matches the checkpoints up to the tip of the store.
func (b *blockManager) getCheckpointedCFHeaders(checkpoints []*chainhash.Hash,
	store *headerfs.FilterHeaderStore, fType wire.FilterType) {

	// We keep going until we've caught up the filter header store with the
	// latest known checkpoint.
	curHeader, curHeight, err := store.ChainTip()
	if err != nil {
		panic("getting chaintip from store")
	}

	initialFilterHeader := curHeader

	log.Infof("Fetching set of checkpointed cfheaders filters from "+
		"height=%v, hash=%v", curHeight, curHeader)

	// The starting interval is the checkpoint index that we'll be starting
	// from based on our current height in the filter header index.
	startingInterval := curHeight / wire.CFCheckptInterval

	log.Infof("Starting to query for cfheaders from "+
		"checkpoint_interval=%v", startingInterval)

	queryMsgs := make([]wire.Message, 0, len(checkpoints))

	// We'll also create an additional set of maps that we'll use to
	// re-order the responses as we get them in.
	queryResponses := make(map[uint32]*wire.MsgCFHeaders)
	stopHashes := make(map[chainhash.Hash]uint32)

	// Generate all of the requests we'll be batching and space to store
	// the responses. Also make a map of stophash to index to make it
	// easier to match against incoming responses.
	//
	// TODO(roasbeef): extract to func to test
	currentInterval := startingInterval
	for currentInterval < uint32(len(checkpoints)) {
		// Each checkpoint is spaced wire.CFCheckptInterval after the
		// prior one, so we'll fetch headers in batches using the
		// checkpoints as a guide.
		startHeightRange := uint32(
			currentInterval*wire.CFCheckptInterval,
		) + 1
		endHeightRange := uint32(
			(currentInterval + 1) * wire.CFCheckptInterval,
		)

		log.Tracef("Checkpointed cfheaders request start_range=%v, "+
			"end_range=%v", startHeightRange, endHeightRange)

		// In order to fetch the range, we'll need the block header for
		// the end of the height range.
		stopHeader, err := b.server.BlockHeaders.FetchHeaderByHeight(
			endHeightRange,
		)
		if err != nil {
			// Try to recover this.
			select {
			case <-b.quit:
				return
			default:
				currentInterval--
				time.Sleep(QueryTimeout)
				continue
			}
		}
		stopHash := stopHeader.BlockHash()

		// Once we have the stop hash, we can construct the query
		// message itself.
		queryMsg := wire.NewMsgGetCFHeaders(
			fType, uint32(startHeightRange), &stopHash,
		)

		// We'll mark that the ith interval is queried by this message,
		// and also map the top hash back to the index of this message.
		queryMsgs = append(queryMsgs, queryMsg)
		stopHashes[stopHash] = currentInterval

		// With the queries for this interval constructed, we'll move
		// onto the next one.
		currentInterval++
	}

	log.Infof("Attempting to query for %v cfheader batches", len(queryMsgs))

	// With the set of messages constructed, we'll now request the batch
	// all at once. This message will distributed the header requests
	// amongst all active peers, effectively sharding each query
	// dynamically.
	b.server.queryBatch(
		queryMsgs,

		// Callback to process potential replies. Always called from
		// the same goroutine as the outer function, so we don't have
		// to worry about synchronization.
		func(sp *ServerPeer, query wire.Message,
			resp wire.Message) bool {

			r, ok := resp.(*wire.MsgCFHeaders)
			if !ok {
				// We are only looking for cfheaders messages.
				return false
			}

			q, ok := query.(*wire.MsgGetCFHeaders)
			if !ok {
				// We sent a getcfheaders message, so that's
				// what we should be comparing against.
				return false
			}

			// The response doesn't match the query.
			if q.FilterType != r.FilterType ||
				q.StopHash != r.StopHash {
				return false
			}

			checkPointIndex, ok := stopHashes[r.StopHash]
			if !ok {
				// We never requested a matching stop hash.
				return false
			}

			// The response doesn't match the checkpoint.
			if !verifyCheckpoint(checkpoints[checkPointIndex], r) {
				log.Warnf("Checkpoints at index %v don't match " +
					"response!!!")
				return false
			}

			// At this point, the response matches the query, and
			// the relevant checkpoint we got earlier, so we should
			// always return true so that the peer looking for the
			// answer to this query can move on to the next query.
			// We still have to check that these headers are next
			// before we write them; otherwise, we cache them if
			// they're too far ahead, or discard them if we don't
			// need them.

			// Find the first and last height for the blocks
			// represented by this message.
			startHeight := checkPointIndex*wire.CFCheckptInterval + 1
			lastHeight := startHeight + wire.CFCheckptInterval

			log.Debugf("Got cfheaders from height=%v to height=%v",
				startHeight, lastHeight)

			// If this is out of order but not yet written, we can
			// verify that the checkpoints match, and then store
			// them.
			if startHeight > curHeight+1 {
				log.Debugf("Got response for headers at "+
					"height=%v, only at height=%v, stashing",
					startHeight, curHeight)

				queryResponses[checkPointIndex] = r

				return true
			}

			// If this is out of order stuff that's already been
			// written, we can ignore it.
			if lastHeight <= curHeight {
				log.Debugf("Received out of order reply "+
					"end_height=%v, already written", lastHeight)
				return true
			}

			// If this is the very first range we've requested, we
			// may already have a portion of the headers written to
			// disk.
			//
			// TODO(roasbeef): can eventually special case handle
			// this at the top
			if bytes.Equal(curHeader[:], initialFilterHeader[:]) {
				// So we'll set the prev header to our best
				// known header, and seek within the header
				// range a bit so we don't write any duplicate
				// headers.
				r.PrevFilterHeader = *curHeader
				offset := curHeight + 1 - startHeight
				r.FilterHashes = r.FilterHashes[offset:]
			}

			curHeader, err = b.writeCFHeadersMsg(r, store)
			if err != nil {
				panic(fmt.Sprintf("couldn't write cfheaders "+
					"msg: %v", err))
			}

			// Then, we cycle through any cached messages, adding
			// them to the batch and deleting them from the cache.
			for {
				checkPointIndex++

				// We'll also update the current height of the
				// last written set of cfheaders.
				curHeight = checkPointIndex * wire.CFCheckptInterval

				// Break if we've gotten to the end of the
				// responses or we don't have the next one.
				if checkPointIndex >= uint32(len(queryResponses)) {
					break
				}

				// If we don't yet have the next response, then
				// we'll break out so we can wait for the peers
				// to respond with this message.
				r := queryResponses[checkPointIndex]
				if r == nil {
					break
				}

				// We have another response to write, so delete
				// it from the cache and write it.
				queryResponses[checkPointIndex] = nil

				log.Debugf("Writing cfheaders at height=%v to "+
					"next checkpoint", curHeight)

				// As we write the set of headers to disk, we
				// also obtain the hash of the last filter
				// header we've written to disk so we can
				// properly set the PrevFilterHeader field of
				// the next message.
				curHeader, err = b.writeCFHeadersMsg(r, store)
				if err != nil {
					panic(fmt.Sprintf("couldn't write "+
						"cfheaders msg: %v", err))
				}
			}

			return true
		},

		// Same quit channel we're watching.
		b.quit,
	)
}

// writeCFHeadersMsg writes a cfheaders message to the specified store. It
// assumes that everything is being written in order. The hints are required to
// store the correct block heights for the filters. We also return final
// constructed cfheader in this range as this lets callers populate the prev
// filter header field in the next message range before writing to disk.
func (b *blockManager) writeCFHeadersMsg(msg *wire.MsgCFHeaders,
	store *headerfs.FilterHeaderStore) (*chainhash.Hash, error) {

	b.newFilterHeadersMtx.Lock()
	defer b.newFilterHeadersMtx.Unlock()

	// Check that the PrevFilterHeader is the same as the last stored so we
	// can prevent misalignment.
	tip, _, err := store.ChainTip()
	if err != nil {
		return nil, err
	}
	if *tip != msg.PrevFilterHeader {
		return nil, fmt.Errorf("attempt to write cfheaders out of order")
	}

	// Cycle through the headers and compute each header based on the prev
	// header and the filter hash from the cfheaders response entries.
	lastHeader := msg.PrevFilterHeader
	headerBatch := make([]headerfs.FilterHeader, 0, wire.CFCheckptInterval)
	for _, hash := range msg.FilterHashes {
		// header = dsha256(filterHash || prevHeader)
		lastHeader = chainhash.DoubleHashH(
			append(hash[:], lastHeader[:]...),
		)

		headerBatch = append(headerBatch, headerfs.FilterHeader{
			FilterHash: lastHeader,
		})
	}

	numHeaders := len(headerBatch)

	// We'll now query for the set of block headers which match each of
	// these filters headers in their corresponding chains. Our query will
	// return the headers for the entire checkpoint interval ending at the
	// designated stop hash.
	blockHeaders := b.server.BlockHeaders
	matchingBlockHeaders, startHeight, err := blockHeaders.FetchHeaderAncestors(
		uint32(numHeaders-1), &msg.StopHash,
	)
	if err != nil {
		return nil, err
	}

	// The final height in our range will be offset to the end of this
	// particular checkpoint interval.
	lastHeight := startHeight + uint32(numHeaders) - 1
	lastBlockHeader := matchingBlockHeaders[numHeaders-1]

	// We only need to set the height and hash of the very last filter
	// header in the range to ensure that the index properly updates the
	// tip of the chain.
	headerBatch[numHeaders-1].HeaderHash = lastBlockHeader.BlockHash()
	headerBatch[numHeaders-1].Height = lastHeight

	log.Debugf("Writing filter headers up to height=%v", lastHeight)

	// Write the header batch.
	err = store.WriteHeaders(headerBatch...)
	if err != nil {
		return nil, err
	}

	// Notify subscribers, and also update the filter header progress
	// logger at the same time.
	msgType := connectBasic
	for i, header := range matchingBlockHeaders {
		header := header

		headerHeight := startHeight + uint32(i)
		b.fltrHeaderProgessLogger.LogBlockHeight(
			header.Timestamp, int32(headerHeight),
		)

		b.server.sendSubscribedMsg(&blockMessage{
			msgType: msgType,
			header:  &header,
		})
	}

	// We'll also set the new header tip and notify any peers that the tip
	// has changed as well. Unlike the set of notifications above, this is
	// for sub-system that only need to know the height has changed rather
	// than know each new header that's been added to the tip.
	b.filterHeaderTip = lastHeight
	b.filterHeaderTipHash = lastBlockHeader.BlockHash()
	b.newFilterHeadersSignal.Broadcast()

	return &lastHeader, nil
}

// verifyHeaderCheckpoint verifies that a CFHeaders message matches the passed
// checkpoint. It assumes everything else has been checked, including filter
// type and stop hash matches, and returns true if matching and false if not.
func verifyCheckpoint(checkpoint *chainhash.Hash, cfheaders *wire.MsgCFHeaders) bool {

	lastHeader := cfheaders.PrevFilterHeader
	for _, hash := range cfheaders.FilterHashes {
		lastHeader = chainhash.DoubleHashH(
			append(hash[:], lastHeader[:]...),
		)
	}

	return lastHeader == *checkpoint
}

// resolveConflict finds the correct checkpoint information, rewinds the header
// store if it's incorrect, and bans any peers giving us incorrect header
// information.
func (b *blockManager) resolveConflict(
	checkpoints map[string][]*chainhash.Hash,
	store *headerfs.FilterHeaderStore, fType wire.FilterType) (
	[]*chainhash.Hash, error) {

	heightDiff, err := checkCFCheckptSanity(checkpoints, store)
	if err != nil {
		return nil, err
	}

	// If we got -1, we have full agreement between all peers and the store.
	if heightDiff == -1 {
		// Take the first peer's checkpoint list and return it.
		for _, checkpts := range checkpoints {
			return checkpts, nil
		}
	}

	log.Warnf("Detected mismatch at index=%v for checkpoints!!!", heightDiff)

	// Delete any responses that have fewer checkpoints than where we see a
	// mismatch.
	for peer, checkpts := range checkpoints {
		if len(checkpts) < heightDiff {
			delete(checkpoints, peer)
		}
	}

	if len(checkpoints) == 0 {
		return nil, fmt.Errorf("no peer is serving good cfheaders")
	}

	// Now we get all of the mismatched CFHeaders from peers, and check
	// which ones are valid.
	startHeight := uint32(heightDiff) * wire.CFCheckptInterval
	headers := b.getCFHeadersForAllPeers(startHeight, fType)

	// Make sure we're working off the same baseline. Otherwise, we want to
	// go back and get checkpoints again.
	var hash chainhash.Hash
	for _, msg := range headers {
		if hash == zeroHash {
			hash = msg.PrevFilterHeader
		} else if hash != msg.PrevFilterHeader {
			return nil, fmt.Errorf("mismatch between filter " +
				"headers expected to be the same")
		}
	}

	// For each header, go through and check whether all headers messages
	// have the same filter hash. If we find a difference, get the block,
	// calculate the filter, and throw out any mismatching peers.
	for i := 0; i < wire.MaxCFHeadersPerMsg; i++ {
		if checkForCFHeaderMismatch(headers, i) {
			// Get the block header for this height, along with the
			// block as well.
			targetHeight := startHeight + uint32(i)

			log.Warnf("Detected cfheader mismatch at "+
				"height=%v!!!", targetHeight)

			header, err := b.server.BlockHeaders.FetchHeaderByHeight(
				targetHeight,
			)
			if err != nil {
				return nil, err
			}
			block, err := b.server.GetBlock(header.BlockHash())
			if err != nil {
				return nil, err
			}

			log.Infof("Attempting to reconcile cfheader mismatch "+
				"amongst %v peers", len(headers))

			// We'll also fetch each of the filters from the peers
			// that reported check points, as we may need this in
			// order to determine which peers are faulty.
			filtersFromPeers := b.fetchFilterFromAllPeers(
				targetHeight, header.BlockHash(), fType,
			)
			badPeers, err := resolveCFHeaderMismatch(
				block.MsgBlock(), fType, filtersFromPeers,
			)
			if err != nil {
				return nil, err
			}

			log.Warnf("Banning %v peers due to invalid filter "+
				"headers", len(badPeers))

			for _, peer := range badPeers {
				log.Infof("Banning peer=%v for invalid filter "+
					"headers", peer)

				sp := b.server.PeerByAddr(peer)
				if sp != nil {
					b.server.BanPeer(sp)
					sp.Disconnect()
				}
				delete(headers, peer)
				delete(checkpoints, peer)
			}
		}
	}

	// Any mismatches have now been thrown out. Delete any checkpoint
	// lists that don't have matching headers, as these are peers that
	// didn't respond, and ban them from future queries.
	for peer := range checkpoints {
		if _, ok := headers[peer]; !ok {
			sp := b.server.PeerByAddr(peer)
			if sp != nil {
				b.server.BanPeer(sp)
				sp.Disconnect()
			}
			delete(checkpoints, peer)
		}
	}

	// Check sanity again. If we're sane, return a matching checkpoint
	// list. If not, return an error and download checkpoints from
	// remaining peers.
	heightDiff, err = checkCFCheckptSanity(checkpoints, store)
	if err != nil {
		return nil, err
	}

	// If we got -1, we have full agreement between all peers and the store.
	if heightDiff == -1 {
		// Take the first peer's checkpoint list and return it.
		for _, checkpts := range checkpoints {
			return checkpts, nil
		}
	}

	// Otherwise, return an error and allow the loop which calls this
	// function to call it again with the new set of peers.
	return nil, fmt.Errorf("got mismatched checkpoints")
}

// checkForCFHeaderMismatch checks all peers' responses at a specific position
// and detects a mismatch. It returns true if a mismatch has occurred.
func checkForCFHeaderMismatch(headers map[string]*wire.MsgCFHeaders,
	idx int) bool {

	// First, see if we have a mismatch.
	hash := zeroHash
	for _, msg := range headers {
		if len(msg.FilterHashes) <= idx {
			continue
		}

		if hash == zeroHash {
			hash = *msg.FilterHashes[idx]
			continue
		}

		if hash != *msg.FilterHashes[idx] {
			// We've found a mismatch!
			return true
		}
	}

	return false
}

// resolveCFHeaderMismatch will attempt to cross-reference each filter received
// by each peer based on what we can reconstruct and verify from the filter in
// question. We'll return all the peers that returned what we believe to in
// invalid filter.
func resolveCFHeaderMismatch(block *wire.MsgBlock, fType wire.FilterType,
	filtersFromPeers map[string]*gcs.Filter) ([]string, error) {

	badPeers := make(map[string]struct{})

	blockHash := block.BlockHash()
	filterKey := builder.DeriveKey(&blockHash)

	log.Infof("Attempting to pinpoint mismatch in cfheaders for block=%v",
		block.Header.BlockHash())

	// Based on the type of filter, our verification algorithm will differ.
	switch fType {

	// With the current set of items that we can fetch from the p2p
	// network, we're forced to only verify what we can at this point. So
	// we'll just ensure that each of the filters returned
	//
	// TODO(roasbeef): update after BLOCK_WITH_PREV_OUTS is a thing
	case wire.GCSFilterRegular:

		// We'll now run through each peer and ensure that each output
		// script is included in the filter that they responded with to
		// our query.
		for peerAddr, filter := range filtersFromPeers {
		peerVerification:

			// We'll ensure that all the filters include every
			// output script within the block.
			//
			// TODO(roasbeef): eventually just do a comparison
			// against decompressed filters
			for _, tx := range block.Transactions {
				for _, txOut := range tx.TxOut {
					switch {
					// If the script itself is blank, then
					// we'll skip this as it doesn't
					// contain any useful information.
					case len(txOut.PkScript) == 0:
						continue

					// We'll also skip any OP_RETURN
					// scripts as well since we don't index
					// these in order to avoid a circular
					// dependency.
					case txOut.PkScript[0] == txscript.OP_RETURN &&
						txscript.IsPushOnlyScript(txOut.PkScript[1:]):
						continue
					}

					match, err := filter.Match(
						filterKey, txOut.PkScript,
					)
					if err != nil {
						// If we're unable to query
						// this filter, then we'll skip
						// this peer all together.
						continue peerVerification
					}

					if match {
						continue
					}

					// If this filter doesn't match, then
					// we'll mark this peer as bad and move
					// on to the next peer.
					badPeers[peerAddr] = struct{}{}
					continue peerVerification
				}
			}
		}

	default:
		return nil, fmt.Errorf("unknown filter: %v", fType)
	}

	// TODO: We can add an after-the-fact countermeasure here against
	// eclipse attacks. If the checkpoints don't match the store, we can
	// check whether the store or the checkpoints we got from the network
	// are correct.

	// With the set of bad peers known, we'll collect a slice of all the
	// faulty peers.
	invalidPeers := make([]string, 0, len(badPeers))
	for peer := range badPeers {
		invalidPeers = append(invalidPeers, peer)
	}

	return invalidPeers, nil
}

// getCFHeadersForAllPeers runs a query for cfheaders at a specific height and
// returns a map of responses from all peers.
func (b *blockManager) getCFHeadersForAllPeers(height uint32,
	fType wire.FilterType) map[string]*wire.MsgCFHeaders {

	// Create the map we're returning.
	headers := make(map[string]*wire.MsgCFHeaders)

	// Get the header we expect at either the tip of the block header store
	// or at the end of the maximum-size response message, whichever is
	// larger.
	stopHeader, stopHeight, err := b.server.BlockHeaders.ChainTip()
	if stopHeight-height >= wire.MaxCFHeadersPerMsg {
		stopHeader, err = b.server.BlockHeaders.FetchHeaderByHeight(
			height + wire.MaxCFHeadersPerMsg - 1,
		)
		if err != nil {
			return headers
		}
	}

	// Calculate the hash and use it to create the query message.
	stopHash := stopHeader.BlockHash()
	msg := wire.NewMsgGetCFHeaders(fType, height, &stopHash)

	// Send the query to all peers and record their responses in the map.
	b.server.queryAllPeers(
		msg,
		func(sp *ServerPeer, resp wire.Message, quit chan<- struct{},
			peerQuit chan<- struct{}) {
			switch m := resp.(type) {
			case *wire.MsgCFHeaders:
				if m.StopHash == stopHash &&
					m.FilterType == fType {
					headers[sp.Addr()] = m

					// We got an answer from this peer so
					// that peer's goroutine can stop.
					close(peerQuit)
				}
			}
		},
	)

	return headers
}

// fetchFilterFromAllPeers attempts to fetch a filter for the target filter
// type and blocks from all peers connected to the block manager. This method
// returns a map which allows the caller to match a peer to the filter it
// responded with.
func (b *blockManager) fetchFilterFromAllPeers(
	height uint32, blockHash chainhash.Hash,
	filterType wire.FilterType) map[string]*gcs.Filter {

	// We'll use this map to collate all responses we receive from each
	// peer.
	filterResponses := make(map[string]*gcs.Filter)

	// We'll now request the target filter from each peer, using a stop
	// hash at the target block hash to ensure we only get a single filter.
	fitlerReqMsg := wire.NewMsgGetCFilters(filterType, height, &blockHash)
	b.server.queryAllPeers(
		fitlerReqMsg,
		func(sp *ServerPeer, resp wire.Message, quit chan<- struct{},
			peerQuit chan<- struct{}) {

			switch response := resp.(type) {
			// We're only interested in "cfilter" messages.
			case *wire.MsgCFilter:
				// If the response doesn't match our request.
				// Ignore this message.
				if blockHash != response.BlockHash ||
					filterType != response.FilterType {
					return
				}

				// Now that we know we have the proper filter,
				// we'll decode it into an object the caller
				// can utilize.
				gcsFilter, err := gcs.FromNBytes(
					builder.DefaultP, builder.DefaultM,
					response.Data,
				)
				if err != nil {
					// Malformed filter data. We can ignore
					// this message.
					return
				}

				// Now that we're able to properly parse this
				// filter, we'll assign it to its source peer,
				// and wait for the next response.
				filterResponses[sp.Addr()] = gcsFilter

			default:
			}
		},
	)

	return filterResponses
}

// getCheckpts runs a query for cfcheckpts against all peers and returns a map
// of responses.
func (b *blockManager) getCheckpts(lastHash *chainhash.Hash,
	fType wire.FilterType) map[string][]*chainhash.Hash {

	checkpoints := make(map[string][]*chainhash.Hash)
	getCheckptMsg := wire.NewMsgGetCFCheckpt(fType, lastHash)
	b.server.queryAllPeers(
		getCheckptMsg,
		func(sp *ServerPeer, resp wire.Message, quit chan<- struct{},
			peerQuit chan<- struct{}) {
			switch m := resp.(type) {
			case *wire.MsgCFCheckpt:
				if m.FilterType == fType &&
					m.StopHash == *lastHash {
					checkpoints[sp.Addr()] = m.FilterHeaders
					close(peerQuit)
				}
			}
		},
	)
	return checkpoints
}

// checkCFCheckptSanity checks whether all peers which have responded agree.
// If so, it returns -1; otherwise, it returns the earliest index at which at
// least one of the peers differs. The checkpoints are also checked against the
// existing store up to the tip of the store. If all of the peers match but
// the store doesn't, the height at which the mismatch occurs is returned.
func checkCFCheckptSanity(cp map[string][]*chainhash.Hash,
	headerStore *headerfs.FilterHeaderStore) (int, error) {

	// Get the known best header to compare against checkpoints.
	_, storeTip, err := headerStore.ChainTip()
	if err != nil {
		return 0, err
	}

	// Determine the maximum length of each peer's checkpoint list. If they
	// differ, we don't return yet because we want to make sure they match
	// up to the shortest one.
	maxLen := 0
	for _, checkpoints := range cp {
		if len(checkpoints) > maxLen {
			maxLen = len(checkpoints)
		}
	}

	// Compare the actual checkpoints against each other and anything
	// stored in the header store.
	for i := 0; i < maxLen; i++ {
		var checkpoint chainhash.Hash
		for _, checkpoints := range cp {
			if i >= len(checkpoints) {
				continue
			}
			if checkpoint == zeroHash {
				checkpoint = *checkpoints[i]
			}
			if checkpoint != *checkpoints[i] {
				log.Warnf("mismatch at %v, expected %v got "+
					"%v", i, checkpoint, checkpoints[i])
				return i, nil
			}
		}

		ckptHeight := uint32((i + 1) * wire.CFCheckptInterval)

		if ckptHeight <= storeTip {
			header, err := headerStore.FetchHeaderByHeight(
				ckptHeight,
			)
			if err != nil {
				return i, err
			}

			if *header != checkpoint {
				log.Warnf("mismatch at height %v, expected %v got "+
					"%v", ckptHeight, header, checkpoint)
				return i, nil
			}
		}
	}

	return -1, nil
}

// blockHandler is the main handler for the block manager.  It must be run as a
// goroutine.  It processes block and inv messages in a separate goroutine from
// the peer handlers so the block (MsgBlock) messages are handled by a single
// thread without needing to lock memory data structures.  This is important
// because the block manager controls which blocks are needed and how
// the fetching should proceed.
func (b *blockManager) blockHandler() {
	candidatePeers := list.New()
out:
	for {
		// Now check peer messages and quit channels.
		select {
		case m := <-b.peerChan:
			switch msg := m.(type) {
			case *newPeerMsg:
				b.handleNewPeerMsg(candidatePeers, msg.peer)

			case *invMsg:
				b.handleInvMsg(msg)

			case *headersMsg:
				b.handleHeadersMsg(msg)

			case *donePeerMsg:
				b.handleDonePeerMsg(candidatePeers, msg.peer)

			default:
				log.Warnf("Invalid message type in block "+
					"handler: %T", msg)
			}

		case <-b.quit:
			break out
		}
	}

	b.wg.Done()
	log.Trace("Block handler done")
}

// SyncPeer returns the current sync peer.
func (b *blockManager) SyncPeer() *ServerPeer {
	b.syncPeerMutex.Lock()
	defer b.syncPeerMutex.Unlock()

	return b.syncPeer
}

// isSyncCandidate returns whether or not the peer is a candidate to consider
// syncing from.
func (b *blockManager) isSyncCandidate(sp *ServerPeer) bool {
	// The peer is not a candidate for sync if it's not a full node.
	return sp.Services()&wire.SFNodeNetwork == wire.SFNodeNetwork
}

// findNextHeaderCheckpoint returns the next checkpoint after the passed height.
// It returns nil when there is not one either because the height is already
// later than the final checkpoint or there are none for the current network.
func (b *blockManager) findNextHeaderCheckpoint(height int32) *chaincfg.Checkpoint {
	// There is no next checkpoint if there are none for this current
	// network.
	checkpoints := b.server.chainParams.Checkpoints
	if len(checkpoints) == 0 {
		return nil
	}

	// There is no next checkpoint if the height is already after the final
	// checkpoint.
	finalCheckpoint := &checkpoints[len(checkpoints)-1]
	if height >= finalCheckpoint.Height {
		return nil
	}

	// Find the next checkpoint.
	nextCheckpoint := finalCheckpoint
	for i := len(checkpoints) - 2; i >= 0; i-- {
		if height >= checkpoints[i].Height {
			break
		}
		nextCheckpoint = &checkpoints[i]
	}
	return nextCheckpoint
}

// findPreviousHeaderCheckpoint returns the last checkpoint before the passed
// height. It returns a checkpoint matching the genesis block when the height
// is earlier than the first checkpoint or there are no checkpoints for the
// current network. This is used for resetting state when a malicious peer
// sends us headers that don't lead up to a known checkpoint.
func (b *blockManager) findPreviousHeaderCheckpoint(height int32) *chaincfg.Checkpoint {
	// Start with the genesis block - earliest checkpoint to which our code
	// will want to reset
	prevCheckpoint := &chaincfg.Checkpoint{
		Height: 0,
		Hash:   b.server.chainParams.GenesisHash,
	}

	// Find the latest checkpoint lower than height or return genesis block
	// if there are none.
	checkpoints := b.server.chainParams.Checkpoints
	for i := 0; i < len(checkpoints); i++ {
		if height <= checkpoints[i].Height {
			break
		}
		prevCheckpoint = &checkpoints[i]
	}

	return prevCheckpoint
}

// startSync will choose the best peer among the available candidate peers to
// download/sync the blockchain from.  When syncing is already running, it
// simply returns.  It also examines the candidates for any which are no longer
// candidates and removes them as needed.
func (b *blockManager) startSync(peers *list.List) {
	// Return now if we're already syncing.
	if b.syncPeer != nil {
		return
	}

	best, err := b.server.BestSnapshot()
	if err != nil {
		log.Errorf("Failed to get hash and height for the "+
			"latest block: %s", err)
		return
	}

	var bestPeer *ServerPeer
	var enext *list.Element
	for e := peers.Front(); e != nil; e = enext {
		enext = e.Next()
		sp := e.Value.(*ServerPeer)

		// Remove sync candidate peers that are no longer candidates
		// due to passing their latest known block.
		//
		// NOTE: The < is intentional as opposed to <=.  While
		// techcnically the peer doesn't have a later block when it's
		// equal, it will likely have one soon so it is a reasonable
		// choice.  It also allows the case where both are at 0 such as
		// during regression test.
		if sp.LastBlock() < best.Height {
			peers.Remove(e)
			continue
		}

		// TODO: Use a better algorithm to choose the best peer.
		// For now, just pick the candidate with the highest last block.
		if bestPeer == nil || sp.LastBlock() > bestPeer.LastBlock() {
			bestPeer = sp
		}
	}

	// Start syncing from the best peer if one was selected.
	if bestPeer != nil {
		locator, err := b.server.BlockHeaders.LatestBlockLocator()
		if err != nil {
			log.Errorf("Failed to get block locator for the "+
				"latest block: %s", err)
			return
		}

		log.Infof("Syncing to block height %d from peer %s",
			bestPeer.LastBlock(), bestPeer.Addr())

		// Now that we know we have a new sync peer, we'll lock it in
		// within the proper attribute.
		b.syncPeerMutex.Lock()
		b.syncPeer = bestPeer
		b.syncPeerMutex.Unlock()

		// By default will use the zero hash as our stop hash to query
		// for all the headers beyond our view of the network based on
		// our latest block locator.
		stopHash := &zeroHash

		// If we're still within the range of the set checkpoints, then
		// we'll use the next checkpoint to guide the set of headers we
		// fetch, setting our stop hash to the next checkpoint hash.
		if b.nextCheckpoint != nil && best.Height < b.nextCheckpoint.Height {
			log.Infof("Downloading headers for blocks %d to "+
				"%d from peer %s", best.Height+1,
				b.nextCheckpoint.Height, bestPeer.Addr())

			stopHash = b.nextCheckpoint.Hash
		} else {
			log.Infof("Fetching set of headers from tip "+
				"(height=%v) from peer %s", best.Height,
				bestPeer.Addr())
		}

		// With our stop hash selected, we'll kick off the sync from
		// this peer with an initial GetHeaders message.
		b.SyncPeer().PushGetHeadersMsg(locator, stopHash)
	} else {
		log.Warnf("No sync peer candidates available")
	}
}

// IsCurrent returns whether or not the block manager believes it is synced
// with the connected peers.
func (b *blockManager) IsCurrent() bool {
	b.syncPeerMutex.RLock()
	defer b.syncPeerMutex.RUnlock()

	// Figure out the latest block we know.
	header, height, err := b.server.BlockHeaders.ChainTip()
	if err != nil {
		return false
	}

	// There is no last checkpoint if checkpoints are disabled or there are
	// none for this current network.
	checkpoints := b.server.chainParams.Checkpoints
	if len(checkpoints) != 0 {
		// We aren't current if the newest block we know of isn't ahead
		// of all checkpoints.
		if checkpoints[len(checkpoints)-1].Height >= int32(height) {
			return false
		}
	}

	// If we have a syncPeer and are below the block we are syncing to, we
	// are not current.
	if b.syncPeer != nil && int32(height) < b.syncPeer.LastBlock() {
		return false
	}

	// If our time source (median times of all the connected peers) is at
	// least 24 hours ahead of our best known block, we aren't current.
	minus24Hours := b.server.timeSource.AdjustedTime().Add(-24 * time.Hour)
	if header.Timestamp.Before(minus24Hours) {
		return false
	}

	// If we have no sync peer, we can assume we're current for now.
	if b.syncPeer == nil {
		return true
	}

	// If we have a syncPeer and the peer reported a higher known block
	// height on connect than we know the peer already has, we're probably
	// not current. If the peer is lying to us, other code will disconnect
	// it and then we'll re-check and notice that we're actually current.
	return b.syncPeer.LastBlock() >= b.syncPeer.StartingHeight()
}

// FilterHeaderTip returns the current height of the filter header chain tip.
//
// NOTE: This method is safe for concurrent access.
func (b *blockManager) FilterHeaderTip() uint32 {
	b.newFilterHeadersMtx.RLock()
	defer b.newFilterHeadersMtx.RUnlock()

	return b.filterHeaderTip
}

// SynchronizeFilterHeaders allows the caller to execute a function closure
// that depends on synchronization with the current set of filter headers. This
// allows the caller to execute an action that depends on the current filter
// header state, thereby ensuring that the state would shift from underneath
// them. Each execution of the closure will have the current filter header tip
// passed in to ensue that the caller gets a consistent view.
func (b *blockManager) SynchronizeFilterHeaders(f func(uint32) error) error {
	b.newFilterHeadersMtx.RLock()
	defer b.newFilterHeadersMtx.RUnlock()

	return f(b.filterHeaderTip)
}

// BlockHeaderTip returns the current height of the block header chain tip.
//
// NOTE: This method is safe for concurrent access.
func (b *blockManager) BlockHeaderTip() uint32 {
	b.newHeadersMtx.RLock()
	defer b.newHeadersMtx.RUnlock()

	return b.headerTip
}

// QueueInv adds the passed inv message and peer to the block handling queue.
func (b *blockManager) QueueInv(inv *wire.MsgInv, sp *ServerPeer) {
	// No channel handling here because peers do not need to block on inv
	// messages.
	if atomic.LoadInt32(&b.shutdown) != 0 {
		return
	}

	select {
	case b.peerChan <- &invMsg{inv: inv, peer: sp}:
	case <-b.quit:
		return
	}
}

// handleInvMsg handles inv messages from all peers.
// We examine the inventory advertised by the remote peer and act accordingly.
func (b *blockManager) handleInvMsg(imsg *invMsg) {
	// Attempt to find the final block in the inventory list.  There may
	// not be one.
	lastBlock := -1
	invVects := imsg.inv.InvList
	for i := len(invVects) - 1; i >= 0; i-- {
		if invVects[i].Type == wire.InvTypeBlock {
			lastBlock = i
			break
		}
	}

	// If this inv contains a block announcement, and this isn't coming from
	// our current sync peer or we're current, then update the last
	// announced block for this peer. We'll use this information later to
	// update the heights of peers based on blocks we've accepted that they
	// previously announced.
	if lastBlock != -1 && (imsg.peer != b.SyncPeer() || b.IsCurrent()) {
		imsg.peer.UpdateLastAnnouncedBlock(&invVects[lastBlock].Hash)
	}

	// Ignore invs from peers that aren't the sync if we are not current.
	// Helps prevent dealing with orphans.
	if imsg.peer != b.SyncPeer() && !b.IsCurrent() {
		return
	}

	// If our chain is current and a peer announces a block we already
	// know of, then update their current block height.
	if lastBlock != -1 && b.IsCurrent() {
		height, err := b.server.BlockHeaders.HeightFromHash(&invVects[lastBlock].Hash)
		if err == nil {
			imsg.peer.UpdateLastBlockHeight(int32(height))
		}
	}

	// Add blocks to the cache of known inventory for the peer.
	for _, iv := range invVects {
		if iv.Type == wire.InvTypeBlock {
			imsg.peer.AddKnownInventory(iv)
		}
	}

	// If this is the sync peer or we're current, get the headers for the
	// announced blocks and update the last announced block.
	if lastBlock != -1 && (imsg.peer == b.SyncPeer() || b.IsCurrent()) {
		lastEl := b.headerList.Back()
		var lastHash chainhash.Hash
		if lastEl != nil {
			lastHash = lastEl.Header.BlockHash()
		}

		// Only send getheaders if we don't already know about the last
		// block hash being announced.
		if lastHash != invVects[lastBlock].Hash && lastEl != nil &&
			b.lastRequested != invVects[lastBlock].Hash {

			// Make a locator starting from the latest known header
			// we've processed.
			locator := make(blockchain.BlockLocator, 0,
				wire.MaxBlockLocatorsPerMsg)
			locator = append(locator, &lastHash)

			// Add locator from the database as backup.
			knownLocator, err := b.server.BlockHeaders.LatestBlockLocator()
			if err == nil {
				locator = append(locator, knownLocator...)
			}

			// Get headers based on locator.
			err = imsg.peer.PushGetHeadersMsg(locator,
				&invVects[lastBlock].Hash)
			if err != nil {
				log.Warnf("Failed to send getheaders message "+
					"to peer %s: %s", imsg.peer.Addr(), err)
				return
			}
			b.lastRequested = invVects[lastBlock].Hash
		}
	}
}

// QueueHeaders adds the passed headers message and peer to the block handling
// queue.
func (b *blockManager) QueueHeaders(headers *wire.MsgHeaders, sp *ServerPeer) {
	// No channel handling here because peers do not need to block on
	// headers messages.
	if atomic.LoadInt32(&b.shutdown) != 0 {
		return
	}

	select {
	case b.peerChan <- &headersMsg{headers: headers, peer: sp}:
	case <-b.quit:
		return
	}
}

// handleHeadersMsg handles headers messages from all peers.
func (b *blockManager) handleHeadersMsg(hmsg *headersMsg) {
	msg := hmsg.headers
	numHeaders := len(msg.Headers)

	// Nothing to do for an empty headers message.
	if numHeaders == 0 {
		return
	}

	// For checking to make sure blocks aren't too far in the future as of
	// the time we receive the headers message.
	maxTimestamp := b.server.timeSource.AdjustedTime().
		Add(maxTimeOffset)

	// We'll attempt to write the entire batch of validated headers
	// atomically in order to improve peformance.
	headerWriteBatch := make([]headerfs.BlockHeader, 0, len(msg.Headers))

	// Process all of the received headers ensuring each one connects to
	// the previous and that checkpoints match.
	receivedCheckpoint := false
	var (
		finalHash   *chainhash.Hash
		finalHeight int32
	)
	for i, blockHeader := range msg.Headers {
		blockHash := blockHeader.BlockHash()
		finalHash = &blockHash

		// Ensure there is a previous header to compare against.
		prevNodeEl := b.headerList.Back()
		if prevNodeEl == nil {
			log.Warnf("Header list does not contain a previous" +
				"element as expected -- disconnecting peer")
			hmsg.peer.Disconnect()
			return
		}

		// Ensure the header properly connects to the previous one,
		// that the proof of work is good, and that the header's
		// timestamp isn't too far in the future, and add it to the
		// list of headers.
		node := headerlist.Node{Header: *blockHeader}
		prevNode := prevNodeEl
		prevHash := prevNode.Header.BlockHash()
		if prevHash.IsEqual(&blockHeader.PrevBlock) {
			err := b.checkHeaderSanity(blockHeader, maxTimestamp,
				false)
			if err != nil {
				log.Warnf("Header doesn't pass sanity check: "+
					"%s -- disconnecting peer", err)
				hmsg.peer.Disconnect()
				return
			}

			node.Height = prevNode.Height + 1
			finalHeight = node.Height

			// This header checks out, so we'll add it to our write
			// batch.
			headerWriteBatch = append(headerWriteBatch, headerfs.BlockHeader{
				BlockHeader: blockHeader,
				Height:      uint32(node.Height),
			})

			hmsg.peer.UpdateLastBlockHeight(node.Height)

			b.blkHeaderProgressLogger.LogBlockHeight(
				blockHeader.Timestamp, node.Height,
			)

			// Finally initialize the header ->
			// map[filterHash]*peer map for filter header
			// validation purposes later.
			e := b.headerList.PushBack(node)
			if b.startHeader == nil {
				b.startHeader = e
			}
		} else {
			// The block doesn't connect to the last block we know.
			// We will need to do some additional checks to process
			// possible reorganizations or incorrect chain on
			// either our or the peer's side.
			//
			// If we got these headers from a peer that's not our
			// sync peer, they might not be aligned correctly or
			// even on the right chain. Just ignore the rest of the
			// message. However, if we're current, this might be a
			// reorg, in which case we'll either change our sync
			// peer or disconnect the peer that sent us these bad
			// headers.
			if hmsg.peer != b.SyncPeer() && !b.IsCurrent() {
				return
			}

			// Check if this is the last block we know of. This is
			// a shortcut for sendheaders so that each redundant
			// header doesn't cause a disk read.
			if blockHash == prevHash {
				continue
			}

			// Check if this block is known. If so, we continue to
			// the next one.
			_, _, err := b.server.BlockHeaders.FetchHeader(&blockHash)
			if err == nil {
				continue
			}

			// Check if the previous block is known. If it is, this
			// is probably a reorg based on the estimated latest
			// block that matches between us and the peer as
			// derived from the block locator we sent to request
			// these headers. Otherwise, the headers don't connect
			// to anything we know and we should disconnect the
			// peer.
			backHead, backHeight, err := b.server.BlockHeaders.FetchHeader(
				&blockHeader.PrevBlock,
			)
			if err != nil {
				log.Warnf("Received block header that does not"+
					" properly connect to the chain from"+
					" peer %s (%s) -- disconnecting",
					hmsg.peer.Addr(), err)
				hmsg.peer.Disconnect()
				return
			}

			// We've found a branch we weren't aware of. If the
			// branch is earlier than the latest synchronized
			// checkpoint, it's invalid and we need to disconnect
			// the reporting peer.
			prevCheckpoint := b.findPreviousHeaderCheckpoint(
				prevNode.Height,
			)
			if backHeight < uint32(prevCheckpoint.Height) {
				log.Errorf("Attempt at a reorg earlier than a "+
					"checkpoint past which we've already "+
					"synchronized -- disconnecting peer "+
					"%s", hmsg.peer.Addr())
				hmsg.peer.Disconnect()
				return
			}

			// Check the sanity of the new branch. If any of the
			// blocks don't pass sanity checks, disconnect the
			// peer.  We also keep track of the work represented by
			// these headers so we can compare it to the work in
			// the known good chain.
			b.reorgList.ResetHeaderState(headerlist.Node{
				Header: *backHead,
				Height: int32(backHeight),
			})
			totalWork := big.NewInt(0)
			for j, reorgHeader := range msg.Headers[i:] {
				err = b.checkHeaderSanity(reorgHeader,
					maxTimestamp, true)
				if err != nil {
					log.Warnf("Header doesn't pass sanity"+
						" check: %s -- disconnecting "+
						"peer", err)
					hmsg.peer.Disconnect()
					return
				}
				totalWork.Add(totalWork,
					blockchain.CalcWork(reorgHeader.Bits))
				b.reorgList.PushBack(headerlist.Node{
					Header: *reorgHeader,
					Height: int32(backHeight+1) + int32(j),
				})
			}
			log.Tracef("Sane reorg attempted. Total work from "+
				"reorg chain: %v", totalWork)

			// All the headers pass sanity checks. Now we calculate
			// the total work for the known chain.
			knownWork := big.NewInt(0)

			// This should NEVER be nil because the most recent
			// block is always pushed back by resetHeaderState
			knownEl := b.headerList.Back()
			var knownHead *wire.BlockHeader
			for j := uint32(prevNode.Height); j > backHeight; j-- {
				if knownEl != nil {
					knownHead = &knownEl.Header
					knownEl = knownEl.Prev()
				} else {
					knownHead, _, err = b.server.BlockHeaders.FetchHeader(
						&knownHead.PrevBlock)
					if err != nil {
						log.Criticalf("Can't get block"+
							"header for hash %s: "+
							"%v",
							knownHead.PrevBlock,
							err)
						// Should we panic here?
					}
				}
				knownWork.Add(knownWork,
					blockchain.CalcWork(knownHead.Bits))
			}

			log.Tracef("Total work from known chain: %v", knownWork)

			// Compare the two work totals and reject the new chain
			// if it doesn't have more work than the previously
			// known chain. Disconnect if it's actually less than
			// the known chain.
			switch knownWork.Cmp(totalWork) {
			case 1:
				log.Warnf("Reorg attempt that has less work "+
					"than known chain from peer %s -- "+
					"disconnecting", hmsg.peer.Addr())
				hmsg.peer.Disconnect()
				fallthrough
			case 0:
				return
			default:
			}

			// At this point, we have a valid reorg, so we roll
			// back the existing chain and add the new block
			// header.  We also change the sync peer. Then we can
			// continue with the rest of the headers in the message
			// as if nothing has happened.
			b.syncPeerMutex.Lock()
			b.syncPeer = hmsg.peer
			b.syncPeerMutex.Unlock()
			_, err = b.server.rollBackToHeight(backHeight)
			if err != nil {
				panic(fmt.Sprintf("Rollback failed: %s", err))
				// Should we panic here?
			}

			hdrs := headerfs.BlockHeader{
				BlockHeader: blockHeader,
				Height:      backHeight + 1,
			}
			err = b.server.BlockHeaders.WriteHeaders(hdrs)
			if err != nil {
				log.Criticalf("Couldn't write block to "+
					"database: %s", err)
				// Should we panic here?
			}

			b.headerList.ResetHeaderState(headerlist.Node{
				Header: *backHead,
				Height: int32(backHeight),
			})
			b.headerList.PushBack(headerlist.Node{
				Header: *blockHeader,
				Height: int32(backHeight + 1),
			})
		}

		// Verify the header at the next checkpoint height matches.
		if b.nextCheckpoint != nil && node.Height == b.nextCheckpoint.Height {
			nodeHash := node.Header.BlockHash()
			if nodeHash.IsEqual(b.nextCheckpoint.Hash) {
				receivedCheckpoint = true
				log.Infof("Verified downloaded block "+
					"header against checkpoint at height "+
					"%d/hash %s", node.Height, nodeHash)
			} else {
				log.Warnf("Block header at height %d/hash "+
					"%s from peer %s does NOT match "+
					"expected checkpoint hash of %s -- "+
					"disconnecting", node.Height,
					nodeHash, hmsg.peer.Addr(),
					b.nextCheckpoint.Hash)

				prevCheckpoint := b.findPreviousHeaderCheckpoint(
					node.Height,
				)

				log.Infof("Rolling back to previous validated "+
					"checkpoint at height %d/hash %s",
					prevCheckpoint.Height,
					prevCheckpoint.Hash)

				_, err := b.server.rollBackToHeight(uint32(
					prevCheckpoint.Height),
				)
				if err != nil {
					log.Criticalf("Rollback failed: %s",
						err)
					// Should we panic here?
				}

				hmsg.peer.Disconnect()
				return
			}
			break
		}
	}

	log.Tracef("Writing header batch of %v block headers",
		len(headerWriteBatch))

	if len(headerWriteBatch) > 0 {
		// With all the headers in this batch validated, we'll write
		// them all in a single transaction such that this entire batch
		// is atomic.
		err := b.server.BlockHeaders.WriteHeaders(headerWriteBatch...)
		if err != nil {
			panic(fmt.Sprintf("unable to write block header: %v",
				err))
		}
	}

	// When this header is a checkpoint, find the next checkpoint.
	if receivedCheckpoint {
		b.nextCheckpoint = b.findNextHeaderCheckpoint(finalHeight)
	}

	// If not current, request the next batch of headers starting from the
	// latest known header and ending with the next checkpoint.
	if !b.IsCurrent() || b.server.chainParams.Net == chaincfg.SimNetParams.Net {
		locator := blockchain.BlockLocator([]*chainhash.Hash{finalHash})
		nextHash := zeroHash
		if b.nextCheckpoint != nil {
			nextHash = *b.nextCheckpoint.Hash
		}
		err := hmsg.peer.PushGetHeadersMsg(locator, &nextHash)
		if err != nil {
			log.Warnf("Failed to send getheaders message to "+
				"peer %s: %s", hmsg.peer.Addr(), err)
			return
		}
	}

	// Since we have a new set of headers written to disk, we'll send out a
	// new signal to notify any waiting sub-systems that they can now maybe
	// proceed do to us extending the header chain.
	b.newHeadersMtx.Lock()
	b.headerTip = uint32(finalHeight)
	b.headerTipHash = *finalHash
	b.newHeadersMtx.Unlock()
	b.newHeadersSignal.Broadcast()
}

// checkHeaderSanity checks the PoW, and timestamp of a block header.
func (b *blockManager) checkHeaderSanity(blockHeader *wire.BlockHeader,
	maxTimestamp time.Time, reorgAttempt bool) error {
	diff, err := b.calcNextRequiredDifficulty(
		blockHeader.Timestamp, reorgAttempt)
	if err != nil {
		return err
	}
	stubBlock := btcutil.NewBlock(&wire.MsgBlock{
		Header: *blockHeader,
	})
	err = blockchain.CheckProofOfWork(stubBlock,
		blockchain.CompactToBig(diff))
	if err != nil {
		return err
	}
	// Ensure the block time is not too far in the future.
	if blockHeader.Timestamp.After(maxTimestamp) {
		return fmt.Errorf("block timestamp of %v is too far in the "+
			"future", blockHeader.Timestamp)
	}
	return nil
}

// calcNextRequiredDifficulty calculates the required difficulty for the block
// after the passed previous block node based on the difficulty retarget rules.
func (b *blockManager) calcNextRequiredDifficulty(newBlockTime time.Time,
	reorgAttempt bool) (uint32, error) {

	hList := b.headerList
	if reorgAttempt {
		hList = b.reorgList
	}

	lastNode := hList.Back()

	// Genesis block.
	if lastNode == nil {
		return b.server.chainParams.PowLimitBits, nil
	}

	// Return the previous block's difficulty requirements if this block
	// is not at a difficulty retarget interval.
	if (lastNode.Height+1)%b.blocksPerRetarget != 0 {
		// For networks that support it, allow special reduction of the
		// required difficulty once too much time has elapsed without
		// mining a block.
		if b.server.chainParams.ReduceMinDifficulty {
			// Return minimum difficulty when more than the desired
			// amount of time has elapsed without mining a block.
			reductionTime := int64(
				b.server.chainParams.MinDiffReductionTime /
					time.Second)
			allowMinTime := lastNode.Header.Timestamp.Unix() +
				reductionTime
			if newBlockTime.Unix() > allowMinTime {
				return b.server.chainParams.PowLimitBits, nil
			}

			// The block was mined within the desired timeframe, so
			// return the difficulty for the last block which did
			// not have the special minimum difficulty rule applied.
			prevBits, err := b.findPrevTestNetDifficulty(hList)
			if err != nil {
				return 0, err
			}
			return prevBits, nil
		}

		// For the main network (or any unrecognized networks), simply
		// return the previous block's difficulty requirements.
		return lastNode.Header.Bits, nil
	}

	// Get the block node at the previous retarget (targetTimespan days
	// worth of blocks).
	firstNode, err := b.server.BlockHeaders.FetchHeaderByHeight(
		uint32(lastNode.Height + 1 - b.blocksPerRetarget),
	)
	if err != nil {
		return 0, err
	}

	// Limit the amount of adjustment that can occur to the previous
	// difficulty.
	actualTimespan := lastNode.Header.Timestamp.Unix() -
		firstNode.Timestamp.Unix()
	adjustedTimespan := actualTimespan
	if actualTimespan < b.minRetargetTimespan {
		adjustedTimespan = b.minRetargetTimespan
	} else if actualTimespan > b.maxRetargetTimespan {
		adjustedTimespan = b.maxRetargetTimespan
	}

	// Calculate new target difficulty as:
	//  currentDifficulty * (adjustedTimespan / targetTimespan)
	// The result uses integer division which means it will be slightly
	// rounded down.  Bitcoind also uses integer division to calculate this
	// result.
	oldTarget := blockchain.CompactToBig(lastNode.Header.Bits)
	newTarget := new(big.Int).Mul(oldTarget, big.NewInt(adjustedTimespan))
	targetTimeSpan := int64(b.server.chainParams.TargetTimespan /
		time.Second)
	newTarget.Div(newTarget, big.NewInt(targetTimeSpan))

	// Limit new value to the proof of work limit.
	if newTarget.Cmp(b.server.chainParams.PowLimit) > 0 {
		newTarget.Set(b.server.chainParams.PowLimit)
	}

	// Log new target difficulty and return it.  The new target logging is
	// intentionally converting the bits back to a number instead of using
	// newTarget since conversion to the compact representation loses
	// precision.
	newTargetBits := blockchain.BigToCompact(newTarget)
	log.Debugf("Difficulty retarget at block height %d", lastNode.Height+1)
	log.Debugf("Old target %08x (%064x)", lastNode.Header.Bits, oldTarget)
	log.Debugf("New target %08x (%064x)", newTargetBits,
		blockchain.CompactToBig(newTargetBits))
	log.Debugf("Actual timespan %v, adjusted timespan %v, target timespan %v",
		time.Duration(actualTimespan)*time.Second,
		time.Duration(adjustedTimespan)*time.Second,
		b.server.chainParams.TargetTimespan)

	return newTargetBits, nil
}

// findPrevTestNetDifficulty returns the difficulty of the previous block which
// did not have the special testnet minimum difficulty rule applied.
func (b *blockManager) findPrevTestNetDifficulty(hList headerlist.Chain) (uint32, error) {
	startNode := hList.Back()

	// Genesis block.
	if startNode == nil {
		return b.server.chainParams.PowLimitBits, nil
	}

	// Search backwards through the chain for the last block without
	// the special rule applied.
	iterEl := startNode
	iterNode := &startNode.Header
	iterHeight := startNode.Height
	for iterNode != nil && iterHeight%b.blocksPerRetarget != 0 &&
		iterNode.Bits == b.server.chainParams.PowLimitBits {

		// Get the previous block node.  This function is used over
		// simply accessing iterNode.parent directly as it will
		// dynamically create previous block nodes as needed.  This
		// helps allow only the pieces of the chain that are needed
		// to remain in memory.
		iterHeight--
		el := iterEl.Prev()
		if el != nil {
			iterNode = &el.Header
		} else {
			node, err := b.server.BlockHeaders.FetchHeaderByHeight(
				uint32(iterHeight),
			)
			if err != nil {
				log.Errorf("GetBlockByHeight: %s", err)
				return 0, err
			}
			iterNode = node
		}
	}

	// Return the found difficulty or the minimum difficulty if no
	// appropriate block was found.
	lastBits := b.server.chainParams.PowLimitBits
	if iterNode != nil {
		lastBits = iterNode.Bits
	}
	return lastBits, nil
}