lotus/chain/sync.go

package chain

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"os"
	"sort"
	"strings"
	"sync"
	"time"

	"github.com/filecoin-project/lotus/chain/actors/builtin"

	"github.com/filecoin-project/lotus/node/modules/dtypes"

	"github.com/filecoin-project/specs-actors/actors/runtime/proof"

	"github.com/Gurpartap/async"
	"github.com/hashicorp/go-multierror"
	blocks "github.com/ipfs/go-block-format"
	"github.com/ipfs/go-cid"
	cbor "github.com/ipfs/go-ipld-cbor"
	logging "github.com/ipfs/go-log/v2"
	"github.com/libp2p/go-libp2p-core/connmgr"
	"github.com/libp2p/go-libp2p-core/peer"
	cbg "github.com/whyrusleeping/cbor-gen"
	"github.com/whyrusleeping/pubsub"
	"go.opencensus.io/stats"
	"go.opencensus.io/trace"
	"golang.org/x/xerrors"

	"github.com/filecoin-project/go-address"
	"github.com/filecoin-project/go-state-types/abi"
	"github.com/filecoin-project/go-state-types/crypto"
	"github.com/filecoin-project/lotus/extern/sector-storage/ffiwrapper"
	blst "github.com/supranational/blst/bindings/go"

	adt0 "github.com/filecoin-project/specs-actors/actors/util/adt"

	"github.com/filecoin-project/lotus/api"
	"github.com/filecoin-project/lotus/build"
	"github.com/filecoin-project/lotus/chain/actors/builtin/power"
	"github.com/filecoin-project/lotus/chain/beacon"
	"github.com/filecoin-project/lotus/chain/exchange"
	"github.com/filecoin-project/lotus/chain/gen"
	"github.com/filecoin-project/lotus/chain/state"
	"github.com/filecoin-project/lotus/chain/stmgr"
	"github.com/filecoin-project/lotus/chain/store"
	"github.com/filecoin-project/lotus/chain/types"
	"github.com/filecoin-project/lotus/chain/vm"
	bstore "github.com/filecoin-project/lotus/lib/blockstore"
	"github.com/filecoin-project/lotus/lib/sigs"
	"github.com/filecoin-project/lotus/lib/sigs/bls"
	"github.com/filecoin-project/lotus/metrics"
)

// Blocks that are more than MaxHeightDrift epochs above
// the theoretical max height based on systime are quickly rejected
const MaxHeightDrift = 5

var (
	// LocalIncoming is the _local_ pubsub (unrelated to libp2p pubsub) topic
	// where the Syncer publishes candidate chain heads to be synced.
	LocalIncoming = "incoming"

	log = logging.Logger("chain")

	concurrentSyncRequests = exchange.ShufflePeersPrefix
	syncRequestBatchSize   = 8
	syncRequestRetries     = 5
)

// Syncer is in charge of running the chain synchronization logic. As such, it
// is tasked with these functions, amongst others:
//
//  * Fast-forwards the chain as it learns of new TipSets from the network via
//    the SyncManager.
//  * Applies the fork choice rule to select the correct side when confronted
//    with a fork in the network.
//  * Requests block headers and messages from other peers when not available
//    in our BlockStore.
//  * Tracks blocks marked as bad in a cache.
//  * Keeps the BlockStore and ChainStore consistent with our view of the world,
//    the latter of which in turn informs other components when a reorg has been
//    committed.
//
// The Syncer does not run workers itself. It's mainly concerned with
// ensuring a consistent state of chain consensus. The reactive and network-
// interfacing processes are part of other components, such as the SyncManager
// (which owns the sync scheduler and sync workers), ChainExchange, the HELLO
// protocol, and the gossipsub block propagation layer.
//
// {hint/concept} The fork-choice rule as it currently stands is: "pick the
// chain with the heaviest weight, so long as it hasn’t deviated one finality
// threshold from our head (900 epochs, parameter determined by spec-actors)".
type Syncer struct {
	// The interface for accessing and putting tipsets into local storage
	store *store.ChainStore

	// handle to the random beacon for verification
	beacon beacon.Schedule

	// the state manager handles making state queries
	sm *stmgr.StateManager

	// The known Genesis tipset
	Genesis *types.TipSet

	// TipSets known to be invalid
	bad *BadBlockCache

	// handle to the block sync service
	Exchange exchange.Client

	self peer.ID

	syncmgr SyncManager

	connmgr connmgr.ConnManager

	incoming *pubsub.PubSub

	receiptTracker *blockReceiptTracker

	verifier ffiwrapper.Verifier

	tickerCtxCancel context.CancelFunc

	checkptLk sync.Mutex

	checkpt types.TipSetKey

	ds dtypes.MetadataDS
}

type SyncManagerCtor func(syncFn SyncFunc) SyncManager

// NewSyncer creates a new Syncer object.
func NewSyncer(ds dtypes.MetadataDS, sm *stmgr.StateManager, exchange exchange.Client, syncMgrCtor SyncManagerCtor, connmgr connmgr.ConnManager, self peer.ID, beacon beacon.Schedule, verifier ffiwrapper.Verifier) (*Syncer, error) {
	gen, err := sm.ChainStore().GetGenesis()
	if err != nil {
		return nil, xerrors.Errorf("getting genesis block: %w", err)
	}

	gent, err := types.NewTipSet([]*types.BlockHeader{gen})
	if err != nil {
		return nil, err
	}

	cp, err := loadCheckpoint(ds)
	if err != nil {
		return nil, xerrors.Errorf("error loading mpool config: %w", err)
	}

	s := &Syncer{
		ds:             ds,
		checkpt:        cp,
		beacon:         beacon,
		bad:            NewBadBlockCache(),
		Genesis:        gent,
		Exchange:       exchange,
		store:          sm.ChainStore(),
		sm:             sm,
		self:           self,
		receiptTracker: newBlockReceiptTracker(),
		connmgr:        connmgr,
		verifier:       verifier,

		incoming: pubsub.New(50),
	}

	if build.InsecurePoStValidation {
		log.Warn("*********************************************************************************************")
		log.Warn(" [INSECURE-POST-VALIDATION] Insecure test validation is enabled. If you see this outside of a test, it is a severe bug! ")
		log.Warn("*********************************************************************************************")
	}

	s.syncmgr = syncMgrCtor(s.Sync)
	return s, nil
}

func (syncer *Syncer) Start() {
	tickerCtx, tickerCtxCancel := context.WithCancel(context.Background())
	syncer.syncmgr.Start()

	syncer.tickerCtxCancel = tickerCtxCancel

	go syncer.runMetricsTricker(tickerCtx)
}

func (syncer *Syncer) runMetricsTricker(tickerCtx context.Context) {
	genesisTime := time.Unix(int64(syncer.Genesis.MinTimestamp()), 0)
	ticker := build.Clock.Ticker(time.Duration(build.BlockDelaySecs) * time.Second)
	defer ticker.Stop()

	for {
		select {
		case <-ticker.C:
			sinceGenesis := build.Clock.Now().Sub(genesisTime)
			expectedHeight := int64(sinceGenesis.Seconds()) / int64(build.BlockDelaySecs)

			stats.Record(tickerCtx, metrics.ChainNodeHeightExpected.M(expectedHeight))
		case <-tickerCtx.Done():
			return
		}
	}
}

func (syncer *Syncer) Stop() {
	syncer.syncmgr.Stop()
	syncer.tickerCtxCancel()
}

// InformNewHead informs the syncer about a new potential tipset
// This should be called when connecting to new peers, and additionally
// when receiving new blocks from the network
func (syncer *Syncer) InformNewHead(from peer.ID, fts *store.FullTipSet) bool {
	defer func() {
		if err := recover(); err != nil {
			log.Errorf("panic in InformNewHead: ", err)
		}
	}()

	ctx := context.Background()
	if fts == nil {
		log.Errorf("got nil tipset in InformNewHead")
		return false
	}

	if syncer.IsEpochBeyondCurrMax(fts.TipSet().Height()) {
		log.Errorf("Received block with impossibly large height %d", fts.TipSet().Height())
		return false
	}

	for _, b := range fts.Blocks {
		if reason, ok := syncer.bad.Has(b.Cid()); ok {
			log.Warnf("InformNewHead called on block marked as bad: %s (reason: %s)", b.Cid(), reason)
			return false
		}
		if err := syncer.ValidateMsgMeta(b); err != nil {
			log.Warnf("invalid block received: %s", err)
			return false
		}
	}

	syncer.incoming.Pub(fts.TipSet().Blocks(), LocalIncoming)

	if from == syncer.self {
		// TODO: this is kindof a hack...
		log.Debug("got block from ourselves")

		if err := syncer.Sync(ctx, fts.TipSet()); err != nil {
			log.Errorf("failed to sync our own block %s: %+v", fts.TipSet().Cids(), err)
			return false
		}

		return true
	}

	// TODO: IMPORTANT(GARBAGE) this needs to be put in the 'temporary' side of
	// the blockstore
	if err := syncer.store.PersistBlockHeaders(fts.TipSet().Blocks()...); err != nil {
		log.Warn("failed to persist incoming block header: ", err)
		return false
	}

	syncer.Exchange.AddPeer(from)

	bestPweight := syncer.store.GetHeaviestTipSet().ParentWeight()
	targetWeight := fts.TipSet().ParentWeight()
	if targetWeight.LessThan(bestPweight) {
		var miners []string
		for _, blk := range fts.TipSet().Blocks() {
			miners = append(miners, blk.Miner.String())
		}
		log.Infof("incoming tipset from %s does not appear to be better than our best chain, ignoring for now", miners)
		return false
	}

	syncer.syncmgr.SetPeerHead(ctx, from, fts.TipSet())
	return true
}

// IncomingBlocks spawns a goroutine that subscribes to the local eventbus to
// receive new block headers as they arrive from the network, and sends them to
// the returned channel.
//
// These blocks have not necessarily been incorporated to our view of the chain.
func (syncer *Syncer) IncomingBlocks(ctx context.Context) (<-chan *types.BlockHeader, error) {
	sub := syncer.incoming.Sub(LocalIncoming)
	out := make(chan *types.BlockHeader, 10)

	go func() {
		defer syncer.incoming.Unsub(sub, LocalIncoming)

		for {
			select {
			case r := <-sub:
				hs := r.([]*types.BlockHeader)
				for _, h := range hs {
					select {
					case out <- h:
					case <-ctx.Done():
						return
					}
				}
			case <-ctx.Done():
				return
			}
		}
	}()

	return out, nil
}

// ValidateMsgMeta performs structural and content hash validation of the
// messages within this block. If validation passes, it stores the messages in
// the underlying IPLD block store.
func (syncer *Syncer) ValidateMsgMeta(fblk *types.FullBlock) error {
	if msgc := len(fblk.BlsMessages) + len(fblk.SecpkMessages); msgc > build.BlockMessageLimit {
		return xerrors.Errorf("block %s has too many messages (%d)", fblk.Header.Cid(), msgc)
	}

	// Collect the CIDs of both types of messages separately: BLS and Secpk.
	var bcids, scids []cid.Cid
	for _, m := range fblk.BlsMessages {
		bcids = append(bcids, m.Cid())
	}

	for _, m := range fblk.SecpkMessages {
		scids = append(scids, m.Cid())
	}

	// TODO: IMPORTANT(GARBAGE). These message puts and the msgmeta
	// computation need to go into the 'temporary' side of the blockstore when
	// we implement that
	blockstore := syncer.store.Blockstore()

	bs := cbor.NewCborStore(blockstore)

	// Compute the root CID of the combined message trie.
	smroot, err := computeMsgMeta(bs, bcids, scids)
	if err != nil {
		return xerrors.Errorf("validating msgmeta, compute failed: %w", err)
	}

	// Check that the message trie root matches with what's in the block.
	if fblk.Header.Messages != smroot {
		return xerrors.Errorf("messages in full block did not match msgmeta root in header (%s != %s)", fblk.Header.Messages, smroot)
	}

	for _, m := range fblk.BlsMessages {
		_, err := store.PutMessage(blockstore, m)
		if err != nil {
			return xerrors.Errorf("putting bls message to blockstore after msgmeta computation: %w", err)
		}
	}

	for _, m := range fblk.SecpkMessages {
		_, err := store.PutMessage(blockstore, m)
		if err != nil {
			return xerrors.Errorf("putting bls message to blockstore after msgmeta computation: %w", err)
		}
	}

	return nil
}

func (syncer *Syncer) LocalPeer() peer.ID {
	return syncer.self
}

func (syncer *Syncer) ChainStore() *store.ChainStore {
	return syncer.store
}

func (syncer *Syncer) InformNewBlock(from peer.ID, blk *types.FullBlock) bool {
	// TODO: search for other blocks that could form a tipset with this block
	// and then send that tipset to InformNewHead

	fts := &store.FullTipSet{Blocks: []*types.FullBlock{blk}}
	return syncer.InformNewHead(from, fts)
}

func copyBlockstore(ctx context.Context, from, to bstore.Blockstore) error {
	ctx, span := trace.StartSpan(ctx, "copyBlockstore")
	defer span.End()

	cids, err := from.AllKeysChan(ctx)
	if err != nil {
		return err
	}

	// TODO: should probably expose better methods on the blockstore for this operation
	var blks []blocks.Block
	for c := range cids {
		b, err := from.Get(c)
		if err != nil {
			return err
		}

		blks = append(blks, b)
	}

	if err := to.PutMany(blks); err != nil {
		return err
	}

	return nil
}

// TODO: this function effectively accepts unchecked input from the network,
// either validate it here, or ensure that its validated elsewhere (maybe make
// sure the blocksync code checks it?)
// maybe this code should actually live in blocksync??
func zipTipSetAndMessages(bs cbor.IpldStore, ts *types.TipSet, allbmsgs []*types.Message, allsmsgs []*types.SignedMessage, bmi, smi [][]uint64) (*store.FullTipSet, error) {
	if len(ts.Blocks()) != len(smi) || len(ts.Blocks()) != len(bmi) {
		return nil, fmt.Errorf("msgincl length didnt match tipset size")
	}

	fts := &store.FullTipSet{}
	for bi, b := range ts.Blocks() {
		if msgc := len(bmi[bi]) + len(smi[bi]); msgc > build.BlockMessageLimit {
			return nil, fmt.Errorf("block %q has too many messages (%d)", b.Cid(), msgc)
		}

		var smsgs []*types.SignedMessage
		var smsgCids []cid.Cid
		for _, m := range smi[bi] {
			smsgs = append(smsgs, allsmsgs[m])
			smsgCids = append(smsgCids, allsmsgs[m].Cid())
		}

		var bmsgs []*types.Message
		var bmsgCids []cid.Cid
		for _, m := range bmi[bi] {
			bmsgs = append(bmsgs, allbmsgs[m])
			bmsgCids = append(bmsgCids, allbmsgs[m].Cid())
		}

		mrcid, err := computeMsgMeta(bs, bmsgCids, smsgCids)
		if err != nil {
			return nil, err
		}

		if b.Messages != mrcid {
			return nil, fmt.Errorf("messages didnt match message root in header for ts %s", ts.Key())
		}

		fb := &types.FullBlock{
			Header:        b,
			BlsMessages:   bmsgs,
			SecpkMessages: smsgs,
		}

		fts.Blocks = append(fts.Blocks, fb)
	}

	return fts, nil
}

// computeMsgMeta computes the root CID of the combined arrays of message CIDs
// of both types (BLS and Secpk).
func computeMsgMeta(bs cbor.IpldStore, bmsgCids, smsgCids []cid.Cid) (cid.Cid, error) {
	// block headers use adt0
	store := adt0.WrapStore(context.TODO(), bs)
	bmArr := adt0.MakeEmptyArray(store)
	smArr := adt0.MakeEmptyArray(store)

	for i, m := range bmsgCids {
		c := cbg.CborCid(m)
		if err := bmArr.Set(uint64(i), &c); err != nil {
			return cid.Undef, err
		}
	}

	for i, m := range smsgCids {
		c := cbg.CborCid(m)
		if err := smArr.Set(uint64(i), &c); err != nil {
			return cid.Undef, err
		}
	}

	bmroot, err := bmArr.Root()
	if err != nil {
		return cid.Undef, err
	}

	smroot, err := smArr.Root()
	if err != nil {
		return cid.Undef, err
	}

	mrcid, err := store.Put(store.Context(), &types.MsgMeta{
		BlsMessages:   bmroot,
		SecpkMessages: smroot,
	})
	if err != nil {
		return cid.Undef, xerrors.Errorf("failed to put msgmeta: %w", err)
	}

	return mrcid, nil
}

// FetchTipSet tries to load the provided tipset from the store, and falls back
// to the network (client) by querying the supplied peer if not found
// locally.
//
// {hint/usage} This is used from the HELLO protocol, to fetch the greeting
// peer's heaviest tipset if we don't have it.
func (syncer *Syncer) FetchTipSet(ctx context.Context, p peer.ID, tsk types.TipSetKey) (*store.FullTipSet, error) {
	if fts, err := syncer.tryLoadFullTipSet(tsk); err == nil {
		return fts, nil
	}

	// fall back to the network.
	return syncer.Exchange.GetFullTipSet(ctx, p, tsk)
}

// tryLoadFullTipSet queries the tipset in the ChainStore, and returns a full
// representation of it containing FullBlocks. If ALL blocks are not found
// locally, it errors entirely with blockstore.ErrNotFound.
func (syncer *Syncer) tryLoadFullTipSet(tsk types.TipSetKey) (*store.FullTipSet, error) {
	ts, err := syncer.store.LoadTipSet(tsk)
	if err != nil {
		return nil, err
	}

	fts := &store.FullTipSet{}
	for _, b := range ts.Blocks() {
		bmsgs, smsgs, err := syncer.store.MessagesForBlock(b)
		if err != nil {
			return nil, err
		}

		fb := &types.FullBlock{
			Header:        b,
			BlsMessages:   bmsgs,
			SecpkMessages: smsgs,
		}
		fts.Blocks = append(fts.Blocks, fb)
	}

	return fts, nil
}

// Sync tries to advance our view of the chain to `maybeHead`. It does nothing
// if our current head is heavier than the requested tipset, or if we're already
// at the requested head, or if the head is the genesis.
//
// Most of the heavy-lifting logic happens in syncer#collectChain. Refer to the
// godocs on that method for a more detailed view.
func (syncer *Syncer) Sync(ctx context.Context, maybeHead *types.TipSet) error {
	ctx, span := trace.StartSpan(ctx, "chain.Sync")
	defer span.End()

	if span.IsRecordingEvents() {
		span.AddAttributes(
			trace.StringAttribute("tipset", fmt.Sprint(maybeHead.Cids())),
			trace.Int64Attribute("height", int64(maybeHead.Height())),
		)
	}

	if syncer.store.GetHeaviestTipSet().ParentWeight().GreaterThan(maybeHead.ParentWeight()) {
		return nil
	}

	if syncer.Genesis.Equals(maybeHead) || syncer.store.GetHeaviestTipSet().Equals(maybeHead) {
		return nil
	}

	if err := syncer.collectChain(ctx, maybeHead); err != nil {
		span.AddAttributes(trace.StringAttribute("col_error", err.Error()))
		span.SetStatus(trace.Status{
			Code:    13,
			Message: err.Error(),
		})
		return xerrors.Errorf("collectChain failed: %w", err)
	}

	// At this point we have accepted and synced to the new `maybeHead`
	// (`StageSyncComplete`).
	if err := syncer.store.PutTipSet(ctx, maybeHead); err != nil {
		span.AddAttributes(trace.StringAttribute("put_error", err.Error()))
		span.SetStatus(trace.Status{
			Code:    13,
			Message: err.Error(),
		})
		return xerrors.Errorf("failed to put synced tipset to chainstore: %w", err)
	}

	peers := syncer.receiptTracker.GetPeers(maybeHead)
	if len(peers) > 0 {
		syncer.connmgr.TagPeer(peers[0], "new-block", 40)

		for _, p := range peers[1:] {
			syncer.connmgr.TagPeer(p, "new-block", 25)
		}
	}

	return nil
}

func isPermanent(err error) bool {
	return !errors.Is(err, ErrTemporal)
}

func (syncer *Syncer) ValidateTipSet(ctx context.Context, fts *store.FullTipSet, useCache bool) error {
	ctx, span := trace.StartSpan(ctx, "validateTipSet")
	defer span.End()

	span.AddAttributes(trace.Int64Attribute("height", int64(fts.TipSet().Height())))

	ts := fts.TipSet()
	if ts.Equals(syncer.Genesis) {
		return nil
	}

	var futures []async.ErrorFuture
	for _, b := range fts.Blocks {
		b := b // rebind to a scoped variable

		futures = append(futures, async.Err(func() error {
			if err := syncer.ValidateBlock(ctx, b, useCache); err != nil {
				if isPermanent(err) {
					syncer.bad.Add(b.Cid(), NewBadBlockReason([]cid.Cid{b.Cid()}, err.Error()))
				}
				return xerrors.Errorf("validating block %s: %w", b.Cid(), err)
			}

			if err := syncer.sm.ChainStore().AddToTipSetTracker(b.Header); err != nil {
				return xerrors.Errorf("failed to add validated header to tipset tracker: %w", err)
			}
			return nil
		}))
	}
	for _, f := range futures {
		if err := f.AwaitContext(ctx); err != nil {
			return err
		}
	}
	return nil
}

func (syncer *Syncer) minerIsValid(ctx context.Context, maddr address.Address, baseTs *types.TipSet) error {
	act, err := syncer.sm.LoadActor(ctx, power.Address, baseTs)
	if err != nil {
		return xerrors.Errorf("failed to load power actor: %w", err)
	}

	powState, err := power.Load(syncer.store.Store(ctx), act)
	if err != nil {
		return xerrors.Errorf("failed to load power actor state: %w", err)
	}

	_, exist, err := powState.MinerPower(maddr)
	if err != nil {
		return xerrors.Errorf("failed to look up miner's claim: %w", err)
	}

	if !exist {
		return xerrors.New("miner isn't valid")
	}

	return nil
}

var ErrTemporal = errors.New("temporal error")

func blockSanityChecks(h *types.BlockHeader) error {
	if h.ElectionProof == nil {
		return xerrors.Errorf("block cannot have nil election proof")
	}

	if h.Ticket == nil {
		return xerrors.Errorf("block cannot have nil ticket")
	}

	if h.BlockSig == nil {
		return xerrors.Errorf("block had nil signature")
	}

	if h.BLSAggregate == nil {
		return xerrors.Errorf("block had nil bls aggregate signature")
	}

	return nil
}

// ValidateBlock should match up with 'Semantical Validation' in validation.md in the spec
func (syncer *Syncer) ValidateBlock(ctx context.Context, b *types.FullBlock, useCache bool) (err error) {
	defer func() {
		// b.Cid() could panic for empty blocks that are used in tests.
		if rerr := recover(); rerr != nil {
			err = xerrors.Errorf("validate block panic: %w", rerr)
			return
		}
	}()

	if useCache {
		isValidated, err := syncer.store.IsBlockValidated(ctx, b.Cid())
		if err != nil {
			return xerrors.Errorf("check block validation cache %s: %w", b.Cid(), err)
		}

		if isValidated {
			return nil
		}
	}

	validationStart := build.Clock.Now()
	defer func() {
		stats.Record(ctx, metrics.BlockValidationDurationMilliseconds.M(metrics.SinceInMilliseconds(validationStart)))
		log.Infow("block validation", "took", time.Since(validationStart), "height", b.Header.Height, "age", time.Since(time.Unix(int64(b.Header.Timestamp), 0)))
	}()

	ctx, span := trace.StartSpan(ctx, "validateBlock")
	defer span.End()

	if err := blockSanityChecks(b.Header); err != nil {
		return xerrors.Errorf("incoming header failed basic sanity checks: %w", err)
	}

	h := b.Header

	baseTs, err := syncer.store.LoadTipSet(types.NewTipSetKey(h.Parents...))
	if err != nil {
		return xerrors.Errorf("load parent tipset failed (%s): %w", h.Parents, err)
	}

	lbts, err := stmgr.GetLookbackTipSetForRound(ctx, syncer.sm, baseTs, h.Height)
	if err != nil {
		return xerrors.Errorf("failed to get lookback tipset for block: %w", err)
	}

	lbst, _, err := syncer.sm.TipSetState(ctx, lbts)
	if err != nil {
		return xerrors.Errorf("failed to compute lookback tipset state (epoch %d): %w", lbts.Height(), err)
	}

	prevBeacon, err := syncer.store.GetLatestBeaconEntry(baseTs)
	if err != nil {
		return xerrors.Errorf("failed to get latest beacon entry: %w", err)
	}

	// fast checks first
	nulls := h.Height - (baseTs.Height() + 1)
	if tgtTs := baseTs.MinTimestamp() + build.BlockDelaySecs*uint64(nulls+1); h.Timestamp != tgtTs {
		return xerrors.Errorf("block has wrong timestamp: %d != %d", h.Timestamp, tgtTs)
	}

	now := uint64(build.Clock.Now().Unix())
	if h.Timestamp > now+build.AllowableClockDriftSecs {
		return xerrors.Errorf("block was from the future (now=%d, blk=%d): %w", now, h.Timestamp, ErrTemporal)
	}
	if h.Timestamp > now {
		log.Warn("Got block from the future, but within threshold", h.Timestamp, build.Clock.Now().Unix())
	}

	msgsCheck := async.Err(func() error {
		if err := syncer.checkBlockMessages(ctx, b, baseTs); err != nil {
			return xerrors.Errorf("block had invalid messages: %w", err)
		}
		return nil
	})

	minerCheck := async.Err(func() error {
		if err := syncer.minerIsValid(ctx, h.Miner, baseTs); err != nil {
			return xerrors.Errorf("minerIsValid failed: %w", err)
		}
		return nil
	})

	baseFeeCheck := async.Err(func() error {
		baseFee, err := syncer.store.ComputeBaseFee(ctx, baseTs)
		if err != nil {
			return xerrors.Errorf("computing base fee: %w", err)
		}
		if types.BigCmp(baseFee, b.Header.ParentBaseFee) != 0 {
			return xerrors.Errorf("base fee doesn't match: %s (header) != %s (computed)",
				b.Header.ParentBaseFee, baseFee)
		}
		return nil
	})
	pweight, err := syncer.store.Weight(ctx, baseTs)
	if err != nil {
		return xerrors.Errorf("getting parent weight: %w", err)
	}

	if types.BigCmp(pweight, b.Header.ParentWeight) != 0 {
		return xerrors.Errorf("parrent weight different: %s (header) != %s (computed)",
			b.Header.ParentWeight, pweight)
	}

	stateRootCheck := async.Err(func() error {
		stateroot, precp, err := syncer.sm.TipSetState(ctx, baseTs)
		if err != nil {
			return xerrors.Errorf("get tipsetstate(%d, %s) failed: %w", h.Height, h.Parents, err)
		}

		if stateroot != h.ParentStateRoot {
			msgs, err := syncer.store.MessagesForTipset(baseTs)
			if err != nil {
				log.Error("failed to load messages for tipset during tipset state mismatch error: ", err)
			} else {
				log.Warn("Messages for tipset with mismatching state:")
				for i, m := range msgs {
					mm := m.VMMessage()
					log.Warnf("Message[%d]: from=%s to=%s method=%d params=%x", i, mm.From, mm.To, mm.Method, mm.Params)
				}
			}

			return xerrors.Errorf("parent state root did not match computed state (%s != %s)", stateroot, h.ParentStateRoot)
		}

		if precp != h.ParentMessageReceipts {
			return xerrors.Errorf("parent receipts root did not match computed value (%s != %s)", precp, h.ParentMessageReceipts)
		}

		return nil
	})

	// Stuff that needs worker address
	waddr, err := stmgr.GetMinerWorkerRaw(ctx, syncer.sm, lbst, h.Miner)
	if err != nil {
		return xerrors.Errorf("GetMinerWorkerRaw failed: %w", err)
	}

	winnerCheck := async.Err(func() error {
		if h.ElectionProof.WinCount < 1 {
			return xerrors.Errorf("block is not claiming to be a winner")
		}

		eligible, err := stmgr.MinerEligibleToMine(ctx, syncer.sm, h.Miner, baseTs, lbts)
		if err != nil {
			return xerrors.Errorf("determining if miner has min power failed: %w", err)
		}

		if !eligible {
			return xerrors.New("block's miner is ineligible to mine")
		}

		rBeacon := *prevBeacon
		if len(h.BeaconEntries) != 0 {
			rBeacon = h.BeaconEntries[len(h.BeaconEntries)-1]
		}
		buf := new(bytes.Buffer)
		if err := h.Miner.MarshalCBOR(buf); err != nil {
			return xerrors.Errorf("failed to marshal miner address to cbor: %w", err)
		}

		vrfBase, err := store.DrawRandomness(rBeacon.Data, crypto.DomainSeparationTag_ElectionProofProduction, h.Height, buf.Bytes())
		if err != nil {
			return xerrors.Errorf("could not draw randomness: %w", err)
		}

		if err := VerifyElectionPoStVRF(ctx, waddr, vrfBase, h.ElectionProof.VRFProof); err != nil {
			return xerrors.Errorf("validating block election proof failed: %w", err)
		}

		slashed, err := stmgr.GetMinerSlashed(ctx, syncer.sm, baseTs, h.Miner)
		if err != nil {
			return xerrors.Errorf("failed to check if block miner was slashed: %w", err)
		}

		if slashed {
			return xerrors.Errorf("received block was from slashed or invalid miner")
		}

		mpow, tpow, _, err := stmgr.GetPowerRaw(ctx, syncer.sm, lbst, h.Miner)
		if err != nil {
			return xerrors.Errorf("failed getting power: %w", err)
		}

		j := h.ElectionProof.ComputeWinCount(mpow.QualityAdjPower, tpow.QualityAdjPower)
		if h.ElectionProof.WinCount != j {
			return xerrors.Errorf("miner claims wrong number of wins: miner: %d, computed: %d", h.ElectionProof.WinCount, j)
		}

		return nil
	})

	blockSigCheck := async.Err(func() error {
		if err := sigs.CheckBlockSignature(ctx, h, waddr); err != nil {
			return xerrors.Errorf("check block signature failed: %w", err)
		}
		return nil
	})

	beaconValuesCheck := async.Err(func() error {
		if os.Getenv("LOTUS_IGNORE_DRAND") == "_yes_" {
			return nil
		}

		if err := beacon.ValidateBlockValues(syncer.beacon, h, baseTs.Height(), *prevBeacon); err != nil {
			return xerrors.Errorf("failed to validate blocks random beacon values: %w", err)
		}
		return nil
	})

	tktsCheck := async.Err(func() error {
		buf := new(bytes.Buffer)
		if err := h.Miner.MarshalCBOR(buf); err != nil {
			return xerrors.Errorf("failed to marshal miner address to cbor: %w", err)
		}

		if h.Height > build.UpgradeSmokeHeight {
			buf.Write(baseTs.MinTicket().VRFProof)
		}

		beaconBase := *prevBeacon
		if len(h.BeaconEntries) != 0 {
			beaconBase = h.BeaconEntries[len(h.BeaconEntries)-1]
		}

		vrfBase, err := store.DrawRandomness(beaconBase.Data, crypto.DomainSeparationTag_TicketProduction, h.Height-build.TicketRandomnessLookback, buf.Bytes())
		if err != nil {
			return xerrors.Errorf("failed to compute vrf base for ticket: %w", err)
		}

		err = VerifyElectionPoStVRF(ctx, waddr, vrfBase, h.Ticket.VRFProof)
		if err != nil {
			return xerrors.Errorf("validating block tickets failed: %w", err)
		}
		return nil
	})

	wproofCheck := async.Err(func() error {
		if err := syncer.VerifyWinningPoStProof(ctx, h, *prevBeacon, lbst, waddr); err != nil {
			return xerrors.Errorf("invalid election post: %w", err)
		}
		return nil
	})

	await := []async.ErrorFuture{
		minerCheck,
		tktsCheck,
		blockSigCheck,
		beaconValuesCheck,
		wproofCheck,
		winnerCheck,
		msgsCheck,
		baseFeeCheck,
		stateRootCheck,
	}

	var merr error
	for _, fut := range await {
		if err := fut.AwaitContext(ctx); err != nil {
			merr = multierror.Append(merr, err)
		}
	}
	if merr != nil {
		mulErr := merr.(*multierror.Error)
		mulErr.ErrorFormat = func(es []error) string {
			if len(es) == 1 {
				return fmt.Sprintf("1 error occurred:\n\t* %+v\n\n", es[0])
			}

			points := make([]string, len(es))
			for i, err := range es {
				points[i] = fmt.Sprintf("* %+v", err)
			}

			return fmt.Sprintf(
				"%d errors occurred:\n\t%s\n\n",
				len(es), strings.Join(points, "\n\t"))
		}
		return mulErr
	}

	if useCache {
		if err := syncer.store.MarkBlockAsValidated(ctx, b.Cid()); err != nil {
			return xerrors.Errorf("caching block validation %s: %w", b.Cid(), err)
		}
	}

	return nil
}

func (syncer *Syncer) VerifyWinningPoStProof(ctx context.Context, h *types.BlockHeader, prevBeacon types.BeaconEntry, lbst cid.Cid, waddr address.Address) error {
	if build.InsecurePoStValidation {
		if len(h.WinPoStProof) == 0 {
			return xerrors.Errorf("[INSECURE-POST-VALIDATION] No winning post proof given")
		}

		if string(h.WinPoStProof[0].ProofBytes) == "valid proof" {
			return nil
		}
		return xerrors.Errorf("[INSECURE-POST-VALIDATION] winning post was invalid")
	}

	buf := new(bytes.Buffer)
	if err := h.Miner.MarshalCBOR(buf); err != nil {
		return xerrors.Errorf("failed to marshal miner address: %w", err)
	}

	rbase := prevBeacon
	if len(h.BeaconEntries) > 0 {
		rbase = h.BeaconEntries[len(h.BeaconEntries)-1]
	}

	rand, err := store.DrawRandomness(rbase.Data, crypto.DomainSeparationTag_WinningPoStChallengeSeed, h.Height, buf.Bytes())
	if err != nil {
		return xerrors.Errorf("failed to get randomness for verifying winning post proof: %w", err)
	}

	mid, err := address.IDFromAddress(h.Miner)
	if err != nil {
		return xerrors.Errorf("failed to get ID from miner address %s: %w", h.Miner, err)
	}

	sectors, err := stmgr.GetSectorsForWinningPoSt(ctx, syncer.verifier, syncer.sm, lbst, h.Miner, rand)
	if err != nil {
		return xerrors.Errorf("getting winning post sector set: %w", err)
	}

	ok, err := ffiwrapper.ProofVerifier.VerifyWinningPoSt(ctx, proof.WinningPoStVerifyInfo{
		Randomness:        rand,
		Proofs:            h.WinPoStProof,
		ChallengedSectors: sectors,
		Prover:            abi.ActorID(mid),
	})
	if err != nil {
		return xerrors.Errorf("failed to verify election post: %w", err)
	}

	if !ok {
		log.Errorf("invalid winning post (block: %s, %x; %v)", h.Cid(), rand, sectors)
		return xerrors.Errorf("winning post was invalid")
	}

	return nil
}

// TODO: We should extract this somewhere else and make the message pool and miner use the same logic
func (syncer *Syncer) checkBlockMessages(ctx context.Context, b *types.FullBlock, baseTs *types.TipSet) error {
	{
		var sigCids []cid.Cid // this is what we get for people not wanting the marshalcbor method on the cid type
		var pubks [][]byte

		for _, m := range b.BlsMessages {
			sigCids = append(sigCids, m.Cid())

			pubk, err := syncer.sm.GetBlsPublicKey(ctx, m.From, baseTs)
			if err != nil {
				return xerrors.Errorf("failed to load bls public to validate block: %w", err)
			}

			pubks = append(pubks, pubk)
		}

		if err := syncer.verifyBlsAggregate(ctx, b.Header.BLSAggregate, sigCids, pubks); err != nil {
			return xerrors.Errorf("bls aggregate signature was invalid: %w", err)
		}
	}

	nonces := make(map[address.Address]uint64)

	stateroot, _, err := syncer.sm.TipSetState(ctx, baseTs)
	if err != nil {
		return err
	}

	cst := cbor.NewCborStore(syncer.store.Blockstore())
	st, err := state.LoadStateTree(cst, stateroot)
	if err != nil {
		return xerrors.Errorf("failed to load base state tree: %w", err)
	}

	pl := vm.PricelistByEpoch(baseTs.Height())
	var sumGasLimit int64
	checkMsg := func(msg types.ChainMsg) error {
		m := msg.VMMessage()

		// Phase 1: syntactic validation, as defined in the spec
		minGas := pl.OnChainMessage(msg.ChainLength())
		if err := m.ValidForBlockInclusion(minGas.Total()); err != nil {
			return err
		}

		// ValidForBlockInclusion checks if any single message does not exceed BlockGasLimit
		// So below is overflow safe
		sumGasLimit += m.GasLimit
		if sumGasLimit > build.BlockGasLimit {
			return xerrors.Errorf("block gas limit exceeded")
		}

		// Phase 2: (Partial) semantic validation:
		// the sender exists and is an account actor, and the nonces make sense
		if _, ok := nonces[m.From]; !ok {
			// `GetActor` does not validate that this is an account actor.
			act, err := st.GetActor(m.From)
			if err != nil {
				return xerrors.Errorf("failed to get actor: %w", err)
			}

			if !builtin.IsAccountActor(act.Code) {
				return xerrors.New("Sender must be an account actor")
			}
			nonces[m.From] = act.Nonce
		}

		if nonces[m.From] != m.Nonce {
			return xerrors.Errorf("wrong nonce (exp: %d, got: %d)", nonces[m.From], m.Nonce)
		}
		nonces[m.From]++

		return nil
	}

	store := adt0.WrapStore(ctx, cst)

	bmArr := adt0.MakeEmptyArray(store)
	for i, m := range b.BlsMessages {
		if err := checkMsg(m); err != nil {
			return xerrors.Errorf("block had invalid bls message at index %d: %w", i, err)
		}

		c := cbg.CborCid(m.Cid())
		if err := bmArr.Set(uint64(i), &c); err != nil {
			return xerrors.Errorf("failed to put bls message at index %d: %w", i, err)
		}
	}

	smArr := adt0.MakeEmptyArray(store)
	for i, m := range b.SecpkMessages {
		if err := checkMsg(m); err != nil {
			return xerrors.Errorf("block had invalid secpk message at index %d: %w", i, err)
		}

		// `From` being an account actor is only validated inside the `vm.ResolveToKeyAddr` call
		// in `StateManager.ResolveToKeyAddress` here (and not in `checkMsg`).
		kaddr, err := syncer.sm.ResolveToKeyAddress(ctx, m.Message.From, baseTs)
		if err != nil {
			return xerrors.Errorf("failed to resolve key addr: %w", err)
		}

		if err := sigs.Verify(&m.Signature, kaddr, m.Message.Cid().Bytes()); err != nil {
			return xerrors.Errorf("secpk message %s has invalid signature: %w", m.Cid(), err)
		}

		c := cbg.CborCid(m.Cid())
		if err := smArr.Set(uint64(i), &c); err != nil {
			return xerrors.Errorf("failed to put secpk message at index %d: %w", i, err)
		}
	}

	bmroot, err := bmArr.Root()
	if err != nil {
		return err
	}

	smroot, err := smArr.Root()
	if err != nil {
		return err
	}

	mrcid, err := cst.Put(ctx, &types.MsgMeta{
		BlsMessages:   bmroot,
		SecpkMessages: smroot,
	})
	if err != nil {
		return err
	}

	if b.Header.Messages != mrcid {
		return fmt.Errorf("messages didnt match message root in header")
	}

	return nil
}

func (syncer *Syncer) verifyBlsAggregate(ctx context.Context, sig *crypto.Signature, msgs []cid.Cid, pubks [][]byte) error {
	_, span := trace.StartSpan(ctx, "syncer.verifyBlsAggregate")
	defer span.End()
	span.AddAttributes(
		trace.Int64Attribute("msgCount", int64(len(msgs))),
	)

	msgsS := make([]blst.Message, len(msgs))
	for i := 0; i < len(msgs); i++ {
		msgsS[i] = msgs[i].Bytes()
	}

	if len(msgs) == 0 {
		return nil
	}

	valid := new(bls.Signature).AggregateVerifyCompressed(sig.Data, pubks,
		msgsS, []byte(bls.DST))
	if !valid {
		return xerrors.New("bls aggregate signature failed to verify")
	}
	return nil
}

type syncStateKey struct{}

func extractSyncState(ctx context.Context) *SyncerState {
	v := ctx.Value(syncStateKey{})
	if v != nil {
		return v.(*SyncerState)
	}
	return nil
}

// collectHeaders collects the headers from the blocks between any two tipsets.
//
// `incoming` is the heaviest/projected/target tipset we have learned about, and
// `known` is usually an anchor tipset we already have in our view of the chain
// (which could be the genesis).
//
// collectHeaders checks if portions of the chain are in our ChainStore; falling
// down to the network to retrieve the missing parts. If during the process, any
// portion we receive is in our denylist (bad list), we short-circuit.
//
// {hint/usage}: This is used by collectChain, which is in turn called from the
// main Sync method (Syncer#Sync), so it's a pretty central method.
//
// {hint/logic}: The logic of this method is as follows:
//
//  1. Check that the from tipset is not linked to a parent block known to be
//     bad.
//  2. Check the consistency of beacon entries in the from tipset. We check
//     total equality of the BeaconEntries in each block.
//  3. Traverse the chain backwards, for each tipset:
//  	3a. Load it from the chainstore; if found, it move on to its parent.
//      3b. Query our peers via client in batches, requesting up to a
//      maximum of 500 tipsets every time.
//
// Once we've concluded, if we find a mismatching tipset at the height where the
// anchor tipset should be, we are facing a fork, and we invoke Syncer#syncFork
// to resolve it. Refer to the godocs there.
//
// All throughout the process, we keep checking if the received blocks are in
// the deny list, and short-circuit the process if so.
func (syncer *Syncer) collectHeaders(ctx context.Context, incoming *types.TipSet, known *types.TipSet) ([]*types.TipSet, error) {
	ctx, span := trace.StartSpan(ctx, "collectHeaders")
	defer span.End()
	ss := extractSyncState(ctx)

	span.AddAttributes(
		trace.Int64Attribute("incomingHeight", int64(incoming.Height())),
		trace.Int64Attribute("knownHeight", int64(known.Height())),
	)

	// Check if the parents of the from block are in the denylist.
	// i.e. if a fork of the chain has been requested that we know to be bad.
	for _, pcid := range incoming.Parents().Cids() {
		if reason, ok := syncer.bad.Has(pcid); ok {
			newReason := reason.Linked("linked to %s", pcid)
			for _, b := range incoming.Cids() {
				syncer.bad.Add(b, newReason)
			}
			return nil, xerrors.Errorf("chain linked to block marked previously as bad (%s, %s) (reason: %s)", incoming.Cids(), pcid, reason)
		}
	}

	{
		// ensure consistency of beacon entires
		targetBE := incoming.Blocks()[0].BeaconEntries
		sorted := sort.SliceIsSorted(targetBE, func(i, j int) bool {
			return targetBE[i].Round < targetBE[j].Round
		})
		if !sorted {
			syncer.bad.Add(incoming.Cids()[0], NewBadBlockReason(incoming.Cids(), "wrong order of beacon entires"))
			return nil, xerrors.Errorf("wrong order of beacon entires")
		}

		for _, bh := range incoming.Blocks()[1:] {
			if len(targetBE) != len(bh.BeaconEntries) {
				// cannot mark bad, I think @Kubuxu
				return nil, xerrors.Errorf("tipset contained different number for beacon entires")
			}
			for i, be := range bh.BeaconEntries {
				if targetBE[i].Round != be.Round || !bytes.Equal(targetBE[i].Data, be.Data) {
					// cannot mark bad, I think @Kubuxu
					return nil, xerrors.Errorf("tipset contained different beacon entires")
				}
			}

		}
	}

	blockSet := []*types.TipSet{incoming}

	// Parent of the new (possibly better) tipset that we need to fetch next.
	at := incoming.Parents()

	// we want to sync all the blocks until the height above our
	// best tipset so far
	untilHeight := known.Height() + 1

	ss.SetHeight(blockSet[len(blockSet)-1].Height())

	var acceptedBlocks []cid.Cid

loop:
	for blockSet[len(blockSet)-1].Height() > untilHeight {
		for _, bc := range at.Cids() {
			if reason, ok := syncer.bad.Has(bc); ok {
				newReason := reason.Linked("change contained %s", bc)
				for _, b := range acceptedBlocks {
					syncer.bad.Add(b, newReason)
				}

				return nil, xerrors.Errorf("chain contained block marked previously as bad (%s, %s) (reason: %s)", incoming.Cids(), bc, reason)
			}
		}

		// If, for some reason, we have a suffix of the chain locally, handle that here
		ts, err := syncer.store.LoadTipSet(at)
		if err == nil {
			acceptedBlocks = append(acceptedBlocks, at.Cids()...)

			blockSet = append(blockSet, ts)
			at = ts.Parents()
			continue
		}
		if !xerrors.Is(err, bstore.ErrNotFound) {
			log.Warn("loading local tipset: %s", err)
		}

		// NB: GetBlocks validates that the blocks are in-fact the ones we
		// requested, and that they are correctly linked to one another. It does
		// not validate any state transitions.
		window := 500
		if gap := int(blockSet[len(blockSet)-1].Height() - untilHeight); gap < window {
			window = gap
		}
		blks, err := syncer.Exchange.GetBlocks(ctx, at, window)
		if err != nil {
			// Most likely our peers aren't fully synced yet, but forwarded
			// new block message (ideally we'd find better peers)

			log.Errorf("failed to get blocks: %+v", err)

			span.AddAttributes(trace.StringAttribute("error", err.Error()))

			// This error will only be logged above,
			return nil, xerrors.Errorf("failed to get blocks: %w", err)
		}
		log.Info("Got blocks: ", blks[0].Height(), len(blks))

		// Check that the fetched segment of the chain matches what we already
		// have. Since we fetch from the head backwards our reassembled chain
		// is sorted in reverse here: we have a child -> parent order, our last
		// tipset then should be child of the first tipset retrieved.
		// FIXME: The reassembly logic should be part of the `client`
		//  service, the consumer should not be concerned with the
		//  `MaxRequestLength` limitation, it should just be able to request
		//  an segment of arbitrary length. The same burden is put on
		//  `syncFork()` which needs to be aware this as well.
		if blockSet[len(blockSet)-1].IsChildOf(blks[0]) == false {
			return nil, xerrors.Errorf("retrieved segments of the chain are not connected at heights %d/%d",
				blockSet[len(blockSet)-1].Height(), blks[0].Height())
			// A successful `GetBlocks()` call is guaranteed to fetch at least
			// one tipset so the acess `blks[0]` is safe.
		}

		for _, b := range blks {
			if b.Height() < untilHeight {
				break loop
			}
			for _, bc := range b.Cids() {
				if reason, ok := syncer.bad.Has(bc); ok {
					newReason := reason.Linked("change contained %s", bc)
					for _, b := range acceptedBlocks {
						syncer.bad.Add(b, newReason)
					}

					return nil, xerrors.Errorf("chain contained block marked previously as bad (%s, %s) (reason: %s)", incoming.Cids(), bc, reason)
				}
			}
			blockSet = append(blockSet, b)
		}

		acceptedBlocks = append(acceptedBlocks, at.Cids()...)

		ss.SetHeight(blks[len(blks)-1].Height())
		at = blks[len(blks)-1].Parents()
	}

	base := blockSet[len(blockSet)-1]
	if base.IsChildOf(known) {
		// common case: receiving blocks that are building on top of our best tipset
		return blockSet, nil
	}

	knownParent, err := syncer.store.LoadTipSet(known.Parents())
	if err != nil {
		return nil, xerrors.Errorf("failed to load next local tipset: %w", err)
	}
	if base.IsChildOf(knownParent) {
		// common case: receiving a block thats potentially part of the same tipset as our best block
		return blockSet, nil
	}

	// We have now ascertained that this is *not* a 'fast forward'
	log.Warnf("(fork detected) synced header chain (%s - %d) does not link to our best block (%s - %d)", incoming.Cids(), incoming.Height(), known.Cids(), known.Height())
	fork, err := syncer.syncFork(ctx, base, known)
	if err != nil {
		if xerrors.Is(err, ErrForkTooLong) || xerrors.Is(err, ErrForkCheckpoint) {
			// TODO: we're marking this block bad in the same way that we mark invalid blocks bad. Maybe distinguish?
			log.Warn("adding forked chain to our bad tipset cache")
			for _, b := range incoming.Blocks() {
				syncer.bad.Add(b.Cid(), NewBadBlockReason(incoming.Cids(), "fork past finality"))
			}
		}
		return nil, xerrors.Errorf("failed to sync fork: %w", err)
	}

	blockSet = append(blockSet, fork...)

	return blockSet, nil
}

var ErrForkTooLong = fmt.Errorf("fork longer than threshold")
var ErrForkCheckpoint = fmt.Errorf("fork would require us to diverge from checkpointed block")

// syncFork tries to obtain the chain fragment that links a fork into a common
// ancestor in our view of the chain.
//
// If the fork is too long (build.ForkLengthThreshold), or would cause us to diverge from the checkpoint (ErrForkCheckpoint),
// we add the entire subchain to the denylist. Else, we find the common ancestor, and add the missing chain
// fragment until the fork point to the returned []TipSet.
func (syncer *Syncer) syncFork(ctx context.Context, incoming *types.TipSet, known *types.TipSet) ([]*types.TipSet, error) {

	chkpt := syncer.GetCheckpoint()
	if known.Key() == chkpt {
		return nil, ErrForkCheckpoint
	}

	// TODO: Does this mean we always ask for ForkLengthThreshold blocks from the network, even if we just need, like, 2?
	// Would it not be better to ask in smaller chunks, given that an ~ForkLengthThreshold is very rare?
	tips, err := syncer.Exchange.GetBlocks(ctx, incoming.Parents(), int(build.ForkLengthThreshold))
	if err != nil {
		return nil, err
	}

	nts, err := syncer.store.LoadTipSet(known.Parents())
	if err != nil {
		return nil, xerrors.Errorf("failed to load next local tipset: %w", err)
	}

	for cur := 0; cur < len(tips); {
		if nts.Height() == 0 {
			if !syncer.Genesis.Equals(nts) {
				return nil, xerrors.Errorf("somehow synced chain that linked back to a different genesis (bad genesis: %s)", nts.Key())
			}
			return nil, xerrors.Errorf("synced chain forked at genesis, refusing to sync; incoming: %s", incoming.Cids())
		}

		if nts.Equals(tips[cur]) {
			return tips[:cur+1], nil
		}

		if nts.Height() < tips[cur].Height() {
			cur++
		} else {
			// We will be forking away from nts, check that it isn't checkpointed
			if nts.Key() == chkpt {
				return nil, ErrForkCheckpoint
			}

			nts, err = syncer.store.LoadTipSet(nts.Parents())
			if err != nil {
				return nil, xerrors.Errorf("loading next local tipset: %w", err)
			}
		}
	}

	return nil, ErrForkTooLong
}

func (syncer *Syncer) syncMessagesAndCheckState(ctx context.Context, headers []*types.TipSet) error {
	ss := extractSyncState(ctx)
	ss.SetHeight(headers[len(headers)-1].Height())

	return syncer.iterFullTipsets(ctx, headers, func(ctx context.Context, fts *store.FullTipSet) error {
		log.Debugw("validating tipset", "height", fts.TipSet().Height(), "size", len(fts.TipSet().Cids()))
		if err := syncer.ValidateTipSet(ctx, fts, true); err != nil {
			log.Errorf("failed to validate tipset: %+v", err)
			return xerrors.Errorf("message processing failed: %w", err)
		}

		stats.Record(ctx, metrics.ChainNodeWorkerHeight.M(int64(fts.TipSet().Height())))
		ss.SetHeight(fts.TipSet().Height())

		return nil
	})
}

// fills out each of the given tipsets with messages and calls the callback with it
func (syncer *Syncer) iterFullTipsets(ctx context.Context, headers []*types.TipSet, cb func(context.Context, *store.FullTipSet) error) error {
	ss := extractSyncState(ctx)
	ctx, span := trace.StartSpan(ctx, "iterFullTipsets")
	defer span.End()

	span.AddAttributes(trace.Int64Attribute("num_headers", int64(len(headers))))

	for i := len(headers) - 1; i >= 0; {
		fts, err := syncer.store.TryFillTipSet(headers[i])
		if err != nil {
			return err
		}
		if fts != nil {
			if err := cb(ctx, fts); err != nil {
				return err
			}
			i--
			continue
		}

		batchSize := concurrentSyncRequests * syncRequestBatchSize
		if i < batchSize {
			batchSize = i + 1
		}

		ss.SetStage(api.StageFetchingMessages)
		startOffset := i + 1 - batchSize
		bstout, batchErr := syncer.fetchMessages(ctx, headers[startOffset:startOffset+batchSize], startOffset)
		ss.SetStage(api.StageMessages)

		if batchErr != nil {
			return xerrors.Errorf("failed to fetch messages: %w", batchErr)
		}

		for bsi := 0; bsi < len(bstout); bsi++ {
			// temp storage so we don't persist data we dont want to
			bs := bstore.NewTemporary()
			blks := cbor.NewCborStore(bs)

			this := headers[i-bsi]
			bstip := bstout[len(bstout)-(bsi+1)]
			fts, err := zipTipSetAndMessages(blks, this, bstip.Bls, bstip.Secpk, bstip.BlsIncludes, bstip.SecpkIncludes)
			if err != nil {
				log.Warnw("zipping failed", "error", err, "bsi", bsi, "i", i,
					"height", this.Height(),
					"next-height", i+batchSize)
				return xerrors.Errorf("message processing failed: %w", err)
			}

			if err := cb(ctx, fts); err != nil {
				return err
			}

			if err := persistMessages(ctx, bs, bstip); err != nil {
				return err
			}

			if err := copyBlockstore(ctx, bs, syncer.store.Blockstore()); err != nil {
				return xerrors.Errorf("message processing failed: %w", err)
			}
		}

		i -= batchSize
	}

	return nil
}

func (syncer *Syncer) fetchMessages(ctx context.Context, headers []*types.TipSet, startOffset int) ([]*exchange.CompactedMessages, error) {
	batchSize := len(headers)
	batch := make([]*exchange.CompactedMessages, batchSize)

	var wg sync.WaitGroup
	var mx sync.Mutex
	var batchErr error

	start := build.Clock.Now()

	for j := 0; j < batchSize; j += syncRequestBatchSize {
		wg.Add(1)
		go func(j int) {
			defer wg.Done()

			nreq := syncRequestBatchSize
			if j+nreq > batchSize {
				nreq = batchSize - j
			}

			failed := false
			for offset := 0; !failed && offset < nreq; {
				nextI := j + offset
				lastI := j + nreq

				var requestErr error
				var requestResult []*exchange.CompactedMessages
				for retry := 0; requestResult == nil && retry < syncRequestRetries; retry++ {
					if retry > 0 {
						log.Infof("fetching messages at %d (retry %d)", startOffset+nextI, retry)
					} else {
						log.Infof("fetching messages at %d", startOffset+nextI)
					}

					result, err := syncer.Exchange.GetChainMessages(ctx, headers[nextI:lastI])
					if err != nil {
						requestErr = multierror.Append(requestErr, err)
					} else {
						requestResult = result
					}
				}

				mx.Lock()
				if requestResult != nil {
					copy(batch[j+offset:], requestResult)
					offset += len(requestResult)
				} else {
					log.Errorf("error fetching messages at %d: %s", nextI, requestErr)
					batchErr = multierror.Append(batchErr, requestErr)
					failed = true
				}
				mx.Unlock()
			}
		}(j)
	}
	wg.Wait()

	if batchErr != nil {
		return nil, batchErr
	}

	log.Infof("fetching messages for %d tipsets at %d done; took %s", batchSize, startOffset, build.Clock.Since(start))

	return batch, nil
}

func persistMessages(ctx context.Context, bs bstore.Blockstore, bst *exchange.CompactedMessages) error {
	_, span := trace.StartSpan(ctx, "persistMessages")
	defer span.End()

	for _, m := range bst.Bls {
		//log.Infof("putting BLS message: %s", m.Cid())
		if _, err := store.PutMessage(bs, m); err != nil {
			log.Errorf("failed to persist messages: %+v", err)
			return xerrors.Errorf("BLS message processing failed: %w", err)
		}
	}
	for _, m := range bst.Secpk {
		if m.Signature.Type != crypto.SigTypeSecp256k1 {
			return xerrors.Errorf("unknown signature type on message %s: %q", m.Cid(), m.Signature.Type)
		}
		//log.Infof("putting secp256k1 message: %s", m.Cid())
		if _, err := store.PutMessage(bs, m); err != nil {
			log.Errorf("failed to persist messages: %+v", err)
			return xerrors.Errorf("secp256k1 message processing failed: %w", err)
		}
	}

	return nil
}

// collectChain tries to advance our view of the chain to the purported head.
//
// It goes through various stages:
//
//  1. StageHeaders: we proceed in the sync process by requesting block headers
//     from our peers, moving back from their heads, until we reach a tipset
//     that we have in common (such a common tipset must exist, thought it may
//     simply be the genesis block).
//
//     If the common tipset is our head, we treat the sync as a "fast-forward",
//     else we must drop part of our chain to connect to the peer's head
//     (referred to as "forking").
//
//	2. StagePersistHeaders: now that we've collected the missing headers,
//     augmented by those on the other side of a fork, we persist them to the
//     BlockStore.
//
//  3. StageMessages: having acquired the headers and found a common tipset,
//     we then move forward, requesting the full blocks, including the messages.
func (syncer *Syncer) collectChain(ctx context.Context, ts *types.TipSet) error {
	ctx, span := trace.StartSpan(ctx, "collectChain")
	defer span.End()
	ss := extractSyncState(ctx)

	ss.Init(syncer.store.GetHeaviestTipSet(), ts)

	headers, err := syncer.collectHeaders(ctx, ts, syncer.store.GetHeaviestTipSet())
	if err != nil {
		ss.Error(err)
		return err
	}

	span.AddAttributes(trace.Int64Attribute("syncChainLength", int64(len(headers))))

	if !headers[0].Equals(ts) {
		log.Errorf("collectChain headers[0] should be equal to sync target. Its not: %s != %s", headers[0].Cids(), ts.Cids())
	}

	ss.SetStage(api.StagePersistHeaders)

	toPersist := make([]*types.BlockHeader, 0, len(headers)*int(build.BlocksPerEpoch))
	for _, ts := range headers {
		toPersist = append(toPersist, ts.Blocks()...)
	}
	if err := syncer.store.PersistBlockHeaders(toPersist...); err != nil {
		err = xerrors.Errorf("failed to persist synced blocks to the chainstore: %w", err)
		ss.Error(err)
		return err
	}
	toPersist = nil

	ss.SetStage(api.StageMessages)

	if err := syncer.syncMessagesAndCheckState(ctx, headers); err != nil {
		err = xerrors.Errorf("collectChain syncMessages: %w", err)
		ss.Error(err)
		return err
	}

	ss.SetStage(api.StageSyncComplete)
	log.Debugw("new tipset", "height", ts.Height(), "tipset", types.LogCids(ts.Cids()))

	return nil
}

func VerifyElectionPoStVRF(ctx context.Context, worker address.Address, rand []byte, evrf []byte) error {
	if build.InsecurePoStValidation {
		return nil
	}
	return gen.VerifyVRF(ctx, worker, rand, evrf)
}

func (syncer *Syncer) State() []SyncerStateSnapshot {
	return syncer.syncmgr.State()
}

// MarkBad manually adds a block to the "bad blocks" cache.
func (syncer *Syncer) MarkBad(blk cid.Cid) {
	syncer.bad.Add(blk, NewBadBlockReason([]cid.Cid{blk}, "manually marked bad"))
}

// UnmarkBad manually adds a block to the "bad blocks" cache.
func (syncer *Syncer) UnmarkBad(blk cid.Cid) {
	syncer.bad.Remove(blk)
}

func (syncer *Syncer) UnmarkAllBad() {
	syncer.bad.Purge()
}

func (syncer *Syncer) CheckBadBlockCache(blk cid.Cid) (string, bool) {
	bbr, ok := syncer.bad.Has(blk)
	return bbr.String(), ok
}

func (syncer *Syncer) getLatestBeaconEntry(_ context.Context, ts *types.TipSet) (*types.BeaconEntry, error) {
	cur := ts
	for i := 0; i < 20; i++ {
		cbe := cur.Blocks()[0].BeaconEntries
		if len(cbe) > 0 {
			return &cbe[len(cbe)-1], nil
		}

		if cur.Height() == 0 {
			return nil, xerrors.Errorf("made it back to genesis block without finding beacon entry")
		}

		next, err := syncer.store.LoadTipSet(cur.Parents())
		if err != nil {
			return nil, xerrors.Errorf("failed to load parents when searching back for latest beacon entry: %w", err)
		}
		cur = next
	}

	return nil, xerrors.Errorf("found NO beacon entries in the 20 latest tipsets")
}

func (syncer *Syncer) IsEpochBeyondCurrMax(epoch abi.ChainEpoch) bool {
	g, err := syncer.store.GetGenesis()
	if err != nil {
		return false
	}

	now := uint64(build.Clock.Now().Unix())
	return epoch > (abi.ChainEpoch((now-g.Timestamp)/build.BlockDelaySecs) + MaxHeightDrift)
}