backport from v1.8.23 (#304)

* dex: backport f6193ad

* dex/downloader: backport accc0fa accc0fab 174083c3

* dex: backport 434dd5b

* dex: backport 42a914a 0983d02

* dex: backport 48b70ec 31b3334 and some modification

* dex/downloader: backport 5f251a6

* dex/downloader: backport 81c3dc7

* dex, dex/downloader: fix typos

12 files changed, 433 insertions, 87 deletions

diff --git a/dex/api_backend.go b/dex/api_backend.go
index 60a874701..7333a9bb1 100644
--- a/dex/api_backend.go
+++ b/dex/api_backend.go
@@ -25,7 +25,6 @@ import (
 	"github.com/dexon-foundation/dexon/common/math"
 	"github.com/dexon-foundation/dexon/core"
 	"github.com/dexon-foundation/dexon/core/bloombits"
-	"github.com/dexon-foundation/dexon/core/rawdb"
 	"github.com/dexon-foundation/dexon/core/state"
 	"github.com/dexon-foundation/dexon/core/types"
 	"github.com/dexon-foundation/dexon/core/vm"
@@ -92,18 +91,11 @@ func (b *DexAPIBackend) GetBlock(ctx context.Context, hash common.Hash) (*types.
 }
 
 func (b *DexAPIBackend) GetReceipts(ctx context.Context, hash common.Hash) (types.Receipts, error) {
-	if number := rawdb.ReadHeaderNumber(b.dex.chainDb, hash); number != nil {
-		return rawdb.ReadReceipts(b.dex.chainDb, hash, *number), nil
-	}
-	return nil, nil
+	return b.dex.blockchain.GetReceiptsByHash(hash), nil
 }
 
 func (b *DexAPIBackend) GetLogs(ctx context.Context, hash common.Hash) ([][]*types.Log, error) {
-	number := rawdb.ReadHeaderNumber(b.dex.chainDb, hash)
-	if number == nil {
-		return nil, nil
-	}
-	receipts := rawdb.ReadReceipts(b.dex.chainDb, hash, *number)
+	receipts := b.dex.blockchain.GetReceiptsByHash(hash)
 	if receipts == nil {
 		return nil, nil
 	}
diff --git a/dex/api_tracer.go b/dex/api_tracer.go
index bb6acd764..d451c376d 100644
--- a/dex/api_tracer.go
+++ b/dex/api_tracer.go
@@ -17,11 +17,13 @@
 package dex
 
 import (
+	"bufio"
 	"bytes"
 	"context"
 	"errors"
 	"fmt"
 	"io/ioutil"
+	"os"
 	"runtime"
 	"sync"
 	"time"
@@ -60,6 +62,13 @@ type TraceConfig struct {
 	Reexec  *uint64
 }
 
+// StdTraceConfig holds extra parameters to standard-json trace functions.
+type StdTraceConfig struct {
+	*vm.LogConfig
+	Reexec *uint64
+	TxHash common.Hash
+}
+
 // txTraceResult is the result of a single transaction trace.
 type txTraceResult struct {
 	Result interface{} `json:"result,omitempty"` // Trace results produced by the tracer
@@ -134,7 +143,7 @@ func (api *PrivateDebugAPI) traceChain(ctx context.Context, start, end *types.Bl
 
 	// Ensure we have a valid starting state before doing any work
 	origin := start.NumberU64()
-	database := state.NewDatabase(api.dex.ChainDb())
+	database := state.NewDatabaseWithCache(api.dex.ChainDb(), 16) // Chain tracing will probably start at genesis
 
 	if number := start.NumberU64(); number > 0 {
 		start = api.dex.blockchain.GetBlock(start.ParentHash(), start.NumberU64()-1)
@@ -360,7 +369,7 @@ func (api *PrivateDebugAPI) TraceBlockByNumber(ctx context.Context, number rpc.B
 func (api *PrivateDebugAPI) TraceBlockByHash(ctx context.Context, hash common.Hash, config *TraceConfig) ([]*txTraceResult, error) {
 	block := api.dex.blockchain.GetBlockByHash(hash)
 	if block == nil {
-		return nil, fmt.Errorf("block #%x not found", hash)
+		return nil, fmt.Errorf("block %#x not found", hash)
 	}
 	return api.traceBlock(ctx, block, config)
 }
@@ -385,6 +394,43 @@ func (api *PrivateDebugAPI) TraceBlockFromFile(ctx context.Context, file string,
 	return api.TraceBlock(ctx, blob, config)
 }
 
+// TraceBadBlockByHash returns the structured logs created during the execution of
+// EVM against a block pulled from the pool of bad ones and returns them as a JSON
+// object.
+func (api *PrivateDebugAPI) TraceBadBlock(ctx context.Context, hash common.Hash, config *TraceConfig) ([]*txTraceResult, error) {
+	blocks := api.dex.blockchain.BadBlocks()
+	for _, block := range blocks {
+		if block.Hash() == hash {
+			return api.traceBlock(ctx, block, config)
+		}
+	}
+	return nil, fmt.Errorf("bad block %#x not found", hash)
+}
+
+// StandardTraceBlockToFile dumps the structured logs created during the
+// execution of EVM to the local file system and returns a list of files
+// to the caller.
+func (api *PrivateDebugAPI) StandardTraceBlockToFile(ctx context.Context, hash common.Hash, config *StdTraceConfig) ([]string, error) {
+	block := api.dex.blockchain.GetBlockByHash(hash)
+	if block == nil {
+		return nil, fmt.Errorf("block %#x not found", hash)
+	}
+	return api.standardTraceBlockToFile(ctx, block, config)
+}
+
+// StandardTraceBadBlockToFile dumps the structured logs created during the
+// execution of EVM against a block pulled from the pool of bad ones to the
+// local file system and returns a list of files to the caller.
+func (api *PrivateDebugAPI) StandardTraceBadBlockToFile(ctx context.Context, hash common.Hash, config *StdTraceConfig) ([]string, error) {
+	blocks := api.dex.blockchain.BadBlocks()
+	for _, block := range blocks {
+		if block.Hash() == hash {
+			return api.standardTraceBlockToFile(ctx, block, config)
+		}
+	}
+	return nil, fmt.Errorf("bad block %#x not found", hash)
+}
+
 // traceBlock configures a new tracer according to the provided configuration, and
 // executes all the transactions contained within. The return value will be one item
 // per transaction, dependent on the requestd tracer.
@@ -395,7 +441,7 @@ func (api *PrivateDebugAPI) traceBlock(ctx context.Context, block *types.Block,
 	}
 	parent := api.dex.blockchain.GetBlock(block.ParentHash(), block.NumberU64()-1)
 	if parent == nil {
-		return nil, fmt.Errorf("parent %x not found", block.ParentHash())
+		return nil, fmt.Errorf("parent %#x not found", block.ParentHash())
 	}
 	reexec := defaultTraceReexec
 	if config != nil && config.Reexec != nil {
@@ -466,6 +512,107 @@ func (api *PrivateDebugAPI) traceBlock(ctx context.Context, block *types.Block,
 	return results, nil
 }
 
+// standardTraceBlockToFile configures a new tracer which uses standard JSON output,
+// and traces either a full block or an individual transaction. The return value will
+// be one filename per transaction traced.
+func (api *PrivateDebugAPI) standardTraceBlockToFile(ctx context.Context, block *types.Block, config *StdTraceConfig) ([]string, error) {
+	// If we're tracing a single transaction, make sure it's present
+	if config != nil && config.TxHash != (common.Hash{}) {
+		var exists bool
+		for _, tx := range block.Transactions() {
+			if exists = (tx.Hash() == config.TxHash); exists {
+				break
+			}
+		}
+		if !exists {
+			return nil, fmt.Errorf("transaction %#x not found in block", config.TxHash)
+		}
+	}
+	// Create the parent state database
+	if err := api.dex.engine.VerifyHeader(api.dex.blockchain, block.Header(), true); err != nil {
+		return nil, err
+	}
+	parent := api.dex.blockchain.GetBlock(block.ParentHash(), block.NumberU64()-1)
+	if parent == nil {
+		return nil, fmt.Errorf("parent %#x not found", block.ParentHash())
+	}
+	reexec := defaultTraceReexec
+	if config != nil && config.Reexec != nil {
+		reexec = *config.Reexec
+	}
+	statedb, err := api.computeStateDB(parent, reexec)
+	if err != nil {
+		return nil, err
+	}
+	// Retrieve the tracing configurations, or use default values
+	var (
+		logConfig vm.LogConfig
+		txHash    common.Hash
+	)
+	if config != nil {
+		if config.LogConfig != nil {
+			logConfig = *config.LogConfig
+		}
+		txHash = config.TxHash
+	}
+	logConfig.Debug = true
+
+	// Execute transaction, either tracing all or just the requested one
+	var (
+		signer = types.MakeSigner(api.config, block.Number())
+		dumps  []string
+	)
+	for i, tx := range block.Transactions() {
+		// Prepare the trasaction for un-traced execution
+		var (
+			msg, _ = tx.AsMessage(signer)
+			vmctx  = core.NewEVMContext(msg, block.Header(), api.dex.blockchain, nil)
+
+			vmConf vm.Config
+			dump   *os.File
+			err    error
+		)
+		// If the transaction needs tracing, swap out the configs
+		if tx.Hash() == txHash || txHash == (common.Hash{}) {
+			// Generate a unique temporary file to dump it into
+			prefix := fmt.Sprintf("block_%#x-%d-%#x-", block.Hash().Bytes()[:4], i, tx.Hash().Bytes()[:4])
+
+			dump, err = ioutil.TempFile(os.TempDir(), prefix)
+			if err != nil {
+				return nil, err
+			}
+			dumps = append(dumps, dump.Name())
+
+			// Swap out the noop logger to the standard tracer
+			vmConf = vm.Config{
+				Debug:                   true,
+				Tracer:                  vm.NewJSONLogger(&logConfig, bufio.NewWriter(dump)),
+				EnablePreimageRecording: true,
+			}
+		}
+		// Execute the transaction and flush any traces to disk
+		vmenv := vm.NewEVM(vmctx, statedb, api.config, vmConf)
+		_, _, _, err = core.ApplyMessage(vmenv, msg, new(core.GasPool).AddGas(msg.Gas()))
+
+		if dump != nil {
+			dump.Close()
+			log.Info("Wrote standard trace", "file", dump.Name())
+		}
+		if err != nil {
+			return dumps, err
+		}
+		// Finalize the state so any modifications are written to the trie
+		// Only delete empty objects if EIP158/161 (a.k.a Spurious Dragon) is in effect
+		statedb.Finalise(vmenv.ChainConfig().IsEIP158(block.Number()))
+
+		// If we've traced the transaction we were looking for, abort
+		if tx.Hash() == txHash {
+			break
+		}
+	}
+	return dumps, nil
+}
+
 // computeStateDB retrieves the state database associated with a certain block.
 // If no state is locally available for the given block, a number of blocks are
 // attempted to be reexecuted to generate the desired state.
@@ -477,7 +624,7 @@ func (api *PrivateDebugAPI) computeStateDB(block *types.Block, reexec uint64) (*
 	}
 	// Otherwise try to reexec blocks until we find a state or reach our limit
 	origin := block.NumberU64()
-	database := state.NewDatabase(api.dex.ChainDb())
+	database := state.NewDatabaseWithCache(api.dex.ChainDb(), 16)
 
 	for i := uint64(0); i < reexec; i++ {
 		block = api.dex.blockchain.GetBlock(block.ParentHash(), block.NumberU64()-1)
@@ -491,7 +638,7 @@ func (api *PrivateDebugAPI) computeStateDB(block *types.Block, reexec uint64) (*
 	if err != nil {
 		switch err.(type) {
 		case *trie.MissingNodeError:
-			return nil, errors.New("required historical state unavailable")
+			return nil, fmt.Errorf("required historical state unavailable (reexec=%d)", reexec)
 		default:
 			return nil, err
 		}
@@ -505,7 +652,7 @@ func (api *PrivateDebugAPI) computeStateDB(block *types.Block, reexec uint64) (*
 	for block.NumberU64() < origin {
 		// Print progress logs if long enough time elapsed
 		if time.Since(logged) > 8*time.Second {
-			log.Info("Regenerating historical state", "block", block.NumberU64()+1, "target", origin, "elapsed", time.Since(start))
+			log.Info("Regenerating historical state", "block", block.NumberU64()+1, "target", origin, "remaining", origin-block.NumberU64()-1, "elapsed", time.Since(start))
 			logged = time.Now()
 		}
 		// Retrieve the next block to regenerate and process it
@@ -514,7 +661,7 @@ func (api *PrivateDebugAPI) computeStateDB(block *types.Block, reexec uint64) (*
 		}
 		_, _, _, err := api.dex.blockchain.Processor().Process(block, statedb, vm.Config{})
 		if err != nil {
-			return nil, err
+			return nil, fmt.Errorf("processing block %d failed: %v", block.NumberU64(), err)
 		}
 		// Finalize the state so any modifications are written to the trie
 		root, err := statedb.Commit(true)
@@ -522,7 +669,7 @@ func (api *PrivateDebugAPI) computeStateDB(block *types.Block, reexec uint64) (*
 			return nil, err
 		}
 		if err := statedb.Reset(root); err != nil {
-			return nil, err
+			return nil, fmt.Errorf("state reset after block %d failed: %v", block.NumberU64(), err)
 		}
 		database.TrieDB().Reference(root, common.Hash{})
 		if proot != (common.Hash{}) {
@@ -541,7 +688,7 @@ func (api *PrivateDebugAPI) TraceTransaction(ctx context.Context, hash common.Ha
 	// Retrieve the transaction and assemble its EVM context
 	tx, blockHash, _, index := rawdb.ReadTransaction(api.dex.ChainDb(), hash)
 	if tx == nil {
-		return nil, fmt.Errorf("transaction %x not found", hash)
+		return nil, fmt.Errorf("transaction %#x not found", hash)
 	}
 	reexec := defaultTraceReexec
 	if config != nil && config.Reexec != nil {
@@ -621,11 +768,11 @@ func (api *PrivateDebugAPI) computeTxEnv(blockHash common.Hash, txIndex int, ree
 	// Create the parent state database
 	block := api.dex.blockchain.GetBlockByHash(blockHash)
 	if block == nil {
-		return nil, vm.Context{}, nil, fmt.Errorf("block %x not found", blockHash)
+		return nil, vm.Context{}, nil, fmt.Errorf("block %#x not found", blockHash)
 	}
 	parent := api.dex.blockchain.GetBlock(block.ParentHash(), block.NumberU64()-1)
 	if parent == nil {
-		return nil, vm.Context{}, nil, fmt.Errorf("parent %x not found", block.ParentHash())
+		return nil, vm.Context{}, nil, fmt.Errorf("parent %#x not found", block.ParentHash())
 	}
 	statedb, err := api.computeStateDB(parent, reexec)
 	if err != nil {
@@ -644,10 +791,10 @@ func (api *PrivateDebugAPI) computeTxEnv(blockHash common.Hash, txIndex int, ree
 		// Not yet the searched for transaction, execute on top of the current state
 		vmenv := vm.NewEVM(context, statedb, api.config, vm.Config{})
 		if _, _, _, err := core.ApplyMessage(vmenv, msg, new(core.GasPool).AddGas(tx.Gas())); err != nil {
-			return nil, vm.Context{}, nil, fmt.Errorf("tx %x failed: %v", tx.Hash(), err)
+			return nil, vm.Context{}, nil, fmt.Errorf("tx %#x failed: %v", tx.Hash(), err)
 		}
 		// Ensure any modifications are committed to the state
 		statedb.Finalise(true)
 	}
-	return nil, vm.Context{}, nil, fmt.Errorf("tx index %d out of range for block %x", txIndex, blockHash)
+	return nil, vm.Context{}, nil, fmt.Errorf("transaction index %d out of range for block %#x", txIndex, blockHash)
 }
diff --git a/dex/backend.go b/dex/backend.go
index 29ac42906..26a0b49eb 100644
--- a/dex/backend.go
+++ b/dex/backend.go
@@ -173,7 +173,7 @@ func New(ctx *node.ServiceContext, config *Config) (*Dexon, error) {
 
 	pm, err := NewProtocolManager(dex.chainConfig, config.SyncMode,
 		config.NetworkId, dex.eventMux, dex.txPool, dex.engine, dex.blockchain,
-		chainDb, config.BlockProposerEnabled, dex.governance, dex.app)
+		chainDb, config.Whitelist, config.BlockProposerEnabled, dex.governance, dex.app)
 	if err != nil {
 		return nil, err
 	}
diff --git a/dex/config.go b/dex/config.go
index d218b35e2..ac3e89969 100644
--- a/dex/config.go
+++ b/dex/config.go
@@ -25,6 +25,7 @@ import (
 	"runtime"
 	"time"
 
+	"github.com/dexon-foundation/dexon/common"
 	"github.com/dexon-foundation/dexon/core"
 	"github.com/dexon-foundation/dexon/dex/downloader"
 	"github.com/dexon-foundation/dexon/eth/gasprice"
@@ -81,6 +82,9 @@ type Config struct {
 	SyncMode  downloader.SyncMode
 	NoPruning bool
 
+	// Whitelist of required block number -> hash values to accept
+	Whitelist map[uint64]common.Hash `toml:"-"`
+
 	// Light client options
 	LightServ  int `toml:",omitempty"` // Maximum percentage of time allowed for serving LES requests
 	LightPeers int `toml:",omitempty"` // Maximum number of LES client peers
@@ -112,7 +116,7 @@ type Config struct {
 	// Miscellaneous options
 	DocRoot string `toml:"-"`
 
-	// Type of the EWASM interpreter ("" for detault)
+	// Type of the EWASM interpreter ("" for default)
 	EWASMInterpreter string
 
 	// Type of the EVM interpreter ("" for default)
diff --git a/dex/downloader/downloader.go b/dex/downloader/downloader.go
index e3960ea30..9d609584b 100644
--- a/dex/downloader/downloader.go
+++ b/dex/downloader/downloader.go
@@ -103,6 +103,7 @@ type Downloader struct {
 	mode SyncMode       // Synchronisation mode defining the strategy used (per sync cycle)
 	mux  *event.TypeMux // Event multiplexer to announce sync operation events
 
+	genesis uint64   // Genesis block number to limit sync to (e.g. light client CHT)
 	queue   *queue   // Scheduler for selecting the hashes to download
 	peers   *peerSet // Set of active peers from which download can proceed
 	stateDB ethdb.Database
@@ -192,6 +193,9 @@ type BlockChain interface {
 	// HasBlock verifies a block's presence in the local chain.
 	HasBlock(common.Hash, uint64) bool
 
+	// HasFastBlock verifies a fast block's presence in the local chain.
+	HasFastBlock(common.Hash, uint64) bool
+
 	// GetBlockByHash retrieves a block from the local chain.
 	GetBlockByHash(common.Hash) *types.Block
 
@@ -442,7 +446,7 @@ func (d *Downloader) syncWithPeer(p *peerConnection, hash common.Hash, number ui
 	}
 	height := latest.Number.Uint64()
 
-	origin, err := d.findAncestor(p, height)
+	origin, err := d.findAncestor(p, latest)
 	if err != nil {
 		return err
 	}
@@ -691,41 +695,107 @@ func (d *Downloader) fetchGovState(p *peerConnection,
 	}
 }
 
-// findAncestor tries to locate the common ancestor link of the local chain and
-// a remote peers blockchain. In the general case when our node was in sync and
-// on the correct chain, checking the top N links should already get us a match.
-// In the rare scenario when we ended up on a long reorganisation (i.e. none of
-// the head links match), we do a binary search to find the common ancestor.
-func (d *Downloader) findAncestor(p *peerConnection, height uint64) (uint64, error) {
-	// Figure out the valid ancestor range to prevent rewrite attacks
-	floor, ceil := int64(-1), d.lightchain.CurrentHeader().Number.Uint64()
-
-	if d.mode == FullSync {
-		ceil = d.blockchain.CurrentBlock().NumberU64()
-	} else if d.mode == FastSync {
-		ceil = d.blockchain.CurrentFastBlock().NumberU64()
+// calculateRequestSpan calculates what headers to request from a peer when trying to determine the
+// common ancestor.
+// It returns parameters to be used for peer.RequestHeadersByNumber:
+//  from - starting block number
+//  count - number of headers to request
+//  skip - number of headers to skip
+// and also returns 'max', the last block which is expected to be returned by the remote peers,
+// given the (from,count,skip)
+func calculateRequestSpan(remoteHeight, localHeight uint64) (int64, int, int, uint64) {
+	var (
+		from     int
+		count    int
+		MaxCount = MaxHeaderFetch / 16
+	)
+	// requestHead is the highest block that we will ask for. If requestHead is not offset,
+	// the highest block that we will get is 16 blocks back from head, which means we
+	// will fetch 14 or 15 blocks unnecessarily in the case the height difference
+	// between us and the peer is 1-2 blocks, which is most common
+	requestHead := int(remoteHeight) - 1
+	if requestHead < 0 {
+		requestHead = 0
+	}
+	// requestBottom is the lowest block we want included in the query
+	// Ideally, we want to include just below own head
+	requestBottom := int(localHeight - 1)
+	if requestBottom < 0 {
+		requestBottom = 0
 	}
-	if ceil >= MaxForkAncestry {
-		floor = int64(ceil - MaxForkAncestry)
+	totalSpan := requestHead - requestBottom
+	span := 1 + totalSpan/MaxCount
+	if span < 2 {
+		span = 2
+	}
+	if span > 16 {
+		span = 16
 	}
-	p.log.Debug("Looking for common ancestor", "local", ceil, "remote", height)
 
-	// Request the topmost blocks to short circuit binary ancestor lookup
-	head := ceil
-	if head > height {
-		head = height
+	count = 1 + totalSpan/span
+	if count > MaxCount {
+		count = MaxCount
+	}
+	if count < 2 {
+		count = 2
 	}
-	from := int64(head) - int64(MaxHeaderFetch)
+	from = requestHead - (count-1)*span
 	if from < 0 {
 		from = 0
 	}
-	// Span out with 15 block gaps into the future to catch bad head reports
-	limit := 2 * MaxHeaderFetch / 16
-	count := 1 + int((int64(ceil)-from)/16)
-	if count > limit {
-		count = limit
+	max := from + (count-1)*span
+	return int64(from), count, span - 1, uint64(max)
+}
+
+// findAncestor tries to locate the common ancestor link of the local chain and
+// a remote peers blockchain. In the general case when our node was in sync and
+// on the correct chain, checking the top N links should already get us a match.
+// In the rare scenario when we ended up on a long reorganisation (i.e. none of
+// the head links match), we do a binary search to find the common ancestor.
+func (d *Downloader) findAncestor(p *peerConnection, remoteHeader *types.Header) (uint64, error) {
+	// Figure out the valid ancestor range to prevent rewrite attacks
+	var (
+		floor        = int64(-1)
+		localHeight  uint64
+		remoteHeight = remoteHeader.Number.Uint64()
+	)
+	switch d.mode {
+	case FullSync:
+		localHeight = d.blockchain.CurrentBlock().NumberU64()
+	case FastSync:
+		localHeight = d.blockchain.CurrentFastBlock().NumberU64()
+	default:
+		localHeight = d.lightchain.CurrentHeader().Number.Uint64()
+	}
+	p.log.Debug("Looking for common ancestor", "local", localHeight, "remote", remoteHeight)
+	if localHeight >= MaxForkAncestry {
+		// We're above the max reorg threshold, find the earliest fork point
+		floor = int64(localHeight - MaxForkAncestry)
+
+		// If we're doing a light sync, ensure the floor doesn't go below the CHT, as
+		// all headers before that point will be missing.
+		if d.mode == LightSync {
+			// If we dont know the current CHT position, find it
+			if d.genesis == 0 {
+				header := d.lightchain.CurrentHeader()
+				for header != nil {
+					d.genesis = header.Number.Uint64()
+					if floor >= int64(d.genesis)-1 {
+						break
+					}
+					header = d.lightchain.GetHeaderByHash(header.ParentHash)
+				}
+			}
+			// We already know the "genesis" block number, cap floor to that
+			if floor < int64(d.genesis)-1 {
+				floor = int64(d.genesis) - 1
+			}
+		}
 	}
-	go p.peer.RequestHeadersByNumber(uint64(from), count, 15, false, false)
+	from, count, skip, max := calculateRequestSpan(remoteHeight, localHeight)
+
+	p.log.Trace("Span searching for common ancestor", "count", count, "from", from, "skip", skip)
+	go p.peer.RequestHeadersByNumber(uint64(from), count, skip, false, false)
 
 	// Wait for the remote response to the head fetch
 	number, hash := uint64(0), common.Hash{}
@@ -751,9 +821,10 @@ func (d *Downloader) findAncestor(p *peerConnection, height uint64) (uint64, err
 				return 0, errEmptyHeaderSet
 			}
 			// Make sure the peer's reply conforms to the request
-			for i := 0; i < len(headers); i++ {
-				if number := headers[i].Number.Int64(); number != from+int64(i)*16 {
-					p.log.Warn("Head headers broke chain ordering", "index", i, "requested", from+int64(i)*16, "received", number)
+			for i, header := range headers {
+				expectNumber := from + int64(i)*int64((skip+1))
+				if number := header.Number.Int64(); number != expectNumber {
+					p.log.Warn("Head headers broke chain ordering", "index", i, "requested", expectNumber, "received", number)
 					return 0, errInvalidChain
 				}
 			}
@@ -761,20 +832,24 @@ func (d *Downloader) findAncestor(p *peerConnection, height uint64) (uint64, err
 			finished = true
 			for i := len(headers) - 1; i >= 0; i-- {
 				// Skip any headers that underflow/overflow our requested set
-				if headers[i].Number.Int64() < from || headers[i].Number.Uint64() > ceil {
+				if headers[i].Number.Int64() < from || headers[i].Number.Uint64() > max {
 					continue
 				}
 				// Otherwise check if we already know the header or not
 				h := headers[i].Hash()
 				n := headers[i].Number.Uint64()
-				if (d.mode == FullSync && d.blockchain.HasBlock(h, n)) || (d.mode != FullSync && d.lightchain.HasHeader(h, n)) {
-					number, hash = n, h
 
-					// If every header is known, even future ones, the peer straight out lied about its head
-					if number > height && i == limit-1 {
-						p.log.Warn("Lied about chain head", "reported", height, "found", number)
-						return 0, errStallingPeer
-					}
+				var known bool
+				switch d.mode {
+				case FullSync:
+					known = d.blockchain.HasBlock(h, n)
+				case FastSync:
+					known = d.blockchain.HasFastBlock(h, n)
+				default:
+					known = d.lightchain.HasHeader(h, n)
+				}
+				if known {
+					number, hash = n, h
 					break
 				}
 			}
@@ -798,10 +873,11 @@ func (d *Downloader) findAncestor(p *peerConnection, height uint64) (uint64, err
 		return number, nil
 	}
 	// Ancestor not found, we need to binary search over our chain
-	start, end := uint64(0), head
+	start, end := uint64(0), remoteHeight
 	if floor > 0 {
 		start = uint64(floor)
 	}
+	p.log.Trace("Binary searching for common ancestor", "start", start, "end", end)
 	for start+1 < end {
 		// Split our chain interval in two, and request the hash to cross check
 		check := (start + end) / 2
@@ -834,7 +910,16 @@ func (d *Downloader) findAncestor(p *peerConnection, height uint64) (uint64, err
 				// Modify the search interval based on the response
 				h := headers[0].Hash()
 				n := headers[0].Number.Uint64()
-				if (d.mode == FullSync && !d.blockchain.HasBlock(h, n)) || (d.mode != FullSync && !d.lightchain.HasHeader(h, n)) {
+				var known bool
+				switch d.mode {
+				case FullSync:
+					known = d.blockchain.HasBlock(h, n)
+				case FastSync:
+					known = d.blockchain.HasFastBlock(h, n)
+				default:
+					known = d.lightchain.HasHeader(h, n)
+				}
+				if !known {
 					end = check
 					break
 				}
@@ -1511,8 +1596,15 @@ func (d *Downloader) importBlockResults(results []*fetchResult) error {
 		blocks[i] = types.NewBlockWithHeader(result.Header).WithBody(result.Transactions, result.Uncles)
 	}
 	if index, err := d.blockchain.InsertDexonChain(blocks); err != nil {
-		log.Debug("Downloaded item processing failed", "number", results[index].Header.Number, "hash", results[index].Header.Hash(), "err", err)
-		return errInvalidChain
+		if index < len(results) {
+			log.Debug("Downloaded item processing failed", "number", results[index].Header.Number, "hash", results[index].Header.Hash(), "err", err)
+		} else {
+			// The InsertChain method in blockchain.go will sometimes return an out-of-bounds index,
+			// when it needs to preprocess blocks to import a sidechain.
+			// The importer will put together a new list of blocks to import, which is a superset
+			// of the blocks delivered from the downloader, and the indexing will be off.
+			log.Debug("Downloaded item processing failed on sidechain import", "index", index, "err", err)
+		}
 	}
 	return nil
 }
diff --git a/dex/downloader/downloader_test.go b/dex/downloader/downloader_test.go
index e8ec0056b..e01e0d96b 100644
--- a/dex/downloader/downloader_test.go
+++ b/dex/downloader/downloader_test.go
@@ -19,6 +19,7 @@ package downloader
 import (
 	"errors"
 	"fmt"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"testing"
@@ -118,6 +119,15 @@ func (dl *downloadTester) HasBlock(hash common.Hash, number uint64) bool {
 	return dl.GetBlockByHash(hash) != nil
 }
 
+// HasFastBlock checks if a block is present in the testers canonical chain.
+func (dl *downloadTester) HasFastBlock(hash common.Hash, number uint64) bool {
+	dl.lock.RLock()
+	defer dl.lock.RUnlock()
+
+	_, ok := dl.ownReceipts[hash]
+	return ok
+}
+
 // GetHeader retrieves a header from the testers canonical chain.
 func (dl *downloadTester) GetHeaderByHash(hash common.Hash) *types.Header {
 	dl.lock.RLock()
@@ -271,6 +281,7 @@ func (dl *downloadTester) InsertDexonChain(blocks types.Blocks) (i int, err erro
 			dl.ownHeaders[block.Hash()] = block.Header()
 		}
 		dl.ownBlocks[block.Hash()] = block
+		dl.ownReceipts[block.Hash()] = make(types.Receipts, 0)
 		dl.stateDb.Put(block.Root().Bytes(), []byte{0x00})
 	}
 	return len(blocks), nil
@@ -416,14 +427,20 @@ func (dlp *downloadTesterPeer) RequestNodeData(hashes []common.Hash) error {
 // assertOwnChain checks if the local chain contains the correct number of items
 // of the various chain components.
 func assertOwnChain(t *testing.T, tester *downloadTester, length int) {
+	// Mark this method as a helper to report errors at callsite, not in here
+	t.Helper()
+
 	assertOwnForkedChain(t, tester, 1, []int{length})
 }
 
 // assertOwnForkedChain checks if the local forked chain contains the correct
 // number of items of the various chain components.
 func assertOwnForkedChain(t *testing.T, tester *downloadTester, common int, lengths []int) {
+	// Mark this method as a helper to report errors at callsite, not in here
+	t.Helper()
+
 	// Initialize the counters for the first fork
-	headers, blocks, receipts := lengths[0], lengths[0], lengths[0]-fsMinFullBlocks
+	headers, blocks, receipts := lengths[0], lengths[0], lengths[0]
 
 	if receipts < 0 {
 		receipts = 1
@@ -432,12 +449,9 @@ func assertOwnForkedChain(t *testing.T, tester *downloadTester, common int, leng
 	for _, length := range lengths[1:] {
 		headers += length - common
 		blocks += length - common
-		receipts += length - common - fsMinFullBlocks
+		receipts += length - common
 	}
-	switch tester.downloader.mode {
-	case FullSync:
-		receipts = 1
-	case LightSync:
+	if tester.downloader.mode == LightSync {
 		blocks, receipts = 1, 1
 	}
 	if hs := len(tester.ownHeaders); hs != headers {
@@ -1105,7 +1119,9 @@ func testSyncProgress(t *testing.T, protocol int, mode SyncMode) {
 }
 
 func checkProgress(t *testing.T, d *Downloader, stage string, want ethereum.SyncProgress) {
+	// Mark this method as a helper to report errors at callsite, not in here
 	t.Helper()
+
 	p := d.Progress()
 	p.KnownStates, p.PulledStates = 0, 0
 	want.KnownStates, want.PulledStates = 0, 0
@@ -1363,3 +1379,78 @@ func (ftp *floodingTestPeer) RequestHeadersByNumber(from uint64, count, skip int
 	}
 	return nil
 }
+
+func TestRemoteHeaderRequestSpan(t *testing.T) {
+	testCases := []struct {
+		remoteHeight uint64
+		localHeight  uint64
+		expected     []int
+	}{
+		// Remote is way higher. We should ask for the remote head and go backwards
+		{1500, 1000,
+			[]int{1323, 1339, 1355, 1371, 1387, 1403, 1419, 1435, 1451, 1467, 1483, 1499},
+		},
+		{15000, 13006,
+			[]int{14823, 14839, 14855, 14871, 14887, 14903, 14919, 14935, 14951, 14967, 14983, 14999},
+		},
+		//Remote is pretty close to us. We don't have to fetch as many
+		{1200, 1150,
+			[]int{1149, 1154, 1159, 1164, 1169, 1174, 1179, 1184, 1189, 1194, 1199},
+		},
+		// Remote is equal to us (so on a fork with higher td)
+		// We should get the closest couple of ancestors
+		{1500, 1500,
+			[]int{1497, 1499},
+		},
+		// We're higher than the remote! Odd
+		{1000, 1500,
+			[]int{997, 999},
+		},
+		// Check some weird edgecases that it behaves somewhat rationally
+		{0, 1500,
+			[]int{0, 2},
+		},
+		{6000000, 0,
+			[]int{5999823, 5999839, 5999855, 5999871, 5999887, 5999903, 5999919, 5999935, 5999951, 5999967, 5999983, 5999999},
+		},
+		{0, 0,
+			[]int{0, 2},
+		},
+	}
+	reqs := func(from, count, span int) []int {
+		var r []int
+		num := from
+		for len(r) < count {
+			r = append(r, num)
+			num += span + 1
+		}
+		return r
+	}
+	for i, tt := range testCases {
+		from, count, span, max := calculateRequestSpan(tt.remoteHeight, tt.localHeight)
+		data := reqs(int(from), count, span)
+
+		if max != uint64(data[len(data)-1]) {
+			t.Errorf("test %d: wrong last value %d != %d", i, data[len(data)-1], max)
+		}
+		failed := false
+		if len(data) != len(tt.expected) {
+			failed = true
+			t.Errorf("test %d: length wrong, expected %d got %d", i, len(tt.expected), len(data))
+		} else {
+			for j, n := range data {
+				if n != tt.expected[j] {
+					failed = true
+					break
+				}
+			}
+		}
+		if failed {
+			res := strings.Replace(fmt.Sprint(data), " ", ",", -1)
+			exp := strings.Replace(fmt.Sprint(tt.expected), " ", ",", -1)
+			fmt.Printf("got: %v\n", res)
+			fmt.Printf("exp: %v\n", exp)
+			t.Errorf("test %d: wrong values", i)
+		}
+	}
+}
diff --git a/dex/downloader/queue.go b/dex/downloader/queue.go
index f3a36ec3c..ea847913f 100644
--- a/dex/downloader/queue.go
+++ b/dex/downloader/queue.go
@@ -325,7 +325,7 @@ func (q *queue) Schedule(headers []*types.HeaderWithGovState, from uint64) []*ty
 		}
 		// Make sure no duplicate requests are executed
 		if _, ok := q.blockTaskPool[hash]; ok {
-			log.Warn("Header  already scheduled for block fetch", "number", header.Number, "hash", hash)
+			log.Warn("Header already scheduled for block fetch", "number", header.Number, "hash", hash)
 			continue
 		}
 		if _, ok := q.receiptTaskPool[hash]; ok {
diff --git a/dex/downloader/statesync.go b/dex/downloader/statesync.go
index 49117abbb..1695ba19c 100644
--- a/dex/downloader/statesync.go
+++ b/dex/downloader/statesync.go
@@ -152,7 +152,7 @@ func (d *Downloader) runStateSync(s *stateSync) *stateSync {
 			finished = append(finished, req)
 			delete(active, pack.PeerId())
 
-			// Handle dropped peer connections:
+		// Handle dropped peer connections:
 		case p := <-peerDrop:
 			// Skip if no request is currently pending
 			req := active[p.id]
@@ -398,9 +398,8 @@ func (s *stateSync) fillTasks(n int, req *stateReq) {
 
 // process iterates over a batch of delivered state data, injecting each item
 // into a running state sync, re-queuing any items that were requested but not
-// delivered.
-// Returns whether the peer actually managed to deliver anything of value,
-// and any error that occurred
+// delivered. Returns whether the peer actually managed to deliver anything of
+// value, and any error that occurred
 func (s *stateSync) process(req *stateReq) (int, error) {
 	// Collect processing stats and update progress if valid data was received
 	duplicate, unexpected, successful := 0, 0, 0
@@ -412,14 +411,12 @@ func (s *stateSync) process(req *stateReq) (int, error) {
 	}(time.Now())
 
 	// Iterate over all the delivered data and inject one-by-one into the trie
-	progress := false
 	for _, blob := range req.response {
-		prog, hash, err := s.processNodeData(blob)
+		_, hash, err := s.processNodeData(blob)
 		switch err {
 		case nil:
 			s.numUncommitted++
 			s.bytesUncommitted += len(blob)
-			progress = progress || prog
 			successful++
 		case trie.ErrNotRequested:
 			unexpected++
diff --git a/dex/handler.go b/dex/handler.go
index ec5704103..20df41709 100644
--- a/dex/handler.go
+++ b/dex/handler.go
@@ -124,6 +124,8 @@ type ProtocolManager struct {
 	recordsCh  chan newRecordsEvent
 	recordsSub event.Subscription
 
+	whitelist map[uint64]common.Hash
+
 	// channels for fetcher, syncer, txsyncLoop
 	newPeerCh    chan *peer
 	txsyncCh     chan *txsync
@@ -161,7 +163,7 @@ type ProtocolManager struct {
 func NewProtocolManager(
 	config *params.ChainConfig, mode downloader.SyncMode, networkID uint64,
 	mux *event.TypeMux, txpool txPool, engine consensus.Engine,
-	blockchain *core.BlockChain, chaindb ethdb.Database,
+	blockchain *core.BlockChain, chaindb ethdb.Database, whitelist map[uint64]common.Hash,
 	isBlockProposer bool, gov governance, app dexconApp) (*ProtocolManager, error) {
 	tab := newNodeTable()
 	// Create the protocol manager with the base fields
@@ -175,6 +177,7 @@ func NewProtocolManager(
 		cache:              newCache(5120, dexDB.NewDatabase(chaindb)),
 		nextPullVote:       &sync.Map{},
 		chainconfig:        config,
+		whitelist:          whitelist,
 		newPeerCh:          make(chan *peer),
 		noMorePeers:        make(chan struct{}),
 		txsyncCh:           make(chan *txsync),
@@ -388,7 +391,13 @@ func (pm *ProtocolManager) handle(p *peer) error {
 	pm.syncTransactions(p)
 	pm.syncNodeRecords(p)
 
-	// main loop. handle incoming messages.
+	// If we have any explicit whitelist block hashes, request them
+	for number := range pm.whitelist {
+		if err := p.RequestWhitelistHeader(number); err != nil {
+			return err
+		}
+	}
+	// Handle incoming messages until the connection is torn down
 	for {
 		if err := pm.handleMsg(p); err != nil {
 			p.Log().Debug("Ethereum message handling failed", "err", err)
@@ -597,6 +606,14 @@ func (pm *ProtocolManager) handleMsg(p *peer) error {
 			if err != nil {
 				log.Debug("Failed to deliver headers", "err", err)
 			}
+		case whitelistReq:
+			if want, ok := pm.whitelist[data.Headers[0].Number.Uint64()]; ok {
+				if hash := data.Headers[0].Hash(); want != hash {
+					p.Log().Info("Whitelist mismatch, dropping peer", "number", data.Headers[0].Number.Uint64(), "hash", hash, "want", want)
+					return errors.New("whitelist block mismatch")
+				}
+				p.Log().Debug("Whitelist block verified", "number", data.Headers[0].Number.Uint64(), "hash", want)
+			}
 		default:
 			log.Debug("Got headers with unexpected flag", "flag", data.Flag)
 		}
@@ -791,7 +808,7 @@ func (pm *ProtocolManager) handleMsg(p *peer) error {
 			p.SetHead(trueHead, trueNumber)
 
 			// Schedule a sync if above ours. Note, this will not fire a sync for a gap of
-			// a singe block (as the true number is below the propagated block), however this
+			// a single block (as the true number is below the propagated block), however this
 			// scenario should easily be covered by the fetcher.
 			currentBlock := pm.blockchain.CurrentBlock()
 			if trueNumber > currentBlock.NumberU64() {
diff --git a/dex/helper_test.go b/dex/helper_test.go
index 4f5541052..1d903a907 100644
--- a/dex/helper_test.go
+++ b/dex/helper_test.go
@@ -133,7 +133,7 @@ func newTestProtocolManager(mode downloader.SyncMode, blocks int, generator func
 		notarySetFunc: func(uint64) (map[string]struct{}, error) { return nil, nil },
 	}
 
-	pm, err := NewProtocolManager(gspec.Config, mode, DefaultConfig.NetworkId, evmux, &testTxPool{added: newtx}, engine, blockchain, db, true, tgov, &testApp{})
+	pm, err := NewProtocolManager(gspec.Config, mode, DefaultConfig.NetworkId, evmux, &testTxPool{added: newtx}, engine, blockchain, db, nil, true, tgov, &testApp{})
 	if err != nil {
 		return nil, nil, err
 	}
diff --git a/dex/peer.go b/dex/peer.go
index 562cbfaca..0fa1ac61d 100644
--- a/dex/peer.go
+++ b/dex/peer.go
@@ -641,6 +641,11 @@ func (p *peer) RequestHeadersByNumber(origin uint64, amount int, skip int, rever
 	return p2p.Send(p.rw, GetBlockHeadersMsg, &getBlockHeadersData{Origin: hashOrNumber{Number: origin}, Amount: uint64(amount), Skip: uint64(skip), Reverse: reverse, WithGov: withGov, Flag: downloaderReq})
 }
 
+func (p *peer) RequestWhitelistHeader(origin uint64) error {
+	p.Log().Debug("Fetching whitelist header", "number", origin, "flag", whitelistReq)
+	return p2p.Send(p.rw, GetBlockHeadersMsg, &getBlockHeadersData{Origin: hashOrNumber{Number: origin}, Amount: 1, Skip: 0, Reverse: false, WithGov: false, Flag: whitelistReq})
+}
+
 func (p *peer) RequestGovStateByHash(hash common.Hash) error {
 	p.Log().Debug("Fetching one gov state", "hash", hash)
 	return p2p.Send(p.rw, GetGovStateMsg, hash)
diff --git a/dex/protocol.go b/dex/protocol.go
index 639925265..287bf0883 100644
--- a/dex/protocol.go
+++ b/dex/protocol.go
@@ -55,7 +55,7 @@ const (
 // ProtocolName is the official short name of the protocol used during capability negotiation.
 var ProtocolName = "dex"
 
-// ProtocolVersions are the upported versions of the eth protocol (first is primary).
+// ProtocolVersions are the supported versions of the eth protocol (first is primary).
 var ProtocolVersions = []uint{dex64}
 
 // ProtocolLengths are the number of implemented message corresponding to different protocol versions.
@@ -116,6 +116,7 @@ const (
 const (
 	fetcherReq = uint8(iota)
 	downloaderReq
+	whitelistReq
 )
 
 func (e errCode) String() string {