Statediff missing parent blocks automatically

Co-authored-by: Roy Crihfield <roy@manteia.ltd>

config.go

@@ -39,7 +39,7 @@ type Config struct {
 	// Whether to enable writing state diffs directly to track blockchain head
 	EnableWriteLoop bool
 	// The maximum number of blocks to backfill when tracking head.
-	BackfillMaxHeadGap uint64
+	BackfillMaxDepth uint64
 	// The maximum number of blocks behind the startup position to check for gaps.
 	BackfillCheckPastBlocks uint64
 	// Size of the worker pool
@@ -60,6 +60,19 @@ type Params struct {
 	watchedAddressesLeafPaths [][]byte
 }
 
+func (p *Params) Copy() Params {
+	ret := Params{
+		IncludeBlock:    p.IncludeBlock,
+		IncludeReceipts: p.IncludeReceipts,
+		IncludeTD:       p.IncludeTD,
+		IncludeCode:     p.IncludeCode,
+	}
+	ret.WatchedAddresses = make([]common.Address, len(p.WatchedAddresses))
+	copy(ret.WatchedAddresses, p.WatchedAddresses)
+
+	return ret
+}
+
 // ComputeWatchedAddressesLeafPaths populates a slice with paths (hex_encoding(Keccak256)) of each of the WatchedAddresses
 func (p *Params) ComputeWatchedAddressesLeafPaths() {
 	p.watchedAddressesLeafPaths = make([][]byte, len(p.WatchedAddresses))
@@ -74,6 +87,14 @@ type ParamsWithMutex struct {
 	sync.RWMutex
 }
 
+// CopyParams returns a defensive copy of the Params
+func (p *ParamsWithMutex) CopyParams() Params {
+	p.RLock()
+	defer p.RUnlock()
+
+	return p.Params.Copy()
+}
+
 // Args bundles the arguments for the state diff builder
 type Args struct {
 	OldStateRoot, NewStateRoot, BlockHash common.Hash

indexer/database/dump/indexer.go

@@ -187,6 +187,7 @@ func (sdi *StateDiffIndexer) processHeader(tx *BatchTx, header *types.Header, he
 		UnclesHash:      header.UncleHash.String(),
 		Timestamp:       header.Time,
 		Coinbase:        header.Coinbase.String(),
+		Canonical:       true,
 	}
 	_, err := fmt.Fprintf(sdi.dump, "%+v\r\n", mod)
 	return headerID, err

indexer/database/file/csv_writer.go

@@ -231,7 +231,8 @@ func (csw *CSVWriter) upsertHeaderCID(header models.HeaderModel) {
 	var values []interface{}
 	values = append(values, header.BlockNumber, header.BlockHash, header.ParentHash, header.CID,
 		header.TotalDifficulty, header.NodeIDs, header.Reward, header.StateRoot, header.TxRoot,
-		header.RctRoot, header.UnclesHash, header.Bloom, strconv.FormatUint(header.Timestamp, 10), header.Coinbase)
+		header.RctRoot, header.UnclesHash, header.Bloom, strconv.FormatUint(header.Timestamp, 10), header.Coinbase,
+		header.Canonical)
 	csw.rows <- tableRow{schema.TableHeader, values}
 	metrics.IndexerMetrics.BlocksCounter.Inc(1)
 }

indexer/database/file/indexer.go

@@ -244,6 +244,7 @@ func (sdi *StateDiffIndexer) processHeader(header *types.Header, headerNode ipld
 		UnclesHash:      header.UncleHash.String(),
 		Timestamp:       header.Time,
 		Coinbase:        header.Coinbase.String(),
+		Canonical:       true,
 	})
 	return headerID
 }

indexer/database/file/sql_writer.go

@@ -140,8 +140,8 @@ const (
 	ipldInsert = "INSERT INTO ipld.blocks (block_number, key, data) VALUES ('%s', '%s', '\\x%x');\n"
 
 	headerInsert = "INSERT INTO eth.header_cids (block_number, block_hash, parent_hash, cid, td, node_ids, reward, " +
-		"state_root, tx_root, receipt_root, uncles_hash, bloom, timestamp, coinbase) VALUES " +
+		"state_root, tx_root, receipt_root, uncles_hash, bloom, timestamp, coinbase, canonical) VALUES " +
-		"('%s', '%s', '%s', '%s', '%s', '%s', '%s', '%s', '%s', '%s', '%s', '\\x%x', %d, '%s');\n"
+		"('%s', '%s', '%s', '%s', '%s', '%s', '%s', '%s', '%s', '%s', '%s', '\\x%x', %d, '%s', %t);\n"
 
 	uncleInsert = "INSERT INTO eth.uncle_cids (block_number, block_hash, header_id, parent_hash, cid, reward, index) VALUES " +
 		"('%s', '%s', '%s', '%s', '%s', '%s', %d);\n"
@@ -189,7 +189,7 @@ func (sqw *SQLWriter) upsertIPLDNode(blockNumber string, i ipld.IPLD) {
 func (sqw *SQLWriter) upsertHeaderCID(header models.HeaderModel) {
 	stmt := fmt.Sprintf(headerInsert, header.BlockNumber, header.BlockHash, header.ParentHash, header.CID,
 		header.TotalDifficulty, formatPostgresStringArray(header.NodeIDs), header.Reward, header.StateRoot, header.TxRoot,
-		header.RctRoot, header.UnclesHash, header.Bloom, header.Timestamp, header.Coinbase)
+		header.RctRoot, header.UnclesHash, header.Bloom, header.Timestamp, header.Coinbase, header.Canonical)
 	sqw.stmts <- []byte(stmt)
 	metrics.IndexerMetrics.BlocksCounter.Inc(1)
 }

indexer/database/sql/indexer.go

@@ -257,6 +257,7 @@ func (sdi *StateDiffIndexer) processHeader(tx *BatchTx, header *types.Header, he
 		UnclesHash:      header.UncleHash.String(),
 		Timestamp:       header.Time,
 		Coinbase:        header.Coinbase.String(),
+		Canonical:       true,
 	})
 }
 

indexer/database/sql/interfaces.go

@@ -49,6 +49,7 @@ type Statements interface {
 	MaxHeaderStm() string
 	ExistsHeaderStm() string
 	InsertHeaderStm() string
+	SetCanonicalHeaderStm() string
 	InsertUncleStm() string
 	InsertTxStm() string
 	InsertRctStm() string

indexer/database/sql/postgres/database.go

@@ -62,6 +62,12 @@ func (db *DB) InsertHeaderStm() string {
 	return schema.TableHeader.ToInsertStatement(db.upsert)
 }
 
+// SetCanonicalHeaderStm satisfies the sql.Statements interface
+// Stm == Statement
+func (db *DB) SetCanonicalHeaderStm() string {
+	return fmt.Sprintf("UPDATE %s SET canonical = false WHERE block_number = $1::BIGINT AND block_hash <> $2::TEXT AND canonical = true", schema.TableHeader.Name)
+}
+
 // InsertUncleStm satisfies the sql.Statements interface
 func (db *DB) InsertUncleStm() string {
 	return schema.TableUncle.ToInsertStatement(db.upsert)

indexer/database/sql/writer.go

@@ -90,6 +90,7 @@ func (w *Writer) maxHeader() (*models.HeaderModel, error) {
 		&model.Bloom,
 		&model.Timestamp,
 		&model.Coinbase,
+		&model.Canonical,
 	)
 	model.BlockNumber = strconv.FormatUint(number, 10)
 	model.TotalDifficulty = strconv.FormatUint(td, 10)
@@ -118,11 +119,22 @@ func (w *Writer) upsertHeaderCID(tx Tx, header models.HeaderModel) error {
 		header.UnclesHash,
 		header.Bloom,
 		header.Timestamp,
-		header.Coinbase)
+		header.Coinbase,
+		header.Canonical,
+	)
 	if err != nil {
 		return insertError{"eth.header_cids", err, w.db.InsertHeaderStm(), header}
 	}
 	metrics.IndexerMetrics.BlocksCounter.Inc(1)
+
+	_, err = tx.Exec(w.db.Context(), w.db.SetCanonicalHeaderStm(),
+		header.BlockNumber,
+		header.BlockHash,
+	)
+	if err != nil {
+		return insertError{"eth.header_cids", err, w.db.SetCanonicalHeaderStm(), header}
+	}
+
 	return nil
 }
 

indexer/models/models.go

@@ -41,6 +41,7 @@ type HeaderModel struct {
 	Bloom           []byte         `db:"bloom"`
 	Timestamp       uint64         `db:"timestamp"`
 	Coinbase        string         `db:"coinbase"`
+	Canonical       bool           `db:"canonical"`
 }
 
 // UncleModel is the db model for eth.uncle_cids

indexer/shared/schema/schema.go

@@ -54,6 +54,7 @@ var TableHeader = Table{
 		{Name: "bloom", Type: Dbytea},
 		{Name: "timestamp", Type: Dnumeric},
 		{Name: "coinbase", Type: Dvarchar},
+		{Name: "canonical", Type: Dboolean},
 	},
 	UpsertClause: OnConflict("block_number", "block_hash").Set(
 		"parent_hash",
@@ -68,6 +69,7 @@ var TableHeader = Table{
 		"bloom",
 		"timestamp",
 		"coinbase",
+		"canonical",
 	)}
 
 var TableStateNode = Table{

main/flags.go

@@ -56,9 +56,9 @@ func init() {
 		"statediff.backfillcheckpastblocks", 7200,
 		"Number of blocks behind the startup statediff position to check (and fill) for gaps when head tracking",
 	)
-	Flags.Uint64Var(&config.BackfillMaxHeadGap,
+	Flags.Uint64Var(&config.BackfillMaxDepth,
-		"statediff.backfillmaxheadgap", 7200,
+		"statediff.backfillmaxdepth", 7200,
-		"Maximum gap between the startup statediff and startup head positions that can be backfilled",
+		"When statediffing head, the maximum number of missing parents that can be backfilled",
 	)
 
 	Flags.Var(&dbType,

service.go

@@ -83,7 +83,7 @@ type Service struct {
 	// Parameters to use in the service write loop, if enabled
 	writeLoopParams ParamsWithMutex
 	// Settings to use for backfilling state diffs (plugging gaps when tracking head)
-	backfillMaxHeadGap      uint64
+	backfillMaxDepth        uint64
 	backfillCheckPastBlocks uint64
 	// Size of the worker pool
 	numWorkers uint
@@ -133,6 +133,12 @@ type jobStatusSubscription struct {
 	quitChan   chan<- bool
 }
 
+// Utility type for showing the relative positions of the blockchain and the statediff indexer.
+type servicePosition struct {
+	chainBlockNumber   uint64
+	indexerBlockNumber uint64
+}
+
 // BlockCache caches the last block for safe access from different service loops
 type BlockCache struct {
 	sync.Mutex
@@ -141,7 +147,7 @@ type BlockCache struct {
 }
 
 type workerParams struct {
-	chainEventCh <-chan core.ChainEvent
+	blockCh <-chan *types.Block
 	wg      *sync.WaitGroup
 	id      uint
 }
@@ -172,7 +178,7 @@ func NewService(cfg Config, blockChain BlockChain, backend plugeth.Backend, inde
 		ShouldWaitForSync:       cfg.WaitForSync,
 		indexer:                 indexer,
 		enableWriteLoop:         cfg.EnableWriteLoop,
-		backfillMaxHeadGap:      cfg.BackfillMaxHeadGap,
+		backfillMaxDepth:        cfg.BackfillMaxDepth,
 		backfillCheckPastBlocks: cfg.BackfillCheckPastBlocks,
 		numWorkers:              workers,
 		maxRetry:                defaultRetryLimit,
@@ -213,16 +219,21 @@ func (lbc *BlockCache) getParentBlock(currentBlock *types.Block, bc BlockChain)
 
 // WriteLoop event loop for progressively processing and writing diffs directly to DB
 func (sds *Service) WriteLoop(chainEventCh chan core.ChainEvent) {
-	log.Info("Starting statediff write loop")
+	initialPos := sds.currentPosition()
+	log.Info(
+		"WriteLoop: initial positions",
+		"chain", initialPos.chainBlockNumber,
+		"indexer", initialPos.indexerBlockNumber,
+	)
 	log := log.New("context", "statediff writing")
 	sub := sds.BlockChain.SubscribeChainEvent(chainEventCh)
 	defer sub.Unsubscribe()
 
 	var wg sync.WaitGroup
-	chainEventFwd := make(chan core.ChainEvent, chainEventChanSize)
+	blockFwd := make(chan *types.Block, chainEventChanSize)
 	defer func() {
 		log.Info("Quitting")
-		close(chainEventFwd)
+		close(blockFwd)
 	}()
 
 	wg.Add(1)
@@ -232,16 +243,30 @@ func (sds *Service) WriteLoop(chainEventCh chan core.ChainEvent) {
 			select {
 			case event := <-chainEventCh:
 				// First process metrics for chain events, then forward to workers
-				lastHeight := defaultStatediffMetrics.lastEventHeight.Value()
+				lastHeight := uint64(defaultStatediffMetrics.lastEventHeight.Value())
+				if lastHeight == 0 {
+					lastHeight = initialPos.indexerBlockNumber
+				}
 				block := event.Block
 				log.Debug("Chain event received", "number", block.Number(), "hash", event.Hash)
-				nextHeight := int64(block.Number().Uint64())
+				nextHeight := block.Number().Uint64()
-				if nextHeight-lastHeight != 1 {
+				if nextHeight > lastHeight {
-					log.Warn("Received block out-of-order", "next", nextHeight, "last", lastHeight)
+					distance := nextHeight - lastHeight
+					if distance == 1 {
+						log.Info("WriteLoop: received expected block",
+							"block number", nextHeight, "last number", lastHeight)
+					} else {
+						log.Warn("WriteLoop: received unexpected block from the future",
+							"block number", nextHeight, "last number", lastHeight)
+					}
+					blockFwd <- block
+					defaultStatediffMetrics.lastEventHeight.Update(int64(nextHeight))
+				} else {
+					log.Warn("WriteLoop: received unexpected block from the past",
+						"block number", nextHeight, "last number", lastHeight)
+					blockFwd <- block
 				}
-				defaultStatediffMetrics.lastEventHeight.Update(nextHeight)
 				defaultStatediffMetrics.writeLoopChannelLen.Update(int64(len(chainEventCh)))
-				chainEventFwd <- event
 			case err := <-sub.Err():
 				if err != nil {
 					log.Error("Error from subscription", "error", err)
@@ -255,7 +280,7 @@ func (sds *Service) WriteLoop(chainEventCh chan core.ChainEvent) {
 	}()
 	wg.Add(int(sds.numWorkers))
 	for worker := uint(0); worker < sds.numWorkers; worker++ {
-		params := workerParams{chainEventCh: chainEventFwd, wg: &wg, id: worker}
+		params := workerParams{blockCh: blockFwd, wg: &wg, id: worker}
 		go sds.writeLoopWorker(params)
 	}
 	wg.Wait()
@@ -264,10 +289,8 @@ func (sds *Service) WriteLoop(chainEventCh chan core.ChainEvent) {
 func (sds *Service) writeGenesisStateDiff(currBlock *types.Block, logger log.Logger) {
 	// For genesis block we need to return the entire state trie hence we diff it with an empty trie.
 	log.Info("Writing genesis state diff", "number", genesisBlockNumber)
-	sds.writeLoopParams.RLock()
-	defer sds.writeLoopParams.RUnlock()
 
-	err := sds.writeStateDiffWithRetry(currBlock, common.Hash{}, sds.writeLoopParams.Params)
+	err := sds.writeStateDiffWithRetry(currBlock, common.Hash{}, sds.writeLoopParams.CopyParams())
 	if err != nil {
 		log.Error("failed to write state diff", "number",
 			genesisBlockNumber, "error", err)
@@ -279,27 +302,71 @@ func (sds *Service) writeGenesisStateDiff(currBlock *types.Block, logger log.Log
 func (sds *Service) writeLoopWorker(params workerParams) {
 	log := log.New("context", "statediff writing", "worker", params.id)
 	defer params.wg.Done()
-	for {
+
-		select {
+	// statediffs the indicated block, and while maxBackfill > 0, backfills missing parent blocks.
-		case event := <-params.chainEventCh:
+	var writeBlockWithParents func(*types.Block, uint64, Params) error
-			block := event.Block
+	writeBlockWithParents = func(block *types.Block, maxBackfill uint64, writeParams Params) error {
-			parent := sds.BlockCache.getParentBlock(block, sds.BlockChain)
+		parentBlock := sds.BlockCache.getParentBlock(block, sds.BlockChain)
-			if parent == nil {
+		if parentBlock == nil {
-				log.Error("Parent block is nil, skipping this block", "number", block.Number())
+			return errors.New("Parent block is nil, skipping this block")
-				continue
 		}
 
+		parentIsGenesis := parentBlock.Number().Uint64() == genesisBlockNumber
+
 		// chainEvent streams block from block 1, but we also need to include data from the genesis block.
-			if parent.Number().Uint64() == genesisBlockNumber {
+		if parentIsGenesis {
-				sds.writeGenesisStateDiff(parent, log)
+			sds.writeGenesisStateDiff(parentBlock, log)
 		}
 
 		log.Info("Writing state diff", "number", block.Number())
-			sds.writeLoopParams.RLock()
+		err := sds.writeStateDiffWithRetry(block, parentBlock.Root(), writeParams)
-			err := sds.writeStateDiffWithRetry(block, parent.Root(), sds.writeLoopParams.Params)
-			sds.writeLoopParams.RUnlock()
 		if err != nil {
-				log.Error("failed to write state diff",
+			return err
+		}
+
+		if parentIsGenesis {
+			return nil
+		}
+		// We do this _after_ indexing the requested block.  This makes sure that if a child of ours arrives for
+		// statediffing while we are still working on missing ancestors, its regress stops at us, and only we
+		// continue working backward.
+		parentIndexed, err := sds.indexedOrInProgress(parentBlock)
+		if err != nil {
+			log.Error("Error checking for indexing status of parent block.",
+				"number", block.Number(), "hash", block.Hash(),
+				"parent number", parentBlock.Number(), "parent hash", parentBlock.Hash(),
+				"error", err)
+			return err
+		}
+		if parentIndexed {
+			return nil
+		}
+		if maxBackfill > 0 {
+			log.Info("Parent block not indexed. Indexing now.",
+				"number", block.Number(), "hash", block.Hash(),
+				"parent number", parentBlock.Number(), "parent hash", parentBlock.Hash())
+			err = writeBlockWithParents(parentBlock, maxBackfill-1, writeParams)
+			if err != nil {
+				log.Error("Error indexing parent block.",
+					"number", block.Number(), "hash", block.Hash(),
+					"parent number", parentBlock.Number(), "parent hash", parentBlock.Hash(),
+					"error", err)
+			}
+		} else {
+			log.Error("Parent block not indexed but max backfill depth exceeded. Index MUST be corrected manually.",
+				"number", block.Number(), "hash", block.Hash(),
+				"parent number", parentBlock.Number(), "parent hash", parentBlock.Hash())
+		}
+		return nil
+	}
+
+	for {
+		select {
+		case block := <-params.blockCh:
+			log.Debug("Block received", "number", block.Number())
+			err := writeBlockWithParents(block, sds.backfillMaxDepth, sds.writeLoopParams.CopyParams())
+			if err != nil {
+				log.Error("Error processing statediff",
 					"number", block.Number(),
 					"hash", block.Hash(),
 					"error", err)
@@ -681,6 +748,23 @@ func (sds *Service) WriteStateDiffFor(blockHash common.Hash, params Params) erro
 	return sds.writeStateDiffWithRetry(currentBlock, parentRoot, sds.maybeReplaceWatchedAddresses(params))
 }
 
+// indexedOrInProgress returns true if the block has already been statediffed or is in progress, else false.
+func (sds *Service) indexedOrInProgress(block *types.Block) (bool, error) {
+	if sds.statediffInProgress(block) {
+		return true, nil
+	}
+	return sds.indexer.HasBlock(block.Hash(), block.NumberU64())
+}
+
+// statediffInProgress returns true if statediffing is currently in progress for the block, else false.
+func (sds *Service) statediffInProgress(block *types.Block) bool {
+	sds.currentBlocksMutex.Lock()
+	defer sds.currentBlocksMutex.Unlock()
+
+	key := fmt.Sprintf("%s,%d", block.Hash().Hex(), block.NumberU64())
+	return sds.currentBlocks[key]
+}
+
 // Claim exclusive access for state diffing the specified block.
 // Returns true and a function to release access if successful, else false, nil.
 func (sds *Service) claimExclusiveAccess(block *types.Block) (bool, func()) {
@@ -928,110 +1012,43 @@ func MapWatchAddressArgsToAddresses(args []types2.WatchAddressArg) ([]common.Add
 
 // Backfill is executed on startup to make sure there are no gaps in the recent past when tracking head.
 func (sds *Service) Backfill() {
-	chainBlock := sds.BlockChain.CurrentBlock()
+	pos := sds.currentPosition()
-	if nil == chainBlock {
+	if pos.chainBlockNumber == 0 {
-		log.Info("Backfill: No previous chain block, nothing to backfill.")
-		return
-	}
-
-	chainBlockNumber := chainBlock.Number.Uint64()
-	if chainBlockNumber == 0 {
 		log.Info("Backfill: At start of chain, nothing to backfill.")
 		return
 	}
 
-	indexerBlock, err := sds.indexer.CurrentBlock()
-	if nil == indexerBlock {
-		log.Info("Backfill: No previous indexer block, nothing to backfill.")
-		return
-	}
-	if nil != err {
-		log.Error("Backfill error", err)
-		return
-	}
-
-	indexerBlockNumber, err := strconv.ParseUint(indexerBlock.BlockNumber, 10, 64)
-	if nil != err {
-		log.Error("Backfill error", err)
-		return
-	}
-
-	headGap := chainBlockNumber - indexerBlockNumber
 	log.Info(
 		"Backfill: initial positions",
-		"chain", chainBlockNumber,
+		"chain", pos.chainBlockNumber,
-		"indexer", indexerBlockNumber,
+		"indexer", pos.indexerBlockNumber,
-		"headGap", headGap,
 	)
 
-	if sds.backfillMaxHeadGap > 0 && headGap > 0 {
-		if headGap < sds.backfillMaxHeadGap {
-			sds.backfillHeadGap(indexerBlockNumber, chainBlockNumber)
-			log.Info("Backfill: all workers done filling headGap.")
-		} else {
-			log.Error("Backfill: headGap too large to fill.")
-		}
-	}
-
 	if sds.backfillCheckPastBlocks > 0 {
 		var gapCheckBeginNumber uint64 = 0
-		if indexerBlockNumber > sds.backfillCheckPastBlocks {
+		if pos.indexerBlockNumber > sds.backfillCheckPastBlocks {
-			gapCheckBeginNumber = indexerBlockNumber - sds.backfillCheckPastBlocks
+			gapCheckBeginNumber = pos.indexerBlockNumber - sds.backfillCheckPastBlocks
 		}
-		blockGaps, err := sds.indexer.DetectGaps(gapCheckBeginNumber, chainBlockNumber)
+		blockGaps, err := sds.indexer.DetectGaps(gapCheckBeginNumber, pos.chainBlockNumber)
 		if nil != err {
-			log.Error("Backfill error", err)
+			log.Error("Backfill error", "error", err)
 			return
 		}
 
 		if nil != blockGaps && len(blockGaps) > 0 {
 			gapsMsg, _ := json.Marshal(blockGaps)
-			log.Info("Backfill: detected gaps in range", "begin", gapCheckBeginNumber, "end", chainBlockNumber, "gaps", string(gapsMsg))
+			log.Info("Backfill: detected gaps in range",
+				"begin", gapCheckBeginNumber, "end", pos.chainBlockNumber, "gaps", string(gapsMsg))
 			sds.backfillDetectedGaps(blockGaps)
-			log.Info("Backfill: done processing detected gaps in range", "begin", gapCheckBeginNumber, "end", chainBlockNumber, "gaps", string(gapsMsg))
+			log.Info("Backfill: done processing detected gaps in range",
+				"begin", gapCheckBeginNumber, "end", pos.chainBlockNumber, "gaps", string(gapsMsg))
 		} else {
-			log.Info("Backfill: no gaps detected in range", "begin", gapCheckBeginNumber, "end", chainBlockNumber)
+			log.Info("Backfill: no gaps detected in range",
+				"begin", gapCheckBeginNumber, "end", pos.chainBlockNumber)
 		}
 	}
 }
 
-// backfillHeadGap fills in any gap between the statediff position and the chain position at startup.
-// A gap can be created in this way if there is some problem in statediffing (eg, DB connectivity is lost,
-// while the chain keeps syncing), if the process is terminated with a statediff in-flight, etc.
-func (sds *Service) backfillHeadGap(indexerBlockNumber uint64, chainBlockNumber uint64) {
-	var ch = make(chan uint64)
-	var wg sync.WaitGroup
-	for i := uint(0); i < sds.numWorkers; i++ {
-		wg.Add(1)
-		go func(w uint) {
-			defer wg.Done()
-			for {
-				select {
-				case num, ok := <-ch:
-					if !ok {
-						log.Info("Backfill: headGap done", "worker", w)
-						return
-					}
-					log.Info("Backfill: backfilling head gap", "block", num, "worker", w)
-					err := sds.writeStateDiffAt(num, sds.writeLoopParams.Params)
-					if err != nil {
-						log.Error("Backfill error", err)
-					}
-				case <-sds.QuitChan:
-					log.Info("Backfill: quitting before finish", "worker", w)
-					return
-				}
-			}
-		}(i)
-	}
-
-	for bn := indexerBlockNumber + 1; bn <= chainBlockNumber; bn++ {
-		ch <- bn
-	}
-	close(ch)
-	wg.Wait()
-}
-
 // backfillDetectedGaps fills gaps which have occurred in the recent past.  These gaps can happen because of
 // transient errors, such as DB errors that are later corrected (so head statediffing continues, but with a hole)
 // a missed ChainEvent (happens sometimes when debugging), or if the process is terminated when an earlier block
@@ -1051,7 +1068,7 @@ func (sds *Service) backfillDetectedGaps(blockGaps []*interfaces.BlockGap) {
 						return
 					}
 					log.Info("Backfill: backfilling detected gap", "block", num, "worker", w)
-					err := sds.writeStateDiffAt(num, sds.writeLoopParams.Params)
+					err := sds.writeStateDiffAt(num, sds.writeLoopParams.CopyParams())
 					if err != nil {
 						log.Error("Backfill error: ", err)
 					}
@@ -1071,3 +1088,24 @@ func (sds *Service) backfillDetectedGaps(blockGaps []*interfaces.BlockGap) {
 	close(ch)
 	wg.Wait()
 }
+
+// currentPosition returns the current block height for both the BlockChain and the statediff indexer.
+func (sds *Service) currentPosition() servicePosition {
+	ret := servicePosition{}
+	chainBlock := sds.BlockChain.CurrentBlock()
+	if nil != chainBlock {
+		ret.chainBlockNumber = chainBlock.Number.Uint64()
+	}
+
+	indexerBlock, _ := sds.indexer.CurrentBlock()
+	if nil != indexerBlock {
+		indexerBlockNumber, err := strconv.ParseUint(indexerBlock.BlockNumber, 10, 64)
+		if nil == err {
+			ret.indexerBlockNumber = indexerBlockNumber
+		} else {
+			log.Error("Error parsing indexer block number", "block", indexerBlock.BlockNumber)
+		}
+	}
+
+	return ret
+}