ledger/tracker.go

// Copyright (C) 2019-2022 Algorand, Inc.
// This file is part of go-algorand
//
// go-algorand is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as
// published by the Free Software Foundation, either version 3 of the
// License, or (at your option) any later version.
//
// go-algorand is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with go-algorand.  If not, see <https://www.gnu.org/licenses/>.

package ledger

import (
	"context"
	"database/sql"
	"errors"
	"fmt"
	"reflect"
	"sync"
	"time"

	"github.com/algorand/go-algorand/config"
	"github.com/algorand/go-algorand/crypto"
	"github.com/algorand/go-algorand/data/basics"
	"github.com/algorand/go-algorand/data/bookkeeping"
	"github.com/algorand/go-algorand/ledger/internal"
	"github.com/algorand/go-algorand/ledger/ledgercore"
	"github.com/algorand/go-algorand/logging"
	"github.com/algorand/go-algorand/logging/telemetryspec"
	"github.com/algorand/go-algorand/util/db"
	"github.com/algorand/go-deadlock"
)

// ledgerTracker defines part of the API for any state machine that
// tracks the ledger's blockchain.  In addition to the API below,
// each ledgerTracker provides a tracker-specific read-only API
// (e.g., querying the balance of an account).
//
// A tracker's read-only API must be indexed by rounds, and the
// tracker must be prepared to answer queries for rounds until a
// subsequent call to committedUpTo().
//
// For example, the rewards AVL tree must be prepared to answer
// queries for old rounds, even if the tree has moved on in response
// to newBlock() calls.  It should do so by remembering the precise
// answer for old rounds, until committedUpTo() allows it to GC
// those old answers.
//
// The ledger provides a RWMutex to ensure that the tracker is invoked
// with at most one modification API call (below), OR zero modification
// calls and any number of read-only calls.  If internally the tracker
// modifies state in response to read-only calls, it is the tracker's
// responsibility to ensure thread-safety.
type ledgerTracker interface {
	// loadFromDisk loads the state of a tracker from persistent
	// storage.  The ledger argument allows loadFromDisk to load
	// blocks from the database, or access its own state.  The
	// ledgerForTracker interface abstracts away the details of
	// ledger internals so that individual trackers can be tested
	// in isolation. The provided round number represents the
	// current accounts storage round number.
	loadFromDisk(ledgerForTracker, basics.Round) error

	// newBlock informs the tracker of a new block along with
	// a given ledgercore.StateDelta as produced by BlockEvaluator.
	newBlock(blk bookkeeping.Block, delta ledgercore.StateDelta)

	// committedUpTo informs the tracker that the block database has
	// committed all blocks up to and including rnd to persistent
	// storage.  This can allow the tracker
	// to garbage-collect state that will not be needed.
	//
	// committedUpTo() returns the round number of the earliest
	// block that this tracker needs to be stored in the block
	// database for subsequent calls to loadFromDisk().
	// All blocks with round numbers before that may be deleted to
	// save space, and the tracker is expected to still function
	// after a restart and a call to loadFromDisk().
	// For example, returning 0 means that no blocks can be deleted.
	// Separetly, the method returns the lookback that is being
	// maintained by the tracker.
	committedUpTo(basics.Round) (minRound, lookback basics.Round)

	// produceCommittingTask prepares a deferredCommitRange; Preparing a deferredCommitRange is a joint
	// effort, and all the trackers contribute to that effort. All the trackers are being handed a
	// pointer to the deferredCommitRange, and have the ability to either modify it, or return a
	// nil. If nil is returned, the commit would be skipped.
	// The contract:
	// offset must not be greater than the received dcr.offset value of non zero
	// oldBase must not be modifed if non zero
	produceCommittingTask(committedRound basics.Round, dbRound basics.Round, dcr *deferredCommitRange) *deferredCommitRange

	// prepareCommit, commitRound and postCommit are called when it is time to commit tracker's data.
	// If an error returned the process is aborted.

	// prepareCommit aligns the data structures stored in the deferredCommitContext with the current
	// state of the tracker. It allows the tracker to decide what data is going to be persisted
	// on the coming commitRound.
	prepareCommit(*deferredCommitContext) error
	// commitRound is called for each of the trackers after a deferredCommitContext was agreed upon
	// by all the prepareCommit calls. The commitRound is being executed within a single transactional
	// context, and so, if any of the tracker's commitRound calls fails, the transaction is rolled back.
	commitRound(context.Context, *sql.Tx, *deferredCommitContext) error
	// postCommit is called only on a successful commitRound. In that case, each of the trackers have
	// the chance to update it's internal data structures, knowing that the given deferredCommitContext
	// has completed. An optional context is provided for long-running operations.
	postCommit(context.Context, *deferredCommitContext)

	// postCommitUnlocked is called only on a successful commitRound. In that case, each of the trackers have
	// the chance to make changes that aren't state-dependent.
	// An optional context is provided for long-running operations.
	postCommitUnlocked(context.Context, *deferredCommitContext)

	// handleUnorderedCommit is a special method for handling deferred commits that are out of order.
	// Tracker might update own state in this case. For example, account updates tracker cancels
	// scheduled catchpoint writing that deferred commit.
	handleUnorderedCommit(*deferredCommitContext)

	// close terminates the tracker, reclaiming any resources
	// like open database connections or goroutines.  close may
	// be called even if loadFromDisk() is not called or does
	// not succeed.
	close()
}

// ledgerForTracker defines the part of the ledger that a tracker can
// access.  This is particularly useful for testing trackers in isolation.
type ledgerForTracker interface {
	trackerDB() db.Pair
	blockDB() db.Pair
	trackerLog() logging.Logger
	trackerEvalVerified(bookkeeping.Block, internal.LedgerForEvaluator) (ledgercore.StateDelta, error)

	Latest() basics.Round
	Block(basics.Round) (bookkeeping.Block, error)
	BlockHdr(basics.Round) (bookkeeping.BlockHeader, error)
	GenesisHash() crypto.Digest
	GenesisProto() config.ConsensusParams
	GenesisAccounts() map[basics.Address]basics.AccountData
}

type trackerRegistry struct {
	trackers []ledgerTracker
	// the accts has some exceptional usages in the tracker registry.
	accts *accountUpdates

	// ctx is the context for the committing go-routine.
	ctx context.Context
	// ctxCancel is the canceling function for canceling the committing go-routine ( i.e. signaling the committing go-routine that it's time to abort )
	ctxCancel context.CancelFunc

	// deferredCommits is the channel of pending deferred commits
	deferredCommits chan *deferredCommitContext

	// commitSyncerClosed is the blocking channel for synchronizing closing the commitSyncer goroutine. Once it's closed, the
	// commitSyncer can be assumed to have aborted.
	commitSyncerClosed chan struct{}

	// accountsWriting provides synchronization around the background writing of account balances.
	accountsWriting sync.WaitGroup

	// dbRound is always exactly accountsRound(),
	// cached to avoid SQL queries.
	dbRound basics.Round

	dbs db.Pair
	log logging.Logger

	// the synchronous mode that would be used for the account database.
	synchronousMode db.SynchronousMode

	// the synchronous mode that would be used while the accounts database is being rebuilt.
	accountsRebuildSynchronousMode db.SynchronousMode

	mu deadlock.RWMutex

	// lastFlushTime is the time we last flushed updates to
	// the accounts DB (bumping dbRound).
	lastFlushTime time.Time
}

// deferredCommitRange is used during the calls to produceCommittingTask, and used as a data structure
// to syncronize the various trackers and create a uniformity around which rounds need to be persisted
// next.
type deferredCommitRange struct {
	offset   uint64
	oldBase  basics.Round
	lookback basics.Round

	// pendingDeltas is the number of accounts that were modified within this commit context.
	// note that in this number we might have the same account being modified several times.
	pendingDeltas int

	isCatchpointRound bool

	// catchpointWriting is a pointer to a variable with the same name in the catchpointTracker.
	// it's used in order to reset the catchpointWriting flag from the acctupdates's
	// prepareCommit/commitRound ( which is called before the corresponding catchpoint tracker method )
	catchpointWriting *int32

	// enableGeneratingCatchpointFiles controls whether the node produces catchpoint files or not.
	enableGeneratingCatchpointFiles bool
}

// deferredCommitContext is used in order to syncornize the persistence of a given deferredCommitRange.
// prepareCommit, commitRound and postCommit are all using it to exchange data.
type deferredCommitContext struct {
	deferredCommitRange

	newBase   basics.Round
	flushTime time.Time

	genesisProto config.ConsensusParams

	deltas                 []ledgercore.AccountDeltas
	roundTotals            ledgercore.AccountTotals
	compactAccountDeltas   compactAccountDeltas
	compactResourcesDeltas compactResourcesDeltas
	compactCreatableDeltas map[basics.CreatableIndex]ledgercore.ModifiedCreatable

	updatedPersistedAccounts  []persistedAccountData
	updatedPersistedResources map[basics.Address][]persistedResourcesData

	committedRoundDigest     crypto.Digest
	trieBalancesHash         crypto.Digest
	updatingBalancesDuration time.Duration
	catchpointLabel          string

	stats       telemetryspec.AccountsUpdateMetrics
	updateStats bool
}

var errMissingAccountUpdateTracker = errors.New("initializeTrackerCaches : called without a valid accounts update tracker")

func (tr *trackerRegistry) initialize(l ledgerForTracker, trackers []ledgerTracker, cfg config.Local) (err error) {
	tr.dbs = l.trackerDB()
	tr.log = l.trackerLog()

	err = tr.dbs.Rdb.Atomic(func(ctx context.Context, tx *sql.Tx) (err error) {
		tr.dbRound, err = accountsRound(tx)
		return err
	})

	if err != nil {
		return err
	}

	tr.ctx, tr.ctxCancel = context.WithCancel(context.Background())
	tr.deferredCommits = make(chan *deferredCommitContext, 1)
	tr.commitSyncerClosed = make(chan struct{})
	tr.synchronousMode = db.SynchronousMode(cfg.LedgerSynchronousMode)
	tr.accountsRebuildSynchronousMode = db.SynchronousMode(cfg.AccountsRebuildSynchronousMode)
	go tr.commitSyncer(tr.deferredCommits)

	tr.trackers = append([]ledgerTracker{}, trackers...)

	for _, tracker := range tr.trackers {
		if accts, ok := tracker.(*accountUpdates); ok {
			tr.accts = accts
			break
		}
	}
	return
}

func (tr *trackerRegistry) loadFromDisk(l ledgerForTracker) error {
	tr.mu.RLock()
	dbRound := tr.dbRound
	tr.mu.RUnlock()

	for _, lt := range tr.trackers {
		err := lt.loadFromDisk(l, dbRound)
		if err != nil {
			// find the tracker name.
			trackerName := reflect.TypeOf(lt).String()
			return fmt.Errorf("tracker %s failed to loadFromDisk : %w", trackerName, err)
		}
	}

	err := tr.initializeTrackerCaches(l)
	if err != nil {
		return fmt.Errorf("initializeTrackerCaches failed : %w", err)
	}

	// the votes have a special dependency on the account updates, so we need to initialize these separetly.
	tr.accts.voters = &votersTracker{}
	err = tr.accts.voters.loadFromDisk(l, tr.accts)
	if err != nil {
		err = fmt.Errorf("voters tracker failed to loadFromDisk : %w", err)
	}
	return err
}

func (tr *trackerRegistry) newBlock(blk bookkeeping.Block, delta ledgercore.StateDelta) {
	for _, lt := range tr.trackers {
		lt.newBlock(blk, delta)
	}
}

func (tr *trackerRegistry) committedUpTo(rnd basics.Round) basics.Round {
	minBlock := rnd
	maxLookback := basics.Round(0)
	for _, lt := range tr.trackers {
		retainRound, lookback := lt.committedUpTo(rnd)
		if retainRound < minBlock {
			minBlock = retainRound
		}
		if lookback > maxLookback {
			maxLookback = lookback
		}
	}

	tr.scheduleCommit(rnd, maxLookback)

	return minBlock
}

func (tr *trackerRegistry) scheduleCommit(blockqRound, maxLookback basics.Round) {
	dcc := &deferredCommitContext{
		deferredCommitRange: deferredCommitRange{
			lookback: maxLookback,
		},
	}
	cdr := &dcc.deferredCommitRange

	tr.mu.RLock()
	dbRound := tr.dbRound
	for _, lt := range tr.trackers {
		base := cdr.oldBase
		offset := cdr.offset
		cdr = lt.produceCommittingTask(blockqRound, dbRound, cdr)
		if cdr == nil {
			break
		}
		if offset > 0 && cdr.offset > offset {
			tr.log.Warnf("tracker %T produced offset %d but expected not greater than %d, dbRound %d, latestRound %d", lt, cdr.offset, offset, dbRound, blockqRound)
		}
		if base > 0 && base != cdr.oldBase {
			tr.log.Warnf("tracker %T modified oldBase %d that expected to be %d, dbRound %d, latestRound %d", lt, cdr.oldBase, base, dbRound, blockqRound)
		}
	}
	if cdr != nil {
		dcc.deferredCommitRange = *cdr
	} else {
		dcc = nil
	}
	// If we recently flushed, wait to aggregate some more blocks.
	// ( unless we're creating a catchpoint, in which case we want to flush it right away
	//   so that all the instances of the catchpoint would contain exactly the same data )
	flushTime := time.Now()
	if dcc != nil && !flushTime.After(tr.lastFlushTime.Add(balancesFlushInterval)) && !dcc.isCatchpointRound && dcc.pendingDeltas < pendingDeltasFlushThreshold {
		dcc = nil
	}
	tr.mu.RUnlock()

	if dcc != nil {
		tr.accountsWriting.Add(1)
		tr.deferredCommits <- dcc
	}
}

// waitAccountsWriting waits for all the pending ( or current ) account writing to be completed.
func (tr *trackerRegistry) waitAccountsWriting() {
	tr.accountsWriting.Wait()
}

func (tr *trackerRegistry) close() {
	if tr.ctxCancel != nil {
		tr.ctxCancel()
	}

	// close() is called from reloadLedger() when and trackerRegistry is not initialized yet
	if tr.commitSyncerClosed != nil {
		tr.waitAccountsWriting()
		// this would block until the commitSyncerClosed channel get closed.
		<-tr.commitSyncerClosed
	}

	for _, lt := range tr.trackers {
		lt.close()
	}
	tr.trackers = nil
	tr.accts = nil
}

// commitSyncer is the syncer go-routine function which perform the database updates. Internally, it dequeues deferredCommits and
// send the tasks to commitRound for completing the operation.
func (tr *trackerRegistry) commitSyncer(deferredCommits chan *deferredCommitContext) {
	defer close(tr.commitSyncerClosed)
	for {
		select {
		case commit, ok := <-deferredCommits:
			if !ok {
				return
			}
			tr.commitRound(commit)
		case <-tr.ctx.Done():
			// drain the pending commits queue:
			drained := false
			for !drained {
				select {
				case <-deferredCommits:
					tr.accountsWriting.Done()
				default:
					drained = true
				}
			}
			return
		}
	}
}

// commitRound commits the given deferredCommitContext via the trackers.
func (tr *trackerRegistry) commitRound(dcc *deferredCommitContext) {
	defer tr.accountsWriting.Done()
	tr.mu.RLock()

	offset := dcc.offset
	dbRound := dcc.oldBase

	// we can exit right away, as this is the result of mis-ordered call to committedUpTo.
	if tr.dbRound < dbRound || offset < uint64(tr.dbRound-dbRound) {
		tr.log.Warnf("out of order deferred commit: offset %d, dbRound %d but current tracker DB round is %d", offset, dbRound, tr.dbRound)
		for _, lt := range tr.trackers {
			lt.handleUnorderedCommit(dcc)
		}
		tr.mu.RUnlock()
		return
	}

	// adjust the offset according to what happened meanwhile..
	offset -= uint64(tr.dbRound - dbRound)

	// if this iteration need to flush out zero rounds, just return right away.
	// this usecase can happen when two subsequent calls to committedUpTo concludes that the same rounds range need to be
	// flush, without the commitRound have a chance of committing these rounds.
	if offset == 0 {
		tr.mu.RUnlock()
		return
	}

	dbRound = tr.dbRound
	newBase := basics.Round(offset) + dbRound

	dcc.offset = offset
	dcc.oldBase = dbRound
	dcc.newBase = newBase
	dcc.flushTime = time.Now()

	for _, lt := range tr.trackers {
		err := lt.prepareCommit(dcc)
		if err != nil {
			tr.log.Errorf(err.Error())
			tr.mu.RUnlock()
			return
		}
	}
	tr.mu.RUnlock()

	start := time.Now()
	ledgerCommitroundCount.Inc(nil)
	err := tr.dbs.Wdb.Atomic(func(ctx context.Context, tx *sql.Tx) (err error) {
		for _, lt := range tr.trackers {
			err0 := lt.commitRound(ctx, tx, dcc)
			if err0 != nil {
				return err0
			}
		}

		err = updateAccountsRound(tx, dbRound+basics.Round(offset))
		if err != nil {
			return err
		}

		return nil
	})
	ledgerCommitroundMicros.AddMicrosecondsSince(start, nil)

	if err != nil {
		tr.log.Warnf("unable to advance tracker db snapshot (%d-%d): %v", dbRound, dbRound+basics.Round(offset), err)
		return
	}

	tr.mu.Lock()
	tr.dbRound = newBase
	for _, lt := range tr.trackers {
		lt.postCommit(tr.ctx, dcc)
	}
	tr.lastFlushTime = dcc.flushTime
	tr.mu.Unlock()

	for _, lt := range tr.trackers {
		lt.postCommitUnlocked(tr.ctx, dcc)
	}

}

// initializeTrackerCaches fills up the accountUpdates cache with the most recent ~320 blocks ( on normal execution ).
// the method also support balances recovery in cases where the difference between the lastBalancesRound and the lastestBlockRound
// is far greater than 320; in these cases, it would flush to disk periodically in order to avoid high memory consumption.
func (tr *trackerRegistry) initializeTrackerCaches(l ledgerForTracker) (err error) {
	lastestBlockRound := l.Latest()
	lastBalancesRound := tr.dbRound

	var blk bookkeeping.Block
	var delta ledgercore.StateDelta

	if tr.accts == nil {
		return errMissingAccountUpdateTracker
	}

	accLedgerEval := accountUpdatesLedgerEvaluator{
		au: tr.accts,
	}

	if lastBalancesRound < lastestBlockRound {
		accLedgerEval.prevHeader, err = l.BlockHdr(lastBalancesRound)
		if err != nil {
			return fmt.Errorf("unable to load block header %d : %w", lastBalancesRound, err)
		}
	}

	skipAccountCacheMessage := make(chan struct{})
	writeAccountCacheMessageCompleted := make(chan struct{})
	defer func() {
		close(skipAccountCacheMessage)
		select {
		case <-writeAccountCacheMessageCompleted:
			if err == nil {
				tr.log.Infof("initializeTrackerCaches completed initializing account data caches")
			}
		default:
		}
	}()

	catchpointInterval := uint64(0)
	for _, tracker := range tr.trackers {
		if catchpointTracker, ok := tracker.(*catchpointTracker); ok {
			catchpointInterval = catchpointTracker.catchpointInterval
			break
		}
	}

	// this goroutine logs a message once if the parent function have not completed in initializingAccountCachesMessageTimeout seconds.
	// the message is important, since we're blocking on the ledger block database here, and we want to make sure that we log a message
	// within the above timeout.
	go func() {
		select {
		case <-time.After(initializingAccountCachesMessageTimeout):
			tr.log.Infof("initializeTrackerCaches is initializing account data caches")
			close(writeAccountCacheMessageCompleted)
		case <-skipAccountCacheMessage:
		}
	}()

	blocksStream := make(chan bookkeeping.Block, initializeCachesReadaheadBlocksStream)
	blockEvalFailed := make(chan struct{}, 1)
	var blockRetrievalError error
	go func() {
		defer close(blocksStream)
		for roundNumber := lastBalancesRound + 1; roundNumber <= lastestBlockRound; roundNumber++ {
			blk, blockRetrievalError = l.Block(roundNumber)
			if blockRetrievalError != nil {
				return
			}
			select {
			case blocksStream <- blk:
			case <-blockEvalFailed:
				return
			}
		}
	}()

	lastFlushedRound := lastBalancesRound
	const accountsCacheLoadingMessageInterval = 5 * time.Second
	lastProgressMessage := time.Now().Add(-accountsCacheLoadingMessageInterval / 2)

	// rollbackSynchronousMode ensures that we switch to "fast writing mode" when we start flushing out rounds to disk, and that
	// we exit this mode when we're done.
	rollbackSynchronousMode := false
	defer func() {
		if rollbackSynchronousMode {
			// restore default synchronous mode
			err0 := tr.dbs.Wdb.SetSynchronousMode(context.Background(), tr.synchronousMode, tr.synchronousMode >= db.SynchronousModeFull)
			// override the returned error only in case there is no error - since this
			// operation has a lower criticality.
			if err == nil {
				err = err0
			}
		}
	}()

	for blk := range blocksStream {
		delta, err = l.trackerEvalVerified(blk, &accLedgerEval)
		if err != nil {
			close(blockEvalFailed)
			err = fmt.Errorf("trackerEvalVerified failed : %w", err)
			return
		}
		tr.newBlock(blk, delta)

		// flush to disk if any of the following applies:
		// 1. if we have loaded up more than initializeCachesRoundFlushInterval rounds since the last time we flushed the data to disk
		// 2. if we completed the loading and we loaded up more than 320 rounds.
		flushIntervalExceed := blk.Round()-lastFlushedRound > initializeCachesRoundFlushInterval
		loadCompleted := (lastestBlockRound == blk.Round() && lastBalancesRound+basics.Round(blk.ConsensusProtocol().MaxBalLookback) < lastestBlockRound)
		if flushIntervalExceed || loadCompleted {
			// adjust the last flush time, so that we would not hold off the flushing due to "working too fast"
			tr.lastFlushTime = time.Now().Add(-balancesFlushInterval)

			if !rollbackSynchronousMode {
				// switch to rebuild synchronous mode to improve performance
				err0 := tr.dbs.Wdb.SetSynchronousMode(context.Background(), tr.accountsRebuildSynchronousMode, tr.accountsRebuildSynchronousMode >= db.SynchronousModeFull)
				if err0 != nil {
					tr.log.Warnf("initializeTrackerCaches was unable to switch to rbuild synchronous mode : %v", err0)
				} else {
					// flip the switch to rollback the synchronous mode once we're done.
					rollbackSynchronousMode = true
				}
			}

			var roundsBehind basics.Round

			// flush the account data
			tr.scheduleCommit(blk.Round(), basics.Round(config.Consensus[blk.BlockHeader.CurrentProtocol].MaxBalLookback))
			// wait for the writing to complete.
			tr.waitAccountsWriting()

			func() {
				tr.mu.RLock()
				defer tr.mu.RUnlock()

				// The au.dbRound after writing should be ~320 behind the block round.
				roundsBehind = blk.Round() - tr.dbRound
			}()

			// are we too far behind ? ( taking into consideration the catchpoint writing, which can stall the writing for quite a bit )
			if roundsBehind > initializeCachesRoundFlushInterval+basics.Round(catchpointInterval) {
				// we're unable to persist changes. This is unexpected, but there is no point in keep trying batching additional changes since any further changes
				// would just accumulate in memory.
				close(blockEvalFailed)
				tr.log.Errorf("initializeTrackerCaches was unable to fill up the account caches accounts round = %d, block round = %d. See above error for more details.", blk.Round()-roundsBehind, blk.Round())
				err = fmt.Errorf("initializeTrackerCaches failed to initialize the account data caches")
				return
			}

			// and once we flushed it to disk, update the lastFlushedRound
			lastFlushedRound = blk.Round()
		}

		// if enough time have passed since the last time we wrote a message to the log file then give the user an update about the progess.
		if time.Since(lastProgressMessage) > accountsCacheLoadingMessageInterval {
			// drop the initial message if we're got to this point since a message saying "still initializing" that comes after "is initializing" doesn't seems to be right.
			select {
			case skipAccountCacheMessage <- struct{}{}:
				// if we got to this point, we should be able to close the writeAccountCacheMessageCompleted channel to have the "completed initializing" message written.
				close(writeAccountCacheMessageCompleted)
			default:
			}
			tr.log.Infof("initializeTrackerCaches is still initializing account data caches, %d rounds loaded out of %d rounds", blk.Round()-lastBalancesRound, lastestBlockRound-lastBalancesRound)
			lastProgressMessage = time.Now()
		}

		// prepare for the next iteration.
		accLedgerEval.prevHeader = *delta.Hdr
	}

	if blockRetrievalError != nil {
		err = blockRetrievalError
	}
	return

}