nimbus-eth2/beacon_chain/eth1/eth1_monitor.nim

# beacon_chain
# Copyright (c) 2018-2021 Status Research & Development GmbH
# Licensed and distributed under either of
#   * MIT license (license terms in the root directory or at https://opensource.org/licenses/MIT).
#   * Apache v2 license (license terms in the root directory or at https://www.apache.org/licenses/LICENSE-2.0).
# at your option. This file may not be copied, modified, or distributed except according to those terms.

{.push raises: [Defect].}

import
  std/[deques, hashes, options, strformat, strutils, sequtils, tables,
       typetraits, uri, json],
  # Nimble packages:
  chronos, json, metrics, chronicles/timings,
  web3, web3/ethtypes as web3Types, web3/ethhexstrings, web3/engine_api,
  eth/common/eth_types,
  eth/async_utils, stew/byteutils,
  # Local modules:
  ../spec/[eth2_merkleization, forks, helpers],
  ../spec/datatypes/[base, merge],
  ../networking/network_metadata,
  ../consensus_object_pools/block_pools_types,
  ".."/[beacon_chain_db, beacon_node_status],
  ./merkle_minimal

export
  web3Types, deques

logScope:
  topics = "eth1"

contract(DepositContract):
  proc deposit(pubkey: Bytes48,
               withdrawalCredentials: Bytes32,
               signature: Bytes96,
               deposit_data_root: FixedBytes[32])

  proc get_deposit_root(): FixedBytes[32]
  proc get_deposit_count(): Bytes8

  proc DepositEvent(pubkey: Bytes48,
                    withdrawalCredentials: Bytes32,
                    amount: Bytes8,
                    signature: Bytes96,
                    index: Bytes8) {.event.}

const
  web3Timeouts = 60.seconds
  hasDepositRootChecks = defined(has_deposit_root_checks)
  hasGenesisDetection* = defined(has_genesis_detection)

type
  Eth1BlockNumber* = uint64
  Eth1BlockTimestamp* = uint64
  Eth1BlockHeader = web3Types.BlockHeader

  Database* = object

  Eth1Block* = ref object
    number*: Eth1BlockNumber
    timestamp*: Eth1BlockTimestamp
    deposits*: seq[DepositData]
    voteData*: Eth1Data

    when hasGenesisDetection:
      activeValidatorsCount*: uint64

  DepositsMerkleizer* = SszMerkleizer[depositContractLimit]

  Eth1Chain* = object
    db: BeaconChainDB
    cfg: RuntimeConfig
    blocks: Deque[Eth1Block]
    blocksByHash: Table[BlockHash, Eth1Block]
    finalizedBlockHash: Eth2Digest
    finalizedDepositsMerkleizer: DepositsMerkleizer
    hasConsensusViolation: bool
      ## The local chain contradicts the observed consensus on the network

  Eth1MonitorState = enum
    Initialized
    Started
    Failed
    Stopping
    Stopped

  Eth1Monitor* = ref object
    state: Eth1MonitorState
    startIdx: int
    web3Urls: seq[string]
    eth1Network: Option[Eth1Network]
    depositContractAddress*: Eth1Address

    dataProvider: Web3DataProviderRef

    eth1Chain: Eth1Chain
    latestEth1BlockNumber: Eth1BlockNumber
    eth1Progress: AsyncEvent

    runFut: Future[void]
    stopFut: Future[void]

    when hasGenesisDetection:
      genesisValidators: seq[ImmutableValidatorData]
      genesisValidatorKeyToIndex: Table[ValidatorPubKey, ValidatorIndex]
      genesisState: NilableBeaconStateRef
      genesisStateFut: Future[void]

  Web3DataProvider* = object
    url: string
    web3: Web3
    ns: Sender[DepositContract]
    blockHeadersSubscription: Subscription

  Web3DataProviderRef* = ref Web3DataProvider

  DataProviderFailure* = object of CatchableError
  CorruptDataProvider* = object of DataProviderFailure
  DataProviderTimeout* = object of DataProviderFailure

  DisconnectHandler* = proc () {.gcsafe, raises: [Defect].}

  DepositEventHandler* = proc (
    pubkey: Bytes48,
    withdrawalCredentials: Bytes32,
    amount: Bytes8,
    signature: Bytes96, merkleTreeIndex: Bytes8, j: JsonNode) {.gcsafe, raises: [Defect].}

  BlockProposalEth1Data* = object
    vote*: Eth1Data
    deposits*: seq[Deposit]
    hasMissingDeposits*: bool

declareCounter failed_web3_requests,
  "Failed web3 requests"

declareGauge eth1_latest_head,
  "The highest Eth1 block number observed on the network"

declareGauge eth1_synced_head,
  "Block number of the highest synchronized block according to follow distance"

declareGauge eth1_finalized_head,
  "Block number of the highest Eth1 block finalized by Eth2 consensus"

declareGauge eth1_finalized_deposits,
  "Number of deposits that were finalized by the Eth2 consensus"

declareGauge eth1_chain_len,
  "The length of the in-memory chain of Eth1 blocks"

func depositCountU64(s: DepositContractState): uint64 =
  for i in 0 .. 23:
    doAssert s.deposit_count[i] == 0

  uint64.fromBytesBE s.deposit_count[24..31]

when hasGenesisDetection:
  import spec/[beaconstate, signatures]

  template hasEnoughValidators(m: Eth1Monitor, blk: Eth1Block): bool =
    blk.activeValidatorsCount >= m.cfg.MIN_GENESIS_ACTIVE_VALIDATOR_COUNT

  func chainHasEnoughValidators(m: Eth1Monitor): bool =
    if m.eth1Chain.blocks.len > 0:
      m.hasEnoughValidators(m.eth1Chain.blocks[^1])
    else:
      m.knownStart.depositContractState.depositCountU64 >=
        m.cfg.MIN_GENESIS_ACTIVE_VALIDATOR_COUNT

  func isAfterMinGenesisTime(m: Eth1Monitor, blk: Eth1Block): bool =
    doAssert blk.timestamp != 0
    let t = genesis_time_from_eth1_timestamp(m.cfg, uint64 blk.timestamp)
    t >= m.cfg.MIN_GENESIS_TIME

  func isGenesisCandidate(m: Eth1Monitor, blk: Eth1Block): bool =
    m.hasEnoughValidators(blk) and m.isAfterMinGenesisTime(blk)

  proc findGenesisBlockInRange(m: Eth1Monitor, startBlock, endBlock: Eth1Block):
                               Future[Eth1Block] {.async, gcsafe.}

  proc signalGenesis(m: Eth1Monitor, genesisState: BeaconStateRef) =
    m.genesisState = genesisState

    if not m.genesisStateFut.isNil:
      m.genesisStateFut.complete()
      m.genesisStateFut = nil

  proc allGenesisDepositsUpTo(m: Eth1Monitor, totalDeposits: uint64): seq[DepositData] =
    for i in 0'u64 ..< totalDeposits:
      result.add m.db.genesisDeposits.get(i)

  proc createGenesisState(m: Eth1Monitor, eth1Block: Eth1Block): BeaconStateRef =
    notice "Generating genesis state",
      blockNum = eth1Block.number,
      blockHash = eth1Block.voteData.block_hash,
      blockTimestamp = eth1Block.timestamp,
      totalDeposits = eth1Block.voteData.deposit_count,
      activeValidators = eth1Block.activeValidatorsCount

    var deposits = m.allGenesisDepositsUpTo(eth1Block.voteData.deposit_count)

    result = initialize_beacon_state_from_eth1(
      m.cfg,
      eth1Block.voteData.block_hash,
      eth1Block.timestamp.uint64,
      deposits, {})

    if eth1Block.activeValidatorsCount != 0:
      doAssert result.validators.lenu64 == eth1Block.activeValidatorsCount

  proc produceDerivedData(m: Eth1Monitor, deposit: DepositData) =
    let htr = hash_tree_root(deposit)

    if verify_deposit_signature(m.cfg, deposit):
      let pubkey = deposit.pubkey
      if pubkey notin m.genesisValidatorKeyToIndex:
        let idx = ValidatorIndex m.genesisValidators.len
        m.genesisValidators.add ImmutableValidatorData(
          pubkey: pubkey,
          withdrawal_credentials: deposit.withdrawal_credentials)
        m.genesisValidatorKeyToIndex[pubkey] = idx

  proc processGenesisDeposit*(m: Eth1Monitor, newDeposit: DepositData) =
    m.db.genesisDeposits.add newDeposit
    m.produceDerivedData(newDeposit)

template blocks*(m: Eth1Monitor): Deque[Eth1Block] =
  m.eth1Chain.blocks

template cfg(m: Eth1Monitor): auto =
  m.eth1Chain.cfg

template finalizedDepositsMerkleizer(m: Eth1Monitor): auto =
  m.eth1Chain.finalizedDepositsMerkleizer

proc fixupWeb3Urls*(web3Url: var string) =
  ## Converts HTTP and HTTPS Infura URLs to their WebSocket equivalents
  ## because we are missing a functional HTTPS client.
  let normalizedUrl = toLowerAscii(web3Url)
  var pos = 0

  template skip(x: string): bool {.dirty.} =
    if normalizedUrl.len - pos >= x.len and
       normalizedUrl.toOpenArray(pos, pos + x.len - 1) == x:
      pos += x.len
      true
    else:
      false

  if not (skip("https://") or skip("http://")):
    if not (skip("ws://") or skip("wss://")):
      web3Url = "ws://" & web3Url
      warn "The Web3 URL does not specify a protocol. Assuming a WebSocket server", web3Url
    return

  block infuraRewrite:
    var pos = pos
    let network = if skip("mainnet"): mainnet
                  elif skip("goerli"): goerli
                  else: break

    if not skip(".infura.io/v3/"):
      break

    template infuraKey: string = normalizedUrl.substr(pos)

    web3Url = "wss://" & $network & ".infura.io/ws/v3/" & infuraKey
    return

  block gethRewrite:
    web3Url = "ws://" & normalizedUrl.substr(pos)
    warn "Only WebSocket web3 providers are supported. Rewriting URL", web3Url

template toGaugeValue(x: Quantity): int64 =
  toGaugeValue(distinctBase x)

# TODO: Add cfg validation
# MIN_GENESIS_ACTIVE_VALIDATOR_COUNT should be larger than SLOTS_PER_EPOCH
#  doAssert SECONDS_PER_ETH1_BLOCK * cfg.ETH1_FOLLOW_DISTANCE < GENESIS_DELAY,
#             "Invalid configuration: GENESIS_DELAY is set too low"

# https://github.com/ethereum/consensus-specs/blob/v1.0.1/specs/phase0/validator.md#get_eth1_data
func compute_time_at_slot(genesis_time: uint64, slot: Slot): uint64 =
  genesis_time + slot * SECONDS_PER_SLOT

# https://github.com/ethereum/consensus-specs/blob/v1.0.1/specs/phase0/validator.md#get_eth1_data
func voting_period_start_time*(state: ForkedHashedBeaconState): uint64 =
  let eth1_voting_period_start_slot =
    getStateField(state, slot) - getStateField(state, slot) mod
      SLOTS_PER_ETH1_VOTING_PERIOD.uint64
  compute_time_at_slot(
    getStateField(state, genesis_time), eth1_voting_period_start_slot)

# https://github.com/ethereum/consensus-specs/blob/v1.0.1/specs/phase0/validator.md#get_eth1_data
func is_candidate_block(cfg: RuntimeConfig,
                        blk: Eth1Block,
                        period_start: uint64): bool =
  (blk.timestamp + cfg.SECONDS_PER_ETH1_BLOCK * cfg.ETH1_FOLLOW_DISTANCE <= period_start) and
  (blk.timestamp + cfg.SECONDS_PER_ETH1_BLOCK * cfg.ETH1_FOLLOW_DISTANCE * 2 >= period_start)

func asEth2Digest*(x: BlockHash): Eth2Digest =
  Eth2Digest(data: array[32, byte](x))

template asBlockHash(x: Eth2Digest): BlockHash =
  BlockHash(x.data)

func shortLog*(b: Eth1Block): string =
  try:
    &"{b.number}:{shortLog b.voteData.block_hash}(deposits = {b.voteData.deposit_count})"
  except ValueError as exc: raiseAssert exc.msg

template findBlock*(chain: Eth1Chain, eth1Data: Eth1Data): Eth1Block =
  getOrDefault(chain.blocksByHash, asBlockHash(eth1Data.block_hash), nil)

func makeSuccessorWithoutDeposits(existingBlock: Eth1Block,
                                  successor: BlockObject): ETh1Block =
  result = Eth1Block(
    number: Eth1BlockNumber successor.number,
    timestamp: Eth1BlockTimestamp successor.timestamp,
    voteData: Eth1Data(
      block_hash: successor.hash.asEth2Digest,
      deposit_count: existingBlock.voteData.deposit_count,
      deposit_root: existingBlock.voteData.deposit_root))

  when hasGenesisDetection:
    result.activeValidatorsCount = existingBlock.activeValidatorsCount

func latestCandidateBlock(chain: Eth1Chain, periodStart: uint64): Eth1Block =
  for i in countdown(chain.blocks.len - 1, 0):
    let blk = chain.blocks[i]
    if is_candidate_block(chain.cfg, blk, periodStart):
      return blk

proc popFirst(chain: var Eth1Chain) =
  let removed = chain.blocks.popFirst
  chain.blocksByHash.del removed.voteData.block_hash.asBlockHash
  eth1_chain_len.set chain.blocks.len.int64

proc addBlock*(chain: var Eth1Chain, newBlock: Eth1Block) =
  chain.blocks.addLast newBlock
  chain.blocksByHash[newBlock.voteData.block_hash.asBlockHash] = newBlock
  eth1_chain_len.set chain.blocks.len.int64

func hash*(x: Eth1Data): Hash =
  hashData(unsafeAddr x, sizeof(x))

template hash*(x: Eth1Block): Hash =
  hash(x.voteData)

template awaitWithRetries*[T](lazyFutExpr: Future[T],
                              retries = 3,
                              timeout = web3Timeouts): untyped =
  const
    reqType = astToStr(lazyFutExpr)

  var
    retryDelayMs = 16000
    f: Future[T]
    attempts = 0

  while true:
    f = lazyFutExpr
    yield f or sleepAsync(timeout)
    if not f.finished:
      await cancelAndWait(f)
    elif f.failed:
      if f.error[] of Defect:
        raise f.error
      else:
        debug "Web3 request failed", req = reqType, err = f.error.msg
        inc failed_web3_requests
    else:
      break

    inc attempts
    if attempts >= retries:
      var errorMsg = reqType & " failed " & $retries & " times"
      if f.failed: errorMsg &= ". Last error: " & f.error.msg
      raise newException(DataProviderFailure, errorMsg)

    await sleepAsync(chronos.milliseconds(retryDelayMs))
    retryDelayMs *= 2

  read(f)

proc close*(p: Web3DataProviderRef): Future[void] {.async.} =
  if p.blockHeadersSubscription != nil:
    try:
      awaitWithRetries(p.blockHeadersSubscription.unsubscribe())
    except CatchableError:
      debug "Failed to clean up block headers subscription properly"

  await p.web3.close()

proc getBlockByHash*(p: Web3DataProviderRef, hash: BlockHash):
                     Future[BlockObject] =
  return p.web3.provider.eth_getBlockByHash(hash, false)

proc getBlockByNumber*(p: Web3DataProviderRef,
                       number: Eth1BlockNumber): Future[BlockObject] =
  let hexNumber = try: &"0x{number:X}" # No leading 0's!
  except ValueError as exc: raiseAssert exc.msg # Never fails
  p.web3.provider.eth_getBlockByNumber(hexNumber, false)

proc preparePayload*(p: Web3DataProviderRef,
                     parentHash: Eth2Digest,
                     timestamp: uint64,
                     randomData: array[32, byte],
                     feeRecipient: Eth1Address): Future[PreparePayloadResponse] =
  p.web3.provider.engine_preparePayload(PayloadAttributes(
    parentHash: parentHash.asBlockHash,
    timestamp: Quantity timestamp,
    random: FixedBytes[32] randomData,
    feeRecipient: feeRecipient))

proc getPayload*(p: Web3DataProviderRef,
                 payloadId: Quantity): Future[engine_api.ExecutionPayload] =
  p.web3.provider.engine_getPayload(payloadId)

proc executePayload*(p: Web3DataProviderRef,
                     payload: engine_api.ExecutionPayload): Future[ExecutePayloadResponse] =
  p.web3.provider.engine_executePayload(payload)

proc consensusValidated*(p: Web3DataProviderRef,
                         blockHash: BlockHash,
                         status: BlockValidationStatus): Future[JsonNode] =
  p.web3.provider.engine_consensusValidated(BlockValidationResult(
    blockHash: blockHash,
    status: $status))

proc forkchoiceUpdated*(p: Web3DataProviderRef,
                        headBlock, finalizedBlock: Eth2Digest): Future[JsonNode] =
  p.web3.provider.engine_forkchoiceUpdated(ForkChoiceUpdate(
    headBlockHash: headBlock.asBlockHash,
    finalizedBlockHash: finalizedBlock.asBlockHash))

template readJsonField(j: JsonNode, fieldName: string, ValueType: type): untyped =
  var res: ValueType
  fromJson(j[fieldName], fieldName, res)
  res

proc depositEventsToBlocks(depositsList: JsonNode): seq[Eth1Block] {.
    raises: [Defect, CatchableError].} =
  if depositsList.kind != JArray:
    raise newException(CatchableError,
      "Web3 provider didn't return a list of deposit events")

  var lastEth1Block: Eth1Block

  for logEvent in depositsList:
    let
      blockNumber = Eth1BlockNumber readJsonField(logEvent, "blockNumber", Quantity)
      blockHash = readJsonField(logEvent, "blockHash", BlockHash)
      logData = strip0xPrefix(logEvent["data"].getStr)

    if lastEth1Block == nil or lastEth1Block.number != blockNumber:
      lastEth1Block = Eth1Block(
        number: blockNumber,
        voteData: Eth1Data(block_hash: blockHash.asEth2Digest))

      result.add lastEth1Block

    var
      pubkey: Bytes48
      withdrawalCredentials: Bytes32
      amount: Bytes8
      signature: Bytes96
      index: Bytes8

    var offset = 0
    offset += decode(logData, offset, pubkey)
    offset += decode(logData, offset, withdrawalCredentials)
    offset += decode(logData, offset, amount)
    offset += decode(logData, offset, signature)
    offset += decode(logData, offset, index)

    lastEth1Block.deposits.add DepositData(
      pubkey: ValidatorPubKey.init(array[48, byte](pubkey)),
      withdrawal_credentials: Eth2Digest(data: array[32, byte](withdrawalCredentials)),
      amount: bytes_to_uint64(array[8, byte](amount)),
      signature: ValidatorSig.init(array[96, byte](signature)))

proc fetchTimestamp(p: Web3DataProviderRef, blk: Eth1Block) {.async.} =
  let web3block = awaitWithRetries(
    p.getBlockByHash(blk.voteData.block_hash.asBlockHash))
  blk.timestamp = Eth1BlockTimestamp web3block.timestamp

type
  DepositContractDataStatus = enum
    Fetched
    VerifiedCorrect
    DepositRootIncorrect
    DepositRootUnavailable
    DepositCountIncorrect
    DepositCountUnavailable

when hasDepositRootChecks:
  const
    contractCallTimeout = seconds(60)

  template awaitOrRaiseOnTimeout[T](fut: Future[T],
                                    timeout: Duration): T =
    awaitWithTimeout(fut, timeout):
      raise newException(DataProviderTimeout, "Timeout")

  proc fetchDepositContractData(p: Web3DataProviderRef, blk: Eth1Block):
                                Future[DepositContractDataStatus] {.async.} =
    let
      depositRoot = p.ns.get_deposit_root.call(blockNumber = blk.number)
      rawCount = p.ns.get_deposit_count.call(blockNumber = blk.number)

    try:
      let fetchedRoot = asEth2Digest(
        awaitOrRaiseOnTimeout(depositRoot, contractCallTimeout))
      if blk.voteData.deposit_root == default(Eth2Digest):
        blk.voteData.deposit_root = fetchedRoot
        result = Fetched
      elif blk.voteData.deposit_root == fetchedRoot:
        result = VerifiedCorrect
      else:
        result = DepositRootIncorrect
    except CatchableError as err:
      debug "Failed to fetch deposits root",
        blockNumber = blk.number,
        err = err.msg
      result = DepositRootUnavailable

    try:
      let fetchedCount = bytes_to_uint64(array[8, byte](
        awaitOrRaiseOnTimeout(rawCount, contractCallTimeout)))
      if blk.voteData.deposit_count == 0:
        blk.voteData.deposit_count = fetchedCount
      elif blk.voteData.deposit_count != fetchedCount:
        result = DepositCountIncorrect
    except CatchableError as err:
      debug "Failed to fetch deposits count",
            blockNumber = blk.number,
            err = err.msg
      result = DepositCountUnavailable

proc onBlockHeaders*(p: Web3DataProviderRef,
                     blockHeaderHandler: BlockHeaderHandler,
                     errorHandler: SubscriptionErrorHandler) {.async.} =
  info "Waiting for new Eth1 block headers"

  p.blockHeadersSubscription = awaitWithRetries(
    p.web3.subscribeForBlockHeaders(blockHeaderHandler, errorHandler))

func getDepositsRoot*(m: DepositsMerkleizer): Eth2Digest =
  mixInLength(m.getFinalHash, int m.totalChunks)

func toDepositContractState*(merkleizer: DepositsMerkleizer): DepositContractState =
  # TODO There is an off by one discrepancy in the size of the arrays here that
  #      need to be investigated. It shouldn't matter as long as the tree is
  #      not populated to its maximum size.
  result.branch[0..31] = merkleizer.getCombinedChunks[0..31]
  result.deposit_count[24..31] = merkleizer.getChunkCount().toBytesBE

func createMerkleizer*(s: DepositContractState): DepositsMerkleizer =
  DepositsMerkleizer.init(s.branch, s.depositCountU64)

func createMerkleizer*(s: DepositContractSnapshot): DepositsMerkleizer =
  createMerkleizer(s.depositContractState)

func eth1DataFromMerkleizer(eth1Block: Eth2Digest,
                            merkleizer: DepositsMerkleizer): Eth1Data =
  Eth1Data(
    block_hash: eth1Block,
    deposit_count: merkleizer.getChunkCount,
    deposit_root: merkleizer.getDepositsRoot)

proc pruneOldBlocks(chain: var Eth1Chain, depositIndex: uint64) =
  let initialChunks = chain.finalizedDepositsMerkleizer.getChunkCount
  var lastBlock: Eth1Block

  while chain.blocks.len > 0:
    let blk = chain.blocks.peekFirst
    if blk.voteData.deposit_count >= depositIndex:
      break
    else:
      for deposit in blk.deposits:
        chain.finalizedDepositsMerkleizer.addChunk hash_tree_root(deposit).data
    chain.popFirst()
    lastBlock = blk

  if chain.finalizedDepositsMerkleizer.getChunkCount > initialChunks:
    chain.finalizedBlockHash = lastBlock.voteData.block_hash
    chain.db.putEth2FinalizedTo DepositContractSnapshot(
      eth1Block: lastBlock.voteData.block_hash,
      depositContractState: chain.finalizedDepositsMerkleizer.toDepositContractState)

    eth1_finalized_head.set lastBlock.number.toGaugeValue
    eth1_finalized_deposits.set lastBlock.voteData.deposit_count.toGaugeValue

    debug "Eth1 blocks pruned",
           newTailBlock = lastBlock.voteData.block_hash,
           depositsCount = lastBlock.voteData.deposit_count

proc advanceMerkleizer(chain: Eth1Chain,
                       merkleizer: var DepositsMerkleizer,
                       depositIndex: uint64): bool =
  if chain.blocks.len == 0:
    return depositIndex == merkleizer.getChunkCount

  if chain.blocks.peekLast.voteData.deposit_count < depositIndex:
    return false

  let
    firstBlock = chain.blocks[0]
    depositsInLastPrunedBlock = firstBlock.voteData.deposit_count -
                                firstBlock.deposits.lenu64

  # advanceMerkleizer should always be called shortly after prunning the chain
  doAssert depositsInLastPrunedBlock == merkleizer.getChunkCount

  for blk in chain.blocks:
    for deposit in blk.deposits:
      if merkleizer.getChunkCount < depositIndex:
        merkleizer.addChunk hash_tree_root(deposit).data
      else:
        return true

  return merkleizer.getChunkCount == depositIndex

proc getDepositsRange(chain: Eth1Chain, first, last: uint64): seq[DepositData] =
  # TODO It's possible to make this faster by performing binary search that
  #      will locate the blocks holding the `first` and `last` indices.
  # TODO There is an assumption here that the requested range will be present
  #      in the Eth1Chain. This should hold true at the single call site right
  #      now, but we need to guard the pre-conditions better.
  for blk in chain.blocks:
    if blk.voteData.deposit_count <= first:
      continue

    let firstDepositIdxInBlk = blk.voteData.deposit_count - blk.deposits.lenu64
    if firstDepositIdxInBlk >= last:
      return

    for i in 0 ..< blk.deposits.lenu64:
      let globalIdx = firstDepositIdxInBlk + i
      if globalIdx >= first and globalIdx < last:
        result.add blk.deposits[i]

proc lowerBound(chain: Eth1Chain, depositCount: uint64): Eth1Block =
  # TODO: This can be replaced with a proper binary search in the
  #       future, but the `algorithm` module currently requires an
  #       `openArray`, which the `deques` module can't provide yet.
  for eth1Block in chain.blocks:
    if eth1Block.voteData.deposit_count > depositCount:
      return
    result = eth1Block

proc trackFinalizedState*(chain: var Eth1Chain,
                          finalizedEth1Data: Eth1Data,
                          finalizedStateDepositIndex: uint64): bool =
  # Returns true if the Eth1Monitor is synced to the finalization point
  if chain.blocks.len == 0:
    debug "Eth1 chain not initialized"
    return false

  let latest = chain.blocks.peekLast
  if latest.voteData.deposit_count < finalizedEth1Data.deposit_count:
    warn "Eth1 chain not synced",
          ourDepositsCount = latest.voteData.deposit_count,
          targetDepositsCount = finalizedEth1Data.deposit_count
    return false

  let matchingBlock = chain.lowerBound(finalizedEth1Data.deposit_count)
  result = if matchingBlock != nil:
    if matchingBlock.voteData.deposit_root == finalizedEth1Data.deposit_root:
      true
    else:
      error "Corrupted deposits history detected",
            ourDepositsCount = matchingBlock.voteData.deposit_count,
            taretDepositsCount = finalizedEth1Data.deposit_count,
            ourDepositsRoot = matchingBlock.voteData.deposit_root,
            targetDepositsRoot = finalizedEth1Data.deposit_root
      chain.hasConsensusViolation = true
      false
  else:
    error "The Eth1 chain is in inconsistent state",
          checkpointHash = finalizedEth1Data.block_hash,
          checkpointDeposits = finalizedEth1Data.deposit_count,
          localChainStart = shortLog(chain.blocks.peekFirst),
          localChainEnd = shortLog(chain.blocks.peekLast)
    chain.hasConsensusViolation = true
    false

  if result:
    chain.pruneOldBlocks(finalizedStateDepositIndex)

template trackFinalizedState*(m: Eth1Monitor,
                              finalizedEth1Data: Eth1Data,
                              finalizedStateDepositIndex: uint64): bool =
  trackFinalizedState(m.eth1Chain, finalizedEth1Data, finalizedStateDepositIndex)

# https://github.com/ethereum/consensus-specs/blob/v1.0.1/specs/phase0/validator.md#get_eth1_data
proc getBlockProposalData*(chain: var Eth1Chain,
                           state: ForkedHashedBeaconState,
                           finalizedEth1Data: Eth1Data,
                           finalizedStateDepositIndex: uint64): BlockProposalEth1Data =
  let
    periodStart = voting_period_start_time(state)
    hasLatestDeposits = chain.trackFinalizedState(finalizedEth1Data,
                                                  finalizedStateDepositIndex)

  var otherVotesCountTable = initCountTable[Eth1Data]()
  for vote in getStateField(state, eth1_data_votes):
    let eth1Block = chain.findBlock(vote)
    if eth1Block != nil and
       eth1Block.voteData.deposit_root == vote.deposit_root and
       vote.deposit_count >= getStateField(state, eth1_data).deposit_count and
       is_candidate_block(chain.cfg, eth1Block, periodStart):
      otherVotesCountTable.inc vote
    else:
      debug "Ignoring eth1 vote",
            root = vote.block_hash,
            deposits = vote.deposit_count,
            depositsRoot = vote.deposit_root,
            localDeposits = getStateField(state, eth1_data).deposit_count

  var pendingDepositsCount =
    getStateField(state, eth1_data).deposit_count -
      getStateField(state, eth1_deposit_index)
  if otherVotesCountTable.len > 0:
    let (winningVote, votes) = otherVotesCountTable.largest
    debug "Voting on eth1 head with majority", votes
    result.vote = winningVote
    if uint64((votes + 1) * 2) > SLOTS_PER_ETH1_VOTING_PERIOD:
      pendingDepositsCount = winningVote.deposit_count -
        getStateField(state, eth1_deposit_index)
  else:
    let latestBlock = chain.latestCandidateBlock(periodStart)
    if latestBlock == nil:
      debug "No acceptable eth1 votes and no recent candidates. Voting no change"
      result.vote = getStateField(state, eth1_data)
    else:
      debug "No acceptable eth1 votes. Voting for latest candidate"
      result.vote = latestBlock.voteData

  if pendingDepositsCount > 0:
    if hasLatestDeposits:
      let
        totalDepositsInNewBlock = min(MAX_DEPOSITS, pendingDepositsCount)
        deposits = chain.getDepositsRange(
          getStateField(state, eth1_deposit_index),
          getStateField(state, eth1_deposit_index) + pendingDepositsCount)
        depositRoots = mapIt(deposits, hash_tree_root(it))

      var scratchMerkleizer = copy chain.finalizedDepositsMerkleizer
      if chain.advanceMerkleizer(
          scratchMerkleizer, getStateField(state, eth1_deposit_index)):
        let proofs = scratchMerkleizer.addChunksAndGenMerkleProofs(depositRoots)
        for i in 0 ..< totalDepositsInNewBlock:
          var proof: array[33, Eth2Digest]
          proof[0..31] = proofs.getProof(i.int)
          proof[32] = default(Eth2Digest)
          proof[32].data[0..7] = toBytesLE uint64(result.vote.deposit_count)
          result.deposits.add Deposit(data: deposits[i], proof: proof)
      else:
        error "The Eth1 chain is in inconsistent state" # This should not really happen
        result.hasMissingDeposits = true
    else:
      result.hasMissingDeposits = true

template getBlockProposalData*(m: Eth1Monitor,
                               state: ForkedHashedBeaconState,
                               finalizedEth1Data: Eth1Data,
                               finalizedStateDepositIndex: uint64): BlockProposalEth1Data =
  getBlockProposalData(m.eth1Chain, state, finalizedEth1Data, finalizedStateDepositIndex)

proc new(T: type Web3DataProvider,
         depositContractAddress: Eth1Address,
         web3Url: string): Future[Result[Web3DataProviderRef, string]] {.async.} =
  let web3Fut = newWeb3(web3Url)
  yield web3Fut or sleepAsync(chronos.seconds(10))
  if (not web3Fut.finished) or web3Fut.failed:
    await cancelAndWait(web3Fut)
    return err "Failed to setup web3 connection"

  let
    web3 = web3Fut.read
    ns = web3.contractSender(DepositContract, depositContractAddress)

  return ok Web3DataProviderRef(url: web3Url, web3: web3, ns: ns)

proc putInitialDepositContractSnapshot*(db: BeaconChainDB,
                                        s: DepositContractSnapshot) =
  let existingStart = db.getEth2FinalizedTo()
  if not existingStart.isOk:
    db.putEth2FinalizedTo(s)

template getOrDefault[T, E](r: Result[T, E]): T =
  type TT = T
  get(r, default(TT))

proc init*(T: type Eth1Chain, cfg: RuntimeConfig, db: BeaconChainDB): T =
  let finalizedDeposits = db.getEth2FinalizedTo().getOrDefault()
  let m = finalizedDeposits.createMerkleizer

  T(db: db,
    cfg: cfg,
    finalizedBlockHash: finalizedDeposits.eth1Block,
    finalizedDepositsMerkleizer: finalizedDeposits.createMerkleizer)

proc init*(T: type Eth1Monitor,
           cfg: RuntimeConfig,
           db: BeaconChainDB,
           web3Urls: seq[string],
           depositContractSnapshot: DepositContractSnapshot,
           eth1Network: Option[Eth1Network]): T =
  doAssert web3Urls.len > 0

  var web3Urls = web3Urls
  for url in mitems(web3Urls):
    fixupWeb3Urls url

  putInitialDepositContractSnapshot(db, depositContractSnapshot)

  T(state: Initialized,
    eth1Chain: Eth1Chain.init(cfg, db),
    depositContractAddress: cfg.DEPOSIT_CONTRACT_ADDRESS,
    web3Urls: web3Urls,
    eth1Network: eth1Network,
    eth1Progress: newAsyncEvent())

proc safeCancel(fut: var Future[void]) =
  if not fut.isNil and not fut.finished:
    fut.cancel()
  fut = nil

proc clear(chain: var Eth1Chain) =
  chain.blocks.clear()
  chain.blocksByHash.clear()
  chain.hasConsensusViolation = false

proc resetState(m: Eth1Monitor) {.async.} =
  safeCancel m.runFut

  m.eth1Chain.clear()
  m.latestEth1BlockNumber = 0

  if m.dataProvider != nil:
    await m.dataProvider.close()
    m.dataProvider = nil

proc stop*(m: Eth1Monitor) {.async.} =
  if m.state == Started:
    m.state = Stopping
    m.stopFut = resetState(m)
    await m.stopFut
    m.state = Stopped
  elif m.state == Stopping:
    await m.stopFut

const
  votedBlocksSafetyMargin = 50

proc earliestBlockOfInterest(m: Eth1Monitor): Eth1BlockNumber =
  m.latestEth1BlockNumber - (2 * m.cfg.ETH1_FOLLOW_DISTANCE) - votedBlocksSafetyMargin



proc syncBlockRange(m: Eth1Monitor,
                    merkleizer: ref DepositsMerkleizer,
                    fromBlock, toBlock,
                    fullSyncFromBlock: Eth1BlockNumber) {.gcsafe, async.} =
  doAssert m.eth1Chain.blocks.len > 0 and m.dataProvider != nil

  var currentBlock = fromBlock
  while currentBlock <= toBlock:
    var
      depositLogs: JsonNode = nil
      blocksPerRequest = 5000'u64 # This is roughly a day of Eth1 blocks
      maxBlockNumberRequested: Eth1BlockNumber
      backoff = 100

    while true:
      maxBlockNumberRequested = min(toBlock, currentBlock + blocksPerRequest - 1)

      template retryOrRaise(err: ref CatchableError) =
        blocksPerRequest = blocksPerRequest div 2
        if blocksPerRequest == 0:
          raise err
        continue

      debug "Obtaining deposit log events",
            fromBlock = currentBlock,
            toBlock = maxBlockNumberRequested,
            backoff

      debug.logTime "Deposit logs obtained":
        # Reduce all request rate until we have a more general solution
        # for dealing with Infura's rate limits
        await sleepAsync(milliseconds(backoff))

        let jsonLogsFut = m.dataProvider.ns.getJsonLogs(
          DepositEvent,
          fromBlock = some blockId(currentBlock),
          toBlock = some blockId(maxBlockNumberRequested))

        depositLogs = try:
          # Downloading large amounts of deposits can be quite slow
          awaitWithTimeout(jsonLogsFut, web3Timeouts):
            retryOrRaise newException(DataProviderTimeout,
              "Request time out while obtaining json logs")
        except CatchableError as err:
          debug "Request for deposit logs failed", err = err.msg
          inc failed_web3_requests
          backoff = (backoff * 3) div 2
          retryOrRaise err

      currentBlock = maxBlockNumberRequested + 1
      break

    let blocksWithDeposits = depositEventsToBlocks(depositLogs)

    for i in 0 ..< blocksWithDeposits.len:
      let blk = blocksWithDeposits[i]

      for deposit in blk.deposits:
        merkleizer[].addChunk hash_tree_root(deposit).data

      blk.voteData.deposit_count = merkleizer[].getChunkCount
      blk.voteData.deposit_root = merkleizer[].getDepositsRoot

      if blk.number > fullSyncFromBlock:
        let lastBlock = m.eth1Chain.blocks.peekLast
        for n in max(lastBlock.number + 1, fullSyncFromBlock) ..< blk.number:
          debug "Obtaining block without deposits", blockNum = n
          let blockWithoutDeposits = awaitWithRetries(
            m.dataProvider.getBlockByNumber(n))

          m.eth1Chain.addBlock(
            lastBlock.makeSuccessorWithoutDeposits(blockWithoutDeposits))
          eth1_synced_head.set blockWithoutDeposits.number.toGaugeValue

      m.eth1Chain.addBlock blk
      eth1_synced_head.set blk.number.toGaugeValue

    if blocksWithDeposits.len > 0:
      let lastIdx = blocksWithDeposits.len - 1
      template lastBlock: auto = blocksWithDeposits[lastIdx]

      let status = when hasDepositRootChecks:
        awaitWithRetries m.dataProvider.fetchDepositContractData(lastBlock)
      else:
        DepositRootUnavailable

      when hasDepositRootChecks:
        debug "Deposit contract state verified",
              status = $status,
              ourCount = lastBlock.voteData.deposit_count,
              ourRoot = lastBlock.voteData.deposit_root

      case status
      of DepositRootIncorrect, DepositCountIncorrect:
        raise newException(CorruptDataProvider,
          "The deposit log events disagree with the deposit contract state")
      else:
        discard

      notice "Eth1 sync progress",
        blockNumber = lastBlock.number,
        depositsProcessed = lastBlock.voteData.deposit_count

    when hasGenesisDetection:
      if blocksWithDeposits.len > 0:
        for blk in blocksWithDeposits:
          for deposit in blk.deposits:
            m.processGenesisDeposit(deposit)
          blk.activeValidatorsCount = m.genesisValidators.lenu64

        let depositContractState = DepositContractSnapshot(
          eth1Block: blocksWithDeposits[^1].voteData.block_hash,
          depositContractState: merkleizer[].toDepositContractState)

        m.db.putEth2FinalizedTo depositContractState

      if m.genesisStateFut != nil and m.chainHasEnoughValidators:
        let lastIdx = m.eth1Chain.blocks.len - 1
        template lastBlock: auto = m.eth1Chain.blocks[lastIdx]

        if maxBlockNumberRequested == toBlock and
           (m.eth1Chain.blocks.len == 0 or lastBlock.number != toBlock):
          let web3Block = awaitWithRetries(
            m.dataProvider.getBlockByNumber(toBlock))

          debug "Latest block doesn't hold deposits. Obtaining it",
                 ts = web3Block.timestamp.uint64,
                 number = web3Block.number.uint64

          m.eth1Chain.addBlock lastBlock.makeSuccessorWithoutDeposits(web3Block)
        else:
          awaitWithRetries m.dataProvider.fetchTimestamp(lastBlock)

        var genesisBlockIdx = m.eth1Chain.blocks.len - 1
        if m.isAfterMinGenesisTime(m.eth1Chain.blocks[genesisBlockIdx]):
          for i in 1 ..< blocksWithDeposits.len:
            let idx = (m.eth1Chain.blocks.len - 1) - i
            let blk = m.eth1Chain.blocks[idx]
            awaitWithRetries m.dataProvider.fetchTimestamp(blk)
            if m.isGenesisCandidate(blk):
              genesisBlockIdx = idx
            else:
              break
          # We have a candidate state on our hands, but our current Eth1Chain
          # may consist only of blocks that have deposits attached to them
          # while the real genesis may have happened in a block without any
          # deposits (triggered by MIN_GENESIS_TIME).
          #
          # This can happen when the beacon node is launched after the genesis
          # event. We take a short cut when constructing the initial Eth1Chain
          # by downloading only deposit log entries. Thus, we'll see all the
          # blocks with deposits, but not the regular blocks in between.
          #
          # We'll handle this special case below by examing whether we are in
          # this potential scenario and we'll use a fast guessing algorith to
          # discover the ETh1 block with minimal valid genesis time.
          var genesisBlock = m.eth1Chain.blocks[genesisBlockIdx]
          if genesisBlockIdx > 0:
            let genesisParent = m.eth1Chain.blocks[genesisBlockIdx - 1]
            if genesisParent.timestamp == 0:
              awaitWithRetries m.dataProvider.fetchTimestamp(genesisParent)
            if m.hasEnoughValidators(genesisParent) and
               genesisBlock.number - genesisParent.number > 1:
              genesisBlock = awaitWithRetries(
                m.findGenesisBlockInRange(genesisParent, genesisBlock))

          m.signalGenesis m.createGenesisState(genesisBlock)

proc startEth1Syncing(m: Eth1Monitor, delayBeforeStart: Duration) {.async.} =
  if m.state == Failed:
    await m.resetState()
  elif m.state == Stopping:
    await m.stopFut

  if delayBeforeStart != ZeroDuration:
    await sleepAsync(delayBeforeStart)

  let web3Url = m.web3Urls[m.startIdx mod m.web3Urls.len]
  inc m.startIdx

  info "Starting Eth1 deposit contract monitoring",
    contract = $m.depositContractAddress, url = web3Url

  let dataProviderRes = await Web3DataProvider.new(
    m.depositContractAddress,
    web3Url)

  m.dataProvider = dataProviderRes.tryGet()
  let web3 = m.dataProvider.web3

  if m.state == Initialized and m.eth1Network.isSome:
    let
      providerNetwork = awaitWithRetries web3.provider.net_version()
      expectedNetwork = case m.eth1Network.get
        of mainnet: "1"
        of rinkeby: "4"
        of goerli:  "5"
    if expectedNetwork != providerNetwork:
      fatal "The specified web3 provider serves data for a different network",
             expectedNetwork, providerNetwork
      quit 1

  m.state = Started

  await m.dataProvider.onBlockHeaders do (blk: Eth1BlockHeader)
                                         {.raises: [Defect], gcsafe.}:
    try:
      if blk.number.uint64 > m.latestEth1BlockNumber:
        eth1_latest_head.set blk.number.toGaugeValue
        m.latestEth1BlockNumber = Eth1BlockNumber blk.number
        m.eth1Progress.fire()
    except Exception:
      # TODO Investigate why this exception is being raised
      raiseAssert "AsyncEvent.fire should not raise exceptions"
  do (err: CatchableError):
    debug "Error while processing Eth1 block headers subscription", err = err.msg

  let startBlock = awaitWithRetries(
    m.dataProvider.getBlockByHash(m.eth1Chain.finalizedBlockHash.asBlockHash))

  doAssert m.eth1Chain.blocks.len == 0
  m.eth1Chain.addBlock Eth1Block(
    number: Eth1BlockNumber startBlock.number,
    timestamp: Eth1BlockTimestamp startBlock.timestamp,
    voteData: eth1DataFromMerkleizer(
      m.eth1Chain.finalizedBlockHash,
      m.eth1Chain.finalizedDepositsMerkleizer))

  var eth1SyncedTo = Eth1BlockNumber startBlock.number
  eth1_synced_head.set eth1SyncedTo.toGaugeValue
  eth1_finalized_head.set eth1SyncedTo.toGaugeValue
  eth1_finalized_deposits.set(
    m.eth1Chain.finalizedDepositsMerkleizer.getChunkCount.toGaugeValue)

  var scratchMerkleizer = newClone(copy m.finalizedDepositsMerkleizer)

  debug "Starting Eth1 syncing", `from` = shortLog(m.eth1Chain.blocks[0])

  while true:
    if bnStatus == BeaconNodeStatus.Stopping:
      when hasGenesisDetection:
        if not m.genesisStateFut.isNil:
          m.genesisStateFut.complete()
          m.genesisStateFut = nil
      await m.stop()
      return

    if m.eth1Chain.hasConsensusViolation:
      raise newException(CorruptDataProvider, "Eth1 chain contradicts Eth2 consensus")

    awaitWithTimeout(m.eth1Progress.wait(), 5.minutes):
      raise newException(CorruptDataProvider, "No eth1 chain progress for too long")

    m.eth1Progress.clear()

    if m.latestEth1BlockNumber <= m.cfg.ETH1_FOLLOW_DISTANCE:
      continue

    let targetBlock = m.latestEth1BlockNumber - m.cfg.ETH1_FOLLOW_DISTANCE
    if targetBlock <= eth1SyncedTo:
      continue

    let earliestBlockOfInterest = m.earliestBlockOfInterest()
    await m.syncBlockRange(scratchMerkleizer,
                           eth1SyncedTo + 1,
                           targetBlock,
                           earliestBlockOfInterest)
    eth1SyncedTo = targetBlock
    eth1_synced_head.set eth1SyncedTo.toGaugeValue

proc start(m: Eth1Monitor, delayBeforeStart: Duration) =
  if m.runFut.isNil:
    let runFut = m.startEth1Syncing(delayBeforeStart)
    m.runFut = runFut
    runFut.addCallback do (p: pointer):
      if runFut.failed:
        if runFut.error[] of CatchableError:
          if runFut == m.runFut:
            error "Eth1 chain monitoring failure, restarting", err = runFut.error.msg
            m.state = Failed
        else:
          fatal "Fatal exception reached", err = runFut.error.msg
          quit 1

      safeCancel m.runFut
      m.start(5.seconds)

proc start*(m: Eth1Monitor) =
  m.start(0.seconds)

proc getEth1BlockHash*(url: string, blockId: RtBlockIdentifier): Future[BlockHash] {.async.} =
  let web3 = await newWeb3(url)
  try:
    let blk = awaitWithRetries(
      web3.provider.eth_getBlockByNumber(blockId, false))
    return blk.hash
  finally:
    await web3.close()

proc testWeb3Provider*(web3Url: Uri,
                       depositContractAddress: Eth1Address) {.async.} =
  template mustSucceed(action: static string, expr: untyped): untyped =
    try: expr
    except CatchableError as err:
      fatal("Failed to " & action, err = err.msg)
      quit 1

  let
    web3 = mustSucceed "connect to web3 provider":
      await newWeb3($web3Url)
    network = mustSucceed "get network version":
      awaitWithRetries web3.provider.net_version()
    latestBlock = mustSucceed "get latest block":
      awaitWithRetries web3.provider.eth_getBlockByNumber(blockId("latest"), false)

  echo "Network: ", network
  echo "Latest block: ", latestBlock.number.uint64

  let ns = web3.contractSender(DepositContract, depositContractAddress)
  try:
    let depositRoot = awaitWithRetries(
      ns.get_deposit_root.call(blockNumber = latestBlock.number.uint64))
    echo "Deposit root: ", depositRoot
  except CatchableError as err:
    echo "Web3 provider is not archive mode: ", err.msg

when hasGenesisDetection:
  proc init*(T: type Eth1Monitor,
             db: BeaconChainDB,
             cfg: RuntimeConfig,
             web3Urls: seq[string],
             depositContractAddress: Eth1Address,
             depositContractDeployedAt: BlockHashOrNumber,
             eth1Network: Option[Eth1Network]): Future[Result[T, string]] {.async.} =
    doAssert web3Urls.len > 0
    try:
      var urlIdx = 0
      let dataProviderRes = await Web3DataProvider.new(depositContractAddress, web3Urls[urlIdx])
      if dataProviderRes.isErr:
        return err(dataProviderRes.error)
      var dataProvider = dataProviderRes.get

      let knownStartBlockHash =
        if depositContractDeployedAt.isHash:
          depositContractDeployedAt.hash
        else:
          var blk: BlockObject
          while true:
            try:
              blk = awaitWithRetries(
                dataProvider.getBlockByNumber(depositContractDeployedAt.number))
              break
            except CatchableError as err:
              error "Failed to obtain details for the starting block " &
                    "of the deposit contract sync. The Web3 provider " &
                    "may still be not fully synced", error = err.msg
            await sleepAsync(chronos.seconds(10))
            # TODO: After a single failure, the web3 object may enter a state
            #       where it's no longer possible to make additional requests.
            #       Until this is fixed upstream, we'll just try to recreate
            #       the web3 provider before retrying. In case this fails,
            #       the Eth1Monitor will be restarted.
            inc urlIdx
            dataProvider = tryGet(
              await Web3DataProvider.new(depositContractAddress,
                                         web3Urls[urlIdx mod web3Urls.len]))
          blk.hash.asEth2Digest

      let depositContractSnapshot = DepositContractSnapshot(
        eth1Block: knownStartBlockHash)

      var monitor = Eth1Monitor.init(
        db,
        cfg,
        web3Urls,
        depositContractAddress,
        depositContractSnapshot,
        eth1Network)

      for i in 0 ..< db.genesisDeposits.len:
        monitor.produceDerivedData db.genesisDeposits.get(i)

      return ok monitor

    except CatchableError as err:
      return err("Failed to initialize the Eth1 monitor")

  proc findGenesisBlockInRange(m: Eth1Monitor, startBlock, endBlock: Eth1Block):
                               Future[Eth1Block] {.async.} =
    doAssert startBlock.timestamp != 0 and not m.isAfterMinGenesisTime(startBlock)
    doAssert endBlock.timestamp != 0 and m.isAfterMinGenesisTime(endBlock)
    doAssert m.hasEnoughValidators(startBlock)
    doAssert m.hasEnoughValidators(endBlock)

    var
      startBlock = startBlock
      endBlock = endBlock
      depositData = startBlock.voteData
      activeValidatorsCountDuringRange = startBlock.activeValidatorsCount

    while startBlock.number + 1 < endBlock.number:
      let
        MIN_GENESIS_TIME = m.cfg.MIN_GENESIS_TIME
        startBlockTime = genesis_time_from_eth1_timestamp(m.cfg, startBlock.timestamp)
        secondsPerBlock = float(endBlock.timestamp - startBlock.timestamp) /
                          float(endBlock.number - startBlock.number)
        blocksToJump = max(float(MIN_GENESIS_TIME - startBlockTime) / secondsPerBlock, 1.0)
        candidateNumber = min(endBlock.number - 1, startBlock.number + blocksToJump.uint64)
        candidateBlock = awaitWithRetries(
          m.dataProvider.getBlockByNumber(candidateNumber))

      var candidateAsEth1Block = Eth1Block(number: candidateBlock.number.uint64,
                                           timestamp: candidateBlock.timestamp.uint64,
                                           voteData: depositData)
      candidateAsEth1Block.voteData.block_hash = candidateBlock.hash.asEth2Digest

      let candidateGenesisTime = genesis_time_from_eth1_timestamp(
        m.cfg, candidateBlock.timestamp.uint64)

      notice "Probing possible genesis block",
        `block` = candidateBlock.number.uint64,
        candidateGenesisTime

      if candidateGenesisTime < MIN_GENESIS_TIME:
        startBlock = candidateAsEth1Block
      else:
        endBlock = candidateAsEth1Block

    if endBlock.activeValidatorsCount == 0:
      endBlock.activeValidatorsCount = activeValidatorsCountDuringRange

    return endBlock

  proc waitGenesis*(m: Eth1Monitor): Future[BeaconStateRef] {.async.} =
    if m.genesisState.isNil:
      m.start()

      if m.genesisStateFut.isNil:
        m.genesisStateFut = newFuture[void]("waitGenesis")

      info "Awaiting genesis event"
      await m.genesisStateFut
      m.genesisStateFut = nil

    if m.genesisState != nil:
      return m.genesisState
    else:
      doAssert bnStatus == BeaconNodeStatus.Stopping
      return new BeaconStateRef # cannot return nil...