nimbus-eth1/nimbus/sync/fast.nim

# nim-eth
# 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.

import
  std/[sets, options, random, hashes, sequtils],
  chronicles,
  chronos,
  eth/[common/eth_types, p2p],
  eth/p2p/[private/p2p_types, peer_pool],
  stew/byteutils,
  "."/[protocol, types]

{.push raises:[Defect].}

logScope:
  topics = "fast-sync"

const
  minPeersToStartSync* = 2 # Wait for consensus of at least this
                           # number of peers before syncing

type
  #SyncStatus = enum
  #  syncSuccess
  #  syncNotEnoughPeers
  #  syncTimeOut

  BlockchainSyncDefect* = object of Defect
    ## Catch and relay exception

  WantedBlocksState = enum
    Initial,
    Requested,
    Received,
    Persisted

  WantedBlocks = object
    startIndex: BlockNumber
    numBlocks: uint
    state: WantedBlocksState
    headers: seq[BlockHeader]
    bodies: seq[BlockBody]

  FastSyncCtx* = ref object
    workQueue: seq[WantedBlocks]
    endBlockNumber: BlockNumber
    finalizedBlock: BlockNumber # Block which was downloaded and verified
    chain: AbstractChainDB
    peerPool: PeerPool
    trustedPeers: HashSet[Peer]
    hasOutOfOrderBlocks: bool

proc hash*(p: Peer): Hash = hash(cast[pointer](p))

proc endIndex(b: WantedBlocks): BlockNumber =
  result = b.startIndex
  result += (b.numBlocks - 1).toBlockNumber

proc availableWorkItem(ctx: FastSyncCtx): int =
  var maxPendingBlock = ctx.finalizedBlock # the last downloaded & processed
  trace "queue len", length = ctx.workQueue.len
  result = -1
  for i in 0 .. ctx.workQueue.high:
    case ctx.workQueue[i].state
    of Initial:
      # When there is a work item at Initial state, immediatly use this one.
      # This usually means a previous work item that failed somewhere in the
      # process, and thus can be reused to work on.
      return i
    of Persisted:
      # In case of Persisted, we can reset this work item to a new one.
      result = i
      # No break here to give work items in Initial state priority and to
      # calculate endBlock.
    else:
      discard

    # Check all endBlocks of all workqueue items to decide on next range of
    # blocks to collect & process.
    let endBlock = ctx.workQueue[i].endIndex
    if endBlock > maxPendingBlock:
      maxPendingBlock = endBlock

  let nextRequestedBlock = maxPendingBlock + 1
  # If this next block doesn't exist yet according to any of our peers, don't
  # return a work item (and sync will be stopped).
  if nextRequestedBlock >= ctx.endBlockNumber:
    return -1

  # Increase queue when there are no free (Initial / Persisted) work items in
  # the queue. At start, queue will be empty.
  if result == -1:
    result = ctx.workQueue.len
    ctx.workQueue.setLen(result + 1)

  # Create new work item when queue was increased, reset when selected work item
  # is at Persisted state.
  var numBlocks = (ctx.endBlockNumber - nextRequestedBlock).truncate(int)
  if numBlocks > maxHeadersFetch:
    numBlocks = maxHeadersFetch
  ctx.workQueue[result] = WantedBlocks(startIndex: nextRequestedBlock, numBlocks: numBlocks.uint, state: Initial)

proc persistWorkItem(ctx: FastSyncCtx, wi: var WantedBlocks): ValidationResult
    {.gcsafe, raises:[Defect,CatchableError].} =
  try:
    result = ctx.chain.persistBlocks(wi.headers, wi.bodies)
  except CatchableError as e:
    error "storing persistent blocks failed",
      error = $e.name, msg = e.msg
    result = ValidationResult.Error
  except Defect as e:
    # Pass through
    raise e
  except Exception as e:
    # Notorious case where the `Chain` reference applied to `persistBlocks()`
    # has the compiler traced a possible `Exception` (i.e. `ctx.chain` could
    # be uninitialised.)
    error "exception while storing persistent blocks",
      error = $e.name, msg = e.msg
    raise (ref Defect)(msg: $e.name & ": " & e.msg)
  case result
  of ValidationResult.OK:
    ctx.finalizedBlock = wi.endIndex
    wi.state = Persisted
  of ValidationResult.Error:
    wi.state = Initial
  # successful or not, we're done with these blocks
  wi.headers = @[]
  wi.bodies = @[]

proc persistPendingWorkItems(ctx: FastSyncCtx): (int, ValidationResult)
    {.gcsafe, raises:[Defect,CatchableError].} =
  var nextStartIndex = ctx.finalizedBlock + 1
  var keepRunning = true
  var hasOutOfOrderBlocks = false
  trace "Looking for out of order blocks"
  while keepRunning:
    keepRunning = false
    hasOutOfOrderBlocks = false
    # Go over the full work queue and check for every work item if it is in
    # Received state and has the next blocks in line to be processed.
    for i in 0 ..< ctx.workQueue.len:
      let start = ctx.workQueue[i].startIndex
      # There should be at least 1 like this, namely the just received work item
      # that initiated this call.
      if ctx.workQueue[i].state == Received:
        if start == nextStartIndex:
          trace "Processing pending work item", number = start
          result = (i, ctx.persistWorkItem(ctx.workQueue[i]))
          # TODO: We can stop here on failure, but have to set
          # hasOutofORderBlocks. Is this always valid?
          nextStartIndex = ctx.finalizedBlock + 1
          keepRunning = true
          break
        else:
          hasOutOfOrderBlocks = true

  ctx.hasOutOfOrderBlocks = hasOutOfOrderBlocks

proc returnWorkItem(ctx: FastSyncCtx, workItem: int): ValidationResult
    {.gcsafe, raises:[Defect,CatchableError].} =
  let wi = addr ctx.workQueue[workItem]
  let askedBlocks = wi.numBlocks.int
  let receivedBlocks = wi.headers.len
  let start = wi.startIndex

  if askedBlocks == receivedBlocks:
    trace "Work item complete",
      start,
      askedBlocks,
      receivedBlocks

    if wi.startIndex != ctx.finalizedBlock + 1:
      trace "Blocks out of order", start, final = ctx.finalizedBlock
      ctx.hasOutOfOrderBlocks = true

    if ctx.hasOutOfOrderBlocks:
      let (index, validation) = ctx.persistPendingWorkItems()
      # Only report an error if it was this peer's work item that failed
      if validation == ValidationResult.Error and index == workItem:
        result = ValidationResult.Error
      # TODO: What about failures on other peers' work items?
      # In that case the peer will probably get disconnected on future erroneous
      # work items, but before this occurs, several more blocks (that will fail)
      # might get downloaded from this peer. This will delay the sync and this
      # should be improved.
    else:
      trace "Processing work item", number = wi.startIndex
      # Validation result needs to be returned so that higher up can be decided
      # to disconnect from this peer in case of error.
      result = ctx.persistWorkItem(wi[])
  else:
    trace "Work item complete but we got fewer blocks than requested, so we're ditching the whole thing.",
      start,
      askedBlocks,
      receivedBlocks
    return ValidationResult.Error

proc handleLostPeer(ctx: FastSyncCtx) =
  # TODO: ask the PeerPool for new connections and then call
  # `obtainBlocksFromPeer`
  discard

proc getBestBlockNumber(p: Peer): Future[BlockNumber] {.async.} =
  let request = BlocksRequest(
    startBlock: HashOrNum(isHash: true,
                          hash: p.state(eth).bestBlockHash),
    maxResults: 1,
    skip: 0,
    reverse: true)

  trace trEthSendSendingGetBlockHeaders, peer=p,
    startBlock=request.startBlock.hash.toHex, max=request.maxResults
  let latestBlock = await p.getBlockHeaders(request)

  if latestBlock.isSome:
    if latestBlock.get.headers.len > 0:
      result = latestBlock.get.headers[0].blockNumber
    trace trEthRecvReceivedBlockHeaders, peer=p,
      count=latestBlock.get.headers.len,
      blockNumber=(if latestBlock.get.headers.len > 0: $result else: "missing")

proc obtainBlocksFromPeer(syncCtx: FastSyncCtx, peer: Peer) {.async.} =
  # Update our best block number
  try:
    let bestBlockNumber = await peer.getBestBlockNumber()
    if bestBlockNumber > syncCtx.endBlockNumber:
      trace "New sync end block number", number = bestBlockNumber
      syncCtx.endBlockNumber = bestBlockNumber
  except TransportError:
    debug "Transport got closed during obtainBlocksFromPeer"
  except CatchableError as e:
    debug "Exception in getBestBlockNumber()", exc = e.name, err = e.msg
    # no need to exit here, because the context might still have blocks to fetch
    # from this peer

  while (let workItemIdx = syncCtx.availableWorkItem(); workItemIdx != -1 and
         peer.connectionState notin {Disconnecting, Disconnected}):
    template workItem: auto = syncCtx.workQueue[workItemIdx]
    workItem.state = Requested
    trace "Requesting block headers", start = workItem.startIndex,
      count = workItem.numBlocks, peer = peer.remote.node
    let request = BlocksRequest(
      startBlock: HashOrNum(isHash: false, number: workItem.startIndex),
      maxResults: workItem.numBlocks,
      skip: 0,
      reverse: false)

    var dataReceived = false
    try:
      trace trEthSendSendingGetBlockHeaders, peer,
        startBlock=request.startBlock.number, max=request.maxResults,
        step=traceStep(request)
      let results = await peer.getBlockHeaders(request)
      if results.isSome:
        trace trEthRecvReceivedBlockHeaders, peer,
          count=results.get.headers.len, requested=request.maxResults
        shallowCopy(workItem.headers, results.get.headers)

        var bodies = newSeqOfCap[BlockBody](workItem.headers.len)
        var hashes = newSeqOfCap[KeccakHash](maxBodiesFetch)
        template fetchBodies() =
          trace trEthSendSendingGetBlockBodies, peer,
            hashes=hashes.len
          let b = await peer.getBlockBodies(hashes)
          if b.isNone:
            raise newException(
              CatchableError, "Was not able to get the block bodies")
          let bodiesLen = b.get.blocks.len
          trace trEthRecvReceivedBlockBodies, peer,
            count=bodiesLen, requested=hashes.len
          if bodiesLen == 0:
            raise newException(CatchableError, "Zero block bodies received for request")
          elif bodiesLen < hashes.len:
            hashes.delete(0, bodiesLen - 1)
          elif bodiesLen == hashes.len:
            hashes.setLen(0)
          else:
            raise newException(CatchableError, "Too many block bodies received for request")
          bodies.add(b.get.blocks)

        var nextIndex = workItem.startIndex
        for i in workItem.headers:
          if i.blockNumber != nextIndex:
            raise newException(CatchableError, "The block numbers are not in sequence. Not processing this workItem.")
          else:
            nextIndex = nextIndex + 1
          hashes.add(blockHash(i))
          if hashes.len == maxBodiesFetch:
            fetchBodies()

        while hashes.len != 0:
          fetchBodies()

        if bodies.len == workItem.headers.len:
          shallowCopy(workItem.bodies, bodies)
          dataReceived = true
        else:
          warn "Bodies len != headers.len", bodies = bodies.len, headers = workItem.headers.len
    except TransportError:
      debug "Transport got closed during obtainBlocksFromPeer"
    except CatchableError as e:
      # the success case sets `dataReceived`, so we can just fall back to the
      # failure path below. If we signal time-outs with exceptions such
      # failures will be easier to handle.
      debug "Exception in obtainBlocksFromPeer()", exc = e.name, err = e.msg

    var giveUpOnPeer = false

    if dataReceived:
      workItem.state = Received
      if syncCtx.returnWorkItem(workItemIdx) != ValidationResult.OK:
        giveUpOnPeer = true
    else:
      giveUpOnPeer = true

    if giveUpOnPeer:
      workItem.state = Initial
      try:
        await peer.disconnect(SubprotocolReason)
      except CatchableError:
        discard
      syncCtx.handleLostPeer()
      break

  trace "Finished obtaining blocks", peer

proc peersAgreeOnChain(a, b: Peer): Future[bool] {.async.} =
  # Returns true if one of the peers acknowledges existence of the best block
  # of another peer.
  var
    a = a
    b = b

  if a.state(eth).bestDifficulty < b.state(eth).bestDifficulty:
    swap(a, b)

  let request = BlocksRequest(
    startBlock: HashOrNum(isHash: true,
                          hash: b.state(eth).bestBlockHash),
    maxResults: 1,
    skip: 0,
    reverse: true)

  trace trEthSendSendingGetBlockHeaders, peer=a,
    startBlock=request.startBlock.hash.toHex, max=request.maxResults
  let latestBlock = await a.getBlockHeaders(request)

  result = latestBlock.isSome and latestBlock.get.headers.len > 0
  if latestBlock.isSome:
    let blockNumber = if result: $latestBlock.get.headers[0].blockNumber
                      else: "missing"
    trace trEthRecvReceivedBlockHeaders, peer=a,
      count=latestBlock.get.headers.len, blockNumber

proc randomTrustedPeer(ctx: FastSyncCtx): Peer =
  var k = rand(ctx.trustedPeers.len - 1)
  var i = 0
  for p in ctx.trustedPeers:
    result = p
    if i == k: return
    inc i

proc startSyncWithPeer(ctx: FastSyncCtx, peer: Peer) {.async.} =
  trace "Start sync", peer, trustedPeers = ctx.trustedPeers.len
  if ctx.trustedPeers.len >= minPeersToStartSync:
    # We have enough trusted peers. Validate new peer against trusted
    if await peersAgreeOnChain(peer, ctx.randomTrustedPeer()):
      ctx.trustedPeers.incl(peer)
      asyncSpawn ctx.obtainBlocksFromPeer(peer)
  elif ctx.trustedPeers.len == 0:
    # Assume the peer is trusted, but don't start sync until we reevaluate
    # it with more peers
    trace "Assume trusted peer", peer
    ctx.trustedPeers.incl(peer)
  else:
    # At this point we have some "trusted" candidates, but they are not
    # "trusted" enough. We evaluate `peer` against all other candidates.
    # If one of the candidates disagrees, we swap it for `peer`. If all
    # candidates agree, we add `peer` to trusted set. The peers in the set
    # will become "fully trusted" (and sync will start) when the set is big
    # enough
    var
      agreeScore = 0
      disagreedPeer: Peer

    for tp in ctx.trustedPeers:
      if await peersAgreeOnChain(peer, tp):
        inc agreeScore
      else:
        disagreedPeer = tp

    let disagreeScore = ctx.trustedPeers.len - agreeScore

    if agreeScore == ctx.trustedPeers.len:
      ctx.trustedPeers.incl(peer) # The best possible outcome
    elif disagreeScore == 1:
      trace "Peer is no longer trusted for sync", peer
      ctx.trustedPeers.excl(disagreedPeer)
      ctx.trustedPeers.incl(peer)
    else:
      trace "Peer not trusted for sync", peer

    if ctx.trustedPeers.len == minPeersToStartSync:
      for p in ctx.trustedPeers:
        asyncSpawn ctx.obtainBlocksFromPeer(p)


proc onPeerConnected(ctx: FastSyncCtx, peer: Peer) =
  trace "New candidate for sync", peer
  try:
    let f = ctx.startSyncWithPeer(peer)
    f.callback = proc(data: pointer) {.gcsafe.} =
      if f.failed:
        if f.error of TransportError:
          debug "Transport got closed during startSyncWithPeer"
        else:
          error "startSyncWithPeer failed", msg = f.readError.msg, peer
  except TransportError:
    debug "Transport got closed during startSyncWithPeer"
  except CatchableError as e:
    debug "Exception in startSyncWithPeer()", exc = e.name, err = e.msg


proc onPeerDisconnected(ctx: FastSyncCtx, p: Peer) =
  trace "peer disconnected ", peer = p
  ctx.trustedPeers.excl(p)

proc new*(T: type FastSyncCtx; ethNode: EthereumNode): T
    {.gcsafe, raises:[Defect,CatchableError].} =
  FastSyncCtx(
    # workQueue:           n/a
    # endBlockNumber:      n/a
    # hasOutOfOrderBlocks: n/a
    chain:          ethNode.chain,
    peerPool:       ethNode.peerPool,
    trustedPeers:   initHashSet[Peer](),
    finalizedBlock: ethNode.chain.getBestBlockHeader.blockNumber)

proc start*(ctx: FastSyncCtx) =
  ## Code for the fast blockchain sync procedure:
  ##   <https://github.com/ethereum/wiki/wiki/Parallel-Block-Downloads>_
  ##   <https://github.com/ethereum/go-ethereum/pull/1889__
  var po = PeerObserver(
    onPeerConnected:
      proc(p: Peer) {.gcsafe.} =
         ctx.onPeerConnected(p),
    onPeerDisconnected:
      proc(p: Peer) {.gcsafe.} =
        ctx.onPeerDisconnected(p))
  po.setProtocol eth
  ctx.peerPool.addObserver(ctx, po)

# End