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# Nimbus - Rapidly converge on and track the canonical chain head of each peer
#
# Copyright (c) 2021 Status Research & Development GmbH
# Licensed under either of
# * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
# * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
# at your option. This file may not be copied, modified, or distributed except according to those terms.
## This module fetches and tracks the canonical chain head of each connected
## peer. (Or in future, each peer we care about; we won't poll them all so
## often.)
##
## This is for when we aren't sure of the block number of a peer's canonical
## chain head. Most of the time, after finding which block, it quietly polls
## to track small updates to the "best" block number and hash of each peer.
##
## But sometimes that can get out of step. If there has been a deeper reorg
## than our tracking window, or a burst of more than a few new blocks, network
## delays, downtime, or the peer is itself syncing. Perhaps we stopped Nimbus
## and restarted a while later, e.g. suspending a laptop or Control-Z. Then
## this will catch up. It is even possible that the best hash the peer gave us
## in the `Status` handshake has disappeared by the time we query for the
## corresponding block number, so we start at zero.
##
## The steps here perform a robust and efficient O(log N) search to rapidly
## converge on the new best block if it's moved out of the polling window no
## matter where it starts, confirm the peer's canonical chain head boundary,
## then track the peer's chain head in real-time by polling. The method is
## robust to peer state changes at any time.
##
## The purpose is to:
##
## - Help with finding a peer common chain prefix ("fast sync pivot") in a
## consistent, fast and explicit way.
##
## - Catch up quickly after any long pauses of network downtime, program not
## running, or deep chain reorgs.
##
## - Be able to display real-time peer states, so they are less mysterious.
##
## - Tell the beam/snap/trie sync processes when to start and what blocks to
## fetch, and keep those fetchers in the head-adjacent window of the
## ever-changing chain.
##
## - Help the sync process bootstrap usefully when we only have one peer,
## speculatively fetching and validating what data we can before we have more
## peers to corroborate the consensus.
##
## - Help detect consensus failures in the network.
##
## We cannot assume a peer's canonical chain stays the same or only gains new
## blocks from one query to the next. There can be reorgs, including deep
## reorgs. When a reorg happens, the best block number can decrease if the new
## canonical chain is shorter than the old one, and the best block hash we
## previously knew can become unavailable on the peer. So we must detect when
## the current best block disappears and be able to reduce block number.
{.push raises: [Defect].}
import
std/bitops,
chronos, stint, chronicles, stew/byteutils,
eth/[common/eth_types, rlp, p2p],
eth/p2p/[rlpx, private/p2p_types],
../p2p/chain/chain_desc,
"."/[sync_types, protocol_ethxx, pie/slicer]
const
syncLockedMinimumReply = 8
## Minimum number of headers we assume any peers will send if they have
## them in contiguous ascending queries. Fewer than this confirms we have
## found the peer's canonical chain head boundary. Must be at least 2, and
## at least `syncLockedQueryOverlap+2` to stay `SyncLocked` when the chain
## extends. Should not be large as that would be stretching assumptions
## about peer implementations. 8 is chosen as it allows 3-deep extensions
## and 3-deep reorgs to be followed in a single round trip.
syncLockedQueryOverlap = 4
## Number of headers to re-query on each poll when `SyncLocked` so that we
## get small reorg updates in one round trip. Must be no more than
## `syncLockedMinimumReply-1`, no more than `syncLockedMinimumReply-2` to
## stay `SyncLocked` when the chain extends, and not too large to avoid
## excessive duplicate fetching. 4 is chosen as it allows 3-deep reorgs
## to be followed in single round trip.
syncLockedQuerySize = 192
## Query size when polling `SyncLocked`. Must be at least
## `syncLockedMinimumReply`. Large is fine, if we get a large reply the
## values are almost always useful.
syncHuntQuerySize = 16
## Query size when hunting for canonical head boundary. Small is good
## because we don't want to keep most of the headers at hunt time.
syncHuntForwardExpandShift = 4
## Expansion factor during `SyncHuntForward` exponential search.
## 16 is chosen for rapid convergence when bootstrapping or catching up.
syncHuntBackwardExpandShift = 1
## Expansion factor during `SyncHuntBackward` exponential search.
## 2 is chosen for better convergence when tracking a chain reorg.
doAssert syncLockedMinimumReply >= 2
doAssert syncLockedMinimumReply >= syncLockedQueryOverlap + 2
doAssert syncLockedQuerySize <= maxHeadersFetch
doAssert syncHuntQuerySize >= 1 and syncHuntQuerySize <= maxHeadersFetch
doAssert syncHuntForwardExpandShift >= 1 and syncHuntForwardExpandShift <= 8
doAssert syncHuntBackwardExpandShift >= 1 and syncHuntBackwardExpandShift <= 8
proc clearSyncStateRoot(sp: SyncPeer) =
if sp.syncStateRoot.isSome:
debug "Sync: Stopping state sync from this peer", peer=sp
sp.syncStateRoot = none(TrieHash)
proc setSyncStateRoot(sp: SyncPeer, blockNumber: BlockNumber,
blockHash: BlockHash, stateRoot: TrieHash) =
if sp.syncStateRoot.isNone:
debug "Sync: Starting state sync from this peer", peer=sp,
`block`=blockNumber, blockHash=($blockHash), stateRoot=($stateRoot)
elif sp.syncStateRoot.unsafeGet != stateRoot:
trace "Sync: Adjusting state sync root from this peer", peer=sp,
`block`=blockNumber, blockHash=($blockHash), stateRoot=($stateRoot)
sp.syncStateRoot = some(stateRoot)
if not sp.startedFetch:
sp.startedFetch = true
trace "Sync: Starting to download block state", peer=sp,
`block`=blockNumber, blockHash=($blockHash), stateRoot=($stateRoot)
asyncSpawn sp.stateFetch()
proc traceSyncLocked(sp: SyncPeer, bestNumber: BlockNumber,
bestHash: BlockHash) =
## Trace messages when peer canonical head is confirmed or updated.
if sp.syncMode != SyncLocked:
debug "Sync: Now tracking chain head of peer",
`block`=bestNumber, blockHash=($bestHash), peer=sp
elif bestNumber > sp.bestBlockNumber:
if bestNumber == sp.bestBlockNumber + 1:
debug "Sync: Peer chain head advanced one block", peer=sp,
advance=1, `block`=bestNumber, blockHash=($bestHash)
else:
debug "Sync: Peer chain head advanced some blocks", peer=sp,
advance=(sp.bestBlockNumber - bestNumber),
`block`=bestNumber, blockHash=($bestHash)
elif bestNumber < sp.bestBlockNumber or bestHash != sp.bestBlockHash:
debug "Sync: Peer chain head reorg detected", peer=sp,
advance=(sp.bestBlockNumber - bestNumber),
`block`=bestNumber, blockHash=($bestHash)
proc setSyncLocked(sp: SyncPeer, bestNumber: BlockNumber,
bestHash: BlockHash) =
## Actions to take when peer canonical head is confirmed or updated.
sp.traceSyncLocked(bestNumber, bestHash)
sp.bestBlockNumber = bestNumber
sp.bestBlockHash = bestHash
sp.syncMode = SyncLocked
proc setHuntBackward(sp: SyncPeer, lowestAbsent: BlockNumber) =
## Start exponential search mode backward due to new uncertainty.
sp.syncMode = SyncHuntBackward
sp.huntStep = 0
# Block zero is always present.
sp.huntLow = 0.toBlockNumber
# Zero `lowestAbsent` is never correct, but an incorrect peer could send it.
sp.huntHigh = if lowestAbsent > 0: lowestAbsent else: 1.toBlockNumber
sp.clearSyncStateRoot()
proc setHuntForward(sp: SyncPeer, highestPresent: BlockNumber) =
## Start exponential search mode forward due to new uncertainty.
sp.syncMode = SyncHuntForward
sp.huntStep = 0
sp.huntLow = highestPresent
sp.huntHigh = high(BlockNumber)
sp.clearSyncStateRoot()
proc updateHuntAbsent(sp: SyncPeer, lowestAbsent: BlockNumber) =
## Converge uncertainty range backward.
if lowestAbsent < sp.huntHigh:
sp.huntHigh = lowestAbsent
# If uncertainty range has moved outside the search window, change to hunt
# backward to block zero. Note that empty uncertainty range is allowed
# (empty range is `huntLow + 1 == huntHigh`).
if sp.huntHigh <= sp.huntLow:
sp.setHuntBackward(lowestAbsent)
sp.clearSyncStateRoot()
proc updateHuntPresent(sp: SyncPeer, highestPresent: BlockNumber) =
## Converge uncertainty range forward.
if highestPresent > sp.huntLow:
sp.huntLow = highestPresent
# If uncertainty range has moved outside the search window, change to hunt
# forward to no upper limit. Note that empty uncertainty range is allowed
# (empty range is `huntLow + 1 == huntHigh`).
if sp.huntLow >= sp.huntHigh:
sp.setHuntForward(highestPresent)
sp.clearSyncStateRoot()
proc peerSyncChainEmptyReply(sp: SyncPeer, request: BlocksRequest) =
## Handle empty `GetBlockHeaders` reply. This means `request.startBlock` is
## absent on the peer. If it was `SyncLocked` there must have been a reorg
## and the previous canonical chain head has disappeared. If hunting, this
## updates the range of uncertainty.
# Treat empty response to a request starting from block 1 as equivalent to
# length 1 starting from block 0 in `peerSyncChainNonEmptyReply`. We treat
# every peer as if it would send genesis for block 0, without asking for it.
if request.skip == 0 and not request.reverse and
not request.startBlock.isHash and
request.startBlock.number == 1.toBlockNumber:
sp.setSyncLocked(0.toBlockNumber, sp.peer.network.chain.genesisHash)
sp.setSyncStateRoot(0.toBlockNumber, sp.peer.network.chain.genesisHash,
sp.peer.network.chain.Chain.genesisStateRoot)
return
if sp.syncMode == SyncLocked or sp.syncMode == SyncOnlyHash:
inc sp.stats.ok.reorgDetected
trace "Sync: Peer reorg detected, best block disappeared", peer=sp,
`block`=request.startBlock
let lowestAbsent = request.startBlock.number
case sp.syncMode:
of SyncLocked:
# If this message doesn't change our knowledge, ignore it.
if lowestAbsent > sp.bestBlockNumber:
return
# Due to a reorg, peer's canonical head has lower block number, outside
# our tracking window. Sync lock is no longer valid. Switch to hunt
# backward to find the new canonical head.
sp.setHuntBackward(lowestAbsent)
of SyncOnlyHash:
# Due to a reorg, peer doesn't have the block hash it originally gave us.
# Switch to hunt forward from block zero to find the canonical head.
sp.setHuntForward(0.toBlockNumber)
of SyncHuntForward, SyncHuntBackward, SyncHuntRange, SyncHuntRangeFinal:
# Update the hunt range.
sp.updateHuntAbsent(lowestAbsent)
# Update best block number. It is invalid except when `SyncLocked`, but
# still useful as a hint of what we knew recently, for example in displays.
if lowestAbsent <= sp.bestBlockNumber:
sp.bestBlockNumber = if lowestAbsent == 0.toBlockNumber: lowestAbsent
else: lowestAbsent - 1.toBlockNumber
sp.bestBlockHash = default(typeof(sp.bestBlockHash))
proc peerSyncChainNonEmptyReply(sp: SyncPeer, request: BlocksRequest,
headers: openArray[BlockHeader]) =
## Handle non-empty `GetBlockHeaders` reply. This means `request.startBlock`
## is present on the peer and in its canonical chain (unless the request was
## made with a hash). If it's a short, contiguous, ascending order reply, it
## reveals the abrupt transition at the end of the chain and we have learned
## or reconfirmed the real-time head block. If hunting, this updates the
## range of uncertainty.
let len = headers.len
let highestIndex = if request.reverse: 0 else: len - 1
# We assume a short enough reply means we've learned the peer's canonical
# head, because it would have replied with another header if not at the head.
# This is not justified when the request used a general hash, because the
# peer doesn't have to reply with its canonical chain in that case, except it
# is still justified if the hash was the known canonical head, which is
# the case in a `SyncOnlyHash` request.
if len < syncLockedMinimumReply and
request.skip == 0 and not request.reverse and
len.uint < request.maxResults:
let blockHash = headers[highestIndex].blockHash
sp.setSyncLocked(headers[highestIndex].blockNumber, blockHash)
sp.setSyncStateRoot(headers[highestIndex].blockNumber, blockHash,
headers[highestIndex].stateRoot)
return
# Be careful, this number is from externally supplied data and arithmetic
# in the upward direction could overflow.
let highestPresent = headers[highestIndex].blockNumber
# A reply that isn't short enough for the canonical head criterion above
# tells us headers up to some number, but it doesn't tell us if there are
# more after it in the peer's canonical chain. We have to request more
# headers to find out.
case sp.syncMode:
of SyncLocked:
# If this message doesn't change our knowledge, ignore it.
if highestPresent <= sp.bestBlockNumber:
return
# Sync lock is no longer valid as we don't have confirmed canonical head.
# Switch to hunt forward to find the new canonical head.
sp.setHuntForward(highestPresent)
of SyncOnlyHash:
# As `SyncLocked` but without the block number check.
sp.setHuntForward(highestPresent)
of SyncHuntForward, SyncHuntBackward, SyncHuntRange, SyncHuntRangeFinal:
# Update the hunt range.
sp.updateHuntPresent(highestPresent)
# Update best block number. It is invalid except when `SyncLocked`, but
# still useful as a hint of what we knew recently, for example in displays.
if highestPresent > sp.bestBlockNumber:
sp.bestBlockNumber = highestPresent
sp.bestBlockHash = headers[highestIndex].blockHash
proc peerSyncChainRequest(sp: SyncPeer, request: var BlocksRequest) =
## Choose `GetBlockHeaders` parameters when hunting or following the canonical
## chain of a peer.
request = BlocksRequest(
startBlock: HashOrNum(isHash: false),
skip: 0,
reverse: false
)
if sp.syncMode == SyncLocked:
# Stable and locked. This is just checking for changes including reorgs.
# `sp.bestBlockNumber` was recently the head of the peer's canonical
# chain. We must include this block number to detect when the canonical
# chain gets shorter versus no change.
request.startBlock.number =
if sp.bestBlockNumber <= syncLockedQueryOverlap:
# Every peer should send genesis for block 0, so don't ask for it.
# `peerSyncChainEmptyReply` has logic to handle this reply as if it
# was for block 0. Aside from saving bytes, this is more robust if
# some client doesn't do genesis reply correctly.
1.toBlockNumber
else:
min(sp.bestBlockNumber - syncLockedQueryOverlap.toBlockNumber,
high(BlockNumber) - (syncLockedQuerySize - 1).toBlockNumber)
request.maxResults = syncLockedQuerySize
return
if sp.syncMode == SyncOnlyHash:
# We only have the hash of the recent head of the peer's canonical chain.
# Like `SyncLocked`, query more than one item to detect when the
# canonical chain gets shorter, no change or longer.
request.startBlock = HashOrNum(isHash: true, hash: sp.bestBlockHash)
request.maxResults = syncLockedQuerySize
return
# Searching for the peers's canonical head. An ascending query is always
# used, regardless of search direction. This is because a descending query
# (`reverse = true` and `maxResults > 1`) is useless for searching: Either
# `startBlock` is present, in which case the extra descending results
# contribute no more information about the canonical head boundary, or
# `startBlock` is absent in which case there are zero results. It's not
# defined in the `eth` specification that there must be zero results (in
# principle peers could return the lower numbered blocks), but in practice
# peers stop at the first absent block in the sequence from `startBlock`.
#
# Guaranteeing O(log N) time convergence in all scenarios requires some
# properties to be true in both exponential search (expanding) and
# quasi-binary search (converging in a range). The most important is that
# the gap to `startBlock` after `huntLow` and also before `huntHigh` are
# proportional to the query step, where the query step is `huntStep`
# exponentially expanding each round, or `maxStep` approximately evenly
# distributed in the range.
#
# `huntLow+1` must not be used consistently as the start, even with a large
# enough query step size, as that will sometimes take O(N) to converge in
# both the exponential and quasi-binary searches. (Ending at `huntHigh-1`
# is fine if `syncHuntQuerySize > 1`. This asymmetry is due to ascending
# queries (see earlier comment), and non-empty truncated query reply being
# proof of presence before the truncation point, but not proof of absence
# after it. A reply can be truncated just because the peer decides to.)
#
# The proportional gap requirement is why we divide by query size here,
# instead of stretching to fit more strictly with `(range-1)/(size-1)`.
const syncHuntFinalSize = max(2, syncHuntQuerySize)
var maxStep: typeof(request.skip)
let fullRangeClamped =
if sp.huntHigh <= sp.huntLow: typeof(maxStep)(0)
else: min(high(typeof(maxStep)).toBlockNumber,
sp.huntHigh - sp.huntLow).truncate(typeof(maxStep)) - 1
if fullRangeClamped >= syncHuntFinalSize: # `SyncHuntRangeFinal` condition.
maxStep = if syncHuntQuerySize == 1:
fullRangeClamped
elif (syncHuntQuerySize and (syncHuntQuerySize-1)) == 0:
fullRangeClamped shr fastLog2(syncHuntQuerySize)
else:
fullRangeClamped div syncHuntQuerySize
doAssert syncHuntFinalSize >= syncHuntQuerySize
doAssert maxStep >= 1 # Ensured by the above assertion.
# Check for exponential search (expanding). Iterate `huntStep`. O(log N)
# requires `startBlock` to be offset from `huntLow`/`huntHigh`.
if sp.syncMode in {SyncHuntForward, SyncHuntBackward} and
fullRangeClamped >= syncHuntFinalSize:
let forward = sp.syncMode == SyncHuntForward
let expandShift = if forward: syncHuntForwardExpandShift
else: syncHuntBackwardExpandShift
# Switches to range search when this condition is no longer true.
if sp.huntStep < maxStep shr expandShift:
# The `if` above means the next line cannot overflow.
sp.huntStep = if sp.huntStep > 0: sp.huntStep shl expandShift else: 1
# Satisfy the O(log N) convergence conditions.
request.startBlock.number =
if forward: sp.huntLow + sp.huntStep.toBlockNumber
else: sp.huntHigh - (sp.huntStep * syncHuntQuerySize).toBlockNumber
request.maxResults = syncHuntQuerySize
request.skip = sp.huntStep - 1
return
# For tracing/display.
sp.huntStep = maxStep
sp.syncMode = SyncHuntRange
if maxStep > 0:
# Quasi-binary search (converging in a range). O(log N) requires
# `startBlock` to satisfy the constraints described above, with the
# proportionality from both ends of the range. The optimal information
# gathering position is tricky and doesn't make much difference, so don't
# bother. We'll centre the query in the range.
var offset = fullRangeClamped - maxStep * (syncHuntQuerySize-1)
# Rounding must bias towards end to ensure `offset >= 1` after this.
offset -= offset shr 1
request.startBlock.number = sp.huntLow + offset.toBlockNumber
request.maxResults = syncHuntQuerySize
request.skip = maxStep - 1
else:
# Small range, final step. At `fullRange == 0` we must query at least one
# block before and after the range to confirm the canonical head boundary,
# or find it has moved. This ensures progress without getting stuck. When
# `fullRange` is small this is also beneficial, to get `SyncLocked` in one
# round trip from hereand it simplifies the other search branches below.
# Ideally the query is similar to `SyncLocked`, enough to get `SyncLocked`
# in one round trip, and accommodate a small reorg or extension.
const afterSoftMax = syncLockedMinimumReply - syncLockedQueryOverlap
const beforeHardMax = syncLockedQueryOverlap
let extra = syncHuntFinalSize - fullRangeClamped
var before = (extra + 1) shr 1
before = max(before + afterSoftMax, extra) - afterSoftMax
before = min(before, beforeHardMax)
# See `SyncLocked` case.
request.startBlock.number =
if sp.bestBlockNumber <= before.toBlockNumber: 1.toBlockNumber
else: min(sp.bestBlockNumber - before.toBlockNumber,
high(BlockNumber) - (syncHuntFinalSize - 1).toBlockNumber)
request.maxResults = syncHuntFinalSize
sp.syncMode = SyncHuntRangeFinal
proc peerSyncChainTrace(sp: SyncPeer) =
## To be called after `peerSyncChainRequest` has updated state.
case sp.syncMode:
of SyncLocked:
trace "Sync: SyncLocked",
bestBlock=sp.bestBlockNumber, bestBlockHash=($sp.bestBlockHash)
of SyncOnlyHash:
trace "Sync: OnlyHash", bestBlockHash=($sp.bestBlockHash)
of SyncHuntForward:
template highMax(n: BlockNumber): string =
if n == high(BlockNumber): "max" else: $n
trace "Sync: HuntForward",
low=sp.huntLow, high=highMax(sp.huntHigh), step=sp.huntStep
of SyncHuntBackward:
trace "Sync: HuntBackward",
low=sp.huntLow, high=sp.huntHigh, step=sp.huntStep
of SyncHuntRange:
trace "Sync: HuntRange",
low=sp.huntLow, high=sp.huntHigh, step=sp.huntStep
of SyncHuntRangeFinal:
trace "Sync: HuntRangeFinal",
low=sp.huntLow, high=sp.huntHigh, step=1
proc peerHuntCanonical*(sp: SyncPeer) {.async.} =
## Query a peer to update our knowledge of its canonical chain and its best
## block, which is its canonical chain head. This can be called at any time
## after a peer has negotiated the connection.
##
## This function is called in an exponential then binary search style
## during initial sync to find the canonical head, real-time polling
## afterwards to check for updates.
##
## All replies to this query are part of the peer's canonical chain at the
## time the peer sends them.
# If we send multiple `GetBlockHeaders` requests, the replies can be out of
# order, and prior to eth/66 there is no request-id. We'll avoid this
# problem by never sending overlapping `GetBlockHeaders` to the same peer.
if sp.pendingGetBlockHeaders:
#trace ">| Blocked overlapping eth.GetBlockHeaders (0x03)", peer=sp
await sleepAsync(chronos.milliseconds(500))
return
sp.pendingGetBlockHeaders = true
var request {.noinit.}: BlocksRequest
sp.peerSyncChainRequest(request)
if tracePackets:
if request.maxResults == 1 and request.startBlock.isHash:
tracePacket ">> Sending eth.GetBlockHeaders/Hash (0x03)", peer=sp,
blockHash=($request.startBlock.hash), count=1
elif request.maxResults == 1:
tracePacket ">> Sending eth.GetBlockHeaders (0x03)", peer=sp,
`block`=request.startBlock, count=1
elif request.startBlock.isHash:
tracePacket ">> Sending eth.GetBlockHeaders/Hash (0x03)", peer=sp,
firstBlockHash=request.startBlock, count=request.maxResults,
step=traceStep(request)
else:
tracePacket ">> Sending eth.GetBlockHeaders (0x03)", peer=sp,
firstBlock=request.startBlock, count=request.maxResults,
step=traceStep(request)
inc sp.stats.ok.getBlockHeaders
var reply: typeof await sp.peer.getBlockHeaders(request)
try:
reply = await sp.peer.getBlockHeaders(request)
except CatchableError as e:
traceNetworkError "<< Error waiting for reply to eth.GetBlockHeaders (0x03)",
peer=sp, error=e.msg
inc sp.stats.major.networkErrors
sp.stopped = true
return
if reply.isNone:
traceTimeout "<< Timeout waiting for reply to eth.GetBlockHeaders (0x03)",
peer=sp
# TODO: Should disconnect?
inc sp.stats.minor.timeoutBlockHeaders
return
let len = reply.get.headers.len
if tracePackets:
if len == 0:
tracePacket "<< Got EMPTY reply eth.BlockHeaders (0x04)", peer=sp,
got=0, requested=request.maxResults
else:
let firstBlock = reply.get.headers[0].blockNumber
let lastBlock = reply.get.headers[len - 1].blockNumber
tracePacket "<< Got reply eth.BlockHeaders (0x04)", peer=sp,
got=len, requested=request.maxResults, firstBlock, lastBlock
sp.pendingGetBlockHeaders = false
if len > request.maxResults.int:
tracePacketError "<< Protocol violation, excess headers in eth.BlockHeaders (0x04)",
peer=sp, got=len, requested=request.maxResults
# TODO: Should disconnect.
inc sp.stats.major.excessBlockHeaders
return
if len > 0:
# TODO: Check this is not copying the `headers`.
sp.peerSyncChainNonEmptyReply(request, reply.get.headers)
else:
sp.peerSyncChainEmptyReply(request)
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