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# beacon_chain
# Copyright (c) 2018-2024 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: [].}
import
# Standard library
std/[tables, typetraits],
# Status libraries
chronicles,
results,
# Internal
../spec/datatypes/base,
../spec/helpers,
# Fork choice
./fork_choice_types
logScope:
topics = "fork_choice"
export results
# https://github.com/ethereum/consensus-specs/blob/v1.3.0/specs/phase0/fork-choice.md
# This is a port of https://github.com/sigp/lighthouse/pull/804
# which is a port of "Proto-Array": https://github.com/protolambda/lmd-ghost
# See also:
# - Protolambda port of Lighthouse: https://github.com/protolambda/eth2-py-hacks/blob/ae286567/proto_array.py
# - Prysmatic writeup: https://hackmd.io/bABJiht3Q9SyV3Ga4FT9lQ#High-level-concept
# - Gasper Whitepaper: https://arxiv.org/abs/2003.03052
# Helpers
# ----------------------------------------------------------------------
func tiebreak(a, b: Eth2Digest): bool =
## Fork-Choice tie-break between 2 digests
## Currently implemented as `>=` (greater or equal)
## on the binary representation
for i in 0 ..< a.data.len:
if a.data[i] < b.data[i]:
return false
elif a.data[i] > b.data[i]:
return true
# else we have equality so far
return true
template unsafeGet*[K, V](table: Table[K, V], key: K): V =
## Get a value from a Nim Table, turning KeyError into
## an AssertionError defect
# Pointer is used to work around the lack of a `var` withValue
try:
table[key]
except KeyError as exc:
raiseAssert(exc.msg)
func `[]`(nodes: ProtoNodes, idx: Index): Option[ProtoNode] =
## Retrieve a ProtoNode at "Index"
if idx < nodes.offset:
return none(ProtoNode)
let i = idx - nodes.offset
if i >= nodes.buf.len:
return none(ProtoNode)
some(nodes.buf[i])
func len*(nodes: ProtoNodes): int =
nodes.buf.len
func add(nodes: var ProtoNodes, node: ProtoNode) =
nodes.buf.add node
# Forward declarations
# ----------------------------------------------------------------------
func maybeUpdateBestChildAndDescendant(self: var ProtoArray,
parentIdx: Index,
childIdx: Index): FcResult[void]
func nodeIsViableForHead(
self: var ProtoArray, node: ProtoNode, nodeIdx: Index): bool
func nodeLeadsToViableHead(
self: var ProtoArray, node: ProtoNode, nodeIdx: Index): FcResult[bool]
# ProtoArray routines
# ----------------------------------------------------------------------
func init*(
T: type ProtoArray, checkpoints: FinalityCheckpoints): T =
let node = ProtoNode(
bid: BlockId(
slot: checkpoints.finalized.epoch.start_slot,
root: checkpoints.finalized.root),
parent: none(int),
checkpoints: checkpoints,
weight: 0,
invalid: false,
bestChild: none(int),
bestDescendant: none(int))
T(checkpoints: checkpoints,
nodes: ProtoNodes(buf: @[node], offset: 0),
indices: {node.bid.root: 0}.toTable())
iterator realizePendingCheckpoints*(
self: var ProtoArray): FinalityCheckpoints =
# Pull-up chain tips from previous epoch
for idx, unrealized in self.currentEpochTips.pairs():
let physicalIdx = idx - self.nodes.offset
if unrealized != self.nodes.buf[physicalIdx].checkpoints:
trace "Pulling up chain tip",
blck = self.nodes.buf[physicalIdx].bid.root,
checkpoints = self.nodes.buf[physicalIdx].checkpoints,
unrealized
self.nodes.buf[physicalIdx].checkpoints = unrealized
yield unrealized
# Reset tip tracking for new epoch
self.currentEpochTips.clear()
# https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.3/specs/phase0/fork-choice.md#get_weight
func calculateProposerBoost(justifiedTotalActiveBalance: Gwei): Gwei =
let committee_weight = justifiedTotalActiveBalance div SLOTS_PER_EPOCH
(committee_weight * PROPOSER_SCORE_BOOST) div 100
func applyScoreChanges*(self: var ProtoArray,
deltas: var openArray[Delta],
currentEpoch: Epoch,
checkpoints: FinalityCheckpoints,
justifiedTotalActiveBalance: Gwei,
proposerBoostRoot: Eth2Digest): FcResult[void] =
## Iterate backwards through the array, touching all nodes and their parents
## and potentially the best-child of each parent.
#
# The structure of `self.nodes` array ensures that the child of each node
# is always touched before its parent.
#
# For each node the following is done:
#
# 1. Update the node's weight with the corresponding delta.
# 2. Backpropagate each node's delta to its parent's delta.
# 3. Compare the current node with the parent's best-child,
# updating if the current node should become the best-child
# 4. If required, update the parent's best-descendant with the current node
# or its best-descendant
doAssert self.indices.len == self.nodes.len # By construction
if deltas.len != self.indices.len:
return err ForkChoiceError(
kind: fcInvalidDeltaLen,
deltasLen: deltas.len,
indicesLen: self.indices.len)
self.currentEpoch = currentEpoch
doAssert checkpoints.finalized.epoch >= self.checkpoints.finalized.epoch
doAssert checkpoints.finalized.epoch > self.checkpoints.finalized.epoch or
checkpoints.finalized.root == self.checkpoints.finalized.root or
self.checkpoints.finalized.epoch == GENESIS_EPOCH
self.checkpoints = checkpoints
## Alias
# This cannot raise the IndexError exception, how to tell compiler?
template node: untyped {.dirty.} =
self.nodes.buf[nodePhysicalIdx]
# Default value, if not otherwise set in first node loop
var proposerBoostScore: Gwei
# Iterate backwards through all the indices in `self.nodes`
for nodePhysicalIdx in countdown(self.nodes.len - 1, 0):
if node.bid.root.isZero:
continue
var nodeDelta = deltas[nodePhysicalIdx]
# If we find the node for which the proposer boost was previously applied,
# decrease the delta by the previous score amount.
if (not self.previousProposerBoostRoot.isZero) and
self.previousProposerBoostRoot == node.bid.root:
if nodeDelta < 0 and
nodeDelta - low(Delta) < self.previousProposerBoostScore.int64:
return err ForkChoiceError(
kind: fcDeltaUnderflow,
index: nodePhysicalIdx)
nodeDelta -= self.previousProposerBoostScore.int64
# If we find the node matching the current proposer boost root, increase
# the delta by the new score amount.
#
# https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.3/specs/phase0/fork-choice.md#get_weight
if (not proposerBoostRoot.isZero) and proposerBoostRoot == node.bid.root:
proposerBoostScore = calculateProposerBoost(justifiedTotalActiveBalance)
if nodeDelta >= 0 and
high(Delta) - nodeDelta < proposerBoostScore.int64:
return err ForkChoiceError(
kind: fcDeltaOverflow,
index: nodePhysicalIdx)
nodeDelta += proposerBoostScore.int64
# Apply the delta to the node
# We fail fast if underflow, which shouldn't happen.
# Note that delta can be negative but weight cannot
let weight = node.weight + nodeDelta
if weight < 0:
return err ForkChoiceError(
kind: fcDeltaUnderflow,
index: nodePhysicalIdx)
node.weight = weight
# If the node has a parent, try to update its best-child and best-descendent
if node.parent.isSome():
let parentLogicalIdx = node.parent.unsafeGet()
let parentPhysicalIdx = parentLogicalIdx - self.nodes.offset
if parentPhysicalIdx < 0:
# Orphan, for example
# 0
# / \
# 2 1
# |
# 3
# |
# 4
# -------pruned here ------
# 5 6
# |
# 7
# |
# 8
# / \
# 9 10
#
# with 5 the canonical chain and 6 a discarded fork
# that will be pruned next.
continue
if parentPhysicalIdx >= deltas.len:
return err ForkChoiceError(
kind: fcInvalidParentDelta,
index: parentPhysicalIdx)
# Back-propagate the nodes delta to its parent.
deltas[parentPhysicalIdx] += nodeDelta
# After applying all deltas, update the `previous_proposer_boost`.
self.previousProposerBoostRoot = proposerBoostRoot
self.previousProposerBoostScore = proposerBoostScore
for nodePhysicalIdx in countdown(self.nodes.len - 1, 0):
if node.bid.root.isZero:
continue
if node.parent.isSome():
let parentLogicalIdx = node.parent.unsafeGet()
let parentPhysicalIdx = parentLogicalIdx - self.nodes.offset
if parentPhysicalIdx < 0:
# Orphan
continue
let nodeLogicalIdx = nodePhysicalIdx + self.nodes.offset
? self.maybeUpdateBestChildAndDescendant(parentLogicalIdx, nodeLogicalIdx)
ok()
func onBlock*(self: var ProtoArray,
bid: BlockId,
parent: Eth2Digest,
checkpoints: FinalityCheckpoints,
unrealized = none(FinalityCheckpoints)): FcResult[void] =
## Register a block with the fork choice
## A block `hasParentInForkChoice` may be false
## on fork choice initialization:
## - either from Genesis
## - or from a finalized state loaded from database
# Note: if parent is an "Option" type, we can run out of stack space.
# If the block is already known, ignore it
if bid.root in self.indices:
return ok()
var parentIdx: Index
self.indices.withValue(parent, index) do:
parentIdx = index[]
do:
return err ForkChoiceError(
kind: fcUnknownParent,
childRoot: bid.root,
parentRoot: parent)
let nodeLogicalIdx = self.nodes.offset + self.nodes.buf.len
let node = ProtoNode(
bid: bid,
parent: some(parentIdx),
checkpoints: checkpoints,
weight: 0,
invalid: false,
bestChild: none(int),
bestDescendant: none(int))
self.indices[node.bid.root] = nodeLogicalIdx
self.nodes.add node
self.currentEpochTips.del parentIdx
if unrealized.isSome:
self.currentEpochTips[nodeLogicalIdx] = unrealized.get
? self.maybeUpdateBestChildAndDescendant(parentIdx, nodeLogicalIdx)
ok()
func findHead*(self: var ProtoArray,
head: var Eth2Digest,
justifiedRoot: Eth2Digest): FcResult[void] =
## Follows the best-descendant links to find the best-block (i.e. head-block)
##
## ️ Warning
## The result may not be accurate if `onBlock` is not followed by
## `applyScoreChanges` as `onBlock` does not update the whole tree.
var justifiedIdx: Index
self.indices.withValue(justifiedRoot, value) do:
justifiedIdx = value[]
do:
return err ForkChoiceError(
kind: fcJustifiedNodeUnknown,
blockRoot: justifiedRoot)
let justifiedNode = self.nodes[justifiedIdx]
if justifiedNode.isNone():
return err ForkChoiceError(
kind: fcInvalidJustifiedIndex,
index: justifiedIdx)
let bestDescendantIdx = justifiedNode.get().bestDescendant.get(justifiedIdx)
let bestNode = self.nodes[bestDescendantIdx]
if bestNode.isNone():
return err ForkChoiceError(
kind: fcInvalidBestDescendant,
index: bestDescendantIdx)
# Perform a sanity check to ensure the node can be head
if not self.nodeIsViableForHead(bestNode.get(), bestDescendantIdx):
return err ForkChoiceError(
kind: fcInvalidBestNode,
startRoot: justifiedRoot,
fkChoiceCheckpoints: self.checkpoints,
headRoot: justifiedNode.get().bid.root,
headCheckpoints: justifiedNode.get().checkpoints)
head = bestNode.get().bid.root
ok()
func prune*(
self: var ProtoArray,
checkpoints: FinalityCheckpoints): FcResult[void] =
## Update the tree with new finalization information.
## The tree is pruned if and only if:
## - `checkpoints.finalized.root` and finalized epoch
## are different from current
##
## Returns error if:
## - The finalized epoch is less than the current one
## - The finalized epoch matches the current one but the f
## inalized root is different
## - Internal error due to invalid indices in `self`
var finalizedIdx: int
self.indices.withValue(checkpoints.finalized.root, value) do:
finalizedIdx = value[]
do:
return err ForkChoiceError(
kind: fcFinalizedNodeUnknown,
blockRoot: checkpoints.finalized.root)
if finalizedIdx == self.nodes.offset:
# Nothing to do
return ok()
if finalizedIdx < self.nodes.offset:
return err ForkChoiceError(
kind: fcPruningFromOutdatedFinalizedRoot,
finalizedRoot: checkpoints.finalized.root)
trace "Pruning blocks from fork choice", checkpoints
let finalPhysicalIdx = finalizedIdx - self.nodes.offset
for nodeIdx in 0 ..< finalPhysicalIdx:
self.currentEpochTips.del nodeIdx
self.indices.del(self.nodes.buf[nodeIdx].bid.root)
# Drop all nodes prior to finalization.
# This is done in-place with `moveMem` to avoid costly reallocations.
static: doAssert ProtoNode.supportsCopyMem(), "ProtoNode must be a trivial type"
let tail = self.nodes.len - finalPhysicalIdx
# TODO: can we have an unallocated `self.nodes`? i.e. self.nodes[0] is nil
moveMem(self.nodes.buf[0].addr, self.nodes.buf[finalPhysicalIdx].addr, tail * sizeof(ProtoNode))
self.nodes.buf.setLen(tail)
# update offset
self.nodes.offset = finalizedIdx
ok()
func maybeUpdateBestChildAndDescendant(self: var ProtoArray,
parentIdx: Index,
childIdx: Index): FcResult[void] =
## Observe the parent at `parentIdx` with respect to the child at `childIdx` and
## potentially modify the `parent.bestChild` and `parent.bestDescendant` values
##
## There are four scenarios:
##
## 1. The child is already the best child
## but it's now invalid due to a FFG change and should be removed.
## 2. The child is already the best child
## and the parent is updated with the new best descendant
## 3. The child is not the best child but becomes the best child
## 4. The child is not the best child and does not become the best child
let child = self.nodes[childIdx]
if child.isNone():
return err ForkChoiceError(
kind: fcInvalidNodeIndex,
index: childIdx)
let parent = self.nodes[parentIdx]
if parent.isNone():
return err ForkChoiceError(
kind: fcInvalidNodeIndex,
index: parentIdx)
let childLeadsToViableHead =
? self.nodeLeadsToViableHead(child.get(), childIdx)
let # Aliases to the 3 possible (bestChild, bestDescendant) tuples
changeToNone = (none(Index), none(Index))
changeToChild = (
some(childIdx),
# Nim `options` module doesn't implement option `or`
if child.get().bestDescendant.isSome(): child.get().bestDescendant
else: some(childIdx)
)
noChange = (parent.get().bestChild, parent.get().bestDescendant)
# TODO: state-machine? The control-flow is messy
let (newBestChild, newBestDescendant) = block:
if parent.get().bestChild.isSome:
let bestChildIdx = parent.get().bestChild.unsafeGet()
if bestChildIdx == childIdx and not childLeadsToViableHead:
# The child is already the best-child of the parent
# but it's not viable to be the head block => remove it
changeToNone
elif bestChildIdx == childIdx:
# If the child is the best-child already, set it again to ensure
# that the best-descendant of the parent is up-to-date.
changeToChild
else:
let bestChild = self.nodes[bestChildIdx]
if bestChild.isNone():
return err ForkChoiceError(
kind: fcInvalidBestDescendant,
index: bestChildIdx)
let bestChildLeadsToViableHead =
? self.nodeLeadsToViableHead(bestChild.get(), bestChildIdx)
if childLeadsToViableHead and not bestChildLeadsToViableHead:
# The child leads to a viable head, but the current best-child doesn't
changeToChild
elif not childLeadsToViableHead and bestChildLeadsToViableHead:
# The best child leads to a viable head, but the child doesn't
noChange
elif child.get().weight == bestChild.get().weight:
# Tie-breaker of equal weights by root
if child.get().bid.root.tiebreak(bestChild.get().bid.root):
changeToChild
else:
noChange
else: # Choose winner by weight
let cw = child.get().weight
let bw = bestChild.get().weight
if cw >= bw:
changeToChild
else:
noChange
else:
if childLeadsToViableHead:
# There is no current best-child and the child is viable
changeToChild
else:
# There is no current best-child but the child is not viable
noChange
self.nodes.buf[parentIdx - self.nodes.offset].bestChild = newBestChild
self.nodes.buf[parentIdx - self.nodes.offset].bestDescendant = newBestDescendant
ok()
func nodeLeadsToViableHead(
self: var ProtoArray, node: ProtoNode, nodeIdx: Index): FcResult[bool] =
## Indicates if the node itself or its best-descendant are viable
## for blockchain head
let bestDescendantIsViableForHead = block:
if node.bestDescendant.isSome():
let bestDescendantIdx = node.bestDescendant.unsafeGet()
let bestDescendant = self.nodes[bestDescendantIdx]
if bestDescendant.isNone:
return err ForkChoiceError(
kind: fcInvalidBestDescendant,
index: bestDescendantIdx)
self.nodeIsViableForHead(bestDescendant.get(), bestDescendantIdx)
else:
false
ok(bestDescendantIsViableForHead or self.nodeIsViableForHead(node, nodeIdx))
func nodeIsViableForHead(
self: var ProtoArray, node: ProtoNode, nodeIdx: Index): bool =
## This is the equivalent of `filter_block_tree` function in consensus specs
## https://github.com/ethereum/consensus-specs/blob/v1.3.0/specs/phase0/fork-choice.md#filter_block_tree
if node.invalid:
return false
# The voting source should be at the same height as the store's
# justified checkpoint
var correctJustified =
self.checkpoints.justified.epoch == GENESIS_EPOCH or
node.checkpoints.justified.epoch == self.checkpoints.justified.epoch
if not correctJustified:
# The voting source should be either at the same height as the store's
# justified checkpoint or not more than two epochs ago
correctJustified =
node.checkpoints.justified.epoch + 2 >= self.currentEpoch
return
if not correctJustified:
false
elif self.checkpoints.finalized.epoch == GENESIS_EPOCH:
true
elif node.sharedFinalizedEpoch == self.checkpoints.finalized.epoch:
# Already checked to share history with `self.checkpoints.finalized`.
# `self.checkpoints.finalized` cannot reorg, see `applyScoreChanges`
true
else:
# Check that this node is not going to be pruned
let
finalizedEpoch = self.checkpoints.finalized.epoch
finalizedSlot = finalizedEpoch.start_slot
var ancestor = some node
while ancestor.isSome and ancestor.unsafeGet.bid.slot > finalizedSlot and
ancestor.unsafeGet.sharedFinalizedEpoch != finalizedEpoch:
if ancestor.unsafeGet.parent.isSome:
ancestor = self.nodes[ancestor.unsafeGet.parent.unsafeGet]
else:
ancestor.reset()
if ancestor.isNone:
false
elif ancestor.unsafeGet.sharedFinalizedEpoch != finalizedEpoch and
ancestor.unsafeGet.bid.root != self.checkpoints.finalized.root:
false
else:
# An ancestor was already checked to share canonical history,
# or we reached the `finalizedSlot` and the root matches expectations
let ancestorSlot = ancestor.unsafeGet.bid.slot
var idx = nodeIdx
template mutableNode: untyped = self.nodes.buf[idx - self.nodes.offset]
while mutableNode.bid.slot > ancestorSlot:
mutableNode.sharedFinalizedEpoch = finalizedEpoch
idx = mutableNode.parent.unsafeGet
mutableNode.sharedFinalizedEpoch = finalizedEpoch
true
func propagateInvalidity*(
self: var ProtoArray, startPhysicalIdx: Index) =
# Called when startPhysicalIdx is updated in a parent role, so the pairs of
# indices generated of (parent, child) where both >= startPhysicalIdx, mean
# the loop in general from the child's perspective starts one index higher.
for nodePhysicalIdx in startPhysicalIdx + 1 ..< self.nodes.len:
let nodeParent = self.nodes.buf[nodePhysicalIdx].parent
if nodeParent.isNone:
continue
let
parentLogicalIdx = nodeParent.unsafeGet()
parentPhysicalIdx = parentLogicalIdx - self.nodes.offset
# Former case is orphaned, latter is invalid, but caught in score updates
if parentPhysicalIdx < 0 or parentPhysicalIdx >= self.nodes.len:
continue
# Invalidity transmits to all descendents
if self.nodes.buf[parentPhysicalIdx].invalid:
self.nodes.buf[nodePhysicalIdx].invalid = true
# Diagnostics
# ----------------------------------------------------------------------
# Helpers to dump internal state
type ProtoArrayItem* = object
bid*: BlockId
parent*: Eth2Digest
checkpoints*: FinalityCheckpoints
unrealized*: Option[FinalityCheckpoints]
weight*: int64
invalid*: bool
bestChild*: Eth2Digest
bestDescendant*: Eth2Digest
func root(self: ProtoNodes, logicalIdx: Option[Index]): Eth2Digest =
if logicalIdx.isNone:
return ZERO_HASH
let node = self[logicalIdx.unsafeGet]
if node.isNone:
return ZERO_HASH
node.unsafeGet.bid.root
iterator items*(self: ProtoArray): ProtoArrayItem =
## Iterate over all nodes known by fork choice.
doAssert self.indices.len == self.nodes.len
for nodePhysicalIdx, node in self.nodes.buf:
if node.bid.root.isZero:
continue
let unrealized = block:
let nodeLogicalIdx = nodePhysicalIdx + self.nodes.offset
if self.currentEpochTips.hasKey(nodeLogicalIdx):
some self.currentEpochTips.unsafeGet(nodeLogicalIdx)
else:
none(FinalityCheckpoints)
yield ProtoArrayItem(
bid: node.bid,
parent: self.nodes.root(node.parent),
checkpoints: node.checkpoints,
unrealized: unrealized,
weight: node.weight,
invalid: node.invalid,
bestChild: self.nodes.root(node.bestChild),
bestDescendant: self.nodes.root(node.bestDescendant))
# Sanity checks
# ----------------------------------------------------------------------
# Sanity checks on internal private procedures
when isMainModule:
import nimcrypto/hash
echo "Sanity checks on fork choice tiebreaks"
block:
const a = Eth2Digest.fromHex("0x0000000000000001000000000000000000000000000000000000000000000000")
const b = Eth2Digest.fromHex("0x0000000000000000000000000000000000000000000000000000000000000000") # sha256(1)
doAssert tiebreak(a, b)
block:
const a = Eth2Digest.fromHex("0x0000000000000002000000000000000000000000000000000000000000000000")
const b = Eth2Digest.fromHex("0x0000000000000001000000000000000000000000000000000000000000000000") # sha256(1)
doAssert tiebreak(a, b)
block:
const a = Eth2Digest.fromHex("0xD86E8112F3C4C4442126F8E9F44F16867DA487F29052BF91B810457DB34209A4") # sha256(2)
const b = Eth2Digest.fromHex("0x7C9FA136D4413FA6173637E883B6998D32E1D675F88CDDFF9DCBCF331820F4B8") # sha256(1)
doAssert tiebreak(a, b)
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