nimbus-eth2/beacon_chain/fork_choice/proto_array.nim

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# 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
# Standard library
std/tables, std/options, std/typetraits,
# Status libraries
chronicles,
stew/results,
# Internal
../spec/datatypes/base,
# Fork choice
./fork_choice_types
logScope:
topics = "fork_choice"
export results
# https://github.com/ethereum/consensus-specs/blob/v0.11.1/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)
return 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: ProtoArray, node: ProtoNode): bool
func nodeLeadsToViableHead(self: ProtoArray, node: ProtoNode): FcResult[bool]
# ProtoArray routines
# ----------------------------------------------------------------------
func init*(T: type ProtoArray,
justifiedEpoch: Epoch,
finalizedRoot: Eth2Digest,
finalizedEpoch: Epoch): T =
let node = ProtoNode(
root: finalizedRoot,
parent: none(int),
justifiedEpoch: justifiedEpoch,
finalizedEpoch: finalizedEpoch,
weight: 0,
bestChild: none(int),
bestDescendant: none(int)
)
T(
justifiedEpoch: justifiedEpoch,
finalizedEpoch: finalizedEpoch,
nodes: ProtoNodes(buf: @[node], offset: 0),
indices: {node.root: 0}.toTable()
)
func applyScoreChanges*(self: var ProtoArray,
deltas: var openArray[Delta],
justifiedEpoch: Epoch,
finalizedEpoch: Epoch): 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 it's aprent.
##
## 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.justifiedEpoch = justifiedEpoch
self.finalizedEpoch = finalizedEpoch
## Alias
# This cannot raise the IndexError exception, how to tell compiler?
template node: untyped {.dirty.} =
self.nodes.buf[nodePhysicalIdx]
# Iterate backwards through all the indices in `self.nodes`
for nodePhysicalIdx in countdown(self.nodes.len - 1, 0):
if node.root == default(Eth2Digest):
continue
let nodeDelta = deltas[nodePhysicalIdx]
# 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-descendant
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
for nodePhysicalIdx in countdown(self.nodes.len - 1, 0):
if node.root == default(Eth2Digest):
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,
root: Eth2Digest,
parent: Eth2Digest,
justifiedEpoch: Epoch,
finalizedEpoch: Epoch): 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 root in self.indices:
return ok()
var parentIdx: Index
self.indices.withValue(parent, index) do:
parentIdx = index[]
do:
return err ForkChoiceError(
kind: fcUnknownParent,
childRoot: root,
parentRoot: parent)
let nodeLogicalIdx = self.nodes.offset + self.nodes.buf.len
let node = ProtoNode(
root: root,
parent: some(parentIdx),
justifiedEpoch: justifiedEpoch,
finalizedEpoch: finalizedEpoch,
weight: 0,
bestChild: none(int),
bestDescendant: none(int)
)
self.indices[node.root] = nodeLogicalIdx
self.nodes.add node
? 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()):
return err ForkChoiceError(
kind: fcInvalidBestNode,
startRoot: justifiedRoot,
justifiedEpoch: self.justifiedEpoch,
finalizedEpoch: self.finalizedEpoch,
headRoot: justifiedNode.get().root,
headJustifiedEpoch: justifiedNode.get().justifiedEpoch,
headFinalizedEpoch: justifiedNode.get().finalizedEpoch)
head = bestNode.get().root
ok()
func prune*(self: var ProtoArray, finalizedRoot: Eth2Digest): FcResult[void] =
## Update the tree with new finalization information.
## The tree is pruned if and only if:
## - The `finalizedRoot` 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 finalized root is different
## - Internal error due to invalid indices in `self`
var finalizedIdx: int
self.indices.withValue(finalizedRoot, value) do:
finalizedIdx = value[]
do:
return err ForkChoiceError(
kind: fcFinalizedNodeUnknown,
blockRoot: finalizedRoot)
if finalizedIdx == self.nodes.offset:
# Nothing to do
return ok()
if finalizedIdx < self.nodes.offset:
return err ForkChoiceError(
kind: fcPruningFromOutdatedFinalizedRoot,
finalizedRoot: finalizedRoot)
trace "Pruning blocks from fork choice",
finalizedRoot = shortlog(finalizedRoot)
let finalPhysicalIdx = finalizedIdx - self.nodes.offset
for nodeIdx in 0 ..< finalPhysicalIdx:
self.indices.del(self.nodes.buf[nodeIdx].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())
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())
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().root.tiebreak(bestChild.get().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: ProtoArray, node: ProtoNode): 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())
else:
false
ok(bestDescendantIsViableForHead or self.nodeIsViableForHead(node))
func nodeIsViableForHead(self: ProtoArray, node: ProtoNode): bool =
## This is the equivalent of `filter_block_tree` function in eth2 spec
## https://github.com/ethereum/consensus-specs/blob/v0.10.0/specs/phase0/fork-choice.md#filter_block_tree
##
## Any node that has a different finalized or justified epoch
## should not be viable for the head.
(
(node.justifiedEpoch == self.justifiedEpoch) or
(self.justifiedEpoch == Epoch(0))
) and (
(node.finalizedEpoch == self.finalizedEpoch) or
(self.finalizedEpoch == Epoch(0))
)
# Sanity checks
# ----------------------------------------------------------------------
# Sanity checks on internal private procedures
when isMainModule:
import nimcrypto/hash
echo "Sanity checks on fork choice tiebreaks"
block:
let a = Eth2Digest.fromHex("0x0000000000000001000000000000000000000000000000000000000000000000")
let b = Eth2Digest.fromHex("0x0000000000000000000000000000000000000000000000000000000000000000") # sha256(1)
doAssert tiebreak(a, b)
block:
let a = Eth2Digest.fromHex("0x0000000000000002000000000000000000000000000000000000000000000000")
let b = Eth2Digest.fromHex("0x0000000000000001000000000000000000000000000000000000000000000000") # sha256(1)
doAssert tiebreak(a, b)
block:
let a = Eth2Digest.fromHex("0xD86E8112F3C4C4442126F8E9F44F16867DA487F29052BF91B810457DB34209A4") # sha256(2)
let b = Eth2Digest.fromHex("0x7C9FA136D4413FA6173637E883B6998D32E1D675F88CDDFF9DCBCF331820F4B8") # sha256(1)
doAssert tiebreak(a, b)