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nimbus-eth2/beacon_chain/fork_choice/proto_array.nim

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# beacon_chain
# Copyright (c) 2018-2020 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, digest],
# Fork choice
./fork_choice_types
logScope:
topics = "fork_choice"
export results
# https://github.com/ethereum/eth2.0-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] {.inline.} =
## 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 {.inline.} =
nodes.buf.len
func add(nodes: var ProtoNodes, node: ProtoNode) {.inline.} =
nodes.buf.add node
# Forward declarations
# ----------------------------------------------------------------------
func maybe_update_best_child_and_descendant(
self: var ProtoArray, parent_index: Index, child_index: Index): FcResult[void]
func node_is_viable_for_head(self: ProtoArray, node: ProtoNode): bool
func node_leads_to_viable_head(self: ProtoArray, node: ProtoNode): FcResult[bool]
# ProtoArray routines
# ----------------------------------------------------------------------
func init*(T: type ProtoArray,
justified_epoch: Epoch,
finalized_root: Eth2Digest,
finalized_epoch: Epoch): T =
let node = ProtoNode(
root: finalized_root,
parent: none(int),
justified_epoch: justified_epoch,
finalized_epoch: finalized_epoch,
weight: 0,
best_child: none(int),
best_descendant: none(int)
)
T(
justified_epoch: justified_epoch,
finalized_epoch: finalized_epoch,
nodes: ProtoNodes(buf: @[node], offset: 0),
indices: {node.root: 0}.toTable()
)
func apply_score_changes*(
self: var ProtoArray,
deltas: var openarray[Delta],
justified_epoch: Epoch,
finalized_epoch: 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.justified_epoch = justified_epoch
self.finalized_epoch = finalized_epoch
# Iterate backwards through all the indices in `self.nodes`
for node_physical_index in countdown(self.nodes.len - 1, 0):
template node: untyped {.dirty.}= self.nodes.buf[node_physical_index]
## Alias
# This cannot raise the IndexError exception, how to tell compiler?
if node.root == default(Eth2Digest):
continue
let node_delta = deltas[node_physical_index]
# 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 + node_delta
if weight < 0:
return err ForkChoiceError(
kind: fcDeltaUnderflow,
index: node_physical_index
)
node.weight = weight
# If the node has a parent, try to update its best-child and best-descendant
if node.parent.isSome():
let parent_logical_index = node.parent.unsafeGet()
let parent_physical_index = parent_logical_index - self.nodes.offset
if parent_physical_index < 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.
break
if parent_physical_index >= deltas.len:
return err ForkChoiceError(
kind: fcInvalidParentDelta,
index: parent_physical_index
)
# Back-propagate the nodes delta to its parent.
deltas[parent_physical_index] += node_delta
let node_logical_index = node_physical_index + self.nodes.offset
? self.maybe_update_best_child_and_descendant(parent_logical_index, node_logical_index)
return ok()
func on_block*(
self: var ProtoArray,
root: Eth2Digest,
parent: Eth2Digest,
justified_epoch: Epoch,
finalized_epoch: 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 parent_index: Index
self.indices.withValue(parent, index) do:
parent_index = index[]
do:
return err ForkChoiceError(
kind: fcUnknownParent,
child_root: root,
parent_root: parent
)
let node_logical_index = self.nodes.offset + self.nodes.buf.len
let node = ProtoNode(
root: root,
parent: some(parent_index),
justified_epoch: justified_epoch,
finalized_epoch: finalized_epoch,
weight: 0,
best_child: none(int),
best_descendant: none(int)
)
self.indices[node.root] = node_logical_index
self.nodes.add node
? self.maybe_update_best_child_and_descendant(parent_index, node_logical_index)
return ok()
func find_head*(
self: var ProtoArray,
head: var Eth2Digest,
justified_root: 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 `on_new_block`
## is not followed by `apply_score_changes` as `on_new_block` does not
## update the whole tree.
var justified_index: Index
self.indices.withValue(justified_root, value) do:
justified_index = value[]
do:
return err ForkChoiceError(
kind: fcJustifiedNodeUnknown,
block_root: justified_root
)
let justified_node = self.nodes[justified_index]
if justified_node.isNone():
return err ForkChoiceError(
kind: fcInvalidJustifiedIndex,
index: justified_index
)
let best_descendant_index = justified_node.get().best_descendant.get(justified_index)
let best_node = self.nodes[best_descendant_index]
if best_node.isNone():
return err ForkChoiceError(
kind: fcInvalidBestDescendant,
index: best_descendant_index
)
# Perform a sanity check to ensure the node can be head
if not self.node_is_viable_for_head(best_node.get()):
return err ForkChoiceError(
kind: fcInvalidBestNode,
start_root: justified_root,
justified_epoch: self.justified_epoch,
finalized_epoch: self.finalized_epoch,
head_root: justified_node.get().root,
head_justified_epoch: justified_node.get().justified_epoch,
head_finalized_epoch: justified_node.get().finalized_epoch
)
head = best_node.get().root
return ok()
func prune*(
self: var ProtoArray,
finalized_root: Eth2Digest
): FcResult[void] =
## Update the tree with new finalization information.
## The tree is pruned if and only if:
## - The `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 finalized root is different
## - Internal error due to invalid indices in `self`
var finalized_index: int
self.indices.withValue(finalized_root, value) do:
finalized_index = value[]
do:
return err ForkChoiceError(
kind: fcFinalizedNodeUnknown,
block_root: finalized_root
)
if finalized_index == self.nodes.offset:
# Nothing to do
return ok()
if finalized_index < self.nodes.offset:
return err ForkChoiceError(
kind: fcPruningFromOutdatedFinalizedRoot,
finalizedRoot: finalized_root
)
trace "Pruning blocks from fork choice",
finalizedRoot = shortlog(finalized_root),
pcs = "prune"
let final_phys_index = finalized_index-self.nodes.offset
for node_index in 0 ..< final_phys_index:
self.indices.del(self.nodes.buf[node_index].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 - final_phys_index
# TODO: can we have an unallocated `self.nodes`? i.e. self.nodes[0] is nil
moveMem(self.nodes.buf[0].addr, self.nodes.buf[final_phys_index].addr, tail * sizeof(ProtoNode))
self.nodes.buf.setLen(tail)
# update offset
self.nodes.offset = finalized_index
return ok()
func maybe_update_best_child_and_descendant(
self: var ProtoArray,
parent_index: Index,
child_index: Index): Result[void, ForkChoiceError] =
## Observe the parent at `parent_index` with respect to the child at `child_index` and
## potentiatlly modify the `parent.best_child` and `parent.best_descendant` 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[child_index]
if child.isNone():
return err ForkChoiceError(
kind: fcInvalidNodeIndex,
index: child_index
)
let parent = self.nodes[parent_index]
if parent.isNone():
return err ForkChoiceError(
kind: fcInvalidNodeIndex,
index: parent_index
)
let child_leads_to_viable_head = ? self.node_leads_to_viable_head(child.get())
let # Aliases to the 3 possible (best_child, best_descendant) tuples
change_to_none = (none(Index), none(Index))
change_to_child = (
some(child_index),
# Nim `options` module doesn't implement option `or`
if child.get().best_descendant.isSome(): child.get().best_descendant
else: some(child_index)
)
no_change = (parent.get().best_child, parent.get().best_descendant)
# TODO: state-machine? The control-flow is messy
let (new_best_child, new_best_descendant) = block:
if parent.get().best_child.isSome:
let best_child_index = parent.get().best_child.unsafeGet()
if best_child_index == child_index and not child_leads_to_viable_head:
# The child is already the best-child of the parent
# but it's not viable to be the head block => remove it
change_to_none
elif best_child_index == child_index:
# If the child is the best-child already, set it again to ensure
# that the best-descendant of the parent is up-to-date.
change_to_child
else:
let best_child = self.nodes[best_child_index]
if best_child.isNone():
return err ForkChoiceError(
kind: fcInvalidBestDescendant,
index: best_child_index
)
let best_child_leads_to_viable_head =
? self.node_leads_to_viable_head(best_child.get())
if child_leads_to_viable_head and not best_child_leads_to_viable_head:
# The child leads to a viable head, but the current best-child doesn't
change_to_child
elif not child_leads_to_viable_head and best_child_leads_to_viable_head:
# The best child leads to a viable head, but the child doesn't
no_change
elif child.get().weight == best_child.get().weight:
# Tie-breaker of equal weights by root
if child.get().root.tiebreak(best_child.get().root):
change_to_child
else:
no_change
else: # Choose winner by weight
let cw = child.get().weight
let bw = best_child.get().weight
if cw >= bw:
change_to_child
else:
no_change
else:
if child_leads_to_viable_head:
# There is no current best-child and the child is viable
change_to_child
else:
# There is no current best-child but the child is not viable
no_change
self.nodes.buf[parent_index - self.nodes.offset].best_child = new_best_child
self.nodes.buf[parent_index - self.nodes.offset].best_descendant = new_best_descendant
return ok()
func node_leads_to_viable_head(
self: ProtoArray, node: ProtoNode
): FcResult[bool] =
## Indicates if the node itself or its best-descendant are viable
## for blockchain head
let best_descendant_is_viable_for_head = block:
if node.best_descendant.isSome():
let best_descendant_index = node.best_descendant.unsafeGet()
let best_descendant = self.nodes[best_descendant_index]
if best_descendant.isNone:
return err ForkChoiceError(
kind: fcInvalidBestDescendant,
index: best_descendant_index
)
self.node_is_viable_for_head(best_descendant.get())
else:
false
return ok(best_descendant_is_viable_for_head or
self.node_is_viable_for_head(node))
func node_is_viable_for_head(self: ProtoArray, node: ProtoNode): bool =
## This is the equivalent of `filter_block_tree` function in eth2 spec
## https://github.com/ethereum/eth2.0-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.justified_epoch == self.justified_epoch) or
(self.justified_epoch == Epoch(0))
) and (
(node.finalized_epoch == self.finalized_epoch) or
(self.finalized_epoch == 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)
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