# 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 func isPreviousEpochJustified(self: ProtoArray): bool = self.checkpoints.justified.epoch + 1 == self.currentEpoch # Forward declarations # ---------------------------------------------------------------------- func maybeUpdateBestChildAndDescendant(self: var ProtoArray, parentIdx: Index, childIdx: Index): FcResult[void] func nodeIsViableForHead( self: ProtoArray, node: ProtoNode, nodeIdx: Index): bool func nodeLeadsToViableHead( self: ProtoArray, node: ProtoNode, nodeIdx: Index): FcResult[bool] # ProtoArray routines # ---------------------------------------------------------------------- func init*( T: type ProtoArray, checkpoints: FinalityCheckpoints, version: ForkChoiceVersion): 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(version: version, 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 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: 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: 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: case self.version of ForkChoiceVersion.Stable: # If the previous epoch is justified, the block should be pulled-up. # In this case, check that unrealized justification is higher than the # store and that the voting source is not more than two epochs ago if self.isPreviousEpochJustified and node.bid.slot.epoch == self.currentEpoch: let unrealized = self.currentEpochTips.getOrDefault(nodeIdx, node.checkpoints) correctJustified = unrealized.justified.epoch >= self.checkpoints.justified.epoch and node.checkpoints.justified.epoch + 2 >= self.currentEpoch of ForkChoiceVersion.Pr3431: # 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 else: let finalizedSlot = self.checkpoints.finalized.epoch.start_slot var ancestor = some node while ancestor.isSome and ancestor.unsafeGet.bid.slot > finalizedSlot: if ancestor.unsafeGet.parent.isSome: ancestor = self.nodes[ancestor.unsafeGet.parent.unsafeGet] else: ancestor.reset() if ancestor.isSome: ancestor.unsafeGet.bid.root == self.checkpoints.finalized.root else: false 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)