# 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, justifiedCheckpoint: Checkpoint, finalizedCheckpoint: Checkpoint): T = let node = ProtoNode( root: finalizedCheckpoint.root, parent: none(int), justifiedCheckpoint: justifiedCheckpoint, finalizedCheckpoint: finalizedCheckpoint, weight: 0, bestChild: none(int), bestDescendant: none(int) ) T( justifiedCheckpoint: justifiedCheckpoint, finalizedCheckpoint: finalizedCheckpoint, nodes: ProtoNodes(buf: @[node], offset: 0), indices: {node.root: 0}.toTable() ) func applyScoreChanges*(self: var ProtoArray, deltas: var openArray[Delta], justifiedCheckpoint: Checkpoint, finalizedCheckpoint: Checkpoint): 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.justifiedCheckpoint = justifiedCheckpoint self.finalizedCheckpoint = finalizedCheckpoint ## 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, justifiedCheckpoint: Checkpoint, finalizedCheckpoint: Checkpoint): 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), justifiedCheckpoint: justifiedCheckpoint, finalizedCheckpoint: finalizedCheckpoint, 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, fkChoiceJustifiedCheckpoint: self.justifiedCheckpoint, fkChoiceFinalizedCheckpoint: self.finalizedCheckpoint, headRoot: justifiedNode.get().root, headJustifiedCheckpoint: justifiedNode.get().justifiedCheckpoint, headFinalizedCheckpoint: justifiedNode.get().finalizedCheckpoint) 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.justifiedCheckpoint == self.justifiedCheckpoint) or (self.justifiedCheckpoint.epoch == Epoch(0)) ) and ( (node.finalizedCheckpoint == self.finalizedCheckpoint) or (self.finalizedCheckpoint.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)