# Bellatrix -- Fork Choice ## Table of contents - [Introduction](#introduction) - [Custom types](#custom-types) - [Protocols](#protocols) - [`ExecutionEngine`](#executionengine) - [`notify_forkchoice_updated`](#notify_forkchoice_updated) - [`safe_block_hash`](#safe_block_hash) - [`should_override_forkchoice_update`](#should_override_forkchoice_update) - [Helpers](#helpers) - [`PayloadAttributes`](#payloadattributes) - [`PowBlock`](#powblock) - [`get_pow_block`](#get_pow_block) - [`is_valid_terminal_pow_block`](#is_valid_terminal_pow_block) - [`validate_merge_block`](#validate_merge_block) - [Updated fork-choice handlers](#updated-fork-choice-handlers) - [`on_block`](#on_block) ## Introduction This is the modification of the fork choice according to the executable beacon chain proposal. *Note*: It introduces the process of transition from the last PoW block to the first PoS block. ## Custom types | Name | SSZ equivalent | Description | | - | - | - | | `PayloadId` | `Bytes8` | Identifier of a payload building process | ## Protocols ### `ExecutionEngine` *Note*: The `notify_forkchoice_updated` function is added to the `ExecutionEngine` protocol to signal the fork choice updates. The body of this function is implementation dependent. The Engine API may be used to implement it with an external execution engine. #### `notify_forkchoice_updated` This function performs three actions *atomically*: * Re-organizes the execution payload chain and corresponding state to make `head_block_hash` the head. * Updates safe block hash with the value provided by `safe_block_hash` parameter. * Applies finality to the execution state: it irreversibly persists the chain of all execution payloads and corresponding state, up to and including `finalized_block_hash`. Additionally, if `payload_attributes` is provided, this function sets in motion a payload build process on top of `head_block_hash` and returns an identifier of initiated process. ```python def notify_forkchoice_updated(self: ExecutionEngine, head_block_hash: Hash32, safe_block_hash: Hash32, finalized_block_hash: Hash32, payload_attributes: Optional[PayloadAttributes]) -> Optional[PayloadId]: ... ``` *Note*: The `(head_block_hash, finalized_block_hash)` values of the `notify_forkchoice_updated` function call maps on the `POS_FORKCHOICE_UPDATED` event defined in the [EIP-3675](https://eips.ethereum.org/EIPS/eip-3675#definitions). As per EIP-3675, before a post-transition block is finalized, `notify_forkchoice_updated` MUST be called with `finalized_block_hash = Hash32()`. *Note*: Client software MUST NOT call this function until the transition conditions are met on the PoW network, i.e. there exists a block for which `is_valid_terminal_pow_block` function returns `True`. *Note*: Client software MUST call this function to initiate the payload build process to produce the merge transition block; the `head_block_hash` parameter MUST be set to the hash of a terminal PoW block in this case. ##### `safe_block_hash` The `safe_block_hash` parameter MUST be set to return value of [`get_safe_execution_payload_hash(store: Store)`](../../fork_choice/safe-block.md#get_safe_execution_payload_hash) function. ##### `should_override_forkchoice_update` If proposer boost re-orgs are implemented and enabled (see `get_proposer_head`) then additional care must be taken to ensure that the proposer is able to build an execution payload. If a beacon node knows it will propose the next block then it SHOULD NOT call `notify_forkchoice_updated` if it detects the current head to be weak and potentially capable of being re-orged. Complete information for evaluating `get_proposer_head` _will not_ be available immediately after the receipt of a new block, so an approximation of those conditions should be used when deciding whether to send or suppress a fork choice notification. The exact conditions used may be implementation-specific, a suggested implementation is below. Let `validator_is_connected(validator_index: ValidatorIndex) -> bool` be a function that indicates whether the validator with `validator_index` is connected to the node (e.g. has sent an unexpired proposer preparation message). ```python def should_override_forkchoice_update(store: Store, head_root: Root) -> bool: head_block = store.blocks[head_root] parent_root = head_block.parent_root parent_block = store.blocks[parent_root] current_slot = get_current_slot(store) proposal_slot = head_block.slot + Slot(1) # Only re-org the head_block block if it arrived later than the attestation deadline. head_late = is_head_late(store, head_root) # Shuffling stable. shuffling_stable = is_shuffling_stable(proposal_slot) # FFG information of the new head_block will be competitive with the current head. ffg_competitive = is_ffg_competitive(store, head_root, parent_root) # Do not re-org if the chain is not finalizing with acceptable frequency. finalization_ok = is_finalization_ok(store, proposal_slot) # Only suppress the fork choice update if we are confident that we will propose the next block. parent_state_advanced = store.block_states[parent_root].copy() process_slots(parent_state_advanced, proposal_slot) proposer_index = get_beacon_proposer_index(parent_state_advanced) proposing_reorg_slot = validator_is_connected(proposer_index) # Single slot re-org. parent_slot_ok = parent_block.slot + 1 == head_block.slot proposing_on_time = is_proposing_on_time(store) # Note that this condition is different from `get_proposer_head` current_time_ok = (head_block.slot == current_slot or (proposal_slot == current_slot and proposing_on_time)) single_slot_reorg = parent_slot_ok and current_time_ok # Check the head weight only if the attestations from the head slot have already been applied. # Implementations may want to do this in different ways, e.g. by advancing # `store.time` early, or by counting queued attestations during the head block's slot. if current_slot > head_block.slot: head_weak = is_head_weak(store, head_root) parent_strong = is_parent_strong(store, parent_root) else: head_weak = True parent_strong = True return all([head_late, shuffling_stable, ffg_competitive, finalization_ok, proposing_reorg_slot, single_slot_reorg, head_weak, parent_strong]) ``` *Note*: The ordering of conditions is a suggestion only. Implementations are free to optimize by re-ordering the conditions from least to most expensive and by returning early if any of the early conditions are `False`. In case `should_override_forkchoice_update` returns `True`, a node SHOULD instead call `notify_forkchoice_updated` with parameters appropriate for building upon the parent block. Care must be taken to compute the correct `payload_attributes`, as they may change depending on the slot of the block to be proposed (due to withdrawals). If `should_override_forkchoice_update` returns `True` but `get_proposer_head` later chooses the canonical head rather than its parent, then this is a misprediction that will cause the node to construct a payload with less notice. The result of `get_proposer_head` MUST be honoured in preference to the heuristic method. ## Helpers ### `PayloadAttributes` Used to signal to initiate the payload build process via `notify_forkchoice_updated`. ```python @dataclass class PayloadAttributes(object): timestamp: uint64 prev_randao: Bytes32 suggested_fee_recipient: ExecutionAddress ``` ### `PowBlock` ```python class PowBlock(Container): block_hash: Hash32 parent_hash: Hash32 total_difficulty: uint256 ``` ### `get_pow_block` Let `get_pow_block(block_hash: Hash32) -> Optional[PowBlock]` be the function that given the hash of the PoW block returns its data. It may result in `None` if the requested block is not yet available. *Note*: The `eth_getBlockByHash` JSON-RPC method may be used to pull this information from an execution client. ### `is_valid_terminal_pow_block` Used by fork-choice handler, `on_block`. ```python def is_valid_terminal_pow_block(block: PowBlock, parent: PowBlock) -> bool: is_total_difficulty_reached = block.total_difficulty >= TERMINAL_TOTAL_DIFFICULTY is_parent_total_difficulty_valid = parent.total_difficulty < TERMINAL_TOTAL_DIFFICULTY return is_total_difficulty_reached and is_parent_total_difficulty_valid ``` ### `validate_merge_block` ```python def validate_merge_block(block: BeaconBlock) -> None: """ Check the parent PoW block of execution payload is a valid terminal PoW block. Note: Unavailable PoW block(s) may later become available, and a client software MAY delay a call to ``validate_merge_block`` until the PoW block(s) become available. """ if TERMINAL_BLOCK_HASH != Hash32(): # If `TERMINAL_BLOCK_HASH` is used as an override, the activation epoch must be reached. assert compute_epoch_at_slot(block.slot) >= TERMINAL_BLOCK_HASH_ACTIVATION_EPOCH assert block.body.execution_payload.parent_hash == TERMINAL_BLOCK_HASH return pow_block = get_pow_block(block.body.execution_payload.parent_hash) # Check if `pow_block` is available assert pow_block is not None pow_parent = get_pow_block(pow_block.parent_hash) # Check if `pow_parent` is available assert pow_parent is not None # Check if `pow_block` is a valid terminal PoW block assert is_valid_terminal_pow_block(pow_block, pow_parent) ``` ## Updated fork-choice handlers ### `on_block` *Note*: The only modification is the addition of the verification of transition block conditions. ```python def on_block(store: Store, signed_block: SignedBeaconBlock) -> None: """ Run ``on_block`` upon receiving a new block. A block that is asserted as invalid due to unavailable PoW block may be valid at a later time, consider scheduling it for later processing in such case. """ block = signed_block.message # Parent block must be known assert block.parent_root in store.block_states # Make a copy of the state to avoid mutability issues pre_state = copy(store.block_states[block.parent_root]) # Blocks cannot be in the future. If they are, their consideration must be delayed until they are in the past. assert get_current_slot(store) >= block.slot # Check that block is later than the finalized epoch slot (optimization to reduce calls to get_ancestor) finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch) assert block.slot > finalized_slot # Check block is a descendant of the finalized block at the checkpoint finalized slot finalized_checkpoint_block = get_checkpoint_block( store, block.parent_root, store.finalized_checkpoint.epoch, ) assert store.finalized_checkpoint.root == finalized_checkpoint_block # Check the block is valid and compute the post-state state = pre_state.copy() block_root = hash_tree_root(block) state_transition(state, signed_block, True) # [New in Bellatrix] if is_merge_transition_block(pre_state, block.body): validate_merge_block(block) # Add new block to the store store.blocks[block_root] = block # Add new state for this block to the store store.block_states[block_root] = state # Add block timeliness to the store time_into_slot = (store.time - store.genesis_time) % SECONDS_PER_SLOT is_before_attesting_interval = time_into_slot < SECONDS_PER_SLOT // INTERVALS_PER_SLOT is_timely = get_current_slot(store) == block.slot and is_before_attesting_interval store.block_timeliness[hash_tree_root(block)] = is_timely # Add proposer score boost if the block is timely and not conflicting with an existing block is_first_block = store.proposer_boost_root == Root() if is_timely and is_first_block: store.proposer_boost_root = hash_tree_root(block) # Update checkpoints in store if necessary update_checkpoints(store, state.current_justified_checkpoint, state.finalized_checkpoint) # Eagerly compute unrealized justification and finality. compute_pulled_up_tip(store, block_root) ```