eth2.0-specs/specs/bellatrix/fork-choice.md

Bellatrix -- Fork Choice

Table of contents

• Introduction
• Custom types
• Protocols
  • ExecutionEngine
    • notify_forkchoice_updated
      • safe_block_hash
      • should_override_forkchoice_update
• Helpers
  • PayloadAttributes
  • PowBlock
  • get_pow_block
  • is_valid_terminal_pow_block
  • validate_merge_block
• Updated fork-choice handlers
  • 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.

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. 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) 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 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).

def validator_is_connected(_validator_index: ValidatorIndex) -> boolean:
    ...

def should_override_forkchoice_update(
    store: Store,
    head_root: Root,
) -> boolean:
    justified_state = store.checkpoint_states[store.justified_checkpoint]
    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 = store.block_timeliness.get(head_root) is False

    # 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]
    process_slots(parent_state_advanced, proposal_slot)
    proposer_index = get_beacon_proposer_index(parent_state_advanced)
    proposing_reorg_slot = validator_is_connected(proposer_index)

    # Do not re-org if the chain is not finalizing with acceptable frequency.
    proposal_epoch = compute_epoch_at_slot(proposal_slot)
    epochs_since_finalization = proposal_epoch - store.finalized_checkpoint.epoch
    finalization_ok = epochs_since_finalization <= REORG_MAX_EPOCHS_SINCE_FINALIZATION

    # Single slot re-org.
    parent_slot_ok = parent_block.slot + 1 == head_block.slot
    time_into_slot = (store.time - store.genesis_time) % SECONDS_PER_SLOT
    current_time_ok = (head_block.slot == current_slot or
                       (proposal_slot == current_slot and
                        time_into_slot <= SECONDS_PER_SLOT // INTERVALS_PER_SLOT // 2))
    single_slot_reorg = parent_slot_ok and current_time_ok

    # Shuffling stable.
    shuffling_stable = proposal_slot % SLOTS_PER_EPOCH != 0

    # FFG information of the new head_block will be competitive with the current head.
    ffg_competitive = (store.unrealized_justifications[parent_root] ==
                       store.unrealized_justifications[head_root])

    # 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_weight = get_weight(store, head_root)
        reorg_threshold = calculate_committee_fraction(justified_state, REORG_WEIGHT_THRESHOLD)
        head_weak = head_weight < reorg_threshold

        parent_weight = get_weight(store, parent_root)
        parent_threshold = calculate_committee_fraction(justified_state, REORG_PARENT_WEIGHT_THRESHOLD)
        parent_strong = parent_weight > parent_threshold
    else:
        head_weak = True
        parent_strong = True

    return all([head_late, proposing_reorg_slot, finalization_ok, single_slot_reorg,
                shuffling_stable, ffg_competitive, head_weak, parent_strong])

Note that 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.

@dataclass
class PayloadAttributes(object):
    timestamp: uint64
    prev_randao: Bytes32
    suggested_fee_recipient: ExecutionAddress

PowBlock

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.

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

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.

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 proposer score boost if the block is timely
    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_first_block = store.proposer_boost_root == Root()
    if get_current_slot(store) == block.slot and is_before_attesting_interval 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)