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Capella -- Fork Choice

Introduction

This is the modification of the fork choice according to the Capella upgrade.

Unless stated explicitly, all prior functionality from Bellatrix is inherited.

Protocols

ExecutionEngine

Note: The notify_forkchoice_updated function is modified in the ExecutionEngine protocol at the Capella upgrade.

notify_forkchoice_updated

The only change made is to the PayloadAttributes container through the addition of withdrawals. Otherwise, notify_forkchoice_updated inherits all prior functionality.

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def notify_forkchoice_updated(
    self: ExecutionEngine,
    head_block_hash: Hash32,
    safe_block_hash: Hash32,
    finalized_block_hash: Hash32,
    payload_attributes: Optional[PayloadAttributes],
) -> Optional[PayloadId]: ...

Helpers

Modified PayloadAttributes

PayloadAttributes is extended with the withdrawals field.

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@dataclass
class PayloadAttributes(object):
    timestamp: uint64
    prev_randao: Bytes32
    suggested_fee_recipient: ExecutionAddress
    # [New in Capella]
    withdrawals: Sequence[Withdrawal]

Updated fork-choice handlers

on_block

Note: The only modification is the deletion of the verification of merge transition block conditions.

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def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
    """
    Run ``on_block`` upon receiving a new block.
    """
    block = signed_block.message
    # Parent block must be known
    assert block.parent_root in store.block_states
    # 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
    # Make a copy of the state to avoid mutability issues
    state = copy(store.block_states[block.parent_root])
    block_root = hash_tree_root(block)
    state_transition(state, signed_block, True)

    # 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)