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Fulu -- Fork Choice

Introduction

This is the modification of the fork choice accompanying Fulu.

Helpers

Modified get_proposer_head

Note: The function is modified to allow proposer re-orgs at epoch boundaries. The proposer lookahead introduced in Fulu fixes proposer assignments before the epoch boundary, so a re-org of a late block at the end of the previous epoch cannot change the current slot's proposer.

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def get_proposer_head(store: Store, head_node: ForkChoiceNode, slot: Slot) -> ForkChoiceNode:
    head_block = store.blocks[head_node.root]
    parent_root = head_block.parent_root
    parent_block = store.blocks[parent_root]
    parent_node = ForkChoiceNode(root=parent_root)

    # Only re-org the head block if it arrived later than the attestation deadline.
    head_late = is_head_late(store, head_node.root)

    # [Modified in Fulu:EIP7917]
    # Removed `shuffling_stable = is_shuffling_stable(slot)`

    # Ensure that the FFG information of the new head will be competitive with the current head.
    ffg_competitive = is_ffg_competitive(store, head_node.root, parent_root)

    # Do not re-org if the chain is not finalizing with acceptable frequency.
    finalization_ok = is_finalization_ok(store, slot)

    # Only re-org if we are proposing on-time.
    proposing_on_time = is_proposing_on_time(store)

    # Only re-org a single slot at most.
    parent_slot_ok = parent_block.slot + 1 == head_block.slot
    current_time_ok = head_block.slot + 1 == slot
    single_slot_reorg = parent_slot_ok and current_time_ok

    # Check that the head has few enough votes to be overpowered by our proposer boost.
    assert store.proposer_boost_root != head_node.root  # ensure boost has worn off
    head_weak = is_head_weak(store, head_node.root)

    # Check that the missing votes are assigned to the parent and not being hoarded.
    parent_strong = is_parent_strong(store, head_node.root)

    # Re-org more aggressively if there is a proposer equivocation in the previous slot.
    proposer_equivocation = is_proposer_equivocation(store, head_node.root)

    if all([
        head_late,
        # [Modified in Fulu:EIP7917]
        # Removed `shuffling_stable`
        ffg_competitive,
        finalization_ok,
        proposing_on_time,
        single_slot_reorg,
        head_weak,
        parent_strong,
    ]):
        # We can re-org the current head by building upon its parent node.
        return parent_node
    elif all([head_weak, current_time_ok, proposer_equivocation]):
        return parent_node
    else:
        return head_node

Modified is_data_available

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def is_data_available(beacon_block_root: Root) -> bool:
    # `retrieve_column_sidecars` is implementation and context dependent, replacing
    # `retrieve_blobs_and_proofs`. For the given block root, it returns all column
    # sidecars to sample, or raises an exception if they are not available.
    # The p2p network does not guarantee sidecar retrieval outside of
    # `MIN_EPOCHS_FOR_DATA_COLUMN_SIDECARS_REQUESTS` epochs.
    column_sidecars = retrieve_column_sidecars(beacon_block_root)
    return all(
        verify_data_column_sidecar(column_sidecar)
        and verify_data_column_sidecar_kzg_proofs(column_sidecar)
        for column_sidecar in column_sidecars
    )

Handlers

Modified on_block

Note: The only modification is that is_data_available does not take blob_kzg_commitments as input.

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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
    block_root = hash_tree_root(block)

    # Return early if the block is already known
    if block_root in store.blocks:
        return

    # Parent block must be known
    assert block.parent_root in store.block_states
    # Make a copy of the state to avoid mutability issues
    state = store.block_states[block.parent_root].copy()
    # 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

    # [Modified in Fulu:EIP7594]
    # Check if blob data is available
    # If not, this payload MAY be queued and subsequently considered when blob data becomes available
    assert is_data_available(block_root)

    # Check the block is valid and compute the post-state
    state_transition(state, signed_block, validate_result=True)

    # Compute head before applying the block
    head = get_head(store)
    # 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

    record_block_timeliness(store, block_root)
    update_proposer_boost_root(store, head.root, block_root)

    # 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)