consensus-specs/specs/deneb/fork-choice.md

Deneb -- Fork Choice

Table of contents

• Introduction
• Containers
• Helpers
  • validate_blobs
  • is_data_available
• Updated fork-choice handlers
  • on_block

Introduction

This is the modification of the fork choice accompanying the Deneb upgrade.

Containers

Helpers

validate_blobs

def validate_blobs(expected_kzg_commitments: Sequence[KZGCommitment],
                   blobs: Sequence[Blob],
                   proofs: Sequence[KZGProof]) -> None:
    assert len(expected_kzg_commitments) == len(blobs)
    assert len(blobs) == len(proofs)

    assert verify_blob_kzg_proof_batch(blobs, expected_kzg_commitments, proofs)

is_data_available

The implementation of is_data_available will become more sophisticated during later scaling upgrades. Initially, verification requires every verifying actor to retrieve all matching Blobs and KZGProofs, and validate them with validate_blobs.

The block MUST NOT be considered valid until all valid Blobs have been downloaded. Blocks that have been previously validated as available SHOULD be considered available even if the associated Blobs have subsequently been pruned.

def is_data_available(beacon_block_root: Root, blob_kzg_commitments: Sequence[KZGCommitment]) -> bool:
    # `retrieve_blobs_and_proofs` is implementation and context dependent
    # It returns all the blobs for the given block root, and raises an exception if not available
    # Note: the p2p network does not guarantee sidecar retrieval outside of `MIN_EPOCHS_FOR_BLOB_SIDECARS_REQUESTS`
    blobs, proofs = retrieve_blobs_and_proofs(beacon_block_root)

    # For testing, `retrieve_blobs_and_proofs` returns ("TEST", "TEST").
    # TODO: Remove it once we have a way to inject `BlobSidecar` into tests.
    if isinstance(blobs, str) or isinstance(proofs, str):
        return True

    validate_blobs(blob_kzg_commitments, blobs, proofs)
    return True

Updated fork-choice handlers

on_block

Note: The only modification is the addition of the blob data availability check.

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
    # 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
    assert get_ancestor(store, block.parent_root, finalized_slot) == store.finalized_checkpoint.root

    # [New in Deneb]
    # Check if blob data is available
    # If not, this block MAY be queued and subsequently considered when blob data becomes available
    assert is_data_available(hash_tree_root(block), block.body.blob_kzg_commitments)

    # 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)

    # Check the merge transition
    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
    if get_current_slot(store) == block.slot and is_before_attesting_interval:
        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)