eth2.0-specs/specs/_features/eip7594/fork-choice.md

# EIP-7594 -- Fork Choice

## Table of contents
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- [Introduction](#introduction)
  - [Helpers](#helpers)
    - [Modified `is_data_available`](#modified-is_data_available)
    - [Modified `get_head`](#modified-get_head)
- [Updated fork-choice handlers](#updated-fork-choice-handlers)
  - [Modified `on_block`](#modified-on_block)

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## Introduction

This is the modification of the fork choice accompanying EIP-7594.

### Helpers

#### Modified `is_data_available`

```python
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_kzg_proofs(column_sidecar)
        for column_sidecar in column_sidecars
    )
```

#### Modified `get_head`

*Note*: children of the current `head` are required to be available in order to be considered by the fork-choice.

```python
def get_head(store: Store) -> Root:
    # Get filtered block tree that only includes viable branches
    blocks = get_filtered_block_tree(store)
    # Execute the LMD-GHOST fork choice
    head = store.justified_checkpoint.root
    while True:
        # Get available children for the current slot
        children = [
            root for (root, block) in blocks.items()
            if (
                block.parent_root == head
                and is_data_available(root)
            )
        ]
        if len(children) == 0:
            return head
        # Sort by latest attesting balance with ties broken lexicographically
        # Ties broken by favoring block with lexicographically higher root
        head = max(children, key=lambda root: (get_weight(store, root), root))
```

## Updated fork-choice handlers

### Modified `on_block`

*Note*: The blob data availability check is removed. We import blocks regardless
of their availability status, and move all filtering to the fork-choice (`get_head`).

```python
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
    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
    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)
```