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

Ethereum 2.0 The Merge

Notice: This document is a work-in-progress for researchers and implementers.

Table of contents

• Introduction
• Protocols
  • ExecutionEngine
    • set_head
    • finalize_block
• Containers
  • PowBlock
• Helper functions
  • get_pow_block
  • is_valid_transition_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.

Protocols

ExecutionEngine

The following methods are added to the ExecutionEngine protocol for use in the fork choice:

set_head

Re-organizes the execution payload chain and corresponding state to make block_hash the head.

The body of this function is implementation dependent. The Consensus API may be used to implement this with an external execution engine.

def set_head(self: ExecutionEngine, block_hash: Hash32) -> bool:
    """
    Returns True if the ``block_hash`` was successfully set as head of the execution payload chain.
    """
    ...

finalize_block

Applies finality to the execution state: it irreversibly persists the chain of all execution payloads and corresponding state, up to and including block_hash.

The body of this function is implementation dependent. The Consensus API may be used to implement this with an external execution engine.

def finalize_block(self: ExecutionEngine, block_hash: Hash32) -> bool:
    """
    Returns True if the data up to and including ``block_hash`` was successfully finalized.
    """
    ...

Containers

PowBlock

class PowBlock(Container):
    block_hash: Hash32
    is_processed: boolean
    is_valid: boolean
    total_difficulty: uint256

Helper functions

get_pow_block

Let get_pow_block(block_hash: Hash32) -> PowBlock be the function that given the hash of the PoW block returns its data.

Note: The eth_getBlockByHash JSON-RPC method does not distinguish invalid blocks from blocks that haven't been processed yet. Either extending this existing method or implementing a new one is required.

is_valid_transition_block

Used by fork-choice handler, on_block.

def is_valid_transition_block(block: PowBlock) -> bool:
    is_total_difficulty_reached = block.total_difficulty >= TRANSITION_TOTAL_DIFFICULTY
    return block.is_valid and is_total_difficulty_reached

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:
    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 the 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 Merge]
    if is_transition_block(pre_state, block):
        # Delay consideration of block until PoW block is processed by the PoW node
        pow_block = get_pow_block(block.body.execution_payload.parent_hash)
        assert pow_block.is_processed
        assert is_valid_transition_block(pow_block)

    # Check the block is valid and compute the post-state
    state = pre_state.copy()
    state_transition(state, signed_block, True)
    # Add new block to the store
    store.blocks[hash_tree_root(block)] = block
    # Add new state for this block to the store
    store.block_states[hash_tree_root(block)] = state

    # Update justified checkpoint
    if state.current_justified_checkpoint.epoch > store.justified_checkpoint.epoch:
        if state.current_justified_checkpoint.epoch > store.best_justified_checkpoint.epoch:
            store.best_justified_checkpoint = state.current_justified_checkpoint
        if should_update_justified_checkpoint(store, state.current_justified_checkpoint):
            store.justified_checkpoint = state.current_justified_checkpoint

    # Update finalized checkpoint
    if state.finalized_checkpoint.epoch > store.finalized_checkpoint.epoch:
        store.finalized_checkpoint = state.finalized_checkpoint
        
        # Potentially update justified if different from store
        if store.justified_checkpoint != state.current_justified_checkpoint:
            # Update justified if new justified is later than store justified
            if state.current_justified_checkpoint.epoch > store.justified_checkpoint.epoch:
                store.justified_checkpoint = state.current_justified_checkpoint
                return

            # Update justified if store justified is not in chain with finalized checkpoint
            finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
            ancestor_at_finalized_slot = get_ancestor(store, store.justified_checkpoint.root, finalized_slot)
            if ancestor_at_finalized_slot != store.finalized_checkpoint.root:
                store.justified_checkpoint = state.current_justified_checkpoint