6.6 KiB
The Merge -- Fork Choice
Notice: This document is a work-in-progress for researchers and implementers.
Table of contents
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.
"""
...
Helpers
TransitionStore
@dataclass
class TransitionStore(object):
terminal_total_difficulty: uint256
PowBlock
@dataclass
class PowBlock(object):
block_hash: Hash32
parent_hash: Hash32
is_processed: boolean
is_valid: boolean
total_difficulty: uint256
difficulty: uint256
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_terminal_pow_block
Used by fork-choice handler, on_block.
def is_valid_terminal_pow_block(transition_store: TransitionStore, block: PowBlock, parent: PowBlock) -> bool:
is_total_difficulty_reached = block.total_difficulty >= transition_store.terminal_total_difficulty
is_parent_total_difficulty_valid = parent.total_difficulty < transition_store.terminal_total_difficulty
return block.is_valid and is_total_difficulty_reached and is_parent_total_difficulty_valid
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, transition_store: TransitionStore=None) -> 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 (transition_store is not None) and is_merge_block(pre_state, block.body):
# Delay consideration of block until PoW block is processed by the PoW node
pow_block = get_pow_block(block.body.execution_payload.parent_hash)
pow_parent = get_pow_block(pow_block.parent_hash)
assert pow_block.is_processed
assert is_valid_terminal_pow_block(transition_store, pow_block, pow_parent)
# 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