Notice: This document is a work-in-progress for researchers and implementers.
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.
The following methods are added to the ExecutionEngine
protocol for use in the fork choice:
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.
"""
...
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.
"""
...
@dataclass
class TransitionStore(object):
transition_total_difficulty: uint256
@dataclass
class PowBlock(object):
block_hash: Hash32
parent_hash: Hash32
is_processed: boolean
is_valid: boolean
total_difficulty: uint256
difficulty: uint256
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.
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.transition_total_difficulty
is_parent_total_difficulty_valid = parent.total_difficulty < transition_store.transition_total_difficulty
return block.is_valid and is_total_difficulty_reached and is_parent_total_difficulty_valid
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):
# 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