7.0 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
Note: The notify_forkchoice_updated function is added to the ExecutionEngine protocol to signal the fork choice updates.
The body of this function is implementation dependent. The Engine API may be used to implement it with an external execution engine.
notify_forkchoice_updated
This function performs two actions atomically:
- Re-organizes the execution payload chain and corresponding state to make
head_block_hashthe head. - Applies finality to the execution state: it irreversibly persists the chain of all execution payloads
and corresponding state, up to and including
finalized_block_hash.
Additionally, if payload_attributes is provided, this function sets in motion a payload build process on top of
head_block_hash with the result to be gathered by a followup call to get_payload.
def notify_forkchoice_updated(self: ExecutionEngine,
head_block_hash: Hash32,
finalized_block_hash: Hash32,
payload_attributes: Optional[PayloadAttributes]) -> None:
...
Note: The call of the notify_forkchoice_updated function maps on the POS_FORKCHOICE_UPDATED event defined in the EIP-3675.
As per EIP-3675, before a post-transition block is finalized, notify_forkchoice_updated must be called with finalized_block_hash = Hash32().
Helpers
PayloadAttributes
Used to signal to initiate the payload build process via notify_forkchoice_updated.
@dataclass
class PayloadAttributes(object):
timestamp: uint64
random: Bytes32
fee_recipient: ExecutionAddress
PowBlock
class PowBlock(Container):
block_hash: Hash32
parent_hash: Hash32
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 may be used to pull this information from an execution client.
is_valid_terminal_pow_block
Used by fork-choice handler, on_block.
def is_valid_terminal_pow_block(block: PowBlock, parent: PowBlock) -> bool:
if TERMINAL_BLOCK_HASH != Hash32():
return block.block_hash == TERMINAL_BLOCK_HASH
is_total_difficulty_reached = block.total_difficulty >= TERMINAL_TOTAL_DIFFICULTY
is_parent_total_difficulty_valid = parent.total_difficulty < TERMINAL_TOTAL_DIFFICULTY
return 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) -> 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
# Check the block is valid and compute the post-state
state = pre_state.copy()
state_transition(state, signed_block, True)
# [New in Merge]
if is_merge_block(pre_state, block.body):
# Check the parent PoW block of execution payload is a valid terminal PoW block.
pow_block = get_pow_block(block.body.execution_payload.parent_hash)
pow_parent = get_pow_block(pow_block.parent_hash)
assert is_valid_terminal_pow_block(pow_block, pow_parent)
# If `TERMINAL_BLOCK_HASH` is used as an override, the activation epoch must be reached.
if TERMINAL_BLOCK_HASH != Hash32():
assert compute_epoch_at_slot(block.slot) >= TERMINAL_BLOCK_HASH_ACTIVATION_EPOCH
# 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