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.
| Name | SSZ equivalent | Description |
|---|---|---|
PayloadId |
Bytes8 |
Identifier of a payload building process |
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.
This function performs three actions atomically:
- Re-organizes the execution payload chain and corresponding state to make
head_block_hashthe head. - Updates safe block hash with the value provided by
safe_block_hashparameter. - 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 and returns an identifier of initiated process.
def notify_forkchoice_updated(self: ExecutionEngine,
head_block_hash: Hash32,
safe_block_hash: Hash32,
finalized_block_hash: Hash32,
payload_attributes: Optional[PayloadAttributes]) -> Optional[PayloadId]:
...Note: The (head_block_hash, finalized_block_hash) values of the notify_forkchoice_updated function call 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().
Note: Client software MUST NOT call this function until the transition conditions are met on the PoW network, i.e. there exists a block for which is_valid_terminal_pow_block function returns True.
Note: Client software MUST call this function to initiate the payload build process to produce the merge transition block; the head_block_hash parameter MUST be set to the hash of a terminal PoW block in this case.
The safe_block_hash parameter MUST be set to return value of
get_safe_execution_payload_hash(store: Store) function.
If proposer boost re-orgs are implemented and enabled (see get_proposer_head) then additional care
must be taken to ensure that the proposer is able to build an execution payload.
If a beacon node knows it will propose the next block then it SHOULD NOT call
notify_forkchoice_updated if it detects the current head to be weak and potentially capable of
being re-orged. Complete information for evaluating get_proposer_head will not be available
immediately after the receipt of a new block, so an approximation of those conditions should be
used when deciding whether to send or suppress a fork choice notification. The exact conditions
used may be implementation-specific, a suggested implementation is below.
Let validator_is_connected(validator_index: ValidatorIndex) -> bool be a function that indicates
whether the validator with validator_index is connected to the node (e.g. has sent an unexpired
proposer preparation message).
def should_override_forkchoice_update(store: Store, head_root: Root) -> bool:
head_block = store.blocks[head_root]
parent_root = head_block.parent_root
parent_block = store.blocks[parent_root]
current_slot = get_current_slot(store)
proposal_slot = head_block.slot + Slot(1)
# Only re-org the head_block block if it arrived later than the attestation deadline.
head_late = is_head_late(store, head_root)
# Shuffling stable.
shuffling_stable = is_shuffling_stable(proposal_slot)
# FFG information of the new head_block will be competitive with the current head.
ffg_competitive = is_ffg_competitive(store, head_root, parent_root)
# Do not re-org if the chain is not finalizing with acceptable frequency.
finalization_ok = is_finalization_ok(store, proposal_slot)
# Only suppress the fork choice update if we are confident that we will propose the next block.
parent_state_advanced = store.block_states[parent_root].copy()
process_slots(parent_state_advanced, proposal_slot)
proposer_index = get_beacon_proposer_index(parent_state_advanced)
proposing_reorg_slot = validator_is_connected(proposer_index)
# Single slot re-org.
parent_slot_ok = parent_block.slot + 1 == head_block.slot
proposing_on_time = is_proposing_on_time(store)
# Note that this condition is different from `get_proposer_head`
current_time_ok = (head_block.slot == current_slot
or (proposal_slot == current_slot and proposing_on_time))
single_slot_reorg = parent_slot_ok and current_time_ok
# Check the head weight only if the attestations from the head slot have already been applied.
# Implementations may want to do this in different ways, e.g. by advancing
# `store.time` early, or by counting queued attestations during the head block's slot.
if current_slot > head_block.slot:
head_weak = is_head_weak(store, head_root)
parent_strong = is_parent_strong(store, parent_root)
else:
head_weak = True
parent_strong = True
return all([head_late, shuffling_stable, ffg_competitive, finalization_ok,
proposing_reorg_slot, single_slot_reorg,
head_weak, parent_strong])Note: The ordering of conditions is a suggestion only. Implementations are free to
optimize by re-ordering the conditions from least to most expensive and by returning early if
any of the early conditions are False.
In case should_override_forkchoice_update returns True, a node SHOULD instead call
notify_forkchoice_updated with parameters appropriate for building upon the parent block. Care
must be taken to compute the correct payload_attributes, as they may change depending on the slot
of the block to be proposed (due to withdrawals).
If should_override_forkchoice_update returns True but get_proposer_head later chooses the
canonical head rather than its parent, then this is a misprediction that will cause the node
to construct a payload with less notice. The result of get_proposer_head MUST be preferred over
the result of should_override_forkchoice_update (when proposer reorgs are enabled).
Used to signal to initiate the payload build process via notify_forkchoice_updated.
@dataclass
class PayloadAttributes(object):
timestamp: uint64
prev_randao: Bytes32
suggested_fee_recipient: ExecutionAddressclass PowBlock(Container):
block_hash: Hash32
parent_hash: Hash32
total_difficulty: uint256Let get_pow_block(block_hash: Hash32) -> Optional[PowBlock] be the function that given the hash of the PoW block returns its data.
It may result in None if the requested block is not yet available.
Note: The eth_getBlockByHash JSON-RPC method may be used to pull this information from an execution client.
Used by fork-choice handler, on_block.
def is_valid_terminal_pow_block(block: PowBlock, parent: PowBlock) -> bool:
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_validdef validate_merge_block(block: BeaconBlock) -> None:
"""
Check the parent PoW block of execution payload is a valid terminal PoW block.
Note: Unavailable PoW block(s) may later become available,
and a client software MAY delay a call to ``validate_merge_block``
until the PoW block(s) become available.
"""
if TERMINAL_BLOCK_HASH != Hash32():
# If `TERMINAL_BLOCK_HASH` is used as an override, the activation epoch must be reached.
assert compute_epoch_at_slot(block.slot) >= TERMINAL_BLOCK_HASH_ACTIVATION_EPOCH
assert block.body.execution_payload.parent_hash == TERMINAL_BLOCK_HASH
return
pow_block = get_pow_block(block.body.execution_payload.parent_hash)
# Check if `pow_block` is available
assert pow_block is not None
pow_parent = get_pow_block(pow_block.parent_hash)
# Check if `pow_parent` is available
assert pow_parent is not None
# Check if `pow_block` is a valid terminal PoW block
assert is_valid_terminal_pow_block(pow_block, pow_parent)Note: The only modification is the addition of the verification of transition block conditions.
def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
"""
Run ``on_block`` upon receiving a new block.
A block that is asserted as invalid due to unavailable PoW block may be valid at a later time,
consider scheduling it for later processing in such case.
"""
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 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
state = pre_state.copy()
block_root = hash_tree_root(block)
state_transition(state, signed_block, True)
# [New in Bellatrix]
if is_merge_transition_block(pre_state, block.body):
validate_merge_block(block)
# 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)