From c8ca26f4253c6ba0876bd45a11b2210faf76d767 Mon Sep 17 00:00:00 2001 From: Potuz Date: Tue, 25 Jun 2024 11:42:30 +0200 Subject: [PATCH] Forkchoice changes --- specs/_features/epbs/fork-choice.md | 500 ++++++++++++++++++++++++++++ 1 file changed, 500 insertions(+) create mode 100644 specs/_features/epbs/fork-choice.md diff --git a/specs/_features/epbs/fork-choice.md b/specs/_features/epbs/fork-choice.md new file mode 100644 index 0000000000..858072e83f --- /dev/null +++ b/specs/_features/epbs/fork-choice.md @@ -0,0 +1,500 @@ +# ePBS -- Fork Choice + +## Table of contents + + + + +- [Introduction](#introduction) +- [Constants](#constants) +- [Containers](#containers) + - [New `ChildNode`](#new-childnode) +- [Helpers](#helpers) + - [Modified `LatestMessage`](#modified-latestmessage) + - [Modified `update_latest_messages`](#modified-update_latest_messages) + - [Modified `Store`](#modified-store) + - [`notify_ptc_messages`](#notify_ptc_messages) + - [`is_payload_present`](#is_payload_present) + - [`is_parent_node_full`](#is_parent_node_full) + - [Modified `get_ancestor`](#modified-get_ancestor) + - [Modified `get_checkpoint_block`](#modified-get_checkpoint_block) + - [`is_supporting_vote`](#is_supporting_vote) + - [New `compute_proposer_boost`](#new-compute_proposer_boost) + - [New `compute_withhold_boost` `](#new-compute_withhold_boost-) + - [New `compute_reveal_boost` `](#new-compute_reveal_boost-) + - [Modified `get_weight`](#modified-get_weight) + - [Modified `get_head`](#modified-get_head) +- [Updated fork-choice handlers](#updated-fork-choice-handlers) + - [`on_block`](#on_block) +- [New fork-choice handlers](#new-fork-choice-handlers) + - [New `on_execution_payload`](#new-on_execution_payload) + - [`seconds_into_slot`](#seconds_into_slot) + - [Modified `on_tick_per_slot`](#modified-on_tick_per_slot) + - [`on_payload_attestation_message`](#on_payload_attestation_message) + + + + +## Introduction + +This is the modification of the fork choice accompanying the ePBS upgrade. + +## Constants + +| Name | Value | +| -------------------- | ----------- | +| `PAYLOAD_TIMELY_THRESHOLD` | `PTC_SIZE/2` (=`uint64(256)`) | +| `PROPOSER_SCORE_BOOST` | `20` # [modified in EIP-XXXX] | +| `PAYLOAD_WITHHOLD_BOOST` | `40` | +| `PAYLOAD_REVEAL_BOOST` | `40` | + +## Containers + +### New `ChildNode` +Auxiliary class to consider `(block, slot, bool)` LMD voting + +```python +class ChildNode(Container): + root: Root + slot: Slot + is_payload_present: bool +``` + +## Helpers + +### Modified `LatestMessage` +**Note:** The class is modified to keep track of the slot instead of the epoch. + +```python +@dataclass(eq=True, frozen=True) +class LatestMessage(object): + slot: Slot + root: Root +``` + +### Modified `update_latest_messages` +**Note:** the function `update_latest_messages` is updated to use the attestation slot instead of target. Notice that this function is only called on validated attestations and validators cannot attest twice in the same epoch without equivocating. Notice also that target epoch number and slot number are validated on `validate_on_attestation`. + +```python +def update_latest_messages(store: Store, attesting_indices: Sequence[ValidatorIndex], attestation: Attestation) -> None: + slot = attestation.data.slot + beacon_block_root = attestation.data.beacon_block_root + non_equivocating_attesting_indices = [i for i in attesting_indices if i not in store.equivocating_indices] + for i in non_equivocating_attesting_indices: + if i not in store.latest_messages or slot > store.latest_messages[i].slot: + store.latest_messages[i] = LatestMessage(slot=slot, root=beacon_block_root) +``` + +### Modified `Store` +**Note:** `Store` is modified to track the intermediate states of "empty" consensus blocks, that is, those consensus blocks for which the corresponding execution payload has not been revealed or has not been included on chain. + +```python +@dataclass +class Store(object): + time: uint64 + genesis_time: uint64 + justified_checkpoint: Checkpoint + finalized_checkpoint: Checkpoint + unrealized_justified_checkpoint: Checkpoint + unrealized_finalized_checkpoint: Checkpoint + proposer_boost_root: Root + equivocating_indices: Set[ValidatorIndex] + blocks: Dict[Root, BeaconBlock] = field(default_factory=dict) + block_states: Dict[Root, BeaconState] = field(default_factory=dict) + checkpoint_states: Dict[Checkpoint, BeaconState] = field(default_factory=dict) + latest_messages: Dict[ValidatorIndex, LatestMessage] = field(default_factory=dict) + unrealized_justifications: Dict[Root, Checkpoint] = field(default_factory=dict) + execution_payload_states: Dict[Root, BeaconState] = field(default_factory=dict) # [New in EIP-XXXX] + ptc_vote: Dict[Root, Vector[uint8, PTC_SIZE]] = field(default_factory=dict) # [New in EIP-XXXX] + payload_withhold_boost_root: Root # [New in EIP-XXXX] + payload_withhold_boost_full: bool # [New in EIP-XXXX] + payload_reveal_boost_root: Root # [New in EIP-XXXX] +``` + +### `notify_ptc_messages` + +```python +def notify_ptc_messages(store: Store, state: BeaconState, payload_attestations: Sequence[PayloadAttestation]) -> None: + """ + Extracts a list of ``PayloadAttestationMessage`` from ``payload_attestations`` and updates the store with them + These Payload attestations are assumed to be in the beacon block hence signature verification is not needed + """ + if state.slot == 0: + return + for payload_attestation in payload_attestations: + indexed_payload_attestation = get_indexed_payload_attestation(state, state.slot - 1, payload_attestation) + for idx in indexed_payload_attestation.attesting_indices: + store.on_payload_attestation_message(PayloadAttestationMessage(validator_index=idx, + data=payload_attestation.data, signature= BLSSignature(), is_from_block=true)) +``` + +### `is_payload_present` + +```python +def is_payload_present(store: Store, beacon_block_root: Root) -> bool: + """ + Return whether the execution payload for the beacon block with root ``beacon_block_root`` was voted as present + by the PTC + """ + # The beacon block root must be known + assert beacon_block_root in store.ptc_vote + return store.ptc_vote[beacon_block_root].count(PAYLOAD_PRESENT) > PAYLOAD_TIMELY_THRESHOLD +``` + +### `is_parent_node_full` + +```python +def is_parent_node_full(store: Store, block: BeaconBlock) -> bool: + parent = store.blocks[block.parent_root] + return block.body.signed_execution_payload_header.message.parent_block_hash == parent.body.signed_execution_payload_header.message.block_hash +``` + +### Modified `get_ancestor` +**Note:** `get_ancestor` is modified to return whether the chain is based on an *empty* or *full* block. + +```python +def get_ancestor(store: Store, root: Root, slot: Slot) -> ChildNode: + """ + Returns the beacon block root, the slot and the payload status of the ancestor of the beacon block + with ``root`` at ``slot``. If the beacon block with ``root`` is already at ``slot`` or we are + requesting an ancestor "in the future" it returns its PTC status instead of the actual payload content. + """ + block = store.blocks[root] + if block.slot <= slot: + return ChildNode(root=root, slot=slot, is_payload_present=is_payload_present(store, root)) + + parent = store.blocks[block.parent_root] + if parent.slot > slot: + return get_ancestor(store, block.parent_root, slot) + return ChildNode(root=block.parent_root, slot=parent.slot, is_payload_present=is_parent_node_full(block)) +``` + +### Modified `get_checkpoint_block` +**Note:** `get_checkpoint_block` is modified to use the new `get_ancestor` + +```python +def get_checkpoint_block(store: Store, root: Root, epoch: Epoch) -> Root: + """ + Compute the checkpoint block for epoch ``epoch`` in the chain of block ``root`` + """ + epoch_first_slot = compute_start_slot_at_epoch(epoch) + return get_ancestor(store, root, epoch_first_slot).root +``` + + +### `is_supporting_vote` + +```python +def is_supporting_vote(store: Store, node: ChildNode, message: LatestMessage) -> bool: + """ + Returns whether a vote for ``message.root`` supports the chain containing the beacon block ``node.root`` with the + payload contents indicated by ``node.is_payload_present`` as head during slot ``node.slot``. + """ + if node.root == message.root: + # an attestation for a given root always counts for that root regardless if full or empty + # as long as the attestation happened after the requested slot. + return node.slot <= message.slot + message_block = store.blocks[message.root] + if node.slot >= message_block.slot: + return False + ancestor = get_ancestor(store, message.root, node.slot) + return (node.root == ancestor.root) and (node.is_payload_present == ancestor.is_payload_present) +``` + +### New `compute_proposer_boost` +This is a helper to compute the proposer boost. It applies the proposer boost to any ancestor of the proposer boost root taking into account the payload presence. There is one exception: if the requested node has the same root and slot as the block with the proposer boost root, then the proposer boost is applied to both empty and full versions of the node. +```python +def compute_proposer_boost(store: Store, state: State, node: ChildNode) -> Gwei: + if store.proposer_boost_root == Root(): + return Gwei(0) + ancestor = get_ancestor(store, store.proposer_boost_root, node.slot) + if ancestor.root != node.root: + return Gwei(0) + proposer_boost_slot = store.blocks[store.proposer_boost_root].slot + # Proposer boost is not applied after skipped slots + if node.slot > proposer_boost_slot: + return Gwei(0) + if (node.slot < proposer_boost_slot) and (ancestor.is_payload_present != node.is_payload_present): + return Gwei(0) + committee_weight = get_total_active_balance(state) // SLOTS_PER_EPOCH + return (committee_weight * PROPOSER_SCORE_BOOST) // 100 +``` + +### New `compute_withhold_boost` ` +This is a similar helper that applies for the withhold boost. In this case this always takes into account the reveal status. + +```python +def compute_withhold_boost(store: Store, state: State, node: ChildNode) -> Gwei: + if store.payload_withhold_boost_root == Root(): + return Gwei(0) + ancestor = get_ancestor(store, store.payload_withold_boost_root, node.slot) + if ancestor.root != node.root: + return Gwei(0) + if node.slot >= store.blocks[store.payload_withhold_boost_root].slot: + ancestor.is_payload_present = store.payload_withhold_boost_full + if ancestor.is_payload_present != node.is_payload_present: + return Gwei(0) + + committee_weight = get_total_active_balance(state) // SLOTS_PER_EPOCH + return (committee_weight * PAYLOAD_WITHHOLD_BOOST) // 100 +``` + +### New `compute_reveal_boost` ` +This is a similar helper to the last two, the only difference is that the reveal boost is only applied to the full version of the node when querying for the same slot as the revealed payload. + +```python +def compute_reveal_boost(store: Store, state: State, node: ChildNode) -> Gwei: + if store.payload_reveal_boost_root == Root(): + return Gwei(0) + ancestor = get_ancestor(store, store.payload_reveal_boost_root, node.slot) + if ancestor.root != node.root: + return Gwei(0) + if node.slot >= store.blocks[store.payload_reveal_boost_root].slot: + ancestor.is_payload_present = True + if is_ancestor_full != node.is_payload_present: + return Gwei(0) + committee_weight = get_total_active_balance(state) // SLOTS_PER_EPOCH + return (committee_weight * PAYLOAD_REVEAL_BOOST) // 100 +``` + +### Modified `get_weight` + +**Note:** `get_weight` is modified to only count votes for descending chains that support the status of a triple `Root, Slot, bool`, where the `bool` indicates if the block was full or not. `Slot` is needed for a correct implementation of `(Block, Slot)` voting. + +```python +def get_weight(store: Store, node: ChildNode) -> Gwei: + state = store.checkpoint_states[store.justified_checkpoint] + unslashed_and_active_indices = [ + i for i in get_active_validator_indices(state, get_current_epoch(state)) + if not is_slashed_attester(state.validators[i]) + ] + attestation_score = Gwei(sum( + state.validators[i].effective_balance for i in unslashed_and_active_indices + if (i in store.latest_messages + and i not in store.equivocating_indices + and is_supporting_vote(store, node, store.latest_messages[i])) + )) + + # Compute boosts + proposer_score = compute_boost(store, state, node) + builder_reveal_score = compute_reveal_boost(store, state, node) + builder_withhold_score = compute_withhold_boost(store, state, node) + + return attestation_score + proposer_score + builder_reveal_score + builder_withhold_score +``` + +### Modified `get_head` + +**Note:** `get_head` is a modified to use the new `get_weight` function. It returns the `ChildNode` object corresponidng to the head block. + +```python +def get_head(store: Store) -> ChildNode: + # Get filtered block tree that only includes viable branches + blocks = get_filtered_block_tree(store) + # Execute the LMD-GHOST fork choice + justified_root = store.justified_checkpoint.root + justified_block = store.blocks[justified_root] + justified_slot = justified_block.slot + justified_full = is_payload_present(store, justified_root) + best_child = ChildNode(root=head_root, slot=head_slot, is_payload_present=head_full) + while True: + children = [ + ChildNode(root=root, slot=block.slot, is_payload_present=present) for (root, block) in blocks.items() + if block.parent_root == best_child.root and + is_parent_node_full(store, block) == best_child.is_payload_present if root != store.justified_root + for present in (True, False) if root in store.execution_payload_states or present == False + ] + if len(children) == 0: + return best_child + # if we have children we consider the current head advanced as a possible head + children += [ChildNode(root=best_child.root, slot=best_child.slot + 1, best_child.is_payload_present)] + # Sort by latest attesting balance with ties broken lexicographically + # Ties broken by favoring full blocks according to the PTC vote + # Ties are then broken by favoring full blocks + # Ties broken then by favoring higher slot numbers + # Ties then broken by favoring block with lexicographically higher root + new_best_child = max(children, key=lambda child: (get_weight(store, child), is_payload_present(store, child.root), child.is_payload_present, child.slot, child.root)) + if new_best_child.root == best_child.root: + return new_best_child + best_child = new_best_child +``` + +## Updated fork-choice handlers + +### `on_block` + +*Note*: The handler `on_block` is modified to consider the pre `state` of the given consensus beacon block depending not only on the parent block root, but also on the parent blockhash. In addition we delay the checking of blob data availability until the processing of the execution payload. + +```python +def on_block(store: Store, signed_block: SignedBeaconBlock) -> None: + """ + Run ``on_block`` upon receiving a new block. + """ + block = signed_block.message + # Parent block must be known + assert block.parent_root in store.block_states + + # Check if this blocks builds on empty or full parent block + parent_block = store.blocks[block.parent_root] + header = block.body.signed_execution_payload_header.message + parent_header = parent_block.body.signed_execution_payload_header.message + # Make a copy of the state to avoid mutability issues + if is_parent_node_full(store, block): + assert block.parent_root in store.execution_payload_states + state = copy(store.execution_payload_states[block.parent_root]) + else: + assert header.parent_block_hash == parent_header.parent_block_hash + 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. + current_slot = get_current_slot(store) + assert current_slot >= 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 + block_root = hash_tree_root(block) + state_transition(state, signed_block, True) + + # 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 a new PTC voting for this block to the store + store.ptc_vote[block_root] = [PAYLOAD_ABSENT]*PTC_SIZE + + # Notify the store about the payload_attestations in the block + store.notify_ptc_messages(state, block.body.payload_attestations) + # Add proposer score boost if the block is timely + time_into_slot = (store.time - store.genesis_time) % SECONDS_PER_SLOT + is_before_attesting_interval = time_into_slot < SECONDS_PER_SLOT // INTERVALS_PER_SLOT + if get_current_slot(store) == block.slot and is_before_attesting_interval: + 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) +``` + +## New fork-choice handlers + +### New `on_execution_payload` + +The handler `on_execution_payload` is called when the node receives a `SignedExecutionPayloadEnvelope` to sync. + +```python +def on_execution_payload(store: Store, signed_envelope: SignedExecutionPayloadEnvelope) -> None: + """ + Run ``on_execution_payload`` upon receiving a new execution payload. + """ + envelope = signed_envelope.message + # The corresponding beacon block root needs to be known + assert envelope.beacon_block_root in store.block_states + + # Check if blob data is available + # If not, this payload MAY be queued and subsequently considered when blob data becomes available + assert is_data_available(envelope.beacon_block_root, envelope.blob_kzg_commitments) + + # Make a copy of the state to avoid mutability issues + state = copy(store.block_states[envelope.beacon_block_root]) + + # Process the execution payload + process_execution_payload(state, signed_envelope, EXECUTION_ENGINE) + + # Add new state for this payload to the store + store.execution_payload_states[envelope.beacon_block_root] = state +``` + +### `seconds_into_slot` + +```python +def seconds_into_slot(store: Store) -> uint64: + return (store.time - store.genesis_time) % SECONDS_PER_SLOT +``` + +### Modified `on_tick_per_slot` + +Modified to reset the payload boost roots + +```python +def on_tick_per_slot(store: Store, time: uint64) -> None: + previous_slot = get_current_slot(store) + + # Update store time + store.time = time + + current_slot = get_current_slot(store) + + # If this is a new slot, reset store.proposer_boost_root + if current_slot > previous_slot: + store.proposer_boost_root = Root() + else: + # Reset the payload boost if this is the attestation time + if seconds_into_slot(store) >= SECONDS_PER_SLOT // INTERVALS_PER_SLOT: + store.payload_withhold_boost_root = Root() + store.payload_withhold_boost_full = False + store.payload_reveal_boost_root = Root() + + # If a new epoch, pull-up justification and finalization from previous epoch + if current_slot > previous_slot and compute_slots_since_epoch_start(current_slot) == 0: + update_checkpoints(store, store.unrealized_justified_checkpoint, store.unrealized_finalized_checkpoint) +``` + +### `on_payload_attestation_message` + +```python +def on_payload_attestation_message(store: Store, + ptc_message: PayloadAttestationMessage, is_from_block: bool=False) -> None: + """ + Run ``on_payload_attestation_message`` upon receiving a new ``ptc_message`` directly on the wire. + """ + # The beacon block root must be known + data = ptc_message.data + # PTC attestation must be for a known block. If block is unknown, delay consideration until the block is found + state = store.block_states[data.beacon_block_root] + ptc = get_ptc(state, data.slot) + # PTC votes can only change the vote for their assigned beacon block, return early otherwise + if data.slot != state.slot: + return + # Check that the attester is from the PTC + assert ptc_message.validator_index in ptc + + # Verify the signature and check that its for the current slot if it is coming from the wire + if not is_from_block: + # Check that the attestation is for the current slot + assert data.slot == get_current_slot(store) + # Verify the signature + assert is_valid_indexed_payload_attestation(state, + IndexedPayloadAttestation(attesting_indices = [ptc_message.validator_index], data = data, + signature = ptc_message.signature)) + # Update the ptc vote for the block + ptc_index = ptc.index(ptc_message.validator_index) + ptc_vote = store.ptc_vote[data.beacon_block_root] + ptc_vote[ptc_index] = data.payload_status + + # Only update payload boosts with attestations from a block if the block is for the current slot and it's early + if is_from_block && data.slot + 1 != get_current_slot(store): + return + time_into_slot = (store.time - store.genesis_time) % SECONDS_PER_SLOT + if is_from_block and time_into_slot >= SECONDS_PER_SLOT // INTERVALS_PER_SLOT: + return + + # Update the payload boosts if threshold has been achieved + if ptc_vote.count(PAYLOAD_PRESENT) > PAYLOAD_TIMELY_THRESHOLD: + store.payload_reveal_boost_root = data.beacon_block_root + if ptc_vote.count(PAYLOAD_WITHHELD) > PAYLOAD_TIMELY_THRESHOLD: + block = store.blocks[data.beacon_block_root] + store.payload_withhold_boost_root = block.parent_root + store.payload_withhold_boost_full = is_parent_node_full(block) +```