lighthouse/beacon_node/beacon_chain/src/fork_choice.rs

242 lines
8.4 KiB
Rust

use crate::{metrics, BeaconChain, BeaconChainTypes};
use lmd_ghost::LmdGhost;
use state_processing::common::get_attesting_indices;
use std::sync::Arc;
use store::{Error as StoreError, Store};
use types::{
Attestation, BeaconBlock, BeaconState, BeaconStateError, Epoch, EthSpec, Hash256, Slot,
};
type Result<T> = std::result::Result<T, Error>;
#[derive(Debug, PartialEq)]
pub enum Error {
MissingBlock(Hash256),
MissingState(Hash256),
BackendError(String),
BeaconStateError(BeaconStateError),
StoreError(StoreError),
}
pub struct ForkChoice<T: BeaconChainTypes> {
store: Arc<T::Store>,
backend: T::LmdGhost,
/// Used for resolving the `0x00..00` alias back to genesis.
///
/// Does not necessarily need to be the _actual_ genesis, it suffices to be the finalized root
/// whenever the struct was instantiated.
genesis_block_root: Hash256,
}
impl<T: BeaconChainTypes> ForkChoice<T> {
/// Instantiate a new fork chooser.
///
/// "Genesis" does not necessarily need to be the absolute genesis, it can be some finalized
/// block.
pub fn new(
store: Arc<T::Store>,
genesis_block: &BeaconBlock<T::EthSpec>,
genesis_block_root: Hash256,
) -> Self {
Self {
store: store.clone(),
backend: T::LmdGhost::new(store, genesis_block, genesis_block_root),
genesis_block_root,
}
}
pub fn find_head(&self, chain: &BeaconChain<T>) -> Result<Hash256> {
let timer = metrics::start_timer(&metrics::FORK_CHOICE_FIND_HEAD_TIMES);
let start_slot = |epoch: Epoch| epoch.start_slot(T::EthSpec::slots_per_epoch());
// From the specification:
//
// Let justified_head be the descendant of finalized_head with the highest epoch that has
// been justified for at least 1 epoch ... If no such descendant exists,
// set justified_head to finalized_head.
let (start_state, start_block_root, start_block_slot) = {
let state = &chain.head().beacon_state;
let (block_root, block_slot) =
if state.current_epoch() + 1 > state.current_justified_checkpoint.epoch {
(
state.current_justified_checkpoint.root,
start_slot(state.current_justified_checkpoint.epoch),
)
} else {
(
state.finalized_checkpoint.root,
start_slot(state.finalized_checkpoint.epoch),
)
};
let block = chain
.store
.get::<BeaconBlock<T::EthSpec>>(&block_root)?
.ok_or_else(|| Error::MissingBlock(block_root))?;
// Resolve the `0x00.. 00` alias back to genesis
let block_root = if block_root == Hash256::zero() {
self.genesis_block_root
} else {
block_root
};
let state = chain
.store
.get::<BeaconState<T::EthSpec>>(&block.state_root)?
.ok_or_else(|| Error::MissingState(block.state_root))?;
(state, block_root, block_slot)
};
// A function that returns the weight for some validator index.
let weight = |validator_index: usize| -> Option<u64> {
start_state
.validators
.get(validator_index)
.map(|v| v.effective_balance)
};
let result = self
.backend
.find_head(start_block_slot, start_block_root, weight)
.map_err(Into::into);
metrics::stop_timer(timer);
result
}
/// Process all attestations in the given `block`.
///
/// Assumes the block (and therefore it's attestations) are valid. It is a logic error to
/// provide an invalid block.
pub fn process_block(
&self,
state: &BeaconState<T::EthSpec>,
block: &BeaconBlock<T::EthSpec>,
block_root: Hash256,
) -> Result<()> {
let timer = metrics::start_timer(&metrics::FORK_CHOICE_PROCESS_BLOCK_TIMES);
// Note: we never count the block as a latest message, only attestations.
//
// I (Paul H) do not have an explicit reference to this, but I derive it from this
// document:
//
// https://github.com/ethereum/eth2.0-specs/blob/v0.7.0/specs/core/0_fork-choice.md
for attestation in &block.body.attestations {
// If the `data.beacon_block_root` block is not known to us, simply ignore the latest
// vote.
if let Some(block) = self
.store
.get::<BeaconBlock<T::EthSpec>>(&attestation.data.beacon_block_root)?
{
self.process_attestation(state, attestation, &block)?;
}
}
// This does not apply a vote to the block, it just makes fork choice aware of the block so
// it can still be identified as the head even if it doesn't have any votes.
//
// A case where a block without any votes can be the head is where it is the only child of
// a block that has the majority of votes applied to it.
self.backend.process_block(block, block_root)?;
metrics::stop_timer(timer);
Ok(())
}
/// Process an attestation which references `block` in `attestation.data.beacon_block_root`.
///
/// Assumes the attestation is valid.
pub fn process_attestation(
&self,
state: &BeaconState<T::EthSpec>,
attestation: &Attestation<T::EthSpec>,
block: &BeaconBlock<T::EthSpec>,
) -> Result<()> {
let timer = metrics::start_timer(&metrics::FORK_CHOICE_PROCESS_ATTESTATION_TIMES);
let block_hash = attestation.data.beacon_block_root;
// Ignore any attestations to the zero hash.
//
// This is an edge case that results from the spec aliasing the zero hash to the genesis
// block. Attesters may attest to the zero hash if they have never seen a block.
//
// We have two options here:
//
// 1. Apply all zero-hash attestations to the zero hash.
// 2. Ignore all attestations to the zero hash.
//
// (1) becomes weird once we hit finality and fork choice drops the genesis block. (2) is
// fine because votes to the genesis block are not useful; all validators implicitly attest
// to genesis just by being present in the chain.
//
// Additionally, don't add any block hash to fork choice unless we have imported the block.
if block_hash != Hash256::zero() {
let validator_indices =
get_attesting_indices(state, &attestation.data, &attestation.aggregation_bits)?;
for validator_index in validator_indices {
self.backend
.process_attestation(validator_index, block_hash, block.slot)?;
}
}
metrics::stop_timer(timer);
Ok(())
}
/// Returns the latest message for a given validator, if any.
///
/// Returns `(block_root, block_slot)`.
pub fn latest_message(&self, validator_index: usize) -> Option<(Hash256, Slot)> {
self.backend.latest_message(validator_index)
}
/// Runs an integrity verification function on the underlying fork choice algorithm.
///
/// Returns `Ok(())` if the underlying fork choice has maintained it's integrity,
/// `Err(description)` otherwise.
pub fn verify_integrity(&self) -> core::result::Result<(), String> {
self.backend.verify_integrity()
}
/// Inform the fork choice that the given block (and corresponding root) have been finalized so
/// it may prune it's storage.
///
/// `finalized_block_root` must be the root of `finalized_block`.
pub fn process_finalization(
&self,
finalized_block: &BeaconBlock<T::EthSpec>,
finalized_block_root: Hash256,
) -> Result<()> {
self.backend
.update_finalized_root(finalized_block, finalized_block_root)
.map_err(Into::into)
}
}
impl From<BeaconStateError> for Error {
fn from(e: BeaconStateError) -> Error {
Error::BeaconStateError(e)
}
}
impl From<StoreError> for Error {
fn from(e: StoreError) -> Error {
Error::StoreError(e)
}
}
impl From<String> for Error {
fn from(e: String) -> Error {
Error::BackendError(e)
}
}