lighthouse/beacon_node/beacon_chain/src/beacon_proposer_cache.rs

//! The `BeaconProposer` cache stores the proposer indices for some epoch.
//!
//! This cache is keyed by `(epoch, block_root)` where `block_root` is the block root at
//! `end_slot(epoch - 1)`. We make the assertion that the proposer shuffling is identical for all
//! blocks in `epoch` which share the common ancestor of `block_root`.
//!
//! The cache is a fairly unintelligent LRU cache that is not pruned after finality. This makes it
//! very simple to reason about, but it might store values that are useless due to finalization. The
//! values it stores are very small, so this should not be an issue.

use crate::{BeaconChain, BeaconChainError, BeaconChainTypes};
use fork_choice::ExecutionStatus;
use lru::LruCache;
use once_cell::sync::OnceCell;
use safe_arith::SafeArith;
use smallvec::SmallVec;
use state_processing::state_advance::partial_state_advance;
use std::num::NonZeroUsize;
use std::sync::Arc;
use types::non_zero_usize::new_non_zero_usize;
use types::{
    BeaconState, BeaconStateError, ChainSpec, Epoch, EthSpec, Fork, Hash256, Slot, Unsigned,
};

/// The number of sets of proposer indices that should be cached.
const CACHE_SIZE: NonZeroUsize = new_non_zero_usize(16);

/// This value is fairly unimportant, it's used to avoid heap allocations. The result of it being
/// incorrect is non-substantial from a consensus perspective (and probably also from a
/// performance perspective).
pub const TYPICAL_SLOTS_PER_EPOCH: usize = 32;

/// For some given slot, this contains the proposer index (`index`) and the `fork` that should be
/// used to verify their signature.
pub struct Proposer {
    pub index: usize,
    pub fork: Fork,
}

/// The list of proposers for some given `epoch`, alongside the `fork` that should be used to verify
/// their signatures.
pub struct EpochBlockProposers {
    /// The epoch to which the proposers pertain.
    pub(crate) epoch: Epoch,
    /// The fork that should be used to verify proposer signatures.
    pub(crate) fork: Fork,
    /// A list of length `T::EthSpec::slots_per_epoch()`, representing the proposers for each slot
    /// in that epoch.
    ///
    /// E.g., if `self.epoch == 1`, then `self.proposers[0]` contains the proposer for slot `32`.
    pub(crate) proposers: SmallVec<[usize; TYPICAL_SLOTS_PER_EPOCH]>,
}

impl EpochBlockProposers {
    pub fn new(epoch: Epoch, fork: Fork, proposers: Vec<usize>) -> Self {
        Self {
            epoch,
            fork,
            proposers: proposers.into(),
        }
    }

    pub fn get_slot<E: EthSpec>(&self, slot: Slot) -> Result<Proposer, BeaconChainError> {
        let epoch = slot.epoch(E::slots_per_epoch());
        if epoch == self.epoch {
            self.proposers
                .get(slot.as_usize() % E::SlotsPerEpoch::to_usize())
                .map(|&index| Proposer {
                    index,
                    fork: self.fork,
                })
                .ok_or(BeaconChainError::ProposerCacheOutOfBounds { slot, epoch })
        } else {
            Err(BeaconChainError::ProposerCacheWrongEpoch {
                request_epoch: epoch,
                cache_epoch: self.epoch,
            })
        }
    }
}

/// A cache to store the proposers for some epoch.
///
/// See the module-level documentation for more information.
pub struct BeaconProposerCache {
    cache: LruCache<(Epoch, Hash256), Arc<OnceCell<EpochBlockProposers>>>,
}

impl Default for BeaconProposerCache {
    fn default() -> Self {
        Self {
            cache: LruCache::new(CACHE_SIZE),
        }
    }
}

impl BeaconProposerCache {
    /// If it is cached, returns the proposer for the block at `slot` where the block has the
    /// ancestor block root of `shuffling_decision_block` at `end_slot(slot.epoch() - 1)`.
    pub fn get_slot<E: EthSpec>(
        &mut self,
        shuffling_decision_block: Hash256,
        slot: Slot,
    ) -> Option<Proposer> {
        let epoch = slot.epoch(E::slots_per_epoch());
        let key = (epoch, shuffling_decision_block);
        let cache = self.cache.get(&key)?.get()?;
        cache.get_slot::<E>(slot).ok()
    }

    /// As per `Self::get_slot`, but returns all proposers in all slots for the given `epoch`.
    ///
    /// The nth slot in the returned `SmallVec` will be equal to the nth slot in the given `epoch`.
    /// E.g., if `epoch == 1` then `smallvec[0]` refers to slot 32 (assuming `SLOTS_PER_EPOCH ==
    /// 32`).
    pub fn get_epoch<E: EthSpec>(
        &mut self,
        shuffling_decision_block: Hash256,
        epoch: Epoch,
    ) -> Option<&SmallVec<[usize; TYPICAL_SLOTS_PER_EPOCH]>> {
        let key = (epoch, shuffling_decision_block);
        self.cache
            .get(&key)
            .and_then(|cache_once_cell| cache_once_cell.get().map(|proposers| &proposers.proposers))
    }

    /// Returns the `OnceCell` for the given `(epoch, shuffling_decision_block)` key,
    /// inserting an empty one if it doesn't exist.
    ///
    /// The returned `OnceCell` allows the caller to initialise the value externally
    /// using `get_or_try_init`, enabling deferred computation without holding a mutable
    /// reference to the cache.
    pub fn get_or_insert_key(
        &mut self,
        epoch: Epoch,
        shuffling_decision_block: Hash256,
    ) -> Arc<OnceCell<EpochBlockProposers>> {
        let key = (epoch, shuffling_decision_block);
        self.cache
            .get_or_insert(key, || Arc::new(OnceCell::new()))
            .clone()
    }

    /// Insert the proposers into the cache.
    ///
    /// See `Self::get` for a description of `shuffling_decision_block`.
    ///
    /// The `fork` value must be valid to verify proposer signatures in `epoch`.
    pub fn insert(
        &mut self,
        epoch: Epoch,
        shuffling_decision_block: Hash256,
        proposers: Vec<usize>,
        fork: Fork,
    ) -> Result<(), BeaconStateError> {
        let key = (epoch, shuffling_decision_block);
        if !self.cache.contains(&key) {
            let epoch_proposers = EpochBlockProposers::new(epoch, fork, proposers);
            self.cache
                .put(key, Arc::new(OnceCell::with_value(epoch_proposers)));
        }

        Ok(())
    }
}

/// Compute the proposer duties using the head state without cache.
pub fn compute_proposer_duties_from_head<T: BeaconChainTypes>(
    request_epoch: Epoch,
    chain: &BeaconChain<T>,
) -> Result<(Vec<usize>, Hash256, ExecutionStatus, Fork), BeaconChainError> {
    // Atomically collect information about the head whilst holding the canonical head `Arc` as
    // short as possible.
    let (mut state, head_state_root, head_block_root) = {
        let head = chain.canonical_head.cached_head();
        // Take a copy of the head state.
        let head_state = head.snapshot.beacon_state.clone();
        let head_state_root = head.head_state_root();
        let head_block_root = head.head_block_root();
        (head_state, head_state_root, head_block_root)
    };

    let execution_status = chain
        .canonical_head
        .fork_choice_read_lock()
        .get_block_execution_status(&head_block_root)
        .ok_or(BeaconChainError::HeadMissingFromForkChoice(head_block_root))?;

    // Advance the state into the requested epoch.
    ensure_state_can_determine_proposers_for_epoch(
        &mut state,
        head_state_root,
        request_epoch,
        &chain.spec,
    )?;

    let indices = state
        .get_beacon_proposer_indices(request_epoch, &chain.spec)
        .map_err(BeaconChainError::from)?;

    let dependent_root = state
        // The only block which decides its own shuffling is the genesis block.
        .proposer_shuffling_decision_root(chain.genesis_block_root, &chain.spec)
        .map_err(BeaconChainError::from)?;

    Ok((indices, dependent_root, execution_status, state.fork()))
}

/// If required, advance `state` to the epoch required to determine proposer indices in `target_epoch`.
///
/// ## Details
///
/// - Returns an error if `state.current_epoch() > target_epoch`.
/// - No-op if `state.current_epoch() == target_epoch`.
/// - It must be the case that `state.canonical_root() == state_root`, but this function will not
///   check that.
pub fn ensure_state_can_determine_proposers_for_epoch<E: EthSpec>(
    state: &mut BeaconState<E>,
    state_root: Hash256,
    target_epoch: Epoch,
    spec: &ChainSpec,
) -> Result<(), BeaconChainError> {
    // The decision slot is the end of an epoch, so we add 1 to reach the first slot of the epoch
    // at which the shuffling is determined.
    let minimum_slot = spec
        .proposer_shuffling_decision_slot::<E>(target_epoch)
        .safe_add(1)?;
    let minimum_epoch = minimum_slot.epoch(E::slots_per_epoch());

    // Before and after Fulu, the oldest epoch reachable from a state at epoch N is epoch N itself,
    // i.e. we can never "look back".
    let maximum_epoch = target_epoch;

    if state.current_epoch() > maximum_epoch {
        Err(BeaconStateError::SlotOutOfBounds.into())
    } else if state.current_epoch() >= minimum_epoch {
        if target_epoch > state.current_epoch() {
            let target_slot = target_epoch.start_slot(E::slots_per_epoch());

            // Advance the state into the same epoch as the block. Use the "partial" method since state
            // roots are not important for proposer/attester shuffling.
            partial_state_advance(state, Some(state_root), target_slot, spec)
                .map_err(BeaconChainError::from)?;
        }
        Ok(())
    } else {
        // State's current epoch is less than the minimum epoch.
        // Advance the state up to the minimum epoch.
        partial_state_advance(state, Some(state_root), minimum_slot, spec)
            .map_err(BeaconChainError::from)
    }
}