lighthouse/eth2/operation_pool/src/lib.rs

mod attestation;
mod attestation_id;
mod max_cover;
mod persistence;

pub use persistence::PersistedOperationPool;

use attestation::{earliest_attestation_validators, AttMaxCover};
use attestation_id::AttestationId;
use max_cover::maximum_cover;
use parking_lot::RwLock;
use state_processing::per_block_processing::errors::{
    AttestationValidationError, AttesterSlashingValidationError, DepositValidationError,
    ExitValidationError, ProposerSlashingValidationError,
};
use state_processing::per_block_processing::{
    get_slashable_indices_modular, verify_attestation_for_block_inclusion,
    verify_attester_slashing, verify_exit, verify_exit_time_independent_only,
    verify_proposer_slashing, VerifySignatures,
};
use std::collections::{btree_map::Entry, hash_map, BTreeMap, HashMap, HashSet};
use std::marker::PhantomData;
use types::{
    typenum::Unsigned, Attestation, AttesterSlashing, BeaconState, ChainSpec, Deposit, EthSpec,
    ProposerSlashing, Validator, VoluntaryExit,
};

#[derive(Default, Debug)]
pub struct OperationPool<T: EthSpec + Default> {
    /// Map from attestation ID (see below) to vectors of attestations.
    attestations: RwLock<HashMap<AttestationId, Vec<Attestation<T>>>>,
    /// Map from deposit index to deposit data.
    // NOTE: We assume that there is only one deposit per index
    // because the Eth1 data is updated (at most) once per epoch,
    // and the spec doesn't seem to accommodate for re-orgs on a time-frame
    // longer than an epoch
    deposits: RwLock<BTreeMap<u64, Deposit>>,
    /// Map from two attestation IDs to a slashing for those IDs.
    attester_slashings: RwLock<HashMap<(AttestationId, AttestationId), AttesterSlashing<T>>>,
    /// Map from proposer index to slashing.
    proposer_slashings: RwLock<HashMap<u64, ProposerSlashing>>,
    /// Map from exiting validator to their exit data.
    voluntary_exits: RwLock<HashMap<u64, VoluntaryExit>>,
    _phantom: PhantomData<T>,
}

#[derive(Debug, PartialEq, Clone)]
pub enum DepositInsertStatus {
    /// The deposit was not already in the pool.
    Fresh,
    /// The deposit already existed in the pool.
    Duplicate,
    /// The deposit conflicted with an existing deposit, which was replaced.
    Replaced(Box<Deposit>),
}

impl<T: EthSpec> OperationPool<T> {
    /// Create a new operation pool.
    pub fn new() -> Self {
        Self::default()
    }

    /// Insert an attestation into the pool, aggregating it with existing attestations if possible.
    ///
    /// ## Note
    ///
    /// This function assumes the given `attestation` is valid.
    pub fn insert_attestation(
        &self,
        attestation: Attestation<T>,
        state: &BeaconState<T>,
        spec: &ChainSpec,
    ) -> Result<(), AttestationValidationError> {
        let id = AttestationId::from_data(&attestation.data, state, spec);

        // Take a write lock on the attestations map.
        let mut attestations = self.attestations.write();

        let existing_attestations = match attestations.entry(id) {
            hash_map::Entry::Vacant(entry) => {
                entry.insert(vec![attestation]);
                return Ok(());
            }
            hash_map::Entry::Occupied(entry) => entry.into_mut(),
        };

        let mut aggregated = false;
        for existing_attestation in existing_attestations.iter_mut() {
            if existing_attestation.signers_disjoint_from(&attestation) {
                existing_attestation.aggregate(&attestation);
                aggregated = true;
            } else if *existing_attestation == attestation {
                aggregated = true;
            }
        }

        if !aggregated {
            existing_attestations.push(attestation);
        }

        Ok(())
    }

    /// Total number of attestations in the pool, including attestations for the same data.
    pub fn num_attestations(&self) -> usize {
        self.attestations.read().values().map(Vec::len).sum()
    }

    /// Get a list of attestations for inclusion in a block.
    pub fn get_attestations(
        &self,
        state: &BeaconState<T>,
        spec: &ChainSpec,
    ) -> Vec<Attestation<T>> {
        // Attestations for the current fork, which may be from the current or previous epoch.
        let prev_epoch = state.previous_epoch();
        let current_epoch = state.current_epoch();
        let prev_domain_bytes = AttestationId::compute_domain_bytes(prev_epoch, state, spec);
        let curr_domain_bytes = AttestationId::compute_domain_bytes(current_epoch, state, spec);
        let reader = self.attestations.read();
        let valid_attestations = reader
            .iter()
            .filter(|(key, _)| {
                key.domain_bytes_match(&prev_domain_bytes)
                    || key.domain_bytes_match(&curr_domain_bytes)
            })
            .flat_map(|(_, attestations)| attestations)
            // That are valid...
            .filter(|attestation| {
                verify_attestation_for_block_inclusion(
                    state,
                    attestation,
                    VerifySignatures::True,
                    spec,
                )
                .is_ok()
            })
            .map(|att| AttMaxCover::new(att, earliest_attestation_validators(att, state)));

        maximum_cover(valid_attestations, T::MaxAttestations::to_usize())
    }

    /// Remove attestations which are too old to be included in a block.
    pub fn prune_attestations(&self, finalized_state: &BeaconState<T>) {
        // We know we can include an attestation if:
        // state.slot <= attestation_slot + SLOTS_PER_EPOCH
        // We approximate this check using the attestation's epoch, to avoid computing
        // the slot or relying on the committee cache of the finalized state.
        self.attestations.write().retain(|_, attestations| {
            // All the attestations in this bucket have the same data, so we only need to
            // check the first one.
            attestations.first().map_or(false, |att| {
                finalized_state.current_epoch() <= att.data.target.epoch + 1
            })
        });
    }

    /// Add a deposit to the pool.
    ///
    /// No two distinct deposits should be added with the same index.
    // TODO: we need to rethink this entirely
    pub fn insert_deposit(
        &self,
        index: u64,
        deposit: Deposit,
    ) -> Result<DepositInsertStatus, DepositValidationError> {
        use DepositInsertStatus::*;

        match self.deposits.write().entry(index) {
            Entry::Vacant(entry) => {
                entry.insert(deposit);
                Ok(Fresh)
            }
            Entry::Occupied(mut entry) => {
                if entry.get() == &deposit {
                    Ok(Duplicate)
                } else {
                    Ok(Replaced(Box::new(entry.insert(deposit))))
                }
            }
        }
    }

    /// Get an ordered list of deposits for inclusion in a block.
    ///
    /// Take at most the maximum number of deposits, beginning from the current deposit index.
    pub fn get_deposits(&self, state: &BeaconState<T>) -> Vec<Deposit> {
        // TODO: We need to update the Merkle proofs for existing deposits as more deposits
        // are added. It probably makes sense to construct the proofs from scratch when forming
        // a block, using fresh info from the ETH1 chain for the current deposit root.
        let start_idx = state.eth1_deposit_index;
        (start_idx..start_idx + T::MaxDeposits::to_u64())
            .map(|idx| self.deposits.read().get(&idx).cloned())
            .take_while(Option::is_some)
            .flatten()
            .collect()
    }

    /// Remove all deposits with index less than the deposit index of the latest finalised block.
    pub fn prune_deposits(&self, state: &BeaconState<T>) -> BTreeMap<u64, Deposit> {
        let deposits_keep = self.deposits.write().split_off(&state.eth1_deposit_index);
        std::mem::replace(&mut self.deposits.write(), deposits_keep)
    }

    /// The number of deposits stored in the pool.
    pub fn num_deposits(&self) -> usize {
        self.deposits.read().len()
    }

    /// Insert a proposer slashing into the pool.
    pub fn insert_proposer_slashing(
        &self,
        slashing: ProposerSlashing,
        state: &BeaconState<T>,
        spec: &ChainSpec,
    ) -> Result<(), ProposerSlashingValidationError> {
        // TODO: should maybe insert anyway if the proposer is unknown in the validator index,
        // because they could *become* known later
        verify_proposer_slashing(&slashing, state, VerifySignatures::True, spec)?;
        self.proposer_slashings
            .write()
            .insert(slashing.proposer_index, slashing);
        Ok(())
    }

    /// Compute the tuple ID that is used to identify an attester slashing.
    ///
    /// Depends on the fork field of the state, but not on the state's epoch.
    fn attester_slashing_id(
        slashing: &AttesterSlashing<T>,
        state: &BeaconState<T>,
        spec: &ChainSpec,
    ) -> (AttestationId, AttestationId) {
        (
            AttestationId::from_data(&slashing.attestation_1.data, state, spec),
            AttestationId::from_data(&slashing.attestation_2.data, state, spec),
        )
    }

    /// Insert an attester slashing into the pool.
    pub fn insert_attester_slashing(
        &self,
        slashing: AttesterSlashing<T>,
        state: &BeaconState<T>,
        spec: &ChainSpec,
    ) -> Result<(), AttesterSlashingValidationError> {
        verify_attester_slashing(state, &slashing, true, VerifySignatures::True, spec)?;
        let id = Self::attester_slashing_id(&slashing, state, spec);
        self.attester_slashings.write().insert(id, slashing);
        Ok(())
    }

    /// Get proposer and attester slashings for inclusion in a block.
    ///
    /// This function computes both types of slashings together, because
    /// attester slashings may be invalidated by proposer slashings included
    /// earlier in the block.
    pub fn get_slashings(
        &self,
        state: &BeaconState<T>,
        spec: &ChainSpec,
    ) -> (Vec<ProposerSlashing>, Vec<AttesterSlashing<T>>) {
        let proposer_slashings = filter_limit_operations(
            self.proposer_slashings.read().values(),
            |slashing| {
                state
                    .validators
                    .get(slashing.proposer_index as usize)
                    .map_or(false, |validator| !validator.slashed)
            },
            T::MaxProposerSlashings::to_usize(),
        );

        // Set of validators to be slashed, so we don't attempt to construct invalid attester
        // slashings.
        let mut to_be_slashed = proposer_slashings
            .iter()
            .map(|s| s.proposer_index)
            .collect::<HashSet<_>>();

        let attester_slashings = self
            .attester_slashings
            .read()
            .iter()
            .filter(|(id, slashing)| {
                // Check the fork.
                Self::attester_slashing_id(slashing, state, spec) == **id
            })
            .filter(|(_, slashing)| {
                // Take all slashings that will slash 1 or more validators.
                let slashed_validators =
                    get_slashable_indices_modular(state, slashing, |index, validator| {
                        validator.slashed || to_be_slashed.contains(&index)
                    });

                // Extend the `to_be_slashed` set so subsequent iterations don't try to include
                // useless slashings.
                if let Ok(validators) = slashed_validators {
                    to_be_slashed.extend(validators);
                    true
                } else {
                    false
                }
            })
            .take(T::MaxAttesterSlashings::to_usize())
            .map(|(_, slashing)| slashing.clone())
            .collect();

        (proposer_slashings, attester_slashings)
    }

    /// Prune proposer slashings for all slashed or withdrawn validators.
    pub fn prune_proposer_slashings(&self, finalized_state: &BeaconState<T>) {
        prune_validator_hash_map(
            &mut self.proposer_slashings.write(),
            |validator| {
                validator.slashed || validator.is_withdrawable_at(finalized_state.current_epoch())
            },
            finalized_state,
        );
    }

    /// Prune attester slashings for all slashed or withdrawn validators, or attestations on another
    /// fork.
    pub fn prune_attester_slashings(&self, finalized_state: &BeaconState<T>, spec: &ChainSpec) {
        self.attester_slashings.write().retain(|id, slashing| {
            let fork_ok = &Self::attester_slashing_id(slashing, finalized_state, spec) == id;
            let curr_epoch = finalized_state.current_epoch();
            let slashing_ok =
                get_slashable_indices_modular(finalized_state, slashing, |_, validator| {
                    validator.slashed || validator.is_withdrawable_at(curr_epoch)
                })
                .is_ok();
            fork_ok && slashing_ok
        });
    }

    /// Insert a voluntary exit, validating it almost-entirely (future exits are permitted).
    pub fn insert_voluntary_exit(
        &self,
        exit: VoluntaryExit,
        state: &BeaconState<T>,
        spec: &ChainSpec,
    ) -> Result<(), ExitValidationError> {
        verify_exit_time_independent_only(state, &exit, VerifySignatures::True, spec)?;
        self.voluntary_exits
            .write()
            .insert(exit.validator_index, exit);
        Ok(())
    }

    /// Get a list of voluntary exits for inclusion in a block.
    pub fn get_voluntary_exits(
        &self,
        state: &BeaconState<T>,
        spec: &ChainSpec,
    ) -> Vec<VoluntaryExit> {
        filter_limit_operations(
            self.voluntary_exits.read().values(),
            |exit| verify_exit(state, exit, VerifySignatures::False, spec).is_ok(),
            T::MaxVoluntaryExits::to_usize(),
        )
    }

    /// Prune if validator has already exited at the last finalized state.
    pub fn prune_voluntary_exits(&self, finalized_state: &BeaconState<T>) {
        prune_validator_hash_map(
            &mut self.voluntary_exits.write(),
            |validator| validator.is_exited_at(finalized_state.current_epoch()),
            finalized_state,
        );
    }

    /// Prune all types of transactions given the latest finalized state.
    pub fn prune_all(&self, finalized_state: &BeaconState<T>, spec: &ChainSpec) {
        self.prune_attestations(finalized_state);
        self.prune_deposits(finalized_state);
        self.prune_proposer_slashings(finalized_state);
        self.prune_attester_slashings(finalized_state, spec);
        self.prune_voluntary_exits(finalized_state);
    }
}

/// Filter up to a maximum number of operations out of an iterator.
fn filter_limit_operations<'a, T: 'a, I, F>(operations: I, filter: F, limit: usize) -> Vec<T>
where
    I: IntoIterator<Item = &'a T>,
    F: Fn(&T) -> bool,
    T: Clone,
{
    operations
        .into_iter()
        .filter(|x| filter(*x))
        .take(limit)
        .cloned()
        .collect()
}

/// Remove all entries from the given hash map for which `prune_if` returns true.
///
/// The keys in the map should be validator indices, which will be looked up
/// in the state's validator registry and then passed to `prune_if`.
/// Entries for unknown validators will be kept.
fn prune_validator_hash_map<T, F, E: EthSpec>(
    map: &mut HashMap<u64, T>,
    prune_if: F,
    finalized_state: &BeaconState<E>,
) where
    F: Fn(&Validator) -> bool,
{
    map.retain(|&validator_index, _| {
        finalized_state
            .validators
            .get(validator_index as usize)
            .map_or(true, |validator| !prune_if(validator))
    });
}

/// Compare two operation pools.
impl<T: EthSpec + Default> PartialEq for OperationPool<T> {
    fn eq(&self, other: &Self) -> bool {
        *self.attestations.read() == *other.attestations.read()
            && *self.deposits.read() == *other.deposits.read()
            && *self.attester_slashings.read() == *other.attester_slashings.read()
            && *self.proposer_slashings.read() == *other.proposer_slashings.read()
            && *self.voluntary_exits.read() == *other.voluntary_exits.read()
    }
}

#[cfg(test)]
mod tests {
    use super::DepositInsertStatus::*;
    use super::*;
    use rand::Rng;
    use types::test_utils::*;
    use types::*;

    #[test]
    fn insert_deposit() {
        let rng = &mut XorShiftRng::from_seed([42; 16]);
        let op_pool = OperationPool::<MinimalEthSpec>::new();
        let deposit1 = make_deposit(rng);
        let deposit2 = make_deposit(rng);
        let index = rng.gen();

        assert_eq!(op_pool.insert_deposit(index, deposit1.clone()), Ok(Fresh));
        assert_eq!(
            op_pool.insert_deposit(index, deposit1.clone()),
            Ok(Duplicate)
        );
        assert_eq!(
            op_pool.insert_deposit(index, deposit2),
            Ok(Replaced(Box::new(deposit1)))
        );
    }

    #[test]
    fn get_deposits_max() {
        let rng = &mut XorShiftRng::from_seed([42; 16]);
        let (_, mut state) = test_state(rng);
        let op_pool = OperationPool::new();
        let start = 10000;
        let max_deposits = <MainnetEthSpec as EthSpec>::MaxDeposits::to_u64();
        let extra = 5;
        let offset = 1;
        assert!(offset <= extra);

        let deposits = dummy_deposits(rng, start, max_deposits + extra);

        for (i, deposit) in &deposits {
            assert_eq!(op_pool.insert_deposit(*i, deposit.clone()), Ok(Fresh));
        }

        state.eth1_deposit_index = start + offset;
        let deposits_for_block = op_pool.get_deposits(&state);

        assert_eq!(deposits_for_block.len() as u64, max_deposits);
        let expected = deposits[offset as usize..(offset + max_deposits) as usize]
            .iter()
            .map(|(_, d)| d.clone())
            .collect::<Vec<_>>();
        assert_eq!(deposits_for_block[..], expected[..]);
    }

    #[test]
    fn prune_deposits() {
        let rng = &mut XorShiftRng::from_seed([42; 16]);
        let op_pool = OperationPool::<MinimalEthSpec>::new();

        let start1 = 100;
        // test is super slow in debug mode if this parameter is too high
        let count = 5;
        let gap = 25;
        let start2 = start1 + count + gap;

        let deposits1 = dummy_deposits(rng, start1, count);
        let deposits2 = dummy_deposits(rng, start2, count);

        for (i, d) in deposits1.into_iter().chain(deposits2) {
            assert!(op_pool.insert_deposit(i, d).is_ok());
        }

        assert_eq!(op_pool.num_deposits(), 2 * count as usize);

        let mut state = BeaconState::random_for_test(rng);
        state.eth1_deposit_index = start1;

        // Pruning the first bunch of deposits in batches of 5 should work.
        let step = 5;
        let mut pool_size = step + 2 * count as usize;
        for i in (start1..=(start1 + count)).step_by(step) {
            state.eth1_deposit_index = i;
            op_pool.prune_deposits(&state);
            pool_size -= step;
            assert_eq!(op_pool.num_deposits(), pool_size);
        }
        assert_eq!(pool_size, count as usize);
        // Pruning in the gap should do nothing.
        for i in (start1 + count..start2).step_by(step) {
            state.eth1_deposit_index = i;
            op_pool.prune_deposits(&state);
            assert_eq!(op_pool.num_deposits(), count as usize);
        }
        // Same again for the later deposits.
        pool_size += step;
        for i in (start2..=(start2 + count)).step_by(step) {
            state.eth1_deposit_index = i;
            op_pool.prune_deposits(&state);
            pool_size -= step;
            assert_eq!(op_pool.num_deposits(), pool_size);
        }
        assert_eq!(op_pool.num_deposits(), 0);
    }

    // Create a random deposit
    fn make_deposit(rng: &mut XorShiftRng) -> Deposit {
        Deposit::random_for_test(rng)
    }

    // Create `count` dummy deposits with sequential deposit IDs beginning from `start`.
    fn dummy_deposits(rng: &mut XorShiftRng, start: u64, count: u64) -> Vec<(u64, Deposit)> {
        let proto_deposit = make_deposit(rng);
        (start..start + count)
            .map(|index| {
                let mut deposit = proto_deposit.clone();
                deposit.data.amount = index * 1000;
                (index, deposit)
            })
            .collect()
    }

    fn test_state(rng: &mut XorShiftRng) -> (ChainSpec, BeaconState<MainnetEthSpec>) {
        let spec = MainnetEthSpec::default_spec();

        let mut state = BeaconState::random_for_test(rng);

        state.fork = Fork::default();

        (spec, state)
    }

    #[cfg(not(debug_assertions))]
    mod release_tests {
        use super::*;

        /// Create a signed attestation for use in tests.
        /// Signed by all validators in `committee[signing_range]` and `committee[extra_signer]`.
        fn signed_attestation<R: std::slice::SliceIndex<[usize], Output = [usize]>, E: EthSpec>(
            committee: &[usize],
            index: u64,
            keypairs: &[Keypair],
            signing_range: R,
            slot: Slot,
            state: &BeaconState<E>,
            spec: &ChainSpec,
            extra_signer: Option<usize>,
        ) -> Attestation<E> {
            let mut builder = TestingAttestationBuilder::new(
                AttestationTestTask::Valid,
                state,
                committee,
                slot,
                index,
                spec,
            );
            let signers = &committee[signing_range];
            let committee_keys = signers.iter().map(|&i| &keypairs[i].sk).collect::<Vec<_>>();
            builder.sign(
                AttestationTestTask::Valid,
                signers,
                &committee_keys,
                &state.fork,
                spec,
            );
            extra_signer.map(|c_idx| {
                let validator_index = committee[c_idx];
                builder.sign(
                    AttestationTestTask::Valid,
                    &[validator_index],
                    &[&keypairs[validator_index].sk],
                    &state.fork,
                    spec,
                )
            });
            builder.build()
        }

        /// Test state for attestation-related tests.
        fn attestation_test_state<E: EthSpec>(
            num_committees: usize,
        ) -> (BeaconState<E>, Vec<Keypair>, ChainSpec) {
            let spec = E::default_spec();

            let num_validators =
                num_committees * E::slots_per_epoch() as usize * spec.target_committee_size;
            let mut state_builder = TestingBeaconStateBuilder::from_default_keypairs_file_if_exists(
                num_validators,
                &spec,
            );
            let slot_offset = 1000 * E::slots_per_epoch() + E::slots_per_epoch() / 2;
            let slot = spec.genesis_slot + slot_offset;
            state_builder.teleport_to_slot(slot);
            state_builder.build_caches(&spec).unwrap();
            let (state, keypairs) = state_builder.build();
            (state, keypairs, MainnetEthSpec::default_spec())
        }

        #[test]
        fn test_earliest_attestation() {
            let (ref mut state, ref keypairs, ref spec) =
                attestation_test_state::<MainnetEthSpec>(1);
            let slot = state.slot - 1;
            let committees = state
                .get_beacon_committees_at_slot(slot)
                .unwrap()
                .into_iter()
                .map(BeaconCommittee::into_owned)
                .collect::<Vec<_>>();

            for bc in committees {
                let att1 = signed_attestation(
                    &bc.committee,
                    bc.index,
                    keypairs,
                    ..2,
                    slot,
                    state,
                    spec,
                    None,
                );
                let att2 = signed_attestation(
                    &bc.committee,
                    bc.index,
                    keypairs,
                    ..,
                    slot,
                    state,
                    spec,
                    None,
                );

                assert_eq!(
                    att1.aggregation_bits.num_set_bits(),
                    earliest_attestation_validators(&att1, state).num_set_bits()
                );
                state
                    .current_epoch_attestations
                    .push(PendingAttestation {
                        aggregation_bits: att1.aggregation_bits.clone(),
                        data: att1.data.clone(),
                        inclusion_delay: 0,
                        proposer_index: 0,
                    })
                    .unwrap();

                assert_eq!(
                    bc.committee.len() - 2,
                    earliest_attestation_validators(&att2, state).num_set_bits()
                );
            }
        }

        /// End-to-end test of basic attestation handling.
        #[test]
        fn attestation_aggregation_insert_get_prune() {
            let (ref mut state, ref keypairs, ref spec) =
                attestation_test_state::<MainnetEthSpec>(1);

            let op_pool = OperationPool::new();

            let slot = state.slot - 1;
            let committees = state
                .get_beacon_committees_at_slot(slot)
                .unwrap()
                .into_iter()
                .map(BeaconCommittee::into_owned)
                .collect::<Vec<_>>();

            assert_eq!(
                committees.len(),
                1,
                "we expect just one committee with this many validators"
            );

            for bc in &committees {
                let step_size = 2;
                for i in (0..bc.committee.len()).step_by(step_size) {
                    let att = signed_attestation(
                        &bc.committee,
                        bc.index,
                        keypairs,
                        i..i + step_size,
                        slot,
                        state,
                        spec,
                        None,
                    );
                    op_pool.insert_attestation(att, state, spec).unwrap();
                }
            }

            assert_eq!(op_pool.attestations.read().len(), committees.len());
            assert_eq!(op_pool.num_attestations(), committees.len());

            // Before the min attestation inclusion delay, get_attestations shouldn't return anything.
            state.slot -= 1;
            assert_eq!(op_pool.get_attestations(state, spec).len(), 0);

            // Then once the delay has elapsed, we should get a single aggregated attestation.
            state.slot += spec.min_attestation_inclusion_delay;

            let block_attestations = op_pool.get_attestations(state, spec);
            assert_eq!(block_attestations.len(), committees.len());

            let agg_att = &block_attestations[0];
            assert_eq!(
                agg_att.aggregation_bits.num_set_bits(),
                spec.target_committee_size as usize
            );

            // Prune attestations shouldn't do anything at this point.
            op_pool.prune_attestations(state);
            assert_eq!(op_pool.num_attestations(), committees.len());

            // But once we advance to more than an epoch after the attestation, it should prune it
            // out of existence.
            state.slot += 2 * MainnetEthSpec::slots_per_epoch();
            op_pool.prune_attestations(state);
            assert_eq!(op_pool.num_attestations(), 0);
        }

        /// Adding an attestation already in the pool should not increase the size of the pool.
        #[test]
        fn attestation_duplicate() {
            let (ref mut state, ref keypairs, ref spec) =
                attestation_test_state::<MainnetEthSpec>(1);

            let op_pool = OperationPool::new();

            let slot = state.slot - 1;
            let committees = state
                .get_beacon_committees_at_slot(slot)
                .unwrap()
                .into_iter()
                .map(BeaconCommittee::into_owned)
                .collect::<Vec<_>>();

            for bc in &committees {
                let att = signed_attestation(
                    &bc.committee,
                    bc.index,
                    keypairs,
                    ..,
                    slot,
                    state,
                    spec,
                    None,
                );
                op_pool
                    .insert_attestation(att.clone(), state, spec)
                    .unwrap();
                op_pool.insert_attestation(att, state, spec).unwrap();
            }

            assert_eq!(op_pool.num_attestations(), committees.len());
        }

        /// Adding lots of attestations that only intersect pairwise should lead to two aggregate
        /// attestations.
        #[test]
        fn attestation_pairwise_overlapping() {
            let (ref mut state, ref keypairs, ref spec) =
                attestation_test_state::<MainnetEthSpec>(1);

            let op_pool = OperationPool::new();

            let slot = state.slot - 1;
            let committees = state
                .get_beacon_committees_at_slot(slot)
                .unwrap()
                .into_iter()
                .map(BeaconCommittee::into_owned)
                .collect::<Vec<_>>();

            let step_size = 2;
            for bc in &committees {
                // Create attestations that overlap on `step_size` validators, like:
                // {0,1,2,3}, {2,3,4,5}, {4,5,6,7}, ...
                for i in (0..bc.committee.len() - step_size).step_by(step_size) {
                    let att = signed_attestation(
                        &bc.committee,
                        bc.index,
                        keypairs,
                        i..i + 2 * step_size,
                        slot,
                        state,
                        spec,
                        None,
                    );
                    op_pool.insert_attestation(att, state, spec).unwrap();
                }
            }

            // The attestations should get aggregated into two attestations that comprise all
            // validators.
            assert_eq!(op_pool.attestations.read().len(), committees.len());
            assert_eq!(op_pool.num_attestations(), 2 * committees.len());
        }

        /// Create a bunch of attestations signed by a small number of validators, and another
        /// bunch signed by a larger number, such that there are at least `max_attestations`
        /// signed by the larger number. Then, check that `get_attestations` only returns the
        /// high-quality attestations. To ensure that no aggregation occurs, ALL attestations
        /// are also signed by the 0th member of the committee.
        #[test]
        fn attestation_get_max() {
            let small_step_size = 2;
            let big_step_size = 4;

            let (ref mut state, ref keypairs, ref spec) =
                attestation_test_state::<MainnetEthSpec>(big_step_size);

            let op_pool = OperationPool::new();

            let slot = state.slot - 1;
            let committees = state
                .get_beacon_committees_at_slot(slot)
                .unwrap()
                .into_iter()
                .map(BeaconCommittee::into_owned)
                .collect::<Vec<_>>();

            let max_attestations = <MainnetEthSpec as EthSpec>::MaxAttestations::to_usize();
            let target_committee_size = spec.target_committee_size as usize;

            let insert_attestations = |bc: &OwnedBeaconCommittee, step_size| {
                for i in (0..target_committee_size).step_by(step_size) {
                    let att = signed_attestation(
                        &bc.committee,
                        bc.index,
                        keypairs,
                        i..i + step_size,
                        slot,
                        state,
                        spec,
                        if i == 0 { None } else { Some(0) },
                    );
                    op_pool.insert_attestation(att, state, spec).unwrap();
                }
            };

            for committee in &committees {
                assert_eq!(committee.committee.len(), target_committee_size);
                // Attestations signed by only 2-3 validators
                insert_attestations(committee, small_step_size);
                // Attestations signed by 4+ validators
                insert_attestations(committee, big_step_size);
            }

            let num_small = target_committee_size / small_step_size;
            let num_big = target_committee_size / big_step_size;

            assert_eq!(op_pool.attestations.read().len(), committees.len());
            assert_eq!(
                op_pool.num_attestations(),
                (num_small + num_big) * committees.len()
            );
            assert!(op_pool.num_attestations() > max_attestations);

            state.slot += spec.min_attestation_inclusion_delay;
            let best_attestations = op_pool.get_attestations(state, spec);
            assert_eq!(best_attestations.len(), max_attestations);

            // All the best attestations should be signed by at least `big_step_size` (4) validators.
            for att in &best_attestations {
                assert!(att.aggregation_bits.num_set_bits() >= big_step_size);
            }
        }
    }

    // TODO: more tests
}