lighthouse/beacon_node/network/src/attestation_service/mod.rs

//! This service keeps track of which shard subnet the beacon node should be subscribed to at any
//! given time. It schedules subscriptions to shard subnets, requests peer discoveries and
//! determines whether attestations should be aggregated and/or passed to the beacon node.

use beacon_chain::{BeaconChain, BeaconChainTypes};
use eth2_libp2p::{types::GossipKind, NetworkGlobals};
use futures::prelude::*;
use hashset_delay::HashSetDelay;
use rand::seq::SliceRandom;
use rest_types::ValidatorSubscription;
use slog::{crit, debug, error, o, trace, warn};
use slot_clock::SlotClock;
use std::collections::VecDeque;
use std::pin::Pin;
use std::sync::Arc;
use std::task::{Context, Poll};
use std::time::{Duration, Instant};
use types::{Attestation, EthSpec, Slot, SubnetId};

mod tests;

/// The minimum number of slots ahead that we attempt to discover peers for a subscription. If the
/// slot is less than this number, skip the peer discovery process.
const MIN_PEER_DISCOVERY_SLOT_LOOK_AHEAD: u64 = 1;
/// The number of slots ahead that we attempt to discover peers for a subscription. If the slot to
/// attest to is greater than this, we queue a discovery request for this many slots prior to
/// subscribing.
const TARGET_PEER_DISCOVERY_SLOT_LOOK_AHEAD: u64 = 6;
/// The time (in slots) before a last seen validator is considered absent and we unsubscribe from the random
/// gossip topics that we subscribed to due to the validator connection.
const LAST_SEEN_VALIDATOR_TIMEOUT: u32 = 150;
// 30 mins at a 12s slot time
/// The fraction of a slot that we subscribe to a subnet before the required slot.
///
/// Note: The time is calculated as `time = milliseconds_per_slot / ADVANCE_SUBSCRIPTION_TIME`.
const ADVANCE_SUBSCRIBE_TIME: u32 = 3;
/// The default number of slots before items in hash delay sets used by this class should expire.
const DEFAULT_EXPIRATION_TIMEOUT: u32 = 3;
// 36s at 12s slot time
/// The default number of slots before items in hash delay sets used by this class should expire.
const DURATION_DIFFERENCE: Duration = Duration::from_millis(1);

#[derive(Debug, Eq, Clone)]
pub enum AttServiceMessage {
    /// Subscribe to the specified subnet id.
    Subscribe(SubnetId),
    /// Unsubscribe to the specified subnet id.
    Unsubscribe(SubnetId),
    /// Add the `SubnetId` to the ENR bitfield.
    EnrAdd(SubnetId),
    /// Remove the `SubnetId` from the ENR bitfield.
    EnrRemove(SubnetId),
    /// Discover peers for a particular subnet.
    /// The includes the `Instant` we need the discovered peer until.
    DiscoverPeers {
        subnet_id: SubnetId,
        min_ttl: Option<Instant>,
    },
}

impl PartialEq for AttServiceMessage {
    fn eq(&self, other: &AttServiceMessage) -> bool {
        match (self, other) {
            (&AttServiceMessage::Subscribe(a), &AttServiceMessage::Subscribe(b)) => a == b,
            (&AttServiceMessage::Unsubscribe(a), &AttServiceMessage::Unsubscribe(b)) => a == b,
            (&AttServiceMessage::EnrAdd(a), &AttServiceMessage::EnrAdd(b)) => a == b,
            (&AttServiceMessage::EnrRemove(a), &AttServiceMessage::EnrRemove(b)) => a == b,
            (
                &AttServiceMessage::DiscoverPeers { subnet_id, min_ttl },
                &AttServiceMessage::DiscoverPeers {
                    subnet_id: other_subnet_id,
                    min_ttl: other_min_ttl,
                },
            ) => match (min_ttl, other_min_ttl) {
                (Some(min_ttl_instant), Some(other_min_ttl_instant)) => {
                    min_ttl_instant.saturating_duration_since(other_min_ttl_instant)
                        < DURATION_DIFFERENCE
                        && other_min_ttl_instant.saturating_duration_since(min_ttl_instant)
                            < DURATION_DIFFERENCE
                        && subnet_id == other_subnet_id
                }
                (None, None) => subnet_id == other_subnet_id,
                _ => false,
            },
            _ => false,
        }
    }
}

/// A particular subnet at a given slot.
#[derive(PartialEq, Eq, Hash, Clone, Debug)]
pub struct ExactSubnet {
    /// The `SubnetId` associated with this subnet.
    pub subnet_id: SubnetId,
    /// The `Slot` associated with this subnet.
    pub slot: Slot,
}

pub struct AttestationService<T: BeaconChainTypes> {
    /// Queued events to return to the driving service.
    events: VecDeque<AttServiceMessage>,

    /// A collection of public network variables.
    network_globals: Arc<NetworkGlobals<T::EthSpec>>,

    /// A reference to the beacon chain to process received attestations.
    beacon_chain: Arc<BeaconChain<T>>,

    /// The collection of currently subscribed random subnets mapped to their expiry deadline.
    random_subnets: HashSetDelay<SubnetId>,

    /// A collection of timeouts for when to start searching for peers for a particular shard.
    discover_peers: HashSetDelay<ExactSubnet>,

    /// A collection of timeouts for when to subscribe to a shard subnet.
    subscriptions: HashSetDelay<ExactSubnet>,

    /// A collection of timeouts for when to unsubscribe from a shard subnet.
    unsubscriptions: HashSetDelay<ExactSubnet>,

    /// A collection timeouts to track the existence of aggregate validator subscriptions at an `ExactSubnet`.
    aggregate_validators_on_subnet: HashSetDelay<ExactSubnet>,

    /// A collection of seen validators. These dictate how many random subnets we should be
    /// subscribed to. As these time out, we unsubscribe for the required random subnets and update
    /// our ENR.
    /// This is a set of validator indices.
    known_validators: HashSetDelay<u64>,

    /// The logger for the attestation service.
    log: slog::Logger,
}

impl<T: BeaconChainTypes> AttestationService<T> {
    /* Public functions */

    pub fn new(
        beacon_chain: Arc<BeaconChain<T>>,
        network_globals: Arc<NetworkGlobals<T::EthSpec>>,
        log: &slog::Logger,
    ) -> Self {
        let log = log.new(o!("service" => "attestation_service"));

        // calculate the random subnet duration from the spec constants
        let spec = &beacon_chain.spec;
        let slot_duration = beacon_chain.slot_clock.slot_duration();
        let random_subnet_duration_millis = spec
            .epochs_per_random_subnet_subscription
            .saturating_mul(T::EthSpec::slots_per_epoch())
            .saturating_mul(slot_duration.as_millis() as u64);

        // Panics on overflow. Ensure LAST_SEEN_VALIDATOR_TIMEOUT is not too large.
        let last_seen_val_timeout = slot_duration
            .checked_mul(LAST_SEEN_VALIDATOR_TIMEOUT)
            .expect("LAST_SEEN_VALIDATOR_TIMEOUT must not be ridiculously large");
        let default_timeout = slot_duration
            .checked_mul(DEFAULT_EXPIRATION_TIMEOUT)
            .expect("DEFAULT_EXPIRATION_TIMEOUT must not be ridiculoustly large");

        AttestationService {
            events: VecDeque::with_capacity(10),
            network_globals,
            beacon_chain,
            random_subnets: HashSetDelay::new(Duration::from_millis(random_subnet_duration_millis)),
            discover_peers: HashSetDelay::new(default_timeout),
            subscriptions: HashSetDelay::new(default_timeout),
            unsubscriptions: HashSetDelay::new(default_timeout),
            aggregate_validators_on_subnet: HashSetDelay::new(default_timeout),
            known_validators: HashSetDelay::new(last_seen_val_timeout),
            log,
        }
    }

    /// Processes a list of validator subscriptions.
    ///
    /// This will:
    /// - Register new validators as being known.
    /// - Subscribe to the required number of random subnets.
    /// - Update the local ENR for new random subnets due to seeing new validators.
    /// - Search for peers for required subnets.
    /// - Request subscriptions for subnets on specific slots when required.
    /// - Build the timeouts for each of these events.
    ///
    /// This returns a result simply for the ergonomics of using ?. The result can be
    /// safely dropped.
    pub fn validator_subscriptions(
        &mut self,
        subscriptions: Vec<ValidatorSubscription>,
    ) -> Result<(), String> {
        for subscription in subscriptions {
            //NOTE: We assume all subscriptions have been verified before reaching this service

            // Registers the validator with the attestation service.
            // This will subscribe to long-lived random subnets if required.
            trace!(self.log,
                "Validator subscription";
                "subscription" => format!("{:?}", subscription),
            );
            self.add_known_validator(subscription.validator_index);

            let subnet_id = match SubnetId::compute_subnet::<T::EthSpec>(
                subscription.slot,
                subscription.attestation_committee_index,
                subscription.committee_count_at_slot,
                &self.beacon_chain.spec,
            ) {
                Ok(subnet_id) => subnet_id,
                Err(e) => {
                    warn!(self.log,
                        "Failed to compute subnet id for validator subscription";
                        "error" => format!("{:?}", e),
                        "validator_index" => subscription.validator_index
                    );
                    continue;
                }
            };

            let exact_subnet = ExactSubnet {
                subnet_id,
                slot: subscription.slot,
            };
            // determine if we should run a discovery lookup request and request it if required
            if let Err(e) = self.discover_peers_request(exact_subnet.clone()) {
                warn!(self.log, "Discovery lookup request error"; "error" => e);
            }

            // determine if the validator is an aggregator. If so, we subscribe to the subnet and
            // if successful add the validator to a mapping of known aggregators for that exact
            // subnet.
            // NOTE: There is a chance that a fork occurs between now and when the validator needs
            // to aggregate attestations. If this happens, the signature will no longer be valid
            // and it could be likely the validator no longer needs to aggregate. More
            // sophisticated logic should be added using known future forks.
            // TODO: Implement

            if subscription.is_aggregator {
                // set the subscription timer to subscribe to the next subnet if required
                if let Err(e) = self.subscribe_to_subnet(exact_subnet.clone()) {
                    warn!(self.log,
                        "Subscription to subnet error";
                        "error" => e,
                        "validator_index" => subscription.validator_index,
                    );
                } else {
                    trace!(self.log,
                        "Subscribed to subnet for aggregator duties";
                        "exact_subnet" => format!("{:?}", exact_subnet),
                        "validator_index" => subscription.validator_index
                    );
                }
            }
        }
        Ok(())
    }

    /// Checks if we have subscribed aggregate validators for the subnet. If not, checks the gossip
    /// verification, re-propagates and returns false.
    pub fn should_process_attestation(
        &mut self,
        subnet: &SubnetId,
        attestation: &Attestation<T::EthSpec>,
    ) -> bool {
        let exact_subnet = ExactSubnet {
            subnet_id: subnet.clone(),
            slot: attestation.data.slot,
        };
        self.aggregate_validators_on_subnet.contains(&exact_subnet)
    }

    /* Internal private functions */

    /// Checks if there are currently queued discovery requests and the time required to make the
    /// request.
    ///
    /// If there is sufficient time and no other request exists, queues a peer discovery request
    /// for the required subnet.
    fn discover_peers_request(&mut self, exact_subnet: ExactSubnet) -> Result<(), &'static str> {
        let current_slot = self
            .beacon_chain
            .slot_clock
            .now()
            .ok_or_else(|| "Could not get the current slot")?;
        let slot_duration = self.beacon_chain.slot_clock.slot_duration();

        // if there is enough time to perform a discovery lookup
        if exact_subnet.slot >= current_slot.saturating_add(MIN_PEER_DISCOVERY_SLOT_LOOK_AHEAD) {
            // check if a discovery request already exists
            if self.discover_peers.get(&exact_subnet).is_some() {
                // already a request queued, end
                return Ok(());
            }

            // if the slot is more than epoch away, add an event to start looking for peers
            if exact_subnet.slot
                < current_slot.saturating_add(TARGET_PEER_DISCOVERY_SLOT_LOOK_AHEAD)
            {
                // add one slot to ensure we keep the peer for the subscription slot
                let min_ttl = self
                    .beacon_chain
                    .slot_clock
                    .duration_to_slot(exact_subnet.slot + 1)
                    .map(|duration| std::time::Instant::now() + duration);

                self.send_or_update_discovery_event(exact_subnet.subnet_id, min_ttl);
            } else {
                // Queue the discovery event to be executed for
                // TARGET_PEER_DISCOVERY_SLOT_LOOK_AHEAD

                let duration_to_discover = {
                    let duration_to_next_slot = self
                        .beacon_chain
                        .slot_clock
                        .duration_to_next_slot()
                        .ok_or_else(|| "Unable to determine duration to next slot")?;
                    // The -1 is done here to exclude the current slot duration, as we will use
                    // `duration_to_next_slot`.
                    let slots_until_discover = exact_subnet
                        .slot
                        .saturating_sub(current_slot)
                        .saturating_sub(1u64)
                        .saturating_sub(TARGET_PEER_DISCOVERY_SLOT_LOOK_AHEAD);

                    duration_to_next_slot + slot_duration * (slots_until_discover.as_u64() as u32)
                };

                self.discover_peers
                    .insert_at(exact_subnet, duration_to_discover);
            }
        } else {
            // TODO: Send the time frame needed to have a peer connected, so that we can
            // maintain peers for a least this duration.
            // We may want to check the global PeerInfo to see estimated timeouts for each
            // peer before they can be removed.
            return Err("Not enough time for a discovery search");
        }
        Ok(())
    }

    /// Checks if we have a discover peers event already and sends a new event if necessary
    ///
    /// If a message exists for the same subnet, compare the `min_ttl` of the current and
    /// existing messages and extend the existing message as necessary.
    fn send_or_update_discovery_event(&mut self, subnet_id: SubnetId, min_ttl: Option<Instant>) {
        // track whether this message already exists in the event queue
        let mut is_duplicate = false;

        self.events.iter_mut().for_each(|event| {
            match event {
                AttServiceMessage::DiscoverPeers {
                    subnet_id: other_subnet_id,
                    min_ttl: other_min_ttl,
                } => {
                    if subnet_id == *other_subnet_id {
                        let other_min_ttl_clone = other_min_ttl.clone();
                        match (min_ttl, other_min_ttl_clone) {
                            (Some(min_ttl_instant), Some(other_min_ttl_instant)) =>
                            // only update the min_ttl if it is greater than the existing min_ttl and a DURATION_DIFFERENCE padding
                            {
                                if min_ttl_instant.saturating_duration_since(other_min_ttl_instant)
                                    > DURATION_DIFFERENCE
                                {
                                    *other_min_ttl = min_ttl;
                                }
                            }
                            (None, Some(_)) => {
                                // Update the min_ttl to None, because the new message is longer-lived.
                                *other_min_ttl = None;
                            }
                            (Some(_), None) => {} // Don't replace this because the existing message is for a longer-lived peer.
                            (None, None) => {}    // Duplicate message, do nothing.
                        }
                        is_duplicate = true;
                        return;
                    }
                }
                _ => {}
            };
        });
        if !is_duplicate {
            self.events
                .push_back(AttServiceMessage::DiscoverPeers { subnet_id, min_ttl });
        }
    }

    /// Checks the current random subnets and subscriptions to determine if a new subscription for this
    /// subnet is required for the given slot.
    ///
    /// If required, adds a subscription event and an associated unsubscription event.
    fn subscribe_to_subnet(&mut self, exact_subnet: ExactSubnet) -> Result<(), &'static str> {
        // initialise timing variables
        let current_slot = self
            .beacon_chain
            .slot_clock
            .now()
            .ok_or_else(|| "Could not get the current slot")?;

        // Calculate the duration to the subscription event and the duration to the end event.
        // There are two main cases. Attempting to subscribe to the current slot and all others.
        let (duration_to_subscribe, expected_end_subscription_duration) = {
            let duration_to_next_slot = self
                .beacon_chain
                .slot_clock
                .duration_to_next_slot()
                .ok_or_else(|| "Unable to determine duration to next slot")?;

            if current_slot >= exact_subnet.slot {
                (Duration::from_secs(0), duration_to_next_slot)
            } else {
                let slot_duration = self.beacon_chain.slot_clock.slot_duration();
                let advance_subscription_duration = slot_duration
                    .checked_div(ADVANCE_SUBSCRIBE_TIME)
                    .expect("ADVANCE_SUBSCRIPTION_TIME cannot be too large");

                // calculate the time to subscribe to the subnet
                let duration_to_subscribe = self
                    .beacon_chain
                    .slot_clock
                    .duration_to_slot(exact_subnet.slot)
                    .ok_or_else(|| "Unable to determine duration to subscription slot")?
                    .checked_sub(advance_subscription_duration)
                    .unwrap_or_else(|| Duration::from_secs(0));

                // the duration until we no longer need this subscription. We assume a single slot is
                // sufficient.
                let expected_end_subscription_duration = duration_to_subscribe
                    + slot_duration
                    + std::cmp::min(advance_subscription_duration, duration_to_next_slot);

                (duration_to_subscribe, expected_end_subscription_duration)
            }
        };

        // Regardless of whether or not we have already subscribed to a subnet, track the expiration
        // of aggregate validator subscriptions to exact subnets so we know whether or not to drop
        // attestations for a given subnet + slot
        self.aggregate_validators_on_subnet
            .insert_at(exact_subnet.clone(), expected_end_subscription_duration);

        // Checks on current subscriptions
        // Note: We may be connected to a long-lived random subnet. In this case we still add the
        // subscription timeout and check this case when the timeout fires. This is because a
        // long-lived random subnet can be unsubscribed at any time when a validator becomes
        // in-active. This case is checked on the subscription event (see `handle_subscriptions`).

        // Return if we already have a subscription for this subnet_id and slot
        if self.subscriptions.contains(&exact_subnet) {
            return Ok(());
        }

        // We are not currently subscribed and have no waiting subscription, create one
        self.subscriptions
            .insert_at(exact_subnet.clone(), duration_to_subscribe);

        // if there is an unsubscription event for the slot prior, we remove it to prevent
        // unsubscriptions immediately after the subscription. We also want to minimize
        // subscription churn and maintain a consecutive subnet subscriptions.
        let to_remove_subnet = ExactSubnet {
            subnet_id: exact_subnet.subnet_id,
            slot: exact_subnet.slot.saturating_sub(1u64),
        };
        self.unsubscriptions.remove(&to_remove_subnet);
        // add an unsubscription event to remove ourselves from the subnet once completed
        self.unsubscriptions
            .insert_at(exact_subnet, expected_end_subscription_duration);
        Ok(())
    }

    /// Updates the `known_validators` mapping and subscribes to a set of random subnets if required.
    ///
    /// This also updates the ENR to indicate our long-lived subscription to the subnet
    fn add_known_validator(&mut self, validator_index: u64) {
        if self.known_validators.get(&validator_index).is_none() {
            // New validator has subscribed
            // Subscribe to random topics and update the ENR if needed.

            let spec = &self.beacon_chain.spec;

            if self.random_subnets.len() < spec.attestation_subnet_count as usize {
                // Still room for subscriptions
                self.subscribe_to_random_subnets(
                    self.beacon_chain.spec.random_subnets_per_validator as usize,
                );
            }
        }
        // add the new validator or update the current timeout for a known validator
        self.known_validators.insert(validator_index);
    }

    /// Subscribe to long-lived random subnets and update the local ENR bitfield.
    fn subscribe_to_random_subnets(&mut self, no_subnets_to_subscribe: usize) {
        let subnet_count = self.beacon_chain.spec.attestation_subnet_count;

        // Build a list of random subnets that we are not currently subscribed to.
        let available_subnets = (0..subnet_count)
            .map(SubnetId::new)
            .filter(|subnet_id| self.random_subnets.get(subnet_id).is_none())
            .collect::<Vec<_>>();

        let to_subscribe_subnets = {
            if available_subnets.len() < no_subnets_to_subscribe {
                debug!(self.log, "Reached maximum random subnet subscriptions");
                available_subnets
            } else {
                // select a random sample of available subnets
                available_subnets
                    .choose_multiple(&mut rand::thread_rng(), no_subnets_to_subscribe)
                    .cloned()
                    .collect::<Vec<_>>()
            }
        };

        for subnet_id in to_subscribe_subnets {
            // remove this subnet from any immediate subscription/un-subscription events
            self.subscriptions
                .retain(|exact_subnet| exact_subnet.subnet_id != subnet_id);
            self.unsubscriptions
                .retain(|exact_subnet| exact_subnet.subnet_id != subnet_id);

            // insert a new random subnet
            self.random_subnets.insert(subnet_id);

            // if we are not already subscribed, then subscribe
            let topic_kind = &GossipKind::Attestation(subnet_id);

            let already_subscribed = self
                .network_globals
                .gossipsub_subscriptions
                .read()
                .iter()
                .find(|topic| topic.kind() == topic_kind)
                .is_some();

            if !already_subscribed {
                // send a discovery request and a subscription
                self.send_or_update_discovery_event(subnet_id, None);
                self.events
                    .push_back(AttServiceMessage::Subscribe(subnet_id));
            }
            // add the subnet to the ENR bitfield
            self.events.push_back(AttServiceMessage::EnrAdd(subnet_id));
        }
    }

    /* A collection of functions that handle the various timeouts */

    /// Request a discovery query to find peers for a particular subnet.
    fn handle_discover_peers(&mut self, exact_subnet: ExactSubnet) {
        debug!(self.log, "Searching for peers for subnet"; "subnet" => *exact_subnet.subnet_id, "target_slot" => exact_subnet.slot);

        // add one slot to ensure we keep the peer for the subscription slot
        let min_ttl = self
            .beacon_chain
            .slot_clock
            .duration_to_slot(exact_subnet.slot + 1)
            .map(|duration| std::time::Instant::now() + duration);

        self.send_or_update_discovery_event(exact_subnet.subnet_id, min_ttl)
    }

    /// A queued subscription is ready.
    ///
    /// We add subscriptions events even if we are already subscribed to a random subnet (as these
    /// can be unsubscribed at any time by inactive validators). If we are
    /// still subscribed at the time the event fires, we don't re-subscribe.
    fn handle_subscriptions(&mut self, exact_subnet: ExactSubnet) {
        // Check if the subnet currently exists as a long-lasting random subnet
        if let Some(expiry) = self.random_subnets.get(&exact_subnet.subnet_id) {
            // we are subscribed via a random subnet, if this is to expire during the time we need
            // to be subscribed, just extend the expiry
            let slot_duration = self.beacon_chain.slot_clock.slot_duration();
            let advance_subscription_duration = slot_duration
                .checked_div(ADVANCE_SUBSCRIBE_TIME)
                .expect("ADVANCE_SUBSCRIPTION_TIME cannot be too large");
            // we require the subnet subscription for at least a slot on top of the initial
            // subscription time
            let expected_end_subscription_duration = slot_duration + advance_subscription_duration;

            if expiry < &(Instant::now() + expected_end_subscription_duration) {
                self.random_subnets
                    .update_timeout(&exact_subnet.subnet_id, expected_end_subscription_duration);
            }
        } else {
            // we are also not un-subscribing from a subnet if the next slot requires us to be
            // subscribed. Therefore there could be the case that we are already still subscribed
            // to the required subnet. In which case we do not issue another subscription request.
            let topic_kind = &GossipKind::Attestation(exact_subnet.subnet_id);
            if self
                .network_globals
                .gossipsub_subscriptions
                .read()
                .iter()
                .find(|topic| topic.kind() == topic_kind)
                .is_none()
            {
                // we are not already subscribed
                debug!(self.log, "Subscribing to subnet"; "subnet" => *exact_subnet.subnet_id, "target_slot" => exact_subnet.slot.as_u64());
                self.events
                    .push_back(AttServiceMessage::Subscribe(exact_subnet.subnet_id));
            }
        }
    }

    /// A queued unsubscription is ready.
    ///
    /// Unsubscription events are added, even if we are subscribed to long-lived random subnets. If
    /// a random subnet is present, we do not unsubscribe from it.
    fn handle_unsubscriptions(&mut self, exact_subnet: ExactSubnet) {
        // Check if the subnet currently exists as a long-lasting random subnet
        if self.random_subnets.contains(&exact_subnet.subnet_id) {
            return;
        }

        debug!(self.log, "Unsubscribing from subnet"; "subnet" => *exact_subnet.subnet_id, "processed_slot" => exact_subnet.slot.as_u64());

        // various logic checks
        if self.subscriptions.contains(&exact_subnet) {
            crit!(self.log, "Unsubscribing from a subnet in subscriptions");
        }
        self.events
            .push_back(AttServiceMessage::Unsubscribe(exact_subnet.subnet_id));
    }

    /// A random subnet has expired.
    ///
    /// This function selects a new subnet to join, or extends the expiry if there are no more
    /// available subnets to choose from.
    fn handle_random_subnet_expiry(&mut self, subnet_id: SubnetId) {
        let subnet_count = self.beacon_chain.spec.attestation_subnet_count;
        if self.random_subnets.len() == (subnet_count - 1) as usize {
            // We are at capacity, simply increase the timeout of the current subnet
            self.random_subnets.insert(subnet_id);
            return;
        }

        // we are not at capacity, unsubscribe from the current subnet, remove the ENR bitfield bit and choose a new random one
        // from the available subnets
        // Note: This should not occur during a required subnet as subscriptions update the timeout
        // to last as long as they are needed.

        debug!(self.log, "Unsubscribing from random subnet"; "subnet_id" => *subnet_id);
        self.events
            .push_back(AttServiceMessage::Unsubscribe(subnet_id));
        self.events
            .push_back(AttServiceMessage::EnrRemove(subnet_id));
        self.subscribe_to_random_subnets(1);
    }

    /// A known validator has not sent a subscription in a while. They are considered offline and the
    /// beacon node no longer needs to be subscribed to the allocated random subnets.
    ///
    /// We don't keep track of a specific validator to random subnet, rather the ratio of active
    /// validators to random subnets. So when a validator goes offline, we can simply remove the
    /// allocated amount of random subnets.
    fn handle_known_validator_expiry(&mut self) -> Result<(), ()> {
        let spec = &self.beacon_chain.spec;
        let subnet_count = spec.attestation_subnet_count;
        let random_subnets_per_validator = spec.random_subnets_per_validator;
        if self.known_validators.len() as u64 * random_subnets_per_validator >= subnet_count {
            // have too many validators, ignore
            return Ok(());
        }

        let subscribed_subnets = self.random_subnets.keys().cloned().collect::<Vec<_>>();
        let to_remove_subnets = subscribed_subnets.choose_multiple(
            &mut rand::thread_rng(),
            random_subnets_per_validator as usize,
        );
        let current_slot = self.beacon_chain.slot_clock.now().ok_or_else(|| {
            warn!(self.log, "Could not get the current slot");
        })?;

        for subnet_id in to_remove_subnets {
            // If a subscription is queued for two slots in the future, it's associated unsubscription
            // will unsubscribe from the expired subnet.
            // If there is no unsubscription for this subnet,slot it is safe to add one, without
            // unsubscribing early from a required subnet
            let subnet = ExactSubnet {
                subnet_id: *subnet_id,
                slot: current_slot + 2,
            };
            if self.subscriptions.get(&subnet).is_none() {
                // set an unsubscribe event
                let duration_to_next_slot = self
                    .beacon_chain
                    .slot_clock
                    .duration_to_next_slot()
                    .ok_or_else(|| {
                        warn!(self.log, "Unable to determine duration to next slot");
                    })?;
                let slot_duration = self.beacon_chain.slot_clock.slot_duration();
                // Set the unsubscription timeout
                let unsubscription_duration = duration_to_next_slot + slot_duration * 2;
                self.unsubscriptions
                    .insert_at(subnet, unsubscription_duration);
            }
            // as the long lasting subnet subscription is being removed, remove the subnet_id from
            // the ENR bitfield
            self.events
                .push_back(AttServiceMessage::EnrRemove(*subnet_id));
            self.random_subnets.remove(subnet_id);
        }
        Ok(())
    }
}

impl<T: BeaconChainTypes> Stream for AttestationService<T> {
    type Item = AttServiceMessage;

    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        // process any peer discovery events
        match self.discover_peers.poll_next_unpin(cx) {
            Poll::Ready(Some(Ok(exact_subnet))) => self.handle_discover_peers(exact_subnet),
            Poll::Ready(Some(Err(e))) => {
                error!(self.log, "Failed to check for peer discovery requests"; "error"=> format ! ("{}", e));
            }
            Poll::Ready(None) | Poll::Pending => {}
        }

        // process any subscription events
        match self.subscriptions.poll_next_unpin(cx) {
            Poll::Ready(Some(Ok(exact_subnet))) => self.handle_subscriptions(exact_subnet),
            Poll::Ready(Some(Err(e))) => {
                error!(self.log, "Failed to check for subnet subscription times"; "error"=> format!("{}", e));
            }
            Poll::Ready(None) | Poll::Pending => {}
        }

        // process any un-subscription events
        match self.unsubscriptions.poll_next_unpin(cx) {
            Poll::Ready(Some(Ok(exact_subnet))) => self.handle_unsubscriptions(exact_subnet),
            Poll::Ready(Some(Err(e))) => {
                error!(self.log, "Failed to check for subnet unsubscription times"; "error"=> format!("{}", e));
            }
            Poll::Ready(None) | Poll::Pending => {}
        }

        // process any random subnet expiries
        match self.random_subnets.poll_next_unpin(cx) {
            Poll::Ready(Some(Ok(subnet))) => self.handle_random_subnet_expiry(subnet),
            Poll::Ready(Some(Err(e))) => {
                error!(self.log, "Failed to check for random subnet cycles"; "error"=> format!("{}", e));
            }
            Poll::Ready(None) | Poll::Pending => {}
        }

        // process any known validator expiries
        match self.known_validators.poll_next_unpin(cx) {
            Poll::Ready(Some(Ok(_validator_index))) => {
                let _ = self.handle_known_validator_expiry();
            }
            Poll::Ready(Some(Err(e))) => {
                error!(self.log, "Failed to check for random subnet cycles"; "error"=> format!("{}", e));
            }
            Poll::Ready(None) | Poll::Pending => {}
        }
        // poll to remove entries on expiration, no need to act on expiration events
        if let Poll::Ready(Some(Err(e))) = self.aggregate_validators_on_subnet.poll_next_unpin(cx) {
            error!(self.log, "Failed to check for aggregate validator on subnet expirations"; "error"=> format!("{}", e));
        }

        // process any generated events
        if let Some(event) = self.events.pop_front() {
            return Poll::Ready(Some(event));
        }

        Poll::Pending
    }
}