lighthouse/beacon_node/network/src/subnet_service/attestation_subnets.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 super::SubnetServiceMessage;
#[cfg(any(test, feature = "deterministic_long_lived_attnets"))]
use std::collections::HashSet;
use std::collections::{HashMap, VecDeque};
use std::pin::Pin;
use std::sync::Arc;
use std::task::{Context, Poll};
use std::time::Duration;

use beacon_chain::{BeaconChain, BeaconChainTypes};
use delay_map::{HashMapDelay, HashSetDelay};
use futures::prelude::*;
use lighthouse_network::{NetworkConfig, Subnet, SubnetDiscovery};
#[cfg(not(feature = "deterministic_long_lived_attnets"))]
use rand::seq::SliceRandom;
use slog::{debug, error, o, trace, warn};
use slot_clock::SlotClock;
use types::{Attestation, EthSpec, Slot, SubnetId, ValidatorSubscription};

use crate::metrics;

/// 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.
/// Subnet discovery query takes at most 30 secs, 2 slots take 24s.
pub(crate) const MIN_PEER_DISCOVERY_SLOT_LOOK_AHEAD: u64 = 2;
/// 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.
#[cfg(not(feature = "deterministic_long_lived_attnets"))]
const LAST_SEEN_VALIDATOR_TIMEOUT_SLOTS: u32 = 150;
/// The fraction of a slot that we subscribe to a subnet before the required slot.
///
/// Currently a whole slot ahead.
const ADVANCE_SUBSCRIBE_SLOT_FRACTION: u32 = 1;

#[derive(Debug, Clone, Copy, Hash, PartialEq, Eq)]
pub(crate) enum SubscriptionKind {
    /// Long lived subscriptions.
    ///
    /// These have a longer duration and are advertised in our ENR.
    LongLived,
    /// Short lived subscriptions.
    ///
    /// Subscribing to these subnets has a short duration and we don't advertise it in our ENR.
    ShortLived,
}

/// A particular subnet at a given slot.
#[derive(PartialEq, Eq, Hash, Clone, Debug, Copy)]
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<SubnetServiceMessage>,

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

    /// Subnets we are currently subscribed to as short lived subscriptions.
    ///
    /// Once they expire, we unsubscribe from these.
    short_lived_subscriptions: HashMapDelay<SubnetId, Slot>,

    /// Subnets we are currently subscribed to as long lived subscriptions.
    ///
    /// We advertise these in our ENR. When these expire, the subnet is removed from our ENR.
    #[cfg(feature = "deterministic_long_lived_attnets")]
    long_lived_subscriptions: HashSet<SubnetId>,
    #[cfg(not(feature = "deterministic_long_lived_attnets"))]
    long_lived_subscriptions: HashMapDelay<SubnetId, Slot>,

    /// Short lived subscriptions that need to be done in the future.
    scheduled_short_lived_subscriptions: HashSetDelay<ExactSubnet>,

    /// A collection timeouts to track the existence of aggregate validator subscriptions at an
    /// `ExactSubnet`.
    aggregate_validators_on_subnet: Option<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.
    #[cfg(not(feature = "deterministic_long_lived_attnets"))]
    known_validators: HashSetDelay<u64>,

    /// The waker for the current thread.
    waker: Option<std::task::Waker>,

    /// The discovery mechanism of lighthouse is disabled.
    discovery_disabled: bool,

    /// We are always subscribed to all subnets.
    subscribe_all_subnets: bool,

    /// For how many slots we subscribe to long lived subnets.
    #[cfg(not(feature = "deterministic_long_lived_attnets"))]
    long_lived_subnet_subscription_slots: u64,

    /// Our Discv5 node_id.
    #[cfg(feature = "deterministic_long_lived_attnets")]
    node_id: ethereum_types::U256,

    /// Future used to manage subscribing and unsubscribing from long lived subnets.
    #[cfg(feature = "deterministic_long_lived_attnets")]
    next_long_lived_subscription_event: Pin<Box<tokio::time::Sleep>>,

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

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

    #[cfg(not(feature = "deterministic_long_lived_attnets"))]
    pub fn new(
        beacon_chain: Arc<BeaconChain<T>>,
        config: &NetworkConfig,
        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 long_lived_subnet_subscription_slots = spec
            .epochs_per_random_subnet_subscription
            .saturating_mul(T::EthSpec::slots_per_epoch());
        let long_lived_subscription_duration = Duration::from_millis(
            slot_duration.as_millis() as u64 * long_lived_subnet_subscription_slots,
        );

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

        let track_validators = !config.import_all_attestations;
        let aggregate_validators_on_subnet =
            track_validators.then(|| HashSetDelay::new(slot_duration));
        AttestationService {
            events: VecDeque::with_capacity(10),
            beacon_chain,
            short_lived_subscriptions: HashMapDelay::new(slot_duration),
            long_lived_subscriptions: HashMapDelay::new(long_lived_subscription_duration),
            scheduled_short_lived_subscriptions: HashSetDelay::default(),
            aggregate_validators_on_subnet,
            known_validators: HashSetDelay::new(last_seen_val_timeout),
            waker: None,
            discovery_disabled: config.disable_discovery,
            subscribe_all_subnets: config.subscribe_all_subnets,
            long_lived_subnet_subscription_slots,
            log,
        }
    }

    #[cfg(feature = "deterministic_long_lived_attnets")]
    pub fn new(
        beacon_chain: Arc<BeaconChain<T>>,
        node_id: ethereum_types::U256,
        config: &NetworkConfig,
        log: &slog::Logger,
    ) -> Self {
        let log = log.new(o!("service" => "attestation_service"));

        // Calculate the random subnet duration from the spec constants.
        let slot_duration = beacon_chain.slot_clock.slot_duration();

        slog::info!(log, "Deterministic long lived subnets enabled"; "subnets_per_node" => beacon_chain.spec.subnets_per_node);

        let track_validators = !config.import_all_attestations;
        let aggregate_validators_on_subnet =
            track_validators.then(|| HashSetDelay::new(slot_duration));
        let mut service = AttestationService {
            events: VecDeque::with_capacity(10),
            beacon_chain,
            short_lived_subscriptions: HashMapDelay::new(slot_duration),
            long_lived_subscriptions: HashSet::default(),
            scheduled_short_lived_subscriptions: HashSetDelay::default(),
            aggregate_validators_on_subnet,
            waker: None,
            discovery_disabled: config.disable_discovery,
            subscribe_all_subnets: config.subscribe_all_subnets,
            node_id,
            next_long_lived_subscription_event: {
                // Set a dummy sleep. Calculating the current subnet subscriptions will update this
                // value with a smarter timing
                Box::pin(tokio::time::sleep(Duration::from_secs(1)))
            },
            log,
        };
        service.recompute_long_lived_subnets();
        service
    }

    /// Return count of all currently subscribed subnets (long-lived **and** short-lived).
    #[cfg(test)]
    pub fn subscription_count(&self) -> usize {
        if self.subscribe_all_subnets {
            self.beacon_chain.spec.attestation_subnet_count as usize
        } else {
            #[cfg(feature = "deterministic_long_lived_attnets")]
            let count = self
                .short_lived_subscriptions
                .keys()
                .chain(self.long_lived_subscriptions.iter())
                .collect::<HashSet<_>>()
                .len();
            #[cfg(not(feature = "deterministic_long_lived_attnets"))]
            let count = self
                .short_lived_subscriptions
                .keys()
                .chain(self.long_lived_subscriptions.keys())
                .collect::<HashSet<_>>()
                .len();
            count
        }
    }

    /// Returns whether we are subscribed to a subnet for testing purposes.
    #[cfg(test)]
    pub(crate) fn is_subscribed(
        &self,
        subnet_id: &SubnetId,
        subscription_kind: SubscriptionKind,
    ) -> bool {
        match subscription_kind {
            #[cfg(feature = "deterministic_long_lived_attnets")]
            SubscriptionKind::LongLived => self.long_lived_subscriptions.contains(subnet_id),
            #[cfg(not(feature = "deterministic_long_lived_attnets"))]
            SubscriptionKind::LongLived => self.long_lived_subscriptions.contains_key(subnet_id),
            SubscriptionKind::ShortLived => self.short_lived_subscriptions.contains_key(subnet_id),
        }
    }

    /// 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> {
        // Maps each subnet_id subscription to it's highest slot
        let mut subnets_to_discover: HashMap<SubnetId, Slot> = HashMap::new();
        for subscription in subscriptions {
            metrics::inc_counter(&metrics::SUBNET_SUBSCRIPTION_REQUESTS);

            // Registers the validator with the attestation service.
            // This will subscribe to long-lived random subnets if required.
            trace!(self.log,
                "Validator subscription";
                "subscription" => ?subscription,
            );
            #[cfg(not(feature = "deterministic_long_lived_attnets"))]
            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" => ?e,
                        "validator_index" => subscription.validator_index
                    );
                    continue;
                }
            };
            // Ensure each subnet_id inserted into the map has the highest slot as it's value.
            // Higher slot corresponds to higher min_ttl in the `SubnetDiscovery` entry.
            if let Some(slot) = subnets_to_discover.get(&subnet_id) {
                if subscription.slot > *slot {
                    subnets_to_discover.insert(subnet_id, subscription.slot);
                }
            } else if !self.discovery_disabled {
                subnets_to_discover.insert(subnet_id, subscription.slot);
            }

            let exact_subnet = ExactSubnet {
                subnet_id,
                slot: subscription.slot,
            };

            // 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.

            if subscription.is_aggregator {
                metrics::inc_counter(&metrics::SUBNET_SUBSCRIPTION_AGGREGATOR_REQUESTS);
                if let Err(e) = self.subscribe_to_subnet(exact_subnet) {
                    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" => ?exact_subnet,
                        "validator_index" => subscription.validator_index
                    );
                }
            }
        }

        // If the discovery mechanism isn't disabled, attempt to set up a peer discovery for the
        // required subnets.
        if !self.discovery_disabled {
            if let Err(e) = self.discover_peers_request(
                subnets_to_discover
                    .into_iter()
                    .map(|(subnet_id, slot)| ExactSubnet { subnet_id, slot }),
            ) {
                warn!(self.log, "Discovery lookup request error"; "error" => e);
            };
        }

        Ok(())
    }

    #[cfg(feature = "deterministic_long_lived_attnets")]
    fn recompute_long_lived_subnets(&mut self) {
        // Ensure the next computation is scheduled even if assigning subnets fails.
        let next_subscription_event = self
            .recompute_long_lived_subnets_inner()
            .unwrap_or_else(|_| self.beacon_chain.slot_clock.slot_duration());

        debug!(self.log, "Recomputing deterministic long lived attnets");
        self.next_long_lived_subscription_event =
            Box::pin(tokio::time::sleep(next_subscription_event));

        if let Some(waker) = self.waker.as_ref() {
            waker.wake_by_ref();
        }
    }

    /// Gets the long lived subnets the node should be subscribed to during the current epoch and
    /// the remaining duration for which they remain valid.
    #[cfg(feature = "deterministic_long_lived_attnets")]
    fn recompute_long_lived_subnets_inner(&mut self) -> Result<Duration, ()> {
        let current_epoch = self.beacon_chain.epoch().map_err(
            |e| error!(self.log, "Failed to get the current epoch from clock"; "err" => ?e),
        )?;

        let (subnets, next_subscription_epoch) = SubnetId::compute_subnets_for_epoch::<T::EthSpec>(
            self.node_id,
            current_epoch,
            &self.beacon_chain.spec,
        )
        .map_err(|e| error!(self.log, "Could not compute subnets for current epoch"; "err" => e))?;

        let next_subscription_slot =
            next_subscription_epoch.start_slot(T::EthSpec::slots_per_epoch());
        let next_subscription_event = self
            .beacon_chain
            .slot_clock
            .duration_to_slot(next_subscription_slot)
            .ok_or_else(|| {
                error!(
                    self.log,
                    "Failed to compute duration to next to long lived subscription event"
                )
            })?;

        self.update_long_lived_subnets(subnets.collect());

        Ok(next_subscription_event)
    }

    #[cfg(all(test, feature = "deterministic_long_lived_attnets"))]
    pub fn update_long_lived_subnets_testing(&mut self, subnets: HashSet<SubnetId>) {
        self.update_long_lived_subnets(subnets)
    }

    /// Updates the long lived subnets.
    ///
    /// New subnets are registered as subscribed, removed subnets as unsubscribed and the Enr
    /// updated accordingly.
    #[cfg(feature = "deterministic_long_lived_attnets")]
    fn update_long_lived_subnets(&mut self, mut subnets: HashSet<SubnetId>) {
        for subnet in &subnets {
            // Add the events for those subnets that are new as long lived subscriptions.
            if !self.long_lived_subscriptions.contains(subnet) {
                // Check if this subnet is new and send the subscription event if needed.
                if !self.short_lived_subscriptions.contains_key(subnet) {
                    debug!(self.log, "Subscribing to subnet";
                        "subnet" => ?subnet,
                        "subscription_kind" => ?SubscriptionKind::LongLived,
                    );
                    self.queue_event(SubnetServiceMessage::Subscribe(Subnet::Attestation(
                        *subnet,
                    )));
                }
                self.queue_event(SubnetServiceMessage::EnrAdd(Subnet::Attestation(*subnet)));
                if !self.discovery_disabled {
                    self.queue_event(SubnetServiceMessage::DiscoverPeers(vec![SubnetDiscovery {
                        subnet: Subnet::Attestation(*subnet),
                        min_ttl: None,
                    }]))
                }
            }
        }

        // Check for subnets that are being removed
        std::mem::swap(&mut self.long_lived_subscriptions, &mut subnets);
        for subnet in subnets {
            if !self.long_lived_subscriptions.contains(&subnet) {
                if !self.short_lived_subscriptions.contains_key(&subnet) {
                    debug!(self.log, "Unsubscribing from subnet"; "subnet" => ?subnet, "subscription_kind" => ?SubscriptionKind::LongLived);
                    self.queue_event(SubnetServiceMessage::Unsubscribe(Subnet::Attestation(
                        subnet,
                    )));
                }

                self.queue_event(SubnetServiceMessage::EnrRemove(Subnet::Attestation(subnet)));
            }
        }
    }

    /// Overwrites the long lived subscriptions for testing.
    #[cfg(all(test, feature = "deterministic_long_lived_attnets"))]
    pub fn set_long_lived_subscriptions(&mut self, subnets: HashSet<SubnetId>) {
        self.long_lived_subscriptions = subnets
    }

    /// 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(
        &self,
        subnet: SubnetId,
        attestation: &Attestation<T::EthSpec>,
    ) -> bool {
        self.aggregate_validators_on_subnet
            .as_ref()
            .map(|tracked_vals| {
                tracked_vals.contains_key(&ExactSubnet {
                    subnet_id: subnet,
                    slot: attestation.data.slot,
                })
            })
            .unwrap_or(true)
    }

    /* Internal private functions */

    /// Adds an event to the event queue and notifies that this service is ready to be polled
    /// again.
    fn queue_event(&mut self, ev: SubnetServiceMessage) {
        self.events.push_back(ev);
        if let Some(waker) = &self.waker {
            waker.wake_by_ref()
        }
    }
    /// Checks if there are currently queued discovery requests and the time required to make the
    /// request.
    ///
    /// If there is sufficient time, queues a peer discovery request for all the required subnets.
    fn discover_peers_request(
        &mut self,
        exact_subnets: impl Iterator<Item = ExactSubnet>,
    ) -> Result<(), &'static str> {
        let current_slot = self
            .beacon_chain
            .slot_clock
            .now()
            .ok_or("Could not get the current slot")?;

        let discovery_subnets: Vec<SubnetDiscovery> = exact_subnets
            .filter_map(|exact_subnet| {
                // Check if there is enough time to perform a discovery lookup.
                if exact_subnet.slot
                    >= current_slot.saturating_add(MIN_PEER_DISCOVERY_SLOT_LOOK_AHEAD)
                {
                    // Send out an event to start looking for peers.
                    // Require the peer for an additional slot to ensure we keep the peer for the
                    // duration of the subscription.
                    let min_ttl = self
                        .beacon_chain
                        .slot_clock
                        .duration_to_slot(exact_subnet.slot + 1)
                        .map(|duration| std::time::Instant::now() + duration);
                    Some(SubnetDiscovery {
                        subnet: Subnet::Attestation(exact_subnet.subnet_id),
                        min_ttl,
                    })
                } else {
                    // We may want to check the global PeerInfo to see estimated timeouts for each
                    // peer before they can be removed.
                    warn!(self.log,
                        "Not enough time for a discovery search";
                        "subnet_id" => ?exact_subnet
                    );
                    None
                }
            })
            .collect();

        if !discovery_subnets.is_empty() {
            self.queue_event(SubnetServiceMessage::DiscoverPeers(discovery_subnets));
        }
        Ok(())
    }

    // Subscribes to the subnet if it should be done immediately, or schedules it if required.
    fn subscribe_to_subnet(
        &mut self,
        ExactSubnet { subnet_id, slot }: ExactSubnet,
    ) -> Result<(), &'static str> {
        let slot_duration = self.beacon_chain.slot_clock.slot_duration();

        // Calculate how long before we need to subscribe to the subnet.
        let time_to_subscription_start = {
            // The short time we schedule the subscription before it's actually required. This
            // ensures we are subscribed on time, and allows consecutive subscriptions to the same
            // subnet to overlap, reducing subnet churn.
            let advance_subscription_duration = slot_duration / ADVANCE_SUBSCRIBE_SLOT_FRACTION;
            // The time to the required slot.
            let time_to_subscription_slot = self
                .beacon_chain
                .slot_clock
                .duration_to_slot(slot)
                .unwrap_or_default(); // If this is a past slot we will just get a 0 duration.
            time_to_subscription_slot.saturating_sub(advance_subscription_duration)
        };

        if let Some(tracked_vals) = self.aggregate_validators_on_subnet.as_mut() {
            tracked_vals.insert(ExactSubnet { subnet_id, slot });
        }

        // If the subscription should be done in the future, schedule it. Otherwise subscribe
        // immediately.
        if time_to_subscription_start.is_zero() {
            // This is a current or past slot, we subscribe immediately.
            self.subscribe_to_subnet_immediately(
                subnet_id,
                #[cfg(not(feature = "deterministic_long_lived_attnets"))]
                SubscriptionKind::ShortLived,
                slot + 1,
            )?;
        } else {
            // This is a future slot, schedule subscribing.
            trace!(self.log, "Scheduling subnet subscription"; "subnet" => ?subnet_id, "time_to_subscription_start" => ?time_to_subscription_start);
            self.scheduled_short_lived_subscriptions
                .insert_at(ExactSubnet { subnet_id, slot }, time_to_subscription_start);
        }

        Ok(())
    }

    /// Updates the `known_validators` mapping and subscribes to long lived subnets if required.
    #[cfg(not(feature = "deterministic_long_lived_attnets"))]
    fn add_known_validator(&mut self, validator_index: u64) {
        let previously_known = self.known_validators.contains_key(&validator_index);
        // Add the new validator or update the current timeout for a known validator.
        self.known_validators.insert(validator_index);
        if !previously_known {
            // New validator has subscribed.
            // Subscribe to random topics and update the ENR if needed.
            self.subscribe_to_random_subnets();
        }
    }

    /// Subscribe to long-lived random subnets and update the local ENR bitfield.
    /// The number of subnets to subscribe depends on the number of active validators and number of
    /// current subscriptions.
    #[cfg(not(feature = "deterministic_long_lived_attnets"))]
    fn subscribe_to_random_subnets(&mut self) {
        if self.subscribe_all_subnets {
            // This case is not handled by this service.
            return;
        }

        let max_subnets = self.beacon_chain.spec.attestation_subnet_count;
        // Calculate how many subnets we need,
        let required_long_lived_subnets = {
            let subnets_for_validators = self
                .known_validators
                .len()
                .saturating_mul(self.beacon_chain.spec.random_subnets_per_validator as usize);
            subnets_for_validators // How many subnets we need
                .min(max_subnets as usize) // Capped by the max
                .saturating_sub(self.long_lived_subscriptions.len()) // Minus those we have
        };

        if required_long_lived_subnets == 0 {
            // Nothing to do.
            return;
        }

        // Build a list of the subnets that we are not currently advertising.
        let available_subnets = (0..max_subnets)
            .map(SubnetId::new)
            .filter(|subnet_id| !self.long_lived_subscriptions.contains_key(subnet_id))
            .collect::<Vec<_>>();

        let subnets_to_subscribe: Vec<_> = available_subnets
            .choose_multiple(&mut rand::thread_rng(), required_long_lived_subnets)
            .cloned()
            .collect();

        // Calculate in which slot does this subscription end.
        let end_slot = match self.beacon_chain.slot_clock.now() {
            Some(slot) => slot + self.long_lived_subnet_subscription_slots,
            None => {
                return debug!(
                    self.log,
                    "Failed to calculate end slot of long lived subnet subscriptions."
                )
            }
        };

        for subnet_id in &subnets_to_subscribe {
            if let Err(e) = self.subscribe_to_subnet_immediately(
                *subnet_id,
                SubscriptionKind::LongLived,
                end_slot,
            ) {
                debug!(self.log, "Failed to subscribe to long lived subnet"; "subnet" => ?subnet_id, "err" => e);
            }
        }
    }

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

    /// Registers a subnet as subscribed.
    ///
    /// Checks that the time in which the subscription would end is not in the past. If we are
    /// already subscribed, extends the timeout if necessary. If this is a new subscription, we send
    /// out the appropriate events.
    ///
    /// On determinist long lived subnets, this is only used for short lived subscriptions.
    fn subscribe_to_subnet_immediately(
        &mut self,
        subnet_id: SubnetId,
        #[cfg(not(feature = "deterministic_long_lived_attnets"))]
        subscription_kind: SubscriptionKind,
        end_slot: Slot,
    ) -> Result<(), &'static str> {
        if self.subscribe_all_subnets {
            // Case not handled by this service.
            return Ok(());
        }

        let time_to_subscription_end = self
            .beacon_chain
            .slot_clock
            .duration_to_slot(end_slot)
            .unwrap_or_default();

        // First check this is worth doing.
        if time_to_subscription_end.is_zero() {
            return Err("Time when subscription would end has already passed.");
        }

        #[cfg(feature = "deterministic_long_lived_attnets")]
        let subscription_kind = SubscriptionKind::ShortLived;

        // We need to check and add a subscription for the right kind, regardless of the presence
        // of the subnet as a subscription of the other kind. This is mainly since long lived
        // subscriptions can be removed at any time when a validator goes offline.
        #[cfg(not(feature = "deterministic_long_lived_attnets"))]
        let (subscriptions, already_subscribed_as_other_kind) = match subscription_kind {
            SubscriptionKind::ShortLived => (
                &mut self.short_lived_subscriptions,
                self.long_lived_subscriptions.contains_key(&subnet_id),
            ),
            SubscriptionKind::LongLived => (
                &mut self.long_lived_subscriptions,
                self.short_lived_subscriptions.contains_key(&subnet_id),
            ),
        };

        #[cfg(feature = "deterministic_long_lived_attnets")]
        let (subscriptions, already_subscribed_as_other_kind) = (
            &mut self.short_lived_subscriptions,
            self.long_lived_subscriptions.contains(&subnet_id),
        );

        match subscriptions.get(&subnet_id) {
            Some(current_end_slot) => {
                // We are already subscribed. Check if we need to extend the subscription.
                if &end_slot > current_end_slot {
                    trace!(self.log, "Extending subscription to subnet";
                        "subnet" => ?subnet_id,
                        "prev_end_slot" => current_end_slot,
                        "new_end_slot" => end_slot,
                        "subscription_kind" => ?subscription_kind,
                    );
                    subscriptions.insert_at(subnet_id, end_slot, time_to_subscription_end);
                }
            }
            None => {
                // This is a new subscription. Add with the corresponding timeout and send the
                // notification.
                subscriptions.insert_at(subnet_id, end_slot, time_to_subscription_end);

                // Inform of the subscription.
                if !already_subscribed_as_other_kind {
                    debug!(self.log, "Subscribing to subnet";
                        "subnet" => ?subnet_id,
                        "end_slot" => end_slot,
                        "subscription_kind" => ?subscription_kind,
                    );
                    self.queue_event(SubnetServiceMessage::Subscribe(Subnet::Attestation(
                        subnet_id,
                    )));
                }

                // If this is a new long lived subscription, send out the appropriate events.
                #[cfg(not(feature = "deterministic_long_lived_attnets"))]
                if SubscriptionKind::LongLived == subscription_kind {
                    let subnet = Subnet::Attestation(subnet_id);
                    // Advertise this subnet in our ENR.
                    self.long_lived_subscriptions.insert_at(
                        subnet_id,
                        end_slot,
                        time_to_subscription_end,
                    );
                    self.queue_event(SubnetServiceMessage::EnrAdd(subnet));

                    if !self.discovery_disabled {
                        self.queue_event(SubnetServiceMessage::DiscoverPeers(vec![
                            SubnetDiscovery {
                                subnet,
                                min_ttl: None,
                            },
                        ]))
                    }
                }
            }
        }

        Ok(())
    }

    /// 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.
    #[cfg(not(feature = "deterministic_long_lived_attnets"))]
    fn handle_random_subnet_expiry(&mut self, subnet_id: SubnetId) {
        self.handle_removed_subnet(subnet_id, SubscriptionKind::LongLived);

        // Remove the ENR bitfield bit and choose a new random on from the available subnets
        // Subscribe to a new random subnet.
        self.subscribe_to_random_subnets();
    }

    // Unsubscribes from a subnet that was removed if it does not continue to exist as a
    // subscription of the other kind. For long lived subscriptions, it also removes the
    // advertisement from our ENR.
    fn handle_removed_subnet(&mut self, subnet_id: SubnetId, subscription_kind: SubscriptionKind) {
        let exists_in_other_subscriptions = match subscription_kind {
            SubscriptionKind::LongLived => self.short_lived_subscriptions.contains_key(&subnet_id),
            #[cfg(feature = "deterministic_long_lived_attnets")]
            SubscriptionKind::ShortLived => self.long_lived_subscriptions.contains(&subnet_id),
            #[cfg(not(feature = "deterministic_long_lived_attnets"))]
            SubscriptionKind::ShortLived => self.long_lived_subscriptions.contains_key(&subnet_id),
        };

        if !exists_in_other_subscriptions {
            // Subscription no longer exists as short lived or long lived.
            debug!(self.log, "Unsubscribing from subnet"; "subnet" => ?subnet_id, "subscription_kind" => ?subscription_kind);
            self.queue_event(SubnetServiceMessage::Unsubscribe(Subnet::Attestation(
                subnet_id,
            )));
        }

        if subscription_kind == SubscriptionKind::LongLived {
            // Remove from our ENR even if we remain subscribed in other way.
            self.queue_event(SubnetServiceMessage::EnrRemove(Subnet::Attestation(
                subnet_id,
            )));
        }
    }

    /// 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.
    #[cfg(not(feature = "deterministic_long_lived_attnets"))]
    fn handle_known_validator_expiry(&mut self) {
        // Calculate how many subnets should we remove.
        let extra_subnet_count = {
            let max_subnets = self.beacon_chain.spec.attestation_subnet_count;
            let subnets_for_validators = self
                .known_validators
                .len()
                .saturating_mul(self.beacon_chain.spec.random_subnets_per_validator as usize)
                .min(max_subnets as usize);

            self.long_lived_subscriptions
                .len()
                .saturating_sub(subnets_for_validators)
        };

        if extra_subnet_count == 0 {
            // Nothing to do
            return;
        }

        let advertised_subnets = self
            .long_lived_subscriptions
            .keys()
            .cloned()
            .collect::<Vec<_>>();
        let to_remove_subnets = advertised_subnets
            .choose_multiple(&mut rand::thread_rng(), extra_subnet_count)
            .cloned();

        for subnet_id in to_remove_subnets {
            self.long_lived_subscriptions.remove(&subnet_id);
            self.handle_removed_subnet(subnet_id, SubscriptionKind::LongLived);
        }
    }
}

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

    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        // Update the waker if needed.
        if let Some(waker) = &self.waker {
            if waker.will_wake(cx.waker()) {
                self.waker = Some(cx.waker().clone());
            }
        } else {
            self.waker = Some(cx.waker().clone());
        }

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

        // Process first any known validator expiries, since these affect how many long lived
        // subnets we need.
        #[cfg(not(feature = "deterministic_long_lived_attnets"))]
        match self.known_validators.poll_next_unpin(cx) {
            Poll::Ready(Some(Ok(_validator_index))) => {
                self.handle_known_validator_expiry();
            }
            Poll::Ready(Some(Err(e))) => {
                error!(self.log, "Failed to check for random subnet cycles"; "error"=> e);
            }
            Poll::Ready(None) | Poll::Pending => {}
        }

        #[cfg(feature = "deterministic_long_lived_attnets")]
        match self.next_long_lived_subscription_event.as_mut().poll(cx) {
            Poll::Ready(_) => self.recompute_long_lived_subnets(),
            Poll::Pending => {}
        }

        // Process scheduled subscriptions that might be ready, since those can extend a soon to
        // expire subscription.
        match self.scheduled_short_lived_subscriptions.poll_next_unpin(cx) {
            Poll::Ready(Some(Ok(ExactSubnet { subnet_id, slot }))) => {
                if let Err(e) = self.subscribe_to_subnet_immediately(
                    subnet_id,
                    #[cfg(not(feature = "deterministic_long_lived_attnets"))]
                    SubscriptionKind::ShortLived,
                    slot + 1,
                ) {
                    debug!(self.log, "Failed to subscribe to short lived subnet"; "subnet" => ?subnet_id, "err" => e);
                }
            }
            Poll::Ready(Some(Err(e))) => {
                error!(self.log, "Failed to check for scheduled subnet subscriptions"; "error"=> e);
            }
            Poll::Ready(None) | Poll::Pending => {}
        }

        // Finally process any expired subscriptions.
        match self.short_lived_subscriptions.poll_next_unpin(cx) {
            Poll::Ready(Some(Ok((subnet_id, _end_slot)))) => {
                self.handle_removed_subnet(subnet_id, SubscriptionKind::ShortLived);
            }
            Poll::Ready(Some(Err(e))) => {
                error!(self.log, "Failed to check for subnet unsubscription times"; "error"=> e);
            }
            Poll::Ready(None) | Poll::Pending => {}
        }

        // Process any random subnet expiries.
        #[cfg(not(feature = "deterministic_long_lived_attnets"))]
        match self.long_lived_subscriptions.poll_next_unpin(cx) {
            Poll::Ready(Some(Ok((subnet_id, _end_slot)))) => {
                self.handle_random_subnet_expiry(subnet_id)
            }
            Poll::Ready(Some(Err(e))) => {
                error!(self.log, "Failed to check for random subnet cycles"; "error"=> e);
            }
            Poll::Ready(None) | Poll::Pending => {}
        }

        // Poll to remove entries on expiration, no need to act on expiration events.
        if let Some(tracked_vals) = self.aggregate_validators_on_subnet.as_mut() {
            if let Poll::Ready(Some(Err(e))) = tracked_vals.poll_next_unpin(cx) {
                error!(self.log, "Failed to check for aggregate validator on subnet expirations"; "error"=> e);
            }
        }

        Poll::Pending
    }
}