pezkuwi-subxt/polkadot/node/network/statement-distribution/src/lib.rs

// Copyright 2021 Parity Technologies (UK) Ltd.
// This file is part of Polkadot.

// Polkadot is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// Polkadot is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.

//! The Statement Distribution Subsystem.
//!
//! This is responsible for distributing signed statements about candidate
//! validity among validators.

#![deny(unused_crate_dependencies)]
#![warn(missing_docs)]

use error::{log_error, FatalResult, JfyiErrorResult};
use parity_scale_codec::Encode;

use polkadot_node_network_protocol::{
	peer_set::{IsAuthority, PeerSet},
	request_response::{v1 as request_v1, IncomingRequestReceiver},
	v1::{self as protocol_v1, StatementMetadata},
	IfDisconnected, PeerId, UnifiedReputationChange as Rep, View,
};
use polkadot_node_primitives::{SignedFullStatement, Statement, UncheckedSignedFullStatement};
use polkadot_node_subsystem_util::{self as util, rand, MIN_GOSSIP_PEERS};

use polkadot_primitives::v2::{
	AuthorityDiscoveryId, CandidateHash, CommittedCandidateReceipt, CompactStatement, Hash,
	SignedStatement, SigningContext, UncheckedSignedStatement, ValidatorId, ValidatorIndex,
	ValidatorSignature,
};
use polkadot_subsystem::{
	jaeger,
	messages::{
		AllMessages, CandidateBackingMessage, NetworkBridgeEvent, NetworkBridgeMessage,
		StatementDistributionMessage,
	},
	overseer, ActiveLeavesUpdate, FromOverseer, OverseerSignal, PerLeafSpan, SpawnedSubsystem,
	SubsystemContext, SubsystemError,
};

use futures::{
	channel::{mpsc, oneshot},
	future::RemoteHandle,
	prelude::*,
};
use indexmap::{map::Entry as IEntry, IndexMap};
use sp_keystore::SyncCryptoStorePtr;
use util::runtime::RuntimeInfo;

use std::collections::{hash_map::Entry, HashMap, HashSet, VecDeque};

use fatality::Nested;

mod error;
pub use error::{Error, FatalError, JfyiError, Result};

/// Background task logic for requesting of large statements.
mod requester;
use requester::{fetch, RequesterMessage};

/// Background task logic for responding for large statements.
mod responder;
use responder::{respond, ResponderMessage};

/// Metrics for the statement distribution
pub(crate) mod metrics;
use metrics::Metrics;

#[cfg(test)]
mod tests;

const COST_UNEXPECTED_STATEMENT: Rep = Rep::CostMinor("Unexpected Statement");
const COST_UNEXPECTED_STATEMENT_MISSING_KNOWLEDGE: Rep =
	Rep::CostMinor("Unexpected Statement, missing knowlege for relay parent");
const COST_UNEXPECTED_STATEMENT_UNKNOWN_CANDIDATE: Rep =
	Rep::CostMinor("Unexpected Statement, unknown candidate");
const COST_UNEXPECTED_STATEMENT_REMOTE: Rep =
	Rep::CostMinor("Unexpected Statement, remote not allowed");

const COST_FETCH_FAIL: Rep =
	Rep::CostMinor("Requesting `CommittedCandidateReceipt` from peer failed");
const COST_INVALID_SIGNATURE: Rep = Rep::CostMajor("Invalid Statement Signature");
const COST_WRONG_HASH: Rep = Rep::CostMajor("Received candidate had wrong hash");
const COST_DUPLICATE_STATEMENT: Rep =
	Rep::CostMajorRepeated("Statement sent more than once by peer");
const COST_APPARENT_FLOOD: Rep = Rep::Malicious("Peer appears to be flooding us with statements");

const BENEFIT_VALID_STATEMENT: Rep = Rep::BenefitMajor("Peer provided a valid statement");
const BENEFIT_VALID_STATEMENT_FIRST: Rep =
	Rep::BenefitMajorFirst("Peer was the first to provide a valid statement");
const BENEFIT_VALID_RESPONSE: Rep =
	Rep::BenefitMajor("Peer provided a valid large statement response");

/// The maximum amount of candidates each validator is allowed to second at any relay-parent.
/// Short for "Validator Candidate Threshold".
///
/// This is the amount of candidates we keep per validator at any relay-parent.
/// Typically we will only keep 1, but when a validator equivocates we will need to track 2.
const VC_THRESHOLD: usize = 2;

const LOG_TARGET: &str = "parachain::statement-distribution";

/// Large statements should be rare.
const MAX_LARGE_STATEMENTS_PER_SENDER: usize = 20;

/// The statement distribution subsystem.
pub struct StatementDistributionSubsystem<R> {
	/// Pointer to a keystore, which is required for determining this node's validator index.
	keystore: SyncCryptoStorePtr,
	/// Receiver for incoming large statement requests.
	req_receiver: Option<IncomingRequestReceiver<request_v1::StatementFetchingRequest>>,
	/// Prometheus metrics
	metrics: Metrics,
	/// Pseudo-random generator for peers selection logic
	rng: R,
}

impl<Context, R: rand::Rng + Send + Sync + 'static> overseer::Subsystem<Context, SubsystemError>
	for StatementDistributionSubsystem<R>
where
	Context: SubsystemContext<Message = StatementDistributionMessage>,
	Context: overseer::SubsystemContext<Message = StatementDistributionMessage>,
{
	fn start(self, ctx: Context) -> SpawnedSubsystem {
		// Swallow error because failure is fatal to the node and we log with more precision
		// within `run`.
		SpawnedSubsystem {
			name: "statement-distribution-subsystem",
			future: self
				.run(ctx)
				.map_err(|e| SubsystemError::with_origin("statement-distribution", e))
				.boxed(),
		}
	}
}

#[derive(Default)]
struct RecentOutdatedHeads {
	buf: VecDeque<Hash>,
}

impl RecentOutdatedHeads {
	fn note_outdated(&mut self, hash: Hash) {
		const MAX_BUF_LEN: usize = 10;

		self.buf.push_back(hash);

		while self.buf.len() > MAX_BUF_LEN {
			let _ = self.buf.pop_front();
		}
	}

	fn is_recent_outdated(&self, hash: &Hash) -> bool {
		self.buf.contains(hash)
	}
}

/// Tracks our impression of a single peer's view of the candidates a validator has seconded
/// for a given relay-parent.
///
/// It is expected to receive at most `VC_THRESHOLD` from us and be aware of at most `VC_THRESHOLD`
/// via other means.
#[derive(Default)]
struct VcPerPeerTracker {
	local_observed: arrayvec::ArrayVec<[CandidateHash; VC_THRESHOLD]>,
	remote_observed: arrayvec::ArrayVec<[CandidateHash; VC_THRESHOLD]>,
}

impl VcPerPeerTracker {
	/// Note that the remote should now be aware that a validator has seconded a given candidate (by hash)
	/// based on a message that we have sent it from our local pool.
	fn note_local(&mut self, h: CandidateHash) {
		if !note_hash(&mut self.local_observed, h) {
			gum::warn!(
				target: LOG_TARGET,
				"Statement distribution is erroneously attempting to distribute more \
				than {} candidate(s) per validator index. Ignoring",
				VC_THRESHOLD,
			);
		}
	}

	/// Note that the remote should now be aware that a validator has seconded a given candidate (by hash)
	/// based on a message that it has sent us.
	///
	/// Returns `true` if the peer was allowed to send us such a message, `false` otherwise.
	fn note_remote(&mut self, h: CandidateHash) -> bool {
		note_hash(&mut self.remote_observed, h)
	}

	/// Returns `true` if the peer is allowed to send us such a message, `false` otherwise.
	fn is_wanted_candidate(&self, h: &CandidateHash) -> bool {
		!self.remote_observed.contains(h) && !self.remote_observed.is_full()
	}
}

fn note_hash(
	observed: &mut arrayvec::ArrayVec<[CandidateHash; VC_THRESHOLD]>,
	h: CandidateHash,
) -> bool {
	if observed.contains(&h) {
		return true
	}

	observed.try_push(h).is_ok()
}

/// knowledge that a peer has about goings-on in a relay parent.
#[derive(Default)]
struct PeerRelayParentKnowledge {
	/// candidates that the peer is aware of because we sent statements to it. This indicates that we can
	/// send other statements pertaining to that candidate.
	sent_candidates: HashSet<CandidateHash>,
	/// candidates that peer is aware of, because we received statements from it.
	received_candidates: HashSet<CandidateHash>,
	/// fingerprints of all statements a peer should be aware of: those that
	/// were sent to the peer by us.
	sent_statements: HashSet<(CompactStatement, ValidatorIndex)>,
	/// fingerprints of all statements a peer should be aware of: those that
	/// were sent to us by the peer.
	received_statements: HashSet<(CompactStatement, ValidatorIndex)>,
	/// How many candidates this peer is aware of for each given validator index.
	seconded_counts: HashMap<ValidatorIndex, VcPerPeerTracker>,
	/// How many statements we've received for each candidate that we're aware of.
	received_message_count: HashMap<CandidateHash, usize>,

	/// How many large statements this peer already sent us.
	///
	/// Flood protection for large statements is rather hard and as soon as we get
	/// `https://github.com/paritytech/polkadot/issues/2979` implemented also no longer necessary.
	/// Reason: We keep messages around until we fetched the payload, but if a node makes up
	/// statements and never provides the data, we will keep it around for the slot duration. Not
	/// even signature checking would help, as the sender, if a validator, can just sign arbitrary
	/// invalid statements and will not face any consequences as long as it won't provide the
	/// payload.
	///
	/// Quick and temporary fix, only accept `MAX_LARGE_STATEMENTS_PER_SENDER` per connected node.
	///
	/// Large statements should be rare, if they were not, we would run into problems anyways, as
	/// we would not be able to distribute them in a timely manner. Therefore
	/// `MAX_LARGE_STATEMENTS_PER_SENDER` can be set to a relatively small number. It is also not
	/// per candidate hash, but in total as candidate hashes can be made up, as illustrated above.
	///
	/// An attacker could still try to fill up our memory, by repeatedly disconnecting and
	/// connecting again with new peer ids, but we assume that the resulting effective bandwidth
	/// for such an attack would be too low.
	large_statement_count: usize,

	/// We have seen a message that that is unexpected from this peer, so note this fact
	/// and stop subsequent logging and peer reputation flood.
	unexpected_count: usize,
}

impl PeerRelayParentKnowledge {
	/// Updates our view of the peer's knowledge with this statement's fingerprint based
	/// on something that we would like to send to the peer.
	///
	/// NOTE: assumes `self.can_send` returned true before this call.
	///
	/// Once the knowledge has incorporated a statement, it cannot be incorporated again.
	///
	/// This returns `true` if this is the first time the peer has become aware of a
	/// candidate with the given hash.
	fn send(&mut self, fingerprint: &(CompactStatement, ValidatorIndex)) -> bool {
		debug_assert!(
			self.can_send(fingerprint),
			"send is only called after `can_send` returns true; qed",
		);

		let new_known = match fingerprint.0 {
			CompactStatement::Seconded(ref h) => {
				self.seconded_counts.entry(fingerprint.1).or_default().note_local(h.clone());

				let was_known = self.is_known_candidate(h);
				self.sent_candidates.insert(h.clone());
				!was_known
			},
			CompactStatement::Valid(_) => false,
		};

		self.sent_statements.insert(fingerprint.clone());

		new_known
	}

	/// This returns `true` if the peer cannot accept this statement, without altering internal
	/// state, `false` otherwise.
	fn can_send(&self, fingerprint: &(CompactStatement, ValidatorIndex)) -> bool {
		let already_known = self.sent_statements.contains(fingerprint) ||
			self.received_statements.contains(fingerprint);

		if already_known {
			return false
		}

		match fingerprint.0 {
			CompactStatement::Valid(ref h) => {
				// The peer can only accept Valid statements for which it is aware
				// of the corresponding candidate.
				self.is_known_candidate(h)
			},
			CompactStatement::Seconded(_) => true,
		}
	}

	/// Attempt to update our view of the peer's knowledge with this statement's fingerprint based on
	/// a message we are receiving from the peer.
	///
	/// Provide the maximum message count that we can receive per candidate. In practice we should
	/// not receive more statements for any one candidate than there are members in the group assigned
	/// to that para, but this maximum needs to be lenient to account for equivocations that may be
	/// cross-group. As such, a maximum of 2 * `n_validators` is recommended.
	///
	/// This returns an error if the peer should not have sent us this message according to protocol
	/// rules for flood protection.
	///
	/// If this returns `Ok`, the internal state has been altered. After `receive`ing a new
	/// candidate, we are then cleared to send the peer further statements about that candidate.
	///
	/// This returns `Ok(true)` if this is the first time the peer has become aware of a
	/// candidate with given hash.
	fn receive(
		&mut self,
		fingerprint: &(CompactStatement, ValidatorIndex),
		max_message_count: usize,
	) -> std::result::Result<bool, Rep> {
		// We don't check `sent_statements` because a statement could be in-flight from both
		// sides at the same time.
		if self.received_statements.contains(fingerprint) {
			return Err(COST_DUPLICATE_STATEMENT)
		}

		let (candidate_hash, fresh) = match fingerprint.0 {
			CompactStatement::Seconded(ref h) => {
				let allowed_remote = self
					.seconded_counts
					.entry(fingerprint.1)
					.or_insert_with(Default::default)
					.note_remote(h.clone());

				if !allowed_remote {
					return Err(COST_UNEXPECTED_STATEMENT_REMOTE)
				}

				(h, !self.is_known_candidate(h))
			},
			CompactStatement::Valid(ref h) => {
				if !self.is_known_candidate(h) {
					return Err(COST_UNEXPECTED_STATEMENT_UNKNOWN_CANDIDATE)
				}

				(h, false)
			},
		};

		{
			let received_per_candidate =
				self.received_message_count.entry(*candidate_hash).or_insert(0);

			if *received_per_candidate >= max_message_count {
				return Err(COST_APPARENT_FLOOD)
			}

			*received_per_candidate += 1;
		}

		self.received_statements.insert(fingerprint.clone());
		self.received_candidates.insert(candidate_hash.clone());
		Ok(fresh)
	}

	/// Note a received large statement metadata.
	fn receive_large_statement(&mut self) -> std::result::Result<(), Rep> {
		if self.large_statement_count >= MAX_LARGE_STATEMENTS_PER_SENDER {
			return Err(COST_APPARENT_FLOOD)
		}
		self.large_statement_count += 1;
		Ok(())
	}

	/// This method does the same checks as `receive` without modifying the internal state.
	/// Returns an error if the peer should not have sent us this message according to protocol
	/// rules for flood protection.
	fn check_can_receive(
		&self,
		fingerprint: &(CompactStatement, ValidatorIndex),
		max_message_count: usize,
	) -> std::result::Result<(), Rep> {
		// We don't check `sent_statements` because a statement could be in-flight from both
		// sides at the same time.
		if self.received_statements.contains(fingerprint) {
			return Err(COST_DUPLICATE_STATEMENT)
		}

		let candidate_hash = match fingerprint.0 {
			CompactStatement::Seconded(ref h) => {
				let allowed_remote = self
					.seconded_counts
					.get(&fingerprint.1)
					.map_or(true, |r| r.is_wanted_candidate(h));

				if !allowed_remote {
					return Err(COST_UNEXPECTED_STATEMENT_REMOTE)
				}

				h
			},
			CompactStatement::Valid(ref h) => {
				if !self.is_known_candidate(&h) {
					return Err(COST_UNEXPECTED_STATEMENT_UNKNOWN_CANDIDATE)
				}

				h
			},
		};

		let received_per_candidate = self.received_message_count.get(candidate_hash).unwrap_or(&0);

		if *received_per_candidate >= max_message_count {
			Err(COST_APPARENT_FLOOD)
		} else {
			Ok(())
		}
	}

	/// Check for candidates that the peer is aware of. This indicates that we can
	/// send other statements pertaining to that candidate.
	fn is_known_candidate(&self, candidate: &CandidateHash) -> bool {
		self.sent_candidates.contains(candidate) || self.received_candidates.contains(candidate)
	}
}

struct PeerData {
	view: View,
	view_knowledge: HashMap<Hash, PeerRelayParentKnowledge>,
	/// Peer might be known as authority with the given ids.
	maybe_authority: Option<HashSet<AuthorityDiscoveryId>>,
}

impl PeerData {
	/// Updates our view of the peer's knowledge with this statement's fingerprint based
	/// on something that we would like to send to the peer.
	///
	/// NOTE: assumes `self.can_send` returned true before this call.
	///
	/// Once the knowledge has incorporated a statement, it cannot be incorporated again.
	///
	/// This returns `true` if this is the first time the peer has become aware of a
	/// candidate with the given hash.
	fn send(
		&mut self,
		relay_parent: &Hash,
		fingerprint: &(CompactStatement, ValidatorIndex),
	) -> bool {
		debug_assert!(
			self.can_send(relay_parent, fingerprint),
			"send is only called after `can_send` returns true; qed",
		);
		self.view_knowledge
			.get_mut(relay_parent)
			.expect("send is only called after `can_send` returns true; qed")
			.send(fingerprint)
	}

	/// This returns `None` if the peer cannot accept this statement, without altering internal
	/// state.
	fn can_send(
		&self,
		relay_parent: &Hash,
		fingerprint: &(CompactStatement, ValidatorIndex),
	) -> bool {
		self.view_knowledge.get(relay_parent).map_or(false, |k| k.can_send(fingerprint))
	}

	/// Attempt to update our view of the peer's knowledge with this statement's fingerprint based on
	/// a message we are receiving from the peer.
	///
	/// Provide the maximum message count that we can receive per candidate. In practice we should
	/// not receive more statements for any one candidate than there are members in the group assigned
	/// to that para, but this maximum needs to be lenient to account for equivocations that may be
	/// cross-group. As such, a maximum of 2 * `n_validators` is recommended.
	///
	/// This returns an error if the peer should not have sent us this message according to protocol
	/// rules for flood protection.
	///
	/// If this returns `Ok`, the internal state has been altered. After `receive`ing a new
	/// candidate, we are then cleared to send the peer further statements about that candidate.
	///
	/// This returns `Ok(true)` if this is the first time the peer has become aware of a
	/// candidate with given hash.
	fn receive(
		&mut self,
		relay_parent: &Hash,
		fingerprint: &(CompactStatement, ValidatorIndex),
		max_message_count: usize,
	) -> std::result::Result<bool, Rep> {
		self.view_knowledge
			.get_mut(relay_parent)
			.ok_or(COST_UNEXPECTED_STATEMENT_MISSING_KNOWLEDGE)?
			.receive(fingerprint, max_message_count)
	}

	/// This method does the same checks as `receive` without modifying the internal state.
	/// Returns an error if the peer should not have sent us this message according to protocol
	/// rules for flood protection.
	fn check_can_receive(
		&self,
		relay_parent: &Hash,
		fingerprint: &(CompactStatement, ValidatorIndex),
		max_message_count: usize,
	) -> std::result::Result<(), Rep> {
		self.view_knowledge
			.get(relay_parent)
			.ok_or(COST_UNEXPECTED_STATEMENT_MISSING_KNOWLEDGE)?
			.check_can_receive(fingerprint, max_message_count)
	}

	/// Receive a notice about out of view statement and returns the value of the old flag
	fn receive_unexpected(&mut self, relay_parent: &Hash) -> usize {
		self.view_knowledge
			.get_mut(relay_parent)
			.map_or(0_usize, |relay_parent_peer_knowledge| {
				let old = relay_parent_peer_knowledge.unexpected_count;
				relay_parent_peer_knowledge.unexpected_count += 1_usize;
				old
			})
	}

	/// Basic flood protection for large statements.
	fn receive_large_statement(&mut self, relay_parent: &Hash) -> std::result::Result<(), Rep> {
		self.view_knowledge
			.get_mut(relay_parent)
			.ok_or(COST_UNEXPECTED_STATEMENT_MISSING_KNOWLEDGE)?
			.receive_large_statement()
	}
}

// A statement stored while a relay chain head is active.
#[derive(Debug, Copy, Clone)]
struct StoredStatement<'a> {
	comparator: &'a StoredStatementComparator,
	statement: &'a SignedFullStatement,
}

// A value used for comparison of stored statements to each other.
//
// The compact version of the statement, the validator index, and the signature of the validator
// is enough to differentiate between all types of equivocations, as long as the signature is
// actually checked to be valid. The same statement with 2 signatures and 2 statements with
// different (or same) signatures wll all be correctly judged to be unequal with this comparator.
#[derive(PartialEq, Eq, Hash, Clone, Debug)]
struct StoredStatementComparator {
	compact: CompactStatement,
	validator_index: ValidatorIndex,
	signature: ValidatorSignature,
}

impl<'a> From<(&'a StoredStatementComparator, &'a SignedFullStatement)> for StoredStatement<'a> {
	fn from(
		(comparator, statement): (&'a StoredStatementComparator, &'a SignedFullStatement),
	) -> Self {
		Self { comparator, statement }
	}
}

impl<'a> StoredStatement<'a> {
	fn compact(&self) -> &'a CompactStatement {
		&self.comparator.compact
	}

	fn fingerprint(&self) -> (CompactStatement, ValidatorIndex) {
		(self.comparator.compact.clone(), self.statement.validator_index())
	}
}

#[derive(Debug)]
enum NotedStatement<'a> {
	NotUseful,
	Fresh(StoredStatement<'a>),
	UsefulButKnown,
}

/// Large statement fetching status.
enum LargeStatementStatus {
	/// We are currently fetching the statement data from a remote peer. We keep a list of other nodes
	/// claiming to have that data and will fallback on them.
	Fetching(FetchingInfo),
	/// Statement data is fetched or we got it locally via `StatementDistributionMessage::Share`.
	FetchedOrShared(CommittedCandidateReceipt),
}

/// Info about a fetch in progress.
struct FetchingInfo {
	/// All peers that send us a `LargeStatement` or a `Valid` statement for the given
	/// `CandidateHash`, together with their originally sent messages.
	///
	/// We use an `IndexMap` here to preserve the ordering of peers sending us messages. This is
	/// desirable because we reward first sending peers with reputation.
	available_peers: IndexMap<PeerId, Vec<protocol_v1::StatementDistributionMessage>>,
	/// Peers left to try in case the background task needs it.
	peers_to_try: Vec<PeerId>,
	/// Sender for sending fresh peers to the fetching task in case of failure.
	peer_sender: Option<oneshot::Sender<Vec<PeerId>>>,
	/// Task taking care of the request.
	///
	/// Will be killed once dropped.
	#[allow(dead_code)]
	fetching_task: RemoteHandle<()>,
}

/// Messages to be handled in this subsystem.
enum MuxedMessage {
	/// Messages from other subsystems.
	Subsystem(FatalResult<FromOverseer<StatementDistributionMessage>>),
	/// Messages from spawned requester background tasks.
	Requester(Option<RequesterMessage>),
	/// Messages from spawned responder background task.
	Responder(Option<ResponderMessage>),
}

impl MuxedMessage {
	async fn receive(
		ctx: &mut (impl SubsystemContext<Message = StatementDistributionMessage>
		          + overseer::SubsystemContext<Message = StatementDistributionMessage>),
		from_requester: &mut mpsc::Receiver<RequesterMessage>,
		from_responder: &mut mpsc::Receiver<ResponderMessage>,
	) -> MuxedMessage {
		// We are only fusing here to make `select` happy, in reality we will quit if one of those
		// streams end:
		let from_overseer = ctx.recv().fuse();
		let from_requester = from_requester.next();
		let from_responder = from_responder.next();
		futures::pin_mut!(from_overseer, from_requester, from_responder);
		futures::select! {
			msg = from_overseer => MuxedMessage::Subsystem(msg.map_err(FatalError::SubsystemReceive)),
			msg = from_requester => MuxedMessage::Requester(msg),
			msg = from_responder => MuxedMessage::Responder(msg),
		}
	}
}

#[derive(Debug, PartialEq, Eq)]
enum DeniedStatement {
	NotUseful,
	UsefulButKnown,
}

struct ActiveHeadData {
	/// All candidates we are aware of for this head, keyed by hash.
	candidates: HashSet<CandidateHash>,
	/// Stored statements for circulation to peers.
	///
	/// These are iterable in insertion order, and `Seconded` statements are always
	/// accepted before dependent statements.
	statements: IndexMap<StoredStatementComparator, SignedFullStatement>,
	/// Large statements we are waiting for with associated meta data.
	waiting_large_statements: HashMap<CandidateHash, LargeStatementStatus>,
	/// The parachain validators at the head's child session index.
	validators: Vec<ValidatorId>,
	/// The current session index of this fork.
	session_index: sp_staking::SessionIndex,
	/// How many `Seconded` statements we've seen per validator.
	seconded_counts: HashMap<ValidatorIndex, usize>,
	/// A Jaeger span for this head, so we can attach data to it.
	span: PerLeafSpan,
}

impl ActiveHeadData {
	fn new(
		validators: Vec<ValidatorId>,
		session_index: sp_staking::SessionIndex,
		span: PerLeafSpan,
	) -> Self {
		ActiveHeadData {
			candidates: Default::default(),
			statements: Default::default(),
			waiting_large_statements: Default::default(),
			validators,
			session_index,
			seconded_counts: Default::default(),
			span,
		}
	}

	/// Note the given statement.
	///
	/// If it was not already known and can be accepted,  returns `NotedStatement::Fresh`,
	/// with a handle to the statement.
	///
	/// If it can be accepted, but we already know it, returns `NotedStatement::UsefulButKnown`.
	///
	/// We accept up to `VC_THRESHOLD` (2 at time of writing) `Seconded` statements
	/// per validator. These will be the first ones we see. The statement is assumed
	/// to have been checked, including that the validator index is not out-of-bounds and
	/// the signature is valid.
	///
	/// Any other statements or those that reference a candidate we are not aware of cannot be accepted
	/// and will return `NotedStatement::NotUseful`.
	fn note_statement(&mut self, statement: SignedFullStatement) -> NotedStatement {
		let validator_index = statement.validator_index();
		let comparator = StoredStatementComparator {
			compact: statement.payload().to_compact(),
			validator_index,
			signature: statement.signature().clone(),
		};

		match comparator.compact {
			CompactStatement::Seconded(h) => {
				let seconded_so_far = self.seconded_counts.entry(validator_index).or_insert(0);
				if *seconded_so_far >= VC_THRESHOLD {
					gum::trace!(
						target: LOG_TARGET,
						?validator_index,
						?statement,
						"Extra statement is ignored"
					);
					return NotedStatement::NotUseful
				}

				self.candidates.insert(h);
				if let Some(old) = self.statements.insert(comparator.clone(), statement) {
					gum::trace!(
						target: LOG_TARGET,
						?validator_index,
						statement = ?old,
						"Known statement"
					);
					NotedStatement::UsefulButKnown
				} else {
					*seconded_so_far += 1;

					gum::trace!(
						target: LOG_TARGET,
						?validator_index,
						statement = ?self.statements.last().expect("Just inserted").1,
						"Noted new statement"
					);
					// This will always return `Some` because it was just inserted.
					let key_value = self
						.statements
						.get_key_value(&comparator)
						.expect("Statement was just inserted; qed");

					NotedStatement::Fresh(key_value.into())
				}
			},
			CompactStatement::Valid(h) => {
				if !self.candidates.contains(&h) {
					gum::trace!(
						target: LOG_TARGET,
						?validator_index,
						?statement,
						"Statement for unknown candidate"
					);
					return NotedStatement::NotUseful
				}

				if let Some(old) = self.statements.insert(comparator.clone(), statement) {
					gum::trace!(
						target: LOG_TARGET,
						?validator_index,
						statement = ?old,
						"Known statement"
					);
					NotedStatement::UsefulButKnown
				} else {
					gum::trace!(
						target: LOG_TARGET,
						?validator_index,
						statement = ?self.statements.last().expect("Just inserted").1,
						"Noted new statement"
					);
					// This will always return `Some` because it was just inserted.
					NotedStatement::Fresh(
						self.statements
							.get_key_value(&comparator)
							.expect("Statement was just inserted; qed")
							.into(),
					)
				}
			},
		}
	}

	/// Returns an error if the statement is already known or not useful
	/// without modifying the internal state.
	fn check_useful_or_unknown(
		&self,
		statement: &UncheckedSignedStatement,
	) -> std::result::Result<(), DeniedStatement> {
		let validator_index = statement.unchecked_validator_index();
		let compact = statement.unchecked_payload();
		let comparator = StoredStatementComparator {
			compact: compact.clone(),
			validator_index,
			signature: statement.unchecked_signature().clone(),
		};

		match compact {
			CompactStatement::Seconded(_) => {
				let seconded_so_far = self.seconded_counts.get(&validator_index).unwrap_or(&0);
				if *seconded_so_far >= VC_THRESHOLD {
					gum::trace!(
						target: LOG_TARGET,
						?validator_index,
						?statement,
						"Extra statement is ignored",
					);
					return Err(DeniedStatement::NotUseful)
				}

				if self.statements.contains_key(&comparator) {
					gum::trace!(
						target: LOG_TARGET,
						?validator_index,
						?statement,
						"Known statement",
					);
					return Err(DeniedStatement::UsefulButKnown)
				}
			},
			CompactStatement::Valid(h) => {
				if !self.candidates.contains(&h) {
					gum::trace!(
						target: LOG_TARGET,
						?validator_index,
						?statement,
						"Statement for unknown candidate",
					);
					return Err(DeniedStatement::NotUseful)
				}

				if self.statements.contains_key(&comparator) {
					gum::trace!(
						target: LOG_TARGET,
						?validator_index,
						?statement,
						"Known statement",
					);
					return Err(DeniedStatement::UsefulButKnown)
				}
			},
		}
		Ok(())
	}

	/// Get an iterator over all statements for the active head. Seconded statements come first.
	fn statements(&self) -> impl Iterator<Item = StoredStatement<'_>> + '_ {
		self.statements.iter().map(Into::into)
	}

	/// Get an iterator over all statements for the active head that are for a particular candidate.
	fn statements_about(
		&self,
		candidate_hash: CandidateHash,
	) -> impl Iterator<Item = StoredStatement<'_>> + '_ {
		self.statements()
			.filter(move |s| s.compact().candidate_hash() == &candidate_hash)
	}
}

/// Check a statement signature under this parent hash.
fn check_statement_signature(
	head: &ActiveHeadData,
	relay_parent: Hash,
	statement: UncheckedSignedStatement,
) -> std::result::Result<SignedStatement, UncheckedSignedStatement> {
	let signing_context =
		SigningContext { session_index: head.session_index, parent_hash: relay_parent };

	head.validators
		.get(statement.unchecked_validator_index().0 as usize)
		.ok_or_else(|| statement.clone())
		.and_then(|v| statement.try_into_checked(&signing_context, v))
}

/// Places the statement in storage if it is new, and then
/// circulates the statement to all peers who have not seen it yet, and
/// sends all statements dependent on that statement to peers who could previously not receive
/// them but now can.
async fn circulate_statement_and_dependents(
	gossip_peers: &HashSet<PeerId>,
	peers: &mut HashMap<PeerId, PeerData>,
	active_heads: &mut HashMap<Hash, ActiveHeadData>,
	ctx: &mut (impl SubsystemContext + overseer::SubsystemContext),
	relay_parent: Hash,
	statement: SignedFullStatement,
	priority_peers: Vec<PeerId>,
	metrics: &Metrics,
	rng: &mut impl rand::Rng,
) {
	let active_head = match active_heads.get_mut(&relay_parent) {
		Some(res) => res,
		None => return,
	};

	let _span = active_head
		.span
		.child("circulate-statement")
		.with_candidate(statement.payload().candidate_hash())
		.with_stage(jaeger::Stage::StatementDistribution);

	// First circulate the statement directly to all peers needing it.
	// The borrow of `active_head` needs to encompass only this (Rust) statement.
	let outputs: Option<(CandidateHash, Vec<PeerId>)> = {
		match active_head.note_statement(statement) {
			NotedStatement::Fresh(stored) => Some((
				*stored.compact().candidate_hash(),
				circulate_statement(
					gossip_peers,
					peers,
					ctx,
					relay_parent,
					stored,
					priority_peers,
					metrics,
					rng,
				)
				.await,
			)),
			_ => None,
		}
	};

	let _span = _span.child("send-to-peers");
	// Now send dependent statements to all peers needing them, if any.
	if let Some((candidate_hash, peers_needing_dependents)) = outputs {
		for peer in peers_needing_dependents {
			if let Some(peer_data) = peers.get_mut(&peer) {
				let _span_loop = _span.child("to-peer").with_peer_id(&peer);
				// defensive: the peer data should always be some because the iterator
				// of peers is derived from the set of peers.
				send_statements_about(
					peer,
					peer_data,
					ctx,
					relay_parent,
					candidate_hash,
					&*active_head,
					metrics,
				)
				.await;
			}
		}
	}
}

/// Create a network message from a given statement.
fn statement_message(
	relay_parent: Hash,
	statement: SignedFullStatement,
	metrics: &Metrics,
) -> protocol_v1::ValidationProtocol {
	let (is_large, size) = is_statement_large(&statement);
	if let Some(size) = size {
		metrics.on_created_message(size);
	}

	let msg = if is_large {
		protocol_v1::StatementDistributionMessage::LargeStatement(StatementMetadata {
			relay_parent,
			candidate_hash: statement.payload().candidate_hash(),
			signed_by: statement.validator_index(),
			signature: statement.signature().clone(),
		})
	} else {
		protocol_v1::StatementDistributionMessage::Statement(relay_parent, statement.into())
	};

	protocol_v1::ValidationProtocol::StatementDistribution(msg)
}

/// Check whether a statement should be treated as large statement.
///
/// Also report size of statement - if it is a `Seconded` statement, otherwise `None`.
fn is_statement_large(statement: &SignedFullStatement) -> (bool, Option<usize>) {
	match &statement.payload() {
		Statement::Seconded(committed) => {
			let size = statement.as_unchecked().encoded_size();
			// Runtime upgrades will always be large and even if not - no harm done.
			if committed.commitments.new_validation_code.is_some() {
				return (true, Some(size))
			}

			// Half max size seems to be a good threshold to start not using notifications:
			let threshold =
				PeerSet::Validation.get_info(IsAuthority::Yes).max_notification_size as usize / 2;

			(size >= threshold, Some(size))
		},
		Statement::Valid(_) => (false, None),
	}
}

/// Circulates a statement to all peers who have not seen it yet, and returns
/// an iterator over peers who need to have dependent statements sent.
async fn circulate_statement<'a>(
	gossip_peers: &HashSet<PeerId>,
	peers: &mut HashMap<PeerId, PeerData>,
	ctx: &mut (impl SubsystemContext + overseer::SubsystemContext),
	relay_parent: Hash,
	stored: StoredStatement<'a>,
	mut priority_peers: Vec<PeerId>,
	metrics: &Metrics,
	rng: &mut impl rand::Rng,
) -> Vec<PeerId> {
	let fingerprint = stored.fingerprint();

	let mut peers_to_send: Vec<PeerId> = peers
		.iter()
		.filter_map(|(peer, data)| {
			if data.can_send(&relay_parent, &fingerprint) {
				Some(peer.clone())
			} else {
				None
			}
		})
		.collect();

	let good_peers: HashSet<&PeerId> = peers_to_send.iter().collect();
	// Only take priority peers we can send data to:
	priority_peers.retain(|p| good_peers.contains(p));

	// Avoid duplicates:
	let priority_set: HashSet<&PeerId> = priority_peers.iter().collect();
	peers_to_send.retain(|p| !priority_set.contains(p));

	util::choose_random_subset_with_rng(
		|e| gossip_peers.contains(e),
		&mut peers_to_send,
		rng,
		MIN_GOSSIP_PEERS,
	);
	// We don't want to use less peers, than we would without any priority peers:
	let min_size = std::cmp::max(peers_to_send.len(), MIN_GOSSIP_PEERS);
	// Make set full:
	let needed_peers = min_size as i64 - priority_peers.len() as i64;
	if needed_peers > 0 {
		peers_to_send.truncate(needed_peers as usize);
		// Order important here - priority peers are placed first, so will be sent first.
		// This gives backers a chance to be among the first in requesting any large statement
		// data.
		priority_peers.append(&mut peers_to_send);
	}
	peers_to_send = priority_peers;
	// We must not have duplicates:
	debug_assert!(
		peers_to_send.len() == peers_to_send.clone().into_iter().collect::<HashSet<_>>().len(),
		"We filter out duplicates above. qed.",
	);
	let peers_to_send: Vec<(PeerId, bool)> = peers_to_send
		.into_iter()
		.map(|peer_id| {
			let new = peers
				.get_mut(&peer_id)
				.expect("a subset is taken above, so it exists; qed")
				.send(&relay_parent, &fingerprint);
			(peer_id, new)
		})
		.collect();

	// Send all these peers the initial statement.
	if !peers_to_send.is_empty() {
		let payload = statement_message(relay_parent, stored.statement.clone(), metrics);
		gum::trace!(
			target: LOG_TARGET,
			?peers_to_send,
			?relay_parent,
			statement = ?stored.statement,
			"Sending statement",
		);
		ctx.send_message(AllMessages::NetworkBridge(NetworkBridgeMessage::SendValidationMessage(
			peers_to_send.iter().map(|(p, _)| p.clone()).collect(),
			payload,
		)))
		.await;
	}

	peers_to_send
		.into_iter()
		.filter_map(|(peer, needs_dependent)| if needs_dependent { Some(peer) } else { None })
		.collect()
}

/// Send all statements about a given candidate hash to a peer.
async fn send_statements_about(
	peer: PeerId,
	peer_data: &mut PeerData,
	ctx: &mut (impl SubsystemContext + overseer::SubsystemContext),
	relay_parent: Hash,
	candidate_hash: CandidateHash,
	active_head: &ActiveHeadData,
	metrics: &Metrics,
) {
	for statement in active_head.statements_about(candidate_hash) {
		let fingerprint = statement.fingerprint();
		if !peer_data.can_send(&relay_parent, &fingerprint) {
			continue
		}
		peer_data.send(&relay_parent, &fingerprint);
		let payload = statement_message(relay_parent, statement.statement.clone(), metrics);

		gum::trace!(
			target: LOG_TARGET,
			?peer,
			?relay_parent,
			?candidate_hash,
			statement = ?statement.statement,
			"Sending statement",
		);
		ctx.send_message(AllMessages::NetworkBridge(NetworkBridgeMessage::SendValidationMessage(
			vec![peer.clone()],
			payload,
		)))
		.await;

		metrics.on_statement_distributed();
	}
}

/// Send all statements at a given relay-parent to a peer.
async fn send_statements(
	peer: PeerId,
	peer_data: &mut PeerData,
	ctx: &mut (impl SubsystemContext + overseer::SubsystemContext),
	relay_parent: Hash,
	active_head: &ActiveHeadData,
	metrics: &Metrics,
) {
	for statement in active_head.statements() {
		let fingerprint = statement.fingerprint();
		if !peer_data.can_send(&relay_parent, &fingerprint) {
			continue
		}
		peer_data.send(&relay_parent, &fingerprint);
		let payload = statement_message(relay_parent, statement.statement.clone(), metrics);

		gum::trace!(
			target: LOG_TARGET,
			?peer,
			?relay_parent,
			statement = ?statement.statement,
			"Sending statement"
		);
		ctx.send_message(AllMessages::NetworkBridge(NetworkBridgeMessage::SendValidationMessage(
			vec![peer.clone()],
			payload,
		)))
		.await;

		metrics.on_statement_distributed();
	}
}

async fn report_peer(
	ctx: &mut (impl SubsystemContext + overseer::SubsystemContext),
	peer: PeerId,
	rep: Rep,
) {
	ctx.send_message(AllMessages::NetworkBridge(NetworkBridgeMessage::ReportPeer(peer, rep)))
		.await
}

/// If message contains a statement, then retrieve it, otherwise fork task to fetch it.
///
/// This function will also return `None` if the message did not pass some basic checks, in that
/// case no statement will be requested, on the flipside you get `ActiveHeadData` in addition to
/// your statement.
///
/// If the message was large, but the result has been fetched already that one is returned.
async fn retrieve_statement_from_message<'a>(
	peer: PeerId,
	message: protocol_v1::StatementDistributionMessage,
	active_head: &'a mut ActiveHeadData,
	ctx: &mut (impl SubsystemContext + overseer::SubsystemContext),
	req_sender: &mpsc::Sender<RequesterMessage>,
	metrics: &Metrics,
) -> Option<UncheckedSignedFullStatement> {
	let fingerprint = message.get_fingerprint();
	let candidate_hash = *fingerprint.0.candidate_hash();

	// Immediately return any Seconded statement:
	let message = if let protocol_v1::StatementDistributionMessage::Statement(h, s) = message {
		if let Statement::Seconded(_) = s.unchecked_payload() {
			return Some(s)
		}
		protocol_v1::StatementDistributionMessage::Statement(h, s)
	} else {
		message
	};

	match active_head.waiting_large_statements.entry(candidate_hash) {
		Entry::Occupied(mut occupied) => {
			match occupied.get_mut() {
				LargeStatementStatus::Fetching(info) => {
					let is_large_statement = message.is_large_statement();

					let is_new_peer = match info.available_peers.entry(peer) {
						IEntry::Occupied(mut occupied) => {
							occupied.get_mut().push(message);
							false
						},
						IEntry::Vacant(vacant) => {
							vacant.insert(vec![message]);
							true
						},
					};

					if is_new_peer & is_large_statement {
						info.peers_to_try.push(peer);
						// Answer any pending request for more peers:
						if let Some(sender) = info.peer_sender.take() {
							let to_send = std::mem::take(&mut info.peers_to_try);
							if let Err(peers) = sender.send(to_send) {
								// Requester no longer interested for now, might want them
								// later:
								info.peers_to_try = peers;
							}
						}
					}
				},
				LargeStatementStatus::FetchedOrShared(committed) => {
					match message {
						protocol_v1::StatementDistributionMessage::Statement(_, s) => {
							// We can now immediately return any statements (should only be
							// `Statement::Valid` ones, but we don't care at this point.)
							return Some(s)
						},
						protocol_v1::StatementDistributionMessage::LargeStatement(metadata) =>
							return Some(UncheckedSignedFullStatement::new(
								Statement::Seconded(committed.clone()),
								metadata.signed_by,
								metadata.signature.clone(),
							)),
					}
				},
			}
		},
		Entry::Vacant(vacant) => {
			match message {
				protocol_v1::StatementDistributionMessage::LargeStatement(metadata) => {
					if let Some(new_status) =
						launch_request(metadata, peer, req_sender.clone(), ctx, metrics).await
					{
						vacant.insert(new_status);
					}
				},
				protocol_v1::StatementDistributionMessage::Statement(_, s) => {
					// No fetch in progress, safe to return any statement immediately (we don't bother
					// about normal network jitter which might cause `Valid` statements to arrive early
					// for now.).
					return Some(s)
				},
			}
		},
	}
	None
}

/// Launch request for a large statement and get tracking status.
///
/// Returns `None` if spawning task failed.
async fn launch_request(
	meta: StatementMetadata,
	peer: PeerId,
	req_sender: mpsc::Sender<RequesterMessage>,
	ctx: &mut (impl SubsystemContext + overseer::SubsystemContext),
	metrics: &Metrics,
) -> Option<LargeStatementStatus> {
	let (task, handle) =
		fetch(meta.relay_parent, meta.candidate_hash, vec![peer], req_sender, metrics.clone())
			.remote_handle();

	let result = ctx.spawn("large-statement-fetcher", task.boxed());
	if let Err(err) = result {
		gum::error!(target: LOG_TARGET, ?err, "Spawning task failed.");
		return None
	}
	let available_peers = {
		let mut m = IndexMap::new();
		m.insert(peer, vec![protocol_v1::StatementDistributionMessage::LargeStatement(meta)]);
		m
	};
	Some(LargeStatementStatus::Fetching(FetchingInfo {
		available_peers,
		peers_to_try: Vec::new(),
		peer_sender: None,
		fetching_task: handle,
	}))
}

/// Handle incoming message and circulate it to peers, if we did not know it already.
///
async fn handle_incoming_message_and_circulate<'a>(
	peer: PeerId,
	gossip_peers: &HashSet<PeerId>,
	peers: &mut HashMap<PeerId, PeerData>,
	active_heads: &'a mut HashMap<Hash, ActiveHeadData>,
	recent_outdated_heads: &RecentOutdatedHeads,
	ctx: &mut (impl SubsystemContext + overseer::SubsystemContext),
	message: protocol_v1::StatementDistributionMessage,
	req_sender: &mpsc::Sender<RequesterMessage>,
	metrics: &Metrics,
	rng: &mut impl rand::Rng,
) {
	let handled_incoming = match peers.get_mut(&peer) {
		Some(data) =>
			handle_incoming_message(
				peer,
				data,
				active_heads,
				recent_outdated_heads,
				ctx,
				message,
				req_sender,
				metrics,
			)
			.await,
		None => None,
	};

	// if we got a fresh message, we need to circulate it to all peers.
	if let Some((relay_parent, statement)) = handled_incoming {
		// we can ignore the set of peers who this function returns as now expecting
		// dependent statements.
		//
		// we have the invariant in this subsystem that we never store a `Valid` or `Invalid`
		// statement before a `Seconded` statement. `Seconded` statements are the only ones
		// that require dependents. Thus, if this is a `Seconded` statement for a candidate we
		// were not aware of before, we cannot have any dependent statements from the candidate.
		let _ = metrics.time_network_bridge_update_v1("circulate_statement");
		let _ = circulate_statement(
			gossip_peers,
			peers,
			ctx,
			relay_parent,
			statement,
			Vec::new(),
			metrics,
			rng,
		)
		.await;
	}
}

// Handle a statement. Returns a reference to a newly-stored statement
// if we were not already aware of it, along with the corresponding relay-parent.
//
// This function checks the signature and ensures the statement is compatible with our
// view. It also notifies candidate backing if the statement was previously unknown.
async fn handle_incoming_message<'a>(
	peer: PeerId,
	peer_data: &mut PeerData,
	active_heads: &'a mut HashMap<Hash, ActiveHeadData>,
	recent_outdated_heads: &RecentOutdatedHeads,
	ctx: &mut (impl SubsystemContext + overseer::SubsystemContext),
	message: protocol_v1::StatementDistributionMessage,
	req_sender: &mpsc::Sender<RequesterMessage>,
	metrics: &Metrics,
) -> Option<(Hash, StoredStatement<'a>)> {
	let relay_parent = message.get_relay_parent();
	let _ = metrics.time_network_bridge_update_v1("handle_incoming_message");

	let active_head = match active_heads.get_mut(&relay_parent) {
		Some(h) => h,
		None => {
			gum::debug!(
				target: LOG_TARGET,
				%relay_parent,
				"our view out-of-sync with active heads; head not found",
			);

			if !recent_outdated_heads.is_recent_outdated(&relay_parent) {
				report_peer(ctx, peer, COST_UNEXPECTED_STATEMENT).await;
			}

			return None
		},
	};

	if let protocol_v1::StatementDistributionMessage::LargeStatement(_) = message {
		if let Err(rep) = peer_data.receive_large_statement(&relay_parent) {
			gum::debug!(target: LOG_TARGET, ?peer, ?message, ?rep, "Unexpected large statement.",);
			report_peer(ctx, peer, rep).await;
			return None
		}
	}

	let fingerprint = message.get_fingerprint();
	let candidate_hash = fingerprint.0.candidate_hash().clone();
	let handle_incoming_span = active_head
		.span
		.child("handle-incoming")
		.with_candidate(candidate_hash)
		.with_peer_id(&peer);

	let max_message_count = active_head.validators.len() * 2;

	// perform only basic checks before verifying the signature
	// as it's more computationally heavy
	if let Err(rep) = peer_data.check_can_receive(&relay_parent, &fingerprint, max_message_count) {
		// This situation can happen when a peer's Seconded message was lost
		// but we have received the Valid statement.
		// So we check it once and then ignore repeated violation to avoid
		// reputation change flood.
		let unexpected_count = peer_data.receive_unexpected(&relay_parent);

		gum::debug!(
			target: LOG_TARGET,
			?relay_parent,
			?peer,
			?message,
			?rep,
			?unexpected_count,
			"Error inserting received statement"
		);

		match rep {
			// This happens when a Valid statement has been received but there is no corresponding Seconded
			COST_UNEXPECTED_STATEMENT_UNKNOWN_CANDIDATE => {
				metrics.on_unexpected_statement_valid();
				// Report peer merely if this is not a duplicate out-of-view statement that
				// was caused by a missing Seconded statement from this peer
				if unexpected_count == 0_usize {
					report_peer(ctx, peer, rep).await;
				}
			},
			// This happens when we have an unexpected remote peer that announced Seconded
			COST_UNEXPECTED_STATEMENT_REMOTE => {
				metrics.on_unexpected_statement_seconded();
				report_peer(ctx, peer, rep).await;
			},
			_ => {
				report_peer(ctx, peer, rep).await;
			},
		}

		return None
	}

	let checked_compact = {
		let (compact, validator_index) = message.get_fingerprint();
		let signature = message.get_signature();

		let unchecked_compact = UncheckedSignedStatement::new(compact, validator_index, signature);

		match active_head.check_useful_or_unknown(&unchecked_compact) {
			Ok(()) => {},
			Err(DeniedStatement::NotUseful) => return None,
			Err(DeniedStatement::UsefulButKnown) => {
				// Note a received statement in the peer data
				peer_data
					.receive(&relay_parent, &fingerprint, max_message_count)
					.expect("checked in `check_can_receive` above; qed");
				report_peer(ctx, peer, BENEFIT_VALID_STATEMENT).await;

				return None
			},
		}

		// check the signature on the statement.
		match check_statement_signature(&active_head, relay_parent, unchecked_compact) {
			Err(statement) => {
				gum::debug!(target: LOG_TARGET, ?peer, ?statement, "Invalid statement signature");
				report_peer(ctx, peer, COST_INVALID_SIGNATURE).await;
				return None
			},
			Ok(statement) => statement,
		}
	};

	// Fetch from the network only after signature and usefulness checks are completed.
	let is_large_statement = message.is_large_statement();
	let statement =
		retrieve_statement_from_message(peer, message, active_head, ctx, req_sender, metrics)
			.await?;

	let payload = statement.unchecked_into_payload();

	// Upgrade the `Signed` wrapper from the compact payload to the full payload.
	// This fails if the payload doesn't encode correctly.
	let statement: SignedFullStatement = match checked_compact.convert_to_superpayload(payload) {
		Err((compact, _)) => {
			gum::debug!(
				target: LOG_TARGET,
				?peer,
				?compact,
				is_large_statement,
				"Full statement had bad payload."
			);
			report_peer(ctx, peer, COST_WRONG_HASH).await;
			return None
		},
		Ok(statement) => statement,
	};

	// Ensure the statement is stored in the peer data.
	//
	// Note that if the peer is sending us something that is not within their view,
	// it will not be kept within their log.
	match peer_data.receive(&relay_parent, &fingerprint, max_message_count) {
		Err(_) => {
			unreachable!("checked in `check_can_receive` above; qed");
		},
		Ok(true) => {
			gum::trace!(target: LOG_TARGET, ?peer, ?statement, "Statement accepted");
			// Send the peer all statements concerning the candidate that we have,
			// since it appears to have just learned about the candidate.
			send_statements_about(
				peer.clone(),
				peer_data,
				ctx,
				relay_parent,
				candidate_hash,
				&*active_head,
				metrics,
			)
			.await;
		},
		Ok(false) => {},
	}

	// Note: `peer_data.receive` already ensures that the statement is not an unbounded equivocation
	// or unpinned to a seconded candidate. So it is safe to place it into the storage.
	match active_head.note_statement(statement) {
		NotedStatement::NotUseful | NotedStatement::UsefulButKnown => {
			unreachable!("checked in `is_useful_or_unknown` above; qed");
		},
		NotedStatement::Fresh(statement) => {
			report_peer(ctx, peer, BENEFIT_VALID_STATEMENT_FIRST).await;

			let mut _span = handle_incoming_span.child("notify-backing");

			// When we receive a new message from a peer, we forward it to the
			// candidate backing subsystem.
			ctx.send_message(CandidateBackingMessage::Statement(
				relay_parent,
				statement.statement.clone(),
			))
			.await;

			Some((relay_parent, statement))
		},
	}
}

/// Update a peer's view. Sends all newly unlocked statements based on the previous
async fn update_peer_view_and_maybe_send_unlocked(
	peer: PeerId,
	gossip_peers: &HashSet<PeerId>,
	peer_data: &mut PeerData,
	ctx: &mut (impl SubsystemContext + overseer::SubsystemContext),
	active_heads: &HashMap<Hash, ActiveHeadData>,
	new_view: View,
	metrics: &Metrics,
	rng: &mut impl rand::Rng,
) {
	let old_view = std::mem::replace(&mut peer_data.view, new_view);

	// Remove entries for all relay-parents in the old view but not the new.
	for removed in old_view.difference(&peer_data.view) {
		let _ = peer_data.view_knowledge.remove(removed);
	}

	let is_gossip_peer = gossip_peers.contains(&peer);
	let lucky = is_gossip_peer ||
		util::gen_ratio_rng(
			util::MIN_GOSSIP_PEERS.saturating_sub(gossip_peers.len()),
			util::MIN_GOSSIP_PEERS,
			rng,
		);

	// Add entries for all relay-parents in the new view but not the old.
	// Furthermore, send all statements we have for those relay parents.
	let new_view = peer_data.view.difference(&old_view).copied().collect::<Vec<_>>();
	for new in new_view.iter().copied() {
		peer_data.view_knowledge.insert(new, Default::default());
		if !lucky {
			continue
		}
		if let Some(active_head) = active_heads.get(&new) {
			send_statements(peer.clone(), peer_data, ctx, new, active_head, metrics).await;
		}
	}
}

async fn handle_network_update(
	peers: &mut HashMap<PeerId, PeerData>,
	gossip_peers: &mut HashSet<PeerId>,
	authorities: &mut HashMap<AuthorityDiscoveryId, PeerId>,
	active_heads: &mut HashMap<Hash, ActiveHeadData>,
	recent_outdated_heads: &RecentOutdatedHeads,
	ctx: &mut (impl SubsystemContext + overseer::SubsystemContext),
	req_sender: &mpsc::Sender<RequesterMessage>,
	update: NetworkBridgeEvent<protocol_v1::StatementDistributionMessage>,
	metrics: &Metrics,
	rng: &mut impl rand::Rng,
) {
	match update {
		NetworkBridgeEvent::PeerConnected(peer, role, maybe_authority) => {
			gum::trace!(target: LOG_TARGET, ?peer, ?role, "Peer connected");
			peers.insert(
				peer,
				PeerData {
					view: Default::default(),
					view_knowledge: Default::default(),
					maybe_authority: maybe_authority.clone(),
				},
			);
			if let Some(authority_ids) = maybe_authority {
				authority_ids.into_iter().for_each(|a| {
					authorities.insert(a, peer);
				});
			}
		},
		NetworkBridgeEvent::PeerDisconnected(peer) => {
			gum::trace!(target: LOG_TARGET, ?peer, "Peer disconnected");
			if let Some(auth_ids) = peers.remove(&peer).and_then(|p| p.maybe_authority) {
				auth_ids.into_iter().for_each(|a| {
					authorities.remove(&a);
				});
			}
		},
		NetworkBridgeEvent::NewGossipTopology(new_peers) => {
			let _ = metrics.time_network_bridge_update_v1("new_gossip_topology");
			let newly_added: Vec<PeerId> = new_peers.difference(gossip_peers).cloned().collect();
			*gossip_peers = new_peers;
			for peer in newly_added {
				if let Some(data) = peers.get_mut(&peer) {
					let view = std::mem::take(&mut data.view);
					update_peer_view_and_maybe_send_unlocked(
						peer,
						gossip_peers,
						data,
						ctx,
						&*active_heads,
						view,
						metrics,
						rng,
					)
					.await
				}
			}
		},
		NetworkBridgeEvent::PeerMessage(peer, message) => {
			handle_incoming_message_and_circulate(
				peer,
				gossip_peers,
				peers,
				active_heads,
				&*recent_outdated_heads,
				ctx,
				message,
				req_sender,
				metrics,
				rng,
			)
			.await;
		},
		NetworkBridgeEvent::PeerViewChange(peer, view) => {
			let _ = metrics.time_network_bridge_update_v1("peer_view_change");
			gum::trace!(target: LOG_TARGET, ?peer, ?view, "Peer view change");
			match peers.get_mut(&peer) {
				Some(data) =>
					update_peer_view_and_maybe_send_unlocked(
						peer,
						gossip_peers,
						data,
						ctx,
						&*active_heads,
						view,
						metrics,
						rng,
					)
					.await,
				None => (),
			}
		},
		NetworkBridgeEvent::OurViewChange(_view) => {
			// handled by `ActiveLeavesUpdate`
		},
	}
}

impl<R: rand::Rng> StatementDistributionSubsystem<R> {
	/// Create a new Statement Distribution Subsystem
	pub fn new(
		keystore: SyncCryptoStorePtr,
		req_receiver: IncomingRequestReceiver<request_v1::StatementFetchingRequest>,
		metrics: Metrics,
		rng: R,
	) -> Self {
		Self { keystore, req_receiver: Some(req_receiver), metrics, rng }
	}

	async fn run(
		mut self,
		mut ctx: (impl SubsystemContext<Message = StatementDistributionMessage>
		     + overseer::SubsystemContext<Message = StatementDistributionMessage>),
	) -> std::result::Result<(), FatalError> {
		let mut peers: HashMap<PeerId, PeerData> = HashMap::new();
		let mut gossip_peers: HashSet<PeerId> = HashSet::new();
		let mut authorities: HashMap<AuthorityDiscoveryId, PeerId> = HashMap::new();
		let mut active_heads: HashMap<Hash, ActiveHeadData> = HashMap::new();
		let mut recent_outdated_heads = RecentOutdatedHeads::default();

		let mut runtime = RuntimeInfo::new(Some(self.keystore.clone()));

		// Sender/Receiver for getting news from our statement fetching tasks.
		let (req_sender, mut req_receiver) = mpsc::channel(1);
		// Sender/Receiver for getting news from our responder task.
		let (res_sender, mut res_receiver) = mpsc::channel(1);

		ctx.spawn(
			"large-statement-responder",
			respond(
				self.req_receiver.take().expect("Mandatory argument to new. qed"),
				res_sender.clone(),
			)
			.boxed(),
		)
		.map_err(FatalError::SpawnTask)?;

		loop {
			let message =
				MuxedMessage::receive(&mut ctx, &mut req_receiver, &mut res_receiver).await;
			match message {
				MuxedMessage::Subsystem(result) => {
					let result = self
						.handle_subsystem_message(
							&mut ctx,
							&mut runtime,
							&mut peers,
							&mut gossip_peers,
							&mut authorities,
							&mut active_heads,
							&mut recent_outdated_heads,
							&req_sender,
							result?,
						)
						.await;
					match result.into_nested()? {
						Ok(true) => break,
						Ok(false) => {},
						Err(jfyi) => gum::debug!(target: LOG_TARGET, error = ?jfyi),
					}
				},
				MuxedMessage::Requester(result) => {
					let result = self
						.handle_requester_message(
							&mut ctx,
							&gossip_peers,
							&mut peers,
							&mut active_heads,
							&recent_outdated_heads,
							&req_sender,
							result.ok_or(FatalError::RequesterReceiverFinished)?,
						)
						.await;
					log_error(result.map_err(From::from), "handle_requester_message")?;
				},
				MuxedMessage::Responder(result) => {
					let result = self
						.handle_responder_message(
							&peers,
							&mut active_heads,
							result.ok_or(FatalError::ResponderReceiverFinished)?,
						)
						.await;
					log_error(result.map_err(From::from), "handle_responder_message")?;
				},
			};
		}
		Ok(())
	}

	/// Handle messages from responder background task.
	async fn handle_responder_message(
		&self,
		peers: &HashMap<PeerId, PeerData>,
		active_heads: &mut HashMap<Hash, ActiveHeadData>,
		message: ResponderMessage,
	) -> JfyiErrorResult<()> {
		match message {
			ResponderMessage::GetData { requesting_peer, relay_parent, candidate_hash, tx } => {
				if !requesting_peer_knows_about_candidate(
					peers,
					&requesting_peer,
					&relay_parent,
					&candidate_hash,
				)? {
					return Err(JfyiError::RequestedUnannouncedCandidate(
						requesting_peer,
						candidate_hash,
					))
				}

				let active_head =
					active_heads.get(&relay_parent).ok_or(JfyiError::NoSuchHead(relay_parent))?;

				let committed = match active_head.waiting_large_statements.get(&candidate_hash) {
					Some(LargeStatementStatus::FetchedOrShared(committed)) => committed.clone(),
					_ =>
						return Err(JfyiError::NoSuchFetchedLargeStatement(
							relay_parent,
							candidate_hash,
						)),
				};

				tx.send(committed).map_err(|_| JfyiError::ResponderGetDataCanceled)?;
			},
		}
		Ok(())
	}

	async fn handle_requester_message(
		&mut self,
		ctx: &mut impl SubsystemContext,
		gossip_peers: &HashSet<PeerId>,
		peers: &mut HashMap<PeerId, PeerData>,
		active_heads: &mut HashMap<Hash, ActiveHeadData>,
		recent_outdated_heads: &RecentOutdatedHeads,
		req_sender: &mpsc::Sender<RequesterMessage>,
		message: RequesterMessage,
	) -> JfyiErrorResult<()> {
		match message {
			RequesterMessage::Finished {
				relay_parent,
				candidate_hash,
				from_peer,
				response,
				bad_peers,
			} => {
				for bad in bad_peers {
					report_peer(ctx, bad, COST_FETCH_FAIL).await;
				}
				report_peer(ctx, from_peer, BENEFIT_VALID_RESPONSE).await;

				let active_head = active_heads
					.get_mut(&relay_parent)
					.ok_or(JfyiError::NoSuchHead(relay_parent))?;

				let status = active_head.waiting_large_statements.remove(&candidate_hash);

				let info = match status {
					Some(LargeStatementStatus::Fetching(info)) => info,
					Some(LargeStatementStatus::FetchedOrShared(_)) => {
						// We are no longer interested in the data.
						return Ok(())
					},
					None =>
						return Err(JfyiError::NoSuchLargeStatementStatus(
							relay_parent,
							candidate_hash,
						)),
				};

				active_head
					.waiting_large_statements
					.insert(candidate_hash, LargeStatementStatus::FetchedOrShared(response));

				// Cache is now populated, send all messages:
				for (peer, messages) in info.available_peers {
					for message in messages {
						handle_incoming_message_and_circulate(
							peer,
							gossip_peers,
							peers,
							active_heads,
							recent_outdated_heads,
							ctx,
							message,
							req_sender,
							&self.metrics,
							&mut self.rng,
						)
						.await;
					}
				}
			},
			RequesterMessage::SendRequest(req) => {
				ctx.send_message(AllMessages::NetworkBridge(NetworkBridgeMessage::SendRequests(
					vec![req],
					IfDisconnected::ImmediateError,
				)))
				.await;
			},
			RequesterMessage::GetMorePeers { relay_parent, candidate_hash, tx } => {
				let active_head = active_heads
					.get_mut(&relay_parent)
					.ok_or(JfyiError::NoSuchHead(relay_parent))?;

				let status = active_head.waiting_large_statements.get_mut(&candidate_hash);

				let info = match status {
					Some(LargeStatementStatus::Fetching(info)) => info,
					Some(LargeStatementStatus::FetchedOrShared(_)) => {
						// This task is going to die soon - no need to send it anything.
						gum::debug!(target: LOG_TARGET, "Zombie task wanted more peers.");
						return Ok(())
					},
					None =>
						return Err(JfyiError::NoSuchLargeStatementStatus(
							relay_parent,
							candidate_hash,
						)),
				};

				if info.peers_to_try.is_empty() {
					info.peer_sender = Some(tx);
				} else {
					let peers_to_try = std::mem::take(&mut info.peers_to_try);
					if let Err(peers) = tx.send(peers_to_try) {
						// No longer interested for now - might want them later:
						info.peers_to_try = peers;
					}
				}
			},
			RequesterMessage::ReportPeer(peer, rep) => report_peer(ctx, peer, rep).await,
		}
		Ok(())
	}

	async fn handle_subsystem_message(
		&mut self,
		ctx: &mut (impl SubsystemContext + overseer::SubsystemContext),
		runtime: &mut RuntimeInfo,
		peers: &mut HashMap<PeerId, PeerData>,
		gossip_peers: &mut HashSet<PeerId>,
		authorities: &mut HashMap<AuthorityDiscoveryId, PeerId>,
		active_heads: &mut HashMap<Hash, ActiveHeadData>,
		recent_outdated_heads: &mut RecentOutdatedHeads,
		req_sender: &mpsc::Sender<RequesterMessage>,
		message: FromOverseer<StatementDistributionMessage>,
	) -> Result<bool> {
		let metrics = &self.metrics;

		match message {
			FromOverseer::Signal(OverseerSignal::ActiveLeaves(ActiveLeavesUpdate {
				activated,
				deactivated,
			})) => {
				let _timer = metrics.time_active_leaves_update();

				for deactivated in deactivated {
					if active_heads.remove(&deactivated).is_some() {
						gum::trace!(
							target: LOG_TARGET,
							hash = ?deactivated,
							"Deactivating leaf",
						);

						recent_outdated_heads.note_outdated(deactivated);
					}
				}

				for activated in activated {
					let relay_parent = activated.hash;
					let span = PerLeafSpan::new(activated.span, "statement-distribution");
					gum::trace!(
						target: LOG_TARGET,
						hash = ?relay_parent,
						"New active leaf",
					);

					// Retrieve the parachain validators at the child of the head we track.
					let session_index =
						runtime.get_session_index_for_child(ctx.sender(), relay_parent).await?;
					let info = runtime
						.get_session_info_by_index(ctx.sender(), relay_parent, session_index)
						.await?;
					let session_info = &info.session_info;

					active_heads.entry(relay_parent).or_insert(ActiveHeadData::new(
						session_info.validators.clone(),
						session_index,
						span,
					));
				}
			},
			FromOverseer::Signal(OverseerSignal::BlockFinalized(..)) => {
				// do nothing
			},
			FromOverseer::Signal(OverseerSignal::Conclude) => return Ok(true),
			FromOverseer::Communication { msg } => match msg {
				StatementDistributionMessage::Share(relay_parent, statement) => {
					let _timer = metrics.time_share();

					// Make sure we have data in cache:
					if is_statement_large(&statement).0 {
						if let Statement::Seconded(committed) = &statement.payload() {
							let active_head = active_heads
								.get_mut(&relay_parent)
								// This should never be out-of-sync with our view if the view
								// updates correspond to actual `StartWork` messages.
								.ok_or(JfyiError::NoSuchHead(relay_parent))?;
							active_head.waiting_large_statements.insert(
								statement.payload().candidate_hash(),
								LargeStatementStatus::FetchedOrShared(committed.clone()),
							);
						}
					}

					let info = runtime.get_session_info(ctx.sender(), relay_parent).await?;
					let session_info = &info.session_info;
					let validator_info = &info.validator_info;

					// Get peers in our group, so we can make sure they get our statement
					// directly:
					let group_peers = {
						if let Some(our_group) = validator_info.our_group {
							let our_group = &session_info.validator_groups[our_group.0 as usize];

							our_group
								.into_iter()
								.filter_map(|i| {
									if Some(*i) == validator_info.our_index {
										return None
									}
									let authority_id = &session_info.discovery_keys[i.0 as usize];
									authorities.get(authority_id).map(|p| *p)
								})
								.collect()
						} else {
							Vec::new()
						}
					};
					circulate_statement_and_dependents(
						gossip_peers,
						peers,
						active_heads,
						ctx,
						relay_parent,
						statement,
						group_peers,
						metrics,
						&mut self.rng,
					)
					.await;
				},
				StatementDistributionMessage::NetworkBridgeUpdateV1(event) => {
					handle_network_update(
						peers,
						gossip_peers,
						authorities,
						active_heads,
						&*recent_outdated_heads,
						ctx,
						req_sender,
						event,
						metrics,
						&mut self.rng,
					)
					.await;
				},
			},
		}
		Ok(false)
	}
}

/// Check whether a peer knows about a candidate from us.
///
/// If not, it is deemed illegal for it to request corresponding data from us.
fn requesting_peer_knows_about_candidate(
	peers: &HashMap<PeerId, PeerData>,
	requesting_peer: &PeerId,
	relay_parent: &Hash,
	candidate_hash: &CandidateHash,
) -> JfyiErrorResult<bool> {
	let peer_data = peers
		.get(requesting_peer)
		.ok_or_else(|| JfyiError::NoSuchPeer(*requesting_peer))?;
	let knowledge = peer_data
		.view_knowledge
		.get(relay_parent)
		.ok_or_else(|| JfyiError::NoSuchHead(*relay_parent))?;
	Ok(knowledge.sent_candidates.get(&candidate_hash).is_some())
}