pezkuwi-sdk/pezbridges/relays/messages/src/message_race_loop.rs

// Copyright 2019-2021 Parity Technologies (UK) Ltd.
// This file is part of Parity Bridges Common.

// Parity Bridges Common 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.

// Parity Bridges Common 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.

//! Loop that is serving single race within message lane. This could be
//! message delivery race, receiving confirmations race or processing
//! confirmations race.
//!
//! The idea of the race is simple - we have `nonce`-s on source and target
//! nodes. We're trying to prove that the source node has this nonce (and
//! associated data - like messages, lane state, etc) to the target node by
//! generating and submitting proof.

use crate::message_lane_loop::{BatchTransaction, ClientState, NoncesSubmitArtifacts};

use async_trait::async_trait;
use futures::{
	future::{FutureExt, TryFutureExt},
	stream::{FusedStream, StreamExt},
};
use pezbp_messages::MessageNonce;
use relay_utils::{
	process_future_result, retry_backoff, FailedClient, MaybeConnectionError,
	TrackedTransactionStatus, TransactionTracker,
};
use std::{
	fmt::Debug,
	ops::RangeInclusive,
	time::{Duration, Instant},
};

/// One of races within lane.
pub trait MessageRace {
	/// Header id of the race source.
	type SourceHeaderId: Debug + Clone + PartialEq + Send + Sync;
	/// Header id of the race source.
	type TargetHeaderId: Debug + Clone + PartialEq + Send + Sync;

	/// Message nonce used in the race.
	type MessageNonce: Debug + Clone;
	/// Proof that is generated and delivered in this race.
	type Proof: Debug + Clone + Send + Sync;

	/// Name of the race source.
	fn source_name() -> String;
	/// Name of the race target.
	fn target_name() -> String;
}

/// State of race source client.
type SourceClientState<P> =
	ClientState<<P as MessageRace>::SourceHeaderId, <P as MessageRace>::TargetHeaderId>;

/// State of race target client.
type TargetClientState<P> =
	ClientState<<P as MessageRace>::TargetHeaderId, <P as MessageRace>::SourceHeaderId>;

/// Inclusive nonces range.
pub trait NoncesRange: Debug + Sized {
	/// Get begin of the range.
	fn begin(&self) -> MessageNonce;
	/// Get end of the range.
	fn end(&self) -> MessageNonce;
	/// Returns new range with current range nonces that are greater than the passed `nonce`.
	/// If there are no such nonces, `None` is returned.
	fn greater_than(self, nonce: MessageNonce) -> Option<Self>;
}

/// Nonces on the race source client.
#[derive(Debug, Clone)]
pub struct SourceClientNonces<NoncesRange> {
	/// New nonces range known to the client. `New` here means all nonces generated after
	/// `prev_latest_nonce` passed to the `SourceClient::nonces` method.
	pub new_nonces: NoncesRange,
	/// The latest nonce that is confirmed to the bridged client. This nonce only makes
	/// sense in some races. In other races it is `None`.
	pub confirmed_nonce: Option<MessageNonce>,
}

/// Nonces on the race target client.
#[derive(Debug, Clone)]
pub struct TargetClientNonces<TargetNoncesData> {
	/// The latest nonce that is known to the target client.
	pub latest_nonce: MessageNonce,
	/// Additional data from target node that may be used by the race.
	pub nonces_data: TargetNoncesData,
}

/// One of message lane clients, which is source client for the race.
#[async_trait]
pub trait SourceClient<P: MessageRace> {
	/// Type of error these clients returns.
	type Error: std::fmt::Debug + MaybeConnectionError;
	/// Type of nonces range returned by the source client.
	type NoncesRange: NoncesRange;
	/// Additional proof parameters required to generate proof.
	type ProofParameters;

	/// Return nonces that are known to the source client.
	async fn nonces(
		&self,
		at_block: P::SourceHeaderId,
		prev_latest_nonce: MessageNonce,
	) -> Result<(P::SourceHeaderId, SourceClientNonces<Self::NoncesRange>), Self::Error>;
	/// Generate proof for delivering to the target client.
	async fn generate_proof(
		&self,
		at_block: P::SourceHeaderId,
		nonces: RangeInclusive<MessageNonce>,
		proof_parameters: Self::ProofParameters,
	) -> Result<(P::SourceHeaderId, RangeInclusive<MessageNonce>, P::Proof), Self::Error>;
}

/// One of message lane clients, which is target client for the race.
#[async_trait]
pub trait TargetClient<P: MessageRace> {
	/// Type of error these clients returns.
	type Error: std::fmt::Debug + MaybeConnectionError;
	/// Type of the additional data from the target client, used by the race.
	type TargetNoncesData: std::fmt::Debug;
	/// Type of batch transaction that submits finality and proof to the target node.
	type BatchTransaction: BatchTransaction<P::SourceHeaderId> + Clone;
	/// Transaction tracker to track submitted transactions.
	type TransactionTracker: TransactionTracker<HeaderId = P::TargetHeaderId>;

	/// Ask headers relay to relay finalized headers up to (and including) given header
	/// from race source to race target.
	///
	/// The client may return `Some(_)`, which means that nothing has happened yet and
	/// the caller must generate and append proof to the batch transaction
	/// to actually send it (along with required header) to the node.
	///
	/// If function has returned `None`, it means that the caller now must wait for the
	/// appearance of the required header `id` at the target client.
	async fn require_source_header(
		&self,
		id: P::SourceHeaderId,
	) -> Result<Option<Self::BatchTransaction>, Self::Error>;

	/// Return nonces that are known to the target client.
	async fn nonces(
		&self,
		at_block: P::TargetHeaderId,
		update_metrics: bool,
	) -> Result<(P::TargetHeaderId, TargetClientNonces<Self::TargetNoncesData>), Self::Error>;
	/// Submit proof to the target client.
	async fn submit_proof(
		&self,
		maybe_batch_tx: Option<Self::BatchTransaction>,
		generated_at_block: P::SourceHeaderId,
		nonces: RangeInclusive<MessageNonce>,
		proof: P::Proof,
	) -> Result<NoncesSubmitArtifacts<Self::TransactionTracker>, Self::Error>;
}

/// Race strategy.
#[async_trait]
pub trait RaceStrategy<SourceHeaderId, TargetHeaderId, Proof>: Debug {
	/// Type of nonces range expected from the source client.
	type SourceNoncesRange: NoncesRange;
	/// Additional proof parameters required to generate proof.
	type ProofParameters;
	/// Additional data expected from the target client.
	type TargetNoncesData;

	/// Return id of source header that is required to be on target to continue synchronization.
	async fn required_source_header_at_target<RS: RaceState<SourceHeaderId, TargetHeaderId>>(
		&self,
		race_state: RS,
	) -> Option<SourceHeaderId>;
	/// Return the best nonce at source node.
	///
	/// `Some` is returned only if we are sure that the value is greater or equal
	/// than the result of `best_at_target`.
	fn best_at_source(&self) -> Option<MessageNonce>;
	/// Return the best nonce at target node.
	///
	/// May return `None` if value is yet unknown.
	fn best_at_target(&self) -> Option<MessageNonce>;

	/// Called when nonces are updated at source node of the race.
	fn source_nonces_updated(
		&mut self,
		at_block: SourceHeaderId,
		nonces: SourceClientNonces<Self::SourceNoncesRange>,
	);
	/// Called when we want to wait until next `best_target_nonces_updated` before selecting
	/// any nonces for delivery.
	fn reset_best_target_nonces(&mut self);
	/// Called when best nonces are updated at target node of the race.
	fn best_target_nonces_updated<RS: RaceState<SourceHeaderId, TargetHeaderId>>(
		&mut self,
		nonces: TargetClientNonces<Self::TargetNoncesData>,
		race_state: &mut RS,
	);
	/// Called when finalized nonces are updated at target node of the race.
	fn finalized_target_nonces_updated<RS: RaceState<SourceHeaderId, TargetHeaderId>>(
		&mut self,
		nonces: TargetClientNonces<Self::TargetNoncesData>,
		race_state: &mut RS,
	);
	/// Should return `Some(nonces)` if we need to deliver proof of `nonces` (and associated
	/// data) from source to target node.
	/// Additionally, parameters required to generate proof are returned.
	async fn select_nonces_to_deliver<RS: RaceState<SourceHeaderId, TargetHeaderId>>(
		&self,
		race_state: RS,
	) -> Option<(RangeInclusive<MessageNonce>, Self::ProofParameters)>;
}

/// State of the race.
pub trait RaceState<SourceHeaderId, TargetHeaderId>: Clone + Send + Sync {
	/// Set best finalized source header id at the best block on the target
	/// client (at the `best_finalized_source_header_id_at_best_target`).
	fn set_best_finalized_source_header_id_at_best_target(&mut self, id: SourceHeaderId);

	/// Best finalized source header id at the best block on the target
	/// client (at the `best_finalized_source_header_id_at_best_target`).
	fn best_finalized_source_header_id_at_best_target(&self) -> Option<SourceHeaderId>;

	/// Returns `true` if we have selected nonces to submit to the target node.
	fn nonces_to_submit(&self) -> Option<RangeInclusive<MessageNonce>>;
	/// Reset our nonces selection.
	fn reset_nonces_to_submit(&mut self);

	/// Returns `true` if we have submitted some nonces to the target node and are
	/// waiting for them to appear there.
	fn nonces_submitted(&self) -> Option<RangeInclusive<MessageNonce>>;
	/// Reset our nonces submission.
	fn reset_nonces_submitted(&mut self);
}

/// State of the race and prepared batch transaction (if available).
#[derive(Debug, Clone)]
pub(crate) struct RaceStateImpl<SourceHeaderId, TargetHeaderId, Proof, BatchTx> {
	/// Best finalized source header id at the source client.
	pub best_finalized_source_header_id_at_source: Option<SourceHeaderId>,
	/// Best finalized source header id at the best block on the target
	/// client (at the `best_finalized_source_header_id_at_best_target`).
	pub best_finalized_source_header_id_at_best_target: Option<SourceHeaderId>,
	/// The best header id at the target client.
	pub best_target_header_id: Option<TargetHeaderId>,
	/// Best finalized header id at the target client.
	pub best_finalized_target_header_id: Option<TargetHeaderId>,
	/// Range of nonces that we have selected to submit.
	pub nonces_to_submit: Option<(SourceHeaderId, RangeInclusive<MessageNonce>, Proof)>,
	/// Batch transaction ready to include and deliver selected `nonces_to_submit` from the
	/// `state`.
	pub nonces_to_submit_batch: Option<BatchTx>,
	/// Range of nonces that is currently submitted.
	pub nonces_submitted: Option<RangeInclusive<MessageNonce>>,
}

impl<SourceHeaderId, TargetHeaderId, Proof, BatchTx> Default
	for RaceStateImpl<SourceHeaderId, TargetHeaderId, Proof, BatchTx>
{
	fn default() -> Self {
		RaceStateImpl {
			best_finalized_source_header_id_at_source: None,
			best_finalized_source_header_id_at_best_target: None,
			best_target_header_id: None,
			best_finalized_target_header_id: None,
			nonces_to_submit: None,
			nonces_to_submit_batch: None,
			nonces_submitted: None,
		}
	}
}

impl<SourceHeaderId, TargetHeaderId, Proof, BatchTx> RaceState<SourceHeaderId, TargetHeaderId>
	for RaceStateImpl<SourceHeaderId, TargetHeaderId, Proof, BatchTx>
where
	SourceHeaderId: Clone + Send + Sync,
	TargetHeaderId: Clone + Send + Sync,
	Proof: Clone + Send + Sync,
	BatchTx: Clone + Send + Sync,
{
	fn set_best_finalized_source_header_id_at_best_target(&mut self, id: SourceHeaderId) {
		self.best_finalized_source_header_id_at_best_target = Some(id);
	}

	fn best_finalized_source_header_id_at_best_target(&self) -> Option<SourceHeaderId> {
		self.best_finalized_source_header_id_at_best_target.clone()
	}

	fn nonces_to_submit(&self) -> Option<RangeInclusive<MessageNonce>> {
		self.nonces_to_submit.clone().map(|(_, nonces, _)| nonces)
	}

	fn reset_nonces_to_submit(&mut self) {
		self.nonces_to_submit = None;
		self.nonces_to_submit_batch = None;
	}

	fn nonces_submitted(&self) -> Option<RangeInclusive<MessageNonce>> {
		self.nonces_submitted.clone()
	}

	fn reset_nonces_submitted(&mut self) {
		self.nonces_submitted = None;
	}
}

/// Run race loop until connection with target or source node is lost.
pub async fn run<P: MessageRace, SC: SourceClient<P>, TC: TargetClient<P>>(
	race_source: SC,
	race_source_updated: impl FusedStream<Item = SourceClientState<P>>,
	race_target: TC,
	race_target_updated: impl FusedStream<Item = TargetClientState<P>>,
	mut strategy: impl RaceStrategy<
		P::SourceHeaderId,
		P::TargetHeaderId,
		P::Proof,
		SourceNoncesRange = SC::NoncesRange,
		ProofParameters = SC::ProofParameters,
		TargetNoncesData = TC::TargetNoncesData,
	>,
) -> Result<(), FailedClient> {
	let mut progress_context = Instant::now();
	let mut race_state = RaceStateImpl::default();

	let mut source_retry_backoff = retry_backoff();
	let mut source_client_is_online = true;
	let mut source_nonces_required = false;
	let mut source_required_header = None;
	let source_nonces = futures::future::Fuse::terminated();
	let source_generate_proof = futures::future::Fuse::terminated();
	let source_go_offline_future = futures::future::Fuse::terminated();

	let mut target_retry_backoff = retry_backoff();
	let mut target_client_is_online = true;
	let mut target_best_nonces_required = false;
	let mut target_finalized_nonces_required = false;
	let mut target_batch_transaction = None;
	let target_require_source_header = futures::future::Fuse::terminated();
	let target_best_nonces = futures::future::Fuse::terminated();
	let target_finalized_nonces = futures::future::Fuse::terminated();
	let target_submit_proof = futures::future::Fuse::terminated();
	let target_tx_tracker = futures::future::Fuse::terminated();
	let target_go_offline_future = futures::future::Fuse::terminated();

	futures::pin_mut!(
		race_source_updated,
		source_nonces,
		source_generate_proof,
		source_go_offline_future,
		race_target_updated,
		target_require_source_header,
		target_best_nonces,
		target_finalized_nonces,
		target_submit_proof,
		target_tx_tracker,
		target_go_offline_future,
	);

	loop {
		futures::select! {
			// when headers ids are updated
			source_state = race_source_updated.next() => {
				if let Some(source_state) = source_state {
					let is_source_state_updated = race_state.best_finalized_source_header_id_at_source.as_ref()
						!= Some(&source_state.best_finalized_self);
					if is_source_state_updated {
						source_nonces_required = true;
						race_state.best_finalized_source_header_id_at_source
							= Some(source_state.best_finalized_self);
					}
				}
			},
			target_state = race_target_updated.next() => {
				if let Some(target_state) = target_state {
					let is_target_best_state_updated = race_state.best_target_header_id.as_ref()
						!= Some(&target_state.best_self);

					if is_target_best_state_updated {
						target_best_nonces_required = true;
						race_state.best_target_header_id = Some(target_state.best_self);
						race_state.best_finalized_source_header_id_at_best_target
							= target_state.best_finalized_peer_at_best_self;
					}

					let is_target_finalized_state_updated = race_state.best_finalized_target_header_id.as_ref()
						!= Some(&target_state.best_finalized_self);
					if  is_target_finalized_state_updated {
						target_finalized_nonces_required = true;
						race_state.best_finalized_target_header_id = Some(target_state.best_finalized_self);
					}
				}
			},

			// when nonces are updated
			nonces = source_nonces => {
				source_nonces_required = false;

				source_client_is_online = process_future_result(
					nonces,
					&mut source_retry_backoff,
					|(at_block, nonces)| {
						tracing::debug!(
							target: "bridge",
							source=%P::source_name(),
							?nonces,
							"Received nonces"
						);

						strategy.source_nonces_updated(at_block, nonces);
					},
					&mut source_go_offline_future,
					async_std::task::sleep,
					|| format!("Error retrieving nonces from {}", P::source_name()),
				).fail_if_connection_error(FailedClient::Source)?;

				// ask for more headers if we have nonces to deliver and required headers are missing
				source_required_header = strategy
					.required_source_header_at_target(race_state.clone())
					.await;
			},
			nonces = target_best_nonces => {
				target_best_nonces_required = false;

				target_client_is_online = process_future_result(
					nonces,
					&mut target_retry_backoff,
					|(_, nonces)| {
						tracing::debug!(
							target: "bridge",
							target=%P::target_name(),
							?nonces,
							"Received best nonces"
						);

						strategy.best_target_nonces_updated(nonces, &mut race_state);
					},
					&mut target_go_offline_future,
					async_std::task::sleep,
					|| format!("Error retrieving best nonces from {}", P::target_name()),
				).fail_if_connection_error(FailedClient::Target)?;
			},
			nonces = target_finalized_nonces => {
				target_finalized_nonces_required = false;

				target_client_is_online = process_future_result(
					nonces,
					&mut target_retry_backoff,
					|(_, nonces)| {
						tracing::debug!(
							target: "bridge",
							target=%P::target_name(),
							?nonces,
							"Received finalized nonces"
						);

						strategy.finalized_target_nonces_updated(nonces, &mut race_state);
					},
					&mut target_go_offline_future,
					async_std::task::sleep,
					|| format!("Error retrieving finalized nonces from {}", P::target_name()),
				).fail_if_connection_error(FailedClient::Target)?;
			},

			// proof generation and submission
			maybe_batch_transaction = target_require_source_header => {
				source_required_header = None;

				target_client_is_online = process_future_result(
					maybe_batch_transaction,
					&mut target_retry_backoff,
					|maybe_batch_transaction: Option<TC::BatchTransaction>| {
						tracing::debug!(
							target: "bridge",
							target=%P::target_name(),
							source=%P::source_name(),
							batch_tx=%maybe_batch_transaction
								.as_ref()
								.map(|bt| format!("yes ({:?})", bt.required_header_id()))
								.unwrap_or_else(|| "no".into()),
							"Target client has been asked for more headers."
						);

						target_batch_transaction = maybe_batch_transaction;
					},
					&mut target_go_offline_future,
					async_std::task::sleep,
					|| format!("Error asking for source headers at {}", P::target_name()),
				).fail_if_connection_error(FailedClient::Target)?;
			},
			proof = source_generate_proof => {
				source_client_is_online = process_future_result(
					proof,
					&mut source_retry_backoff,
					|(at_block, nonces_range, proof, batch_transaction)| {
						tracing::debug!(
							target: "bridge",
							source=%P::source_name(),
							?nonces_range,
							"Received proof"
						);

						race_state.nonces_to_submit = Some((at_block, nonces_range, proof));
						race_state.nonces_to_submit_batch = batch_transaction;
					},
					&mut source_go_offline_future,
					async_std::task::sleep,
					|| format!("Error generating proof at {}", P::source_name()),
				).fail_if_error(FailedClient::Source).map(|_| true)?;
			},
			proof_submit_result = target_submit_proof => {
				target_client_is_online = process_future_result(
					proof_submit_result,
					&mut target_retry_backoff,
					|artifacts: NoncesSubmitArtifacts<TC::TransactionTracker>| {
						tracing::debug!(
							target: "bridge",
							target=%P::target_name(),
							nonces=?artifacts.nonces,
							"Successfully submitted proof"
						);

						race_state.nonces_submitted = Some(artifacts.nonces);
						target_tx_tracker.set(artifacts.tx_tracker.wait().fuse());
					},
					&mut target_go_offline_future,
					async_std::task::sleep,
					|| format!("Error submitting proof {}", P::target_name()),
				).fail_if_connection_error(FailedClient::Target)?;

				// in any case - we don't need to retry submitting the same nonces again until
				// we read nonces from the target client
				race_state.reset_nonces_to_submit();
				// if we have failed to submit transaction AND that is not the connection issue,
				// then we need to read best target nonces before selecting nonces again
				if !target_client_is_online {
					strategy.reset_best_target_nonces();
				}
			},
			target_transaction_status = target_tx_tracker => {
				match (target_transaction_status, race_state.nonces_submitted.as_ref()) {
					(TrackedTransactionStatus::Finalized(at_block), Some(nonces_submitted)) => {
						// our transaction has been mined, but was it successful or not? let's check the best
						// nonce at the target node.
						let _ = race_target.nonces(at_block, false)
							.await
							.map_err(|e| format!("failed to read nonces from target node: {e:?}"))
							.and_then(|(_, nonces_at_target)| {
								if nonces_at_target.latest_nonce < *nonces_submitted.end() {
									Err(format!(
										"best nonce at target after tx is {:?} and we've submitted {:?}",
										nonces_at_target.latest_nonce,
										nonces_submitted.end(),
									))
								} else {
									Ok(())
								}
							})
							.map_err(|e| {
								tracing::error!(
									target: "bridge",
									error=%e,
									source=%P::source_name(),
									target=%P::target_name(),
									"Source -> target race transaction failed"
								);

								race_state.reset_nonces_submitted();
							});
					},
					(TrackedTransactionStatus::Lost, _) => {
						tracing::warn!(
							target: "bridge",
							source=%P::source_name(),
							target=%P::target_name(),
							state=?race_state,
							?strategy,
							"Source -> target race transaction has been lost."
						);

						race_state.reset_nonces_submitted();
					},
					_ => (),
				}
			},

			// when we're ready to retry request
			_ = source_go_offline_future => {
				source_client_is_online = true;
			},
			_ = target_go_offline_future => {
				target_client_is_online = true;
			},
		}

		progress_context = print_race_progress::<P, _>(progress_context, &strategy);

		if source_client_is_online {
			source_client_is_online = false;

			// if we've started to submit batch transaction, let's prioritize it
			//
			// we're using `take` here, because we don't need batch transaction (i.e. some
			// underlying finality proof) anymore for our future calls - we were unable to
			// use it for our current state, so why would we need to keep an obsolete proof
			// for the future?
			let target_batch_transaction = target_batch_transaction.take();
			let expected_race_state =
				if let Some(ref target_batch_transaction) = target_batch_transaction {
					// when selecting nonces for the batch transaction, we assume that the required
					// source header is already at the target chain
					let required_source_header_at_target =
						target_batch_transaction.required_header_id();
					let mut expected_race_state = race_state.clone();
					expected_race_state.best_finalized_source_header_id_at_best_target =
						Some(required_source_header_at_target);
					expected_race_state
				} else {
					race_state.clone()
				};

			let nonces_to_deliver = select_nonces_to_deliver(expected_race_state, &strategy).await;
			let best_at_source = strategy.best_at_source();

			if let Some((at_block, nonces_range, proof_parameters)) = nonces_to_deliver {
				tracing::debug!(
					target: "bridge",
					source=P::source_name(),
					?nonces_range,
					?at_block,
					"Asking to prove"
				);

				source_generate_proof.set(
					race_source
						.generate_proof(at_block, nonces_range, proof_parameters)
						.and_then(|(at_source_block, nonces, proof)| async {
							Ok((at_source_block, nonces, proof, target_batch_transaction))
						})
						.fuse(),
				);
			} else if let (true, Some(best_at_source)) = (source_nonces_required, best_at_source) {
				tracing::debug!(target: "bridge", source=%P::source_name(), "Asking about message nonces");
				let at_block = race_state
					.best_finalized_source_header_id_at_source
					.as_ref()
					.expect(
						"source_nonces_required is only true when \
						best_finalized_source_header_id_at_source is Some; qed",
					)
					.clone();
				source_nonces.set(race_source.nonces(at_block, best_at_source).fuse());
			} else {
				source_client_is_online = true;
			}
		}

		if target_client_is_online {
			target_client_is_online = false;

			if let Some((at_block, nonces_range, proof)) = race_state.nonces_to_submit.as_ref() {
				tracing::debug!(
					target: "bridge",
					target=%P::target_name(),
					?nonces_range,
					"Going to submit proof of messages in range to node{}",
					race_state.nonces_to_submit_batch.as_ref().map(|tx| format!(
						". This transaction is batched with sending the proof for header {:?}.",
						tx.required_header_id())
					).unwrap_or_default(),
				);

				target_submit_proof.set(
					race_target
						.submit_proof(
							race_state.nonces_to_submit_batch.clone(),
							at_block.clone(),
							nonces_range.clone(),
							proof.clone(),
						)
						.fuse(),
				);
			} else if let Some(source_required_header) = source_required_header.clone() {
				tracing::debug!(
					target: "bridge",
					source=%P::source_name(),
					target=%P::target_name(),
					?source_required_header,
					"Going to require header"
				);
				target_require_source_header
					.set(race_target.require_source_header(source_required_header).fuse());
			} else if target_best_nonces_required {
				tracing::debug!(target: "bridge", target=%P::target_name(), "Asking about best message nonces");
				let at_block = race_state
					.best_target_header_id
					.as_ref()
					.expect("target_best_nonces_required is only true when best_target_header_id is Some; qed")
					.clone();
				target_best_nonces.set(race_target.nonces(at_block, false).fuse());
			} else if target_finalized_nonces_required {
				tracing::debug!(target: "bridge", target=%P::target_name(), "Asking about finalized message nonces");
				let at_block = race_state
					.best_finalized_target_header_id
					.as_ref()
					.expect(
						"target_finalized_nonces_required is only true when \
						best_finalized_target_header_id is Some; qed",
					)
					.clone();
				target_finalized_nonces.set(race_target.nonces(at_block, true).fuse());
			} else {
				target_client_is_online = true;
			}
		}
	}
}

/// Print race progress.
fn print_race_progress<P, S>(prev_time: Instant, strategy: &S) -> Instant
where
	P: MessageRace,
	S: RaceStrategy<P::SourceHeaderId, P::TargetHeaderId, P::Proof>,
{
	let now_time = Instant::now();

	let need_update = now_time.saturating_duration_since(prev_time) > Duration::from_secs(10);
	if !need_update {
		return prev_time;
	}

	let now_best_nonce_at_source = strategy.best_at_source();
	let now_best_nonce_at_target = strategy.best_at_target();
	tracing::info!(
		target: "bridge",
		source=%P::source_name(),
		target=%P::target_name(),
		?now_best_nonce_at_target,
		?now_best_nonce_at_source,
		"Synced nonces in source -> target race"

	);
	now_time
}

async fn select_nonces_to_deliver<SourceHeaderId, TargetHeaderId, Proof, Strategy>(
	race_state: impl RaceState<SourceHeaderId, TargetHeaderId>,
	strategy: &Strategy,
) -> Option<(SourceHeaderId, RangeInclusive<MessageNonce>, Strategy::ProofParameters)>
where
	SourceHeaderId: Clone,
	Strategy: RaceStrategy<SourceHeaderId, TargetHeaderId, Proof>,
{
	let best_finalized_source_header_id_at_best_target =
		race_state.best_finalized_source_header_id_at_best_target()?;
	strategy
		.select_nonces_to_deliver(race_state)
		.await
		.map(|(nonces_range, proof_parameters)| {
			(best_finalized_source_header_id_at_best_target, nonces_range, proof_parameters)
		})
}

#[cfg(test)]
mod tests {
	use super::*;
	use crate::message_race_strategy::BasicStrategy;
	use relay_utils::HeaderId;

	#[async_std::test]
	async fn proof_is_generated_at_best_block_known_to_target_node() {
		const GENERATED_AT: u64 = 6;
		const BEST_AT_SOURCE: u64 = 10;
		const BEST_AT_TARGET: u64 = 8;

		// target node only knows about source' BEST_AT_TARGET block
		// source node has BEST_AT_SOURCE > BEST_AT_TARGET block
		let mut race_state = RaceStateImpl::<_, _, (), ()> {
			best_finalized_source_header_id_at_source: Some(HeaderId(
				BEST_AT_SOURCE,
				BEST_AT_SOURCE,
			)),
			best_finalized_source_header_id_at_best_target: Some(HeaderId(
				BEST_AT_TARGET,
				BEST_AT_TARGET,
			)),
			best_target_header_id: Some(HeaderId(0, 0)),
			best_finalized_target_header_id: Some(HeaderId(0, 0)),
			nonces_to_submit: None,
			nonces_to_submit_batch: None,
			nonces_submitted: None,
		};

		// we have some nonces to deliver and they're generated at GENERATED_AT < BEST_AT_SOURCE
		let mut strategy = BasicStrategy::<_, _, _, _, _, ()>::new();
		strategy.source_nonces_updated(
			HeaderId(GENERATED_AT, GENERATED_AT),
			SourceClientNonces { new_nonces: 0..=10, confirmed_nonce: None },
		);
		strategy.best_target_nonces_updated(
			TargetClientNonces { latest_nonce: 5u64, nonces_data: () },
			&mut race_state,
		);

		// the proof will be generated on source, but using BEST_AT_TARGET block
		assert_eq!(
			select_nonces_to_deliver(race_state, &strategy).await,
			Some((HeaderId(BEST_AT_TARGET, BEST_AT_TARGET), 6..=10, (),))
		);
	}
}