Extract (headers, exchange, messages) relay loops into separate crates (#357)

* extracted relay crates

* moved metrics to utils

* exchange-relay compilation

* fix compilation of headers-relay

* fixed messages-relay compilation

* fixed ethereum-poa-relay compilation

* cargo lock

* cargo fmt --all

* clippy

* cargo fmt --all

* fix tests compilation

* clippy

* eof

* module level docs

* removed obsolete comment

* #![warn(missing_docs)]

* .0 -> Deref

* post-merge fix

* cargo fmt

* Update relays/headers-relay/src/headers.rs

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>

* Update relays/headers-relay/src/headers.rs

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>

* Update relays/headers-relay/src/lib.rs

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>

bridges/relays/messages-relay/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "messages-relay"
+version = "0.1.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"
+license = "GPL-3.0-or-later WITH Classpath-exception-2.0"
+
+[dependencies]
+async-std = "1.6.2"
+async-trait = "0.1.40"
+futures = "0.3.5"
+log = "0.4.11"
+num-traits = "0.2"
+parking_lot = "0.11.0"
+relay-utils = { path = "../utils" }

bridges/relays/messages-relay/src/lib.rs

@@ -0,0 +1,32 @@
+// Copyright 2019-2020 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.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Relaying [`message-lane`](../pallet_message_lane/index.html) application specific
+//! data. Message lane allows sending arbitrary messages between bridged chains. This
+//! module provides entrypoint that starts reading messages from given message lane
+//! of source chain and submits proof-of-message-at-source-chain transactions to the
+//! target chain. Additionaly, proofs-of-messages-delivery are sent back from the
+//! target chain to the source chain.
+
+// required for futures::select!
+#![recursion_limit = "1024"]
+#![warn(missing_docs)]
+
+pub mod message_lane;
+pub mod message_lane_loop;
+pub mod message_race_delivery;
+pub mod message_race_loop;
+pub mod message_race_receiving;

bridges/relays/messages-relay/src/message_lane.rs

@@ -0,0 +1,65 @@
+// Copyright 2019-2020 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.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+//! One-way message lane types. Within single one-way lane we have three 'races' where we try to:
+//!
+//! 1) relay new messages from source to target node;
+//! 2) relay proof-of-receiving from target to source node.
+
+use relay_utils::HeaderId;
+
+use num_traits::{One, Zero};
+use std::fmt::Debug;
+
+/// One-way message lane.
+pub trait MessageLane {
+	/// Name of the messages source.
+	const SOURCE_NAME: &'static str;
+	/// Name of the messages target.
+	const TARGET_NAME: &'static str;
+
+	/// Message nonce type.
+	type MessageNonce: Clone
+		+ Copy
+		+ Debug
+		+ Default
+		+ From<u32>
+		+ Ord
+		+ std::ops::Add<Output = Self::MessageNonce>
+		+ One
+		+ Zero;
+
+	/// Messages proof.
+	type MessagesProof: Clone;
+	/// Messages receiving proof.
+	type MessagesReceivingProof: Clone;
+
+	/// Number of the source header.
+	type SourceHeaderNumber: Clone + Debug + Default + Ord + PartialEq;
+	/// Hash of the source header.
+	type SourceHeaderHash: Clone + Debug + Default + PartialEq;
+
+	/// Number of the target header.
+	type TargetHeaderNumber: Clone + Debug + Default + Ord + PartialEq;
+	/// Hash of the target header.
+	type TargetHeaderHash: Clone + Debug + Default + PartialEq;
+}
+
+/// Source header id within given one-way message lane.
+pub type SourceHeaderIdOf<P> = HeaderId<<P as MessageLane>::SourceHeaderHash, <P as MessageLane>::SourceHeaderNumber>;
+
+/// Target header id within given one-way message lane.
+pub type TargetHeaderIdOf<P> = HeaderId<<P as MessageLane>::TargetHeaderHash, <P as MessageLane>::TargetHeaderNumber>;

bridges/relays/messages-relay/src/message_lane_loop.rs

@@ -0,0 +1,670 @@
+// Copyright 2019-2020 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.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Message delivery loop. Designed to work with message-lane pallet.
+//!
+//! Single relay instance delivers messages of single lane in single direction.
+//! To serve two-way lane, you would need two instances of relay.
+//! To serve N two-way lanes, you would need N*2 instances of relay.
+//!
+//! Please keep in mind that the best header in this file is actually best
+//! finalized header. I.e. when talking about headers in lane context, we
+//! only care about finalized headers.
+
+// Until there'll be actual message-lane in the runtime.
+#![allow(dead_code)]
+
+use crate::message_lane::{MessageLane, SourceHeaderIdOf, TargetHeaderIdOf};
+use crate::message_race_delivery::run as run_message_delivery_race;
+use crate::message_race_receiving::run as run_message_receiving_race;
+
+use async_trait::async_trait;
+use futures::{channel::mpsc::unbounded, future::FutureExt, stream::StreamExt};
+use relay_utils::{interval, process_future_result, retry_backoff, FailedClient, MaybeConnectionError};
+use std::{fmt::Debug, future::Future, ops::RangeInclusive, time::Duration};
+
+/// Source client trait.
+#[async_trait(?Send)]
+pub trait SourceClient<P: MessageLane>: Clone {
+	/// Type of error this clients returns.
+	type Error: std::fmt::Debug + MaybeConnectionError;
+
+	/// Try to reconnect to source node.
+	fn reconnect(self) -> Self;
+
+	/// Returns state of the client.
+	async fn state(&self) -> Result<SourceClientState<P>, Self::Error>;
+
+	/// Get nonce of instance of latest generated message.
+	async fn latest_generated_nonce(
+		&self,
+		id: SourceHeaderIdOf<P>,
+	) -> Result<(SourceHeaderIdOf<P>, P::MessageNonce), Self::Error>;
+	/// Get nonce of the latest message, which receiving has been confirmed by the target chain.
+	async fn latest_confirmed_received_nonce(
+		&self,
+		id: SourceHeaderIdOf<P>,
+	) -> Result<(SourceHeaderIdOf<P>, P::MessageNonce), Self::Error>;
+
+	/// Prove messages in inclusive range [begin; end].
+	async fn prove_messages(
+		&self,
+		id: SourceHeaderIdOf<P>,
+		nonces: RangeInclusive<P::MessageNonce>,
+	) -> Result<(SourceHeaderIdOf<P>, RangeInclusive<P::MessageNonce>, P::MessagesProof), Self::Error>;
+
+	/// Submit messages receiving proof.
+	async fn submit_messages_receiving_proof(
+		&self,
+		generated_at_block: TargetHeaderIdOf<P>,
+		proof: P::MessagesReceivingProof,
+	) -> Result<RangeInclusive<P::MessageNonce>, Self::Error>;
+}
+
+/// Target client trait.
+#[async_trait(?Send)]
+pub trait TargetClient<P: MessageLane>: Clone {
+	/// Type of error this clients returns.
+	type Error: std::fmt::Debug + MaybeConnectionError;
+
+	/// Try to reconnect to source node.
+	fn reconnect(self) -> Self;
+
+	/// Returns state of the client.
+	async fn state(&self) -> Result<TargetClientState<P>, Self::Error>;
+
+	/// Get nonce of latest received message.
+	async fn latest_received_nonce(
+		&self,
+		id: TargetHeaderIdOf<P>,
+	) -> Result<(TargetHeaderIdOf<P>, P::MessageNonce), Self::Error>;
+
+	/// Prove messages receiving at given block.
+	async fn prove_messages_receiving(
+		&self,
+		id: TargetHeaderIdOf<P>,
+	) -> Result<(TargetHeaderIdOf<P>, P::MessagesReceivingProof), Self::Error>;
+
+	/// Submit messages proof.
+	async fn submit_messages_proof(
+		&self,
+		generated_at_header: SourceHeaderIdOf<P>,
+		nonces: RangeInclusive<P::MessageNonce>,
+		proof: P::MessagesProof,
+	) -> Result<RangeInclusive<P::MessageNonce>, Self::Error>;
+}
+
+/// State of the client.
+#[derive(Clone, Debug, Default, PartialEq)]
+pub struct ClientState<SelfHeaderId, PeerHeaderId> {
+	/// Best header id of this chain.
+	pub best_self: SelfHeaderId,
+	/// Best header id of the peer chain.
+	pub best_peer: PeerHeaderId,
+}
+
+/// State of source client in one-way message lane.
+pub type SourceClientState<P> = ClientState<SourceHeaderIdOf<P>, TargetHeaderIdOf<P>>;
+
+/// State of target client in one-way message lane.
+pub type TargetClientState<P> = ClientState<TargetHeaderIdOf<P>, SourceHeaderIdOf<P>>;
+
+/// Both clients state.
+#[derive(Debug, Default)]
+pub struct ClientsState<P: MessageLane> {
+	/// Source client state.
+	pub source: Option<SourceClientState<P>>,
+	/// Target client state.
+	pub target: Option<TargetClientState<P>>,
+}
+
+/// Run message lane service loop.
+pub fn run<P: MessageLane>(
+	mut source_client: impl SourceClient<P>,
+	source_tick: Duration,
+	mut target_client: impl TargetClient<P>,
+	target_tick: Duration,
+	reconnect_delay: Duration,
+	stall_timeout: Duration,
+	exit_signal: impl Future<Output = ()>,
+) {
+	let mut local_pool = futures::executor::LocalPool::new();
+	let exit_signal = exit_signal.shared();
+
+	local_pool.run_until(async move {
+		loop {
+			let result = run_until_connection_lost(
+				source_client.clone(),
+				source_tick,
+				target_client.clone(),
+				target_tick,
+				stall_timeout,
+				exit_signal.clone(),
+			)
+			.await;
+
+			match result {
+				Ok(()) => break,
+				Err(failed_client) => {
+					async_std::task::sleep(reconnect_delay).await;
+					if failed_client == FailedClient::Both || failed_client == FailedClient::Source {
+						source_client = source_client.reconnect();
+					}
+					if failed_client == FailedClient::Both || failed_client == FailedClient::Target {
+						target_client = target_client.reconnect();
+					}
+				}
+			}
+
+			log::debug!(
+				target: "bridge",
+				"Restarting lane {} -> {}",
+				P::SOURCE_NAME,
+				P::TARGET_NAME,
+			);
+		}
+	});
+}
+
+/// Run one-way message delivery loop until connection with target or source node is lost, or exit signal is received.
+async fn run_until_connection_lost<P: MessageLane, SC: SourceClient<P>, TC: TargetClient<P>>(
+	source_client: SC,
+	source_tick: Duration,
+	target_client: TC,
+	target_tick: Duration,
+	stall_timeout: Duration,
+	exit_signal: impl Future<Output = ()>,
+) -> Result<(), FailedClient> {
+	let mut source_retry_backoff = retry_backoff();
+	let mut source_client_is_online = false;
+	let mut source_state_required = true;
+	let source_state = source_client.state().fuse();
+	let source_go_offline_future = futures::future::Fuse::terminated();
+	let source_tick_stream = interval(source_tick).fuse();
+
+	let mut target_retry_backoff = retry_backoff();
+	let mut target_client_is_online = false;
+	let mut target_state_required = true;
+	let target_state = target_client.state().fuse();
+	let target_go_offline_future = futures::future::Fuse::terminated();
+	let target_tick_stream = interval(target_tick).fuse();
+
+	let (
+		(delivery_source_state_sender, delivery_source_state_receiver),
+		(delivery_target_state_sender, delivery_target_state_receiver),
+	) = (unbounded(), unbounded());
+	let delivery_race_loop = run_message_delivery_race(
+		source_client.clone(),
+		delivery_source_state_receiver,
+		target_client.clone(),
+		delivery_target_state_receiver,
+		stall_timeout,
+	)
+	.fuse();
+
+	let (
+		(receiving_source_state_sender, receiving_source_state_receiver),
+		(receiving_target_state_sender, receiving_target_state_receiver),
+	) = (unbounded(), unbounded());
+	let receiving_race_loop = run_message_receiving_race(
+		source_client.clone(),
+		receiving_source_state_receiver,
+		target_client.clone(),
+		receiving_target_state_receiver,
+		stall_timeout,
+	)
+	.fuse();
+
+	let exit_signal = exit_signal.fuse();
+
+	futures::pin_mut!(
+		source_state,
+		source_go_offline_future,
+		source_tick_stream,
+		target_state,
+		target_go_offline_future,
+		target_tick_stream,
+		delivery_race_loop,
+		receiving_race_loop,
+		exit_signal
+	);
+
+	loop {
+		futures::select! {
+			new_source_state = source_state => {
+				source_state_required = false;
+
+				source_client_is_online = process_future_result(
+					new_source_state,
+					&mut source_retry_backoff,
+					|new_source_state| {
+						log::debug!(
+							target: "bridge",
+							"Received state from {} node: {:?}",
+							P::SOURCE_NAME,
+							new_source_state,
+						);
+						let _ = delivery_source_state_sender.unbounded_send(new_source_state.clone());
+						let _ = receiving_source_state_sender.unbounded_send(new_source_state.clone());
+					},
+					&mut source_go_offline_future,
+					|delay| async_std::task::sleep(delay),
+					|| format!("Error retrieving state from {} node", P::SOURCE_NAME),
+				).fail_if_connection_error(FailedClient::Source)?;
+			},
+			_ = source_go_offline_future => {
+				source_client_is_online = true;
+			},
+			_ = source_tick_stream.next() => {
+				source_state_required = true;
+			},
+			new_target_state = target_state => {
+				target_state_required = false;
+
+				target_client_is_online = process_future_result(
+					new_target_state,
+					&mut target_retry_backoff,
+					|new_target_state| {
+						log::debug!(
+							target: "bridge",
+							"Received state from {} node: {:?}",
+							P::TARGET_NAME,
+							new_target_state,
+						);
+						let _ = delivery_target_state_sender.unbounded_send(new_target_state.clone());
+						let _ = receiving_target_state_sender.unbounded_send(new_target_state.clone());
+					},
+					&mut target_go_offline_future,
+					|delay| async_std::task::sleep(delay),
+					|| format!("Error retrieving state from {} node", P::TARGET_NAME),
+				).fail_if_connection_error(FailedClient::Target)?;
+			},
+			_ = target_go_offline_future => {
+				target_client_is_online = true;
+			},
+			_ = target_tick_stream.next() => {
+				target_state_required = true;
+			},
+
+			delivery_error = delivery_race_loop => {
+				match delivery_error {
+					Ok(_) => unreachable!("only ends with error; qed"),
+					Err(err) => return Err(err),
+				}
+			},
+			receiving_error = receiving_race_loop => {
+				match receiving_error {
+					Ok(_) => unreachable!("only ends with error; qed"),
+					Err(err) => return Err(err),
+				}
+			},
+
+			() = exit_signal => {
+				return Ok(());
+			}
+		}
+
+		if source_client_is_online && source_state_required {
+			log::debug!(target: "bridge", "Asking {} node about its state", P::SOURCE_NAME);
+			source_state.set(source_client.state().fuse());
+			source_client_is_online = false;
+		}
+
+		if target_client_is_online && target_state_required {
+			log::debug!(target: "bridge", "Asking {} node about its state", P::TARGET_NAME);
+			target_state.set(target_client.state().fuse());
+			target_client_is_online = false;
+		}
+	}
+}
+
+#[cfg(test)]
+pub(crate) mod tests {
+	use super::*;
+	use futures::stream::StreamExt;
+	use parking_lot::Mutex;
+	use relay_utils::HeaderId;
+	use std::sync::Arc;
+
+	pub fn header_id(number: TestSourceHeaderNumber) -> HeaderId<TestSourceHeaderNumber, TestSourceHeaderHash> {
+		HeaderId(number, number)
+	}
+
+	pub type TestMessageNonce = u64;
+	pub type TestMessagesProof = RangeInclusive<TestMessageNonce>;
+	pub type TestMessagesReceivingProof = TestMessageNonce;
+
+	pub type TestSourceHeaderNumber = u64;
+	pub type TestSourceHeaderHash = u64;
+
+	pub type TestTargetHeaderNumber = u64;
+	pub type TestTargetHeaderHash = u64;
+
+	#[derive(Debug)]
+	pub enum TestError {
+		Logic,
+		Connection,
+	}
+
+	impl MaybeConnectionError for TestError {
+		fn is_connection_error(&self) -> bool {
+			match *self {
+				TestError::Logic => false,
+				TestError::Connection => true,
+			}
+		}
+	}
+
+	pub struct TestMessageLane;
+
+	impl MessageLane for TestMessageLane {
+		const SOURCE_NAME: &'static str = "TestSource";
+		const TARGET_NAME: &'static str = "TestTarget";
+
+		type MessageNonce = TestMessageNonce;
+
+		type MessagesProof = TestMessagesProof;
+		type MessagesReceivingProof = TestMessagesReceivingProof;
+
+		type SourceHeaderNumber = TestSourceHeaderNumber;
+		type SourceHeaderHash = TestSourceHeaderHash;
+
+		type TargetHeaderNumber = TestTargetHeaderNumber;
+		type TargetHeaderHash = TestTargetHeaderHash;
+	}
+
+	#[derive(Debug, Default, Clone)]
+	pub struct TestClientData {
+		is_source_fails: bool,
+		is_source_reconnected: bool,
+		source_state: SourceClientState<TestMessageLane>,
+		source_latest_generated_nonce: TestMessageNonce,
+		source_latest_confirmed_received_nonce: TestMessageNonce,
+		submitted_messages_receiving_proofs: Vec<TestMessagesReceivingProof>,
+		is_target_fails: bool,
+		is_target_reconnected: bool,
+		target_state: SourceClientState<TestMessageLane>,
+		target_latest_received_nonce: TestMessageNonce,
+		submitted_messages_proofs: Vec<TestMessagesProof>,
+	}
+
+	#[derive(Clone)]
+	pub struct TestSourceClient {
+		data: Arc<Mutex<TestClientData>>,
+		tick: Arc<dyn Fn(&mut TestClientData)>,
+	}
+
+	#[async_trait(?Send)]
+	impl SourceClient<TestMessageLane> for TestSourceClient {
+		type Error = TestError;
+
+		fn reconnect(self) -> Self {
+			{
+				let mut data = self.data.lock();
+				(self.tick)(&mut *data);
+				data.is_source_reconnected = true;
+			}
+			self
+		}
+
+		async fn state(&self) -> Result<SourceClientState<TestMessageLane>, Self::Error> {
+			let mut data = self.data.lock();
+			(self.tick)(&mut *data);
+			if data.is_source_fails {
+				return Err(TestError::Connection);
+			}
+			Ok(data.source_state.clone())
+		}
+
+		async fn latest_generated_nonce(
+			&self,
+			id: SourceHeaderIdOf<TestMessageLane>,
+		) -> Result<(SourceHeaderIdOf<TestMessageLane>, TestMessageNonce), Self::Error> {
+			let mut data = self.data.lock();
+			(self.tick)(&mut *data);
+			if data.is_source_fails {
+				return Err(TestError::Connection);
+			}
+			Ok((id, data.source_latest_generated_nonce))
+		}
+
+		async fn latest_confirmed_received_nonce(
+			&self,
+			id: SourceHeaderIdOf<TestMessageLane>,
+		) -> Result<(SourceHeaderIdOf<TestMessageLane>, TestMessageNonce), Self::Error> {
+			let mut data = self.data.lock();
+			(self.tick)(&mut *data);
+			Ok((id, data.source_latest_confirmed_received_nonce))
+		}
+
+		async fn prove_messages(
+			&self,
+			id: SourceHeaderIdOf<TestMessageLane>,
+			nonces: RangeInclusive<TestMessageNonce>,
+		) -> Result<
+			(
+				SourceHeaderIdOf<TestMessageLane>,
+				RangeInclusive<TestMessageNonce>,
+				TestMessagesProof,
+			),
+			Self::Error,
+		> {
+			Ok((id, nonces.clone(), nonces))
+		}
+
+		async fn submit_messages_receiving_proof(
+			&self,
+			_generated_at_block: TargetHeaderIdOf<TestMessageLane>,
+			proof: TestMessagesReceivingProof,
+		) -> Result<RangeInclusive<TestMessageNonce>, Self::Error> {
+			let mut data = self.data.lock();
+			(self.tick)(&mut *data);
+			data.submitted_messages_receiving_proofs.push(proof);
+			data.source_latest_confirmed_received_nonce = proof;
+			Ok(proof..=proof)
+		}
+	}
+
+	#[derive(Clone)]
+	pub struct TestTargetClient {
+		data: Arc<Mutex<TestClientData>>,
+		tick: Arc<dyn Fn(&mut TestClientData)>,
+	}
+
+	#[async_trait(?Send)]
+	impl TargetClient<TestMessageLane> for TestTargetClient {
+		type Error = TestError;
+
+		fn reconnect(self) -> Self {
+			{
+				let mut data = self.data.lock();
+				(self.tick)(&mut *data);
+				data.is_target_reconnected = true;
+			}
+			self
+		}
+
+		async fn state(&self) -> Result<TargetClientState<TestMessageLane>, Self::Error> {
+			let mut data = self.data.lock();
+			(self.tick)(&mut *data);
+			if data.is_target_fails {
+				return Err(TestError::Connection);
+			}
+			Ok(data.target_state.clone())
+		}
+
+		async fn latest_received_nonce(
+			&self,
+			id: TargetHeaderIdOf<TestMessageLane>,
+		) -> Result<(TargetHeaderIdOf<TestMessageLane>, TestMessageNonce), Self::Error> {
+			let mut data = self.data.lock();
+			(self.tick)(&mut *data);
+			if data.is_target_fails {
+				return Err(TestError::Connection);
+			}
+			Ok((id, data.target_latest_received_nonce))
+		}
+
+		async fn prove_messages_receiving(
+			&self,
+			id: TargetHeaderIdOf<TestMessageLane>,
+		) -> Result<(TargetHeaderIdOf<TestMessageLane>, TestMessagesReceivingProof), Self::Error> {
+			Ok((id, self.data.lock().target_latest_received_nonce))
+		}
+
+		async fn submit_messages_proof(
+			&self,
+			_generated_at_header: SourceHeaderIdOf<TestMessageLane>,
+			nonces: RangeInclusive<TestMessageNonce>,
+			proof: TestMessagesProof,
+		) -> Result<RangeInclusive<TestMessageNonce>, Self::Error> {
+			let mut data = self.data.lock();
+			(self.tick)(&mut *data);
+			if data.is_target_fails {
+				return Err(TestError::Connection);
+			}
+			data.target_state.best_self =
+				HeaderId(data.target_state.best_self.0 + 1, data.target_state.best_self.1 + 1);
+			data.target_latest_received_nonce = *proof.end();
+			data.submitted_messages_proofs.push(proof);
+			Ok(nonces)
+		}
+	}
+
+	fn run_loop_test(
+		data: TestClientData,
+		source_tick: Arc<dyn Fn(&mut TestClientData)>,
+		target_tick: Arc<dyn Fn(&mut TestClientData)>,
+		exit_signal: impl Future<Output = ()>,
+	) -> TestClientData {
+		async_std::task::block_on(async {
+			let data = Arc::new(Mutex::new(data));
+
+			let source_client = TestSourceClient {
+				data: data.clone(),
+				tick: source_tick,
+			};
+			let target_client = TestTargetClient {
+				data: data.clone(),
+				tick: target_tick,
+			};
+			run(
+				source_client,
+				Duration::from_millis(100),
+				target_client,
+				Duration::from_millis(100),
+				Duration::from_millis(0),
+				Duration::from_secs(60),
+				exit_signal,
+			);
+
+			let result = data.lock().clone();
+			result
+		})
+	}
+
+	#[test]
+	fn message_lane_loop_is_able_to_recover_from_connection_errors() {
+		// with this configuration, source client will return Err, making source client
+		// reconnect. Then the target client will fail with Err + reconnect. Then we finally
+		// able to deliver messages.
+		let (exit_sender, exit_receiver) = unbounded();
+		let result = run_loop_test(
+			TestClientData {
+				is_source_fails: true,
+				source_state: ClientState {
+					best_self: HeaderId(0, 0),
+					best_peer: HeaderId(0, 0),
+				},
+				source_latest_generated_nonce: 1,
+				target_state: ClientState {
+					best_self: HeaderId(0, 0),
+					best_peer: HeaderId(0, 0),
+				},
+				target_latest_received_nonce: 0,
+				..Default::default()
+			},
+			Arc::new(|data: &mut TestClientData| {
+				if data.is_source_reconnected {
+					data.is_source_fails = false;
+					data.is_target_fails = true;
+				}
+			}),
+			Arc::new(move |data: &mut TestClientData| {
+				if data.is_target_reconnected {
+					data.is_target_fails = false;
+				}
+				if data.target_state.best_peer.0 < 10 {
+					data.target_state.best_peer =
+						HeaderId(data.target_state.best_peer.0 + 1, data.target_state.best_peer.0 + 1);
+				}
+				if !data.submitted_messages_proofs.is_empty() {
+					exit_sender.unbounded_send(()).unwrap();
+				}
+			}),
+			exit_receiver.into_future().map(|(_, _)| ()),
+		);
+
+		assert_eq!(result.submitted_messages_proofs, vec![1..=1],);
+	}
+
+	#[test]
+	fn message_lane_loop_works() {
+		// with this configuration, target client must first sync headers [1; 10] and
+		// then submit proof-of-messages [0; 10] at once
+		let (exit_sender, exit_receiver) = unbounded();
+		let result = run_loop_test(
+			TestClientData {
+				source_state: ClientState {
+					best_self: HeaderId(10, 10),
+					best_peer: HeaderId(0, 0),
+				},
+				source_latest_generated_nonce: 10,
+				target_state: ClientState {
+					best_self: HeaderId(0, 0),
+					best_peer: HeaderId(0, 0),
+				},
+				target_latest_received_nonce: 0,
+				..Default::default()
+			},
+			Arc::new(|_: &mut TestClientData| {}),
+			Arc::new(move |data: &mut TestClientData| {
+				// syncing source headers -> target chain (by one)
+				if data.target_state.best_peer.0 < data.source_state.best_self.0 {
+					data.target_state.best_peer =
+						HeaderId(data.target_state.best_peer.0 + 1, data.target_state.best_peer.0 + 1);
+				}
+				// syncing source headers -> target chain (all at once)
+				if data.source_state.best_peer.0 < data.target_state.best_self.0 {
+					data.source_state.best_peer = data.target_state.best_self;
+				}
+				// if target has received all messages => increase target block so that confirmations may be sent
+				if data.target_latest_received_nonce == 10 {
+					data.target_state.best_self =
+						HeaderId(data.source_state.best_self.0 + 1, data.source_state.best_self.0 + 1);
+				}
+				// if source has received all messages receiving confirmations => increase source block so that confirmations may be sent
+				if data.source_latest_confirmed_received_nonce == 10 {
+					exit_sender.unbounded_send(()).unwrap();
+				}
+			}),
+			exit_receiver.into_future().map(|(_, _)| ()),
+		);
+
+		assert_eq!(result.submitted_messages_proofs, vec![1..=4, 5..=8, 9..=10],);
+		assert!(!result.submitted_messages_receiving_proofs.is_empty());
+	}
+}

bridges/relays/messages-relay/src/message_race_delivery.rs

@@ -0,0 +1,425 @@
+// Copyright 2019-2020 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.
+
+//! Message delivery race delivers proof-of-messages from lane.source to lane.target.
+
+use crate::message_lane::{MessageLane, SourceHeaderIdOf, TargetHeaderIdOf};
+use crate::message_lane_loop::{
+	SourceClient as MessageLaneSourceClient, SourceClientState, TargetClient as MessageLaneTargetClient,
+	TargetClientState,
+};
+use crate::message_race_loop::{MessageRace, RaceState, RaceStrategy, SourceClient, TargetClient};
+
+use async_trait::async_trait;
+use futures::stream::FusedStream;
+use num_traits::{One, Zero};
+use relay_utils::{FailedClient, HeaderId};
+use std::{collections::VecDeque, marker::PhantomData, ops::RangeInclusive, time::Duration};
+
+/// Maximal number of messages to relay in single transaction.
+const MAX_MESSAGES_TO_RELAY_IN_SINGLE_TX: u32 = 4;
+
+/// Run message delivery race.
+pub async fn run<P: MessageLane>(
+	source_client: impl MessageLaneSourceClient<P>,
+	source_state_updates: impl FusedStream<Item = SourceClientState<P>>,
+	target_client: impl MessageLaneTargetClient<P>,
+	target_state_updates: impl FusedStream<Item = TargetClientState<P>>,
+	stall_timeout: Duration,
+) -> Result<(), FailedClient> {
+	crate::message_race_loop::run(
+		MessageDeliveryRaceSource {
+			client: source_client,
+			_phantom: Default::default(),
+		},
+		source_state_updates,
+		MessageDeliveryRaceTarget {
+			client: target_client,
+			_phantom: Default::default(),
+		},
+		target_state_updates,
+		stall_timeout,
+		MessageDeliveryStrategy::<P>::new(MAX_MESSAGES_TO_RELAY_IN_SINGLE_TX.into()),
+	)
+	.await
+}
+
+/// Message delivery race.
+struct MessageDeliveryRace<P>(std::marker::PhantomData<P>);
+
+impl<P: MessageLane> MessageRace for MessageDeliveryRace<P> {
+	type SourceHeaderId = SourceHeaderIdOf<P>;
+	type TargetHeaderId = TargetHeaderIdOf<P>;
+
+	type MessageNonce = P::MessageNonce;
+	type Proof = P::MessagesProof;
+
+	fn source_name() -> String {
+		format!("{}::MessagesDelivery", P::SOURCE_NAME)
+	}
+
+	fn target_name() -> String {
+		format!("{}::MessagesDelivery", P::TARGET_NAME)
+	}
+}
+
+/// Message delivery race source, which is a source of the lane.
+struct MessageDeliveryRaceSource<P: MessageLane, C> {
+	client: C,
+	_phantom: PhantomData<P>,
+}
+
+#[async_trait(?Send)]
+impl<P, C> SourceClient<MessageDeliveryRace<P>> for MessageDeliveryRaceSource<P, C>
+where
+	P: MessageLane,
+	C: MessageLaneSourceClient<P>,
+{
+	type Error = C::Error;
+
+	async fn latest_nonce(
+		&self,
+		at_block: SourceHeaderIdOf<P>,
+	) -> Result<(SourceHeaderIdOf<P>, P::MessageNonce), Self::Error> {
+		self.client.latest_generated_nonce(at_block).await
+	}
+
+	async fn generate_proof(
+		&self,
+		at_block: SourceHeaderIdOf<P>,
+		nonces: RangeInclusive<P::MessageNonce>,
+	) -> Result<(SourceHeaderIdOf<P>, RangeInclusive<P::MessageNonce>, P::MessagesProof), Self::Error> {
+		self.client.prove_messages(at_block, nonces).await
+	}
+}
+
+/// Message delivery race target, which is a target of the lane.
+struct MessageDeliveryRaceTarget<P: MessageLane, C> {
+	client: C,
+	_phantom: PhantomData<P>,
+}
+
+#[async_trait(?Send)]
+impl<P, C> TargetClient<MessageDeliveryRace<P>> for MessageDeliveryRaceTarget<P, C>
+where
+	P: MessageLane,
+	C: MessageLaneTargetClient<P>,
+{
+	type Error = C::Error;
+
+	async fn latest_nonce(
+		&self,
+		at_block: TargetHeaderIdOf<P>,
+	) -> Result<(TargetHeaderIdOf<P>, P::MessageNonce), Self::Error> {
+		self.client.latest_received_nonce(at_block).await
+	}
+
+	async fn submit_proof(
+		&self,
+		generated_at_block: SourceHeaderIdOf<P>,
+		nonces: RangeInclusive<P::MessageNonce>,
+		proof: P::MessagesProof,
+	) -> Result<RangeInclusive<P::MessageNonce>, Self::Error> {
+		self.client
+			.submit_messages_proof(generated_at_block, nonces, proof)
+			.await
+	}
+}
+
+/// Messages delivery strategy.
+type MessageDeliveryStrategy<P> = DeliveryStrategy<
+	<P as MessageLane>::SourceHeaderNumber,
+	<P as MessageLane>::SourceHeaderHash,
+	<P as MessageLane>::TargetHeaderNumber,
+	<P as MessageLane>::TargetHeaderHash,
+	<P as MessageLane>::MessageNonce,
+	<P as MessageLane>::MessagesProof,
+>;
+
+/// Nonces delivery strategy.
+#[derive(Debug)]
+pub struct DeliveryStrategy<SourceHeaderNumber, SourceHeaderHash, TargetHeaderNumber, TargetHeaderHash, Nonce, Proof> {
+	/// All queued nonces.
+	source_queue: VecDeque<(HeaderId<SourceHeaderHash, SourceHeaderNumber>, Nonce)>,
+	/// Best nonce known to target node.
+	target_nonce: Nonce,
+	/// Max nonces to relay in single transaction.
+	max_nonces_to_relay_in_single_tx: Nonce,
+	/// Unused generic types dump.
+	_phantom: PhantomData<(TargetHeaderNumber, TargetHeaderHash, Proof)>,
+}
+
+impl<SourceHeaderNumber, SourceHeaderHash, TargetHeaderNumber, TargetHeaderHash, Nonce: Default, Proof>
+	DeliveryStrategy<SourceHeaderNumber, SourceHeaderHash, TargetHeaderNumber, TargetHeaderHash, Nonce, Proof>
+{
+	/// Create new delivery strategy.
+	pub fn new(max_nonces_to_relay_in_single_tx: Nonce) -> Self {
+		DeliveryStrategy {
+			source_queue: VecDeque::new(),
+			target_nonce: Default::default(),
+			max_nonces_to_relay_in_single_tx,
+			_phantom: Default::default(),
+		}
+	}
+}
+
+impl<SourceHeaderNumber, SourceHeaderHash, TargetHeaderNumber, TargetHeaderHash, Nonce, Proof>
+	RaceStrategy<
+		HeaderId<SourceHeaderHash, SourceHeaderNumber>,
+		HeaderId<TargetHeaderHash, TargetHeaderNumber>,
+		Nonce,
+		Proof,
+	> for DeliveryStrategy<SourceHeaderNumber, SourceHeaderHash, TargetHeaderNumber, TargetHeaderHash, Nonce, Proof>
+where
+	SourceHeaderHash: Clone,
+	SourceHeaderNumber: Clone + Ord,
+	Nonce: Clone + Copy + From<u32> + Ord + std::ops::Add<Output = Nonce> + One + Zero,
+{
+	fn is_empty(&self) -> bool {
+		self.source_queue.is_empty()
+	}
+
+	fn source_nonce_updated(&mut self, at_block: HeaderId<SourceHeaderHash, SourceHeaderNumber>, nonce: Nonce) {
+		if nonce <= self.target_nonce {
+			return;
+		}
+
+		match self.source_queue.back() {
+			Some((_, prev_nonce)) if *prev_nonce < nonce => (),
+			Some(_) => return,
+			None => (),
+		}
+
+		self.source_queue.push_back((at_block, nonce))
+	}
+
+	fn target_nonce_updated(
+		&mut self,
+		nonce: Nonce,
+		race_state: &mut RaceState<
+			HeaderId<SourceHeaderHash, SourceHeaderNumber>,
+			HeaderId<TargetHeaderHash, TargetHeaderNumber>,
+			Nonce,
+			Proof,
+		>,
+	) {
+		if nonce < self.target_nonce {
+			return;
+		}
+
+		while let Some(true) = self
+			.source_queue
+			.front()
+			.map(|(_, source_nonce)| *source_nonce <= nonce)
+		{
+			self.source_queue.pop_front();
+		}
+
+		let need_to_select_new_nonces = race_state
+			.nonces_to_submit
+			.as_ref()
+			.map(|(_, nonces, _)| *nonces.end() <= nonce)
+			.unwrap_or(false);
+		if need_to_select_new_nonces {
+			race_state.nonces_to_submit = None;
+		}
+
+		let need_new_nonces_to_submit = race_state
+			.nonces_submitted
+			.as_ref()
+			.map(|nonces| *nonces.end() <= nonce)
+			.unwrap_or(false);
+		if need_new_nonces_to_submit {
+			race_state.nonces_submitted = None;
+		}
+
+		self.target_nonce = nonce;
+	}
+
+	fn select_nonces_to_deliver(
+		&mut self,
+		race_state: &RaceState<
+			HeaderId<SourceHeaderHash, SourceHeaderNumber>,
+			HeaderId<TargetHeaderHash, TargetHeaderNumber>,
+			Nonce,
+			Proof,
+		>,
+	) -> Option<RangeInclusive<Nonce>> {
+		// if we have already selected nonces that we want to submit, do nothing
+		if race_state.nonces_to_submit.is_some() {
+			return None;
+		}
+
+		// if we already submitted some nonces, do nothing
+		if race_state.nonces_submitted.is_some() {
+			return None;
+		}
+
+		// 1) we want to deliver all nonces, starting from `target_nonce + 1`
+		// 2) we want to deliver at most `self.max_nonces_to_relay_in_single_tx` nonces in this batch
+		// 3) we can't deliver new nonce until header, that has emitted this nonce, is finalized
+		// by target client
+		let nonces_begin = self.target_nonce + 1.into();
+		let best_header_at_target = &race_state.target_state.as_ref()?.best_peer;
+		let mut nonces_end = None;
+		let mut i = Zero::zero();
+		while i < self.max_nonces_to_relay_in_single_tx {
+			let nonce = nonces_begin + i;
+
+			// if queue is empty, we don't need to prove anything
+			let (first_queued_at, first_queued_nonce) = match self.source_queue.front() {
+				Some((first_queued_at, first_queued_nonce)) => ((*first_queued_at).clone(), *first_queued_nonce),
+				None => break,
+			};
+
+			// if header that has queued the message is not yet finalized at bridged chain,
+			// we can't prove anything
+			if first_queued_at.0 > best_header_at_target.0 {
+				break;
+			}
+
+			// ok, we may deliver this nonce
+			nonces_end = Some(nonce);
+
+			// probably remove it from the queue?
+			if nonce == first_queued_nonce {
+				self.source_queue.pop_front();
+			}
+
+			i = i + One::one();
+		}
+
+		nonces_end.map(|nonces_end| RangeInclusive::new(nonces_begin, nonces_end))
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+	use crate::message_lane_loop::{
+		tests::{header_id, TestMessageLane, TestMessageNonce, TestMessagesProof},
+		ClientState,
+	};
+
+	#[test]
+	fn strategy_is_empty_works() {
+		let mut strategy = MessageDeliveryStrategy::<TestMessageLane>::new(4);
+		assert_eq!(strategy.is_empty(), true);
+		strategy.source_nonce_updated(header_id(1), 1);
+		assert_eq!(strategy.is_empty(), false);
+	}
+
+	#[test]
+	fn source_nonce_is_never_lower_than_known_target_nonce() {
+		let mut strategy = MessageDeliveryStrategy::<TestMessageLane>::new(4);
+		strategy.target_nonce_updated(10, &mut Default::default());
+		strategy.source_nonce_updated(header_id(1), 5);
+		assert_eq!(strategy.source_queue, vec![]);
+	}
+
+	#[test]
+	fn source_nonce_is_never_lower_than_latest_known_source_nonce() {
+		let mut strategy = MessageDeliveryStrategy::<TestMessageLane>::new(4);
+		strategy.source_nonce_updated(header_id(1), 5);
+		strategy.source_nonce_updated(header_id(2), 3);
+		strategy.source_nonce_updated(header_id(2), 5);
+		assert_eq!(strategy.source_queue, vec![(header_id(1), 5)]);
+	}
+
+	#[test]
+	fn target_nonce_is_never_lower_than_latest_known_target_nonce() {
+		let mut strategy = MessageDeliveryStrategy::<TestMessageLane>::new(4);
+		strategy.target_nonce_updated(10, &mut Default::default());
+		strategy.target_nonce_updated(5, &mut Default::default());
+		assert_eq!(strategy.target_nonce, 10);
+	}
+
+	#[test]
+	fn updated_target_nonce_removes_queued_entries() {
+		let mut strategy = MessageDeliveryStrategy::<TestMessageLane>::new(4);
+		strategy.source_nonce_updated(header_id(1), 5);
+		strategy.source_nonce_updated(header_id(2), 10);
+		strategy.source_nonce_updated(header_id(3), 15);
+		strategy.source_nonce_updated(header_id(4), 20);
+		strategy.target_nonce_updated(15, &mut Default::default());
+		assert_eq!(strategy.source_queue, vec![(header_id(4), 20)]);
+	}
+
+	#[test]
+	fn selected_nonces_are_dropped_on_target_nonce_update() {
+		let mut state = RaceState::default();
+		let mut strategy = MessageDeliveryStrategy::<TestMessageLane>::new(4);
+		state.nonces_to_submit = Some((header_id(1), 5..=10, 5..=10));
+		strategy.target_nonce_updated(7, &mut state);
+		assert!(state.nonces_to_submit.is_some());
+		strategy.target_nonce_updated(10, &mut state);
+		assert!(state.nonces_to_submit.is_none());
+	}
+
+	#[test]
+	fn submitted_nonces_are_dropped_on_target_nonce_update() {
+		let mut state = RaceState::default();
+		let mut strategy = MessageDeliveryStrategy::<TestMessageLane>::new(4);
+		state.nonces_submitted = Some(5..=10);
+		strategy.target_nonce_updated(7, &mut state);
+		assert!(state.nonces_submitted.is_some());
+		strategy.target_nonce_updated(10, &mut state);
+		assert!(state.nonces_submitted.is_none());
+	}
+
+	#[test]
+	fn nothing_is_selected_if_something_is_already_selected() {
+		let mut state = RaceState::default();
+		let mut strategy = MessageDeliveryStrategy::<TestMessageLane>::new(4);
+		state.nonces_to_submit = Some((header_id(1), 1..=10, 1..=10));
+		strategy.source_nonce_updated(header_id(1), 10);
+		assert_eq!(strategy.select_nonces_to_deliver(&state), None);
+	}
+
+	#[test]
+	fn nothing_is_selected_if_something_is_already_submitted() {
+		let mut state = RaceState::default();
+		let mut strategy = MessageDeliveryStrategy::<TestMessageLane>::new(4);
+		state.nonces_submitted = Some(1..=10);
+		strategy.source_nonce_updated(header_id(1), 10);
+		assert_eq!(strategy.select_nonces_to_deliver(&state), None);
+	}
+
+	#[test]
+	fn select_nonces_to_deliver_works() {
+		let mut state = RaceState::<_, _, TestMessageNonce, TestMessagesProof>::default();
+		let mut strategy = MessageDeliveryStrategy::<TestMessageLane>::new(4);
+		strategy.source_nonce_updated(header_id(1), 1);
+		strategy.source_nonce_updated(header_id(2), 2);
+		strategy.source_nonce_updated(header_id(3), 6);
+		strategy.source_nonce_updated(header_id(5), 8);
+
+		state.target_state = Some(ClientState {
+			best_self: header_id(0),
+			best_peer: header_id(4),
+		});
+		assert_eq!(strategy.select_nonces_to_deliver(&state), Some(1..=4));
+		strategy.target_nonce_updated(4, &mut state);
+		assert_eq!(strategy.select_nonces_to_deliver(&state), Some(5..=6));
+		strategy.target_nonce_updated(6, &mut state);
+		assert_eq!(strategy.select_nonces_to_deliver(&state), None);
+
+		state.target_state = Some(ClientState {
+			best_self: header_id(0),
+			best_peer: header_id(5),
+		});
+		assert_eq!(strategy.select_nonces_to_deliver(&state), Some(7..=8));
+		strategy.target_nonce_updated(8, &mut state);
+		assert_eq!(strategy.select_nonces_to_deliver(&state), None);
+	}
+}

bridges/relays/messages-relay/src/message_race_loop.rs

@@ -0,0 +1,359 @@
+// Copyright 2019-2020 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.
+
+// Until there'll be actual message-lane in the runtime.
+#![allow(dead_code)]
+
+use crate::message_lane_loop::ClientState;
+
+use async_trait::async_trait;
+use futures::{
+	future::FutureExt,
+	stream::{FusedStream, StreamExt},
+};
+use relay_utils::{process_future_result, retry_backoff, FailedClient, MaybeConnectionError};
+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;
+	/// Header id of the race source.
+	type TargetHeaderId: Debug + Clone + PartialEq;
+
+	/// Message nonce used in the race.
+	type MessageNonce: Debug + Clone;
+	/// Proof that is generated and delivered in this race.
+	type Proof: Clone;
+
+	/// 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>;
+
+/// One of message lane clients, which is source client for the race.
+#[async_trait(?Send)]
+pub trait SourceClient<P: MessageRace> {
+	/// Type of error this clients returns.
+	type Error: std::fmt::Debug + MaybeConnectionError;
+
+	/// Return latest nonce that is known to the source client.
+	async fn latest_nonce(
+		&self,
+		at_block: P::SourceHeaderId,
+	) -> Result<(P::SourceHeaderId, P::MessageNonce), Self::Error>;
+	/// Generate proof for delivering to the target client.
+	async fn generate_proof(
+		&self,
+		at_block: P::SourceHeaderId,
+		nonces: RangeInclusive<P::MessageNonce>,
+	) -> Result<(P::SourceHeaderId, RangeInclusive<P::MessageNonce>, P::Proof), Self::Error>;
+}
+
+/// One of message lane clients, which is target client for the race.
+#[async_trait(?Send)]
+pub trait TargetClient<P: MessageRace> {
+	/// Type of error this clients returns.
+	type Error: std::fmt::Debug + MaybeConnectionError;
+
+	/// Return latest nonce that is known to the target client.
+	async fn latest_nonce(
+		&self,
+		at_block: P::TargetHeaderId,
+	) -> Result<(P::TargetHeaderId, P::MessageNonce), Self::Error>;
+	/// Submit proof to the target client.
+	async fn submit_proof(
+		&self,
+		generated_at_block: P::SourceHeaderId,
+		nonces: RangeInclusive<P::MessageNonce>,
+		proof: P::Proof,
+	) -> Result<RangeInclusive<P::MessageNonce>, Self::Error>;
+}
+
+/// Race strategy.
+pub trait RaceStrategy<SourceHeaderId, TargetHeaderId, MessageNonce, Proof> {
+	/// Should return true if nothing has to be synced.
+	fn is_empty(&self) -> bool;
+	/// Called when latest nonce is updated at source node of the race.
+	fn source_nonce_updated(&mut self, at_block: SourceHeaderId, nonce: MessageNonce);
+	/// Called when latest nonce is updated at target node of the race.
+	fn target_nonce_updated(
+		&mut self,
+		nonce: MessageNonce,
+		race_state: &mut RaceState<SourceHeaderId, TargetHeaderId, MessageNonce, Proof>,
+	);
+	/// Should return `Some(nonces)` if we need to deliver proof of `nonces` (and associated
+	/// data) from source to target node.
+	fn select_nonces_to_deliver(
+		&mut self,
+		race_state: &RaceState<SourceHeaderId, TargetHeaderId, MessageNonce, Proof>,
+	) -> Option<RangeInclusive<MessageNonce>>;
+}
+
+/// State of the race.
+pub struct RaceState<SourceHeaderId, TargetHeaderId, MessageNonce, Proof> {
+	/// Source state, if known.
+	pub source_state: Option<ClientState<SourceHeaderId, TargetHeaderId>>,
+	/// Target state, if known.
+	pub target_state: Option<ClientState<TargetHeaderId, SourceHeaderId>>,
+	/// Range of nonces that we have selected to submit.
+	pub nonces_to_submit: Option<(SourceHeaderId, RangeInclusive<MessageNonce>, Proof)>,
+	/// Range of nonces that is currently submitted.
+	pub nonces_submitted: Option<RangeInclusive<MessageNonce>>,
+}
+
+/// Run race loop until connection with target or source node is lost.
+pub async fn run<P: MessageRace>(
+	race_source: impl SourceClient<P>,
+	race_source_updated: impl FusedStream<Item = SourceClientState<P>>,
+	race_target: impl TargetClient<P>,
+	race_target_updated: impl FusedStream<Item = TargetClientState<P>>,
+	stall_timeout: Duration,
+	mut strategy: impl RaceStrategy<P::SourceHeaderId, P::TargetHeaderId, P::MessageNonce, P::Proof>,
+) -> Result<(), FailedClient> {
+	let mut race_state = RaceState::default();
+	let mut stall_countdown = Instant::now();
+
+	let mut source_retry_backoff = retry_backoff();
+	let mut source_client_is_online = true;
+	let mut source_latest_nonce_required = false;
+	let source_latest_nonce = 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_latest_nonce_required = false;
+	let target_latest_nonce = futures::future::Fuse::terminated();
+	let target_submit_proof = futures::future::Fuse::terminated();
+	let target_go_offline_future = futures::future::Fuse::terminated();
+
+	futures::pin_mut!(
+		race_source_updated,
+		source_latest_nonce,
+		source_generate_proof,
+		source_go_offline_future,
+		race_target_updated,
+		target_latest_nonce,
+		target_submit_proof,
+		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 {
+					if race_state.source_state.as_ref() != Some(&source_state) {
+						source_latest_nonce_required = true;
+						race_state.source_state = Some(source_state);
+					}
+				}
+			},
+			target_state = race_target_updated.next() => {
+				if let Some(target_state) = target_state {
+					if race_state.target_state.as_ref() != Some(&target_state) {
+						target_latest_nonce_required = true;
+						race_state.target_state = Some(target_state);
+					}
+				}
+			},
+
+			// when nonces are updated
+			latest_nonce = source_latest_nonce => {
+				source_latest_nonce_required = false;
+
+				source_client_is_online = process_future_result(
+					latest_nonce,
+					&mut source_retry_backoff,
+					|(at_block, latest_nonce)| {
+						log::debug!(
+							target: "bridge",
+							"Received latest nonce from {}: {:?}",
+							P::source_name(),
+							latest_nonce,
+						);
+
+						strategy.source_nonce_updated(at_block, latest_nonce);
+					},
+					&mut source_go_offline_future,
+					|delay| async_std::task::sleep(delay),
+					|| format!("Error retrieving latest nonce from {}", P::source_name()),
+				).fail_if_connection_error(FailedClient::Source)?;
+			},
+			latest_nonce = target_latest_nonce => {
+				target_latest_nonce_required = false;
+
+				target_client_is_online = process_future_result(
+					latest_nonce,
+					&mut target_retry_backoff,
+					|(_, latest_nonce)| {
+						log::debug!(
+							target: "bridge",
+							"Received latest nonce from {}: {:?}",
+							P::target_name(),
+							latest_nonce,
+						);
+
+						strategy.target_nonce_updated(latest_nonce, &mut race_state);
+					},
+					&mut target_go_offline_future,
+					|delay| async_std::task::sleep(delay),
+					|| format!("Error retrieving latest nonce from {}", P::target_name()),
+				).fail_if_connection_error(FailedClient::Target)?;
+			},
+
+			// proof generation and submission
+			proof = source_generate_proof => {
+				source_client_is_online = process_future_result(
+					proof,
+					&mut source_retry_backoff,
+					|(at_block, nonces_range, proof)| {
+						log::debug!(
+							target: "bridge",
+							"Received proof for nonces in range {:?} from {}",
+							nonces_range,
+							P::source_name(),
+						);
+
+						race_state.nonces_to_submit = Some((at_block, nonces_range, proof));
+					},
+					&mut source_go_offline_future,
+					|delay| async_std::task::sleep(delay),
+					|| format!("Error generating proof at {}", P::source_name()),
+				).fail_if_connection_error(FailedClient::Source)?;
+			},
+			proof_submit_result = target_submit_proof => {
+				target_client_is_online = process_future_result(
+					proof_submit_result,
+					&mut target_retry_backoff,
+					|nonces_range| {
+						log::debug!(
+							target: "bridge",
+							"Successfully submitted proof of nonces {:?} to {}",
+							nonces_range,
+							P::target_name(),
+						);
+
+						race_state.nonces_to_submit = None;
+						race_state.nonces_submitted = Some(nonces_range);
+					},
+					&mut target_go_offline_future,
+					|delay| async_std::task::sleep(delay),
+					|| format!("Error submitting proof {}", P::target_name()),
+				).fail_if_connection_error(FailedClient::Target)?;
+			}
+		}
+
+		if stall_countdown.elapsed() > stall_timeout {
+			return Err(FailedClient::Both);
+		} else if race_state.nonces_to_submit.is_none() && race_state.nonces_submitted.is_none() && strategy.is_empty()
+		{
+			stall_countdown = Instant::now();
+		}
+
+		if source_client_is_online {
+			source_client_is_online = false;
+
+			let nonces_to_deliver = race_state.source_state.as_ref().and_then(|source_state| {
+				strategy
+					.select_nonces_to_deliver(&race_state)
+					.map(|nonces_range| (source_state.best_self.clone(), nonces_range))
+			});
+
+			if let Some((at_block, nonces_range)) = nonces_to_deliver {
+				log::debug!(
+					target: "bridge",
+					"Asking {} to prove nonces in range {:?}",
+					P::source_name(),
+					nonces_range,
+				);
+				source_generate_proof.set(race_source.generate_proof(at_block, nonces_range).fuse());
+			} else if source_latest_nonce_required {
+				log::debug!(target: "bridge", "Asking {} about latest generated message nonce", P::source_name());
+				let at_block = race_state
+					.source_state
+					.as_ref()
+					.expect("source_latest_nonce_required is only true when source_state is Some; qed")
+					.best_self
+					.clone();
+				source_latest_nonce.set(race_source.latest_nonce(at_block).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() {
+				log::debug!(
+					target: "bridge",
+					"Going to submit proof of messages in range {:?} to {} node",
+					nonces_range,
+					P::target_name(),
+				);
+				target_submit_proof.set(
+					race_target
+						.submit_proof(at_block.clone(), nonces_range.clone(), proof.clone())
+						.fuse(),
+				);
+			}
+			if target_latest_nonce_required {
+				log::debug!(target: "bridge", "Asking {} about latest nonce", P::target_name());
+				let at_block = race_state
+					.target_state
+					.as_ref()
+					.expect("target_latest_nonce_required is only true when target_state is Some; qed")
+					.best_self
+					.clone();
+				target_latest_nonce.set(race_target.latest_nonce(at_block).fuse());
+			} else {
+				target_client_is_online = true;
+			}
+		}
+	}
+}
+
+impl<SourceHeaderId, TargetHeaderId, MessageNonce, Proof> Default
+	for RaceState<SourceHeaderId, TargetHeaderId, MessageNonce, Proof>
+{
+	fn default() -> Self {
+		RaceState {
+			source_state: None,
+			target_state: None,
+			nonces_to_submit: None,
+			nonces_submitted: None,
+		}
+	}
+}

bridges/relays/messages-relay/src/message_race_receiving.rs

@@ -0,0 +1,154 @@
+// Copyright 2019-2020 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.
+
+//! Message receiving race delivers proof-of-messages-delivery from lane.target to lane.source.
+
+use crate::message_lane::{MessageLane, SourceHeaderIdOf, TargetHeaderIdOf};
+use crate::message_lane_loop::{
+	SourceClient as MessageLaneSourceClient, SourceClientState, TargetClient as MessageLaneTargetClient,
+	TargetClientState,
+};
+use crate::message_race_delivery::DeliveryStrategy;
+use crate::message_race_loop::{MessageRace, SourceClient, TargetClient};
+
+use async_trait::async_trait;
+use futures::stream::FusedStream;
+use relay_utils::FailedClient;
+use std::{marker::PhantomData, ops::RangeInclusive, time::Duration};
+
+/// Message receiving confirmations delivery strategy.
+type ReceivingConfirmationsDeliveryStrategy<P> = DeliveryStrategy<
+	<P as MessageLane>::TargetHeaderNumber,
+	<P as MessageLane>::TargetHeaderHash,
+	<P as MessageLane>::SourceHeaderNumber,
+	<P as MessageLane>::SourceHeaderHash,
+	<P as MessageLane>::MessageNonce,
+	<P as MessageLane>::MessagesReceivingProof,
+>;
+
+/// Run receiving confirmations race.
+pub async fn run<P: MessageLane>(
+	source_client: impl MessageLaneSourceClient<P>,
+	source_state_updates: impl FusedStream<Item = SourceClientState<P>>,
+	target_client: impl MessageLaneTargetClient<P>,
+	target_state_updates: impl FusedStream<Item = TargetClientState<P>>,
+	stall_timeout: Duration,
+) -> Result<(), FailedClient> {
+	crate::message_race_loop::run(
+		ReceivingConfirmationsRaceSource {
+			client: target_client,
+			_phantom: Default::default(),
+		},
+		target_state_updates,
+		ReceivingConfirmationsRaceTarget {
+			client: source_client,
+			_phantom: Default::default(),
+		},
+		source_state_updates,
+		stall_timeout,
+		ReceivingConfirmationsDeliveryStrategy::<P>::new(std::u32::MAX.into()),
+	)
+	.await
+}
+
+/// Messages receiving confirmations race.
+struct ReceivingConfirmationsRace<P>(std::marker::PhantomData<P>);
+
+impl<P: MessageLane> MessageRace for ReceivingConfirmationsRace<P> {
+	type SourceHeaderId = TargetHeaderIdOf<P>;
+	type TargetHeaderId = SourceHeaderIdOf<P>;
+
+	type MessageNonce = P::MessageNonce;
+	type Proof = P::MessagesReceivingProof;
+
+	fn source_name() -> String {
+		format!("{}::ReceivingConfirmationsDelivery", P::SOURCE_NAME)
+	}
+
+	fn target_name() -> String {
+		format!("{}::ReceivingConfirmationsDelivery", P::TARGET_NAME)
+	}
+}
+
+/// Message receiving confirmations race source, which is a target of the lane.
+struct ReceivingConfirmationsRaceSource<P: MessageLane, C> {
+	client: C,
+	_phantom: PhantomData<P>,
+}
+
+#[async_trait(?Send)]
+impl<P, C> SourceClient<ReceivingConfirmationsRace<P>> for ReceivingConfirmationsRaceSource<P, C>
+where
+	P: MessageLane,
+	C: MessageLaneTargetClient<P>,
+{
+	type Error = C::Error;
+
+	async fn latest_nonce(
+		&self,
+		at_block: TargetHeaderIdOf<P>,
+	) -> Result<(TargetHeaderIdOf<P>, P::MessageNonce), Self::Error> {
+		self.client.latest_received_nonce(at_block).await
+	}
+
+	async fn generate_proof(
+		&self,
+		at_block: TargetHeaderIdOf<P>,
+		nonces: RangeInclusive<P::MessageNonce>,
+	) -> Result<
+		(
+			TargetHeaderIdOf<P>,
+			RangeInclusive<P::MessageNonce>,
+			P::MessagesReceivingProof,
+		),
+		Self::Error,
+	> {
+		self.client
+			.prove_messages_receiving(at_block)
+			.await
+			.map(|(at_block, proof)| (at_block, nonces, proof))
+	}
+}
+
+/// Message receiving confirmations race target, which is a source of the lane.
+struct ReceivingConfirmationsRaceTarget<P: MessageLane, C> {
+	client: C,
+	_phantom: PhantomData<P>,
+}
+
+#[async_trait(?Send)]
+impl<P, C> TargetClient<ReceivingConfirmationsRace<P>> for ReceivingConfirmationsRaceTarget<P, C>
+where
+	P: MessageLane,
+	C: MessageLaneSourceClient<P>,
+{
+	type Error = C::Error;
+
+	async fn latest_nonce(
+		&self,
+		at_block: SourceHeaderIdOf<P>,
+	) -> Result<(SourceHeaderIdOf<P>, P::MessageNonce), Self::Error> {
+		self.client.latest_confirmed_received_nonce(at_block).await
+	}
+
+	async fn submit_proof(
+		&self,
+		generated_at_block: TargetHeaderIdOf<P>,
+		_nonces: RangeInclusive<P::MessageNonce>,
+		proof: P::MessagesReceivingProof,
+	) -> Result<RangeInclusive<P::MessageNonce>, Self::Error> {
+		self.client
+			.submit_messages_receiving_proof(generated_at_block, proof)
+			.await
+	}
+}