Relay Millau && Rialto headers using (future) finality verifier API (#761)

* finality proofs relay

* SyncHeader::is_mandatory

* empty ancestry proof

* logs

* fixed submit condition

* fixed wrong split index

* tick comment

* recent_finality_proofs

* basic finality loop tests

* removed obsolete files

* rename files in substrate relay

* fmt

* clippy

* fixed TODOs

* clippy

* stop syncing if target node is out of sync

* more clippy

* more clippy

* Update relays/finality-relay/src/finality_loop.rs

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

* Update relays/finality-relay/src/finality_loop.rs

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

* Update relays/finality-relay/src/finality_loop.rs

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

* docs

* moved doc

* typo

* Update relays/finality-relay/src/finality_loop_tests.rs

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

* Update relays/finality-relay/src/finality_loop_tests.rs

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

* header_and_finality_proof_by_number -> header_and_finality_proof

* VecDeque isn't required (because of make_contiguous)

* fixed wrong expect

* Update relays/finality-relay/src/finality_loop.rs

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

* Update relays/substrate/src/rialto_headers_to_millau.rs

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

* Update relays/substrate/src/rialto_headers_to_millau.rs

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

* RialtoSyncHeader

* Update relays/finality-relay/src/finality_loop.rs

Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

* Update relays/finality-relay/src/finality_loop.rs

Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

* removed wrong comment

* Update relays/finality-relay/src/finality_loop.rs

Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

* fix used runtime methods names

* fix for new jsonrpsee

* fix comment

* initialize finality verifier pallet

* fmt

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

bridges/relays/finality-relay/Cargo.toml

@@ -0,0 +1,20 @@
+[package]
+name = "finality-relay"
+version = "0.1.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"
+license = "GPL-3.0-or-later WITH Classpath-exception-2.0"
+description = "Finality proofs relay"
+
+[dependencies]
+async-std = "1.6.5"
+async-trait = "0.1.40"
+backoff = "0.2"
+futures = "0.3.5"
+headers-relay = { path = "../headers-relay" }
+log = "0.4.11"
+num-traits = "0.2"
+relay-utils = { path = "../utils" }
+
+[dev-dependencies]
+parking_lot = "0.11.0"

bridges/relays/finality-relay/src/finality_loop.rs

@@ -0,0 +1,581 @@
+// 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.
+
+// 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/>.
+
+//! The loop basically reads all missing headers and their finality proofs from the source client.
+//! The proof for the best possible header is then submitted to the target node. The only exception
+//! is the mandatory headers, which we always submit to the target node. For such headers, we
+//! assume that the persistent proof either exists, or will eventually become available.
+
+use crate::{FinalityProof, FinalitySyncPipeline, SourceHeader};
+
+use async_trait::async_trait;
+use backoff::backoff::Backoff;
+use futures::{select, Future, FutureExt, Stream, StreamExt};
+use headers_relay::sync_loop_metrics::SyncLoopMetrics;
+use num_traits::{One, Saturating};
+use relay_utils::{
+	metrics::{start as metrics_start, GlobalMetrics, MetricsParams},
+	relay_loop::Client as RelayClient,
+	retry_backoff, FailedClient, MaybeConnectionError,
+};
+use std::{
+	pin::Pin,
+	time::{Duration, Instant},
+};
+
+/// Finality proof synchronization loop parameters.
+#[derive(Debug, Clone)]
+pub struct FinalitySyncParams {
+	/// Interval at which we check updates on both clients. Normally should be larger than
+	/// `min(source_block_time, target_block_time)`.
+	///
+	/// This parameter may be used to limit transactions rate. Increase the value && you'll get
+	/// infrequent updates => sparse headers => potential slow down of bridge applications, but pallet storage
+	/// won't be super large. Decrease the value to near `source_block_time` and you'll get
+	/// transaction for (almost) every block of the source chain => all source headers will be known
+	/// to the target chain => bridge applications will run faster, but pallet storage may explode
+	/// (but if pruning is there, then it's fine).
+	pub tick: Duration,
+	/// Number of finality proofs to keep in internal buffer between loop wakeups.
+	///
+	/// While in "major syncing" state, we still read finality proofs from the stream. They're stored
+	/// in the internal buffer between loop wakeups. When we're close to the tip of the chain, we may
+	/// meet finality delays if headers are not finalized frequently. So instead of waiting for next
+	/// finality proof to appear in the stream, we may use existing proof from that buffer.
+	pub recent_finality_proofs_limit: usize,
+	/// Timeout before we treat our transactions as lost and restart the whole sync process.
+	pub stall_timeout: Duration,
+}
+
+/// Source client used in finality synchronization loop.
+#[async_trait]
+pub trait SourceClient<P: FinalitySyncPipeline>: RelayClient {
+	/// Stream of new finality proofs. The stream is allowed to miss proofs for some
+	/// headers, even if those headers are mandatory.
+	type FinalityProofsStream: Stream<Item = P::FinalityProof>;
+
+	/// Get best finalized block number.
+	async fn best_finalized_block_number(&self) -> Result<P::Number, Self::Error>;
+
+	/// Get canonical header and its finality proof by number.
+	async fn header_and_finality_proof(
+		&self,
+		number: P::Number,
+	) -> Result<(P::Header, Option<P::FinalityProof>), Self::Error>;
+
+	/// Subscribe to new finality proofs.
+	async fn finality_proofs(&self) -> Result<Self::FinalityProofsStream, Self::Error>;
+}
+
+/// Target client used in finality synchronization loop.
+#[async_trait]
+pub trait TargetClient<P: FinalitySyncPipeline>: RelayClient {
+	/// Get best finalized source block number.
+	async fn best_finalized_source_block_number(&self) -> Result<P::Number, Self::Error>;
+
+	/// Submit header finality proof.
+	async fn submit_finality_proof(&self, header: P::Header, proof: P::FinalityProof) -> Result<(), Self::Error>;
+}
+
+/// Run finality proofs synchronization loop.
+pub fn run<P: FinalitySyncPipeline>(
+	source_client: impl SourceClient<P>,
+	target_client: impl TargetClient<P>,
+	sync_params: FinalitySyncParams,
+	metrics_params: Option<MetricsParams>,
+	exit_signal: impl Future<Output = ()>,
+) {
+	let exit_signal = exit_signal.shared();
+
+	let metrics_global = GlobalMetrics::default();
+	let metrics_sync = SyncLoopMetrics::default();
+	let metrics_enabled = metrics_params.is_some();
+	metrics_start(
+		format!("{}_to_{}_Sync", P::SOURCE_NAME, P::TARGET_NAME),
+		metrics_params,
+		&metrics_global,
+		&metrics_sync,
+	);
+
+	relay_utils::relay_loop::run(
+		relay_utils::relay_loop::RECONNECT_DELAY,
+		source_client,
+		target_client,
+		|source_client, target_client| {
+			run_until_connection_lost(
+				source_client,
+				target_client,
+				sync_params.clone(),
+				if metrics_enabled {
+					Some(metrics_global.clone())
+				} else {
+					None
+				},
+				if metrics_enabled {
+					Some(metrics_sync.clone())
+				} else {
+					None
+				},
+				exit_signal.clone(),
+			)
+		},
+	);
+}
+
+/// Unjustified headers container. Ordered by header number.
+pub(crate) type UnjustifiedHeaders<P> = Vec<<P as FinalitySyncPipeline>::Header>;
+/// Finality proofs container. Ordered by target header number.
+pub(crate) type FinalityProofs<P> = Vec<(
+	<P as FinalitySyncPipeline>::Number,
+	<P as FinalitySyncPipeline>::FinalityProof,
+)>;
+
+/// Error that may happen inside finality synchronization loop.
+#[derive(Debug)]
+enum Error<P: FinalitySyncPipeline, SourceError, TargetError> {
+	/// Source client request has failed with given error.
+	Source(SourceError),
+	/// Target client request has failed with given error.
+	Target(TargetError),
+	/// Finality proof for mandatory header is missing from the source node.
+	MissingMandatoryFinalityProof(P::Number),
+	/// The synchronization has stalled.
+	Stalled,
+}
+
+impl<P, SourceError, TargetError> Error<P, SourceError, TargetError>
+where
+	P: FinalitySyncPipeline,
+	SourceError: MaybeConnectionError,
+	TargetError: MaybeConnectionError,
+{
+	fn fail_if_connection_error(&self) -> Result<(), FailedClient> {
+		match *self {
+			Error::Source(ref error) if error.is_connection_error() => Err(FailedClient::Source),
+			Error::Target(ref error) if error.is_connection_error() => Err(FailedClient::Target),
+			Error::Stalled => Err(FailedClient::Both),
+			_ => Ok(()),
+		}
+	}
+}
+
+/// Information about transaction that we have submitted.
+#[derive(Debug, Clone)]
+struct Transaction<Number> {
+	/// Time when we have submitted this transaction.
+	pub time: Instant,
+	/// The number of the header we have submitted.
+	pub submitted_header_number: Number,
+}
+
+/// Finality proofs stream that may be restarted.
+struct RestartableFinalityProofsStream<S> {
+	/// Flag that the stream needs to be restarted.
+	needs_restart: bool,
+	/// The stream itself.
+	stream: Pin<Box<S>>,
+}
+
+/// Finality synchronization loop state.
+struct FinalityLoopState<'a, P: FinalitySyncPipeline, FinalityProofsStream> {
+	/// Synchronization loop progress.
+	progress: &'a mut (Instant, Option<P::Number>),
+	/// Finality proofs stream.
+	finality_proofs_stream: &'a mut RestartableFinalityProofsStream<FinalityProofsStream>,
+	/// Recent finality proofs that we have read from the stream.
+	recent_finality_proofs: &'a mut FinalityProofs<P>,
+	/// Last transaction that we have submitted to the target node.
+	last_transaction: Option<Transaction<P::Number>>,
+}
+
+async fn run_until_connection_lost<P: FinalitySyncPipeline>(
+	source_client: impl SourceClient<P>,
+	target_client: impl TargetClient<P>,
+	sync_params: FinalitySyncParams,
+	metrics_global: Option<GlobalMetrics>,
+	metrics_sync: Option<SyncLoopMetrics>,
+	exit_signal: impl Future<Output = ()>,
+) -> Result<(), FailedClient> {
+	let restart_finality_proofs_stream = || async {
+		source_client.finality_proofs().await.map_err(|error| {
+			log::error!(
+				target: "bridge",
+				"Failed to subscribe to {} justifications: {:?}. Going to reconnect",
+				P::SOURCE_NAME,
+				error,
+			);
+
+			FailedClient::Source
+		})
+	};
+
+	let exit_signal = exit_signal.fuse();
+	futures::pin_mut!(exit_signal);
+
+	let mut finality_proofs_stream = RestartableFinalityProofsStream {
+		needs_restart: false,
+		stream: Box::pin(restart_finality_proofs_stream().await?),
+	};
+	let mut recent_finality_proofs = Vec::new();
+
+	let mut progress = (Instant::now(), None);
+	let mut retry_backoff = retry_backoff();
+	let mut last_transaction = None;
+
+	loop {
+		// run loop iteration
+		let iteration_result = run_loop_iteration(
+			&source_client,
+			&target_client,
+			FinalityLoopState {
+				progress: &mut progress,
+				finality_proofs_stream: &mut finality_proofs_stream,
+				recent_finality_proofs: &mut recent_finality_proofs,
+				last_transaction: last_transaction.clone(),
+			},
+			&sync_params,
+			&metrics_sync,
+		)
+		.await;
+
+		// update global metrics
+		if let Some(ref metrics_global) = metrics_global {
+			metrics_global.update().await;
+		}
+
+		// deal with errors
+		let next_tick = match iteration_result {
+			Ok(updated_last_transaction) => {
+				last_transaction = updated_last_transaction;
+				retry_backoff.reset();
+				sync_params.tick
+			}
+			Err(error) => {
+				log::error!(target: "bridge", "Finality sync loop iteration has failed with error: {:?}", error);
+				error.fail_if_connection_error()?;
+				retry_backoff
+					.next_backoff()
+					.unwrap_or(relay_utils::relay_loop::RECONNECT_DELAY)
+			}
+		};
+		if finality_proofs_stream.needs_restart {
+			finality_proofs_stream.needs_restart = false;
+			finality_proofs_stream.stream = Box::pin(restart_finality_proofs_stream().await?);
+		}
+
+		// wait till exit signal, or new source block
+		select! {
+			_ = async_std::task::sleep(next_tick).fuse() => {},
+			_ = exit_signal => return Ok(()),
+		}
+	}
+}
+
+async fn run_loop_iteration<P, SC, TC>(
+	source_client: &SC,
+	target_client: &TC,
+	state: FinalityLoopState<'_, P, SC::FinalityProofsStream>,
+	sync_params: &FinalitySyncParams,
+	metrics_sync: &Option<SyncLoopMetrics>,
+) -> Result<Option<Transaction<P::Number>>, Error<P, SC::Error, TC::Error>>
+where
+	P: FinalitySyncPipeline,
+	SC: SourceClient<P>,
+	TC: TargetClient<P>,
+{
+	// read best source headers ids from source and target nodes
+	let best_number_at_source = source_client
+		.best_finalized_block_number()
+		.await
+		.map_err(Error::Source)?;
+	let best_number_at_target = target_client
+		.best_finalized_source_block_number()
+		.await
+		.map_err(Error::Target)?;
+	if let Some(ref metrics_sync) = *metrics_sync {
+		metrics_sync.update_best_block_at_source(best_number_at_source);
+		metrics_sync.update_best_block_at_target(best_number_at_target);
+	}
+	*state.progress = print_sync_progress::<P>(*state.progress, best_number_at_source, best_number_at_target);
+
+	// if we have already submitted header, then we just need to wait for it
+	// if we're waiting too much, then we believe our transaction has been lost and restart sync
+	if let Some(last_transaction) = state.last_transaction {
+		if best_number_at_target >= last_transaction.submitted_header_number {
+			// transaction has been mined && we can continue
+		} else if last_transaction.time.elapsed() > sync_params.stall_timeout {
+			log::error!(
+				target: "bridge",
+				"Finality synchronization from {} to {} has stalled. Going to restart",
+				P::SOURCE_NAME,
+				P::TARGET_NAME,
+			);
+
+			return Err(Error::Stalled);
+		} else {
+			return Ok(Some(last_transaction));
+		}
+	}
+
+	// submit new header if we have something new
+	match select_header_to_submit(
+		source_client,
+		target_client,
+		state.finality_proofs_stream,
+		state.recent_finality_proofs,
+		best_number_at_source,
+		best_number_at_target,
+		sync_params,
+	)
+	.await?
+	{
+		Some((header, justification)) => {
+			let new_transaction = Transaction {
+				time: Instant::now(),
+				submitted_header_number: header.number(),
+			};
+
+			log::debug!(
+				target: "bridge",
+				"Going to submit finality proof of {} header #{:?} to {}",
+				P::SOURCE_NAME,
+				new_transaction.submitted_header_number,
+				P::TARGET_NAME,
+			);
+
+			target_client
+				.submit_finality_proof(header, justification)
+				.await
+				.map_err(Error::Target)?;
+			Ok(Some(new_transaction))
+		}
+		None => Ok(None),
+	}
+}
+
+async fn select_header_to_submit<P, SC, TC>(
+	source_client: &SC,
+	_target_client: &TC,
+	finality_proofs_stream: &mut RestartableFinalityProofsStream<SC::FinalityProofsStream>,
+	recent_finality_proofs: &mut FinalityProofs<P>,
+	best_number_at_source: P::Number,
+	best_number_at_target: P::Number,
+	sync_params: &FinalitySyncParams,
+) -> Result<Option<(P::Header, P::FinalityProof)>, Error<P, SC::Error, TC::Error>>
+where
+	P: FinalitySyncPipeline,
+	SC: SourceClient<P>,
+	TC: TargetClient<P>,
+{
+	let mut selected_finality_proof = None;
+	let mut unjustified_headers = Vec::new();
+
+	// to see that the loop is progressing
+	log::trace!(
+		target: "bridge",
+		"Considering range of headers ({:?}; {:?}]",
+		best_number_at_target,
+		best_number_at_source,
+	);
+
+	// read missing headers. if we see that the header schedules GRANDPA change, we need to
+	// submit this header
+	let mut header_number = best_number_at_target + One::one();
+	while header_number <= best_number_at_source {
+		let (header, finality_proof) = source_client
+			.header_and_finality_proof(header_number)
+			.await
+			.map_err(Error::Source)?;
+		let is_mandatory = header.is_mandatory();
+
+		match (is_mandatory, finality_proof) {
+			(true, Some(finality_proof)) => {
+				log::trace!(target: "bridge", "Header {:?} is mandatory", header_number);
+				return Ok(Some((header, finality_proof)));
+			}
+			(true, None) => return Err(Error::MissingMandatoryFinalityProof(header.number())),
+			(false, Some(finality_proof)) => {
+				log::trace!(target: "bridge", "Header {:?} has persistent finality proof", header_number);
+				selected_finality_proof = Some((header, finality_proof));
+				prune_unjustified_headers::<P>(header_number, &mut unjustified_headers);
+			}
+			(false, None) => {
+				unjustified_headers.push(header);
+			}
+		}
+
+		header_number = header_number + One::one();
+	}
+
+	// see if we can improve finality by using recent finality proofs
+	if !unjustified_headers.is_empty() && !recent_finality_proofs.is_empty() {
+		const NOT_EMPTY_PROOF: &str = "we have checked that the vec is not empty; qed";
+
+		// we need proofs for headers in range unjustified_range_begin..=unjustified_range_end
+		let unjustified_range_begin = unjustified_headers.first().expect(NOT_EMPTY_PROOF).number();
+		let unjustified_range_end = unjustified_headers.last().expect(NOT_EMPTY_PROOF).number();
+
+		// we have proofs for headers in range buffered_range_begin..=buffered_range_end
+		let buffered_range_begin = recent_finality_proofs.first().expect(NOT_EMPTY_PROOF).0;
+		let buffered_range_end = recent_finality_proofs.last().expect(NOT_EMPTY_PROOF).0;
+
+		// we have two ranges => find intersection
+		let intersection_begin = std::cmp::max(unjustified_range_begin, buffered_range_begin);
+		let intersection_end = std::cmp::min(unjustified_range_end, buffered_range_end);
+		let intersection = intersection_begin..=intersection_end;
+
+		// find last proof from intersection
+		let selected_finality_proof_index = recent_finality_proofs
+			.binary_search_by_key(intersection.end(), |(number, _)| *number)
+			.unwrap_or_else(|index| index.saturating_sub(1));
+		let (selected_header_number, finality_proof) = &recent_finality_proofs[selected_finality_proof_index];
+		if intersection.contains(selected_header_number) {
+			// now remove all obsolete headers and extract selected header
+			let selected_header = prune_unjustified_headers::<P>(*selected_header_number, &mut unjustified_headers)
+				.expect("unjustified_headers contain all headers from intersection; qed");
+			selected_finality_proof = Some((selected_header, finality_proof.clone()));
+		}
+	}
+
+	// read all proofs from the stream, probably selecting updated proof that we're going to submit
+	loop {
+		let next_proof = finality_proofs_stream.stream.next();
+		let finality_proof = match next_proof.now_or_never() {
+			Some(Some(finality_proof)) => finality_proof,
+			Some(None) => {
+				finality_proofs_stream.needs_restart = true;
+				break;
+			}
+			None => break,
+		};
+		let finality_proof_target_header_number = match finality_proof.target_header_number() {
+			Some(target_header_number) => target_header_number,
+			None => {
+				continue;
+			}
+		};
+
+		let justified_header =
+			prune_unjustified_headers::<P>(finality_proof_target_header_number, &mut unjustified_headers);
+		if let Some(justified_header) = justified_header {
+			recent_finality_proofs.clear();
+			selected_finality_proof = Some((justified_header, finality_proof));
+		} else {
+			// the number of proofs read during single wakeup is expected to be low, so we aren't pruning
+			// `recent_finality_proofs` collection too often
+			recent_finality_proofs.push((finality_proof_target_header_number, finality_proof));
+		}
+	}
+
+	// remove obsolete 'recent' finality proofs + keep its size under certain limit
+	let oldest_finality_proof_to_keep = selected_finality_proof
+		.as_ref()
+		.map(|(header, _)| header.number())
+		.unwrap_or(best_number_at_target);
+	prune_recent_finality_proofs::<P>(
+		oldest_finality_proof_to_keep,
+		recent_finality_proofs,
+		sync_params.recent_finality_proofs_limit,
+	);
+
+	Ok(selected_finality_proof)
+}
+
+/// Remove headers from `unjustified_headers` collection with number lower or equal than `justified_header_number`.
+///
+/// Returns the header that matches `justified_header_number` (if any).
+pub(crate) fn prune_unjustified_headers<P: FinalitySyncPipeline>(
+	justified_header_number: P::Number,
+	unjustified_headers: &mut UnjustifiedHeaders<P>,
+) -> Option<P::Header> {
+	prune_ordered_vec(justified_header_number, unjustified_headers, usize::MAX, |header| {
+		header.number()
+	})
+}
+
+pub(crate) fn prune_recent_finality_proofs<P: FinalitySyncPipeline>(
+	justified_header_number: P::Number,
+	recent_finality_proofs: &mut FinalityProofs<P>,
+	recent_finality_proofs_limit: usize,
+) {
+	prune_ordered_vec(
+		justified_header_number,
+		recent_finality_proofs,
+		recent_finality_proofs_limit,
+		|(header_number, _)| *header_number,
+	);
+}
+
+fn prune_ordered_vec<T, Number: relay_utils::BlockNumberBase>(
+	header_number: Number,
+	ordered_vec: &mut Vec<T>,
+	maximal_vec_size: usize,
+	extract_header_number: impl Fn(&T) -> Number,
+) -> Option<T> {
+	let position = ordered_vec.binary_search_by_key(&header_number, extract_header_number);
+
+	// first extract element we're interested in
+	let extracted_element = match position {
+		Ok(position) => {
+			let updated_vec = ordered_vec.split_off(position + 1);
+			let extracted_element = ordered_vec.pop().expect(
+				"binary_search_by_key has returned Ok(); so there's element at `position`;\
+					we're splitting vec at `position+1`; so we have pruned at least 1 element;\
+					qed",
+			);
+			*ordered_vec = updated_vec;
+			Some(extracted_element)
+		}
+		Err(position) => {
+			*ordered_vec = ordered_vec.split_off(position);
+			None
+		}
+	};
+
+	// now - limit vec by size
+	let split_index = ordered_vec.len().saturating_sub(maximal_vec_size);
+	*ordered_vec = ordered_vec.split_off(split_index);
+
+	extracted_element
+}
+
+fn print_sync_progress<P: FinalitySyncPipeline>(
+	progress_context: (Instant, Option<P::Number>),
+	best_number_at_source: P::Number,
+	best_number_at_target: P::Number,
+) -> (Instant, Option<P::Number>) {
+	let (prev_time, prev_best_number_at_target) = progress_context;
+	let now = Instant::now();
+
+	let need_update = now - prev_time > Duration::from_secs(10)
+		|| prev_best_number_at_target
+			.map(|prev_best_number_at_target| {
+				best_number_at_target.saturating_sub(prev_best_number_at_target) > 10.into()
+			})
+			.unwrap_or(true);
+
+	if !need_update {
+		return (prev_time, prev_best_number_at_target);
+	}
+
+	log::info!(
+		target: "bridge",
+		"Synced {:?} of {:?} headers",
+		best_number_at_target,
+		best_number_at_source,
+	);
+	(now, Some(best_number_at_target))
+}

bridges/relays/finality-relay/src/finality_loop_tests.rs

@@ -0,0 +1,339 @@
+// 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.
+
+// 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/>.
+
+//! Tests for finality synchronization loop.
+
+#![cfg(test)]
+
+use crate::finality_loop::{
+	prune_recent_finality_proofs, prune_unjustified_headers, run, FinalityProofs, FinalitySyncParams, SourceClient,
+	TargetClient, UnjustifiedHeaders,
+};
+use crate::{FinalityProof, FinalitySyncPipeline, SourceHeader};
+
+use async_trait::async_trait;
+use futures::{FutureExt, Stream, StreamExt};
+use parking_lot::Mutex;
+use relay_utils::{relay_loop::Client as RelayClient, MaybeConnectionError};
+use std::{collections::HashMap, pin::Pin, sync::Arc, time::Duration};
+
+type IsMandatory = bool;
+type TestNumber = u64;
+
+#[derive(Debug, Clone)]
+enum TestError {
+	NonConnection,
+}
+
+impl MaybeConnectionError for TestError {
+	fn is_connection_error(&self) -> bool {
+		false
+	}
+}
+
+#[derive(Debug, Clone)]
+struct TestFinalitySyncPipeline;
+
+impl FinalitySyncPipeline for TestFinalitySyncPipeline {
+	const SOURCE_NAME: &'static str = "TestSource";
+	const TARGET_NAME: &'static str = "TestTarget";
+
+	type Hash = u64;
+	type Number = TestNumber;
+	type Header = TestSourceHeader;
+	type FinalityProof = TestFinalityProof;
+}
+
+#[derive(Debug, Clone, PartialEq)]
+struct TestSourceHeader(IsMandatory, TestNumber);
+
+impl SourceHeader<TestNumber> for TestSourceHeader {
+	fn number(&self) -> TestNumber {
+		self.1
+	}
+
+	fn is_mandatory(&self) -> bool {
+		self.0
+	}
+}
+
+#[derive(Debug, Clone, PartialEq)]
+struct TestFinalityProof(Option<TestNumber>);
+
+impl FinalityProof<TestNumber> for TestFinalityProof {
+	fn target_header_number(&self) -> Option<TestNumber> {
+		self.0
+	}
+}
+
+#[derive(Debug, Clone, Default)]
+struct ClientsData {
+	source_best_block_number: TestNumber,
+	source_headers: HashMap<TestNumber, (TestSourceHeader, Option<TestFinalityProof>)>,
+	source_proofs: Vec<TestFinalityProof>,
+
+	target_best_block_number: TestNumber,
+	target_headers: Vec<(TestSourceHeader, TestFinalityProof)>,
+}
+
+#[derive(Clone)]
+struct TestSourceClient {
+	on_method_call: Arc<dyn Fn(&mut ClientsData) + Send + Sync>,
+	data: Arc<Mutex<ClientsData>>,
+}
+
+#[async_trait]
+impl RelayClient for TestSourceClient {
+	type Error = TestError;
+
+	async fn reconnect(&mut self) -> Result<(), TestError> {
+		unreachable!()
+	}
+}
+
+#[async_trait]
+impl SourceClient<TestFinalitySyncPipeline> for TestSourceClient {
+	type FinalityProofsStream = Pin<Box<dyn Stream<Item = TestFinalityProof>>>;
+
+	async fn best_finalized_block_number(&self) -> Result<TestNumber, TestError> {
+		let mut data = self.data.lock();
+		(self.on_method_call)(&mut *data);
+		Ok(data.source_best_block_number)
+	}
+
+	async fn header_and_finality_proof(
+		&self,
+		number: TestNumber,
+	) -> Result<(TestSourceHeader, Option<TestFinalityProof>), TestError> {
+		let mut data = self.data.lock();
+		(self.on_method_call)(&mut *data);
+		data.source_headers
+			.get(&number)
+			.cloned()
+			.ok_or(TestError::NonConnection)
+	}
+
+	async fn finality_proofs(&self) -> Result<Self::FinalityProofsStream, TestError> {
+		let mut data = self.data.lock();
+		(self.on_method_call)(&mut *data);
+		Ok(futures::stream::iter(data.source_proofs.clone()).boxed())
+	}
+}
+
+#[derive(Clone)]
+struct TestTargetClient {
+	on_method_call: Arc<dyn Fn(&mut ClientsData) + Send + Sync>,
+	data: Arc<Mutex<ClientsData>>,
+}
+
+#[async_trait]
+impl RelayClient for TestTargetClient {
+	type Error = TestError;
+
+	async fn reconnect(&mut self) -> Result<(), TestError> {
+		unreachable!()
+	}
+}
+
+#[async_trait]
+impl TargetClient<TestFinalitySyncPipeline> for TestTargetClient {
+	async fn best_finalized_source_block_number(&self) -> Result<TestNumber, TestError> {
+		let mut data = self.data.lock();
+		(self.on_method_call)(&mut *data);
+		Ok(data.target_best_block_number)
+	}
+
+	async fn submit_finality_proof(&self, header: TestSourceHeader, proof: TestFinalityProof) -> Result<(), TestError> {
+		let mut data = self.data.lock();
+		(self.on_method_call)(&mut *data);
+		data.target_best_block_number = header.number();
+		data.target_headers.push((header, proof));
+		Ok(())
+	}
+}
+
+fn run_sync_loop(state_function: impl Fn(&mut ClientsData) -> bool + Send + Sync + 'static) -> ClientsData {
+	let (exit_sender, exit_receiver) = futures::channel::mpsc::unbounded();
+	let internal_state_function: Arc<dyn Fn(&mut ClientsData) + Send + Sync> = Arc::new(move |data| {
+		if state_function(data) {
+			exit_sender.unbounded_send(()).unwrap();
+		}
+	});
+	let clients_data = Arc::new(Mutex::new(ClientsData {
+		source_best_block_number: 10,
+		source_headers: vec![
+			(6, (TestSourceHeader(false, 6), None)),
+			(7, (TestSourceHeader(false, 7), Some(TestFinalityProof(Some(7))))),
+			(8, (TestSourceHeader(true, 8), Some(TestFinalityProof(Some(8))))),
+			(9, (TestSourceHeader(false, 9), Some(TestFinalityProof(Some(9))))),
+			(10, (TestSourceHeader(false, 10), None)),
+		]
+		.into_iter()
+		.collect(),
+		source_proofs: vec![TestFinalityProof(Some(12)), TestFinalityProof(Some(14))],
+
+		target_best_block_number: 5,
+		target_headers: vec![],
+	}));
+	let source_client = TestSourceClient {
+		on_method_call: internal_state_function.clone(),
+		data: clients_data.clone(),
+	};
+	let target_client = TestTargetClient {
+		on_method_call: internal_state_function,
+		data: clients_data.clone(),
+	};
+	let sync_params = FinalitySyncParams {
+		tick: Duration::from_secs(0),
+		recent_finality_proofs_limit: 1024,
+		stall_timeout: Duration::from_secs(1),
+	};
+
+	run(
+		source_client,
+		target_client,
+		sync_params,
+		None,
+		exit_receiver.into_future().map(|(_, _)| ()),
+	);
+
+	let clients_data = clients_data.lock().clone();
+	clients_data
+}
+
+#[test]
+fn finality_sync_loop_works() {
+	let client_data = run_sync_loop(|data| {
+		// header#7 has persistent finality proof, but it isn't mandatory => it isn't submitted, because
+		// header#8 has persistent finality proof && it is mandatory => it is submitted
+		// header#9 has persistent finality proof, but it isn't mandatory => it is submitted, because
+		//   there are no more persistent finality proofs
+		//
+		// once this ^^^ is done, we generate more blocks && read proof for blocks 12, 14 and 16 from the stream
+		// but we only submit proof for 16
+		//
+		// proof for block 15 is ignored - we haven't managed to decode it
+		if data.target_best_block_number == 9 {
+			data.source_best_block_number = 17;
+			data.source_headers.insert(11, (TestSourceHeader(false, 11), None));
+			data.source_headers
+				.insert(12, (TestSourceHeader(false, 12), Some(TestFinalityProof(Some(12)))));
+			data.source_headers.insert(13, (TestSourceHeader(false, 13), None));
+			data.source_headers
+				.insert(14, (TestSourceHeader(false, 14), Some(TestFinalityProof(Some(14)))));
+			data.source_headers
+				.insert(15, (TestSourceHeader(false, 15), Some(TestFinalityProof(None))));
+			data.source_headers
+				.insert(16, (TestSourceHeader(false, 16), Some(TestFinalityProof(Some(16)))));
+			data.source_headers.insert(17, (TestSourceHeader(false, 17), None));
+		}
+
+		data.target_best_block_number == 16
+	});
+
+	assert_eq!(
+		client_data.target_headers,
+		vec![
+			(TestSourceHeader(true, 8), TestFinalityProof(Some(8))),
+			(TestSourceHeader(false, 9), TestFinalityProof(Some(9))),
+			(TestSourceHeader(false, 16), TestFinalityProof(Some(16))),
+		],
+	);
+}
+
+#[test]
+fn prune_unjustified_headers_works() {
+	let original_unjustified_headers: UnjustifiedHeaders<TestFinalitySyncPipeline> = vec![
+		TestSourceHeader(false, 10),
+		TestSourceHeader(false, 13),
+		TestSourceHeader(false, 15),
+		TestSourceHeader(false, 17),
+		TestSourceHeader(false, 19),
+	]
+	.into_iter()
+	.collect();
+
+	// when header is in the collection
+	let mut unjustified_headers = original_unjustified_headers.clone();
+	assert_eq!(
+		prune_unjustified_headers::<TestFinalitySyncPipeline>(10, &mut unjustified_headers),
+		Some(TestSourceHeader(false, 10)),
+	);
+	assert_eq!(&original_unjustified_headers[1..], unjustified_headers,);
+
+	// when the header doesn't exist in the collection
+	let mut unjustified_headers = original_unjustified_headers.clone();
+	assert_eq!(
+		prune_unjustified_headers::<TestFinalitySyncPipeline>(11, &mut unjustified_headers),
+		None,
+	);
+	assert_eq!(&original_unjustified_headers[1..], unjustified_headers,);
+
+	// when last entry is pruned
+	let mut unjustified_headers = original_unjustified_headers.clone();
+	assert_eq!(
+		prune_unjustified_headers::<TestFinalitySyncPipeline>(19, &mut unjustified_headers),
+		Some(TestSourceHeader(false, 19)),
+	);
+
+	assert_eq!(&original_unjustified_headers[5..], unjustified_headers,);
+
+	// when we try and prune past last entry
+	let mut unjustified_headers = original_unjustified_headers.clone();
+	assert_eq!(
+		prune_unjustified_headers::<TestFinalitySyncPipeline>(20, &mut unjustified_headers),
+		None,
+	);
+	assert_eq!(&original_unjustified_headers[5..], unjustified_headers,);
+}
+
+#[test]
+fn prune_recent_finality_proofs_works() {
+	let original_recent_finality_proofs: FinalityProofs<TestFinalitySyncPipeline> = vec![
+		(10, TestFinalityProof(Some(10))),
+		(13, TestFinalityProof(Some(13))),
+		(15, TestFinalityProof(Some(15))),
+		(17, TestFinalityProof(Some(17))),
+		(19, TestFinalityProof(Some(19))),
+	]
+	.into_iter()
+	.collect();
+
+	// when there's proof for justified header in the vec
+	let mut recent_finality_proofs = original_recent_finality_proofs.clone();
+	prune_recent_finality_proofs::<TestFinalitySyncPipeline>(10, &mut recent_finality_proofs, 1024);
+	assert_eq!(&original_recent_finality_proofs[1..], recent_finality_proofs,);
+
+	// when there are no proof for justified header in the vec
+	let mut recent_finality_proofs = original_recent_finality_proofs.clone();
+	prune_recent_finality_proofs::<TestFinalitySyncPipeline>(11, &mut recent_finality_proofs, 1024);
+	assert_eq!(&original_recent_finality_proofs[1..], recent_finality_proofs,);
+
+	// when there are too many entries after initial prune && they also need to be pruned
+	let mut recent_finality_proofs = original_recent_finality_proofs.clone();
+	prune_recent_finality_proofs::<TestFinalitySyncPipeline>(10, &mut recent_finality_proofs, 2);
+	assert_eq!(&original_recent_finality_proofs[3..], recent_finality_proofs,);
+
+	// when last entry is pruned
+	let mut recent_finality_proofs = original_recent_finality_proofs.clone();
+	prune_recent_finality_proofs::<TestFinalitySyncPipeline>(19, &mut recent_finality_proofs, 2);
+	assert_eq!(&original_recent_finality_proofs[5..], recent_finality_proofs,);
+
+	// when post-last entry is pruned
+	let mut recent_finality_proofs = original_recent_finality_proofs.clone();
+	prune_recent_finality_proofs::<TestFinalitySyncPipeline>(20, &mut recent_finality_proofs, 2);
+	assert_eq!(&original_recent_finality_proofs[5..], recent_finality_proofs,);
+}

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

@@ -0,0 +1,60 @@
+// 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.
+
+// 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/>.
+
+//! This crate has single entrypoint to run synchronization loop that is built around finality
+//! proofs, as opposed to headers synchronization loop, which is built around headers. The headers
+//! are still submitted to the target node, but are treated as auxiliary data as we are not trying
+//! to submit all source headers to the target node.
+
+pub use crate::finality_loop::{run, FinalitySyncParams, SourceClient, TargetClient};
+
+use std::fmt::Debug;
+
+mod finality_loop;
+mod finality_loop_tests;
+
+/// Finality proofs synchronization pipeline.
+pub trait FinalitySyncPipeline: Clone + Debug + Send + Sync {
+	/// Name of the finality proofs source.
+	const SOURCE_NAME: &'static str;
+	/// Name of the finality proofs target.
+	const TARGET_NAME: &'static str;
+
+	/// Headers we're syncing are identified by this hash.
+	type Hash: Eq + Clone + Copy + Send + Sync + Debug;
+	/// Headers we're syncing are identified by this number.
+	type Number: relay_utils::BlockNumberBase;
+	/// Type of header that we're syncing.
+	type Header: SourceHeader<Self::Number>;
+	/// Finality proof type.
+	type FinalityProof: FinalityProof<Self::Number>;
+}
+
+/// Header that we're receiving from source node.
+pub trait SourceHeader<Number>: Clone + Debug + PartialEq + Send + Sync {
+	/// Returns number of header.
+	fn number(&self) -> Number;
+	/// Returns true if this header needs to be submitted to target node.
+	fn is_mandatory(&self) -> bool;
+}
+
+/// Abstract finality proof that is justifying block finality.
+pub trait FinalityProof<Number>: Clone + Send + Sync + Debug {
+	/// Return header id that this proof is generated for.
+	///
+	/// None is returned if proof is invalid from relayer PoV.
+	fn target_header_number(&self) -> Option<Number>;
+}