Equivocation detection loop: Reorganize block checking logic as state…

… machine (#2555) (#2557)

* Split `reconnect_failed_client()` logic

* Reorganize block checking logic as state machine

This way we'll be able to save the state in case of a failure

bridges/relays/equivocation/src/block_checker.rs

@@ -0,0 +1,252 @@
+// Copyright (C) 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/>.
+
+use crate::{
+	handle_client_error, reporter::EquivocationsReporter, EquivocationDetectionPipeline,
+	EquivocationReportingContext, HeaderFinalityInfo, SourceClient, TargetClient,
+};
+
+use bp_header_chain::{FinalityProof, FindEquivocations as FindEquivocationsT};
+use finality_relay::FinalityProofsBuf;
+use futures::future::{BoxFuture, FutureExt};
+use num_traits::Saturating;
+
+/// First step in the block checking state machine.
+///
+/// Getting the finality info associated to the source headers synced with the target chain
+/// at the specified block.
+pub struct ReadSyncedHeaders<P: EquivocationDetectionPipeline> {
+	pub target_block_num: P::TargetNumber,
+}
+
+impl<P: EquivocationDetectionPipeline> ReadSyncedHeaders<P> {
+	pub async fn next<TC: TargetClient<P>>(
+		self,
+		target_client: &mut TC,
+	) -> Result<ReadContext<P>, Self> {
+		match target_client.synced_headers_finality_info(self.target_block_num).await {
+			Ok(synced_headers) =>
+				Ok(ReadContext { target_block_num: self.target_block_num, synced_headers }),
+			Err(e) => {
+				log::error!(
+					target: "bridge",
+					"Could not get {} headers synced to {} at block {}: {e:?}",
+					P::SOURCE_NAME,
+					P::TARGET_NAME,
+					self.target_block_num
+				);
+
+				// Reconnect target client in case of a connection error.
+				handle_client_error(target_client, e).await;
+
+				Err(self)
+			},
+		}
+	}
+}
+
+/// Second step in the block checking state machine.
+///
+/// Reading the equivocation reporting context from the target chain.
+pub struct ReadContext<P: EquivocationDetectionPipeline> {
+	target_block_num: P::TargetNumber,
+	synced_headers: Vec<HeaderFinalityInfo<P>>,
+}
+
+impl<P: EquivocationDetectionPipeline> ReadContext<P> {
+	pub async fn next<TC: TargetClient<P>>(
+		self,
+		target_client: &mut TC,
+	) -> Result<Option<FindEquivocations<P>>, Self> {
+		match EquivocationReportingContext::try_read_from_target::<TC>(
+			target_client,
+			self.target_block_num.saturating_sub(1.into()),
+		)
+		.await
+		{
+			Ok(Some(context)) => Ok(Some(FindEquivocations {
+				target_block_num: self.target_block_num,
+				synced_headers: self.synced_headers,
+				context,
+			})),
+			Ok(None) => Ok(None),
+			Err(e) => {
+				log::error!(
+					target: "bridge",
+					"Could not read {} `EquivocationReportingContext` from {} at block {}: {e:?}",
+					P::SOURCE_NAME,
+					P::TARGET_NAME,
+					self.target_block_num.saturating_sub(1.into()),
+				);
+
+				// Reconnect target client in case of a connection error.
+				handle_client_error(target_client, e).await;
+
+				Err(self)
+			},
+		}
+	}
+}
+
+/// Third step in the block checking state machine.
+///
+/// Searching for equivocations in the source headers synced with the target chain.
+pub struct FindEquivocations<P: EquivocationDetectionPipeline> {
+	target_block_num: P::TargetNumber,
+	synced_headers: Vec<HeaderFinalityInfo<P>>,
+	context: EquivocationReportingContext<P>,
+}
+
+impl<P: EquivocationDetectionPipeline> FindEquivocations<P> {
+	pub fn next(
+		mut self,
+		finality_proofs_buf: &mut FinalityProofsBuf<P>,
+	) -> Vec<ReportEquivocations<P>> {
+		let mut result = vec![];
+		for synced_header in self.synced_headers {
+			match P::EquivocationsFinder::find_equivocations(
+				&self.context.synced_verification_context,
+				&synced_header.finality_proof,
+				finality_proofs_buf.buf().as_slice(),
+			) {
+				Ok(equivocations) => result.push(ReportEquivocations {
+					source_block_hash: self.context.synced_header_hash,
+					equivocations,
+				}),
+				Err(e) => {
+					log::error!(
+						target: "bridge",
+						"Could not search for equivocations in the finality proof \
+						for source header {:?} synced at target block {}: {e:?}",
+						synced_header.finality_proof.target_header_hash(),
+						self.target_block_num
+					);
+				},
+			};
+
+			finality_proofs_buf.prune(synced_header.finality_proof.target_header_number(), None);
+			self.context.update(synced_header);
+		}
+
+		result
+	}
+}
+
+/// Fourth step in the block checking state machine.
+///
+/// Reporting the detected equivocations (if any).
+pub struct ReportEquivocations<P: EquivocationDetectionPipeline> {
+	source_block_hash: P::Hash,
+	equivocations: Vec<P::EquivocationProof>,
+}
+
+impl<P: EquivocationDetectionPipeline> ReportEquivocations<P> {
+	pub async fn next<SC: SourceClient<P>>(
+		mut self,
+		source_client: &mut SC,
+		reporter: &mut EquivocationsReporter<P, SC>,
+	) -> Result<(), Self> {
+		let mut unprocessed_equivocations = vec![];
+		for equivocation in self.equivocations {
+			match reporter
+				.submit_report(source_client, self.source_block_hash, equivocation.clone())
+				.await
+			{
+				Ok(_) => {},
+				Err(e) => {
+					log::error!(
+						target: "bridge",
+						"Could not submit equivocation report to {} for {equivocation:?}: {e:?}",
+						P::SOURCE_NAME,
+					);
+
+					// Mark the equivocation as unprocessed
+					unprocessed_equivocations.push(equivocation);
+					// Reconnect source client in case of a connection error.
+					handle_client_error(source_client, e).await;
+				},
+			}
+		}
+
+		self.equivocations = unprocessed_equivocations;
+		if !self.equivocations.is_empty() {
+			return Err(self)
+		}
+
+		Ok(())
+	}
+}
+
+/// Block checking state machine.
+pub enum BlockChecker<P: EquivocationDetectionPipeline> {
+	ReadSyncedHeaders(ReadSyncedHeaders<P>),
+	ReadContext(ReadContext<P>),
+	ReportEquivocations(Vec<ReportEquivocations<P>>),
+}
+
+impl<P: EquivocationDetectionPipeline> BlockChecker<P> {
+	pub fn new(target_block_num: P::TargetNumber) -> Self {
+		Self::ReadSyncedHeaders(ReadSyncedHeaders { target_block_num })
+	}
+
+	pub fn run<'a, SC: SourceClient<P>, TC: TargetClient<P>>(
+		self,
+		source_client: &'a mut SC,
+		target_client: &'a mut TC,
+		finality_proofs_buf: &'a mut FinalityProofsBuf<P>,
+		reporter: &'a mut EquivocationsReporter<P, SC>,
+	) -> BoxFuture<'a, Result<(), Self>> {
+		async move {
+			match self {
+				Self::ReadSyncedHeaders(state) => {
+					let read_context =
+						state.next(target_client).await.map_err(Self::ReadSyncedHeaders)?;
+					Self::ReadContext(read_context)
+						.run(source_client, target_client, finality_proofs_buf, reporter)
+						.await
+				},
+				Self::ReadContext(state) => {
+					let maybe_find_equivocations =
+						state.next(target_client).await.map_err(Self::ReadContext)?;
+					let find_equivocations = match maybe_find_equivocations {
+						Some(find_equivocations) => find_equivocations,
+						None => return Ok(()),
+					};
+					Self::ReportEquivocations(find_equivocations.next(finality_proofs_buf))
+						.run(source_client, target_client, finality_proofs_buf, reporter)
+						.await
+				},
+				Self::ReportEquivocations(state) => {
+					let mut failures = vec![];
+					for report_equivocations in state {
+						if let Err(failure) =
+							report_equivocations.next(source_client, reporter).await
+						{
+							failures.push(failure);
+						}
+					}
+
+					if !failures.is_empty() {
+						return Err(Self::ReportEquivocations(failures))
+					}
+
+					Ok(())
+				},
+			}
+		}
+		.boxed()
+	}
+}