TransactionInvalidationTracker (#1544)

* TransactionInvalidationTracker

* TransacitonInvalidationTracker -> TransactionTracker

* change sign_transaction method

* clippy and spelling

* removed comment

* more transactiontracker tests

* stalls_when_transaction_tracker_returns_error

* remove test code

* remove "impl TransactionTracker for ()"

* enum TrackedTransactionStatus

* test TransactionTracker in on_transaction_status

* do_wait

bridges/relays/bin-substrate/src/cli/register_parachain.rs

@@ -26,10 +26,8 @@ use polkadot_runtime_common::{
 	paras_registrar::Call as ParaRegistrarCall, slots::Call as ParaSlotsCall,
 };
 use polkadot_runtime_parachains::paras::ParaLifecycle;
-use relay_substrate_client::{
-	AccountIdOf, CallOf, Chain, Client, HashOf, SignParam, Subscription, TransactionStatusOf,
-	UnsignedTransaction,
-};
+use relay_substrate_client::{AccountIdOf, CallOf, Chain, Client, SignParam, UnsignedTransaction};
+use relay_utils::{TrackedTransactionStatus, TransactionTracker};
 use rialto_runtime::SudoCall;
 use sp_core::{
 	storage::{well_known_keys::CODE, StorageKey},
@@ -116,26 +114,30 @@ impl RegisterParachain {
 				ParaRegistrarCall::reserve {}.into();
 			let reserve_parachain_signer = relay_sign.clone();
 			let (spec_version, transaction_version) = relay_client.simple_runtime_version().await?;
-			wait_until_transaction_is_finalized::<Relaychain>(
-				relay_client
-					.submit_and_watch_signed_extrinsic(
-						relay_sudo_account.clone(),
-						SignParam::<Relaychain> {
-							spec_version,
-							transaction_version,
-							genesis_hash: relay_genesis_hash,
-							signer: reserve_parachain_signer,
-						},
-						move |_, transaction_nonce| {
-							Ok(UnsignedTransaction::new(
-								reserve_parachain_id_call.into(),
-								transaction_nonce,
-							))
-						},
-					)
-					.await?,
-			)
-			.await?;
+			let reserve_result = relay_client
+				.submit_and_watch_signed_extrinsic(
+					relay_sudo_account.clone(),
+					SignParam::<Relaychain> {
+						spec_version,
+						transaction_version,
+						genesis_hash: relay_genesis_hash,
+						signer: reserve_parachain_signer,
+					},
+					move |_, transaction_nonce| {
+						Ok(UnsignedTransaction::new(
+							reserve_parachain_id_call.into(),
+							transaction_nonce,
+						))
+					},
+				)
+				.await?
+				.wait()
+				.await;
+			if reserve_result == TrackedTransactionStatus::Lost {
+				return Err(anyhow::format_err!(
+					"Failed to finalize `reserve-parachain-id` transaction"
+				))
+			}
 			log::info!(target: "bridge", "Reserved parachain id: {:?}", para_id);
 
 			// step 2: register parathread
@@ -161,26 +163,30 @@ impl RegisterParachain {
 			}
 			.into();
 			let register_parathread_signer = relay_sign.clone();
-			wait_until_transaction_is_finalized::<Relaychain>(
-				relay_client
-					.submit_and_watch_signed_extrinsic(
-						relay_sudo_account.clone(),
-						SignParam::<Relaychain> {
-							spec_version,
-							transaction_version,
-							genesis_hash: relay_genesis_hash,
-							signer: register_parathread_signer,
-						},
-						move |_, transaction_nonce| {
-							Ok(UnsignedTransaction::new(
-								register_parathread_call.into(),
-								transaction_nonce,
-							))
-						},
-					)
-					.await?,
-			)
-			.await?;
+			let register_result = relay_client
+				.submit_and_watch_signed_extrinsic(
+					relay_sudo_account.clone(),
+					SignParam::<Relaychain> {
+						spec_version,
+						transaction_version,
+						genesis_hash: relay_genesis_hash,
+						signer: register_parathread_signer,
+					},
+					move |_, transaction_nonce| {
+						Ok(UnsignedTransaction::new(
+							register_parathread_call.into(),
+							transaction_nonce,
+						))
+					},
+				)
+				.await?
+				.wait()
+				.await;
+			if register_result == TrackedTransactionStatus::Lost {
+				return Err(anyhow::format_err!(
+					"Failed to finalize `register-parathread` transaction"
+				))
+			}
 			log::info!(target: "bridge", "Registered parachain: {:?}. Waiting for onboarding", para_id);
 
 			// wait until parathread is onboarded
@@ -256,46 +262,6 @@ impl RegisterParachain {
 	}
 }
 
-/// Wait until transaction is included into finalized block.
-///
-/// Returns the hash of the finalized block with transaction.
-pub(crate) async fn wait_until_transaction_is_finalized<C: Chain>(
-	subscription: Subscription<TransactionStatusOf<C>>,
-) -> anyhow::Result<HashOf<C>> {
-	loop {
-		let transaction_status = subscription.next().await?;
-		match transaction_status {
-			Some(TransactionStatusOf::<C>::FinalityTimeout(_)) |
-			Some(TransactionStatusOf::<C>::Usurped(_)) |
-			Some(TransactionStatusOf::<C>::Dropped) |
-			Some(TransactionStatusOf::<C>::Invalid) |
-			None =>
-				return Err(anyhow::format_err!(
-					"We've been waiting for finalization of {} transaction, but it now has the {:?} status",
-					C::NAME,
-					transaction_status,
-				)),
-			Some(TransactionStatusOf::<C>::Finalized(block_hash)) => {
-				log::trace!(
-					target: "bridge",
-					"{} transaction has been finalized at block {}",
-					C::NAME,
-					block_hash,
-				);
-				return Ok(block_hash)
-			},
-			_ => {
-				log::trace!(
-					target: "bridge",
-					"Received intermediate status of {} transaction: {:?}",
-					C::NAME,
-					transaction_status,
-				);
-			},
-		}
-	}
-}
-
 /// Wait until parachain state is changed.
 async fn wait_para_state<Relaychain: Chain>(
 	relay_client: &Client<Relaychain>,

bridges/relays/client-substrate/src/client.rs

@@ -23,8 +23,8 @@ use crate::{
 		SubstrateGrandpaClient, SubstrateStateClient, SubstrateSystemClient,
 		SubstrateTransactionPaymentClient,
 	},
-	ConnectionParams, Error, HashOf, HeaderIdOf, Result, SignParam, TransactionSignScheme,
-	TransactionStatusOf, UnsignedTransaction,
+	transaction_stall_timeout, ConnectionParams, Error, HashOf, HeaderIdOf, Result, SignParam,
+	TransactionSignScheme, TransactionTracker, UnsignedTransaction,
 };
 
 use async_std::sync::{Arc, Mutex};
@@ -40,7 +40,7 @@ use jsonrpsee::{
 use num_traits::{Bounded, Zero};
 use pallet_balances::AccountData;
 use pallet_transaction_payment::InclusionFee;
-use relay_utils::relay_loop::RECONNECT_DELAY;
+use relay_utils::{relay_loop::RECONNECT_DELAY, STALL_TIMEOUT};
 use sp_core::{
 	storage::{StorageData, StorageKey},
 	Bytes, Hasher,
@@ -58,7 +58,7 @@ const SUB_API_TXPOOL_VALIDATE_TRANSACTION: &str = "TaggedTransactionQueue_valida
 const MAX_SUBSCRIPTION_CAPACITY: usize = 4096;
 
 /// Opaque justifications subscription type.
-pub struct Subscription<T>(Mutex<futures::channel::mpsc::Receiver<Option<T>>>);
+pub struct Subscription<T>(pub(crate) Mutex<futures::channel::mpsc::Receiver<Option<T>>>);
 
 /// Opaque GRANDPA authorities set.
 pub type OpaqueGrandpaAuthoritiesSet = Vec<u8>;
@@ -467,14 +467,20 @@ impl<C: Chain> Client<C> {
 		prepare_extrinsic: impl FnOnce(HeaderIdOf<C>, C::Index) -> Result<UnsignedTransaction<C>>
 			+ Send
 			+ 'static,
-	) -> Result<Subscription<TransactionStatusOf<C>>> {
+	) -> Result<TransactionTracker<C>> {
 		let _guard = self.submit_signed_extrinsic_lock.lock().await;
 		let transaction_nonce = self.next_account_index(extrinsic_signer).await?;
 		let best_header = self.best_header().await?;
 		let best_header_id = best_header.id();
-		let subscription = self
+		let (sender, receiver) = futures::channel::mpsc::channel(MAX_SUBSCRIPTION_CAPACITY);
+		let (tracker, subscription) = self
 			.jsonrpsee_execute(move |client| async move {
 				let extrinsic = prepare_extrinsic(best_header_id, transaction_nonce)?;
+				let stall_timeout = transaction_stall_timeout(
+					extrinsic.era.mortality_period(),
+					C::AVERAGE_BLOCK_INTERVAL,
+					STALL_TIMEOUT,
+				);
 				let signed_extrinsic = S::sign_transaction(signing_data, extrinsic)?.encode();
 				let tx_hash = C::Hasher::hash(&signed_extrinsic);
 				let subscription = SubstrateAuthorClient::<C>::submit_and_watch_extrinsic(
@@ -487,17 +493,21 @@ impl<C: Chain> Client<C> {
 					e
 				})?;
 				log::trace!(target: "bridge", "Sent transaction to {} node: {:?}", C::NAME, tx_hash);
-				Ok(subscription)
+				let tracker = TransactionTracker::new(
+					stall_timeout,
+					tx_hash,
+					Subscription(Mutex::new(receiver)),
+				);
+				Ok((tracker, subscription))
 			})
 			.await?;
-		let (sender, receiver) = futures::channel::mpsc::channel(MAX_SUBSCRIPTION_CAPACITY);
 		self.tokio.spawn(Subscription::background_worker(
 			C::NAME.into(),
 			"extrinsic".into(),
 			subscription,
 			sender,
 		));
-		Ok(Subscription(Mutex::new(receiver)))
+		Ok(tracker)
 	}
 
 	/// Returns pending extrinsics from transaction pool.
@@ -669,6 +679,14 @@ impl<C: Chain> Client<C> {
 }
 
 impl<T: DeserializeOwned> Subscription<T> {
+	/// Consumes subscription and returns future statuses stream.
+	pub fn into_stream(self) -> impl futures::Stream<Item = T> {
+		futures::stream::unfold(self, |this| async {
+			let item = this.0.lock().await.next().await.unwrap_or(None);
+			item.map(|i| (i, this))
+		})
+	}
+
 	/// Return next item from the subscription.
 	pub async fn next(&self) -> Result<Option<T>> {
 		let mut receiver = self.0.lock().await;

bridges/relays/client-substrate/src/lib.rs

@@ -23,6 +23,7 @@ mod client;
 mod error;
 mod rpc;
 mod sync_header;
+mod transaction_tracker;
 
 pub mod guard;
 pub mod metrics;
@@ -39,6 +40,7 @@ pub use crate::{
 	client::{ChainRuntimeVersion, Client, OpaqueGrandpaAuthoritiesSet, Subscription},
 	error::{Error, Result},
 	sync_header::SyncHeader,
+	transaction_tracker::TransactionTracker,
 };
 pub use bp_runtime::{
 	AccountIdOf, AccountPublicOf, BalanceOf, BlockNumberOf, Chain as ChainBase, HashOf, HeaderOf,

bridges/relays/client-substrate/src/transaction_tracker.rs

@@ -0,0 +1,322 @@
+// 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/>.
+
+//! Helper for tracking transaction invalidation events.
+
+use crate::{Chain, HashOf, Subscription, TransactionStatusOf};
+
+use async_trait::async_trait;
+use futures::{Stream, StreamExt};
+use relay_utils::TrackedTransactionStatus;
+use std::time::Duration;
+
+/// Substrate transaction tracker implementation.
+///
+/// Substrate node provides RPC API to submit and watch for transaction events. This way
+/// we may know when transaction is included into block, finalized or rejected. There are
+/// some edge cases, when we can't fully trust this mechanism - e.g. transaction may broadcasted
+/// and then dropped out of node transaction pool (some other cases are also possible - node
+/// restarts, connection lost, ...). Then we can't know for sure - what is currently happening
+/// with our transaction. Is the transaction really lost? Is it still alive on the chain network?
+///
+/// We have several options to handle such cases:
+///
+/// 1) hope that the transaction is still alive and wait for its mining until it is spoiled;
+///
+/// 2) assume that the transaction is lost and resubmit another transaction instantly;
+///
+/// 3) wait for some time (if transaction is mortal - then until block where it dies; if it is
+///    immortal - then for some time that we assume is long enough to mine it) and assume that
+///    it is lost.
+///
+/// This struct implements third option as it seems to be the most optimal.
+pub struct TransactionTracker<C: Chain> {
+	transaction_hash: HashOf<C>,
+	stall_timeout: Duration,
+	subscription: Subscription<TransactionStatusOf<C>>,
+}
+
+impl<C: Chain> TransactionTracker<C> {
+	/// Create transaction tracker.
+	pub fn new(
+		stall_timeout: Duration,
+		transaction_hash: HashOf<C>,
+		subscription: Subscription<TransactionStatusOf<C>>,
+	) -> Self {
+		Self { stall_timeout, transaction_hash, subscription }
+	}
+
+	/// Wait for final transaction status and return it along with last known internal invalidation
+	/// status.
+	async fn do_wait(self) -> (TrackedTransactionStatus, InvalidationStatus) {
+		let invalidation_status = watch_transaction_status::<C, _>(
+			self.transaction_hash,
+			self.subscription.into_stream(),
+		)
+		.await;
+		match invalidation_status {
+			InvalidationStatus::Finalized =>
+				(TrackedTransactionStatus::Finalized, invalidation_status),
+			InvalidationStatus::Invalid => (TrackedTransactionStatus::Lost, invalidation_status),
+			InvalidationStatus::Lost => {
+				async_std::task::sleep(self.stall_timeout).await;
+				// if someone is still watching for our transaction, then we're reporting
+				// an error here (which is treated as "transaction lost")
+				log::trace!(
+					target: "bridge",
+					"{} transaction {:?} is considered lost after timeout",
+					C::NAME,
+					self.transaction_hash,
+				);
+
+				(TrackedTransactionStatus::Lost, invalidation_status)
+			},
+		}
+	}
+}
+
+#[async_trait]
+impl<C: Chain> relay_utils::TransactionTracker for TransactionTracker<C> {
+	async fn wait(self) -> TrackedTransactionStatus {
+		self.do_wait().await.0
+	}
+}
+
+/// Transaction invalidation status.
+///
+/// Note that in places where the `TransactionTracker` is used, the finalization event will be
+/// ignored - relay loops are detecting the mining/finalization using their own
+/// techniques. That's why we're using `InvalidationStatus` here.
+#[derive(Debug, PartialEq)]
+enum InvalidationStatus {
+	/// Transaction has been included into block and finalized.
+	Finalized,
+	/// Transaction has been invalidated.
+	Invalid,
+	/// We have lost track of transaction status.
+	Lost,
+}
+
+/// Watch for transaction status until transaction is finalized or we lose track of its status.
+async fn watch_transaction_status<C: Chain, S: Stream<Item = TransactionStatusOf<C>>>(
+	transaction_hash: HashOf<C>,
+	subscription: S,
+) -> InvalidationStatus {
+	futures::pin_mut!(subscription);
+
+	loop {
+		match subscription.next().await {
+			Some(TransactionStatusOf::<C>::Finalized(block_hash)) => {
+				// the only "successful" outcome of this method is when the block with transaction
+				// has been finalized
+				log::trace!(
+					target: "bridge",
+					"{} transaction {:?} has been finalized at block: {:?}",
+					C::NAME,
+					transaction_hash,
+					block_hash,
+				);
+				return InvalidationStatus::Finalized
+			},
+			Some(TransactionStatusOf::<C>::Invalid) => {
+				// if node says that the transaction is invalid, there are still chances that
+				// it is not actually invalid - e.g. if the block where transaction has been
+				// revalidated is retracted and transaction (at some other node pool) becomes
+				// valid again on other fork. But let's assume that the chances of this event
+				// are almost zero - there's a lot of things that must happen for this to be the
+				// case.
+				log::trace!(
+					target: "bridge",
+					"{} transaction {:?} has been invalidated",
+					C::NAME,
+					transaction_hash,
+				);
+				return InvalidationStatus::Invalid
+			},
+			Some(TransactionStatusOf::<C>::Future) |
+			Some(TransactionStatusOf::<C>::Ready) |
+			Some(TransactionStatusOf::<C>::Broadcast(_)) => {
+				// nothing important (for us) has happened
+			},
+			Some(TransactionStatusOf::<C>::InBlock(block_hash)) => {
+				// TODO: read matching system event (ExtrinsicSuccess or ExtrinsicFailed), log it
+				// here and use it later (on finality) for reporting invalid transaction
+				// https://github.com/paritytech/parity-bridges-common/issues/1464
+				log::trace!(
+					target: "bridge",
+					"{} transaction {:?} has been included in block: {:?}",
+					C::NAME,
+					transaction_hash,
+					block_hash,
+				);
+			},
+			Some(TransactionStatusOf::<C>::Retracted(block_hash)) => {
+				log::trace!(
+					target: "bridge",
+					"{} transaction {:?} at block {:?} has been retracted",
+					C::NAME,
+					transaction_hash,
+					block_hash,
+				);
+			},
+			Some(TransactionStatusOf::<C>::FinalityTimeout(block_hash)) => {
+				// finality is lagging? let's wait a bit more and report a stall
+				log::trace!(
+					target: "bridge",
+					"{} transaction {:?} block {:?} has not been finalized for too long",
+					C::NAME,
+					transaction_hash,
+					block_hash,
+				);
+				return InvalidationStatus::Lost
+			},
+			Some(TransactionStatusOf::<C>::Usurped(new_transaction_hash)) => {
+				// this may be result of our transaction resubmitter work or some manual
+				// intervention. In both cases - let's start stall timeout, because the meaning
+				// of transaction may have changed
+				log::trace!(
+					target: "bridge",
+					"{} transaction {:?} has been usurped by new transaction: {:?}",
+					C::NAME,
+					transaction_hash,
+					new_transaction_hash,
+				);
+				return InvalidationStatus::Lost
+			},
+			Some(TransactionStatusOf::<C>::Dropped) => {
+				// the transaction has been removed from the pool because of its limits. Let's wait
+				// a bit and report a stall
+				log::trace!(
+					target: "bridge",
+					"{} transaction {:?} has been dropped from the pool",
+					C::NAME,
+					transaction_hash,
+				);
+				return InvalidationStatus::Lost
+			},
+			None => {
+				// the status of transaction is unknown to us (the subscription has been closed?).
+				// Let's wait a bit and report a stall
+				return InvalidationStatus::Lost
+			},
+		}
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+	use crate::test_chain::TestChain;
+	use futures::{FutureExt, SinkExt};
+	use sc_transaction_pool_api::TransactionStatus;
+
+	async fn on_transaction_status(
+		status: TransactionStatus<HashOf<TestChain>, HashOf<TestChain>>,
+	) -> Option<(TrackedTransactionStatus, InvalidationStatus)> {
+		let (mut sender, receiver) = futures::channel::mpsc::channel(1);
+		let tx_tracker = TransactionTracker::<TestChain>::new(
+			Duration::from_secs(0),
+			Default::default(),
+			Subscription(async_std::sync::Mutex::new(receiver)),
+		);
+
+		sender.send(Some(status)).await.unwrap();
+		tx_tracker.do_wait().now_or_never()
+	}
+
+	#[async_std::test]
+	async fn returns_finalized_on_finalized() {
+		assert_eq!(
+			on_transaction_status(TransactionStatus::Finalized(Default::default())).await,
+			Some((TrackedTransactionStatus::Finalized, InvalidationStatus::Finalized)),
+		);
+	}
+
+	#[async_std::test]
+	async fn returns_invalid_on_invalid() {
+		assert_eq!(
+			on_transaction_status(TransactionStatus::Invalid).await,
+			Some((TrackedTransactionStatus::Lost, InvalidationStatus::Invalid)),
+		);
+	}
+
+	#[async_std::test]
+	async fn waits_on_future() {
+		assert_eq!(on_transaction_status(TransactionStatus::Future).await, None,);
+	}
+
+	#[async_std::test]
+	async fn waits_on_ready() {
+		assert_eq!(on_transaction_status(TransactionStatus::Ready).await, None,);
+	}
+
+	#[async_std::test]
+	async fn waits_on_broadcast() {
+		assert_eq!(
+			on_transaction_status(TransactionStatus::Broadcast(Default::default())).await,
+			None,
+		);
+	}
+
+	#[async_std::test]
+	async fn waits_on_in_block() {
+		assert_eq!(
+			on_transaction_status(TransactionStatus::InBlock(Default::default())).await,
+			None,
+		);
+	}
+
+	#[async_std::test]
+	async fn waits_on_retracted() {
+		assert_eq!(
+			on_transaction_status(TransactionStatus::Retracted(Default::default())).await,
+			None,
+		);
+	}
+
+	#[async_std::test]
+	async fn lost_on_finality_timeout() {
+		assert_eq!(
+			on_transaction_status(TransactionStatus::FinalityTimeout(Default::default())).await,
+			Some((TrackedTransactionStatus::Lost, InvalidationStatus::Lost)),
+		);
+	}
+
+	#[async_std::test]
+	async fn lost_on_usurped() {
+		assert_eq!(
+			on_transaction_status(TransactionStatus::Usurped(Default::default())).await,
+			Some((TrackedTransactionStatus::Lost, InvalidationStatus::Lost)),
+		);
+	}
+
+	#[async_std::test]
+	async fn lost_on_dropped() {
+		assert_eq!(
+			on_transaction_status(TransactionStatus::Dropped).await,
+			Some((TrackedTransactionStatus::Lost, InvalidationStatus::Lost)),
+		);
+	}
+
+	#[async_std::test]
+	async fn lost_on_subscription_error() {
+		assert_eq!(
+			watch_transaction_status::<TestChain, _>(Default::default(), futures::stream::iter([]))
+				.now_or_never(),
+			Some(InvalidationStatus::Lost),
+		);
+	}
+}

bridges/relays/finality/src/finality_loop.rs

@@ -29,7 +29,7 @@ use futures::{select, Future, FutureExt, Stream, StreamExt};
 use num_traits::{One, Saturating};
 use relay_utils::{
 	metrics::MetricsParams, relay_loop::Client as RelayClient, retry_backoff, FailedClient,
-	HeaderId, MaybeConnectionError,
+	HeaderId, MaybeConnectionError, TrackedTransactionStatus, TransactionTracker,
 };
 use std::{
 	pin::Pin,
@@ -86,6 +86,9 @@ pub trait SourceClient<P: FinalitySyncPipeline>: RelayClient {
 /// Target client used in finality synchronization loop.
 #[async_trait]
 pub trait TargetClient<P: FinalitySyncPipeline>: RelayClient {
+	/// Transaction tracker to track submitted transactions.
+	type TransactionTracker: TransactionTracker;
+
 	/// Get best finalized source block number.
 	async fn best_finalized_source_block_id(
 		&self,
@@ -96,7 +99,7 @@ pub trait TargetClient<P: FinalitySyncPipeline>: RelayClient {
 		&self,
 		header: P::Header,
 		proof: P::FinalityProof,
-	) -> Result<(), Self::Error>;
+	) -> Result<Self::TransactionTracker, Self::Error>;
 }
 
 /// Return prefix that will be used by default to expose Prometheus metrics of the finality proofs
@@ -153,8 +156,6 @@ pub(crate) enum Error<P: FinalitySyncPipeline, SourceError, TargetError> {
 	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>
@@ -167,7 +168,6 @@ where
 		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(()),
 		}
 	}
@@ -175,9 +175,9 @@ where
 
 /// Information about transaction that we have submitted.
 #[derive(Debug, Clone)]
-pub(crate) struct Transaction<Number> {
-	/// Time when we have submitted this transaction.
-	pub time: Instant,
+pub(crate) struct Transaction<Tracker, Number> {
+	/// Submitted transaction tracker.
+	pub tracker: Tracker,
 	/// The number of the header we have submitted.
 	pub submitted_header_number: Number,
 }
@@ -206,11 +206,12 @@ pub(crate) struct FinalityLoopState<'a, P: FinalitySyncPipeline, FinalityProofsS
 		&'a mut RestartableFinalityProofsStream<FinalityProofsStream>,
 	/// Recent finality proofs that we have read from the stream.
 	pub(crate) recent_finality_proofs: &'a mut FinalityProofs<P>,
-	/// Last transaction that we have submitted to the target node.
-	pub(crate) last_transaction: Option<Transaction<P::Number>>,
+	/// Number of the last header, submitted to the target node.
+	pub(crate) submitted_header_number: Option<P::Number>,
 }
 
-async fn run_until_connection_lost<P: FinalitySyncPipeline>(
+/// Run finality relay loop until connection to one of nodes is lost.
+pub(crate) async fn run_until_connection_lost<P: FinalitySyncPipeline>(
 	source_client: impl SourceClient<P>,
 	target_client: impl TargetClient<P>,
 	sync_params: FinalitySyncParams,
@@ -230,8 +231,9 @@ async fn run_until_connection_lost<P: FinalitySyncPipeline>(
 		})
 	};
 
+	let last_transaction_tracker = futures::future::Fuse::terminated();
 	let exit_signal = exit_signal.fuse();
-	futures::pin_mut!(exit_signal);
+	futures::pin_mut!(last_transaction_tracker, exit_signal);
 
 	let mut finality_proofs_stream = RestartableFinalityProofsStream {
 		needs_restart: false,
@@ -241,7 +243,7 @@ async fn run_until_connection_lost<P: FinalitySyncPipeline>(
 
 	let mut progress = (Instant::now(), None);
 	let mut retry_backoff = retry_backoff();
-	let mut last_transaction = None;
+	let mut last_submitted_header_number = None;
 
 	loop {
 		// run loop iteration
@@ -252,7 +254,7 @@ async fn run_until_connection_lost<P: FinalitySyncPipeline>(
 				progress: &mut progress,
 				finality_proofs_stream: &mut finality_proofs_stream,
 				recent_finality_proofs: &mut recent_finality_proofs,
-				last_transaction: last_transaction.clone(),
+				submitted_header_number: last_submitted_header_number,
 			},
 			&sync_params,
 			&metrics_sync,
@@ -261,8 +263,14 @@ async fn run_until_connection_lost<P: FinalitySyncPipeline>(
 
 		// deal with errors
 		let next_tick = match iteration_result {
-			Ok(updated_last_transaction) => {
-				last_transaction = updated_last_transaction;
+			Ok(Some(updated_last_transaction)) => {
+				last_transaction_tracker.set(updated_last_transaction.tracker.wait().fuse());
+				last_submitted_header_number =
+					Some(updated_last_transaction.submitted_header_number);
+				retry_backoff.reset();
+				sync_params.tick
+			},
+			Ok(None) => {
 				retry_backoff.reset();
 				sync_params.tick
 			},
@@ -281,6 +289,18 @@ async fn run_until_connection_lost<P: FinalitySyncPipeline>(
 
 		// wait till exit signal, or new source block
 		select! {
+			transaction_status = last_transaction_tracker => {
+				if transaction_status == TrackedTransactionStatus::Lost {
+					log::error!(
+						target: "bridge",
+						"Finality synchronization from {} to {} has stalled. Going to restart",
+						P::SOURCE_NAME,
+						P::TARGET_NAME,
+					);
+
+					return Err(FailedClient::Both);
+				}
+			},
 			_ = async_std::task::sleep(next_tick).fuse() => {},
 			_ = exit_signal => return Ok(()),
 		}
@@ -293,7 +313,7 @@ pub(crate) async fn run_loop_iteration<P, SC, TC>(
 	state: FinalityLoopState<'_, P, SC::FinalityProofsStream>,
 	sync_params: &FinalitySyncParams,
 	metrics_sync: &Option<SyncLoopMetrics>,
-) -> Result<Option<Transaction<P::Number>>, Error<P, SC::Error, TC::Error>>
+) -> Result<Option<Transaction<TC::TransactionTracker, P::Number>>, Error<P, SC::Error, TC::Error>>
 where
 	P: FinalitySyncPipeline,
 	SC: SourceClient<P>,
@@ -333,20 +353,11 @@ where
 
 	// 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 {
+	if let Some(submitted_header_number) = state.submitted_header_number {
+		if best_number_at_target >= 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))
+			return Ok(None)
 		}
 	}
 
@@ -363,22 +374,20 @@ where
 	.await?
 	{
 		Some((header, justification)) => {
-			let new_transaction =
-				Transaction { time: Instant::now(), submitted_header_number: header.number() };
-
+			let 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,
+				submitted_header_number,
 				P::TARGET_NAME,
 			);
 
-			target_client
+			let tracker = target_client
 				.submit_finality_proof(header, justification)
 				.await
 				.map_err(Error::Target)?;
-			Ok(Some(new_transaction))
+			Ok(Some(Transaction { tracker, submitted_header_number }))
 		},
 		None => Ok(None),
 	}

bridges/relays/finality/src/finality_loop_tests.rs

@@ -20,10 +20,10 @@
 
 use crate::{
 	finality_loop::{
-		prune_recent_finality_proofs, read_finality_proofs_from_stream, run, run_loop_iteration,
-		select_better_recent_finality_proof, select_header_to_submit, FinalityLoopState,
-		FinalityProofs, FinalitySyncParams, RestartableFinalityProofsStream, SourceClient,
-		TargetClient,
+		prune_recent_finality_proofs, read_finality_proofs_from_stream, run_loop_iteration,
+		run_until_connection_lost, select_better_recent_finality_proof, select_header_to_submit,
+		FinalityLoopState, FinalityProofs, FinalitySyncParams, RestartableFinalityProofsStream,
+		SourceClient, TargetClient,
 	},
 	sync_loop_metrics::SyncLoopMetrics,
 	FinalityProof, FinalitySyncPipeline, SourceHeader,
@@ -33,7 +33,8 @@ use async_trait::async_trait;
 use futures::{FutureExt, Stream, StreamExt};
 use parking_lot::Mutex;
 use relay_utils::{
-	metrics::MetricsParams, relay_loop::Client as RelayClient, HeaderId, MaybeConnectionError,
+	relay_loop::Client as RelayClient, FailedClient, HeaderId, MaybeConnectionError,
+	TrackedTransactionStatus, TransactionTracker,
 };
 use std::{
 	collections::HashMap,
@@ -46,6 +47,22 @@ type IsMandatory = bool;
 type TestNumber = u64;
 type TestHash = u64;
 
+#[derive(Clone, Debug)]
+struct TestTransactionTracker(TrackedTransactionStatus);
+
+impl Default for TestTransactionTracker {
+	fn default() -> TestTransactionTracker {
+		TestTransactionTracker(TrackedTransactionStatus::Finalized)
+	}
+}
+
+#[async_trait]
+impl TransactionTracker for TestTransactionTracker {
+	async fn wait(self) -> TrackedTransactionStatus {
+		self.0
+	}
+}
+
 #[derive(Debug, Clone)]
 enum TestError {
 	NonConnection,
@@ -104,6 +121,7 @@ struct ClientsData {
 
 	target_best_block_id: HeaderId<TestHash, TestNumber>,
 	target_headers: Vec<(TestSourceHeader, TestFinalityProof)>,
+	target_transaction_tracker: TestTransactionTracker,
 }
 
 #[derive(Clone)]
@@ -164,6 +182,8 @@ impl RelayClient for TestTargetClient {
 
 #[async_trait]
 impl TargetClient<TestFinalitySyncPipeline> for TestTargetClient {
+	type TransactionTracker = TestTransactionTracker;
+
 	async fn best_finalized_source_block_id(
 		&self,
 	) -> Result<HeaderId<TestHash, TestNumber>, TestError> {
@@ -176,12 +196,13 @@ impl TargetClient<TestFinalitySyncPipeline> for TestTargetClient {
 		&self,
 		header: TestSourceHeader,
 		proof: TestFinalityProof,
-	) -> Result<(), TestError> {
+	) -> Result<TestTransactionTracker, TestError> {
 		let mut data = self.data.lock();
 		(self.on_method_call)(&mut data);
 		data.target_best_block_id = HeaderId(header.number(), header.hash());
 		data.target_headers.push((header, proof));
-		Ok(())
+		(self.on_method_call)(&mut data);
+		Ok(data.target_transaction_tracker.clone())
 	}
 }
 
@@ -203,6 +224,7 @@ fn prepare_test_clients(
 
 		target_best_block_id: HeaderId(5, 5),
 		target_headers: vec![],
+		target_transaction_tracker: TestTransactionTracker(TrackedTransactionStatus::Finalized),
 	}));
 	(
 		TestSourceClient {
@@ -224,7 +246,7 @@ fn test_sync_params() -> FinalitySyncParams {
 
 fn run_sync_loop(
 	state_function: impl Fn(&mut ClientsData) -> bool + Send + Sync + 'static,
-) -> ClientsData {
+) -> (ClientsData, Result<(), FailedClient>) {
 	let (exit_sender, exit_receiver) = futures::channel::mpsc::unbounded();
 	let (source_client, target_client) = prepare_test_clients(
 		exit_sender,
@@ -243,21 +265,21 @@ fn run_sync_loop(
 	let sync_params = test_sync_params();
 
 	let clients_data = source_client.data.clone();
-	let _ = async_std::task::block_on(run(
+	let result = async_std::task::block_on(run_until_connection_lost(
 		source_client,
 		target_client,
 		sync_params,
-		MetricsParams::disabled(),
+		None,
 		exit_receiver.into_future().map(|(_, _)| ()),
 	));
 
 	let clients_data = clients_data.lock().clone();
-	clients_data
+	(clients_data, result)
 }
 
 #[test]
 fn finality_sync_loop_works() {
-	let client_data = run_sync_loop(|data| {
+	let (client_data, result) = 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,
@@ -286,6 +308,7 @@ fn finality_sync_loop_works() {
 		data.target_best_block_id.0 == 16
 	});
 
+	assert_eq!(result, Ok(()));
 	assert_eq!(
 		client_data.target_headers,
 		vec![
@@ -538,7 +561,7 @@ fn different_forks_at_source_and_at_target_are_detected() {
 			progress: &mut progress,
 			finality_proofs_stream: &mut finality_proofs_stream,
 			recent_finality_proofs: &mut recent_finality_proofs,
-			last_transaction: None,
+			submitted_header_number: None,
 		},
 		&test_sync_params(),
 		&Some(metrics_sync.clone()),
@@ -547,3 +570,14 @@ fn different_forks_at_source_and_at_target_are_detected() {
 
 	assert!(!metrics_sync.is_using_same_fork());
 }
+
+#[test]
+fn stalls_when_transaction_tracker_returns_error() {
+	let (_, result) = run_sync_loop(|data| {
+		data.target_transaction_tracker = TestTransactionTracker(TrackedTransactionStatus::Lost);
+		data.target_best_block_id = HeaderId(5, 5);
+		data.target_best_block_id.0 == 16
+	});
+
+	assert_eq!(result, Err(FailedClient::Both));
+}

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

@@ -28,7 +28,7 @@ use async_trait::async_trait;
 use finality_relay::TargetClient;
 use relay_substrate_client::{
 	AccountIdOf, AccountKeyPairOf, Chain, Client, Error, HeaderIdOf, HeaderOf, SignParam,
-	SyncHeader, TransactionEra, TransactionSignScheme, UnsignedTransaction,
+	SyncHeader, TransactionEra, TransactionSignScheme, TransactionTracker, UnsignedTransaction,
 };
 use relay_utils::relay_loop::Client as RelayClient;
 use sp_core::Pair;
@@ -89,6 +89,8 @@ where
 	AccountIdOf<P::TargetChain>: From<<AccountKeyPairOf<P::TransactionSignScheme> as Pair>::Public>,
 	P::TransactionSignScheme: TransactionSignScheme<Chain = P::TargetChain>,
 {
+	type TransactionTracker = TransactionTracker<P::TargetChain>;
+
 	async fn best_finalized_source_block_id(&self) -> Result<HeaderIdOf<P::SourceChain>, Error> {
 		// we can't continue to relay finality if target node is out of sync, because
 		// it may have already received (some of) headers that we're going to relay
@@ -109,14 +111,14 @@ where
 		&self,
 		header: SyncHeader<HeaderOf<P::SourceChain>>,
 		proof: SubstrateFinalityProof<P>,
-	) -> Result<(), Error> {
+	) -> Result<Self::TransactionTracker, Error> {
 		let genesis_hash = *self.client.genesis_hash();
 		let transaction_params = self.transaction_params.clone();
 		let call =
 			P::SubmitFinalityProofCallBuilder::build_submit_finality_proof_call(header, proof);
 		let (spec_version, transaction_version) = self.client.simple_runtime_version().await?;
 		self.client
-			.submit_signed_extrinsic(
+			.submit_and_watch_signed_extrinsic(
 				self.transaction_params.signer.public().into(),
 				SignParam::<P::TransactionSignScheme> {
 					spec_version,
@@ -130,6 +132,5 @@ where
 				},
 			)
 			.await
-			.map(drop)
 	}
 }

bridges/relays/utils/src/lib.rs

@@ -20,6 +20,7 @@ pub use bp_runtime::HeaderId;
 pub use error::Error;
 pub use relay_loop::{relay_loop, relay_metrics};
 
+use async_trait::async_trait;
 use backoff::{backoff::Backoff, ExponentialBackoff};
 use futures::future::FutureExt;
 use std::time::Duration;
@@ -119,6 +120,22 @@ pub trait MaybeConnectionError {
 	fn is_connection_error(&self) -> bool;
 }
 
+/// Final status of the tracked transaction.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum TrackedTransactionStatus {
+	/// Transaction has been lost.
+	Lost,
+	/// Transaction has been mined and finalized.
+	Finalized,
+}
+
+/// Transaction tracker.
+#[async_trait]
+pub trait TransactionTracker: Send {
+	/// Wait until transaction is either finalized or invalidated/lost.
+	async fn wait(self) -> TrackedTransactionStatus;
+}
+
 /// Stringified error that may be either connection-related or not.
 #[derive(Error, Debug)]
 pub enum StringifiedMaybeConnectionError {