pause relays(s) when node is syncing (#605)

bridges/relays/substrate/src/headers_target.rs

@@ -73,6 +73,10 @@ where
 	P: SubstrateHeadersSyncPipeline<Completion = Justification, Extra = ()>,
 {
 	async fn best_header_id(&self) -> Result<HeaderIdOf<P>, SubstrateError> {
+		// we can't continue to relay headers if target node is out of sync, because
+		// it may have already received (some of) headers that we're going to relay
+		self.client.ensure_synced().await?;
+
 		let call = P::BEST_BLOCK_METHOD.into();
 		let data = Bytes(Vec::new());
 

bridges/relays/substrate/src/messages_source.rs

@@ -99,6 +99,10 @@ where
 	P::TargetHeaderHash: Decode,
 {
 	async fn state(&self) -> Result<SourceClientState<P>, SubstrateError> {
+		// we can't continue to deliver confirmations if source node is out of sync, because
+		// it may have already received confirmations that we're going to deliver
+		self.client.ensure_synced().await?;
+
 		read_client_state::<_, P::TargetHeaderHash, P::TargetHeaderNumber>(
 			&self.client,
 			P::BEST_FINALIZED_TARGET_HEADER_ID_AT_SOURCE,

bridges/relays/substrate/src/messages_target.rs

@@ -95,6 +95,10 @@ where
 	P::SourceHeaderHash: Decode,
 {
 	async fn state(&self) -> Result<TargetClientState<P>, SubstrateError> {
+		// we can't continue to deliver messages if target node is out of sync, because
+		// it may have already received (some of) messages that we're going to deliver
+		self.client.ensure_synced().await?;
+
 		read_client_state::<_, P::SourceHeaderHash, P::SourceHeaderNumber>(
 			&self.client,
 			P::BEST_FINALIZED_SOURCE_HEADER_ID_AT_TARGET,