Always run on-demand headers relay in complex relay (#975)

* always run on-demand headers relay in complex relay

* fix compilation

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

@@ -21,6 +21,7 @@ use crate::client::Client;
 use crate::error::Error;
 use crate::sync_header::SyncHeader;
 
+use async_std::sync::{Arc, Mutex};
 use async_trait::async_trait;
 use bp_header_chain::justification::GrandpaJustification;
 use codec::Decode;
@@ -30,26 +31,41 @@ use relay_utils::relay_loop::Client as RelayClient;
 use sp_runtime::traits::Header as HeaderT;
 use std::{marker::PhantomData, pin::Pin};
 
+/// Shared updatable reference to the maximal header number that we want to sync from the source.
+pub type RequiredHeaderNumberRef<C> = Arc<Mutex<<C as bp_runtime::Chain>::BlockNumber>>;
+
 /// Substrate node as finality source.
 pub struct FinalitySource<C: Chain, P> {
 	client: Client<C>,
+	maximal_header_number: Option<RequiredHeaderNumberRef<C>>,
 	_phantom: PhantomData<P>,
 }
 
 impl<C: Chain, P> FinalitySource<C, P> {
 	/// Create new headers source using given client.
-	pub fn new(client: Client<C>) -> Self {
+	pub fn new(client: Client<C>, maximal_header_number: Option<RequiredHeaderNumberRef<C>>) -> Self {
 		FinalitySource {
 			client,
+			maximal_header_number,
 			_phantom: Default::default(),
 		}
 	}
+
+	/// Returns best finalized block number.
+	pub async fn on_chain_best_finalized_block_number(&self) -> Result<C::BlockNumber, Error> {
+		// we **CAN** continue to relay finality proofs if source node is out of sync, because
+		// target node may be missing proofs that are already available at the source
+		let finalized_header_hash = self.client.best_finalized_header_hash().await?;
+		let finalized_header = self.client.header_by_hash(finalized_header_hash).await?;
+		Ok(*finalized_header.number())
+	}
 }
 
 impl<C: Chain, P> Clone for FinalitySource<C, P> {
 	fn clone(&self) -> Self {
 		FinalitySource {
 			client: self.client.clone(),
+			maximal_header_number: self.maximal_header_number.clone(),
 			_phantom: Default::default(),
 		}
 	}
@@ -80,11 +96,16 @@ where
 	type FinalityProofsStream = Pin<Box<dyn Stream<Item = GrandpaJustification<C::Header>> + Send>>;
 
 	async fn best_finalized_block_number(&self) -> Result<P::Number, Error> {
-		// we **CAN** continue to relay finality proofs if source node is out of sync, because
-		// target node may be missing proofs that are already available at the source
-		let finalized_header_hash = self.client.best_finalized_header_hash().await?;
-		let finalized_header = self.client.header_by_hash(finalized_header_hash).await?;
-		Ok(*finalized_header.number())
+		let mut finalized_header_number = self.on_chain_best_finalized_block_number().await?;
+		// never return block number larger than requested. This way we'll never sync headers
+		// past `maximal_header_number`
+		if let Some(ref maximal_header_number) = self.maximal_header_number {
+			let maximal_header_number = *maximal_header_number.lock().await;
+			if finalized_header_number > maximal_header_number {
+				finalized_header_number = maximal_header_number;
+			}
+		}
+		Ok(finalized_header_number)
 	}
 
 	async fn header_and_finality_proof(