client/beefy: fix on-demand justifications sync for old blocks (#12767)

* client/beefy: fix on-demand justif sync for old blocks

When receiving BEEFY justifications for old blocks the state might
be pruned for them, in which case justification verification fails
because BEEFY validator set cannot be retrieved from runtime state.

Fix this by having the voter give the validator set to the
`OnDemandJustificationsEngine` as request information. On receiving
a BEEFY justification for requested block, the provided validator
set will be used to validate the justification.

Signed-off-by: acatangiu <adrian@parity.io>

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <git@kchr.de>

* impl review suggestions

* client/beefy: fail initialization if state unavailable

* beefy: remove spammy log

Signed-off-by: acatangiu <adrian@parity.io>
Co-authored-by: parity-processbot <>
Co-authored-by: Bastian Köcher <git@kchr.de>

substrate/client/beefy/src/lib.rs

@@ -244,7 +244,6 @@ where
 	// The `GossipValidator` adds and removes known peers based on valid votes and network events.
 	let on_demand_justifications = OnDemandJustificationsEngine::new(
 		network.clone(),
-		runtime.clone(),
 		justifications_protocol_name,
 		known_peers,
 	);
@@ -295,7 +294,7 @@ where
 		persisted_state,
 	};
 
-	let worker = worker::BeefyWorker::<_, _, _, _, _>::new(worker_params);
+	let worker = worker::BeefyWorker::<_, _, _, _>::new(worker_params);
 
 	futures::future::join(
 		worker.run(block_import_justif, finality_notifications),
@@ -377,17 +376,8 @@ where
 			break state
 		}
 
-		// Check if we should move up the chain.
-		let parent_hash = *header.parent_hash();
-		if *header.number() == One::one() ||
-			runtime
-				.runtime_api()
-				.validator_set(&BlockId::hash(parent_hash))
-				.ok()
-				.flatten()
-				.is_none()
-		{
-			// We've reached pallet genesis, initialize voter here.
+		if *header.number() == One::one() {
+			// We've reached chain genesis, initialize voter here.
 			let genesis_num = *header.number();
 			let genesis_set = expect_validator_set(runtime, BlockId::hash(header.hash()))
 				.and_then(genesis_set_sanity_check)?;
@@ -408,6 +398,19 @@ where
 			sessions.push_front(Rounds::new(*header.number(), active));
 		}
 
+		// Check if state is still available if we move up the chain.
+		let parent_hash = *header.parent_hash();
+		runtime
+			.runtime_api()
+			.validator_set(&BlockId::hash(parent_hash))
+			.ok()
+			.flatten()
+			.ok_or_else(|| {
+				let msg = format!("{}. Could not initialize BEEFY voter.", parent_hash);
+				error!(target: "beefy", "🥩 {}", msg);
+				ClientError::Consensus(sp_consensus::Error::StateUnavailable(msg))
+			})?;
+
 		// Move up the chain.
 		header = blockchain.expect_header(BlockId::Hash(parent_hash))?;
 	};