Fixed shared cache race on import (#4194)

* Fixed is_best race on import

* Take import lock outside of backend

* Actually take the lock

substrate/client/src/light/backend.rs

@@ -19,7 +19,7 @@
 
 use std::collections::HashMap;
 use std::sync::Arc;
-use parking_lot::{RwLock, Mutex};
+use parking_lot::RwLock;
 
 use state_machine::{Backend as StateBackend, TrieBackend, backend::InMemory as InMemoryState, ChangesTrieTransaction};
 use primitives::offchain::storage::InMemOffchainStorage;
@@ -49,7 +49,7 @@ const IN_MEMORY_EXPECT_PROOF: &str = "InMemory state backend has Void error type
 pub struct Backend<S, H: Hasher> {
 	blockchain: Arc<Blockchain<S>>,
 	genesis_state: RwLock<Option<InMemoryState<H>>>,
-	import_lock: Mutex<()>,
+	import_lock: RwLock<()>,
 }
 
 /// Light block (header and justification) import operation.
@@ -216,7 +216,7 @@ impl<S, Block, H> ClientBackend<Block, H> for Backend<S, H> where
 		Err(ClientError::NotAvailableOnLightClient)
 	}
 
-	fn get_import_lock(&self) -> &Mutex<()> {
+	fn get_import_lock(&self) -> &RwLock<()> {
 		&self.import_lock
 	}
 }