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/api/src/backend.rs

@@ -33,7 +33,7 @@ use crate::{
 };
 use consensus::BlockOrigin;
 use hash_db::Hasher;
-use parking_lot::Mutex;
+use parking_lot::RwLock;
 
 /// In memory array of storage values.
 pub type StorageCollection = Vec<(Vec<u8>, Option<Vec<u8>>)>;
@@ -310,7 +310,7 @@ pub trait Backend<Block, H>: AuxStore + Send + Sync where
 	/// the using components should acquire and hold the lock whenever they do
 	/// something that the import of a block would interfere with, e.g. importing
 	/// a new block or calculating the best head.
-	fn get_import_lock(&self) -> &Mutex<()>;
+	fn get_import_lock(&self) -> &RwLock<()>;
 }
 
 /// Changes trie storage that supports pruning.

substrate/client/api/src/blockchain.rs

@@ -22,7 +22,7 @@ use sr_primitives::traits::{Block as BlockT, Header as HeaderT, NumberFor};
 use sr_primitives::generic::BlockId;
 use sr_primitives::Justification;
 use log::warn;
-use parking_lot::Mutex;
+use parking_lot::RwLock;
 
 use header_metadata::HeaderMetadata;
 
@@ -109,7 +109,7 @@ pub trait Backend<Block: BlockT>: HeaderBackend<Block> + HeaderMetadata<Block, E
 		&self,
 		target_hash: Block::Hash,
 		maybe_max_number: Option<NumberFor<Block>>,
-		import_lock: &Mutex<()>,
+		import_lock: &RwLock<()>,
 	) -> Result<Option<Block::Hash>> {
 		let target_header = {
 			match self.header(BlockId::Hash(target_hash))? {
@@ -130,7 +130,7 @@ pub trait Backend<Block: BlockT>: HeaderBackend<Block> + HeaderMetadata<Block, E
 			// ensure no blocks are imported during this code block.
 			// an import could trigger a reorg which could change the canonical chain.
 			// we depend on the canonical chain staying the same during this code block.
-			let _import_guard = import_lock.lock();
+			let _import_guard = import_lock.read();
 
 			let info = self.info();