client.rs

				header: import_headers.into_post(),
				is_new_best,
				storage_changes,
				retracted,
			})
		}

		Ok(ImportResult::imported(is_new_best))
	}

	fn block_execution(
		&self,
		transaction: &B::BlockImportOperation,
		import_headers: &PrePostHeader<Block::Header>,
		origin: BlockOrigin,
		hash: Block::Hash,
		body: Option<Vec<Block::Extrinsic>>,
	) -> error::Result<(
		Option<StorageUpdate<B, Block>>,
		Option<Option<ChangesUpdate<Block>>>,
		Option<(
			Vec<(Vec<u8>, Option<Vec<u8>>)>,
			Vec<(Vec<u8>, Vec<(Vec<u8>, Option<Vec<u8>>)>)>
		)>
	)>
		where
			E: CallExecutor<Block, Blake2Hasher> + Send + Sync + Clone,
	{
		match transaction.state()? {
			Some(transaction_state) => {
				let mut overlay = Default::default();
				let get_execution_manager = |execution_strategy: ExecutionStrategy| {
					match execution_strategy {
						ExecutionStrategy::NativeElseWasm => ExecutionManager::NativeElseWasm,
						ExecutionStrategy::AlwaysWasm => ExecutionManager::AlwaysWasm(BackendTrustLevel::Trusted),
						ExecutionStrategy::NativeWhenPossible => ExecutionManager::NativeWhenPossible,
						ExecutionStrategy::Both => ExecutionManager::Both(|wasm_result, native_result| {
							let header = import_headers.post();
							warn!("Consensus error between wasm and native block execution at block {}", hash);
							warn!("   Header {:?}", header);
							warn!("   Native result {:?}", native_result);
							warn!("   Wasm result {:?}", wasm_result);
							telemetry!(SUBSTRATE_INFO; "block.execute.consensus_failure";
								"hash" => ?hash,
								"origin" => ?origin,
								"header" => ?header
							);
							wasm_result
						}),
					}
				};

				let encoded_block = <Block as BlockT>::encode_from(
					import_headers.pre(),
					&body.unwrap_or_default()
				);

				let (_, storage_update, changes_update) = self.executor
					.call_at_state::<_, _, NeverNativeValue, fn() -> _>(
						transaction_state,
						&mut overlay,
						"Core_execute_block",
						&encoded_block,
						match origin {
							BlockOrigin::NetworkInitialSync => get_execution_manager(
								self.execution_strategies().syncing,
							),
							_ => get_execution_manager(self.execution_strategies().importing),
						},
						None,
						None,
					)?;

				overlay.commit_prospective();

				let (top, children) = overlay.into_committed();
				let children = children.map(|(sk, it)| (sk, it.collect())).collect();
				if import_headers.post().state_root() != &storage_update.1 {
					return Err(error::Error::InvalidStateRoot);
				}

				Ok((Some(storage_update.0), Some(changes_update), Some((top.collect(), children))))
			},
			None => Ok((None, None, None))
		}
	}

	fn apply_finality_with_block_hash(
		&self,
		operation: &mut ClientImportOperation<Block, Blake2Hasher, B>,
		block: Block::Hash,
		justification: Option<Justification>,
		best_block: Block::Hash,
		notify: bool,
	) -> error::Result<()> {
		// find tree route from last finalized to given block.
		let last_finalized = self.backend.blockchain().last_finalized()?;

		if block == last_finalized {
			warn!("Possible safety violation: attempted to re-finalize last finalized block {:?} ", last_finalized);
			return Ok(());
		}

		let route_from_finalized = header_metadata::tree_route(self.backend.blockchain(), last_finalized, block)?;

		if let Some(retracted) = route_from_finalized.retracted().get(0) {
			warn!("Safety violation: attempted to revert finalized block {:?} which is not in the \
				same chain as last finalized {:?}", retracted, last_finalized);

			return Err(error::Error::NotInFinalizedChain);
		}

		let route_from_best = header_metadata::tree_route(self.backend.blockchain(), best_block, block)?;

		// if the block is not a direct ancestor of the current best chain,
		// then some other block is the common ancestor.
		if route_from_best.common_block().hash != block {
			// NOTE: we're setting the finalized block as best block, this might
			// be slightly inaccurate since we might have a "better" block
			// further along this chain, but since best chain selection logic is
			// pluggable we cannot make a better choice here. usages that need
			// an accurate "best" block need to go through `SelectChain`
			// instead.
			operation.op.mark_head(BlockId::Hash(block))?;
		}

		let enacted = route_from_finalized.enacted();
		assert!(enacted.len() > 0);
		for finalize_new in &enacted[..enacted.len() - 1] {
			operation.op.mark_finalized(BlockId::Hash(finalize_new.hash), None)?;
		}

		assert_eq!(enacted.last().map(|e| e.hash), Some(block));
		operation.op.mark_finalized(BlockId::Hash(block), justification)?;

		if notify {
			// sometimes when syncing, tons of blocks can be finalized at once.
			// we'll send notifications spuriously in that case.
			const MAX_TO_NOTIFY: usize = 256;
			let enacted = route_from_finalized.enacted();
			let start = enacted.len() - ::std::cmp::min(enacted.len(), MAX_TO_NOTIFY);
			for finalized in &enacted[start..] {
				operation.notify_finalized.push(finalized.hash);
			}
		}

		Ok(())
	}

	fn notify_finalized(
		&self,
		notify_finalized: Vec<Block::Hash>,
	) -> error::Result<()> {
		let mut sinks = self.finality_notification_sinks.lock();

		for finalized_hash in notify_finalized {
			let header = self.header(&BlockId::Hash(finalized_hash))?
				.expect("header already known to exist in DB because it is indicated in the tree route; qed");

			telemetry!(SUBSTRATE_INFO; "notify.finalized";
				"height" => format!("{}", header.number()),
				"best" => ?finalized_hash,
			);

			let notification = FinalityNotification {
				header,
				hash: finalized_hash,
			};

			sinks.retain(|sink| sink.unbounded_send(notification.clone()).is_ok());
		}

		Ok(())
	}

	fn notify_imported(&self, notify_import: ImportSummary<Block>) -> error::Result<()> {
		if let Some(storage_changes) = notify_import.storage_changes {
			// TODO [ToDr] How to handle re-orgs? Should we re-emit all storage changes?
			self.storage_notifications.lock()
				.trigger(
					&notify_import.hash,
					storage_changes.0.into_iter(),
					storage_changes.1.into_iter().map(|(sk, v)| (sk, v.into_iter())),
				);
		}

		let notification = BlockImportNotification::<Block> {
			hash: notify_import.hash,
			origin: notify_import.origin,
			header: notify_import.header,
			is_new_best: notify_import.is_new_best,
			retracted: notify_import.retracted,
		};

		self.import_notification_sinks.lock()
			.retain(|sink| sink.unbounded_send(notification.clone()).is_ok());

		Ok(())
	}

	/// Attempts to revert the chain by `n` blocks. Returns the number of blocks that were
	/// successfully reverted.
	pub fn revert(&self, n: NumberFor<Block>) -> error::Result<NumberFor<Block>> {
		Ok(self.backend.revert(n)?)
	}

	/// Get blockchain info.
	pub fn info(&self) -> ClientInfo<Block> {
		let info = self.backend.blockchain().info();
		ClientInfo {
			chain: info,
			used_state_cache_size: self.backend.used_state_cache_size(),
		}
	}

	/// Get block status.
	pub fn block_status(&self, id: &BlockId<Block>) -> error::Result<BlockStatus> {
		// this can probably be implemented more efficiently
		if let BlockId::Hash(ref h) = id {
			if self.importing_block.read().as_ref().map_or(false, |importing| h == importing) {
				return Ok(BlockStatus::Queued);
			}
		}
		let hash_and_number = match id.clone() {
			BlockId::Hash(hash) => self.backend.blockchain().number(hash)?.map(|n| (hash, n)),
			BlockId::Number(n) => self.backend.blockchain().hash(n)?.map(|hash| (hash, n)),
		};
		match hash_and_number {
			Some((hash, number)) => {
				if self.backend.have_state_at(&hash, number) {
					Ok(BlockStatus::InChainWithState)
				} else {
					Ok(BlockStatus::InChainPruned)
				}
			}
			None => Ok(BlockStatus::Unknown),
		}
	}

	/// Get block header by id.
	pub fn header(&self, id: &BlockId<Block>) -> error::Result<Option<<Block as BlockT>::Header>> {
		self.backend.blockchain().header(*id)
	}

	/// Get block body by id.
	pub fn body(&self, id: &BlockId<Block>) -> error::Result<Option<Vec<<Block as BlockT>::Extrinsic>>> {
		self.backend.blockchain().body(*id)
	}

	/// Get block justification set by id.
	pub fn justification(&self, id: &BlockId<Block>) -> error::Result<Option<Justification>> {
		self.backend.blockchain().justification(*id)
	}

	/// Get full block by id.
	pub fn block(&self, id: &BlockId<Block>)
		-> error::Result<Option<SignedBlock<Block>>>
	{
		Ok(match (self.header(id)?, self.body(id)?, self.justification(id)?) {
			(Some(header), Some(extrinsics), justification) =>
				Some(SignedBlock { block: Block::new(header, extrinsics), justification }),
			_ => None,
		})
	}

	/// Gets the uncles of the block with `target_hash` going back `max_generation` ancestors.
	pub fn uncles(&self, target_hash: Block::Hash, max_generation: NumberFor<Block>) -> error::Result<Vec<Block::Hash>> {
		let load_header = |id: Block::Hash| -> error::Result<Block::Header> {
			match self.backend.blockchain().header(BlockId::Hash(id))? {
				Some(hdr) => Ok(hdr),
				None => Err(Error::UnknownBlock(format!("{:?}", id))),
			}
		};

		let genesis_hash = self.backend.blockchain().info().genesis_hash;
		if genesis_hash == target_hash { return Ok(Vec::new()); }

		let mut current_hash = target_hash;
		let mut current = load_header(current_hash)?;
		let mut ancestor_hash = *current.parent_hash();
		let mut ancestor = load_header(ancestor_hash)?;
		let mut uncles = Vec::new();

		for _generation in 0..max_generation.saturated_into() {
			let children = self.backend.blockchain().children(ancestor_hash)?;
			uncles.extend(children.into_iter().filter(|h| h != &current_hash));
			current_hash = ancestor_hash;
			if genesis_hash == current_hash { break; }
			current = ancestor;
			ancestor_hash = *current.parent_hash();
			ancestor = load_header(ancestor_hash)?;
		}
		trace!("Collected {} uncles", uncles.len());
		Ok(uncles)
	}

	fn changes_trie_config(&self) -> Result<Option<ChangesTrieConfiguration>, Error> {
		Ok(self.backend.state_at(BlockId::Number(self.backend.blockchain().info().best_number))?
			.storage(well_known_keys::CHANGES_TRIE_CONFIG)
			.map_err(|e| error::Error::from_state(Box::new(e)))?
			.and_then(|c| Decode::decode(&mut &*c).ok()))
	}

	/// Prepare in-memory header that is used in execution environment.
	fn prepare_environment_block(&self, parent: &BlockId<Block>) -> error::Result<Block::Header> {
		let parent_header = self.backend.blockchain().expect_header(*parent)?;
		Ok(<<Block as BlockT>::Header as HeaderT>::new(
			self.backend.blockchain().expect_block_number_from_id(parent)? + One::one(),
			Default::default(),
			Default::default(),
			parent_header.hash(),
			Default::default(),
		))
	}
}

impl<B, E, Block, RA> HeaderMetadata<Block> for Client<B, E, Block, RA> where
	B: backend::Backend<Block, Blake2Hasher>,
	E: CallExecutor<Block, Blake2Hasher>,
	Block: BlockT<Hash=H256>,
{
	type Error = error::Error;

	fn header_metadata(&self, hash: Block::Hash) -> Result<CachedHeaderMetadata<Block>, Self::Error> {
		self.backend.blockchain().header_metadata(hash)
	}

	fn insert_header_metadata(&self, hash: Block::Hash, metadata: CachedHeaderMetadata<Block>) {
		self.backend.blockchain().insert_header_metadata(hash, metadata)
	}

	fn remove_header_metadata(&self, hash: Block::Hash) {
		self.backend.blockchain().remove_header_metadata(hash)
	}
}

impl<B, E, Block, RA> ProvideUncles<Block> for Client<B, E, Block, RA> where
	B: backend::Backend<Block, Blake2Hasher>,
	E: CallExecutor<Block, Blake2Hasher>,
	Block: BlockT<Hash=H256>,
{
	fn uncles(&self, target_hash: Block::Hash, max_generation: NumberFor<Block>) -> error::Result<Vec<Block::Header>> {
		Ok(Client::uncles(self, target_hash, max_generation)?
			.into_iter()
			.filter_map(|hash| Client::header(self, &BlockId::Hash(hash)).unwrap_or(None))
			.collect()
		)
	}
}

impl<B, E, Block, RA> ChainHeaderBackend<Block> for Client<B, E, Block, RA> where
	B: backend::Backend<Block, Blake2Hasher>,
	E: CallExecutor<Block, Blake2Hasher> + Send + Sync,
	Block: BlockT<Hash=H256>,
	RA: Send + Sync
{
	fn header(&self, id: BlockId<Block>) -> error::Result<Option<Block::Header>> {
		self.backend.blockchain().header(id)
	}

	fn info(&self) -> blockchain::Info<Block> {
		self.backend.blockchain().info()
	}

	fn status(&self, id: BlockId<Block>) -> error::Result<blockchain::BlockStatus> {
		self.backend.blockchain().status(id)
	}

	fn number(&self, hash: Block::Hash) -> error::Result<Option<<<Block as BlockT>::Header as HeaderT>::Number>> {
		self.backend.blockchain().number(hash)
	}

	fn hash(&self, number: NumberFor<Block>) -> error::Result<Option<Block::Hash>> {
		self.backend.blockchain().hash(number)
	}
}

impl<B, E, Block, RA> ChainHeaderBackend<Block> for &Client<B, E, Block, RA> where
	B: backend::Backend<Block, Blake2Hasher>,
	E: CallExecutor<Block, Blake2Hasher> + Send + Sync,
	Block: BlockT<Hash=H256>,
	RA: Send + Sync,
{
	fn header(&self, id: BlockId<Block>) -> error::Result<Option<Block::Header>> {
		(**self).backend.blockchain().header(id)
	}

	fn info(&self) -> blockchain::Info<Block> {
		(**self).backend.blockchain().info()
	}

	fn status(&self, id: BlockId<Block>) -> error::Result<blockchain::BlockStatus> {
		(**self).status(id)
	}

	fn number(&self, hash: Block::Hash) -> error::Result<Option<<<Block as BlockT>::Header as HeaderT>::Number>> {
		(**self).number(hash)
	}

	fn hash(&self, number: NumberFor<Block>) -> error::Result<Option<Block::Hash>> {
		(**self).hash(number)
	}
}

impl<B, E, Block, RA> ProvideCache<Block> for Client<B, E, Block, RA> where
	B: backend::Backend<Block, Blake2Hasher>,
	Block: BlockT<Hash=H256>,
{
	fn cache(&self) -> Option<Arc<dyn Cache<Block>>> {
		self.backend.blockchain().cache()
	}
}

impl<B, E, Block, RA> ProvideRuntimeApi for Client<B, E, Block, RA> where
	B: backend::Backend<Block, Blake2Hasher>,
	E: CallExecutor<Block, Blake2Hasher> + Clone + Send + Sync,
	Block: BlockT<Hash=H256>,
	RA: ConstructRuntimeApi<Block, Self>
{
	type Api = <RA as ConstructRuntimeApi<Block, Self>>::RuntimeApi;

	fn runtime_api<'a>(&'a self) -> ApiRef<'a, Self::Api> {
		RA::construct_runtime_api(self)
	}
}

impl<B, E, Block, RA> CallRuntimeAt<Block> for Client<B, E, Block, RA> where
	B: backend::Backend<Block, Blake2Hasher>,
	E: CallExecutor<Block, Blake2Hasher> + Clone + Send + Sync,
	Block: BlockT<Hash=H256>,
{
	fn call_api_at<
		'a,
		R: Encode + Decode + PartialEq,
		NC: FnOnce() -> result::Result<R, String> + UnwindSafe,
		C: CoreApi<Block>,
	>(
		&self,
		core_api: &C,
		at: &BlockId<Block>,
		function: &'static str,
		args: Vec<u8>,
		changes: &RefCell<OverlayedChanges>,
		initialize_block: InitializeBlock<'a, Block>,
		native_call: Option<NC>,
		context: ExecutionContext,
		recorder: &Option<Rc<RefCell<ProofRecorder<Block>>>>,
	) -> error::Result<NativeOrEncoded<R>> {
		let manager = match context {
			ExecutionContext::BlockConstruction =>
				self.execution_strategies.block_construction.get_manager(),
			ExecutionContext::Syncing =>
				self.execution_strategies.syncing.get_manager(),
			ExecutionContext::Importing =>
				self.execution_strategies.importing.get_manager(),
			ExecutionContext::OffchainCall(Some((_, capabilities))) if capabilities.has_all() =>
				self.execution_strategies.offchain_worker.get_manager(),
			ExecutionContext::OffchainCall(_) =>
				self.execution_strategies.other.get_manager(),
		};

		let capabilities = context.capabilities();
		let offchain_extensions = if let ExecutionContext::OffchainCall(Some(ext)) = context {
			Some(OffchainExt::new(offchain::LimitedExternalities::new(capabilities, ext.0)))
		} else {
			None
		};

		self.executor.contextual_call::<_, fn(_,_) -> _,_,_>(
			|| core_api.initialize_block(at, &self.prepare_environment_block(at)?),
			at,
			function,
			&args,
			changes,
			initialize_block,
			manager,
			native_call,
			offchain_extensions,
			recorder,
			capabilities.has(offchain::Capability::Keystore),
		)
	}

	fn runtime_version_at(&self, at: &BlockId<Block>) -> error::Result<RuntimeVersion> {
		self.runtime_version_at(at)
	}
}

/// NOTE: only use this implementation when you are sure there are NO consensus-level BlockImport
/// objects. Otherwise, importing blocks directly into the client would be bypassing
/// important verification work.
impl<'a, B, E, Block, RA> consensus::BlockImport<Block> for &'a Client<B, E, Block, RA> where
	B: backend::Backend<Block, Blake2Hasher>,
	E: CallExecutor<Block, Blake2Hasher> + Clone + Send + Sync,
	Block: BlockT<Hash=H256>,
{
	type Error = ConsensusError;

	/// Import a checked and validated block. If a justification is provided in
	/// `BlockImportParams` then `finalized` *must* be true.
	///
	/// NOTE: only use this implementation when there are NO consensus-level BlockImport
	/// objects. Otherwise, importing blocks directly into the client would be bypassing
	/// important verification work.
	///
	/// If you are not sure that there are no BlockImport objects provided by the consensus
	/// algorithm, don't use this function.
	fn import_block(
		&mut self,
		import_block: BlockImportParams<Block>,
		new_cache: HashMap<CacheKeyId, Vec<u8>>,
	) -> Result<ImportResult, Self::Error> {
		self.lock_import_and_run(|operation| {
			self.apply_block(operation, import_block, new_cache)
		}).map_err(|e| {
			warn!("Block import error:\n{:?}", e);
			ConsensusError::ClientImport(e.to_string()).into()
		})
	}

	/// Check block preconditions.
	fn check_block(
		&mut self,
		block: BlockCheckParams<Block>,
	) -> Result<ImportResult, Self::Error> {
		let BlockCheckParams { hash, number, parent_hash } = block;

		if let Some(h) = self.fork_blocks.as_ref().and_then(|x| x.get(&number)) {
			if &hash != h  {
				trace!(
					"Rejecting block from known invalid fork. Got {:?}, expected: {:?} at height {}",
					hash,
					h,
					number
				);
				return Ok(ImportResult::KnownBad);
			}
		}

		match self.block_status(&BlockId::Hash(parent_hash))
			.map_err(|e| ConsensusError::ClientImport(e.to_string()))?
		{
			BlockStatus::InChainWithState | BlockStatus::Queued => {},
			BlockStatus::Unknown | BlockStatus::InChainPruned => return Ok(ImportResult::UnknownParent),
			BlockStatus::KnownBad => return Ok(ImportResult::KnownBad),
		}

		match self.block_status(&BlockId::Hash(hash))
			.map_err(|e| ConsensusError::ClientImport(e.to_string()))?
		{
			BlockStatus::InChainWithState | BlockStatus::Queued => return Ok(ImportResult::AlreadyInChain),
			BlockStatus::Unknown | BlockStatus::InChainPruned => {},
			BlockStatus::KnownBad => return Ok(ImportResult::KnownBad),
		}


		Ok(ImportResult::imported(false))
	}
}

impl<B, E, Block, RA> consensus::BlockImport<Block> for Client<B, E, Block, RA> where
	B: backend::Backend<Block, Blake2Hasher>,
	E: CallExecutor<Block, Blake2Hasher> + Clone + Send + Sync,
	Block: BlockT<Hash=H256>,
{
	type Error = ConsensusError;

	fn import_block(
		&mut self,
		import_block: BlockImportParams<Block>,
		new_cache: HashMap<CacheKeyId, Vec<u8>>,
	) -> Result<ImportResult, Self::Error> {
		(&*self).import_block(import_block, new_cache)
	}

	fn check_block(
		&mut self,
		block: BlockCheckParams<Block>,
	) -> Result<ImportResult, Self::Error> {
		(&*self).check_block(block)
	}
}

impl<B, E, Block, RA> Finalizer<Block, Blake2Hasher, B> for Client<B, E, Block, RA> where
	B: backend::Backend<Block, Blake2Hasher>,
	E: CallExecutor<Block, Blake2Hasher>,
	Block: BlockT<Hash=H256>,
{
	fn apply_finality(
		&self,
		operation: &mut ClientImportOperation<Block, Blake2Hasher, B>,
		id: BlockId<Block>,
		justification: Option<Justification>,
		notify: bool,
	) -> error::Result<()> {
		let last_best = self.backend.blockchain().info().best_hash;
		let to_finalize_hash = self.backend.blockchain().expect_block_hash_from_id(&id)?;
		self.apply_finality_with_block_hash(operation, to_finalize_hash, justification, last_best, notify)
	}

	fn finalize_block(&self, id: BlockId<Block>, justification: Option<Justification>, notify: bool) -> error::Result<()> {
		self.lock_import_and_run(|operation| {
			self.apply_finality(operation, id, justification, notify)
		})
	}
}

impl<B, E, Block, RA> Finalizer<Block, Blake2Hasher, B> for &Client<B, E, Block, RA> where
	B: backend::Backend<Block, Blake2Hasher>,
	E: CallExecutor<Block, Blake2Hasher>,
	Block: BlockT<Hash=H256>,
{
	fn apply_finality(
		&self,
		operation: &mut ClientImportOperation<Block, Blake2Hasher, B>,
		id: BlockId<Block>,
		justification: Option<Justification>,
		notify: bool,
	) -> error::Result<()> {
		(**self).apply_finality(operation, id, justification, notify)
	}

	fn finalize_block(&self, id: BlockId<Block>, justification: Option<Justification>, notify: bool) -> error::Result<()> {
		(**self).finalize_block(id, justification, notify)
	}
}

impl<B, E, Block, RA> BlockchainEvents<Block> for Client<B, E, Block, RA>
where
	E: CallExecutor<Block, Blake2Hasher>,
	Block: BlockT<Hash=H256>,
{
	/// Get block import event stream.
	fn import_notification_stream(&self) -> ImportNotifications<Block> {
		let (sink, stream) = mpsc::unbounded();
		self.import_notification_sinks.lock().push(sink);
		stream
	}

	fn finality_notification_stream(&self) -> FinalityNotifications<Block> {
		let (sink, stream) = mpsc::unbounded();
		self.finality_notification_sinks.lock().push(sink);
		stream
	}

	/// Get storage changes event stream.
	fn storage_changes_notification_stream(
		&self,
		filter_keys: Option<&[StorageKey]>,
		child_filter_keys: Option<&[(StorageKey, Option<Vec<StorageKey>>)]>,
	) -> error::Result<StorageEventStream<Block::Hash>> {
		Ok(self.storage_notifications.lock().listen(filter_keys, child_filter_keys))
	}
}

/// Implement Longest Chain Select implementation
/// where 'longest' is defined as the highest number of blocks
pub struct LongestChain<B, Block> {
	backend: Arc<B>,
	_phantom: PhantomData<Block>
}

impl<B, Block> Clone for LongestChain<B, Block> {
	fn clone(&self) -> Self {
		let backend = self.backend.clone();
		LongestChain {
			backend,
			_phantom: Default::default()
		}
	}
}

impl<B, Block> LongestChain<B, Block>
where
	B: backend::Backend<Block, Blake2Hasher>,
	Block: BlockT<Hash=H256>,
{
	/// Instantiate a new LongestChain for Backend B
	pub fn new(backend: Arc<B>) -> Self {
		LongestChain {
			backend,
			_phantom: Default::default()
		}
	}

	fn best_block_header(&self) -> error::Result<<Block as BlockT>::Header> {
		let info = self.backend.blockchain().info();
		let import_lock = self.backend.get_import_lock();
		let best_hash = self.backend.blockchain().best_containing(info.best_hash, None, import_lock)?
			.unwrap_or(info.best_hash);

		Ok(self.backend.blockchain().header(BlockId::Hash(best_hash))?
			.expect("given block hash was fetched from block in db; qed"))
	}

	fn leaves(&self) -> Result<Vec<<Block as BlockT>::Hash>, error::Error> {
		self.backend.blockchain().leaves()
	}
}

impl<B, Block> SelectChain<Block> for LongestChain<B, Block>
where
	B: backend::Backend<Block, Blake2Hasher>,
	Block: BlockT<Hash=H256>,
{

	fn leaves(&self) -> Result<Vec<<Block as BlockT>::Hash>, ConsensusError> {
		LongestChain::leaves(self)
			.map_err(|e| ConsensusError::ChainLookup(e.to_string()).into())
	}

	fn best_chain(&self)
		-> Result<<Block as BlockT>::Header, ConsensusError>
	{
		LongestChain::best_block_header(&self)
			.map_err(|e| ConsensusError::ChainLookup(e.to_string()).into())
	}

	fn finality_target(
		&self,
		target_hash: Block::Hash,
		maybe_max_number: Option<NumberFor<Block>>
	) -> Result<Option<Block::Hash>, ConsensusError> {
		let import_lock = self.backend.get_import_lock();
		self.backend.blockchain().best_containing(target_hash, maybe_max_number, import_lock)
			.map_err(|e| ConsensusError::ChainLookup(e.to_string()).into())
	}
}

impl<B, E, Block, RA> BlockBody<Block> for Client<B, E, Block, RA>
	where
		B: backend::Backend<Block, Blake2Hasher>,
		E: CallExecutor<Block, Blake2Hasher>,
		Block: BlockT<Hash=H256>,
{
	fn block_body(&self, id: &BlockId<Block>) -> error::Result<Option<Vec<<Block as BlockT>::Extrinsic>>> {
		self.body(id)
	}
}

impl<B, E, Block, RA> backend::AuxStore for Client<B, E, Block, RA>
	where
		B: backend::Backend<Block, Blake2Hasher>,
		E: CallExecutor<Block, Blake2Hasher>,
		Block: BlockT<Hash=H256>,
{
	/// Insert auxiliary data into key-value store.
	fn insert_aux<
		'a,
		'b: 'a,
		'c: 'a,
		I: IntoIterator<Item=&'a(&'c [u8], &'c [u8])>,
		D: IntoIterator<Item=&'a &'b [u8]>,
	>(&self, insert: I, delete: D) -> error::Result<()> {
		// Import is locked here because we may have other block import
		// operations that tries to set aux data. Note that for consensus
		// layer, one can always use atomic operations to make sure
		// import is only locked once.
		self.lock_import_and_run(|operation| {
			apply_aux(operation, insert, delete)
		})
	}
	/// Query auxiliary data from key-value store.
	fn get_aux(&self, key: &[u8]) -> error::Result<Option<Vec<u8>>> {
		crate::backend::AuxStore::get_aux(&*self.backend, key)
	}
}


impl<B, E, Block, RA> backend::AuxStore for &Client<B, E, Block, RA>
	where
		B: backend::Backend<Block, Blake2Hasher>,
		E: CallExecutor<Block, Blake2Hasher>,
		Block: BlockT<Hash=H256>,
{

	fn insert_aux<
		'a,
		'b: 'a,
		'c: 'a,
		I: IntoIterator<Item=&'a(&'c [u8], &'c [u8])>,
		D: IntoIterator<Item=&'a &'b [u8]>,
	>(&self, insert: I, delete: D) -> error::Result<()> {
		(**self).insert_aux(insert, delete)
	}

	fn get_aux(&self, key: &[u8]) -> error::Result<Option<Vec<u8>>> {
		(**self).get_aux(key)
	}
}

/// Helper function to apply auxiliary data insertion into an operation.
pub fn apply_aux<'a, 'b: 'a, 'c: 'a, B, Block, H, D, I>(
	operation: &mut ClientImportOperation<Block, H, B>,
	insert: I,
	delete: D
) -> error::Result<()>
where
	Block: BlockT,
	H: Hasher<Out=Block::Hash>,
	B: backend::Backend<Block, H>,
	I: IntoIterator<Item=&'a(&'c [u8], &'c [u8])>,
	D: IntoIterator<Item=&'a &'b [u8]>,
{
	operation.op.insert_aux(
		insert.into_iter()
			.map(|(k, v)| (k.to_vec(), Some(v.to_vec())))
			.chain(delete.into_iter().map(|k| (k.to_vec(), None)))
	)
}

/// Utility methods for the client.
pub mod utils {
	use super::*;
	use crate::error;
	use primitives::H256;
	use std::borrow::Borrow;

	/// Returns a function for checking block ancestry, the returned function will
	/// return `true` if the given hash (second parameter) is a descendent of the
	/// base (first parameter). If the `current` parameter is defined, it should
	/// represent the current block `hash` and its `parent hash`, if given the
	/// function that's returned will assume that `hash` isn't part of the local DB
	/// yet, and all searches in the DB will instead reference the parent.
	pub fn is_descendent_of<'a, Block: BlockT<Hash=H256>, T, H: Borrow<H256> + 'a>(
		client: &'a T,
		current: Option<(H, H)>,
	) -> impl Fn(&H256, &H256) -> Result<bool, error::Error> + 'a
		where T: ChainHeaderBackend<Block> + HeaderMetadata<Block, Error=error::Error>,
	{
		move |base, hash| {
			if base == hash { return Ok(false); }

			let current = current.as_ref().map(|(c, p)| (c.borrow(), p.borrow()));

			let mut hash = hash;
			if let Some((current_hash, current_parent_hash)) = current {
				if base == current_hash { return Ok(false); }
				if hash == current_hash {
					if base == current_parent_hash {
						return Ok(true);
					} else {
						hash = current_parent_hash;
					}
				}
			}

			let ancestor = header_metadata::lowest_common_ancestor(client, *hash, *base)?;

			Ok(ancestor.hash == *base)
		}
	}
}

impl<BE, E, B, RA> consensus::block_validation::Chain<B> for Client<BE, E, B, RA>
	where BE: backend::Backend<B, Blake2Hasher>,
		  E: CallExecutor<B, Blake2Hasher>,
		  B: BlockT<Hash = H256>
{
	fn block_status(&self, id: &BlockId<B>) -> Result<BlockStatus, Box<dyn std::error::Error + Send>> {
		Client::block_status(self, id).map_err(|e| Box::new(e) as Box<_>)
	}
}

#[cfg(test)]
pub(crate) mod tests {
	use std::collections::HashMap;
	use super::*;
	use primitives::blake2_256;
	use sr_primitives::DigestItem;
	use consensus::{BlockOrigin, SelectChain};
	use test_client::{
		prelude::*,
		client_db::{Backend, DatabaseSettings, DatabaseSettingsSrc, PruningMode},
		runtime::{self, Block, Transfer, RuntimeApi, TestAPI},
	};

	/// Returns tuple, consisting of:
	/// 1) test client pre-filled with blocks changing balances;
	/// 2) roots of changes tries for these blocks
	/// 3) test cases in form (begin, end, key, vec![(block, extrinsic)]) that are required to pass
	pub fn prepare_client_with_key_changes() -> (
		test_client::client::Client<test_client::Backend, test_client::Executor, Block, RuntimeApi>,
		Vec<H256>,
		Vec<(u64, u64, Vec<u8>, Vec<(u64, u32)>)>,
	) {
		// prepare block structure
		let blocks_transfers = vec![
			vec![(AccountKeyring::Alice, AccountKeyring::Dave), (AccountKeyring::Bob, AccountKeyring::Dave)],
			vec![(AccountKeyring::Charlie, AccountKeyring::Eve)],
			vec![],
			vec![(AccountKeyring::Alice, AccountKeyring::Dave)],
		];

		// prepare client ang import blocks
		let mut local_roots = Vec::new();
		let remote_client = TestClientBuilder::new().set_support_changes_trie(true).build();
		let mut nonces: HashMap<_, u64> = Default::default();
		for (i, block_transfers) in blocks_transfers.into_iter().enumerate() {
			let mut builder = remote_client.new_block(Default::default()).unwrap();
			for (from, to) in block_transfers {
				builder.push_transfer(Transfer {
					from: from.into(),
					to: to.into(),
					amount: 1,
					nonce: *nonces.entry(from).and_modify(|n| { *n = *n + 1 }).or_default(),
				}).unwrap();
			}
			remote_client.import(BlockOrigin::Own, builder.bake().unwrap()).unwrap();

			let header = remote_client.header(&BlockId::Number(i as u64 + 1)).unwrap().unwrap();
			let trie_root = header.digest().log(DigestItem::as_changes_trie_root)
				.map(|root| H256::from_slice(root.as_ref()))
				.unwrap();
			local_roots.push(trie_root);
		}

		// prepare test cases
		let alice = blake2_256(&runtime::system::balance_of_key(AccountKeyring::Alice.into())).to_vec();
		let bob = blake2_256(&runtime::system::balance_of_key(AccountKeyring::Bob.into())).to_vec();
		let charlie = blake2_256(&runtime::system::balance_of_key(AccountKeyring::Charlie.into())).to_vec();
		let dave = blake2_256(&runtime::system::balance_of_key(AccountKeyring::Dave.into())).to_vec();
		let eve = blake2_256(&runtime::system::balance_of_key(AccountKeyring::Eve.into())).to_vec();
		let ferdie = blake2_256(&runtime::system::balance_of_key(AccountKeyring::Ferdie.into())).to_vec();
		let test_cases = vec![
			(1, 4, alice.clone(), vec![(4, 0), (1, 0)]),
			(1, 3, alice.clone(), vec![(1, 0)]),
			(2, 4, alice.clone(), vec![(4, 0)]),
			(2, 3, alice.clone(), vec![]),

			(1, 4, bob.clone(), vec![(1, 1)]),
			(1, 1, bob.clone(), vec![(1, 1)]),
			(2, 4, bob.clone(), vec![]),

			(1, 4, charlie.clone(), vec![(2, 0)]),

			(1, 4, dave.clone(), vec![(4, 0), (1, 1), (1, 0)]),
			(1, 1, dave.clone(), vec![(1, 1), (1, 0)]),
			(3, 4, dave.clone(), vec![(4, 0)]),

			(1, 4, eve.clone(), vec![(2, 0)]),
			(1, 1, eve.clone(), vec![]),
			(3, 4, eve.clone(), vec![]),

			(1, 4, ferdie.clone(), vec![]),
		];

		(remote_client, local_roots, test_cases)
	}

	#[test]
	fn client_initializes_from_genesis_ok() {
		let client = test_client::new();

		assert_eq!(
			client.runtime_api().balance_of(
				&BlockId::Number(client.info().chain.best_number),
				AccountKeyring::Alice.into()
			).unwrap(),
			1000
		);
		assert_eq!(
			client.runtime_api().balance_of(
				&BlockId::Number(client.info().chain.best_number),
				AccountKeyring::Ferdie.into()
			).unwrap(),
			0
		);
	}

	#[test]
	fn block_builder_works_with_no_transactions() {
		let client = test_client::new();

		let builder = client.new_block(Default::default()).unwrap();

		client.import(BlockOrigin::Own, builder.bake().unwrap()).unwrap();

		assert_eq!(client.info().chain.best_number, 1);
	}

	#[test]
	fn block_builder_works_with_transactions() {
		let client = test_client::new();

		let mut builder = client.new_block(Default::default()).unwrap();

		builder.push_transfer(Transfer {
			from: AccountKeyring::Alice.into(),
			to: AccountKeyring::Ferdie.into(),
			amount: 42,
			nonce: 0,
		}).unwrap();

		client.import(BlockOrigin::Own, builder.bake().unwrap()).unwrap();

		assert_eq!(client.info().chain.best_number, 1);
		assert_ne!(
			client.state_at(&BlockId::Number(1)).unwrap().pairs(),
			client.state_at(&BlockId::Number(0)).unwrap().pairs()