lib.rs

				meta.best_hash,
				route_to,
			)?;

			// uncanonicalize: check safety violations and ensure the numbers no longer
			// point to these block hashes in the key mapping.
			for r in tree_route.retracted() {
				if r.hash == meta.finalized_hash {
					warn!(
						"Potential safety failure: reverting finalized block {:?}",
						(&r.number, &r.hash)
					);

					return Err(::sp_blockchain::Error::NotInFinalizedChain.into());
				}

				retracted.push(r.hash.clone());
				utils::remove_number_to_key_mapping(
					transaction,
					columns::KEY_LOOKUP,
					r.number
				)?;
			}

			// canonicalize: set the number lookup to map to this block's hash.
			for e in tree_route.enacted() {
				enacted.push(e.hash.clone());
				utils::insert_number_to_key_mapping(
					transaction,
					columns::KEY_LOOKUP,
					e.number,
					e.hash
				)?;
			}
		}

		let lookup_key = utils::number_and_hash_to_lookup_key(best_to.0, &best_to.1)?;
		transaction.put(columns::META, meta_keys::BEST_BLOCK, &lookup_key);
		utils::insert_number_to_key_mapping(
			transaction,
			columns::KEY_LOOKUP,
			best_to.0,
			best_to.1,
		)?;

		Ok((enacted, retracted))
	}

	fn ensure_sequential_finalization(
		&self,
		header: &Block::Header,
		last_finalized: Option<Block::Hash>,
	) -> ClientResult<()> {
		let last_finalized = last_finalized.unwrap_or_else(|| self.blockchain.meta.read().finalized_hash);
		if *header.parent_hash() != last_finalized {
			return Err(::sp_blockchain::Error::NonSequentialFinalization(
				format!("Last finalized {:?} not parent of {:?}", last_finalized, header.hash()),
			).into());
		}
		Ok(())
	}

	fn finalize_block_with_transaction(
		&self,
		transaction: &mut DBTransaction,
		hash: &Block::Hash,
		header: &Block::Header,
		last_finalized: Option<Block::Hash>,
		justification: Option<Justification>,
		finalization_displaced: &mut Option<FinalizationDisplaced<Block::Hash, NumberFor<Block>>>,
	) -> ClientResult<(Block::Hash, <Block::Header as HeaderT>::Number, bool, bool)> {
		// TODO: ensure best chain contains this block.
		let number = *header.number();
		self.ensure_sequential_finalization(header, last_finalized)?;
		self.note_finalized(
			transaction,
			header,
			*hash,
			finalization_displaced,
		)?;

		if let Some(justification) = justification {
			transaction.put(
				columns::JUSTIFICATION,
				&utils::number_and_hash_to_lookup_key(number, hash)?,
				&justification.encode(),
			);
		}
		Ok((*hash, number, false, true))
	}

	// performs forced canonicaliziation with a delay after importing a non-finalized block.
	fn force_delayed_canonicalize(
		&self,
		transaction: &mut DBTransaction,
		hash: Block::Hash,
		number: NumberFor<Block>,
	)
		-> ClientResult<()>
	{
		let number_u64 = number.saturated_into::<u64>();
		if number_u64 > self.canonicalization_delay {
			let new_canonical = number_u64 - self.canonicalization_delay;

			if new_canonical <= self.storage.state_db.best_canonical().unwrap_or(0) {
				return Ok(())
			}

			let hash = if new_canonical == number_u64 {
				hash
			} else {
				::sc_client::blockchain::HeaderBackend::hash(&self.blockchain, new_canonical.saturated_into())?
					.expect("existence of block with number `new_canonical` \
						implies existence of blocks with all numbers before it; qed")
			};

			trace!(target: "db", "Canonicalize block #{} ({:?})", new_canonical, hash);
			let commit = self.storage.state_db.canonicalize_block(&hash)
				.map_err(|e: sc_state_db::Error<io::Error>| sp_blockchain::Error::from(format!("State database error: {:?}", e)))?;
			apply_state_commit(transaction, commit);
		};

		Ok(())
	}

	fn try_commit_operation(&self, mut operation: BlockImportOperation<Block, Blake2Hasher>)
		-> ClientResult<()>
	{
		let mut transaction = DBTransaction::new();
		let mut finalization_displaced_leaves = None;

		operation.apply_aux(&mut transaction);

		let mut meta_updates = Vec::with_capacity(operation.finalized_blocks.len());
		let mut last_finalized_hash = self.blockchain.meta.read().finalized_hash;

		for (block, justification) in operation.finalized_blocks {
			let block_hash = self.blockchain.expect_block_hash_from_id(&block)?;
			let block_header = self.blockchain.expect_header(BlockId::Hash(block_hash))?;

			meta_updates.push(self.finalize_block_with_transaction(
				&mut transaction,
				&block_hash,
				&block_header,
				Some(last_finalized_hash),
				justification,
				&mut finalization_displaced_leaves,
			)?);
			last_finalized_hash = block_hash;
		}

		let imported = if let Some(pending_block) = operation.pending_block {
			let hash = pending_block.header.hash();
			let parent_hash = *pending_block.header.parent_hash();
			let number = pending_block.header.number().clone();

			// blocks are keyed by number + hash.
			let lookup_key = utils::number_and_hash_to_lookup_key(number, hash)?;

			let (enacted, retracted) = if pending_block.leaf_state.is_best() {
				self.set_head_with_transaction(&mut transaction, parent_hash, (number, hash))?
			} else {
				(Default::default(), Default::default())
			};

			utils::insert_hash_to_key_mapping(
				&mut transaction,
				columns::KEY_LOOKUP,
				number,
				hash,
			)?;

			let header_metadata = CachedHeaderMetadata::from(&pending_block.header);
			self.blockchain.insert_header_metadata(
				header_metadata.hash,
				header_metadata,
			);

			transaction.put(columns::HEADER, &lookup_key, &pending_block.header.encode());
			if let Some(body) = &pending_block.body {
				transaction.put(columns::BODY, &lookup_key, &body.encode());
			}
			if let Some(justification) = pending_block.justification {
				transaction.put(columns::JUSTIFICATION, &lookup_key, &justification.encode());
			}

			if number.is_zero() {
				transaction.put(columns::META, meta_keys::FINALIZED_BLOCK, &lookup_key);
				transaction.put(columns::META, meta_keys::GENESIS_HASH, hash.as_ref());
			}

			let finalized = if operation.commit_state {
				let mut changeset: sc_state_db::ChangeSet<Vec<u8>> = sc_state_db::ChangeSet::default();
				for (key, (val, rc)) in operation.db_updates.drain() {
					if rc > 0 {
						changeset.inserted.push((key, val.to_vec()));
					} else if rc < 0 {
						changeset.deleted.push(key);
					}
				}
				let number_u64 = number.saturated_into::<u64>();
				let commit = self.storage.state_db.insert_block(&hash, number_u64, &pending_block.header.parent_hash(), changeset)
					.map_err(|e: sc_state_db::Error<io::Error>| sp_blockchain::Error::from(format!("State database error: {:?}", e)))?;
				apply_state_commit(&mut transaction, commit);

				// Check if need to finalize. Genesis is always finalized instantly.
				let finalized = number_u64 == 0 || pending_block.leaf_state.is_final();
				finalized
			} else {
				false
			};

			let header = &pending_block.header;
			let is_best = pending_block.leaf_state.is_best();
			let changes_trie_updates = operation.changes_trie_updates;

			self.changes_tries_storage.commit(&mut transaction, changes_trie_updates);
			let cache = operation.old_state.release(); // release state reference so that it can be finalized

			if finalized {
				// TODO: ensure best chain contains this block.
				self.ensure_sequential_finalization(header, Some(last_finalized_hash))?;
				self.note_finalized(
					&mut transaction,
					header,
					hash,
					&mut finalization_displaced_leaves,
				)?;
			} else {
				// canonicalize blocks which are old enough, regardless of finality.
				self.force_delayed_canonicalize(&mut transaction, hash, *header.number())?
			}

			debug!(target: "db", "DB Commit {:?} ({}), best = {}", hash, number, is_best);

			let displaced_leaf = {
				let mut leaves = self.blockchain.leaves.write();
				let displaced_leaf = leaves.import(hash, number, parent_hash);
				leaves.prepare_transaction(&mut transaction, columns::META, meta_keys::LEAF_PREFIX);

				displaced_leaf
			};

			let mut children = children::read_children(&*self.storage.db, columns::META, meta_keys::CHILDREN_PREFIX, parent_hash)?;
			children.push(hash);
			children::write_children(&mut transaction, columns::META, meta_keys::CHILDREN_PREFIX, parent_hash, children);

			meta_updates.push((hash, number, pending_block.leaf_state.is_best(), finalized));

			Some((number, hash, enacted, retracted, displaced_leaf, is_best, cache))
		} else {
			None
		};

		let cache_update = if let Some(set_head) = operation.set_head {
			if let Some(header) = ::sc_client::blockchain::HeaderBackend::header(&self.blockchain, set_head)? {
				let number = header.number();
				let hash = header.hash();

				let (enacted, retracted) = self.set_head_with_transaction(
					&mut transaction,
					hash.clone(),
					(number.clone(), hash.clone())
				)?;
				meta_updates.push((hash, *number, true, false));
				Some((enacted, retracted))
			} else {
				return Err(sp_blockchain::Error::UnknownBlock(format!("Cannot set head {:?}", set_head)))
			}
		} else {
			None
		};

		let write_result = self.storage.db.write(transaction).map_err(db_err);

		if let Some(changes_trie_cache_update) = operation.changes_trie_cache_update {
			self.changes_tries_storage.commit_cache(changes_trie_cache_update);
		}

		if let Some((number, hash, enacted, retracted, displaced_leaf, is_best, mut cache)) = imported {
			if let Err(e) = write_result {
				let mut leaves = self.blockchain.leaves.write();
				let mut undo = leaves.undo();
				if let Some(displaced_leaf) = displaced_leaf {
					undo.undo_import(displaced_leaf);
				}

				if let Some(finalization_displaced) = finalization_displaced_leaves {
					undo.undo_finalization(finalization_displaced);
				}

				return Err(e)
			}

			cache.sync_cache(
				&enacted,
				&retracted,
				operation.storage_updates,
				operation.child_storage_updates,
				Some(hash),
				Some(number),
				is_best,
			);
		}

		if let Some((enacted, retracted)) = cache_update {
			self.shared_cache.lock().sync(&enacted, &retracted);
		}

		for (hash, number, is_best, is_finalized) in meta_updates {
			self.blockchain.update_meta(hash, number, is_best, is_finalized);
		}

		Ok(())
	}


	// write stuff to a transaction after a new block is finalized.
	// this canonicalizes finalized blocks. Fails if called with a block which
	// was not a child of the last finalized block.
	fn note_finalized(
		&self,
		transaction: &mut DBTransaction,
		f_header: &Block::Header,
		f_hash: Block::Hash,
		displaced: &mut Option<FinalizationDisplaced<Block::Hash, NumberFor<Block>>>
	) -> ClientResult<()> where
		Block: BlockT<Hash=H256>,
	{
		let f_num = f_header.number().clone();

		if self.storage.state_db.best_canonical().map(|c| f_num.saturated_into::<u64>() > c).unwrap_or(true) {
			let parent_hash = f_header.parent_hash().clone();

			let lookup_key = utils::number_and_hash_to_lookup_key(f_num, f_hash.clone())?;
			transaction.put(columns::META, meta_keys::FINALIZED_BLOCK, &lookup_key);

			let commit = self.storage.state_db.canonicalize_block(&f_hash)
				.map_err(|e: sc_state_db::Error<io::Error>| sp_blockchain::Error::from(format!("State database error: {:?}", e)))?;
			apply_state_commit(transaction, commit);

			let changes_trie_config = self.changes_trie_config(parent_hash)?;
			if let Some(changes_trie_config) = changes_trie_config {
				self.changes_tries_storage.prune(&changes_trie_config, transaction, f_hash, f_num);
			}
		}

		let new_displaced = self.blockchain.leaves.write().finalize_height(f_num);
		match displaced {
			x @ &mut None => *x = Some(new_displaced),
			&mut Some(ref mut displaced) => displaced.merge(new_displaced),
		}

		Ok(())
	}
}

fn apply_state_commit(transaction: &mut DBTransaction, commit: sc_state_db::CommitSet<Vec<u8>>) {
	for (key, val) in commit.data.inserted.into_iter() {
		transaction.put(columns::STATE, &key[..], &val);
	}
	for key in commit.data.deleted.into_iter() {
		transaction.delete(columns::STATE, &key[..]);
	}
	for (key, val) in commit.meta.inserted.into_iter() {
		transaction.put(columns::STATE_META, &key[..], &val);
	}
	for key in commit.meta.deleted.into_iter() {
		transaction.delete(columns::STATE_META, &key[..]);
	}
}

impl<Block> sc_client_api::backend::AuxStore for Backend<Block> where 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) -> ClientResult<()> {
		let mut transaction = DBTransaction::new();
		for (k, v) in insert {
			transaction.put(columns::AUX, k, v);
		}
		for k in delete {
			transaction.delete(columns::AUX, k);
		}
		self.storage.db.write(transaction).map_err(db_err)?;
		Ok(())
	}

	fn get_aux(&self, key: &[u8]) -> ClientResult<Option<Vec<u8>>> {
		Ok(self.storage.db.get(columns::AUX, key).map(|r| r.map(|v| v.to_vec())).map_err(db_err)?)
	}
}

impl<Block> sc_client_api::backend::Backend<Block, Blake2Hasher> for Backend<Block> where Block: BlockT<Hash=H256> {
	type BlockImportOperation = BlockImportOperation<Block, Blake2Hasher>;
	type Blockchain = BlockchainDb<Block>;
	type State = CachingState<Blake2Hasher, RefTrackingState<Block>, Block>;
	type ChangesTrieStorage = DbChangesTrieStorage<Block>;
	type OffchainStorage = offchain::LocalStorage;

	fn begin_operation(&self) -> ClientResult<Self::BlockImportOperation> {
		let old_state = self.state_at(BlockId::Hash(Default::default()))?;
		Ok(BlockImportOperation {
			pending_block: None,
			old_state,
			db_updates: PrefixedMemoryDB::default(),
			storage_updates: Default::default(),
			child_storage_updates: Default::default(),
			changes_trie_updates: MemoryDB::default(),
			changes_trie_cache_update: None,
			aux_ops: Vec::new(),
			finalized_blocks: Vec::new(),
			set_head: None,
			commit_state: false,
		})
	}

	fn begin_state_operation(
		&self,
		operation: &mut Self::BlockImportOperation,
		block: BlockId<Block>,
	) -> ClientResult<()> {
		operation.old_state = self.state_at(block)?;
		operation.commit_state = true;
		Ok(())
	}

	fn commit_operation(&self, operation: Self::BlockImportOperation)
		-> ClientResult<()>
	{
		match self.try_commit_operation(operation) {
			Ok(_) => {
				self.storage.state_db.apply_pending();
				Ok(())
			},
			e @ Err(_) => {
				self.storage.state_db.revert_pending();
				e
			}
		}
	}

	fn finalize_block(&self, block: BlockId<Block>, justification: Option<Justification>)
		-> ClientResult<()>
	{
		let mut transaction = DBTransaction::new();
		let hash = self.blockchain.expect_block_hash_from_id(&block)?;
		let header = self.blockchain.expect_header(block)?;
		let mut displaced = None;
		let commit = |displaced| {
			let (hash, number, is_best, is_finalized) = self.finalize_block_with_transaction(
				&mut transaction,
				&hash,
				&header,
				None,
				justification,
				displaced,
			)?;
			self.storage.db.write(transaction).map_err(db_err)?;
			self.blockchain.update_meta(hash, number, is_best, is_finalized);
			Ok(())
		};
		match commit(&mut displaced) {
			Ok(()) => self.storage.state_db.apply_pending(),
			e @ Err(_) => {
				self.storage.state_db.revert_pending();
				if let Some(displaced) = displaced {
					self.blockchain.leaves.write().undo().undo_finalization(displaced);
				}
				return e;
			}
		}
		Ok(())
	}

	fn changes_trie_storage(&self) -> Option<&Self::ChangesTrieStorage> {
		Some(&self.changes_tries_storage)
	}

	fn offchain_storage(&self) -> Option<Self::OffchainStorage> {
		Some(self.offchain_storage.clone())
	}

	fn revert(&self, n: NumberFor<Block>) -> ClientResult<NumberFor<Block>> {
		let mut best = self.blockchain.info().best_number;
		let finalized = self.blockchain.info().finalized_number;
		let revertible = best - finalized;
		let n = if revertible < n { revertible } else { n };

		for c in 0 .. n.saturated_into::<u64>() {
			if best.is_zero() {
				return Ok(c.saturated_into::<NumberFor<Block>>())
			}
			let mut transaction = DBTransaction::new();
			match self.storage.state_db.revert_one() {
				Some(commit) => {
					apply_state_commit(&mut transaction, commit);
					let removed = self.blockchain.header(BlockId::Number(best))?.ok_or_else(
						|| sp_blockchain::Error::UnknownBlock(
							format!("Error reverting to {}. Block hash not found.", best)))?;

					best -= One::one();	// prev block
					let hash = self.blockchain.hash(best)?.ok_or_else(
						|| sp_blockchain::Error::UnknownBlock(
							format!("Error reverting to {}. Block hash not found.", best)))?;
					let key = utils::number_and_hash_to_lookup_key(best.clone(), &hash)?;
					transaction.put(columns::META, meta_keys::BEST_BLOCK, &key);
					transaction.delete(columns::KEY_LOOKUP, removed.hash().as_ref());
					children::remove_children(&mut transaction, columns::META, meta_keys::CHILDREN_PREFIX, hash);
					self.storage.db.write(transaction).map_err(db_err)?;
					self.blockchain.update_meta(hash, best, true, false);
					self.blockchain.leaves.write().revert(removed.hash().clone(), removed.number().clone(), removed.parent_hash().clone());
				}
				None => return Ok(c.saturated_into::<NumberFor<Block>>())
			}
		}
		Ok(n)
	}

	fn blockchain(&self) -> &BlockchainDb<Block> {
		&self.blockchain
	}

	fn used_state_cache_size(&self) -> Option<usize> {
		let used = (*&self.shared_cache).lock().used_storage_cache_size();
		Some(used)
	}

	fn state_at(&self, block: BlockId<Block>) -> ClientResult<Self::State> {
		use sc_client::blockchain::HeaderBackend as BcHeaderBackend;

		// special case for genesis initialization
		match block {
			BlockId::Hash(h) if h == Default::default() => {
				let genesis_storage = DbGenesisStorage::new();
				let root = genesis_storage.0.clone();
				let db_state = DbState::new(Arc::new(genesis_storage), root);
				let state = RefTrackingState::new(db_state, self.storage.clone(), None);
				return Ok(CachingState::new(state, self.shared_cache.clone(), None));
			},
			_ => {}
		}

		match self.blockchain.header(block) {
			Ok(Some(ref hdr)) => {
				let hash = hdr.hash();
				if !self.have_state_at(&hash, *hdr.number()) {
					return Err(sp_blockchain::Error::UnknownBlock(format!("State already discarded for {:?}", block)))
				}
				if let Ok(()) = self.storage.state_db.pin(&hash) {
					let root = H256::from_slice(hdr.state_root().as_ref());
					let db_state = DbState::new(self.storage.clone(), root);
					let state = RefTrackingState::new(db_state, self.storage.clone(), Some(hash.clone()));
					Ok(CachingState::new(state, self.shared_cache.clone(), Some(hash)))
				} else {
					Err(sp_blockchain::Error::UnknownBlock(format!("State already discarded for {:?}", block)))
				}
			},
			Ok(None) => Err(sp_blockchain::Error::UnknownBlock(format!("Unknown state for block {:?}", block))),
			Err(e) => Err(e),
		}
	}

	fn have_state_at(&self, hash: &Block::Hash, number: NumberFor<Block>) -> bool {
		if self.is_archive {
			match self.blockchain.header(BlockId::Hash(hash.clone())) {
				Ok(Some(header)) => {
					sp_state_machine::Storage::get(self.storage.as_ref(), &header.state_root(), (&[], None)).unwrap_or(None).is_some()
				},
				_ => false,
			}
		} else {
			!self.storage.state_db.is_pruned(hash, number.saturated_into::<u64>())
		}
	}

	fn destroy_state(&self, state: Self::State) -> ClientResult<()> {
		if let Some(hash) = state.cache.parent_hash.clone() {
			let is_best = self.blockchain.meta.read().best_hash == hash;
			state.release().sync_cache(&[], &[], vec![], vec![], None, None, is_best);
		}
		Ok(())
	}

	fn get_import_lock(&self) -> &RwLock<()> {
		&self.import_lock
	}
}

impl<Block> sc_client_api::backend::LocalBackend<Block, Blake2Hasher> for Backend<Block>
where Block: BlockT<Hash=H256> {}

/// TODO: remove me in #3201
pub fn unused_sink<Block: BlockT>(cache_tx: crate::cache::DbCacheTransaction<Block>) {
	cache_tx.on_block_revert(&crate::cache::ComplexBlockId::new(Default::default(), 0.into())).unwrap();
	unimplemented!()
}

#[cfg(test)]
mod tests {
	use hash_db::{HashDB, EMPTY_PREFIX};
	use super::*;
	use crate::columns;
	use sc_client_api::backend::{Backend as BTrait, BlockImportOperation as Op};
	use sc_client::blockchain::Backend as BLBTrait;
	use sp_runtime::testing::{Header, Block as RawBlock, ExtrinsicWrapper};
	use sp_runtime::traits::{Hash, BlakeTwo256};
	use sp_state_machine::{TrieMut, TrieDBMut, ChangesTrieRootsStorage, ChangesTrieStorage};
	use sp_blockchain::{lowest_common_ancestor, tree_route};

	type Block = RawBlock<ExtrinsicWrapper<u64>>;

	fn prepare_changes(changes: Vec<(Vec<u8>, Vec<u8>)>) -> (H256, MemoryDB<Blake2Hasher>) {
		let mut changes_root = H256::default();
		let mut changes_trie_update = MemoryDB::<Blake2Hasher>::default();
		{
			let mut trie = TrieDBMut::<Blake2Hasher>::new(
				&mut changes_trie_update,
				&mut changes_root
			);
			for (key, value) in changes {
				trie.insert(&key, &value).unwrap();
			}
		}

		(changes_root, changes_trie_update)
	}

	fn insert_header(
		backend: &Backend<Block>,
		number: u64,
		parent_hash: H256,
		changes: Vec<(Vec<u8>, Vec<u8>)>,
		extrinsics_root: H256,
	) -> H256 {
		use sp_runtime::testing::Digest;
		let (changes_root, changes_trie_update) = prepare_changes(changes);
		let digest = Digest {
			logs: vec![
				DigestItem::ChangesTrieRoot(changes_root),
			],
		};
		let header = Header {
			number,
			parent_hash,
			state_root: BlakeTwo256::trie_root(Vec::new()),
			digest,
			extrinsics_root,
		};
		let header_hash = header.hash();

		let block_id = if number == 0 {
			BlockId::Hash(Default::default())
		} else {
			BlockId::Number(number - 1)
		};
		let mut op = backend.begin_operation().unwrap();
		backend.begin_state_operation(&mut op, block_id).unwrap();
		op.set_block_data(header, Some(Vec::new()), None, NewBlockState::Best).unwrap();
		op.update_changes_trie((changes_trie_update, ChangesTrieCacheAction::Clear)).unwrap();
		backend.commit_operation(op).unwrap();

		header_hash
	}

	#[test]
	fn block_hash_inserted_correctly() {
		let backing = {
			let db = Backend::<Block>::new_test(1, 0);
			for i in 0..10 {
				assert!(db.blockchain().hash(i).unwrap().is_none());

				{
					let id = if i == 0 {
						BlockId::Hash(Default::default())
					} else {
						BlockId::Number(i - 1)
					};

					let mut op = db.begin_operation().unwrap();
					db.begin_state_operation(&mut op, id).unwrap();
					let header = Header {
						number: i,
						parent_hash: if i == 0 {
							Default::default()
						} else {
							db.blockchain.hash(i - 1).unwrap().unwrap()
						},
						state_root: Default::default(),
						digest: Default::default(),
						extrinsics_root: Default::default(),
					};

					op.set_block_data(
						header,
						Some(vec![]),
						None,
						NewBlockState::Best,
					).unwrap();
					db.commit_operation(op).unwrap();
				}

				assert!(db.blockchain().hash(i).unwrap().is_some())
			}
			db.storage.db.clone()
		};

		let backend = Backend::<Block>::new(DatabaseSettings {
			state_cache_size: 16777216,
			state_cache_child_ratio: Some((50, 100)),
			pruning: PruningMode::keep_blocks(1),
			source: DatabaseSettingsSrc::Custom(backing),
		}, 0).unwrap();
		assert_eq!(backend.blockchain().info().best_number, 9);
		for i in 0..10 {
			assert!(backend.blockchain().hash(i).unwrap().is_some())
		}
	}

	#[test]
	fn set_state_data() {
		let db = Backend::<Block>::new_test(2, 0);
		let hash = {
			let mut op = db.begin_operation().unwrap();
			db.begin_state_operation(&mut op, BlockId::Hash(Default::default())).unwrap();
			let mut header = Header {
				number: 0,
				parent_hash: Default::default(),
				state_root: Default::default(),
				digest: Default::default(),
				extrinsics_root: Default::default(),
			};

			let storage = vec![
				(vec![1, 3, 5], vec![2, 4, 6]),
				(vec![1, 2, 3], vec![9, 9, 9]),
			];

			header.state_root = op.old_state.storage_root(storage
				.iter()
				.cloned()
				.map(|(x, y)| (x, Some(y)))
			).0.into();
			let hash = header.hash();

			op.reset_storage(Storage {
				top: storage.iter().cloned().collect(),
				children: Default::default(),
			}).unwrap();
			op.set_block_data(
				header.clone(),
				Some(vec![]),
				None,
				NewBlockState::Best,
			).unwrap();

			db.commit_operation(op).unwrap();

			let state = db.state_at(BlockId::Number(0)).unwrap();

			assert_eq!(state.storage(&[1, 3, 5]).unwrap(), Some(vec![2, 4, 6]));
			assert_eq!(state.storage(&[1, 2, 3]).unwrap(), Some(vec![9, 9, 9]));
			assert_eq!(state.storage(&[5, 5, 5]).unwrap(), None);

			hash
		};

		{
			let mut op = db.begin_operation().unwrap();
			db.begin_state_operation(&mut op, BlockId::Number(0)).unwrap();
			let mut header = Header {
				number: 1,
				parent_hash: hash,
				state_root: Default::default(),
				digest: Default::default(),
				extrinsics_root: Default::default(),
			};

			let storage = vec![
				(vec![1, 3, 5], None),
				(vec![5, 5, 5], Some(vec![4, 5, 6])),
			];

			let (root, overlay) = op.old_state.storage_root(storage.iter().cloned());
			op.update_db_storage(overlay).unwrap();
			header.state_root = root.into();

			op.set_block_data(
				header,
				Some(vec![]),
				None,
				NewBlockState::Best,
			).unwrap();

			db.commit_operation(op).unwrap();

			let state = db.state_at(BlockId::Number(1)).unwrap();

			assert_eq!(state.storage(&[1, 3, 5]).unwrap(), None);
			assert_eq!(state.storage(&[1, 2, 3]).unwrap(), Some(vec![9, 9, 9]));
			assert_eq!(state.storage(&[5, 5, 5]).unwrap(), Some(vec![4, 5, 6]));
		}
	}

	#[test]
	fn delete_only_when_negative_rc() {
		let _ = ::env_logger::try_init();
		let key;
		let backend = Backend::<Block>::new_test(1, 0);

		let hash = {
			let mut op = backend.begin_operation().unwrap();
			backend.begin_state_operation(&mut op, BlockId::Hash(Default::default())).unwrap();
			let mut header = Header {
				number: 0,
				parent_hash: Default::default(),
				state_root: Default::default(),
				digest: Default::default(),
				extrinsics_root: Default::default(),
			};

			let storage: Vec<(_, _)> = vec![];

			header.state_root = op.old_state.storage_root(storage
				.iter()
				.cloned()
				.map(|(x, y)| (x, Some(y)))
			).0.into();
			let hash = header.hash();

			op.reset_storage(Storage {
				top: storage.iter().cloned().collect(),
				children: Default::default(),
			}).unwrap();

			key = op.db_updates.insert(EMPTY_PREFIX, b"hello");
			op.set_block_data(
				header,
				Some(vec![]),
				None,
				NewBlockState::Best,
			).unwrap();

			backend.commit_operation(op).unwrap();
			assert_eq!(backend.storage.db.get(
				columns::STATE,
				&sp_trie::prefixed_key::<Blake2Hasher>(&key, EMPTY_PREFIX)
			).unwrap().unwrap(), &b"hello"[..]);
			hash
		};

		let hash = {
			let mut op = backend.begin_operation().unwrap();
			backend.begin_state_operation(&mut op, BlockId::Number(0)).unwrap();
			let mut header = Header {
				number: 1,
				parent_hash: hash,
				state_root: Default::default(),
				digest: Default::default(),
				extrinsics_root: Default::default(),
			};

			let storage: Vec<(_, _)> = vec![];

			header.state_root = op.old_state.storage_root(storage
				.iter()
				.cloned()
				.map(|(x, y)| (x, Some(y)))
			).0.into();
			let hash = header.hash();

			op.db_updates.insert(EMPTY_PREFIX, b"hello");
			op.db_updates.remove(&key, EMPTY_PREFIX);
			op.set_block_data(
				header,
				Some(vec![]),
				None,
				NewBlockState::Best,
			).unwrap();

			backend.commit_operation(op).unwrap();
			assert_eq!(backend.storage.db.get(
				columns::STATE,
				&sp_trie::prefixed_key::<Blake2Hasher>(&key, EMPTY_PREFIX)
			).unwrap().unwrap(), &b"hello"[..]);
			hash
		};

		let hash = {
			let mut op = backend.begin_operation().unwrap();
			backend.begin_state_operation(&mut op, BlockId::Number(1)).unwrap();
			let mut header = Header {
				number: 2,
				parent_hash: hash,
				state_root: Default::default(),
				digest: Default::default(),
				extrinsics_root: Default::default(),
			};

			let storage: Vec<(_, _)> = vec![];

			header.state_root = op.old_state.storage_root(storage
				.iter()
				.cloned()
				.map(|(x, y)| (x, Some(y)))
			).0.into();
			let hash = header.hash();

			op.db_updates.remove(&key, EMPTY_PREFIX);
			op.set_block_data(
				header,
				Some(vec![]),
				None,
				NewBlockState::Best,
			).unwrap();

			backend.commit_operation(op).unwrap();


			assert!(backend.storage.db.get(
				columns::STATE,
				&sp_trie::prefixed_key::<Blake2Hasher>(&key, EMPTY_PREFIX)
			).unwrap().is_some());
			hash
		};

		{
			let mut op = backend.begin_operation().unwrap();
			backend.begin_state_operation(&mut op, BlockId::Number(2)).unwrap();
			let mut header = Header {
				number: 3,
				parent_hash: hash,
				state_root: Default::default(),
				digest: Default::default(),
				extrinsics_root: Default::default(),
			};

			let storage: Vec<(_, _)> = vec![];

			header.state_root = op.old_state.storage_root(storage
				.iter()
				.cloned()
				.map(|(x, y)| (x, Some(y)))
			).0.into();

			op.set_block_data(
				header,
				Some(vec![]),
				None,
				NewBlockState::Best,
			).unwrap();

			backend.commit_operation(op).unwrap();
			assert!(backend.storage.db.get(
				columns::STATE,
				&sp_trie::prefixed_key::<Blake2Hasher>(&key, EMPTY_PREFIX)
			).unwrap().is_none());
		}

		backend.finalize_block(BlockId::Number(1), None).unwrap();
		backend.finalize_block(BlockId::Number(2), None).unwrap();
		backend.finalize_block(BlockId::Number(3), None).unwrap();
		assert!(backend.storage.db.get(
			columns::STATE,
			&sp_trie::prefixed_key::<Blake2Hasher>(&key, EMPTY_PREFIX)
		).unwrap().is_none());
	}

	#[test]
	fn changes_trie_storage_works() {
		let backend = Backend::<Block>::new_test(1000, 100);
		backend.changes_tries_storage.meta.write().finalized_number = 1000;


		let check_changes = |backend: &Backend<Block>, block: u64, changes: Vec<(Vec<u8>, Vec<u8>)>| {
			let (changes_root, mut changes_trie_update) = prepare_changes(changes);
			let anchor = sp_state_machine::ChangesTrieAnchorBlockId {
				hash: backend.blockchain().header(BlockId::Number(block)).unwrap().unwrap().hash(),
				number: block
			};
			assert_eq!(backend.changes_tries_storage.root(&anchor, block), Ok(Some(changes_root)));

			for (key, (val, _)) in changes_trie_update.drain() {
				assert_eq!(backend.changes_trie_storage().unwrap().get(&key, EMPTY_PREFIX), Ok(Some(val)));
			}
		};

		let changes0 = vec![(b"key_at_0".to_vec(), b"val_at_0".to_vec())];
		let changes1 = vec![
			(b"key_at_1".to_vec(), b"val_at_1".to_vec()),
			(b"another_key_at_1".to_vec(), b"another_val_at_1".to_vec()),
		];
		let changes2 = vec![(b"key_at_2".to_vec(), b"val_at_2".to_vec())];

		let block0 = insert_header(&backend, 0, Default::default(), changes0.clone(), Default::default());
		let block1 = insert_header(&backend, 1, block0, changes1.clone(), Default::default());
		let _ = insert_header(&backend, 2, block1, changes2.clone(), Default::default());