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    // Copyright 2015-2017 Parity Technologies (UK) Ltd.
    // This file is part of Parity.

    // Parity is free software: you can redistribute it and/or modify
    // it under the terms of the GNU General Public License as published by
    // the Free Software Foundation, either version 3 of the License, or
    // (at your option) any later version.

    // Parity is distributed in the hope that it will be useful,
    // but WITHOUT ANY WARRANTY; without even the implied warranty of
    // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    // GNU General Public License for more details.

    // You should have received a copy of the GNU General Public License
    // along with Parity.  If not, see <http://www.gnu.org/licenses/>.

    //! Snapshot creation, restoration, and network service.
    //!
    //! Documentation of the format can be found at
    //! https://github.com/ethcore/parity/wiki/%22PV64%22-Snapshot-Format

    use std::collections::{HashMap, HashSet, VecDeque};
    use std::sync::Arc;
    use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};

    use account_db::{AccountDB, AccountDBMut};
    use blockchain::{BlockChain, BlockProvider};
    use engines::Engine;
    use header::Header;
    use ids::BlockId;
    use views::BlockView;

    use util::{Bytes, Hashable, HashDB, DBValue, snappy, U256, Uint};
    use util::Mutex;
    use util::hash::{FixedHash, H256};
    use util::journaldb::{self, Algorithm, JournalDB};
    use util::kvdb::Database;
    use util::trie::{TrieDB, TrieDBMut, Trie, TrieMut};
    use util::sha3::SHA3_NULL_RLP;
    use rlp::{RlpStream, Stream, UntrustedRlp, View};
    use bloom_journal::Bloom;

    use self::block::AbridgedBlock;
    use self::io::SnapshotWriter;

    use super::state_db::StateDB;
    use super::state::Account as StateAccount;

    use crossbeam::scope;
    use rand::{Rng, OsRng};

    pub use self::error::Error;

    pub use self::service::{Service, DatabaseRestore};
    pub use self::traits::SnapshotService;
    pub use self::watcher::Watcher;
    pub use types::snapshot_manifest::ManifestData;
    pub use types::restoration_status::RestorationStatus;

    pub mod io;
    pub mod service;

    mod account;
    mod block;
    mod error;
    mod watcher;

    #[cfg(test)]
    mod tests;

    /// IPC interfaces
    #[cfg(feature="ipc")]
    pub mod remote {
    	pub use super::traits::RemoteSnapshotService;
    }

    mod traits {
    	#![allow(dead_code, unused_assignments, unused_variables, missing_docs)] // codegen issues
    	include!(concat!(env!("OUT_DIR"), "/snapshot_service_trait.rs"));
    }

    // Try to have chunks be around 4MB (before compression)
    const PREFERRED_CHUNK_SIZE: usize = 4 * 1024 * 1024;

    // How many blocks to include in a snapshot, starting from the head of the chain.
    const SNAPSHOT_BLOCKS: u64 = 30000;

    /// A progress indicator for snapshots.
    #[derive(Debug, Default)]
    pub struct Progress {
    	accounts: AtomicUsize,
    	blocks: AtomicUsize,
    	size: AtomicUsize, // Todo [rob] use Atomicu64 when it stabilizes.
    	done: AtomicBool,
    }

    impl Progress {
    	/// Reset the progress.
    	pub fn reset(&self) {
    		self.accounts.store(0, Ordering::Release);
    		self.blocks.store(0, Ordering::Release);
    		self.size.store(0, Ordering::Release);

    		// atomic fence here to ensure the others are written first?
    		// logs might very rarely get polluted if not.
    		self.done.store(false, Ordering::Release);
    	}

    	/// Get the number of accounts snapshotted thus far.
    	pub fn accounts(&self) -> usize { self.accounts.load(Ordering::Acquire) }

    	/// Get the number of blocks snapshotted thus far.
    	pub fn blocks(&self) -> usize { self.blocks.load(Ordering::Acquire) }

    	/// Get the written size of the snapshot in bytes.
    	pub fn size(&self) -> usize { self.size.load(Ordering::Acquire) }

    	/// Whether the snapshot is complete.
    	pub fn done(&self) -> bool  { self.done.load(Ordering::Acquire) }

    }
    /// Take a snapshot using the given blockchain, starting block hash, and database, writing into the given writer.
    pub fn take_snapshot<W: SnapshotWriter + Send>(
    	chain: &BlockChain,
    	block_at: H256,
    	state_db: &HashDB,
    	writer: W,
    	p: &Progress
    ) -> Result<(), Error> {
    	let start_header = chain.block_header(&block_at)
    		.ok_or(Error::InvalidStartingBlock(BlockId::Hash(block_at)))?;
    	let state_root = start_header.state_root();
    	let number = start_header.number();

    	info!("Taking snapshot starting at block {}", number);

    	let writer = Mutex::new(writer);
    	let (state_hashes, block_hashes) = scope(|scope| {
    		let block_guard = scope.spawn(|| chunk_blocks(chain, block_at, &writer, p));
    		let state_res = chunk_state(state_db, state_root, &writer, p);

    		state_res.and_then(|state_hashes| {
    			block_guard.join().map(|block_hashes| (state_hashes, block_hashes))
    		})
    	})?;

    	info!("produced {} state chunks and {} block chunks.", state_hashes.len(), block_hashes.len());

    	let manifest_data = ManifestData {
    		state_hashes: state_hashes,
    		block_hashes: block_hashes,
    		state_root: *state_root,
    		block_number: number,
    		block_hash: block_at,
    	};

    	writer.into_inner().finish(manifest_data)?;

    	p.done.store(true, Ordering::SeqCst);

    	Ok(())
    }

    /// Used to build block chunks.
    struct BlockChunker<'a> {
    	chain: &'a BlockChain,
    	// block, receipt rlp pairs.
    	rlps: VecDeque<Bytes>,
    	current_hash: H256,
    	hashes: Vec<H256>,
    	snappy_buffer: Vec<u8>,
    	writer: &'a Mutex<SnapshotWriter + 'a>,
    	progress: &'a Progress,
    }

    impl<'a> BlockChunker<'a> {
    	// Repeatedly fill the buffers and writes out chunks, moving backwards from starting block hash.
    	// Loops until we reach the first desired block, and writes out the remainder.
    	fn chunk_all(&mut self) -> Result<(), Error> {
    		let mut loaded_size = 0;
    		let mut last = self.current_hash;

    		let genesis_hash = self.chain.genesis_hash();

    		for _ in 0..SNAPSHOT_BLOCKS {
    			if self.current_hash == genesis_hash { break }

    			let (block, receipts) = self.chain.block(&self.current_hash)
    				.and_then(|b| self.chain.block_receipts(&self.current_hash).map(|r| (b, r)))
    				.ok_or(Error::BlockNotFound(self.current_hash))?;

    			let abridged_rlp = AbridgedBlock::from_block_view(&block.view()).into_inner();

    			let pair = {
    				let mut pair_stream = RlpStream::new_list(2);
    				pair_stream.append_raw(&abridged_rlp, 1).append(&receipts);
    				pair_stream.out()
    			};

    			let new_loaded_size = loaded_size + pair.len();

    			// cut off the chunk if too large.

    			if new_loaded_size > PREFERRED_CHUNK_SIZE && !self.rlps.is_empty() {
    				self.write_chunk(last)?;
    				loaded_size = pair.len();
    			} else {
    				loaded_size = new_loaded_size;
    			}

    			self.rlps.push_front(pair);

    			last = self.current_hash;
    			self.current_hash = block.header_view().parent_hash();
    		}

    		if loaded_size != 0 {
    			self.write_chunk(last)?;
    		}

    		Ok(())
    	}

    	// write out the data in the buffers to a chunk on disk
    	//
    	// we preface each chunk with the parent of the first block's details,
    	// obtained from the details of the last block written.
    	fn write_chunk(&mut self, last: H256) -> Result<(), Error> {
    		trace!(target: "snapshot", "prepared block chunk with {} blocks", self.rlps.len());

    		let (last_header, last_details) = self.chain.block_header(&last)
    			.and_then(|n| self.chain.block_details(&last).map(|d| (n, d)))
    			.ok_or(Error::BlockNotFound(last))?;

    		let parent_number = last_header.number() - 1;
    		let parent_hash = last_header.parent_hash();
    		let parent_total_difficulty = last_details.total_difficulty - *last_header.difficulty();

    		trace!(target: "snapshot", "parent last written block: {}", parent_hash);

    		let num_entries = self.rlps.len();
    		let mut rlp_stream = RlpStream::new_list(3 + num_entries);
    		rlp_stream.append(&parent_number).append(parent_hash).append(&parent_total_difficulty);

    		for pair in self.rlps.drain(..) {
    			rlp_stream.append_raw(&pair, 1);
    		}

    		let raw_data = rlp_stream.out();

    		let size = snappy::compress_into(&raw_data, &mut self.snappy_buffer);
    		let compressed = &self.snappy_buffer[..size];
    		let hash = compressed.sha3();

    		self.writer.lock().write_block_chunk(hash, compressed)?;
    		trace!(target: "snapshot", "wrote block chunk. hash: {}, size: {}, uncompressed size: {}", hash.hex(), size, raw_data.len());

    		self.progress.size.fetch_add(size, Ordering::SeqCst);
    		self.progress.blocks.fetch_add(num_entries, Ordering::SeqCst);

    		self.hashes.push(hash);
    		Ok(())
    	}
    }

    /// Create and write out all block chunks to disk, returning a vector of all
    /// the hashes of block chunks created.
    ///
    /// The path parameter is the directory to store the block chunks in.
    /// This function assumes the directory exists already.
    /// Returns a list of chunk hashes, with the first having the blocks furthest from the genesis.
    pub fn chunk_blocks<'a>(chain: &'a BlockChain, start_hash: H256, writer: &Mutex<SnapshotWriter + 'a>, progress: &'a Progress) -> Result<Vec<H256>, Error> {
    	let mut chunker = BlockChunker {
    		chain: chain,
    		rlps: VecDeque::new(),
    		current_hash: start_hash,
    		hashes: Vec::new(),
    		snappy_buffer: vec![0; snappy::max_compressed_len(PREFERRED_CHUNK_SIZE)],
    		writer: writer,
    		progress: progress,
    	};

    	chunker.chunk_all()?;

    	Ok(chunker.hashes)
    }

    /// State trie chunker.
    struct StateChunker<'a> {
    	hashes: Vec<H256>,
    	rlps: Vec<Bytes>,
    	cur_size: usize,
    	snappy_buffer: Vec<u8>,
    	writer: &'a Mutex<SnapshotWriter + 'a>,
    	progress: &'a Progress,
    }

    impl<'a> StateChunker<'a> {
    	// Push a key, value pair to be encoded.
    	//
    	// If the buffer is greater than the desired chunk size,
    	// this will write out the data to disk.
    	fn push(&mut self, account_hash: Bytes, data: Bytes) -> Result<(), Error> {
    		let pair = {
    			let mut stream = RlpStream::new_list(2);
    			stream.append(&account_hash).append_raw(&data, 1);
    			stream.out()
    		};

    		if self.cur_size + pair.len() >= PREFERRED_CHUNK_SIZE {
    			self.write_chunk()?;
    		}

    		self.cur_size += pair.len();
    		self.rlps.push(pair);

    		Ok(())
    	}

    	// Write out the buffer to disk, pushing the created chunk's hash to
    	// the list.
    	fn write_chunk(&mut self) -> Result<(), Error> {
    		let num_entries = self.rlps.len();
    		let mut stream = RlpStream::new_list(num_entries);
    		for rlp in self.rlps.drain(..) {
    			stream.append_raw(&rlp, 1);
    		}

    		let raw_data = stream.out();

    		let compressed_size = snappy::compress_into(&raw_data, &mut self.snappy_buffer);
    		let compressed = &self.snappy_buffer[..compressed_size];
    		let hash = compressed.sha3();

    		self.writer.lock().write_state_chunk(hash, compressed)?;
    		trace!(target: "snapshot", "wrote state chunk. size: {}, uncompressed size: {}", compressed_size, raw_data.len());

    		self.progress.accounts.fetch_add(num_entries, Ordering::SeqCst);
    		self.progress.size.fetch_add(compressed_size, Ordering::SeqCst);

    		self.hashes.push(hash);
    		self.cur_size = 0;

    		Ok(())
    	}
    }

    /// Walk the given state database starting from the given root,
    /// creating chunks and writing them out.
    ///
    /// Returns a list of hashes of chunks created, or any error it may
    /// have encountered.
    pub fn chunk_state<'a>(db: &HashDB, root: &H256, writer: &Mutex<SnapshotWriter + 'a>, progress: &'a Progress) -> Result<Vec<H256>, Error> {
    	let account_trie = TrieDB::new(db, &root)?;

    	let mut chunker = StateChunker {
    		hashes: Vec::new(),
    		rlps: Vec::new(),
    		cur_size: 0,
    		snappy_buffer: vec![0; snappy::max_compressed_len(PREFERRED_CHUNK_SIZE)],
    		writer: writer,
    		progress: progress,
    	};

    	let mut used_code = HashSet::new();

    	// account_key here is the address' hash.
    	for item in account_trie.iter()? {
    		let (account_key, account_data) = item?;
    		let account = ::rlp::decode(&*account_data);
    		let account_key_hash = H256::from_slice(&account_key);

    		let account_db = AccountDB::from_hash(db, account_key_hash);

    		let fat_rlp = account::to_fat_rlp(&account, &account_db, &mut used_code)?;
    		chunker.push(account_key, fat_rlp)?;
    	}

    	if chunker.cur_size != 0 {
    		chunker.write_chunk()?;
    	}

    	Ok(chunker.hashes)
    }

    /// Used to rebuild the state trie piece by piece.
    pub struct StateRebuilder {
    	db: Box<JournalDB>,
    	state_root: H256,
    	known_code: HashMap<H256, H256>, // code hashes mapped to first account with this code.
    	missing_code: HashMap<H256, Vec<H256>>, // maps code hashes to lists of accounts missing that code.
    	bloom: Bloom,
    }

    impl StateRebuilder {
    	/// Create a new state rebuilder to write into the given backing DB.
    	pub fn new(db: Arc<Database>, pruning: Algorithm) -> Self {
    		StateRebuilder {
    			db: journaldb::new(db.clone(), pruning, ::db::COL_STATE),
    			state_root: SHA3_NULL_RLP,
    			known_code: HashMap::new(),
    			missing_code: HashMap::new(),
    			bloom: StateDB::load_bloom(&*db),
    		}
    	}

    	/// Feed an uncompressed state chunk into the rebuilder.
    	pub fn feed(&mut self, chunk: &[u8], flag: &AtomicBool) -> Result<(), ::error::Error> {
    		let rlp = UntrustedRlp::new(chunk);
    		let empty_rlp = StateAccount::new_basic(U256::zero(), U256::zero()).rlp();
    		let mut pairs = Vec::with_capacity(rlp.item_count());

    		// initialize the pairs vector with empty values so we have slots to write into.
    		pairs.resize(rlp.item_count(), (H256::new(), Vec::new()));

    		let status = rebuild_accounts(
    			self.db.as_hashdb_mut(),
    			rlp,
    			&mut pairs,
    			&self.known_code,
    			flag
    		)?;

    		for (addr_hash, code_hash) in status.missing_code {
    			self.missing_code.entry(code_hash).or_insert_with(Vec::new).push(addr_hash);
    		}

    		// patch up all missing code. must be done after collecting all new missing code entries.
    		for (code_hash, code, first_with) in status.new_code {
    			for addr_hash in self.missing_code.remove(&code_hash).unwrap_or_else(Vec::new) {
    				let mut db = AccountDBMut::from_hash(self.db.as_hashdb_mut(), addr_hash);
    				db.emplace(code_hash, DBValue::from_slice(&code));
    			}

    			self.known_code.insert(code_hash, first_with);
    		}

    		let backing = self.db.backing().clone();

    		// batch trie writes
    		{
    			let mut account_trie = if self.state_root != SHA3_NULL_RLP {
    				TrieDBMut::from_existing(self.db.as_hashdb_mut(), &mut self.state_root)?
    			} else {
    				TrieDBMut::new(self.db.as_hashdb_mut(), &mut self.state_root)
    			};

    			for (hash, thin_rlp) in pairs {
    				if !flag.load(Ordering::SeqCst) { return Err(Error::RestorationAborted.into()) }

    				if &thin_rlp[..] != &empty_rlp[..] {
    					self.bloom.set(&*hash);
    				}
    				account_trie.insert(&hash, &thin_rlp)?;
    			}
    		}

    		let bloom_journal = self.bloom.drain_journal();
    		let mut batch = backing.transaction();
    		StateDB::commit_bloom(&mut batch, bloom_journal)?;
    		self.db.inject(&mut batch)?;
    		backing.write_buffered(batch);
    		trace!(target: "snapshot", "current state root: {:?}", self.state_root);
    		Ok(())
    	}

    	/// Check for accounts missing code. Once all chunks have been fed, there should
    	/// be none.
    	pub fn check_missing(self) -> Result<(), Error> {
    		let missing = self.missing_code.keys().cloned().collect::<Vec<_>>();
    		match missing.is_empty() {
    			true => Ok(()),
    			false => Err(Error::MissingCode(missing)),
    		}
    	}

    	/// Get the state root of the rebuilder.
    	pub fn state_root(&self) -> H256 { self.state_root }
    }

    #[derive(Default)]
    struct RebuiltStatus {
    	// new code that's become available. (code_hash, code, addr_hash)
    	new_code: Vec<(H256, Bytes, H256)>,
    	missing_code: Vec<(H256, H256)>, // accounts that are missing code.
    }

    // rebuild a set of accounts and their storage.
    // returns a status detailing newly-loaded code and accounts missing code.
    fn rebuild_accounts(
    	db: &mut HashDB,
    	account_fat_rlps: UntrustedRlp,
    	out_chunk: &mut [(H256, Bytes)],
    	known_code: &HashMap<H256, H256>,
    	abort_flag: &AtomicBool,
    ) -> Result<RebuiltStatus, ::error::Error> {
    	let mut status = RebuiltStatus::default();
    	for (account_rlp, out) in account_fat_rlps.into_iter().zip(out_chunk) {
    		if !abort_flag.load(Ordering::SeqCst) { return Err(Error::RestorationAborted.into()) }

    		let hash: H256 = account_rlp.val_at(0)?;
    		let fat_rlp = account_rlp.at(1)?;

    		let thin_rlp = {

    			// fill out the storage trie and code while decoding.
    			let (acc, maybe_code) = {
    				let mut acct_db = AccountDBMut::from_hash(db, hash);
    				account::from_fat_rlp(&mut acct_db, fat_rlp)?
    			};

    			let code_hash = acc.code_hash.clone();
    			match maybe_code {
    				// new inline code
    				Some(code) => status.new_code.push((code_hash, code, hash)),
    				None => {
    					if code_hash != ::util::SHA3_EMPTY {
    						// see if this code has already been included inline
    						match known_code.get(&code_hash) {
    							Some(&first_with) => {
    								// if so, load it from the database.
    								let code = AccountDB::from_hash(db, first_with)
    									.get(&code_hash)
    									.ok_or_else(|| Error::MissingCode(vec![first_with]))?;

    								// and write it again under a different mangled key
    								AccountDBMut::from_hash(db, hash).emplace(code_hash, code);
    							}
    							// if not, queue it up to be filled later
    							None => status.missing_code.push((hash, code_hash)),
    						}
    					}
    				}
    			}

    			::rlp::encode(&acc).to_vec()
    		};

    		*out = (hash, thin_rlp);
    	}
    	Ok(status)
    }

    /// Proportion of blocks which we will verify `PoW` for.
    const POW_VERIFY_RATE: f32 = 0.02;

    /// Verify an old block with the given header, engine, blockchain, body. If `always` is set, it will perform
    /// the fullest verification possible. If not, it will take a random sample to determine whether it will
    /// do heavy or light verification.
    pub fn verify_old_block(rng: &mut OsRng, header: &Header, engine: &Engine, chain: &BlockChain, body: Option<&[u8]>, always: bool) -> Result<(), ::error::Error> {
    	if always || rng.gen::<f32>() <= POW_VERIFY_RATE {
    		match chain.block_header(header.parent_hash()) {
    			Some(parent) => engine.verify_block_family(header, &parent, body),
    			None => engine.verify_block_seal(header),
    		}
    	} else {
    		engine.verify_block_basic(header, body)
    	}
    }

    /// Rebuilds the blockchain from chunks.
    ///
    /// Does basic verification for all blocks, but `PoW` verification for some.
    /// Blocks must be fed in-order.
    ///
    /// The first block in every chunk is disconnected from the last block in the
    /// chunk before it, as chunks may be submitted out-of-order.
    ///
    /// After all chunks have been submitted, we "glue" the chunks together.
    pub struct BlockRebuilder {
    	chain: BlockChain,
    	db: Arc<Database>,
    	rng: OsRng,
    	disconnected: Vec<(u64, H256)>,
    	best_number: u64,
    	best_hash: H256,
    	best_root: H256,
    	fed_blocks: u64,
    }

    impl BlockRebuilder {
    	/// Create a new BlockRebuilder.
    	pub fn new(chain: BlockChain, db: Arc<Database>, manifest: &ManifestData) -> Result<Self, ::error::Error> {
    		Ok(BlockRebuilder {
    			chain: chain,
    			db: db,
    			rng: OsRng::new()?,
    			disconnected: Vec::new(),
    			best_number: manifest.block_number,
    			best_hash: manifest.block_hash,
    			best_root: manifest.state_root,
    			fed_blocks: 0,
    		})
    	}

    	/// Feed the rebuilder an uncompressed block chunk.
    	/// Returns the number of blocks fed or any errors.
    	pub fn feed(&mut self, chunk: &[u8], engine: &Engine, abort_flag: &AtomicBool) -> Result<u64, ::error::Error> {
    		use basic_types::Seal::With;
    		use util::U256;
    		use util::triehash::ordered_trie_root;

    		let rlp = UntrustedRlp::new(chunk);
    		let item_count = rlp.item_count();
    		let num_blocks = (item_count - 3) as u64;

    		trace!(target: "snapshot", "restoring block chunk with {} blocks.", item_count - 3);

    		if self.fed_blocks + num_blocks > SNAPSHOT_BLOCKS {
    			return Err(Error::TooManyBlocks(SNAPSHOT_BLOCKS, self.fed_blocks).into())
    		}

    		// todo: assert here that these values are consistent with chunks being in order.
    		let mut cur_number = rlp.val_at::<u64>(0)? + 1;
    		let mut parent_hash = rlp.val_at::<H256>(1)?;
    		let parent_total_difficulty = rlp.val_at::<U256>(2)?;

    		for idx in 3..item_count {
    			if !abort_flag.load(Ordering::SeqCst) { return Err(Error::RestorationAborted.into()) }

    			let pair = rlp.at(idx)?;
    			let abridged_rlp = pair.at(0)?.as_raw().to_owned();
    			let abridged_block = AbridgedBlock::from_raw(abridged_rlp);
    			let receipts: Vec<::receipt::Receipt> = pair.val_at(1)?;
    			let receipts_root = ordered_trie_root(
    				pair.at(1)?.iter().map(|r| r.as_raw().to_owned())
    			);

    			let block = abridged_block.to_block(parent_hash, cur_number, receipts_root)?;
    			let block_bytes = block.rlp_bytes(With);
    			let is_best = cur_number == self.best_number;

    			if is_best {
    				if block.header.hash() != self.best_hash {
    					return Err(Error::WrongBlockHash(cur_number, self.best_hash, block.header.hash()).into())
    				}

    				if block.header.state_root() != &self.best_root {
    					return Err(Error::WrongStateRoot(self.best_root, *block.header.state_root()).into())
    				}
    			}

    			verify_old_block(
    				&mut self.rng,
    				&block.header,
    				engine,
    				&self.chain,
    				Some(&block_bytes),
    				is_best
    			)?;

    			let mut batch = self.db.transaction();

    			// special-case the first block in each chunk.
    			if idx == 3 {
    				if self.chain.insert_unordered_block(&mut batch, &block_bytes, receipts, Some(parent_total_difficulty), is_best, false) {
    					self.disconnected.push((cur_number, block.header.hash()));
    				}
    			} else {
    				self.chain.insert_unordered_block(&mut batch, &block_bytes, receipts, None, is_best, false);
    			}
    			self.db.write_buffered(batch);
    			self.chain.commit();

    			parent_hash = BlockView::new(&block_bytes).hash();
    			cur_number += 1;
    		}

    		self.fed_blocks += num_blocks;

    		Ok(num_blocks)
    	}

    	/// Glue together any disconnected chunks and check that the chain is complete.
    	pub fn finalize(self, canonical: HashMap<u64, H256>) -> Result<(), Error> {
    		let mut batch = self.db.transaction();

    		for (first_num, first_hash) in self.disconnected {
    			let parent_num = first_num - 1;

    			// check if the parent is even in the chain.
    			// since we don't restore every single block in the chain,
    			// the first block of the first chunks has nothing to connect to.
    			if let Some(parent_hash) = self.chain.block_hash(parent_num) {
    				// if so, add the child to it.
    				self.chain.add_child(&mut batch, parent_hash, first_hash);
    			}
    		}
    		self.db.write_buffered(batch);

    		let best_number = self.best_number;
    		for num in (0..self.fed_blocks).map(|x| best_number - x) {

    			let hash = self.chain.block_hash(num).ok_or(Error::IncompleteChain)?;

    			if let Some(canon_hash) = canonical.get(&num).cloned() {
    				if canon_hash != hash {
    					return Err(Error::WrongBlockHash(num, canon_hash, hash));
    				}
    			}
    		}

    		Ok(())
    	}
    }

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