Implement hardcoded sync in the light client (#8075)

* Implement hardcoded sync

* Fix concerns

* Remove artifact

* Fix cli tests

* Fix compilation

* Update hardcoded sync block

* Don't use any data fetch for the light service

ethcore/light/src/client/header_chain.rs

@@ -31,11 +31,11 @@ use std::sync::Arc;
 use cht;
 
 use ethcore::block_status::BlockStatus;
-use ethcore::error::{BlockImportError, BlockError};
+use ethcore::error::{Error, BlockImportError, BlockError};
 use ethcore::encoded;
 use ethcore::header::Header;
 use ethcore::ids::BlockId;
-use ethcore::spec::Spec;
+use ethcore::spec::{Spec, SpecHardcodedSync};
 use ethcore::engines::epoch::{
 	Transition as EpochTransition,
 	PendingTransition as PendingEpochTransition
@@ -45,7 +45,7 @@ use rlp::{Encodable, Decodable, DecoderError, RlpStream, Rlp, UntrustedRlp};
 use heapsize::HeapSizeOf;
 use ethereum_types::{H256, H264, U256};
 use plain_hasher::H256FastMap;
-use kvdb::{self, DBTransaction, KeyValueDB};
+use kvdb::{DBTransaction, KeyValueDB};
 
 use cache::Cache;
 use parking_lot::{Mutex, RwLock};
@@ -180,6 +180,12 @@ pub struct PendingChanges {
 	best_block: Option<BlockDescriptor>, // new best block.
 }
 
+/// Whether or not the hardcoded sync feature is allowed.
+pub enum HardcodedSync {
+	Allow,
+	Deny,
+}
+
 /// Header chain. See module docs for more details.
 pub struct HeaderChain {
 	genesis_header: encoded::Header, // special-case the genesis.
@@ -198,7 +204,8 @@ impl HeaderChain {
 		col: Option<u32>,
 		spec: &Spec,
 		cache: Arc<Mutex<Cache>>,
-	) -> Result<Self, kvdb::Error> {
+		allow_hs: HardcodedSync,
+	) -> Result<Self, Error> {
 		let mut live_epoch_proofs = ::std::collections::HashMap::default();
 
 		let genesis = ::rlp::encode(&spec.genesis_header()).into_vec();
@@ -240,7 +247,7 @@ impl HeaderChain {
 			let best_block = {
 				let era = match candidates.get(&best_number) {
 					Some(era) => era,
-					None => return Err("Database corrupt: highest block referenced but no data.".into()),
+					None => return Err(Error::Database("Database corrupt: highest block referenced but no data.".into())),
 				};
 
 				let best = &era.candidates[0];
@@ -260,8 +267,9 @@ impl HeaderChain {
 				col: col,
 				cache: cache,
 			}
+
 		} else {
-			HeaderChain {
+			let chain = HeaderChain {
 				genesis_header: encoded::Header::new(genesis),
 				best_block: RwLock::new(BlockDescriptor {
 					hash: decoded_header.hash(),
@@ -270,15 +278,49 @@ impl HeaderChain {
 				}),
 				candidates: RwLock::new(BTreeMap::new()),
 				live_epoch_proofs: RwLock::new(live_epoch_proofs),
-				db: db,
+				db: db.clone(),
 				col: col,
 				cache: cache,
+			};
+
+			// insert the hardcoded sync into the database.
+			if let (&Some(ref hardcoded_sync), HardcodedSync::Allow) = (&spec.hardcoded_sync, allow_hs) {
+				let mut batch = db.transaction();
+
+				// insert the hardcoded CHT roots into the database.
+				for (cht_num, cht_root) in hardcoded_sync.chts.iter().enumerate() {
+					batch.put(col, cht_key(cht_num as u64).as_bytes(), &::rlp::encode(cht_root));
 				}
+
+				let decoded_header = hardcoded_sync.header.decode();
+				let decoded_header_num = decoded_header.number();
+
+				// write the block in the DB.
+				info!(target: "chain", "Inserting hardcoded block #{} in chain",
+					  decoded_header_num);
+				let pending = chain.insert_with_td(&mut batch, decoded_header,
+												hardcoded_sync.total_difficulty, None)?;
+
+				// check that we have enough hardcoded CHT roots. avoids panicking later.
+				let cht_num = cht::block_to_cht_number(decoded_header_num - 1)
+					.expect("specs provided a hardcoded block with height 0");
+				if cht_num >= hardcoded_sync.chts.len() as u64 {
+					warn!(target: "chain", "specs didn't provide enough CHT roots for its \
+											hardcoded block ; falling back to non-hardcoded sync \
+											mode");
+				} else {
+					db.write_buffered(batch);
+					chain.apply_pending(pending);
+				}
+			}
+
+			chain
 		};
 
 		// instantiate genesis epoch data if it doesn't exist.
 		if let None = chain.db.get(col, LAST_CANONICAL_TRANSITION)? {
-			let genesis_data = spec.genesis_epoch_data()?;
+			let genesis_data = spec.genesis_epoch_data()
+				.map_err(|s| Error::Database(s.into()))?;
 
 			{
 				let mut batch = chain.db.transaction();
@@ -304,6 +346,29 @@ impl HeaderChain {
 		transaction: &mut DBTransaction,
 		header: Header,
 		transition_proof: Option<Vec<u8>>,
+	) -> Result<PendingChanges, BlockImportError> {
+		self.insert_inner(transaction, header, None, transition_proof)
+	}
+
+	/// Insert a pre-verified header, with a known total difficulty. Similary to `insert`.
+	///
+	/// This blindly trusts that the data given to it is sensible.
+	pub fn insert_with_td(
+		&self,
+		transaction: &mut DBTransaction,
+		header: Header,
+		total_difficulty: U256,
+		transition_proof: Option<Vec<u8>>,
+	) -> Result<PendingChanges, BlockImportError> {
+		self.insert_inner(transaction, header, Some(total_difficulty), transition_proof)
+	}
+
+	fn insert_inner(
+		&self,
+		transaction: &mut DBTransaction,
+		header: Header,
+		total_difficulty: Option<U256>,
+		transition_proof: Option<Vec<u8>>,
 	) -> Result<PendingChanges, BlockImportError> {
 		let hash = header.hash();
 		let number = header.number();
@@ -321,7 +386,10 @@ impl HeaderChain {
 		// hold candidates the whole time to guard import order.
 		let mut candidates = self.candidates.write();
 
-		// find parent details.
+		// find total difficulty.
+		let total_difficulty = match total_difficulty {
+			Some(td) => td,
+			None => {
 				let parent_td =
 					if number == 1 {
 						self.genesis_header.difficulty()
@@ -333,7 +401,9 @@ impl HeaderChain {
 							.map_err(BlockImportError::Block)?
 					};
 
-		let total_difficulty = parent_td + *header.difficulty();
+				parent_td + *header.difficulty()
+			},
+		};
 
 		// insert headers and candidates entries and write era to disk.
 		{
@@ -479,6 +549,65 @@ impl HeaderChain {
 		Ok(pending)
 	}
 
+	/// Generates the specifications for hardcoded sync. This is typically only called manually
+	/// from time to time by a Parity developer in order to update the chain specifications.
+	///
+	/// Returns `None` if we are at the genesis block, or if an error happens .
+	pub fn read_hardcoded_sync(&self) -> Result<Option<SpecHardcodedSync>, Error> {
+		let mut chts = Vec::new();
+		let mut cht_num = 0;
+
+		loop {
+			let cht = match self.cht_root(cht_num) {
+				Some(cht) => cht,
+				None if cht_num != 0 => {
+					// end of the iteration
+					let h_num = 1 + cht_num as u64 * cht::SIZE;
+					let header = if let Some(header) = self.block_header(BlockId::Number(h_num)) {
+						header
+					} else {
+						let msg = format!("header of block #{} not found in DB ; database in an \
+											inconsistent state", h_num);
+						return Err(Error::Database(msg.into()));
+					};
+
+					let decoded = header.decode();
+
+					let entry: Entry = {
+						let bytes = self.db.get(self.col, era_key(h_num).as_bytes())?
+							.ok_or_else(|| {
+								let msg = format!("entry for era #{} not found in DB ; database \
+													in an inconsistent state", h_num);
+								Error::Database(msg.into())
+							})?;
+						::rlp::decode(&bytes)
+					};
+
+					let total_difficulty = entry.candidates.iter()
+						.find(|c| c.hash == decoded.hash())
+						.ok_or_else(|| {
+							let msg = "no candidate matching block found in DB ; database in an \
+										inconsistent state";
+							Error::Database(msg.into())
+						})?
+						.total_difficulty;
+
+					break Ok(Some(SpecHardcodedSync {
+						header: header,
+						total_difficulty: total_difficulty,
+						chts: chts,
+					}));
+				},
+				None => {
+					break Ok(None);
+				},
+			};
+
+			chts.push(cht);
+			cht_num += 1;
+		}
+	}
+
 	/// Apply pending changes from a previous `insert` operation.
 	/// Must be done before the next `insert` call.
 	pub fn apply_pending(&self, pending: PendingChanges) {
@@ -721,7 +850,7 @@ impl<'a> Iterator for AncestryIter<'a> {
 
 #[cfg(test)]
 mod tests {
-	use super::HeaderChain;
+	use super::{HeaderChain, HardcodedSync};
 	use std::sync::Arc;
 
 	use ethereum_types::U256;
@@ -747,7 +876,7 @@ mod tests {
 
 		let cache = Arc::new(Mutex::new(Cache::new(Default::default(), Duration::from_secs(6 * 3600))));
 
-		let chain = HeaderChain::new(db.clone(), None, &spec, cache).unwrap();
+		let chain = HeaderChain::new(db.clone(), None, &spec, cache, HardcodedSync::Allow).unwrap();
 
 		let mut parent_hash = genesis_header.hash();
 		let mut rolling_timestamp = genesis_header.timestamp();
@@ -780,7 +909,7 @@ mod tests {
 		let db = make_db();
 		let cache = Arc::new(Mutex::new(Cache::new(Default::default(), Duration::from_secs(6 * 3600))));
 
-		let chain = HeaderChain::new(db.clone(), None, &spec, cache).unwrap();
+		let chain = HeaderChain::new(db.clone(), None, &spec, cache, HardcodedSync::Allow).unwrap();
 
 		let mut parent_hash = genesis_header.hash();
 		let mut rolling_timestamp = genesis_header.timestamp();
@@ -862,7 +991,7 @@ mod tests {
 		let db = make_db();
 		let cache = Arc::new(Mutex::new(Cache::new(Default::default(), Duration::from_secs(6 * 3600))));
 
-		let chain = HeaderChain::new(db.clone(), None, &spec, cache).unwrap();
+		let chain = HeaderChain::new(db.clone(), None, &spec, cache, HardcodedSync::Allow).unwrap();
 
 		assert!(chain.block_header(BlockId::Earliest).is_some());
 		assert!(chain.block_header(BlockId::Latest).is_some());
@@ -876,7 +1005,8 @@ mod tests {
 		let cache = Arc::new(Mutex::new(Cache::new(Default::default(), Duration::from_secs(6 * 3600))));
 
 		{
-			let chain = HeaderChain::new(db.clone(), None, &spec, cache.clone()).unwrap();
+			let chain = HeaderChain::new(db.clone(), None, &spec, cache.clone(),
+										HardcodedSync::Allow).unwrap();
 			let mut parent_hash = genesis_header.hash();
 			let mut rolling_timestamp = genesis_header.timestamp();
 			for i in 1..10000 {
@@ -896,7 +1026,8 @@ mod tests {
 			}
 		}
 
-		let chain = HeaderChain::new(db.clone(), None, &spec, cache.clone()).unwrap();
+		let chain = HeaderChain::new(db.clone(), None, &spec, cache.clone(),
+									HardcodedSync::Allow).unwrap();
 		assert!(chain.block_header(BlockId::Number(10)).is_none());
 		assert!(chain.block_header(BlockId::Number(9000)).is_some());
 		assert!(chain.cht_root(2).is_some());
@@ -912,7 +1043,8 @@ mod tests {
 		let cache = Arc::new(Mutex::new(Cache::new(Default::default(), Duration::from_secs(6 * 3600))));
 
 		{
-			let chain = HeaderChain::new(db.clone(), None, &spec, cache.clone()).unwrap();
+			let chain = HeaderChain::new(db.clone(), None, &spec, cache.clone(),
+										HardcodedSync::Allow).unwrap();
 			let mut parent_hash = genesis_header.hash();
 			let mut rolling_timestamp = genesis_header.timestamp();
 
@@ -954,7 +1086,8 @@ mod tests {
 		}
 
 		// after restoration, non-canonical eras should still be loaded.
-		let chain = HeaderChain::new(db.clone(), None, &spec, cache.clone()).unwrap();
+		let chain = HeaderChain::new(db.clone(), None, &spec, cache.clone(),
+									HardcodedSync::Allow).unwrap();
 		assert_eq!(chain.block_header(BlockId::Latest).unwrap().number(), 10);
 		assert!(chain.candidates.read().get(&100).is_some())
 	}
@@ -966,7 +1099,8 @@ mod tests {
 		let db = make_db();
 		let cache = Arc::new(Mutex::new(Cache::new(Default::default(), Duration::from_secs(6 * 3600))));
 
-		let chain = HeaderChain::new(db.clone(), None, &spec, cache.clone()).unwrap();
+		let chain = HeaderChain::new(db.clone(), None, &spec, cache.clone(),
+									HardcodedSync::Allow).unwrap();
 
 		assert!(chain.block_header(BlockId::Earliest).is_some());
 		assert!(chain.block_header(BlockId::Number(0)).is_some());
@@ -980,7 +1114,7 @@ mod tests {
 		let db = make_db();
 		let cache = Arc::new(Mutex::new(Cache::new(Default::default(), Duration::from_secs(6 * 3600))));
 
-		let chain = HeaderChain::new(db.clone(), None, &spec, cache).unwrap();
+		let chain = HeaderChain::new(db.clone(), None, &spec, cache, HardcodedSync::Allow).unwrap();
 
 		let mut parent_hash = genesis_header.hash();
 		let mut rolling_timestamp = genesis_header.timestamp();
@@ -1038,4 +1172,45 @@ mod tests {
 		assert!(chain.live_epoch_proofs.read().is_empty());
 		assert_eq!(chain.epoch_transition_for(parent_hash).unwrap().1, vec![1, 2, 3, 4]);
 	}
+
+	#[test]
+	fn hardcoded_sync_gen() {
+		let spec = Spec::new_test();
+		let genesis_header = spec.genesis_header();
+		let db = make_db();
+
+		let cache = Arc::new(Mutex::new(Cache::new(Default::default(), Duration::from_secs(6 * 3600))));
+
+		let chain = HeaderChain::new(db.clone(), None, &spec, cache, HardcodedSync::Allow).unwrap();
+
+		let mut parent_hash = genesis_header.hash();
+		let mut rolling_timestamp = genesis_header.timestamp();
+		let mut total_difficulty = *genesis_header.difficulty();
+		let h_num = 3 * ::cht::SIZE + 1;
+		for i in 1..10000 {
+			let mut header = Header::new();
+			header.set_parent_hash(parent_hash);
+			header.set_number(i);
+			header.set_timestamp(rolling_timestamp);
+			let diff = *genesis_header.difficulty() * i as u32;
+			header.set_difficulty(diff);
+			if i <= h_num {
+				total_difficulty = total_difficulty + diff;
+			}
+			parent_hash = header.hash();
+
+			let mut tx = db.transaction();
+			let pending = chain.insert(&mut tx, header, None).unwrap();
+			db.write(tx).unwrap();
+			chain.apply_pending(pending);
+
+			rolling_timestamp += 10;
+		}
+
+		let hardcoded_sync = chain.read_hardcoded_sync().unwrap().unwrap();
+		assert_eq!(hardcoded_sync.chts.len(), 3);
+		assert_eq!(hardcoded_sync.total_difficulty, total_difficulty);
+		let decoded: Header = hardcoded_sync.header.decode();
+		assert_eq!(decoded.number(), h_num);
+	}
 }

ethcore/light/src/client/mod.rs

@@ -22,22 +22,22 @@ use ethcore::block_status::BlockStatus;
 use ethcore::client::{ClientReport, EnvInfo, ClientIoMessage};
 use ethcore::engines::{epoch, EthEngine, EpochChange, EpochTransition, Proof};
 use ethcore::machine::EthereumMachine;
-use ethcore::error::BlockImportError;
+use ethcore::error::{Error, BlockImportError};
 use ethcore::ids::BlockId;
 use ethcore::header::{BlockNumber, Header};
 use ethcore::verification::queue::{self, HeaderQueue};
 use ethcore::blockchain_info::BlockChainInfo;
-use ethcore::spec::Spec;
+use ethcore::spec::{Spec, SpecHardcodedSync};
 use ethcore::encoded;
 use io::IoChannel;
 use parking_lot::{Mutex, RwLock};
 use ethereum_types::{H256, U256};
 use futures::{IntoFuture, Future};
 
-use kvdb::{self, KeyValueDB};
+use kvdb::KeyValueDB;
 
 use self::fetch::ChainDataFetcher;
-use self::header_chain::{AncestryIter, HeaderChain};
+use self::header_chain::{AncestryIter, HeaderChain, HardcodedSync};
 
 use cache::Cache;
 
@@ -59,6 +59,8 @@ pub struct Config {
 	pub verify_full: bool,
 	/// Should it check the seal of blocks?
 	pub check_seal: bool,
+	/// Disable hardcoded sync.
+	pub no_hardcoded_sync: bool,
 }
 
 impl Default for Config {
@@ -68,6 +70,7 @@ impl Default for Config {
 			chain_column: None,
 			verify_full: true,
 			check_seal: true,
+			no_hardcoded_sync: false,
 		}
 	}
 }
@@ -175,11 +178,14 @@ impl<T: ChainDataFetcher> Client<T> {
 		fetcher: T,
 		io_channel: IoChannel<ClientIoMessage>,
 		cache: Arc<Mutex<Cache>>
-	) -> Result<Self, kvdb::Error> {
+	) -> Result<Self, Error> {
 		Ok(Client {
 			queue: HeaderQueue::new(config.queue, spec.engine.clone(), io_channel, config.check_seal),
 			engine: spec.engine.clone(),
-			chain: HeaderChain::new(db.clone(), chain_col, &spec, cache)?,
+			chain: {
+				let hs_cfg = if config.no_hardcoded_sync { HardcodedSync::Deny } else { HardcodedSync::Allow };
+				HeaderChain::new(db.clone(), chain_col, &spec, cache, hs_cfg)?
+			},
 			report: RwLock::new(ClientReport::default()),
 			import_lock: Mutex::new(()),
 			db: db,
@@ -189,6 +195,14 @@ impl<T: ChainDataFetcher> Client<T> {
 		})
 	}
 
+	/// Generates the specifications for hardcoded sync. This is typically only called manually
+	/// from time to time by a Parity developer in order to update the chain specifications.
+	///
+	/// Returns `None` if we are at the genesis block.
+	pub fn read_hardcoded_sync(&self) -> Result<Option<SpecHardcodedSync>, Error> {
+		self.chain.read_hardcoded_sync()
+	}
+
 	/// Adds a new `LightChainNotify` listener.
 	pub fn add_listener(&self, listener: Weak<LightChainNotify>) {
 		self.listeners.write().push(listener);

ethcore/light/src/client/service.rs

@@ -22,9 +22,10 @@ use std::sync::Arc;
 
 use ethcore::client::ClientIoMessage;
 use ethcore::db;
+use ethcore::error::Error as CoreError;
 use ethcore::spec::Spec;
 use io::{IoContext, IoError, IoHandler, IoService};
-use kvdb::{self, KeyValueDB};
+use kvdb::KeyValueDB;
 
 use cache::Cache;
 use parking_lot::Mutex;
@@ -34,16 +35,23 @@ use super::{ChainDataFetcher, Client, Config as ClientConfig};
 /// Errors on service initialization.
 #[derive(Debug)]
 pub enum Error {
-	/// Database error.
+	/// Core error.
-	Database(kvdb::Error),
+	Core(CoreError),
 	/// I/O service error.
 	Io(IoError),
 }
 
+impl From<CoreError> for Error {
+	#[inline]
+	fn from(err: CoreError) -> Error {
+		Error::Core(err)
+	}
+}
+
 impl fmt::Display for Error {
 	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 		match *self {
-			Error::Database(ref msg) => write!(f, "Database error: {}", msg),
+			Error::Core(ref msg) => write!(f, "Core error: {}", msg),
 			Error::Io(ref err) => write!(f, "I/O service error: {}", err),
 		}
 	}
@@ -67,7 +75,7 @@ impl<T: ChainDataFetcher> Service<T> {
 			fetcher,
 			io_service.channel(),
 			cache,
-		).map_err(Error::Database)?);
+		)?);
 
 		io_service.register_handler(Arc::new(ImportBlocks(client.clone()))).map_err(Error::Io)?;
 		spec.engine.register_client(Arc::downgrade(&client) as _);

ethcore/src/spec/mod.rs

@@ -21,4 +21,4 @@ mod seal;
 mod spec;
 
 pub use self::genesis::Genesis;
-pub use self::spec::{Spec, SpecParams, CommonParams, OptimizeFor};
+pub use self::spec::{Spec, SpecHardcodedSync, SpecParams, CommonParams, OptimizeFor};

ethcore/src/spec/spec.rs

@@ -28,10 +28,11 @@ use hash::{KECCAK_NULL_RLP, keccak};
 use memorydb::MemoryDB;
 use parking_lot::RwLock;
 use rlp::{Rlp, RlpStream};
-use rustc_hex::FromHex;
+use rustc_hex::{FromHex, ToHex};
 use vm::{EnvInfo, CallType, ActionValue, ActionParams, ParamsType};
 
 use builtin::Builtin;
+use encoded;
 use engines::{EthEngine, NullEngine, InstantSeal, BasicAuthority, AuthorityRound, Tendermint, DEFAULT_BLOCKHASH_CONTRACT};
 use error::Error;
 use executive::Executive;
@@ -319,6 +320,9 @@ pub struct Spec {
 	/// Each seal field, expressed as RLP, concatenated.
 	pub seal_rlp: Bytes,
 
+	/// Hardcoded synchronization. Allows the light client to immediately jump to a specific block.
+	pub hardcoded_sync: Option<SpecHardcodedSync>,
+
 	/// Contract constructors to be executed on genesis.
 	constructors: Vec<(Address, Bytes)>,
 
@@ -347,6 +351,7 @@ impl Clone for Spec {
 			timestamp: self.timestamp.clone(),
 			extra_data: self.extra_data.clone(),
 			seal_rlp: self.seal_rlp.clone(),
+			hardcoded_sync: self.hardcoded_sync.clone(),
 			constructors: self.constructors.clone(),
 			state_root_memo: RwLock::new(*self.state_root_memo.read()),
 			genesis_state: self.genesis_state.clone(),
@@ -354,6 +359,45 @@ impl Clone for Spec {
 	}
 }
 
+/// Part of `Spec`. Describes the hardcoded synchronization parameters.
+pub struct SpecHardcodedSync {
+	/// Header of the block to jump to for hardcoded sync, and total difficulty.
+	pub header: encoded::Header,
+	/// Total difficulty of the block to jump to.
+	pub total_difficulty: U256,
+	/// List of hardcoded CHTs, in order. If `hardcoded_sync` is set, the CHTs should include the
+	/// header of `hardcoded_sync`.
+	pub chts: Vec<H256>,
+}
+
+impl SpecHardcodedSync {
+	/// Turns this specifications back into JSON. Useful for pretty printing.
+	pub fn to_json(self) -> ethjson::spec::HardcodedSync {
+		self.into()
+	}
+}
+
+#[cfg(test)]
+impl Clone for SpecHardcodedSync {
+	fn clone(&self) -> SpecHardcodedSync {
+		SpecHardcodedSync {
+			header: self.header.clone(),
+			total_difficulty: self.total_difficulty.clone(),
+			chts: self.chts.clone(),
+		}
+	}
+}
+
+impl From<SpecHardcodedSync> for ethjson::spec::HardcodedSync {
+	fn from(sync: SpecHardcodedSync) -> ethjson::spec::HardcodedSync {
+		ethjson::spec::HardcodedSync {
+			header: sync.header.into_inner().to_hex(),
+			total_difficulty: ethjson::uint::Uint(sync.total_difficulty),
+			chts: sync.chts.into_iter().map(Into::into).collect(),
+		}
+	}
+}
+
 fn load_machine_from(s: ethjson::spec::Spec) -> EthereumMachine {
 	let builtins = s.accounts.builtins().into_iter().map(|p| (p.0.into(), From::from(p.1))).collect();
 	let params = CommonParams::from(s.params);
@@ -372,6 +416,23 @@ fn load_from(spec_params: SpecParams, s: ethjson::spec::Spec) -> Result<Spec, Er
 	let GenericSeal(seal_rlp) = g.seal.into();
 	let params = CommonParams::from(s.params);
 
+	let hardcoded_sync = if let Some(ref hs) = s.hardcoded_sync {
+		if let Ok(header) = hs.header.from_hex() {
+			Some(SpecHardcodedSync {
+				header: encoded::Header::new(header),
+				total_difficulty: hs.total_difficulty.into(),
+				chts: s.hardcoded_sync
+					.as_ref()
+					.map(|s| s.chts.iter().map(|c| c.clone().into()).collect())
+					.unwrap_or(Vec::new()),
+			})
+		} else {
+			None
+		}
+	} else {
+		None
+	};
+
 	let mut s = Spec {
 		name: s.name.clone().into(),
 		engine: Spec::engine(spec_params, s.engine, params, builtins),
@@ -387,6 +448,7 @@ fn load_from(spec_params: SpecParams, s: ethjson::spec::Spec) -> Result<Spec, Er
 		timestamp: g.timestamp,
 		extra_data: g.extra_data,
 		seal_rlp: seal_rlp,
+		hardcoded_sync: hardcoded_sync,
 		constructors: s.accounts
 			.constructors()
 			.into_iter()

json/src/spec/hardcoded_sync.rs

@@ -0,0 +1,63 @@
+// Copyright 2018 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/>.
+
+//! Spec hardcoded synchronization deserialization for the light client.
+
+use hash::H256;
+use uint::Uint;
+
+/// Spec hardcoded sync.
+#[derive(Debug, PartialEq, Serialize, Deserialize)]
+pub struct HardcodedSync {
+	/// Hexadecimal of the RLP encoding of the header of the block to start synchronization from.
+	pub header: String,
+	/// Total difficulty including the block of `header`.
+	#[serde(rename="totalDifficulty")]
+	pub total_difficulty: Uint,
+	/// Ordered trie roots of blocks before and including `header`.
+	#[serde(rename="CHTs")]
+	pub chts: Vec<H256>,
+}
+
+#[cfg(test)]
+mod tests {
+	use serde_json;
+	use uint::Uint;
+	use ethereum_types::{U256, H256 as Eth256};
+	use hash::H256;
+	use spec::hardcoded_sync::HardcodedSync;
+
+	#[test]
+	fn hardcoded_sync_deserialization() {
+		let s = r#"{
+			"header": "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",
+			"totalDifficulty": "0x400000000",
+			"CHTs": [
+				"0x11bbe8db4e347b4e8c937c1c8370e4b5ed33adb3db69cbdb7a38e1e50b1b82fa",
+				"0xd7f8974fb5ac78d9ac099b9ad5018bedc2ce0a72dad1827a1709da30580f0544"
+			]
+		}"#;
+		let deserialized: HardcodedSync = serde_json::from_str(s).unwrap();
+		assert_eq!(deserialized, HardcodedSync {
+			header: String::from("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"),
+			total_difficulty: Uint(U256::from(0x400000000u64)),
+			chts: vec![
+				H256(Eth256::from("0x11bbe8db4e347b4e8c937c1c8370e4b5ed33adb3db69cbdb7a38e1e50b1b82fa")),
+				H256(Eth256::from("0xd7f8974fb5ac78d9ac099b9ad5018bedc2ce0a72dad1827a1709da30580f0544")),
+			]
+		});
+	}
+}

json/src/spec/mod.rs

@@ -30,6 +30,7 @@ pub mod basic_authority;
 pub mod authority_round;
 pub mod tendermint;
 pub mod null_engine;
+pub mod hardcoded_sync;
 
 pub use self::account::Account;
 pub use self::builtin::{Builtin, Pricing, Linear};
@@ -45,3 +46,4 @@ pub use self::basic_authority::{BasicAuthority, BasicAuthorityParams};
 pub use self::authority_round::{AuthorityRound, AuthorityRoundParams};
 pub use self::tendermint::{Tendermint, TendermintParams};
 pub use self::null_engine::{NullEngine, NullEngineParams};
+pub use self::hardcoded_sync::HardcodedSync;

json/src/spec/spec.rs

@@ -19,7 +19,7 @@
 use std::io::Read;
 use serde_json;
 use serde_json::Error;
-use spec::{Params, Genesis, Engine, State};
+use spec::{Params, Genesis, Engine, State, HardcodedSync};
 
 /// Spec deserialization.
 #[derive(Debug, PartialEq, Deserialize)]
@@ -39,6 +39,9 @@ pub struct Spec {
 	pub accounts: State,
 	/// Boot nodes.
 	pub nodes: Option<Vec<String>>,
+	/// Hardcoded synchronization for the light client.
+	#[serde(rename="hardcodedSync")]
+	pub hardcoded_sync: Option<HardcodedSync>,
 }
 
 impl Spec {
@@ -104,6 +107,14 @@ mod tests {
 		"0000000000000000000000000000000000000003": { "balance": "1", "nonce": "1048576", "builtin": { "name": "ripemd160", "pricing": { "linear": { "base": 600, "word": 120 } } } },
 		"0000000000000000000000000000000000000004": { "balance": "1", "nonce": "1048576", "builtin": { "name": "identity", "pricing": { "linear": { "base": 15, "word": 3 } } } },
 		"102e61f5d8f9bc71d0ad4a084df4e65e05ce0e1c": { "balance": "1606938044258990275541962092341162602522202993782792835301376", "nonce": "1048576" }
+	},
+	"hardcodedSync": {
+		"header": "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",
+		"totalDifficulty": "0x400000000",
+		"CHTs": [
+			"0x11bbe8db4e347b4e8c937c1c8370e4b5ed33adb3db69cbdb7a38e1e50b1b82fa",
+			"0xd7f8974fb5ac78d9ac099b9ad5018bedc2ce0a72dad1827a1709da30580f0544"
+		]
 	}
 		}"#;
 		let _deserialized: Spec = serde_json::from_str(s).unwrap();

json/src/uint.rs

@@ -18,7 +18,7 @@
 
 use std::fmt;
 use std::str::FromStr;
-use serde::{Deserialize, Deserializer};
+use serde::{Deserialize, Deserializer, Serialize, Serializer};
 use serde::de::{Error, Visitor, Unexpected};
 use ethereum_types::U256;
 
@@ -50,6 +50,13 @@ impl Into<u8> for Uint {
 	}
 }
 
+impl Serialize for Uint {
+	fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+		where S: Serializer {
+		self.0.to_string().serialize(serializer)
+	}
+}
+
 impl<'a> Deserialize<'a> for Uint {
 	fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
 		where D: Deserializer<'a> {

parity/blockchain.rs

@@ -201,6 +201,7 @@ fn execute_import_light(cmd: ImportBlockchain) -> Result<(), String> {
 		chain_column: ::ethcore::db::COL_LIGHT_CHAIN,
 		verify_full: true,
 		check_seal: cmd.check_seal,
+		no_hardcoded_sync: true,
 	};
 
 	config.queue.max_mem_use = cmd.cache_config.queue() as usize * 1024 * 1024;

parity/cli/mod.rs

@@ -231,6 +231,11 @@ usage! {
 				"Clean the database",
 			}
 		}
+
+		CMD cmd_export_hardcoded_sync
+		{
+			"Export the hardcoded sync JSON file from the existing light client database",
+		}
 	}
 	{
 		// Global flags and arguments
@@ -251,6 +256,10 @@ usage! {
 			"--light",
 			"Experimental: run in light client mode. Light clients synchronize a bare minimum of data and fetch necessary data on-demand from the network. Much lower in storage, potentially higher in bandwidth. Has no effect with subcommands.",
 
+			FLAG flag_no_hardcoded_sync: (bool) = false, or |c: &Config| c.parity.as_ref()?.no_hardcoded_sync,
+			"--no-hardcoded-sync",
+			"By default, if there is no existing database the light client will automatically jump to a block hardcoded in the chain's specifications. This disables this feature.",
+
 			FLAG flag_force_direct: (bool) = false, or |_| None,
 			"--force-direct",
 			"Run the originally installed version of Parity, ignoring any updates that have since been installed.",
@@ -1006,6 +1015,7 @@ struct Operating {
 	identity: Option<String>,
 	light: Option<bool>,
 	no_persistent_txqueue: Option<bool>,
+	no_hardcoded_sync: Option<bool>,
 }
 
 #[derive(Default, Debug, PartialEq, Deserialize)]
@@ -1409,6 +1419,7 @@ mod tests {
 			cmd_tools_hash: false,
 			cmd_db: false,
 			cmd_db_kill: false,
+			cmd_export_hardcoded_sync: false,
 
 			// Arguments
 			arg_daemon_pid_file: None,
@@ -1443,6 +1454,7 @@ mod tests {
 			arg_keys_path: "$HOME/.parity/keys".into(),
 			arg_identity: "".into(),
 			flag_light: false,
+			flag_no_hardcoded_sync: false,
 			flag_no_persistent_txqueue: false,
 			flag_force_direct: false,
 
@@ -1697,6 +1709,7 @@ mod tests {
 				keys_path: None,
 				identity: None,
 				light: None,
+				no_hardcoded_sync: None,
 				no_persistent_txqueue: None,
 			}),
 			account: Some(Account {

parity/configuration.rs

@@ -49,6 +49,7 @@ use secretstore::{NodeSecretKey, Configuration as SecretStoreConfiguration, Cont
 use updater::{UpdatePolicy, UpdateFilter, ReleaseTrack};
 use run::RunCmd;
 use blockchain::{BlockchainCmd, ImportBlockchain, ExportBlockchain, KillBlockchain, ExportState, DataFormat};
+use export_hardcoded_sync::ExportHsyncCmd;
 use presale::ImportWallet;
 use account::{AccountCmd, NewAccount, ListAccounts, ImportAccounts, ImportFromGethAccounts};
 use snapshot::{self, SnapshotCommand};
@@ -79,6 +80,7 @@ pub enum Cmd {
 	},
 	Snapshot(SnapshotCommand),
 	Hash(Option<String>),
+	ExportHardcodedSync(ExportHsyncCmd),
 }
 
 pub struct Execute {
@@ -317,6 +319,16 @@ impl Configuration {
 				block_at: to_block_id("latest")?, // unimportant.
 			};
 			Cmd::Snapshot(restore_cmd)
+		} else if self.args.cmd_export_hardcoded_sync {
+			let export_hs_cmd = ExportHsyncCmd {
+				cache_config: cache_config,
+				dirs: dirs,
+				spec: spec,
+				pruning: pruning,
+				compaction: compaction,
+				wal: wal,
+			};
+			Cmd::ExportHardcodedSync(export_hs_cmd)
 		} else {
 			let daemon = if self.args.cmd_daemon {
 				Some(self.args.arg_daemon_pid_file.clone().expect("CLI argument is required; qed"))
@@ -375,6 +387,7 @@ impl Configuration {
 				light: self.args.flag_light,
 				no_persistent_txqueue: self.args.flag_no_persistent_txqueue,
 				whisper: whisper_config,
+				no_hardcoded_sync: self.args.flag_no_hardcoded_sync,
 			};
 			Cmd::Run(run_cmd)
 		};
@@ -1422,6 +1435,7 @@ mod tests {
 			verifier_settings: Default::default(),
 			serve_light: true,
 			light: false,
+			no_hardcoded_sync: false,
 			no_persistent_txqueue: false,
 			whisper: Default::default(),
 		};

parity/export_hardcoded_sync.rs

@@ -0,0 +1,117 @@
+// Copyright 2018 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/>.
+
+use std::sync::Arc;
+use std::time::Duration;
+
+use ethcore::client::DatabaseCompactionProfile;
+use ethcore::db::NUM_COLUMNS;
+use ethcore::spec::{SpecParams, OptimizeFor};
+use kvdb_rocksdb::{Database, DatabaseConfig};
+use light::client::fetch::Unavailable as UnavailableDataFetcher;
+use light::Cache as LightDataCache;
+
+use params::{SpecType, Pruning};
+use helpers::execute_upgrades;
+use dir::Directories;
+use cache::CacheConfig;
+use user_defaults::UserDefaults;
+
+// Number of minutes before a given gas price corpus should expire.
+// Light client only.
+const GAS_CORPUS_EXPIRATION_MINUTES: u64 = 60 * 6;
+
+#[derive(Debug, PartialEq)]
+pub struct ExportHsyncCmd {
+	pub cache_config: CacheConfig,
+	pub dirs: Directories,
+	pub spec: SpecType,
+	pub pruning: Pruning,
+	pub compaction: DatabaseCompactionProfile,
+	pub wal: bool,
+}
+
+pub fn execute(cmd: ExportHsyncCmd) -> Result<String, String> {
+	use light::client as light_client;
+	use parking_lot::Mutex;
+
+	// load spec
+	let spec = cmd.spec.spec(SpecParams::new(cmd.dirs.cache.as_ref(), OptimizeFor::Memory))?;
+
+	// load genesis hash
+	let genesis_hash = spec.genesis_header().hash();
+
+	// database paths
+	let db_dirs = cmd.dirs.database(genesis_hash, cmd.spec.legacy_fork_name(), spec.data_dir.clone());
+
+	// user defaults path
+	let user_defaults_path = db_dirs.user_defaults_path();
+
+	// load user defaults
+	let user_defaults = UserDefaults::load(&user_defaults_path)?;
+
+	// select pruning algorithm
+	let algorithm = cmd.pruning.to_algorithm(&user_defaults);
+
+	let compaction = cmd.compaction.compaction_profile(db_dirs.db_root_path().as_path());
+
+	// execute upgrades
+	execute_upgrades(&cmd.dirs.base, &db_dirs, algorithm, compaction.clone())?;
+
+	// create dirs used by parity
+	cmd.dirs.create_dirs(false, false, false)?;
+
+	// TODO: configurable cache size.
+	let cache = LightDataCache::new(Default::default(), Duration::from_secs(60 * GAS_CORPUS_EXPIRATION_MINUTES));
+	let cache = Arc::new(Mutex::new(cache));
+
+	// start client and create transaction queue.
+	let mut config = light_client::Config {
+		queue: Default::default(),
+		chain_column: ::ethcore::db::COL_LIGHT_CHAIN,
+		verify_full: true,
+		check_seal: true,
+		no_hardcoded_sync: true,
+	};
+
+	config.queue.max_mem_use = cmd.cache_config.queue() as usize * 1024 * 1024;
+
+	// initialize database.
+	let db = {
+		let db_config = DatabaseConfig {
+			memory_budget: Some(cmd.cache_config.blockchain() as usize * 1024 * 1024),
+			compaction: compaction,
+			wal: cmd.wal,
+			.. DatabaseConfig::with_columns(NUM_COLUMNS)
+		};
+
+		Arc::new(Database::open(
+			&db_config,
+			&db_dirs.client_path(algorithm).to_str().expect("DB path could not be converted to string.")
+		).map_err(|e| format!("Error opening database: {}", e))?)
+	};
+
+	let service = light_client::Service::start(config, &spec, UnavailableDataFetcher, db, cache)
+		.map_err(|e| format!("Error starting light client: {}", e))?;
+
+	let hs = service.client().read_hardcoded_sync()
+		.map_err(|e| format!("Error reading hardcoded sync: {}", e))?;
+	if let Some(hs) = hs {
+		Ok(::serde_json::to_string_pretty(&hs.to_json()).expect("generated JSON is always valid"))
+	} else {
+		Err("Error: cannot generate hardcoded sync because the database is empty.".into())
+	}
+}

parity/main.rs

@@ -106,6 +106,7 @@ mod cache;
 mod cli;
 mod configuration;
 mod dapps;
+mod export_hardcoded_sync;
 mod ipfs;
 mod deprecated;
 mod helpers;
@@ -175,6 +176,7 @@ fn execute(command: Execute, can_restart: bool) -> Result<PostExecutionAction, S
 		Cmd::SignerList { port, authfile } => rpc_cli::signer_list(port, authfile).map(|s| PostExecutionAction::Print(s)),
 		Cmd::SignerReject { id, port, authfile } => rpc_cli::signer_reject(id, port, authfile).map(|s| PostExecutionAction::Print(s)),
 		Cmd::Snapshot(snapshot_cmd) => snapshot::execute(snapshot_cmd).map(|s| PostExecutionAction::Print(s)),
+		Cmd::ExportHardcodedSync(export_hs_cmd) => export_hardcoded_sync::execute(export_hs_cmd).map(|s| PostExecutionAction::Print(s)),
 	}
 }
 

parity/run.rs

@@ -130,7 +130,8 @@ pub struct RunCmd {
 	pub serve_light: bool,
 	pub light: bool,
 	pub no_persistent_txqueue: bool,
-	pub whisper: ::whisper::Config
+	pub whisper: ::whisper::Config,
+	pub no_hardcoded_sync: bool,
 }
 
 pub fn open_ui(ws_conf: &rpc::WsConfiguration, ui_conf: &rpc::UiConfiguration, logger_config: &LogConfig) -> Result<(), String> {
@@ -226,6 +227,7 @@ fn execute_light_impl(cmd: RunCmd, can_restart: bool, logger: Arc<RotatingLogger
 		chain_column: ::ethcore::db::COL_LIGHT_CHAIN,
 		verify_full: true,
 		check_seal: cmd.check_seal,
+		no_hardcoded_sync: cmd.no_hardcoded_sync,
 	};
 
 	config.queue.max_mem_use = cmd.cache_config.queue() as usize * 1024 * 1024;
@@ -567,6 +569,11 @@ pub fn execute_impl(cmd: RunCmd, can_restart: bool, logger: Arc<RotatingLogger>)
 		}
 	}
 
+	// display warning if using --no-hardcoded-sync
+	if !cmd.no_hardcoded_sync {
+		warn!("The --no-hardcoded-sync flag has no effect if you don't use --light");
+	}
+
 	// create client config
 	let mut client_config = to_client_config(
 		&cmd.cache_config,