PoA: Wait for transition finality before applying (#5774)

* final engine changes

* migration to v13

* adding and removing pending transitions

* epoch_transition_for

* port snapshot to new engine methods

* final validator set interface

* fix compiler errors

* revert v13/epoch_depth transition

* make call on new epoch

* rolling finality checker

* tests for finality checker

* constructing finality proof upon pending transition

* fix warnings and finality proof checking

* fix compiler warnings in tests

* test fixes

* don't include genesis in finality checking

* change snapshot test chain building logic

* minor refactorings

* fetch epoch transition based on parent, fix divide-by-zero in SimpleList

* fix formatting

* fix ABIs and finality checking in snapshot restoration

* encode signal number in proof

* create more blocks at the end of tests

* update gist to accurate contract code

* test for epoch_transition_for

* fix tests with immediateTransitions parameter

* disable force flag after forcing

* rename ValidatorsChanged to InitiateChange and finalizeSignal to finalizeChange

* a few more validator set tests

ethcore/native_contracts/build.rs

@@ -28,7 +28,7 @@ const SECRETSTORE_ACL_STORAGE_ABI: &'static str = include_str!("res/secretstore_
 const VALIDATOR_SET_ABI: &'static str = include_str!("res/validator_set.json");
 const VALIDATOR_REPORT_ABI: &'static str = include_str!("res/validator_report.json");
 
-const TEST_VALIDATOR_SET_ABI: &'static str = r#"[{"constant":true,"inputs":[],"name":"transitionNonce","outputs":[{"name":"n","type":"uint256"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"newValidators","type":"address[]"}],"name":"setValidators","outputs":[],"payable":false,"type":"function"},{"constant":true,"inputs":[],"name":"getValidators","outputs":[{"name":"vals","type":"address[]"}],"payable":false,"type":"function"},{"inputs":[],"payable":false,"type":"constructor"},{"anonymous":false,"inputs":[{"indexed":true,"name":"_parent_hash","type":"bytes32"},{"indexed":true,"name":"_nonce","type":"uint256"},{"indexed":false,"name":"_new_set","type":"address[]"}],"name":"ValidatorsChanged","type":"event"}]"#;
+const TEST_VALIDATOR_SET_ABI: &'static str = include_str!("res/test_validator_set.json");
 
 fn build_file(name: &str, abi: &str, filename: &str) {
 	let code = ::native_contract_generator::generate_module(name, abi).unwrap();

ethcore/native_contracts/generator/src/lib.rs

@@ -108,7 +108,7 @@ fn generate_functions(contract: &Contract) -> Result<String, Error> {
 /// Outputs: {abi_outputs:?}
 pub fn {snake_name}<F, U>(&self, call: F, {params}) -> BoxFuture<{output_type}, String>
 	where
-	    F: Fn(util::Address, Vec<u8>) -> U,
+	    F: FnOnce(util::Address, Vec<u8>) -> U,
 	    U: IntoFuture<Item=Vec<u8>, Error=String>,
 		U::Future: Send + 'static
 {{

ethcore/native_contracts/res/test_validator_set.json

@@ -0,0 +1,8 @@
+[
+	{"constant":false,"inputs":[{"name":"_validators","type":"address[]"}],"name":"setValidators","outputs":[],"payable":false,"type":"function"},
+	{"constant":false,"inputs":[{"name":"","type":"address"},{"name":"","type":"bytes"}],"name":"reportMalicious","outputs":[],"payable":false,"type":"function"},
+	{"constant":false,"inputs":[],"name":"finalizeChange","outputs":[],"payable":false,"type":"function"},
+	{"constant":true,"inputs":[],"name":"getValidators","outputs":[{"name":"_validators","type":"address[]"}],"payable":false,"type":"function"},
+	{"constant":false,"inputs":[{"name":"","type":"address"}],"name":"reportBenign","outputs":[],"payable":false,"type":"function"},
+	{"anonymous":false,"inputs":[{"indexed":true,"name":"_parent_hash","type":"bytes32"},{"indexed":false,"name":"_new_set","type":"address[]"}],"name":"InitiateChange","type":"event"}
+]

ethcore/native_contracts/res/validator_set.json

@@ -1,5 +1,5 @@
 [
-	{"constant":true,"inputs":[],"name":"transitionNonce","outputs":[{"name":"nonce","type":"uint256"}],"payable":false,"type":"function"},
+	{"constant":false,"inputs":[],"name":"finalizeChange","outputs":[],"payable":false,"type":"function"},
 	{"constant":true,"inputs":[],"name":"getValidators","outputs":[{"name":"validators","type":"address[]"}],"payable":false,"type":"function"},
-	{"anonymous":false,"inputs":[{"indexed":true,"name":"_parent_hash","type":"bytes32"},{"indexed":true,"name":"_nonce","type":"uint256"},{"indexed":false,"name":"_new_set","type":"address[]"}],"name":"ValidatorsChanged","type":"event"}
+	{"anonymous":false,"inputs":[{"indexed":true,"name":"_parent_hash","type":"bytes32"},{"indexed":false,"name":"_new_set","type":"address[]"}],"name":"InitiateChange","type":"event"}
 ]

ethcore/res/authority_round.json

@@ -11,7 +11,8 @@
 						"0x7d577a597b2742b498cb5cf0c26cdcd726d39e6e",
 						"0x82a978b3f5962a5b0957d9ee9eef472ee55b42f1"
 					]
-				}
+				},
+				"immediateTransitions": true
 			}
 		}
 	},

ethcore/res/validator_contract.json

@@ -8,7 +8,8 @@
 				"startStep": 2,
 				"validators": {
 					"contract": "0x0000000000000000000000000000000000000005"
-				}
+				},
+				"immediateTransitions": true
 			}
 		}
 	},

ethcore/src/block.rs

@@ -260,6 +260,7 @@ impl<'x> OpenBlock<'x> {
 		author: Address,
 		gas_range_target: (U256, U256),
 		extra_data: Bytes,
+		is_epoch_begin: bool,
 	) -> Result<Self, Error> {
 		let number = parent.number() + 1;
 		let state = State::from_existing(db, parent.state_root().clone(), engine.account_start_nonce(number), factories)?;
@@ -279,7 +280,8 @@ impl<'x> OpenBlock<'x> {
 		let gas_floor_target = cmp::max(gas_range_target.0, engine.params().min_gas_limit);
 		let gas_ceil_target = cmp::max(gas_range_target.1, gas_floor_target);
 		engine.populate_from_parent(&mut r.block.header, parent, gas_floor_target, gas_ceil_target);
-		engine.on_new_block(&mut r.block, last_hashes)?;
+		engine.on_new_block(&mut r.block, last_hashes, is_epoch_begin)?;
+
 		Ok(r)
 	}
 
@@ -430,7 +432,7 @@ impl<'x> OpenBlock<'x> {
 
 	#[cfg(test)]
 	/// Return mutable block reference. To be used in tests only.
-	pub fn block_mut (&mut self) -> &mut ExecutedBlock { &mut self.block }
+	pub fn block_mut(&mut self) -> &mut ExecutedBlock { &mut self.block }
 }
 
 impl<'x> IsBlock for OpenBlock<'x> {
@@ -554,6 +556,7 @@ pub fn enact(
 	parent: &Header,
 	last_hashes: Arc<LastHashes>,
 	factories: Factories,
+	is_epoch_begin: bool,
 ) -> Result<LockedBlock, Error> {
 	{
 		if ::log::max_log_level() >= ::log::LogLevel::Trace {
@@ -563,7 +566,19 @@ pub fn enact(
 		}
 	}
 
-	let mut b = OpenBlock::new(engine, factories, tracing, db, parent, last_hashes, Address::new(), (3141562.into(), 31415620.into()), vec![])?;
+	let mut b = OpenBlock::new(
+		engine,
+		factories,
+		tracing,
+		db,
+		parent,
+		last_hashes,
+		Address::new(),
+		(3141562.into(), 31415620.into()),
+		vec![],
+		is_epoch_begin,
+	)?;
+
 	b.set_difficulty(*header.difficulty());
 	b.set_gas_limit(*header.gas_limit());
 	b.set_timestamp(header.timestamp());
@@ -618,9 +633,22 @@ pub fn enact_verified(
 	parent: &Header,
 	last_hashes: Arc<LastHashes>,
 	factories: Factories,
+	is_epoch_begin: bool,
 ) -> Result<LockedBlock, Error> {
 	let view = BlockView::new(&block.bytes);
-	enact(&block.header, &block.transactions, &view.uncles(), engine, tracing, db, parent, last_hashes, factories)
+
+	enact(
+		&block.header,
+		&block.transactions,
+		&view.uncles(),
+		engine,
+		tracing,
+		db,
+		parent,
+		last_hashes,
+		factories,
+		is_epoch_begin,
+	)
 }
 
 #[cfg(test)]
@@ -653,7 +681,7 @@ mod tests {
 		let header = block.header();
 		let transactions: Result<Vec<_>, Error> = block.transactions().into_iter().map(SignedTransaction::new).collect();
 		let transactions = transactions?;
-		enact(&header, &transactions, &block.uncles(), engine, tracing, db, parent, last_hashes, factories)
+		enact(&header, &transactions, &block.uncles(), engine, tracing, db, parent, last_hashes, factories, false)
 	}
 
 	/// Enact the block given by `block_bytes` using `engine` on the database `db` with given `parent` block header. Seal the block aferwards
@@ -678,7 +706,7 @@ mod tests {
 		let genesis_header = spec.genesis_header();
 		let db = spec.ensure_db_good(get_temp_state_db(), &Default::default()).unwrap();
 		let last_hashes = Arc::new(vec![genesis_header.hash()]);
-		let b = OpenBlock::new(&*spec.engine, Default::default(), false, db, &genesis_header, last_hashes, Address::zero(), (3141562.into(), 31415620.into()), vec![]).unwrap();
+		let b = OpenBlock::new(&*spec.engine, Default::default(), false, db, &genesis_header, last_hashes, Address::zero(), (3141562.into(), 31415620.into()), vec![], false).unwrap();
 		let b = b.close_and_lock();
 		let _ = b.seal(&*spec.engine, vec![]);
 	}
@@ -692,7 +720,7 @@ mod tests {
 
 		let db = spec.ensure_db_good(get_temp_state_db(), &Default::default()).unwrap();
 		let last_hashes = Arc::new(vec![genesis_header.hash()]);
-		let b = OpenBlock::new(engine, Default::default(), false, db, &genesis_header, last_hashes.clone(), Address::zero(), (3141562.into(), 31415620.into()), vec![]).unwrap()
+		let b = OpenBlock::new(engine, Default::default(), false, db, &genesis_header, last_hashes.clone(), Address::zero(), (3141562.into(), 31415620.into()), vec![], false).unwrap()
 			.close_and_lock().seal(engine, vec![]).unwrap();
 		let orig_bytes = b.rlp_bytes();
 		let orig_db = b.drain();
@@ -716,7 +744,7 @@ mod tests {
 
 		let db = spec.ensure_db_good(get_temp_state_db(), &Default::default()).unwrap();
 		let last_hashes = Arc::new(vec![genesis_header.hash()]);
-		let mut open_block = OpenBlock::new(engine, Default::default(), false, db, &genesis_header, last_hashes.clone(), Address::zero(), (3141562.into(), 31415620.into()), vec![]).unwrap();
+		let mut open_block = OpenBlock::new(engine, Default::default(), false, db, &genesis_header, last_hashes.clone(), Address::zero(), (3141562.into(), 31415620.into()), vec![], false).unwrap();
 		let mut uncle1_header = Header::new();
 		uncle1_header.set_extra_data(b"uncle1".to_vec());
 		let mut uncle2_header = Header::new();

ethcore/src/blockchain/blockchain.rs

@@ -35,6 +35,7 @@ use blockchain::{CacheSize, ImportRoute, Config};
 use db::{self, Writable, Readable, CacheUpdatePolicy};
 use cache_manager::CacheManager;
 use encoded;
+use engines::epoch::{Transition as EpochTransition, PendingTransition as PendingEpochTransition};
 
 const LOG_BLOOMS_LEVELS: usize = 3;
 const LOG_BLOOMS_ELEMENTS_PER_INDEX: usize = 16;
@@ -145,6 +146,10 @@ pub trait BlockProvider {
 		where F: Fn(&LogEntry) -> bool, Self: Sized;
 }
 
+macro_rules! otry {
+	($e:expr) => { match $e { Some(x) => x, None => return None } }
+}
+
 #[derive(Debug, Hash, Eq, PartialEq, Clone)]
 enum CacheId {
 	BlockHeader(H256),
@@ -508,7 +513,7 @@ impl BlockChain {
 					number: header.number(),
 					total_difficulty: header.difficulty(),
 					parent: header.parent_hash(),
-					children: vec![]
+					children: vec![],
 				};
 
 				let mut batch = DBTransaction::new();
@@ -698,10 +703,6 @@ impl BlockChain {
 	/// If the tree route verges into pruned or unknown blocks,
 	/// `None` is returned.
 	pub fn tree_route(&self, from: H256, to: H256) -> Option<TreeRoute> {
-		macro_rules! otry {
-			($e:expr) => { match $e { Some(x) => x, None => return None } }
-		}
-
 		let mut from_branch = vec![];
 		let mut to_branch = vec![];
 
@@ -878,16 +879,60 @@ impl BlockChain {
 		}
 	}
 
-	/// Get a specific epoch transition by epoch number and provided block hash.
-	pub fn epoch_transition(&self, epoch_num: u64, block_hash: H256) -> Option<EpochTransition> {
-		trace!(target: "blockchain", "Loading epoch {} transition at block {}",
-			epoch_num, block_hash);
+	/// Get a specific epoch transition by block number and provided block hash.
+	pub fn epoch_transition(&self, block_num: u64, block_hash: H256) -> Option<EpochTransition> {
+		trace!(target: "blockchain", "Loading epoch transition at block {}, {}",
+			block_num, block_hash);
 
-		self.db.read(db::COL_EXTRA, &epoch_num).and_then(|transitions: EpochTransitions| {
+		self.db.read(db::COL_EXTRA, &block_num).and_then(|transitions: EpochTransitions| {
 			transitions.candidates.into_iter().find(|c| c.block_hash == block_hash)
 		})
 	}
 
+	/// Get the transition to the epoch the given parent hash is part of
+	/// or transitions to.
+	/// This will give the epoch that any children of this parent belong to.
+	///
+	/// The block corresponding the the parent hash must be stored already.
+	pub fn epoch_transition_for(&self, parent_hash: H256) -> Option<EpochTransition> {
+		// slow path: loop back block by block
+		for hash in otry!(self.ancestry_iter(parent_hash)) {
+			let details = otry!(self.block_details(&hash));
+
+			// look for transition in database.
+			if let Some(transition) = self.epoch_transition(details.number, hash) {
+				return Some(transition)
+			}
+
+			// canonical hash -> fast breakout:
+			// get the last epoch transition up to this block.
+			//
+			// if `block_hash` is canonical it will only return transitions up to
+			// the parent.
+			if otry!(self.block_hash(details.number)) == hash {
+				return self.epoch_transitions()
+					.map(|(_, t)| t)
+					.take_while(|t| t.block_number <= details.number)
+					.last()
+			}
+		}
+
+		// should never happen as the loop will encounter genesis before concluding.
+		None
+	}
+
+	/// Write a pending epoch transition by block hash.
+	pub fn insert_pending_transition(&self, batch: &mut DBTransaction, hash: H256, t: PendingEpochTransition) {
+		batch.write(db::COL_EXTRA, &hash, &t);
+	}
+
+	/// Get a pending epoch transition by block hash.
+	// TODO: implement removal safely: this can only be done upon finality of a block
+	// that _uses_ the pending transition.
+	pub fn get_pending_transition(&self, hash: H256) -> Option<PendingEpochTransition> {
+		self.db.read(db::COL_EXTRA, &hash)
+	}
+
 	/// Add a child to a given block. Assumes that the block hash is in
 	/// the chain and the child's parent is this block.
 	///
@@ -1165,12 +1210,12 @@ impl BlockChain {
 		let mut parent_details = self.block_details(&parent_hash).unwrap_or_else(|| panic!("Invalid parent hash: {:?}", parent_hash));
 		parent_details.children.push(info.hash.clone());
 
-		// create current block details
+		// create current block details.
 		let details = BlockDetails {
 			number: header.number(),
 			total_difficulty: info.total_difficulty,
 			parent: parent_hash.clone(),
-			children: vec![]
+			children: vec![],
 		};
 
 		// write to batch
@@ -2201,7 +2246,7 @@ mod tests {
 
 	#[test]
 	fn epoch_transitions_iter() {
-		use blockchain::extras::EpochTransition;
+		use ::engines::EpochTransition;
 
 		let mut canon_chain = ChainGenerator::default();
 		let mut finalizer = BlockFinalizer::default();
@@ -2223,7 +2268,6 @@ mod tests {
 					block_hash: hash,
 					block_number: i + 1,
 					proof: vec![],
-					state_proof: vec![],
 				});
 				bc.commit();
 			}
@@ -2236,7 +2280,6 @@ mod tests {
 				block_hash: hash,
 				block_number: 1,
 				proof: vec![],
-				state_proof: vec![]
 			});
 
 			db.write(batch).unwrap();
@@ -2252,4 +2295,85 @@ mod tests {
 		assert_eq!(bc.best_block_number(), 5);
 		assert_eq!(bc.epoch_transitions().map(|(i, _)| i).collect::<Vec<_>>(), vec![0, 1, 2, 3, 4]);
 	}
+
+	#[test]
+	fn epoch_transition_for() {
+		use ::engines::EpochTransition;
+
+		let mut canon_chain = ChainGenerator::default();
+		let mut finalizer = BlockFinalizer::default();
+		let genesis = canon_chain.generate(&mut finalizer).unwrap();
+
+		let db = new_db();
+
+		let bc = new_chain(&genesis, db.clone());
+
+		let mut batch = db.transaction();
+		bc.insert_epoch_transition(&mut batch, 0, EpochTransition {
+			block_hash: bc.genesis_hash(),
+			block_number: 0,
+			proof: vec![],
+		});
+		db.write(batch).unwrap();
+
+		// set up a chain where we have a canonical chain of 10 blocks
+		// and a non-canonical fork of 8 from genesis.
+		let fork_hash = {
+			let mut fork_chain = canon_chain.fork(1);
+			let mut fork_finalizer = finalizer.fork();
+
+			for _ in 0..7 {
+				let mut batch = db.transaction();
+				let fork_block = fork_chain.generate(&mut fork_finalizer).unwrap();
+
+				bc.insert_block(&mut batch, &fork_block, vec![]);
+				bc.commit();
+				db.write(batch).unwrap();
+			}
+
+			assert_eq!(bc.best_block_number(), 7);
+			bc.chain_info().best_block_hash
+		};
+
+		for _ in 0..10 {
+			let mut batch = db.transaction();
+			let canon_block = canon_chain.generate(&mut finalizer).unwrap();
+
+			bc.insert_block(&mut batch, &canon_block, vec![]);
+			bc.commit();
+
+			db.write(batch).unwrap();
+		}
+
+		assert_eq!(bc.best_block_number(), 10);
+
+		let mut batch = db.transaction();
+		bc.insert_epoch_transition(&mut batch, 4, EpochTransition {
+			block_hash: bc.block_hash(4).unwrap(),
+			block_number: 4,
+			proof: vec![],
+		});
+		db.write(batch).unwrap();
+
+		// blocks where the parent is one of the first 4 will be part of genesis epoch.
+		for i in 0..4 {
+			let hash = bc.block_hash(i).unwrap();
+			assert_eq!(bc.epoch_transition_for(hash).unwrap().block_number, 0);
+		}
+
+		// blocks where the parent is the transition at 4 or after will be
+		// part of that epoch.
+		for i in 4..11 {
+			let hash = bc.block_hash(i).unwrap();
+			assert_eq!(bc.epoch_transition_for(hash).unwrap().block_number, 4);
+		}
+
+		let fork_hashes = bc.ancestry_iter(fork_hash).unwrap().collect::<Vec<_>>();
+		assert_eq!(fork_hashes.len(), 8);
+
+		// non-canonical fork blocks should all have genesis transition
+		for fork_hash in fork_hashes {
+			assert_eq!(bc.epoch_transition_for(fork_hash).unwrap().block_number, 0);
+		}
+	}
 }

ethcore/src/blockchain/extras.rs

@@ -17,13 +17,16 @@
 //! Blockchain DB extras.
 
 use bloomchain;
-use util::*;
-use util::kvdb::PREFIX_LEN as DB_PREFIX_LEN;
-use rlp::*;
+use blooms::{GroupPosition, BloomGroup};
+use db::Key;
+use engines::epoch::{Transition as EpochTransition};
 use header::BlockNumber;
 use receipt::Receipt;
-use db::Key;
-use blooms::{GroupPosition, BloomGroup};
+
+use rlp::*;
+use util::*;
+use util::kvdb::PREFIX_LEN as DB_PREFIX_LEN;
+
 
 /// Represents index of extra data in database
 #[derive(Copy, Debug, Hash, Eq, PartialEq, Clone)]
@@ -40,6 +43,8 @@ pub enum ExtrasIndex {
 	BlockReceipts = 4,
 	/// Epoch transition data index.
 	EpochTransitions = 5,
+	/// Pending epoch transition data index.
+	PendingEpochTransition = 6,
 }
 
 fn with_index(hash: &H256, i: ExtrasIndex) -> H264 {
@@ -137,6 +142,14 @@ impl Key<BlockReceipts> for H256 {
 	}
 }
 
+impl Key<::engines::epoch::PendingTransition> for H256 {
+	type Target = H264;
+
+	fn key(&self) -> H264 {
+		with_index(self, ExtrasIndex::PendingEpochTransition)
+	}
+}
+
 /// length of epoch keys.
 pub const EPOCH_KEY_LEN: usize = DB_PREFIX_LEN + 16;
 
@@ -296,41 +309,6 @@ impl Decodable for EpochTransitions {
 	}
 }
 
-#[derive(Debug, Clone)]
-pub struct EpochTransition {
-	pub block_hash: H256, // block hash at which the transition occurred.
-	pub block_number: BlockNumber, // block number at which the tranition occurred.
-	pub proof: Vec<u8>, // "transition/epoch" proof from the engine.
-	pub state_proof: Vec<DBValue>, // state items necessary to regenerate proof.
-}
-
-impl Encodable for EpochTransition {
-	fn rlp_append(&self, s: &mut RlpStream) {
-		s.begin_list(4)
-			.append(&self.block_hash)
-			.append(&self.block_number)
-			.append(&self.proof)
-			.begin_list(self.state_proof.len());
-
-		for item in &self.state_proof {
-			s.append(&&**item);
-		}
-	}
-}
-
-impl Decodable for EpochTransition {
-	fn decode(rlp: &UntrustedRlp) -> Result<Self, DecoderError> {
-		Ok(EpochTransition {
-			block_hash: rlp.val_at(0)?,
-			block_number: rlp.val_at(1)?,
-			proof: rlp.val_at(2)?,
-			state_proof: rlp.at(3)?.iter().map(|x| {
-				Ok(DBValue::from_slice(x.data()?))
-			}).collect::<Result<Vec<_>, _>>()?,
-		})
-	}
-}
-
 #[cfg(test)]
 mod tests {
 	use rlp::*;

ethcore/src/blockchain/mod.rs

@@ -31,6 +31,5 @@ pub mod generator;
 pub use self::blockchain::{BlockProvider, BlockChain};
 pub use self::cache::CacheSize;
 pub use self::config::Config;
-pub use self::extras::EpochTransition;
 pub use types::tree_route::TreeRoute;
 pub use self::import_route::ImportRoute;

ethcore/src/client/ancient_import.rs

@@ -18,8 +18,8 @@
 
 use std::sync::Arc;
 
-use engines::{Engine, EpochVerifier, EpochChange};
-use error::Error;
+use blockchain::BlockChain;
+use engines::{Engine, EpochVerifier};
 use header::Header;
 
 use rand::Rng;
@@ -46,21 +46,21 @@ impl AncientVerifier {
 
 	/// Verify the next block header, randomly choosing whether to do heavy or light
 	/// verification. If the block is the end of an epoch, updates the epoch verifier.
-	pub fn verify<R: Rng, F: Fn(u64) -> Result<Box<EpochVerifier>, Error>>(
+	pub fn verify<R: Rng>(
 		&self,
 		rng: &mut R,
 		header: &Header,
-		block: &[u8],
-		receipts: &[::receipt::Receipt],
-		load_verifier: F,
+		chain: &BlockChain,
 	) -> Result<(), ::error::Error> {
 		match rng.gen::<f32>() <= HEAVY_VERIFY_RATE {
 			true => self.cur_verifier.read().verify_heavy(header)?,
 			false => self.cur_verifier.read().verify_light(header)?,
 		}
 
-		if let EpochChange::Yes(num) = self.engine.is_epoch_end(header, Some(block), Some(receipts)) {
-			*self.cur_verifier.write() = load_verifier(num)?;
+		// ancient import will only use transitions obtained from the snapshot.
+		if let Some(transition) = chain.epoch_transition(header.number(), header.hash()) {
+			let v = self.engine.epoch_verifier(&header, &transition.proof).known_confirmed()?;
+			*self.cur_verifier.write() = v;
 		}
 
 		Ok(())

ethcore/src/client/client.rs

@@ -32,7 +32,7 @@ use util::kvdb::*;
 // other
 use basic_types::Seal;
 use block::*;
-use blockchain::{BlockChain, BlockProvider, EpochTransition, TreeRoute, ImportRoute};
+use blockchain::{BlockChain, BlockProvider,  TreeRoute, ImportRoute};
 use blockchain::extras::TransactionAddress;
 use client::ancient_import::AncientVerifier;
 use client::Error as ClientError;
@@ -42,7 +42,7 @@ use client::{
 	ChainNotify, PruningInfo, ProvingBlockChainClient,
 };
 use encoded;
-use engines::Engine;
+use engines::{Engine, EpochTransition};
 use env_info::EnvInfo;
 use env_info::LastHashes;
 use error::{ImportError, ExecutionError, CallError, BlockError, ImportResult, Error as EthcoreError};
@@ -256,9 +256,31 @@ impl Client {
 		{
 			let chain = client.chain.read();
 			let gh = spec.genesis_header();
-			if chain.epoch_transition(0, spec.genesis_header().hash()).is_none() {
+			if chain.epoch_transition(0, gh.hash()).is_none() {
 				trace!(target: "client", "No genesis transition found.");
-				client.generate_epoch_proof(&gh, 0, &*chain);
+
+				let proof = client.with_proving_caller(
+					BlockId::Number(0),
+					|call| client.engine.genesis_epoch_data(&gh, call)
+				);
+				let proof = match proof {
+					Ok(proof) => proof,
+					Err(e) => {
+						warn!(target: "client", "Error generating genesis epoch data: {}. Snapshots generated may not be complete.", e);
+						Vec::new()
+					}
+				};
+
+				debug!(target: "client", "Obtained genesis transition proof: {:?}", proof);
+
+				let mut batch = DBTransaction::new();
+				chain.insert_epoch_transition(&mut batch, 0, EpochTransition {
+					block_hash: gh.hash(),
+					block_number: 0,
+					proof: proof,
+				});
+
+				client.db.read().write_buffered(batch);
 			}
 		}
 
@@ -405,7 +427,16 @@ impl Client {
 			let last_hashes = self.build_last_hashes(header.parent_hash().clone());
 			let db = self.state_db.lock().boxed_clone_canon(header.parent_hash());
 
-			let enact_result = enact_verified(block, engine, self.tracedb.read().tracing_enabled(), db, &parent, last_hashes, self.factories.clone());
+			let is_epoch_begin = chain.epoch_transition(parent.number(), *header.parent_hash()).is_some();
+			let enact_result = enact_verified(block,
+				engine,
+				self.tracedb.read().tracing_enabled(),
+				db,
+				&parent,
+				last_hashes,
+				self.factories.clone(),
+				is_epoch_begin,
+			);
 			let mut locked_block = enact_result.map_err(|e| {
 				warn!(target: "client", "Block import failed for #{} ({})\nError: {:?}", header.number(), header.hash(), e);
 			})?;
@@ -555,18 +586,9 @@ impl Client {
 			{
 				// closure for verifying a block.
 				let verify_with = |verifier: &AncientVerifier| -> Result<(), ::error::Error> {
-					// verify the block, passing a closure used to load an epoch verifier
-					// by number.
-					verifier.verify(
-						&mut *self.rng.lock(),
-						&header,
-						&block_bytes,
-						&receipts,
-						|epoch_num| chain.epoch_transition(epoch_num, hash)
-							.ok_or(BlockError::UnknownEpochTransition(epoch_num))
-							.map_err(Into::into)
-							.and_then(|t| self.engine.epoch_verifier(&header, &t.proof))
-					)
+					// verify the block, passing the chain for updating the epoch
+					// verifier.
+					verifier.verify(&mut *self.rng.lock(), &header, &chain)
 				};
 
 				// initialize the ancient block verifier if we don't have one already.
@@ -583,7 +605,8 @@ impl Client {
 							.map(|(_, t)| t.proof)
 							.expect("At least one epoch entry (genesis) always stored; qed");
 
-						let current_verifier = self.engine.epoch_verifier(&header, &current_epoch_data)?;
+						let current_verifier = self.engine.epoch_verifier(&header, &current_epoch_data)
+							.known_confirmed()?;
 						let current_verifier = AncientVerifier::new(self.engine.clone(), current_verifier);
 
 						verify_with(&current_verifier)?;
@@ -606,6 +629,7 @@ impl Client {
 	fn commit_block<B>(&self, block: B, hash: &H256, block_data: &[u8]) -> ImportRoute where B: IsBlock + Drain {
 		let number = block.header().number();
 		let parent = block.header().parent_hash().clone();
+		let header = block.header().clone(); // TODO: optimize and avoid copy.
 		let chain = self.chain.read();
 
 		// Commit results
@@ -619,28 +643,13 @@ impl Client {
 
 		let mut batch = DBTransaction::new();
 
-		// generate validation proof if the engine requires them.
-		// TODO: make conditional?
-		let entering_new_epoch = {
-			use engines::EpochChange;
-			match self.engine.is_epoch_end(block.header(), Some(block_data), Some(&receipts)) {
-				EpochChange::Yes(e) => Some((block.header().clone(), e)),
-				EpochChange::No => None,
-				EpochChange::Unsure(_) => {
-					warn!(target: "client", "Detected invalid engine implementation.");
-					warn!(target: "client", "Engine claims to require more block data, but everything provided.");
-					None
-				}
-			}
-		};
-
 		// CHECK! I *think* this is fine, even if the state_root is equal to another
 		// already-imported block of the same number.
 		// TODO: Prove it with a test.
 		let mut state = block.drain();
 
 		state.journal_under(&mut batch, number, hash).expect("DB commit failed");
-		let route = chain.insert_block(&mut batch, block_data, receipts);
+		let route = chain.insert_block(&mut batch, block_data, receipts.clone());
 
 		self.tracedb.read().import(&mut batch, TraceImportRequest {
 			traces: traces.into(),
@@ -655,65 +664,102 @@ impl Client {
 		// Final commit to the DB
 		self.db.read().write_buffered(batch);
 		chain.commit();
+
+		// check for epoch end. do this after writing first batch so we can prove
+		// transactions on the block's state.
+		// TODO: work these changes into the existing DBTransaction.
+		self.check_epoch_end_signal(&header, block_data, &receipts, &chain);
+		self.check_epoch_end(&header, &chain);
+
 		self.update_last_hashes(&parent, hash);
 
 		if let Err(e) = self.prune_ancient(state, &chain) {
 			warn!("Failed to prune ancient state data: {}", e);
 		}
 
-		if let Some((header, epoch)) = entering_new_epoch {
-			self.generate_epoch_proof(&header, epoch, &chain);
-		}
-
 		route
 	}
 
-	// generate an epoch transition proof at the given block, and write it into the given blockchain.
-	fn generate_epoch_proof(&self, header: &Header, epoch_number: u64, chain: &BlockChain) {
-		use std::cell::RefCell;
-		use std::collections::BTreeSet;
+	// check for epoch end signal and write pending transition if it occurs.
+	// state for the given block must be available.
+	fn check_epoch_end_signal(&self, header: &Header, block: &[u8], receipts: &[Receipt], chain: &BlockChain) {
+		use engines::EpochChange;
 
-		let mut batch = DBTransaction::new();
 		let hash = header.hash();
-		debug!(target: "client", "Generating validation proof for epoch {} at block {}",
-			epoch_number, hash);
+		match self.engine.signals_epoch_end(header, Some(block), Some(&receipts)) {
+			EpochChange::Yes(proof) => {
+				use engines::epoch::PendingTransition;
+				use engines::Proof;
 
-		// proof is two-part. state items read in lexicographical order,
-		// and the secondary "proof" part.
-		let read_values = RefCell::new(BTreeSet::new());
-		let block_id = BlockId::Hash(hash.clone());
-		let proof = {
-			let call = |a, d| {
-				let tx = self.contract_call_tx(block_id, a, d);
-				let (result, items) = self.prove_transaction(tx, block_id)
-					.ok_or_else(|| format!("Unable to make call to generate epoch proof."))?;
+				let proof = match proof {
+					Proof::Known(proof) => proof,
+					Proof::WithState(with_state) =>
+						match self.with_proving_caller(BlockId::Hash(hash), move |c| with_state(c)) {
+							Ok(proof) => proof,
+							Err(e) => {
+								warn!(target: "client", "Failed to generate transition proof for block {}: {}", hash, e);
+								warn!(target: "client", "Snapshots produced by this client may be incomplete");
 
-				read_values.borrow_mut().extend(items);
-				Ok(result)
-			};
+								Vec::new()
+							}
+						},
+				};
 
-			self.engine.epoch_proof(&header, &call)
-		};
+				debug!(target: "client", "Block {} signals epoch end.", hash);
 
-		// insert into database, using the generated proof.
-		match proof {
-			Ok(proof) => {
-				chain.insert_epoch_transition(&mut batch, epoch_number, EpochTransition {
-					block_hash: hash.clone(),
-					block_number: header.number(),
-					proof: proof,
-					state_proof: read_values.into_inner().into_iter().collect(),
-				});
+				// write pending transition to DB.
+				let mut batch = DBTransaction::new();
+
+				let pending = PendingTransition { proof: proof };
+				chain.insert_pending_transition(&mut batch, hash, pending);
 
 				self.db.read().write_buffered(batch);
-			}
-			Err(e) => {
-				warn!(target: "client", "Error generating epoch change proof for block {}: {}", hash, e);
-				warn!(target: "client", "Snapshots generated by this node will be incomplete.");
+			},
+			EpochChange::No => {},
+			EpochChange::Unsure(_) => {
+				warn!(target: "client", "Detected invalid engine implementation.");
+				warn!(target: "client", "Engine claims to require more block data, but everything provided.");
 			}
 		}
 	}
 
+	// check for ending of epoch and write transition if it occurs.
+	fn check_epoch_end<'a>(&self, header: &'a Header, chain: &BlockChain) {
+		let is_epoch_end = self.engine.is_epoch_end(
+			header,
+			&(|hash| chain.block_header(&hash)),
+			&(|hash| chain.get_pending_transition(hash)), // TODO: limit to current epoch.
+		);
+
+		if let Some(proof) = is_epoch_end {
+			debug!(target: "client", "Epoch transition at block {}", header.hash());
+
+			let mut batch = DBTransaction::new();
+			chain.insert_epoch_transition(&mut batch, header.number(), EpochTransition {
+				block_hash: header.hash(),
+				block_number: header.number(),
+				proof: proof,
+			});
+			self.db.read().write_buffered(batch);
+		}
+	}
+
+	// use a state-proving closure for the given block.
+	fn with_proving_caller<F, T>(&self, id: BlockId, with_call: F) -> T
+		where F: FnOnce(&::engines::Call) -> T
+	{
+		let call = |a, d| {
+			let tx = self.contract_call_tx(id, a, d);
+			let (result, items) = self.prove_transaction(tx, id)
+				.ok_or_else(|| format!("Unable to make call. State unavailable?"))?;
+
+			let items = items.into_iter().map(|x| x.to_vec()).collect();
+			Ok((result, items))
+		};
+
+		with_call(&call)
+	}
+
 	// prune ancient states until below the memory limit or only the minimum amount remain.
 	fn prune_ancient(&self, mut state_db: StateDB, chain: &BlockChain) -> Result<(), ClientError> {
 		let number = match state_db.journal_db().latest_era() {
@@ -1622,17 +1668,21 @@ impl MiningBlockChainClient for Client {
 		let engine = &*self.engine;
 		let chain = self.chain.read();
 		let h = chain.best_block_hash();
+		let best_header = &chain.block_header(&h)
+			.expect("h is best block hash: so its header must exist: qed");
 
+		let is_epoch_begin = chain.epoch_transition(best_header.number(), h).is_some();
 		let mut open_block = OpenBlock::new(
 			engine,
 			self.factories.clone(),
 			false,	// TODO: this will need to be parameterised once we want to do immediate mining insertion.
 			self.state_db.lock().boxed_clone_canon(&h),
-			&chain.block_header(&h).expect("h is best block hash: so its header must exist: qed"),
+			best_header,
 			self.build_last_hashes(h.clone()),
 			author,
 			gas_range_target,
 			extra_data,
+			is_epoch_begin,
 		).expect("OpenBlock::new only fails if parent state root invalid; state root of best block's header is never invalid; qed");
 
 		// Add uncles
@@ -1717,6 +1767,10 @@ impl EngineClient for Client {
 	fn broadcast_consensus_message(&self, message: Bytes) {
 		self.notify(|notify| notify.broadcast(message.clone()));
 	}
+
+	fn epoch_transition_for(&self, parent_hash: H256) -> Option<::engines::EpochTransition> {
+		self.chain.read().epoch_transition_for(parent_hash)
+	}
 }
 
 impl ProvingBlockChainClient for Client {
@@ -1746,7 +1800,10 @@ impl ProvingBlockChainClient for Client {
 
 		match res {
 			Err(ExecutionError::Internal(_)) => None,
-			Err(_) => Some((Vec::new(), state.drop().1.extract_proof())),
+			Err(e) => {
+				trace!(target: "client", "Proved call failed: {}", e);
+				Some((Vec::new(), state.drop().1.extract_proof()))
+			}
 			Ok(res) => Some((res.output, state.drop().1.extract_proof())),
 		}
 	}

ethcore/src/client/test_client.rs

@@ -372,7 +372,8 @@ impl MiningBlockChainClient for TestBlockChainClient {
 			Arc::new(last_hashes),
 			author,
 			gas_range_target,
-			extra_data
+			extra_data,
+			false,
 		).expect("Opening block for tests will not fail.");
 		// TODO [todr] Override timestamp for predictability (set_timestamp_now kind of sucks)
 		open_block.set_timestamp(*self.latest_block_timestamp.read());
@@ -786,4 +787,8 @@ impl EngineClient for TestBlockChainClient {
 	}
 
 	fn broadcast_consensus_message(&self, _message: Bytes) {}
+
+	fn epoch_transition_for(&self, _block_hash: H256) -> Option<::engines::EpochTransition> {
+		None
+	}
 }

ethcore/src/client/traits.rs

@@ -311,6 +311,13 @@ pub trait EngineClient: MiningBlockChainClient {
 
 	/// Broadcast a consensus message to the network.
 	fn broadcast_consensus_message(&self, message: Bytes);
+
+	/// Get the transition to the epoch the given parent hash is part of
+	/// or transitions to.
+	/// This will give the epoch that any children of this parent belong to.
+	///
+	/// The block corresponding the the parent hash must be stored already.
+	fn epoch_transition_for(&self, parent_hash: H256) -> Option<::engines::EpochTransition>;
 }
 
 /// Extended client interface for providing proofs of the state.

ethcore/src/engines/authority_round/finality.rs

@@ -0,0 +1,187 @@
+// 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/>.
+
+//! Finality proof generation and checking.
+
+use std::collections::{VecDeque};
+use std::collections::hash_map::{HashMap, Entry};
+
+use util::{Address, H256};
+
+use engines::validator_set::SimpleList;
+
+/// Error indicating unknown validator.
+#[derive(Debug, PartialEq, Eq, Clone, Copy)]
+pub struct UnknownValidator;
+
+/// Rolling finality checker for authority round consensus.
+/// Stores a chain of unfinalized hashes that can be pushed onto.
+pub struct RollingFinality {
+	headers: VecDeque<(H256, Address)>,
+	signers: SimpleList,
+	sign_count: HashMap<Address, usize>,
+	last_pushed: Option<H256>,
+}
+
+impl RollingFinality {
+	/// Create a blank finality checker under the given validator set.
+	pub fn blank(signers: Vec<Address>) -> Self {
+		RollingFinality {
+			headers: VecDeque::new(),
+			signers: SimpleList::new(signers),
+			sign_count: HashMap::new(),
+			last_pushed: None,
+		}
+	}
+
+	/// Extract unfinalized subchain from ancestry iterator.
+	/// Clears the current subchain.
+	///
+	/// Fails if any provided signature isn't part of the signers set.
+	pub fn build_ancestry_subchain<I>(&mut self, iterable: I) -> Result<(), UnknownValidator>
+		where I: IntoIterator<Item=(H256, Address)>
+	{
+		self.clear();
+		for (hash, signer) in iterable {
+			if !self.signers.contains(&signer) { return Err(UnknownValidator) }
+			if self.last_pushed.is_none() { self.last_pushed = Some(hash) }
+
+			// break when we've got our first finalized block.
+			{
+				let current_signed = self.sign_count.len();
+				let would_be_finalized = (current_signed + 1) * 2 > self.signers.len();
+
+				let entry = self.sign_count.entry(signer);
+				if let (true, &Entry::Vacant(_)) = (would_be_finalized, &entry) {
+					break
+				}
+
+				*entry.or_insert(0) += 1;
+			}
+
+			self.headers.push_front((hash, signer));
+		}
+
+		Ok(())
+	}
+
+	/// Clear the finality status, but keeps the validator set.
+	pub fn clear(&mut self) {
+		self.headers.clear();
+		self.sign_count.clear();
+		self.last_pushed = None;
+	}
+
+	/// Returns the last pushed hash.
+	pub fn subchain_head(&self) -> Option<H256> {
+		self.last_pushed
+	}
+
+	/// Get an iterator over stored hashes in order.
+	pub fn unfinalized_hashes(&self) -> Iter { Iter(self.headers.iter()) }
+
+	/// Get the validator set.
+	pub fn validators(&self) -> &SimpleList { &self.signers }
+
+	/// Push a hash onto the rolling finality checker (implying `subchain_head` == head.parent)
+	///
+	/// Fails if `signer` isn't a member of the active validator set.
+	/// Returns a list of all newly finalized headers.
+	// TODO: optimize with smallvec.
+	pub fn push_hash(&mut self, head: H256, signer: Address) -> Result<Vec<H256>, UnknownValidator> {
+		if !self.signers.contains(&signer) { return Err(UnknownValidator) }
+
+		self.headers.push_back((head, signer));
+		*self.sign_count.entry(signer).or_insert(0) += 1;
+
+		let mut newly_finalized = Vec::new();
+
+		while self.sign_count.len() * 2 > self.signers.len() {
+			let (hash, signer) = self.headers.pop_front()
+				.expect("headers length always greater than sign count length; qed");
+
+			newly_finalized.push(hash);
+
+			match self.sign_count.entry(signer) {
+				Entry::Occupied(mut entry) => {
+					// decrement count for this signer and purge on zero.
+					*entry.get_mut() -= 1;
+
+					if *entry.get() == 0 {
+						entry.remove();
+					}
+				}
+				Entry::Vacant(_) => panic!("all hashes in `header` should have an entry in `sign_count` for their signer; qed"),
+			}
+		}
+
+		Ok(newly_finalized)
+	}
+}
+
+pub struct Iter<'a>(::std::collections::vec_deque::Iter<'a, (H256, Address)>);
+
+impl<'a> Iterator for Iter<'a> {
+	type Item = H256;
+
+	fn next(&mut self) -> Option<H256> {
+		self.0.next().map(|&(h, _)| h)
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use util::{Address, H256};
+	use super::RollingFinality;
+
+	#[test]
+	fn rejects_unknown_signer() {
+		let signers = (0..3).map(|_| Address::random()).collect();
+		let mut finality = RollingFinality::blank(signers);
+		assert!(finality.push_hash(H256::random(), Address::random()).is_err());
+	}
+
+	#[test]
+	fn finalize_multiple() {
+		let signers: Vec<_> = (0..6).map(|_| Address::random()).collect();
+
+		let mut finality = RollingFinality::blank(signers.clone());
+		let hashes: Vec<_> = (0..7).map(|_| H256::random()).collect();
+
+		// 3 / 6 signers is < 51% so no finality.
+		for (i, hash) in hashes.iter().take(6).cloned().enumerate() {
+			let i = i % 3;
+			assert!(finality.push_hash(hash, signers[i]).unwrap().len() == 0);
+		}
+
+		// after pushing a block signed by a fourth validator, the first four
+		// blocks of the unverified chain become verified.
+		assert_eq!(finality.push_hash(hashes[6], signers[4]).unwrap(),
+			vec![hashes[0], hashes[1], hashes[2], hashes[3]]);
+	}
+
+	#[test]
+	fn from_ancestry() {
+		let signers: Vec<_> = (0..6).map(|_| Address::random()).collect();
+		let hashes: Vec<_> = (0..12).map(|i| (H256::random(), signers[i % 6])).collect();
+
+		let mut finality = RollingFinality::blank(signers.clone());
+		finality.build_ancestry_subchain(hashes.iter().rev().cloned()).unwrap();
+
+		assert_eq!(finality.unfinalized_hashes().count(), 3);
+		assert_eq!(finality.subchain_head(), Some(hashes[11].0));
+	}
+}

ethcore/src/engines/authority_round/mod.rs

@@ -19,24 +19,32 @@
 use std::sync::atomic::{AtomicUsize, AtomicBool, Ordering as AtomicOrdering};
 use std::sync::Weak;
 use std::time::{UNIX_EPOCH, Duration};
-use util::*;
-use ethkey::{verify_address, Signature};
-use rlp::{UntrustedRlp, encode};
+
 use account_provider::AccountProvider;
 use block::*;
-use spec::CommonParams;
-use engines::{Call, Engine, Seal, EngineError};
-use header::Header;
+use builtin::Builtin;
+use client::{Client, EngineClient};
+use engines::{Call, Engine, Seal, EngineError, ConstructedVerifier};
 use error::{Error, TransactionError, BlockError};
 use ethjson;
-use io::{IoContext, IoHandler, TimerToken, IoService};
-use builtin::Builtin;
-use transaction::UnverifiedTransaction;
-use client::{Client, EngineClient};
+use header::{Header, BlockNumber};
+use spec::CommonParams;
 use state::CleanupMode;
+use transaction::UnverifiedTransaction;
+
 use super::signer::EngineSigner;
 use super::validator_set::{ValidatorSet, SimpleList, new_validator_set};
 
+use self::finality::RollingFinality;
+
+use ethkey::{verify_address, Signature};
+use io::{IoContext, IoHandler, TimerToken, IoService};
+use itertools::{self, Itertools};
+use rlp::{UntrustedRlp, encode};
+use util::*;
+
+mod finality;
+
 /// `AuthorityRound` params.
 pub struct AuthorityRoundParams {
 	/// Gas limit divisor.
@@ -57,6 +65,8 @@ pub struct AuthorityRoundParams {
 	pub eip155_transition: u64,
 	/// Monotonic step validation transition block.
 	pub validate_step_transition: u64,
+	/// Immediate transitions.
+	pub immediate_transitions: bool,
 }
 
 impl From<ethjson::spec::AuthorityRoundParams> for AuthorityRoundParams {
@@ -71,6 +81,7 @@ impl From<ethjson::spec::AuthorityRoundParams> for AuthorityRoundParams {
 			validate_score_transition: p.validate_score_transition.map_or(0, Into::into),
 			eip155_transition: p.eip155_transition.map_or(0, Into::into),
 			validate_step_transition: p.validate_step_transition.map_or(0, Into::into),
+			immediate_transitions: p.immediate_transitions.unwrap_or(false),
 		}
 	}
 }
@@ -114,6 +125,92 @@ impl Step {
 	}
 }
 
+struct EpochManager {
+	epoch_transition_hash: H256,
+	epoch_transition_number: BlockNumber,
+	finality_checker: RollingFinality,
+	force: bool,
+}
+
+impl EpochManager {
+	fn blank() -> Self {
+		EpochManager {
+			epoch_transition_hash: H256::default(),
+			epoch_transition_number: 0,
+			finality_checker: RollingFinality::blank(Vec::new()),
+			force: true,
+		}
+	}
+
+	// zoom to epoch for given header. returns true if succeeded, false otherwise.
+	fn zoom_to(&mut self, client: &EngineClient, engine: &Engine, validators: &ValidatorSet, header: &Header) -> bool {
+		let last_was_parent = self.finality_checker.subchain_head() == Some(header.parent_hash().clone());
+
+		// early exit for current target == chain head, but only if the epochs are
+		// the same.
+		if last_was_parent && !self.force {
+			return true;
+		}
+
+		self.force = false;
+		debug!(target: "engine", "Zooming to epoch for block {}", header.hash());
+
+		// epoch_transition_for can be an expensive call, but in the absence of
+		// forks it will only need to be called for the block directly after
+		// epoch transition, in which case it will be O(1) and require a single
+		// DB lookup.
+		let last_transition = match client.epoch_transition_for(*header.parent_hash()) {
+			Some(t) => t,
+			None => {
+				// this really should never happen unless the block passed
+				// hasn't got a parent in the database.
+				debug!(target: "engine", "No genesis transition found.");
+				return false;
+			}
+		};
+
+
+		// extract other epoch set if it's not the same as the last.
+		if last_transition.block_hash != self.epoch_transition_hash {
+			let (signal_number, set_proof, _) = destructure_proofs(&last_transition.proof)
+				.expect("proof produced by this engine; therefore it is valid; qed");
+
+			trace!(target: "engine", "extracting epoch set for epoch ({}, {}) signalled at #{}",
+				last_transition.block_number, last_transition.block_hash, signal_number);
+
+			let first = signal_number == 0;
+			let epoch_set = validators.epoch_set(
+				first,
+				engine,
+				signal_number, // use signal number so multi-set first calculation is correct.
+				set_proof,
+			)
+				.ok()
+				.map(|(list, _)| list.into_inner())
+				.expect("proof produced by this engine; therefore it is valid; qed");
+
+			self.finality_checker = RollingFinality::blank(epoch_set);
+		}
+
+		self.epoch_transition_hash = last_transition.block_hash;
+		self.epoch_transition_number = last_transition.block_number;
+
+		true
+	}
+
+	// note new epoch hash. this will force the next block to re-load
+	// the epoch set
+	// TODO: optimize and don't require re-loading after epoch change.
+	fn note_new_epoch(&mut self) {
+		self.force = true;
+	}
+
+	/// Get validator set. Zoom to the correct epoch first.
+	fn validators(&self) -> &SimpleList {
+		self.finality_checker.validators()
+	}
+}
+
 /// Engine using `AuthorityRound` proof-of-authority BFT consensus.
 pub struct AuthorityRound {
 	params: CommonParams,
@@ -130,22 +227,63 @@ pub struct AuthorityRound {
 	validate_score_transition: u64,
 	eip155_transition: u64,
 	validate_step_transition: u64,
+	epoch_manager: Mutex<EpochManager>,
+	immediate_transitions: bool,
 }
 
 // header-chain validator.
 struct EpochVerifier {
-	epoch_number: u64,
 	step: Arc<Step>,
 	subchain_validators: SimpleList,
 }
 
 impl super::EpochVerifier for EpochVerifier {
-	fn epoch_number(&self) -> u64 { self.epoch_number.clone() }
 	fn verify_light(&self, header: &Header) -> Result<(), Error> {
 		// always check the seal since it's fast.
 		// nothing heavier to do.
-		verify_external(header, &self.subchain_validators, &*self.step)
+		verify_external(header, &self.subchain_validators, &*self.step, |_| {})
 	}
+
+	fn check_finality_proof(&self, proof: &[u8]) -> Option<Vec<H256>> {
+		macro_rules! otry {
+			($e: expr) => {
+				match $e {
+					Some(x) => x,
+					None => return None,
+				}
+			}
+		}
+
+		let mut finality_checker = RollingFinality::blank(self.subchain_validators.clone().into_inner());
+		let mut finalized = Vec::new();
+
+		let headers: Vec<Header> = otry!(UntrustedRlp::new(proof).as_list().ok());
+
+
+		for header in &headers {
+			// ensure all headers have correct number of seal fields so we can `verify_external`
+			// without panic.
+			//
+			// `verify_external` checks that signature is correct and author == signer.
+			if header.seal().len() != 2 { return None }
+			otry!(verify_external(header, &self.subchain_validators, &*self.step, |_| {}).ok());
+
+			let newly_finalized = otry!(finality_checker.push_hash(header.hash(), header.author().clone()).ok());
+			finalized.extend(newly_finalized);
+		}
+
+		if finalized.is_empty() { None } else { Some(finalized) }
+	}
+}
+
+// Report misbehavior
+#[derive(Debug)]
+#[allow(dead_code)]
+enum Report {
+	// Malicious behavior
+	Malicious(Address, BlockNumber, Bytes),
+	// benign misbehavior
+	Benign(Address, BlockNumber),
 }
 
 fn header_step(header: &Header) -> Result<usize, ::rlp::DecoderError> {
@@ -156,13 +294,25 @@ fn header_signature(header: &Header) -> Result<Signature, ::rlp::DecoderError> {
 	UntrustedRlp::new(&header.seal().get(1).expect("was checked with verify_block_basic; has 2 fields; qed")).as_val::<H520>().map(Into::into)
 }
 
-fn verify_external(header: &Header, validators: &ValidatorSet, step: &Step) -> Result<(), Error> {
+fn step_proposer(validators: &ValidatorSet, bh: &H256, step: usize) -> Address {
+	let proposer = validators.get(bh, step);
+	trace!(target: "engine", "Fetched proposer for step {}: {}", step, proposer);
+	proposer
+}
+
+fn is_step_proposer(validators: &ValidatorSet, bh: &H256, step: usize, address: &Address) -> bool {
+	step_proposer(validators, bh, step) == *address
+}
+
+fn verify_external<F: Fn(Report)>(header: &Header, validators: &ValidatorSet, step: &Step, report: F)
+	-> Result<(), Error>
+{
 	let header_step = header_step(header)?;
 
 	// Give one step slack if step is lagging, double vote is still not possible.
 	if step.is_future(header_step) {
 		trace!(target: "engine", "verify_block_unordered: block from the future");
-		validators.report_benign(header.author(), header.number());
+		report(Report::Benign(*header.author(), header.number()));
 		Err(BlockError::InvalidSeal)?
 	} else {
 		let proposer_signature = header_signature(header)?;
@@ -179,6 +329,21 @@ fn verify_external(header: &Header, validators: &ValidatorSet, step: &Step) -> R
 	}
 }
 
+fn combine_proofs(signal_number: BlockNumber, set_proof: &[u8], finality_proof: &[u8]) -> Vec<u8> {
+	let mut stream = ::rlp::RlpStream::new_list(3);
+	stream.append(&signal_number).append(&set_proof).append(&finality_proof);
+	stream.out()
+}
+
+fn destructure_proofs(combined: &[u8]) -> Result<(BlockNumber, &[u8], &[u8]), Error> {
+	let rlp = UntrustedRlp::new(combined);
+	Ok((
+		rlp.at(0)?.as_val()?,
+		rlp.at(1)?.data()?,
+		rlp.at(2)?.data()?,
+	))
+}
+
 trait AsMillis {
 	fn as_millis(&self) -> u64;
 }
@@ -214,7 +379,10 @@ impl AuthorityRound {
 				validate_score_transition: our_params.validate_score_transition,
 				eip155_transition: our_params.eip155_transition,
 				validate_step_transition: our_params.validate_step_transition,
+				epoch_manager: Mutex::new(EpochManager::blank()),
+				immediate_transitions: our_params.immediate_transitions,
 			});
+
 		// Do not initialize timeouts for tests.
 		if should_timeout {
 			let handler = TransitionHandler { engine: Arc::downgrade(&engine) };
@@ -222,14 +390,6 @@ impl AuthorityRound {
 		}
 		Ok(engine)
 	}
-
-	fn step_proposer(&self, bh: &H256, step: usize) -> Address {
-		self.validators.get(bh, step)
-	}
-
-	fn is_step_proposer(&self, bh: &H256, step: usize, address: &Address) -> bool {
-		self.step_proposer(bh, step) == *address
-	}
 }
 
 fn unix_now() -> Duration {
@@ -325,7 +485,33 @@ impl Engine for AuthorityRound {
 
 		let header = block.header();
 		let step = self.step.load();
-		if self.is_step_proposer(header.parent_hash(), step, header.author()) {
+
+		// fetch correct validator set for current epoch, taking into account
+		// finality of previous transitions.
+		let active_set;
+
+		let validators = if self.immediate_transitions {
+			&*self.validators
+		} else {
+			let mut epoch_manager = self.epoch_manager.lock();
+			let client = match self.client.read().as_ref().and_then(|weak| weak.upgrade()) {
+				Some(client) => client,
+				None => {
+					warn!(target: "engine", "Unable to generate seal: missing client ref.");
+					return Seal::None;
+				}
+			};
+
+			if !epoch_manager.zoom_to(&*client, self, &*self.validators, header) {
+				debug!(target: "engine", "Unable to zoom to epoch.");
+				return Seal::None;
+			}
+
+			active_set = epoch_manager.validators().clone();
+			&active_set as &_
+		};
+
+		if is_step_proposer(validators, header.parent_hash(), step, header.author()) {
 			if let Ok(signature) = self.signer.sign(header.bare_hash()) {
 				trace!(target: "engine", "generate_seal: Issuing a block for step {}.", step);
 
@@ -337,11 +523,43 @@ impl Engine for AuthorityRound {
 				warn!(target: "engine", "generate_seal: FAIL: Accounts secret key unavailable.");
 			}
 		} else {
-			trace!(target: "engine", "generate_seal: Not a proposer for step {}.", step);
+			trace!(target: "engine", "generate_seal: {} not a proposer for step {}.",
+				header.author(), step);
 		}
 		Seal::None
 	}
 
+	fn on_new_block(
+		&self,
+		block: &mut ExecutedBlock,
+		last_hashes: Arc<::env_info::LastHashes>,
+		epoch_begin: bool,
+	) -> Result<(), Error> {
+		let parent_hash = block.fields().header.parent_hash().clone();
+		::engines::common::push_last_hash(block, last_hashes.clone(), self, &parent_hash)?;
+
+		if !epoch_begin { return Ok(()) }
+
+		// genesis is never a new block, but might as well check.
+		let header = block.fields().header.clone();
+		let first = header.number() == 0;
+
+		let mut call = |to, data| {
+			let result = ::engines::common::execute_as_system(
+				block,
+				last_hashes.clone(),
+				self,
+				to,
+				U256::max_value(), // unbounded gas? maybe make configurable.
+				Some(data),
+			);
+
+			result.map_err(|e| format!("{}", e))
+		};
+
+		self.validators.on_epoch_begin(first, &header, &mut call)
+	}
+
 	/// Apply the block reward on finalisation of the block.
 	fn on_close_block(&self, block: &mut ExecutedBlock) -> Result<(), Error> {
 		let fields = block.fields_mut();
@@ -386,19 +604,26 @@ impl Engine for AuthorityRound {
 		}
 
 		let parent_step = header_step(parent)?;
+
 		// Ensure header is from the step after parent.
 		if step == parent_step
 			|| (header.number() >= self.validate_step_transition && step <= parent_step) {
 			trace!(target: "engine", "Multiple blocks proposed for step {}.", parent_step);
-			self.validators.report_malicious(header.author(), header.number(), Default::default());
+
+			self.validators.report_malicious(header.author(), header.number(), header.number(), Default::default());
 			Err(EngineError::DoubleVote(header.author().clone()))?;
 		}
 		// Report skipped primaries.
 		if step > parent_step + 1 {
+			// TODO: use epochmanager to get correct validator set for reporting?
+			// or just rely on the fact that in general these will be the same
+			// and some reports might go missing?
+			trace!(target: "engine", "Author {} built block with step gap. current step: {}, parent step: {}",
+				header.author(), step, parent_step);
+
 			for s in parent_step + 1..step {
-				let skipped_primary = self.step_proposer(&parent.hash(), s);
-				trace!(target: "engine", "Author {} did not build his block on top of the intermediate designated primary {}.", header.author(), skipped_primary);
-				self.validators.report_benign(&skipped_primary, header.number());
+				let skipped_primary = step_proposer(&*self.validators, &parent.hash(), s);
+				self.validators.report_benign(&skipped_primary, header.number(), header.number());
 			}
 		}
 
@@ -413,30 +638,164 @@ impl Engine for AuthorityRound {
 
 	// Check the validators.
 	fn verify_block_external(&self, header: &Header, _block: Option<&[u8]>) -> Result<(), Error> {
-		verify_external(header, &*self.validators, &*self.step)
+		// fetch correct validator set for current epoch, taking into account
+		// finality of previous transitions.
+		let active_set;
+
+		let (validators, set_number) = if self.immediate_transitions {
+			(&*self.validators, header.number())
+		} else {
+			// get correct validator set for epoch.
+			let client = match self.client.read().as_ref().and_then(|weak| weak.upgrade()) {
+				Some(client) => client,
+				None => {
+					debug!(target: "engine", "Unable to verify sig: missing client ref.");
+					return Err(EngineError::RequiresClient.into())
+				}
+			};
+
+			let mut epoch_manager = self.epoch_manager.lock();
+			if !epoch_manager.zoom_to(&*client, self, &*self.validators, header) {
+				debug!(target: "engine", "Unable to zoom to epoch.");
+				return Err(EngineError::RequiresClient.into())
+			}
+
+			active_set = epoch_manager.validators().clone();
+			(&active_set as &_, epoch_manager.epoch_transition_number)
+		};
+
+		let report = |report| match report {
+			Report::Benign(address, block_number) =>
+				self.validators.report_benign(&address, set_number, block_number),
+			Report::Malicious(address, block_number, proof) =>
+				self.validators.report_malicious(&address, set_number, block_number, proof),
+		};
+
+		// verify signature against fixed list, but reports should go to the
+		// contract itself.
+		verify_external(header, validators, &*self.step, report)
 	}
 
-	// the proofs we need just allow us to get the full validator set.
-	fn epoch_proof(&self, header: &Header, caller: &Call) -> Result<Bytes, Error> {
-		self.validators.epoch_proof(header, caller)
-			.map_err(|e| EngineError::InsufficientProof(e).into())
+	fn genesis_epoch_data(&self, header: &Header, call: &Call) -> Result<Vec<u8>, String> {
+		self.validators.genesis_epoch_data(header, call)
+			.map(|set_proof| combine_proofs(0, &set_proof, &[]))
 	}
 
-	fn is_epoch_end(&self, header: &Header, block: Option<&[u8]>, receipts: Option<&[::receipt::Receipt]>)
+	fn signals_epoch_end(&self, header: &Header, block: Option<&[u8]>, receipts: Option<&[::receipt::Receipt]>)
 		-> super::EpochChange
 	{
-		self.validators.is_epoch_end(header, block, receipts)
+		if self.immediate_transitions { return super::EpochChange::No }
+
+		let first = header.number() == 0;
+		self.validators.signals_epoch_end(first, header, block, receipts)
 	}
 
-	fn epoch_verifier(&self, header: &Header, proof: &[u8]) -> Result<Box<super::EpochVerifier>, Error> {
-		// extract a simple list from the proof.
-		let (num, simple_list) = self.validators.epoch_set(header, proof)?;
+	fn is_epoch_end(
+		&self,
+		chain_head: &Header,
+		chain: &super::Headers,
+		transition_store: &super::PendingTransitionStore,
+	) -> Option<Vec<u8>> {
+		// epochs only matter if we want to support light clients.
+		if self.immediate_transitions { return None }
 
-		Ok(Box::new(EpochVerifier {
-			epoch_number: num,
-			step: self.step.clone(),
-			subchain_validators: simple_list,
-		}))
+		let first = chain_head.number() == 0;
+
+		// apply immediate transitions.
+		if let Some(change) = self.validators.is_epoch_end(first, chain_head) {
+			return Some(change)
+		}
+
+		let client = match self.client.read().as_ref().and_then(|weak| weak.upgrade()) {
+			Some(client) => client,
+			None => {
+				warn!(target: "engine", "Unable to check for epoch end: missing client ref.");
+				return None;
+			}
+		};
+
+		// find most recently finalized blocks, then check transition store for pending transitions.
+		let mut epoch_manager = self.epoch_manager.lock();
+		if !epoch_manager.zoom_to(&*client, self, &*self.validators, chain_head) {
+			return None;
+		}
+
+		if epoch_manager.finality_checker.subchain_head() != Some(*chain_head.parent_hash()) {
+			// build new finality checker from ancestry of chain head,
+			// not including chain head itself yet.
+			let mut hash = chain_head.parent_hash().clone();
+			let epoch_transition_hash = epoch_manager.epoch_transition_hash;
+
+			// walk the chain within current epoch backwards.
+			// author == ec_recover(sig) known since
+			// the blocks are in the DB.
+			let ancestry_iter = itertools::repeat_call(move || {
+				chain(hash).and_then(|header| {
+					if header.number() == 0 { return None }
+
+					let res = (hash, header.author().clone());
+					hash = header.parent_hash().clone();
+					Some(res)
+				})
+			})
+				.while_some()
+				.take_while(|&(h, _)| h != epoch_transition_hash);
+
+			if let Err(_) = epoch_manager.finality_checker.build_ancestry_subchain(ancestry_iter) {
+				debug!(target: "engine", "inconsistent validator set within epoch");
+				return None;
+			}
+		}
+
+		{
+			if let Ok(finalized) = epoch_manager.finality_checker.push_hash(chain_head.hash(), *chain_head.author()) {
+				let mut finalized = finalized.into_iter();
+				while let Some(hash) = finalized.next() {
+					if let Some(pending) = transition_store(hash) {
+						let finality_proof = ::std::iter::once(hash)
+							.chain(finalized)
+							.chain(epoch_manager.finality_checker.unfinalized_hashes())
+							.map(|hash| chain(hash)
+								.expect("these headers fetched before when constructing finality checker; qed"))
+							.collect::<Vec<Header>>();
+
+						// this gives us the block number for `hash`, assuming it's ancestry.
+						let signal_number = chain_head.number()
+							- finality_proof.len() as BlockNumber
+							+ 1;
+						let finality_proof = ::rlp::encode_list(&finality_proof);
+						epoch_manager.note_new_epoch();
+
+						return Some(combine_proofs(signal_number, &pending.proof, &*finality_proof));
+					}
+				}
+			}
+		}
+
+		None
+	}
+
+	fn epoch_verifier<'a>(&self, _header: &Header, proof: &'a [u8]) -> ConstructedVerifier<'a> {
+		let (signal_number, set_proof, finality_proof) = match destructure_proofs(proof) {
+			Ok(x) => x,
+			Err(e) => return ConstructedVerifier::Err(e),
+		};
+
+		let first = signal_number == 0;
+		match self.validators.epoch_set(first, self, signal_number, set_proof) {
+			Ok((list, finalize)) => {
+				let verifier = Box::new(EpochVerifier {
+					step: self.step.clone(),
+					subchain_validators: list,
+				});
+
+				match finalize {
+					Some(finalize) => ConstructedVerifier::Unconfirmed(verifier, finality_proof, finalize),
+					None => ConstructedVerifier::Trusted(verifier),
+				}
+			}
+			Err(e) => ConstructedVerifier::Err(e),
+		}
 	}
 
 	fn verify_transaction_basic(&self, t: &UnverifiedTransaction, header: &Header) -> result::Result<(), Error> {
@@ -465,7 +824,11 @@ impl Engine for AuthorityRound {
 	}
 
 	fn snapshot_components(&self) -> Option<Box<::snapshot::SnapshotComponents>> {
-		Some(Box::new(::snapshot::PoaSnapshot))
+		if self.immediate_transitions {
+			None
+		} else {
+			Some(Box::new(::snapshot::PoaSnapshot))
+		}
 	}
 }
 
@@ -535,9 +898,9 @@ mod tests {
 		let db1 = spec.ensure_db_good(get_temp_state_db(), &Default::default()).unwrap();
 		let db2 = spec.ensure_db_good(get_temp_state_db(), &Default::default()).unwrap();
 		let last_hashes = Arc::new(vec![genesis_header.hash()]);
-		let b1 = OpenBlock::new(engine, Default::default(), false, db1, &genesis_header, last_hashes.clone(), addr1, (3141562.into(), 31415620.into()), vec![]).unwrap();
+		let b1 = OpenBlock::new(engine, Default::default(), false, db1, &genesis_header, last_hashes.clone(), addr1, (3141562.into(), 31415620.into()), vec![], false).unwrap();
 		let b1 = b1.close_and_lock();
-		let b2 = OpenBlock::new(engine, Default::default(), false, db2, &genesis_header, last_hashes, addr2, (3141562.into(), 31415620.into()), vec![]).unwrap();
+		let b2 = OpenBlock::new(engine, Default::default(), false, db2, &genesis_header, last_hashes, addr2, (3141562.into(), 31415620.into()), vec![], false).unwrap();
 		let b2 = b2.close_and_lock();
 
 		engine.set_signer(tap.clone(), addr1, "1".into());
@@ -643,6 +1006,7 @@ mod tests {
 			validate_score_transition: 0,
 			validate_step_transition: 0,
 			eip155_transition: 0,
+			immediate_transitions: true,
 		};
 		let aura = AuthorityRound::new(Default::default(), params, Default::default()).unwrap();
 

ethcore/src/engines/basic_authority.rs

@@ -23,7 +23,7 @@ use account_provider::AccountProvider;
 use block::*;
 use builtin::Builtin;
 use spec::CommonParams;
-use engines::{Engine, EngineError, Seal, Call, EpochChange};
+use engines::{Engine, Seal, Call, ConstructedVerifier, EngineError};
 use error::{BlockError, Error};
 use evm::Schedule;
 use ethjson;
@@ -51,12 +51,10 @@ impl From<ethjson::spec::BasicAuthorityParams> for BasicAuthorityParams {
 }
 
 struct EpochVerifier {
-	epoch_number: u64,
 	list: SimpleList,
 }
 
 impl super::EpochVerifier for EpochVerifier {
-	fn epoch_number(&self) -> u64 { self.epoch_number.clone() }
 	fn verify_light(&self, header: &Header) -> Result<(), Error> {
 		verify_external(header, &self.list)
 	}
@@ -187,26 +185,54 @@ impl Engine for BasicAuthority {
 		verify_external(header, &*self.validators)
 	}
 
-	// the proofs we need just allow us to get the full validator set.
-	fn epoch_proof(&self, header: &Header, caller: &Call) -> Result<Bytes, Error> {
-		self.validators.epoch_proof(header, caller)
-			.map_err(|e| EngineError::InsufficientProof(e).into())
+	fn genesis_epoch_data(&self, header: &Header, call: &Call) -> Result<Vec<u8>, String> {
+		self.validators.genesis_epoch_data(header, call)
 	}
 
-	fn is_epoch_end(&self, header: &Header, block: Option<&[u8]>, receipts: Option<&[::receipt::Receipt]>)
-		-> EpochChange
+	#[cfg(not(test))]
+	fn signals_epoch_end(&self, _header: &Header, _block: Option<&[u8]>, _receipts: Option<&[::receipt::Receipt]>)
+		-> super::EpochChange
 	{
-		self.validators.is_epoch_end(header, block, receipts)
+		// don't bother signalling even though a contract might try.
+		super::EpochChange::No
 	}
 
-	fn epoch_verifier(&self, header: &Header, proof: &[u8]) -> Result<Box<super::EpochVerifier>, Error> {
-		// extract a simple list from the proof.
-		let (num, simple_list) = self.validators.epoch_set(header, proof)?;
+	#[cfg(test)]
+	fn signals_epoch_end(&self, header: &Header, block: Option<&[u8]>, receipts: Option<&[::receipt::Receipt]>)
+		-> super::EpochChange
+	{
+		// in test mode, always signal even though they don't be finalized.
+		let first = header.number() == 0;
+		self.validators.signals_epoch_end(first, header, block, receipts)
+	}
 
-		Ok(Box::new(EpochVerifier {
-			epoch_number: num,
-			list: simple_list,
-		}))
+	fn is_epoch_end(
+		&self,
+		chain_head: &Header,
+		_chain: &super::Headers,
+		_transition_store: &super::PendingTransitionStore,
+	) -> Option<Vec<u8>> {
+		let first = chain_head.number() == 0;
+
+		// finality never occurs so only apply immediate transitions.
+		self.validators.is_epoch_end(first, chain_head)
+	}
+
+	fn epoch_verifier<'a>(&self, header: &Header, proof: &'a [u8]) -> ConstructedVerifier<'a> {
+		let first = header.number() == 0;
+
+		match self.validators.epoch_set(first, self, header.number(), proof) {
+			Ok((list, finalize)) => {
+				let verifier = Box::new(EpochVerifier { list: list });
+
+				// our epoch verifier will ensure no unverified verifier is ever verified.
+				match finalize {
+					Some(finalize) => ConstructedVerifier::Unconfirmed(verifier, proof, finalize),
+					None => ConstructedVerifier::Trusted(verifier),
+				}
+			}
+			Err(e) => ConstructedVerifier::Err(e),
+		}
 	}
 
 	fn register_client(&self, client: Weak<Client>) {
@@ -222,7 +248,7 @@ impl Engine for BasicAuthority {
 	}
 
 	fn snapshot_components(&self) -> Option<Box<::snapshot::SnapshotComponents>> {
-		Some(Box::new(::snapshot::PoaSnapshot))
+		None
 	}
 }
 
@@ -292,7 +318,7 @@ mod tests {
 		let genesis_header = spec.genesis_header();
 		let db = spec.ensure_db_good(get_temp_state_db(), &Default::default()).unwrap();
 		let last_hashes = Arc::new(vec![genesis_header.hash()]);
-		let b = OpenBlock::new(engine, Default::default(), false, db, &genesis_header, last_hashes, addr, (3141562.into(), 31415620.into()), vec![]).unwrap();
+		let b = OpenBlock::new(engine, Default::default(), false, db, &genesis_header, last_hashes, addr, (3141562.into(), 31415620.into()), vec![], false).unwrap();
 		let b = b.close_and_lock();
 		if let Seal::Regular(seal) = engine.generate_seal(b.block()) {
 			assert!(b.try_seal(engine, seal).is_ok());

ethcore/src/engines/epoch.rs

@@ -0,0 +1,102 @@
+// 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/>.
+
+//! Epoch verifiers and transitions.
+
+use error::Error;
+use header::Header;
+
+use rlp::{Encodable, Decodable, DecoderError, RlpStream, UntrustedRlp};
+use util::H256;
+
+/// A full epoch transition.
+#[derive(Debug, Clone)]
+pub struct Transition {
+	/// Block hash at which the transition occurred.
+	pub block_hash: H256,
+	/// Block number at which the transition occurred.
+	pub block_number: u64,
+	/// "transition/epoch" proof from the engine combined with a finality proof.
+	pub proof: Vec<u8>,
+}
+
+impl Encodable for Transition {
+	fn rlp_append(&self, s: &mut RlpStream) {
+		s.begin_list(3)
+			.append(&self.block_hash)
+			.append(&self.block_number)
+			.append(&self.proof);
+	}
+}
+
+impl Decodable for Transition {
+	fn decode(rlp: &UntrustedRlp) -> Result<Self, DecoderError> {
+		Ok(Transition {
+			block_hash: rlp.val_at(0)?,
+			block_number: rlp.val_at(1)?,
+			proof: rlp.val_at(2)?,
+		})
+	}
+}
+
+/// An epoch transition pending a finality proof.
+/// Not all transitions need one.
+pub struct PendingTransition {
+	/// "transition/epoch" proof from the engine.
+	pub proof: Vec<u8>,
+}
+
+impl Encodable for PendingTransition {
+	fn rlp_append(&self, s: &mut RlpStream) {
+		s.append(&self.proof);
+	}
+}
+
+impl Decodable for PendingTransition {
+	fn decode(rlp: &UntrustedRlp) -> Result<Self, DecoderError> {
+		Ok(PendingTransition {
+			proof: rlp.as_val()?,
+		})
+	}
+}
+
+/// Verifier for all blocks within an epoch with self-contained state.
+///
+/// See docs on `Engine` relating to proving functions for more details.
+pub trait EpochVerifier: Send + Sync {
+	/// Lightly verify the next block header.
+	/// This may not be a header belonging to a different epoch.
+	fn verify_light(&self, header: &Header) -> Result<(), Error>;
+
+	/// Perform potentially heavier checks on the next block header.
+	fn verify_heavy(&self, header: &Header) -> Result<(), Error> {
+		self.verify_light(header)
+	}
+
+	/// Check a finality proof against this epoch verifier.
+	/// Returns `Some(hashes)` if the proof proves finality of these hashes.
+	/// Returns `None` if the proof doesn't prove anything.
+	fn check_finality_proof(&self, _proof: &[u8]) -> Option<Vec<H256>> {
+		None
+	}
+}
+
+/// Special "no-op" verifier for stateless, epoch-less engines.
+pub struct NoOp;
+
+impl EpochVerifier for NoOp {
+	fn verify_light(&self, _header: &Header) -> Result<(), Error> { Ok(()) }
+}

ethcore/src/engines/epoch_verifier.rs

@@ -1,45 +0,0 @@
-// 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/>.
-
-// Epoch verifiers.
-
-use error::Error;
-use header::Header;
-
-/// Verifier for all blocks within an epoch with self-contained state.
-///
-/// See docs on `Engine` relating to proving functions for more details.
-pub trait EpochVerifier: Send + Sync {
-	/// Get the epoch number.
-	fn epoch_number(&self) -> u64;
-
-	/// Lightly verify the next block header.
-	/// This may not be a header belonging to a different epoch.
-	fn verify_light(&self, header: &Header) -> Result<(), Error>;
-
-	/// Perform potentially heavier checks on the next block header.
-	fn verify_heavy(&self, header: &Header) -> Result<(), Error> {
-		self.verify_light(header)
-	}
-}
-
-/// Special "no-op" verifier for stateless, epoch-less engines.
-pub struct NoOp;
-
-impl EpochVerifier for NoOp {
-	fn epoch_number(&self) -> u64 { 0 }
-	fn verify_light(&self, _header: &Header) -> Result<(), Error> { Ok(()) }
-}

ethcore/src/engines/instant_seal.rs

@@ -79,7 +79,7 @@ mod tests {
 		let db = spec.ensure_db_good(get_temp_state_db(), &Default::default()).unwrap();
 		let genesis_header = spec.genesis_header();
 		let last_hashes = Arc::new(vec![genesis_header.hash()]);
-		let b = OpenBlock::new(engine, Default::default(), false, db, &genesis_header, last_hashes, Address::default(), (3141562.into(), 31415620.into()), vec![]).unwrap();
+		let b = OpenBlock::new(engine, Default::default(), false, db, &genesis_header, last_hashes, Address::default(), (3141562.into(), 31415620.into()), vec![], false).unwrap();
 		let b = b.close_and_lock();
 		if let Seal::Regular(seal) = engine.generate_seal(b.block()) {
 			assert!(b.try_seal(engine, seal).is_ok());

ethcore/src/engines/mod.rs

@@ -18,7 +18,6 @@
 
 mod authority_round;
 mod basic_authority;
-mod epoch_verifier;
 mod instant_seal;
 mod null_engine;
 mod signer;
@@ -27,15 +26,19 @@ mod transition;
 mod validator_set;
 mod vote_collector;
 
+pub mod epoch;
+
 pub use self::authority_round::AuthorityRound;
 pub use self::basic_authority::BasicAuthority;
-pub use self::epoch_verifier::EpochVerifier;
+pub use self::epoch::{EpochVerifier, Transition as EpochTransition};
 pub use self::instant_seal::InstantSeal;
 pub use self::null_engine::NullEngine;
 pub use self::tendermint::Tendermint;
 
 use std::sync::Weak;
 
+use self::epoch::PendingTransition;
+
 use account_provider::AccountProvider;
 use block::ExecutedBlock;
 use builtin::Builtin;
@@ -72,6 +75,10 @@ pub enum EngineError {
 	BadSealFieldSize(OutOfBounds<usize>),
 	/// Validation proof insufficient.
 	InsufficientProof(String),
+	/// Failed system call.
+	FailedSystemCall(String),
+	/// Requires client ref, but none registered.
+	RequiresClient,
 }
 
 impl fmt::Display for EngineError {
@@ -84,6 +91,8 @@ impl fmt::Display for EngineError {
 			UnexpectedMessage => "This Engine should not be fed messages.".into(),
 			BadSealFieldSize(ref oob) => format!("Seal field has an unexpected length: {}", oob),
 			InsufficientProof(ref msg) => format!("Insufficient validation proof: {}", msg),
+			FailedSystemCall(ref msg) => format!("Failed to make system call: {}", msg),
+			RequiresClient => format!("Call requires client but none registered"),
 		};
 
 		f.write_fmt(format_args!("Engine error ({})", msg))
@@ -102,17 +111,53 @@ pub enum Seal {
 }
 
 /// Type alias for a function we can make calls through synchronously.
-pub type Call<'a> = Fn(Address, Bytes) -> Result<Bytes, String> + 'a;
+/// Returns the call result and state proof for each call.
+pub type Call<'a> = Fn(Address, Bytes) -> Result<(Bytes, Vec<Vec<u8>>), String> + 'a;
+
+/// Type alias for a function we can get headers by hash through.
+pub type Headers<'a> = Fn(H256) -> Option<Header> + 'a;
+
+/// Type alias for a function we can query pending transitions by block hash through.
+pub type PendingTransitionStore<'a> = Fn(H256) -> Option<PendingTransition> + 'a;
+
+/// Proof generated on epoch change.
+pub enum Proof {
+	/// Known proof (exctracted from signal)
+	Known(Vec<u8>),
+	/// Extract proof from caller.
+	WithState(Box<Fn(&Call) -> Result<Vec<u8>, String>>),
+}
+
+/// Generated epoch verifier.
+pub enum ConstructedVerifier<'a> {
+	/// Fully trusted verifier.
+	Trusted(Box<EpochVerifier>),
+	/// Verifier unconfirmed. Check whether given finality proof finalizes given hash
+	/// under previous epoch.
+	Unconfirmed(Box<EpochVerifier>, &'a [u8], H256),
+	/// Error constructing verifier.
+	Err(Error),
+}
+
+impl<'a> ConstructedVerifier<'a> {
+	/// Convert to a result, indicating that any necessary confirmation has been done
+	/// already.
+	pub fn known_confirmed(self) -> Result<Box<EpochVerifier>, Error> {
+		match self {
+			ConstructedVerifier::Trusted(v) | ConstructedVerifier::Unconfirmed(v, _, _) => Ok(v),
+			ConstructedVerifier::Err(e) => Err(e),
+		}
+	}
+}
 
 /// Results of a query of whether an epoch change occurred at the given block.
-#[derive(Debug, Clone, PartialEq)]
 pub enum EpochChange {
 	/// Cannot determine until more data is passed.
 	Unsure(Unsure),
 	/// No epoch change.
 	No,
-	/// Validation proof required, and the new epoch number.
-	Yes(u64),
+	/// The epoch will change, with proof.
+	Yes(Proof),
 }
 
 /// More data required to determine if an epoch change occurred at a given block.
@@ -171,7 +216,13 @@ pub trait Engine : Sync + Send {
 	}
 
 	/// Block transformation functions, before the transactions.
-	fn on_new_block(&self, block: &mut ExecutedBlock, last_hashes: Arc<LastHashes>) -> Result<(), Error> {
+	/// `epoch_begin` set to true if this block kicks off an epoch.
+	fn on_new_block(
+		&self,
+		block: &mut ExecutedBlock,
+		last_hashes: Arc<LastHashes>,
+		_epoch_begin: bool,
+	) -> Result<(), Error> {
 		let parent_hash = block.fields().header.parent_hash().clone();
 		common::push_last_hash(block, last_hashes, self, &parent_hash)
 	}
@@ -233,46 +284,44 @@ pub trait Engine : Sync + Send {
 		self.verify_block_basic(header, None).and_then(|_| self.verify_block_unordered(header, None))
 	}
 
-	/// Generate epoch change proof.
-	///
-	/// This will be used to generate proofs of epoch change as well as verify them.
-	/// Must be called on blocks that have already passed basic verification.
-	///
-	/// Return the "epoch proof" generated.
-	/// This must be usable to generate a `EpochVerifier` for verifying all blocks
-	/// from the supplied header up to the next one where proof is required.
-	///
-	/// For example, for PoA chains the proof will be a validator set,
-	/// and the corresponding `EpochVerifier` can be used to correctly validate
-	/// all blocks produced under that `ValidatorSet`
-	///
-	/// It must be possible to generate an epoch proof for any block in an epoch,
-	/// and it should always be equivalent to the proof of the transition block.
-	fn epoch_proof(&self, _header: &Header, _caller: &Call)
-		-> Result<Vec<u8>, Error>
-	{
-		Ok(Vec::new())
-	}
+	/// Genesis epoch data.
+	fn genesis_epoch_data(&self, _header: &Header, _call: &Call) -> Result<Vec<u8>, String> { Ok(Vec::new()) }
 
-	/// Whether an epoch change occurred at the given header.
+	/// Whether an epoch change is signalled at the given header but will require finality.
+	/// If a change can be enacted immediately then return `No` from this function but
+	/// `Yes` from `is_epoch_end`.
 	///
 	/// If the block or receipts are required, return `Unsure` and the function will be
 	/// called again with them.
 	/// Return `Yes` or `No` when the answer is definitively known.
 	///
 	/// Should not interact with state.
-	fn is_epoch_end(&self, _header: &Header, _block: Option<&[u8]>, _receipts: Option<&[Receipt]>)
+	fn signals_epoch_end(&self, _header: &Header, _block: Option<&[u8]>, _receipts: Option<&[Receipt]>)
 		-> EpochChange
 	{
 		EpochChange::No
 	}
 
-	/// Create an epoch verifier from validation proof.
+	/// Whether a block is the end of an epoch.
 	///
-	/// The proof should be one generated by `epoch_proof`.
-	/// See docs of `epoch_proof` for description.
-	fn epoch_verifier(&self, _header: &Header, _proof: &[u8]) -> Result<Box<EpochVerifier>, Error> {
-		Ok(Box::new(self::epoch_verifier::NoOp))
+	/// This either means that an immediate transition occurs or a block signalling transition
+	/// has reached finality. The `Headers` given are not guaranteed to return any blocks
+	/// from any epoch other than the current.
+	///
+	/// Return optional transition proof.
+	fn is_epoch_end(
+		&self,
+		_chain_head: &Header,
+		_chain: &Headers,
+		_transition_store: &PendingTransitionStore,
+	) -> Option<Vec<u8>> {
+		None
+	}
+
+	/// Create an epoch verifier from validation proof and a flag indicating
+	/// whether finality is required.
+	fn epoch_verifier<'a>(&self, _header: &Header, _proof: &'a [u8]) -> ConstructedVerifier<'a> {
+		ConstructedVerifier::Trusted(Box::new(self::epoch::NoOp))
 	}
 
 	/// Populate a header's fields based on its parent's header.
@@ -329,7 +378,11 @@ pub trait Engine : Sync + Send {
 
 	/// Returns new contract address generation scheme at given block number.
 	fn create_address_scheme(&self, number: BlockNumber) -> CreateContractAddress {
-		if number >= self.params().eip86_transition { CreateContractAddress::FromCodeHash } else { CreateContractAddress::FromSenderAndNonce }
+		if number >= self.params().eip86_transition {
+			CreateContractAddress::FromCodeHash
+		} else {
+			CreateContractAddress::FromSenderAndNonce
+		}
 	}
 }
 
@@ -349,6 +402,52 @@ pub mod common {
 	use util::*;
 	use super::Engine;
 
+	/// Execute a call as the system address.
+	pub fn execute_as_system<E: Engine + ?Sized>(
+		block: &mut ExecutedBlock,
+		last_hashes: Arc<LastHashes>,
+		engine: &E,
+		contract_address: Address,
+		gas: U256,
+		data: Option<Bytes>,
+	) -> Result<Bytes, Error> {
+		let env_info = {
+			let header = block.fields().header;
+			EnvInfo {
+				number: header.number(),
+				author: header.author().clone(),
+				timestamp: header.timestamp(),
+				difficulty: header.difficulty().clone(),
+				last_hashes: last_hashes,
+				gas_used: U256::zero(),
+				gas_limit: gas,
+			}
+		};
+
+		let mut state = block.fields_mut().state;
+		let params = ActionParams {
+			code_address: contract_address.clone(),
+			address: contract_address.clone(),
+			sender: SYSTEM_ADDRESS.clone(),
+			origin: SYSTEM_ADDRESS.clone(),
+			gas: gas,
+			gas_price: 0.into(),
+			value: ActionValue::Transfer(0.into()),
+			code: state.code(&contract_address)?,
+			code_hash: state.code_hash(&contract_address)?,
+			data: data,
+			call_type: CallType::Call,
+		};
+		let mut ex = Executive::new(&mut state, &env_info, engine);
+		let mut substate = Substate::new();
+		let mut output = Vec::new();
+		if let Err(e) = ex.call(params, &mut substate, BytesRef::Flexible(&mut output), &mut NoopTracer, &mut NoopVMTracer) {
+			warn!("Encountered error on making system call: {}", e);
+		}
+
+		Ok(output)
+	}
+
 	/// Push last known block hash to the state.
 	pub fn push_last_hash<E: Engine + ?Sized>(block: &mut ExecutedBlock, last_hashes: Arc<LastHashes>, engine: &E, hash: &H256) -> Result<(), Error> {
 		if block.fields().header.number() == engine.params().eip210_transition {
@@ -356,39 +455,14 @@ pub mod common {
 			state.init_code(&engine.params().eip210_contract_address, engine.params().eip210_contract_code.clone())?;
 		}
 		if block.fields().header.number() >= engine.params().eip210_transition {
-			let env_info = {
-				let header = block.fields().header;
-				EnvInfo {
-					number: header.number(),
-					author: header.author().clone(),
-					timestamp: header.timestamp(),
-					difficulty: header.difficulty().clone(),
-					last_hashes: last_hashes,
-					gas_used: U256::zero(),
-					gas_limit: engine.params().eip210_contract_gas,
-				}
-			};
-			let mut state = block.fields_mut().state;
-			let contract_address = engine.params().eip210_contract_address;
-			let params = ActionParams {
-				code_address: contract_address.clone(),
-				address: contract_address.clone(),
-				sender: SYSTEM_ADDRESS.clone(),
-				origin: SYSTEM_ADDRESS.clone(),
-				gas: engine.params().eip210_contract_gas,
-				gas_price: 0.into(),
-				value: ActionValue::Transfer(0.into()),
-				code: state.code(&contract_address)?,
-				code_hash: state.code_hash(&contract_address)?,
-				data: Some(hash.to_vec()),
-				call_type: CallType::Call,
-			};
-			let mut ex = Executive::new(&mut state, &env_info, engine);
-			let mut substate = Substate::new();
-			let mut output = [];
-			if let Err(e) = ex.call(params, &mut substate, BytesRef::Fixed(&mut output), &mut NoopTracer, &mut NoopVMTracer) {
-				warn!("Encountered error on updating last hashes: {}", e);
-			}
+			let _ = execute_as_system(
+				block,
+				last_hashes,
+				engine,
+				engine.params().eip210_contract_address,
+				engine.params().eip210_contract_gas,
+				Some(hash.to_vec()),
+			)?;
 		}
 		Ok(())
 	}

ethcore/src/engines/tendermint/mod.rs

@@ -466,7 +466,8 @@ impl Engine for Tendermint {
 			}
 			self.broadcast_message(rlp.as_raw().to_vec());
 			if let Some(double) = self.votes.vote(message.clone(), &sender) {
-				self.validators.report_malicious(&sender, message.vote_step.height as BlockNumber, ::rlp::encode(&double).to_vec());
+				let height = message.vote_step.height as BlockNumber;
+				self.validators.report_malicious(&sender, height, height, ::rlp::encode(&double).to_vec());
 				return Err(EngineError::DoubleVote(sender).into());
 			}
 			trace!(target: "engine", "Handling a valid {:?} from {}.", message, sender);
@@ -555,7 +556,7 @@ impl Engine for Tendermint {
 		let min_gas = parent.gas_limit().clone() - parent.gas_limit().clone() / gas_limit_divisor;
 		let max_gas = parent.gas_limit().clone() + parent.gas_limit().clone() / gas_limit_divisor;
 		if header.gas_limit() <= &min_gas || header.gas_limit() >= &max_gas {
-			self.validators.report_malicious(header.author(), header.number(), Default::default());
+			self.validators.report_malicious(header.author(), header.number(), header.number(), Default::default());
 			return Err(BlockError::InvalidGasLimit(OutOfBounds { min: Some(min_gas), max: Some(max_gas), found: header.gas_limit().clone() }).into());
 		}
 
@@ -607,7 +608,7 @@ impl Engine for Tendermint {
 					// Report the proposer if no proposal was received.
 					let height = self.height.load(AtomicOrdering::SeqCst);
 					let current_proposer = self.view_proposer(&*self.proposal_parent.read(), height, self.view.load(AtomicOrdering::SeqCst));
-					self.validators.report_benign(&current_proposer, height as BlockNumber);
+					self.validators.report_benign(&current_proposer, height as BlockNumber, height as BlockNumber);
 				}
 				Step::Prevote
 			},
@@ -674,7 +675,7 @@ mod tests {
 		let db = spec.ensure_db_good(db, &Default::default()).unwrap();
 		let genesis_header = spec.genesis_header();
 		let last_hashes = Arc::new(vec![genesis_header.hash()]);
-		let b = OpenBlock::new(spec.engine.as_ref(), Default::default(), false, db.boxed_clone(), &genesis_header, last_hashes, proposer, (3141562.into(), 31415620.into()), vec![]).unwrap();
+		let b = OpenBlock::new(spec.engine.as_ref(), Default::default(), false, db.boxed_clone(), &genesis_header, last_hashes, proposer, (3141562.into(), 31415620.into()), vec![], false).unwrap();
 		let b = b.close();
 		if let Seal::Proposal(seal) = spec.engine.generate_seal(b.block()) {
 			(b, seal)

ethcore/src/engines/transition.rs

@@ -36,7 +36,7 @@ pub struct TransitionHandler<S: Sync + Send + Clone>  {
 	timeouts: Box<Timeouts<S>>,
 }
 
-impl <S> TransitionHandler<S> where S: Sync + Send + Clone {
+impl<S> TransitionHandler<S> where S: Sync + Send + Clone {
 	/// New step caller by timeouts.
 	pub fn new(engine: Weak<Engine>, timeouts: Box<Timeouts<S>>) -> Self {
 		TransitionHandler {
@@ -54,7 +54,7 @@ fn set_timeout<S: Sync + Send + Clone>(io: &IoContext<S>, timeout: Duration) {
 		.unwrap_or_else(|e| warn!(target: "engine", "Failed to set consensus step timeout: {}.", e))
 }
 
-impl <S> IoHandler<S> for TransitionHandler<S> where S: Sync + Send + Clone + 'static {
+impl<S> IoHandler<S> for TransitionHandler<S> where S: Sync + Send + Clone + 'static {
 	fn initialize(&self, io: &IoContext<S>) {
 		let initial = self.timeouts.initial();
 		trace!(target: "engine", "Setting the initial timeout to {}.", initial);

ethcore/src/engines/validator_set/contract.rs

@@ -24,10 +24,10 @@ use futures::Future;
 use native_contracts::ValidatorReport as Provider;
 
 use client::{Client, BlockChainClient};
-use engines::Call;
+use engines::{Call, Engine};
 use header::{Header, BlockNumber};
 
-use super::ValidatorSet;
+use super::{ValidatorSet, SimpleList, SystemCall};
 use super::safe_contract::ValidatorSafeContract;
 
 /// A validator contract with reporting.
@@ -51,7 +51,7 @@ impl ValidatorContract {
 	// could be `impl Trait`.
 	// note: dispatches transactions to network as well as execute.
 	// TODO [keorn]: Make more general.
-	fn transact(&self) -> Box<Call> {
+	fn transact(&self) -> Box<Fn(Address, Bytes) -> Result<Bytes, String>> {
 		let client = self.client.read().clone();
 		Box::new(move |a, d| client.as_ref()
 			.and_then(Weak::upgrade)
@@ -66,18 +66,30 @@ impl ValidatorSet for ValidatorContract {
 		self.validators.default_caller(id)
 	}
 
-	fn is_epoch_end(&self, header: &Header, block: Option<&[u8]>, receipts: Option<&[::receipt::Receipt]>)
-		-> ::engines::EpochChange
-	{
-		self.validators.is_epoch_end(header, block, receipts)
+	fn on_epoch_begin(&self, first: bool, header: &Header, call: &mut SystemCall) -> Result<(), ::error::Error> {
+		self.validators.on_epoch_begin(first, header, call)
 	}
 
-	fn epoch_proof(&self, header: &Header, caller: &Call) -> Result<Vec<u8>, String> {
-		self.validators.epoch_proof(header, caller)
+	fn genesis_epoch_data(&self, header: &Header, call: &Call) -> Result<Vec<u8>, String> {
+		self.validators.genesis_epoch_data(header, call)
 	}
 
-	fn epoch_set(&self, header: &Header, proof: &[u8]) -> Result<(u64, super::SimpleList), ::error::Error> {
-		self.validators.epoch_set(header, proof)
+	fn is_epoch_end(&self, first: bool, chain_head: &Header) -> Option<Vec<u8>> {
+		self.validators.is_epoch_end(first, chain_head)
+	}
+
+	fn signals_epoch_end(
+		&self,
+		first: bool,
+		header: &Header,
+		block: Option<&[u8]>,
+		receipts: Option<&[::receipt::Receipt]>,
+	) -> ::engines::EpochChange {
+		self.validators.signals_epoch_end(first, header, block, receipts)
+	}
+
+	fn epoch_set(&self, first: bool, engine: &Engine, number: BlockNumber, proof: &[u8]) -> Result<(SimpleList, Option<H256>), ::error::Error> {
+		self.validators.epoch_set(first, engine, number, proof)
 	}
 
 	fn contains_with_caller(&self, bh: &H256, address: &Address, caller: &Call) -> bool {
@@ -92,14 +104,14 @@ impl ValidatorSet for ValidatorContract {
 		self.validators.count_with_caller(bh, caller)
 	}
 
-	fn report_malicious(&self, address: &Address, block: BlockNumber, proof: Bytes) {
+	fn report_malicious(&self, address: &Address, _set_block: BlockNumber, block: BlockNumber, proof: Bytes) {
 		match self.provider.report_malicious(&*self.transact(), *address, block.into(), proof).wait() {
 			Ok(_) => warn!(target: "engine", "Reported malicious validator {}", address),
 			Err(s) => warn!(target: "engine", "Validator {} could not be reported {}", address, s),
 		}
 	}
 
-	fn report_benign(&self, address: &Address, block: BlockNumber) {
+	fn report_benign(&self, address: &Address, _set_block: BlockNumber, block: BlockNumber) {
 		match self.provider.report_benign(&*self.transact(), *address, block.into()).wait() {
 			Ok(_) => warn!(target: "engine", "Reported benign validator misbehaviour {}", address),
 			Err(s) => warn!(target: "engine", "Validator {} could not be reported {}", address, s),

ethcore/src/engines/validator_set/mod.rs

@@ -37,7 +37,10 @@ use self::contract::ValidatorContract;
 use self::safe_contract::ValidatorSafeContract;
 use self::multi::Multi;
 
-use super::Call;
+use super::{Call, Engine};
+
+/// A system-calling closure. Enacts calls on a block's state from the system address.
+pub type SystemCall<'a> = FnMut(Address, Bytes) -> Result<Bytes, String> + 'a;
 
 /// Creates a validator set from spec.
 pub fn new_validator_set(spec: ValidatorSpec) -> Box<ValidatorSet> {
@@ -64,6 +67,7 @@ pub trait ValidatorSet: Send + Sync {
 		let default = self.default_caller(BlockId::Hash(*parent));
 		self.contains_with_caller(parent, address, &*default)
 	}
+
 	/// Draws an validator nonce modulo number of validators.
 	fn get(&self, parent: &H256, nonce: usize) -> Address {
 		let default = self.default_caller(BlockId::Hash(*parent));
@@ -76,48 +80,66 @@ pub trait ValidatorSet: Send + Sync {
 		self.count_with_caller(parent, &*default)
 	}
 
+	/// Signalling that a new epoch has begun.
+	///
+	/// All calls here will be from the `SYSTEM_ADDRESS`: 2^160 - 2
+	/// and will have an effect on the block's state.
+	/// The caller provided here may not generate proofs.
+	///
+	/// `first` is true if this is the first block in the set.
+	fn on_epoch_begin(&self, _first: bool, _header: &Header, _call: &mut SystemCall) -> Result<(), ::error::Error> {
+		Ok(())
+	}
+
+	/// Extract genesis epoch data from the genesis state and header.
+	fn genesis_epoch_data(&self, _header: &Header, _call: &Call) -> Result<Vec<u8>, String> { Ok(Vec::new()) }
+
 	/// Whether this block is the last one in its epoch.
-	/// Usually indicates that the validator set changed at the given block.
 	///
-	/// Should not inspect state! This is used in situations where
-	/// state is not generally available.
+	/// Indicates that the validator set changed at the given block in a manner
+	/// that doesn't require finality.
 	///
-	/// Return `Yes` or `No` indicating whether it changed at the given header,
-	/// or `Unsure` indicating a need for more information.
-	///
-	/// If block or receipts are provided, do not return `Unsure` indicating
-	/// need for them.
-	fn is_epoch_end(&self, header: &Header, block: Option<&[u8]>, receipts: Option<&[::receipt::Receipt]>)
-		-> super::EpochChange;
+	/// `first` is true if this is the first block in the set.
+	fn is_epoch_end(&self, first: bool, chain_head: &Header) -> Option<Vec<u8>>;
 
-	/// Generate epoch proof.
-	/// Must interact with state only through the given caller!
-	/// Otherwise, generated proofs may be wrong.
-	fn epoch_proof(&self, header: &Header, caller: &Call) -> Result<Vec<u8>, String>;
+	/// Whether the given block signals the end of an epoch, but change won't take effect
+	/// until finality.
+	///
+	/// Engine should set `first` only if the header is genesis. Multiplexing validator
+	/// sets can set `first` to internal changes.
+	fn signals_epoch_end(
+		&self,
+		first: bool,
+		header: &Header,
+		block: Option<&[u8]>,
+		receipts: Option<&[::receipt::Receipt]>,
+	) -> ::engines::EpochChange;
 
-	/// Recover the validator set for all
+	/// Recover the validator set from the given proof, the block number, and
+	/// whether this header is first in its set.
 	///
 	/// May fail if the given header doesn't kick off an epoch or
 	/// the proof is invalid.
 	///
-	/// Returns the epoch number and proof.
-	fn epoch_set(&self, header: &Header, proof: &[u8]) -> Result<(u64, SimpleList), ::error::Error>;
+	/// Returns the set, along with a flag indicating whether finality of a specific
+	/// hash should be proven.
+	fn epoch_set(&self, first: bool, engine: &Engine, number: BlockNumber, proof: &[u8])
+		-> Result<(SimpleList, Option<H256>), ::error::Error>;
 
 	/// Checks if a given address is a validator, with the given function
 	/// for executing synchronous calls to contracts.
 	fn contains_with_caller(&self, parent_block_hash: &H256, address: &Address, caller: &Call) -> bool;
 
 	/// Draws an validator nonce modulo number of validators.
-	///
 	fn get_with_caller(&self, parent_block_hash: &H256, nonce: usize, caller: &Call) -> Address;
 
 	/// Returns the current number of validators.
 	fn count_with_caller(&self, parent_block_hash: &H256, caller: &Call) -> usize;
 
 	/// Notifies about malicious behaviour.
-	fn report_malicious(&self, _validator: &Address, _block: BlockNumber, _proof: Bytes) {}
+	fn report_malicious(&self, _validator: &Address, _set_block: BlockNumber, _block: BlockNumber, _proof: Bytes) {}
 	/// Notifies about benign misbehaviour.
-	fn report_benign(&self, _validator: &Address, _block: BlockNumber) {}
+	fn report_benign(&self, _validator: &Address, _set_block: BlockNumber, _block: BlockNumber) {}
 	/// Allows blockchain state access.
 	fn register_contract(&self, _client: Weak<Client>) {}
 }

ethcore/src/engines/validator_set/multi.rs

@@ -18,12 +18,12 @@
 
 use std::collections::BTreeMap;
 use std::sync::Weak;
-use engines::{Call, EpochChange};
+use engines::{Call, Engine};
 use util::{Bytes, H256, Address, RwLock};
 use ids::BlockId;
 use header::{BlockNumber, Header};
 use client::{Client, BlockChainClient};
-use super::ValidatorSet;
+use super::{SystemCall, ValidatorSet};
 
 type BlockNumberLookup = Box<Fn(BlockId) -> Result<BlockNumber, String> + Send + Sync + 'static>;
 
@@ -72,49 +72,38 @@ impl ValidatorSet for Multi {
 			.unwrap_or(Box::new(|_, _| Err("No validator set for given ID.".into())))
 	}
 
-	fn is_epoch_end(&self, header: &Header, block: Option<&[u8]>, receipts: Option<&[::receipt::Receipt]>)
-		-> EpochChange
+	fn on_epoch_begin(&self, _first: bool, header: &Header, call: &mut SystemCall) -> Result<(), ::error::Error> {
+		let (set_block, set) = self.correct_set_by_number(header.number());
+		let first = set_block == header.number();
+
+		set.on_epoch_begin(first, header, call)
+	}
+
+	fn genesis_epoch_data(&self, header: &Header, call: &Call) -> Result<Vec<u8>, String> {
+		self.correct_set_by_number(0).1.genesis_epoch_data(header, call)
+	}
+
+	fn is_epoch_end(&self, _first: bool, chain_head: &Header) -> Option<Vec<u8>> {
+		let (set_block, set) = self.correct_set_by_number(chain_head.number());
+		let first = set_block == chain_head.number();
+
+		set.is_epoch_end(first, chain_head)
+	}
+
+	fn signals_epoch_end(&self, _first: bool, header: &Header, block: Option<&[u8]>, receipts: Option<&[::receipt::Receipt]>)
+		-> ::engines::EpochChange
 	{
 		let (set_block, set) = self.correct_set_by_number(header.number());
-		let (next_set_block, _) = self.correct_set_by_number(header.number() + 1);
+		let first = set_block == header.number();
 
-		// multi-set transitions require epoch changes.
-		if next_set_block != set_block {
-			return EpochChange::Yes(next_set_block);
-		}
-
-		match set.is_epoch_end(header, block, receipts) {
-			EpochChange::Yes(num) => EpochChange::Yes(set_block + num),
-			other => other,
-		}
+		set.signals_epoch_end(first, header, block, receipts)
 	}
 
-	fn epoch_proof(&self, header: &Header, caller: &Call) -> Result<Vec<u8>, String> {
-		let (set_block, set) = self.correct_set_by_number(header.number());
-		let (next_set_block, next_set) = self.correct_set_by_number(header.number() + 1);
+	fn epoch_set(&self, _first: bool, engine: &Engine, number: BlockNumber, proof: &[u8]) -> Result<(super::SimpleList, Option<H256>), ::error::Error> {
+		let (set_block, set) = self.correct_set_by_number(number);
+		let first = set_block == number;
 
-		if next_set_block != set_block {
-			return next_set.epoch_proof(header, caller);
-		}
-
-		set.epoch_proof(header, caller)
-	}
-
-	fn epoch_set(&self, header: &Header, proof: &[u8]) -> Result<(u64, super::SimpleList), ::error::Error> {
-		// "multi" epoch is the inner set's epoch plus the transition block to that set.
-		// ensures epoch increases monotonically.
-		let (set_block, set) = self.correct_set_by_number(header.number());
-		let (next_set_block, next_set) = self.correct_set_by_number(header.number() + 1);
-
-		// this block kicks off a new validator set -- get the validator set
-		// starting there.
-		if next_set_block != set_block {
-			let (inner_epoch, list) = next_set.epoch_set(header, proof)?;
-			Ok((next_set_block + inner_epoch, list))
-		} else {
-			let (inner_epoch, list) = set.epoch_set(header, proof)?;
-			Ok((set_block + inner_epoch, list))
-		}
+		set.epoch_set(first, engine, number, proof)
 	}
 
 	fn contains_with_caller(&self, bh: &H256, address: &Address, caller: &Call) -> bool {
@@ -132,12 +121,12 @@ impl ValidatorSet for Multi {
 			.map_or_else(usize::max_value, |set| set.count_with_caller(bh, caller))
 	}
 
-	fn report_malicious(&self, validator: &Address, block: BlockNumber, proof: Bytes) {
-		self.correct_set_by_number(block).1.report_malicious(validator, block, proof);
+	fn report_malicious(&self, validator: &Address, set_block: BlockNumber, block: BlockNumber, proof: Bytes) {
+		self.correct_set_by_number(set_block).1.report_malicious(validator, set_block, block, proof);
 	}
 
-	fn report_benign(&self, validator: &Address, block: BlockNumber) {
-		self.correct_set_by_number(block).1.report_benign(validator, block);
+	fn report_benign(&self, validator: &Address, set_block: BlockNumber, block: BlockNumber) {
+		self.correct_set_by_number(set_block).1.report_benign(validator, set_block, block);
 	}
 
 	fn register_contract(&self, client: Weak<Client>) {
@@ -153,18 +142,24 @@ impl ValidatorSet for Multi {
 
 #[cfg(test)]
 mod tests {
-	use util::*;
-	use types::ids::BlockId;
-	use spec::Spec;
 	use account_provider::AccountProvider;
 	use client::{BlockChainClient, EngineClient};
+	use engines::EpochChange;
+	use engines::validator_set::ValidatorSet;
 	use ethkey::Secret;
+	use header::Header;
 	use miner::MinerService;
+	use spec::Spec;
 	use tests::helpers::{generate_dummy_client_with_spec_and_accounts, generate_dummy_client_with_spec_and_data};
+	use types::ids::BlockId;
+	use util::*;
+
+	use super::Multi;
 
 	#[test]
 	fn uses_current_set() {
-		::env_logger::init().unwrap();
+		let _ = ::env_logger::init();
+
 		let tap = Arc::new(AccountProvider::transient_provider());
 		let s0: Secret = "0".sha3().into();
 		let v0 = tap.insert_account(s0.clone(), "").unwrap();
@@ -205,4 +200,39 @@ mod tests {
 		sync_client.flush_queue();
 		assert_eq!(sync_client.chain_info().best_block_number, 3);
 	}
+
+	#[test]
+	fn transition_to_fixed_list_instant() {
+		use super::super::SimpleList;
+
+		let mut map: BTreeMap<_, Box<ValidatorSet>> = BTreeMap::new();
+		let list1: Vec<_> = (0..10).map(|_| Address::random()).collect();
+		let list2 = {
+			let mut list = list1.clone();
+			list.push(Address::random());
+			list
+		};
+
+		map.insert(0, Box::new(SimpleList::new(list1)));
+		map.insert(500, Box::new(SimpleList::new(list2)));
+
+		let multi = Multi::new(map);
+
+		let mut header = Header::new();
+		header.set_number(499);
+
+		match multi.signals_epoch_end(false, &header, None, None) {
+			EpochChange::No => {},
+			_ => panic!("Expected no epoch signal change."),
+		}
+		assert!(multi.is_epoch_end(false, &header).is_none());
+
+		header.set_number(500);
+
+		match multi.signals_epoch_end(false, &header, None, None) {
+			EpochChange::No => {},
+			_ => panic!("Expected no epoch signal change."),
+		}
+		assert!(multi.is_epoch_end(false, &header).is_some());
+	}
 }

ethcore/src/engines/validator_set/safe_contract.rs

@@ -22,21 +22,23 @@ use native_contracts::ValidatorSet as Provider;
 
 use util::*;
 use util::cache::MemoryLruCache;
+use rlp::{UntrustedRlp, RlpStream};
 
 use basic_types::LogBloom;
 use client::{Client, BlockChainClient};
-use engines::Call;
+use engines::{Call, Engine};
 use header::Header;
 use ids::BlockId;
 use log_entry::LogEntry;
+use receipt::Receipt;
 
-use super::ValidatorSet;
+use super::{SystemCall, ValidatorSet};
 use super::simple_list::SimpleList;
 
 const MEMOIZE_CAPACITY: usize = 500;
 
 // TODO: ethabi should be able to generate this.
-const EVENT_NAME: &'static [u8] = &*b"ValidatorsChanged(bytes32,uint256,address[])";
+const EVENT_NAME: &'static [u8] = &*b"InitiateChange(bytes32,address[])";
 
 lazy_static! {
 	static ref EVENT_NAME_HASH: H256 = EVENT_NAME.sha3();
@@ -50,14 +52,68 @@ pub struct ValidatorSafeContract {
 	client: RwLock<Option<Weak<Client>>>, // TODO [keorn]: remove
 }
 
-fn encode_proof(nonce: U256, validators: &[Address]) -> Bytes {
-	use rlp::RlpStream;
-
+// first proof is just a state proof call of `getValidators` at header's state.
+fn encode_first_proof(header: &Header, state_items: &[Vec<u8>]) -> Bytes {
 	let mut stream = RlpStream::new_list(2);
-	stream.append(&nonce).append_list(validators);
+	stream.append(header).begin_list(state_items.len());
+	for item in state_items {
+		stream.append(item);
+	}
+
+	stream.out()
+}
+
+fn decode_first_proof(rlp: &UntrustedRlp) -> Result<(Header, Vec<DBValue>), ::error::Error> {
+	let header = rlp.val_at(0)?;
+	let state_items = rlp.at(1)?.iter().map(|x| {
+		let mut val = DBValue::new();
+		val.append_slice(x.data()?);
+		Ok(val)
+	}).collect::<Result<_, ::error::Error>>()?;
+
+	Ok((header, state_items))
+}
+
+// inter-contract proofs are a header and receipts.
+// checking will involve ensuring that the receipts match the header and
+// extracting the validator set from the receipts.
+fn encode_proof(header: &Header, receipts: &[Receipt]) -> Bytes {
+	let mut stream = RlpStream::new_list(2);
+	stream.append(header).append_list(receipts);
 	stream.drain().to_vec()
 }
 
+fn decode_proof(rlp: &UntrustedRlp) -> Result<(Header, Vec<Receipt>), ::error::Error> {
+	Ok((rlp.val_at(0)?, rlp.list_at(1)?))
+}
+
+// given a provider and caller, generate proof. this will just be a state proof
+// of `getValidators`.
+fn prove_initial(provider: &Provider, header: &Header, caller: &Call) -> Result<Vec<u8>, String> {
+	use std::cell::RefCell;
+
+	let epoch_proof = RefCell::new(None);
+	let res = {
+		let caller = |a, d| {
+			let (result, proof) = caller(a, d)?;
+			*epoch_proof.borrow_mut() = Some(encode_first_proof(header, &proof));
+			Ok(result)
+		};
+
+		provider.get_validators(caller)
+			.wait()
+	};
+
+	res.map(|validators| {
+		let proof = epoch_proof.into_inner().expect("epoch_proof always set after call; qed");
+
+		trace!(target: "engine", "obtained proof for initial set: {} validators, {} bytes",
+			validators.len(), proof.len());
+
+		proof
+	})
+}
+
 impl ValidatorSafeContract {
 	pub fn new(contract_address: Address) -> Self {
 		ValidatorSafeContract {
@@ -70,6 +126,7 @@ impl ValidatorSafeContract {
 
 	/// Queries the state and gets the set of validators.
 	fn get_list(&self, caller: &Call) -> Option<SimpleList> {
+		let caller = move |a, d| caller(a, d).map(|x| x.0);
 		match self.provider.get_validators(caller).wait() {
 			Ok(new) => {
 				debug!(target: "engine", "Set of validators obtained: {:?}", new);
@@ -82,17 +139,6 @@ impl ValidatorSafeContract {
 		}
 	}
 
-	/// Queries for the current validator set transition nonce.
-	fn get_nonce(&self, caller: &Call) -> Option<::util::U256> {
-		match self.provider.transition_nonce(caller).wait() {
-			Ok(nonce) => Some(nonce),
-			Err(s) => {
-				debug!(target: "engine", "Unable to fetch transition nonce: {}", s);
-				None
-			}
-		}
-	}
-
 	// Whether the header matches the expected bloom.
 	//
 	// The expected log should have 3 topics:
@@ -109,12 +155,70 @@ impl ValidatorSafeContract {
 	//
 	// The log data is an array of all new validator addresses.
 	fn expected_bloom(&self, header: &Header) -> LogBloom {
+		let topics = vec![*EVENT_NAME_HASH, *header.parent_hash()];
+
+		debug!(target: "engine", "Expected topics for header {}: {:?}",
+			header.hash(), topics);
+
 		LogEntry {
 			address: self.address,
-			topics: vec![*EVENT_NAME_HASH, *header.parent_hash()],
+			topics: topics,
 			data: Vec::new(), // irrelevant for bloom.
 		}.bloom()
 	}
+
+	// check receipts for log event. bloom should be `expected_bloom` for the
+	// header the receipts correspond to.
+	fn extract_from_event(&self, bloom: LogBloom, header: &Header, receipts: &[Receipt]) -> Option<SimpleList> {
+		let check_log = |log: &LogEntry| {
+			log.address == self.address &&
+				log.topics.len() == 2 &&
+				log.topics[0] == *EVENT_NAME_HASH &&
+				log.topics[1] == *header.parent_hash()
+		};
+
+		let event = Provider::contract(&self.provider)
+			.event("InitiateChange".into())
+			.expect("Contract known ahead of time to have `InitiateChange` event; qed");
+
+		// iterate in reverse because only the _last_ change in a given
+		// block actually has any effect.
+		// the contract should only increment the nonce once.
+		let mut decoded_events = receipts.iter()
+			.rev()
+			.filter(|r| &bloom & &r.log_bloom == bloom)
+			.flat_map(|r| r.logs.iter())
+			.filter(move |l| check_log(l))
+			.filter_map(|log| {
+				let topics = log.topics.iter().map(|x| x.0.clone()).collect();
+				event.decode_log(topics, log.data.clone()).ok()
+			});
+
+		match decoded_events.next() {
+			None => None,
+			Some(matched_event) => {
+
+				// decode log manually until the native contract generator is
+				// good enough to do it for us.
+				let &(_, _, ref validators_token) = &matched_event.params[1];
+
+				let validators = validators_token.clone().to_array()
+					.and_then(|a| a.into_iter()
+						.map(|x| x.to_address().map(H160))
+						.collect::<Option<Vec<_>>>()
+					)
+					.map(SimpleList::new);
+
+				if validators.is_none() {
+					debug!(target: "engine", "Successfully decoded log turned out to be bad.");
+				}
+
+				trace!(target: "engine", "decoded log. validators: {:?}", validators);
+
+				validators
+			}
+		}
+	}
 }
 
 impl ValidatorSet for ValidatorSafeContract {
@@ -123,97 +227,144 @@ impl ValidatorSet for ValidatorSafeContract {
 		Box::new(move |addr, data| client.as_ref()
 			.and_then(Weak::upgrade)
 			.ok_or("No client!".into())
-			.and_then(|c| c.call_contract(id, addr, data)))
+			.and_then(|c| c.call_contract(id, addr, data))
+			.map(|out| (out, Vec::new()))) // generate no proofs in general
 	}
 
-	fn is_epoch_end(&self, header: &Header, _block: Option<&[u8]>, receipts: Option<&[::receipt::Receipt]>)
+	fn on_epoch_begin(&self, first: bool, _header: &Header, caller: &mut SystemCall) -> Result<(), ::error::Error> {
+		if first { return Ok(()) } // only signalled changes need to be noted.
+
+		self.provider.finalize_change(caller)
+			.wait()
+			.map_err(::engines::EngineError::FailedSystemCall)
+			.map_err(Into::into)
+	}
+
+	fn genesis_epoch_data(&self, header: &Header, call: &Call) -> Result<Vec<u8>, String> {
+		prove_initial(&self.provider, header, call)
+	}
+
+	fn is_epoch_end(&self, _first: bool, _chain_head: &Header) -> Option<Vec<u8>> {
+		None // no immediate transitions to contract.
+	}
+
+	fn signals_epoch_end(&self, first: bool, header: &Header, _block: Option<&[u8]>, receipts: Option<&[Receipt]>)
 		-> ::engines::EpochChange
 	{
+		// transition to the first block of a contract requires finality but has no log event.
+		if first {
+			debug!(target: "engine", "signalling transition to fresh contract.");
+			let (provider, header) = (self.provider.clone(), header.clone());
+			let with_caller: Box<Fn(&Call) -> _> = Box::new(move |caller| prove_initial(&provider, &header, caller));
+			return ::engines::EpochChange::Yes(::engines::Proof::WithState(with_caller))
+		}
+
+		// otherwise, we're checking for logs.
 		let bloom = self.expected_bloom(header);
 		let header_bloom = header.log_bloom();
 
 		if &bloom & header_bloom != bloom { return ::engines::EpochChange::No }
 
+		trace!(target: "engine", "detected epoch change event bloom");
+
 		match receipts {
 			None => ::engines::EpochChange::Unsure(::engines::Unsure::NeedsReceipts),
-			Some(receipts) => {
-				let check_log = |log: &LogEntry| {
-					log.address == self.address &&
-						log.topics.len() == 3 &&
-						log.topics[0] == *EVENT_NAME_HASH &&
-						log.topics[1] == *header.parent_hash()
-						// don't have anything to compare nonce to yet.
-				};
+			Some(receipts) => match self.extract_from_event(bloom, header, receipts) {
+				None => ::engines::EpochChange::No,
+				Some(_) => {
+					debug!(target: "engine", "signalling transition within contract");
 
-				let event = Provider::contract(&self.provider)
-					.event("ValidatorsChanged".into())
-					.expect("Contract known ahead of time to have `ValidatorsChanged` event; qed");
-
-				// iterate in reverse because only the _last_ change in a given
-				// block actually has any effect.
-				// the contract should only increment the nonce once.
-				let mut decoded_events = receipts.iter()
-					.rev()
-					.filter(|r| &bloom & &r.log_bloom == bloom)
-					.flat_map(|r| r.logs.iter())
-					.filter(move |l| check_log(l))
-					.filter_map(|log| {
-						let topics = log.topics.iter().map(|x| x.0.clone()).collect();
-						match event.decode_log(topics, log.data.clone()) {
-							Ok(decoded) => Some(decoded),
-							Err(_) => None,
-						}
-					});
-
-				match decoded_events.next() {
-					None => ::engines::EpochChange::No,
-					Some(matched_event) => {
-						// decode log manually until the native contract generator is
-						// good enough to do it for us.
-						let &(_, _, ref nonce_token) = &matched_event.params[1];
-						let &(_, _, ref validators_token) = &matched_event.params[2];
-
-						let nonce: Option<U256> = nonce_token.clone().to_uint()
-							.map(H256).map(Into::into);
-						let validators = validators_token.clone().to_array()
-							.and_then(|a| a.into_iter()
-								.map(|x| x.to_address().map(H160))
-								.collect::<Option<Vec<_>>>()
-							);
-
-						match (nonce, validators) {
-							(Some(nonce), Some(_)) => {
-								let new_epoch = nonce.low_u64();
-								::engines::EpochChange::Yes(new_epoch)
-							}
-							_ => {
-								debug!(target: "engine", "Successfully decoded log turned out to be bad.");
-								::engines::EpochChange::No
-							}
-						}
-					}
+					let proof = encode_proof(&header, receipts);
+					::engines::EpochChange::Yes(::engines::Proof::Known(proof))
 				}
-			}
+			},
 		}
 	}
 
-	// the proof we generate is an RLP list containing two parts.
-	//   (nonce, validators)
-	fn epoch_proof(&self, _header: &Header, caller: &Call) -> Result<Vec<u8>, String> {
-		match (self.get_nonce(caller), self.get_list(caller)) {
-			(Some(nonce), Some(list)) => Ok(encode_proof(nonce, &list.into_inner())),
-			_ => Err("Caller insufficient to generate validator proof.".into()),
-		}
-	}
-
-	fn epoch_set(&self, _header: &Header, proof: &[u8]) -> Result<(u64, SimpleList), ::error::Error> {
-		use rlp::UntrustedRlp;
+	fn epoch_set(&self, first: bool, engine: &Engine, _number: ::header::BlockNumber, proof: &[u8])
+		-> Result<(SimpleList, Option<H256>), ::error::Error>
+	{
+		use transaction::{Action, Transaction};
 
 		let rlp = UntrustedRlp::new(proof);
-		let nonce: u64 = rlp.val_at(0)?;
-		let validators: Vec<Address> = rlp.list_at(1)?;
 
-		Ok((nonce, SimpleList::new(validators)))
+		if first {
+			trace!(target: "engine", "Recovering initial epoch set");
+
+			// TODO: match client contract_call_tx more cleanly without duplication.
+			const PROVIDED_GAS: u64 = 50_000_000;
+
+			let (old_header, state_items) = decode_first_proof(&rlp)?;
+			let old_hash = old_header.hash();
+
+			let env_info = ::env_info::EnvInfo {
+				number: old_header.number(),
+				author: *old_header.author(),
+				difficulty: *old_header.difficulty(),
+				gas_limit: PROVIDED_GAS.into(),
+				timestamp: old_header.timestamp(),
+				last_hashes: {
+					// this will break if we don't inclue all 256 last hashes.
+					let mut last_hashes: Vec<_> = (0..256).map(|_| H256::default()).collect();
+					last_hashes[255] = *old_header.parent_hash();
+					Arc::new(last_hashes)
+				},
+				gas_used: 0.into(),
+			};
+
+			// check state proof using given engine.
+			let number = old_header.number();
+			let addresses = self.provider.get_validators(move |a, d| {
+				let from = Address::default();
+				let tx = Transaction {
+					nonce: engine.account_start_nonce(number),
+					action: Action::Call(a),
+					gas: PROVIDED_GAS.into(),
+					gas_price: U256::default(),
+					value: U256::default(),
+					data: d,
+				}.fake_sign(from);
+
+				let res = ::state::check_proof(
+					&state_items,
+					*old_header.state_root(),
+					&tx,
+					engine,
+					&env_info,
+				);
+
+				match res {
+					::state::ProvedExecution::BadProof => Err("Bad proof".into()),
+					::state::ProvedExecution::Failed(e) => Err(format!("Failed call: {}", e)),
+					::state::ProvedExecution::Complete(e) => Ok(e.output),
+				}
+			}).wait().map_err(::engines::EngineError::InsufficientProof)?;
+
+			trace!(target: "engine", "extracted epoch set at #{}: {} addresses",
+				number, addresses.len());
+
+			Ok((SimpleList::new(addresses), Some(old_hash)))
+		} else {
+			let (old_header, receipts) = decode_proof(&rlp)?;
+
+			// ensure receipts match header.
+			// TODO: optimize? these were just decoded.
+			let found_root = ::util::triehash::ordered_trie_root(
+				receipts.iter().map(::rlp::encode).map(|x| x.to_vec())
+			);
+			if found_root != *old_header.receipts_root() {
+				return Err(::error::BlockError::InvalidReceiptsRoot(
+					Mismatch { expected: *old_header.receipts_root(), found: found_root }
+				).into());
+			}
+
+			let bloom = self.expected_bloom(&old_header);
+
+			match self.extract_from_event(bloom, &old_header, &receipts) {
+				Some(list) => Ok((list, Some(old_header.hash()))),
+				None => Err(::engines::EngineError::InsufficientProof("No log event in proof.".into()).into()),
+			}
+		}
 	}
 
 	fn contains_with_caller(&self, block_hash: &H256, address: &Address, caller: &Call) -> bool {
@@ -371,6 +522,7 @@ mod tests {
 		let last_hash = client.best_block_header().hash();
 		let mut new_header = Header::default();
 		new_header.set_parent_hash(last_hash);
+		new_header.set_number(1); // so the validator set looks for a log.
 
 		// first, try without the parent hash.
 		let mut event = LogEntry {
@@ -380,12 +532,35 @@ mod tests {
 		};
 
 		new_header.set_log_bloom(event.bloom());
-		assert_eq!(engine.is_epoch_end(&new_header, None, None), EpochChange::No);
+		match engine.signals_epoch_end(&new_header, None, None) {
+			EpochChange::No => {},
+			_ => panic!("Expected bloom to be unrecognized."),
+		};
 
 		// with the last hash, it should need the receipts.
 		event.topics.push(last_hash);
 		new_header.set_log_bloom(event.bloom());
-		assert_eq!(engine.is_epoch_end(&new_header, None, None),
-			EpochChange::Unsure(Unsure::NeedsReceipts));
+
+		match engine.signals_epoch_end(&new_header, None, None) {
+			EpochChange::Unsure(Unsure::NeedsReceipts) => {},
+			_ => panic!("Expected bloom to be recognized."),
+		};
+	}
+
+	#[test]
+	fn initial_contract_is_signal() {
+		use header::Header;
+		use engines::{EpochChange, Proof};
+
+		let client = generate_dummy_client_with_spec_and_accounts(Spec::new_validator_safe_contract, None);
+		let engine = client.engine().clone();
+
+		let mut new_header = Header::default();
+		new_header.set_number(0); // so the validator set doesn't look for a log
+
+		match engine.signals_epoch_end(&new_header, None, None) {
+			EpochChange::Yes(Proof::WithState(_)) => {},
+			_ => panic!("Expected state to be required to prove initial signal"),
+		};
 	}
 }

ethcore/src/engines/validator_set/simple_list.rs

@@ -18,8 +18,8 @@
 
 use util::{H256, Address, HeapSizeOf};
 
-use engines::Call;
-use header::Header;
+use engines::{Call, Engine};
+use header::{BlockNumber, Header};
 use super::ValidatorSet;
 
 /// Validator set containing a known set of addresses.
@@ -42,6 +42,20 @@ impl SimpleList {
 	}
 }
 
+impl ::std::ops::Deref for SimpleList {
+	type Target = [Address];
+
+	fn deref(&self) -> &[Address] { &self.validators }
+}
+
+impl From<Vec<Address>> for SimpleList {
+	fn from(validators: Vec<Address>) -> Self {
+		SimpleList {
+			validators: validators,
+		}
+	}
+}
+
 impl HeapSizeOf for SimpleList {
 	fn heap_size_of_children(&self) -> usize {
 		self.validators.heap_size_of_children()
@@ -53,18 +67,21 @@ impl ValidatorSet for SimpleList {
 		Box::new(|_, _| Err("Simple list doesn't require calls.".into()))
 	}
 
-	fn is_epoch_end(&self, _header: &Header, _block: Option<&[u8]>, _receipts: Option<&[::receipt::Receipt]>)
+	fn is_epoch_end(&self, first: bool, _chain_head: &Header) -> Option<Vec<u8>> {
+		match first {
+			true => Some(Vec::new()), // allow transition to fixed list, and instantly
+			false => None,
+		}
+	}
+
+	fn signals_epoch_end(&self, _: bool, _: &Header, _: Option<&[u8]>, _: Option<&[::receipt::Receipt]>)
 		-> ::engines::EpochChange
 	{
 		::engines::EpochChange::No
 	}
 
-	fn epoch_proof(&self, _header: &Header, _caller: &Call) -> Result<Vec<u8>, String> {
-		Ok(Vec::new())
-	}
-
-	fn epoch_set(&self, _header: &Header, _: &[u8]) -> Result<(u64, SimpleList), ::error::Error> {
-		Ok((0, self.clone()))
+	fn epoch_set(&self, _first: bool, _: &Engine, _: BlockNumber, _: &[u8]) -> Result<(SimpleList, Option<H256>), ::error::Error> {
+		Ok((self.clone(), None))
 	}
 
 	fn contains_with_caller(&self, _bh: &H256, address: &Address, _: &Call) -> bool {
@@ -73,6 +90,11 @@ impl ValidatorSet for SimpleList {
 
 	fn get_with_caller(&self, _bh: &H256, nonce: usize, _: &Call) -> Address {
 		let validator_n = self.validators.len();
+
+		if validator_n == 0 {
+			panic!("Cannot operate with an empty validator set.");
+		}
+
 		self.validators.get(nonce % validator_n).expect("There are validator_n authorities; taking number modulo validator_n gives number in validator_n range; qed").clone()
 	}
 

ethcore/src/engines/validator_set/test.rs

@@ -20,7 +20,7 @@ use std::str::FromStr;
 use std::sync::atomic::{AtomicUsize, Ordering as AtomicOrdering};
 use util::{Arc, Bytes, H256, Address, HeapSizeOf};
 
-use engines::Call;
+use engines::{Call, Engine};
 use header::{Header, BlockNumber};
 use super::{ValidatorSet, SimpleList};
 
@@ -52,18 +52,16 @@ impl ValidatorSet for TestSet {
 		Box::new(|_, _| Err("Test set doesn't require calls.".into()))
 	}
 
-	fn is_epoch_end(&self, _header: &Header, _block: Option<&[u8]>, _receipts: Option<&[::receipt::Receipt]>)
+	fn is_epoch_end(&self, _first: bool, _chain_head: &Header) -> Option<Vec<u8>> { None }
+
+	fn signals_epoch_end(&self, _: bool, _: &Header, _: Option<&[u8]>, _: Option<&[::receipt::Receipt]>)
 		-> ::engines::EpochChange
 	{
 		::engines::EpochChange::No
 	}
 
-	fn epoch_proof(&self, _header: &Header, _caller: &Call) -> Result<Vec<u8>, String> {
-		Ok(Vec::new())
-	}
-
-	fn epoch_set(&self, _header: &Header, _: &[u8]) -> Result<(u64, SimpleList), ::error::Error> {
-		Ok((0, self.validator.clone()))
+	fn epoch_set(&self, _: bool, _: &Engine, _: BlockNumber, _: &[u8]) -> Result<(SimpleList, Option<H256>), ::error::Error> {
+		Ok((self.validator.clone(), None))
 	}
 
 	fn contains_with_caller(&self, bh: &H256, address: &Address, _: &Call) -> bool {
@@ -78,11 +76,11 @@ impl ValidatorSet for TestSet {
 		1
 	}
 
-	fn report_malicious(&self, _validator: &Address, block: BlockNumber, _proof: Bytes) {
+	fn report_malicious(&self, _validator: &Address, _set_block: BlockNumber, block: BlockNumber, _proof: Bytes) {
 		self.last_malicious.store(block as usize, AtomicOrdering::SeqCst)
 	}
 
-	fn report_benign(&self, _validator: &Address, block: BlockNumber) {
+	fn report_benign(&self, _validator: &Address, _set_block: BlockNumber, block: BlockNumber) {
 		self.last_benign.store(block as usize, AtomicOrdering::SeqCst)
 	}
 }

ethcore/src/ethereum/ethash.rs

@@ -166,7 +166,6 @@ impl Ethash {
 // in the future, we might move the Ethash epoch
 // caching onto this mechanism as well.
 impl ::engines::EpochVerifier for Arc<Ethash> {
-	fn epoch_number(&self) -> u64 { 0 }
 	fn verify_light(&self, _header: &Header) -> Result<(), Error> { Ok(()) }
 	fn verify_heavy(&self, header: &Header) -> Result<(), Error> {
 		self.verify_block_unordered(header, None)
@@ -256,7 +255,12 @@ impl Engine for Arc<Ethash> {
 //		info!("ethash: populate_from_parent #{}: difficulty={} and gas_limit={}", header.number(), header.difficulty(), header.gas_limit());
 	}
 
-	fn on_new_block(&self, block: &mut ExecutedBlock, last_hashes: Arc<LastHashes>) -> Result<(), Error> {
+	fn on_new_block(
+		&self,
+		block: &mut ExecutedBlock,
+		last_hashes: Arc<LastHashes>,
+		_begins_epoch: bool,
+	) -> Result<(), Error> {
 		let parent_hash = block.fields().header.parent_hash().clone();
 		::engines::common::push_last_hash(block, last_hashes, self, &parent_hash)?;
 		if block.fields().header.number() == self.ethash_params.dao_hardfork_transition {
@@ -400,8 +404,8 @@ impl Engine for Arc<Ethash> {
 		Ok(())
 	}
 
-	fn epoch_verifier(&self, _header: &Header, _proof: &[u8]) -> Result<Box<::engines::EpochVerifier>, Error> {
-		Ok(Box::new(self.clone()))
+	fn epoch_verifier<'a>(&self, _header: &Header, _proof: &'a [u8]) -> ::engines::ConstructedVerifier<'a> {
+		::engines::ConstructedVerifier::Trusted(Box::new(self.clone()))
 	}
 
 	fn snapshot_components(&self) -> Option<Box<::snapshot::SnapshotComponents>> {
@@ -559,7 +563,7 @@ mod tests {
 		let genesis_header = spec.genesis_header();
 		let db = spec.ensure_db_good(get_temp_state_db(), &Default::default()).unwrap();
 		let last_hashes = Arc::new(vec![genesis_header.hash()]);
-		let b = OpenBlock::new(engine, Default::default(), false, db, &genesis_header, last_hashes, Address::zero(), (3141562.into(), 31415620.into()), vec![]).unwrap();
+		let b = OpenBlock::new(engine, Default::default(), false, db, &genesis_header, last_hashes, Address::zero(), (3141562.into(), 31415620.into()), vec![], false).unwrap();
 		let b = b.close();
 		assert_eq!(b.state().balance(&Address::zero()).unwrap(), U256::from_str("4563918244f40000").unwrap());
 	}
@@ -571,7 +575,7 @@ mod tests {
 		let genesis_header = spec.genesis_header();
 		let db = spec.ensure_db_good(get_temp_state_db(), &Default::default()).unwrap();
 		let last_hashes = Arc::new(vec![genesis_header.hash()]);
-		let mut b = OpenBlock::new(engine, Default::default(), false, db, &genesis_header, last_hashes, Address::zero(), (3141562.into(), 31415620.into()), vec![]).unwrap();
+		let mut b = OpenBlock::new(engine, Default::default(), false, db, &genesis_header, last_hashes, Address::zero(), (3141562.into(), 31415620.into()), vec![], false).unwrap();
 		let mut uncle = Header::new();
 		let uncle_author: Address = "ef2d6d194084c2de36e0dabfce45d046b37d1106".into();
 		uncle.set_author(uncle_author);

ethcore/src/migrations/v9.rs

@@ -51,7 +51,6 @@ impl ToV9 {
 }
 
 impl Migration for ToV9 {
-
 	fn columns(&self) -> Option<u32> { Some(5) }
 
 	fn version(&self) -> u32 { 9 }

ethcore/src/snapshot/consensus/authority.rs

@@ -24,34 +24,28 @@ use super::{SnapshotComponents, Rebuilder, ChunkSink};
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::Arc;
 
-use blockchain::{BlockChain, BlockProvider, EpochTransition};
-use engines::{Engine, EpochVerifier};
-use env_info::EnvInfo;
+use blockchain::{BlockChain, BlockProvider};
+use engines::{Engine, EpochVerifier, EpochTransition};
 use ids::BlockId;
 use header::Header;
 use receipt::Receipt;
 use snapshot::{Error, ManifestData};
-use state_db::StateDB;
 
 use itertools::{Position, Itertools};
 use rlp::{RlpStream, UntrustedRlp};
-use util::{Address, Bytes, H256, KeyValueDB, DBValue};
+use util::{Bytes, H256, KeyValueDB};
 
 /// Snapshot creation and restoration for PoA chains.
 /// Chunk format:
 ///
-/// [FLAG, [header, epoch_number, epoch data, state proof, last hashes], ...]
+/// [FLAG, [header, epoch data], ...]
 ///   - Header data at which transition occurred,
-///   - epoch data (usually list of validators)
-///   - state items required to check epoch data
-///   - last 256 hashes before the transition; required for checking state changes.
+///   - epoch data (usually list of validators and proof of change)
 ///
 /// FLAG is a bool: true for last chunk, false otherwise.
 ///
 /// The last item of the last chunk will be a list containing data for the warp target block:
-/// [header, transactions, uncles, receipts, last_hashes, parent_td].
-/// If this block is not a transition block, the epoch data should be the same as that
-/// for the last transition.
+/// [header, transactions, uncles, receipts, parent_td].
 pub struct PoaSnapshot;
 
 impl SnapshotComponents for PoaSnapshot {
@@ -68,35 +62,23 @@ impl SnapshotComponents for PoaSnapshot {
 		let mut pending_size = 0;
 		let mut rlps = Vec::new();
 
-		// TODO: this will become irrelevant after recent block hashes are moved into
-		// the state. can we optimize it out in that case?
-		let make_last_hashes = |parent_hash| chain.ancestry_iter(parent_hash)
-			.into_iter()
-			.flat_map(|inner| inner)
-			.take(255)
-			.collect::<Vec<_>>();
-
-		for (epoch_number, transition) in chain.epoch_transitions()
+		for (_, transition) in chain.epoch_transitions()
 			.take_while(|&(_, ref t)| t.block_number <= number)
 		{
+			// this can happen when our starting block is non-canonical.
+			if transition.block_number == number && transition.block_hash != block_at {
+				break
+			}
+
 			let header = chain.block_header_data(&transition.block_hash)
 				.ok_or(Error::BlockNotFound(transition.block_hash))?;
 
-			let last_hashes: Vec<_> = make_last_hashes(header.parent_hash());
-
 			let entry = {
-				let mut entry_stream = RlpStream::new_list(5);
+				let mut entry_stream = RlpStream::new_list(2);
 				entry_stream
 					.append_raw(&header.into_inner(), 1)
-					.append(&epoch_number)
 					.append(&transition.proof);
 
-				entry_stream.begin_list(transition.state_proof.len());
-				for item in transition.state_proof {
-					entry_stream.append(&&*item);
-				}
-
-				entry_stream.append_list(&last_hashes);
 				entry_stream.out()
 			};
 
@@ -121,16 +103,13 @@ impl SnapshotComponents for PoaSnapshot {
 			.map(|d| d.total_difficulty)
 			.ok_or(Error::BlockNotFound(block_at))?;
 
-		let last_hashes = make_last_hashes(*block.header.parent_hash());
-
 		rlps.push({
-			let mut stream = RlpStream::new_list(6);
+			let mut stream = RlpStream::new_list(5);
 			stream
 				.append(&block.header)
 				.append_list(&block.transactions)
 				.append_list(&block.uncles)
 				.append(&receipts)
-				.append_list(&last_hashes)
 				.append(&parent_td);
 			stream.out()
 		});
@@ -153,7 +132,7 @@ impl SnapshotComponents for PoaSnapshot {
 			db: db,
 			had_genesis: false,
 			unverified_firsts: Vec::new(),
-			last_proofs: Vec::new(),
+			last_epochs: Vec::new(),
 		}))
 	}
 
@@ -178,7 +157,7 @@ fn write_chunk(last: bool, chunk_data: &mut Vec<Bytes>, sink: &mut ChunkSink) ->
 // transition header is verifiable from the epoch data of the one prior.
 struct ChunkRebuilder {
 	manifest: ManifestData,
-	warp_target: Option<(Header, Vec<H256>)>,
+	warp_target: Option<Header>,
 	chain: BlockChain,
 	db: Arc<KeyValueDB>,
 	had_genesis: bool,
@@ -186,52 +165,16 @@ struct ChunkRebuilder {
 	// sorted vectors of unverified first blocks in a chunk
 	// and epoch data from last blocks in chunks.
 	// verification for these will be done at the end.
-	unverified_firsts: Vec<(u64, Header)>,
-	last_proofs: Vec<(u64, Header, Bytes)>,
+	unverified_firsts: Vec<(Header, Bytes, H256)>,
+	last_epochs: Vec<(Header, Box<EpochVerifier>)>,
 }
 
 // verified data.
 struct Verified {
-	epoch_number: u64,
 	epoch_transition: EpochTransition,
 	header: Header,
 }
 
-// make a transaction and env info.
-// TODO: hardcoded 50M to match constants in client.
-// would be nice to extract magic numbers, or better yet
-// off-chain transaction execution, into its own module.
-fn make_tx_and_env(
-	engine: &Engine,
-	addr: Address,
-	data: Bytes,
-	header: &Header,
-	last_hashes: Arc<Vec<H256>>,
-) -> (::transaction::SignedTransaction, EnvInfo) {
-	use transaction::{Action, Transaction};
-
-	let transaction = Transaction {
-		nonce: engine.account_start_nonce(header.number()),
-		action: Action::Call(addr),
-		gas: 50_000_000.into(),
-		gas_price: 0.into(),
-		value: 0.into(),
-		data: data,
-	}.fake_sign(Default::default());
-
-	let env = EnvInfo {
-		number: header.number(),
-		author: *header.author(),
-		timestamp: header.timestamp(),
-		difficulty: *header.difficulty(),
-		gas_limit: 50_000_000.into(),
-		last_hashes: last_hashes,
-		gas_used: 0.into(),
-	};
-
-	(transaction, env)
-}
-
 impl ChunkRebuilder {
 	fn verify_transition(
 		&mut self,
@@ -239,69 +182,49 @@ impl ChunkRebuilder {
 		transition_rlp: UntrustedRlp,
 		engine: &Engine,
 	) -> Result<Verified, ::error::Error> {
+		use engines::ConstructedVerifier;
+
 		// decode.
 		let header: Header = transition_rlp.val_at(0)?;
-		let epoch_number: u64 = transition_rlp.val_at(1)?;
-		let epoch_data: Bytes = transition_rlp.val_at(2)?;
-		let state_proof: Vec<DBValue> = transition_rlp.at(3)?
-			.iter()
-			.map(|x| Ok(DBValue::from_slice(x.data()?)))
-			.collect::<Result<_, ::rlp::DecoderError>>()?;
-		let last_hashes: Vec<H256> = transition_rlp.list_at(4)?;
-		let last_hashes = Arc::new(last_hashes);
+		let epoch_data: Bytes = transition_rlp.val_at(1)?;
 
-		trace!(target: "snapshot", "verifying transition to epoch {}", epoch_number);
+		trace!(target: "snapshot", "verifying transition to epoch at block {}", header.number());
 
 		// check current transition against validators of last epoch.
-		if let Some(verifier) = last_verifier.as_ref() {
-			verifier.verify_heavy(&header)?;
-		}
+		let new_verifier = match engine.epoch_verifier(&header, &epoch_data) {
+			ConstructedVerifier::Trusted(v) => v,
+			ConstructedVerifier::Unconfirmed(v, finality_proof, hash) => {
+				match *last_verifier {
+					Some(ref last) =>
+						if last.check_finality_proof(finality_proof).map_or(true, |hashes| !hashes.contains(&hash))
+					{
+						return Err(Error::BadEpochProof(header.number()).into());
+					},
+					None if header.number() != 0 => {
+						// genesis never requires additional validation.
 
-		{
-			// check the provided state proof actually leads to the
-			// given epoch data.
-			let caller = |addr, data| {
-				use state::{check_proof, ProvedExecution};
+						let idx = self.unverified_firsts
+							.binary_search_by_key(&header.number(), |&(ref h, _, _)| h.number())
+							.unwrap_or_else(|x| x);
 
-				let (transaction, env_info) = make_tx_and_env(
-					engine,
-					addr,
-					data,
-					&header,
-					last_hashes.clone(),
-				);
-
-				let result = check_proof(
-					&state_proof,
-					header.state_root().clone(),
-					&transaction,
-					engine,
-					&env_info,
-				);
-
-				match result {
-					ProvedExecution::Complete(executed) => Ok(executed.output),
-					_ => Err("Bad state proof".into()),
+						let entry = (header.clone(), finality_proof.to_owned(), hash);
+						self.unverified_firsts.insert(idx, entry);
+					}
+					None => {}
 				}
-			};
 
-			let extracted_proof = engine.epoch_proof(&header, &caller)
-				.map_err(|_| Error::BadEpochProof(epoch_number))?;
-
-			if extracted_proof != epoch_data {
-				return Err(Error::BadEpochProof(epoch_number).into());
+				v
 			}
-		}
+			ConstructedVerifier::Err(e) => return Err(e),
+		};
 
 		// create new epoch verifier.
-		*last_verifier = Some(engine.epoch_verifier(&header, &epoch_data)?);
+		*last_verifier = Some(new_verifier);
 
 		Ok(Verified {
-			epoch_number: epoch_number,
 			epoch_transition: EpochTransition {
 				block_hash: header.hash(),
 				block_number: header.number(),
-				state_proof: state_proof,
 				proof: epoch_data,
 			},
 			header: header,
@@ -327,6 +250,10 @@ impl Rebuilder for ChunkRebuilder {
 			num_items - 1
 		};
 
+		if num_transitions == 0 && !is_last_chunk {
+			return Err(Error::WrongChunkFormat("Found non-last chunk without any data.".into()).into());
+		}
+
 		let mut last_verifier = None;
 		let mut last_number = None;
 		for transition_rlp in rlp.iter().skip(1).take(num_transitions).with_position() {
@@ -356,41 +283,33 @@ impl Rebuilder for ChunkRebuilder {
 			if is_first {
 				// make sure the genesis transition was included,
 				// but it doesn't need verification later.
-				if verified.epoch_number == 0 && verified.header.number() == 0 {
+				if verified.header.number() == 0 {
 					if verified.header.hash() != self.chain.genesis_hash() {
 						return Err(Error::WrongBlockHash(0, verified.header.hash(), self.chain.genesis_hash()).into());
 					}
 
 					self.had_genesis = true;
-				} else {
-					let idx = self.unverified_firsts
-						.binary_search_by_key(&verified.epoch_number, |&(a, _)| a)
-						.unwrap_or_else(|x| x);
-
-					let entry = (verified.epoch_number, verified.header.clone());
-					self.unverified_firsts.insert(idx, entry);
 				}
 			}
 			if is_last {
-				let idx = self.last_proofs
-					.binary_search_by_key(&verified.epoch_number, |&(a, _, _)| a)
+				let idx = self.last_epochs
+					.binary_search_by_key(&verified.header.number(), |&(ref h, _)| h.number())
 					.unwrap_or_else(|x| x);
 
 				let entry = (
-					verified.epoch_number,
 					verified.header.clone(),
-					verified.epoch_transition.proof.clone()
+					last_verifier.take().expect("last_verifier always set after verify_transition; qed"),
 				);
-				self.last_proofs.insert(idx, entry);
+				self.last_epochs.insert(idx, entry);
 			}
 
 			// write epoch transition into database.
 			let mut batch = self.db.transaction();
-			self.chain.insert_epoch_transition(&mut batch, verified.epoch_number,
+			self.chain.insert_epoch_transition(&mut batch, verified.header.number(),
 				verified.epoch_transition);
 			self.db.write_buffered(batch);
 
-			trace!(target: "snapshot", "Verified epoch transition for epoch {}", verified.epoch_number);
+			trace!(target: "snapshot", "Verified epoch transition for epoch at block {}", verified.header.number());
 		}
 
 		if is_last_chunk {
@@ -413,84 +332,57 @@ impl Rebuilder for ChunkRebuilder {
 				}
 			}
 
-			let last_hashes: Vec<H256> = last_rlp.list_at(4)?;
-			let parent_td: ::util::U256 = last_rlp.val_at(5)?;
+			let parent_td: ::util::U256 = last_rlp.val_at(4)?;
 
 			let mut batch = self.db.transaction();
 			self.chain.insert_unordered_block(&mut batch, &block_data, receipts, Some(parent_td), true, false);
 			self.db.write_buffered(batch);
 
-			self.warp_target = Some((block.header, last_hashes));
+			self.warp_target = Some(block.header);
 		}
 
 		Ok(())
 	}
 
-	fn finalize(&mut self, db: StateDB, engine: &Engine) -> Result<(), ::error::Error> {
-		use state::State;
-
+	fn finalize(&mut self, _engine: &Engine) -> Result<(), ::error::Error> {
 		if !self.had_genesis {
 			return Err(Error::WrongChunkFormat("No genesis transition included.".into()).into());
 		}
 
-		let (target_header, target_last_hashes) = match self.warp_target.take() {
+		let target_header = match self.warp_target.take() {
 			Some(x) => x,
 			None => return Err(Error::WrongChunkFormat("Warp target block not included.".into()).into()),
 		};
 
-		// we store the last data even for the last chunk for easier verification
-		// of warp target, but we don't store genesis transition data.
-		// other than that, there should be a one-to-one correspondence of
-		// chunk ends to chunk beginnings.
-		if self.last_proofs.len() != self.unverified_firsts.len() + 1 {
-			return Err(Error::WrongChunkFormat("More than one 'last' chunk".into()).into());
-		}
-
 		// verify the first entries of chunks we couldn't before.
-		let lasts_iter = self.last_proofs.iter().map(|&(_, ref hdr, ref proof)| (hdr, &proof[..]));
-		let firsts_iter = self.unverified_firsts.iter().map(|&(_, ref hdr)| hdr);
+		// we store all last verifiers, but not all firsts.
+		// match each unverified first epoch with a last epoch verifier.
+		let mut lasts_reversed = self.last_epochs.iter().rev();
+		for &(ref header, ref finality_proof, hash) in self.unverified_firsts.iter().rev() {
+			let mut found = false;
+			while let Some(&(ref last_header, ref last_verifier)) = lasts_reversed.next() {
+				if last_header.number() < header.number() {
+					if last_verifier.check_finality_proof(&finality_proof).map_or(true, |hashes| !hashes.contains(&hash)) {
+						return Err(Error::BadEpochProof(header.number()).into());
+					}
+					found = true;
+					break;
+				}
+			}
 
-		for ((last_hdr, last_proof), first_hdr) in lasts_iter.zip(firsts_iter) {
-			let verifier = engine.epoch_verifier(&last_hdr, &last_proof)?;
-			verifier.verify_heavy(&first_hdr)?;
+			if !found {
+				return Err(Error::WrongChunkFormat("Inconsistent chunk ordering.".into()).into());
+			}
 		}
 
-		// verify that the validator set of the warp target is the same as that of the
-		// most recent transition. if the warp target was a transition itself,
-		// `last_data` will still be correct
-		let &(_, _, ref last_data) = self.last_proofs.last()
-			.expect("last_proofs known to have at least one element by the check above; qed");
+		// verify that the warp target verifies correctly the
+		// most recent epoch. if the warp target was a transition itself,
+		// it's already verified and doesn't need any more verification.
+		let &(ref header, ref last_epoch) = self.last_epochs.last()
+			.expect("last_epochs known to have at least one element by the check above; qed");
 
-		let target_last_hashes = Arc::new(target_last_hashes);
-		let caller = |addr, data| {
-			use executive::{Executive, TransactOptions};
-
-			let factories = ::factory::Factories::default();
-			let mut state = State::from_existing(
-				db.boxed_clone(),
-				self.manifest.state_root.clone(),
-				engine.account_start_nonce(target_header.number()),
-				factories,
-			).map_err(|e| format!("State root mismatch: {}", e))?;
-
-			let (tx, env_info) = make_tx_and_env(
-				engine,
-				addr,
-				data,
-				&target_header,
-				target_last_hashes.clone(),
-			);
-
-			let options = TransactOptions { tracing: false, vm_tracing: false, check_nonce: false };
-			Executive::new(&mut state, &env_info, engine)
-				.transact_virtual(&tx, options)
-				.map(|e| e.output)
-				.map_err(|e| format!("Error executing: {}", e))
-		};
-
-		let data = engine.epoch_proof(&target_header, &caller)?;
-		if &data[..] != &last_data[..] {
-			return Err(Error::WrongChunkFormat("Warp target has different epoch data than epoch transition.".into()).into())
+		if header != &target_header {
+			last_epoch.verify_heavy(&target_header)?;
 		}
 
 		Ok(())

ethcore/src/snapshot/consensus/mod.rs

@@ -93,7 +93,5 @@ pub trait Rebuilder: Send {
 	///
 	/// This should apply the necessary "glue" between chunks,
 	/// and verify against the restored state.
-	///
-	/// The database passed contains the state for the warp target block.
-	fn finalize(&mut self, db: ::state_db::StateDB, engine: &Engine) -> Result<(), ::error::Error>;
+	fn finalize(&mut self, engine: &Engine) -> Result<(), ::error::Error>;
 }

ethcore/src/snapshot/consensus/work.rs

@@ -283,7 +283,7 @@ impl Rebuilder for PowRebuilder {
 	}
 
 	/// Glue together any disconnected chunks and check that the chain is complete.
-	fn finalize(&mut self, _: ::state_db::StateDB, _: &Engine) -> Result<(), ::error::Error> {
+	fn finalize(&mut self, _: &Engine) -> Result<(), ::error::Error> {
 		let mut batch = self.db.transaction();
 
 		for (first_num, first_hash) in self.disconnected.drain(..) {
@@ -299,12 +299,12 @@ impl Rebuilder for PowRebuilder {
 		}
 
 		let genesis_hash = self.chain.genesis_hash();
-		self.chain.insert_epoch_transition(&mut batch, 0, ::blockchain::EpochTransition {
+		self.chain.insert_epoch_transition(&mut batch, 0, ::engines::EpochTransition {
 			block_number: 0,
 			block_hash: genesis_hash,
 			proof: vec![],
-			state_proof: vec![],
 		});
+
 		self.db.write_buffered(batch);
 		Ok(())
 	}

ethcore/src/snapshot/service.rs

@@ -164,11 +164,10 @@ impl Restoration {
 		}
 
 		// check for missing code.
-		let db = self.state.finalize(self.manifest.block_number, self.manifest.block_hash)?;
-		let db = ::state_db::StateDB::new(db, 0);
+		self.state.finalize(self.manifest.block_number, self.manifest.block_hash)?;
 
 		// connect out-of-order chunks and verify chain integrity.
-		self.secondary.finalize(db, engine)?;
+		self.secondary.finalize(engine)?;
 
 		if let Some(writer) = self.writer {
 			writer.finish(self.manifest)?;

ethcore/src/snapshot/tests/helpers.rs

@@ -26,7 +26,6 @@ use client::{BlockChainClient, Client};
 use engines::Engine;
 use snapshot::{StateRebuilder};
 use snapshot::io::{SnapshotReader, PackedWriter, PackedReader};
-use state_db::StateDB;
 
 use devtools::{RandomTempPath, GuardedTempResult};
 use rand::Rng;
@@ -195,8 +194,7 @@ pub fn restore(
 		secondary.feed(&snappy_buffer[..len], engine, &flag)?;
 	}
 
-	let jdb = state.finalize(manifest.block_number, manifest.block_hash)?;
-	let state_db = StateDB::new(jdb, 0);
-
-	secondary.finalize(state_db, engine)
+	trace!(target: "snapshot", "finalizing");
+	state.finalize(manifest.block_number, manifest.block_hash)?;
+	secondary.finalize(engine)
 }

ethcore/src/snapshot/tests/proof_of_authority.rs

@@ -21,11 +21,9 @@ use std::sync::Arc;
 use std::str::FromStr;
 
 use account_provider::AccountProvider;
-use client::{Client, BlockChainClient, MiningBlockChainClient};
+use client::{Client, BlockChainClient};
 use ethkey::Secret;
-use engines::Seal;
 use futures::Future;
-use miner::MinerService;
 use native_contracts::test_contracts::ValidatorSet;
 use snapshot::tests::helpers as snapshot_helpers;
 use spec::Spec;
@@ -37,7 +35,7 @@ use util::kvdb;
 
 const PASS: &'static str = "";
 const TRANSITION_BLOCK_1: usize = 2; // block at which the contract becomes activated.
-const TRANSITION_BLOCK_2: usize = 6; // block at which the second contract activates.
+const TRANSITION_BLOCK_2: usize = 10; // block at which the second contract activates.
 
 macro_rules! secret {
 	($e: expr) => { Secret::from_slice(&$e.sha3()) }
@@ -54,10 +52,10 @@ lazy_static! {
 }
 
 
-/// Contract code used here: https://gist.github.com/rphmeier/2de14fd365a969e3a9e10d77eb9a1e37
+/// Contract code used here: https://gist.github.com/anonymous/2a43783647e0f0dfcc359bd6fd81d6d9
 /// Account with secrets "1".sha3() is initially the validator.
 /// Transitions to the contract at block 2, initially same validator set.
-/// Create a new Spec with BasicAuthority which uses a contract at address 5 to determine the current validators using `getValidators`.
+/// Create a new Spec with AuthorityRound which uses a contract at address 5 to determine the current validators using `getValidators`.
 /// `native_contracts::test_contracts::ValidatorSet` provides a native wrapper for the ABi.
 fn spec_fixed_to_contract() -> Spec {
 	let data = include_bytes!("test_validator_contract.json");
@@ -101,45 +99,41 @@ fn make_chain(accounts: Arc<AccountProvider>, blocks_beyond: usize, transitions:
 	{
 		// push a block with given number, signed by one of the signers, with given transactions.
 		let push_block = |signers: &[Address], n, txs: Vec<SignedTransaction>| {
-			use block::IsBlock;
+			use miner::MinerService;
+
+			let idx = n as usize % signers.len();
+			trace!(target: "snapshot", "Pushing block #{}, {} txs, author={}",
+				n, txs.len(), signers[idx]);
+
+			client.miner().set_author(signers[idx]);
+			client.miner().import_external_transactions(&*client,
+				txs.into_iter().map(Into::into).collect());
 
 			let engine = client.engine();
-			let idx = n as usize % signers.len();
 			engine.set_signer(accounts.clone(), signers[idx], PASS.to_owned());
+			engine.step();
 
-			trace!(target: "snapshot", "Pushing block #{}, {} txs, author={}", n, txs.len(), signers[idx]);
-
-			let mut open_block = client.prepare_open_block(signers[idx], (5_000_000.into(), 5_000_000.into()), Vec::new());
-			for tx in txs {
-				open_block.push_transaction(tx, None).unwrap();
-			}
-			let block = open_block.close_and_lock();
-			let seal = match engine.generate_seal(block.block()) {
-				Seal::Regular(seal) => seal,
-				_ => panic!("Unable to generate seal for dummy chain block #{}", n),
-			};
-			let block = block.seal(&*engine, seal).unwrap();
-
-			client.import_sealed_block(block).unwrap();
+			assert_eq!(client.chain_info().best_block_number, n);
 		};
 
 		// execution callback for native contract: push transaction to be sealed.
 		let nonce = RefCell::new(client.engine().account_start_nonce(0));
-		let exec = |addr, data| {
+
+		// create useless transactions vector so we don't have to dig in
+		// and force sealing.
+		let make_useless_transactions = || {
 			let mut nonce = nonce.borrow_mut();
 			let transaction = Transaction {
 				nonce: *nonce,
-				gas_price: 0.into(),
-				gas: 1_000_000.into(),
-				action: Action::Call(addr),
-				value: 0.into(),
-				data: data,
+				gas_price: 1.into(),
+				gas: 21_000.into(),
+				action: Action::Call(Address::new()),
+				value: 1.into(),
+				data: Vec::new(),
 			}.sign(&*RICH_SECRET, client.signing_network_id());
 
-			client.miner().import_own_transaction(&*client, transaction.into()).unwrap();
-
 			*nonce = *nonce + 1.into();
-			Ok(Vec::new())
+			vec![transaction]
 		};
 
 		let contract_1 = ValidatorSet::new(*CONTRACT_ADDR_1);
@@ -156,8 +150,12 @@ fn make_chain(accounts: Arc<AccountProvider>, blocks_beyond: usize, transitions:
 				panic!("Bad test: issued epoch change before transition to contract.");
 			}
 
+			if (num as u64) < client.chain_info().best_block_number {
+				panic!("Bad test: issued epoch change before previous transition finalized.");
+			}
+
 			for number in client.chain_info().best_block_number + 1 .. num as u64 {
-				push_block(&cur_signers, number, vec![]);
+				push_block(&cur_signers, number, make_useless_transactions());
 			}
 
 			let pending = if manual {
@@ -167,22 +165,48 @@ fn make_chain(accounts: Arc<AccountProvider>, blocks_beyond: usize, transitions:
 					false => &contract_1,
 				};
 
-				contract.set_validators(&exec, new_set.clone()).wait().unwrap();
-				client.ready_transactions()
-					.into_iter()
-					.map(|x| x.transaction)
-					.collect()
+				let mut pending = Vec::new();
+				{
+					let mut exec = |addr, data| {
+						let mut nonce = nonce.borrow_mut();
+						let transaction = Transaction {
+							nonce: *nonce,
+							gas_price: 0.into(),
+							gas: 1_000_000.into(),
+							action: Action::Call(addr),
+							value: 0.into(),
+							data: data,
+						}.sign(&*RICH_SECRET, client.signing_network_id());
+
+						pending.push(transaction);
+
+						*nonce = *nonce + 1.into();
+						Ok(Vec::new())
+					};
+
+					contract.set_validators(&mut exec, new_set.clone()).wait().unwrap();
+				}
+
+				pending
 			} else {
-				Vec::new()
+				make_useless_transactions()
 			};
 
+			// push transition block.
 			push_block(&cur_signers, num as u64, pending);
+
+			// push blocks to finalize transition
+			for finalization_count in 1.. {
+				if finalization_count * 2 > cur_signers.len() { break }
+				push_block(&cur_signers, (num + finalization_count) as u64, make_useless_transactions());
+			}
+
 			cur_signers = new_set;
 		}
 
 		// make blocks beyond.
 		for number in (client.chain_info().best_block_number..).take(blocks_beyond) {
-			push_block(&cur_signers, number + 1, vec![]);
+			push_block(&cur_signers, number + 1, make_useless_transactions());
 		}
 	}
 
@@ -190,7 +214,7 @@ fn make_chain(accounts: Arc<AccountProvider>, blocks_beyond: usize, transitions:
 }
 
 #[test]
-fn fixed_to_contract() {
+fn fixed_to_contract_only() {
 	let (provider, addrs) = make_accounts(&[
 		RICH_SECRET.clone(),
 		secret!("foo"),
@@ -204,17 +228,21 @@ fn fixed_to_contract() {
 
 	assert!(provider.has_account(*RICH_ADDR).unwrap());
 
-	let client = make_chain(provider, 1, vec![
+	let client = make_chain(provider, 3, vec![
 		Transition::Manual(3, vec![addrs[2], addrs[3], addrs[5], addrs[7]]),
-		Transition::Manual(4, vec![addrs[0], addrs[1], addrs[4], addrs[6]]),
+		Transition::Manual(6, vec![addrs[0], addrs[1], addrs[4], addrs[6]]),
 	]);
 
-	assert_eq!(client.chain_info().best_block_number, 5);
+	// 6, 7, 8 prove finality for transition at 6.
+	// 3 beyond gets us to 11.
+	assert_eq!(client.chain_info().best_block_number, 11);
 	let reader = snapshot_helpers::snap(&*client);
 
 	let new_db = kvdb::in_memory(::db::NUM_COLUMNS.unwrap_or(0));
 	let spec = spec_fixed_to_contract();
 
+	// ensure fresh engine's step matches.
+	for _ in 0..11 { spec.engine.step() }
 	snapshot_helpers::restore(Arc::new(new_db), &*spec.engine, &**reader, &spec.genesis_block()).unwrap();
 }
 
@@ -233,17 +261,18 @@ fn fixed_to_contract_to_contract() {
 
 	assert!(provider.has_account(*RICH_ADDR).unwrap());
 
-	let client = make_chain(provider, 2, vec![
+	let client = make_chain(provider, 3, vec![
 		Transition::Manual(3, vec![addrs[2], addrs[3], addrs[5], addrs[7]]),
-		Transition::Manual(4, vec![addrs[0], addrs[1], addrs[4], addrs[6]]),
-		Transition::Implicit(5, vec![addrs[0]]),
-		Transition::Manual(8, vec![addrs[2], addrs[4], addrs[6], addrs[7]]),
+		Transition::Manual(6, vec![addrs[0], addrs[1], addrs[4], addrs[6]]),
+		Transition::Implicit(10, vec![addrs[0]]),
+		Transition::Manual(13, vec![addrs[2], addrs[4], addrs[6], addrs[7]]),
 	]);
 
-	assert_eq!(client.chain_info().best_block_number, 10);
+	assert_eq!(client.chain_info().best_block_number, 16);
 	let reader = snapshot_helpers::snap(&*client);
 	let new_db = kvdb::in_memory(::db::NUM_COLUMNS.unwrap_or(0));
 	let spec = spec_fixed_to_contract();
 
+	for _ in 0..16 { spec.engine.step() }
 	snapshot_helpers::restore(Arc::new(new_db), &*spec.engine, &**reader, &spec.genesis_block()).unwrap();
 }

ethcore/src/snapshot/tests/proof_of_work.rs

@@ -23,10 +23,8 @@ use blockchain::generator::{ChainGenerator, ChainIterator, BlockFinalizer};
 use blockchain::BlockChain;
 use snapshot::{chunk_secondary, Error as SnapshotError, Progress, SnapshotComponents};
 use snapshot::io::{PackedReader, PackedWriter, SnapshotReader, SnapshotWriter};
-use state_db::StateDB;
 
 use util::{Mutex, snappy};
-use util::journaldb::{self, Algorithm};
 use util::kvdb::{self, KeyValueDB, DBTransaction};
 
 use std::sync::Arc;
@@ -83,7 +81,6 @@ fn chunk_and_restore(amount: u64) {
 	// restore it.
 	let new_db = Arc::new(kvdb::in_memory(::db::NUM_COLUMNS.unwrap_or(0)));
 	let new_chain = BlockChain::new(Default::default(), &genesis, new_db.clone());
-	let new_state = StateDB::new(journaldb::new(new_db.clone(), Algorithm::Archive, None), 0);
 	let mut rebuilder = SNAPSHOT_MODE.rebuilder(new_chain, new_db.clone(), &manifest).unwrap();
 
 	let reader = PackedReader::new(&snapshot_path).unwrap().unwrap();
@@ -94,7 +91,7 @@ fn chunk_and_restore(amount: u64) {
 		rebuilder.feed(&chunk, engine.as_ref(), &flag).unwrap();
 	}
 
-	rebuilder.finalize(new_state, engine.as_ref()).unwrap();
+	rebuilder.finalize(engine.as_ref()).unwrap();
 	drop(rebuilder);
 
 	// and test it.

ethcore/src/snapshot/tests/test_validator_contract.json

@@ -1,15 +1,16 @@
 {
 	"name": "TestValidatorContract",
 	"engine": {
-		"basicAuthority": {
+		"authorityRound": {
 			"params": {
 				"gasLimitBoundDivisor": "0x0400",
-				"durationLimit": "0x0d",
+				"stepDuration": 1,
+				"startStep": 0,
 				"validators": {
 					"multi": {
 						"0": { "list": ["0x7d577a597b2742b498cb5cf0c26cdcd726d39e6e"] },
 						"2": { "contract": "0x0000000000000000000000000000000000000005" },
-						"6": { "contract": "0x0000000000000000000000000000000000000006" }
+						"10": { "contract": "0x0000000000000000000000000000000000000006" }
 					}
 				}
 			}
@@ -23,7 +24,10 @@
 	},
 	"genesis": {
 		"seal": {
-			"generic": "0xc180"
+			"authorityRound": {
+				"step": "0x0",
+				"signature": "0x0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
+			}
 		},
 		"difficulty": "0x20000",
 		"author": "0x0000000000000000000000000000000000000000",
@@ -39,11 +43,11 @@
 		"0000000000000000000000000000000000000004": { "balance": "1", "builtin": { "name": "identity", "pricing": { "linear": { "base": 15, "word": 3 } } } },
 		"0000000000000000000000000000000000000005": {
 			"balance": "1",
-			"constructor": "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"
+			"constructor": "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"
 		},
 		"0000000000000000000000000000000000000006": {
 			"balance": "1",
-			"constructor": "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"
+			"constructor": "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"
 		},
 		"0x7d577a597b2742b498cb5cf0c26cdcd726d39e6e": { "balance": "1606938044258990275541962092341162602522202993782792835301376" },
 		"0x82a978b3f5962a5b0957d9ee9eef472ee55b42f1": { "balance": "1606938044258990275541962092341162602522202993782792835301376" }

ethcore/src/tests/helpers.rs

@@ -193,7 +193,8 @@ pub fn generate_dummy_client_with_spec_accounts_and_data<F>(get_test_spec: F, ac
 			Arc::new(last_hashes.clone()),
 			author.clone(),
 			(3141562.into(), 31415620.into()),
-			vec![]
+			vec![],
+			false,
 		).unwrap();
 		b.set_difficulty(U256::from(0x20000));
 		rolling_timestamp += 10;

json/src/spec/authority_round.rs

@@ -49,6 +49,9 @@ pub struct AuthorityRoundParams {
 	/// Block from which monotonic steps start.
 	#[serde(rename="validateStepTransition")]
 	pub validate_step_transition: Option<Uint>,
+	/// Whether transitions should be immediate.
+	#[serde(rename="immediateTransitions")]
+	pub immediate_transitions: Option<bool>,
 }
 
 /// Authority engine deserialization.
@@ -92,5 +95,6 @@ mod tests {
 		assert!(deserialized.params.registrar.is_none());
 		assert_eq!(deserialized.params.start_step, Some(Uint(U256::from(24))));
 		assert_eq!(deserialized.params.eip155_transition, Some(Uint(U256::from(0x42))));
+		assert_eq!(deserialized.params.immediate_transitions, None);
 	}
 }