Merge pull request #5454 from paritytech/aura-contract-warp

Groundwork for generalized warp sync

Cargo.lock

@@ -339,8 +339,8 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "elastic-array"
-version = "0.6.0"
+version = "0.7.0"
-source = "git+https://github.com/paritytech/elastic-array#346f1ba5982576dab9d0b8fa178b50e1db0a21cd"
+source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "heapsize 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
@@ -670,7 +670,7 @@ version = "1.7.0"
 dependencies = [
  "ansi_term 0.9.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "clippy 0.0.103 (registry+https://github.com/rust-lang/crates.io-index)",
- "elastic-array 0.6.0 (git+https://github.com/paritytech/elastic-array)",
+ "elastic-array 0.7.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "env_logger 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "eth-secp256k1 0.5.6 (git+https://github.com/paritytech/rust-secp256k1)",
  "ethcore-bigint 0.1.2",
@@ -2030,7 +2030,7 @@ name = "rlp"
 version = "0.1.0"
 dependencies = [
  "byteorder 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
- "elastic-array 0.6.0 (git+https://github.com/paritytech/elastic-array)",
+ "elastic-array 0.7.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "ethcore-bigint 0.1.2",
  "lazy_static 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustc-serialize 0.3.19 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -2817,7 +2817,7 @@ dependencies = [
 "checksum docopt 0.7.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ab32ea6e284d87987066f21a9e809a73c14720571ef34516f0890b3d355ccfd8"
 "checksum dtoa 0.4.0 (registry+https://github.com/rust-lang/crates.io-index)" = "5edd69c67b2f8e0911629b7e6b8a34cb3956613cd7c6e6414966dee349c2db4f"
 "checksum either 1.0.2 (registry+https://github.com/rust-lang/crates.io-index)" = "3d2b503c86dad62aaf414ecf2b8c527439abedb3f8d812537f0b12bfd6f32a91"
-"checksum elastic-array 0.6.0 (git+https://github.com/paritytech/elastic-array)" = "<none>"
+"checksum elastic-array 0.7.0 (registry+https://github.com/rust-lang/crates.io-index)" = "71a64decd4b8cd06654a4e643c45cb558ad554abbffd82a7e16e34f45f51b605"
 "checksum env_logger 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)" = "e3856f1697098606fc6cb97a93de88ca3f3bc35bb878c725920e6e82ecf05e83"
 "checksum eth-secp256k1 0.5.6 (git+https://github.com/paritytech/rust-secp256k1)" = "<none>"
 "checksum ethabi 1.0.4 (registry+https://github.com/rust-lang/crates.io-index)" = "63df67d0af5e3cb906b667ca1a6e00baffbed87d0d8f5f78468a1f5eb3a66345"

ethcore/light/src/provider.rs

@@ -258,7 +258,7 @@ impl<T: ProvingBlockChainClient + ?Sized> Provider for T {
 		}.fake_sign(req.from);
 
 		self.prove_transaction(transaction, id)
-			.map(|proof| ::request::ExecutionResponse { items: proof })
+			.map(|(_, proof)| ::request::ExecutionResponse { items: proof })
 	}
 
 	fn ready_transactions(&self) -> Vec<PendingTransaction> {

ethcore/native_contracts/build.rs

@@ -22,9 +22,17 @@ use std::io::Write;
 
 // TODO: `include!` these from files where they're pretty-printed?
 const REGISTRY_ABI: &'static str = r#"[{"constant":true,"inputs":[{"name":"_data","type":"address"}],"name":"canReverse","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_new","type":"address"}],"name":"setOwner","outputs":[],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_name","type":"bytes32"},{"name":"_key","type":"string"},{"name":"_value","type":"bytes32"}],"name":"setData","outputs":[{"name":"success","type":"bool"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_name","type":"string"}],"name":"confirmReverse","outputs":[{"name":"success","type":"bool"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_name","type":"bytes32"}],"name":"reserve","outputs":[{"name":"success","type":"bool"}],"payable":true,"type":"function"},{"constant":false,"inputs":[{"name":"_name","type":"bytes32"}],"name":"drop","outputs":[{"name":"success","type":"bool"}],"payable":false,"type":"function"},{"constant":true,"inputs":[{"name":"_name","type":"bytes32"},{"name":"_key","type":"string"}],"name":"getAddress","outputs":[{"name":"","type":"address"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_amount","type":"uint256"}],"name":"setFee","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_name","type":"bytes32"},{"name":"_to","type":"address"}],"name":"transfer","outputs":[{"name":"success","type":"bool"}],"payable":false,"type":"function"},{"constant":true,"inputs":[],"name":"owner","outputs":[{"name":"","type":"address"}],"payable":false,"type":"function"},{"constant":true,"inputs":[{"name":"_name","type":"bytes32"},{"name":"_key","type":"string"}],"name":"getData","outputs":[{"name":"","type":"bytes32"}],"payable":false,"type":"function"},{"constant":true,"inputs":[{"name":"_name","type":"bytes32"}],"name":"reserved","outputs":[{"name":"reserved","type":"bool"}],"payable":false,"type":"function"},{"constant":false,"inputs":[],"name":"drain","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_name","type":"string"},{"name":"_who","type":"address"}],"name":"proposeReverse","outputs":[{"name":"success","type":"bool"}],"payable":false,"type":"function"},{"constant":true,"inputs":[{"name":"_name","type":"bytes32"}],"name":"hasReverse","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function"},{"constant":true,"inputs":[{"name":"_name","type":"bytes32"},{"name":"_key","type":"string"}],"name":"getUint","outputs":[{"name":"","type":"uint256"}],"payable":false,"type":"function"},{"constant":true,"inputs":[],"name":"fee","outputs":[{"name":"","type":"uint256"}],"payable":false,"type":"function"},{"constant":true,"inputs":[{"name":"_name","type":"bytes32"}],"name":"getOwner","outputs":[{"name":"","type":"address"}],"payable":false,"type":"function"},{"constant":true,"inputs":[{"name":"_name","type":"bytes32"}],"name":"getReverse","outputs":[{"name":"","type":"address"}],"payable":false,"type":"function"},{"constant":true,"inputs":[{"name":"_data","type":"address"}],"name":"reverse","outputs":[{"name":"","type":"string"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_name","type":"bytes32"},{"name":"_key","type":"string"},{"name":"_value","type":"uint256"}],"name":"setUint","outputs":[{"name":"success","type":"bool"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_name","type":"string"},{"name":"_who","type":"address"}],"name":"confirmReverseAs","outputs":[{"name":"success","type":"bool"}],"payable":false,"type":"function"},{"constant":false,"inputs":[],"name":"removeReverse","outputs":[],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_name","type":"bytes32"},{"name":"_key","type":"string"},{"name":"_value","type":"address"}],"name":"setAddress","outputs":[{"name":"success","type":"bool"}],"payable":false,"type":"function"}]"#;
+
 const SERVICE_TRANSACTION_ABI: &'static str = r#"[{"constant":false,"inputs":[{"name":"_new","type":"address"}],"name":"setOwner","outputs":[],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_who","type":"address"}],"name":"certify","outputs":[],"payable":false,"type":"function"},{"constant":true,"inputs":[{"name":"_who","type":"address"},{"name":"_field","type":"string"}],"name":"getAddress","outputs":[{"name":"","type":"address"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_who","type":"address"}],"name":"revoke","outputs":[],"payable":false,"type":"function"},{"constant":true,"inputs":[],"name":"owner","outputs":[{"name":"","type":"address"}],"payable":false,"type":"function"},{"constant":true,"inputs":[],"name":"delegate","outputs":[{"name":"","type":"address"}],"payable":false,"type":"function"},{"constant":true,"inputs":[{"name":"_who","type":"address"},{"name":"_field","type":"string"}],"name":"getUint","outputs":[{"name":"","type":"uint256"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_new","type":"address"}],"name":"setDelegate","outputs":[],"payable":false,"type":"function"},{"constant":true,"inputs":[{"name":"_who","type":"address"}],"name":"certified","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function"},{"constant":true,"inputs":[{"name":"_who","type":"address"},{"name":"_field","type":"string"}],"name":"get","outputs":[{"name":"","type":"bytes32"}],"payable":false,"type":"function"}]"#;
+
 const SECRETSTORE_ACL_STORAGE_ABI: &'static str = r#"[{"constant":true,"inputs":[{"name":"user","type":"address"},{"name":"document","type":"bytes32"}],"name":"checkPermissions","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function"}]"#;
 
+// be very careful changing these: ensure `ethcore/engines` validator sets have corresponding
+// changes.
+const VALIDATOR_SET_ABI: &'static str = r#"[{"constant":true,"inputs":[],"name":"transitionNonce","outputs":[{"name":"nonce","type":"uint256"}],"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"}]"#;
+
+const VALIDATOR_REPORT_ABI: &'static str = r#"[{"constant":false,"inputs":[{"name":"validator","type":"address"}],"name":"reportMalicious","outputs":[],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"validator","type":"address"}],"name":"reportBenign","outputs":[],"payable":false,"type":"function"}]"#;
+
 fn build_file(name: &str, abi: &str, filename: &str) {
 	let code = ::native_contract_generator::generate_module(name, abi).unwrap();
 
@@ -39,4 +47,6 @@ fn main() {
 	build_file("Registry", REGISTRY_ABI, "registry.rs");
 	build_file("ServiceTransactionChecker", SERVICE_TRANSACTION_ABI, "service_transaction.rs");
 	build_file("SecretStoreAclStorage", SECRETSTORE_ACL_STORAGE_ABI, "secretstore_acl_storage.rs");
+	build_file("ValidatorSet", VALIDATOR_SET_ABI, "validator_set.rs");
+	build_file("ValidatorReport", VALIDATOR_REPORT_ABI, "validator_report.rs");
 }

ethcore/native_contracts/generator/src/lib.rs

@@ -46,8 +46,8 @@ pub fn generate_module(struct_name: &str, abi: &str) -> Result<String, Error> {
 
 	Ok(format!(r##"
 use byteorder::{{BigEndian, ByteOrder}};
-use futures::{{future, Future, BoxFuture}};
+use futures::{{future, Future, IntoFuture, BoxFuture}};
-use ethabi::{{Contract, Interface, Token}};
+use ethabi::{{Contract, Interface, Token, Event}};
 use util::{{self, Uint}};
 
 pub struct {name} {{
@@ -70,6 +70,11 @@ impl {name} {{
 		}}
 	}}
 
+	/// Access the underlying `ethabi` contract.
+	pub fn contract(this: &Self) -> &Contract {{
+		&this.contract
+	}}
+
 	{functions}
 }}
 "##,
@@ -99,7 +104,10 @@ fn generate_functions(contract: &Contract) -> Result<String, Error> {
 /// Inputs: {abi_inputs:?}
 /// 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, U: Future<Item=Vec<u8>, Error=String> + Send + 'static
+	where
+	    F: Fn(util::Address, Vec<u8>) -> U,
+	    U: IntoFuture<Item=Vec<u8>, Error=String>,
+		U::Future: Send + 'static
 {{
 	let function = self.contract.function(r#"{abi_name}"#.to_string())
 		.expect("function existence checked at compile-time; qed");
@@ -111,6 +119,7 @@ pub fn {snake_name}<F, U>(&self, call: F, {params}) -> BoxFuture<{output_type},
 	}};
 
 	call_future
+		.into_future()
 		.and_then(move |out| function.decode_output(out).map_err(|e| format!("{{:?}}", e)))
 		.map(::std::collections::VecDeque::from)
 		.and_then(|mut outputs| {decode_outputs})
@@ -299,10 +308,10 @@ fn detokenize(name: &str, output_type: ParamType) -> String {
 		ParamType::Bool => format!("{}.to_bool()", name),
 		ParamType::String => format!("{}.to_string()", name),
 		ParamType::Array(kind) => {
-			let read_array = format!("x.into_iter().map(|a| {{ {} }}).collect::<Option<Vec<_>>()",
+			let read_array = format!("x.into_iter().map(|a| {{ {} }}).collect::<Option<Vec<_>>>()",
 				detokenize("a", *kind));
 
-			format!("{}.to_array().and_then(|x| {})",
+			format!("{}.to_array().and_then(|x| {{ {} }})",
 				name, read_array)
 		}
 		ParamType::FixedArray(_, _) => panic!("Fixed-length arrays not supported.")

ethcore/native_contracts/src/lib.rs

@@ -26,7 +26,11 @@ extern crate ethcore_util as util;
 mod registry;
 mod service_transaction;
 mod secretstore_acl_storage;
+mod validator_set;
+mod validator_report;
 
 pub use self::registry::Registry;
 pub use self::service_transaction::ServiceTransactionChecker;
 pub use self::secretstore_acl_storage::SecretStoreAclStorage;
+pub use self::validator_set::ValidatorSet;
+pub use self::validator_report::ValidatorReport;

ethcore/native_contracts/src/validator_report.rs

@@ -0,0 +1,22 @@
+// 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/>.
+
+#![allow(unused_mut, unused_variables, unused_imports)]
+
+//! Validator reporting.
+// TODO: testing.
+
+include!(concat!(env!("OUT_DIR"), "/validator_report.rs"));

ethcore/native_contracts/src/validator_set.rs

@@ -0,0 +1,22 @@
+// 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/>.
+
+#![allow(unused_mut, unused_variables, unused_imports)]
+
+//! Validator set contract.
+// TODO: testing.
+
+include!(concat!(env!("OUT_DIR"), "/validator_set.rs"));

ethcore/src/block.rs

@@ -484,7 +484,11 @@ impl LockedBlock {
 	/// Provide a valid seal in order to turn this into a `SealedBlock`.
 	/// This does check the validity of `seal` with the engine.
 	/// Returns the `ClosedBlock` back again if the seal is no good.
-	pub fn try_seal(self, engine: &Engine, seal: Vec<Bytes>) -> Result<SealedBlock, (Error, LockedBlock)> {
+	pub fn try_seal(
+		self,
+		engine: &Engine,
+		seal: Vec<Bytes>,
+	) -> Result<SealedBlock, (Error, LockedBlock)> {
 		let mut s = self;
 		s.block.header.set_seal(seal);
 		match engine.verify_block_seal(&s.block.header) {

ethcore/src/blockchain/blockchain.rs

@@ -419,6 +419,45 @@ impl<'a> Iterator for AncestryIter<'a> {
 	}
 }
 
+/// An iterator which walks all epoch transitions.
+/// Returns epoch transitions.
+pub struct EpochTransitionIter<'a> {
+	chain: &'a BlockChain,
+	prefix_iter: Box<Iterator<Item=(Box<[u8]>, Box<[u8]>)> + 'a>,
+}
+
+impl<'a> Iterator for EpochTransitionIter<'a> {
+	type Item = (u64, EpochTransition);
+
+	fn next(&mut self) -> Option<Self::Item> {
+		loop {
+			match self.prefix_iter.next() {
+				Some((key, val)) => {
+					// iterator may continue beyond values beginning with this
+					// prefix.
+					if !key.starts_with(&EPOCH_KEY_PREFIX[..]) { return None }
+
+					let transitions: EpochTransitions = ::rlp::decode(&val[..]);
+
+					// if there are multiple candidates, at most one will be on the
+					// canon chain.
+					for transition in transitions.candidates.into_iter() {
+						let is_in_canon_chain = self.chain.block_hash(transition.block_number)
+							.map_or(false, |hash| hash == transition.block_hash);
+
+						if is_in_canon_chain {
+							return Some((transitions.number, transition))
+						}
+					}
+
+					// some epochs never occurred on the main chain.
+				}
+				None => return None,
+			}
+		}
+	}
+}
+
 impl BlockChain {
 	/// Create new instance of blockchain from given Genesis.
 	pub fn new(config: Config, genesis: &[u8], db: Arc<KeyValueDB>) -> BlockChain {
@@ -804,6 +843,35 @@ impl BlockChain {
 		}
 	}
 
+	/// Insert an epoch transition. Provide an epoch number being transitioned to
+	/// and epoch transition object.
+	///
+	/// The block the transition occurred at should have already been inserted into the chain.
+	pub fn insert_epoch_transition(&self, batch: &mut DBTransaction, epoch_num: u64, transition: EpochTransition) {
+		let mut transitions = match self.db.read(db::COL_EXTRA, &epoch_num) {
+			Some(existing) => existing,
+			None => EpochTransitions {
+				number: epoch_num,
+				candidates: Vec::with_capacity(1),
+			}
+		};
+
+		// ensure we don't write any duplicates.
+		if transitions.candidates.iter().find(|c| c.block_hash == transition.block_hash).is_none() {
+			transitions.candidates.push(transition);
+			batch.write(db::COL_EXTRA, &epoch_num, &transitions);
+		}
+	}
+
+	/// Iterate over all epoch transitions.
+	pub fn epoch_transitions(&self) -> EpochTransitionIter {
+		let iter = self.db.iter_from_prefix(db::COL_EXTRA, &EPOCH_KEY_PREFIX[..]);
+		EpochTransitionIter {
+			chain: self,
+			prefix_iter: iter,
+		}
+	}
+
 	/// Add a child to a given block. Assumes that the block hash is in
 	/// the chain and the child's parent is this block.
 	///
@@ -2114,4 +2182,58 @@ mod tests {
 		assert_eq!(bc.rewind(), Some(genesis_hash.clone()));
 		assert_eq!(bc.rewind(), None);
 	}
+
+	#[test]
+	fn epoch_transitions_iter() {
+		use blockchain::extras::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 uncle = canon_chain.fork(1).generate(&mut finalizer.fork()).unwrap();
+
+			let mut batch = db.transaction();
+			// create a longer fork
+			for i in 0..5 {
+				let canon_block = canon_chain.generate(&mut finalizer).unwrap();
+				let hash = BlockView::new(&canon_block).header_view().sha3();
+
+				bc.insert_block(&mut batch, &canon_block, vec![]);
+				bc.insert_epoch_transition(&mut batch, i, EpochTransition {
+					block_hash: hash,
+					block_number: i + 1,
+					proof: vec![],
+					state_proof: vec![],
+				});
+				bc.commit();
+			}
+
+			assert_eq!(bc.best_block_number(), 5);
+
+			let hash = BlockView::new(&uncle).header_view().sha3();
+			bc.insert_block(&mut batch, &uncle, vec![]);
+			bc.insert_epoch_transition(&mut batch, 999, EpochTransition {
+				block_hash: hash,
+				block_number: 1,
+				proof: vec![],
+				state_proof: vec![]
+			});
+
+			db.write(batch).unwrap();
+			bc.commit();
+
+			// epoch 999 not in canonical chain.
+			assert_eq!(bc.epoch_transitions().map(|(i, _)| i).collect::<Vec<_>>(), vec![0, 1, 2, 3, 4]);
+		}
+
+		// re-loading the blockchain should load the correct best block.
+		let bc = new_chain(&genesis, db);
+
+		assert_eq!(bc.best_block_number(), 5);
+		assert_eq!(bc.epoch_transitions().map(|(i, _)| i).collect::<Vec<_>>(), vec![0, 1, 2, 3, 4]);
+	}
 }

ethcore/src/blockchain/extras.rs

@@ -18,6 +18,7 @@
 
 use bloomchain;
 use util::*;
+use util::kvdb::PREFIX_LEN as DB_PREFIX_LEN;
 use rlp::*;
 use header::BlockNumber;
 use receipt::Receipt;
@@ -37,6 +38,8 @@ pub enum ExtrasIndex {
 	BlocksBlooms = 3,
 	/// Block receipts index
 	BlockReceipts = 4,
+	/// Epoch transition data index.
+	EpochTransitions = 5,
 }
 
 fn with_index(hash: &H256, i: ExtrasIndex) -> H264 {
@@ -134,6 +137,36 @@ impl Key<BlockReceipts> for H256 {
 	}
 }
 
+/// length of epoch keys.
+pub const EPOCH_KEY_LEN: usize = DB_PREFIX_LEN + 16;
+
+/// epoch key prefix.
+/// used to iterate over all epoch transitions in order from genesis.
+pub const EPOCH_KEY_PREFIX: &'static [u8; DB_PREFIX_LEN] = &[
+	ExtrasIndex::EpochTransitions as u8, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+];
+
+pub struct EpochTransitionsKey([u8; EPOCH_KEY_LEN]);
+impl Deref for EpochTransitionsKey {
+	type Target = [u8];
+
+	fn deref(&self) -> &[u8] { &self.0[..] }
+}
+
+impl Key<EpochTransitions> for u64 {
+	type Target = EpochTransitionsKey;
+
+	fn key(&self) -> Self::Target {
+		let mut arr = [0u8; EPOCH_KEY_LEN];
+		arr[..DB_PREFIX_LEN].copy_from_slice(&EPOCH_KEY_PREFIX[..]);
+
+		write!(&mut arr[DB_PREFIX_LEN..], "{:016x}", self)
+			.expect("format arg is valid; no more than 16 chars will be written; qed");
+
+		EpochTransitionsKey(arr)
+	}
+}
+
 /// Familial details concerning a block
 #[derive(Debug, Clone)]
 pub struct BlockDetails {
@@ -144,7 +177,7 @@ pub struct BlockDetails {
 	/// Parent block hash
 	pub parent: H256,
 	/// List of children block hashes
-	pub children: Vec<H256>
+	pub children: Vec<H256>,
 }
 
 impl HeapSizeOf for BlockDetails {
@@ -241,6 +274,63 @@ impl HeapSizeOf for BlockReceipts {
 	}
 }
 
+/// Candidate transitions to an epoch with specific number.
+#[derive(Clone)]
+pub struct EpochTransitions {
+	pub number: u64,
+	pub candidates: Vec<EpochTransition>,
+}
+
+impl Encodable for EpochTransitions {
+	fn rlp_append(&self, s: &mut RlpStream) {
+		s.begin_list(2).append(&self.number).append_list(&self.candidates);
+	}
+}
+
+impl Decodable for EpochTransitions {
+	fn decode(rlp: &UntrustedRlp) -> Result<Self, DecoderError> {
+		Ok(EpochTransitions {
+			number: rlp.val_at(0)?,
+			candidates: rlp.list_at(1)?,
+		})
+	}
+}
+
+#[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

@@ -31,5 +31,6 @@ 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/client.rs

@@ -32,13 +32,13 @@ use util::kvdb::*;
 // other
 use basic_types::Seal;
 use block::*;
-use blockchain::{BlockChain, BlockProvider, TreeRoute, ImportRoute};
+use blockchain::{BlockChain, BlockProvider, EpochTransition, TreeRoute, ImportRoute};
 use blockchain::extras::TransactionAddress;
 use client::Error as ClientError;
 use client::{
 	BlockId, TransactionId, UncleId, TraceId, ClientConfig, BlockChainClient,
 	MiningBlockChainClient, EngineClient, TraceFilter, CallAnalytics, BlockImportError, Mode,
-	ChainNotify, PruningInfo,
+	ChainNotify, PruningInfo, ProvingBlockChainClient,
 };
 use encoded;
 use engines::Engine;
@@ -49,7 +49,7 @@ use evm::{Factory as EvmFactory, Schedule};
 use executive::{Executive, Executed, TransactOptions, contract_address};
 use factory::Factories;
 use futures::{future, Future};
-use header::BlockNumber;
+use header::{BlockNumber, Header};
 use io::*;
 use log_entry::LocalizedLogEntry;
 use miner::{Miner, MinerService, TransactionImportResult};
@@ -247,17 +247,27 @@ impl Client {
 			exit_handler: Mutex::new(None),
 		});
 
+		// prune old states.
 		{
 			let state_db = client.state_db.lock().boxed_clone();
 			let chain = client.chain.read();
 			client.prune_ancient(state_db, &chain)?;
 		}
 
+		// ensure genesis epoch proof in the DB.
+		{
+			let chain = client.chain.read();
+			client.generate_epoch_proof(&spec.genesis_header(), 0, &*chain);
+		}
+
 		if let Some(reg_addr) = client.additional_params().get("registrar").and_then(|s| Address::from_str(s).ok()) {
 			trace!(target: "client", "Found registrar at {}", reg_addr);
 			let registrar = Registry::new(reg_addr);
 			*client.registrar.lock() = Some(registrar);
 		}
+
+		// ensure buffered changes are flushed.
+		client.db.read().flush().map_err(ClientError::Database)?;
 		Ok(client)
 	}
 
@@ -380,6 +390,12 @@ impl Client {
 			return Err(());
 		};
 
+		let verify_external_result = self.verifier.verify_block_external(header, &block.bytes, engine);
+		if let Err(e) = verify_external_result {
+			warn!(target: "client", "Stage 4 block verification failed for #{} ({})\nError: {:?}", header.number(), header.hash(), e);
+			return Err(());
+		};
+
 		// Check if Parent is in chain
 		let chain_has_parent = chain.block_header(header.parent_hash());
 		if let Some(parent) = chain_has_parent {
@@ -398,7 +414,7 @@ impl Client {
 
 			// Final Verification
 			if let Err(e) = self.verifier.verify_block_final(header, locked_block.block().header()) {
-				warn!(target: "client", "Stage 4 block verification failed for #{} ({})\nError: {:?}", header.number(), header.hash(), e);
+				warn!(target: "client", "Stage 5 block verification failed for #{} ({})\nError: {:?}", header.number(), header.hash(), e);
 				return Err(());
 			}
 
@@ -569,6 +585,22 @@ impl Client {
 		//let traces = From::from(block.traces().clone().unwrap_or_else(Vec::new));
 
 		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.
@@ -576,6 +608,7 @@ impl Client {
 
 		state.journal_under(&mut batch, number, hash).expect("DB commit failed");
 		let route = chain.insert_block(&mut batch, block_data, receipts);
+
 		self.tracedb.read().import(&mut batch, TraceImportRequest {
 			traces: traces.into(),
 			block_hash: hash.clone(),
@@ -595,9 +628,58 @@ impl Client {
 			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;
+
+		let mut batch = DBTransaction::new();
+		let hash = header.hash();
+		debug!(target: "client", "Generating validation proof for block {}", hash);
+
+		// 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."))?;
+
+				read_values.borrow_mut().extend(items);
+				Ok(result)
+			};
+
+			self.engine.epoch_proof(&header, &call)
+		};
+
+		// 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(),
+				});
+
+				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.");
+			}
+		}
+	}
+
 	// 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() {
@@ -814,7 +896,7 @@ impl Client {
 			},
 		};
 
-		snapshot::take_snapshot(&self.chain.read(), start_hash, db.as_hashdb(), writer, p)?;
+		snapshot::take_snapshot(&*self.engine, &self.chain.read(), start_hash, db.as_hashdb(), writer, p)?;
 
 		Ok(())
 	}
@@ -865,6 +947,20 @@ impl Client {
 			}
 		}
 	}
+
+	// transaction for calling contracts from services like engine.
+	// from the null sender, with 50M gas.
+	fn contract_call_tx(&self, block_id: BlockId, address: Address, data: Bytes) -> SignedTransaction {
+		let from = Address::default();
+		Transaction {
+			nonce: self.nonce(&from, block_id).unwrap_or_else(|| self.engine.account_start_nonce()),
+			action: Action::Call(address),
+			gas: U256::from(50_000_000),
+			gas_price: U256::default(),
+			value: U256::default(),
+			data: data,
+		}.fake_sign(from)
+	}
 }
 
 impl snapshot::DatabaseRestore for Client {
@@ -1456,15 +1552,7 @@ impl BlockChainClient for Client {
 	}
 
 	fn call_contract(&self, block_id: BlockId, address: Address, data: Bytes) -> Result<Bytes, String> {
-		let from = Address::default();
+		let transaction = self.contract_call_tx(block_id, address, data);
-		let transaction = Transaction {
-			nonce: self.latest_nonce(&from),
-			action: Action::Call(address),
-			gas: U256::from(50_000_000),
-			gas_price: U256::default(),
-			value: U256::default(),
-			data: data,
-		}.fake_sign(from);
 
 		self.call(&transaction, block_id, Default::default())
 			.map_err(|e| format!("{:?}", e))
@@ -1620,7 +1708,7 @@ impl MayPanic for Client {
 	}
 }
 
-impl ::client::ProvingBlockChainClient for Client {
+impl ProvingBlockChainClient for Client {
 	fn prove_storage(&self, key1: H256, key2: H256, id: BlockId) -> Option<(Vec<Bytes>, H256)> {
 		self.state_at(id)
 			.and_then(move |state| state.prove_storage(key1, key2).ok())
@@ -1631,7 +1719,7 @@ impl ::client::ProvingBlockChainClient for Client {
 			.and_then(move |state| state.prove_account(key1).ok())
 	}
 
-	fn prove_transaction(&self, transaction: SignedTransaction, id: BlockId) -> Option<Vec<DBValue>> {
+	fn prove_transaction(&self, transaction: SignedTransaction, id: BlockId) -> Option<(Bytes, Vec<DBValue>)> {
 		let (state, mut env_info) = match (self.state_at(id), self.env_info(id)) {
 			(Some(s), Some(e)) => (s, e),
 			_ => return None,
@@ -1646,8 +1734,9 @@ impl ::client::ProvingBlockChainClient for Client {
 		let res = Executive::new(&mut state, &env_info, &*self.engine, &self.factories.vm).transact(&transaction, options);
 
 		match res {
-			Err(ExecutionError::Internal(_)) => return None,
+			Err(ExecutionError::Internal(_)) => None,
-			_ => return Some(state.drop().1.extract_proof()),
+			Err(_) => Some((Vec::new(), state.drop().1.extract_proof())),
+			Ok(res) => Some((res.output, state.drop().1.extract_proof())),
 		}
 	}
 }

ethcore/src/client/test_client.rs

@@ -769,7 +769,7 @@ impl ProvingBlockChainClient for TestBlockChainClient {
 		None
 	}
 
-	fn prove_transaction(&self, _: SignedTransaction, _: BlockId) -> Option<Vec<DBValue>> {
+	fn prove_transaction(&self, _: SignedTransaction, _: BlockId) -> Option<(Bytes, Vec<DBValue>)> {
 		None
 	}
 }

ethcore/src/client/traits.rs

@@ -327,5 +327,7 @@ pub trait ProvingBlockChainClient: BlockChainClient {
 	fn prove_account(&self, key1: H256, id: BlockId) -> Option<(Vec<Bytes>, BasicAccount)>;
 
 	/// Prove execution of a transaction at the given block.
-	fn prove_transaction(&self, transaction: SignedTransaction, id: BlockId) -> Option<Vec<DBValue>>;
+	/// Returns the output of the call and a vector of database items necessary
+	/// to reproduce it.
+	fn prove_transaction(&self, transaction: SignedTransaction, id: BlockId) -> Option<(Bytes, Vec<DBValue>)>;
 }

ethcore/src/engines/authority_round.rs

@@ -25,7 +25,7 @@ use rlp::{UntrustedRlp, encode};
 use account_provider::AccountProvider;
 use block::*;
 use spec::CommonParams;
-use engines::{Engine, Seal, EngineError};
+use engines::{Call, Engine, Seal, EngineError};
 use header::{Header, BlockNumber};
 use error::{Error, TransactionError, BlockError};
 use evm::Schedule;
@@ -36,7 +36,7 @@ use transaction::UnverifiedTransaction;
 use client::{Client, EngineClient};
 use state::CleanupMode;
 use super::signer::EngineSigner;
-use super::validator_set::{ValidatorSet, new_validator_set};
+use super::validator_set::{ValidatorSet, SimpleList, new_validator_set};
 
 /// `AuthorityRound` params.
 #[derive(Debug, PartialEq)]
@@ -74,27 +74,78 @@ impl From<ethjson::spec::AuthorityRoundParams> for AuthorityRoundParams {
 	}
 }
 
-/// Engine using `AuthorityRound` proof-of-work consensus algorithm, suitable for Ethereum
+// Helper for managing the step.
-/// mainnet chains in the Olympic, Frontier and Homestead eras.
+#[derive(Debug)]
+struct Step {
+	calibrate: bool, // whether calibration is enabled.
+	inner: AtomicUsize,
+	duration: Duration,
+}
+
+impl Step {
+	fn load(&self) -> usize { self.inner.load(AtomicOrdering::SeqCst) }
+	fn duration_remaining(&self) -> Duration {
+		let now = unix_now();
+		let step_end = self.duration * (self.load() as u32 + 1);
+		if step_end > now {
+			step_end - now
+		} else {
+			Duration::from_secs(0)
+		}
+	}
+	fn increment(&self) {
+		self.inner.fetch_add(1, AtomicOrdering::SeqCst);
+	}
+	fn calibrate(&self) {
+		if self.calibrate {
+			let new_step = unix_now().as_secs() / self.duration.as_secs();
+			self.inner.store(new_step as usize, AtomicOrdering::SeqCst);
+		}
+	}
+	fn is_future(&self, given: usize) -> bool {
+		if given > self.load() + 1 {
+			// Make absolutely sure that the given step is correct.
+			self.calibrate();
+			given > self.load() + 1
+		} else {
+			false
+		}
+	}
+}
+
+/// Engine using `AuthorityRound` proof-of-authority BFT consensus.
 pub struct AuthorityRound {
 	params: CommonParams,
 	gas_limit_bound_divisor: U256,
 	block_reward: U256,
 	registrar: Address,
-	step_duration: Duration,
 	builtins: BTreeMap<Address, Builtin>,
 	transition_service: IoService<()>,
-	step: AtomicUsize,
+	step: Arc<Step>,
 	proposed: AtomicBool,
 	client: RwLock<Option<Weak<EngineClient>>>,
 	signer: EngineSigner,
 	validators: Box<ValidatorSet>,
-	/// Is this Engine just for testing (prevents step calibration).
-	calibrate_step: bool,
 	validate_score_transition: u64,
 	eip155_transition: u64,
 }
 
+// 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)
+	}
+}
+
 fn header_step(header: &Header) -> Result<usize, ::rlp::DecoderError> {
 	UntrustedRlp::new(&header.seal().get(0).expect("was either checked with verify_block_basic or is genesis; has 2 fields; qed (Make sure the spec file has a correct genesis seal)")).as_val()
 }
@@ -103,6 +154,26 @@ 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> {
+	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());
+		Err(BlockError::InvalidSeal)?
+	} else {
+		let proposer_signature = header_signature(header)?;
+		let correct_proposer = validators.get(header.parent_hash(), header_step);
+		if !verify_address(&correct_proposer, &proposer_signature, &header.bare_hash())? {
+			trace!(target: "engine", "verify_block_unordered: bad proposer for step: {}", header_step);
+			Err(EngineError::NotProposer(Mismatch { expected: correct_proposer, found: header.author().clone() }))?
+		} else {
+			Ok(())
+		}
+	}
+}
+
 trait AsMillis {
 	fn as_millis(&self) -> u64;
 }
@@ -124,15 +195,17 @@ impl AuthorityRound {
 				gas_limit_bound_divisor: our_params.gas_limit_bound_divisor,
 				block_reward: our_params.block_reward,
 				registrar: our_params.registrar,
-				step_duration: our_params.step_duration,
 				builtins: builtins,
 				transition_service: IoService::<()>::start()?,
-				step: AtomicUsize::new(initial_step),
+				step: Arc::new(Step {
+					inner: AtomicUsize::new(initial_step),
+					calibrate: our_params.start_step.is_none(),
+					duration: our_params.step_duration,
+				}),
 				proposed: AtomicBool::new(false),
 				client: RwLock::new(None),
 				signer: Default::default(),
 				validators: new_validator_set(our_params.validators),
-				calibrate_step: our_params.start_step.is_none(),
 				validate_score_transition: our_params.validate_score_transition,
 				eip155_transition: our_params.eip155_transition,
 			});
@@ -144,22 +217,6 @@ impl AuthorityRound {
 		Ok(engine)
 	}
 
-	fn calibrate_step(&self) {
-		if self.calibrate_step {
-			self.step.store((unix_now().as_secs() / self.step_duration.as_secs()) as usize, AtomicOrdering::SeqCst);
-		}
-	}
-
-	fn remaining_step_duration(&self) -> Duration {
-		let now = unix_now();
-		let step_end = self.step_duration * (self.step.load(AtomicOrdering::SeqCst) as u32 + 1);
-		if step_end > now {
-			step_end - now
-		} else {
-			Duration::from_secs(0)
-		}
-	}
-
 	fn step_proposer(&self, bh: &H256, step: usize) -> Address {
 		self.validators.get(bh, step)
 	}
@@ -167,16 +224,6 @@ impl AuthorityRound {
 	fn is_step_proposer(&self, bh: &H256, step: usize, address: &Address) -> bool {
 		self.step_proposer(bh, step) == *address
 	}
-
-	fn is_future_step(&self, step: usize) -> bool {
-		if step > self.step.load(AtomicOrdering::SeqCst) + 1 {
-			// Make absolutely sure that the step is correct.
-			self.calibrate_step();
-			step > self.step.load(AtomicOrdering::SeqCst) + 1
-		} else {
-			false
-		}
-	}
 }
 
 fn unix_now() -> Duration {
@@ -192,7 +239,8 @@ const ENGINE_TIMEOUT_TOKEN: TimerToken = 23;
 impl IoHandler<()> for TransitionHandler {
 	fn initialize(&self, io: &IoContext<()>) {
 		if let Some(engine) = self.engine.upgrade() {
-			io.register_timer_once(ENGINE_TIMEOUT_TOKEN, engine.remaining_step_duration().as_millis())
+			let remaining = engine.step.duration_remaining();
+			io.register_timer_once(ENGINE_TIMEOUT_TOKEN, remaining.as_millis())
 				.unwrap_or_else(|e| warn!(target: "engine", "Failed to start consensus step timer: {}.", e))
 		}
 	}
@@ -201,7 +249,8 @@ impl IoHandler<()> for TransitionHandler {
 		if timer == ENGINE_TIMEOUT_TOKEN {
 			if let Some(engine) = self.engine.upgrade() {
 				engine.step();
-				io.register_timer_once(ENGINE_TIMEOUT_TOKEN, engine.remaining_step_duration().as_millis())
+				let remaining = engine.step.duration_remaining();
+				io.register_timer_once(ENGINE_TIMEOUT_TOKEN, remaining.as_millis())
 					.unwrap_or_else(|e| warn!(target: "engine", "Failed to restart consensus step timer: {}.", e))
 			}
 		}
@@ -223,7 +272,7 @@ impl Engine for AuthorityRound {
 	fn builtins(&self) -> &BTreeMap<Address, Builtin> { &self.builtins }
 
 	fn step(&self) {
-		self.step.fetch_add(1, AtomicOrdering::SeqCst);
+		self.step.increment();
 		self.proposed.store(false, AtomicOrdering::SeqCst);
 		if let Some(ref weak) = *self.client.read() {
 			if let Some(c) = weak.upgrade() {
@@ -247,7 +296,7 @@ impl Engine for AuthorityRound {
 
 	fn populate_from_parent(&self, header: &mut Header, parent: &Header, gas_floor_target: U256, _gas_ceil_target: U256) {
 		// Chain scoring: total weight is sqrt(U256::max_value())*height - step
-		let new_difficulty = U256::from(U128::max_value()) + header_step(parent).expect("Header has been verified; qed").into() - self.step.load(AtomicOrdering::SeqCst).into();
+		let new_difficulty = U256::from(U128::max_value()) + header_step(parent).expect("Header has been verified; qed").into() - self.step.load().into();
 		header.set_difficulty(new_difficulty);
 		header.set_gas_limit({
 			let gas_limit = parent.gas_limit().clone();
@@ -271,7 +320,7 @@ impl Engine for AuthorityRound {
 	fn generate_seal(&self, block: &ExecutedBlock) -> Seal {
 		if self.proposed.load(AtomicOrdering::SeqCst) { return Seal::None; }
 		let header = block.header();
-		let step = self.step.load(AtomicOrdering::SeqCst);
+		let step = self.step.load();
 		if self.is_step_proposer(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);
@@ -319,32 +368,19 @@ impl Engine for AuthorityRound {
 		Ok(())
 	}
 
-	/// Do the validator and gas limit validation.
+	/// Do the step and gas limit validation.
 	fn verify_block_family(&self, header: &Header, parent: &Header, _block: Option<&[u8]>) -> Result<(), Error> {
 		let step = header_step(header)?;
-		// Give one step slack if step is lagging, double vote is still not possible.
-		if self.is_future_step(step) {
-			trace!(target: "engine", "verify_block_unordered: block from the future");
-			self.validators.report_benign(header.author());
-			Err(BlockError::InvalidSeal)?
-		} else {
-			let proposer_signature = header_signature(header)?;
-			let correct_proposer = self.step_proposer(header.parent_hash(), step);
-			if !verify_address(&correct_proposer, &proposer_signature, &header.bare_hash())? {
-				trace!(target: "engine", "verify_block_unordered: bad proposer for step: {}", step);
-				Err(EngineError::NotProposer(Mismatch { expected: correct_proposer, found: header.author().clone() }))?
-			}
-		}
 
 		// Do not calculate difficulty for genesis blocks.
 		if header.number() == 0 {
 			return Err(From::from(BlockError::RidiculousNumber(OutOfBounds { min: Some(1), max: None, found: header.number() })));
 		}
 
-		// Check if parent is from a previous step.
+		// Ensure header is from the step after parent.
 		let parent_step = header_step(parent)?;
-		if step == parent_step {
+		if step <= parent_step {
-			trace!(target: "engine", "Multiple blocks proposed for step {}.", step);
+			trace!(target: "engine", "Multiple blocks proposed for step {}.", parent_step);
 			self.validators.report_malicious(header.author());
 			Err(EngineError::DoubleVote(header.author().clone()))?;
 		}
@@ -358,6 +394,34 @@ impl Engine for AuthorityRound {
 		Ok(())
 	}
 
+	// Check the validators.
+	fn verify_block_external(&self, header: &Header, _block: Option<&[u8]>) -> Result<(), Error> {
+		verify_external(header, &*self.validators, &*self.step)
+	}
+
+	// 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 is_epoch_end(&self, header: &Header, block: Option<&[u8]>, receipts: Option<&[::receipt::Receipt]>)
+		-> super::EpochChange
+	{
+		self.validators.is_epoch_end(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)?;
+
+		Ok(Box::new(EpochVerifier {
+			epoch_number: num,
+			step: self.step.clone(),
+			subchain_validators: simple_list,
+		}))
+	}
+
 	fn verify_transaction_basic(&self, t: &UnverifiedTransaction, header: &Header) -> result::Result<(), Error> {
 		t.check_low_s()?;
 
@@ -432,7 +496,7 @@ mod tests {
 		let mut header: Header = Header::default();
 		header.set_seal(vec![encode(&H520::default()).to_vec()]);
 
-		let verify_result = engine.verify_block_family(&header, &Default::default(), None);
+		let verify_result = engine.verify_block_external(&header, None);
 		assert!(verify_result.is_err());
 	}
 
@@ -486,9 +550,11 @@ mod tests {
 		// Two validators.
 		// Spec starts with step 2.
 		header.set_seal(vec![encode(&2usize).to_vec(), encode(&(&*signature as &[u8])).to_vec()]);
-		assert!(engine.verify_block_family(&header, &parent_header, None).is_err());
+		assert!(engine.verify_block_family(&header, &parent_header, None).is_ok());
+		assert!(engine.verify_block_external(&header, None).is_err());
 		header.set_seal(vec![encode(&1usize).to_vec(), encode(&(&*signature as &[u8])).to_vec()]);
 		assert!(engine.verify_block_family(&header, &parent_header, None).is_ok());
+		assert!(engine.verify_block_external(&header, None).is_ok());
 	}
 
 	#[test]
@@ -511,7 +577,33 @@ mod tests {
 		// Spec starts with step 2.
 		header.set_seal(vec![encode(&1usize).to_vec(), encode(&(&*signature as &[u8])).to_vec()]);
 		assert!(engine.verify_block_family(&header, &parent_header, None).is_ok());
+		assert!(engine.verify_block_external(&header, None).is_ok());
 		header.set_seal(vec![encode(&5usize).to_vec(), encode(&(&*signature as &[u8])).to_vec()]);
+		assert!(engine.verify_block_family(&header, &parent_header, None).is_ok());
+		assert!(engine.verify_block_external(&header, None).is_err());
+	}
+
+	#[test]
+	fn rejects_step_backwards() {
+		let tap = AccountProvider::transient_provider();
+		let addr = tap.insert_account(Secret::from_slice(&"0".sha3()).unwrap(), "0").unwrap();
+
+		let mut parent_header: Header = Header::default();
+		parent_header.set_seal(vec![encode(&4usize).to_vec()]);
+		parent_header.set_gas_limit(U256::from_str("222222").unwrap());
+		let mut header: Header = Header::default();
+		header.set_number(1);
+		header.set_gas_limit(U256::from_str("222222").unwrap());
+		header.set_author(addr);
+
+		let engine = Spec::new_test_round().engine;
+
+		let signature = tap.sign(addr, Some("0".into()), header.bare_hash()).unwrap();
+		// Two validators.
+		// Spec starts with step 2.
+		header.set_seal(vec![encode(&5usize).to_vec(), encode(&(&*signature as &[u8])).to_vec()]);
+		assert!(engine.verify_block_family(&header, &parent_header, None).is_ok());
+		header.set_seal(vec![encode(&3usize).to_vec(), encode(&(&*signature as &[u8])).to_vec()]);
 		assert!(engine.verify_block_family(&header, &parent_header, None).is_err());
 	}
 }

ethcore/src/engines/basic_authority.rs

@@ -23,14 +23,14 @@ use account_provider::AccountProvider;
 use block::*;
 use builtin::Builtin;
 use spec::CommonParams;
-use engines::{Engine, Seal};
+use engines::{Engine, EngineError, Seal, Call, EpochChange};
 use error::{BlockError, Error};
 use evm::Schedule;
 use ethjson;
 use header::{Header, BlockNumber};
 use client::Client;
 use super::signer::EngineSigner;
-use super::validator_set::{ValidatorSet, new_validator_set};
+use super::validator_set::{ValidatorSet, SimpleList, new_validator_set};
 
 /// `BasicAuthority` params.
 #[derive(Debug, PartialEq)]
@@ -50,8 +50,32 @@ impl From<ethjson::spec::BasicAuthorityParams> for BasicAuthorityParams {
 	}
 }
 
-/// Engine using `BasicAuthority` proof-of-work consensus algorithm, suitable for Ethereum
+struct EpochVerifier {
-/// mainnet chains in the Olympic, Frontier and Homestead eras.
+	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)
+	}
+}
+
+fn verify_external(header: &Header, validators: &ValidatorSet) -> Result<(), Error> {
+	use rlp::UntrustedRlp;
+
+	// Check if the signature belongs to a validator, can depend on parent state.
+	let sig = UntrustedRlp::new(&header.seal()[0]).as_val::<H520>()?;
+	let signer = public_to_address(&recover(&sig.into(), &header.bare_hash())?);
+
+	match validators.contains(header.parent_hash(), &signer) {
+		false => Err(BlockError::InvalidSeal.into()),
+		true => Ok(())
+	}
+}
+
+/// Engine using `BasicAuthority`, trivial proof-of-authority consensus.
 pub struct BasicAuthority {
 	params: CommonParams,
 	gas_limit_bound_divisor: U256,
@@ -137,14 +161,6 @@ impl Engine for BasicAuthority {
 	}
 
 	fn verify_block_family(&self, header: &Header, parent: &Header, _block: Option<&[u8]>) -> result::Result<(), Error> {
-		use rlp::UntrustedRlp;
-		// Check if the signature belongs to a validator, can depend on parent state.
-		let sig = UntrustedRlp::new(&header.seal()[0]).as_val::<H520>()?;
-		let signer = public_to_address(&recover(&sig.into(), &header.bare_hash())?);
-		if !self.validators.contains(header.parent_hash(), &signer) {
-			return Err(BlockError::InvalidSeal)?;
-		}
-
 		// Do not calculate difficulty for genesis blocks.
 		if header.number() == 0 {
 			return Err(From::from(BlockError::RidiculousNumber(OutOfBounds { min: Some(1), max: None, found: header.number() })));
@@ -163,6 +179,32 @@ impl Engine for BasicAuthority {
 		Ok(())
 	}
 
+	fn verify_block_external(&self, header: &Header, _block: Option<&[u8]>) -> Result<(), Error> {
+		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 is_epoch_end(&self, header: &Header, block: Option<&[u8]>, receipts: Option<&[::receipt::Receipt]>)
+		-> EpochChange
+	{
+		self.validators.is_epoch_end(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)?;
+
+		Ok(Box::new(EpochVerifier {
+			epoch_number: num,
+			list: simple_list,
+		}))
+	}
+
 	fn register_client(&self, client: Weak<Client>) {
 		self.validators.register_contract(client);
 	}

ethcore/src/engines/epoch_verifier.rs

@@ -0,0 +1,45 @@
+// 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: 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/mod.rs

@@ -16,35 +16,41 @@
 
 //! Consensus engine specification and basic implementations.
 
-mod transition;
-mod vote_collector;
-mod null_engine;
-mod instant_seal;
-mod basic_authority;
 mod authority_round;
-mod tendermint;
+mod basic_authority;
-mod validator_set;
+mod epoch_verifier;
+mod instant_seal;
+mod null_engine;
 mod signer;
+mod tendermint;
+mod transition;
+mod validator_set;
+mod vote_collector;
 
-pub use self::null_engine::NullEngine;
-pub use self::instant_seal::InstantSeal;
-pub use self::basic_authority::BasicAuthority;
 pub use self::authority_round::AuthorityRound;
+pub use self::basic_authority::BasicAuthority;
+pub use self::epoch_verifier::EpochVerifier;
+pub use self::instant_seal::InstantSeal;
+pub use self::null_engine::NullEngine;
 pub use self::tendermint::Tendermint;
 
 use std::sync::Weak;
-use util::*;
+
-use ethkey::Signature;
 use account_provider::AccountProvider;
 use block::ExecutedBlock;
 use builtin::Builtin;
+use client::Client;
 use env_info::EnvInfo;
 use error::Error;
-use spec::CommonParams;
 use evm::Schedule;
 use header::{Header, BlockNumber};
+use receipt::Receipt;
+use snapshot::SnapshotComponents;
+use spec::CommonParams;
 use transaction::{UnverifiedTransaction, SignedTransaction};
-use client::Client;
+
+use ethkey::Signature;
+use util::*;
 
 /// Voting errors.
 #[derive(Debug)]
@@ -59,6 +65,8 @@ pub enum EngineError {
 	UnexpectedMessage,
 	/// Seal field has an unexpected size.
 	BadSealFieldSize(OutOfBounds<usize>),
+	/// Validation proof insufficient.
+	InsufficientProof(String),
 }
 
 impl fmt::Display for EngineError {
@@ -70,6 +78,7 @@ impl fmt::Display for EngineError {
 			NotAuthorized(ref address) => format!("Signer {} is not authorized.", address),
 			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),
 		};
 
 		f.write_fmt(format_args!("Engine error ({})", msg))
@@ -87,6 +96,31 @@ pub enum Seal {
 	None,
 }
 
+/// Type alias for a function we can make calls through synchronously.
+pub type Call<'a> = Fn(Address, Bytes) -> Result<Bytes, String> + 'a;
+
+/// 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 and expected proof.
+	Yes(u64, Bytes),
+}
+
+/// More data required to determine if an epoch change occurred at a given block.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum Unsure {
+	/// Needs the body.
+	NeedsBody,
+	/// Needs the receipts.
+	NeedsReceipts,
+	/// Needs both body and receipts.
+	NeedsBoth,
+}
+
 /// A consensus mechanism for the chain. Generally either proof-of-work or proof-of-stake-based.
 /// Provides hooks into each of the major parts of block import.
 pub trait Engine : Sync + Send {
@@ -152,6 +186,9 @@ pub trait Engine : Sync + Send {
 	/// may be provided for additional checks. Returns either a null `Ok` or a general error detailing the problem with import.
 	fn verify_block_family(&self, _header: &Header, _parent: &Header, _block: Option<&[u8]>) -> Result<(), Error> { Ok(()) }
 
+	/// Phase 4 verification. Verify block header against potentially external data.
+	fn verify_block_external(&self, _header: &Header, _block: Option<&[u8]>) -> Result<(), Error> { Ok(()) }
+
 	/// Additional verification for transactions in blocks.
 	// TODO: Add flags for which bits of the transaction to check.
 	// TODO: consider including State in the params.
@@ -177,6 +214,40 @@ 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`
+	fn epoch_proof(&self, _header: &Header, _caller: &Call)
+		-> Result<Vec<u8>, Error>
+	{
+		Ok(Vec::new())
+	}
+
+	/// Whether an epoch change occurred at the given header.
+	/// Should not interact with state.
+	fn is_epoch_end(&self, _header: &Header, _block: Option<&[u8]>, _receipts: Option<&[Receipt]>)
+		-> EpochChange
+	{
+		EpochChange::No
+	}
+
+	/// Create an epoch verifier from validation proof.
+	///
+	/// 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))
+	}
+
 	/// Populate a header's fields based on its parent's header.
 	/// Usually implements the chain scoring rule based on weight.
 	/// The gas floor target must not be lower than the engine's minimum gas limit.
@@ -217,4 +288,10 @@ pub trait Engine : Sync + Send {
 
 	/// Stops any services that the may hold the Engine and makes it safe to drop.
 	fn stop(&self) {}
+
+	/// Create a factory for building snapshot chunks and restoring from them.
+	/// Returning `None` indicates that this engine doesn't support snapshot creation.
+	fn snapshot_components(&self) -> Option<Box<SnapshotComponents>> {
+		None
+	}
 }

ethcore/src/engines/null_engine.rs

@@ -60,4 +60,8 @@ impl Engine for NullEngine {
 	fn schedule(&self, _block_number: BlockNumber) -> Schedule {
 		Schedule::new_homestead()
 	}
+
+	fn snapshot_components(&self) -> Option<Box<::snapshot::SnapshotComponents>> {
+		Some(Box::new(::snapshot::PowSnapshot(10000)))
+	}
 }

ethcore/src/engines/validator_set/contract.rs

@@ -19,119 +19,96 @@
 
 use std::sync::Weak;
 use util::*;
+
+use futures::Future;
+use native_contracts::ValidatorReport as Provider;
+
 use client::{Client, BlockChainClient};
+use engines::Call;
+use header::Header;
+
 use super::ValidatorSet;
 use super::safe_contract::ValidatorSafeContract;
 
-/// The validator contract should have the following interface:
+/// A validator contract with reporting.
-/// [{"constant":true,"inputs":[],"name":"getValidators","outputs":[{"name":"","type":"address[]"}],"payable":false,"type":"function"}]
 pub struct ValidatorContract {
 	validators: ValidatorSafeContract,
-	provider: RwLock<Option<provider::Contract>>,
+	provider: Provider,
+	client: RwLock<Option<Weak<Client>>>, // TODO [keorn]: remove
 }
 
 impl ValidatorContract {
 	pub fn new(contract_address: Address) -> Self {
 		ValidatorContract {
 			validators: ValidatorSafeContract::new(contract_address),
-			provider: RwLock::new(None),
+			provider: Provider::new(contract_address),
+			client: RwLock::new(None),
 		}
 	}
 }
 
+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> {
+		let client = self.client.read().clone();
+		Box::new(move |a, d| client.as_ref()
+			.and_then(Weak::upgrade)
+			.ok_or("No client!".into())
+			.and_then(|c| c.transact_contract(a, d).map_err(|e| format!("Transaction import error: {}", e)))
+			.map(|_| Default::default()))
+	}
+}
+
 impl ValidatorSet for ValidatorContract {
-	fn contains(&self, bh: &H256, address: &Address) -> bool {
+	fn default_caller(&self, id: ::ids::BlockId) -> Box<Call> {
-		self.validators.contains(bh, address)
+		self.validators.default_caller(id)
 	}
 
-	fn get(&self, bh: &H256, nonce: usize) -> Address {
+	fn is_epoch_end(&self, header: &Header, block: Option<&[u8]>, receipts: Option<&[::receipt::Receipt]>)
-		self.validators.get(bh, nonce)
+		-> ::engines::EpochChange
+	{
+		self.validators.is_epoch_end(header, block, receipts)
 	}
 
-	fn count(&self, bh: &H256) -> usize {
+	fn epoch_proof(&self, header: &Header, caller: &Call) -> Result<Vec<u8>, String> {
-		self.validators.count(bh)
+		self.validators.epoch_proof(header, caller)
+	}
+
+	fn epoch_set(&self, header: &Header, proof: &[u8]) -> Result<(u64, super::SimpleList), ::error::Error> {
+		self.validators.epoch_set(header, proof)
+	}
+
+	fn contains_with_caller(&self, bh: &H256, address: &Address, caller: &Call) -> bool {
+		self.validators.contains_with_caller(bh, address, caller)
+	}
+
+	fn get_with_caller(&self, bh: &H256, nonce: usize, caller: &Call) -> Address {
+		self.validators.get_with_caller(bh, nonce, caller)
+	}
+
+	fn count_with_caller(&self, bh: &H256, caller: &Call) -> usize {
+		self.validators.count_with_caller(bh, caller)
 	}
 
 	fn report_malicious(&self, address: &Address) {
-		if let Some(ref provider) = *self.provider.read() {
+		match self.provider.report_malicious(&*self.transact(), *address).wait() {
-			match provider.report_malicious(address) {
+			Ok(_) => warn!(target: "engine", "Reported malicious validator {}", address),
-				Ok(_) => warn!(target: "engine", "Reported malicious validator {}", address),
+			Err(s) => warn!(target: "engine", "Validator {} could not be reported {}", address, s),
-				Err(s) => warn!(target: "engine", "Validator {} could not be reported {}", address, s),
-			}
-		} else {
-			warn!(target: "engine", "Malicious behaviour could not be reported: no provider contract.")
 		}
 	}
 
 	fn report_benign(&self, address: &Address) {
-		if let Some(ref provider) = *self.provider.read() {
+		match self.provider.report_benign(&*self.transact(), *address).wait() {
-			match provider.report_benign(address) {
+			Ok(_) => warn!(target: "engine", "Reported benign validator misbehaviour {}", address),
-				Ok(_) => warn!(target: "engine", "Reported benign validator misbehaviour {}", address),
+			Err(s) => warn!(target: "engine", "Validator {} could not be reported {}", address, s),
-				Err(s) => warn!(target: "engine", "Validator {} could not be reported {}", address, s),
-			}
-		} else {
-			warn!(target: "engine", "Benign misbehaviour could not be reported: no provider contract.")
 		}
 	}
 
 	fn register_contract(&self, client: Weak<Client>) {
 		self.validators.register_contract(client.clone());
-		let transact = move |a, d| client
+		*self.client.write() = Some(client);
-			.upgrade()
-			.ok_or("No client!".into())
-			.and_then(|c| c.transact_contract(a, d).map_err(|e| format!("Transaction import error: {}", e)))
-			.map(|_| Default::default());
-		*self.provider.write() = Some(provider::Contract::new(self.validators.address, transact));
-	}
-}
-
-mod provider {
-	// Autogenerated from JSON contract definition using Rust contract convertor.
-	#![allow(unused_imports)]
-	use std::string::String;
-	use std::result::Result;
-	use std::fmt;
-	use {util, ethabi};
-	use util::{Uint};
-
-	pub struct Contract {
-		contract: ethabi::Contract,
-		address: util::Address,
-		do_call: Box<Fn(util::Address, Vec<u8>) -> Result<Vec<u8>, String> + Send + Sync + 'static>,
-	}
-	impl Contract {
-		pub fn new<F>(address: util::Address, do_call: F) -> Self where F: Fn(util::Address, Vec<u8>) -> Result<Vec<u8>, String> + Send + Sync + 'static {
-			Contract {
-				contract: ethabi::Contract::new(ethabi::Interface::load(b"[{\"constant\":false,\"inputs\":[{\"name\":\"validator\",\"type\":\"address\"}],\"name\":\"reportMalicious\",\"outputs\":[],\"payable\":false,\"type\":\"function\"},{\"constant\":false,\"inputs\":[{\"name\":\"validator\",\"type\":\"address\"}],\"name\":\"reportBenign\",\"outputs\":[],\"payable\":false,\"type\":\"function\"}]").expect("JSON is autogenerated; qed")),
-				address: address,
-				do_call: Box::new(do_call),
-			}
-		}
-		fn as_string<T: fmt::Debug>(e: T) -> String { format!("{:?}", e) }
-
-		/// Auto-generated from: `{"constant":false,"inputs":[{"name":"validator","type":"address"}],"name":"reportMalicious","outputs":[],"payable":false,"type":"function"}`
-		#[allow(dead_code)]
-		pub fn report_malicious(&self, validator: &util::Address) -> Result<(), String> {
-			let call = self.contract.function("reportMalicious".into()).map_err(Self::as_string)?;
-			let data = call.encode_call(
-				vec![ethabi::Token::Address(validator.clone().0)]
-			).map_err(Self::as_string)?;
-			call.decode_output((self.do_call)(self.address.clone(), data)?).map_err(Self::as_string)?;
-
-			Ok(())
-		}
-
-		/// Auto-generated from: `{"constant":false,"inputs":[{"name":"validator","type":"address"}],"name":"reportBenign","outputs":[],"payable":false,"type":"function"}`
-		#[allow(dead_code)]
-		pub fn report_benign(&self, validator: &util::Address) -> Result<(), String> {
-			let call = self.contract.function("reportBenign".into()).map_err(Self::as_string)?;
-			let data = call.encode_call(
-				vec![ethabi::Token::Address(validator.clone().0)]
-			).map_err(Self::as_string)?;
-			call.decode_output((self.do_call)(self.address.clone(), data)?).map_err(Self::as_string)?;
-
-			Ok(())
-		}
 	}
 }
 
@@ -180,7 +157,7 @@ mod tests {
 		header.set_parent_hash(client.chain_info().best_block_hash);
 
 		// `reportBenign` when the designated proposer releases block from the future (bad clock).
-		assert!(client.engine().verify_block_family(&header, &header, None).is_err());
+		assert!(client.engine().verify_block_external(&header, None).is_err());
 		// Seal a block.
 		client.engine().step();
 		assert_eq!(client.chain_info().best_block_number, 1);

ethcore/src/engines/validator_set/mod.rs

@@ -22,14 +22,19 @@ mod contract;
 mod multi;
 
 use std::sync::Weak;
+use ids::BlockId;
 use util::{Address, H256};
 use ethjson::spec::ValidatorSet as ValidatorSpec;
 use client::Client;
-use self::simple_list::SimpleList;
+use header::Header;
+
+pub use self::simple_list::SimpleList;
 use self::contract::ValidatorContract;
 use self::safe_contract::ValidatorSafeContract;
 use self::multi::Multi;
 
+use super::Call;
+
 /// Creates a validator set from spec.
 pub fn new_validator_set(spec: ValidatorSpec) -> Box<ValidatorSet> {
 	match spec {
@@ -42,13 +47,69 @@ pub fn new_validator_set(spec: ValidatorSpec) -> Box<ValidatorSet> {
 	}
 }
 
+/// A validator set.
 pub trait ValidatorSet: Send + Sync {
-	/// Checks if a given address is a validator.
+	/// Get the default "Call" helper, for use in general operation.
-	fn contains(&self, parent_block_hash: &H256, address: &Address) -> bool;
+	// TODO [keorn]: this is a hack intended to migrate off of
+	// a strict dependency on state always being available.
+	fn default_caller(&self, block_id: BlockId) -> Box<Call>;
+
+	/// Checks if a given address is a validator,
+	/// using underlying, default call mechanism.
+	fn contains(&self, parent: &H256, address: &Address) -> bool {
+		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_block_hash: &H256, nonce: usize) -> Address;
+	fn get(&self, parent: &H256, nonce: usize) -> Address {
+		let default = self.default_caller(BlockId::Hash(*parent));
+		self.get_with_caller(parent, nonce, &*default)
+	}
+
 	/// Returns the current number of validators.
-	fn count(&self, parent_block_hash: &H256) -> usize;
+	fn count(&self, parent: &H256) -> usize {
+		let default = self.default_caller(BlockId::Hash(*parent));
+		self.count_with_caller(parent, &*default)
+	}
+
+	/// 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.
+	///
+	/// 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;
+
+	/// 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>;
+
+	/// Recover the validator set for all
+	///
+	/// 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>;
+
+	/// 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) {}
 	/// Notifies about benign misbehaviour.

ethcore/src/engines/validator_set/multi.rs

@@ -18,13 +18,14 @@
 
 use std::collections::BTreeMap;
 use std::sync::Weak;
+use engines::{Call, EpochChange};
 use util::{H256, Address, RwLock};
 use ids::BlockId;
-use header::BlockNumber;
+use header::{BlockNumber, Header};
 use client::{Client, BlockChainClient};
 use super::ValidatorSet;
 
-type BlockNumberLookup = Box<Fn(&H256) -> Result<BlockNumber, String> + Send + Sync + 'static>;
+type BlockNumberLookup = Box<Fn(BlockId) -> Result<BlockNumber, String> + Send + Sync + 'static>;
 
 pub struct Multi {
 	sets: BTreeMap<BlockNumber, Box<ValidatorSet>>,
@@ -40,42 +41,73 @@ impl Multi {
 		}
 	}
 
-	fn correct_set(&self, bh: &H256) -> Option<&Box<ValidatorSet>> {
+	fn correct_set(&self, id: BlockId) -> Option<&ValidatorSet> {
-		match self
+		match self.block_number.read()(id).map(|parent_block| self.correct_set_by_number(parent_block)) {
-			.block_number
+			Ok((_, set)) => Some(set),
-			.read()(bh)
-			.map(|parent_block| self
-					 .sets
-					 .iter()
-					 .rev()
-					 .find(|&(block, _)| *block <= parent_block + 1)
-					 .expect("constructor validation ensures that there is at least one validator set for block 0;
-									 block 0 is less than any uint;
-									 qed")
-				) {
-			Ok((block, set)) => {
-				trace!(target: "engine", "Multi ValidatorSet retrieved for block {}.", block);
-				Some(set)
-			},
 			Err(e) => {
 				debug!(target: "engine", "ValidatorSet could not be recovered: {}", e);
 				None
 			},
 		}
 	}
+
+	// get correct set by block number, along with block number at which
+	// this set was activated.
+	fn correct_set_by_number(&self, parent_block: BlockNumber) -> (BlockNumber, &ValidatorSet) {
+		let (block, set) = self.sets.iter()
+			.rev()
+			.find(|&(block, _)| *block <= parent_block + 1)
+			.expect("constructor validation ensures that there is at least one validator set for block 0;
+					 block 0 is less than any uint;
+					 qed");
+
+		trace!(target: "engine", "Multi ValidatorSet retrieved for block {}.", block);
+		(*block, &**set)
+	}
 }
 
 impl ValidatorSet for Multi {
-	fn contains(&self, bh: &H256, address: &Address) -> bool {
+	fn default_caller(&self, block_id: BlockId) -> Box<Call> {
-		self.correct_set(bh).map_or(false, |set| set.contains(bh, address))
+		self.correct_set(block_id).map(|set| set.default_caller(block_id))
+			.unwrap_or(Box::new(|_, _| Err("No validator set for given ID.".into())))
 	}
 
-	fn get(&self, bh: &H256, nonce: usize) -> Address {
+	fn is_epoch_end(&self, header: &Header, block: Option<&[u8]>, receipts: Option<&[::receipt::Receipt]>)
-		self.correct_set(bh).map_or_else(Default::default, |set| set.get(bh, nonce))
+		-> EpochChange
+	{
+		let (set_block, set) = self.correct_set_by_number(header.number());
+
+		match set.is_epoch_end(header, block, receipts) {
+			EpochChange::Yes(num, proof) => EpochChange::Yes(set_block + num, proof),
+			other => other,
+		}
 	}
 
-	fn count(&self, bh: &H256) -> usize {
+	fn epoch_proof(&self, header: &Header, caller: &Call) -> Result<Vec<u8>, String> {
-		self.correct_set(bh).map_or_else(usize::max_value, |set| set.count(bh))
+		self.correct_set_by_number(header.number()).1.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 (inner_epoch, list) = set.epoch_set(header, proof)?;
+		Ok((set_block + inner_epoch, list))
+	}
+
+	fn contains_with_caller(&self, bh: &H256, address: &Address, caller: &Call) -> bool {
+		self.correct_set(BlockId::Hash(*bh))
+			.map_or(false, |set| set.contains_with_caller(bh, address, caller))
+	}
+
+	fn get_with_caller(&self, bh: &H256, nonce: usize, caller: &Call) -> Address {
+		self.correct_set(BlockId::Hash(*bh))
+			.map_or_else(Default::default, |set| set.get_with_caller(bh, nonce, caller))
+	}
+
+	fn count_with_caller(&self, bh: &H256, caller: &Call) -> usize {
+		self.correct_set(BlockId::Hash(*bh))
+			.map_or_else(usize::max_value, |set| set.count_with_caller(bh, caller))
 	}
 
 	fn report_malicious(&self, validator: &Address) {
@@ -94,10 +126,10 @@ impl ValidatorSet for Multi {
 		for set in self.sets.values() {
 			set.register_contract(client.clone());
 		}
-		*self.block_number.write() = Box::new(move |hash| client
+		*self.block_number.write() = Box::new(move |id| client
 			.upgrade()
 			.ok_or("No client!".into())
-			.and_then(|c| c.block_number(BlockId::Hash(*hash)).ok_or("Unknown block".into())));
+			.and_then(|c| c.block_number(id).ok_or("Unknown block".into())));
 	}
 }
 

ethcore/src/engines/validator_set/safe_contract.rs

@@ -17,24 +17,45 @@
 /// Validator set maintained in a contract, updated using `getValidators` method.
 
 use std::sync::Weak;
-use ethabi;
+use futures::Future;
+use native_contracts::ValidatorSet as Provider;
+
 use util::*;
 use util::cache::MemoryLruCache;
-use types::ids::BlockId;
+
+use basic_types::LogBloom;
 use client::{Client, BlockChainClient};
+use engines::Call;
+use header::Header;
+use ids::BlockId;
+use log_entry::LogEntry;
+
 use super::ValidatorSet;
 use super::simple_list::SimpleList;
 
 const MEMOIZE_CAPACITY: usize = 500;
-const CONTRACT_INTERFACE: &'static [u8] = b"[{\"constant\":true,\"inputs\":[],\"name\":\"getValidators\",\"outputs\":[{\"name\":\"\",\"type\":\"address[]\"}],\"payable\":false,\"type\":\"function\"}]";
+
-const GET_VALIDATORS: &'static str = "getValidators";
+// TODO: ethabi should be able to generate this.
+const EVENT_NAME: &'static [u8] = &*b"ValidatorsChanged(bytes32,uint256,address[])";
+
+lazy_static! {
+	static ref EVENT_NAME_HASH: H256 = EVENT_NAME.sha3();
+}
 
 /// The validator contract should have the following interface:
-/// [{"constant":true,"inputs":[],"name":"getValidators","outputs":[{"name":"","type":"address[]"}],"payable":false,"type":"function"}]
 pub struct ValidatorSafeContract {
 	pub address: Address,
 	validators: RwLock<MemoryLruCache<H256, SimpleList>>,
-	provider: RwLock<Option<provider::Contract>>,
+	provider: Provider,
+	client: RwLock<Option<Weak<Client>>>, // TODO [keorn]: remove
+}
+
+fn encode_proof(nonce: U256, validators: &[Address]) -> Bytes {
+	use rlp::RlpStream;
+
+	let mut stream = RlpStream::new_list(2);
+	stream.append(&nonce).append_list(validators);
+	stream.drain().to_vec()
 }
 
 impl ValidatorSafeContract {
@@ -42,39 +63,168 @@ impl ValidatorSafeContract {
 		ValidatorSafeContract {
 			address: contract_address,
 			validators: RwLock::new(MemoryLruCache::new(MEMOIZE_CAPACITY)),
-			provider: RwLock::new(None),
+			provider: Provider::new(contract_address),
+			client: RwLock::new(None),
 		}
 	}
 
 	/// Queries the state and gets the set of validators.
-	fn get_list(&self, block_hash: H256) -> Option<SimpleList> {
+	fn get_list(&self, caller: &Call) -> Option<SimpleList> {
-		if let Some(ref provider) = *self.provider.read() {
+		match self.provider.get_validators(caller).wait() {
-			match provider.get_validators(BlockId::Hash(block_hash)) {
+			Ok(new) => {
-				Ok(new) => {
+				debug!(target: "engine", "Set of validators obtained: {:?}", new);
-					debug!(target: "engine", "Set of validators obtained: {:?}", new);
+				Some(SimpleList::new(new))
-					Some(SimpleList::new(new))
+			},
-				},
+			Err(s) => {
-				Err(s) => {
+				debug!(target: "engine", "Set of validators could not be updated: {}", s);
-					debug!(target: "engine", "Set of validators could not be updated: {}", s);
+				None
-					None
+			},
-				},
-			}
-		} else {
-			warn!(target: "engine", "Set of validators could not be updated: no provider contract.");
-			None
 		}
 	}
+
+	/// 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:
+	//   1. ETHABI-encoded log name.
+	//   2. the block's parent hash.
+	//   3. the "nonce": n for the nth transition in history.
+	//
+	// We can only search for the first 2, since we don't have the third
+	// just yet.
+	//
+	// The parent hash is included to prevent
+	// malicious actors from brute forcing other logs that would
+	// produce the same bloom.
+	//
+	// The log data is an array of all new validator addresses.
+	fn expected_bloom(&self, header: &Header) -> LogBloom {
+		LogEntry {
+			address: self.address,
+			topics: vec![*EVENT_NAME_HASH, *header.parent_hash()],
+			data: Vec::new(), // irrelevant for bloom.
+		}.bloom()
+	}
 }
 
 impl ValidatorSet for ValidatorSafeContract {
-	fn contains(&self, block_hash: &H256, address: &Address) -> bool {
+	fn default_caller(&self, id: BlockId) -> Box<Call> {
+		let client = self.client.read().clone();
+		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)))
+	}
+
+	fn is_epoch_end(&self, header: &Header, _block: Option<&[u8]>, receipts: Option<&[::receipt::Receipt]>)
+		-> ::engines::EpochChange
+	{
+		let bloom = self.expected_bloom(header);
+		let header_bloom = header.log_bloom();
+
+		if &bloom & header_bloom != bloom { return ::engines::EpochChange::No }
+
+		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.
+				};
+
+				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(validators)) => {
+								let proof = encode_proof(nonce, &validators);
+								let new_epoch = nonce.low_u64();
+								::engines::EpochChange::Yes(new_epoch, proof)
+							}
+							_ => {
+								debug!(target: "engine", "Successfully decoded log turned out to be bad.");
+								::engines::EpochChange::No
+							}
+						}
+					}
+				}
+			}
+		}
+	}
+
+	// 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;
+
+		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)))
+	}
+
+	fn contains_with_caller(&self, block_hash: &H256, address: &Address, caller: &Call) -> bool {
 		let mut guard = self.validators.write();
 		let maybe_existing = guard
 			.get_mut(block_hash)
 			.map(|list| list.contains(block_hash, address));
 		maybe_existing
 			.unwrap_or_else(|| self
-				.get_list(block_hash.clone())
+				.get_list(caller)
 				.map_or(false, |list| {
 					let contains = list.contains(block_hash, address);
 					guard.insert(block_hash.clone(), list);
@@ -82,14 +232,14 @@ impl ValidatorSet for ValidatorSafeContract {
 				 }))
 	}
 
-	fn get(&self, block_hash: &H256, nonce: usize) -> Address {
+	fn get_with_caller(&self, block_hash: &H256, nonce: usize, caller: &Call) -> Address {
 		let mut guard = self.validators.write();
 		let maybe_existing = guard
 			.get_mut(block_hash)
 			.map(|list| list.get(block_hash, nonce));
 		maybe_existing
 			.unwrap_or_else(|| self
-				.get_list(block_hash.clone())
+				.get_list(caller)
 				.map_or_else(Default::default, |list| {
 					let address = list.get(block_hash, nonce);
 					guard.insert(block_hash.clone(), list);
@@ -97,14 +247,14 @@ impl ValidatorSet for ValidatorSafeContract {
 				 }))
 	}
 
-	fn count(&self, block_hash: &H256) -> usize {
+	fn count_with_caller(&self, block_hash: &H256, caller: &Call) -> usize {
 		let mut guard = self.validators.write();
 		let maybe_existing = guard
 			.get_mut(block_hash)
 			.map(|list| list.count(block_hash));
 		maybe_existing
 			.unwrap_or_else(|| self
-				.get_list(block_hash.clone())
+				.get_list(caller)
 				.map_or_else(usize::max_value, |list| {
 					let address = list.count(block_hash);
 					guard.insert(block_hash.clone(), list);
@@ -114,55 +264,7 @@ impl ValidatorSet for ValidatorSafeContract {
 
 	fn register_contract(&self, client: Weak<Client>) {
 		trace!(target: "engine", "Setting up contract caller.");
-		let contract = ethabi::Contract::new(ethabi::Interface::load(CONTRACT_INTERFACE).expect("JSON interface is valid; qed"));
+		*self.client.write() = Some(client);
-		let call = contract.function(GET_VALIDATORS.into()).expect("Method name is valid; qed");
-		let data = call.encode_call(vec![]).expect("get_validators does not take any arguments; qed");
-		let contract_address = self.address.clone();
-		let do_call = move |id| client
-			.upgrade()
-			.ok_or("No client!".into())
-			.and_then(|c| c.call_contract(id, contract_address.clone(), data.clone()))
-			.map(|raw_output| call.decode_output(raw_output).expect("ethabi is correct; qed"));
-		*self.provider.write() = Some(provider::Contract::new(do_call));
-	}
-}
-
-mod provider {
-	use std::string::String;
-	use std::result::Result;
-	use {util, ethabi};
-	use types::ids::BlockId;
-
-	pub struct Contract {
-		do_call: Box<Fn(BlockId) -> Result<Vec<ethabi::Token>, String> + Send + Sync + 'static>,
-	}
-
-	impl Contract {
-		pub fn new<F>(do_call: F) -> Self where F: Fn(BlockId) -> Result<Vec<ethabi::Token>, String> + Send + Sync + 'static {
-			Contract {
-				do_call: Box::new(do_call),
-			}
-		}
-
-		/// Gets validators from contract with interface: `{"constant":true,"inputs":[],"name":"getValidators","outputs":[{"name":"","type":"address[]"}],"payable":false,"type":"function"}`
-		pub fn get_validators(&self, id: BlockId) -> Result<Vec<util::Address>, String> {
-			Ok((self.do_call)(id)?
-				 .into_iter()
-				 .rev()
-				 .collect::<Vec<_>>()
-				 .pop()
-				 .expect("get_validators returns one argument; qed")
-				 .to_array()
-				 .and_then(|v| v
-					 .into_iter()
-					 .map(|a| a.to_address())
-					 .collect::<Option<Vec<[u8; 20]>>>())
-				 .expect("get_validators returns a list of addresses; qed")
-				 .into_iter()
-				 .map(util::Address::from)
-				 .collect::<Vec<_>>()
-			)
-		}
 	}
 }
 
@@ -178,7 +280,7 @@ mod tests {
 	use miner::MinerService;
 	use tests::helpers::{generate_dummy_client_with_spec_and_accounts, generate_dummy_client_with_spec_and_data};
 	use super::super::ValidatorSet;
-	use super::ValidatorSafeContract;
+	use super::{ValidatorSafeContract, EVENT_NAME_HASH};
 
 	#[test]
 	fn fetches_validators() {
@@ -256,4 +358,35 @@ mod tests {
 		sync_client.flush_queue();
 		assert_eq!(sync_client.chain_info().best_block_number, 3);
 	}
+
+	#[test]
+	fn detects_bloom() {
+		use header::Header;
+		use engines::{EpochChange, Unsure};
+		use log_entry::LogEntry;
+
+		let client = generate_dummy_client_with_spec_and_accounts(Spec::new_validator_safe_contract, None);
+		let engine = client.engine().clone();
+		let validator_contract = Address::from_str("0000000000000000000000000000000000000005").unwrap();
+
+		let last_hash = client.best_block_header().hash();
+		let mut new_header = Header::default();
+		new_header.set_parent_hash(last_hash);
+
+		// first, try without the parent hash.
+		let mut event = LogEntry {
+			address: validator_contract,
+			topics: vec![*EVENT_NAME_HASH],
+			data: Vec::new(),
+		};
+
+		new_header.set_log_bloom(event.bloom());
+		assert_eq!(engine.is_epoch_end(&new_header, None, None), EpochChange::No);
+
+		// 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));
+	}
 }

ethcore/src/engines/validator_set/simple_list.rs

@@ -17,40 +17,67 @@
 /// Preconfigured validator list.
 
 use util::{H256, Address, HeapSizeOf};
+
+use engines::Call;
+use header::Header;
 use super::ValidatorSet;
 
-#[derive(Debug, PartialEq, Eq, Default)]
+/// Validator set containing a known set of addresses.
+#[derive(Clone, Debug, PartialEq, Eq, Default)]
 pub struct SimpleList {
 	validators: Vec<Address>,
-	validator_n: usize,
 }
 
 impl SimpleList {
+	/// Create a new `SimpleList`.
 	pub fn new(validators: Vec<Address>) -> Self {
 		SimpleList {
-			validator_n: validators.len(),
 			validators: validators,
 		}
 	}
+
+	/// Convert into inner representation.
+	pub fn into_inner(self) -> Vec<Address> {
+		self.validators
+	}
 }
 
 impl HeapSizeOf for SimpleList {
 	fn heap_size_of_children(&self) -> usize {
-		self.validators.heap_size_of_children() + self.validator_n.heap_size_of_children()
+		self.validators.heap_size_of_children()
 	}
 }
 
 impl ValidatorSet for SimpleList {
-	fn contains(&self, _bh: &H256, address: &Address) -> bool {
+	fn default_caller(&self, _block_id: ::ids::BlockId) -> Box<Call> {
+		Box::new(|_, _| Err("Simple list doesn't require calls.".into()))
+	}
+
+	fn is_epoch_end(&self, _header: &Header, _block: Option<&[u8]>, _receipts: 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 contains_with_caller(&self, _bh: &H256, address: &Address, _: &Call) -> bool {
 		self.validators.contains(address)
 	}
 
-	fn get(&self, _bh: &H256, nonce: usize) -> Address {
+	fn get_with_caller(&self, _bh: &H256, nonce: usize, _: &Call) -> Address {
-		self.validators.get(nonce % self.validator_n).expect("There are validator_n authorities; taking number modulo validator_n gives number in validator_n range; qed").clone()
+		let validator_n = self.validators.len();
+		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()
 	}
 
-	fn count(&self, _bh: &H256) -> usize {
+	fn count_with_caller(&self, _bh: &H256, _: &Call) -> usize {
-		self.validator_n
+		self.validators.len()
 	}
 }
 

ethcore/src/ethereum/ethash.rs

@@ -32,6 +32,10 @@ use rlp::{self, UntrustedRlp};
 /// Parity tries to round block.gas_limit to multiple of this constant
 pub const PARITY_GAS_LIMIT_DETERMINANT: U256 = U256([37, 0, 0, 0]);
 
+/// Number of blocks in an ethash snapshot.
+// make dependent on difficulty incrment divisor?
+const SNAPSHOT_BLOCKS: u64 = 30000;
+
 /// Ethash params.
 #[derive(Debug, PartialEq)]
 pub struct EthashParams {
@@ -139,17 +143,33 @@ pub struct Ethash {
 
 impl Ethash {
 	/// Create a new instance of Ethash engine
-	pub fn new(params: CommonParams, ethash_params: EthashParams, builtins: BTreeMap<Address, Builtin>) -> Self {
+	pub fn new(params: CommonParams, ethash_params: EthashParams, builtins: BTreeMap<Address, Builtin>) -> Arc<Self> {
-		Ethash {
+		Arc::new(Ethash {
 			params: params,
 			ethash_params: ethash_params,
 			builtins: builtins,
 			pow: EthashManager::new(),
-		}
+		})
 	}
 }
 
-impl Engine for Ethash {
+// TODO [rphmeier]
+//
+// for now, this is different than Ethash's own epochs, and signal
+// "consensus epochs".
+// in this sense, `Ethash` is epochless: the same `EpochVerifier` can be used
+// for any block in the chain.
+// 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)
+	}
+}
+
+impl Engine for Arc<Ethash> {
 	fn name(&self) -> &str { "Ethash" }
 	fn version(&self) -> SemanticVersion { SemanticVersion::new(1, 0, 0) }
 	// Two fields - mix
@@ -379,6 +399,14 @@ impl Engine for Ethash {
 		t.verify_basic(check_low_s, network_id, false)?;
 		Ok(())
 	}
+
+	fn epoch_verifier(&self, _header: &Header, _proof: &[u8]) -> Result<Box<::engines::EpochVerifier>, Error> {
+		Ok(Box::new(self.clone()))
+	}
+
+	fn snapshot_components(&self) -> Option<Box<::snapshot::SnapshotComponents>> {
+		Some(Box::new(::snapshot::PowSnapshot(SNAPSHOT_BLOCKS)))
+	}
 }
 
 // Try to round gas_limit a bit so that:

ethcore/src/migrations/state/v7.rs

@@ -238,7 +238,7 @@ impl Migration for OverlayRecentV7 {
 		}
 
 		let mut count = 0;
-		for (key, value) in source.iter(None) {
+		for (key, value) in source.iter(None).into_iter().flat_map(|inner| inner) {
 			count += 1;
 			if count == 100_000 {
 				count = 0;

ethcore/src/migrations/v10.rs

@@ -102,7 +102,7 @@ impl Migration for ToV10 {
 
 	fn migrate(&mut self, source: Arc<Database>, config: &Config, dest: &mut Database, col: Option<u32>) -> Result<(), Error> {
 		let mut batch = Batch::new(config, col);
-		for (key, value) in source.iter(col) {
+		for (key, value) in source.iter(col).into_iter().flat_map(|inner| inner) {
 			self.progress.tick();
 			batch.insert(key.to_vec(), value.to_vec(), dest)?;
 		}

ethcore/src/migrations/v9.rs

@@ -59,7 +59,7 @@ impl Migration for ToV9 {
 	fn migrate(&mut self, source: Arc<Database>, config: &Config, dest: &mut Database, col: Option<u32>) -> Result<(), Error> {
 		let mut batch = Batch::new(config, self.column);
 
-		for (key, value) in source.iter(col) {
+		for (key, value) in source.iter(col).into_iter().flat_map(|inner| inner) {
 			self.progress.tick();
 			match self.extract {
 				Extract::Header => {

ethcore/src/snapshot/consensus/mod.rs

@@ -0,0 +1,352 @@
+// 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/>.
+
+//! Secondary chunk creation and restoration, implementations for different consensus
+//! engines.
+
+use std::collections::VecDeque;
+use std::io;
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Arc;
+
+use blockchain::{BlockChain, BlockProvider};
+use engines::Engine;
+use snapshot::{Error, ManifestData};
+use snapshot::block::AbridgedBlock;
+
+use util::{Bytes, H256};
+use util::kvdb::KeyValueDB;
+use rand::OsRng;
+use rlp::{RlpStream, UntrustedRlp};
+
+
+/// A sink for produced chunks.
+pub type ChunkSink<'a> = FnMut(&[u8]) -> io::Result<()> + 'a;
+
+/// Components necessary for snapshot creation and restoration.
+pub trait SnapshotComponents: Send {
+	/// Create secondary snapshot chunks; these corroborate the state data
+	/// in the state chunks.
+	///
+	/// Chunks shouldn't exceed the given preferred size, and should be fed
+	/// uncompressed into the sink.
+	///
+	/// This will vary by consensus engine, so it's exposed as a trait.
+	fn chunk_all(
+		&mut self,
+		chain: &BlockChain,
+		block_at: H256,
+		chunk_sink: &mut ChunkSink,
+		preferred_size: usize,
+	) -> Result<(), Error>;
+
+	/// Create a rebuilder, which will have chunks fed into it in aribtrary
+	/// order and then be finalized.
+	///
+	/// The manifest, a database, and fresh `BlockChain` are supplied.
+	// TODO: supply anything for state?
+	fn rebuilder(
+		&self,
+		chain: BlockChain,
+		db: Arc<KeyValueDB>,
+		manifest: &ManifestData,
+	) -> Result<Box<Rebuilder>, ::error::Error>;
+}
+
+
+/// Restore from secondary snapshot chunks.
+pub trait Rebuilder: Send {
+	/// Feed a chunk, potentially out of order.
+	///
+	/// Check `abort_flag` periodically while doing heavy work. If set to `false`, should bail with
+	/// `Error::RestorationAborted`.
+	fn feed(
+		&mut self,
+		chunk: &[u8],
+		engine: &Engine,
+		abort_flag: &AtomicBool,
+	) -> Result<(), ::error::Error>;
+
+	/// Finalize the restoration. Will be done after all chunks have been
+	/// fed successfully.
+	/// This will apply the necessary "glue" between chunks.
+	fn finalize(&mut self) -> Result<(), Error>;
+}
+
+/// Snapshot creation and restoration for PoW chains.
+/// This includes blocks from the head of the chain as a
+/// loose assurance that the chain is valid.
+///
+/// The field is the number of blocks from the head of the chain
+/// to include in the snapshot.
+#[derive(Clone, Copy, PartialEq)]
+pub struct PowSnapshot(pub u64);
+
+impl SnapshotComponents for PowSnapshot {
+	fn chunk_all(
+		&mut self,
+		chain: &BlockChain,
+		block_at: H256,
+		chunk_sink: &mut ChunkSink,
+		preferred_size: usize,
+	) -> Result<(), Error> {
+		PowWorker {
+			chain: chain,
+			rlps: VecDeque::new(),
+			current_hash: block_at,
+			writer: chunk_sink,
+			preferred_size: preferred_size,
+		}.chunk_all(self.0)
+	}
+
+	fn rebuilder(
+		&self,
+		chain: BlockChain,
+		db: Arc<KeyValueDB>,
+		manifest: &ManifestData,
+	) -> Result<Box<Rebuilder>, ::error::Error> {
+		PowRebuilder::new(chain, db, manifest, self.0).map(|r| Box::new(r) as Box<_>)
+	}
+}
+
+/// Used to build block chunks.
+struct PowWorker<'a> {
+	chain: &'a BlockChain,
+	// block, receipt rlp pairs.
+	rlps: VecDeque<Bytes>,
+	current_hash: H256,
+	writer: &'a mut ChunkSink<'a>,
+	preferred_size: usize,
+}
+
+impl<'a> PowWorker<'a> {
+	// Repeatedly fill the buffers and writes out chunks, moving backwards from starting block hash.
+	// Loops until we reach the first desired block, and writes out the remainder.
+	fn chunk_all(&mut self, snapshot_blocks: u64) -> Result<(), Error> {
+		let mut loaded_size = 0;
+		let mut last = self.current_hash;
+
+		let genesis_hash = self.chain.genesis_hash();
+
+		for _ in 0..snapshot_blocks {
+			if self.current_hash == genesis_hash { break }
+
+			let (block, receipts) = self.chain.block(&self.current_hash)
+				.and_then(|b| self.chain.block_receipts(&self.current_hash).map(|r| (b, r)))
+				.ok_or(Error::BlockNotFound(self.current_hash))?;
+
+			let abridged_rlp = AbridgedBlock::from_block_view(&block.view()).into_inner();
+
+			let pair = {
+				let mut pair_stream = RlpStream::new_list(2);
+				pair_stream.append_raw(&abridged_rlp, 1).append(&receipts);
+				pair_stream.out()
+			};
+
+			let new_loaded_size = loaded_size + pair.len();
+
+			// cut off the chunk if too large.
+
+			if new_loaded_size > self.preferred_size && !self.rlps.is_empty() {
+				self.write_chunk(last)?;
+				loaded_size = pair.len();
+			} else {
+				loaded_size = new_loaded_size;
+			}
+
+			self.rlps.push_front(pair);
+
+			last = self.current_hash;
+			self.current_hash = block.header_view().parent_hash();
+		}
+
+		if loaded_size != 0 {
+			self.write_chunk(last)?;
+		}
+
+		Ok(())
+	}
+
+	// write out the data in the buffers to a chunk on disk
+	//
+	// we preface each chunk with the parent of the first block's details,
+	// obtained from the details of the last block written.
+	fn write_chunk(&mut self, last: H256) -> Result<(), Error> {
+		trace!(target: "snapshot", "prepared block chunk with {} blocks", self.rlps.len());
+
+		let (last_header, last_details) = self.chain.block_header(&last)
+			.and_then(|n| self.chain.block_details(&last).map(|d| (n, d)))
+			.ok_or(Error::BlockNotFound(last))?;
+
+		let parent_number = last_header.number() - 1;
+		let parent_hash = last_header.parent_hash();
+		let parent_total_difficulty = last_details.total_difficulty - *last_header.difficulty();
+
+		trace!(target: "snapshot", "parent last written block: {}", parent_hash);
+
+		let num_entries = self.rlps.len();
+		let mut rlp_stream = RlpStream::new_list(3 + num_entries);
+		rlp_stream.append(&parent_number).append(parent_hash).append(&parent_total_difficulty);
+
+		for pair in self.rlps.drain(..) {
+			rlp_stream.append_raw(&pair, 1);
+		}
+
+		let raw_data = rlp_stream.out();
+
+		(self.writer)(&raw_data)?;
+
+		Ok(())
+	}
+}
+
+/// Rebuilder for proof-of-work chains.
+/// Does basic verification for all blocks, but `PoW` verification for some.
+/// Blocks must be fed in-order.
+///
+/// The first block in every chunk is disconnected from the last block in the
+/// chunk before it, as chunks may be submitted out-of-order.
+///
+/// After all chunks have been submitted, we "glue" the chunks together.
+pub struct PowRebuilder {
+	chain: BlockChain,
+	db: Arc<KeyValueDB>,
+	rng: OsRng,
+	disconnected: Vec<(u64, H256)>,
+	best_number: u64,
+	best_hash: H256,
+	best_root: H256,
+	fed_blocks: u64,
+	snapshot_blocks: u64,
+}
+
+impl PowRebuilder {
+	/// Create a new PowRebuilder.
+	fn new(chain: BlockChain, db: Arc<KeyValueDB>, manifest: &ManifestData, snapshot_blocks: u64) -> Result<Self, ::error::Error> {
+		Ok(PowRebuilder {
+			chain: chain,
+			db: db,
+			rng: OsRng::new()?,
+			disconnected: Vec::new(),
+			best_number: manifest.block_number,
+			best_hash: manifest.block_hash,
+			best_root: manifest.state_root,
+			fed_blocks: 0,
+			snapshot_blocks: snapshot_blocks,
+		})
+	}
+}
+
+impl Rebuilder for PowRebuilder {
+	/// Feed the rebuilder an uncompressed block chunk.
+	/// Returns the number of blocks fed or any errors.
+	fn feed(&mut self, chunk: &[u8], engine: &Engine, abort_flag: &AtomicBool) -> Result<(), ::error::Error> {
+		use basic_types::Seal::With;
+		use views::BlockView;
+		use snapshot::verify_old_block;
+		use util::U256;
+		use util::triehash::ordered_trie_root;
+
+		let rlp = UntrustedRlp::new(chunk);
+		let item_count = rlp.item_count()?;
+		let num_blocks = (item_count - 3) as u64;
+
+		trace!(target: "snapshot", "restoring block chunk with {} blocks.", item_count - 3);
+
+		if self.fed_blocks + num_blocks > self.snapshot_blocks {
+			return Err(Error::TooManyBlocks(self.snapshot_blocks, self.fed_blocks).into())
+		}
+
+		// todo: assert here that these values are consistent with chunks being in order.
+		let mut cur_number = rlp.val_at::<u64>(0)? + 1;
+		let mut parent_hash = rlp.val_at::<H256>(1)?;
+		let parent_total_difficulty = rlp.val_at::<U256>(2)?;
+
+		for idx in 3..item_count {
+			if !abort_flag.load(Ordering::SeqCst) { return Err(Error::RestorationAborted.into()) }
+
+			let pair = rlp.at(idx)?;
+			let abridged_rlp = pair.at(0)?.as_raw().to_owned();
+			let abridged_block = AbridgedBlock::from_raw(abridged_rlp);
+			let receipts: Vec<::receipt::Receipt> = pair.list_at(1)?;
+			let receipts_root = ordered_trie_root(
+				pair.at(1)?.iter().map(|r| r.as_raw().to_owned())
+			);
+
+			let block = abridged_block.to_block(parent_hash, cur_number, receipts_root)?;
+			let block_bytes = block.rlp_bytes(With);
+			let is_best = cur_number == self.best_number;
+
+			if is_best {
+				if block.header.hash() != self.best_hash {
+					return Err(Error::WrongBlockHash(cur_number, self.best_hash, block.header.hash()).into())
+				}
+
+				if block.header.state_root() != &self.best_root {
+					return Err(Error::WrongStateRoot(self.best_root, *block.header.state_root()).into())
+				}
+			}
+
+			verify_old_block(
+				&mut self.rng,
+				&block.header,
+				engine,
+				&self.chain,
+				Some(&block_bytes),
+				is_best
+			)?;
+
+			let mut batch = self.db.transaction();
+
+			// special-case the first block in each chunk.
+			if idx == 3 {
+				if self.chain.insert_unordered_block(&mut batch, &block_bytes, receipts, Some(parent_total_difficulty), is_best, false) {
+					self.disconnected.push((cur_number, block.header.hash()));
+				}
+			} else {
+				self.chain.insert_unordered_block(&mut batch, &block_bytes, receipts, None, is_best, false);
+			}
+			self.db.write_buffered(batch);
+			self.chain.commit();
+
+			parent_hash = BlockView::new(&block_bytes).hash();
+			cur_number += 1;
+		}
+
+		self.fed_blocks += num_blocks;
+
+		Ok(())
+	}
+
+	/// Glue together any disconnected chunks and check that the chain is complete.
+	fn finalize(&mut self) -> Result<(), Error> {
+		let mut batch = self.db.transaction();
+
+		for (first_num, first_hash) in self.disconnected.drain(..) {
+			let parent_num = first_num - 1;
+
+			// check if the parent is even in the chain.
+			// since we don't restore every single block in the chain,
+			// the first block of the first chunks has nothing to connect to.
+			if let Some(parent_hash) = self.chain.block_hash(parent_num) {
+				// if so, add the child to it.
+				self.chain.add_child(&mut batch, parent_hash, first_hash);
+			}
+		}
+		self.db.write_buffered(batch);
+		Ok(())
+	}
+}

ethcore/src/snapshot/error.rs

@@ -57,6 +57,8 @@ pub enum Error {
 	VersionNotSupported(u64),
 	/// Max chunk size is to small to fit basic account data.
 	ChunkTooSmall,
+	/// Snapshots not supported by the consensus engine.
+	SnapshotsUnsupported,
 }
 
 impl fmt::Display for Error {
@@ -79,6 +81,7 @@ impl fmt::Display for Error {
 			Error::Trie(ref err) => err.fmt(f),
 			Error::VersionNotSupported(ref ver) => write!(f, "Snapshot version {} is not supprted.", ver),
 			Error::ChunkTooSmall => write!(f, "Chunk size is too small."),
+			Error::SnapshotsUnsupported => write!(f, "Snapshots unsupported by consensus engine."),
 		}
 	}
 }

ethcore/src/snapshot/mod.rs

@@ -17,9 +17,9 @@
 //! Snapshot creation, restoration, and network service.
 //!
 //! Documentation of the format can be found at
-//! https://github.com/paritytech/parity/wiki/%22PV64%22-Snapshot-Format
+//! https://github.com/paritytech/parity/wiki/Warp-Sync-Snapshot-Format
 
-use std::collections::{HashMap, HashSet, VecDeque};
+use std::collections::{HashMap, HashSet};
 use std::sync::Arc;
 use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
 
@@ -28,7 +28,6 @@ use blockchain::{BlockChain, BlockProvider};
 use engines::Engine;
 use header::Header;
 use ids::BlockId;
-use views::BlockView;
 
 use util::{Bytes, Hashable, HashDB, DBValue, snappy, U256, Uint};
 use util::Mutex;
@@ -40,7 +39,6 @@ use util::sha3::SHA3_NULL_RLP;
 use rlp::{RlpStream, UntrustedRlp};
 use bloom_journal::Bloom;
 
-use self::block::AbridgedBlock;
 use self::io::SnapshotWriter;
 
 use super::state_db::StateDB;
@@ -51,6 +49,7 @@ use rand::{Rng, OsRng};
 
 pub use self::error::Error;
 
+pub use self::consensus::*;
 pub use self::service::{Service, DatabaseRestore};
 pub use self::traits::SnapshotService;
 pub use self::watcher::Watcher;
@@ -63,6 +62,7 @@ pub mod service;
 
 mod account;
 mod block;
+mod consensus;
 mod error;
 mod watcher;
 
@@ -83,9 +83,6 @@ mod traits {
 // Try to have chunks be around 4MB (before compression)
 const PREFERRED_CHUNK_SIZE: usize = 4 * 1024 * 1024;
 
-// How many blocks to include in a snapshot, starting from the head of the chain.
-const SNAPSHOT_BLOCKS: u64 = 30000;
-
 /// A progress indicator for snapshots.
 #[derive(Debug, Default)]
 pub struct Progress {
@@ -122,6 +119,7 @@ impl Progress {
 }
 /// Take a snapshot using the given blockchain, starting block hash, and database, writing into the given writer.
 pub fn take_snapshot<W: SnapshotWriter + Send>(
+	engine: &Engine,
 	chain: &BlockChain,
 	block_at: H256,
 	state_db: &HashDB,
@@ -136,9 +134,11 @@ pub fn take_snapshot<W: SnapshotWriter + Send>(
 	info!("Taking snapshot starting at block {}", number);
 
 	let writer = Mutex::new(writer);
+	let chunker = engine.snapshot_components().ok_or(Error::SnapshotsUnsupported)?;
 	let (state_hashes, block_hashes) = scope(|scope| {
-		let block_guard = scope.spawn(|| chunk_blocks(chain, block_at, &writer, p));
+		let writer = &writer;
-		let state_res = chunk_state(state_db, state_root, &writer, p);
+		let block_guard = scope.spawn(move || chunk_secondary(chunker, chain, block_at, writer, p));
+		let state_res = chunk_state(state_db, state_root, writer, p);
 
 		state_res.and_then(|state_hashes| {
 			block_guard.join().map(|block_hashes| (state_hashes, block_hashes))
@@ -163,128 +163,41 @@ pub fn take_snapshot<W: SnapshotWriter + Send>(
 	Ok(())
 }
 
-/// Used to build block chunks.
+/// Create and write out all secondary chunks to disk, returning a vector of all
-struct BlockChunker<'a> {
+/// the hashes of secondary chunks created.
-	chain: &'a BlockChain,
-	// block, receipt rlp pairs.
-	rlps: VecDeque<Bytes>,
-	current_hash: H256,
-	hashes: Vec<H256>,
-	snappy_buffer: Vec<u8>,
-	writer: &'a Mutex<SnapshotWriter + 'a>,
-	progress: &'a Progress,
-}
-
-impl<'a> BlockChunker<'a> {
-	// Repeatedly fill the buffers and writes out chunks, moving backwards from starting block hash.
-	// Loops until we reach the first desired block, and writes out the remainder.
-	fn chunk_all(&mut self) -> Result<(), Error> {
-		let mut loaded_size = 0;
-		let mut last = self.current_hash;
-
-		let genesis_hash = self.chain.genesis_hash();
-
-		for _ in 0..SNAPSHOT_BLOCKS {
-			if self.current_hash == genesis_hash { break }
-
-			let (block, receipts) = self.chain.block(&self.current_hash)
-				.and_then(|b| self.chain.block_receipts(&self.current_hash).map(|r| (b, r)))
-				.ok_or(Error::BlockNotFound(self.current_hash))?;
-
-			let abridged_rlp = AbridgedBlock::from_block_view(&block.view()).into_inner();
-
-			let pair = {
-				let mut pair_stream = RlpStream::new_list(2);
-				pair_stream.append_raw(&abridged_rlp, 1).append(&receipts);
-				pair_stream.out()
-			};
-
-			let new_loaded_size = loaded_size + pair.len();
-
-			// cut off the chunk if too large.
-
-			if new_loaded_size > PREFERRED_CHUNK_SIZE && !self.rlps.is_empty() {
-				self.write_chunk(last)?;
-				loaded_size = pair.len();
-			} else {
-				loaded_size = new_loaded_size;
-			}
-
-			self.rlps.push_front(pair);
-
-			last = self.current_hash;
-			self.current_hash = block.header_view().parent_hash();
-		}
-
-		if loaded_size != 0 {
-			self.write_chunk(last)?;
-		}
-
-		Ok(())
-	}
-
-	// write out the data in the buffers to a chunk on disk
-	//
-	// we preface each chunk with the parent of the first block's details,
-	// obtained from the details of the last block written.
-	fn write_chunk(&mut self, last: H256) -> Result<(), Error> {
-		trace!(target: "snapshot", "prepared block chunk with {} blocks", self.rlps.len());
-
-		let (last_header, last_details) = self.chain.block_header(&last)
-			.and_then(|n| self.chain.block_details(&last).map(|d| (n, d)))
-			.ok_or(Error::BlockNotFound(last))?;
-
-		let parent_number = last_header.number() - 1;
-		let parent_hash = last_header.parent_hash();
-		let parent_total_difficulty = last_details.total_difficulty - *last_header.difficulty();
-
-		trace!(target: "snapshot", "parent last written block: {}", parent_hash);
-
-		let num_entries = self.rlps.len();
-		let mut rlp_stream = RlpStream::new_list(3 + num_entries);
-		rlp_stream.append(&parent_number).append(parent_hash).append(&parent_total_difficulty);
-
-		for pair in self.rlps.drain(..) {
-			rlp_stream.append_raw(&pair, 1);
-		}
-
-		let raw_data = rlp_stream.out();
-
-		let size = snappy::compress_into(&raw_data, &mut self.snappy_buffer);
-		let compressed = &self.snappy_buffer[..size];
-		let hash = compressed.sha3();
-
-		self.writer.lock().write_block_chunk(hash, compressed)?;
-		trace!(target: "snapshot", "wrote block chunk. hash: {}, size: {}, uncompressed size: {}", hash.hex(), size, raw_data.len());
-
-		self.progress.size.fetch_add(size, Ordering::SeqCst);
-		self.progress.blocks.fetch_add(num_entries, Ordering::SeqCst);
-
-		self.hashes.push(hash);
-		Ok(())
-	}
-}
-
-/// Create and write out all block chunks to disk, returning a vector of all
-/// the hashes of block chunks created.
 ///
-/// The path parameter is the directory to store the block chunks in.
+/// Secondary chunks are engine-specific, but they intend to corroborate the state data
-/// This function assumes the directory exists already.
+/// in the state chunks.
 /// Returns a list of chunk hashes, with the first having the blocks furthest from the genesis.
-pub fn chunk_blocks<'a>(chain: &'a BlockChain, start_hash: H256, writer: &Mutex<SnapshotWriter + 'a>, progress: &'a Progress) -> Result<Vec<H256>, Error> {
+pub fn chunk_secondary<'a>(mut chunker: Box<SnapshotComponents>, chain: &'a BlockChain, start_hash: H256, writer: &Mutex<SnapshotWriter + 'a>, progress: &'a Progress) -> Result<Vec<H256>, Error> {
-	let mut chunker = BlockChunker {
+	let mut chunk_hashes = Vec::new();
-		chain: chain,
+	let mut snappy_buffer = vec![0; snappy::max_compressed_len(PREFERRED_CHUNK_SIZE)];
-		rlps: VecDeque::new(),
-		current_hash: start_hash,
-		hashes: Vec::new(),
-		snappy_buffer: vec![0; snappy::max_compressed_len(PREFERRED_CHUNK_SIZE)],
-		writer: writer,
-		progress: progress,
-	};
 
-	chunker.chunk_all()?;
+	{
+		let mut chunk_sink = |raw_data: &[u8]| {
+			let compressed_size = snappy::compress_into(raw_data, &mut snappy_buffer);
+			let compressed = &snappy_buffer[..compressed_size];
+			let hash = compressed.sha3();
+			let size = compressed.len();
 
-	Ok(chunker.hashes)
+			writer.lock().write_block_chunk(hash, compressed)?;
+			trace!(target: "snapshot", "wrote secondary chunk. hash: {}, size: {}, uncompressed size: {}",
+				hash.hex(), size, raw_data.len());
+
+			progress.size.fetch_add(size, Ordering::SeqCst);
+			chunk_hashes.push(hash);
+			Ok(())
+		};
+
+		chunker.chunk_all(
+			chain,
+			start_hash,
+			&mut chunk_sink,
+			PREFERRED_CHUNK_SIZE,
+		)?;
+	}
+
+	Ok(chunk_hashes)
 }
 
 /// State trie chunker.
@@ -564,158 +477,15 @@ const POW_VERIFY_RATE: f32 = 0.02;
 /// the fullest verification possible. If not, it will take a random sample to determine whether it will
 /// do heavy or light verification.
 pub fn verify_old_block(rng: &mut OsRng, header: &Header, engine: &Engine, chain: &BlockChain, body: Option<&[u8]>, always: bool) -> Result<(), ::error::Error> {
+	engine.verify_block_basic(header, body)?;
+
 	if always || rng.gen::<f32>() <= POW_VERIFY_RATE {
+		engine.verify_block_unordered(header, body)?;
 		match chain.block_header(header.parent_hash()) {
 			Some(parent) => engine.verify_block_family(header, &parent, body),
-			None => engine.verify_block_seal(header),
+			None => Ok(()),
 		}
 	} else {
-		engine.verify_block_basic(header, body)
-	}
-}
-
-/// Rebuilds the blockchain from chunks.
-///
-/// Does basic verification for all blocks, but `PoW` verification for some.
-/// Blocks must be fed in-order.
-///
-/// The first block in every chunk is disconnected from the last block in the
-/// chunk before it, as chunks may be submitted out-of-order.
-///
-/// After all chunks have been submitted, we "glue" the chunks together.
-pub struct BlockRebuilder {
-	chain: BlockChain,
-	db: Arc<Database>,
-	rng: OsRng,
-	disconnected: Vec<(u64, H256)>,
-	best_number: u64,
-	best_hash: H256,
-	best_root: H256,
-	fed_blocks: u64,
-}
-
-impl BlockRebuilder {
-	/// Create a new BlockRebuilder.
-	pub fn new(chain: BlockChain, db: Arc<Database>, manifest: &ManifestData) -> Result<Self, ::error::Error> {
-		Ok(BlockRebuilder {
-			chain: chain,
-			db: db,
-			rng: OsRng::new()?,
-			disconnected: Vec::new(),
-			best_number: manifest.block_number,
-			best_hash: manifest.block_hash,
-			best_root: manifest.state_root,
-			fed_blocks: 0,
-		})
-	}
-
-	/// Feed the rebuilder an uncompressed block chunk.
-	/// Returns the number of blocks fed or any errors.
-	pub fn feed(&mut self, chunk: &[u8], engine: &Engine, abort_flag: &AtomicBool) -> Result<u64, ::error::Error> {
-		use basic_types::Seal::With;
-		use util::U256;
-		use util::triehash::ordered_trie_root;
-
-		let rlp = UntrustedRlp::new(chunk);
-		let item_count = rlp.item_count()?;
-		let num_blocks = (item_count - 3) as u64;
-
-		trace!(target: "snapshot", "restoring block chunk with {} blocks.", item_count - 3);
-
-		if self.fed_blocks + num_blocks > SNAPSHOT_BLOCKS {
-			return Err(Error::TooManyBlocks(SNAPSHOT_BLOCKS, self.fed_blocks).into())
-		}
-
-		// todo: assert here that these values are consistent with chunks being in order.
-		let mut cur_number = rlp.val_at::<u64>(0)? + 1;
-		let mut parent_hash = rlp.val_at::<H256>(1)?;
-		let parent_total_difficulty = rlp.val_at::<U256>(2)?;
-
-		for idx in 3..item_count {
-			if !abort_flag.load(Ordering::SeqCst) { return Err(Error::RestorationAborted.into()) }
-
-			let pair = rlp.at(idx)?;
-			let abridged_rlp = pair.at(0)?.as_raw().to_owned();
-			let abridged_block = AbridgedBlock::from_raw(abridged_rlp);
-			let receipts: Vec<::receipt::Receipt> = pair.list_at(1)?;
-			let receipts_root = ordered_trie_root(
-				pair.at(1)?.iter().map(|r| r.as_raw().to_owned())
-			);
-
-			let block = abridged_block.to_block(parent_hash, cur_number, receipts_root)?;
-			let block_bytes = block.rlp_bytes(With);
-			let is_best = cur_number == self.best_number;
-
-			if is_best {
-				if block.header.hash() != self.best_hash {
-					return Err(Error::WrongBlockHash(cur_number, self.best_hash, block.header.hash()).into())
-				}
-
-				if block.header.state_root() != &self.best_root {
-					return Err(Error::WrongStateRoot(self.best_root, *block.header.state_root()).into())
-				}
-			}
-
-			verify_old_block(
-				&mut self.rng,
-				&block.header,
-				engine,
-				&self.chain,
-				Some(&block_bytes),
-				is_best
-			)?;
-
-			let mut batch = self.db.transaction();
-
-			// special-case the first block in each chunk.
-			if idx == 3 {
-				if self.chain.insert_unordered_block(&mut batch, &block_bytes, receipts, Some(parent_total_difficulty), is_best, false) {
-					self.disconnected.push((cur_number, block.header.hash()));
-				}
-			} else {
-				self.chain.insert_unordered_block(&mut batch, &block_bytes, receipts, None, is_best, false);
-			}
-			self.db.write_buffered(batch);
-			self.chain.commit();
-
-			parent_hash = BlockView::new(&block_bytes).hash();
-			cur_number += 1;
-		}
-
-		self.fed_blocks += num_blocks;
-
-		Ok(num_blocks)
-	}
-
-	/// Glue together any disconnected chunks and check that the chain is complete.
-	pub fn finalize(self, canonical: HashMap<u64, H256>) -> Result<(), Error> {
-		let mut batch = self.db.transaction();
-
-		for (first_num, first_hash) in self.disconnected {
-			let parent_num = first_num - 1;
-
-			// check if the parent is even in the chain.
-			// since we don't restore every single block in the chain,
-			// the first block of the first chunks has nothing to connect to.
-			if let Some(parent_hash) = self.chain.block_hash(parent_num) {
-				// if so, add the child to it.
-				self.chain.add_child(&mut batch, parent_hash, first_hash);
-			}
-		}
-		self.db.write_buffered(batch);
-
-		let best_number = self.best_number;
-		for num in (0..self.fed_blocks).map(|x| best_number - x) {
-
-			let hash = self.chain.block_hash(num).ok_or(Error::IncompleteChain)?;
-
-			if let Some(canon_hash) = canonical.get(&num).cloned() {
-				if canon_hash != hash {
-					return Err(Error::WrongBlockHash(num, canon_hash, hash));
-				}
-			}
-		}
-
 		Ok(())
 	}
 }

ethcore/src/snapshot/service.rs

@@ -16,14 +16,14 @@
 
 //! Snapshot network service implementation.
 
-use std::collections::{HashMap, HashSet};
+use std::collections::HashSet;
 use std::io::ErrorKind;
 use std::fs;
 use std::path::PathBuf;
 use std::sync::Arc;
 use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
 
-use super::{ManifestData, StateRebuilder, BlockRebuilder, RestorationStatus, SnapshotService};
+use super::{ManifestData, StateRebuilder, Rebuilder, RestorationStatus, SnapshotService};
 use super::io::{SnapshotReader, LooseReader, SnapshotWriter, LooseWriter};
 
 use blockchain::BlockChain;
@@ -69,12 +69,11 @@ struct Restoration {
 	state_chunks_left: HashSet<H256>,
 	block_chunks_left: HashSet<H256>,
 	state: StateRebuilder,
-	blocks: BlockRebuilder,
+	secondary: Box<Rebuilder>,
 	writer: Option<LooseWriter>,
 	snappy_buffer: Bytes,
 	final_state_root: H256,
 	guard: Guard,
-	canonical_hashes: HashMap<u64, H256>,
 	db: Arc<Database>,
 }
 
@@ -86,6 +85,7 @@ struct RestorationParams<'a> {
 	writer: Option<LooseWriter>, // writer for recovered snapshot.
 	genesis: &'a [u8], // genesis block of the chain.
 	guard: Guard, // guard for the restoration directory.
+	engine: &'a Engine,
 }
 
 impl Restoration {
@@ -100,7 +100,10 @@ impl Restoration {
 			.map_err(UtilError::SimpleString)?);
 
 		let chain = BlockChain::new(Default::default(), params.genesis, raw_db.clone());
-		let blocks = BlockRebuilder::new(chain, raw_db.clone(), &manifest)?;
+		let components = params.engine.snapshot_components()
+			.ok_or_else(|| ::snapshot::Error::SnapshotsUnsupported)?;
+
+		let secondary = components.rebuilder(chain, raw_db.clone(), &manifest)?;
 
 		let root = manifest.state_root.clone();
 		Ok(Restoration {
@@ -108,12 +111,11 @@ impl Restoration {
 			state_chunks_left: state_chunks,
 			block_chunks_left: block_chunks,
 			state: StateRebuilder::new(raw_db.clone(), params.pruning),
-			blocks: blocks,
+			secondary: secondary,
 			writer: params.writer,
 			snappy_buffer: Vec::new(),
 			final_state_root: root,
 			guard: params.guard,
-			canonical_hashes: HashMap::new(),
 			db: raw_db,
 		})
 	}
@@ -138,7 +140,7 @@ impl Restoration {
 		if self.block_chunks_left.remove(&hash) {
 			let len = snappy::decompress_into(chunk, &mut self.snappy_buffer)?;
 
-			self.blocks.feed(&self.snappy_buffer[..len], engine, flag)?;
+			self.secondary.feed(&self.snappy_buffer[..len], engine, flag)?;
 			if let Some(ref mut writer) = self.writer.as_mut() {
 				 writer.write_block_chunk(hash, chunk)?;
 			}
@@ -147,13 +149,8 @@ impl Restoration {
 		Ok(())
 	}
 
-	// note canonical hashes.
-	fn note_canonical(&mut self, hashes: &[(u64, H256)]) {
-		self.canonical_hashes.extend(hashes.iter().cloned());
-	}
-
 	// finish up restoration.
-	fn finalize(self) -> Result<(), Error> {
+	fn finalize(mut self) -> Result<(), Error> {
 		use util::trie::TrieError;
 
 		if !self.is_done() { return Ok(()) }
@@ -169,7 +166,7 @@ impl Restoration {
 		self.state.finalize(self.manifest.block_number, self.manifest.block_hash)?;
 
 		// connect out-of-order chunks and verify chain integrity.
-		self.blocks.finalize(self.canonical_hashes)?;
+		self.secondary.finalize()?;
 
 		if let Some(writer) = self.writer {
 			writer.finish(self.manifest)?;
@@ -425,6 +422,7 @@ impl Service {
 			writer: writer,
 			genesis: &self.genesis_block,
 			guard: Guard::new(rest_dir),
+			engine: &*self.engine,
 		};
 
 		let state_chunks = params.manifest.state_hashes.len();
@@ -593,14 +591,6 @@ impl SnapshotService for Service {
 			trace!("Error sending snapshot service message: {:?}", e);
 		}
 	}
-
-	fn provide_canon_hashes(&self, canonical: &[(u64, H256)]) {
-		let mut rest = self.restoration.lock();
-
-		if let Some(ref mut rest) = rest.as_mut() {
-			rest.note_canonical(canonical);
-		}
-	}
 }
 
 impl Drop for Service {

ethcore/src/snapshot/snapshot_service_trait.rs

@@ -48,10 +48,6 @@ pub trait SnapshotService : Sync + Send {
 	/// Feed a raw block chunk to the service to be processed asynchronously.
 	/// no-op if currently restoring.
 	fn restore_block_chunk(&self, hash: H256, chunk: Bytes);
-
-	/// Give the restoration in-progress some canonical block hashes for
-	/// extra verification (performed at the end)
-	fn provide_canon_hashes(&self, canonical: &[(u64, H256)]);
 }
 
 impl IpcConfig for SnapshotService { }

ethcore/src/snapshot/tests/blocks.rs

@@ -21,33 +21,32 @@ use error::Error;
 
 use blockchain::generator::{ChainGenerator, ChainIterator, BlockFinalizer};
 use blockchain::BlockChain;
-use snapshot::{chunk_blocks, BlockRebuilder, Error as SnapshotError, Progress};
+use snapshot::{chunk_secondary, Error as SnapshotError, Progress, SnapshotComponents};
 use snapshot::io::{PackedReader, PackedWriter, SnapshotReader, SnapshotWriter};
 
 use util::{Mutex, snappy};
-use util::kvdb::{Database, DatabaseConfig};
+use util::kvdb::{self, KeyValueDB, DBTransaction};
 
-use std::collections::HashMap;
 use std::sync::Arc;
 use std::sync::atomic::AtomicBool;
 
+const SNAPSHOT_MODE: ::snapshot::PowSnapshot = ::snapshot::PowSnapshot(30000);
+
 fn chunk_and_restore(amount: u64) {
 	let mut canon_chain = ChainGenerator::default();
 	let mut finalizer = BlockFinalizer::default();
 	let genesis = canon_chain.generate(&mut finalizer).unwrap();
-	let db_cfg = DatabaseConfig::with_columns(::db::NUM_COLUMNS);
 
 	let engine = Arc::new(::engines::NullEngine::default());
-	let orig_path = RandomTempPath::create_dir();
 	let new_path = RandomTempPath::create_dir();
 	let mut snapshot_path = new_path.as_path().to_owned();
 	snapshot_path.push("SNAP");
 
-	let old_db = Arc::new(Database::open(&db_cfg, orig_path.as_str()).unwrap());
+	let old_db = Arc::new(kvdb::in_memory(::db::NUM_COLUMNS.unwrap_or(0)));
 	let bc = BlockChain::new(Default::default(), &genesis, old_db.clone());
 
 	// build the blockchain.
-	let mut batch = old_db.transaction();
+	let mut batch = DBTransaction::new();
 	for _ in 0..amount {
 		let block = canon_chain.generate(&mut finalizer).unwrap();
 		bc.insert_block(&mut batch, &block, vec![]);
@@ -56,12 +55,18 @@ fn chunk_and_restore(amount: u64) {
 
 	old_db.write(batch).unwrap();
 
-
 	let best_hash = bc.best_block_hash();
 
 	// snapshot it.
 	let writer = Mutex::new(PackedWriter::new(&snapshot_path).unwrap());
-	let block_hashes = chunk_blocks(&bc, best_hash, &writer, &Progress::default()).unwrap();
+	let block_hashes = chunk_secondary(
+		Box::new(SNAPSHOT_MODE),
+		&bc,
+		best_hash,
+		&writer,
+		&Progress::default()
+	).unwrap();
+
 	let manifest = ::snapshot::ManifestData {
 		version: 2,
 		state_hashes: Vec::new(),
@@ -74,9 +79,10 @@ fn chunk_and_restore(amount: u64) {
 	writer.into_inner().finish(manifest.clone()).unwrap();
 
 	// restore it.
-	let new_db = Arc::new(Database::open(&db_cfg, new_path.as_str()).unwrap());
+	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 mut rebuilder = BlockRebuilder::new(new_chain, new_db.clone(), &manifest).unwrap();
+	let mut rebuilder = SNAPSHOT_MODE.rebuilder(new_chain, new_db.clone(), &manifest).unwrap();
+
 	let reader = PackedReader::new(&snapshot_path).unwrap().unwrap();
 	let flag = AtomicBool::new(true);
 	for chunk_hash in &reader.manifest().block_hashes {
@@ -85,7 +91,8 @@ fn chunk_and_restore(amount: u64) {
 		rebuilder.feed(&chunk, engine.as_ref(), &flag).unwrap();
 	}
 
-	rebuilder.finalize(HashMap::new()).unwrap();
+	rebuilder.finalize().unwrap();
+	drop(rebuilder);
 
 	// and test it.
 	let new_chain = BlockChain::new(Default::default(), &genesis, new_db);
@@ -118,10 +125,8 @@ fn checks_flag() {
 	};
 
 	let chunk = stream.out();
-	let path = RandomTempPath::create_dir();
 
-	let db_cfg = DatabaseConfig::with_columns(::db::NUM_COLUMNS);
+	let db = Arc::new(kvdb::in_memory(::db::NUM_COLUMNS.unwrap_or(0)));
-	let db = Arc::new(Database::open(&db_cfg, path.as_str()).unwrap());
 	let engine = Arc::new(::engines::NullEngine::default());
 	let chain = BlockChain::new(Default::default(), &genesis, db.clone());
 
@@ -134,7 +139,7 @@ fn checks_flag() {
 		block_hash: H256::default(),
 	};
 
-	let mut rebuilder = BlockRebuilder::new(chain, db.clone(), &manifest).unwrap();
+	let mut rebuilder = SNAPSHOT_MODE.rebuilder(chain, db.clone(), &manifest).unwrap();
 
 	match rebuilder.feed(&chunk, engine.as_ref(), &AtomicBool::new(false)) {
 		Err(Error::Snapshot(SnapshotError::RestorationAborted)) => {}

ethcore/src/tests/client.rs

@@ -350,7 +350,7 @@ fn transaction_proof() {
 		data: Vec::new(),
 	}.fake_sign(address);
 
-	let proof = client.prove_transaction(transaction.clone(), BlockId::Latest).unwrap();
+	let proof = client.prove_transaction(transaction.clone(), BlockId::Latest).unwrap().1;
 	let backend = state::backend::ProofCheck::new(&proof);
 
 	let mut factories = ::factory::Factories::default();

ethcore/src/verification/canon_verifier.rs

@@ -34,4 +34,8 @@ impl Verifier for CanonVerifier {
 	fn verify_block_final(&self, expected: &Header, got: &Header) -> Result<(), Error> {
 		verification::verify_block_final(expected, got)
 	}
+
+	fn verify_block_external(&self, header: &Header, bytes: &[u8], engine: &Engine) -> Result<(), Error> {
+		engine.verify_block_external(header, Some(bytes))
+	}
 }

ethcore/src/verification/noop_verifier.rs

@@ -34,4 +34,8 @@ impl Verifier for NoopVerifier {
 	fn verify_block_final(&self, _expected: &Header, _got: &Header) -> Result<(), Error> {
 		Ok(())
 	}
+
+	fn verify_block_external(&self, _header: &Header, _bytes: &[u8], _engine: &Engine) -> Result<(), Error> {
+		Ok(())
+	}
 }

ethcore/src/verification/verifier.rs

@@ -27,4 +27,6 @@ pub trait Verifier: Send + Sync {
 	fn verify_block_family(&self, header: &Header, bytes: &[u8], engine: &Engine, bc: &BlockProvider) -> Result<(), Error>;
 	/// Do a final verification check for an enacted header vs its expected counterpart.
 	fn verify_block_final(&self, expected: &Header, got: &Header) -> Result<(), Error>;
+	/// Verify a block, inspecing external state.
+	fn verify_block_external(&self, header: &Header, bytes: &[u8], engine: &Engine) -> Result<(), Error>;
 }

rpc/src/v1/tests/helpers/snapshot_service.rs

@@ -47,5 +47,4 @@ impl SnapshotService for TestSnapshotService {
 	fn abort_restore(&self) { }
 	fn restore_state_chunk(&self, _hash: H256, _chunk: Bytes) { }
 	fn restore_block_chunk(&self, _hash: H256, _chunk: Bytes) { }
-	fn provide_canon_hashes(&self, _hashes: &[(u64, H256)]) { }
+}
-}

sync/src/tests/snapshot.rs

@@ -24,7 +24,6 @@ use SyncConfig;
 pub struct TestSnapshotService {
 	manifest: Option<ManifestData>,
 	chunks: HashMap<H256, Bytes>,
-	canon_hashes: Mutex<HashMap<u64, H256>>,
 
 	restoration_manifest: Mutex<Option<ManifestData>>,
 	state_restoration_chunks: Mutex<HashMap<H256, Bytes>>,
@@ -36,7 +35,6 @@ impl TestSnapshotService {
 		TestSnapshotService {
 			manifest: None,
 			chunks: HashMap::new(),
-			canon_hashes: Mutex::new(HashMap::new()),
 			restoration_manifest: Mutex::new(None),
 			state_restoration_chunks: Mutex::new(HashMap::new()),
 			block_restoration_chunks: Mutex::new(HashMap::new()),
@@ -61,7 +59,6 @@ impl TestSnapshotService {
 		TestSnapshotService {
 			manifest: Some(manifest),
 			chunks: chunks,
-			canon_hashes: Mutex::new(HashMap::new()),
 			restoration_manifest: Mutex::new(None),
 			state_restoration_chunks: Mutex::new(HashMap::new()),
 			block_restoration_chunks: Mutex::new(HashMap::new()),
@@ -115,10 +112,6 @@ impl SnapshotService for TestSnapshotService {
 			self.block_restoration_chunks.lock().insert(hash, chunk);
 		}
 	}
-
-	fn provide_canon_hashes(&self, hashes: &[(u64, H256)]) {
-		self.canon_hashes.lock().extend(hashes.iter().cloned());
-	}
 }
 
 #[test]

util/Cargo.toml

@@ -16,7 +16,7 @@ time = "0.1.34"
 rocksdb = { git = "https://github.com/paritytech/rust-rocksdb" }
 eth-secp256k1 = { git = "https://github.com/paritytech/rust-secp256k1" }
 rust-crypto = "0.2.34"
-elastic-array = { git = "https://github.com/paritytech/elastic-array" }
+elastic-array = "0.7.0"
 rlp = { path = "rlp" }
 heapsize = { version = "0.3", features = ["unstable"] }
 itertools = "0.5"

util/rlp/Cargo.toml

@@ -6,7 +6,7 @@ version = "0.1.0"
 authors = ["Parity Technologies <admin@parity.io>"]
 
 [dependencies]
-elastic-array = { git = "https://github.com/paritytech/elastic-array" }
+elastic-array = "0.7.0"
 ethcore-bigint = { path = "../bigint" }
 lazy_static = "0.2"
 rustc-serialize = "0.3"

util/src/journaldb/mod.rs

@@ -125,8 +125,8 @@ pub fn new(backing: Arc<::kvdb::KeyValueDB>, algorithm: Algorithm, col: Option<u
 }
 
 // all keys must be at least 12 bytes
-const DB_PREFIX_LEN : usize = 12;
+const DB_PREFIX_LEN : usize = ::kvdb::PREFIX_LEN;
-const LATEST_ERA_KEY : [u8; DB_PREFIX_LEN] = [ b'l', b'a', b's', b't', 0, 0, 0, 0, 0, 0, 0, 0 ];
+const LATEST_ERA_KEY : [u8; ::kvdb::PREFIX_LEN] = [ b'l', b'a', b's', b't', 0, 0, 0, 0, 0, 0, 0, 0 ];
 
 #[cfg(test)]
 mod tests {

util/src/kvdb.rs

@@ -36,6 +36,9 @@ use std::fs::File;
 const DB_BACKGROUND_FLUSHES: i32 = 2;
 const DB_BACKGROUND_COMPACTIONS: i32 = 2;
 
+/// Required length of prefixes.
+pub const PREFIX_LEN: usize = 12;
+
 /// Write transaction. Batches a sequence of put/delete operations for efficiency.
 #[derive(Default, Clone, PartialEq)]
 pub struct DBTransaction {
@@ -167,6 +170,10 @@ pub trait KeyValueDB: Sync + Send {
 	/// Iterate over flushed data for a given column.
 	fn iter<'a>(&'a self, col: Option<u32>) -> Box<Iterator<Item=(Box<[u8]>, Box<[u8]>)> + 'a>;
 
+	/// Iterate over flushed data for a given column, starting from a given prefix.
+	fn iter_from_prefix<'a>(&'a self, col: Option<u32>, prefix: &'a [u8])
+		-> Box<Iterator<Item=(Box<[u8]>, Box<[u8]>)> + 'a>;
+
 	/// Attempt to replace this database with a new one located at the given path.
 	fn restore(&self, new_db: &str) -> Result<(), UtilError>;
 }
@@ -247,7 +254,21 @@ impl KeyValueDB for InMemory {
 					.into_iter()
 					.map(|(k, v)| (k.into_boxed_slice(), v.to_vec().into_boxed_slice()))
 			),
-			None => Box::new(None.into_iter())
+			None => Box::new(None.into_iter()),
+		}
+	}
+
+	fn iter_from_prefix<'a>(&'a self, col: Option<u32>, prefix: &'a [u8])
+		-> Box<Iterator<Item=(Box<[u8]>, Box<[u8]>)> + 'a>
+	{
+		match self.columns.read().get(&col) {
+			Some(map) => Box::new(
+				map.clone()
+					.into_iter()
+					.skip_while(move |&(ref k, _)| !k.starts_with(prefix))
+					.map(|(k, v)| (k.into_boxed_slice(), v.to_vec().into_boxed_slice()))
+			),
+			None => Box::new(None.into_iter()),
 		}
 	}
 
@@ -691,23 +712,17 @@ impl Database {
 	/// Get value by partial key. Prefix size should match configured prefix size. Only searches flushed values.
 	// TODO: support prefix seek for unflushed data
 	pub fn get_by_prefix(&self, col: Option<u32>, prefix: &[u8]) -> Option<Box<[u8]>> {
-		match *self.db.read() {
+		self.iter_from_prefix(col, prefix).and_then(|mut iter| {
-			Some(DBAndColumns { ref db, ref cfs }) => {
+			match iter.next() {
-				let mut iter = col.map_or_else(|| db.iterator_opt(IteratorMode::From(prefix, Direction::Forward), &self.read_opts),
+				// TODO: use prefix_same_as_start read option (not availabele in C API currently)
-					|c| db.iterator_cf_opt(cfs[c as usize], IteratorMode::From(prefix, Direction::Forward), &self.read_opts)
+				Some((k, v)) => if k[0 .. prefix.len()] == prefix[..] { Some(v) } else { None },
-						.expect("iterator params are valid; qed"));
+				_ => None
-				match iter.next() {
+			}
-					// TODO: use prefix_same_as_start read option (not availabele in C API currently)
+		})
-					Some((k, v)) => if k[0 .. prefix.len()] == prefix[..] { Some(v) } else { None },
-					_ => None
-				}
-			},
-			None => None,
-		}
 	}
 
 	/// Get database iterator for flushed data.
-	pub fn iter(&self, col: Option<u32>) -> DatabaseIterator {
+	pub fn iter(&self, col: Option<u32>) -> Option<DatabaseIterator> {
 		//TODO: iterate over overlay
 		match *self.db.read() {
 			Some(DBAndColumns { ref db, ref cfs }) => {
@@ -717,12 +732,28 @@ impl Database {
 						.expect("iterator params are valid; qed")
 				);
 
-				DatabaseIterator {
+				Some(DatabaseIterator {
 					iter: iter,
 					_marker: PhantomData,
-				}
+				})
 			},
-			None => panic!("Not supported yet") //TODO: return an empty iterator or change return type
+			None => None,
+		}
+	}
+
+	fn iter_from_prefix(&self, col: Option<u32>, prefix: &[u8]) -> Option<DatabaseIterator> {
+		match *self.db.read() {
+			Some(DBAndColumns { ref db, ref cfs }) => {
+				let iter = col.map_or_else(|| db.iterator_opt(IteratorMode::From(prefix, Direction::Forward), &self.read_opts),
+					|c| db.iterator_cf_opt(cfs[c as usize], IteratorMode::From(prefix, Direction::Forward), &self.read_opts)
+						.expect("iterator params are valid; qed"));
+
+				Some(DatabaseIterator {
+					iter: iter,
+					_marker: PhantomData,
+				})
+			},
+			None => None,
 		}
 	}
 
@@ -836,7 +867,14 @@ impl KeyValueDB for Database {
 
 	fn iter<'a>(&'a self, col: Option<u32>) -> Box<Iterator<Item=(Box<[u8]>, Box<[u8]>)> + 'a> {
 		let unboxed = Database::iter(self, col);
-		Box::new(unboxed)
+		Box::new(unboxed.into_iter().flat_map(|inner| inner))
+	}
+
+	fn iter_from_prefix<'a>(&'a self, col: Option<u32>, prefix: &'a [u8])
+		-> Box<Iterator<Item=(Box<[u8]>, Box<[u8]>)> + 'a>
+	{
+		let unboxed = Database::iter_from_prefix(self, col, prefix);
+		Box::new(unboxed.into_iter().flat_map(|inner| inner))
 	}
 
 	fn restore(&self, new_db: &str) -> Result<(), UtilError> {
@@ -872,7 +910,7 @@ mod tests {
 
 		assert_eq!(&*db.get(None, &key1).unwrap().unwrap(), b"cat");
 
-		let contents: Vec<_> = db.iter(None).collect();
+		let contents: Vec<_> = db.iter(None).into_iter().flat_map(|inner| inner).collect();
 		assert_eq!(contents.len(), 2);
 		assert_eq!(&*contents[0].0, &*key1);
 		assert_eq!(&*contents[0].1, b"cat");

util/src/migration/mod.rs

@@ -157,7 +157,12 @@ impl<T: SimpleMigration> Migration for T {
 	fn migrate(&mut self, source: Arc<Database>, config: &Config, dest: &mut Database, col: Option<u32>) -> Result<(), Error> {
 		let mut batch = Batch::new(config, col);
 
-		for (key, value) in source.iter(col) {
+		let iter = match source.iter(col) {
+			Some(iter) => iter,
+			None => return Ok(()),
+		};
+
+		for (key, value) in iter {
 			if let Some((key, value)) = self.simple_migrate(key.to_vec(), value.to_vec()) {
 				batch.insert(key, value, dest)?;
 			}

util/src/migration/tests.rs

@@ -94,7 +94,7 @@ impl Migration for AddsColumn {
 	fn migrate(&mut self, source: Arc<Database>, config: &Config, dest: &mut Database, col: Option<u32>) -> Result<(), Error> {
 		let mut batch = Batch::new(config, col);
 
-		for (key, value) in source.iter(col) {
+		for (key, value) in source.iter(col).into_iter().flat_map(|inner| inner) {
 			batch.insert(key.to_vec(), value.to_vec(), dest)?;
 		}