Merge branch 'master' into fo-6418-dont-export-bigint

ethcore/light/src/client/fetch.rs

@@ -0,0 +1,74 @@
+// 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/>.
+
+//! Trait for fetching chain data.
+
+use std::sync::Arc;
+
+use ethcore::encoded;
+use ethcore::engines::{Engine, StateDependentProof};
+use ethcore::header::Header;
+use ethcore::receipt::Receipt;
+use futures::future::IntoFuture;
+use util::H256;
+
+/// Provides full chain data.
+pub trait ChainDataFetcher: Send + Sync + 'static {
+	/// Error type when data unavailable.
+	type Error: ::std::fmt::Debug;
+
+	/// Future for fetching block body.
+	type Body: IntoFuture<Item=encoded::Block, Error=Self::Error>;
+	/// Future for fetching block receipts.
+	type Receipts: IntoFuture<Item=Vec<Receipt>, Error=Self::Error>;
+	/// Future for fetching epoch transition
+	type Transition: IntoFuture<Item=Vec<u8>, Error=Self::Error>;
+
+	/// Fetch a block body.
+	fn block_body(&self, header: &Header) -> Self::Body;
+
+	/// Fetch block receipts.
+	fn block_receipts(&self, header: &Header) -> Self::Receipts;
+
+	/// Fetch epoch transition proof at given header.
+	fn epoch_transition(&self, hash: H256, engine: Arc<Engine>, checker: Arc<StateDependentProof>) -> Self::Transition;
+}
+
+/// Fetcher implementation which cannot fetch anything.
+pub struct Unavailable;
+
+/// Create a fetcher which has all data unavailable.
+pub fn unavailable() -> Unavailable { Unavailable }
+
+impl ChainDataFetcher for Unavailable {
+	type Error = &'static str;
+
+	type Body = Result<encoded::Block, &'static str>;
+	type Receipts = Result<Vec<Receipt>, &'static str>;
+	type Transition = Result<Vec<u8>, &'static str>;
+
+	fn block_body(&self, _header: &Header) -> Self::Body {
+		Err("fetching block bodies unavailable")
+	}
+
+	fn block_receipts(&self, _header: &Header) -> Self::Receipts {
+		Err("fetching block receipts unavailable")
+	}
+
+	fn epoch_transition(&self, _h: H256, _e: Arc<Engine>, _check: Arc<StateDependentProof>) -> Self::Transition {
+		Err("fetching epoch transition proofs unavailable")
+	}
+}

ethcore/light/src/client/header_chain.rs

@@ -18,11 +18,12 @@
 //!
 //! Unlike a full node's `BlockChain` this doesn't store much in the database.
 //! It stores candidates for the last 2048-4096 blocks as well as CHT roots for
-//! historical blocks all the way to the genesis.
+//! historical blocks all the way to the genesis. If the engine makes use
+//! of epoch transitions, those are stored as well.
 //!
 //! This is separate from the `BlockChain` for two reasons:
 //!   - It stores only headers (and a pruned subset of them)
-//!   - To allow for flexibility in the database layout once that's incorporated.
+//!   - To allow for flexibility in the database layout..
 
 use std::collections::BTreeMap;
 use std::sync::Arc;
@@ -30,15 +31,20 @@ use std::sync::Arc;
 use cht;
 
 use ethcore::block_status::BlockStatus;
-use ethcore::error::BlockError;
+use ethcore::error::{BlockImportError, BlockError};
 use ethcore::encoded;
 use ethcore::header::Header;
 use ethcore::ids::BlockId;
+use ethcore::spec::Spec;
+use ethcore::engines::epoch::{
+	Transition as EpochTransition,
+	PendingTransition as PendingEpochTransition
+};
 
 use rlp::{Encodable, Decodable, DecoderError, RlpStream, Rlp, UntrustedRlp};
 use heapsize::HeapSizeOf;
 use bigint::prelude::U256;
-use bigint::hash::H256;
+use bigint::hash::{H256, H256FastMap, H264};
 use util::kvdb::{DBTransaction, KeyValueDB};
 
 use cache::Cache;
@@ -54,6 +60,9 @@ const HISTORY: u64 = 2048;
 /// The best block key. Maps to an RLP list: [best_era, last_era]
 const CURRENT_KEY: &'static [u8] = &*b"best_and_latest";
 
+/// Key storing the last canonical epoch transition.
+const LAST_CANONICAL_TRANSITION: &'static [u8] = &*b"canonical_transition";
+
 /// Information about a block.
 #[derive(Debug, Clone)]
 pub struct BlockDescriptor {
@@ -101,7 +110,6 @@ impl Encodable for Entry {
 
 impl Decodable for Entry {
 	fn decode(rlp: &UntrustedRlp) -> Result<Self, DecoderError> {
-
 		let mut candidates = SmallVec::<[Candidate; 3]>::new();
 
 		for item in rlp.iter() {
@@ -131,6 +139,42 @@ fn era_key(number: u64) -> String {
 	format!("candidates_{}", number)
 }
 
+fn pending_transition_key(block_hash: H256) -> H264 {
+	const LEADING: u8 = 1;
+
+	let mut key = H264::default();
+
+	key[0] = LEADING;
+	key.0[1..].copy_from_slice(&block_hash.0[..]);
+
+	key
+}
+
+fn transition_key(block_hash: H256) -> H264 {
+	const LEADING: u8 = 2;
+
+	let mut key = H264::default();
+
+	key[0] = LEADING;
+	key.0[1..].copy_from_slice(&block_hash.0[..]);
+
+	key
+}
+
+// encode last canonical transition entry: header and proof.
+fn encode_canonical_transition(header: &Header, proof: &[u8]) -> Vec<u8> {
+	let mut stream = RlpStream::new_list(2);
+	stream.append(header).append(&proof);
+	stream.out()
+}
+
+// decode last canonical transition entry.
+fn decode_canonical_transition(t: &[u8]) -> Result<(Header, &[u8]), DecoderError> {
+	let rlp = UntrustedRlp::new(t);
+
+	Ok((rlp.val_at(0)?, rlp.at(1)?.data()?))
+}
+
 /// Pending changes from `insert` to be applied after the database write has finished.
 pub struct PendingChanges {
 	best_block: Option<BlockDescriptor>, // new best block.
@@ -141,6 +185,7 @@ pub struct HeaderChain {
 	genesis_header: encoded::Header, // special-case the genesis.
 	candidates: RwLock<BTreeMap<u64, Entry>>,
 	best_block: RwLock<BlockDescriptor>,
+	live_epoch_proofs: RwLock<H256FastMap<EpochTransition>>,
 	db: Arc<KeyValueDB>,
 	col: Option<u32>,
 	cache: Arc<Mutex<Cache>>,
@@ -148,8 +193,16 @@ pub struct HeaderChain {
 
 impl HeaderChain {
 	/// Create a new header chain given this genesis block and database to read from.
-	pub fn new(db: Arc<KeyValueDB>, col: Option<u32>, genesis: &[u8], cache: Arc<Mutex<Cache>>) -> Result<Self, String> {
-		use ethcore::views::HeaderView;
+	pub fn new(
+		db: Arc<KeyValueDB>,
+		col: Option<u32>,
+		spec: &Spec,
+		cache: Arc<Mutex<Cache>>,
+	) -> Result<Self, String> {
+		let mut live_epoch_proofs = ::std::collections::HashMap::default();
+
+		let genesis = ::rlp::encode(&spec.genesis_header()).into_vec();
+		let decoded_header = spec.genesis_header();
 
 		let chain = if let Some(current) = db.get(col, CURRENT_KEY)? {
 			let (best_number, highest_number) = {
@@ -160,12 +213,24 @@ impl HeaderChain {
 			let mut cur_number = highest_number;
 			let mut candidates = BTreeMap::new();
 
-			// load all era entries and referenced headers within them.
+			// load all era entries, referenced headers within them,
+			// and live epoch proofs.
 			while let Some(entry) = db.get(col, era_key(cur_number).as_bytes())? {
 				let entry: Entry = ::rlp::decode(&entry);
 				trace!(target: "chain", "loaded header chain entry for era {} with {} candidates",
 					cur_number, entry.candidates.len());
 
+				for c in &entry.candidates {
+					let key = transition_key(c.hash);
+
+					if let Some(proof) = db.get(col, &*key)? {
+						live_epoch_proofs.insert(c.hash, EpochTransition {
+							block_hash: c.hash,
+							block_number: cur_number,
+							proof: proof.into_vec(),
+						});
+					}
+				}
 				candidates.insert(cur_number, entry);
 
 				cur_number -= 1;
@@ -187,29 +252,42 @@ impl HeaderChain {
 			};
 
 			HeaderChain {
-				genesis_header: encoded::Header::new(genesis.to_owned()),
+				genesis_header: encoded::Header::new(genesis),
 				best_block: RwLock::new(best_block),
 				candidates: RwLock::new(candidates),
+				live_epoch_proofs: RwLock::new(live_epoch_proofs),
 				db: db,
 				col: col,
 				cache: cache,
 			}
 		} else {
-			let g_view = HeaderView::new(genesis);
 			HeaderChain {
-				genesis_header: encoded::Header::new(genesis.to_owned()),
+				genesis_header: encoded::Header::new(genesis),
 				best_block: RwLock::new(BlockDescriptor {
-					hash: g_view.hash(),
+					hash: decoded_header.hash(),
 					number: 0,
-					total_difficulty: g_view.difficulty(),
+					total_difficulty: *decoded_header.difficulty(),
 				}),
 				candidates: RwLock::new(BTreeMap::new()),
+				live_epoch_proofs: RwLock::new(live_epoch_proofs),
 				db: db,
 				col: col,
 				cache: cache,
 			}
 		};
 
+		// instantiate genesis epoch data if it doesn't exist.
+		if let None = chain.db.get(col, LAST_CANONICAL_TRANSITION)? {
+			let genesis_data = spec.genesis_epoch_data()?;
+
+			{
+				let mut batch = chain.db.transaction();
+				let data = encode_canonical_transition(&decoded_header, &genesis_data);
+				batch.put_vec(col, LAST_CANONICAL_TRANSITION, data);
+				chain.db.write(batch)?;
+			}
+		}
+
 		Ok(chain)
 	}
 
@@ -218,10 +296,24 @@ impl HeaderChain {
 	/// This blindly trusts that the data given to it is sensible.
 	/// Returns a set of pending changes to be applied with `apply_pending`
 	/// before the next call to insert and after the transaction has been written.
-	pub fn insert(&self, transaction: &mut DBTransaction, header: Header) -> Result<PendingChanges, BlockError> {
+	///
+	/// If the block is an epoch transition, provide the transition along with
+	/// the header.
+	pub fn insert(
+		&self,
+		transaction: &mut DBTransaction,
+		header: Header,
+		transition_proof: Option<Vec<u8>>,
+	) -> Result<PendingChanges, BlockImportError> {
 		let hash = header.hash();
 		let number = header.number();
 		let parent_hash = *header.parent_hash();
+		let transition = transition_proof.map(|proof| EpochTransition {
+			block_hash: hash,
+			block_number: number,
+			proof: proof,
+		});
+
 		let mut pending = PendingChanges {
 			best_block: None,
 		};
@@ -237,7 +329,8 @@ impl HeaderChain {
 				candidates.get(&(number - 1))
 					.and_then(|entry| entry.candidates.iter().find(|c| c.hash == parent_hash))
 					.map(|c| c.total_difficulty)
-					.ok_or_else(|| BlockError::UnknownParent(parent_hash))?
+					.ok_or_else(|| BlockError::UnknownParent(parent_hash))
+					.map_err(BlockImportError::Block)?
 			};
 
 		let total_difficulty = parent_td + *header.difficulty();
@@ -262,8 +355,13 @@ impl HeaderChain {
 			transaction.put(self.col, era_key(number).as_bytes(), &::rlp::encode(&*cur_era))
 		}
 
-		let raw = ::rlp::encode(&header);
-		transaction.put(self.col, &hash[..], &*raw);
+		if let Some(transition) = transition {
+			transaction.put(self.col, &*transition_key(hash), &transition.proof);
+			self.live_epoch_proofs.write().insert(hash, transition);
+		}
+
+		let raw = header.encoded().into_inner();
+		transaction.put_vec(self.col, &hash[..], raw);
 
 		let (best_num, is_new_best) = {
 			let cur_best = self.best_block.read();
@@ -316,8 +414,10 @@ impl HeaderChain {
 				let cht_num = cht::block_to_cht_number(earliest_era)
 					.expect("fails only for number == 0; genesis never imported; qed");
 
+				let mut last_canonical_transition = None;
 				let cht_root = {
 					let mut i = earliest_era;
+					let mut live_epoch_proofs = self.live_epoch_proofs.write();
 
 					// iterable function which removes the candidates as it goes
 					// along. this will only be called until the CHT is complete.
@@ -328,7 +428,25 @@ impl HeaderChain {
 
 						i += 1;
 
+						// prune old blocks and epoch proofs.
 						for ancient in &era_entry.candidates {
+							let maybe_transition = live_epoch_proofs.remove(&ancient.hash);
+							if let Some(epoch_transition) = maybe_transition {
+								transaction.delete(self.col, &*transition_key(ancient.hash));
+
+								if ancient.hash == era_entry.canonical_hash {
+									last_canonical_transition = match self.db.get(self.col, &ancient.hash) {
+										Err(e) => {
+											warn!(target: "chain", "Error reading from DB: {}\n
+												", e);
+											None
+										}
+										Ok(None) => panic!("stored candidates always have corresponding headers; qed"),
+										Ok(Some(header)) => Some((epoch_transition, ::rlp::decode(&header))),
+									};
+								}
+							}
+
 							transaction.delete(self.col, &ancient.hash);
 						}
 
@@ -342,6 +460,12 @@ impl HeaderChain {
 				// write the CHT root to the database.
 				debug!(target: "chain", "Produced CHT {} root: {:?}", cht_num, cht_root);
 				transaction.put(self.col, cht_key(cht_num).as_bytes(), &::rlp::encode(&cht_root));
+
+				// update the last canonical transition proof
+				if let Some((epoch_transition, header)) = last_canonical_transition {
+					let x = encode_canonical_transition(&header, &epoch_transition.proof);
+					transaction.put_vec(self.col, LAST_CANONICAL_TRANSITION, x);
+				}
 			}
 		}
 
@@ -367,7 +491,7 @@ impl HeaderChain {
 	/// will be returned.
 	pub fn block_hash(&self, id: BlockId) -> Option<H256> {
 		match id {
-			BlockId::Earliest => Some(self.genesis_hash()),
+			BlockId::Earliest | BlockId::Number(0) => Some(self.genesis_hash()),
 			BlockId::Hash(hash) => Some(hash),
 			BlockId::Number(num) => {
 				if self.best_block.read().number < num { return None }
@@ -518,6 +642,56 @@ impl HeaderChain {
 			false => BlockStatus::Unknown,
 		}
 	}
+
+	/// Insert a pending transition.
+	pub fn insert_pending_transition(&self, batch: &mut DBTransaction, hash: H256, t: PendingEpochTransition) {
+		let key = pending_transition_key(hash);
+		batch.put(self.col, &*key, &*::rlp::encode(&t));
+	}
+
+	/// Get pending transition for a specific block hash.
+	pub fn pending_transition(&self, hash: H256) -> Option<PendingEpochTransition> {
+		let key = pending_transition_key(hash);
+		match self.db.get(self.col, &*key) {
+			Ok(val) => val.map(|x| ::rlp::decode(&x)),
+			Err(e) => {
+				warn!(target: "chain", "Error reading from database: {}", e);
+				None
+			}
+		}
+	}
+
+	/// Get the transition to the epoch the given parent hash is part of
+	/// or transitions to.
+	/// This will give the epoch that any children of this parent belong to.
+	///
+	/// The header corresponding the the parent hash must be stored already.
+	pub fn epoch_transition_for(&self, parent_hash: H256) -> Option<(Header, Vec<u8>)> {
+		// slow path: loop back block by block
+		let live_proofs = self.live_epoch_proofs.read();
+
+		for hdr in self.ancestry_iter(BlockId::Hash(parent_hash)) {
+			if let Some(transition) = live_proofs.get(&hdr.hash()).cloned() {
+				return Some((hdr.decode(), transition.proof))
+			}
+		}
+
+		// any blocks left must be descendants of the last canonical transition block.
+		match self.db.get(self.col, LAST_CANONICAL_TRANSITION) {
+			Ok(x) => {
+				let x = x.expect("last canonical transition always instantiated; qed");
+
+				let (hdr, proof) = decode_canonical_transition(&x)
+					.expect("last canonical transition always encoded correctly; qed");
+
+				Some((hdr, proof.to_vec()))
+			}
+			Err(e) => {
+				warn!("Error reading from DB: {}", e);
+				None
+			}
+		}
+	}
 }
 
 impl HeapSizeOf for HeaderChain {
@@ -570,7 +744,7 @@ mod tests {
 
 		let cache = Arc::new(Mutex::new(Cache::new(Default::default(), Duration::hours(6))));
 
-		let chain = HeaderChain::new(db.clone(), None, &::rlp::encode(&genesis_header), cache).unwrap();
+		let chain = HeaderChain::new(db.clone(), None, &spec, cache).unwrap();
 
 		let mut parent_hash = genesis_header.hash();
 		let mut rolling_timestamp = genesis_header.timestamp();
@@ -583,7 +757,7 @@ mod tests {
 			parent_hash = header.hash();
 
 			let mut tx = db.transaction();
-			let pending = chain.insert(&mut tx, header).unwrap();
+			let pending = chain.insert(&mut tx, header, None).unwrap();
 			db.write(tx).unwrap();
 			chain.apply_pending(pending);
 
@@ -603,7 +777,7 @@ mod tests {
 		let db = make_db();
 		let cache = Arc::new(Mutex::new(Cache::new(Default::default(), Duration::hours(6))));
 
-		let chain = HeaderChain::new(db.clone(), None, &::rlp::encode(&genesis_header), cache).unwrap();
+		let chain = HeaderChain::new(db.clone(), None, &spec, cache).unwrap();
 
 		let mut parent_hash = genesis_header.hash();
 		let mut rolling_timestamp = genesis_header.timestamp();
@@ -616,7 +790,7 @@ mod tests {
 			parent_hash = header.hash();
 
 			let mut tx = db.transaction();
-			let pending = chain.insert(&mut tx, header).unwrap();
+			let pending = chain.insert(&mut tx, header, None).unwrap();
 			db.write(tx).unwrap();
 			chain.apply_pending(pending);
 
@@ -635,7 +809,7 @@ mod tests {
 				parent_hash = header.hash();
 
 				let mut tx = db.transaction();
-				let pending = chain.insert(&mut tx, header).unwrap();
+				let pending = chain.insert(&mut tx, header, None).unwrap();
 				db.write(tx).unwrap();
 				chain.apply_pending(pending);
 
@@ -659,7 +833,7 @@ mod tests {
 				parent_hash = header.hash();
 
 				let mut tx = db.transaction();
-				let pending = chain.insert(&mut tx, header).unwrap();
+				let pending = chain.insert(&mut tx, header, None).unwrap();
 				db.write(tx).unwrap();
 				chain.apply_pending(pending);
 
@@ -682,12 +856,10 @@ mod tests {
 	#[test]
 	fn earliest_is_latest() {
 		let spec = Spec::new_test();
-		let genesis_header = spec.genesis_header();
 		let db = make_db();
 		let cache = Arc::new(Mutex::new(Cache::new(Default::default(), Duration::hours(6))));
 
-		let chain = HeaderChain::new(db.clone(), None, &::rlp::encode(&genesis_header), cache).unwrap();
-
+		let chain = HeaderChain::new(db.clone(), None, &spec, cache).unwrap();
 
 		assert!(chain.block_header(BlockId::Earliest).is_some());
 		assert!(chain.block_header(BlockId::Latest).is_some());
@@ -702,7 +874,7 @@ mod tests {
 		let cache = Arc::new(Mutex::new(Cache::new(Default::default(), Duration::hours(6))));
 
 		{
-			let chain = HeaderChain::new(db.clone(), None, &::rlp::encode(&genesis_header), cache.clone()).unwrap();
+			let chain = HeaderChain::new(db.clone(), None, &spec, cache.clone()).unwrap();
 			let mut parent_hash = genesis_header.hash();
 			let mut rolling_timestamp = genesis_header.timestamp();
 			for i in 1..10000 {
@@ -714,7 +886,7 @@ mod tests {
 				parent_hash = header.hash();
 
 				let mut tx = db.transaction();
-				let pending = chain.insert(&mut tx, header).unwrap();
+				let pending = chain.insert(&mut tx, header, None).unwrap();
 				db.write(tx).unwrap();
 				chain.apply_pending(pending);
 
@@ -722,7 +894,7 @@ mod tests {
 			}
 		}
 
-		let chain = HeaderChain::new(db.clone(), None, &::rlp::encode(&genesis_header), cache.clone()).unwrap();
+		let chain = HeaderChain::new(db.clone(), None, &spec, cache.clone()).unwrap();
 		assert!(chain.block_header(BlockId::Number(10)).is_none());
 		assert!(chain.block_header(BlockId::Number(9000)).is_some());
 		assert!(chain.cht_root(2).is_some());
@@ -738,7 +910,7 @@ mod tests {
 		let cache = Arc::new(Mutex::new(Cache::new(Default::default(), Duration::hours(6))));
 
 		{
-			let chain = HeaderChain::new(db.clone(), None, &::rlp::encode(&genesis_header), cache.clone()).unwrap();
+			let chain = HeaderChain::new(db.clone(), None, &spec, cache.clone()).unwrap();
 			let mut parent_hash = genesis_header.hash();
 			let mut rolling_timestamp = genesis_header.timestamp();
 
@@ -752,7 +924,7 @@ mod tests {
 				parent_hash = header.hash();
 
 				let mut tx = db.transaction();
-				let pending = chain.insert(&mut tx, header).unwrap();
+				let pending = chain.insert(&mut tx, header, None).unwrap();
 				db.write(tx).unwrap();
 				chain.apply_pending(pending);
 
@@ -769,7 +941,7 @@ mod tests {
 				parent_hash = header.hash();
 
 				let mut tx = db.transaction();
-				let pending = chain.insert(&mut tx, header).unwrap();
+				let pending = chain.insert(&mut tx, header, None).unwrap();
 				db.write(tx).unwrap();
 				chain.apply_pending(pending);
 
@@ -780,7 +952,7 @@ mod tests {
 		}
 
 		// after restoration, non-canonical eras should still be loaded.
-		let chain = HeaderChain::new(db.clone(), None, &::rlp::encode(&genesis_header), cache.clone()).unwrap();
+		let chain = HeaderChain::new(db.clone(), None, &spec, cache.clone()).unwrap();
 		assert_eq!(chain.block_header(BlockId::Latest).unwrap().number(), 10);
 		assert!(chain.candidates.read().get(&100).is_some())
 	}
@@ -792,10 +964,76 @@ mod tests {
 		let db = make_db();
 		let cache = Arc::new(Mutex::new(Cache::new(Default::default(), Duration::hours(6))));
 
-		let chain = HeaderChain::new(db.clone(), None, &::rlp::encode(&genesis_header), cache.clone()).unwrap();
+		let chain = HeaderChain::new(db.clone(), None, &spec, cache.clone()).unwrap();
 
 		assert!(chain.block_header(BlockId::Earliest).is_some());
 		assert!(chain.block_header(BlockId::Number(0)).is_some());
 		assert!(chain.block_header(BlockId::Hash(genesis_header.hash())).is_some());
 	}
+
+	#[test]
+	fn epoch_transitions_available_after_cht() {
+		let spec = Spec::new_test();
+		let genesis_header = spec.genesis_header();
+		let db = make_db();
+		let cache = Arc::new(Mutex::new(Cache::new(Default::default(), Duration::hours(6))));
+
+		let chain = HeaderChain::new(db.clone(), None, &spec, cache).unwrap();
+
+		let mut parent_hash = genesis_header.hash();
+		let mut rolling_timestamp = genesis_header.timestamp();
+		for i in 1..6 {
+			let mut header = Header::new();
+			header.set_parent_hash(parent_hash);
+			header.set_number(i);
+			header.set_timestamp(rolling_timestamp);
+			header.set_difficulty(*genesis_header.difficulty() * i.into());
+			parent_hash = header.hash();
+
+			let mut tx = db.transaction();
+			let epoch_proof = if i == 3 {
+				Some(vec![1, 2, 3, 4])
+			} else {
+				None
+			};
+
+			let pending = chain.insert(&mut tx, header, epoch_proof).unwrap();
+			db.write(tx).unwrap();
+			chain.apply_pending(pending);
+
+			rolling_timestamp += 10;
+		}
+
+		// these 3 should end up falling back to the genesis epoch proof in DB
+		for i in 0..3 {
+			let hash = chain.block_hash(BlockId::Number(i)).unwrap();
+			assert_eq!(chain.epoch_transition_for(hash).unwrap().1, Vec::<u8>::new());
+		}
+
+		// these are live.
+		for i in 3..6 {
+			let hash = chain.block_hash(BlockId::Number(i)).unwrap();
+			assert_eq!(chain.epoch_transition_for(hash).unwrap().1, vec![1, 2, 3, 4]);
+		}
+
+		for i in 6..10000 {
+			let mut header = Header::new();
+			header.set_parent_hash(parent_hash);
+			header.set_number(i);
+			header.set_timestamp(rolling_timestamp);
+			header.set_difficulty(*genesis_header.difficulty() * i.into());
+			parent_hash = header.hash();
+
+			let mut tx = db.transaction();
+			let pending = chain.insert(&mut tx, header, None).unwrap();
+			db.write(tx).unwrap();
+			chain.apply_pending(pending);
+
+			rolling_timestamp += 10;
+		}
+
+		// no live blocks have associated epoch proofs -- make sure we aren't leaking memory.
+		assert!(chain.live_epoch_proofs.read().is_empty());
+		assert_eq!(chain.epoch_transition_for(parent_hash).unwrap().1, vec![1, 2, 3, 4]);
+	}
 }

ethcore/light/src/client/mod.rs

@@ -19,11 +19,11 @@
 use std::sync::{Weak, Arc};
 
 use ethcore::block_status::BlockStatus;
-use ethcore::client::{ClientReport, EnvInfo};
-use ethcore::engines::Engine;
-use ethcore::error::BlockImportError;
+use ethcore::client::{TransactionImportResult, ClientReport, EnvInfo};
+use ethcore::engines::{epoch, Engine, EpochChange, EpochTransition, Proof, Unsure};
+use ethcore::error::{TransactionError, BlockImportError, Error as EthcoreError};
 use ethcore::ids::BlockId;
-use ethcore::header::Header;
+use ethcore::header::{BlockNumber, Header};
 use ethcore::verification::queue::{self, HeaderQueue};
 use ethcore::blockchain_info::BlockChainInfo;
 use ethcore::spec::Spec;
@@ -31,11 +31,14 @@ use ethcore::service::ClientIoMessage;
 use ethcore::encoded;
 use io::IoChannel;
 use parking_lot::{Mutex, RwLock};
-
 use bigint::prelude::U256;
 use bigint::hash::H256;
+use futures::{IntoFuture, Future};
+
+use util::Address;
 use util::kvdb::{KeyValueDB, CompactionProfile};
 
+use self::fetch::ChainDataFetcher;
 use self::header_chain::{AncestryIter, HeaderChain};
 
 use cache::Cache;
@@ -45,6 +48,8 @@ pub use self::service::Service;
 mod header_chain;
 mod service;
 
+pub mod fetch;
+
 /// Configuration for the light client.
 #[derive(Debug, Clone)]
 pub struct Config {
@@ -80,6 +85,9 @@ impl Default for Config {
 
 /// Trait for interacting with the header chain abstractly.
 pub trait LightChainClient: Send + Sync {
+	/// Adds a new `LightChainNotify` listener.
+	fn add_listener(&self, listener: Weak<LightChainNotify>);
+
 	/// Get chain info.
 	fn chain_info(&self) -> BlockChainInfo;
 
@@ -128,7 +136,7 @@ pub trait LightChainClient: Send + Sync {
 	fn cht_root(&self, i: usize) -> Option<H256>;
 
 	/// Get the EIP-86 transition block number.
-	fn eip86_transition(&self) -> u64;
+	fn eip86_transition(&self) -> BlockNumber;
 
 	/// Get a report of import activity since the last call.
 	fn report(&self) -> ClientReport;
@@ -156,7 +164,7 @@ impl<T: LightChainClient> AsLightClient for T {
 }
 
 /// Light client implementation.
-pub struct Client {
+pub struct Client<T> {
 	queue: HeaderQueue,
 	engine: Arc<Engine>,
 	chain: HeaderChain,
@@ -164,22 +172,30 @@ pub struct Client {
 	import_lock: Mutex<()>,
 	db: Arc<KeyValueDB>,
 	listeners: RwLock<Vec<Weak<LightChainNotify>>>,
+	fetcher: T,
 	verify_full: bool,
 }
 
-impl Client {
+impl<T: ChainDataFetcher> Client<T> {
 	/// Create a new `Client`.
-	pub fn new(config: Config, db: Arc<KeyValueDB>, chain_col: Option<u32>, spec: &Spec, io_channel: IoChannel<ClientIoMessage>, cache: Arc<Mutex<Cache>>) -> Result<Self, String> {
-		let gh = ::rlp::encode(&spec.genesis_header());
-
+	pub fn new(
+		config: Config,
+		db: Arc<KeyValueDB>,
+		chain_col: Option<u32>,
+		spec: &Spec,
+		fetcher: T,
+		io_channel: IoChannel<ClientIoMessage>,
+		cache: Arc<Mutex<Cache>>
+	) -> Result<Self, String> {
 		Ok(Client {
 			queue: HeaderQueue::new(config.queue, spec.engine.clone(), io_channel, config.check_seal),
 			engine: spec.engine.clone(),
-			chain: HeaderChain::new(db.clone(), chain_col, &gh, cache)?,
+			chain: HeaderChain::new(db.clone(), chain_col, &spec, cache)?,
 			report: RwLock::new(ClientReport::default()),
 			import_lock: Mutex::new(()),
 			db: db,
 			listeners: RwLock::new(vec![]),
+			fetcher: fetcher,
 			verify_full: config.verify_full,
 		})
 	}
@@ -191,10 +207,24 @@ impl Client {
 
 	/// Create a new `Client` backed purely in-memory.
 	/// This will ignore all database options in the configuration.
-	pub fn in_memory(config: Config, spec: &Spec, io_channel: IoChannel<ClientIoMessage>, cache: Arc<Mutex<Cache>>) -> Self {
+	pub fn in_memory(
+		config: Config,
+		spec: &Spec,
+		fetcher: T,
+		io_channel: IoChannel<ClientIoMessage>,
+		cache: Arc<Mutex<Cache>>
+	) -> Self {
 		let db = ::util::kvdb::in_memory(0);
 
-		Client::new(config, Arc::new(db), None, spec, io_channel, cache).expect("New DB creation infallible; qed")
+		Client::new(
+			config,
+			Arc::new(db),
+			None,
+			spec,
+			fetcher,
+			io_channel,
+			cache
+		).expect("New DB creation infallible; qed")
 	}
 
 	/// Import a header to the queue for additional verification.
@@ -293,19 +323,33 @@ impl Client {
 				continue
 			}
 
-			// TODO: `epoch_end_signal`, `is_epoch_end`.
-			// proofs we get from the network would be _complete_, whereas we need
-			// _incomplete_ signals
+			let write_proof_result = match self.check_epoch_signal(&verified_header) {
+				Ok(Some(proof)) => self.write_pending_proof(&verified_header, proof),
+				Ok(None) => Ok(()),
+				Err(e) =>
+					panic!("Unable to fetch epoch transition proof: {:?}", e),
+			};
+
+			if let Err(e) = write_proof_result {
+				warn!(target: "client", "Error writing pending transition proof to DB: {:?} \
+					The node may not be able to synchronize further.", e);
+			}
+
+			let epoch_proof =  self.engine.is_epoch_end(
+				&verified_header,
+				&|h| self.chain.block_header(BlockId::Hash(h)).map(|hdr| hdr.decode()),
+				&|h| self.chain.pending_transition(h),
+			);
 
 			let mut tx = self.db.transaction();
-			let pending = match self.chain.insert(&mut tx, verified_header) {
+			let pending = match self.chain.insert(&mut tx, verified_header, epoch_proof) {
 				Ok(pending) => {
 					good.push(hash);
 					self.report.write().blocks_imported += 1;
 					pending
 				}
 				Err(e) => {
-					debug!(target: "client", "Error importing header {:?}: {}", (num, hash), e);
+					debug!(target: "client", "Error importing header {:?}: {:?}", (num, hash), e);
 					bad.push(hash);
 					continue;
 				}
@@ -421,9 +465,76 @@ impl Client {
 
 		true
 	}
+
+	fn check_epoch_signal(&self, verified_header: &Header) -> Result<Option<Proof>, T::Error> {
+		let (mut block, mut receipts) = (None, None);
+
+		// First, check without providing auxiliary data.
+		match self.engine.signals_epoch_end(verified_header, None, None) {
+			EpochChange::No => return Ok(None),
+			EpochChange::Yes(proof) => return Ok(Some(proof)),
+			EpochChange::Unsure(unsure) => {
+				let (b, r) = match unsure {
+					Unsure::NeedsBody =>
+						(Some(self.fetcher.block_body(verified_header)), None),
+					Unsure::NeedsReceipts =>
+						(None, Some(self.fetcher.block_receipts(verified_header))),
+					Unsure::NeedsBoth => (
+						Some(self.fetcher.block_body(verified_header)),
+						Some(self.fetcher.block_receipts(verified_header)),
+					),
+				};
+
+				if let Some(b) = b {
+					block = Some(b.into_future().wait()?.into_inner());
+				}
+
+				if let Some(r) = r {
+					receipts = Some(r.into_future().wait()?);
+				}
+			}
+		}
+
+		let block = block.as_ref().map(|x| &x[..]);
+		let receipts = receipts.as_ref().map(|x| &x[..]);
+
+		// Check again now that required data has been fetched.
+		match self.engine.signals_epoch_end(verified_header, block, receipts) {
+			EpochChange::No => return Ok(None),
+			EpochChange::Yes(proof) => return Ok(Some(proof)),
+			EpochChange::Unsure(_) =>
+				panic!("Detected faulty engine implementation: requests additional \
+					data to check epoch end signal when everything necessary provided"),
+		}
+	}
+
+	// attempts to fetch the epoch proof from the network until successful.
+	fn write_pending_proof(&self, header: &Header, proof: Proof) -> Result<(), T::Error> {
+		let proof = match proof {
+			Proof::Known(known) => known,
+			Proof::WithState(state_dependent) => {
+				self.fetcher.epoch_transition(
+					header.hash(),
+					self.engine.clone(),
+					state_dependent
+				).into_future().wait()?
+			}
+		};
+
+		let mut batch = self.db.transaction();
+		self.chain.insert_pending_transition(&mut batch, header.hash(), epoch::PendingTransition {
+			proof: proof,
+		});
+		self.db.write_buffered(batch);
+		Ok(())
+	}
 }
 
-impl LightChainClient for Client {
+impl<T: ChainDataFetcher> LightChainClient for Client<T> {
+	fn add_listener(&self, listener: Weak<LightChainNotify>) {
+		Client::add_listener(self, listener)
+	}
+
 	fn chain_info(&self) -> BlockChainInfo { Client::chain_info(self) }
 
 	fn queue_header(&self, header: Header) -> Result<H256, BlockImportError> {
@@ -482,7 +593,7 @@ impl LightChainClient for Client {
 		Client::cht_root(self, i)
 	}
 
-	fn eip86_transition(&self) -> u64 {
+	fn eip86_transition(&self) -> BlockNumber {
 		self.engine().params().eip86_transition
 	}
 
@@ -490,3 +601,38 @@ impl LightChainClient for Client {
 		Client::report(self)
 	}
 }
+
+impl<T: ChainDataFetcher> ::ethcore::client::EngineClient for Client<T> {
+	fn update_sealing(&self) { }
+	fn submit_seal(&self, _block_hash: H256, _seal: Vec<Vec<u8>>) { }
+	fn broadcast_consensus_message(&self, _message: Vec<u8>) { }
+
+	fn epoch_transition_for(&self, parent_hash: H256) -> Option<EpochTransition> {
+		self.chain.epoch_transition_for(parent_hash).map(|(hdr, proof)| EpochTransition {
+			block_hash: hdr.hash(),
+			block_number: hdr.number(),
+			proof: proof,
+		})
+	}
+
+	fn chain_info(&self) -> BlockChainInfo {
+		Client::chain_info(self)
+	}
+
+	fn call_contract(&self, _id: BlockId, _address: Address, _data: Vec<u8>) -> Result<Vec<u8>, String> {
+		Err("Contract calling not supported by light client".into())
+	}
+
+	fn transact_contract(&self, _address: Address, _data: Vec<u8>)
+		-> Result<TransactionImportResult, EthcoreError>
+	{
+		// TODO: these are only really used for misbehavior reporting.
+		// no relevant clients will be running light clients, but maybe
+		// they could be at some point?
+		Err(TransactionError::LimitReached.into())
+	}
+
+	fn block_number(&self, id: BlockId) -> Option<BlockNumber> {
+		self.block_header(id).map(|hdr| hdr.number())
+	}
+}

ethcore/light/src/client/service.rs

@@ -30,7 +30,7 @@ use util::kvdb::{Database, DatabaseConfig};
 use cache::Cache;
 use parking_lot::Mutex;
 
-use super::{Client, Config as ClientConfig};
+use super::{ChainDataFetcher, Client, Config as ClientConfig};
 
 /// Errors on service initialization.
 #[derive(Debug)]
@@ -51,14 +51,14 @@ impl fmt::Display for Error {
 }
 
 /// Light client service.
-pub struct Service {
-	client: Arc<Client>,
+pub struct Service<T> {
+	client: Arc<Client<T>>,
 	io_service: IoService<ClientIoMessage>,
 }
 
-impl Service {
+impl<T: ChainDataFetcher> Service<T> {
 	/// Start the service: initialize I/O workers and client itself.
-	pub fn start(config: ClientConfig, spec: &Spec, path: &Path, cache: Arc<Mutex<Cache>>) -> Result<Self, Error> {
+	pub fn start(config: ClientConfig, spec: &Spec, fetcher: T, path: &Path, cache: Arc<Mutex<Cache>>) -> Result<Self, Error> {
 
 		// initialize database.
 		let mut db_config = DatabaseConfig::with_columns(db::NUM_COLUMNS);
@@ -81,10 +81,14 @@ impl Service {
 			db,
 			db::COL_LIGHT_CHAIN,
 			spec,
+			fetcher,
 			io_service.channel(),
 			cache,
 		).map_err(Error::Database)?);
+
 		io_service.register_handler(Arc::new(ImportBlocks(client.clone()))).map_err(Error::Io)?;
+		spec.engine.register_client(Arc::downgrade(&client) as _);
+
 		Ok(Service {
 			client: client,
 			io_service: io_service,
@@ -97,14 +101,14 @@ impl Service {
 	}
 
 	/// Get a handle to the client.
-	pub fn client(&self) -> &Arc<Client> {
+	pub fn client(&self) -> &Arc<Client<T>> {
 		&self.client
 	}
 }
 
-struct ImportBlocks(Arc<Client>);
+struct ImportBlocks<T>(Arc<Client<T>>);
 
-impl IoHandler<ClientIoMessage> for ImportBlocks {
+impl<T: ChainDataFetcher> IoHandler<ClientIoMessage> for ImportBlocks<T> {
 	fn message(&self, _io: &IoContext<ClientIoMessage>, message: &ClientIoMessage) {
 		if let ClientIoMessage::BlockVerified = *message {
 			self.0.import_verified();
@@ -120,6 +124,7 @@ mod tests {
 
 	use std::sync::Arc;
 	use cache::Cache;
+	use client::fetch;
 	use time::Duration;
 	use parking_lot::Mutex;
 
@@ -129,6 +134,6 @@ mod tests {
 		let temp_path = RandomTempPath::new();
 		let cache = Arc::new(Mutex::new(Cache::new(Default::default(), Duration::hours(6))));
 
-		Service::start(Default::default(), &spec, temp_path.as_path(), cache).unwrap();
+		Service::start(Default::default(), &spec, fetch::unavailable(), temp_path.as_path(), cache).unwrap();
 	}
 }

ethcore/light/src/net/load_timer.rs

@@ -62,6 +62,7 @@ fn hardcoded_serve_time(kind: Kind) -> u64 {
 		Kind::Storage => 2_000_000,
 		Kind::Code => 1_500_000,
 		Kind::Execution => 250, // per gas.
+		Kind::Signal => 500_000,
 	}
 }
 

ethcore/light/src/net/mod.rs

@@ -104,9 +104,8 @@ mod packet {
 	// relay transactions to peers.
 	pub const SEND_TRANSACTIONS: u8 = 0x06;
 
-	// request and respond with epoch transition proof
-	pub const REQUEST_EPOCH_PROOF: u8 = 0x07;
-	pub const EPOCH_PROOF: u8 = 0x08;
+	// two packets were previously meant to be reserved for epoch proofs.
+	// these have since been moved to requests.
 }
 
 // timeouts for different kinds of requests. all values are in milliseconds.
@@ -124,6 +123,7 @@ mod timeout {
 	pub const CONTRACT_CODE: i64 = 100;
 	pub const HEADER_PROOF: i64 = 100;
 	pub const TRANSACTION_PROOF: i64 = 1000; // per gas?
+	pub const EPOCH_SIGNAL: i64 = 200;
 }
 
 /// A request id.
@@ -584,12 +584,6 @@ impl LightProtocol {
 
 			packet::SEND_TRANSACTIONS => self.relay_transactions(peer, io, rlp),
 
-			packet::REQUEST_EPOCH_PROOF | packet::EPOCH_PROOF => {
-				// ignore these for now, but leave them specified.
-				debug!(target: "pip", "Ignoring request/response for epoch proof");
-				Ok(())
-			}
-
 			other => {
 				Err(Error::UnrecognizedPacket(other))
 			}
@@ -952,6 +946,7 @@ impl LightProtocol {
 				CompleteRequest::Storage(req) => self.provider.storage_proof(req).map(Response::Storage),
 				CompleteRequest::Code(req) => self.provider.contract_code(req).map(Response::Code),
 				CompleteRequest::Execution(req) => self.provider.transaction_proof(req).map(Response::Execution),
+				CompleteRequest::Signal(req) => self.provider.epoch_signal(req).map(Response::Signal),
 			}
 		});
 

ethcore/light/src/net/request_credits.rs

@@ -91,6 +91,7 @@ pub struct CostTable {
 	code: U256,
 	header_proof: U256,
 	transaction_proof: U256, // cost per gas.
+	epoch_signal: U256,
 }
 
 impl Default for CostTable {
@@ -107,6 +108,7 @@ impl Default for CostTable {
 			code: 20000.into(),
 			header_proof: 15000.into(),
 			transaction_proof: 2.into(),
+			epoch_signal: 10000.into(),
 		}
 	}
 }
@@ -121,7 +123,7 @@ impl Encodable for CostTable {
 			s.append(cost);
 		}
 
-		s.begin_list(10).append(&self.base);
+		s.begin_list(11).append(&self.base);
 		append_cost(s, &self.headers, request::Kind::Headers);
 		append_cost(s, &self.transaction_index, request::Kind::TransactionIndex);
 		append_cost(s, &self.body, request::Kind::Body);
@@ -131,6 +133,7 @@ impl Encodable for CostTable {
 		append_cost(s, &self.code, request::Kind::Code);
 		append_cost(s, &self.header_proof, request::Kind::HeaderProof);
 		append_cost(s, &self.transaction_proof, request::Kind::Execution);
+		append_cost(s, &self.epoch_signal, request::Kind::Signal);
 	}
 }
 
@@ -147,6 +150,7 @@ impl Decodable for CostTable {
 		let mut code = None;
 		let mut header_proof = None;
 		let mut transaction_proof = None;
+		let mut epoch_signal = None;
 
 		for cost_list in rlp.iter().skip(1) {
 			let cost = cost_list.val_at(1)?;
@@ -160,6 +164,7 @@ impl Decodable for CostTable {
 				request::Kind::Code => code = Some(cost),
 				request::Kind::HeaderProof => header_proof = Some(cost),
 				request::Kind::Execution => transaction_proof = Some(cost),
+				request::Kind::Signal => epoch_signal = Some(cost),
 			}
 		}
 
@@ -176,6 +181,7 @@ impl Decodable for CostTable {
 			code: unwrap_cost(code)?,
 			header_proof: unwrap_cost(header_proof)?,
 			transaction_proof: unwrap_cost(transaction_proof)?,
+			epoch_signal: unwrap_cost(epoch_signal)?,
 		})
 	}
 }
@@ -238,6 +244,7 @@ impl FlowParams {
 			code: cost_for_kind(Kind::Code),
 			header_proof: cost_for_kind(Kind::HeaderProof),
 			transaction_proof: cost_for_kind(Kind::Execution),
+			epoch_signal: cost_for_kind(Kind::Signal),
 		};
 
 		FlowParams {
@@ -263,7 +270,8 @@ impl FlowParams {
 				storage: free_cost.clone(),
 				code: free_cost.clone(),
 				header_proof: free_cost.clone(),
-				transaction_proof: free_cost,
+				transaction_proof: free_cost.clone(),
+				epoch_signal: free_cost,
 			}
 		}
 	}
@@ -293,6 +301,7 @@ impl FlowParams {
 			Request::Storage(_) => self.costs.storage,
 			Request::Code(_) => self.costs.code,
 			Request::Execution(ref req) => self.costs.transaction_proof * req.gas,
+			Request::Signal(_) => self.costs.epoch_signal,
 		}
 	}
 

ethcore/light/src/net/request_set.rs

@@ -139,6 +139,7 @@ fn compute_timeout(reqs: &Requests) -> Duration {
 			Request::Storage(_) => timeout::PROOF,
 			Request::Code(_) => timeout::CONTRACT_CODE,
 			Request::Execution(_) => timeout::TRANSACTION_PROOF,
+			Request::Signal(_) => timeout::EPOCH_SIGNAL,
 		}
 	}))
 }

ethcore/light/src/net/tests/mod.rs

@@ -158,6 +158,12 @@ impl Provider for TestProvider {
 		None
 	}
 
+	fn epoch_signal(&self, _req: request::CompleteSignalRequest) -> Option<request::SignalResponse> {
+		Some(request::SignalResponse {
+			signal: vec![1, 2, 3, 4],
+		})
+	}
+
 	fn ready_transactions(&self) -> Vec<PendingTransaction> {
 		self.0.client.ready_transactions()
 	}
@@ -523,6 +529,50 @@ fn get_contract_code() {
 	proto.handle_packet(&expected, &1, packet::REQUEST, &request_body);
 }
 
+#[test]
+fn epoch_signal() {
+	let capabilities = capabilities();
+
+	let (provider, proto) = setup(capabilities.clone());
+	let flow_params = proto.flow_params.read().clone();
+
+	let cur_status = status(provider.client.chain_info());
+
+	{
+		let packet_body = write_handshake(&cur_status, &capabilities, &proto);
+		proto.on_connect(&1, &Expect::Send(1, packet::STATUS, packet_body.clone()));
+		proto.handle_packet(&Expect::Nothing, &1, packet::STATUS, &packet_body);
+	}
+
+	let req_id = 112;
+	let request = Request::Signal(request::IncompleteSignalRequest {
+		block_hash: H256([1; 32]).into(),
+	});
+
+	let requests = encode_single(request.clone());
+	let request_body = make_packet(req_id, &requests);
+
+	let response = {
+		let response = vec![Response::Signal(SignalResponse {
+			signal: vec![1, 2, 3, 4],
+		})];
+
+		let limit = *flow_params.limit();
+		let cost = flow_params.compute_cost_multi(requests.requests());
+
+		println!("limit = {}, cost = {}", limit, cost);
+		let new_creds = limit - cost;
+
+		let mut response_stream = RlpStream::new_list(3);
+		response_stream.append(&req_id).append(&new_creds).append_list(&response);
+
+		response_stream.out()
+	};
+
+	let expected = Expect::Respond(packet::RESPONSE, response);
+	proto.handle_packet(&expected, &1, packet::REQUEST, &request_body);
+}
+
 #[test]
 fn proof_of_execution() {
 	let capabilities = capabilities();

ethcore/light/src/on_demand/mod.rs

@@ -195,6 +195,8 @@ fn guess_capabilities(requests: &[CheckedRequest]) -> Capabilities {
 				caps.serve_headers = true,
 			CheckedRequest::HeaderByHash(_, _) =>
 				caps.serve_headers = true,
+			CheckedRequest::Signal(_, _) =>
+				caps.serve_headers = true,
 			CheckedRequest::Body(ref req, _) => if let Ok(ref hdr) = req.0.as_ref() {
 				update_since(&mut caps.serve_chain_since, hdr.number());
 			},

ethcore/light/src/on_demand/request.rs

@@ -20,7 +20,7 @@ use std::sync::Arc;
 
 use ethcore::basic_account::BasicAccount;
 use ethcore::encoded;
-use ethcore::engines::Engine;
+use ethcore::engines::{Engine, StateDependentProof};
 use ethcore::receipt::Receipt;
 use ethcore::state::{self, ProvedExecution};
 use ethcore::transaction::SignedTransaction;
@@ -56,6 +56,8 @@ pub enum Request {
 	Code(Code),
 	/// A request for proof of execution.
 	Execution(TransactionProof),
+	/// A request for epoch change signal.
+	Signal(Signal),
 }
 
 /// A request argument.
@@ -136,6 +138,7 @@ impl_single!(Body, Body, encoded::Block);
 impl_single!(Account, Account, Option<BasicAccount>);
 impl_single!(Code, Code, Bytes);
 impl_single!(Execution, TransactionProof, super::ExecutionResult);
+impl_single!(Signal, Signal, Vec<u8>);
 
 macro_rules! impl_args {
 	() => {
@@ -244,6 +247,7 @@ pub enum CheckedRequest {
 	Account(Account, net_request::IncompleteAccountRequest),
 	Code(Code, net_request::IncompleteCodeRequest),
 	Execution(TransactionProof, net_request::IncompleteExecutionRequest),
+	Signal(Signal, net_request::IncompleteSignalRequest)
 }
 
 impl From<Request> for CheckedRequest {
@@ -302,6 +306,12 @@ impl From<Request> for CheckedRequest {
 				};
 				CheckedRequest::Execution(req, net_req)
 			}
+			Request::Signal(req) => {
+				let net_req = net_request::IncompleteSignalRequest {
+					block_hash: req.hash.into(),
+				};
+				CheckedRequest::Signal(req, net_req)
+			}
 		}
 	}
 }
@@ -319,6 +329,7 @@ impl CheckedRequest {
 			CheckedRequest::Account(_, req) => NetRequest::Account(req),
 			CheckedRequest::Code(_, req) => NetRequest::Code(req),
 			CheckedRequest::Execution(_, req) => NetRequest::Execution(req),
+			CheckedRequest::Signal(_, req) => NetRequest::Signal(req),
 		}
 	}
 
@@ -446,6 +457,7 @@ macro_rules! match_me {
 			CheckedRequest::Account($check, $req) => $e,
 			CheckedRequest::Code($check, $req) => $e,
 			CheckedRequest::Execution($check, $req) => $e,
+			CheckedRequest::Signal($check, $req) => $e,
 		}
 	}
 }
@@ -473,6 +485,7 @@ impl IncompleteRequest for CheckedRequest {
 			CheckedRequest::Account(_, ref req) => req.check_outputs(f),
 			CheckedRequest::Code(_, ref req) => req.check_outputs(f),
 			CheckedRequest::Execution(_, ref req) => req.check_outputs(f),
+			CheckedRequest::Signal(_, ref req) => req.check_outputs(f),
 		}
 	}
 
@@ -493,6 +506,7 @@ impl IncompleteRequest for CheckedRequest {
 			CheckedRequest::Account(_, req) => req.complete().map(CompleteRequest::Account),
 			CheckedRequest::Code(_, req) => req.complete().map(CompleteRequest::Code),
 			CheckedRequest::Execution(_, req) => req.complete().map(CompleteRequest::Execution),
+			CheckedRequest::Signal(_, req) => req.complete().map(CompleteRequest::Signal),
 		}
 	}
 
@@ -544,6 +558,9 @@ impl net_request::CheckedRequest for CheckedRequest {
 			CheckedRequest::Execution(ref prover, _) =>
 				expect!((&NetResponse::Execution(ref res), _) =>
 					prover.check_response(cache, &res.items).map(Response::Execution)),
+			CheckedRequest::Signal(ref prover, _) =>
+				expect!((&NetResponse::Signal(ref res), _) =>
+					prover.check_response(cache, &res.signal).map(Response::Signal)),
 		}
 	 }
 }
@@ -567,6 +584,8 @@ pub enum Response {
 	Code(Vec<u8>),
 	/// Response to a request for proved execution.
 	Execution(super::ExecutionResult),
+	/// Response to a request for epoch change signal.
+	Signal(Vec<u8>),
 }
 
 impl net_request::ResponseLike for Response {
@@ -850,6 +869,27 @@ impl TransactionProof {
 	}
 }
 
+/// Request for epoch signal.
+/// Provide engine and state-dependent proof checker.
+#[derive(Clone)]
+pub struct Signal {
+	/// Block hash and number to fetch proof for.
+	pub hash: H256,
+	/// Consensus engine, used to check the proof.
+	pub engine: Arc<Engine>,
+	/// Special checker for the proof.
+	pub proof_check: Arc<StateDependentProof>,
+}
+
+impl Signal {
+	/// Check the signal, returning the signal or indicate that it's bad.
+	pub fn check_response(&self, _: &Mutex<::cache::Cache>, signal: &[u8]) -> Result<Vec<u8>, Error> {
+		self.proof_check.check_proof(&*self.engine, signal)
+			.map(|_| signal.to_owned())
+			.map_err(|_| Error::BadProof)
+	}
+}
+
 #[cfg(test)]
 mod tests {
 	use super::*;

ethcore/light/src/provider.rs

@@ -127,6 +127,9 @@ pub trait Provider: Send + Sync {
 	/// Provide a proof-of-execution for the given transaction proof request.
 	/// Returns a vector of all state items necessary to execute the transaction.
 	fn transaction_proof(&self, req: request::CompleteExecutionRequest) -> Option<request::ExecutionResponse>;
+
+	/// Provide epoch signal data at given block hash. This should be just the
+	fn epoch_signal(&self, req: request::CompleteSignalRequest) -> Option<request::SignalResponse>;
 }
 
 // Implementation of a light client data provider for a client.
@@ -265,6 +268,12 @@ impl<T: ProvingBlockChainClient + ?Sized> Provider for T {
 	fn ready_transactions(&self) -> Vec<PendingTransaction> {
 		BlockChainClient::ready_transactions(self)
 	}
+
+	fn epoch_signal(&self, req: request::CompleteSignalRequest) -> Option<request::SignalResponse> {
+		self.epoch_signal(req.block_hash).map(|signal| request::SignalResponse {
+			signal: signal,
+		})
+	}
 }
 
 /// The light client "provider" implementation. This wraps a `LightClient` and
@@ -330,6 +339,10 @@ impl<L: AsLightClient + Send + Sync> Provider for LightProvider<L> {
 		None
 	}
 
+	fn epoch_signal(&self, _req: request::CompleteSignalRequest) -> Option<request::SignalResponse> {
+		None
+	}
+
 	fn ready_transactions(&self) -> Vec<PendingTransaction> {
 		let chain_info = self.chain_info();
 		self.txqueue.read().ready_transactions(chain_info.best_block_number, chain_info.best_block_timestamp)

ethcore/light/src/types/request/mod.rs

@@ -67,6 +67,11 @@ pub use self::execution::{
 	Incomplete as IncompleteExecutionRequest,
 	Response as ExecutionResponse,
 };
+pub use self::epoch_signal::{
+	Complete as CompleteSignalRequest,
+	Incomplete as IncompleteSignalRequest,
+	Response as SignalResponse,
+};
 
 pub use self::builder::{RequestBuilder, Requests};
 
@@ -261,6 +266,8 @@ pub enum Request {
 	Code(IncompleteCodeRequest),
 	/// A request for proof of execution,
 	Execution(IncompleteExecutionRequest),
+	/// A request for an epoch signal.
+	Signal(IncompleteSignalRequest),
 }
 
 /// All request types, in an answerable state.
@@ -284,6 +291,8 @@ pub enum CompleteRequest {
 	Code(CompleteCodeRequest),
 	/// A request for proof of execution,
 	Execution(CompleteExecutionRequest),
+	/// A request for an epoch signal.
+	Signal(CompleteSignalRequest),
 }
 
 impl CompleteRequest {
@@ -299,6 +308,7 @@ impl CompleteRequest {
 			CompleteRequest::Storage(_) => Kind::Storage,
 			CompleteRequest::Code(_) => Kind::Code,
 			CompleteRequest::Execution(_) => Kind::Execution,
+			CompleteRequest::Signal(_) => Kind::Signal,
 		}
 	}
 }
@@ -316,6 +326,7 @@ impl Request {
 			Request::Storage(_) => Kind::Storage,
 			Request::Code(_) => Kind::Code,
 			Request::Execution(_) => Kind::Execution,
+			Request::Signal(_) => Kind::Signal,
 		}
 	}
 }
@@ -332,6 +343,7 @@ impl Decodable for Request {
 			Kind::Storage => Ok(Request::Storage(rlp.val_at(1)?)),
 			Kind::Code => Ok(Request::Code(rlp.val_at(1)?)),
 			Kind::Execution => Ok(Request::Execution(rlp.val_at(1)?)),
+			Kind::Signal => Ok(Request::Signal(rlp.val_at(1)?)),
 		}
 	}
 }
@@ -353,6 +365,7 @@ impl Encodable for Request {
 			Request::Storage(ref req) => s.append(req),
 			Request::Code(ref req) => s.append(req),
 			Request::Execution(ref req) => s.append(req),
+			Request::Signal(ref req) => s.append(req),
 		};
 	}
 }
@@ -374,6 +387,7 @@ impl IncompleteRequest for Request {
 			Request::Storage(ref req) => req.check_outputs(f),
 			Request::Code(ref req) => req.check_outputs(f),
 			Request::Execution(ref req) => req.check_outputs(f),
+			Request::Signal(ref req) => req.check_outputs(f),
 		}
 	}
 
@@ -388,6 +402,7 @@ impl IncompleteRequest for Request {
 			Request::Storage(ref req) => req.note_outputs(f),
 			Request::Code(ref req) => req.note_outputs(f),
 			Request::Execution(ref req) => req.note_outputs(f),
+			Request::Signal(ref req) => req.note_outputs(f),
 		}
 	}
 
@@ -402,6 +417,7 @@ impl IncompleteRequest for Request {
 			Request::Storage(ref mut req) => req.fill(oracle),
 			Request::Code(ref mut req) => req.fill(oracle),
 			Request::Execution(ref mut req) => req.fill(oracle),
+			Request::Signal(ref mut req) => req.fill(oracle),
 		}
 	}
 
@@ -416,6 +432,7 @@ impl IncompleteRequest for Request {
 			Request::Storage(req) => req.complete().map(CompleteRequest::Storage),
 			Request::Code(req) => req.complete().map(CompleteRequest::Code),
 			Request::Execution(req) => req.complete().map(CompleteRequest::Execution),
+			Request::Signal(req) => req.complete().map(CompleteRequest::Signal),
 		}
 	}
 
@@ -430,6 +447,7 @@ impl IncompleteRequest for Request {
 			Request::Storage(ref mut req) => req.adjust_refs(mapping),
 			Request::Code(ref mut req) => req.adjust_refs(mapping),
 			Request::Execution(ref mut req) => req.adjust_refs(mapping),
+			Request::Signal(ref mut req) => req.adjust_refs(mapping),
 		}
 	}
 }
@@ -471,6 +489,8 @@ pub enum Kind {
 	Code = 7,
 	/// A request for transaction execution + state proof.
 	Execution = 8,
+	/// A request for epoch transition signal.
+	Signal = 9,
 }
 
 impl Decodable for Kind {
@@ -485,6 +505,7 @@ impl Decodable for Kind {
 			6 => Ok(Kind::Storage),
 			7 => Ok(Kind::Code),
 			8 => Ok(Kind::Execution),
+			9 => Ok(Kind::Signal),
 			_ => Err(DecoderError::Custom("Unknown PIP request ID.")),
 		}
 	}
@@ -517,6 +538,8 @@ pub enum Response {
 	Code(CodeResponse),
 	/// A response for proof of execution,
 	Execution(ExecutionResponse),
+	/// A response for epoch change signal.
+	Signal(SignalResponse),
 }
 
 impl ResponseLike for Response {
@@ -532,6 +555,7 @@ impl ResponseLike for Response {
 			Response::Storage(ref res) => res.fill_outputs(f),
 			Response::Code(ref res) => res.fill_outputs(f),
 			Response::Execution(ref res) => res.fill_outputs(f),
+			Response::Signal(ref res) => res.fill_outputs(f),
 		}
 	}
 }
@@ -549,6 +573,7 @@ impl Response {
 			Response::Storage(_) => Kind::Storage,
 			Response::Code(_) => Kind::Code,
 			Response::Execution(_) => Kind::Execution,
+			Response::Signal(_) => Kind::Signal,
 		}
 	}
 }
@@ -565,6 +590,7 @@ impl Decodable for Response {
 			Kind::Storage => Ok(Response::Storage(rlp.val_at(1)?)),
 			Kind::Code => Ok(Response::Code(rlp.val_at(1)?)),
 			Kind::Execution => Ok(Response::Execution(rlp.val_at(1)?)),
+			Kind::Signal => Ok(Response::Signal(rlp.val_at(1)?)),
 		}
 	}
 }
@@ -586,6 +612,7 @@ impl Encodable for Response {
 			Response::Storage(ref res) => s.append(res),
 			Response::Code(ref res) => s.append(res),
 			Response::Execution(ref res) => s.append(res),
+			Response::Signal(ref res) => s.append(res),
 		};
 	}
 }
@@ -1509,6 +1536,104 @@ pub mod execution {
 	}
 }
 
+/// A request for epoch signal data.
+pub mod epoch_signal {
+	use super::{Field, NoSuchOutput, OutputKind, Output};
+	use rlp::{Encodable, Decodable, DecoderError, RlpStream, UntrustedRlp};
+	use util::{Bytes, H256};
+
+	/// Potentially incomplete epoch signal request.
+	#[derive(Debug, Clone, PartialEq, Eq)]
+	pub struct Incomplete {
+		/// The block hash to request the signal for.
+		pub block_hash: Field<H256>,
+	}
+
+	impl Decodable for Incomplete {
+		fn decode(rlp: &UntrustedRlp) -> Result<Self, DecoderError> {
+			Ok(Incomplete {
+				block_hash: rlp.val_at(0)?,
+			})
+		}
+	}
+
+	impl Encodable for Incomplete {
+		fn rlp_append(&self, s: &mut RlpStream) {
+			s.begin_list(1).append(&self.block_hash);
+		}
+	}
+
+	impl super::IncompleteRequest for Incomplete {
+		type Complete = Complete;
+		type Response = Response;
+
+		fn check_outputs<F>(&self, mut f: F) -> Result<(), NoSuchOutput>
+			where F: FnMut(usize, usize, OutputKind) -> Result<(), NoSuchOutput>
+		{
+			if let Field::BackReference(req, idx) = self.block_hash {
+				f(req, idx, OutputKind::Hash)?;
+			}
+
+			Ok(())
+		}
+
+		fn note_outputs<F>(&self, _: F) where F: FnMut(usize, OutputKind) {}
+
+		fn fill<F>(&mut self, oracle: F) where F: Fn(usize, usize) -> Result<Output, NoSuchOutput> {
+			if let Field::BackReference(req, idx) = self.block_hash {
+				self.block_hash = match oracle(req, idx) {
+					Ok(Output::Hash(block_hash)) => Field::Scalar(block_hash.into()),
+					_ => Field::BackReference(req, idx),
+				}
+			}
+		}
+
+		fn complete(self) -> Result<Self::Complete, NoSuchOutput> {
+			Ok(Complete {
+				block_hash: self.block_hash.into_scalar()?,
+			})
+		}
+
+		fn adjust_refs<F>(&mut self, mut mapping: F) where F: FnMut(usize) -> usize {
+			self.block_hash.adjust_req(&mut mapping);
+		}
+	}
+
+	/// A complete request.
+	#[derive(Debug, Clone, PartialEq, Eq)]
+	pub struct Complete {
+		/// The block hash to request the epoch signal for.
+		pub block_hash: H256,
+	}
+
+	/// The output of a request for an epoch signal.
+	#[derive(Debug, Clone, PartialEq, Eq)]
+	pub struct Response {
+		/// The requested epoch signal.
+		pub signal: Bytes,
+	}
+
+	impl super::ResponseLike for Response {
+		/// Fill reusable outputs by providing them to the function.
+		fn fill_outputs<F>(&self, _: F) where F: FnMut(usize, Output) {}
+	}
+
+	impl Decodable for Response {
+		fn decode(rlp: &UntrustedRlp) -> Result<Self, DecoderError> {
+
+			Ok(Response {
+				signal: rlp.as_val()?,
+			})
+		}
+	}
+
+	impl Encodable for Response {
+		fn rlp_append(&self, s: &mut RlpStream) {
+			s.append(&self.signal);
+		}
+	}
+}
+
 #[cfg(test)]
 mod tests {
 	use super::*;
@@ -1797,4 +1922,22 @@ mod tests {
 		let raw = ::rlp::encode_list(&reqs);
 		assert_eq!(::rlp::decode_list::<Response>(&raw), reqs);
 	}
+
+	#[test]
+	fn epoch_signal_roundtrip() {
+		let req = IncompleteSignalRequest {
+			block_hash: Field::Scalar(Default::default()),
+		};
+
+		let full_req = Request::Signal(req.clone());
+		let res = SignalResponse {
+			signal: vec![1, 2, 3, 4, 5, 6, 7, 6, 5, 4],
+		};
+		let full_res = Response::Signal(res.clone());
+
+		check_roundtrip(req);
+		check_roundtrip(full_req);
+		check_roundtrip(res);
+		check_roundtrip(full_res);
+	}
 }