serve epoch signals over network and check them

ethcore/light/src/client/fetch.rs

@@ -16,9 +16,13 @@
 
 //! Trait for fetching chain data.
 
+use std::sync::Arc;
+
+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 {
@@ -39,7 +43,7 @@ pub trait ChainDataFetcher: Send + Sync + 'static {
 	fn block_receipts(&self, header: &Header) -> Self::Receipts;
 
 	/// Fetch epoch transition proof at given header.
-	fn epoch_transition(&self, header: &Header) -> Self::Transition;
+	fn epoch_transition(&self, hash: H256, engine: Arc<Engine>, checker: Arc<StateDependentProof>) -> Self::Transition;
 }
 
 /// Fetcher implementation which cannot fetch anything.
@@ -63,7 +67,7 @@ impl ChainDataFetcher for Unavailable {
 		Err("fetching block receipts unavailable")
 	}
 
-	fn epoch_transition(&self, _header: &Header) -> Self::Body {
+	fn epoch_transition(&self, _h: H256, _e: Arc<Engine>, _check: Arc<StateDependentProof>) -> Self::Transition {
 		Err("fetching epoch transition proofs unavailable")
 	}
 }

ethcore/light/src/client/mod.rs

@@ -508,14 +508,11 @@ impl<T: ChainDataFetcher> Client<T> {
 		let proof = match proof {
 			Proof::Known(known) => known,
 			Proof::WithState(state_dependent) => {
-				loop {
-					let proof = self.fetcher.epoch_transition(header).into_future().wait()?;
-					match state_dependent.check_proof(&*self.engine, &proof) {
-						Ok(()) => break proof,
-						Err(e) =>
-							debug!(target: "client", "Fetched bad epoch transition proof from network: {}", e),
-					}
-				}
+				self.fetcher.epoch_transition(
+					header.hash(),
+					self.engine.clone(),
+					state_dependent
+				).into_future().wait()?
 			}
 		};
 

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

@@ -102,9 +102,8 @@ 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.
@@ -122,6 +121,7 @@ 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.
@@ -582,12 +582,6 @@ 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))
 			}
@@ -950,6 +944,7 @@ 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

@@ -156,6 +156,12 @@ 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()
 	}
@@ -521,6 +527,50 @@ 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;
@@ -53,6 +53,8 @@ pub enum Request {
 	Code(Code),
 	/// A request for proof of execution.
 	Execution(TransactionProof),
+	/// A request for epoch change signal.
+	Signal(Signal),
 }
 
 /// A request argument.
@@ -133,6 +135,7 @@ 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 {
 	() => {
@@ -241,6 +244,7 @@ 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 {
@@ -299,6 +303,12 @@ 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)
+			}
 		}
 	}
 }
@@ -316,6 +326,7 @@ 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),
 		}
 	}
 
@@ -443,6 +454,7 @@ macro_rules! match_me {
 			CheckedRequest::Account($check, $req) => $e,
 			CheckedRequest::Code($check, $req) => $e,
 			CheckedRequest::Execution($check, $req) => $e,
+			CheckedRequest::Signal($check, $req) => $e,
 		}
 	}
 }
@@ -470,6 +482,7 @@ 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),
 		}
 	}
 
@@ -490,6 +503,7 @@ 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),
 		}
 	}
 
@@ -541,6 +555,9 @@ 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)),
 		}
 	 }
 }
@@ -564,6 +581,8 @@ 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 {
@@ -847,6 +866,27 @@ 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

@@ -126,6 +126,9 @@ 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.
@@ -264,6 +267,12 @@ 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
@@ -329,6 +338,10 @@ 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),
 		};
 	}
 }
@@ -1756,6 +1783,104 @@ 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::*;
@@ -2044,4 +2169,22 @@ 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);
+	}
 }

ethcore/src/client/client.rs

@@ -1918,6 +1918,12 @@ impl ProvingBlockChainClient for Client {
 			false,
 		)
 	}
+
+	fn epoch_signal(&self, hash: H256) -> Option<Vec<u8>> {
+		// pending transitions are never deleted, and do not contain
+		// finality proofs by definition.
+		self.chain.read().get_pending_transition(hash).map(|pending| pending.proof)
+	}
 }
 
 impl Drop for Client {

ethcore/src/client/test_client.rs

@@ -778,6 +778,10 @@ impl ProvingBlockChainClient for TestBlockChainClient {
 	fn prove_transaction(&self, _: SignedTransaction, _: BlockId) -> Option<(Bytes, Vec<DBValue>)> {
 		None
 	}
+
+	fn epoch_signal(&self, _: H256) -> Option<Vec<u8>> {
+		None
+	}
 }
 
 impl super::traits::EngineClient for TestBlockChainClient {

ethcore/src/client/traits.rs

@@ -353,4 +353,7 @@ pub trait ProvingBlockChainClient: BlockChainClient {
 	/// 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>)>;
+
+	/// Get an epoch change signal by block hash.
+	fn epoch_signal(&self, hash: H256) -> Option<Vec<u8>>;
 }

ethcore/src/engines/mod.rs

@@ -121,7 +121,7 @@ pub type Headers<'a> = Fn(H256) -> Option<Header> + 'a;
 pub type PendingTransitionStore<'a> = Fn(H256) -> Option<PendingTransition> + 'a;
 
 /// Proof dependent on state.
-pub trait StateDependentProof {
+pub trait StateDependentProof: Send + Sync {
 	/// Generate a proof, given the state.
 	fn generate_proof(&self, caller: &Call) -> Result<Vec<u8>, String>;
 	/// Check a proof generated elsewhere (potentially by a peer).
@@ -135,7 +135,7 @@ pub enum Proof {
 	/// Known proof (extracted from signal)
 	Known(Vec<u8>),
 	/// State dependent proof.
-	WithState(Box<StateDependentProof>),
+	WithState(Arc<StateDependentProof>),
 }
 
 /// Generated epoch verifier.

ethcore/src/engines/validator_set/safe_contract.rs

@@ -47,19 +47,19 @@ lazy_static! {
 // state-dependent proofs for the safe contract:
 // only "first" proofs are such.
 struct StateProof {
-	header: Header,
+	header: Mutex<Header>,
 	provider: Provider,
 }
 
 impl ::engines::StateDependentProof for StateProof {
 	fn generate_proof(&self, caller: &Call) -> Result<Vec<u8>, String> {
-		prove_initial(&self.provider, &self.header, caller)
+		prove_initial(&self.provider, &*self.header.lock(), caller)
 	}
 
 	fn check_proof(&self, engine: &Engine, proof: &[u8]) -> Result<(), String> {
 		let (header, state_items) = decode_first_proof(&UntrustedRlp::new(proof))
 			.map_err(|e| format!("proof incorrectly encoded: {}", e))?;
-		if header != self.header {
+		if &header != &*self.header.lock(){
 			return Err("wrong header in proof".into());
 		}
 
@@ -330,11 +330,11 @@ impl ValidatorSet for ValidatorSafeContract {
 		// transition to the first block of a contract requires finality but has no log event.
 		if first {
 			debug!(target: "engine", "signalling transition to fresh contract.");
-			let state_proof = Box::new(StateProof {
-				header: header.clone(),
+			let state_proof = Arc::new(StateProof {
+				header: Mutex::new(header.clone()),
 				provider: self.provider.clone(),
 			});
-			return ::engines::EpochChange::Yes(::engines::Proof::WithState(state_proof as Box<_>));
+			return ::engines::EpochChange::Yes(::engines::Proof::WithState(state_proof as Arc<_>));
 		}
 
 		// otherwise, we're checking for logs.