serve epoch signals over network and check them

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>>;
 }