PoA warp implementation (#5488)

* separate modules for consensus snapshot chunks

* bulk of authority warp logic

* finish authority warp implementation

* squash warnings and enable authority snapshot mode

* test harness for PoA

* fiddle with harness

* epoch generation proof fixes

* update constructor code

* store epoch transition proof after block commit

* basic snap and restore test

* use keyvaluedb in state restoration

* decompress chunks

* fix encoding issues

* fixed-to-contract-to-contract test

* implement ancient block import

* restore genesis transition in PoW snapshot

* add format version method to snapshot components

* supported version numbers in snapshot_components

* allow returning of ancient epoch transitions

* genesis hash mismatch check

* remove commented code

ethcore/src/client/ancient_import.rs

@@ -0,0 +1,68 @@
+// 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/>.
+
+//! Helper for ancient block import.
+
+use std::sync::Arc;
+
+use engines::{Engine, EpochVerifier, EpochChange};
+use error::Error;
+use header::Header;
+
+use rand::Rng;
+use util::RwLock;
+
+// do "heavy" verification on ~1/50 blocks, randomly sampled.
+const HEAVY_VERIFY_RATE: f32 = 0.02;
+
+/// Ancient block verifier: import an ancient sequence of blocks in order from a starting
+/// epoch.
+pub struct AncientVerifier {
+	cur_verifier: RwLock<Box<EpochVerifier>>,
+	engine: Arc<Engine>,
+}
+
+impl AncientVerifier {
+	/// Create a new ancient block verifier with the given engine and initial verifier.
+	pub fn new(engine: Arc<Engine>, start_verifier: Box<EpochVerifier>) -> Self {
+		AncientVerifier {
+			cur_verifier: RwLock::new(start_verifier),
+			engine: engine,
+		}
+	}
+
+	/// Verify the next block header, randomly choosing whether to do heavy or light
+	/// verification. If the block is the end of an epoch, updates the epoch verifier.
+	pub fn verify<R: Rng, F: Fn(u64) -> Result<Box<EpochVerifier>, Error>>(
+		&self,
+		rng: &mut R,
+		header: &Header,
+		block: &[u8],
+		receipts: &[::receipt::Receipt],
+		load_verifier: F,
+	) -> Result<(), ::error::Error> {
+		match rng.gen::<f32>() <= HEAVY_VERIFY_RATE {
+			true => self.cur_verifier.read().verify_heavy(header)?,
+			false => self.cur_verifier.read().verify_light(header)?,
+		}
+
+		if let EpochChange::Yes(num) = self.engine.is_epoch_end(header, Some(block), Some(receipts)) {
+			*self.cur_verifier.write() = load_verifier(num)?;
+		}
+
+		Ok(())
+	}
+}