Consensus: Proof of Work (#3473)

* consensus-pow: init primtives and verifier

* consensus-pow: add total difficulty auxiliary

* consensus-pow: implement total difficulty chain selection

* consensus-pow: implement pow import queue

* consensus-pow-primitives: add mine into PowApi

* consensus-pow: implement mining

* Update lock file

* Style fixes

No run-on expressions allowed.

* consensus-pow: refactor register_pow_inherent_data_provider

* consensus-pow: make PowApi::mine yieldable

* consensus-pow: better mining loop

* Add missing license header

* consensus-pow-primitives: clarify the meaning of None for PowApi::verify

* consensus-pow: changing total difficulty addition to use saturating add

* consensus-pow: change mine-loop error to log on error! level

* consensus-pow: allow inserting arbitrary preruntime digest for pow

The preruntime digest can be intepreted by the runtime as the block author/coinbase.

* Fix line width

* More line width fixes

* consensus-pow: separate difficulty, verify API

This makes it more apparent that currently in PoW engine, `difficulty` should
be input, not output.

* srml-pow: implementation of average_span difficulty adjustment

* srml-pow: basic blake2 algo example

* srml-pow-average-span: make it not require genesis config

* srml-pow: add support for authorship

* Missing license headers

* consensus-pow: PowAlgorithm trait generalization

* Missing docs for consensus-pow

* More docs

* node-runtime: bump impl_version

* Add rationale for difficulty type

* consensus-pow: refactor aux_key

* Update lock file

* Update core/consensus/pow/src/lib.rs

Co-Authored-By: Sergei Pepyakin <sergei@parity.io>

* Update core/consensus/pow/src/lib.rs

Co-Authored-By: Sergei Pepyakin <sergei@parity.io>

* Update core/consensus/pow/src/lib.rs

Co-Authored-By: Sergei Pepyakin <sergei@parity.io>

* Update core/consensus/pow/src/lib.rs

Co-Authored-By: Sergei Pepyakin <sergei@parity.io>

* Update core/consensus/pow/src/lib.rs

Co-Authored-By: Sergei Pepyakin <sergei@parity.io>

* Update core/consensus/pow/src/lib.rs

Co-Authored-By: Sergei Pepyakin <sergei@parity.io>

* Update core/consensus/pow/src/lib.rs

Co-Authored-By: Sergei Pepyakin <sergei@parity.io>

* Update core/consensus/pow/primitives/src/lib.rs

Co-Authored-By: Sergei Pepyakin <sergei@parity.io>

* Update core/consensus/pow/primitives/src/lib.rs

Co-Authored-By: Sergei Pepyakin <sergei@parity.io>

* Remove PowRuntimeAlgorithm

* block_id -> parent_block_id

* Auxiliary data -> auxiliary storage data

* Fix error message

* Fix compile

* Update core/consensus/pow/primitives/src/lib.rs

Co-Authored-By: DemiMarie-parity <48690212+DemiMarie-parity@users.noreply.github.com>

* Update core/consensus/pow/src/lib.rs

Co-Authored-By: DemiMarie-parity <48690212+DemiMarie-parity@users.noreply.github.com>

* Update core/consensus/pow/primitives/src/lib.rs

Co-Authored-By: DemiMarie-parity <48690212+DemiMarie-parity@users.noreply.github.com>

* Update core/consensus/pow/src/lib.rs

Co-Authored-By: DemiMarie-parity <48690212+DemiMarie-parity@users.noreply.github.com>

* Fix crate description

* More docs

* Address grumbles

1. Make preruntime Optional.
2. Add more docs on what is `preruntie` and `round`.
3. Replace `Default::default` with the approriate type.

substrate/core/consensus/pow/primitives/src/lib.rs

@@ -0,0 +1,36 @@
+// Copyright 2017-2019 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+
+// Substrate 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.
+
+// Substrate 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 Substrate.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Primitives for Substrate Proof-of-Work (PoW) consensus.
+
+#![cfg_attr(not(feature = "std"), no_std)]
+
+use rstd::vec::Vec;
+use sr_primitives::ConsensusEngineId;
+
+/// The `ConsensusEngineId` of PoW.
+pub const POW_ENGINE_ID: ConsensusEngineId = [b'p', b'o', b'w', b'_'];
+
+/// Type of difficulty.
+///
+/// For runtime designed for Substrate, it's always possible to fit its total
+/// difficulty range under `u128::max_value()` because it can be freely scaled
+/// up or scaled down. Very few PoW chains use difficulty values
+/// larger than `u128::max_value()`.
+pub type Difficulty = u128;
+
+/// Type of seal.
+pub type Seal = Vec<u8>;