Allow for customisation of chain selection systems (#2240)

* move SelectChain trait out of client

* Extend SelectChain, move longest chain implementation into it

* Bring SelectChain into service

* implement LongestChain SelectChain

* implement longest chain for node

* update Cargo.lock's

* in between erroring tests

* deprecate ::backend and ::import_lock

* Remove unneded space

Co-Authored-By: gnunicorn <ben.kampmann@googlemail.com>

* Remove unneded space

Co-Authored-By: gnunicorn <ben.kampmann@googlemail.com>

* Fixes test compilation

* remove todo

* re-enable client test

* add doc

* fixing tests

* Clarify SelectChain Interface, intended implementation and usage

* minor components cleanups

* minor cleanups

* Update lock files

* Implement cleaner interface for SelectChain

* addressing comments

* Updating tests

* bump node runtime impl version

* address grumbles

substrate/core/consensus/common/src/error.rs

@@ -105,5 +105,11 @@ error_chain! {
 			description("Import failed"),
 			display("Import failed: {}", reason),
 		}
+
+		/// Error from the client while importing
+		ChainLookup(reason: String) {
+			description("Looking up chain failed"),
+			display("Chain lookup failed: {}", reason),
+		}
 	}
 }

substrate/core/consensus/common/src/lib.rs

@@ -40,6 +40,7 @@ pub use inherents::InherentData;
 pub mod offline_tracker;
 pub mod error;
 mod block_import;
+mod select_chain;
 pub mod import_queue;
 pub mod evaluation;
 
@@ -50,6 +51,7 @@ pub use self::error::{Error, ErrorKind};
 pub use block_import::{
 	BlockImport, BlockOrigin, ForkChoiceStrategy, ImportedAux, ImportBlock, ImportResult, JustificationImport,
 };
+pub use select_chain::SelectChain;
 
 /// Trait for getting the authorities at a given block.
 pub trait Authorities<B: Block> {

substrate/core/consensus/common/src/select_chain.rs

@@ -0,0 +1,54 @@
+// Copyright 2019 Parity Technologies (UK) Ltd.
+// This file is part of Substrate Consensus Common.
+
+// Substrate Demo 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 Consensus Common 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 Consensus Common.  If not, see <http://www.gnu.org/licenses/>.
+
+use crate::error::Error;
+use runtime_primitives::traits::{Block as BlockT, NumberFor};
+
+
+/// The SelectChain trait defines the strategy upon which the head is chosen
+/// if multiple forks are present for an opaque definition of "best" in the 
+/// specific chain build.
+///
+/// The Strategy can be customised for the two use cases of authoring new blocks
+/// upon the best chain or which fork to finalise. Unless implemented differently
+/// by default finalisation methods fall back to use authoring, so as a minimum
+/// `_authoring`-functions must be implemented. 
+///
+/// Any particular user must make explicit, however, whether they intend to finalise
+/// or author through the using the right function call, as these might differ in
+/// some implementations.
+///
+/// Non-deterministicly finalising chains may only use the `_authoring` functions.
+pub trait SelectChain<Block: BlockT>: Sync + Send + Clone {
+
+	/// Get all leaves of the chain: block hashes that have no children currently.
+	/// Leaves that can never be finalized will not be returned.
+	fn leaves(&self) -> Result<Vec<<Block as BlockT>::Hash>, Error>;
+
+	/// Among those `leaves` deterministically pick one chain as the generally
+	/// best chain to author new blocks upon and probably finalize.
+	fn best_chain(&self) -> Result<<Block as BlockT>::Header, Error>;
+
+	/// Get the best ancestor of `target_hash` that we should attempt
+	/// to finalize next.
+	fn finality_target(
+		&self,
+		target_hash: <Block as BlockT>::Hash,
+		_maybe_max_number: Option<NumberFor<Block>>
+	) -> Result<Option<<Block as BlockT>::Hash>, Error> {
+		Ok(Some(target_hash))
+	}
+}