Introduce notion of finality to substrate (#760)

* finalization for in_mem

* fetch last finalized block

* pruning: use canonical term instead of final

* finalize blocks in full node

* begin to port light client DB

* add tree-route

* keep number index consistent in full nodes

* fix tests

* disable cache and finish porting light client

* add AsMut to system module

* final leaf is always best

* fix all tests

* Fix comment and trace

* removed unused Into call

* add comment on behavior of `finalize_block`

substrate/core/client/src/light/blockchain.rs

@@ -25,6 +25,7 @@ use primitives::AuthorityId;
 use runtime_primitives::{bft::Justification, generic::BlockId};
 use runtime_primitives::traits::{Block as BlockT, Header as HeaderT, NumberFor, Zero};
 
+use backend::NewBlockState;
 use blockchain::{Backend as BlockchainBackend, BlockStatus, Cache as BlockchainCache,
 	HeaderBackend as BlockchainHeaderBackend, Info as BlockchainInfo};
 use cht;
@@ -33,14 +34,20 @@ use light::fetcher::{Fetcher, RemoteHeaderRequest};
 
 /// Light client blockchain storage.
 pub trait Storage<Block: BlockT>: BlockchainHeaderBackend<Block> {
-	/// Store new header.
+	/// Store new header. Should refuse to revert any finalized blocks.
 	fn import_header(
 		&self,
-		is_new_best: bool,
 		header: Block::Header,
-		authorities: Option<Vec<AuthorityId>>
+		authorities: Option<Vec<AuthorityId>>,
+		state: NewBlockState,
 	) -> ClientResult<()>;
 
+	/// Mark historic header as finalized.
+	fn finalize_header(&self, block: BlockId<Block>) -> ClientResult<()>;
+
+	/// Get last finalized header.
+	fn last_finalized(&self) -> ClientResult<Block::Hash>;
+
 	/// Get CHT root for given block. Fails if the block is not pruned (not a part of any CHT).
 	fn cht_root(&self, cht_size: u64, block: NumberFor<Block>) -> ClientResult<Block::Hash>;
 
@@ -136,6 +143,10 @@ impl<S, F, Block> BlockchainBackend<Block> for Blockchain<S, F> where Block: Blo
 		Ok(None)
 	}
 
+	fn last_finalized(&self) -> ClientResult<Block::Hash> {
+		self.storage.last_finalized()
+	}
+
 	fn cache(&self) -> Option<&BlockchainCache<Block>> {
 		self.storage.cache()
 	}