DB-based light client backend (#250)

* use db in light clients

* fixed comment

* fixed grumbles

substrate/substrate/client/src/light/fetcher.rs

@@ -0,0 +1,90 @@
+// Copyright 2017 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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 Polkadot.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Light client data fetcher. Fetches requested data from remote full nodes.
+
+use std::sync::Arc;
+use futures::IntoFuture;
+
+use runtime_primitives::traits::{As, Block as BlockT, Header as HeaderT};
+use state_machine::CodeExecutor;
+
+use call_executor::CallResult;
+use error::{Error as ClientError, Result as ClientResult};
+use light::blockchain::{Blockchain, Storage as BlockchainStorage};
+use light::call_executor::check_execution_proof;
+
+/// Remote call request.
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct RemoteCallRequest<Hash: ::std::fmt::Display> {
+	/// Call at state of given block.
+	pub block: Hash,
+	/// Method to call.
+	pub method: String,
+	/// Call data.
+	pub call_data: Vec<u8>,
+}
+
+/// Light client data fetcher. Implementations of this trait must check if remote data
+/// is correct (see FetchedDataChecker) and return already checked data.
+pub trait Fetcher<Block: BlockT>: Send + Sync {
+	/// Remote call result future.
+	type RemoteCallResult: IntoFuture<Item=CallResult, Error=ClientError>;
+
+	/// Fetch remote call result.
+	fn remote_call(&self, request: RemoteCallRequest<Block::Hash>) -> Self::RemoteCallResult;
+}
+
+/// Light client remote data checker.
+pub trait FetchChecker<Block: BlockT>: Send + Sync {
+	/// Check remote method execution proof.
+	fn check_execution_proof(&self, request: &RemoteCallRequest<Block::Hash>, remote_proof: Vec<Vec<u8>>) -> ClientResult<CallResult>;
+}
+
+/// Remote data checker.
+pub struct LightDataChecker<S, E, F> {
+	blockchain: Arc<Blockchain<S, F>>,
+	executor: E,
+}
+
+impl<S, E, F> LightDataChecker<S, E, F> {
+	/// Create new light data checker.
+	pub fn new(blockchain: Arc<Blockchain<S, F>>, executor: E) -> Self {
+		Self {
+			blockchain,
+			executor,
+		}
+	}
+
+	/// Get blockchain reference.
+	pub fn blockchain(&self) -> &Arc<Blockchain<S, F>> {
+		&self.blockchain
+	}
+}
+
+impl<S, E, F, Block> FetchChecker<Block> for LightDataChecker<S, E, F>
+	where
+		Block: BlockT,
+		<Block as BlockT>::Hash: From<[u8; 32]> + Into<[u8; 32]>, // TODO: remove when patricia_trie generic.
+		<<Block as BlockT>::Header as HeaderT>::Number: As<u32>,
+		S: BlockchainStorage<Block>,
+		E: CodeExecutor,
+		F: Fetcher<Block>,
+{
+	fn check_execution_proof(&self, request: &RemoteCallRequest<Block::Hash>, remote_proof: Vec<Vec<u8>>) -> ClientResult<CallResult> {
+		check_execution_proof(&*self.blockchain, &self.executor, request, remote_proof)
+	}
+}