RPC api for offchain storage (#4694)

* Rpc api for offchain storage

* Replace Vec<u8> to Bytes, replace Mutex to RwLock

* Remove pub

* Modify copyright year

substrate/client/rpc-api/src/offchain/error.rs

@@ -0,0 +1,51 @@
+// Copyright 2020 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/>.
+
+//! Offchain RPC errors.
+
+use jsonrpc_core as rpc;
+
+/// Offchain RPC Result type.
+pub type Result<T> = std::result::Result<T, Error>;
+
+/// Offchain RPC errors.
+#[derive(Debug, derive_more::Display, derive_more::From)]
+pub enum Error {
+	/// Unavailable storage kind error.
+	#[display(fmt="This storage kind is not available yet.")]
+	UnavailableStorageKind,
+}
+
+impl std::error::Error for Error {
+	fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+		None
+	}
+}
+
+/// Base error code for all offchain errors.
+const BASE_ERROR: i64 = 5000;
+
+impl From<Error> for rpc::Error {
+	fn from(e: Error) -> Self {
+		match e {
+			Error::UnavailableStorageKind => rpc::Error {
+				code: rpc::ErrorCode::ServerError(BASE_ERROR + 1),
+				message: "This storage kind is not available yet" .into(),
+				data: None,
+			},
+		}
+	}
+}