Mixnet integration (#1346)

See #1345, <https://github.com/paritytech/substrate/pull/14207>.

This adds all the necessary mixnet components, and puts them together in
the "kitchen-sink" node/runtime. The components added are:

- A pallet (`frame/mixnet`). This is responsible for determining the
current mixnet session and phase, and the mixnodes to use in each
session. It provides a function that validators can call to register a
mixnode for the next session. The logic of this pallet is very similar
to that of the `im-online` pallet.
- A service (`client/mixnet`). This implements the core mixnet logic,
building on the `mixnet` crate. The service communicates with other
nodes using notifications sent over the "mixnet" protocol.
- An RPC interface. This currently only supports sending transactions
over the mixnet.

---------

Co-authored-by: David Emett <dave@sp4m.net>
Co-authored-by: Javier Viola <javier@parity.io>

substrate/client/rpc/src/lib.rs

@@ -34,6 +34,7 @@ pub use sc_rpc_api::DenyUnsafe;
 pub mod author;
 pub mod chain;
 pub mod dev;
+pub mod mixnet;
 pub mod offchain;
 pub mod state;
 pub mod statement;

substrate/client/rpc/src/mixnet/mod.rs

@@ -0,0 +1,47 @@
+// This file is part of Substrate.
+
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: GPL-3.0-or-later WITH Classpath-exception-2.0
+
+// This program 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.
+
+// This program 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 this program. If not, see <https://www.gnu.org/licenses/>.
+
+//! Substrate mixnet API.
+
+use jsonrpsee::core::{async_trait, RpcResult};
+use sc_mixnet::Api;
+use sc_rpc_api::mixnet::error::Error;
+pub use sc_rpc_api::mixnet::MixnetApiServer;
+use sp_core::Bytes;
+
+/// Mixnet API.
+pub struct Mixnet(futures::lock::Mutex<Api>);
+
+impl Mixnet {
+	/// Create a new mixnet API instance.
+	pub fn new(api: Api) -> Self {
+		Self(futures::lock::Mutex::new(api))
+	}
+}
+
+#[async_trait]
+impl MixnetApiServer for Mixnet {
+	async fn submit_extrinsic(&self, extrinsic: Bytes) -> RpcResult<()> {
+		// We only hold the lock while pushing the request into the requests channel
+		let fut = {
+			let mut api = self.0.lock().await;
+			api.submit_extrinsic(extrinsic).await
+		};
+		Ok(fut.await.map_err(Error)?)
+	}
+}