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/mixnet/src/api.rs

@@ -0,0 +1,69 @@
+// 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/>.
+
+use super::{config::Config, error::Error, request::Request};
+use futures::{
+	channel::{mpsc, oneshot},
+	SinkExt,
+};
+use sp_core::Bytes;
+use std::future::Future;
+
+/// The other end of an [`Api`]. This should be passed to [`run`](super::run::run).
+pub struct ApiBackend {
+	pub(super) request_receiver: mpsc::Receiver<Request>,
+}
+
+/// Interface to the mixnet service.
+#[derive(Clone)]
+pub struct Api {
+	request_sender: mpsc::Sender<Request>,
+}
+
+impl Api {
+	/// Create a new `Api`. The [`ApiBackend`] should be passed to [`run`](super::run::run).
+	pub fn new(config: &Config) -> (Self, ApiBackend) {
+		let (request_sender, request_receiver) = mpsc::channel(config.substrate.request_buffer);
+		(Self { request_sender }, ApiBackend { request_receiver })
+	}
+
+	/// Submit an extrinsic via the mixnet.
+	///
+	/// Returns a [`Future`] which returns another `Future`.
+	///
+	/// The first `Future` resolves as soon as there is space in the mixnet service queue. The
+	/// second `Future` resolves once a reply is received over the mixnet (or sooner if there is an
+	/// error).
+	///
+	/// The first `Future` references `self`, but the second does not. This makes it possible to
+	/// submit concurrent mixnet requests using a single `Api` instance.
+	pub async fn submit_extrinsic(
+		&mut self,
+		extrinsic: Bytes,
+	) -> impl Future<Output = Result<(), Error>> {
+		let (reply_sender, reply_receiver) = oneshot::channel();
+		let res = self
+			.request_sender
+			.feed(Request::SubmitExtrinsic { extrinsic, reply_sender })
+			.await;
+		async move {
+			res.map_err(|_| Error::ServiceUnavailable)?;
+			reply_receiver.await.map_err(|_| Error::ServiceUnavailable)?
+		}
+	}
+}