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/error.rs

@@ -0,0 +1,56 @@
+// 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 codec::{Decode, Encode};
+use mixnet::core::PostErr;
+
+/// Error handling a request. Sent in replies over the mixnet.
+#[derive(Debug, thiserror::Error, Decode, Encode)]
+pub enum RemoteErr {
+	/// An error that doesn't map to any of the other variants.
+	#[error("{0}")]
+	Other(String),
+	/// Failed to decode the request.
+	#[error("Failed to decode the request: {0}")]
+	Decode(String),
+}
+
+/// Mixnet error.
+#[derive(Debug, thiserror::Error)]
+pub enum Error {
+	/// Failed to communicate with the mixnet service. Possibly it panicked. The node probably
+	/// needs to be restarted.
+	#[error(
+		"Failed to communicate with the mixnet service; the node probably needs to be restarted"
+	)]
+	ServiceUnavailable,
+	/// Did not receive a reply after the configured number of attempts.
+	#[error("Did not receive a reply from the mixnet after the configured number of attempts")]
+	NoReply,
+	/// Received a malformed reply.
+	#[error("Received a malformed reply from the mixnet")]
+	BadReply,
+	/// Failed to post the request to the mixnet. Note that some [`PostErr`] variants, eg
+	/// [`PostErr::NotEnoughSpaceInQueue`], are handled internally and will never be returned from
+	/// the top-level API.
+	#[error("Failed to post the request to the mixnet: {0}")]
+	Post(#[from] PostErr),
+	/// Error reported by destination mixnode.
+	#[error("Error reported by the destination mixnode: {0}")]
+	Remote(#[from] RemoteErr),
+}