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/protocol.rs

@@ -0,0 +1,42 @@
+// 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;
+use mixnet::core::PACKET_SIZE;
+use sc_network::{config::NonDefaultSetConfig, ProtocolName};
+
+/// Returns the protocol name to use for the mixnet controlled by the given chain.
+pub fn protocol_name(genesis_hash: &[u8], fork_id: Option<&str>) -> ProtocolName {
+	let name = if let Some(fork_id) = fork_id {
+		format!("/{}/{}/mixnet/1", array_bytes::bytes2hex("", genesis_hash), fork_id)
+	} else {
+		format!("/{}/mixnet/1", array_bytes::bytes2hex("", genesis_hash))
+	};
+	name.into()
+}
+
+/// Returns the peers set configuration for the mixnet protocol.
+pub fn peers_set_config(name: ProtocolName, config: &Config) -> NonDefaultSetConfig {
+	let mut set_config = NonDefaultSetConfig::new(name, PACKET_SIZE as u64);
+	if config.substrate.num_gateway_slots != 0 {
+		// out_peers is always 0; we are only interested in connecting to mixnodes, which we do by
+		// setting them as reserved nodes
+		set_config.allow_non_reserved(config.substrate.num_gateway_slots, 0);
+	}
+	set_config
+}