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/bin/node/runtime/src/lib.rs

@@ -589,6 +589,7 @@ impl_opaque_keys! {
 		pub babe: Babe,
 		pub im_online: ImOnline,
 		pub authority_discovery: AuthorityDiscovery,
+		pub mixnet: Mixnet,
 	}
 }
 
@@ -1048,7 +1049,7 @@ impl pallet_democracy::Config for Runtime {
 		pallet_collective::EnsureProportionAtLeast<AccountId, TechnicalCollective, 2, 3>;
 	type InstantOrigin =
 		pallet_collective::EnsureProportionAtLeast<AccountId, TechnicalCollective, 1, 1>;
-	type InstantAllowed = frame_support::traits::ConstBool<true>;
+	type InstantAllowed = ConstBool<true>;
 	type FastTrackVotingPeriod = FastTrackVotingPeriod;
 	// To cancel a proposal which has been passed, 2/3 of the council must agree to it.
 	type CancellationOrigin =
@@ -2028,6 +2029,29 @@ impl pallet_broker::Config for Runtime {
 	type PriceAdapter = pallet_broker::Linear;
 }
 
+parameter_types! {
+	pub const MixnetNumCoverToCurrentBlocks: BlockNumber = 3;
+	pub const MixnetNumRequestsToCurrentBlocks: BlockNumber = 3;
+	pub const MixnetNumCoverToPrevBlocks: BlockNumber = 3;
+	pub const MixnetNumRegisterStartSlackBlocks: BlockNumber = 3;
+	pub const MixnetNumRegisterEndSlackBlocks: BlockNumber = 3;
+	pub const MixnetRegistrationPriority: TransactionPriority = ImOnlineUnsignedPriority::get() - 1;
+}
+
+impl pallet_mixnet::Config for Runtime {
+	type MaxAuthorities = MaxAuthorities;
+	type MaxExternalAddressSize = ConstU32<128>;
+	type MaxExternalAddressesPerMixnode = ConstU32<16>;
+	type NextSessionRotation = Babe;
+	type NumCoverToCurrentBlocks = MixnetNumCoverToCurrentBlocks;
+	type NumRequestsToCurrentBlocks = MixnetNumRequestsToCurrentBlocks;
+	type NumCoverToPrevBlocks = MixnetNumCoverToPrevBlocks;
+	type NumRegisterStartSlackBlocks = MixnetNumRegisterStartSlackBlocks;
+	type NumRegisterEndSlackBlocks = MixnetNumRegisterEndSlackBlocks;
+	type RegistrationPriority = MixnetRegistrationPriority;
+	type MinMixnodes = ConstU32<7>; // Low to allow small testing networks
+}
+
 construct_runtime!(
 	pub struct Runtime
 	{
@@ -2104,6 +2128,7 @@ construct_runtime!(
 		SafeMode: pallet_safe_mode,
 		Statement: pallet_statement,
 		Broker: pallet_broker,
+		Mixnet: pallet_mixnet,
 	}
 );
 
@@ -2663,6 +2688,24 @@ impl_runtime_apis! {
 		}
 	}
 
+	impl sp_mixnet::runtime_api::MixnetApi<Block> for Runtime {
+		fn session_status() -> sp_mixnet::types::SessionStatus {
+			Mixnet::session_status()
+		}
+
+		fn prev_mixnodes() -> Result<Vec<sp_mixnet::types::Mixnode>, sp_mixnet::types::MixnodesErr> {
+			Mixnet::prev_mixnodes()
+		}
+
+		fn current_mixnodes() -> Result<Vec<sp_mixnet::types::Mixnode>, sp_mixnet::types::MixnodesErr> {
+			Mixnet::current_mixnodes()
+		}
+
+		fn maybe_register(session_index: sp_mixnet::types::SessionIndex, mixnode: sp_mixnet::types::Mixnode) -> bool {
+			Mixnet::maybe_register(session_index, mixnode)
+		}
+	}
+
 	impl sp_session::SessionKeys<Block> for Runtime {
 		fn generate_session_keys(seed: Option<Vec<u8>>) -> Vec<u8> {
 			SessionKeys::generate(seed)