client/authority-discovery: Introduce AuthorityDiscoveryService (#6760)

* client/authority-discovery: Rename AuthorityDiscovery to XXXWorker

* client/authority-discovery: Introduce AuthorityDiscoveryService

Add a basic `AuthorityDiscoveryService` implementation which enables
callers to get the addresses for a given `AuthorityId` from the local
cache.

* client/authority-discovery: Split into worker and service mod

Move `Service` and `Worker` to their own Rust modules resulting in the
following file structure.

├── build.rs
├── Cargo.toml
└── src
    ├── error.rs
    ├── lib.rs
    ├── service.rs
    ├── tests.rs
    ├── worker
    │   ├── addr_cache.rs
    │   ├── schema
    │   │   └── dht.proto
    │   └── tests.rs
    └── worker.rs

* client/authority-discovery: Cache PeerId -> AuthorityId mapping

* client/authority-discovery: Update priority group on interval

Instead of updating the authority discovery peerset priority group each
time a new DHT value is found, update it regularly on an interval.

This removes the need for deterministic random selection. Instead of
trying to return a random stable set of `Multiaddr`s, the `AddrCache`
now returns a random set on each call.

* client/authority-discovery: Implement Service::get_authority_id

* client/authority-discovery: Use HashMap instead of BTreeMap

* client/authority-discovery: Rework priority group interval

* client/authority-discovery: Fix comment

* bin/node/cli: Update authority discovery constructor

* client/authority-discovery: Fuse from_service receiver

* client/authority-discovery: Remove Rng import

* client/authority-discovery: Ignore Multiaddr without PeerId

* client/authority-discovery/service: Add note on returned None

* client/authority-discovery/addr_cache: Replace double clone with deref

substrate/client/authority-discovery/src/service.rs

@@ -0,0 +1,70 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+
+// Substrate 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.
+
+// Substrate 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 Substrate.  If not, see <http://www.gnu.org/licenses/>.
+
+use crate::ServicetoWorkerMsg;
+
+use futures::channel::{mpsc, oneshot};
+use futures::SinkExt;
+
+use sc_network::{Multiaddr, PeerId};
+use sp_authority_discovery::AuthorityId;
+
+/// Service to interact with the [`Worker`].
+#[derive(Clone)]
+pub struct Service {
+	to_worker: mpsc::Sender<ServicetoWorkerMsg>,
+}
+
+/// A [`Service`] allows to interact with a [`Worker`], e.g. by querying the
+/// [`Worker`]'s local address cache for a given [`AuthorityId`].
+impl Service {
+	pub(crate) fn new(to_worker: mpsc::Sender<ServicetoWorkerMsg>) -> Self {
+		Self {
+			to_worker,
+		}
+	}
+
+	/// Get the addresses for the given [`AuthorityId`] from the local address cache.
+	///
+	/// Returns `None` if no entry was present or connection to the [`crate::Worker`] failed.
+	///
+	/// [`Multiaddr`]s returned always include a [`libp2p::core::multiaddr:Protocol::P2p`]
+	/// component.
+	pub async fn get_addresses_by_authority_id(&mut self, authority: AuthorityId) -> Option<Vec<Multiaddr>> {
+		let (tx, rx) = oneshot::channel();
+
+		self.to_worker
+			.send(ServicetoWorkerMsg::GetAddressesByAuthorityId(authority, tx))
+			.await
+			.ok()?;
+
+		rx.await.ok().flatten()
+	}
+
+	/// Get the [`AuthorityId`] for the given [`PeerId`] from the local address cache.
+	///
+	/// Returns `None` if no entry was present or connection to the [`crate::Worker`] failed.
+	pub async fn get_authority_id_by_peer_id(&mut self, peer_id: PeerId) -> Option<AuthorityId> {
+		let (tx, rx) = oneshot::channel();
+
+		self.to_worker
+			.send(ServicetoWorkerMsg::GetAuthorityIdByPeerId(peer_id, tx))
+			.await
+			.ok()?;
+
+		rx.await.ok().flatten()
+	}
+}