Introduce ChainSyncInterface (#12489)

* Introduce `ChainSyncInterface`

`ChainSyncInterface` provides an asynchronous interface for other
subsystems to submit calls to `ChainSync`. This allows `NetworkService`
to delegate calls to `ChainSync` while still providing the same API
for other subsystems (for now). This makes it possible to move the
syncing code in piecemeal fashion out of `protocol.rs` as the calls
are just forwarded to `ChainSync`.

* Apply review comments

* Fix tests

substrate/client/network/sync/src/tests.rs

@@ -0,0 +1,59 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2017-2022 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 crate::{ChainSync, ForkTarget};
+
+use libp2p::PeerId;
+use sc_network_common::{service::NetworkSyncForkRequest, sync::ChainSync as ChainSyncT};
+use sp_consensus::block_validation::DefaultBlockAnnounceValidator;
+use sp_core::H256;
+use std::{sync::Arc, task::Poll};
+use substrate_test_runtime_client::{TestClientBuilder, TestClientBuilderExt as _};
+
+// verify that the fork target map is empty, then submit a new sync fork request,
+// poll `ChainSync` and verify that a new sync fork request has been registered
+#[async_std::test]
+async fn delegate_to_chainsync() {
+	let (mut chain_sync, chain_sync_service) = ChainSync::new(
+		sc_network_common::sync::SyncMode::Full,
+		Arc::new(TestClientBuilder::with_default_backend().build_with_longest_chain().0),
+		Box::new(DefaultBlockAnnounceValidator),
+		1u32,
+		None,
+	)
+	.unwrap();
+
+	let hash = H256::random();
+	let in_number = 1337u64;
+	let peers = (0..3).map(|_| PeerId::random()).collect::<Vec<_>>();
+
+	assert!(chain_sync.fork_targets.is_empty());
+	chain_sync_service.set_sync_fork_request(peers, hash, in_number);
+
+	futures::future::poll_fn(|cx| {
+		let _ = chain_sync.poll(cx);
+		Poll::Ready(())
+	})
+	.await;
+
+	if let Some(ForkTarget { number, .. }) = chain_sync.fork_targets.get(&hash) {
+		assert_eq!(number, &in_number);
+	} else {
+		panic!("expected to contain `ForkTarget`");
+	}
+}