Whole subsystem test for new availability-distribution (#2552)

* WIP: Whole subsystem test.

* New tests compile.

* Avoid needless runtime queries for no validator nodes.

* Make tx and rx publicly accessible in virtual overseer.

This simplifies mocking in some cases, as tx can be cloned, but rx can
not.

* Whole subsystem test working.

* Update node/network/availability-distribution/src/session_cache.rs

Co-authored-by: Andronik Ordian <write@reusable.software>

* Update node/network/availability-distribution/src/session_cache.rs

Co-authored-by: Andronik Ordian <write@reusable.software>

* Document better what `None` return value means.

* Get rid of BitVec dependency.

* Update Cargo.lock

* Hopefully fixed implementers guide build.

Co-authored-by: Andronik Ordian <write@reusable.software>

polkadot/node/network/availability-distribution/src/tests/mod.rs

@@ -0,0 +1,63 @@
+// Copyright 2021 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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 Polkadot.  If not, see <http://www.gnu.org/licenses/>.
+
+use futures::{executor, future, Future};
+
+use sp_keystore::SyncCryptoStorePtr;
+
+use polkadot_subsystem_testhelpers as test_helpers;
+
+use super::*;
+
+mod state;
+/// State for test harnesses.
+use state::{TestState, TestHarness};
+
+/// Mock data useful for testing.
+pub(crate) mod mock;
+
+fn test_harness<T: Future<Output = ()>>(
+	keystore: SyncCryptoStorePtr,
+	test_fx: impl FnOnce(TestHarness) -> T,
+) {
+	sp_tracing::try_init_simple();
+
+	let pool = sp_core::testing::TaskExecutor::new();
+	let (context, virtual_overseer) = test_helpers::make_subsystem_context(pool.clone());
+
+	let subsystem = AvailabilityDistributionSubsystem::new(keystore, Default::default());
+	{
+		let subsystem = subsystem.run(context);
+
+		let test_fut = test_fx(TestHarness { virtual_overseer, pool });
+
+		futures::pin_mut!(test_fut);
+		futures::pin_mut!(subsystem);
+
+		executor::block_on(future::select(test_fut, subsystem));
+	}
+}
+
+/// Simple basic check, whether the subsystem works as expected.
+///
+/// Exceptional cases are tested as unit tests in `fetch_task`.
+#[test]
+fn check_basic() {
+	let state = TestState::default();
+	test_harness(state.keystore.clone(), move |harness| {
+		state.run(harness)
+	});
+}