Approve block announcements of backed blocks (#394)

* Approve block announcements of backed blocks

If we receive a block announcement without a statement attached that
matches the latest backed block, it is valid and we need to approve the
block announcement to download the block.

* Fix tests

* Approve block announcement if it comes from the best known block

* Fetch backed block only when required

cumulus/client/network/src/tests.rs

@@ -15,7 +15,7 @@
 // along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
 
 use super::*;
-use cumulus_test_service::runtime::{Block, Header};
+use cumulus_test_service::runtime::{Block, Header, Hash};
 use futures::{executor::block_on, poll, task::Poll};
 use polkadot_node_primitives::{SignedFullStatement, Statement};
 use polkadot_primitives::v1::{
@@ -37,6 +37,7 @@ use sp_keyring::Sr25519Keyring;
 use sp_keystore::{testing::KeyStore, SyncCryptoStore, SyncCryptoStorePtr};
 use sp_runtime::RuntimeAppPublic;
 use std::collections::BTreeMap;
+use parking_lot::Mutex;
 
 fn check_error(error: crate::BoxedError, check_error: impl Fn(&BlockAnnounceError) -> bool) {
 	let error = *error
@@ -173,6 +174,7 @@ fn invalid_if_no_data_exceeds_best_known_number() {
 	let mut validator = make_validator_and_api().0;
 	let header = Header {
 		number: 1,
+		state_root: Hash::random(),
 		..default_header()
 	};
 	let res = block_on(validator.validate(&header, &[]));
@@ -184,6 +186,18 @@ fn invalid_if_no_data_exceeds_best_known_number() {
 	);
 }
 
+#[test]
+fn valid_if_no_data_and_block_matches_best_known_block() {
+	let mut validator = make_validator_and_api().0;
+	let res = block_on(validator.validate(&default_header(), &[]));
+
+	assert_eq!(
+		res.unwrap(),
+		Validation::Success { is_new_best: true },
+		"validation is successful when the block hash matches the best known block",
+	);
+}
+
 #[test]
 fn check_statement_is_encoded_correctly() {
 	let mut validator = make_validator_and_api().0;
@@ -331,13 +345,33 @@ fn relay_parent_not_imported_when_block_announce_is_processed() {
 	});
 }
 
+/// Ensures that when we receive a block announcement without a statement included, while the block
+/// is not yet included by the node checking the announcement, but the node is already backed.
+#[test]
+fn block_announced_without_statement_and_block_only_backed() {
+	block_on(async move {
+		let (mut validator, api) = make_validator_and_api();
+		api.data.lock().has_pending_availability = true;
+
+		let header = default_header();
+
+		let validation = validator.validate(&header, &[]);
+
+		assert!(matches!(
+			validation.await,
+			Ok(Validation::Success { is_new_best: true })
+		));
+	});
+}
+
 #[derive(Default)]
 struct ApiData {
 	validators: Vec<ValidatorId>,
+	has_pending_availability: bool,
 }
 
 struct TestApi {
-	data: Arc<ApiData>,
+	data: Arc<Mutex<ApiData>>,
 	relay_client: Arc<PClient>,
 	relay_backend: Arc<PBackend>,
 }
@@ -348,9 +382,10 @@ impl TestApi {
 		let relay_backend = builder.backend();
 
 		Self {
-			data: Arc::new(ApiData {
+			data: Arc::new(Mutex::new(ApiData {
 				validators: vec![Sr25519Keyring::Alice.public().into()],
-			}),
+				has_pending_availability: false,
+			})),
 			relay_client: Arc::new(builder.build()),
 			relay_backend,
 		}
@@ -359,7 +394,7 @@ impl TestApi {
 
 #[derive(Default)]
 struct RuntimeApi {
-	data: Arc<ApiData>,
+	data: Arc<Mutex<ApiData>>,
 }
 
 impl ProvideRuntimeApi<PBlock> for TestApi {
@@ -376,7 +411,7 @@ impl ProvideRuntimeApi<PBlock> for TestApi {
 sp_api::mock_impl_runtime_apis! {
 	impl ParachainHost<PBlock> for RuntimeApi {
 		fn validators(&self) -> Vec<ValidatorId> {
-			self.data.validators.clone()
+			self.data.lock().validators.clone()
 		}
 
 		fn validator_groups(&self) -> (Vec<Vec<ValidatorIndex>>, GroupRotationInfo<BlockNumber>) {
@@ -407,7 +442,19 @@ sp_api::mock_impl_runtime_apis! {
 		}
 
 		fn candidate_pending_availability(&self, _: ParaId) -> Option<CommittedCandidateReceipt<PHash>> {
-			None
+			if self.data.lock().has_pending_availability {
+				Some(CommittedCandidateReceipt {
+					descriptor: CandidateDescriptor {
+						para_head: polkadot_parachain::primitives::HeadData(
+							default_header().encode(),
+						).hash(),
+						..Default::default()
+					},
+					..Default::default()
+				})
+			} else {
+				None
+			}
 		}
 
 		fn candidate_events(&self) -> Vec<CandidateEvent<PHash>> {