Runtime: allow backing multiple candidates of same parachain on diffe…

…rent cores (#3231)

Fixes #3144

Builds on top of #3229

### Summary
Some preparations for Runtime to support elastic scaling, guarded by
config node features bit `FeatureIndex::ElasticScalingMVP`. This PR
introduces a per-candidate `CoreIndex` but does it in a hacky way to
avoid changing `CandidateCommitments`, `CandidateReceipts` primitives
and networking protocols.

#### Including `CoreIndex` in `BackedCandidate`
If the `ElasticScalingMVP` feature bit is enabled then
`BackedCandidate::validator_indices` is extended by 8 bits.
The value stored in these bits represents the assumed core index for the
candidate.

It is temporary solution which works by creating a mapping from
`BackedCandidate` to `CoreIndex` by assuming the `CoreIndex` can be
discovered by checking in which validator group the validator that
signed the statement is.

TODO:
- [x] fix tests
- [x] add new tests
- [x] Bump runtime API for Kusama, so we have that node features thing!
-> polkadot-fellows/runtimes#194

---------

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>
Signed-off-by: alindima <alin@parity.io>
Co-authored-by: alindima <alin@parity.io>

.gitlab/pipeline/zombienet/polkadot.yml

@@ -158,6 +158,14 @@ zombienet-polkadot-functional-0011-async-backing-6-seconds-rate:
       --local-dir="${LOCAL_DIR}/functional"
       --test="0011-async-backing-6-seconds-rate.zndsl"
 
+zombienet-polkadot-functional-0012-elastic-scaling-mvp:
+  extends:
+    - .zombienet-polkadot-common
+  script:
+    - /home/nonroot/zombie-net/scripts/ci/run-test-local-env-manager.sh
+      --local-dir="${LOCAL_DIR}/functional"
+      --test="0012-elastic-scaling-mvp.zndsl"
+
 zombienet-polkadot-smoke-0001-parachains-smoke-test:
   extends:
     - .zombienet-polkadot-common

polkadot/node/core/backing/Cargo.toml

@@ -32,5 +32,6 @@ sc-keystore = { path = "../../../../substrate/client/keystore" }
 sp-tracing = { path = "../../../../substrate/primitives/tracing" }
 futures = { version = "0.3.21", features = ["thread-pool"] }
 assert_matches = "1.4.0"
+rstest = "0.18.2"
 polkadot-node-subsystem-test-helpers = { path = "../../subsystem-test-helpers" }
 test-helpers = { package = "polkadot-primitives-test-helpers", path = "../../../primitives/test-helpers" }

polkadot/node/core/backing/src/lib.rs

@@ -70,7 +70,7 @@ use std::{
 	sync::Arc,
 };
 
-use bitvec::{order::Lsb0 as BitOrderLsb0, vec::BitVec};
+use bitvec::vec::BitVec;
 use futures::{
 	channel::{mpsc, oneshot},
 	future::BoxFuture,
@@ -494,20 +494,15 @@ fn table_attested_to_backed(
 	}
 	vote_positions.sort_by_key(|(_orig, pos_in_group)| *pos_in_group);
 
-	if inject_core_index {
-		let core_index_to_inject: BitVec<u8, BitOrderLsb0> =
-			BitVec::from_vec(vec![core_index.0 as u8]);
-		validator_indices.extend(core_index_to_inject);
-	}
-
-	Some(BackedCandidate {
+	Some(BackedCandidate::new(
 		candidate,
-		validity_votes: vote_positions
+		vote_positions
 			.into_iter()
 			.map(|(pos_in_votes, _pos_in_group)| validity_votes[pos_in_votes].clone())
 			.collect(),
 		validator_indices,
-	})
+		inject_core_index.then_some(core_index),
+	))
 }
 
 async fn store_available_data(
@@ -1775,7 +1770,7 @@ async fn post_import_statement_actions<Context>(
 				&rp_state.table_context,
 				rp_state.inject_core_index,
 			) {
-				let para_id = backed.candidate.descriptor.para_id;
+				let para_id = backed.candidate().descriptor.para_id;
 				gum::debug!(
 					target: LOG_TARGET,
 					candidate_hash = ?candidate_hash,
@@ -1796,7 +1791,7 @@ async fn post_import_statement_actions<Context>(
 					// notify collator protocol.
 					ctx.send_message(CollatorProtocolMessage::Backed {
 						para_id,
-						para_head: backed.candidate.descriptor.para_head,
+						para_head: backed.candidate().descriptor.para_head,
 					})
 					.await;
 					// Notify statement distribution of backed candidate.

polkadot/node/core/backing/src/tests/mod.rs

@@ -33,9 +33,10 @@ use polkadot_node_subsystem::{
 };
 use polkadot_node_subsystem_test_helpers as test_helpers;
 use polkadot_primitives::{
-	CandidateDescriptor, GroupRotationInfo, HeadData, PersistedValidationData, PvfExecKind,
-	ScheduledCore, SessionIndex, LEGACY_MIN_BACKING_VOTES,
+	vstaging::node_features, CandidateDescriptor, GroupRotationInfo, HeadData,
+	PersistedValidationData, PvfExecKind, ScheduledCore, SessionIndex, LEGACY_MIN_BACKING_VOTES,
 };
+use rstest::rstest;
 use sp_application_crypto::AppCrypto;
 use sp_keyring::Sr25519Keyring;
 use sp_keystore::Keystore;
@@ -79,6 +80,7 @@ pub(crate) struct TestState {
 	relay_parent: Hash,
 	minimum_backing_votes: u32,
 	disabled_validators: Vec<ValidatorIndex>,
+	node_features: NodeFeatures,
 }
 
 impl TestState {
@@ -157,6 +159,7 @@ impl Default for TestState {
 			relay_parent,
 			minimum_backing_votes: LEGACY_MIN_BACKING_VOTES,
 			disabled_validators: Vec::new(),
+			node_features: Default::default(),
 		}
 	}
 }
@@ -298,7 +301,7 @@ async fn test_startup(virtual_overseer: &mut VirtualOverseer, test_state: &TestS
 		AllMessages::RuntimeApi(
 			RuntimeApiMessage::Request(_parent, RuntimeApiRequest::NodeFeatures(_session_index, tx))
 		) => {
-			tx.send(Ok(Default::default())).unwrap();
+			tx.send(Ok(test_state.node_features.clone())).unwrap();
 		}
 	);
 
@@ -494,9 +497,20 @@ fn backing_second_works() {
 }
 
 // Test that the candidate reaches quorum successfully.
-#[test]
-fn backing_works() {
-	let test_state = TestState::default();
+#[rstest]
+#[case(true)]
+#[case(false)]
+fn backing_works(#[case] elastic_scaling_mvp: bool) {
+	let mut test_state = TestState::default();
+	if elastic_scaling_mvp {
+		test_state
+			.node_features
+			.resize((node_features::FeatureIndex::ElasticScalingMVP as u8 + 1) as usize, false);
+		test_state
+			.node_features
+			.set(node_features::FeatureIndex::ElasticScalingMVP as u8 as usize, true);
+	}
+
 	test_harness(test_state.keystore.clone(), |mut virtual_overseer| async move {
 		test_startup(&mut virtual_overseer, &test_state).await;
 
@@ -647,6 +661,31 @@ fn backing_works() {
 
 		virtual_overseer.send(FromOrchestra::Communication { msg: statement }).await;
 
+		let (tx, rx) = oneshot::channel();
+		let msg = CandidateBackingMessage::GetBackedCandidates(
+			vec![(candidate_a_hash, test_state.relay_parent)],
+			tx,
+		);
+
+		virtual_overseer.send(FromOrchestra::Communication { msg }).await;
+
+		let candidates = rx.await.unwrap();
+		assert_eq!(1, candidates.len());
+		assert_eq!(candidates[0].validity_votes().len(), 3);
+
+		let (validator_indices, maybe_core_index) =
+			candidates[0].validator_indices_and_core_index(elastic_scaling_mvp);
+		if elastic_scaling_mvp {
+			assert_eq!(maybe_core_index.unwrap(), CoreIndex(0));
+		} else {
+			assert!(maybe_core_index.is_none());
+		}
+
+		assert_eq!(
+			validator_indices,
+			bitvec::bitvec![u8, bitvec::order::Lsb0; 1, 1, 0, 1].as_bitslice()
+		);
+
 		virtual_overseer
 			.send(FromOrchestra::Signal(OverseerSignal::ActiveLeaves(
 				ActiveLeavesUpdate::stop_work(test_state.relay_parent),
@@ -919,20 +958,20 @@ fn backing_works_while_validation_ongoing() {
 
 		let candidates = rx.await.unwrap();
 		assert_eq!(1, candidates.len());
-		assert_eq!(candidates[0].validity_votes.len(), 3);
+		assert_eq!(candidates[0].validity_votes().len(), 3);
 
 		assert!(candidates[0]
-			.validity_votes
+			.validity_votes()
 			.contains(&ValidityAttestation::Implicit(signed_a.signature().clone())));
 		assert!(candidates[0]
-			.validity_votes
+			.validity_votes()
 			.contains(&ValidityAttestation::Explicit(signed_b.signature().clone())));
 		assert!(candidates[0]
-			.validity_votes
+			.validity_votes()
 			.contains(&ValidityAttestation::Explicit(signed_c.signature().clone())));
 		assert_eq!(
-			candidates[0].validator_indices,
-			bitvec::bitvec![u8, bitvec::order::Lsb0; 1, 0, 1, 1],
+			candidates[0].validator_indices_and_core_index(false),
+			(bitvec::bitvec![u8, bitvec::order::Lsb0; 1, 0, 1, 1].as_bitslice(), None)
 		);
 
 		virtual_overseer
@@ -1604,8 +1643,11 @@ fn candidate_backing_reorders_votes() {
 	let expected_attestations =
 		vec![fake_attestation(1).into(), fake_attestation(3).into(), fake_attestation(5).into()];
 
-	assert_eq!(backed.validator_indices, expected_bitvec);
-	assert_eq!(backed.validity_votes, expected_attestations);
+	assert_eq!(
+		backed.validator_indices_and_core_index(false),
+		(expected_bitvec.as_bitslice(), None)
+	);
+	assert_eq!(backed.validity_votes(), expected_attestations);
 }
 
 // Test whether we retry on failed PoV fetching.

polkadot/node/core/provisioner/src/lib.rs

@@ -766,7 +766,7 @@ async fn select_candidates(
 	// keep only one candidate with validation code.
 	let mut with_validation_code = false;
 	candidates.retain(|c| {
-		if c.candidate.commitments.new_validation_code.is_some() {
+		if c.candidate().commitments.new_validation_code.is_some() {
 			if with_validation_code {
 				return false
 			}