sc-consensus-beefy: fix initialization when state is unavailable (#1888)

Fix situation where BEEFY initial validator set could not be determined.

If state is unavailable at BEEFY genesis block to get initial validator
set, get the info from header digests. For this, we need to walk back
the chain starting from BEEFY genesis looking for the BEEFY digest
announcing the active validator set for that respective session.

This commit fixes a silly bug where walking back the chain was stopped
when reaching BEEFY genesis block, which is incorrect when BEEFY genesis
is not session boundary block. When BEEFY genesis is set to some random
block within a session, we need to walk back to the start of the session
to see the validator set announcement.

Added regression test for this fix.

Fixes https://github.com/paritytech/polkadot-sdk/issues/1885

Signed-off-by: Adrian Catangiu <adrian@parity.io>

substrate/client/consensus/beefy/src/tests.rs

@@ -247,7 +247,7 @@ impl TestNetFactory for BeefyTestNet {
 #[derive(Clone)]
 pub(crate) struct TestApi {
 	pub beefy_genesis: u64,
-	pub validator_set: BeefyValidatorSet,
+	pub validator_set: Option<BeefyValidatorSet>,
 	pub mmr_root_hash: MmrRootHash,
 	pub reported_equivocations:
 		Option<Arc<Mutex<Vec<EquivocationProof<NumberFor<Block>, AuthorityId, Signature>>>>>,
@@ -261,7 +261,7 @@ impl TestApi {
 	) -> Self {
 		TestApi {
 			beefy_genesis,
-			validator_set: validator_set.clone(),
+			validator_set: Some(validator_set.clone()),
 			mmr_root_hash,
 			reported_equivocations: None,
 		}
@@ -270,7 +270,7 @@ impl TestApi {
 	pub fn with_validator_set(validator_set: &BeefyValidatorSet) -> Self {
 		TestApi {
 			beefy_genesis: 1,
-			validator_set: validator_set.clone(),
+			validator_set: Some(validator_set.clone()),
 			mmr_root_hash: GOOD_MMR_ROOT,
 			reported_equivocations: None,
 		}
@@ -300,7 +300,7 @@ sp_api::mock_impl_runtime_apis! {
 		}
 
 		fn validator_set() -> Option<BeefyValidatorSet> {
-			Some(self.inner.validator_set.clone())
+			self.inner.validator_set.clone()
 		}
 
 		fn submit_report_equivocation_unsigned_extrinsic(
@@ -1188,6 +1188,54 @@ async fn should_initialize_voter_at_latest_finalized() {
 	assert_eq!(state, persisted_state);
 }
 
+#[tokio::test]
+async fn should_initialize_voter_at_custom_genesis_when_state_unavailable() {
+	let keys = &[BeefyKeyring::Alice];
+	let validator_set = ValidatorSet::new(make_beefy_ids(keys), 0).unwrap();
+	let mut net = BeefyTestNet::new(1);
+	let backend = net.peer(0).client().as_backend();
+	// custom pallet genesis is block number 7
+	let custom_pallet_genesis = 7;
+	let mut api = TestApi::new(custom_pallet_genesis, &validator_set, GOOD_MMR_ROOT);
+	// remove validator set from `TestApi`, practically simulating unavailable/pruned runtime state
+	api.validator_set = None;
+
+	// push 30 blocks with `AuthorityChange` digests every 5 blocks
+	let hashes = net.generate_blocks_and_sync(30, 5, &validator_set, false).await;
+	let mut finality = net.peer(0).client().as_client().finality_notification_stream().fuse();
+	// finalize 30 without justifications
+	net.peer(0).client().as_client().finalize_block(hashes[30], None).unwrap();
+
+	// load persistent state - nothing in DB, should init at genesis
+	let persisted_state = voter_init_setup(&mut net, &mut finality, &api).await.unwrap();
+
+	// Test initialization at session boundary.
+	// verify voter initialized with all sessions pending, first one starting at block 5 (start of
+	// session containing `custom_pallet_genesis`).
+	let sessions = persisted_state.voting_oracle().sessions();
+	// should have enqueued 6 sessions (every 5 blocks from 5 to 30)
+	assert_eq!(sessions.len(), 6);
+	assert_eq!(sessions[0].session_start(), 7);
+	assert_eq!(sessions[1].session_start(), 10);
+	assert_eq!(sessions[2].session_start(), 15);
+	assert_eq!(sessions[3].session_start(), 20);
+	assert_eq!(sessions[4].session_start(), 25);
+	assert_eq!(sessions[5].session_start(), 30);
+	let rounds = persisted_state.active_round().unwrap();
+	assert_eq!(rounds.session_start(), custom_pallet_genesis);
+	assert_eq!(rounds.validator_set_id(), validator_set.id());
+
+	// verify next vote target is mandatory block 7 (genesis)
+	assert_eq!(persisted_state.best_beefy_block(), 0);
+	assert_eq!(persisted_state.best_grandpa_number(), 30);
+	assert_eq!(persisted_state.voting_oracle().voting_target(), Some(custom_pallet_genesis));
+
+	// verify state also saved to db
+	assert!(verify_persisted_version(&*backend));
+	let state = load_persistent(&*backend).unwrap().unwrap();
+	assert_eq!(state, persisted_state);
+}
+
 #[tokio::test]
 async fn beefy_finalizing_after_pallet_genesis() {
 	sp_tracing::try_init_simple();