BEEFY voter bugfixes (#11335)

* beefy: gadget should always use current validator set

The gadget/client-voter was using previous' session validator set
to sign the 1st block in the new session (to have chained validator
set handoffs).

This is not necessary because:
1. BEEFY piggy-backs on GRANDPA and only works on canonical chain,
   so it need not concern itself with the validity of the block header
   (which contains digest with the new session's validator set). It
   can safely assume header is valid and simply use new validator set.
2. The BEEFY payload itself already contains a merkle root for the
   next validator set keys. So at the BEEFY-payload layer we already
   have a validated/trusted hand-off of authority.

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

* beefy: buffer votes for not yet finalized blocks

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

* beefy: add buffered votes regression test

substrate/client/beefy/src/round.rs

@@ -73,7 +73,6 @@ pub(crate) struct Rounds<Payload, B: Block> {
 	best_done: Option<NumberFor<B>>,
 	session_start: NumberFor<B>,
 	validator_set: ValidatorSet<Public>,
-	prev_validator_set: ValidatorSet<Public>,
 }
 
 impl<P, B> Rounds<P, B>
@@ -81,18 +80,8 @@ where
 	P: Ord + Hash + Clone,
 	B: Block,
 {
-	pub(crate) fn new(
-		session_start: NumberFor<B>,
-		validator_set: ValidatorSet<Public>,
-		prev_validator_set: ValidatorSet<Public>,
-	) -> Self {
-		Rounds {
-			rounds: BTreeMap::new(),
-			best_done: None,
-			session_start,
-			validator_set,
-			prev_validator_set,
-		}
+	pub(crate) fn new(session_start: NumberFor<B>, validator_set: ValidatorSet<Public>) -> Self {
+		Rounds { rounds: BTreeMap::new(), best_done: None, session_start, validator_set }
 	}
 }
 
@@ -101,24 +90,12 @@ where
 	P: Ord + Hash + Clone,
 	B: Block,
 {
-	pub(crate) fn validator_set_id_for(&self, block_number: NumberFor<B>) -> ValidatorSetId {
-		if block_number > self.session_start {
-			self.validator_set.id()
-		} else {
-			self.prev_validator_set.id()
-		}
+	pub(crate) fn validator_set_id(&self) -> ValidatorSetId {
+		self.validator_set.id()
 	}
 
-	pub(crate) fn validators_for(&self, block_number: NumberFor<B>) -> &[Public] {
-		if block_number > self.session_start {
-			self.validator_set.validators()
-		} else {
-			self.prev_validator_set.validators()
-		}
-	}
-
-	pub(crate) fn validator_set(&self) -> &ValidatorSet<Public> {
-		&self.validator_set
+	pub(crate) fn validators(&self) -> &[Public] {
+		self.validator_set.validators()
 	}
 
 	pub(crate) fn session_start(&self) -> &NumberFor<B> {
@@ -143,7 +120,7 @@ where
 				round.1
 			);
 			false
-		} else if !self.validator_set.validators().iter().any(|id| vote.0 == *id) {
+		} else if !self.validators().iter().any(|id| vote.0 == *id) {
 			debug!(
 				target: "beefy",
 				"🥩 received vote {:?} from validator that is not in the validator set, ignoring",
@@ -170,12 +147,11 @@ where
 			// remove this and older (now stale) rounds
 			let signatures = self.rounds.remove(round)?.votes;
 			self.rounds.retain(|&(_, number), _| number > round.1);
-			self.best_done = self.best_done.clone().max(Some(round.1.clone()));
+			self.best_done = self.best_done.max(Some(round.1));
 			debug!(target: "beefy", "🥩 Concluded round #{}", round.1);
 
 			Some(
-				self.validator_set
-					.validators()
+				self.validators()
 					.iter()
 					.map(|authority_id| signatures.get(authority_id).cloned())
 					.collect(),
@@ -247,13 +223,13 @@ mod tests {
 		.unwrap();
 
 		let session_start = 1u64.into();
-		let rounds = Rounds::<H256, Block>::new(session_start, validators.clone(), validators);
+		let rounds = Rounds::<H256, Block>::new(session_start, validators);
 
-		assert_eq!(42, rounds.validator_set_id_for(session_start));
+		assert_eq!(42, rounds.validator_set_id());
 		assert_eq!(1, *rounds.session_start());
 		assert_eq!(
 			&vec![Keyring::Alice.public(), Keyring::Bob.public(), Keyring::Charlie.public()],
-			rounds.validators_for(session_start)
+			rounds.validators()
 		);
 	}
 
@@ -274,7 +250,7 @@ mod tests {
 		let round = (H256::from_low_u64_le(1), 1);
 
 		let session_start = 1u64.into();
-		let mut rounds = Rounds::<H256, Block>::new(session_start, validators.clone(), validators);
+		let mut rounds = Rounds::<H256, Block>::new(session_start, validators);
 
 		// no self vote yet, should self vote
 		assert!(rounds.should_self_vote(&round));
@@ -347,7 +323,7 @@ mod tests {
 		.unwrap();
 
 		let session_start = 1u64.into();
-		let mut rounds = Rounds::<H256, Block>::new(session_start, validators.clone(), validators);
+		let mut rounds = Rounds::<H256, Block>::new(session_start, validators);
 
 		// round 1
 		assert!(rounds.add_vote(