pallet-beefy-mmr: add API for BEEFY Authority Sets (#11406)

* pallet-beefy: add Config::OnNewValidatorSet type

Add a hook to pallet-beefy for doing specific work when
BEEFY validator set changes.

For example, this can be used by pallet-beefy-mmr to cache
a lightweight MMR root over validators and make it available
to light clients.

* pallet-beefy-mmr: implement OnNewValidatorSet

Implement pallet-beefy::OnNewValidatorSet to be notified of BEEFY
validator set changes. Use the notifications to compute and cache
a light weight 'BEEFY authority set' which is an MMR root over
BEEFY validator set plus some extra info.

Previously, pallet-beefy-mmr was interogating pallet-beefy about
validator set id on every block to find out when it needs to recompute
the authority set.
By using the event-driven approach in this commit, we also save one
extra state interogation per block.

* pallet-beefy-mmr: add new authority_set() API

Expose current and next BEEFY authority sets through runtime API.
These can be directly used by light clients to avoid having them
compute them themselves based on BEEFY validator sets.

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

* rename BeefyMmr exposed runtime api

substrate/frame/beefy/src/lib.rs

@@ -32,7 +32,9 @@ use sp_runtime::{
 };
 use sp_std::prelude::*;
 
-use beefy_primitives::{AuthorityIndex, ConsensusLog, ValidatorSet, BEEFY_ENGINE_ID};
+use beefy_primitives::{
+	AuthorityIndex, ConsensusLog, OnNewValidatorSet, ValidatorSet, BEEFY_ENGINE_ID,
+};
 
 #[cfg(test)]
 mod mock;
@@ -58,6 +60,13 @@ pub mod pallet {
 
 		/// The maximum number of authorities that can be added.
 		type MaxAuthorities: Get<u32>;
+
+		/// A hook to act on the new BEEFY validator set.
+		///
+		/// For some applications it might be beneficial to make the BEEFY validator set available
+		/// externally apart from having it in the storage. For instance you might cache a light
+		/// weight MMR root over validators and make it available for Light Clients.
+		type OnNewValidatorSet: OnNewValidatorSet<<Self as Config>::BeefyId>;
 	}
 
 	#[pallet::pallet]
@@ -118,20 +127,29 @@ impl<T: Config> Pallet<T> {
 	) {
 		<Authorities<T>>::put(&new);
 
-		let next_id = Self::validator_set_id() + 1u64;
-		<ValidatorSetId<T>>::put(next_id);
-		if let Some(validator_set) = ValidatorSet::<T::BeefyId>::new(new, next_id) {
-			let log = DigestItem::Consensus(
-				BEEFY_ENGINE_ID,
-				ConsensusLog::AuthoritiesChange(validator_set).encode(),
-			);
-			<frame_system::Pallet<T>>::deposit_log(log);
-		}
+		let new_id = Self::validator_set_id() + 1u64;
+		<ValidatorSetId<T>>::put(new_id);
 
 		<NextAuthorities<T>>::put(&queued);
+
+		if let Some(validator_set) = ValidatorSet::<T::BeefyId>::new(new, new_id) {
+			let log = DigestItem::Consensus(
+				BEEFY_ENGINE_ID,
+				ConsensusLog::AuthoritiesChange(validator_set.clone()).encode(),
+			);
+			<frame_system::Pallet<T>>::deposit_log(log);
+
+			let next_id = new_id + 1;
+			if let Some(next_validator_set) = ValidatorSet::<T::BeefyId>::new(queued, next_id) {
+				<T::OnNewValidatorSet as OnNewValidatorSet<_>>::on_new_validator_set(
+					&validator_set,
+					&next_validator_set,
+				);
+			}
+		}
 	}
 
-	fn initialize_authorities(authorities: &[T::BeefyId]) -> Result<(), ()> {
+	fn initialize_authorities(authorities: &Vec<T::BeefyId>) -> Result<(), ()> {
 		if authorities.is_empty() {
 			return Ok(())
 		}
@@ -141,12 +159,25 @@ impl<T: Config> Pallet<T> {
 		}
 
 		let bounded_authorities =
-			BoundedSlice::<T::BeefyId, T::MaxAuthorities>::try_from(authorities)?;
+			BoundedSlice::<T::BeefyId, T::MaxAuthorities>::try_from(authorities.as_slice())?;
 
+		let id = 0;
 		<Authorities<T>>::put(bounded_authorities);
-		<ValidatorSetId<T>>::put(0);
+		<ValidatorSetId<T>>::put(id);
 		// Like `pallet_session`, initialize the next validator set as well.
 		<NextAuthorities<T>>::put(bounded_authorities);
+
+		if let Some(validator_set) = ValidatorSet::<T::BeefyId>::new(authorities.clone(), id) {
+			let next_id = id + 1;
+			if let Some(next_validator_set) =
+				ValidatorSet::<T::BeefyId>::new(authorities.clone(), next_id)
+			{
+				<T::OnNewValidatorSet as OnNewValidatorSet<_>>::on_new_validator_set(
+					&validator_set,
+					&next_validator_set,
+				);
+			}
+		}
 		Ok(())
 	}
 }

substrate/frame/beefy/src/mock.rs

@@ -87,6 +87,7 @@ impl frame_system::Config for Test {
 impl pallet_beefy::Config for Test {
 	type BeefyId = BeefyId;
 	type MaxAuthorities = ConstU32<100>;
+	type OnNewValidatorSet = ();
 }
 
 parameter_types! {