Configurable maximum validators (#2586)

* guide: max_validators

* guide: new approach

* restrict validators based on configurable max

* add some tests

* fix wasm build (Vec)

* clean up warnings

* add logging

* set validator indices in tests as well

* fix common tests

* Update runtime/parachains/src/util.rs

Co-authored-by: Andronik Ordian <write@reusable.software>

Co-authored-by: Andronik Ordian <write@reusable.software>

polkadot/runtime/parachains/src/shared.rs

@@ -19,12 +19,17 @@
 //! To avoid cyclic dependencies, it is important that this module is not
 //! dependent on any of the other modules.
 
-use primitives::v1::SessionIndex;
+use primitives::v1::{SessionIndex, ValidatorId, ValidatorIndex};
 use frame_support::{
 	decl_storage, decl_module, decl_error,
 	weights::Weight,
 };
-use crate::initializer::SessionChangeNotification;
+use sp_std::vec::Vec;
+
+use rand::{SeedableRng, seq::SliceRandom};
+use rand_chacha::ChaCha20Rng;
+
+use crate::configuration::HostConfiguration;
 
 pub trait Config: frame_system::Config { }
 
@@ -37,6 +42,12 @@ decl_storage! {
 	trait Store for Module<T: Config> as ParasShared {
 		/// The current session index.
 		CurrentSessionIndex get(fn session_index): SessionIndex;
+		/// All the validators actively participating in parachain consensus.
+		/// Indices are into the broader validator set.
+		ActiveValidatorIndices get(fn active_validator_indices): Vec<ValidatorIndex>;
+		/// The parachain attestation keys of the validators actively participating in parachain consensus.
+		/// This should be the same length as `ActiveValidatorIndices`.
+		ActiveValidatorKeys get(fn active_validator_keys): Vec<ValidatorId>;
 	}
 }
 
@@ -63,8 +74,35 @@ impl<T: Config> Module<T> {
 	/// Called by the initializer to note that a new session has started.
 	///
 	/// Returns the list of outgoing paras from the actions queue.
-	pub(crate) fn initializer_on_new_session(notification: &SessionChangeNotification<T::BlockNumber>) {
-		CurrentSessionIndex::set(notification.session_index);
+	pub(crate) fn initializer_on_new_session(
+		session_index: SessionIndex,
+		random_seed: [u8; 32],
+		new_config: &HostConfiguration<T::BlockNumber>,
+		all_validators: Vec<ValidatorId>,
+	) -> Vec<ValidatorId> {
+		CurrentSessionIndex::set(session_index);
+		let mut rng: ChaCha20Rng = SeedableRng::from_seed(random_seed);
+
+		let mut shuffled_indices: Vec<_> = (0..all_validators.len())
+			.enumerate()
+			.map(|(i, _)| ValidatorIndex(i as _))
+			.collect();
+
+		shuffled_indices.shuffle(&mut rng);
+
+		if let Some(max) = new_config.max_validators {
+			shuffled_indices.truncate(max as usize);
+		}
+
+		let active_validator_keys = crate::util::take_active_subset(
+			&shuffled_indices,
+			&all_validators,
+		);
+
+		ActiveValidatorIndices::set(shuffled_indices);
+		ActiveValidatorKeys::set(active_validator_keys.clone());
+
+		active_validator_keys
 	}
 
 	/// Return the session index that should be used for any future scheduled changes.
@@ -76,4 +114,122 @@ impl<T: Config> Module<T> {
 	pub(crate) fn set_session_index(index: SessionIndex) {
 		CurrentSessionIndex::set(index);
 	}
+
+	#[cfg(test)]
+	pub(crate) fn set_active_validators(active: Vec<ValidatorId>) {
+		ActiveValidatorIndices::set(
+			(0..active.len()).map(|i| ValidatorIndex(i as _)).collect()
+		);
+		ActiveValidatorKeys::set(active);
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+	use crate::configuration::HostConfiguration;
+	use crate::mock::{new_test_ext, MockGenesisConfig, Shared};
+	use keyring::Sr25519Keyring;
+
+	fn validator_pubkeys(val_ids: &[Sr25519Keyring]) -> Vec<ValidatorId> {
+		val_ids.iter().map(|v| v.public().into()).collect()
+	}
+
+	#[test]
+	fn sets_and_shuffles_validators() {
+		let validators = vec![
+			Sr25519Keyring::Alice,
+			Sr25519Keyring::Bob,
+			Sr25519Keyring::Charlie,
+			Sr25519Keyring::Dave,
+			Sr25519Keyring::Ferdie,
+		];
+
+		let mut config = HostConfiguration::default();
+		config.max_validators = None;
+
+		let pubkeys = validator_pubkeys(&validators);
+
+		new_test_ext(MockGenesisConfig::default()).execute_with(|| {
+			let validators = Shared::initializer_on_new_session(
+				1,
+				[1; 32],
+				&config,
+				pubkeys,
+			);
+
+			assert_eq!(
+				validators,
+				validator_pubkeys(&[
+					Sr25519Keyring::Ferdie,
+					Sr25519Keyring::Bob,
+					Sr25519Keyring::Charlie,
+					Sr25519Keyring::Dave,
+					Sr25519Keyring::Alice,
+				])
+			);
+
+			assert_eq!(
+				Shared::active_validator_keys(),
+				validators,
+			);
+
+			assert_eq!(
+				Shared::active_validator_indices(),
+				vec![
+					ValidatorIndex(4),
+					ValidatorIndex(1),
+					ValidatorIndex(2),
+					ValidatorIndex(3),
+					ValidatorIndex(0),
+				]
+			);
+		});
+	}
+
+	#[test]
+	fn sets_truncates_and_shuffles_validators() {
+		let validators = vec![
+			Sr25519Keyring::Alice,
+			Sr25519Keyring::Bob,
+			Sr25519Keyring::Charlie,
+			Sr25519Keyring::Dave,
+			Sr25519Keyring::Ferdie,
+		];
+
+		let mut config = HostConfiguration::default();
+		config.max_validators = Some(2);
+
+		let pubkeys = validator_pubkeys(&validators);
+
+		new_test_ext(MockGenesisConfig::default()).execute_with(|| {
+			let validators = Shared::initializer_on_new_session(
+				1,
+				[1; 32],
+				&config,
+				pubkeys,
+			);
+
+			assert_eq!(
+				validators,
+				validator_pubkeys(&[
+					Sr25519Keyring::Ferdie,
+					Sr25519Keyring::Bob,
+				])
+			);
+
+			assert_eq!(
+				Shared::active_validator_keys(),
+				validators,
+			);
+
+			assert_eq!(
+				Shared::active_validator_indices(),
+				vec![
+					ValidatorIndex(4),
+					ValidatorIndex(1),
+				]
+			);
+		});
+	}
 }