Fix collators not producing blocks when selected candidates is zero

pallets/parachain-staking/src/lib.rs

@@ -1716,7 +1716,9 @@ pub mod pallet {
 		}
 
 		/// Compute the top `TotalSelected` candidates in the CandidatePool and return
-		/// a vec of their AccountIds (sorted by AccountId)
+		/// a vec of their AccountIds (sorted by AccountId).
+		///
+		/// If the returned vec is empty, the previous candidates should be used.
 		pub fn compute_top_candidates() -> Vec<T::AccountId> {
 			let top_n = <TotalSelected<T>>::get() as usize;
 			if top_n == 0 {

runtime/common/src/apis.rs

@@ -420,10 +420,17 @@ macro_rules! impl_runtime_apis_plus_common {
 							// return false if author mapping not registered like in can_author impl
 							return false
 						};
+						let candidates = pallet_parachain_staking::Pallet::<Self>::compute_top_candidates();
+						if candidates.is_empty() {
+							// If there are zero selected candidates, we use the same eligibility
+							// as the previous round
+							return AuthorInherent::can_author(&author, &slot);
+						}
+
 						// predict eligibility post-selection by computing selection results now
 						let (eligible, _) =
 							pallet_author_slot_filter::compute_pseudo_random_subset::<Self>(
-								pallet_parachain_staking::Pallet::<Self>::compute_top_candidates(),
+								candidates,
 								&slot
 							);
 						eligible.contains(&author_account_id)

runtime/moonbase/tests/common/mod.rs

@@ -27,9 +27,9 @@ use moonbase_runtime::{asset_config::AssetRegistrarMetadata, xcm_config::AssetTy
 pub use moonbase_runtime::{
 	currency::{GIGAWEI, SUPPLY_FACTOR, UNIT, WEI},
 	AccountId, AssetId, AssetManager, Assets, AuthorInherent, Balance, Balances, CrowdloanRewards,
-	Ethereum, Executive, InflationInfo, LocalAssets, ParachainStaking, Range, Runtime, RuntimeCall,
-	RuntimeEvent, System, TransactionConverter, TransactionPaymentAsGasPrice, UncheckedExtrinsic,
-	HOURS, WEEKS,
+	Ethereum, Executive, Header, InflationInfo, LocalAssets, ParachainStaking, Range, Runtime,
+	RuntimeCall, RuntimeEvent, System, TransactionConverter, TransactionPaymentAsGasPrice,
+	UncheckedExtrinsic, HOURS, WEEKS,
 };
 use nimbus_primitives::{NimbusId, NIMBUS_ENGINE_ID};
 use sp_core::{Encode, H160};

runtime/moonbase/tests/runtime_apis.rs

@@ -20,12 +20,14 @@ mod common;
 use common::*;
 
 use fp_evm::GenesisAccount;
+use frame_support::assert_ok;
 use nimbus_primitives::NimbusId;
 use pallet_evm::{Account as EVMAccount, AddressMapping, FeeCalculator};
 use sp_core::{ByteArray, H160, H256, U256};
 
 use fp_rpc::runtime_decl_for_EthereumRuntimeRPCApi::EthereumRuntimeRPCApi;
 use moonbeam_rpc_primitives_txpool::runtime_decl_for_TxPoolRuntimeApi::TxPoolRuntimeApi;
+use nimbus_primitives::runtime_decl_for_NimbusApi::NimbusApi;
 use std::{collections::BTreeMap, str::FromStr};
 
 #[test]
@@ -298,6 +300,95 @@ fn txpool_runtime_api_extrinsic_filter() {
 	});
 }
 
+#[test]
+fn can_author_when_selected_is_empty() {
+	ExtBuilder::default()
+		.with_balances(vec![
+			(AccountId::from(ALICE), 20_000_000 * UNIT),
+			(AccountId::from(BOB), 10_000_000 * UNIT),
+		])
+		.with_collators(vec![(AccountId::from(ALICE), 2_000_000 * UNIT)])
+		.with_mappings(vec![(
+			NimbusId::from_slice(&ALICE_NIMBUS).unwrap(),
+			AccountId::from(ALICE),
+		)])
+		.build()
+		.execute_with(|| {
+			set_parachain_inherent_data();
+			run_to_block(2, Some(NimbusId::from_slice(&ALICE_NIMBUS).unwrap()));
+
+			assert_eq!(ParachainStaking::candidate_pool().0.len(), 1);
+
+			let slot_number = 0;
+			let parent = Header {
+				digest: Default::default(),
+				extrinsics_root: Default::default(),
+				number: Default::default(),
+				parent_hash: Default::default(),
+				state_root: Default::default(),
+			};
+
+			// Base case: ALICE can author blocks when she is the only candidate
+			let can_author_block = Runtime::can_author(
+				NimbusId::from_slice(&ALICE_NIMBUS).unwrap(),
+				slot_number,
+				&parent,
+			);
+
+			assert!(can_author_block);
+
+			// Remove ALICE from candidate pool, leaving the candidate_pool empty
+			assert_ok!(ParachainStaking::go_offline(origin_of(AccountId::from(
+				ALICE
+			))));
+
+			// Need to fast forward to right before the next session, which is when selected candidates
+			// will be updated. We want to test the creation of the first block of the next session.
+			run_to_block(1799, Some(NimbusId::from_slice(&ALICE_NIMBUS).unwrap()));
+
+			assert_eq!(ParachainStaking::candidate_pool().0.len(), 0);
+
+			let slot_number = 0;
+			let parent = Header {
+				digest: Default::default(),
+				extrinsics_root: Default::default(),
+				number: 1799,
+				parent_hash: Default::default(),
+				state_root: Default::default(),
+			};
+
+			let can_author_block = Runtime::can_author(
+				NimbusId::from_slice(&ALICE_NIMBUS).unwrap(),
+				slot_number,
+				&parent,
+			);
+
+			assert!(can_author_block);
+
+			// Check that it works as expected after session update
+			run_to_block(1800, Some(NimbusId::from_slice(&ALICE_NIMBUS).unwrap()));
+
+			assert_eq!(ParachainStaking::candidate_pool().0.len(), 0);
+
+			let slot_number = 0;
+			let parent = Header {
+				digest: Default::default(),
+				extrinsics_root: Default::default(),
+				number: 1800,
+				parent_hash: Default::default(),
+				state_root: Default::default(),
+			};
+
+			let can_author_block = Runtime::can_author(
+				NimbusId::from_slice(&ALICE_NIMBUS).unwrap(),
+				slot_number,
+				&parent,
+			);
+
+			assert!(can_author_block);
+		});
+}
+
 // Some Priority-related test ideas
 // 1. Eth balance transfer with various gas prices. Priority == gas price
 // 2. Eth contract call with various gas prices. Priority == gas price

runtime/moonbeam/tests/common/mod.rs

@@ -28,9 +28,9 @@ pub use moonbeam_runtime::{
 	currency::{GIGAWEI, GLMR, SUPPLY_FACTOR, WEI},
 	xcm_config::AssetType,
 	AccountId, AssetId, AssetManager, Assets, AuthorInherent, Balance, Balances, CrowdloanRewards,
-	Ethereum, Executive, InflationInfo, LocalAssets, ParachainStaking, Range, Runtime, RuntimeCall,
-	RuntimeEvent, System, TransactionConverter, TransactionPaymentAsGasPrice, UncheckedExtrinsic,
-	HOURS, WEEKS,
+	Ethereum, Executive, Header, InflationInfo, LocalAssets, ParachainStaking, Range, Runtime,
+	RuntimeCall, RuntimeEvent, System, TransactionConverter, TransactionPaymentAsGasPrice,
+	UncheckedExtrinsic, HOURS, WEEKS,
 };
 use nimbus_primitives::{NimbusId, NIMBUS_ENGINE_ID};
 use sp_core::{Encode, H160};
@@ -132,7 +132,7 @@ pub struct ExtBuilder {
 	delegations: Vec<(AccountId, AccountId, Balance, Percent)>,
 	// per-round inflation config
 	inflation: InflationInfo<Balance>,
-	// AuthorId -> AccoutId mappings
+	// AuthorId -> AccountId mappings
 	mappings: Vec<(NimbusId, AccountId)>,
 	// Crowdloan fund
 	crowdloan_fund: Balance,

runtime/moonbeam/tests/runtime_apis.rs

@@ -20,12 +20,14 @@ mod common;
 use common::*;
 
 use fp_evm::GenesisAccount;
+use frame_support::assert_ok;
 use nimbus_primitives::NimbusId;
 use pallet_evm::{Account as EVMAccount, AddressMapping, FeeCalculator};
 use sp_core::{ByteArray, H160, H256, U256};
 
 use fp_rpc::runtime_decl_for_EthereumRuntimeRPCApi::EthereumRuntimeRPCApi;
 use moonbeam_rpc_primitives_txpool::runtime_decl_for_TxPoolRuntimeApi::TxPoolRuntimeApi;
+use nimbus_primitives::runtime_decl_for_NimbusApi::NimbusApi;
 use std::{collections::BTreeMap, str::FromStr};
 
 #[test]
@@ -300,3 +302,92 @@ fn txpool_runtime_api_extrinsic_filter() {
 		assert_eq!(txpool.future.len(), 1);
 	});
 }
+
+#[test]
+fn can_author_when_selected_is_empty() {
+	ExtBuilder::default()
+		.with_balances(vec![
+			(AccountId::from(ALICE), 20_000_000 * GLMR),
+			(AccountId::from(BOB), 10_000_000 * GLMR),
+		])
+		.with_collators(vec![(AccountId::from(ALICE), 2_000_000 * GLMR)])
+		.with_mappings(vec![(
+			NimbusId::from_slice(&ALICE_NIMBUS).unwrap(),
+			AccountId::from(ALICE),
+		)])
+		.build()
+		.execute_with(|| {
+			set_parachain_inherent_data();
+			run_to_block(2, Some(NimbusId::from_slice(&ALICE_NIMBUS).unwrap()));
+
+			assert_eq!(ParachainStaking::candidate_pool().0.len(), 1);
+
+			let slot_number = 0;
+			let parent = Header {
+				digest: Default::default(),
+				extrinsics_root: Default::default(),
+				number: Default::default(),
+				parent_hash: Default::default(),
+				state_root: Default::default(),
+			};
+
+			// Base case: ALICE can author blocks when she is the only candidate
+			let can_author_block = Runtime::can_author(
+				NimbusId::from_slice(&ALICE_NIMBUS).unwrap(),
+				slot_number,
+				&parent,
+			);
+
+			assert!(can_author_block);
+
+			// Remove ALICE from candidate pool, leaving the candidate_pool empty
+			assert_ok!(ParachainStaking::go_offline(origin_of(AccountId::from(
+				ALICE
+			))));
+
+			// Need to fast forward to right before the next session, which is when selected candidates
+			// will be updated. We want to test the creation of the first block of the next session.
+			run_to_block(1799, Some(NimbusId::from_slice(&ALICE_NIMBUS).unwrap()));
+
+			assert_eq!(ParachainStaking::candidate_pool().0.len(), 0);
+
+			let slot_number = 0;
+			let parent = Header {
+				digest: Default::default(),
+				extrinsics_root: Default::default(),
+				number: 1799,
+				parent_hash: Default::default(),
+				state_root: Default::default(),
+			};
+
+			let can_author_block = Runtime::can_author(
+				NimbusId::from_slice(&ALICE_NIMBUS).unwrap(),
+				slot_number,
+				&parent,
+			);
+
+			assert!(can_author_block);
+
+			// Check that it works as expected after session update
+			run_to_block(1800, Some(NimbusId::from_slice(&ALICE_NIMBUS).unwrap()));
+
+			assert_eq!(ParachainStaking::candidate_pool().0.len(), 0);
+
+			let slot_number = 0;
+			let parent = Header {
+				digest: Default::default(),
+				extrinsics_root: Default::default(),
+				number: 1800,
+				parent_hash: Default::default(),
+				state_root: Default::default(),
+			};
+
+			let can_author_block = Runtime::can_author(
+				NimbusId::from_slice(&ALICE_NIMBUS).unwrap(),
+				slot_number,
+				&parent,
+			);
+
+			assert!(can_author_block);
+		});
+}

runtime/moonriver/tests/common/mod.rs

@@ -28,9 +28,9 @@ pub use moonriver_runtime::{
 	currency::{GIGAWEI, MOVR, SUPPLY_FACTOR, WEI},
 	xcm_config::AssetType,
 	AccountId, AssetId, AssetManager, Assets, AuthorInherent, Balance, Balances, CrowdloanRewards,
-	Ethereum, Executive, InflationInfo, LocalAssets, ParachainStaking, Range, Runtime, RuntimeCall,
-	RuntimeEvent, System, TransactionConverter, TransactionPaymentAsGasPrice, UncheckedExtrinsic,
-	HOURS, WEEKS,
+	Ethereum, Executive, Header, InflationInfo, LocalAssets, ParachainStaking, Range, Runtime,
+	RuntimeCall, RuntimeEvent, System, TransactionConverter, TransactionPaymentAsGasPrice,
+	UncheckedExtrinsic, HOURS, WEEKS,
 };
 use nimbus_primitives::{NimbusId, NIMBUS_ENGINE_ID};
 use sp_core::{Encode, H160};