Decouple the session validators from im-online

bin/node/runtime/src/lib.rs

@@ -744,9 +744,16 @@ impl<C> frame_system::offchain::SendTransactionTypes<C> for Runtime where
 	type OverarchingCall = Call;
 }
 
+impl pallet_im_online::ValidatorSet<<Self as pallet_session::Trait>::ValidatorId> for Runtime {
+	fn validators() -> Vec<<Self as pallet_session::Trait>::ValidatorId> {
+		Session::validators()
+	}
+}
+
 impl pallet_im_online::Trait for Runtime {
 	type AuthorityId = ImOnlineId;
 	type Event = Event;
+	type ValidatorSet = Self;
 	type SessionDuration = SessionDuration;
 	type ReportUnresponsiveness = Offences;
 	type UnsignedPriority = ImOnlineUnsignedPriority;

frame/im-online/src/lib.rs

@@ -231,6 +231,12 @@ pub trait WeightInfo {
 	fn validate_unsigned_and_then_heartbeat(k: u32, e: u32, ) -> Weight;
 }
 
+/// The set of validators that are considered to be running an authority node.
+pub trait ValidatorSet<ValidatorId> {
+	/// Returns all the validators ought to be online.
+	fn validators() -> Vec<ValidatorId>;
+}
+
 pub trait Trait: SendTransactionTypes<Call<Self>> + pallet_session::historical::Trait {
 	/// The identifier type for an authority.
 	type AuthorityId: Member + Parameter + RuntimeAppPublic + Default + Ord;
@@ -246,6 +252,9 @@ pub trait Trait: SendTransactionTypes<Call<Self>> + pallet_session::historical::
 	/// there is a chance the authority will produce a block and they won't be necessary.
 	type SessionDuration: Get<Self::BlockNumber>;
 
+	/// A set of validators expected to be online.
+	type ValidatorSet: ValidatorSet<<Self as pallet_session::Trait>::ValidatorId>;
+
 	/// A type that gives us the ability to submit unresponsiveness offence reports.
 	type ReportUnresponsiveness:
 		ReportOffence<
@@ -417,7 +426,7 @@ impl<T: Trait> Module<T> {
 	/// authored at least one block, during the current session. Otherwise
 	/// `false`.
 	pub fn is_online(authority_index: AuthIndex) -> bool {
-		let current_validators = <pallet_session::Module<T>>::validators();
+		let current_validators = T::ValidatorSet::validators();
 
 		if authority_index >= current_validators.len() as u32 {
 			return false;
@@ -465,7 +474,7 @@ impl<T: Trait> Module<T> {
 		}
 
 		let session_index = <pallet_session::Module<T>>::current_index();
-		let validators_len = <pallet_session::Module<T>>::validators().len() as u32;
+		let validators_len = T::ValidatorSet::validators().len() as u32;
 
 		Ok(Self::local_authority_keys()
 			.map(move |(authority_index, key)|
@@ -645,7 +654,7 @@ impl<T: Trait> pallet_session::OneSessionHandler<T::AccountId> for Module<T> {
 	fn on_before_session_ending() {
 		let session_index = <pallet_session::Module<T>>::current_index();
 		let keys = Keys::<T>::get();
-		let current_validators = <pallet_session::Module<T>>::validators();
+		let current_validators = T::ValidatorSet::validators();
 
 		let offenders = current_validators.into_iter().enumerate()
 			.filter(|(index, id)|

frame/im-online/src/mock.rs

@@ -21,7 +21,7 @@
 
 use std::cell::RefCell;
 
-use crate::{Module, Trait};
+use crate::{Module, Trait, ValidatorSet};
 use sp_runtime::Perbill;
 use sp_staking::{SessionIndex, offence::{ReportOffence, OffenceError}};
 use sp_runtime::testing::{Header, UintAuthorityId, TestXt};
@@ -179,10 +179,17 @@ parameter_types! {
 	pub const UnsignedPriority: u64 = 1 << 20;
 }
 
+impl ValidatorSet<<Self as pallet_session::Trait>::ValidatorId> for Runtime {
+	fn validators() -> Vec<<Self as pallet_session::Trait>::ValidatorId> {
+		Session::validators()
+	}
+}
+
 impl Trait for Runtime {
 	type AuthorityId = UintAuthorityId;
 	type Event = ();
 	type ReportUnresponsiveness = OffenceHandler;
+	type ValidatorSet = Self;
 	type SessionDuration = Period;
 	type UnsignedPriority = UnsignedPriority;
 	type WeightInfo = ();

frame/offences/benchmarking/src/mock.rs

@@ -186,9 +186,16 @@ impl pallet_staking::Trait for Test {
 	type WeightInfo = ();
 }
 
+impl pallet_im_online::ValidatorSet<<Self as pallet_session::Trait>::ValidatorId> for Test {
+	fn validators() -> Vec<<Self as pallet_session::Trait>::ValidatorId> {
+		Session::validators()
+	}
+}
+
 impl pallet_im_online::Trait for Test {
 	type AuthorityId = UintAuthorityId;
 	type Event = Event;
+	type ValidatorSet = Self;
 	type SessionDuration = Period;
 	type ReportUnresponsiveness = Offences;
 	type UnsignedPriority = ();