Override RuntimeApi::authorities() to include past sessions (#2494)

* override authorities runtime call

* addressing feedback

* addressing feedback and restoring semantics

* updated helper function

* naming convention + comment

Co-authored-by: Parity <parity2305@gmail.com>

polkadot/runtime/parachains/src/runtime_api_impl/v1.rs

@@ -25,11 +25,12 @@ use primitives::v1::{
 	Id as ParaId, OccupiedCoreAssumption, SessionIndex, ValidationCode,
 	CommittedCandidateReceipt, ScheduledCore, OccupiedCore, CoreOccupied, CoreIndex,
 	GroupIndex, CandidateEvent, PersistedValidationData, SessionInfo,
-	InboundDownwardMessage, InboundHrmpMessage, Hash,
+	InboundDownwardMessage, InboundHrmpMessage, Hash, AuthorityDiscoveryId
 };
 use frame_support::debug;
 use crate::{initializer, inclusion, scheduler, configuration, paras, session_info, dmp, hrmp, shared};
 
+
 /// Implementation for the `validators` function of the runtime API.
 pub fn validators<T: initializer::Config>() -> Vec<ValidatorId> {
 	<inclusion::Module<T>>::validators()
@@ -231,6 +232,27 @@ pub fn session_index_for_child<T: initializer::Config>() -> SessionIndex {
 	<shared::Module<T>>::session_index()
 }
 
+/// Implementation for the `AuthorityDiscoveryApi::authorities()` function of the runtime API.
+/// It is a heavy call, but currently only used for authority discovery, so it is fine.
+/// Gets next, current and some historical authority ids using session_info module.
+pub fn relevant_authority_ids<T: initializer::Config + pallet_authority_discovery::Config>() -> Vec<AuthorityDiscoveryId> {
+	let current_session_index = session_index_for_child::<T>();
+	let earliest_stored_session = <session_info::Module<T>>::earliest_stored_session();
+	let mut authority_ids = <pallet_authority_discovery::Module<T>>::next_authorities();
+
+	for session_index in earliest_stored_session..=current_session_index {
+		let info = <session_info::Module<T>>::session_info(session_index);
+		if let Some(mut info) = info {
+			authority_ids.append(&mut info.discovery_keys);
+		}
+	}
+
+	authority_ids.sort();
+	authority_ids.dedup();
+
+	authority_ids
+}
+
 /// Implementation for the `validation_code` function of the runtime API.
 pub fn validation_code<T: initializer::Config>(
 	para_id: ParaId,

polkadot/runtime/rococo/src/lib.rs

@@ -849,7 +849,7 @@ sp_api::impl_runtime_apis! {
 
 	impl authority_discovery_primitives::AuthorityDiscoveryApi<Block> for Runtime {
 		fn authorities() -> Vec<AuthorityDiscoveryId> {
-			AuthorityDiscovery::authorities()
+			runtime_api_impl::relevant_authority_ids::<Runtime>()
 		}
 	}