Move SessionInterface to sp-session and impl it in session pallet

Ref #7127 (comment)

bin/node/runtime/src/lib.rs

@@ -759,7 +759,7 @@ impl<C> frame_system::offchain::SendTransactionTypes<C> for Runtime where
 impl pallet_im_online::Trait for Runtime {
 	type AuthorityId = ImOnlineId;
 	type Event = Event;
-	type SessionInterface = Self;
+	type SessionInterface = Session;
 	type SessionDuration = SessionDuration;
 	type ReportUnresponsiveness = Offences;
 	type UnsignedPriority = ImOnlineUnsignedPriority;

frame/im-online/Cargo.toml

@@ -23,6 +23,7 @@ pallet-session = { version = "2.0.0", default-features = false, path = "../sessi
 sp-io = { version = "2.0.0", default-features = false, path = "../../primitives/io" }
 sp-runtime = { version = "2.0.0", default-features = false, path = "../../primitives/runtime" }
 sp-staking = { version = "2.0.0", default-features = false, path = "../../primitives/staking" }
+sp-session = { version = "2.0.0", default-features = false, path = "../../primitives/session" }
 frame-support = { version = "2.0.0", default-features = false, path = "../support" }
 frame-system = { version = "2.0.0", default-features = false, path = "../system" }
 
@@ -41,6 +42,7 @@ std = [
 	"sp-io/std",
 	"sp-runtime/std",
 	"sp-staking/std",
+	"sp-session/std",
 	"frame-support/std",
 	"frame-system/std",
 ]

frame/im-online/src/lib.rs

@@ -92,6 +92,7 @@ use sp_staking::{
 	SessionIndex,
 	offence::{ReportOffence, Offence, Kind},
 };
+use sp_session::SessionInterface;
 use frame_support::{
 	decl_module, decl_event, decl_storage, Parameter, debug, decl_error,
 	traits::Get,
@@ -231,34 +232,6 @@ pub trait WeightInfo {
 	fn validate_unsigned_and_then_heartbeat(k: u32, e: u32, ) -> Weight;
 }
 
-/// Trait for retrieving the session info needed for online node inspection.
-///
-/// This trait is used for decouple the pallet-session dependency from im-online
-/// module so that the user of im-online & offences modules can pass any list of
-/// validators that are considered to be online in each session, particularly useful
-/// for the Substrate-based projects having their own staking implementation
-/// instead of using pallet-staking directly.
-pub trait SessionInterface<ValidatorId> {
-	/// Returns current session index.
-	fn current_index() -> SessionIndex;
-
-	/// Returns all the validators ought to be online in a session.
-	///
-	/// The returned validators are all expected to be running an authority node.
-	fn validators() -> Vec<ValidatorId>;
-}
-
-impl<T: pallet_session::Trait>
-	SessionInterface<<Self as ValidatorIdentification<<Self as frame_system::Trait>::AccountId>>::ValidatorId> for T
-{
-	fn current_index() -> sp_staking::SessionIndex {
-		pallet_session::Module::<T>::current_index()
-	}
-	fn validators() -> Vec<<Self as ValidatorIdentification<<Self as frame_system::Trait>::AccountId>>::ValidatorId> {
-		pallet_session::Module::<T>::validators()
-	}
-}
-
 pub trait Trait:
 	SendTransactionTypes<Call<Self>>
 	+ ValidatorIdentification<<Self as frame_system::Trait>::AccountId>

frame/im-online/src/mock.rs

@@ -185,7 +185,7 @@ impl Trait for Runtime {
 	type AuthorityId = UintAuthorityId;
 	type Event = ();
 	type ReportUnresponsiveness = OffenceHandler;
-	type SessionInterface = Self;
+	type SessionInterface = Session;
 	type SessionDuration = Period;
 	type UnsignedPriority = UnsignedPriority;
 	type WeightInfo = ();

frame/offences/benchmarking/src/mock.rs

@@ -190,7 +190,7 @@ impl pallet_staking::Trait for Test {
 impl pallet_im_online::Trait for Test {
 	type AuthorityId = UintAuthorityId;
 	type Event = Event;
-	type SessionInterface = Self;
+	type SessionInterface = Session;
 	type SessionDuration = Period;
 	type ReportUnresponsiveness = Offences;
 	type UnsignedPriority = ();

frame/session/src/lib.rs

@@ -826,3 +826,16 @@ impl<T: Trait> EstimateNextNewSession<T::BlockNumber> for Module<T> {
 		T::NextSessionRotation::weight(now)
 	}
 }
+
+impl<T: Trait>
+	sp_session::SessionInterface<
+		<T as ValidatorIdentification<<T as frame_system::Trait>::AccountId>>::ValidatorId
+	> for Module<T>
+{
+	fn current_index() -> sp_staking::SessionIndex {
+		Module::<T>::current_index()
+	}
+	fn validators() -> Vec<<T as ValidatorIdentification<<T as frame_system::Trait>::AccountId>>::ValidatorId> {
+		Module::<T>::validators()
+	}
+}

primitives/session/src/lib.rs

@@ -106,6 +106,23 @@ impl GetValidatorCount for MembershipProof {
 	}
 }
 
+/// Trait for retrieving the session info needed for online node inspection.
+///
+/// This trait is used for decouple the pallet-session dependency from im-online
+/// module so that the user of im-online & offences modules can pass any list of
+/// validators that are considered to be online in each session, particularly useful
+/// for the Substrate-based projects having their own staking implementation
+/// instead of using pallet-staking directly.
+pub trait SessionInterface<ValidatorId> {
+	/// Returns current session index.
+	fn current_index() -> SessionIndex;
+
+	/// Returns all the validators ought to be online in a session.
+	///
+	/// The returned validators are all expected to be running an authority node.
+	fn validators() -> Vec<ValidatorId>;
+}
+
 /// Generate the initial session keys with the given seeds, at the given block and store them in
 /// the client's keystore.
 #[cfg(feature = "std")]