srml: staking: track session index of current era start (#3203)

* srml: staking: track session index of current era start

* node: bump spec_version

substrate/node/runtime/src/lib.rs

@@ -79,8 +79,8 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	// and set impl_version to equal spec_version. If only runtime
 	// implementation changes and behavior does not, then leave spec_version as
 	// is and increment impl_version.
-	spec_version: 120,
-	impl_version: 120,
+	spec_version: 121,
+	impl_version: 121,
 	apis: RUNTIME_API_VERSIONS,
 };
 

substrate/srml/staking/src/lib.rs

@@ -590,6 +590,9 @@ decl_storage! {
 		/// The start of the current era.
 		pub CurrentEraStart get(current_era_start): MomentOf<T>;
 
+		/// The session index at which the current era started.
+		pub CurrentEraStartSessionIndex get(current_era_start_session_index): SessionIndex;
+
 		/// Rewards for the current era. Using indices of current elected set.
 		pub CurrentEraRewards: EraRewards;
 
@@ -1169,6 +1172,9 @@ impl<T: Trait> Module<T> {
 
 		// Increment current era.
 		let current_era = CurrentEra::mutate(|s| { *s += 1; *s });
+		CurrentEraStartSessionIndex::mutate(|v| {
+			*v = start_session_index;
+		});
 		let bonding_duration = T::BondingDuration::get();
 
 		if current_era > bonding_duration {