[Staking] Fix LastValidatorEra false positive

prdoc/pr_11647.prdoc

@@ -0,0 +1,15 @@
+title: 'Fix try-state warning for LastValidatorEra in staking-async'
+doc:
+- audience: Runtime Dev
+  description: |-
+    Fixes a false try-state warning where `LastValidatorEra` was flagged as incorrect for active
+    validators. After the election for the next era completes but before that era becomes active,
+    `LastValidatorEra` is correctly set to `active_era + 1`. The previous check only accepted
+    `active_era`, causing spurious warnings.
+
+    Adds a test verifying `LastValidatorEra` transitions from `active_era` to `active_era + 1`
+    once the next era's election results are stored.
+
+crates:
+- name: pallet-staking-async
+  bump: patch

substrate/frame/staking-async/src/pallet/impls.rs

@@ -2011,14 +2011,16 @@ impl<T: Config> Pallet<T> {
 		let Some(era) = ActiveEra::<T>::get().map(|a| a.index) else { return Ok(()) };
 		let overview_and_pages = ErasStakersOverview::<T>::iter_prefix(era)
 			.map(|(validator, metadata)| {
-				// ensure `LastValidatorEra` is correctly set
-				if LastValidatorEra::<T>::get(&validator) != Some(era) {
+				let last_validator_era = LastValidatorEra::<T>::get(&validator).unwrap_or_default();
+				// If election for next era is finished, last_validator_era is set to next era.
+				if last_validator_era != era && last_validator_era != era + 1 {
 					log!(
-						warn,
-						"Validator {:?} has incorrect LastValidatorEra (expected {:?}, got {:?})",
+						error,
+						"Validator {:?} has incorrect LastValidatorEra (expected {:?} or {:?}, got {:?})",
 						validator,
 						era,
-						LastValidatorEra::<T>::get(&validator)
+						era + 1,
+						last_validator_era
 					);
 				}
 

substrate/frame/staking-async/src/tests/try_state.rs

@@ -88,6 +88,36 @@ fn try_state_bad_exposure() {
 	});
 }
 
+#[test]
+fn last_validator_era_can_be_one_greater_than_active_era() {
+	// When the election for the next era has finished but the era is not yet active,
+	// `LastValidatorEra` is set to `active_era + 1`.
+	ExtBuilder::default().try_state(false).build_and_execute(|| {
+		Session::roll_until_active_era(1);
+		let era = active_era();
+
+		// Before election, `LastValidatorEra` equals the active era.
+		for (validator, _) in ErasStakersOverview::<T>::iter_prefix(era) {
+			assert_eq!(LastValidatorEra::<T>::get(&validator), Some(era));
+		}
+
+		// Roll session by session until the election for the next era has been stored, i.e.
+		// `ErasStakersOverview` for the next era is populated.
+		while ErasStakersOverview::<T>::iter_prefix(era + 1).next().is_none() {
+			Session::roll_to_next_session();
+		}
+
+		// Election for era 2 is stored but era 2 is not yet active.
+		assert_eq!(active_era(), 1);
+		assert_eq!(current_era(), 2);
+
+		// After election, `LastValidatorEra` is now 1 greater than active era.
+		for (validator, _) in ErasStakersOverview::<T>::iter_prefix(era) {
+			assert_eq!(LastValidatorEra::<T>::get(&validator), Some(era + 1));
+		}
+	});
+}
+
 #[test]
 fn try_state_bad_eras_total_stake() {
 	ExtBuilder::default().try_state(false).build_and_execute(|| {