Adds integration test for slashed/chilled validator with subsequent validation intention (#13717)

* Adds integration test for slashed/chilled validator with subsequent validation intention

* Removes unecessary comment

* Update frame/election-provider-multi-phase/test-staking-e2e/src/lib.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Update frame/election-provider-multi-phase/test-staking-e2e/src/lib.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Addresses PR review comments

* Fixes after conflict resolved

---------

Co-authored-by: parity-processbot <>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

substrate/frame/election-provider-multi-phase/test-staking-e2e/src/lib.rs

@@ -20,6 +20,7 @@ mod mock;
 
 pub(crate) const LOG_TARGET: &str = "tests::e2e-epm";
 
+use frame_support::assert_ok;
 use mock::*;
 use sp_core::Get;
 use sp_npos_elections::{to_supports, StakedAssignment};
@@ -204,3 +205,72 @@ fn continous_slashes_below_offending_threshold() {
 			}
 		});
 }
+
+#[test]
+/// When validators are slashed, they are chilled and removed from the current `VoterList`. Thus,
+/// the slashed validator should not be considered in the next validator set. However, if the
+/// slashed validator sets its intention to validate again in the same era when it was slashed and
+/// chilled, the validator may not be removed from the active validator set across eras, provided
+/// it would selected in the subsequent era if there was no slash. Nominators of the slashed
+/// validator will also be slashed and chilled, as expected, but the nomination intentions will
+/// remain after the validator re-set the intention to be validating again.
+///
+/// This behaviour is due to removing implicit chill upon slash
+/// <https://github.com/paritytech/substrate/pull/12420>.
+///
+/// Related to <https://github.com/paritytech/substrate/issues/13714>.
+fn set_validation_intention_after_chilled() {
+	use frame_election_provider_support::SortedListProvider;
+	use pallet_staking::{Event, Forcing, Nominators};
+
+	let staking_builder = StakingExtBuilder::default();
+	let epm_builder = EpmExtBuilder::default();
+
+	ExtBuilder::default()
+		.staking(staking_builder)
+		.epm(epm_builder)
+		.build_and_execute(|| {
+			assert_eq!(active_era(), 0);
+			// validator is part of the validator set.
+			assert!(Session::validators().contains(&81));
+			assert!(<Runtime as pallet_staking::Config>::VoterList::contains(&81));
+
+			// nominate validator 81.
+			assert_ok!(Staking::nominate(RuntimeOrigin::signed(21), vec![81]));
+			assert_eq!(Nominators::<Runtime>::get(21).unwrap().targets, vec![81]);
+
+			// validator is slashed. it is removed from the `VoterList` through chilling but in the
+			// current era, the validator is still part of the active validator set.
+			add_slash(&81);
+			assert!(Session::validators().contains(&81));
+			assert!(!<Runtime as pallet_staking::Config>::VoterList::contains(&81));
+			assert_eq!(
+				staking_events(),
+				[
+					Event::Chilled { stash: 81 },
+					Event::ForceEra { mode: Forcing::ForceNew },
+					Event::SlashReported {
+						validator: 81,
+						slash_era: 0,
+						fraction: Perbill::from_percent(10)
+					}
+				],
+			);
+
+			// after the nominator is slashed and chilled, the nominations remain.
+			assert_eq!(Nominators::<Runtime>::get(21).unwrap().targets, vec![81]);
+
+			// validator sets intention to stake again in the same era it was chilled.
+			assert_ok!(Staking::validate(RuntimeOrigin::signed(81), Default::default()));
+
+			// progress era and check that the slashed validator is still part of the validator
+			// set.
+			assert!(start_next_active_era().is_ok());
+			assert_eq!(active_era(), 1);
+			assert!(Session::validators().contains(&81));
+			assert!(<Runtime as pallet_staking::Config>::VoterList::contains(&81));
+
+			// nominations are still active as before the slash.
+			assert_eq!(Nominators::<Runtime>::get(21).unwrap().targets, vec![81]);
+		})
+}