Staking: store minimum active stake on-chain

frame/staking/src/pallet/impls.rs

@@ -704,6 +704,8 @@ impl<T: Config> Pallet<T> {
 	///
 	/// `maybe_max_len` can imposes a cap on the number of voters returned;
 	///
+	/// Sets `LastMinimumActiveBond` to the minimum active bond in the returned set of nominators.
+	///
 	/// This function is self-weighing as [`DispatchClass::Mandatory`].
 	///
 	/// ### Slashing
@@ -727,6 +729,7 @@ impl<T: Config> Pallet<T> {
 		let mut nominators_taken = 0u32;
 
 		let mut sorted_voters = T::VoterList::iter();
+		let mut min_active_bond = None;
 		while all_voters.len() < max_allowed_len &&
 			voters_seen < (NPOS_MAX_ITERATIONS_COEFFICIENT * max_allowed_len as u32)
 		{
@@ -747,10 +750,19 @@ impl<T: Config> Pallet<T> {
 						.get(stash)
 						.map_or(true, |spans| submitted_in >= spans.last_nonzero_slash())
 				});
+				let voter_weight = weight_of(&voter);
 				if !targets.len().is_zero() {
-					all_voters.push((voter.clone(), weight_of(&voter), targets));
+					all_voters.push((voter.clone(), voter_weight, targets));
 					nominators_taken.saturating_inc();
 				}
+
+				min_active_bond = min_active_bond.map_or(Some(voter_weight), |current_min| {
+					if voter_weight < current_min {
+						Some(voter_weight)
+					} else {
+						Some(current_min)
+					}
+				});
 			} else if Validators::<T>::contains_key(&voter) {
 				// if this voter is a validator:
 				let self_vote = (
@@ -785,6 +797,10 @@ impl<T: Config> Pallet<T> {
 			slashing_spans.len() as u32,
 		));
 
+		LastMinimumActiveBond::<T>::put::<T::CurrencyBalance>(
+			min_active_bond.map_or(0, |v| v).into(),
+		);
+
 		log!(
 			info,
 			"generated {} npos voters, {} from validators and {} nominators",

frame/staking/src/pallet/mod.rs

@@ -295,6 +295,10 @@ pub mod pallet {
 	#[pallet::storage]
 	pub type MinValidatorBond<T: Config> = StorageValue<_, BalanceOf<T>, ValueQuery>;
 
+	/// The minimum active bond of the last successful election.
+	#[pallet::storage]
+	pub type LastMinimumActiveBond<T> = StorageValue<_, BalanceOf<T>, ValueQuery>;
+
 	/// The minimum amount of commission that validators can set.
 	///
 	/// If set to `0`, no limit exists.

frame/staking/src/tests.rs

@@ -4398,6 +4398,24 @@ mod election_data_provider {
 		);
 	}
 
+	#[test]
+	fn set_last_minimum_nominator_bond_is_correct() {
+		ExtBuilder::default()
+			.nominate(false)
+			.add_staker(61, 60, 2_000, StakerStatus::<AccountId>::Nominator(vec![21]))
+			.add_staker(71, 70, 10, StakerStatus::<AccountId>::Nominator(vec![21]))
+			.add_staker(81, 80, 50, StakerStatus::<AccountId>::Nominator(vec![21]))
+			.build_and_execute(|| {
+				assert_ok!(<Staking as ElectionDataProvider>::electing_voters(None));
+				assert_eq!(LastMinimumActiveBond::<Test>::get(), 10);
+
+				// remove staker with lower bond by limiting the number of voters and check
+				// `LastMinimumActiveBond` again after electing voters.
+				assert_ok!(<Staking as ElectionDataProvider>::electing_voters(Some(5)));
+				assert_eq!(LastMinimumActiveBond::<Test>::get(), 50);
+			});
+	}
+
 	#[test]
 	fn voters_include_self_vote() {
 		ExtBuilder::default().nominate(false).build_and_execute(|| {