Staking: store last min-active-bond on-chain (#12889)

* Staking: store last min-active-bond on-chain

Storing the `min-active-bond` onchain helps the UIs with minimal on-chain costs.

Closes https://github.com/paritytech/substrate/issues/12746

* Avoid relying on  sorting to set the

* Addresses PR comments

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

@@ -704,6 +704,9 @@ impl<T: Config> Pallet<T> {
 	///
 	/// `maybe_max_len` can imposes a cap on the number of voters returned;
 	///
+	/// Sets `MinimumActiveStake` to the minimum active nominator stake in the returned set of
+	/// nominators.
+	///
 	/// This function is self-weighing as [`DispatchClass::Mandatory`].
 	pub fn get_npos_voters(maybe_max_len: Option<usize>) -> Vec<VoterOf<Self>> {
 		let max_allowed_len = {
@@ -719,6 +722,7 @@ impl<T: Config> Pallet<T> {
 		let mut voters_seen = 0u32;
 		let mut validators_taken = 0u32;
 		let mut nominators_taken = 0u32;
+		let mut min_active_stake = u64::MAX;
 
 		let mut sorted_voters = T::VoterList::iter();
 		while all_voters.len() < max_allowed_len &&
@@ -733,12 +737,15 @@ impl<T: Config> Pallet<T> {
 			};
 
 			if let Some(Nominations { targets, .. }) = <Nominators<T>>::get(&voter) {
+				let voter_weight = weight_of(&voter);
 				if !targets.is_empty() {
-					all_voters.push((voter.clone(), weight_of(&voter), targets));
+					all_voters.push((voter.clone(), voter_weight, targets));
 					nominators_taken.saturating_inc();
 				} else {
 					// Technically should never happen, but not much we can do about it.
 				}
+				min_active_stake =
+					if voter_weight < min_active_stake { voter_weight } else { min_active_stake };
 			} else if Validators::<T>::contains_key(&voter) {
 				// if this voter is a validator:
 				let self_vote = (
@@ -769,6 +776,11 @@ impl<T: Config> Pallet<T> {
 
 		Self::register_weight(T::WeightInfo::get_npos_voters(validators_taken, nominators_taken));
 
+		let min_active_stake: T::CurrencyBalance =
+			if all_voters.len() == 0 { 0u64.into() } else { min_active_stake.into() };
+
+		MinimumActiveStake::<T>::put(min_active_stake);
+
 		log!(
 			info,
 			"generated {} npos voters, {} from validators and {} nominators",

substrate/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 nominator stake of the last successful election.
+	#[pallet::storage]
+	pub type MinimumActiveStake<T> = StorageValue<_, BalanceOf<T>, ValueQuery>;
+
 	/// The minimum amount of commission that validators can set.
 	///
 	/// If set to `0`, no limit exists.

substrate/frame/staking/src/tests.rs

@@ -4463,6 +4463,32 @@ mod election_data_provider {
 		);
 	}
 
+	#[test]
+	fn set_minimum_active_stake_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!(MinimumActiveStake::<Test>::get(), 10);
+
+				// remove staker with lower bond by limiting the number of voters and check
+				// `MinimumActiveStake` again after electing voters.
+				assert_ok!(<Staking as ElectionDataProvider>::electing_voters(Some(5)));
+				assert_eq!(MinimumActiveStake::<Test>::get(), 50);
+			});
+	}
+
+	#[test]
+	fn set_minimum_active_stake_zero_correct() {
+		ExtBuilder::default().has_stakers(false).build_and_execute(|| {
+			assert_ok!(<Staking as ElectionDataProvider>::electing_voters(None));
+			assert_eq!(MinimumActiveStake::<Test>::get(), 0);
+		});
+	}
+
 	#[test]
 	fn voters_include_self_vote() {
 		ExtBuilder::default().nominate(false).build_and_execute(|| {