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

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

Closes #12746

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
@@ -793,6 +795,12 @@ impl<T: Config> Pallet<T> {
 			nominators_taken
 		);
 
+		// `all_voters` is sorted by vote weight, so we use the last element to set the current
+		// minimum active bond. Note that depending on the `T::VoterList` list, the sorting may be
+		// inconsistent as the sorting may be lazy.
+		let min_active_bond: T::CurrencyBalance = all_voters.last().map_or(0, |v| v.1).into();
+		LastMinimumActiveBond::<T>::put(min_active_bond);
+
 		all_voters
 	}
 

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(|| {