Implements a % cap on staking rewards from era inflation

polkadot/runtime/common/src/impls.rs

@@ -19,7 +19,7 @@
 use crate::NegativeImbalance;
 use frame_support::traits::{Currency, Imbalance, OnUnbalanced};
 use primitives::Balance;
-use sp_runtime::Perquintill;
+use sp_runtime::{Percent, Perquintill};
 
 /// Logic for the author to get a portion of fees.
 pub struct ToAuthor<R>(sp_std::marker::PhantomData<R>);
@@ -64,6 +64,7 @@ pub fn era_payout(
 	total_stakable: Balance,
 	max_annual_inflation: Perquintill,
 	period_fraction: Perquintill,
+	min_fraction_remainder: Percent,
 	auctioned_slots: u64,
 ) -> (Balance, Balance) {
 	use pallet_staking_reward_fn::compute_inflation;
@@ -86,8 +87,12 @@ pub fn era_payout(
 		min_annual_inflation.saturating_add(delta_annual_inflation * adjustment);
 
 	let max_payout = period_fraction * max_annual_inflation * total_stakable;
-	let staking_payout = (period_fraction * staking_inflation) * total_stakable;
-	let rest = max_payout.saturating_sub(staking_payout);
+	let min_remainder = min_fraction_remainder * max_payout;
+	let staking_payout =
+		((period_fraction * staking_inflation) * total_stakable).min(max_payout - min_remainder);
+	let other_remainder = max_payout - (min_remainder + staking_payout);
+
+	let rest = min_remainder + other_remainder;
 
 	let other_issuance = total_stakable.saturating_sub(total_staked);
 	if total_staked > other_issuance {
@@ -282,11 +287,56 @@ mod tests {
 	#[test]
 	fn era_payout_should_give_sensible_results() {
 		assert_eq!(
-			era_payout(75, 100, Perquintill::from_percent(10), Perquintill::one(), 0,),
+			era_payout(
+				75,
+				100,
+				Perquintill::from_percent(10),
+				Perquintill::one(),
+				Percent::zero(),
+				0
+			),
 			(10, 0)
 		);
 		assert_eq!(
-			era_payout(80, 100, Perquintill::from_percent(10), Perquintill::one(), 0,),
+			era_payout(
+				80,
+				100,
+				Perquintill::from_percent(10),
+				Perquintill::one(),
+				Percent::zero(),
+				0
+			),
+			(6, 4)
+		);
+	}
+
+	#[test]
+	fn era_payout_min_remainder_works() {
+		// minimum remainder is set to 50% of total payout for the era, so the validator payout
+		// takes a cut.
+		assert_eq!(
+			era_payout(
+				75,
+				100,
+				Perquintill::from_percent(10),
+				Perquintill::one(),
+				Percent::from_parts(50),
+				0
+			),
+			(5, 5)
+		);
+		// minimum remainder is 10%. the validator payout does not take a cut because the payout
+		// remainder (`payout_remainder = max_payout - validator_payout`) is larger than the set
+		// minimum remainder.
+		assert_eq!(
+			era_payout(
+				80,
+				100,
+				Perquintill::from_percent(10),
+				Perquintill::one(),
+				Percent::from_parts(10),
+				0
+			),
 			(6, 4)
 		);
 	}

polkadot/runtime/kusama/src/lib.rs

@@ -643,6 +643,7 @@ impl pallet_staking::EraPayout<Balance> for EraPayout {
 	fn era_payout(
 		total_staked: Balance,
 		_total_issuance: Balance,
+		min_fraction_remainder: Percent,
 		era_duration_millis: u64,
 	) -> (Balance, Balance) {
 		// all para-ids that are currently active.
@@ -661,6 +662,7 @@ impl pallet_staking::EraPayout<Balance> for EraPayout {
 			Nis::issuance().other,
 			MAX_ANNUAL_INFLATION,
 			Perquintill::from_rational(era_duration_millis, MILLISECONDS_PER_YEAR),
+			min_fraction_remainder,
 			auctioned_slots,
 		)
 	}

polkadot/runtime/kusama/src/weights/pallet_staking.rs

@@ -793,4 +793,9 @@ impl<T: frame_system::Config> pallet_staking::WeightInfo for WeightInfo<T> {
 			.saturating_add(Weight::from_parts(0, 0))
 			.saturating_add(T::DbWeight::get().writes(1))
 	}
+
+	// TODO(gpestana): run bench bot
+	fn set_min_treasury_fraction() -> Weight {
+		Weight::default()
+	}
 }

polkadot/runtime/polkadot/src/lib.rs

@@ -561,6 +561,7 @@ impl pallet_staking::EraPayout<Balance> for EraPayout {
 	fn era_payout(
 		total_staked: Balance,
 		total_issuance: Balance,
+		min_fraction_remainder: Percent,
 		era_duration_millis: u64,
 	) -> (Balance, Balance) {
 		// all para-ids that are not active.
@@ -579,6 +580,7 @@ impl pallet_staking::EraPayout<Balance> for EraPayout {
 			total_issuance,
 			MAX_ANNUAL_INFLATION,
 			Perquintill::from_rational(era_duration_millis, MILLISECONDS_PER_YEAR),
+			min_fraction_remainder,
 			auctioned_slots,
 		)
 	}

polkadot/runtime/polkadot/src/weights/pallet_staking.rs

@@ -793,4 +793,9 @@ impl<T: frame_system::Config> pallet_staking::WeightInfo for WeightInfo<T> {
 			.saturating_add(Weight::from_parts(0, 0))
 			.saturating_add(T::DbWeight::get().writes(1))
 	}
+
+	// TODO(gpestana): run bench bot
+	fn set_min_treasury_fraction() -> Weight {
+		Weight::default()
+	}
 }

polkadot/runtime/westend/src/weights/pallet_staking.rs

@@ -793,4 +793,9 @@ impl<T: frame_system::Config> pallet_staking::WeightInfo for WeightInfo<T> {
 			.saturating_add(Weight::from_parts(0, 0))
 			.saturating_add(T::DbWeight::get().writes(1))
 	}
+
+	// TODO(gpestana): run bench bot
+	fn set_min_treasury_fraction() -> Weight {
+		Weight::default()
+	}
 }

substrate/frame/staking/src/benchmarking.rs

@@ -939,6 +939,13 @@ benchmarks! {
 		assert_eq!(MinCommission::<T>::get(), Perbill::from_percent(100));
 	}
 
+	set_min_treasury_fraction {
+		let treasury_fraction = Percent::max_value();
+	}: _(RawOrigin::Root, treasury_fraction)
+	verify {
+		assert_eq!(MinRemainderPayout::<T>::get(), Percent::from_parts(100));
+	}
+
 	impl_benchmark_test_suite!(
 		Staking,
 		crate::mock::ExtBuilder::default().has_stakers(true),

substrate/frame/staking/src/lib.rs

@@ -200,6 +200,13 @@
 //! ```nocompile
 //! remaining_payout = max_yearly_inflation * total_tokens / era_per_year - staker_payout
 //! ```
+//!
+//! Note, however, that it is possible to set a minimum `remaining_payout` through the
+//! `MinRemainderPayout` storage type. The `era_payout` implementor must ensure that the
+//! `max_payout >= min_remainder_payout + staker_payout`. The excess should be taken from the
+//! `staker_payout` to ensure that the `max_payout` per era is not exceeded and the
+//! `min_remainder_payout` is respected.
+//!
 //! The remaining reward is send to the configurable end-point
 //! [`Config::RewardRemainder`].
 //!
@@ -310,7 +317,7 @@ use scale_info::TypeInfo;
 use sp_runtime::{
 	curve::PiecewiseLinear,
 	traits::{AtLeast32BitUnsigned, Convert, Saturating, StaticLookup, Zero},
-	Perbill, Perquintill, Rounding, RuntimeDebug,
+	Perbill, Percent, Perquintill, Rounding, RuntimeDebug,
 };
 pub use sp_staking::StakerStatus;
 use sp_staking::{
@@ -845,6 +852,7 @@ pub trait EraPayout<Balance> {
 	fn era_payout(
 		total_staked: Balance,
 		total_issuance: Balance,
+		min_fraction_remainder: Percent,
 		era_duration_millis: u64,
 	) -> (Balance, Balance);
 }
@@ -853,6 +861,7 @@ impl<Balance: Default> EraPayout<Balance> for () {
 	fn era_payout(
 		_total_staked: Balance,
 		_total_issuance: Balance,
+		_min_fraction_remainder: Percent,
 		_era_duration_millis: u64,
 	) -> (Balance, Balance) {
 		(Default::default(), Default::default())
@@ -862,12 +871,15 @@ impl<Balance: Default> EraPayout<Balance> for () {
 /// Adaptor to turn a `PiecewiseLinear` curve definition into an `EraPayout` impl, used for
 /// backwards compatibility.
 pub struct ConvertCurve<T>(sp_std::marker::PhantomData<T>);
-impl<Balance: AtLeast32BitUnsigned + Clone, T: Get<&'static PiecewiseLinear<'static>>>
-	EraPayout<Balance> for ConvertCurve<T>
+impl<Balance, T> EraPayout<Balance> for ConvertCurve<T>
+where
+	Balance: AtLeast32BitUnsigned + Clone + Copy,
+	T: Get<&'static PiecewiseLinear<'static>>,
 {
 	fn era_payout(
 		total_staked: Balance,
 		total_issuance: Balance,
+		min_fraction_remainder: Percent,
 		era_duration_millis: u64,
 	) -> (Balance, Balance) {
 		let (validator_payout, max_payout) = inflation::compute_total_payout(
@@ -877,7 +889,10 @@ impl<Balance: AtLeast32BitUnsigned + Clone, T: Get<&'static PiecewiseLinear<'sta
 			// Duration of era; more than u64::MAX is rewarded as u64::MAX.
 			era_duration_millis,
 		);
-		let rest = max_payout.saturating_sub(validator_payout.clone());
+		let min_remainder = min_fraction_remainder * max_payout;
+		let validator_payout = validator_payout.min(max_payout - min_remainder);
+		let other_remainder = max_payout - (min_remainder + validator_payout);
+		let rest = min_remainder + other_remainder;
 		(validator_payout, rest)
 	}
 }

substrate/frame/staking/src/mock.rs

@@ -213,6 +213,7 @@ parameter_types! {
 
 parameter_types! {
 	pub static RewardRemainderUnbalanced: u128 = 0;
+	pub static MinRewardRemainder: Percent = Percent::default();
 }
 
 pub struct RewardRemainderMock;
@@ -345,6 +346,7 @@ pub struct ExtBuilder {
 	initialize_first_session: bool,
 	pub min_nominator_bond: Balance,
 	min_validator_bond: Balance,
+	min_remainder: Percent,
 	balance_factor: Balance,
 	status: BTreeMap<AccountId, StakerStatus<AccountId>>,
 	stakes: BTreeMap<AccountId, Balance>,
@@ -363,6 +365,7 @@ impl Default for ExtBuilder {
 			initialize_first_session: true,
 			min_nominator_bond: ExistentialDeposit::get(),
 			min_validator_bond: ExistentialDeposit::get(),
+			min_remainder: Percent::default(),
 			status: Default::default(),
 			stakes: Default::default(),
 			stakers: Default::default(),
@@ -445,6 +448,10 @@ impl ExtBuilder {
 		self.balance_factor = factor;
 		self
 	}
+	pub fn min_remainder(mut self, fraction: u8) -> Self {
+		self.min_remainder = Percent::from_parts(fraction);
+		self
+	}
 	fn build(self) -> sp_io::TestExternalities {
 		sp_tracing::try_init_simple();
 		let mut storage = frame_system::GenesisConfig::<Test>::default().build_storage().unwrap();
@@ -532,6 +539,7 @@ impl ExtBuilder {
 			slash_reward_fraction: Perbill::from_percent(10),
 			min_nominator_bond: self.min_nominator_bond,
 			min_validator_bond: self.min_validator_bond,
+			min_remainder: self.min_remainder,
 			..Default::default()
 		}
 		.assimilate_storage(&mut storage);
@@ -661,6 +669,7 @@ pub(crate) fn current_total_payout_for_duration(duration: u64) -> Balance {
 	let (payout, _rest) = <Test as Config>::EraPayout::era_payout(
 		Staking::eras_total_stake(active_era()),
 		Balances::total_issuance(),
+		MinRewardRemainder::get(),
 		duration,
 	);
 	assert!(payout > 0);
@@ -671,6 +680,7 @@ pub(crate) fn maximum_payout_for_duration(duration: u64) -> Balance {
 	let (payout, rest) = <Test as Config>::EraPayout::era_payout(
 		Staking::eras_total_stake(active_era()),
 		Balances::total_issuance(),
+		MinRewardRemainder::get(),
 		duration,
 	);
 	payout + rest

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

@@ -457,8 +457,13 @@ impl<T: Config> Pallet<T> {
 			let era_duration = (now_as_millis_u64 - active_era_start).saturated_into::<u64>();
 			let staked = Self::eras_total_stake(&active_era.index);
 			let issuance = T::Currency::total_issuance();
-			let (validator_payout, remainder) =
-				T::EraPayout::era_payout(staked, issuance, era_duration);
+
+			let (validator_payout, remainder) = T::EraPayout::era_payout(
+				staked,
+				issuance,
+				MinRemainderPayout::<T>::get(),
+				era_duration,
+			);
 
 			Self::deposit_event(Event::<T>::EraPaid {
 				era_index: active_era.index,

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

@@ -485,6 +485,12 @@ pub mod pallet {
 	#[pallet::getter(fn force_era)]
 	pub type ForceEra<T> = StorageValue<_, Forcing, ValueQuery>;
 
+	/// Minimum of the reward remainder per era, i.e. the percentage of the era inflation that
+	/// is not used for rewards.
+	/// See [Era payout](./index.html#era-payout).
+	#[pallet::storage]
+	pub type MinRemainderPayout<T> = StorageValue<_, Percent, ValueQuery>;
+
 	/// The percentage of the slash that is distributed to reporters.
 	///
 	/// The rest of the slashed value is handled by the `Slash`.
@@ -591,6 +597,7 @@ pub mod pallet {
 		pub canceled_payout: BalanceOf<T>,
 		pub stakers:
 			Vec<(T::AccountId, T::AccountId, BalanceOf<T>, crate::StakerStatus<T::AccountId>)>,
+		pub min_remainder: Percent,
 		pub min_nominator_bond: BalanceOf<T>,
 		pub min_validator_bond: BalanceOf<T>,
 		pub max_validator_count: Option<u32>,
@@ -606,6 +613,7 @@ pub mod pallet {
 			ForceEra::<T>::put(self.force_era);
 			CanceledSlashPayout::<T>::put(self.canceled_payout);
 			SlashRewardFraction::<T>::put(self.slash_reward_fraction);
+			MinRemainderPayout::<T>::put(self.min_remainder);
 			MinNominatorBond::<T>::put(self.min_nominator_bond);
 			MinValidatorBond::<T>::put(self.min_validator_bond);
 			if let Some(x) = self.max_validator_count {
@@ -1780,6 +1788,16 @@ pub mod pallet {
 			MinCommission::<T>::put(new);
 			Ok(())
 		}
+
+		/// Sets the minimum fraction of the era's inflation that should be minted directly into
+		/// the treasury.
+		#[pallet::call_index(26)]
+		#[pallet::weight(T::WeightInfo::set_min_treasury_fraction())]
+		pub fn set_min_treasury_fraction(origin: OriginFor<T>, new: Percent) -> DispatchResult {
+			T::AdminOrigin::ensure_origin(origin)?;
+			MinRemainderPayout::<T>::put(new);
+			Ok(())
+		}
 	}
 }