feat: introduce pallet-parameters to Westend to parameterize inflation

polkadot/runtime/common/src/impls.rs

@@ -67,29 +67,51 @@ where
 	}
 }
 
-pub fn era_payout(
-	total_staked: Balance,
-	total_stakable: Balance,
-	max_annual_inflation: Perquintill,
-	period_fraction: Perquintill,
-	auctioned_slots: u64,
-) -> (Balance, Balance) {
-	use pallet_staking_reward_fn::compute_inflation;
+/// Parameters passed into [`relay_era_payout`] function.
+pub struct EraPayoutParams {
+	/// Total staked amount.
+	pub total_staked: Balance,
+	/// Total stakable amount.
+	///
+	/// Usually, this is equal to the total issuance, except if a large part of the issuance is
+	/// locked in another sub-system.
+	pub total_stakable: Balance,
+	/// Ideal stake ratio, which is deducted by `legacy_auction_proportion` if not `None`.
+	pub ideal_stake: Perquintill,
+	/// Maximum inflation rate.
+	pub max_annual_inflation: Perquintill,
+	/// Minimum inflation rate.
+	pub min_annual_inflation: Perquintill,
+	/// Falloff used to calculate era payouts.
+	pub falloff: Perquintill,
+	/// Fraction of the era period used to calculate era payouts.
+	pub period_fraction: Perquintill,
+	/// Legacy auction proportion, which substracts from `ideal_stake` if not `None`.
+	pub legacy_auction_proportion: Option<Perquintill>,
+}
+
+/// A specialized function to compute the inflation of the staking system, tailored for polkadot
+/// relay chains, such as Polkadot, Kusama and Westend.
+pub fn relay_era_payout(params: EraPayoutParams) -> (Balance, Balance) {
 	use sp_runtime::traits::Saturating;
 
-	let min_annual_inflation = Perquintill::from_rational(25u64, 1000u64);
-	let delta_annual_inflation = max_annual_inflation.saturating_sub(min_annual_inflation);
+	let EraPayoutParams {
+		total_staked,
+		total_stakable,
+		ideal_stake,
+		max_annual_inflation,
+		min_annual_inflation,
+		falloff,
+		period_fraction,
+		legacy_auction_proportion,
+	} = params;
 
-	// 30% reserved for up to 60 slots.
-	let auction_proportion = Perquintill::from_rational(auctioned_slots.min(60), 200u64);
+	let delta_annual_inflation = max_annual_inflation.saturating_sub(min_annual_inflation);
 
-	// Therefore the ideal amount at stake (as a percentage of total issuance) is 75% less the
-	// amount that we expect to be taken up with auctions.
-	let ideal_stake = Perquintill::from_percent(75).saturating_sub(auction_proportion);
+	let ideal_stake = ideal_stake.saturating_sub(legacy_auction_proportion.unwrap_or_default());
 
 	let stake = Perquintill::from_rational(total_staked, total_stakable);
-	let falloff = Perquintill::from_percent(5);
-	let adjustment = compute_inflation(stake, ideal_stake, falloff);
+	let adjustment = pallet_staking_reward_fn::compute_inflation(stake, ideal_stake, falloff);
 	let staking_inflation =
 		min_annual_inflation.saturating_add(delta_annual_inflation * adjustment);
 
@@ -371,6 +393,46 @@ mod tests {
 		t.into()
 	}
 
+	pub fn deprecated_era_payout(
+		total_staked: Balance,
+		total_stakable: Balance,
+		max_annual_inflation: Perquintill,
+		period_fraction: Perquintill,
+		auctioned_slots: u64,
+	) -> (Balance, Balance) {
+		use pallet_staking_reward_fn::compute_inflation;
+		use sp_runtime::traits::Saturating;
+
+		let min_annual_inflation = Perquintill::from_rational(25u64, 1000u64);
+		let delta_annual_inflation = max_annual_inflation.saturating_sub(min_annual_inflation);
+
+		// 30% reserved for up to 60 slots.
+		let auction_proportion = Perquintill::from_rational(auctioned_slots.min(60), 200u64);
+
+		// Therefore the ideal amount at stake (as a percentage of total issuance) is 75% less the
+		// amount that we expect to be taken up with auctions.
+		let ideal_stake = Perquintill::from_percent(75).saturating_sub(auction_proportion);
+
+		let stake = Perquintill::from_rational(total_staked, total_stakable);
+		let falloff = Perquintill::from_percent(5);
+		let adjustment = compute_inflation(stake, ideal_stake, falloff);
+		let staking_inflation =
+			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 other_issuance = total_stakable.saturating_sub(total_staked);
+		if total_staked > other_issuance {
+			let _cap_rest =
+				Perquintill::from_rational(other_issuance, total_staked) * staking_payout;
+			// We don't do anything with this, but if we wanted to, we could introduce a cap on the
+			// treasury amount with: `rest = rest.min(cap_rest);`
+		}
+		(staking_payout, rest)
+	}
+
 	#[test]
 	fn test_fees_and_tip_split() {
 		new_test_ext().execute_with(|| {
@@ -425,13 +487,99 @@ mod tests {
 
 	#[test]
 	fn era_payout_should_give_sensible_results() {
-		assert_eq!(
-			era_payout(75, 100, Perquintill::from_percent(10), Perquintill::one(), 0,),
-			(10, 0)
+		let payout =
+			deprecated_era_payout(75, 100, Perquintill::from_percent(10), Perquintill::one(), 0);
+		assert_eq!(payout, (10, 0));
+
+		let payout =
+			deprecated_era_payout(80, 100, Perquintill::from_percent(10), Perquintill::one(), 0);
+		assert_eq!(payout, (6, 4));
+	}
+
+	#[test]
+	fn relay_era_payout_should_give_sensible_results() {
+		let params = EraPayoutParams {
+			total_staked: 75,
+			total_stakable: 100,
+			ideal_stake: Perquintill::from_percent(75),
+			max_annual_inflation: Perquintill::from_percent(10),
+			min_annual_inflation: Perquintill::from_rational(25u64, 1000u64),
+			falloff: Perquintill::from_percent(5),
+			period_fraction: Perquintill::one(),
+			legacy_auction_proportion: None,
+		};
+		assert_eq!(relay_era_payout(params), (10, 0));
+
+		let params = EraPayoutParams {
+			total_staked: 80,
+			total_stakable: 100,
+			ideal_stake: Perquintill::from_percent(75),
+			max_annual_inflation: Perquintill::from_percent(10),
+			min_annual_inflation: Perquintill::from_rational(25u64, 1000u64),
+			falloff: Perquintill::from_percent(5),
+			period_fraction: Perquintill::one(),
+			legacy_auction_proportion: None,
+		};
+		assert_eq!(relay_era_payout(params), (6, 4));
+	}
+
+	#[test]
+	fn relay_era_payout_should_give_same_results_as_era_payout() {
+		let total_staked = 1_000_000;
+		let total_stakable = 2_000_000;
+		let max_annual_inflation = Perquintill::from_percent(10);
+		let period_fraction = Perquintill::from_percent(25);
+		let auctioned_slots = 30;
+
+		let params = EraPayoutParams {
+			total_staked,
+			total_stakable,
+			ideal_stake: Perquintill::from_percent(75),
+			max_annual_inflation,
+			min_annual_inflation: Perquintill::from_rational(25u64, 1000u64),
+			falloff: Perquintill::from_percent(5),
+			period_fraction,
+			legacy_auction_proportion: Some(Perquintill::from_rational(
+				auctioned_slots.min(60),
+				200u64,
+			)),
+		};
+
+		let payout = deprecated_era_payout(
+			total_staked,
+			total_stakable,
+			max_annual_inflation,
+			period_fraction,
+			auctioned_slots,
 		);
-		assert_eq!(
-			era_payout(80, 100, Perquintill::from_percent(10), Perquintill::one(), 0,),
-			(6, 4)
+		assert_eq!(relay_era_payout(params), payout);
+
+		let total_staked = 1_900_000;
+		let total_stakable = 2_000_000;
+		let auctioned_slots = 60;
+
+		let params = EraPayoutParams {
+			total_staked,
+			total_stakable,
+			ideal_stake: Perquintill::from_percent(75),
+			max_annual_inflation,
+			min_annual_inflation: Perquintill::from_rational(25u64, 1000u64),
+			falloff: Perquintill::from_percent(5),
+			period_fraction,
+			legacy_auction_proportion: Some(Perquintill::from_rational(
+				auctioned_slots.min(60),
+				200u64,
+			)),
+		};
+
+		let payout = deprecated_era_payout(
+			total_staked,
+			total_stakable,
+			max_annual_inflation,
+			period_fraction,
+			auctioned_slots,
 		);
+
+		assert_eq!(relay_era_payout(params), payout);
 	}
 }

polkadot/runtime/westend/Cargo.toml

@@ -71,6 +71,7 @@ pallet-multisig = { workspace = true }
 pallet-nomination-pools = { workspace = true }
 pallet-conviction-voting = { workspace = true }
 pallet-offences = { workspace = true }
+pallet-parameters = { workspace = true }
 pallet-preimage = { workspace = true }
 pallet-proxy = { workspace = true }
 pallet-recovery = { workspace = true }
@@ -79,7 +80,6 @@ pallet-scheduler = { workspace = true }
 pallet-session = { workspace = true }
 pallet-society = { workspace = true }
 pallet-staking = { workspace = true }
-pallet-staking-reward-curve = { workspace = true, default-features = true }
 pallet-staking-runtime-api = { workspace = true }
 pallet-delegated-staking = { workspace = true }
 pallet-state-trie-migration = { workspace = true }
@@ -173,6 +173,7 @@ std = [
 	"pallet-nomination-pools/std",
 	"pallet-offences-benchmarking?/std",
 	"pallet-offences/std",
+	"pallet-parameters/std",
 	"pallet-preimage/std",
 	"pallet-proxy/std",
 	"pallet-recovery/std",
@@ -260,6 +261,7 @@ runtime-benchmarks = [
 	"pallet-nomination-pools/runtime-benchmarks",
 	"pallet-offences-benchmarking/runtime-benchmarks",
 	"pallet-offences/runtime-benchmarks",
+	"pallet-parameters/runtime-benchmarks",
 	"pallet-preimage/runtime-benchmarks",
 	"pallet-proxy/runtime-benchmarks",
 	"pallet-recovery/runtime-benchmarks",
@@ -318,6 +320,7 @@ try-runtime = [
 	"pallet-multisig/try-runtime",
 	"pallet-nomination-pools/try-runtime",
 	"pallet-offences/try-runtime",
+	"pallet-parameters/try-runtime",
 	"pallet-preimage/try-runtime",
 	"pallet-proxy/try-runtime",
 	"pallet-recovery/try-runtime",