Westend: Constant yearly emission (#5999)

Testing the approach of this before it goes live on Polkadot
polkadot-fellows/runtimes#471

---------

Signed-off-by: Oliver Tale-Yazdi <[email protected]>
Co-authored-by: Dónal Murray <[email protected]>

Cargo.toml

@@ -587,6 +587,7 @@ alloy-primitives = { version = "0.4.2", default-features = false }
 alloy-sol-types = { version = "0.4.2", default-features = false }
 always-assert = { version = "0.1" }
 anyhow = { version = "1.0.81", default-features = false }
+approx = { version = "0.5.1" }
 aquamarine = { version = "0.5.0" }
 arbitrary = { version = "1.3.2" }
 ark-bls12-377 = { version = "0.4.0", default-features = false }

polkadot/runtime/westend/Cargo.toml

@@ -118,6 +118,7 @@ xcm-builder = { workspace = true }
 xcm-runtime-apis = { workspace = true }
 
 [dev-dependencies]
+approx = { workspace = true }
 tiny-keccak = { features = ["keccak"], workspace = true }
 sp-keyring = { workspace = true, default-features = true }
 serde_json = { workspace = true, default-features = true }

polkadot/runtime/westend/src/lib.rs

@@ -65,8 +65,8 @@ use polkadot_runtime_common::{
 	elections::OnChainAccuracy,
 	identity_migrator, impl_runtime_weights,
 	impls::{
-		relay_era_payout, ContainsParts, EraPayoutParams, LocatableAssetConverter, ToAuthor,
-		VersionedLocatableAsset, VersionedLocationConverter,
+		ContainsParts, LocatableAssetConverter, ToAuthor, VersionedLocatableAsset,
+		VersionedLocationConverter,
 	},
 	paras_registrar, paras_sudo_wrapper, prod_or_fast, slots,
 	traits::OnSwap,
@@ -681,33 +681,26 @@ impl pallet_bags_list::Config<VoterBagsListInstance> for Runtime {
 pub struct EraPayout;
 impl pallet_staking::EraPayout<Balance> for EraPayout {
 	fn era_payout(
-		total_staked: Balance,
-		total_issuance: Balance,
+		_total_staked: Balance,
+		_total_issuance: Balance,
 		era_duration_millis: u64,
 	) -> (Balance, Balance) {
-		const MILLISECONDS_PER_YEAR: u64 = 1000 * 3600 * 24 * 36525 / 100;
-
-		let params = EraPayoutParams {
-			total_staked,
-			total_stakable: total_issuance,
-			ideal_stake: dynamic_params::inflation::IdealStake::get(),
-			max_annual_inflation: dynamic_params::inflation::MaxInflation::get(),
-			min_annual_inflation: dynamic_params::inflation::MinInflation::get(),
-			falloff: dynamic_params::inflation::Falloff::get(),
-			period_fraction: Perquintill::from_rational(era_duration_millis, MILLISECONDS_PER_YEAR),
-			legacy_auction_proportion: if dynamic_params::inflation::UseAuctionSlots::get() {
-				let auctioned_slots = parachains_paras::Parachains::<Runtime>::get()
-					.into_iter()
-					// all active para-ids that do not belong to a system chain is the number of
-					// parachains that we should take into account for inflation.
-					.filter(|i| *i >= 2000.into())
-					.count() as u64;
-				Some(Perquintill::from_rational(auctioned_slots.min(60), 200u64))
-			} else {
-				None
-			},
-		};
-		relay_era_payout(params)
+		const MILLISECONDS_PER_YEAR: u64 = (1000 * 3600 * 24 * 36525) / 100;
+		// A normal-sized era will have 1 / 365.25 here:
+		let relative_era_len =
+			FixedU128::from_rational(era_duration_millis.into(), MILLISECONDS_PER_YEAR.into());
+
+		// Fixed total TI that we use as baseline for the issuance.
+		let fixed_total_issuance: i128 = 5_216_342_402_773_185_773;
+		let fixed_inflation_rate = FixedU128::from_rational(8, 100);
+		let yearly_emission = fixed_inflation_rate.saturating_mul_int(fixed_total_issuance);
+
+		let era_emission = relative_era_len.saturating_mul_int(yearly_emission);
+		// 15% to treasury, as per Polkadot ref 1139.
+		let to_treasury = FixedU128::from_rational(15, 100).saturating_mul_int(era_emission);
+		let to_stakers = era_emission.saturating_sub(to_treasury);
+
+		(to_stakers.saturated_into(), to_treasury.saturated_into())
 	}
 }
 

polkadot/runtime/westend/src/tests.rs

@@ -19,11 +19,15 @@
 use std::collections::HashSet;
 
 use crate::{xcm_config::LocationConverter, *};
+use approx::assert_relative_eq;
 use frame_support::traits::WhitelistedStorageKeys;
+use pallet_staking::EraPayout;
 use sp_core::{crypto::Ss58Codec, hexdisplay::HexDisplay};
 use sp_keyring::AccountKeyring::Alice;
 use xcm_runtime_apis::conversions::LocationToAccountHelper;
 
+const MILLISECONDS_PER_HOUR: u64 = 60 * 60 * 1000;
+
 #[test]
 fn remove_keys_weight_is_sensible() {
 	use polkadot_runtime_common::crowdloan::WeightInfo;
@@ -311,3 +315,94 @@ fn location_conversion_works() {
 		assert_eq!(got, expected, "{}", tc.description);
 	}
 }
+
+#[test]
+fn staking_inflation_correct_single_era() {
+	let (to_stakers, to_treasury) = super::EraPayout::era_payout(
+		123, // ignored
+		456, // ignored
+		MILLISECONDS_PER_HOUR,
+	);
+
+	assert_relative_eq!(to_stakers as f64, (4_046 * CENTS) as f64, max_relative = 0.01);
+	assert_relative_eq!(to_treasury as f64, (714 * CENTS) as f64, max_relative = 0.01);
+	// Total per hour is ~47.6 WND
+	assert_relative_eq!(
+		(to_stakers as f64 + to_treasury as f64),
+		(4_760 * CENTS) as f64,
+		max_relative = 0.001
+	);
+}
+
+#[test]
+fn staking_inflation_correct_longer_era() {
+	// Twice the era duration means twice the emission:
+	let (to_stakers, to_treasury) = super::EraPayout::era_payout(
+		123, // ignored
+		456, // ignored
+		2 * MILLISECONDS_PER_HOUR,
+	);
+
+	assert_relative_eq!(to_stakers as f64, (4_046 * CENTS) as f64 * 2.0, max_relative = 0.001);
+	assert_relative_eq!(to_treasury as f64, (714 * CENTS) as f64 * 2.0, max_relative = 0.001);
+}
+
+#[test]
+fn staking_inflation_correct_whole_year() {
+	let (to_stakers, to_treasury) = super::EraPayout::era_payout(
+		123,                                        // ignored
+		456,                                        // ignored
+		(36525 * 24 * MILLISECONDS_PER_HOUR) / 100, // 1 year
+	);
+
+	// Our yearly emissions is about 417k WND:
+	let yearly_emission = 417_307 * UNITS;
+	assert_relative_eq!(
+		to_stakers as f64 + to_treasury as f64,
+		yearly_emission as f64,
+		max_relative = 0.001
+	);
+
+	assert_relative_eq!(to_stakers as f64, yearly_emission as f64 * 0.85, max_relative = 0.001);
+	assert_relative_eq!(to_treasury as f64, yearly_emission as f64 * 0.15, max_relative = 0.001);
+}
+
+// 10 years into the future, our values do not overflow.
+#[test]
+fn staking_inflation_correct_not_overflow() {
+	let (to_stakers, to_treasury) = super::EraPayout::era_payout(
+		123,                                       // ignored
+		456,                                       // ignored
+		(36525 * 24 * MILLISECONDS_PER_HOUR) / 10, // 10 years
+	);
+	let initial_ti: i128 = 5_216_342_402_773_185_773;
+	let projected_total_issuance = (to_stakers as i128 + to_treasury as i128) + initial_ti;
+
+	// In 2034, there will be about 9.39 million WND in existence.
+	assert_relative_eq!(
+		projected_total_issuance as f64,
+		(9_390_000 * UNITS) as f64,
+		max_relative = 0.001
+	);
+}
+
+// Print percent per year, just as convenience.
+#[test]
+fn staking_inflation_correct_print_percent() {
+	let (to_stakers, to_treasury) = super::EraPayout::era_payout(
+		123,                                        // ignored
+		456,                                        // ignored
+		(36525 * 24 * MILLISECONDS_PER_HOUR) / 100, // 1 year
+	);
+	let yearly_emission = to_stakers + to_treasury;
+	let mut ti: i128 = 5_216_342_402_773_185_773;
+
+	for y in 0..10 {
+		let new_ti = ti + yearly_emission as i128;
+		let inflation = 100.0 * (new_ti - ti) as f64 / ti as f64;
+		println!("Year {y} inflation: {inflation}%");
+		ti = new_ti;
+
+		assert!(inflation <= 8.0 && inflation > 2.0, "sanity check");
+	}
+}

prdoc/pr_5999.prdoc

@@ -0,0 +1,15 @@
+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: "Westend: Constant yearly emission"
+
+doc:
+  - audience: Runtime User
+    description: |
+      Integrating the new inflation approach from https://github.com/polkadot-fellows/runtimes/pull/471
+      into Westend first to check that it is working.
+
+
+crates:
+  - name: westend-runtime
+    bump: patch