Kusama: Make the current inflation formula adjustable.

CHANGELOG.md

@@ -20,8 +20,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Add the Polkadot Coretime Chain runtime ([polkadot-fellows/runtimes#410](https://github.com/polkadot-fellows/runtimes/pull/410))
 - Kusama: Add a "Spokesperson" proxy type only allowed to send remarks ([polkadot-fellows/runtimes#430](https://github.com/polkadot-fellows/runtimes/pull/430))
 - Add the Polkadot and Kusama Coretime Chain specs ([polkadot-fellows/runtimes#432](https://github.com/polkadot-fellows/runtimes/pull/432))
+- Kusama: Make the current inflation formula adjustable ([polkadot-fellows/runtimes#364](https://github.com/polkadot-fellows/runtimes/pull/364))
 - Port Agile Coretime migration from polkadot-sdk in order to fix leases with gaps handling([polkadot-fellows/runtimes#426](https://github.com/polkadot-fellows/runtimes/pull/426))
 
+
 #### From [#322](https://github.com/polkadot-fellows/runtimes/pull/322):
 
 - Add `claim_assets` extrinsic to `pallet-xcm` ([SDK v1.9 #3403](https://github.com/paritytech/polkadot-sdk/pull/3403)).

Cargo.toml

@@ -137,6 +137,7 @@ pallet-nomination-pools-benchmarking = { version = "34.0.0", default-features =
 pallet-nomination-pools-runtime-api = { version = "31.0.0", default-features = false }
 pallet-offences = { version = "35.0.0", default-features = false }
 pallet-offences-benchmarking = { version = "36.0.0", default-features = false }
+pallet-parameters = { version = "0.7.0", default-features = false }
 pallet-preimage = { version = "36.0.0", default-features = false }
 pallet-proxy = { version = "36.0.0", default-features = false }
 pallet-ranked-collective = { version = "36.0.0", default-features = false }
@@ -243,6 +244,8 @@ subxt = { version = "0.35.0", default-features = false }
 tracing-subscriber = { version = "0.3.18" }
 zombienet-sdk = { version = "0.2.4" }
 tuplex = { version = "0.1.0", default-features = false }
+relay-common = { path = "relay/common", default-features = false }
+ss58-registry = { version = "1.47.0" }
 
 [workspace]
 resolver = "2"
@@ -275,6 +278,7 @@ members = [
 	"integration-tests/emulated/tests/people/people-kusama",
 	"integration-tests/emulated/tests/people/people-polkadot",
 	"integration-tests/zombienet",
+	"relay/common",
 	"relay/kusama",
 	"relay/kusama/constants",
 	"relay/polkadot",

relay/common/Cargo.toml

@@ -0,0 +1,30 @@
+[package]
+authors.workspace = true
+description = "Shared utilities between relay runtimes"
+edition.workspace = true
+license.workspace = true
+name = "relay-common"
+repository.workspace = true
+version.workspace = true
+
+[dependencies]
+codec = { features = ["derive", "max-encoded-len"], workspace = true }
+scale-info = { features = ["derive"], workspace = true }
+
+sp-api ={ workspace = true }
+sp-runtime = { workspace = true }
+polkadot-primitives = { workspace = true }
+pallet-staking-reward-fn ={ workspace = true }
+
+
+[features]
+default = ["std"]
+std = [
+	"codec/std",
+	"scale-info/std",
+
+	"sp-api/std",
+	"sp-runtime/std",
+	"polkadot-primitives/std",
+	"pallet-staking-reward-fn/std",
+]

relay/common/src/lib.rs

@@ -0,0 +1,115 @@
+// Copyright (C) Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Polkadot is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Polkadot. If not, see <http://www.gnu.org/licenses/>.
+
+//! Shared code between the Kusama nd Polkadot RC Runtimes.
+#![cfg_attr(not(feature = "std"), no_std)]
+
+use polkadot_primitives::Balance;
+use sp_runtime::{Perquintill, Saturating};
+
+/// Extra runtime APIs for kusama runtime.
+pub mod apis {
+	/// Information about the current inflation rate of the system.
+	///
+	/// Both fields should be treated as best-effort, given that the inflation rate might not be
+	/// fully predict-able.
+	#[derive(scale_info::TypeInfo, codec::Encode, codec::Decode)]
+	pub struct InflationInfo {
+		/// The rate of inflation estimated per annum.
+		pub inflation: sp_runtime::Perquintill,
+		/// Next amount that we anticipate to mint.
+		///
+		/// First item is the amount that goes to stakers, second is the leftover that is usually
+		/// forwarded to the treasury.
+		pub next_mint: (polkadot_primitives::Balance, polkadot_primitives::Balance),
+	}
+
+	sp_api::decl_runtime_apis! {
+		pub trait Inflation {
+			/// Return the current estimates of the inflation amount.
+			///
+			/// This is marked as experimental in light of RFC#89. Nonetheless, its usage is highly
+			/// recommended over trying to read-storage, or re-create the onchain logic.
+			fn experimental_inflation_prediction_info() -> InflationInfo;
+		}
+	}
+}
+
+// ---- TODO: Below is copy pasted from sdk, remove once we pull the version containing
+// https://github.com/paritytech/polkadot-sdk/pull/4938
+
+#[derive(Debug, Clone)]
+/// 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) {
+	let EraPayoutParams {
+		total_staked,
+		total_stakable,
+		ideal_stake,
+		max_annual_inflation,
+		min_annual_inflation,
+		falloff,
+		period_fraction,
+		legacy_auction_proportion,
+	} = params;
+
+	let delta_annual_inflation = max_annual_inflation.saturating_sub(min_annual_inflation);
+
+	let ideal_stake = ideal_stake.saturating_sub(legacy_auction_proportion.unwrap_or_default());
+
+	let stake = Perquintill::from_rational(total_staked, total_stakable);
+	let adjustment = pallet_staking_reward_fn::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)
+}
+
+// ---- TODO: Above is copy pasted from sdk, remove once we pull the version containing
+// https://github.com/paritytech/polkadot-sdk/pull/4938

relay/kusama/Cargo.toml

@@ -85,6 +85,7 @@ pallet-whitelist = { workspace = true }
 pallet-xcm = { workspace = true }
 pallet-xcm-benchmarks = { optional = true, workspace = true }
 frame-election-provider-support = { workspace = true }
+pallet-parameters = { workspace = true }
 
 frame-benchmarking = { optional = true, workspace = true }
 frame-try-runtime = { optional = true, workspace = true }
@@ -105,6 +106,7 @@ xcm-builder = { workspace = true }
 xcm-runtime-apis = { workspace = true }
 
 sp-debug-derive = { workspace = true }
+relay-common = { workspace = true }
 
 [dev-dependencies]
 sp-keyring = { workspace = true }
@@ -113,6 +115,7 @@ separator = { workspace = true }
 remote-externalities = { workspace = true }
 tokio = { features = ["macros"], workspace = true }
 sp-tracing = { workspace = true }
+ss58-registry = { workspace = true }
 
 [build-dependencies]
 substrate-wasm-builder = { workspace = true, optional = true }
@@ -166,6 +169,7 @@ std = [
 	"pallet-offences/std",
 	"pallet-preimage/std",
 	"pallet-proxy/std",
+	"pallet-parameters/std",
 	"pallet-ranked-collective/std",
 	"pallet-recovery/std",
 	"pallet-referenda/std",
@@ -186,6 +190,7 @@ std = [
 	"pallet-xcm/std",
 	"polkadot-primitives/std",
 	"polkadot-runtime-common/std",
+	"relay-common/std",
 	"runtime-parachains/std",
 	"scale-info/std",
 	"serde_json/std",
@@ -241,6 +246,7 @@ 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-ranked-collective/runtime-benchmarks",
@@ -314,6 +320,7 @@ try-runtime = [
 	"polkadot-runtime-common/try-runtime",
 	"runtime-parachains/try-runtime",
 	"sp-runtime/try-runtime",
+	"pallet-parameters/try-runtime",
 ]
 
 # Enable metadata hash generation at compile time for the `CheckMetadataHash` extension.