lib.rs

// This file is part of Substrate.

// Copyright (C) 2017-2021 Parity Technologies (UK) Ltd.
// SPDX-License-Identifier: Apache-2.0

// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// 	http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//! # Staking Pallet
//!
//! The Staking pallet is used to manage funds at stake by network maintainers.
//!
//! - [`Config`]
//! - [`Call`]
//! - [`Pallet`]
//!
//! ## Overview
//!
//! The Staking pallet is the means by which a set of network maintainers (known as _authorities_ in
//! some contexts and _validators_ in others) are chosen based upon those who voluntarily place
//! funds under deposit. Under deposit, those funds are rewarded under normal operation but are held
//! at pain of _slash_ (expropriation) should the staked maintainer be found not to be discharging
//! its duties properly.
//!
//! ### Terminology
//! <!-- Original author of paragraph: @gavofyork -->
//!
//! - Staking: The process of locking up funds for some time, placing them at risk of slashing
//!   (loss) in order to become a rewarded maintainer of the network.
//! - Validating: The process of running a node to actively maintain the network, either by
//!   producing blocks or guaranteeing finality of the chain.
//! - Nominating: The process of placing staked funds behind one or more validators in order to
//!   share in any reward, and punishment, they take.
//! - Stash account: The account holding an owner's funds used for staking.
//! - Controller account: The account that controls an owner's funds for staking.
//! - Era: A (whole) number of sessions, which is the period that the validator set (and each
//!   validator's active nominator set) is recalculated and where rewards are paid out.
//! - Slash: The punishment of a staker by reducing its funds.
//!
//! ### Goals
//! <!-- Original author of paragraph: @gavofyork -->
//!
//! The staking system in Substrate NPoS is designed to make the following possible:
//!
//! - Stake funds that are controlled by a cold wallet.
//! - Withdraw some, or deposit more, funds without interrupting the role of an entity.
//! - Switch between roles (nominator, validator, idle) with minimal overhead.
//!
//! ### Scenarios
//!
//! #### Staking
//!
//! Almost any interaction with the Staking pallet requires a process of _**bonding**_ (also known
//! as being a _staker_). To become *bonded*, a fund-holding account known as the _stash account_,
//! which holds some or all of the funds that become frozen in place as part of the staking process,
//! is paired with an active **controller** account, which issues instructions on how they shall be
//! used.
//!
//! An account pair can become bonded using the [`bond`](Call::bond) call.
//!
//! Stash accounts can change their associated controller using the
//! [`set_controller`](Call::set_controller) call.
//!
//! There are three possible roles that any staked account pair can be in: `Validator`, `Nominator`
//! and `Idle` (defined in [`StakerStatus`]). There are three
//! corresponding instructions to change between roles, namely:
//! [`validate`](Call::validate),
//! [`nominate`](Call::nominate), and [`chill`](Call::chill).
//!
//! #### Validating
//!
//! A **validator** takes the role of either validating blocks or ensuring their finality,
//! maintaining the veracity of the network. A validator should avoid both any sort of malicious
//! misbehavior and going offline. Bonded accounts that state interest in being a validator do NOT
//! get immediately chosen as a validator. Instead, they are declared as a _candidate_ and they
//! _might_ get elected at the _next era_ as a validator. The result of the election is determined
//! by nominators and their votes.
//!
//! An account can become a validator candidate via the
//! [`validate`](Call::validate) call.
//!
//! #### Nomination
//!
//! A **nominator** does not take any _direct_ role in maintaining the network, instead, it votes on
//! a set of validators  to be elected. Once interest in nomination is stated by an account, it
//! takes effect at the next election round. The funds in the nominator's stash account indicate the
//! _weight_ of its vote. Both the rewards and any punishment that a validator earns are shared
//! between the validator and its nominators. This rule incentivizes the nominators to NOT vote for
//! the misbehaving/offline validators as much as possible, simply because the nominators will also
//! lose funds if they vote poorly.
//!
//! An account can become a nominator via the [`nominate`](Call::nominate) call.
//!
//! #### Rewards and Slash
//!
//! The **reward and slashing** procedure is the core of the Staking pallet, attempting to _embrace
//! valid behavior_ while _punishing any misbehavior or lack of availability_.
//!
//! Rewards must be claimed for each era before it gets too old by `$HISTORY_DEPTH` using the
//! `payout_stakers` call. Any account can call `payout_stakers`, which pays the reward to the
//! validator as well as its nominators. Only the [`Config::MaxNominatorRewardedPerValidator`]
//! biggest stakers can claim their reward. This is to limit the i/o cost to mutate storage for each
//! nominator's account.
//!
//! Slashing can occur at any point in time, once misbehavior is reported. Once slashing is
//! determined, a value is deducted from the balance of the validator and all the nominators who
//! voted for this validator (values are deducted from the _stash_ account of the slashed entity).
//!
//! Slashing logic is further described in the documentation of the `slashing` pallet.
//!
//! Similar to slashing, rewards are also shared among a validator and its associated nominators.
//! Yet, the reward funds are not always transferred to the stash account and can be configured. See
//! [Reward Calculation](#reward-calculation) for more details.
//!
//! #### Chilling
//!
//! Finally, any of the roles above can choose to step back temporarily and just chill for a while.
//! This means that if they are a nominator, they will not be considered as voters anymore and if
//! they are validators, they will no longer be a candidate for the next election.
//!
//! An account can step back via the [`chill`](Call::chill) call.
//!
//! ### Session managing
//!
//! The pallet implement the trait `SessionManager`. Which is the only API to query new validator
//! set and allowing these validator set to be rewarded once their era is ended.
//!
//! ## Interface
//!
//! ### Dispatchable Functions
//!
//! The dispatchable functions of the Staking pallet enable the steps needed for entities to accept
//! and change their role, alongside some helper functions to get/set the metadata of the pallet.
//!
//! ### Public Functions
//!
//! The Staking pallet contains many public storage items and (im)mutable functions.
//!
//! ## Usage
//!
//! ### Example: Rewarding a validator by id.
//!
//! ```
//! use frame_support::{decl_module, dispatch};
//! use frame_system::ensure_signed;
//! use pallet_staking::{self as staking};
//!
//! pub trait Config: staking::Config {}
//!
//! decl_module! {
//!     pub struct Module<T: Config> for enum Call where origin: T::Origin {
//!         /// Reward a validator.
//!         #[weight = 0]
//!         pub fn reward_myself(origin) -> dispatch::DispatchResult {
//!             let reported = ensure_signed(origin)?;
//!             <staking::Pallet<T>>::reward_by_ids(vec![(reported, 10)]);
//!             Ok(())
//!         }
//!     }
//! }
//! # fn main() { }
//! ```
//!
//! ## Implementation Details
//!
//! ### Era payout
//!
//! The era payout is computed using yearly inflation curve defined at
//! [`Config::EraPayout`] as such:
//!
//! ```nocompile
//! staker_payout = yearly_inflation(npos_token_staked / total_tokens) * total_tokens / era_per_year
//! ```
//! This payout is used to reward stakers as defined in next section
//!
//! ```nocompile
//! remaining_payout = max_yearly_inflation * total_tokens / era_per_year - staker_payout
//! ```
//! The remaining reward is send to the configurable end-point
//! [`Config::RewardRemainder`].
//!
//! ### Reward Calculation
//!
//! Validators and nominators are rewarded at the end of each era. The total reward of an era is
//! calculated using the era duration and the staking rate (the total amount of tokens staked by
//! nominators and validators, divided by the total token supply). It aims to incentivize toward a
//! defined staking rate. The full specification can be found
//! [here](https://research.web3.foundation/en/latest/polkadot/Token%20Economics.html#inflation-model).
//!
//! Total reward is split among validators and their nominators depending on the number of points
//! they received during the era. Points are added to a validator using
//! [`reward_by_ids`](Pallet::reward_by_ids).
//!
//! [`Pallet`] implements
//! [`pallet_authorship::EventHandler`] to add reward
//! points to block producer and block producer of referenced uncles.
//!
//! The validator and its nominator split their reward as following:
//!
//! The validator can declare an amount, named
//! [`commission`](ValidatorPrefs::commission), that does not get shared
//! with the nominators at each reward payout through its
//! [`ValidatorPrefs`]. This value gets deducted from the total reward
//! that is paid to the validator and its nominators. The remaining portion is split among the
//! validator and all of the nominators that nominated the validator, proportional to the value
//! staked behind this validator (_i.e._ dividing the
//! [`own`](Exposure::own) or
//! [`others`](Exposure::others) by
//! [`total`](Exposure::total) in [`Exposure`]).
//!
//! All entities who receive a reward have the option to choose their reward destination through the
//! [`Payee`] storage item (see
//! [`set_payee`](Call::set_payee)), to be one of the following:
//!
//! - Controller account, (obviously) not increasing the staked value.
//! - Stash account, not increasing the staked value.
//! - Stash account, also increasing the staked value.
//!
//! ### Additional Fund Management Operations
//!
//! Any funds already placed into stash can be the target of the following operations:
//!
//! The controller account can free a portion (or all) of the funds using the
//! [`unbond`](Call::unbond) call. Note that the funds are not immediately
//! accessible. Instead, a duration denoted by
//! [`Config::BondingDuration`] (in number of eras) must
//! pass until the funds can actually be removed. Once the `BondingDuration` is over, the
//! [`withdraw_unbonded`](Call::withdraw_unbonded) call can be used to actually
//! withdraw the funds.
//!
//! Note that there is a limitation to the number of fund-chunks that can be scheduled to be
//! unlocked in the future via [`unbond`](Call::unbond). In case this maximum
//! (`MAX_UNLOCKING_CHUNKS`) is reached, the bonded account _must_ first wait until a successful
//! call to `withdraw_unbonded` to remove some of the chunks.
//!
//! ### Election Algorithm
//!
//! The current election algorithm is implemented based on Phragmén. The reference implementation
//! can be found [here](https://github.com/w3f/consensus/tree/master/NPoS).
//!
//! The election algorithm, aside from electing the validators with the most stake value and votes,
//! tries to divide the nominator votes among candidates in an equal manner. To further assure this,
//! an optional post-processing can be applied that iteratively normalizes the nominator staked
//! values until the total difference among votes of a particular nominator are less than a
//! threshold.
//!
//! ## GenesisConfig
//!
//! The Staking pallet depends on the [`GenesisConfig`]. The
//! `GenesisConfig` is optional and allow to set some initial stakers.
//!
//! ## Related Modules
//!
//! - [Balances](../pallet_balances/index.html): Used to manage values at stake.
//! - [Session](../pallet_session/index.html): Used to manage sessions. Also, a list of new
//!   validators is stored in the Session pallet's `Validators` at the end of each era.

#![recursion_limit = "128"]
#![cfg_attr(not(feature = "std"), no_std)]

#[cfg(any(feature = "runtime-benchmarks", test))]
pub mod benchmarking;
#[cfg(test)]
mod mock;
#[cfg(any(feature = "runtime-benchmarks", test))]
pub mod testing_utils;
#[cfg(test)]
mod tests;

pub mod inflation;
pub mod slashing;
pub mod weights;

use codec::{Decode, Encode, HasCompact};
use frame_election_provider_support::{data_provider, ElectionProvider, Supports, VoteWeight};
use frame_support::{
	pallet_prelude::*,
	traits::{
		Currency, CurrencyToVote, EnsureOrigin, EstimateNextNewSession, Get, Imbalance,
		LockIdentifier, LockableCurrency, OnUnbalanced, UnixTime, WithdrawReasons,
	},
	weights::{
		constants::{WEIGHT_PER_MICROS, WEIGHT_PER_NANOS},
		Weight, WithPostDispatchInfo,
	},
};
use frame_system::{ensure_root, ensure_signed, offchain::SendTransactionTypes, pallet_prelude::*};
pub use pallet::*;
use pallet_session::historical;
use sp_runtime::{
	curve::PiecewiseLinear,
	traits::{
		AtLeast32BitUnsigned, Bounded, CheckedSub, Convert, SaturatedConversion, Saturating,
		StaticLookup, Zero,
	},
	DispatchError, Perbill, Percent, RuntimeDebug,
};
use sp_staking::{
	offence::{Offence, OffenceDetails, OffenceError, OnOffenceHandler, ReportOffence},
	SessionIndex,
};
use sp_std::{collections::btree_map::BTreeMap, convert::From, prelude::*, result};
pub use weights::WeightInfo;

const STAKING_ID: LockIdentifier = *b"staking ";
pub(crate) const LOG_TARGET: &'static str = "runtime::staking";

// syntactic sugar for logging.
#[macro_export]
macro_rules! log {
	($level:tt, $patter:expr $(, $values:expr)* $(,)?) => {
		log::$level!(
			target: crate::LOG_TARGET,
			concat!("[{:?}] 💸 ", $patter), <frame_system::Pallet<T>>::block_number() $(, $values)*
		)
	};
}

pub const MAX_UNLOCKING_CHUNKS: usize = 32;

/// Counter for the number of eras that have passed.
pub type EraIndex = u32;

/// Counter for the number of "reward" points earned by a given validator.
pub type RewardPoint = u32;

/// The balance type of this pallet.
pub type BalanceOf<T> =
	<<T as Config>::Currency as Currency<<T as frame_system::Config>::AccountId>>::Balance;

type PositiveImbalanceOf<T> = <<T as Config>::Currency as Currency<
	<T as frame_system::Config>::AccountId,
>>::PositiveImbalance;
type NegativeImbalanceOf<T> = <<T as Config>::Currency as Currency<
	<T as frame_system::Config>::AccountId,
>>::NegativeImbalance;

/// Information regarding the active era (era in used in session).
#[derive(Encode, Decode, RuntimeDebug)]
pub struct ActiveEraInfo {
	/// Index of era.
	pub index: EraIndex,
	/// Moment of start expressed as millisecond from `$UNIX_EPOCH`.
	///
	/// Start can be none if start hasn't been set for the era yet,
	/// Start is set on the first on_finalize of the era to guarantee usage of `Time`.
	start: Option<u64>,
}

/// Reward points of an era. Used to split era total payout between validators.
///
/// This points will be used to reward validators and their respective nominators.
#[derive(PartialEq, Encode, Decode, Default, RuntimeDebug)]
pub struct EraRewardPoints<AccountId: Ord> {
	/// Total number of points. Equals the sum of reward points for each validator.
	total: RewardPoint,
	/// The reward points earned by a given validator.
	individual: BTreeMap<AccountId, RewardPoint>,
}

/// Indicates the initial status of the staker.
#[derive(RuntimeDebug)]
#[cfg_attr(feature = "std", derive(serde::Serialize, serde::Deserialize))]
pub enum StakerStatus<AccountId> {
	/// Chilling.
	Idle,
	/// Declared desire in validating or already participating in it.
	Validator,
	/// Nominating for a group of other stakers.
	Nominator(Vec<AccountId>),
}

/// A destination account for payment.
#[derive(PartialEq, Eq, Copy, Clone, Encode, Decode, RuntimeDebug)]
pub enum RewardDestination<AccountId> {
	/// Pay into the stash account, increasing the amount at stake accordingly.
	Staked,
	/// Pay into the stash account, not increasing the amount at stake.
	Stash,
	/// Pay into the controller account.
	Controller,
	/// Pay into a specified account.
	Account(AccountId),
	/// Receive no reward.
	None,
}

impl<AccountId> Default for RewardDestination<AccountId> {
	fn default() -> Self {
		RewardDestination::Staked
	}
}

/// Preference of what happens regarding validation.
#[derive(PartialEq, Eq, Clone, Encode, Decode, RuntimeDebug)]
pub struct ValidatorPrefs {
	/// Reward that validator takes up-front; only the rest is split between themselves and
	/// nominators.
	#[codec(compact)]
	pub commission: Perbill,
	/// Whether or not this validator is accepting more nominations. If `true`, then no nominator
	/// who is not already nominating this validator may nominate them. By default, validators
	/// are accepting nominations.
	pub blocked: bool,
}

impl Default for ValidatorPrefs {
	fn default() -> Self {
		ValidatorPrefs { commission: Default::default(), blocked: false }
	}
}

/// Just a Balance/BlockNumber tuple to encode when a chunk of funds will be unlocked.
#[derive(PartialEq, Eq, Clone, Encode, Decode, RuntimeDebug)]
pub struct UnlockChunk<Balance: HasCompact> {
	/// Amount of funds to be unlocked.
	#[codec(compact)]
	value: Balance,
	/// Era number at which point it'll be unlocked.
	#[codec(compact)]
	era: EraIndex,
}

/// The ledger of a (bonded) stash.
#[derive(PartialEq, Eq, Clone, Encode, Decode, RuntimeDebug)]
pub struct StakingLedger<AccountId, Balance: HasCompact> {
	/// The stash account whose balance is actually locked and at stake.
	pub stash: AccountId,
	/// The total amount of the stash's balance that we are currently accounting for.
	/// It's just `active` plus all the `unlocking` balances.
	#[codec(compact)]
	pub total: Balance,
	/// The total amount of the stash's balance that will be at stake in any forthcoming
	/// rounds.
	#[codec(compact)]
	pub active: Balance,
	/// Any balance that is becoming free, which may eventually be transferred out
	/// of the stash (assuming it doesn't get slashed first).
	pub unlocking: Vec<UnlockChunk<Balance>>,
	/// List of eras for which the stakers behind a validator have claimed rewards. Only updated
	/// for validators.
	pub claimed_rewards: Vec<EraIndex>,
}

impl<AccountId, Balance: HasCompact + Copy + Saturating + AtLeast32BitUnsigned>
	StakingLedger<AccountId, Balance>
{
	/// Remove entries from `unlocking` that are sufficiently old and reduce the
	/// total by the sum of their balances.
	fn consolidate_unlocked(self, current_era: EraIndex) -> Self {
		let mut total = self.total;
		let unlocking = self
			.unlocking
			.into_iter()
			.filter(|chunk| {
				if chunk.era > current_era {
					true
				} else {
					total = total.saturating_sub(chunk.value);
					false
				}
			})
			.collect();

		Self {
			stash: self.stash,
			total,
			active: self.active,
			unlocking,
			claimed_rewards: self.claimed_rewards,
		}
	}

	/// Re-bond funds that were scheduled for unlocking.
	fn rebond(mut self, value: Balance) -> Self {
		let mut unlocking_balance: Balance = Zero::zero();

		while let Some(last) = self.unlocking.last_mut() {
			if unlocking_balance + last.value <= value {
				unlocking_balance += last.value;
				self.active += last.value;
				self.unlocking.pop();
			} else {
				let diff = value - unlocking_balance;

				unlocking_balance += diff;
				self.active += diff;
				last.value -= diff;
			}

			if unlocking_balance >= value {
				break
			}
		}

		self
	}
}

impl<AccountId, Balance> StakingLedger<AccountId, Balance>
where
	Balance: AtLeast32BitUnsigned + Saturating + Copy,
{
	/// Slash the validator for a given amount of balance. This can grow the value
	/// of the slash in the case that the validator has less than `minimum_balance`
	/// active funds. Returns the amount of funds actually slashed.
	///
	/// Slashes from `active` funds first, and then `unlocking`, starting with the
	/// chunks that are closest to unlocking.
	fn slash(&mut self, mut value: Balance, minimum_balance: Balance) -> Balance {
		let pre_total = self.total;
		let total = &mut self.total;
		let active = &mut self.active;

		let slash_out_of =
			|total_remaining: &mut Balance, target: &mut Balance, value: &mut Balance| {
				let mut slash_from_target = (*value).min(*target);

				if !slash_from_target.is_zero() {
					*target -= slash_from_target;

					// Don't leave a dust balance in the staking system.
					if *target <= minimum_balance {
						slash_from_target += *target;
						*value += sp_std::mem::replace(target, Zero::zero());
					}

					*total_remaining = total_remaining.saturating_sub(slash_from_target);
					*value -= slash_from_target;
				}
			};

		slash_out_of(total, active, &mut value);

		let i = self
			.unlocking
			.iter_mut()
			.map(|chunk| {
				slash_out_of(total, &mut chunk.value, &mut value);
				chunk.value
			})
			.take_while(|value| value.is_zero()) // Take all fully-consumed chunks out.
			.count();

		// Kill all drained chunks.
		let _ = self.unlocking.drain(..i);

		pre_total.saturating_sub(*total)
	}
}

/// A record of the nominations made by a specific account.
#[derive(PartialEq, Eq, Clone, Encode, Decode, RuntimeDebug)]
pub struct Nominations<AccountId> {
	/// The targets of nomination.
	pub targets: Vec<AccountId>,
	/// The era the nominations were submitted.
	///
	/// Except for initial nominations which are considered submitted at era 0.
	pub submitted_in: EraIndex,
	/// Whether the nominations have been suppressed. This can happen due to slashing of the
	/// validators, or other events that might invalidate the nomination.
	///
	/// NOTE: this for future proofing and is thus far not used.
	pub suppressed: bool,
}

/// The amount of exposure (to slashing) than an individual nominator has.
#[derive(PartialEq, Eq, PartialOrd, Ord, Clone, Encode, Decode, RuntimeDebug)]
pub struct IndividualExposure<AccountId, Balance: HasCompact> {
	/// The stash account of the nominator in question.
	pub who: AccountId,
	/// Amount of funds exposed.
	#[codec(compact)]
	pub value: Balance,
}

/// A snapshot of the stake backing a single validator in the system.
#[derive(PartialEq, Eq, PartialOrd, Ord, Clone, Encode, Decode, Default, RuntimeDebug)]
pub struct Exposure<AccountId, Balance: HasCompact> {
	/// The total balance backing this validator.
	#[codec(compact)]
	pub total: Balance,
	/// The validator's own stash that is exposed.
	#[codec(compact)]
	pub own: Balance,
	/// The portions of nominators stashes that are exposed.
	pub others: Vec<IndividualExposure<AccountId, Balance>>,
}

/// A pending slash record. The value of the slash has been computed but not applied yet,
/// rather deferred for several eras.
#[derive(Encode, Decode, Default, RuntimeDebug)]
pub struct UnappliedSlash<AccountId, Balance: HasCompact> {
	/// The stash ID of the offending validator.
	validator: AccountId,
	/// The validator's own slash.
	own: Balance,
	/// All other slashed stakers and amounts.
	others: Vec<(AccountId, Balance)>,
	/// Reporters of the offence; bounty payout recipients.
	reporters: Vec<AccountId>,
	/// The amount of payout.
	payout: Balance,
}

/// Means for interacting with a specialized version of the `session` trait.
///
/// This is needed because `Staking` sets the `ValidatorIdOf` of the `pallet_session::Config`
pub trait SessionInterface<AccountId>: frame_system::Config {
	/// Disable a given validator by stash ID.
	///
	/// Returns `true` if new era should be forced at the end of this session.
	/// This allows preventing a situation where there is too many validators
	/// disabled and block production stalls.
	fn disable_validator(validator: &AccountId) -> Result<bool, ()>;
	/// Get the validators from session.
	fn validators() -> Vec<AccountId>;
	/// Prune historical session tries up to but not including the given index.
	fn prune_historical_up_to(up_to: SessionIndex);
}

impl<T: Config> SessionInterface<<T as frame_system::Config>::AccountId> for T
where
	T: pallet_session::Config<ValidatorId = <T as frame_system::Config>::AccountId>,
	T: pallet_session::historical::Config<
		FullIdentification = Exposure<<T as frame_system::Config>::AccountId, BalanceOf<T>>,
		FullIdentificationOf = ExposureOf<T>,
	>,
	T::SessionHandler: pallet_session::SessionHandler<<T as frame_system::Config>::AccountId>,
	T::SessionManager: pallet_session::SessionManager<<T as frame_system::Config>::AccountId>,
	T::ValidatorIdOf: Convert<
		<T as frame_system::Config>::AccountId,
		Option<<T as frame_system::Config>::AccountId>,
	>,
{
	fn disable_validator(validator: &<T as frame_system::Config>::AccountId) -> Result<bool, ()> {
		<pallet_session::Pallet<T>>::disable(validator)
	}

	fn validators() -> Vec<<T as frame_system::Config>::AccountId> {
		<pallet_session::Pallet<T>>::validators()
	}

	fn prune_historical_up_to(up_to: SessionIndex) {
		<pallet_session::historical::Pallet<T>>::prune_up_to(up_to);
	}
}

/// Handler for determining how much of a balance should be paid out on the current era.
pub trait EraPayout<Balance> {
	/// Determine the payout for this era.
	///
	/// Returns the amount to be paid to stakers in this era, as well as whatever else should be
	/// paid out ("the rest").
	fn era_payout(
		total_staked: Balance,
		total_issuance: Balance,
		era_duration_millis: u64,
	) -> (Balance, Balance);
}

impl<Balance: Default> EraPayout<Balance> for () {
	fn era_payout(
		_total_staked: Balance,
		_total_issuance: Balance,
		_era_duration_millis: u64,
	) -> (Balance, Balance) {
		(Default::default(), Default::default())
	}
}

/// 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>
{
	fn era_payout(
		total_staked: Balance,
		total_issuance: Balance,
		era_duration_millis: u64,
	) -> (Balance, Balance) {
		let (validator_payout, max_payout) = inflation::compute_total_payout(
			&T::get(),
			total_staked,
			total_issuance,
			// Duration of era; more than u64::MAX is rewarded as u64::MAX.
			era_duration_millis,
		);
		let rest = max_payout.saturating_sub(validator_payout.clone());
		(validator_payout, rest)
	}
}

/// Mode of era-forcing.
#[derive(Copy, Clone, PartialEq, Eq, Encode, Decode, RuntimeDebug)]
#[cfg_attr(feature = "std", derive(serde::Serialize, serde::Deserialize))]
pub enum Forcing {
	/// Not forcing anything - just let whatever happen.
	NotForcing,
	/// Force a new era, then reset to `NotForcing` as soon as it is done.
	/// Note that this will force to trigger an election until a new era is triggered, if the
	/// election failed, the next session end will trigger a new election again, until success.
	ForceNew,
	/// Avoid a new era indefinitely.
	ForceNone,
	/// Force a new era at the end of all sessions indefinitely.
	ForceAlways,
}

impl Default for Forcing {
	fn default() -> Self {
		Forcing::NotForcing
	}
}

// A value placed in storage that represents the current version of the Staking storage. This value
// is used by the `on_runtime_upgrade` logic to determine whether we run storage migration logic.
// This should match directly with the semantic versions of the Rust crate.
#[derive(Encode, Decode, Clone, Copy, PartialEq, Eq, RuntimeDebug)]
enum Releases {
	V1_0_0Ancient,
	V2_0_0,
	V3_0_0,
	V4_0_0,
	V5_0_0, // blockable validators.
	V6_0_0, // removal of all storage associated with offchain phragmen.
	V7_0_0, // keep track of number of nominators / validators in map
}

impl Default for Releases {
	fn default() -> Self {
		Releases::V7_0_0
	}
}

pub mod migrations {
	use super::*;

	pub mod v7 {
		use super::*;

		pub fn pre_migrate<T: Config>() -> Result<(), &'static str> {
			assert!(
				CounterForValidators::<T>::get().is_zero(),
				"CounterForValidators already set."
			);
			assert!(
				CounterForNominators::<T>::get().is_zero(),
				"CounterForNominators already set."
			);
			assert!(StorageVersion::<T>::get() == Releases::V6_0_0);
			Ok(())
		}

		pub fn migrate<T: Config>() -> Weight {
			log!(info, "Migrating staking to Releases::V7_0_0");
			let validator_count = Validators::<T>::iter().count() as u32;
			let nominator_count = Nominators::<T>::iter().count() as u32;

			CounterForValidators::<T>::put(validator_count);
			CounterForNominators::<T>::put(nominator_count);

			StorageVersion::<T>::put(Releases::V7_0_0);
			log!(info, "Completed staking migration to Releases::V7_0_0");

			T::DbWeight::get()
				.reads_writes(validator_count.saturating_add(nominator_count).into(), 2)
		}
	}

	pub mod v6 {
		use super::*;
		use frame_support::{generate_storage_alias, traits::Get, weights::Weight};

		// NOTE: value type doesn't matter, we just set it to () here.
		generate_storage_alias!(Staking, SnapshotValidators => Value<()>);
		generate_storage_alias!(Staking, SnapshotNominators => Value<()>);
		generate_storage_alias!(Staking, QueuedElected => Value<()>);
		generate_storage_alias!(Staking, QueuedScore => Value<()>);
		generate_storage_alias!(Staking, EraElectionStatus => Value<()>);
		generate_storage_alias!(Staking, IsCurrentSessionFinal => Value<()>);

		/// check to execute prior to migration.
		pub fn pre_migrate<T: Config>() -> Result<(), &'static str> {
			// these may or may not exist.
			log!(info, "SnapshotValidators.exits()? {:?}", SnapshotValidators::exists());
			log!(info, "SnapshotNominators.exits()? {:?}", SnapshotNominators::exists());
			log!(info, "QueuedElected.exits()? {:?}", QueuedElected::exists());
			log!(info, "QueuedScore.exits()? {:?}", QueuedScore::exists());
			// these must exist.
			assert!(
				IsCurrentSessionFinal::exists(),
				"IsCurrentSessionFinal storage item not found!"
			);
			assert!(EraElectionStatus::exists(), "EraElectionStatus storage item not found!");
			Ok(())
		}

		/// Migrate storage to v6.
		pub fn migrate<T: Config>() -> Weight {
			log!(info, "Migrating staking to Releases::V6_0_0");

			SnapshotValidators::kill();
			SnapshotNominators::kill();
			QueuedElected::kill();
			QueuedScore::kill();
			EraElectionStatus::kill();
			IsCurrentSessionFinal::kill();

			StorageVersion::<T>::put(Releases::V6_0_0);
			log!(info, "Done.");
			T::DbWeight::get().writes(6 + 1)
		}
	}
}

#[frame_support::pallet]
pub mod pallet {
	use super::*;

	#[pallet::pallet]
	#[pallet::generate_store(pub(super) trait Store)]
	pub struct Pallet<T>(_);

	#[pallet::config]
	pub trait Config: frame_system::Config + SendTransactionTypes<Call<Self>> {
		/// The staking balance.
		type Currency: LockableCurrency<Self::AccountId, Moment = Self::BlockNumber>;

		/// Time used for computing era duration.
		///
		/// It is guaranteed to start being called from the first `on_finalize`. Thus value at genesis
		/// is not used.
		type UnixTime: UnixTime;

		/// Convert a balance into a number used for election calculation. This must fit into a `u64`
		/// but is allowed to be sensibly lossy. The `u64` is used to communicate with the
		/// [`sp_npos_elections`] crate which accepts u64 numbers and does operations in 128.
		/// Consequently, the backward convert is used convert the u128s from sp-elections back to a
		/// [`BalanceOf`].
		type CurrencyToVote: CurrencyToVote<BalanceOf<Self>>;

		/// Something that provides the election functionality.
		type ElectionProvider: frame_election_provider_support::ElectionProvider<
			Self::AccountId,
			Self::BlockNumber,
			// we only accept an election provider that has staking as data provider.
			DataProvider = Pallet<Self>,
		>;

		/// Something that provides the election functionality at genesis.
		type GenesisElectionProvider: frame_election_provider_support::ElectionProvider<
			Self::AccountId,
			Self::BlockNumber,
			DataProvider = Pallet<Self>,
		>;

		/// Maximum number of nominations per nominator.
		const MAX_NOMINATIONS: u32;

		/// Tokens have been minted and are unused for validator-reward.
		/// See [Era payout](./index.html#era-payout).
		type RewardRemainder: OnUnbalanced<NegativeImbalanceOf<Self>>;

		/// The overarching event type.
		type Event: From<Event<Self>> + IsType<<Self as frame_system::Config>::Event>;

		/// Handler for the unbalanced reduction when slashing a staker.
		type Slash: OnUnbalanced<NegativeImbalanceOf<Self>>;

		/// Handler for the unbalanced increment when rewarding a staker.
		type Reward: OnUnbalanced<PositiveImbalanceOf<Self>>;

		/// Number of sessions per era.
		#[pallet::constant]
		type SessionsPerEra: Get<SessionIndex>;

		/// Number of eras that staked funds must remain bonded for.
		#[pallet::constant]
		type BondingDuration: Get<EraIndex>;

		/// Number of eras that slashes are deferred by, after computation.
		///
		/// This should be less than the bonding duration. Set to 0 if slashes
		/// should be applied immediately, without opportunity for intervention.
		#[pallet::constant]
		type SlashDeferDuration: Get<EraIndex>;

		/// The origin which can cancel a deferred slash. Root can always do this.
		type SlashCancelOrigin: EnsureOrigin<Self::Origin>;

		/// Interface for interacting with a session pallet.
		type SessionInterface: self::SessionInterface<Self::AccountId>;

		/// The payout for validators and the system for the current era.
		/// See [Era payout](./index.html#era-payout).
		type EraPayout: EraPayout<BalanceOf<Self>>;

		/// Something that can estimate the next session change, accurately or as a best effort guess.
		type NextNewSession: EstimateNextNewSession<Self::BlockNumber>;

		/// The maximum number of nominators rewarded for each validator.
		///
		/// For each validator only the `$MaxNominatorRewardedPerValidator` biggest stakers can claim
		/// their reward. This used to limit the i/o cost for the nominator payout.
		#[pallet::constant]
		type MaxNominatorRewardedPerValidator: Get<u32>;

		/// Weight information for extrinsics in this pallet.
		type WeightInfo: WeightInfo;
	}

	#[pallet::extra_constants]
	impl<T: Config> Pallet<T> {
		// TODO: rename to snake case after https://github.com/paritytech/substrate/issues/8826 fixed.
		#[allow(non_snake_case)]
		fn MaxNominations() -> u32 {
			T::MAX_NOMINATIONS
		}
	}

	#[pallet::type_value]
	pub(crate) fn HistoryDepthOnEmpty() -> u32 {
		84u32
	}

	/// Number of eras to keep in history.
	///
	/// Information is kept for eras in `[current_era - history_depth; current_era]`.
	///
	/// Must be more than the number of eras delayed by session otherwise. I.e. active era must
	/// always be in history. I.e. `active_era > current_era - history_depth` must be
	/// guaranteed.
	#[pallet::storage]
	#[pallet::getter(fn history_depth)]
	pub(crate) type HistoryDepth<T> = StorageValue<_, u32, ValueQuery, HistoryDepthOnEmpty>;

	/// The ideal number of staking participants.
	#[pallet::storage]
	#[pallet::getter(fn validator_count)]
	pub type ValidatorCount<T> = StorageValue<_, u32, ValueQuery>;

	/// Minimum number of staking participants before emergency conditions are imposed.
	#[pallet::storage]
	#[pallet::getter(fn minimum_validator_count)]
	pub type MinimumValidatorCount<T> = StorageValue<_, u32, ValueQuery>;

	/// Any validators that may never be slashed or forcibly kicked. It's a Vec since they're
	/// easy to initialize and the performance hit is minimal (we expect no more than four
	/// invulnerables) and restricted to testnets.
	#[pallet::storage]
	#[pallet::getter(fn invulnerables)]
	pub type Invulnerables<T: Config> = StorageValue<_, Vec<T::AccountId>, ValueQuery>;

	/// Map from all locked "stash" accounts to the controller account.
	#[pallet::storage]
	#[pallet::getter(fn bonded)]
	pub type Bonded<T: Config> = StorageMap<_, Twox64Concat, T::AccountId, T::AccountId>;

	/// The minimum active bond to become and maintain the role of a nominator.
	#[pallet::storage]
	pub type MinNominatorBond<T: Config> = StorageValue<_, BalanceOf<T>, ValueQuery>;

	/// The minimum active bond to become and maintain the role of a validator.
	#[pallet::storage]
	pub type MinValidatorBond<T: Config> = StorageValue<_, BalanceOf<T>, ValueQuery>;

	/// Map from all (unlocked) "controller" accounts to the info regarding the staking.
	#[pallet::storage]
	#[pallet::getter(fn ledger)]
	pub type Ledger<T: Config> =
		StorageMap<_, Blake2_128Concat, T::AccountId, StakingLedger<T::AccountId, BalanceOf<T>>>;

	/// Where the reward payment should be made. Keyed by stash.
	#[pallet::storage]
	#[pallet::getter(fn payee)]
	pub type Payee<T: Config> =
		StorageMap<_, Twox64Concat, T::AccountId, RewardDestination<T::AccountId>, ValueQuery>;

	/// The map from (wannabe) validator stash key to the preferences of that validator.
	///
	/// When updating this storage item, you must also update the `CounterForValidators`.
	#[pallet::storage]
	#[pallet::getter(fn validators)]
	pub type Validators<T: Config> =
		StorageMap<_, Twox64Concat, T::AccountId, ValidatorPrefs, ValueQuery>;

	/// A tracker to keep count of the number of items in the `Validators` map.
	#[pallet::storage]