lib.rs

// Copyright 2017-2020 Parity Technologies (UK) Ltd.
// This file is part of Substrate.

// Substrate 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.

// Substrate 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 Substrate.  If not, see <http://www.gnu.org/licenses/>.

//! # Staking Module
//!
//! The Staking module is used to manage funds at stake by network maintainers.
//!
//! - [`staking::Trait`](./trait.Trait.html)
//! - [`Call`](./enum.Call.html)
//! - [`Module`](./struct.Module.html)
//!
//! ## Overview
//!
//! The Staking module 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 module 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`](./enum.Call.html#variant.bond) call.
//!
//! Stash accounts can change their associated controller using the
//! [`set_controller`](./enum.Call.html#variant.set_controller) call.
//!
//! There are three possible roles that any staked account pair can be in: `Validator`, `Nominator`
//! and `Idle` (defined in [`StakerStatus`](./enum.StakerStatus.html)). There are three
//! corresponding instructions to change between roles, namely:
//! [`validate`](./enum.Call.html#variant.validate),
//! [`nominate`](./enum.Call.html#variant.nominate), and [`chill`](./enum.Call.html#variant.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`](./enum.Call.html#variant.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`](enum.Call.html#variant.nominate) call.
//!
//! #### Rewards and Slash
//!
//! The **reward and slashing** procedure is the core of the Staking module, 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 [`T::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` module.
//!
//! 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`](enum.Call.html#variant.chill) call.
//!
//! ### Session managing
//!
//! The module 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 module enable the steps needed for entities to accept
//! and change their role, alongside some helper functions to get/set the metadata of the module.
//!
//! ### Public Functions
//!
//! The Staking module 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::{self as system, ensure_signed};
//! use pallet_staking::{self as staking};
//!
//! pub trait Trait: staking::Trait {}
//!
//! decl_module! {
//! 	pub struct Module<T: Trait> for enum Call where origin: T::Origin {
//!			/// Reward a validator.
//! 		#[weight = frame_support::weights::SimpleDispatchInfo::default()]
//! 		pub fn reward_myself(origin) -> dispatch::DispatchResult {
//! 			let reported = ensure_signed(origin)?;
//! 			<staking::Module<T>>::reward_by_ids(vec![(reported, 10)]);
//! 			Ok(())
//! 		}
//! 	}
//! }
//! # fn main() { }
//! ```
//!
//! ## Implementation Details
//!
//! ### 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`](./enum.Call.html#variant.reward_by_ids) or
//! [`reward_by_indices`](./enum.Call.html#variant.reward_by_indices).
//!
//! [`Module`](./struct.Module.html) implements
//! [`pallet_authorship::EventHandler`](../pallet_authorship/trait.EventHandler.html) 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`](./struct.ValidatorPrefs.html#structfield.commission), that does not
//! get shared with the nominators at each reward payout through its
//! [`ValidatorPrefs`](./struct.ValidatorPrefs.html). 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`](./struct.Exposure.html#structfield.own) or
//! [`others`](./struct.Exposure.html#structfield.others) by
//! [`total`](./struct.Exposure.html#structfield.total) in [`Exposure`](./struct.Exposure.html)).
//!
//! All entities who receive a reward have the option to choose their reward destination
//! through the [`Payee`](./struct.Payee.html) storage item (see
//! [`set_payee`](enum.Call.html#variant.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`](enum.Call.html#variant.unbond) call. Note that the funds are not immediately
//! accessible. Instead, a duration denoted by [`BondingDuration`](./struct.BondingDuration.html)
//! (in number of eras) must pass until the funds can actually be removed. Once the
//! `BondingDuration` is over, the [`withdraw_unbonded`](./enum.Call.html#variant.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`](enum.Call.html#variant.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 module depends on the [`GenesisConfig`](./struct.GenesisConfig.html).
//! 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 module's `Validators` at the end of each era.

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

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

pub mod slashing;
pub mod offchain_election;
pub mod inflation;

use sp_std::{
	result,
	prelude::*,
	collections::btree_map::BTreeMap,
	convert::{TryInto, From},
	mem::size_of,
};
use codec::{HasCompact, Encode, Decode};
use frame_support::{
	decl_module, decl_event, decl_storage, ensure, decl_error, debug,
	weights::{SimpleDispatchInfo, Weight},
	storage::IterableStorageMap,
	dispatch::{IsSubType, DispatchResult},
	traits::{
		Currency, LockIdentifier, LockableCurrency, WithdrawReasons, OnUnbalanced, Imbalance, Get,
		UnixTime, EstimateNextNewSession, EnsureOrigin,
	}
};
use pallet_session::historical;
use sp_runtime::{
	Perbill, PerU16, PerThing, RuntimeDebug,
	curve::PiecewiseLinear,
	traits::{
		Convert, Zero, StaticLookup, CheckedSub, Saturating, SaturatedConversion, AtLeast32Bit,
		SignedExtension,
	},
	transaction_validity::{
		TransactionValidityError, TransactionValidity, ValidTransaction, InvalidTransaction,
		TransactionSource,
	},
};
use sp_staking::{
	SessionIndex,
	offence::{OnOffenceHandler, OffenceDetails, Offence, ReportOffence, OffenceError},
};
#[cfg(feature = "std")]
use sp_runtime::{Serialize, Deserialize};
use frame_system::{
	self as system, ensure_signed, ensure_root, ensure_none,
	offchain::SubmitUnsignedTransaction,
};
use sp_phragmen::{
	ExtendedBalance, Assignment, PhragmenScore, PhragmenResult, build_support_map, evaluate_support,
	elect, generate_compact_solution_type, is_score_better, VotingLimit, SupportMap,
};

const DEFAULT_MINIMUM_VALIDATOR_COUNT: u32 = 4;
const STAKING_ID: LockIdentifier = *b"staking ";
pub const MAX_UNLOCKING_CHUNKS: usize = 32;
pub const MAX_NOMINATIONS: usize = <CompactAssignments as VotingLimit>::LIMIT;

/// Data type used to index nominators in the compact type
pub type NominatorIndex = u32;

/// Data type used to index validators in the compact type.
pub type ValidatorIndex = u16;

// Ensure the size of both ValidatorIndex and NominatorIndex. They both need to be well below usize.
static_assertions::const_assert!(size_of::<ValidatorIndex>() <= size_of::<usize>());
static_assertions::const_assert!(size_of::<NominatorIndex>() <= size_of::<usize>());

/// Maximum number of stakers that can be stored in a snapshot.
pub(crate) const MAX_VALIDATORS: usize = ValidatorIndex::max_value() as usize;
pub(crate) const MAX_NOMINATORS: usize = NominatorIndex::max_value() as usize;

/// 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;

// Note: Maximum nomination limit is set here -- 16.
generate_compact_solution_type!(pub GenericCompactAssignments, 16);

/// Information regarding the active era (era in used in session).
#[derive(Encode, Decode, RuntimeDebug)]
pub struct ActiveEraInfo {
	/// Index of era.
	index: EraIndex,
	/// Moment of start expresed 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>,
}

/// Accuracy used for on-chain phragmen.
pub type ChainAccuracy = Perbill;

/// Accuracy used for off-chain phragmen. This better be small.
pub type OffchainAccuracy = PerU16;

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

/// The compact type for election solutions.
pub type CompactAssignments =
	GenericCompactAssignments<NominatorIndex, ValidatorIndex, OffchainAccuracy>;

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

/// 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(Serialize, 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 {
	/// 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,
}

impl Default for RewardDestination {
	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,
}

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

/// 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 + AtLeast32Bit,
> 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: AtLeast32Bit + 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.
	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,
}

/// Indicate how an election round was computed.
#[derive(PartialEq, Eq, Clone, Copy, Encode, Decode, RuntimeDebug)]
pub enum ElectionCompute {
	/// Result was forcefully computed on chain at the end of the session.
	OnChain,
	/// Result was submitted and accepted to the chain via a signed transaction.
	Signed,
	/// Result was submitted and accepted to the chain via an unsigned transaction (by an
	/// authority).
	Unsigned,
}

/// The result of an election round.
#[derive(PartialEq, Eq, Clone, Encode, Decode, RuntimeDebug)]
pub struct ElectionResult<AccountId, Balance: HasCompact> {
	/// Flat list of validators who have been elected.
	elected_stashes: Vec<AccountId>,
	/// Flat list of new exposures, to be updated in the [`Exposure`] storage.
	exposures: Vec<(AccountId, Exposure<AccountId, Balance>)>,
	/// Type of the result. This is kept on chain only to track and report the best score's
	/// submission type. An optimisation could remove this.
	compute: ElectionCompute,
}

/// The status of the upcoming (offchain) election.
#[derive(PartialEq, Eq, Clone, Encode, Decode, RuntimeDebug)]
pub enum ElectionStatus<BlockNumber> {
	/// Nothing has and will happen for now. submission window is not open.
	Closed,
	/// The submission window has been open since the contained block number.
	Open(BlockNumber),
}

impl<BlockNumber: PartialEq> ElectionStatus<BlockNumber> {
	fn is_open_at(&self, n: BlockNumber) -> bool {
		*self == Self::Open(n)
	}

	fn is_closed(&self) -> bool {
		match self {
			Self::Closed => true,
			_ => false
		}
	}

	fn is_open(&self) -> bool {
		!self.is_closed()
	}
}

impl<BlockNumber> Default for ElectionStatus<BlockNumber> {
	fn default() -> Self {
		Self::Closed
	}
}

/// Means for interacting with a specialized version of the `session` trait.
///
/// This is needed because `Staking` sets the `ValidatorIdOf` of the `pallet_session::Trait`
pub trait SessionInterface<AccountId>: frame_system::Trait {
	/// 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: Trait> SessionInterface<<T as frame_system::Trait>::AccountId> for T where
	T: pallet_session::Trait<ValidatorId = <T as frame_system::Trait>::AccountId>,
	T: pallet_session::historical::Trait<
		FullIdentification = Exposure<<T as frame_system::Trait>::AccountId, BalanceOf<T>>,
		FullIdentificationOf = ExposureOf<T>,
	>,
	T::SessionHandler: pallet_session::SessionHandler<<T as frame_system::Trait>::AccountId>,
	T::SessionManager: pallet_session::SessionManager<<T as frame_system::Trait>::AccountId>,
	T::ValidatorIdOf:
		Convert<<T as frame_system::Trait>::AccountId, Option<<T as frame_system::Trait>::AccountId>>,
{
	fn disable_validator(validator: &<T as frame_system::Trait>::AccountId) -> Result<bool, ()> {
		<pallet_session::Module<T>>::disable(validator)
	}

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

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

pub trait Trait: frame_system::Trait {
	/// 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.
	/// TODO: #1377
	/// The backward convert should be removed as the new Phragmen API returns ratio.
	/// The post-processing needs it but will be moved to off-chain. TODO: #2908
	type CurrencyToVote: Convert<BalanceOf<Self>, u64> + Convert<u128, BalanceOf<Self>>;

	/// Tokens have been minted and are unused for validator-reward.
	type RewardRemainder: OnUnbalanced<NegativeImbalanceOf<Self>>;

	/// The overarching event type.
	type Event: From<Event<Self>> + Into<<Self as frame_system::Trait>::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.
	type SessionsPerEra: Get<SessionIndex>;

	/// Number of eras that staked funds must remain bonded for.
	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.
	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 module.
	type SessionInterface: self::SessionInterface<Self::AccountId>;

	/// The NPoS reward curve to use.
	type RewardCurve: Get<&'static PiecewiseLinear<'static>>;

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

	/// How many blocks ahead of the era, within the last do we try to run the phragmen offchain?
	/// Setting this to zero will disable the offchain compute and only on-chain seq-phragmen will
	/// be used.
	type ElectionLookahead: Get<Self::BlockNumber>;

	/// The overarching call type.
	type Call: From<Call<Self>> + IsSubType<Module<Self>, Self> + Clone;

	/// A transaction submitter.
	type SubmitTransaction: SubmitUnsignedTransaction<Self, <Self as Trait>::Call>;

	/// 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.
	type MaxNominatorRewardedPerValidator: Get<u32>;
}

/// Mode of era-forcing.
#[derive(Copy, Clone, PartialEq, Eq, Encode, Decode, RuntimeDebug)]
#[cfg_attr(feature = "std", derive(Serialize, 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.
	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,
}

impl Default for Releases {
	fn default() -> Self {
		Releases::V3_0_0
	}
}

decl_storage! {
	trait Store for Module<T: Trait> as Staking {
		/// 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.
		HistoryDepth get(fn history_depth) config(): u32 = 84;

		/// The ideal number of staking participants.
		pub ValidatorCount get(fn validator_count) config(): u32;

		/// Minimum number of staking participants before emergency conditions are imposed.
		pub MinimumValidatorCount get(fn minimum_validator_count) config():
			u32 = DEFAULT_MINIMUM_VALIDATOR_COUNT;

		/// 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.
		pub Invulnerables get(fn invulnerables) config(): Vec<T::AccountId>;

		/// Map from all locked "stash" accounts to the controller account.
		pub Bonded get(fn bonded): map hasher(twox_64_concat) T::AccountId => Option<T::AccountId>;

		/// Map from all (unlocked) "controller" accounts to the info regarding the staking.
		pub Ledger get(fn ledger):
			map hasher(blake2_128_concat) T::AccountId
			=> Option<StakingLedger<T::AccountId, BalanceOf<T>>>;

		/// Where the reward payment should be made. Keyed by stash.
		pub Payee get(fn payee): map hasher(twox_64_concat) T::AccountId => RewardDestination;

		/// The map from (wannabe) validator stash key to the preferences of that validator.
		pub Validators get(fn validators):
			map hasher(twox_64_concat) T::AccountId => ValidatorPrefs;

		/// The map from nominator stash key to the set of stash keys of all validators to nominate.
		pub Nominators get(fn nominators):
			map hasher(twox_64_concat) T::AccountId => Option<Nominations<T::AccountId>>;

		/// The current era index.
		///
		/// This is the latest planned era, depending on how the Session pallet queues the validator
		/// set, it might be active or not.
		pub CurrentEra get(fn current_era): Option<EraIndex>;

		/// The active era information, it holds index and start.
		///
		/// The active era is the era currently rewarded.
		/// Validator set of this era must be equal to `SessionInterface::validators`.
		pub ActiveEra get(fn active_era): Option<ActiveEraInfo>;

		/// The session index at which the era start for the last `HISTORY_DEPTH` eras.
		pub ErasStartSessionIndex get(fn eras_start_session_index):
			map hasher(twox_64_concat) EraIndex => Option<SessionIndex>;

		/// Exposure of validator at era.
		///
		/// This is keyed first by the era index to allow bulk deletion and then the stash account.
		///
		/// Is it removed after `HISTORY_DEPTH` eras.
		/// If stakers hasn't been set or has been removed then empty exposure is returned.
		pub ErasStakers get(fn eras_stakers):
			double_map hasher(twox_64_concat) EraIndex, hasher(twox_64_concat) T::AccountId
			=> Exposure<T::AccountId, BalanceOf<T>>;

		/// Clipped Exposure of validator at era.
		///
		/// This is similar to [`ErasStakers`] but number of nominators exposed is reduced to the
		/// `T::MaxNominatorRewardedPerValidator` biggest stakers.
		/// (Note: the field `total` and `own` of the exposure remains unchanged).
		/// This is used to limit the i/o cost for the nominator payout.
		///
		/// This is keyed fist by the era index to allow bulk deletion and then the stash account.
		///
		/// Is it removed after `HISTORY_DEPTH` eras.
		/// If stakers hasn't been set or has been removed then empty exposure is returned.
		pub ErasStakersClipped get(fn eras_stakers_clipped):
			double_map hasher(twox_64_concat) EraIndex, hasher(twox_64_concat) T::AccountId
			=> Exposure<T::AccountId, BalanceOf<T>>;

		/// Similar to `ErasStakers`, this holds the preferences of validators.
		///
		/// This is keyed first by the era index to allow bulk deletion and then the stash account.
		///
		/// Is it removed after `HISTORY_DEPTH` eras.
		// If prefs hasn't been set or has been removed then 0 commission is returned.
		pub ErasValidatorPrefs get(fn eras_validator_prefs):
			double_map hasher(twox_64_concat) EraIndex, hasher(twox_64_concat) T::AccountId
			=> ValidatorPrefs;

		/// The total validator era payout for the last `HISTORY_DEPTH` eras.
		///
		/// Eras that haven't finished yet or has been removed doesn't have reward.
		pub ErasValidatorReward get(fn eras_validator_reward):
			map hasher(twox_64_concat) EraIndex => Option<BalanceOf<T>>;

		/// Rewards for the last `HISTORY_DEPTH` eras.
		/// If reward hasn't been set or has been removed then 0 reward is returned.
		pub ErasRewardPoints get(fn eras_reward_points):
			map hasher(twox_64_concat) EraIndex => EraRewardPoints<T::AccountId>;

		/// The total amount staked for the last `HISTORY_DEPTH` eras.
		/// If total hasn't been set or has been removed then 0 stake is returned.
		pub ErasTotalStake get(fn eras_total_stake):
			map hasher(twox_64_concat) EraIndex => BalanceOf<T>;

		/// Mode of era forcing.
		pub ForceEra get(fn force_era) config(): Forcing;

		/// The percentage of the slash that is distributed to reporters.
		///
		/// The rest of the slashed value is handled by the `Slash`.
		pub SlashRewardFraction get(fn slash_reward_fraction) config(): Perbill;

		/// The amount of currency given to reporters of a slash event which was
		/// canceled by extraordinary circumstances (e.g. governance).
		pub CanceledSlashPayout get(fn canceled_payout) config(): BalanceOf<T>;

		/// All unapplied slashes that are queued for later.
		pub UnappliedSlashes:
			map hasher(twox_64_concat) EraIndex => Vec<UnappliedSlash<T::AccountId, BalanceOf<T>>>;

		/// A mapping from still-bonded eras to the first session index of that era.
		///
		/// Must contains information for eras for the range:
		/// `[active_era - bounding_duration; active_era]`
		BondedEras: Vec<(EraIndex, SessionIndex)>;

		/// All slashing events on validators, mapped by era to the highest slash proportion
		/// and slash value of the era.
		ValidatorSlashInEra:
			double_map hasher(twox_64_concat) EraIndex, hasher(twox_64_concat) T::AccountId
			=> Option<(Perbill, BalanceOf<T>)>;

		/// All slashing events on nominators, mapped by era to the highest slash value of the era.
		NominatorSlashInEra:
			double_map hasher(twox_64_concat) EraIndex, hasher(twox_64_concat) T::AccountId
			=> Option<BalanceOf<T>>;

		/// Slashing spans for stash accounts.
		SlashingSpans: map hasher(twox_64_concat) T::AccountId => Option<slashing::SlashingSpans>;

		/// Records information about the maximum slash of a stash within a slashing span,
		/// as well as how much reward has been paid out.
		SpanSlash:
			map hasher(twox_64_concat) (T::AccountId, slashing::SpanIndex)
			=> slashing::SpanRecord<BalanceOf<T>>;

		/// The earliest era for which we have a pending, unapplied slash.
		EarliestUnappliedSlash: Option<EraIndex>;

		/// Snapshot of validators at the beginning of the current election window. This should only
		/// have a value when [`EraElectionStatus`] == `ElectionStatus::Open(_)`.
		pub SnapshotValidators get(fn snapshot_validators): Option<Vec<T::AccountId>>;

		/// Snapshot of nominators at the beginning of the current election window. This should only
		/// have a value when [`EraElectionStatus`] == `ElectionStatus::Open(_)`.
		pub SnapshotNominators get(fn snapshot_nominators): Option<Vec<T::AccountId>>;

		/// The next validator set. At the end of an era, if this is available (potentially from the
		/// result of an offchain worker), it is immediately used. Otherwise, the on-chain election
		/// is executed.
		pub QueuedElected get(fn queued_elected): Option<ElectionResult<T::AccountId, BalanceOf<T>>>;

		/// The score of the current [`QueuedElected`].
		pub QueuedScore get(fn queued_score): Option<PhragmenScore>;

		/// Flag to control the execution of the offchain election. When `Open(_)`, we accept
		/// solutions to be submitted.
		pub EraElectionStatus get(fn era_election_status): ElectionStatus<T::BlockNumber>;

		/// True if the current planned session is final.
		pub IsCurrentSessionFinal get(fn is_current_session_final): bool = false;

		/// True if network has been upgraded to this version.
		/// Storage version of the pallet.
		///