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.

//! # Session Pallet
//!
//! The Session pallet allows validators to manage their session keys, provides a function for
//! changing the session length, and handles session rotation.
//!
//! - [`Config`]
//! - [`Call`]
//! - [`Pallet`]
//!
//! ## Overview
//!
//! ### Terminology
//! <!-- Original author of paragraph: @gavofyork -->
//!
//! - **Session:** A session is a period of time that has a constant set of validators. Validators
//!   can only join or exit the validator set at a session change. It is measured in block numbers.
//!   The block where a session is ended is determined by the `ShouldEndSession` trait. When the
//!   session is ending, a new validator set can be chosen by `OnSessionEnding` implementations.
//!
//! - **Session key:** A session key is actually several keys kept together that provide the various
//!   signing functions required by network authorities/validators in pursuit of their duties.
//! - **Validator ID:** Every account has an associated validator ID. For some simple staking
//!   systems, this may just be the same as the account ID. For staking systems using a
//!   stash/controller model, the validator ID would be the stash account ID of the controller.
//!
//! - **Session key configuration process:** Session keys are set using `set_keys` for use not in
//!   the next session, but the session after next. They are stored in `NextKeys`, a mapping between
//!   the caller's `ValidatorId` and the session keys provided. `set_keys` allows users to set their
//!   session key prior to being selected as validator. It is a public call since it uses
//!   `ensure_signed`, which checks that the origin is a signed account. As such, the account ID of
//!   the origin stored in `NextKeys` may not necessarily be associated with a block author or a
//!   validator. The session keys of accounts are removed once their account balance is zero.
//!
//! - **Session length:** This pallet does not assume anything about the length of each session.
//!   Rather, it relies on an implementation of `ShouldEndSession` to dictate a new session's start.
//!   This pallet provides the `PeriodicSessions` struct for simple periodic sessions.
//!
//! - **Session rotation configuration:** Configure as either a 'normal' (rewardable session where
//!   rewards are applied) or 'exceptional' (slashable) session rotation.
//!
//! - **Session rotation process:** At the beginning of each block, the `on_initialize` function
//!   queries the provided implementation of `ShouldEndSession`. If the session is to end the newly
//!   activated validator IDs and session keys are taken from storage and passed to the
//!   `SessionHandler`. The validator set supplied by `SessionManager::new_session` and the
//!   corresponding session keys, which may have been registered via `set_keys` during the previous
//!   session, are written to storage where they will wait one session before being passed to the
//!   `SessionHandler` themselves.
//!
//! ### Goals
//!
//! The Session pallet is designed to make the following possible:
//!
//! - Set session keys of the validator set for upcoming sessions.
//! - Control the length of sessions.
//! - Configure and switch between either normal or exceptional session rotations.
//!
//! ## Interface
//!
//! ### Dispatchable Functions
//!
//! - `set_keys` - Set a validator's session keys for upcoming sessions.
//!
//! ### Public Functions
//!
//! - `rotate_session` - Change to the next session. Register the new authority set. Queue changes
//!   for next session rotation.
//! - `disable_index` - Disable a validator by index.
//! - `disable` - Disable a validator by Validator ID
//!
//! ## Usage
//!
//! ### Example from the FRAME
//!
//! The [Staking pallet](../pallet_staking/index.html) uses the Session pallet to get the validator
//! set.
//!
//! ```
//! use pallet_session as session;
//!
//! fn validators<T: pallet_session::Config>() -> Vec<<T as pallet_session::Config>::ValidatorId> {
//! 	<pallet_session::Pallet<T>>::validators()
//! }
//! # fn main(){}
//! ```
//!
//! ## Related Pallets
//!
//! - [Staking](../pallet_staking/index.html)

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

#[cfg(feature = "historical")]
pub mod historical;
pub mod migrations;
#[cfg(test)]
mod mock;
#[cfg(test)]
mod tests;
pub mod weights;

use frame_support::{
	codec::{Decode, MaxEncodedLen},
	dispatch::{DispatchError, DispatchResult},
	ensure,
	traits::{
		EstimateNextNewSession, EstimateNextSessionRotation, FindAuthor, Get, OneSessionHandler,
		StorageVersion, ValidatorRegistration, ValidatorSet,
	},
	weights::Weight,
	Parameter,
};
use sp_runtime::{
	traits::{AtLeast32BitUnsigned, Convert, Member, One, OpaqueKeys, Zero},
	ConsensusEngineId, KeyTypeId, Permill, RuntimeAppPublic,
};
use sp_staking::SessionIndex;
use sp_std::{
	marker::PhantomData,
	ops::{Rem, Sub},
	prelude::*,
};

pub use pallet::*;
pub use weights::WeightInfo;

/// Decides whether the session should be ended.
pub trait ShouldEndSession<BlockNumber> {
	/// Return `true` if the session should be ended.
	fn should_end_session(now: BlockNumber) -> bool;
}

/// Ends the session after a fixed period of blocks.
///
/// The first session will have length of `Offset`, and
/// the following sessions will have length of `Period`.
/// This may prove nonsensical if `Offset` >= `Period`.
pub struct PeriodicSessions<Period, Offset>(PhantomData<(Period, Offset)>);

impl<
		BlockNumber: Rem<Output = BlockNumber> + Sub<Output = BlockNumber> + Zero + PartialOrd,
		Period: Get<BlockNumber>,
		Offset: Get<BlockNumber>,
	> ShouldEndSession<BlockNumber> for PeriodicSessions<Period, Offset>
{
	fn should_end_session(now: BlockNumber) -> bool {
		let offset = Offset::get();
		now >= offset && ((now - offset) % Period::get()).is_zero()
	}
}

impl<
		BlockNumber: AtLeast32BitUnsigned + Clone,
		Period: Get<BlockNumber>,
		Offset: Get<BlockNumber>,
	> EstimateNextSessionRotation<BlockNumber> for PeriodicSessions<Period, Offset>
{
	fn average_session_length() -> BlockNumber {
		Period::get()
	}

	fn estimate_current_session_progress(now: BlockNumber) -> (Option<Permill>, Weight) {
		let offset = Offset::get();
		let period = Period::get();

		// NOTE: we add one since we assume that the current block has already elapsed,
		// i.e. when evaluating the last block in the session the progress should be 100%
		// (0% is never returned).
		let progress = if now >= offset {
			let current = (now - offset) % period.clone() + One::one();
			Some(Permill::from_rational(current.clone(), period.clone()))
		} else {
			Some(Permill::from_rational(now + One::one(), offset))
		};

		// Weight note: `estimate_current_session_progress` has no storage reads and trivial
		// computational overhead. There should be no risk to the chain having this weight value be
		// zero for now. However, this value of zero was not properly calculated, and so it would be
		// reasonable to come back here and properly calculate the weight of this function.
		(progress, Zero::zero())
	}

	fn estimate_next_session_rotation(now: BlockNumber) -> (Option<BlockNumber>, Weight) {
		let offset = Offset::get();
		let period = Period::get();

		let next_session = if now > offset {
			let block_after_last_session = (now.clone() - offset) % period.clone();
			if block_after_last_session > Zero::zero() {
				now.saturating_add(period.saturating_sub(block_after_last_session))
			} else {
				// this branch happens when the session is already rotated or will rotate in this
				// block (depending on being called before or after `session::on_initialize`). Here,
				// we assume the latter, namely that this is called after `session::on_initialize`,
				// and thus we add period to it as well.
				now + period
			}
		} else {
			offset
		};

		// Weight note: `estimate_next_session_rotation` has no storage reads and trivial
		// computational overhead. There should be no risk to the chain having this weight value be
		// zero for now. However, this value of zero was not properly calculated, and so it would be
		// reasonable to come back here and properly calculate the weight of this function.
		(Some(next_session), Zero::zero())
	}
}

/// A trait for managing creation of new validator set.
pub trait SessionManager<ValidatorId> {
	/// Plan a new session, and optionally provide the new validator set.
	///
	/// Even if the validator-set is the same as before, if any underlying economic conditions have
	/// changed (i.e. stake-weights), the new validator set must be returned. This is necessary for
	/// consensus engines making use of the session pallet to issue a validator-set change so
	/// misbehavior can be provably associated with the new economic conditions as opposed to the
	/// old. The returned validator set, if any, will not be applied until `new_index`. `new_index`
	/// is strictly greater than from previous call.
	///
	/// The first session start at index 0.
	///
	/// `new_session(session)` is guaranteed to be called before `end_session(session-1)`. In other
	/// words, a new session must always be planned before an ongoing one can be finished.
	fn new_session(new_index: SessionIndex) -> Option<Vec<ValidatorId>>;
	/// Same as `new_session`, but it this should only be called at genesis.
	///
	/// The session manager might decide to treat this in a different way. Default impl is simply
	/// using [`new_session`](Self::new_session).
	fn new_session_genesis(new_index: SessionIndex) -> Option<Vec<ValidatorId>> {
		Self::new_session(new_index)
	}
	/// End the session.
	///
	/// Because the session pallet can queue validator set the ending session can be lower than the
	/// last new session index.
	fn end_session(end_index: SessionIndex);
	/// Start an already planned session.
	///
	/// The session start to be used for validation.
	fn start_session(start_index: SessionIndex);
}

impl<A> SessionManager<A> for () {
	fn new_session(_: SessionIndex) -> Option<Vec<A>> {
		None
	}
	fn start_session(_: SessionIndex) {}
	fn end_session(_: SessionIndex) {}
}

/// Handler for session life cycle events.
pub trait SessionHandler<ValidatorId> {
	/// All the key type ids this session handler can process.
	///
	/// The order must be the same as it expects them in
	/// [`on_new_session`](Self::on_new_session<Ks>) and
	/// [`on_genesis_session`](Self::on_genesis_session<Ks>).
	const KEY_TYPE_IDS: &'static [KeyTypeId];

	/// The given validator set will be used for the genesis session.
	/// It is guaranteed that the given validator set will also be used
	/// for the second session, therefore the first call to `on_new_session`
	/// should provide the same validator set.
	fn on_genesis_session<Ks: OpaqueKeys>(validators: &[(ValidatorId, Ks)]);

	/// Session set has changed; act appropriately. Note that this can be called
	/// before initialization of your pallet.
	///
	/// `changed` is true whenever any of the session keys or underlying economic
	/// identities or weightings behind those keys has changed.
	fn on_new_session<Ks: OpaqueKeys>(
		changed: bool,
		validators: &[(ValidatorId, Ks)],
		queued_validators: &[(ValidatorId, Ks)],
	);

	/// A notification for end of the session.
	///
	/// Note it is triggered before any [`SessionManager::end_session`] handlers,
	/// so we can still affect the validator set.
	fn on_before_session_ending() {}

	/// A validator got disabled. Act accordingly until a new session begins.
	fn on_disabled(validator_index: u32);
}

#[impl_trait_for_tuples::impl_for_tuples(1, 30)]
#[tuple_types_custom_trait_bound(OneSessionHandler<AId>)]
impl<AId> SessionHandler<AId> for Tuple {
	for_tuples!(
		const KEY_TYPE_IDS: &'static [KeyTypeId] = &[ #( <Tuple::Key as RuntimeAppPublic>::ID ),* ];
	);

	fn on_genesis_session<Ks: OpaqueKeys>(validators: &[(AId, Ks)]) {
		for_tuples!(
			#(
				let our_keys: Box<dyn Iterator<Item=_>> = Box::new(validators.iter()
					.map(|k| (&k.0, k.1.get::<Tuple::Key>(<Tuple::Key as RuntimeAppPublic>::ID)
						.unwrap_or_default())));

				Tuple::on_genesis_session(our_keys);
			)*
		)
	}

	fn on_new_session<Ks: OpaqueKeys>(
		changed: bool,
		validators: &[(AId, Ks)],
		queued_validators: &[(AId, Ks)],
	) {
		for_tuples!(
			#(
				let our_keys: Box<dyn Iterator<Item=_>> = Box::new(validators.iter()
					.map(|k| (&k.0, k.1.get::<Tuple::Key>(<Tuple::Key as RuntimeAppPublic>::ID)
						.unwrap_or_default())));
				let queued_keys: Box<dyn Iterator<Item=_>> = Box::new(queued_validators.iter()
					.map(|k| (&k.0, k.1.get::<Tuple::Key>(<Tuple::Key as RuntimeAppPublic>::ID)
						.unwrap_or_default())));
				Tuple::on_new_session(changed, our_keys, queued_keys);
			)*
		)
	}

	fn on_before_session_ending() {
		for_tuples!( #( Tuple::on_before_session_ending(); )* )
	}

	fn on_disabled(i: u32) {
		for_tuples!( #( Tuple::on_disabled(i); )* )
	}
}

/// `SessionHandler` for tests that use `UintAuthorityId` as `Keys`.
pub struct TestSessionHandler;
impl<AId> SessionHandler<AId> for TestSessionHandler {
	const KEY_TYPE_IDS: &'static [KeyTypeId] = &[sp_runtime::key_types::DUMMY];
	fn on_genesis_session<Ks: OpaqueKeys>(_: &[(AId, Ks)]) {}
	fn on_new_session<Ks: OpaqueKeys>(_: bool, _: &[(AId, Ks)], _: &[(AId, Ks)]) {}
	fn on_before_session_ending() {}
	fn on_disabled(_: u32) {}
}

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

	/// The current storage version.
	const STORAGE_VERSION: StorageVersion = StorageVersion::new(0);

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

	#[pallet::config]
	pub trait Config: frame_system::Config {
		/// The overarching event type.
		type Event: From<Event> + IsType<<Self as frame_system::Config>::Event>;

		/// A stable ID for a validator.
		type ValidatorId: Member
			+ Parameter
			+ MaybeSerializeDeserialize
			+ MaxEncodedLen
			+ TryFrom<Self::AccountId>;

		/// A conversion from account ID to validator ID.
		///
		/// Its cost must be at most one storage read.
		type ValidatorIdOf: Convert<Self::AccountId, Option<Self::ValidatorId>>;

		/// Indicator for when to end the session.
		type ShouldEndSession: ShouldEndSession<Self::BlockNumber>;

		/// Something that can predict the next session rotation. This should typically come from
		/// the same logical unit that provides [`ShouldEndSession`], yet, it gives a best effort
		/// estimate. It is helpful to implement [`EstimateNextNewSession`].
		type NextSessionRotation: EstimateNextSessionRotation<Self::BlockNumber>;

		/// Handler for managing new session.
		type SessionManager: SessionManager<Self::ValidatorId>;

		/// Handler when a session has changed.
		type SessionHandler: SessionHandler<Self::ValidatorId>;

		/// The keys.
		type Keys: OpaqueKeys + Member + Parameter + Default + MaybeSerializeDeserialize;

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

	#[pallet::genesis_config]
	pub struct GenesisConfig<T: Config> {
		pub keys: Vec<(T::AccountId, T::ValidatorId, T::Keys)>,
	}

	#[cfg(feature = "std")]
	impl<T: Config> Default for GenesisConfig<T> {
		fn default() -> Self {
			Self { keys: Default::default() }
		}
	}

	#[pallet::genesis_build]
	impl<T: Config> GenesisBuild<T> for GenesisConfig<T> {
		fn build(&self) {
			if T::SessionHandler::KEY_TYPE_IDS.len() != T::Keys::key_ids().len() {
				panic!("Number of keys in session handler and session keys does not match");
			}

			T::SessionHandler::KEY_TYPE_IDS
				.iter()
				.zip(T::Keys::key_ids())
				.enumerate()
				.for_each(|(i, (sk, kk))| {
					if sk != kk {
						panic!(
							"Session handler and session key expect different key type at index: {}",
							i,
						);
					}
				});

			for (account, val, keys) in self.keys.iter().cloned() {
				<Pallet<T>>::inner_set_keys(&val, keys)
					.expect("genesis config must not contain duplicates; qed");
				if frame_system::Pallet::<T>::inc_consumers(&account).is_err() {
					// This will leak a provider reference, however it only happens once (at
					// genesis) so it's really not a big deal and we assume that the user wants to
					// do this since it's the only way a non-endowed account can contain a session
					// key.
					frame_system::Pallet::<T>::inc_providers(&account);
				}
			}

			let initial_validators_0 =
				T::SessionManager::new_session_genesis(0).unwrap_or_else(|| {
					frame_support::print(
						"No initial validator provided by `SessionManager`, use \
						session config keys to generate initial validator set.",
					);
					self.keys.iter().map(|x| x.1.clone()).collect()
				});
			assert!(
				!initial_validators_0.is_empty(),
				"Empty validator set for session 0 in genesis block!"
			);

			let initial_validators_1 = T::SessionManager::new_session_genesis(1)
				.unwrap_or_else(|| initial_validators_0.clone());
			assert!(
				!initial_validators_1.is_empty(),
				"Empty validator set for session 1 in genesis block!"
			);

			let queued_keys: Vec<_> = initial_validators_1
				.iter()
				.cloned()
				.map(|v| (v.clone(), <Pallet<T>>::load_keys(&v).unwrap_or_default()))
				.collect();

			// Tell everyone about the genesis session keys
			T::SessionHandler::on_genesis_session::<T::Keys>(&queued_keys);

			<Validators<T>>::put(initial_validators_0);
			<QueuedKeys<T>>::put(queued_keys);

			T::SessionManager::start_session(0);
		}
	}

	/// The current set of validators.
	#[pallet::storage]
	#[pallet::getter(fn validators)]
	pub type Validators<T: Config> = StorageValue<_, Vec<T::ValidatorId>, ValueQuery>;

	/// Current index of the session.
	#[pallet::storage]
	#[pallet::getter(fn current_index)]
	pub type CurrentIndex<T> = StorageValue<_, SessionIndex, ValueQuery>;

	/// True if the underlying economic identities or weighting behind the validators
	/// has changed in the queued validator set.
	#[pallet::storage]
	pub type QueuedChanged<T> = StorageValue<_, bool, ValueQuery>;

	/// The queued keys for the next session. When the next session begins, these keys
	/// will be used to determine the validator's session keys.
	#[pallet::storage]
	#[pallet::getter(fn queued_keys)]
	pub type QueuedKeys<T: Config> = StorageValue<_, Vec<(T::ValidatorId, T::Keys)>, ValueQuery>;

	/// Indices of disabled validators.
	///
	/// The vec is always kept sorted so that we can find whether a given validator is
	/// disabled using binary search. It gets cleared when `on_session_ending` returns
	/// a new set of identities.
	#[pallet::storage]
	#[pallet::getter(fn disabled_validators)]
	pub type DisabledValidators<T> = StorageValue<_, Vec<u32>, ValueQuery>;

	/// The next session keys for a validator.
	#[pallet::storage]
	pub type NextKeys<T: Config> =
		StorageMap<_, Twox64Concat, T::ValidatorId, T::Keys, OptionQuery>;

	/// The owner of a key. The key is the `KeyTypeId` + the encoded key.
	#[pallet::storage]
	pub type KeyOwner<T: Config> =
		StorageMap<_, Twox64Concat, (KeyTypeId, Vec<u8>), T::ValidatorId, OptionQuery>;

	#[pallet::event]
	#[pallet::generate_deposit(pub(super) fn deposit_event)]
	pub enum Event {
		/// New session has happened. Note that the argument is the \[session_index\], not the
		/// block number as the type might suggest.
		NewSession(SessionIndex),
	}

	/// Old name generated by `decl_event`.
	#[deprecated(note = "use `Event` instead")]
	pub type RawEvent = Event;

	/// Error for the session pallet.
	#[pallet::error]
	pub enum Error<T> {
		/// Invalid ownership proof.
		InvalidProof,
		/// No associated validator ID for account.
		NoAssociatedValidatorId,
		/// Registered duplicate key.
		DuplicatedKey,
		/// No keys are associated with this account.
		NoKeys,
		/// Key setting account is not live, so it's impossible to associate keys.
		NoAccount,
	}

	#[pallet::hooks]
	impl<T: Config> Hooks<BlockNumberFor<T>> for Pallet<T> {
		/// Called when a block is initialized. Will rotate session if it is the last
		/// block of the current session.
		fn on_initialize(n: T::BlockNumber) -> Weight {
			if T::ShouldEndSession::should_end_session(n) {
				Self::rotate_session();
				T::BlockWeights::get().max_block
			} else {
				// NOTE: the non-database part of the weight for `should_end_session(n)` is
				// included as weight for empty block, the database part is expected to be in
				// cache.
				0
			}
		}
	}

	#[pallet::call]
	impl<T: Config> Pallet<T> {
		/// Sets the session key(s) of the function caller to `keys`.
		/// Allows an account to set its session key prior to becoming a validator.
		/// This doesn't take effect until the next session.
		///
		/// The dispatch origin of this function must be signed.
		///
		/// # <weight>
		/// - Complexity: `O(1)`. Actual cost depends on the number of length of
		///   `T::Keys::key_ids()` which is fixed.
		/// - DbReads: `origin account`, `T::ValidatorIdOf`, `NextKeys`
		/// - DbWrites: `origin account`, `NextKeys`
		/// - DbReads per key id: `KeyOwner`
		/// - DbWrites per key id: `KeyOwner`
		/// # </weight>
		#[pallet::weight(T::WeightInfo::set_keys())]
		pub fn set_keys(origin: OriginFor<T>, keys: T::Keys, proof: Vec<u8>) -> DispatchResult {
			let who = ensure_signed(origin)?;
			ensure!(keys.ownership_proof_is_valid(&proof), Error::<T>::InvalidProof);

			Self::do_set_keys(&who, keys)?;
			Ok(())
		}

		/// Removes any session key(s) of the function caller.
		///
		/// This doesn't take effect until the next session.
		///
		/// The dispatch origin of this function must be Signed and the account must be either be
		/// convertible to a validator ID using the chain's typical addressing system (this usually
		/// means being a controller account) or directly convertible into a validator ID (which
		/// usually means being a stash account).
		///
		/// # <weight>
		/// - Complexity: `O(1)` in number of key types. Actual cost depends on the number of length
		///   of `T::Keys::key_ids()` which is fixed.
		/// - DbReads: `T::ValidatorIdOf`, `NextKeys`, `origin account`
		/// - DbWrites: `NextKeys`, `origin account`
		/// - DbWrites per key id: `KeyOwner`
		/// # </weight>
		#[pallet::weight(T::WeightInfo::purge_keys())]
		pub fn purge_keys(origin: OriginFor<T>) -> DispatchResult {
			let who = ensure_signed(origin)?;
			Self::do_purge_keys(&who)?;
			Ok(())
		}
	}
}

impl<T: Config> Pallet<T> {
	/// Move on to next session. Register new validator set and session keys. Changes to the
	/// validator set have a session of delay to take effect. This allows for equivocation
	/// punishment after a fork.
	pub fn rotate_session() {
		let session_index = <CurrentIndex<T>>::get();
		log::trace!(target: "runtime::session", "rotating session {:?}", session_index);

		let changed = <QueuedChanged<T>>::get();

		// Inform the session handlers that a session is going to end.
		T::SessionHandler::on_before_session_ending();
		T::SessionManager::end_session(session_index);

		// Get queued session keys and validators.
		let session_keys = <QueuedKeys<T>>::get();
		let validators =
			session_keys.iter().map(|(validator, _)| validator.clone()).collect::<Vec<_>>();
		<Validators<T>>::put(&validators);

		if changed {
			// reset disabled validators
			<DisabledValidators<T>>::take();
		}

		// Increment session index.
		let session_index = session_index + 1;
		<CurrentIndex<T>>::put(session_index);

		T::SessionManager::start_session(session_index);

		// Get next validator set.
		let maybe_next_validators = T::SessionManager::new_session(session_index + 1);
		let (next_validators, next_identities_changed) =
			if let Some(validators) = maybe_next_validators {
				// NOTE: as per the documentation on `OnSessionEnding`, we consider
				// the validator set as having changed even if the validators are the
				// same as before, as underlying economic conditions may have changed.
				(validators, true)
			} else {
				(<Validators<T>>::get(), false)
			};

		// Queue next session keys.
		let (queued_amalgamated, next_changed) = {
			// until we are certain there has been a change, iterate the prior
			// validators along with the current and check for changes
			let mut changed = next_identities_changed;

			let mut now_session_keys = session_keys.iter();
			let mut check_next_changed = |keys: &T::Keys| {
				if changed {
					return
				}
				// since a new validator set always leads to `changed` starting
				// as true, we can ensure that `now_session_keys` and `next_validators`
				// have the same length. this function is called once per iteration.
				if let Some(&(_, ref old_keys)) = now_session_keys.next() {
					if old_keys != keys {
						changed = true;
						return
					}
				}
			};
			let queued_amalgamated = next_validators
				.into_iter()
				.map(|a| {
					let k = Self::load_keys(&a).unwrap_or_default();
					check_next_changed(&k);
					(a, k)
				})
				.collect::<Vec<_>>();

			(queued_amalgamated, changed)
		};

		<QueuedKeys<T>>::put(queued_amalgamated.clone());
		<QueuedChanged<T>>::put(next_changed);

		// Record that this happened.
		Self::deposit_event(Event::NewSession(session_index));

		// Tell everyone about the new session keys.
		T::SessionHandler::on_new_session::<T::Keys>(changed, &session_keys, &queued_amalgamated);
	}

	/// Disable the validator of index `i`, returns `false` if the validator was already disabled.
	pub fn disable_index(i: u32) -> bool {
		if i >= Validators::<T>::decode_len().unwrap_or(0) as u32 {
			return false
		}

		<DisabledValidators<T>>::mutate(|disabled| {
			if let Err(index) = disabled.binary_search(&i) {
				disabled.insert(index, i);
				T::SessionHandler::on_disabled(i);
				return true
			}

			false
		})
	}

	/// Disable the validator identified by `c`. (If using with the staking pallet,
	/// this would be their *stash* account.)
	///
	/// Returns `false` either if the validator could not be found or it was already
	/// disabled.
	pub fn disable(c: &T::ValidatorId) -> bool {
		Self::validators()
			.iter()
			.position(|i| i == c)
			.map(|i| Self::disable_index(i as u32))
			.unwrap_or(false)
	}

	/// Upgrade the key type from some old type to a new type. Supports adding
	/// and removing key types.
	///
	/// This function should be used with extreme care and only during an
	/// `on_runtime_upgrade` block. Misuse of this function can put your blockchain
	/// into an unrecoverable state.
	///
	/// Care should be taken that the raw versions of the
	/// added keys are unique for every `ValidatorId, KeyTypeId` combination.
	/// This is an invariant that the session pallet typically maintains internally.
	///
	/// As the actual values of the keys are typically not known at runtime upgrade,
	/// it's recommended to initialize the keys to a (unique) dummy value with the expectation
	/// that all validators should invoke `set_keys` before those keys are actually
	/// required.
	pub fn upgrade_keys<Old, F>(upgrade: F)
	where
		Old: OpaqueKeys + Member + Decode,
		F: Fn(T::ValidatorId, Old) -> T::Keys,
	{
		let old_ids = Old::key_ids();
		let new_ids = T::Keys::key_ids();

		// Translate NextKeys, and key ownership relations at the same time.
		<NextKeys<T>>::translate::<Old, _>(|val, old_keys| {
			// Clear all key ownership relations. Typically the overlap should
			// stay the same, but no guarantees by the upgrade function.
			for i in old_ids.iter() {
				Self::clear_key_owner(*i, old_keys.get_raw(*i));
			}

			let new_keys = upgrade(val.clone(), old_keys);

			// And now set the new ones.
			for i in new_ids.iter() {
				Self::put_key_owner(*i, new_keys.get_raw(*i), &val);
			}

			Some(new_keys)
		});

		let _ = <QueuedKeys<T>>::translate::<Vec<(T::ValidatorId, Old)>, _>(|k| {
			k.map(|k| {
				k.into_iter()
					.map(|(val, old_keys)| (val.clone(), upgrade(val, old_keys)))
					.collect::<Vec<_>>()
			})
		});
	}

	/// Perform the set_key operation, checking for duplicates. Does not set `Changed`.
	///
	/// This ensures that the reference counter in system is incremented appropriately and as such
	/// must accept an account ID, rather than a validator ID.
	fn do_set_keys(account: &T::AccountId, keys: T::Keys) -> DispatchResult {
		let who = T::ValidatorIdOf::convert(account.clone())
			.ok_or(Error::<T>::NoAssociatedValidatorId)?;

		ensure!(frame_system::Pallet::<T>::can_inc_consumer(&account), Error::<T>::NoAccount);
		let old_keys = Self::inner_set_keys(&who, keys)?;
		if old_keys.is_none() {
			let assertion = frame_system::Pallet::<T>::inc_consumers(&account).is_ok();
			debug_assert!(assertion, "can_inc_consumer() returned true; no change since; qed");
		}

		Ok(())
	}

	/// Perform the set_key operation, checking for duplicates. Does not set `Changed`.
	///
	/// The old keys for this validator are returned, or `None` if there were none.
	///
	/// This does not ensure that the reference counter in system is incremented appropriately, it
	/// must be done by the caller or the keys will be leaked in storage.
	fn inner_set_keys(
		who: &T::ValidatorId,
		keys: T::Keys,
	) -> Result<Option<T::Keys>, DispatchError> {
		let old_keys = Self::load_keys(who);

		for id in T::Keys::key_ids() {
			let key = keys.get_raw(*id);

			// ensure keys are without duplication.
			ensure!(
				Self::key_owner(*id, key).map_or(true, |owner| &owner == who),
				Error::<T>::DuplicatedKey,
			);
		}

		for id in T::Keys::key_ids() {
			let key = keys.get_raw(*id);

			if let Some(old) = old_keys.as_ref().map(|k| k.get_raw(*id)) {
				if key == old {
					continue
				}

				Self::clear_key_owner(*id, old);
			}

			Self::put_key_owner(*id, key, who);
		}

		Self::put_keys(who, &keys);
		Ok(old_keys)
	}

	fn do_purge_keys(account: &T::AccountId) -> DispatchResult {
		let who = T::ValidatorIdOf::convert(account.clone())
			// `purge_keys` may not have a controller-stash pair any more. If so then we expect the
			// stash account to be passed in directly and convert that to a `ValidatorId` using the
			// `TryFrom` trait if supported.
			.or_else(|| T::ValidatorId::try_from(account.clone()).ok())
			.ok_or(Error::<T>::NoAssociatedValidatorId)?;

		let old_keys = Self::take_keys(&who).ok_or(Error::<T>::NoKeys)?;
		for id in T::Keys::key_ids() {
			let key_data = old_keys.get_raw(*id);
			Self::clear_key_owner(*id, key_data);
		}
		frame_system::Pallet::<T>::dec_consumers(&account);

		Ok(())
	}

	fn load_keys(v: &T::ValidatorId) -> Option<T::Keys> {
		<NextKeys<T>>::get(v)
	}

	fn take_keys(v: &T::ValidatorId) -> Option<T::Keys> {
		<NextKeys<T>>::take(v)
	}

	fn put_keys(v: &T::ValidatorId, keys: &T::Keys) {
		<NextKeys<T>>::insert(v, keys);
	}

	/// Query the owner of a session key by returning the owner's validator ID.
	pub fn key_owner(id: KeyTypeId, key_data: &[u8]) -> Option<T::ValidatorId> {
		<KeyOwner<T>>::get((id, key_data))
	}

	fn put_key_owner(id: KeyTypeId, key_data: &[u8], v: &T::ValidatorId) {
		<KeyOwner<T>>::insert((id, key_data), v)
	}

	fn clear_key_owner(id: KeyTypeId, key_data: &[u8]) {
		<KeyOwner<T>>::remove((id, key_data));
	}
}

impl<T: Config> ValidatorRegistration<T::ValidatorId> for Pallet<T> {
	fn is_registered(id: &T::ValidatorId) -> bool {
		Self::load_keys(id).is_some()
	}
}

impl<T: Config> ValidatorSet<T::AccountId> for Pallet<T> {
	type ValidatorId = T::ValidatorId;
	type ValidatorIdOf = T::ValidatorIdOf;

	fn session_index() -> sp_staking::SessionIndex {
		Pallet::<T>::current_index()
	}

	fn validators() -> Vec<Self::ValidatorId> {
		Pallet::<T>::validators()
	}
}

impl<T: Config> EstimateNextNewSession<T::BlockNumber> for Pallet<T> {
	fn average_session_length() -> T::BlockNumber {
		T::NextSessionRotation::average_session_length()
	}

	/// This session pallet always calls new_session and next_session at the same time, hence we
	/// do a simple proxy and pass the function to next rotation.
	fn estimate_next_new_session(now: T::BlockNumber) -> (Option<T::BlockNumber>, Weight) {
		T::NextSessionRotation::estimate_next_session_rotation(now)
	}
}

impl<T: Config> frame_support::traits::DisabledValidators for Pallet<T> {
	fn is_disabled(index: u32) -> bool {
		<Pallet<T>>::disabled_validators().binary_search(&index).is_ok()
	}
}

/// Wraps the author-scraping logic for consensus engines that can recover
/// the canonical index of an author. This then transforms it into the
/// registering account-ID of that session key index.
pub struct FindAccountFromAuthorIndex<T, Inner>(sp_std::marker::PhantomData<(T, Inner)>);

impl<T: Config, Inner: FindAuthor<u32>> FindAuthor<T::ValidatorId>
	for FindAccountFromAuthorIndex<T, Inner>
{
	fn find_author<'a, I>(digests: I) -> Option<T::ValidatorId>
	where
		I: 'a + IntoIterator<Item = (ConsensusEngineId, &'a [u8])>,
	{
		let i = Inner::find_author(digests)?;

		let validators = <Pallet<T>>::validators();
		validators.get(i as usize).map(|k| k.clone())
	}
}