git.delta.rocks / unique-network / refs/commits / 70abe578b643

1// Copyright 2019-2022 Unique Network (Gibraltar) Ltd.2// This file is part of Unique Network.34// Unique Network is free software: you can redistribute it and/or modify5// it under the terms of the GNU General Public License as published by6// the Free Software Foundation, either version 3 of the License, or7// (at your option) any later version.89// Unique Network is distributed in the hope that it will be useful,10// but WITHOUT ANY WARRANTY; without even the implied warranty of11// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the12// GNU General Public License for more details.1314// You should have received a copy of the GNU General Public License15// along with Unique Network. If not, see <http://www.gnu.org/licenses/>.1617// Original license:18// Copyright (C) 2021 Parity Technologies (UK) Ltd.19// SPDX-License-Identifier: Apache-2.02021// Licensed under the Apache License, Version 2.0 (the "License");22// you may not use this file except in compliance with the License.23// You may obtain a copy of the License at24//25// 	http://www.apache.org/licenses/LICENSE-2.026//27// Unless required by applicable law or agreed to in writing, software28// distributed under the License is distributed on an "AS IS" BASIS,29// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.30// See the License for the specific language governing permissions and31// limitations under the License.3233// todo:collator documentation34//! Collator Selection pallet.35//!36//! A pallet to manage collators in a parachain.37//!38//! ## Overview39//!40//! The Collator Selection pallet manages the collators of a parachain. **Collation is _not_ a41//! secure activity** and this pallet does not implement any game-theoretic mechanisms to meet BFT42//! safety assumptions of the chosen set.43//!44//! ## Terminology45//!46//! - Collator: A parachain block producer.47//! - Bond: An amount of `Balance` _reserved_ for candidate registration.48//! - Invulnerable: An account guaranteed to be in the collator set.49//!50//! ## Implementation51//!52//! The final `Collators` are aggregated from two individual lists:53//!54//! 1. [`Invulnerables`]: a set of collators appointed by governance. These accounts will always be55//!    collators.56//! 2. [`Candidates`]: these are *candidates to the collation task* and may or may not be elected as57//!    a final collator.58//!59//! The current implementation resolves congestion of [`Candidates`] in a first-come-first-serve60//! manner.61//!62//! Candidates will not be allowed to get kicked or leave_intent if the total number of candidates63//! fall below MinCandidates. This is for potential disaster recovery scenarios.64//!65//! ### Rewards66//!67//! The Collator Selection pallet maintains an on-chain account (the "Pot"). In each block, the68//! collator who authored it receives:69//!70//! - Half the value of the Pot.71//! - Half the value of the transaction fees within the block. The other half of the transaction72//!   fees are deposited into the Pot.73//!74//! To initiate rewards an ED needs to be transferred to the pot address.75//!76//! Note: Eventually the Pot distribution may be modified as discussed in77//! [this issue](https://github.com/paritytech/statemint/issues/21#issuecomment-810481073).7879#![cfg_attr(not(feature = "std"), no_std)]8081pub use pallet::*;8283#[cfg(test)]84mod mock;8586#[cfg(test)]87mod tests;8889#[cfg(feature = "runtime-benchmarks")]90mod benchmarking;91pub mod weights;9293#[frame_support::pallet]94pub mod pallet {95	pub use crate::weights::WeightInfo;96	use core::ops::Div;97	use frame_support::{98		dispatch::{DispatchClass, DispatchResultWithPostInfo},99		inherent::Vec,100		pallet_prelude::*,101		sp_runtime::traits::{AccountIdConversion, CheckedSub, Saturating, Zero},102		traits::{103			Currency, EnsureOrigin, ExistenceRequirement::KeepAlive, ReservableCurrency,104			ValidatorRegistration,105		},106		BoundedVec, PalletId,107	};108	use frame_system::{pallet_prelude::*, Config as SystemConfig};109	use pallet_session::SessionManager;110	use sp_runtime::{111		Perbill,112		traits::{One, Convert},113	};114	use sp_staking::SessionIndex;115116	type BalanceOf<T> =117		<<T as Config>::Currency as Currency<<T as SystemConfig>::AccountId>>::Balance;118119	/// A convertor from collators id. Since this pallet does not have stash/controller, this is120	/// just identity.121	pub struct IdentityCollator;122	impl<T> sp_runtime::traits::Convert<T, Option<T>> for IdentityCollator {123		fn convert(t: T) -> Option<T> {124			Some(t)125		}126	}127128	/// Configure the pallet by specifying the parameters and types on which it depends.129	#[pallet::config]130	pub trait Config: frame_system::Config {131		/// Overarching event type.132		type RuntimeEvent: From<Event<Self>> + IsType<<Self as frame_system::Config>::RuntimeEvent>;133134		/// The currency mechanism.135		type Currency: ReservableCurrency<Self::AccountId>;136137		/// Origin that can dictate updating parameters of this pallet.138		type UpdateOrigin: EnsureOrigin<Self::RuntimeOrigin>;139140		/// Account Identifier that holds the chain's treasury.141		type TreasuryAccountId: Get<Self::AccountId>;142143		/// Account Identifier from which the internal Pot is generated.144		type PotId: Get<PalletId>;145146		/// Maximum number of candidates and invulnerables that we should have. This is enforced in code.147		type MaxCollators: Get<u32>;148149		/// If kicked, how much of the collator's deposit will be slashed and sent to the slash destination.150		type SlashRatio: Get<Perbill>;151152		/// A stable ID for a validator.153		type ValidatorId: Member + Parameter;154155		/// A conversion from account ID to validator ID.156		///157		/// Its cost must be at most one storage read.158		type ValidatorIdOf: Convert<Self::AccountId, Option<Self::ValidatorId>>;159160		/// Validate a user is registered161		type ValidatorRegistration: ValidatorRegistration<Self::ValidatorId>;162163		/// The weight information of this pallet.164		type WeightInfo: WeightInfo;165	}166167	#[pallet::pallet]168	#[pallet::generate_store(pub(super) trait Store)]169	pub struct Pallet<T>(_);170171	/// The invulnerable, fixed collators.172	#[pallet::storage]173	#[pallet::getter(fn invulnerables)]174	pub type Invulnerables<T: Config> =175		StorageValue<_, BoundedVec<T::AccountId, T::MaxCollators>, ValueQuery>;176177	/// The (community) collation license holders.178	#[pallet::storage]179	#[pallet::getter(fn licenses)]180	pub type Licenses<T: Config> =181		StorageMap<_, Blake2_128Concat, T::AccountId, BalanceOf<T>, ValueQuery>;182183	/// The (community, limited) collation candidates.184	#[pallet::storage]185	#[pallet::getter(fn candidates)]186	pub type Candidates<T: Config> =187		StorageValue<_, BoundedVec<T::AccountId, T::MaxCollators>, ValueQuery>;188189	/// Collator will be kicked if it does not produce a block within the threshold (does not apply to invulnerables).190	///191	/// Should be a multiple of session or things will get inconsistent. todo:collator reword?192	#[pallet::storage]193	#[pallet::getter(fn kick_threshold)]194	pub type KickThreshold<T: Config> = StorageValue<_, T::BlockNumber, ValueQuery>;195196	/// Last block authored by collator.197	#[pallet::storage]198	#[pallet::getter(fn last_authored_block)]199	pub type LastAuthoredBlock<T: Config> =200		StorageMap<_, Twox64Concat, T::AccountId, T::BlockNumber, ValueQuery>;201202	/// Desired number of candidates.203	///204	/// This should ideally always be less than [`Config::MaxCollators`] for weights to be correct.205	#[pallet::storage]206	#[pallet::getter(fn desired_collators)]207	pub type DesiredCollators<T> = StorageValue<_, u32, ValueQuery>;208209	/// Fixed amount to deposit to become a collator.210	///211	/// When a collator calls `leave_intent` they immediately receive the deposit back.212	#[pallet::storage]213	#[pallet::getter(fn license_bond)]214	pub type LicenseBond<T> = StorageValue<_, BalanceOf<T>, ValueQuery>;215216	#[pallet::genesis_config]217	pub struct GenesisConfig<T: Config> {218		pub invulnerables: Vec<T::AccountId>,219		pub license_bond: BalanceOf<T>,220		pub kick_threshold: T::BlockNumber,221		pub desired_collators: u32,222	}223224	#[cfg(feature = "std")]225	impl<T: Config> Default for GenesisConfig<T> {226		fn default() -> Self {227			Self {228				invulnerables: Default::default(),229				license_bond: Default::default(),230				kick_threshold: T::BlockNumber::one(),231				desired_collators: Default::default(),232			}233		}234	}235236	#[pallet::genesis_build]237	impl<T: Config> GenesisBuild<T> for GenesisConfig<T> {238		fn build(&self) {239			let duplicate_invulnerables = self240				.invulnerables241				.iter()242				.collect::<std::collections::BTreeSet<_>>();243			assert!(244				duplicate_invulnerables.len() == self.invulnerables.len(),245				"duplicate invulnerables in genesis."246			);247248			let bounded_invulnerables =249				BoundedVec::<_, T::MaxCollators>::try_from(self.invulnerables.clone())250					.expect("genesis invulnerables are more than T::MaxCollators");251			assert!(252				T::MaxCollators::get() >= self.desired_collators,253				"genesis desired_collators are more than T::MaxCollators",254			);255256			<DesiredCollators<T>>::put(self.desired_collators);257			<LicenseBond<T>>::put(self.license_bond);258			<KickThreshold<T>>::put(self.kick_threshold);259			<Invulnerables<T>>::put(bounded_invulnerables);260		}261	}262263	#[pallet::event]264	#[pallet::generate_deposit(pub(super) fn deposit_event)]265	pub enum Event<T: Config> {266		NewDesiredCollators {267			desired_collators: u32,268		},269		NewLicenseBond {270			bond_amount: BalanceOf<T>,271		},272		NewKickThreshold {273			length_in_blocks: T::BlockNumber,274		},275		InvulnerableAdded {276			invulnerable: T::AccountId,277		},278		InvulnerableRemoved {279			invulnerable: T::AccountId,280		},281		LicenseObtained {282			account_id: T::AccountId,283			deposit: BalanceOf<T>,284		},285		LicenseForfeited {286			account_id: T::AccountId,287			deposit_returned: BalanceOf<T>,288		},289		CandidateAdded {290			account_id: T::AccountId,291		},292		CandidateRemoved {293			account_id: T::AccountId,294		},295	}296297	// Errors inform users that something went wrong.298	#[pallet::error]299	pub enum Error<T> {300		/// Too many candidates301		TooManyCandidates,302		/// Unknown error303		Unknown,304		/// Permission issue305		Permission,306		/// User already holds license to collate307		AlreadyHoldingLicense,308		/// User does not hold a license to collate309		NoLicense,310		/// User is already a candidate311		AlreadyCandidate,312		/// User is not a candidate313		NotCandidate,314		/// Too many invulnerables315		TooManyInvulnerables,316		/// Too few invulnerables317		TooFewInvulnerables,318		/// User is already an Invulnerable319		AlreadyInvulnerable,320		/// User is not an Invulnerable321		NotInvulnerable,322		/// Account has no associated validator ID323		NoAssociatedValidatorId,324		/// Validator ID is not yet registered325		ValidatorNotRegistered,326	}327328	#[pallet::hooks]329	impl<T: Config> Hooks<BlockNumberFor<T>> for Pallet<T> {}330331	#[pallet::call]332	impl<T: Config> Pallet<T> {333		/// Add a collator to the list of invulnerable (fixed) collators.334		#[pallet::weight(T::WeightInfo::set_invulnerables(1u32))] // todo:collator weight335		pub fn add_invulnerable(336			origin: OriginFor<T>,337			new: T::AccountId,338		) -> DispatchResultWithPostInfo {339			T::UpdateOrigin::ensure_origin(origin)?;340341			// check if the new invulnerable has associated validator keys before it is added342			let validator_key = T::ValidatorIdOf::convert(new.clone())343				.ok_or(Error::<T>::NoAssociatedValidatorId)?;344			ensure!(345				T::ValidatorRegistration::is_registered(&validator_key),346				Error::<T>::ValidatorNotRegistered347			);348			if Self::invulnerables().contains(&new) {349				return Ok(().into());350			}351352			<Invulnerables<T>>::try_append(new.clone())353				.map_err(|_| Error::<T>::TooManyInvulnerables)?;354355			// try to offboard the new invulnerable if it was a collator candidate before356			let _ = Self::try_remove_candidate(&new);357358			Self::deposit_event(Event::InvulnerableAdded { invulnerable: new });359			Ok(().into())360		}361362		/// Remove a collator from the list of invulnerable (fixed) collators.363		#[pallet::weight(T::WeightInfo::set_invulnerables(1))] // todo:collator weight364		pub fn remove_invulnerable(365			origin: OriginFor<T>,366			who: T::AccountId,367		) -> DispatchResultWithPostInfo {368			T::UpdateOrigin::ensure_origin(origin)?;369370			<Invulnerables<T>>::try_mutate(|invulnerables| -> DispatchResult {371				if invulnerables.len() <= 1 {372					return Err(Error::<T>::TooFewInvulnerables.into());373				}374375				let index = invulnerables376					.into_iter()377					.position(|r| *r == who)378					.ok_or(Error::<T>::NotInvulnerable)?;379				invulnerables.remove(index);380				Ok(())381			})?;382			Self::deposit_event(Event::InvulnerableRemoved { invulnerable: who });383			Ok(().into())384		}385386		/// Set the ideal number of collators. If lowering this number,387		/// then the number of running collators could be higher than this figure.388		/// Aside from that edge case, there should be no other way to have more collators than the desired number.389		#[pallet::weight(T::WeightInfo::set_desired_collators())]390		pub fn set_desired_collators(origin: OriginFor<T>, max: u32) -> DispatchResultWithPostInfo {391			T::UpdateOrigin::ensure_origin(origin)?;392			// we trust origin calls, this is just a for more accurate benchmarking393			if max > T::MaxCollators::get() {394				log::warn!("max > T::MaxCollators; you might need to run benchmarks again");395			}396			<DesiredCollators<T>>::put(max);397			Self::deposit_event(Event::NewDesiredCollators {398				desired_collators: max,399			});400			Ok(().into())401		}402403		/// Set the candidacy bond amount.404		#[pallet::weight(T::WeightInfo::set_license_bond())]405		pub fn set_license_bond(406			origin: OriginFor<T>,407			bond: BalanceOf<T>,408		) -> DispatchResultWithPostInfo {409			T::UpdateOrigin::ensure_origin(origin)?;410			<LicenseBond<T>>::put(bond);411			Self::deposit_event(Event::NewLicenseBond { bond_amount: bond });412			Ok(().into())413		}414415		/// Set the length of the kick threshold.416		/// Note that if the length is not a multiple of the session period, it might get inconsistent.417		#[pallet::weight(T::WeightInfo::set_license_bond())] // todo:collator weight418		pub fn set_kick_threshold(419			origin: OriginFor<T>,420			kick_threshold: T::BlockNumber,421		) -> DispatchResultWithPostInfo {422			T::UpdateOrigin::ensure_origin(origin)?;423			// todo:collator insert something to guarantee consistency?424			<KickThreshold<T>>::put(kick_threshold);425			Self::deposit_event(Event::NewKickThreshold {426				length_in_blocks: kick_threshold,427			});428			Ok(().into())429		}430431		/// Purchase a license on block collation for this account.432		/// It does not make it a collator candidate, use `onboard` afterward. The account must433		/// (a) already have registered session keys and (b) be able to reserve the `LicenseBond`.434		///435		/// This call is not available to `Invulnerable` collators.436		#[pallet::weight(T::WeightInfo::register_as_candidate(T::MaxCollators::get()))] // todo:collator weight437		pub fn get_license(origin: OriginFor<T>) -> DispatchResultWithPostInfo {438			// register_as_candidate439			let who = ensure_signed(origin)?;440441			if Licenses::<T>::contains_key(&who) {442				return Err(Error::<T>::AlreadyHoldingLicense.into());443			}444445			let validator_key = T::ValidatorIdOf::convert(who.clone())446				.ok_or(Error::<T>::NoAssociatedValidatorId)?;447			ensure!(448				T::ValidatorRegistration::is_registered(&validator_key),449				Error::<T>::ValidatorNotRegistered450			);451452			let deposit = Self::license_bond();453454			T::Currency::reserve(&who, deposit)?;455			Licenses::<T>::insert(who.clone(), deposit);456457			Self::deposit_event(Event::LicenseObtained {458				account_id: who,459				deposit,460			});461			Ok(().into()) // Some(T::WeightInfo::register_as_candidate(current_count as u32)).into())462		}463464		/// Register this account as a candidate for collators for next sessions.465		/// The account must already hold a license, and cannot offboard immediately during a session.466		///467		/// This call is not available to `Invulnerable` collators.468		#[pallet::weight(T::WeightInfo::register_as_candidate(T::MaxCollators::get()))] // todo:collator weight469		pub fn onboard(origin: OriginFor<T>) -> DispatchResultWithPostInfo {470			// register_as_candidate471			let who = ensure_signed(origin)?;472473			// ensure the user obtained the license.474			ensure!(Licenses::<T>::contains_key(&who), Error::<T>::NoLicense);475			// ensure we are below limit.476			let length = <Candidates<T>>::decode_len().unwrap_or_default()477				+ <Invulnerables<T>>::decode_len().unwrap_or_default();478			ensure!(479				(length as u32) < Self::desired_collators(),480				Error::<T>::TooManyCandidates481			);482			ensure!(483				!Self::invulnerables().contains(&who),484				Error::<T>::AlreadyInvulnerable485			);486487			let current_count =488				<Candidates<T>>::try_mutate(|candidates| -> Result<usize, DispatchError> {489					if candidates.iter().any(|candidate| *candidate == who) {490						Err(Error::<T>::AlreadyCandidate)?491					} else {492						candidates493							.try_push(who.clone())494							.map_err(|_| Error::<T>::TooManyCandidates)?;495						// First authored block is current block plus kick threshold to handle session delay496						<LastAuthoredBlock<T>>::insert(497							who.clone(),498							frame_system::Pallet::<T>::block_number() + Self::kick_threshold(),499						);500						Ok(candidates.len())501					}502				})?;503504			Self::deposit_event(Event::CandidateAdded { account_id: who });505			Ok(Some(T::WeightInfo::register_as_candidate(current_count as u32)).into())506		}507508		/// Deregister `origin` as a collator candidate. Note that the collator can only leave on509		/// session change. The license to `onboard` later at any other time will remain.510		#[pallet::weight(T::WeightInfo::leave_intent(T::MaxCollators::get()))] // todo:collator weight511		pub fn offboard(origin: OriginFor<T>) -> DispatchResultWithPostInfo {512			// leave_intent513			let who = ensure_signed(origin)?;514			let current_count = Self::try_remove_candidate(&who)?;515516			Ok(Some(T::WeightInfo::leave_intent(current_count as u32)).into()) // todo:collator weight517		}518519		/// Forfeit `origin`'s own license. The `LicenseBond` will be unreserved immediately.520		///521		/// This call is not available to `Invulnerable` collators.522		#[pallet::weight(T::WeightInfo::leave_intent(T::MaxCollators::get()))] // todo:collator weight523		pub fn release_license(origin: OriginFor<T>) -> DispatchResultWithPostInfo {524			// leave_intent525			let who = ensure_signed(origin)?;526527			let current_count = Self::try_remove_candidate_and_release_license(&who, false, true)?;528529			Ok(Some(T::WeightInfo::leave_intent(current_count as u32)).into()) // todo:collator weight530		}531532		/// Force deregister `origin` as a collator candidate as a governing authority, and revoke its license.533		/// Note that the collator can only leave on session change.534		/// The `LicenseBond` will be unreserved and returned immediately.535		///536		/// This call is, of course, not applicable to `Invulnerable` collators.537		#[pallet::weight(T::WeightInfo::leave_intent(T::MaxCollators::get()))] // todo:collator weight538		pub fn force_revoke_license(539			origin: OriginFor<T>,540			who: T::AccountId,541		) -> DispatchResultWithPostInfo {542			// leave_intent543			T::UpdateOrigin::ensure_origin(origin)?;544545			let current_count = Self::try_remove_candidate_and_release_license(&who, false, true)?;546547			Ok(Some(T::WeightInfo::leave_intent(current_count as u32)).into()) // todo:collator weight548		}549	}550551	impl<T: Config> Pallet<T> {552		/// Get a unique, inaccessible account id from the `PotId`.553		pub fn account_id() -> T::AccountId {554			T::PotId::get().into_account_truncating()555		}556557		/// Removes a candidate and their license, optionally slashed and optionally ignoring,558		/// whether or not they actually are a candidate.559		fn try_remove_candidate_and_release_license(560			who: &T::AccountId,561			should_slash: bool,562			ignore_if_not_candidate: bool,563		) -> Result<usize, DispatchError> {564			let current_count = Self::try_remove_candidate(who);565			let current_count = if ignore_if_not_candidate566				&& current_count == Err(Error::<T>::NotCandidate.into())567			{568				<Candidates<T>>::decode_len().unwrap_or_default()569			} else {570				current_count?571			};572			Self::try_release_license(who, should_slash)?;573			Ok(current_count)574		}575576		/// Removes a candidate from the collator pool for the next session if they exist.577		fn try_remove_candidate(who: &T::AccountId) -> Result<usize, DispatchError> {578			let current_count =579				<Candidates<T>>::try_mutate(|candidates| -> Result<usize, DispatchError> {580					let index = candidates581						.iter()582						.position(|candidate| *candidate == *who)583						.ok_or(Error::<T>::NotCandidate)?;584					candidates.remove(index);585					<LastAuthoredBlock<T>>::remove(who.clone());586					Ok(candidates.len())587				})?;588			Self::deposit_event(Event::CandidateRemoved {589				account_id: who.clone(),590			});591			Ok(current_count)592		}593594		/// Removes a candidate if they exist and sends them back their deposit, optionally slashed.595		fn try_release_license(who: &T::AccountId, should_slash: bool) -> DispatchResult {596			let mut deposit_returned = BalanceOf::<T>::default();597			Licenses::<T>::try_mutate_exists(who, |deposit| -> DispatchResult {598				if let Some(deposit) = deposit.take() {599					if should_slash {600						let slashed = T::SlashRatio::get() * deposit;601						let remaining = deposit - slashed;602603						let (imbalance, _) = T::Currency::slash_reserved(who, slashed);604						//T::Currency::unreserve(who, remaining);605						deposit_returned = remaining;606607						T::Currency::resolve_creating(&T::TreasuryAccountId::get(), imbalance);608					} else {609						//T::Currency::unreserve(who, deposit);610						deposit_returned = deposit;611					}612613					T::Currency::unreserve(who, deposit_returned);614					Ok(())615				} else {616					Err(Error::<T>::NoLicense.into())617				}618			})?;619			Self::deposit_event(Event::LicenseForfeited {620				account_id: who.clone(),621				deposit_returned,622			});623			Ok(())624		}625626		/// Assemble the current set of candidates and invulnerables into the next collator set.627		///628		/// This is done on the fly, as frequent as we are told to do so, as the session manager.629		pub fn assemble_collators(630			candidates: BoundedVec<T::AccountId, T::MaxCollators>,631		) -> Vec<T::AccountId> {632			let mut collators = Self::invulnerables().to_vec();633			collators.extend(candidates);634			collators635		}636637		/// Kicks out candidates that did not produce a block in the kick threshold638		/// and **confiscates** their deposits to the treasury.639		pub fn kick_stale_candidates(640			candidates: BoundedVec<T::AccountId, T::MaxCollators>,641		) -> BoundedVec<T::AccountId, T::MaxCollators> {642			let now = frame_system::Pallet::<T>::block_number();643			let kick_threshold = Self::kick_threshold();644			candidates645				.into_iter()646				.filter_map(|c| {647					let last_block = <LastAuthoredBlock<T>>::get(c.clone());648					let since_last = now.saturating_sub(last_block);649					if since_last < kick_threshold {650						Some(c)651					} else {652						let outcome = Self::try_remove_candidate_and_release_license(&c, true, false);653						if let Err(why) = outcome {654							log::warn!("Failed to kick collator and release license {:?}", why);655							debug_assert!(false, "failed to kick collator and release license {why:?}");656						}657						None658					}659				})660				.collect::<Vec<_>>()661				.try_into()662				.expect("filter_map operation can't result in a bounded vec larger than its original; qed")663		}664	}665666	/// Keep track of number of authored blocks per authority, uncles are counted as well since667	/// they're a valid proof of being online.668	impl<T: Config + pallet_authorship::Config>669		pallet_authorship::EventHandler<T::AccountId, T::BlockNumber> for Pallet<T>670	{671		fn note_author(author: T::AccountId) {672			let pot = Self::account_id();673			// assumes an ED will be sent to pot.674			let reward = T::Currency::free_balance(&pot)675				.checked_sub(&T::Currency::minimum_balance())676				.unwrap_or_else(Zero::zero)677				.div(2u32.into());678			// `reward` is half of pot account minus ED, this should never fail.679			let _success = T::Currency::transfer(&pot, &author, reward, KeepAlive);680			debug_assert!(_success.is_ok());681			<LastAuthoredBlock<T>>::insert(author, frame_system::Pallet::<T>::block_number());682683			frame_system::Pallet::<T>::register_extra_weight_unchecked(684				T::WeightInfo::note_author(),685				DispatchClass::Mandatory,686			);687		}688689		fn note_uncle(_author: T::AccountId, _age: T::BlockNumber) {690			//TODO can we ignore this?691		}692	}693694	/// Play the role of the session manager.695	impl<T: Config> SessionManager<T::AccountId> for Pallet<T> {696		fn new_session(index: SessionIndex) -> Option<Vec<T::AccountId>> {697			log::info!(698				"assembling new collators for new session {} at #{:?}",699				index,700				<frame_system::Pallet<T>>::block_number(),701			);702703			let candidates = Self::candidates();704			let candidates_len_before = candidates.len();705			let active_candidates = Self::kick_stale_candidates(candidates);706			let removed = candidates_len_before - active_candidates.len();707			let result = Self::assemble_collators(active_candidates);708709			frame_system::Pallet::<T>::register_extra_weight_unchecked(710				T::WeightInfo::new_session(candidates_len_before as u32, removed as u32),711				DispatchClass::Mandatory,712			);713			Some(result)714		}715		fn start_session(_: SessionIndex) {716			// we don't care.717		}718		fn end_session(_: SessionIndex) {719			// we don't care.720		}721	}722}