parachains.rs

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

// Polkadot is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// Polkadot is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.

//! Main parachains logic. For now this is just the determination of which validators do what.

use sp_std::prelude::*;
use sp_std::result;
use codec::{Decode, Encode};

use sp_runtime::{
	KeyTypeId, Perbill, RuntimeDebug,
	traits::{
		Hash as HashT, BlakeTwo256, Saturating, One, Dispatchable,
		AccountIdConversion, BadOrigin, Convert, SignedExtension, AppVerify,
	},
	transaction_validity::{TransactionValidityError, ValidTransaction, TransactionValidity},
};
use sp_staking::{
	SessionIndex,
	offence::{ReportOffence, Offence, Kind},
};
use frame_support::{
	traits::KeyOwnerProofSystem,
	dispatch::{IsSubType},
	weights::{DispatchInfo, SimpleDispatchInfo, Weight, WeighData},
};
use primitives::{
	Balance,
	parachain::{
		self, Id as ParaId, Chain, DutyRoster, AttestedCandidate, Statement, ParachainDispatchOrigin,
		UpwardMessage, ValidatorId, ActiveParas, CollatorId, Retriable, OmittedValidationData,
		CandidateReceipt, GlobalValidationSchedule, AbridgedCandidateReceipt,
		LocalValidationData, ValidityAttestation, NEW_HEADS_IDENTIFIER, PARACHAIN_KEY_TYPE_ID,
		ValidatorSignature, SigningContext,
	},
};
use frame_support::{
	Parameter, dispatch::DispatchResult, decl_storage, decl_module, decl_error, ensure,
	traits::{Currency, Get, WithdrawReason, ExistenceRequirement, Randomness},
};
use sp_runtime::transaction_validity::InvalidTransaction;

use inherents::{ProvideInherent, InherentData, MakeFatalError, InherentIdentifier};

use system::{ensure_none, ensure_signed};
use crate::attestations::{self, IncludedBlocks};
use crate::registrar::Registrar;

// ranges for iteration of general block number don't work, so this
// is a utility to get around that.
struct BlockNumberRange<N> {
	low: N,
	high: N,
}

impl<N: Saturating + One + PartialOrd + PartialEq + Clone> Iterator for BlockNumberRange<N> {
	type Item = N;

	fn next(&mut self) -> Option<N> {
		if self.low >= self.high {
			return None
		}

		let item = self.low.clone();
		self.low = self.low.clone().saturating_add(One::one());
		Some(item)
	}
}

// wrapper trait because an associated type of `Currency<Self::AccountId,Balance=Balance>`
// doesn't work.`
pub trait ParachainCurrency<AccountId> {
	fn free_balance(para_id: ParaId) -> Balance;
	fn deduct(para_id: ParaId, amount: Balance) -> DispatchResult;
}

impl<AccountId, T: Currency<AccountId>> ParachainCurrency<AccountId> for T where
	T::Balance: From<Balance> + Into<Balance>,
	ParaId: AccountIdConversion<AccountId>,
{
	fn free_balance(para_id: ParaId) -> Balance {
		let para_account = para_id.into_account();
		T::free_balance(&para_account).into()
	}

	fn deduct(para_id: ParaId, amount: Balance) -> DispatchResult {
		let para_account = para_id.into_account();

		// burn the fee.
		let _ = T::withdraw(
			&para_account,
			amount.into(),
			WithdrawReason::Fee.into(),
			ExistenceRequirement::KeepAlive,
		)?;

		Ok(())
	}
}

/// Interface to the persistent (stash) identities of the current validators.
pub struct ValidatorIdentities<T>(sp_std::marker::PhantomData<T>);

/// A structure used to report conflicting votes by validators.
///
/// It is generic over two parameters:
/// `Proof` - proof of historical ownership of a key by some validator.
/// `Hash` - a type of a hash used in the runtime.
#[derive(RuntimeDebug, Encode, Decode)]
#[derive(Clone, Eq, PartialEq)]
pub struct DoubleVoteReport<Proof> {
	/// Identity of the double-voter.
	pub identity: ValidatorId,
	/// First vote of the double-vote.
	pub first: (Statement, ValidatorSignature),
	/// Second vote of the double-vote.
	pub second: (Statement, ValidatorSignature),
	/// Proof that the validator with `identity` id was actually a validator at `parent_hash`.
	pub proof: Proof,
	/// A `SigningContext` with a session and a parent hash of the moment this offence was commited.
	pub signing_context: SigningContext,
}

impl<Proof: Parameter + GetSessionNumber> DoubleVoteReport<Proof> {
	fn verify<T: Trait<Proof = Proof>>(
		&self,
	) -> Result<(), DoubleVoteValidityError> {
		let first = self.first.clone();
		let second = self.second.clone();
		let id = self.identity.encode();

		T::KeyOwnerProofSystem::check_proof((PARACHAIN_KEY_TYPE_ID, id), self.proof.clone())
			.ok_or(DoubleVoteValidityError::InvalidProof)?;

		if self.proof.session() != self.signing_context.session_index {
			return Err(DoubleVoteValidityError::InvalidReport);
		}

		// Check signatures.
		Self::verify_vote(
			&first,
			&self.signing_context,
			&self.identity,
		)?;
		Self::verify_vote(
			&second,
			&self.signing_context,
			&self.identity,
		)?;

		match (&first.0, &second.0) {
			// If issuing a `Candidate` message on a parachain block, neither a `Valid` or
			// `Invalid` vote cannot be issued on that parachain block, as the `Candidate`
			// message is an implicit validity vote.
			(Statement::Candidate(candidate_hash), Statement::Valid(hash)) |
			(Statement::Candidate(candidate_hash), Statement::Invalid(hash)) |
			(Statement::Valid(hash), Statement::Candidate(candidate_hash)) |
			(Statement::Invalid(hash), Statement::Candidate(candidate_hash))
			if *candidate_hash == *hash => {},
			// Otherwise, it is illegal to cast both a `Valid` and
			// `Invalid` vote on a given parachain block.
			(Statement::Valid(hash_1), Statement::Invalid(hash_2)) |
			(Statement::Invalid(hash_1), Statement::Valid(hash_2))
			if *hash_1 == *hash_2 => {},
			_ => {
				return Err(DoubleVoteValidityError::NotDoubleVote);
			}
		}

		Ok(())
	}

	fn verify_vote(
		vote: &(Statement, ValidatorSignature),
		signing_context: &SigningContext,
		authority: &ValidatorId,
	) -> Result<(), DoubleVoteValidityError> {
		let payload = localized_payload(vote.0.clone(), signing_context);

		if !vote.1.verify(&payload[..], authority) {
			return Err(DoubleVoteValidityError::InvalidSignature);
		}

		Ok(())
	}
}

impl<T: session::Trait> Get<Vec<T::ValidatorId>> for ValidatorIdentities<T> {
	fn get() -> Vec<T::ValidatorId> {
		<session::Module<T>>::validators()
	}
}

/// A trait to get a session number the `Proof` belongs to.
pub trait GetSessionNumber {
	fn session(&self) -> SessionIndex;
}

impl GetSessionNumber for session::historical::Proof {
	fn session(&self) -> SessionIndex {
		self.session()
	}
}

pub trait Trait: attestations::Trait + session::historical::Trait {
	/// The outer origin type.
	type Origin: From<Origin> + From<system::RawOrigin<Self::AccountId>>;

	/// The outer call dispatch type.
	type Call: Parameter + Dispatchable<Origin=<Self as Trait>::Origin>;

	/// Some way of interacting with balances for fees.
	type ParachainCurrency: ParachainCurrency<Self::AccountId>;

	/// Something that provides randomness in the runtime.
	type Randomness: Randomness<Self::Hash>;

	/// Means to determine what the current set of active parachains are.
	type ActiveParachains: ActiveParas;

	/// The way that we are able to register parachains.
	type Registrar: Registrar<Self::AccountId>;

	/// Maximum code size for parachains, in bytes. Note that this is not
	/// the entire storage burden of the parachain, as old code is stored for
	/// `SlashPeriod` blocks.
	type MaxCodeSize: Get<u32>;

	/// Max head data size.
	type MaxHeadDataSize: Get<u32>;

	/// Proof type.
	///
	/// We need this type to bind the `KeyOwnerProofSystem::Proof` to necessary bounds.
	/// As soon as https://rust-lang.github.io/rfcs/2289-associated-type-bounds.html
	/// gets in this can be simplified.
	type Proof: Parameter + GetSessionNumber;

	/// Compute and check proofs of historical key owners.
	type KeyOwnerProofSystem: KeyOwnerProofSystem<
		(KeyTypeId, Vec<u8>),
		Proof = Self::Proof,
		IdentificationTuple = Self::IdentificationTuple,
	>;

	/// An identification tuple type bound to `Parameter`.
	type IdentificationTuple: Parameter;

	/// Report an offence.
	type ReportOffence: ReportOffence<
		Self::AccountId,
		Self::IdentificationTuple,
		DoubleVoteOffence<Self::IdentificationTuple>
	>;

	/// A type that converts the opaque hash type to exact one.
	type BlockHashConversion: Convert<Self::Hash, primitives::Hash>;
}

/// Origin for the parachains module.
#[derive(PartialEq, Eq, Clone)]
#[cfg_attr(feature = "std", derive(Debug))]
pub enum Origin {
	/// It comes from a parachain.
	Parachain(ParaId),
}

/// An offence that is filed if the validator has submitted a double vote.
#[derive(RuntimeDebug)]
#[cfg_attr(feature = "std", derive(Clone, PartialEq, Eq))]
pub struct DoubleVoteOffence<Offender> {
	/// The current session index in which we report a validator.
	session_index: SessionIndex,
	/// The size of the validator set in current session/era.
	validator_set_count: u32,
	/// An offender that has submitted two conflicting votes.
	offender: Offender,
}

impl<Offender: Clone> Offence<Offender> for DoubleVoteOffence<Offender> {
	const ID: Kind = *b"para:double-vote";
	type TimeSlot = SessionIndex;

	fn offenders(&self) -> Vec<Offender> {
		vec![self.offender.clone()]
	}

	fn session_index(&self) -> SessionIndex {
		self.session_index
	}

	fn validator_set_count(&self) -> u32 {
		self.validator_set_count
	}

	fn time_slot(&self) -> Self::TimeSlot {
		self.session_index
	}

	fn slash_fraction(_offenders_count: u32, _validator_set_count: u32) -> Perbill {
		// Slash 100%.
		Perbill::from_percent(100)
	}
}

/// Total number of individual messages allowed in the parachain -> relay-chain message queue.
const MAX_QUEUE_COUNT: usize = 100;
/// Total size of messages allowed in the parachain -> relay-chain message queue before which no
/// further messages may be added to it. If it exceeds this then the queue may contain only a
/// single message.
const WATERMARK_QUEUE_SIZE: usize = 20000;

decl_storage! {
	trait Store for Module<T: Trait> as Parachains
	{
		/// All authorities' keys at the moment.
		pub Authorities get(fn authorities): Vec<ValidatorId>;
		/// The parachains registered at present.
		pub Code get(fn parachain_code): map hasher(twox_64_concat) ParaId => Option<Vec<u8>>;
		/// The heads of the parachains registered at present.
		pub Heads get(fn parachain_head): map hasher(twox_64_concat) ParaId => Option<Vec<u8>>;
		/// Messages ready to be dispatched onto the relay chain. It is subject to
		/// `MAX_MESSAGE_COUNT` and `WATERMARK_MESSAGE_SIZE`.
		pub RelayDispatchQueue: map hasher(twox_64_concat) ParaId => Vec<UpwardMessage>;
		/// Size of the dispatch queues. Separated from actual data in order to avoid costly
		/// decoding when checking receipt validity. First item in tuple is the count of messages
		///	second if the total length (in bytes) of the message payloads.
		pub RelayDispatchQueueSize: map hasher(twox_64_concat) ParaId => (u32, u32);
		/// The ordered list of ParaIds that have a `RelayDispatchQueue` entry.
		NeedsDispatch: Vec<ParaId>;

		/// `Some` if the parachain heads get updated in this block, along with the parachain IDs
		/// that did update. Ordered in the same way as `registrar::Active` (i.e. by ParaId).
		///
		/// `None` if not yet updated.
		pub DidUpdate: Option<Vec<ParaId>>;
	}
	add_extra_genesis {
		config(authorities): Vec<ValidatorId>;
		build(|config| Module::<T>::initialize_authorities(&config.authorities))
	}
}

decl_error! {
	pub enum Error for Module<T: Trait> {
		/// Parachain heads must be updated only once in the block.
		TooManyHeadUpdates,
		/// Too many parachain candidates.
		TooManyParaCandidates,
		/// Proposed heads must be ascending order by parachain ID without duplicate.
		HeadsOutOfOrder,
		/// Candidate is for an unregistered parachain.
		UnregisteredPara,
		/// Invalid collator.
		InvalidCollator,
		/// The message queue is full. Messages will be added when there is space.
		QueueFull,
		/// The message origin is invalid.
		InvalidMessageOrigin,
		/// No validator group for parachain.
		NoValidatorGroup,
		/// Not enough validity votes for candidate.
		NotEnoughValidityVotes,
		/// The number of attestations exceeds the number of authorities.
		VotesExceedsAuthorities,
		/// Attesting validator not on this chain's validation duty.
		WrongValidatorAttesting,
		/// Invalid signature from attester.
		InvalidSignature,
		/// Extra untagged validity votes along with candidate.
		UntaggedVotes,
		/// Wrong parent head for parachain receipt.
		ParentMismatch,
		/// Head data was too large.
		HeadDataTooLarge,
		/// Para does not have enough balance to pay fees.
		CannotPayFees,
		/// Unexpected relay-parent for a candidate receipt.
		UnexpectedRelayParent,
	}
}

decl_module! {
	/// Parachains module.
	pub struct Module<T: Trait> for enum Call where origin: <T as system::Trait>::Origin {
		type Error = Error<T>;

		/// Provide candidate receipts for parachains, in ascending order by id.
		#[weight = SimpleDispatchInfo::FixedNormal(1_000_000)]
		pub fn set_heads(origin, heads: Vec<AttestedCandidate>) -> DispatchResult {
			ensure_none(origin)?;
			ensure!(!<DidUpdate>::exists(), Error::<T>::TooManyHeadUpdates);

			let active_parachains = Self::active_parachains();

			let parachain_count = active_parachains.len();
			ensure!(heads.len() <= parachain_count, Error::<T>::TooManyParaCandidates);

			let mut proceeded = Vec::with_capacity(heads.len());

			let schedule = GlobalValidationSchedule {
				max_code_size: T::MaxCodeSize::get(),
				max_head_data_size: T::MaxHeadDataSize::get(),
			};

			if !active_parachains.is_empty() {
				// perform integrity checks before writing to storage.
				{
					let mut last_id = None;

					let mut iter = active_parachains.iter();
					for head in &heads {
						let id = head.parachain_index();
						// proposed heads must be ascending order by parachain ID without duplicate.
						ensure!(
							last_id.as_ref().map_or(true, |x| x < &id),
							Error::<T>::HeadsOutOfOrder
						);

						// must be unknown since active parachains are always sorted.
						let (_, maybe_required_collator) = iter.find(|para| para.0 == id)
							.ok_or(Error::<T>::UnregisteredPara)?;

						if let Some((required_collator, _)) = maybe_required_collator {
							ensure!(required_collator == &head.candidate.collator, Error::<T>::InvalidCollator);
						}

						Self::check_upward_messages(
							id,
							&head.candidate.commitments.upward_messages,
							MAX_QUEUE_COUNT,
							WATERMARK_QUEUE_SIZE,
						)?;

						let id = head.parachain_index();
						proceeded.push(id);
						last_id = Some(id);
					}
				}

				let para_blocks = Self::check_candidates(
					&schedule,
					&heads,
					&active_parachains,
				)?;

				<attestations::Module<T>>::note_included(&heads, para_blocks);

				Self::update_routing(
					&heads,
				);

				// note: we dispatch new messages _after_ the call to `check_candidates`
				// which deducts any fees. if that were not the case, an upward message
				// could be dispatched and spend money that invalidated a candidate.
				Self::dispatch_upward_messages(
					MAX_QUEUE_COUNT,
					WATERMARK_QUEUE_SIZE,
					Self::dispatch_message,
				);
			}

			DidUpdate::put(proceeded);

			Ok(())
		}

		/// Provide a proof that some validator has commited a double-vote.
		///
		/// The weight is 0; in order to avoid DoS a `SignedExtension` validation
		/// is implemented.
		#[weight = SimpleDispatchInfo::FixedNormal(0)]
		pub fn report_double_vote(
			origin,
			report: DoubleVoteReport<
				<T::KeyOwnerProofSystem as KeyOwnerProofSystem<(KeyTypeId, Vec<u8>)>>::Proof,
			>,
		) -> DispatchResult {
			let reporter = ensure_signed(origin)?;

			let validators = <session::Module<T>>::validators();
			let validator_set_count = validators.len() as u32;

			let session_index = report.proof.session();
			let DoubleVoteReport { identity, proof, .. } = report;

			// We have already checked this proof in `SignedExtension`, but we need
			// this here to get the full identification of the offender.
			let offender = T::KeyOwnerProofSystem::check_proof(
					(PARACHAIN_KEY_TYPE_ID, identity.encode()),
					proof,
				).ok_or("Invalid/outdated key ownership proof.")?;

			let offence = DoubleVoteOffence {
				session_index,
				validator_set_count,
				offender,
			};

			// Checks if this is actually a double vote are
			// implemented in `ValidateDoubleVoteReports::validete`.
			T::ReportOffence::report_offence(vec![reporter], offence)
				.map_err(|_| "Failed to report offence")?;

			Ok(())
		}

		fn on_initialize() -> Weight {
			<Self as Store>::DidUpdate::kill();

			SimpleDispatchInfo::default().weigh_data(())
		}

		fn on_finalize() {
			assert!(<Self as Store>::DidUpdate::exists(), "Parachain heads must be updated once in the block");
		}
	}
}

fn majority_of(list_len: usize) -> usize {
	list_len / 2 + list_len % 2
}

fn localized_payload(
	statement: Statement,
	signing_context: &SigningContext,
) -> Vec<u8> {
	let mut encoded = statement.encode();
	signing_context.using_encoded(|s| encoded.extend(s));
	encoded
}

impl<T: Trait> Module<T> {
	/// Initialize the state of a new parachain/parathread.
	pub fn initialize_para(
		id: ParaId,
		code: Vec<u8>,
		initial_head_data: Vec<u8>,
	) {
		<Code>::insert(id, code);
		<Heads>::insert(id, initial_head_data);
	}

	pub fn cleanup_para(
		id: ParaId,
	) {
		<Code>::remove(id);
		<Heads>::remove(id);
	}

	/// Get a `SigningContext` with a current `SessionIndex` and parent hash.
	pub fn signing_context() -> SigningContext {
		let session_index = <session::Module<T>>::current_index();
		let parent_hash = <system::Module<T>>::parent_hash();

		SigningContext {
			session_index,
			parent_hash: T::BlockHashConversion::convert(parent_hash),
		}
	}

	/// Dispatch some messages from a parachain.
	fn dispatch_message(
		id: ParaId,
		origin: ParachainDispatchOrigin,
		data: &[u8],
	) {
		if let Ok(message_call) = <T as Trait>::Call::decode(&mut &data[..]) {
			let origin: <T as Trait>::Origin = match origin {
				ParachainDispatchOrigin::Signed =>
					system::RawOrigin::Signed(id.into_account()).into(),
				ParachainDispatchOrigin::Parachain =>
					Origin::Parachain(id).into(),
				ParachainDispatchOrigin::Root =>
					system::RawOrigin::Root.into(),
			};
			let _ok = message_call.dispatch(origin).is_ok();
			// Not much to do with the result as it is. It's up to the parachain to ensure that the
			// message makes sense.
		}
	}

	/// Ensure all is well with the upward messages.
	fn check_upward_messages(
		id: ParaId,
		upward_messages: &[UpwardMessage],
		max_queue_count: usize,
		watermark_queue_size: usize,
	) -> DispatchResult {
		// Either there are no more messages to add...
		if !upward_messages.is_empty() {
			let (count, size) = <RelayDispatchQueueSize>::get(id);
			ensure!(
				// ...or we are appending one message onto an empty queue...
				upward_messages.len() + count as usize == 1
				// ...or...
				|| (
				// ...the total messages in the queue ends up being no greater than the
				// limit...
					upward_messages.len() + count as usize <= max_queue_count
				&&
					// ...and the total size of the payloads in the queue ends up being no
					// greater than the limit.
					upward_messages.iter()
						.fold(size as usize, |a, x| a + x.data.len())
					<= watermark_queue_size
				),
				Error::<T>::QueueFull
			);
			if !id.is_system() {
				for m in upward_messages.iter() {
					ensure!(m.origin != ParachainDispatchOrigin::Root, Error::<T>::InvalidMessageOrigin);
				}
			}
		}
		Ok(())
	}

	/// Update routing information from the parachain heads. This queues upwards
	/// messages to the relay chain as well.
	fn update_routing(
		heads: &[AttestedCandidate],
	) {
		// we sort them in order to provide a fast lookup to ensure we can avoid duplicates in the
		// needs_dispatch queue.
		let mut ordered_needs_dispatch = NeedsDispatch::get();

		for head in heads.iter() {
			let id = head.parachain_index();

			// Queue up upwards messages (from parachains to relay chain).
			Self::queue_upward_messages(
				id,
				&head.candidate.commitments.upward_messages,
				&mut ordered_needs_dispatch,
			);
		}

		NeedsDispatch::put(ordered_needs_dispatch);
	}

	/// Place any new upward messages into our queue for later dispatch.
	///
	/// `ordered_needs_dispatch` is mutated to ensure it reflects the new value of
	/// `RelayDispatchQueueSize`. It is up to the caller to guarantee that it gets written into
	/// storage after this call.
	fn queue_upward_messages(
		id: ParaId,
		upward_messages: &[UpwardMessage],
		ordered_needs_dispatch: &mut Vec<ParaId>,
	) {
		if !upward_messages.is_empty() {
			RelayDispatchQueueSize::mutate(id, |&mut(ref mut count, ref mut len)| {
				*count += upward_messages.len() as u32;
				*len += upward_messages.iter()
					.fold(0, |a, x| a + x.data.len()) as u32;
			});
			// Should never be able to fail assuming our state is uncorrupted, but best not
			// to panic, even if it does.
			let _ = RelayDispatchQueue::append(id, upward_messages);
			if let Err(i) = ordered_needs_dispatch.binary_search(&id) {
				// same.
				ordered_needs_dispatch.insert(i, id);
			} else {
				sp_runtime::print("ordered_needs_dispatch contains id?!");
			}
		}
	}

	/// Simple FIFO dispatcher. This must be called after parachain fees are checked,
	/// as dispatched messages may spend parachain funds.
	fn dispatch_upward_messages(
		max_queue_count: usize,
		watermark_queue_size: usize,
		mut dispatch_message: impl FnMut(ParaId, ParachainDispatchOrigin, &[u8]),
	) {
		let queueds = NeedsDispatch::get();
		let mut drained_count = 0usize;
		let mut dispatched_count = 0usize;
		let mut dispatched_size = 0usize;
		for id in queueds.iter() {
			drained_count += 1;

			let (count, size) = <RelayDispatchQueueSize>::get(id);
			let count = count as usize;
			let size = size as usize;
			if dispatched_count == 0 || (
				dispatched_count + count <= max_queue_count
					&& dispatched_size + size <= watermark_queue_size
			) {
				if count > 0 {
					// still dispatching messages...
					RelayDispatchQueueSize::remove(id);
					let messages = RelayDispatchQueue::take(id);
					for UpwardMessage { origin, data } in messages.into_iter() {
						dispatch_message(*id, origin, &data);
					}
					dispatched_count += count;
					dispatched_size += size;
					if dispatched_count >= max_queue_count
						|| dispatched_size >= watermark_queue_size
					{
						break
					}
				}
			}
		}
		NeedsDispatch::put(&queueds[drained_count..]);
	}

	/// Calculate the current block's duty roster using system's random seed.
	/// Returns the duty roster along with the random seed.
	pub fn calculate_duty_roster() -> (DutyRoster, [u8; 32]) {
		let parachains = Self::active_parachains();
		let parachain_count = parachains.len();

		// TODO: use decode length. substrate #2794
		let validator_count = Self::authorities().len();
		let validators_per_parachain =
			if parachain_count == 0 {
				0
			} else {
				(validator_count - 1) / parachain_count
			};

		let mut roles_val = (0..validator_count).map(|i| match i {
			i if i < parachain_count * validators_per_parachain => {
				let idx = i / validators_per_parachain;
				Chain::Parachain(parachains[idx].0.clone())
			}
			_ => Chain::Relay,
		}).collect::<Vec<_>>();

		let mut seed = {
			let phrase = b"validator_role_pairs";
			let seed = T::Randomness::random(&phrase[..]);
			let seed_len = seed.as_ref().len();
			let needed_bytes = validator_count * 4;

			// hash only the needed bits of the random seed.
			// if earlier bits are influencable, they will not factor into
			// the seed used here.
			let seed_off = if needed_bytes >= seed_len {
				0
			} else {
				seed_len - needed_bytes
			};

			BlakeTwo256::hash(&seed.as_ref()[seed_off..])
		};

		let orig_seed = seed.clone().to_fixed_bytes();

		// shuffle
		for i in 0..(validator_count.saturating_sub(1)) {
			// 4 bytes of entropy used per cycle, 32 bytes entropy per hash
			let offset = (i * 4 % 32) as usize;

			// number of roles remaining to select from.
			let remaining = sp_std::cmp::max(1, (validator_count - i) as usize);

			// 8 32-bit ints per 256-bit seed.
			let val_index = u32::decode(&mut &seed[offset..offset + 4])
				.expect("using 4 bytes for a 32-bit quantity") as usize % remaining;

			if offset == 28 {
				// into the last 4 bytes - rehash to gather new entropy
				seed = BlakeTwo256::hash(seed.as_ref());
			}

			// exchange last item with randomly chosen first.
			roles_val.swap(remaining - 1, val_index);
		}

		(DutyRoster { validator_duty: roles_val, }, orig_seed)
	}

	/// Get the global validation schedule for all parachains.
	pub fn global_validation_schedule() -> GlobalValidationSchedule {
		GlobalValidationSchedule {
			max_code_size: T::MaxCodeSize::get(),
			max_head_data_size: T::MaxHeadDataSize::get(),
		}
	}

	/// Get the local validation schedule for a particular parachain.
	pub fn local_validation_data(id: &parachain::Id) -> Option<LocalValidationData> {
		Self::parachain_head(id).map(|parent_head| LocalValidationData {
			parent_head: primitives::parachain::HeadData(parent_head),
			balance: T::ParachainCurrency::free_balance(*id),
		})
	}

	/// Get the currently active set of parachains.
	pub fn active_parachains() -> Vec<(ParaId, Option<(CollatorId, Retriable)>)> {
		T::ActiveParachains::active_paras()
	}

	// check the attestations on these candidates. The candidates should have been checked
	// that each candidates' chain ID is valid.
	fn check_candidates(
		schedule: &GlobalValidationSchedule,
		attested_candidates: &[AttestedCandidate],
		active_parachains: &[(ParaId, Option<(CollatorId, Retriable)>)]
	) -> sp_std::result::Result<IncludedBlocks<T>, sp_runtime::DispatchError>
	{
		// returns groups of slices that have the same chain ID.
		// assumes the inner slice is sorted by id.
		struct GroupedDutyIter<'a> {
			next_idx: usize,
			inner: &'a [(usize, ParaId)],
		}

		impl<'a> GroupedDutyIter<'a> {
			fn new(inner: &'a [(usize, ParaId)]) -> Self {
				GroupedDutyIter { next_idx: 0, inner }
			}

			fn group_for(&mut self, wanted_id: ParaId) -> Option<&'a [(usize, ParaId)]> {
				while let Some((id, keys)) = self.next() {
					if wanted_id == id {
						return Some(keys)
					}
				}

				None
			}
		}

		impl<'a> Iterator for GroupedDutyIter<'a> {
			type Item = (ParaId, &'a [(usize, ParaId)]);

			fn next(&mut self) -> Option<Self::Item> {
				if self.next_idx == self.inner.len() { return None }
				let start_idx = self.next_idx;
				self.next_idx += 1;
				let start_id = self.inner[start_idx].1;

				while self.inner.get(self.next_idx).map_or(false, |&(_, ref id)| id == &start_id) {
					self.next_idx += 1;
				}

				Some((start_id, &self.inner[start_idx..self.next_idx]))
			}
		}

		let authorities = Self::authorities();
		let (duty_roster, random_seed) = Self::calculate_duty_roster();

		// convert a duty roster, which is originally a Vec<Chain>, where each
		// item corresponds to the same position in the session keys, into
		// a list containing (index, parachain duty) where indices are into the session keys.
		// this list is sorted ascending by parachain duty, just like the
		// parachain candidates are.
		let make_sorted_duties = |duty: &[Chain]| {
			let mut sorted_duties = Vec::with_capacity(duty.len());
			for (val_idx, duty) in duty.iter().enumerate() {
				let id = match duty {
					Chain::Relay => continue,
					Chain::Parachain(id) => id,
				};

				let idx = sorted_duties.binary_search_by_key(&id, |&(_, ref id)| id)
					.unwrap_or_else(|idx| idx);

				sorted_duties.insert(idx, (val_idx, *id));
			}

			sorted_duties
		};

		// computes the omitted validation data for a particular parachain.
		let full_candidate = |abridged: &AbridgedCandidateReceipt|
			-> sp_std::result::Result<CandidateReceipt, sp_runtime::DispatchError>
		{
			let para_id = abridged.parachain_index;
			let parent_head = match Self::parachain_head(&para_id)
				.map(primitives::parachain::HeadData)
			{
				Some(p) => p,
				None => Err(Error::<T>::ParentMismatch)?,
			};

			let omitted = OmittedValidationData {
				global_validation: schedule.clone(),
				local_validation: LocalValidationData {
					parent_head,
					balance: T::ParachainCurrency::free_balance(para_id),
				},
			};

			Ok(abridged.clone().complete(omitted))
		};

		let sorted_validators = make_sorted_duties(&duty_roster.validator_duty);

		let parent_hash = <system::Module<T>>::parent_hash();
		let signing_context = Self::signing_context();
		let localized_payload = |statement: Statement| localized_payload(statement, &signing_context);

		let mut validator_groups = GroupedDutyIter::new(&sorted_validators[..]);

		let mut para_block_hashes = Vec::new();

		for candidate in attested_candidates {
			let para_id = candidate.parachain_index();
			let validator_group = validator_groups.group_for(para_id)
				.ok_or(Error::<T>::NoValidatorGroup)?;

			// NOTE: when changing this to allow older blocks,
			// care must be taken in the availability store pruning to ensure that
			// data is stored correctly. A block containing a candidate C can be
			// orphaned before a block containing C is finalized. Care must be taken
			// not to prune the data for C simply because an orphaned block contained
			// it.
			ensure!(
				candidate.candidate().relay_parent.as_ref() == parent_hash.as_ref(),
				Error::<T>::UnexpectedRelayParent,
			);

			ensure!(
				candidate.validity_votes.len() >= majority_of(validator_group.len()),
				Error::<T>::NotEnoughValidityVotes,
			);

			ensure!(
				candidate.validity_votes.len() <= authorities.len(),
				Error::<T>::VotesExceedsAuthorities,
			);

			ensure!(
				schedule.max_head_data_size >= candidate.candidate().head_data.0.len() as _,
				Error::<T>::HeadDataTooLarge,
			);

			let full_candidate = full_candidate(candidate.candidate())?;
			let fees = full_candidate.commitments.fees;

			ensure!(
				full_candidate.local_validation.balance >= full_candidate.commitments.fees,
				Error::<T>::CannotPayFees,
			);

			T::ParachainCurrency::deduct(para_id, fees)?;

			let candidate_hash = candidate.candidate().hash();
			let mut encoded_implicit = None;
			let mut encoded_explicit = None;

			let mut expected_votes_len = 0;
			for (vote_index, (auth_index, _)) in candidate.validator_indices
				.iter()
				.enumerate()
				.filter(|(_, bit)| **bit)
				.enumerate()
			{
				let validity_attestation = match candidate.validity_votes.get(vote_index) {
					None => Err(Error::<T>::NotEnoughValidityVotes)?,
					Some(v) => {
						expected_votes_len = vote_index + 1;
						v
					}
				};

				if validator_group.iter().find(|&(idx, _)| *idx == auth_index).is_none() {
					Err(Error::<T>::WrongValidatorAttesting)?
				}

				let (payload, sig) = match validity_attestation {
					ValidityAttestation::Implicit(sig) => {
						let payload = encoded_implicit.get_or_insert_with(|| localized_payload(
							Statement::Candidate(candidate_hash),
						));

						(payload, sig)
					}
					ValidityAttestation::Explicit(sig) => {
						let payload = encoded_explicit.get_or_insert_with(|| localized_payload(
							Statement::Valid(candidate_hash),
						));

						(payload, sig)
					}
				};

				ensure!(
					sig.verify(&payload[..], &authorities[auth_index]),