v0.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/>.

//! Primitives which are necessary for parachain execution from a relay-chain
//! perspective.

use sp_std::prelude::*;
#[cfg(feature = "std")]
use sp_std::convert::TryInto;
use sp_std::cmp::Ordering;

use parity_scale_codec::{Encode, Decode};
use bitvec::vec::BitVec;
#[cfg(feature = "std")]
use serde::{Serialize, Deserialize};
#[cfg(feature = "std")]
use parity_util_mem::{MallocSizeOf, MallocSizeOfOps};

#[cfg(feature = "std")]
use sp_keystore::{CryptoStore, SyncCryptoStorePtr, Error as KeystoreError};
use primitives::RuntimeDebug;
use runtime_primitives::traits::{AppVerify, Block as BlockT};
use inherents::InherentIdentifier;
#[cfg(feature = "std")]
use application_crypto::AppKey;
use application_crypto::KeyTypeId;

pub use runtime_primitives::traits::{BlakeTwo256, Hash as HashT, Verify, IdentifyAccount};
pub use polkadot_core_primitives::*;
pub use parity_scale_codec::Compact;

pub use polkadot_parachain::primitives::{
	Id, LOWEST_USER_ID, UpwardMessage, HeadData, BlockData,
	ValidationCode,
};

/// The key type ID for a collator key.
pub const COLLATOR_KEY_TYPE_ID: KeyTypeId = KeyTypeId(*b"coll");

/// An identifier for inherent data that provides new minimally-attested
/// parachain heads.
pub const NEW_HEADS_IDENTIFIER: InherentIdentifier = *b"newheads";

mod collator_app {
	use application_crypto::{app_crypto, sr25519};
	app_crypto!(sr25519, super::COLLATOR_KEY_TYPE_ID);
}

/// Identity that collators use.
pub type CollatorId = collator_app::Public;

#[cfg(feature = "std")]
impl MallocSizeOf for CollatorId {
	fn size_of(&self, _ops: &mut MallocSizeOfOps) -> usize {
		0
	}
	fn constant_size() -> Option<usize> {
		Some(0)
	}
}

/// A Parachain collator keypair.
#[cfg(feature = "std")]
pub type CollatorPair = collator_app::Pair;

/// Signature on candidate's block data by a collator.
pub type CollatorSignature = collator_app::Signature;

#[cfg(feature = "std")]
impl MallocSizeOf for CollatorSignature {
	fn size_of(&self, _ops: &mut MallocSizeOfOps) -> usize {
		0
	}
	fn constant_size() -> Option<usize> {
		Some(0)
	}
}

/// The key type ID for a parachain validator key.
pub const PARACHAIN_KEY_TYPE_ID: KeyTypeId = KeyTypeId(*b"para");

mod validator_app {
	use application_crypto::{app_crypto, sr25519};
	app_crypto!(sr25519, super::PARACHAIN_KEY_TYPE_ID);
}

/// Identity that parachain validators use when signing validation messages.
///
/// For now we assert that parachain validator set is exactly equivalent to the authority set, and
/// so we define it to be the same type as `SessionKey`. In the future it may have different crypto.
pub type ValidatorId = validator_app::Public;

#[cfg(feature = "std")]
impl MallocSizeOf for ValidatorId {
	fn size_of(&self, _ops: &mut MallocSizeOfOps) -> usize {
		0
	}
	fn constant_size() -> Option<usize> {
		Some(0)
	}
}

/// Index of the validator is used as a lightweight replacement of the `ValidatorId` when appropriate.
#[derive(Eq, Ord, PartialEq, PartialOrd, Copy, Clone, Encode, Decode)]
#[cfg_attr(feature = "std", derive(Serialize, Deserialize, Debug, Hash, MallocSizeOf))]
pub struct ValidatorIndex(pub u32);

// We should really get https://github.com/paritytech/polkadot/issues/2403 going ..
impl From<u32> for ValidatorIndex {
	fn from(n: u32) -> Self {
		ValidatorIndex(n)
	}
}

application_crypto::with_pair! {
	/// A Parachain validator keypair.
	pub type ValidatorPair = validator_app::Pair;
}

/// Signature with which parachain validators sign blocks.
///
/// For now we assert that parachain validator set is exactly equivalent to the authority set, and
/// so we define it to be the same type as `SessionKey`. In the future it may have different crypto.
pub type ValidatorSignature = validator_app::Signature;

#[cfg(feature = "std")]
impl MallocSizeOf for ValidatorSignature {
	fn size_of(&self, _ops: &mut MallocSizeOfOps) -> usize {
		0
	}
	fn constant_size() -> Option<usize> {
		Some(0)
	}
}

/// Retriability for a given active para.
#[derive(Clone, Eq, PartialEq, Encode, Decode)]
#[cfg_attr(feature = "std", derive(Debug))]
pub enum Retriable {
	/// Ineligible for retry. This means it's either a parachain that is always scheduled anyway or
	/// has been removed/swapped.
	Never,
	/// Eligible for retry; the associated value is the number of retries that the para already had.
	WithRetries(u32),
}

/// Type determining the active set of parachains in current block.
pub trait ActiveParas {
	/// Return the active set of parachains in current block. This attempts to keep any IDs in the
	/// same place between sequential blocks. It is therefore unordered. The second item in the
	/// tuple is the required collator ID, if any. If `Some`, then it is invalid to include any
	/// other collator's block.
	///
	/// NOTE: The initial implementation simply concatenates the (ordered) set of (permanent)
	/// parachain IDs with the (unordered) set of parathread IDs selected for this block.
	fn active_paras() -> Vec<(Id, Option<(CollatorId, Retriable)>)>;
}

/// Description of how often/when this parachain is scheduled for progression.
#[derive(Encode, Decode, Clone, PartialEq, Eq, RuntimeDebug)]
pub enum Scheduling {
	/// Scheduled every block.
	Always,
	/// Scheduled dynamically (i.e. a parathread).
	Dynamic,
}

/// Information regarding a deployed parachain/thread.
#[derive(Encode, Decode, Clone, PartialEq, Eq, RuntimeDebug)]
pub struct Info {
	/// Scheduling info.
	pub scheduling: Scheduling,
}

/// An `Info` value for a standard leased parachain.
pub const PARACHAIN_INFO: Info = Info {
	scheduling: Scheduling::Always,
};

/// Auxilliary for when there's an attempt to swap two parachains/parathreads.
pub trait SwapAux {
	/// Result describing whether it is possible to swap two parachains. Doesn't mutate state.
	fn ensure_can_swap(one: Id, other: Id) -> Result<(), &'static str>;

	/// Updates any needed state/references to enact a logical swap of two parachains. Identity,
	/// code and `head_data` remain equivalent for all parachains/threads, however other properties
	/// such as leases, deposits held and thread/chain nature are swapped.
	///
	/// May only be called on a state that `ensure_can_swap` has previously returned `Ok` for: if this is
	/// not the case, the result is undefined. May only return an error if `ensure_can_swap` also returns
	/// an error.
	fn on_swap(one: Id, other: Id) -> Result<(), &'static str>;
}

impl SwapAux for () {
	fn ensure_can_swap(_: Id, _: Id) -> Result<(), &'static str> { Err("Swapping disabled") }
	fn on_swap(_: Id, _: Id) -> Result<(), &'static str> { Err("Swapping disabled") }
}

/// Identifier for a chain, either one of a number of parachains or the relay chain.
#[derive(Copy, Clone, PartialEq, Encode, Decode)]
#[cfg_attr(feature = "std", derive(Debug))]
pub enum Chain {
	/// The relay chain.
	Relay,
	/// A parachain of the given index.
	Parachain(Id),
}

/// The duty roster specifying what jobs each validator must do.
#[derive(Clone, PartialEq, Encode, Decode)]
#[cfg_attr(feature = "std", derive(Default, Debug))]
pub struct DutyRoster {
	/// Lookup from validator index to chain on which that validator has a duty to validate.
	pub validator_duty: Vec<Chain>,
}

/// Extra data that is needed along with the other fields in a `CandidateReceipt`
/// to fully validate the candidate.
///
/// These are global parameters that apply to all parachain candidates in a block.
#[derive(PartialEq, Eq, Clone, Encode, Decode)]
#[cfg_attr(feature = "std", derive(Debug, Default))]
pub struct GlobalValidationData<N = BlockNumber> {
	/// The maximum code size permitted, in bytes.
	pub max_code_size: u32,
	/// The maximum head-data size permitted, in bytes.
	pub max_head_data_size: u32,
	/// The relay-chain block number this is in the context of.
	pub block_number: N,
}

/// Extra data that is needed along with the other fields in a `CandidateReceipt`
/// to fully validate the candidate. These fields are parachain-specific.
#[derive(PartialEq, Eq, Clone, Encode, Decode)]
#[cfg_attr(feature = "std", derive(Debug, Default))]
pub struct LocalValidationData<N = BlockNumber> {
	/// The parent head-data.
	pub parent_head: HeadData,
	/// The balance of the parachain at the moment of validation.
	pub balance: Balance,
	/// Whether the parachain is allowed to upgrade its validation code.
	///
	/// This is `Some` if so, and contains the number of the minimum relay-chain
	/// height at which the upgrade will be applied, if an upgrade is signaled
	/// now.
	///
	/// A parachain should enact its side of the upgrade at the end of the first
	/// parablock executing in the context of a relay-chain block with at least this
	/// height. This may be equal to the current perceived relay-chain block height, in
	/// which case the code upgrade should be applied at the end of the signaling
	/// block.
	pub code_upgrade_allowed: Option<N>,
}

/// Commitments made in a `CandidateReceipt`. Many of these are outputs of validation.
#[derive(PartialEq, Eq, Clone, Encode, Decode)]
#[cfg_attr(feature = "std", derive(Debug, Default))]
pub struct CandidateCommitments<H = Hash> {
	/// Fees paid from the chain to the relay chain validators.
	pub fees: Balance,
	/// Messages destined to be interpreted by the Relay chain itself.
	pub upward_messages: Vec<UpwardMessage>,
	/// The root of a block's erasure encoding Merkle tree.
	pub erasure_root: H,
	/// New validation code.
	pub new_validation_code: Option<ValidationCode>,
	/// Number of `DownwardMessage`'s that were processed by the Parachain.
	///
	/// It is expected that the Parachain processes them from first to last.
	pub processed_downward_messages: u32,
}

/// Get a collator signature payload on a relay-parent, block-data combo.
pub fn collator_signature_payload<H: AsRef<[u8]>>(
	relay_parent: &H,
	parachain_index: &Id,
	pov_block_hash: &H,
) -> [u8; 68] {
	// 32-byte hash length is protected in a test below.
	let mut payload = [0u8; 68];

	payload[0..32].copy_from_slice(relay_parent.as_ref());
	u32::from(*parachain_index).using_encoded(|s| payload[32..32 + s.len()].copy_from_slice(s));
	payload[36..68].copy_from_slice(pov_block_hash.as_ref());

	payload
}

fn check_collator_signature<H: AsRef<[u8]>>(
	relay_parent: &H,
	parachain_index: &Id,
	pov_block_hash: &H,
	collator: &CollatorId,
	signature: &CollatorSignature,
) -> Result<(),()> {
	let payload = collator_signature_payload(relay_parent, parachain_index, pov_block_hash);
	if signature.verify(&payload[..], collator) {
		Ok(())
	} else {
		Err(())
	}
}

/// All data pertaining to the execution of a parachain candidate.
#[derive(PartialEq, Eq, Clone, Encode, Decode)]
#[cfg_attr(feature = "std", derive(Debug, Default))]
pub struct CandidateReceipt<H = Hash, N = BlockNumber> {
	/// The ID of the parachain this is a candidate for.
	pub parachain_index: Id,
	/// The hash of the relay-chain block this should be executed in
	/// the context of.
	pub relay_parent: H,
	/// The head-data
	pub head_data: HeadData,
	/// The collator's relay-chain account ID
	pub collator: CollatorId,
	/// Signature on blake2-256 of the block data by collator.
	pub signature: CollatorSignature,
	/// The hash of the PoV-block.
	pub pov_block_hash: H,
	/// The global validation schedule.
	pub global_validation: GlobalValidationData<N>,
	/// The local validation data.
	pub local_validation: LocalValidationData<N>,
	/// Commitments made as a result of validation.
	pub commitments: CandidateCommitments<H>,
}

impl<H: AsRef<[u8]>, N> CandidateReceipt<H, N> {
	/// Check integrity vs. provided block data.
	pub fn check_signature(&self) -> Result<(), ()> {
		check_collator_signature(
			&self.relay_parent,
			&self.parachain_index,
			&self.pov_block_hash,
			&self.collator,
			&self.signature,
		)
	}

	/// Abridge this `CandidateReceipt`, splitting it into an `AbridgedCandidateReceipt`
	/// and its omitted component.
	pub fn abridge(self) -> (AbridgedCandidateReceipt<H>, OmittedValidationData<N>) {
		let CandidateReceipt {
			parachain_index,
			relay_parent,
			head_data,
			collator,
			signature,
			pov_block_hash,
			global_validation,
			local_validation,
			commitments,
		} = self;

		let abridged = AbridgedCandidateReceipt {
			parachain_index,
			relay_parent,
			head_data,
			collator,
			signature,
			pov_block_hash,
			commitments,
		};

		let omitted = OmittedValidationData {
			global_validation,
			local_validation,
		};

		(abridged, omitted)
	}
}

impl PartialOrd for CandidateReceipt {
	fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
		Some(self.cmp(other))
	}
}

impl Ord for CandidateReceipt {
	fn cmp(&self, other: &Self) -> Ordering {
		// TODO: compare signatures or something more sane
		// https://github.com/paritytech/polkadot/issues/222
		self.parachain_index.cmp(&other.parachain_index)
			.then_with(|| self.head_data.cmp(&other.head_data))
	}
}

/// All the data which is omitted in an `AbridgedCandidateReceipt`, but that
/// is necessary for validation of the parachain candidate.
#[derive(PartialEq, Eq, Clone, Encode, Decode)]
#[cfg_attr(feature = "std", derive(Debug, Default))]
pub struct OmittedValidationData<N = BlockNumber> {
	/// The global validation schedule.
	pub global_validation: GlobalValidationData<N>,
	/// The local validation data.
	pub local_validation: LocalValidationData<N>,
}

/// An abridged candidate-receipt.
///
/// Much info in a candidate-receipt is duplicated from the relay-chain state.
/// When submitting to the relay-chain, this data should be omitted as it can
/// be re-generated from relay-chain state.
#[derive(PartialEq, Eq, Clone, Encode, Decode)]
#[cfg_attr(feature = "std", derive(Debug, Default))]
pub struct AbridgedCandidateReceipt<H = Hash> {
	/// The ID of the parachain this is a candidate for.
	pub parachain_index: Id,
	/// The hash of the relay-chain block this should be executed in
	/// the context of.
	// NOTE: the fact that the hash includes this value means that code depends
	// on this for deduplication. Removing this field is likely to break things.
	pub relay_parent: H,
	/// The head-data
	pub head_data: HeadData,
	/// The collator's relay-chain account ID
	pub collator: CollatorId,
	/// Signature on blake2-256 of the block data by collator.
	pub signature: CollatorSignature,
	/// The hash of the pov-block.
	pub pov_block_hash: H,
	/// Commitments made as a result of validation.
	pub commitments: CandidateCommitments<H>,
}

/// A candidate-receipt with commitments directly included.
pub struct CommitedCandidateReceipt<H = Hash> {
	/// The descriptor of the candidae.
	pub descriptor: CandidateDescriptor,

	/// The commitments of the candidate receipt.
	pub commitments: CandidateCommitments<H>
}

impl<H: AsRef<[u8]> + Encode> AbridgedCandidateReceipt<H> {
	/// Check integrity vs. provided block data.
	pub fn check_signature(&self) -> Result<(), ()> {
		check_collator_signature(
			&self.relay_parent,
			&self.parachain_index,
			&self.pov_block_hash,
			&self.collator,
			&self.signature,
		)
	}

	/// Compute the hash of the abridged candidate receipt.
	///
	/// This is often used as the canonical hash of the receipt, rather than
	/// the hash of the full receipt. The reason being that all data in the full
	/// receipt is committed to in the abridged receipt; this receipt references
	/// the relay-chain block in which context it should be executed, which implies
	/// any blockchain state that must be referenced.
	pub fn hash(&self) -> Hash {
		BlakeTwo256::hash_of(self)
	}
}

impl AbridgedCandidateReceipt {
	/// Combine the abridged candidate receipt with the omitted data,
	/// forming a full `CandidateReceipt`.
	pub fn complete(self, omitted: OmittedValidationData) -> CandidateReceipt {
		let AbridgedCandidateReceipt {
			parachain_index,
			relay_parent,
			head_data,
			collator,
			signature,
			pov_block_hash,
			commitments,
		} = self;

		let OmittedValidationData {
			global_validation,
			local_validation,
		} = omitted;

		CandidateReceipt {
			parachain_index,
			relay_parent,
			head_data,
			collator,
			signature,
			pov_block_hash,
			local_validation,
			global_validation,
			commitments,
		}
	}

	/// Clone the relevant portions of the `CandidateReceipt` to form a `CollationInfo`.
	pub fn to_collation_info(&self) -> CollationInfo {
		let AbridgedCandidateReceipt {
			parachain_index,
			relay_parent,
			head_data,
			collator,
			signature,
			pov_block_hash,
			commitments: _commitments,
		} = self;

		CollationInfo {
			parachain_index: *parachain_index,
			relay_parent: *relay_parent,
			head_data: head_data.clone(),
			collator: collator.clone(),
			signature: signature.clone(),
			pov_block_hash: *pov_block_hash,
		}
	}

	/// Clone the relevant portions of the `AbridgedCandidateReceipt` to form a `CandidateDescriptor`.
	pub fn to_descriptor(&self) -> CandidateDescriptor {
		CandidateDescriptor {
			para_id: self.parachain_index,
			relay_parent: self.relay_parent,
			collator: self.collator.clone(),
			signature: self.signature.clone(),
			pov_hash: self.pov_block_hash.clone(),
		}
	}
}

impl PartialOrd for AbridgedCandidateReceipt {
	fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
		Some(self.cmp(other))
	}
}

impl Ord for AbridgedCandidateReceipt {
	fn cmp(&self, other: &Self) -> Ordering {
		// TODO: compare signatures or something more sane
		// https://github.com/paritytech/polkadot/issues/222
		self.parachain_index.cmp(&other.parachain_index)
			.then_with(|| self.head_data.cmp(&other.head_data))
	}
}

/// A unique descriptor of the candidate receipt, in a lightweight format.
#[derive(PartialEq, Eq, Clone, Encode, Decode)]
#[cfg_attr(feature = "std", derive(Debug, Default))]
pub struct CandidateDescriptor<H = Hash> {
	/// The ID of the para this is a candidate for.
	pub para_id: Id,
	/// The hash of the relay-chain block this should be executed in
	/// the context of.
	// NOTE: the fact that the hash includes this value means that code depends
	// on this for deduplication. Removing this field is likely to break things.
	pub relay_parent: H,
	/// The collator's relay-chain account ID
	pub collator: CollatorId,
	/// Signature on blake2-256 of components of this receipt:
	/// The para ID, the relay parent, and the pov_hash.
	pub signature: CollatorSignature,
	/// The hash of the pov-block.
	pub pov_hash: H,
}

/// A collation sent by a collator.
#[derive(PartialEq, Eq, Clone, Encode, Decode)]
#[cfg_attr(feature = "std", derive(Debug, Default))]
pub struct CollationInfo {
	/// The ID of the parachain this is a candidate for.
	pub parachain_index: Id,
	/// The relay-chain block hash this block should execute in the
	/// context of.
	pub relay_parent: Hash,
	/// The collator's relay-chain account ID
	pub collator: CollatorId,
	/// Signature on blake2-256 of the block data by collator.
	pub signature: CollatorSignature,
	/// The head-data
	pub head_data: HeadData,
	/// blake2-256 Hash of the pov-block
	pub pov_block_hash: Hash,
}

impl CollationInfo {
	/// Check integrity vs. a pov-block.
	pub fn check_signature(&self) -> Result<(), ()> {
		check_collator_signature(
			&self.relay_parent,
			&self.parachain_index,
			&self.pov_block_hash,
			&self.collator,
			&self.signature,
		)
	}

	/// Turn this into an `AbridgedCandidateReceipt` by supplying a set of commitments.
	pub fn into_receipt(self, commitments: CandidateCommitments) -> AbridgedCandidateReceipt {
		let CollationInfo {
			parachain_index,
			relay_parent,
			collator,
			signature,
			head_data,
			pov_block_hash,
		} = self;

		AbridgedCandidateReceipt {
			parachain_index,
			relay_parent,
			collator,
			signature,
			head_data,
			pov_block_hash,
			commitments,
		}
	}
}

/// A full collation.
#[derive(PartialEq, Eq, Clone)]
#[cfg_attr(feature = "std", derive(Debug, Encode, Decode))]
pub struct Collation {
	/// Candidate receipt itself.
	pub info: CollationInfo,
	/// A proof-of-validation for the receipt.
	pub pov: PoVBlock,
}

/// A Proof-of-Validation block.
#[derive(PartialEq, Eq, Clone)]
#[cfg_attr(feature = "std", derive(Debug, Encode, Decode))]
pub struct PoVBlock {
	/// Block data.
	pub block_data: BlockData,
}

impl PoVBlock {
	/// Compute hash of block data.
	#[cfg(feature = "std")]
	pub fn hash(&self) -> Hash {
		BlakeTwo256::hash_of(&self)
	}
}

/// The data that is kept available about a particular parachain block.
#[derive(PartialEq, Eq, Clone)]
#[cfg_attr(feature = "std", derive(Debug, Encode, Decode))]
pub struct AvailableData {
	/// The PoV block.
	pub pov_block: PoVBlock,
	/// Data that is omitted from an abridged candidate receipt
	/// that is necessary for validation.
	pub omitted_validation: OmittedValidationData,
	// In the future, outgoing messages as well.
}

/// A chunk of erasure-encoded block data.
#[derive(PartialEq, Eq, Clone, Encode, Decode)]
#[cfg_attr(feature = "std", derive(Serialize, Deserialize, Debug, Hash))]
pub struct ErasureChunk {
	/// The erasure-encoded chunk of data belonging to the candidate block.
	pub chunk: Vec<u8>,
	/// The index of this erasure-encoded chunk of data.
	pub index: ValidatorIndex,
	/// Proof for this chunk's branch in the Merkle tree.
	pub proof: Vec<Vec<u8>>,
}

const BACKING_STATEMENT_MAGIC: [u8; 4] = *b"BKNG";

/// Statements that can be made about parachain candidates. These are the
/// actual values that are signed.
#[derive(Clone, PartialEq, Eq)]
#[cfg_attr(feature = "std", derive(Debug, Hash))]
pub enum CompactStatement {
	/// Proposal of a parachain candidate.
	Seconded(CandidateHash),
	/// State that a parachain candidate is valid.
	Valid(CandidateHash),
}

// Inner helper for codec on `CompactStatement`.
#[derive(Encode, Decode)]
enum CompactStatementInner {
	#[codec(index = 1)]
	Seconded(CandidateHash),
	#[codec(index = 2)]
	Valid(CandidateHash),
}

impl From<CompactStatement> for CompactStatementInner {
	fn from(s: CompactStatement) -> Self {
		match s {
			CompactStatement::Seconded(h) => CompactStatementInner::Seconded(h),
			CompactStatement::Valid(h) => CompactStatementInner::Valid(h),
		}
	}
}

impl parity_scale_codec::Encode for CompactStatement {
	fn size_hint(&self) -> usize {
		// magic + discriminant + payload
		4 + 1 + 32
	}

	fn encode_to<T: parity_scale_codec::Output + ?Sized>(&self, dest: &mut T) {
		dest.write(&BACKING_STATEMENT_MAGIC);
		CompactStatementInner::from(self.clone()).encode_to(dest)
	}
}

impl parity_scale_codec::Decode for CompactStatement {
	fn decode<I: parity_scale_codec::Input>(input: &mut I) -> Result<Self, parity_scale_codec::Error> {
		let maybe_magic = <[u8; 4]>::decode(input)?;
		if maybe_magic != BACKING_STATEMENT_MAGIC {
			return Err(parity_scale_codec::Error::from("invalid magic string"));
		}

		Ok(match CompactStatementInner::decode(input)? {
			CompactStatementInner::Seconded(h) => CompactStatement::Seconded(h),
			CompactStatementInner::Valid(h) => CompactStatement::Valid(h),
		})
	}
}

impl CompactStatement {
	/// Get the underlying candidate hash this references.
	pub fn candidate_hash(&self) -> &CandidateHash {
		match *self {
			CompactStatement::Seconded(ref h) | CompactStatement::Valid(ref h) => h,
		}
	}
}

/// A signed compact statement, suitable to be sent to the chain.
pub type SignedStatement = Signed<CompactStatement>;

/// An either implicit or explicit attestation to the validity of a parachain
/// candidate.
#[derive(Clone, Eq, PartialEq, Decode, Encode, RuntimeDebug)]
pub enum ValidityAttestation {
	/// Implicit validity attestation by issuing.
	/// This corresponds to issuance of a `Candidate` statement.
	#[codec(index = 1)]
	Implicit(ValidatorSignature),
	/// An explicit attestation. This corresponds to issuance of a
	/// `Valid` statement.
	#[codec(index = 2)]
	Explicit(ValidatorSignature),
}

impl ValidityAttestation {
	/// Get a reference to the signature.
	pub fn signature(&self) -> &ValidatorSignature {
		match *self {
			ValidityAttestation::Implicit(ref sig) => sig,
			ValidityAttestation::Explicit(ref sig) => sig,
		}
	}

	/// Produce the underlying signed payload of the attestation, given the hash of the candidate,
	/// which should be known in context.
	pub fn signed_payload<H: Encode>(
		&self,
		candidate_hash: CandidateHash,
		signing_context: &SigningContext<H>,
	) -> Vec<u8> {
		match *self {
			ValidityAttestation::Implicit(_) => (
				CompactStatement::Seconded(candidate_hash),
				signing_context,
			).encode(),
			ValidityAttestation::Explicit(_) => (
				CompactStatement::Valid(candidate_hash),
				signing_context,
			).encode(),
		}
	}
}

/// A type returned by runtime with current session index and a parent hash.
#[derive(Clone, Eq, PartialEq, Default, Decode, Encode, RuntimeDebug)]
pub struct SigningContext<H = Hash> {
	/// Current session index.
	pub session_index: sp_staking::SessionIndex,
	/// Hash of the parent.
	pub parent_hash: H,
}

/// An attested candidate. This is submitted to the relay chain by a block author.
#[derive(Clone, PartialEq, Decode, Encode, RuntimeDebug)]
pub struct AttestedCandidate {
	/// The candidate data. This is abridged, because the omitted data
	/// is already present within the relay chain state.
	pub candidate: AbridgedCandidateReceipt,
	/// Validity attestations.
	pub validity_votes: Vec<ValidityAttestation>,
	/// Indices of the corresponding validity votes.
	pub validator_indices: BitVec<bitvec::order::Lsb0, u8>,
}

impl AttestedCandidate {
	/// Get the candidate.
	pub fn candidate(&self) -> &AbridgedCandidateReceipt {
		&self.candidate
	}

	/// Get the group ID of the candidate.
	pub fn parachain_index(&self) -> Id {
		self.candidate.parachain_index
	}
}

/// A fee schedule for messages. This is a linear function in the number of bytes of a message.
#[derive(PartialEq, Eq, PartialOrd, Hash, Default, Clone, Copy, Encode, Decode)]
#[cfg_attr(feature = "std", derive(Serialize, Deserialize, Debug))]
pub struct FeeSchedule {
	/// The base fee charged for all messages.
	pub base: Balance,
	/// The per-byte fee for messages charged on top of that.
	pub per_byte: Balance,
}

impl FeeSchedule {
	/// Compute the fee for a message of given size.
	pub fn compute_message_fee(&self, n_bytes: usize) -> Balance {
		use sp_std::mem;
		debug_assert!(mem::size_of::<Balance>() >= mem::size_of::<usize>());

		let n_bytes = n_bytes as Balance;
		self.base.saturating_add(n_bytes.saturating_mul(self.per_byte))
	}
}

sp_api::decl_runtime_apis! {
	/// The API for querying the state of parachains on-chain.
	#[api_version(3)]
	pub trait ParachainHost {
		/// Get the current validators.
		fn validators() -> Vec<ValidatorId>;
		/// Get the current duty roster.
		fn duty_roster() -> DutyRoster;
		/// Get the currently active parachains.
		fn active_parachains() -> Vec<(Id, Option<(CollatorId, Retriable)>)>;
		/// Get the global validation schedule that all parachains should
		/// be validated under.
		fn global_validation_data() -> GlobalValidationData;
		/// Get the local validation data for a particular parachain.
		fn local_validation_data(id: Id) -> Option<LocalValidationData>;
		/// Get the given parachain's head code blob.
		fn parachain_code(id: Id) -> Option<ValidationCode>;
		/// Extract the abridged head that was set in the extrinsics.
		fn get_heads(extrinsics: Vec<<Block as BlockT>::Extrinsic>)
			-> Option<Vec<AbridgedCandidateReceipt>>;
		/// Get a `SigningContext` with current `SessionIndex` and parent hash.
		fn signing_context() -> SigningContext;
		/// Get the `DownwardMessage`'s for the given parachain.
		fn downward_messages(id: Id) -> Vec<DownwardMessage>;
	}
}

/// Runtime ID module.
pub mod id {
	use sp_version::ApiId;

	/// Parachain host runtime API id.
	pub const PARACHAIN_HOST: ApiId = *b"parahost";
}

/// This helper trait ensures that we can encode Statement as CompactStatement,
/// and anything as itself.
///
/// This resembles `parity_scale_codec::EncodeLike`, but it's distinct:
/// EncodeLike is a marker trait which asserts at the typesystem level that
/// one type's encoding is a valid encoding for another type. It doesn't
/// perform any type conversion when encoding.
///
/// This trait, on the other hand, provides a method which can be used to
/// simultaneously convert and encode one type as another.
pub trait EncodeAs<T> {
	/// Convert Self into T, then encode T.
	///
	/// This is useful when T is a subset of Self, reducing encoding costs;
	/// its signature also means that we do not need to clone Self in order
	/// to retain ownership, as we would if we were to do
	/// `self.clone().into().encode()`.
	fn encode_as(&self) -> Vec<u8>;
}

impl<T: Encode> EncodeAs<T> for T {
	fn encode_as(&self) -> Vec<u8> {
		self.encode()
	}
}

/// A signed type which encapsulates the common desire to sign some data and validate a signature.
///
/// Note that the internal fields are not public; they are all accessable by immutable getters.
/// This reduces the chance that they are accidentally mutated, invalidating the signature.
#[derive(Clone, PartialEq, Eq, Encode, Decode, RuntimeDebug)]
pub struct Signed<Payload, RealPayload = Payload> {
	/// The payload is part of the signed data. The rest is the signing context,
	/// which is known both at signing and at validation.
	payload: Payload,
	/// The index of the validator signing this statement.
	validator_index: ValidatorIndex,
	/// The signature by the validator of the signed payload.
	signature: ValidatorSignature,
	/// This ensures the real payload is tracked at the typesystem level.
	real_payload: sp_std::marker::PhantomData<RealPayload>,
}

// We can't bound this on `Payload: Into<RealPayload>` beacuse that conversion consumes
// the payload, and we don't want that. We can't bound it on `Payload: AsRef<RealPayload>`
// because there's no blanket impl of `AsRef<T> for T`. In the end, we just invent our
// own trait which does what we need: EncodeAs.
impl<Payload: EncodeAs<RealPayload>, RealPayload: Encode> Signed<Payload, RealPayload> {
	fn payload_data<H: Encode>(payload: &Payload, context: &SigningContext<H>) -> Vec<u8> {
		// equivalent to (real_payload, context).encode()
		let mut out = payload.encode_as();
		out.extend(context.encode());
		out
	}

	/// Used to create a `Signed` from already existing parts.
	#[cfg(feature = "std")]
	pub fn new<H: Encode>(
		payload: Payload,
		validator_index: ValidatorIndex,
		signature: ValidatorSignature,
		context: &SigningContext<H>,
		key: &ValidatorId,
	) -> Option<Self> {
		let s = Self {
			payload,
			validator_index,
			signature,
			real_payload: std::marker::PhantomData,
		};

		s.check_signature(context, key).ok()?;

		Some(s)
	}

	/// Sign this payload with the given context and key, storing the validator index.
	#[cfg(feature = "std")]
	pub async fn sign<H: Encode>(
		keystore: &SyncCryptoStorePtr,
		payload: Payload,
		context: &SigningContext<H>,
		validator_index: ValidatorIndex,
		key: &ValidatorId,
	) -> Result<Option<Self>, KeystoreError> {
		let data = Self::payload_data(&payload, context);
		let signature = CryptoStore::sign_with(
			&**keystore,
			ValidatorId::ID,
			&key.into(),
			&data,
		).await?;

		let signature = match signature {
			Some(sig) => sig.try_into().map_err(|_| KeystoreError::KeyNotSupported(ValidatorId::ID))?,
			None => return Ok(None),
		};

		Ok(Some(Self {
			payload,
			validator_index,
			signature,
			real_payload: std::marker::PhantomData,
		}))
	}

	/// Validate the payload given the context and public key.
	pub fn check_signature<H: Encode>(&self, context: &SigningContext<H>, key: &ValidatorId) -> Result<(), ()> {
		let data = Self::payload_data(&self.payload, context);
		if self.signature.verify(data.as_slice(), key) { Ok(()) } else { Err(()) }
	}

	/// Immutably access the payload.
	#[inline]
	pub fn payload(&self) -> &Payload {
		&self.payload
	}

	/// Immutably access the validator index.
	#[inline]