lib.rs

// Copyright 2019 Parity Technologies (UK) Ltd.
// This file is part of Substrate.

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

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

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

//! Consensus extension module for BABE consensus.

#![cfg_attr(not(feature = "std"), no_std)]
#![forbid(unused_must_use, unsafe_code, unused_variables, dead_code)]

pub use timestamp;

use rstd::{result, prelude::*};
use srml_support::{decl_storage, decl_module, StorageValue, traits::FindAuthor, traits::Get};
use timestamp::{OnTimestampSet};
use sr_primitives::{generic::DigestItem, ConsensusEngineId};
use sr_primitives::traits::{IsMember, SaturatedConversion, Saturating, RandomnessBeacon, Convert};
#[cfg(feature = "std")]
use timestamp::TimestampInherentData;
use parity_codec::{Encode, Decode};
use inherents::{RuntimeString, InherentIdentifier, InherentData, ProvideInherent, MakeFatalError};
#[cfg(feature = "std")]
use inherents::{InherentDataProviders, ProvideInherentData};
use babe_primitives::{BABE_ENGINE_ID, ConsensusLog, BabeWeight, Epoch, RawBabePreDigest};
pub use babe_primitives::{AuthorityId, VRF_OUTPUT_LENGTH, PUBLIC_KEY_LENGTH};

/// The BABE inherent identifier.
pub const INHERENT_IDENTIFIER: InherentIdentifier = *b"babeslot";

/// The type of the BABE inherent.
pub type InherentType = u64;
/// Auxiliary trait to extract BABE inherent data.
pub trait BabeInherentData {
	/// Get BABE inherent data.
	fn babe_inherent_data(&self) -> result::Result<InherentType, RuntimeString>;
	/// Replace BABE inherent data.
	fn babe_replace_inherent_data(&mut self, new: InherentType);
}

impl BabeInherentData for InherentData {
	fn babe_inherent_data(&self) -> result::Result<InherentType, RuntimeString> {
		self.get_data(&INHERENT_IDENTIFIER)
			.and_then(|r| r.ok_or_else(|| "BABE inherent data not found".into()))
	}

	fn babe_replace_inherent_data(&mut self, new: InherentType) {
		self.replace_data(INHERENT_IDENTIFIER, &new);
	}
}

/// Provides the slot duration inherent data for BABE.
#[cfg(feature = "std")]
pub struct InherentDataProvider {
	slot_duration: u64,
}

#[cfg(feature = "std")]
impl InherentDataProvider {
	/// Constructs `Self`
	pub fn new(slot_duration: u64) -> Self {
		Self {
			slot_duration
		}
	}
}

#[cfg(feature = "std")]
impl ProvideInherentData for InherentDataProvider {
	fn on_register(
		&self,
		providers: &InherentDataProviders,
	) -> result::Result<(), RuntimeString> {
		if !providers.has_provider(&timestamp::INHERENT_IDENTIFIER) {
			// Add the timestamp inherent data provider, as we require it.
			providers.register_provider(timestamp::InherentDataProvider)
		} else {
			Ok(())
		}
	}

	fn inherent_identifier(&self) -> &'static inherents::InherentIdentifier {
		&INHERENT_IDENTIFIER
	}

	fn provide_inherent_data(
		&self,
		inherent_data: &mut InherentData,
	) -> result::Result<(), RuntimeString> {
		let timestamp = inherent_data.timestamp_inherent_data()?;
		let slot_number = timestamp / self.slot_duration;
		inherent_data.put_data(INHERENT_IDENTIFIER, &slot_number)
	}

	fn error_to_string(&self, error: &[u8]) -> Option<String> {
		RuntimeString::decode(&mut &error[..]).map(Into::into)
	}
}

pub trait Trait: timestamp::Trait {
	type EpochDuration: Get<u64>;
	type ExpectedBlockTime: Get<Self::Moment>;
}

/// The length of the BABE randomness
pub const RANDOMNESS_LENGTH: usize = 32;

decl_storage! {
	trait Store for Module<T: Trait> as Babe {
		/// Current epoch index.
		pub EpochIndex get(epoch_index): u64;

		/// Current epoch authorities.
		pub Authorities get(authorities) config(): Vec<(AuthorityId, BabeWeight)>;

		/// Slot at which the current epoch started. It is possible that no
		/// block was authored at the given slot and the epoch change was
		/// signalled later than this.
		pub EpochStartSlot get(epoch_start_slot): u64;

		/// Current slot number.
		pub CurrentSlot get(current_slot): u64;

		/// The epoch randomness for the *current* epoch.
		///
		/// # Security
		///
		/// This MUST NOT be used for gambling, as it can be influenced by a
		/// malicious validator in the short term. It MAY be used in many
		/// cryptographic protocols, however, so long as one remembers that this
		/// (like everything else on-chain) it is public. For example, it can be
		/// used where a number is needed that cannot have been chosen by an
		/// adversary, for purposes such as public-coin zero-knowledge proofs.
		// NOTE: the following fields don't use the constants to define the
		// array size because the metadata API currently doesn't resolve the
		// variable to its underlying value.
		pub Randomness get(randomness): [u8; 32 /* RANDOMNESS_LENGTH */];

		/// Next epoch randomness.
		NextRandomness: [u8; 32 /* RANDOMNESS_LENGTH */];

		/// Randomness under construction.
		UnderConstruction: [u8; 32 /* VRF_OUTPUT_LENGTH */];
	}
}

decl_module! {
	/// The BABE SRML module
	pub struct Module<T: Trait> for enum Call where origin: T::Origin {
		/// The number of **slots** that an epoch takes. We couple sessions to
		/// epochs, i.e. we start a new session once the new epoch begins.
		const EpochDuration: u64 = T::EpochDuration::get();

		/// The expected average block time at which BABE should be creating
		/// blocks. Since BABE is probabilistic it is not trivial to figure out
		/// what the expected average block time should be based on the slot
		/// duration and the security parameter `c` (where `1 - c` represents
		/// the probability of a slot being empty).
		const ExpectedBlockTime: T::Moment = T::ExpectedBlockTime::get();

		/// Initialization
		fn on_initialize() {
			for digest in Self::get_inherent_digests()
				.logs
				.iter()
				.filter_map(|s| s.as_pre_runtime())
				.filter_map(|(id, mut data)| if id == BABE_ENGINE_ID {
					RawBabePreDigest::decode(&mut data)
				} else {
					None
				})
			{
				if EpochStartSlot::get() == 0 {
					EpochStartSlot::put(digest.slot_number);
				}

				CurrentSlot::put(digest.slot_number);
				Self::deposit_vrf_output(&digest.vrf_output);

				return;
			}
		}
	}
}

impl<T: Trait> RandomnessBeacon for Module<T> {
	fn random() -> [u8; VRF_OUTPUT_LENGTH] {
		Self::randomness()
	}
}

/// A BABE public key
pub type BabeKey = [u8; PUBLIC_KEY_LENGTH];

impl<T: Trait> FindAuthor<u64> for Module<T> {
	fn find_author<'a, I>(digests: I) -> Option<u64> where
		I: 'a + IntoIterator<Item=(ConsensusEngineId, &'a [u8])>
	{
		for (id, mut data) in digests.into_iter() {
			if id == BABE_ENGINE_ID {
				return Some(RawBabePreDigest::decode(&mut data)?.authority_index);
			}
		}
		return None;
	}
}

impl<T: Trait> IsMember<AuthorityId> for Module<T> {
	fn is_member(authority_id: &AuthorityId) -> bool {
		<Module<T>>::authorities()
			.iter()
			.any(|id| &id.0 == authority_id)
	}
}

impl<T: Trait> session::ShouldEndSession<T::BlockNumber> for Module<T> {
	fn should_end_session(_: T::BlockNumber) -> bool {
		let diff = CurrentSlot::get().saturating_sub(EpochStartSlot::get());
		diff >= T::EpochDuration::get()
	}
}

impl<T: Trait> Module<T> {
	/// Determine the BABE slot duration based on the Timestamp module configuration.
	pub fn slot_duration() -> T::Moment {
		// we double the minimum block-period so each author can always propose within
		// the majority of their slot.
		<T as timestamp::Trait>::MinimumPeriod::get().saturating_mul(2.into())
	}

	fn deposit_consensus<U: Encode>(new: U) {
		let log: DigestItem<T::Hash> = DigestItem::Consensus(BABE_ENGINE_ID, new.encode());
		<system::Module<T>>::deposit_log(log.into())
	}

	fn get_inherent_digests() -> system::DigestOf<T> {
		<system::Module<T>>::digest()
	}

	fn deposit_vrf_output(vrf_output: &[u8; VRF_OUTPUT_LENGTH]) {
		UnderConstruction::mutate(|z| z.iter_mut().zip(vrf_output).for_each(|(x, y)| *x^=y))
	}

	/// Call this function exactly once when an epoch changes, to update the
	/// randomness. Returns the new randomness.
	fn randomness_change_epoch(next_epoch_index: u64) -> [u8; RANDOMNESS_LENGTH] {
		let this_randomness = NextRandomness::get();
		let next_randomness = compute_randomness(
			this_randomness,
			next_epoch_index,
			UnderConstruction::get(),
		);
		UnderConstruction::put(&[0; RANDOMNESS_LENGTH]);
		NextRandomness::put(&next_randomness);
		this_randomness
	}

}

impl<T: Trait> OnTimestampSet<T::Moment> for Module<T> {
	fn on_timestamp_set(_moment: T::Moment) { }
}

impl<T: Trait + staking::Trait> session::OneSessionHandler<T::AccountId> for Module<T> {
	type Key = AuthorityId;
	fn on_new_session<'a, I: 'a>(_changed: bool, validators: I, queued_validators: I)
		where I: Iterator<Item=(&'a T::AccountId, AuthorityId)>
	{
		use staking::BalanceOf;
		let to_votes = |b: BalanceOf<T>| {
			<T::CurrencyToVote as Convert<BalanceOf<T>, u64>>::convert(b)
		};

		// Update epoch index
		let epoch_index = EpochIndex::get()
			.checked_add(1)
			.expect("epoch indices will never reach 2^64 before the death of the universe; qed");

		EpochIndex::put(epoch_index);

		// Update authorities.
		let authorities = validators.map(|(account, k)| {
			(k, to_votes(staking::Module::<T>::stakers(account).total))
		}).collect::<Vec<_>>();

		Authorities::put(authorities);

		// Update epoch start slot.
		let now = CurrentSlot::get();
		EpochStartSlot::mutate(|previous| {
			loop {
				// on the first epoch we must account for skipping at least one
				// whole epoch, in case the first block is authored with a slot
				// number far in the past.
				if now.saturating_sub(*previous) < T::EpochDuration::get() {
					break;
				}

				*previous = previous.saturating_add(T::EpochDuration::get());
			}
		});

		// Update epoch randomness.
		let next_epoch_index = epoch_index
			.checked_add(1)
			.expect("epoch indices will never reach 2^64 before the death of the universe; qed");

		// Returns randomness for the current epoch and computes the *next*
		// epoch randomness.
		let randomness = Self::randomness_change_epoch(next_epoch_index);
		Randomness::put(randomness);

		// After we update the current epoch, we signal the *next* epoch change
		// so that nodes can track changes.
		let next_authorities = queued_validators.map(|(account, k)| {
			(k, to_votes(staking::Module::<T>::stakers(account).total))
		}).collect::<Vec<_>>();

		let next_epoch_start_slot = EpochStartSlot::get().saturating_add(T::EpochDuration::get());
		let next_randomness = NextRandomness::get();

		let next = Epoch {
			epoch_index: next_epoch_index,
			start_slot: next_epoch_start_slot,
			duration: T::EpochDuration::get(),
			authorities: next_authorities,
			randomness: next_randomness,
		};

		Self::deposit_consensus(ConsensusLog::NextEpochData(next))
	}

	fn on_disabled(i: usize) {
		Self::deposit_consensus(ConsensusLog::OnDisabled(i as u64))
	}
}

fn compute_randomness(
	last_epoch_randomness: [u8; RANDOMNESS_LENGTH],
	epoch_index: u64,
	rho: [u8; VRF_OUTPUT_LENGTH],
) -> [u8; RANDOMNESS_LENGTH] {
	let mut s = [0; 40 + VRF_OUTPUT_LENGTH];
	s[..32].copy_from_slice(&last_epoch_randomness);
	s[32..40].copy_from_slice(&epoch_index.to_le_bytes());
	s[40..].copy_from_slice(&rho);
	runtime_io::blake2_256(&s)
}

impl<T: Trait> ProvideInherent for Module<T> {
	type Call = timestamp::Call<T>;
	type Error = MakeFatalError<RuntimeString>;
	const INHERENT_IDENTIFIER: InherentIdentifier = INHERENT_IDENTIFIER;

	fn create_inherent(_: &InherentData) -> Option<Self::Call> {
		None
	}

	fn check_inherent(call: &Self::Call, data: &InherentData) -> result::Result<(), Self::Error> {
		let timestamp = match call {
			timestamp::Call::set(ref timestamp) => timestamp.clone(),
			_ => return Ok(()),
		};

		let timestamp_based_slot = (timestamp / Self::slot_duration()).saturated_into::<u64>();
		let seal_slot = data.babe_inherent_data()?;

		if timestamp_based_slot == seal_slot {
			Ok(())
		} else {
			Err(RuntimeString::from("timestamp set in block doesn't match slot in seal").into())
		}
	}
}