Repository: paritytech/substrate
File: frame/aura/src/lib.rs (Rust)

Code before:
				};
				Self::change_authorities(bounded);
			}
		}
	}

	fn on_disabled(i: usize) {
		let log: DigestItem<T::Hash> = DigestItem::Consensus(
			AURA_ENGINE_ID,
			ConsensusLog::<T::AuthorityId>::OnDisabled(i as AuthorityIndex).encode(),
		);

		<frame_system::Pallet<T>>::deposit_log(log.into());
	}
}

impl<T: Config> FindAuthor<u32> for Pallet<T> {
	fn find_author<'a, I>(digests: I) -> Option<u32> where
		I: 'a + IntoIterator<Item=(ConsensusEngineId, &'a [u8])>
	{
		for (id, mut data) in digests.into_iter() {
			if id == AURA_ENGINE_ID {
				let slot = Slot::decode(&mut data).ok()?;
				let author_index = *slot % Self::authorities().len() as u64;
				return Some(author_index as u32)
			}
		}

		None
	}
}

/// We can not implement `FindAuthor` twice, because the compiler does not know if
/// `u32 == T::AuthorityId` and thus, prevents us to implement the trait twice.
#[doc(hidden)]
pub struct FindAccountFromAuthorIndex<T, Inner>(sp_std::marker::PhantomData<(T, Inner)>);

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

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

/// Find the authority ID of the Aura authority who authored the current block.

Review comment (security):
I haven't read deeply, but should we make the users of `on_new_session` check to see if the `validators` param respects `MaxAuthorities? Or have a proof here that it is ok to simply not use the new validators if there are too many?

Code after:

impl<T: Config> OneSessionHandler<T::AccountId> for Pallet<T> {
	type Key = T::AuthorityId;

	fn on_genesis_session<'a, I: 'a>(validators: I)
	where
		I: Iterator<Item = (&'a T::AccountId, T::AuthorityId)>,
	{
		let authorities = validators.map(|(_, k)| k).collect::<Vec<_>>();
		Self::initialize_authorities(&authorities);
	}

	fn on_new_session<'a, I: 'a>(changed: bool, validators: I, _queued_validators: I)
	where
		I: Iterator<Item = (&'a T::AccountId, T::AuthorityId)>,
	{
		// instant changes
		if changed {
			let next_authorities = validators.map(|(_, k)| k).collect::<Vec<_>>();
			let last_authorities = Self::authorities();
			if last_authorities != next_authorities {
				let bounded = <WeakBoundedVec<_, T::MaxAuthorities>>::force_from(
					next_authorities,
					Some("AuRa new session"),
				);
				Self::change_authorities(bounded);
			}
		}
	}

	fn on_disabled(i: usize) {
		let log: DigestItem<T::Hash> = DigestItem::Consensus(
			AURA_ENGINE_ID,
			ConsensusLog::<T::AuthorityId>::OnDisabled(i as AuthorityIndex).encode(),
		);

		<frame_system::Pallet<T>>::deposit_log(log.into());
	}
}

impl<T: Config> FindAuthor<u32> for Pallet<T> {
	fn find_author<'a, I>(digests: I) -> Option<u32>
	where
		I: 'a + IntoIterator<Item = (ConsensusEngineId, &'a [u8])>,
	{
		for (id, mut data) in digests.into_iter() {
			if id == AURA_ENGINE_ID {
				let slot = Slot::decode(&mut data).ok()?;
				let author_index = *slot % Self::authorities().len() as u64;
				return Some(author_index as u32)
			}