Generic authorship and uncle inclusion module (#2941)

* generalized uncle processing

* add some uncle tests

* set author and do event handling

* OnePerAuthorPerHeight no longer O(n^2) and test

* bump impl_version of node

* Documentation and style fixes

Co-Authored-By: DemiMarie-parity <48690212+DemiMarie-parity@users.noreply.github.com>

* fix #2949: index-based FindAuthor wrapper for srml-session

* use for_each_tuple

substrate/Cargo.lock

@@ -3474,6 +3474,19 @@ dependencies = [
  "substrate-primitives 2.0.0",
 ]
 
+[[package]]
+name = "srml-authorship"
+version = "0.1.0"
+dependencies = [
+ "parity-codec 3.5.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "sr-io 2.0.0",
+ "sr-primitives 2.0.0",
+ "sr-std 2.0.0",
+ "srml-support 2.0.0",
+ "srml-system 2.0.0",
+ "substrate-primitives 2.0.0",
+]
+
 [[package]]
 name = "srml-babe"
 version = "2.0.0"

substrate/Cargo.toml

@@ -61,6 +61,7 @@ members = [
 	"srml/support/procedural/tools",
 	"srml/support/procedural/tools/derive",
 	"srml/support/test",
+	"srml/authorship",
 	"srml/assets",
 	"srml/aura",
 	"srml/balances",

substrate/core/sr-std/with_std.rs

@@ -38,4 +38,5 @@ pub use std::vec;
 pub mod collections {
 	pub use std::collections::btree_map;
 	pub use std::collections::btree_set;
+	pub use std::collections::vec_deque;
 }

substrate/core/sr-std/without_std.rs

@@ -74,4 +74,5 @@ pub use core::str;
 pub mod collections {
 	pub use alloc::collections::btree_map;
 	pub use alloc::collections::btree_set;
+	pub use alloc::collections::vec_deque;
 }

substrate/srml/authorship/Cargo.toml

@@ -0,0 +1,27 @@
+[package]
+name = "srml-authorship"
+version = "0.1.0"
+description = "Block and Uncle Author tracking for the SRML"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"
+
+[dependencies]
+substrate-primitives = { path = "../../core/primitives", default-features = false }
+parity-codec = { version = "3.3", default-features = false, features = ["derive"] }
+rstd = { package = "sr-std", path = "../../core/sr-std", default-features = false }
+primitives = { package = "sr-primitives", path = "../../core/sr-primitives", default-features = false }
+srml-support = { path = "../support", default-features = false }
+system = { package = "srml-system", path = "../system", default-features = false }
+runtime_io = { package = "sr-io", path = "../../core/sr-io", default-features = false }
+
+[features]
+default = ["std"]
+std = [
+	"parity-codec/std",
+	"substrate-primitives/std",
+	"rstd/std",
+	"srml-support/std",
+	"primitives/std",
+	"system/std",
+	"runtime_io/std",
+]

substrate/srml/authorship/src/lib.rs

@@ -0,0 +1,622 @@
+// 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/>.
+
+//! Authorship tracking for SRML runtimes.
+//!
+//! This tracks the current author of the block and recent uncles.
+
+use rstd::prelude::*;
+use rstd::collections::btree_set::BTreeSet;
+use srml_support::{decl_module, decl_storage, for_each_tuple, StorageValue};
+use srml_support::traits::{FindAuthor, VerifySeal, Get};
+use srml_support::dispatch::Result as DispatchResult;
+use parity_codec::{Encode, Decode};
+use system::ensure_none;
+use primitives::traits::{SimpleArithmetic, Header as HeaderT, One, Zero};
+
+pub trait Trait: system::Trait {
+	/// Find the author of a block.
+	type FindAuthor: FindAuthor<Self::AccountId>;
+	/// The number of blocks back we should accept uncles.
+	/// This means that we will deal with uncle-parents that are
+	/// `UncleGenerations + 1` before `now`.
+	type UncleGenerations: Get<Self::BlockNumber>;
+	/// A filter for uncles within a block. This is for implementing
+	/// further constraints on what uncles can be included, other than their ancestry.
+	///
+	/// For PoW, as long as the seals are checked, there is no need to use anything
+	/// but the `VerifySeal` implementation as the filter. This is because the cost of making many equivocating
+	/// uncles is high.
+	///
+	/// For PoS, there is no such limitation, so a further constraint must be imposed
+	/// beyond a seal check in order to prevent an arbitrary number of
+	/// equivocating uncles from being included.
+	///
+	/// The `OnePerAuthorPerHeight` filter is good for many slot-based PoS
+	/// engines.
+	type FilterUncle: FilterUncle<Self::Header, Self::AccountId>;
+	/// An event handler for authored blocks.
+	type EventHandler: EventHandler<Self::AccountId, Self::BlockNumber>;
+}
+
+/// An event handler for the authorship module. There is a dummy implementation
+/// for `()`, which does nothing.
+pub trait EventHandler<Author, BlockNumber> {
+	/// Note that the given account ID is the author of the current block.
+	fn note_author(author: Author);
+
+	/// Note that the given account ID authored the given uncle, and how many
+	/// blocks older than the current block it is (age >= 0, so siblings are allowed)
+	fn note_uncle(author: Author, age: BlockNumber);
+}
+
+macro_rules! impl_event_handler {
+	() => (
+		impl<A, B> EventHandler<A, B> for () {
+			fn note_author(_author: A) { }
+			fn note_uncle(_author: A, _age: B) { }
+		}
+	);
+
+	( $($t:ident)* ) => {
+		impl<Author: Clone, BlockNumber: Clone, $($t: EventHandler<Author, BlockNumber>),*>
+			EventHandler<Author, BlockNumber> for ($($t,)*)
+		{
+			fn note_author(author: Author) {
+				$($t::note_author(author.clone());)*
+			}
+			fn note_uncle(author: Author, age: BlockNumber) {
+				$($t::note_uncle(author.clone(), age.clone());)*
+			}
+		}
+	}
+}
+
+for_each_tuple!(impl_event_handler);
+
+/// Additional filtering on uncles that pass preliminary ancestry checks.
+///
+/// This should do work such as checking seals
+pub trait FilterUncle<Header, Author> {
+	/// An accumulator of data about uncles included.
+	///
+	/// In practice, this is used to validate uncles against others in the same block.
+	type Accumulator: Default;
+
+	/// Do additional filtering on a seal-checked uncle block, with the accumulated
+	/// filter.
+	fn filter_uncle(header: &Header, acc: Self::Accumulator)
+		-> Result<(Option<Author>, Self::Accumulator), &'static str>;
+}
+
+/// A filter on uncles which verifies seals and does no additional checks.
+/// This is well-suited to consensus modes such as PoW where the cost of
+/// equivocating is high.
+pub struct SealVerify<T>(rstd::marker::PhantomData<T>);
+
+impl<Header, Author, T: VerifySeal<Header, Author>> FilterUncle<Header, Author>
+	for SealVerify<T>
+{
+	type Accumulator = ();
+
+	fn filter_uncle(header: &Header, _acc: ())
+		-> Result<(Option<Author>, ()), &'static str>
+	{
+		T::verify_seal(header).map(|author| (author, ()))
+	}
+}
+
+/// A filter on uncles which verifies seals and ensures that there is only
+/// one uncle included per author per height.
+///
+/// This does O(n log n) work in the number of uncles included.
+pub struct OnePerAuthorPerHeight<T, N>(rstd::marker::PhantomData<(T, N)>);
+
+impl<Header, Author, T> FilterUncle<Header, Author>
+	for OnePerAuthorPerHeight<T, Header::Number>
+where
+	Header: HeaderT + PartialEq,
+	Header::Number: Ord,
+	Author: Clone + PartialEq + Ord,
+	T: VerifySeal<Header, Author>,
+{
+	type Accumulator = BTreeSet<(Header::Number, Author)>;
+
+	fn filter_uncle(header: &Header, mut acc: Self::Accumulator)
+		-> Result<(Option<Author>, Self::Accumulator), &'static str>
+	{
+		let author = T::verify_seal(header)?;
+		let number = header.number();
+
+		if let Some(ref author) = author {
+			if !acc.insert((number.clone(), author.clone())) {
+				return Err("more than one uncle per number per author included");
+			}
+		}
+
+		Ok((author, acc))
+	}
+}
+
+#[derive(Encode, Decode)]
+#[cfg_attr(any(feature = "std", test), derive(PartialEq, Debug))]
+enum UncleEntryItem<BlockNumber, Hash, Author> {
+	InclusionHeight(BlockNumber),
+	Uncle(Hash, Option<Author>),
+}
+
+decl_storage! {
+	trait Store for Module<T: Trait> as Authorship {
+		/// Uncles
+		Uncles: Vec<UncleEntryItem<T::BlockNumber, T::Hash, T::AccountId>>;
+		/// Author of current block.
+		Author: Option<T::AccountId>;
+		/// Whether uncles were already set in this block.
+		DidSetUncles: bool;
+	}
+}
+
+fn prune_old_uncles<BlockNumber, Hash, Author>(
+	minimum_height: BlockNumber,
+	uncles: &mut Vec<UncleEntryItem<BlockNumber, Hash, Author>>
+) where BlockNumber: SimpleArithmetic {
+	let prune_entries = uncles.iter().take_while(|item| match item {
+		UncleEntryItem::Uncle(_, _) => true,
+		UncleEntryItem::InclusionHeight(height) => height < &minimum_height,
+	});
+	let prune_index = prune_entries.count();
+
+	let _ = uncles.drain(..prune_index);
+}
+
+decl_module! {
+	pub struct Module<T: Trait> for enum Call where origin: T::Origin {
+		fn on_initialize(now: T::BlockNumber) {
+			let uncle_generations = T::UncleGenerations::get();
+			let mut uncles = <Self as Store>::Uncles::get();
+
+			// prune uncles that are older than the allowed number of generations.
+			if uncle_generations <= now {
+				let minimum_height = now - uncle_generations;
+				prune_old_uncles(minimum_height, &mut uncles)
+			}
+
+			<Self as Store>::DidSetUncles::put(false);
+
+			T::EventHandler::note_author(Self::author());
+		}
+
+		fn on_finalize() {
+			// ensure we never go to trie with these values.
+			let _ = <Self as Store>::Author::take();
+			let _ = <Self as Store>::DidSetUncles::take();
+		}
+
+		/// Provide a set of uncles.
+		fn set_uncles(origin, new_uncles: Vec<T::Header>) -> DispatchResult {
+			ensure_none(origin)?;
+
+			if <Self as Store>::DidSetUncles::get() {
+				return Err("Uncles already set in block.");
+			}
+			<Self as Store>::DidSetUncles::put(true);
+
+			Self::verify_and_import_uncles(new_uncles)
+		}
+	}
+}
+
+impl<T: Trait> Module<T> {
+	/// Fetch the author of the block.
+	///
+	/// This is safe to invoke in `on_initialize` implementations, as well
+	/// as afterwards.
+	pub fn author() -> T::AccountId {
+		// Check the memoized storage value.
+		if let Some(author) = <Self as Store>::Author::get() {
+			return author;
+		}
+
+		let digest = <system::Module<T>>::digest();
+		let pre_runtime_digests = digest.logs.iter().filter_map(|d| d.as_pre_runtime());
+		if let Some(author) = T::FindAuthor::find_author(pre_runtime_digests) {
+			<Self as Store>::Author::put(&author);
+			author
+		} else {
+			Default::default()
+		}
+	}
+
+	fn verify_and_import_uncles(new_uncles: Vec<T::Header>) -> DispatchResult {
+		let now = <system::Module<T>>::block_number();
+
+		let (minimum_height, maximum_height) = {
+			let uncle_generations = T::UncleGenerations::get();
+			let min = if now >= uncle_generations {
+				now - uncle_generations
+			} else {
+				Zero::zero()
+			};
+
+			(min, now)
+		};
+
+		let mut uncles = <Self as Store>::Uncles::get();
+		uncles.push(UncleEntryItem::InclusionHeight(now));
+
+		let mut acc: <T::FilterUncle as FilterUncle<_, _>>::Accumulator = Default::default();
+
+		for uncle in new_uncles {
+			let hash = uncle.hash();
+
+			if uncle.number() < &One::one() {
+				return Err("uncle is genesis");
+			}
+
+			if uncle.number() > &maximum_height {
+				return Err("uncles too high in chain");
+			}
+
+			{
+				let parent_number = uncle.number().clone() - One::one();
+				let parent_hash = <system::Module<T>>::block_hash(&parent_number);
+				if &parent_hash != uncle.parent_hash() {
+					return Err("uncle parent not in chain");
+				}
+			}
+
+			if uncle.number() < &minimum_height {
+				return Err("uncle not recent enough to be included");
+			}
+
+			let duplicate = uncles.iter().find(|entry| match entry {
+				UncleEntryItem::InclusionHeight(_) => false,
+				UncleEntryItem::Uncle(h, _) => h == &hash,
+			}).is_some();
+
+			let in_chain = <system::Module<T>>::block_hash(uncle.number()) == hash;
+
+			if duplicate || in_chain { return Err("uncle already included") }
+
+			// check uncle validity.
+			let (author, temp_acc) = T::FilterUncle::filter_uncle(&uncle, acc)?;
+			acc = temp_acc;
+
+			T::EventHandler::note_uncle(
+				author.clone().unwrap_or_default(),
+				now - uncle.number().clone(),
+			);
+			uncles.push(UncleEntryItem::Uncle(hash, author));
+		}
+
+		<Self as Store>::Uncles::put(&uncles);
+		Ok(())
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+	use runtime_io::with_externalities;
+	use substrate_primitives::{H256, Blake2Hasher};
+	use primitives::BuildStorage;
+	use primitives::traits::{BlakeTwo256, IdentityLookup};
+	use primitives::testing::Header;
+	use primitives::generic::DigestItem;
+	use srml_support::{parameter_types, impl_outer_origin, ConsensusEngineId};
+
+	impl_outer_origin!{
+		pub enum Origin for Test {}
+	}
+
+	#[derive(Clone, Eq, PartialEq)]
+	pub struct Test;
+
+	impl system::Trait for Test {
+		type Origin = Origin;
+		type Index = u64;
+		type BlockNumber = u64;
+		type Hash = H256;
+		type Hashing = BlakeTwo256;
+		type AccountId = u64;
+		type Lookup = IdentityLookup<Self::AccountId>;
+		type Header = Header;
+		type Event = ();
+	}
+
+	impl Trait for Test {
+		type FindAuthor = AuthorGiven;
+		type UncleGenerations = UncleGenerations;
+		type FilterUncle = SealVerify<VerifyBlock>;
+		type EventHandler = ();
+	}
+
+	type System = system::Module<Test>;
+	type Authorship = Module<Test>;
+
+	const TEST_ID: ConsensusEngineId = [1, 2, 3, 4];
+
+	pub struct AuthorGiven;
+
+	impl FindAuthor<u64> for AuthorGiven {
+		fn find_author<'a, I>(digests: I) -> Option<u64>
+			where I: 'a + IntoIterator<Item=(ConsensusEngineId, &'a [u8])>
+		{
+			for (id, data) in digests {
+				if id == TEST_ID {
+					return u64::decode(&mut &data[..]);
+				}
+			}
+
+			None
+		}
+	}
+
+	parameter_types! {
+		pub const UncleGenerations: u64 = 5;
+	}
+
+	pub struct VerifyBlock;
+
+	impl VerifySeal<Header, u64> for VerifyBlock {
+		fn verify_seal(header: &Header) -> Result<Option<u64>, &'static str> {
+			let pre_runtime_digests = header.digest.logs.iter().filter_map(|d| d.as_pre_runtime());
+			let seals = header.digest.logs.iter().filter_map(|d| d.as_seal());
+
+			let author = match AuthorGiven::find_author(pre_runtime_digests) {
+				None => return Err("no author"),
+				Some(author) => author,
+			};
+
+			for (id, seal) in seals {
+				if id == TEST_ID {
+					match u64::decode(&mut &seal[..]) {
+						None => return Err("wrong seal"),
+						Some(a) => {
+							if a != author {
+								return Err("wrong author in seal");
+							}
+							break
+						}
+					}
+				}
+			}
+
+			Ok(Some(author))
+		}
+	}
+
+	fn seal_header(mut header: Header, author: u64) -> Header {
+		{
+			let digest = header.digest_mut();
+			digest.logs.push(DigestItem::PreRuntime(TEST_ID, author.encode()));
+			digest.logs.push(DigestItem::Seal(TEST_ID, author.encode()));
+		}
+
+		header
+	}
+
+
+	fn create_header(number: u64, parent_hash: H256, state_root: H256) -> Header {
+		Header::new(
+			number,
+			Default::default(),
+			state_root,
+			parent_hash,
+			Default::default(),
+		)
+	}
+
+	fn new_test_ext() -> runtime_io::TestExternalities<Blake2Hasher> {
+		let t = system::GenesisConfig::<Test>::default().build_storage().unwrap().0;
+		t.into()
+	}
+
+	#[test]
+	fn prune_old_uncles_works() {
+		use UncleEntryItem::*;
+		let mut uncles = vec![
+			InclusionHeight(1u32), Uncle((), Some(())), Uncle((), None), Uncle((), None),
+			InclusionHeight(2u32), Uncle((), None),
+			InclusionHeight(3u32), Uncle((), None),
+		];
+
+		prune_old_uncles(3, &mut uncles);
+
+		assert_eq!(uncles, vec![InclusionHeight(3), Uncle((), None)]);
+	}
+
+	#[test]
+	fn rejects_bad_uncles() {
+		with_externalities(&mut new_test_ext(), || {
+			let author_a = 69;
+
+			struct CanonChain {
+				inner: Vec<Header>,
+			}
+
+			impl CanonChain {
+				fn best_hash(&self) -> H256 {
+					self.inner.last().unwrap().hash()
+				}
+
+				fn canon_hash(&self, index: usize) -> H256 {
+					self.inner[index].hash()
+				}
+
+				fn header(&self, index: usize) -> &Header {
+					&self.inner[index]
+				}
+
+				fn push(&mut self, header: Header) {
+					self.inner.push(header)
+				}
+			}
+
+			let mut canon_chain = CanonChain {
+				inner: vec![seal_header(create_header(0, Default::default(), Default::default()), 999)],
+			};
+
+			for number in 1..8 {
+				System::initialize(&number, &canon_chain.best_hash(), &Default::default(), &Default::default());
+				let header = seal_header(System::finalize(), author_a);
+				canon_chain.push(header);
+			}
+
+			// initialize so system context is set up correctly.
+			System::initialize(&8, &canon_chain.best_hash(), &Default::default(), &Default::default());
+
+			// 2 of the same uncle at once
+			{
+				let uncle_a = seal_header(
+					create_header(3, canon_chain.canon_hash(2), [1; 32].into()),
+					author_a,
+				);
+				assert_eq!(
+					Authorship::verify_and_import_uncles(vec![uncle_a.clone(), uncle_a.clone()]),
+					Err("uncle already included"),
+				);
+			}
+
+			// 2 of the same uncle at different times.
+			{
+				let uncle_a = seal_header(
+					create_header(3, canon_chain.canon_hash(2), [1; 32].into()),
+					author_a,
+				);
+
+				assert!(
+					Authorship::verify_and_import_uncles(vec![uncle_a.clone()]).is_ok();
+				);
+
+				assert_eq!(
+					Authorship::verify_and_import_uncles(vec![uncle_a.clone()]),
+					Err("uncle already included"),
+				);
+			}
+
+			// same uncle as ancestor.
+			{
+				let uncle_clone = canon_chain.header(5).clone();
+
+				assert_eq!(
+					Authorship::verify_and_import_uncles(vec![uncle_clone]),
+					Err("uncle already included"),
+				);
+			}
+
+			// uncle without valid seal.
+			{
+				let unsealed = create_header(3, canon_chain.canon_hash(2), [2; 32].into());
+				assert_eq!(
+					Authorship::verify_and_import_uncles(vec![unsealed]),
+					Err("no author"),
+				);
+			}
+
+			// old uncles can't get in.
+			{
+				assert_eq!(System::block_number(), 8);
+				assert_eq!(<Test as Trait>::UncleGenerations::get(), 5);
+
+				let gen_2 = seal_header(
+					create_header(2, canon_chain.canon_hash(1), [3; 32].into()),
+					author_a,
+				);
+
+				assert_eq!(
+					Authorship::verify_and_import_uncles(vec![gen_2]),
+					Err("uncle not recent enough to be included"),
+				);
+			}
+
+			// siblings are also allowed
+			{
+				let other_8 = seal_header(
+					create_header(8, canon_chain.canon_hash(7), [1; 32].into()),
+					author_a,
+				);
+
+				assert!(
+					Authorship::verify_and_import_uncles(vec![other_8]).is_ok();
+				);
+			}
+		});
+	}
+
+	#[test]
+	fn sets_author_lazily() {
+		with_externalities(&mut new_test_ext(), || {
+			let author = 42;
+			let mut header = seal_header(
+				create_header(1, Default::default(), [1; 32].into()),
+				author,
+			);
+
+			header.digest_mut().pop(); // pop the seal off.
+			System::initialize(&1, &Default::default(), &Default::default(), header.digest());
+
+			assert_eq!(Authorship::author(), author);
+		});
+	}
+
+	#[test]
+	fn one_uncle_per_author_per_number() {
+		type Filter = OnePerAuthorPerHeight<VerifyBlock, u64>;
+
+		let author_a = 42;
+		let author_b = 43;
+
+		let mut acc: Option<<Filter as FilterUncle<Header, u64>>::Accumulator> = Some(Default::default());
+		let header_a1 = seal_header(
+			create_header(1, Default::default(), [1; 32].into()),
+			author_a,
+		);
+		let header_b1 = seal_header(
+			create_header(1, Default::default(), [1; 32].into()),
+			author_b,
+		);
+
+		let header_a2_1 = seal_header(
+			create_header(2, Default::default(), [1; 32].into()),
+			author_a,
+		);
+		let header_a2_2 = seal_header(
+			create_header(2, Default::default(), [2; 32].into()),
+			author_a,
+		);
+
+		let mut check_filter = move |uncle| {
+			match Filter::filter_uncle(uncle, acc.take().unwrap()) {
+				Ok((author, a)) => {
+					acc = Some(a);
+					Ok(author)
+				}
+				Err(e) => Err(e),
+			}
+		};
+
+		// same height, different author is OK.
+		assert_eq!(check_filter(&header_a1), Ok(Some(author_a)));
+		assert_eq!(check_filter(&header_b1), Ok(Some(author_b)));
+
+		// same author, different height.
+		assert_eq!(check_filter(&header_a2_1), Ok(Some(author_a)));
+
+		// same author, same height (author a, height 2)
+		assert!(check_filter(&header_a2_2).is_err());
+	}
+}

substrate/srml/session/src/lib.rs

@@ -34,7 +34,7 @@
 //! - **Session key:** A session key is actually several keys kept together that provide the various signing
 //! functions required by network authorities/validators in pursuit of their duties.
 //! - **Session key configuration process:** A session key is set using `set_key` for use in the
-//! next session. It is stored in `NextKeyFor`, a mapping between the caller's `AccountID` and the session
+//! next session. It is stored in `NextKeyFor`, a mapping between the caller's `AccountId` and the session
 //! key provided. `set_key` allows users to set their session key prior to becoming a validator.
 //! It is a public call since it uses `ensure_signed`, which checks that the origin is a signed account.
 //! As such, the account ID of the origin stored in in `NextKeyFor` may not necessarily be associated with
@@ -120,8 +120,11 @@ use rstd::{prelude::*, marker::PhantomData, ops::Rem};
 use rstd::alloc::borrow::ToOwned;
 use parity_codec::Decode;
 use primitives::traits::{Zero, Saturating, Member, OpaqueKeys};
-use srml_support::{StorageValue, StorageMap, for_each_tuple, decl_module, decl_event, decl_storage};
-use srml_support::{ensure, traits::{OnFreeBalanceZero, Get}, Parameter, print};
+use srml_support::{
+	ConsensusEngineId, StorageValue, StorageMap, for_each_tuple, decl_module,
+	decl_event, decl_storage,
+};
+use srml_support::{ensure, traits::{OnFreeBalanceZero, Get, FindAuthor}, Parameter, print};
 use system::ensure_signed;
 
 /// Simple index type with which we can count sessions.
@@ -377,6 +380,24 @@ impl<T: Trait> OnFreeBalanceZero<T::AccountId> for Module<T> {
 	}
 }
 
+/// Wraps the author-scraping logic for consensus engines that can recover
+/// the canonical index of an author. This then transforms it into the
+/// registering account-ID of that session key index.
+pub struct FindAccountFromAuthorIndex<T, Inner>(rstd::marker::PhantomData<(T, Inner)>);
+
+impl<T: Trait, Inner: FindAuthor<u32>> FindAuthor<T::AccountId>
+	for FindAccountFromAuthorIndex<T, Inner>
+{
+	fn find_author<'a, I>(digests: I) -> Option<T::AccountId>
+		where I: 'a + IntoIterator<Item=(ConsensusEngineId, &'a [u8])>
+	{
+		let i = Inner::find_author(digests)?;
+
+		let validators = <Module<T>>::validators();
+		validators.get(i as usize).map(|k| k.clone())
+	}
+}
+
 #[cfg(test)]
 mod tests {
 	use super::*;

substrate/srml/support/src/lib.rs

@@ -64,6 +64,7 @@ pub use self::hashable::Hashable;
 pub use self::dispatch::{Parameter, Dispatchable, Callable, IsSubType};
 pub use self::double_map::StorageDoubleMapWithHasher;
 pub use runtime_io::{print, storage_root};
+pub use runtime_primitives::ConsensusEngineId;
 
 /// Macro for easily creating a new implementation of the `Get` trait. Use similarly to
 /// how you would declare a `const`:

substrate/srml/support/src/traits.rs

@@ -23,6 +23,7 @@ use crate::codec::{Codec, Encode, Decode};
 use crate::runtime_primitives::traits::{
 	MaybeSerializeDebug, SimpleArithmetic
 };
+use crate::runtime_primitives::ConsensusEngineId;
 
 use super::for_each_tuple;
 
@@ -102,6 +103,20 @@ impl<T> MakePayment<T> for () {
 	fn make_payment(_: &T, _: usize) -> Result<(), &'static str> { Ok(()) }
 }
 
+/// A trait for finding the author of a block header based on the `PreRuntime` digests contained
+/// within it.
+pub trait FindAuthor<Author> {
+	/// Find the author of a block based on the pre-runtime digests.
+	fn find_author<'a, I>(digests: I) -> Option<Author>
+		where I: 'a + IntoIterator<Item=(ConsensusEngineId, &'a [u8])>;
+}
+
+/// A trait for verifying the seal of a header and returning the author.
+pub trait VerifySeal<Header, Author> {
+	/// Verify a header and return the author, if any.
+	fn verify_seal(header: &Header) -> Result<Option<Author>, &'static str>;
+}
+
 /// Handler for when some currency "account" decreased in balance for
 /// some reason.
 ///