pallet_authority_discovery: introduce current_authorities and next_authorities methods (#7892)

* split authorities discovery keys for the current and next session

* Revert "split authorities discovery keys for the current and next session"

This reverts commit 0a40b8b4c14e85d95357a27f6db30199cbe0aa4d.

* pallet_authority_discovery: introduce a next_authorities method

* address feedback

* amend the doccomments

substrate/frame/authority-discovery/src/lib.rs

@@ -17,13 +17,13 @@
 
 //! # Authority discovery module.
 //!
-//! This module is used by the `client/authority-discovery` to retrieve the
-//! current set of authorities.
+//! This module is used by the `client/authority-discovery` and by polkadot's parachain logic
+//! to retrieve the current and the next set of authorities.
 
 // Ensure we're `no_std` when compiling for Wasm.
 #![cfg_attr(not(feature = "std"), no_std)]
 
-use sp_std::{collections::btree_set::BTreeSet, prelude::*};
+use sp_std::prelude::*;
 use frame_support::{decl_module, decl_storage};
 use sp_authority_discovery::AuthorityId;
 
@@ -32,8 +32,10 @@ pub trait Config: frame_system::Config + pallet_session::Config {}
 
 decl_storage! {
 	trait Store for Module<T: Config> as AuthorityDiscovery {
-		/// Keys of the current and next authority set.
+		/// Keys of the current authority set.
 		Keys get(fn keys): Vec<AuthorityId>;
+		/// Keys of the next authority set.
+		NextKeys get(fn next_keys): Vec<AuthorityId>;
 	}
 	add_extra_genesis {
 		config(keys): Vec<AuthorityId>;
@@ -47,15 +49,34 @@ decl_module! {
 }
 
 impl<T: Config> Module<T> {
-	/// Retrieve authority identifiers of the current and next authority set.
+	/// Retrieve authority identifiers of the current and next authority set
+	/// sorted and deduplicated.
 	pub fn authorities() -> Vec<AuthorityId> {
+		let mut keys = Keys::get();
+		let next = NextKeys::get();
+
+		keys.extend(next);
+		keys.sort();
+		keys.dedup();
+
+		keys
+	}
+
+	/// Retrieve authority identifiers of the current authority set in the original order.
+	pub fn current_authorities() -> Vec<AuthorityId> {
 		Keys::get()
 	}
 
+	/// Retrieve authority identifiers of the next authority set in the original order.
+	pub fn next_authorities() -> Vec<AuthorityId> {
+		NextKeys::get()
+	}
+
 	fn initialize_keys(keys: &[AuthorityId]) {
 		if !keys.is_empty() {
 			assert!(Keys::get().is_empty(), "Keys are already initialized!");
 			Keys::put(keys);
+			NextKeys::put(keys);
 		}
 	}
 }
@@ -80,8 +101,10 @@ impl<T: Config> pallet_session::OneSessionHandler<T::AccountId> for Module<T> {
 	{
 		// Remember who the authorities are for the new and next session.
 		if changed {
-			let keys = validators.chain(queued_validators).map(|x| x.1).collect::<BTreeSet<_>>();
-			Keys::put(keys.into_iter().collect::<Vec<_>>());
+			let keys = validators.map(|x| x.1);
+			Keys::put(keys.collect::<Vec<_>>());
+			let next_keys = queued_validators.map(|x| x.1);
+			NextKeys::put(next_keys.collect::<Vec<_>>());
 		}
 	}
 
@@ -250,8 +273,7 @@ mod tests {
 				second_authorities_and_account_ids.clone().into_iter(),
 				third_authorities_and_account_ids.clone().into_iter(),
 			);
-			let mut authorities_returned = AuthorityDiscovery::authorities();
-			authorities_returned.sort();
+			let authorities_returned = AuthorityDiscovery::authorities();
 			assert_eq!(
 				first_authorities,
 				authorities_returned,