grandpa: Storage migration for srml-grandpa module.

srml/grandpa/Cargo.toml

@@ -35,3 +35,4 @@ std = [
 	"session/std",
 	"finality-tracker/std",
 ]
+migrate-authorities = []

srml/grandpa/src/lib.rs

@@ -137,6 +137,13 @@ decl_event!(
 
 decl_storage! {
 	trait Store for Module<T: Trait> as GrandpaFinality {
+		/// DEPRECATED
+		///
+		/// This used to store the current authority set, which has been migrated to the well-known
+		/// GRANDPA_AUTHORITES_KEY unhashed key.
+		#[cfg(feature = "migrate-authorities")]
+		pub(crate) Authorities get(authorities): AuthorityList;
+
 		/// State of the current authority set.
 		State get(state): StoredState<T::BlockNumber> = StoredState::Live;
 
@@ -172,6 +179,11 @@ decl_module! {
 			// FIXME: https://github.com/paritytech/substrate/issues/1112
 		}
 
+		fn on_initialize() {
+			#[cfg(feature = "migrate-authorities")]
+			Self::migrate_authorities();
+		}
+
 		fn on_finalize(block_number: T::BlockNumber) {
 			// check for scheduled pending authority set changes
 			if let Some(pending_change) = <PendingChange<T>>::get() {
@@ -341,6 +353,13 @@ impl<T: Trait> Module<T> {
 			Self::set_grandpa_authorities(authorities);
 		}
 	}
+
+	#[cfg(feature = "migrate-authorities")]
+	fn migrate_authorities() {
+		if Authorities::exists() {
+			Self::set_grandpa_authorities(&Authorities::take());
+		}
+	}
 }
 
 impl<T: Trait> Module<T> {

srml/grandpa/src/tests.rs

@@ -310,3 +310,21 @@ fn time_slot_have_sane_ord() {
 	];
 	assert!(FIXTURE.windows(2).all(|f| f[0] < f[1]));
 }
+
+#[test]
+#[cfg(feature = "migrate-authorities")]
+fn authorities_migration() {
+	use sr_primitives::traits::OnInitialize;
+
+	with_externalities(&mut new_test_ext(vec![]), || {
+		let authorities = to_authorities(vec![(1, 1), (2, 1), (3, 1)]);
+
+		Authorities::put(authorities.clone());
+		assert!(Grandpa::grandpa_authorities().is_empty());
+
+		Grandpa::on_initialize(1);
+
+		assert!(!Authorities::exists());
+		assert_eq!(Grandpa::grandpa_authorities(), authorities);
+	});
+}