add authorities_len for aura (#2040)

substrate/frame/aura/src/lib.rs

@@ -230,6 +230,11 @@ impl<T: Config> Pallet<T> {
 		}
 	}
 
+	/// Return current authorities length.
+	pub fn authorities_len() -> usize {
+		Authorities::<T>::decode_len().unwrap_or(0)
+	}
+
 	/// Get the current slot from the pre-runtime digests.
 	fn current_slot_from_digests() -> Option<Slot> {
 		let digest = frame_system::Pallet::<T>::digest();
@@ -363,7 +368,7 @@ impl<T: Config> FindAuthor<u32> for Pallet<T> {
 		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;
+				let author_index = *slot % Self::authorities_len() as u64;
 				return Some(author_index as u32)
 			}
 		}