Make sure that srml-collective does not initialize Members twice (#3379)

* Make sure that `srml-collective` does not initialize `Members` twice

* Implement trait for `()`

* Fix test

substrate/srml/collective/src/lib.rs

@@ -28,8 +28,9 @@ use primitives::u32_trait::Value as U32;
 use sr_primitives::traits::{Hash, EnsureOrigin};
 use sr_primitives::weights::SimpleDispatchInfo;
 use srml_support::{
-	dispatch::{Dispatchable, Parameter}, codec::{Encode, Decode}, traits::ChangeMembers,
-	StorageValue, StorageMap, decl_module, decl_event, decl_storage, ensure
+	dispatch::{Dispatchable, Parameter}, codec::{Encode, Decode},
+	traits::{ChangeMembers, InitializeMembers}, StorageValue, StorageMap, decl_module, decl_event,
+	decl_storage, ensure,
 };
 use system::{self, ensure_signed, ensure_root};
 
@@ -93,10 +94,20 @@ decl_storage! {
 		/// Proposals so far.
 		pub ProposalCount get(proposal_count): u32;
 		/// The current members of the collective. This is stored sorted (just by value).
-		pub Members get(members) config(): Vec<T::AccountId>;
+		pub Members get(members): Vec<T::AccountId>;
 	}
 	add_extra_genesis {
 		config(phantom): rstd::marker::PhantomData<I>;
+		config(members): Vec<T::AccountId>;
+		build(|
+			storage: &mut (sr_primitives::StorageOverlay, sr_primitives::ChildrenStorageOverlay),
+			config: &Self,
+		| {
+			runtime_io::with_storage(
+				storage,
+				|| Module::<T, I>::initialize_members(&config.members),
+			);
+		})
 	}
 }
 
@@ -282,6 +293,15 @@ impl<T: Trait<I>, I: Instance> ChangeMembers<T::AccountId> for Module<T, I> {
 	}
 }
 
+impl<T: Trait<I>, I: Instance> InitializeMembers<T::AccountId> for Module<T, I> {
+	fn initialize_members(members: &[T::AccountId]) {
+		if !members.is_empty() {
+			assert!(<Members<T, I>>::get().is_empty(), "Members are already initialized!");
+			<Members<T, I>>::put_ref(members);
+		}
+	}
+}
+
 /// Ensure that the origin `o` represents at least `n` members. Returns `Ok` or an `Err`
 /// otherwise.
 pub fn ensure_members<OuterOrigin, AccountId, I>(o: OuterOrigin, n: MemberCount)