pallet-aura: Allow multiple blocks per slot

bin/node-template/runtime/src/lib.rs

@@ -27,7 +27,8 @@ use sp_version::RuntimeVersion;
 pub use frame_support::{
 	construct_runtime, parameter_types,
 	traits::{
-		ConstU128, ConstU32, ConstU64, ConstU8, KeyOwnerProofSystem, Randomness, StorageInfo,
+		ConstBool, ConstU128, ConstU32, ConstU64, ConstU8, KeyOwnerProofSystem, Randomness,
+		StorageInfo,
 	},
 	weights::{
 		constants::{
@@ -207,6 +208,7 @@ impl pallet_aura::Config for Runtime {
 	type AuthorityId = AuraId;
 	type DisabledValidators = ();
 	type MaxAuthorities = ConstU32<32>;
+	type AllowMultipleBlocksPerSlot = ConstBool<false>;
 }
 
 impl pallet_grandpa::Config for Runtime {

frame/aura/src/lib.rs

@@ -81,6 +81,20 @@ pub mod pallet {
 		/// Blocks authored by a disabled validator will lead to a panic as part of this module's
 		/// initialization.
 		type DisabledValidators: DisabledValidators;
+
+		/// Whether to allow block authors to create multiple blocks per slot.
+		///
+		/// If this is `true`, the pallet will allow slots to stay the same across sequential
+		/// blocks. If this is `false`, the pallet will require that subsequent blocks always have
+		/// higher slots than previous ones.
+		///
+		/// Regardless of the setting of this storage value, the pallet will always enforce the
+		/// invariant that slots don't move backwards as the chain progresses.
+		///
+		/// The typical value for this should be 'false' unless this pallet is being augmented by
+		/// another pallet which enforces some limitation on the number of blocks authors can create
+		/// using the same slot.
+		type AllowMultipleBlocksPerSlot: Get<bool>;
 	}
 
 	#[pallet::pallet]
@@ -92,7 +106,12 @@ pub mod pallet {
 			if let Some(new_slot) = Self::current_slot_from_digests() {
 				let current_slot = CurrentSlot::<T>::get();
 
-				assert!(current_slot < new_slot, "Slot must increase");
+				if T::AllowMultipleBlocksPerSlot::get() {
+					assert!(current_slot <= new_slot, "Slot must not decrease");
+				} else {
+					assert!(current_slot < new_slot, "Slot must increase");
+				}
+
 				CurrentSlot::<T>::put(new_slot);
 
 				if let Some(n_authorities) = <Authorities<T>>::decode_len() {

frame/aura/src/mock.rs

@@ -82,6 +82,7 @@ impl pallet_timestamp::Config for Test {
 
 parameter_types! {
 	static DisabledValidatorTestValue: Vec<AuthorityIndex> = Default::default();
+	pub static AllowMultipleBlocksPerSlot: bool = false;
 }
 
 pub struct MockDisabledValidators;
@@ -106,6 +107,7 @@ impl pallet_aura::Config for Test {
 	type AuthorityId = AuthorityId;
 	type DisabledValidators = MockDisabledValidators;
 	type MaxAuthorities = ConstU32<10>;
+	type AllowMultipleBlocksPerSlot = AllowMultipleBlocksPerSlot;
 }
 
 pub fn new_test_ext(authorities: Vec<u64>) -> sp_io::TestExternalities {

frame/aura/src/tests.rs

@@ -54,3 +54,64 @@ fn disabled_validators_cannot_author_blocks() {
 		Aura::on_initialize(42);
 	});
 }
+
+#[test]
+#[should_panic(expected = "Slot must increase")]
+fn pallet_requires_slot_to_increase_unless_allowed() {
+	new_test_ext(vec![0, 1, 2, 3]).execute_with(|| {
+		crate::mock::AllowMultipleBlocksPerSlot::set(false);
+
+		let slot = Slot::from(1);
+		let pre_digest =
+			Digest { logs: vec![DigestItem::PreRuntime(AURA_ENGINE_ID, slot.encode())] };
+
+		System::reset_events();
+		System::initialize(&42, &System::parent_hash(), &pre_digest);
+
+		// and we should not be able to initialize the block with the same slot a second time.
+		Aura::on_initialize(42);
+		Aura::on_initialize(42);
+	});
+}
+
+#[test]
+fn pallet_can_allow_unchanged_slot() {
+	new_test_ext(vec![0, 1, 2, 3]).execute_with(|| {
+		let slot = Slot::from(1);
+		let pre_digest =
+			Digest { logs: vec![DigestItem::PreRuntime(AURA_ENGINE_ID, slot.encode())] };
+
+		System::reset_events();
+		System::initialize(&42, &System::parent_hash(), &pre_digest);
+
+		crate::mock::AllowMultipleBlocksPerSlot::set(true);
+
+		// and we should be able to initialize the block with the same slot a second time.
+		Aura::on_initialize(42);
+		Aura::on_initialize(42);
+	});
+}
+
+#[test]
+#[should_panic(expected = "Slot must not decrease")]
+fn pallet_always_rejects_decreasing_slot() {
+	new_test_ext(vec![0, 1, 2, 3]).execute_with(|| {
+		let slot = Slot::from(2);
+		let pre_digest =
+			Digest { logs: vec![DigestItem::PreRuntime(AURA_ENGINE_ID, slot.encode())] };
+
+		System::reset_events();
+		System::initialize(&42, &System::parent_hash(), &pre_digest);
+
+		crate::mock::AllowMultipleBlocksPerSlot::set(true);
+
+		Aura::on_initialize(42);
+		System::finalize();
+
+		let earlier_slot = Slot::from(1);
+		let pre_digest =
+			Digest { logs: vec![DigestItem::PreRuntime(AURA_ENGINE_ID, earlier_slot.encode())] };
+		System::initialize(&43, &System::parent_hash(), &pre_digest);
+		Aura::on_initialize(43);
+	});
+}