pallet-aura: Allow multiple blocks per slot (#14024)

* pallet-aura: Allow multiple blocks per slot

* run fmt

* rework as associated type

* fix fallout

* fmt

* use constbool

* fmt

substrate/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() {