process enqueued messages on idle (#3844)

This will make it possible to use remaining weight on idle for
processing enqueued messages.
More context here https://github.com/paritytech/polkadot-sdk/issues/3709

---------

Co-authored-by: Adrian Catangiu <adrian@parity.io>

substrate/frame/message-queue/src/lib.rs

@@ -525,12 +525,21 @@ pub mod pallet {
 		type MaxStale: Get<u32>;
 
 		/// The amount of weight (if any) which should be provided to the message queue for
-		/// servicing enqueued items.
+		/// servicing enqueued items `on_initialize`.
 		///
 		/// This may be legitimately `None` in the case that you will call
-		/// `ServiceQueues::service_queues` manually.
+		/// `ServiceQueues::service_queues` manually or set [`Self::IdleMaxServiceWeight`] to have
+		/// it run in `on_idle`.
 		#[pallet::constant]
 		type ServiceWeight: Get<Option<Weight>>;
+
+		/// The maximum amount of weight (if any) to be used from remaining weight `on_idle` which
+		/// should be provided to the message queue for servicing enqueued items `on_idle`.
+		/// Useful for parachains to process messages at the same block they are received.
+		///
+		/// If `None`, it will not call `ServiceQueues::service_queues` in `on_idle`.
+		#[pallet::constant]
+		type IdleMaxServiceWeight: Get<Option<Weight>>;
 	}
 
 	#[pallet::event]
@@ -643,6 +652,15 @@ pub mod pallet {
 			}
 		}
 
+		fn on_idle(_n: BlockNumberFor<T>, remaining_weight: Weight) -> Weight {
+			if let Some(weight_limit) = T::IdleMaxServiceWeight::get() {
+				// Make use of the remaining weight to process enqueued messages.
+				Self::service_queues(weight_limit.min(remaining_weight))
+			} else {
+				Weight::zero()
+			}
+		}
+
 		#[cfg(feature = "try-runtime")]
 		fn try_state(_: BlockNumberFor<T>) -> Result<(), sp_runtime::TryRuntimeError> {
 			Self::do_try_state()