[FRAME] Use 'ready' pages in XCMP suspend logic (#2393)

Changes:
- `QueueFootprint` gets a new field; `ready_pages` that contains the
non-overweight and not yet processed pages.
- `XCMP` queue pallet is change to use the `ready_pages` instead of
`pages` to calculate the channel suspension thresholds.

This should give the XCMP queue pallet a more correct view of when to
suspend channels.

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

cumulus/pallets/xcmp-queue/src/lib.rs

@@ -600,7 +600,7 @@ impl<T: Config> Pallet<T> {
 		let QueueConfigData { drop_threshold, .. } = <QueueConfig<T>>::get();
 		let fp = T::XcmpQueue::footprint(sender);
 		// Assume that it will not fit into the current page:
-		let new_pages = fp.pages.saturating_add(1);
+		let new_pages = fp.ready_pages.saturating_add(1);
 		if new_pages > drop_threshold {
 			// This should not happen since the channel should have been suspended in
 			// [`on_queue_changed`].
@@ -663,12 +663,12 @@ impl<T: Config> OnQueueChanged<ParaId> for Pallet<T> {
 		let mut suspended_channels = <InboundXcmpSuspended<T>>::get();
 		let suspended = suspended_channels.contains(&para);
 
-		if suspended && fp.pages <= resume_threshold {
+		if suspended && fp.ready_pages <= resume_threshold {
 			Self::send_signal(para, ChannelSignal::Resume);
 
 			suspended_channels.remove(&para);
 			<InboundXcmpSuspended<T>>::put(suspended_channels);
-		} else if !suspended && fp.pages >= suspend_threshold {
+		} else if !suspended && fp.ready_pages >= suspend_threshold {
 			log::warn!("XCMP queue for sibling {:?} is full; suspending channel.", para);
 			Self::send_signal(para, ChannelSignal::Suspend);
 

cumulus/pallets/xcmp-queue/src/mock.rs

@@ -247,6 +247,7 @@ impl<T: OnQueueChanged<ParaId>> EnqueueMessage<ParaId> for EnqueueToLocalStorage
 			}
 		}
 		footprint.pages = footprint.storage.size as u32 / 16; // Number does not matter
+		footprint.ready_pages = footprint.pages;
 		footprint
 	}
 }

prdoc/pr_2393.prdoc

@@ -0,0 +1,16 @@
+title: "[XCMP] Use the number of 'ready' pages in XCMP suspend logic"
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      Semantics of the suspension logic in the XCMP queue pallet change from using the number of
+      total pages to the number of 'ready' pages. The number of ready pages is now also exposed by
+      the `MessageQueue` pallet to downstream via the queue `footprint`.
+
+crates:
+  - name: cumulus-pallet-xcmp-queue
+    bump: patch
+  - name: pallet-message-queue
+    bump: patch
+  - name: frame-support
+    bump: major

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

@@ -330,6 +330,11 @@ fn process_some_messages(num_msgs: u32) {
 	ServiceWeight::set(Some(weight));
 	let consumed = next_block();
 
+	for origin in BookStateFor::<Test>::iter_keys() {
+		let fp = MessageQueue::footprint(origin);
+		assert_eq!(fp.pages, fp.ready_pages);
+	}
+
 	assert_eq!(consumed, weight, "\n{}", MessageQueue::debug_info());
 	assert_eq!(NumMessagesProcessed::take(), num_msgs as usize);
 }

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

@@ -208,8 +208,9 @@ use frame_support::{
 	defensive,
 	pallet_prelude::*,
 	traits::{
-		Defensive, DefensiveTruncateFrom, EnqueueMessage, ExecuteOverweightError, Footprint,
-		ProcessMessage, ProcessMessageError, QueueFootprint, QueuePausedQuery, ServiceQueues,
+		Defensive, DefensiveSaturating, DefensiveTruncateFrom, EnqueueMessage,
+		ExecuteOverweightError, Footprint, ProcessMessage, ProcessMessageError, QueueFootprint,
+		QueuePausedQuery, ServiceQueues,
 	},
 	BoundedSlice, CloneNoBound, DefaultNoBound,
 };
@@ -442,6 +443,7 @@ impl<MessageOrigin> From<BookState<MessageOrigin>> for QueueFootprint {
 	fn from(book: BookState<MessageOrigin>) -> Self {
 		QueueFootprint {
 			pages: book.count,
+			ready_pages: book.end.defensive_saturating_sub(book.begin),
 			storage: Footprint { count: book.message_count, size: book.size },
 		}
 	}
@@ -1281,6 +1283,9 @@ impl<T: Config> Pallet<T> {
 			ensure!(book.message_count < 1 << 30, "Likely overflow or corruption");
 			ensure!(book.size < 1 << 30, "Likely overflow or corruption");
 			ensure!(book.count < 1 << 30, "Likely overflow or corruption");
+
+			let fp: QueueFootprint = book.into();
+			ensure!(fp.ready_pages <= fp.pages, "There cannot be more ready than total pages");
 		}
 
 		//loop around this origin

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

@@ -355,8 +355,8 @@ pub fn num_overweight_enqueued_events() -> u32 {
 		.count() as u32
 }
 
-pub fn fp(pages: u32, count: u64, size: u64) -> QueueFootprint {
-	QueueFootprint { storage: Footprint { count, size }, pages }
+pub fn fp(pages: u32, ready_pages: u32, count: u64, size: u64) -> QueueFootprint {
+	QueueFootprint { storage: Footprint { count, size }, pages, ready_pages }
 }
 
 /// A random seed that can be overwritten with `MQ_SEED`.

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

@@ -1064,13 +1064,13 @@ fn footprint_num_pages_works() {
 		MessageQueue::enqueue_message(msg("weight=2"), Here);
 		MessageQueue::enqueue_message(msg("weight=3"), Here);
 
-		assert_eq!(MessageQueue::footprint(Here), fp(2, 2, 16));
+		assert_eq!(MessageQueue::footprint(Here), fp(2, 2, 2, 16));
 
 		// Mark the messages as overweight.
 		assert_eq!(MessageQueue::service_queues(1.into_weight()), 0.into_weight());
 		assert_eq!(System::events().len(), 2);
-		// Overweight does not change the footprint.
-		assert_eq!(MessageQueue::footprint(Here), fp(2, 2, 16));
+		// `ready_pages` decreases but `page` count does not.
+		assert_eq!(MessageQueue::footprint(Here), fp(2, 0, 2, 16));
 
 		// Now execute the second message.
 		assert_eq!(
@@ -1078,7 +1078,7 @@ fn footprint_num_pages_works() {
 				.unwrap(),
 			3.into_weight()
 		);
-		assert_eq!(MessageQueue::footprint(Here), fp(1, 1, 8));
+		assert_eq!(MessageQueue::footprint(Here), fp(1, 0, 1, 8));
 		// And the first one:
 		assert_eq!(
 			<MessageQueue as ServiceQueues>::execute_overweight(2.into_weight(), (Here, 0, 0))
@@ -1086,6 +1086,11 @@ fn footprint_num_pages_works() {
 			2.into_weight()
 		);
 		assert_eq!(MessageQueue::footprint(Here), Default::default());
+		assert_eq!(MessageQueue::footprint(Here), fp(0, 0, 0, 0));
+
+		// `ready_pages` and normal `pages` increases again:
+		MessageQueue::enqueue_message(msg("weight=3"), Here);
+		assert_eq!(MessageQueue::footprint(Here), fp(1, 1, 1, 8));
 	})
 }
 

substrate/frame/support/src/traits/messages.rs

@@ -123,6 +123,8 @@ impl<OverweightAddr> ServiceQueues for NoopServiceQueues<OverweightAddr> {
 pub struct QueueFootprint {
 	/// The number of pages in the queue (including overweight pages).
 	pub pages: u32,
+	/// The number of pages that are ready (not yet processed and also not overweight).
+	pub ready_pages: u32,
 	/// The storage footprint of the queue (including overweight messages).
 	pub storage: Footprint,
 }