collation-generation: Avoid using para_backing_state if runtime is ancient (#4070)

fixes https://github.com/paritytech/polkadot-sdk/issues/4067

Also add an early bail out for look ahead collator such that we don't
waste time if a CollatorFn is not set.

TODO:
- [x] add test.
- [x] Polkadot System Parachain burn-in.

---------

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

polkadot/node/collation-generation/src/lib.rs

@@ -206,9 +206,12 @@ async fn handle_new_activations<Context>(
 	// follow the procedure from the guide:
 	// https://paritytech.github.io/polkadot-sdk/book/node/collators/collation-generation.html
 
+	// If there is no collation function provided, bail out early.
+	// Important: Lookahead collator and slot based collator do not use `CollatorFn`.
 	if config.collator.is_none() {
 		return Ok(())
 	}
+
 	let para_id = config.para_id;
 
 	let _overall_timer = metrics.time_new_activations();
@@ -232,9 +235,14 @@ async fn handle_new_activations<Context>(
 		// The loop bellow will fill in cores that the para is allowed to build on.
 		let mut cores_to_build_on = Vec::new();
 
+		// This assumption refers to all cores of the parachain, taking elastic scaling
+		// into account.
+		let mut para_assumption = None;
 		for (core_idx, core) in availability_cores.into_iter().enumerate() {
-			let scheduled_core = match core {
-				CoreState::Scheduled(scheduled_core) => scheduled_core,
+			// This nested assumption refers only to the core being iterated.
+			let (core_assumption, scheduled_core) = match core {
+				CoreState::Scheduled(scheduled_core) =>
+					(OccupiedCoreAssumption::Free, scheduled_core),
 				CoreState::Occupied(occupied_core) => match async_backing_params {
 					Some(params) if params.max_candidate_depth >= 1 => {
 						// maximum candidate depth when building on top of a block
@@ -257,7 +265,7 @@ async fn handle_new_activations<Context>(
 						};
 
 						match res {
-							Some(res) => res,
+							Some(res) => (OccupiedCoreAssumption::Included, res),
 							None => continue,
 						}
 					},
@@ -291,6 +299,10 @@ async fn handle_new_activations<Context>(
 					"core is not assigned to our para. Keep going.",
 				);
 			} else {
+				// This does not work for elastic scaling, but it should be enough for single
+				// core parachains. If async backing runtime is available we later override
+				// the assumption based on the `para_backing_state` API response.
+				para_assumption = Some(core_assumption);
 				// Accumulate cores for building collation(s) outside the loop.
 				cores_to_build_on.push(CoreIndex(core_idx as u32));
 			}
@@ -301,34 +313,43 @@ async fn handle_new_activations<Context>(
 			continue
 		}
 
+		// If at least one core is assigned to us, `para_assumption` is `Some`.
+		let Some(mut para_assumption) = para_assumption else { continue };
+
+		// If it is none it means that neither async backing or elastic scaling (which
+		// depends on it) are supported. We'll use the `para_assumption` we got from
+		// iterating cores.
+		if async_backing_params.is_some() {
+			// We are being very optimistic here, but one of the cores could pend availability some
+			// more block, ore even time out.
+			// For timeout assumption the collator can't really know because it doesn't receive
+			// bitfield gossip.
 			let para_backing_state =
 				request_para_backing_state(relay_parent, config.para_id, ctx.sender())
 					.await
 					.await??
 					.ok_or(crate::error::Error::MissingParaBackingState)?;
 
-		// We are being very optimistic here, but one of the cores could pend availability some more
-		// block, ore even time out.
-		// For timeout assumption the collator can't really know because it doesn't receive bitfield
-		// gossip.
-		let assumption = if para_backing_state.pending_availability.is_empty() {
+			// Override the assumption about the para's assigned cores.
+			para_assumption = if para_backing_state.pending_availability.is_empty() {
 				OccupiedCoreAssumption::Free
 			} else {
 				OccupiedCoreAssumption::Included
-		};
+			}
+		}
 
 		gum::debug!(
 			target: LOG_TARGET,
 			relay_parent = ?relay_parent,
-			our_para = %config.para_id,
-			?assumption,
+			our_para = %para_id,
+			?para_assumption,
 			"Occupied core(s) assumption",
 		);
 
 		let mut validation_data = match request_persisted_validation_data(
 			relay_parent,
-			config.para_id,
-			assumption,
+			para_id,
+			para_assumption,
 			ctx.sender(),
 		)
 		.await
@@ -339,7 +360,7 @@ async fn handle_new_activations<Context>(
 				gum::debug!(
 					target: LOG_TARGET,
 					relay_parent = ?relay_parent,
-					our_para = %config.para_id,
+					our_para = %para_id,
 					"validation data is not available",
 				);
 				continue
@@ -348,8 +369,8 @@ async fn handle_new_activations<Context>(
 
 		let validation_code_hash = match obtain_validation_code_hash_with_assumption(
 			relay_parent,
-			config.para_id,
-			assumption,
+			para_id,
+			para_assumption,
 			ctx.sender(),
 		)
 		.await?
@@ -359,7 +380,7 @@ async fn handle_new_activations<Context>(
 				gum::debug!(
 					target: LOG_TARGET,
 					relay_parent = ?relay_parent,
-					our_para = %config.para_id,
+					our_para = %para_id,
 					"validation code hash is not found.",
 				);
 				continue

polkadot/node/collation-generation/src/tests.rs

@@ -807,6 +807,56 @@ fn distribute_collation_for_occupied_core_with_async_backing_enabled(#[case] run
 			OccupiedCoreAssumption::Included,
 			1,
 			pending_availability,
+			runtime_version,
+		)
+		.await;
+
+		virtual_overseer
+	});
+}
+
+#[test]
+fn distribute_collation_for_occupied_core_pre_async_backing() {
+	let activated_hash: Hash = [1; 32].into();
+	let para_id = ParaId::from(5);
+	let total_cores = 3;
+
+	// Use runtime version before async backing
+	let runtime_version = RuntimeApiRequest::ASYNC_BACKING_STATE_RUNTIME_REQUIREMENT - 1;
+
+	let cores = (0..total_cores)
+		.into_iter()
+		.map(|_idx| CoreState::Scheduled(ScheduledCore { para_id, collator: None }))
+		.collect::<Vec<_>>();
+
+	let claim_queue = cores
+		.iter()
+		.enumerate()
+		.map(|(idx, _core)| (CoreIndex::from(idx as u32), VecDeque::from([para_id])))
+		.collect::<BTreeMap<_, _>>();
+
+	test_harness(|mut virtual_overseer| async move {
+		helpers::initialize_collator(&mut virtual_overseer, para_id).await;
+		helpers::activate_new_head(&mut virtual_overseer, activated_hash).await;
+		helpers::handle_runtime_calls_on_new_head_activation(
+			&mut virtual_overseer,
+			activated_hash,
+			AsyncBackingParams { max_candidate_depth: 1, allowed_ancestry_len: 1 },
+			cores,
+			runtime_version,
+			claim_queue,
+		)
+		.await;
+
+		helpers::handle_cores_processing_for_a_leaf(
+			&mut virtual_overseer,
+			activated_hash,
+			para_id,
+			// `CoreState` is `Free` => `OccupiedCoreAssumption` is `Free`
+			OccupiedCoreAssumption::Free,
+			total_cores,
+			vec![],
+			runtime_version,
 		)
 		.await;
 
@@ -826,6 +876,8 @@ fn distribute_collation_for_occupied_cores_with_async_backing_enabled_and_elasti
 ) {
 	let activated_hash: Hash = [1; 32].into();
 	let para_id = ParaId::from(5);
+	// Using latest runtime with the fancy claim queue exposed.
+	let runtime_version = RuntimeApiRequest::CLAIM_QUEUE_RUNTIME_REQUIREMENT;
 
 	let cores = (0..3)
 		.into_iter()
@@ -863,8 +915,7 @@ fn distribute_collation_for_occupied_cores_with_async_backing_enabled_and_elasti
 			activated_hash,
 			AsyncBackingParams { max_candidate_depth: 1, allowed_ancestry_len: 1 },
 			cores,
-			// Using latest runtime with the fancy claim queue exposed.
-			RuntimeApiRequest::CLAIM_QUEUE_RUNTIME_REQUIREMENT,
+			runtime_version,
 			claim_queue,
 		)
 		.await;
@@ -882,6 +933,7 @@ fn distribute_collation_for_occupied_cores_with_async_backing_enabled_and_elasti
 			},
 			total_cores,
 			pending_availability,
+			runtime_version,
 		)
 		.await;
 
@@ -901,6 +953,8 @@ fn distribute_collation_for_free_cores_with_async_backing_enabled_and_elastic_sc
 ) {
 	let activated_hash: Hash = [1; 32].into();
 	let para_id = ParaId::from(5);
+	// Using latest runtime with the fancy claim queue exposed.
+	let runtime_version = RuntimeApiRequest::CLAIM_QUEUE_RUNTIME_REQUIREMENT;
 
 	let cores = (0..total_cores)
 		.into_iter()
@@ -921,8 +975,7 @@ fn distribute_collation_for_free_cores_with_async_backing_enabled_and_elastic_sc
 			activated_hash,
 			AsyncBackingParams { max_candidate_depth: 1, allowed_ancestry_len: 1 },
 			cores,
-			// Using latest runtime with the fancy claim queue exposed.
-			RuntimeApiRequest::CLAIM_QUEUE_RUNTIME_REQUIREMENT,
+			runtime_version,
 			claim_queue,
 		)
 		.await;
@@ -935,6 +988,7 @@ fn distribute_collation_for_free_cores_with_async_backing_enabled_and_elastic_sc
 			OccupiedCoreAssumption::Free,
 			total_cores,
 			vec![],
+			runtime_version,
 		)
 		.await;
 
@@ -1074,6 +1128,13 @@ mod helpers {
 			}
 		);
 
+		let async_backing_response =
+			if runtime_version >= RuntimeApiRequest::ASYNC_BACKING_STATE_RUNTIME_REQUIREMENT {
+				Ok(async_backing_params)
+			} else {
+				Err(RuntimeApiError::NotSupported { runtime_api_name: "async_backing_params" })
+			};
+
 		assert_matches!(
 			overseer_recv(virtual_overseer).await,
 			AllMessages::RuntimeApi(RuntimeApiMessage::Request(
@@ -1083,7 +1144,7 @@ mod helpers {
 								),
 							)) => {
 				assert_eq!(hash, activated_hash);
-				let _ = tx.send(Ok(async_backing_params));
+				let _ = tx.send(async_backing_response);
 			}
 		);
 
@@ -1121,6 +1182,7 @@ mod helpers {
 		expected_occupied_core_assumption: OccupiedCoreAssumption,
 		cores_assigned: usize,
 		pending_availability: Vec<CandidatePendingAvailability>,
+		runtime_version: u32,
 	) {
 		// Expect no messages if no cores is assigned to the para
 		if cores_assigned == 0 {
@@ -1138,6 +1200,7 @@ mod helpers {
 			max_pov_size: 1024,
 		};
 
+		if runtime_version >= RuntimeApiRequest::ASYNC_BACKING_STATE_RUNTIME_REQUIREMENT {
 			assert_matches!(
 				overseer_recv(virtual_overseer).await,
 				AllMessages::RuntimeApi(
@@ -1146,6 +1209,7 @@ mod helpers {
 					tx.send(Ok(Some(dummy_backing_state(pending_availability)))).unwrap();
 				}
 			);
+		}
 
 		assert_matches!(
 			overseer_recv(virtual_overseer).await,

prdoc/pr_4070.prdoc

@@ -0,0 +1,12 @@
+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: Avoid using `para_backing_state` if runtime doesn't support async backing
+
+doc:
+  - audience: Node Operator
+    description: |
+      Fixes https://github.com/paritytech/polkadot-sdk/issues/4067 which prevents collators to 
+      upgrade to latest release (v1.10.0)
+
+crates: [ ]