runtime: make the candidate relay parent progression check more strict (#5113)

Previously, we were checking if the relay parent of a new candidate does
not move backwards from the latest included on-chain candidate. This was
fine prior to elastic scaling. We now need to also check that the relay
parent progresses from the latest pending availability candidate, as
well as check the progression within the candidate chain in the inherent
data.

Prospective-parachains is already doing this check but we should also
add it in the runtime

Author: alindima

polkadot/runtime/parachains/src/inclusion/mod.rs

@@ -639,6 +639,8 @@ impl<T: Config> Pallet<T> {
 			for (candidate, core) in para_candidates.iter() {
 				let candidate_hash = candidate.candidate().hash();
 
+				// The previous context is None, as it's already checked during candidate
+				// sanitization.
 				let check_ctx = CandidateCheckContext::<T>::new(None);
 				let relay_parent_number = check_ctx.verify_backed_candidate(
 					&allowed_relay_parents,
@@ -718,7 +720,7 @@ impl<T: Config> Pallet<T> {
 		})
 	}
 
-	// Get the latest backed output head data of this para.
+	// Get the latest backed output head data of this para (including pending availability).
 	pub(crate) fn para_latest_head_data(para_id: &ParaId) -> Option<HeadData> {
 		match PendingAvailability::<T>::get(para_id).and_then(|pending_candidates| {
 			pending_candidates.back().map(|x| x.commitments.head_data.clone())
@@ -728,6 +730,16 @@ impl<T: Config> Pallet<T> {
 		}
 	}
 
+	// Get the relay parent number of the most recent candidate (including pending availability).
+	pub(crate) fn para_most_recent_context(para_id: &ParaId) -> Option<BlockNumberFor<T>> {
+		match PendingAvailability::<T>::get(para_id)
+			.and_then(|pending_candidates| pending_candidates.back().map(|x| x.relay_parent_number))
+		{
+			Some(relay_parent_number) => Some(relay_parent_number),
+			None => paras::MostRecentContext::<T>::get(para_id),
+		}
+	}
+
 	fn check_backing_votes(
 		backed_candidate: &BackedCandidate<T::Hash>,
 		validators: &[ValidatorId],
@@ -797,7 +809,7 @@ impl<T: Config> Pallet<T> {
 		relay_parent_number: BlockNumberFor<T>,
 		validation_outputs: polkadot_primitives::CandidateCommitments,
 	) -> bool {
-		let prev_context = paras::MostRecentContext::<T>::get(para_id);
+		let prev_context = Self::para_most_recent_context(&para_id);
 		let check_ctx = CandidateCheckContext::<T>::new(prev_context);
 
 		if check_ctx

polkadot/runtime/parachains/src/paras_inherent/mod.rs

@@ -1243,22 +1243,27 @@ fn filter_unchained_candidates<T: inclusion::Config + paras::Config + inclusion:
 	candidates: &mut BTreeMap<ParaId, Vec<BackedCandidate<T::Hash>>>,
 	allowed_relay_parents: &AllowedRelayParentsTracker<T::Hash, BlockNumberFor<T>>,
 ) {
-	let mut para_latest_head_data: BTreeMap<ParaId, HeadData> = BTreeMap::new();
+	let mut para_latest_context: BTreeMap<ParaId, (HeadData, BlockNumberFor<T>)> = BTreeMap::new();
 	for para_id in candidates.keys() {
-		let latest_head_data = match inclusion::Pallet::<T>::para_latest_head_data(&para_id) {
-			None => {
-				defensive!("Latest included head data for paraid {:?} is None", para_id);
-				continue
-			},
-			Some(latest_head_data) => latest_head_data,
+		let Some(latest_head_data) = inclusion::Pallet::<T>::para_latest_head_data(&para_id) else {
+			defensive!("Latest included head data for paraid {:?} is None", para_id);
+			continue
 		};
-		para_latest_head_data.insert(*para_id, latest_head_data);
+		let Some(latest_relay_parent) = inclusion::Pallet::<T>::para_most_recent_context(&para_id)
+		else {
+			defensive!("Latest relay parent for paraid {:?} is None", para_id);
+			continue
+		};
+		para_latest_context.insert(*para_id, (latest_head_data, latest_relay_parent));
 	}
 
 	let mut para_visited_candidates: BTreeMap<ParaId, BTreeSet<CandidateHash>> = BTreeMap::new();
 
 	retain_candidates::<T, _, _>(candidates, |para_id, candidate| {
-		let Some(latest_head_data) = para_latest_head_data.get(&para_id) else { return false };
+		let Some((latest_head_data, latest_relay_parent)) = para_latest_context.get(&para_id)
+		else {
+			return false
+		};
 		let candidate_hash = candidate.candidate().hash();
 
 		let visited_candidates =
@@ -1277,15 +1282,23 @@ fn filter_unchained_candidates<T: inclusion::Config + paras::Config + inclusion:
 			visited_candidates.insert(candidate_hash);
 		}
 
-		let prev_context = paras::MostRecentContext::<T>::get(para_id);
-		let check_ctx = CandidateCheckContext::<T>::new(prev_context);
+		let check_ctx = CandidateCheckContext::<T>::new(Some(*latest_relay_parent));
 
-		let res = match check_ctx.verify_backed_candidate(
+		match check_ctx.verify_backed_candidate(
 			&allowed_relay_parents,
 			candidate.candidate(),
 			latest_head_data.clone(),
 		) {
-			Ok(_) => true,
+			Ok(relay_parent_block_number) => {
+				para_latest_context.insert(
+					para_id,
+					(
+						candidate.candidate().commitments.head_data.clone(),
+						relay_parent_block_number,
+					),
+				);
+				true
+			},
 			Err(err) => {
 				log::debug!(
 					target: LOG_TARGET,
@@ -1296,14 +1309,7 @@ fn filter_unchained_candidates<T: inclusion::Config + paras::Config + inclusion:
 				);
 				false
 			},
-		};
-
-		if res {
-			para_latest_head_data
-				.insert(para_id, candidate.candidate().commitments.head_data.clone());
 		}
-
-		res
 	});
 }