Ensure relay chain block number strictly increases

pallets/parachain-system/src/lib.rs

@@ -232,7 +232,7 @@ pub mod pallet {
 			}
 
 			// Remove the validation from the old block.
-			<ValidationData<T>>::kill();
+			ValidationData::<T>::kill();
 			ProcessedDownwardMessages::<T>::kill();
 			HrmpWatermark::<T>::kill();
 			UpwardMessages::<T>::kill();
@@ -307,6 +307,20 @@ pub mod pallet {
 
 			Self::validate_validation_data(&vfp);
 
+			// Ensure that the associated relay chain parent is strictly increasing
+			//
+			// If this isn't the case, a collator tried to build multiple parachain blocks
+			// against the same relay chain block. This is a safeguard for when the relay chain
+			// supports async backing. Then this needs to be revisited and made modular for chains
+			// that may support multiple parachain blocks per relay chain block.
+			if LastRelayChainBlockNumber::<T>::get() < vfp.relay_parent_number {
+				LastRelayChainBlockNumber::<T>::put(vfp.relay_parent_number);
+			} else {
+				panic!(
+					"Relay chain block number needs to strictly increase between Parachain blocks!"
+				)
+			}
+
 			let relay_state_proof = RelayChainStateProof::new(
 				T::SelfParaId::get(),
 				vfp.relay_parent_storage_root,
@@ -474,6 +488,11 @@ pub mod pallet {
 	#[pallet::storage]
 	pub(super) type DidSetValidationCode<T: Config> = StorageValue<_, bool, ValueQuery>;
 
+	/// The relay chain block number associated to the last parachain block.
+	#[pallet::storage]
+	pub(super) type LastRelayChainBlockNumber<T: Config> =
+		StorageValue<_, RelayChainBlockNumber, ValueQuery>;
+
 	/// An option which indicates if the relay-chain restricts signalling a validation code upgrade.
 	/// In other words, if this is `Some` and [`NewValidationCode`] is `Some` then the produced
 	/// candidate will be invalid.

pallets/parachain-system/src/tests.rs

@@ -226,8 +226,7 @@ struct BlockTests {
 	ran: bool,
 	relay_sproof_builder_hook:
 		Option<Box<dyn Fn(&BlockTests, RelayChainBlockNumber, &mut RelayStateSproofBuilder)>>,
-	persisted_validation_data_hook:
-		Option<Box<dyn Fn(&BlockTests, RelayChainBlockNumber, &mut PersistedValidationData)>>,
+	persisted_validation_data_hook: Option<Box<dyn Fn(&BlockTests, &mut PersistedValidationData)>>,
 	inherent_data_hook:
 		Option<Box<dyn Fn(&BlockTests, RelayChainBlockNumber, &mut ParachainInherentData)>>,
 }
@@ -274,10 +273,9 @@ impl BlockTests {
 		self
 	}
 
-	#[allow(dead_code)] // might come in handy in future. If now is future and it still hasn't - feel free.
 	fn with_validation_data<F>(mut self, f: F) -> Self
 	where
-		F: 'static + Fn(&BlockTests, RelayChainBlockNumber, &mut PersistedValidationData),
+		F: 'static + Fn(&BlockTests, &mut PersistedValidationData),
 	{
 		self.persisted_validation_data_hook = Some(Box::new(f));
 		self
@@ -319,7 +317,7 @@ impl BlockTests {
 					..Default::default()
 				};
 				if let Some(ref hook) = self.persisted_validation_data_hook {
-					hook(self, *n as RelayChainBlockNumber, &mut vfp);
+					hook(self, &mut vfp);
 				}
 
 				<ValidationData<Test>>::put(&vfp);
@@ -961,3 +959,14 @@ fn receive_hrmp_after_pause() {
 			});
 		});
 }
+
+#[test]
+#[should_panic = "Relay chain block number needs to strictly increase between Parachain blocks!"]
+fn test() {
+	BlockTests::new()
+		.with_validation_data(|_, data| {
+			data.relay_parent_number = 1;
+		})
+		.add(1, || {})
+		.add(2, || {});
+}