pvf-precheck: Candidate Validation Changes (#4409)

* pvf-precheck: Candidate Validation Changes

Co-authored-by: Chris Sosnin <chris125_@live.com>

* Rename `MockValidationBackend` and specialize it

* Add pre-check tests

Co-authored-by: Chris Sosnin <chris125_@live.com>

polkadot/node/core/candidate-validation/src/lib.rs

@@ -24,7 +24,7 @@
 #![warn(missing_docs)]
 
 use polkadot_node_core_pvf::{
-	InvalidCandidate as WasmInvalidCandidate, Pvf, ValidationError, ValidationHost,
+	InvalidCandidate as WasmInvalidCandidate, PrepareError, Pvf, ValidationError, ValidationHost,
 };
 use polkadot_node_primitives::{
 	BlockData, InvalidCandidate, PoV, ValidationResult, POV_BOMB_LIMIT, VALIDATION_CODE_BOMB_LIMIT,
@@ -32,7 +32,8 @@ use polkadot_node_primitives::{
 use polkadot_node_subsystem::{
 	errors::RuntimeApiError,
 	messages::{
-		CandidateValidationMessage, RuntimeApiMessage, RuntimeApiRequest, ValidationFailed,
+		CandidateValidationMessage, PreCheckOutcome, RuntimeApiMessage, RuntimeApiRequest,
+		ValidationFailed,
 	},
 	overseer, FromOverseer, OverseerSignal, SpawnedSubsystem, SubsystemContext, SubsystemError,
 	SubsystemResult, SubsystemSender,
@@ -194,6 +195,30 @@ where
 
 					ctx.spawn("validate-from-exhaustive", bg.boxed())?;
 				},
+				CandidateValidationMessage::PreCheck(
+					relay_parent,
+					validation_code_hash,
+					response_sender,
+				) => {
+					let bg = {
+						let mut sender = ctx.sender().clone();
+						let validation_host = validation_host.clone();
+
+						async move {
+							let precheck_result = precheck_pvf(
+								&mut sender,
+								validation_host,
+								relay_parent,
+								validation_code_hash,
+							)
+							.await;
+
+							let _ = response_sender.send(precheck_result);
+						}
+					};
+
+					ctx.spawn("candidate-validation-pre-check", bg.boxed())?;
+				},
 			},
 		}
 	}
@@ -235,6 +260,73 @@ where
 		})
 }
 
+async fn request_validation_code_by_hash<Sender>(
+	sender: &mut Sender,
+	relay_parent: Hash,
+	validation_code_hash: ValidationCodeHash,
+) -> Result<Option<ValidationCode>, RuntimeRequestFailed>
+where
+	Sender: SubsystemSender,
+{
+	let (tx, rx) = oneshot::channel();
+	runtime_api_request(
+		sender,
+		relay_parent,
+		RuntimeApiRequest::ValidationCodeByHash(validation_code_hash, tx),
+		rx,
+	)
+	.await
+}
+
+async fn precheck_pvf<Sender>(
+	sender: &mut Sender,
+	mut validation_backend: impl ValidationBackend,
+	relay_parent: Hash,
+	validation_code_hash: ValidationCodeHash,
+) -> PreCheckOutcome
+where
+	Sender: SubsystemSender,
+{
+	let validation_code =
+		match request_validation_code_by_hash(sender, relay_parent, validation_code_hash).await {
+			Ok(Some(code)) => code,
+			_ => {
+				// The reasoning why this is "failed" and not invalid is because we assume that
+				// during pre-checking voting the relay-chain will pin the code. In case the code
+				// actually is not there, we issue failed since this looks more like a bug. This
+				// leads to us abstaining.
+				tracing::warn!(
+					target: LOG_TARGET,
+					?relay_parent,
+					?validation_code_hash,
+					"precheck: requested validation code is not found on-chain!",
+				);
+				return PreCheckOutcome::Failed
+			},
+		};
+
+	let validation_code = match sp_maybe_compressed_blob::decompress(
+		&validation_code.0,
+		VALIDATION_CODE_BOMB_LIMIT,
+	) {
+		Ok(code) => Pvf::from_code(code.into_owned()),
+		Err(e) => {
+			tracing::debug!(target: LOG_TARGET, err=?e, "precheck: cannot decompress validation code");
+			return PreCheckOutcome::Invalid
+		},
+	};
+
+	match validation_backend.precheck_pvf(validation_code).await {
+		Ok(_) => PreCheckOutcome::Valid,
+		Err(prepare_err) => match prepare_err {
+			PrepareError::Prevalidation(_) |
+			PrepareError::Preparation(_) |
+			PrepareError::Panic(_) => PreCheckOutcome::Invalid,
+			PrepareError::TimedOut | PrepareError::DidNotMakeIt => PreCheckOutcome::Failed,
+		},
+	}
+}
+
 #[derive(Debug)]
 enum AssumptionCheckOutcome {
 	Matches(PersistedValidationData, ValidationCode),
@@ -487,6 +579,8 @@ trait ValidationBackend {
 		timeout: Duration,
 		params: ValidationParams,
 	) -> Result<WasmValidationResult, ValidationError>;
+
+	async fn precheck_pvf(&mut self, pvf: Pvf) -> Result<(), PrepareError>;
 }
 
 #[async_trait]
@@ -520,6 +614,17 @@ impl ValidationBackend for ValidationHost {
 
 		validation_result
 	}
+
+	async fn precheck_pvf(&mut self, pvf: Pvf) -> Result<(), PrepareError> {
+		let (tx, rx) = oneshot::channel();
+		if let Err(_) = self.precheck_pvf(pvf, tx).await {
+			return Err(PrepareError::DidNotMakeIt)
+		}
+
+		let precheck_result = rx.await.or(Err(PrepareError::DidNotMakeIt))?;
+
+		precheck_result
+	}
 }
 
 /// Does basic checks of a candidate. Provide the encoded PoV-block. Returns `Ok` if basic checks