Move artifacts states into memory in PVF validation host (#3907)

* pvf host: store only compiled artifacts on disk

* Correctly handle failed artifacts

* Serialize result of PVF preparation uniquely

* Set the artifact state depending on the result

* Return the result of PVF preparation directly

* Move PrepareError to the error module

* Update doc comments

* Update misleading comment

* Cleanup docs

* Conclude a test job with an error

Co-authored-by: Sergei Shulepov <sergei@parity.io>

polkadot/node/core/pvf/src/artifacts.rs

@@ -14,6 +14,7 @@
 // You should have received a copy of the GNU General Public License
 // along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
 
+use crate::error::PrepareError;
 use always_assert::always;
 use async_std::path::{Path, PathBuf};
 use parity_scale_codec::{Decode, Encode};
@@ -23,30 +24,19 @@ use std::{
 	time::{Duration, SystemTime},
 };
 
-/// A final product of preparation process. Contains either a ready to run compiled artifact or
+/// A wrapper for the compiled PVF code.
-/// a description what went wrong.
 #[derive(Encode, Decode)]
-pub enum Artifact {
+pub struct CompiledArtifact(Vec<u8>);
-	/// During the prevalidation stage of preparation an issue was found with the PVF.
+
-	PrevalidationErr(String),
+impl CompiledArtifact {
-	/// Compilation failed for the given PVF.
+	pub fn new(code: Vec<u8>) -> Self {
-	PreparationErr(String),
+		Self(code)
-	/// This state indicates that the process assigned to prepare the artifact wasn't responsible
+	}
-	/// or were killed. This state is reported by the validation host (not by the worker).
-	DidntMakeIt,
-	/// The PVF passed all the checks and is ready for execution.
-	Compiled { compiled_artifact: Vec<u8> },
 }
 
-impl Artifact {
+impl AsRef<[u8]> for CompiledArtifact {
-	/// Serializes this struct into a byte buffer.
+	fn as_ref(&self) -> &[u8] {
-	pub fn serialize(&self) -> Vec<u8> {
+		self.0.as_slice()
-		self.encode()
-	}
-
-	/// Deserialize the given byte buffer to an artifact.
-	pub fn deserialize(mut bytes: &[u8]) -> Result<Self, String> {
-		Artifact::decode(&mut bytes).map_err(|e| format!("{:?}", e))
 	}
 }
 
@@ -117,6 +107,9 @@ pub enum ArtifactState {
 	},
 	/// A task to prepare this artifact is scheduled.
 	Preparing,
+	/// The code couldn't be compiled due to an error. Such artifacts
+	/// never reach the executor and stay in the host's memory.
+	FailedToProcess(PrepareError),
 }
 
 /// A container of all known artifact ids and their states.

polkadot/node/core/pvf/src/error.rs

@@ -14,6 +14,20 @@
 // You should have received a copy of the GNU General Public License
 // along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
 
+use parity_scale_codec::{Decode, Encode};
+
+/// An error that occurred during the prepare part of the PVF pipeline.
+#[derive(Debug, Clone, Encode, Decode)]
+pub enum PrepareError {
+	/// During the prevalidation stage of preparation an issue was found with the PVF.
+	Prevalidation(String),
+	/// Compilation failed for the given PVF.
+	Preparation(String),
+	/// This state indicates that the process assigned to prepare the artifact wasn't responsible
+	/// or were killed. This state is reported by the validation host (not by the worker).
+	DidNotMakeIt,
+}
+
 /// A error raised during validation of the candidate.
 #[derive(Debug, Clone)]
 pub enum ValidationError {
@@ -54,3 +68,14 @@ pub enum InvalidCandidate {
 	/// PVF execution (compilation is not included) took more time than was allotted.
 	HardTimeout,
 }
+
+impl From<PrepareError> for ValidationError {
+	fn from(error: PrepareError) -> Self {
+		let error_str = match error {
+			PrepareError::Prevalidation(err) => err,
+			PrepareError::Preparation(err) => err,
+			PrepareError::DidNotMakeIt => "preparation timeout".to_owned(),
+		};
+		ValidationError::InvalidCandidate(InvalidCandidate::WorkerReportedError(error_str))
+	}
+}

polkadot/node/core/pvf/src/execute/worker.rs

@@ -15,7 +15,7 @@
 // along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
 
 use crate::{
-	artifacts::{Artifact, ArtifactPathId},
+	artifacts::{ArtifactPathId, CompiledArtifact},
 	executor_intf::TaskExecutor,
 	worker_common::{
 		bytes_to_path, framed_recv, framed_send, path_to_bytes, spawn_with_program_path,
@@ -49,8 +49,8 @@ pub enum Outcome {
 	/// PVF execution completed successfully and the result is returned. The worker is ready for
 	/// another job.
 	Ok { result_descriptor: ValidationResult, duration_ms: u64, idle_worker: IdleWorker },
-	/// The candidate validation failed. It may be for example because the preparation process
+	/// The candidate validation failed. It may be for example because the wasm execution triggered a trap.
-	/// produced an error or the wasm execution triggered a trap.
+	/// Errors related to the preparation process are not expected to be encountered by the execution workers.
 	InvalidCandidate { err: String, idle_worker: IdleWorker },
 	/// An internal error happened during the validation. Such an error is most likely related to
 	/// some transient glitch.
@@ -216,18 +216,12 @@ async fn validate_using_artifact(
 		Ok(b) => b,
 	};
 
-	let artifact = match Artifact::deserialize(&artifact_bytes) {
+	let artifact = match CompiledArtifact::decode(&mut artifact_bytes.as_slice()) {
 		Err(e) => return Response::InternalError(format!("artifact deserialization: {:?}", e)),
 		Ok(a) => a,
 	};
 
-	let compiled_artifact = match &artifact {
+	let compiled_artifact = artifact.as_ref();
-		Artifact::PrevalidationErr(msg) => return Response::format_invalid("prevalidation", msg),
-		Artifact::PreparationErr(msg) => return Response::format_invalid("preparation", msg),
-		Artifact::DidntMakeIt => return Response::format_invalid("preparation timeout", ""),
-
-		Artifact::Compiled { compiled_artifact } => compiled_artifact,
-	};
 
 	let validation_started_at = Instant::now();
 	let descriptor_bytes = match unsafe {

polkadot/node/core/pvf/src/host.rs

@@ -344,8 +344,7 @@ async fn run(
 				.await);
 			},
 			from_prepare_queue = from_prepare_queue_rx.next() => {
-				let prepare::FromQueue::Prepared(artifact_id)
+				let from_queue = break_if_fatal!(from_prepare_queue.ok_or(Fatal));
-					= break_if_fatal!(from_prepare_queue.ok_or(Fatal));
 
 				// Note that preparation always succeeds.
 				//
@@ -361,7 +360,7 @@ async fn run(
 					&mut artifacts,
 					&mut to_execute_queue_tx,
 					&mut awaiting_prepare,
-					artifact_id,
+					from_queue,
 				).await);
 			},
 		}
@@ -439,6 +438,9 @@ async fn handle_execute_pvf(
 
 				awaiting_prepare.add(artifact_id, execution_timeout, params, result_tx);
 			},
+			ArtifactState::FailedToProcess(error) => {
+				let _ = result_tx.send(Err(ValidationError::from(error.clone())));
+			},
 		}
 	} else {
 		// Artifact is unknown: register it and enqueue a job with the corresponding priority and
@@ -470,6 +472,7 @@ async fn handle_heads_up(
 					// Already preparing. We don't need to send a priority amend either because
 					// it can't get any lower than the background.
 				},
+				ArtifactState::FailedToProcess(_) => {},
 			}
 		} else {
 			// The artifact is unknown: register it and put a background job into the prepare queue.
@@ -491,8 +494,10 @@ async fn handle_prepare_done(
 	artifacts: &mut Artifacts,
 	execute_queue: &mut mpsc::Sender<execute::ToQueue>,
 	awaiting_prepare: &mut AwaitingPrepare,
-	artifact_id: ArtifactId,
+	from_queue: prepare::FromQueue,
 ) -> Result<(), Fatal> {
+	let prepare::FromQueue { artifact_id, result } = from_queue;
+
 	// Make some sanity checks and extract the current state.
 	let state = match artifacts.artifact_state_mut(&artifact_id) {
 		None => {
@@ -513,6 +518,12 @@ async fn handle_prepare_done(
 			never!("the artifact is already prepared: {:?}", artifact_id);
 			return Ok(())
 		},
+		Some(ArtifactState::FailedToProcess(_)) => {
+			// The reasoning is similar to the above, the artifact cannot be
+			// processed at this point.
+			never!("the artifact is already processed unsuccessfully: {:?}", artifact_id);
+			return Ok(())
+		},
 		Some(state @ ArtifactState::Preparing) => state,
 	};
 
@@ -526,6 +537,12 @@ async fn handle_prepare_done(
 			continue
 		}
 
+		// Don't send failed artifacts to the execution's queue.
+		if let Err(ref error) = result {
+			let _ = result_tx.send(Err(ValidationError::from(error.clone())));
+			continue
+		}
+
 		send_execute(
 			execute_queue,
 			execute::ToQueue::Enqueue {
@@ -538,8 +555,10 @@ async fn handle_prepare_done(
 		.await?;
 	}
 
-	// Now consider the artifact prepared.
+	*state = match result {
-	*state = ArtifactState::Prepared { last_time_needed: SystemTime::now() };
+		Ok(()) => ArtifactState::Prepared { last_time_needed: SystemTime::now() },
+		Err(error) => ArtifactState::FailedToProcess(error.clone()),
+	};
 
 	Ok(())
 }
@@ -937,7 +956,7 @@ mod tests {
 		);
 
 		test.from_prepare_queue_tx
-			.send(prepare::FromQueue::Prepared(artifact_id(1)))
+			.send(prepare::FromQueue { artifact_id: artifact_id(1), result: Ok(()) })
 			.await
 			.unwrap();
 		let result_tx_pvf_1_1 = assert_matches!(
@@ -950,7 +969,7 @@ mod tests {
 		);
 
 		test.from_prepare_queue_tx
-			.send(prepare::FromQueue::Prepared(artifact_id(2)))
+			.send(prepare::FromQueue { artifact_id: artifact_id(2), result: Ok(()) })
 			.await
 			.unwrap();
 		let result_tx_pvf_2 = assert_matches!(
@@ -1005,7 +1024,7 @@ mod tests {
 		);
 
 		test.from_prepare_queue_tx
-			.send(prepare::FromQueue::Prepared(artifact_id(1)))
+			.send(prepare::FromQueue { artifact_id: artifact_id(1), result: Ok(()) })
 			.await
 			.unwrap();
 

polkadot/node/core/pvf/src/prepare/pool.rs

@@ -16,6 +16,7 @@
 
 use super::worker::{self, Outcome};
 use crate::{
+	error::PrepareError,
 	metrics::Metrics,
 	worker_common::{IdleWorker, WorkerHandle},
 	LOG_TARGET,
@@ -78,9 +79,16 @@ pub enum FromPool {
 	/// The given worker was just spawned and is ready to be used.
 	Spawned(Worker),
 
-	/// The given worker either succeeded or failed the given job. Under any circumstances the
+	/// The given worker either succeeded or failed the given job.
-	/// artifact file has been written. The `bool` says whether the worker ripped.
+	Concluded {
-	Concluded(Worker, bool),
+		/// A key for retrieving the worker data from the pool.
+		worker: Worker,
+		/// Indicates whether the worker process was killed.
+		rip: bool,
+		/// [`Ok`] indicates that compiled artifact is successfully stored on disk.
+		/// Otherwise, an [error](PrepareError) is supplied.
+		result: Result<(), PrepareError>,
+	},
 
 	/// The given worker ceased to exist.
 	Rip(Worker),
@@ -295,7 +303,7 @@ fn handle_mux(
 		},
 		PoolEvent::StartWork(worker, outcome) => {
 			match outcome {
-				Outcome::Concluded(idle) => {
+				Outcome::Concluded { worker: idle, result } => {
 					let data = match spawned.get_mut(worker) {
 						None => {
 							// Perhaps the worker was killed meanwhile and the result is no longer
@@ -310,7 +318,7 @@ fn handle_mux(
 					let old = data.idle.replace(idle);
 					assert_matches!(old, None, "attempt to overwrite an idle worker");
 
-					reply(from_pool, FromPool::Concluded(worker, false))?;
+					reply(from_pool, FromPool::Concluded { worker, rip: false, result })?;
 
 					Ok(())
 				},
@@ -321,9 +329,16 @@ fn handle_mux(
 
 					Ok(())
 				},
-				Outcome::DidntMakeIt => {
+				Outcome::DidNotMakeIt => {
 					if attempt_retire(metrics, spawned, worker) {
-						reply(from_pool, FromPool::Concluded(worker, true))?;
+						reply(
+							from_pool,
+							FromPool::Concluded {
+								worker,
+								rip: true,
+								result: Err(PrepareError::DidNotMakeIt),
+							},
+						)?;
 					}
 
 					Ok(())

polkadot/node/core/pvf/src/prepare/queue.rs

@@ -17,7 +17,9 @@
 //! A queue that handles requests for PVF preparation.
 
 use super::pool::{self, Worker};
-use crate::{artifacts::ArtifactId, metrics::Metrics, Priority, Pvf, LOG_TARGET};
+use crate::{
+	artifacts::ArtifactId, error::PrepareError, metrics::Metrics, Priority, Pvf, LOG_TARGET,
+};
 use always_assert::{always, never};
 use async_std::path::PathBuf;
 use futures::{channel::mpsc, stream::StreamExt as _, Future, SinkExt};
@@ -29,7 +31,7 @@ pub enum ToQueue {
 	/// This schedules preparation of the given PVF.
 	///
 	/// Note that it is incorrect to enqueue the same PVF again without first receiving the
-	/// [`FromQueue::Prepared`] response. In case there is a need to bump the priority, use
+	/// [`FromQueue`] response. In case there is a need to bump the priority, use
 	/// [`ToQueue::Amend`].
 	Enqueue { priority: Priority, pvf: Pvf },
 	/// Amends the priority for the given [`ArtifactId`] if it is running. If it's not, then it's noop.
@@ -37,9 +39,13 @@ pub enum ToQueue {
 }
 
 /// A response from queue.
-#[derive(Debug, PartialEq, Eq)]
+#[derive(Debug)]
-pub enum FromQueue {
+pub struct FromQueue {
-	Prepared(ArtifactId),
+	/// Identifier of an artifact.
+	pub(crate) artifact_id: ArtifactId,
+	/// Outcome of the PVF processing. [`Ok`] indicates that compiled artifact
+	/// is successfully stored on disk. Otherwise, an [error](PrepareError) is supplied.
+	pub(crate) result: Result<(), PrepareError>,
 }
 
 #[derive(Default)]
@@ -299,7 +305,8 @@ async fn handle_from_pool(queue: &mut Queue, from_pool: pool::FromPool) -> Resul
 	use pool::FromPool::*;
 	match from_pool {
 		Spawned(worker) => handle_worker_spawned(queue, worker).await?,
-		Concluded(worker, rip) => handle_worker_concluded(queue, worker, rip).await?,
+		Concluded { worker, rip, result } =>
+			handle_worker_concluded(queue, worker, rip, result).await?,
 		Rip(worker) => handle_worker_rip(queue, worker).await?,
 	}
 	Ok(())
@@ -320,6 +327,7 @@ async fn handle_worker_concluded(
 	queue: &mut Queue,
 	worker: Worker,
 	rip: bool,
+	result: Result<(), PrepareError>,
 ) -> Result<(), Fatal> {
 	queue.metrics.prepare_concluded();
 
@@ -377,7 +385,7 @@ async fn handle_worker_concluded(
 		"prepare worker concluded",
 	);
 
-	reply(&mut queue.from_queue_tx, FromQueue::Prepared(artifact_id))?;
+	reply(&mut queue.from_queue_tx, FromQueue { artifact_id, result })?;
 
 	// Figure out what to do with the worker.
 	if rip {
@@ -641,12 +649,9 @@ mod tests {
 
 		let w = test.workers.insert(());
 		test.send_from_pool(pool::FromPool::Spawned(w));
-		test.send_from_pool(pool::FromPool::Concluded(w, false));
+		test.send_from_pool(pool::FromPool::Concluded { worker: w, rip: false, result: Ok(()) });
 
-		assert_eq!(
+		assert_eq!(test.poll_and_recv_from_queue().await.artifact_id, pvf(1).as_artifact_id());
-			test.poll_and_recv_from_queue().await,
-			FromQueue::Prepared(pvf(1).as_artifact_id())
-		);
 	}
 
 	#[async_std::test]
@@ -671,7 +676,7 @@ mod tests {
 		assert_matches!(test.poll_and_recv_to_pool().await, pool::ToPool::StartWork { .. });
 		assert_matches!(test.poll_and_recv_to_pool().await, pool::ToPool::StartWork { .. });
 
-		test.send_from_pool(pool::FromPool::Concluded(w1, false));
+		test.send_from_pool(pool::FromPool::Concluded { worker: w1, rip: false, result: Ok(()) });
 
 		assert_matches!(test.poll_and_recv_to_pool().await, pool::ToPool::StartWork { .. });
 
@@ -704,7 +709,7 @@ mod tests {
 		// That's a bit silly in this context, but in production there will be an entire pool up
 		// to the `soft_capacity` of workers and it doesn't matter which one to cull. Either way,
 		// we just check that edge case of an edge case works.
-		test.send_from_pool(pool::FromPool::Concluded(w1, false));
+		test.send_from_pool(pool::FromPool::Concluded { worker: w1, rip: false, result: Ok(()) });
 		assert_eq!(test.poll_and_recv_to_pool().await, pool::ToPool::Kill(w1));
 	}
 
@@ -749,15 +754,12 @@ mod tests {
 		assert_matches!(test.poll_and_recv_to_pool().await, pool::ToPool::StartWork { .. });
 
 		// Conclude worker 1 and rip it.
-		test.send_from_pool(pool::FromPool::Concluded(w1, true));
+		test.send_from_pool(pool::FromPool::Concluded { worker: w1, rip: true, result: Ok(()) });
 
 		// Since there is still work, the queue requested one extra worker to spawn to handle the
 		// remaining enqueued work items.
 		assert_eq!(test.poll_and_recv_to_pool().await, pool::ToPool::Spawn);
-		assert_eq!(
+		assert_eq!(test.poll_and_recv_from_queue().await.artifact_id, pvf(1).as_artifact_id());
-			test.poll_and_recv_from_queue().await,
-			FromQueue::Prepared(pvf(1).as_artifact_id())
-		);
 	}
 
 	#[async_std::test]
@@ -773,7 +775,11 @@ mod tests {
 
 		assert_matches!(test.poll_and_recv_to_pool().await, pool::ToPool::StartWork { .. });
 
-		test.send_from_pool(pool::FromPool::Concluded(w1, true));
+		test.send_from_pool(pool::FromPool::Concluded {
+			worker: w1,
+			rip: true,
+			result: Err(PrepareError::DidNotMakeIt),
+		});
 		test.poll_ensure_to_pool_is_empty().await;
 	}
 

polkadot/node/core/pvf/src/prepare/worker.rs

@@ -15,7 +15,8 @@
 // along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
 
 use crate::{
-	artifacts::Artifact,
+	artifacts::CompiledArtifact,
+	error::PrepareError,
 	worker_common::{
 		bytes_to_path, framed_recv, framed_send, path_to_bytes, spawn_with_program_path,
 		tmpfile_in, worker_event_loop, IdleWorker, SpawnErr, WorkerHandle,
@@ -29,6 +30,8 @@ use async_std::{
 };
 use futures::FutureExt as _;
 use futures_timer::Delay;
+use parity_scale_codec::{Decode, Encode};
+use sp_core::hexdisplay::HexDisplay;
 use std::{sync::Arc, time::Duration};
 
 const NICENESS_BACKGROUND: i32 = 10;
@@ -48,7 +51,7 @@ pub async fn spawn(
 
 pub enum Outcome {
 	/// The worker has finished the work assigned to it.
-	Concluded(IdleWorker),
+	Concluded { worker: IdleWorker, result: Result<(), PrepareError> },
 	/// The host tried to reach the worker but failed. This is most likely because the worked was
 	/// killed by the system.
 	Unreachable,
@@ -59,7 +62,7 @@ pub enum Outcome {
 	/// the artifact).
 	///
 	/// This doesn't return an idle worker instance, thus this worker is no longer usable.
-	DidntMakeIt,
+	DidNotMakeIt,
 }
 
 /// Given the idle token of a worker and parameters of work, communicates with the worker and
@@ -99,13 +102,11 @@ pub async fn start_work(
 		// Wait for the result from the worker, keeping in mind that there may be a timeout, the
 		// worker may get killed, or something along these lines.
 		//
-		// In that case we should handle these gracefully by writing the artifact file by ourselves.
+		// In that case we should propagate the error to the pool.
-		// We may potentially overwrite the artifact in rare cases where the worker didn't make
-		// it to report back the result.
 
 		#[derive(Debug)]
 		enum Selected {
-			Done,
+			Done(Result<(), PrepareError>),
 			IoErr,
 			Deadline,
 		}
@@ -113,41 +114,48 @@ pub async fn start_work(
 		let selected = futures::select! {
 			res = framed_recv(&mut stream).fuse() => {
 				match res {
-					Ok(x) if x == &[1u8] => {
+					Ok(response_bytes) => {
-						tracing::debug!(
+						// By convention we expect encoded `Result<(), PrepareError>`.
-							target: LOG_TARGET,
+						if let Ok(result) =
-							worker_pid = %pid,
+							<Result<(), PrepareError>>::decode(&mut response_bytes.clone().as_slice())
-							"promoting WIP artifact {} to {}",
+						{
-							tmp_file.display(),
+							if result.is_ok() {
-							artifact_path.display(),
+								tracing::debug!(
-						);
-
-						async_std::fs::rename(&tmp_file, &artifact_path)
-							.await
-							.map(|_| Selected::Done)
-							.unwrap_or_else(|err| {
-								tracing::warn!(
 									target: LOG_TARGET,
 									worker_pid = %pid,
-									"failed to rename the artifact from {} to {}: {:?}",
+									"promoting WIP artifact {} to {}",
 									tmp_file.display(),
 									artifact_path.display(),
-									err,
 								);
-								Selected::IoErr
+
-							})
+								async_std::fs::rename(&tmp_file, &artifact_path)
-					}
+									.await
-					Ok(response_bytes) => {
+									.map(|_| Selected::Done(result))
-						use sp_core::hexdisplay::HexDisplay;
+									.unwrap_or_else(|err| {
-						let bound_bytes =
+										tracing::warn!(
-							&response_bytes[..response_bytes.len().min(4)];
+											target: LOG_TARGET,
-						tracing::warn!(
+											worker_pid = %pid,
-							target: LOG_TARGET,
+											"failed to rename the artifact from {} to {}: {:?}",
-							worker_pid = %pid,
+											tmp_file.display(),
-							"received unexpected response from the prepare worker: {}",
+											artifact_path.display(),
-							HexDisplay::from(&bound_bytes),
+											err,
-						);
+										);
-						Selected::IoErr
+										Selected::IoErr
+									})
+							} else {
+								Selected::Done(result)
+							}
+						} else {
+							// We received invalid bytes from the worker.
+							let bound_bytes = &response_bytes[..response_bytes.len().min(4)];
+							tracing::warn!(
+								target: LOG_TARGET,
+								worker_pid = %pid,
+								"received unexpected response from the prepare worker: {}",
+								HexDisplay::from(&bound_bytes),
+							);
+							Selected::IoErr
+						}
 					},
 					Err(err) => {
 						tracing::warn!(
@@ -164,24 +172,11 @@ pub async fn start_work(
 		};
 
 		match selected {
-			Selected::Done => {
+			Selected::Done(result) => {
 				renice(pid, NICENESS_FOREGROUND);
-				Outcome::Concluded(IdleWorker { stream, pid })
+				Outcome::Concluded { worker: IdleWorker { stream, pid }, result }
-			},
-			Selected::IoErr | Selected::Deadline => {
-				let bytes = Artifact::DidntMakeIt.serialize();
-				// best effort: there is nothing we can do here if the write fails.
-				if let Err(err) = async_std::fs::write(&artifact_path, &bytes).await {
-					tracing::warn!(
-						target: LOG_TARGET,
-						worker_pid = %pid,
-						"preparation didn't make it, because of `{:?}`: {:?}",
-						selected,
-						err,
-					);
-				}
-				Outcome::DidntMakeIt
 			},
+			Selected::IoErr | Selected::Deadline => Outcome::DidNotMakeIt,
 		}
 	})
 	.await
@@ -205,7 +200,7 @@ where
 				"failed to create a temp file for the artifact: {:?}",
 				err,
 			);
-			return Outcome::DidntMakeIt
+			return Outcome::DidNotMakeIt
 		},
 	};
 
@@ -288,31 +283,47 @@ pub fn worker_entrypoint(socket_path: &str) {
 				worker_pid = %std::process::id(),
 				"worker: preparing artifact",
 			);
-			let artifact_bytes = prepare_artifact(&code).serialize();
 
-			// Write the serialized artifact into into a temp file.
+			let result = match prepare_artifact(&code) {
-			tracing::debug!(
+				Err(err) => {
-				target: LOG_TARGET,
+					// Serialized error will be written into the socket.
-				worker_pid = %std::process::id(),
+					Err(err)
-				"worker: writing artifact to {}",
+				},
-				dest.display(),
+				Ok(compiled_artifact) => {
-			);
+					// Write the serialized artifact into a temp file.
-			async_std::fs::write(&dest, &artifact_bytes).await?;
+					// PVF host only keeps artifacts statuses in its memory,
+					// successfully compiled code gets stored on the disk (and
+					// consequently deserialized by execute-workers). The prepare
+					// worker is only required to send an empty `Ok` to the pool
+					// to indicate the success.
 
-			// Return back a byte that signals finishing the work.
+					let artifact_bytes = compiled_artifact.encode();
-			framed_send(&mut stream, &[1u8]).await?;
+
+					tracing::debug!(
+						target: LOG_TARGET,
+						worker_pid = %std::process::id(),
+						"worker: writing artifact to {}",
+						dest.display(),
+					);
+					async_std::fs::write(&dest, &artifact_bytes).await?;
+
+					Ok(())
+				},
+			};
+
+			framed_send(&mut stream, result.encode().as_slice()).await?;
 		}
 	});
 }
 
-fn prepare_artifact(code: &[u8]) -> Artifact {
+fn prepare_artifact(code: &[u8]) -> Result<CompiledArtifact, PrepareError> {
 	let blob = match crate::executor_intf::prevalidate(code) {
-		Err(err) => return Artifact::PrevalidationErr(format!("{:?}", err)),
+		Err(err) => return Err(PrepareError::Prevalidation(format!("{:?}", err))),
 		Ok(b) => b,
 	};
 
 	match crate::executor_intf::prepare(blob) {
-		Ok(compiled_artifact) => Artifact::Compiled { compiled_artifact },
+		Ok(compiled_artifact) => Ok(CompiledArtifact::new(compiled_artifact)),
-		Err(err) => Artifact::PreparationErr(format!("{:?}", err)),
+		Err(err) => Err(PrepareError::Preparation(format!("{:?}", err))),
 	}
 }