PVF preparation: do not conflate errors (#6384)

* PVF preparation: do not conflate errors

+ Adds some more granularity to the prepare errors.
+ Better distinguish whether errors occur on the host side or the worker.
+ Do not kill the worker if the error happened on the host side.
+ Do not retry preparation if the error was `Panic`.
+ Removes unnecessary indirection with `Selected` type.

* Add missing docs, resolve TODOs

* Address review comments and remove TODOs

* Fix error in CI

* Undo unnecessary change

* Update couple of comments

* Don't return error for stream shutdown

* Update node/core/pvf/src/worker_common.rs

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

@@ -320,12 +320,12 @@ where
 
 	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,
-		},
+		Err(prepare_err) =>
+			if prepare_err.is_deterministic() {
+				PreCheckOutcome::Invalid
+			} else {
+				PreCheckOutcome::Failed
+			},
 	}
 }
 
@@ -667,10 +667,11 @@ impl ValidationBackend for ValidationHost {
 	async fn precheck_pvf(&mut self, pvf: Pvf) -> Result<Duration, PrepareError> {
 		let (tx, rx) = oneshot::channel();
 		if let Err(_) = self.precheck_pvf(pvf, tx).await {
-			return Err(PrepareError::DidNotMakeIt)
+			// Return an IO error if there was an error communicating with the host.
+			return Err(PrepareError::IoErr)
 		}
 
-		let precheck_result = rx.await.or(Err(PrepareError::DidNotMakeIt))?;
+		let precheck_result = rx.await.or(Err(PrepareError::IoErr))?;
 
 		precheck_result
 	}

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

@@ -1053,5 +1053,5 @@ fn precheck_properly_classifies_outcomes() {
 	inner(Err(PrepareError::Panic("baz".to_owned())), PreCheckOutcome::Invalid);
 
 	inner(Err(PrepareError::TimedOut), PreCheckOutcome::Failed);
-	inner(Err(PrepareError::DidNotMakeIt), PreCheckOutcome::Failed);
+	inner(Err(PrepareError::IoErr), PreCheckOutcome::Failed);
 }