prepare worker: Catch unexpected unwinds (#4304)

* prepare worker: Catch unexpected unwinds

* Use more specific wording for unknown panic payload

node/core/pvf/src/error.rs

@@ -26,6 +26,8 @@ pub enum PrepareError {
 	Prevalidation(String),
 	/// Compilation failed for the given PVF.
 	Preparation(String),
+	/// An unexpected panic has occured in the preparation worker.
+	Panic(String),
 	/// Failed to prepare the PVF due to the time limit.
 	TimedOut,
 	/// This state indicates that the process assigned to prepare the artifact wasn't responsible
@@ -77,8 +79,9 @@ pub enum InvalidCandidate {
 impl From<PrepareError> for ValidationError {
 	fn from(error: PrepareError) -> Self {
 		let error_str = match error {
-			PrepareError::Prevalidation(err) => err,
-			PrepareError::Preparation(err) => err,
+			PrepareError::Prevalidation(err) => format!("prevalidation: {}", err),
+			PrepareError::Preparation(err) => format!("preparation: {}", err),
+			PrepareError::Panic(err) => format!("panic: {}", err),
 			PrepareError::TimedOut => "preparation timeout".to_owned(),
 			PrepareError::DidNotMakeIt => "communication error".to_owned(),
 		};

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

@@ -30,7 +30,7 @@ use async_std::{
 };
 use parity_scale_codec::{Decode, Encode};
 use sp_core::hexdisplay::HexDisplay;
-use std::{sync::Arc, time::Duration};
+use std::{any::Any, panic, sync::Arc, time::Duration};
 
 const NICENESS_BACKGROUND: i32 = 10;
 const NICENESS_FOREGROUND: i32 = 0;
@@ -318,13 +318,31 @@ pub fn worker_entrypoint(socket_path: &str) {
 }
 
 fn prepare_artifact(code: &[u8]) -> Result<CompiledArtifact, PrepareError> {
-	let blob = match crate::executor_intf::prevalidate(code) {
-		Err(err) => return Err(PrepareError::Prevalidation(format!("{:?}", err))),
-		Ok(b) => b,
-	};
+	panic::catch_unwind(|| {
+		let blob = match crate::executor_intf::prevalidate(code) {
+			Err(err) => return Err(PrepareError::Prevalidation(format!("{:?}", err))),
+			Ok(b) => b,
+		};
+
+		match crate::executor_intf::prepare(blob) {
+			Ok(compiled_artifact) => Ok(CompiledArtifact::new(compiled_artifact)),
+			Err(err) => Err(PrepareError::Preparation(format!("{:?}", err))),
+		}
+	})
+	.map_err(|panic_payload| PrepareError::Panic(stringify_panic_payload(panic_payload)))
+	.and_then(|inner_result| inner_result)
+}
 
-	match crate::executor_intf::prepare(blob) {
-		Ok(compiled_artifact) => Ok(CompiledArtifact::new(compiled_artifact)),
-		Err(err) => Err(PrepareError::Preparation(format!("{:?}", err))),
+/// Attempt to convert an opaque panic payload to a string.
+///
+/// This is a best effort, and is not guaranteed to provide the most accurate value.
+fn stringify_panic_payload(payload: Box<dyn Any + Send + 'static>) -> String {
+	match payload.downcast::<&'static str>() {
+		Ok(msg) => msg.to_string(),
+		Err(payload) => match payload.downcast::<String>() {
+			Ok(msg) => *msg,
+			// At least we tried...
+			Err(_) => "unkown panic payload".to_string(),
+		},
 	}
 }