Make candidate validation bounded again (#2125)

This PR aims to channel the backpressure of the PVF host's preparation
and execution queues to the candidate validation subsystem consumers.

Related: #708

polkadot/node/malus/src/interceptor.rs

@@ -22,7 +22,7 @@
 
 use polkadot_node_subsystem::*;
 pub use polkadot_node_subsystem::{messages::*, overseer, FromOrchestra};
-use std::{future::Future, pin::Pin};
+use std::{collections::VecDeque, future::Future, pin::Pin};
 
 /// Filter incoming and outgoing messages.
 pub trait MessageInterceptor<Sender>: Send + Sync + Clone + 'static
@@ -170,6 +170,7 @@ where
 	inner: Context,
 	message_filter: Fil,
 	sender: InterceptedSender<<Context as overseer::SubsystemContext>::Sender, Fil>,
+	message_buffer: VecDeque<FromOrchestra<<Context as overseer::SubsystemContext>::Message>>,
 }
 
 impl<Context, Fil> InterceptedContext<Context, Fil>
@@ -189,7 +190,7 @@ where
 			inner: inner.sender().clone(),
 			message_filter: message_filter.clone(),
 		};
-		Self { inner, message_filter, sender }
+		Self { inner, message_filter, sender, message_buffer: VecDeque::new() }
 	}
 }
 
@@ -233,6 +234,9 @@ where
 	}
 
 	async fn recv(&mut self) -> SubsystemResult<FromOrchestra<Self::Message>> {
+		if let Some(msg) = self.message_buffer.pop_front() {
+			return Ok(msg)
+		}
 		loop {
 			let msg = self.inner.recv().await?;
 			if let Some(msg) = self.message_filter.intercept_incoming(self.inner.sender(), msg) {
@@ -241,6 +245,19 @@ where
 		}
 	}
 
+	async fn recv_signal(&mut self) -> SubsystemResult<Self::Signal> {
+		loop {
+			let msg = self.inner.recv().await?;
+			if let Some(msg) = self.message_filter.intercept_incoming(self.inner.sender(), msg) {
+				if let FromOrchestra::Signal(sig) = msg {
+					return Ok(sig)
+				} else {
+					self.message_buffer.push_back(msg)
+				}
+			}
+		}
+	}
+
 	fn spawn(
 		&mut self,
 		name: &'static str,