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/core/pvf/src/host.rs

@@ -60,6 +60,9 @@ pub const PREPARE_BINARY_NAME: &str = "polkadot-prepare-worker";
 /// The name of binary spawned to execute a PVF
 pub const EXECUTE_BINARY_NAME: &str = "polkadot-execute-worker";
 
+/// The size of incoming message queue
+pub const HOST_MESSAGE_QUEUE_SIZE: usize = 10;
+
 /// An alias to not spell the type for the oneshot sender for the PVF execution result.
 pub(crate) type ResultSender = oneshot::Sender<Result<ValidationResult, ValidationError>>;
 
@@ -227,7 +230,7 @@ pub async fn start(
 		Err(err) => return Err(SubsystemError::Context(err)),
 	};
 
-	let (to_host_tx, to_host_rx) = mpsc::channel(10);
+	let (to_host_tx, to_host_rx) = mpsc::channel(HOST_MESSAGE_QUEUE_SIZE);
 
 	let validation_host = ValidationHost { to_host_tx, security_status: security_status.clone() };
 

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

@@ -104,7 +104,10 @@ mod worker_interface;
 pub mod testing;
 
 pub use error::{InvalidCandidate, PossiblyInvalidError, ValidationError};
-pub use host::{start, Config, ValidationHost, EXECUTE_BINARY_NAME, PREPARE_BINARY_NAME};
+pub use host::{
+	start, Config, ValidationHost, EXECUTE_BINARY_NAME, HOST_MESSAGE_QUEUE_SIZE,
+	PREPARE_BINARY_NAME,
+};
 pub use metrics::Metrics;
 pub use priority::Priority;
 pub use worker_interface::{framed_recv, framed_send, JOB_TIMEOUT_WALL_CLOCK_FACTOR};