Check unbounded channel first when polling for subsystem mesages (#5566)

* Prefer unbounded channel when selecting rx events

* Fix tests

* Forgotten fmt recursion

* Extract strategy functor to allow easier modifications

polkadot/node/orchestra/proc-macro/src/impl_builder.rs

@@ -605,9 +605,10 @@ pub(crate) fn impl_builder(info: &OrchestraInfo) -> proc_macro2::TokenStream {
 					};
 
 					let unbounded_meter = #channel_name_unbounded_rx.meter().clone();
-
-					let message_rx: SubsystemIncomingMessages< #consumes > = #support_crate ::select(
-						#channel_name_rx, #channel_name_unbounded_rx
+					// Prefer unbounded channel when selecting
+					let message_rx: SubsystemIncomingMessages< #consumes > = #support_crate ::select_with_strategy(
+						#channel_name_rx, #channel_name_unbounded_rx,
+						#support_crate ::select_message_channel_strategy
 					);
 					let (signal_tx, signal_rx) = #support_crate ::metered::channel(
 						self.signal_capacity.unwrap_or(SIGNAL_CHANNEL_CAPACITY)

polkadot/node/orchestra/src/lib.rs

@@ -76,7 +76,7 @@ pub use futures::{
 	channel::{mpsc, oneshot},
 	future::{BoxFuture, Fuse, Future},
 	poll, select,
-	stream::{self, select, FuturesUnordered},
+	stream::{self, select, select_with_strategy, FuturesUnordered, PollNext},
 	task::{Context, Poll},
 	FutureExt, StreamExt,
 };
@@ -203,10 +203,17 @@ pub fn make_packet<T>(signals_received: usize, message: T) -> MessagePacket<T> {
 	MessagePacket { signals_received, message }
 }
 
+/// A functor to specify strategy of the channels selection in the `SubsystemIncomingMessages`
+pub fn select_message_channel_strategy(_: &mut ()) -> PollNext {
+	PollNext::Right
+}
+
 /// Incoming messages from both the bounded and unbounded channel.
-pub type SubsystemIncomingMessages<M> = self::stream::Select<
+pub type SubsystemIncomingMessages<M> = self::stream::SelectWithStrategy<
 	self::metered::MeteredReceiver<MessagePacket<M>>,
 	self::metered::UnboundedMeteredReceiver<MessagePacket<M>>,
+	fn(&mut ()) -> self::stream::PollNext,
+	(),
 >;
 
 /// Watermark to track the received signals.

polkadot/node/overseer/src/tests.rs

@@ -1129,7 +1129,11 @@ fn context_holds_onto_message_until_enough_signals_received() {
 
 	let mut ctx = OverseerSubsystemContext::new(
 		signal_rx,
-		stream::select(bounded_rx, unbounded_rx),
+		stream::select_with_strategy(
+			bounded_rx,
+			unbounded_rx,
+			orchestra::select_message_channel_strategy,
+		),
 		channels_out,
 		to_overseer_tx,
 		"test",