prospective-parachains: respond with multiple backable candidates (#3160)

Fixes https://github.com/paritytech/polkadot-sdk/issues/3129

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

@@ -806,15 +806,18 @@ async fn get_backable_candidate(
 ) -> Result<Option<(CandidateHash, Hash)>, Error> {
 	let (tx, rx) = oneshot::channel();
 	sender
-		.send_message(ProspectiveParachainsMessage::GetBackableCandidate(
+		.send_message(ProspectiveParachainsMessage::GetBackableCandidates(
 			relay_parent,
 			para_id,
+			1, // core count hardcoded to 1, until elastic scaling is implemented and enabled.
 			required_path,
 			tx,
 		))
 		.await;
 
-	rx.await.map_err(Error::CanceledBackableCandidate)
+	rx.await
+		.map_err(Error::CanceledBackableCandidate)
+		.map(|res| res.get(0).copied())
 }
 
 /// The availability bitfield for a given core is the transpose

polkadot/node/core/provisioner/src/tests.rs

@@ -373,13 +373,18 @@ mod select_candidates {
 					let _ = sender.send(response);
 				},
 				AllMessages::ProspectiveParachains(
-					ProspectiveParachainsMessage::GetBackableCandidate(.., tx),
-				) => match prospective_parachains_mode {
-					ProspectiveParachainsMode::Enabled { .. } => {
-						let _ = tx.send(candidates_iter.next());
-					},
-					ProspectiveParachainsMode::Disabled =>
-						panic!("unexpected prospective parachains request"),
+					ProspectiveParachainsMessage::GetBackableCandidates(_, _, count, _, tx),
+				) => {
+					assert_eq!(count, 1);
+
+					match prospective_parachains_mode {
+						ProspectiveParachainsMode::Enabled { .. } => {
+							let _ =
+								tx.send(candidates_iter.next().map_or_else(Vec::new, |c| vec![c]));
+						},
+						ProspectiveParachainsMode::Disabled =>
+							panic!("unexpected prospective parachains request"),
+					}
 				},
 				_ => panic!("Unexpected message: {:?}", from_job),
 			}