Report back when candidate is rejected. (#11463)

We should tell the collator protocol when we reject a candidate, so the
collator can be punished accordingly.

---------

Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: eskimor

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

@@ -2150,6 +2150,11 @@ async fn handle_second_message<Context>(
 			"Not seconding candidate: {}",
 			reason,
 		);
+		ctx.send_message(CollatorProtocolMessage::Invalid(
+			candidate.descriptor().scheduling_parent(),
+			candidate,
+		))
+		.await;
 		return Ok(());
 	}
 

polkadot/node/core/backing/src/tests/mod.rs

@@ -4234,8 +4234,11 @@ fn ambiguous_candidate_rejected_on_second() {
 
 		virtual_overseer.send(FromOrchestra::Communication { msg: second }).await;
 
-		// The candidate should be silently rejected — no validation work issued.
-		assert_matches!(virtual_overseer.recv().timeout(Duration::from_secs(1)).await, None);
+		// The candidate should be rejected and reported as invalid to collator protocol.
+		assert_matches!(
+			virtual_overseer.recv().await,
+			AllMessages::CollatorProtocol(CollatorProtocolMessage::Invalid(_, _))
+		);
 
 		virtual_overseer
 	});
@@ -4408,7 +4411,10 @@ fn version_acceptance_before_and_after_v3_activation_on_second() {
 				},
 			})
 			.await;
-		assert_matches!(virtual_overseer.recv().timeout(Duration::from_secs(1)).await, None);
+		assert_matches!(
+			virtual_overseer.recv().await,
+			AllMessages::CollatorProtocol(CollatorProtocolMessage::Invalid(_, _))
+		);
 
 		// 2. V3 candidate: rejected (V3NotEnabled).
 		let v3_candidate = CommittedCandidateReceipt {
@@ -4440,7 +4446,10 @@ fn version_acceptance_before_and_after_v3_activation_on_second() {
 				},
 			})
 			.await;
-		assert_matches!(virtual_overseer.recv().timeout(Duration::from_secs(1)).await, None);
+		assert_matches!(
+			virtual_overseer.recv().await,
+			AllMessages::CollatorProtocol(CollatorProtocolMessage::Invalid(_, _))
+		);
 
 		// --- Activate V3 ---
 		activate_v3_via_block_finalized(&mut virtual_overseer).await;
@@ -4458,7 +4467,10 @@ fn version_acceptance_before_and_after_v3_activation_on_second() {
 				},
 			})
 			.await;
-		assert_matches!(virtual_overseer.recv().timeout(Duration::from_secs(1)).await, None);
+		assert_matches!(
+			virtual_overseer.recv().await,
+			AllMessages::CollatorProtocol(CollatorProtocolMessage::Invalid(_, _))
+		);
 
 		// 4. V3 candidate: NOW ACCEPTED — passes check_version_acceptance.
 		// It proceeds past the version check into the normal seconding flow.

prdoc/pr_11463.prdoc

@@ -0,0 +1,11 @@
+title: Report back when candidate is rejected.
+doc:
+- audience: Node Dev
+  description: |
+    When the backing subsystem rejects a candidate due to descriptor version
+    acceptance failure, it now sends `CollatorProtocolMessage::Invalid` back to
+    the collator protocol so the collator's reputation is penalized. Previously
+    this case silently dropped the candidate with no feedback.
+crates:
+- name: polkadot-node-core-backing
+  bump: patch