Aura: Ensure we are building on each relay chain fork (#5352)

We only want to build one block per slot for Aura on parachains.
However, we still need to build on each relay chain fork, which is using
the same slot.

Closes: https://github.com/paritytech/polkadot-sdk/issues/5349

---------

Co-authored-by: Davide Galassi <davxy@datawok.net>
Co-authored-by: Sebastian Kunert <skunert49@gmail.com>

cumulus/client/consensus/aura/src/collators/basic.rs

@@ -138,6 +138,7 @@ where
 		};
 
 		let mut last_processed_slot = 0;
+		let mut last_relay_chain_block = Default::default();
 
 		while let Some(request) = collation_requests.next().await {
 			macro_rules! reject_with_error {
@@ -215,11 +216,13 @@ where
 			//
 			// Most parachains currently run with 12 seconds slots and thus, they would try to
 			// produce multiple blocks per slot which very likely would fail on chain. Thus, we have
-			// this "hack" to only produce on block per slot.
+			// this "hack" to only produce one block per slot per relay chain fork.
 			//
 			// With https://github.com/paritytech/polkadot-sdk/issues/3168 this implementation will be
 			// obsolete and also the underlying issue will be fixed.
-			if last_processed_slot >= *claim.slot() {
+			if last_processed_slot >= *claim.slot() &&
+				last_relay_chain_block < *relay_parent_header.number()
+			{
 				continue
 			}
 
@@ -261,6 +264,7 @@ where
 			}
 
 			last_processed_slot = *claim.slot();
+			last_relay_chain_block = *relay_parent_header.number();
 		}
 	}
 }

prdoc/pr_5352.prdoc

+title: "Aura: Ensure parachains are building on all relay chain forks"
+
+doc:
+  - audience: Node Dev
+    description: |
+      Ensure that parachains using the `basic` collator are building on all relay chain forks.
+
+crates:
+  - name: cumulus-client-consensus-aura
+    bump: patch