proposer: return optional block (#2834)

This opens up the proposer to only optionally create blocks. Nodes may
only make blocks when there are transactions or the chain is scheduled.

---------

Co-authored-by: command-bot <>

cumulus/client/consensus/aura/src/collator.rs

@@ -172,12 +172,14 @@ where
 		inherent_data: (ParachainInherentData, InherentData),
 		proposal_duration: Duration,
 		max_pov_size: usize,
-	) -> Result<(Collation, ParachainBlockData<Block>, Block::Hash), Box<dyn Error + Send + 'static>>
-	{
+	) -> Result<
+		Option<(Collation, ParachainBlockData<Block>, Block::Hash)>,
+		Box<dyn Error + Send + 'static>,
+	> {
 		let mut digest = additional_pre_digest.into().unwrap_or_default();
 		digest.push(slot_claim.pre_digest.clone());
 
-		let proposal = self
+		let maybe_proposal = self
 			.proposer
 			.propose(
 				&parent_header,
@@ -190,6 +192,11 @@ where
 			.await
 			.map_err(|e| Box::new(e) as Box<dyn Error + Send>)?;
 
+		let proposal = match maybe_proposal {
+			None => return Ok(None),
+			Some(p) => p,
+		};
+
 		let sealed_importable = seal::<_, P>(
 			proposal.block,
 			proposal.storage_changes,
@@ -234,7 +241,7 @@ where
 				);
 			}
 
-			Ok((collation, block_data, post_hash))
+			Ok(Some((collation, block_data, post_hash)))
 		} else {
 			Err(Box::<dyn Error + Send + Sync>::from("Unable to produce collation")
 				as Box<dyn Error + Send>)

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

@@ -203,7 +203,7 @@ where
 					.await
 			);
 
-			let (collation, _, post_hash) = try_request!(
+			let maybe_collation = try_request!(
 				collator
 					.collate(
 						&parent_header,
@@ -220,8 +220,14 @@ where
 					.await
 			);
 
-			let result_sender = Some(collator.collator_service().announce_with_barrier(post_hash));
-			request.complete(Some(CollationResult { collation, result_sender }));
+			if let Some((collation, _, post_hash)) = maybe_collation {
+				let result_sender =
+					Some(collator.collator_service().announce_with_barrier(post_hash));
+				request.complete(Some(CollationResult { collation, result_sender }));
+			} else {
+				request.complete(None);
+				tracing::debug!(target: crate::LOG_TARGET, "No block proposal");
+			}
 		}
 	}
 }

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

@@ -359,7 +359,7 @@ where
 					)
 					.await
 				{
-					Ok((collation, block_data, new_block_hash)) => {
+					Ok(Some((collation, block_data, new_block_hash))) => {
 						// Here we are assuming that the import logic protects against equivocations
 						// and provides sybil-resistance, as it should.
 						collator.collator_service().announce_block(new_block_hash, None);
@@ -387,6 +387,10 @@ where
 						parent_hash = new_block_hash;
 						parent_header = block_data.into_header();
 					},
+					Ok(None) => {
+						tracing::debug!(target: crate::LOG_TARGET, "No block proposal");
+						break
+					},
 					Err(err) => {
 						tracing::error!(target: crate::LOG_TARGET, ?err);
 						break