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>)