Async backing: minor fixes

node/core/backing/src/lib.rs

@@ -1494,7 +1494,7 @@ async fn import_statement<Context>(
 	// we need to create an entry in the `PerCandidateState` map.
 	//
 	// If the relay parent supports prospective parachains, we also need
-	// to inform the prospective parachains subsystem of the seconded candidate
+	// to inform the prospective parachains subsystem of the seconded candidate.
 	// If `ProspectiveParachainsMessage::Second` fails, then we return
 	// Error::RejectedByProspectiveParachains.
 	//

node/core/prospective-parachains/src/fragment_tree.rs

@@ -381,8 +381,8 @@ impl Scope {
 	}
 }
 
-// We use indices into a flat vector to refer to nodes in the tree.
-// Every tree also has an implicit root.
+/// We use indices into a flat vector to refer to nodes in the tree.
+/// Every tree also has an implicit root.
 #[derive(Debug, Clone, Copy, PartialEq)]
 enum NodePointer {
 	Root,
@@ -618,8 +618,8 @@ impl FragmentTree {
 		let max_depth = self.scope.max_depth;
 		let mut depths = bitvec![u16, Msb0; 0; max_depth + 1];
 
-		// iterate over all nodes < max_depth where parent head-data matches,
-		// relay-parent number is <= candidate, and depth < max_depth.
+		// iterate over all nodes where parent head-data matches,
+		// relay-parent number is <= candidate, and depth <= max_depth.
 		let node_pointers = (0..self.nodes.len()).map(NodePointer::Storage);
 		for parent_pointer in std::iter::once(NodePointer::Root).chain(node_pointers) {
 			let (modifications, child_depth, earliest_rp) = match parent_pointer {
@@ -665,46 +665,46 @@ impl FragmentTree {
 				};
 
 			let parent_head_hash = candidate.parent_head_data_hash();
-			if parent_head_hash == child_constraints.required_parent.hash() {
-				// We do additional checks for complete candidates.
-				if let HypotheticalCandidate::Complete {
-					ref receipt,
-					ref persisted_validation_data,
-				} = candidate
-				{
-					let prospective_candidate = ProspectiveCandidate {
-						commitments: Cow::Borrowed(&receipt.commitments),
-						collator: receipt.descriptor().collator.clone(),
-						collator_signature: receipt.descriptor().signature.clone(),
-						persisted_validation_data: persisted_validation_data.as_ref().clone(),
-						pov_hash: receipt.descriptor().pov_hash,
-						validation_code_hash: receipt.descriptor().validation_code_hash,
-					};
+			if parent_head_hash != child_constraints.required_parent.hash() {
+				continue
+			}
 
-					if Fragment::new(
-						candidate_relay_parent.clone(),
-						child_constraints,
-						prospective_candidate,
-					)
-					.is_err()
-					{
-						continue
-					}
-				}
+			// We do additional checks for complete candidates.
+			if let HypotheticalCandidate::Complete { ref receipt, ref persisted_validation_data } =
+				candidate
+			{
+				let prospective_candidate = ProspectiveCandidate {
+					commitments: Cow::Borrowed(&receipt.commitments),
+					collator: receipt.descriptor().collator.clone(),
+					collator_signature: receipt.descriptor().signature.clone(),
+					persisted_validation_data: persisted_validation_data.as_ref().clone(),
+					pov_hash: receipt.descriptor().pov_hash,
+					validation_code_hash: receipt.descriptor().validation_code_hash,
+				};
 
-				// Check that the path only contains backed candidates, if necessary.
-				if !backed_in_path_only ||
-					self.path_contains_backed_only_candidates(parent_pointer, candidate_storage)
+				if Fragment::new(
+					candidate_relay_parent.clone(),
+					child_constraints,
+					prospective_candidate,
+				)
+				.is_err()
 				{
-					depths.set(child_depth, true);
+					continue
 				}
 			}
+
+			// Check that the path only contains backed candidates, if necessary.
+			if !backed_in_path_only ||
+				self.path_contains_backed_only_candidates(parent_pointer, candidate_storage)
+			{
+				depths.set(child_depth, true);
+			}
 		}
 
 		depths.iter_ones().collect()
 	}
 
-	/// Select a candidate after the given `required_path` which pass
+	/// Select a candidate after the given `required_path` which passes
 	/// the predicate.
 	///
 	/// If there are multiple possibilities, this will select the first one.
@@ -1160,6 +1160,7 @@ mod tests {
 		assert!(storage.head_data_by_hash(&output_head_hash).is_none());
 	}
 
+	// [`FragmentTree::populate`] should pick up candidates that build on other candidates.
 	#[test]
 	fn populate_works_recursively() {
 		let mut storage = CandidateStorage::new();

node/subsystem-types/src/messages.rs

@@ -529,7 +529,7 @@ pub enum ChainApiMessage {
 	/// Request the last finalized block number.
 	/// This request always succeeds.
 	FinalizedBlockNumber(ChainApiResponseChannel<BlockNumber>),
-	/// Request the `k` ancestors block hashes of a block with the given hash.
+	/// Request the `k` ancestor block hashes of a block with the given hash.
 	/// The response channel may return a `Vec` of size up to `k`
 	/// filled with ancestors hashes with the following order:
 	/// `parent`, `grandparent`, ... up to the hash of genesis block

node/subsystem-util/src/backing_implicit_view.rs

@@ -115,7 +115,7 @@ impl View {
 	///
 	/// This returns a list of para-ids which are relevant to the leaf,
 	/// and the allowed relay parents for these paras under this leaf can be
-	/// queried with [`known_allowed_relay_parents_under`].
+	/// queried with [`View::known_allowed_relay_parents_under`].
 	///
 	/// No-op for known leaves.
 	pub async fn activate_leaf<Sender>(

node/subsystem-util/src/inclusion_emulator/staging.rs

@@ -71,19 +71,20 @@
 //!
 //! ### Pruning Fragment Trees
 //!
-//! When the relay-chain advances, we want to compare the new constraints
-//!    of that relay-parent to the roots of the fragment trees we have. There are 3 cases.
+//! When the relay-chain advances, we want to compare the new constraints of that relay-parent to
+//! the roots of the fragment trees we have. There are 3 cases:
 //!
-//! 1. The root fragment is still valid under the new constraints. In this case, we do nothing.
-//!    This is the "prediction still uncertain" case.
-//! 2. The root fragment is invalid under the new constraints because it has been subsumed by the relay-chain.
-//!    in this case, we can discard the root and split & re-root the fragment tree
-//!    under its descendents and compare to the new constraints again.
-//!    This is the "prediction came true" case.
-//! 3. The root fragment is invalid under the new constraints because a competing parachain block has been included
-//!    or it would never be accepted for some other reason. In this case we can discard the entire
-//!    fragment tree.
-//!    This is the "prediction came false" case.
+//! 1. The root fragment is still valid under the new constraints. In this case, we do nothing. This
+//! is the "prediction still uncertain" case.
+//!
+//! 2. The root fragment is invalid under the new constraints because it has been subsumed by the
+//! relay-chain. In this case, we can discard the root and split & re-root the fragment tree under
+//! its descendents and compare to the new constraints again. This is the "prediction came true"
+//! case.
+//!
+//! 3. The root fragment is invalid under the new constraints because a competing parachain block
+//! has been included or it would never be accepted for some other reason. In this case we can
+//! discard the entire fragment tree. This is the "prediction came false" case.
 //!
 //! This is all a bit of a simplification because it assumes that the relay-chain advances without
 //! forks and is finalized instantly. In practice, the set of fragment-trees needs to be observable

roadmap/implementers-guide/src/node/backing/statement-distribution.md

@@ -22,8 +22,7 @@ reasons. As a result, all validators must have a up to date view of all possible
 parachain candidates + backing statements that could be placed on-chain in the
 next block.
 
-[This blog
-post](https://polkadot.network/blog/polkadot-v1-0-sharding-and-economic-security)
+[This blog post](https://polkadot.network/blog/polkadot-v1-0-sharding-and-economic-security)
 puts it another way: "Validators who aren't assigned to the parachain still
 listen for the attestations [statements] because whichever validator ends up
 being the author of the relay-chain block needs to bundle up attested parachain
@@ -155,11 +154,11 @@ backing subsystem itself.
 - Note that requesting is not an implicit acknowledgement, and an explicit
   acknowledgement must be sent upon receipt.
 
-## Statement distribution messages
+## Messages
 
-### Input
+### Incoming
 
-- `ActiveLeavesUpdate`
+- `ActiveLeaves`
   - Notification of a change in the set of active leaves.
 - `StatementDistributionMessage::Share`
   - Notification of a locally-originating statement. That is, this statement
@@ -186,7 +185,7 @@ backing subsystem itself.
     - Acknowledgement.
     - Handled by `handle_incoming_acknowledgement`
 
-### Output
+### Outgoing
 
 - `NetworkBridgeTxMessage::SendValidationMessages`
   - Sends a peer all pending messages / acknowledgements / statements for a
@@ -202,7 +201,7 @@ backing subsystem itself.
   - Gets the hypothetical frontier membership of candidates under active leaves'
     fragment trees.
 - `NetworkBridgeTxMessage::SendRequests`
-  - Sends requests, initiating request/response protocol.
+  - Sends requests, initiating the request/response protocol.
 
 ## Request/Response