node/approval-voting: Address final comments.

node/core/approval-voting/src/backend.rs

@@ -14,7 +14,12 @@
 // You should have received a copy of the GNU General Public License
 // along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
 
+//! An abstraction over storage used by the chain selection subsystem.
 //!
+//! This provides both a [`Backend`] trait and an [`OverlayedBackend`]
+//! struct which allows in-memory changes to be applied on top of a
+//! [`Backend`], maintaining consistency between queries and temporary writes,
+//! before any commit to the underlying storage is made.
 
 use polkadot_node_subsystem::{SubsystemResult};
 use polkadot_primitives::v1::{BlockNumber, CandidateHash, Hash};

node/core/approval-voting/src/ops.rs

@@ -265,14 +265,14 @@ pub fn add_block_entry(
 
 			candidate_entry.block_assignments.insert(
 				entry.block_hash(),
-				ApprovalEntry {
-					tranches: Vec::new(),
+				ApprovalEntry::new(
+					Vec::new(),
 					backing_group,
-					our_assignment: our_assignment.map(|v| v.into()),
-					our_approval_sig: None,
-					assignments: bitvec::bitvec![BitOrderLsb0, u8; 0; n_validators],
-					approved: false,
-				}
+					our_assignment.map(|v| v.into()),
+					None,
+					bitvec::bitvec![BitOrderLsb0, u8; 0; n_validators],
+					false,
+				)
 			);
 
 			store.write_candidate_entry(candidate_entry.clone());

node/core/approval-voting/src/persisted_entries.rs

@@ -76,16 +76,29 @@ impl From<TrancheEntry> for crate::approval_db::v1::TrancheEntry {
 /// particular block.
 #[derive(Debug, Clone, PartialEq)]
 pub struct ApprovalEntry {
-	pub tranches: Vec<TrancheEntry>,
-	pub backing_group: GroupIndex,
-	pub our_assignment: Option<OurAssignment>,
-	pub our_approval_sig: Option<ValidatorSignature>,
+	tranches: Vec<TrancheEntry>,
+	backing_group: GroupIndex,
+	our_assignment: Option<OurAssignment>,
+	our_approval_sig: Option<ValidatorSignature>,
 	// `n_validators` bits.
-	pub assignments: BitVec<BitOrderLsb0, u8>,
-	pub approved: bool,
+	assignments: BitVec<BitOrderLsb0, u8>,
+	approved: bool,
 }
 
 impl ApprovalEntry {
+	/// Convenience constructor
+	pub fn new(
+		tranches: Vec<TrancheEntry>,
+		backing_group: GroupIndex,
+		our_assignment: Option<OurAssignment>,
+		our_approval_sig: Option<ValidatorSignature>,
+		// `n_validators` bits.
+		assignments: BitVec<BitOrderLsb0, u8>,
+		approved: bool,
+	) -> Self {
+		Self { tranches, backing_group, our_assignment, our_approval_sig, assignments, approved }
+	}
+
 	// Access our assignment for this approval entry.
 	pub fn our_assignment(&self) -> Option<&OurAssignment> {
 		self.our_assignment.as_ref()