chainHead/storage: Fix storage iteration using the query key (#1665)

This PR ensures that all storage keys under a prefix are returned by the
`chainHead_storage` method.

Before this PR, the `storage_keys` was used with just the `start_key`.
Before the pagination event was generated, the last reported key
`last_key` was saved internally.
When the pagination is resumed, the `last_key` will serve as the next
`start_key` to the `storage_keys` API.

However, this behavior does not function properly for non-prefixed
storage keys.

Entry keys `a`, `ab`, `abc` share a common prefix and therefore the `ab`
key leads to `abc`.
However, for `a`, `ab`, `aB` and `abc`, the `aB` key does not
immediately lead to `abc`.

To mitigate this, the PR saves the start key of the query, together with
the next pagination key.
Improve testing to ensure we have a key entry that doesn't share the
prefix with the descendant key.

@paritytech/subxt-team

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

substrate/client/rpc-spec-v2/src/chain_head/chain_head_storage.rs

@@ -71,8 +71,10 @@ impl<Client, Block, BE> ChainHeadStorage<Client, Block, BE> {
 
 /// Query to iterate over storage.
 struct QueryIter {
-	/// The next key from which the iteration should continue.
-	next_key: StorageKey,
+	/// The key from which the iteration was started.
+	query_key: StorageKey,
+	/// The key after which pagination should resume.
+	pagination_start_key: Option<StorageKey>,
 	/// The type of the query (either value or hash).
 	ty: IterQueryType,
 }
@@ -184,20 +186,27 @@ where
 		hash: Block::Hash,
 		child_key: Option<&ChildInfo>,
 	) -> QueryIterResult {
-		let QueryIter { next_key, ty } = query;
+		let QueryIter { ty, query_key, pagination_start_key } = query;
 
 		let mut keys_iter = if let Some(child_key) = child_key {
-			self.client
-				.child_storage_keys(hash, child_key.to_owned(), Some(&next_key), None)
+			self.client.child_storage_keys(
+				hash,
+				child_key.to_owned(),
+				Some(&query_key),
+				pagination_start_key.as_ref(),
+			)
 		} else {
-			self.client.storage_keys(hash, Some(&next_key), None)
+			self.client.storage_keys(hash, Some(&query_key), pagination_start_key.as_ref())
 		}
 		.map_err(|err| err.to_string())?;
 
 		let mut ret = Vec::with_capacity(self.operation_max_storage_items);
+		let mut next_pagination_key = None;
 		for _ in 0..self.operation_max_storage_items {
 			let Some(key) = keys_iter.next() else { break };
 
+			next_pagination_key = Some(key.clone());
+
 			let result = match ty {
 				IterQueryType::Value => self.query_storage_value(hash, &key, child_key),
 				IterQueryType::Hash => self.query_storage_hash(hash, &key, child_key),
@@ -209,7 +218,11 @@ where
 		}
 
 		// Save the next key if any to continue the iteration.
-		let maybe_next_query = keys_iter.next().map(|next_key| QueryIter { next_key, ty });
+		let maybe_next_query = keys_iter.next().map(|_| QueryIter {
+			ty,
+			query_key,
+			pagination_start_key: next_pagination_key,
+		});
 		Ok((ret, maybe_next_query))
 	}
 
@@ -325,12 +338,16 @@ where
 							return
 						},
 					},
-				StorageQueryType::DescendantsValues => self
-					.iter_operations
-					.push_back(QueryIter { next_key: item.key, ty: IterQueryType::Value }),
-				StorageQueryType::DescendantsHashes => self
-					.iter_operations
-					.push_back(QueryIter { next_key: item.key, ty: IterQueryType::Hash }),
+				StorageQueryType::DescendantsValues => self.iter_operations.push_back(QueryIter {
+					query_key: item.key,
+					ty: IterQueryType::Value,
+					pagination_start_key: None,
+				}),
+				StorageQueryType::DescendantsHashes => self.iter_operations.push_back(QueryIter {
+					query_key: item.key,
+					ty: IterQueryType::Hash,
+					pagination_start_key: None,
+				}),
 			};
 		}
 

substrate/client/rpc-spec-v2/src/chain_head/tests.rs

@@ -2352,6 +2352,7 @@ async fn check_continue_operation() {
 	builder.push_storage_change(b":m".to_vec(), Some(b"a".to_vec())).unwrap();
 	builder.push_storage_change(b":mo".to_vec(), Some(b"ab".to_vec())).unwrap();
 	builder.push_storage_change(b":moc".to_vec(), Some(b"abc".to_vec())).unwrap();
+	builder.push_storage_change(b":moD".to_vec(), Some(b"abcmoD".to_vec())).unwrap();
 	builder.push_storage_change(b":mock".to_vec(), Some(b"abcd".to_vec())).unwrap();
 	let block = builder.build().unwrap().block;
 	let block_hash = format!("{:?}", block.header.hash());
@@ -2430,6 +2431,25 @@ async fn check_continue_operation() {
 			res.items[0].result == StorageResultType::Value(hex_string(b"ab"))
 	);
 
+	// Pagination event.
+	assert_matches!(
+		get_next_event::<FollowEvent<String>>(&mut sub).await,
+		FollowEvent::OperationWaitingForContinue(res) if res.operation_id == operation_id
+	);
+	does_not_produce_event::<FollowEvent<String>>(
+		&mut sub,
+		std::time::Duration::from_secs(DOES_NOT_PRODUCE_EVENTS_SECONDS),
+	)
+	.await;
+	let _res: () = api.call("chainHead_unstable_continue", [&sub_id, &operation_id]).await.unwrap();
+	assert_matches!(
+		get_next_event::<FollowEvent<String>>(&mut sub).await,
+		FollowEvent::OperationStorageItems(res) if res.operation_id == operation_id &&
+			res.items.len() == 1 &&
+			res.items[0].key == hex_string(b":moD") &&
+			res.items[0].result == StorageResultType::Value(hex_string(b"abcmoD"))
+	);
+
 	// Pagination event.
 	assert_matches!(
 		get_next_event::<FollowEvent<String>>(&mut sub).await,