Return number of keys removed when calling storage_kill on child trie (#8166)

* Initial piping of returning amount of keys killed

* One more test for `None` limit

* forgot to update

* fix return value

* use version 3

* Update to return `KillOutcome`

* Update name to KillChildStorageResult

substrate/primitives/state-machine/src/basic.rs

@@ -211,9 +211,9 @@ impl Externalities for BasicExternalities {
 		&mut self,
 		child_info: &ChildInfo,
 		_limit: Option<u32>,
-	) -> bool {
-		self.inner.children_default.remove(child_info.storage_key());
-		true
+	) -> (bool, u32) {
+		let num_removed = self.inner.children_default.remove(child_info.storage_key()).map(|c| c.data.len()).unwrap_or(0);
+		(true, num_removed as u32)
 	}
 
 	fn clear_prefix(&mut self, prefix: &[u8]) {
@@ -411,6 +411,29 @@ mod tests {
 		assert_eq!(ext.child_storage(child_info, b"doe"), None);
 	}
 
+	#[test]
+	fn kill_child_storage_returns_num_elements_removed() {
+		let child_info = ChildInfo::new_default(b"storage_key");
+		let child_info = &child_info;
+		let mut ext = BasicExternalities::new(Storage {
+			top: Default::default(),
+			children_default: map![
+				child_info.storage_key().to_vec() => StorageChild {
+					data: map![
+						b"doe".to_vec() => b"reindeer".to_vec(),
+						b"dog".to_vec() => b"puppy".to_vec(),
+						b"hello".to_vec() => b"world".to_vec(),
+					],
+					child_info: child_info.to_owned(),
+				}
+			]
+		});
+
+
+		let res = ext.kill_child_storage(child_info, None);
+		assert_eq!(res, (true, 3));
+	}
+
 	#[test]
 	fn basic_externalities_is_empty() {
 		// Make sure no values are set by default in `BasicExternalities`.

substrate/primitives/state-machine/src/ext.rs

@@ -391,7 +391,7 @@ where
 		&mut self,
 		child_info: &ChildInfo,
 		limit: Option<u32>,
-	) -> bool {
+	) -> (bool, u32) {
 		trace!(target: "state", "{:04x}: KillChild({})",
 			self.id,
 			HexDisplay::from(&child_info.storage_key()),
@@ -399,9 +399,9 @@ where
 		let _guard = guard();
 		self.mark_dirty();
 		self.overlay.clear_child_storage(child_info);
+		let mut num_deleted: u32 = 0;
 
 		if let Some(limit) = limit {
-			let mut num_deleted: u32 = 0;
 			let mut all_deleted = true;
 			self.backend.apply_to_child_keys_while(child_info, |key| {
 				if num_deleted == limit {
@@ -417,13 +417,14 @@ where
 				self.overlay.set_child_storage(child_info, key.to_vec(), None);
 				true
 			});
-			all_deleted
+			(all_deleted, num_deleted)
 		} else {
 			self.backend.apply_to_child_keys_while(child_info, |key| {
+				num_deleted = num_deleted.saturating_add(1);
 				self.overlay.set_child_storage(child_info, key.to_vec(), None);
 				true
 			});
-			true
+			(true, num_deleted)
 		}
 	}
 

substrate/primitives/state-machine/src/lib.rs

@@ -1159,7 +1159,7 @@ mod tests {
 				changes_trie::disabled_state::<_, u64>(),
 				None,
 			);
-			assert_eq!(ext.kill_child_storage(&child_info, Some(2)), false);
+			assert_eq!(ext.kill_child_storage(&child_info, Some(2)), (false, 2));
 		}
 
 		assert_eq!(
@@ -1199,12 +1199,14 @@ mod tests {
 			changes_trie::disabled_state::<_, u64>(),
 			None,
 		);
-		assert_eq!(ext.kill_child_storage(&child_info, Some(0)), false);
-		assert_eq!(ext.kill_child_storage(&child_info, Some(1)), false);
-		assert_eq!(ext.kill_child_storage(&child_info, Some(2)), false);
-		assert_eq!(ext.kill_child_storage(&child_info, Some(3)), false);
-		assert_eq!(ext.kill_child_storage(&child_info, Some(4)), true);
-		assert_eq!(ext.kill_child_storage(&child_info, Some(5)), true);
+		assert_eq!(ext.kill_child_storage(&child_info, Some(0)), (false, 0));
+		assert_eq!(ext.kill_child_storage(&child_info, Some(1)), (false, 1));
+		assert_eq!(ext.kill_child_storage(&child_info, Some(2)), (false, 2));
+		assert_eq!(ext.kill_child_storage(&child_info, Some(3)), (false, 3));
+		assert_eq!(ext.kill_child_storage(&child_info, Some(4)), (true, 4));
+		// Only 4 items to remove
+		assert_eq!(ext.kill_child_storage(&child_info, Some(5)), (true, 4));
+		assert_eq!(ext.kill_child_storage(&child_info, None), (true, 4));
 	}
 
 	#[test]

substrate/primitives/state-machine/src/read_only.rs

@@ -132,7 +132,7 @@ impl<'a, H: Hasher, B: 'a + Backend<H>> Externalities for ReadOnlyExternalities<
 		&mut self,
 		_child_info: &ChildInfo,
 		_limit: Option<u32>,
-	) -> bool {
+	) -> (bool, u32) {
 		unimplemented!("kill_child_storage is not supported in ReadOnlyExternalities")
 	}
 

substrate/primitives/state-machine/src/testing.rs

@@ -326,7 +326,7 @@ mod tests {
 		{
 			let mut ext = ext.ext();
 
-			assert!(!ext.kill_child_storage(&child_info, Some(2)), "Should not delete all keys");
+			assert!(!ext.kill_child_storage(&child_info, Some(2)).0, "Should not delete all keys");
 
 			assert!(ext.child_storage(&child_info, &b"doe"[..]).is_none());
 			assert!(ext.child_storage(&child_info, &b"dog"[..]).is_none());