Allow capping the amount of work performed when deleting a child trie (#7671)

* Allow Backend::for_keys_in_child_storage to be aborted by the closure

* Ext::kill_child_storage now takes an upper limit for backend deletion

* Add Storage::storage_kill_limited() runtime interface

* review: Use a new version of kill_storage instead of a new interface

* review: Simplify boolean expression

Co-authored-by: cheme <emericchevalier.pro@gmail.com>

* review: Rename for_keys_in_child_storage

Co-authored-by: cheme <emericchevalier.pro@gmail.com>

substrate/primitives/io/src/lib.rs

@@ -279,7 +279,35 @@ pub trait DefaultChildStorage {
 		storage_key: &[u8],
 	) {
 		let child_info = ChildInfo::new_default(storage_key);
-		self.kill_child_storage(&child_info);
+		self.kill_child_storage(&child_info, None);
+	}
+
+	/// Clear a child storage key.
+	///
+	/// Deletes all keys from the overlay and up to `limit` keys from the backend if
+	/// it is set to `Some`. No limit is applied when `limit` is set to `None`.
+	///
+	/// The limit can be used to partially delete a child trie in case it is too large
+	/// to delete in one go (block).
+	///
+	/// It returns false iff some keys are remaining in
+	/// the child trie after the functions returns.
+	///
+	/// # Note
+	///
+	/// Please note that keys that are residing in the overlay for that child trie when
+	/// issuing this call are all deleted without counting towards the `limit`. Only keys
+	/// written during the current block are part of the overlay. Deleting with a `limit`
+	/// mostly makes sense with an empty overlay for that child trie.
+	///
+	/// Calling this function multiple times per block for the same `storage_key` does
+	/// not make much sense because it is not cumulative when called inside the same block.
+	/// Use this function to distribute the deletion of a single child trie across multiple
+	/// blocks.
+	#[version(2)]
+	fn storage_kill(&mut self, storage_key: &[u8], limit: Option<u32>) -> bool {
+		let child_info = ChildInfo::new_default(storage_key);
+		self.kill_child_storage(&child_info, limit)
 	}
 
 	/// Check a child storage key.