Integration of TOS: Add DirectoryStorage interface.

hadoop-cloud-storage-project/hadoop-tos/src/main/java/org/apache/hadoop/fs/tosfs/object/DirectoryStorage.java

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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.hadoop.fs.tosfs.object;
+
+public interface DirectoryStorage extends ObjectStorage {
+
+  /**
+   * List the objects under the given dir key. Does not guarantee to return the list of objects in a
+   * sorted order.
+   *
+   * @param key       the dir key.
+   * @param recursive indicate whether list dir recursively or not.
+   * @return the sub objects of the given dir key, not include the dir key.
+   */
+  Iterable<ObjectInfo> listDir(String key, boolean recursive);
+
+  /**
+   * The given key could be a file or a directory, if a directory, it can an empty or non-empty
+   * directory.
+   * Deleting a non-empty dir is only allowed if recursive is true.
+   *
+   * @param key       the dir key.
+   * @param recursive indicate whether delete dir recursively or not.
+   */
+  void deleteDir(String key, boolean recursive);
+
+  /**
+   * Whether the directory is empty.
+   *
+   * @param key the dir key.
+   * @return true if the dir is empty or doesn't exist.
+   */
+  boolean isEmptyDir(String key);
+}
+