Github user tillrohrmann commented on a diff in the pull request:

    https://github.com/apache/flink/pull/5239#discussion_r169728016
  
    --- Diff: 
flink-runtime/src/main/java/org/apache/flink/runtime/state/SnapshotDirectory.java
 ---
    @@ -0,0 +1,172 @@
    +/*
    + * 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.flink.runtime.state;
    +
    +import org.apache.flink.core.fs.FileStatus;
    +import org.apache.flink.core.fs.FileSystem;
    +import org.apache.flink.core.fs.Path;
    +
    +import javax.annotation.Nonnull;
    +
    +import java.io.IOException;
    +import java.util.concurrent.atomic.AtomicReference;
    +
    +/**
    + * This class represents a directory that is the target for a state 
snapshot. This class provides some method that
    + * simplify resource management when dealing with such directories, e.g. 
it can produce a {@link DirectoryStateHandle}
    + * when the snapshot is completed and disposal considers whether or not a 
snapshot was already completed. For a
    + * completed snapshot, the ownership for cleanup is transferred to the 
created directory state handle. For incomplete
    + * snapshots, calling {@link #deleteIfIncompleteSnapshot()} will delete 
the underlying directory resource.
    + */
    +public class SnapshotDirectory {
    +
    +   /**
    +    * Lifecycle stages of a snapshot directory.
    +    */
    +   enum State {
    +           ONGOING, COMPLETED, DELETED
    +   }
    +
    +   /** This path describes the underlying directory for the snapshot. */
    +   @Nonnull
    +   private final Path directory;
    +
    +   /** The filesystem that contains the snapshot directory. */
    +   @Nonnull
    +   private final FileSystem fileSystem;
    +
    +   /** This reference tracks the lifecycle state of the snapshot 
directory. */
    +   @Nonnull
    +   private AtomicReference<State> state;
    +
    +   public SnapshotDirectory(@Nonnull Path directory, @Nonnull FileSystem 
fileSystem) {
    +           this.directory = directory;
    +           this.fileSystem = fileSystem;
    +           this.state = new AtomicReference<>(State.ONGOING);
    +   }
    +
    +   public SnapshotDirectory(@Nonnull Path directory) throws IOException {
    +           this(directory, directory.getFileSystem());
    +   }
    +
    +   @Nonnull
    +   public Path getDirectory() {
    +           return directory;
    +   }
    +
    +   public boolean mkdirs() throws IOException {
    +           return fileSystem.mkdirs(directory);
    +   }
    +
    +   @Nonnull
    +   public FileSystem getFileSystem() {
    +           return fileSystem;
    +   }
    +
    +   public boolean exists() throws IOException {
    +           return fileSystem.exists(directory);
    +   }
    +
    +   /**
    +    * List the statuses of the files/directories in the snapshot directory.
    +    *
    +    * @return the statuses of the files/directories in the given path.
    +    * @throws IOException if there is a problem creating the file statuses.
    +    */
    +   public FileStatus[] listStatus() throws IOException {
    +           return fileSystem.listStatus(directory);
    +   }
    +
    +   /**
    +    * Calling this method completes the snapshot into the snapshot 
directory and creates a corresponding
    +    * {@link DirectoryStateHandle} that points to the snapshot directory. 
Calling this method will also change the
    +    * lifecycle state from "ongoing" to "completed". If the state was 
already deleted, an {@link IOException} is
    +    * thrown.
    +    *
    +    * @return a directory state handle that points to the snapshot 
directory.
    +    * @throws IOException if the state of this snapshot directory object 
is different from "ongoing".
    +    */
    +   public DirectoryStateHandle completeSnapshotAndGetHandle() throws 
IOException {
    +           if (state.compareAndSet(State.ONGOING, State.COMPLETED)) {
    +                   return new DirectoryStateHandle(directory, fileSystem);
    +           } else {
    +                   throw new IOException("Expected state " + State.ONGOING 
+ " but found state " + state.get());
    +           }
    +   }
    +
    +   /**
    +    * Calling this method will attempt delete the underlying snapshot 
directory recursively, if the state was
    +    * "ongoing". In this case, the state will be set to "deleted" as a 
result of this call.
    +    *
    +    * @return <code>true</code> if delete is successful, 
<code>false</code> otherwise.
    +    * @throws IOException if an exception happens during the delete.
    +    */
    +   public boolean deleteIfIncompleteSnapshot() throws IOException {
    +           return state.compareAndSet(State.ONGOING, State.DELETED) && 
fileSystem.delete(directory, true);
    +   }
    +
    +   public boolean isSnapshotOngoing() {
    +           return State.ONGOING.equals(state.get());
    +   }
    +
    +   @Override
    +   public boolean equals(Object o) {
    +           if (this == o) {
    +                   return true;
    +           }
    +           if (o == null || getClass() != o.getClass()) {
    +                   return false;
    +           }
    +
    +           SnapshotDirectory that = (SnapshotDirectory) o;
    +
    +           return directory.equals(that.directory);
    +   }
    +
    +   @Override
    +   public int hashCode() {
    +           return directory.hashCode();
    +   }
    +
    +   @Override
    +   public String toString() {
    +           return "SnapshotDirectory{" +
    +                   "directory=" + directory +
    +                   ", state=" + state +
    +                   '}';
    +   }
    +
    +   /**
    +    * Helper method that creates a temporary snapshot directory for the 
given path. This subclass of snapshot directory
    +    * will always return "null" as result of {@link 
#completeSnapshotAndGetHandle()} and can never reach the lifecycle
    +    * state "completed".
    +    */
    +   public static SnapshotDirectory temporary(Path path) throws IOException 
{
    +
    +           return new SnapshotDirectory(path) {
    --- End diff --
    
    Not sure whether an anonymous class is strictly necessary here.


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