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From bowenli86 <...@git.apache.org>
Subject [GitHub] flink pull request #4764: [FLINK-7757] [checkpointing] Introduce resource gu...
Date Wed, 04 Oct 2017 06:21:18 GMT
Github user bowenli86 commented on a diff in the pull request:

    https://github.com/apache/flink/pull/4764#discussion_r142588647
  
    --- Diff: flink-core/src/main/java/org/apache/flink/util/ResourceGuard.java ---
    @@ -0,0 +1,129 @@
    +/*
    + * 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.util;
    +
    +import java.io.IOException;
    +import java.io.Serializable;
    +
    +/**
    + * This class is a guard for shared resources with the following invariants. The resource
can be acquired by multiple
    + * clients in parallel through the {@link #acquireResource()} call. Clients must release
the resource after usage
    + * with a matching call to {@link #releaseResource()}. The resource can only be disposed
once the guard is closed, but
    + * the guard can only be closed once all clients that acquired the resource have released
it. Before this is happened,
    + * the call to {@link #close()} will block until the condition is triggered. After the
guard is closed, calls to
    + * {@link #acquireResource()} will fail with exception. Notice that, obviously clients
are responsible to release the
    + * resource after usage. All clients are considered equal, i.e. there is only a global
count maintained how many
    + * times the resource was acquired but not by whom.
    + */
    +public class ResourceGuard implements AutoCloseable, Serializable {
    +
    +	private static final long serialVersionUID = 1L;
    +
    +	/**
    +	 * Introduced additional checks of invariants.
    +	 */
    +	private static final boolean STRICT_CHECKS = true;
    +
    +	/**
    +	 * The object that serves as lock for count and the closed-flag.
    +	 */
    +	private final SerializableObject lock;
    +
    +	/**
    +	 * Number of clients that have ongoing access to the resource.
    +	 */
    +	private int clientCount;
    +
    +	/**
    +	 * This flag indicated if it is still possible to acquire access to the resource.
    +	 */
    +	private volatile boolean closed;
    +
    +	public ResourceGuard() {
    +		this.lock = new SerializableObject();
    +		this.clientCount = 0;
    +		this.closed = false;
    +	}
    +
    +	/**
    +	 * Acquired access from one new client for the guarded resource.
    +	 *
    +	 * @throws IOException when the resource guard is already closed.
    +	 */
    +	public void acquireResource() throws IOException {
    +
    +		synchronized (lock) {
    +
    +			if (closed) {
    +				throw new IOException("Resource guard was already closed.");
    +			}
    +
    +			++clientCount;
    +		}
    +	}
    +
    +	/**
    +	 * Releases access for one client of the guarded resource. This method must only be
called after a matching call to
    +	 * {@link #acquireResource()}.
    +	 */
    +	public void releaseResource() {
    +
    +		synchronized (lock) {
    +
    +			if (STRICT_CHECKS && clientCount <= 0) {
    --- End diff --
    
    What is the `STRICT_CHECKS` used for here? sorry, I haven't used invariant much in promgramming
before


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