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etcd3 java client and utilities
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/*
 * Copyright 2017, 2018 IBM Corp. All Rights Reserved.
 *
 * Licensed 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 com.ibm.etcd.client;

import java.util.Queue;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.Executor;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.locks.ReentrantLock;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Delegates submitted tasks to the shared threadpool but ensures they
 * are executed in order and in serial
 * 
 */
public class SerializingExecutor implements Executor {

    private static final Logger logger = LoggerFactory.getLogger(SerializingExecutor.class);

    private final Executor sharedPool;
    private final Queue workQueue;
    private volatile boolean scheduled = false;

    public SerializingExecutor(Executor parentPool) {
        this(parentPool, 0);
    }

    public SerializingExecutor(Executor parentPool, int capacity) {
        if (parentPool == null) {
            throw new NullPointerException();
        }
        this.sharedPool = parentPool;
        this.workQueue = capacity > 0 ? new LinkedBlockingQueue<>(capacity)
                : new ConcurrentLinkedQueue<>();
    }

    protected void logTaskUncheckedException(Throwable t) {
        logger.error("Uncaught task exception: " + t, t);
    }

    @SuppressWarnings("serial")
    class TaskRun extends ReentrantLock implements Runnable {
        @Override
        public void run() {
            try {
                for (;;) {
                    Queue wq = workQueue;
                    Runnable next;
                    if ((next = wq.poll()) == null) {
                        lock();
                        try {
                            scheduled = false;
                            if ((next = wq.poll()) == null) {
                                return;
                            }
                            scheduled = true;
                        } finally {
                            unlock();
                        }
                    }
                    try {
                        next.run();
                    } catch (RuntimeException e) {
                        logTaskUncheckedException(e);
                    }
                }
            } catch (Throwable t) {
                dispatch();
                logTaskUncheckedException(t);
                throw t;
            }
        }
    }

    private final TaskRun runner = new TaskRun();

    @Override
    public void execute(Runnable command) {
        if (!workQueue.offer(command)) {
            throw new RejectedExecutionException("SerializingExecutor work queue full");
        }

        if (!scheduled) {
            boolean doit = false;
            runner.lock();
            try {
                if (!scheduled) {
                    scheduled = true;
                    doit = true;
                }
            } finally {
                runner.unlock();
            }
            if (doit) {
                dispatch();
            }
        }
    }

    private void dispatch() {
        boolean ok = false;
        try {
            sharedPool.execute(runner);
            ok = true;
        } finally {
            if (!ok) {
                runner.lock();
                try {
                    scheduled = false; // bad situation
                } finally {
                    runner.unlock();
                }
            }
        }
    }

}