/*

 * Copyright (c) 2019, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

/**

 * @test

 * @summary enumerate(list,n,recurse) may return 0 if the group is being destroyed,

 *          whereas it should never return a value < n. This lead to inconsistent

 *          results if ThreadGroup::enumerate is called concurrently at the same

 *          time that a child group is being destroyed. This is a race condition,

 *          and this test will not always fail without the fix, but it does fail

 *          often enough.

 * @bug 8219197

 *

 */

import java.util.concurrent.CountDownLatch;

import java.util.concurrent.Semaphore;

import java.util.concurrent.atomic.AtomicInteger;

public class Destroy {

    static final class Task implements Runnable {

        final Semaphore sem;

        final CountDownLatch count;

        public Task(Semaphore sem, CountDownLatch count) {

            this.sem = sem;

            this.count = count;

        }

        @Override

        public void run() {

            try {

                count.countDown();

                sem.acquire();

            } catch (Throwable t) {

                t.printStackTrace();

            } finally {

                System.out.println(Thread.currentThread().getName()

                        + " exiting");

            }

        }

    }

    public static void main(String[] args) throws Exception {

        testDestroyChild();

    }

    public static void testDestroyChild() throws Exception {

        ThreadGroup root = new ThreadGroup("root");

        ThreadGroup parent = new ThreadGroup(root,"parent");

        ThreadGroup child1 = new ThreadGroup(parent, "child1");

        CountDownLatch count = new CountDownLatch(2);

        Semaphore sem1 = new Semaphore(1);

        Semaphore sem2 = new Semaphore(1);

        Thread t1 = new Thread(parent, new Task(sem1, count), "PT1");

        Thread t2 = new Thread(parent, new Task(sem2, count), "PT2");

        sem1.acquire();

        sem2.acquire();

        try {

            t1.start();

            t2.start();

            System.out.println("\nAwaiting parent threads...");

            count.await();

            Thread[] threads = new Thread[2];

            int nb = root.enumerate(threads, true);

            if (nb != 2) {

                throw new AssertionError("wrong number of threads: " + nb);

            }

            Thread t3 = new Thread(child1::destroy, "destroy");

            AtomicInteger nbr = new AtomicInteger();

            Thread t4 = new Thread("enumerate") {

                public void run() {

                    Thread[] threads = new Thread[42];

                    nbr.addAndGet(root.enumerate(threads, true));

                }

            };

            t4.start();

            t3.start();

            t4.join();

            t3.join();

            if (nbr.get() != nb) {

                throw new AssertionError("wrong number of threads: " + nbr.get());

            }

        } finally {

            sem1.release();

            sem2.release();

        }

        t1.join();

        t2.join();

    }

}