/*

 * Copyright 2008-2009 Sun Microsystems, Inc.  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.  Sun designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Sun in the LICENSE file that accompanied this code.

 *

 * 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

 * CA 95054 USA or visit www.sun.com if you need additional information or

 * have any questions.

 */

package sun.nio.fs;

import sun.misc.Unsafe;

import java.util.concurrent.ExecutionException;

/**

 * Base implementation of a task (typically native) that polls a memory location

 * during execution so that it may be aborted/cancelled before completion. The

 * task is executed by invoking the {@link runInterruptibly} method defined

 * here and cancelled by invoking Thread.interrupt.

 */

abstract class Cancellable implements Runnable {

    private static final Unsafe unsafe = Unsafe.getUnsafe();

    private final long pollingAddress;

    private final Object lock = new Object();

    // the following require lock when examining or changing

    private boolean completed;

    private Throwable exception;

    protected Cancellable() {

        pollingAddress = unsafe.allocateMemory(4);

        unsafe.putIntVolatile(null, pollingAddress, 0);

    }

    /**

     * Returns the memory address of a 4-byte int that should be polled to

     * detect cancellation.

     */

    protected long addressToPollForCancel() {

        return pollingAddress;

    }

    /**

     * The value to write to the polled memory location to indicate that the

     * task has been cancelled. If this method is not overridden then it

     * defaults to MAX_VALUE.

     */

    protected int cancelValue() {

        return Integer.MAX_VALUE;

    }

    /**

     * "cancels" the task by writing bits into memory location that it polled

     * by the task.

     */

    final void cancel() {

        synchronized (lock) {

            if (!completed) {

                unsafe.putIntVolatile(null, pollingAddress, cancelValue());

            }

        }

    }

    /**

     * Returns the exception thrown by the task or null if the task completed

     * successfully.

     */

    private Throwable exception() {

        synchronized (lock) {

            return exception;

        }

    }

    @Override

    public final void run() {

        try {

            implRun();

        } catch (Throwable t) {

            synchronized (lock) {

                exception = t;

            }

        } finally {

            synchronized (lock) {

                completed = true;

                unsafe.freeMemory(pollingAddress);

            }

        }

    }

    /**

     * The task body. This should periodically poll the memory location

     * to check for cancellation.

     */

    abstract void implRun() throws Throwable;

    /**

     * Invokes the given task in its own thread. If this (meaning the current)

     * thread is interrupted then an attempt is make to cancel the background

     * thread by writing into the memory location that it polls cooperatively.

     */

    static void runInterruptibly(Cancellable task) throws ExecutionException {

        Thread t = new Thread(task);

        t.start();

        boolean cancelledByInterrupt = false;

        while (t.isAlive()) {

            try {

                t.join();

            } catch (InterruptedException e) {

                cancelledByInterrupt = true;

                task.cancel();

            }

        }

        if (cancelledByInterrupt)

            Thread.currentThread().interrupt();

        Throwable exc = task.exception();

        if (exc != null)

            throw new ExecutionException(exc);

    }

}