/*

 * 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.  Oracle designates this

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

 * by Oracle 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 */

/*

 * This file is available under and governed by the GNU General Public

 * License version 2 only, as published by the Free Software Foundation.

 * However, the following notice accompanied the original version of this

 * file:

 *

 * Written by Doug Lea with assistance from members of JCP JSR-166

 * Expert Group and released to the public domain, as explained at

 */

package java.util.concurrent;

import java.io.Serializable;

import java.util.Collection;

import java.util.Collections;

import java.util.List;

import java.util.RandomAccess;

import java.util.Map;

import java.lang.ref.WeakReference;

import java.lang.ref.ReferenceQueue;

import java.util.concurrent.Callable;

import java.util.concurrent.CancellationException;

import java.util.concurrent.ExecutionException;

import java.util.concurrent.Executor;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Future;

import java.util.concurrent.RejectedExecutionException;

import java.util.concurrent.RunnableFuture;

import java.util.concurrent.TimeUnit;

import java.util.concurrent.TimeoutException;

import java.util.concurrent.locks.ReentrantLock;

import java.lang.reflect.Constructor;

/**

 * Abstract base class for tasks that run within a {@link ForkJoinPool}.

 * A {@code ForkJoinTask} is a thread-like entity that is much

 * lighter weight than a normal thread.  Huge numbers of tasks and

 * subtasks may be hosted by a small number of actual threads in a

 * ForkJoinPool, at the price of some usage limitations.

 *

 * <p>A "main" {@code ForkJoinTask} begins execution when submitted

 * to a {@link ForkJoinPool}.  Once started, it will usually in turn

 * start other subtasks.  As indicated by the name of this class,

 * many programs using {@code ForkJoinTask} employ only methods

 * {@link #fork} and {@link #join}, or derivatives such as {@link

 * #invokeAll(ForkJoinTask...) invokeAll}.  However, this class also

 * provides a number of other methods that can come into play in

 * advanced usages, as well as extension mechanics that allow

 * support of new forms of fork/join processing.

 *

 * <p>A {@code ForkJoinTask} is a lightweight form of {@link Future}.

 * The efficiency of {@code ForkJoinTask}s stems from a set of

 * restrictions (that are only partially statically enforceable)

 * reflecting their intended use as computational tasks calculating

 * pure functions or operating on purely isolated objects.  The

 * primary coordination mechanisms are {@link #fork}, that arranges

 * asynchronous execution, and {@link #join}, that doesn't proceed

 * until the task's result has been computed.  Computations should

 * avoid {@code synchronized} methods or blocks, and should minimize

 * other blocking synchronization apart from joining other tasks or

 * using synchronizers such as Phasers that are advertised to

 * cooperate with fork/join scheduling. Tasks should also not perform

 * blocking IO, and should ideally access variables that are

 * completely independent of those accessed by other running

 * tasks. Minor breaches of these restrictions, for example using

 * shared output streams, may be tolerable in practice, but frequent

 * use may result in poor performance, and the potential to

 * indefinitely stall if the number of threads not waiting for IO or

 * other external synchronization becomes exhausted. This usage

 * restriction is in part enforced by not permitting checked

 * exceptions such as {@code IOExceptions} to be thrown. However,

 * computations may still encounter unchecked exceptions, that are

 * rethrown to callers attempting to join them. These exceptions may

 * additionally include {@link RejectedExecutionException} stemming

 * from internal resource exhaustion, such as failure to allocate

 * internal task queues. Rethrown exceptions behave in the same way as

 * regular exceptions, but, when possible, contain stack traces (as

 * displayed for example using {@code ex.printStackTrace()}) of both

 * the thread that initiated the computation as well as the thread

 * actually encountering the exception; minimally only the latter.

 *

 * <p>The primary method for awaiting completion and extracting

 * results of a task is {@link #join}, but there are several variants:

 * The {@link Future#get} methods support interruptible and/or timed

 * waits for completion and report results using {@code Future}

 * conventions. Method {@link #invoke} is semantically

 * equivalent to {@code fork(); join()} but always attempts to begin

 * execution in the current thread. The "<em>quiet</em>" forms of

 * these methods do not extract results or report exceptions. These

 * may be useful when a set of tasks are being executed, and you need

 * to delay processing of results or exceptions until all complete.

 * Method {@code invokeAll} (available in multiple versions)

 * performs the most common form of parallel invocation: forking a set

 * of tasks and joining them all.

 *

 * <p>The execution status of tasks may be queried at several levels

 * of detail: {@link #isDone} is true if a task completed in any way

 * (including the case where a task was cancelled without executing);

 * {@link #isCompletedNormally} is true if a task completed without

 * cancellation or encountering an exception; {@link #isCancelled} is

 * true if the task was cancelled (in which case {@link #getException}

 * returns a {@link java.util.concurrent.CancellationException}); and

 * {@link #isCompletedAbnormally} is true if a task was either

 * cancelled or encountered an exception, in which case {@link

 * #getException} will return either the encountered exception or

 * {@link java.util.concurrent.CancellationException}.

 *

 * <p>The ForkJoinTask class is not usually directly subclassed.

 * Instead, you subclass one of the abstract classes that support a

 * particular style of fork/join processing, typically {@link

 * RecursiveAction} for computations that do not return results, or

 * {@link RecursiveTask} for those that do.  Normally, a concrete

 * ForkJoinTask subclass declares fields comprising its parameters,

 * established in a constructor, and then defines a {@code compute}

 * method that somehow uses the control methods supplied by this base

 * class. While these methods have {@code public} access (to allow

 * instances of different task subclasses to call each other's

 * methods), some of them may only be called from within other

 * ForkJoinTasks (as may be determined using method {@link

 * #inForkJoinPool}).  Attempts to invoke them in other contexts

 * result in exceptions or errors, possibly including

 * {@code ClassCastException}.

 *

 * <p>Method {@link #join} and its variants are appropriate for use

 * only when completion dependencies are acyclic; that is, the

 * parallel computation can be described as a directed acyclic graph

 * (DAG). Otherwise, executions may encounter a form of deadlock as

 * tasks cyclically wait for each other.  However, this framework

 * supports other methods and techniques (for example the use of

 * {@link Phaser}, {@link #helpQuiesce}, and {@link #complete}) that

 * may be of use in constructing custom subclasses for problems that

 * are not statically structured as DAGs.

 *

 * <p>Most base support methods are {@code final}, to prevent

 * overriding of implementations that are intrinsically tied to the

 * underlying lightweight task scheduling framework.  Developers

 * creating new basic styles of fork/join processing should minimally

 * implement {@code protected} methods {@link #exec}, {@link

 * #setRawResult}, and {@link #getRawResult}, while also introducing

 * an abstract computational method that can be implemented in its

 * subclasses, possibly relying on other {@code protected} methods

 * provided by this class.

 *

 * <p>ForkJoinTasks should perform relatively small amounts of

 * computation. Large tasks should be split into smaller subtasks,

 * usually via recursive decomposition. As a very rough rule of thumb,

 * a task should perform more than 100 and less than 10000 basic

 * computational steps, and should avoid indefinite looping. If tasks

 * are too big, then parallelism cannot improve throughput. If too

 * small, then memory and internal task maintenance overhead may

 * overwhelm processing.

 *

 * <p>This class provides {@code adapt} methods for {@link Runnable}

 * and {@link Callable}, that may be of use when mixing execution of

 * {@code ForkJoinTasks} with other kinds of tasks. When all tasks are

 * of this form, consider using a pool constructed in <em>asyncMode</em>.

 *

 * <p>ForkJoinTasks are {@code Serializable}, which enables them to be

 * used in extensions such as remote execution frameworks. It is

 * sensible to serialize tasks only before or after, but not during,

 * execution. Serialization is not relied on during execution itself.

 *

 * @since 1.7

 * @author Doug Lea

 */

public abstract class ForkJoinTask<V> implements Future<V>, Serializable {

    /*

     * See the internal documentation of class ForkJoinPool for a

     * general implementation overview.  ForkJoinTasks are mainly

     * responsible for maintaining their "status" field amidst relays

     * to methods in ForkJoinWorkerThread and ForkJoinPool. The

     * methods of this class are more-or-less layered into (1) basic

     * status maintenance (2) execution and awaiting completion (3)

     * user-level methods that additionally report results. This is

     * sometimes hard to see because this file orders exported methods

     * in a way that flows well in javadocs.

     */

    /*

     * The status field holds run control status bits packed into a

     * single int to minimize footprint and to ensure atomicity (via

     * CAS).  Status is initially zero, and takes on nonnegative

     * values until completed, upon which status holds value

     * NORMAL, CANCELLED, or EXCEPTIONAL. Tasks undergoing blocking

     * waits by other threads have the SIGNAL bit set.  Completion of

     * a stolen task with SIGNAL set awakens any waiters via

     * notifyAll. Even though suboptimal for some purposes, we use

     * basic builtin wait/notify to take advantage of "monitor

     * inflation" in JVMs that we would otherwise need to emulate to

     * avoid adding further per-task bookkeeping overhead.  We want

     * these monitors to be "fat", i.e., not use biasing or thin-lock

     * techniques, so use some odd coding idioms that tend to avoid

     * them.

     */

    /** The run status of this task */

    volatile int status; // accessed directly by pool and workers

    private static final int NORMAL      = -1;

    private static final int CANCELLED   = -2;

    private static final int EXCEPTIONAL = -3;

    private static final int SIGNAL      =  1;

    /**

     * Marks completion and wakes up threads waiting to join this task,

     * also clearing signal request bits.

     *

     * @param completion one of NORMAL, CANCELLED, EXCEPTIONAL

     * @return completion status on exit

     */

    private int setCompletion(int completion) {

        for (int s;;) {

            if ((s = status) < 0)

                return s;

            if (UNSAFE.compareAndSwapInt(this, statusOffset, s, completion)) {

                if (s != 0)

                    synchronized (this) { notifyAll(); }

                return completion;

            }

        }

    }

    /**

     * Tries to block a worker thread until completed or timed out.

     * Uses Object.wait time argument conventions.

     * May fail on contention or interrupt.

     *

     * @param millis if > 0, wait time.

     */

    final void tryAwaitDone(long millis) {

        int s;

        try {

            if (((s = status) > 0 ||

                 (s == 0 &&

                  UNSAFE.compareAndSwapInt(this, statusOffset, 0, SIGNAL))) &&

                status > 0) {

                synchronized (this) {

                    if (status > 0)

                        wait(millis);

                }

            }

        } catch (InterruptedException ie) {

            // caller must check termination

        }

    }

    /**

     * Blocks a non-worker-thread until completion.

     * @return status upon completion

     */

    private int externalAwaitDone() {

        int s;

        if ((s = status) >= 0) {

            boolean interrupted = false;

            synchronized (this) {

                while ((s = status) >= 0) {

                    if (s == 0)

                        UNSAFE.compareAndSwapInt(this, statusOffset,

                                                 0, SIGNAL);

                    else {

                        try {

                            wait();

                        } catch (InterruptedException ie) {

                            interrupted = true;

                        }

                    }

                }

            }

            if (interrupted)

                Thread.currentThread().interrupt();

        }

        return s;

    }

    /**

     * Blocks a non-worker-thread until completion or interruption or timeout.

     */

    private int externalInterruptibleAwaitDone(long millis)

        throws InterruptedException {

        int s;

        if (Thread.interrupted())

            throw new InterruptedException();

        if ((s = status) >= 0) {

            synchronized (this) {

                while ((s = status) >= 0) {

                    if (s == 0)

                        UNSAFE.compareAndSwapInt(this, statusOffset,

                                                 0, SIGNAL);

                    else {

                        wait(millis);

                        if (millis > 0L)

                            break;

                    }

                }

            }

        }

        return s;

    }

    /**

     * Primary execution method for stolen tasks. Unless done, calls

     * exec and records status if completed, but doesn't wait for

     * completion otherwise.

     */

    final void doExec() {

        if (status >= 0) {

            boolean completed;

            try {

                completed = exec();

            } catch (Throwable rex) {

                setExceptionalCompletion(rex);

                return;

            }

            if (completed)

                setCompletion(NORMAL); // must be outside try block

        }

    }

    /**

     * Primary mechanics for join, get, quietlyJoin.

     * @return status upon completion

     */

    private int doJoin() {

        Thread t; ForkJoinWorkerThread w; int s; boolean completed;

        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {

            if ((s = status) < 0)

                return s;

            if ((w = (ForkJoinWorkerThread)t).unpushTask(this)) {

                try {

                    completed = exec();

                } catch (Throwable rex) {

                    return setExceptionalCompletion(rex);

                }

                if (completed)

                    return setCompletion(NORMAL);

            }

            return w.joinTask(this);

        }

        else

            return externalAwaitDone();

    }

    /**

     * Primary mechanics for invoke, quietlyInvoke.

     * @return status upon completion

     */

    private int doInvoke() {

        int s; boolean completed;

        if ((s = status) < 0)

            return s;

        try {

            completed = exec();

        } catch (Throwable rex) {

            return setExceptionalCompletion(rex);

        }

        if (completed)

            return setCompletion(NORMAL);

        else

            return doJoin();

    }

    // Exception table support

    /**

     * Table of exceptions thrown by tasks, to enable reporting by

     * callers. Because exceptions are rare, we don't directly keep

     * them with task objects, but instead use a weak ref table.  Note

     * that cancellation exceptions don't appear in the table, but are

     * instead recorded as status values.

     *

     * Note: These statics are initialized below in static block.

     */

    private static final ExceptionNode[] exceptionTable;

    private static final ReentrantLock exceptionTableLock;

    private static final ReferenceQueue<Object> exceptionTableRefQueue;

    /**

     * Fixed capacity for exceptionTable.

     */

    private static final int EXCEPTION_MAP_CAPACITY = 32;

    /**

     * Key-value nodes for exception table.  The chained hash table

     * uses identity comparisons, full locking, and weak references

     * for keys. The table has a fixed capacity because it only

     * maintains task exceptions long enough for joiners to access

     * them, so should never become very large for sustained

     * periods. However, since we do not know when the last joiner

     * completes, we must use weak references and expunge them. We do

     * so on each operation (hence full locking). Also, some thread in

     * any ForkJoinPool will call helpExpungeStaleExceptions when its

     * pool becomes isQuiescent.

     */

    static final class ExceptionNode extends WeakReference<ForkJoinTask<?>>{

        final Throwable ex;

        ExceptionNode next;

        final long thrower;  // use id not ref to avoid weak cycles

        ExceptionNode(ForkJoinTask<?> task, Throwable ex, ExceptionNode next) {

            super(task, exceptionTableRefQueue);

            this.ex = ex;

            this.next = next;

            this.thrower = Thread.currentThread().getId();

        }

    }

    /**

     * Records exception and sets exceptional completion.

     *

     * @return status on exit

     */

    private int setExceptionalCompletion(Throwable ex) {

        int h = System.identityHashCode(this);

        final ReentrantLock lock = exceptionTableLock;

        lock.lock();

        try {

            expungeStaleExceptions();

            ExceptionNode[] t = exceptionTable;

            int i = h & (t.length - 1);

            for (ExceptionNode e = t[i]; ; e = e.next) {

                if (e == null) {

                    t[i] = new ExceptionNode(this, ex, t[i]);

                    break;

                }

                if (e.get() == this) // already present

                    break;

            }

        } finally {

            lock.unlock();

        }

        return setCompletion(EXCEPTIONAL);

    }

    /**

     * Removes exception node and clears status

     */

    private void clearExceptionalCompletion() {

        int h = System.identityHashCode(this);

        final ReentrantLock lock = exceptionTableLock;

        lock.lock();

        try {

            ExceptionNode[] t = exceptionTable;

            int i = h & (t.length - 1);

            ExceptionNode e = t[i];

            ExceptionNode pred = null;

            while (e != null) {

                ExceptionNode next = e.next;

                if (e.get() == this) {

                    if (pred == null)

                        t[i] = next;

                    else

                        pred.next = next;

                    break;

                }

                pred = e;

                e = next;

            }

            expungeStaleExceptions();

            status = 0;

        } finally {

            lock.unlock();

        }

    }

    /**

     * Returns a rethrowable exception for the given task, if

     * available. To provide accurate stack traces, if the exception

     * was not thrown by the current thread, we try to create a new

     * exception of the same type as the one thrown, but with the

     * recorded exception as its cause. If there is no such

     * constructor, we instead try to use a no-arg constructor,

     * followed by initCause, to the same effect. If none of these

     * apply, or any fail due to other exceptions, we return the

     * recorded exception, which is still correct, although it may

     * contain a misleading stack trace.

     *

     * @return the exception, or null if none