/*

 * 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

 * http://creativecommons.org/publicdomain/zero/1.0/

 */

package java.util.concurrent;

import java.io.Serializable;

import java.lang.invoke.MethodHandles;

import java.lang.invoke.VarHandle;

import java.lang.ref.ReferenceQueue;

import java.lang.ref.WeakReference;

import java.lang.reflect.Constructor;

import java.util.Collection;

import java.util.List;

import java.util.RandomAccess;

import java.util.concurrent.locks.ReentrantLock;

/**

 * 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 it is

 * explicitly submitted to a {@link ForkJoinPool}, or, if not already

 * engaged in a ForkJoin computation, commenced in the {@link

 * ForkJoinPool#commonPool()} via {@link #fork}, {@link #invoke}, or

 * related methods.  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 main 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

 * ideally 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. Subdividable tasks should also

 * not perform blocking I/O, and should ideally access variables that

 * are completely independent of those accessed by other running

 * tasks. These guidelines are loosely 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>It is possible to define and use ForkJoinTasks that may block,

 * but doing so requires three further considerations: (1) Completion

 * of few if any <em>other</em> tasks should be dependent on a task

 * that blocks on external synchronization or I/O. Event-style async

 * tasks that are never joined (for example, those subclassing {@link

 * CountedCompleter}) often fall into this category.  (2) To minimize

 * resource impact, tasks should be small; ideally performing only the

 * (possibly) blocking action. (3) Unless the {@link

 * ForkJoinPool.ManagedBlocker} API is used, or the number of possibly

 * blocked tasks is known to be less than the pool's {@link

 * ForkJoinPool#getParallelism} level, the pool cannot guarantee that

 * enough threads will be available to ensure progress or good

 * performance.

 *

 * <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>In the most typical usages, a fork-join pair act like a call

 * (fork) and return (join) from a parallel recursive function. As is

 * the case with other forms of recursive calls, returns (joins)

 * should be performed innermost-first. For example, {@code a.fork();

 * b.fork(); b.join(); a.join();} is likely to be substantially more

 * efficient than joining {@code a} before {@code b}.

 *

 * <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 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 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 most computations that do not return results,

 * {@link RecursiveTask} for those that do, and {@link

 * CountedCompleter} for those in which completed actions trigger

 * other actions.  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.

 *

 * <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. To support such usages, a

 * ForkJoinTask may be atomically <em>tagged</em> with a {@code short}

 * value using {@link #setForkJoinTaskTag} or {@link

 * #compareAndSetForkJoinTaskTag} and checked using {@link

 * #getForkJoinTaskTag}. The ForkJoinTask implementation does not use

 * these {@code protected} methods or tags for any purpose, but they

 * may be of use in the construction of specialized subclasses.  For

 * example, parallel graph traversals can use the supplied methods to

 * avoid revisiting nodes/tasks that have already been processed.

 * (Method names for tagging are bulky in part to encourage definition

 * of methods that reflect their usage patterns.)

 *

 * <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 ensure atomicity.  Status is initially zero, and

     * takes on nonnegative values until completed, upon which it

     * holds (sign bit) DONE, possibly with ABNORMAL (cancelled or

     * exceptional) and THROWN (in which case an exception has been

     * stored). Tasks with dependent blocked waiting joiners have the

     * SIGNAL bit set.  Completion of a task with SIGNAL set awakens

     * any waiters via notifyAll. (Waiters also help signal others

     * upon completion.)

     *

     * These control bits occupy only (some of) the upper half (16

     * bits) of status field. The lower bits are used for user-defined

     * tags.

     */

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

    private static final int DONE     = 1 << 31; // must be negative

    private static final int ABNORMAL = 1 << 18; // set atomically with DONE

    private static final int THROWN   = 1 << 17; // set atomically with ABNORMAL

    private static final int SIGNAL   = 1 << 16; // true if joiner waiting

    private static final int SMASK    = 0xffff;  // short bits for tags

    static boolean isExceptionalStatus(int s) {  // needed by subclasses

        return (s & THROWN) != 0;

    }

    /**

     * Sets DONE status and wakes up threads waiting to join this task.

     *

     * @return status on exit

     */

    private int setDone() {

        int s;

        if (((s = (int)STATUS.getAndBitwiseOr(this, DONE)) & SIGNAL) != 0)

            synchronized (this) { notifyAll(); }

        return s | DONE;

    }

    /**

     * Marks cancelled or exceptional completion unless already done.

     *

     * @param completion must be DONE | ABNORMAL, ORed with THROWN if exceptional

     * @return status on exit

     */

    private int abnormalCompletion(int completion) {

        for (int s, ns;;) {

            if ((s = status) < 0)

                return s;

            else if (STATUS.weakCompareAndSet(this, s, ns = s | completion)) {

                if ((s & SIGNAL) != 0)

                    synchronized (this) { notifyAll(); }

                return ns;

            }

        }

    }

    /**

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

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

     * completion otherwise.

     *

     * @return status on exit from this method

     */

    final int doExec() {

        int s; boolean completed;

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

            try {

                completed = exec();

            } catch (Throwable rex) {

                completed = false;

                s = setExceptionalCompletion(rex);

            }

            if (completed)

                s = setDone();

        }

        return s;

    }

    /**

     * If not done, sets SIGNAL status and performs Object.wait(timeout).

     * This task may or may not be done on exit. Ignores interrupts.

     *

     * @param timeout using Object.wait conventions.

     */

    final void internalWait(long timeout) {

        if ((int)STATUS.getAndBitwiseOr(this, SIGNAL) >= 0) {

            synchronized (this) {

                if (status >= 0)

                    try { wait(timeout); } catch (InterruptedException ie) { }

                else

                    notifyAll();

            }

        }

    }

    /**

     * Blocks a non-worker-thread until completion.

     * @return status upon completion

     */

    private int externalAwaitDone() {

        int s = tryExternalHelp();

        if (s >= 0 && (s = (int)STATUS.getAndBitwiseOr(this, SIGNAL)) >= 0) {

            boolean interrupted = false;

            synchronized (this) {

                for (;;) {

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

                        try {

                            wait(0L);

                        } catch (InterruptedException ie) {

                            interrupted = true;

                        }

                    }

                    else {

                        notifyAll();

                        break;

                    }

                }

            }

            if (interrupted)

                Thread.currentThread().interrupt();

        }

        return s;

    }

    /**

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

     */

    private int externalInterruptibleAwaitDone() throws InterruptedException {

        int s = tryExternalHelp();

        if (s >= 0 && (s = (int)STATUS.getAndBitwiseOr(this, SIGNAL)) >= 0) {

            synchronized (this) {

                for (;;) {

                    if ((s = status) >= 0)

                        wait(0L);

                    else {

                        notifyAll();

                        break;

                    }

                }

            }

        }

        else if (Thread.interrupted())

            throw new InterruptedException();

        return s;

    }

    /**

     * Tries to help with tasks allowed for external callers.

     *

     * @return current status

     */

    private int tryExternalHelp() {

        int s;

        return ((s = status) < 0 ? s:

                (this instanceof CountedCompleter) ?

                ForkJoinPool.common.externalHelpComplete(

                    (CountedCompleter<?>)this, 0) :

                ForkJoinPool.common.tryExternalUnpush(this) ?

                doExec() : 0);

    }

    /**

     * Implementation for join, get, quietlyJoin. Directly handles

     * only cases of already-completed, external wait, and

     * unfork+exec.  Others are relayed to ForkJoinPool.awaitJoin.

     *

     * @return status upon completion

     */

    private int doJoin() {

        int s; Thread t; ForkJoinWorkerThread wt; ForkJoinPool.WorkQueue w;

        return (s = status) < 0 ? s :

            ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?

            (w = (wt = (ForkJoinWorkerThread)t).workQueue).

            tryUnpush(this) && (s = doExec()) < 0 ? s :

            wt.pool.awaitJoin(w, this, 0L) :

            externalAwaitDone();

    }

    /**

     * Implementation for invoke, quietlyInvoke.

     *

     * @return status upon completion

     */

    private int doInvoke() {

        int s; Thread t; ForkJoinWorkerThread wt;

        return (s = doExec()) < 0 ? s :

            ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?

            (wt = (ForkJoinWorkerThread)t).pool.

            awaitJoin(wt.workQueue, this, 0L) :

            externalAwaitDone();

    }

    // Exception table support

    /**

     * Hash 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.

     *

     * The exception table has a fixed capacity.

     */

    private static final ExceptionNode[] exceptionTable

        = new ExceptionNode[32];

    /** Lock protecting access to exceptionTable. */

    private static final ReentrantLock exceptionTableLock

        = new ReentrantLock();

    /** Reference queue of stale exceptionally completed tasks. */

    private static final ReferenceQueue<ForkJoinTask<?>> exceptionTableRefQueue

        = new ReferenceQueue<>();

    /**

     * 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

        final int hashCode;  // store task hashCode before weak ref disappears

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

                      ReferenceQueue<ForkJoinTask<?>> exceptionTableRefQueue) {

            super(task, exceptionTableRefQueue);

            this.ex = ex;

            this.next = next;

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

            this.hashCode = System.identityHashCode(task);

        }

    }

    /**

     * Records exception and sets status.

     *

     * @return status on exit

     */

    final int recordExceptionalCompletion(Throwable ex) {

        int s;

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

            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],

                                                 exceptionTableRefQueue);

                        break;

                    }

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

                        break;

                }

            } finally {

                lock.unlock();

            }

            s = abnormalCompletion(DONE | ABNORMAL | THROWN);

        }

        return s;

    }