content: /*
   * 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 alluxio.concurrent.jsr;
  
  import java.io.Serializable;
  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.Objects;
  import java.util.RandomAccess;
  import java.util.concurrent.Callable;
  import java.util.concurrent.CancellationException;
  import java.util.concurrent.ExecutionException;
  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;
  
  /**
   * 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 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 java.util.concurrent.RecursiveAction} for most computations that do not return results,
   * {@link java.util.concurrent.RecursiveTask} for those that do, and
   * {@link java9.util.concurrent.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 java.util.concurrent.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 {
    // CVS rev. 1.117
  
    /*
     * 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 (anded with DONE_MASK) 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, mainly by arranging that
     * every synchronized block performs a wait, notifyAll or both.
     *
     * These control bits occupy only (some of) the upper half (16 bits) of status field. The lower
     * bits are used for user-defined tags.
     */
  
    static final int DONE_MASK = 0xf0000000; // mask out non-completion bits
    static final int NORMAL = 0xf0000000; // must be negative
    static final int CANCELLED = 0xc0000000; // must be < NORMAL
    static final int EXCEPTIONAL = 0x80000000; // must be < CANCELLED
    static final int SIGNAL = 0x00010000; // must be >= 1 << 16
    static final int SMASK = 0x0000ffff; // short bits for tags
    /**
     * 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<>();
    private static final long serialVersionUID = -7721805057305804111L;
    // Unsafe mechanics
    private static final sun.misc.Unsafe U = UnsafeAccess.unsafe;
    private static final long STATUS;
  
    static {
      try {
        STATUS = U.objectFieldOffset(ForkJoinTask.class.getDeclaredField("status"));
      } catch (Exception e) {
        throw new ExceptionInInitializerError(e);
      }
    }
  
    /** The run status of this task */
    volatile int status; // accessed directly by pool and workers
  
    // Exception table support
  
    /**
     * Cancels, ignoring any exceptions thrown by cancel. Used during worker and pool shutdown. Cancel
     * is spec'ed not to throw any exceptions, but if it does anyway, we have no recourse during
     * shutdown, so guard against this case.
     */
    static final void cancelIgnoringExceptions(ForkJoinTask<?> t) {
      if (t != null && t.status >= 0) {
        try {
          t.cancel(false);
        } catch (Throwable ignore) {
        }
      }
    }
  
    /**
     * Polls stale refs and removes them. Call only while holding lock.
     */
    private static void expungeStaleExceptions() {
      for (Object x; (x = exceptionTableRefQueue.poll()) != null;) {
        if (x instanceof ExceptionNode) {
          ExceptionNode[] t = exceptionTable;
          int i = ((ExceptionNode) x).hashCode & (t.length - 1);
          ExceptionNode e = t[i];
          ExceptionNode pred = null;
          while (e != null) {
            ExceptionNode next = e.next;
            if (e == x) {
              if (pred == null)
                t[i] = next;
              else
                pred.next = next;
              break;
            }
            pred = e;
            e = next;
          }
        }
      }
    }
  
    /**
     * If lock is available, polls stale refs and removes them. Called from ForkJoinPool when pools
     * become quiescent.
     */
    static final void helpExpungeStaleExceptions() {
      final ReentrantLock lock = exceptionTableLock;
      if (lock.tryLock()) {
        try {
          expungeStaleExceptions();
        } finally {
          lock.unlock();
        }
      }
    }
  
    /**
     * A version of "sneaky throw" to relay exceptions.
     */
    static void rethrow(Throwable ex) {
      ForkJoinTask.<RuntimeException>uncheckedThrow(ex);
    }
  
    /**
     * The sneaky part of sneaky throw, relying on generics limitations to evade compiler complaints
     * about rethrowing unchecked exceptions.
     */
    @SuppressWarnings("unchecked")
    static <T extends Throwable> void uncheckedThrow(Throwable t) throws T {
      if (t != null)
        throw (T) t; // rely on vacuous cast
      else
        throw new Error("Unknown Exception");
    }
  
    /**
     * Forks the given tasks, returning when {@code isDone} holds for each task or an (unchecked)
     * exception is encountered, in which case the exception is rethrown. If more than one task
     * encounters an exception, then this method throws any one of these exceptions. If any task
     * encounters an exception, the other may be cancelled. However, the execution status of
     * individual tasks is not guaranteed upon exceptional return. The status of each task may be
     * obtained using {@link #getException()} and related methods to check if they have been
     * cancelled, completed normally or exceptionally, or left unprocessed.
     *
     * @param t1 the first task
     * @param t2 the second task
     * @throws NullPointerException if any task is null
     */
    public static void invokeAll(ForkJoinTask<?> t1, ForkJoinTask<?> t2) {
      int s1, s2;
      t2.fork();
      if ((s1 = t1.doInvoke() & DONE_MASK) != NORMAL)
        t1.reportException(s1);
      if ((s2 = t2.doJoin() & DONE_MASK) != NORMAL)
        t2.reportException(s2);
    }
  
    /**
     * Forks the given tasks, returning when {@code isDone} holds for each task or an (unchecked)
     * exception is encountered, in which case the exception is rethrown. If more than one task
     * encounters an exception, then this method throws any one of these exceptions. If any task
     * encounters an exception, others may be cancelled. However, the execution status of individual
     * tasks is not guaranteed upon exceptional return. The status of each task may be obtained using
     * {@link #getException()} and related methods to check if they have been cancelled, completed
     * normally or exceptionally, or left unprocessed.
     *
     * @param tasks the tasks
     * @throws NullPointerException if any task is null
     */
    public static void invokeAll(ForkJoinTask<?>... tasks) {
      Throwable ex = null;
      int last = tasks.length - 1;
      for (int i = last; i >= 0; --i) {
        ForkJoinTask<?> t = tasks[i];
        if (t == null) {
          if (ex == null)
            ex = new NullPointerException();
        } else if (i != 0)
          t.fork();
        else if (t.doInvoke() < NORMAL && ex == null)
          ex = t.getException();
      }
      for (int i = 1; i <= last; ++i) {
        ForkJoinTask<?> t = tasks[i];
        if (t != null) {
          if (ex != null)
            t.cancel(false);
          else if (t.doJoin() < NORMAL)
            ex = t.getException();
        }
      }
      if (ex != null)
        rethrow(ex);
    }
  
    /**
     * Forks all tasks in the specified collection, returning when {@code isDone} holds for each task
     * or an (unchecked) exception is encountered, in which case the exception is rethrown. If more
     * than one task encounters an exception, then this method throws any one of these exceptions. If
     * any task encounters an exception, others may be cancelled. However, the execution status of
     * individual tasks is not guaranteed upon exceptional return. The status of each task may be
     * obtained using {@link #getException()} and related methods to check if they have been
     * cancelled, completed normally or exceptionally, or left unprocessed.
     *
     * @param tasks the collection of tasks
     * @param <T> the type of the values returned from the tasks
     * @return the tasks argument, to simplify usage
     * @throws NullPointerException if tasks or any element are null
     */
    public static <T extends ForkJoinTask<?>> Collection<T> invokeAll(Collection<T> tasks) {
      if (!(tasks instanceof RandomAccess) || !(tasks instanceof List<?>)) {
        invokeAll(tasks.toArray(new ForkJoinTask<?>[0]));
        return tasks;
      }
      List<? extends ForkJoinTask<?>> ts = (List<? extends ForkJoinTask<?>>) tasks;
      Throwable ex = null;
      int last = ts.size() - 1;
      for (int i = last; i >= 0; --i) {
        ForkJoinTask<?> t = ts.get(i);
        if (t == null) {
          if (ex == null)
            ex = new NullPointerException();
        } else if (i != 0)
          t.fork();
        else if (t.doInvoke() < NORMAL && ex == null)
          ex = t.getException();
      }
      for (int i = 1; i <= last; ++i) {
        ForkJoinTask<?> t = ts.get(i);
        if (t != null) {
          if (ex != null)
            t.cancel(false);
          else if (t.doJoin() < NORMAL)
            ex = t.getException();
        }
      }
      if (ex != null)
        rethrow(ex);
      return tasks;
    }
  
    /**
     * Possibly executes tasks until the pool hosting the current task
     * {@linkplain ForkJoinPool#isQuiescent is quiescent}. This method may be of use in designs in
     * which many tasks are forked, but none are explicitly joined, instead executing them until all
     * are processed.
     */
    public static void helpQuiesce() {
      Thread t;
      if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
        ForkJoinWorkerThread wt = (ForkJoinWorkerThread) t;
        wt.pool.helpQuiescePool(wt.workQueue);
      } else
        ForkJoinPool.quiesceCommonPool();
    }
  
    /**
     * Returns the pool hosting the current thread, or {@code null} if the current thread is executing
     * outside of any ForkJoinPool.
     *
     * <p>
     * This method returns {@code null} if and only if {@link #inForkJoinPool} returns {@code false}.
     *
     * @return the pool, or {@code null} if none
     */
    public static ForkJoinPool getPool() {
      Thread t = Thread.currentThread();
      return (t instanceof ForkJoinWorkerThread) ? ((ForkJoinWorkerThread) t).pool : null;
    }
  
    /**
     * Returns {@code true} if the current thread is a {@link ForkJoinWorkerThread} executing as a
     * ForkJoinPool computation.
     *
     * @return {@code true} if the current thread is a {@link ForkJoinWorkerThread} executing as a
     *         ForkJoinPool computation, or {@code false} otherwise
     */
    public static boolean inForkJoinPool() {
      return Thread.currentThread() instanceof ForkJoinWorkerThread;
    }
  
    /**
     * Returns an estimate of the number of tasks that have been forked by the current worker thread
     * but not yet executed. This value may be useful for heuristic decisions about whether to fork
     * other tasks.
     *
     * @return the number of tasks
     */
    public static int getQueuedTaskCount() {
      Thread t;
      ForkJoinPool.WorkQueue q;
      if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
        q = ((ForkJoinWorkerThread) t).workQueue;
      else
        q = ForkJoinPool.commonSubmitterQueue();
      return (q == null) ? 0 : q.queueSize();
    }
  
    /**
     * Returns an estimate of how many more locally queued tasks are held by the current worker thread
     * than there are other worker threads that might steal them, or zero if this thread is not
     * operating in a ForkJoinPool. This value may be useful for heuristic decisions about whether to
     * fork other tasks. In many usages of ForkJoinTasks, at steady state, each worker should aim to
     * maintain a small constant surplus (for example, 3) of tasks, and to process computations
     * locally if this threshold is exceeded.
     *
     * @return the surplus number of tasks, which may be negative
     */
    public static int getSurplusQueuedTaskCount() {
      return ForkJoinPool.getSurplusQueuedTaskCount();
    }
  
    /**
     * Returns, but does not unschedule or execute, a task queued by the current thread but not yet
     * executed, if one is immediately available. There is no guarantee that this task will actually
     * be polled or executed next. Conversely, this method may return null even if a task exists but
     * cannot be accessed without contention with other threads. This method is designed primarily to
     * support extensions, and is unlikely to be useful otherwise.
     *
     * @return the next task, or {@code null} if none are available
     */
    protected static ForkJoinTask<?> peekNextLocalTask() {
      Thread t;
      ForkJoinPool.WorkQueue q;
      if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
        q = ((ForkJoinWorkerThread) t).workQueue;
      else
        q = ForkJoinPool.commonSubmitterQueue();
      return (q == null) ? null : q.peek();
    }
  
    /**
     * Unschedules and returns, without executing, the next task queued by the current thread but not
     * yet executed, if the current thread is operating in a ForkJoinPool. This method is designed
     * primarily to support extensions, and is unlikely to be useful otherwise.
     *
     * @return the next task, or {@code null} if none are available
     */
    protected static ForkJoinTask<?> pollNextLocalTask() {
      Thread t;
      return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
          ? ((ForkJoinWorkerThread) t).workQueue.nextLocalTask()
          : null;
    }
  
    // public methods
  
    /**
     * If the current thread is operating in a ForkJoinPool, unschedules and returns, without
     * executing, the next task queued by the current thread but not yet executed, if one is
     * available, or if not available, a task that was forked by some other thread, if available.
     * Availability may be transient, so a {@code null} result does not necessarily imply quiescence
     * of the pool this task is operating in. This method is designed primarily to support extensions,
     * and is unlikely to be useful otherwise.
     *
     * @return a task, or {@code null} if none are available
     */
    protected static ForkJoinTask<?> pollTask() {
      Thread t;
      ForkJoinWorkerThread wt;
      return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
          ? (wt = (ForkJoinWorkerThread) t).pool.nextTaskFor(wt.workQueue)
          : null;
    }
  
    /**
     * If the current thread is operating in a ForkJoinPool, unschedules and returns, without
     * executing, a task externally submitted to the pool, if one is available. Availability may be
     * transient, so a {@code null} result does not necessarily imply quiescence of the pool. This
     * method is designed primarily to support extensions, and is unlikely to be useful otherwise.
     *
     * @return a task, or {@code null} if none are available
     * @since 9
     */
    protected static ForkJoinTask<?> pollSubmission() {
      Thread t;
      return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
          ? ((ForkJoinWorkerThread) t).pool.pollSubmission()
          : null;
    }
  
    /**
     * Returns a new {@code ForkJoinTask} that performs the {@code run} method of the given
     * {@code Runnable} as its action, and returns a null result upon {@link #join}.
     *
     * @param runnable the runnable action
     * @return the task
     */
    public static ForkJoinTask<?> adapt(Runnable runnable) {
      return new AdaptedRunnableAction(runnable);
    }
  
    /**
     * Returns a new {@code ForkJoinTask} that performs the {@code run} method of the given
     * {@code Runnable} as its action, and returns the given result upon {@link #join}.
     *
     * @param runnable the runnable action
     * @param result the result upon completion
     * @param <T> the type of the result
     * @return the task
     */
    public static <T> ForkJoinTask<T> adapt(Runnable runnable, T result) {
      return new AdaptedRunnable<T>(runnable, result);
    }
  
    /**
     * Returns a new {@code ForkJoinTask} that performs the {@code call} method of the given
     * {@code Callable} as its action, and returns its result upon {@link #join}, translating any
     * checked exceptions encountered into {@code RuntimeException}.
     *
     * @param callable the callable action
     * @param <T> the type of the callable's result
     * @return the task
     */
    public static <T> ForkJoinTask<T> adapt(Callable<? extends T> callable) {
      return new AdaptedCallable<T>(callable);
    }
  
    /**
     * Marks completion and wakes up threads waiting to join this task.
     *
     * @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 (U.compareAndSwapInt(this, STATUS, s, s | completion)) {
          if ((s >>> 16) != 0)
            synchronized (this) {
              notifyAll();
            }
          return completion;
        }
      }
    }
  
    /**
     * 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) {
          return setExceptionalCompletion(rex);
        }
        if (completed)
          s = setCompletion(NORMAL);
      }
      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) {
      int s;
      if ((s = status) >= 0 && // force completer to issue notify
          U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
        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 = ((this instanceof CountedCompleter) ? // try helping
          ForkJoinPool.common.externalHelpComplete((CountedCompleter<?>) this, 0)
          : ForkJoinPool.common.tryExternalUnpush(this) ? doExec() : 0);
      if (s >= 0 && (s = status) >= 0) {
        boolean interrupted = false;
        do {
          if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
            synchronized (this) {
              if (status >= 0) {
                try {
                  wait(0L);
                } catch (InterruptedException ie) {
                  interrupted = true;
                }
              } else
                notifyAll();
            }
          }
        } while ((s = status) >= 0);
        if (interrupted)
          Thread.currentThread().interrupt();
      }
      return s;
    }
  
    /**
     * Blocks a non-worker-thread until completion or interruption.
     */
    private int externalInterruptibleAwaitDone() throws InterruptedException {
      int s;
      if (Thread.interrupted())
        throw new InterruptedException();
      if ((s = status) >= 0 && (s = ((this instanceof CountedCompleter)
          ? ForkJoinPool.common.externalHelpComplete((CountedCompleter<?>) this, 0)
          : ForkJoinPool.common.tryExternalUnpush(this) ? doExec() : 0)) >= 0) {
        while ((s = status) >= 0) {
          if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
            synchronized (this) {
              if (status >= 0)
                wait(0L);
              else
                notifyAll();
            }
          }
        }
      }
      return s;
    }
  
    /**
     * 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();
    }
  
    /**
     * 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 = setCompletion(EXCEPTIONAL);
      }
      return s;
    }
  
    /**
     * Records exception and possibly propagates.
     *
     * @return status on exit
     */
    private int setExceptionalCompletion(Throwable ex) {
      int s = recordExceptionalCompletion(ex);
      if ((s & DONE_MASK) == EXCEPTIONAL)
        internalPropagateException(ex);
      return s;
    }
  
    /**
     * Hook for exception propagation support for tasks with completers.
     */
    void internalPropagateException(Throwable ex) {}
  
    /**
     * 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 this 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
     */
    private Throwable getThrowableException() {
      int h = System.identityHashCode(this);
      ExceptionNode e;
      final ReentrantLock lock = exceptionTableLock;
      lock.lock();
      try {
        expungeStaleExceptions();
        ExceptionNode[] t = exceptionTable;
        e = t[h & (t.length - 1)];
        while (e != null && e.get() != this)
          e = e.next;
      } finally {
        lock.unlock();
      }
      Throwable ex;
      if (e == null || (ex = e.ex) == null)
        return null;
      if (e.thrower != Thread.currentThread().getId()) {
        try {
          Constructor<?> noArgCtor = null;
          // public ctors only
          for (Constructor<?> c : ex.getClass().getConstructors()) {
            Class<?>[] ps = c.getParameterTypes();
            if (ps.length == 0)
              noArgCtor = c;
            else if (ps.length == 1 && ps[0] == Throwable.class)
              return (Throwable) c.newInstance(ex);
          }
          if (noArgCtor != null) {
            Throwable wx = (Throwable) noArgCtor.newInstance();
            wx.initCause(ex);
            return wx;
          }
        } catch (Exception ignore) {
        }
      }
      return ex;
    }
  
    /**
     * Throws exception, if any, associated with the given status.
     */
    private void reportException(int s) {
      if (s == CANCELLED)
        throw new CancellationException();
      if (s == EXCEPTIONAL)
        rethrow(getThrowableException());
    }
  
    /**
     * Arranges to asynchronously execute this task in the pool the current task is running in, if
     * applicable, or using the {@link ForkJoinPool#commonPool()} if not {@link #inForkJoinPool}.
     * While it is not necessarily enforced, it is a usage error to fork a task more than once unless
     * it has completed and been reinitialized. Subsequent modifications to the state of this task or
     * any data it operates on are not necessarily consistently observable by any thread other than
     * the one executing it unless preceded by a call to {@link #join} or related methods, or a call
     * to {@link #isDone} returning {@code
     * true}.
     *
     * @return {@code this}, to simplify usage
     */
    public final ForkJoinTask<V> fork() {
      Thread t;
      if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
        ((ForkJoinWorkerThread) t).workQueue.push(this);
      else
        ForkJoinPool.common.externalPush(this);
      return this;
    }
  
    /**
     * Returns the result of the computation when it {@linkplain #isDone is done}. This method differs
     * from {@link #get()} in that abnormal completion results in {@code RuntimeException} or
     * {@code Error}, not {@code ExecutionException}, and that interrupts of the calling thread do
     * <em>not</em> cause the method to abruptly return by throwing {@code InterruptedException}.
     *
     * @return the computed result
     */
    public final V join() {
      int s;
      if ((s = doJoin() & DONE_MASK) != NORMAL)
        reportException(s);
      return getRawResult();
    }
  
    /**
     * Commences performing this task, awaits its completion if necessary, and returns its result, or
     * throws an (unchecked) {@code RuntimeException} or {@code Error} if the underlying computation
     * did so.
     *
     * @return the computed result
     */
    public final V invoke() {
      int s;
      if ((s = doInvoke() & DONE_MASK) != NORMAL)
        reportException(s);
      return getRawResult();
    }
  
    /**
     * Attempts to cancel execution of this task. This attempt will fail if the task has already
     * completed or could not be cancelled for some other reason. If successful, and this task has not
     * started when {@code cancel} is called, execution of this task is suppressed. After this method
     * returns successfully, unless there is an intervening call to {@link #reinitialize}, subsequent
     * calls to {@link #isCancelled}, {@link #isDone}, and {@code cancel} will return {@code true} and
     * calls to {@link #join} and related methods will result in {@code CancellationException}.
     *
     * <p>
     * This method may be overridden in subclasses, but if so, must still ensure that these properties
     * hold. In particular, the {@code cancel} method itself must not throw exceptions.
     *
     * <p>
     * This method is designed to be invoked by <em>other</em> tasks. To terminate the current task,
     * you can just return or throw an unchecked exception from its computation method, or invoke
     * {@link #completeExceptionally(Throwable)}.
     *
     * @param mayInterruptIfRunning this value has no effect in the default implementation because
     *        interrupts are not used to control cancellation.
     *
     * @return {@code true} if this task is now cancelled
     */
    public boolean cancel(boolean mayInterruptIfRunning) {
      return (setCompletion(CANCELLED) & DONE_MASK) == CANCELLED;
    }
  
    public final boolean isDone() {
      return status < 0;
    }
  
    public final boolean isCancelled() {
      return (status & DONE_MASK) == CANCELLED;
    }
  
    /**
     * Returns {@code true} if this task threw an exception or was cancelled.
     *
     * @return {@code true} if this task threw an exception or was cancelled
     */
    public final boolean isCompletedAbnormally() {
      return status < NORMAL;
    }
  
    /**
     * Returns {@code true} if this task completed without throwing an exception and was not
     * cancelled.
     *
     * @return {@code true} if this task completed without throwing an exception and was not cancelled
     */
    public final boolean isCompletedNormally() {
      return (status & DONE_MASK) == NORMAL;
    }
  
    // Extension methods
  
    /**
     * Returns the exception thrown by the base computation, or a {@code CancellationException} if
     * cancelled, or {@code null} if none or if the method has not yet completed.
     *
     * @return the exception, or {@code null} if none
     */
    public final Throwable getException() {
      int s = status & DONE_MASK;
      return ((s >= NORMAL) ? null
          : (s == CANCELLED) ? new CancellationException() : getThrowableException());
    }
  
    /**
     * Completes this task abnormally, and if not already aborted or cancelled, causes it to throw the
     * given exception upon {@code join} and related operations. This method may be used to induce
     * exceptions in asynchronous tasks, or to force completion of tasks that would not otherwise
     * complete. Its use in other situations is discouraged. This method is overridable, but
     * overridden versions must invoke {@code super} implementation to maintain guarantees.
     *
     * @param ex the exception to throw. If this exception is not a {@code RuntimeException} or
     *        {@code Error}, the actual exception thrown will be a {@code RuntimeException} with cause
     *        {@code ex}.
     */
    public void completeExceptionally(Throwable ex) {
      setExceptionalCompletion(
          (ex instanceof RuntimeException) || (ex instanceof Error) ? ex : new RuntimeException(ex));
    }
  
    /**
     * Completes this task, and if not already aborted or cancelled, returning the given value as the
     * result of subsequent invocations of {@code join} and related operations. This method may be
     * used to provide results for asynchronous tasks, or to provide alternative handling for tasks
     * that would not otherwise complete normally. Its use in other situations is discouraged. This
     * method is overridable, but overridden versions must invoke {@code super} implementation to
     * maintain guarantees.
     *
     * @param value the result value for this task
     */
    public void complete(V value) {
      try {
        setRawResult(value);
      } catch (Throwable rex) {
        setExceptionalCompletion(rex);
        return;
      }
      setCompletion(NORMAL);
    }
  
    /**
     * Completes this task normally without setting a value. The most recent value established by
     * {@link #setRawResult} (or {@code
     * null} by default) will be returned as the result of subsequent invocations of {@code join} and
     * related operations.
     *
     * @since 1.8
     */
    public final void quietlyComplete() {
      setCompletion(NORMAL);
    }
  
    /**
     * Waits if necessary for the computation to complete, and then retrieves its result.
     *
     * @return the computed result
     * @throws CancellationException if the computation was cancelled
     * @throws ExecutionException if the computation threw an exception
     * @throws InterruptedException if the current thread is not a member of a ForkJoinPool and was
     *         interrupted while waiting
     */
    public final V get() throws InterruptedException, ExecutionException {
      int s = (Thread.currentThread() instanceof ForkJoinWorkerThread) ? doJoin()
          : externalInterruptibleAwaitDone();
      if ((s &= DONE_MASK) == CANCELLED)
        throw new CancellationException();
      if (s == EXCEPTIONAL)
        throw new ExecutionException(getThrowableException());
      return getRawResult();
    }
  
    /**
     * Waits if necessary for at most the given time for the computation to complete, and then
     * retrieves its result, if available.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @return the computed result
     * @throws CancellationException if the computation was cancelled
     * @throws ExecutionException if the computation threw an exception
     * @throws InterruptedException if the current thread is not a member of a ForkJoinPool and was
     *         interrupted while waiting
     * @throws TimeoutException if the wait timed out
     */
    public final V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
      int s;
      long nanos = unit.toNanos(timeout);
      if (Thread.interrupted())
        throw new InterruptedException();
      if ((s = status) >= 0 && nanos > 0L) {
        long d = System.nanoTime() + nanos;
        long deadline = (d == 0L) ? 1L : d; // avoid 0
        Thread t = Thread.currentThread();
        if (t instanceof ForkJoinWorkerThread) {
          ForkJoinWorkerThread wt = (ForkJoinWorkerThread) t;
          s = wt.pool.awaitJoin(wt.workQueue, this, deadline);
        } else if ((s = ((this instanceof CountedCompleter)
            ? ForkJoinPool.common.externalHelpComplete((CountedCompleter<?>) this, 0)
            : ForkJoinPool.common.tryExternalUnpush(this) ? doExec() : 0)) >= 0) {
          long ns, ms; // measure in nanosecs, but wait in millisecs
          while ((s = status) >= 0 && (ns = deadline - System.nanoTime()) > 0L) {
            if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) > 0L
                && U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
              synchronized (this) {
                if (status >= 0)
                  wait(ms); // OK to throw InterruptedException
                else
                  notifyAll();
              }
            }
          }
        }
      }
      if (s >= 0)
        s = status;
      if ((s &= DONE_MASK) != NORMAL) {
        if (s == CANCELLED)
          throw new CancellationException();
        if (s != EXCEPTIONAL)
          throw new TimeoutException();
        throw new ExecutionException(getThrowableException());
      }
      return getRawResult();
    }
  
    /**
     * Joins this task, without returning its result or throwing its exception. This method may be
     * useful when processing collections of tasks when some have been cancelled or otherwise known to
     * have aborted.
     */
    public final void quietlyJoin() {
      doJoin();
    }
  
    // tag operations
  
    /**
     * Commences performing this task and awaits its completion if necessary, without returning its
     * result or throwing its exception.
     */
    public final void quietlyInvoke() {
      doInvoke();
    }
  
    /**
     * Resets the internal bookkeeping state of this task, allowing a subsequent {@code fork}. This
     * method allows repeated reuse of this task, but only if reuse occurs when this task has either
     * never been forked, or has been forked, then completed and all outstanding joins of this task
     * have also completed. Effects under any other usage conditions are not guaranteed. This method
     * may be useful when executing pre-constructed trees of subtasks in loops.
     *
     * <p>
     * Upon completion of this method, {@code isDone()} reports {@code false}, and
     * {@code getException()} reports {@code
     * null}. However, the value returned by {@code getRawResult} is unaffected. To clear this value,
     * you can invoke {@code
     * setRawResult(null)}.
     */
    public void reinitialize() {
      if ((status & DONE_MASK) == EXCEPTIONAL)
        clearExceptionalCompletion();
      else
        status = 0;
    }
  
    /**
     * Tries to unschedule this task for execution. This method will typically (but is not guaranteed
     * to) succeed if this task is the most recently forked task by the current thread, and has not
     * commenced executing in another thread. This method may be useful when arranging alternative
     * local processing of tasks that could have been, but were not, stolen.
     *
     * @return {@code true} if unforked
     */
    public boolean tryUnfork() {
      Thread t;
      return (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
          ? ((ForkJoinWorkerThread) t).workQueue.tryUnpush(this)
          : ForkJoinPool.common.tryExternalUnpush(this));
    }
  
    /**
     * Returns the result that would be returned by {@link #join}, even if this task completed
     * abnormally, or {@code null} if this task is not known to have been completed. This method is
     * designed to aid debugging, as well as to support extensions. Its use in any other context is
     * discouraged.
     *
     * @return the result, or {@code null} if not completed
     */
    public abstract V getRawResult();
  
    /**
     * Forces the given value to be returned as a result. This method is designed to support
     * extensions, and should not in general be called otherwise.
     *
     * @param value the value
     */
    protected abstract void setRawResult(V value);
  
    /**
     * Immediately performs the base action of this task and returns true if, upon return from this
     * method, this task is guaranteed to have completed normally. This method may return false
     * otherwise, to indicate that this task is not necessarily complete (or is not known to be
     * complete), for example in asynchronous actions that require explicit invocations of completion
     * methods. This method may also throw an (unchecked) exception to indicate abnormal exit. This
     * method is designed to support extensions, and should not in general be called otherwise.
     *
     * @return {@code true} if this task is known to have completed normally
     */
    protected abstract boolean exec();
  
    /**
     * Returns the tag for this task.
     *
     * @return the tag for this task
     * @since 1.8
     */
    public final short getForkJoinTaskTag() {
      return (short) status;
    }
  
    /**
     * Atomically sets the tag value for this task and returns the old value.
     *
     * @param newValue the new tag value
     * @return the previous value of the tag
     * @since 1.8
     */
    public final short setForkJoinTaskTag(short newValue) {
      for (int s;;) {
        if (U.compareAndSwapInt(this, STATUS, s = status, (s & ~SMASK) | (newValue & SMASK)))
          return (short) s;
      }
    }
  
    /**
     * Atomically conditionally sets the tag value for this task. Among other applications, tags can
     * be used as visit markers in tasks operating on graphs, as in methods that check: {@code
     * if (task.compareAndSetForkJoinTaskTag((short)0, (short)1))} before processing, otherwise
     * exiting because the node has already been visited.
     *
     * @param expect the expected tag value
     * @param update the new tag value
     * @return {@code true} if successful; i.e., the current value was equal to {@code expect} and was
     *         changed to {@code update}.
     * @since 1.8
     */
    public final boolean compareAndSetForkJoinTaskTag(short expect, short update) {
      for (int s;;) {
        if ((short) (s = status) != expect)
          return false;
        if (U.compareAndSwapInt(this, STATUS, s, (s & ~SMASK) | (update & SMASK)))
          return true;
      }
    }
  
    /**
     * Saves this task to a stream (that is, serializes it).
     *
     * @param s the stream
     * @throws java.io.IOException if an I/O error occurs
     * @serialData the current run status and the exception thrown during execution, or {@code null}
     *             if none
     */
    private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException {
      s.defaultWriteObject();
      s.writeObject(getException());
    }
  
    // Serialization support
  
    /**
     * Reconstitutes this task from a stream (that is, deserializes it).
     *
     * @param s the stream
     * @throws ClassNotFoundException if the class of a serialized object could not be found
     * @throws java.io.IOException if an I/O error occurs
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
      s.defaultReadObject();
      Object ex = s.readObject();
      if (ex != null)
        setExceptionalCompletion((Throwable) ex);
    }
  
    /**
     * 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;
      final long thrower; // use id not ref to avoid weak cycles
      final int hashCode; // store task hashCode before weak ref disappears
      ExceptionNode next;
  
      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);
      }
    }
  
    /**
     * Adapter for Runnables. This implements RunnableFuture to be compliant with
     * AbstractExecutorService constraints when used in ForkJoinPool.
     */
    static final class AdaptedRunnable<T> extends ForkJoinTask<T> implements RunnableFuture<T> {
      private static final long serialVersionUID = 5232453952276885070L;
      final Runnable runnable;
      T result;
  
      AdaptedRunnable(Runnable runnable, T result) {
        this.runnable = Objects.requireNonNull(runnable);
        this.result = result; // OK to set this even before completion
      }
  
      public final T getRawResult() {
        return result;
      }
  
      public final void setRawResult(T v) {
        result = v;
      }
  
      public final boolean exec() {
        runnable.run();
        return true;
      }
  
      public final void run() {
        invoke();
      }
  
      public String toString() {
        return super.toString() + "[Wrapped task = " + runnable + "]";
      }
    }
  
    /**
     * Adapter for Runnables without results.
     */
    static final class AdaptedRunnableAction extends ForkJoinTask<Void>
        implements RunnableFuture<Void> {
      private static final long serialVersionUID = 5232453952276885070L;
      final Runnable runnable;
  
      AdaptedRunnableAction(Runnable runnable) {
        this.runnable = Objects.requireNonNull(runnable);
      }
  
      public final Void getRawResult() {
        return null;
      }
  
      public final void setRawResult(Void v) {}
  
      public final boolean exec() {
        runnable.run();
        return true;
      }
  
      public final void run() {
        invoke();
      }
  
      public String toString() {
        return super.toString() + "[Wrapped task = " + runnable + "]";
      }
    }
  
    /**
     * Adapter for Runnables in which failure forces worker exception.
     */
    static final class RunnableExecuteAction extends ForkJoinTask<Void> {
      private static final long serialVersionUID = 5232453952276885070L;
      final Runnable runnable;
  
      RunnableExecuteAction(Runnable runnable) {
        this.runnable = Objects.requireNonNull(runnable);
      }
  
      public final Void getRawResult() {
        return null;
      }
  
      public final void setRawResult(Void v) {}
  
      public final boolean exec() {
        runnable.run();
        return true;
      }
  
      void internalPropagateException(Throwable ex) {
        rethrow(ex); // rethrow outside exec() catches.
      }
    }
  
    /**
     * Adapter for Callables.
     */
    static final class AdaptedCallable<T> extends ForkJoinTask<T> implements RunnableFuture<T> {
      private static final long serialVersionUID = 2838392045355241008L;
      final Callable<? extends T> callable;
      T result;
  
      AdaptedCallable(Callable<? extends T> callable) {
        this.callable = Objects.requireNonNull(callable);
      }
  
      public final T getRawResult() {
        return result;
      }
  
      public final void setRawResult(T v) {
        result = v;
      }
  
      public final boolean exec() {
        try {
          result = callable.call();
          return true;
        } catch (RuntimeException rex) {
          throw rex;
        } catch (Exception ex) {
          throw new RuntimeException(ex);
        }
      }
  
      public final void run() {
        invoke();
      }
  
      public String toString() {
        return super.toString() + "[Wrapped task = " + callable + "]";
      }
    }
  }
  