util/concurrent/ForkJoinTask.java

/*
 * 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.LockSupport;

/**
 * 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.
     *
     * Revision notes: The use of "Aux" field replaces previous
     * reliance on a table to hold exceptions and synchronized blocks
     * and monitors to wait for completion.
     */

    /**
     * Nodes for threads waiting for completion, or holding a thrown
     * exception (never both). Waiting threads prepend nodes
     * Treiber-stack-style.  Signallers detach and unpark
     * waiters. Cancelled waiters try to unsplice.
     */
    static final class Aux {
        final Thread thread;
        final Throwable ex;  // null if a waiter
        Aux next;            // accessed only via memory-acquire chains
        Aux(Thread thread, Throwable ex) {
            this.thread = thread;
            this.ex = ex;
        }
        final boolean casNext(Aux c, Aux v) { // used only in cancellation
            return NEXT.compareAndSet(this, c, v);
        }
        private static final VarHandle NEXT;
        static {
            try {
                NEXT = MethodHandles.lookup()
                    .findVarHandle(Aux.class, "next", Aux.class);
            } catch (ReflectiveOperationException e) {
                throw new ExceptionInInitializerError(e);
            }
        }
    }

    /*
     * The status field holds 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).  These control
     * bits occupy only (some of) the upper half (16 bits) of status
     * field. The lower bits are used for user-defined tags.
     */
    private static final int DONE     = 1 << 31; // must be negative
    private static final int ABNORMAL = 1 << 16; // set atomically with DONE
    private static final int THROWN   = 1 << 17; // set atomically with ABNORMAL
    private static final int SMASK    = 0xffff;  // short bits for tags
    // sentinels can be any positive upper half value:
    private static final int INTRPT   = 1 << 16; // awaitDone interrupt return
    static final         int ADJUST   = 1 << 16; // uncompensate after block

    // Fields
    volatile int status;                // accessed directly by pool and workers
    private transient volatile Aux aux; // either waiters or thrown Exception
    // Support for atomic operations
    private static final VarHandle STATUS;
    private static final VarHandle AUX;
    private int getAndBitwiseOrStatus(int v) {
        return (int)STATUS.getAndBitwiseOr(this, v);
    }
    private boolean casStatus(int c, int v) {
        return STATUS.weakCompareAndSet(this, c, v);
    }
    private boolean casAux(Aux c, Aux v) {
        return AUX.compareAndSet(this, c, v);
    }

    /** Removes and unparks waiters */
    private void signalWaiters() {
        for (Aux a; (a = aux) != null && a.ex == null; ) {
            if (casAux(a, null)) {             // detach entire list
                for (Thread t; a != null; a = a.next) {
                    if ((t = a.thread) != Thread.currentThread() && t != null)
                        LockSupport.unpark(t); // don't self-signal
                }
                break;
            }
        }
    }

    /**
     * Possibly blocks until task is done or interrupted or timed out.
     *
     * @param interruptible true if wait can be cancelled by interrupt
     * @param deadline if non-zero use timed waits and possibly timeout
     * @param pool if nonull, pool to uncompensate when unblocking
     * @return status on exit, or INTRPT if interrupted while waiting
     */
    final int awaitDone(boolean interruptible, long deadline,
                        ForkJoinPool pool) {
        int s; Aux node = null; boolean interrupted = false, queued = false;
        for (;;) {
            Aux a; long nanos;
            if ((s = status) < 0)
                break;
            else if (node == null)
                node = new Aux(Thread.currentThread(), null);
            else if (!queued) {
                if ((a = aux) != null && a.ex != null)
                    Thread.onSpinWait(); // exception in progress
                else if (queued = casAux(node.next = a, node))
                    LockSupport.setCurrentBlocker(this);
            }
            else {
                if (deadline == 0L)
                    LockSupport.park();
                else if ((nanos = deadline - System.nanoTime()) > 0L)
                    LockSupport.parkNanos(nanos);
                else {
                    s = 0;               // timeout
                    break;
                }
                if ((interrupted |= Thread.interrupted()) && interruptible) {
                    s = INTRPT;
                    break;
                }
            }
        }
        if (pool != null)
            pool.uncompensate();
        if (s >= 0) {                     // try to unsplice after cancellation
            outer: for (Aux a; (a = aux) != null && a.ex == null; ) {
                for (Aux trail = null;;) {
                    Aux next = a.next;
                    if (a == node) {
                        if (trail != null)
                            trail.casNext(trail, next);
                        else if (casAux(a, next))
                            break outer; // cannot be re-encountered
                        break;           // restart
                    } else {
                        trail = a;
                        if ((a = next) == null)
                            break outer;
                    }
                }
            }
        }
        else if (interrupted)
            Thread.currentThread().interrupt();
        if (queued) {
            LockSupport.setCurrentBlocker(null);
            signalWaiters();             // help clean or signal
        }
        return s;
    }

    /**
     * Sets DONE status and wakes up threads waiting to join this task.
     * @return status on exit
     */
    private int setDone() {
        int s = getAndBitwiseOrStatus(DONE) | DONE;
        signalWaiters();
        return s;
    }

    /**
     * Sets ABNORMAL DONE status unless already done, and wakes up threads
     * waiting to join this task.
     * @return status on exit
     */
    private int trySetCancelled() {
        int s;
        do {} while ((s = status) >= 0 && !casStatus(s, s |= (DONE | ABNORMAL)));
        signalWaiters();
        return s;
    }

    /**
     * Records exception and sets ABNORMAL THROWN DONE status unless
     * already done, and wakes up threads waiting to join this task.
     * If losing a race with setDone or trySetCancelled, the exception
     * may be recorded but not reported.
     *
     * @return status on exit
     */
    final int trySetThrown(Throwable ex) {
        Aux h = new Aux(Thread.currentThread(), ex), p = null;
        boolean installed = false;
        int s;
        while ((s = status) >= 0) {
            Aux a;
            if (!installed && ((a = aux) == null || a.ex == null) &&
                (installed = casAux(a, h)))
                p = a; // list of waiters replaced by h
            if (installed && casStatus(s, s |= (DONE | ABNORMAL | THROWN)))
                break;
        }
        for (; p != null; p = p.next)
            LockSupport.unpark(p.thread);
        return s;
    }

    /**
     * Records exception unless already done. Overridable in subclasses.
     *
     * @return status on exit
     */
    int trySetException(Throwable ex) {
        return trySetThrown(ex);
    }

    static boolean isExceptionalStatus(int s) {  // needed by subclasses
        return (s & THROWN) != 0;
    }

    /**
     * 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) {
                s = trySetException(rex);
                completed = false;
            }
            if (completed)
                s = setDone();
        }
        return s;
    }

    /**
     * Helps and/or waits for completion. Overridable in subclasses.
     *
     * @param interruptible true if wait can be cancelled by interrupt
     * @param ran true if task known to be invoked
     * @return status on exit, or INTRPT if interruptible and interrupted
     */
    int awaitJoin(boolean interruptible, boolean ran) {
        Thread t; ForkJoinWorkerThread wt;
        ForkJoinPool.WorkQueue q = null;
        ForkJoinPool p = null;
        boolean unforked = false;
        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
            p = (wt = (ForkJoinWorkerThread)t).pool;
            q = wt.workQueue;
            if (!ran && q != null && q.tryRemove(this))
                unforked = true;
        }
        else if (!ran && (q = ForkJoinPool.commonQueue()) != null &&
                 q.externalTryUnpush(this))
            unforked = true;
        int s;
        if (unforked && (s = doExec()) < 0)
            return s;
        if (p != null) {
            if ((s = p.helpJoin(this, q)) < 0)
                return s;
            if (s != ADJUST) // uncompensated
                p = null;
        }
        return awaitDone(interruptible, 0L, p);
    }

    /**
     * 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) {
            try {
                t.cancel(false);
            } catch (Throwable ignore) {
            }
        }
    }

    /**
     * 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() {
        Throwable ex; Aux a;
        if ((a = aux) == null)
            ex = null;
        else if ((ex = a.ex) != null && a.thread != Thread.currentThread()) {
            try {
                Constructor<?> noArgCtor = null, oneArgCtor = null;
                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) {
                        oneArgCtor = c;
                        break;
                    }
                }
                if (oneArgCtor != null)
                    ex = (Throwable)oneArgCtor.newInstance(ex);
                else if (noArgCtor != null) {
                    Throwable rx = (Throwable)noArgCtor.newInstance();
                    rx.initCause(ex);
                    ex = rx;
                }
            } catch (Exception ignore) {
            }
        }
        return ex;
    }

    /**
     * Throws exception associated with the given status, or
     * CancellationException if none recorded.
     */
    private void reportException(int s) {
        ForkJoinTask.<RuntimeException>uncheckedThrow(
            (s & THROWN) != 0 ? getThrowableException() : null);
    }

    /**
     * A version of "sneaky throw" to relay exceptions in other
     * contexts.
     */
    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. If argument null, throws
     * CancellationException.
     */
    @SuppressWarnings("unchecked") static <T extends Throwable>
    void uncheckedThrow(Throwable t) throws T {
        if (t == null)
            t = new CancellationException();
        throw (T)t; // rely on vacuous cast
    }

    // public methods

    /**
     * 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; ForkJoinWorkerThread w;
        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
            (w = (ForkJoinWorkerThread)t).workQueue.push(this, w.pool);
        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 = status) >= 0)
            s = awaitJoin(false, false);
        if ((s & ABNORMAL) != 0)
            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 = doExec()) >= 0)
            s = awaitJoin(false, true);
        if ((s & ABNORMAL) != 0)
            reportException(s);
        return getRawResult();
    }

    /**
     * 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;
        if (t1 == null || t2 == null)
            throw new NullPointerException();
        t2.fork();
        if ((s1 = t1.doExec()) >= 0)
            s1 = t1.awaitJoin(false, true);
        if ((s1 & ABNORMAL) != 0) {
            t2.cancel(false);
            t1.reportException(s1);
        }
        else {
            if ((s2 = t2.status) >= 0)
                s2 = t2.awaitJoin(false, false);
            if ((s2 & ABNORMAL) != 0)
                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, s; i >= 0; --i) {
            ForkJoinTask<?> t;
            if ((t = tasks[i]) == null) {
                ex = new NullPointerException();
                break;
            }
            if (i == 0) {
                if ((s = t.doExec()) >= 0)
                    s = t.awaitJoin(false, true);
                if ((s & ABNORMAL) != 0)
                    ex = t.getException();
                break;
            }
            t.fork();
        }
        if (ex == null) {
            for (int i = 1, s; i <= last; ++i) {
                ForkJoinTask<?> t;
                if ((t = tasks[i]) != null) {
                    if ((s = t.status) >= 0)
                        s = t.awaitJoin(false, false);
                    if ((s & ABNORMAL) != 0) {
                        ex = t.getException();
                        break;
                    }
                }
            }
        }
        if (ex != null) { // try to cancel others
            for (int i = 0, s; i <= last; ++i) {
                ForkJoinTask<?> t;
                if ((t = tasks[i]) != null)
                    t.cancel(false);
            }
            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;
        }
        @SuppressWarnings("unchecked")
        List<? extends ForkJoinTask<?>> ts =
            (List<? extends ForkJoinTask<?>>) tasks;
        Throwable ex = null;
        int last = ts.size() - 1;  // nearly same as array version
        for (int i = last, s; i >= 0; --i) {
            ForkJoinTask<?> t;
            if ((t = ts.get(i)) == null) {
                ex = new NullPointerException();
                break;
            }
            if (i == 0) {
                if ((s = t.doExec()) >= 0)
                    s = t.awaitJoin(false, true);
                if ((s & ABNORMAL) != 0)
                    ex = t.getException();
                break;
            }
            t.fork();
        }
        if (ex == null) {
            for (int i = 1, s; i <= last; ++i) {
                ForkJoinTask<?> t;
                if ((t = ts.get(i)) != null) {
                    if ((s = t.status) >= 0)
                        s = t.awaitJoin(false, false);
                    if ((s & ABNORMAL) != 0) {
                        ex = t.getException();
                        break;
                    }
                }
            }
        }
        if (ex != null) {
            for (int i = 0, s; i <= last; ++i) {
                ForkJoinTask<?> t;
                if ((t = ts.get(i)) != null)
                    t.cancel(false);
            }
            rethrow(ex);
        }
        return tasks;
    }

    /**
     * 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 (trySetCancelled() & (ABNORMAL | THROWN)) == ABNORMAL;
    }

    public final boolean isDone() {
        return status < 0;
    }

    public final boolean isCancelled() {
        return (status & (ABNORMAL | THROWN)) == ABNORMAL;
    }

    /**
     * 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 & ABNORMAL) != 0;
    }

    /**
     * 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 | ABNORMAL)) == DONE;
    }

    /**
     * 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;
        return ((s & ABNORMAL) == 0 ? null :
                (s & THROWN)   == 0 ? 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) {
        trySetException((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) {
            trySetException(rex);
            return;
        }
        setDone();
    }

    /**
     * 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() {
        setDone();
    }

    /**
     * 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;
        if (Thread.interrupted())
            s = INTRPT;
        else if ((s = status) >= 0)
            s = awaitJoin(true, false);
        if (s == INTRPT)
            throw new InterruptedException();
        else if ((s & THROWN) != 0)
            throw new ExecutionException(getThrowableException());
        else if ((s & ABNORMAL) != 0)
            throw new CancellationException();
        else
            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 {
        long nanos = unit.toNanos(timeout);
        int s;
        if (Thread.interrupted())
            s = INTRPT;
        else if ((s = status) >= 0 && nanos > 0L) {
            long d = nanos + System.nanoTime();
            long deadline = (d == 0L) ? 1L : d; // avoid 0
            Thread t = Thread.currentThread();
            ForkJoinPool p = (t instanceof ForkJoinWorkerThread) ?
                ((ForkJoinWorkerThread)t).pool : ForkJoinPool.common;
            if (p != null && p.preCompensate() == 0)
                p = null;
            s = awaitDone(true, deadline, p);
        }

        if (s == INTRPT)
            throw new InterruptedException();
        else if (s >= 0)
            throw new TimeoutException();
        else if ((s & THROWN) != 0)
            throw new ExecutionException(getThrowableException());
        else if ((s & ABNORMAL) != 0)
            throw new CancellationException();
        else
            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() {
        if (status >= 0)
            awaitJoin(false, false);
    }

    /**
     * Commences performing this task and awaits its completion if
     * necessary, without returning its result or throwing its
     * exception.
     */
    public final void quietlyInvoke() {
        if (doExec() >= 0)
            awaitJoin(false, true);
    }

    /**
     * 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; ForkJoinWorkerThread w; ForkJoinPool p;
        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread &&
            (p = (w = (ForkJoinWorkerThread)t).pool) != null)
            p.helpQuiescePool(w.workQueue);
        else
            ForkJoinPool.quiesceCommonPool();
    }

    /**
     * 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() {
        aux = null;
        status = 0;
    }

    /**
     * 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;
        return (((t = Thread.currentThread()) 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;
    }

    /**
     * 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; ForkJoinPool.WorkQueue q;
        return (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
                (q = ((ForkJoinWorkerThread)t).workQueue) != null &&
                q.tryUnpush(this) :
                (q = ForkJoinPool.commonQueue()) != null &&
                q.externalTryUnpush(this));
    }

    /**
     * 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.commonQueue();
        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();
    }

    // Extension methods

    /**
     * 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. 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, 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.commonQueue();
        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);
    }

    /**
     * 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 w;
        return (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
                (w = (ForkJoinWorkerThread)t).pool.nextTaskFor(w.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);
    }

    // tag operations

    /**
     * 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 (casStatus(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 (casStatus(s, (s & ~SMASK) | (update & SMASK)))
                return true;
        }
    }

    /**
     * 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> {
        @SuppressWarnings("serial") // Conditionally serializable
        final Runnable runnable;
        @SuppressWarnings("serial") // Conditionally serializable
        T result;
        AdaptedRunnable(Runnable runnable, T result) {
            if (runnable == null) throw new NullPointerException();
            this.runnable = 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 + "]";
        }
        private static final long serialVersionUID = 5232453952276885070L;
    }

    /**
     * Adapter for Runnables without results.
     */
    static final class AdaptedRunnableAction extends ForkJoinTask<Void>
        implements RunnableFuture<Void> {
        @SuppressWarnings("serial") // Conditionally serializable
        final Runnable runnable;
        AdaptedRunnableAction(Runnable runnable) {
            if (runnable == null) throw new NullPointerException();
            this.runnable = 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 + "]";
        }
        private static final long serialVersionUID = 5232453952276885070L;
    }

    /**
     * Adapter for Runnables in which failure forces worker exception.
     */
    static final class RunnableExecuteAction extends ForkJoinTask<Void> {
        @SuppressWarnings("serial") // Conditionally serializable
        final Runnable runnable;
        RunnableExecuteAction(Runnable runnable) {
            if (runnable == null) throw new NullPointerException();
            this.runnable = runnable;
        }
        public final Void getRawResult() { return null; }
        public final void setRawResult(Void v) { }
        public final boolean exec() { runnable.run(); return true; }
        int trySetException(Throwable ex) {
            int s;
            if (isExceptionalStatus(s = trySetThrown(ex)))
                rethrow(ex); // rethrow outside exec() catches.
            return s;
        }
        private static final long serialVersionUID = 5232453952276885070L;
    }

    /**
     * Adapter for Callables.
     */
    static final class AdaptedCallable<T> extends ForkJoinTask<T>
        implements RunnableFuture<T> {
        @SuppressWarnings("serial") // Conditionally serializable
        final Callable<? extends T> callable;
        @SuppressWarnings("serial") // Conditionally serializable
        T result;
        AdaptedCallable(Callable<? extends T> callable) {
            if (callable == null) throw new NullPointerException();
            this.callable = 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 + "]";
        }
        private static final long serialVersionUID = 2838392045355241008L;
    }

    /**
     * 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);
    }

    // Serialization support

    private static final long serialVersionUID = -7721805057305804111L;

    /**
     * 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 {
        Aux a;
        s.defaultWriteObject();
        s.writeObject((a = aux) == null ? null : a.ex);
    }

    /**
     * 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)
            trySetThrown((Throwable)ex);
    }

    static {
        try {
            MethodHandles.Lookup l = MethodHandles.lookup();
            STATUS = l.findVarHandle(ForkJoinTask.class, "status", int.class);
            AUX = l.findVarHandle(ForkJoinTask.class, "aux", Aux.class);
        } catch (ReflectiveOperationException e) {
            throw new ExceptionInInitializerError(e);
        }
    }

}
Revision:	1.125
Committed:	Sat Jan 18 12:30:04 2020 UTC (4 years, 4 months ago) by dl
Branch:	MAIN
Changes since 1.124:	+4 -6 lines
Log Message:	more consistent parallelism-0 handling; doc touchups