src/java.base/share/classes/java/util/concurrent/StructuredTaskScope.java - master - niceSOFT/openjdk21

src/java.base/share/classes/java/util/concurrent/StructuredTaskScope.java
1 323 строки57 KB
Daniel Hu
8313290: Misleading exception message from STS.Subtask::get when task forked after shutdown
10 май 2025, 05:56
10 май 2025, 05:56ad6f24f
/*
 * Copyright (c) 2021, 2023, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */
package java.util.concurrent;

import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.time.Duration;
import java.time.Instant;
import java.util.Objects;
import java.util.Optional;
import java.util.concurrent.locks.ReentrantLock;
import java.util.function.Function;
import java.util.function.Supplier;
import jdk.internal.javac.PreviewFeature;
import jdk.internal.misc.ThreadFlock;

/**
 * A basic API for <em>structured concurrency</em>. {@code StructuredTaskScope} supports
 * cases where a task splits into several concurrent subtasks, and where the subtasks must
 * complete before the main task continues. A {@code StructuredTaskScope} can be used to
 * ensure that the lifetime of a concurrent operation is confined by a <em>syntax block</em>,
 * just like that of a sequential operation in structured programming.
 *
 * <h2>Basic operation</h2>
 *
 * A {@code StructuredTaskScope} is created with one of its public constructors. It defines
 * the {@link #fork(Callable) fork} method to start a thread to execute a subtask, the {@link
 * #join() join} method to wait for all subtasks to finish, and the {@link #close() close}
 * method to close the task scope. The API is intended to be used with the {@code
 * try-with-resources} statement. The intention is that code in the try <em>block</em>
 * uses the {@code fork} method to fork threads to execute the subtasks, wait for the
 * subtasks to finish with the {@code join} method, and then <em>process the results</em>.
 * A call to the {@code fork} method returns a {@link Subtask Subtask} to representing
 * the <em>forked subtask</em>. Once {@code join} is called, the {@code Subtask} can be
 * used to get the result completed successfully, or the exception if the subtask failed.
 * {@snippet lang=java :
 *     Callable<String> task1 = ...
 *     Callable<Integer> task2 = ...
 *
 *     try (var scope = new StructuredTaskScope<Object>()) {
 *
 *         Subtask<String> subtask1 = scope.fork(task1);   // @highlight substring="fork"
 *         Subtask<Integer> subtask2 = scope.fork(task2);  // @highlight substring="fork"
 *
 *         scope.join();                                   // @highlight substring="join"
 *
 *         ... process results/exceptions ...
 *
 *     } // close                                          // @highlight substring="close"
 * }
 * <p> The following example forks a collection of homogeneous subtasks, waits for all of
 * them to complete with the {@code join} method, and uses the {@link Subtask.State
 * Subtask.State} to partition the subtasks into a set of the subtasks that completed
 * successfully and another for the subtasks that failed.
 * {@snippet lang=java :
 *     List<Callable<String>> callables = ...
 *
 *     try (var scope = new StructuredTaskScope<String>()) {
 *
 *         List<Subtask<String>> subtasks = callables.stream().map(scope::fork).toList();
 *
 *         scope.join();
 *
 *         Map<Boolean, Set<Subtask<String>>> map = subtasks.stream()
 *                 .collect(Collectors.partitioningBy(h -> h.state() == Subtask.State.SUCCESS,
 *                                                    Collectors.toSet()));
 *
 *     } // close
 * }
 *
 * <p> To ensure correct usage, the {@code join} and {@code close} methods may only be
 * invoked by the <em>owner</em> (the thread that opened/created the task scope), and the
 * {@code close} method throws an exception after closing if the owner did not invoke the
 * {@code join} method after forking.
 *
 * <p> {@code StructuredTaskScope} defines the {@link #shutdown() shutdown} method to shut
 * down a task scope without closing it. The {@code shutdown()} method <em>cancels</em> all
 * unfinished subtasks by {@linkplain Thread#interrupt() interrupting} the threads. It
 * prevents new threads from starting in the task scope. If the owner is waiting in the
 * {@code join} method then it will wakeup.
 *
 * <p> Shutdown is used for <em>short-circuiting</em> and allow subclasses to implement
 * <em>policy</em> that does not require all subtasks to finish.
 *
 * <h2>Subclasses with policies for common cases</h2>
 *
 * Two subclasses of {@code StructuredTaskScope} are defined to implement policy for
 * common cases:
 * <ol>
 *   <li> {@link ShutdownOnSuccess ShutdownOnSuccess} captures the result of the first
 *   subtask to complete successfully. Once captured, it shuts down the task scope to
 *   interrupt unfinished threads and wakeup the owner. This class is intended for cases
 *   where the result of any subtask will do ("invoke any") and where there is no need to
 *   wait for results of other unfinished subtasks. It defines methods to get the first
 *   result or throw an exception if all subtasks fail.
 *   <li> {@link ShutdownOnFailure ShutdownOnFailure} captures the exception of the first
 *   subtask to fail. Once captured, it shuts down the task scope to interrupt unfinished
 *   threads and wakeup the owner. This class is intended for cases where the results of all
 *   subtasks are required ("invoke all"); if any subtask fails then the results of other
 *   unfinished subtasks are no longer needed. If defines methods to throw an exception if
 *   any of the subtasks fail.
 * </ol>
 *
 * <p> The following are two examples that use the two classes. In both cases, a pair of
 * subtasks are forked to fetch resources from two URL locations "left" and "right". The
 * first example creates a ShutdownOnSuccess object to capture the result of the first
 * subtask to complete successfully, cancelling the other by way of shutting down the task
 * scope. The main task waits in {@code join} until either subtask completes with a result
 * or both subtasks fail. It invokes {@link ShutdownOnSuccess#result(Function)
 * result(Function)} method to get the captured result. If both subtasks fail then this
 * method throws a {@code WebApplicationException} with the exception from one of the
 * subtasks as the cause.
 * {@snippet lang=java :
 *     try (var scope = new StructuredTaskScope.ShutdownOnSuccess<String>()) {
 *
 *         scope.fork(() -> fetch(left));
 *         scope.fork(() -> fetch(right));
 *
 *         scope.join();
 *
 *         // @link regex="result(?=\()" target="ShutdownOnSuccess#result" :
 *         String result = scope.result(e -> new WebApplicationException(e));
 *
 *         ...
 *     }
 * }
 * The second example creates a ShutdownOnFailure object to capture the exception of the
 * first subtask to fail, cancelling the other by way of shutting down the task scope. The
 * main task waits in {@link #joinUntil(Instant)} until both subtasks complete with a
 * result, either fails, or a deadline is reached. It invokes {@link
 * ShutdownOnFailure#throwIfFailed(Function) throwIfFailed(Function)} to throw an exception
 * if either subtask fails. This method is a no-op if both subtasks complete successfully.
 * The example uses {@link Supplier#get()} to get the result of each subtask. Using
 * {@code Supplier} instead of {@code Subtask} is preferred for common cases where the
 * object returned by fork is only used to get the result of a subtask that completed
 * successfully.
 * {@snippet lang=java :
 *    Instant deadline = ...
 *
 *    try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
 *
 *         Supplier<String> supplier1 = scope.fork(() -> query(left));
 *         Supplier<String> supplier2 = scope.fork(() -> query(right));
 *
 *         scope.joinUntil(deadline);
 *
 *         // @link substring="throwIfFailed" target="ShutdownOnFailure#throwIfFailed" :
 *         scope.throwIfFailed(e -> new WebApplicationException(e));
 *
 *         // both subtasks completed successfully
 *         String result = Stream.of(supplier1, supplier2)
 *                 .map(Supplier::get)
 *                 .collect(Collectors.joining(", ", "{ ", " }"));
 *
 *         ...
 *     }
 * }
 *
 * <h2>Extending StructuredTaskScope</h2>
 *
 * {@code StructuredTaskScope} can be extended, and the {@link #handleComplete(Subtask)
 * handleComplete} method overridden, to implement policies other than those implemented
 * by {@code ShutdownOnSuccess} and {@code ShutdownOnFailure}. A subclass may, for example,
 * collect the results of subtasks that complete successfully and ignore subtasks that
 * fail. It may collect exceptions when subtasks fail. It may invoke the {@link #shutdown()
 * shutdown} method to shut down and cause {@link #join() join} to wakeup when some
 * condition arises.
 *
 * <p> A subclass will typically define methods to make available results, state, or other
 * outcome to code that executes after the {@code join} method. A subclass that collects
 * results and ignores subtasks that fail may define a method that returns the results.
 * A subclass that implements a policy to shut down when a subtask fails may define a
 * method to get the exception of the first subtask to fail.
 *
 * <p> The following is an example of a simple {@code StructuredTaskScope} implementation
 * that collects homogenous subtasks that complete successfully. It defines the method
 * "{@code completedSuccessfully()}" that the main task can invoke after it joins.
 * {@snippet lang=java :
 *     class CollectingScope<T> extends StructuredTaskScope<T> {
 *         private final Queue<Subtask<? extends T>> subtasks = new LinkedTransferQueue<>();
 *
 *         @Override
 *         protected void handleComplete(Subtask<? extends T> subtask) {
 *             if (subtask.state() == Subtask.State.SUCCESS) {
 *                 subtasks.add(subtask);
 *             }
 *         }
 *
 *         @Override
 *         public CollectingScope<T> join() throws InterruptedException {
 *             super.join();
 *             return this;
 *         }
 *
 *         public Stream<Subtask<? extends T>> completedSuccessfully() {
 *             // @link substring="ensureOwnerAndJoined" target="ensureOwnerAndJoined" :
 *             super.ensureOwnerAndJoined();
 *             return subtasks.stream();
 *         }
 *     }
 * }
 * <p> The implementations of the {@code completedSuccessfully()} method in the example
 * invokes {@link #ensureOwnerAndJoined()} to ensure that the method can only be invoked
 * by the owner thread and only after it has joined.
 *
 * <h2><a id="TreeStructure">Tree structure</a></h2>
 *
 * Task scopes form a tree where parent-child relations are established implicitly when
 * opening a new task scope:
 * <ul>
 *   <li> A parent-child relation is established when a thread started in a task scope
 *   opens its own task scope. A thread started in task scope "A" that opens task scope
 *   "B" establishes a parent-child relation where task scope "A" is the parent of task
 *   scope "B".
 *   <li> A parent-child relation is established with nesting. If a thread opens task
 *   scope "B", then opens task scope "C" (before it closes "B"), then the enclosing task
 *   scope "B" is the parent of the nested task scope "C".
 * </ul>
 *
 * The <i>descendants</i> of a task scope are the child task scopes that it is a parent
 * of, plus the descendants of the child task scopes, recursively.
 *
 * <p> The tree structure supports:
 * <ul>
 *   <li> Inheritance of {@linkplain ScopedValue scoped values} across threads.
 *   <li> Confinement checks. The phrase "threads contained in the task scope" in method
 *   descriptions means threads started in the task scope or descendant scopes.
 * </ul>
 *
 * <p> The following example demonstrates the inheritance of a scoped value. A scoped
 * value {@code USERNAME} is bound to the value "{@code duke}". A {@code StructuredTaskScope}
 * is created and its {@code fork} method invoked to start a thread to execute {@code
 * childTask}. The thread inherits the scoped value <em>bindings</em> captured when
 * creating the task scope. The code in {@code childTask} uses the value of the scoped
 * value and so reads the value "{@code duke}".
 * {@snippet lang=java :
 *     private static final ScopedValue<String> USERNAME = ScopedValue.newInstance();
 *
 *     // @link substring="runWhere" target="ScopedValue#runWhere(ScopedValue, Object, Runnable)" :
 *     ScopedValue.runWhere(USERNAME, "duke", () -> {
 *         try (var scope = new StructuredTaskScope<String>()) {
 *
 *             scope.fork(() -> childTask());           // @highlight substring="fork"
 *             ...
 *          }
 *     });
 *
 *     ...
 *
 *     String childTask() {
 *         // @link substring="get" target="ScopedValue#get()" :
 *         String name = USERNAME.get();   // "duke"
 *         ...
 *     }
 * }
 *
 * <p> {@code StructuredTaskScope} does not define APIs that exposes the tree structure
 * at this time.
 *
 * <p> Unless otherwise specified, passing a {@code null} argument to a constructor
 * or method in this class will cause a {@link NullPointerException} to be thrown.
 *
 * <h2>Memory consistency effects</h2>
 *
 * <p> Actions in the owner thread of, or a thread contained in, the task scope prior to
 * {@linkplain #fork forking} of a subtask
 * <a href="{@docRoot}/java.base/java/util/concurrent/package-summary.html#MemoryVisibility">
 * <i>happen-before</i></a> any actions taken by that subtask, which in turn <i>happen-before</i>
 * the subtask result is {@linkplain Subtask#get() retrieved} or <i>happen-before</i> any
 * actions taken in a thread after {@linkplain #join() joining} of the task scope.
 *
 * @jls 17.4.5 Happens-before Order
 *
 * @param <T> the result type of tasks executed in the task scope
 * @since 21
 */
@PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
public class StructuredTaskScope<T> implements AutoCloseable {
    private final ThreadFactory factory;
    private final ThreadFlock flock;
    private final ReentrantLock shutdownLock = new ReentrantLock();

    // states: OPEN -> SHUTDOWN -> CLOSED
    private static final int OPEN     = 0;   // initial state
    private static final int SHUTDOWN = 1;
    private static final int CLOSED   = 2;

    // state: set to SHUTDOWN by any thread, set to CLOSED by owner, read by any thread
    private volatile int state;

    // Counters to support checking that the task scope owner joins before processing
    // results and attempts join before closing the task scope. These counters are
    // accessed only by the owner thread.
    private int forkRound;         // incremented when the first subtask is forked after join
    private int lastJoinAttempted; // set to the current fork round when join is attempted
    private int lastJoinCompleted; // set to the current fork round when join completes

    /**
     * Represents a subtask forked with {@link #fork(Callable)}.
     * @param <T> the result type
     * @since 21
     */
    @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
    public sealed interface Subtask<T> extends Supplier<T> permits SubtaskImpl {
        /**
         * {@return the value returning task provided to the {@code fork} method}
         *
         * @apiNote Task objects with unique identity may be used for correlation by
         * implementations of {@link #handleComplete(Subtask) handleComplete}.
         */
        Callable<? extends T> task();

        /**
         * Represents the state of a subtask.
         * @see Subtask#state()
         * @since 21
         */
        @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
        enum State {
            /**
             * The subtask result or exception is not available. This state indicates that
             * the subtask was forked but has not completed, it completed after the task
             * scope was {@linkplain #shutdown() shut down}, or it was forked after the
             * task scope was shut down.
             */
            UNAVAILABLE,
            /**
             * The subtask completed successfully with a result. The {@link Subtask#get()
             * Subtask.get()} method can be used to obtain the result. This is a terminal
             * state.
             */
            SUCCESS,
            /**
             * The subtask failed with an exception. The {@link Subtask#exception()
             * Subtask.exception()} method can be used to obtain the exception. This is a
             * terminal state.
             */
            FAILED,
        }

        /**
         * {@return the state of the subtask}
         */
        State state();

        /**
         * Returns the result of the subtask.
         *
         * <p> To ensure correct usage, if the scope owner {@linkplain #fork(Callable) forks}
         * a subtask, then it must join (with {@link #join() join} or {@link #joinUntil(Instant)
         * joinUntil}) before it can obtain the result of the subtask.
         *
         * @return the possibly-null result
         * @throws IllegalStateException if the subtask has not completed, did not complete
         * successfully, or the current thread is the task scope owner and did not join
         * after forking
         * @see State#SUCCESS
         */
        T get();

        /**
         * {@return the exception thrown by the subtask}
         *
         * <p> To ensure correct usage, if the scope owner {@linkplain #fork(Callable) forks}
         * a subtask, then it must join (with {@link #join() join} or {@link #joinUntil(Instant)
         * joinUntil}) before it can obtain the exception thrown by the subtask.
         *
         * @throws IllegalStateException if the subtask has not completed, completed with
         * a result, or the current thread is the task scope owner and did not join after
         * forking
         * @see State#FAILED
         */
        Throwable exception();
    }

    /**
     * Creates a structured task scope with the given name and thread factory. The task
     * scope is optionally named for the purposes of monitoring and management. The thread
     * factory is used to {@link ThreadFactory#newThread(Runnable) create} threads when
     * subtasks are {@linkplain #fork(Callable) forked}. The task scope is owned by the
     * current thread.
     *
     * <p> Construction captures the current thread's {@linkplain ScopedValue scoped value}
     * bindings for inheritance by threads started in the task scope. The
     * <a href="#TreeStructure">Tree Structure</a> section in the class description details
     * how parent-child relations are established implicitly for the purpose of inheritance
     * of scoped value bindings.
     *
     * @param name the name of the task scope, can be null
     * @param factory the thread factory
     */
    public StructuredTaskScope(String name, ThreadFactory factory) {
        this.factory = Objects.requireNonNull(factory, "'factory' is null");
        if (name == null)
            name = Objects.toIdentityString(this);
        this.flock = ThreadFlock.open(name);
    }

    /**
     * Creates an unnamed structured task scope that creates virtual threads. The task
     * scope is owned by the current thread.
     *
     * @implSpec This constructor is equivalent to invoking the 2-arg constructor with a
     * name of {@code null} and a thread factory that creates virtual threads.
     */
    public StructuredTaskScope() {
        this(null, Thread.ofVirtual().factory());
    }

    private IllegalStateException newIllegalStateExceptionScopeClosed() {
        return new IllegalStateException("Task scope is closed");
    }

    private IllegalStateException newIllegalStateExceptionNoJoin() {
        return new IllegalStateException("Owner did not join after forking subtasks");
    }

    /**
     * Throws IllegalStateException if the scope is closed, returning the state if not
     * closed.
     */
    private int ensureOpen() {
        int s = state;
        if (s == CLOSED)
            throw newIllegalStateExceptionScopeClosed();
        return s;
    }

    /**
     * Throws WrongThreadException if the current thread is not the owner.
     */
    private void ensureOwner() {
        if (Thread.currentThread() != flock.owner())
            throw new WrongThreadException("Current thread not owner");
    }

    /**
     * Throws WrongThreadException if the current thread is not the owner
     * or a thread contained in the tree.
     */
    private void ensureOwnerOrContainsThread() {
        Thread currentThread = Thread.currentThread();
        if (currentThread != flock.owner() && !flock.containsThread(currentThread))
            throw new WrongThreadException("Current thread not owner or thread in the tree");
    }

    /**
     * Throws IllegalStateException if the current thread is the owner, and the owner did
     * not join after forking a subtask in the given fork round.
     */
    private void ensureJoinedIfOwner(int round) {
        if (Thread.currentThread() == flock.owner() && (round > lastJoinCompleted)) {
            throw newIllegalStateExceptionNoJoin();
        }
    }

    /**
     * Ensures that the current thread is the owner of this task scope and that it joined
     * (with {@link #join()} or {@link #joinUntil(Instant)}) after {@linkplain #fork(Callable)
     * forking} subtasks.
     *
     * @apiNote This method can be used by subclasses that define methods to make available
     * results, state, or other outcome to code intended to execute after the join method.
     *
     * @throws WrongThreadException if the current thread is not the task scope owner
     * @throws IllegalStateException if the task scope is open and task scope owner did
     * not join after forking
     */
    protected final void ensureOwnerAndJoined() {
        ensureOwner();
        if (forkRound > lastJoinCompleted) {
            throw newIllegalStateExceptionNoJoin();
        }
    }

    /**
     * Invoked by a subtask when it completes successfully or fails in this task scope.
     * This method is not invoked if a subtask completes after the task scope is
     * {@linkplain #shutdown() shut down}.
     *
     * @implSpec The default implementation throws {@code NullPointerException} if the
     * subtask is {@code null}. It throws {@link IllegalArgumentException} if the subtask
     * has not completed.
     *
     * @apiNote The {@code handleComplete} method should be thread safe. It may be
     * invoked by several threads concurrently.
     *
     * @param subtask the subtask
     *
     * @throws IllegalArgumentException if called with a subtask that has not completed
     */
    protected void handleComplete(Subtask<? extends T> subtask) {
        if (subtask.state() == Subtask.State.UNAVAILABLE)
            throw new IllegalArgumentException();
    }

    /**
     * Starts a new thread in this task scope to execute a value-returning task, thus
     * creating a <em>subtask</em> of this task scope.
     *
     * <p> The value-returning task is provided to this method as a {@link Callable}, the
     * thread executes the task's {@link Callable#call() call} method. The thread is
     * created with the task scope's {@link ThreadFactory}. It inherits the current thread's
     * {@linkplain ScopedValue scoped value} bindings. The bindings must match the bindings
     * captured when the task scope was created.
     *
     * <p> This method returns a {@link Subtask Subtask} to represent the <em>forked
     * subtask</em>. The {@code Subtask} object can be used to obtain the result when
     * the subtask completes successfully, or the exception when the subtask fails. To
     * ensure correct usage, the {@link Subtask#get() get()} and {@link Subtask#exception()
     * exception()} methods may only be called by the task scope owner after it has waited
     * for all threads to finish with the {@link #join() join} or {@link #joinUntil(Instant)}
     * methods. When the subtask completes, the thread invokes the {@link
     * #handleComplete(Subtask) handleComplete} method to consume the completed subtask.
     * If the task scope is {@linkplain #shutdown() shut down} before the subtask completes
     * then the {@code handleComplete} method will not be invoked.
     *
     * <p> If this task scope is {@linkplain #shutdown() shutdown} (or in the process of
     * shutting down) then the subtask will not run and the {@code handleComplete} method
     * will not be invoked.
     *
     * <p> This method may only be invoked by the task scope owner or threads contained
     * in the task scope.
     *
     * @implSpec This method may be overridden for customization purposes, wrapping tasks
     * for example. If overridden, the subclass must invoke {@code super.fork} to start a
     * new thread in this task scope.
     *
     * @param task the value-returning task for the thread to execute
     * @param <U> the result type
     * @return the subtask
     * @throws IllegalStateException if this task scope is closed
     * @throws WrongThreadException if the current thread is not the task scope owner or a
     * thread contained in the task scope
     * @throws StructureViolationException if the current scoped value bindings are not
     * the same as when the task scope was created
     * @throws RejectedExecutionException if the thread factory rejected creating a
     * thread to run the subtask
     */
    public <U extends T> Subtask<U> fork(Callable<? extends U> task) {
        Objects.requireNonNull(task, "'task' is null");
        int s = ensureOpen();   // throws ISE if closed

        // when forked by the owner, the subtask is forked in the current or next round
        int round = -1;
        if (Thread.currentThread() == flock.owner()) {
            round = forkRound;
            if (forkRound == lastJoinCompleted) {
                // new round if first fork after join
                round++;
            }
        }

        SubtaskImpl<U> subtask = new SubtaskImpl<>(this, task, round);
        if (s < SHUTDOWN) {
            // create thread to run task
            Thread thread = factory.newThread(subtask);
            if (thread == null) {
                throw new RejectedExecutionException("Rejected by thread factory");
            }

            // attempt to start the thread
            try {
                flock.start(thread);
            } catch (IllegalStateException e) {
                // shutdown by another thread, or underlying flock is shutdown due
                // to unstructured use
            }
        }

        // force owner to join if this is the first fork in the round
        if (Thread.currentThread() == flock.owner() && round > forkRound) {
            forkRound = round;
        }

        // return forked subtask or a subtask that did not run
        return subtask;
    }

    /**
     * Wait for all threads to finish or the task scope to shut down.
     */
    private void implJoin(Duration timeout)
        throws InterruptedException, TimeoutException
    {
        ensureOwner();
        lastJoinAttempted = forkRound;
        int s = ensureOpen();  // throws ISE if closed
        if (s == OPEN) {
            // wait for all threads, wakeup, interrupt, or timeout
            if (timeout != null) {
                flock.awaitAll(timeout);
            } else {
                flock.awaitAll();
            }
        }
        lastJoinCompleted = forkRound;
    }

    /**
     * Wait for all subtasks started in this task scope to finish or the task scope to
     * shut down.
     *
     * <p> This method waits for all subtasks by waiting for all threads {@linkplain
     * #fork(Callable) started} in this task scope to finish execution. It stops waiting
     * when all threads finish, the task scope is {@linkplain #shutdown() shut down}, or
     * the current thread is {@linkplain Thread#interrupt() interrupted}.
     *
     * <p> This method may only be invoked by the task scope owner.
     *
     * @implSpec This method may be overridden for customization purposes or to return a
     * more specific return type. If overridden, the subclass must invoke {@code
     * super.join} to ensure that the method waits for threads in this task scope to
     * finish.
     *
     * @return this task scope
     * @throws IllegalStateException if this task scope is closed
     * @throws WrongThreadException if the current thread is not the task scope owner
     * @throws InterruptedException if interrupted while waiting
     */
    public StructuredTaskScope<T> join() throws InterruptedException {
        try {
            implJoin(null);
        } catch (TimeoutException e) {
            throw new InternalError();
        }
        return this;
    }

    /**
     * Wait for all subtasks started in this task scope to finish or the task scope to
     * shut down, up to the given deadline.
     *
     * <p> This method waits for all subtasks by waiting for all threads {@linkplain
     * #fork(Callable) started} in this task scope to finish execution. It stops waiting
     * when all threads finish, the task scope is {@linkplain #shutdown() shut down}, the
     * deadline is reached, or the current thread is {@linkplain Thread#interrupt()
     * interrupted}.
     *
     * <p> This method may only be invoked by the task scope owner.
     *
     * @implSpec This method may be overridden for customization purposes or to return a
     * more specific return type. If overridden, the subclass must invoke {@code
     * super.joinUntil} to ensure that the method waits for threads in this task scope to
     * finish.
     *
     * @param deadline the deadline
     * @return this task scope
     * @throws IllegalStateException if this task scope is closed
     * @throws WrongThreadException if the current thread is not the task scope owner
     * @throws InterruptedException if interrupted while waiting
     * @throws TimeoutException if the deadline is reached while waiting
     */
    public StructuredTaskScope<T> joinUntil(Instant deadline)
        throws InterruptedException, TimeoutException
    {
        Duration timeout = Duration.between(Instant.now(), deadline);
        implJoin(timeout);
        return this;
    }

    /**
     * Interrupt all unfinished threads.
     */
    private void implInterruptAll() {
        flock.threads()
            .filter(t -> t != Thread.currentThread())
            .forEach(t -> {
                try {
                    t.interrupt();
                } catch (Throwable ignore) { }
            });
    }

    @SuppressWarnings("removal")
    private void interruptAll() {
        if (System.getSecurityManager() == null) {
            implInterruptAll();
        } else {
            PrivilegedAction<Void> pa = () -> {
                implInterruptAll();
                return null;
            };
            AccessController.doPrivileged(pa);
        }
    }

    /**
     * Shutdown the task scope if not already shutdown. Return true if this method
     * shutdowns the task scope, false if already shutdown.
     */
    private boolean implShutdown() {
        shutdownLock.lock();
        try {
            if (state < SHUTDOWN) {
                // prevent new threads from starting
                flock.shutdown();

                // set status before interrupting tasks
                state = SHUTDOWN;

                // interrupt all unfinished threads
                interruptAll();

                return true;
            } else {
                // already shutdown
                return false;
            }
        } finally {
            shutdownLock.unlock();
        }
    }

    /**
     * Shut down this task scope without closing it. Shutting down a task scope prevents
     * new threads from starting, interrupts all unfinished threads, and causes the
     * {@link #join() join} method to wakeup. Shutdown is useful for cases where the
     * results of unfinished subtasks are no longer needed. It will typically be called
     * by the {@link #handleComplete(Subtask)} implementation of a subclass that
     * implements a policy to discard unfinished tasks once some outcome is reached.
     *
     * <p> More specifically, this method:
     * <ul>
     * <li> {@linkplain Thread#interrupt() Interrupts} all unfinished threads in the
     * task scope (except the current thread).
     * <li> Wakes up the task scope owner if it is waiting in {@link #join()} or {@link
     * #joinUntil(Instant)}. If the task scope owner is not waiting then its next call to
     * {@code join} or {@code joinUntil} will return immediately.
     * </ul>
     *
     * <p> This method may only be invoked by the task scope owner or threads contained
     * in the task scope.
     *
     * @implSpec This method may be overridden for customization purposes. If overridden,
     * the subclass must invoke {@code super.shutdown} to ensure that the method shuts
     * down the task scope.
     *
     * @apiNote
     * There may be threads that have not finished because they are executing code that
     * did not respond (or respond promptly) to thread interrupt. This method does not wait
     * for these threads. When the owner invokes the {@link #close() close} method
     * to close the task scope then it will wait for the remaining threads to finish.
     *
     * @throws IllegalStateException if this task scope is closed
     * @throws WrongThreadException if the current thread is not the task scope owner or
     * a thread contained in the task scope
     * @see #isShutdown()
     */
    public void shutdown() {
        ensureOwnerOrContainsThread();
        int s = ensureOpen();  // throws ISE if closed
        if (s < SHUTDOWN && implShutdown())
            flock.wakeup();
    }

    /**
     * {@return true if this task scope is shutdown, otherwise false}
     * @see #shutdown()
     */
    public final boolean isShutdown() {
        return state >= SHUTDOWN;
    }

    /**
     * Closes this task scope.
     *
     * <p> This method first shuts down the task scope (as if by invoking the {@link
     * #shutdown() shutdown} method). It then waits for the threads executing any
     * unfinished tasks to finish. If interrupted, this method will continue to wait for
     * the threads to finish before completing with the interrupt status set.
     *
     * <p> This method may only be invoked by the task scope owner. If the task scope
     * is already closed then the task scope owner invoking this method has no effect.
     *
     * <p> A {@code StructuredTaskScope} is intended to be used in a <em>structured
     * manner</em>. If this method is called to close a task scope before nested task
     * scopes are closed then it closes the underlying construct of each nested task scope
     * (in the reverse order that they were created in), closes this task scope, and then
     * throws {@link StructureViolationException}.
     * Similarly, if this method is called to close a task scope while executing with
     * {@linkplain ScopedValue scoped value} bindings, and the task scope was created
     * before the scoped values were bound, then {@code StructureViolationException} is
     * thrown after closing the task scope.
     * If a thread terminates without first closing task scopes that it owns then
     * termination will cause the underlying construct of each of its open tasks scopes to
     * be closed. Closing is performed in the reverse order that the task scopes were
     * created in. Thread termination may therefore be delayed when the task scope owner
     * has to wait for threads forked in these task scopes to finish.
     *
     * @implSpec This method may be overridden for customization purposes. If overridden,
     * the subclass must invoke {@code super.close} to close the task scope.
     *
     * @throws IllegalStateException thrown after closing the task scope if the task scope
     * owner did not attempt to join after forking
     * @throws WrongThreadException if the current thread is not the task scope owner
     * @throws StructureViolationException if a structure violation was detected
     */
    @Override
    public void close() {
        ensureOwner();
        int s = state;
        if (s == CLOSED)
            return;

        try {
            if (s < SHUTDOWN)
                implShutdown();
            flock.close();
        } finally {
            state = CLOSED;
        }

        // throw ISE if the owner didn't attempt to join after forking
        if (forkRound > lastJoinAttempted) {
            lastJoinCompleted = forkRound;
            throw newIllegalStateExceptionNoJoin();
        }
    }

    @Override
    public String toString() {
        String name = flock.name();
        return switch (state) {
            case OPEN     -> name;
            case SHUTDOWN -> name + "/shutdown";
            case CLOSED   -> name + "/closed";
            default -> throw new InternalError();
        };
    }

    /**
     * Subtask implementation, runs the task specified to the fork method.
     */
    private static final class SubtaskImpl<T> implements Subtask<T>, Runnable {
        private static final AltResult RESULT_NULL = new AltResult(Subtask.State.SUCCESS);

        private record AltResult(Subtask.State state, Throwable exception) {
            AltResult(Subtask.State state) {
                this(state, null);
            }
        }

        private final StructuredTaskScope<? super T> scope;
        private final Callable<? extends T> task;
        private final int round;
        private volatile Object result;

        SubtaskImpl(StructuredTaskScope<? super T> scope,
                    Callable<? extends T> task,
                    int round) {
            this.scope = scope;
            this.task = task;
            this.round = round;
        }

        @Override
        public void run() {
            T result = null;
            Throwable ex = null;
            try {
                result = task.call();
            } catch (Throwable e) {
                ex = e;
            }

            // nothing to do if task scope is shutdown
            if (scope.isShutdown())
                return;

            // capture result or exception, invoke handleComplete
            if (ex == null) {
                this.result = (result != null) ? result : RESULT_NULL;
            } else {
                this.result = new AltResult(State.FAILED, ex);
            }
            scope.handleComplete(this);
        }

        @Override
        public Callable<? extends T> task() {
            return task;
        }

        @Override
        public Subtask.State state() {
            Object result = this.result;
            if (result == null) {
                return State.UNAVAILABLE;
            } else if (result instanceof AltResult alt) {
                // null or failed
                return alt.state();
            } else {
                return State.SUCCESS;
            }
        }

        @Override
        public T get() {
            scope.ensureJoinedIfOwner(round);
            Object result = this.result;
            if (result instanceof AltResult) {
                if (result == RESULT_NULL) return null;
            } else if (result != null) {
                @SuppressWarnings("unchecked")
                T r = (T) result;
                return r;
            }
            throw new IllegalStateException(
                    "Result is unavailable or subtask did not complete successfully");
        }

        @Override
        public Throwable exception() {
            scope.ensureJoinedIfOwner(round);
            Object result = this.result;
            if (result instanceof AltResult alt && alt.state() == State.FAILED) {
                return alt.exception();
            }
            throw new IllegalStateException(
                    "Exception is unavailable or subtask did not complete with exception");
        }

        @Override
        public String toString() {
            String stateAsString = switch (state()) {
                case UNAVAILABLE -> "[Unavailable]";
                case SUCCESS     -> "[Completed successfully]";
                case FAILED      -> {
                    Throwable ex = ((AltResult) result).exception();
                    yield "[Failed: " + ex + "]";
                }
            };
            return Objects.toIdentityString(this) + stateAsString;
        }
    }

    /**
     * A {@code StructuredTaskScope} that captures the result of the first subtask to
     * complete {@linkplain Subtask.State#SUCCESS successfully}. Once captured, it
     * {@linkplain #shutdown() shuts down} the task scope to interrupt unfinished threads
     * and wakeup the task scope owner. The policy implemented by this class is intended
     * for cases where the result of any subtask will do ("invoke any") and where the
     * results of other unfinished subtasks are no longer needed.
     *
     * <p> Unless otherwise specified, passing a {@code null} argument to a method
     * in this class will cause a {@link NullPointerException} to be thrown.
     *
     * @apiNote This class implements a policy to shut down the task scope when a subtask
     * completes successfully. There shouldn't be any need to directly shut down the task
     * scope with the {@link #shutdown() shutdown} method.
     *
     * @param <T> the result type
     * @since 21
     */
    @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
    public static final class ShutdownOnSuccess<T> extends StructuredTaskScope<T> {
        private static final Object RESULT_NULL = new Object();
        private static final VarHandle FIRST_RESULT;
        private static final VarHandle FIRST_EXCEPTION;
        static {
            try {
                MethodHandles.Lookup l = MethodHandles.lookup();
                FIRST_RESULT = l.findVarHandle(ShutdownOnSuccess.class, "firstResult", Object.class);
                FIRST_EXCEPTION = l.findVarHandle(ShutdownOnSuccess.class, "firstException", Throwable.class);
            } catch (Exception e) {
                throw new ExceptionInInitializerError(e);
            }
        }
        private volatile Object firstResult;
        private volatile Throwable firstException;

        /**
         * Constructs a new {@code ShutdownOnSuccess} with the given name and thread factory.
         * The task scope is optionally named for the purposes of monitoring and management.
         * The thread factory is used to {@link ThreadFactory#newThread(Runnable) create}
         * threads when subtasks are {@linkplain #fork(Callable) forked}. The task scope
         * is owned by the current thread.
         *
         * <p> Construction captures the current thread's {@linkplain ScopedValue scoped
         * value} bindings for inheritance by threads started in the task scope. The
         * <a href="#TreeStructure">Tree Structure</a> section in the class description
         * details how parent-child relations are established implicitly for the purpose
         * of inheritance of scoped value bindings.
         *
         * @param name the name of the task scope, can be null
         * @param factory the thread factory
         */
        public ShutdownOnSuccess(String name, ThreadFactory factory) {
            super(name, factory);
        }

        /**
         * Constructs a new unnamed {@code ShutdownOnSuccess} that creates virtual threads.
         *
         * @implSpec This constructor is equivalent to invoking the 2-arg constructor with
         * a name of {@code null} and a thread factory that creates virtual threads.
         */
        public ShutdownOnSuccess() {
            this(null, Thread.ofVirtual().factory());
        }

        @Override
        protected void handleComplete(Subtask<? extends T> subtask) {
            if (firstResult != null) {
                // already captured a result
                return;
            }

            if (subtask.state() == Subtask.State.SUCCESS) {
                // task succeeded
                T result = subtask.get();
                Object r = (result != null) ? result : RESULT_NULL;
                if (FIRST_RESULT.compareAndSet(this, null, r)) {
                    super.shutdown();
                }
            } else if (firstException == null) {
                // capture the exception thrown by the first subtask that failed
                FIRST_EXCEPTION.compareAndSet(this, null, subtask.exception());
            }
        }

        /**
         * Wait for a subtask started in this task scope to complete {@linkplain
         * Subtask.State#SUCCESS successfully} or all subtasks to complete.
         *
         * <p> This method waits for all subtasks by waiting for all threads {@linkplain
         * #fork(Callable) started} in this task scope to finish execution. It stops waiting
         * when all threads finish, a subtask completes successfully, or the current
         * thread is {@linkplain Thread#interrupt() interrupted}. It also stops waiting
         * if the {@link #shutdown() shutdown} method is invoked directly to shut down
         * this task scope.
         *
         * <p> This method may only be invoked by the task scope owner.
         *
         * @throws IllegalStateException {@inheritDoc}
         * @throws WrongThreadException {@inheritDoc}
         */
        @Override
        public ShutdownOnSuccess<T> join() throws InterruptedException {
            super.join();
            return this;
        }

        /**
         * Wait for a subtask started in this task scope to complete {@linkplain
         * Subtask.State#SUCCESS successfully} or all subtasks to complete, up to the
         * given deadline.
         *
         * <p> This method waits for all subtasks by waiting for all threads {@linkplain
         * #fork(Callable) started} in this task scope to finish execution. It stops waiting
         * when all threads finish, a subtask completes successfully, the deadline is
         * reached, or the current thread is {@linkplain Thread#interrupt() interrupted}.
         * It also stops waiting if the {@link #shutdown() shutdown} method is invoked
         * directly to shut down this task scope.
         *
         * <p> This method may only be invoked by the task scope owner.
         *
         * @throws IllegalStateException {@inheritDoc}
         * @throws WrongThreadException {@inheritDoc}
         */
        @Override
        public ShutdownOnSuccess<T> joinUntil(Instant deadline)
            throws InterruptedException, TimeoutException
        {
            super.joinUntil(deadline);
            return this;
        }

        /**
         * {@return the result of the first subtask that completed {@linkplain
         * Subtask.State#SUCCESS successfully}}
         *
         * <p> When no subtask completed successfully, but a subtask {@linkplain
         * Subtask.State#FAILED failed} then {@code ExecutionException} is thrown with
         * the subtask's exception as the {@linkplain Throwable#getCause() cause}.
         *
         * @throws ExecutionException if no subtasks completed successfully but at least
         * one subtask failed
         * @throws IllegalStateException if no subtasks completed or the task scope owner
         * did not join after forking
         * @throws WrongThreadException if the current thread is not the task scope owner
         */
        public T result() throws ExecutionException {
            return result(ExecutionException::new);
        }

        /**
         * Returns the result of the first subtask that completed {@linkplain
         * Subtask.State#SUCCESS successfully}, otherwise throws an exception produced
         * by the given exception supplying function.
         *
         * <p> When no subtask completed successfully, but a subtask {@linkplain
         * Subtask.State#FAILED failed}, then the exception supplying function is invoked
         * with subtask's exception.
         *
         * @param esf the exception supplying function
         * @param <X> type of the exception to be thrown
         * @return the result of the first subtask that completed with a result
         *
         * @throws X if no subtasks completed successfully but at least one subtask failed
         * @throws IllegalStateException if no subtasks completed or the task scope owner
         * did not join after forking
         * @throws WrongThreadException if the current thread is not the task scope owner
         */
        public <X extends Throwable> T result(Function<Throwable, ? extends X> esf) throws X {
            Objects.requireNonNull(esf);
            ensureOwnerAndJoined();

            Object result = firstResult;
            if (result == RESULT_NULL) {
                return null;
            } else if (result != null) {
                @SuppressWarnings("unchecked")
                T r = (T) result;
                return r;
            }

            Throwable exception = firstException;
            if (exception != null) {
                X ex = esf.apply(exception);
                Objects.requireNonNull(ex, "esf returned null");
                throw ex;
            }

            throw new IllegalStateException("No completed subtasks");
        }
    }

    /**
     * A {@code StructuredTaskScope} that captures the exception of the first subtask to
     * {@linkplain Subtask.State#FAILED fail}. Once captured, it {@linkplain #shutdown()
     * shuts down} the task scope to interrupt unfinished threads and wakeup the task
     * scope owner. The policy implemented by this class is intended for cases where the
     * results for all subtasks are required ("invoke all"); if any subtask fails then the
     * results of other unfinished subtasks are no longer needed.
     *
     * <p> Unless otherwise specified, passing a {@code null} argument to a method
     * in this class will cause a {@link NullPointerException} to be thrown.
     *
     * @apiNote This class implements a policy to shut down the task scope when a subtask
     * fails. There shouldn't be any need to directly shut down the task scope with the
     * {@link #shutdown() shutdown} method.
     *
     * @since 21
     */
    @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
    public static final class ShutdownOnFailure extends StructuredTaskScope<Object> {
        private static final VarHandle FIRST_EXCEPTION;
        static {
            try {
                MethodHandles.Lookup l = MethodHandles.lookup();
                FIRST_EXCEPTION = l.findVarHandle(ShutdownOnFailure.class, "firstException", Throwable.class);
            } catch (Exception e) {
                throw new ExceptionInInitializerError(e);
            }
        }
        private volatile Throwable firstException;

        /**
         * Constructs a new {@code ShutdownOnFailure} with the given name and thread factory.
         * The task scope is optionally named for the purposes of monitoring and management.
         * The thread factory is used to {@link ThreadFactory#newThread(Runnable) create}
         * threads when subtasks are {@linkplain #fork(Callable) forked}. The task scope
         * is owned by the current thread.
         *
         * <p> Construction captures the current thread's {@linkplain ScopedValue scoped
         * value} bindings for inheritance by threads started in the task scope. The
         * <a href="#TreeStructure">Tree Structure</a> section in the class description
         * details how parent-child relations are established implicitly for the purpose
         * of inheritance of scoped value bindings.
         *
         * @param name the name of the task scope, can be null
         * @param factory the thread factory
         */
        public ShutdownOnFailure(String name, ThreadFactory factory) {
            super(name, factory);
        }

        /**
         * Constructs a new unnamed {@code ShutdownOnFailure} that creates virtual threads.
         *
         * @implSpec This constructor is equivalent to invoking the 2-arg constructor with
         * a name of {@code null} and a thread factory that creates virtual threads.
         */
        public ShutdownOnFailure() {
            this(null, Thread.ofVirtual().factory());
        }

        @Override
        protected void handleComplete(Subtask<?> subtask) {
            if (subtask.state() == Subtask.State.FAILED
                    && firstException == null
                    && FIRST_EXCEPTION.compareAndSet(this, null, subtask.exception())) {
                super.shutdown();
            }
        }

        /**
         * Wait for all subtasks started in this task scope to complete or for a subtask
         * to {@linkplain Subtask.State#FAILED fail}.
         *
         * <p> This method waits for all subtasks by waiting for all threads {@linkplain
         * #fork(Callable) started} in this task scope to finish execution. It stops waiting
         * when all threads finish, a subtask fails, or the current thread is {@linkplain
         * Thread#interrupt() interrupted}. It also stops waiting if the {@link #shutdown()
         * shutdown} method is invoked directly to shut down this task scope.
         *
         * <p> This method may only be invoked by the task scope owner.
         *
         * @throws IllegalStateException {@inheritDoc}
         * @throws WrongThreadException {@inheritDoc}
         */
        @Override
        public ShutdownOnFailure join() throws InterruptedException {
            super.join();
            return this;
        }

        /**
         * Wait for all subtasks started in this task scope to complete or for a subtask
         * to {@linkplain Subtask.State#FAILED fail}, up to the given deadline.
         *
         * <p> This method waits for all subtasks by waiting for all threads {@linkplain
         * #fork(Callable) started} in this task scope to finish execution. It stops waiting
         * when all threads finish, a subtask fails, the deadline is reached, or the current
         * thread is {@linkplain Thread#interrupt() interrupted}. It also stops waiting
         * if the {@link #shutdown() shutdown} method is invoked directly to shut down
         * this task scope.
         *
         * <p> This method may only be invoked by the task scope owner.
         *
         * @throws IllegalStateException {@inheritDoc}
         * @throws WrongThreadException {@inheritDoc}
         */
        @Override
        public ShutdownOnFailure joinUntil(Instant deadline)
            throws InterruptedException, TimeoutException
        {
            super.joinUntil(deadline);
            return this;
        }

        /**
         * Returns the exception of the first subtask that {@linkplain Subtask.State#FAILED
         * failed}. If no subtasks failed then an empty {@code Optional} is returned.
         *
         * @return the exception for the first subtask to fail or an empty optional if no
         * subtasks failed
         *
         * @throws WrongThreadException if the current thread is not the task scope owner
         * @throws IllegalStateException if the task scope owner did not join after forking
         */
        public Optional<Throwable> exception() {
            ensureOwnerAndJoined();
            return Optional.ofNullable(firstException);
        }

        /**
         * Throws if a subtask {@linkplain Subtask.State#FAILED failed}.
         * If any subtask failed with an exception then {@code ExecutionException} is
         * thrown with the exception of the first subtask to fail as the {@linkplain
         * Throwable#getCause() cause}. This method does nothing if no subtasks failed.
         *
         * @throws ExecutionException if a subtask failed
         * @throws WrongThreadException if the current thread is not the task scope owner
         * @throws IllegalStateException if the task scope owner did not join after forking
         */
        public void throwIfFailed() throws ExecutionException {
            throwIfFailed(ExecutionException::new);
        }

        /**
         * Throws the exception produced by the given exception supplying function if a
         * subtask {@linkplain Subtask.State#FAILED failed}. If any subtask failed with
         * an exception then the function is invoked with the exception of the first
         * subtask to fail. The exception returned by the function is thrown. This method
         * does nothing if no subtasks failed.
         *
         * @param esf the exception supplying function
         * @param <X> type of the exception to be thrown
         *
         * @throws X produced by the exception supplying function
         * @throws WrongThreadException if the current thread is not the task scope owner
         * @throws IllegalStateException if the task scope owner did not join after forking
         */
        public <X extends Throwable>
        void throwIfFailed(Function<Throwable, ? extends X> esf) throws X {
            ensureOwnerAndJoined();
            Objects.requireNonNull(esf);
            Throwable exception = firstException;
            if (exception != null) {
                X ex = esf.apply(exception);
                Objects.requireNonNull(ex, "esf returned null");
                throw ex;
            }
        }
    }
}