1 /*
   2  * Copyright (c) 2021, 2023, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 package java.util.concurrent;
  26 
  27 import java.lang.invoke.MethodHandles;
  28 import java.lang.invoke.VarHandle;
  29 import java.security.AccessController;
  30 import java.security.PrivilegedAction;
  31 import java.time.Duration;
  32 import java.time.Instant;
  33 import java.util.Objects;
  34 import java.util.Optional;
  35 import java.util.concurrent.locks.ReentrantLock;
  36 import java.util.function.Function;
  37 import java.util.function.Supplier;
  38 import jdk.internal.javac.PreviewFeature;
  39 import jdk.internal.misc.ThreadFlock;
  40 
  41 /**
  42  * A basic API for <em>structured concurrency</em>. {@code StructuredTaskScope} supports
  43  * cases where a task splits into several concurrent subtasks, and where the subtasks must
  44  * complete before the main task continues. A {@code StructuredTaskScope} can be used to
  45  * ensure that the lifetime of a concurrent operation is confined by a <em>syntax block</em>,
  46  * just like that of a sequential operation in structured programming.
  47  *
  48  * <h2>Basic operation</h2>
  49  *
  50  * A {@code StructuredTaskScope} is created with one of its public constructors. It defines
  51  * the {@link #fork(Callable) fork} method to start a thread to execute a subtask, the {@link
  52  * #join() join} method to wait for all subtasks to finish, and the {@link #close() close}
  53  * method to close the task scope. The API is intended to be used with the {@code
  54  * try-with-resources} statement. The intention is that code in the try <em>block</em>
  55  * uses the {@code fork} method to fork threads to execute the subtasks, wait for the
  56  * subtasks to finish with the {@code join} method, and then <em>process the results</em>.
  57  * A call to the {@code fork} method returns a {@link Subtask Subtask} to representing
  58  * the <em>forked subtask</em>. Once {@code join} is called, the {@code Subtask} can be
  59  * used to get the result completed successfully, or the exception if the subtask failed.
  60  * {@snippet lang=java :
  61  *     Callable<String> task1 = ...
  62  *     Callable<Integer> task2 = ...
  63  *
  64  *     try (var scope = new StructuredTaskScope<Object>()) {
  65  *
  66  *         Subtask<String> subtask1 = scope.fork(task1);   // @highlight substring="fork"
  67  *         Subtask<Integer> subtask2 = scope.fork(task2);  // @highlight substring="fork"
  68  *
  69  *         scope.join();                                   // @highlight substring="join"
  70  *
  71  *         ... process results/exceptions ...
  72  *
  73  *     } // close                                          // @highlight substring="close"
  74  * }
  75  * <p> The following example forks a collection of homogeneous subtasks, waits for all of
  76  * them to complete with the {@code join} method, and uses the {@link Subtask.State
  77  * Subtask.State} to partition the subtasks into a set of the subtasks that completed
  78  * successfully and another for the subtasks that failed.
  79  * {@snippet lang=java :
  80  *     List<Callable<String>> callables = ...
  81  *
  82  *     try (var scope = new StructuredTaskScope<String>()) {
  83  *
  84  *         List<Subtask<String>> subtasks = callables.stream().map(scope::fork).toList();
  85  *
  86  *         scope.join();
  87  *
  88  *         Map<Boolean, Set<Subtask<String>>> map = subtasks.stream()
  89  *                 .collect(Collectors.partitioningBy(h -> h.state() == Subtask.State.SUCCESS,
  90  *                                                    Collectors.toSet()));
  91  *
  92  *     } // close
  93  * }
  94  *
  95  * <p> To ensure correct usage, the {@code join} and {@code close} methods may only be
  96  * invoked by the <em>owner</em> (the thread that opened/created the task scope), and the
  97  * {@code close} method throws an exception after closing if the owner did not invoke the
  98  * {@code join} method after forking.
  99  *
 100  * <p> {@code StructuredTaskScope} defines the {@link #shutdown() shutdown} method to shut
 101  * down a task scope without closing it. The {@code shutdown()} method <em>cancels</em> all
 102  * unfinished subtasks by {@linkplain Thread#interrupt() interrupting} the threads. It
 103  * prevents new threads from starting in the task scope. If the owner is waiting in the
 104  * {@code join} method then it will wakeup.
 105  *
 106  * <p> Shutdown is used for <em>short-circuiting</em> and allow subclasses to implement
 107  * <em>policy</em> that does not require all subtasks to finish.
 108  *
 109  * <h2>Subclasses with policies for common cases</h2>
 110  *
 111  * Two subclasses of {@code StructuredTaskScope} are defined to implement policy for
 112  * common cases:
 113  * <ol>
 114  *   <li> {@link ShutdownOnSuccess ShutdownOnSuccess} captures the result of the first
 115  *   subtask to complete successfully. Once captured, it shuts down the task scope to
 116  *   interrupt unfinished threads and wakeup the owner. This class is intended for cases
 117  *   where the result of any subtask will do ("invoke any") and where there is no need to
 118  *   wait for results of other unfinished subtasks. It defines methods to get the first
 119  *   result or throw an exception if all subtasks fail.
 120  *   <li> {@link ShutdownOnFailure ShutdownOnFailure} captures the exception of the first
 121  *   subtask to fail. Once captured, it shuts down the task scope to interrupt unfinished
 122  *   threads and wakeup the owner. This class is intended for cases where the results of all
 123  *   subtasks are required ("invoke all"); if any subtask fails then the results of other
 124  *   unfinished subtasks are no longer needed. If defines methods to throw an exception if
 125  *   any of the subtasks fail.
 126  * </ol>
 127  *
 128  * <p> The following are two examples that use the two classes. In both cases, a pair of
 129  * subtasks are forked to fetch resources from two URL locations "left" and "right". The
 130  * first example creates a ShutdownOnSuccess object to capture the result of the first
 131  * subtask to complete successfully, cancelling the other by way of shutting down the task
 132  * scope. The main task waits in {@code join} until either subtask completes with a result
 133  * or both subtasks fail. It invokes {@link ShutdownOnSuccess#result(Function)
 134  * result(Function)} method to get the captured result. If both subtasks fail then this
 135  * method throws a {@code WebApplicationException} with the exception from one of the
 136  * subtasks as the cause.
 137  * {@snippet lang=java :
 138  *     try (var scope = new StructuredTaskScope.ShutdownOnSuccess<String>()) {
 139  *
 140  *         scope.fork(() -> fetch(left));
 141  *         scope.fork(() -> fetch(right));
 142  *
 143  *         scope.join();
 144  *
 145  *         // @link regex="result(?=\()" target="ShutdownOnSuccess#result" :
 146  *         String result = scope.result(e -> new WebApplicationException(e));
 147  *
 148  *         ...
 149  *     }
 150  * }
 151  * The second example creates a ShutdownOnFailure object to capture the exception of the
 152  * first subtask to fail, cancelling the other by way of shutting down the task scope. The
 153  * main task waits in {@link #joinUntil(Instant)} until both subtasks complete with a
 154  * result, either fails, or a deadline is reached. It invokes {@link
 155  * ShutdownOnFailure#throwIfFailed(Function) throwIfFailed(Function)} to throw an exception
 156  * if either subtask fails. This method is a no-op if both subtasks complete successfully.
 157  * The example uses {@link Supplier#get()} to get the result of each subtask. Using
 158  * {@code Supplier} instead of {@code Subtask} is preferred for common cases where the
 159  * object returned by fork is only used to get the result of a subtask that completed
 160  * successfully.
 161  * {@snippet lang=java :
 162  *    Instant deadline = ...
 163  *
 164  *    try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
 165  *
 166  *         Supplier<String> supplier1 = scope.fork(() -> query(left));
 167  *         Supplier<String> supplier2 = scope.fork(() -> query(right));
 168  *
 169  *         scope.joinUntil(deadline);
 170  *
 171  *         // @link substring="throwIfFailed" target="ShutdownOnFailure#throwIfFailed" :
 172  *         scope.throwIfFailed(e -> new WebApplicationException(e));
 173  *
 174  *         // both subtasks completed successfully
 175  *         String result = Stream.of(supplier1, supplier2)
 176  *                 .map(Supplier::get)
 177  *                 .collect(Collectors.joining(", ", "{ ", " }"));
 178  *
 179  *         ...
 180  *     }
 181  * }
 182  *
 183  * <h2>Extending StructuredTaskScope</h2>
 184  *
 185  * {@code StructuredTaskScope} can be extended, and the {@link #handleComplete(Subtask)
 186  * handleComplete} method overridden, to implement policies other than those implemented
 187  * by {@code ShutdownOnSuccess} and {@code ShutdownOnFailure}. A subclass may, for example,
 188  * collect the results of subtasks that complete successfully and ignore subtasks that
 189  * fail. It may collect exceptions when subtasks fail. It may invoke the {@link #shutdown()
 190  * shutdown} method to shut down and cause {@link #join() join} to wakeup when some
 191  * condition arises.
 192  *
 193  * <p> A subclass will typically define methods to make available results, state, or other
 194  * outcome to code that executes after the {@code join} method. A subclass that collects
 195  * results and ignores subtasks that fail may define a method that returns the results.
 196  * A subclass that implements a policy to shut down when a subtask fails may define a
 197  * method to get the exception of the first subtask to fail.
 198  *
 199  * <p> The following is an example of a simple {@code StructuredTaskScope} implementation
 200  * that collects homogenous subtasks that complete successfully. It defines the method
 201  * "{@code completedSuccessfully()}" that the main task can invoke after it joins.
 202  * {@snippet lang=java :
 203  *     class CollectingScope<T> extends StructuredTaskScope<T> {
 204  *         private final Queue<Subtask<? extends T>> subtasks = new LinkedTransferQueue<>();
 205  *
 206  *         @Override
 207  *         protected void handleComplete(Subtask<? extends T> subtask) {
 208  *             if (subtask.state() == Subtask.State.SUCCESS) {
 209  *                 subtasks.add(subtask);
 210  *             }
 211  *         }
 212  *
 213  *         @Override
 214  *         public CollectingScope<T> join() throws InterruptedException {
 215  *             super.join();
 216  *             return this;
 217  *         }
 218  *
 219  *         public Stream<Subtask<? extends T>> completedSuccessfully() {
 220  *             // @link substring="ensureOwnerAndJoined" target="ensureOwnerAndJoined" :
 221  *             super.ensureOwnerAndJoined();
 222  *             return subtasks.stream();
 223  *         }
 224  *     }
 225  * }
 226  * <p> The implementations of the {@code completedSuccessfully()} method in the example
 227  * invokes {@link #ensureOwnerAndJoined()} to ensure that the method can only be invoked
 228  * by the owner thread and only after it has joined.
 229  *
 230  * <h2><a id="TreeStructure">Tree structure</a></h2>
 231  *
 232  * Task scopes form a tree where parent-child relations are established implicitly when
 233  * opening a new task scope:
 234  * <ul>
 235  *   <li> A parent-child relation is established when a thread started in a task scope
 236  *   opens its own task scope. A thread started in task scope "A" that opens task scope
 237  *   "B" establishes a parent-child relation where task scope "A" is the parent of task
 238  *   scope "B".
 239  *   <li> A parent-child relation is established with nesting. If a thread opens task
 240  *   scope "B", then opens task scope "C" (before it closes "B"), then the enclosing task
 241  *   scope "B" is the parent of the nested task scope "C".
 242  * </ul>
 243  *
 244  * The <i>descendants</i> of a task scope are the child task scopes that it is a parent
 245  * of, plus the descendants of the child task scopes, recursively.
 246  *
 247  * <p> The tree structure supports:
 248  * <ul>
 249  *   <li> Inheritance of {@linkplain ScopedValue scoped values} across threads.
 250  *   <li> Confinement checks. The phrase "threads contained in the task scope" in method
 251  *   descriptions means threads started in the task scope or descendant scopes.
 252  * </ul>
 253  *
 254  * <p> The following example demonstrates the inheritance of a scoped value. A scoped
 255  * value {@code USERNAME} is bound to the value "{@code duke}". A {@code StructuredTaskScope}
 256  * is created and its {@code fork} method invoked to start a thread to execute {@code
 257  * childTask}. The thread inherits the scoped value <em>bindings</em> captured when
 258  * creating the task scope. The code in {@code childTask} uses the value of the scoped
 259  * value and so reads the value "{@code duke}".
 260  * {@snippet lang=java :
 261  *     private static final ScopedValue<String> USERNAME = ScopedValue.newInstance();
 262  *
 263  *     // @link substring="runWhere" target="ScopedValue#runWhere(ScopedValue, Object, Runnable)" :
 264  *     ScopedValue.runWhere(USERNAME, "duke", () -> {
 265  *         try (var scope = new StructuredTaskScope<String>()) {
 266  *
 267  *             scope.fork(() -> childTask());           // @highlight substring="fork"
 268  *             ...
 269  *          }
 270  *     });
 271  *
 272  *     ...
 273  *
 274  *     String childTask() {
 275  *         // @link substring="get" target="ScopedValue#get()" :
 276  *         String name = USERNAME.get();   // "duke"
 277  *         ...
 278  *     }
 279  * }
 280  *
 281  * <p> {@code StructuredTaskScope} does not define APIs that exposes the tree structure
 282  * at this time.
 283  *
 284  * <p> Unless otherwise specified, passing a {@code null} argument to a constructor
 285  * or method in this class will cause a {@link NullPointerException} to be thrown.
 286  *
 287  * <h2>Memory consistency effects</h2>
 288  *
 289  * <p> Actions in the owner thread of, or a thread contained in, the task scope prior to
 290  * {@linkplain #fork forking} of a subtask
 291  * <a href="{@docRoot}/java.base/java/util/concurrent/package-summary.html#MemoryVisibility">
 292  * <i>happen-before</i></a> any actions taken by that subtask, which in turn <i>happen-before</i>
 293  * the subtask result is {@linkplain Subtask#get() retrieved} or <i>happen-before</i> any
 294  * actions taken in a thread after {@linkplain #join() joining} of the task scope.
 295  *
 296  * @jls 17.4.5 Happens-before Order
 297  *
 298  * @param <T> the result type of tasks executed in the task scope
 299  * @since 21
 300  */
 301 @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
 302 public class StructuredTaskScope<T> implements AutoCloseable {
 303     private final ThreadFactory factory;
 304     private final ThreadFlock flock;
 305     private final ReentrantLock shutdownLock = new ReentrantLock();
 306 
 307     // states: OPEN -> SHUTDOWN -> CLOSED
 308     private static final int OPEN     = 0;   // initial state
 309     private static final int SHUTDOWN = 1;
 310     private static final int CLOSED   = 2;
 311 
 312     // state: set to SHUTDOWN by any thread, set to CLOSED by owner, read by any thread
 313     private volatile int state;
 314 
 315     // Counters to support checking that the task scope owner joins before closing the task
 316     // scope. These counters are accessed only by the owner thread.
 317     private int forkRound;         // incremented when the first subtask is forked after join
 318     private int lastJoinAttempted; // set to the current fork round when join is attempted
 319     private int lastJoinCompleted; // set to the current fork round when join completes
 320 
 321     /**
 322      * Represents a subtask forked with {@link #fork(Callable)}.
 323      * @param <T> the result type
 324      * @since 21
 325      */
 326     @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
 327     public sealed interface Subtask<T> extends Supplier<T> permits SubtaskImpl {
 328         /**
 329          * {@return the value returning task provided to the {@code fork} method}
 330          *
 331          * @apiNote Task objects with unique identity may be used for correlation by
 332          * implementations of {@link #handleComplete(Subtask) handleComplete}.
 333          */
 334         Callable<? extends T> task();
 335 
 336         /**
 337          * Represents the state of a subtask.
 338          * @see Subtask#state()
 339          * @since 21
 340          */
 341         @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
 342         enum State {
 343             /**
 344              * The subtask result or exception is not available. This state indicates that
 345              * the subtask was forked but has not completed, it completed after the task
 346              * scope was {@linkplain #shutdown() shut down}, or it was forked after the
 347              * task scope was shut down.
 348              */
 349             UNAVAILABLE,
 350             /**
 351              * The subtask completed successfully with a result. The {@link Subtask#get()
 352              * Subtask.get()} method can be used to obtain the result. This is a terminal
 353              * state.
 354              */
 355             SUCCESS,
 356             /**
 357              * The subtask failed with an exception. The {@link Subtask#exception()
 358              * Subtask.exception()} method can be used to obtain the exception. This is a
 359              * terminal state.
 360              */
 361             FAILED,
 362         }
 363 
 364         /**
 365          * {@return the state of the subtask}
 366          */
 367         State state();
 368 
 369         /**
 370          * Returns the result of the subtask.
 371          *
 372          * @return the possibly-null result
 373          * @throws IllegalStateException if the subtask has not completed or did not
 374          * complete successfully
 375          * @see State#SUCCESS
 376          */
 377         T get();
 378 
 379         /**
 380          * {@return the exception thrown by the subtask}
 381          *
 382          * @throws IllegalStateException if the subtask has not completed or completed
 383          * with a result rather than an exception
 384          * @see State#FAILED
 385          */
 386         Throwable exception();
 387     }
 388 
 389     /**
 390      * Creates a structured task scope with the given name and thread factory. The task
 391      * scope is optionally named for the purposes of monitoring and management. The thread
 392      * factory is used to {@link ThreadFactory#newThread(Runnable) create} threads when
 393      * subtasks are {@linkplain #fork(Callable) forked}. The task scope is owned by the
 394      * current thread.
 395      *
 396      * <p> Construction captures the current thread's {@linkplain ScopedValue scoped value}
 397      * bindings for inheritance by threads started in the task scope. The
 398      * <a href="#TreeStructure">Tree Structure</a> section in the class description details
 399      * how parent-child relations are established implicitly for the purpose of inheritance
 400      * of scoped value bindings.
 401      *
 402      * @param name the name of the task scope, can be null
 403      * @param factory the thread factory
 404      */
 405     public StructuredTaskScope(String name, ThreadFactory factory) {
 406         this.factory = Objects.requireNonNull(factory, "'factory' is null");
 407         if (name == null)
 408             name = Objects.toIdentityString(this);
 409         this.flock = ThreadFlock.open(name);
 410     }
 411 
 412     /**
 413      * Creates an unnamed structured task scope that creates virtual threads. The task
 414      * scope is owned by the current thread.
 415      *
 416      * @implSpec This constructor is equivalent to invoking the 2-arg constructor with a
 417      * name of {@code null} and a thread factory that creates virtual threads.
 418      */
 419     public StructuredTaskScope() {
 420         this(null, Thread.ofVirtual().factory());
 421     }
 422 
 423     private IllegalStateException newIllegalStateExceptionScopeClosed() {
 424         return new IllegalStateException("Task scope is closed");
 425     }
 426 
 427     private IllegalStateException newIllegalStateExceptionNoJoin() {
 428         return new IllegalStateException("Owner did not join after forking subtasks");
 429     }
 430 
 431     /**
 432      * Throws IllegalStateException if the scope is closed, returning the state if not
 433      * closed.
 434      */
 435     private int ensureOpen() {
 436         int s = state;
 437         if (s == CLOSED)
 438             throw newIllegalStateExceptionScopeClosed();
 439         return s;
 440     }
 441 
 442     /**
 443      * Throws WrongThreadException if the current thread is not the owner.
 444      */
 445     private void ensureOwner() {
 446         if (Thread.currentThread() != flock.owner())
 447             throw new WrongThreadException("Current thread not owner");
 448     }
 449 
 450     /**
 451      * Throws WrongThreadException if the current thread is not the owner
 452      * or a thread contained in the tree.
 453      */
 454     private void ensureOwnerOrContainsThread() {
 455         Thread currentThread = Thread.currentThread();
 456         if (currentThread != flock.owner() && !flock.containsThread(currentThread))
 457             throw new WrongThreadException("Current thread not owner or thread in the tree");
 458     }
 459 
 460     /**
 461      * Ensures that the current thread is the owner of this task scope and that it joined
 462      * (with {@link #join()} or {@link #joinUntil(Instant)}) after {@linkplain #fork(Callable)
 463      * forking} subtasks.
 464      *
 465      * @apiNote This method can be used by subclasses that define methods to make available
 466      * results, state, or other outcome to code intended to execute after the join method.
 467      *
 468      * @throws WrongThreadException if the current thread is not the task scope owner
 469      * @throws IllegalStateException if the task scope is open and task scope owner did
 470      * not join after forking
 471      */
 472     protected final void ensureOwnerAndJoined() {
 473         ensureOwner();
 474         if (forkRound > lastJoinCompleted) {
 475             throw newIllegalStateExceptionNoJoin();
 476         }
 477     }
 478 
 479     /**
 480      * Invoked by a subtask when it completes successfully or fails in this task scope.
 481      * This method is not invoked if a subtask completes after the task scope is
 482      * {@linkplain #shutdown() shut down}.
 483      *
 484      * @implSpec The default implementation throws {@code NullPointerException} if the
 485      * subtask is {@code null}. It throws {@link IllegalArgumentException} if the subtask
 486      * has not completed.
 487      *
 488      * @apiNote The {@code handleComplete} method should be thread safe. It may be
 489      * invoked by several threads concurrently.
 490      *
 491      * @param subtask the subtask
 492      *
 493      * @throws IllegalArgumentException if called with a subtask that has not completed
 494      */
 495     protected void handleComplete(Subtask<? extends T> subtask) {
 496         if (subtask.state() == Subtask.State.UNAVAILABLE)
 497             throw new IllegalArgumentException();
 498     }
 499 
 500     /**
 501      * Starts a new thread in this task scope to execute a value-returning task, thus
 502      * creating a <em>subtask</em> of this task scope.
 503      *
 504      * <p> The value-returning task is provided to this method as a {@link Callable}, the
 505      * thread executes the task's {@link Callable#call() call} method. The thread is
 506      * created with the task scope's {@link ThreadFactory}. It inherits the current thread's
 507      * {@linkplain ScopedValue scoped value} bindings. The bindings must match the bindings
 508      * captured when the task scope was created.
 509      *
 510      * <p> This method returns a {@link Subtask Subtask} to represent the <em>forked
 511      * subtask</em>. The {@code Subtask} object can be used to obtain the result when
 512      * the subtask completes successfully, or the exception when the subtask fails. To
 513      * ensure correct usage, the {@link Subtask#get() get()} and {@link Subtask#exception()
 514      * exception()} methods may only be called by the task scope owner after it has waited
 515      * for all threads to finish with the {@link #join() join} or {@link #joinUntil(Instant)}
 516      * methods. When the subtask completes, the thread invokes the {@link
 517      * #handleComplete(Subtask) handleComplete} method to consume the completed subtask.
 518      * If the task scope is {@linkplain #shutdown() shut down} before the subtask completes
 519      * then the {@code handleComplete} method will not be invoked.
 520      *
 521      * <p> If this task scope is {@linkplain #shutdown() shutdown} (or in the process of
 522      * shutting down) then the subtask will not run and the {@code handleComplete} method
 523      * will not be invoked.
 524      *
 525      * <p> This method may only be invoked by the task scope owner or threads contained
 526      * in the task scope.
 527      *
 528      * @implSpec This method may be overridden for customization purposes, wrapping tasks
 529      * for example. If overridden, the subclass must invoke {@code super.fork} to start a
 530      * new thread in this task scope.
 531      *
 532      * @param task the value-returning task for the thread to execute
 533      * @param <U> the result type
 534      * @return the subtask
 535      * @throws IllegalStateException if this task scope is closed
 536      * @throws WrongThreadException if the current thread is not the task scope owner or a
 537      * thread contained in the task scope
 538      * @throws StructureViolationException if the current scoped value bindings are not
 539      * the same as when the task scope was created
 540      * @throws RejectedExecutionException if the thread factory rejected creating a
 541      * thread to run the subtask
 542      */
 543     public <U extends T> Subtask<U> fork(Callable<? extends U> task) {
 544         Objects.requireNonNull(task, "'task' is null");
 545         int s = ensureOpen();   // throws ISE if closed
 546 
 547         // when forked by the owner, the subtask is forked in the current or next round
 548         int round = -1;
 549         if (Thread.currentThread() == flock.owner()) {
 550             round = forkRound;
 551             if (forkRound == lastJoinCompleted) {
 552                 // new round if first fork after join
 553                 round++;
 554             }
 555         }
 556 
 557         var subtask = new SubtaskImpl<U>(this, task);
 558         if (s < SHUTDOWN) {
 559             // create thread to run task
 560             Thread thread = factory.newThread(subtask);
 561             if (thread == null) {
 562                 throw new RejectedExecutionException("Rejected by thread factory");
 563             }
 564 
 565             // attempt to start the thread
 566             try {
 567                 flock.start(thread);
 568             } catch (IllegalStateException e) {
 569                 // shutdown by another thread, or underlying flock is shutdown due
 570                 // to unstructured use
 571             }
 572         }
 573 
 574         // force owner to join if this is the first fork in the round
 575         if (Thread.currentThread() == flock.owner() && round > forkRound) {
 576             forkRound = round;
 577         }
 578 
 579         // return forked subtask or a subtask that did not run
 580         return subtask;
 581     }
 582 
 583     /**
 584      * Wait for all threads to finish or the task scope to shut down.
 585      */
 586     private void implJoin(Duration timeout)
 587         throws InterruptedException, TimeoutException
 588     {
 589         ensureOwner();
 590         lastJoinAttempted = forkRound;
 591         int s = ensureOpen();  // throws ISE if closed
 592         if (s == OPEN) {
 593             // wait for all threads, wakeup, interrupt, or timeout
 594             if (timeout != null) {
 595                 flock.awaitAll(timeout);
 596             } else {
 597                 flock.awaitAll();
 598             }
 599         }
 600         lastJoinCompleted = forkRound;
 601     }
 602 
 603     /**
 604      * Wait for all subtasks started in this task scope to finish or the task scope to
 605      * shut down.
 606      *
 607      * <p> This method waits for all subtasks by waiting for all threads {@linkplain
 608      * #fork(Callable) started} in this task scope to finish execution. It stops waiting
 609      * when all threads finish, the task scope is {@linkplain #shutdown() shut down}, or
 610      * the current thread is {@linkplain Thread#interrupt() interrupted}.
 611      *
 612      * <p> This method may only be invoked by the task scope owner.
 613      *
 614      * @implSpec This method may be overridden for customization purposes or to return a
 615      * more specific return type. If overridden, the subclass must invoke {@code
 616      * super.join} to ensure that the method waits for threads in this task scope to
 617      * finish.
 618      *
 619      * @return this task scope
 620      * @throws IllegalStateException if this task scope is closed
 621      * @throws WrongThreadException if the current thread is not the task scope owner
 622      * @throws InterruptedException if interrupted while waiting
 623      */
 624     public StructuredTaskScope<T> join() throws InterruptedException {
 625         try {
 626             implJoin(null);
 627         } catch (TimeoutException e) {
 628             throw new InternalError();
 629         }
 630         return this;
 631     }
 632 
 633     /**
 634      * Wait for all subtasks started in this task scope to finish or the task scope to
 635      * shut down, up to the given deadline.
 636      *
 637      * <p> This method waits for all subtasks by waiting for all threads {@linkplain
 638      * #fork(Callable) started} in this task scope to finish execution. It stops waiting
 639      * when all threads finish, the task scope is {@linkplain #shutdown() shut down}, the
 640      * deadline is reached, or the current thread is {@linkplain Thread#interrupt()
 641      * interrupted}.
 642      *
 643      * <p> This method may only be invoked by the task scope owner.
 644      *
 645      * @implSpec This method may be overridden for customization purposes or to return a
 646      * more specific return type. If overridden, the subclass must invoke {@code
 647      * super.joinUntil} to ensure that the method waits for threads in this task scope to
 648      * finish.
 649      *
 650      * @param deadline the deadline
 651      * @return this task scope
 652      * @throws IllegalStateException if this task scope is closed
 653      * @throws WrongThreadException if the current thread is not the task scope owner
 654      * @throws InterruptedException if interrupted while waiting
 655      * @throws TimeoutException if the deadline is reached while waiting
 656      */
 657     public StructuredTaskScope<T> joinUntil(Instant deadline)
 658         throws InterruptedException, TimeoutException
 659     {
 660         Duration timeout = Duration.between(Instant.now(), deadline);
 661         implJoin(timeout);
 662         return this;
 663     }
 664 
 665     /**
 666      * Interrupt all unfinished threads.
 667      */
 668     private void implInterruptAll() {
 669         flock.threads()
 670             .filter(t -> t != Thread.currentThread())
 671             .forEach(t -> {
 672                 try {
 673                     t.interrupt();
 674                 } catch (Throwable ignore) { }
 675             });
 676     }
 677 
 678     @SuppressWarnings("removal")
 679     private void interruptAll() {
 680         if (System.getSecurityManager() == null) {
 681             implInterruptAll();
 682         } else {
 683             PrivilegedAction<Void> pa = () -> {
 684                 implInterruptAll();
 685                 return null;
 686             };
 687             AccessController.doPrivileged(pa);
 688         }
 689     }
 690 
 691     /**
 692      * Shutdown the task scope if not already shutdown. Return true if this method
 693      * shutdowns the task scope, false if already shutdown.
 694      */
 695     private boolean implShutdown() {
 696         shutdownLock.lock();
 697         try {
 698             if (state < SHUTDOWN) {
 699                 // prevent new threads from starting
 700                 flock.shutdown();
 701 
 702                 // set status before interrupting tasks
 703                 state = SHUTDOWN;
 704 
 705                 // interrupt all unfinished threads
 706                 interruptAll();
 707 
 708                 return true;
 709             } else {
 710                 // already shutdown
 711                 return false;
 712             }
 713         } finally {
 714             shutdownLock.unlock();
 715         }
 716     }
 717 
 718     /**
 719      * Shut down this task scope without closing it. Shutting down a task scope prevents
 720      * new threads from starting, interrupts all unfinished threads, and causes the
 721      * {@link #join() join} method to wakeup. Shutdown is useful for cases where the
 722      * results of unfinished subtasks are no longer needed. It will typically be called
 723      * by the {@link #handleComplete(Subtask)} implementation of a subclass that
 724      * implements a policy to discard unfinished tasks once some outcome is reached.
 725      *
 726      * <p> More specifically, this method:
 727      * <ul>
 728      * <li> {@linkplain Thread#interrupt() Interrupts} all unfinished threads in the
 729      * task scope (except the current thread).
 730      * <li> Wakes up the task scope owner if it is waiting in {@link #join()} or {@link
 731      * #joinUntil(Instant)}. If the task scope owner is not waiting then its next call to
 732      * {@code join} or {@code joinUntil} will return immediately.
 733      * </ul>
 734      *
 735      * <p> The {@linkplain Subtask.State state} of unfinished subtasks that complete at
 736      * around the time that the task scope is shutdown is not defined. A subtask that
 737      * completes successfully with a result, or fails with an exception, at around
 738      * the time that the task scope is shutdown may or may not <i>transition</i> to a
 739      * terminal state.
 740      *
 741      * <p> This method may only be invoked by the task scope owner or threads contained
 742      * in the task scope.
 743      *
 744      * @implSpec This method may be overridden for customization purposes. If overridden,
 745      * the subclass must invoke {@code super.shutdown} to ensure that the method shuts
 746      * down the task scope.
 747      *
 748      * @apiNote
 749      * There may be threads that have not finished because they are executing code that
 750      * did not respond (or respond promptly) to thread interrupt. This method does not wait
 751      * for these threads. When the owner invokes the {@link #close() close} method
 752      * to close the task scope then it will wait for the remaining threads to finish.
 753      *
 754      * @throws IllegalStateException if this task scope is closed
 755      * @throws WrongThreadException if the current thread is not the task scope owner or
 756      * a thread contained in the task scope
 757      * @see #isShutdown()
 758      */
 759     public void shutdown() {
 760         ensureOwnerOrContainsThread();
 761         int s = ensureOpen();  // throws ISE if closed
 762         if (s < SHUTDOWN && implShutdown())
 763             flock.wakeup();
 764     }
 765 
 766     /**
 767      * {@return true if this task scope is shutdown, otherwise false}
 768      * @see #shutdown()
 769      */
 770     public final boolean isShutdown() {
 771         return state >= SHUTDOWN;
 772     }
 773 
 774     /**
 775      * Closes this task scope.
 776      *
 777      * <p> This method first shuts down the task scope (as if by invoking the {@link
 778      * #shutdown() shutdown} method). It then waits for the threads executing any
 779      * unfinished tasks to finish. If interrupted, this method will continue to wait for
 780      * the threads to finish before completing with the interrupt status set.
 781      *
 782      * <p> This method may only be invoked by the task scope owner. If the task scope
 783      * is already closed then the task scope owner invoking this method has no effect.
 784      *
 785      * <p> A {@code StructuredTaskScope} is intended to be used in a <em>structured
 786      * manner</em>. If this method is called to close a task scope before nested task
 787      * scopes are closed then it closes the underlying construct of each nested task scope
 788      * (in the reverse order that they were created in), closes this task scope, and then
 789      * throws {@link StructureViolationException}.
 790      * Similarly, if this method is called to close a task scope while executing with
 791      * {@linkplain ScopedValue scoped value} bindings, and the task scope was created
 792      * before the scoped values were bound, then {@code StructureViolationException} is
 793      * thrown after closing the task scope.
 794      * If a thread terminates without first closing task scopes that it owns then
 795      * termination will cause the underlying construct of each of its open tasks scopes to
 796      * be closed. Closing is performed in the reverse order that the task scopes were
 797      * created in. Thread termination may therefore be delayed when the task scope owner
 798      * has to wait for threads forked in these task scopes to finish.
 799      *
 800      * @implSpec This method may be overridden for customization purposes. If overridden,
 801      * the subclass must invoke {@code super.close} to close the task scope.
 802      *
 803      * @throws IllegalStateException thrown after closing the task scope if the task scope
 804      * owner did not attempt to join after forking
 805      * @throws WrongThreadException if the current thread is not the task scope owner
 806      * @throws StructureViolationException if a structure violation was detected
 807      */
 808     @Override
 809     public void close() {
 810         ensureOwner();
 811         int s = state;
 812         if (s == CLOSED)
 813             return;
 814 
 815         try {
 816             if (s < SHUTDOWN)
 817                 implShutdown();
 818             flock.close();
 819         } finally {
 820             state = CLOSED;
 821         }
 822 
 823         // throw ISE if the owner didn't attempt to join after forking
 824         if (forkRound > lastJoinAttempted) {
 825             lastJoinCompleted = forkRound;  // ensureOwnerAndJoined is a no-op after close
 826             throw newIllegalStateExceptionNoJoin();
 827         }
 828     }
 829 
 830     @Override
 831     public String toString() {
 832         String name = flock.name();
 833         return switch (state) {
 834             case OPEN     -> name;
 835             case SHUTDOWN -> name + "/shutdown";
 836             case CLOSED   -> name + "/closed";
 837             default -> throw new InternalError();
 838         };
 839     }
 840 
 841     /**
 842      * Subtask implementation, runs the task specified to the fork method.
 843      */
 844     private static final class SubtaskImpl<T> implements Subtask<T>, Runnable {
 845         private static final AltResult RESULT_NULL = new AltResult(Subtask.State.SUCCESS);
 846 
 847         private record AltResult(Subtask.State state, Throwable exception) {
 848             AltResult(Subtask.State state) {
 849                 this(state, null);
 850             }
 851         }
 852 
 853         private final StructuredTaskScope<? super T> scope;
 854         private final Callable<? extends T> task;
 855         private volatile Object result;
 856 
 857         SubtaskImpl(StructuredTaskScope<? super T> scope, Callable<? extends T> task) {
 858             this.scope = scope;
 859             this.task = task;
 860         }
 861 
 862         @Override
 863         public void run() {
 864             T result = null;
 865             Throwable ex = null;
 866             try {
 867                 result = task.call();
 868             } catch (Throwable e) {
 869                 ex = e;
 870             }
 871 
 872             // nothing to do if task scope is shutdown
 873             if (scope.isShutdown())
 874                 return;
 875 
 876             // capture result or exception, invoke handleComplete
 877             if (ex == null) {
 878                 this.result = (result != null) ? result : RESULT_NULL;
 879             } else {
 880                 this.result = new AltResult(State.FAILED, ex);
 881             }
 882             scope.handleComplete(this);
 883         }
 884 
 885         @Override
 886         public Callable<? extends T> task() {
 887             return task;
 888         }
 889 
 890         @Override
 891         public Subtask.State state() {
 892             Object result = this.result;
 893             if (result == null) {
 894                 return State.UNAVAILABLE;
 895             } else if (result instanceof AltResult alt) {
 896                 // null or failed
 897                 return alt.state();
 898             } else {
 899                 return State.SUCCESS;
 900             }
 901         }
 902 
 903         @Override
 904         public T get() {
 905             Object result = this.result;
 906             if (result instanceof AltResult) {
 907                 if (result == RESULT_NULL) return null;
 908             } else if (result != null) {
 909                 @SuppressWarnings("unchecked")
 910                 T r = (T) result;
 911                 return r;
 912             }
 913             throw new IllegalStateException(
 914                     "Result is unavailable or subtask did not complete successfully");
 915         }
 916 
 917         @Override
 918         public Throwable exception() {
 919             Object result = this.result;
 920             if (result instanceof AltResult alt && alt.state() == State.FAILED) {
 921                 return alt.exception();
 922             }
 923             throw new IllegalStateException(
 924                     "Exception is unavailable or subtask did not complete with exception");
 925         }
 926 
 927         @Override
 928         public String toString() {
 929             String stateAsString = switch (state()) {
 930                 case UNAVAILABLE -> "[Unavailable]";
 931                 case SUCCESS     -> "[Completed successfully]";
 932                 case FAILED      -> {
 933                     Throwable ex = ((AltResult) result).exception();
 934                     yield "[Failed: " + ex + "]";
 935                 }
 936             };
 937             return Objects.toIdentityString(this) + stateAsString;
 938         }
 939     }
 940 
 941     /**
 942      * A {@code StructuredTaskScope} that captures the result of the first subtask to
 943      * complete {@linkplain Subtask.State#SUCCESS successfully}. Once captured, it
 944      * {@linkplain #shutdown() shuts down} the task scope to interrupt unfinished threads
 945      * and wakeup the task scope owner. The policy implemented by this class is intended
 946      * for cases where the result of any subtask will do ("invoke any") and where the
 947      * results of other unfinished subtasks are no longer needed.
 948      *
 949      * <p> Unless otherwise specified, passing a {@code null} argument to a method
 950      * in this class will cause a {@link NullPointerException} to be thrown.
 951      *
 952      * @apiNote This class implements a policy to shut down the task scope when a subtask
 953      * completes successfully. There shouldn't be any need to directly shut down the task
 954      * scope with the {@link #shutdown() shutdown} method.
 955      *
 956      * @param <T> the result type
 957      * @since 21
 958      */
 959     @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
 960     public static final class ShutdownOnSuccess<T> extends StructuredTaskScope<T> {
 961         private static final Object RESULT_NULL = new Object();
 962         private static final VarHandle FIRST_RESULT;
 963         private static final VarHandle FIRST_EXCEPTION;
 964         static {
 965             try {
 966                 MethodHandles.Lookup l = MethodHandles.lookup();
 967                 FIRST_RESULT = l.findVarHandle(ShutdownOnSuccess.class, "firstResult", Object.class);
 968                 FIRST_EXCEPTION = l.findVarHandle(ShutdownOnSuccess.class, "firstException", Throwable.class);
 969             } catch (Exception e) {
 970                 throw new ExceptionInInitializerError(e);
 971             }
 972         }
 973         private volatile Object firstResult;
 974         private volatile Throwable firstException;
 975 
 976         /**
 977          * Constructs a new {@code ShutdownOnSuccess} with the given name and thread factory.
 978          * The task scope is optionally named for the purposes of monitoring and management.
 979          * The thread factory is used to {@link ThreadFactory#newThread(Runnable) create}
 980          * threads when subtasks are {@linkplain #fork(Callable) forked}. The task scope
 981          * is owned by the current thread.
 982          *
 983          * <p> Construction captures the current thread's {@linkplain ScopedValue scoped
 984          * value} bindings for inheritance by threads started in the task scope. The
 985          * <a href="#TreeStructure">Tree Structure</a> section in the class description
 986          * details how parent-child relations are established implicitly for the purpose
 987          * of inheritance of scoped value bindings.
 988          *
 989          * @param name the name of the task scope, can be null
 990          * @param factory the thread factory
 991          */
 992         public ShutdownOnSuccess(String name, ThreadFactory factory) {
 993             super(name, factory);
 994         }
 995 
 996         /**
 997          * Constructs a new unnamed {@code ShutdownOnSuccess} that creates virtual threads.
 998          *
 999          * @implSpec This constructor is equivalent to invoking the 2-arg constructor with
1000          * a name of {@code null} and a thread factory that creates virtual threads.
1001          */
1002         public ShutdownOnSuccess() {
1003             this(null, Thread.ofVirtual().factory());
1004         }
1005 
1006         @Override
1007         protected void handleComplete(Subtask<? extends T> subtask) {
1008             if (firstResult != null) {
1009                 // already captured a result
1010                 return;
1011             }
1012 
1013             if (subtask.state() == Subtask.State.SUCCESS) {
1014                 // task succeeded
1015                 T result = subtask.get();
1016                 Object r = (result != null) ? result : RESULT_NULL;
1017                 if (FIRST_RESULT.compareAndSet(this, null, r)) {
1018                     super.shutdown();
1019                 }
1020             } else if (firstException == null) {
1021                 // capture the exception thrown by the first subtask that failed
1022                 FIRST_EXCEPTION.compareAndSet(this, null, subtask.exception());
1023             }
1024         }
1025 
1026         /**
1027          * Wait for a subtask started in this task scope to complete {@linkplain
1028          * Subtask.State#SUCCESS successfully} or all subtasks to complete.
1029          *
1030          * <p> This method waits for all subtasks by waiting for all threads {@linkplain
1031          * #fork(Callable) started} in this task scope to finish execution. It stops waiting
1032          * when all threads finish, a subtask completes successfully, or the current
1033          * thread is {@linkplain Thread#interrupt() interrupted}. It also stops waiting
1034          * if the {@link #shutdown() shutdown} method is invoked directly to shut down
1035          * this task scope.
1036          *
1037          * <p> This method may only be invoked by the task scope owner.
1038          *
1039          * @throws IllegalStateException {@inheritDoc}
1040          * @throws WrongThreadException {@inheritDoc}
1041          */
1042         @Override
1043         public ShutdownOnSuccess<T> join() throws InterruptedException {
1044             super.join();
1045             return this;
1046         }
1047 
1048         /**
1049          * Wait for a subtask started in this task scope to complete {@linkplain
1050          * Subtask.State#SUCCESS successfully} or all subtasks to complete, up to the
1051          * given deadline.
1052          *
1053          * <p> This method waits for all subtasks by waiting for all threads {@linkplain
1054          * #fork(Callable) started} in this task scope to finish execution. It stops waiting
1055          * when all threads finish, a subtask completes successfully, the deadline is
1056          * reached, or the current thread is {@linkplain Thread#interrupt() interrupted}.
1057          * It also stops waiting if the {@link #shutdown() shutdown} method is invoked
1058          * directly to shut down this task scope.
1059          *
1060          * <p> This method may only be invoked by the task scope owner.
1061          *
1062          * @throws IllegalStateException {@inheritDoc}
1063          * @throws WrongThreadException {@inheritDoc}
1064          */
1065         @Override
1066         public ShutdownOnSuccess<T> joinUntil(Instant deadline)
1067             throws InterruptedException, TimeoutException
1068         {
1069             super.joinUntil(deadline);
1070             return this;
1071         }
1072 
1073         /**
1074          * {@return the result of the first subtask that completed {@linkplain
1075          * Subtask.State#SUCCESS successfully}}
1076          *
1077          * <p> When no subtask completed successfully, but a subtask {@linkplain
1078          * Subtask.State#FAILED failed} then {@code ExecutionException} is thrown with
1079          * the subtask's exception as the {@linkplain Throwable#getCause() cause}.
1080          *
1081          * @throws ExecutionException if no subtasks completed successfully but at least
1082          * one subtask failed
1083          * @throws IllegalStateException if no subtasks completed or the task scope owner
1084          * did not join after forking
1085          * @throws WrongThreadException if the current thread is not the task scope owner
1086          */
1087         public T result() throws ExecutionException {
1088             return result(ExecutionException::new);
1089         }
1090 
1091         /**
1092          * Returns the result of the first subtask that completed {@linkplain
1093          * Subtask.State#SUCCESS successfully}, otherwise throws an exception produced
1094          * by the given exception supplying function.
1095          *
1096          * <p> When no subtask completed successfully, but a subtask {@linkplain
1097          * Subtask.State#FAILED failed}, then the exception supplying function is invoked
1098          * with subtask's exception.
1099          *
1100          * @param esf the exception supplying function
1101          * @param <X> type of the exception to be thrown
1102          * @return the result of the first subtask that completed with a result
1103          *
1104          * @throws X if no subtasks completed successfully but at least one subtask failed
1105          * @throws IllegalStateException if no subtasks completed or the task scope owner
1106          * did not join after forking
1107          * @throws WrongThreadException if the current thread is not the task scope owner
1108          */
1109         public <X extends Throwable> T result(Function<Throwable, ? extends X> esf) throws X {
1110             Objects.requireNonNull(esf);
1111             ensureOwnerAndJoined();
1112 
1113             Object result = firstResult;
1114             if (result == RESULT_NULL) {
1115                 return null;
1116             } else if (result != null) {
1117                 @SuppressWarnings("unchecked")
1118                 T r = (T) result;
1119                 return r;
1120             }
1121 
1122             Throwable exception = firstException;
1123             if (exception != null) {
1124                 X ex = esf.apply(exception);
1125                 Objects.requireNonNull(ex, "esf returned null");
1126                 throw ex;
1127             }
1128 
1129             throw new IllegalStateException("No completed subtasks");
1130         }
1131     }
1132 
1133     /**
1134      * A {@code StructuredTaskScope} that captures the exception of the first subtask to
1135      * {@linkplain Subtask.State#FAILED fail}. Once captured, it {@linkplain #shutdown()
1136      * shuts down} the task scope to interrupt unfinished threads and wakeup the task
1137      * scope owner. The policy implemented by this class is intended for cases where the
1138      * results for all subtasks are required ("invoke all"); if any subtask fails then the
1139      * results of other unfinished subtasks are no longer needed.
1140      *
1141      * <p> Unless otherwise specified, passing a {@code null} argument to a method
1142      * in this class will cause a {@link NullPointerException} to be thrown.
1143      *
1144      * @apiNote This class implements a policy to shut down the task scope when a subtask
1145      * fails. There shouldn't be any need to directly shut down the task scope with the
1146      * {@link #shutdown() shutdown} method.
1147      *
1148      * @since 21
1149      */
1150     @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
1151     public static final class ShutdownOnFailure extends StructuredTaskScope<Object> {
1152         private static final VarHandle FIRST_EXCEPTION;
1153         static {
1154             try {
1155                 MethodHandles.Lookup l = MethodHandles.lookup();
1156                 FIRST_EXCEPTION = l.findVarHandle(ShutdownOnFailure.class, "firstException", Throwable.class);
1157             } catch (Exception e) {
1158                 throw new ExceptionInInitializerError(e);
1159             }
1160         }
1161         private volatile Throwable firstException;
1162 
1163         /**
1164          * Constructs a new {@code ShutdownOnFailure} with the given name and thread factory.
1165          * The task scope is optionally named for the purposes of monitoring and management.
1166          * The thread factory is used to {@link ThreadFactory#newThread(Runnable) create}
1167          * threads when subtasks are {@linkplain #fork(Callable) forked}. The task scope
1168          * is owned by the current thread.
1169          *
1170          * <p> Construction captures the current thread's {@linkplain ScopedValue scoped
1171          * value} bindings for inheritance by threads started in the task scope. The
1172          * <a href="#TreeStructure">Tree Structure</a> section in the class description
1173          * details how parent-child relations are established implicitly for the purpose
1174          * of inheritance of scoped value bindings.
1175          *
1176          * @param name the name of the task scope, can be null
1177          * @param factory the thread factory
1178          */
1179         public ShutdownOnFailure(String name, ThreadFactory factory) {
1180             super(name, factory);
1181         }
1182 
1183         /**
1184          * Constructs a new unnamed {@code ShutdownOnFailure} that creates virtual threads.
1185          *
1186          * @implSpec This constructor is equivalent to invoking the 2-arg constructor with
1187          * a name of {@code null} and a thread factory that creates virtual threads.
1188          */
1189         public ShutdownOnFailure() {
1190             this(null, Thread.ofVirtual().factory());
1191         }
1192 
1193         @Override
1194         protected void handleComplete(Subtask<?> subtask) {
1195             if (subtask.state() == Subtask.State.FAILED
1196                     && firstException == null
1197                     && FIRST_EXCEPTION.compareAndSet(this, null, subtask.exception())) {
1198                 super.shutdown();
1199             }
1200         }
1201 
1202         /**
1203          * Wait for all subtasks started in this task scope to complete or for a subtask
1204          * to {@linkplain Subtask.State#FAILED fail}.
1205          *
1206          * <p> This method waits for all subtasks by waiting for all threads {@linkplain
1207          * #fork(Callable) started} in this task scope to finish execution. It stops waiting
1208          * when all threads finish, a subtask fails, or the current thread is {@linkplain
1209          * Thread#interrupt() interrupted}. It also stops waiting if the {@link #shutdown()
1210          * shutdown} method is invoked directly to shut down this task scope.
1211          *
1212          * <p> This method may only be invoked by the task scope owner.
1213          *
1214          * @throws IllegalStateException {@inheritDoc}
1215          * @throws WrongThreadException {@inheritDoc}
1216          */
1217         @Override
1218         public ShutdownOnFailure join() throws InterruptedException {
1219             super.join();
1220             return this;
1221         }
1222 
1223         /**
1224          * Wait for all subtasks started in this task scope to complete or for a subtask
1225          * to {@linkplain Subtask.State#FAILED fail}, up to the given deadline.
1226          *
1227          * <p> This method waits for all subtasks by waiting for all threads {@linkplain
1228          * #fork(Callable) started} in this task scope to finish execution. It stops waiting
1229          * when all threads finish, a subtask fails, the deadline is reached, or the current
1230          * thread is {@linkplain Thread#interrupt() interrupted}. It also stops waiting
1231          * if the {@link #shutdown() shutdown} method is invoked directly to shut down
1232          * this task scope.
1233          *
1234          * <p> This method may only be invoked by the task scope owner.
1235          *
1236          * @throws IllegalStateException {@inheritDoc}
1237          * @throws WrongThreadException {@inheritDoc}
1238          */
1239         @Override
1240         public ShutdownOnFailure joinUntil(Instant deadline)
1241             throws InterruptedException, TimeoutException
1242         {
1243             super.joinUntil(deadline);
1244             return this;
1245         }
1246 
1247         /**
1248          * Returns the exception of the first subtask that {@linkplain Subtask.State#FAILED
1249          * failed}. If no subtasks failed then an empty {@code Optional} is returned.
1250          *
1251          * @return the exception for the first subtask to fail or an empty optional if no
1252          * subtasks failed
1253          *
1254          * @throws WrongThreadException if the current thread is not the task scope owner
1255          * @throws IllegalStateException if the task scope owner did not join after forking
1256          */
1257         public Optional<Throwable> exception() {
1258             ensureOwnerAndJoined();
1259             return Optional.ofNullable(firstException);
1260         }
1261 
1262         /**
1263          * Throws if a subtask {@linkplain Subtask.State#FAILED failed}.
1264          * If any subtask failed with an exception then {@code ExecutionException} is
1265          * thrown with the exception of the first subtask to fail as the {@linkplain
1266          * Throwable#getCause() cause}. This method does nothing if no subtasks failed.
1267          *
1268          * @throws ExecutionException if a subtask failed
1269          * @throws WrongThreadException if the current thread is not the task scope owner
1270          * @throws IllegalStateException if the task scope owner did not join after forking
1271          */
1272         public void throwIfFailed() throws ExecutionException {
1273             throwIfFailed(ExecutionException::new);
1274         }
1275 
1276         /**
1277          * Throws the exception produced by the given exception supplying function if a
1278          * subtask {@linkplain Subtask.State#FAILED failed}. If any subtask failed with
1279          * an exception then the function is invoked with the exception of the first
1280          * subtask to fail. The exception returned by the function is thrown. This method
1281          * does nothing if no subtasks failed.
1282          *
1283          * @param esf the exception supplying function
1284          * @param <X> type of the exception to be thrown
1285          *
1286          * @throws X produced by the exception supplying function
1287          * @throws WrongThreadException if the current thread is not the task scope owner
1288          * @throws IllegalStateException if the task scope owner did not join after forking
1289          */
1290         public <X extends Throwable>
1291         void throwIfFailed(Function<Throwable, ? extends X> esf) throws X {
1292             ensureOwnerAndJoined();
1293             Objects.requireNonNull(esf);
1294             Throwable exception = firstException;
1295             if (exception != null) {
1296                 X ex = esf.apply(exception);
1297                 Objects.requireNonNull(ex, "esf returned null");
1298                 throw ex;
1299             }
1300         }
1301     }
1302 }