/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.util.concurrent;

import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Throwables.throwIfUnchecked;
import static java.util.concurrent.atomic.AtomicReferenceFieldUpdater.newUpdater;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Strings;
import com.google.common.util.concurrent.internal.InternalFutureFailureAccess;
import com.google.common.util.concurrent.internal.InternalFutures;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.ForOverride;
import com.google.j2objc.annotations.ReflectionSupport;
import java.security.AccessController;
import java.security.PrivilegedActionException;
import java.security.PrivilegedExceptionAction;
import java.util.Locale;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.Future;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
import java.util.concurrent.locks.LockSupport;
import java.util.logging.Level;
import java.util.logging.Logger;
import org.checkerframework.checker.nullness.compatqual.NullableDecl;

/**
 * An abstract implementation of {@link ListenableFuture}, intended for advanced users only. More
 * common ways to create a {@code ListenableFuture} include instantiating a {@link SettableFuture},
 * submitting a task to a {@link ListeningExecutorService}, and deriving a {@code Future} from an
 * existing one, typically using methods like {@link Futures#transform(ListenableFuture,
 * com.google.common.base.Function, java.util.concurrent.Executor) Futures.transform} and {@link
 * Futures#catching(ListenableFuture, Class, com.google.common.base.Function,
 * java.util.concurrent.Executor) Futures.catching}.
 *
 * <p>This class implements all methods in {@code ListenableFuture}. Subclasses should provide a way
 * to set the result of the computation through the protected methods {@link #set(Object)}, {@link
 * #setFuture(ListenableFuture)} and {@link #setException(Throwable)}. Subclasses may also override
 * {@link #afterDone()}, which will be invoked automatically when the future completes. Subclasses
 * should rarely override other methods.
 *
 * @author Sven Mawson
 * @author Luke Sandberg
 * @since 1.0
 */
@SuppressWarnings("ShortCircuitBoolean") // we use non-short circuiting comparisons intentionally
@GwtCompatible(emulated = true)
@ReflectionSupport(value = ReflectionSupport.Level.FULL)
public abstract class AbstractFuture<V> extends InternalFutureFailureAccess
    implements ListenableFuture<V> {
  // NOTE: Whenever both tests are cheap and functional, it's faster to use &, | instead of &&, ||

  private static final boolean GENERATE_CANCELLATION_CAUSES;

  static {
    // System.getProperty may throw if the security policy does not permit access.
    boolean generateCancellationCauses;
    try {
      generateCancellationCauses =
          Boolean.parseBoolean(
              System.getProperty("guava.concurrent.generate_cancellation_cause", "false"));
    } catch (SecurityException e) {
      generateCancellationCauses = false;
    }
    GENERATE_CANCELLATION_CAUSES = generateCancellationCauses;
  }

  /**
   * Tag interface marking trusted subclasses. This enables some optimizations. The implementation
   * of this interface must also be an AbstractFuture and must not override or expose for overriding
   * any of the public methods of ListenableFuture.
   */
  interface Trusted<V> extends ListenableFuture<V> {}

  /**
   * A less abstract subclass of AbstractFuture. This can be used to optimize setFuture by ensuring
   * that {@link #get} calls exactly the implementation of {@link AbstractFuture#get}.
   */
  abstract static class TrustedFuture<V> extends AbstractFuture<V> implements Trusted<V> {
    @CanIgnoreReturnValue
    @Override
    public final V get() throws InterruptedException, ExecutionException {
      return super.get();
    }

    @CanIgnoreReturnValue
    @Override
    public final V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
      return super.get(timeout, unit);
    }

    @Override
    public final boolean isDone() {
      return super.isDone();
    }

    @Override
    public final boolean isCancelled() {
      return super.isCancelled();
    }

    @Override
    public final void addListener(Runnable listener, Executor executor) {
      super.addListener(listener, executor);
    }

    @CanIgnoreReturnValue
    @Override
    public final boolean cancel(boolean mayInterruptIfRunning) {
      return super.cancel(mayInterruptIfRunning);
    }
  }

  // Logger to log exceptions caught when running listeners.
  private static final Logger log = Logger.getLogger(AbstractFuture.class.getName());

  // A heuristic for timed gets. If the remaining timeout is less than this, spin instead of
  // blocking. This value is what AbstractQueuedSynchronizer uses.
  private static final long SPIN_THRESHOLD_NANOS = 1000L;

  private static final AtomicHelper ATOMIC_HELPER;

  static {
    AtomicHelper helper;
    Throwable thrownUnsafeFailure = null;
    Throwable thrownAtomicReferenceFieldUpdaterFailure = null;

    try {
      helper = new UnsafeAtomicHelper();
    } catch (Throwable unsafeFailure) {
      thrownUnsafeFailure = unsafeFailure;
      // catch absolutely everything and fall through to our 'SafeAtomicHelper'
      // The access control checks that ARFU does means the caller class has to be AbstractFuture
      // instead of SafeAtomicHelper, so we annoyingly define these here
      try {
        helper =
            new SafeAtomicHelper(
                newUpdater(Waiter.class, Thread.class, "thread"),
                newUpdater(Waiter.class, Waiter.class, "next"),
                newUpdater(AbstractFuture.class, Waiter.class, "waiters"),
                newUpdater(AbstractFuture.class, Listener.class, "listeners"),
                newUpdater(AbstractFuture.class, Object.class, "value"));
      } catch (Throwable atomicReferenceFieldUpdaterFailure) {
        // Some Android 5.0.x Samsung devices have bugs in JDK reflection APIs that cause
        // getDeclaredField to throw a NoSuchFieldException when the field is definitely there.
        // For these users fallback to a suboptimal implementation, based on synchronized. This will
        // be a definite performance hit to those users.
        thrownAtomicReferenceFieldUpdaterFailure = atomicReferenceFieldUpdaterFailure;
        helper = new SynchronizedHelper();
      }
    }
    ATOMIC_HELPER = helper;

    // Prevent rare disastrous classloading in first call to LockSupport.park.
    // See: https://bugs.openjdk.java.net/browse/JDK-8074773
    @SuppressWarnings("unused")
    Class<?> ensureLoaded = LockSupport.class;

    // Log after all static init is finished; if an installed logger uses any Futures methods, it
    // shouldn't break in cases where reflection is missing/broken.
    if (thrownAtomicReferenceFieldUpdaterFailure != null) {
      log.log(Level.SEVERE, "UnsafeAtomicHelper is broken!", thrownUnsafeFailure);
      log.log(
          Level.SEVERE, "SafeAtomicHelper is broken!", thrownAtomicReferenceFieldUpdaterFailure);
    }
  }

  /** Waiter links form a Treiber stack, in the {@link #waiters} field. */
  private static final class Waiter {
    static final Waiter TOMBSTONE = new Waiter(false /* ignored param */);

    @NullableDecl volatile Thread thread;
    @NullableDecl volatile Waiter next;

    /**
     * Constructor for the TOMBSTONE, avoids use of ATOMIC_HELPER in case this class is loaded
     * before the ATOMIC_HELPER. Apparently this is possible on some android platforms.
     */
    Waiter(boolean unused) {}

    Waiter() {
      // avoid volatile write, write is made visible by subsequent CAS on waiters field
      ATOMIC_HELPER.putThread(this, Thread.currentThread());
    }

    // non-volatile write to the next field. Should be made visible by subsequent CAS on waiters
    // field.
    void setNext(Waiter next) {
      ATOMIC_HELPER.putNext(this, next);
    }

    void unpark() {
      // This is racy with removeWaiter. The consequence of the race is that we may spuriously call
      // unpark even though the thread has already removed itself from the list. But even if we did
      // use a CAS, that race would still exist (it would just be ever so slightly smaller).
      Thread w = thread;
      if (w != null) {
        thread = null;
        LockSupport.unpark(w);
      }
    }
  }

  /**
   * Marks the given node as 'deleted' (null waiter) and then scans the list to unlink all deleted
   * nodes. This is an O(n) operation in the common case (and O(n^2) in the worst), but we are saved
   * by two things.
   *
   * <ul>
   *   <li>This is only called when a waiting thread times out or is interrupted. Both of which
   *       should be rare.
   *   <li>The waiters list should be very short.
   * </ul>
   */
  private void removeWaiter(Waiter node) {
    node.thread = null; // mark as 'deleted'
    restart:
    while (true) {
      Waiter pred = null;
      Waiter curr = waiters;
      if (curr == Waiter.TOMBSTONE) {
        return; // give up if someone is calling complete
      }
      Waiter succ;
      while (curr != null) {
        succ = curr.next;
        if (curr.thread != null) { // we aren't unlinking this node, update pred.
          pred = curr;
        } else if (pred != null) { // We are unlinking this node and it has a predecessor.
          pred.next = succ;
          if (pred.thread == null) { // We raced with another node that unlinked pred. Restart.
            continue restart;
          }
        } else if (!ATOMIC_HELPER.casWaiters(this, curr, succ)) { // We are unlinking head
          continue restart; // We raced with an add or complete
        }
        curr = succ;
      }
      break;
    }
  }

  /** Listeners also form a stack through the {@link #listeners} field. */
  private static final class Listener {
    static final Listener TOMBSTONE = new Listener(null, null);
    final Runnable task;
    final Executor executor;

    // writes to next are made visible by subsequent CAS's on the listeners field
    @NullableDecl Listener next;

    Listener(Runnable task, Executor executor) {
      this.task = task;
      this.executor = executor;
    }
  }

  /** A special value to represent {@code null}. */
  private static final Object NULL = new Object();

  /** A special value to represent failure, when {@link #setException} is called successfully. */
  private static final class Failure {
    static final Failure FALLBACK_INSTANCE =
        new Failure(
            new Throwable("Failure occurred while trying to finish a future.") {
              @Override
              public synchronized Throwable fillInStackTrace() {
                return this; // no stack trace
              }
            });
    final Throwable exception;

    Failure(Throwable exception) {
      this.exception = checkNotNull(exception);
    }
  }

  /** A special value to represent cancellation and the 'wasInterrupted' bit. */
  private static final class Cancellation {
    // constants to use when GENERATE_CANCELLATION_CAUSES = false
    static final Cancellation CAUSELESS_INTERRUPTED;
    static final Cancellation CAUSELESS_CANCELLED;

    static {
      if (GENERATE_CANCELLATION_CAUSES) {
        CAUSELESS_CANCELLED = null;
        CAUSELESS_INTERRUPTED = null;
      } else {
        CAUSELESS_CANCELLED = new Cancellation(false, null);
        CAUSELESS_INTERRUPTED = new Cancellation(true, null);
      }
    }

    final boolean wasInterrupted;
    @NullableDecl final Throwable cause;

    Cancellation(boolean wasInterrupted, @NullableDecl Throwable cause) {
      this.wasInterrupted = wasInterrupted;
      this.cause = cause;
    }
  }

  /** A special value that encodes the 'setFuture' state. */
  private static final class SetFuture<V> implements Runnable {
    final AbstractFuture<V> owner;
    final ListenableFuture<? extends V> future;

    SetFuture(AbstractFuture<V> owner, ListenableFuture<? extends V> future) {
      this.owner = owner;
      this.future = future;
    }

    @Override
    public void run() {
      if (owner.value != this) {
        // nothing to do, we must have been cancelled, don't bother inspecting the future.
        return;
      }
      Object valueToSet = getFutureValue(future);
      if (ATOMIC_HELPER.casValue(owner, this, valueToSet)) {
        complete(owner);
      }
    }
  }

  // TODO(lukes): investigate using the @Contended annotation on these fields when jdk8 is
  // available.
  /**
   * This field encodes the current state of the future.
   *
   * <p>The valid values are:
   *
   * <ul>
   *   <li>{@code null} initial state, nothing has happened.
   *   <li>{@link Cancellation} terminal state, {@code cancel} was called.
   *   <li>{@link Failure} terminal state, {@code setException} was called.
   *   <li>{@link SetFuture} intermediate state, {@code setFuture} was called.
   *   <li>{@link #NULL} terminal state, {@code set(null)} was called.
   *   <li>Any other non-null value, terminal state, {@code set} was called with a non-null
   *       argument.
   * </ul>
   */
  @NullableDecl private volatile Object value;

  /** All listeners. */
  @NullableDecl private volatile Listener listeners;

  /** All waiting threads. */
  @NullableDecl private volatile Waiter waiters;

  /** Constructor for use by subclasses. */
  protected AbstractFuture() {}

  // Gets and Timed Gets
  //
  // * Be responsive to interruption
  // * Don't create Waiter nodes if you aren't going to park, this helps reduce contention on the
  //   waiters field.
  // * Future completion is defined by when #value becomes non-null/non SetFuture
  // * Future completion can be observed if the waiters field contains a TOMBSTONE

  // Timed Get
  // There are a few design constraints to consider
  // * We want to be responsive to small timeouts, unpark() has non trivial latency overheads (I
  //   have observed 12 micros on 64 bit linux systems to wake up a parked thread). So if the
  //   timeout is small we shouldn't park(). This needs to be traded off with the cpu overhead of
  //   spinning, so we use SPIN_THRESHOLD_NANOS which is what AbstractQueuedSynchronizer uses for
  //   similar purposes.
  // * We want to behave reasonably for timeouts of 0
  // * We are more responsive to completion than timeouts. This is because parkNanos depends on
  //   system scheduling and as such we could either miss our deadline, or unpark() could be delayed
  //   so that it looks like we timed out even though we didn't. For comparison FutureTask respects
  //   completion preferably and AQS is non-deterministic (depends on where in the queue the waiter
  //   is). If we wanted to be strict about it, we could store the unpark() time in the Waiter node
  //   and we could use that to make a decision about whether or not we timed out prior to being
  //   unparked.

  /**
   * {@inheritDoc}
   *
   * <p>The default {@link AbstractFuture} implementation throws {@code InterruptedException} if the
   * current thread is interrupted during the call, even if the value is already available.
   *
   * @throws CancellationException {@inheritDoc}
   */
  @CanIgnoreReturnValue
  @Override
  public V get(long timeout, TimeUnit unit)
      throws InterruptedException, TimeoutException, ExecutionException {
    // NOTE: if timeout < 0, remainingNanos will be < 0 and we will fall into the while(true) loop
    // at the bottom and throw a timeoutexception.
    final long timeoutNanos = unit.toNanos(timeout); // we rely on the implicit null check on unit.
    long remainingNanos = timeoutNanos;
    if (Thread.interrupted()) {
      throw new InterruptedException();
    }
    Object localValue = value;
    if (localValue != null & !(localValue instanceof SetFuture)) {
      return getDoneValue(localValue);
    }
    // we delay calling nanoTime until we know we will need to either park or spin
    final long endNanos = remainingNanos > 0 ? System.nanoTime() + remainingNanos : 0;
    long_wait_loop:
    if (remainingNanos >= SPIN_THRESHOLD_NANOS) {
      Waiter oldHead = waiters;
      if (oldHead != Waiter.TOMBSTONE) {
        Waiter node = new Waiter();
        do {
          node.setNext(oldHead);
          if (ATOMIC_HELPER.casWaiters(this, oldHead, node)) {
            while (true) {
              LockSupport.parkNanos(this, remainingNanos);
              // Check interruption first, if we woke up due to interruption we need to honor that.
              if (Thread.interrupted()) {
                removeWaiter(node);
                throw new InterruptedException();
              }

              // Otherwise re-read and check doneness. If we loop then it must have been a spurious
              // wakeup
              localValue = value;
              if (localValue != null & !(localValue instanceof SetFuture)) {
                return getDoneValue(localValue);
              }

              // timed out?
              remainingNanos = endNanos - System.nanoTime();
              if (remainingNanos < SPIN_THRESHOLD_NANOS) {
                // Remove the waiter, one way or another we are done parking this thread.
                removeWaiter(node);
                break long_wait_loop; // jump down to the busy wait loop
              }
            }
          }
          oldHead = waiters; // re-read and loop.
        } while (oldHead != Waiter.TOMBSTONE);
      }
      // re-read value, if we get here then we must have observed a TOMBSTONE while trying to add a
      // waiter.
      return getDoneValue(value);
    }
    // If we get here then we have remainingNanos < SPIN_THRESHOLD_NANOS and there is no node on the
    // waiters list
    while (remainingNanos > 0) {
      localValue = value;
      if (localValue != null & !(localValue instanceof SetFuture)) {
        return getDoneValue(localValue);
      }
      if (Thread.interrupted()) {
        throw new InterruptedException();
      }
      remainingNanos = endNanos - System.nanoTime();
    }

    String futureToString = toString();
    final String unitString = unit.toString().toLowerCase(Locale.ROOT);
    String message = "Waited " + timeout + " " + unit.toString().toLowerCase(Locale.ROOT);
    // Only report scheduling delay if larger than our spin threshold - otherwise it's just noise
    if (remainingNanos + SPIN_THRESHOLD_NANOS < 0) {
      // We over-waited for our timeout.
      message += " (plus ";
      long overWaitNanos = -remainingNanos;
      long overWaitUnits = unit.convert(overWaitNanos, TimeUnit.NANOSECONDS);
      long overWaitLeftoverNanos = overWaitNanos - unit.toNanos(overWaitUnits);
      boolean shouldShowExtraNanos =
          overWaitUnits == 0 || overWaitLeftoverNanos > SPIN_THRESHOLD_NANOS;
      if (overWaitUnits > 0) {
        message += overWaitUnits + " " + unitString;
        if (shouldShowExtraNanos) {
          message += ",";
        }
        message += " ";
      }
      if (shouldShowExtraNanos) {
        message += overWaitLeftoverNanos + " nanoseconds ";
      }

      message += "delay)";
    }
    // It's confusing to see a completed future in a timeout message; if isDone() returns false,
    // then we know it must have given a pending toString value earlier. If not, then the future
    // completed after the timeout expired, and the message might be success.
    if (isDone()) {
      throw new TimeoutException(message + " but future completed as timeout expired");
    }
    throw new TimeoutException(message + " for " + futureToString);
  }

  /**
   * {@inheritDoc}
   *
   * <p>The default {@link AbstractFuture} implementation throws {@code InterruptedException} if the
   * current thread is interrupted during the call, even if the value is already available.
   *
   * @throws CancellationException {@inheritDoc}
   */
  @CanIgnoreReturnValue
  @Override
  public V get() throws InterruptedException, ExecutionException {
    if (Thread.interrupted()) {
      throw new InterruptedException();
    }
    Object localValue = value;
    if (localValue != null & !(localValue instanceof SetFuture)) {
      return getDoneValue(localValue);
    }
    Waiter oldHead = waiters;
    if (oldHead != Waiter.TOMBSTONE) {
      Waiter node = new Waiter();
      do {
        node.setNext(oldHead);
        if (ATOMIC_HELPER.casWaiters(this, oldHead, node)) {
          // we are on the stack, now wait for completion.
          while (true) {
            LockSupport.park(this);
            // Check interruption first, if we woke up due to interruption we need to honor that.
            if (Thread.interrupted()) {
              removeWaiter(node);
              throw new InterruptedException();
            }
            // Otherwise re-read and check doneness. If we loop then it must have been a spurious
            // wakeup
            localValue = value;
            if (localValue != null & !(localValue instanceof SetFuture)) {
              return getDoneValue(localValue);
            }
          }
        }
        oldHead = waiters; // re-read and loop.
      } while (oldHead != Waiter.TOMBSTONE);
    }
    // re-read value, if we get here then we must have observed a TOMBSTONE while trying to add a
    // waiter.
    return getDoneValue(value);
  }

  /** Unboxes {@code obj}. Assumes that obj is not {@code null} or a {@link SetFuture}. */
  private V getDoneValue(Object obj) throws ExecutionException {
    // While this seems like it might be too branch-y, simple benchmarking proves it to be
    // unmeasurable (comparing done AbstractFutures with immediateFuture)
    if (obj instanceof Cancellation) {
      throw cancellationExceptionWithCause("Task was cancelled.", ((Cancellation) obj).cause);
    } else if (obj instanceof Failure) {
      throw new ExecutionException(((Failure) obj).exception);
    } else if (obj == NULL) {
      return null;
    } else {
      @SuppressWarnings("unchecked") // this is the only other option
      V asV = (V) obj;
      return asV;
    }
  }

  @Override
  public boolean isDone() {
    final Object localValue = value;
    return localValue != null & !(localValue instanceof SetFuture);
  }

  @Override
  public boolean isCancelled() {
    final Object localValue = value;
    return localValue instanceof Cancellation;
  }

  /**
   * {@inheritDoc}
   *
   * <p>If a cancellation attempt succeeds on a {@code Future} that had previously been {@linkplain
   * #setFuture set asynchronously}, then the cancellation will also be propagated to the delegate
   * {@code Future} that was supplied in the {@code setFuture} call.
   *
   * <p>Rather than override this method to perform additional cancellation work or cleanup,
   * subclasses should override {@link #afterDone}, consulting {@link #isCancelled} and {@link
   * #wasInterrupted} as necessary. This ensures that the work is done even if the future is
   * cancelled without a call to {@code cancel}, such as by calling {@code
   * setFuture(cancelledFuture)}.
   */
  @CanIgnoreReturnValue
  @Override
  public boolean cancel(boolean mayInterruptIfRunning) {
    Object localValue = value;
    boolean rValue = false;
    if (localValue == null | localValue instanceof SetFuture) {
      // Try to delay allocating the exception. At this point we may still lose the CAS, but it is
      // certainly less likely.
      Object valueToSet =
          GENERATE_CANCELLATION_CAUSES
              ? new Cancellation(
                  mayInterruptIfRunning, new CancellationException("Future.cancel() was called."))
              : (mayInterruptIfRunning
                  ? Cancellation.CAUSELESS_INTERRUPTED
                  : Cancellation.CAUSELESS_CANCELLED);
      AbstractFuture<?> abstractFuture = this;
      while (true) {
        if (ATOMIC_HELPER.casValue(abstractFuture, localValue, valueToSet)) {
          rValue = true;
          // We call interuptTask before calling complete(), which is consistent with
          // FutureTask
          if (mayInterruptIfRunning) {
            abstractFuture.interruptTask();
          }
          complete(abstractFuture);
          if (localValue instanceof SetFuture) {
            // propagate cancellation to the future set in setfuture, this is racy, and we don't
            // care if we are successful or not.
            ListenableFuture<?> futureToPropagateTo = ((SetFuture) localValue).future;
            if (futureToPropagateTo instanceof Trusted) {
              // If the future is a TrustedFuture then we specifically avoid calling cancel()
              // this has 2 benefits
              // 1. for long chains of futures strung together with setFuture we consume less stack
              // 2. we avoid allocating Cancellation objects at every level of the cancellation
              //    chain
              // We can only do this for TrustedFuture, because TrustedFuture.cancel is final and
              // does nothing but delegate to this method.
              AbstractFuture<?> trusted = (AbstractFuture<?>) futureToPropagateTo;
              localValue = trusted.value;
              if (localValue == null | localValue instanceof SetFuture) {
                abstractFuture = trusted;
                continue; // loop back up and try to complete the new future
              }
            } else {
              // not a TrustedFuture, call cancel directly.
              futureToPropagateTo.cancel(mayInterruptIfRunning);
            }
          }
          break;
        }
        // obj changed, reread
        localValue = abstractFuture.value;
        if (!(localValue instanceof SetFuture)) {
          // obj cannot be null at this point, because value can only change from null to non-null.
          // So if value changed (and it did since we lost the CAS), then it cannot be null and
          // since it isn't a SetFuture, then the future must be done and we should exit the loop
          break;
        }
      }
    }
    return rValue;
  }

  /**
   * Subclasses can override this method to implement interruption of the future's computation. The
   * method is invoked automatically by a successful call to {@link #cancel(boolean) cancel(true)}.
   *
   * <p>The default implementation does nothing.
   *
   * <p>This method is likely to be deprecated. Prefer to override {@link #afterDone}, consulting
   * {@link #wasInterrupted} to decide whether to interrupt your task.
   *
   * @since 10.0
   */
  protected void interruptTask() {}

  /**
   * Returns true if this future was cancelled with {@code mayInterruptIfRunning} set to {@code
   * true}.
   *
   * @since 14.0
   */
  protected final boolean wasInterrupted() {
    final Object localValue = value;
    return (localValue instanceof Cancellation) && ((Cancellation) localValue).wasInterrupted;
  }

  /**
   * {@inheritDoc}
   *
   * @since 10.0
   */
  @Override
  public void addListener(Runnable listener, Executor executor) {
    checkNotNull(listener, "Runnable was null.");
    checkNotNull(executor, "Executor was null.");
    // Checking isDone and listeners != TOMBSTONE may seem redundant, but our contract for
    // addListener says that listeners execute 'immediate' if the future isDone(). However, our
    // protocol for completing a future is to assign the value field (which sets isDone to true) and
    // then to release waiters, followed by executing afterDone(), followed by releasing listeners.
    // That means that it is possible to observe that the future isDone and that your listeners
    // don't execute 'immediately'.  By checking isDone here we avoid that.
    // A corollary to all that is that we don't need to check isDone inside the loop because if we
    // get into the loop we know that we weren't done when we entered and therefore we aren't under
    // an obligation to execute 'immediately'.
    if (!isDone()) {
      Listener oldHead = listeners;
      if (oldHead != Listener.TOMBSTONE) {
        Listener newNode = new Listener(listener, executor);
        do {
          newNode.next = oldHead;
          if (ATOMIC_HELPER.casListeners(this, oldHead, newNode)) {
            return;
          }
          oldHead = listeners; // re-read
        } while (oldHead != Listener.TOMBSTONE);
      }
    }
    // If we get here then the Listener TOMBSTONE was set, which means the future is done, call
    // the listener.
    executeListener(listener, executor);
  }

  /**
   * Sets the result of this {@code Future} unless this {@code Future} has already been cancelled or
   * set (including {@linkplain #setFuture set asynchronously}). When a call to this method returns,
   * the {@code Future} is guaranteed to be {@linkplain #isDone done} <b>only if</b> the call was
   * accepted (in which case it returns {@code true}). If it returns {@code false}, the {@code
   * Future} may have previously been set asynchronously, in which case its result may not be known
   * yet. That result, though not yet known, cannot be overridden by a call to a {@code set*}
   * method, only by a call to {@link #cancel}.
   *
   * @param value the value to be used as the result
   * @return true if the attempt was accepted, completing the {@code Future}
   */
  @CanIgnoreReturnValue
  protected boolean set(@NullableDecl V value) {
    Object valueToSet = value == null ? NULL : value;
    if (ATOMIC_HELPER.casValue(this, null, valueToSet)) {
      complete(this);
      return true;
    }
    return false;
  }

  /**
   * Sets the failed result of this {@code Future} unless this {@code Future} has already been
   * cancelled or set (including {@linkplain #setFuture set asynchronously}). When a call to this
   * method returns, the {@code Future} is guaranteed to be {@linkplain #isDone done} <b>only if</b>
   * the call was accepted (in which case it returns {@code true}). If it returns {@code false}, the
   * {@code Future} may have previously been set asynchronously, in which case its result may not be
   * known yet. That result, though not yet known, cannot be overridden by a call to a {@code set*}
   * method, only by a call to {@link #cancel}.
   *
   * @param throwable the exception to be used as the failed result
   * @return true if the attempt was accepted, completing the {@code Future}
   */
  @CanIgnoreReturnValue
  protected boolean setException(Throwable throwable) {
    Object valueToSet = new Failure(checkNotNull(throwable));
    if (ATOMIC_HELPER.casValue(this, null, valueToSet)) {
      complete(this);
      return true;
    }
    return false;
  }

  /**
   * Sets the result of this {@code Future} to match the supplied input {@code Future} once the
   * supplied {@code Future} is done, unless this {@code Future} has already been cancelled or set
   * (including "set asynchronously," defined below).
   *
   * <p>If the supplied future is {@linkplain #isDone done} when this method is called and the call
   * is accepted, then this future is guaranteed to have been completed with the supplied future by
   * the time this method returns. If the supplied future is not done and the call is accepted, then
   * the future will be <i>set asynchronously</i>. Note that such a result, though not yet known,
   * cannot be overridden by a call to a {@code set*} method, only by a call to {@link #cancel}.
   *
   * <p>If the call {@code setFuture(delegate)} is accepted and this {@code Future} is later
   * cancelled, cancellation will be propagated to {@code delegate}. Additionally, any call to
   * {@code setFuture} after any cancellation will propagate cancellation to the supplied {@code
   * Future}.
   *
   * <p>Note that, even if the supplied future is cancelled and it causes this future to complete,
   * it will never trigger interruption behavior. In particular, it will not cause this future to
   * invoke the {@link #interruptTask} method, and the {@link #wasInterrupted} method will not
   * return {@code true}.
   *
   * @param future the future to delegate to
   * @return true if the attempt was accepted, indicating that the {@code Future} was not previously
   *     cancelled or set.
   * @since 19.0
   */
  @CanIgnoreReturnValue
  protected boolean setFuture(ListenableFuture<? extends V> future) {
    checkNotNull(future);
    Object localValue = value;
    if (localValue == null) {
      if (future.isDone()) {
        Object value = getFutureValue(future);
        if (ATOMIC_HELPER.casValue(this, null, value)) {
          complete(this);
          return true;
        }
        return false;
      }
      SetFuture valueToSet = new SetFuture<V>(this, future);
      if (ATOMIC_HELPER.casValue(this, null, valueToSet)) {
        // the listener is responsible for calling completeWithFuture, directExecutor is appropriate
        // since all we are doing is unpacking a completed future which should be fast.
        try {
          future.addListener(valueToSet, DirectExecutor.INSTANCE);
        } catch (Throwable t) {
          // addListener has thrown an exception! SetFuture.run can't throw any exceptions so this
          // must have been caused by addListener itself. The most likely explanation is a
          // misconfigured mock. Try to switch to Failure.
          Failure failure;
          try {
            failure = new Failure(t);
          } catch (Throwable oomMostLikely) {
            failure = Failure.FALLBACK_INSTANCE;
          }
          // Note: The only way this CAS could fail is if cancel() has raced with us. That is ok.
          boolean unused = ATOMIC_HELPER.casValue(this, valueToSet, failure);
        }
        return true;
      }
      localValue = value; // we lost the cas, fall through and maybe cancel
    }
    // The future has already been set to something. If it is cancellation we should cancel the
    // incoming future.
    if (localValue instanceof Cancellation) {
      // we don't care if it fails, this is best-effort.
      future.cancel(((Cancellation) localValue).wasInterrupted);
    }
    return false;
  }

  /**
   * Returns a value that satisfies the contract of the {@link #value} field based on the state of
   * given future.
   *
   * <p>This is approximately the inverse of {@link #getDoneValue(Object)}
   */
  private static Object getFutureValue(ListenableFuture<?> future) {
    if (future instanceof Trusted) {
      // Break encapsulation for TrustedFuture instances since we know that subclasses cannot
      // override .get() (since it is final) and therefore this is equivalent to calling .get()
      // and unpacking the exceptions like we do below (just much faster because it is a single
      // field read instead of a read, several branches and possibly creating exceptions).
      Object v = ((AbstractFuture<?>) future).value;
      if (v instanceof Cancellation) {
        // If the other future was interrupted, clear the interrupted bit while preserving the cause
        // this will make it consistent with how non-trustedfutures work which cannot propagate the
        // wasInterrupted bit
        Cancellation c = (Cancellation) v;
        if (c.wasInterrupted) {
          v =
              c.cause != null
                  ? new Cancellation(/* wasInterrupted= */ false, c.cause)
                  : Cancellation.CAUSELESS_CANCELLED;
        }
      }
      return v;
    }
    if (future instanceof InternalFutureFailureAccess) {
      Throwable throwable =
          InternalFutures.tryInternalFastPathGetFailure((InternalFutureFailureAccess) future);
      if (throwable != null) {
        return new Failure(throwable);
      }
    }
    boolean wasCancelled = future.isCancelled();
    // Don't allocate a CancellationException if it's not necessary
    if (!GENERATE_CANCELLATION_CAUSES & wasCancelled) {
      return Cancellation.CAUSELESS_CANCELLED;
    }
    // Otherwise calculate the value by calling .get()
    try {
      Object v = getUninterruptibly(future);
      if (wasCancelled) {
        return new Cancellation(
            false,
            new IllegalArgumentException(
                "get() did not throw CancellationException, despite reporting "
                    + "isCancelled() == true: "
                    + future));
      }
      return v == null ? NULL : v;
    } catch (ExecutionException exception) {
      if (wasCancelled) {
        return new Cancellation(
            false,
            new IllegalArgumentException(
                "get() did not throw CancellationException, despite reporting "
                    + "isCancelled() == true: "
                    + future,
                exception));
      }
      return new Failure(exception.getCause());
    } catch (CancellationException cancellation) {
      if (!wasCancelled) {
        return new Failure(
            new IllegalArgumentException(
                "get() threw CancellationException, despite reporting isCancelled() == false: "
                    + future,
                cancellation));
      }
      return new Cancellation(false, cancellation);
    } catch (Throwable t) {
      return new Failure(t);
    }
  }

  /**
   * An inlined private copy of {@link Uninterruptibles#getUninterruptibly} used to break an
   * internal dependency on other /util/concurrent classes.
   */
  private static <V> V getUninterruptibly(Future<V> future) throws ExecutionException {
    boolean interrupted = false;
    try {
      while (true) {
        try {
          return future.get();
        } catch (InterruptedException e) {
          interrupted = true;
        }
      }
    } finally {
      if (interrupted) {
        Thread.currentThread().interrupt();
      }
    }
  }

  /** Unblocks all threads and runs all listeners. */
  private static void complete(AbstractFuture<?> future) {
    Listener next = null;
    outer:
    while (true) {
      future.releaseWaiters();
      // We call this before the listeners in order to avoid needing to manage a separate stack data
      // structure for them.  Also, some implementations rely on this running prior to listeners
      // so that the cleanup work is visible to listeners.
      // afterDone() should be generally fast and only used for cleanup work... but in theory can
      // also be recursive and create StackOverflowErrors
      future.afterDone();
      // push the current set of listeners onto next
      next = future.clearListeners(next);
      future = null;
      while (next != null) {
        Listener curr = next;
        next = next.next;
        Runnable task = curr.task;
        if (task instanceof SetFuture) {
          SetFuture<?> setFuture = (SetFuture<?>) task;
          // We unwind setFuture specifically to avoid StackOverflowErrors in the case of long
          // chains of SetFutures
          // Handling this special case is important because there is no way to pass an executor to
          // setFuture, so a user couldn't break the chain by doing this themselves.  It is also
          // potentially common if someone writes a recursive Futures.transformAsync transformer.
          future = setFuture.owner;
          if (future.value == setFuture) {
            Object valueToSet = getFutureValue(setFuture.future);
            if (ATOMIC_HELPER.casValue(future, setFuture, valueToSet)) {
              continue outer;
            }
          }
          // other wise the future we were trying to set is already done.
        } else {
          executeListener(task, curr.executor);
        }
      }
      break;
    }
  }

  /**
   * Callback method that is called exactly once after the future is completed.
   *
   * <p>If {@link #interruptTask} is also run during completion, {@link #afterDone} runs after it.
   *
   * <p>The default implementation of this method in {@code AbstractFuture} does nothing. This is
   * intended for very lightweight cleanup work, for example, timing statistics or clearing fields.
   * If your task does anything heavier consider, just using a listener with an executor.
   *
   * @since 20.0
   */
  @Beta
  @ForOverride
  protected void afterDone() {}

  // TODO(b/114236866): Inherit doc from InternalFutureFailureAccess. Also, -link to its URL.
  /**
   * Usually returns {@code null} but, if this {@code Future} has failed, may <i>optionally</i>
   * return the cause of the failure. "Failure" means specifically "completed with an exception"; it
   * does not include "was cancelled." To be explicit: If this method returns a non-null value,
   * then:
   *
   * <ul>
   *   <li>{@code isDone()} must return {@code true}
   *   <li>{@code isCancelled()} must return {@code false}
   *   <li>{@code get()} must not block, and it must throw an {@code ExecutionException} with the
   *       return value of this method as its cause
   * </ul>
   *
   * <p>This method is {@code protected} so that classes like {@code
   * com.google.common.util.concurrent.SettableFuture} do not expose it to their users as an
   * instance method. In the unlikely event that you need to call this method, call {@link
   * InternalFutures#tryInternalFastPathGetFailure(InternalFutureFailureAccess)}.
   *
   * @since 27.0
   */
  @Override
  @NullableDecl
  protected final Throwable tryInternalFastPathGetFailure() {
    if (this instanceof Trusted) {
      Object obj = value;
      if (obj instanceof Failure) {
        return ((Failure) obj).exception;
      }
    }
    return null;
  }

  /**
   * If this future has been cancelled (and possibly interrupted), cancels (and possibly interrupts)
   * the given future (if available).
   */
  final void maybePropagateCancellationTo(@NullableDecl Future<?> related) {
    if (related != null & isCancelled()) {
      related.cancel(wasInterrupted());
    }
  }

  /** Releases all threads in the {@link #waiters} list, and clears the list. */
  private void releaseWaiters() {
    Waiter head;
    do {
      head = waiters;
    } while (!ATOMIC_HELPER.casWaiters(this, head, Waiter.TOMBSTONE));
    for (Waiter currentWaiter = head; currentWaiter != null; currentWaiter = currentWaiter.next) {
      currentWaiter.unpark();
    }
  }

  /**
   * Clears the {@link #listeners} list and prepends its contents to {@code onto}, least recently
   * added first.
   */
  private Listener clearListeners(Listener onto) {
    // We need to
    // 1. atomically swap the listeners with TOMBSTONE, this is because addListener uses that to
    //    to synchronize with us
    // 2. reverse the linked list, because despite our rather clear contract, people depend on us
    //    executing listeners in the order they were added
    // 3. push all the items onto 'onto' and return the new head of the stack
    Listener head;
    do {
      head = listeners;
    } while (!ATOMIC_HELPER.casListeners(this, head, Listener.TOMBSTONE));
    Listener reversedList = onto;
    while (head != null) {
      Listener tmp = head;
      head = head.next;
      tmp.next = reversedList;
      reversedList = tmp;
    }
    return reversedList;
  }

  // TODO(user): move parts into a default method on ListenableFuture?
  @Override
  public String toString() {
    StringBuilder builder = new StringBuilder().append(super.toString()).append("[status=");
    if (isCancelled()) {
      builder.append("CANCELLED");
    } else if (isDone()) {
      addDoneString(builder);
    } else {
      addPendingString(builder); // delegates to addDoneString if future completes mid-way
    }
    return builder.append("]").toString();
  }

  /**
   * Provide a human-readable explanation of why this future has not yet completed.
   *
   * @return null if an explanation cannot be provided (e.g. because the future is done).
   * @since 23.0
   */
  @NullableDecl
  protected String pendingToString() {
    // TODO(diamondm) consider moving this into addPendingString so it's always in the output
    if (this instanceof ScheduledFuture) {
      return "remaining delay=["
          + ((ScheduledFuture) this).getDelay(TimeUnit.MILLISECONDS)
          + " ms]";
    }
    return null;
  }

  private void addPendingString(StringBuilder builder) {
    // Capture current builder length so it can be truncated if this future ends up completing while
    // the toString is being calculated
    int truncateLength = builder.length();

    builder.append("PENDING");

    Object localValue = value;
    if (localValue instanceof SetFuture) {
      builder.append(", setFuture=[");
      appendUserObject(builder, ((SetFuture) localValue).future);
      builder.append("]");
    } else {
      String pendingDescription;
      try {
        pendingDescription = Strings.emptyToNull(pendingToString());
      } catch (RuntimeException | StackOverflowError e) {
        // Don't call getMessage or toString() on the exception, in case the exception thrown by the
        // subclass is implemented with bugs similar to the subclass.
        pendingDescription = "Exception thrown from implementation: " + e.getClass();
      }
      if (pendingDescription != null) {
        builder.append(", info=[").append(pendingDescription).append("]");
      }
    }

    // The future may complete while calculating the toString, so we check once more to see if the
    // future is done
    if (isDone()) {
      // Truncate anything that was appended before realizing this future is done
      builder.delete(truncateLength, builder.length());
      addDoneString(builder);
    }
  }

  private void addDoneString(StringBuilder builder) {
    try {
      V value = getUninterruptibly(this);
      builder.append("SUCCESS, result=[");
      appendUserObject(builder, value);
      builder.append("]");
    } catch (ExecutionException e) {
      builder.append("FAILURE, cause=[").append(e.getCause()).append("]");
    } catch (CancellationException e) {
      builder.append("CANCELLED"); // shouldn't be reachable
    } catch (RuntimeException e) {
      builder.append("UNKNOWN, cause=[").append(e.getClass()).append(" thrown from get()]");
    }
  }

  /** Helper for printing user supplied objects into our toString method. */
  private void appendUserObject(StringBuilder builder, Object o) {
    // This is some basic recursion detection for when people create cycles via set/setFuture or
    // when deep chains of futures exist resulting in a StackOverflowException. We could detect
    // arbitrary cycles using a thread local but this should be a good enough solution (it is also
    // what jdk collections do in these cases)
    try {
      if (o == this) {
        builder.append("this future");
      } else {
        builder.append(o);
      }
    } catch (RuntimeException | StackOverflowError e) {
      // Don't call getMessage or toString() on the exception, in case the exception thrown by the
      // user object is implemented with bugs similar to the user object.
      builder.append("Exception thrown from implementation: ").append(e.getClass());
    }
  }

  /**
   * Submits the given runnable to the given {@link Executor} catching and logging all {@linkplain
   * RuntimeException runtime exceptions} thrown by the executor.
   */
  private static void executeListener(Runnable runnable, Executor executor) {
    try {
      executor.execute(runnable);
    } catch (RuntimeException e) {
      // Log it and keep going -- bad runnable and/or executor. Don't punish the other runnables if
      // we're given a bad one. We only catch RuntimeException because we want Errors to propagate
      // up.
      log.log(
          Level.SEVERE,
          "RuntimeException while executing runnable " + runnable + " with executor " + executor,
          e);
    }
  }

  private abstract static class AtomicHelper {
    /** Non volatile write of the thread to the {@link Waiter#thread} field. */
    abstract void putThread(Waiter waiter, Thread newValue);

    /** Non volatile write of the waiter to the {@link Waiter#next} field. */
    abstract void putNext(Waiter waiter, Waiter newValue);

    /** Performs a CAS operation on the {@link #waiters} field. */
    abstract boolean casWaiters(AbstractFuture<?> future, Waiter expect, Waiter update);

    /** Performs a CAS operation on the {@link #listeners} field. */
    abstract boolean casListeners(AbstractFuture<?> future, Listener expect, Listener update);

    /** Performs a CAS operation on the {@link #value} field. */
    abstract boolean casValue(AbstractFuture<?> future, Object expect, Object update);
  }

  /**
   * {@link AtomicHelper} based on {@link sun.misc.Unsafe}.
   *
   * <p>Static initialization of this class will fail if the {@link sun.misc.Unsafe} object cannot
   * be accessed.
   */
  private static final class UnsafeAtomicHelper extends AtomicHelper {
    static final sun.misc.Unsafe UNSAFE;
    static final long LISTENERS_OFFSET;
    static final long WAITERS_OFFSET;
    static final long VALUE_OFFSET;
    static final long WAITER_THREAD_OFFSET;
    static final long WAITER_NEXT_OFFSET;

    static {
      sun.misc.Unsafe unsafe = null;
      try {
        unsafe = sun.misc.Unsafe.getUnsafe();
      } catch (SecurityException tryReflectionInstead) {
        try {
          unsafe =
              AccessController.doPrivileged(
                  new PrivilegedExceptionAction<sun.misc.Unsafe>() {
                    @Override
                    public sun.misc.Unsafe run() throws Exception {
                      Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
                      for (java.lang.reflect.Field f : k.getDeclaredFields()) {
                        f.setAccessible(true);
                        Object x = f.get(null);
                        if (k.isInstance(x)) {
                          return k.cast(x);
                        }
                      }
                      throw new NoSuchFieldError("the Unsafe");
                    }
                  });
        } catch (PrivilegedActionException e) {
          throw new RuntimeException("Could not initialize intrinsics", e.getCause());
        }
      }
      try {
        Class<?> abstractFuture = AbstractFuture.class;
        WAITERS_OFFSET = unsafe.objectFieldOffset(abstractFuture.getDeclaredField("waiters"));
        LISTENERS_OFFSET = unsafe.objectFieldOffset(abstractFuture.getDeclaredField("listeners"));
        VALUE_OFFSET = unsafe.objectFieldOffset(abstractFuture.getDeclaredField("value"));
        WAITER_THREAD_OFFSET = unsafe.objectFieldOffset(Waiter.class.getDeclaredField("thread"));
        WAITER_NEXT_OFFSET = unsafe.objectFieldOffset(Waiter.class.getDeclaredField("next"));
        UNSAFE = unsafe;
      } catch (Exception e) {
        throwIfUnchecked(e);
        throw new RuntimeException(e);
      }
    }

    @Override
    void putThread(Waiter waiter, Thread newValue) {
      UNSAFE.putObject(waiter, WAITER_THREAD_OFFSET, newValue);
    }

    @Override
    void putNext(Waiter waiter, Waiter newValue) {
      UNSAFE.putObject(waiter, WAITER_NEXT_OFFSET, newValue);
    }

    /** Performs a CAS operation on the {@link #waiters} field. */
    @Override
    boolean casWaiters(AbstractFuture<?> future, Waiter expect, Waiter update) {
      return UNSAFE.compareAndSwapObject(future, WAITERS_OFFSET, expect, update);
    }

    /** Performs a CAS operation on the {@link #listeners} field. */
    @Override
    boolean casListeners(AbstractFuture<?> future, Listener expect, Listener update) {
      return UNSAFE.compareAndSwapObject(future, LISTENERS_OFFSET, expect, update);
    }

    /** Performs a CAS operation on the {@link #value} field. */
    @Override
    boolean casValue(AbstractFuture<?> future, Object expect, Object update) {
      return UNSAFE.compareAndSwapObject(future, VALUE_OFFSET, expect, update);
    }
  }

  /** {@link AtomicHelper} based on {@link AtomicReferenceFieldUpdater}. */
  private static final class SafeAtomicHelper extends AtomicHelper {
    final AtomicReferenceFieldUpdater<Waiter, Thread> waiterThreadUpdater;
    final AtomicReferenceFieldUpdater<Waiter, Waiter> waiterNextUpdater;
    final AtomicReferenceFieldUpdater<AbstractFuture, Waiter> waitersUpdater;
    final AtomicReferenceFieldUpdater<AbstractFuture, Listener> listenersUpdater;
    final AtomicReferenceFieldUpdater<AbstractFuture, Object> valueUpdater;

    SafeAtomicHelper(
        AtomicReferenceFieldUpdater<Waiter, Thread> waiterThreadUpdater,
        AtomicReferenceFieldUpdater<Waiter, Waiter> waiterNextUpdater,
        AtomicReferenceFieldUpdater<AbstractFuture, Waiter> waitersUpdater,
        AtomicReferenceFieldUpdater<AbstractFuture, Listener> listenersUpdater,
        AtomicReferenceFieldUpdater<AbstractFuture, Object> valueUpdater) {
      this.waiterThreadUpdater = waiterThreadUpdater;
      this.waiterNextUpdater = waiterNextUpdater;
      this.waitersUpdater = waitersUpdater;
      this.listenersUpdater = listenersUpdater;
      this.valueUpdater = valueUpdater;
    }

    @Override
    void putThread(Waiter waiter, Thread newValue) {
      waiterThreadUpdater.lazySet(waiter, newValue);
    }

    @Override
    void putNext(Waiter waiter, Waiter newValue) {
      waiterNextUpdater.lazySet(waiter, newValue);
    }

    @Override
    boolean casWaiters(AbstractFuture<?> future, Waiter expect, Waiter update) {
      return waitersUpdater.compareAndSet(future, expect, update);
    }

    @Override
    boolean casListeners(AbstractFuture<?> future, Listener expect, Listener update) {
      return listenersUpdater.compareAndSet(future, expect, update);
    }

    @Override
    boolean casValue(AbstractFuture<?> future, Object expect, Object update) {
      return valueUpdater.compareAndSet(future, expect, update);
    }
  }

  /**
   * {@link AtomicHelper} based on {@code synchronized} and volatile writes.
   *
   * <p>This is an implementation of last resort for when certain basic VM features are broken (like
   * AtomicReferenceFieldUpdater).
   */
  private static final class SynchronizedHelper extends AtomicHelper {
    @Override
    void putThread(Waiter waiter, Thread newValue) {
      waiter.thread = newValue;
    }

    @Override
    void putNext(Waiter waiter, Waiter newValue) {
      waiter.next = newValue;
    }

    @Override
    boolean casWaiters(AbstractFuture<?> future, Waiter expect, Waiter update) {
      synchronized (future) {
        if (future.waiters == expect) {
          future.waiters = update;
          return true;
        }
        return false;
      }
    }

    @Override
    boolean casListeners(AbstractFuture<?> future, Listener expect, Listener update) {
      synchronized (future) {
        if (future.listeners == expect) {
          future.listeners = update;
          return true;
        }
        return false;
      }
    }

    @Override
    boolean casValue(AbstractFuture<?> future, Object expect, Object update) {
      synchronized (future) {
        if (future.value == expect) {
          future.value = update;
          return true;
        }
        return false;
      }
    }
  }

  private static CancellationException cancellationExceptionWithCause(
      @NullableDecl String message, @NullableDecl Throwable cause) {
    CancellationException exception = new CancellationException(message);
    exception.initCause(cause);
    return exception;
  }
}