/*
 * 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.
 */

/**
 * Some portions of this class have been modified to make it functional in this
 * package.
 */
package org.apache.hadoop.hdfs.server.datanode.checker;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Preconditions;
import static com.google.common.base.Preconditions.checkNotNull;
import com.google.common.util.concurrent.ListenableFuture;
import com.google.common.util.concurrent.Uninterruptibles;
import static java.util.concurrent.atomic.AtomicReferenceFieldUpdater
    .newUpdater;

import javax.annotation.Nullable;
import java.security.AccessController;
import java.security.PrivilegedActionException;
import java.security.PrivilegedExceptionAction;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.Future;
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;

/**
 * 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) Futures.transform}
 * and {@link Futures#catching(ListenableFuture, Class,
 * com.google.common.base.Function, java.util.concurrent.Executor)
 * Futures.catching}.
 * <p>
 * <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 #interruptTask()}, which will be
 * invoked automatically if a call to {@link #cancel(boolean) cancel(true)}
 * succeeds in canceling the future. Subclasses should rarely override other
 * methods.
 */

@GwtCompatible(emulated = true)
public abstract class AbstractFuture<V> 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 =
      Boolean.parseBoolean(
          System.getProperty("guava.concurrent.generate_cancellation_cause",
              "false"));

  /**
   * 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> {
    @Override
    public final V get() throws InterruptedException, ExecutionException {
      return super.get();
    }

    @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);
    }

    @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;

    try {
      helper = new UnsafeAtomicHelper();
    } catch (Throwable 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.
        log.log(Level.SEVERE, "UnsafeAtomicHelper is broken!", unsafeFailure);
        log.log(
            Level.SEVERE, "SafeAtomicHelper is broken!",
            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;
  }

  /**
   * 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 */);

    @Nullable volatile Thread thread;
    @Nullable 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
    @Nullable 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 {
    final boolean wasInterrupted;
    @Nullable final Throwable cause;

    Cancellation(boolean wasInterrupted, @Nullable 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);
      }
    }
  }

  /**
   * This field encodes the current state of the future.
   * <p>
   * <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>
   */
  private volatile Object value;

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

  /**
   * All waiting threads.
   */
  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.

  /*
   * Improve the documentation of when InterruptedException is thrown. Our
   * behavior matches the JDK's, but the JDK's documentation is misleading.
   */

  /**
   * {@inheritDoc}
   * <p>
   * <p>The default {@link AbstractFuture} implementation throws {@code
   * InterruptedException} if the current thread is interrupted before or
   * during the call, even if the value is already available.
   *
   * @throws InterruptedException  if the current thread was interrupted
   * before or during the call
   *                               (optional but recommended).
   * @throws CancellationException {@inheritDoc}
   */
  @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.
    long remainingNanos = unit
        .toNanos(timeout); // we rely on the implicit null check on unit.
    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();
    }
    throw new TimeoutException();
  }

  /*
   * Improve the documentation of when InterruptedException is thrown. Our
   * behavior matches the JDK's, but the JDK's documentation is misleading.
   */

  /**
   * {@inheritDoc}
   * <p>
   * <p>The default {@link AbstractFuture} implementation throws {@code
   * InterruptedException} if the current thread is interrupted before or
   * during the call, even if the value is already available.
   *
   * @throws InterruptedException  if the current thread was interrupted
   * before or during the call
   *                               (optional but recommended).
   * @throws CancellationException {@inheritDoc}
   */
  @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>
   * <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.
   */
  @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.
      Throwable cause =
          GENERATE_CANCELLATION_CAUSES
              ? new CancellationException("Future.cancel() was called.")
              : null;
      Object valueToSet = new Cancellation(mayInterruptIfRunning, cause);
      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 TrustedFuture) {
              // 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>
   * <p>The default implementation does nothing.
   *
   * @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.");
    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}
   */
  protected boolean set(@Nullable 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}
   */
  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>
   * <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>
   * <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}.
   *
   * @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
   */
  @Beta
  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());
        } 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, suitable for storing in the {@link #value} field. From
   * the given future, which is assumed to be done.
   * <p>
   * <p>This is approximately the inverse of {@link #getDoneValue(Object)}
   */
  private static Object getFutureValue(ListenableFuture<?> future) {
    Object valueToSet;
    if (future instanceof TrustedFuture) {
      // 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).
      return ((AbstractFuture<?>) future).value;
    } else {
      // Otherwise calculate valueToSet by calling .get()
      try {
        Object v = getDone(future);
        valueToSet = v == null ? NULL : v;
      } catch (ExecutionException exception) {
        valueToSet = new Failure(exception.getCause());
      } catch (CancellationException cancellation) {
        valueToSet = new Cancellation(false, cancellation);
      } catch (Throwable t) {
        valueToSet = new Failure(t);
      }
    }
    return valueToSet;
  }

  /**
   * 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. 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;
    }
  }

  public static <V> V getDone(Future<V> future) throws ExecutionException {
    /*
     * We throw IllegalStateException, since the call could succeed later.
     * Perhaps we "should" throw IllegalArgumentException, since the call
     * could succeed with a different argument. Those exceptions' docs
     * suggest that either is acceptable. Google's Java Practices page
     * recommends IllegalArgumentException here, in part to keep its
     * recommendation simple: Static methods should throw
     * IllegalStateException only when they use static state.
     *
     *
     * Why do we deviate here? The answer: We want for fluentFuture.getDone()
      * to throw the same exception as Futures.getDone(fluentFuture).
     */
    Preconditions.checkState(future.isDone(), "Future was expected to be " +
        "done:" +
        " %s", future);
    return Uninterruptibles.getUninterruptibly(future);
  }

  /**
   * Callback method that is called exactly once after the future is completed.
   * <p>
   * <p>If {@link #interruptTask} is also run during completion,
   * {@link #afterDone} runs after it.
   * <p>
   * <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
  protected void afterDone() {
  }

  /**
   * If this future has been cancelled (and possibly interrupted), cancels
   * (and possibly interrupts) the given future (if available).
   * <p>
   * <p>This method should be used only when this future is completed. It is
   * designed to be called from {@code done}.
   */
  final void maybePropagateCancellation(@Nullable 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;
  }

  /**
   * 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>
   * <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);
      }
    }

    public static void throwIfUnchecked(Throwable throwable) {
      checkNotNull(throwable);
      if (throwable instanceof RuntimeException) {
        throw (RuntimeException) throwable;
      }
      if (throwable instanceof Error) {
        throw (Error) throwable;
      }
    }

    @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>
   * <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(
      @Nullable String message, @Nullable Throwable cause) {
    CancellationException exception = new CancellationException(message);
    exception.initCause(cause);
    return exception;
  }

  /**
   * Returns an {@link Executor} that runs each task in the thread that invokes
   * {@link Executor#execute execute}, as in {@link CallerRunsPolicy}.
   * <p>
   * <p>This instance is equivalent to: <pre>   {@code
   *   final class DirectExecutor implements Executor {
   *     public void execute(Runnable r) {
   *       r.run();
   *     }
   *   }}</pre>
   * <p>
   * <p>This should be preferred to {@link #newDirectExecutorService()}
   * because implementing the {@link ExecutorService} subinterface
   * necessitates significant performance overhead.
   *
   * @since 18.0
   */
  public static Executor directExecutor() {
    return DirectExecutor.INSTANCE;
  }

  /**
   * See {@link #directExecutor} for behavioral notes.
   */
  private enum DirectExecutor implements Executor {
    INSTANCE;

    @Override
    public void execute(Runnable command) {
      command.run();
    }

    @Override
    public String toString() {
      return "MoreExecutors.directExecutor()";
    }
  }

}