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/*
* Copyright (C) 2006 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.util.concurrent.MoreExecutors.directExecutor;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.base.Preconditions;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import javax.annotation.Nullable;
/**
* Implementation of {@code Futures#withTimeout}.
*
* Future that delegates to another but will finish early (via a {@link TimeoutException} wrapped
* in an {@link ExecutionException}) if the specified duration expires. The delegate future is
* interrupted and cancelled if it times out.
*/
@GwtIncompatible
final class TimeoutFuture extends AbstractFuture.TrustedFuture {
static ListenableFuture create(
ListenableFuture delegate,
long time,
TimeUnit unit,
ScheduledExecutorService scheduledExecutor) {
TimeoutFuture result = new TimeoutFuture(delegate);
TimeoutFuture.Fire fire = new TimeoutFuture.Fire(result);
result.timer = scheduledExecutor.schedule(fire, time, unit);
delegate.addListener(fire, directExecutor());
return result;
}
/*
* Memory visibility of these fields. There are two cases to consider.
*
* 1. visibility of the writes to these fields to Fire.run:
*
* The initial write to delegateRef is made definitely visible via the semantics of
* addListener/SES.schedule. The later racy write in cancel() is not guaranteed to be observed,
* however that is fine since the correctness is based on the atomic state in our base class. The
* initial write to timer is never definitely visible to Fire.run since it is assigned after
* SES.schedule is called. Therefore Fire.run has to check for null. However, it should be visible
* if Fire.run is called by delegate.addListener since addListener is called after the assignment
* to timer, and importantly this is the main situation in which we need to be able to see the
* write.
*
* 2. visibility of the writes to an afterDone() call triggered by cancel():
*
* Since these fields are non-final that means that TimeoutFuture is not being 'safely published',
* thus a motivated caller may be able to expose the reference to another thread that would then
* call cancel() and be unable to cancel the delegate.
* There are a number of ways to solve this, none of which are very pretty, and it is currently
* believed to be a purely theoretical problem (since the other actions should supply sufficient
* write-barriers).
*/
@Nullable private ListenableFuture delegateRef;
@Nullable private Future timer;
private TimeoutFuture(ListenableFuture delegate) {
this.delegateRef = Preconditions.checkNotNull(delegate);
}
/** A runnable that is called when the delegate or the timer completes. */
private static final class Fire implements Runnable {
@Nullable TimeoutFuture timeoutFutureRef;
Fire(TimeoutFuture timeoutFuture) {
this.timeoutFutureRef = timeoutFuture;
}
@Override
public void run() {
// If either of these reads return null then we must be after a successful cancel or another
// call to this method.
TimeoutFuture timeoutFuture = timeoutFutureRef;
if (timeoutFuture == null) {
return;
}
ListenableFuture delegate = timeoutFuture.delegateRef;
if (delegate == null) {
return;
}
/*
* If we're about to complete the TimeoutFuture, we want to release our reference to it.
* Otherwise, we'll pin it (and its result) in memory until the timeout task is GCed. (The
* need to clear our reference to the TimeoutFuture is the reason we use a *static* nested
* class with a manual reference back to the "containing" class.)
*
* This has the nice-ish side effect of limiting reentrancy: run() calls
* timeoutFuture.setException() calls run(). That reentrancy would already be harmless, since
* timeoutFuture can be set (and delegate cancelled) only once. (And "set only once" is
* important for other reasons: run() can still be invoked concurrently in different threads,
* even with the above null checks.)
*/
timeoutFutureRef = null;
if (delegate.isDone()) {
timeoutFuture.setFuture(delegate);
} else {
try {
// TODO(lukes): this stack trace is particularly useless (all it does is point at the
// scheduledexecutorservice thread), consider eliminating it altogether?
timeoutFuture.setException(new TimeoutException("Future timed out: " + delegate));
} finally {
delegate.cancel(true);
}
}
}
}
@Override
protected void afterDone() {
maybePropagateCancellation(delegateRef);
Future localTimer = timer;
// Try to cancel the timer as an optimization.
// timer may be null if this call to run was by the timer task since there is no happens-before
// edge between the assignment to timer and an execution of the timer task.
if (localTimer != null) {
localTimer.cancel(false);
}
delegateRef = null;
timer = null;
}
}