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/*
* Copyright (C) 2015 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.NullnessCasts.uncheckedCastNullableTToT;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.j2objc.annotations.ReflectionSupport;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.AbstractOwnableSynchronizer;
import java.util.concurrent.locks.LockSupport;
import org.checkerframework.checker.nullness.qual.Nullable;
@GwtCompatible(emulated = true)
@ReflectionSupport(value = ReflectionSupport.Level.FULL)
@ElementTypesAreNonnullByDefault
// 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.
// Since this class only needs CAS on one field, we can avoid this bug by extending AtomicReference
// instead of using an AtomicReferenceFieldUpdater. This reference stores Thread instances
// and DONE/INTERRUPTED - they have a common ancestor of Runnable.
abstract class InterruptibleTask
extends AtomicReference<@Nullable Runnable> implements Runnable {
static {
// 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;
}
private static final class DoNothingRunnable implements Runnable {
@Override
public void run() {}
}
// The thread executing the task publishes itself to the superclass' reference and the thread
// interrupting sets DONE when it has finished interrupting.
private static final Runnable DONE = new DoNothingRunnable();
private static final Runnable PARKED = new DoNothingRunnable();
// Why 1000? WHY NOT!
private static final int MAX_BUSY_WAIT_SPINS = 1000;
@SuppressWarnings("ThreadPriorityCheck") // The cow told me to
@Override
public final void run() {
/*
* Set runner thread before checking isDone(). If we were to check isDone() first, the task
* might be cancelled before we set the runner thread. That would make it impossible to
* interrupt, yet it will still run, since interruptTask will leave the runner value null,
* allowing the CAS below to succeed.
*/
Thread currentThread = Thread.currentThread();
if (!compareAndSet(null, currentThread)) {
return; // someone else has run or is running.
}
boolean run = !isDone();
T result = null;
Throwable error = null;
try {
if (run) {
result = runInterruptibly();
}
} catch (Throwable t) {
error = t;
} finally {
// Attempt to set the task as done so that further attempts to interrupt will fail.
if (!compareAndSet(currentThread, DONE)) {
waitForInterrupt(currentThread);
}
if (run) {
if (error == null) {
// The cast is safe because of the `run` and `error` checks.
afterRanInterruptiblySuccess(uncheckedCastNullableTToT(result));
} else {
afterRanInterruptiblyFailure(error);
}
}
}
}
private void waitForInterrupt(Thread currentThread) {
/*
* If someone called cancel(true), it is possible that the interrupted bit hasn't been set yet.
* Wait for the interrupting thread to set DONE. (See interruptTask().) We want to wait so that
* the interrupting thread doesn't interrupt the _next_ thing to run on this thread.
*
* Note: We don't reset the interrupted bit, just wait for it to be set. If this is a thread
* pool thread, the thread pool will reset it for us. Otherwise, the interrupted bit may have
* been intended for something else, so don't clear it.
*/
boolean restoreInterruptedBit = false;
int spinCount = 0;
// Interrupting Cow Says:
// ______
// < Spin >
// ------
// \ ^__^
// \ (oo)\_______
// (__)\ )\/\
// ||----w |
// || ||
Runnable state = get();
Blocker blocker = null;
while (state instanceof Blocker || state == PARKED) {
if (state instanceof Blocker) {
blocker = (Blocker) state;
}
spinCount++;
if (spinCount > MAX_BUSY_WAIT_SPINS) {
/*
* If we have spun a lot, just park ourselves. This will save CPU while we wait for a slow
* interrupting thread. In theory, interruptTask() should be very fast, but due to
* InterruptibleChannel and JavaLangAccess.blockedOn(Thread, Interruptible), it isn't
* predictable what work might be done. (e.g., close a file and flush buffers to disk). To
* protect ourselves from this, we park ourselves and tell our interrupter that we did so.
*/
if (state == PARKED || compareAndSet(state, PARKED)) {
// Interrupting Cow Says:
// ______
// < Park >
// ------
// \ ^__^
// \ (oo)\_______
// (__)\ )\/\
// ||----w |
// || ||
// We need to clear the interrupted bit prior to calling park and maintain it in case we
// wake up spuriously.
restoreInterruptedBit = Thread.interrupted() || restoreInterruptedBit;
LockSupport.park(blocker);
}
} else {
Thread.yield();
}
state = get();
}
if (restoreInterruptedBit) {
currentThread.interrupt();
}
/*
* TODO(cpovirk): Clear interrupt status here? We currently don't, which means that an interrupt
* before, during, or after runInterruptibly() (unless it produced an InterruptedException
* caught above) can linger and affect listeners.
*/
}
/**
* Called before runInterruptibly - if true, runInterruptibly and afterRanInterruptibly will not
* be called.
*/
abstract boolean isDone();
/**
* Do interruptible work here - do not complete Futures here, as their listeners could be
* interrupted.
*/
@ParametricNullness
abstract T runInterruptibly() throws Exception;
/**
* Any interruption that happens as a result of calling interruptTask will arrive before this
* method is called. Complete Futures here.
*/
abstract void afterRanInterruptiblySuccess(@ParametricNullness T result);
/**
* Any interruption that happens as a result of calling interruptTask will arrive before this
* method is called. Complete Futures here.
*/
abstract void afterRanInterruptiblyFailure(Throwable error);
/**
* Interrupts the running task. Because this internally calls {@link Thread#interrupt()} which can
* in turn invoke arbitrary code it is not safe to call while holding a lock.
*/
final void interruptTask() {
// Since the Thread is replaced by DONE before run() invokes listeners or returns, if we succeed
// in this CAS, there's no risk of interrupting the wrong thread or interrupting a thread that
// isn't currently executing this task.
Runnable currentRunner = get();
if (currentRunner instanceof Thread) {
Blocker blocker = new Blocker(this);
blocker.setOwner(Thread.currentThread());
if (compareAndSet(currentRunner, blocker)) {
// Thread.interrupt can throw arbitrary exceptions due to the nio InterruptibleChannel API
// This will make sure that tasks don't get stuck busy waiting.
// Some of this is fixed in jdk11 (see https://bugs.openjdk.java.net/browse/JDK-8198692) but
// not all. See the test cases for examples on how this can happen.
try {
((Thread) currentRunner).interrupt();
} finally {
Runnable prev = getAndSet(DONE);
if (prev == PARKED) {
LockSupport.unpark((Thread) currentRunner);
}
}
}
}
}
/**
* Using this as the blocker object allows introspection and debugging tools to see that the
* currentRunner thread is blocked on the progress of the interruptor thread, which can help
* identify deadlocks.
*/
@VisibleForTesting
static final class Blocker extends AbstractOwnableSynchronizer implements Runnable {
private final InterruptibleTask task;
private Blocker(InterruptibleTask task) {
this.task = task;
}
@Override
public void run() {}
private void setOwner(Thread thread) {
super.setExclusiveOwnerThread(thread);
}
@Override
public String toString() {
return task.toString();
}
}
@Override
public final String toString() {
Runnable state = get();
String result;
if (state == DONE) {
result = "running=[DONE]";
} else if (state instanceof Blocker) {
result = "running=[INTERRUPTED]";
} else if (state instanceof Thread) {
// getName is final on Thread, no need to worry about exceptions
result = "running=[RUNNING ON " + ((Thread) state).getName() + "]";
} else {
result = "running=[NOT STARTED YET]";
}
return result + ", " + toPendingString();
}
abstract String toPendingString();
}