/*
 * Copyright 2012 The Netty Project
 *
 * The Netty Project licenses this file to you 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 io.netty.util.concurrent;

import io.netty.util.internal.logging.InternalLogger;
import io.netty.util.internal.logging.InternalLoggerFactory;

import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.Queue;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.Executors;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicBoolean;

Single-thread singleton EventExecutor. It starts the thread automatically and stops it when there is no task pending in the task queue for 1 second. Please note it is not scalable to schedule large number of tasks to this executor; use a dedicated executor. /**
 * Single-thread singleton {@link EventExecutor}.  It starts the thread automatically and stops it when there is no
 * task pending in the task queue for 1 second.  Please note it is not scalable to schedule large number of tasks to
 * this executor; use a dedicated executor.
 */
public final class GlobalEventExecutor extends AbstractScheduledEventExecutor {

    private static final InternalLogger logger = InternalLoggerFactory.getInstance(GlobalEventExecutor.class);

    private static final long SCHEDULE_QUIET_PERIOD_INTERVAL = TimeUnit.SECONDS.toNanos(1);

    public static final GlobalEventExecutor INSTANCE = new GlobalEventExecutor();

    final BlockingQueue<Runnable> taskQueue = new LinkedBlockingQueue<Runnable>();
    final ScheduledFutureTask<Void> quietPeriodTask = new ScheduledFutureTask<Void>(
            this, Executors.<Void>callable(new Runnable() {
        @Override
        public void run() {
            // NOOP
        }
    }, null), ScheduledFutureTask.deadlineNanos(SCHEDULE_QUIET_PERIOD_INTERVAL), -SCHEDULE_QUIET_PERIOD_INTERVAL);

    // because the GlobalEventExecutor is a singleton, tasks submitted to it can come from arbitrary threads and this
    // can trigger the creation of a thread from arbitrary thread groups; for this reason, the thread factory must not
    // be sticky about its thread group
    // visible for testing
    final ThreadFactory threadFactory =
            new DefaultThreadFactory(DefaultThreadFactory.toPoolName(getClass()), false, Thread.NORM_PRIORITY, null);
    private final TaskRunner taskRunner = new TaskRunner();
    private final AtomicBoolean started = new AtomicBoolean();
    volatile Thread thread;

    private final Future<?> terminationFuture = new FailedFuture<Object>(this, new UnsupportedOperationException());

    private GlobalEventExecutor() {
        scheduledTaskQueue().add(quietPeriodTask);
    }

    
Take the next Runnable from the task queue and so will block if no task is currently present. 
Returns:
null if the executor thread has been interrupted or waken up./**
     * Take the next {@link Runnable} from the task queue and so will block if no task is currently present.
     *
     * @return {@code null} if the executor thread has been interrupted or waken up.
     */
    Runnable takeTask() {
        BlockingQueue<Runnable> taskQueue = this.taskQueue;
        for (;;) {
            ScheduledFutureTask<?> scheduledTask = peekScheduledTask();
            if (scheduledTask == null) {
                Runnable task = null;
                try {
                    task = taskQueue.take();
                } catch (InterruptedException e) {
                    // Ignore
                }
                return task;
            } else {
                long delayNanos = scheduledTask.delayNanos();
                Runnable task;
                if (delayNanos > 0) {
                    try {
                        task = taskQueue.poll(delayNanos, TimeUnit.NANOSECONDS);
                    } catch (InterruptedException e) {
                        // Waken up.
                        return null;
                    }
                } else {
                    task = taskQueue.poll();
                }

                if (task == null) {
                    fetchFromScheduledTaskQueue();
                    task = taskQueue.poll();
                }

                if (task != null) {
                    return task;
                }
            }
        }
    }

    private void fetchFromScheduledTaskQueue() {
        long nanoTime = AbstractScheduledEventExecutor.nanoTime();
        Runnable scheduledTask = pollScheduledTask(nanoTime);
        while (scheduledTask != null) {
            taskQueue.add(scheduledTask);
            scheduledTask = pollScheduledTask(nanoTime);
        }
    }

    
Return the number of tasks that are pending for processing.
Be aware that this operation may be expensive as it depends on the internal implementation of the
SingleThreadEventExecutor. So use it was care!
/**
     * Return the number of tasks that are pending for processing.
     *
     * <strong>Be aware that this operation may be expensive as it depends on the internal implementation of the
     * SingleThreadEventExecutor. So use it was care!</strong>
     */
    public int pendingTasks() {
        return taskQueue.size();
    }

    
Add a task to the task queue, or throws a RejectedExecutionException if this instance was shutdown before. /**
     * Add a task to the task queue, or throws a {@link RejectedExecutionException} if this instance was shutdown
     * before.
     */
    private void addTask(Runnable task) {
        if (task == null) {
            throw new NullPointerException("task");
        }
        taskQueue.add(task);
    }

    @Override
    public boolean inEventLoop(Thread thread) {
        return thread == this.thread;
    }

    @Override
    public Future<?> shutdownGracefully(long quietPeriod, long timeout, TimeUnit unit) {
        return terminationFuture();
    }

    @Override
    public Future<?> terminationFuture() {
        return terminationFuture;
    }

    @Override
    @Deprecated
    public void shutdown() {
        throw new UnsupportedOperationException();
    }

    @Override
    public boolean isShuttingDown() {
        return false;
    }

    @Override
    public boolean isShutdown() {
        return false;
    }

    @Override
    public boolean isTerminated() {
        return false;
    }

    @Override
    public boolean awaitTermination(long timeout, TimeUnit unit) {
        return false;
    }

    
Waits until the worker thread of this executor has no tasks left in its task queue and terminates itself.
Because a new worker thread will be started again when a new task is submitted, this operation is only useful
when you want to ensure that the worker thread is terminated after your application is shut
down and there's no chance of submitting a new task afterwards.
Returns:
true if and only if the worker thread has been terminated/**
     * Waits until the worker thread of this executor has no tasks left in its task queue and terminates itself.
     * Because a new worker thread will be started again when a new task is submitted, this operation is only useful
     * when you want to ensure that the worker thread is terminated <strong>after</strong> your application is shut
     * down and there's no chance of submitting a new task afterwards.
     *
     * @return {@code true} if and only if the worker thread has been terminated
     */
    public boolean awaitInactivity(long timeout, TimeUnit unit) throws InterruptedException {
        if (unit == null) {
            throw new NullPointerException("unit");
        }

        final Thread thread = this.thread;
        if (thread == null) {
            throw new IllegalStateException("thread was not started");
        }
        thread.join(unit.toMillis(timeout));
        return !thread.isAlive();
    }

    @Override
    public void execute(Runnable task) {
        if (task == null) {
            throw new NullPointerException("task");
        }

        addTask(task);
        if (!inEventLoop()) {
            startThread();
        }
    }

    private void startThread() {
        if (started.compareAndSet(false, true)) {
            final Thread t = threadFactory.newThread(taskRunner);
            // Set to null to ensure we not create classloader leaks by holds a strong reference to the inherited
            // classloader.
            // See:
            // - https://github.com/netty/netty/issues/7290
            // - https://bugs.openjdk.java.net/browse/JDK-7008595
            AccessController.doPrivileged(new PrivilegedAction<Void>() {
                @Override
                public Void run() {
                    t.setContextClassLoader(null);
                    return null;
                }
            });

            // Set the thread before starting it as otherwise inEventLoop() may return false and so produce
            // an assert error.
            // See https://github.com/netty/netty/issues/4357
            thread = t;
            t.start();
        }
    }

    final class TaskRunner implements Runnable {
        @Override
        public void run() {
            for (;;) {
                Runnable task = takeTask();
                if (task != null) {
                    try {
                        task.run();
                    } catch (Throwable t) {
                        logger.warn("Unexpected exception from the global event executor: ", t);
                    }

                    if (task != quietPeriodTask) {
                        continue;
                    }
                }

                Queue<ScheduledFutureTask<?>> scheduledTaskQueue = GlobalEventExecutor.this.scheduledTaskQueue;
                // Terminate if there is no task in the queue (except the noop task).
                if (taskQueue.isEmpty() && (scheduledTaskQueue == null || scheduledTaskQueue.size() == 1)) {
                    // Mark the current thread as stopped.
                    // The following CAS must always success and must be uncontended,
                    // because only one thread should be running at the same time.
                    boolean stopped = started.compareAndSet(true, false);
                    assert stopped;

                    // Check if there are pending entries added by execute() or schedule*() while we do CAS above.
                    if (taskQueue.isEmpty() && (scheduledTaskQueue == null || scheduledTaskQueue.size() == 1)) {
                        // A) No new task was added and thus there's nothing to handle
                        //    -> safe to terminate because there's nothing left to do
                        // B) A new thread started and handled all the new tasks.
                        //    -> safe to terminate the new thread will take care the rest
                        break;
                    }

                    // There are pending tasks added again.
                    if (!started.compareAndSet(false, true)) {
                        // startThread() started a new thread and set 'started' to true.
                        // -> terminate this thread so that the new thread reads from taskQueue exclusively.
                        break;
                    }

                    // New tasks were added, but this worker was faster to set 'started' to true.
                    // i.e. a new worker thread was not started by startThread().
                    // -> keep this thread alive to handle the newly added entries.
                }
            }
        }
    }
}