/*
    * 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:
    *
    *   https://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.ObjectUtil;
  import io.netty.util.internal.PlatformDependent;
  import io.netty.util.internal.SystemPropertyUtil;
  import io.netty.util.internal.ThreadExecutorMap;
  import io.netty.util.internal.logging.InternalLogger;
  import io.netty.util.internal.logging.InternalLoggerFactory;
  import org.jetbrains.annotations.Async.Schedule;
  
  import java.lang.Thread.State;
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.LinkedHashSet;
  import java.util.List;
  import java.util.Queue;
  import java.util.Set;
  import java.util.concurrent.BlockingQueue;
  import java.util.concurrent.Callable;
  import java.util.concurrent.CountDownLatch;
  import java.util.concurrent.ExecutionException;
  import java.util.concurrent.Executor;
  import java.util.concurrent.LinkedBlockingQueue;
  import java.util.concurrent.RejectedExecutionException;
  import java.util.concurrent.ThreadFactory;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.TimeoutException;
  import java.util.concurrent.atomic.AtomicInteger;
  import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
  import java.util.concurrent.atomic.AtomicLong;
  import java.util.concurrent.atomic.AtomicLongFieldUpdater;
  import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
  import java.util.concurrent.locks.Lock;
  import java.util.concurrent.locks.ReentrantLock;
  
  /**
   * Abstract base class for {@link OrderedEventExecutor}'s that execute all its submitted tasks in a single thread.
   *
   */
  public abstract class SingleThreadEventExecutor extends AbstractScheduledEventExecutor implements OrderedEventExecutor {
  
      static final int DEFAULT_MAX_PENDING_EXECUTOR_TASKS = Math.max(16,
              SystemPropertyUtil.getInt("io.netty.eventexecutor.maxPendingTasks", Integer.MAX_VALUE));
  
      private static final InternalLogger logger =
              InternalLoggerFactory.getInstance(SingleThreadEventExecutor.class);
  
      private static final int ST_NOT_STARTED = 1;
      private static final int ST_SUSPENDING = 2;
      private static final int ST_SUSPENDED = 3;
      private static final int ST_STARTED = 4;
      private static final int ST_SHUTTING_DOWN = 5;
      private static final int ST_SHUTDOWN = 6;
      private static final int ST_TERMINATED = 7;
  
      private static final Runnable NOOP_TASK = new Runnable() {
          @Override
          public void run() {
              // Do nothing.
          }
      };
  
      private static final AtomicIntegerFieldUpdater<SingleThreadEventExecutor> STATE_UPDATER =
              AtomicIntegerFieldUpdater.newUpdater(SingleThreadEventExecutor.class, "state");
      private static final AtomicReferenceFieldUpdater<SingleThreadEventExecutor, ThreadProperties> PROPERTIES_UPDATER =
              AtomicReferenceFieldUpdater.newUpdater(
                      SingleThreadEventExecutor.class, ThreadProperties.class, "threadProperties");
      private static final AtomicLongFieldUpdater<SingleThreadEventExecutor> ACCUMULATED_ACTIVE_TIME_NANOS_UPDATER =
              AtomicLongFieldUpdater.newUpdater(SingleThreadEventExecutor.class, "accumulatedActiveTimeNanos");
      private static final AtomicIntegerFieldUpdater<SingleThreadEventExecutor> CONSECUTIVE_IDLE_CYCLES_UPDATER =
              AtomicIntegerFieldUpdater.newUpdater(SingleThreadEventExecutor.class, "consecutiveIdleCycles");
      private static final AtomicIntegerFieldUpdater<SingleThreadEventExecutor> CONSECUTIVE_BUSY_CYCLES_UPDATER =
              AtomicIntegerFieldUpdater.newUpdater(SingleThreadEventExecutor.class, "consecutiveBusyCycles");
      private final Queue<Runnable> taskQueue;
  
      private volatile Thread thread;
      @SuppressWarnings("unused")
      private volatile ThreadProperties threadProperties;
      private final Executor executor;
      private volatile boolean interrupted;
  
      private final Lock processingLock = new ReentrantLock();
      private final CountDownLatch threadLock = new CountDownLatch(1);
      private final Set<Runnable> shutdownHooks = new LinkedHashSet<Runnable>();
     private final boolean addTaskWakesUp;
     private final int maxPendingTasks;
     private final RejectedExecutionHandler rejectedExecutionHandler;
     private final boolean supportSuspension;
 
     // A running total of nanoseconds this executor has spent in an "active" state.
     private volatile long accumulatedActiveTimeNanos;
     // Timestamp of the last recorded activity (tasks + I/O).
     private volatile long lastActivityTimeNanos;
     /**
      * Tracks the number of consecutive monitor cycles this executor's
      * utilization has been below the scale-down threshold.
      */
     private volatile int consecutiveIdleCycles;
 
     /**
      * Tracks the number of consecutive monitor cycles this executor's
      * utilization has been above the scale-up threshold.
      */
     private volatile int consecutiveBusyCycles;
     private long lastExecutionTime;
 
     @SuppressWarnings({ "FieldMayBeFinal", "unused" })
     private volatile int state = ST_NOT_STARTED;
 
     private volatile long gracefulShutdownQuietPeriod;
     private volatile long gracefulShutdownTimeout;
     private long gracefulShutdownStartTime;
 
     private final Promise<?> terminationFuture = new DefaultPromise<Void>(GlobalEventExecutor.INSTANCE);
 
     /**
      * Create a new instance
      *
      * @param parent            the {@link EventExecutorGroup} which is the parent of this instance and belongs to it
      * @param threadFactory     the {@link ThreadFactory} which will be used for the used {@link Thread}
      * @param addTaskWakesUp    {@code true} if and only if invocation of {@link #addTask(Runnable)} will wake up the
      *                          executor thread
      */
     protected SingleThreadEventExecutor(
             EventExecutorGroup parent, ThreadFactory threadFactory, boolean addTaskWakesUp) {
         this(parent, new ThreadPerTaskExecutor(threadFactory), addTaskWakesUp);
     }
 
     /**
      * Create a new instance
      *
      * @param parent            the {@link EventExecutorGroup} which is the parent of this instance and belongs to it
      * @param threadFactory     the {@link ThreadFactory} which will be used for the used {@link Thread}
      * @param addTaskWakesUp    {@code true} if and only if invocation of {@link #addTask(Runnable)} will wake up the
      *                          executor thread
      * @param maxPendingTasks   the maximum number of pending tasks before new tasks will be rejected.
      * @param rejectedHandler   the {@link RejectedExecutionHandler} to use.
      */
     protected SingleThreadEventExecutor(
             EventExecutorGroup parent, ThreadFactory threadFactory,
             boolean addTaskWakesUp, int maxPendingTasks, RejectedExecutionHandler rejectedHandler) {
         this(parent, new ThreadPerTaskExecutor(threadFactory), addTaskWakesUp, maxPendingTasks, rejectedHandler);
     }
 
     /**
      * Create a new instance
      *
      * @param parent            the {@link EventExecutorGroup} which is the parent of this instance and belongs to it
      * @param threadFactory     the {@link ThreadFactory} which will be used for the used {@link Thread}
      * @param addTaskWakesUp    {@code true} if and only if invocation of {@link #addTask(Runnable)} will wake up the
      *                          executor thread
      * @param supportSuspension {@code true} if suspension of this {@link SingleThreadEventExecutor} is supported.
      * @param maxPendingTasks   the maximum number of pending tasks before new tasks will be rejected.
      * @param rejectedHandler   the {@link RejectedExecutionHandler} to use.
      */
     protected SingleThreadEventExecutor(
             EventExecutorGroup parent, ThreadFactory threadFactory,
             boolean addTaskWakesUp, boolean supportSuspension,
             int maxPendingTasks, RejectedExecutionHandler rejectedHandler) {
         this(parent, new ThreadPerTaskExecutor(threadFactory), addTaskWakesUp, supportSuspension,
                 maxPendingTasks, rejectedHandler);
     }
 
     /**
      * Create a new instance
      *
      * @param parent            the {@link EventExecutorGroup} which is the parent of this instance and belongs to it
      * @param executor          the {@link Executor} which will be used for executing
      * @param addTaskWakesUp    {@code true} if and only if invocation of {@link #addTask(Runnable)} will wake up the
      *                          executor thread
      */
     protected SingleThreadEventExecutor(EventExecutorGroup parent, Executor executor, boolean addTaskWakesUp) {
         this(parent, executor, addTaskWakesUp, DEFAULT_MAX_PENDING_EXECUTOR_TASKS, RejectedExecutionHandlers.reject());
     }
 
     /**
      * Create a new instance
      *
      * @param parent            the {@link EventExecutorGroup} which is the parent of this instance and belongs to it
      * @param executor          the {@link Executor} which will be used for executing
      * @param addTaskWakesUp    {@code true} if and only if invocation of {@link #addTask(Runnable)} will wake up the
      *                          executor thread
      * @param maxPendingTasks   the maximum number of pending tasks before new tasks will be rejected.
      * @param rejectedHandler   the {@link RejectedExecutionHandler} to use.
      */
     protected SingleThreadEventExecutor(EventExecutorGroup parent, Executor executor,
                                         boolean addTaskWakesUp, int maxPendingTasks,
                                         RejectedExecutionHandler rejectedHandler) {
         this(parent, executor, addTaskWakesUp, false, maxPendingTasks, rejectedHandler);
     }
 
     /**
      * Create a new instance
      *
      * @param parent            the {@link EventExecutorGroup} which is the parent of this instance and belongs to it
      * @param executor          the {@link Executor} which will be used for executing
      * @param addTaskWakesUp    {@code true} if and only if invocation of {@link #addTask(Runnable)} will wake up the
      *                          executor thread
      * @param supportSuspension {@code true} if suspension of this {@link SingleThreadEventExecutor} is supported.
      * @param maxPendingTasks   the maximum number of pending tasks before new tasks will be rejected.
      * @param rejectedHandler   the {@link RejectedExecutionHandler} to use.
      */
     protected SingleThreadEventExecutor(EventExecutorGroup parent, Executor executor,
                                         boolean addTaskWakesUp, boolean supportSuspension,
                                         int maxPendingTasks, RejectedExecutionHandler rejectedHandler) {
         super(parent);
         this.addTaskWakesUp = addTaskWakesUp;
         this.supportSuspension = supportSuspension;
         this.maxPendingTasks = Math.max(16, maxPendingTasks);
         this.executor = ThreadExecutorMap.apply(executor, this);
         taskQueue = newTaskQueue(this.maxPendingTasks);
         rejectedExecutionHandler = ObjectUtil.checkNotNull(rejectedHandler, "rejectedHandler");
         lastActivityTimeNanos = ticker().nanoTime();
     }
 
     protected SingleThreadEventExecutor(EventExecutorGroup parent, Executor executor,
                                         boolean addTaskWakesUp, Queue<Runnable> taskQueue,
                                         RejectedExecutionHandler rejectedHandler) {
         this(parent, executor, addTaskWakesUp, false, taskQueue, rejectedHandler);
     }
 
     protected SingleThreadEventExecutor(EventExecutorGroup parent, Executor executor,
                                         boolean addTaskWakesUp, boolean supportSuspension,
                                         Queue<Runnable> taskQueue, RejectedExecutionHandler rejectedHandler) {
         super(parent);
         this.addTaskWakesUp = addTaskWakesUp;
         this.supportSuspension = supportSuspension;
         this.maxPendingTasks = DEFAULT_MAX_PENDING_EXECUTOR_TASKS;
         this.executor = ThreadExecutorMap.apply(executor, this);
         this.taskQueue = ObjectUtil.checkNotNull(taskQueue, "taskQueue");
         this.rejectedExecutionHandler = ObjectUtil.checkNotNull(rejectedHandler, "rejectedHandler");
     }
 
     /**
      * @deprecated Please use and override {@link #newTaskQueue(int)}.
      */
     @Deprecated
     protected Queue<Runnable> newTaskQueue() {
         return newTaskQueue(maxPendingTasks);
     }
 
     /**
      * Create a new {@link Queue} which will holds the tasks to execute. This default implementation will return a
      * {@link LinkedBlockingQueue} but if your sub-class of {@link SingleThreadEventExecutor} will not do any blocking
      * calls on the this {@link Queue} it may make sense to {@code @Override} this and return some more performant
      * implementation that does not support blocking operations at all.
      */
     protected Queue<Runnable> newTaskQueue(int maxPendingTasks) {
         return new LinkedBlockingQueue<Runnable>(maxPendingTasks);
     }
 
     /**
      * Interrupt the current running {@link Thread}.
      */
     protected void interruptThread() {
         Thread currentThread = thread;
         if (currentThread == null) {
             interrupted = true;
         } else {
             currentThread.interrupt();
         }
     }
 
     /**
      * @see Queue#poll()
      */
     protected Runnable pollTask() {
         assert inEventLoop();
         return pollTaskFrom(taskQueue);
     }
 
     protected static Runnable pollTaskFrom(Queue<Runnable> taskQueue) {
         for (;;) {
             Runnable task = taskQueue.poll();
             if (task != WAKEUP_TASK) {
                 return task;
             }
         }
     }
 
     /**
      * Take the next {@link Runnable} from the task queue and so will block if no task is currently present.
      * <p>
      * Be aware that this method will throw an {@link UnsupportedOperationException} if the task queue, which was
      * created via {@link #newTaskQueue()}, does not implement {@link BlockingQueue}.
      * </p>
      *
      * @return {@code null} if the executor thread has been interrupted or waken up.
      */
     protected Runnable takeTask() {
         assert inEventLoop();
         if (!(taskQueue instanceof BlockingQueue)) {
             throw new UnsupportedOperationException();
         }
 
         BlockingQueue<Runnable> taskQueue = (BlockingQueue<Runnable>) this.taskQueue;
         for (;;) {
             ScheduledFutureTask<?> scheduledTask = peekScheduledTask();
             if (scheduledTask == null) {
                 Runnable task = null;
                 try {
                     task = taskQueue.take();
                     if (task == WAKEUP_TASK) {
                         task = null;
                     }
                 } catch (InterruptedException e) {
                     // Ignore
                 }
                 return task;
             } else {
                 long delayNanos = scheduledTask.delayNanos();
                 Runnable task = null;
                 if (delayNanos > 0) {
                     try {
                         task = taskQueue.poll(delayNanos, TimeUnit.NANOSECONDS);
                     } catch (InterruptedException e) {
                         // Waken up.
                         return null;
                     }
                 }
                 if (task == null) {
                     // We need to fetch the scheduled tasks now as otherwise there may be a chance that
                     // scheduled tasks are never executed if there is always one task in the taskQueue.
                     // This is for example true for the read task of OIO Transport
                     // See https://github.com/netty/netty/issues/1614
                     fetchFromScheduledTaskQueue();
                     task = taskQueue.poll();
                 }
 
                 if (task != null) {
                     if (task == WAKEUP_TASK) {
                         return null;
                     }
                     return task;
                 }
             }
         }
     }
 
     private boolean fetchFromScheduledTaskQueue() {
         return fetchFromScheduledTaskQueue(taskQueue);
     }
 
     /**
      * @return {@code true} if at least one scheduled task was executed.
      */
     private boolean executeExpiredScheduledTasks() {
         if (scheduledTaskQueue == null || scheduledTaskQueue.isEmpty()) {
             return false;
         }
         long nanoTime = getCurrentTimeNanos();
         Runnable scheduledTask = pollScheduledTask(nanoTime);
         if (scheduledTask == null) {
             return false;
         }
         do {
             safeExecute(scheduledTask);
         } while ((scheduledTask = pollScheduledTask(nanoTime)) != null);
         return true;
     }
 
     /**
      * @see Queue#peek()
      */
     protected Runnable peekTask() {
         assert inEventLoop();
         return taskQueue.peek();
     }
 
     /**
      * @see Queue#isEmpty()
      */
     protected boolean hasTasks() {
         assert inEventLoop();
         return !taskQueue.isEmpty();
     }
 
     /**
      * Return the number of tasks that are pending for processing.
      */
     public int pendingTasks() {
         return taskQueue.size();
     }
 
     /**
      * Add a task to the task queue, or throws a {@link RejectedExecutionException} if this instance was shutdown
      * before.
      */
     protected void addTask(Runnable task) {
         ObjectUtil.checkNotNull(task, "task");
         if (!offerTask(task)) {
             reject(task);
         }
     }
 
     final boolean offerTask(Runnable task) {
         if (isShutdown()) {
             reject();
         }
         return taskQueue.offer(task);
     }
 
     /**
      * @see Queue#remove(Object)
      */
     protected boolean removeTask(Runnable task) {
         return taskQueue.remove(ObjectUtil.checkNotNull(task, "task"));
     }
 
     /**
      * Poll all tasks from the task queue and run them via {@link Runnable#run()} method.
      *
      * @return {@code true} if and only if at least one task was run
      */
     protected boolean runAllTasks() {
         assert inEventLoop();
         boolean fetchedAll;
         boolean ranAtLeastOne = false;
 
         do {
             fetchedAll = fetchFromScheduledTaskQueue(taskQueue);
             if (runAllTasksFrom(taskQueue)) {
                 ranAtLeastOne = true;
             }
         } while (!fetchedAll); // keep on processing until we fetched all scheduled tasks.
 
         if (ranAtLeastOne) {
             lastExecutionTime = getCurrentTimeNanos();
         }
         afterRunningAllTasks();
         return ranAtLeastOne;
     }
 
     /**
      * Execute all expired scheduled tasks and all current tasks in the executor queue until both queues are empty,
      * or {@code maxDrainAttempts} has been exceeded.
      * @param maxDrainAttempts The maximum amount of times this method attempts to drain from queues. This is to prevent
      *                         continuous task execution and scheduling from preventing the EventExecutor thread to
      *                         make progress and return to the selector mechanism to process inbound I/O events.
      * @return {@code true} if at least one task was run.
      */
     protected final boolean runScheduledAndExecutorTasks(final int maxDrainAttempts) {
         assert inEventLoop();
         boolean ranAtLeastOneTask;
         int drainAttempt = 0;
         do {
             // We must run the taskQueue tasks first, because the scheduled tasks from outside the EventLoop are queued
             // here because the taskQueue is thread safe and the scheduledTaskQueue is not thread safe.
             ranAtLeastOneTask = runExistingTasksFrom(taskQueue) | executeExpiredScheduledTasks();
         } while (ranAtLeastOneTask && ++drainAttempt < maxDrainAttempts);
 
         if (drainAttempt > 0) {
             lastExecutionTime = getCurrentTimeNanos();
         }
         afterRunningAllTasks();
 
         return drainAttempt > 0;
     }
 
     /**
      * Runs all tasks from the passed {@code taskQueue}.
      *
      * @param taskQueue To poll and execute all tasks.
      *
      * @return {@code true} if at least one task was executed.
      */
     protected final boolean runAllTasksFrom(Queue<Runnable> taskQueue) {
         Runnable task = pollTaskFrom(taskQueue);
         if (task == null) {
             return false;
         }
         for (;;) {
             safeExecute(task);
             task = pollTaskFrom(taskQueue);
             if (task == null) {
                 return true;
             }
         }
     }
 
     /**
      * What ever tasks are present in {@code taskQueue} when this method is invoked will be {@link Runnable#run()}.
      * @param taskQueue the task queue to drain.
      * @return {@code true} if at least {@link Runnable#run()} was called.
      */
     private boolean runExistingTasksFrom(Queue<Runnable> taskQueue) {
         Runnable task = pollTaskFrom(taskQueue);
         if (task == null) {
             return false;
         }
         int remaining = Math.min(maxPendingTasks, taskQueue.size());
         safeExecute(task);
         // Use taskQueue.poll() directly rather than pollTaskFrom() since the latter may
         // silently consume more than one item from the queue (skips over WAKEUP_TASK instances)
         while (remaining-- > 0 && (task = taskQueue.poll()) != null) {
             safeExecute(task);
         }
         return true;
     }
 
     /**
      * Poll all tasks from the task queue and run them via {@link Runnable#run()} method.  This method stops running
      * the tasks in the task queue and returns if it ran longer than {@code timeoutNanos}.
      */
     @SuppressWarnings("NonAtomicOperationOnVolatileField")
     protected boolean runAllTasks(long timeoutNanos) {
         fetchFromScheduledTaskQueue(taskQueue);
         Runnable task = pollTask();
         if (task == null) {
             afterRunningAllTasks();
             return false;
         }
 
         final long deadline = timeoutNanos > 0 ? getCurrentTimeNanos() + timeoutNanos : 0;
         long runTasks = 0;
         long lastExecutionTime;
 
         long workStartTime = ticker().nanoTime();
         for (;;) {
             safeExecute(task);
 
             runTasks ++;
 
             // Check timeout every 64 tasks because nanoTime() is relatively expensive.
             // XXX: Hard-coded value - will make it configurable if it is really a problem.
             if ((runTasks & 0x3F) == 0) {
                 lastExecutionTime = getCurrentTimeNanos();
                 if (lastExecutionTime >= deadline) {
                     break;
                 }
             }
 
             task = pollTask();
             if (task == null) {
                 lastExecutionTime = getCurrentTimeNanos();
                 break;
             }
         }
 
         long workEndTime = ticker().nanoTime();
         accumulatedActiveTimeNanos += workEndTime - workStartTime;
         lastActivityTimeNanos = workEndTime;
 
         afterRunningAllTasks();
         this.lastExecutionTime = lastExecutionTime;
         return true;
     }
 
     /**
      * Invoked before returning from {@link #runAllTasks()} and {@link #runAllTasks(long)}.
      */
     protected void afterRunningAllTasks() { }
 
     /**
      * Returns the amount of time left until the scheduled task with the closest dead line is executed.
      */
     protected long delayNanos(long currentTimeNanos) {
         currentTimeNanos -= ticker().initialNanoTime();
 
         ScheduledFutureTask<?> scheduledTask = peekScheduledTask();
         if (scheduledTask == null) {
             return SCHEDULE_PURGE_INTERVAL;
         }
 
         return scheduledTask.delayNanos(currentTimeNanos);
     }
 
     /**
      * Returns the absolute point in time (relative to {@link #getCurrentTimeNanos()}) at which the next
      * closest scheduled task should run.
      */
     protected long deadlineNanos() {
         ScheduledFutureTask<?> scheduledTask = peekScheduledTask();
         if (scheduledTask == null) {
             return getCurrentTimeNanos() + SCHEDULE_PURGE_INTERVAL;
         }
         return scheduledTask.deadlineNanos();
     }
 
     /**
      * Updates the internal timestamp that tells when a submitted task was executed most recently.
      * {@link #runAllTasks()} and {@link #runAllTasks(long)} updates this timestamp automatically, and thus there's
      * usually no need to call this method.  However, if you take the tasks manually using {@link #takeTask()} or
      * {@link #pollTask()}, you have to call this method at the end of task execution loop for accurate quiet period
      * checks.
      */
     protected void updateLastExecutionTime() {
         long now = getCurrentTimeNanos();
         lastExecutionTime = now;
         lastActivityTimeNanos = now;
     }
 
     /**
      * Returns the number of registered channels for auto-scaling related decisions.
      * This is intended to be used by {@link MultithreadEventExecutorGroup} for dynamic scaling.
      *
      * @return The number of registered channels, or {@code -1} if not applicable.
      */
     protected int getNumOfRegisteredChannels() {
         return -1;
     }
 
     /**
      * Adds the given duration to the total active time for the current measurement window.
      * <p>
      * <strong>Note:</strong> This method is not thread-safe and must only be called from the
      * {@link #inEventLoop() event loop thread}.
      *
      * @param nanos The active time in nanoseconds to add.
      */
     @SuppressWarnings("NonAtomicOperationOnVolatileField")
     protected void reportActiveIoTime(long nanos) {
         assert inEventLoop();
         if (nanos > 0) {
             accumulatedActiveTimeNanos += nanos;
             lastActivityTimeNanos = ticker().nanoTime();
         }
     }
 
     /**
      * Returns the accumulated active time since the last call and resets the counter.
      */
     protected long getAndResetAccumulatedActiveTimeNanos() {
         return ACCUMULATED_ACTIVE_TIME_NANOS_UPDATER.getAndSet(this, 0);
     }
 
     /**
      * Returns the timestamp of the last known activity (tasks + I/O).
      */
     protected long getLastActivityTimeNanos() {
         return lastActivityTimeNanos;
     }
 
     /**
      * Atomically increments the counter for consecutive monitor cycles where utilization was below the
      * scale-down threshold. This is used by the auto-scaling monitor to track sustained idleness.
      *
      * @return The number of consecutive idle cycles before the increment.
      */
     protected int getAndIncrementIdleCycles() {
         return CONSECUTIVE_IDLE_CYCLES_UPDATER.getAndIncrement(this);
     }
 
     /**
      * Resets the counter for consecutive idle cycles to zero. This is typically called when the
      * executor's utilization is no longer considered idle, breaking the streak.
      */
     protected void resetIdleCycles() {
         CONSECUTIVE_IDLE_CYCLES_UPDATER.set(this, 0);
     }
 
     /**
      * Atomically increments the counter for consecutive monitor cycles where utilization was above the
      * scale-up threshold. This is used by the auto-scaling monitor to track a sustained high load.
      *
      * @return The number of consecutive busy cycles before the increment.
      */
     protected int getAndIncrementBusyCycles() {
         return CONSECUTIVE_BUSY_CYCLES_UPDATER.getAndIncrement(this);
     }
 
     /**
      * Resets the counter for consecutive busy cycles to zero. This is typically called when the
      * executor's utilization is no longer considered busy, breaking the streak.
      */
     protected void resetBusyCycles() {
         CONSECUTIVE_BUSY_CYCLES_UPDATER.set(this, 0);
     }
 
     /**
      * Returns {@code true} if this {@link SingleThreadEventExecutor} supports suspension.
      */
     protected boolean isSuspensionSupported() {
         return supportSuspension;
     }
 
     /**
      * Run the tasks in the {@link #taskQueue}
      */
     protected abstract void run();
 
     /**
      * Do nothing, sub-classes may override
      */
     protected void cleanup() {
         // NOOP
     }
 
     protected void wakeup(boolean inEventLoop) {
         if (!inEventLoop) {
             // Use offer as we actually only need this to unblock the thread and if offer fails we do not care as there
             // is already something in the queue.
             taskQueue.offer(WAKEUP_TASK);
         }
     }
 
     @Override
     public boolean inEventLoop(Thread thread) {
         return thread == this.thread;
     }
 
     /**
      * Add a {@link Runnable} which will be executed on shutdown of this instance
      */
     public void addShutdownHook(final Runnable task) {
         if (inEventLoop()) {
             shutdownHooks.add(task);
         } else {
             execute(new Runnable() {
                 @Override
                 public void run() {
                     shutdownHooks.add(task);
                 }
             });
         }
     }
 
     /**
      * Remove a previous added {@link Runnable} as a shutdown hook
      */
     public void removeShutdownHook(final Runnable task) {
         if (inEventLoop()) {
             shutdownHooks.remove(task);
         } else {
             execute(new Runnable() {
                 @Override
                 public void run() {
                     shutdownHooks.remove(task);
                 }
             });
         }
     }
 
     private boolean runShutdownHooks() {
         boolean ran = false;
         // Note shutdown hooks can add / remove shutdown hooks.
         while (!shutdownHooks.isEmpty()) {
             List<Runnable> copy = new ArrayList<Runnable>(shutdownHooks);
             shutdownHooks.clear();
             for (Runnable task: copy) {
                 try {
                     runTask(task);
                 } catch (Throwable t) {
                     logger.warn("Shutdown hook raised an exception.", t);
                 } finally {
                     ran = true;
                 }
             }
         }
 
         if (ran) {
             lastExecutionTime = getCurrentTimeNanos();
         }
 
         return ran;
     }
 
     private void shutdown0(long quietPeriod, long timeout, int shutdownState) {
         if (isShuttingDown()) {
             return;
         }
 
         boolean inEventLoop = inEventLoop();
         boolean wakeup;
         int oldState;
         for (;;) {
             if (isShuttingDown()) {
                 return;
             }
             int newState;
             wakeup = true;
             oldState = state;
             if (inEventLoop) {
                 newState = shutdownState;
             } else {
                 switch (oldState) {
                     case ST_NOT_STARTED:
                     case ST_STARTED:
                     case ST_SUSPENDING:
                     case ST_SUSPENDED:
                         newState = shutdownState;
                         break;
                     default:
                         newState = oldState;
                         wakeup = false;
                 }
             }
             if (STATE_UPDATER.compareAndSet(this, oldState, newState)) {
                 break;
             }
         }
         if (quietPeriod != -1) {
             gracefulShutdownQuietPeriod = quietPeriod;
         }
         if (timeout != -1) {
             gracefulShutdownTimeout = timeout;
         }
 
         if (ensureThreadStarted(oldState)) {
             return;
         }
 
         if (wakeup) {
             taskQueue.offer(WAKEUP_TASK);
             if (!addTaskWakesUp) {
                 wakeup(inEventLoop);
             }
         }
     }
 
     @Override
     public Future<?> shutdownGracefully(long quietPeriod, long timeout, TimeUnit unit) {
         ObjectUtil.checkPositiveOrZero(quietPeriod, "quietPeriod");
         if (timeout < quietPeriod) {
             throw new IllegalArgumentException(
                     "timeout: " + timeout + " (expected >= quietPeriod (" + quietPeriod + "))");
         }
         ObjectUtil.checkNotNull(unit, "unit");
 
         shutdown0(unit.toNanos(quietPeriod), unit.toNanos(timeout), ST_SHUTTING_DOWN);
         return terminationFuture();
     }
 
     @Override
     public Future<?> terminationFuture() {
         return terminationFuture;
     }
 
     @Override
     @Deprecated
     public void shutdown() {
         shutdown0(-1, -1, ST_SHUTDOWN);
     }
 
     @Override
     public boolean isShuttingDown() {
         return state >= ST_SHUTTING_DOWN;
     }
 
     @Override
     public boolean isShutdown() {
         return state >= ST_SHUTDOWN;
     }
 
     @Override
     public boolean isTerminated() {
         return state == ST_TERMINATED;
     }
 
     @Override
     public boolean isSuspended() {
         int currentState = state;
         return currentState == ST_SUSPENDED || currentState == ST_SUSPENDING;
     }
 
     @Override
     public boolean trySuspend() {
         if (supportSuspension) {
             if (STATE_UPDATER.compareAndSet(this, ST_STARTED, ST_SUSPENDING)) {
                 wakeup(inEventLoop());
                 return true;
             }
             int currentState = state;
             return currentState == ST_SUSPENDED || currentState == ST_SUSPENDING;
         }
         return false;
     }
 
     /**
      * Returns {@code true} if this {@link SingleThreadEventExecutor} can be suspended at the moment, {@code false}
      * otherwise.
      *
      * @return  if suspension is possible at the moment.
      */
     protected boolean canSuspend() {
         return canSuspend(state);
     }
 
     /**
      * Returns {@code true} if this {@link SingleThreadEventExecutor} can be suspended at the moment, {@code false}
      * otherwise.
      *
      * Subclasses might override this method to add extra checks.
      *
      * @param   state   the current internal state of the {@link SingleThreadEventExecutor}.
      * @return          if suspension is possible at the moment.
      */
     protected boolean canSuspend(int state) {
         assert inEventLoop();
         return supportSuspension && (state == ST_SUSPENDED || state == ST_SUSPENDING)
                 && !hasTasks() && nextScheduledTaskDeadlineNanos() == -1;
     }
 
     /**
      * Confirm that the shutdown if the instance should be done now!
      */
     protected boolean confirmShutdown() {
         if (!isShuttingDown()) {
             return false;
         }
 
         if (!inEventLoop()) {
             throw new IllegalStateException("must be invoked from an event loop");
         }
 
         cancelScheduledTasks();
 
         if (gracefulShutdownStartTime == 0) {
             gracefulShutdownStartTime = getCurrentTimeNanos();
         }
 
         if (runAllTasks() || runShutdownHooks()) {
             if (isShutdown()) {
                 // Executor shut down - no new tasks anymore.
                 return true;
             }
 
             // There were tasks in the queue. Wait a little bit more until no tasks are queued for the quiet period or
             // terminate if the quiet period is 0.
             // See https://github.com/netty/netty/issues/4241
             if (gracefulShutdownQuietPeriod == 0) {
                 return true;
             }
             taskQueue.offer(WAKEUP_TASK);
             return false;
         }
 
         final long nanoTime = getCurrentTimeNanos();
 
         if (isShutdown() || nanoTime - gracefulShutdownStartTime > gracefulShutdownTimeout) {
             return true;
         }
 
         if (nanoTime - lastExecutionTime <= gracefulShutdownQuietPeriod) {
             // Check if any tasks were added to the queue every 100ms.
             // TODO: Change the behavior of takeTask() so that it returns on timeout.
             taskQueue.offer(WAKEUP_TASK);
             try {
                 Thread.sleep(100);
             } catch (InterruptedException e) {
                 // Ignore
             }
 
             return false;
         }
 
         // No tasks were added for last quiet period - hopefully safe to shut down.
         // (Hopefully because we really cannot make a guarantee that there will be no execute() calls by a user.)
         return true;
     }
 
     @Override
     public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException {
         ObjectUtil.checkNotNull(unit, "unit");
         if (inEventLoop()) {
             throw new IllegalStateException("cannot await termination of the current thread");
         }
 
         threadLock.await(timeout, unit);
 
         return isTerminated();
     }
 
     @Override
     public void execute(Runnable task) {
         execute0(task);
     }
 
     @Override
     public void lazyExecute(Runnable task) {
         lazyExecute0(task);
     }
 
     private void execute0(@Schedule Runnable task) {
         ObjectUtil.checkNotNull(task, "task");
         execute(task, wakesUpForTask(task));
     }
 
     private void lazyExecute0(@Schedule Runnable task) {
         execute(ObjectUtil.checkNotNull(task, "task"), false);
     }
 
     @Override
     void scheduleRemoveScheduled(final ScheduledFutureTask<?> task) {
         ObjectUtil.checkNotNull(task, "task");
         int currentState = state;
         if (supportSuspension && currentState == ST_SUSPENDED) {
             // In the case of scheduling for removal we need to also ensure we will recover the "suspend" state
             // after it if it was set before. Otherwise we will always end up "unsuspending" things on cancellation
             // which is not optimal.
             execute(new Runnable() {
                 @Override
                 public void run() {
                     task.run();
                     if (canSuspend(ST_SUSPENDED)) {
                         // Try suspending again to recover the state before we submitted the new task that will
                         // handle cancellation itself.
                         trySuspend();
                     }
                 }
             }, true);
         } else {
             // task will remove itself from scheduled task queue when it runs
             execute(task, false);
         }
     }
 
     private void execute(Runnable task, boolean immediate) {
         boolean inEventLoop = inEventLoop();
         addTask(task);
         if (!inEventLoop) {
             startThread();
             if (isShutdown()) {
                 boolean reject = false;
                 try {
                     if (removeTask(task)) {
                         reject = true;
                     }
                 } catch (UnsupportedOperationException e) {
                     // The task queue does not support removal so the best thing we can do is to just move on and
                     // hope we will be able to pick-up the task before its completely terminated.
                     // In worst case we will log on termination.
                 }
                 if (reject) {
                     reject();
                 }
             }
         }
 
         if (!addTaskWakesUp && immediate) {
             wakeup(inEventLoop);
         }
     }
 
     @Override
     public <T> T invokeAny(Collection<? extends Callable<T>> tasks) throws InterruptedException, ExecutionException {
         throwIfInEventLoop("invokeAny");
         return super.invokeAny(tasks);
     }
 
     @Override
     public <T> T invokeAny(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit)
             throws InterruptedException, ExecutionException, TimeoutException {
         throwIfInEventLoop("invokeAny");
         return super.invokeAny(tasks, timeout, unit);
     }
 
     @Override
     public <T> List<java.util.concurrent.Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
             throws InterruptedException {
         throwIfInEventLoop("invokeAll");
         return super.invokeAll(tasks);
     }
 
     @Override
     public <T> List<java.util.concurrent.Future<T>> invokeAll(
             Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit) throws InterruptedException {
         throwIfInEventLoop("invokeAll");
         return super.invokeAll(tasks, timeout, unit);
     }
 
     private void throwIfInEventLoop(String method) {
         if (inEventLoop()) {
             throw new RejectedExecutionException("Calling " + method + " from within the EventLoop is not allowed");
         }
     }
 
     /**
      * Returns the {@link ThreadProperties} of the {@link Thread} that powers the {@link SingleThreadEventExecutor}.
      * If the {@link SingleThreadEventExecutor} is not started yet, this operation will start it and block until
      * it is fully started.
      */
     public final ThreadProperties threadProperties() {
         ThreadProperties threadProperties = this.threadProperties;
         if (threadProperties == null) {
             Thread thread = this.thread;
             if (thread == null) {
                 assert !inEventLoop();
                 submit(NOOP_TASK).syncUninterruptibly();
                 thread = this.thread;
                 assert thread != null;
             }
 
             threadProperties = new DefaultThreadProperties(thread);
             if (!PROPERTIES_UPDATER.compareAndSet(this, null, threadProperties)) {
                 threadProperties = this.threadProperties;
             }
         }
 
         return threadProperties;
     }
 
     /**
      * @deprecated override {@link SingleThreadEventExecutor#wakesUpForTask} to re-create this behaviour
      */
     @Deprecated
     protected interface NonWakeupRunnable extends LazyRunnable { }
 
     /**
      * Can be overridden to control which tasks require waking the {@link EventExecutor} thread
      * if it is waiting so that they can be run immediately.
      */
     protected boolean wakesUpForTask(Runnable task) {
         return true;
     }
 
     protected static void reject() {
         throw new RejectedExecutionException("event executor terminated");
     }
 
     /**
      * Offers the task to the associated {@link RejectedExecutionHandler}.
      *
      * @param task to reject.
      */
     protected final void reject(Runnable task) {
         rejectedExecutionHandler.rejected(task, this);
     }
 
     // ScheduledExecutorService implementation
 
     private static final long SCHEDULE_PURGE_INTERVAL = TimeUnit.SECONDS.toNanos(1);
 
     private void startThread() {
         int currentState = state;
         if (currentState == ST_NOT_STARTED || currentState == ST_SUSPENDED) {
             if (STATE_UPDATER.compareAndSet(this, currentState, ST_STARTED)) {
                 resetIdleCycles();
                 resetBusyCycles();
                 boolean success = false;
                 try {
                     doStartThread();
                     success = true;
                 } finally {
                     if (!success) {
                         STATE_UPDATER.compareAndSet(this, ST_STARTED, ST_NOT_STARTED);
                     }
                 }
             }
         }
     }
 
     private boolean ensureThreadStarted(int oldState) {
         if (oldState == ST_NOT_STARTED || oldState == ST_SUSPENDED) {
             try {
                 doStartThread();
             } catch (Throwable cause) {
                 STATE_UPDATER.set(this, ST_TERMINATED);
                 terminationFuture.tryFailure(cause);
 
                 if (!(cause instanceof Exception)) {
                     // Also rethrow as it may be an OOME for example
                     PlatformDependent.throwException(cause);
                 }
                 return true;
             }
         }
         return false;
     }
 
     private void doStartThread() {
         executor.execute(new Runnable() {
             @Override
             public void run() {
                 processingLock.lock();
                 assert thread == null;
                 thread = Thread.currentThread();
                 if (interrupted) {
                     thread.interrupt();
                     interrupted = false;
                 }
                 boolean success = false;
                 Throwable unexpectedException = null;
                 updateLastExecutionTime();
                 boolean suspend = false;
                 try {
                     for (;;) {
                         SingleThreadEventExecutor.this.run();
                         success = true;
 
                         int currentState = state;
                         if (canSuspend(currentState)) {
                             if (!STATE_UPDATER.compareAndSet(SingleThreadEventExecutor.this,
                                     ST_SUSPENDING, ST_SUSPENDED)) {
                                 // Try again as the CAS failed.
                                 continue;
                             }
 
                             if (!canSuspend(ST_SUSPENDED) && STATE_UPDATER.compareAndSet(SingleThreadEventExecutor.this,
                                         ST_SUSPENDED, ST_STARTED)) {
                                 // Seems like there was something added to the task queue again in the meantime but we
                                 // were able to re-engage this thread as the event loop thread.
                                 continue;
                             }
                             suspend = true;
                         }
                         break;
                     }
                 } catch (Throwable t) {
                     unexpectedException = t;
                     logger.warn("Unexpected exception from an event executor: ", t);
                 } finally {
                     boolean shutdown = !suspend;
                     if (shutdown) {
                         for (;;) {
                             // We are re-fetching the state as it might have been shutdown in the meantime.
                             int oldState = state;
                             if (oldState >= ST_SHUTTING_DOWN || STATE_UPDATER.compareAndSet(
                                     SingleThreadEventExecutor.this, oldState, ST_SHUTTING_DOWN)) {
                                 break;
                             }
                         }
                         if (success && gracefulShutdownStartTime == 0) {
                             // Check if confirmShutdown() was called at the end of the loop.
                             if (logger.isErrorEnabled()) {
                                 logger.error("Buggy " + EventExecutor.class.getSimpleName() + " implementation; " +
                                         SingleThreadEventExecutor.class.getSimpleName() + ".confirmShutdown() must " +
                                         "be called before run() implementation terminates.");
                             }
                         }
                     }
 
                     try {
                         if (shutdown) {
                             // Run all remaining tasks and shutdown hooks. At this point the event loop
                             // is in ST_SHUTTING_DOWN state still accepting tasks which is needed for
                             // graceful shutdown with quietPeriod.
                             for (;;) {
                                 if (confirmShutdown()) {
                                     break;
                                 }
                             }
 
                             // Now we want to make sure no more tasks can be added from this point. This is
                             // achieved by switching the state. Any new tasks beyond this point will be rejected.
                             for (;;) {
                                 int currentState = state;
                                 if (currentState >= ST_SHUTDOWN || STATE_UPDATER.compareAndSet(
                                         SingleThreadEventExecutor.this, currentState, ST_SHUTDOWN)) {
                                     break;
                                 }
                             }
 
                             // We have the final set of tasks in the queue now, no more can be added, run all remaining.
                             // No need to loop here, this is the final pass.
                             confirmShutdown();
                         }
                     } finally {
                         try {
                             if (shutdown) {
                                 try {
                                     cleanup();
                                 } finally {
                                     // Lets remove all FastThreadLocals for the Thread as we are about to terminate and
                                     // notify the future. The user may block on the future and once it unblocks the JVM
                                     // may terminate and start unloading classes.
                                     // See https://github.com/netty/netty/issues/6596.
                                     FastThreadLocal.removeAll();
 
                                     STATE_UPDATER.set(SingleThreadEventExecutor.this, ST_TERMINATED);
                                     threadLock.countDown();
                                     int numUserTasks = drainTasks();
                                     if (numUserTasks > 0 && logger.isWarnEnabled()) {
                                         logger.warn("An event executor terminated with " +
                                                 "non-empty task queue (" + numUserTasks + ')');
                                     }
                                     if (unexpectedException == null) {
                                         terminationFuture.setSuccess(null);
                                     } else {
                                         terminationFuture.setFailure(unexpectedException);
                                     }
                                 }
                             } else {
                                 // Lets remove all FastThreadLocals for the Thread as we are about to terminate it.
                                 FastThreadLocal.removeAll();
 
                                 // Reset the stored threadProperties in case of suspension.
                                 threadProperties = null;
                             }
                         } finally {
                             thread = null;
                             // Let the next thread take over if needed.
                             processingLock.unlock();
                         }
                     }
                 }
             }
         });
     }
 
     final int drainTasks() {
         int numTasks = 0;
         for (;;) {
             Runnable runnable = taskQueue.poll();
             if (runnable == null) {
                 break;
             }
             // WAKEUP_TASK should be just discarded as these are added internally.
             // The important bit is that we not have any user tasks left.
             if (WAKEUP_TASK != runnable) {
                 numTasks++;
             }
         }
         return numTasks;
     }
 
     private static final class DefaultThreadProperties implements ThreadProperties {
         private final Thread t;
 
         DefaultThreadProperties(Thread t) {
             this.t = t;
         }
 
         @Override
         public State state() {
             return t.getState();
         }
 
         @Override
         public int priority() {
             return t.getPriority();
         }
 
         @Override
         public boolean isInterrupted() {
             return t.isInterrupted();
         }
 
         @Override
         public boolean isDaemon() {
             return t.isDaemon();
         }
 
         @Override
         public String name() {
             return t.getName();
         }
 
         @Override
         public long id() {
             return t.getId();
         }
 
         @Override
         public StackTraceElement[] stackTrace() {
             return t.getStackTrace();
         }
 
         @Override
         public boolean isAlive() {
             return t.isAlive();
         }
     }
 }