/*
    * 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.channel.nio;
  
  import io.netty.channel.Channel;
  import io.netty.channel.ChannelException;
  import io.netty.channel.EventLoop;
  import io.netty.channel.EventLoopException;
  import io.netty.channel.SelectStrategy;
  import io.netty.channel.SingleThreadEventLoop;
  import io.netty.channel.nio.AbstractNioChannel.NioUnsafe;
  import io.netty.util.IntSupplier;
  import io.netty.util.concurrent.RejectedExecutionHandler;
  import io.netty.util.internal.PlatformDependent;
  import io.netty.util.internal.ReflectionUtil;
  import io.netty.util.internal.SystemPropertyUtil;
  import io.netty.util.internal.logging.InternalLogger;
  import io.netty.util.internal.logging.InternalLoggerFactory;
  
  import java.io.IOException;
  import java.lang.reflect.Field;
  import java.nio.channels.CancelledKeyException;
  import java.nio.channels.SelectableChannel;
  import java.nio.channels.SelectionKey;
  import java.nio.channels.Selector;
  import java.nio.channels.spi.SelectorProvider;
  import java.security.AccessController;
  import java.security.PrivilegedAction;
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.Iterator;
  import java.util.Queue;
  import java.util.Set;
  import java.util.concurrent.ThreadFactory;
  import java.util.concurrent.Callable;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.atomic.AtomicBoolean;
  
  /**
   * {@link SingleThreadEventLoop} implementation which register the {@link Channel}'s to a
   * {@link Selector} and so does the multi-plexing of these in the event loop.
   *
   */
  public final class NioEventLoop extends SingleThreadEventLoop {
  
      private static final InternalLogger logger = InternalLoggerFactory.getInstance(NioEventLoop.class);
  
      private static final int CLEANUP_INTERVAL = 256; // XXX Hard-coded value, but won't need customization.
  
      private static final boolean DISABLE_KEYSET_OPTIMIZATION =
              SystemPropertyUtil.getBoolean("io.netty.noKeySetOptimization", false);
  
      private static final int MIN_PREMATURE_SELECTOR_RETURNS = 3;
      private static final int SELECTOR_AUTO_REBUILD_THRESHOLD;
  
      private final IntSupplier selectNowSupplier = new IntSupplier() {
          @Override
          public int get() throws Exception {
              return selectNow();
          }
      };
      private final Callable<Integer> pendingTasksCallable = new Callable<Integer>() {
          @Override
          public Integer call() throws Exception {
              return NioEventLoop.super.pendingTasks();
          }
      };
  
      // Workaround for JDK NIO bug.
      //
      // See:
      // - http://bugs.sun.com/view_bug.do?bug_id=6427854
      // - https://github.com/netty/netty/issues/203
      static {
          final String key = "sun.nio.ch.bugLevel";
          final String buglevel = SystemPropertyUtil.get(key);
          if (buglevel == null) {
              try {
                  AccessController.doPrivileged(new PrivilegedAction<Void>() {
                      @Override
                      public Void run() {
                          System.setProperty(key, "");
                          return null;
                      }
                  });
              } catch (final SecurityException e) {
                 logger.debug("Unable to get/set System Property: " + key, e);
             }
         }
 
         int selectorAutoRebuildThreshold = SystemPropertyUtil.getInt("io.netty.selectorAutoRebuildThreshold", 512);
         if (selectorAutoRebuildThreshold < MIN_PREMATURE_SELECTOR_RETURNS) {
             selectorAutoRebuildThreshold = 0;
         }
 
         SELECTOR_AUTO_REBUILD_THRESHOLD = selectorAutoRebuildThreshold;
 
         if (logger.isDebugEnabled()) {
             logger.debug("-Dio.netty.noKeySetOptimization: {}", DISABLE_KEYSET_OPTIMIZATION);
             logger.debug("-Dio.netty.selectorAutoRebuildThreshold: {}", SELECTOR_AUTO_REBUILD_THRESHOLD);
         }
     }
 
     /**
      * The NIO {@link Selector}.
      */
     private Selector selector;
     private Selector unwrappedSelector;
     private SelectedSelectionKeySet selectedKeys;
 
     private final SelectorProvider provider;
 
     /**
      * Boolean that controls determines if a blocked Selector.select should
      * break out of its selection process. In our case we use a timeout for
      * the select method and the select method will block for that time unless
      * waken up.
      */
     private final AtomicBoolean wakenUp = new AtomicBoolean();
 
     private final SelectStrategy selectStrategy;
 
     private volatile int ioRatio = 50;
     private int cancelledKeys;
     private boolean needsToSelectAgain;
 
     NioEventLoop(NioEventLoopGroup parent, ThreadFactory threadFactory, SelectorProvider selectorProvider,
                  SelectStrategy strategy, RejectedExecutionHandler rejectedExecutionHandler) {
         super(parent, threadFactory, false, DEFAULT_MAX_PENDING_TASKS, rejectedExecutionHandler);
         if (selectorProvider == null) {
             throw new NullPointerException("selectorProvider");
         }
         if (strategy == null) {
             throw new NullPointerException("selectStrategy");
         }
         provider = selectorProvider;
         final SelectorTuple selectorTuple = openSelector();
         selector = selectorTuple.selector;
         unwrappedSelector = selectorTuple.unwrappedSelector;
         selectStrategy = strategy;
     }
 
     private static final class SelectorTuple {
         final Selector unwrappedSelector;
         final Selector selector;
 
         SelectorTuple(Selector unwrappedSelector) {
             this.unwrappedSelector = unwrappedSelector;
             this.selector = unwrappedSelector;
         }
 
         SelectorTuple(Selector unwrappedSelector, Selector selector) {
             this.unwrappedSelector = unwrappedSelector;
             this.selector = selector;
         }
     }
 
     private SelectorTuple openSelector() {
         final Selector unwrappedSelector;
         try {
             unwrappedSelector = provider.openSelector();
         } catch (IOException e) {
             throw new ChannelException("failed to open a new selector", e);
         }
 
         if (DISABLE_KEYSET_OPTIMIZATION) {
             return new SelectorTuple(unwrappedSelector);
         }
 
         final SelectedSelectionKeySet selectedKeySet = new SelectedSelectionKeySet();
 
         Object maybeSelectorImplClass = AccessController.doPrivileged(new PrivilegedAction<Object>() {
             @Override
             public Object run() {
                 try {
                     return Class.forName(
                             "sun.nio.ch.SelectorImpl",
                             false,
                             PlatformDependent.getSystemClassLoader());
                 } catch (Throwable cause) {
                     return cause;
                 }
             }
         });
 
         if (!(maybeSelectorImplClass instanceof Class) ||
                 // ensure the current selector implementation is what we can instrument.
                 !((Class<?>) maybeSelectorImplClass).isAssignableFrom(unwrappedSelector.getClass())) {
             if (maybeSelectorImplClass instanceof Throwable) {
                 Throwable t = (Throwable) maybeSelectorImplClass;
                 logger.trace("failed to instrument a special java.util.Set into: {}", unwrappedSelector, t);
             }
             return new SelectorTuple(unwrappedSelector);
         }
 
         final Class<?> selectorImplClass = (Class<?>) maybeSelectorImplClass;
 
         Object maybeException = AccessController.doPrivileged(new PrivilegedAction<Object>() {
             @Override
             public Object run() {
                 try {
                     Field selectedKeysField = selectorImplClass.getDeclaredField("selectedKeys");
                     Field publicSelectedKeysField = selectorImplClass.getDeclaredField("publicSelectedKeys");
 
                     Throwable cause = ReflectionUtil.trySetAccessible(selectedKeysField, true);
                     if (cause != null) {
                         return cause;
                     }
                     cause = ReflectionUtil.trySetAccessible(publicSelectedKeysField, true);
                     if (cause != null) {
                         return cause;
                     }
 
                     selectedKeysField.set(unwrappedSelector, selectedKeySet);
                     publicSelectedKeysField.set(unwrappedSelector, selectedKeySet);
                     return null;
                 } catch (NoSuchFieldException e) {
                     return e;
                 } catch (IllegalAccessException e) {
                     return e;
                 }
             }
         });
 
         if (maybeException instanceof Exception) {
             selectedKeys = null;
             Exception e = (Exception) maybeException;
             logger.trace("failed to instrument a special java.util.Set into: {}", unwrappedSelector, e);
             return new SelectorTuple(unwrappedSelector);
         }
         selectedKeys = selectedKeySet;
         logger.trace("instrumented a special java.util.Set into: {}", unwrappedSelector);
         return new SelectorTuple(unwrappedSelector,
                                  new SelectedSelectionKeySetSelector(unwrappedSelector, selectedKeySet));
     }
 
     /**
      * Returns the {@link SelectorProvider} used by this {@link NioEventLoop} to obtain the {@link Selector}.
      */
     public SelectorProvider selectorProvider() {
         return provider;
     }
 
     @Override
     protected Queue<Runnable> newTaskQueue(int maxPendingTasks) {
         // This event loop never calls takeTask()
         return maxPendingTasks == Integer.MAX_VALUE ? PlatformDependent.<Runnable>newMpscQueue()
                                                     : PlatformDependent.<Runnable>newMpscQueue(maxPendingTasks);
     }
 
     @Override
     public int pendingTasks() {
         // As we use a MpscQueue we need to ensure pendingTasks() is only executed from within the EventLoop as
         // otherwise we may see unexpected behavior (as size() is only allowed to be called by a single consumer).
         // See https://github.com/netty/netty/issues/5297
         if (inEventLoop()) {
             return super.pendingTasks();
         } else {
             return submit(pendingTasksCallable).syncUninterruptibly().getNow();
         }
     }
 
     /**
      * Registers an arbitrary {@link SelectableChannel}, not necessarily created by Netty, to the {@link Selector}
      * of this event loop.  Once the specified {@link SelectableChannel} is registered, the specified {@code task} will
      * be executed by this event loop when the {@link SelectableChannel} is ready.
      */
     public void register(final SelectableChannel ch, final int interestOps, final NioTask<?> task) {
         if (ch == null) {
             throw new NullPointerException("ch");
         }
         if (interestOps == 0) {
             throw new IllegalArgumentException("interestOps must be non-zero.");
         }
         if ((interestOps & ~ch.validOps()) != 0) {
             throw new IllegalArgumentException(
                     "invalid interestOps: " + interestOps + "(validOps: " + ch.validOps() + ')');
         }
         if (task == null) {
             throw new NullPointerException("task");
         }
 
         if (isShutdown()) {
             throw new IllegalStateException("event loop shut down");
         }
 
         try {
             ch.register(selector, interestOps, task);
         } catch (Exception e) {
             throw new EventLoopException("failed to register a channel", e);
         }
     }
 
     /**
      * Returns the percentage of the desired amount of time spent for I/O in the event loop.
      */
     public int getIoRatio() {
         return ioRatio;
     }
 
     /**
      * Sets the percentage of the desired amount of time spent for I/O in the event loop.  The default value is
      * {@code 50}, which means the event loop will try to spend the same amount of time for I/O as for non-I/O tasks.
      */
     public void setIoRatio(int ioRatio) {
         if (ioRatio <= 0 || ioRatio > 100) {
             throw new IllegalArgumentException("ioRatio: " + ioRatio + " (expected: 0 < ioRatio <= 100)");
         }
         this.ioRatio = ioRatio;
     }
 
     /**
      * Replaces the current {@link Selector} of this event loop with newly created {@link Selector}s to work
      * around the infamous epoll 100% CPU bug.
      */
     public void rebuildSelector() {
         if (!inEventLoop()) {
             execute(new Runnable() {
                 @Override
                 public void run() {
                     rebuildSelector0();
                 }
             });
             return;
         }
         rebuildSelector0();
     }
 
     private void rebuildSelector0() {
         final Selector oldSelector = selector;
         final SelectorTuple newSelectorTuple;
 
         if (oldSelector == null) {
             return;
         }
 
         try {
             newSelectorTuple = openSelector();
         } catch (Exception e) {
             logger.warn("Failed to create a new Selector.", e);
             return;
         }
 
         // Register all channels to the new Selector.
         int nChannels = 0;
         for (SelectionKey key: oldSelector.keys()) {
             Object a = key.attachment();
             try {
                 if (!key.isValid() || key.channel().keyFor(newSelectorTuple.unwrappedSelector) != null) {
                     continue;
                 }
 
                 int interestOps = key.interestOps();
                 key.cancel();
                 SelectionKey newKey = key.channel().register(newSelectorTuple.unwrappedSelector, interestOps, a);
                 if (a instanceof AbstractNioChannel) {
                     // Update SelectionKey
                     ((AbstractNioChannel) a).selectionKey = newKey;
                 }
                 nChannels ++;
             } catch (Exception e) {
                 logger.warn("Failed to re-register a Channel to the new Selector.", e);
                 if (a instanceof AbstractNioChannel) {
                     AbstractNioChannel ch = (AbstractNioChannel) a;
                     ch.unsafe().close(ch.unsafe().voidPromise());
                 } else {
                     @SuppressWarnings("unchecked")
                     NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
                     invokeChannelUnregistered(task, key, e);
                 }
             }
         }
 
         selector = newSelectorTuple.selector;
         unwrappedSelector = newSelectorTuple.unwrappedSelector;
 
         try {
             // time to close the old selector as everything else is registered to the new one
             oldSelector.close();
         } catch (Throwable t) {
             if (logger.isWarnEnabled()) {
                 logger.warn("Failed to close the old Selector.", t);
             }
         }
 
         logger.info("Migrated " + nChannels + " channel(s) to the new Selector.");
     }
 
     @Override
     protected void run() {
         for (;;) {
             try {
                 switch (selectStrategy.calculateStrategy(selectNowSupplier, hasTasks())) {
                     case SelectStrategy.CONTINUE:
                         continue;
                     case SelectStrategy.SELECT:
                         select(wakenUp.getAndSet(false));
 
                         // 'wakenUp.compareAndSet(false, true)' is always evaluated
                         // before calling 'selector.wakeup()' to reduce the wake-up
                         // overhead. (Selector.wakeup() is an expensive operation.)
                         //
                         // However, there is a race condition in this approach.
                         // The race condition is triggered when 'wakenUp' is set to
                         // true too early.
                         //
                         // 'wakenUp' is set to true too early if:
                         // 1) Selector is waken up between 'wakenUp.set(false)' and
                         //    'selector.select(...)'. (BAD)
                         // 2) Selector is waken up between 'selector.select(...)' and
                         //    'if (wakenUp.get()) { ... }'. (OK)
                         //
                         // In the first case, 'wakenUp' is set to true and the
                         // following 'selector.select(...)' will wake up immediately.
                         // Until 'wakenUp' is set to false again in the next round,
                         // 'wakenUp.compareAndSet(false, true)' will fail, and therefore
                         // any attempt to wake up the Selector will fail, too, causing
                         // the following 'selector.select(...)' call to block
                         // unnecessarily.
                         //
                         // To fix this problem, we wake up the selector again if wakenUp
                         // is true immediately after selector.select(...).
                         // It is inefficient in that it wakes up the selector for both
                         // the first case (BAD - wake-up required) and the second case
                         // (OK - no wake-up required).
 
                         if (wakenUp.get()) {
                             selector.wakeup();
                         }
                         // fall through
                     default:
                 }
 
                 cancelledKeys = 0;
                 needsToSelectAgain = false;
                 final int ioRatio = this.ioRatio;
                 if (ioRatio == 100) {
                     try {
                         processSelectedKeys();
                     } finally {
                         // Ensure we always run tasks.
                         runAllTasks();
                     }
                 } else {
                     final long ioStartTime = System.nanoTime();
                     try {
                         processSelectedKeys();
                     } finally {
                         // Ensure we always run tasks.
                         final long ioTime = System.nanoTime() - ioStartTime;
                         runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
                     }
                 }
             } catch (Throwable t) {
                 handleLoopException(t);
             }
             // Always handle shutdown even if the loop processing threw an exception.
             try {
                 if (isShuttingDown()) {
                     closeAll();
                     if (confirmShutdown()) {
                         return;
                     }
                 }
             } catch (Throwable t) {
                 handleLoopException(t);
             }
         }
     }
 
     private static void handleLoopException(Throwable t) {
         logger.warn("Unexpected exception in the selector loop.", t);
 
         // Prevent possible consecutive immediate failures that lead to
         // excessive CPU consumption.
         try {
             Thread.sleep(1000);
         } catch (InterruptedException e) {
             // Ignore.
         }
     }
 
     private void processSelectedKeys() {
         if (selectedKeys != null) {
             processSelectedKeysOptimized();
         } else {
             processSelectedKeysPlain(selector.selectedKeys());
         }
     }
 
     @Override
     protected void cleanup() {
         try {
             selector.close();
         } catch (IOException e) {
             logger.warn("Failed to close a selector.", e);
         }
     }
 
     void cancel(SelectionKey key) {
         key.cancel();
         cancelledKeys ++;
         if (cancelledKeys >= CLEANUP_INTERVAL) {
             cancelledKeys = 0;
             needsToSelectAgain = true;
         }
     }
 
     @Override
     protected Runnable pollTask() {
         Runnable task = super.pollTask();
         if (needsToSelectAgain) {
             selectAgain();
         }
         return task;
     }
 
     private void processSelectedKeysPlain(Set<SelectionKey> selectedKeys) {
         // check if the set is empty and if so just return to not create garbage by
         // creating a new Iterator every time even if there is nothing to process.
         // See https://github.com/netty/netty/issues/597
         if (selectedKeys.isEmpty()) {
             return;
         }
 
         Iterator<SelectionKey> i = selectedKeys.iterator();
         for (;;) {
             final SelectionKey k = i.next();
             final Object a = k.attachment();
             i.remove();
 
             if (a instanceof AbstractNioChannel) {
                 processSelectedKey(k, (AbstractNioChannel) a);
             } else {
                 @SuppressWarnings("unchecked")
                 NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
                 processSelectedKey(k, task);
             }
 
             if (!i.hasNext()) {
                 break;
             }
 
             if (needsToSelectAgain) {
                 selectAgain();
                 selectedKeys = selector.selectedKeys();
 
                 // Create the iterator again to avoid ConcurrentModificationException
                 if (selectedKeys.isEmpty()) {
                     break;
                 } else {
                     i = selectedKeys.iterator();
                 }
             }
         }
     }
 
     private void processSelectedKeysOptimized() {
         for (int i = 0; i < selectedKeys.size; ++i) {
             final SelectionKey k = selectedKeys.keys[i];
             // null out entry in the array to allow to have it GC'ed once the Channel close
             // See https://github.com/netty/netty/issues/2363
             selectedKeys.keys[i] = null;
 
             final Object a = k.attachment();
 
             if (a instanceof AbstractNioChannel) {
                 processSelectedKey(k, (AbstractNioChannel) a);
             } else {
                 @SuppressWarnings("unchecked")
                 NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
                 processSelectedKey(k, task);
             }
 
             if (needsToSelectAgain) {
                 // null out entries in the array to allow to have it GC'ed once the Channel close
                 // See https://github.com/netty/netty/issues/2363
                 selectedKeys.reset(i + 1);
 
                 selectAgain();
                 i = -1;
             }
         }
     }
 
     private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
         final NioUnsafe unsafe = ch.unsafe();
         if (!k.isValid()) {
             final EventLoop eventLoop;
             try {
                 eventLoop = ch.eventLoop();
             } catch (Throwable ignored) {
                 // If the channel implementation throws an exception because there is no event loop, we ignore this
                 // because we are only trying to determine if ch is registered to this event loop and thus has authority
                 // to close ch.
                 return;
             }
             // Only close ch if ch is still registered to this EventLoop. ch could have deregistered from the event loop
             // and thus the SelectionKey could be cancelled as part of the deregistration process, but the channel is
             // still healthy and should not be closed.
             // See https://github.com/netty/netty/issues/5125
             if (eventLoop != this || eventLoop == null) {
                 return;
             }
             // close the channel if the key is not valid anymore
             unsafe.close(unsafe.voidPromise());
             return;
         }
 
         try {
             int readyOps = k.readyOps();
             // We first need to call finishConnect() before try to trigger a read(...) or write(...) as otherwise
             // the NIO JDK channel implementation may throw a NotYetConnectedException.
             if ((readyOps & SelectionKey.OP_CONNECT) != 0) {
                 // remove OP_CONNECT as otherwise Selector.select(..) will always return without blocking
                 // See https://github.com/netty/netty/issues/924
                 int ops = k.interestOps();
                 ops &= ~SelectionKey.OP_CONNECT;
                 k.interestOps(ops);
 
                 unsafe.finishConnect();
             }
 
             // Process OP_WRITE first as we may be able to write some queued buffers and so free memory.
             if ((readyOps & SelectionKey.OP_WRITE) != 0) {
                 // Call forceFlush which will also take care of clear the OP_WRITE once there is nothing left to write
                 ch.unsafe().forceFlush();
             }
 
             // Also check for readOps of 0 to workaround possible JDK bug which may otherwise lead
             // to a spin loop
             if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
                 unsafe.read();
             }
         } catch (CancelledKeyException ignored) {
             unsafe.close(unsafe.voidPromise());
         }
     }
 
     private static void processSelectedKey(SelectionKey k, NioTask<SelectableChannel> task) {
         int state = 0;
         try {
             task.channelReady(k.channel(), k);
             state = 1;
         } catch (Exception e) {
             k.cancel();
             invokeChannelUnregistered(task, k, e);
             state = 2;
         } finally {
             switch (state) {
             case 0:
                 k.cancel();
                 invokeChannelUnregistered(task, k, null);
                 break;
             case 1:
                 if (!k.isValid()) { // Cancelled by channelReady()
                     invokeChannelUnregistered(task, k, null);
                 }
                 break;
             }
         }
     }
 
     private void closeAll() {
         selectAgain();
         Set<SelectionKey> keys = selector.keys();
         Collection<AbstractNioChannel> channels = new ArrayList<AbstractNioChannel>(keys.size());
         for (SelectionKey k: keys) {
             Object a = k.attachment();
             if (a instanceof AbstractNioChannel) {
                 channels.add((AbstractNioChannel) a);
             } else {
                 k.cancel();
                 @SuppressWarnings("unchecked")
                 NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
                 invokeChannelUnregistered(task, k, null);
             }
         }
 
         for (AbstractNioChannel ch: channels) {
             ch.unsafe().close(ch.unsafe().voidPromise());
         }
     }
 
     private static void invokeChannelUnregistered(NioTask<SelectableChannel> task, SelectionKey k, Throwable cause) {
         try {
             task.channelUnregistered(k.channel(), cause);
         } catch (Exception e) {
             logger.warn("Unexpected exception while running NioTask.channelUnregistered()", e);
         }
     }
 
     @Override
     protected void wakeup(boolean inEventLoop) {
         if (!inEventLoop && wakenUp.compareAndSet(false, true)) {
             selector.wakeup();
         }
     }
 
     Selector unwrappedSelector() {
         return unwrappedSelector;
     }
 
     int selectNow() throws IOException {
         try {
             return selector.selectNow();
         } finally {
             // restore wakeup state if needed
             if (wakenUp.get()) {
                 selector.wakeup();
             }
         }
     }
 
     private void select(boolean oldWakenUp) throws IOException {
         Selector selector = this.selector;
         try {
             int selectCnt = 0;
             long currentTimeNanos = System.nanoTime();
             long selectDeadLineNanos = currentTimeNanos + delayNanos(currentTimeNanos);
             for (;;) {
                 long timeoutMillis = (selectDeadLineNanos - currentTimeNanos + 500000L) / 1000000L;
                 if (timeoutMillis <= 0) {
                     if (selectCnt == 0) {
                         selector.selectNow();
                         selectCnt = 1;
                     }
                     break;
                 }
 
                 // If a task was submitted when wakenUp value was true, the task didn't get a chance to call
                 // Selector#wakeup. So we need to check task queue again before executing select operation.
                 // If we don't, the task might be pended until select operation was timed out.
                 // It might be pended until idle timeout if IdleStateHandler existed in pipeline.
                 if (hasTasks() && wakenUp.compareAndSet(false, true)) {
                     selector.selectNow();
                     selectCnt = 1;
                     break;
                 }
 
                 int selectedKeys = selector.select(timeoutMillis);
                 selectCnt ++;
 
                 if (selectedKeys != 0 || oldWakenUp || wakenUp.get() || hasTasks() || hasScheduledTasks()) {
                     // - Selected something,
                     // - waken up by user, or
                     // - the task queue has a pending task.
                     // - a scheduled task is ready for processing
                     break;
                 }
                 if (Thread.interrupted()) {
                     // Thread was interrupted so reset selected keys and break so we not run into a busy loop.
                     // As this is most likely a bug in the handler of the user or it's client library we will
                     // also log it.
                     //
                     // See https://github.com/netty/netty/issues/2426
                     if (logger.isDebugEnabled()) {
                         logger.debug("Selector.select() returned prematurely because " +
                                 "Thread.currentThread().interrupt() was called. Use " +
                                 "NioEventLoop.shutdownGracefully() to shutdown the NioEventLoop.");
                     }
                     selectCnt = 1;
                     break;
                 }
 
                 long time = System.nanoTime();
                 if (time - TimeUnit.MILLISECONDS.toNanos(timeoutMillis) >= currentTimeNanos) {
                     // timeoutMillis elapsed without anything selected.
                     selectCnt = 1;
                 } else if (SELECTOR_AUTO_REBUILD_THRESHOLD > 0 &&
                         selectCnt >= SELECTOR_AUTO_REBUILD_THRESHOLD) {
                     // The selector returned prematurely many times in a row.
                     // Rebuild the selector to work around the problem.
                     logger.warn(
                             "Selector.select() returned prematurely {} times in a row; rebuilding Selector {}.",
                             selectCnt, selector);
 
                     rebuildSelector();
                     selector = this.selector;
 
                     // Select again to populate selectedKeys.
                     selector.selectNow();
                     selectCnt = 1;
                     break;
                 }
 
                 currentTimeNanos = time;
             }
 
             if (selectCnt > MIN_PREMATURE_SELECTOR_RETURNS) {
                 if (logger.isDebugEnabled()) {
                     logger.debug("Selector.select() returned prematurely {} times in a row for Selector {}.",
                             selectCnt - 1, selector);
                 }
             }
         } catch (CancelledKeyException e) {
             if (logger.isDebugEnabled()) {
                 logger.debug(CancelledKeyException.class.getSimpleName() + " raised by a Selector {} - JDK bug?",
                         selector, e);
             }
             // Harmless exception - log anyway
         }
     }
 
     private void selectAgain() {
         needsToSelectAgain = false;
         try {
             selector.selectNow();
         } catch (Throwable t) {
             logger.warn("Failed to update SelectionKeys.", t);
         }
     }
 }