/*
    * 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.netty5.channel.nio;
  
  import io.netty5.channel.Channel;
  import io.netty5.channel.ChannelException;
  import io.netty5.channel.DefaultSelectStrategyFactory;
  import io.netty5.channel.IoExecutionContext;
  import io.netty5.channel.IoHandle;
  import io.netty5.channel.IoHandler;
  import io.netty5.channel.IoHandlerFactory;
  import io.netty5.channel.SelectStrategy;
  import io.netty5.channel.SelectStrategyFactory;
  import io.netty5.util.internal.PlatformDependent;
  import io.netty5.util.internal.ReflectionUtil;
  import io.netty5.util.internal.StringUtil;
  import io.netty5.util.internal.SystemPropertyUtil;
  import io.netty5.util.internal.logging.InternalLogger;
  import io.netty5.util.internal.logging.InternalLoggerFactory;
  
  import java.io.IOException;
  import java.io.UncheckedIOException;
  import java.lang.reflect.Field;
  import java.nio.channels.CancelledKeyException;
  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.Set;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.atomic.AtomicBoolean;
  import java.util.function.IntSupplier;
  
  import static java.util.Objects.requireNonNull;
  
  /**
   * {@link IoHandler} 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 NioHandler implements IoHandler {
  
      private static final InternalLogger logger = InternalLoggerFactory.getInstance(NioHandler.class);
  
      private static final int CLEANUP_INTERVAL = 256; // XXX Hard-coded value, but won't need customization.
  
      private static final boolean DISABLE_KEY_SET_OPTIMIZATION =
              SystemPropertyUtil.getBoolean("io.netty5.noKeySetOptimization", false);
  
      private static final int MIN_PREMATURE_SELECTOR_RETURNS = 3;
      private static final int SELECTOR_AUTO_REBUILD_THRESHOLD;
  
      private final IntSupplier selectNowSupplier = () -> {
          try {
              return selectNow();
          } catch (IOException e) {
              throw new UncheckedIOException(e);
          }
      };
  
      static {
          int selectorAutoRebuildThreshold = SystemPropertyUtil.getInt("io.netty5.selectorAutoRebuildThreshold", 512);
          if (selectorAutoRebuildThreshold < MIN_PREMATURE_SELECTOR_RETURNS) {
              selectorAutoRebuildThreshold = 0;
          }
  
          SELECTOR_AUTO_REBUILD_THRESHOLD = selectorAutoRebuildThreshold;
  
          if (logger.isDebugEnabled()) {
              logger.debug("-Dio.netty5.noKeySetOptimization: {}", DISABLE_KEY_SET_OPTIMIZATION);
              logger.debug("-Dio.netty5.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 int cancelledKeys;
     private boolean needsToSelectAgain;
 
     private NioHandler() {
         this(SelectorProvider.provider(), DefaultSelectStrategyFactory.INSTANCE.newSelectStrategy());
     }
 
     private NioHandler(SelectorProvider selectorProvider, SelectStrategy strategy) {
         provider = selectorProvider;
         final SelectorTuple selectorTuple = openSelector();
         selector = selectorTuple.selector;
         unwrappedSelector = selectorTuple.unwrappedSelector;
         selectStrategy = strategy;
     }
 
     /**
      * Returns a new {@link IoHandlerFactory} that creates {@link NioHandler} instances.
      */
     public static IoHandlerFactory newFactory() {
         return NioHandler::new;
     }
 
     /**
      * Returns a new {@link IoHandlerFactory} that creates {@link NioHandler} instances.
      */
     public static IoHandlerFactory newFactory(final SelectorProvider selectorProvider,
                                               final SelectStrategyFactory selectStrategyFactory) {
         requireNonNull(selectorProvider, "selectorProvider");
         requireNonNull(selectStrategyFactory, "selectStrategyFactory");
         return () -> new NioHandler(selectorProvider, selectStrategyFactory.newSelectStrategy());
     }
 
     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_KEY_SET_OPTIMIZATION) {
             return new SelectorTuple(unwrappedSelector);
         }
 
         Object maybeSelectorImplClass = AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
             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;
         final SelectedSelectionKeySet selectedKeySet = new SelectedSelectionKeySet();
 
         Object maybeException = AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
             try {
                 Field selectedKeysField = selectorImplClass.getDeclaredField("selectedKeys");
                 Field publicSelectedKeysField = selectorImplClass.getDeclaredField("publicSelectedKeys");
 
                 if (PlatformDependent.hasUnsafe()) {
                     // Let us try to use sun.misc.Unsafe to replace the SelectionKeySet.
                     // This allows us to also do this in Java9+ without any extra flags.
                     long selectedKeysFieldOffset = PlatformDependent.objectFieldOffset(selectedKeysField);
                     long publicSelectedKeysFieldOffset =
                             PlatformDependent.objectFieldOffset(publicSelectedKeysField);
 
                     if (selectedKeysFieldOffset != -1 && publicSelectedKeysFieldOffset != -1) {
                         PlatformDependent.putObject(
                                 unwrappedSelector, selectedKeysFieldOffset, selectedKeySet);
                         PlatformDependent.putObject(
                                 unwrappedSelector, publicSelectedKeysFieldOffset, selectedKeySet);
                         return null;
                     }
                     // We could not retrieve the offset, lets try reflection as last-resort.
                 }
 
                 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 | 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 NioHandler} to obtain the {@link Selector}.
      */
     public SelectorProvider selectorProvider() {
         return provider;
     }
 
     /**
      * Replaces the current {@link Selector} of this event loop with newly created {@link Selector}s to work
      * around the infamous epoll 100% CPU bug.
      */
     void rebuildSelector() {
         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()) {
             NioProcessor handle = (NioProcessor) key.attachment();
             try {
                 if (!key.isValid() || key.channel().keyFor(newSelectorTuple.unwrappedSelector) != null) {
                     continue;
                 }
                 handle.register(newSelectorTuple.unwrappedSelector);
                 nChannels ++;
             } catch (Exception e) {
                 logger.warn("Failed to re-register a NioHandle to the new Selector.", e);
                 handle.close();
             }
         }
 
         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);
             }
         }
 
         if (logger.isInfoEnabled()) {
             logger.info("Migrated " + nChannels + " channel(s) to the new Selector.");
         }
     }
 
     private static NioProcessor nioHandle(IoHandle handle) {
         if (handle instanceof AbstractNioChannel) {
             return ((AbstractNioChannel<?, ?, ?>) handle).nioProcessor();
         }
         if (handle instanceof NioSelectableChannelHandle) {
             return ((NioSelectableChannelHandle<?>) handle).nioProcessor();
         }
         throw new IllegalArgumentException("IoHandle of type " + StringUtil.simpleClassName(handle) + " not supported");
     }
 
     @Override
     public void register(IoHandle handle) throws Exception {
         NioProcessor nioProcessor = nioHandle(handle);
         boolean selected = false;
         for (;;) {
             try {
                 nioProcessor.register(unwrappedSelector());
                 return;
             } catch (CancelledKeyException e) {
                 if (!selected) {
                     // Force the Selector to select now as the "canceled" SelectionKey may still be
                     // cached and not removed because no Select.select(..) operation was called yet.
                     selectNow();
                     selected = true;
                 } else {
                     // We forced a select operation on the selector before but the SelectionKey is still cached
                     // for whatever reason. JDK bug ?
                     throw e;
                 }
             }
         }
     }
 
     @Override
     public void deregister(IoHandle handle) {
         NioProcessor nioProcessor = nioHandle(handle);
         nioProcessor.deregister();
         cancelledKeys ++;
         if (cancelledKeys >= CLEANUP_INTERVAL) {
             cancelledKeys = 0;
             needsToSelectAgain = true;
         }
     }
 
     @Override
     public int run(IoExecutionContext runner) {
         int handled = 0;
         try {
             try {
                 switch (selectStrategy.calculateStrategy(selectNowSupplier, !runner.canBlock())) {
                     case SelectStrategy.CONTINUE:
                         return 0;
 
                     case SelectStrategy.BUSY_WAIT:
                         // fall-through to SELECT since the busy-wait is not supported with NIO
 
                     case SelectStrategy.SELECT:
                         select(runner, 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:
                 }
             } catch (IOException e) {
                 // If we receive an IOException here its because the Selector is messed up. Let's rebuild
                 // the selector and retry. https://github.com/netty/netty/issues/8566
                 rebuildSelector();
                 handleLoopException(e);
                 return 0;
             }
 
             cancelledKeys = 0;
             needsToSelectAgain = false;
             handled = processSelectedKeys();
         } catch (Error e) {
             throw e;
         } catch (Throwable t) {
             handleLoopException(t);
         }
         return handled;
     }
 
     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 int processSelectedKeys() {
         if (selectedKeys != null) {
             return processSelectedKeysOptimized();
         } else {
             return processSelectedKeysPlain(selector.selectedKeys());
         }
     }
 
     @Override
     public void destroy() {
         try {
             selector.close();
         } catch (IOException e) {
             logger.warn("Failed to close a selector.", e);
         }
     }
 
     private int 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 0;
         }
 
         Iterator<SelectionKey> i = selectedKeys.iterator();
         int handled = 0;
         for (;;) {
             final SelectionKey k = i.next();
             i.remove();
 
             processSelectedKey(k);
             ++handled;
 
             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();
                 }
             }
         }
         return handled;
     }
 
     private int processSelectedKeysOptimized() {
         int handled = 0;
         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;
 
             processSelectedKey(k);
             ++handled;
 
             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;
             }
         }
         return handled;
     }
 
     private void processSelectedKey(SelectionKey k) {
         final NioProcessor handle = (NioProcessor) k.attachment();
         handle.handle(k);
     }
 
     @Override
     public void prepareToDestroy() {
         selectAgain();
         Set<SelectionKey> keys = selector.keys();
         Collection<NioProcessor> handles = new ArrayList<>(keys.size());
         for (SelectionKey k: keys) {
             NioProcessor handle = (NioProcessor) k.attachment();
             handles.add(handle);
         }
 
         for (NioProcessor h: handles) {
             h.close();
         }
     }
 
     @Override
     public void wakeup(boolean inEventLoop) {
         if (!inEventLoop && wakenUp.compareAndSet(false, true)) {
             selector.wakeup();
         }
     }
 
     @Override
     public boolean isCompatible(Class<? extends IoHandle> handleType) {
         return AbstractNioChannel.class.isAssignableFrom(handleType);
     }
 
     Selector unwrappedSelector() {
         return unwrappedSelector;
     }
 
     private int selectNow() throws IOException {
         try {
             return selector.selectNow();
         } finally {
             // restore wakeup state if needed
             if (wakenUp.get()) {
                 selector.wakeup();
             }
         }
     }
 
     private void select(IoExecutionContext runner, boolean oldWakenUp) throws IOException {
         Selector selector = this.selector;
         try {
             int selectCnt = 0;
             long currentTimeNanos = System.nanoTime();
             long selectDeadLineNanos = currentTimeNanos + runner.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 (!runner.canBlock() && wakenUp.compareAndSet(false, true)) {
                     selector.selectNow();
                     selectCnt = 1;
                     break;
                 }
 
                 int selectedKeys = selector.select(timeoutMillis);
                 selectCnt ++;
 
                 if (selectedKeys != 0 || oldWakenUp || wakenUp.get() || !runner.canBlock()) {
                     // - 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 " +
                                 "NioHandler.shutdownGracefully() to shutdown the NioHandler.");
                     }
                     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 code exists in an extra method to ensure the method is not too big to inline as this
                     // branch is not very likely to get hit very frequently.
                     selector = selectRebuildSelector(selectCnt);
                     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 Selector selectRebuildSelector(int selectCnt) throws IOException {
         // 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 selector = this.selector;
 
         // Select again to populate selectedKeys.
         selector.selectNow();
         return selector;
     }
 
     private void selectAgain() {
         needsToSelectAgain = false;
         try {
             selector.selectNow();
         } catch (Throwable t) {
             logger.warn("Failed to update SelectionKeys.", t);
         }
     }
 }