/*
    * Copyright 2016 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.kqueue;
  
  import io.netty5.buffer.api.Buffer;
  import io.netty5.buffer.api.BufferAllocator;
  import io.netty5.channel.AdaptiveRecvBufferAllocator;
  import io.netty5.channel.ChannelException;
  import io.netty5.channel.ChannelMetadata;
  import io.netty5.channel.ChannelOption;
  import io.netty5.channel.ChannelOutboundBuffer;
  import io.netty5.channel.ChannelPipeline;
  import io.netty5.channel.ChannelShutdownDirection;
  import io.netty5.channel.DefaultFileRegion;
  import io.netty5.channel.EventLoop;
  import io.netty5.channel.FileRegion;
  import io.netty5.channel.RecvBufferAllocator;
  import io.netty5.channel.internal.ChannelUtils;
  import io.netty5.channel.socket.SocketChannel;
  import io.netty5.channel.socket.SocketProtocolFamily;
  import io.netty5.channel.unix.DomainSocketReadMode;
  import io.netty5.channel.unix.FileDescriptor;
  import io.netty5.channel.unix.IntegerUnixChannelOption;
  import io.netty5.channel.unix.IovArray;
  import io.netty5.channel.unix.PeerCredentials;
  import io.netty5.channel.unix.RawUnixChannelOption;
  import io.netty5.channel.unix.SocketWritableByteChannel;
  import io.netty5.channel.unix.UnixChannel;
  import io.netty5.channel.unix.UnixChannelOption;
  import io.netty5.channel.unix.UnixChannelUtil;
  import io.netty5.util.Resource;
  import io.netty5.util.concurrent.Future;
  import io.netty5.util.concurrent.GlobalEventExecutor;
  import io.netty5.util.internal.PlatformDependent;
  import io.netty5.util.internal.StringUtil;
  import io.netty5.util.internal.UnstableApi;
  
  import java.io.IOException;
  import java.net.InetSocketAddress;
  import java.net.ProtocolFamily;
  import java.net.SocketAddress;
  import java.nio.ByteBuffer;
  import java.nio.channels.NotYetConnectedException;
  import java.nio.channels.WritableByteChannel;
  import java.util.Set;
  import java.util.concurrent.Executor;
  import java.util.function.Predicate;
  
  import static io.netty5.channel.ChannelOption.IP_TOS;
  import static io.netty5.channel.ChannelOption.SO_KEEPALIVE;
  import static io.netty5.channel.ChannelOption.SO_LINGER;
  import static io.netty5.channel.ChannelOption.SO_RCVBUF;
  import static io.netty5.channel.ChannelOption.SO_REUSEADDR;
  import static io.netty5.channel.ChannelOption.SO_SNDBUF;
  import static io.netty5.channel.ChannelOption.TCP_NODELAY;
  import static io.netty5.channel.internal.ChannelUtils.MAX_BYTES_PER_GATHERING_WRITE_ATTEMPTED_LOW_THRESHOLD;
  import static io.netty5.channel.internal.ChannelUtils.WRITE_STATUS_SNDBUF_FULL;
  import static io.netty5.channel.kqueue.KQueueChannelOption.SO_SNDLOWAT;
  import static io.netty5.channel.kqueue.KQueueChannelOption.TCP_NOPUSH;
  import static io.netty5.channel.unix.UnixChannelOption.DOMAIN_SOCKET_READ_MODE;
  import static java.util.Objects.requireNonNull;
  
  /**
   * {@link SocketChannel} implementation that uses KQueue.
   *
   * <h3>Available options</h3>
   *
   * In addition to the options provided by {@link SocketChannel} and {@link UnixChannel},
   * {@link KQueueSocketChannel} allows the following options in the option map:
   * <table border="1" cellspacing="0" cellpadding="6">
   * <tr>
   * <th>{@link ChannelOption}</th>
   * <th>{@code INET}</th>
   * <th>{@code INET6}</th>
   * <th>{@code UNIX}</th>
   * </tr><tr>
   * <td>{@link IntegerUnixChannelOption}</td><td>X</td><td>X</td><td>X</td>
   * </tr><tr>
   * <td>{@link RawUnixChannelOption}</td><td>X</td><td>X</td><td>X</td>
   * </tr><tr>
   * <td>{@link KQueueChannelOption#SO_SNDLOWAT}</td><td>X</td><td>X</td><td>-</td>
   * </tr><tr>
   * <td>{@link KQueueChannelOption#TCP_NOPUSH}</td><td>X</td><td>X</td><td>-</td>
   * </tr><tr>
   * <td>{@link ChannelOption#TCP_FASTOPEN_CONNECT}</td><td>X</td><td>X</td><td>-</td>
   * </tr>
  * </table>
  */
 @UnstableApi
 public final class KQueueSocketChannel
         extends AbstractKQueueChannel<KQueueServerSocketChannel>
         implements SocketChannel {
     private static final Set<ChannelOption<?>> SUPPORTED_OPTIONS = supportedOptions();
     private static final Set<ChannelOption<?>> SUPPORTED_OPTIONS_DOMAIN_SOCKET = supportedOptionsDomainSocket();
 
     private static final ChannelMetadata METADATA = new ChannelMetadata(false, 16);
     private static final String EXPECTED_TYPES =
             " (expected: " + StringUtil.simpleClassName(Buffer.class) + ", " +
                     StringUtil.simpleClassName(DefaultFileRegion.class) + ')';
     private WritableByteChannel byteChannel;
 
     // Calling flush0 directly to ensure we not try to flush messages that were added via write(...) in the
     // meantime.
     private final Runnable flushTask = this::writeFlushed;
 
     private volatile DomainSocketReadMode mode = DomainSocketReadMode.BYTES;
 
     private volatile boolean tcpFastopen;
 
     public KQueueSocketChannel(EventLoop eventLoop) {
         this(eventLoop, (ProtocolFamily) null);
     }
 
     public KQueueSocketChannel(EventLoop eventLoop, ProtocolFamily protocolFamily) {
         super(null, eventLoop, METADATA, new AdaptiveRecvBufferAllocator(), BsdSocket.newSocket(protocolFamily), false);
         enableTcpNoDelayIfSupported();
         calculateMaxBytesPerGatheringWrite();
     }
 
     public KQueueSocketChannel(EventLoop eventLoop, int fd, ProtocolFamily protocolFamily) {
         this(eventLoop, new BsdSocket(fd, SocketProtocolFamily.of(protocolFamily)));
     }
 
     private KQueueSocketChannel(EventLoop eventLoop, BsdSocket fd) {
         super(null, eventLoop, METADATA, new AdaptiveRecvBufferAllocator(), fd, isSoErrorZero(fd));
         enableTcpNoDelayIfSupported();
         calculateMaxBytesPerGatheringWrite();
     }
 
     KQueueSocketChannel(KQueueServerSocketChannel parent, EventLoop eventLoop,
                         BsdSocket fd, SocketAddress remoteAddress) {
         super(parent, eventLoop, METADATA, new AdaptiveRecvBufferAllocator(), fd, remoteAddress);
         enableTcpNoDelayIfSupported();
         calculateMaxBytesPerGatheringWrite();
     }
 
     private void enableTcpNoDelayIfSupported() {
         if (socket.protocolFamily() != SocketProtocolFamily.UNIX && PlatformDependent.canEnableTcpNoDelayByDefault()) {
             setTcpNoDelay(true);
         }
     }
 
     @SuppressWarnings("unchecked")
     @Override
     protected <T> T getExtendedOption(ChannelOption<T> option) {
         if (isSupported(socket.protocolFamily(), option)) {
             if (option == SO_RCVBUF) {
                 return (T) Integer.valueOf(getReceiveBufferSize());
             }
             if (option == SO_SNDBUF) {
                 return (T) Integer.valueOf(getSendBufferSize());
             }
             if (option == TCP_NODELAY) {
                 return (T) Boolean.valueOf(isTcpNoDelay());
             }
             if (option == SO_KEEPALIVE) {
                 return (T) Boolean.valueOf(isKeepAlive());
             }
             if (option == SO_REUSEADDR) {
                 return (T) Boolean.valueOf(isReuseAddress());
             }
             if (option == SO_LINGER) {
                 return (T) Integer.valueOf(getSoLinger());
             }
             if (option == IP_TOS) {
                 return (T) Integer.valueOf(getTrafficClass());
             }
             if (option == SO_SNDLOWAT) {
                 return (T) Integer.valueOf(getSndLowAt());
             }
             if (option == TCP_NOPUSH) {
                 return (T) Boolean.valueOf(isTcpNoPush());
             }
             if (option == ChannelOption.TCP_FASTOPEN_CONNECT) {
                 return (T) Boolean.valueOf(isTcpFastOpenConnect());
             }
             if (option == DOMAIN_SOCKET_READ_MODE) {
                 return (T) getReadMode();
             }
             if (option == UnixChannelOption.SO_PEERCRED) {
                 return (T) getPeerCredentials();
             }
         }
         return super.getExtendedOption(option);
     }
 
     @Override
     protected <T> void setExtendedOption(ChannelOption<T> option, T value) {
         if (isSupported(socket.protocolFamily(), option)) {
             if (option == SO_RCVBUF) {
                 setReceiveBufferSize((Integer) value);
             } else if (option == SO_SNDBUF) {
                 setSendBufferSize((Integer) value);
             } else if (option == TCP_NODELAY) {
                 setTcpNoDelay((Boolean) value);
             } else if (option == SO_KEEPALIVE) {
                 setKeepAlive((Boolean) value);
             } else if (option == SO_REUSEADDR) {
                 setReuseAddress((Boolean) value);
             } else if (option == SO_LINGER) {
                 setSoLinger((Integer) value);
             } else if (option == IP_TOS) {
                 setTrafficClass((Integer) value);
             } else if (option == SO_SNDLOWAT) {
                 setSndLowAt((Integer) value);
             } else if (option == TCP_NOPUSH) {
                 setTcpNoPush((Boolean) value);
             } else if (option == ChannelOption.TCP_FASTOPEN_CONNECT) {
                 setTcpFastOpenConnect((Boolean) value);
             } else if (option == DOMAIN_SOCKET_READ_MODE) {
                 setReadMode((DomainSocketReadMode) value);
             } else if (option == UnixChannelOption.SO_PEERCRED) {
                 throw new UnsupportedOperationException("read-only option: " + option);
             }
         } else {
             super.setExtendedOption(option, value);
         }
     }
 
     private boolean isSupported(SocketProtocolFamily protocolFamily, ChannelOption<?> option) {
         if (protocolFamily == SocketProtocolFamily.UNIX) {
             return SUPPORTED_OPTIONS_DOMAIN_SOCKET.contains(option);
         }
         return SUPPORTED_OPTIONS.contains(option);
     }
 
     @Override
     protected boolean isExtendedOptionSupported(ChannelOption<?> option) {
         return isSupported(socket.protocolFamily(), option) || super.isExtendedOptionSupported(option);
     }
 
     private static Set<ChannelOption<?>> supportedOptions() {
         return newSupportedIdentityOptionsSet(SO_RCVBUF, SO_SNDBUF, TCP_NODELAY,
                 SO_KEEPALIVE, SO_REUSEADDR, SO_LINGER, IP_TOS, SO_SNDLOWAT, TCP_NOPUSH,
                 ChannelOption.TCP_FASTOPEN_CONNECT);
     }
 
     private static Set<ChannelOption<?>> supportedOptionsDomainSocket() {
         return newSupportedIdentityOptionsSet(SO_RCVBUF, SO_SNDBUF, DOMAIN_SOCKET_READ_MODE,
                 UnixChannelOption.SO_PEERCRED);
     }
 
     private void setReadMode(DomainSocketReadMode mode) {
         requireNonNull(mode, "mode");
         this.mode = mode;
     }
 
     private DomainSocketReadMode getReadMode() {
         return mode;
     }
 
     private int getReceiveBufferSize() {
         try {
             return socket.getReceiveBufferSize();
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     private int getSendBufferSize() {
         try {
             return socket.getSendBufferSize();
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     private int getSoLinger() {
         try {
             return socket.getSoLinger();
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     private int getTrafficClass() {
         try {
             return socket.getTrafficClass();
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     private boolean isKeepAlive() {
         try {
             return socket.isKeepAlive();
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     private boolean isReuseAddress() {
         try {
             return socket.isReuseAddress();
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     private boolean isTcpNoDelay() {
         try {
             return socket.isTcpNoDelay();
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     private int getSndLowAt() {
         try {
             return socket.getSndLowAt();
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     private void setSndLowAt(int sndLowAt)  {
         try {
             socket.setSndLowAt(sndLowAt);
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     private boolean isTcpNoPush() {
         try {
             return socket.isTcpNoPush();
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     private void setTcpNoPush(boolean tcpNoPush)  {
         try {
             socket.setTcpNoPush(tcpNoPush);
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     private void setKeepAlive(boolean keepAlive) {
         try {
             socket.setKeepAlive(keepAlive);
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     private void setReceiveBufferSize(int receiveBufferSize) {
         try {
             socket.setReceiveBufferSize(receiveBufferSize);
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     private void setReuseAddress(boolean reuseAddress) {
         try {
             socket.setReuseAddress(reuseAddress);
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     private void setSendBufferSize(int sendBufferSize) {
         try {
             socket.setSendBufferSize(sendBufferSize);
             calculateMaxBytesPerGatheringWrite();
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     private void setSoLinger(int soLinger) {
         try {
             socket.setSoLinger(soLinger);
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     private void setTcpNoDelay(boolean tcpNoDelay) {
         try {
             socket.setTcpNoDelay(tcpNoDelay);
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     private void setTrafficClass(int trafficClass) {
         try {
             socket.setTrafficClass(trafficClass);
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     /**
      * Enables client TCP fast open, if available.
      */
     private void setTcpFastOpenConnect(boolean fastOpenConnect) {
         tcpFastopen = fastOpenConnect;
     }
 
     /**
      * Returns {@code true} if TCP fast open is enabled, {@code false} otherwise.
      */
     private boolean isTcpFastOpenConnect() {
         return tcpFastopen;
     }
 
     private void calculateMaxBytesPerGatheringWrite() {
         // Multiply by 2 to give some extra space in case the OS can process write data faster than we can provide.
         int newSendBufferSize = getSendBufferSize() << 1;
         if (newSendBufferSize > 0) {
             setMaxBytesPerGatheringWrite(getSendBufferSize() << 1);
         }
     }
 
     @Override
     protected Object filterOutboundMessage(Object msg) {
         if (socket.protocolFamily() == SocketProtocolFamily.UNIX && msg instanceof FileDescriptor) {
             return msg;
         }
         if (msg instanceof Buffer) {
             Buffer buf = (Buffer) msg;
             return UnixChannelUtil.isBufferCopyNeededForWrite(buf)? newDirectBuffer(buf) : buf;
         }
 
         if (msg instanceof FileRegion) {
             return msg;
         }
 
         throw new UnsupportedOperationException(
                 "unsupported message type: " + StringUtil.simpleClassName(msg) + EXPECTED_TYPES);
     }
 
     @Override
     protected boolean doConnect0(SocketAddress remoteAddress, SocketAddress localAddress) throws Exception {
         if (isTcpFastOpenConnect()) {
             ChannelOutboundBuffer outbound = outboundBuffer();
             outbound.addFlush();
             Object curr;
             if ((curr = outbound.current()) instanceof Buffer) {
                 Buffer initialData = (Buffer) curr;
                 // Don't bother with TCP FastOpen if we don't have any initial data to send anyway.
                 if (initialData.readableBytes() > 0) {
                     IovArray iov = new IovArray();
                     try {
                         initialData.forEachReadable(0, iov);
                         int bytesSent = socket.connectx(
                                 (InetSocketAddress) localAddress, (InetSocketAddress) remoteAddress, iov, true);
                         writeFilter(true);
                         outbound.removeBytes(Math.abs(bytesSent));
                         // The `connectx` method returns a negative number if connection is in-progress.
                         // So we should return `true` to indicate that connection was established, if it's positive.
                         return bytesSent > 0;
                     } finally {
                         iov.release();
                     }
                 }
             }
         }
         return super.doConnect0(remoteAddress, localAddress);
     }
 
     @Override
     protected Future<Executor> prepareToClose() {
         if (socket.protocolFamily() != SocketProtocolFamily.UNIX) {
             try {
                 // Check isOpen() first as otherwise it will throw a RuntimeException
                 // when call getSoLinger() as the fd is not valid anymore.
                 if (isOpen() && getSoLinger() > 0) {
                     // We need to cancel this key of the channel so we may not end up in a eventloop spin
                     // because we try to read or write until the actual close happens which may be later due
                     // SO_LINGER handling.
                     // See https://github.com/netty/netty/issues/4449
                     return executor().deregisterForIo(this).map(v -> GlobalEventExecutor.INSTANCE);
                 }
             } catch (Throwable ignore) {
                 // Ignore the error as the underlying channel may be closed in the meantime and so
                 // getSoLinger() may produce an exception. In this case we just return null.
                 // See https://github.com/netty/netty/issues/4449
             }
         }
         return null;
     }
 
     @Override
     void readReady(RecvBufferAllocator.Handle allocHandle, BufferAllocator recvBufferAllocator,
                    Predicate<RecvBufferAllocator.Handle> maybeMoreData) {
         if (socket.protocolFamily() == SocketProtocolFamily.UNIX &&
                 getReadMode() == DomainSocketReadMode.FILE_DESCRIPTORS) {
             readReadyFd(allocHandle);
         } else {
             readReadyBytes(allocHandle, recvBufferAllocator, maybeMoreData);
         }
     }
 
     private void readReadyFd(RecvBufferAllocator.Handle allocHandle) {
         final ChannelPipeline pipeline = pipeline();
         try {
             readLoop: do {
                 // lastBytesRead represents the fd. We use lastBytesRead because it must be set so that the
                 // KQueueRecvBufferAllocatorHandle knows if it should try to read again or not when autoRead is
                 // enabled.
                 int recvFd = socket.recvFd();
                 switch(recvFd) {
                     case 0:
                         allocHandle.lastBytesRead(0);
                         break readLoop;
                     case -1:
                         allocHandle.lastBytesRead(-1);
                         closeTransportNow();
                         return;
                     default:
                         allocHandle.lastBytesRead(1);
                         allocHandle.incMessagesRead(1);
                         readPending = false;
                         pipeline.fireChannelRead(new FileDescriptor(recvFd));
                         break;
                 }
             } while (allocHandle.continueReading(isAutoRead()) && !isShutdown(ChannelShutdownDirection.Inbound));
 
             allocHandle.readComplete();
             pipeline.fireChannelReadComplete();
         } catch (Throwable t) {
             allocHandle.readComplete();
             pipeline.fireChannelReadComplete();
             pipeline.fireChannelExceptionCaught(t);
         } finally {
             readIfIsAutoRead();
         }
     }
 
     private PeerCredentials getPeerCredentials() {
         try {
             return socket.getPeerCredentials();
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     /**
      * Write bytes form the given {@link Buffer} to the underlying {@link java.nio.channels.Channel}.
      * @param in the collection which contains objects to write.
      * @param buf the {@link Buffer} from which the bytes should be written
      * @return The value that should be decremented from the write-quantum which starts at
      * {@link #getWriteSpinCount()}. The typical use cases are as follows:
      * <ul>
      *     <li>0 - if no write was attempted. This is appropriate if an empty {@link Buffer} (or other empty content)
      *     is encountered</li>
      *     <li>1 - if a single call to write data was made to the OS</li>
      *     <li>{@link ChannelUtils#WRITE_STATUS_SNDBUF_FULL} - if an attempt to write data was made to the OS, but
      *     no data was accepted</li>
      * </ul>
      */
     private int writeBytes(ChannelOutboundBuffer in, Buffer buf) throws Exception {
         int readableBytes = buf.readableBytes();
         if (readableBytes == 0) {
             in.remove();
             return 0;
         }
 
         int readableComponents = buf.countReadableComponents();
         if (readableComponents == 1) {
             return doWriteBytes(in, buf);
         }
         ByteBuffer[] nioBuffers = new ByteBuffer[readableComponents];
         buf.forEachReadable(0, (index, component) -> {
             nioBuffers[index] = component.readableBuffer();
             return true;
         });
         return writeBytesMultiple(in, nioBuffers, nioBuffers.length, readableBytes,
                 getMaxBytesPerGatheringWrite());
     }
 
     private void adjustMaxBytesPerGatheringWrite(long attempted, long written, long oldMaxBytesPerGatheringWrite) {
         // By default we track the SO_SNDBUF when ever it is explicitly set. However some OSes may dynamically change
         // SO_SNDBUF (and other characteristics that determine how much data can be written at once) so we should try
         // make a best effort to adjust as OS behavior changes.
         if (attempted == written) {
             if (attempted << 1 > oldMaxBytesPerGatheringWrite) {
                 setMaxBytesPerGatheringWrite(attempted << 1);
             }
         } else if (attempted > MAX_BYTES_PER_GATHERING_WRITE_ATTEMPTED_LOW_THRESHOLD && written < attempted >>> 1) {
             setMaxBytesPerGatheringWrite(attempted >>> 1);
         }
     }
 
     /**
      * Write multiple bytes via {@link IovArray}.
      * @param in the collection which contains objects to write.
      * @param array The array which contains the content to write.
      * @return The value that should be decremented from the write quantum which starts at
      * {@link #getWriteSpinCount()}. The typical use cases are as follows:
      * <ul>
      *     <li>0 - if no write was attempted. This is appropriate if an empty {@link Buffer} (or other empty content)
      *     is encountered</li>
      *     <li>1 - if a single call to write data was made to the OS</li>
      *     <li>{@link ChannelUtils#WRITE_STATUS_SNDBUF_FULL} - if an attempt to write data was made to the OS, but
      *     no data was accepted</li>
      * </ul>
      * @throws IOException If an I/O exception occurs during write.
      */
     private int writeBytesMultiple(ChannelOutboundBuffer in, IovArray array) throws IOException {
         final long expectedWrittenBytes = array.size();
         assert expectedWrittenBytes != 0;
         final int cnt = array.count();
         assert cnt != 0;
 
         final long localWrittenBytes = socket.writevAddresses(array.memoryAddress(0), cnt);
         if (localWrittenBytes > 0) {
             adjustMaxBytesPerGatheringWrite(expectedWrittenBytes, localWrittenBytes, array.maxBytes());
             in.removeBytes(localWrittenBytes);
             return 1;
         }
         return WRITE_STATUS_SNDBUF_FULL;
     }
 
     /**
      * Write multiple bytes via {@link ByteBuffer} array.
      * @param in the collection which contains objects to write.
      * @param nioBuffers The buffers to write.
      * @param nioBufferCnt The number of buffers to write.
      * @param expectedWrittenBytes The number of bytes we expect to write.
      * @param maxBytesPerGatheringWrite The maximum number of bytes we should attempt to write.
      * @return The value that should be decremented from the write quantum which starts at
      * {@link #getWriteSpinCount()}. The typical use cases are as follows:
      * <ul>
      *     <li>0 - if no write was attempted. This is appropriate if an empty {@link Buffer} (or other empty content)
      *     is encountered</li>
      *     <li>1 - if a single call to write data was made to the OS</li>
      *     <li>{@link ChannelUtils#WRITE_STATUS_SNDBUF_FULL} - if an attempt to write data was made to the OS, but
      *     no data was accepted</li>
      * </ul>
      * @throws IOException If an I/O exception occurs during write.
      */
     private int writeBytesMultiple(
             ChannelOutboundBuffer in, ByteBuffer[] nioBuffers, int nioBufferCnt, long expectedWrittenBytes,
             long maxBytesPerGatheringWrite) throws IOException {
         assert expectedWrittenBytes != 0;
         if (expectedWrittenBytes > maxBytesPerGatheringWrite) {
             expectedWrittenBytes = maxBytesPerGatheringWrite;
         }
 
         final long localWrittenBytes = socket.writev(nioBuffers, 0, nioBufferCnt, expectedWrittenBytes);
         if (localWrittenBytes > 0) {
             adjustMaxBytesPerGatheringWrite(expectedWrittenBytes, localWrittenBytes, maxBytesPerGatheringWrite);
             in.removeBytes(localWrittenBytes);
             return 1;
         }
         return WRITE_STATUS_SNDBUF_FULL;
     }
 
     /**
      * Write a {@link DefaultFileRegion}
      * @param in the collection which contains objects to write.
      * @param region the {@link DefaultFileRegion} from which the bytes should be written
      * @return The value that should be decremented from the write quantum which starts at
      * {@link #getWriteSpinCount()}. The typical use cases are as follows:
      * <ul>
      *     <li>0 - if no write was attempted. This is appropriate if an empty {@link Buffer} (or other empty content)
      *     is encountered</li>
      *     <li>1 - if a single call to write data was made to the OS</li>
      *     <li>{@link ChannelUtils#WRITE_STATUS_SNDBUF_FULL} - if an attempt to write data was made to the OS, but
      *     no data was accepted</li>
      * </ul>
      */
     private int writeDefaultFileRegion(ChannelOutboundBuffer in, DefaultFileRegion region) throws Exception {
         final long regionCount = region.count();
         final long offset = region.transferred();
 
         if (offset >= regionCount) {
             in.remove();
             return 0;
         }
 
         final long flushedAmount = socket.sendFile(region, region.position(), offset, regionCount - offset);
         if (flushedAmount > 0) {
             in.progress(flushedAmount);
             if (region.transferred() >= regionCount) {
                 in.remove();
             }
             return 1;
         }
         if (flushedAmount == 0) {
             validateFileRegion(region, offset);
         }
         return WRITE_STATUS_SNDBUF_FULL;
     }
 
     /**
      * Write a {@link FileRegion}
      * @param in the collection which contains objects to write.
      * @param region the {@link FileRegion} from which the bytes should be written
      * @return The value that should be decremented from the write quantum which starts at
      * {@link #getWriteSpinCount()}. The typical use cases are as follows:
      * <ul>
      *     <li>0 - if no write was attempted. This is appropriate if an empty {@link Buffer} (or other empty content)
      *     is encountered</li>
      *     <li>1 - if a single call to write data was made to the OS</li>
      *     <li>{@link ChannelUtils#WRITE_STATUS_SNDBUF_FULL} - if an attempt to write data was made to the OS, but no
      *     data was accepted</li>
      * </ul>
      */
     private int writeFileRegion(ChannelOutboundBuffer in, FileRegion region) throws Exception {
         if (region.transferred() >= region.count()) {
             in.remove();
             return 0;
         }
 
         if (byteChannel == null) {
             byteChannel = new KQueueSocketWritableByteChannel();
         }
         final long flushedAmount = region.transferTo(byteChannel, region.transferred());
         if (flushedAmount > 0) {
             in.progress(flushedAmount);
             if (region.transferred() >= region.count()) {
                 in.remove();
             }
             return 1;
         }
         return WRITE_STATUS_SNDBUF_FULL;
     }
 
     @Override
     protected void doWrite(ChannelOutboundBuffer in) throws Exception {
         int writeSpinCount = getWriteSpinCount();
         do {
             final int msgCount = in.size();
             // Do gathering write if the outbound buffer entries start with more than one Buffer.
             if (msgCount > 1 && in.current() instanceof Buffer) {
                 writeSpinCount -= doWriteMultiple(in);
             } else if (msgCount == 0) {
                 // Wrote all messages.
                 writeFilter(false);
                 // Return here so we don't set the WRITE flag.
                 return;
             } else { // msgCount == 1
                 writeSpinCount -= doWriteSingle(in);
             }
 
             // We do not break the loop here even if the outbound buffer was flushed completely,
             // because a user might have triggered another write and flush when we notify his or her
             // listeners.
         } while (writeSpinCount > 0);
 
         if (writeSpinCount == 0) {
             // It is possible that we have set the write filter, woken up by KQUEUE because the socket is writable, and
             // then use our write quantum. In this case we no longer want to set the write filter because the socket is
             // still writable (as far as we know). We will find out next time we attempt to write if the socket is
             // writable and set the write filter if necessary.
             writeFilter(false);
 
             // We used our writeSpin quantum, and should try to write again later.
             executor().execute(flushTask);
         } else {
             // Underlying descriptor can not accept all data currently, so set the WRITE flag to be woken up
             // when it can accept more data.
             writeFilter(true);
         }
     }
 
     /**
      * Attempt to write a single object.
      * @param in the collection which contains objects to write.
      * @return The value that should be decremented from the write quantum which starts at
      * {@link #getWriteSpinCount()}. The typical use cases are as follows:
      * <ul>
      *     <li>0 - if no write was attempted. This is appropriate if an empty {@link Buffer} (or other empty content)
      *     is encountered</li>
      *     <li>1 - if a single call to write data was made to the OS</li>
      *     <li>{@link ChannelUtils#WRITE_STATUS_SNDBUF_FULL} - if an attempt to write data was made to the OS, but no
      *     data was accepted</li>
      * </ul>
      * @throws Exception If an I/O error occurs.
      */
     private int doWriteSingle(ChannelOutboundBuffer in) throws Exception {
         // The outbound buffer contains only one message or it contains a file region.
         Object msg = in.current();
         if (socket.protocolFamily() == SocketProtocolFamily.UNIX) {
             if (msg instanceof FileDescriptor && socket.sendFd(((FileDescriptor) msg).intValue()) > 0) {
                 // File descriptor was written, so remove it.
                 in.remove();
                 return 1;
             }
         }
 
         if (msg instanceof Buffer) {
             return writeBytes(in, (Buffer) msg);
         } else if (msg instanceof DefaultFileRegion) {
             return writeDefaultFileRegion(in, (DefaultFileRegion) msg);
         } else if (msg instanceof FileRegion) {
             return writeFileRegion(in, (FileRegion) msg);
         } else {
             // Should never reach here.
             throw new Error();
         }
     }
 
     /**
      * Attempt to write multiple {@link Buffer} objects.
      * @param in the collection which contains objects to write.
      * @return The value that should be decremented from the write quantum which starts at
      * {@link #getWriteSpinCount()}. The typical use cases are as follows:
      * <ul>
      *     <li>0 - if no write was attempted. This is appropriate if an empty {@link Buffer} (or other empty content)
      *     is encountered</li>
      *     <li>1 - if a single call to write data was made to the OS</li>
      *     <li>{@link ChannelUtils#WRITE_STATUS_SNDBUF_FULL} - if an attempt to write data was made to the OS, but no
      *     data was accepted</li>
      * </ul>
      * @throws Exception If an I/O error occurs.
      */
     private int doWriteMultiple(ChannelOutboundBuffer in) throws Exception {
         final long maxBytesPerGatheringWrite = getMaxBytesPerGatheringWrite();
         IovArray array = registration().cleanArray();
         array.maxBytes(maxBytesPerGatheringWrite);
         in.forEachFlushedMessage(array);
 
         if (array.count() >= 1) {
             return writeBytesMultiple(in, array);
         }
         // cnt == 0, which means the outbound buffer contained empty buffers only.
         in.removeBytes(0);
         return 0;
     }
 
     @Override
     protected void doShutdown(ChannelShutdownDirection direction) throws Exception {
         switch (direction) {
             case Outbound:
                 socket.shutdown(false, true);
                 break;
             case Inbound:
                 try {
                     socket.shutdown(true, false);
                 } catch (NotYetConnectedException ignore) {
                     // We attempted to shutdown and failed, which means the input has already effectively been
                     // shutdown.
                 }
                 break;
             default:
                 throw new AssertionError();
         }
     }
 
     @Override
     public boolean isShutdown(ChannelShutdownDirection direction) {
         if (!isActive()) {
             return true;
         }
         switch (direction) {
             case Outbound:
                 return socket.isOutputShutdown();
             case Inbound:
                 return socket.isInputShutdown();
             default:
                 throw new AssertionError();
         }
     }
 
     private void readReadyBytes(RecvBufferAllocator.Handle allocHandle, BufferAllocator recvBufferAllocator,
                                 Predicate<RecvBufferAllocator.Handle> maybeMoreData) {
         final ChannelPipeline pipeline = pipeline();
         allocHandle.reset();
         Buffer buffer = null;
         boolean close = false;
         try {
             do {
                 // we use a direct buffer here as the native implementations only be able
                 // to handle direct buffers.
                 buffer = allocHandle.allocate(recvBufferAllocator);
                 doReadBytes(buffer);
                 if (allocHandle.lastBytesRead() <= 0) {
                     // nothing was read, release the buffer.
                     Resource.dispose(buffer);
                     buffer = null;
                     close = allocHandle.lastBytesRead() < 0;
                     if (close) {
                         // There is nothing left to read as we received an EOF.
                         readPending = false;
                     }
                     break;
                 }
                 allocHandle.incMessagesRead(1);
                 readPending = false;
                 pipeline.fireChannelRead(buffer);
                 buffer = null;
 
                 if (shouldBreakReadReady()) {
                     // We need to do this for two reasons:
                     //
                     // - If the input was shutdown in between (which may be the case when the user did it in the
                     //   fireChannelRead(...) method we should not try to read again to not produce any
                     //   miss-leading exceptions.
                     //
                     // - If the user closes the channel we need to ensure we not try to read from it again as
                     //   the filedescriptor may be re-used already by the OS if the system is handling a lot of
                     //   concurrent connections and so needs a lot of filedescriptors. If not do this we risk
                     //   reading data from a filedescriptor that belongs to another socket then the socket that
                     //   was "wrapped" by this Channel implementation.
                     break;
                 }
             } while (allocHandle.continueReading(isAutoRead(), maybeMoreData)
                     && !isShutdown(ChannelShutdownDirection.Inbound));
 
             allocHandle.readComplete();
             pipeline.fireChannelReadComplete();
 
             if (close) {
                 shutdownInput(false);
             } else {
                 readIfIsAutoRead();
             }
         } catch (Throwable t) {
             handleReadException(pipeline, buffer, t, close, allocHandle);
         }
     }
 
     private void handleReadException(ChannelPipeline pipeline, Buffer buffer, Throwable cause, boolean close,
                                      RecvBufferAllocator.Handle allocHandle) {
         if (buffer.readableBytes() > 0) {
             readPending = false;
             pipeline.fireChannelRead(buffer);
         } else {
             buffer.close();
         }
         if (isConnectPending()) {
             finishConnect();
         } else {
             allocHandle.readComplete();
             pipeline.fireChannelReadComplete();
             pipeline.fireChannelExceptionCaught(cause);
 
             // If oom will close the read event, release connection.
             // See https://github.com/netty/netty/issues/10434
             if (close || cause instanceof OutOfMemoryError || cause instanceof IOException) {
                 shutdownInput(false);
             } else {
                 readIfIsAutoRead();
             }
         }
     }
 
     private final class KQueueSocketWritableByteChannel extends SocketWritableByteChannel {
         KQueueSocketWritableByteChannel() {
             super(socket);
         }
 
         @Override
         protected BufferAllocator alloc() {
             return bufferAllocator();
         }
     }
 }