/*
    * Copyright 2014 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.epoll;
  
  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.StringUtil;
  
  import java.io.IOException;
  import java.net.InetAddress;
  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.Collection;
  import java.util.Collections;
  import java.util.Map;
  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.epoll.Native.IS_SUPPORTING_TCP_FASTOPEN_CLIENT;
  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.unix.Limits.SSIZE_MAX;
  import static io.netty5.channel.unix.UnixChannelOption.DOMAIN_SOCKET_READ_MODE;
  import static java.util.Objects.requireNonNull;
  
  /**
   * {@link SocketChannel} implementation that uses linux EPOLL Edge-Triggered Mode for
   * maximal performance.
   *
   * <h3>Available options</h3>
   *
   * In addition to the options provided by {@link SocketChannel} and {@link UnixChannel},
   * {@link EpollSocketChannel} 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 EpollChannelOption#TCP_CORK}</td><td>X</td><td>X</td><td>-</td>
   * </tr><tr>
   * <td>{@link EpollChannelOption#TCP_NOTSENT_LOWAT}</td><td>X</td><td>X</td><td>-</td>
   * </tr><tr>
  * <td>{@link EpollChannelOption#TCP_KEEPCNT}</td><td>X</td><td>X</td><td>-</td>
  * </tr><tr>
  * <td>{@link EpollChannelOption#TCP_KEEPIDLE}</td><td>X</td><td>X</td><td>-</td>
  * </tr><tr>
  * <td>{@link EpollChannelOption#TCP_KEEPINTVL}</td><td>X</td><td>X</td><td>-</td>
  * </tr><tr>
  * <td>{@link EpollChannelOption#TCP_MD5SIG}</td><td>X</td><td>X</td><td>-</td>
  * </tr><tr>
  * <td>{@link EpollChannelOption#TCP_QUICKACK}</td><td>X</td><td>X</td><td>-</td>
  * </tr><tr>
  * <td>{@link EpollChannelOption#TCP_INFO}</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><tr>
  * <td>{@link EpollChannelOption#IP_TRANSPARENT}</td><td>X</td><td>X</td><td>-</td>
  * </tr><tr>
  * <td>{@link EpollChannelOption#SO_BUSY_POLL}</td><td>X</td><td>X</td><td>-</td>
  * </tr><tr>
  * <td>{@link UnixChannelOption#SO_PEERCRED}</td><td></td><td></td><td>X</td>
  * </tr><tr>
  * <td>{@link UnixChannelOption#DOMAIN_SOCKET_READ_MODE}</td><td></td><td></td><td>X</td>
  * </tr>
  * </table>
  */
 public final class EpollSocketChannel
         extends AbstractEpollChannel<EpollServerSocketChannel>
         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) + ')';
     // 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 WritableByteChannel byteChannel;
     private volatile long maxBytesPerGatheringWrite = SSIZE_MAX;
 
     private volatile Collection<InetAddress> tcpMd5SigAddresses = Collections.emptyList();
 
     private volatile DomainSocketReadMode mode = DomainSocketReadMode.BYTES;
 
     private volatile boolean tcpFastopen;
 
     private static final Predicate<RecvBufferAllocator.Handle> MAYBE_MORE_DATA = h ->
          h.lastBytesRead() == h.attemptedBytesRead();
 
     private static final Predicate<RecvBufferAllocator.Handle> MAYBE_MORE_DATA_RDHUP = h -> true;
 
     public EpollSocketChannel(EventLoop eventLoop) {
         this(eventLoop, (ProtocolFamily) null);
     }
 
     public EpollSocketChannel(EventLoop eventLoop, ProtocolFamily protocolFamily) {
         // Add EPOLLRDHUP so we are notified once the remote peer close the connection.
         super(null, eventLoop, METADATA, Native.EPOLLRDHUP, new AdaptiveRecvBufferAllocator(),
                 LinuxSocket.newSocket(protocolFamily), false);
     }
 
     public EpollSocketChannel(EventLoop eventLoop, int fd, ProtocolFamily family) {
         this(eventLoop, new LinuxSocket(fd, SocketProtocolFamily.of(family)));
     }
 
     private EpollSocketChannel(EventLoop eventLoop, LinuxSocket socket) {
         // Add EPOLLRDHUP so we are notified once the remote peer close the connection.
         super(null, eventLoop, METADATA, Native.EPOLLRDHUP, new AdaptiveRecvBufferAllocator(),
                 socket, isSoErrorZero(socket));
     }
 
     EpollSocketChannel(EpollServerSocketChannel parent, EventLoop eventLoop,
                        LinuxSocket fd, SocketAddress remoteAddress) {
         // Add EPOLLRDHUP so we are notified once the remote peer close the connection.
         super(parent, eventLoop, METADATA, Native.EPOLLRDHUP, new AdaptiveRecvBufferAllocator(), fd, remoteAddress);
 
         if (fd.protocolFamily() != SocketProtocolFamily.UNIX && parent != null) {
             tcpMd5SigAddresses = parent.tcpMd5SigAddresses();
         }
     }
 
     /**
      * 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);
         } else {
             ByteBuffer[] nioBuffers = new ByteBuffer[readableComponents];
             buf.forEachReadable(0, (index, component) -> {
                 nioBuffers[index] = component.readableBuffer();
                 return true;
             });
             return writeBytesMultiple(in, nioBuffers, nioBuffers.length, readableBytes,
                     getMaxBytesPerGatheringWrite());
         }
     }
 
     void setMaxBytesPerGatheringWrite(long maxBytesPerGatheringWrite) {
         this.maxBytesPerGatheringWrite = maxBytesPerGatheringWrite;
     }
 
     long getMaxBytesPerGatheringWrite() {
         return maxBytesPerGatheringWrite;
     }
 
     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 offset = region.transferred();
         final long regionCount = region.count();
         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 EpollSocketWritableByteChannel();
         }
         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.
                 clearFlag(Native.EPOLLOUT);
                 // Return here so we not set the EPOLLOUT 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 EPOLLOUT, woken up by EPOLL because the socket is writable, and then use
             // our write quantum. In this case we no longer want to set the EPOLLOUT flag 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 EPOLLOUT if necessary.
             clearFlag(Native.EPOLLOUT);
 
             // 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 EPOLLOUT flag to be woken up
             // when it can accept more data.
             setFlag(Native.EPOLLOUT);
         }
     }
 
     /**
      * 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 (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().cleanIovArray();
         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 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 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 handleReadException(ChannelPipeline pipeline, Buffer buffer, Throwable cause, boolean close,
                                      RecvBufferAllocator.Handle allocHandle) {
         if (buffer.readableBytes() > 0) {
             readPending = false;
             pipeline.fireChannelRead(buffer);
         } else {
             buffer.close();
         }
         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();
         }
     }
 
     @Override
     protected void epollInReady(RecvBufferAllocator.Handle handle, BufferAllocator recvBufferAllocator,
                                 boolean receivedRdHup) {
         if (socket.protocolFamily() == SocketProtocolFamily.UNIX
                 && getReadMode() == DomainSocketReadMode.FILE_DESCRIPTORS) {
             epollInReadFd(handle, receivedRdHup);
         } else {
             epollInReadyBytes(handle, recvBufferAllocator, receivedRdHup);
         }
     }
 
     private static Predicate<RecvBufferAllocator.Handle> maybeMoreData(boolean receivedRdHup) {
         return receivedRdHup ? MAYBE_MORE_DATA_RDHUP : MAYBE_MORE_DATA;
     }
 
     private void epollInReadyBytes(RecvBufferAllocator.Handle recvAlloc, BufferAllocator bufferAllocator,
                                    boolean receivedRdHup) {
         final ChannelPipeline pipeline = pipeline();
         Predicate<RecvBufferAllocator.Handle> maybeMoreData = maybeMoreData(receivedRdHup);
 
         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 = recvAlloc.allocate(bufferAllocator);
                 doReadBytes(buffer);
                 if (recvAlloc.lastBytesRead() <= 0) {
                     // nothing was read, release the buffer.
                     Resource.dispose(buffer);
                     buffer = null;
                     close = recvAlloc.lastBytesRead() < 0;
                     if (close) {
                         // There is nothing left to read as we received an EOF.
                         readPending = false;
                     }
                     break;
                 }
                 recvAlloc.incMessagesRead(1);
                 readPending = false;
                 pipeline.fireChannelRead(buffer);
                 buffer = null;
 
                 if (shouldBreakEpollInReady()) {
                     // 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 (recvAlloc.continueReading(isAutoRead(), maybeMoreData)
                     && !isShutdown(ChannelShutdownDirection.Inbound));
 
             recvAlloc.readComplete();
             pipeline.fireChannelReadComplete();
 
             if (close) {
                 shutdownInput(false);
             } else {
                 readIfIsAutoRead();
             }
         } catch (Throwable t) {
             handleReadException(pipeline, buffer, t, close, recvAlloc);
         }
     }
 
     private final class EpollSocketWritableByteChannel extends SocketWritableByteChannel {
         EpollSocketWritableByteChannel() {
             super(socket);
         }
 
         @Override
         protected BufferAllocator alloc() {
             return bufferAllocator();
         }
     }
 
     @SuppressWarnings("unchecked")
     @Override
     protected <T> T getExtendedOption(ChannelOption<T> option) {
         if (isOptionSupported(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 == EpollChannelOption.TCP_CORK) {
                 return (T) Boolean.valueOf(isTcpCork());
             }
             if (option == EpollChannelOption.TCP_NOTSENT_LOWAT) {
                 return (T) Long.valueOf(getTcpNotSentLowAt());
             }
             if (option == EpollChannelOption.TCP_KEEPIDLE) {
                 return (T) Integer.valueOf(getTcpKeepIdle());
             }
             if (option == EpollChannelOption.TCP_KEEPINTVL) {
                 return (T) Integer.valueOf(getTcpKeepIntvl());
             }
             if (option == EpollChannelOption.TCP_KEEPCNT) {
                 return (T) Integer.valueOf(getTcpKeepCnt());
             }
             if (option == EpollChannelOption.TCP_USER_TIMEOUT) {
                 return (T) Integer.valueOf(getTcpUserTimeout());
             }
             if (option == EpollChannelOption.TCP_QUICKACK) {
                 return (T) Boolean.valueOf(isTcpQuickAck());
             }
             if (option == EpollChannelOption.IP_TRANSPARENT) {
                 return (T) Boolean.valueOf(isIpTransparent());
             }
             if (option == ChannelOption.TCP_FASTOPEN_CONNECT) {
                 return (T) Boolean.valueOf(isTcpFastOpenConnect());
             }
             if (option == EpollChannelOption.SO_BUSY_POLL) {
                 return (T) Integer.valueOf(getSoBusyPoll());
             }
             if (option == DOMAIN_SOCKET_READ_MODE) {
                 return (T) getReadMode();
             }
             if (option == EpollChannelOption.TCP_INFO) {
                 return (T) getTcpInfo();
             }
             if (option == UnixChannelOption.SO_PEERCRED) {
                 return (T) getPeerCredentials();
             }
         }
         return super.getExtendedOption(option);
     }
 
     @SuppressWarnings("unchecked")
     @Override
     protected <T> void setExtendedOption(ChannelOption<T> option, T value) {
         if (isOptionSupported(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 == EpollChannelOption.TCP_CORK) {
                 setTcpCork((Boolean) value);
             } else if (option == EpollChannelOption.TCP_NOTSENT_LOWAT) {
                 setTcpNotSentLowAt((Long) value);
             } else if (option == EpollChannelOption.TCP_KEEPIDLE) {
                 setTcpKeepIdle((Integer) value);
             } else if (option == EpollChannelOption.TCP_KEEPCNT) {
                 setTcpKeepCnt((Integer) value);
             } else if (option == EpollChannelOption.TCP_KEEPINTVL) {
                 setTcpKeepIntvl((Integer) value);
             } else if (option == EpollChannelOption.TCP_USER_TIMEOUT) {
                 setTcpUserTimeout((Integer) value);
             } else if (option == EpollChannelOption.IP_TRANSPARENT) {
                 setIpTransparent((Boolean) value);
             } else if (option == EpollChannelOption.TCP_MD5SIG) {
                 @SuppressWarnings("unchecked")
                 final Map<InetAddress, byte[]> m = (Map<InetAddress, byte[]>) value;
                 setTcpMd5Sig(m);
             } else if (option == EpollChannelOption.TCP_QUICKACK) {
                 setTcpQuickAck((Boolean) value);
             } else if (option == ChannelOption.TCP_FASTOPEN_CONNECT) {
                 setTcpFastOpenConnect((Boolean) value);
             } else if (option == EpollChannelOption.SO_BUSY_POLL) {
                 setSoBusyPoll((Integer) value);
             } else if (option == DOMAIN_SOCKET_READ_MODE) {
                 setReadMode((DomainSocketReadMode) value);
             } else if (option == EpollChannelOption.TCP_INFO) {
                 throw new UnsupportedOperationException("read-only option: " + option);
             } else if (option == UnixChannelOption.SO_PEERCRED) {
                 throw new UnsupportedOperationException("read-only option: " + option);
             }
         } else {
             super.setExtendedOption(option, value);
         }
     }
 
     private static boolean isOptionSupported(SocketProtocolFamily family, ChannelOption<?> option) {
         if (family == SocketProtocolFamily.UNIX) {
             return SUPPORTED_OPTIONS_DOMAIN_SOCKET.contains(option);
         }
         return SUPPORTED_OPTIONS.contains(option);
     }
 
     @Override
     protected boolean isExtendedOptionSupported(ChannelOption<?> option) {
         return isOptionSupported(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, EpollChannelOption.TCP_CORK, EpollChannelOption.TCP_KEEPIDLE, EpollChannelOption.TCP_KEEPCNT,
                 EpollChannelOption.TCP_KEEPINTVL, EpollChannelOption.TCP_USER_TIMEOUT,
                 EpollChannelOption.IP_TRANSPARENT, EpollChannelOption.TCP_MD5SIG, EpollChannelOption.TCP_QUICKACK,
                 ChannelOption.TCP_FASTOPEN_CONNECT, EpollChannelOption.SO_BUSY_POLL,
                 EpollChannelOption.TCP_NOTSENT_LOWAT, EpollChannelOption.TCP_INFO);
     }
 
     private static Set<ChannelOption<?>> supportedOptionsDomainSocket() {
         return newSupportedIdentityOptionsSet(SO_RCVBUF, SO_SNDBUF, DOMAIN_SOCKET_READ_MODE,
                 UnixChannelOption.SO_PEERCRED);
     }
 
     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);
         }
     }
 
     /**
      * Get the {@code TCP_CORK} option on the socket. See {@code man 7 tcp} for more details.
      */
     private boolean isTcpCork() {
         try {
             return socket.isTcpCork();
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     /**
      * Get the {@code SO_BUSY_POLL} option on the socket. See {@code man 7 tcp} for more details.
      */
     private int getSoBusyPoll() {
         try {
             return socket.getSoBusyPoll();
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     /**
      * Get the {@code TCP_NOTSENT_LOWAT} option on the socket. See {@code man 7 tcp} for more details.
      * @return value is a uint32_t
      */
     private long getTcpNotSentLowAt() {
         try {
             return socket.getTcpNotSentLowAt();
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     /**
      * Get the {@code TCP_KEEPIDLE} option on the socket. See {@code man 7 tcp} for more details.
      */
     private int getTcpKeepIdle() {
         try {
             return socket.getTcpKeepIdle();
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     /**
      * Get the {@code TCP_KEEPINTVL} option on the socket. See {@code man 7 tcp} for more details.
      */
     private int getTcpKeepIntvl() {
         try {
             return socket.getTcpKeepIntvl();
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     /**
      * Get the {@code TCP_KEEPCNT} option on the socket. See {@code man 7 tcp} for more details.
      */
     private int getTcpKeepCnt() {
         try {
             return socket.getTcpKeepCnt();
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     /**
      * Get the {@code TCP_USER_TIMEOUT} option on the socket. See {@code man 7 tcp} for more details.
      */
     private int getTcpUserTimeout() {
         try {
             return socket.getTcpUserTimeout();
         } 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);
         }
     }
 
     /**
      * Set the {@code TCP_CORK} option on the socket. See {@code man 7 tcp} for more details.
      */
     private void setTcpCork(boolean tcpCork) {
         try {
             socket.setTcpCork(tcpCork);
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     /**
      * Set the {@code SO_BUSY_POLL} option on the socket. See {@code man 7 tcp} for more details.
      */
     private void setSoBusyPoll(int loopMicros) {
         try {
             socket.setSoBusyPoll(loopMicros);
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     /**
      * Set the {@code TCP_NOTSENT_LOWAT} option on the socket. See {@code man 7 tcp} for more details.
      * @param tcpNotSentLowAt is a uint32_t
      */
     private void setTcpNotSentLowAt(long tcpNotSentLowAt) {
         try {
             socket.setTcpNotSentLowAt(tcpNotSentLowAt);
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     private void setTrafficClass(int trafficClass) {
         try {
             socket.setTrafficClass(trafficClass);
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     /**
      * Set the {@code TCP_KEEPIDLE} option on the socket. See {@code man 7 tcp} for more details.
      */
     private void setTcpKeepIdle(int seconds) {
         try {
             socket.setTcpKeepIdle(seconds);
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     /**
      * Set the {@code TCP_KEEPINTVL} option on the socket. See {@code man 7 tcp} for more details.
      */
     private void setTcpKeepIntvl(int seconds) {
         try {
             socket.setTcpKeepIntvl(seconds);
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     /**
      * Set the {@code TCP_KEEPCNT} option on the socket. See {@code man 7 tcp} for more details.
      */
     private void setTcpKeepCnt(int probes) {
         try {
             socket.setTcpKeepCnt(probes);
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     /**
      * Set the {@code TCP_USER_TIMEOUT} option on the socket. See {@code man 7 tcp} for more details.
      */
     private void setTcpUserTimeout(int milliseconds) {
         try {
             socket.setTcpUserTimeout(milliseconds);
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     /**
      * Returns {@code true} if <a href="https://man7.org/linux/man-pages/man7/ip.7.html">IP_TRANSPARENT</a> is enabled,
      * {@code false} otherwise.
      */
     public boolean isIpTransparent() {
         try {
             return socket.isIpTransparent();
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     /**
      * If {@code true} is used <a href="https://man7.org/linux/man-pages/man7/ip.7.html">IP_TRANSPARENT</a> is enabled,
      * {@code false} for disable it. Default is disabled.
      */
     private void setIpTransparent(boolean transparent) {
         try {
             socket.setIpTransparent(transparent);
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     /**
      * Set the {@code TCP_QUICKACK} option on the socket.
      * See <a href="https://linux.die.net//man/7/tcp">TCP_QUICKACK</a>
      * for more details.
      */
     private void setTcpQuickAck(boolean quickAck) {
         try {
             socket.setTcpQuickAck(quickAck);
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     /**
      * Returns {@code true} if <a href="https://linux.die.net//man/7/tcp">TCP_QUICKACK</a> is enabled,
      * {@code false} otherwise.
      */
     private boolean isTcpQuickAck() {
         try {
             return socket.isTcpQuickAck();
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     private void setReadMode(DomainSocketReadMode mode) {
         requireNonNull(mode, "mode");
         this.mode = mode;
     }
 
     private DomainSocketReadMode getReadMode() {
         return mode;
     }
 
     /**
      * Enables client TCP fast open. {@code TCP_FASTOPEN_CONNECT} normally
      * requires Linux kernel 4.11 or later, so instead we use the traditional fast open
      * client socket mechanics that work with kernel 3.6 and later. See this
      * <a href="https://lwn.net/Articles/508865/">LWN article</a> for more info.
      */
     private void setTcpFastOpenConnect(boolean fastOpenConnect) {
         this.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(newSendBufferSize);
         }
     }
 
     /**
      * Updates and returns the {@code TCP_INFO} for the current socket.
      * See <a href="https://linux.die.net//man/7/tcp">man 7 tcp</a>.
      */
     private EpollTcpInfo getTcpInfo() {
         try {
             EpollTcpInfo info = new EpollTcpInfo();
             socket.getTcpInfo(info);
             return info;
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     @Override
     protected boolean doConnect0(SocketAddress remote) throws Exception {
         if (IS_SUPPORTING_TCP_FASTOPEN_CLIENT && socket.protocolFamily() != SocketProtocolFamily.UNIX &&
                 isTcpFastOpenConnect()) {
             ChannelOutboundBuffer outbound = outboundBuffer();
             outbound.addFlush();
             Object curr = outbound.current();
             if (curr instanceof Buffer) {
                 // If no cookie is present, the write fails with EINPROGRESS and this call basically
                 // becomes a normal async connect. All writes will be sent normally afterwards.
                 final long localFlushedAmount;
                 Buffer initialData = (Buffer) curr;
                 localFlushedAmount = doWriteOrSendBytes(initialData, remote, true);
                 if (localFlushedAmount > 0) {
                     // We had a cookie and our fast-open proceeded. Remove written data
                     // then continue with normal TCP operation.
                     outbound.removeBytes(localFlushedAmount);
                     return true;
                 }
             }
         }
         return super.doConnect0(remote);
     }
 
     @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
                     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;
     }
 
     /**
      * Set the {@code TCP_MD5SIG} option on the socket. See {@code linux/tcp.h} for more details.
      * Keys can only be set on, not read to prevent a potential leak, as they are confidential.
      * Allowing them being read would mean anyone with access to the channel could get them.
      */
     private void setTcpMd5Sig(Map<InetAddress, byte[]> keys) {
         // Add synchronized as newTcpMp5Sigs might do multiple operations on the socket itself.
         synchronized (this) {
             try {
                 tcpMd5SigAddresses = TcpMd5Util.newTcpMd5Sigs(this, tcpMd5SigAddresses, keys);
             } catch (IOException e) {
                 throw new ChannelException(e);
             }
         }
     }
 
     private PeerCredentials getPeerCredentials() {
         try {
             return socket.getPeerCredentials();
         } catch (IOException e) {
             throw new ChannelException(e);
         }
     }
 
     private void epollInReadFd(RecvBufferAllocator.Handle allocHandle, boolean receivedRdHup) {
         final ChannelPipeline pipeline = pipeline();
         Predicate<RecvBufferAllocator.Handle> maybeMoreData = maybeMoreData(receivedRdHup);
         try {
             readLoop: do {
                 // lastBytesRead represents the fd. We use lastBytesRead because it must be set so that the
                 // EpollRecvBufferAllocatorHandle knows if it should try to read again or not when autoRead is
                 // enabled.
                 allocHandle.lastBytesRead(socket.recvFd());
                 switch(allocHandle.lastBytesRead()) {
                     case 0:
                         break readLoop;
                     case -1:
                         closeTransport(newPromise());
                         return;
                     default:
                         allocHandle.incMessagesRead(1);
                         readPending = false;
                         pipeline.fireChannelRead(new FileDescriptor(allocHandle.lastBytesRead()));
                         break;
                 }
             } while (allocHandle.continueReading(isAutoRead(), maybeMoreData)
                     && !isShutdown(ChannelShutdownDirection.Inbound));
 
             allocHandle.readComplete();
             pipeline.fireChannelReadComplete();
         } catch (Throwable t) {
             allocHandle.readComplete();
             pipeline.fireChannelReadComplete();
             pipeline.fireChannelExceptionCaught(t);
         }
     }
 
     @Override
     protected boolean maybeMoreDataToRead(RecvBufferAllocator.Handle handle) {
         return handle.lastBytesRead() == handle.attemptedBytesRead();
     }
 }