/*
    * 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.netty.handler.codec.http2;
  
  import io.netty.buffer.ByteBuf;
  import io.netty.buffer.ByteBufUtil;
  import io.netty.buffer.Unpooled;
  import io.netty.channel.ChannelFuture;
  import io.netty.channel.ChannelFutureListener;
  import io.netty.channel.ChannelHandlerContext;
  import io.netty.channel.ChannelOutboundHandler;
  import io.netty.channel.ChannelPromise;
  import io.netty.handler.codec.ByteToMessageDecoder;
  import io.netty.handler.codec.http.HttpResponseStatus;
  import io.netty.handler.codec.http2.Http2Exception.CompositeStreamException;
  import io.netty.handler.codec.http2.Http2Exception.StreamException;
  import io.netty.util.CharsetUtil;
  import io.netty.util.concurrent.Future;
  import io.netty.util.internal.logging.InternalLogger;
  import io.netty.util.internal.logging.InternalLoggerFactory;
  
  import java.net.SocketAddress;
  import java.util.HashSet;
  import java.util.List;
  import java.util.Set;
  import java.util.concurrent.TimeUnit;
  
  import static io.netty.buffer.ByteBufUtil.hexDump;
  import static io.netty.buffer.Unpooled.EMPTY_BUFFER;
  import static io.netty.handler.codec.http2.Http2CodecUtil.HTTP_UPGRADE_STREAM_ID;
  import static io.netty.handler.codec.http2.Http2CodecUtil.connectionPrefaceBuf;
  import static io.netty.handler.codec.http2.Http2CodecUtil.getEmbeddedHttp2Exception;
  import static io.netty.handler.codec.http2.Http2Error.INTERNAL_ERROR;
  import static io.netty.handler.codec.http2.Http2Error.NO_ERROR;
  import static io.netty.handler.codec.http2.Http2Error.PROTOCOL_ERROR;
  import static io.netty.handler.codec.http2.Http2Exception.connectionError;
  import static io.netty.handler.codec.http2.Http2Exception.isStreamError;
  import static io.netty.handler.codec.http2.Http2FrameTypes.SETTINGS;
  import static io.netty.handler.codec.http2.Http2Stream.State.IDLE;
  import static io.netty.util.CharsetUtil.UTF_8;
  import static io.netty.util.internal.ObjectUtil.checkNotNull;
  import static java.lang.Math.min;
  import static java.util.concurrent.TimeUnit.MILLISECONDS;
  
  /**
   * Provides the default implementation for processing inbound frame events and delegates to a
   * {@link Http2FrameListener}
   * <p>
   * This class will read HTTP/2 frames and delegate the events to a {@link Http2FrameListener}
   * <p>
   * This interface enforces inbound flow control functionality through
   * {@link Http2LocalFlowController}
   */
  public class Http2ConnectionHandler extends ByteToMessageDecoder implements Http2LifecycleManager,
                                                                              ChannelOutboundHandler {
  
      private static final InternalLogger logger = InternalLoggerFactory.getInstance(Http2ConnectionHandler.class);
  
      private static final Http2Headers HEADERS_TOO_LARGE_HEADERS = ReadOnlyHttp2Headers.serverHeaders(false,
              HttpResponseStatus.REQUEST_HEADER_FIELDS_TOO_LARGE.codeAsText());
      private static final ByteBuf HTTP_1_X_BUF = Unpooled.unreleasableBuffer(
          Unpooled.wrappedBuffer(new byte[] {'H', 'T', 'T', 'P', '/', '1', '.'})).asReadOnly();
  
      private final Http2ConnectionDecoder decoder;
      private final Http2ConnectionEncoder encoder;
      private final Http2Settings initialSettings;
      private final boolean decoupleCloseAndGoAway;
      private final boolean flushPreface;
      private ChannelFutureListener closeListener;
      private BaseDecoder byteDecoder;
      private long gracefulShutdownTimeoutMillis;
      private boolean inFlush;
      private boolean flushAgain;
  
      protected Http2ConnectionHandler(Http2ConnectionDecoder decoder, Http2ConnectionEncoder encoder,
                                       Http2Settings initialSettings) {
          this(decoder, encoder, initialSettings, false);
      }
  
      protected Http2ConnectionHandler(Http2ConnectionDecoder decoder, Http2ConnectionEncoder encoder,
                                       Http2Settings initialSettings, boolean decoupleCloseAndGoAway) {
          this(decoder, encoder, initialSettings, decoupleCloseAndGoAway, true);
      }
  
      protected Http2ConnectionHandler(Http2ConnectionDecoder decoder, Http2ConnectionEncoder encoder,
                                       Http2Settings initialSettings, boolean decoupleCloseAndGoAway,
                                       boolean flushPreface) {
         this.initialSettings = checkNotNull(initialSettings, "initialSettings");
         this.decoder = checkNotNull(decoder, "decoder");
         this.encoder = checkNotNull(encoder, "encoder");
         this.decoupleCloseAndGoAway = decoupleCloseAndGoAway;
         this.flushPreface = flushPreface;
         if (encoder.connection() != decoder.connection()) {
             throw new IllegalArgumentException("Encoder and Decoder do not share the same connection object");
         }
     }
 
     /**
      * Get the amount of time (in milliseconds) this endpoint will wait for all streams to be closed before closing
      * the connection during the graceful shutdown process. Returns -1 if this connection is configured to wait
      * indefinitely for all streams to close.
      */
     public long gracefulShutdownTimeoutMillis() {
         return gracefulShutdownTimeoutMillis;
     }
 
     /**
      * Set the amount of time (in milliseconds) this endpoint will wait for all streams to be closed before closing
      * the connection during the graceful shutdown process.
      * @param gracefulShutdownTimeoutMillis the amount of time (in milliseconds) this endpoint will wait for all
      * streams to be closed before closing the connection during the graceful shutdown process.
      */
     public void gracefulShutdownTimeoutMillis(long gracefulShutdownTimeoutMillis) {
         if (gracefulShutdownTimeoutMillis < -1) {
             throw new IllegalArgumentException("gracefulShutdownTimeoutMillis: " + gracefulShutdownTimeoutMillis +
                                                " (expected: -1 for indefinite or >= 0)");
         }
         this.gracefulShutdownTimeoutMillis = gracefulShutdownTimeoutMillis;
     }
 
     public Http2Connection connection() {
         return encoder.connection();
     }
 
     public Http2ConnectionDecoder decoder() {
         return decoder;
     }
 
     public Http2ConnectionEncoder encoder() {
         return encoder;
     }
 
     private boolean prefaceSent() {
         return byteDecoder != null && byteDecoder.prefaceSent();
     }
 
     /**
      * Handles the client-side (cleartext) upgrade from HTTP to HTTP/2.
      * Reserves local stream 1 for the HTTP/2 response.
      */
     public void onHttpClientUpgrade() throws Http2Exception {
         if (connection().isServer()) {
             throw connectionError(PROTOCOL_ERROR, "Client-side HTTP upgrade requested for a server");
         }
         if (!prefaceSent()) {
             // If the preface was not sent yet it most likely means the handler was not added to the pipeline before
             // calling this method.
             throw connectionError(INTERNAL_ERROR, "HTTP upgrade must occur after preface was sent");
         }
         if (decoder.prefaceReceived()) {
             throw connectionError(PROTOCOL_ERROR, "HTTP upgrade must occur before HTTP/2 preface is received");
         }
 
         // Create a local stream used for the HTTP cleartext upgrade.
         connection().local().createStream(HTTP_UPGRADE_STREAM_ID, true);
     }
 
     /**
      * Handles the server-side (cleartext) upgrade from HTTP to HTTP/2.
      * @param settings the settings for the remote endpoint.
      */
     public void onHttpServerUpgrade(Http2Settings settings) throws Http2Exception {
         if (!connection().isServer()) {
             throw connectionError(PROTOCOL_ERROR, "Server-side HTTP upgrade requested for a client");
         }
         if (!prefaceSent()) {
             // If the preface was not sent yet it most likely means the handler was not added to the pipeline before
             // calling this method.
             throw connectionError(INTERNAL_ERROR, "HTTP upgrade must occur after preface was sent");
         }
         if (decoder.prefaceReceived()) {
             throw connectionError(PROTOCOL_ERROR, "HTTP upgrade must occur before HTTP/2 preface is received");
         }
 
         // Apply the settings but no ACK is necessary.
         encoder.remoteSettings(settings);
 
         // Create a stream in the half-closed state.
         connection().remote().createStream(HTTP_UPGRADE_STREAM_ID, true);
     }
 
     @Override
     public void flush(ChannelHandlerContext ctx) {
         if (inFlush) {
             // Reentrant flush (e.g. from a synchronous writability change) must not recurse — that livelocks
             // (#17256); the in-progress flush loop picks it up.
             flushAgain = true;
             return;
         }
         inFlush = true;
         try {
             do {
                 flushAgain = false;
                 // Trigger pending writes in the remote flow controller.
                 encoder.flowController().writePendingBytes();
                 ctx.flush();
                 // Honor any flush re-requested while we were flushing (a resumed write, or a direct flush from
                 // elsewhere in the pipeline) regardless of writability — flushing already-written data doesn't
                 // need a writable channel.
             } while (flushAgain);
         } catch (Http2Exception e) {
             onError(ctx, true, e);
         } catch (Throwable cause) {
             onError(ctx, true, connectionError(INTERNAL_ERROR, cause, "Error flushing"));
         } finally {
             inFlush = false;
         }
     }
 
     private boolean hasPendingData() {
         final Http2RemoteFlowController flowController = encoder.flowController();
         try {
             // Stop at the first stream that still has a flow-controlled frame queued. Frame-based, so it counts
             // zero-length frames (e.g. trailing headers) that carry no flow-control bytes.
             return connection().forEachActiveStream(new Http2StreamVisitor() {
                 @Override
                 public boolean visit(Http2Stream stream) {
                     return !flowController.hasFlowControlled(stream);
                 }
             }) != null;
         } catch (Http2Exception e) {
             return false;
         }
     }
 
     private abstract class BaseDecoder {
         public abstract void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception;
         public void handlerRemoved(ChannelHandlerContext ctx) throws Exception { }
         public void channelActive(ChannelHandlerContext ctx) throws Exception { }
 
         public void channelInactive(ChannelHandlerContext ctx) throws Exception {
             // Connection has terminated, close the encoder and decoder.
             encoder().close();
             decoder().close();
 
             // We need to remove all streams (not just the active ones).
             // See https://github.com/netty/netty/issues/4838.
             connection().close(ctx.voidPromise());
         }
 
         /**
          * Determine if the HTTP/2 connection preface been sent.
          */
         public boolean prefaceSent() {
             return true;
         }
 
         /**
          * Send the preface if needed.
          *
          * @param ctx           the {@link ChannelHandlerContext} to use.
          * @throws Exception    thrown on error.
          */
         public void sendPrefaceIfNeeded(ChannelHandlerContext ctx) throws Exception {
             // Noop by default.
         }
     }
 
     private final class PrefaceDecoder extends BaseDecoder {
         private ByteBuf clientPrefaceString;
         private boolean prefaceSent;
 
         PrefaceDecoder(ChannelHandlerContext ctx) throws Exception {
             clientPrefaceString = clientPrefaceString(encoder.connection());
             // This handler was just added to the context. In case it was handled after
             // the connection became active, send the connection preface now.
             sendPrefaceIfNeeded(ctx);
         }
 
         @Override
         public boolean prefaceSent() {
             return prefaceSent;
         }
 
         @Override
         public void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
             try {
                 if (ctx.channel().isActive() && readClientPrefaceString(in) && verifyFirstFrameIsSettings(in)) {
                     // After the preface is read, it is time to hand over control to the post initialized decoder.
                     byteDecoder = new FrameDecoder();
                     byteDecoder.decode(ctx, in, out);
                 }
             } catch (Throwable e) {
                 if (byteDecoder != null) {
                     // Skip all bytes before we report the exception as
                     in.skipBytes(in.readableBytes());
                 }
                 onError(ctx, false, e);
             }
         }
 
         @Override
         public void channelActive(ChannelHandlerContext ctx) throws Exception {
             // The channel just became active - send the connection preface to the remote endpoint.
             sendPrefaceIfNeeded(ctx);
         }
 
         @Override
         public void channelInactive(ChannelHandlerContext ctx) throws Exception {
             cleanup();
             super.channelInactive(ctx);
         }
 
         /**
          * Releases the {@code clientPrefaceString}. Any active streams will be left in the open.
          */
         @Override
         public void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
             cleanup();
         }
 
         /**
          * Releases the {@code clientPrefaceString}. Any active streams will be left in the open.
          */
         private void cleanup() {
             if (clientPrefaceString != null) {
                 clientPrefaceString.release();
                 clientPrefaceString = null;
             }
         }
 
         /**
          * Decodes the client connection preface string from the input buffer.
          *
          * @return {@code true} if processing of the client preface string is complete. Since client preface strings can
          *         only be received by servers, returns true immediately for client endpoints.
          */
         private boolean readClientPrefaceString(ByteBuf in) throws Http2Exception {
             if (clientPrefaceString == null) {
                 return true;
             }
 
             int prefaceRemaining = clientPrefaceString.readableBytes();
             int bytesRead = min(in.readableBytes(), prefaceRemaining);
 
             // If the input so far doesn't match the preface, break the connection.
             if (bytesRead == 0 || !ByteBufUtil.equals(in, in.readerIndex(),
                                                       clientPrefaceString, clientPrefaceString.readerIndex(),
                                                       bytesRead)) {
                 int maxSearch = 1024; // picked because 512 is too little, and 2048 too much
                 int http1Index =
                     ByteBufUtil.indexOf(HTTP_1_X_BUF, in.slice(in.readerIndex(), min(in.readableBytes(), maxSearch)));
                 if (http1Index != -1) {
                     String chunk = in.toString(in.readerIndex(), http1Index - in.readerIndex(), CharsetUtil.US_ASCII);
                     throw connectionError(PROTOCOL_ERROR, "Unexpected HTTP/1.x request: %s", chunk);
                 }
                 String receivedBytes = hexDump(in, in.readerIndex(),
                                                min(in.readableBytes(), clientPrefaceString.readableBytes()));
                 throw connectionError(PROTOCOL_ERROR, "HTTP/2 client preface string missing or corrupt. " +
                                                       "Hex dump for received bytes: %s", receivedBytes);
             }
             in.skipBytes(bytesRead);
             clientPrefaceString.skipBytes(bytesRead);
 
             if (!clientPrefaceString.isReadable()) {
                 // Entire preface has been read.
                 clientPrefaceString.release();
                 clientPrefaceString = null;
                 return true;
             }
             return false;
         }
 
         /**
          * Peeks at that the next frame in the buffer and verifies that it is a non-ack {@code SETTINGS} frame.
          *
          * @param in the inbound buffer.
          * @return {@code true} if the next frame is a non-ack {@code SETTINGS} frame, {@code false} if more
          * data is required before we can determine the next frame type.
          * @throws Http2Exception thrown if the next frame is NOT a non-ack {@code SETTINGS} frame.
          */
         private boolean verifyFirstFrameIsSettings(ByteBuf in) throws Http2Exception {
             if (in.readableBytes() < 5) {
                 // Need more data before we can see the frame type for the first frame.
                 return false;
             }
 
             short frameType = in.getUnsignedByte(in.readerIndex() + 3);
             if (frameType != SETTINGS) {
                 throw connectionError(PROTOCOL_ERROR, "First received frame was not SETTINGS. " +
                                                       "Hex dump for first 5 bytes: %s",
                                       hexDump(in, in.readerIndex(), 5));
             }
             short flags = in.getUnsignedByte(in.readerIndex() + 4);
             if ((flags & Http2Flags.ACK) != 0) {
                 throw connectionError(PROTOCOL_ERROR, "First received frame was SETTINGS frame but had ACK flag set. " +
                         "Hex dump for first 5 bytes: %s",
                         hexDump(in, in.readerIndex(), 5));
             }
             return true;
         }
 
         /**
          * Sends the HTTP/2 connection preface upon establishment of the connection, if not already sent.
          */
         @Override
         public void sendPrefaceIfNeeded(ChannelHandlerContext ctx) throws Exception {
             if (prefaceSent || !ctx.channel().isActive()) {
                 return;
             }
 
             prefaceSent = true;
 
             final boolean isClient = !connection().isServer();
             if (isClient) {
                 // Clients must send the preface string as the first bytes on the connection.
                 ctx.write(connectionPrefaceBuf()).addListener(ChannelFutureListener.CLOSE_ON_FAILURE);
             }
 
             // Both client and server must send their initial settings.
             encoder.writeSettings(ctx, initialSettings, ctx.newPromise()).addListener(
                     ChannelFutureListener.CLOSE_ON_FAILURE);
 
             try {
                 if (isClient) {
                     // If this handler is extended by the user and we directly fire the userEvent from this context then
                     // the user will not see the event. We should fire the event starting with this handler so this
                     // class (and extending classes) have a chance to process the event.
                     userEventTriggered(ctx, Http2ConnectionPrefaceAndSettingsFrameWrittenEvent.INSTANCE);
                 }
             } finally {
                 if (flushPreface) {
                     // As we don't know if any channelReadComplete() events will be triggered at all we need to ensure
                     // we also flush. Otherwise the remote peer might never see the preface / settings frame.
                     // See https://github.com/netty/netty/issues/12089
                     ctx.flush();
                 }
             }
         }
     }
 
     private final class FrameDecoder extends BaseDecoder {
         @Override
         public void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
             try {
                 decoder.decodeFrame(ctx, in, out);
             } catch (Throwable e) {
                 onError(ctx, false, e);
             }
         }
     }
 
     @Override
     public void handlerAdded(final ChannelHandlerContext ctx) throws Exception {
         // Initialize the encoder, decoder, flow controllers, and internal state.
         encoder.lifecycleManager(this);
         decoder.lifecycleManager(this);
         encoder.flowController().channelHandlerContext(ctx);
         decoder.flowController().channelHandlerContext(ctx);
         byteDecoder = new PrefaceDecoder(ctx);
         // Streams are not only closed via closeStream(...): the connection also closes them itself, for example
         // when a GOAWAY is received or the channel becomes inactive. Check if a pending graceful shutdown can
         // complete whenever a stream is closed, whatever the reason.
         connection().addListener(new Http2ConnectionAdapter() {
             @Override
             public void onStreamClosed(Http2Stream stream) {
                 checkCloseConnection(ctx.newSucceededFuture());
             }
         });
     }
 
     @Override
     protected void handlerRemoved0(ChannelHandlerContext ctx) throws Exception {
         if (byteDecoder != null) {
             byteDecoder.handlerRemoved(ctx);
             byteDecoder = null;
         }
     }
 
     @Override
     public void channelActive(ChannelHandlerContext ctx) throws Exception {
         if (byteDecoder == null) {
             byteDecoder = new PrefaceDecoder(ctx);
         }
         byteDecoder.channelActive(ctx);
         super.channelActive(ctx);
     }
 
     @Override
     public void channelInactive(ChannelHandlerContext ctx) throws Exception {
         // Call super class first, as this may result in decode being called.
         super.channelInactive(ctx);
         if (byteDecoder != null) {
             byteDecoder.channelInactive(ctx);
             byteDecoder = null;
         }
     }
 
     @Override
     public void channelWritabilityChanged(ChannelHandlerContext ctx) throws Exception {
         try {
             // Only flush on a writability change if the flow controller has frames queued. A toggle with
             // nothing to write (e.g. SslHandler during setup) must not flush, or the flush -> writability ->
             // flush cycle livelocks (#17256).
             if (ctx.channel().isWritable() && hasPendingData()) {
                 flush(ctx);
             }
             encoder.flowController().channelWritabilityChanged();
         } finally {
             super.channelWritabilityChanged(ctx);
         }
     }
 
     @Override
     protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
         byteDecoder.decode(ctx, in, out);
     }
 
     @Override
     public void bind(ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise) throws Exception {
         // Ensure we send the preface before we notify the bind promise as the user might try to write
         // directly in the listener attached to the promise and we need to ensure the preface is always the first
         // thing that is written.
         ctx.bind(localAddress, ctx.newPromise()).addListener(new PrefaceSendListener(ctx, promise));
     }
 
     @Override
     public void connect(ChannelHandlerContext ctx, SocketAddress remoteAddress, SocketAddress localAddress,
                         ChannelPromise promise) throws Exception {
         // Ensure we send the preface before we notify the connect promise as the user might try to write
         // directly in the listener attached to the promise and we need to ensure the preface is always the first
         // thing that is written.
         ctx.connect(remoteAddress, localAddress, ctx.newPromise()).addListener(new PrefaceSendListener(ctx, promise));
     }
 
     @Override
     public void disconnect(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception {
         ctx.disconnect(promise);
     }
 
     @Override
     public void close(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception {
         if (decoupleCloseAndGoAway) {
             ctx.close(promise);
             return;
         }
         promise = promise.unvoid();
         // Avoid NotYetConnectedException and avoid sending before connection preface
         if (!ctx.channel().isActive() || !prefaceSent()) {
             ctx.close(promise);
             return;
         }
 
         // If the user has already sent a GO_AWAY frame they may be attempting to do a graceful shutdown which requires
         // sending multiple GO_AWAY frames. We should only send a GO_AWAY here if one has not already been sent. If
         // a GO_AWAY has been sent we send a empty buffer just so we can wait to close until all other data has been
         // flushed to the OS.
         // https://github.com/netty/netty/issues/5307
         ChannelFuture f = connection().goAwaySent() ? ctx.write(EMPTY_BUFFER) : goAway(ctx, null, ctx.newPromise());
         ctx.flush();
         doGracefulShutdown(ctx, f, promise);
     }
 
     private ChannelFutureListener newClosingChannelFutureListener(
             ChannelHandlerContext ctx, ChannelPromise promise) {
         long gracefulShutdownTimeoutMillis = this.gracefulShutdownTimeoutMillis;
         return gracefulShutdownTimeoutMillis < 0 ?
                 new ClosingChannelFutureListener(ctx, promise) :
                 new ClosingChannelFutureListener(ctx, promise, gracefulShutdownTimeoutMillis, MILLISECONDS);
     }
 
     private void doGracefulShutdown(ChannelHandlerContext ctx, ChannelFuture future, final ChannelPromise promise) {
         final ChannelFutureListener listener = newClosingChannelFutureListener(ctx, promise);
         if (isGracefulShutdownComplete()) {
             // If there are no active streams, close immediately after the GO_AWAY write completes or the timeout
             // elapsed.
             future.addListener(listener);
         } else {
             // If there are active streams we should wait until they are all closed before closing the connection.
 
             // The ClosingChannelFutureListener will cascade promise completion. We need to always notify the
             // new ClosingChannelFutureListener when the graceful close completes if the promise is not null.
             if (closeListener == null) {
                 closeListener = listener;
             } else if (promise != null) {
                 final ChannelFutureListener oldCloseListener = closeListener;
                 closeListener = new ChannelFutureListener() {
                     @Override
                     public void operationComplete(ChannelFuture future) throws Exception {
                         try {
                             oldCloseListener.operationComplete(future);
                         } finally {
                             listener.operationComplete(future);
                         }
                     }
                 };
             }
         }
     }
 
     @Override
     public void deregister(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception {
         ctx.deregister(promise);
     }
 
     @Override
     public void read(ChannelHandlerContext ctx) throws Exception {
         ctx.read();
     }
 
     @Override
     public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
         ctx.write(msg, promise);
     }
 
     @Override
     public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
         // Trigger flush after read on the assumption that flush is cheap if there is nothing to write and that
         // for flow-control the read may release window that causes data to be written that can now be flushed.
         try {
             // First call channelReadComplete0(...) as this may produce more data that we want to flush
             channelReadComplete0(ctx);
         } finally {
             flush(ctx);
         }
     }
 
     final void channelReadComplete0(ChannelHandlerContext ctx) {
         // Discard bytes of the cumulation buffer if needed.
         discardSomeReadBytes();
 
         // Ensure we never stale the HTTP/2 Channel. Flow-control is enforced by HTTP/2.
         //
         // See https://tools.ietf.org/html/rfc7540#section-5.2.2
         if (!ctx.channel().config().isAutoRead()) {
             ctx.read();
         }
 
         ctx.fireChannelReadComplete();
     }
 
     /**
      * Handles {@link Http2Exception} objects that were thrown from other handlers. Ignores all other exceptions.
      */
     @Override
     public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
         if (getEmbeddedHttp2Exception(cause) != null) {
             // Some exception in the causality chain is an Http2Exception - handle it.
             onError(ctx, false, cause);
         } else {
             super.exceptionCaught(ctx, cause);
         }
     }
 
     /**
      * Closes the local side of the given stream. If this causes the stream to be closed, adds a
      * hook to close the channel after the given future completes.
      *
      * @param stream the stream to be half closed.
      * @param future If closing, the future after which to close the channel.
      */
     @Override
     public void closeStreamLocal(Http2Stream stream, ChannelFuture future) {
         switch (stream.state()) {
             case HALF_CLOSED_LOCAL:
             case OPEN:
                 stream.closeLocalSide();
                 break;
             default:
                 closeStream(stream, future);
                 break;
         }
     }
 
     /**
      * Closes the remote side of the given stream. If this causes the stream to be closed, adds a
      * hook to close the channel after the given future completes.
      *
      * @param stream the stream to be half closed.
      * @param future If closing, the future after which to close the channel.
      */
     @Override
     public void closeStreamRemote(Http2Stream stream, ChannelFuture future) {
         switch (stream.state()) {
             case HALF_CLOSED_REMOTE:
             case OPEN:
                 stream.closeRemoteSide();
                 break;
             default:
                 closeStream(stream, future);
                 break;
         }
     }
 
     @Override
     public void closeStream(final Http2Stream stream, ChannelFuture future) {
         if (future.isDone()) {
             doCloseStream(stream, future);
         } else {
             future.addListener(new ChannelFutureListener() {
                 @Override
                 public void operationComplete(ChannelFuture future) {
                     doCloseStream(stream, future);
                 }
             });
         }
     }
 
     /**
      * Central handler for all exceptions caught during HTTP/2 processing.
      */
     @Override
     public void onError(ChannelHandlerContext ctx, boolean outbound, Throwable cause) {
         Http2Exception embedded = getEmbeddedHttp2Exception(cause);
         if (isStreamError(embedded)) {
             onStreamError(ctx, outbound, cause, (StreamException) embedded);
         } else if (embedded instanceof CompositeStreamException) {
             CompositeStreamException compositException = (CompositeStreamException) embedded;
             // Each contained StreamException is generally an independent error for a distinct stream (e.g.
             // one per active stream that overflowed its flow-control window when SETTINGS_INITIAL_WINDOW_SIZE
             // changed), so every affected stream must still be reset individually; otherwise it would be left
             // open with a corrupted flow-control window. Should the composite ever contain more than one
             // exception for the same stream id, only report the first one, per RFC 9113, Section 5.4's
             // guidance to report at most one stream error per stream.
             Set<Integer> handledStreamIds = new HashSet<Integer>();
             for (StreamException streamException : compositException) {
                 if (handledStreamIds.add(streamException.streamId())) {
                     onStreamError(ctx, outbound, cause, streamException);
                 }
             }
         } else {
             onConnectionError(ctx, outbound, cause, embedded);
         }
         ctx.flush();
     }
 
     /**
      * Called by the graceful shutdown logic to determine when it is safe to close the connection. Returns {@code true}
      * if the graceful shutdown has completed and the connection can be safely closed. This implementation just
      * guarantees that there are no active streams. Subclasses may override to provide additional checks.
      */
     protected boolean isGracefulShutdownComplete() {
         return connection().numActiveStreams() == 0;
     }
 
     /**
      * Handler for a connection error. Sends a GO_AWAY frame to the remote endpoint. Once all
      * streams are closed, the connection is shut down.
      *
      * @param ctx the channel context
      * @param outbound {@code true} if the error was caused by an outbound operation.
      * @param cause the exception that was caught
      * @param http2Ex the {@link Http2Exception} that is embedded in the causality chain. This may
      *            be {@code null} if it's an unknown exception.
      */
     protected void onConnectionError(ChannelHandlerContext ctx, boolean outbound,
                                      Throwable cause, Http2Exception http2Ex) {
         if (http2Ex == null) {
             http2Ex = new Http2Exception(INTERNAL_ERROR, cause.getMessage(), cause);
         }
 
         ChannelPromise promise = ctx.newPromise();
         ChannelFuture future = goAway(ctx, http2Ex, ctx.newPromise());
         if (http2Ex.shutdownHint() == Http2Exception.ShutdownHint.GRACEFUL_SHUTDOWN) {
             doGracefulShutdown(ctx, future, promise);
         } else {
             future.addListener(newClosingChannelFutureListener(ctx, promise));
         }
     }
 
     /**
      * Handler for a stream error. Sends a {@code RST_STREAM} frame to the remote endpoint and closes the
      * stream.
      *
      * @param ctx the channel context
      * @param outbound {@code true} if the error was caused by an outbound operation.
      * @param cause the exception that was caught
      * @param http2Ex the {@link StreamException} that is embedded in the causality chain.
      */
     protected void onStreamError(ChannelHandlerContext ctx, boolean outbound,
                                  @SuppressWarnings("unused") Throwable cause, StreamException http2Ex) {
         final int streamId = http2Ex.streamId();
         Http2Stream stream = connection().stream(streamId);
 
         //if this is caused by reading headers that are too large, send a header with status 431
         if (http2Ex instanceof Http2Exception.HeaderListSizeException &&
             ((Http2Exception.HeaderListSizeException) http2Ex).duringDecode() &&
             connection().isServer()) {
 
             // NOTE We have to check to make sure that a stream exists before we send our reply.
             // We likely always create the stream below as the stream isn't created until the
             // header block is completely processed.
 
             // The case of a streamId referring to a stream which was already closed is handled
             // by createStream and will land us in the catch block below
             if (stream == null) {
                 try {
                     stream = encoder.connection().remote().createStream(streamId, true);
                 } catch (Http2Exception e) {
                     encoder().writeRstStream(ctx, streamId, http2Ex.error().code(), ctx.newPromise());
                     return;
                 }
             }
 
             // ensure that we have not already sent headers on this stream
             if (stream != null && !stream.isHeadersSent()) {
                 try {
                     handleServerHeaderDecodeSizeError(ctx, stream);
                 } catch (Throwable cause2) {
                     onError(ctx, outbound, connectionError(INTERNAL_ERROR, cause2, "Error DecodeSizeError"));
                 }
             }
         }
 
         if (stream == null) {
             if (!outbound || connection().local().mayHaveCreatedStream(streamId)) {
                 encoder().writeRstStream(ctx, streamId, http2Ex.error().code(), ctx.newPromise());
             }
         } else {
             encoder().writeRstStream(ctx, streamId, http2Ex.error().code(), ctx.newPromise());
         }
     }
 
     /**
      * Notifies client that this server has received headers that are larger than what it is
      * willing to accept. Override to change behavior.
      *
      * @param ctx the channel context
      * @param stream the Http2Stream on which the header was received
      */
     protected void handleServerHeaderDecodeSizeError(ChannelHandlerContext ctx, Http2Stream stream) {
         encoder().writeHeaders(ctx, stream.id(), HEADERS_TOO_LARGE_HEADERS, 0, true, ctx.newPromise());
     }
 
     protected Http2FrameWriter frameWriter() {
         return encoder().frameWriter();
     }
 
     /**
      * Sends a {@code RST_STREAM} frame even if we don't know about the stream. This error condition is most likely
      * triggered by the first frame of a stream being invalid. That is, there was an error reading the frame before
      * we could create a new stream.
      */
     private ChannelFuture resetUnknownStream(final ChannelHandlerContext ctx, int streamId, long errorCode,
                                              ChannelPromise promise) {
         ChannelFuture future = frameWriter().writeRstStream(ctx, streamId, errorCode, promise);
         if (future.isDone()) {
             closeConnectionOnError(ctx, future);
         } else {
             future.addListener(new ChannelFutureListener() {
                 @Override
                 public void operationComplete(ChannelFuture future) throws Exception {
                     closeConnectionOnError(ctx, future);
                 }
             });
         }
         return future;
     }
 
     @Override
     public ChannelFuture resetStream(final ChannelHandlerContext ctx, int streamId, long errorCode,
                                      ChannelPromise promise) {
         final Http2Stream stream = connection().stream(streamId);
         if (stream == null) {
             return resetUnknownStream(ctx, streamId, errorCode, promise.unvoid());
         }
 
        return resetStream(ctx, stream, errorCode, promise);
     }
 
     private ChannelFuture resetStream(final ChannelHandlerContext ctx, final Http2Stream stream,
                                       long errorCode, ChannelPromise promise) {
         promise = promise.unvoid();
         if (stream.isResetSent()) {
             // Don't write a RST_STREAM frame if we have already written one.
             return promise.setSuccess();
         }
         // Synchronously set the resetSent flag to prevent any subsequent calls
         // from resulting in multiple reset frames being sent.
         //
         // This needs to be done before we notify the promise as the promise may have a listener attached that
         // call resetStream(...) again.
         stream.resetSent();
 
         final ChannelFuture future;
         // If the remote peer is not aware of the steam, then we are not allowed to send a RST_STREAM
         // https://tools.ietf.org/html/rfc7540#section-6.4.
         if (stream.state() == IDLE ||
             connection().local().created(stream) && !stream.isHeadersSent() && !stream.isPushPromiseSent()) {
             future = promise.setSuccess();
         } else {
             future = frameWriter().writeRstStream(ctx, stream.id(), errorCode, promise);
         }
         if (future.isDone()) {
             processRstStreamWriteResult(ctx, stream, future);
         } else {
             future.addListener(new ChannelFutureListener() {
                 @Override
                 public void operationComplete(ChannelFuture future) throws Exception {
                     processRstStreamWriteResult(ctx, stream, future);
                 }
             });
         }
 
         return future;
     }
 
     @Override
     public ChannelFuture goAway(final ChannelHandlerContext ctx, final int lastStreamId, final long errorCode,
                                 final ByteBuf debugData, ChannelPromise promise) {
         promise = promise.unvoid();
         final Http2Connection connection = connection();
         try {
             if (!connection.goAwaySent(lastStreamId, errorCode, debugData)) {
                 debugData.release();
                 promise.trySuccess();
                 return promise;
             }
         } catch (Throwable cause) {
             debugData.release();
             promise.tryFailure(cause);
             return promise;
         }
 
         // Need to retain before we write the buffer because if we do it after the refCnt could already be 0 and
         // result in an IllegalRefCountException.
         debugData.retain();
         ChannelFuture future = frameWriter().writeGoAway(ctx, lastStreamId, errorCode, debugData, promise);
 
         if (future.isDone()) {
             processGoAwayWriteResult(ctx, lastStreamId, errorCode, debugData, future);
         } else {
             future.addListener(new ChannelFutureListener() {
                 @Override
                 public void operationComplete(ChannelFuture future) throws Exception {
                     processGoAwayWriteResult(ctx, lastStreamId, errorCode, debugData, future);
                 }
             });
         }
 
         return future;
     }
 
     /**
      * Closes the connection if the graceful shutdown process has completed.
      * @param future Represents the status that will be passed to the {@link #closeListener}.
      */
     private void checkCloseConnection(ChannelFuture future) {
         // If this connection is closing and the graceful shutdown has completed, close the connection
         // once this operation completes.
         if (closeListener != null && isGracefulShutdownComplete()) {
             ChannelFutureListener closeListener = this.closeListener;
             // This method could be called multiple times
             // and we don't want to notify the closeListener multiple times.
             this.closeListener = null;
             try {
                 closeListener.operationComplete(future);
             } catch (Exception e) {
                 throw new IllegalStateException("Close listener threw an unexpected exception", e);
             }
         }
     }
 
     /**
      * Close the remote endpoint with a {@code GO_AWAY} frame. Does <strong>not</strong> flush
      * immediately, this is the responsibility of the caller.
      */
     private ChannelFuture goAway(ChannelHandlerContext ctx, Http2Exception cause, ChannelPromise promise) {
         long errorCode = cause != null ? cause.error().code() : NO_ERROR.code();
         int lastKnownStream;
         if (cause != null && cause.shutdownHint() == Http2Exception.ShutdownHint.HARD_SHUTDOWN) {
             // The hard shutdown could have been triggered during header processing, before updating
             // lastStreamCreated(). Specifically, any connection errors encountered by Http2FrameReader or HPACK
             // decoding will fail to update the last known stream. So we must be pessimistic.
             // https://github.com/netty/netty/issues/10670
             lastKnownStream = Integer.MAX_VALUE;
         } else {
             lastKnownStream = connection().remote().lastStreamCreated();
         }
         return goAway(ctx, lastKnownStream, errorCode, Http2CodecUtil.toByteBuf(ctx, cause), promise);
     }
 
     private void processRstStreamWriteResult(ChannelHandlerContext ctx, Http2Stream stream, ChannelFuture future) {
         if (future.isSuccess()) {
             closeStream(stream, future);
         } else {
             // The connection will be closed and so no need to change the resetSent flag to false.
             onConnectionError(ctx, true, future.cause(), null);
         }
     }
 
     private void closeConnectionOnError(ChannelHandlerContext ctx, ChannelFuture future) {
         if (!future.isSuccess()) {
             onConnectionError(ctx, true, future.cause(), null);
         }
     }
 
     private void doCloseStream(final Http2Stream stream, ChannelFuture future) {
         stream.close();
         checkCloseConnection(future);
     }
 
     /**
      * Returns the client preface string if this is a client connection, otherwise returns {@code null}.
      */
     private static ByteBuf clientPrefaceString(Http2Connection connection) {
         return connection.isServer() ? connectionPrefaceBuf() : null;
     }
 
     private static void processGoAwayWriteResult(final ChannelHandlerContext ctx, final int lastStreamId,
                                                  final long errorCode, final ByteBuf debugData, ChannelFuture future) {
         try {
             if (future.isSuccess()) {
                 if (errorCode != NO_ERROR.code()) {
                     if (logger.isDebugEnabled()) {
                         logger.debug("{} Sent GOAWAY: lastStreamId '{}', errorCode '{}', " +
                                      "debugData '{}'. Forcing shutdown of the connection.",
                                      ctx.channel(), lastStreamId, errorCode, debugData.toString(UTF_8));
                     }
                     ctx.close();
                 }
             } else {
                 if (logger.isDebugEnabled()) {
                     logger.debug("{} Sending GOAWAY failed: lastStreamId '{}', errorCode '{}', " +
                                  "debugData '{}'. Forcing shutdown of the connection.",
                                  ctx.channel(), lastStreamId, errorCode, debugData.toString(UTF_8), future.cause());
                 }
                 ctx.close();
             }
         } finally {
             // We're done with the debug data now.
             debugData.release();
         }
     }
 
     /**
      * Closes the channel when the future completes.
      */
     private static final class ClosingChannelFutureListener implements ChannelFutureListener {
         private final ChannelHandlerContext ctx;
         private final ChannelPromise promise;
         private final Future<?> timeoutTask;
         private boolean closed;
 
         ClosingChannelFutureListener(ChannelHandlerContext ctx, ChannelPromise promise) {
             this.ctx = ctx;
             this.promise = promise;
             timeoutTask = null;
         }
 
         ClosingChannelFutureListener(final ChannelHandlerContext ctx, final ChannelPromise promise,
                                      long timeout, TimeUnit unit) {
             this.ctx = ctx;
             this.promise = promise;
             timeoutTask = ctx.executor().schedule(new Runnable() {
                 @Override
                 public void run() {
                     doClose();
                 }
             }, timeout, unit);
         }
 
         @Override
         public void operationComplete(ChannelFuture sentGoAwayFuture) {
             if (timeoutTask != null) {
                 timeoutTask.cancel(false);
             }
             doClose();
         }
 
         private void doClose() {
             // We need to guard against multiple calls as the timeout may trigger close() first and then it will be
             // triggered again because of operationComplete(...) is called.
             if (closed) {
                 // This only happens if we also scheduled a timeout task.
                 assert timeoutTask != null;
                 return;
             }
             closed = true;
             if (promise == null) {
                 ctx.close();
             } else {
                 ctx.close(promise);
             }
         }
     }
 
     private final class PrefaceSendListener implements ChannelFutureListener {
         private final ChannelHandlerContext ctx;
         private final ChannelPromise promise;
 
         PrefaceSendListener(ChannelHandlerContext ctx, ChannelPromise promise) {
             this.ctx = ctx;
             this.promise = promise;
         }
 
         @Override
         public void operationComplete(ChannelFuture f) {
             if (f.isSuccess()) {
                 try {
                     if (byteDecoder != null) {
                         byteDecoder.sendPrefaceIfNeeded(ctx);
                     }
                 } catch (Throwable e) {
                     promise.setFailure(e);
                     return;
                 }
                 promise.setSuccess();
             } else {
                 promise.setFailure(f.cause());
             }
         }
     }
 }