/*
 * 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.UnstableApi;
import io.netty.util.internal.logging.InternalLogger;
import io.netty.util.internal.logging.InternalLoggerFactory;

import java.net.SocketAddress;
import java.util.List;
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}
 */
@UnstableApi
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;

    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) {
        try {
            // Trigger pending writes in the remote flow controller.
            encoder.flowController().writePendingBytes();
            ctx.flush();
        } catch (Http2Exception e) {
            onError(ctx, true, e);
        } catch (Throwable cause) {
            onError(ctx, true, connectionError(INTERNAL_ERROR, cause, "Error flushing"));
        }
    }

    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;
        }
    }

    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.
            sendPreface(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) {
                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.
            sendPreface(ctx);

            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();
            }
        }

        @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);
            short flags = in.getUnsignedByte(in.readerIndex() + 4);
            if (frameType != SETTINGS || (flags & Http2Flags.ACK) != 0) {
                throw connectionError(PROTOCOL_ERROR, "First received frame was not SETTINGS. " +
                                                      "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.
         */
        private void sendPreface(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);

            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);
            }
        }
    }

    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(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);
    }

    @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 {
        // Writability is expected to change while we are writing. We cannot allow this event to trigger reentering
        // the allocation and write loop. Reentering the event loop will lead to over or illegal allocation.
        try {
            if (ctx.channel().isWritable()) {
                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 {
        ctx.bind(localAddress, promise);
    }

    @Override
    public void connect(ChannelHandlerContext ctx, SocketAddress remoteAddress, SocketAddress localAddress,
                        ChannelPromise promise) throws Exception {
        ctx.connect(remoteAddress, localAddress, 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;
            for (StreamException streamException : compositException) {
                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) {
                    resetUnknownStream(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)) {
                resetUnknownStream(ctx, streamId, http2Ex.error().code(), ctx.newPromise());
            }
        } else {
            resetStream(ctx, stream, 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), future.cause());
                    }
                    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);
            }
        }
    }
}
