/*
 * Copyright (c) 2015, 2018, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package jdk.internal.net.http;

import java.io.EOFException;
import java.io.IOException;
import java.io.UncheckedIOException;
import java.net.InetSocketAddress;
import java.net.URI;
import java.nio.ByteBuffer;
import java.nio.charset.StandardCharsets;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Locale;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.CompletableFuture;
import java.util.ArrayList;
import java.util.Objects;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.Flow;
import java.util.function.Function;
import java.util.function.Supplier;
import javax.net.ssl.SSLEngine;
import javax.net.ssl.SSLException;
import java.net.http.HttpClient;
import java.net.http.HttpHeaders;
import jdk.internal.net.http.HttpConnection.HttpPublisher;
import jdk.internal.net.http.common.FlowTube;
import jdk.internal.net.http.common.FlowTube.TubeSubscriber;
import jdk.internal.net.http.common.HttpHeadersBuilder;
import jdk.internal.net.http.common.Log;
import jdk.internal.net.http.common.Logger;
import jdk.internal.net.http.common.MinimalFuture;
import jdk.internal.net.http.common.SequentialScheduler;
import jdk.internal.net.http.common.Utils;
import jdk.internal.net.http.frame.ContinuationFrame;
import jdk.internal.net.http.frame.DataFrame;
import jdk.internal.net.http.frame.ErrorFrame;
import jdk.internal.net.http.frame.FramesDecoder;
import jdk.internal.net.http.frame.FramesEncoder;
import jdk.internal.net.http.frame.GoAwayFrame;
import jdk.internal.net.http.frame.HeaderFrame;
import jdk.internal.net.http.frame.HeadersFrame;
import jdk.internal.net.http.frame.Http2Frame;
import jdk.internal.net.http.frame.MalformedFrame;
import jdk.internal.net.http.frame.OutgoingHeaders;
import jdk.internal.net.http.frame.PingFrame;
import jdk.internal.net.http.frame.PushPromiseFrame;
import jdk.internal.net.http.frame.ResetFrame;
import jdk.internal.net.http.frame.SettingsFrame;
import jdk.internal.net.http.frame.WindowUpdateFrame;
import jdk.internal.net.http.hpack.Encoder;
import jdk.internal.net.http.hpack.Decoder;
import jdk.internal.net.http.hpack.DecodingCallback;
import static java.nio.charset.StandardCharsets.UTF_8;
import static jdk.internal.net.http.frame.SettingsFrame.*;

An Http2Connection. Encapsulates the socket(channel) and any SSLEngine used
over it. Contains an HttpConnection which hides the SocketChannel SSL stuff.
Http2Connections belong to a Http2ClientImpl, (one of) which belongs
to a HttpClientImpl.
Creation cases:
1) upgraded HTTP/1.1 plain tcp connection
2) prior knowledge directly created plain tcp connection
3) directly created HTTP/2 SSL connection which uses ALPN.
Sending is done by writing directly to underlying HttpConnection object which
is operating in async mode. No flow control applies on output at this level
and all writes are just executed as puts to an output Q belonging to HttpConnection
Flow control is implemented by HTTP/2 protocol itself.
Hpack header compression
and outgoing stream creation is also done here, because these operations
must be synchronized at the socket level. Stream objects send frames simply
by placing them on the connection's output Queue. sendFrame() is called
from a higher level (Stream) thread.
asyncReceive(ByteBuffer) is always called from the selector thread. It assembles
incoming Http2Frames, and directs them to the appropriate Stream.incoming()
or handles them directly itself. This thread performs hpack decompression
and incoming stream creation (Server push). Incoming frames destined for a
stream are provided by calling Stream.incoming().
/**
 * An Http2Connection. Encapsulates the socket(channel) and any SSLEngine used
 * over it. Contains an HttpConnection which hides the SocketChannel SSL stuff.
 *
 * Http2Connections belong to a Http2ClientImpl, (one of) which belongs
 * to a HttpClientImpl.
 *
 * Creation cases:
 * 1) upgraded HTTP/1.1 plain tcp connection
 * 2) prior knowledge directly created plain tcp connection
 * 3) directly created HTTP/2 SSL connection which uses ALPN.
 *
 * Sending is done by writing directly to underlying HttpConnection object which
 * is operating in async mode. No flow control applies on output at this level
 * and all writes are just executed as puts to an output Q belonging to HttpConnection
 * Flow control is implemented by HTTP/2 protocol itself.
 *
 * Hpack header compression
 * and outgoing stream creation is also done here, because these operations
 * must be synchronized at the socket level. Stream objects send frames simply
 * by placing them on the connection's output Queue. sendFrame() is called
 * from a higher level (Stream) thread.
 *
 * asyncReceive(ByteBuffer) is always called from the selector thread. It assembles
 * incoming Http2Frames, and directs them to the appropriate Stream.incoming()
 * or handles them directly itself. This thread performs hpack decompression
 * and incoming stream creation (Server push). Incoming frames destined for a
 * stream are provided by calling Stream.incoming().
 */
class Http2Connection  {

    final Logger debug = Utils.getDebugLogger(this::dbgString, Utils.DEBUG);
    final static Logger DEBUG_LOGGER =
            Utils.getDebugLogger("Http2Connection"::toString, Utils.DEBUG);
    private final Logger debugHpack =
            Utils.getHpackLogger(this::dbgString, Utils.DEBUG_HPACK);
    static final ByteBuffer EMPTY_TRIGGER = ByteBuffer.allocate(0);

    static private final int MAX_CLIENT_STREAM_ID = Integer.MAX_VALUE; // 2147483647
    static private final int MAX_SERVER_STREAM_ID = Integer.MAX_VALUE - 1; // 2147483646

    
Flag set when no more streams to be opened on this connection.
Two cases where it is used.
1. Two connections to the same server were opened concurrently, in which
   case one of them will be put in the cache, and the second will expire
   when all its opened streams (which usually should be a single client
   stream + possibly some additional push-promise server streams) complete.
2. A cached connection reaches its maximum number of streams (~ 2^31-1)
   either server / or client allocated, in which case it will be taken
   out of the cache - allowing a new connection to replace it. It will
   expire when all its still open streams (which could be many) eventually
   complete.
/**
     * Flag set when no more streams to be opened on this connection.
     * Two cases where it is used.
     *
     * 1. Two connections to the same server were opened concurrently, in which
     *    case one of them will be put in the cache, and the second will expire
     *    when all its opened streams (which usually should be a single client
     *    stream + possibly some additional push-promise server streams) complete.
     * 2. A cached connection reaches its maximum number of streams (~ 2^31-1)
     *    either server / or client allocated, in which case it will be taken
     *    out of the cache - allowing a new connection to replace it. It will
     *    expire when all its still open streams (which could be many) eventually
     *    complete.
     */
    private boolean finalStream;

    /*
     * ByteBuffer pooling strategy for HTTP/2 protocol.
     *
     * In general there are 4 points where ByteBuffers are used:
     *  - incoming/outgoing frames from/to ByteBuffers plus incoming/outgoing
     *    encrypted data in case of SSL connection.
     *
     * 1. Outgoing frames encoded to ByteBuffers.
     *
     *  Outgoing ByteBuffers are created with required size and frequently
     *  small (except DataFrames, etc). At this place no pools at all. All
     *  outgoing buffers should eventually be collected by GC.
     *
     * 2. Incoming ByteBuffers (decoded to frames).
     *
     *  Here, total elimination of BB pool is not a good idea.
     *  We don't know how many bytes we will receive through network.
     *
     *  A possible future improvement ( currently not implemented ):
     *  Allocate buffers of reasonable size. The following life of the BB:
     *   - If all frames decoded from the BB are other than DataFrame and
     *     HeaderFrame (and HeaderFrame subclasses) BB is returned to pool,
     *   - If a DataFrame is decoded from the BB. In that case DataFrame refers
     *     to sub-buffer obtained by slice(). Such a BB is never returned to the
     *     pool and will eventually be GC'ed.
     *   - If a HeadersFrame is decoded from the BB. Then header decoding is
     *     performed inside processFrame method and the buffer could be release
     *     back to pool.
     *
     * 3. SSL encrypted buffers ( received ).
     *
     *  The current implementation recycles encrypted buffers read from the
     *  channel. The pool of buffers has a maximum size of 3, SocketTube.MAX_BUFFERS,
     *  direct buffers which are shared by all connections on a given client.
     *  The pool is used by all SSL connections - whether HTTP/1.1 or HTTP/2,
     *  but only for SSL encrypted buffers that circulate between the SocketTube
     *  Publisher and the SSLFlowDelegate Reader. Limiting the pool to this
     *  particular segment allows the use of direct buffers, thus avoiding any
     *  additional copy in the NIO socket channel implementation. See
     *  HttpClientImpl.SSLDirectBufferSupplier, SocketTube.SSLDirectBufferSource,
     *  and SSLTube.recycler.
     */


    // A small class that allows to control frames with respect to the state of
    // the connection preface. Any data received before the connection
    // preface is sent will be buffered.
    private final class FramesController {
        volatile boolean prefaceSent;
        volatile List<ByteBuffer> pending;

        boolean processReceivedData(FramesDecoder decoder, ByteBuffer buf)
                throws IOException
        {
            // if preface is not sent, buffers data in the pending list
            if (!prefaceSent) {
                if (debug.on())
                    debug.log("Preface not sent: buffering %d", buf.remaining());
                synchronized (this) {
                    if (!prefaceSent) {
                        if (pending == null) pending = new ArrayList<>();
                        pending.add(buf);
                        if (debug.on())
                            debug.log("there are now %d bytes buffered waiting for preface to be sent"
                                    + Utils.remaining(pending)
                            );
                        return false;
                    }
                }
            }

            // Preface is sent. Checks for pending data and flush it.
            // We rely on this method being called from within the Http2TubeSubscriber
            // scheduler, so we know that no other thread could execute this method
            // concurrently while we're here.
            // This ensures that later incoming buffers will not
            // be processed before we have flushed the pending queue.
            // No additional synchronization is therefore necessary here.
            List<ByteBuffer> pending = this.pending;
            this.pending = null;
            if (pending != null) {
                // flush pending data
                if (debug.on()) debug.log(() -> "Processing buffered data: "
                      + Utils.remaining(pending));
                for (ByteBuffer b : pending) {
                    decoder.decode(b);
                }
            }
            // push the received buffer to the frames decoder.
            if (buf != EMPTY_TRIGGER) {
                if (debug.on()) debug.log("Processing %d", buf.remaining());
                decoder.decode(buf);
            }
            return true;
        }

        // Mark that the connection preface is sent
        void markPrefaceSent() {
            assert !prefaceSent;
            synchronized (this) {
                prefaceSent = true;
            }
        }
    }

    volatile boolean closed;

    //-------------------------------------
    final HttpConnection connection;
    private final Http2ClientImpl client2;
    private final Map<Integer,Stream<?>> streams = new ConcurrentHashMap<>();
    private int nextstreamid;
    private int nextPushStream = 2;
    // actual stream ids are not allocated until the Headers frame is ready
    // to be sent. The following two fields are updated as soon as a stream
    // is created and assigned to a connection. They are checked before
    // assigning a stream to a connection.
    private int lastReservedClientStreamid = 1;
    private int lastReservedServerStreamid = 0;
    private int numReservedClientStreams = 0; // count of current streams
    private int numReservedServerStreams = 0; // count of current streams
    private final Encoder hpackOut;
    private final Decoder hpackIn;
    final SettingsFrame clientSettings;
    private volatile SettingsFrame serverSettings;
    private final String key; // for HttpClientImpl.connections map
    private final FramesDecoder framesDecoder;
    private final FramesEncoder framesEncoder = new FramesEncoder();

    
Send Window controller for both connection and stream windows.
Each of this connection's Streams MUST use this controller.
/**
     * Send Window controller for both connection and stream windows.
     * Each of this connection's Streams MUST use this controller.
     */
    private final WindowController windowController = new WindowController();
    private final FramesController framesController = new FramesController();
    private final Http2TubeSubscriber subscriber;
    final ConnectionWindowUpdateSender windowUpdater;
    private volatile Throwable cause;
    private volatile Supplier<ByteBuffer> initial;

    static final int DEFAULT_FRAME_SIZE = 16 * 1024;


    // TODO: need list of control frames from other threads
    // that need to be sent

    private Http2Connection(HttpConnection connection,
                            Http2ClientImpl client2,
                            int nextstreamid,
                            String key) {
        this.connection = connection;
        this.client2 = client2;
        this.subscriber = new Http2TubeSubscriber(client2.client());
        this.nextstreamid = nextstreamid;
        this.key = key;
        this.clientSettings = this.client2.getClientSettings();
        this.framesDecoder = new FramesDecoder(this::processFrame,
                clientSettings.getParameter(SettingsFrame.MAX_FRAME_SIZE));
        // serverSettings will be updated by server
        this.serverSettings = SettingsFrame.defaultRFCSettings();
        this.hpackOut = new Encoder(serverSettings.getParameter(HEADER_TABLE_SIZE));
        this.hpackIn = new Decoder(clientSettings.getParameter(HEADER_TABLE_SIZE));
        if (debugHpack.on()) {
            debugHpack.log("For the record:" + super.toString());
            debugHpack.log("Decoder created: %s", hpackIn);
            debugHpack.log("Encoder created: %s", hpackOut);
        }
        this.windowUpdater = new ConnectionWindowUpdateSender(this,
                client2.getConnectionWindowSize(clientSettings));
    }

    
Case 1) Create from upgraded HTTP/1.1 connection.
Is ready to use. Can't be SSL. exchange is the Exchange
that initiated the connection, whose response will be delivered
on a Stream.
/**
     * Case 1) Create from upgraded HTTP/1.1 connection.
     * Is ready to use. Can't be SSL. exchange is the Exchange
     * that initiated the connection, whose response will be delivered
     * on a Stream.
     */
    private Http2Connection(HttpConnection connection,
                    Http2ClientImpl client2,
                    Exchange<?> exchange,
                    Supplier<ByteBuffer> initial)
        throws IOException, InterruptedException
    {
        this(connection,
                client2,
                3, // stream 1 is registered during the upgrade
                keyFor(connection));
        reserveStream(true);
        Log.logTrace("Connection send window size {0} ", windowController.connectionWindowSize());

        Stream<?> initialStream = createStream(exchange);
        initialStream.registerStream(1);
        windowController.registerStream(1, getInitialSendWindowSize());
        initialStream.requestSent();
        // Upgrading:
        //    set callbacks before sending preface - makes sure anything that
        //    might be sent by the server will come our way.
        this.initial = initial;
        connectFlows(connection);
        sendConnectionPreface();
    }

    // Used when upgrading an HTTP/1.1 connection to HTTP/2 after receiving
    // agreement from the server. Async style but completes immediately, because
    // the connection is already connected.
    static CompletableFuture<Http2Connection> createAsync(HttpConnection connection,
                                                          Http2ClientImpl client2,
                                                          Exchange<?> exchange,
                                                          Supplier<ByteBuffer> initial)
    {
        return MinimalFuture.supply(() -> new Http2Connection(connection, client2, exchange, initial));
    }

    // Requires TLS handshake. So, is really async
    static CompletableFuture<Http2Connection> createAsync(HttpRequestImpl request,
                                                          Http2ClientImpl h2client,
                                                          Exchange<?> exchange) {
        assert request.secure();
        AbstractAsyncSSLConnection connection = (AbstractAsyncSSLConnection)
        HttpConnection.getConnection(request.getAddress(),
                                     h2client.client(),
                                     request,
                                     HttpClient.Version.HTTP_2);

        // Expose the underlying connection to the exchange's aborter so it can
        // be closed if a timeout occurs.
        exchange.connectionAborter.connection(connection);

        return connection.connectAsync(exchange)
                  .thenCompose(unused -> connection.finishConnect())
                  .thenCompose(unused -> checkSSLConfig(connection))
                  .thenCompose(notused-> {
                      CompletableFuture<Http2Connection> cf = new MinimalFuture<>();
                      try {
                          Http2Connection hc = new Http2Connection(request, h2client, connection);
                          cf.complete(hc);
                      } catch (IOException e) {
                          cf.completeExceptionally(e);
                      }
                      return cf; } );
    }

    
Cases 2) 3)
request is request to be sent.
/**
     * Cases 2) 3)
     *
     * request is request to be sent.
     */
    private Http2Connection(HttpRequestImpl request,
                            Http2ClientImpl h2client,
                            HttpConnection connection)
        throws IOException
    {
        this(connection,
             h2client,
             1,
             keyFor(request.uri(), request.proxy()));

        Log.logTrace("Connection send window size {0} ", windowController.connectionWindowSize());

        // safe to resume async reading now.
        connectFlows(connection);
        sendConnectionPreface();
    }

    private void connectFlows(HttpConnection connection) {
        FlowTube tube =  connection.getConnectionFlow();
        // Connect the flow to our Http2TubeSubscriber:
        tube.connectFlows(connection.publisher(), subscriber);
    }

    final HttpClientImpl client() {
        return client2.client();
    }

    // call these before assigning a request/stream to a connection
    // if false returned then a new Http2Connection is required
    // if true, the the stream may be assigned to this connection
    // for server push, if false returned, then the stream should be cancelled
    synchronized boolean reserveStream(boolean clientInitiated) throws IOException {
        if (finalStream) {
            return false;
        }
        if (clientInitiated && (lastReservedClientStreamid + 2) >= MAX_CLIENT_STREAM_ID) {
            setFinalStream();
            client2.deleteConnection(this);
            return false;
        } else if (!clientInitiated && (lastReservedServerStreamid + 2) >= MAX_SERVER_STREAM_ID) {
            setFinalStream();
            client2.deleteConnection(this);
            return false;
        }
        if (clientInitiated)
            lastReservedClientStreamid+=2;
        else
            lastReservedServerStreamid+=2;

        assert numReservedClientStreams >= 0;
        assert numReservedServerStreams >= 0;
        if (clientInitiated &&numReservedClientStreams >= maxConcurrentClientInitiatedStreams()) {
            throw new IOException("too many concurrent streams");
        } else if (clientInitiated) {
            numReservedClientStreams++;
        }
        if (!clientInitiated && numReservedServerStreams >= maxConcurrentServerInitiatedStreams()) {
            return false;
        } else if (!clientInitiated) {
            numReservedServerStreams++;
        }
        return true;
    }

    
Throws an IOException if h2 was not negotiated
/**
     * Throws an IOException if h2 was not negotiated
     */
    private static CompletableFuture<?> checkSSLConfig(AbstractAsyncSSLConnection aconn) {
        assert aconn.isSecure();

        Function<String, CompletableFuture<Void>> checkAlpnCF = (alpn) -> {
            CompletableFuture<Void> cf = new MinimalFuture<>();
            SSLEngine engine = aconn.getEngine();
            assert Objects.equals(alpn, engine.getApplicationProtocol());

            DEBUG_LOGGER.log("checkSSLConfig: alpn: %s", alpn );

            if (alpn == null || !alpn.equals("h2")) {
                String msg;
                if (alpn == null) {
                    Log.logSSL("ALPN not supported");
                    msg = "ALPN not supported";
                } else {
                    switch (alpn) {
                        case "":
                            Log.logSSL(msg = "No ALPN negotiated");
                            break;
                        case "http/1.1":
                            Log.logSSL( msg = "HTTP/1.1 ALPN returned");
                            break;
                        default:
                            Log.logSSL(msg = "Unexpected ALPN: " + alpn);
                            cf.completeExceptionally(new IOException(msg));
                    }
                }
                cf.completeExceptionally(new ALPNException(msg, aconn));
                return cf;
            }
            cf.complete(null);
            return cf;
        };

        return aconn.getALPN()
                .whenComplete((r,t) -> {
                    if (t != null && t instanceof SSLException) {
                        // something went wrong during the initial handshake
                        // close the connection
                        aconn.close();
                    }
                })
                .thenCompose(checkAlpnCF);
    }

    synchronized boolean finalStream() {
        return finalStream;
    }

    
Mark this connection so no more streams created on it and it will close when
all are complete.
/**
     * Mark this connection so no more streams created on it and it will close when
     * all are complete.
     */
    synchronized void setFinalStream() {
        finalStream = true;
    }

    static String keyFor(HttpConnection connection) {
        boolean isProxy = connection.isProxied(); // tunnel or plain clear connection through proxy
        boolean isSecure = connection.isSecure();
        InetSocketAddress addr = connection.address();

        return keyString(isSecure, isProxy, addr.getHostString(), addr.getPort());
    }

    static String keyFor(URI uri, InetSocketAddress proxy) {
        boolean isSecure = uri.getScheme().equalsIgnoreCase("https");
        boolean isProxy = proxy != null;

        String host;
        int port;

        if (proxy != null && !isSecure) {
            // clear connection through proxy: use
            // proxy host / proxy port
            host = proxy.getHostString();
            port = proxy.getPort();
        } else {
            // either secure tunnel connection through proxy
            // or direct connection to host, but in either
            // case only that host can be reached through
            // the connection: use target host / target port
            host = uri.getHost();
            port = uri.getPort();
        }
        return keyString(isSecure, isProxy, host, port);
    }

    // {C,S}:{H:P}:host:port
    // C indicates clear text connection "http"
    // S indicates secure "https"
    // H indicates host (direct) connection
    // P indicates proxy
    // Eg: "S:H:foo.com:80"
    static String keyString(boolean secure, boolean proxy, String host, int port) {
        if (secure && port == -1)
            port = 443;
        else if (!secure && port == -1)
            port = 80;
        return (secure ? "S:" : "C:") + (proxy ? "P:" : "H:") + host + ":" + port;
    }

    String key() {
        return this.key;
    }

    boolean offerConnection() {
        return client2.offerConnection(this);
    }

    private HttpPublisher publisher() {
        return connection.publisher();
    }

    private void decodeHeaders(HeaderFrame frame, DecodingCallback decoder)
            throws IOException
    {
        if (debugHpack.on()) debugHpack.log("decodeHeaders(%s)", decoder);

        boolean endOfHeaders = frame.getFlag(HeaderFrame.END_HEADERS);

        List<ByteBuffer> buffers = frame.getHeaderBlock();
        int len = buffers.size();
        for (int i = 0; i < len; i++) {
            ByteBuffer b = buffers.get(i);
            hpackIn.decode(b, endOfHeaders && (i == len - 1), decoder);
        }
    }

    final int getInitialSendWindowSize() {
        return serverSettings.getParameter(INITIAL_WINDOW_SIZE);
    }

    final int maxConcurrentClientInitiatedStreams() {
        return serverSettings.getParameter(MAX_CONCURRENT_STREAMS);
    }

    final int maxConcurrentServerInitiatedStreams() {
        return clientSettings.getParameter(MAX_CONCURRENT_STREAMS);
    }

    void close() {
        Log.logTrace("Closing HTTP/2 connection: to {0}", connection.address());
        GoAwayFrame f = new GoAwayFrame(0,
                                        ErrorFrame.NO_ERROR,
                                        "Requested by user".getBytes(UTF_8));
        // TODO: set last stream. For now zero ok.
        sendFrame(f);
    }

    long count;
    final void asyncReceive(ByteBuffer buffer) {
        // We don't need to read anything and
        // we don't want to send anything back to the server
        // until the connection preface has been sent.
        // Therefore we're going to wait if needed before reading
        // (and thus replying) to anything.
        // Starting to reply to something (e.g send an ACK to a
        // SettingsFrame sent by the server) before the connection
        // preface is fully sent might result in the server
        // sending a GOAWAY frame with 'invalid_preface'.
        //
        // Note: asyncReceive is only called from the Http2TubeSubscriber
        //       sequential scheduler.
        try {
            Supplier<ByteBuffer> bs = initial;
            // ensure that we always handle the initial buffer first,
            // if any.
            if (bs != null) {
                initial = null;
                ByteBuffer b = bs.get();
                if (b.hasRemaining()) {
                    long c = ++count;
                    if (debug.on())
                        debug.log(() -> "H2 Receiving Initial(" + c +"): " + b.remaining());
                    framesController.processReceivedData(framesDecoder, b);
                }
            }
            ByteBuffer b = buffer;
            // the Http2TubeSubscriber scheduler ensures that the order of incoming
            // buffers is preserved.
            if (b == EMPTY_TRIGGER) {
                if (debug.on()) debug.log("H2 Received EMPTY_TRIGGER");
                boolean prefaceSent = framesController.prefaceSent;
                assert prefaceSent;
                // call framesController.processReceivedData to potentially
                // trigger the processing of all the data buffered there.
                framesController.processReceivedData(framesDecoder, buffer);
                if (debug.on()) debug.log("H2 processed buffered data");
            } else {
                long c = ++count;
                if (debug.on())
                    debug.log("H2 Receiving(%d): %d", c, b.remaining());
                framesController.processReceivedData(framesDecoder, buffer);
                if (debug.on()) debug.log("H2 processed(%d)", c);
            }
        } catch (Throwable e) {
            String msg = Utils.stackTrace(e);
            Log.logTrace(msg);
            shutdown(e);
        }
    }

    Throwable getRecordedCause() {
        return cause;
    }

    void shutdown(Throwable t) {
        if (debug.on()) debug.log(() -> "Shutting down h2c (closed="+closed+"): " + t);
        if (closed == true) return;
        synchronized (this) {
            if (closed == true) return;
            closed = true;
        }
        if (Log.errors()) {
            if (!(t instanceof EOFException) || isActive()) {
                Log.logError(t);
            } else if (t != null) {
                Log.logError("Shutting down connection: {0}", t.getMessage());
            }
        }
        Throwable initialCause = this.cause;
        if (initialCause == null) this.cause = t;
        client2.deleteConnection(this);
        List<Stream<?>> c = new LinkedList<>(streams.values());
        for (Stream<?> s : c) {
            try {
                s.connectionClosing(t);
            } catch (Throwable e) {
                Log.logError("Failed to close stream {0}: {1}", s.streamid, e);
            }
        }
        connection.close();
    }

    
Streams initiated by a client MUST use odd-numbered stream
identifiers; those initiated by the server MUST use even-numbered
stream identifiers.
/**
     * Streams initiated by a client MUST use odd-numbered stream
     * identifiers; those initiated by the server MUST use even-numbered
     * stream identifiers.
     */
    private static final boolean isServerInitiatedStream(int streamid) {
        return (streamid & 0x1) == 0;
    }

    
Handles stream 0 (common) frames that apply to whole connection and passes
other stream specific frames to that Stream object.
Invokes Stream.incoming() which is expected to process frame without
blocking.
/**
     * Handles stream 0 (common) frames that apply to whole connection and passes
     * other stream specific frames to that Stream object.
     *
     * Invokes Stream.incoming() which is expected to process frame without
     * blocking.
     */
    void processFrame(Http2Frame frame) throws IOException {
        Log.logFrames(frame, "IN");
        int streamid = frame.streamid();
        if (frame instanceof MalformedFrame) {
            Log.logError(((MalformedFrame) frame).getMessage());
            if (streamid == 0) {
                framesDecoder.close("Malformed frame on stream 0");
                protocolError(((MalformedFrame) frame).getErrorCode(),
                        ((MalformedFrame) frame).getMessage());
            } else {
                if (debug.on())
                    debug.log(() -> "Reset stream: " + ((MalformedFrame) frame).getMessage());
                resetStream(streamid, ((MalformedFrame) frame).getErrorCode());
            }
            return;
        }
        if (streamid == 0) {
            handleConnectionFrame(frame);
        } else {
            if (frame instanceof SettingsFrame) {
                // The stream identifier for a SETTINGS frame MUST be zero
                framesDecoder.close(
                        "The stream identifier for a SETTINGS frame MUST be zero");
                protocolError(GoAwayFrame.PROTOCOL_ERROR);
                return;
            }

            Stream<?> stream = getStream(streamid);
            if (stream == null) {
                // Should never receive a frame with unknown stream id

                if (frame instanceof HeaderFrame) {
                    // always decode the headers as they may affect
                    // connection-level HPACK decoding state
                    DecodingCallback decoder = new ValidatingHeadersConsumer();
                    try {
                        decodeHeaders((HeaderFrame) frame, decoder);
                    } catch (UncheckedIOException e) {
                        protocolError(ResetFrame.PROTOCOL_ERROR, e.getMessage());
                        return;
                    }
                }

                if (!(frame instanceof ResetFrame)) {
                    if (frame instanceof DataFrame) {
                        dropDataFrame((DataFrame)frame);
                    }
                    if (isServerInitiatedStream(streamid)) {
                        if (streamid < nextPushStream) {
                            // trailing data on a cancelled push promise stream,
                            // reset will already have been sent, ignore
                            Log.logTrace("Ignoring cancelled push promise frame " + frame);
                        } else {
                            resetStream(streamid, ResetFrame.PROTOCOL_ERROR);
                        }
                    } else if (streamid >= nextstreamid) {
                        // otherwise the stream has already been reset/closed
                        resetStream(streamid, ResetFrame.PROTOCOL_ERROR);
                    }
                }
                return;
            }
            if (frame instanceof PushPromiseFrame) {
                PushPromiseFrame pp = (PushPromiseFrame)frame;
                try {
                    handlePushPromise(stream, pp);
                } catch (UncheckedIOException e) {
                    protocolError(ResetFrame.PROTOCOL_ERROR, e.getMessage());
                    return;
                }
            } else if (frame instanceof HeaderFrame) {
                // decode headers (or continuation)
                try {
                    decodeHeaders((HeaderFrame) frame, stream.rspHeadersConsumer());
                } catch (UncheckedIOException e) {
                    protocolError(ResetFrame.PROTOCOL_ERROR, e.getMessage());
                    return;
                }
                stream.incoming(frame);
            } else {
                stream.incoming(frame);
            }
        }
    }

    final void dropDataFrame(DataFrame df) {
        if (closed) return;
        if (debug.on()) {
            debug.log("Dropping data frame for stream %d (%d payload bytes)",
                    df.streamid(), df.payloadLength());
        }
        ensureWindowUpdated(df);
    }

    final void ensureWindowUpdated(DataFrame df) {
        try {
            if (closed) return;
            int length = df.payloadLength();
            if (length > 0) {
                windowUpdater.update(length);
            }
        } catch(Throwable t) {
            Log.logError("Unexpected exception while updating window: {0}", (Object)t);
        }
    }

    private <T> void handlePushPromise(Stream<T> parent, PushPromiseFrame pp)
        throws IOException
    {
        // always decode the headers as they may affect connection-level HPACK
        // decoding state
        HeaderDecoder decoder = new HeaderDecoder();
        decodeHeaders(pp, decoder);

        HttpRequestImpl parentReq = parent.request;
        int promisedStreamid = pp.getPromisedStream();
        if (promisedStreamid != nextPushStream) {
            resetStream(promisedStreamid, ResetFrame.PROTOCOL_ERROR);
            return;
        } else if (!reserveStream(false)) {
            resetStream(promisedStreamid, ResetFrame.REFUSED_STREAM);
            return;
        } else {
            nextPushStream += 2;
        }

        HttpHeaders headers = decoder.headers();
        HttpRequestImpl pushReq = HttpRequestImpl.createPushRequest(parentReq, headers);
        Exchange<T> pushExch = new Exchange<>(pushReq, parent.exchange.multi);
        Stream.PushedStream<T> pushStream = createPushStream(parent, pushExch);
        pushExch.exchImpl = pushStream;
        pushStream.registerStream(promisedStreamid);
        parent.incoming_pushPromise(pushReq, pushStream);
    }

    private void handleConnectionFrame(Http2Frame frame)
        throws IOException
    {
        switch (frame.type()) {
          case SettingsFrame.TYPE:
              handleSettings((SettingsFrame)frame);
              break;
          case PingFrame.TYPE:
              handlePing((PingFrame)frame);
              break;
          case GoAwayFrame.TYPE:
              handleGoAway((GoAwayFrame)frame);
              break;
          case WindowUpdateFrame.TYPE:
              handleWindowUpdate((WindowUpdateFrame)frame);
              break;
          default:
            protocolError(ErrorFrame.PROTOCOL_ERROR);
        }
    }

    void resetStream(int streamid, int code) throws IOException {
        try {
            if (connection.channel().isOpen()) {
                // no need to try & send a reset frame if the
                // connection channel is already closed.
                Log.logError(
                        "Resetting stream {0,number,integer} with error code {1,number,integer}",
                        streamid, code);
                ResetFrame frame = new ResetFrame(streamid, code);
                sendFrame(frame);
            } else if (debug.on()) {
                debug.log("Channel already closed, no need to reset stream %d",
                          streamid);
            }
        } finally {
            decrementStreamsCount(streamid);
            closeStream(streamid);
        }
    }

    // reduce count of streams by 1 if stream still exists
    synchronized void decrementStreamsCount(int streamid) {
        Stream<?> s = streams.get(streamid);
        if (s == null || !s.deRegister())
            return;
        if (streamid % 2 == 1) {
            numReservedClientStreams--;
            assert numReservedClientStreams >= 0 :
                    "negative client stream count for stream=" + streamid;
        } else {
            numReservedServerStreams--;
            assert numReservedServerStreams >= 0 :
                    "negative server stream count for stream=" + streamid;
        }
    }

    void closeStream(int streamid) {
        if (debug.on()) debug.log("Closed stream %d", streamid);
        boolean isClient = (streamid % 2) == 1;
        Stream<?> s = streams.remove(streamid);
        if (s != null) {
            // decrement the reference count on the HttpClientImpl
            // to allow the SelectorManager thread to exit if no
            // other operation is pending and the facade is no
            // longer referenced.
            client().streamUnreference();
        }
        // ## Remove s != null. It is a hack for delayed cancellation,reset
        if (s != null && !(s instanceof Stream.PushedStream)) {
            // Since PushStreams have no request body, then they have no
            // corresponding entry in the window controller.
            windowController.removeStream(streamid);
        }
        if (finalStream() && streams.isEmpty()) {
            // should be only 1 stream, but there might be more if server push
            close();
        }
    }

    
Increments this connection's send Window by the amount in the given frame.
/**
     * Increments this connection's send Window by the amount in the given frame.
     */
    private void handleWindowUpdate(WindowUpdateFrame f)
        throws IOException
    {
        int amount = f.getUpdate();
        if (amount <= 0) {
            // ## temporarily disable to workaround a bug in Jetty where it
            // ## sends Window updates with a 0 update value.
            //protocolError(ErrorFrame.PROTOCOL_ERROR);
        } else {
            boolean success = windowController.increaseConnectionWindow(amount);
            if (!success) {
                protocolError(ErrorFrame.FLOW_CONTROL_ERROR);  // overflow
            }
        }
    }

    private void protocolError(int errorCode)
        throws IOException
    {
        protocolError(errorCode, null);
    }

    private void protocolError(int errorCode, String msg)
        throws IOException
    {
        GoAwayFrame frame = new GoAwayFrame(0, errorCode);
        sendFrame(frame);
        shutdown(new IOException("protocol error" + (msg == null?"":(": " + msg))));
    }

    private void handleSettings(SettingsFrame frame)
        throws IOException
    {
        assert frame.streamid() == 0;
        if (!frame.getFlag(SettingsFrame.ACK)) {
            int newWindowSize = frame.getParameter(INITIAL_WINDOW_SIZE);
            if (newWindowSize != -1) {
                int oldWindowSize = serverSettings.getParameter(INITIAL_WINDOW_SIZE);
                int diff = newWindowSize - oldWindowSize;
                if (diff != 0) {
                    windowController.adjustActiveStreams(diff);
                }
            }

            serverSettings.update(frame);
            sendFrame(new SettingsFrame(SettingsFrame.ACK));
        }
    }

    private void handlePing(PingFrame frame)
        throws IOException
    {
        frame.setFlag(PingFrame.ACK);
        sendUnorderedFrame(frame);
    }

    private void handleGoAway(GoAwayFrame frame)
        throws IOException
    {
        shutdown(new IOException(
                        String.valueOf(connection.channel().getLocalAddress())
                        +": GOAWAY received"));
    }

    
Max frame size we are allowed to send
/**
     * Max frame size we are allowed to send
     */
    public int getMaxSendFrameSize() {
        int param = serverSettings.getParameter(MAX_FRAME_SIZE);
        if (param == -1) {
            param = DEFAULT_FRAME_SIZE;
        }
        return param;
    }

    
Max frame size we will receive
/**
     * Max frame size we will receive
     */
    public int getMaxReceiveFrameSize() {
        return clientSettings.getParameter(MAX_FRAME_SIZE);
    }

    private static final String CLIENT_PREFACE = "PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n";

    private static final byte[] PREFACE_BYTES =
        CLIENT_PREFACE.getBytes(StandardCharsets.ISO_8859_1);

    
Sends Connection preface and Settings frame with current preferred
values
/**
     * Sends Connection preface and Settings frame with current preferred
     * values
     */
    private void sendConnectionPreface() throws IOException {
        Log.logTrace("{0}: start sending connection preface to {1}",
                     connection.channel().getLocalAddress(),
                     connection.address());
        SettingsFrame sf = new SettingsFrame(clientSettings);
        ByteBuffer buf = framesEncoder.encodeConnectionPreface(PREFACE_BYTES, sf);
        Log.logFrames(sf, "OUT");
        // send preface bytes and SettingsFrame together
        HttpPublisher publisher = publisher();
        publisher.enqueueUnordered(List.of(buf));
        publisher.signalEnqueued();
        // mark preface sent.
        framesController.markPrefaceSent();
        Log.logTrace("PREFACE_BYTES sent");
        Log.logTrace("Settings Frame sent");

        // send a Window update for the receive buffer we are using
        // minus the initial 64 K -1 specified in protocol:
        // RFC 7540, Section 6.9.2:
        // "[...] the connection flow-control window is set to the default
        // initial window size until a WINDOW_UPDATE frame is received."
        //
        // Note that the default initial window size, not to be confused
        // with the initial window size, is defined by RFC 7540 as
        // 64K -1.
        final int len = windowUpdater.initialWindowSize - DEFAULT_INITIAL_WINDOW_SIZE;
        if (len != 0) {
            if (Log.channel()) {
                Log.logChannel("Sending initial connection window update frame: {0} ({1} - {2})",
                        len, windowUpdater.initialWindowSize, DEFAULT_INITIAL_WINDOW_SIZE);
            }
            windowUpdater.sendWindowUpdate(len);
        }
        // there will be an ACK to the windows update - which should
        // cause any pending data stored before the preface was sent to be
        // flushed (see PrefaceController).
        Log.logTrace("finished sending connection preface");
        if (debug.on())
            debug.log("Triggering processing of buffered data"
                      + " after sending connection preface");
        subscriber.onNext(List.of(EMPTY_TRIGGER));
    }

    
Returns an existing Stream with given id, or null if doesn't exist
/**
     * Returns an existing Stream with given id, or null if doesn't exist
     */
    @SuppressWarnings("unchecked")
    <T> Stream<T> getStream(int streamid) {
        return (Stream<T>)streams.get(streamid);
    }

    
Creates Stream with given id.
/**
     * Creates Stream with given id.
     */
    final <T> Stream<T> createStream(Exchange<T> exchange) {
        Stream<T> stream = new Stream<>(this, exchange, windowController);
        return stream;
    }

    <T> Stream.PushedStream<T> createPushStream(Stream<T> parent, Exchange<T> pushEx) {
        PushGroup<T> pg = parent.exchange.getPushGroup();
        return new Stream.PushedStream<>(pg, this, pushEx);
    }

    <T> void putStream(Stream<T> stream, int streamid) {
        // increment the reference count on the HttpClientImpl
        // to prevent the SelectorManager thread from exiting until
        // the stream is closed.
        client().streamReference();
        streams.put(streamid, stream);
    }

    
Encode the headers into a List and then create HEADERS
and CONTINUATION frames from the list and return the List.
/**
     * Encode the headers into a List<ByteBuffer> and then create HEADERS
     * and CONTINUATION frames from the list and return the List<Http2Frame>.
     */
    private List<HeaderFrame> encodeHeaders(OutgoingHeaders<Stream<?>> frame) {
        List<ByteBuffer> buffers = encodeHeadersImpl(
                getMaxSendFrameSize(),
                frame.getAttachment().getRequestPseudoHeaders(),
                frame.getUserHeaders(),
                frame.getSystemHeaders());

        List<HeaderFrame> frames = new ArrayList<>(buffers.size());
        Iterator<ByteBuffer> bufIterator = buffers.iterator();
        HeaderFrame oframe = new HeadersFrame(frame.streamid(), frame.getFlags(), bufIterator.next());
        frames.add(oframe);
        while(bufIterator.hasNext()) {
            oframe = new ContinuationFrame(frame.streamid(), bufIterator.next());
            frames.add(oframe);
        }
        oframe.setFlag(HeaderFrame.END_HEADERS);
        return frames;
    }

    // Dedicated cache for headers encoding ByteBuffer.
    // There can be no concurrent access to this  buffer as all access to this buffer
    // and its content happen within a single critical code block section protected
    // by the sendLock. / (see sendFrame())
    // private final ByteBufferPool headerEncodingPool = new ByteBufferPool();

    private ByteBuffer getHeaderBuffer(int maxFrameSize) {
        ByteBuffer buf = ByteBuffer.allocate(maxFrameSize);
        buf.limit(maxFrameSize);
        return buf;
    }

    /*
     * Encodes all the headers from the given HttpHeaders into the given List
     * of buffers.
     *
     * From https://tools.ietf.org/html/rfc7540#section-8.1.2 :
     *
     *     ...Just as in HTTP/1.x, header field names are strings of ASCII
     *     characters that are compared in a case-insensitive fashion.  However,
     *     header field names MUST be converted to lowercase prior to their
     *     encoding in HTTP/2...
     */
    private List<ByteBuffer> encodeHeadersImpl(int maxFrameSize, HttpHeaders... headers) {
        ByteBuffer buffer = getHeaderBuffer(maxFrameSize);
        List<ByteBuffer> buffers = new ArrayList<>();
        for(HttpHeaders header : headers) {
            for (Map.Entry<String, List<String>> e : header.map().entrySet()) {
                String lKey = e.getKey().toLowerCase(Locale.US);
                List<String> values = e.getValue();
                for (String value : values) {
                    hpackOut.header(lKey, value);
                    while (!hpackOut.encode(buffer)) {
                        buffer.flip();
                        buffers.add(buffer);
                        buffer =  getHeaderBuffer(maxFrameSize);
                    }
                }
            }
        }
        buffer.flip();
        buffers.add(buffer);
        return buffers;
    }

    private List<ByteBuffer> encodeHeaders(OutgoingHeaders<Stream<?>> oh, Stream<?> stream) {
        oh.streamid(stream.streamid);
        if (Log.headers()) {
            StringBuilder sb = new StringBuilder("HEADERS FRAME (stream=");
            sb.append(stream.streamid).append(")\n");
            Log.dumpHeaders(sb, "    ", oh.getAttachment().getRequestPseudoHeaders());
            Log.dumpHeaders(sb, "    ", oh.getSystemHeaders());
            Log.dumpHeaders(sb, "    ", oh.getUserHeaders());
            Log.logHeaders(sb.toString());
        }
        List<HeaderFrame> frames = encodeHeaders(oh);
        return encodeFrames(frames);
    }

    private List<ByteBuffer> encodeFrames(List<HeaderFrame> frames) {
        if (Log.frames()) {
            frames.forEach(f -> Log.logFrames(f, "OUT"));
        }
        return framesEncoder.encodeFrames(frames);
    }

    private Stream<?> registerNewStream(OutgoingHeaders<Stream<?>> oh) {
        Stream<?> stream = oh.getAttachment();
        assert stream.streamid == 0;
        int streamid = nextstreamid;
        nextstreamid += 2;
        stream.registerStream(streamid);
        // set outgoing window here. This allows thread sending
        // body to proceed.
        windowController.registerStream(streamid, getInitialSendWindowSize());
        return stream;
    }

    private final Object sendlock = new Object();

    void sendFrame(Http2Frame frame) {
        try {
            HttpPublisher publisher = publisher();
            synchronized (sendlock) {
                if (frame instanceof OutgoingHeaders) {
                    @SuppressWarnings("unchecked")
                    OutgoingHeaders<Stream<?>> oh = (OutgoingHeaders<Stream<?>>) frame;
                    Stream<?> stream = registerNewStream(oh);
                    // provide protection from inserting unordered frames between Headers and Continuation
                    publisher.enqueue(encodeHeaders(oh, stream));
                } else {
                    publisher.enqueue(encodeFrame(frame));
                }
            }
            publisher.signalEnqueued();
        } catch (IOException e) {
            if (!closed) {
                Log.logError(e);
                shutdown(e);
            }
        }
    }

    private List<ByteBuffer> encodeFrame(Http2Frame frame) {
        Log.logFrames(frame, "OUT");
        return framesEncoder.encodeFrame(frame);
    }

    void sendDataFrame(DataFrame frame) {
        try {
            HttpPublisher publisher = publisher();
            publisher.enqueue(encodeFrame(frame));
            publisher.signalEnqueued();
        } catch (IOException e) {
            if (!closed) {
                Log.logError(e);
                shutdown(e);
            }
        }
    }

    /*
     * Direct call of the method bypasses synchronization on "sendlock" and
     * allowed only of control frames: WindowUpdateFrame, PingFrame and etc.
     * prohibited for such frames as DataFrame, HeadersFrame, ContinuationFrame.
     */
    void sendUnorderedFrame(Http2Frame frame) {
        try {
            HttpPublisher publisher = publisher();
            publisher.enqueueUnordered(encodeFrame(frame));
            publisher.signalEnqueued();
        } catch (IOException e) {
            if (!closed) {
                Log.logError(e);
                shutdown(e);
            }
        }
    }

    
A simple tube subscriber for reading from the connection flow.
/**
     * A simple tube subscriber for reading from the connection flow.
     */
    final class Http2TubeSubscriber implements TubeSubscriber {
        private volatile Flow.Subscription subscription;
        private volatile boolean completed;
        private volatile boolean dropped;
        private volatile Throwable error;
        private final ConcurrentLinkedQueue<ByteBuffer> queue
                = new ConcurrentLinkedQueue<>();
        private final SequentialScheduler scheduler =
                SequentialScheduler.synchronizedScheduler(this::processQueue);
        private final HttpClientImpl client;

        Http2TubeSubscriber(HttpClientImpl client) {
            this.client = Objects.requireNonNull(client);
        }

        final void processQueue() {
            try {
                while (!queue.isEmpty() && !scheduler.isStopped()) {
                    ByteBuffer buffer = queue.poll();
                    if (debug.on())
                        debug.log("sending %d to Http2Connection.asyncReceive",
                                  buffer.remaining());
                    asyncReceive(buffer);
                }
            } catch (Throwable t) {
                Throwable x = error;
                if (x == null) error = t;
            } finally {
                Throwable x = error;
                if (x != null) {
                    if (debug.on()) debug.log("Stopping scheduler", x);
                    scheduler.stop();
                    Http2Connection.this.shutdown(x);
                }
            }
        }

        private final void runOrSchedule() {
            if (client.isSelectorThread()) {
                scheduler.runOrSchedule(client.theExecutor());
            } else scheduler.runOrSchedule();
        }

        @Override
        public void onSubscribe(Flow.Subscription subscription) {
            // supports being called multiple time.
            // doesn't cancel the previous subscription, since that is
            // most probably the same as the new subscription.
            assert this.subscription == null || dropped == false;
            this.subscription = subscription;
            dropped = false;
            // TODO FIXME: request(1) should be done by the delegate.
            if (!completed) {
                if (debug.on())
                    debug.log("onSubscribe: requesting Long.MAX_VALUE for reading");
                subscription.request(Long.MAX_VALUE);
            } else {
                if (debug.on()) debug.log("onSubscribe: already completed");
            }
        }

        @Override
        public void onNext(List<ByteBuffer> item) {
            if (debug.on()) debug.log(() -> "onNext: got " + Utils.remaining(item)
                    + " bytes in " + item.size() + " buffers");
            queue.addAll(item);
            runOrSchedule();
        }

        @Override
        public void onError(Throwable throwable) {
            if (debug.on()) debug.log(() -> "onError: " + throwable);
            error = throwable;
            completed = true;
            runOrSchedule();
        }

        @Override
        public void onComplete() {
            String msg = isActive()
                    ? "EOF reached while reading"
                    : "Idle connection closed by HTTP/2 peer";
            if (debug.on()) debug.log(msg);
            error = new EOFException(msg);
            completed = true;
            runOrSchedule();
        }

        @Override
        public void dropSubscription() {
            if (debug.on()) debug.log("dropSubscription");
            // we could probably set subscription to null here...
            // then we might not need the 'dropped' boolean?
            dropped = true;
        }
    }

    synchronized boolean isActive() {
        return numReservedClientStreams > 0 || numReservedServerStreams > 0;
    }

    @Override
    public final String toString() {
        return dbgString();
    }

    final String dbgString() {
        return "Http2Connection("
                    + connection.getConnectionFlow() + ")";
    }

    static class HeaderDecoder extends ValidatingHeadersConsumer {

        HttpHeadersBuilder headersBuilder;

        HeaderDecoder() {
            this.headersBuilder = new HttpHeadersBuilder();
        }

        @Override
        public void onDecoded(CharSequence name, CharSequence value) {
            String n = name.toString();
            String v = value.toString();
            super.onDecoded(n, v);
            headersBuilder.addHeader(n, v);
        }

        HttpHeaders headers() {
            return headersBuilder.build();
        }
    }

    /*
     * Checks RFC 7540 rules (relaxed) compliance regarding pseudo-headers.
     */
    static class ValidatingHeadersConsumer implements DecodingCallback {

        private static final Set<String> PSEUDO_HEADERS =
                Set.of(":authority", ":method", ":path", ":scheme", ":status");

        
Used to check that if there are pseudo-headers, they go first /** Used to check that if there are pseudo-headers, they go first */
        private boolean pseudoHeadersEnded;

        
Called when END_HEADERS was received. This consumer may be invoked
again after reset() is called, but for a whole new set of headers.
/**
         * Called when END_HEADERS was received. This consumer may be invoked
         * again after reset() is called, but for a whole new set of headers.
         */
        void reset() {
            pseudoHeadersEnded = false;
        }

        @Override
        public void onDecoded(CharSequence name, CharSequence value)
                throws UncheckedIOException
        {
            String n = name.toString();
            if (n.startsWith(":")) {
                if (pseudoHeadersEnded) {
                    throw newException("Unexpected pseudo-header '%s'", n);
                } else if (!PSEUDO_HEADERS.contains(n)) {
                    throw newException("Unknown pseudo-header '%s'", n);
                }
            } else {
                pseudoHeadersEnded = true;
                if (!Utils.isValidName(n)) {
                    throw newException("Bad header name '%s'", n);
                }
            }
            String v = value.toString();
            if (!Utils.isValidValue(v)) {
                throw newException("Bad header value '%s'", v);
            }
        }

        private UncheckedIOException newException(String message, String header)
        {
            return new UncheckedIOException(
                    new IOException(String.format(message, header)));
        }
    }

    static final class ConnectionWindowUpdateSender extends WindowUpdateSender {

        final int initialWindowSize;
        public ConnectionWindowUpdateSender(Http2Connection connection,
                                            int initialWindowSize) {
            super(connection, initialWindowSize);
            this.initialWindowSize = initialWindowSize;
        }

        @Override
        int getStreamId() {
            return 0;
        }
    }

    
Thrown when https handshake negotiates http/1.1 alpn instead of h2
/**
     * Thrown when https handshake negotiates http/1.1 alpn instead of h2
     */
    static final class ALPNException extends IOException {
        private static final long serialVersionUID = 0L;
        final transient AbstractAsyncSSLConnection connection;

        ALPNException(String msg, AbstractAsyncSSLConnection connection) {
            super(msg);
            this.connection = connection;
        }

        AbstractAsyncSSLConnection getConnection() {
            return connection;
        }
    }
}