netty: HTTP/2 Prior Knowledge does not appear to work with Http2Codec and Http2ServerDowngrader

Expected behavior

The CleartextHttp2ServerUpgradeHandler should work with both HTTP Upgrade and Prior Knowledge when downgrading HTTP/2 requests internally.

Actual behavior

HTTP Upgrade works, but not Prior Knowledge. In the latter case, nghttp reports: (last_stream_id=0, error_code=PROTOCOL_ERROR(0x01), opaque_data(96)=[HTTP/2 client preface string missing or corrupt. Hex dump for received bytes: 000000040100000000])

Steps to reproduce

1. Fire up io.netty.example.http2.helloworld.server.Http2Server with the modifications described in the next section
2. Install nghttp
3. Run nghttp --upgrade --verbose --no-dep http://localhost:8080/; it should succeed
4. Run nghttp --verbose --no-dep http://localhost:8080/; it fails like so:

[  0.002] Connected
[  0.002] send SETTINGS frame <length=12, flags=0x00, stream_id=0>
          (niv=2)
          [SETTINGS_MAX_CONCURRENT_STREAMS(0x03):100]
          [SETTINGS_INITIAL_WINDOW_SIZE(0x04):65535]
[  0.002] send HEADERS frame <length=33, flags=0x05, stream_id=1>
          ; END_STREAM | END_HEADERS
          (padlen=0)
          ; Open new stream
          :method: GET
          :path: /
          :scheme: http
          :authority: localhost:8080
          accept: */*
          accept-encoding: gzip, deflate
          user-agent: nghttp2/1.23.1
[  0.005] recv SETTINGS frame <length=6, flags=0x00, stream_id=0>
          (niv=1)
          [SETTINGS_MAX_HEADER_LIST_SIZE(0x06):8192]
[  0.005] send SETTINGS frame <length=0, flags=0x01, stream_id=0>
          ; ACK
          (niv=0)
[  0.006] recv GOAWAY frame <length=104, flags=0x00, stream_id=0>
          (last_stream_id=0, error_code=PROTOCOL_ERROR(0x01), opaque_data(96)=[HTTP/2 client preface string missing or corrupt. Hex dump for received bytes: 000000040100000000])
Some requests were not processed. total=1, processed=0

Minimal yet complete reproducer code (or URL to code)

Run the Http2Server example code, using the following modified configureClearText method:

    private void configureClearText(SocketChannel ch) {
        final ChannelPipeline p = ch.pipeline();
        final HttpServerCodec sourceCodec = new HttpServerCodec();

        final Http2Codec http2Codec = new Http2CodecBuilder(true, new ChannelInitializer<Channel>() {
            @Override
            protected void initChannel(Channel channel) throws Exception {
                ChannelPipeline p = channel.pipeline();
                p.addLast("downgrader", new Http2ServerDowngrader(true));
                p.addLast("chunk-aggregator", new HttpObjectAggregator(1048576));
                p.addLast("http1-handler", new HelloWorldHttp1Handler("Downgrade"));
            }
        }).build();

        final UpgradeCodecFactory upgradeCodecFactory = new UpgradeCodecFactory() {
            @Override
            public UpgradeCodec newUpgradeCodec(CharSequence protocol) {
                if (AsciiString.contentEquals(Http2CodecUtil.HTTP_UPGRADE_PROTOCOL_NAME, protocol)) {
                    return new Http2ServerUpgradeCodec("server-upgrade-codec", http2Codec);
                } else {
                    return null;
                }
            }
        };

        final HttpServerUpgradeHandler upgradeHandler = new HttpServerUpgradeHandler(sourceCodec, upgradeCodecFactory);

        final CleartextHttp2ServerUpgradeHandler cleartextHttp2ServerUpgradeHandler =
            new CleartextHttp2ServerUpgradeHandler(sourceCodec, upgradeHandler, http2Codec);

        p.addLast(cleartextHttp2ServerUpgradeHandler);

        p.addLast(new SimpleChannelInboundHandler<HttpMessage>() {
            @Override
            protected void channelRead0(ChannelHandlerContext ctx, HttpMessage msg) throws Exception {
                // If this handler is hit then no upgrade has been attempted and the client is just talking HTTP.
                System.err.println("Directly talking: " + msg.protocolVersion() + " (no upgrade was attempted)");
                ChannelPipeline pipeline = ctx.pipeline();
                ChannelHandlerContext thisCtx = pipeline.context(this);
                pipeline.addAfter(thisCtx.name(), null, new HelloWorldHttp1Handler("Direct. No Upgrade Attempted."));
                pipeline.replace(this, null, new HttpObjectAggregator(maxHttpContentLength));
                ctx.fireChannelRead(ReferenceCountUtil.retain(msg));
            }
        });

        p.addLast(new UserEventLogger());
    }

Netty version

Latest 4.1 branch

JVM version (e.g. java -version)

java 8u131

OS version (e.g. uname -a)

macOS 10.12.4