Write/Flush fails when encoder is NOT last handler and runs in UnorderedThreadPoolEventExecutor with size > 1

[andilem]

When there is an encoder in a channel pipeline which is NOT the last handler AND runs in an UnorderedThreadPoolEventExecutor with size > 1, channel.writeAndFlush(...) fails to flush (from my simple investigation probably because the encoded bytes are not yet written - there is a FLUSH event, but no data). When flushing manually after a small delay, it works.

• When setting the executor size to 1, it works.
• When the encoder it is the last handler, it works

Minimal yet complete reproducer code

import java.nio.charset.StandardCharsets;

import io.netty.bootstrap.Bootstrap;
import io.netty.bootstrap.ServerBootstrap;
import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelFutureListener;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelOption;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;
import io.netty.channel.socket.nio.NioSocketChannel;
import io.netty.handler.codec.MessageToByteEncoder;
import io.netty.handler.logging.LogLevel;
import io.netty.handler.logging.LoggingHandler;
import io.netty.util.concurrent.UnorderedThreadPoolEventExecutor;

public class NettyBug {
    static {
        System.setProperty("org.slf4j.simpleLogger.defaultLogLevel", "trace");
    }
    
    private static class Codec extends MessageToByteEncoder<String> {
        @Override
        protected void encode(ChannelHandlerContext ctx, String msg, ByteBuf out) throws Exception {
            out.writeCharSequence(msg, StandardCharsets.UTF_8);
        }
    }
    
    public static void main(String[] args) throws InterruptedException {
        // ===== discard + close server =====
        var serverGroup = new NioEventLoopGroup(1);
        var serverBootstrap = new ServerBootstrap().group(serverGroup);
        serverBootstrap.channel(NioServerSocketChannel.class).option(ChannelOption.SO_BACKLOG, Integer.valueOf(128))
            .handler(new LoggingHandler("ServerAcceptor"));
        serverBootstrap.childHandler(new ChannelInboundHandlerAdapter() {
            @Override
            public void channelRead(ChannelHandlerContext ctx, Object msg) {
                // Discard the received data silently.
                System.err.println("OK! Received " + msg);
                ((ByteBuf) msg).release();
                System.err.println("Close");
                ctx.close();
            }
            
            @Override
            public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
                // Close the connection when an exception is raised.
                cause.printStackTrace();
                ctx.close();
            }
        });
        serverBootstrap.bind(50005).sync();
        
        // ===== CLIENT =====
        var clientIOGroup = new NioEventLoopGroup(1);
        var clientWorkerGroup = new UnorderedThreadPoolEventExecutor(2); // works with size = 1 !!!
        var bootstrap = new Bootstrap().group(clientIOGroup).channel(NioSocketChannel.class);
        bootstrap.handler(new ChannelInitializer<SocketChannel>() {
            @Override
            protected void initChannel(SocketChannel ch) throws Exception {
                var pipeline = ch.pipeline();
                pipeline.addLast("frameLogger", new LoggingHandler("first", LogLevel.DEBUG)); // no impact, just for debugging
                pipeline.addLast(clientWorkerGroup, new Codec());
                pipeline.addLast("finalLogger", new LoggingHandler("second", LogLevel.DEBUG)); // works WITHOUT this handler !!!
            }
        });
        bootstrap.connect("localhost", 50005).addListener(new ChannelFutureListener() {
            @Override
            public void operationComplete(ChannelFuture f) throws Exception {
                f.channel().writeAndFlush("hello").addListener(ChannelFutureListener.CLOSE);
            }
        });
    }
}

Netty version

4.1.106

JVM version (e.g. java -version)

17

OS version (e.g. uname -a)

Windows

[normanmaurer]

let me check

[normanmaurer]

So this is kind of expected in the sense that each handler methods will be called in an "unordered" fashion. As writeAndFlush is just a shortcut for calling write(...) and flush separately you can end up with such a situation

[andilem]

That's what I thought. Big problem, because it prevents running the de/encoder in a separate thread pool if there is anything in the pipeline afterwards.