LEAK: ByteBuf.release() was not called - how do we solve this?

Question

We have a netty-based network traffic intensive Java app/server.

Side note: I mainly support this app, I didn't build it so I don't know it in full.

We sometimes get this error as shown below. Previously we used to get this error after the server has been up for 3-4 days. Now I noticed that we are getting this error even just 10-15 mins after rebooting the server/app.

I don't understand how this is possible. Is this error something worrying and how can we fix it? I recall doing extensive research in the past on this same error, back then I even tried upgrading and patching netty but nothing helped to fully resolve the issue.

OS: Linux
Java version: 1.8
Netty version: netty-all-4.1.30.Final.jar

This is the only line of app-specific code, everything else happens in Netty.

com.company.japp.protocol.http.decoders.ConditionalHttpChunkAggregator.channelRead

Is this a bug of some sort in Netty itself? Would Netty upgrade or any other configuration tuning help here?

[2020-09-04 08:33:53,072]

ERROR

io.netty.util.ResourceLeakDetector

LEAK: ByteBuf.release() was not called before it's garbage-collected. 

See https://netty.io/wiki/reference-counted-objects.html for more information.

Recent access records: 
Created at:
    io.netty.buffer.AbstractByteBufAllocator.compositeDirectBuffer(AbstractByteBufAllocator.java:221)
    io.netty.buffer.AbstractByteBufAllocator.compositeBuffer(AbstractByteBufAllocator.java:199)
    io.netty.handler.codec.MessageAggregator.decode(MessageAggregator.java:255)
    io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:88)
    com.company.japp.protocol.http.decoders.ConditionalHttpChunkAggregator.channelRead(ConditionalHttpChunkAggregator.java:112)
    io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
    io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
    io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
    io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:323)
    io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:297)
    io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
    io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
    io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
    io.netty.handler.timeout.IdleStateHandler.channelRead(IdleStateHandler.java:286)
    io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
    io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
    io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
    io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1434)
    io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
    io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
    io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:965)
    io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:163)
    io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:644)
    io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:579)
    io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:496)
    io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:458)
    io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:897)
    io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
    java.lang.Thread.run(Thread.java:748)

Here is the code for the ConditionalHttpChunkAggregator.

package com.company.japp.protocol.http.decoders;

import com.company.japp.IHttpProxyServer;
import io.netty.channel.ChannelDuplexHandler;
import io.netty.channel.ChannelHandler;
import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.http.HttpHeaders;
import io.netty.handler.codec.http.HttpMessage;
import io.netty.handler.codec.http.HttpObjectAggregator;
import io.netty.handler.codec.http.HttpResponse;
import io.netty.util.internal.logging.InternalLogger;
import io.netty.util.internal.logging.InternalLoggerFactory;

import java.util.HashSet;

@ChannelHandler.Sharable
public class ConditionalHttpChunkAggregator extends HttpObjectAggregator {
    private static final InternalLogger logger = InternalLoggerFactory.getInstance(ConditionalHttpChunkAggregator.class);

    private volatile boolean        sendaschunked;
    private volatile int        maxContentLength;

    private static IHttpProxyServer         iHttpProxyServer;

    public static void initialize(IHttpProxyServer iHttpProxyServer) {
        ConditionalHttpChunkAggregator.iHttpProxyServer = iHttpProxyServer;
    }

    public ConditionalHttpChunkAggregator(int maxContentLength) {
        super(maxContentLength);
        this.maxContentLength = maxContentLength;
        sendaschunked = false;
    }

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg){
        if ((msg instanceof HttpResponse)) {
            HttpResponse response = (HttpResponse)msg;
            if ((msg instanceof HttpMessage)) {
                HttpMessage httpmessage= (HttpMessage)msg;

                try  {
                    // If the content length exceeds the threshhold, then send it as chunked
                    // It's too large to process substitutions
                    Long contentlength = 
                            httpmessage.headers().get(HttpHeaders.Names.CONTENT_LENGTH) != null ? 
                            Long.valueOf(httpmessage.headers().get(HttpHeaders.Names.CONTENT_LENGTH)) : -1;
                    if (contentlength >= maxContentLength) {
                        sendaschunked = true;
                    } else {
                    // Check content types
                         HashSet<String> chunkabletypes = iHttpProxyServer.getConfig().getProperty("chunkabletypes");
                        if (!chunkabletypes.isEmpty() && response.headers().contains(HttpHeaders.Names.CONTENT_TYPE)) {
                            String contentType = response.headers().get(HttpHeaders.Names.CONTENT_TYPE).toLowerCase().trim();
                            if (contentType.length()>0) {
                                sendaschunked = chunkabletypes.contains(contentType);
                                if (!sendaschunked) {
                                    for (String chunkabletype: chunkabletypes) {
                                        // Begins with
                                        if (contentType.indexOf(chunkabletype)==0) {
                                            sendaschunked = true;
                                            break;
                                        }
                                    }
                                }
                            }
                        }
                    }
                    if (sendaschunked) {
                        ctx.fireChannelRead(msg);
                        return;
                    }
                }
                catch(Exception ex) {
                    logger.error("error determining chunkable viability", ex);
                }
            }
        }
        if (sendaschunked) {
            ctx.fireChannelRead(msg);
            return;
        }

        try {
            super.channelRead(ctx, msg);
            
        } catch (Exception e) {
            logger.error("error determining chunkable viability", e);
            e.printStackTrace();
        }
    }
}

And this is the value for the chunkable types property:

video/x-ms-wvx,video/x-flv,application/x-shockwave-flash,video/quicktime,video/,audio/

Edit

I think this is bug in netty 4.1.30 around this line 255 in io.netty.handler.codec.MessageAggregator. Seems this CompositeByteBuf is allocated but not released. Am I right? I am hoping for some authoritative answer to either confirm or reject this idea.

        // A streamed message - initialize the cumulative buffer, and wait for incoming chunks.
        CompositeByteBuf content = ctx.alloc().compositeBuffer(maxCumulationBufferComponents); // LINE 255 
        if (m instanceof ByteBufHolder) {
            appendPartialContent(content, ((ByteBufHolder) m).content());
        }
        currentMessage = beginAggregation(m, content);

Answer 1

In our case it was a problem in the body of the response not being read which resulted into byte buf never being released.

Original code:

WebClient client = WebClient.create("some_url");
return client.get().exchange().flatMap(response -> Mono.just(builder.up().build()));

Fixed code:

WebClient client = WebClient.create("some_url");
return client.get().retrieve().toBodilessEntity().flatMap(response -> Mono.just(builder.up().build()));

Even the javadoc for exchange says it:

Deprecated since 5.3 due to the possibility to leak memory and/or connections; please, use exchangeToMono(Function), exchangeToFlux(Function); consider also using retrieve() which provides access to the response status and headers via ResponseEntity along with error status handling.

Kudos: https://stackoverflow.com/a/51321602/3242721

To reproduce the issue on localhost I used -Dio.netty.leakDetectionLevel=paranoid vm args.

Answer 2

You need to release Bytebuf allocated by you. It's not a Netty bug.

ByteBuf is at com.company.japp.protocol.http.decoders.ConditionalHttpChunkAggregator.channelRead(ConditionalHttpChunkAggregator.java:112)