Deadlock while closing Netty channel

Question

I'm working on an application that uses Netty to send/receive RTP traffic. During a load test, we noticed that our single work thread ran into a deadlock while attempting to wait for Netty to close a channel.

Context:

• There is a shared NioEventLoopGroup among all RTP Endpoints (=> UDP sockets)
• The channel might get closed while there is still traffic being received
• The closing of the channel definitely happens outside of the Netty threads from a dedicated worker thread
• Netty version is 4.1.65

Bootstrapping code:

EventLoopGroup eventLoopGroup = RtpEventLoopGroup.acquireEventLoopGroup();
Bootstrap b = new Bootstrap();
b.group(eventLoopGroup);
b.channel(NioDatagramChannel.class);
b.option(ChannelOption.SO_BROADCAST, true);
b.option(ChannelOption.ALLOCATOR, PooledByteBufAllocator.DEFAULT);
b.handler(new ChannelInitializer<DatagramChannel>() {
    @Override
    public void initChannel(final DatagramChannel ch) {
        ch.pipeline().addLast(new RtpInboundChannelHandler(rtpReceiver));
    }
});
Channel channel = b.bind(localAddress).syncUninterruptibly().channel();

Shutdown code:

channel.disconnect().syncUninterruptibly();
channel.close().syncUninterruptibly();
RtpEventLoopGroup.releaseEventLoopGroup();

Thread dump of the stuck worker thread (NettyDatagramSocket:107 points to the channel.close() method):

Worker Thread
  at java.lang.Object.wait(J)V (Native Method)
  at java.lang.Object.wait()V (Object.java:328)
  at io.netty.util.concurrent.DefaultPromise.awaitUninterruptibly()Lio/netty/util/concurrent/Promise; (DefaultPromise.java:275)
  at io.netty.channel.DefaultChannelPromise.awaitUninterruptibly()Lio/netty/channel/ChannelPromise; (DefaultChannelPromise.java:137)
  at io.netty.channel.DefaultChannelPromise.awaitUninterruptibly()Lio/netty/util/concurrent/Promise; (DefaultChannelPromise.java:30)
  at io.netty.util.concurrent.DefaultPromise.syncUninterruptibly()Lio/netty/util/concurrent/Promise; (DefaultPromise.java:411)
  at io.netty.channel.DefaultChannelPromise.syncUninterruptibly()Lio/netty/channel/ChannelPromise; (DefaultChannelPromise.java:125)
  at io.netty.channel.DefaultChannelPromise.syncUninterruptibly()Lio/netty/channel/ChannelFuture; (DefaultChannelPromise.java:30)
  at mypackage.NettyDatagramSocket.close()V (NettyDatagramSocket.java:107)

There are 8 NIO EventLoopGroup threads, all are [alive, runnable]

• Some are currently processing a received packet
• None of them seems to be handling the Channel closing
• Some show the following thread stack:

NIO EventLoopGroup-1
  at sun.nio.ch.WindowsSelectorImpl$SubSelector.poll0(JI[I[I[IJJ)I (Native Method)
  at sun.nio.ch.WindowsSelectorImpl$SubSelector.poll()I (WindowsSelectorImpl.java:357)
  at sun.nio.ch.WindowsSelectorImpl.doSelect(Ljava/util/function/Consumer;J)I (WindowsSelectorImpl.java:182)
  at sun.nio.ch.SelectorImpl.lockAndDoSelect(Ljava/util/function/Consumer;J)I (SelectorImpl.java:124)
  at sun.nio.ch.SelectorImpl.select()I (SelectorImpl.java:141)
  at io.netty.channel.nio.SelectedSelectionKeySetSelector.select()I (SelectedSelectionKeySetSelector.java:68)
  at io.netty.channel.nio.NioEventLoop.select(J)I (NioEventLoop.java:810)
  at io.netty.channel.nio.NioEventLoop.run()V (NioEventLoop.java:457)
  at io.netty.util.concurrent.SingleThreadEventExecutor$4.run()V (SingleThreadEventExecutor.java:989)
  at io.netty.util.internal.ThreadExecutorMap$2.run()V (ThreadExecutorMap.java:74)
  at java.lang.Thread.run()V (Thread.java:829)

Does this seem like an error in my application or can I asume this is a bug in Netty? Is there anything else that could be relevant and I might have missed?