Make AbstractChannel.outboundBuffer volatile.

Motivation:

The field can be read from arbitrary threads via Channel.(isWritable()|bytesBeforeWritable()|bytesBeforeUnwritable()), WriteAndFlushTask.newInstance(), PendingWriteQueue, etc.

Modifications:

Make AbstractChannel.outboundBuffer volatile.

Result:

More correct in a concurrent use case.

transport/src/main/java/io/netty/channel/AbstractChannel.java

@@ -413,7 +413,7 @@ public final ChannelPromise voidPromise() {
      */
     protected abstract class AbstractUnsafe implements Unsafe {
 
-        private ChannelOutboundBuffer outboundBuffer = new ChannelOutboundBuffer(AbstractChannel.this);
+        private volatile ChannelOutboundBuffer outboundBuffer = new ChannelOutboundBuffer(AbstractChannel.this);
         private RecvByteBufAllocator.Handle recvHandle;
         private boolean inFlush0;
         /** true if the channel has never been registered, false otherwise */