Github user NicoK commented on the issue:
https://github.com/apache/flink/pull/4529
FYI:
- newer netty 4.0.xx versions also use `buffer.slice(index, length)` inside
their `extractFrame()` method (see
https://github.com/netty/netty/commit/891be30a28c6dc5a1edf1cb5a3690644cf4ff66e)
and despite the comment on `extractFrame()`, this is also safe when used
outside the `decode()` method as long as the buffer is retained (and then
released later). By letting `NettyMessageDecoder` inherit from
`LengthFieldBasedFrameDecoder` though, we also gain a smaller channel handler
pipeline.
- netty 4.1 offers an even improved `buffer.retainedSlice(index, length)`
implementation (see
https://github.com/netty/netty/commit/3a9f47216143082bdfba62e8940160856767d672)
but we can't go that way yet due to our netty-router dependency
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