TCP, FramedTCP message send is not truly async

[PPakalns]

As seen in TODO comment and behaviour on io::ErrorKind::WouldBlock.

In server case this blocks communication with other clients.

I stumbled upon a situation when a faulty virtual network tunnel doesn't process sent data and whole program gets stuck in this loop and has 100% CPU utilization. It causes issues and delays in other situations too with large messages.

https://github.com/lemunozm/message-io/blob/master/src/adapters/tcp.rs#L187

    fn send(&self, data: &[u8]) -> SendStatus {
        // TODO: The current implementation implies an active waiting,
        // improve it using POLLIN instead to avoid active waiting.
        // Note: Despite the fear that an active waiting could generate,
        // this only occurs in the case when the receiver is full because reads slower that it sends.
        let mut total_bytes_sent = 0;
        loop {
            let mut stream = &self.stream;
            match stream.write(&data[total_bytes_sent..]) {
                Ok(bytes_sent) => {
                    total_bytes_sent += bytes_sent;
                    if total_bytes_sent == data.len() {
                        break SendStatus::Sent
                    }
                }
                Err(ref err) if err.kind() == io::ErrorKind::WouldBlock => continue,

                // Others errors are considered fatal for the connection.
                // a Event::Disconnection will be generated later.
                Err(err) => {
                    log::error!("TCP receive error: {}", err);
                    break SendStatus::ResourceNotFound // should not happen
                }
            }
        }
    }

[lemunozm]

Hi, thanks for bringing this issue to light!

Yes, if the way of unlocking the receiver queue is in the same thread as this sender, then it will cause 100% of the CPU without the possibility of unlocking it. That should definitely be fixed using a POLLIN

[PPakalns]

Workaround implementation of AsyncFrameAdapter: https://github.com/lemunozm/message-io/compare/master...PPakalns:message-io:async_framed_tcp

Of course, this adds additional cost of storing queue of buffers to be sent over the network.

[lemunozm]

Great! Feel free to open a PR with this new adapter. It can be interesting for users using tcp as you.