I ran into this particular problem in @mvoidex's hsdev server while tracking down why hsdev would send incomplete responses. I was able to track the problem down to a reimplementation of sendAll:

            sendAll :: Socket -> BS.ByteString -> IO ()
            sendAll sock bs
                    | BS.null bs = return ()
                    | otherwise = do
                            sent <- Net.send sock bs
                            when (sent > 0) $ sendAll sock (BS.drop sent bs)

The subtle bug in this code is the send > 0: send can return 0 because the OS is waiting for a buffer to drain and the underlying descriptors are in non-blocking mode (GHC runtime "feature".)

One way around the send == 0 issue is to call Control.Concurrent.threadDelay. Optimally, you'd really want to call Control.Concurrent.threadWaitWrite, but you can't because converting the socket to a file descriptor is a one way process.

Is there a way to call threadWaitWrite without converting the socket to a descriptor?