`send` == 0: how to wait for socket to drain?

[bscottm]

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?

[eborden]

Hmm, this would possibly require dropping down to GHC.Prim and using waitWrite# on the Socket file descriptor.

[eborden]

@bscottm Are you able to produce a minimal test case for this failure?

[bscottm]

@eborden: I'll see what I can craft... Probably the best test case would be to take the echo server and send 66Kb buffers (at least for MacOS -- localhost/::1 buffering is 64Kb.)

Receiver side is a bit trickier because the receiver's call to recv needs to consume less than 64kb at a time. In the SublimeHaskell plugin, the receiver consumes 10Kb at a time, looking for a newline (it's a JSON stream, newline is the separator between responses...)

/cc @mvoidex

[kazu-yamamoto]

What about this?

sendAll :: Socket      -- ^ Connected socket
        -> ByteString  -- ^ Data to send
        -> IO ()
sendAll sock bs
  | B.null bs = return ()
  | otherwise = do
      sent <- send sock bs
      if sent == 0 then do
          threadWaitWrite $ fromIntegral $ sockFd sock
          sendAll sock bs
        else if sent > 0 then
          sendAll sock (B.drop sent bs)
        else
          return ()

[bscottm]

I have not been able to successfully reproduce the send == 0 issue.

[kazu-yamamoto]

@bscottm Thanks!
I will merge this into each versions.

[kazu-yamamoto]

• sendAll
• sendAllTo
• sendMany
• sendManyTo
• sendAll for Lazy

[kazu-yamamoto]

I did many git push -f. Sorry for your confusion.