Haskell tcp server, fd is too big error - sockets

Question

I've been trying to write a Haskell server for a Go client. For the Haskell TCP server I'm simply using Network.Socket. Whenever I try to run hWaitForInput, I am getting this error:
fdReady: fd is too big.
Here is the server code -
connHandler :: (Socket, SockAddr) -> IO()
connHandler (sock, _) = do
putStrLn "Starting Handler"
handle <- socketToHandle sock ReadWriteMode
hSetBuffering handle LineBuffering
hPutStrLn handle "Hello Client!"
putStrLn "Waiting for Input"
success <- hWaitForInput handle (1000*10)
putStrLn "Wait done"
if success
then do
putStrLn "Client timed out"
else do
msg <- hGetLine handle
putStrLn msg
hClose handle
The Go client is receiving and printing the server's message("Hello Client!") but the haskell server throws the error right after printing "Waiting for Input"

Answer 1

You aren't doing anything wrong. The specific error message you're seeing only shows up with GHC >=8.0.2 running on Windows and represents a bug/limitation in an internal GHC function fdReady that they've tried to address on non-Windows architectures but have left unfixed on Windows. (Don't feel too jealous, though -- the "fix" on non-Windows architectures is currently broken and crashes, too.) Trying an earlier version of GHC probably wouldn't help -- it would still cause an error, but the error message would be different.
Here's the problem: on Windows, the internal function fdReady uses the select() system call to poll file descriptors for sockets, and select is limited to a certain maximum numerical value for the file descriptors it can poll. It looks like the Windows default for this value is quite low (64) but can be increased at compile time (the time GHC is compiled, unfortunately, not the time when GHC compiles your program).
If you add the line:
hShow handle >>= putStrLn
just before your hWaitForInput, you should see some debug info printed for the socket, including something like loc=<socket: nnn> where nnn is the file descriptor. This may help you verify that you're seeing a file descriptor greater than 64 that's causing the problem.
If this is the case, I would suggest filing a GHC bug to see if you can get it fixed.

Answer 2

As an alternative/workaround, you could try reading a line in one thread and a timer in another thread.
putStrLn "Waiting for Input"
msgMVar <- newEmptyMVar
tid <- forkIO $ hGetLine handle >>= putMVar msgMVar
maybeExn <- waitTimeout tid (1000*10)
case maybeExn of
Nothing -> do
killThread tid
putStrLn "Client timed out"
Just (Just _) ->
putStrLn "Exception"
_ -> do
msg <- takeMVar msgMVar
putStrLn msg
hClose handle
This does have different behavior (can time out in the middle of reading a line) than your code (never times out if a single byte can be read, even if the line is not complete), though.