I've had a strange bug that's baffled me for a long time. I finally got serious about tracking it down, and managed to reduce it to a small program that exhibits the unexpected behaviour, namely that a createProcess seems to block writing to and closing a socket.
Here's the example program: --- import Control.Monad import qualified Network import qualified System.Environment as Environment import qualified System.IO as IO import qualified System.Process as Process main :: IO () main = Network.withSocketsDo $ do args <- Environment.getArgs case args of ["server"] -> server ["client"] -> client _ -> error $ show args server :: IO () server = do socket <- Network.listenOn port forever $ do putStrLn "accept" (hdl, _host, _port) <- Network.accept socket msg <- IO.hGetLine hdl putStrLn $ "from client: " ++ show msg sleep putStrLn "send response" IO.hPutStr hdl "response" IO.hClose hdl client :: IO () client = do hdl <- Network.connectTo "localhost" port IO.hPutStr hdl "hi from client\n" IO.hFlush hdl resp <- IO.hGetContents hdl print resp port = Network.UnixSocket "port" sleep = Process.createProcess (Process.proc "sleep" ["5"]) --- You can test with: % ghc --make Test.hs && rm -f port && ./Test server then on another window: % ./Test client Since the createProcess is async (it doesn't wait on the pid), I expect the response to come back to the client immediately. And the server immediately says "send response" then "accept", so it doesn't get stuck on the sleep. But the client waits 5 seconds before displaying "response", so evidently even though the handle has already been written to and closed from the server, the client waits until the subprocess (of the server!) completes before getting the response. Comment out the "sleep" line and everything is fast. Can anyone else repro this? I'm guessing it has to do with the ghc IO manager, and the fork implied by a createProcess is causing it the socket close to block, but I haven't dug any deeper yet, in case this is a know issue, or I'm just doing something wrong. This is OS X 10.8.3. thanks! _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe