Error when using `interruptProcessGroupOf` on Mac OS

[asr]

Running the code below in Ubuntu I got

$ ./Test
The Glorious Glasgow Haskell Compilation System, version 8.0.1
Just ExitSuccess

but in Mac OS my student @jonaprieto got

$ ./Test
The Glorious Glasgow Haskell Compilation System, version 8.0.1
Test.hs: getProcessGroupIDOf: does not exist (No such process)

The error disappears if I replace interruptProcessGroupOf by terminateProcess.

$ cat Test.hs

module Main where

import Data.Maybe
import System.IO
import System.Process

foo :: IO (ProcessHandle, String)
foo = do
  (_, oh, _, ph) <-
    createProcess CreateProcess
      { cmdspec            = RawCommand "ghc" ["-V"]
      , cwd                = Nothing
      , env                = Nothing
      , std_in             = Inherit
      , std_out            = CreatePipe
      , std_err            = Inherit
      , close_fds          = False
      , create_group       = True
      , delegate_ctlc      = False
      , create_new_console = False
      , detach_console     = False
      , new_session        = False
      , child_group        = Nothing
      , child_user         = Nothing
      }

  o <- hGetContents $ fromMaybe (error "foo") oh
  return (ph, o)

main :: IO ()
main = do
  (ph, o) <- foo
  putStr o

  _     <- interruptProcessGroupOf ph
  e     <- getProcessExitCode ph
  print e

[snoyberg]

The issue is that the process has already exited by the time getProcessGroupIDOf is called, and therefore the call fails. I don't know why that call does not fail on Linux. Perhaps reasonable behavior would be to simply swallow that exception and perform a no-op, on the assumption that if getProcessGroupIDOf fails, it's because the process has already exited.

@simonmar It looks like you last touched the getProcessGroupIDOf call, how do you feel about that change?