    {-
    Copyright (C) 2013-2016 John MacFarlane <jgm@berkeley.edu>

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
    -}

    {- |
       Module      : Text.Pandoc.Process
       Copyright   : Copyright (C) 2013-2016 John MacFarlane
       License     : GNU GPL, version 2 or above

       Maintainer  : John MacFarlane <jgm@berkeley.edu>
       Stability   : alpha
       Portability : portable

    ByteString variant of 'readProcessWithExitCode'.
    -}
    module Text.Pandoc.Process (pipeProcess)
    where
    import System.Process
    import System.Exit (ExitCode (..))
    import Control.Exception
    import System.IO (hClose, hFlush)
    import Control.Concurrent (putMVar, takeMVar, newEmptyMVar, forkIO)
    import Control.Monad (unless)
    import qualified Data.ByteString.Lazy as BL

    {- |
    Version of 'System.Process.readProcessWithExitCode' that uses lazy bytestrings
    instead of strings and allows setting environment variables.

    @readProcessWithExitCode@ creates an external process, reads its
    standard output and standard error strictly, waits until the process
    terminates, and then returns the 'ExitCode' of the process,
    the standard output, and the standard error.

    If an asynchronous exception is thrown to the thread executing
    @readProcessWithExitCode@, the forked process will be terminated and
    @readProcessWithExitCode@ will wait (block) until the process has been
    terminated.
    -}

    pipeProcess
        :: Maybe [(String, String)] -- ^ environment variables
        -> FilePath                 -- ^ Filename of the executable (see 'proc' for details)
        -> [String]                 -- ^ any arguments
        -> BL.ByteString            -- ^ standard input
        -> IO (ExitCode,BL.ByteString,BL.ByteString) -- ^ exitcode, stdout, stderr
    pipeProcess mbenv cmd args input =
        mask $ \restore -> do
          (Just inh, Just outh, Just errh, pid) <- createProcess (proc cmd args)
                                                       { env     = mbenv,
                                                         std_in  = CreatePipe,
                                                         std_out = CreatePipe,
                                                         std_err = CreatePipe }
          flip onException
            (do hClose inh; hClose outh; hClose errh;
                terminateProcess pid; waitForProcess pid) $ restore $ do
            -- fork off a thread to start consuming stdout
            out <- BL.hGetContents outh
            waitOut <- forkWait $ evaluate $ BL.length out

            -- fork off a thread to start consuming stderr
            err <- BL.hGetContents errh
            waitErr <- forkWait $ evaluate $ BL.length err

            -- now write and flush any input
            let writeInput = do
                  unless (BL.null input) $ do
                    BL.hPutStr inh input
                    hFlush inh
                  hClose inh

            writeInput

            -- wait on the output
            waitOut
            waitErr

            hClose outh
            hClose errh

            -- wait on the process
            ex <- waitForProcess pid

            return (ex, out, err)

    forkWait :: IO a -> IO (IO a)
    forkWait a = do
      res <- newEmptyMVar
      _ <- mask $ \restore -> forkIO $ try (restore a) >>= putMVar res
      return (takeMVar res >>= either (\ex -> throwIO (ex :: SomeException)) return)