[GHC] #3231: Permission denied error with runProcess/openFile

[GHC]

#3231: Permission denied error with runProcess/openFile
---------------------------------+------------------------------------------
    Reporter:  NeilMitchell      |        Owner:  simonmar
        Type:  bug               |       Status:  closed  
    Priority:  normal            |    Milestone:  6.12.1  
   Component:  Runtime System    |      Version:  6.10.4  
    Severity:  normal            |   Resolution:  invalid 
    Keywords:                    |   Difficulty:  Unknown 
    Testcase:                    |           Os:  Windows 
Architecture:  Unknown/Multiple  |  
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Changes (by simonmar):

  * status:  new => closed
  * resolution:  => invalid

Comment:

 Repeating the program, to get the markup right:

 {{{
 module Main where
 import System
 import System.IO
 import System.Process
 import System.Directory
 import Control.Monad
 import Data.List
 import Control.Concurrent

 tempFile = "mytempfile.txt"

 run :: FilePath -> IO String
 run exe = do
     h <- openFile tempFile WriteMode
     pid <- runProcess exe [] Nothing Nothing Nothing (Just h) Nothing
     exitCode <- waitForProcess pid
     hClose h

     if exitCode /= ExitSuccess
         then error $ exe ++ " failed"
         else readFile tempFile

 main = replicateM_ 10 $ run "ls"
 }}}

 The program does indeed fail with `openFile: permission denied`, but it's
 not a problem with `runProcess`.  In the `else` branch of the `if` you
 have a `readFile` which opens a lazy stream to read `tempfile`, and then
 the next iteration attempts to open the file for writing.  It's illegal to
 have the same file open for both reading and writing, hence the error.

 Unfortunately neither myself nor Duncan Coutts spotted the problem
 immediately, because we were looking in the wrong place.  `readFile`
 considered harmful, IMO!

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3231#comment:27>
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