[Haskell-cafe] hGetContents Illegal byte sequence / ghc-pkg

[David Fox]

On Sat, Aug 11, 2012 at 4:13 AM, Benjamin Edwards <edwards.benj at gmail.com>wrote:

> Hello café,
>
> I have a program that is crashing, and I have no idea why:
>
> module Main
>   where
>
> import System.Process (readProcessWithExitCode)
>
>
> main :: IO ()
> main = do _ <- readProcessWithExitCode "ghc-pkg" ["describe", "hoopl"] ""
>           putStrLn "Should never get here"
>
> this is using the process package from hackage. The program crashes with
>
> minimal-test: fd:5: hGetContents: invalid argument (invalid byte sequence)
> minimal-test: thread blocked indefinitely in an MVar operation
>
> inspecting the source of readProcessWithExitCode yields an obvious
> explanation to the MVar problem, but I don't understand why hGetContents is
> so offended.
>
> For the lazy it is defined as follows:
>
> readProcessWithExitCode
>     :: FilePath                 -- ^ command to run
>     -> [String]                 -- ^ any arguments
>     -> String                   -- ^ standard input
>     -> IO (ExitCode,String,String) -- ^ exitcode, stdout, stderr
> readProcessWithExitCode cmd args input = do
>     (Just inh, Just outh, Just errh, pid) <-
>         createProcess (proc cmd args){ std_in  = CreatePipe,
>                                        std_out = CreatePipe,
>                                        std_err = CreatePipe }
>
>     outMVar <- newEmptyMVar
>
>     -- fork off a thread to start consuming stdout
>     out  <- hGetContents outh
>     _ <- forkIO $ C.evaluate (length out) >> putMVar outMVar ()
>
>     -- fork off a thread to start consuming stderr
>     err  <- hGetContents errh
>     _ <- forkIO $ C.evaluate (length err) >> putMVar outMVar ()
>
>     -- now write and flush any input
>     when (not (null input)) $ do hPutStr inh input; hFlush inh
>     hClose inh -- done with stdin
>
>     -- wait on the output
>     takeMVar outMVar
>     takeMVar outMVar
>     hClose outh
>     hClose errh
>
>     -- wait on the process
>     ex <- waitForProcess pid
>
>     return (ex, out, err)
>
> Now having looked at the source of ghc-pkg it is dumping it's output using
> putStr and friends, so that should be using my local encoding on the
> system, right? and so should hGetContents in my program..?
>
> Now, for the curious: the reason I care is that this problem has
> effectively prevented me from using virthualenv. Sadness and woe.
>

I would recommend using ByteStrings.  There is a link to a version of
readProcessWithExitCode that uses ByteString instead of String here:
http://www.haskell.org/pipermail/libraries/2012-August/018263.html
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