Re: [Haskell-cafe] Can't figure out source of race condition when using System.Process
David Roundy wrote: On Sun, Nov 02, 2008 at 09:15:25PM +0100, Marc Weber wrote: On Mon, Nov 03, 2008 at 01:02:12AM +1100, Rafal Kolanski wrote: Rafal Kolanski. [...] Why do you use forkIO here? It's not necessary. The process will run in background on its own. ? It looks to me like this code requires a forkIO, not in the writing to inp, but rather in the reading of out and err. Otherwise, those buffers may fill up and your process will hang... It seems to be as you say. Trying for a single-threaded solution to this problem always locked up the program, until I found someone's code snippet online using forkIO and extrapolated from that. Sincerely, Rafal Kolanski. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Can't figure out source of race condition when using System.Process
Rafal Kolanski wrote: ..., until I found someone's code snippet online ... and extrapolated from that. Oh yes, I love that kind of programming. Hardly possible in other languages than Haskell. :-) -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Can't figure out source of race condition when using System.Process
Bryan O'Sullivan wrote:
What is the it that segfaults? The Haskell program shouldn't, at least.
The Haskell program. It does so rarely, however, and I'm unable to
reproduce it with any consistency, only enough to notice something is
wrong with what I've written.
Upon closer examination my code also returns an error exit code for
subprocesses that should've succeeded (pdflatex, but once in a blue moon
also pstoedit) similarly rarely.
That said, your code for reading from child processes is wrong. You
should read all of a child's output strictly (i.e. hGetContents alone
will not do the trick) before you wait for its exit status. Your current
scheme reverses this order, and there is hence no guarantee that it will
read anything from the defunct child process. There's some possibility
that this might be the cause of your segfault, which to me would suggest
the presence of a bug in the runtime system.
Well, if there's a bug in the runtime system, I'll try to make sure I
can reproduce it before filing a report.
Meanwhile, you are absolutely right. How would you propose I properly
perform that sort of interaction? It's often useful to use external
programs to do things for you by throwing data at them and reading (or
ignoring) the result until they die. Is there a Haskell idiom for this?
Overall, I find Haskell to be very nice. I just keep getting bitten by
laziness, at least until I get used to it.
Sincerely,
Rafal Kolanski.
PS. The file I attached is the only real place where concurrency happens
in my code. The rest of the uses are simple ... when rendering a slide
using cairo:
slideNaive = do
...
paintLatex \\textbf{datatype} ref = Ref int $\\mid$ Null 50 0
...
which calls:
paintLatex text x y = do
svg - liftIO $ updateSVG text
paintSVG svg 3 x y
updateSVG is defined in SVGLatex.hs which I attached earlier, and the rest:
paintSVG :: FilePath - Double - Double - Double - Render ()
paintSVG file scale' x y = do
save
svg - liftIO $ svgNewFromFile file
let (w,h) = svgGetSize svg
translate x y
scale scale' scale'
moveTo 0 0
svgRender svg
restore
which invokes functions defined in:
import Graphics.Rendering.Cairo
import Graphics.Rendering.Cairo.SVG
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[Haskell-cafe] Can't figure out source of race condition when using System.Process
Greetings Gentlemen (and Ladies),
As part of my small and simple framework for making presentations in
Haskell I have a module which, given some text, makes a .tex file and
then converts it to .svg for loading into a cairo canvas.
It features simple caching (performs md5 on the contents of the .tex
file and uses that as the file name... if an .svg by that name exists,
it'll get loaded rather than regenerated).
Unfortunately, it also segfaults once in a while, probably indicating I
have some kind of race condition ... but I can't figure out why. This is
the only point in my code that I think I'm using any concurrency,
although I'm compiling with ghc --make -threaded. Compiling without
-threaded results in a deadlock.
Ubuntu 8.10, 64-bit, GHC 6.8.2.
Any advice? I'm attaching the module in question (79 lines).
Sincerely,
Rafal Kolanski.
module LatexSVG (updateSVG, updateSVG') where
import System.Process
import Control.Concurrent (forkIO)
import Control.Monad (when)
import System.IO
import System.Directory (doesFileExist)
import System.Exit
workingDir = tex/
prefix = unlines $
[\\documentclass{minimal} ,
\\usepackage{graphicx,latexsym,amsmath,url,color},
\\usepackage{amssymb,amsmath,wasysym, pst-node, color},
\\usepackage{url},
\\usepackage{isabelle,isabellesym},
%\\usepackage{mathpartir},
\\definecolor{dblue}{rgb}{0,0,0.4},
\\definecolor{blue}{rgb}{0,0,0.6},
\\definecolor{green}{rgb}{0,0.6,0},
\\definecolor{gray}{rgb}{0.7,0.7,0.7},
\\definecolor{dblue}{rgb}{0,0,0.6},
\\definecolor{darkgray}{rgb}{0.4,0.4,0.4},
\\definecolor{red}{rgb}{0.6,0,0},
\\begin{document}]
suffix = \n\\end{document}
-- Pass text to md5sum and drop the final - when returning
hashMD5 text = do
(inp,out,err,pid) - runInteractiveProcess md5sum [] Nothing Nothing
forkIO $ do
hPutStrLn inp text
hClose inp
exit - waitForProcess pid
case exit of ExitFailure _ - error md5sum fail
_ - return ()
md5hash - hGetContents out
return $ takeWhile (\x - x /= ' ') md5hash
blindExecWithFail cmd args dir = do
(inp,out,err,pid) - runInteractiveProcess cmd args (Just dir) Nothing
exit - waitForProcess pid
case exit of ExitFailure _ - error $ cmd ++ ++ (show args) ++ fail
_ - return ()
generateSVG text filebase = do
-- Create a .tex file
writeFile (workingDir ++ filebase ++ .tex) text
-- Run pdflatex over it
blindExecWithFail pdflatex [filebase] workingDir
-- Run pstoedit on resulting .pdf
blindExecWithFail pstoedit
[-page 1,-dt,-psarg,-r9600x9600,
filebase ++ .pdf, filebase ++ .sk]
workingDir
-- Run skconvert on resulting .sk
blindExecWithFail skconvert
[filebase ++ .sk, filebase ++ .svg]
workingDir
updateSVG' :: [Char] - [Char] - [Char] - IO [Char]
updateSVG' prefix suffix text = do
-- Hash the tex
let tex = prefix ++ text ++ suffix
md5hash - hashMD5 tex
-- Filenames are the hash prefixed with hash and ending with extension
let filebase = hash ++ md5hash
-- See if an svg file by that name exists
svge - doesFileExist $ workingDir ++ filebase ++ .svg
when (not svge) $! do
putStrLn $ filebase ++ .svg does not exist, creating
generateSVG tex filebase
return $ workingDir ++ filebase ++ .svg
updateSVG :: [Char] - IO [Char]
updateSVG = updateSVG' prefix suffix
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Re: [Haskell-cafe] Can't figure out source of race condition when using System.Process
2008/11/2 Rafal Kolanski [EMAIL PROTECTED] Unfortunately, it also segfaults once in a while, probably indicating I have some kind of race condition ... but I can't figure out why. What is the it that segfaults? The Haskell program shouldn't, at least. That said, your code for reading from child processes is wrong. You should read all of a child's output strictly (i.e. hGetContents alone will not do the trick) before you wait for its exit status. Your current scheme reverses this order, and there is hence no guarantee that it will read anything from the defunct child process. There's some possibility that this might be the cause of your segfault, which to me would suggest the presence of a bug in the runtime system. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Can't figure out source of race condition when using System.Process
On Mon, Nov 03, 2008 at 01:02:12AM +1100, Rafal Kolanski wrote: Rafal Kolanski. -- Pass text to md5sum and drop the final - when returning hashMD5 text = do (inp,out,err,pid) - runInteractiveProcess md5sum [] Nothing Nothing forkIO $ do hPutStrLn inp text hClose inp exit - waitForProcess pid case exit of ExitFailure _ - error md5sum fail _ - return () [...] Why do you use forkIO here? It's not necessary. The process will run in background on its own. ? It would help me tracking the problem down having a full compilable example.. Sincerly Marc Weber ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Can't figure out source of race condition when using System.Process
On Sun, Nov 02, 2008 at 09:15:25PM +0100, Marc Weber wrote: On Mon, Nov 03, 2008 at 01:02:12AM +1100, Rafal Kolanski wrote: Rafal Kolanski. -- Pass text to md5sum and drop the final - when returning hashMD5 text = do (inp,out,err,pid) - runInteractiveProcess md5sum [] Nothing Nothing forkIO $ do hPutStrLn inp text hClose inp exit - waitForProcess pid case exit of ExitFailure _ - error md5sum fail _ - return () [...] Why do you use forkIO here? It's not necessary. The process will run in background on its own. ? It looks to me like this code requires a forkIO, not in the writing to inp, but rather in the reading of out and err. Otherwise, those buffers may fill up and your process will hang... -- David Roundy http://www.darcs.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
