HGL indefinite pause with "thread blocked indefinitely" message

[calvins]

As discussed on haskell-cafe < http://article.gmane.org/gmane.comp.lang.haskell.cafe/18064>, the program below sometimes pauses indefinitely when run, and seems to be a result of some kind of deadlock issue.

Behavior has been observed on the following platforms/versions:

1) GNU/Linux (2.6.18 kernel, gcc-4.1.1), ghc-6.6, hgl-3.1, xorg-x11-7.1 (on both single core and multi-core x32).

2) Mac OS X, ghc-6.4.2, x11-1.1 and hgl-3.1.

module Main where

import Graphics.SOE

main = 
  runGraphics (
  do w <- openWindow "Snowflake Fractal" (600, 600)
     fillStar w 300 125 256 (cycle $ enumFrom Blue)
     spaceClose w
  )

spaceClose w =
  do k <- getKey w
     if k == ' ' 
        then closeWindow w
        else spaceClose w


minSize = 2 :: Int

fillStar :: Window -> Int -> Int -> Int -> [Color] -> IO ()
fillStar w x y h clrs | h <= minSize 
  = return ()
fillStar w x y h clrs 
 = do drawInWindow w 
                  (withColor (head clrs) 
                             (polygon [t1p1,t1p2,t1p3,t1p1]))
      drawInWindow w 
                  (withColor (head clrs) 
                             (polygon [t2p1,t2p2,t2p3,t2p1]))
      sequence_ $ map recur [t1p1,t1p2,t1p3,t2p1,t2p2,t2p3]                  
   where tanPiOverSix = tan(pi/6) :: Float
         halfSide = truncate $ tanPiOverSix * fromIntegral h
         hFrag = truncate $ tanPiOverSix * tanPiOverSix * fromIntegral h
         (t1p1,t1p2,t1p3) = 
           ((x, y), (x-halfSide, y+h),(x+halfSide, y+h))
         (t2p1,t2p2,t2p3) = 
           ((x-halfSide, y+hFrag),(x, y+h+hFrag),(x+halfSide, y+hFrag))
         reVert y = y - ((h - hFrag) `div` 3)
         recur pnt = 
           fillStar w (fst pnt) (reVert (snd pnt)) (h`div`3) (tail clrs)

The problematic behavior, observed with ghci and also a ghc-compiled binary -- but not with Hugs (March 2005) -- is that sometimes the rendering pauses at various places and does not continue rendering until some kind of ui event occurs. Moving the mouse or typing any key but space (which exits) causes the rendering to continue.

When the pause occurs, the following message is printed to the console: "thread blocked indefinitely".

Please let me know if you need any further information.

[calvins]

I meant "single core and multi-core x86 (32-bit)" instead of "single core and multi-core x32" above.

[simonmar]

I believe I've fixed some of the issues here, but there may be more.

• The X11 library didn't provide a way to call XInitThreads, required for multithreaded clients. (now added, and called from HGL.runGraphics if necessary).

• The XNextEvent function wasn't imported safe, so it blocked the other threads.

However, with these fixes, the program still hangs on startup until the first event is received. I'm not sure why - the two active threads both appear to be waiting in XNextEvent, so the program really is waiting for an event. Maybe an HGL expert is needed here (is there one?).

[simonmar]

These might be related: #471, #742

[simonmar]

Note that Graphics.HGL.Gtk is probably a good workaround:

 http://www.haskell.org/pipermail/haskell/2006-August/018398.html

[duncan]

That would be Graphics.SOE.Gtk

We have a reimplementation of the SOE subset of HGL, not the whole HGL.

[igloo]

No longer an extralib, so closing the bug