Restore old signal handler after each computation.

src/GF/System/Signal.hs

@@ -5,9 +5,9 @@
 -- Stability   : (stability)
 -- Portability : (portability)
 --
--- > CVS $Date: 2005/11/07 20:15:05 $ 
+-- > CVS $Date: 2005/11/07 22:27:13 $ 
 -- > CVS $Author: bringert $
--- > CVS $Revision: 1.1 $
+-- > CVS $Revision: 1.2 $
 --
 -- Allows SIGINT (Ctrl-C) to interrupt computations.
 -----------------------------------------------------------------------------
@@ -26,18 +26,19 @@ import System.Posix.Signals
 --   normally.
 --   NOTES: 
 --   * This will replace any existing SIGINT
---     handlers, and after the computation has completed
---     the default handler will be installed for SIGINT.
+--     handler during the action. After the computation 
+--     has completed the existing handler will be restored.
 --   * If the IO action is lazy (e.g. using readFile,
 --     unsafeInterleaveIO etc.) the lazy computation will
 --     not be interruptible, as it will be performed
 --     after the signal handler has been removed.
 runInterruptibly :: IO a -> IO (Either Exception a)
-runInterruptibly a = do t <- myThreadId
-                        installHandler sigINT (Catch (killThread t)) Nothing
-                        x <- p `catch` h
-                        installHandler sigINT Default Nothing
-                        return x
+runInterruptibly a = 
+    do t <- myThreadId
+       oldH <- installHandler sigINT (Catch (killThread t)) Nothing
+       x <- p `catch` h
+       installHandler sigINT oldH Nothing
+       return x
   where p = a >>= \x -> return $! Right $! x
         h e = return $ Left e