[Haskell-cafe] forkProcess, forkIO, and multithreaded runtime

[Alexander V Vershilov]

Mon, Oct 15, 2012 at 09:47:35AM +0200, Michael Snoyman wrote
> Hi all,
> 
> I think I have a misunderstanding of how forkProcess should be working.
> Ultimately this relates to some bugs in the development version of keter, but
> I've found some behavior in a simple test program which I wouldn't have
> expected either, which may or may not be related.
> 
> With the program at the end of this email, I would expect that, once per
> second, I would get a message printed from each forkIO'd green thread, the
> forked process, and the master process. And if I spawn 8 or less child threads
> that's precisely what happens. However, as soon as I up that number to 9, the
> child process is never run. The process is, however, created, as can be
> confirmed by looking at the process table.
> 
> This only occurs when using the multithreaded runtime. In other words,
> compiling with "ghc --make test.hs" seems to always produce the expected
> output, whereas "ghc --make -threaded test.hs" causes the behavior described
> above. Having looked through the code for the process package a bit, my initial
> guess is that this is being caused by a signal being sent to the child process,
> but I'm not familiar enough with the inner workings to confirm or disprove this
> guess.
> 
> If anyone has any ideas on this, I'd appreciate it.
> 
> Michael
> 
> import System.Posix.Process (forkProcess, getProcessID)
> import Control.Concurrent (forkIO, threadDelay)
> import System.IO (hFlush, stdout)
> import System.Posix.Signals (signalProcess, sigKILL)
> import Control.Exception (finally)
> 
> main :: IO ()
> main = do
>     mapM_ spawnChild [1..9]
>     child <- forkProcess $ do
>         putStrLn "starting child"
>         hFlush stdout
>         loop "child" 0
>     print ("child pid", child)
>     hFlush stdout
> 
>     -- I've commented out the "finally" so that the zombie process stays alive,
>     -- to prove that it was actually created.
>     loop "parent" 0 -- `finally` signalProcess sigKILL child
> 
> spawnChild :: Int -> IO ()
> spawnChild i = do
>     _ <- forkIO $ loop ("spawnChild " ++ show i) 0
>     return ()
> 
> loop :: String -> Int -> IO ()
> loop msg i = do
>     pid <- getProcessID
>     print (pid, msg, i)
>     hFlush stdout
>     threadDelay 1000000
>     loop msg (i + 1)

Sounds like a bug and is not reproducing on ghc-7.6

--
Alexander Vershilov
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