ghci and ghc -threaded broken with pipes & forking
simonmarhaskell at gmail.com
Thu Mar 1 10:06:22 EST 2007
Ok, what happens here is that in the forked process there is only a single
thread, the runtime kills all the other threads (as advertised). Unfortunately
this includes the I/O manager thread, so as soon as you do some I/O in the
forked process, you block.
It might be possible to fix this, but not easily I'm afraid, because the I/O
manager doesn't currently have a way to restart after it's been killed. We
could implement that, though. I'll create a bug report.
On a more general note, forkProcess is known to be hairy - simply the fact that
it kills all the other threads in the system in the forked process means that
there's a good supply of means to shoot yourself in the foot, even accidentally.
John - perhaps there's another way to achieve what you want?
Jeremy Shaw wrote:
> Here is a simplified example that seems to exhibit the same behaviour,
> unless I screwed up:
> module Main where
> import System.Posix
> import System.IO
> import System.Exit
> main =
> do putStrLn "running..."
> (stdinr, stdinw) <- createPipe
> (stdoutr, stdoutw) <- createPipe
> pid <- forkProcess $ do hw <- fdToHandle stdoutw
> hr <- fdToHandle stdinr
> closeFd stdinw
> hGetContents hr >>= hPutStr hw
> hClose hr
> hClose hw
> exitImmediately ExitSuccess
> closeFd stdoutw
> closeFd stdinw
> hr2 <- fdToHandle stdoutr
> hGetContents hr2 >>= putStr
> getProcessStatus True False pid >>= print
> Compiling with:
> ghc --make -no-recomp test3.hs -o test3 && ./test3
> works. But compiling with:
> ghc --make -no-recomp -threaded test3.hs -o test3 && ./test3
> results in a hang. If you comment out the "hGetContents hr >>=" and
> change 'hPutStr hw' to 'hPutStr hw "hi"', then it seems to work ok.
> As you suggested, it seems that hGetContents is not ever seeing the
> EOF when -threaded is enabled. I think it gets 'Resource temporarily
> unavailable' instead. So, it keeps retrying.
> Assuming I have recreated the same bug, we at least have a simpiler
> test case now...
> At Wed, 28 Feb 2007 11:15:04 -0600,
> John Goerzen wrote:
>> I've been hitting my head against a wall for the past couple of days
>> trying to figure out why my shell-like pipeline code kept hanging. I
>> found fd leakage (file descriptors not being closed), which disrupts EOF
>> detection and can lead to deadlocks. I just couldn't find the problem.
>> I finally tried compiling my test with ghc instead of running it in
>> And poof, it worked fine the first time.
>> I tried asking on #haskell, and got the suggestion that ghci uses
>> -threaded. I tried compiling my test program with ghc -threaded, and
>> again, same deadlock. My program never calls forkIO or forkOS or any
>> other threading code.
>> You can see my test case with:
>> darcs get '--tag=glasgow ml' http://darcs.complete.org/hsh
>> ghc -fglasgow-exts --make -o test2 test2.hs
>> That'll run fine. If you add -threaded, it will hang.
>> -- John
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