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?
Cheers,
Simon
Jeremy Shaw wrote:
Hello,
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...
j.
At Wed, 28 Feb 2007 11:15:04 -0600,
John Goerzen wrote:
Hi,
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
ghci.
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.
Ideas?
Thanks,
-- John
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