#5212: waitForProcess: does not exist (No child processes)
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Reporter: chrisdone | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Runtime System | Version: 7.4.2
Resolution: | Keywords:
Os: Linux | Architecture: x86_64 (amd64)
Failure: Incorrect result at runtime | Difficulty:
Testcase: | Blockedby:
Blocking: | Related:
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Changes (by chrisdone):
* status: closed => new
* cc: chrisdone@… (added)
* component: GHC API => Runtime System
* version: 6.12.3 => 7.4.2
* owner: chrisdone =>
* resolution: worksforme =>
Comment:
Oddly, two years later I am experiencing the same problem. I was never
able to reproduce it outside of production, which was a machine I had no
control over. In the end I worked around it by running: catch
(waitForProcess pid >>= return) (\_ -> return $ ExitSuccess) However, that
was a workaround, not a solution.
This time with a completely different codebase, XMonad, I can reproduce it
in few lines of code:
Here's a test-case:
{{{
import Control.Concurrent
import System.Process
import XMonad
import XMonad.Config
main = do
forkIO $ xmonad defaultConfig
readProcess "ghc" ["--version"] "" >>= putStrLn
}}}
Compile and run like this (make sure you don't have xmonad already
running):
{{{
chris@retina:~$ ghc --make X.hs -threaded
[1 of 1] Compiling Main ( X.hs, X.o )
Linking X ...
chris@retina:~$ ./X
X: waitForProcess: does not exist (No child processes)
}}}
The GHC version is 7.4.2, but it perhaps it's not so much a GHC bug but
some behaviour of the runtime system and interaction with exceptions that
is subtle or unclear to me.
Can anyone reproduce, and if so, why does this happen? If you comment out
the forkIO line, the program succeeds.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5212#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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