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waitForProcess: does not exist (No child processes) #226
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It's a bit hard to help without seeing any code. You should be able to mix
asyncandprocess, there's no fundamental limitation I think. #107 is probably not related.I assume the code is private but if you could strip out the process invocation/waiting/cancel parts and post them, it'd be a lot easier to give help. Preferably the non-
asyncversion, just fewer moving blocks.Sure I understand that, I will try to strip out some of the important parts:
After the input is parsed I call all elements of
solverson the input with the following parallelization:ecParP :: Input -> Maybe Int -> Maybe Int -> IO [Result] ecParP p i n = do var <- newEmptyMVar threads <- mapM (forkIO . mVarFun null var . run') solvers waitThreadSet [] success (S.fromList threads) var where (_, c) = getMain p run' solv = run solv c success Nothing = False success (Just ress) = any succ' ress where succ' (Failure _ _) = False succ' (Success _ _) = TruemVarFun :: (a -> Bool) -> MVar (ThreadId, Maybe a) -> IO a -> IO () mVarFun noResult var comp = do res <- comp id <- myThreadId if noResult res then putMVar var (id, Nothing) else putMVar var (id, Just res)-- Wait for threads created with forkIO waitThreadSet :: a -> (Maybe a -> Bool) -> S.Set ThreadId -> MVar (ThreadId, Maybe a) -> IO a waitThreadSet def success threads var = go S.empty where go finished = if threads == finished then return def else do (id,res) <- takeMVar var if success res then do mapM_ killThread (threads S.\\ finished) return $ fromJust res else go (S.insert id finished)Within a solver an external tool is e.g. called with:
readProcess "matlab" ["-nosplash", "-nodisplay", "-r", file] ""I simplified the functions a bit and hope that this helps.
Cheers
Doesn't look like it should be broken. Maybe
waitForProcessshould be checking ECHILD just likegetProcessExitCodedoes? Perhaps someone else has to weigh in.You should write down your
processversion and OS too I gues.I found out that also
readProcessWithExitCodeis used in a local external tool I am using.$ uname -a
Linux arch-work 5.15.13-arch1-1 #1 SMP PREEMPT Wed, 05 Jan 2022 16:20:59 +0000 x86_64 GNU/Linux$ cabal info process
* process (library) Synopsis: Process libraries Versions available: 1.4.2.0, 1.4.3.0, 1.5.0.0, 1.6.10.0, 1.6.11.0, 1.6.12.0, (1.6.13.0), 1.6.13.1, 1.6.13.2 (and 29 others) Versions installed: 1.6.13.2 Homepage: [ Not specified ] Bug reports: https://github.com/haskell/process/issues Description: This package contains libraries for dealing with system processes. The typed-process package is a more recent take on a process API, which uses this package internally. It features better binary support, easier concurrency, and a more composable API. You can read more about it at <https://github.com/fpco/typed-process/#readme>. Category: System License: BSD3 Maintainer: [email protected] Source repo: https://github.com/haskell/process.git Dependencies: base >=4.10 && <4.17, directory >=1.1 && <1.4, filepath >=1.2 && <1.5, deepseq >=1.1 && <1.5, Win32 >=2.4 && <2.13, unix >=2.5 && <2.8, base >=4 && <5, bytestring, directory, process Documentation: ~/.ghcup/ghc/8.10.7/share/doc/ghc-8.10.7/html/libraries/process-1.6.13.2 Cached: No Modules: System.Cmd System.Process System.Process.InternalsreadProcess and readProcessWithExitCode end up going through same function in the end so I expect both to have the same behaviour fwiw.
Ahh yeah, then it was not necessary to mention that :)
But I still can't figure out what is going wrong here...Compare the discussion in fpco/typed-process#38 and #204. It does appear the
processisn't async-safe.Could we close this? The bug should be fixed in 1.6.14.0.
I believe so, closing.
- added 2 commits that reference this issue
on Feb 16, 2023 - added a commit that references this issue
on Feb 21, 2023
Hi all!
I hope that this is the right place to ask such a question.
I try to parallize parts of a program of mine, but get a strange runtime error and as far as I know it is related to something in the process package.
In my tool, I get some specific input, build a constraint from it which can be solved in 3 different ways (3 solvers; involving external tools).
Thus I would like to run these three different solvers on the constraint in parallel and return the result of the fastest one.
In simple words it looks as follows:
A solver
shas a types :: constraint -> IO [a]and I apply the constraint to all solvers to get a list of possible solutions[IO [a]]. After that I callasync0 on them and wait withwaitAnyfor a result (if I find a correct result, i.e. a non-empty list is returned, then I kill all the other jobs withcancel). Within a solver external tools are called withreadProcessandreadProcessWithExitCode. The problem is that I get quite often the following runtime errorwaitForProcess: does not exist (No child processes).Is mixing
asyncandprocesscausing this?Is this related to something similar as 1?
Does anybody has an idea what could cause this?
In the meantime I replaced the
asyncpackage functions completely byforkIOetc., but still this exception occurs.However, I never experienced this exception before I parallelized my program.
Thank you very much and cheers!