Re: [process] Process.Spawn leaves the child in zombie state on posix

From: Klemens Morgenstern (klemensdavidmorgenstern_at_[hidden])
Date: 2022-07-07 11:21:37

I don't think this is a solvable problem.
The reason is that you need to set SIGCHLD to SIGIGN for the whole
application in order to prevent zombies. Doesn't really work as a general
solution, which is why I'll deprecate that function at some point.

You can probably `waitpid(-1, &status, 0); from time to time to reap the
zombie processes or you can put them in a process group; but I couldn't
come up with a satisfying solution, which is why the only way will be to
just hold a handle to the child process.

On Thu, Jul 7, 2022 at 7:08 PM Benedek Tass via Boost <boost_at_[hidden]>
wrote:

> I was planning to send an example project, I'm sending it now as an
> attachment.
> As the demo sketch shows, I would absolutely call waitpid() in the parent
> process, the sig_init_-like class' on_success() method would be a
> reasonable place for it. The current implementation doesn't lend itself to
> an easy fix in case of this bug, I don't have a fully fletched idea how to
> do it.
>
>
> Andrey Semashev via Boost <boost_at_[hidden]> ezt írta (időpont: 2022.
> júl. 6., Sze, 17:28):
>
> > On 7/6/22 16:28, Benedek Tass via Boost wrote:
> > > Hi, everyone!
> > >
> > > This bug was observed in 2022.07.05, using boost version 1.79.0.
> > > Similar behaviour was commented on before on online messageboards.
> > >
> > >
> > > Bug description
> > >
> > > When starting a new, detached process with Process.Spawn in a posix
> > system,
> > > if the parent process outlives the child, the child process remains in
> > > zombie state for the parent process' lifetime.
> > > The bug described above is demonstrated in the CMake project attached
> to
> > > this document.
> > >
> > >
> > > Analysis
> > >
> > > The spawn function injects the syscall "signal(SIGCHLD, SIG_IGN)" with
> a
> > > functor of type boost::process::detail::posix::sig_init_. This is done
> to
> > > the forked child process, to no avail. And this is not done (and should
> > not
> > > be done) in the parent process being too intrusive.
> > >
> > >
> > > Possible mitigation
> > >
> > > Introducing an in-between forked process that serves as the parent of
> the
> > > to-be-spawned process with SIGCHLD set to SIG_IGN would prevent the
> > spawned
> > > process to become zombie, and simultaneously does not disturb the
> parent
> > > process' signal handlers.
> > > Introducing another class alternative to sig_init_ with the
> functionality
> > > described above would be a reasonable approach. An implementation
> sketch
> > of
> > > the double fork method can be found in the attached project.
> > >
> > >
> > > System used
> > >
> > > OS: Ubuntu 18.04.6 LTS
> > > arch: x86_64
> > > compiler: gcc-7.5.0
> > > libc: libc-2.27
> > > boost: 1.79.0 (built from source)
> >
> > You should probably report bugs on GitHub:
> >
> > https://github.com/boostorg/process/issues
> >
> > In the bug, it is always desirable to post a small compilable code
> > sample that reproduces the issue.
> >
> > Regarding the proposed fix, it's not clear how introducing an
> > intermediate process would fix the parent not calling waitpid() or
> > equivalent. You'd just get a different zombie process.
> >
> > _______________________________________________
> > Unsubscribe & other changes:
> > http://lists.boost.org/mailman/listinfo.cgi/boost
> >
>
> _______________________________________________
> Unsubscribe & other changes:
> http://lists.boost.org/mailman/listinfo.cgi/boost
>