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

8 Jul 2022

      The following figure may clarify the idea i propose:

PARENT  -->fork()----------------------------------waitpid()--->
              \                                        /
               \         signal(SIGCHLD,SIG_IGN)      /
CHILD          fork()----signal(SIGHUP,SIG_IGN)----exit()   # this process
takes the burden of ignoring SIGCHLD from parent
                 \              setsid()
                  \
GRANDCHILD       execve()------>

Benedek Tass  ezt írta (időpont: 2022. júl. 7., Cs,
13:44):
...
Have you checked out the sketch i provided? It seems to me that it solves
the problem reasonably well.
Klemens Morgenstern via Boost  ezt írta (időpont:
2022. júl. 7., Cs, 13:22):
...
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@lists.boost.org>
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  ezt írta (időpont:
...
júl. 6., Sze, 17:28):
...
...
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
...
to-be-spawned process with SIGCHLD set to SIG_IGN would prevent the
spawned
process to become zombie, and simultaneously does not disturb the
On 7/6/22 16:28, Benedek Tass via Boost wrote:
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.
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