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

From: Benedek Tass (tass.benedek_at_[hidden])
Date: 2022-07-06 13:28:48

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)

Best regards,
Benedek Tass