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
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.
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