[C++-sig] subprocess fork() sometimes hangs when called from within boost::python::exec_file

[Stefan Ring]

On Thu, Apr 2, 2015 at 9:12 PM, Peter Schüller <schueller.p at gmail.com> wrote:
> Dear all,
>
> In the hexhex project on github [1] we execute a python program using
> boost::python::exec_file, the respective code is in
> src/PythonPlugin.cpp in the function PythonPlugin::runPythonMain(...).
>
> In this python program I use subprocess.call() to run a shell
> application (actually graphviz).
>
> Sometimes it works, sometimes it hangs using 100% CPU. It seems to
> depend on how much the C++ program had to do before it executed the
> python part. In both cases the C++ program uses threads (is that
> relevant wrt. fork() in python? I found some hints online that it
> could be relevant).
>
> I traced it down to os.fork() in subprocess.py which returns in one
> case and does not return at all (neither parent nor child) in the
> other case.
>
> If I do strace -f on the process I get the following (the
> SIGPROF/rt_sigreturn/clone repeats until the disk is full):
>
> [pid 18646] clone(child_stack=0,
> flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD,
> child_tidptr=0x7f96a5fa2a50) = ? ERESTARTNOINTR (To be restarted)
> [pid 18646] --- SIGPROF {si_signo=SIGPROF, si_code=SI_KERNEL} ---
> [pid 18646] rt_sigreturn()              = 56
> [pid 18646] clone(child_stack=0,
> flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD,
> child_tidptr=0x7f96a5fa2a50) = ? ERESTARTNOINTR (To be restarted)
> [pid 18646] --- SIGPROF {si_signo=SIGPROF, si_code=SI_KERNEL} ---
> [pid 18646] rt_sigreturn()              = 56
> [pid 18646] clone(child_stack=0,
> flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD,
> child_tidptr=0x7f96a5fa2a50) = ? ERESTARTNOINTR (To be restarted)
> [pid 18646] --- SIGPROF {si_signo=SIGPROF, si_code=SI_KERNEL} ---
> [pid 18646] rt_sigreturn()              = 56
> [pid 18646] clone(child_stack=0,
> flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD,
> child_tidptr=0x7f96a5fa2a50) = ? ERESTARTNOINTR (To be restarted)
> [pid 18646] --- SIGPROF {si_signo=SIGPROF, si_code=SI_KERNEL} ---

There is no loop in Python's fork code, so it cannot really be blamed.
It sure looks like there is a pending signal which interferes for some
reason.