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Re: Problem with zombie processes

On Tue, Feb 21, 2017 at 12:58 PM Erik Bray wrote:
> On Mon, Feb 20, 2017 at 11:54 PM, Mark Geisert wrote:
> > Erik Bray wrote:
> >>
> >> On Mon, Feb 20, 2017 at 11:54 AM, Mark Geisert wrote:
> >>>>
> >>>> So my guess was that Cygwin might try to hold on to a handle to a
> >>>> child process at least until it's been explicitly wait()ed.  But that
> >>>> does not seem to be the case after all.
> >>>
> >>>
> >>>
> >>> You might have missed a subtlety in what I said above.  The Python
> >>> interpreter itself is calling wait4() to reap your child process.  Cygwin
> >>> has told Python one of its children has died.  You won't get the chance
> >>> to
> >>> wait() for it yourself.  Cygwin *does* have a handle to the process, but
> >>> it
> >>> gets closed as part of Python calling wait4().
> >>
> >>
> >> To be clear, wait4() is not called from Python until the script
> >> explicitly calls p.wait().
> >> In other words, when run this step by step (e.g. in gdb) I don't see a
> >> wait4() call until the point where the script explicitly waits().  I
> >> don't see any reason Python would do this behind the scenes.
> >
> >
> > You're right.  I missed the wait in your script and ASSumed too much of the
> > Python interpreter :-( .
> >
> >
> >>>> Anyways, I think it would be nicer if /proc returned at least partial
> >>>> information on zombie processes, rather than an error.  I have a patch
> >>>> to this effect for /proc/<pid>/stat, and will add a few more as well.
> >>>> To me /proc/<pid>/stat was the most important because that's the
> >>>> easiest way to check the process's state in the first place!  Now I
> >>>> also have to catch EINVAL as well and assume that means a zombie
> >>>> process.
> >>>
> >>>
> >>>
> >>> The file /proc/<pid>/stat is there until Cygwin finishes cleanup of the
> >>> child due to Python having wait()ed for it.  When you run your test
> >>> script,
> >>> pay attention to the process state character in those cases where you
> >>> successfully read the stat file.  It's often S (stopped, I think) or R
> >>> (running) but I also see Z (zombie) sometimes.  Your script is in a race
> >>> with Cygwin, and you cannot guarantee you'll see a killed process's state
> >>> before Cygwin cleans it up.
> >>>
> >>> One way around this *might* be to install a SIGCHLD handler in your
> >>> Python
> >>> script.  If that's possible, that should tell you when your child exits.
> >>
> >>
> >> Perhaps the Python script is a red herring.  I just wrote it to
> >> demonstrate the problem.  The difference between where I send stdout
> >> to is strange, but you're likely right that it just comes down to
> >> subtle timing differences.  Here's a C program that demonstrates the
> >> same issue more reliably.  Interestingly, it works when I run it in
> >> strace (probably just because of the strace overhead) but not when I
> >> run it normally.
> >>
> >> My point in all this is I'm confused why Cygwin would give up its
> >> handles to the Windows process before wait() has been called.
> >>
> >> (In fact, it's pretty confusing to have fopen returning EINVAL which
> >> according to [1] it should only be doing if the mode string were
> >> invalid.)
> >>
> >> Thanks,
> >> Erik
> >>
> >> [1] http://pubs.opengroup.org/onlinepubs/9699919799/functions/fopen.html
> >
> >
> > O.K., you may be on to something amiss in the Cygwin DLL.  Thanks for the
> > STC in C; that'll help somebody looking further at this.  I'm out of ideas.
> > It might be possible to reduce strace overhead somewhat by selecting a
> > smaller set of trace options than the default.
> Note: My previous test program had a bug in do_child() (not correctly
> terminating the argv array).  The attached program fixes the bug.
> I've also attached a (truncated) strace log from it.

With apologies for re-raising a 2 year old thread; I've finally been
back to working on my port of psutil [1].  I was getting some
confusing errors reading the /proc/[pid]/stat files of recently
created processes that had quickly become zombified.  I had completely
forgotten about this issue until I saw that trying to read the stat
file was resulting in EINVAL ("invalid argument") and something about
that ringed a bell.

So, I can confirm that this is still an issue.  Apparently I wrote
that I had a patch to Cygwin for this.  I have no idea where that
patch is but I'll look for it, or try to reproduce it.  I think the
idea for the patch was to at least make a zombie process's stat file
readable so that the status flag ("Z") can be read, and maybe fill the
remaining fields with 0.

Once I find and/or reproduce that patch I'll submit it to cygwin-patches.

[1] https://psutil.readthedocs.io/en/latest/

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