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Re: Problem with zombie processes

Erik Bray wrote:
On Fri, Feb 17, 2017 at 11:06 PM, Mark Geisert <REMOVED> wrote:
Erik Bray wrote:

On Tue, Feb 14, 2017 at 8:25 PM, Mark Geisert <XXXX@XXXXXX.XXX> wrote:

Please don't quote raw email addresses.  We try to avoid feeding spammers.

Sorry--normally replies on this ML are just back to the ML itself (my
preference as well) so I wasn't expecting it.

Reiterating this: Please manually remove quoted email addresses from your replies.

Erik Bray wrote:

The attached Python script


D'oh!  Here is the script.  It at least demonstrates the problem.


Thanks!  Running this script repeatedly on my system (Win7, 2 cores / 4 HT
threads) showed no differences between your Test 1 and Test 2.  Each Test
concludes in one of three ways, seemingly randomly: (1) read of
/proc/<pid>/stat succeeds and process status is displayed, (2) read fails
with Python IOError, (3) read apparently succeeds but there's no process
data displayed.

An strace of your script shows Python itself is calling wait4() to reap the
child process.  So, as Doug suggested on another thread, the script's
actions are just subject to the whims of process scheduling and vary from
run to run.

You're right.  The first time I was testing this, for whatever reason,
I was getting *very* consistent results.  Test 1 *always* succeeded
and test 2 always fails.  But trying it now, I am getting similar

What I was going by was the docs for ExitProcess [1] which states:

"Exiting a process does not necessarily remove the process object from
the operating system. A process object is deleted when the last handle
to the process is closed."

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().

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

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.


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