Problem with zombie processes

[Erik Bray]

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.
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