[Python-Dev] popen2.py strangeness

[Guido van Rossum]

> I just noticed the following behavior (current CVS):
> 
> + python
> Python 2.3b1+ (#37, Jun  2 2003, 11:34:36)
> [GCC 3.2 20020903 (Red Hat Linux 8.0 3.2-7)] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
> >>> import popen2
> >>> a = popen2.Popen3('sleep 1', 1, -1)
> >>> # wait more than one second to give the process time to exit
> ...
> >>> b = popen2.Popen3('sleep 1', 1, -1)
> >>> a.wait()
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
>   File "/ufs/sjoerd/src/python/Lib/popen2.py", line 86, in wait
>     pid, sts = os.waitpid(self.pid, 0)
> OSError: [Errno 10] No child processes
> >>>
> 
> I would expect the a.wait() to return the exit status (or None) of the
> first "sleep 1" command, but instead I get a traceback.
> 
> The reason is clear: in popen2.Popen3.__init__ the very first thing that
> happens is a call to _cleanup() which does a poll() call on all active
> instances.  If the process has died, the os.waitpid() call that poll()
> does removes it from the OS's process table, so that a next os.waitpid()
> on the same process id will fail.
> 
> I understand why the call to _cleanup() happens: defunct processes need
> to be cleaned up, and if you only ever use the popen[234] factory
> functions, the library needs to call os.waitpid for the created
> processes.
> 
> However, it seems to me the above sequence of events is legitimate and
> should be supported.  The question is: how?

Call os.waitpid() with an explicit pid argument?

--Guido van Rossum (home page: http://www.python.org/~guido/)