Re: [Qemu-devel] [PATCH v3 1/3] qemu.py: fix is_running()

[Markus Armbruster <armbru@redhat.com>]

Amador Pahim <apahim@redhat.com> writes:

> On Thu, Jul 20, 2017 at 1:49 PM, Markus Armbruster <armbru@redhat.com> wrote:
>> Amador Pahim <apahim@redhat.com> writes:
>>
>>> Current implementation is broken. It does not really test if the child
>>> process is running.
>>
>> What usage exactly is broken by this?  Got a reproducer for me?
>
> Problem is that 'returncode' is not set without a calling
> poll()/wait()/communicate(), so it's only useful to test if the
> process is running after such calls. But if we use 'poll()' instead,
> it will, according to the docs, "Check if child process has
> terminated. Set and return returncode attribute."
>
> Reproducer is:
>
>  >>> import subprocess
>  >>> devnull = open('/dev/null', 'rb')
>  >>> p = subprocess.Popen(['qemu-system-x86_64', '-broken'],
> stdin=devnull, stdout=devnull, stderr=devnull, shell=False)
>  >>> print p.returncode
>  None
>  >>> print p.poll()
>  1
>  >>> print p.returncode
>  1
>
>>> The Popen.returncode will only be set after by a poll(), wait() or
>>> communicate(). If the Popen fails to launch a VM, the Popen.returncode
>>> will not turn to None by itself.
>>
>> Hmm.  What is the value of .returncode then?
>
> returncode starts with None and becomes the process exit code when the
> process is over and one of that three methods is called (poll(),
> wait() or communicate()).
>
> There's an error in my description though. The correct would be: "The
> Popen.returncode will only be set after a call to poll(), wait() or
> communicate(). If the Popen fails to launch a VM, the Popen.returncode
> will not turn from None to the actual return code by itself."

Suggest to add ", and is_running() continues to report True".

>>> Instead of using Popen.returncode, let's use Popen.poll(), which
>>> actually checks if child process has terminated.
>>>
>>> Signed-off-by: Amador Pahim <apahim@redhat.com>
>>> Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
>>> Reviewed-by: Fam Zheng <famz@redhat.com>
>>> ---
>>>  scripts/qemu.py | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/scripts/qemu.py b/scripts/qemu.py
>>> index 880e3e8219..f0fade32bd 100644
>>> --- a/scripts/qemu.py
>>> +++ b/scripts/qemu.py
>>> @@ -86,7 +86,7 @@ class QEMUMachine(object):
>>>              raise
>>>
>>>      def is_running(self):
>>> -        return self._popen and (self._popen.returncode is None)
>>> +        return self._popen and (self._popen.poll() is None)
>>>
>>>      def exitcode(self):
>>>          if self._popen is None:
>>                return None
>>            return self._popen.returncode
>>
>> Why is this one safe?
>
> Here it's used just to retrieve the value from the Popen.returncode.
> It's not being used to check whether the process is running or not.

If self._popen is not None, we return self._popen.returncode.  It's None
if .poll() etc. haven't been called.  Can this happen?  If not, why not?
If yes, why is returning None then okay?