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

[Amador Pahim]

On Thu, Jul 20, 2017 at 5:09 PM, Markus Armbruster <address@hidden> wrote:
> Amador Pahim <address@hidden> writes:
>
>> On Thu, Jul 20, 2017 at 1:49 PM, Markus Armbruster <address@hidden> wrote:
>>> Amador Pahim <address@hidden> 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 <address@hidden>
>>>> Reviewed-by: Eduardo Habkost <address@hidden>
>>>> Reviewed-by: Fam Zheng <address@hidden>
>>>> ---
>>>>  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?

Yes, that can happen. This method is not returning an up-to-date
returncode, it's serving just as a wrapper to the attribute, being the
attribute updated or not.
I lack the background on why it was coded that way, but considering
the API-user perspective, I agree with you. We should return
self._popen.poll() here indeed. Fixing that.