[libvirt] [PATCH python] override: Return NULL on python failure in getCPUModelNames

[Eric Blake]

On 03/20/2014 11:58 AM, Cole Robinson wrote:
> Eric pointed this out on the last patch, but I pushed it before noticing
> his message.
> ---
>  libvirt-override.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/libvirt-override.c b/libvirt-override.c
> index 71e241e..2532400 100644
> --- a/libvirt-override.c
> +++ b/libvirt-override.c
> @@ -2350,7 +2350,7 @@ done:
>  
>  error:
>      Py_XDECREF(rv);
> -    rv = VIR_PY_NONE;
> +    rv = NULL;
>      goto done;

ACK.  We probably have other code not quite doing the right thing; maybe
it's time to audit the code base.

The rule of thumb (which took me some time to learn) is:

If libvirt failed, we have a thread-local libvirt error set.  Return a
non-NULL sentinel value (-1 or None, depending on whether the interface
normally returns an integer or a python struct) so that the wrapper code
will then raise a libvirt exception class object.

If python failed, we either encountered OOM, or there was some other
problem with the user's input (perhaps the user passed in a python
object that can't be converted to the type we expect), and we don't have
any thread-local libvirt error set.  Return NULL so that python will
immediately raise a python exception class object without even getting
to our wrapper.

Returning NULL when there is no thread-local python exception ready to
raise, or returning non-NULL when there is no thread-local libvirt error
to convert to a libvirt exception object, or returning the wrong type of
sentinel, results in broken code.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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