By the new Py_TPFLAGS_METHOD_DESCRIPTOR flag, a new code path is
activated, and when extension types like PySide create a new
class, we observe negative refcounts.

The reason is that the code in typeobject.c fkt. type_mro_modified
calls lookup_maybe_method which returns a _borrowed_ reference.
This happens in the "if (custom) {" branch.

Removing all Py_XDECREF calls from the function fixes that.

From the PR:

Christian, can you open the PR against master instead? We will backport the change to 3.8 after is merged.

No, this appears to be impossible.

The function "type_mro_modified" exists as well, but there is no
"if (custom) {" branch at all!

On 10.12.19 17:21, Pablo Galindo Salgado wrote:
> 
> Pablo Galindo Salgado <pablogsal@gmail.com> added the comment:
> 
>>From the PR:
> 
> Christian, can you open the PR against master instead? We will backport the change to 3.8 after is merged.
> 
> ----------
> nosy: +pablogsal
> 
> _______________________________________
> Python tracker <report@bugs.python.org>
> <https://bugs.python.org/issue39016>
> _______________________________________
>

> No, this appears to be impossible.

Oh, I see. Apologies then for the misunderstunding.

On 10.12.19 17:57, Pablo Galindo Salgado wrote:
> 
> Pablo Galindo Salgado <pablogsal@gmail.com> added the comment:
> 
>> No, this appears to be impossible.
> 
> Oh, I see. Apologies then for the misunderstunding.

Well, but I think that is weird, too!
Why should that custom clause be in 3.8 but not in master?

On 10.12.19 17:57, Pablo Galindo Salgado wrote:
> 
> Pablo Galindo Salgado <pablogsal@gmail.com> added the comment:
> 
>> No, this appears to be impossible.
> 
> Oh, I see. Apologies then for the misunderstunding.

No problem! You could as well have been right.
I tried to move the patch to master and realized only then
that there was nothing similar. Interesting btw., I should
investigate when this divergence was introduced.

Best -- Chris

lookup_maybe_method should not return a borrowed reference. It increfs its return value.

On 11.12.19 14:13, Petr Viktorin wrote:
> 
> Petr Viktorin <encukou@gmail.com> added the comment:
> 
> lookup_maybe_method should not return a borrowed reference. It increfs its return value.

At second sight, this seems to be true. No idea why I
was so convinced. Need to sleep and look again..

The goal now should be to find a reasonably small reproducer.

I'm trying to compile PySide to see what it does, but it's a big project and I'm a bit lost.
Christian, you said in an e-mail that you have a workaround -- unsetting the Py_TPFLAGS_METHOD_DESCRIPTOR. Could you point me to the commit with that workaround, so I could try exploring the code in context?

On 11.12.19 17:37, Petr Viktorin wrote:
> 
> Petr Viktorin <encukou@gmail.com> added the comment:
> 
> The goal now should be to find a reasonably small reproducer.
> 
> I'm trying to compile PySide to see what it does, but it's a big project and I'm a bit lost.
> Christian, you said in an e-mail that you have a workaround -- unsetting the Py_TPFLAGS_METHOD_DESCRIPTOR. Could you point me to the commit with that workaround, so I could try exploring the code in context?

Yes, it went into gerrit here:

https://codereview.qt-project.org/c/pyside/pyside-setup/+/282705

The problem is that during PySide type creation the PyType_Ready
function looks into the mro() method, which uses this new
flag. When I temporary remove that, everything works.

Cheers -- Chris

Thanks!

(note: unfortunately I'll probably not have time for CPython until next week, but further investigation is on my list.)

Sorry, I believe I was wrong and lookup_maybe_method does not return a borrowed reference. _PyType_Lookup does and I was confused.
Closing that for now.