On Mon, Feb 20, 2023 at 09:08:08PM +0200, Nir Soffer wrote:
On Mon, Feb 20, 2023 at 10:45 AM Laszlo Ersek
<lersek(a)redhat.com> wrote:
>
> On 2/17/23 17:52, Eric Blake wrote:
> > On Thu, Feb 16, 2023 at 03:09:02PM +0100, Laszlo Ersek wrote:
>
> >> - Py_BuildValue with the "O" format specifier transfers the new
list's
> >> *sole* reference (= ownership) to the just-built higher-level object
"args"
> >
> > Reference transfer is done with "N", not "O". That would
be an
> > alternative to decreasing the refcount of py_array on success, but not
> > eliminate the need to decrease the refcount on Py_BuildValue failure.
> >
> >>
> >> - when "args" is killed (decref'd), it takes care of
"py_array".
> >>
> >> Consequently, if Py_BuildValue fails, "py_array" continues owning
the
> >> new list -- and I believe that, if we take the new error branch, we leak
> >> the object pointed-to by "py_array". Is that the case?
> >
> > Not quite. "O" is different than "N".
>
> I agree with you *now*, looking up the "O" specification at
> <
https://docs.python.org/3/c-api/arg.html#building-values>.
>
> However, when I was writing my email, I looked up Py_BuildValue at that
> time as well, just elsewhere. I don't know where. Really. And then that
> documentation said that the reference count would *not* be increased. I
> distinctly remember that, because it surprised me -- I actually recalled
> an *even earlier* experience reading the documentation, which had again
> stated that "O" would increase the reference count.
Maybe here:
https://docs.python.org/2/c-api/arg.html#building-values
Looks like another incompatibility between python 2 and 3.
Or maybe misreading the wrong part of the page. PyArg_ParseTuple()
and Py_BuildValue() are listed on the same page, and intentionally use
similar-looking format strings, so you have to check _which_ part of
the page the "O" you are reading about is associated with. The first
"O" is under PyArg_ParseTuple() and friends, and intentionally does
not increase reference count (the usesr passed us an Object, we are
parsing it into a placeholder where we don't need to clean up after
ourselves unless we want to add a reference to the object to make it
last beyond the caller), the latter says that "O" does increase the
reference count (we are building up a larger object that will now be
an additional reference path into the object we are passing in).
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
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