Re: [Libguestfs] [PATCH 1/2] python: Avoid crash if callback parameters cannot be built

On Mon, Feb 20, 2023 at 10:45 AM Laszlo Ersek <lersek(a)redhat.com&gt; 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.