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

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. So right now, I have three (inconsistent) memories: - original (old) memory: "O" increments the refcount - recent memory: "O" does not increment the refcount - your reminder: "O" does increment the refcount

I guess I must have misread something (I can't find the document now!). Sorry about that; I agree we need to drop the original py_array reference unconditionally. Laszlo