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

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&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. 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).