Fix ref counting of Python objects.


Fix ref counting of Python objects.

PythonObjects were being incorrectly ref-counted. This problem was
pervasive throughout the codebase, leading to an unknown number of memory
leaks and potentially use-after-free.

The issue stems from the fact that Python native methods can either return
"borrowed" references or "owned" references. For the former category, you
*must* incref it prior to decrefing it. And for the latter category, you
should not incref it before decrefing it. This is mostly an issue when a
Python C API method returns a PyObject to you, but it can also happen with
a method accepts a PyObject. Notably, this happens in PyList_SetItem,
which is documented to "steal" the reference that you give it. So if you
pass something to PyList_SetItem, you cannot hold onto it unless you
incref it first. But since this is one of only two exceptions in the
entire API, it's confusing and difficult to remember.

Our PythonObject class was indiscriminantely increfing every object it
received, which means that if you passed it an owned reference, you now
have a dangling reference since owned references should not be increfed.
We were doing this in quite a few places.

There was also a fair amount of manual increfing and decrefing prevalent
throughout the codebase, which is easy to get wrong.

This patch solves the problem by making any construction of a
PythonObject from a PyObject take a flag which indicates whether it is
an owned reference or a borrowed reference. There is no way to construct a
PythonObject without this flag, and it does not offer a default value,
forcing the user to make an explicit decision every time.

All manual uses of PyObject have been cleaned up throughout the codebase
and replaced with PythonObject in order to make RAII the predominant
pattern when dealing with native Python objects.

Differential Revision: http://reviews.llvm.org/D13617
Reviewed By: Greg Clayton


zturnerOct 13 2015, 11:16 AM
Differential Revision
D13617: Fix ref-counting of Python objects