class IntWithDict:
    def __init__(self, **kwargs):
        self.kwargs = kwargs
    def __index__(self):
        self.kwargs.clear()
        L = [2**i for i in range(10000)]
        return 0
x = IntWithDict(dont_inherit=float())
compile("", "", "", x, **x.kwargs)


The above crashes CPython due to the usage of borrowed references in _PyStack_UnpackDict(): the dict x.kwargs contains the only reference to the float() object stored in x.kwargs

When parsing the arguments, x.__int__() is called, which clears the dict, removing the only reference to that float()

Ideally, this would be fixed together with #36904.

The idea of #36904 could be used here: define a special kind of tuple, which is like an ordinary tuple followed by a C array of PyObject* entries (all refcounted), terminated by a NULL to know where it ends. A special deallocation function would decref all entries.

New changeset 77aa396bb9415428de09112ddf6b34bb843811eb by Petr Viktorin (Jeroen Demeyer) in branch 'master':
bpo-36907: fix refcount bug in _PyStack_UnpackDict() (GH-13381)
https://github.com/python/cpython/commit/77aa396bb9415428de09112ddf6b34bb843811eb

Jeroen, do you want to also do a backport for 3.7?

> Jeroen, do you want to also do a backport for 3.7?

Don't we have a bot for that?

We do, but here the test will need to be changed:

Python 3.7.3+ (heads/3.7:791e5fcbab, May 22 2019, 13:37:27) 
[GCC 9.1.1 20190503 (Red Hat 9.1.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> class IntWithDict:
...     def __init__(self, **kwargs):
...         self.kwargs = kwargs
...     def __index__(self):
...         self.kwargs.clear()
...         return 0
... 
>>> x = IntWithDict(dont_inherit=float())
>>> compile("", "", "", x, **x.kwargs)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: an integer is required (got type IntWithDict)

Using __int__ instead of __index__ works. PR coming right away.

New changeset d092caf096fa48baadfc0900792206bb5aa0192d by Petr Viktorin (Jeroen Demeyer) in branch '3.7':
bpo-36907: fix refcount bug in _PyStack_UnpackDict() (GH-13381) (GH-13493)
https://github.com/python/cpython/commit/d092caf096fa48baadfc0900792206bb5aa0192d