Current refcount tracking idea seems to be incomplete for borrowed references

[JustAMan]

I think that I understand correctly that currently what you changed is basically removed global locking and made inc- and decref's be atomic operations. If that is true then there's (I think) a possible data race issue when an object is borrowed.

Consider the following idea of example:

1. Code borrows an object from some other object, say, takes a reference to the first item of a list, e.g. x = PyList_GetItem(lst, 0)
2. Before that code could call an incref on x some other parallel thread at that moment replaces first item of that list with something else, e.g. PyList_SetItem(lst, 0, y).
3. That setting calls a decref on x which might be called before first thread calls its incref.
4. This could lead to x reaching zero refcount and being destroyed, and then first thread tries to increase refcount for x. Bam! Segfault :(

I think that probability for this issue is really low, but it's possible to exist.
I don't think I have any ideas on how to fix this issue, though, so I'll just drop a note here and maybe someone else would have some idea.

To sum up, there's a possibility of a data race leading to possible crash because using some API that borrows a reference and then incrementing a reference is not atomic.