[Python-Dev] Showstopper

[Michael Hudson]

On Sun, 15 Apr 2001, Tim Peters wrote:

> We've got a showstopper bug involving gc and dicts.  It's not brand
> new, but was probably introduced when we fiddled PyDict_Next() to stop
> the dict resizing problems plaguing Jeremy.

Crap.

Two ideas suggest themselves: (1) don't have the resize check in
PyDict_Next (it could be in PyDict_SetItem instead, though the fact that
this is safe is delicate to say the least) (2) don't use PyDict_Next in
dict_traverse.

OTOH, the GC runs __del__ methods, right?  So what if a __del__ method
mutates the dictionary that .items() is being called on?  If allocating
memory can execute arbitrary Python code, I dread to think how many bugs
of this form are hiding in Python (actually it's only allocating
containers that's the worry, but still...).  On the third hand, I can't
trigger one deliberately, so maybe I'm talking nonsense.

To fix items/keys/values, you could build up the list of tuples first,
check you still have the right amount, then fill them in.

not-sure-this-is-helping-ly y'rs
M.